Patent Publication Number: US-9893351-B2

Title: Battery, negative electrode active material, and electric tool

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 14/726,811, filed on Jun. 1, 2015 now U.S. Pat. No. 9,225,014, which is a continuation of U.S. patent application Ser. No. 13/526,133, filed on Jun. 18, 2012, now U.S. Pat. No. 9,048,485, which claims priority to Japanese Priority Patent Application JP 2011-141005 filed in the Japan Patent Office on Jun. 24, 2011, the entire content of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     The present application relates to a negative electrode for a lithium-ion secondary battery, the negative electrode containing a negative-electrode active material capable of occluding and releasing lithium ions, a lithium-ion secondary battery including the negative electrode, a battery pack, an electric vehicle, a power storage system, an electric tool, and an electronic device, which include the secondary battery. 
     In recent years, electronic devices typified by, for example, cellular phones and personal digital assistants (PDAs) have been widely used. Further size and weight reduction and longer life of electronic devices have been strongly demanded. Thus, there have been advances in the development of batteries serving as power sources, in particular, small and lightweight secondary batteries having a high energy density. Recently, various applications of secondary batteries to, for example, battery packs, electric vehicles, such as electric automobiles, power storage systems, such as power servers for household use, and electric tools, such as electric drills, as well as electronic devices described above have been studied. 
     Secondary batteries using various charge-discharge principles have been reported. In particular, lithium-ion secondary batteries that utilize the occlusion and release of lithium ions hold great promise because they have higher energy densities than lead-acid batteries, nickel-cadmium batteries, and other batteries. 
     A lithium-ion secondary battery includes a positive electrode, a negative electrode, and an electrolytic solution. The negative electrode contains a negative-electrode active material capable of occluding and releasing lithium ions. As the negative-electrode active material, a carbon material, such as graphite, is widely used. Recently, secondary batteries have been required to have higher battery capacities. Thus, the use of Si has been studied. The theoretical capacity of Si (4199 mAh/g) is much higher than the theoretical capacity of graphite (372 mAh/g), so the battery capacity should be significantly improved. 
     However, the use of Si as a negative-electrode active material causes extreme expansion and contraction of the negative-electrode active material during charge and discharge, so that the negative-electrode active material is liable to be cracked mainly in the vicinity of its surface. When the negative-electrode active material is cracked, a high-reactive newly-formed surface (an active surface) is formed, thereby increasing the surface area (reactive area) of the negative-electrode active material. As a result, the decomposition reaction of an electrolytic solution occurs on the newly-formed surface. The electrolytic solution is consumed to form a coating film derived from the electrolytic solution on the newly-formed surface. Thus, battery characteristics, such as cycle characteristics, are liable to decrease. 
     Thus, in order to improve battery characteristics, such as cycle characteristics, various configurations of the lithium-ion secondary batteries have been studied. 
     Specifically, to improve cycle characteristics and safety, Si and amorphous SiO 2  are simultaneously deposited by a sputtering method (for example, see Japanese Unexamined Patent Application Publication No. 2001-185127). To obtain excellent battery capacity and safety performance, electron-conductive material layers (carbon material) are arranged on surfaces of SiO x  particles (for example, see Japanese Unexamined Patent Application Publication No. 2002-042806). To improve high rate charge-discharge characteristics and cycle characteristics, a negative-electrode active material layer containing Si and O is formed in such a manner that the oxygen content is increased with decreasing distance from a negative-electrode collector (for example, see Japanese Unexamined Patent Application Publication No. 2006-164954). To improve cycle characteristics, a negative-electrode active material layer containing Si and O is formed in such a manner that the average oxygen content in the whole negative-electrode active material layer is 40 atomic percent or less and that the average oxygen content is increased with decreasing distance from a negative-electrode collector (for example, see Japanese Unexamined Patent Application Publication No. 2006-114454). In this case, a difference in average oxygen content between a portion near the negative-electrode collector and a portion remote from the negative-electrode collector is in the range of 4 atomic percent to 30 atomic percent. 
     To improve initial charge-discharge characteristics and the like, a nano-composite including a Si phase, SiO 2 , and metal oxide M y O is used (for example, see Japanese Unexamined Patent Application Publication No. 2009-070825). To improve cycle characteristics, powdered SiO x  (0.8≦x≦1.5, particle size range: 1 μm to 50 μm) and a carbonaceous material are mixed and fired at 800° C. to 1600° C. for 3 hours to 12 hours (for example, see Japanese Unexamined Patent Application Publication No. 2008-282819). To shorten an initial charge time, a negative-electrode active material expressed as Li a SiO x  (0.5≦a−x≦1.1 and 0.2≦x≦1.2) is used (for example, see International Publication No. WO2007/010922). In this case, Li is deposited by evaporation on an active material precursor containing Si and O. To improve charge-discharge cycle characteristics, the composition of SiO x  is controlled in such a manner that the molar ratio of the 0 content to the Si content of a negative-electrode active material is in the range of 0.1 to 1.2 and that a difference between the maximum value and the minimum value of the molar ratio of the 0 content to the Si content in the vicinity of a boundary between the negative-electrode active material and a current collector is 0.4 or less (for example, see Japanese Unexamined Patent Application Publication No. 2008-251369). To improve load characteristics, a Li-containing porous metal oxide (Li x SiO: 2.1≦x≦4) is used (for example, Japanese Unexamined Patent Application Publication No. 2008-177346). 
     To improve charge-discharge cycle characteristics, a hydrophobic layer of a silane compound, a siloxane compound, or the like is formed on a thin film containing Si (for example, see Japanese Unexamined Patent Application Publication No. 2007-234255). To improve cycle characteristics, a conductive powder in which surfaces of SiO x  (0.5≦x&lt;1.6) particles are covered with graphite coating films is used (for example, see Japanese Unexamined Patent Application Publication No. 2009-212074). In this case, on Raman spectroscopy analysis, each graphite coating film develops broad peaks at 1330 cm −1  and 1580 cm −1  Raman shift, and an intensity ratio I 1330 /I 1580  is 1.5&lt;I 1330 /I 1580 &lt;3. To improve a battery capacity and cycle characteristics, a powder including 1% by mass to 30% by mass of particles is used, the particles each having a structure in which Si microcrystals (crystal size: 1 nm to 500 nm) are dispersed in SiO 2  (for example, see Japanese Unexamined Patent Application Publication No. 2009-205950). In this case, in a particle size distribution by a laser diffraction/scattering type particle size distribution measurement method, the 90% accumulated diameter (D90) of the power is 50 μm or less, and the particle diameters of the particles are less than 2 To improve cycle characteristics, SiO x  (0.3≦x≦1.6) is used, and an electrode unit is pressurized at a pressure of 3 kgf/cm 2  or more during charge and discharge (for example, see Japanese Unexamined Patent Application Publication No. 2009-076373). To improve overcharge characteristics, over-discharge characteristics, and the like, an oxide of silicon with a silicon-oxygen atomic ratio of 1:y (0&lt;y&lt;2) is used (for example, see Japanese Patent No. 2997741). 
     Furthermore, in order to electrochemically accumulate or release a large amount of lithium ions, an amorphous metal oxide is provided on surfaces of primary particles of Si or the like (for example, see Japanese Unexamined Patent Application Publication No. 2009-164104). The Gibbs free energy when the metal oxide is formed by oxidation of a metal is lower than the Gibbs free energy when Si or the like is oxidized. To achieve a high capacity, high efficiency, a high operating voltage, and long lifetime, it is reported that a negative-electrode material in which the oxidation numbers of Si atoms satisfy predetermined requirements is used (for example, see Japanese Unexamined Patent Application Publication No. 2005-183264). The negative-electrode material contains Si with an oxidation number of zero, a Si compound having a Si atom with an oxidation number of +4, and a lower oxide of Si having a silicon atoms with oxidation numbers of more than zero and less than +4. 
     SUMMARY 
     Electronic devices and so force have higher performance and more functions and are more frequently used. Thus, lithium-ion secondary batteries tend to be frequently charged and discharged. Hence, lithium-ion secondary batteries are required to have further improved battery characteristics. 
     It is desirable to provide a negative electrode for a lithium-ion secondary battery, the negative electrode providing excellent battery characteristics, a lithium-ion secondary battery, a battery pack, an electric vehicle, a power storage system, an electric tool, and an electronic device. 
     A negative electrode for a lithium-ion secondary battery according to an embodiment of the present application includes an active material, in which the active material includes a core portion capable of occluding and releasing lithium ions, and a covering portion arranged on at least part of a surface of the core portion, in which the covering portion contains, as constituent elements, Si, O, and at least one element M1 selected from Li, C, Mg, Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Ge, Zr, Mo, Ag, Sn, Ba, W, Ta, Na, and K, and the atomic ratio y (O/Si) of O to Si is 0.5≦y≦1.8. A lithium-ion secondary battery according to an embodiment of the present application includes a positive electrode, a negative electrode, and an electrolytic solution, in which the negative electrode contains the same active material as that of the foregoing negative electrode for a lithium-ion secondary battery. A battery pack, an electric vehicle, a power storage system, an electric tool, or an electronic device according to an embodiment of the present application includes the lithium-ion secondary battery according to an embodiment of the present application. 
     In the negative electrode for a lithium-ion secondary battery or the lithium-ion secondary battery according to an embodiment of the present application, the active material of the negative electrode includes the covering portion on the surface of the core portion, in which the covering portion contains, as constituent elements, Si, O, and element M1, such as Li, and the atomic ratio y of O to Si is 0.5≦y≦1.8. It is thus possible to obtain excellent battery characteristics. Furthermore, for the battery pack, the electric vehicle, the power storage system, the electric tool, or the electronic device according to an embodiment of the present application, it is possible to obtain the same effect. 
     Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a sectional view illustrating a structure of a negative electrode for a lithium-ion secondary battery according to an embodiment of the present application; 
         FIG. 2  is a photograph illustrating a sectional structure of a negative-electrode active material, the photograph being taken with a scanning electron microscope (SEM); 
         FIG. 3  is a sectional view illustrating a structure of a lithium-ion secondary battery (prismatic type) according to an embodiment of the present application; 
         FIG. 4  is a sectional view taken along line IV-IV in  FIG. 3  that illustrates the lithium-ion secondary battery; 
         FIG. 5  is a schematic plan view illustrating structures of a positive electrode and a negative electrode illustrated in  FIG. 4 ; 
         FIG. 6  is a sectional view illustrating a structure of a lithium-ion secondary battery (cylindrical type) according to an embodiment of the present application; 
         FIG. 7  is a partially enlarged sectional view of a spirally wound electrode illustrated in  FIG. 6 ; 
         FIG. 8  is an exploded perspective view illustrating a structure of a lithium-ion secondary battery (laminated-film type) according to an embodiment of the present application; 
         FIG. 9  is an enlarged cross-sectional view taken along line IX-IX in  FIG. 8  that illustrates a spirally wound electrode; 
         FIG. 10  is a block diagram illustrating the configuration of an application example (battery pack) of a lithium-ion secondary battery; 
         FIG. 11  is a block diagram illustrating the configuration of an application example (electric vehicle) of a lithium-ion secondary battery; 
         FIG. 12  is a block diagram illustrating the configuration of an application example (power storage system) of a lithium-ion secondary battery; and 
         FIG. 13  is a block diagram illustrating the configuration of an application example (electric tool) of a lithium-ion secondary battery. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present application will be described in detail below with reference to the attached drawings. Descriptions are made in the order listed below: 
     1. negative electrode for lithium-ion secondary battery, 
     2. lithium-ion secondary battery, 
     2-1. prismatic type, 
     2-2. cylindrical type, 
     2-3. laminated-film type, 
     3. application of lithium-ion secondary battery, 
     3-1. battery pack, 
     3-2. electric vehicle, 
     3-3. power storage system, and 
     3-4. electric tool. 
     1. Negative Electrode for Lithium-Ion Secondary Battery 
       FIG. 1  illustrates a sectional structure of a negative electrode for a lithium-ion secondary battery (hereinafter, referred to simply as a “negative electrode”) according to an embodiment of the present application.  FIG. 2  is a SEM photograph illustrating the cross-sectional structure of an active material contained in the negative electrode (negative-electrode active material). 
     Overall Structure of Negative Electrode 
     The negative electrode includes, for example, as illustrated in  FIG. 1 , negative-electrode active material layers  2  on a negative-electrode collector  1 . For this negative electrode, the negative-electrode active material layers  2  may be arranged on both surfaces of the negative-electrode collector  1 . Alternatively, one negative-electrode active material layer may be arranged on only one surface of the collector. Furthermore, the negative-electrode collector  1  may not be arranged. 
     Negative-Electrode Collector 
     The negative-electrode collector  1  is composed of, for example, a conductive material having excellent electrochemical stability, electrical conductivity, and mechanical strength. Examples of the conductive material include Cu, Ni, and stainless steel. In particular, a material which does not form an intermetallic compound with Li and which can be alloyed with a material constituting the negative-electrode active material layers  2  is preferred. 
     The negative-electrode collector  1  preferably contains, as constituent elements, C and S. The reason for this is that the physical strength of the negative-electrode collector  1  is improved, so that the negative-electrode collector  1  is less likely to be deformed even if the negative-electrode active material layers  2  expand and contract during charge and discharge. An example of the negative-electrode collector  1  is metal foil doped with C and S. The C content and the S content are not particularly limited and are each preferably 100 ppm or less because a higher effect is obtained. 
     The negative-electrode collector  1  may have a roughened surface or may not have a roughened surface. An example of the negative-electrode collector  1  having an unroughened surface is rolled metal foil. An example of the negative-electrode collector  1  having a roughened surface is metal foil that has been subjected to electrolytic treatment or sandblasting. The electrolytic treatment is a method in which fine particles are formed on a surface of metal foil or the like in an electrolytic bath by an electrolytic process to produce irregularities. Metal foil produced by the electrolytic process is commonly referred to as electrolytic foil (e.g., electrolytic Cu foil). 
     The negative-electrode collector  1  preferably has a roughened surface because the adhesion of the negative-electrode active material layers  2  to the negative-electrode collector  1  is improved by an anchor effect. The surface roughness (e.g., ten-point height of irregularities Rz) of the negative-electrode collector  1  is not particularly limited and is preferably maximized in order to improve the adhesion of the negative-electrode active material layers  2  by the anchor effect. However, an excessively high surface roughness may result in a reduction in the adhesion of the negative-electrode active material layers  2 . 
     Negative-Electrode Active Material Layer 
     Each of the negative-electrode active material layers  2  contains a plurality of particles of a negative-electrode active material  200  capable of occluding and releasing lithium ions as illustrated in  FIG. 2 . If necessary, each of the negative-electrode active material layers  2  may further contain an additional material, for example, a negative-electrode binder or a negative-electrode conductive agent. 
     The negative-electrode active material  200  includes a core portion  201  capable of occluding and releasing lithium ions and a covering portion  202  arranged on the core portion  201 . The state in which the core portion  201  is covered with the covering portion  202  can be confirmed by SEM observation as illustrated in  FIG. 2 . 
     Core Portion 
     The composition of the core portion  201  is not particularly limited as long as the core portion  201  is capable of occluding and releasing lithium ions. In particular, the core portion  201  preferably contains, as a constituent element, at least one of Si and Sn because a high energy density is provided. The core portion  201  may contain Si in elemental form, a Si compound, a Si alloy, or two or more thereof. Similarly, Sn in elemental form, a Sn compound, or a Sn alloy may be contained. The term “elemental form” refers to an elemental form in a general sense (the element may contain minute quantities of impurities (elements other than oxygen)) and does not necessarily indicates a purity of 100%. 
     For example, the Si alloy contains Si and one or two or more elements selected from Sn, Ni, Cu, Fe, Co, Mn, Zn, In, Ag, Ti, Ge, Bi, Sb, Cr, and so forth. For example, the Si compound contains Si and one or two or more elements selected from C, O, and so forth. The Si compound may further contain one or two or more elements described in the Si alloy. Examples of the Si alloy and the Si compound include SiB 4 , SiB 6 , Mg 2 Si, Ni 2 Si, TiSi 2 , MoSi 2 , CoSi 2 , NiSi 2 , CaSi 2 , CrSi 2 , Cu 5 Si, FeSi 2 , MnSi 2 , NbSi 2 , TaSi 2 , VSi 2 , WSi 2 , ZnSi 2 , SiC, Si 3 N 4 , Si 2 N 2 O, SiO v  (0&lt;v≦2), and LiSiO. 
     For example, the Sn alloy contains Sn and one or two or more elements selected from Si, Ni, Cu, Fe, Co, Mn, Zn, In, Ag, Ti, Ge, Bi, Sb, Cr, and so forth. For example, the Sn compound contains Sn and one or two or more elements selected from C, O, and so forth. The Sn compound may further contain one or two or more elements described in the Sn alloy. Examples of the Sn alloy and the Sn compound include SnO w  (0&lt;w≦2), SnSiO 3 , LiSnO, Mg 2 Sn, SnCo, SnCoTi, and SnFeCo. 
     Preferably, the core portion  201  contains, for example, Si and O serving as constituent elements, and the atomic ratio x of O to Si, i.e., O/Si, is 0≦x&lt;0.5. The reason for this is that the core portion  201  easily occludes and releases lithium ions during charge and discharge and that a high battery capacity is obtained owing to a reduction in irreversible capacity, compared with the case where the atomic ratio x is outside the range (0.5≦x). 
     As is apparent from the foregoing composition (the atomic ratio x), the core portion  201  may be composed of elemental Si (x=0) or SiO x  (0&lt;x&lt;0.5). Note that x is preferably minimized. More preferably, x=0 (elemental Si). The reason for this is that a higher energy density is obtained and that the discharge capacity is less likely to decrease from the early stage of the charge-discharge cycle because the degradation of the core portion  201  is inhibited. 
     The core portion  201  may have a crystal structure (with high or low crystallinity) or an amorphous structure. The core portion  201  preferably has a crystal structure with high or low crystallinity and more preferably with high crystallinity. This is because the core portion  201  easily occludes and releases lithium ions during charge and discharge to achieve high battery capacity and so forth and because the core portion  201  is less likely to expand and contract during charge and discharge. In particular, in the core portion  201 , the half-width (2θ) of a diffraction peak attributed to the silicon (111) crystal face observed by X-ray diffraction is preferably 20° or less, and the size of a crystallite attribute to the (111) crystal face is preferably 10 nm or more. This is because a higher effect is provided. 
     The median diameter of the core portion  201  is not particularly limited. In particular, the core portion  201  preferably has a median diameter of 0.3 μm to 20 μm because the core portion  201  easily occludes and releases lithium ions during charge and discharge and because the core portion  201  is not easily broken. More particularly, a median diameter of less than 0.3 μm can facilitate expansion and contraction during charge and discharge because of an excessively large total surface area of the core portion  201 . A median diameter exceeding 20 μm is liable to lead to a break of the core portion  201  during charge and discharge. 
     The core portion  201  may contain, as a constituent element, one or two or more additional elements (excluding Si and Sn), together with Si and Sn. 
     Specifically, the core portion  201  preferably contains, as a constituent element, at least one element M2 selected from Fe and Al. Note that the ratio of M2 to Si and O, i.e., M2/(Si+O), is preferably in the range of 0.01 atomic percent to 50 atomic percent because the electrical resistance of the core portion  201  is reduced and because the diffusibility of is improved. 
     In the core portion  201 , M2 may be present (in the free state) independently of Si and O or may be combined with at least one of Si and O to form an alloy or a compound. The composition (e.g., the bonding state of M2) of the core portion  201  including M2 can be identified by, for example, energy dispersive x-ray analysis (EDX). The bonding state and the identification method of M3 and M4 described below are the same as described above. 
     In particular, the core portion  201  preferably contains Al because the core portion  201  has low crystallinity, so that the core portion  201  is less likely to expand and contract during charge and discharge and the diffusibility of lithium ions is further improved. In the core portion  201  containing Al, the half-width (2θ) of a diffraction peak attributed to the Si(111) crystal face observed by X-ray diffraction is preferably 0.6° or more. The size of a crystallite attribute to the (111) crystal face is preferably 90 nm or less. In the case where the half-width is investigated, preferably, the covering portion  202  is removed by dissolution with HF or the like, and then the core portion  201  is analyzed. 
     More particularly, in the case where the core portion  201  does not contain Al and where the core portion  201  has high crystallinity, the core portion  201  easily expands and contracts during charge and discharge. In contrast, in the case where the core portion  201  contains Al, the core portion  201  is less likely to expand and contract during charge and discharge regardless of whether the core portion  201  has high or low crystallinity. In this case, when the core portion  201  has low crystallinity, the expansion and contraction of the core portion  201  are inhibited, and the diffusibility of lithium ions is improved. 
     The core portion  201  preferably contains, as a constituent element, at least one element M3 selected from Cr and Ni. Note that the ratio of M3 to Si and O, i.e., M3/(Si+O), is preferably in the range of 1 atomic percent to 50 atomic percent. Also in this case, the electrical resistance of the core portion  201  is reduced, and the diffusibility of lithium ions is improved. 
     The core portion  201  preferably contains, as a constituent element, at least one element M4 selected from B, Mg, Ca, Ti, V, Mn, Co, Cu, Ge, Y, Zr, Mo, Ag, In, Sn, Sb, Ta, W, Pb, La, Ce, Pr, and Nd. Note that the ratio of M4 to Si and O, i.e., M4/(Si+O), is preferably in the range of 0.01 atomic percent to 30 atomic percent. Also in this case, the electrical resistance of the core portion  201  is reduced, and the diffusibility of lithium ions is improved. 
     Covering Portion 
     The covering portion  202  is arranged on at least part of a surface of the core portion  201 . Thus, the covering portion  202  may cover part of the surface of the core portion  201  or may cover the entire surface of the core portion  201 . In the case of the former, the covering portion  202  may be arranged on a plurality of portions of a surface of the core portion  201  and may cover the portions. 
     The covering portion  202  contains, as constituent elements, Si, O, and element M1 that can form an alloy with Si. Element M1 is at least one selected from Li, C, Mg, Al, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Ge, Zr, Mo, Ag, Sn, Ba, W, Ta, Na, and K. The atomic ratio y of O to Si, i.e., O/Si, is 0.5≦y≦1.8, preferably 0.7≦y≦1.3, and more preferably y=1.2. This is because the protective function of the covering portion  202  described below is effectively provided. 
     The reason the atomic ratio y is within the range described above is that the degradation of the negative-electrode active material  200  is suppressed when charge and discharge are repeated compared with the case where the atomic ratio y is outside the range (i.e., y&lt;0.5, or y&gt;1.8). In this case, the core portion  201  chemically and physically protects the core portion  201  while the entry and exit of lithium ions from the core portion  201  are ensured. 
     More particularly, in the case where the covering portion  202  intervenes between the core portion  201  and an electrolytic solution, the highly reactive core portion  201  is less likely to come into contact with the electrolytic solution, thus inhibiting the decomposition reaction of the electrolytic solution. In this case, when the covering portion  202  is composed of a material similar to a material, which contains common Si as a constituent element, constituting the core portion  201 , the adhesion of the covering portion  202  to the core portion  201  is increased. 
     The covering portion  202  is flexible (easily deformable). Thus, when the core portion  201  expands and contracts during charge and discharge, the covering portion  202  follows the deformation to expand and contract (extend and shrink) easily. Hence, if the core portion  201  expands and contracts, the covering portion  202  is less likely to be damaged (broken). As a result, the state of the core portion  201  covered with the covering portion  202  is maintained even if charge and discharge are repeated. Thus, even if the core portion  201  is broken during charge and discharge, a newly formed surface is less likely to be exposed. Furthermore, the newly formed surface is less likely to come into contact with an electrolytic solution, thus inhibiting the decomposition reaction of the electrolytic solution. 
     The reason the covering portion  202  contains M1 together with Si and O is that when the atomic ratio y is within the range described above, a compound (Si-M1-O) of Si, O, and M1 is easily formed in the covering portion  202 . This results in a reduction in irreversible capacity and a reduction in the electrical resistance of the negative-electrode active material  200 . In the covering portion  202 , at least one of the atoms of M1 may form Si-M1-O. Also in this case, the foregoing advantages are provided. The remaining M1 may be present in a free elemental form, may form an alloy with Si, or may be combined with O to form a compound. 
     More particularly, with respect to the bonding states (valence) of Si atoms bonded to O atoms in the covering portion  202 , five valence states are known: zero valence (Si 0+ ), monovalence (Si 1+ ), divalence (Si 2+ ), trivalence (Si 3+ ), and tetravalence (Si 4+ ). The presence or absence of Si atoms in these bonding states and their proportions (atomic ratios) can be determined by analysis of the covering portion  202  by, for example, X-ray photoelectron spectroscopy (XPS). Note that in the case where the outermost layer of the covering portion  202  is unintentionally oxidized (SiO 2  is formed), the analysis is preferably performed after SiO 2  is removed by dissolution with HF or the like. 
     In the case where Si-M1-O is formed in the covering portion  202 , in the zero-valent to tetra-valent bonding states, the proportion of the tetravalent silicon which is liable to lead to irreversible capacity during charge and discharge and which has high resistance is relatively reduced. Thus, even if the covering portion  202  is arranged on the surface of the core portion  201 , the presence of the covering portion  202  is less likely to lead to irreversible capacity, and the electrical resistance of the covering portion  202  is reduced. 
     The ratio of M1 to Si and O, i.e., M1/(Si+O), is not particularly limited and is preferably 50 atomic percent or less and more preferably 20 atomic percent or less. This is because the series of advantages of the covering portion  202  described above is obtained while suppressing a reduction in battery capacity due to the presence of M1. 
     As with the core portion  201 , the covering portion  202  is preferably capable of occluding and releasing lithium ions. This is because the covering portion  202  is less likely to inhibit the occlusion and release of lithium ions, so that the core portion  201  easily occludes and releases lithium ions. 
     Furthermore, the covering portion  202  is preferably noncrystalline (amorphous) or preferably has low crystallinity. The reason for this is as follows: lithium ions are easily diffused compared with the case where the covering portion  202  is crystalline (with high crystallinity), so that even when the surface of the core portion  201  is covered with the covering portion  202 , the core portion  201  easily and smoothly occludes and releases lithium ions. 
     In particular, the covering portion  202  is preferably noncrystalline. This is because the covering portion  202  has improved flexibility and thus easily follows the expansion and contraction of the core portion  201  during charge and discharge. Furthermore, the covering portion  202  is less likely to trap lithium ions, so that the entry and exit of lithium ions from the core portion  201  are less likely to be inhibited. 
     The term “low crystallinity” indicates that a material contained in the covering portion  202  includes a noncrystalline region and a crystalline region, the material being different from a “noncrystalline” material including a noncrystalline region alone. To identify whether the covering portion  202  has low crystallinity or not, for example, the covering portion  202  may be observed with a high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM). If a TEM photograph reveals that a noncrystalline region and a crystalline region are both present, the covering portion  202  has low crystallinity. In the case where a noncrystalline region and a crystalline region are both present, the crystalline region is observed as a region (crystal grain) having a granular contour. A striped pattern (crystal lattice pattern) attributed to the crystallinity is observed inside the crystal grain. It is thus possible to distinguish the crystal grain from the noncrystalline region. 
     The covering portion  202  may have a single-layer structure or a multilayer structure. In particular, the covering portion  202  preferably has a multilayer structure. This is because the covering portion  202  is less likely to be broken even if the core portion  201  expands and contracts during charge and discharge. More particularly, for the covering portion  202  having a single-layer structure, internal stress in the covering portion  202  is not easily relaxed, depending on its thickness. Thus, the covering portion  202  can be broken (e.g., fracture or separation) by the effect of the expanded and contracted core portion  201  during charge and discharge. In contrast, for the covering portion  202  having a multilayer structure, a minute gap between layers functions as a gap that relaxes stress. Thus, the internal stress is relaxed, so that the covering portion  202  is not easily broken. 
     For the covering portion  202  having the single-layer structure, Si, O, and M1 are contained in the single layer. For the covering portion  202  having the multilayer structure, layers containing Si, O, and M1 may be stacked. Alternatively, a layer containing Si and O and a layer containing Si, O, and M1 may be stacked. Furthermore, these layers may be mixed. Also in this case, the same effect is provided. Of course, any stacking order of the layers may be used in the multilayer structure. 
     The average thickness of the covering portion  202  is not particularly limited. In particular, the average thickness is preferably minimized. The covering portion  202  preferably has an average thickness of 1 nm to 10,000 nm and more preferably 100 nm to 10,000 nm. This is because the core portion  201  easily occludes and releases lithium ions and because the protective function of the covering portion  202  is effectively provided. More particularly, an average thickness of less than 1 nm can cause the covering portion  202  to be less likely to protect the core portion  201 . An average thickness exceeding 10,000 nm can increase the electrical resistance and can cause the core portion  201  to be less likely to occlude and release lithium ions during charge and discharge. The reason for this is that in the case where the covering portion  202  is composed of SiO y , SiO y  easily occludes lithium ions but does not easily release lithium ions that have been occluded. 
     The average thickness of the covering portion  202  is calculated by the following procedure. As illustrated in  FIG. 2 , one particle of the negative-electrode active material  200  is observed with a scanning electron microscope (SEM). To measure the thickness T of the covering portion  202 , the observation is preferably performed at a magnification such that the boundary between the core portion  201  and the covering portion  202  can be visually identified (determined). Subsequently, thicknesses of the covering portion  202  are measured at 10 random positions. Then the average value of the thicknesses (average thickness T of one particle) is calculated. In this case, the measurement positions are preferably set in such a manner that the measurement positions are not localized at a specific site but are widely distributed to the extent possible. Next, the foregoing calculation of the average value is repeated until the total number of particles observed with the SEM reaches 100. Finally, the average value (average value of the average thicknesses T) of the calculated average thicknesses T (corresponding to the respective particles) of 100 particles of the negative-electrode active material  200  is calculated and defined as the average thickness of the covering portion  202 . 
     The average coverage of the covering portion  202  on the core portion  201  is not particularly limited and is preferably maximized. More preferably, the average coverage of the covering portion  202  is preferably in the range of 30% to 100% because the protective function of the covering portion  202  is further improved. 
     The average coverage of the covering portion  202  is calculated by the following procedure. As with the case where the average thickness is calculated, one particle of the negative-electrode active material  200  is observed with a scanning electron microscope (SEM). The observation is preferably performed at a magnification such that in the core portion  201 , a portion that is covered with the covering portion  202  and a portion that is not covered with the covering portion  202  can be visually distinguished. With respect to the outer edge (contour) of the core portion  201 , the length of a portion that is covered with the covering portion  202  and the length of a portion that is not covered with the covering portion  202  are measured. Then the following calculation is performed: coverage (coverage for one particle: %)=(length of portion covered with covering portion  202 /length of outer edge of core portion  201 )×100. Next, the foregoing calculation of the coverage is repeated until the total number of particles observed with the SEM reaches 100. Finally, the average value of the calculated coverage values (corresponding to the respective particles) of 100 particles of the negative-electrode active material  200  is calculated and defined as the average coverage of the covering portion  202 . 
     The covering portion  202  is preferably adjacent to the core portion  201  and may be present on the surface of the core portion  201  with a natural oxide film (SiO 2 ) provided therebetween. The natural oxide film is formed by, for example, oxidation of a surface portion of the core portion  201  in air. In the case where the core portion  201  is present in the middle of each particle of the negative-electrode active material  200  and where the covering portion  202  is present outside the particle, the presence of the natural oxide film has little effect on functions of the core portion  201  and the covering portion  202 . 
     To check the fact that the negative-electrode active material  200  includes the core portion  201  and the covering portion  202 , the negative-electrode active material  200  may be analyzed by, for example, X-ray photoelectron spectroscopy (XPS) or energy-dispersive X-ray analysis (EDX) in addition to the SEM observation described above. 
     In this case, for example, the compositions of the core portion  201  and the covering portion  202  can be identified by measuring the degrees of oxidation (atomic ratios x and y) at the central portion and the surface portion of each particle of the negative-electrode active material  200 . To investigate the composition of the core portion  201  covered with the covering portion  202 , the covering portion  202  may be removed by dissolution with HF or the like. 
     An exemplary procedure for measuring the degree of oxidation will be described in detail below. First, the negative-electrode active material  200  (the core portion  201  covered with the covering portion  202 ) is quantified by a combustion method to calculate the total amount of Si and the total amount of O. Next, the covering portion  202  is removed by rinsing with HF, and then the core portion  201  is quantified to calculate the amount of Si and the amount of O. Finally, the amount of Si and the amount of O in the covering portion  202  are determined by subtracting the amount of Si and the amount of O in the core portion  201  from the total amount of Si and the total amount of O. As a result, the amounts of Si and the amounts of O in the core portion  201  and the covering portion  202  are determined, thus determining the degrees of oxidation therein. In place of the removal of the covering portion  202  by rinsing, the degrees of oxidation may be measured by the use of the core portion  201  covered with the covering portion  202  and a portion of the core portion  201  that is not covered therewith. 
     Conductive Portion 
     In particular, the negative-electrode active material  200  may include a conductive portion on a surface of the covering portion  202 . The conductive portion is arranged on at least part of a surface of the covering portion  202  and has a lower electrical resistance than those of the core portion  201  and the covering portion  202 . In this case, the core portion  201  does not easily come into contact with an electrolytic solution, thus inhibiting the decomposition reaction of the electrolytic solution. Furthermore, the electrical resistance of the negative-electrode active material  200  is further reduced. The conductive portion contains, for example, one or two or more of carbon materials, metal materials, and inorganic compounds. An example of carbon materials is graphite. Examples of metal materials include Fe, Cu, and Al. An example of inorganic compounds is SiO 2 . Among these materials, carbon materials or metal materials are preferred. Carbon materials are more preferred. This is because the electrical resistance of the negative-electrode active material  200  is further reduced. Note that the conductive portion may have any average coverage and any average thickness. The average coverage and the average thickness are calculated in the same ways as those used for the covering portion  202 . 
     The negative-electrode binder contains, for example, one or two or more of synthetic rubber and polymeric materials. Examples of synthetic rubber include styrene-butadiene-based rubber, fluorocarbon rubber, and ethylene-propylene-diene. Examples of polymeric materials include polyvinylidene fluoride, polyimide, polyamide, polyamide-imide, polyacrylic acid, lithium polyacrylate, sodium polyacrylate, polymaleic acid, and copolymers thereof. Further examples of polymeric materials include carboxymethyl cellulose, styrene-butadiene rubber, and polyvinyl alcohol. 
     The negative-electrode conductive agent contains, for example, one or two or more of carbon materials, such as graphite, carbon black, acetylene black, and Ketjenblack. The negative-electrode conductive agent may also be a conductive material, for example, a metal material or a conductive polymer. 
     Each of the negative-electrode active material layers  2  may contain another negative-electrode active material in addition to the negative-electrode active material  200  including the core portion  201  and the covering portion  202  described above, if necessary. 
     An example of another negative-electrode active material is a carbon material. This is because the electrical resistance of the negative-electrode active material layers  2  is reduced and because the negative-electrode active material layers  2  are less likely to expand and contract during charge and discharge. Examples of the carbon material include graphitizable carbon, non-graphitizable carbon in which the interplanar spacing of the (002) planes is 0.37 nm or more, and graphite in which the interplanar spacing of the (002) planes is 0.34 nm or less. Specific examples thereof include pyrolytic carbon, coke, glassy carbon fibers, a burned organic polymeric compound, activated carbon, and carbon black. Examples of coke include pitch coke, needle coke, and petroleum coke. The burned organic polymeric compound refers to a material formed by burning a phenolic resin or a furan resin at an appropriate temperature into carbon. The carbon material may have any shape selected from fibrous shapes, spherical shapes, granular shapes, and flaky shapes. The carbon material content of the negative-electrode active material layers  2  is not particularly limited and is preferably 60% by weight or less and more preferably 10% by weight to 60% by weight. 
     Furthermore, another negative-electrode active material may be a metal oxide or a polymeric compound. Examples of the metal oxide include iron oxide, ruthenium oxide, and molybdenum oxide. Examples of the polymeric compound include polyacetylene, polyaniline, and polypyrrole. 
     The negative-electrode active material layers  2  are formed by, for example, an application method, a firing method (a sintering method), or a combination of two or more thereof. The application method refers to a method in which, for example, a negative-electrode active material is mixed with a negative-electrode binder, the resulting mixture is dispersed in an organic solvent, and application is performed. The firing method refers to a method in which, for example, after application is performed in the same way as the application method, heat treatment is performed at a temperature higher than the melting point of the negative-electrode binder or the like. As the firing method, a method of the related art may be employed. Examples thereof include an atmosphere firing method, a reactive firing method, and a hot-press firing method. 
     Production Method of Negative Electrode 
     A negative electrode is produced by, for example, a procedure described below. The materials constituting the negative-electrode collector  1  and the negative-electrode active material layers  2  have been described in detail. Thus, the descriptions are appropriately omitted. 
     First, the granular (powdery) core portion  201  having the foregoing composition is formed by, for example, a gas atomization method, a water atomization method, or a melt pulverization method. 
     Next, the covering portion  202  having the foregoing composition is formed on the surface of the core portion  201  by a vapor deposition method, for example, an evaporation method or a sputtering method. In the case where a material constituting the covering portion  202  is deposited by the vapor deposition method as described above, the covering portion  202  tends to be noncrystalline. In this case, the material constituting the covering portion  202  may be deposited while being heated by, for example, induction heating, resistance heating, or electron-beam heating. Alternatively, after the formation of the covering portion  202 , the covering portion  202  may be heated so as to have low crystallinity. The degree of the low crystallinity is controlled, depending on, for example, heating conditions, such as temperature and time. The heat treatment results in the removal of water in the covering portion  202  and results in improvement in the adhesion of the covering portion  202  to the core portion  201 . 
     In particular, in the case where the vapor deposition method is employed, Si-M1-O is easily formed in the covering portion  202  by heating not only the material constituting the covering portion  202  but also a substrate used for the deposition. For example, the substrate temperature is preferably 200° C. or higher and lower than 900° C. When the covering portion  202  is formed, the proportions of the bonding states of Si atoms bonded to O atoms can be controlled by adjusting the flow rates of oxygen (O 2 ), hydrogen (H 2 ), and so forth introduced into a chamber and by adjusting the temperature of the core portion  201 . As a result, the core portion  201  is covered with the core portion  201 , resulting in the negative-electrode active material  200 . 
     In the case where the negative-electrode active material  200  is formed, a conductive portion may be formed on the surface of the covering portion  202  by, for example, a vapor deposition method, such as an evaporation method, a sputtering method, or a chemical vapor deposition (CVD) method, or a wet coating method. 
     In the case where the evaporation method is employed, for example, vapor is allowed to impinge directly on the surface of the negative-electrode active material  200 . In the case where the sputtering method is employed, for example, a conductive portion is formed by a powder sputtering method while Ar gas is introduced. In the case where the CVD method is employed, for example, a gas formed by subliming a metal chloride and a mixed gas of H 2 , N 2 , and so forth are mixed in such a manner that the mole fraction of the metal chloride is in the range of 0.03 to 0.3, and then the resulting gas is heated to 1000° C. or higher to form a conductive portion on the surface of the covering portion  202 . In the case where the wet coating method is employed, for example, an alkali solution is added to a slurry containing the negative-electrode active material  200  while a metal-containing solution is added to the slurry to form a metal hydroxide. Then reduction treatment with H 2  is performed at 450° C. to form a conductive portion on the surface of the covering portion  202 . In the case where a carbon material is used as a material constituting the conductive portion, the negative-electrode active material  200  is placed in a chamber. An organic gas is introduced into the chamber. Heat treatment is performed at 10,000 Pa and 1000° C. or higher for 5 hours to form a conductive portion on the surface of the covering portion  202 . The organic gas is not particularly limited as long as it is thermally decomposed to form carbon. Examples of the organic gas include methane, ethane, ethylene, acetylene, and propane. 
     Next, the negative-electrode active material  200  and other materials, such as the negative-electrode binder, are mixed to form a negative-electrode mixture. The resulting negative-electrode mixture is dissolved in a solvent, such as an organic solvent, to form a slurry containing the negative-electrode mixture. Finally, the slurry containing the negative-electrode mixture is applied to the surface of the negative-electrode collector  1  and dried to form the negative-electrode active material layers  2 . If necessary, the negative-electrode active material layers  2  may be subjected to compression forming and heated (fired). 
     Function and Effect of Embodiment 
     In the negative electrode, the negative-electrode active material  200  includes the covering portion  202  on the surface of the core portion  201 . The covering portion  202  contains, as constituent elements, Si, O, and element M1, such as Li. The atomic ratio y of O to Si is 0.5≦y≦1.8. Thus, the core portion  201  easily and smoothly occludes and releases lithium ions. Furthermore, the core portion  201  is protected by the covering portion  202  so as not to expose a newly-formed surface during charge and discharge while the smooth occlusion and release are maintained. Moreover, Si-M-O is easily formed in the covering portion  202 . Thus, the presence of the covering portion  202  is less likely to lead to irreversible capacity, and the electrical resistance of the covering portion  202  is reduced. Accordingly, the negative electrode contributes to improvement in the performance of a lithium-ion secondary battery including the negative electrode. Specifically, the negative electrode contributes to improvement in cycle characteristics, initial charge-discharge characteristics, load characteristics, and so forth. 
     In particular, in the case where the ratio of M1 to Si and O in the covering portion  202  is preferably 50 atomic percent or less and more preferably 20 atomic percent or less, a higher effect can be provided. In the case where the covering portion  202  on the core portion  201  has an average coverage of 30% to 100% or where the covering portion  202  has an average thickness of 1 nm to 10,000 nm, a higher effect can be provided. Moreover, the covering portion  202  has a multilayer structure, a higher effect can be provided. 
     2. Lithium-Ion Secondary Battery 
     A lithium-ion secondary battery including the negative electrode for a lithium-ion secondary battery (hereinafter, referred to simply as “secondary battery”) will be described below. 
     2-1. Prismatic Type 
       FIGS. 3 and 4  are sectional views illustrating a structure of a prismatic lithium-ion secondary battery.  FIG. 4  is a sectional view taken along line Iv-Iv in  FIG. 3 .  FIG. 5  is a schematic plan view illustrating structures of a positive electrode  21  and a negative electrode  22  illustrated in  FIG. 4 . 
     Entire Structure of Secondary Battery 
     The prismatic secondary battery mainly includes a battery element  20  in a battery can  11 . The battery element  20  is formed of a spirally wound laminate in which the positive electrode  21  and the negative electrode  22  are stacked with a separator  23  provided therebetween and spirally wound, the battery element  20  having a flattened shape in response to the shape of the battery can  11 . 
     The battery can  11  is, for example, a prismatic package member. As illustrated in  FIG. 4 , the prismatic package member has a rectangular or substantially rectangular (partially curved) shape in longitudinal section. The prismatic package member can be used to not only a prismatic battery with a rectangular shape but also a prismatic battery with an oval shape. In other words, the prismatic package member is a vessel-shaped member having a rectangular closed end or an oval closed end and having an opening portion with a rectangular shape or a substantially rectangular (oval) shape formed by connecting arcs with straight lines.  FIG. 4  illustrates the battery can  11  having a rectangular section. 
     The battery can  11  is composed of a conductive material, for example, iron, aluminum, or an alloy thereof. The battery can  11  may function as an electrode terminal. Among these materials, Fe, which is harder than Al, is preferred in order to prevent swelling of the battery can  11  during charge and discharge with use of the hardness (resistance to deformation) of the battery can  11 . In the case where the battery can  11  is composed of Fe, the surface of the battery can  11  may be plated with Ni or the like. 
     The battery can  11  has a hollow structure having an open end portion and a closed end portion. The battery can  11  is sealed with an insulating plate  12  and a battery cover  13  attached to the open end portion. The insulating plate  12  is arranged between the battery element  20  and the battery cover  13 . The insulating plate  12  is composed of an insulating material, such as polypropylene. The battery cover  13  is composed of, for example, a material the same as that of the battery can  11 . Similarly to the battery can  11 , the battery cover  13  may function as an electrode terminal. 
     A terminal plate  14  serving as a positive-electrode terminal is arranged outside the battery cover  13 . The terminal plate  14  is electrically insulated from the battery cover  13  with an insulating case  16  provided therebetween. The insulating case  16  is composed of an insulating material, such as polybutylene terephthalate. A through hole is arranged in the substantially middle of the battery cover  13 . A positive-electrode pin  15  is interposed in the through hole so as to be electrically connected to the terminal plate  14  and so as to be electrically insulated from the battery cover  13  with a gasket  17 . The gasket  17  is composed of, for example, an insulating material. The surface of the gasket  17  is coated with asphalt. 
     A cleavage valve  18  and an injection hole  19  are arranged at the outer edge of the battery cover  13 . The cleavage valve  18  is electrically connected to the battery cover  13 . If the internal pressure of the battery is increased to a predetermined value or higher by an internal short-circuit or externally applied heat, the cleavage valve  18  is separated from the battery cover  13  to release the internal pressure. The injection hole  19  is capped with a sealing member  19 A formed of, for example, a stainless-steel ball. 
     A positive-electrode lead  24  composed of a conductive material, such as Al, is attached to an end portion (for example, an inner end portion) of the positive electrode  21 . A negative-electrode lead  25  composed of a conductive material, such as Ni, is attached to an end portion (for example, an outer end portion) of the negative electrode  22 . The positive-electrode lead  24  is welded to an end of the positive-electrode pin  15  and is electrically connected to the terminal plate  14 . The negative-electrode lead  25  is welded and electrically connected to the battery can  11 . 
     Positive Electrode 
     For example, the positive electrode  21  includes a positive-electrode active material layer  21 B provided on each surface of a positive-electrode collector  21 A. Alternatively, the positive-electrode active material layer  21 B may be arranged on only one surface of the positive-electrode collector  21 A. 
     The positive-electrode collector  21 A is composed of a conductive material, e.g., Al, Ni, or stainless steel. 
     Each of the positive-electrode active material layers  21 B contains one or two or more positive-electrode materials which serve as positive-electrode active materials and which are capable of occluding and releasing lithium ions. If necessary, each of the positive-electrode active material layers  21 B may further contain an additional material, for example, a positive-electrode binder or a positive-electrode conductive agent. Details of the positive-electrode binder and the positive-electrode conductive agent are the same as those of the negative-electrode binder and the negative-electrode conductive agent described above. 
     As the positive-electrode material, a Li-containing compound is preferred because a high energy density is provided. Examples of the Li-containing compound include a complex oxide that contains, as constituent elements, Li and a transition metal element; and a phosphate compound that contains, as constituent elements, Li and a transition metal element. It is preferred that the transition metal element be one or two or more of Co, Ni, Mn, and Fe. This is because a higher voltage is provided. The complex oxide and the phosphate compound are expressed as, for example, Li x M 11 O 2  and Li y M 12 PO 4 , respectively, wherein M11 and M12 each represent one or more transition metal elements, and the values of x and y vary depending on a charge-discharge state of the battery and are usually 0.05≦x≦1.10 and 0.05≦y≦1.10. In particular, when the positive-electrode material contains Ni or Mn, the volume stability tends to be improved. 
     Examples of the complex oxide containing Li and a transition metal element include Li x CoO 2 , Li x NiO 2 , and a LiNi-based complex oxide represented by formula (1). Examples of the phosphate compound containing Li and a transition metal element include LiFePO 4  and LiFe 1-u Mn u PO 4  (u&lt;1). In this case, a high battery capacity and excellent cycle characteristics are provided. The positive-electrode material may be a material other than the foregoing materials. Examples thereof include materials represented by Li x M 14y O 2  (wherein M14 represents at least one selected from Ni and M13 described in formula (1); x&gt;1; and y represents any value).
 
LiNi 1-x M13 x O 2   (1)
 
     (wherein M13 represents at least one selected from Co, Mn, Fe, Al, V, Sn, Mg, Ti, Sr, Ca, Zr, Mo, Tc, Ru, Ta, W, Re, Y, Cu, Zn, Ba, B, Cr, Si, Ga, P, Sb, and Nb; and x is 0.005&lt;x&lt;0.5). 
     Further examples of the positive-electrode material include oxides, disulfides, chalcogenides, and conductive polymers. Examples of oxides include titanium oxide, vanadium oxide, and manganese dioxide. Examples of disulfides include titanium disulfide and molybdenum sulfide. An example of chalcogenides is niobium selenide. Examples of conductive polymers include sulfur, polyaniline, and polythiophene. 
     Negative Electrode 
     The negative electrode  22  has the same structure as that of the foregoing negative electrode for a lithium-ion secondary battery. For example, the negative electrode  22  includes a negative-electrode active material layer  22 B provided on each surface of a negative-electrode collector  22 A. The structures of the negative-electrode collector  22 A and the negative-electrode active material layers  22 B are the same as those of the negative-electrode collector  1  and the negative-electrode active material layers  2 . The chargeable capacity of the negative-electrode material capable of occluding and releasing lithium ions is preferably higher than the discharge capacity of the positive electrode  21  in order to prevent metallic Li from being unintentionally deposited during charge and discharge. 
     As illustrated in  FIG. 5 , for example, each of the positive-electrode active material layers  21 B is arranged on a portion (for example, a middle region in the longitudinal direction) of a corresponding one of the surfaces of the positive-electrode collector  21 A. Meanwhile, for example, each of the negative-electrode active material layers  22 B is arranged on the whole of a corresponding one of the surfaces of the negative-electrode collector  22 A. Thus, each of the negative-electrode active material layers  22 B is arranged in a region (opposed region R 1 ) of the negative-electrode collector  22 A which is opposed to a corresponding one of the positive-electrode active material layers  21 B, and a region (non-opposed region R 2 ) of the negative-electrode collector  22 A which is not opposed to the corresponding positive-electrode active material layer  21 B. In this case, a portion of each negative-electrode active material layer  22 B arranged in the opposed region R 1  is associated with charge and discharge, whereas a portion arranged in the non-opposed region R 2  has little association with charge and discharge. In  FIG. 5 , shaded areas indicate the positive-electrode active material layer  21 B and the negative-electrode active material layer  22 B. 
     As described above, the negative-electrode active material  200  (see  FIG. 2 ) contained in the negative-electrode active material layers  22 B includes the core portion  201  and the covering portion  202 . However, the negative-electrode active material layers  22 B might be deformed or broken by expansion and contraction during charge and discharge. Thus, the formation states of the core portion  201  and the covering portion  202  may be changed from states at the time of the formation of the negative-electrode active material layers  22 B. However, in the non-opposed region R 2 , the formation states of the negative-electrode active material layers  22 B are little affected by charge and discharge and are maintained. Thus, the foregoing parameters, such as the presence or absence of the core portion  201  and the covering portion  202 , the compositions (atomic ratios x and y) of the core portion  201  and the covering portion  202 , and the proportion of M1, are preferably investigated in the negative-electrode active material layers  22 B in the non-opposed region R 2 . This is because the presence or absence of the core portion  201  and the covering portion  202 , the compositions of the core portion  201  and the covering portion  202 , and so forth can be investigated reproducibly and accurately independent of charge-discharge history (e.g., whether charge and discharge are performed or not, and the number of times of charge and discharge). 
     The maximum utilization factor in a fully charged state of the negative electrode  22  (hereinafter, referred to simply as a “negative-electrode utilization factor”) is not particularly limited. Any negative-electrode utilization factor may be set in response to the ratio of the capacity of the positive electrode  21  to the capacity of the negative electrode  22 . 
     The foregoing “negative-electrode utilization factor” is expressed as utilization factor Z (%)=(X/Y)×100, where X represents the amount of lithium ions occluded in the negative electrode  22  per unit area in a fully charged state, and Y represents the amount of lithium ions that can be electrochemically occluded in the negative electrode  22  per unit area. 
     For example, the amount X occluded can be determined by the following procedure. First, the secondary battery is charged to a fully charged state. The secondary battery is then disassembled to cut out a portion (test piece of the negative electrode) of the negative electrode  22  opposed to the positive electrode  21 . Next, an evaluation battery including metallic lithium serving as a counter electrode is assembled using the test piece of the negative electrode. Finally, the evaluation battery is discharged to measure the discharge capacity at the time of initial discharge. The discharge capacity is divided by the area of the test piece of the negative electrode to determine the amount X occluded. In this case, the term “discharge” indicates that a current flows in a direction where lithium ions are released from the test piece of the negative electrode. For example, the battery is subjected to constant current discharge at a constant current, for example, at a current density of 0.1 mA/cm 2 , until the battery voltage reaches 1.5 V. 
     Meanwhile, for example, the amount Y occluded is determined by subjecting the foregoing discharged evaluation battery to constant voltage and constant current charge until the battery voltage reaches 0 V, measuring the charge capacity, and dividing the charge capacity by the area of the test piece of the negative electrode. In this case, the term “charge” indicates that a current flows in a direction where lithium ions are occluded in the test piece of the negative electrode. For example, the battery is subjected to constant voltage charge at a current density of 0.1 mA/cm 2  and a battery voltage of 0 V until the current density reaches 0.02 mA/cm 2 . 
     In particular, the negative-electrode utilization factor is preferably in the range of 35% to 80% because excellent cycle characteristics, initial charge-discharge characteristics, and load characteristics are provided. 
     Separator 
     The separator  23  isolates the positive electrode  21  from the negative electrode  22  to prevent the occurrence of a short circuit due to the contact of both electrodes and allows lithium ions to pass therethrough. The separator  23  is formed of, for example, a porous film of a synthetic resin or a ceramic material. The separator  23  may be formed of a laminated film in which two or more porous films are stacked. Examples of the synthetic resin include polytetrafluoroethylene, polypropylene, and polyethylene. 
     Electrolytic Solution 
     The separator  23  is impregnated with an electrolytic solution, which is a liquid electrolyte. The electrolytic solution is formed by dissolution of an electrolyte salt in a solvent. The electrolytic solution may contain an additional material, such as an additive, if necessary. 
     The solvent contains one or two or more of nonaqueous solvents, such as organic solvents. Examples of nonaqueous solvents include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, γ-butyrolactone, γ-valerolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,3-dioxane, 1,4-dioxane, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, methyl butyrate, methyl isobutyrate, methyl trimethylacetate, ethyl trimethylacetate, acetonitrile, glutaronitrile, adiponitrile, methoxyacetonitrile, 3-methoxypropionitrile, N,N-dimethylformamide, N-methylpyrrolidinone, N-methyloxazolidinone, N,N′-dimethylimidazolidinone, nitromethane, nitroethane, sulfolane, trimethyl phosphate, and dimethyl sulfoxide. In this case, excellent battery capacity, cycle characteristics, and storage characteristics are provided. 
     In particular, at least one selected from ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate is preferred because superior characteristics are provided. In this case, a combination of a high-viscosity (high dielectric constant) solvent (for example, dielectric constant ∈≧30), e.g., ethylene carbonate or propylene carbonate, and a low-viscosity solvent (for example, viscosity ≦1 mPa·s), e.g., dimethyl carbonate, ethyl methyl carbonate, or diethyl carbonate, is more preferred because the dissociation property of the electrolyte salt and ion mobility are improved. 
     In particular, the nonaqueous solvent preferably contains at least one of a halogenated chain carbonate and a halogenated cyclic carbonate. This is because a stable coating film is formed on a surface of the negative electrode  22  during charge and discharge, thereby inhibiting the decomposition reaction of the electrolytic solution. The halogenated chain carbonate indicates a chain carbonate containing a halogen serving as a constituent element. In other words, the halogenated chain carbonate indicates a chain carbonate in which at least one hydrogen atom is substituted with a halogen. The halogenated cyclic carbonate indicates a cyclic carbonate containing a halogen serving as a constituent element. In other words, the halogenated cyclic carbonate indicates a cyclic carbonate in which at least one H is substituted with a halogen. 
     The type of halogen is not particularly limited. In particular, F, Cl, or Br is preferred, and F is more preferred. This is because F provides a higher effect than those of other halogens. With respect to the number of halogen atoms, two halogen atoms are more preferable than one halogen atom. Furthermore, three or more halogen atoms may be used. The reason for this is that the ability to form a protective film is increased and a stronger and stabler coating film is formed, thereby further inhibiting the decomposition reaction of the electrolytic solution. 
     Examples of the halogenated chain carbonate include fluoromethyl methyl carbonate, bis(fluoromethyl) carbonate, and difluoromethyl methyl carbonate. Examples of the halogenated cyclic carbonate include 4-fluoro-1,3-dioxolan-2-one, and 4,5-difluoro-1,3-dioxolan-2-one. For the halogenated cyclic carbonate, geometrical isomers are included. The proportions of the halogenated chain carbonate and the halogenated cyclic carbonate in the nonaqueous solvent are each in the range of, for example, 0.01% by weight to 50% by weight. 
     The nonaqueous solvent preferably contains an unsaturated carbon bond cyclic carbonate. This is because a stable coating film is formed on a surface of the negative electrode  22  during charge and discharge to inhibit the decomposition reaction of the electrolytic solution. The unsaturated carbon bond cyclic carbonate indicates a cyclic carbonate having one or two or more unsaturated carbon bonds. In other words, the unsaturated carbon bond cyclic carbonate indicates a cyclic carbonate in which an unsaturated carbon bond is introduced into any portion. Examples of the unsaturated carbon bond cyclic carbonate include vinylene carbonate and vinyl ethylene carbonate. The proportion of the unsaturated carbon bond cyclic carbonate in the nonaqueous solvent is in the range of, for example, 0.01% by weight to 10% by weight. 
     The nonaqueous solvent preferably contains a sultone (cyclic sulfonate) because the chemical stability of the electrolytic solution is improved. Examples of the sultone include propane sultone and propene sultone. The proportion of the sultone is in the range of, for example, 0.5% by weight to 5% by weight. 
     The nonaqueous solvent preferably contains an acid anhydride because the chemical stability of the electrolytic solution is improved. Examples of the acid anhydride include carboxylic anhydrides, disulfonic anhydrides, and carboxylic-sulfonic anhydrides. Examples of carboxylic anhydrides include succinic anhydride, glutaric anhydride, and maleic anhydride. Examples of disulfonic anhydrides include ethanedisulfonic anhydride and propanedisulfonic anhydride. Examples of carboxylic-sulfonic anhydrides include sulfobenzoic anhydride, sulfopropionic anhydride, and sulfobutyric anhydride. The proportion of the acid anhydride in the nonaqueous solvent is in the range of, for example, 0.5% by weight to 5% by weight. 
     The electrolyte salt contains one or two or more light metal salts, such as Li salts. Examples of Li salts include LiPF 6 , LiBF 4 , LiClO 4 , LiAsF 6 , LiB(C 6 H 5 ) 4 , LiCH 3 SO 3 , LiCF 3 SO 3 , LiAlCl 4 , Li 2 SiF 6 , LiCl, and LiBr. Another Li salt may be used. This is because excellent battery capacity, cyclic characteristics, storage characteristics, and so forth are provided. 
     Among these compounds, one or two or more of LiPF 6 , LiBF 4 , LiClO 4 , and LiAsF 6  are preferred. LiPF 6  or LiBF 4  is preferred. LiPF 6  is more preferred. This is because the internal resistance is reduced to achieve superior characteristics. 
     The electrolyte salt content is preferably in the range of 0.3 mol/kg to 3.0 mol/kg with respect to the solvent because a high ionic conductivity is provided. 
     Operation of Secondary Battery 
     When the prismatic secondary battery is charged, for example, lithium ions released from the positive electrode  21  are occluded in the negative electrode  22  through the electrolytic solution. When the prismatic secondary battery is discharged, for example, lithium ions released from the negative electrode  22  are occluded in the positive electrode  21  through the electrolytic solution. 
     Method for Producing Secondary Battery 
     The secondary battery is produced by, for example, a procedure described below. 
     First, the positive electrode  21  is formed. The positive-electrode active material and, if necessary, the positive-electrode binder, the positive-electrode conductive agent, and so forth are mixed together to form a positive-electrode mixture. The positive-electrode mixture is dispersed in an organic solvent or the like to form a paste-like slurry of the positive-electrode mixture. Next, the paste-like slurry of the positive-electrode mixture is applied to the positive-electrode collector  21 A with a coating apparatus, for example, a doctor blade or a bar coater, and dried to form the positive-electrode active material layers  21 B. Finally, the positive-electrode active material layers  21 B are subjected to compression forming with a roll press or the like while being heated, if necessary. In this case, the compression forming may be repeated plural times. 
     Next, the negative-electrode active material layers  22 B are formed on the negative-electrode collector  22 A by a procedure the same as the foregoing procedure for forming the negative electrode  22  for a lithium-ion secondary battery. 
     Next, the battery element  20  is formed. First, the positive-electrode lead  24  is attached to the positive-electrode collector  21 A by a welding method or the like. The negative-electrode lead  25  is attached to the negative-electrode collector  22 A by a welding method or the like. Then the positive electrode  21  and the negative electrode  22  are stacked with the separator  23  provided therebetween. The resulting stack is spirally wound in a longitudinal direction. Finally, the resulting spirally wound body is formed so as to have a flattened shape. 
     Finally, the secondary battery is assembled. First, the battery element  20  is placed in the battery can  11 . The insulating plate  12  is placed on the battery element  20 . Next, the positive-electrode lead  24  is attached to the positive-electrode pin  15  by a welding method or the like. The negative-electrode lead  25  is attached to the battery can  11  by a welding method or the like. In this case, the battery cover  13  is fixed to an open end portion of the battery can  11  by a laser welding method or the like. Finally, the electrolytic solution is injected into the battery can  11  from the injection hole  19  to impregnate the separator  23  with the electrolytic solution, and then the injection hole  19  is sealed with the sealing member  19 A. 
     Function and Effect of Secondary Battery 
     For the prismatic secondary battery, the negative electrode  22  has the same structure as that of the foregoing negative electrode for a lithium-ion secondary battery, thereby providing the same effects. It is thus possible to provide excellent battery characteristics, such as cycle characteristics, initial charge-discharge characteristics, and load characteristics. Effects other than these effects are the same as those of the negative electrode for a lithium-ion secondary battery. 
     2-2. Cylindrical Type 
       FIGS. 6 and 7  are sectional views illustrating a structure of a cylindrical lithium-ion secondary battery.  FIG. 7  is a partially enlarged view of a spirally wound electrode  40  illustrated in  FIG. 6 . The cylindrical secondary battery will be described below with reference to components of the foregoing prismatic secondary battery, as necessary. 
     Structure of Secondary Battery 
     The cylindrical secondary battery mainly includes the spirally wound electrode  40  and a pair of insulating plates  32  and  33  in a substantially hollow cylindrical-shaped battery can  31 . The spirally wound electrode  40  is formed of a spirally wound laminate in which a positive electrode  41  and a negative electrode  42  are stacked with a separator  43  provided therebetween and spirally wound. 
     The battery can  31  has a hollow structure in which an end portion of the battery can  31  is closed and the other end portion thereof is opened. The battery can  31  is composed of, for example, a material the same as that of the battery can  11 . The pair of insulating plates  32  and  33  are arranged in such a manner that the spirally wound electrode  40  is sandwiched therebetween at the top and the bottom of the spirally wound electrode body  40  and that the pair of insulating plates  32  and  33  extend in a direction perpendicular to a peripheral winding surface. 
     In the open end portion of the battery can  31 , a battery cover  34 , a safety valve mechanism  35 , and a positive temperature coefficient device (PTC device)  36  are caulked with a gasket  37 . The battery can  31  is sealed. The battery cover  34  is composed of, for example, a material the same as that of the battery can  31 . The safety valve mechanism  35  and the positive temperature coefficient device  36  are arranged inside the battery cover  34 . The safety valve mechanism  35  is electrically connected to the battery cover  34  through the positive temperature coefficient device  36 . In the safety valve mechanism  35 , if the internal pressure of the secondary battery is increased to a predetermined value or higher by an internal short-circuit or externally applied heat, a disk plate  35 A is reversed to disconnect the electrical connection between the battery cover  34  and the spirally wound electrode  40 . The positive temperature coefficient device  36  exhibits an increase in resistance with increasing temperature and thus prevents abnormal heat generation due to a large current. The gasket  37  is composed of, for example, an insulating material. The surface of the gasket  37  may be coated with asphalt. 
     A center pin  44  may be interposed in the center of the spirally wound electrode  40 . A positive-electrode lead  45  composed of a conductive material, such as Al, is connected to the positive electrode  41 . A negative-electrode lead  46  composed of a conductive material, such as Ni, is connected to the negative electrode  42 . The positive-electrode lead  45  is attached to the safety valve mechanism  35  by welding or the like to establish electrical connection with the battery cover  34 . The negative-electrode lead  46  is attached to the battery can  31  by welding or the like to establish electrical connection with the battery can  31 . 
     The positive electrode  41  includes, for example, a positive-electrode active material layer  41 B provided on each surface of a positive-electrode collector  41 A. The negative electrode  42  has the same structure as the foregoing negative electrode for a lithium-ion secondary battery. For example, the negative electrode  42  includes a negative-electrode active material layer  42 B provided on each surface of a negative-electrode collector  42 A. The structures of the positive-electrode collector  41 A, the positive-electrode active material layers  41 B, the negative-electrode collector  42 A, the negative-electrode active material layers  42 B, and the separator  43  are the same as those of the positive-electrode collector  21 A, the positive-electrode active material layers  21 B, the negative-electrode collector  22 A, the negative-electrode active material layers  22 B, and the separator  23 , respectively. The composition of an electrolytic solution with which the separator  43  is impregnated is the same as that of the electrolytic solution used in the prismatic secondary battery. 
     Operation of Secondary Battery 
     When the cylindrical secondary battery is charged, for example, lithium ions released from the positive electrode  41  are occluded in the negative electrode  42  through the electrolytic solution. When the cylindrical secondary battery is discharged, for example, lithium ions released from the negative electrode  42  are occluded in the positive electrode  41  through the electrolytic solution. 
     Method for Producing Secondary Battery 
     The cylindrical secondary battery is produced by, for example, a procedure described below. First, for example, the positive-electrode active material layer  41 B is formed on each of the surfaces of the positive-electrode collector  41 A to form the positive electrode  41  in the same way as the procedure for forming the positive electrode  21 . The negative-electrode active material layer  42 B is formed on each of the surfaces of the negative-electrode collector  42 A to form the negative electrode  42  in the same way as the procedure for forming the negative electrode  22 . Next, the positive-electrode lead  45  is attached to the positive electrode  41  by a welding method or the like. The negative-electrode lead  46  is attached to the negative electrode  42  by a welding method or the like. Then the positive electrode  41  and the negative electrode  42  are stacked with the separator  43  provided therebetween. The resulting stack is spirally wound to form a spirally wound electrode  40 . The center pin  44  is inserted into the center of the spirally wound electrode  40 . The spirally wound electrode  40  is placed in the battery can  31  while being sandwiched between the pair of insulating plates  32  and  33 . In this case, the positive-electrode lead  45  is attached to the safety valve mechanism  35  by a welding method or the like. An end portion of the negative-electrode lead  46  is attached to the battery can  31  by a welding method or the like. Subsequently, the electrolytic solution is injected into the battery can  31 , thereby impregnating the separator  43  with the electrolytic solution. Finally, the battery cover  34 , the safety valve mechanism  35 , and the positive temperature coefficient device  36  are attached to the open end portion of the battery can  31  and then are caulked with the gasket  37 . 
     Function and Effect of Secondary Battery 
     For the cylindrical secondary battery, the negative electrode  42  has the same structure as that of the foregoing negative electrode for a lithium-ion secondary battery, thereby providing the same effects as those of the prismatic secondary battery. 
     2-3. Laminated-Film Type 
       FIG. 8  is an exploded perspective view illustrating a structure of a laminated-film-type lithium-ion secondary battery.  FIG. 9  is an enlarged cross-sectional view taken along line IX-IX in  FIG. 8  that illustrates a spirally wound electrode  50 . 
     Structure of Secondary Battery 
     The laminated-film-type secondary battery mainly includes the spirally wound electrode  50  in film-shaped package members  59 . The spirally wound electrode  50  is formed of a spirally wound laminate in which a positive electrode  53  and a negative electrode  54  are stacked with separators  55  and electrolyte layers  56  provided therebetween and spirally wound. A positive-electrode lead  51  is attached to the positive electrode  53 . A negative-electrode lead  52  is attached to the negative electrode  54 . The outermost portion of the spirally wound electrode  50  is protected by a protective tape  57 . 
     For example, the positive-electrode lead  51  and the negative-electrode lead  52  extend from the inside to the outside of the package members  59  in one direction. The positive-electrode lead  51  is composed of a conductive material, e.g., Al. The negative-electrode lead  52  is composed of a conductive material, e.g., Cu, Ni, stainless steel. These materials each have a sheet shape or a mesh shape. 
     For example, each of the package members  59  is formed of a laminated film in which a bonding layer, a metal layer, and a surface protective layer are stacked in that order. For the laminated films, for example, peripheral portions of the bonding layers of two laminated films are bonded together by fusion bonding or with an adhesive in such a manner that the bonding layers face the spirally wound electrode  50 . Each of the bonding layers is formed of a film of polyethylene, polypropylene, or the like. The metal layer is formed of Al foil or the like. The surface protective layer is formed of a film of nylon, polyethylene terephthalate, or the like. 
     In particular, as each package member  59 , an aluminum-laminated film in which a polyethylene film, aluminum foil, and a nylon film are stacked in that order is preferred. However, each package member  59  may be formed of a laminated film having another stacking structure. Alternatively, each package member  59  may be formed of a polymer film of polypropylene or a metal film. 
     Contact films  58  configured to prevent the entry of outside air are arranged between the positive-electrode lead  51  and the package members  59  and between the negative-electrode lead  52  and the package members  59 . Each of the contact films  58  is composed of a material adhesive to the positive-electrode lead  51  and the negative-electrode lead  52 . Examples of the material include polyolefin resins, such as polyethylene, polypropylene, modified polyethylene, and modified polypropylene. 
     The positive electrode  53  includes, for example, a positive-electrode active material layer  53 B provided on each surface of a positive-electrode collector  53 A. The negative electrode  54  has the same structure as the foregoing negative electrode for a lithium-ion secondary battery. For example, the negative electrode  54  includes a negative-electrode active material layer  54 B provided on each surface of a negative-electrode collector  54 A. The structures of the positive-electrode collector  53 A, the positive-electrode active material layers  53 B, the negative-electrode collector  54 A, and the negative-electrode active material layers  54 B are the same as those of the positive-electrode collector  21 A, the positive-electrode active material layers  21 B, the negative-electrode collector  22 A, and the negative-electrode active material layers  22 B, respectively. The structure of each of the separators  55  is the same as that of the separator  23 . 
     Each of the electrolyte layers  56  is formed of a component in which an electrolytic solution is held by a polymeric compound. Each electrolyte layer  56  may contain additional material, such as an additive, if necessary. The electrolyte layer  56  is composed of what is called a gel-like electrolyte. The gel-like electrolyte is preferred because a high ionic conductivity (e.g., 1 mS/cm or more at room temperature) is obtained and the leakage of the electrolytic solution from the battery is prevented. 
     The polymeric compound contains one or two or more compounds described below. Examples thereof include polyacrylonitrile, polyvinylidene fluoride, polytetrafluoroethylene, polyhexafluoropropylene, polyethylene oxide, polypropylene oxide, polyphosphazene, polysiloxane, polyvinyl fluoride, polyvinyl acetate, polyvinyl alcohol, polymethyl methacrylate, polyacrylic acid, polymethacrylic acid, styrene-butadiene rubber, nitrile-butadiene rubber, polystyrene, polycarbonate, and copolymers of vinylidene fluoride and hexafluoropropylene. Among these compounds, polyvinylidene fluoride or copolymers of vinylidene fluoride and hexafluoropropylene are preferred because of their good electrochemical stability. 
     For example, the electrolytic solution has the same composition as the composition of the electrolytic solution used in the prismatic secondary battery. However, in the electrolyte layers  56  composed of a gel-like electrolyte, the solvent of the electrolytic solution indicates a wide concept including not only a liquid solvent but also a material which is capable of dissociating an electrolyte salt and which has ionic conductivity. Thus, in the case where a polymeric compound having ionic conductivity is used, the polymeric compound is included in the concept of the solvent. 
     Instead of the gel-like electrolyte layers  56 , an electrolytic solution may be used. In this case, the separators  55  are impregnated with the electrolytic solution. 
     Operation of Secondary Battery 
     When the laminated-film-type secondary battery is charged, lithium ions released from the positive electrode  53  are occluded in the negative electrode  54  through the electrolyte layers  56 . When the laminated-film-type secondary battery is discharged, lithium ions released from the negative electrode  54  are occluded in the positive electrode  53  through the electrolyte layers  56 . 
     Method for Producing Secondary Battery 
     The laminated-film-type secondary battery including the gel-like electrolyte layers  56  is produced by, for example, three types of procedures described below. 
     In a first procedure, first, the positive electrode  53  and the negative electrode  54  are formed in the same ways as the procedures for forming the positive electrode  21  and the negative electrode  22 . In this case, the positive-electrode active material layer  53 B is formed on each surface of the positive-electrode collector  53 A to form the positive electrode  53 . The negative-electrode active material layer  54 B is formed on each of the surfaces of the negative-electrode collector  54 A to form the negative electrode  54 . Next, a precursor solution including an electrolytic solution, a polymer compound, an organic solvent, and the like is prepared. The precursor solution is applied to the positive electrode  53  and the negative electrode  54  to form the gel-like electrolyte layers  56 . Then the positive-electrode lead  51  is attached to the positive-electrode collector  53 A by a welding method or the like. The negative-electrode lead  52  is attached to the negative-electrode collector  54 A by a welding method or the like. Subsequently, the positive electrode  53  and the negative electrode  54  including the electrolyte layers  56  are stacked with the separators  55  and spirally wound to form the spirally wound electrode  50 . The protective tape  57  is bonded to the outermost portion of the spirally wound electrode  50 . Finally, the spirally wound electrode  50  is sandwiched between two film-shaped package members  59 . The peripheral portions of the package members  59  are bonded together by a heat fusion method or the like to seal the spirally wound electrode  50  in the package members  59 . In this case, the contact films  58  are interposed between the positive-electrode lead  51  and the package members  59  and between the negative-electrode lead  52  and the package members  59 . 
     In a second procedure, first, the positive-electrode lead  51  is attached to the positive electrode  53 . The negative-electrode lead  52  is attached to the negative electrode  54 . Next, the positive electrode  53  and the negative electrode  54  are stacked with the separators  55  provided therebetween and spirally wound to form a spirally wound component serving as a precursor of the spirally wound electrode  50 . The protective tape  57  is bonded to the outermost portion of the spirally wound component. Then the spirally wound component is sandwiched between two film-shaped package members  59 . The peripheral portions except for a peripheral portion on one side are bonded by a heat fusion method or the like to accommodate the spirally wound component in the pouch-like package members  59 . Next, an electrolytic composition containing an electrolytic solution, a monomer serving as a raw material of a polymeric compound, a polymerization initiator, and, optionally, an additional material, such as a polymerization inhibitor, is prepared. The resulting electrolytic composition is injected into the pouch-like package members  59 . The opening portion of the package members  59  is sealed by a heat fusion method or the like. Finally, the monomer is thermally polymerized to form a polymeric compound, thereby resulting in the gel-like electrolyte layers  56 . 
     In a third procedure, first, a spirally wound component is formed and accommodated in the pouch-like package members  59  in the same way as the second procedure, except that the separators  55  each having both surfaces coated with a polymeric compound is used. Examples of the polymeric compound applied to the separators  55  include polymers from vinylidene fluoride (homopolymers, copolymers, and multi-component copolymers). Specific examples thereof include polyvinylidene fluoride; two-component copolymers of vinylidene fluoride and hexafluoropropylene; and three-component copolymers of vinylidene fluoride, hexafluoropropylene, and chlorotrifluoroethylene. In combination with a polymer from vinylidene fluoride, one or two or more other polymeric compounds may be used. Next, the electrolytic solution is prepared and injected into the package members  59 . The opening portion of the package members  59  is sealed by a heat fusion method or the like. Finally, the package members  59  are heated under load to bring the separators  55  into close contact with the positive electrode  53  and the negative electrode  54  with the polymeric compound. Thus, the polymeric compound is impregnated with the electrolytic solution. Thereby, the polymeric compound gels to form the electrolyte layers  56 . 
     In the third procedure, swelling of the battery is suppressed, compared with the first procedure. A negligible amount of a monomer serving as a raw material for the polymeric compound, an organic solvent, or the like is left, compared with the second procedure, thereby satisfactorily controlling a step of forming the polymeric compound. Thus, the electrolyte layers  56  have sufficient adhesion to the positive electrode  53 , the negative electrode  54 , and the separators  55 . 
     Function and Effect of Secondary Battery 
     For the laminated-film-type secondary battery, the negative electrode  54  has the same structure as that of the foregoing negative electrode for a lithium-ion secondary battery, thereby providing the same effects as those of the prismatic secondary battery. 
     3. Application of Lithium-Ion Secondary Battery 
     Application examples of the foregoing lithium-ion secondary battery will be described below. 
     The application of the lithium-ion secondary battery is not particularly limited as long as the lithium-ion secondary battery is applied to machines, devices, appliances, apparatuses, systems (combinations of a plurality of devices), and the like which can use the lithium-ion secondary battery as a power source for operation or a power storage source for accumulation of power. In the case where the lithium-ion secondary battery is used as a power source, the power source may be used as a main power source (a power source to be preferentially used) or an auxiliary power source (a power source to be used instead of the main power source or by switching from the main power source). The type of the main power source is not limited to the lithium-ion secondary battery. 
     The lithium-ion secondary battery is applied to, for example, the following applications. Examples of the applications include portable electronic devices, such as video cameras, digital still cameras, cellular phones, notebook personal computers, cordless telephones, headphone stereos, portable radios, portable television sets, and personal digital assistants; portable home appliances, such as electric shavers; backup power sources; storage devices, such as memory cards; electric tools, such as electric drills and electric saws; battery packs used as power sources for notebook personal computers and so forth; medical electronic devices, such as pacemakers and hearing aids; vehicles, such as electric vehicles (including hybrid vehicles); and energy storage systems, such as household battery systems storing power in case of emergency or the like. The lithium-ion secondary battery may be applied to applications other than the foregoing applications. 
     In particular, the lithium-ion secondary battery is effectively applied to, for example, a battery pack, an electric vehicle, a power storage system, an electric tool, or an electronic device. This is because such an application demands excellent battery characteristics; hence, the use of the lithium-ion secondary battery according to an embodiment of the present application effectively improves the characteristics. The battery pack refers to a power source including the lithium-ion secondary batteries and is what is called a set of batteries or the like. The electric vehicle refers to a vehicle that operates (runs) using the lithium-ion secondary battery as a power source for operation. As described above, the electric vehicle may include a vehicle (e.g., a hybrid vehicle) with a driving source in addition to the lithium-ion secondary battery. The power storage system refers to a system including the lithium-ion secondary battery as a power storage source. For example, in a household energy storage system, electric power is stored in the lithium-ion secondary battery serving as a power storage source. Electric power is consumed when necessary, so home appliances can be used. The electric tool refers to a tool having a moving part (such as a drill) that is movable using the lithium-ion secondary battery as a power source for operation. The electronic device refers to a device that performs various functions using the lithium-ion secondary battery for operation. 
     Some application examples of the lithium-ion secondary battery will be specifically described below. Configurations of the application examples described below are merely examples and thus can be appropriately changed. 
     3-1. Battery Pack 
       FIG. 10  illustrates a block configuration of a battery pack. For example, as illustrated in  FIG. 10 , the battery pack includes a controller  61 , a power source  62 , a switching unit  63 , a current measurement unit  64 , a temperature detecting unit  65 , a voltage detecting unit  66 , a switching controller  67 , memory  68 , a temperature detecting element  69 , a current detecting resistance  70 , a positive-electrode terminal  71 , and a negative-electrode terminal  72  in a housing  60  composed of, for example, a plastic material. 
     The controller  61  controls the overall operation of the battery pack (including the usage state of the power source  62 ) and includes, for example, a central processing unit (CPU). The power source  62  includes one or two or more lithium-ion secondary batteries (not illustrated). The power source  62  refers to, for example, a set of batteries including two or more lithium-ion secondary batteries. These lithium-ion secondary batteries may be connected in series, in parallel, or in combination thereof. For example, the power source  62  includes six lithium-ion secondary batteries in which three sets of two batteries connected in parallel are connected in series. 
     The switching unit  63  is configured to switch the usage state of the power source  62  (availability of connection between the power source  62  and external equipment) in response to instructions from the controller  61 . The switching unit  63  includes, for example, a charge control switch, a discharge control switch, a diode for charge, and a diode for discharge (all elements are not illustrated). Examples of the charge control switch and the discharge control switch include semiconductor switches formed of, for example, metal oxide semiconductor field-effect transistors (MOSFETs). 
     The current measurement unit  64  is configured to measure a current with the current detecting resistance  70  and to send the measurement results to the controller  61 . The temperature detecting unit  65  is configured to measure a temperature with the temperature detecting element  69  and to send the measurement results to the controller  61 . For example, the temperature measurement results are used when the controller  61  controls charge and discharge at the time of abnormal heat generation and when the controller  61  performs correction at the time of the calculation of remaining battery capacity. The voltage detecting unit  66  is configured to measure the voltage of the lithium-ion secondary batteries in the power source  62 , subject the measured voltage to analog-to-digital (A/D) conversion, and send the resulting digital output to the controller  61 . 
     The switching controller  67  is configured to control the operation of the switching unit  63  in response to signals from the current measurement unit  64  and the voltage detecting unit  66 . 
     The switching controller  67  controls the switching unit  63  in such a manner that, for example, when the battery voltage reaches an overcharge detection voltage, the switching unit  63  (charge control switch) is disconnected so as not to allow a charging current to flow through the current path of the power source  62 . This permits the power source  62  only to discharge with the diode for discharge. For example, the switching controller  67  is configured to interrupt a charging current when a large current flows during charging. 
     Furthermore, the switching controller  67  controls the switching unit  63  in such a manner that, for example, when the battery voltage reaches an over-discharge detection voltage, the switching unit  63  (discharge control switch) is disconnected so as not to allow a discharge current to flow through the current path of the power source  62 . This permits the power source  62  only to charge with the diode for charge. For example, the switching controller  67  is configured to interrupt a discharge current when a large current flows during discharging. 
     In the lithium-ion secondary battery, for example, the overcharge detection voltage is 4.20 V±0.05 V, and the over-discharge detection voltage is 2.4 V±0.1 V. 
     An example of the memory  68  is electrically erasable programmable read-only memory (EEPROM), which is nonvolatile memory. For example, the memory  68  stores a numerical value calculated by the controller  61  and information about the lithium-ion secondary batteries (for example, initial internal resistance) measured in the production process. In the case where the full charge capacity of the lithium-ion secondary batteries is stored in the memory  68 , the controller  61  can obtain information about remaining capacity and so forth. 
     The temperature detecting element  69  is configured to measure the temperature of the power source  62  and to send the measurement results to the controller  61 . An example of the temperature detecting element  69  is a thermistor. 
     The positive-electrode terminal  71  and the negative-electrode terminal  72  are terminals for connection to external equipment (e.g., a notebook personal computer) operated by the battery pack or to external equipment (e.g., a charger) used to charge the battery pack. The charge and discharge of the power source  62  are performed through the positive-electrode terminal  71  and the negative-electrode terminal  72 . 
     3-2. Electric Vehicle 
       FIG. 11  illustrates a block configuration of a hybrid vehicle as an example of an electric vehicle. For example, as illustrated in  FIG. 11 , the electric vehicle includes a controller  74 , an engine  75 , a power source  76 , a driving motor  77 , a differential device  78 , a dynamo  79 , a transmission  80 , a clutch  81 , inverters  82  and  83 , and various sensors  84  in a metal chassis  73 . The electric vehicle further includes, for example, an axle shaft  85  for front wheels, the axle shaft  85  being connected to the differential device  78  and the transmission  80 , front wheels  86 , an axle shaft  87  for rear wheels, and rear wheels  88 . 
     The electric vehicle can run using any one of the engine  75  and the motor  77  as a driving source. The engine  75  serves as a main driving source, such as a gasoline engine. In the case where the engine  75  is used as a driving source, for example, the driving force (torque) of the engine  75  is transmitted to the front wheels  86  or the rear wheels  88  through the differential device  78 , the transmission  80 , and clutch  81  serving as drive members. The torque of the engine  75  is also transmitted to the dynamo  79  and allows the dynamo  79  to generate alternating-current (AC) power. The AC power is converted into direct-current (DC) power by the inverter  83 . The resulting DC power is stored in the power source  76 . Meanwhile, in the case where the motor  77  serving as a convertor is used as a driving source, power (DC power) supplied from the power source  76  is converted into AC power by the inverter  82 . The motor  77  is driven by the AC power. The driving force (torque) obtained by conversion of electric power using the motor  77  is transmitted to, for example, the front wheels  86  or the rear wheels  88  through the differential device  78 , the transmission  80 , and clutch  81  serving as drive members. 
     When the electric vehicle slows down with a brake mechanism (not illustrated), the resistance during the slowing down may be transmitted to the motor  77  in the form of torque, and the motor  77  may generate AC power using the torque. Preferably, the resulting AC power is converted into DC power by the inverter  82 , and the regenerative DC power is stored in the power source  76 . 
     The controller  74  controls the operation of the overall electric vehicle and includes, for example, a central processing unit (CPU). The power source  76  includes one or two or more lithium-ion secondary batteries (not illustrated). The power source  76  may be configured to be capable of storing power by establishing connection with an external power source and receiving power from the external power source. For example, the various sensors  84  are used to control the number of revolutions of the engine  75  and to control the position (throttle position) of a throttle valve (not illustrated). The various sensors  84  include, for example, a speed sensor, an acceleration sensor, and an engine speed sensor. 
     As described above, the hybrid vehicle has been described as an electric vehicle. The electric vehicle may be a vehicle (electric vehicle) operated by the power source  76  and the motor  77  without the engine  75 . 
     3-3. Power Storage System 
       FIG. 12  illustrates a block configuration of a power storage system. For example, as illustrated in  FIG. 12 , the power storage system includes a controller  90 , a power source  91 , a smart meter  92 , and a power hub  93  in a house  89 , e.g., a general house or commercial building. 
     Here, for example, the power source  91  is connected to electric equipment  94  installed in the house  89  and can be connected to an electric vehicle  96  parked outside the house  89 . Furthermore, for example, the power source  91  is connected to a private generator  95  mounted on the house  89  via the power hub  93  and can be connected to an external centralized power grid  97  via the smart meter  92  and the power hub  93 . 
     The electric equipment  94  includes one or two or more home appliances, such as refrigerators, television sets, and water heaters. The private generator  95  includes one or two or more generators, such as solar photovoltaic generators and wind generators. The electric vehicle  96  includes one or two or more vehicles, such as electric vehicles, electric motorcycles, and hybrid vehicles. The centralized power grid  97  includes one or two or more power grids connected to, for example, thermal power plants, nuclear power plants, hydroelectric power stations, and wind farms. 
     The controller  90  controls the operation of the overall power storage system (including the usage state of the power source  91 ) and includes, for example, a CPU. The power source  91  includes one or two or more lithium-ion secondary batteries (not illustrated). The smart meter  92  is, for example, a network-ready electrical meter installed in the house  89 , which is on the power demand side and can communicate with the power supply side. Thus, for example, the smart meter  92  is configured to control a balance between supply and demand in the house  89  while communicating with the outside, if necessary, thereby efficiently and stably supplying energy. 
     In this power storage system, for example, power is stored in the power source  91  from the centralized power grid  97 , which is an external power source, via the smart meter  92  and the power hub  93 . Furthermore, power is stored in the power source  91  from the private generator  95 , which is an independent power source, via the power hub  93 . The power stored in the power source  91  is supplied to the electric equipment  94  or the electric vehicle  96 , if necessary, in response to instructions from the controller  90 . Thus, the electric equipment  94  can be operated, and the electric vehicle  96  can be charged. That is, the power storage system is a system capable of storing and supplying power in the house  89  using the power source  91 . 
     Power stored in the power source  91  can be desirably used. For example, power can be stored in the power source  91  from the centralized power grid  97  during late-night hours in which the market price of electricity is low. The power stored in the power source  91  can be used during daytime hours in which the market price of electricity is high. 
     The foregoing power storage system may be installed for each house (family) or may be installed for each set of a plurality of houses (a plurality of families). 
     3-4. Electric Tool 
       FIG. 13  illustrates a block configuration of an electric tool. For example, as illustrated in  FIG. 13 , the electric tool is an electric drill that includes a controller  99  and a power source  100  in a main body  98  composed of, for example, a plastic material. For example, a drill portion  101 , which is a moving part, is rotatably attached to the main body  98 . 
     The controller  99  controls the overall operation of the electric tool (including the usage state of the power source  100 ) and includes, for example, a CPU. The power source  100  includes one or two or more lithium-ion secondary batteries (not illustrated). The controller  99  is configured to appropriately supply power from the power source  100  to the drill portion  101  in response to the operation of an operation switch (not illustrated) to drive the drill portion  101 . 
     EXAMPLES 
     Examples of the present application will be described in detail below. 
     Experimental Example 1-1 to 1-9 
     Laminated-film-type secondary batteries illustrated in  FIGS. 8 and 9  were produced by a procedure described below. 
     The positive electrode  53  was formed. First, 91 parts by mass of a positive-electrode active material (LiCoO 2 ), 6 parts by mass of a positive-electrode conductive agent (graphite), and 3 parts by mass of a positive-electrode binder (polyvinylidene fluoride: PVDF) were mixed together to form a positive-electrode mixture. The positive-electrode mixture was dispersed in an organic solvent (N-methyl-2-pyrrolidone:NMP) to form a paste-like slurry of the positive-electrode mixture. Then the slurry of the positive-electrode mixture was applied to both surface of the positive-electrode collector  53 A with a coating apparatus and dried to form the positive-electrode active material layers  53 B. As the positive-electrode collector  53 A, strip-shaped aluminum foil (thickness: 12 μm) was used. Finally, the positive-electrode active material layers  53 B were subjected to compression forming with a roll press. The thickness of each of the positive-electrode active material layers  53 B was adjusted so as to prevent the deposition of metallic Li on the negative electrode  54  in a fully charged state. 
     Next, the negative electrode  54  was formed. First, a core portion (SiO x ) was formed by a gas atomization method. A single-layer covering portion (SiO y +M1 (Ni)) was formed on a surface of the core portion by a powder evaporation method. The compositions of the core portion and the covering portion (atomic ratios x and y, ratio (M1/(Si+O))) were described in Table 1. In this case, the core portion had a half-width of 0.6°, a crystallite size of 90 nm, and a median diameter of 4 μm. The covering portion had an average thickness of 200 nm and an average coverage of 70%. 
     When the core portion was formed, the oxygen flow rate was adjusted during the melt-solidification of the raw material (Si) to control the composition (atomic ratio x). When the covering portion was formed, powdered SiO y  and powdered metal M1 were co-deposited. During the deposition of the raw materials, the flow rate of O 2  or H 2  was adjusted to control the composition (atomic ratio y). Simultaneously, the input power was adjusted to control the ratio (M1/(Si+O)). In the powder evaporation method, a deflective electron beam evaporation source was used. The raw material Si powder had a median diameter of 0.2 μm to 30 μm. The deposition rate was 2 nm/sec. A vacuum state, i.e., a pressure of 1×10 −3  Pa, was used with a turbo-molecular pump. 
     Next, a negative-electrode active material and a precursor of a negative-electrode binder were mixed together in a dry weight ratio of 90:10. The resulting mixture was diluted with NMP to form a paste-like slurry of a negative-electrode mixture. In this case, a polyamic acid containing NMP and N,N-dimethylacetamide (DMAC) was used. Then the slurry of the negative-electrode mixture was applied to both surface of the negative-electrode collector  54 A with a coating apparatus and dried. As the negative-electrode collector  54 A, rolled Cu foil (thickness: 15 μm, ten-point height of irregularities Rz&lt;0.5 μm) was used. Finally, in order to enhance binding properties, the resulting coating films were hot-pressed and baked in a vacuum atmosphere at 400° C. for 1 hour. Thereby, the negative-electrode binder (polyamide-imide) was formed, thus resulting in the negative-electrode active material layers  54 B containing the negative-electrode active material and the negative-electrode binder. The thickness of each of the negative-electrode active material layers  54 B was adjusted in such a manner that the negative-electrode utilization factor was 65%. 
     Next, an electrolyte salt (LiPF 6 ) was dissolved in a mixed solvent (ethylene carbonate (EC) and diethyl carbonate (DEC)) to prepare an electrolytic solution. In this case, with respect to the composition of the mixed solvent, the ratio by weight of EC to DEC was 50 to 50, and the proportion of the electrolyte salt was 1 mol/kg with respect to the mixed solvent. 
     Finally, the secondary battery was assembled. First, the positive-electrode lead  51  composed of Al was welded to an end of the positive-electrode collector  53 A. The negative-electrode lead  52  composed of Ni was welded to an end of the negative-electrode collector  54 A. Next, the positive electrode  53 , the separator  55 , the negative electrode  54 , and the separator  55  were stacked in that order. The resulting stack was spirally wound in a longitudinal direction to form a spirally wound component serving as a precursor of the spirally wound electrode  50 . The outermost portion of the spirally wound component was fixed with the protective tape  57  (adhesive tape). In this case, as each of the separators  55 , a laminated film (thickness: 20 μm) in which a film mainly containing porous polyethylene was sandwiched between films mainly composed of porous polypropylene was used. Next, the spirally wound component was sandwiched between the package members  59 . The peripheral portions except for a peripheral portion on one side were bonded by heat fusion to accommodate the spirally wound component in the pouch-like package members  59 . In this case, as each of the package members  59 , an aluminum-laminated film in which a nylon film (thickness: 30 μm), Al foil (thickness: 40 μm), and a non-stretched polypropylene film (thickness: 30 μm) were stacked in that order from the outside was used. Then the electrolytic solution was injected from an opening portion of the package members  59  to impregnate the separators  55  with the electrolytic solution, thereby forming the spirally wound electrode  50 . Finally, the opening portion of the package members  59  was sealed by heat fusion in a vacuum atmosphere. 
     The cycle characteristics, the initial charge-discharge characteristics, and the load characteristics of the secondary batteries were investigated. Table 1 illustrates the results. 
     In the case where the cycle characteristics were investigated, first, one charge-discharge cycle was performed in an atmosphere with a temperature of 23° C. in order to stabilize the battery state. Subsequently, another charge-discharge cycle was performed to measure the discharge capacity. Next, the charge-discharge cycle was repeated until the number of cycles reached 100 cycles, and then the discharge capacity was measured. Finally, the cycle retention rate was calculated from the following expression: cycle retention rate (%)=(discharge capacity at 100th cycle/discharge capacity at second cycle)×100. In the case of charging, each secondary battery was charged at a constant current density of 3 mA/cm 2  until the voltage reached 4.2 V, and then the battery was charged at a constant voltage of 4.2 V until the current density reached 0.3 mA/cm 2 . In the case of discharging, the battery was discharged at a constant current density of 3 mA/cm 2  until the voltage reached 2.5 V. 
     In the case where the initial charge-discharge characteristics were investigated, first, one charge-discharge cycle was performed in order to stabilize the battery state. Subsequently, each secondary battery was charged again to measure the charge capacity. Then the battery was discharged to measure the discharge capacity. Finally, the initial efficiency was calculated from the following expression: initial efficiency (%)=(discharge capacity/charge capacity)×100. The atmospheric temperature and charge-discharge conditions were the same as those in the case of investigating the cycle characteristics. 
     In the case of investigating the load characteristics, first, one charge-discharge cycle was performed in order to stabilize the battery state. Subsequently, the second cycle of the charge-discharge operation was performed to measure the discharge capacity. Then the third cycle of the charge-discharge operation was performed to measure the discharge capacity. Finally, the load retention rate was calculated from the following expression: load retention rate (%)=(discharge capacity at third cycle/discharge capacity at second cycle)×100. The atmospheric temperature and charge-discharge conditions were the same as those in the case of investigating the cycle characteristics, except that the discharge current density at the second cycle was changed to 0.2 mA/cm 2  and the discharge current density at the third cycle was changed to 1 mA/cm 2 . 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Covering portion 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                 Proportion 
                 rate 
                 efficiency 
                 rate 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 example 
                 Composition 
                 x 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 1-1 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ni 
                 1 
                 80.1 
                 80.2 
                 94.0 
               
               
                 1-2 
                   
                   
                   
                   
                   
                 5 
                 82.2 
                 82.2 
                 96.0 
               
               
                 1-3 
                   
                   
                   
                   
                   
                 10 
                 84.0 
                 83.6 
                 97.0 
               
               
                 1-4 
                   
                   
                   
                   
                   
                 20 
                 85.0 
                 84.0 
                 97.5 
               
               
                 1-5 
                   
                   
                   
                   
                   
                 30 
                 84.0 
                 83.2 
                 98.0 
               
               
                 1-6 
                   
                   
                   
                   
                   
                 50 
                 83.5 
                 82.5 
                 98.0 
               
               
                 1-7 
                   
                   
                   
                   
                   
                 60 
                 82.0 
                 81.5 
                 98.0 
               
               
                 1-8 
                 SiO x   
                 0.1 
                 — 
                 — 
                 — 
                 — 
                 33.0 
                 85.0 
                 98.0 
               
               
                 1-9 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 — 
                 — 
                 75.5 
                 78.5 
                 92.0 
               
               
                   
               
            
           
         
       
     
     In the case where the covering portion (Si+O+Ni) was formed on the surface of the core portion (Si+O), the cycle retention rate was significantly increased while maintaining high initial efficiency and a high load retention rate, as compared with the cases where the covering portion was not formed and where the covering portion did not contain Ni. 
     Specifically, the formation of the covering portion (Si+O) on the surface of the core portion (Si+O) resulted in a significant increase in cycle retention rate but resulted in reductions in initial efficiency and load retention rate, as compared with the case of the absence of the covering portion. In contrast, the formation of the covering portion (Si+O+Ni) on the surface of the core portion (Si+O) resulted in a further increase in cycle retention rate while maintaining an initial efficiency exceeding 80% and a load retention rate exceeding 90%, as compared with the case of the absence of the covering portion. The advantageous tendency to a further increase in cycle retention rate while minimizing reductions in initial efficiency and load retention rate is a specific tendency first accomplished by the formation of the covering portion (Si+O+Ni). 
     In particular, in the case where the covering portion (Si+O+Ni) was formed, an M1 ratio of 50 atomic percent or less resulted in inhibition of a reduction in battery capacity, thus providing a high battery capacity. In this case, an M1 ratio of 20 atomic percent or less resulted in a higher battery capacity. 
     Experimental Example 2-1 to 2-94 
     As illustrated in Tables 2 to 7, secondary batteries were produced by the same procedure as in Experimental Examples 1-1 to 1-7, except that different types and combinations of M1 metals were used. Characteristics of each of the resulting secondary batteries were investigated. In this case, in order to perform co-deposition with powdered SiO y , each powdered metal M1 was used. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Covering portion 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                 Proportion 
                 rate 
                 efficiency 
                 rate 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 example 
                 Composition 
                 x 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 2-1 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Al 
                 1 
                 80.0 
                 80.1 
                 94.0 
               
               
                 2-2 
                   
                   
                   
                   
                   
                 10 
                 83.2 
                 82.6 
                 96.0 
               
               
                 2-3 
                   
                   
                   
                   
                   
                 20 
                 83.6 
                 82.9 
                 97.0 
               
               
                 2-4 
                   
                   
                   
                   
                   
                 50 
                 81.5 
                 81.0 
                 98.0 
               
               
                 2-5 
                   
                   
                   
                   
                   
                 60 
                 81.0 
                 80.2 
                 98.0 
               
               
                 2-6 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Fe 
                 10 
                 83.6 
                 83.0 
                 96.0 
               
               
                 2-7 
                   
                   
                   
                   
                   
                 20 
                 84.0 
                 83.6 
                 97.0 
               
               
                 2-8 
                   
                   
                   
                   
                   
                 50 
                 82.5 
                 81.5 
                 98.0 
               
               
                 2-9 
                   
                   
                   
                   
                   
                 60 
                 82.0 
                 80.6 
                 98.0 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Covering portion 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                 Proportion 
                 rate 
                 efficiency 
                 rate 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 example 
                 Composition 
                 x 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
               
                 2-10 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Cu 
                 10 
                 81.6 
                 81.2 
                 96.0 
               
               
                 2-11 
                   
                   
                   
                   
                   
                 20 
                 81.8 
                 81.6 
                 97.0 
               
               
                 2-12 
                   
                   
                   
                   
                   
                 50 
                 81.0 
                 81.0 
                 98.0 
               
               
                 2-13 
                   
                   
                   
                   
                   
                 60 
                 80.9 
                 80.6 
                 98.0 
               
               
                 2-14 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 C 
                 10 
                 82.1 
                 81.6 
                 96.0 
               
               
                 2-15 
                   
                   
                   
                   
                   
                 20 
                 82.5 
                 81.7 
                 97.0 
               
               
                 2-16 
                   
                   
                   
                   
                   
                 50 
                 82.0 
                 81.5 
                 98.0 
               
               
                 2-17 
                   
                   
                   
                   
                   
                 60 
                 81.5 
                 81.3 
                 98.0 
               
               
                 2-18 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Mg 
                 10 
                 81.6 
                 81.5 
                 96.0 
               
               
                 2-19 
                   
                   
                   
                   
                   
                 20 
                 81.7 
                 81.7 
                 97.0 
               
               
                 2-20 
                   
                   
                   
                   
                   
                 50 
                 81.0 
                 81.0 
                 98.0 
               
               
                 2-21 
                   
                   
                   
                   
                   
                 60 
                 80.5 
                 80.4 
                 98.0 
               
               
                 2-22 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ca 
                 10 
                 82.1 
                 81.0 
                 96.0 
               
               
                 2-23 
                   
                   
                   
                   
                   
                 20 
                 82.5 
                 81.5 
                 97.0 
               
               
                 2-24 
                   
                   
                   
                   
                   
                 50 
                 82.0 
                 81.0 
                 98.0 
               
               
                 2-25 
                   
                   
                   
                   
                   
                 60 
                 81.6 
                 80.9 
                 98.0 
               
               
                 2-26 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ti 
                 10 
                 82.5 
                 81.5 
                 96.0 
               
               
                 2-27 
                   
                   
                   
                   
                   
                 20 
                 83.0 
                 82.0 
                 97.0 
               
               
                 2-28 
                   
                   
                   
                   
                   
                 50 
                 82.6 
                 82.0 
                 98.0 
               
               
                 2-29 
                   
                   
                   
                   
                   
                 60 
                 82.0 
                 81.6 
                 98.0 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
             
            
               
                   
                   
               
               
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Covering portion 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                 Proportion 
                 rate 
                 efficiency 
                 rate 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 example 
                 Composition 
                 x 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
               
                 2-30 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Cr 
                 10 
                 82.0 
                 81.0 
                 96.0 
               
               
                 2-31 
                   
                   
                   
                   
                   
                 20 
                 82.5 
                 81.5 
                 97.0 
               
               
                 2-32 
                   
                   
                   
                   
                   
                 50 
                 81.6 
                 81.2 
                 98.0 
               
               
                 2-33 
                   
                   
                   
                   
                   
                 60 
                 81.5 
                 80.6 
                 98.0 
               
               
                 2-34 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Mn 
                 10 
                 81.5 
                 81.0 
                 96.0 
               
               
                 2-35 
                   
                   
                   
                   
                   
                 20 
                 81.9 
                 81.5 
                 97.0 
               
               
                 2-36 
                   
                   
                   
                   
                   
                 50 
                 81.4 
                 81.6 
                 98.0 
               
               
                 2-37 
                   
                   
                   
                   
                   
                 60 
                 81.2 
                 81.0 
                 98.0 
               
               
                 2-38 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Co 
                 10 
                 83.5 
                 82.5 
                 96.0 
               
               
                 2-39 
                   
                   
                   
                   
                   
                 20 
                 83.6 
                 83.0 
                 97.0 
               
               
                 2-40 
                   
                   
                   
                   
                   
                 50 
                 82.8 
                 83.1 
                 98.0 
               
               
                 2-41 
                   
                   
                   
                   
                   
                 60 
                 82.1 
                 82.9 
                 98.0 
               
               
                 2-42 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ge 
                 10 
                 83.0 
                 82.0 
                 97.0 
               
               
                 2-43 
                   
                   
                   
                   
                   
                 20 
                 83.1 
                 82.2 
                 97.0 
               
               
                 2-44 
                   
                   
                   
                   
                   
                 50 
                 83.1 
                 82.1 
                 98.0 
               
               
                 2-45 
                   
                   
                   
                   
                   
                 60 
                 83.0 
                 82.1 
                 98.0 
               
               
                 2-46 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Zr 
                 10 
                 82.1 
                 81.5 
                 96.0 
               
               
                 2-47 
                   
                   
                   
                   
                   
                 20 
                 82.5 
                 81.6 
                 97.0 
               
               
                 2-48 
                   
                   
                   
                   
                   
                 50 
                 82.1 
                 81.5 
                 98.0 
               
               
                 2-49 
                   
                   
                   
                   
                   
                 60 
                 81.6 
                 81.3 
                 98.0 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 5 
               
             
            
               
                   
                   
               
               
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Covering portion 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                 Proportion 
                 rate 
                 efficiency 
                 rate 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 example 
                 Composition 
                 x 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
               
                 2-50 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Mo 
                 10 
                 82.1 
                 82.0 
                 97.0 
               
               
                 2-51 
                   
                   
                   
                   
                   
                 20 
                 82.6 
                 82.1 
                 97.0 
               
               
                 2-52 
                   
                   
                   
                   
                   
                 50 
                 81.1 
                 82.2 
                 98.0 
               
               
                 2-53 
                   
                   
                   
                   
                   
                 60 
                 80.5 
                 82.1 
                 98.0 
               
               
                 2-54 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ag 
                 10 
                 80.6 
                 81.5 
                 96.0 
               
               
                 2-55 
                   
                   
                   
                   
                   
                 20 
                 80.9 
                 81.7 
                 96.0 
               
               
                 2-56 
                   
                   
                   
                   
                   
                 50 
                 80.5 
                 81.5 
                 97.0 
               
               
                 2-57 
                   
                   
                   
                   
                   
                 60 
                 80.3 
                 81.4 
                 98.0 
               
               
                 2-58 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Sn 
                 10 
                 82.2 
                 82.5 
                 97.0 
               
               
                 2-59 
                   
                   
                   
                   
                   
                 20 
                 82.3 
                 82.4 
                 97.0 
               
               
                 2-60 
                   
                   
                   
                   
                   
                 50 
                 82.3 
                 82.5 
                 98.0 
               
               
                 2-61 
                   
                   
                   
                   
                   
                 60 
                 82.2 
                 82.5 
                 98.0 
               
               
                 2-62 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ba 
                 10 
                 80.2 
                 81.5 
                 96.0 
               
               
                 2-63 
                   
                   
                   
                   
                   
                 20 
                 82.6 
                 81.3 
                 96.0 
               
               
                 2-64 
                   
                   
                   
                   
                   
                 50 
                 82.6 
                 81.2 
                 97.0 
               
               
                 2-65 
                   
                   
                   
                   
                   
                 60 
                 82.5 
                 81.0 
                 98.0 
               
               
                 2-66 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 W 
                 10 
                 81.5 
                 82.0 
                 96.0 
               
               
                 2-67 
                   
                   
                   
                   
                   
                 20 
                 81.6 
                 82.1 
                 97.0 
               
               
                 2-68 
                   
                   
                   
                   
                   
                 50 
                 81.2 
                 82.0 
                 98.0 
               
               
                 2-69 
                   
                   
                   
                   
                   
                 60 
                 81.0 
                 81.8 
                 98.0 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 6 
               
             
            
               
                   
                   
               
               
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Covering portion 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                 Proportion 
                 rate 
                 efficiency 
                 rate 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 example 
                 Composition 
                 x 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 2-70 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ta 
                 10 
                 82.0 
                 82.0 
                 96.0 
               
               
                 2-71 
                   
                   
                   
                   
                   
                 20 
                 82.1 
                 82.3 
                 97.0 
               
               
                 2-72 
                   
                   
                   
                   
                   
                 50 
                 82.1 
                 82.3 
                 98.0 
               
               
                 2-73 
                   
                   
                   
                   
                   
                 60 
                 80.5 
                 82.1 
                 98.0 
               
               
                 2-74 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Na 
                 10 
                 81.5 
                 81.5 
                 96.0 
               
               
                 2-75 
                   
                   
                   
                   
                   
                 20 
                 81.8 
                 81.6 
                 96.0 
               
               
                 2-76 
                   
                   
                   
                   
                   
                 50 
                 81.0 
                 81.5 
                 97.0 
               
               
                 2-77 
                   
                   
                   
                   
                   
                 60 
                 81.0 
                 81.3 
                 98.0 
               
               
                 2-78 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 K 
                 10 
                 81.2 
                 81.3 
                 96.0 
               
               
                 2-79 
                   
                   
                   
                   
                   
                 20 
                 81.6 
                 81.5 
                 96.0 
               
               
                 2-80 
                   
                   
                   
                   
                   
                 50 
                 81.2 
                 81.4 
                 97.0 
               
               
                 2-81 
                   
                   
                   
                   
                   
                 60 
                 81.0 
                 81.2 
                 98.0 
               
               
                 2-82 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Li 
                 1 
                 82.5 
                 80.3 
                 97.0 
               
               
                 2-83 
                   
                   
                   
                   
                   
                 5 
                 82.4 
                 83.6 
                 98.0 
               
               
                 2-84 
                   
                   
                   
                   
                   
                 10 
                 82.4 
                 85.5 
                 98.0 
               
               
                 2-85 
                   
                   
                   
                   
                   
                 20 
                 82.1 
                 87.5 
                 98.0 
               
               
                 2-86 
                   
                   
                   
                   
                   
                 40 
                 82.1 
                 90.2 
                 98.0 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 7 
               
             
            
               
                   
                   
               
               
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Covering portion 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                 Proportion 
                 rate 
                 efficiency 
                 rate 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 example 
                 Composition 
                 x 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
               
                 2-87 
                   
                   
                   
                   
                   
                 5 + 5 
                 83.2 
                 84.0 
                 97.0 
               
               
                 2-88 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ni + Sn 
                 10 + 10 
                 83.3 
                 84.2 
                 97.0 
               
               
                 2-89 
                   
                   
                   
                   
                   
                 25 + 25 
                 83.4 
                 84.1 
                 98.0 
               
               
                 2-90 
                   
                   
                   
                   
                   
                 30 + 30 
                 83.0 
                 84.1 
                 98.0 
               
               
                 2-91 
                   
                   
                   
                   
                   
                 5 + 5 
                 83.0 
                 85.0 
                 97.0 
               
               
                 2-92 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ni + Li 
                 10 + 5  
                 83.1 
                 85.5 
                 98.0 
               
               
                 2-93 
                   
                   
                   
                   
                   
                 25 + 10 
                 83.1 
                 87.5 
                 98.0 
               
               
                 2-94 
                   
                   
                   
                   
                   
                 30 + 10 
                 83.0 
                 88.0 
                 98.0 
               
               
                   
               
            
           
         
       
     
     Even when different types and combinations of M1 metals were used, high cycle retention rates, high initial efficiency, and high load retention rates were obtained as with the results illustrated in Table 1. 
     Experimental Examples 3-1 to 3-7 
     As illustrated in Table 8, secondary batteries were produced by the same procedure as in Experimental Examples 1-1 to 1-7, except that the covering portions had different compositions (different atomic ratios y). Characteristics of each of the resulting secondary batteries were investigated. In this case, the oxygen flow rate was adjusted during the melt-solidification of the raw material (Si) to control the atomic ratio y. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 8 
               
             
            
               
                   
                   
               
               
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Covering portion 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                 Proportion 
                 rate 
                 efficiency 
                 rate 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 example 
                 Composition 
                 x 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 3-1 
                   
                   
                   
                 0.2 
                   
                   
                 48.0 
                 84.9 
                 98.0 
               
               
                 3-2 
                   
                   
                   
                 0.5 
                   
                   
                 76.0 
                 84.5 
                 98.0 
               
               
                 3-3 
                   
                   
                   
                 0.7 
                   
                   
                 79.0 
                 84.3 
                 97.0 
               
               
                 3-4 
                 SiO x   
                 0.1 
                 SiO y   
                 1 
                 Ni 
                 10 
                 81.0 
                 84.0 
                 97.0 
               
               
                 3-5 
                   
                   
                   
                 1.4 
                   
                   
                 82.0 
                 83.1 
                 97.0 
               
               
                 3-6 
                   
                   
                   
                 1.8 
                   
                   
                 81.0 
                 82.9 
                 96.0 
               
               
                 3-7 
                   
                   
                   
                 2 
                   
                   
                 35.0 
                 84.0 
                 86.0 
               
               
                   
               
            
           
         
       
     
     When the atomic ratio y was 0.5≦y≦1.5, a high cycle retention rate was obtained. 
     Experimental Examples 4-1 to 4-9 and 5-1 to 5-10 
     As illustrated in Tables 9 and 10, secondary batteries were produced by the same procedure as in Experimental Examples 1-1 to 1-7, except that the covering portions had different average coverage values and average thickness values. Characteristics of each of the resulting secondary batteries were investigated. In this case, during the formation of the covering portion, the input power and the deposition time were changed to control the average coverage, and the deposition rate and the deposition time were changed to control the average thickness. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 9 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Covering portion 
                 Average 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                 Proportion 
                 coverage 
                 rate 
                 efficiency 
                 rate 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 example 
                 Composition 
                 x 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (%) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 4-1 
                   
                   
                   
                   
                   
                   
                 10 
                 76.0 
                 86.0 
                 95.0 
               
               
                 4-2 
                   
                   
                   
                   
                   
                   
                 20 
                 78.0 
                 85.6 
                 95.0 
               
               
                 4-3 
                   
                   
                   
                   
                   
                   
                 30 
                 81.0 
                 85.2 
                 96.0 
               
               
                 4-4 
                   
                   
                   
                   
                   
                   
                 40 
                 82.0 
                 84.5 
                 96.0 
               
               
                 4-5 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 50 
                 83.0 
                 84.2 
                 96.0 
               
               
                 4-6 
                   
                   
                   
                   
                   
                   
                 60 
                 83.5 
                 84.0 
                 96.0 
               
               
                 4-7 
                   
                   
                   
                   
                   
                   
                 80 
                 84.5 
                 83.0 
                 97.0 
               
               
                 4-8 
                   
                   
                   
                   
                   
                   
                 90 
                 85.0 
                 82.5 
                 97.0 
               
               
                 4-9 
                   
                   
                   
                   
                   
                   
                 100 
                 85.0 
                 82.0 
                 98.0 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 10 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Covering portion 
                 Average 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                 Proportion 
                 thickness 
                 rate 
                 efficiency 
                 rate 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 example 
                 Composition 
                 x 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (nm) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 5-1 
                   
                   
                   
                   
                   
                   
                 1 
                 75.6 
                 84.5 
                 95.0 
               
               
                 5-2 
                   
                   
                   
                   
                   
                   
                 10 
                 78.0 
                 84.0 
                 95.0 
               
               
                 5-3 
                   
                   
                   
                   
                   
                   
                 100 
                 82.0 
                 83.8 
                 96.0 
               
               
                 5-4 
                   
                   
                   
                   
                   
                   
                 500 
                 85.0 
                 83.4 
                 97.0 
               
               
                 5-5 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 1000 
                 85.5 
                 82.5 
                 98.0 
               
               
                 5-6 
                   
                   
                   
                   
                   
                   
                 2000 
                 85.6 
                 82.0 
                 97.0 
               
               
                 5-7 
                   
                   
                   
                   
                   
                   
                 3000 
                 85.6 
                 81.5 
                 97.0 
               
               
                 5-8 
                   
                   
                   
                   
                   
                   
                 5000 
                 85.6 
                 80.7 
                 96.0 
               
               
                 5-9 
                   
                   
                   
                   
                   
                   
                 10000 
                 85.7 
                 80.2 
                 95.0 
               
               
                  5-10 
                   
                   
                   
                   
                   
                   
                 15000 
                 85.7 
                 79.0 
                 95.0 
               
               
                   
               
            
           
         
       
     
     In the case where the average coverage was 30% or more and where the average thickness was in the range of 1 nm to 10,000 nm, a high cycle retention rate was obtained. 
     Experimental Examples 6-1 to 6-5 
     As illustrated in Table 11, secondary batteries were produced by the same procedure as in Experimental Examples 1-1 to 1-7, except that the covering portions had different layer structures. Characteristics of each of the resulting secondary batteries were investigated. In this case, a process for forming the covering portion was performed in two divided steps, thereby providing the multilayer covering portion. Furthermore, when the covering portion was formed, the substrate temperature during code position was changed, thereby controlling the bonding state in the covering portion. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 11 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Covering portion 
                   
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                 Proportion 
                 Layer 
                 Bonding 
                 rate 
                 efficiency 
                 rate 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 example 
                 Composition 
                 x 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 structure 
                 state 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
               
                 6-1 
                   
                   
                   
                   
                   
                   
                 multilayer 
                 SiO + Ni 
                 84.0 
                 83.5 
                 97.0 
               
               
                 6-2 
                   
                   
                   
                   
                   
                   
                 single 
                 SiNiO + Ni 
                 84.0 
                 83.6 
                 98.0 
               
               
                   
                   
                   
                   
                   
                   
                   
                 layer 
               
               
                 6-3 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 single 
                 SiO + Ni 
                 84.1 
                 83.5 
                 98.0 
               
               
                   
                   
                   
                   
                   
                   
                   
                 layer 
               
               
                 6-4 
                   
                   
                   
                   
                   
                   
                 multilayer 
                 SiNiO + Ni 
                 84.1 
                 83.5 
                 98.0 
               
               
                 6-5 
                   
                   
                   
                   
                   
                   
                 multilayer 
                 SiNiO + Ni/ 
                 84.0 
                 83.6 
                 98.0 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 SiO + Ni 
               
               
                   
               
            
           
         
       
     
     The presence of the multilayer covering portion increased the cycle retention rate. Furthermore, the presence of SiNiO in the covering portion further increased the cycle retention rate. 
     Experimental Examples 7-1 to 7-5 
     As illustrated in Table 12, secondary batteries were produced by the same procedure as in Experimental Examples 1-1 to 1-7, except that the core portions had different compositions (atomic ratios x). Characteristics of each of the resulting secondary batteries were investigated. In this case, the oxygen flow rate was adjusted during the melt-solidification of the raw material (Si) to control the atomic ratio x. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 12 
               
             
            
               
                   
                   
               
               
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Covering portion 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                 Proportion 
                 rate 
                 efficiency 
                 rate 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 example 
                 Composition 
                 x 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 7-1 
                   
                 0 
                   
                   
                   
                   
                 83.0 
                 84.5 
                 97.0 
               
               
                 7-2 
                   
                 0.05 
                   
                   
                   
                   
                 83.5 
                 84.0 
                 97.0 
               
               
                 7-3 
                 SiO x   
                 0.3 
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 84.2 
                 82.0 
                 97.0 
               
               
                 7-4 
                   
                 0.5 
                   
                   
                   
                   
                 84.5 
                 80.0 
                 97.0 
               
               
                 7-5 
                   
                 0.7 
                   
                   
                   
                   
                 84.9 
                 78.7 
                 96.0 
               
               
                   
               
            
           
         
       
     
     When the atomic ratio x was 0≦x&lt;0.5, the cycle retention rate and the initial efficiency were further increased. 
     Experimental Examples 8-1 to 8-3 
     As illustrated in Table 13, secondary batteries were produced by the same procedure as in Experimental Examples 1-1 to 1-7, except that the core portions were composed of different materials. Characteristics of each of the resulting secondary batteries were investigated. As the material constituting the core portion, a Sn alloy was used. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 13 
               
             
            
               
                   
                   
               
               
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Covering portion 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                   
                   
                 Proportion 
                 rate 
                 efficiency 
                 rate 
               
               
                 example 
                 Composition 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
               
                 8-1 
                 SnCo 
                   
                   
                   
                   
                 83.2 
                 82.0 
                 95.0 
               
               
                 8-2 
                 SnCoTi 
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 83.4 
                 82.1 
                 96.0 
               
               
                 8-3 
                 SnFeCo 
                   
                   
                   
                   
                 83.4 
                 82.0 
                 96.0 
               
               
                   
               
            
           
         
       
     
     Even when the core portion was composed of elemental Sn or the Sn alloy, a high cycle retention rate, high initial efficiency, and a high load retention rate were obtained. 
     Experimental Examples 9-1 to 9-14 
     As illustrated in Table 14, secondary batteries were produced by the same procedure as in Experimental Examples 1-1 to 1-7, except that element M2 (e.g., Fe) was incorporated into each core portion. Characteristics of each of the resulting secondary batteries were investigated. In this case, the core portion was formed by a gas atomization method using powdered SiO x  and powdered metal M2 (e.g., Al) as raw materials. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 14 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Core portion 
                 Covering portion 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Experimental 
                   
                   
                   
                 Proportion 
                   
                   
                   
                 Proportion 
                 rate 
                 efficiency 
                 rate 
               
               
                 example 
                 Composition 
                 x 
                 M2 
                 (at. %) 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 9-1 
                   
                   
                   
                 0.01 
                   
                   
                   
                   
                 84.2 
                 83.6 
                 97.0 
               
               
                 9-2 
                   
                   
                   
                 0.1 
                   
                   
                   
                   
                 84.3 
                 83.5 
                 97.0 
               
               
                 9-3 
                   
                   
                   
                 1 
                   
                   
                   
                   
                 84.6 
                 83.5 
                 98.0 
               
               
                 9-4 
                 SiO x   
                 0.1 
                 Al 
                 10 
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 85.0 
                 83.5 
                 98.0 
               
               
                 9-5 
                   
                   
                   
                 30 
                   
                   
                   
                   
                 85.2 
                 83.4 
                 98.0 
               
               
                 9-6 
                   
                   
                   
                 50 
                   
                   
                   
                   
                 85.6 
                 83.4 
                 98.0 
               
               
                 9-7 
                   
                   
                   
                 60 
                   
                   
                   
                   
                 85.6 
                 83.3 
                 98.0 
               
               
                 9-8 
                   
                   
                   
                 0.01 
                   
                   
                   
                   
                 84.5 
                 83.8 
                 97.0 
               
               
                 9-9 
                   
                   
                   
                 0.1 
                   
                   
                   
                   
                 84.6 
                 83.7 
                 97.0 
               
               
                 9-10 
                   
                   
                   
                 1 
                   
                   
                   
                   
                 84.8 
                 83.5 
                 98.0 
               
               
                 9-11 
                 SiO x   
                 0.1 
                 Fe 
                 10 
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 85.0 
                 83.5 
                 98.0 
               
               
                 9-12 
                   
                   
                   
                 30 
                   
                   
                   
                   
                 85.3 
                 84.0 
                 98.0 
               
               
                 9-13 
                   
                   
                   
                 50 
                   
                   
                   
                   
                 85.3 
                 84.0 
                 98.0 
               
               
                 9-14 
                   
                   
                   
                 60 
                   
                   
                   
                   
                 85.4 
                 83.6 
                 98.0 
               
               
                   
               
            
           
         
       
     
     The incorporation of M2 into the core portion resulted in a further increase in cycle retention rate. In this case, an M2 ratio of 0.01 atomic percent to 50 atomic percent resulted in a higher battery capacity. 
     Experimental Examples 10-1 to 10-60 
     As illustrated in Tables 15 to 17, secondary batteries were produced by the same procedure as in Experimental Examples 1-1 to 1-7, except that element M3 (e.g., Cr) or element M4 (e.g., B) was incorporated into each core portion. Characteristics of each of the resulting secondary batteries were investigated. In this case, the core portion was formed by a gas atomization method using powdered SiO x  and, for example, powdered metal M3 (Cr or the like) as raw materials. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 15 
               
             
            
               
                   
                   
               
               
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Covering portion 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                   
                   
                 Proportion 
                 rate 
                 efficiency 
                 rate 
               
               
                 example 
                 (M3, M4) 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
               
                 10-1 
                 Si 50 Al 49 Cr 1   
                   
                   
                   
                   
                 86.1 
                 83.8 
                 98.0 
               
               
                 10-2 
                 Si 50 Al 49 Ni 1   
                   
                   
                   
                   
                 86.0 
                 83.7 
                 97.0 
               
               
                 10-3 
                 Si 50 Al 49 Fe 1   
                   
                   
                   
                   
                 86.2 
                 83.5 
                 98.0 
               
               
                 10-4 
                 Si 40 Al 41 Cr 19   
                   
                   
                   
                   
                 86.3 
                 83.5 
                 97.0 
               
               
                 10-5 
                 Si 40 Al 41 Ni 19   
                   
                   
                   
                   
                 86.2 
                 83.6 
                 98.0 
               
               
                 10-6 
                 Si 40 Al 41 Fe 19   
                   
                   
                   
                   
                 86.4 
                 83.5 
                 97.0 
               
               
                 10-7 
                 Si 35 Al 46 Cr 19   
                   
                   
                   
                   
                 86.3 
                 83.5 
                 97.0 
               
               
                 10-8 
                 Si 35 Al 46 Ni 19   
                   
                   
                   
                   
                 86.2 
                 83.5 
                 97.0 
               
               
                 10-9 
                 Si 35 Al 46 Fe 19   
                   
                   
                   
                   
                 86.2 
                 83.4 
                 97.0 
               
               
                 10-10 
                 Si 30 Al 20 Cr 50   
                   
                   
                   
                   
                 86.1 
                 83.4 
                 97.0 
               
               
                 10-11 
                 Si 30 Al 20 Ni 50   
                   
                   
                   
                   
                 86.4 
                 83.3 
                 97.0 
               
               
                 10-12 
                 Si 30 Al 20 Fe 50   
                   
                   
                   
                   
                 86.2 
                 83.5 
                 98.0 
               
               
                 10-13 
                 Si 30 Al 10 Cr 60   
                   
                   
                   
                   
                 86.1 
                 83.6 
                 97.0 
               
               
                 10-14 
                 Si 30 Al 10 Ni 60   
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 86.0 
                 83.5 
                 98.0 
               
               
                 10-15 
                 Si 30 Al 10 Fe 60   
                   
                   
                   
                   
                 86.2 
                 83.5 
                 97.0 
               
               
                 10-16 
                 Si 30 Al 47.5 Cr 22.49 Cu 0.01   
                   
                   
                   
                   
                 86.3 
                 83.5 
                 97.0 
               
               
                 10-17 
                 Si 30 Al 47.5 Ni 22.49 Cu 0.01   
                   
                   
                   
                   
                 86.2 
                 83.7 
                 98.0 
               
               
                 10-18 
                 Si 30 Al 47.5 Fe 22.49 Cu 0.01   
                   
                   
                   
                   
                 86.4 
                 83.5 
                 97.0 
               
               
                 10-19 
                 Si 30 Al 47.5 Cr 12.5 Cu 10   
                   
                   
                   
                   
                 86.3 
                 83.5 
                 97.0 
               
               
                 10-20 
                 Si 30 Al 47.5 Ni 12.5 Cu 10   
                   
                   
                   
                   
                 86.2 
                 83.6 
                 97.0 
               
               
                 10-21 
                 Si 30 Al 47.5 Fe 12.5 Cu 10   
                   
                   
                   
                   
                 86.2 
                 83.5 
                 97.0 
               
               
                 10-22 
                 Si 30 Al 25 Cr 25 Cu 20   
                   
                   
                   
                   
                 86.2 
                 83.5 
                 97.0 
               
               
                 10-23 
                 Si 30 Al 25 Ni 25 Cu 20   
                   
                   
                   
                   
                 86.1 
                 83.5 
                 97.0 
               
               
                 10-24 
                 Si 30 Al 25 Fe 25 Cu 20   
                   
                   
                   
                   
                 86.4 
                 83.4 
                 98.0 
               
               
                 10-25 
                 Si 30 Al 20 Cr 30 Cu 20   
                   
                   
                   
                   
                 86.2 
                 83.4 
                 97.0 
               
               
                 10-26 
                 Si 30 Al 20 Ni 30 Cu 20   
                   
                   
                   
                   
                 86.1 
                 83.3 
                 98.0 
               
               
                 10-27 
                 Si 30 Al 20 Fe 30 Cu 20   
                   
                   
                   
                   
                 86.0 
                 83.5 
                 97.0 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 16 
               
             
            
               
                   
                   
               
               
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Covering portion 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                   
                   
                 Proportion 
                 rate 
                 efficiency 
                 rate 
               
               
                 example 
                 (M3, M4) 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
               
                 10-28 
                 Si 30 Al 27.5 Cr 12.5 Cu 30   
                   
                   
                   
                   
                 86.2 
                 83.5 
                 97.0 
               
               
                 10-29 
                 Si 30 Al 27.5 Ni 12.5 Cu 30   
                   
                   
                   
                   
                 86.3 
                 83.4 
                 98.0 
               
               
                 10-30 
                 Si 30 Al 27.5 Fe 12.5 Cu 30   
                   
                   
                   
                   
                 86.2 
                 83.4 
                 97.0 
               
               
                 10-31 
                 Si 30 Al 20 Cr 12.5 Cu 37.5   
                   
                   
                   
                   
                 86.3 
                 83.3 
                 97.0 
               
               
                 10-32 
                 Si 30 Al 20 Ni 12.5 Cu 37.5   
                   
                   
                   
                   
                 86.2 
                 83.5 
                 97.0 
               
               
                 10-33 
                 Si 30 Al 20 Fe 12.5 Cu 37.5   
                   
                   
                   
                   
                 86.4 
                 83.6 
                 97.0 
               
               
                 10-34 
                 Si 30 Al 47.5 Cr 12.5 B 10   
                   
                   
                   
                   
                 86.3 
                 83.5 
                 97.0 
               
               
                 10-35 
                 Si 30 Al 47.5 Cr 12.5 Mg 10   
                   
                   
                   
                   
                 86.2 
                 83.5 
                 97.0 
               
               
                 10-36 
                 Si 30 Al 47.5 Cr 12.5 Ca 10   
                   
                   
                   
                   
                 86.2 
                 83.5 
                 97.0 
               
               
                 10-37 
                 Si 30 Al 47.5 Cr 12.5 Ti 10   
                   
                   
                   
                   
                 86.2 
                 83.7 
                 97.0 
               
               
                 10-38 
                 Si 30 Al 47.5 Cr 12.5 V 10   
                   
                   
                   
                   
                 86.4 
                 83.5 
                 98.0 
               
               
                 10-39 
                 Si 30 Al 47.5 Cr 12.5 Mn 10   
                   
                   
                   
                   
                 86.2 
                 83.5 
                 97.0 
               
               
                 10-40 
                 Si 30 Al 47.5 Cr 12.5 Co 10   
                   
                   
                   
                   
                 86.1 
                 83.6 
                 98.0 
               
               
                 10-41 
                 Si 30 Al 47.5 Cr 12.5 Ge 10   
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 86.0 
                 83.5 
                 97.0 
               
               
                 10-42 
                 Si 30 Al 47.5 Cr 12.5 Y 10   
                   
                   
                   
                   
                 86.2 
                 83.5 
                 97.0 
               
               
                 10-43 
                 Si 30 Al 47.5 Cr 12.5 Zr 10   
                   
                   
                   
                   
                 86.3 
                 83.4 
                 98.0 
               
               
                 10-44 
                 Si 30 Al 47.5 Cr 12.5 Mo 10   
                   
                   
                   
                   
                 86.2 
                 83.4 
                 97.0 
               
               
                 10-45 
                 Si 30 Al 47.5 Cr 12.5 Ag 10   
                   
                   
                   
                   
                 86.3 
                 83.3 
                 97.0 
               
               
                 10-46 
                 Si 30 Al 47.5 Cr 12.5 In 10   
                   
                   
                   
                   
                 86.4 
                 83.5 
                 97.0 
               
               
                 10-47 
                 Si 30 Al 47.5 Cr 12.5 Sn 10   
                   
                   
                   
                   
                 86.3 
                 83.5 
                 98.0 
               
               
                 10-48 
                 Si 30 Al 47.5 Cr 12.5 Sb 10   
                   
                   
                   
                   
                 86.2 
                 83.4 
                 97.0 
               
               
                 10-49 
                 Si 30 Al 47.5 Cr 12.5 Ta 10   
                   
                   
                   
                   
                 86.2 
                 83.4 
                 98.0 
               
               
                 10-50 
                 Si 30 Al 47.5 Cr 12.5 W 10   
                   
                   
                   
                   
                 86.2 
                 83.3 
                 97.0 
               
               
                 10-51 
                 Si 30 Al 47.5 Cr 12.5 Pb 10   
                   
                   
                   
                   
                 86.4 
                 83.5 
                 97.0 
               
               
                 10-52 
                 Si 30 Al 47.5 Cr 12.5 La 10   
                   
                   
                   
                   
                 86.2 
                 83.5 
                 98.0 
               
               
                 10-53 
                 Si 30 Al 47.5 Cr 12.5 Ce 10   
                   
                   
                   
                   
                 86.1 
                 83.5 
                 97.0 
               
               
                 10-54 
                 Si 30 Al 47.5 Cr 12.5 Pr 10   
                   
                   
                   
                   
                 86.4 
                 83.7 
                 98.0 
               
               
                 10-55 
                 Si 30 Al 47.5 Cr 12.5 Nd 10   
                   
                   
                   
                   
                 86.2 
                 83.5 
                 97.0 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 17 
               
             
            
               
                   
                   
               
               
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Covering portion 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                   
                   
                 Proportion 
                 rate 
                 efficiency 
                 rate 
               
               
                 example 
                 (M3, M4) 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
               
                 10-56 
                 Si 10 Al 47.5 Cr 12.5 Cu 30   
                   
                   
                   
                   
                 86.4 
                 83.5 
                 98.0 
               
               
                 10-57 
                 Si 20 Al 47.5 Cr 7.5 Cu 5   
                   
                   
                   
                   
                 86.2 
                 83.6 
                 97.0 
               
               
                 10-58 
                 Si 80 Al 10 Cr 5 Cu 5   
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 86.1 
                 83.5 
                 97.0 
               
               
                 10-59 
                 Si 85 Al 5 Cr 5 Cu 5   
                   
                   
                   
                   
                 86.0 
                 83.3 
                 98.0 
               
               
                 10-60 
                 Si 30 Al 47.5 Cr 12.5 Cu 10   
                   
                   
                   
                   
                 86.2 
                 83.3 
                 97.0 
               
               
                   
               
            
           
         
       
     
     The incorporation of M3 or M4 into the core portion resulted in a further increase in cycle retention rate. In this case, when the M3 ratio was in the range of 1 atomic percent to 50 atomic percent and when the M4 ratio was in the range of 0.01 atomic percent to 30 atomic percent, a higher battery capacity was obtained. 
     Experimental Examples 11-1 to 11-8 
     As illustrated in Table 18, secondary batteries were produced by the same procedure as in Experimental Examples 1-1 to 1-7, except that a conductive portion was formed on the surface of each core portion. Characteristics of each of the resulting secondary batteries were investigated. In this case, each conductive portion was formed by the same procedure as the procedure for forming the covering portion. The average thickness and the average coverage of each conductive portion are described in Table 18. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 18 
               
             
            
               
                   
                   
               
               
                   
                 Conductive portion 
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Covering portion 
                   
                 Average 
                 Average 
                 Cycle retention 
                 Initial 
                 Load retention 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Experimental 
                 Core portion 
                   
                 Proportion 
                   
                 thickness 
                 coverage 
                 rate 
                 efficiency 
                 rate 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 example 
                 Composition 
                 x 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 Type 
                 (nm) 
                 (%) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 11-1 
                   
                   
                   
                   
                   
                   
                   
                   
                 5 
                 84.0 
                 83.6 
                 97.0 
               
               
                 11-2 
                   
                   
                   
                   
                   
                   
                   
                   
                 10 
                 84.0 
                 83.6 
                 98.0 
               
               
                 11-3 
                   
                   
                   
                   
                   
                   
                   
                   
                 15 
                 84.1 
                 83.6 
                 98.0 
               
               
                 11-4 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 C 
                 100 
                 30 
                 84.1 
                 83.7 
                 98.0 
               
               
                 11-5 
                   
                   
                   
                   
                   
                   
                   
                   
                 50 
                 84.1 
                 83.7 
                 99.0 
               
               
                 11-6 
                   
                   
                   
                   
                   
                   
                   
                   
                 70 
                 84.1 
                 83.7 
                 99.0 
               
               
                 11-7 
                   
                   
                   
                   
                   
                   
                   
                   
                 90 
                 84.1 
                 83.8 
                 99.0 
               
               
                 11-8 
                   
                   
                   
                   
                   
                   
                   
                   
                 99 
                 84.2 
                 83.8 
                 99.0 
               
               
                   
               
            
           
         
       
     
     The formation of the conductive portion provided better results. 
     Experimental Examples 12-1 to 12-6 
     As illustrated in Table 19, secondary batteries were produced by the same procedure as in Experimental Examples 1-1 to 1-7, except that the core portions had different median diameters. Characteristics of each of the resulting secondary batteries were investigated. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 19 
               
             
            
               
                   
                   
               
               
                   
                 Core portion 
                   
                 Cycle 
                   
                 Load 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Median 
                 Covering portion 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Experimental 
                   
                   
                 diameter 
                   
                   
                   
                 Proportion 
                 rate 
                 efficiency 
                 rate 
               
               
                 example 
                 Composition 
                 x 
                 (nm) 
                 Composition 
                 y 
                 M1 
                 (at. %) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 12-1 
                   
                   
                 0.1 
                   
                   
                   
                   
                 82.6 
                 79.2 
                 97.0 
               
               
                 12-2 
                   
                   
                 0.3 
                   
                   
                   
                   
                 83.0 
                 80.5 
                 97.0 
               
               
                 12-3 
                 SiO x   
                 0.1 
                 1 
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 83.3 
                 82.3 
                 97.0 
               
               
                 12-4 
                   
                   
                 10 
                   
                   
                   
                   
                 82.0 
                 84.5 
                 98.0 
               
               
                 12-5 
                   
                   
                 20 
                   
                   
                   
                   
                 81.0 
                 83.0 
                 98.0 
               
               
                 12-6 
                   
                   
                 30 
                   
                   
                   
                   
                 76.0 
                 79.0 
                 96.0 
               
               
                   
               
            
           
         
       
     
     A median diameter of the core portion of 0.3 μm to 20 μm resulted in a high cycle retention rate, high initial efficiency, and a high battery capacity. 
     Experimental Examples 13-1 to 13-18 
     As illustrated in Table 20, secondary batteries were produced by the same procedure as in Experimental Examples 1-1 to 1-7, except that different negative-electrode binders were used. Characteristics of each of the resulting secondary batteries were investigated. In this case, as the negative-electrode binders, polyimide (PI), polyvinylidene fluoride (PVDF), polyamide (PA), polyacrylic acid (PAA), lithium polyacrylate (PAAL), and carbonized polyimide (carbonized PI) were used. In the case where PAA or PAAL was used, a slurry of a negative-electrode mixture was prepared with an aqueous solution containing 17% by volume PAA or PAAL. Hot-pressing was performed to form the negative-electrode active material layers  54 B without baking. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 20 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Core 
                 Covering portion 
                 Negative- 
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Experimental 
                 portion 
                   
                 Proportion 
                 electrode 
                 rate 
                 efficiency 
                 rate 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 example 
                 Type 
                 x 
                 Type 
                 y 
                 M1 
                 (at. %) 
                 binder 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
               
                 13-1 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 PI 
                 83.5 
                 83.4 
                 97.0 
               
               
                 13-2 
                   
                   
                   
                   
                   
                 20 
                   
                 84.6 
                 83.5 
                 97.0 
               
               
                 13-3 
                   
                   
                   
                   
                   
                 50 
                   
                 83.3 
                 82.1 
                 97.0 
               
               
                 13-4 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 PVDF 
                 82.8 
                 83.1 
                 97.0 
               
               
                 13-5 
                   
                   
                   
                   
                   
                 20 
                   
                 84.0 
                 83.3 
                 97.0 
               
               
                 13-6 
                   
                   
                   
                   
                   
                 50 
                   
                 83.0 
                 83.0 
                 97.0 
               
               
                 13-7 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 PA 
                 83.0 
                 83.0 
                 97.0 
               
               
                 13-8 
                   
                   
                   
                   
                   
                 20 
                   
                 84.0 
                 83.5 
                 97.0 
               
               
                 13-9 
                   
                   
                   
                   
                   
                 50 
                   
                 83.0 
                 83.1 
                 97.0 
               
               
                 13-10 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 PAA 
                 82.6 
                 83.1 
                 97.0 
               
               
                 13-11 
                   
                   
                   
                   
                   
                 20 
                   
                 83.1 
                 83.3 
                 97.0 
               
               
                 13-12 
                   
                   
                   
                   
                   
                 50 
                   
                 82.8 
                 83.3 
                 97.0 
               
               
                 13-13 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 PAAL 
                 83.5 
                 84.1 
                 97.0 
               
               
                 13-14 
                   
                   
                   
                   
                   
                 20 
                   
                 84.2 
                 84.2 
                 97.0 
               
               
                 13-15 
                   
                   
                   
                   
                   
                 50 
                   
                 83.5 
                 84.1 
                 97.0 
               
               
                 13-16 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 carbonized PI 
                 83.6 
                 84.5 
                 97.0 
               
               
                 13-17 
                   
                   
                   
                   
                   
                 20 
                   
                 84.5 
                 84.6 
                 97.0 
               
               
                 13-18 
                   
                   
                   
                   
                   
                 50 
                   
                 83.9 
                 84.4 
                 97.0 
               
               
                   
               
            
           
         
       
     
     Even when different negative-electrode binders were used, high cycle retention rates, high initial efficiency, and high load retention rates were obtained. 
     Experimental Examples 14-1 to 14-12 
     As illustrated in Table 21, secondary batteries were produced by the same procedure as in Experimental Examples 1-1 to 1-7, except that different positive-electrode active materials were used. Characteristics of each of the resulting secondary batteries were investigated. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 21 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                   
                 Cycle 
                   
                 Load 
               
               
                   
                 Core 
                 Covering portion 
                   
                 retention 
                 Initial 
                 retention 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Experimental 
                 portion 
                   
                 Proportion 
                 Positive electrode active 
                 rate 
                 efficiency 
                 rate 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 example 
                 Type 
                 x 
                 Type 
                 y 
                 M1 
                 (at. %) 
                 material 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
               
                 14-1 
                   
                   
                   
                   
                   
                   
                 LiNi 0.70 Co 0.25 Al 0.05 O 2   
                 84.1 
                 83.5 
                 97.0 
               
               
                 14-2 
                   
                   
                   
                   
                   
                   
                 LiNi 0.79 Co 0.14 Al 0.07 O 2   
                 84.0 
                 83.6 
                 97.0 
               
               
                 14-3 
                   
                   
                   
                   
                   
                   
                 LiNi 0.70 Co 0.25 Mg 0.05 O 2   
                 84.2 
                 83.7 
                 97.0 
               
               
                 14-4 
                   
                   
                   
                   
                   
                   
                 LiNi 0.70 Co 0.25 Fe 0.05 O 2   
                 84.1 
                 83.6 
                 97.0 
               
               
                 14-5 
                   
                   
                   
                   
                   
                   
                 LiNiO 2   
                 84.1 
                 83.7 
                 97.0 
               
               
                 14-6 
                 SiO x   
                 0.1 
                 SiO y   
                 1.2 
                 Ni 
                 10 
                 LiNi 0.33 Co 0.33 Mn 0.33 O 2   
                 84.2 
                 83.6 
                 97.0 
               
               
                 14-7 
                   
                   
                   
                   
                   
                   
                 LiNi 0.13 Co 0.60 Mn 0.27 O 2   
                 84.1 
                 83.7 
                 97.0 
               
               
                 14-8 
                   
                   
                   
                   
                   
                   
                 Li 1.13 [Ni 0.22 Co 0.18 Mn 0.60 ] 0.87 O 2   
                 84.2 
                 83.5 
                 97.0 
               
               
                 14-9 
                   
                   
                   
                   
                   
                   
                 Li 1.13 [Ni 0.20 Co 0.20 Mn 0.60 ] 0.87 O 2   
                 84.0 
                 83.6 
                 97.0 
               
               
                 14-10 
                   
                   
                   
                   
                   
                   
                 Li 1.13 [Ni 0.18 Co 0.22 Mn 0.60 ] 0.87 O 2   
                 84.2 
                 83.6 
                 97.0 
               
               
                 14-11 
                   
                   
                   
                   
                   
                   
                 Li 1.13 [Ni 0.25 Co 0.25 Mn 0.50 ] 0.87 O 2   
                 84.2 
                 83.5 
                 97.0 
               
               
                 14-12 
                   
                   
                   
                   
                   
                   
                 Li 2 Ni 0.40 Cu 0.60 O 2   
                 84.1 
                 83.7 
                 97.0 
               
               
                   
               
            
           
         
       
     
     Even when different positive-electrode active materials were used, high cycle retention rates, high initial efficiency, and high load retention rates were obtained. 
     The results illustrated in Tables 1 to 21 demonstrated that when the negative-electrode active material included the covering portion having a predetermined composition on the surface of the core portion, high cycle characteristics, high initial charge-discharge characteristics, and high load characteristics were obtained. 
     While the present application has been described above with reference to the embodiments and examples, the present application is not limited to these embodiments and examples. Various modifications may be made. For example, while the capacity of the negative electrode has been represented on the basis of the occlusion and release of lithium ions in the above-described embodiments, the present application is not necessarily limited thereto. The present application is also applicable to the case where the capacity of a negative electrode includes a capacity on the basis of the occlusion and release of lithium ions and a capacity on the basis of the deposition and dissolution of metallic Li, and is expressed as the sum of the capacities. In this case, as the negative-electrode active material, a negative-electrode material capable of occluding and releasing lithium ions is used, and the chargeable capacity of the negative-electrode material is set to be higher than the discharge capacity of the positive electrode. 
     While the case where the battery has a prismatic shape, a cylindrical shape, or a laminated-film shape and where the battery element has a spirally wound structure has been described above, the present application is not necessarily limited thereto. The present application is also applicable to a battery having a prismatic shape, a button shape, or the like, or to a battery element having a laminated structure. 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.