Patent Publication Number: US-2022223873-A1

Title: Negative electrode for secondary battery, and secondary battery

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of PCT patent application no. PCT/JP2020/033531, filed on Sep. 4, 2020, which claims priority to Japanese patent application no. JP2019-178789 filed on Sep. 30, 2019, the entire contents of which are being incorporated herein by reference. 
    
    
     BACKGROUND 
     The present technology generally relates to a negative electrode for a secondary battery, and a secondary battery. 
     Various kinds of electronic equipment, including mobile phones, have been widely used. Such widespread use has promoted development of a secondary battery as a power source that is smaller in size and lighter in weight and allows for a higher energy density. The secondary battery includes a positive electrode, a negative electrode, and an electrolytic solution. A configuration of the secondary battery influences a battery characteristic. Accordingly, the configuration of the secondary battery has been considered in various ways. 
     Specifically, in order to improve durability to withstand charging and discharging, a ceramic coat layer including ceramic particles as a main component is provided on a surface of a negative electrode active material layer. In order to improve a characteristic such as a high-temperature cyclability characteristic, a covering layer including filler particles and a binder is provided on a surface of a negative electrode active material layer. In order to improve safety, a porous insulating layer including an inorganic oxide filler and a resin binder is provided on a surface of an active material layer. In order to improve safety, an organic-inorganic composite porous coat layer including inorganic particles and a binder polymer is provided on a surface of an electrode. In order to suppress gas generation upon storing at high temperatures, a separator is provided with a coating layer including an inorganic filler. 
     SUMMARY 
     The present technology generally relates to a negative electrode for a secondary battery, and a secondary battery. 
     Although consideration has been given in various ways to solve problems of a secondary battery, the secondary battery has not yet achieved sufficient electrochemical performance or sufficient safety, and there is still room for improvement in terms thereof. 
     The present technology has been made in view of such an issue and it is an object of the technology to provide a negative electrode for a secondary battery, and a secondary battery that each make it possible to achieve both ensuring of electrochemical performance and improvement of safety. 
     A negative electrode for a secondary battery according to an embodiment of the present technology includes a negative electrode active material layer and a covering layer. The covering layer covers a surface of the negative electrode active material layer and includes inorganic particles and a binder. The covering layer includes a first covering part that is located closer to the negative electrode active material layer in a thickness direction and a second covering part that is located farther from the negative electrode active material layer in the thickness direction, and a weight ratio of the inorganic particles to the binder in the second covering part is greater than a weight ratio of the inorganic particles to the binder in the first covering part. The covering layer may be divided equally into the first covering part and the second covering part according to an embodiment of the present technology. 
     A secondary battery according to an embodiment of the present technology includes a positive electrode, a negative electrode, and an intermediate layer. The positive electrode and the negative electrode are opposed to each other with a separator interposed therebetween. The intermediate layer is disposed between the negative electrode and the separator and includes inorganic particles and a binder. The intermediate layer includes a first intermediate part that is located closer to the negative electrode in a thickness direction and a second intermediate part that is located farther from the negative electrode in the thickness direction, and a weight ratio of the inorganic particles to the binder in the second intermediate part is greater than a weight ratio of the inorganic particles to the binder in the first intermediate part. The intermediate layer may be divided equally into the first covering part and the second covering part according to an embodiment of the present technology. 
     According to the negative electrode for a secondary battery of an embodiment of the present technology, the covering layer including the inorganic particles and the binder covers the surface of the negative electrode active material layer. Where the covering layer is divided equally into the first covering part that is located closer to the negative electrode active material layer in the thickness direction and the second covering part that is located farther from the negative electrode active material layer in the thickness direction, the weight ratio of the inorganic particles to the binder is greater in the second covering part than in the first covering part. This makes it possible to achieve both ensuring of electrochemical performance and improvement of safety. 
     According to the secondary battery of an embodiment of the present technology, the intermediate layer including the inorganic particles and the binder is disposed between the negative electrode and the separator. Where the intermediate layer is divided equally into the first intermediate part that is located closer to the negative electrode in the thickness direction and the second intermediate part that is located farther from the negative electrode in the thickness direction, the weight ratio of the inorganic particles to the binder is greater in the second intermediate part than in the first intermediate part. This makes it possible to achieve both ensuring of electrochemical performance and improvement of safety. 
     It should be understood that effects of the technology are not necessarily limited to those described above and may include any of a series of effects described below in relation to the technology. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a perspective view of a configuration of a secondary battery according to an embodiment of the present technology. 
         FIG. 2  is a sectional view of a configuration of a wound electrode body illustrated in  FIG. 1 . 
         FIG. 3  is a sectional view of a configuration of a main part of the wound electrode body illustrated in  FIG. 2 . 
         FIG. 4  is a sectional view of a configuration of a negative electrode illustrated in  FIG. 2 . 
         FIG. 5  is a capacity potential curve (charge voltage Ec=4.30 V) of a secondary battery according to a reference example. 
         FIG. 6  is another capacity potential curve (charge voltage Ec=4.45 V) of the secondary battery according to the reference example. 
         FIG. 7  is a capacity potential curve (charge voltage Ec=4.38 V) of the secondary battery according to an embodiment of the present technology. 
         FIG. 8  is another capacity potential curve (charge voltage Ec=4.45 V) of the secondary battery according to an embodiment of the present technology. 
         FIG. 9  is a sectional view of a configuration of a separator in a secondary battery according to an embodiment of the present technology. 
         FIG. 10  is a block diagram illustrating a configuration of an application example of the secondary battery according to an embodiment, which is a battery pack including a single battery. 
         FIG. 11  is a block diagram illustrating a configuration of an application example of the secondary battery according to an embodiment, which is a battery pack including an assembled battery. 
         FIG. 12  is a block diagram illustrating a configuration of an application example of the secondary battery according to an embodiment, which is an electric vehicle. 
     
    
    
     DETAILED DESCRIPTION 
     As described herein, the present disclosure will be described based on examples with reference to the drawings, but the present disclosure is not to be considered limited to the examples, and various numerical values and materials in the examples are considered by way of example. 
     A description is given first of a secondary battery according to an embodiment of the present technology. A negative electrode for a secondary battery according to an embodiment of the technology is a portion or a component of the secondary battery, and is thus described together below. Hereinafter, the negative electrode for a secondary battery is simply referred to as a “negative electrode”. 
     The secondary battery to be described herein is a secondary battery that obtains a battery capacity using insertion and extraction of an electrode reactant, and includes a positive electrode, a negative electrode, and an electrolytic solution. In the secondary battery, to prevent precipitation of the electrode reactant on a surface of the negative electrode during charging, a charge capacity of the negative electrode is greater than a discharge capacity of the positive electrode. In other words, an electrochemical capacity per unit area of the negative electrode is set to be greater than an electrochemical capacity per unit area of the positive electrode. 
     Although not particularly limited in kind, the electrode reactant is a light metal, such as an alkali metal or an alkaline earth metal. Examples of the alkali metal include lithium, sodium, and potassium. Examples of the alkaline earth metal include beryllium, magnesium, and calcium. 
     Examples are given below of a case where the electrode reactant is lithium. A secondary battery that obtains a battery capacity using insertion and extraction of lithium is a so-called lithium-ion secondary battery. In the lithium-ion secondary battery, lithium is inserted and extracted in an ionic state. 
       FIG. 1  illustrates a perspective configuration of the secondary battery.  FIG. 2  illustrates a sectional configuration of a wound electrode body  10  illustrated in  FIG. 1 .  FIG. 3  illustrates a sectional configuration of a main part of the wound electrode body  10  illustrated in  FIG. 2 . 
     It should be understood that  FIG. 1  illustrates a state in which the wound electrode body  10  and an outer package film  20  are separated from each other.  FIG. 2  illustrates only a portion of the wound electrode body  10 .  FIG. 3  illustrates a negative electrode active material layer  12 B, a separator  13 , and an intermediate layer  14  in the wound electrode body  10 . 
     In the secondary battery, as illustrated in  FIG. 1 , a battery device of a wound type, i.e., the wound electrode body  10 , is contained inside the outer package film  20  having a pouch shape. A positive electrode lead  15  and a negative electrode lead  16  are coupled to the wound electrode body  10 . The positive electrode lead  15  and the negative electrode lead  16  are led out in respective directions that are common to each other, from inside to outside the outer package film  20 . 
     In other words, the secondary battery described here is a secondary battery of a laminated-film type in which the outer package film  20  is used as an outer package member to contain the battery device, i.e., the wound electrode body  10  therein. The outer package film  20  has flexibility or softness. 
     [Outer Package Film] 
     The outer package film  20  is, for example, a single film-shaped member that is foldable in a direction of an arrow R (an arrowed dash-dotted line), as illustrated in  FIG. 1 . The outer package film  20  has a depression part  20 U to place the wound electrode body  10  therein. The depression part  20 U is a so-called deep drawn part. 
     Specifically, the outer package film  20  is a three-layered laminated film including a fusion-bonding layer, a metal layer, and a surface protective layer that are stacked in this order from an inner side. In a state in which the outer package film  20  is folded, outer edges of the fusion-bonding layer are fusion-bonded to each other. The fusion-bonding layer includes a polymer compound such as polypropylene. The metal layer includes a metal material such as aluminum. The surface protective layer includes a polymer compound such as nylon. The number of layers of the outer package film  20  as a laminated film is not limited to three, and may be two, or four or more. The outer package film  20  is not limited to a multilayered laminated film, and may be single-layered. 
     A sealing film  21  is interposed between the outer package film  20  and the positive electrode lead  15 . A sealing film  22  is interposed between the outer package film  20  and the negative electrode lead  16 . Each of the sealing films  21  and  22  is a member that prevents entry of outside air, and includes one or more of materials having adherence to both the positive electrode lead  15  and the negative electrode lead  16 , such as a polyolefin resin. Examples of the polyolefin resin include polyethylene, polypropylene, modified polyethylene, and modified polypropylene. It should be understood that one or both of the sealing films  21  and  22  may be omitted. 
     As illustrated in  FIGS. 1 and 2 , the wound electrode body  10  includes a positive electrode  11 , a negative electrode  12 , the separator  13 , the intermediate layer  14 , and an electrolytic solution. The electrolytic solution is a liquid electrolyte. The wound electrode body  10  has a structure in which the positive electrode  11  and the negative electrode  12  are stacked on each other with the separator  13  and the intermediate layer  14  interposed therebetween, and the stack of the positive electrode  11 , the negative electrode  12 , the separator  13 , and the intermediate layer  14  is wound. Mainly, the positive electrode  11 , the negative electrode  12 , and the separator  13  are each impregnated with the electrolytic solution. It should be understood that  FIG. 2  omits the illustration of the electrolytic solution. 
     As illustrated in  FIG. 2 , the positive electrode  11  includes a positive electrode current collector  11 A, and two positive electrode active material layers  11 B provided on respective opposite sides of the positive electrode current collector  11 A. However, the positive electrode active material layer  11 B may be provided only on one of the opposite sides of the positive electrode current collector  11 A. 
     The positive electrode current collector  11 A includes one or more of electrically conductive materials including, without limitation, aluminum, nickel, and stainless steel. The positive electrode active material layer  11 B includes one or more of positive electrode active materials into which lithium is insertable and from which lithium is extractable, that is, materials that allow lithium to be inserted thereinto and extracted therefrom in an ionic state. The positive electrode active material layer  11 B may further include, without limitation, a positive electrode binder and a positive electrode conductor. 
     Although not particularly limited in kind, the positive electrode active material is a lithium-containing compound such as a lithium-containing transition metal compound. The lithium-containing transition metal compound includes lithium and one or more of transition metal elements, and may further include one or more of other elements. The other elements may be any elements other than transition metal elements, and are not particularly limited in kind. In particular, the other elements are preferably those belong to groups 2 to 15 in the long period periodic table of elements. It should be understood that the lithium-containing transition metal compound may be an oxide or may be any other compound such as a phosphoric acid compound, a silicic acid compound, or a boric acid compound. 
     Specific examples of the oxide include LiNiO 2 , LiCoO 2 , LiCO 0.98 Al 0.01 Mg 0.01 O 2 , LiNi 0.5 Co 0.2 Mn 0.3 O 2 , LiNi 0.8 Co 0.15 Al 0.05 O 2 , LiNi 0.33 Co 0.33 Mn 0.33 O 2 , Li 1.2 Mn 0.52 Co 0.175 Ni 0.1 O 2 , Li 1.15 (Mn 0.65 Ni 0.22 Co 0.13 )O 2 , and LiMn 2 O 4 . Specific examples of the phosphoric acid compound include LiFePO 4 , LiMnPO 4 , LiFe 0.5 Mn 0.5 PO 4 , and LiFe 0.3 Mn 0.7 PO 4 . 
     The positive electrode binder includes one or more of materials including, without limitation, a synthetic rubber and a polymer compound. Examples of the synthetic rubber include a styrene-butadiene-based rubber, a fluorine-based rubber, and ethylene propylene diene. Examples of the polymer compound include polyvinylidene difluoride, polyimide, and carboxymethyl cellulose. 
     The positive electrode conductor includes one or more of electrically conductive materials including, without limitation, a carbon material. Examples of the carbon material include graphite, carbon black, acetylene black, and Ketjen black. The positive electrode conductor may be a material such as a metal material or an electrically conductive polymer as long as the material has an electrically conductive property. 
     As illustrated in  FIG. 2 , the negative electrode  12  includes a negative electrode current collector  12 A, and a negative electrode active material layer  12 B provided on each of opposite sides of the negative electrode current collector  12 A. However, the negative electrode active material layer  12 B may be provided only on one of the opposite sides of the negative electrode current collector  12 A. 
     The negative electrode current collector  12 A includes one or more of electrically conductive materials including, without limitation, copper, aluminum, nickel, and stainless steel. The negative electrode active material layer  12 B includes one or more of negative electrode active materials into which lithium is insertable and from which lithium is extractable, that is, materials that allow lithium to be inserted thereinto and extracted therefrom in an ionic state. The negative electrode active material layer  12 B may further include, without limitation, a negative electrode binder and a negative electrode conductor. Details of the negative electrode binder and the negative electrode conductor are similar to the details of the positive electrode binder and the positive electrode conductor described above, respectively. 
     The negative electrode active material is not particularly limited in kind, and examples thereof include a carbon material and a metal-based material. Examples of the carbon material include graphitizable carbon, non-graphitizable carbon, and graphite. The metal-based material is a material including one or more among metal elements and metalloid elements that are each able to form an alloy with lithium. Specifically, the metal-based material includes one or more of elements including, without limitation, silicon and tin, as a constituent element or constituent elements. The metal-based material may be, for example, a simple substance, an alloy, a compound, or a mixture of two or more thereof. 
     Specific examples of the metal-based material 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 or 0.2&lt;v&lt;1.4), LiSiO, SnO w  (0&lt;w≤2), SnSiO 3 , LiSnO, and Mg 2 Sn. 
     (Separator) 
     As illustrated in  FIG. 2 , the separator  13  is interposed between the positive electrode  11  and the negative electrode  12 . The positive electrode  11  and the negative electrode  12  are thus opposed to each other with the separator  13  interposed therebetween. 
     The separator  13  is an insulating porous film that allows lithium to pass therethrough while preventing a contact or a short-circuit between the positive electrode  11  and the negative electrode  12 . The porous film may be single-layered or multilayered. The porous film includes one or more of polymer compounds including, without limitation, polytetrafluoroethylene, polypropylene, and polyethylene. 
     Although not particularly limited, an air permeability of the separator  13  is preferably within a range from 100 sec/cm 3  (=100 sec/ml) to 1000 sec/cm 3  (=1000 sec/ml) both inclusive, in particular. A reason for this is that this secures permeability to lithium and thus improves mobility of lithium between the positive electrode  11  and the negative electrode  12  during insertion and extraction. 
     It should be understood that the air permeability of the separator  13  described here is not the air permeability of the separator  13  to be used in a process of manufacturing the secondary battery (i.e., the separator  13  before being adhered to the negative electrode  12 ), but the air permeability of the separator  13  collected from the completed secondary battery (i.e., the separator  13  after being adhered to the negative electrode  12 ). A procedure to measure the air permeability is as described below. First, the secondary battery is disassembled to thereby collect the separator  13 . Thereafter, the air permeability of the separator  13  is measured at each of ten different locations by means of an air permeability tester (a Gurley type densometer available from Toyo Seiki Co., Ltd.). Lastly, an average value of ten air permeabilities measured at the ten respective locations is calculated as the air permeability of the separator  13 . 
     The air permeability of the separator  13  is adjustable by varying a condition such as a treatment temperature at which an activation treatment is to be performed in the process of manufacturing the secondary battery, that is, in an activation process described later. 
     Although not particularly limited, a thickness of the separator  13  is preferably within a range from 3 μm to 12 μm both inclusive, in particular. A reason for this is that such a thickness allows for compatibility between energy density of the secondary battery and physical strength of the separator  13 . The thickness here is an average value of ten thicknesses measured at ten respective different locations. 
     The intermediate layer  14  is disposed between the negative electrode  12  and the separator  13 , and is thus adhered to both the negative electrode  12  and the separator  13 . The intermediate layer  14  includes inorganic particles and an intermediate binder. The intermediate binder is a binder included in the intermediate layer  14 . Details of the intermediate binder are similar to the details of the positive electrode binder. It should be understood that the intermediate layer  14  may further include one or more of materials including, without limitation, any additives on an as-needed basis. 
     In the intermediate layer  14 , as will be described later, a distribution of the inorganic particles is optimized; more specifically, a dispersion state of the inorganic particles is set to allow a weight ratio RN to be greater than a weight ratio RM. This improves safety of the secondary battery while securing electrochemical performance of the secondary battery. A detailed description will be given later of an advantage that is brought about by the optimization of the distribution of the inorganic particles described here. 
     The inorganic particles include one or more of inorganic materials. Although the inorganic material is not particularly limited in kind, examples thereof include a metal oxide, a metal nitride, and a metal hydroxide. 
     Specific examples of the metal oxide include aluminum oxide, silicon oxide, titanium oxide, magnesium oxide, and zirconium oxide. Specific examples of the metal nitride include aluminum nitride. Specific examples of the metal hydroxide include magnesium hydroxide. 
     The inorganic material preferably includes one or more among the metal oxides and metal hydroxides, in particular, and more preferably includes one or more of materials including, without limitation, aluminum oxide and magnesium hydroxide. A reason for this is that this further improves the safety while securing the electrochemical performance. 
     Although not particularly limited, a thickness of the intermediate layer  14  is preferably within a range from 0.1 μm to 5 μm both inclusive, in particular. A reason for this is that such a thickness reduces inhibition of insertion and extraction of lithium into and from the negative electrode  12 , thus allowing the above-described advantage to be achieved while securing insertion and extraction of lithium. The thickness of the intermediate layer  14  is a dimension in a Z-axis direction in  FIGS. 2 and 3 , that is, a dimension in a direction in which the positive electrode  11  and the negative electrode  12  are opposed to each other with the separator  13  interposed therebetween. 
     A procedure to calculate the thickness of the intermediate layer  14  is as described below. First, the secondary battery is disassembled to thereby collect the negative electrode  12 . Thereafter, a section of the negative electrode  12  ( FIG. 3 ) is observed by means of a microscope such as a scanning electron microscope (SEM). Observation conditions including magnification may be freely chosen. Thereafter, the thickness of the intermediate layer  14  is measured at each of ten different locations on the basis of a result of the observation (a micrograph) of the section of the negative electrode  12 . Lastly, an average value of ten thicknesses measured at the ten respective locations is calculated as the thickness of the intermediate layer  14 . 
     A detailed description will now be given of the distribution of the inorganic particles described above. In the intermediate layer  14  described here, the distribution of the inorganic particles, that is, the dispersion state (a weight ratio R) of the inorganic particles is optimized as described below. 
     Specifically, as illustrated in  FIG. 3 , the intermediate layer  14  is divided equally into two parts in a thickness direction (the Z-axis direction) of the intermediate layer  14 . Thus, the intermediate layer  14  is divided into a lower layer  14 M and an upper layer  14 N. The lower layer  14 M corresponds to a first intermediate part located closer to the negative electrode  12  (the negative electrode active material layer  12 B), in other words, a lower part of the intermediate layer  14 . The upper layer  14 N corresponds to a second intermediate part located farther from the negative electrode  12 , in other words, an upper part of the intermediate layer  14 . In  FIG. 3 , a dashed line L represents a border between the lower layer  14 M and the upper layer  14 N. 
     The lower layer  14 M and the upper layer  14 N each include the inorganic particles and the intermediate binder. Accordingly, the weight ratio R is defined in the lower layer  14 M as the weight ratio RM, and is defined in the upper layer  14 N as the weight ratio RN. The weight ratio RM is a ratio of a weight M 2  of the inorganic particles to a weight M 1  of the intermediate binder in the lower layer  14 M, and is therefore calculated by the following formula: RM=M 2 /M 1 . The weight ratio RN is a ratio of a weight M 4  of the inorganic particles to a weight M 3  of the intermediate binder in the upper layer  14 N, and is therefore calculated by the following formula: RN=M 4 /M 3 . 
     In this case, the weight ratio RN is set to be greater than the weight ratio RM. In other words, a distribution amount, i.e., a dispersion amount, of the inorganic particles is greater in the upper layer  14 N than in the lower layer  14 M. This optimizes the distribution of the inorganic particles in the intermediate layer  14 , thus improving the safety of the secondary battery while securing the electrochemical performance of the secondary battery as described above. 
     A plurality of formation methods is possible for forming the intermediate layer  14  to cause the weight ratio RN to be greater than the weight ratio RM. Such formation methods of the intermediate layer  14  will be described in detail later. 
     Respective ranges of the weight ratios RM and RN are not particularly limited. The weight ratio RM is preferably within a range from 0.1 to 10 both inclusive, and the weight ratio RN is preferably within a range from 0.2 to 20 both inclusive, in particular. A reason for this is that this optimizes the respective ranges of the weight ratios RM and RN, thus allowing for sufficient improvement in safety of the secondary battery while securing the electrochemical performance of the secondary battery. 
     A procedure to calculate the weight ratio RN is as described below. First, the secondary battery is disassembled to thereby collect the intermediate layer  14 . Thereafter, a portion of the intermediate layer  14  is cut by means of cutting equipment (Surface and Interfacial Cutting Analysis System (SAICAS) NN, a diagonal cutting apparatus available from Daipla Wintes Co., Ltd., “SAICAS” being a registered trademark) to thereby separate the upper layer  14 N from the lower layer  14 M. Thereafter, the upper layer  14 N is analyzed by means of a simultaneous thermogravimeter-differential thermal analyzer (TG-DTA STA7000 available from Hitachi High-Tech Science Corporation) to thereby measure each of the weight M 3  of the intermediate binder and the weight M 4  of the inorganic particles. In this case, the temperature is changed within a range from room temperature (=23° C.) to 1000° C. both inclusive, with a temperature rise rate set to 10° C./min. Lastly, the weight ratio RN of the upper layer  14 N is calculated on the basis of the weight M 3  of the intermediate binder and the weight M 4  of the inorganic particles. 
     A procedure to calculate the weight ratio RM is similar to the above-described procedure to calculate the weight ratio RN, except that the lower layer  14 M is used instead of the upper layer  14 N. 
     The electrolytic solution includes a solvent and an electrolyte salt. The electrolytic solution may include only one solvent or may include two or more solvents. The electrolytic solution may include only one electrolyte salt or may include two or more electrolyte salts. 
     The solvent includes a non-aqueous solvent (an organic solvent). An electrolytic solution including a non-aqueous solvent is a so-called non-aqueous electrolytic solution. 
     Examples of the non-aqueous solvent include esters and ethers. More specific examples of the non-aqueous solvent include a carbonic-acid-ester-based compound, a carboxylic-acid-ester-based compound, and a lactone-based compound. 
     Examples of the carbonic-acid-ester-based compound include a cyclic carbonic acid ester and a chain carbonic acid ester. Examples of the cyclic carbonic acid ester include ethylene carbonate and propylene carbonate. Examples of the chain carbonic acid ester include dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. Examples of the carboxylic-acid-ester-based compound include ethyl acetate, ethyl propionate, and ethyl trimethyl acetate. Examples of the lactone-based compound include γ-butyrolactone and γ-valerolactone. Examples of the ethers other than the lactone-based compounds described above include 1,2-dimethoxy ethane, tetrahydrofuran, 1,3-dioxolane, and 1,4-dioxane. 
     Further, examples of the non-aqueous solvent include an unsaturated cyclic carbonic acid ester, a halogenated carbonic acid ester, a sulfonic acid ester, a phosphoric acid ester, an acid anhydride, a nitrile compound, and an isocyanate compound. A reason for this is that chemical stability of the electrolytic solution improves. 
     Specific examples of the unsaturated cyclic carbonic acid ester include vinylene carbonate, vinylethylene carbonate, and methylene ethylene carbonate. Examples of the halogenated carbonic acid ester include fluoroethylene carbonate and difluoroethylene carbonate. Examples of the sulfonic acid ester include 1,3-propane sultone. Examples of the phosphoric acid ester include trimethyl phosphate. Examples of the acid anhydride include a cyclic carboxylic acid anhydride, a cyclic disulfonic acid anhydride, and a cyclic carboxylic acid sulfonic acid anhydride. Examples of the cyclic carboxylic acid anhydride include succinic anhydride, glutaric anhydride, and maleic anhydride. Examples of the cyclic disulfonic acid anhydride include ethane disulfonic anhydride and propane disulfonic anhydride. Examples of the cyclic carboxylic acid sulfonic acid anhydride include sulfobenzoic anhydride, sulfopropionic anhydride, and sulfobutyric anhydride. Examples of the nitrile compound include acetonitrile and succinonitrile. Examples of the isocyanate compound include hexamethylene diisocyanate. 
     The electrolyte salt includes one or more of light metal salts including, without limitation, a lithium salt. Examples of the lithium salt include lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), lithium trifluoromethanesulfonate (LiCF 3 SO 3 ), lithium bis(fluorosulfonyl)imide (LiN(FSO 2 ) 2 ), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF 3 SO 2 ) 2 ), lithium tris(trifluoromethanesulfonyl)methide (LiC(CF 3 SO 2 ) 3 ), and lithium bis(oxalato)borate (LiB(C 2 O 4 ) 2 ). Although not particularly limited, a content of the electrolyte salt is within a range from 0.3 mol/kg to 3.0 mol/kg both inclusive with respect to the solvent. A reason for this is that a high ion conductivity is obtainable. 
     The positive electrode lead  15  is coupled to the positive electrode  11  (the positive electrode current collector  11 A), and the negative electrode lead  16  is coupled to the negative electrode  12  (the negative electrode current collector  12 A). The positive electrode lead  15  includes a material similar to that included in the positive electrode current collector  11 A. The negative electrode lead  16  includes a material similar to that included in the negative electrode current collector  12 A. The positive electrode lead  15  and the negative electrode lead  16  each have a shape such as a thin plate shape or a meshed shape. 
     It is sufficient that the intermediate layer  14  is interposed between the negative electrode  12  and the separator  13 , and therefore a joining relationship between the intermediate layer  14  and another component is not particularly limited. 
       FIG. 4  illustrates a sectional configuration of the negative electrode  12  illustrated in  FIG. 2 , and corresponds to  FIG. 3 . It should be understood that the negative electrode  12  illustrated in  FIG. 4  is that to be used in the process of manufacturing the secondary battery. In the following,  FIGS. 2 and 3  will be referred to as necessary. 
     Here, the intermediate layer  14  is provided on a surface of the negative electrode  12  (the negative electrode active material layer  12 B) on a side opposed to the separator  13 . Accordingly, the intermediate layer  14  is joined to the negative electrode  12 , thus serving as a covering layer covering the surface of the negative electrode active material layer  12 B. The intermediate layer  14  serving as the covering layer includes the lower layer  14 M and the upper layer  14 N as described above. The lower layer  14 M corresponds to a first covering part. The upper layer  14 N corresponds to a second covering part. 
     In this case, the intermediate layer  14  is provided integrally with the negative electrode  12 . This secures adherence of the intermediate layer  14  to the negative electrode  12 . Further, owing to the negative electrode  12  and the intermediate layer  14  constituting a single member as a whole, handleability of the negative electrode  12  and the intermediate layer  14  improves as compared with a case where the negative electrode  12  and the intermediate layer  14  are separated from each other, i.e., a case where the negative electrode  12  and the intermediate layer  14  are two separate members. This makes it easier to manufacture the secondary battery. 
     Although not particularly limited, a coverage of the intermediate layer  14  is preferably within a range from 20% to 100%, both inclusive, of the surface of the negative electrode active material layer  12 B, in particular. A reason for this is that such a coverage allows for sufficient adhesion of the negative electrode  12  to the separator  13 , thus allowing for sufficient improvement in electrochemical performance of the secondary battery and also sufficient improvement in safety of the secondary battery. 
     The coverage is adjustable by changing, for example, respective solids concentrations of a first intermediate mixture slurry and a second intermediate mixture slurry described later in a process of forming the intermediate layer  14 . 
     A procedure to measure the coverage of the intermediate layer  14  is as described below. First, the secondary battery is disassembled to thereby collect the negative electrode  12  with the intermediate layer  14 . Thereafter, elemental analysis is performed on ten different locations in a predetermined analysis range or analysis area of the surface of the negative electrode active material layer  12 B by means of an energy dispersive X-ray spectrometer (EDX) to thereby identify a formation range or formation area of the intermediate layer  14 . This elemental analysis is performed for a constituent element of the inorganic particles included in the intermediate layer  14 . Specifically, in a case where the inorganic particles include magnesium hydroxide, the elemental analysis is performed for magnesium. Thereafter, the following is calculated: coverage (%)=(formation area of the intermediate layer  14 /analysis area of the negative electrode active material layer  12 B)×100. For example, EDX-7000, an energy dispersive X-ray fluorescence spectrometer available from Shimadzu Corporation is usable as the EDX. An analysis condition is that a degree of vacuum is within a range from 10 −5  to 10 −6  both inclusive, although not particularly limited thereto. Lastly, an average value of ten coverages calculated for the ten respective locations is calculated as the coverage of the intermediate layer  14 . The value of the coverage is rounded off to the nearest whole number. 
     In the case where the intermediate layer  14  is provided on the surface of the negative electrode  12 , the negative electrode  12  is adhered to the separator  13  via the intermediate layer  14 . Although not particularly limited, an adhesion strength of the negative electrode  12  to the separator  13  is preferably within a range from 3 mN/mm to 30 mN/mm both inclusive, in particular. A reason for this is that such a strength allows for uniform adhesion of the negative electrode  12  to the separator  13 , thus suppressing variations in distance between the negative electrode  12  and the separator  13  and also variations in electrical resistance of the negative electrode  12 . 
     A procedure to measure the adhesion strength of the negative electrode  12  is as described below. First, the secondary battery is disassembled to thereby collect a staked body including the negative electrode  12 , the separator  13 , and the intermediate layer  14  stacked on each other. Thereafter, the intermediate layer  14  is peeled off in a direction at 180° with respect to the separator  13  by means of a tensile tester (Tensilon RTF, a universal testing instrument available from A&amp;D Company, Limited) to thereby measure the adhesion strength of the negative electrode  12  to the separator  13 . Lastly, an average value of ten adhesion strengths calculated at ten respective locations is calculated as the adhesion strength of the negative electrode  12 . The value of the adhesion strength is rounded off to the nearest whole number. 
     To cause the secondary battery to be chargeable and dischargeable under a high charge voltage condition, the negative electrode  12  preferably satisfies predetermined configuration conditions and predetermined physical property conditions described below. 
       FIGS. 5 and 6  each represent a capacity potential curve related to a secondary battery according to a reference example for the secondary battery according to the present embodiment.  FIGS. 7 and 8  each represent a capacity potential curve related to the secondary battery according to the present embodiment. 
     In each of  FIGS. 5 to 8 , a horizontal axis represents a capacity C (mAh) and a vertical axis represents a potential E (V). The potential E is an open circuit potential to be measured with lithium metal as a reference electrode, i.e., a potential versus a lithium reference electrode.  FIGS. 5 to 8  each represent a capacity potential curve L 1  of the positive electrode  11  and a capacity potential curve L 2  of the negative electrode  12 . It should be understood that a position of a dashed line indicated as “charged” represents a full charge state, and a position of a dashed line indicated as “discharged” represents a full discharge state. 
     A charge voltage Ec (V) and a discharge voltage Ed (V) are set as follows. In  FIG. 5 , the charge voltage Ec is set to 4.30 V and the discharge voltage Ed is set to 3.00 V. In  FIG. 6 , the charge voltage Ec is set to 4.45 V and the discharge voltage Ed is set to 3.00 V. In  FIG. 7 , the charge voltage Ec is set to 4.38 V and the discharge voltage Ed is set to 3.00 V. In  FIG. 8 , the charge voltage Ec is set to 4.45 V and the discharge voltage Ed is set to 3.00 V. Upon charging and discharging, the secondary battery is charged until a battery voltage (a closed circuit voltage) reaches the charge voltage Ec and then discharged until the battery voltage reaches the discharge voltage Ed. 
     In the following, a description is given of a premise, i.e., the configuration conditions, for describing a charge and discharge principle and the physical property conditions of the secondary battery according to the present embodiment, and thereafter, a description is given of the charge and discharge principle and also the physical property conditions for achieving the charge and discharge principle. 
     Here, the positive electrode active material of the positive electrode  11 , i.e., a lithium-containing transition metal compound, includes one or more of lithium-cobalt composite oxides having a layered rock-salt crystal structure (hereinafter referred to as “layered rock-salt lithium-cobalt composite oxides”) that are represented by Formula (1) below. A reason for this is that a high energy density is stably achievable. 
       Li x Co 1-y M y O 2-z X z   (1)
 
     where:
 
M is at least one of titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), nickel (Ni), copper (Cu), sodium (Na), magnesium (Mg), aluminum (Al), silicon (Si), tin (Sn), potassium (K), calcium (Ca), zinc (Zn), gallium (Ga), strontium (Sr), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), barium (Ba), lanthanum (La), tungsten (W), or boron (B); X is at least one of fluorine (F), chlorine (Cl), bromine (Br), iodine (I), or sulfur (S); and
 
x, y, and z satisfy 0.8&lt;x&lt;1.2, 0&lt;y≤0.15, and 0≤z&lt;0.05.
 
     It should be understood that a composition of lithium differs depending on a charging state and a discharging state. A value of x included in Formula (1) represents a value in a state in which the positive electrode  11  taken out of the secondary battery has been discharged until the potential has reached 3.0 V (versus a lithium reference electrode). 
     As is apparent from Formula (1), the layered rock-salt lithium-cobalt composite oxide is a cobalt-based lithium composite oxide. The layered rock-salt lithium-cobalt composite oxide may further include one or more of first additional elements (M), and may further include one or more of second additional elements (X). Details of each of the first additional element (M) and the second additional element (X) are as described above. 
     In other words, as is apparent from a value range that y can take, the layered rock-salt lithium-cobalt composite oxide may include no first additional element (M). Similarly, as is apparent from a value range that z can take, the layered rock-salt lithium-cobalt composite oxide may include no second additional element (X). 
     The layered rock-salt lithium-cobalt composite oxide is not particularly limited in kind and may be any compound represented by Formula (1). Specific examples of the layered rock-salt lithium-cobalt composite oxide include LiCoO 2 , LiCo 0.98 Al 0.02 O 2 , LiCo 0.98 Mn 0.02 O 2 , and LiCo 0.98 Mg 0.02 O 2 . 
     The negative electrode active material of the negative electrode  12 , i.e., a carbon material, includes graphite. The graphite is not particularly limited in kind. The graphite may be artificial graphite, natural graphite, or both. 
     To achieve an improved energy density of the secondary battery including the positive electrode  11  in which the positive electrode active material is the layered rock-salt lithium-cobalt composite oxide and the negative electrode  12  in which the negative electrode active material is graphite, it is conceivable to increase the charge voltage Ec, i.e., a so-called end-of-charge voltage. Increasing the charge voltage Ec raises the potential E of the positive electrode  11  in an end stage of charging, and eventually, at an end of charging. This raises a use range of the potential E, i.e., a potential range to be used in the positive electrode  11  during charging. 
     In a case where the layered rock-salt lithium-cobalt composite oxide is used as the positive electrode active material, a potential constant region P 2  associated with a phase transition (O3/H1-3 transition) generally exists. Increasing the charge voltage Ec also increases the potential E of the positive electrode  11  in the end stage of charging, thus causing the potential E of the positive electrode  11  to reach inside the potential constant region P 2  described above. Accordingly, the capacity potential curve L 1  of the positive electrode  11  has a potential varying region P 1  and the potential constant region P 2  as indicated in  FIGS. 5 to 8 . The potential varying region P 1  is a region in which the potential E varies as the capacity C varies. The potential constant region P 2  is a region in the capacity potential curve located to the left of the potential constant region P 1  and is a region in which the potential E hardly varies even if the capacity C varies as a result of the phase transition. 
     If the secondary battery including the layered rock-salt lithium-cobalt composite oxide is charged and discharged in such a manner that the potential E of the positive electrode  11  reaches inside the potential constant region P 2  associated with the phase transition, or the potential E of the positive electrode  11  passes through the potential constant region P 2  associated with the phase transition, there is tendency that a capacity loss easily occurs and gas generation also easily occurs. Such a tendency is noticeable in a case where the secondary battery is used and stored in a high temperature environment. In particular, if the charge voltage Ec is 4.38 V or higher, it becomes easier for the potential E of the positive electrode  11  to reach the potential constant region P 2  associated with the phase transition, or it becomes easier for the potential E of the positive electrode  11  to pass through the potential constant region P 2  associated with the phase transition. 
     In contrast, if the charge voltage Ec is increased in a case where graphite is used as the negative electrode active material, a two-phase coexistence reaction of an intercalation compound stage  1  and an intercalation compound stage  2  proceeds in the graphite. As a result, the capacity potential curve L 2  of the negative electrode  12  has a potential constant region P 3  as indicated in  FIGS. 5 to 8 . The potential constant region P 3  is a region in which the potential E hardly varies even if the capacity C varies in association with the two-phase coexistence reaction. The potential E of the negative electrode  12  in the potential constant region P 3  is about 90 mV to about 100 mV. 
     It should be understood that if the charge voltage Ec is further increased, the potential E of the negative electrode  12  exceeds the potential constant region P 3 , and thus the potential E varies markedly. In association with the increase in the charge voltage Ec that causes the potential E to exceed the potential constant region P 3 , the capacity potential curve L 2  of the negative electrode  12  has a potential varying region P 4 , as indicated in  FIGS. 5 to 8 . In  FIGS. 5 to 8 , the potential varying region P 4  is a region in the capacity potential curve located to the left of the potential constant region P 3  and is a region in which the potential E varies (decreases) markedly if the capacity C varies. The potential E of the negative electrode  12  in the potential varying region P 4  is lower than about 90 mV. 
     In the secondary battery according to the present embodiment in which the positive electrode  11  includes the positive electrode active material (the layered rock-salt lithium-cobalt composite oxide) and the negative electrode  12  includes the negative electrode active material (graphite), charging and discharging are performed as described below on the basis of the premise described above. In the following, the charge and discharge principle of the secondary battery according to the present embodiment ( FIGS. 7 and 8 ) will be described in comparison with the charge and discharge principle of the secondary battery according to the reference example ( FIGS. 5 and 6 ). 
     In the secondary battery according to the reference example, as indicated in  FIG. 5 , the potential E of the negative electrode  12  at the end of charging (charge voltage Ec=4.30 V) is set to cause the charging to be completed in the potential constant region P 3 , in order to prevent a battery capacity from decreasing due to precipitation of lithium metal on the negative electrode  12 . 
     However, in the secondary battery according to the reference example, in a case where the charge voltage Ec is increased to 4.38 V or higher, more specifically, to 4.45 V, the potential E of the positive electrode  11  reaches 4.50 V or higher as indicated in  FIG. 6  in association with the increase in the potential E of the negative electrode  12  at the end of charging. As a result, the potential E of the positive electrode  11  at the end of charging (charge voltage Ec=4.45 V) reaches the potential constant region P 2  associated with the phase transition or passes through the potential constant region P 2  associated with the phase transition. 
     Thus, in the secondary battery according to the reference example, the increase in the charge voltage Ec to 4.38 V or higher makes it easier for the potential E of the positive electrode  11  to reach the potential constant region P 2  associated with the phase transition, or for the potential E of the positive electrode  11  to pass through the potential constant region P 2  associated with the phase transition. This generates a tendency that the capacity loss easily occurs and the gas generation also easily occurs, making it easier for a battery characteristic to deteriorate. As described above, the tendency that the battery characteristic easily deteriorates is noticeable in the case where the secondary battery is used and stored in a high temperature environment. 
     Moreover, in the secondary battery according to the reference example, the battery capacity is easily influenced by, for example, an active material ratio, i.e., a ratio between the amount of the positive electrode active material and the amount of the negative electrode active material, and the charge voltage Ec. Thus, the battery capacity easily varies in association with, for example, variations in the active material ratio (amount) and a setting error of the charge voltage Ec by a charging device. Accordingly, the variation in the capacity C of the positive electrode  11  makes it easier for the potential E of the positive electrode  11  to reach the potential constant region P 2  associated with the phase transition, or the potential E of the positive electrode  11  to pass through the potential constant region P 2  associated with phase transition. As a result, the battery capacity easily varies, and an operable time of, for example, equipment or an apparatus that operates using the secondary battery as a power source is shortened due to decrease in the battery capacity. In addition, if the battery capacity varies, lithium metal is generated on the negative electrode  12  more easily. 
     In contrast, in the secondary battery according to the present embodiment, the potential E of the negative electrode  12  is set to help to prevent the potential E of the positive electrode  11  (the layered rock-salt lithium-cobalt composite oxide) from reaching the potential constant region P 2  associated with the phase transition or the potential E of the positive electrode  11  from passing through the potential constant region P 2  associated with the phase transition, and also to suppress the precipitation of lithium metal on the negative electrode  12 . Specifically, as indicated in  FIG. 7 , the potential E of the negative electrode  12  at the end of charging (charge voltage Ec=4.38 V) is set to cause the charging not to be completed in the potential constant region P 3  but to be completed in the potential varying region P 4 . Further, as indicated in  FIG. 8 , the potential E of the negative electrode  12  at the end of charging (charge voltage Ec=4.45 V) is similarly set to cause the charging not to be completed in the potential constant region P 3  but to be completed in the potential varying region P 4 . 
     In this case, because the potential E of the negative electrode  12  at the end of charging decreases, the potential E of the positive electrode  11  at the end of charging also decreases. Specifically, in the secondary battery according to the present embodiment, in association with the decrease in the potential E of the negative electrode  12  at the end of charging, the potential E of the positive electrode  11  does not reach 4.50 V or above even if the charge voltage Ec is increased to 4.38 V or higher, more specifically to 4.45 V, as indicated in  FIGS. 7 and 8 . Thus, the potential E of the positive electrode  11  at the end of charging (charge voltage Ec=4.38 V or 4.45 V) is set not to reach the potential constant region P 2  associated with the phase transition, or not to pass through the potential constant region P 2  associated with the phase transition. 
     Upon charging, as is apparent from  FIGS. 7 and 8 , when the secondary battery is charged up to the charge voltage Ec of 4.38 V or higher, the potential E of the negative electrode  12  markedly decreases in the potential varying region P 4 , and thus a charging reaction is completed. Thus, the potential E of the positive electrode  11  is controlled at the end stage of charging as described above. This prevents the potential E of the positive electrode  11  from easily reaching the potential constant region P 2  associated with the phase transition, or prevents the potential E of the positive electrode  11  from easily passing through the potential constant region P 2  associated with the phase transition. In addition, if the potential E of the negative electrode  12  markedly decreases in the potential varying region P 4 , the charging reaction is immediately terminated. This prevents the charging reaction from proceeding to an extent where the precipitation of lithium metal occurs on the negative electrode  12 . 
     Accordingly, in the secondary battery according to the present embodiment, even if the charge voltage Ec is increased to 4.38 V or higher, the potential E of the positive electrode  11  is prevented from easily reaching the potential constant region P 2  associated with the phase transition, or the potential E of the positive electrode  11  is prevented from easily passing through the potential constant region P 2  associated with the phase transition. This generates a tendency that the capacity loss is suppressed and the gas generation is also suppressed. In addition, even if the charge voltage Ec is increased to 4.38 V or higher, the precipitation of lithium metal on the negative electrode  12  is suppressed, and a decrease in the battery capacity is thus suppressed. 
     Moreover, in the secondary battery according to the present embodiment, the battery capacity is less influenced by, for example, an active material ratio and the charge voltage Ec. This helps to suppress variation in the battery capacity, and secures the operable time of, for example, equipment or an apparatus that operates using the secondary battery as a power source. In addition, even if the battery capacity varies, generation of lithium metal is suppressed on the negative electrode  12 . 
     In the secondary battery according to the present embodiment, two physical property conditions described below are satisfied in order to achieve the charge and discharge principle described above. 
     Firstly, a state in which the secondary battery is charged with a constant voltage of a closed circuit voltage (OCV (Open Circuit Voltage)) of 4.38 V or higher for 24 hours is referred to as a full charge state. The potential E of the negative electrode  12  measured in the secondary battery in the full charge state, i.e., a negative electrode potential Ef, is within a range from 19 mV to 86 mV both inclusive. It should be understood that a value of a current at the time of charging the secondary battery until the closed circuit voltage reaches 4.38 V or higher is not particularly limited, and may thus be set to any value. 
     That is, as described above, the potential E of the negative electrode  12  is set to cause the charging not to be completed in the potential constant region P 3  but to be completed in the potential varying region P 4 . Accordingly, if the secondary battery is charged to the full charge state, the negative electrode potential Ef becomes lower in a case where the charging is completed in the potential varying region P 4  than in a case where the charging is completed in the potential constant region P 3 . Thus, the negative electrode potential Ef becomes lower than about 90 mV, and more specifically, falls within the range from 19 mV to 86 mV both inclusive, as described above. 
     Secondly, a discharge capacity obtained when the secondary battery is discharged with a constant current from the full charge state until the closed circuit voltage reaches 3.00 V and thereafter the secondary battery is discharged with a constant voltage of the closed circuit voltage of 3.00 V for 24 hours is referred to as a maximum discharge capacity (mAh). In this case, when the secondary battery is discharged from the full charge state by a capacity corresponding to 1% of the maximum discharge capacity, a variation of the potential E of the negative electrode  12 , i.e., a negative electrode potential variation Ev, represented by Formula (2) below is 1 mV or greater. As is apparent from Formula (2), the negative electrode potential variation Ev is a difference between a potential E 1  (a first negative electrode potential) and a potential E 2  (a second negative electrode potential). It should be understood that the current value at the time of discharging the secondary battery from the full charge state until the closed circuit voltage reaches 3.00 V is not particularly limited and may be set to any value that falls within a typical range, because the secondary battery is discharged with a constant voltage for 24 hours. 
       Negative electrode potential variation  Ev  (mV)=potential  E 2 (mV)−potential  E 1 (mV)  (2)
 
     where:
 
the potential E 1  is an open circuit potential (versus a lithium reference electrode) of the negative electrode  12  measured in the secondary battery in the full charge state; and
 
the potential E 2  is an open circuit potential (versus a lithium reference electrode) of the negative electrode  12  measured in the secondary battery in a state in which the secondary battery is discharged from the full charge state by the capacity corresponding to 1% of the maximum discharge capacity.
 
     That is, as described above, in a case where the potential E of the negative electrode  12  is set to cause the charging to be completed in the potential varying region P 4 , the potential E of the negative electrode  12  increases markedly upon discharging the secondary battery in the full charge state by the capacity corresponding to 1% of the maximum discharge capacity, as is apparent from  FIGS. 7 and 8 . Thus, the potential E of the negative electrode  12  after the discharging, i.e., the potential E 2 , sufficiently increases as compared with the potential E of the negative electrode  12  before the discharging (the full charge state), i.e., the potential E 1 . Accordingly, the negative electrode potential variation Ev, i.e., the difference between the potential E 1  and the potential E 2 , is 1 mV or greater as described above. 
     The secondary battery operates as described below. Upon charging, in the wound electrode body  10 , lithium is extracted from the positive electrode  11 , and the extracted lithium is inserted into the negative electrode  12  via the electrolytic solution. Upon discharging, in the wound electrode body  10 , lithium is extracted from the negative electrode  12 , and the extracted lithium is inserted into the positive electrode  11  via the electrolytic solution. Upon charging and discharging, lithium is inserted and extracted in an ionic state. 
     In a case of manufacturing the secondary battery, the following processes are performed by a procedure described below: a process of fabricating the positive electrode  11 , a process of fabricating the negative electrode  12 , a process of forming the intermediate layer  14 , a process of preparing the electrolytic solution, a process of assembling the secondary battery, and an activation process. 
     First, the positive electrode active material is mixed with materials including, without limitation, the positive electrode binder and the positive electrode conductor on an as-needed basis to thereby obtain a positive electrode mixture. Thereafter, the positive electrode mixture is put into a solvent such as an organic solvent to thereby prepare a paste positive electrode mixture slurry. Lastly, the positive electrode mixture slurry is applied on opposite sides of the positive electrode current collector  11 A to thereby form the positive electrode active material layers  11 B. Thereafter, the positive electrode active material layers  11 B may be compression-molded by means of a machine such as a roll pressing machine. In this case, the positive electrode active material layers  11 B may be heated. The positive electrode active material layers  11 B may be compression-molded multiple times. In this manner, the positive electrode active material layers  11 B are formed on the respective opposite sides of the positive electrode current collector  11 A. Thus, the positive electrode  11  is fabricated. 
     The negative electrode active material layers  12 B are formed on respective opposite sides of the negative electrode current collector  12 A by a procedure similar to the fabrication procedure of the positive electrode  11  described above. Specifically, the negative electrode active material is mixed with materials including, without limitation, the negative electrode binder and the negative electrode conductor on an as-needed basis to thereby obtain a negative electrode mixture. Thereafter, the negative electrode mixture is put into a solvent such as an organic solvent to thereby prepare a paste negative electrode mixture slurry. Thereafter, the negative electrode mixture slurry is applied on the opposite sides of the negative electrode current collector  12 A to thereby form the negative electrode active material layers  12 B. Thereafter, the negative electrode active material layers  12 B may be compression-molded. In this manner, the negative electrode active material layers  12 B are formed on the respective opposite sides of the negative electrode current collector  12 A. Thus, the negative electrode  12  is fabricated. 
     First, an intermediate mixture slurry is prepared in which the inorganic particles are dispersed and the intermediate binder is dissolved in a solvent such as an organic solvent, following which the intermediate mixture slurry is applied on a surface of the negative electrode  12  (the negative electrode active material layer  12 B) to thereby form the intermediate layer  14  serving as a covering layer covering the negative electrode active material layer  12 B. 
     In the case of forming the intermediate layer  14 , as described above, the weight ratio RN in the upper layer  14 N is set to be greater than the weight ratio RM in the lower layer  14 M. Specific examples of methods of forming the intermediate layer  14  include the following two formation methods. 
     A first formation method uses two paste intermediate mixture slurries, i.e., a first intermediate mixture slurry and a second intermediate mixture slurry that each include the inorganic particles and the intermediate binder and that are different from each other in solids concentration. 
     In this case, first, the inorganic particles and the intermediate binder are mixed at a mixture ratio corresponding to the weight ratio RM, and the mixture is put into a solvent such as an organic solvent to thereby prepare the first intermediate mixture slurry having a relatively low solids concentration. Thereafter, the inorganic particles and the intermediate binder are mixed at a mixture ratio corresponding to the weight ratio RN, and the mixture is put into a solvent such as an organic solvent to thereby prepare the second intermediate mixture slurry having a relatively high solids concentration. Thereafter, the first intermediate mixture slurry is applied on the surface of the negative electrode  12  (the negative electrode active material layer  12 B) to thereby form the lower layer  14 M. Lastly, the second intermediate mixture slurry is applied on the surface of the lower layer  14 M to thereby form the upper layer  14 N. 
     The lower layer  14 M and the upper layer  14 N are thereby stacked in this order on the surface of the negative electrode  12 . In this manner, the intermediate layer  14  is formed. As is apparent from the formation procedure described above, the lower layer  14 M and the upper layer  14 N formed here are physically separated from each other. Thus, the intermediate layer  14  is formed into a two-layered structure including the lower layer  14 M and the upper layer  14 N. As long as the weight ratio RN in the upper layer  14 N is greater than the weight ratio RM in the lower layer  14 M, respective thicknesses of the lower layer  14 M and the upper layer  14 N may be equal to each other or different from each other. 
     In the case where the intermediate layer  14  is formed using the two intermediate mixture slurries described above, the weight ratio R varies intermittently in the thickness direction of the intermediate layer  14 . Specifically, in a direction from the negative electrode  12  (the negative electrode active material layer  12 B) toward the separator  13 , the weight ratio R increases intermittently to change from the weight ration RM to the weight ratio RN with the line L as the border therebetween. 
     A second formation method uses a single paste precursor mixture slurry including only the intermediate binder with no inorganic particles. 
     In this case, first, the intermediate binder is put into a solvent such as an organic solvent to thereby prepare the precursor mixture slurry. Thereafter, the precursor mixture slurry is applied on the surface of the negative electrode  12  by supplying the precursor mixture slurry continuously to the surface of the negative electrode  12  (the negative electrode active material layer  12 B) through the use of a coating apparatus with a tank containing the precursor mixture slurry. In this case, while stirring the precursor mixture slurry contained in the tank, the inorganic particles are added, in the course of applying the mixture slurry, to the precursor mixture slurry in the tank in such a manner that the amount of addition gradually increases. The intermediate layer  14  including the intermediate binder and the inorganic particles is thereby formed on the surface of the negative electrode  12 . As is apparent from the formation procedure described above, this intermediate layer  14  is not physically separated into two in the course of formation, and is thus formed into a single-layered structure. As long as the weight ratio RN in the upper layer  14 N is greater than the weight ratio RM in the lower layer  14 M, conditions including the amount of addition of the inorganic particles and the rate of addition thereof may be freely chosen. 
     In the case where the intermediate layer  14  is formed using the single precursor mixture slurry described above, the weight ratio R varies continuously in the thickness direction of the intermediate layer  14 . Specifically, in the direction from the negative electrode  12  toward the separator  13 , the weight ratio R increases continuously to change from the weight ratio RM to the weight ratio RN. 
     The electrolyte salt is put into a solvent such as an organic solvent. The electrolyte salt is thereby dispersed or dissolved in the solvent. In this manner, the electrolytic solution is prepared. 
     First, by a method such as a welding method, the positive electrode lead  15  is coupled to the positive electrode  11  (the positive electrode current collector  11 A) and the negative electrode lead  16  is coupled to the negative electrode  12  (the negative electrode current collector  12 A). Thereafter, the positive electrode  11  and the negative electrode  12  are stacked on each other with the separator  13  and the intermediate layer  14  interposed therebetween, following which the stack of the positive electrode  11 , the negative electrode  12 , the separator  13 , and the intermediate layer  14  is wound to thereby fabricate a wound body. Thereafter, the wound body is placed inside the depression  20 U and the outer package film  20  is folded, following which outer edges of two sides of the outer package film  20  (the fusion-bonding layer) are bonded to each other by a method such as a thermal fusion bonding method. The wound body is thereby contained in the pouch-shaped outer package film  20 . Lastly, the electrolytic solution is injected into the pouch-shaped outer package film  20 , following which the outer edges of the remaining one side of the outer package film  20  (the fusion-bonding layer) are bonded to each other by a method such as a thermal fusion bonding method. In this case, the sealing film  21  is interposed between the outer package film  20  and the positive electrode lead  15 , and the sealing film  22  is interposed between the outer package film  20  and the negative electrode lead  16 . The wound body is thereby impregnated with the electrolytic solution. Thus, the wound electrode body  10  is fabricated. In this manner, the wound electrode body  10  is sealed in the pouch-shaped outer package film  20 . As a result, the secondary battery of the laminated-film type is assembled. 
     The secondary battery is charged and discharged in a high temperature environment by using, for example, a thermostatic chamber to thereby apply an activation treatment to the secondary battery. The activation treatment causes a solid electrolyte interphase (SEI) film to be formed on the surface of a component such as the negative electrode  12 , thereby stabilizing an electrochemical state of the wound electrode body  10 . The secondary battery is thus completed. 
     Conditions including a treatment temperature and the number of times of charging and discharging during the activation treatment may be freely chosen. In particular, the treatment temperature is preferably within a range from 50° C. to 95° C. both inclusive, and more preferably within a range from 70° C. to 85° C. both inclusive, although not particularly limited thereto. The number of times of charging and discharging is at least once and is not particularly limited further. 
     According to the secondary battery, the intermediate layer  14  including the inorganic particles and the intermediate binder is disposed between the negative electrode  12  (the negative electrode active material layer  12 B) and the separator  13 . Further, where the intermediate layer  14  is divided equally into the lower layer  14 M and the upper layer  14 N in the thickness direction, the weight ratio RN in the upper layer  14 N is greater than the weight ratio RM in the lower layer  14 M. 
     In this case, an optimized distribution or dispersion state of the inorganic particles is achieved in the intermediate layer  14 , and it thus becomes easier to cause the negative electrode  12  to adhere to the separator  13  via the intermediate layer  14 . The negative electrode  12  is thereby firmly fixed to the separator  13 , and is thus prevented from easily becoming misaligned with respect to the separator  13  even in a case where the secondary battery is subjected to an external load such as vibrations or a drop. This makes it easier to maintain a state in which the positive electrode  11  and the negative electrode  12  are opposed to each other with the separator  13  interposed therebetween, thus improving the wound electrode body  10  in physical stability (robustness). Further, owing to the negative electrode  12  being disposed at a substantially uniform distance from the separator  13 , variations in distance between the positive electrode  11  and the negative electrode  12  are suppressed, and also variations in electrical resistance between the positive electrode  11  and the negative electrode  12  are suppressed. This suppresses precipitation of lithium caused by a local overvoltage increase upon charging and discharging, and thus stabilizes the operations, i.e., charging and discharging, of the wound electrode body  10 . 
     Based upon the above, the secondary battery improves in safety while securing electrochemical performance of the secondary battery. Accordingly, it is possible to achieve both ensuring of the electrochemical performance and improvement of the safety. 
     In particular, the inorganic particles may include, for example, a metal oxide. This further improves the safety of the secondary battery while ensuring the electrochemical performance of the secondary battery. Accordingly, it is possible to achieve higher effects. In this case, the metal oxide may include, for example, aluminum oxide, the metal nitride may include, for example, aluminum nitride, and the metal hydroxide may include, for example, magnesium hydroxide. This improves the safety of the secondary battery even further, making it possible to achieve even higher effects. 
     Further, the intermediate layer  14  may have a thickness within the range from 0.1 μm to 5 μm both inclusive. This makes it possible to obtain the above-described advantages while securing insertion and extraction of lithium. Accordingly, it is possible to achieve higher effects. 
     Further, the intermediate layer  14  may be provided on the surface of the negative electrode  12  on the side opposed to the separator  13 . This secures adherence of the intermediate layer  14  to the negative electrode  12 , making it possible to achieve higher effects. In this case, the coverage of the intermediate layer  14  may be within the range from 20% to 100% both inclusive. This allows for sufficient adhesion of the negative electrode  12  to the separator  13 , making it possible to achieve higher effects. 
     Further, the separator  13  may have an air permeability within the range from 100 sec/cm 3  to 1000 sec/cm 3  both inclusive. This improves mobility of lithium during insertion and extraction, making it possible to achieve higher effects. 
     Further, in a case where the positive electrode  11  includes the lithium-cobalt composite oxide having the layered rock-salt crystal structure and where the negative electrode  12  includes graphite, the negative electrode potential Ef may be within the range from 19 mV to 86 mV both inclusive, and the negative electrode potential variation Ev may be 1 mV or greater. In such a case, even if the charge voltage Ec is increased to 4.38 V or higher, the potential E of the positive electrode  11  is prevented from easily reaching the potential constant region P 2  associated with the phase transition, or the potential E of the positive electrode  11  is prevented from easily passing through the potential constant region P 2  associated with the phase transition, and moreover, precipitation of lithium metal on the negative electrode  12  is suppressed. This sufficiently improves the secondary battery in safety while securing the electrochemical performance of the secondary battery. Accordingly, it is possible to achieve higher effects. 
     Further, the secondary battery may be a lithium-ion secondary battery. This makes it possible to obtain a sufficient battery capacity stably through the use of insertion and extraction of lithium. Accordingly, it is possible to achieve higher effects. 
     Other than the above, according to the negative electrode  12 , the intermediate layer  14  serving as a covering layer covers the surface of the negative electrode active material layer  12 B, and satisfies the condition that the weight ratio RN in the upper layer  14 N is greater than the weight ratio RM in the lower layer  14 M described above in relation to the configuration of the intermediate layer  14 . Accordingly, for the reasons described above, it is possible for a secondary battery including the negative electrode  12  to achieve a superior battery characteristic. 
     Next, a description is given of modifications of the above-described secondary battery. The configuration of the secondary battery is appropriately modifiable as described below. It should be understood that any two or more of the following series of modifications may be combined. 
     [Modification 1] 
     In  FIG. 4 , the intermediate layer  14  is provided on the surface of the negative electrode  12 . Thus, the intermediate layer  14  is joined to the negative electrode  12 , and is therefore provided integrally with the negative electrode  12 . Alternatively, the intermediate layer  14  may be provided on a surface of the separator  13 , not the negative electrode  12 . 
     Specifically, as illustrated in  FIG. 9  corresponding to  FIG. 4 , the intermediate layer  14  may be provided on a surface of the separator  13  on a side opposed to the negative electrode  12 .  FIG. 9  illustrates the separator  13  to be used in the process of manufacturing the secondary battery. 
     Thus, the intermediate layer  14  is joined to the separator  13 , and is therefore provided integrally with the separator  13 . The intermediate layer  14  joined to the separator  13  has a configuration similar to that of the intermediate layer  14  joined to the negative electrode  12 , except for being joined to the separator  13  instead of being joined to the negative electrode  12 . Accordingly, the above-described condition that the weight ratio RN in the upper layer  14 N is greater than the weight ratio RM in the lower layer  14 M is satisfied also in relation to the intermediate layer  14  joined to the separator  13 . In this case, the upper layer  14 N and the lower layer  14 M are formed in this order on the separator  13 . 
     A procedure to form the intermediate layer  14  joined to the separator  13  is similar to the procedure to form the intermediate layer  14  joined to the negative electrode  12 , except that the intermediate layer  14  is formed on the surface of the separator  13 , not on the surface of the negative electrode  12 . In summary, the intermediate mixture slurry is prepared in which the inorganic particles are dispersed and the intermediate binder is dissolved in a solvent such as an organic solvent, following which the intermediate mixture slurry is applied on the surface of the separator  13  to thereby form the intermediate layer  14 . In this case, as described above, the first formation method may be used, or the second formation method may be used. 
     In a case of using the first formation method, the second intermediate mixture slurry and the first intermediate mixture slurry are applied in this order on the surface of the separator  13  to thereby stack the upper layer  14 N and the lower layer  14 M in this order on the surface of the separator  13 . In a case of using the second formation method, the inorganic particles are added, in the course of applying the precursor mixture slurry, to the precursor mixture slurry in the tank in such a manner that the amount of addition gradually decreases. The upper layer  14 N and the lower layer  14 M are thereby stacked in this order on the surface of the separator  13 . 
     In this case also, the intermediate layer  14  is interposed between the negative electrode  12  and the separator  13  in the completed secondary battery. Accordingly, it is possible to achieve similar effects. 
     [Modification 2] 
     In  FIG. 1 , the single positive electrode lead  15  is coupled to the wound electrode body  10 . However, the positive electrode lead  15  is not limited to one in number, and two or more positive electrode leads  15  may be provided. Increasing the number of the positive electrode leads  15  results in a decrease in electrical resistance of the wound electrode body  10 , making it possible to achieve higher effects. The description given here in relation to the positive electrode lead  15  also applies to the negative electrode lead  16 . Thus, for a reason similar to that described in relation to the positive electrode lead  15 , the negative electrode lead  16  is not limited to one in number, and two or more negative electrode leads  16  may be provided. 
     [Modification 3] 
     In  FIG. 2 , the separator  13  which is a porous film is used. However, although not specifically illustrated here, a separator of a stacked type including a polymer compound layer may be used instead of the separator  13  which is the porous film. 
     Specifically, the separator of the stacked type includes a base layer which is the above-described porous film, and a polymer compound layer provided on one of or each of opposite sides of the base layer. A reason for this is that adherence of the separator to each of the positive electrode  11  and the negative electrode  12  improves to suppress the occurrence of misalignment of the wound electrode body  10 . This helps to prevent the secondary battery from easily swelling even if, for example, a decomposition reaction of the electrolytic solution occurs. The polymer compound layer includes a polymer compound such as polyvinylidene difluoride. A reason for this is that such a polymer compound has superior physical strength and is electrochemically stable. 
     It should be understood that the base layer, the polymer compound layer, or both may each include one or more kinds of particles including, for example, inorganic particles and resin particles. A reason for this is that such particles, including inorganic particles, dissipate heat upon heat generation by the secondary battery, thus improving heat resistance and safety of the secondary battery. The inorganic particles are not particularly limited in kind, and examples thereof include aluminum oxide (alumina), aluminum nitride, boehmite, silicon oxide (silica), titanium oxide (titania), magnesium oxide (magnesia), and zirconium oxide (zirconia). 
     In a case of fabricating the separator of the stacked type, a precursor solution including, without limitation, the polymer compound and an organic solvent, is prepared and thereafter the precursor solution is applied on one of or each of the opposite sides of the base layer. 
     Similar effects are obtainable also in the case where the separator of the stacked type is used, as lithium is movable between the positive electrode  11  and the negative electrode  12 . 
     [Modification 4] 
     The electrolytic solution which is a liquid electrolyte is used in  FIG. 1 . However, although not specifically illustrated here, an electrolyte layer which is a gel electrolyte may be used instead of the electrolytic solution. 
     In the wound electrode body  10  including the electrolyte layer, the positive electrode  11  and the negative electrode  12  are stacked on each other with the separator  13 , the intermediate layer  14 , and the electrolyte layer interposed therebetween, and the stack of the positive electrode  11 , the negative electrode  12 , the separator  13 , the intermediate layer  14 , and the electrolyte layer is wound. The electrolyte layer is interposed between the positive electrode  11  and the separator  13 , and between the intermediate layer  14  and the separator  13 . 
     Specifically, the electrolyte layer includes a polymer compound together with the electrolytic solution. The electrolytic solution is held by the polymer compound in the electrolyte layer. The configuration of the electrolytic solution is as described above. The polymer compound includes, for example, polyvinylidene difluoride. In a case of forming the electrolyte layer, a precursor solution including, without limitation, the electrolytic solution, the polymer compound, and an organic solvent, is prepared and thereafter the precursor solution is applied on opposite sides of each of the positive electrode  11  and the negative electrode  12 . 
     Similar effects are obtainable also in the case where the electrolyte layer is used, as lithium is movable between the positive electrode  11  and the negative electrode  12  via the electrolyte layer. 
     Next, a description is given of applications (application examples) of the above-described secondary battery. 
     The applications of the secondary battery are not particularly limited as long as they are, for example, machines, equipment, instruments, apparatuses, or systems (an assembly of a plurality of pieces of equipment, for example) in which the secondary battery is usable mainly as a driving power source, an electric power storage source for electric power accumulation, or any other source. The secondary battery used as a power source may serve as a main power source or an auxiliary power source. The main power source is preferentially used regardless of the presence of any other power source. The auxiliary power source may be used in place of the main power source, or may be switched from the main power source on an as-needed basis. In a case where the secondary battery is used as the auxiliary power source, the kind of the main power source is not limited to the secondary battery. 
     Specific examples of the applications of the secondary battery include: electronic equipment including portable electronic equipment; portable life appliances; apparatuses for data storage; electric power tools; battery packs to be mounted as detachable power sources on, for example, laptop personal computers; medical electronic equipment; electric vehicles; and electric power storage systems. Examples of the electronic equipment include video cameras, digital still cameras, mobile phones, laptop personal computers, cordless phones, headphone stereos, portable radios, portable televisions, and portable information terminals. Examples of the portable life appliances include electric shavers. Examples of the apparatuses for data storage include backup power sources and memory cards. Examples of the electric power tools include electric drills and electric saws. Examples of the medical electronic equipment include pacemakers and hearing aids. Examples of the electric vehicles include electric automobiles including hybrid automobiles. Examples of the electric power storage systems include home battery systems for accumulation of electric power for a situation such as emergency. It should be understood that the secondary battery may have a battery structure of the above-described laminated-film type, a cylindrical type, or any other type. Further, multiple secondary batteries may be used, for example, as a battery pack or a battery module. 
     In particular, the battery pack and the battery module are each effectively applied to relatively large-sized equipment, etc., including an electric vehicle, an electric power storage system, and an electric power tool. The battery pack, as will be described later, may include a single battery, or may include an assembled battery. The electric vehicle is a vehicle that operates (travels) using the secondary battery as a driving power source, and may be an automobile that is additionally provided with a driving source other than the secondary battery as described above, such as a hybrid automobile. The electric power storage system is a system that uses the secondary battery as an electric power storage source. An electric power storage system for home use accumulates electric power in the secondary battery which is an electric power storage source, and the accumulated electric power may thus be utilized for using, for example, home appliances. 
     Some application examples of the secondary battery will now be described in detail. The configurations of the application examples described below are merely examples, and are appropriately modifiable. 
       FIG. 10  illustrates a block configuration of a battery pack including a single battery. The battery pack described here is a simple battery pack (a so-called soft pack) including one secondary battery, and is to be mounted on, for example, electronic equipment typified by a smartphone. 
     As illustrated in  FIG. 10 , the battery pack includes an electric power source  61  and a circuit board  62 . The circuit board  62  is coupled to the electric power source  61 , and includes a positive electrode terminal  63 , a negative electrode terminal  64 , and a temperature detection terminal (a so-called T terminal)  65 . 
     The electric power source  61  includes one secondary battery. The secondary battery has a positive electrode lead coupled to the positive electrode terminal  63  and a negative electrode lead coupled to the negative electrode terminal  64 . The electric power source  61  is couplable to outside via the positive electrode terminal  63  and the negative electrode terminal  64 , and is thus chargeable and dischargeable via the positive electrode terminal  63  and the negative electrode terminal  64 . The circuit board  62  includes a controller  66 , a switch  67 , a PTC device  68 , and a temperature detector  69 . However, the PTC device  68  may be omitted. 
     The controller  66  includes, for example, a central processing unit (CPU) and a memory, and controls an overall operation of the battery pack. The controller  66  detects and controls a use state of the electric power source  61  on an as-needed basis. 
     If a battery voltage of the electric power source  61  (the secondary battery) reaches an overcharge detection voltage or an overdischarge detection voltage, the controller  66  turns off the switch  67 . This prevents a charging current from flowing into a current path of the electric power source  61 . In addition, if a large current flows upon charging or discharging, the controller  66  turns off the switch  67  to block the charging current. The overcharge detection voltage and the overdischarge detection voltage are not particularly limited. For example, the overcharge detection voltage is 4.2 V±0.05 V and the overdischarge detection voltage is 2.4 V±0.1 V. 
     The switch  67  includes, for example, a charge control switch, a discharge control switch, a charging diode, and a discharging diode. The switch  67  performs switching between coupling and decoupling between the electric power source  61  and external equipment in accordance with an instruction from the controller  66 . The switch  67  includes, for example, a metal-oxide-semiconductor field-effect transistor (MOSFET) including a metal-oxide semiconductor. The charging and discharging currents are detected on the basis of an ON-resistance of the switch  67 . 
     The temperature detector  69  includes a temperature detection device such as a thermistor. The temperature detector  69  measures a temperature of the electric power source  61  using the temperature detection terminal  65 , and outputs a result of the temperature measurement to the controller  66 . The result of the temperature measurement to be obtained by the temperature detector  69  is used, for example, in a case where the controller  66  performs charge/discharge control upon abnormal heat generation or in a case where the controller  66  performs a correction process upon calculating a remaining capacity. 
       FIG. 11  illustrates a block configuration of a battery pack including an assembled battery. In the following description, reference will be made as necessary to the components of the battery pack including the single battery ( FIG. 10 ). 
     As illustrated in  FIG. 11 , the battery pack includes a positive electrode terminal  81  and a negative electrode terminal  82 . Specifically, the battery pack includes, inside a housing  70 , the following components: a controller  71 , an electric power source  72 , a switch  73 , a current measurement unit  74 , a temperature detector  75 , a voltage detector  76 , a switch controller  77 , a memory  78 , a temperature detection device  79 , and a current detection resistor  80 . 
     The electric power source  72  includes an assembled battery in which two or more secondary batteries are coupled to each other, and a type of the coupling of the two or more secondary batteries is not particularly limited. Accordingly, the coupling scheme may be in series, in parallel, or of a mixed type of both. For example, the electric power source  72  includes six secondary batteries coupled to each other in two parallel and three series. 
     Configurations of the controller  71 , the switch  73 , the temperature detector  75 , and the temperature detection device  79  are similar to those of the controller  66 , the switch  67 , and the temperature detector  69  (the temperature detection device). The current measurement unit  74  measures a current using the current detection resistor  80 , and outputs a result of the measurement of the current to the controller  71 . The voltage detector  76  measures a battery voltage of the electric power source  72  (the secondary battery) and provides the controller  71  with a result of the measurement of the voltage that has been subjected to analog-to-digital conversion. 
     The switch controller  77  controls an operation of the switch  73  in response to signals supplied by the current measurement unit  74  and the voltage detector  76 . If a battery voltage reaches an overcharge detection voltage or an overdischarge detection voltage, the switch controller  77  turns off the switch  73  (the charge control switch). This prevents a charging current from flowing into a current path of the electric power source  72 . This enables the electric power source  72  to perform only discharging via the discharging diode, or only charging via the charging diode. In addition, if a large current flows upon charging or discharging, the switch controller  77  blocks the charging current or the discharging current. 
     The switch controller  77  may be omitted and the controller  71  may thus also serve as the switch controller  77 . The overcharge detection voltage and the overdischarge detection voltage are not particularly limited, and are similar to those described above in relation to the battery pack including the single battery. 
     The memory  78  includes, for example, an electrically erasable programmable read-only memory (EEPROM) which is a non-volatile memory, and the memory  78  stores, for example, a numeric value calculated by the controller  71  and data (e.g., an initial internal resistance, a full charge capacity, and a remaining capacity) of the secondary battery measured in the manufacturing process. 
     The positive electrode terminal  81  and the negative electrode terminal  82  are terminals coupled to, for example, external equipment that operates using the battery pack, such as a laptop personal computer, or external equipment that is used to charge the battery pack, such as a charger. The electric power source  72  (the secondary battery) is chargeable and dischargeable via the positive electrode terminal  81  and the negative electrode terminal  82 . 
       FIG. 12  illustrates a block configuration of a hybrid automobile which is an example of the electric vehicle. As illustrated in  FIG. 12 , the electric vehicle includes, inside a housing  83 , the following components: a controller  84 , an engine  85 , an electric power source  86 , a motor  87 , a differential  88 , an electric generator  89 , a transmission  90 , a clutch  91 , inverters  92  and  93 , and sensors  94 . The electric vehicle also includes a front wheel drive shaft  95 , a pair of front wheels  96 , a rear wheel drive shaft  97 , and a pair of rear wheels  98 . The front wheel drive shaft  95  and the pair of front wheels  96  are coupled to the differential  88  and the transmission  90 . 
     The electric vehicle is configured to travel by using one of the engine  85  and the motor  87  as a driving source. The engine  85  is a major power source, such as a gasoline engine. In a case where the engine  85  is used as a power source, a driving force (a rotational force) of the engine  85  is transmitted to the front wheels  96  and the rear wheels  98  via the differential  88 , the transmission  90 , and the clutch  91 , which are driving parts. It should be understood that the rotational force of the engine  85  is transmitted to the electric generator  89 , and the electric generator  89  thus generates alternating-current power by utilizing the rotational force. In addition, the alternating-current power is converted into direct-current power via the inverter  93 , and the direct-current power is thus accumulated in the electric power source  86 . In contrast, in a case where the motor  87  which is a converter is used as a power source, electric power (direct-current power) supplied from the electric power source  86  is converted into alternating-current power via the inverter  92 . Thus, the motor  87  is driven by utilizing the alternating-current power. A driving force (a rotational force) converted from the electric power by the motor  87  is transmitted to the front wheels  96  and the rear wheels  98  via the differential  88 , the transmission  90 , and the clutch  91 , which are the driving parts. 
     When the electric vehicle is decelerated by means of a brake mechanism, a resistance force at the time of the deceleration is transmitted as a rotational force to the motor  87 . Thus, the motor  87  may generate alternating-current power by utilizing the rotational force. The alternating-current power is converted into direct-current power via the inverter  92 , and direct-current regenerative power is thus accumulated in the electric power source  86 . 
     The controller  84  includes, for example, a CPU, and controls an overall operation of the electric vehicle. The electric power source  86  includes one or more secondary batteries and is coupled to an external electric power source. In this case, the electric power source  86  may be supplied with electric power from the external electric power source and thereby accumulate the electric power. The sensors  94  are used to control the number of revolutions of the engine  85  and to control an angle of a throttle valve (a throttle angle). The sensors  94  include one or more of sensors including, without limitation, a speed sensor, an acceleration sensor, and an engine speed sensor. 
     The case where the electric vehicle is a hybrid automobile has been described as an example; however, the electric vehicle may be a vehicle that operates using only the electric power source  86  and the motor  87  and not using the engine  85 , such as an electric automobile. 
     Although not specifically illustrated here, other application examples are also conceivable as application examples of the secondary battery. 
     Specifically, the secondary battery is applicable to an electric power storage system. The electric power storage system includes, inside a building such as a residential house or a commercial building, the following components: a controller, an electric power source including one or more secondary batteries, a smart meter, and a power hub. 
     The electric power source is coupled to electric equipment such as a refrigerator installed inside the building, and is couplable to an electric vehicle such as a hybrid automobile stopped outside the building. Further, the electric power source is coupled, via the power hub, to a home power generator such as a solar power generator installed at the building, and is also coupled, via the smart meter and the power hub, to a centralized power system of an external power station such as a thermal power station. 
     Alternatively, the secondary battery is applicable to an electric power tool such as an electric drill or an electric saw. The electric power tool includes, inside a housing to which a movable part such as a drilling part or a saw blade part is attached, the following components: a controller, and an electric power source including one or more secondary batteries. 
     EXAMPLES 
     A description is given of Examples of the technology below. 
     Experiment Examples 1-1 to 1-11 
     Secondary batteries (lithium-ion secondary batteries) of the laminated-film type illustrated in  FIGS. 1 to 3  were fabricated, following which the secondary batteries were evaluated for their respective battery characteristics as described below. 
     The secondary batteries were fabricated in accordance with the following procedure. 
     First, 91 parts by mass of the positive electrode active material (lithium cobalt oxide (LiCoO 2 ) as the layered rock-salt lithium-cobalt composite oxide), 3 parts by mass of the positive electrode binder (polyvinylidene difluoride (PVDF)), and 6 parts by mass of the positive electrode conductor (graphite) were mixed with each other to thereby obtain a positive electrode mixture. Thereafter, the positive electrode mixture was put into an organic solvent (N-methyl-2-pyrrolidone), following which the organic solvent was stirred to thereby prepare a paste positive electrode mixture slurry. Thereafter, the positive electrode mixture slurry was applied on opposite sides of the positive electrode current collector  11 A (a band-shaped aluminum foil having a thickness of 12 μm) by means of a coating apparatus, following which the applied positive electrode mixture slurry was dried to thereby form the positive electrode active material layers  11 B. Lastly, the positive electrode active material layers  11 B were compression-molded by means of a roll pressing machine. In this manner, the positive electrode active material layers  11 B were formed on respective opposite sides of the positive electrode current collector  11 A. Thus, the positive electrode  11  was fabricated. 
     First, 93 parts by mass of the negative electrode active material (artificial graphite) and 7 parts by mass of the positive electrode binder (PVDF) were mixed with each other to thereby obtain a negative electrode mixture. Thereafter, the negative electrode mixture was put into an organic solvent (N-methyl-2-pyrrolidone), following the organic solvent was stirred to thereby prepare a paste negative electrode mixture slurry. Thereafter, the negative electrode mixture slurry was applied on opposite sides of the negative electrode current collector  12 A (a band-shaped copper foil having a thickness of 15 μm) by means of a coating apparatus, following which the applied negative electrode mixture slurry was dried to thereby form the negative electrode active material layers  12 B. Lastly, the negative electrode active material layers  12 B were compression-molded by means of a roll pressing machine. In this manner, the negative electrode active material layers  12 B were formed on the respective opposite sides of the negative electrode current collector  12 A. Thus, the negative electrode  12  was fabricated. 
     The negative electrode potential Ef (mV) and the negative electrode potential variation Ev (mV) in a case where the charge voltage Ec was set to 4.45 V were as listed in Table 1. Here, the maximum discharge capacity was set to be within a range from 1950 mAh to 2050 mAh both inclusive. 
     The intermediate layer  14  having the two-layered structure including the lower layer  14 M and the upper layer  14 N was formed by the foregoing first formation method. 
     Specifically, first, a mixture of the inorganic particles and the intermediate binder (PVDF) was put into an organic solvent (N-methyl-2-pyrrolidone), following which the organic solvent was stirred. Thus, the inorganic particles were dispersed and the intermediate binder was dissolved in the organic solvent to thereby prepare the first intermediate mixture slurry having a relatively low solids concentration. In this case, the mixture ratio (the weight ratio) between the inorganic particles and the intermediate binder was set to 10:20. Materials of the inorganic particles, that is, materials included in the inorganic particles were magnesium hydroxide (Mg(OH) 2 ), aluminum oxide (Al 2 O 3 ), silicon oxide (SiO 2 ), and aluminum nitride (AlN). 
     Thereafter, the second intermediate mixture slurry having a relatively high solids concentration was prepared by a procedure similar to the above-described procedure by which the first intermediate mixture slurry was prepared, except that the mixture ratio (the weight ratio) between the inorganic particles and the intermediate binder was changed to 10:2. 
     Thereafter, the first intermediate mixture slurry was applied on a surface of the negative electrode  12  (the negative electrode active material layer  12 B) by means of a coating apparatus, following which the first intermediate mixture slurry was dried to thereby form the lower layer  14 M. 
     Lastly, the second intermediate mixture slurry was applied on the surface of the lower layer  14 M by means of a coating apparatus, following which the second intermediate mixture slurry was dried to thereby form the upper layer  14 N. The lower layer  14 M and the upper layer  14 N were thereby stacked in this order on the surface of the negative electrode  12 . Thus, the intermediate layer  14  having the two-layered structure was formed on the surface of the negative electrode  12  in such a manner that the weight ratio RN in the upper layer  14 N was greater than the weight ratio RM in the lower layer  14 M. 
     The thickness (μm) and coverage (%) of the intermediate layer  14  were as listed in Table 1. In the case of forming the intermediate layer  14 , the thickness of the lower layer  14 M and the thickness of the upper layer  14 N were set to be equal to each other. 
     In the case of forming the intermediate layer  14 , the intermediate layer  14  was also formed on a surface of the separator  13  by a similar procedure except that the second intermediate mixture slurry and the first intermediate mixture slurry were applied in this order on the surface of the separator  13  instead of the surface of the negative electrode  12 . The location where the intermediate layer  14  was formed, that is, which of the negative electrode  12  and the separator  13  the intermediate layer  14  was formed on, is listed in the “Formation Location” column in Table 1. The formation location of the intermediate layer  14  described here may refer to the location where the intermediate layer  14  was formed in the secondary battery under manufacturing, that is, in the secondary battery before completion, or may be the location where the intermediate layer  14  had been formed in the completed secondary battery, that is, as of a point in time when the secondary battery was disassembled. 
     For the sake of comparison, no intermediate layer  14  was formed. Further, for the sake of comparison, the intermediate layer  14  was formed on the surface of the negative electrode  12  by a similar procedure except that the order of use of the first intermediate mixture slurry and the second intermediate mixture slurry was reversed. In this case, the intermediate layer  14  having the two-layered structure was formed in which the weight ratio RN in the upper layer  14 N was smaller than the weight ratio RM in the lower layer  14 M. 
     The electrolyte salt (lithium hexafluorophosphate (LiPF 6 )) was added to a solvent (ethylene carbonate which is a cyclic carbonic acid ester and diethyl carbonate which is a chain carbonic acid ester), following which the solvent was stirred. A mixture ratio (a weight ratio) of the solvent was set as follows: ethylene carbonate/diethyl carbonate=50:50. The content of the electrolyte salt with respect to the solvent was set to 1 mol/kg. 
     First, the positive electrode lead  15  including aluminum was welded to the positive electrode current collector  11 A, and the negative electrode lead  16  including copper was welded to the negative electrode current collector  12 A. Thereafter, the positive electrode  11  and the negative electrode  12  were stacked on each other with the separator  13  (a fine-porous polyethylene film having a thickness of 15 μm) and the intermediate layer  14  interposed therebetween, following which the stack of the positive electrode  11 , the negative electrode  12 , the separator  13 , and the intermediate layer  14  was wound to thereby fabricate a wound body. 
     Thereafter, the outer package film  20  was folded in such a manner as to sandwich the wound body placed in the depression part  20 U, following which the outer edges of two sides of the outer package film  20  were thermal fusion bonded to each other to thereby allow the wound body to be contained inside the pouch-shaped outer package film  20 . As the outer package film  20 , an aluminum laminated film was used in which a fusion-bonding layer (a polypropylene film having a thickness of 30 μm), a metal layer (an aluminum foil having a thickness of 40 μm), and a surface protective layer (a nylon film having a thickness of 25 μm) were stacked in this order from the inner side. 
     Lastly, the electrolytic solution was injected into the pouch-shaped outer package film  20  and thereafter, the outer edges of the remaining one side of the outer package film  20  were thermal fusion bonded to each other in a reduced-pressure environment. In this case, the sealing film  21  (a polypropylene film having a thickness of 5 μm) was interposed between the outer package film  20  and the positive electrode lead  15 , and the sealing film  22  (a polypropylene film having a thickness of 5 μm) was interposed between the outer package film  20  and the negative electrode lead  16 . The wound body was thereby impregnated with the electrolytic solution. Thus, the wound electrode body  10  was formed. In this manner, the wound electrode body  10  was sealed in the outer package film  20 . As a result, the secondary battery of the laminated-film type was assembled. 
     Lastly, the assembled secondary battery was charged and discharged for one cycle in a thermostatic chamber (at a temperature of 80° C.) to thereby apply an activation treatment to the secondary battery. Upon charging, the secondary battery was charged with a constant current of 0.1 C until a voltage reached 4.43 V, following which the secondary battery was charged with a constant voltage of that value until a current reached 0.05 C. Upon discharging, the secondary battery was discharged with a constant current of 0.1 C until a voltage reached 2.50 V. It should be understood that 0.1 C is a value of a current that causes a battery capacity (a theoretical capacity) to be completely discharged in 10 hours, and 0.05 C is a value of a current that causes the above-described battery capacity to be completely discharged in 20 hours. 
     The electrochemical state of the wound electrode body  10  was thereby stabilized. Thus, the secondary battery of the laminated-film type was completed. 
     Evaluation of the secondary batteries for their battery characteristics (a safety characteristic, a cyclability characteristic, and an electrical resistance characteristic) revealed the results presented in Table 1. 
     In a case of examining the safety characteristic, a collision test was performed on the secondary battery and the state (durability) of the secondary battery after the collision test was visually determined. In the collision test, the secondary battery was placed on the floor, and thereafter a circular-column-shaped weight of SUS having an outer diameter of 15.8 mm and a length of 340 mm was dropped onto the secondary battery. In this case, a drop height of the weight, i.e., a distance between the weight before dropping and the secondary battery, was set to 61 cm. 
     A case where neither smoking nor ignition occurred as a result of the collision test was determined as “A” indicating that sufficient durability was achieved. A case where smoking occurred but no ignition occurred was determined as “B” indicating that acceptable level of durability was achieved. A case where ignition occurred was determined as “C” indicating that acceptable level of durability was not achieved. 
     In a case of examining the cyclability characteristic, first, the secondary battery was charged and discharged in an ambient temperature environment (at a temperature of 23° C.) to thereby measure a discharge capacity (a first-cycle discharge capacity). Thereafter, the secondary battery was repeatedly charged and discharged in the same environment until the total number of cycles reached 400 to thereby measure the discharge capacity (a 400th-cycle discharge capacity). Lastly, the following was calculated: capacity retention rate (%)=(400th-cycle discharge capacity/first-cycle discharge capacity)×100. Charging and discharging conditions were similar to the charging and discharging conditions in the activation process described above. 
     In a case of examining the electrical resistance characteristic, when examining the cyclability characteristic described above, an electrical resistance (a first-cycle electrical resistance) of the secondary battery was measured with a battery tester after the first-cycle charging and discharging, and thereafter an electrical resistance (a 400th-cycle electrical resistance) of the secondary battery was measured with the battery tester after the 400th-cycle charging and discharging. The following was thereby calculated: resistance increase rate (%)=[(400th-cycle electrical resistance−first-cycle electrical resistance)/first-cycle electrical resistance]×100. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Positive electrode active material: lithium cobalt oxide; Negative electrode active material: artificial graphite 
               
            
           
           
               
               
               
            
               
                   
                 Negative electrode 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Negative 
                 Separator 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Intermediate layer 
                 Negative 
                 electrode 
                 Air 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Inter- 
                   
                 Thick- 
                 Cover- 
                 electrode 
                 potential 
                 perme- 
                   
                 Capacity 
                 Resistance 
               
               
                 Experiment 
                 Formation 
                 Inorganic 
                 mediate 
                 Distri- 
                 ness 
                 age 
                 potential 
                 variation 
                 ability 
                   
                 retention 
                 increase 
               
               
                 example 
                 location 
                 particles 
                 binder 
                 bution 
                 (μm) 
                 (%) 
                 Ef (mV) 
                 Ev (mV) 
                 (sec/cm 3 ) 
                 Durability 
                 rate (%) 
                 rate (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1-1 
                 Negative 
                 Mg(OH) 2   
                 PVDF 
                 RN &gt; RM 
                 3 
                 50 
                 50 
                 17 
                 300 
                 A 
                 93 
                 12 
               
               
                 1-2 
                 electrode 
                 Al 2 O 3   
                   
                   
                   
                   
                   
                   
                   
                 A 
                 93 
                 11 
               
               
                 1-3 
                   
                 SiO 2   
                   
                   
                   
                   
                   
                   
                   
                 B 
                 89 
                 15 
               
               
                 1-4 
                   
                 AlN 
                   
                   
                   
                   
                   
                   
                   
                 B 
                 90 
                 17 
               
               
                 1-5 
                 Separator 
                 Mg(OH) 2   
                 PVDF 
                 RN &gt; RM 
                 3 
                 50 
                 50 
                 17 
                 300 
                 A 
                 88 
                 19 
               
               
                 1-6 
                   
                 Al 2 O 3   
                   
                   
                   
                   
                   
                   
                   
                 A 
                 87 
                 18 
               
               
                 1-7 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 50 
                 17 
                 300 
                 C 
                 68 
                 45 
               
               
                 1-8 
                 Negative 
                 Mg(OH) 2   
                 PVDF 
                 RN &lt; RM 
                 3 
                 50 
                 50 
                 17 
                 300 
                 A 
                 71 
                 46 
               
               
                 1-9 
                 electrode 
                 Al 2 O 3   
                   
                   
                   
                   
                   
                   
                   
                 A 
                 72 
                 49 
               
               
                 1-10 
                   
                 SiO 2   
                   
                   
                   
                   
                   
                   
                   
                 C 
                 67 
                 52 
               
               
                 1-11 
                   
                 AlN 
                   
                   
                   
                   
                   
                   
                   
                 C 
                 64 
                 44 
               
               
                   
               
            
           
         
       
     
     As indicated in Table 1, the durability, the capacity retention rate, and the resistance increase rate each varied greatly depending on the configuration of the secondary battery (the presence or absence of the intermediate layer  14  and the configuration thereof). 
     Specifically, in a case where the weight ratio RN was greater than the weight ratio RM (Experiment examples 1-1 to 1-6), a satisfactory durability was achieved together with a high capacity retention rate while the resistance increase rate was suppressed to be low, in contrast to a case where no intermediate layer  14  was formed (Experiment example 1-7) and a case where the weight ratio RN was smaller than the weight ratio RM (Experiment examples 1-8 to 1-11). Such a favorable tendency was achieved independently of the formation location of the intermediate layer  14  (i.e., the negative electrode  12  or the separator  13 ). 
     In the case where the weight ratio RN was greater than the weight ratio RM, the durability further improved particularly if magnesium hydroxide or aluminum oxide was used as the material of the inorganic particles. 
     Experiment Examples 2-1 to 2-5 
     As described in Table 2, secondary batteries were fabricated and were evaluated for their respective battery characteristics by similar procedures except that the thickness of the intermediate layer  14  was varied. In order to vary the thickness of the intermediate layer  14 , respective application amounts of the first intermediate mixture slurry and the second intermediate mixture slurry were adjusted. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Positive electrode active material: lithium cobalt oxide; Negative electrode active material: artificial graphite 
               
            
           
           
               
               
               
            
               
                   
                 Negative electrode 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Negative 
                 Separator 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Intermediate layer 
                 Negative 
                 electrode 
                 Air 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Inter- 
                   
                 Thick- 
                 Cover- 
                 electrode 
                 potential 
                 perme- 
                   
                 Capacity 
                 Resistance 
               
               
                 Experiment 
                 Formation 
                 Inorganic 
                 mediate 
                 Distri- 
                 ness 
                 age 
                 potential 
                 variation 
                 ability 
                   
                 retention 
                 increase 
               
               
                 example 
                 location 
                 particles 
                 binder 
                 bution 
                 (μm) 
                 (%) 
                 Ef (mV) 
                 Ev (mV) 
                 (sec/cm 3 ) 
                 Durability 
                 rate (%) 
                 rate (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 2-1 
                 Negative 
                 Mg(OH) 2   
                 PVDF 
                 RN &gt; RM 
                 0.05 
                 50 
                 50 
                 17 
                 300 
                 B 
                 93 
                 12 
               
               
                 2-2 
                 electrode 
                   
                   
                   
                 0.1 
                   
                   
                   
                   
                 A 
                 93 
                 12 
               
               
                 2-3 
                   
                   
                   
                   
                 1 
                   
                   
                   
                   
                 A 
                 92 
                 11 
               
               
                 1-1 
                   
                   
                   
                   
                 3 
                   
                   
                   
                   
                 A 
                 93 
                 12 
               
               
                 2-4 
                   
                   
                   
                   
                 5 
                   
                   
                   
                   
                 A 
                 86 
                 18 
               
               
                 2-5 
                   
                   
                   
                   
                 10 
                   
                   
                   
                   
                 A 
                 82 
                 26 
               
               
                   
               
            
           
         
       
     
     As indicated in Table 2, even if the thickness of the intermediate layer  14  was varied, a satisfactory durability was achieved together with a high capacity retention rate while the resistance increase rate was suppressed to be low. In this case, particularly if the thickness of the intermediate layer  14  was within a range from 0.1 μm to 5 μm both inclusive, the durability further improved while the capacity retention rate further increased and the resistance increase rate further decreased. 
     Experiment Examples 3-1 to 3-4 
     As described in Table 3, secondary batteries were fabricated and were evaluated for their respective battery characteristics by similar procedures except that the coverage of the intermediate layer  14  was varied. In order to vary the coverage of the intermediate layer  14 , respective solids concentrations of the first intermediate mixture slurry and the second intermediate mixture slurry were adjusted. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Positive electrode active material: lithium cobalt oxide; Negative electrode active material: artificial graphite 
               
            
           
           
               
               
               
            
               
                   
                 Negative electrode 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Negative 
                 Separator 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Intermediate layer 
                 Negative 
                 electrode 
                 Air 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Inter- 
                   
                 Thick- 
                 Cover- 
                 electrode 
                 potential 
                 perme- 
                   
                 Capacity 
                 Resistance 
               
               
                 Experiment 
                 Formation 
                 Inorganic 
                 mediate 
                 Distri- 
                 ness 
                 age 
                 potential 
                 variation 
                 ability 
                   
                 retention 
                 increase 
               
               
                 example 
                 location 
                 particles 
                 binder 
                 bution 
                 (μm) 
                 (%) 
                 Ef (mV) 
                 Ev (mV) 
                 (sec/cm 3 ) 
                 Durability 
                 rate (%) 
                 rate (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 3-1 
                 Negative 
                 Mg(OH) 2   
                 PVDF 
                 RN &gt; RM 
                 3 
                 10 
                 50 
                 17 
                 300 
                 B 
                 91 
                 13 
               
               
                 3-2 
                 electrode 
                   
                   
                   
                   
                 20 
                   
                   
                   
                 A 
                 92 
                 11 
               
               
                 1-1 
                   
                   
                   
                   
                   
                 50 
                   
                   
                   
                 A 
                 93 
                 12 
               
               
                 3-3 
                   
                   
                   
                   
                   
                 80 
                   
                   
                   
                 A 
                 90 
                 17 
               
               
                 3-4 
                   
                   
                   
                   
                   
                 100 
                   
                   
                   
                 A 
                 88 
                 22 
               
               
                   
               
            
           
         
       
     
     As indicated in Table 3, even if the coverage of the intermediate layer  14  was varied, a satisfactory durability was achieved together with a high capacity retention rate while the resistance increase rate was suppressed to be low. In this case, particularly if the coverage of the intermediate layer  14  was within a range from 20% to 100% both inclusive, the durability further improved while the high capacity retention rate and the low resistance increase rate were retained. 
     Experiment Examples 4-1 to 4-4 
     As described in Table 4, secondary batteries were fabricated and were evaluated for their respective battery characteristics by similar procedures except that the air permeability of the separator  13  was varied. In order to vary the air permeability of the separator  13 , the temperature at the time of the activation treatment was adjusted within a range from 50° C. to 95° C. both inclusive. In this case, the air permeability of the separator  13  exhibited a tendency to increase with increasing temperature at the time of the activation treatment. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Positive electrode active material: lithium cobalt oxide; Negative electrode active material: artificial graphite 
               
            
           
           
               
               
               
            
               
                   
                 Negative electrode 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Negative 
                 Separator 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Intermediate layer 
                 Negative 
                 electrode 
                 Air 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Inter- 
                   
                 Thick- 
                 Cover- 
                 electrode 
                 potential 
                 perme- 
                   
                 Capacity 
                 Resistance 
               
               
                 Experiment 
                 Formation 
                 Inorganic 
                 mediate 
                 Distri- 
                 ness 
                 age 
                 potential 
                 variation 
                 ability 
                   
                 retention 
                 increase 
               
               
                 example 
                 location 
                 particles 
                 binder 
                 bution 
                 (μm) 
                 (%) 
                 Ef (mV) 
                 Ev (mV) 
                 (sec/cm 3 ) 
                 Durability 
                 rate (%) 
                 rate (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 4-1 
                 Negative 
                 Mg(OH) 2   
                 PVDF 
                 RN &gt; RM 
                 3 
                 50 
                 50 
                 17 
                 50 
                 B 
                 92 
                 10 
               
               
                 4-2 
                 electrode 
                   
                   
                   
                   
                   
                   
                   
                 100 
                 A 
                 92 
                 11 
               
               
                 1-1 
                   
                   
                   
                   
                   
                   
                   
                   
                 300 
                 A 
                 93 
                 12 
               
               
                 4-3 
                   
                   
                   
                   
                   
                   
                   
                   
                 1000 
                 A 
                 88 
                 19 
               
               
                 4-4 
                   
                   
                   
                   
                   
                   
                   
                   
                 1500 
                 A 
                 82 
                 26 
               
               
                   
               
            
           
         
       
     
     As indicated in Table 4, even if the air permeability of the separator  13  was varied, a satisfactory durability was achieved together with a high capacity retention rate while the resistance increase rate was suppressed to be low. In this case, particularly if the air permeability of the separator  13  was within a range from 100 sec/cm 3  to 1000 sec/cm 3  both inclusive, the durability further improved while the capacity retention rate further increased and the resistance increase rate further decreased. 
     Experiment Examples 5-1 to 5-6 
     As described in Table 5, secondary batteries were fabricated and were evaluated for their respective battery characteristics by similar procedures except that the negative electrode potential Ef and the negative electrode potential variation Ev were each varied. In order to vary each of the negative electrode potential Ef and the negative electrode potential variation Ev, a mixture ratio (a weight ratio) between the positive electrode active material and the negative electrode active material was adjusted. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Positive electrode active material: lithium cobalt oxide; Negative electrode active material: artificial graphite 
               
            
           
           
               
               
               
            
               
                   
                 Negative electrode 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Negative 
                 Separator 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Intermediate layer 
                 Negative 
                 electrode 
                 Air 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Inter- 
                   
                 Thick- 
                 Cover- 
                 electrode 
                 potential 
                 perme- 
                   
                 Capacity 
                 Resistance 
               
               
                 Experiment 
                 Formation 
                 Inorganic 
                 mediate 
                 Distri- 
                 ness 
                 age 
                 potential 
                 variation 
                 ability 
                   
                 retention 
                 increase 
               
               
                 example 
                 location 
                 particles 
                 binder 
                 bution 
                 (μm) 
                 (%) 
                 Ef (mV) 
                 Ev (mV) 
                 (sec/cm 3 ) 
                 Durability 
                 rate (%) 
                 rate (%) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 5-1 
                 Negative 
                 Mg(OH) 2   
                 PVDF 
                 RN &gt; RM 
                 3 
                 50 
                 90 
                 &lt;1 
                 300 
                 A 
                 77 
                 28 
               
               
                 5-2 
                 electrode 
                   
                   
                   
                   
                   
                 86 
                 1 
                   
                 A 
                 89 
                 13 
               
               
                 5-3 
                   
                   
                   
                   
                   
                   
                 80 
                 3 
                   
                 A 
                 90 
                 14 
               
               
                 5-4 
                   
                   
                   
                   
                   
                   
                 68 
                 9 
                   
                 A 
                 91 
                 11 
               
               
                 1-1 
                   
                   
                   
                   
                   
                   
                 50 
                 17 
                   
                 A 
                 93 
                 12 
               
               
                 5-5 
                   
                   
                   
                   
                   
                   
                 19 
                 28 
                   
                 A 
                 90 
                 13 
               
               
                 5-6 
                   
                   
                   
                   
                   
                   
                 12 
                 &lt;1 
                   
                 A 
                 78 
                 25 
               
               
                   
               
            
           
         
       
     
     As indicated in Table 5, even if the negative electrode potential Ef and the negative electrode potential variation Ev were each varied, a satisfactory durability was achieved together with a high capacity retention rate while the resistance increase rate was suppressed to be low. In this case, particularly if the negative electrode potential Ef was within a range from 19 mV to 86 mV both inclusive and the negative electrode potential variation Ev was 1 mV or greater, the capacity retention rate further increased and the resistance increase rate further decreased while the high durability was retained. 
     Based upon the results presented in Tables 1 to 5, in the case where the intermediate layer  14  (the inorganic particles and the intermediate binder) was interposed between the negative electrode  12  and the separator  13  and where the weight ratio RN in the upper layer  14 N was greater than the weight ratio RM in the lower layer  14 M in the intermediate layer  14 , the safety characteristic improved while the cyclability characteristic and the electrical resistance characteristic were each secured. Accordingly, a superior battery characteristic of the secondary battery was obtained. 
     Although the technology has been described above with reference to the embodiments and Examples, configurations of the technology are not limited to those described with reference to the embodiments and Examples above and are modifiable in a variety of ways. 
     Specifically, although the description has been given of the case of using a liquid electrolyte (an electrolytic solution) and the case of using a gel electrolyte (an electrolyte layer), the electrolyte is not particularly limited in kind. Thus, an electrolyte in a solid form (a solid electrolyte) may be used. 
     Further, although the description has been given of the case where the secondary battery has a battery structure of the laminated-film type, the battery structure is not particularly limited. Accordingly, the battery structure of the secondary battery may be of any other type, such as the cylindrical type, a prismatic type, a coin type, or a button type. 
     Further, although the description has been given of the case where the battery device has a device structure of the wound type, the device structure of the battery device is not particularly limited. Accordingly, the device structure of the battery device may be of any other type, such as a stacked type in which the positive electrodes and the negative electrodes are alternately stacked, or a zigzag folded type in which the positive electrode and the negative electrode are each folded in a zigzag manner. 
     Further, although the description has been given of the case where the electrode reactant is lithium, the electrode reactant is not particularly limited. Specifically, the electrode reactant may be another alkali metal such as sodium or potassium, or may be an alkaline earth metal such as beryllium, magnesium, or calcium, as described above. In addition, the electrode reactant may be another light metal such as aluminum. 
     It should be understood that the effects described herein are mere examples, and effects of the technology are therefore not limited to those described herein. Accordingly, the present technology may achieve any other effect. 
     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.