Patent Publication Number: US-2022216457-A1

Title: Positive electrode active material and nonaqueous electrolyte secondary battery using the positive electrode active material

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the priority based on Japanese Patent Application No. 2021-000514 filed on Jan. 5, 2021, the entire contents of which are incorporated by reference in the present specification. 
     BACKGROUND OF THE DISCLOSURE 
     1. Technical Field 
     The present disclosure relates to a positive electrode active material. The present disclosure also relates to a nonaqueous electrolyte secondary battery using the positive electrode active material. 
     2. Description of Background 
     In recent years, a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery has been preferably used as a portable power supply for a personal computer, a portable terminal, or the like, or a power supply for driving automobiles such as a battery electric vehicle (BEV), a hybrid electric vehicle (HEV), and a plug-in hybrid electric vehicle (PHEV). 
     More and more nonaqueous electrolyte secondary batteries are used and have been required to be further enhanced in performances with the spread thereof. For the positive electrode of such a nonaqueous electrolyte secondary battery, generally, a positive electrode active material capable of occluding and releasing ions serving as electric charge carriers is used. In order to improve the performances of the nonaqueous electrolyte secondary battery, coating is provided on the surface of the positive electrode active material. 
     As one example of such coating, Japanese Patent Application Publication No. 2015-99646 discloses the technology of coating the surface of the positive electrode active material with titanium dioxide (TiO 2 ) in order to improve the output characteristic, and the like. Further, Japanese Patent Application Publication No. 2019-19047 discloses the technology of concentrating the compound including tungsten and lithium on the surface layer of the primary particle present on the surface or the inside of the secondary particle of a positive electrode active material (lithium metal composite oxide) and the grain boundary among primary particles in order to reduce the reaction resistance, and to generate a positive electrode active material with high crystallinity. 
     SUMMARY 
     However, the present inventors conducted a close study, and as a result, found the following: with a nonaqueous electrolyte secondary battery using the positive electrode active material provided with coating by the related art, the internal resistance of the battery is still large, and the output characteristic is insufficient. Further, the present inventors found that the resistance increase rate after repeating charging and discharging the nonaqueous electrolyte secondary battery is large, and the cycle characteristic is insufficient. Still further, the present inventors found the following: coating of the positive electrode active material with tungsten provides the effect of improving the output characteristic and the cycle characteristic of the nonaqueous electrolyte secondary battery; on the other hand, when the concentration of tungsten is too high, tungsten is eluted from the positive electrode, and is precipitated at the negative electrode. The present inventors also found that precipitation of tungsten at the negative electrode facilitates precipitation of lithium on the negative electrode, which can cause an increase in internal resistance of the battery. 
     The present disclosure was completed in view of such circumstances. It is a main object thereof to provide a positive electrode active material capable of imparting a nonaqueous electrolyte secondary battery with excellent output characteristic and cycle characteristic. Further, it is another object thereof to provide a nonaqueous electrolyte secondary battery using such a positive electrode active material. 
     In order to achieve the foregoing objects, the positive electrode active material herein disclosed is provided. The positive electrode active material herein disclosed includes: a core part including a lithium composite oxide; a first region formed on at least a partial surface of the core part, and including tungsten (W); and a second region formed on at least a partial surface of the first region, and including titanium (Ti). Herein, the W concentration of the first region with TEM-EDX analysis is higher than the W concentration of the core part. 
     As described above, inclusion of the first region having a higher tungsten concentration than that of the core part can impart a nonaqueous electrolyte secondary battery with excellent output characteristic and cycle characteristic. Further, inclusion of the second region including Ti can suppress elution of tungsten, and can further exhibit the effect of addition of high-concentration tungsten. With such a configuration, it is possible to provide a positive electrode active material for imparting a nonaqueous electrolyte secondary battery with excellent output characteristic and cycle characteristic. Incidentally, in the present specification, the “output characteristic” is evaluated by the magnitude of the initial resistance of a secondary battery, and the “cycle characteristic” is evaluated by the resistance increase rate after charging and discharging cycle. 
     With a preferable one aspect of the positive electrode active material herein disclosed, the atomic ratio (Ti/W) of Ti to W in the overall positive electrode active material is 0.2 or more and 9 or less. 
     With such a configuration, it is possible to provide a positive electrode active material for imparting a nonaqueous electrolyte secondary battery with further excellent output characteristic and cycle characteristic. 
     With another preferable one aspect of the positive electrode active material herein disclosed, the second region includes TiO 2  and/or a composite oxide including Li and Ti as titanium-containing compounds. Further, with a still other preferable one aspect, the second region includes TiO 2  as the titanium-containing compound, and the Ti concentration of TiO 2  identified as brookite type TiO 2  based on Ti peak analysis of XAFS with the Ti concentration of the titanium-containing compound taken as 100 mol % is 10 mol % or more. 
     With such a configuration, the effect of inclusion of the second region including Ti is exhibited at a higher level. 
     Further, in order to achieve the foregoing other object, a nonaqueous electrolyte secondary battery is provided which includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. Herein the positive electrode includes the positive electrode active material. 
     With such a configuration, it is possible to provide a nonaqueous electrolyte secondary battery having an excellent output characteristic and an excellent cycle characteristic. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view schematically showing the internal structure of a lithium ion secondary battery in accordance with one embodiment; 
         FIG. 2  is view schematically showing the configuration of a wound electrode body of a lithium ion secondary battery in accordance with one embodiment; and 
         FIG. 3  is a view schematically showing one example of a positive electrode active material for use in a lithium ion secondary battery in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Below, referring to the accompanying drawings, preferable embodiments of the present disclosure will be described. It is naturally understood that the embodiments herein described should not be construed as restring the present disclosure. Incidentally, matters necessary for executing the present disclosure, except for matters specifically referred to in the present specification (e.g., a general configuration of a nonaqueous electrolyte secondary battery not characterizing the present disclosure) can be grasped as design matters of those skilled in the art based on the related art in the present field. The present disclosure can be executed based on the contents disclosed in the present specification, and the technical common sense in the present field. Further, a reference sign X in each figure represents the “width direction”, and a reference sign Z represents the “thickness direction”. Incidentally, the dimensional relation (such as length, width, or thickness) does not reflect the actual dimensional relation. 
     Further, it is assumed that the expression of “A to B (provided that A or B is a given value) showing the range in the present specification means A or more and B or less. 
     In the present specification, the term “battery” is a term denoting an electric storage device capable of extracting the electric energy in general, and is a concept including a primary battery and a secondary battery. The term “secondary battery” is a term denoting an electric storage device capable of repeatedly charging and discharging in general, and including a so-called storage battery, and an electric storage element such as an electric double layer capacitor. Further, in the present specification, the term “lithium ion secondary battery” denotes a secondary battery using lithium ions as electric charge carriers, and implementing charging and discharging by the transfer of electric charges accompanying lithium ions between the positive and negative electrodes. 
       FIG. 1  is a cross sectional view schematically showing a lithium ion secondary battery in accordance with one embodiment. As shown in  FIG. 1 , a lithium ion secondary battery  1  is a closed type battery constructed by accommodating a wound electrode body  20  in a flat shape and a nonaqueous electrolyte (not shown) in a flat battery case (i.e., an exterior container)  30 . The battery case  30  is provided with a positive electrode terminal  42  and a negative electrode terminal  44  for external connection, and a thin-walled safety valve  32  set for releasing the internal pressure when the internal pressure of the battery case  30  increases to a prescribed level, or higher. The positive electrode terminal  42  is electrically connected with a positive electrode collector sheet  42   a . The negative electrode terminal  44  is electrically connected with a negative electrode collector sheet  44   a . As the material for the battery case  30 , for example, a metal material which is lightweight and has good thermal conductivity such as aluminum is used. 
     The wound electrode body  20  has a form in which rectangular sheet-shaped positive electrode (which will be hereinafter referred to as a “positive electrode sheet  50 ”) and a rectangular sheet-shaped negative electrode (which will be hereinafter referred to as a “negative electrode sheet  60 ”) are stacked one on another via two long separators (which will be hereinafter referred to as separator sheets  70 ), and are wound in the longitudinal direction as shown in  FIGS. 1 and 2 . The positive electrode sheet  50  has a configuration in which a positive electrode active material layer  54  is formed along the longitudinal direction on one surface or both surfaces of a long positive electrode collector  52 . The negative electrode sheet  60  has a configuration in which a negative electrode active material layer  64  is formed along the longitudinal direction on one surface or both surfaces of a long negative electrode collector  62 . One edge in the width direction of the positive electrode collector  52  is provided with a portion at which the positive electrode active material layer  54  is not formed along the edge, and the positive electrode collector  52  is exposed (i.e., a positive electrode collector exposed part  52   a ). The other edge in the width direction of the negative electrode collector  62  is provided with a portion at which the negative electrode active material layer  64  is not formed along the edge, and the negative electrode collector  62  is exposed (i.e., a negative electrode collector exposed part  62   a ). The positive electrode collector exposed part  52   a  and the negative electrode collector exposed part  62   a  are joined with the positive electrode collector sheet  42   a  and the negative electrode collector sheet  44   a , respectively. 
     As the positive electrode collector  52 , a conventionally known positive electrode collector for use in a lithium ion secondary battery may be used. As one example thereof, mention may be made of a sheet or foil made of a metal with good electric conductivity (e.g., aluminum, nickel, titanium, or stainless steel). The positive electrode collector  52  is preferably, for example, aluminum foil. The dimension of the positive electrode collector  52  has no particular restriction, and may be appropriately determined according to the battery design. When aluminum foil is used as the positive electrode collector  52 , the dimension is, for example, preferably 5 μm or more and 35 μm or less, and more preferably 7 μm or more and 20 μm or less. 
     The positive electrode active material layer  54  includes the positive electrode active material herein disclosed. The positive electrode active material layer  54  may include other components than the positive electrode active material, for example, trilithium phosphate, a conductive material, and a binder. As the conductive material, for example, carbon black such as acetylene black (AB), or other carbon materials (e.g., graphite) can be preferably used. As the binder, for example, polyvinylidene fluoride (PVDF) can be used. Further, other materials than the foregoing ones (e.g., various additives) may be included therein unless the effects of the present disclosure are impaired. 
     Although not particularly restricted, the content of the positive electrode active material in the positive electrode active material layer  54  (i.e., the content of the positive electrode active material based on the total mass of the positive electrode active material layer  54 ) is preferably 70 mass % or more, more preferably 80 mass % or more and 97 mass % or less, and further preferably 85 mass % or more and 96 mass % or less. The content of trilithium phosphate in the positive electrode active material layer  54  is preferably, for example, 1 mass % or more and 15 mass % or less, and more preferably 2 mass % or more and 12 mass % or less. The content of the conductive material in the positive electrode active material layer  54  is preferably, for example, 1 mass % or more and 15 mass % or less, and more preferably 3 mass % or more and 13 mass % or less. The content of the binder in the positive electrode active material layer  54  is, for example, preferably, 1 mass % or more and 15 mass % or less, and more preferably 1.5 mass % or more and 10 mass % or less. 
     The thickness of the positive electrode active material layer  54  has no particular restriction, and is for example, preferably, 10 μm or more and 300 μm or less, and more preferably 20 μm or more and 200 μm or less. 
     As the negative electrode collector  62 , a conventionally known negative electrode collector for use in a lithium ion secondary battery may be used. As one example thereof, mention may be made of a sheet or foil made of a metal with good electrical conductivity (e.g., copper, nickel, titanium, or stainless steel). The negative electrode collector  62  is preferably, for example, copper foil. The dimension of the negative electrode collector  62  has no particular restriction, and may be appropriately determined according to the battery design. When copper foil is used as the negative electrode collector  62 , the dimension is, for example, preferably 5 μm or more and 35 μm or less, and more preferably 7 μm or more 20 μm or less. 
     The negative electrode active material layer  64  includes a negative electrode active material. As the negative electrode active material, for example, a carbon material such as graphite, hard carbon, or soft carbon can be preferably used. Graphite may be natural graphite or artificial graphite, and may be amorphous carbon coated graphite in a form in which graphite is coated with an amorphous carbon material. 
     The average particle diameter (median diameter: D50) of the negative electrode active material has no particular restriction, and is, for example, preferably 0.1 μm or more and 50 μm or less, and more preferably 1 μm or more and 25 μm or less. Incidentally, the average particle diameter (D50) of the negative electrode active material can be determined by, for example, the laser diffraction scattering method. 
     As the additive materials other than the negative electrode active material included in the negative electrode active material layer  64 , mention may be made of a binder, a thickener, or the like. As the binder, for example, styrene butadiene rubber (SBR) can be preferably used. As the thickener, for example, carboxymethyl cellulose (CMC), or methyl cellulose (MC) can be preferably used. Further, other materials than the foregoing ones (e.g., various additives) may be included unless the effects of the present disclosure are impaired. 
     Although not particularly restricted, the content of the negative electrode active material in the negative electrode active material layer  64  is preferably 90 mass % or more, and more preferably 95 mass % or more and 99 mass % or less. The content of the binder in the negative electrode active material layer  64  is, for example, preferably 0.1 mass % or more and 8 mass % or less, and more preferably 0.5 mass % or more and 3 mass % or less. The content of the thickener in the negative electrode active material layer  64  is, for example, preferably 0.3 mass % or more and 3 mass % or less, and more preferably 0.5 mass % or more and 2 mass % or less. 
     The thickness of the negative electrode active material layer  64  has no particular restriction, and is, for example, preferably 10 μm or more and 300 μm or less, and more preferably 20 μm or more and 200 μm or less. 
     Examples of the separator sheet  70  may include a porous sheet (film) including a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide. Such a porous sheet may be a monolayered structure, or a lamination structure of two or more layers (e.g., a three-layered structure in which PP layers are stacked on both surfaces of a PE layer). The separator sheet  70  may be provided with a heat resistant layer (HRL). 
     For the nonaqueous electrolyte, the same one as that of a conventional lithium ion secondary battery can be used. Typically, the one obtained by allowing a support salt to be included in an organic solvent (nonaqueous solvent) can be used. As the nonaqueous solvents, organic solvents such as carbonates, esters, ethers, nitriles, sulfones, and lactones can be used without particular restriction. Specifically, for example, nonaqueous solvents such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), and trifluoro dimethyl carbonate (TFDMC) can be preferably used. Such nonaqueous solvents can be used singly alone, or in appropriate combination of two or more thereof. 
     As the support salt, for example, a lithium salt such as LiPF 6 , LiBF 4 , or LiClO 4  can be preferably used. The concentration of the support salt has no particular restriction, and is preferably about 0.7 mol/L or more and 1.3 mol/L or less. 
     Incidentally, the nonaqueous electrolyte may include other components than the nonaqueous solvent and the support salt, for example, various additives including a film forming agent such as oxalate complex; a gas generator such as biphenyl (BP) or cyclohexyl benzene (CHB); a dispersant; and a thickener; and the like unless the effects of the present disclosure are remarkably impaired. 
     A lithium ion secondary battery  1  can be manufactured in the same manner as the conventionally known method, except for using the positive electrode active material herein disclosed as the positive electrode active material. 
     Then, the positive electrode active material for use in the present embodiment will be described.  FIG. 3  is a view schematically showing one example of a positive electrode active material  10  for use in a lithium ion secondary battery  1  in accordance with the present embodiment. As shown in  FIG. 3 , the positive electrode active material  10  includes a core part  12 , a first region  14 , and a second region  16 . Below, each will be described. 
     (a) Core Part 
     The core part  12  is a particle including a lithium composite oxide. The crystal structure of the lithium composite oxide has no particular restriction, and may be a layered structure, a spinel structure, an olivine structure, or the like. The lithium composite oxide is preferably a lithium transition metal composite oxide including at least one of Ni, Co, and Mn as a transition metal element. Specifically, mention may be made of a lithium manganese type composite oxide, a lithium nickel manganese type composite oxide, a lithium nickel cobalt manganese type composite oxide, a lithium nickel cobalt aluminum type composite oxide, a lithium iron nickel manganese type composite oxide, or the like. 
     The lithium composite oxide preferably has a layered structure because the initial resistance is small, and is more preferably a lithium nickel cobalt manganese type composite oxide of a layered structure. 
     Incidentally, in the present specification, the “lithium nickel cobalt manganese type composite oxide” is a term including, other than an oxide having Li, Ni, Co, Mn, or O as a constituent element, also an oxide including one or two or more additive elements other than these. Examples of such an additive element may be metal elements such as W, Mg, Ca, Al, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, Na, Fe, Zn, and Sn, and non-metal elements such as S, F, Cl, Br, and I. This also applies to the case where the lithium manganese type composite oxide, lithium nickel manganese type composite oxide, lithium nickel cobalt aluminum type composite oxide, lithium iron nickel manganese type composite oxide, or the like is used as the core part  12 . 
     From the viewpoint of the reduction of the initial resistance and the suppression of the resistance increase rate of the secondary battery, the additive element to the core part  12  is in particular preferably tungsten (W). On the other hand, when the W concentration in the core part  12  becomes too high, tungsten may be eluted from the positive electrode, and may be precipitated at the negative electrode. This facilitates precipitation of lithium on the negative electrode, which undesirably may cause deterioration of the negative electrode. In consideration of such points, the W concentration of the core part  12  is preferably 1 mol % or less, preferably 0.8 mol % or less, and preferably 0.5 mol % or less. Incidentally, the presence or absence of tungsten in the core part  12  does not restrict the present disclosure. Namely, the W concentration in the core part may be 0 mol %. 
     The “W concentration in the core part” in the present specification can be determined by the following procedure. First, using energy dispersive X-ray spectroscopy (TEM-EDX), a plurality of (e.g., 20) given positive electrode active materials  10  are photographed. Measurement sites each with a radius of 3 nm with the position 150 nm from the surface of the first region  14  toward the core part  12  as the center are set. Then, for each positive electrode active material  10 , a plurality of (e.g., 5) measurement site are set. Thus, the number of atoms of the transition metal element (such as Ni, Co, Mn, or W) forming the positive electrode active material in each measurement site is measured. Then, the proportion of the number of atoms of W with the total number of atoms of the transition metal elements forming the positive electrode active material measured at each measurement site taken as 100 mol % is calculated. The average value of such values is herein referred to as the “W concentration in the core part”. 
     The shape of the core part  12  has no particular restriction, and may be a spherical shape, a sheet shape, a needle shape, an amorphous shape, or the like. Alternatively, the core part  12  may be in a form of a secondary particle including primary particles aggregated therein, or in a hollow form. The average particle diameter (median diameter: D50) of the core part  12  is, for example, 0.05 μm or more and 20 μm or less, preferably 1 μm or more and 20 μm or less, and more preferably 3 μm or more and 15 μm or less. Incidentally, the average particle diameter (D50) of the core part  12  can be determined by, for example, the laser diffraction scattering method. 
     Incidentally, the core part  12  can be manufactured by, for example, in the following manner. The precursor (e.g., metal hydroxide) of the lithium composite oxide is manufactured by the crystallization method, or the like, and lithium is introduced to the precursor. As a result, the core part  12  can be manufactured. 
     (b) First Region 
     A first region  14  is formed on at least a partial surface of the core part  12 . Such a first region  14  is a region including tungsten (W). The W concentration in the first region  14  is higher than the W concentration in the core part  12  (i.e., tungsten is concentrated). With such a configuration, it is possible to prevent the deterioration of the positive electrode active material  10  due to elimination of oxygen of the core part  12 , and to improve the cycle characteristic of the battery. Further, the diffusibility of Li ions of the positive electrode active material  10  can be improved, which can also contribute to the improvement of the output characteristic of the battery. 
     Typically, the first region  14  includes a metal oxide including W and other elements. The “other elements” in such a first region  14  may be the same kind of elements as the constituent elements of the core part  12 . Namely, the first region  14  may include Li, Mg, Ca, Al, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, Na, Fe, Zn, Sn, S, F, Cl, Br, I, or the like other than W. Incidentally, the constituent elements of the first region  14  may be the same as, or may be not the same as the constituent elements of the core part  12 . For example, when the core part  12  includes a tungsten-containing lithium nickel cobalt manganese type composite oxide, the first region  14  may include a tungsten-containing lithium nickel cobalt manganese type composite oxide, or may include a tungsten-containing lithium cobalt type composite oxide, a tungsten-containing lithium manganese type composite oxide, a tungsten-containing lithium nickel type composite oxide, or the like. However, in consideration of the ease of the formation thereof on the core part  12 , the constituent elements of the first region  14  are preferably the same as the constituent elements of the core part  12 . 
     The first region  14  is formed on at least a partial surface of the core part  12 . However, the first region  14  may be formed on the entire surface of the core part  12  as shown in  FIG. 3 . The reference of the W concentration of the core part  12  is set at the W concentration at the position 150 nm from the first region  14  toward the core part  12 . Accordingly, the thickness of the first region  14  is less than 150 nm. The thickness of the first region  14  is, for example, preferably 10 nm or more and 100 nm or less, and preferably 15 nm or more and 80 nm or less. The thickness of the first region  14  can be determined by, for example, observing the cross section of the positive electrode active material  10  by TEM-EDX. 
     The lower limit of the W concentration in the first region  14  may only be a higher concentration than the W concentration in the core part  12 , and for example, may be 0.1 mol % or more, may be 0.5 mol % or more, and may be 1 mol % or more. As described above, when the W concentration is too high, tungsten may be eluted from the positive electrode, which may cause deterioration of the negative electrode. For this reason, the W concentration is, for example, preferably 3 mol % or less, preferably 2.5 mol % or less, and preferably 2 mol % or less. 
     Incidentally, the “W concentration in the first region” in the present specification can be measured by the same procedure as with the “W concentration in the core part” described above. Namely, using TEM-EDX, a plurality of (e.g., 20) given positive electrode active materials  10  are photographed. A plurality of measurement sites are set in the vicinity of the surface of each positive electrode active material  10 , and the W concentrations (mol %) at the plurality of measurement sites are calculated, and then, the average value thereof is determined. Incidentally, the measurement site for determining the “W concentration in the first region” is set at a position with a radius of 3 nm with the position 5 nm from the surface of the first region  14  toward the core part  12  as the center. 
     Incidentally, the particle having the first region  14  at at least a partial surface of the core part  12  can be manufactured by, for example, the following procedure. With the precursor of the core part  12  dispersed in a solution having a higher composition ratio of W (e.g., W/(W+Ni+Co+Mn)) than that of the solution for use in forming the core part  12 , the crystallization method is performed. This can result in the formation of a precursor particle in which the precursor of the first region  14  is formed on the surface of the precursor of the core part  12 . Then, lithium is introduced into such a precursor particle. As a result, it is possible to form the first region  14  in which tungsten is concentrated on at least a partial surface of the core part  12 . 
     (c) Second Region 
     A second region  16  is formed on at least partial surface of the first region  14 . Such a second region  16  is a region including titanium (Ti). With such a configuration, it is possible to suppress the elution of tungsten from the tungsten-concentrated first region  14 . As a result of this, it is possible to achieve provision of the positive electrode active material for imparting a nonaqueous electrolyte secondary battery with excellent output characteristic and cycle characteristic. The second region  16  includes titanium dioxide (TiO 2 ) and/or a composite oxide including Li and Ti as titanium-containing compounds. 
     As the crystal structures of TiO 2 , generally, anatase type (tetragonal), rutile type (tetragonal), brookite type (rhombic), and the like are known. The brookite type crystal structure is very unstable as compared with the anatase type and rutile type crystal structures. For example, when the brookite type TiO 2  is heated to 650° C. or more, transition to the most stable rutile type TiO 2  is caused. 
     When the second region  16  includes TiO 2  as a titanium-containing compound, as TiO 2 , those selected from the group consisting of the brookite type, the anatase type, and the rutile type can be used singly alone, or in mixture of two or more thereof. Particularly, from the viewpoint of improving the output characteristic, the brookite type TiO 2  is preferably used. 
     The reason why the inclusion of the brookite type TiO 2  in the second region  16  improves the output characteristic of a secondary battery has no particular restriction, and can be considered as follow. The brookite type TiO 2  has an unstable crystal structure, and hence tends to form a complex with Li ions. For this reason, at least a partial surface of the first region  14  is provided with coating including the brookite type TiO 2 , which speeds up the insertion/extraction of Li ions into/from the positive electrode active material  10 . Therefore, the activation energy can be reduced, which can reduce the resistance. As a result of this, it is possible to improve the output characteristic of a secondary battery. 
     Incidentally, the inclusion of the brookite type TiO 2  in the second region  16  can be confirmed by a conventionally known method. For example, the inclusion of the brookite type TiO 2  in the second region  16  can be confirmed by performing X-ray absorption fine structure (XAFS) analysis on the second region  16 , and analyzing the Ti peak. 
     The Ti concentration of the brookite type TiO 2  in the second region  16  is, for example, preferably 1 mol % or more, more preferably 5 mol % or more, and further preferably 10 mol % or more with the Ti concentration of the titanium-containing compound included in the second region  16  taken as 100 mol %. The Ti concentration of the brookite type TiO 2  in the second region  16  may be, for example, 100 mol % (i.e., the overall titanium-containing compound includes the brookite type TiO 2 ). 
     Incidentally, the “Ti concentration of the brookite type TiO 2  in the second region” can be determined by Ti peak analysis by XAFS. Although specific measurement device and measurement conditions will be described in examples described later, for example, the Ti peaks of standard samples of Ti included in the titanium-containing compound are respectively determined by XAFS analysis. Then, the Ti peak of the positive electrode active material  10  (target sample) including the second region  16  is determined by XAFS analysis. The Ti peak of such a standard sample, and the Ti peak of the target sample are subjected to fitting using the analysis software of XAFS (such as Athena or Artemis), thereby to be quantified. This value is herein referred to as the “Ti concentration of the brookite type TiO 2  in the second region”. 
     The brookite type TiO 2  has a very unstable crystal structure, and hence has been difficult to use as a coating. However, the present inventors conducted a close study thereon, and as a result, found that a mechanochemical treatment can form the second region  16  including the brookite type TiO 2  on at least a part of the first region  14 . 
     When the second region  16  includes a composite oxide including Li and Ti as a titanium-containing compound, for example, a lithium titanium composite oxide (or lithium titanate) such as Li 2 TiO 3  or Li 4 Ti 5 O 12  can be used. As the composite oxide including Li and Ti, lithium titanium composite oxides (or lithium titanate) having different atomic ratios (Li/Ti) of Li to Ti can be used in mixture of two or more thereof. 
     Incidentally, the lithium titanium composite oxide (or lithium titanate) may be synthesized by a conventionally known synthesis method, or may be prepared by purchasing a commercially available product. 
     The second region  16  is preferably formed so that the atomic ratio (Ti/W) of Ti with respect to the number of atoms of W included in the overall positive electrode active material  10  may become 0.01 or more and 10 or less, more preferably 0.03 or more and 10 or less, further preferably 0.2 or more and 9 or less, and in particular preferably 0.6 or more and 3.5 or less. 
     Incidentally, the number of atoms of W included in the positive electrode active material  10  and the number of atoms of Ti included in the second region  16  can be determined by, for example, ICP analysis. 
     The second region  16  may only be typically present (i.e., be interspersed) on at least a partial surface of the first region  14  as shown in  FIG. 3 , and has no particular restriction on the form. For example, the second region  16  may be formed on the entire surface of the first region  14 . However, from the viewpoint of the battery characteristics, preferably, the second region  16  is interspersed on at least a partial surface of the first region  14 , and partially coats the first region  14 . The thickness of the second region  16  has no particular restriction, and is, for example, 0.1 nm or more and 100 nm or less. The thickness of the second region  16  can be determined by, for example, observing the cross section of the positive electrode active material  10  by TEM-EDX. 
     Incidentally, the positive electrode active material particle including the core part  12 , the first region  14 , and the second region  16  can be manufactured by, for example, the following procedure. A particle including the core part  12  and the first region  14 , and a titanium-containing compound particle are charged into a mechanochemical device, and are subjected to a mechanochemical treatment for a prescribed time. As a result, it is possible to manufacture the positive electrode active material  10  including the core part  12 , the first region  14 , and the second region  16 . 
     The lithium ion secondary battery  1  configured as described up to this point is usable for various uses. For example, the lithium ion secondary battery  1  can be preferably used as a high output power source for a motor to be mounted on an automobile (driving power supply). The kind of the automobile has no particular restriction. Typically, mention may be made of a vehicle, for example, a plug-in hybrid electric vehicle (PHEV), a hybrid electric vehicle (HEV), or a battery electric vehicle (BEV). The lithium ion secondary battery  1  can also be used in a form of an assembled battery including a plurality of batteries electrically connected to one another. 
     Below, test examples regarding the present disclosure will be described. However, it is not intended that the present disclosure is limited to such test examples. 
     Example 1 
     First, an aqueous solution obtained by dissolving a sulfuric acid salt of a metal other than Li in water was prepared. Specifically, a first raw material aqueous solution including Ni, Co, Mn, and W so as to achieve molar ratios of Ni, Co, Mn, and W of 1:1:1:0.0015 was prepared in a reaction vessel. NaOH and aqueous ammonia were added to the first raw material aqueous solution in the reaction vessel for neutralization. As a result, a composite hydroxide including metals (Ni, Co, Mn, and W) other than Li to be the precursor of the core part was crystallized. 
     Then, a raw material aqueous solution (second raw material aqueous solution) with a higher W concentration than that of the first raw material aqueous solution was prepared. The second raw material aqueous solution was specifically prepared so that the molar ratios of Ni, Co, Mn, and W became 1:1:1:0.02. Such a second raw material aqueous solution was supplied into the mixed aqueous solution including the precursor of the core part crystallized therein, thereby forming a composite hydroxide particle (precursor particle) in which the precursor of the first region was formed on the surface of the precursor of the core part. 
     The formed precursor particle was washed with water, followed by filtration, and drying. Then, the precursor particle after drying and lithium carbonate were mixed so as to achieve molar ratios of (Ni+Co+Mn):Li of 1.16:1. The mixture was burnt at a temperature of 870° C. for 15 hours. Such a mixture was cooled to room temperature (25° C.±5° C.), and was subjected to a disaggregation treatment. As a result of this, a spheroidal particle (burnt powder) including primary particles each having the core part and the first region aggregated therein was obtained. 
     The manufactured burnt powder, and a titanium-containing compound powder (a mixed powder of a TiO 2  powder and a Li 2 TiO 3  powder) were charged into a mechanochemical device, and were subjected to a mechanochemical treatment at 6000 rpm for 30 minutes. 
     Incidentally, the atomic ratio (Ti/W) of Ti with respect to W in the manufactured positive electrode active material was confirmed by quantitative analysis using an ICP emission analyzer (ion chromatograph ISC-5000 manufactured by Nippon Dionex K.K.). 
     Examples 2 to 5 
     By changing the molar ratios of Ni, Co, Mn, and W of the first raw material aqueous solution and the second raw material aqueous solution of Example 1, the W concentrations of the core part and the first region were changed. Further, the mechanochemical treatment was performed under the foregoing conditions. At this step, by changing the amount of the titanium-containing compound powder based on the amount of the burnt powder, the atomic ratio (Ti/W) of Ti with respect to W was changed. Specifically, the results are shown in Table 1. 
     Example 6 
     First, an aqueous solution obtained by dissolving a sulfuric acid salt of a metal other than Li in water was prepared. Specifically, a first raw material aqueous solution including Ni, Co, and Mn so as to achieve molar ratios of 1:1:1 was prepared in a reaction vessel. NaOH and aqueous ammonia were added to the first raw material aqueous solution in the reaction vessel for neutralization. As a result, a composite hydroxide including metals other than Li to be the precursor of the core part was crystallized. Then, the second raw material aqueous solution obtained by adding W to the first raw material aqueous solution was supplied into the mixed aqueous solution including the precursor of the core part crystallized therein, thereby forming a composite hydroxide particle (precursor particle) in which the precursor of the first region was formed on the surface of the precursor of the core part. Namely, the composite oxide particle not including W at the core part was formed. As described above, Li was introduced, resulting in a spheroidal particle (burnt powder) including primary particles aggregated therein. 
     Such a burnt powder and a titanium-containing compound powder were charged into a mechanochemical device, and were subjected to a mechanochemical treatment under the foregoing conditions, resulting in an active material of Example 6. 
     Comparative Example 1 
     In the same manner as in Example 6, a composite hydroxide including metals other than Li to be the precursor of the positive electrode active material was crystallized. The resulting composite hydroxide and lithium carbonate were mixed so as to achieve molar ratios of (Ni+Co+Mn):Li of 1:1. The mixture was burnt at a temperature of 870° C. for 15 hours. Such a mixture was cooled to room temperature (25° C.±5° C.), and was subjected to a disaggregation treatment. As a result of this, a spheroidal particle (burnt powder) including primary particles aggregated therein was obtained. Such a burnt powder was used as it was (i.e., without being subjected to a mechanochemical treatment) as an active material of Comparative Example 1. The active material of Comparative Example 1 is an active material not including the first region and the second region. 
     Comparative Example 2 
     The burnt powder manufactured by the same procedure as with Examples 1 to 5 was used without being subjected to a mechanochemical treatment (i.e., without being provided with the second region) as an active material of Comparative Example 2. 
     Comparative Example 3 
     The burnt powder of Comparative Example 1 and a titanium-containing compound powder were charged into a mechanochemical device, and were subjected to a mechanochemical treatment under the foregoing conditions, resulting in an active material of Comparative Example 3. 
     Comparative Example 4 
     Using the second raw material aqueous solution with a lower W concentration than that of the first raw material aqueous solution, a spheroidal particle (burnt powder) including primary particles in each of which the W concentration of the first region was lower than that of the core part aggregated therein was manufactured. Such a burnt powder and a titanium-containing compound powder were charged into a mechanochemical device, and were subjected to a mechanochemical treatment under the foregoing conditions, resulting in an active material of Comparative Example 4. 
     Measurement of W Concentration 
     Using TEM-EDX (TEM device: JFE-ARM300F, EDX device: JED-2300T manufactured by JEOL), 10 positive electrode active materials of each example were photographed, and the site having a radius of 3 nm with the position 150 nm from the surface of the first region toward the core part was set as the measurement site. Then, in each of the positive electrode active materials to be measured, 5 such measurement sites were set, and each number of atoms of the transition metal elements (Ni, Co, Mn, and W) at each measurement site was measured. Then, the proportion of the number of atoms of W with the total number of atoms of the transition metal elements measured at each measurement site taken as 100 mol % was calculated, thereby calculating the “W concentration in the core part”. Further, the “W concentration in the first region” was calculated in the same manner except for setting the measurement site at the position 5 nm from the surface of the first region toward the core part. The results are shown in Table 1. 
     Manufacturing of Evaluating Lithium Ion Secondary Battery 
     Each active material of respective Examples and respective Comparative Examples manufactured, acetylene black (AB) as a conductive material, polyvinylidene fluoride (PVDF) as a binder, and N-methyl pyrrolidone (NMP) as a disperse medium were mixed using a planetary mixer, thereby preparing a positive electrode active material layer forming paste. At this step, the mass ratios of the active material, AB, and PVDF were set at 90:8:2, and the solid content concentration was set at 56%. Using a die coater, such a paste was applied onto both surfaces of aluminum foil, and was dried, followed by pressing, thereby manufacturing a positive electrode sheet. 
     Further, natural graphite (C) as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxyl methyl cellulose (CMC) as a thickener were mixed at mass ratios of C:SBR:CMC=98:1:1 in ion exchanged water, thereby preparing a negative electrode active material layer forming paste. Using a die coater, such a paste was applied onto both surfaces of copper foil, and was dried, followed by pressing, thereby manufacturing a negative electrode sheet. 
     As the separator sheets, two porous polyolefin sheets each having a three-layered structure of PP/PE/PP, and having a thickness of 24 μm were prepared. 
     The manufactured positive electrode sheet and negative electrode sheet, and the prepared two separator sheets were stacked one on another, and were wound, thereby manufacturing a wound electrode body. Electrode terminals were mounted on the positive electrode sheet and the negative electrode sheet of the manufactured wound electrode body by welding, respectively. This was accommodated in a battery case having a liquid injection port. 
     A nonaqueous electrolyte was injected from such a liquid injection port, and the liquid injection port was hermetically sealed by a sealing lid. Incidentally, as the nonaqueous electrolyte, the one obtained by dissolving LiPF 6  as a support salt in a mixed solvent including ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) at volume ratios of 1:1:1 in a concentration of 1.0 mol/L was used. In the manner described up to this point, an evaluating lithium ion secondary battery was obtained. 
     Activating Treatment 
     Under 25° C. environment, each evaluating lithium ion secondary battery was subjected to an activating treatment (first charging). The activating treatment was performed in the following manner: with the constant current—constant voltage system, constant current charging was performed at a current value of ⅓ C up to 4.2 V, and then, constant voltage charging was performed until the current value became 1/50 C, resulting in a fully charged state. Subsequently, constant current discharging was performed until the voltage became 3.0 V at a current value of ⅓ C. 
     Output Characteristic Evaluation 
     Each evaluating lithium ion secondary battery after the activating treatment was adjusted to an open voltage of 3.80 V, and then was placed under temperature environment of −30° C. Discharging was performed at a current value of 1 C for 2 seconds, thereby determining the voltage drop amount (A V). Such a voltage drop amount A V was divided by the discharge current value (1 C), thereby calculating the battery resistance, which was referred to as the initial resistance. The initial resistance ratios of other Comparative Examples and Examples with the initial resistance of Comparative Example 1 taken as 1 were determined. Incidentally, it can be evaluated that the output characteristic becomes better with a decrease in initial resistance ratio. The results are shown in Table 1. 
     Cycle Characteristic Evaluation 
     Each evaluating lithium ion secondary battery measured for the initial resistance was placed under 0° C. environment. Thus, the charging and discharging cycle including constant current charging at 30 C to 4.3 V, and constant current discharging at 30 C to 3.1 V as one cycle was repeated for 500 cycles. The battery resistance at the 500th cycle was measured in the same manner as described above. As the indicator of the resistance increase, the resistance increase rate was determined by the formula: (battery resistance at 500th charging and discharging cycle—initial resistance)/initial resistance. The ratios of the resistance increase rates of other Comparative Examples and Examples with the resistance increase rate of Comparative Example 1 taken as 1 were determined. Incidentally, it can be evaluated that the cycle characteristic becomes better with a decrease in such a ratio of the resistance increase rate. The results are shown in Table 1. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 W 
                 Presence 
                   
                   
                 Ratio of 
               
               
                   
                 W concentration 
                 concentration 
                 or absence 
                   
                 Initial 
                 resistance 
               
               
                   
                 of core part 
                 of first region 
                 of second 
                   
                 resistance 
                 increase 
               
               
                   
                 (mol %) 
                 (mol %) 
                 region 
                 Ti/W 
                 ratio 
                 rate 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Comparative 
                 0 
                 0 
                 None 
                 — 
                 1 
                 1 
               
               
                 Example 1 
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 0.2 
                 0.7 
                 None 
                 — 
                 0.94 
                 0.90 
               
               
                 Example 2 
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 0 
                 0 
                 Present 
                 — 
                 0.89 
                 0.86 
               
               
                 Example 3 
                   
                   
                   
                   
                   
                   
               
               
                 Comparative 
                 0.5 
                 0.4 
                 Present 
                 0.9 
                 0.91 
                 0.91 
               
               
                 Example 4 
                   
                   
                   
                   
                   
                   
               
               
                 Example 1 
                 0.8 
                 1.5 
                 Present 
                 0.03 
                 0.86 
                 0.84 
               
               
                 Example 2 
                 0.3 
                 1 
                 Present 
                 0.2 
                 0.67 
                 0.73 
               
               
                 Example 3 
                 0.2 
                 0.6 
                 Present 
                 0.6 
                 0.55 
                 0.70 
               
               
                 Example 4 
                 0.2 
                 0.5 
                 Present 
                 0.9 
                 0.56 
                 0.67 
               
               
                 Example 5 
                 0.1 
                 0.3 
                 Present 
                 3.5 
                 0.60 
                 0.64 
               
               
                 Example 6 
                 0 
                 0.1 
                 Present 
                 9 
                 0.75 
                 0.74 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, it is indicated as follows: when the positive electrode active material has the core part including a lithium composite oxide, a first region formed on at least a partial surface of the core part, and including W, and a second region formed on at least a part of the first region, and including Ti, and the W concentration of the first region is higher than that of the core part, the initial resistance ratio and the ratio of the resistance increase rate are remarkably small. Therefore, the positive electrode active material herein disclosed can impart excellent output characteristic and cycle characteristic to a nonaqueous electrolyte secondary battery. 
     Further, the results of Examples 1 to 6 indicate as follows: when the atomic ratio (Ti/W) of Ti to W is 0.2 or more and 9 or less, the initial resistance ratio and the ratio of the resistance increase rate are very small; and when the atomic ratio (Ti/W) of Ti to W is 0.6 or more and 3.5 or less, the initial resistance ratio and the ratio of the resistance increase rate are particularly small. 
     Examples 7 to 11 
     The molar ratios of Ni, Co, Mn, and W of the first raw material aqueous solution and the second raw material aqueous solution were changed, resulting in a spheroidal particle (burnt powder) including primary particles in each of which the W concentrations of the core part and the first region were different, aggregated therein. Such a burnt powder and a titanium-containing compound powder (mixed powder of a TiO 2  powder and a Li 2 TiO 3  powder) were charged into a mechanochemical device, and were subjected to a mechanochemical treatment under the foregoing conditions. Each TiO 2  powder used in Examples 7 to 11 includes a brookite type TiO 2  powder. By changing the amount of the brookite type TiO 2  powder based on the amount of the titanium-containing compound powder, the Ti concentration (mol %) of the brookite type TiO 2  in the second region was changed. Incidentally, the atomic ratio (Ti/W) of Ti to W of the overall positive electrode active material was adjusted so as to be constant. 
     The W concentrations of the core part and the first region of each of the active materials, and the atomic ratio (Ti/W) of Ti to W of the overall positive electrode active material were measured in the same manner as described above. The results are shown in Table 2. 
     The Ti concentration (mol %) of the brookite type TiO 2  in the second region was determined by XAFS analysis. The analysis conditions were set as follows. 
     Analyzer: Aichi Synchrotron Radiation Center BL5S1 
     Measurement method: transmission method (standard sample) and fluorescence method 
     EXAMPLE 
     Monochrometer: double crystal spectrometer
 
Analyzing crystal: Si (111)
 
Measurement absorption edge: Ti-K absorption edge
 
Measurement energy region: 4800 eV to 6000 eV
 
Analysis software: Athena (Demeter ver. 0.9.26)
 
     A standard sample of the brookite type TiO 2  and boron nitride were mixed at a molar ratio of 1:99 using a dancing mill. Further, a standard sample of Li 2 TiO 3  and boron nitride were mixed at a molar ratio of 1:99 using a dancing mill. Such mixtures were pressed under a press pressure of 30 kN, thereby manufacturing each analyzing specimen. Such specimens were analyzed under the conditions of XAFS, thereby determining the Ti peak of the brookite type TiO 2  and the Ti peak of Li 2 TiO 3 . 
     Then, the active materials of Examples 7 to 10 were pressed under a press pressure of 30 kN, thereby manufacturing respective analyzing specimens. These were measured under the conditions of XAFS, and their respective Ti peaks were determined. The Ti peak of the brookite type TiO 2  and the Ti peak of Li 2 TiO 3  determined by the measurement of the standard samples and the Ti peaks of respective examples (around 5000 eV) were subjected to fitting using analysis software Athena for quantification. The results are shown in Table 2. 
     Using the active materials, each evaluating lithium ion secondary battery was manufactured in the same manner as described above, and the output characteristic and the cycle characteristic were evaluated in the same manner as described above. Incidentally, the results shown in Table 2 are the initial resistance ratio and the ratio of the resistance increase rate of each Example when the result of Comparative Example 1 is assumed to be 1. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                 Ti 
                   
                   
               
               
                   
                 W 
                 W 
                   
                 concentration 
                   
                 Ratio of 
               
               
                   
                 concentration 
                 concentration 
                   
                 of brookite 
                 Initial 
                 resistance 
               
               
                   
                 of core part 
                 of first region 
                   
                 type TiO 2   
                 resistance 
                 increase 
               
               
                   
                 (mol %) 
                 (mol %) 
                 Ti/W 
                 (mol %) 
                 ratio 
                 rate 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Comparative 
                 0 
                 0 
                 — 
                 — 
                 1 
                 1 
               
               
                 Example 1 
                   
                   
                   
                   
                   
                   
               
               
                 Example 7 
                 0.2 
                 0.6 
                 0.9 
                 6 
                 0.80 
                 0.72 
               
               
                 Example 8 
                 0.1 
                 0.5 
                 0.9 
                 11 
                 0.69 
                 0.70 
               
               
                 Example 9 
                 0.1 
                 0.5 
                 0.9 
                 58 
                 0.58 
                 0.66 
               
               
                 Example 10 
                 0.2 
                 0.5 
                 0.9 
                 72 
                 0.56 
                 0.67 
               
               
                 Example 11 
                 0.2 
                 0.7 
                 0.9 
                 100 
                 0.58 
                 0.67 
               
               
                   
               
            
           
         
       
     
     Table 2 also shows the results of Comparative Example 1 together. It is indicated as follows: for any of Examples 7 to 11, as compared with Comparative Example 1, the initial resistance ratio and the ratio of the resistance increase rate are smaller. The results of Examples 7 to 11 indicate as follows: when the Ti concentration of the brookite type TiO 2  is 11 mol % or more, the initial resistance ratio and the ratio of the resistance increase rate are very small; and when the Ti concentration of the brookite type TiO 2  is 58 mol % or more, the initial resistance ratio and the ratio of the resistance increase rate are particularly small. 
     Up to this point, specific examples of the present disclosure were described in details. However, these are merely illustrative, and should not be construed as limiting the scope of the appended claims. The technology described in the appended claims includes various modifications and changes of the specific examples exemplified up to this point.