Positive electrode for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

Provided is a positive electrode for a non-aqueous electrolyte secondary battery comprising a positive electrode mixture that includes a positive electrode active material and an adhesive having a polymer structure derived from vinylidene fluoride, the ATR-IR spectrum of the positive electrode mixture having an α peak that belongs to an α-crystal of the polymer structure in a wavelength range of 760-764 cm−1, and a β peak that belongs to a β-crystal of the polymer structure in a wavelength range of 838-842 cm−1, and the maximum absorption intensity H(α) of the α peak and the maximum absorption intensity H(β) of the β peak satisfying the relationship 0.2≤H(α) / H(β)≤5. This makes it possible to improve the capacity retention of the non-aqueous electrolyte secondary battery while retaining the capacity when a ferroelectric substance is used.

Description

Positive electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

[0001] The present invention relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery.

[0002] Nonaqueous electrolyte secondary batteries, typified by lithium ion secondary batteries, are used as power sources for electronic devices such as mobile terminals, and as power sources for vehicles such as electric vehicles. The positive electrode comprises a positive electrode mixture containing a positive electrode active material and a binder.

[0003] Patent Document 1 proposes a "positive electrode for a non-aqueous electrolyte secondary battery, comprising: a positive electrode current collector sheet; and a positive electrode mixture layer supported on the positive electrode current collector sheet, the positive electrode mixture layer including a positive electrode active material, a binder, and a conductive agent, the positive electrode active material having a layered rock salt crystal structure and including a composite oxide containing lithium and an element A other than the lithium, the element A including at least nickel, the atomic ratio of the nickel to the element A: Ni / A being 0.8 or more and 1.0 or less, the binder including a polymer binder having a three-dimensional network structure, and the mass of the positive electrode mixture layer supported per 1 m2 of the positive electrode current collector sheet being 280 g or more."

[0004] Patent Document 2 proposes "a lithium ion secondary battery comprising: an electrode mixture layer containing an electrode active material and an organic ferroelectric having a relative dielectric constant of 25 or more; and an electrolyte solution containing lithium-bis(fluorosulfonyl)imide and a non-aqueous solvent, wherein the content of the organic ferroelectric is 0.5 parts by mass or more and 10 parts by mass or less relative to 100 parts by mass of the electrode active material; and the proportion of a highly polar solvent having a relative dielectric constant of 10 or more in the non-aqueous solvent is 10% by volume or less."

[0005] Patent Document 3 proposes "a lithium ion secondary battery comprising a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, and a non-aqueous electrolyte, wherein at least one of the positive electrode active material and the negative electrode active material contains a ferroelectric ceramic having a Curie temperature equal to or lower than the ambient temperature for use."

[0006] International Publication No. 2022 / 070891 Japanese Patent Application Laid-Open No. 2016-164832 International Publication No. 2018 / 168241

[0007] Patent Documents 2 and 3 propose incorporating a ferroelectric material into a battery to improve input / output characteristics. For example, sintering an inorganic ferroelectric material onto the surface of a positive electrode active material is said to promote ionization of Li salt at the interface between the positive electrode active material and the electrolyte, thereby improving the battery's output characteristics. However, incorporating a ferroelectric material into the positive electrode can reduce the electronic conductivity within the positive electrode mixture. In this case, the capacity retention rate decreases. Furthermore, since the ferroelectric material does not contribute to battery capacity, incorporating a ferroelectric material into a battery reduces the battery capacity accordingly.

[0008] One aspect of the present invention provides a cathode mixture including a cathode active material and a binder having a polymerization structure derived from vinylidene fluoride, wherein the ATR-IR spectrum of the cathode mixture is 760 to 764 cm －１ In the wavelength region of 838 to 842 cm －１ and a β peak attributable to β-type crystals of the polymer structure in a wavelength region of 0.2≦H(α) / H(β), wherein the maximum absorption intensity H(α) of the α peak and the maximum absorption intensity H(β) of the β peak satisfy 0.2≦H(α) / H(β).

[0009] Another aspect of the present invention relates to a non-aqueous electrolyte secondary battery including the above-mentioned positive electrode for a non-aqueous electrolyte secondary battery, a negative electrode, and a non-aqueous electrolyte.

[0010] According to the present disclosure, when a ferroelectric substance is used, it is possible to improve the capacity retention rate while maintaining the capacity of a nonaqueous electrolyte secondary battery.

[0011] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present application, will be better understood from the following detailed description taken in conjunction with the drawings.

[0012] 1 is a longitudinal sectional view of a nonaqueous electrolyte secondary battery according to one embodiment of the present invention.

[0013] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values, materials, etc. may be exemplified. However, other numerical values, materials, etc. may be applied as long as the effects of the present disclosure are obtained. Note that components other than those characteristic of the present disclosure may be those of known secondary batteries. In this specification, when a "range of numerical value A to numerical value B" is used, the range includes numerical value A and numerical value B. For example, "A to B mol %" is synonymous with "A mol % or more and B mol % or less." In the following description, when lower and upper limits for specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not greater than the upper limit. When multiple materials are exemplified, one material may be selected from the materials and used alone, or two or more materials may be used in combination.

[0014] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0015] Non-aqueous electrolyte secondary batteries include lithium ion secondary batteries that use a material that reversibly absorbs and releases at least lithium ions as a negative electrode active material, lithium secondary batteries in which lithium metal precipitates at the negative electrode during charging and dissolves during discharging, and all-solid-state batteries.

[0016] The non-aqueous electrolyte secondary battery according to the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. A separator is typically disposed between the positive electrode and the negative electrode. The non-aqueous electrolyte typically has lithium ion conductivity.

[0017] [Positive Electrode] A positive electrode for a nonaqueous electrolyte secondary battery according to one embodiment of the present invention includes, for example, a positive electrode current collector sheet and a positive electrode mixture layer supported on the positive electrode current collector sheet. The positive electrode mixture layer is composed of a positive electrode mixture including a positive electrode active material and a binder having a polymerized structure derived from vinylidene fluoride (VDF) (hereinafter also referred to as a "PVDF structure"). The positive electrode mixture may further include a conductive agent. Here, the polymerized structure refers to a repeating structure of one or more types of monomer units, and here refers to a repeating structure of vinylidene fluoride units.

[0018] The ATR-IR spectrum of the positive electrode mixture (a spectrum obtained by infrared spectroscopy (IR) using the attenuated-total-reflection method) includes a spectrum specific to the binder. The ATR-IR spectrum refers to a spectrum obtained by infrared spectroscopy (IR) using the attenuated-total-reflection method. The ATR-IR spectrum specific to the binder includes a spectrum from 760 to 764 cm －１ In the wavelength region of 838-842 cm －１ A β peak attributable to β-type crystals of the PVDF structure is observed in the wavelength region of β-type crystals. β-type crystals act as ferroelectrics. When the binder contains β-type crystals, ionization of salts in the non-aqueous electrolyte near the positive electrode active material is promoted, reducing the activation energy of the Faraday reaction. As a result, the internal resistance of the positive electrode is reduced and the reaction efficiency is improved. These phenomena are reflected in an improved capacity retention rate. Furthermore, when part of the binder is used as a ferroelectric, the capacity (initial capacity) of the non-aqueous electrolyte secondary battery can be maintained, unlike when a separate ferroelectric is used.

[0019] An example of preferred measurement conditions for the ATR-IR spectrum is shown below: Measurement device: Fourier transform infrared spectrometer (FT-IR), ALPHA (manufactured by Bruker Corporation) Measurement method: ATR method (Di) Measurement wavenumber range: 4000 cm －１ ~400cm －１ Resolution: 4cm －１

[0020] The measurement sample can be a powder of the positive electrode mixture obtained by peeling off the positive electrode mixture layer from the positive electrode. In such powder, the binder adheres to the surface of the positive electrode active material particles. The measurement sample can be prepared by placing the powder on a crystal and pressing it with a jig.

[0021] However, the ratio of the maximum absorption intensity H(α) of the α peak to the maximum absorption intensity H(β) of the β peak: H(α) / H(β) must satisfy 0.2≦H(α) / H(β)≦5. If the H(α) / H(β) ratio exceeds 5, the β-type crystals of the ferroelectric are not sufficiently formed, and the aforementioned effects cannot be obtained. On the other hand, if the H(α) / H(β) ratio is less than 0.2, the binding strength of the binder decreases, the electron conduction path in the positive electrode mixture is easily deteriorated, and the capacity retention rate decreases due to a decrease in electronic conductivity. H(α) / H(β) may satisfy 0.2≦H(α) / H(β)≦3 or 0.2≦H(α) / H(β)≦2.

[0022] The binder having a PVDF structure may be a mixture of multiple polymers. The binder having a PVDF structure may include a first polymer having a copolymer structure of vinylidene fluoride (VDF) units and trifluoroethylene (TrFE) units (hereinafter also referred to as a "VDF-TrFE copolymer structure"), and a second polymer different from the first polymer. The first polymer is likely to form ferroelectric β-type crystals. The first polymer may contain a VDF-TrFE copolymer structure as a main component. The VDF-TrFE copolymer structure may be a block copolymer structure or a random copolymer structure. In particular, in a random copolymer structure, the polarity of TrFE has a stronger influence on the orientation of VDF, which tends to increase the dielectric constant of the copolymer structure.

[0023] The term "unit" refers to the smallest structure derived from a monomer before polymerization. For example, a vinylidene fluoride unit is vinylidene fluoride (CF ２ = H ２ ) is the smallest structure derived from -(CF ２ CH ２ )- means a divalent group represented by the formula:

[0024] The main component refers to a component that accounts for 50 mol % or more of all monomer units constituting the polymer. The main component may account for 70 mol % or more, or may account for 80 mol % or more of all monomer units.

[0025] The first polymer may be composed of only vinylidene fluoride units and trifluoroethylene units, or may contain a monomer unit other than vinylidene fluoride and trifluoroethylene units (hereinafter also referred to as a "third monomer unit"). The first polymer may contain a VDF-TrFE copolymer structure as a main component, and it is desirable that 90 mol % or more of all the monomer units contained in the first polymer be composed of vinylidene fluoride units and trifluoroethylene units.

[0026] The ratio of the number of moles of vinylidene fluoride units (mVDF) to the number of moles of trifluoroethylene units (mTrFE) contained in the first polymer (mVDF / mTrFE ratio) is, for example, 50 / 50 to 90 / 10, or may be 60 / 40 to 80 / 20. In this case, the H(α) / H(β) ratio is easily controlled to satisfy 0.2≦H(α) / H(β)≦5, and is also easily controlled to satisfy 0.2≦H(α) / H(β)≦3 or 0.2≦H(α) / H(β)≦2. The mVDF / mTrFE ratio can be estimated from the ATR-IR spectrum described above.

[0027] The second polymer may be at least one selected from the group consisting of polyvinylidene fluoride (PVDF), modified polyvinylidene fluoride (modified PVDF), vinylidene fluoride copolymers, and modified vinylidene fluoride copolymers. The second polymer has excellent binding strength. By using the first polymer and the second polymer in combination in a balanced manner, the above-mentioned effects of the ferroelectric are fully exhibited and the electron conduction path in the positive electrode mixture is less likely to deteriorate, thereby improving the capacity retention rate.

[0028] Polyvinylidene fluoride may be composed only of vinylidene fluoride units, and it is desirable that 50 mol % or more, further 90 mol % or more, or 98 mol % or more of all monomer units contained in polyvinylidene fluoride are composed of vinylidene fluoride units.

[0029] Modified polyvinylidene fluoride refers to a polymer in which the fluorine atoms or hydrogen atoms of the vinylidene fluoride units constituting polyvinylidene fluoride are substituted with other substituents. The modified vinylidene fluoride units may account for, for example, 10 mol% or less of the total monomer units. That is, it is desirable that 50 mol% or more, and even 90 mol% or more of the total monomer units contained in the modified polyvinylidene fluoride are composed of ordinary vinylidene fluoride units. Examples of other substituents include methacrylic acid, maleic acid, acrylic acid, fumaric acid, and itaconic acid. However, the modified polyvinylidene fluoride may be composed only of modified vinylidene fluoride units, or modified vinylidene fluoride units may constitute 50 mol% or more.

[0030] A vinylidene fluoride copolymer refers to a copolymer containing vinylidene fluoride units as a main component and containing monomer units other than vinylidene fluoride and trifluoroethylene (hereinafter also referred to as "fourth monomer units"). The vinylidene fluoride copolymer may be modified, such as modified polyvinylidene fluoride, by substituting 10 mol % or less of the monomer units with other substituents such as fluorine atoms and hydrogen atoms. It is desirable that 50 mol % or more of all the monomer units contained in the vinylidene fluoride copolymer are composed of vinylidene fluoride units.

[0031] The modified vinylidene fluoride copolymer refers to a copolymer containing modified vinylidene fluoride units as a main component and containing a fourth monomer unit, and it is desirable that 50 mol % or more of all the monomer units contained in the modified vinylidene fluoride copolymer are composed of modified vinylidene fluoride units.

[0032] The third monomer unit and the fourth monomer unit may each be a vinyl monomer such as hexafluoropropylene (HFP), tetrafluoroethylene (TFE), trifluorochloroethylene, ethylene, or propylene. These may be used alone or in combination of two or more. Among these, hexafluoropropylene (HFP), tetrafluoroethylene (TFE), trifluorochloroethylene, and the like are preferred.

[0033] In general, the second polymer can be said to be a polymer whose main components are vinylidene fluoride and monomer units derived from vinylidene fluoride, to the extent that it does not overlap with the first polymer.

[0034] The ratio (M1 / M2 ratio) of the mass M1 of the first polymer to the mass M2 of the second polymer contained in the binder is, for example, 20 / 80 to 60 / 40, or may be 40 / 60 to 60 / 40. In this case, it is easy to control the H(α) / H(β) ratio to satisfy 0.2≦H(α) / H(β)≦5, and it is also easy to control it to satisfy 0.2≦H(α) / H(β)≦3 or 0.2≦H(α) / H(β)≦2.

[0035] The M1 / M2 ratio can be estimated by peeling the positive electrode mixture from the positive electrode, roughly separating the binder and the positive electrode active material, then performing 19F-MAS-NMR analysis of the binder and calculating the ratio of VDF to TrFE from the spectrum. Alternatively, multiple compatible resins of PVDF and PVDF-TrFE copolymers with varying ratios of VDF and TrFE may be prepared, a film may be created with the compatible resin, and a calibration curve for viscoelastic behavior may be created to estimate the mixture ratio. Separation of the binder and the positive electrode active material is possible by heating the positive electrode mixture in a solvent such as NMP to dilute and dissolve the binder, followed by centrifugation.

[0036] The number average molecular weight of the first polymer may be, for example, 100,000 to 500,000. The number average molecular weight of the second polymer may be, for example, 300,000 to 2,000,000. Here, the number average molecular weight is a polystyrene-equivalent value determined by gel permeation chromatography (GPC).

[0037] The positive electrode mixture may contain a binder other than a binder having a PVDF structure, as long as the effects described above are not significantly impaired. The proportion of the binder having a PVDF structure in the entire binder is desirably 80% by mass or more, and all of the binders may be binders having a PVDF structure. Specific examples of binders other than binders having a PVDF structure include polytetrafluoroethylene (PTFE), polyethylene, polypropylene, polyacrylic acid, polymethyl acrylate, ethylene-acrylic acid copolymer, and the like. These may be used alone or in combination of two or more.

[0038] The content of the binder having a PVDF structure in the positive electrode mixture is, for example, 0.5 parts by mass or more and 2 parts by mass or less, or may be 0.5 parts by mass or more and 1.5 parts by mass or less, per 100 parts by mass of the positive electrode active material. In this case, the positive electrode capacity is increased, and the binding strength of the positive electrode mixture and the adhesion strength between the positive electrode mixture layer and the positive electrode current collector sheet are also likely to be improved.

[0039] The positive electrode active material includes, for example, a composite oxide having a layered rock salt crystal structure and containing lithium and an element A other than lithium. To achieve high capacity, the element A preferably contains at least nickel. The atomic ratio of nickel to element A, Ni / A, may be, for example, 0.8 or more and 1.0 or less. Composite oxides containing such high concentrations of Ni are prone to the formation of high-resistance components on the surface. On the other hand, when the binder contains β-type crystals and satisfies 0.2≦H(α) / H(β)≦5, ionization of salt in the non-aqueous electrolyte is promoted near the positive electrode active material, reducing the activation energy of the Faraday reaction and suppressing an increase in the internal resistance of the positive electrode. In other words, the influence of the high-resistance components can be reduced.

[0040] Hereinafter, a composite oxide having a layered rock salt type crystal structure, containing lithium and element A, and having an atomic ratio of nickel to element A, Ni / A, of 0.8 to 1.0, will also be referred to as a "composite oxide HN." The proportion of the composite oxide HN in the entire positive electrode active material is, for example, 80 mass % or more, and the entire positive electrode active material may be the composite oxide HN. The positive electrode active material may contain a composite oxide other than the composite oxide HN (for example, LiCoO ２ , Li ２ NiO ２ , Li ５ FeO ４ It may also contain small amounts of

[0041] The element A contains at least Ni, and may further contain at least one element selected from the group consisting of cobalt (Co), manganese (Mn), aluminum (Al), magnesium (Mg), calcium (Ca), iron (Fe), copper (Cu), zinc (Zn), chromium (Cr), titanium (Ti), niobium (Nb), zirconium (Zr), vanadium (V), tantalum (Ta), molybdenum (Mo), tungsten (W), strontium (Sr), silicon (Si), and boron (B).

[0042] Among these, element A preferably contains Ni and at least one selected from the group consisting of Co, Mn, and Al, and more preferably contains Ni, Co, and at least one of Mn and Al. When element A contains Co, the phase transition of the composite oxide containing Li and Ni is suppressed during charge and discharge, the stability of the crystal structure is improved, and the capacity retention rate is likely to be improved. When element A contains at least one of Mn and Al, thermal stability is improved.

[0043] The composite oxide HN has the general formula: Li ａ Ni ｘ Co ｙ M １－ｘ－ｙ O ２ In the formula, 0.97≦a≦1.2, 0.8≦x≦1.0, and 0≦y≦0.2, and M is preferably at least one element selected from the group consisting of Mn, Al, B, W, Sr, Mg, Mo, Nb, Ti, Si, and Zr.

[0044] When a, which indicates the Li composition ratio, is 0.97 or more and 1.2 or less, cation mixing, in which Ni ions enter the Li site, is less likely to occur, and output characteristics are likely to improve. When x, which indicates the Ni composition ratio, is 0.8 or more and 1 or less, the proportion of Ni in element A is large, and high capacity is likely to be achieved. y may be greater than 0 and 0.2 or less. When the composite oxide HN contains Co, the stability of the crystal structure is likely to be improved, and the capacity retention rate is likely to be improved. The element M may be Al, and 0<y≦0.2, 0<(1-x-y)≦0.05. When the composite oxide HN contains Al, the thermal stability of the composite oxide is likely to be improved. The value of a changes during charge and discharge.

[0045] The density of the positive electrode mixture layer is 3.45 g / cm ３ Above, 3.75g / cm ３ or less, 3.5 g / cm ３ Above, 3.75g / cm ３ The density of the positive electrode mixture layer may be 3.45 g / cm or less. ３ In this case, the number of contact points between the composite oxide and the conductive agent and binder increases. Furthermore, contact points between the composite oxide particles are easily formed. This allows for sufficient formation of electron conduction paths, making it easier to obtain high capacity and improve capacity retention. Furthermore, the adhesion between the positive electrode mixture layer and the positive electrode current collector sheet is also improved.

[0046] The composite oxide particles usually contain secondary particles formed by agglomeration of multiple primary particles. The average particle size (D50) of the secondary particles is, for example, 5 μm or more and 20 μm or less. The average particle size (D50) here refers to the median diameter at which the volume cumulative value is 50% in the volume-based particle size distribution. The average particle size (D50) of the secondary particles can be determined by measuring the particle size distribution using a laser diffraction method.

[0047] The positive electrode mixture may contain a conductive agent. The conductive agent sufficiently forms conductive paths between the positive electrode active materials and between the positive electrode active material and the positive electrode current collector sheet. The conductive agent preferably contains carbon nanotubes (CNTs). CNTs easily become entangled with the binder, and the binder firmly maintains the contact points between the CNTs and the positive electrode active material during charge and discharge.

[0048] The average length of the CNTs is preferably 0.5 μm or more, more preferably 0.5 μm or more and 10.0 μm or less, and even more preferably 0.5 μm or more and 5.0 μm or less. In this case, the CNTs are likely to be interposed between particles of the positive electrode active material, and the CNTs are likely to form sufficient electron conduction paths between the particles of the positive electrode active material.

[0049] From the viewpoint of improving the capacity retention rate, the average diameter of the CNTs may be 0.5 nm or more and 30 nm or less, or 0.5 nm or more and 20 nm or less. When the average diameter of the CNTs is 0.5 nm or more, the strength of the CNTs is sufficiently ensured, and the CNTs are likely to maintain an electron conduction path during charge and discharge. In addition, the CNTs are likely to be interposed between particles of the positive electrode active material.

[0050] The average length and average diameter of the CNTs are determined by obtaining an image of the cross section of the positive electrode mixture layer or the CNTs using a scanning electron microscope (SEM) or a transmission electron microscope (TEM), measuring the lengths and diameters of a number of randomly selected CNTs (e.g., about 50 to 200) using the image, and averaging the lengths and diameters. Note that the length of the CNTs refers to the length when they are linear.

[0051] The conductive agent may contain a conductive material other than CNT. Examples of conductive materials other than CNT include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black; graphene sheets; metal fibers; and metal powders such as aluminum. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0052] From the viewpoint of achieving high capacity, the content of the conductive agent in the positive electrode mixture layer is preferably, for example, 0.01 part by mass or more and 1.0 part by mass or less per 100 parts by mass of the positive electrode active material. In this case, the entire conductive agent may be CNT.

[0053] The positive electrode current collector sheet may be, for example, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the positive electrode current collector sheet include stainless steel, aluminum, an aluminum alloy, and titanium. The thickness of the positive electrode current collector sheet is, for example, 3 to 50 μm.

[0054] A method for producing a positive electrode includes, for example, a step of preparing a positive electrode slurry by dispersing a positive electrode mixture containing a positive electrode active material and a binder having a PVDF structure as essential components and a conductive agent as an optional component in a dispersion medium, and a step of applying the positive electrode slurry to the surface of a positive electrode current collector sheet and drying it to form a positive electrode mixture layer. The dried coating may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector sheet. Examples of the dispersion medium that can be used include water, alcohols such as ethanol, and N-methyl-2-pyrrolidone (NMP).

[0055] [Negative Electrode] The negative electrode may include, for example, a negative electrode current collector sheet and a negative electrode mixture layer supported on the negative electrode current collector sheet. The negative electrode may be obtained, for example, by applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector sheet and drying the slurry to form a negative electrode mixture layer. The dried coating may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector sheet.

[0056] The negative electrode mixture contains a negative electrode active material as an essential component, and may optionally contain a binder, a conductive agent, a thickener, and the like. Examples of binders include fluororesins, acrylic resins, and rubber materials. Examples of fluororesins include tetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), modified PVDF, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), and tetrafluoroethylene-hexafluoropropylene copolymer (FEP). Examples of acrylic resins include polyacrylic acid and acrylic acid-methacrylic acid copolymer. Examples of rubber materials include styrene butadiene rubber. Water is preferably used as the dispersion medium. Conductive agents, except for graphite, can be used as those exemplified for the positive electrode. Examples of thickeners that can be used include carboxymethyl cellulose (CMC), CMC salts, and the like.

[0057] A carbon material can be used as the negative electrode active material. Examples of carbon materials include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon). Among them, graphite is preferred because it has excellent charge / discharge stability and low irreversible capacity. Graphite refers to a material having a graphite-type crystal structure, and includes, for example, natural graphite, artificial graphite, graphitized mesophase carbon particles, and the like. One type of carbon material may be used alone, or two or more types may be used in combination.

[0058] From the viewpoint of increasing capacity, the negative electrode active material preferably contains a silicon-containing material. Examples of the silicon-containing material include a material containing a lithium ion conductive phase and a silicon phase dispersed in the lithium ion conductive phase. The lithium ion conductive phase is a silicate phase containing at least one of an alkali metal element and a Group 2 element (e.g., Li ２ｕ SiO ｕ＋２ (0<u<2)), SiO ２ phase, amorphous carbon phase, etc. ２ The silicon-containing material, in which the silicon phase is dispersed in the phase, is generally, for example, SiO ｘ (0.5<x<1.5).

[0059] By using a carbon material and a silicon-containing material in combination, it is possible to obtain a good balance of good cycle characteristics and high capacity. The proportion of the silicon-containing material in the total of the carbon material and the silicon-containing material is, for example, 0.5 mass% or more, more preferably 1 mass% or more, and even more preferably 2 mass% or more. Furthermore, from the viewpoint of improving cycle characteristics, the proportion of the silicon-containing material in the total of the silicon-containing material and the carbon material is, for example, 30 mass% or less, more preferably 20 mass% or less, and even more preferably 15 mass% or less.

[0060] Examples of the negative electrode current collector sheet include a sheet having the same form as the positive electrode current collector sheet (e.g., metal foil) and made of a material such as stainless steel, nickel, a nickel alloy, copper, a copper alloy, etc. The thickness of the negative electrode current collector sheet is, for example, 1 to 50 μm.

[0061] [Non-aqueous electrolyte] The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution), a gel electrolyte, or a solid electrolyte. The gel electrolyte includes a lithium salt and a matrix polymer, or a lithium salt, a non-aqueous solvent, and a matrix polymer. For example, a polymer material that absorbs the non-aqueous solvent and gels is used as the matrix polymer. Examples of the polymer material include fluororesin, acrylic resin, polyether resin, and polyethylene oxide. The solid electrolyte may be an inorganic solid electrolyte. For example, a material known in all-solid-state lithium ion secondary batteries (e.g., oxide-based solid electrolyte, sulfide-based solid electrolyte, halide-based solid electrolyte, etc.) is used as the inorganic solid electrolyte. For example, the liquid electrolyte (electrolytic solution) includes a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent.

[0062] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Cyclic carbonates having an unsaturated bond, such as vinylene carbonate (VC), may also be used. Cyclic carbonates having a fluorine atom, such as fluoroethylene carbonate (FEC), may also be used. Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0063] Examples of lithium salts include LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ Examples of the lithium salts include lithium carboxylates, lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, and imide salts. Examples of the borates include lithium bis(oxalato)borate and lithium difluorooxalatoborate. Examples of the imide salts include lithium bis(fluorosulfonylimide) (LiN(FSO ２ ) ２ ), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonate nonafluorobutanesulfonate imide (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), lithium bispentafluoroethanesulfonyl imide (LiN(C ２ F ５ SO ２ ) ２ The lithium salt may be used alone or in combination of two or more. The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0064] [Separator] It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0065] <Non-aqueous electrolyte secondary battery> An example of the structure of a non-aqueous electrolyte secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween is housed in an exterior body together with a non-aqueous electrolyte. However, this is not limited thereto, and other forms of electrode groups may also be applied. For example, a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be used. The shape of the secondary battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0066] The structure of a nonaqueous electrolyte secondary battery will be described below with reference to Fig. 1. Fig. 1 is a longitudinal cross-sectional view of a cylindrical secondary battery as an example of this embodiment. However, the present disclosure is not limited to the following configuration.

[0067] The nonaqueous electrolyte secondary battery (hereinafter referred to as battery 10) includes an electrode group 18, a nonaqueous electrolyte (not shown), and a cylindrical battery can 22 with a bottom that accommodates these. A sealing body 11 is crimped to the opening of the battery can 22 via a gasket 21, thereby sealing the battery. The sealing body 11 includes a valve body 12, a metal plate 13, and an annular insulating member 14 interposed between the valve body 12 and the metal plate 13. The valve body 12 and the metal plate 13 are connected to each other at their respective centers. A positive electrode lead 15a extending from a positive electrode 15 is connected to the metal plate 13. Thus, the valve body 12 functions as an external terminal for the positive electrode. A negative electrode lead 16a extending from a negative electrode 16 is connected to the inner bottom surface of the battery can 22. An annular groove 22a is formed near the open end of the battery can 22. A first insulating plate 23 is disposed between one end face of the electrode group 18 and the annular groove portion 22a. A second insulating plate 24 is disposed between the other end face of the electrode group 18 and the bottom of the battery can 22. The electrode group 18 is formed by winding a positive electrode 15 and a negative electrode 16 with a separator 17 interposed therebetween.

[0068] (Additional Note) The above description discloses the following technology: (Technology 1) A cathode mixture containing a cathode active material and a binder having a polymer structure derived from vinylidene fluoride is provided, and the ATR-IR spectrum of the cathode mixture is －１an α peak attributable to the α-type crystal of the polymer structure in the wavelength region of 838 to 842 cm －１ and a β-peak attributable to β-type crystals of the polymer structure in a wavelength region of 0.25 μm, wherein the maximum absorption intensity H(α) of the α-peak and the maximum absorption intensity H(β) of the β-peak satisfy 0.2≦H(α) / H(β)≦5. (Technology 2) The positive electrode for a non-aqueous electrolyte secondary battery according to Technology 1, wherein the binder includes a first polymer and a second polymer, the first polymer having a copolymer structure of vinylidene fluoride units and trifluoroethylene units, and the second polymer includes at least one selected from the group consisting of polyvinylidene fluoride, modified polyvinylidene fluoride, and vinylidene fluoride copolymers. (Technology 3) The positive electrode for a non-aqueous electrolyte secondary battery according to Technology 1 or 2, wherein the ratio M1 / M2 of the mass M1 of the first polymer to the mass M2 of the second polymer contained in the binder is 20 / 80 to 60 / 40. (Technology 4) A positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technologies 1 to 3, wherein the ratio of the number of moles of vinylidene fluoride units mVDF to the number of moles of trifluoroethylene units mTrFE contained in the first polymer, mVDF / mTrFE, is 50 / 50 to 90 / 10. (Technology 5) A positive electrode for a non-aqueous electrolyte secondary battery according to any one of Technologies 1 to 4, wherein the positive electrode active material has a layered rock salt crystal structure and contains a composite oxide containing lithium and an element A other than lithium, the element A containing at least nickel, and the atomic ratio of nickel to the element A, Ni / A, is 0.8 or more and 1.0 or less. (Technology 6) The composite oxide is a compound represented by the general formula: Li ａ Ni ｘ Co ｙ M １－ｘ－ｙ O ２ wherein 0.97≦a≦1.2, 0.8≦x≦1.0, and 0≦y≦0.2, and M is at least one element selected from the group consisting of Mn, Al, B, W, Sr, Mg, Mo, Nb, Ti, Si, and Zr. (Technology 7) A nonaqueous electrolyte secondary battery comprising the positive electrode for a nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 6, a negative electrode, and a nonaqueous electrolyte.

[0069] EXAMPLES The present invention will be specifically described below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0070] Comparative Example 1 (Preparation of composite oxide HN) Ni obtained by coprecipitation method ０．８ Co ０．１７ Al ０．０３ (OH) ２ And Li ２ CO ３ The above was mixed so that the atomic ratio of Li to the total of Ni, Co, and Al, Li / (Ni+Co+Al), was 1.05 / 1, and the mixture was fired in an oxygen atmosphere to obtain a composite oxide. The composition of the obtained composite oxide was as follows: １．０５ Ni ０．８ Co ０．１７ Al ０．０３ O ２ The composition of the composite oxide was determined by ICP emission spectroscopy. The composite oxide HN powder having an average particle size of 12 μm was obtained by pulverization and classification using a sieve.

[0071] (Preparation of Positive Electrode) 100 parts by mass of composite oxide HN, which is the positive electrode active material (PAM), was added with 1 part by mass of a binder having a PVDF polymer structure, 1 part by mass of a conductive agent, and an appropriate amount of N-methyl-2-pyrrolidone (NMP), and then stirred to prepare a positive electrode slurry. Polyvinylidene fluoride (PVDF), which is a second polymer, was used alone (M1 / M2 = 0 / 100). CNT (average length 1 μm, average diameter 10 nm) was used as the conductive agent.

[0072] The positive electrode slurry was applied to the surface of an aluminum foil (positive electrode current collector sheet), the coating was dried, and then the aluminum foil was rolled to form a positive electrode mixture layer (density 3.6 g / cm ) on both sides of the aluminum foil. ３ In this way, a positive electrode was obtained. ２ The amount of the positive electrode slurry applied was adjusted so that the amount of the positive electrode mixture layer carried per electrode was 280 g.

[0073] (Preparation of Negative Electrode) To 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), 1 part by mass of carboxymethyl cellulose sodium (CMC-Na), and an appropriate amount of water were added, and the mixture was stirred to prepare a negative electrode slurry.

[0074] The negative electrode active material was a mixture of a silicon-containing material and graphite (average particle size (D50) 25 μm). The mass ratio of the silicon-containing material to the graphite in the negative electrode active material was 10:90. The silicon-containing material was a SiO 2 composite whose surface was coated with a conductive layer containing conductive carbon. ｘ Particles (x=1, average particle size (D50) 5 μm) were used. The coating amount of the conductive layer was SiO ｘ The amount was 5 parts by mass per 100 parts by mass of the total of the particles and the conductive layer.

[0075] The negative electrode slurry was applied to the surface of a copper foil (negative electrode current collector sheet), the coating was dried, and then rolled to form a negative electrode mixture layer (thickness: 200 μm, density: 1.5 g / cm ) on both sides of the copper foil. ３ ) was formed. In this way, a negative electrode was obtained.

[0076] (Preparation of non-aqueous electrolyte) LiPF 6 was dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 3:7). ６ was dissolved in a concentration of 1.0 mol / L to obtain a non-aqueous electrolyte.

[0077] (Preparation of Non-Aqueous Electrolyte Secondary Battery) One end of an aluminum positive electrode lead was attached to the obtained positive electrode. One end of a nickel negative electrode lead was attached to the obtained negative electrode. The positive and negative electrodes were wound with a polyethylene separator interposed therebetween to prepare a wound electrode assembly. The electrode assembly was vacuum dried at 105°C for 2 hours and then housed in a cylindrical battery case with a bottom that also served as a negative electrode terminal. An iron case (outer diameter 18 mm, height 65 mm) was used as the battery case. The other end of the positive electrode lead was connected to a sealing member, and the other end of the negative electrode lead was connected to the inner bottom surface of the battery case. After non-aqueous electrolyte was injected into the battery case, the opening of the battery case was closed with a metal sealing member that also served as a positive electrode terminal. A resin gasket was interposed between the sealing member and the open end of the battery case. In this manner, an 18650-type cylindrical non-aqueous electrolyte secondary battery (Battery C1) was prepared.

[0078] Example 1 In preparing the positive electrode, 0.8 parts by mass of PVDF, which is the second polymer, and 0.2 parts by mass of a copolymer (P(VDF-TrFE)) of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) in a molar ratio of 75:25, which is the first polymer, were used as binders having a PVDF polymer structure. That is, M1 / M2 was 20 / 80, and the mVDF / mTrFE ratio of the first polymer was 75 / 25. Battery A1 was prepared in the same manner as in Comparative Example 1, except for the above.

[0079] Example 2 In preparing the positive electrode, 0.6 parts by mass of PVDF, which is the second polymer, and 0.4 parts by mass of a copolymer (P(VDF-TrFE)) of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) in a molar ratio of 75:25, which is the first polymer, were used as binders having a PVDF polymer structure. That is, M1 / M2 was 40 / 60, and the mVDF / mTrFE ratio of the first polymer was 75 / 25. Battery A2 was prepared in the same manner as in Comparative Example 1, except for the above.

[0080] Example 3 In preparing the positive electrode, 0.4 parts by mass of PVDF, which is the second polymer, and 0.6 parts by mass of a copolymer (P(VDF-TrFE)) of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) in a molar ratio of 75:25, which is the first polymer, were used as the binder having a PVDF polymer structure. That is, M1 / M2 was 60 / 40, and the mVDF / mTrFE ratio of the first polymer was 75 / 25. Battery A3 was prepared in the same manner as in Comparative Example 1, except for the above.

[0081] Comparative Example 2 In preparing the positive electrode, 0.2 parts by mass of PVDF, which is the second polymer, and 0.8 parts by mass of a copolymer (P(VDF-TrFE)) of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) in a molar ratio of 75:25, which is the first polymer, were used as the binder having a PVDF polymer structure. That is, M1 / M2 was 80 / 20, and the mVDF / mTrFE ratio of the first polymer was 75 / 25. Battery C2 was prepared in the same manner as in Comparative Example 1, except for the above.

[0082] Comparative Example 3 In preparing the positive electrode, 1.0 part by mass of a copolymer (P(VDF-TrFE)) of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) in a molar ratio of 75:25, which was the first polymer, was used alone as the binder having a PVDF polymer structure. That is, M1 / M2 was 100 / 0, and the mVDF / mTrFE ratio of the first polymer was 75 / 25. Battery C3 was prepared in the same manner as in Comparative Example 1, except for the above.

[0083] Comparative Example 4 In preparing the positive electrode, 0.8 parts by mass of PVDF, which is the second polymer, and 0.2 parts by mass of a copolymer (P(VDF-TrFE)) of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) in a molar ratio of 30:70, which is the first polymer, were used as the binder having a PVDF polymer structure. That is, M1 / M2 was 20 / 80, and the mVDF / mTrFE ratio of the first polymer was 30 / 70. Battery C4 was prepared in the same manner as in Comparative Example 1, except for the above.

[0084] Comparative Example 5 In the production of a positive electrode, 0.8 parts by mass of PVDF, which is a second polymer, was used as a binder having a PVDF polymer structure, and 0.2 parts by mass of BaTiO ３ A battery C5 was fabricated in the same manner as in Comparative Example 1 except for the above. ３ The powder was mixed with the positive electrode active material and stirred at 2700 rpm for 3 minutes, and then sintered at 500° C. for 4 hours to be supported on the surface of the positive electrode active material.

[0085] Comparative Example 6 In the preparation of the positive electrode, 1.2 parts by mass of PVDF, which is a second polymer, was used as a binder having a PVDF polymer structure, and 0.3 parts by mass of BaTiO ３ A battery C6 was fabricated in the same manner as in Comparative Example 1 except for the above. ３ The powder was supported on the surface of the positive electrode active material in the same manner as in Comparative Example 5.

[0086] The following evaluations were carried out on the batteries A1 to A3 and C1 to C6. The results are shown in Table 1.

[0087] [Evaluation 1: ATR-IR] The completed battery was charged to 4.2 V at a constant current equivalent to 0.3 C, and then discharged to 2.5 V at a constant current equivalent to 0.3 C, thereby obtaining a battery in a discharged state (SOC = 10% or less) equivalent to the initial state. 1 C is the current value at which an amount of electricity corresponding to the rated capacity flows in 1 hour when charged or discharged at a constant current.

[0088] The battery in the initial state was disassembled to remove the positive electrode, the positive electrode was washed with DMC, and after vacuum drying, only the positive electrode mixture layer was peeled off to obtain a sample of the positive electrode mixture. The sample was analyzed by ATR-IR. －１ The α peak attributable to the α-type crystal of the PVDF polymer structure observed in the wavelength region of 838 to 842 cm －１ The maximum absorption intensities H(α) and H(β) of the β peaks attributable to β-type crystals of the PVDF polymer structure observed in the wavelength region were measured, and the H(α) / H(β) ratio was calculated. Each maximum absorption intensity H is the height of the peak from the baseline.

[0089] [Evaluation 2: Peel Strength Between Positive Electrode Mixture Layer and Positive Electrode Current Collector Sheet] The initial battery was disassembled, the positive electrode removed, the positive electrode washed with DMC, and vacuum dried. The peel strength of the positive electrode mix layer from the positive electrode current collector sheet (aluminum foil) was measured using a measuring device conforming to JIS Z0237 (2009). Specifically, the positive electrode was formed into a strip-shaped sample having a width of 10 mm and a length of 50 mm or more. One side of a 20 mm wide x 130 mm long double-sided tape (e.g., No. 5606 manufactured by Nitto Denko Corporation) was attached to the positive electrode mix layer of the sample. The other side of the double-sided tape was attached to a horizontal table with a flat surface. One end of the longitudinal direction of the positive electrode current collector sheet was fixed with a force gauge and pulled vertically at a rate of 50 mm / min. The positive electrode mix layer attached to the double-sided tape was peeled off from the positive electrode current collector sheet. The tension at this time was measured for 15 seconds or more, and the average tension in a continuous 15-second interval was calculated. When the average tension was 5.0 N / mm or more, the peel strength was evaluated as OK, and when it was less than that, it was evaluated as NG.

[0090] [Evaluation 3: Initial Capacity] The initial battery was subjected to constant current charging at a current of 0.5 C until the voltage reached 4.2 V, followed by constant voltage charging at a voltage of 4.2 V until the current reached 0.05 C. Thereafter, constant current discharging was performed at a current of 0.1 C until the voltage reached 2.5 V, and the discharge capacity at this time was recorded as the initial capacity. The rest time between charging and discharging was 10 minutes. The charging and discharging were performed in an environment of 25°C. The initial capacity was expressed as a relative value when the initial capacity of Battery C1 of Comparative Example 1 was set to 100.

[0091] [Evaluation 4: Capacity Retention Rate] A charge / discharge cycle test was carried out under the following conditions: The rest time between charge and discharge was 10 minutes, and the charge / discharge was carried out in an environment of 25°C.

[0092] <Charging> The battery was charged at a constant current of 0.5 C until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.05 C.

[0093] <Discharge> Constant current discharge was carried out at a current of 0.5 C until the voltage reached 2.5 V.

[0094] Charge and discharge were repeated under the above conditions. The ratio (percentage) of the discharge capacity at the 100th cycle to the discharge capacity at the 1st cycle was calculated as the capacity retention rate (%). The capacity retention rate was expressed as a relative value when the capacity retention rate of Battery C1 of Comparative Example 1 was set to 100.

[0095]

[0096] Batteries A1 to A3, which satisfied 0.2≦H(α) / H(β)≦5, were able to improve their capacity retention rates while maintaining a high capacity equivalent to that of Battery B1. On the other hand, Batteries B2 to B5, which did not satisfy 0.2≦H(α) / H(β)≦5, showed a decrease in their capacity retention rates. Battery B6 showed an improvement in its capacity retention rate, but a significant decrease in its initial capacity.

[0097] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0098] The positive electrode for a non-aqueous electrolyte secondary battery according to the present disclosure is suitably used, for example, in a non-aqueous electrolyte secondary battery that requires high capacity and high capacity retention rate (cycle characteristics).

[0099] 10: Secondary battery, 11: Sealing body, 12: Valve body, 13: Metal plate, 14: Insulating member, 15: Positive electrode, 15a: Positive electrode lead, 16: Negative electrode, 16a: Negative electrode lead, 17: Separator, 18: Electrode group, 21: Gasket, 22: Battery can, 22a: Groove, 23: First insulating plate, 24: Second insulating plate, 16: Negative electrode

Claims

1. A positive electrode mixture including a positive electrode active material and a binder having a polymerization structure derived from vinylidene fluoride, The ATR-IR spectrum of the positive electrode mixture is 760-764cm －１ In the wavelength region of 838 to 842 cm －１ and a β peak attributable to β-type crystals of the polymer structure in the wavelength region of a maximum absorption intensity H(α) of the α peak and a maximum absorption intensity H(β) of the β peak satisfy 0.2≦H(α) / H(β)≦5;

2. The binder includes a first polymer and a second polymer, the first polymer has a copolymer structure of vinylidene fluoride units and ethylene trifluoride units, 2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the second polymer comprises at least one selected from the group consisting of polyvinylidene fluoride, modified polyvinylidene fluoride, and vinylidene fluoride-based copolymers.

3. The binder The mass M1 of the first polymer; The mass M2 of the second polymer; 3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 2, wherein the ratio of M1 / M2 is 20 / 80 to 60 / 40.

4. Included in the first polymer the number of moles of vinylidene fluoride units mVDF; the number of moles of the trifluoroethylene unit, mTrFE; 3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 2, wherein the ratio of mVDF / mTrFE is 50 / 50 to 90 / 10.

5. the positive electrode active material includes a composite oxide having a layered rock salt type crystal structure and including lithium and an element A other than lithium, The element A includes at least nickel, 2 . The positive electrode for a nonaqueous electrolyte secondary battery in accordance with claim 1 , wherein an atomic ratio of said nickel to said element A: Ni / A is 0.8 or more and 1.0 or less.

6. The composite oxide has the general formula: Li ａ Ni ｘ Co ｙ M １－ｘ－ｙ O ２ It is expressed as 6. The positive electrode for a nonaqueous electrolyte secondary battery according to claim 5, wherein in the formula, 0.97≦a≦1.2, 0.8≦x≦1.0, and 0≦y≦0.2, and M is at least one selected from the group consisting of Mn, Al, B, W, Sr, Mg, Mo, Nb, Ti, Si, and Zr.

7. A non-aqueous electrolyte secondary battery comprising the positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 , a negative electrode, and a non-aqueous electrolyte.