Non-aqueous electrolyte secondary battery

Abstract

To provide a non-aqueous electrolyte secondary battery capable of suppressing increase in battery resistance during a high-temperature cycle.SOLUTION: In a non-aqueous electrolyte secondary battery in an embodiment, the positive electrode has a positive electrode active material A. The positive electrode active material A is composed of Lithium transition metal composite oxides represented by a general formula LiaNibCocMndAleMfOg (wherein M is at least one element selected from groups 4, 5, and 6, 0.8≤a≤1.2, b≥0.82, 0<c≤0.08, 0.05≤d≤0.12, 0≤e≤0.05, 0.01≤f≤0.05, 1≤g≤2) and Lithium metal compounds represented by a general formula LixMyOz (wherein 1≤x≤4, 1≤y≤5, 1≤z≤12). The non-aqueous electrolyte secondary battery includes a first layer formed on a particle surface of lithium transition metal composite oxide and a second layer composed of boron compounds and formed on the first layer. The first layer is formed over its entire area on the particle surface of the lithium transition metal composite oxide without the second layer interposed therebetween.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This disclosure relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery comprising a lithium transition metal composite oxide as the positive electrode active material. [Background technology]

[0002] Conventionally, positive electrode active materials have been known in which other compounds are present on the particle surface of lithium transition metal composite oxides in order to improve battery performance such as storage characteristics. For example, Patent Document 1 discloses a positive electrode active material manufactured by firing lithium transition metal composite oxide particles with a compound of a predetermined element (such as TiO2) from groups 4 to 6 whose oxide has a melting point of 750°C or higher present on the particle surface. Patent Document 2 also discloses a positive electrode active material manufactured by firing lithium transition metal composite oxide particles with a borate compound present on the particle surface, wherein the carbonate ion content is 0.15% by weight or less and the borate ion content is 0.01% by weight to 5.0% by weight. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2004-253305 [Patent Document 2] Japanese Patent Publication No. 2010-040382 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] Incidentally, in non-aqueous electrolyte secondary batteries, it is required to reduce the charge transfer resistance at the positive electrode and keep the initial resistance of the battery low. Furthermore, when non-aqueous electrolyte secondary batteries are charged and discharged in a high-temperature environment, resistance tends to increase, and suppressing such resistance increase is an important issue. The purpose of this disclosure is to provide a non-aqueous electrolyte secondary battery that has low initial resistance and can suppress resistance increase during high-temperature cycling.

Means for Solving the Problem

[0005] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is a non-aqueous electrolyte secondary battery including an electrode body including a positive electrode, a negative electrode, and a separator, and a non-aqueous electrolyte. The positive electrode has at least a positive electrode active material A and a positive electrode active material B. The positive electrode active materials A and B are secondary particles in which primary particles are aggregated. The average primary particle diameter of the positive electrode active material B is 0.5 μm or more and larger than the average primary particle diameter of the positive electrode active material A. The average secondary particle diameter of the positive electrode active material B is 2 μm to 7 μm and smaller than the average secondary particle diameter of the positive electrode active material A. The positive electrode active material A contains Ni, Co, and Mn, and contains a metal M composed of at least one selected from Ti, Nb, W, and Zr. The content rate of Ni is 82 mol% or more of the total molar number of metal elements excluding Li, and is composed of a lithium transition metal composite oxide A and a lithium metal oxide containing the metal M. The first layer formed on the particle surface of the lithium transition metal composite oxide A and the second layer formed on the first layer and composed of a boron compound are included. The first layer is formed on the particle surface of the lithium transition metal composite oxide A without passing through the second layer over the entire area.

Advantages of the Invention

[0006] According to the non-aqueous electrolyte secondary battery according to one aspect of the present disclosure, an increase in battery resistance during high-temperature cycling can be suppressed.

Brief Description of the Drawings

[0007] [Figure 1] It is a perspective view of a non-aqueous electrolyte secondary battery which is an example of an embodiment. [Figure 2] It is a perspective view of an electrode body which is an example of an embodiment.

Modes for Carrying Out the Invention

[0008] Conventionally, on the particle surface of a lithium transition metal composite oxide, the general formula Li x M y Oz It is known that the initial resistance of a battery can be reduced by the presence of a lithium metal compound represented by []. The lithium metal compound is considered to function as a lithium ion conductor and contribute to the reduction of the charge transfer resistance of the positive electrode. On the other hand, the presence of a lithium metal compound on the particle surface of a lithium transition metal composite oxide cannot suppress the increase in battery resistance during high-temperature cycling, and may instead increase the resistance.

[0009] The inventors have succeeded in reducing the initial resistance while suppressing the increase in resistance during high-temperature cycling by forming a first layer composed of a lithium metal compound and a second layer composed of a boron compound that covers the first layer on the particle surface of the lithium transition metal composite oxide. The presence of the second layer of the boron compound that covers the first layer forms a strong film containing M and boron on the particle surface of the positive electrode active material during high-temperature cycling. Thus, it is considered that the side reaction of the non-aqueous electrolyte in the positive electrode and the elution of the metal in the positive electrode active material are suppressed, and the increase in battery resistance is suppressed.

[0010] Hereinafter, an example of an embodiment of a non-aqueous electrolyte secondary battery according to the present disclosure will be described in detail. Hereinafter, a non-aqueous electrolyte secondary battery 10 in which a wound electrode body 14 is housed in an exterior body 11 made of a laminate sheet will be exemplified, but the exterior body is not limited thereto, and may be an exterior can such as a cylindrical shape, a rectangular shape, or a coin shape. Further, the electrode body may be a laminated electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated via a separator.

[0011] Figure 1 is a perspective view showing the external appearance of a non-aqueous electrolyte secondary battery 10, which is an example of an embodiment. As illustrated in Figure 1, the non-aqueous electrolyte secondary battery 10 comprises an outer casing 11 composed of two laminate films 11A and 11B. The non-aqueous electrolyte secondary battery 10 also comprises an electrode body 14 housed in the outer casing 11 and a non-aqueous electrolyte. The outer casing 11 has, for example, a substantially rectangular shape in plan view and includes a housing portion 12 in which the electrode body 14 and the non-aqueous electrolyte are housed, and a sealing portion 13 formed around the housing portion 12. The laminate films 11A and 11B are generally composed of resin films containing a metal layer such as aluminum.

[0012] The housing portion 12 can be provided by forming a recess in at least one of the laminate films 11A and 11B that can accommodate the electrode body 14. In the example shown in Figure 1, the recess is formed only in the laminate film 11A. The sealing portion 13 is formed by joining the peripheral edges of the laminate films 11A and 11B. In the example shown in Figure 1, the sealing portion 13 is formed in a frame shape with approximately the same width so as to surround the housing portion 12.

[0013] The non-aqueous electrolyte secondary battery 10 includes a pair of electrode leads (positive electrode lead 15 and negative electrode lead 16) connected to an electrode body 14. In the example shown in Figure 1, the positive electrode lead 15 and the negative electrode lead 16 are led out of the casing 11 from the same end of the casing 11.

[0014] A non-aqueous electrolyte comprises a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and mixtures of two or more of these. The non-aqueous solvent may also contain halogen-substituted solvents, in which some of the hydrogen atoms in the solvent are replaced with halogen atoms such as fluorine. The non-aqueous electrolyte is not limited to a liquid electrolyte; it may also be a solid electrolyte using a gel-like polymer. Examples of lithium salts, such as LiPF6, are used as the electrolyte salt.

[0015] Figure 2 is a perspective view of an electrode body 14, which is an example of an embodiment. As illustrated in Figure 2, the electrode body 14 includes a positive electrode 20, a negative electrode 30, and a separator 40, and is a wound-type electrode body in which the positive electrode 20 and the negative electrode 30 are wound in a spiral shape via the separator 40 and formed into a flat shape. The positive electrode 20 has a positive electrode tab 21, which is a convex portion of the electrode plate that protrudes in the axial direction of the electrode body 14. Similarly, the negative electrode 30 has a negative electrode tab 31 that protrudes in the same direction as the positive electrode tab 21. Multiple positive electrode tabs 21 and negative electrode tabs 31 are formed at regular intervals in the longitudinal direction of each electrode plate.

[0016] The electrode body 14 is formed by overlapping and winding a positive electrode 20 and a negative electrode 30 via a separator 40 so that the positive electrode tabs 21 and negative electrode tabs 31 are alternately arranged in the longitudinal direction of the electrode plate. In the electrode body 14, the positive electrode tabs 21 overlap each other and the negative electrode tabs 31 overlap each other, forming a positive electrode tab stacked portion 22 at one end in the width direction of the electrode body 14 and a negative electrode tab stacked portion 32 at the other end in the width direction. A positive electrode lead 15 is welded to the positive electrode tab stacked portion 22, and a negative electrode lead 16 is welded to the negative electrode tab stacked portion 32.

[0017] The following will provide a detailed explanation of the positive electrode 20, negative electrode 30, and separator 40 that constitute the electrode body 14, with particular emphasis on the positive electrode 20.

[0018] [Positive electrode] The positive electrode 20 comprises a positive electrode core and a positive electrode composite layer provided on the surface of the positive electrode core. The positive electrode core can be made of a metal foil that is stable within the potential range of the positive electrode 20, such as aluminum, or a film with the metal arranged on its surface. The positive electrode composite layer contains a positive electrode active material, a conductive material, and a binder, and is preferably provided on both sides of the positive electrode core, excluding the portion to which the positive electrode lead 15 is connected. The positive electrode 20 can be manufactured, for example, by applying a positive electrode composite slurry containing a positive electrode active material, a conductive material, and a binder to the surface of the positive electrode core, drying the coating, and then compressing it to form the positive electrode composite layer on both sides of the positive electrode core.

[0019] Examples of the conductive material contained in the positive electrode composite layer include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of the binder contained in the positive electrode composite layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, polyethylene oxide (PEO), etc.

[0020] The positive electrode composite layer has at least positive electrode active material A as the positive electrode active material. The positive electrode active material A is composed of a lithium transition metal composite oxide and a lithium metal compound, includes a first layer formed on the particle surface of the lithium transition metal composite oxide, and includes a second layer composed of a boron compound and formed on the first layer. The positive electrode active material A is secondary particles in which primary particles are aggregated. The first layer is formed on the particle surface of the lithium transition metal composite oxide without passing through the second layer over the entire area.

[0021] The positive electrode active material A includes, in order from the inside of the particles, a lithium transition metal composite oxide / the first layer / the second layer. That is, the positive electrode active material A can be said to be core-shell particles in which a shell composed of the first layer and the second layer is formed on the surface of core particles composed of a lithium transition metal composite oxide. By forming the first layer made of a lithium metal compound on the surface of the secondary particles of the lithium transition metal composite oxide, the initial resistance of the battery can be reduced, and by forming the second layer made of a boron compound covering the first layer, an increase in the battery resistance during high-temperature cycling can be suppressed.

[0022] The lithium transition metal composite oxide (hereinafter sometimes referred to as "lithium transition metal composite oxide A") constituting the positive electrode active material A has the general formula Li a Ni b Co c Mn d Al e M f O g(In the formula, M is at least one element selected from Group 4, Group 5, and Group 6 elements, 0.8 ≦ a ≦ 1.2, b ≧ 0.82, 0 < c ≦ 0.08, 0.05 ≦ d ≦ 0.12, 0 ≦ e ≦ 0.05, 0.01 ≦ f ≦ 0.05, 1 ≦ g ≦ 2), which is a composite oxide. The content of Ni is preferably 82 to 92 mol%, more preferably 82 to 90 mol%, based on the total number of moles of metal elements excluding Li.)

[0023] In the lithium transition metal composite oxide A, the content of Co is preferably 3 to 8 mol%, more preferably 5 to 8 mol%, based on the total number of moles of metal elements excluding Li. When the content of Co exceeds 8 mol%, the increase in resistance during high-temperature cycling cannot be suppressed. Also, the content of Mn is preferably 6 to 10 mol% based on the total number of moles of metal elements excluding Li. When the content of Mn is less than 5 mol%, the increase in resistance during high-temperature cycling cannot be suppressed. Incidentally, the lithium transition metal composite oxide A may contain elements other than Li, Ni, Co, Mn, and M as long as the object of the present disclosure is not impaired.)

[0024] The first layer is composed of a lithium metal compound represented by the general formula Li x M y O z (where 1 ≦ x ≦ 4, 1 ≦ y ≦ 5, 1 ≦ z ≦ 12). The first layer may be formed so as to cover the entire surface of the secondary particles of the lithium transition metal composite oxide A, or may be scattered on the particle surface.)

[0025] M in the above general formula is at least one element selected from Group 4, Group 5, and Group 6 elements, preferably at least one selected from Ti, Nb, W, and Zr. That is, the lithium transition metal composite oxide A preferably contains at least one selected from Ti, Nb, W, and Zr. Also, the lithium metal compound constituting the first layer preferably contains at least one selected from Ti, Nb, W, and Zr. Suitable lithium metal compounds are, for example, Li2TiO3, Li4Ti5O 12, LiTiO4, Li2Ti2O5, LiTiO2, Li3NbO4, LiNbO3, Li4Nb2O7, Li8Nb6O 19 These are Li2ZrO3, LiZrO2, Li4ZrO4, Li2WO4, and Li4WO5.

[0026] The content of the first layer is preferably 0.001 to 1 mol%, and more preferably 0.01 to 0.5 mol%, based on the element M in the general formula above, relative to the total number of moles of metal elements in the positive electrode active material A excluding Li. If the content of the first layer is within this range, it becomes easier to suppress the increase in battery resistance during high-temperature cycles.

[0027] As described above, the second layer is composed of a boron compound and is formed on the first layer. Preferably, the second layer covers the entire area of ​​the first layer. That is, it is preferable that the first layer is not exposed on the surface of the positive electrode active material A. If the first layer is scattered on the particle surface of the lithium transition metal composite oxide A, a portion of the second layer may be formed directly on the particle surface of the lithium transition metal composite oxide A. The second layer may be formed to cover the entire area of ​​the secondary particle surface of the lithium transition metal composite oxide A, including the region where the first layer is formed.

[0028] The second layer is not formed between the secondary particle surface of lithium transition metal composite oxide A and the first layer, but only on the surface of the first layer facing away from lithium transition metal composite oxide A. Furthermore, the lithium metal compound constituting the first layer and the boron compound constituting the second layer do not mix with each other, and the boundary between the first and second layers can be confirmed, for example, by XPS.

[0029] The boron compound constituting the second layer may be any compound containing B, and is not particularly limited, but is preferably an oxide or lithium oxide. Examples of boron compounds include boron oxide (B2O3) and lithium borate (Li2B4O7). The content of the second layer is preferably 0.1 to 1.5 mol%, and more preferably 0.5 to 1.0 mol%, based on the boron element relative to the total number of moles of metal elements excluding Li in the positive electrode active material A. If the content of the second layer is within this range, it becomes easier to suppress the increase in battery resistance during high-temperature cycling.

[0030] The average primary particle diameter of positive electrode active material A is, for example, 100 nm to 1000 nm. The average particle diameter (average secondary particle diameter) of positive electrode active material A is, for example, 8 μm to 15 μm. The particle size of positive electrode active material A is approximately equal to the particle size of lithium transition metal composite oxide A.

[0031] The average primary particle diameter of the positive electrode active material can be determined by analyzing SEM images of particle cross-sections observed using a scanning electron microscope (SEM). For example, the positive electrode 20 or positive electrode active material is embedded in a resin, a cross-section is prepared by cross-section polishing (CP), and this cross-section is photographed with an SEM. From the SEM image, 30 primary particles are randomly selected, and the grain boundaries of the primary particles are observed. After determining the external shape of the primary particles, the major axis (longest diameter) of each of the 30 primary particles is determined, and the average value of these is taken as the average primary particle diameter.

[0032] The average secondary particle diameter can also be determined from SEM images of the particle cross-section. Specifically, 30 secondary particles are randomly selected from the above SEM images, and the grain boundaries of these 30 secondary particles are observed. After determining the external shape of the secondary particles, the major axis (longest diameter) of each of the 30 secondary particles is determined, and the average value of these is taken as the average secondary particle diameter.

[0033] The positive electrode active material A is manufactured, for example, by the following process. (1) Nickel-cobalt-manganese composite hydroxide is calcined at 400°C to 600°C to obtain nickel-cobalt-manganese composite oxide. (2) Mix the composite oxide, a lithium compound such as lithium hydroxide, and a compound containing a metal element selected from Group 4, Group 5, and Group 6 in a predetermined molar ratio, and calcine under the conditions of 700 °C to 900 °C in an oxygen atmosphere to obtain a precursor in which a lithium metal compound (the first layer) represented by Li x M y O z is fixed on the particle surface of the lithium transition metal composite oxide. (3) Mix the precursor and a boron compound in a predetermined molar ratio, and calcine under the conditions of 150 °C to 400 °C in an oxygen atmosphere.

[0034] The positive electrode 20 preferably has a positive electrode active material A and a positive electrode active material B as the positive electrode active material. The positive electrode active material B is preferably secondary particles in which primary particles are aggregated, similar to the positive electrode active material A. The average primary particle diameter of the positive electrode active material B is 0.5 μm or more and larger than the average primary particle diameter of the positive electrode active material A. The average primary particle diameter of the positive electrode active material B is, for example, 0.5 μm to 4 μm. Also, the average secondary particle diameter of the positive electrode active material B is 2 μm to 7 μm and smaller than the average secondary particle diameter of the positive electrode active material A. The positive electrode active material B may be composed of only primary particles instead of secondary particles. By using the positive electrode active material B in combination, the increase in resistance during high-temperature cycling can be further suppressed.

[0035] The lithium transition metal composite oxide (hereinafter sometimes referred to as "lithium transition metal composite oxide B") constituting the positive electrode active material B is a composite oxide represented by the general formula Li a Ni b Co c Mn d M e O f (where M is at least one element selected from Group 4, Group 5, and Group 6, 0.8 ≤ a ≤ 1.2, b ≥ 0.80, 0 < c ≤ 0.15, 0 < d ≤ 0.15, 0 ≤ e ≤ 0.05, 1 ≤ f ≤ 2). The lithium transition metal composite oxide B may have the same composition as the lithium transition metal composite oxide A. In addition, the amount of Co in the positive electrode active material B is preferably equal to or more than the amount of Co in the positive electrode active material A.

[0036] The positive electrode active material B is Li x M y O z It is preferable that the lithium metal compound is composed of a lithium metal compound represented by the formula (wherein 1≦x≦4, 1≦y≦5, 1≦z≦12) and includes a surface layer formed on the surface of the secondary particles of the lithium transition metal composite oxide B. This surface layer is a layer corresponding to the first layer of the positive electrode active material A and may be formed to cover the entire surface of the secondary particles of the lithium transition metal composite oxide B, or it may be scattered on the particle surface. In the above general formula, M is at least one element selected from groups 4, 5, and 6, and preferably at least one selected from Ti, Nb, W, and Zr. Suitable lithium metal compounds are Li2TiO3 and Li4Ti5O 12 , LiTiO4, Li2Ti2O5, LiTiO2, Li3NbO4, LiNbO3, Li4Nb2O7, Li8Nb6O 19 These are Li2ZrO3, LiZrO2, Li4ZrO4, Li2WO4, and Li4WO5.

[0037] The content of the surface layer in positive electrode active material B is preferably lower than the content of the first layer in positive electrode active material A. The content of the surface layer is preferably 0.001 to 1.0 mol%, and more preferably 0.01 to 0.5 mol%, based on the element M in the above general formula, relative to the total number of moles of metal elements excluding Li in positive electrode active material B. The ratio of the content of the first layer in positive electrode active material B to the content of the first layer in positive electrode active material A is preferably 1.1 or higher.

[0038] The positive electrode active material B preferably further includes a second surface layer formed on the above surface layer. The second surface layer is a layer corresponding to the second layer of the positive electrode active material A and is composed of a boron compound. The second surface layer preferably covers the entire area of ​​the above surface layer (hereinafter referred to as the "first surface layer"). If the first surface layer is scattered on the particle surface of the lithium transition metal composite oxide B, a part of the second surface layer may be formed directly on the particle surface of the lithium transition metal composite oxide B.

[0039] The second surface layer is not formed between the secondary particle surface of lithium transition metal composite oxide B and the first surface layer, but only on the surface of the first surface layer facing away from lithium transition metal composite oxide A. In other words, the first surface layer is formed on the particle surface of lithium transition metal composite oxide B throughout its entire surface without going through the second surface layer.

[0040] The boron compound constituting the second surface layer may be any compound containing B and is not particularly limited, but it is preferably an oxide or lithium oxide. Examples of boron compounds include boron oxide (B2O3) and lithium borate (Li2B4O7). The content of the second surface layer in positive electrode active material B may be lower than the content of the second layer in positive electrode active material A. The content of the second layer is preferably 0.1 to 1.5 mol%, and more preferably 0.5 to 1.0 mol%, based on the boron element, relative to the total number of moles of metal elements excluding Li in positive electrode active material B.

[0041] The positive electrode active material B is manufactured, for example, by the following process. (1) Nickel-cobalt-manganese composite hydroxide is calcined at 400°C to 600°C to obtain nickel-cobalt-manganese composite oxide. (2) The composite oxide is mixed with a lithium compound such as lithium hydroxide and a compound containing a metal element selected from Group 4, Group 5, and Group 6 in a predetermined molar ratio, and an alkaline component such as potassium hydroxide is added at a predetermined concentration, and the mixture is fired in an oxygen atmosphere at a temperature of 650°C to 850°C to form Li on the particle surface of the lithium transition metal composite oxide. x M y O z A precursor is obtained to which a lithium metal compound (first surface layer) represented by [formula] is fixed. (3) The precursor and the boron compound are mixed in a predetermined molar ratio and calcined in an oxygen atmosphere at a temperature of 150°C to 400°C.

[0042] [Negative electrode] The negative electrode 30 comprises a negative electrode core and a negative electrode composite layer provided on the surface of the negative electrode core. The negative electrode core can be made of a metal foil that is stable in the potential range of the negative electrode 30, such as copper, or a film with the metal arranged on its surface. The negative electrode composite layer contains a negative electrode active material and a binder, and is preferably provided on both sides of the negative electrode core, excluding the portion to which the negative electrode lead 16 is connected. The negative electrode 30 can be manufactured, for example, by applying a negative electrode composite slurry containing a negative electrode active material and a binder to the surface of the negative electrode core, drying the coating, and then compressing it to form the negative electrode composite layer on both sides of the negative electrode core.

[0043] The negative electrode composite layer contains, as the negative electrode active material, a carbon-based active material that reversibly intercepts and releases lithium ions, for example. Suitable carbon-based active materials include natural graphite such as flake graphite, lumpy graphite, and clay-like graphite, as well as artificial graphite such as lumpy artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). In addition, a Si-based active material composed of at least one of Si and a Si-containing compound may be used as the negative electrode active material, and carbon-based active materials and Si-based active materials may be used in combination.

[0044] The binder included in the negative electrode composite layer may be fluororesin, PAN, polyimide, acrylic resin, polyolefin, etc., as in the case of the positive electrode 20, but it is preferable to use styrene-butadiene rubber (SBR). Furthermore, it is preferable that the negative electrode composite layer also contains CMC or its salt, polyacrylic acid (PAA) or its salt, polyvinyl alcohol (PVA), etc. Among these, it is preferable to use SBR in combination with CMC or its salt, or PAA or its salt.

[0045] [Separator] The separator 40 is made of a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include microporous thin films, woven fabrics, and nonwoven fabrics. Suitable materials for the separator 40 include polyethylene, polyolefins such as polypropylene, and cellulose. The separator 40 may have either a single-layer structure or a laminated structure. A heat-resistant layer or the like may be formed on the surface of the separator. [Examples]

[0046] The present disclosure will be further illustrated below with reference to examples, but the present disclosure is not limited to these examples.

[0047] <Example 1> [Synthesis of positive electrode active material A] Nickel-cobalt-manganese composite hydroxide obtained by coprecipitation was calcined at 500°C to obtain nickel-cobalt-manganese composite oxide. Next, this composite oxide, lithium hydroxide, and zirconium oxide (ZrO2) were mixed so that the total amount of Ni, Co, and Mn and the molar ratio of Li and Zr were 1:1.08:0.01. This mixture was calcined in an oxygen atmosphere at 800°C for 20 hours and then pulverized to obtain a positive electrode active material precursor. This precursor and boric acid (H3BO3) were mixed so that the total amount of Ni, Co, and Mn and the molar ratio of B were 1:0.01. This mixture was calcined in an oxygen atmosphere at 300°C for 3 hours to obtain positive electrode active material A in which the surface of the lithium metal compound (first layer) was covered with a boron compound (second layer).

[0048] According to ICP, the composition of positive electrode active material A is Li 1.03 Ni 0.85 Co 0.08 Mn 0.07 Zr 0.01 It was confirmed to be O2. The average primary particle diameter of positive electrode active material A was 800 nm, and the average particle diameter (average secondary particle diameter) was 12.1 μm.

[0049] [Fabrication of the positive electrode] A cathode composite slurry was prepared by mixing cathode active material A, acetylene black, and polyvinylidene fluoride (PVdF) in a mass ratio of 96.3:2.5:1.2, and using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. Next, the cathode composite slurry was applied to both sides of a cathode core made of aluminum foil, the coating was dried and compressed, and then cut to a predetermined electrode size to produce a cathode in which cathode composite layers were formed on both sides of the cathode core.

[0050] [Fabrication of the negative electrode] Natural graphite was used as the negative electrode active material. The negative electrode active material, sodium carboxymethylcellulose (CMC-Na), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 100:1:1, and water was used as the dispersion medium to prepare a negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to both sides of a negative electrode core made of copper foil, the coating was dried and compressed, and then cut to a predetermined electrode size to produce a negative electrode in which a negative electrode mixture layer was formed on both sides of the negative electrode core.

[0051] [Preparation of non-aqueous electrolyte solution] LiPF6 was dissolved at a concentration of 1 mol / L in a mixed solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 3:3:4. Furthermore, vinylene carbonate (VC) was dissolved at a concentration of 2% by mass in this mixed solvent to prepare a non-aqueous electrolyte.

[0052] [Battery construction] A positive electrode with an aluminum positive lead and a negative electrode with a nickel negative lead were wound in a spiral shape via a polyethylene separator and then flattened to create a wound electrode body. This electrode body was housed in an aluminum laminate casing, the non-aqueous electrolyte was injected, and the opening of the casing was sealed to create a 650mAh non-aqueous electrolyte secondary battery.

[0053] <Example 2> In the synthesis of the positive electrode active material A, titanium dioxide (TiO2) was used instead of ZrO2, and a nickel-cobalt-manganese composite oxide, lithium hydroxide, and titanium dioxide (TiO2) were mixed so that the total amount of Ni, Co, and Mn and the molar ratio of Li and Ti were 1:1.08:0.03. Otherwise, a non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1.

[0054] <Example 3> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that niobium oxide (Nb2O5) was used instead of ZrO2 in the synthesis of the positive electrode active material A.

[0055] <Example 4> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that tungsten oxide (WO3) was used instead of ZrO2 in the synthesis of the positive electrode active material A.

[0056] <Example 5> [Synthesis of positive electrode active material B] The nickel-cobalt-manganese composite hydroxide obtained by coprecipitation was calcined at 500°C to obtain a nickel-cobalt-manganese composite oxide. Next, this composite oxide, lithium hydroxide, and TiO2 were mixed so that the total amount of Ni, Co, and Mn and the molar ratio of Li to Ti were 1:1.08:0.03. Furthermore, 10% by mass of potassium hydroxide was added to this mixture, and after calcination in an oxygen atmosphere at 750°C for 40 hours, the mixture was crushed, washed with water, and dried to obtain positive electrode active material B.

[0057] The composition of positive electrode active material B is determined by ICP, Li 1.03 Ni 0.85 Co 0.08 Mn 0.07 Ti 0.03 It was confirmed to be O2. The average primary particle diameter of positive electrode active material B was 2 μm, and the average secondary particle diameter was 5 μm.

[0058] A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, except that, in the preparation of the positive electrode, a mixture of positive electrode active material A and positive electrode active material B in a mass ratio of 7:3 was used as the positive electrode active material.

[0059] <Example 6> A non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 4, except that in the synthesis of the positive electrode active material B, nickel-cobalt-manganese composite oxide, lithium hydroxide, and titanium oxide were mixed so that the total amount of Ni, Co, and Mn and the molar ratio of Li and Ti were 1:1.08:0.01.

[0060] <Example 7> [Synthesis of positive electrode active material B] Nickel-cobalt-manganese composite hydroxide obtained by coprecipitation was calcined at 500°C to obtain nickel-cobalt-manganese composite oxide. Next, this composite oxide, lithium hydroxide, and TiO2 were mixed so that the total amount of Ni, Co, and Mn and the molar ratio of Li and Ti were 1:1.08:0.01. Furthermore, 10% by mass of potassium hydroxide was added to this mixture, and after calcination in an oxygen atmosphere at 750°C for 40 hours, the mixture was crushed, washed with water, and dried to obtain a positive electrode active material precursor. This precursor and H3BO3 were mixed so that the total amount of Ni, Co, and Mn and the molar ratio of B were 1:0.01, and this mixture was calcined in an oxygen atmosphere at 300°C for 3 hours to obtain positive electrode active material B in which the surface of the lithium metal compound (first surface layer) was covered with a boron compound (second surface layer). The average primary particle diameter of the positive electrode active material B was 2 μm, and the average secondary particle diameter was 5 μm.

[0061] A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, except that, in the preparation of the positive electrode, a mixture of positive electrode active material A and positive electrode active material B in a mass ratio of 7:3 was used as the positive electrode active material.

[0062] <Comparative Example 1> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, except that TiO2 was not mixed into the positive electrode active material A, and H3BO3 was not mixed and subsequently calcined. The average primary particle diameter of the positive electrode active material A was 740 nm, and the average secondary particle diameter was 11.1 μm.

[0063] <Comparative Example 2> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, except that TiO2 was not mixed in the synthesis of the positive electrode active material A. The average primary particle diameter of the positive electrode active material A was 740 nm, and the average secondary particle diameter was 11.1 μm.

[0064] <Comparative Example 3> A non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 2, except that the mixing of H3BO3 and subsequent calcination were omitted in the synthesis of the positive electrode active material A. The average primary particle diameter of the positive electrode active material A was 740 nm, and the average secondary particle diameter was 12.1 μm.

[0065] <Comparative Example 4> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, except that in the synthesis of the positive electrode active material A, a nickel-cobalt-manganese composite hydroxide was synthesized such that the molar ratio of Ni, Co, and Mn was 0.82:0.12:0.06.

[0066] <Comparative Example 5> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, except that lithium nickel cobalt manganese composite oxide, TiO2, and H3BO3 were mixed and calcined in an oxygen atmosphere at 300°C for 3 hours in the synthesis of positive electrode active material A. The average primary particle diameter of positive electrode active material A was 700 nm, and the average secondary particle diameter was 11.8 μm.

[0067] <Comparative Example 6> In the synthesis of positive electrode active material A, nickel-cobalt-manganese composite oxide, lithium hydroxide, and H3BO3 were mixed such that the total amount of Ni, Co, and Mn and the molar ratio of Li and B were 1:1.08:0.01. This mixture was then calcined in an oxygen atmosphere at 300°C for 3 hours to obtain a positive electrode active material precursor in which a boron compound was fixed to the particle surface of lithium transition metal composite oxide. This precursor and titanium oxide were mixed such that the total amount of Ni, Co, and Mn and the molar ratio of Ti was 1:0.03. This mixture was then calcined in an oxygen atmosphere at 300°C for 3 hours to obtain positive electrode active material A. A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 2, except that the positive electrode was made using this positive electrode active material A.

[0068] <Comparative Example 7> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 3, except that tungsten oxide (WO3) was used instead of TiO2 in the synthesis of positive electrode active material A, and nickel-cobalt-manganese composite oxide, lithium hydroxide, and tungsten oxide (WO3) were mixed so that the total amount of Ni, Co, and Mn and the molar ratio of Li and W were 1:1.08:0.01.

[0069] <Comparative Example 8> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 4, except that tungsten oxide (WO3) was used instead of TiO2 in the synthesis of positive electrode active material A.

[0070] <Comparative Example 9> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 5, except that tungsten oxide (WO3) was used instead of TiO2 in the synthesis of positive electrode active material A.

[0071] <Comparative Example 10> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 6, except that tungsten oxide (WO3) was used instead of TiO2 in the synthesis of positive electrode active material A.

[0072] <Comparative Example 11> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 3, except that niobium oxide (Nb2O5) was used instead of TiO2 in the synthesis of positive electrode active material A.

[0073] <Comparative Example 12> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 4, except that niobium oxide (Nb2O5) was used instead of TiO2 in the synthesis of positive electrode active material A.

[0074] <Comparative Example 13> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 5, except that niobium oxide (Nb2O5) was used instead of TiO2 in the synthesis of positive electrode active material A.

[0075] <Comparative Example 14> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 6, except that niobium oxide (Nb2O5) was used instead of TiO2 in the synthesis of positive electrode active material A.

[0076] <Comparative Example 15> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 3, except that zirconium oxide (ZrO2) was used instead of TiO2 in the synthesis of positive electrode active material A.

[0077] <Comparative Example 16> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 4, except that zirconium oxide (ZrO2) was used instead of TiO2 in the synthesis of positive electrode active material A.

[0078] <Comparative Example 17> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 5, except that zirconium oxide (ZrO2) was used instead of TiO2 in the synthesis of positive electrode active material A.

[0079] <Comparative Example 18> A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 6, except that zirconium oxide (ZrO2) was used instead of TiO2 in the synthesis of positive electrode active material A.

[0080] <Comparative Example 19> In the synthesis of positive electrode active material A, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that nickel-cobalt-manganese composite oxide, lithium hydroxide, and titanium oxide (TiO2) were mixed so that the total amount of Ni, Co, and Mn and the molar ratio of Li and Ti were 1:1.08:0.1. XRD measurements of positive electrode active material A confirmed that Li2TiO3 was attached to the particle surface of the lithium transition metal composite oxide.

[0081] <Comparative Example 20> In the synthesis of positive electrode active material A, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that nickel-cobalt-manganese composite oxide, lithium hydroxide, and niobium oxide (NbO2) were mixed so that the total amount of Ni, Co, and Mn and the molar ratio of Li to Nb were 1:1.08:0.1. XRD measurements of positive electrode active material A confirmed that Li3NiO4 was attached to the particle surface of the lithium transition metal composite oxide.

[0082] <Comparative Example 21> In the synthesis of positive electrode active material A, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that nickel-cobalt-manganese composite oxide, lithium hydroxide, and zirconium oxide (ZrO2) were mixed so that the total amount of Ni, Co, and Mn and the molar ratio of Li to Zr were 1:1.08:0.1. XRD measurements of positive electrode active material A confirmed that Li2ZrO3 was attached to the particle surface of the lithium transition metal composite oxide.

[0083] [Evaluation of resistance increase rate after high-temperature cycle testing] For each battery in the examples and comparative examples, charging was performed at a constant current of 0.5 It at a temperature of 25°C until it reached half of its initial capacity. Charging was then stopped and the battery was left for 15 minutes. Afterward, it was charged again at a constant current of 0.1 It for 10 seconds, the voltage was measured, and then the battery was discharged to the equivalent of the 10-second charge. This charging / discharging and voltage measurement process was repeated with current values ​​ranging from 0.1 It to 2 It. The resistance value was determined from the relationship between the measured voltage and current values ​​and used as the resistance value before the cycle test.

[0084] A cycle test was conducted under the following conditions, and the resistance value after 150 cycles was determined using the method described above. The rate of increase in resistance after 150 cycles relative to the resistance value before the cycle test was then calculated. The evaluation results are shown in Table 1 as relative values, with the rate of increase for the battery in Example 1 set to 100.

[0085] (Cycle testing) Each battery was charged with a constant current of 0.5It at a temperature of 60°C until the battery voltage reached 4.2V, and then charged with a constant voltage until the current was reduced to 1 / 50It. After that, the batteries were discharged with a constant current of 0.5It until the battery voltage reached 2.5V. This charge-discharge cycle was repeated 150 times.

[0086] [Table 1]

[0087] As shown in Table 1, all of the batteries in the examples showed a lower resistance increase rate after high-temperature cycling tests compared to the batteries in the comparative examples. Furthermore, when positive electrode active materials A and B were used in combination (see Examples 4-6), the increase in resistance could be further suppressed. On the other hand, when at least one of the first and second layers was absent on the particle surface of the lithium transition metal composite oxide (Comparative Examples 1-3, 7, 11, 15), when the particle / first layer / second layer configuration was absent (Comparative Examples 5, 6, 9, 10, 13, 14, 17, 18), and when the lithium transition metal composite oxide did not have a predetermined composition (Comparative Examples 4, 8, 12, 16), the battery resistance increased significantly after high-temperature cycling tests. [Explanation of Symbols]

[0088] 10 Nonaqueous electrolyte secondary battery 11 Exterior 12 Storage Unit 13 Sealing part 14 Electrode body 15 Positive lead 16 Negative lead 20 positive electrode 21 Positive Tab 22 Positive electrode tab stacked section 30 negative electrode 31 Negative electrode tab 32 Negative electrode tab laminated section 40 Separators

Claims

1. A non-aqueous electrolyte secondary battery comprising an electrode body including a positive electrode, a negative electrode, and a separator, and a non-aqueous electrolyte, The positive electrode comprises positive electrode active material A and positive electrode active material B. The positive electrode active materials A and B are secondary particles formed by the aggregation of primary particles, The average primary particle diameter of the positive electrode active material B is 0.5 μm or larger, and is greater than the average primary particle diameter of the positive electrode active material A. The average secondary particle diameter of the positive electrode active material B is 2 μm to 7 μm, and is smaller than the average secondary particle diameter of the positive electrode active material A. The positive electrode active material A is A lithium transition metal composite oxide A contains Ni, Co, and Mn, and also contains a metal M selected from Ti, Nb, W, and Zr, wherein the content of Ni is 82 mol% or more and the content of Co is 8 mol% or less relative to the total number of moles of metal elements excluding Li, A first layer formed on the particle surface of the lithium transition metal composite oxide A, which is composed of a lithium metal oxide containing the aforementioned metal M, A second layer formed on the first layer, composed of a boron compound, Includes, A non-aqueous electrolyte secondary battery, wherein the first layer is formed on the particle surface of the lithium transition metal composite oxide A over its entire surface without intervening through the second layer.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the second layer covers the entire area of ​​the first layer.

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the positive electrode active material B contains Ni, Co, and Mn, and also contains a metal M consisting of at least one selected from Ti, Nb, W, and Zr, and comprises a lithium transition metal composite oxide B in which the Ni content is 80 mol% or more relative to the total number of moles of metal elements excluding Li.

4. The positive electrode active material B includes a surface layer formed on the surface of the secondary particles of the lithium transition metal composite oxide B. The surface layer is composed of a lithium metal oxide containing the metal M. The non-aqueous electrolyte secondary battery according to claim 3, wherein the content of the surface layer in the positive electrode active material B is lower than the content of the first layer in the positive electrode active material A.

5. The positive electrode active material B includes a second surface layer formed on the surface layer, The non-aqueous electrolyte secondary battery according to claim 4, wherein the second surface layer is composed of a boron compound.

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