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

Abstract

The positive electrode for a non-aqueous electrolyte secondary battery includes a positive electrode active material and a conductive material. The positive electrode active material contains lithium composite oxide particles containing 80 mol% or more and less than 100 mol% of Ni relative to the total molar amount of metal elements other than Li. The lithium composite oxide particles include particles having a stepped structure with a plane having an outer edge stacked in 3 or more layers and a length of 1 μm or more. The average particle diameter of the conductive material is 30 nm or less.

Description

Technical Field

[0001] The present invention relates to the technology of non-aqueous electrolyte secondary batteries. Background Art

[0002] In recent years, as secondary batteries with high output and high energy density, non-aqueous electrolyte secondary batteries having a positive electrode, a negative electrode, and a non-aqueous electrolyte, and allowing lithium ions or the like to move between the positive electrode and the negative electrode for charge and discharge have been widely used.

[0003] As the positive electrode active material used in the positive electrode of the non-aqueous electrolyte secondary battery, for example, the following substances are known.

[0004] For example, Patent Document 1 discloses a positive electrode active material represented by the compositional formula: Li x Ni 1-y M y O 2+α (In the above formula, M is one or more selected from Sc, Ti, V, Cr, Mn, Fe, Co, Cu, Zn, Ga, Ge, Al, Bi, Sn, Mg, Ca, B, and Zr, 0.9 ≤ x ≤ 1.2, 0 < y ≤ 0.7, α ≥ 0.) When observing the cross section of the particles by SIM (scanning ion microscope) image, the average grain diameter is 1.2 to 5.0 μm.

[0005] In addition, for example, Patent Document 2 discloses a positive electrode active material represented by the compositional formula: Li x Ni 1-y M y O 2+α (In the above formula, M is one or more selected from Sc, Ti, V, Cr, Mn, Fe, Co, Cu, Zn, Ga, Ge, Bi, Sn, Mg, Ca, B, and Zr, 0.9 ≤ x ≤ 1.2, 0 < y ≤ 0.7, α > 0.1.) The average particle diameter D50 of the primary particles is 1.6 to 2.3 μm, the alkali amount on the particle surface measured by two-stage neutralization titration is 1.2 mass% or less, and when lithium hydroxide is set as A mass% and lithium carbonate is set as B mass% among the alkali amount on the particle surface, A / B is 1 or less.

[0006] Prior Art Documents

[0007] Patent Documents

[0008] Patent Document 1: Japanese Patent No. 5876739

[0009] Patent Document 2: Japanese Patent No. 6030546 Summary of the Invention

[0010] However, when a lithium composite oxide in which the proportion of Ni is 80 mol% or more and less than 100 mol% relative to the total number of moles of metal elements other than Li is used as a positive electrode active material, high capacity of a non-aqueous electrolyte secondary battery can be achieved. On the other hand, there is a problem of deterioration of charge-discharge cycle characteristics.

[0011] Therefore, an object of the present disclosure is to suppress deterioration of charge-discharge cycle characteristics of a non-aqueous electrolyte secondary battery when a lithium composite oxide in which the proportion of Ni is 80 mol% or more and less than 100 mol% relative to the total number of moles of metal elements other than Li is used as a positive electrode active material of the non-aqueous electrolyte secondary battery.

[0012] One aspect of the present disclosure is a positive electrode for a non-aqueous electrolyte secondary battery, which includes a positive electrode active material and a conductive material. The positive electrode active material includes: lithium composite oxide particles containing 80 mol% or more and less than 100 mol% of Ni relative to the total number of moles of metal elements other than Li. The lithium composite oxide particles include: particles having a stepped structure with a plane having an outer edge laminated with 3 or more layers and a length of 1 μm or more. The average particle diameter of the conductive material is 30 nm or less.

[0013] According to one aspect of the present disclosure, when a lithium composite oxide in which the proportion of Ni is 80 mol% or more and less than 100 mol% relative to the total number of moles of metal elements other than Li is used as a positive electrode active material of a non-aqueous electrolyte secondary battery, deterioration of charge-discharge cycle characteristics of the non-aqueous electrolyte secondary battery can be suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery as an example of an embodiment.

[0015] Figure 2 is a schematic diagram showing an example of the shape of lithium composite oxide particles.

[0016] Figure 3 is a backscattered electron compositional image (3000 times) of an SEM of a positive electrode of an example. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] The present inventors proposed in a prior application (Japanese Patent Application No. 2018-071818) that by forming lithium composite oxide particles containing 80 mol% or more and less than 100 mol% of Ni relative to the total number of moles of metal elements other than Li into a stepped structure with a plane having an outer edge laminated with 3 or more layers and a length of 1 μm or more, deterioration of charge-discharge cycle characteristics of a non-aqueous electrolyte secondary battery can be suppressed.

[0018] The inventors of the present invention conducted further research and found that: by using a positive electrode comprising lithium composite oxide particles having the above-described stepped structure and a conductive material with an average particle size of 30 nm or less, it is possible to further suppress a decrease in the charge-discharge cycle characteristics of a non-aqueous electrolyte secondary battery. As one of the reasons for exerting the above effects, it is speculated as follows. By dispersing lithium composite oxide particles having the above-described stepped structure and a conductive material with an average particle size of 30 nm or less in the positive electrode active material layer constituting the positive electrode, a good conductive network is formed in the positive electrode active material layer. As a result, the insertion and extraction reaction of lithium ions in the lithium composite oxide particles having the above-described stepped structure can proceed smoothly, thereby suppressing a decrease in the charge-discharge cycle characteristics.

[0019] Hereinafter, an example of a non-aqueous electrolyte secondary battery as one aspect of the present disclosure will be described.

[0020] Figure 1 It is a cross-sectional view of a non-aqueous electrolyte secondary battery as an example of an embodiment. Figure 1 The non-aqueous electrolyte secondary battery 10 shown includes: a wound electrode body 14 formed by winding a positive electrode 11 and a negative electrode 12 with a separator 13 interposed therebetween, a non-aqueous electrolyte, insulating plates 18 and 19 disposed above and below the electrode body 14, respectively, and a battery case 15 that houses the above components. The battery case 15 is composed of a bottomed cylindrical case body 16 and a sealing body 17 that closes the opening of the case body 16. It should be noted that other forms of electrode bodies, such as a laminated electrode body in which the positive electrode and the negative electrode are alternately laminated with a separator interposed therebetween, can also be applied instead of the wound electrode body 14. In addition, as the battery case 15, examples include metal cases such as cylindrical, square, coin-shaped, and button-shaped cases, and resin cases (laminated batteries) formed by laminating resin sheets.

[0021] The case body 16 is, for example, a bottomed cylindrical metal container. A gasket 28 is provided between the case body 16 and the sealing body 17 to ensure the airtightness inside the battery. The case body 16 has, for example, a protruding portion 22 in which a part of the side surface protrudes inward and supports the sealing body 17. The protruding portion 22 is preferably formed in a ring shape along the circumferential direction of the case body 16 and supports the sealing body 17 with its upper surface.

[0022] The sealing body 17 has a structure in which a partially open metal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a lid 27 are laminated in sequence from the side of the electrode body 14. Each member constituting the sealing body 17 has, for example, a disc shape or an annular shape, and the members other than the insulating member 25 are electrically connected to each other. The lower valve body 24 and the upper valve body 26 are connected to each other at their respective central portions, and the insulating member 25 is interposed between their respective peripheral portions. When the internal pressure of the non-aqueous electrolyte secondary battery 10 rises due to heat generation caused by internal short circuit or the like, for example, the lower valve body 24 deforms and breaks in such a way as to push the upper valve body 26 toward the lid 27 side, and the current path between the lower valve body 24 and the upper valve body 26 is cut off. Further, when the internal pressure rises, the upper valve body 26 breaks, and gas is discharged from the opening portion of the lid 27.

[0023] Figure 1 In the non-aqueous electrolyte secondary battery 10 shown, the positive electrode lead 20 mounted on the positive electrode 11 passes through the through hole of the insulating plate 18 and extends toward the sealing body 17 side, and the negative electrode lead 21 mounted on the negative electrode 12 passes outside the insulating plate 19 and extends toward the bottom side of the battery case main body 16. The positive electrode lead 20 is connected to the lower surface of the partially open metal plate 23 which is the bottom plate of the sealing body 17 by welding or the like, and the lid 27 which is the top plate of the sealing body 17 electrically connected to the partially open metal plate 23 is the positive electrode terminal. The negative electrode lead 21 is connected to the inner surface of the bottom of the battery case main body 16 by welding or the like, and the battery case main body 16 is the negative electrode terminal.

[0024] The positive electrode 11, the negative electrode 12, the non-aqueous electrolyte, and the separator 13 will be described in detail below.

[0025] <Positive Electrode>

[0026] The positive electrode 11 is composed of, for example, a positive electrode current collector such as a metal foil and a positive electrode active material layer formed on the positive electrode current collector. As the positive electrode current collector, a foil of a metal stable in the potential range of the positive electrode such as aluminum, a thin film having the metal disposed on the surface layer, or the like can be used. The positive electrode active material layer contains a positive electrode active material and a conductive material. From the viewpoint of adhesiveness to the positive electrode current collector and the like, the positive electrode active material layer preferably contains a binder material and the like.

[0027] The positive electrode 11 can be obtained, for example, by coating / drying a positive electrode composite paste containing a positive electrode active material, a binder material, a conductive material, etc. on the positive electrode current collector, thereby forming a positive electrode active material layer on the positive electrode current collector, and rolling the positive electrode active material layer.

[0028] The positive electrode active material contains lithium composite oxide particles containing 80 mol% or more and less than 100 mol% of Ni relative to the total number of moles of metal elements other than Li. The lithium composite oxide particles may also contain other elements in addition to Ni and Li, and examples thereof include at least one element selected from Al, Co, Mn, Ti, Nb, Si, Mo, Zr, V, Fe, Mg, Cr, Cu, Sn, Ta, W, Na, K, Ba, Sr, Bi, Be, Zn, Ca, and B. Among these, from the viewpoint of suppressing the deterioration of charge-discharge cycle characteristics and the like, at least one element selected from Al, Mn, and Co is preferred.

[0029] Figure 2 (A) and (B) of are schematic views showing an example of the shape of the lithium composite oxide particles. The lithium composite oxide particles containing 80 mol% or more and less than 100 mol% of Ni relative to the total number of moles of metal elements other than Li are as Figure 2 shown in (A) and (B) of

[0029] , and include particles 32 having a stepped structure in which 3 or more planes 30 with an outer edge length (A) of 1 μm or more are stacked. It should be noted that the lithium composite oxide particles containing 80 mol% or more and less than 100 mol% of Ni relative to the total number of moles of metal elements other than Li may be composed only of particles 32, or may be composed of particles 32 and particles having other known shapes in addition to particles 32.

[0030] The outer edge length (A) of the plane 30 may be 1 μm or more, and is preferably 2 μm or more, for example. The upper limit of the outer edge length (A) also depends on the particle size, and is 15 μm or less, for example. The shape of the outer edge of the plane 30 is not particularly limited to a polygonal shape, a curved shape, or the like. The number of stacked planes 30 may be 3 or more, and from the viewpoint of further suppressing the deterioration of charge-discharge cycle characteristics, 5 or more is preferred. The upper limit of the number of stacked planes 30 also depends on the particle size, and is 15 or less, for example. From the viewpoint of further suppressing the deterioration of charge-discharge cycle characteristics, the height difference (height (B) from one plane 30 to the plane 30 on one of its layers) of the stepped structure is preferably in the range of 0.01 μm or more and 1 μm or less, and more preferably in the range of 0.03 μm or more and 0.2 μm or less, for example.

[0031] The proportion of the particles 32 in the total lithium composite oxide particles is preferably 3% or more, and more preferably 5% or more, in the lithium composite oxide particles containing 80 mol% or more and less than 100 mol% of Ni relative to the total number of moles of metal elements other than Li. When the proportion of the particles 32 satisfies the above range, the deterioration of charge-discharge cycle characteristics is more suppressed than in the case where the above range is not satisfied.

[0032] The ratio of the particles 32 is determined by the following method. Using an electron microscope, randomly observe 20 fields of view at a magnification that can sufficiently observe the surface of the lithium composite oxide particles. Measure the total number of particles (M) observed in the field of view and the number of particles 32 (N). This measurement is performed for 20 fields of view, and the ratio (N / M) of the number of particles 32 (N) to the total number of particles (M) is obtained. The average value of this value is set as the ratio of the particles 32.

[0033] From the viewpoints of charge-discharge cycle characteristics and the like, the particle diameter of the particles 32 is preferably 1.5 μm or more, more preferably 3 μm or more, for example. The upper limit of the particle diameter of the particles 32 is preferably 20 μm or less, more preferably 15 μm or less, for example. When the particle diameter of the particles 32 is too large, the battery capacity may sometimes decrease. For the particle diameter of the particles 32, using an electron microscope, randomly specify 20 particles 32, perform image analysis on the specified particles 32, respectively obtain the longest diameters of the 20 particles 32, and set their average value as the particle diameter of the particles 32.

[0034] The particle fracture strength of the particles 32 is preferably 230 MPa or more, more preferably 300 MPa or more, for example. When the particle fracture strength of the particles 32 satisfies the above range, compared with the case where it does not satisfy the above range, cracks in the particles 32 caused by charge and discharge are suppressed, and sometimes a decrease in charge-discharge cycle characteristics is more suppressed. The upper limit value of the particle fracture strength of the particles 32 is not particularly limited, and is preferably 1000 MPa or less, for example. The particle fracture strength is measured by the method specified in JIS-R1639-5. However, in JIS-R1639-5, α (a dimensionless number that changes according to the position inside the particle) is set to 2.48, and in the present application, it is set to 2.8.

[0035] As the content of the lithium composite oxide particles containing 80 mol% or more and less than 100 mol% of Ni relative to the total molar amount of metal elements other than Li, from the viewpoints of improving the battery capacity and the like, it is preferably 90 mass% or more, preferably 99 mass% or more, relative to the total mass of the positive electrode active material.

[0036] In addition to the lithium composite oxide particles containing 80 mol% or more and less than 100 mol% of Ni relative to the total molar amount of metal elements other than Li, the positive electrode active material may also contain other lithium composite oxide particles. As the other lithium composite oxide particles, for example, lithium composite oxide particles in which the proportion of Ni is less than 80 mol% can be cited.

[0037] Lithium composite oxide particles containing 80 mol% or more and less than 100 mol% of Ni relative to the total molar amount of metal elements other than Li can be obtained by mixing a Ni-containing oxide and a Li compound and firing the mixture. Here, particles 32 having a stepped structure of a plane 30 with an outer edge length (A) of 1 μm or more and laminated with 3 or more layers can be obtained, for example, by adding an alkali or alkaline earth compound having a melting point of 600°C or lower, such as potassium hydroxide, to the mixture of the Ni-containing oxide and the Li compound, or by subjecting the mixture to two-stage firing. The conditions for the two-stage firing are appropriately set according to the composition of the Ni-containing oxide, the mixing ratio of the Ni-containing oxide and the Li compound, the presence or absence of addition of potassium hydroxide, etc. For example, it is preferable that the firing temperature in the second stage is lower than the firing temperature in the first stage. The firing temperature in the first stage is, for example, in the range of 700°C to 1000°C, and the firing temperature in the second stage is, for example, in the range of 600°C to 800°C. In addition, the firing time in the first stage and the second stage is, for example, 1 to 10 hours each. It should be noted that when growing the particles of the positive electrode active material, the particle diameter of the added potassium hydroxide is preferably 3 to 20 mm. In addition, potassium hydroxide reacts with carbon dioxide in the atmosphere to become potassium carbonate, so it is preferably added at the time of mixing the Ni-containing oxide and the Li compound.

[0038] As the conductive material contained in the positive electrode active material layer, materials having higher conductivity than the positive electrode active material are, for example, carbon powders such as carbon black, acetylene black, Ketjen black, and graphite. These can be used alone or in combination of two or more.

[0039] The average particle diameter of the conductive material can be 30 nm or less. From the viewpoint of further suppressing the deterioration of charge-discharge cycle characteristics and the like, it is preferably 25 nm or less, and more preferably 23 nm or less. From the viewpoint of processability and the like, the lower limit value of the average particle diameter of the conductive material is, for example, preferably 1 nm or more, and more preferably 5 nm or more. The average particle diameter of the conductive material is obtained as follows. Using an electron microscope, 20 conductive materials are randomly specified, image analysis is performed on the specified conductive materials, the longest diameters of the 20 conductive materials are respectively obtained, and their average value is calculated.

[0040] From the viewpoint of further suppressing the deterioration of charge-discharge cycle characteristics and the like, the BET specific surface area of the conductive material is, for example, preferably 80 m 2 / g or more, and more preferably 100 m 2 / g or more and 300 m 2 / g or less, and more preferably 100 m 2 / g or more and 250 m 2 / g or less. The BET specific surface area is measured according to the BET method (nitrogen adsorption method) described in JIS R1626.

[0041] From the aspect of further suppressing the reduction of charge-discharge cycle characteristics, etc., for example, relative to 100 parts by mass of the positive electrode active material, the content of the conductive material is preferably 0.1% by mass or more and 5% by mass or less, more preferably 0.2% by mass or more and 3% by mass or less.

[0042] Examples of the binder material contained in the positive electrode active material layer include fluorine-based polymers, rubber-based polymers, PAN, polyimide-based resins, acrylic resins, polyolefin resins, etc. Examples of the fluorine-based polymer include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), or modified products thereof, etc. Examples of the rubber-based polymer include ethylene-propylene-isoprene copolymer, ethylene-propylene-butadiene copolymer, etc. These can be used alone or in combination of two or more.

[0043] <Negative electrode>

[0044] The negative electrode 12 includes, for example, a negative electrode current collector such as a metal foil and a negative electrode active material layer formed on the negative electrode current collector. As the negative electrode current collector, a foil of a metal stable in the potential range of the negative electrode such as copper or a thin film having the metal disposed on the surface can be used. The negative electrode active material layer contains, for example, a negative electrode active material, a binder material, a thickening material, etc.

[0045] The negative electrode 12 can be obtained, for example, by coating / drying a negative electrode composite slurry containing a negative electrode active material and a binder material on the negative electrode current collector, thereby forming a negative electrode active material layer on the negative electrode current collector and rolling the negative electrode active material layer.

[0046] The negative electrode active material contained in the negative electrode active material layer is not particularly limited as long as it is a material capable of storing / releasing lithium ions, and examples thereof include carbon materials, metals capable of forming an alloy with lithium, or alloy compounds containing the metal. As the carbon material, graphite-based materials such as natural graphite, non-graphitizable carbon, artificial graphite, and coke-based materials can be used. As the alloy compound, a compound containing at least one metal capable of forming an alloy with lithium can be cited. As the element capable of forming an alloy with lithium, silicon and tin are preferred, and silicon oxides, tin oxides, etc. formed by their combination with oxygen can also be used. In addition, a substance obtained by mixing the above carbon material with a compound of silicon and tin can be used. In addition to the above, a material having a charge-discharge potential higher than that of the carbon material relative to metallic lithium, such as lithium titanate, can also be used.

[0047] As the binder material contained in the negative electrode active material layer, for example, the same as in the case of the positive electrode, fluorine-based polymers, rubber-based polymers, PAN, polyimide-based resins, acrylic resins, polyolefin-based resins, etc. can be used. It should be noted that when preparing the negative electrode composite slurry using an aqueous solvent, etc., styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts (PAA-Na, PAA-K, etc., and partial neutralized salts can also be used), polyvinyl alcohol (PVA), polyethylene oxide (PEO), etc. are preferably used.

[0048] <Non-aqueous electrolyte>

[0049] The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. The non-aqueous electrolyte is not limited to a liquid electrolyte (non-aqueous electrolyte solution), and can also be a solid electrolyte using a gel-like polymer, etc. As the non-aqueous solvent, for example, esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these can be used. The non-aqueous solvent may also contain halogenated substituents in which at least a part of the hydrogen of these solvents is substituted by halogen atoms such as fluorine.

[0050] Examples of the above esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, cyclic carboxylic acid esters such as γ-butyrolactone (GBL), γ-valerolactone (GVL), chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate, γ-butyrolactone, etc.

[0051] Examples of the above ethers include cyclic ethers such as 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-epoxybutane, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ethers, chain ethers such as 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, etc.

[0052] As the above-mentioned halogenated compound, fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, fluorinated chain carboxylates such as methyl fluoropropionate (FMP), etc. are preferably used.

[0053] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt include LiBF 4 , LiClO 4 , LiPF 6 , LiAsF 6 , LiSbF 6 , LiAlCl 4 , LiSCN, LiCF 3 SO 3 , LiCF 3 CO 2 , Li(P(C 2 O 4 )F 4 ), LiPF 6-x (C n F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, Li 2 B 4 O 7 , Li(B(C 2 O 4 )F 2 ) and other borate salts, LiN(SO 2 CF 3 ) 2 , LiN(C 1 F 2l+1 SO 2 )(C m F 2m+1 SO 2 ) {l, m are integers of 0 or more} and other imide salts, etc. For the lithium salt, one kind can be used alone or multiple kinds can be used in combination. Among these, from the viewpoints of ionic conductivity, electrochemical stability, etc., LiPF 6 is preferably used. The concentration of the lithium salt is preferably set to 0.8 to 1.8 mol per 1 L of non-aqueous solvent.

[0054] <Separator>

[0055] The separator 13 can be, for example, a porous sheet having ion permeability and insulation. Specific examples of the porous sheet include microporous films, woven fabrics, non-woven fabrics, etc. As the material of the separator, olefin resins such as polyethylene and polypropylene, cellulose, etc. are suitable.

[0056] Example

[0057] Hereinafter, the present invention will be further described using examples, but the present invention is not limited to these examples.

[0058] <Example 1>

[0059] [Production of Ni-containing lithium composite oxide particles]

[0060] The [Ni 0.88 Co 0.09 Al 0.03 (OH) 2 shown composite hydroxide was calcined at 500 °C for 10 hours to obtain a composite oxide containing Ni, Co and Al. The average particle size (D50) of this composite oxide was 12 μm. Then, LiOH and the composite oxide containing Ni, Co and Al were mixed so that the molar ratio of Li to the total amount of Ni, Co and Al became 1.03:1, and then KOH in an amount of 10% by mass based on this mixture was added. After calcining this mixture at 750 °C for 40 hours in a 90% oxygen gas stream, impurities were removed by washing with water to obtain Ni-containing lithium composite oxide particles.

[0061] [Production of positive electrode]

[0062] The above-mentioned Ni-containing lithium composite oxide particles as the positive electrode active material were mixed to be 100% by mass, acetylene black (average particle size 23 nm) as the conductive material was 1% by mass, and polyvinylidene fluoride as the binder material was 0.9% by mass. After that, N-methyl-2-pyrrolidone was added in an appropriate amount to prepare a positive electrode composite material slurry. Then, the positive electrode composite material slurry was coated on both sides of an aluminum positive electrode current collector with a thickness of 15 μm, and the coating film was rolled to form a positive electrode active material layer with a thickness of 70 μm on both sides of the positive electrode current collector. This was used as the positive electrode of the example.

[0063] Figure 3 is a backscattered electron compositional image (3000 times) of the SEM of the positive electrode obtained in the example. As Figure 3 shown, the Ni-containing lithium composite oxide particles obtained in the example contain particles having a stepped structure with a plane having a length of 1 μm or more and having three or more stacked outer peripheries. In addition, the proportion of the particles having this stepped structure in the total Ni-containing lithium composite oxide particles obtained in the example was 90%. The measurement method was as described above.

[0064] [Production of negative electrode]

[0065] The graphite as the negative electrode active material was mixed to be 100% by mass, and styrene-butadiene copolymer (SBR) as the binder material was mixed to be 1% by mass. Then, an appropriate amount of water was added to prepare a negative electrode composite material slurry. Next, the negative electrode composite material slurry was coated on both sides of a copper negative electrode current collector with a thickness of 10 μm, and the coating film was rolled to form a negative electrode active material layer with a thickness of 100 μm on both sides of the negative electrode current collector. This was used as the negative electrode.

[0066] [Preparation of electrolyte solution]

[0067] In a mixed solvent in which fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed at a volume ratio of 15:45:40, lithium hexafluorophosphate (LiPF 6 ) was dissolved to a concentration of 1.3 mol / L to prepare an electrolyte solution.

[0068] [Fabrication of non-aqueous electrolyte secondary battery]

[0069] The above positive electrode and the above negative electrode were each cut into a specified size. After installing the electrode sheets, a separator was inserted between these electrodes and wound to fabricate a wound electrode body. The wound electrode body was housed in a Ni-plated stainless steel case body (diameter 18 mm, height 65 mm) with insulating plates arranged above and below the electrode body. The negative electrode sheet was welded to the bottom of the case body, and the positive electrode sheet was welded to the sealing body. Then, the above electrolyte solution was injected into the case body, and the case body was sealed with the sealing body to fabricate a non-aqueous electrolyte secondary battery.

[0070] [Example 2]

[0071] The composite hydroxide shown as [Ni 0.91 Co 0.045 Al 0.045 (OH) 2 obtained by the coprecipitation method was used. Except for this, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in the example (Example 1).

[0072] [Comparative Example 1]

[0073] In the fabrication of the positive electrode, acetylene black with an average particle size of 35 nm was used as the conductive material. Except for this, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in the example.

[0074] [Comparative Example 2]

[0075] In the production of Ni-containing lithium composite oxide particles, KOH was not added. Except for this, a non-aqueous electrolyte secondary battery was produced in the same manner as in the examples. It should be noted that the Ni-containing lithium composite oxide particles obtained in Comparative Example 2 were confirmed by an electron microscope, and as a result, particles having a stepped structure as in the examples were not confirmed.

[0076] <Comparative Example 3>

[0077] In the production of Ni-containing lithium composite oxide particles, KOH was not added. In the production of the positive electrode, acetylene black with an average particle size of 35 nm was used as the conductive material. Except for this, a non-aqueous electrolyte secondary battery was produced in the same manner as in the examples. It should be noted that the Ni-containing lithium composite oxide particles obtained in Comparative Example 3 were confirmed by an electron microscope, and as a result, particles having a stepped structure as in the examples were not confirmed.

[0078] [Measurement of capacity retention rate in charge-discharge cycle characteristics]

[0079] At an ambient temperature of 45 °C, the non-aqueous electrolyte secondary batteries of the examples and each comparative example were charged at a constant current of 0.5C until the battery voltage reached 4.15V, and then discharged at a constant current of 0.5C until the battery voltage reached 3.0V. This charge-discharge cycle was performed 100 times, and the capacity retention rate in the charge-discharge cycle of the non-aqueous electrolyte secondary batteries of the examples and each comparative example was calculated using the following formula. It should be noted that the higher the capacity retention rate, the more the decrease in charge-discharge cycle characteristics is suppressed.

[0080] Capacity retention rate = (discharge capacity of the 100th cycle / discharge capacity of the 1st cycle) × 100

[0081] [Table 1]

[0082]

[0083] As shown in Table 1, the capacity retention rate of the non-aqueous electrolyte secondary battery of Example 1 was 98.8%, the capacity retention rate of the non-aqueous electrolyte secondary battery of Example 2 was 98.2%, the capacity retention rate of the non-aqueous electrolyte secondary battery of Comparative Example 1 was 95.7%, the capacity retention rate of the non-aqueous electrolyte secondary battery of Comparative Example 2 was 95.3%, and the capacity retention rate of the non-aqueous electrolyte secondary battery of Comparative Example 3 was 95.4%. That is, it can be considered that by using Ni-containing lithium composite oxide particles containing particles having a stepped structure with a plane having an outer edge laminated with 3 or more layers and a length of 1 μm or more, and a positive electrode having a conductive material with an average particle size of 30 nm or less, the decrease in charge-discharge cycle characteristics of the non-aqueous electrolyte secondary battery can be suppressed.

[0084] Description of the reference numerals

[0085] 10 Non-aqueous electrolyte secondary battery

[0086] 11 Positive electrode

[0087] 12 Negative electrode

[0088] 13 Separator

[0089] 14 Electrode body

[0090] 15 Battery case

[0091] 16 Case body

[0092] 17 Sealing body

[0093] 18, 19 Insulating plate

[0094] 20 Positive electrode lead

[0095] 21 Negative electrode lead

[0096] 22 Protrusion

[0097] 23 Locally opened metal plate

[0098] 24 Lower valve body

[0099] 25 Insulating member

[0100] 26 Upper valve body

[0101] 27 Cover

[0102] 28 Gasket

Claims

1. A positive electrode for a non-aqueous electrolyte secondary battery, which includes a positive electrode active material and a conductive material, and the positive electrode active material includes: lithium-containing composite oxide particles containing 80 mol% or more and less than 100 mol% of Ni relative to the total molar amount of metal elements other than Li, the lithium-containing composite oxide particles include: particles having a stepped structure with a plane having an outer edge stacked with 3 or more layers and a length of 2 μm or more, the particle size of the particles having the stepped structure is 3 μm or more, the average particle size of the conductive material is 30 nm or less, The BET specific surface area of the conductive material is 80 m 2 / g or more and 300 m 2 / g or less, the content of the conductive material is 0.1 mass% or more and 3 mass% or less relative to 100 mass parts of the positive electrode active material.

2. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein, the height difference of the stepped structure is 0.01 μm or more and 1 μm or less.

3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein, the particle breaking strength of the particles having the stepped structure is 230 MPa or more.

4. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein, the average particle size of the conductive material is 1 nm or more and 25 nm or less.

5. A non-aqueous electrolyte secondary battery, which includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, the positive electrode is the positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 4.

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