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

Abstract

The purpose of the present disclosure is to provide a nonaqueous electrolyte secondary battery that has high capacity and excellent cycle characteristics. A nonaqueous electrolyte secondary battery according to one embodiment of the present invention is provided with a positive electrode, a negative electrode and a nonaqueous electrolyte. An electrode mixture layer of the positive electrode contains: first lithium metal composite oxide particles that are non-aggregated particles which have a volume-based median diameter of from 2 μm to 10 μm; and second lithium metal composite oxide particles that are secondary particles, in each of which primary particles having an average particle diameter of from 50 nm to 2 μm aggregate, and which have a volume-based median diameter of from 10 μm to 30 μm. If the positive electrode mixture layer is divided into equal halves in the thickness direction and the halves are defined as the first region and the second region sequentially from the surface side of the positive electrode mixture layer, the content of the first lithium metal composite oxide particles in the first region is higher than the content of the first lithium metal composite oxide particles in the second region.

Description

[Technical Field]

[0001] The present disclosure relates to a positive electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery using the positive electrode. [Background technology]

[0002] In recent years, with the widespread use of non-aqueous electrolyte secondary batteries in automotive and power storage applications, there has been a demand for non-aqueous electrolyte secondary batteries with high capacity and excellent cycle characteristics. Because the positive electrode has a significant effect on battery characteristics, including battery capacity and cycle characteristics, much research has been done on the positive electrode. For example, Patent Document 1 discloses a non-aqueous electrolyte secondary battery that uses, as a positive electrode active material, single-crystal particles that are non-aggregated particles and / or secondary particles formed by aggregating multiple primary particles. Patent Document 1 describes the effect of improving the cycle characteristics of the battery. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2020-53386 Summary of the Invention [Problem to be solved by the invention]

[0004] However, as a result of the inventors' investigations, it was found that when non-aggregated particles are used as the positive electrode active material, although the cycle characteristics are improved, the battery capacity is reduced due to a decrease in the packing density of the positive electrode active material. On the other hand, it was found that when secondary particles are used as the positive electrode active material, the cycle characteristics are reduced compared to when non-aggregated particles are used. Furthermore, it was found that when a mixture of non-aggregated particles and secondary particles is used, it is not possible to achieve both high capacity and good cycle characteristics.

[0005] An object of the present disclosure is to provide a non-aqueous electrolyte secondary battery that has a high capacity and excellent cycle characteristics. [Means for solving the problem]

[0006] A positive electrode for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a positive electrode core material and a positive electrode mixture layer formed on the surface of the positive electrode core material. The positive electrode mixture layer includes first lithium metal composite oxide particles, which are non-aggregated particles with a volumetric median diameter of 2 to 10 μm, and second lithium metal composite oxide particles, which are secondary particles formed by agglomeration of primary particles with an average particle diameter of 50 nm to 2 μm and have a volumetric median diameter of 10 to 30 μm. When the positive electrode mixture layer is divided into two equal parts in the thickness direction and defined as a first region and a second region in order from the surface side of the positive electrode mixture layer, the content of the first lithium metal composite oxide particles in the first region is greater than the content of the first lithium metal composite oxide particles in the second region.

[0007] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes the above positive electrode, a negative electrode, and a non-aqueous electrolyte. [Effects of the Invention]

[0008] According to one aspect of the present disclosure, a nonaqueous electrolyte secondary battery with high capacity and excellent cycle characteristics can be provided. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view of a positive electrode according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] As described above, achieving both high capacity and excellent cycle characteristics is an important challenge for non-aqueous electrolyte secondary batteries. As a result of extensive research aimed at solving this challenge, the present inventors have found that a non-aqueous electrolyte secondary battery with high capacity and excellent cycle characteristics can be obtained by using a combination of specific non-aggregated particles and specific secondary particles as a positive electrode active material and increasing the content (mass) of the non-aggregated particles on the surface side of the positive electrode mixture layer compared to the core side.

[0011] When non-aggregated particles are used alone as the positive electrode active material, the cycle characteristics are improved, but the battery capacity is reduced due to a decrease in the packing density of the positive electrode active material. On the other hand, when secondary particles are used, particle cracking is likely to occur during the compression process of the positive electrode, and the gaps between particles that serve as paths for the electrolyte are blocked by the cracked particles. This increases the packing density of the active material, but reduces the cycle characteristics. Furthermore, when a mixture of non-aggregated particles and secondary particles is used, these properties cannot be achieved simultaneously.

[0012] In other words, high capacity and excellent cycle characteristics are uniquely achieved when the non-aggregated particles in the positive electrode mixture layer satisfy the relationship that the content of the non-aggregated particles in the first region on the surface side of the positive electrode mixture layer is greater than the content of the non-aggregated particles in the second region on the core side of the positive electrode mixture layer. In this case, it is believed that it is possible to secure a flow path for the electrolyte in the positive electrode mixture layer to improve cycle characteristics, while increasing the packing density of the active material to achieve high capacity.

[0013] Hereinafter, with reference to the drawings, an example of an embodiment of a positive electrode for a non-aqueous electrolyte secondary battery according to the present disclosure and a non-aqueous electrolyte secondary battery using the positive electrode will be described in detail. Note that it is initially anticipated that multiple embodiments and modifications described below may be selectively combined.

[0014] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is exemplified, but the outer can of the battery is not limited to a cylindrical outer can and may be, for example, a prismatic outer can (prismatic battery) or a coin-shaped outer can (coin battery), or may be an outer can (pouch battery) made of a laminate sheet including a metal layer and a resin layer. Also, the electrode assembly may be a laminated electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0015] FIG. 1 is a cross-sectional view of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an outer can 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container that is open on one axial side and has a bottom, and the opening of the outer can 16 is closed by a sealing member 17. For ease of explanation, the sealing member 17 side of the battery will be referred to as the top, and the bottom side of the outer can 16 will be referred to as the bottom.

[0016] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all strip-shaped, long bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the longitudinal direction and width direction (short direction). The two separators 13 are formed to be at least slightly larger than the positive electrode 11 and are arranged, for example, to sandwich the positive electrode 11. The electrode assembly 14 includes a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0017] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1 , the positive electrode lead 20 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0018] A gasket 28 is provided between the exterior can 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior can 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior can 16 by the grooved portion 22 and the open end of the exterior can 16 that is crimped to the sealing body 17.

[0019] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, the lower valve body 24 deforms and breaks, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0020] The positive electrode 11, the negative electrode 12, the separator 13, and the non-aqueous electrolyte, particularly the positive electrode 11, will be described in detail below.

[0021] [Positive electrode] The positive electrode 11 includes a positive electrode core material 30 and a positive electrode mixture layer 31 formed on the surface of the positive electrode core material 30. The positive electrode core material 30 can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on the surface. An example of the positive electrode core material 30 is an aluminum or aluminum alloy foil having a thickness of 10 to 20 μm. The positive electrode mixture layer 31 contains a positive electrode active material, a conductive agent, and a binder, and is preferably formed on both sides of the positive electrode core material 30. The thickness of the positive electrode mixture layer 31 is, for example, 30 to 100 μm on one side of the positive electrode core material 30. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, etc., onto the positive electrode core material 30, drying the coating, and then compressing it to form the positive electrode mixture layer 31 on both sides of the positive electrode core material 30.

[0022] Examples of the conductive agent contained in the positive electrode mixture layer 31 include carbon materials such as carbon black, acetylene black, ketjen black, graphite, carbon nanotubes, carbon nanofibers, graphene, etc. The content of the conductive agent is, for example, 0.01 to 10 parts by mass, and preferably 0.05 to 5 parts by mass, per 100 parts by mass of the positive electrode active material.

[0023] Examples of the binder contained in the positive electrode mixture layer 31 include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, etc. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), etc. The content of the binder is, for example, 0.1 to 10 parts by mass, and preferably 0.5 to 5 parts by mass, per 100 parts by mass of the positive electrode active material.

[0024] The positive electrode mixture layer 31 contains particulate lithium metal composite oxide as a positive electrode active material. The lithium metal composite oxide is a composite oxide containing metal elements such as Co, Mn, Ni, and Al in addition to Li. The metal element constituting the lithium metal composite oxide is, for example, at least one selected from Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, and Bi. Among these, it is preferable to contain at least one selected from Co, Ni, and Mn. Examples of suitable composite oxides include lithium metal composite oxides containing Ni, Co, and Mn, and lithium metal composite oxides containing Ni, Co, and Al.

[0025] The positive electrode mixture layer 31 contains two types of lithium metal composite oxide particles. When the positive electrode mixture layer 31 is divided into two equal parts in the thickness direction and defined as a first region 31a and a second region 31b in that order from the surface side of the positive electrode mixture layer 31, the materials contained in the first region 31a and the second region 31b are different from each other. In this embodiment, the first region 31a and the second region 31b contain different types of lithium metal composite oxide particles. When two types of lithium metal composite oxide particles are contained in each region, the mass ratios of the lithium metal composite oxide particles are different. The types and contents of the conductive agent and binder may be the same or different between the first region 31a and the second region 31b.

[0026] The positive electrode mixture layer 31 contains first lithium metal composite oxide particles, which are non-aggregated particles, and second lithium metal composite oxide particles, which are secondary particles formed by aggregation of primary particles having an average particle size of 50 nm to 2 μm. The positive electrode mixture layer 31 may contain only the first and second lithium metal composite oxide particles as the positive electrode active material, or may also contain third lithium metal composite oxide particles as long as the object of the present disclosure is not impaired. An example of the third lithium metal composite oxide particles is composite oxide particles that do not satisfy the particle size condition described below.

[0027] The volume-based median diameter (hereinafter sometimes referred to as "D50") of the first lithium metal composite oxide particles is 2 to 10 μm, preferably 3 to 8 μm. The D50 of the second lithium metal composite oxide particles is 10 to 30 μm, preferably 12 to 20 μm. D50 refers to the particle size at which the cumulative frequency of the smallest particle size in the volume-based particle size distribution is 50%. The particle size distribution of the lithium metal composite oxide particles can be measured using a laser diffraction particle size distribution analyzer (for example, MT3000II manufactured by Microtrack Bell Corporation) with water as the dispersion medium.

[0028] The first lithium metal composite oxide particles are particles that do not have grain boundaries inside, and are, for example, single-crystal primary particles. The crystallinity of the lithium metal composite oxide particles can be confirmed using a scanning ion microscope. However, the first lithium metal composite oxide particles synthesized by the manufacturing method described below may contain particles that contain, for example, five or less primary particles.

[0029] The second lithium metal composite oxide particles are secondary particles formed by agglomeration of primary particles having an average particle size of 50 nm to 2 μm, preferably 500 nm to 2 μm. The second lithium metal composite oxide particles have grain boundaries between the primary particles. The primary particles can be confirmed by observing the second lithium metal composite oxide particles with a scanning electron microscope (SEM). The primary particles are adhered to each other with such strength that they do not break apart even when a strong force is applied, for example, during pulverization after synthesis of the second lithium metal composite oxide particles or during preparation of the positive electrode mixture slurry.

[0030] The average particle size of the primary particles constituting the second lithium metal composite oxide particles can be determined by analyzing SEM images of the particle cross sections. For example, the positive electrode 11 is embedded in a resin, a cross section is prepared by cross-section polishing (CP), and this cross section is photographed with an SEM. 30 primary particles are randomly selected from the SEM image, and the grain boundaries are observed. The diameters of the circumscribed circles of each of the 30 primary particles are calculated, and the average diameter is taken as the average particle size.

[0031] Each lithium metal composite oxide particle can be synthesized by the method described in the examples below. The first lithium metal composite oxide particle can be synthesized, for example, by increasing the pH of the alkaline aqueous solution used when synthesizing a precursor (metal composite hydroxide) containing Ni, Co, Mn, Al, etc. and / or increasing the firing temperature of the precursor, compared to the case of synthesizing the second lithium metal composite oxide particle. An example of a suitable pH of the alkaline aqueous solution is 10 to 11, and an example of a suitable firing temperature is 950 to 1100°C. When synthesizing the second lithium metal composite oxide particle, for example, an alkaline aqueous solution with a pH of 9 to 10 is used and the firing temperature is set to 950°C or lower.

[0032] Each lithium metal composite oxide particle is composed of a lithium metal composite oxide having a hexagonal crystal structure belonging to the space group R-3m, preferably LiNi Co y Mn z O2 (0.3 < x < 0.6, x + y + z = 1), or LiNi x Co y Al z O2 (0.8 < x < 0.95, x + y + z = 1) as the main component. Here, the main component means the component with the largest mass among the components constituting the lithium metal composite oxide particle. Note that the compositions of the lithium metal composite oxide particles may be the same or different from each other.

[0033] As described above, the first and second lithium metal composite oxide particles are contained in the positive electrode mixture layer 31, but the materials contained in the first region 31a and the second region 31b are different from each other. The content of the first lithium metal composite oxide particles in the positive electrode mixture layer 31 is not uniform and is higher in the first region 31a than in the second region 31b.

[0034] [[ID=The first lithium metal composite oxide particles are less likely to crack during the manufacturing process of the positive electrode 11 than the second lithium metal composite oxide particles, and so by having a large number of these particles in the first region 31a, which is the surface side of the positive electrode mixture layer 31, a flow path for the electrolyte is secured in the first region 31a. In this case, it is believed that the electrolyte can penetrate well into the second region 31b, improving the cycle characteristics.

[0035] Furthermore, the ratio of the mass of the first lithium metal composite oxide particles to the mass of the positive electrode active material in the first region 31a is preferably higher than the ratio of the mass of the first lithium metal composite oxide particles to the mass of the positive electrode active material in the second region 31b. The content of the positive electrode active material in the first region 31a and the second region 31b may be different, but is preferably substantially the same. The content of the positive electrode active material is, for example, 90 to 99.9 mass% and preferably 95 to 99 mass% with respect to the mass of the positive electrode mixture layer 31.

[0036] The first region 31a may contain substantially only first lithium metal composite oxide particles as the positive electrode active material. Alternatively, the first lithium metal composite oxide particles may be contained only in the first region 31a, and not present in the second region 31b. When second lithium metal composite oxide particles are present in the first region 31a, it is preferable that the content of each composite oxide particle in the first region 31a satisfy the relationship: content of the first lithium metal composite oxide particles > content of the second lithium metal composite oxide particles.

[0037] The content of the second lithium metal composite oxide particles is higher in the second region 31b than in the first region 31a. The ratio of the mass of the second lithium metal composite oxide particles to the mass of the positive electrode active material in the second region 31b is preferably higher than the ratio of the mass of the second lithium metal composite oxide particles to the mass of the positive electrode active material in the first region 31a.

[0038] Since the second lithium metal composite oxide particles can be packed in larger amounts per unit volume than the first lithium metal composite oxide particles, their use can increase the density of the positive electrode mixture layer 31, contributing to higher battery capacity. By having more second lithium metal composite oxide particles on the positive electrode core material 30 side, which has less effect on the permeability of the electrolyte solution into the positive electrode mixture layer 31, it is possible to achieve higher capacity while maintaining good cycle characteristics.

[0039] The second region 31b may contain substantially only second lithium metal composite oxide particles as the positive electrode active material. Alternatively, the second lithium metal composite oxide particles may be contained only in the second region 31b, and not present in the first region 31a. When the first lithium metal composite oxide particles are present in the second region 31b, it is preferable that the content of the first lithium metal composite oxide particles in the second region 31b be less than the content of the second lithium metal composite oxide particles.

[0040] As described above, the first region 31a may contain first and second lithium metal composite oxide particles. In this case, the mass ratio of the first lithium metal composite oxide particles to the second lithium metal composite oxide particles in the first region 31a is preferably 60:40 to 90:10, and more preferably 65:35 to 80:20. If the mass ratio of the composite oxide particles is within this range, it becomes easy to achieve both high capacity and excellent cycle characteristics.

[0041] In the second region 31b, the contents of the first and second lithium metal composite oxide particles can be the same, but preferably the content of the first lithium metal composite oxide particles is less than the content of the second lithium metal composite oxide particles, or only the second lithium metal composite oxide particles are used as the positive electrode active material. When the second region 31b contains the first lithium metal composite oxide particles, the mass ratio of the first lithium metal composite oxide particles to the second lithium metal composite oxide particles in the second region 31b is preferably 10:90 to 40:60, and more preferably 20:80 to 35:65. When the mass ratio of the composite oxide particles is within this range, it becomes easy to achieve both high capacity and excellent cycle characteristics.

[0042] [Negative electrode] The negative electrode 12 includes a negative electrode core material and a negative electrode mixture layer formed on the surface of the negative electrode core material. The negative electrode core material can be a foil of a metal, such as copper or a copper alloy, that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on the surface layer. An example of the negative electrode core material is a copper or copper alloy foil having a thickness of 5 to 15 μm. The negative electrode mixture layer contains a negative electrode active material and a binder, and is preferably formed on both sides of the negative electrode core material. The thickness of the negative electrode mixture layer is, for example, 30 to 100 μm on one side of the negative electrode core material. The negative electrode 12 can be produced by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, etc., onto the negative electrode core material, drying the coating, and then compressing it to form a negative electrode mixture layer on both sides of the negative electrode core material.

[0043] The negative electrode mixture layer contains, as the negative electrode active material, for example, a carbon-based active material that reversibly absorbs and releases lithium ions. Suitable carbon-based active materials include natural graphite such as flake graphite, lump graphite, and amorphous graphite, and artificial graphite such as lump artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB). The negative electrode active material may be a Si-based active material composed of at least one of Si and a Si-containing compound, or a combination of a carbon-based active material and a Si-based active material.

[0044] The binder contained in the negative electrode mixture layer can be a fluorine-containing resin such as PTFE or PVdF, PAN, polyimide, acrylic resin, polyolefin, or styrene-butadiene rubber (SBR). The negative electrode mixture layer may also contain CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, or polyvinyl alcohol (PVA). The binder content is, for example, 0.1 to 10 parts by mass, and preferably 0.5 to 5 parts by mass, per 100 parts by mass of the negative electrode active material. A conductive agent such as carbon black, acetylene black, or ketjen black may also be added to the negative electrode mixture layer.

[0045] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator include polyolefins such as polyethylene and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a laminated structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13.

[0046] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides and phosphate compounds containing metals such as Ti, Al, Si, and Mg. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0047] [Non-aqueous electrolyte] The non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with halogen atoms such as fluorine. Examples of the halogen-substituted compound include fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, and fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP).

[0048] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone (GBL) and γ-valerolactone (GVL); and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate.

[0049] Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ether, etc. Examples of chain ethers include 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-dimethoxybenzyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzyl ether, methyl phenyl ether, ethyl phenyl ether, di ... Examples include zen, 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.

[0050] The electrolyte salt is preferably a lithium salt. Examples of lithium salts include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), 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, borate salts such as Li2B4O7, Li(B(C2O4)F2), imide salts such as LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 SO2){l, m are integers of 0 or more}, etc. The lithium salt may be used alone or in combination of multiple kinds. Among these, from the viewpoints of ionic conductivity, electrochemical stability, etc., it is preferable to use LiPF6. The concentration of the lithium salt is, for example, 0.8 mol to 1.8 mol per 1 L of the non - aqueous solvent.

Examples

[0051] Hereinafter, the present disclosure will be further described in detail by examples, but the present disclosure is not limited to these examples.

[0052] <Example 1> [Synthesis of the first lithium metal composite oxide particles] Nickel sulfate, cobalt sulfate, and manganese sulfate were mixed in a predetermined ratio and uniformly mixed in an alkaline aqueous solution of pH 10 to 11 to prepare a precursor. Next, the precursor was mixed with lithium carbonate, and the mixture was baked at a temperature of 1000°C for 15 hours, and then pulverized to obtain first lithium metal composite oxide particles, which are non-aggregated particles. The composition and D50 of the particles are as follows: Composition: LiNi 0.5 Co 0.2 Mn 0.3 O2 D50: 4.5 μm

[0053] [Synthesis of second lithium metal composite oxide particles] Secondary particles formed by aggregation of primary particles were obtained in the same manner as the first lithium metal composite oxide particles, except that the pH of the alkaline aqueous solution was changed to 9 to 10 and the firing temperature was changed to 900° C. The composition of the particles, the average particle size of the primary particles, and the D50 of the secondary particles were as follows: Composition: LiNi 0.5 Co 0.2 Mn 0.3 O2 Average primary particle size: 1.6 μm D50 of secondary particles (second lithium metal composite oxide particles): 14.1 μm

[0054] [Preparation of positive electrode] First lithium metal composite oxide particles, acetylene black (AB), and polyvinylidene fluoride (PVdF) were mixed in a mass ratio of 98:1:1, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added as a dispersion medium to prepare a first positive electrode mixture slurry with a solids concentration of 70% by mass. A second positive electrode mixture slurry was prepared in the same manner as the first positive electrode mixture slurry, except that second lithium metal composite oxide particles were used instead of the first lithium metal composite oxide particles. Next, the second positive electrode mixture slurry was applied to both sides of a positive electrode core material made of aluminum foil, and then the first positive electrode mixture slurry was applied to the coating film of the second positive electrode mixture slurry. The coating film was dried and compressed (linear pressure 3000 N / m), and then cut to a predetermined electrode size to produce a positive electrode with a positive electrode mixture layer formed on both sides of the positive electrode core material. The application amounts of the first positive electrode mixture slurry and the second positive electrode mixture slurry were set to the same value.

[0055] [Preparation of negative electrode] As the negative electrode active material, 95 parts by mass of graphite powder and 5 parts by mass of SiO x A mixture of 100 parts by mass of the negative electrode active material, 1 part by mass of sodium carboxymethyl cellulose (CMC-Na), and water was mixed, and then 1.2 parts by mass of a dispersion of styrene-butadiene rubber (SBR) was mixed to prepare a negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to both sides of a negative electrode core material made of copper foil, and 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 material.

[0056] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 1:3 (25°C). Five parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of the mixed solvent, and LiPF6 was dissolved therein to a concentration of 1 mol / L to prepare a non-aqueous electrolyte solution.

[0057] [Battery construction] Lead terminals were attached to the positive electrode and the negative electrode, respectively, and the positive electrode and the negative electrode were spirally wound with a separator interposed therebetween to produce a wound electrode assembly. The electrode assembly was housed in a cylindrical outer can with a bottom, the negative electrode lead was welded to the inner bottom surface of the outer can, and the positive electrode lead was welded to the internal terminal plate of the sealing body. The nonaqueous electrolyte was then poured into the outer can, and the opening edge of the outer can was crimped and fixed to the sealing body to produce a cylindrical secondary battery with a battery capacity of 2500 mAh.

[0058] <Example 2> A mixture of first lithium metal composite oxide particles and second lithium metal composite oxide particles in a mass ratio of 7:3 was mixed with AB and PVdF in a mass ratio of 98:1:1, and an appropriate amount of NMP was added as a dispersion medium to prepare a first positive electrode mixture slurry with a solid content of 70 mass%. A mixture of first lithium metal composite oxide particles and second lithium metal composite oxide particles in a mass ratio of 3:7 was mixed with AB and PVdF in a mass ratio of 98:1:1, and an appropriate amount of NMP was added as a dispersion medium to prepare a second positive electrode mixture slurry with a solid content of 70 mass%. A positive electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 1, except that these two types of positive electrode mixture slurries were used in the fabrication of the positive electrode.

[0059] <Comparative Example 1> A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 1, except that in fabricating the positive electrode, the first positive electrode mixture slurry was applied first and then the second positive electrode mixture slurry was applied.

[0060] <Comparative Example 2> A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 1, except that only the first positive electrode mixture slurry was used in fabricating the positive electrode.

[0061] <Comparative Example 3> A positive electrode and a non-aqueous electrolyte secondary battery were fabricated in the same manner as in Example 1, except that only the second positive electrode mixture slurry was used in fabricating the positive electrode.

[0062] <Comparative Example 4> A mixture of first lithium metal composite oxide particles and second lithium metal composite oxide particles in a mass ratio of 1:1 was mixed with AB and PVdF in a mass ratio of 98:1:1, and an appropriate amount of NMP was added as a dispersion medium to prepare a positive electrode mixture slurry with a solids concentration of 70 mass%. A positive electrode and a nonaqueous electrolyte secondary battery were fabricated in the same manner as in Example 1, except that this positive electrode mixture slurry was used in the fabrication of the positive electrode.

[0063] The positive electrodes and batteries of each example and comparative example were evaluated by the following methods. The evaluation results are shown in Table 1.

[0064] [Positive electrode mixture layer density (filling density)] The thickness and mass of the positive electrode were measured to calculate the density of the positive electrode mixture layer.

[0065] [Cycle characteristics (capacity retention)] At a temperature of 25°C, the battery was charged at a constant current of 0.7 It until the battery voltage reached 4.2 V. It was then charged at a constant voltage of 4.2 V until the current reached 0.05 It. It was then discharged at a constant current of 0.7 It until the battery voltage reached 2.5 V. This charge / discharge cycle was repeated 300 times, and the capacity retention rate was calculated using the following formula: It (A) = rated capacity (Ah) / 1 (h). Capacity retention rate = (discharge capacity at 300th cycle / discharge capacity at 1st cycle) x 100

[0066] [Table 1]

[0067] As shown in Table 1, the batteries of the examples have high capacity retention and excellent cycle characteristics. Furthermore, the packing density of the positive electrode active material is high, allowing for a high battery capacity. Although the positive electrode of Comparative Example 1 has the same packing density as the positive electrodes of the examples, the battery of Comparative Example 1 has a significantly lower capacity retention rate than the batteries of the examples. This is presumably due mainly to the fact that cracks in the secondary particles occur when the positive electrode is compressed, blocking the gaps between particles that serve as paths for the electrolyte in the first region on the surface side of the positive electrode mixture layer, thereby inhibiting the supply of electrolyte to the second region on the core side of the positive electrode mixture layer.

[0068] In the case of Comparative Example 2, although the cycle characteristics are good, the packing density of the positive electrode active material is significantly reduced, making it difficult to achieve a high capacity. In the case of Comparative Example 3, although the packing density of the positive electrode active material is high, the blockage of the electrolyte flow path significantly reduces the cycle characteristics. In Comparative Example 3, it is possible to adjust the packing density to 3.5 g / cc by reducing the linear pressure when compressing the positive electrode, but the capacity retention rate is not improved in this case either.

[0069] The capacity retention rate of the battery of Comparative Example 4 was equivalent to that of the batteries of Comparative Examples 1 and 3, and the cycle characteristics were inferior to those of the batteries of the Examples. In other words, simply mixing non-aggregated particles and secondary particles does not improve the cycle characteristics of the battery. When the non-aggregated particles satisfy the relationship of the content of non-aggregated particles in the first region of the positive electrode mixture layer being greater than the content of non-aggregated particles in the second region, as in the Examples, the packing density of the positive electrode active material can be increased while ensuring a flow path for the electrolyte, and a non-aqueous electrolyte secondary battery with high capacity and excellent cycle characteristics can be realized. [Explanation of symbols]

[0070] 10 non-aqueous electrolyte secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 16 outer can, 17 sealing body, 18, 19 insulating plate, 20 positive electrode lead, 21 negative electrode lead, 22 grooved portion, 23 internal terminal plate, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket, 30 positive electrode core material, 31 positive electrode mixture layer, 31a first region, 31b second region

Claims

1. A positive electrode for a non-aqueous electrolyte secondary battery comprising: a positive electrode core material; and a positive electrode mixture layer formed on a surface of the positive electrode core material, the positive electrode mixture layer includes first lithium metal composite oxide particles which are non-aggregated particles having a volume-based median diameter of 2 to 10 μm, and second lithium metal composite oxide particles which are secondary particles having a volume-based median diameter of 10 to 30 μm and formed by aggregation of primary particles having an average particle diameter of 50 nm to 2 μm; when the positive electrode mixture layer is divided into two equal parts in a thickness direction and defined as a first region and a second region in that order from the surface side of the positive electrode mixture layer, the content of the first lithium metal composite oxide particles in the first region is greater than the content of the first lithium metal composite oxide particles in the second region, the first region contains the first and second lithium-metal composite oxide particles, a mass ratio of the first lithium metal composite oxide particles to the second lithium metal composite oxide particles in the first region is 60:40 to 90:10;

2. 2 . The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1 , wherein a content of the second lithium metal composite oxide particles in the second region is greater than a content of the second lithium metal composite oxide particles in the first region.

3. the second region contains the first and second lithium-metal composite oxide particles, 3. The positive electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein a mass ratio of the first lithium metal composite oxide particles to the second lithium metal composite oxide particles in the second region is 10:90 to 40:

60.

4. A non-aqueous electrolyte secondary battery comprising the positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 3, a negative electrode, and a non-aqueous electrolyte.

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