Non-aqueous electrolyte secondary batteries

Abstract

The purpose of the present disclosure is to provide a nonaqueous electrolyte secondary battery which has high capacity, while being suppressed in decrease in the high-rate charge / discharge cycle characteristics. A nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure is provided with: an electrode body wherein a positive electrode and a negative electrode are arranged to face each other, with a porous separator being interposed therebetween; a nonaqueous electrolyte; and an outer package that contains the electrode body and the nonaqueous electrolyte. The negative electrode comprises a negative electrode collector, a first negative electrode mixture layer that is provided on the surface of the negative electrode collector, and a second negative electrode mixture layer that faces the positive electrode, with the separator being interposed therebetween; the first negative electrode mixture layer and the second negative electrode mixture layer contain graphite particles; the ratio of the void fraction (S2) among the graphite particles in the second negative electrode mixture layer to the void fraction (S1) among the graphite particles in the first negative electrode mixture layer, namely S2 / S1 is from 1.1 to 2.0; the ratio of the packing density (D2) of the second negative electrode mixture layer to the packing density (D1) of the first negative electrode mixture layer, namely D2 / D1 is from 0.9 to 1.1; and the separator has a thickness of 10 μm or less, while having a porosity of from 25% to 45%.

Description

[Technical Field]

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries using graphite particles as the negative electrode active material are widely used as high-capacity secondary batteries. Although the battery capacity can be increased by increasing the packing density per unit volume of the negative electrode active material in the negative electrode mixture layer, increasing the packing density of the negative electrode active material reduces the voids between the negative electrode active material, impairing the circulation of the electrolyte, and resulting in a decrease in battery capacity with repeated charge-discharge cycles (rapid charge-discharge cycles).

[0003] Patent Document 1 discloses an ultrathin, low-porosity polyethylene microporous membrane. However, such a microporous membrane has poor liquid retention and is therefore unable to improve the liquid circulation in the negative electrode, and is therefore unable to improve the rapid charge-discharge cycle characteristics of secondary batteries.

[0004] On the other hand, in the inventions disclosed in Patent Documents 2 to 4, the packing density of the negative electrode active material in the negative electrode mixture layer is lower on the outer surface side than on the current collector side, thereby increasing the voids between the negative electrode active material on the outer surface side and improving the circulation of the electrolyte. However, this reduces the amount of negative electrode active material per unit volume in the negative electrode mixture layer, resulting in a problem of reduced battery capacity. Therefore, even if the polyethylene microporous membrane disclosed in Patent Document 1 is applied to the inventions disclosed in Patent Documents 2 to 4, it is not possible to achieve both high capacity and excellent rapid charge / discharge cycle characteristics. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 11-60790 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-77463 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-196457 [Patent Document 4] Special Publication No. 2015-511389 Summary of the Invention [Problem to be solved by the invention]

[0006] Therefore, an object of the present disclosure is to provide a nonaqueous electrolyte secondary battery that has a high capacity and suppresses deterioration in rapid charge / discharge cycle characteristics. [Means for solving the problem]

[0007] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes an electrode assembly in which a positive electrode and a negative electrode face each other with a porous separator interposed therebetween, a non-aqueous electrolyte, and an exterior housing that houses the electrode assembly and the non-aqueous electrolyte. The negative electrode has a negative electrode current collector, a first negative electrode mixture layer provided on the surface of the negative electrode current collector, and a second negative electrode mixture layer facing the positive electrode via a separator, the first negative electrode mixture layer and the second negative electrode mixture layer containing graphite particles, a ratio (S2 / S1) of the porosity (S2) between the graphite particles in the second negative electrode mixture layer to the porosity (S1) between the graphite particles in the first negative electrode mixture layer is 1.1 to 2.0, a ratio (D2 / D1) of the packing density (D2) of the second negative electrode mixture layer to the packing density (D1) of the first negative electrode mixture layer is 0.9 to 1.1, and the separator has a thickness of 10 μm or less and a porosity of 25% to 45%. [Effects of the Invention]

[0008] According to one aspect of the present disclosure, it is possible to provide a nonaqueous electrolyte secondary battery that has a high capacity and is capable of suppressing a decrease in rapid charge / discharge cycle characteristics. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is an axial cross-sectional view of a cylindrical secondary battery as an example of the embodiment. [Figure 2] FIG. 2 is a cross-sectional view of a negative electrode according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes an electrode assembly in which a positive electrode and a negative electrode face each other with a porous separator interposed therebetween, a non-aqueous electrolyte, and an exterior housing that houses the electrode assembly and the non-aqueous electrolyte. The negative electrode has a negative electrode current collector, a first negative electrode mixture layer provided on the surface of the negative electrode current collector, and a second negative electrode mixture layer facing the positive electrode via a separator, the first negative electrode mixture layer and the second negative electrode mixture layer containing graphite particles, a ratio (S2 / S1) of the porosity (S2) between the graphite particles in the second negative electrode mixture layer to the porosity (S1) between the graphite particles in the first negative electrode mixture layer is 1.1 to 2.0, a ratio (D2 / D1) of the packing density (D2) of the second negative electrode mixture layer to the packing density (D1) of the first negative electrode mixture layer is 0.9 to 1.1, and the separator has a thickness of 10 μm or less and a porosity of 25% to 45%.

[0011] An example of an embodiment of a cylindrical secondary battery according to the present disclosure will be described in detail below with reference to the drawings. In the following description, specific shapes, materials, numerical values, directions, etc. are examples for facilitating understanding of the present invention and can be appropriately changed according to the specifications of the cylindrical secondary battery. Furthermore, the exterior body is not limited to a cylindrical shape and may be, for example, a rectangular shape. Furthermore, when the following description includes multiple embodiments and modified examples, it is initially assumed that their characteristic features can be appropriately combined and used.

[0012] Fig. 1 is an axial cross-sectional view of a cylindrical secondary battery 10 that is one example of an embodiment. In the secondary battery 10 shown in Fig. 1, an electrode assembly 14 and a non-aqueous electrolyte (not shown) are housed in an exterior body 15. The electrode assembly 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a porous separator 13 interposed therebetween. For ease of explanation, the following description will refer to the sealing body 16 side as the "top" and the bottom side of the exterior body 15 as the "bottom."

[0013] The open end of the exterior body 15 is sealed with the sealing body 16, thereby sealing the interior of the secondary battery 10. Insulating plates 17 and 18 are provided above and below the electrode body 14. The positive electrode lead 19 extends upward through a through hole in the insulating plate 17 and is welded to the underside of a filter 22, which is the bottom plate of the sealing body 16. In the secondary battery 10, a cap 26, which is the top plate of the sealing body 16 and is electrically connected to the filter 22, serves as the positive electrode terminal. On the other hand, the negative electrode lead 20 extends through a through hole in the insulating plate 18 toward the bottom of the exterior body 15 and is welded to the inner bottom surface of the exterior body 15. In the secondary battery 10, the exterior body 15 serves as the negative electrode terminal. Note that if the negative electrode lead 20 is installed at the terminal end, the negative electrode lead 20 passes outside the insulating plate 18, extends toward the bottom of the exterior body 15, and is welded to the inner bottom surface of the exterior body 15.

[0014] Exterior body 15 is, for example, a cylindrical metal exterior can with a bottom. A gasket 27 is provided between exterior body 15 and sealing body 16, ensuring the airtightness of the interior of secondary battery 10. Exterior body 15 has a grooved portion 21 that supports sealing body 16, formed, for example, by pressing the side surface from the outside. Grooved portion 21 is preferably formed in an annular shape along the circumferential direction of exterior body 15, and supports sealing body 16 on its upper surface via gasket 27.

[0015] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26, which are stacked in this order from the electrode body 14 side. Each component of the sealing body 16 has, for example, a disk or ring shape, and all components except for the insulating member 24 are electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, with the insulating member 24 interposed between their respective peripheral edges. If the internal pressure of the battery increases due to abnormal heat generation, for example, the lower valve body 23 may break, causing the upper valve body 25 to bulge toward the cap 26 and separate from the lower valve body 25, thereby cutting off the electrical connection between them. If the internal pressure continues to increase, the upper valve body 25 may break, and gas may be released from the opening 26a of the cap 26.

[0016] The positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte that constitute the secondary battery 10 will be described in detail below, particularly the negative electrode active material contained in the negative electrode mixture layer 32 that constitutes the negative electrode 12.

[0017] [Negative electrode] 2 is a cross-sectional view of a negative electrode 12 according to an embodiment. The negative electrode 12 includes a negative electrode current collector 30, a first negative electrode mixture layer 32a provided on the surface of the negative electrode current collector 30, and a second negative electrode mixture layer 32b provided on the surface of the first negative electrode mixture layer 32a. The first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b may have the same thickness or different thicknesses. The thickness ratio of the first negative electrode mixture layer 32a to the second negative electrode mixture layer 32b is, for example, 3:7 to 7:3, preferably 4:6 to 6:4, and more preferably 5:5 to 6:4.

[0018] A foil of a metal such as copper that is stable in the potential range of the negative electrode, or a film with such a metal disposed on the surface layer, is used for the negative electrode current collector 30. The thickness of the negative electrode current collector 30 is, for example, 5 μm to 30 μm.

[0019] The first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b (hereinafter, the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b may be collectively referred to as the negative electrode mixture layer 32) contain graphite particles. The negative electrode mixture layer 32 preferably contains a binder or the like. Examples of binders include fluorine-based resins, polyacrylonitrile (PAN), polyimide-based resins, acrylic-based resins, polyolefin-based resins, styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof (such as PAA-Na or PAA-K, or a partially neutralized salt), and polyvinyl alcohol (PVA). These may be used alone or in combination of two or more.

[0020] The graphite particles used in this embodiment include natural graphite and artificial graphite. The interplanar spacing (d 002) is, for example, preferably 0.3354 nm or more, more preferably 0.3357 nm or more, and preferably less than 0.340 nm, more preferably 0.338 nm or less. The crystallite size (Lc(002)) of the graphite particles used in this embodiment, determined by X-ray diffraction, is, for example, preferably 5 nm or more, more preferably 10 nm or more, and preferably 300 nm or less, more preferably 200 nm or less. The interplanar spacing (d 002 When the thickness (Lc(002)) and the crystallite size (Lc(002)) satisfy the above ranges, the battery capacity of the secondary battery 10 tends to be larger than when the thickness (Lc(002)) and the crystallite size (Lc(002)) do not satisfy the above ranges.

[0021] The graphite particles contained in the first negative electrode mixture layer 32a can be produced, for example, as follows. Coke (precursor), the main raw material, is crushed to a predetermined size, agglomerated with a binder, and then pressed into a block shape. This block shape is then fired at a temperature of 2600°C or higher to graphitize it. The graphitized block shape is crushed and sieved to obtain graphite particles of a desired size. The internal porosity of the graphite particles can be adjusted by the particle size of the crushed precursor or the particle size of the agglomerated precursor. For example, the average particle size (volume-equivalent median diameter D50, the same applies hereinafter) of the crushed precursor is preferably in the range of 12 μm to 20 μm. The internal porosity of the graphite particles can also be adjusted by the amount of volatile components added to the block shape. If a portion of the binder added to the coke (precursor) volatilizes during firing, the binder can be used as the volatile component. Pitch is an example of such a binder.

[0022] The graphite particles contained in the second negative electrode mixture layer 32b can be produced, for example, as follows. Coke (precursor), which is the main raw material, is crushed to a predetermined size, and the crushed coke is agglomerated with a binder such as pitch. The crushed coke is then fired at a temperature of 2600°C or higher to form graphite. The graphite is then sieved to obtain graphite particles of a desired size. The internal porosity of the graphite particles can be adjusted by adjusting the particle size of the crushed precursor or the particle size of the agglomerated precursor. For example, the average particle size of the crushed precursor is preferably in the range of 12 μm to 20 μm.

[0023] The ratio (S2 / S1) of the porosity (S2) between graphite particles in the second negative electrode mixture layer 32b to the porosity (S1) between graphite particles in the first negative electrode mixture layer 32a is 1.1 to 2.0, preferably 1.1 to 1.7, and more preferably 1.1 to 1.5. If S2 / S1 is less than 1.1, the circulation of the electrolyte solution is poor, resulting in a decrease in battery capacity after repeated rapid charging. If S2 / S1 is more than 2.0, the packing density of the second negative electrode mixture layer 32b (described later) cannot be made substantially equal to the packing density of the first negative electrode mixture layer 32a, resulting in a decrease in battery capacity. Here, the porosity between graphite particles is a two-dimensional value calculated from the ratio of the area of ​​voids between graphite particles to the cross-sectional area of ​​the negative electrode mixture layer 32. S2 / S1 is found by calculating the porosity (S1) between the graphite particles in the first negative electrode mixture layer 32a and the porosity (S2) between the graphite particles in the second negative electrode mixture layer 32b in the following procedure.

[0024] <Method for measuring void ratio between graphite particles> (1) Exposing the cross section of the negative electrode mixture layer. For example, a method for exposing the cross section includes cutting out a part of the negative electrode and processing it with an ion milling device (e.g., IM4000PLUS manufactured by Hitachi High-Technologies Corporation) to expose the cross section of the negative electrode mixture layer. (2) Using a scanning electron microscope, backscattered electron images of the cross sections of the exposed negative electrode mixture layers are taken for each of the first negative electrode mixture layer 32 a and the second negative electrode mixture layer 32 b. The magnification for taking the backscattered electron images is, for example, 800 times. (3) The cross-sectional image obtained as described above is imported into a computer and binarized using image analysis software (e.g., ImageJ, manufactured by the National Institutes of Health, USA) to obtain a binarized image in which the particle cross-sections in the cross-sectional image are colored black and the voids present in the particle cross-sections are colored white. (4) In the binarized images of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b, the voids converted to white are those inside the graphite particles (pores not connected to the particle surfaces) and those pores connected to the graphite particle surfaces with a width of 3 μm or less. These voids are then considered to be the voids between the graphite particles, and the area of ​​the voids between the graphite particles is calculated. The void ratio between the graphite particles can be calculated using the following formula: Porosity between graphite particles = Area of ​​voids between graphite particles / Area of ​​cross section of negative electrode mixture layer × 100

[0025] S1 and S2 are each determined as the average value of the three measurements, and S1 / S2 can be calculated from these values.

[0026] The ratio (D2 / D1) of the packing density (D2) of the second negative electrode mixture layer 32b to the packing density (D1) of the first negative electrode mixture layer 32a is 0.9 to 1.1. When D2 / D1 is in this range while S2 / S1 is 1.1 to 2.0, a decrease in battery capacity can be suppressed. For example, by making the internal porosity of the graphite particles contained in the first negative electrode mixture layer 32a higher than the internal porosity of the graphite particles contained in the second negative electrode mixture layer 32b, the above ranges for S2 / S1 and D2 / D1 can be satisfied.

[0027] The packing density (D1) of the first negative electrode mixture layer 32a and the packing density (D2) of the second negative electrode mixture layer 32b are, for example, 1.3 g / m 3 ~2.0g / m 3 It can be said that:

[0028] The packing density of the negative electrode mixture layer 32 is the mass per unit volume of the negative electrode mixture layer 32. First, using the negative electrode 12, the mix mass per unit area of ​​each of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b is measured. The mix layer thickness of each of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b is measured from the cross-sectional image obtained when calculating the inter-particle porosity. When the mix layer thickness is not stable, measurements are taken at 10 points on the image, and the average value can be used as the mix layer thickness. The packing density (D1) of the first negative electrode mixture layer 32a and the packing density (D2) of the second negative electrode mixture layer 32b can be calculated by dividing the mix mass per unit area by the mix layer thickness. From these values, the ratio (D2 / D1) of the packing density (D2) of the second negative electrode mixture layer 32b to the packing density (D1) of the first negative electrode mixture layer 32a is obtained.

[0029] Next, a specific method for forming the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b will be described. For example, first, a negative electrode active material containing graphite particles (hereinafter sometimes referred to as first graphite particles), a binder, and a solvent such as water are mixed to prepare a first negative electrode mixture slurry. Separately, a negative electrode active material containing graphite particles different from the first graphite particles (hereinafter sometimes referred to as second graphite particles), a binder, and a solvent such as water are mixed to prepare a second negative electrode mixture slurry. Then, the first negative electrode mixture slurry is applied to both sides of the negative electrode current collector and dried. After that, the second negative electrode mixture slurry is applied to both sides of the coating of the first negative electrode mixture slurry and dried. Furthermore, the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b are rolled using a rolling roller to form the negative electrode mixture layer 32.

[0030] Even if the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b are simultaneously rolled as described above, the packing densities of the first graphite particles and the second graphite particles are not necessarily the same during rolling. For example, the packing densities of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b can be adjusted by changing the particle size distribution of the first graphite particles and the second graphite particles. Furthermore, by making the internal porosity of the second graphite particles lower than that of the first graphite particles, the interparticle porosity can be increased without excessively reducing the packing density of the second negative electrode mixture layer 32b. In the above method, the first negative electrode mixture slurry is applied and dried, and then the second negative electrode mixture slurry is applied. However, the second negative electrode mixture slurry may be applied after the first negative electrode mixture slurry is applied and before drying. Alternatively, the first negative electrode mixture slurry may be applied, dried, and rolled, and then the second negative electrode mixture slurry may be applied onto the first negative electrode mixture layer 32a. By changing the rolling conditions for the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b, the packing densities of the respective layers can be more freely adjusted.

[0031] At least one of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b may contain a Si-based material. The Si-based material is a material that can reversibly store and release lithium ions and functions as a negative electrode active material. Examples of the Si-based material include Si, an alloy containing Si, and SiO x (x is 0.8 to 1.6), and the like. Si-based materials are negative electrode materials that can improve battery capacity more than graphite particles. From the viewpoints of improving battery capacity and suppressing deterioration of rapid charge / discharge cycle characteristics, the content of the Si-based material is, for example, preferably 1% by mass to 10% by mass, and more preferably 3% by mass to 7% by mass, relative to the mass of the negative electrode active material.

[0032] Other examples of the other material capable of reversibly absorbing and releasing lithium ions include metals that alloy with lithium, such as tin (Sn), or alloys or oxides containing metal elements such as Sn. The negative electrode active material may contain the other material, and the content of the other material is preferably, for example, 10 mass % or less relative to the mass of the negative electrode active material.

[0033] [Positive electrode] The positive electrode 11 is composed of, for example, a positive electrode current collector such as a metal foil, and a positive electrode mixture layer formed on the positive electrode current collector. As the positive electrode current collector, a foil of a metal stable within the potential range of the positive electrode such as aluminum, a film having the metal disposed on the surface layer, or the like can be used. The positive electrode mixture layer contains, for example, a positive electrode active material, a binder, a conductive agent, and the like.

[0034] The positive electrode 11 can be manufactured by, for example, applying a positive electrode mixture slurry containing a positive electrode active material, a binder, a conductive agent, and the like onto the positive electrode current collector, drying to form a positive electrode mixture layer, and then rolling this positive electrode mixture layer.

[0035] Examples of the positive electrode active material include lithium transition metal oxides containing transition metal elements such as Co, Mn, and Ni. The lithium transition metal oxides are, for example, Li x CoO2, Li x NiO2, Li x MnO2, Li x Co y Ni 1-y O2, Li x Co y M 1-y O z , Li x Ni 1-y M y O z , Li x Mn2O4, Li x Mn 2-y M y O4, LiMPO4, Li2MPO4F (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3). These may be used alone or in combination of multiple kinds. In terms of achieving a higher capacity of the non-aqueous electrolyte secondary battery, the positive electrode active material is Li x NiO2, Li x Co y Ni 1-y O2, Li x Ni<000​​​​It is preferable to contain a lithium nickel composite oxide such as (M; at least one of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B, 0 < x ≤ 1.2, 0 < y ≤ 0.9, 2.0 ≤ z ≤ 2.3), etc.

[0036] Examples of the conductive agent include carbon-based particles such as carbon black (CB), acetylene black (AB), ketjen black, and graphite. These may be used alone or in combination of two or more.

[0037] Examples of the binder include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic resins, and polyolefin-based resins. These may be used alone or in combination of two or more.

[0038] [Separator] For the separator 13, for example, a porous sheet having ion permeability and insulation properties is used. Specific examples of the porous sheet include microporous membranes, woven fabrics, non-woven fabrics, etc. As the material of the separator, olefin-based resins such as polyethylene and polypropylene, and cellulose are suitable. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Also, a multilayer separator including a polyethylene layer and a polypropylene layer may be used, or a separator 13 with a material such as an aramid-based resin or ceramic coated on its surface may be used.

[0039] The thickness of the separator 13 is 10 μm or less. Thereby, the battery capacity can be improved. Also, from the viewpoint of strength, the thickness of the separator 13 is preferably 6 μm or more.

[0040] The porosity of the separator 13 is 25% to 45%. Within this range, even if the separator 13 is thin, it is possible to maintain strength and improve circulation of the electrolyte, resulting in a battery with high capacity and suppressed deterioration of rapid charge / discharge cycle characteristics. The porosity of the separator 13 can be calculated using the following formula. Porosity of separator = [1 - {mass of separator / (thickness of separator × area of ​​main surface of separator × true density of material constituting separator)}] × 100

[0041] [Non-aqueous electrolyte] The non-aqueous electrolyte includes 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 (electrolytic solution) and may be a solid electrolyte using a gel polymer or the like. 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 product in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0042] 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 and γ-valerolactone; and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate.

[0043] 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-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ether, etc., and 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.

[0044] As the above halogenated derivatives, it is preferable to use fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP), etc.

[0045] The electrolyte salt is preferably a lithium salt. Examples of the lithium salt 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 <#cod# 10, LiCl, LiBr, LiI, lithium chloroborane, lower aliphatic lithium carboxylates, borates such as Li2B4O7, Li(B(C2O4)F2), LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 Examples of the lithium salt include imide salts such as SiO2) (where l and m are integers of 1 or more). The lithium salt may be used alone or in combination. Among these, LiPF6 is preferred from the viewpoints of ionic conductivity, electrochemical stability, etc. The concentration of the lithium salt is preferably 0.8 to 1.8 mol per 1 L of solvent. [Example]

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

[0047] Example 1 [Preparation of positive electrode] The positive electrode active material is aluminum-containing lithium nickel cobalt oxide (LiNi 0.88 Co 0.09 Al 0.03 O2) was used. 100 parts by mass of the above positive electrode active material, 1 part by mass of graphite as a conductive agent, and 0.9 parts by mass of polyvinylidene fluoride powder as a binder were mixed together, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry. This slurry was applied to both sides of a positive electrode current collector made of aluminum foil (thickness 15 μm) by the doctor blade method, and after drying the coating, the coating was rolled with a rolling roller to produce a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode current collector.

[0048] [Preparation of graphite particles A] Coke with an average particle size of 17 μm was agglomerated with the addition of pitch as a binder. Isotropic pressure of 1.6 g / cm was applied to the agglomerates. 3 ~1.9g / cm 3This block-shaped compact was graphitized by firing at a temperature of 2800°C, and then crushed and sieved to produce graphite particles A with an average particle size of 23 μm.

[0049] [Preparation of graphite particles B] Pitch was added as a binder to coke with an average particle size of 13 μm, and the mixture was agglomerated until the average particle size reached 18 μm. The agglomerates were graphitized by firing at 2800°C and then sieved to produce graphite particles B with an average particle size of 23 μm. When producing graphite particles B, the amount of pitch added to the coke was reduced from the amount of pitch used in producing graphite particles A, and the average particle size of the agglomerates was adjusted to produce graphite particles B with a smaller internal porosity than graphite particles A.

[0050] [Preparation of negative electrode] Graphite particles A were mixed to 95 parts by mass and SiO 5 parts by mass, which was designated as negative electrode active material A. Negative electrode active material A: carboxymethyl cellulose (CMC): styrene-butadiene copolymer rubber (SBR) were mixed so that the mass ratio thereof was 100:1:1, and the mixture was kneaded in water to prepare a first negative electrode mixture slurry. Graphite particles B were mixed to 95 parts by mass and SiO 5 parts by mass, which was designated as negative electrode active material B. Negative electrode active material B: carboxymethyl cellulose (CMC): styrene-butadiene copolymer rubber (SBR) were mixed so that the mass ratio thereof was 100:1:1, and the mixture was kneaded in water to prepare a second negative electrode mixture slurry.

[0051] The first negative electrode mixture slurry was applied to both sides of a copper foil negative electrode current collector by a doctor blade method and dried to form a first negative electrode mixture layer. Furthermore, the second negative electrode mixture slurry was applied to the first negative electrode mixture layer and dried to form a second negative electrode mixture layer. The applied mass ratio per unit area of ​​the first negative electrode mixture slurry to the second negative electrode mixture slurry was 5:5. The first negative electrode mixture layer and the second negative electrode mixture layer were rolled using a rolling roller to produce a negative electrode.

[0052] [Preparation of non-aqueous electrolyte] Five parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of a non-aqueous solvent in which ethylene carbonate (EC) and dimethyl carbonate were mixed in a volume ratio of 1:3, and LiPF6 was dissolved therein at a concentration of 1.5 mol / L to prepare a non-aqueous electrolyte.

[0053] [Fabrication of Non-Aqueous Electrolyte Secondary Battery] (1) A microporous polypropylene film having a thickness of 6 μm and a porosity of 35% was used as the separator. (2) A positive electrode lead was attached to the positive electrode current collector, and a negative electrode lead was attached to the negative electrode current collector. After that, the positive electrode and the negative electrode were wound with a separator interposed between them to prepare a wound electrode body. (3) Insulating plates were placed above and below the electrode body, the negative electrode lead was welded to the exterior body, and the positive electrode lead was welded to the sealing member, and the electrode body was housed within the exterior body. (4) After the non-aqueous electrolyte was injected into the exterior body by a reduced pressure method, the opening of the exterior body was sealed with a sealing member via a gasket, completing a non-aqueous electrolyte secondary battery.

[0054] [Calculation of void ratio between graphite particles] At an ambient temperature of 25°C, the nonaqueous electrolyte secondary batteries were charged at a constant current of 0.2 C (920 mA) to 4.2 V, and then charged at a constant voltage of 4.2 V to C / 50. They were then discharged at a constant current of 0.2 C to 2.5 V. This cycle of charge and discharge was counted as one cycle, and five cycles were performed. After five cycles, the negative electrodes were removed from the nonaqueous electrolyte secondary batteries of each Example and Comparative Example, and the porosity between the graphite particles was calculated.

[0055] [Capacity retention rate measurement during rapid charge / discharge cycles] At an ambient temperature of 25°C, the nonaqueous electrolyte secondary batteries of each Example and Comparative Example were charged at a constant current of 1 C (4600 mA) to 4.2 V, and then charged at a constant voltage of 1 / 50 C from 4.2 V. They were then discharged at a constant current of 0.5 C to 2.5 V. This cycle of charge and discharge was counted as one cycle, and 100 cycles were performed. The capacity retention rate of the nonaqueous electrolyte secondary batteries of each Example and Comparative Example in rapid charge and discharge cycles was calculated using the following formula. Capacity retention rate = (discharge capacity at 100th cycle / discharge capacity at 1st cycle) x 100

[0056] <Example 2> A non-aqueous electrolyte secondary battery was fabricated and evaluated in the same manner as in Example 1, except that the porosity of the separator was changed to 45%.

[0057] Example 3 A non-aqueous electrolyte secondary battery was fabricated and evaluated in the same manner as in Example 1, except that the thickness of the separator was changed to 10 μm and the porosity was changed to 45%.

[0058] <Comparative Example 1> A nonaqueous electrolyte secondary battery was fabricated and evaluated in the same manner as in Example 1, except that the negative electrode active material A contained in the first negative electrode mixture slurry and the negative electrode active material B contained in the second negative electrode mixture slurry were both mixed so that the graphite particles A were 47.5 parts by mass, the graphite particles B were 47.5 parts by mass, and SiO was 5 parts by mass.

[0059] <Comparative Example 2> A nonaqueous electrolyte secondary battery was fabricated and evaluated in the same manner as in Example 2, except that the negative electrode active material A contained in the first negative electrode mixture slurry and the negative electrode active material B contained in the second negative electrode mixture slurry were both mixed so that the graphite particles A were 47.5 parts by mass, the graphite particles B were 47.5 parts by mass, and SiO was 5 parts by mass.

[0060] <Comparative Example 3> A nonaqueous electrolyte secondary battery was fabricated and evaluated in the same manner as in Example 3, except that the negative electrode active material A contained in the first negative electrode mixture slurry and the negative electrode active material B contained in the second negative electrode mixture slurry were both mixed so that the graphite particles A were 47.5 parts by mass, the graphite particles B were 47.5 parts by mass, and SiO was 5 parts by mass.

[0061] <Comparative Example 4> A non-aqueous electrolyte secondary battery was fabricated and evaluated in the same manner as in Example 1, except that the thickness of the separator was changed to 10 μm and the porosity was changed to 45%.

[0062] <Comparative Example 5> A nonaqueous electrolyte secondary battery was fabricated and evaluated in the same manner as in Comparative Example 4, except that the negative electrode active material A contained in the first negative electrode mixture slurry and the negative electrode active material B contained in the second negative electrode mixture slurry were both mixed so that the graphite particles A were 47.5 parts by mass, the graphite particles B were 47.5 parts by mass, and SiO was 5 parts by mass.

[0063] Table 1 summarizes the results of the capacity retention rate and battery capacity in rapid charge-discharge cycles for the nonaqueous electrolyte secondary batteries of Examples and Comparative Examples. Regarding battery capacity, the thinner the separator, which does not contribute to charge-discharge, the higher the rating. Table 1 also shows the separator porosity, D1, D2, D2 / D1, and S2 / S1. Note that a higher value of the capacity retention rate in rapid charge-discharge cycles indicates that the deterioration of rapid charge-discharge cycle characteristics was suppressed.

[0064] [Table 1]

[0065] In the examples, a higher capacity retention rate was obtained compared to the comparative examples, and both high capacity and excellent rapid charge / discharge cycle characteristics were achieved. The reason for the improved rapid charge / discharge cycle characteristics is thought to be that the increase in interparticle voids in the second negative electrode mixture layer improved the circulation of the electrolyte solution in the negative electrode. In addition, the fact that the packing density of the second negative electrode mixture layer was not excessively reduced and that a separator with a predetermined thickness and porosity was used contributed to the high capacity of the secondary battery. [Explanation of symbols]

[0066] 10 secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 15 exterior body, 16 sealing body, 17, 18 insulating plate, 19 positive electrode lead, 20 negative electrode lead, 21 grooved portion, 22 filter, 23 lower valve body, 24 insulating member, 25 upper valve body, 26 cap, 26a opening, 27 gasket, 30 negative electrode current collector, 32 negative electrode mixture layer, 32a first negative electrode mixture layer, 32b second negative electrode mixture layer

Claims

1. A non-aqueous electrolyte secondary battery comprising: an electrode assembly in which a positive electrode and a negative electrode face each other with a porous separator interposed therebetween; a non-aqueous electrolyte; and an exterior body that accommodates the electrode assembly and the non-aqueous electrolyte, the negative electrode includes a negative electrode current collector, a first negative electrode mixture layer provided on a surface of the negative electrode current collector, and a second negative electrode mixture layer facing the positive electrode with the separator interposed therebetween; the first negative electrode mixture layer and the second negative electrode mixture layer contain graphite particles having the same average particle size, and the internal porosity of the graphite particles contained in the first negative electrode mixture layer is higher than the internal porosity of the graphite particles contained in the second negative electrode mixture layer; a ratio (S2 / S1) of the porosity (S2) between the graphite particles in the second negative electrode mixture layer to the porosity (S1) between the graphite particles in the first negative electrode mixture layer is 1.1 to 2.0; a ratio (D2 / D1) of the packing density (D2) of the second negative electrode mixture layer to the packing density (D1) of the first negative electrode mixture layer is 0.9 to 1.1; The separator has a thickness of 10 μm or less and a porosity of 25% to 45%.

2. The packing density (D1) of the first negative electrode mixture layer and the packing density (D2) of the second negative electrode mixture layer are 1.3 g / m ３ ~2.0 g / m ３ 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein

3. 3 . The nonaqueous electrolyte secondary battery according to claim 1 , wherein at least one of the first negative electrode mixture layer and the second negative electrode mixture layer contains a Si-based material.

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