Non-aqueous electrolyte secondary battery

Abstract

The purpose of the present disclosure is to provide a non-aqueous electrolyte secondary battery that has a high capacity and that achieves excellent suppression of deterioration in quick charge cycle characteristics. A non-aqueous electrolyte secondary battery according to an aspect of the present disclosure is provided with a negative electrode having: a negative electrode collector; a first negative electrode mixture layer provided on the surface of the negative electrode collector; and a second negative electrode mixture layer provided on the surface of the first negative electrode mixture layer. Each of the first negative electrode mixture layer and the second negative electrode mixture layer contains graphite particles. The ratio (S2 / S1) of the inter-particle porosity (S2) of the graphite particles in the second negative electrode mixture layer to the inter-particle porosity (S1) of the graphite particles in the first negative electrode mixture layer is 1.1-2.0. The ratio (D2 / D1) of the filling density (D2) of the second negative electrode mixture layer to the filling density (D1) of the first negative electrode mixture layer is 0.9-1.1.

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 a negative electrode active material are widely used as secondary batteries with high energy density. The battery capacity can be increased by increasing the packing density of the negative electrode active material in the negative electrode mixture layer, but if the amount of the negative electrode active material per unit volume is increased to increase the packing density, the gaps between the negative electrode active materials become smaller, which leads to poor permeability of the electrolyte, resulting in a problem of a decrease in battery capacity due to repeated rapid charging.

[0003] For example, in the inventions disclosed in Patent Documents 1 to 3, the packing density of the negative electrode active material in the negative electrode mixture layer is made lower on the outer surface side than on the current collector side, thereby increasing the gaps between the negative electrode active material on the outer surface side and improving the liquid permeability of the electrolyte. However, there is a problem in that the amount of negative electrode active material per unit volume in the negative electrode mixture layer is reduced, resulting in a reduced battery capacity. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2003-77463 A [Patent Document 2] JP 2006-196457 A [Patent Document 3] Special Publication No. 2015-511389 Summary of the Invention [Problem to be solved by the invention]

[0005] Therefore, an object of the present disclosure is to provide a nonaqueous electrolyte secondary battery that has a high capacity and is excellent in suppressing the deterioration of rapid charging cycle characteristics. [Means for solving the problem]

[0006] A nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a negative electrode having 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 provided on a surface of the first negative electrode mixture layer, 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, and 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. Effect of the Invention

[0007] 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 excellent in suppressing deterioration of rapid charging cycle characteristics. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is an axial cross-sectional view of a cylindrical secondary battery as one example of the embodiment. [Diagram 2] FIG. 2 is a cross-sectional view of a negative electrode according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] As described above, it was difficult for conventional batteries to achieve both high capacity and excellent suppression of deterioration in rapid charging cycle characteristics. As a result of intensive research, the present inventors discovered that by including graphite particles with low internal porosity on the outer surface side of the negative electrode mixture layer, the porosity between the graphite particles on the outer surface side can be increased while maintaining the packing density approximately equal to that on the negative electrode current collector side, and came up with the following embodiment of a nonaqueous electrolyte secondary battery that has high capacity and is excellent in suppressing deterioration in rapid charging cycle characteristics.

[0010] A nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a negative electrode having 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 provided on a surface of the first negative electrode mixture layer, 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, and 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.

[0011] Hereinafter, an example of an embodiment of a cylindrical secondary battery according to the present disclosure will be described in detail with reference to the drawings. In the following description, specific shapes, materials, 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. In addition, the exterior body is not limited to a cylindrical shape, and may be, for example, a rectangular shape. In addition, in the following description, when multiple embodiments and modified examples are included, it is assumed from the beginning that the characteristic parts of these embodiments and modified examples will be used in appropriate combination.

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

[0013] The opening end of the outer package 15 is sealed by the sealing body 16, so that the interior of the secondary battery 10 is sealed. Insulating plates 17 and 18 are respectively provided above and below the electrode body 14. The positive electrode lead 19 extends upward through the through-hole of the insulating plate 17 and is welded to the lower surface of the filter 22 which is the bottom plate of the sealing body 16. In the secondary battery 10, the cap 26 which is the top plate of the sealing body 16 electrically connected to the filter 22 serves as the positive electrode terminal. On the other hand, the negative electrode lead 20 extends through the through-hole of the insulating plate 18 to the bottom side of the outer package 15 and is welded to the inner surface of the bottom of the outer package 15. In the secondary battery 10, the outer package 15 serves as the negative electrode terminal. When the end portion of the negative electrode lead 20 is installed, the negative electrode lead 20 extends through the outside of the insulating plate 18 to the bottom side of the outer package 15 and is welded to the inner surface of the bottom of the outer package 15.

[0014] The outer package 15 is, for example, a bottomed cylindrical metal outer can. A gasket 27 is provided between the outer package 15 and the sealing body 16 to ensure the sealing property inside the secondary battery 10. The outer package 15 has, for example, a groove portion 21 formed by pressing the side surface from the outside to support the sealing body 16. The groove portion 21 is preferably formed in an annular shape along the circumferential direction of the outer package 15 and supports the sealing body 16 via the gasket 27 on its upper surface.

[0015] The sealing body 16 has a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26 laminated in order from the side of the electrode body 14. Each member constituting the sealing body 16 has, for example, a disc shape or a ring shape, and each member except the insulating member 24 is electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their central portions, and an insulating member 24 is interposed between the peripheral edges of each. When the internal pressure of the battery rises due to abnormal heat generation, for example, the lower valve body 23 breaks, and as a result, the upper valve body 25 bulges toward the cap 26 side and separates from the lower valve body 23, thereby cutting off their electrical connection. When the internal pressure further rises, the upper valve body 25 breaks, and gas is discharged 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, and in particular the negative electrode active material contained in the negative electrode mixture layer 32 that constitutes the negative electrode 12, will be described in detail below.

[0017] [Negative electrode] 2 is a cross-sectional view of the 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 may have 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] For example, a foil of a metal such as copper that is stable in the potential range of the negative electrode, or a film having 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. In addition, the negative electrode mixture layer 32 preferably contains a binder or the like. Examples of the binder include fluorine-based resin, PAN, polyimide-based resin, acrylic-based resin, polyolefin-based resin, styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof (PAA-Na, PAA-K, etc., or a partially neutralized salt), polyvinyl alcohol (PVA), and the like. These may be used alone or in combination of two or more kinds.

[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 crystallite size (Lc(002)) satisfies the above range, the battery capacity of the secondary battery 10 tends to be larger than when the crystallite size (Lc(002)) does not satisfy the above range.

[0021] The graphite particles contained in the first negative electrode mixture layer 32a can be prepared, for example, as follows. The main raw material coke (precursor) is pulverized to a predetermined size, aggregated with a binder, and then pressed into a block shape, which is then fired at a temperature of 2600°C or higher to be graphitized. The graphitized block-shaped body is pulverized and sieved to obtain graphite particles of a desired size. Here, the internal porosity of the graphite particles can be adjusted by the particle size of the pulverized precursor or the particle size of the aggregated precursor. For example, the average particle size (volume-equivalent median diameter D50, the same applies below) of the pulverized 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-shaped body. When a part of the binder added to the coke (precursor) volatilizes during firing, the binder can be used as a 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) as a main raw material is pulverized to a predetermined size, and in this state, the coke is aggregated with a binder such as pitch, baked at a temperature of 2600°C or higher, graphitized, and then sieved to obtain graphite particles of a desired size. Here, the internal porosity of the graphite particles can be adjusted by the particle size of the precursor after pulverization or the particle size of the precursor in an aggregated state. For example, the average particle size of the precursor after pulverization is preferably in the range of 12 μm to 20 μm.

[0023] The ratio (S2 / S1) of the void ratio (S2) between the graphite particles in the second negative electrode mixture layer 32b to the void ratio (S1) between the 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 liquid circulation and permeability of the electrolyte is deteriorated, and the battery capacity decreases due to 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, and the battery capacity decreases. Here, the void ratio between the graphite particles is a two-dimensional value calculated from the ratio of the area of ​​the gap between the 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 a 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, a reflected electron image of the cross section of the exposed negative electrode mixture layer is taken for each of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b. The magnification when taking the reflected electron image 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 binary processed image in which the particle cross-sections in the cross-sectional image are colored black and 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, among the voids converted to white, the voids inside the graphite particles (pores not connected to the particle surface) and the pores connected to the graphite particle surface and having a width of 3 μm or less are regarded as 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 based on 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 calculated 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. By making S2 / S1 1.1 to 2.0 and D2 / D1 within this range, it is possible to obtain an excellent battery that has a high capacity but reduces the inhibition of rapid charging cycle characteristics. 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, it is possible to satisfy the above ranges for S2 / S1 and D2 / D1.

[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 1.3 g / c m 3 ~2.0g / c m 3 This allows the battery to have a high capacity.

[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, the mass of the mixture per unit area of ​​each of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b is measured using the negative electrode 12. Also, the thickness of each of the mixture layer 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 interparticle porosity. When the mixture layer thickness is not stable, 10 points are measured in the image, and the average value can be used as the mixture 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 mixture mass per unit area by the mixture 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 (hereinafter sometimes referred to as second graphite particles) different from the first 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, and then the second negative electrode mixture slurry is applied to both sides on the coating film 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 by 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 properties of the first graphite particles and the second graphite particles during rolling are not necessarily the same. For example, the packing density 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. In addition, by making the internal porosity of the second graphite particles lower than the internal porosity 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 conditions for rolling the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b, the packing densities of the respective layers can be adjusted more freely.

[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 absorb 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. The Si-based material is a negative electrode material capable of improving the battery capacity more than graphite particles. The content of the Si-based material is, for example, preferably 1 mass % to 10 mass %, and more preferably 3 mass % to 7 mass %, relative to the mass of the negative electrode active material, from the viewpoints of improving the battery capacity and suppressing the deterioration of the rapid charging cycle characteristics.

[0032] Other examples of the other material capable of reversibly absorbing and releasing lithium ions include metals that are alloyed 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 materials, and the content of the other materials is desirably, for example, 10% by mass or less with respect 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 produced, for example, by 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 oxide is, for example, Li x CoO 2 , Li x NiO 2 , Li x MnO 2 , Li x Co y Ni 1-y O 2 , Li x Co y M 1-y O z , Li x Ni 1-y M y O z , Li x Mn 2 O 4 , Li x Mn 2-y M y O 4 , LiMPO 4 , Li 2 MPO 4 F(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 types. 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 O 2 、 Li x Ni 1-y M y O z It is preferable to contain a lithium nickel composite oxide such as z (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).

[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 is used. Specific examples of the porous sheet include microporous thin films, 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. Further, it may be a multilayer separator including a polyethylene layer and a polypropylene layer, or a separator in which a material such as an aramid-based resin or ceramic is applied to the surface of the separator 13 may be used.

[0039] [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-like polymer or the like. The non-aqueous solvent may be, for example, esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, or a mixed solvent of two or more of these. The non-aqueous solvent may contain a halogen-substituted product in which at least a part of the hydrogen of these solvents is substituted with a halogen atom such as fluorine.

[0040] 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 carboxylates such as γ-butyrolactone and γ-valerolactone; and chain carboxylates such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), ethyl propionate, and γ-butyrolactone.

[0041] Examples of the 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, cyclic ethers such as crown ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, and the like. and chain ethers such as 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, and tetraethylene glycol dimethyl ether.

[0042] As the halogen-substituted compound, it is preferable to use a fluorinated cyclic carbonate such as fluoroethylene carbonate (FEC), a fluorinated chain carbonate, a fluorinated chain carboxylate such as methyl fluoropropionate (FMP), or the like.

[0043] The electrolyte salt is preferably a lithium salt. An example of the lithium salt is LiBF 4 , LiClO 4 , LiPF 6 , LiAsF 6 , LiSbF 6 , LiAlCl 4 , LiSCN, LiCF 3 SO 3 , LiCF 3 CO 2 , Li(P(C 2 O 4 )F 4 ), LiPF 6-x (Cn F 2n+1 ) x (1 < x < 6, n is 1 or 2), LiB 10 Cl 10 , LiCl, LiBr, LiI, lithium chloroborane, lithium lower aliphatic carboxylate, Li 2 B 4 O 7 , Li(B(C 2 O 4 )F 2 ) and other borates, LiN(SO 2 CF 3 ) 2 , LiN(C 1 F 2l+1 SO 2 )(C m F 2m+1 SO 2 ) {l, m are integers of 1 or more} and other imide salts and the like can be mentioned. The lithium salt may be used alone or in combination of plural kinds. Among these, from the viewpoints of ionic conductivity, electrochemical stability, etc., it is preferable to use LiPF 6 . The concentration of the lithium salt is preferably 0.8 to 1.8 mol per 1 L of the solvent.

Examples

[0044] Hereinafter, the present disclosure will be further described by way of examples, but the present disclosure is not limited to these examples.

[0045] <Example 1> [Fabrication of positive electrode] As the positive electrode active material, lithium nickel cobalt aluminate containing aluminum and cobalt (LiNi 0.88 Co 0.09 Al 0.03 O 2) was used. The positive electrode active material was mixed in an amount of 100 parts by mass, graphite as a conductive agent was 1 part by mass, and polyvinylidene fluoride powder as a binder was 0.9 parts by mass, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further 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 a doctor blade method, and after drying the coating, the coating was rolled with a rolling roller to prepare a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode current collector.

[0046] [Preparation of graphite particles A] The coke was crushed to an average particle size of 17 μm, and pitch was added as a binder to the crushed coke to cause agglomeration. The agglomerates were subjected to isotropic pressure of 1.6 g / cm 3 ~1.9g / cm 3 A block-shaped compact having a density of 1000 nm was produced. This block-shaped compact was graphitized by firing at a temperature of 2800° C. Next, the graphitized block-shaped compact was pulverized and sieved using a 250 mesh sieve to obtain graphite particles A having an average particle size of 23 μm.

[0047] [Preparation of graphite particles B] Coke was pulverized to an average particle size of 13 μm, and pitch was added as a binder to the pulverized coke, and the coke was aggregated to an average particle size of 18 μm. The aggregate was graphitized by firing at a temperature of 2800° C., and then sieved using a 250 mesh sieve to obtain 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 aggregate was adjusted, thereby producing graphite particles B having a smaller internal porosity than graphite particles A.

[0048] [Preparation of negative electrode] Graphite particles A were mixed to 95 parts by mass and SiO was mixed to 5 parts by mass, which was used as negative electrode active material A. Negative electrode active material A: carboxymethylcellulose (CMC): styrene-butadiene copolymer rubber (SBR) were mixed to a mass ratio of 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 was mixed to 5 parts by mass, which was used as negative electrode active material B. Negative electrode active material B: carboxymethylcellulose (CMC): styrene-butadiene copolymer rubber (SBR) were mixed to a mass ratio of 100:1:1, and the mixture was kneaded in water to prepare a second negative electrode mixture slurry.

[0049] The first negative electrode mixture slurry was applied to both sides of a negative electrode current collector made of copper foil by a doctor blade method, and then dried to form a first negative electrode mixture layer. Furthermore, the above-mentioned second negative electrode mixture slurry was applied to the first negative electrode mixture layer, and then dried to form a second negative electrode mixture layer. At this time, the application mass ratio per unit area of ​​the first negative electrode mixture slurry and the second negative electrode mixture slurry was 5:5. The first negative electrode mixture layer and the second negative electrode mixture layer were rolled by a rolling roller to produce a negative electrode.

[0050] [Preparation of non-aqueous electrolyte] 5 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 LiPF 6 was dissolved at a concentration of 1.5 mol / L to prepare a non-aqueous electrolyte.

[0051] [Preparation of non-aqueous electrolyte secondary battery] (1) 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, a separator made of a microporous polyethylene film was placed between the positive electrode and the negative electrode, and the electrodes were wound to prepare a wound electrode body. (2) 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. (3) 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.

[0052] <Example 2> The same procedure was carried out as in Example 1, except that graphite particles C prepared as follows were used instead of graphite particles B.

[0053] [Preparation of graphite particles C] The coke was pulverized until the average particle size was 12 μm. Pitch was added as a binder to the pulverized coke, and the coke was aggregated until the average particle size was 18 μm. The aggregate was fired at a temperature of 2800 ° C., graphitized, and then sieved using a 250 mesh sieve to produce graphite particles C with an average particle size of 23 μm. The amount of pitch used to produce graphite particles C was less than the amount of pitch used to produce graphite particles A and more than the amount of pitch used to produce graphite particles B. In this way, by adjusting the average particle size of the coke and the amount of pitch, the internal porosity of graphite particles C was lower than the internal porosity of graphite particles A and higher than the internal porosity of graphite particles B.

[0054] <Example 3> The same procedure as in Example 1 was followed except that the coating mass ratio per unit area of ​​the first negative electrode mixture slurry and the second negative electrode mixture slurry was set to 6:4.

[0055] <Example 4> The same procedure as in Example 1 was followed except that the coating mass ratio per unit area of ​​the first negative electrode mixture slurry and the second negative electrode mixture slurry was set to 4:6.

[0056] <Comparative Example 1> The same procedure as in Example 1 was carried out except that graphite particles C were used instead of graphite particles A and graphite particles A were used instead of graphite particles B.

[0057] <Comparative Example 2> The same procedure was carried out as in Example 1, except that instead of the graphite particles A and B, a mixture of graphite particles A and graphite particles C in a mass ratio of 1:1 was used.

[0058] [Calculation of void ratio between graphite particles] 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 0.2C (920mA) to 4.2V, and then charged at a constant voltage of 4.2V to C / 50. Thereafter, the batteries were discharged at a constant current of 0.2C to 2.5V. This charge / discharge cycle 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. Table 1 shows the results of each Example and Comparative Example.

[0059] [Measurement of capacity retention rate during rapid charging cycles] At an ambient temperature of 25°C, the non-aqueous electrolyte secondary batteries of each Example and Comparative Example were charged at a constant current of 1C (4600mA) to 4.2V, and then charged at a constant voltage of 1 / 50C at 4.2V. Thereafter, the batteries were discharged at a constant current of 0.5C to 2.5V. This charge and discharge constituted one cycle, and 100 cycles were performed. The capacity retention rate in the rapid charge cycle of the non-aqueous electrolyte secondary batteries of each Example and Comparative Example was calculated by the following formula. The discharge capacity in the first cycle was taken as the battery capacity. Capacity retention rate = (discharge capacity at 100th cycle / discharge capacity at 1st cycle) x 100

[0060] The results of the capacity retention rate and the battery capacity in the rapid charging cycle of the nonaqueous electrolyte secondary batteries of each Example and Comparative Example are summarized in Table 1. D1, D2, D2 / D1, and S2 / S1 are also shown in Table 1. Note that the higher the value of the capacity retention rate in the rapid charging cycle, the more excellent the suppression of the deterioration of the rapid charging cycle characteristics.

[0061] [Table 1]

[0062] In the examples in which the interparticle porosity of the second negative electrode mixture layer was higher than that of the first negative electrode mixture layer, the rapid charging cycle characteristics were improved. This is believed to be because the increase in the interparticle porosity of the second negative electrode mixture layer made it easier for the electrolyte to permeate from the surface of the mixture layer facing the positive electrode to the current collector side, and the electrolyte became easier to permeate into the entire negative electrode mixture layer. In addition, from the results of Examples 1, 3, and 4, it was found that the rapid charging cycle characteristics were improved when the coating mass per unit area was greater in the first negative electrode mixture layer than in the second negative electrode mixture layer. This is believed to be because the amount of electrolyte permeating into the negative electrode mixture layer was optimized. In addition, it was confirmed that a high-capacity battery was obtained in Examples 1 to 4. [Explanation of symbols]

[0063] REFERENCE SIGNS LIST 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 negative electrode including 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 provided on a surface of the first negative electrode mixture layer; the first negative electrode mixture layer and the second negative electrode mixture layer contain 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, and the porosities (S1 and S2) between the graphite particles are determined from a ratio of an area of ​​the gaps between the graphite particles to a cross-sectional area of ​​the first negative electrode mixture layer and the second negative electrode mixture layer, a ratio (D2 / D1) of a packing density (D2) of the second negative electrode mixture layer to a packing density (D1) of the first negative electrode mixture layer is 0.9 to 1.

1.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein 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 1.

7.

3. 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 / cm ３ ~2.0g / cm ３ 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein

4. 4. 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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