Negative 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 having a high capacity and satisfactory rapid charge / discharge performance. A negative electrode for nonaqueous electrolyte secondary batteries according to one aspect of the present disclosure is provided with: a negative electrode current collector; a first negative electrode mix layer arranged on a surface of the negative electrode current collector; and a second negative electrode mix layer arranged on a surface of the first negative electrode mix layer. The first negative electrode mix layer contains a first carbon material having a true density of 2.1 g / cm3 to 2.3 g / cm3, the second negative electrode mix layer contains a second carbon material having a true density of 1.5 g / cm3 to 2.0 g / cm3, the inter-particle porosity of the second carbon material in the second negative electrode mix layer is larger than that of the first carbon material in the first negative electrode mix layer, and the ratio of the mass of the first negative electrode mix layer to that of the second negative electrode mix layer is 95:5 to 80:20.

Description

[Technical field]

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

[0002] In order to increase the energy density of non-aqueous electrolyte secondary batteries, graphite with high true density is sometimes used as the negative electrode active material. However, when the negative electrode active material is made only of graphite with high true density, there is a problem that the battery capacity decreases with repeated charging and discharging.

[0003] For example, Patent Documents 1 and 2 disclose a method of suppressing the decrease in battery capacity due to charge / discharge cycles by using a mixed material of graphite and a non-graphitic carbon material having a lower true density than graphite as the negative electrode active material. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 7-192724 [Patent Document 2] Japanese Patent Application Publication No. 11-250936 Summary of the Invention [Problem to be solved by the invention]

[0005] Meanwhile, secondary batteries for in-vehicle and power storage applications are required to be capable of rapid charging and discharging in addition to high capacity. The methods disclosed in Patent Documents 1 and 2 still have room for improvement in terms of rapid charging and discharging performance.

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

[0007] A negative electrode for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes 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 provided on the surface of the first negative electrode mixture layer. The first negative electrode mixture layer has a true density of 2.1 g / cm 3 ~2.3g / cm 3 The second negative electrode mixture layer has a true density of 1.5 g / cm 3 ~2.0g / cm 3 the interparticle porosity of the second carbon material in the second negative electrode mixture layer is greater than the interparticle porosity of the first carbon material in the first negative electrode mixture layer, and the mass ratio of the first negative electrode mixture layer to the second negative electrode mixture layer is 95:5 to 80:20.

[0008] A nonaqueous electrolyte secondary battery according to one aspect of the present disclosure includes the above-described negative electrode for nonaqueous electrolyte secondary batteries, a positive electrode, and a nonaqueous electrolyte. Effect of the Invention

[0009] According to one aspect of the present disclosure, it is possible to provide a nonaqueous electrolyte secondary battery having high capacity and good rapid charge / discharge performance. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a longitudinal 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

[0011] A negative electrode for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes 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 provided on the surface of the first negative electrode mixture layer. The first negative electrode mixture layer has a true density of 2.1 g / cm 3 ~2.3g / cm 3 The second negative electrode mixture layer has a true density of 1.5 g / cm 3~2.0g / cm 3 the interparticle porosity of the second carbon material in the second negative electrode mixture layer is greater than the interparticle porosity of the first carbon material in the first negative electrode mixture layer, and the mass ratio of the first negative electrode mixture layer to the second negative electrode mixture layer is 95:5 to 80:20.

[0012] 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.

[0013] Fig. 1 is a longitudinal 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".

[0014] The opening end of the exterior body 15 is closed with the sealing body 16, so that the inside of the secondary battery 10 is sealed. Insulating plates 17 and 18 are provided above and below the electrode body 14, respectively. The positive electrode lead 19 extends upward through the through hole of the insulating plate 17 and is welded to the underside 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 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. In addition, when the negative electrode lead 20 is installed at the terminal portion, the negative electrode lead 20 passes outside the insulating plate 18, extends to the bottom side of the exterior body 15, and is welded to the inner bottom surface of the exterior body 15.

[0015] The exterior body 15 is, for example, a cylindrical metal exterior can with a bottom. A gasket 27 is provided between the exterior body 15 and the sealing body 16 to ensure the sealing of the inside of the secondary battery 10. The exterior body 15 has a grooved portion 21 that supports the sealing body 16 and is formed, for example, by pressing a side portion from the outside. The grooved portion 21 is preferably formed in an annular shape along the circumferential direction of the exterior body 15, and supports the sealing body 16 on its upper surface via the gasket 27.

[0016] 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 member constituting the sealing body 16 has, for example, a disk 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 respective centers, and the insulating member 24 is interposed between their respective peripheral edges. When the internal pressure of the battery increases due to abnormal heat generation, for example, the lower valve body 23 breaks, and as a result, the upper valve body 25 swells toward the cap 26 and separates from the lower valve body 23, thereby cutting off the electrical connection between them. When the internal pressure further increases, the upper valve body 25 breaks, and gas is discharged from the opening 26a of the cap 26.

[0017] Hereinafter, the positive electrode 11, negative electrode 12, separator 13, and nonaqueous electrolyte that constitute the secondary battery 10 will be described in detail, in particular the configurations of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b that constitute the negative electrode 12 (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).

[0018] [Negative electrode] FIG. 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 mass ratio of the first negative electrode mixture layer to the second negative electrode mixture layer is 95:5 to 80:20. If the mass ratio of the second negative electrode mixture layer to the first negative electrode mixture layer is less than 5 / 95, the permeability of the electrolyte is too low, resulting in poor rapid charge / discharge performance. If the mass ratio of the second negative electrode mixture layer to the first negative electrode mixture layer exceeds 20 / 80, the battery capacity is reduced.

[0019] 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.

[0020] The first negative electrode mixture layer has a true density of 2.1 g / cm 3 ~2.3g / cm 3 The first carbon material contains a first carbon material having a structure in which the first carbon material is a carbon material having a high energy density and the secondary battery 10 has a high capacity. Here, the true density is calculated by dividing the mass by the volume not including the internal voids and pores, and can be measured by, for example, a pycnometer method. The true density of the second carbon material can be measured in the same manner.

[0021] The first carbon material is, for example, graphite particles. Examples of graphite particles include natural graphite and artificial graphite. Natural graphite is obtained, for example, by spheroidizing naturally produced flake-like graphite and molding it into a suitable particle size. Artificial graphite is obtained, for example, by using petroleum or coal-based pitch material having a specific particle shape as a starting material, calcining it at about 900°C to 1100°C to temporarily remove volatile components, and further calcining it at a high temperature of 2500°C or higher. The true density can be controlled by the calcination temperature, time, and starting material.

[0022] The second negative electrode mixture layer has a true density of 1.5 g / cm 3 ~2.0g / cm 3The second carbon material may be, for example, easy-carbonized graphite or difficult-carbonized graphite. The second carbon material is obtained by firing a petroleum or coal-based pitch material having a specific particle shape at about 900°C to 1100°C, and is a material having a graphite structure in at least a part of its crystal structure. The true density can be controlled by the firing temperature, time, and starting materials.

[0023] The interparticle porosity of the second carbon material in the second negative electrode mixture layer is greater than the interparticle porosity of the first carbon material in the first negative electrode mixture layer. The second carbon material has a true density of 2.1 g / cm 3 ~2.3g / cm 3 Since the carbon material is harder than the first carbon material, the negative electrode mixture layer 32 is formed The shape does not change even after compression, and many voids remain between the particles according to the particle shape. The high interparticle porosity improves the permeability of the electrolyte, and facilitates the diffusion of lithium ions during the expansion and contraction of the negative electrode accompanying charge and discharge, improving the rapid charge and discharge performance. In this specification, the interparticle porosity of the carbon material is a two-dimensional value calculated from the ratio of the area of ​​the interparticle voids of the carbon material to the cross-sectional area of ​​the negative electrode mixture layer, and can be calculated as follows.

[0024] <Method for measuring interparticle void ratio of carbon materials> (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, the voids converted to white are excluded from the voids inside the particles of the carbon material (pores not connected to the particle surface) and the pores connected to the particle surface of the carbon material with a width of 3 μm or less, and the area of ​​the voids between the particles of the carbon material is calculated. The interparticle void ratio of the carbon material can be calculated based on the following formula. Interparticle void ratio of carbon material = area of ​​interparticle void of carbon material / cross-sectional area of ​​negative electrode mixture layer × 100 (5) The interparticle porosity of the first carbon material in the first negative electrode mixture layer 32a and the interparticle porosity of the second carbon material in the second negative electrode mixture layer 32b are each calculated as an average value of the three measurements.

[0025] 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 a first carbon material, 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 a second carbon material different from the first carbon material, 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. In the above method, the first negative electrode mixture slurry is applied and dried, and then the second negative electrode mixture slurry is applied, but the second negative electrode mixture slurry may be applied after the first negative electrode mixture slurry is applied and before drying. Also, the second negative electrode mixture slurry may be applied onto the first negative electrode mixture layer 32a after the first negative electrode mixture slurry is applied, dried, and rolled.

[0026] 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 that can improve 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.

[0027] 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 preferably, for example, 10% by mass or less with respect to the mass of the negative electrode active material.

[0028] In addition, the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b preferably contain 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), etc. These may be used alone or in combination of two or more kinds.

[0029] [Positive electrode] The positive electrode 11 is composed of a positive electrode current collector such as a metal foil and a positive electrode mixture layer formed on the positive electrode current collector. The positive electrode current collector may be a foil of a metal such as aluminum that is stable in the potential range of the positive electrode, or a film having the metal disposed on the surface layer. The positive electrode mixture layer contains, for example, a positive electrode active material, a binder, a conductive agent, etc.

[0030] 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, etc. onto a positive electrode current collector, drying it to form a positive electrode mixture layer, and then rolling this positive electrode mixture layer.

[0031] 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 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 two or more. In terms of achieving high capacity of the non-aqueous electrolyte secondary battery, the positive electrode active material preferably contains lithium nickel composite oxides such as Li x NiO2, Li x Co y Ni 1-y O2, Li x Ni 1-y M y O 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), etc.

[0032] 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.

[0033] Examples of the binder include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic-based resins, polyolefin-based resins, etc. These may be used alone or in combination of two or more kinds.

[0034] [Separator] For example, a porous sheet having ion permeability and insulation is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. The separator is preferably made of an olefin resin such as polyethylene or polypropylene, or cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin resin. The separator 13 may also be a multilayer separator including a polyethylene layer and a polypropylene layer, and may have a material such as an aramid resin or ceramic applied to the surface of the separator 13.

[0035] [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.

[0036] 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), and ethyl propionate.

[0037] 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.

[0038] 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.

[0039] 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 1 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 preferably 0.8 to 1.8 mol per 1 L of the solvent.

Examples

[0040] Hereinafter, the present disclosure will be further described by way of examples, but the present disclosure is not limited to these examples.

[0041] <Example 1> [Fabrication of positive electrode] As the positive electrode active material, lithium cobaltate containing zirconium, magnesium, and aluminum (LiCo 0.979 Zr 0.001 Mg 0.01 Al 0.01O2) was used. The positive electrode active material was mixed in an amount of 95 parts by mass, carbon powder as a conductive agent was 2.5 parts by mass, and polyvinylidene fluoride powder as a binder was 2.5 parts by mass, 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 prepare a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode current collector.

[0042] [Preparation of carbon material A] Coke was crushed to an average particle size of 13 μm, and pitch was added as a binder to the crushed coke and pressed into a block shape. This block-shaped molded body was graphitized by firing at a temperature of 2500°C or higher, and then crushed and sieved using a 250 mesh sieve to obtain carbon material A with an average particle size of 23 μm. The true density of the produced carbon material A was measured by the pycnometer method and found to be 2.2 g / cm 3 It was.

[0043] [Preparation of carbon material 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 fired at a temperature of 1000° C., and then sieved using a 250 mesh sieve to obtain carbon material B with an average particle size of 18 μm. The true density of the produced carbon material B was measured by the pycnometer method and found to be 1.7 g / cm 3 It was.

[0044] [Preparation of negative electrode] Carbon material A was mixed to 94 parts by mass and SiO was mixed to 6 parts by mass, which was used as negative electrode active material A. Negative electrode active material A: Na salt of carboxymethyl cellulose (CMC-Na): 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. Carbon material B was mixed to 94 parts by mass and SiO was mixed to 6 parts by mass, which was used as negative electrode active material B. Negative electrode active material B: CMC-Na: 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.

[0045] 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 to form a first negative electrode mixture layer. Furthermore, the second negative electrode mixture slurry was applied to the first negative electrode mixture layer 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 10:90. After drying the first negative electrode mixture layer and the second negative electrode mixture layer, they were rolled with a rolling roller to prepare a negative electrode. The application amount of the slurry on the positive electrode and the negative electrode was adjusted so that the charge capacity ratio (negative electrode charge capacity / positive electrode charge capacity), which is the ratio of the negative electrode charge capacity to the positive electrode charge capacity at 4.2V, was 1.1.

[0046] [Preparation of non-aqueous electrolyte] LiPF6 was dissolved in a non-aqueous solvent, which was a mixture of ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC) in a volume ratio of 10:10:80, at a concentration of 1.0 mol / L to prepare a non-aqueous electrolyte.

[0047] [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, and then the positive and negative electrodes were wound with a separator made of a microporous polyethylene film between them, and a polypropylene tape was attached to the outermost circumference to prepare a cylindrical electrode body. The electrode body was then pressed to form a flat electrode body. (2) A sheet-shaped laminate material having a five-layer structure of a resin layer (polypropylene) / adhesive layer / aluminum alloy layer / adhesive layer / resin layer (polypropylene) was prepared, a cup-shaped storage section for storing an electrode body was provided in the laminate material, and the laminate material was folded to cover the storage section to form a pouch-shaped exterior body. In a glove box under an argon atmosphere, the flat electrode body and the nonaqueous electrolyte were inserted into the storage section of the exterior body. Thereafter, the pressure inside the exterior body was reduced to impregnate the separator with the nonaqueous electrolyte, and the open end of the exterior body was sealed to produce a pouch-shaped nonaqueous electrolyte secondary battery with a height of 62 mm, a width of 35 mm, and a thickness of 3.6 mm.

[0048] <Examples 2 and 4, Comparative Examples 5 to 7> As shown in Table 1, a nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that the mass ratio between the first negative electrode mixture layer and the second negative electrode mixture layer was changed.

[0049] <Example 3> A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 2, except that negative electrode active material B was replaced with negative electrode active material C prepared by mixing carbon material C and SiO prepared as described below.

[0050] [Preparation of carbon material C] 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 fired at a temperature of 1050° C., and then sieved using a 250 mesh sieve to obtain carbon material C with an average particle size of 18 μm. The true density of the produced carbon material C was measured by the pycnometer method and found to be 1.9 g / cm 3 It was.

[0051] <Comparative Examples 1 to 4> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that one negative electrode mixture layer containing the negative electrode active material shown in Table 1 was used instead of the two negative electrode mixture layers.

[0052] [Calculation of interparticle void ratio of carbon materials] 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 0.2C (920mA) to 4.2V, and then charged at a constant voltage of 4.2V to C / 50. Then, the batteries were discharged at a constant current of 0.5C to 2.5V. This charge / discharge cycle was repeated for 5 cycles. After 5 cycles, the negative electrodes were removed from the non-aqueous electrolyte secondary batteries of each Example and Comparative Example, and the interparticle porosity of the carbon material in the first negative electrode mixture layer and the second negative electrode mixture layer was compared. As a result, it was confirmed that the interparticle porosity of the second carbon material in the second negative electrode mixture layer was larger than the interparticle porosity of the first carbon material in the first negative electrode mixture layer in all of Examples 1 to 4 and Comparative Examples 5 to 7. Note that the interparticle porosity of the carbon material was not measured for Comparative Examples 1 to 4.

[0053] [Measurement of absorption time] The negative electrodes of the examples and comparative examples were dried for 10 hours in a thermostatic chamber heated to 200°C under a nitrogen atmosphere, and each negative electrode was cut into a size of 2 cm x 5 cm to prepare a sample. 3 μL of polypropylene carbonate (PC) was dropped vertically onto the surface of each sample, and the time until the PC was absorbed into the inside of the sample was measured visually. Measurements were performed six times for each sample, and the average value was taken as the liquid absorption time. The shorter the liquid absorption time, the better the permeability of the electrolyte into the negative electrode.

[0054] [Measurement of battery capacity and discharge load characteristics] At an environmental temperature of 25°C, the non-aqueous electrolyte secondary batteries of the examples and comparative examples were charged at a constant current of 1C (800mA) to 4.2V, and then charged at a constant voltage of 1 / 50C at 4.2V. The discharge capacity at the time of constant current discharge at 1C (800mA) to 2.75V was taken as the 1C discharge capacity. Similarly, the non-aqueous electrolyte secondary batteries were charged at a constant current of 4.2V, then charged at a constant voltage of 1 / 50C at 4.2V, and then discharged at a constant current of 3C (2400mA) to 2.75V was taken as the 3C discharge capacity. The discharge load characteristics of the non-aqueous electrolyte secondary batteries of the examples and comparative examples were calculated by the following formula. The higher the discharge load characteristics, the better the discharge characteristics. The 1C discharge capacity was taken as the battery capacity. Discharge load characteristics = (3C discharge capacity / 1C discharge capacity) × 100

[0055] The results of the electrolyte permeability, discharge load characteristics, and battery capacity of the nonaqueous electrolyte secondary batteries of the Examples and Comparative Examples are summarized in Table 1. In Table 1, the values ​​of the electrolyte absorption time, discharge load characteristics, and battery capacity of Comparative Example 1 are set to 100, and the values ​​of the other Examples and Comparative Examples are shown as relative values. Table 1 also shows the type and mass ratio of the negative electrode active material of the first negative electrode mixture layer and the second negative electrode mixture layer.

[0056] [Table 1]

[0057] In Comparative Example 2, the discharge load characteristics are improved compared to Comparative Example 1, but the battery capacity is significantly reduced. This result suggests that when the negative electrode active material B is used, the battery capacity is reduced but the discharge load characteristics are improved. However, in Comparative Examples 3 and 4, which use a mixture of the negative electrode active material A and the negative electrode active material B, the decrease in the battery capacity is suppressed compared to Comparative Example 2, but the effect of improving the discharge load characteristics is not sufficient. In this way, when a mixture of the negative electrode active material A and the negative electrode active material B is used in a range in which the decrease in the battery capacity is suppressed, the discharge load characteristics cannot be sufficiently improved. On the other hand, in Examples 1 to 4, the decrease in the battery capacity is suppressed as in Comparative Examples 3 and 4, and the discharge load characteristics are equal to or higher than those of Comparative Example 2. In this way, by using a two-layer negative electrode mixture layer that satisfies the predetermined conditions, a nonaqueous electrolyte secondary battery having high capacity and good rapid charge and discharge characteristics can be realized. [Explanation of symbols]

[0058] 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 current collector; and a negative electrode mixture layer including 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 negative electrode mixture layer contains a negative electrode active material made of a carbon material and a Si-based material, At least one of the first negative electrode mixture layer and the second negative electrode mixture layer contains 1 mass % to 10 mass % of a Si-based material with respect to the mass of the negative electrode active material, The first negative electrode mixture layer has a true density of 2.1 g / cm ３ ~2.3g / cm ３ The negative electrode active material includes a first carbon material, The second negative electrode mixture layer has a true density of 1.5 g / cm ３ ~2.0g / cm ３ The negative electrode active material includes a second carbon material, an interparticle porosity of the second carbon material in the second negative electrode mixture layer is greater than an interparticle porosity of the first carbon material in the first negative electrode mixture layer; a mass ratio of the first negative electrode mixture layer to the second negative electrode mixture layer is 95:5 to 80:

20.

2. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1 , A positive electrode and A non-aqueous electrolyte secondary battery comprising:

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