Non-aqueous electrolyte secondary battery

Abstract

Provided is a nonaqueous electrolyte secondary battery having high capacity and suppressed reduction in rapid charge / discharge cycle characteristics. A nonaqueous electrolyte secondary battery of one embodiment of the present application includes an electrode body in which a positive electrode and a negative electrode face each other with a porous separator therebetween, a nonaqueous electrolyte, and a container that accommodates the electrode body and the nonaqueous 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 therebetween. The first negative electrode mixture layer and the second negative electrode mixture layer each include graphite particles. The ratio (S2 / S1) of the porosity between the graphite particles in the second negative electrode mixture layer (S2) to the porosity between the graphite particles in the first negative electrode mixture layer (S1) is 1.1 to 2.0. The ratio (D2 / D1) of the packing density of the second negative electrode mixture layer (D2) to the packing density of the first negative electrode mixture layer (D1) is 0.9 to 1.1. The thickness of the separator is less than or equal to 10 µm. The porosity of the separator is 25% to 45%.

Description

TECHNICAL FIELD

[0001] The present application relates to a nonaqueous electrolyte secondary battery. BACKGROUND

[0002] A nonaqueous electrolyte secondary battery using graphite particles as a negative electrode active material is widely used as a high-capacity secondary battery. By increasing the packing density of the negative electrode active material per unit volume in the negative electrode mixture layer, it is possible to increase the battery capacity, but if the packing density of the negative electrode active material is increased, the voids between the negative electrode active materials become small, the liquid circulation of the electrolyte solution becomes poor, and there is a problem that the battery capacity decreases with repeated charge-discharge cycles of rapid charging and discharging (rapid charge-discharge cycles).

[0003] A polyethylene microporous film that is extremely thin and has a low porosity is disclosed in Patent Literature 1. However, such a microporous film cannot improve the liquid circulation of the negative electrode and cannot improve the rapid charge-discharge cycle characteristics of the secondary battery because of the lack of liquid retention.

[0004] On the other hand, in the inventions disclosed in Patent Literatures 2 to 4, by making the packing density of the negative electrode active material lower on the outer surface side than on the current collector side in the negative electrode mixture layer, the voids between the negative electrode active materials on the outer surface side are increased, and the liquid circulation of the electrolyte solution is improved. However, since the amount of the negative electrode active material per unit volume in the negative electrode mixture layer is reduced, there is a problem that the battery capacity decreases. Therefore, even if the polyethylene microporous film disclosed in Patent Literature 1 is applied to the inventions disclosed in Patent Literatures 2 to 4, it is not possible to achieve both high capacity and excellent rapid charge-discharge cycle characteristics.

[0005] Prior Art Documents

[0006] Patent Literature

[0007] Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 11-60790

[0008] Patent Literature 2: Japanese Patent Application Laid-Open (JP-A) No. 2003-77463

[0009] Patent Literature 3: Japanese Patent Application Laid-Open (JP-A) No. 2006-196457

[0010] Patent Literature 4: Japanese Patent Application Laid-Open (JP-A) No. 2015-511389 SUMMARY

[0011] Problems to be Solved by the Invention

[0012] To achieve the above object, the present application provides a nonaqueous electrolyte secondary battery having high capacity and in which the decrease in rapid charge-discharge cycle characteristics is suppressed.

[0013] Means for Solving the Problems

[0014] The nonaqueous electrolyte secondary battery according to one embodiment of the present application includes: an electrode body in which a positive electrode and a negative electrode face each other with a porous separator interposed therebetween, a nonaqueous electrolyte, and an exterior body that accommodates the electrode body and the nonaqueous 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. A ratio (S2 / S1) of a porosity (S2) between the graphite particles in the second negative electrode mixture layer to a porosity (S1) between the graphite particles in the first negative electrode mixture layer is 1.1 to 2.0. 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. A thickness of the separator is 10 μm or less. A porosity of the separator is 25% to 45%.

[0015] Effects of Invention

[0016] According to one embodiment of the present application, a nonaqueous electrolyte secondary battery with high capacity and reduced degradation of rapid charge-discharge cycle characteristics can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 FIG. 1 is an axial cross-sectional view of a cylindrical secondary battery according to one embodiment.

[0018] Figure 2 FIG. 3 is a cross-sectional view of a negative electrode according to one embodiment. DETAILED DESCRIPTION

[0019] The nonaqueous electrolyte secondary battery according to one embodiment of the present application includes: an electrode body in which a positive electrode and a negative electrode face each other with a porous separator interposed therebetween, a nonaqueous electrolyte, and an exterior body that accommodates the electrode body and the nonaqueous 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. A ratio (S2 / S1) of a porosity (S2) between the graphite particles in the second negative electrode mixture layer to a porosity (S1) between the graphite particles in the first negative electrode mixture layer is 1.1 to 2.0. 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. A thickness of the separator is 10 μm or less. A porosity of the separator is 25% to 45%.

[0020] Hereinafter, an example of an embodiment of the cylindrical secondary battery of the present application will be described in detail with reference to the drawings. In the following description, specific shapes, materials, numerical values, directions, and the like are examples for making the present application easy to understand, and can be appropriately changed according to the specifications of the cylindrical secondary battery. In addition, the outer body is not limited to the cylindrical shape, and can be, for example, a square shape or the like. In addition, in the following description, when a plurality of embodiments or modified examples are included, it is assumed from the beginning that the characteristic parts thereof are appropriately combined and used.

[0021] Figure 1 is an axial cross-sectional view of the cylindrical secondary battery 10 as an example of an embodiment. Figure 1 In the secondary battery 10 shown, an electrode body 14 and a nonaqueous electrolyte (not shown) are housed in an outer body 15. The electrode body 14 has a jelly-roll structure in which a positive electrode 11 and a negative electrode 12 are wound with a porous separator 13 interposed therebetween. Note that, hereinafter, for the convenience of explanation, the sealing body 16 side will be referred to as "upper", and the bottom side of the outer body 15 will be referred to as "lower".

[0022] The inside of the secondary battery 10 is sealed by closing the open end portion of the outer body 15 with the sealing body 16. Insulating plates 17 and 18 are provided above and below the electrode body 14, respectively. A positive electrode lead 19 extends upward through a through-hole of the insulating plate 17, and is welded to the lower surface of a bottom plate, i.e., a filter 22, of the sealing body 16. In the secondary battery 10, a top plate, i.e., a lid 26, of the sealing body 16 that is electrically connected to the filter 22 serves as a positive electrode terminal. On the other hand, a negative electrode lead 20 extends toward the bottom side of the outer body 15 through a through-hole of the insulating plate 18, and is welded to the inner surface of the bottom of the outer body 15. In the secondary battery 10, the outer body 15 serves as a negative electrode terminal. Note that, in the case where the negative electrode lead 20 is provided at the terminal end portion, the negative electrode lead 20 extends toward the bottom side of the outer body 15 through the outside of the insulating plate 18, and is welded to the inner surface of the bottom of the outer body 15.

[0023] The outer body 15 is, for example, a bottomed cylindrical metal outer can. A sealing gasket 27 is provided between the outer body 15 and the sealing body 16 to ensure the airtightness of the inside of the secondary battery 10. The outer body 15 has, for example, a groove portion 21 that supports the sealing body 16 by pressurizing the side surface portion from the outside. The groove portion 21 is preferably formed in a ring shape along the circumferential direction of the outer body 15, and supports the sealing body 16 on the upper surface thereof with the sealing gasket 27 interposed therebetween.

[0024] The sealing body 16 comprises a filter element 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cover 26, stacked sequentially from the electrode body 14 side. Each component constituting the sealing body 16 has, for example, a circular or annular shape, and all components except 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 central portions, with the insulating member 24 sandwiched between their respective peripheral portions. If the internal pressure of the battery rises due to abnormal heating, for example, the lower valve body 23 breaks, causing the upper valve body 25 to bulge towards the cover 26 and separate from the lower valve body 23, thereby severing the electrical connection between them. If the internal pressure rises further, the upper valve body 25 breaks, and gas is discharged from the opening 26a of the cover 26.

[0025] The following provides a detailed description of the negative electrode active material contained in the positive electrode 11, negative electrode 12, spacer 13, and non-aqueous electrolyte constituting the secondary battery 10, particularly the negative electrode compound layer 32 constituting the negative electrode 12.

[0026] [negative electrode]

[0027] Figure 2 This is a cross-sectional view of the negative electrode 12 as an example of an embodiment. The negative electrode 12 includes: a negative electrode current collector 30, a first negative electrode mixture layer 32a disposed on the surface of the negative electrode current collector 30, and a second negative electrode mixture layer 32b disposed on the surface of the first negative electrode mixture layer 32a. The thicknesses of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b may be the same or different. The ratio of the thicknesses 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.

[0028] The negative current collector 30 may be made of a foil of a metal such as copper that is stable within the potential range of the negative electrode, or a film of the same metal disposed on its surface. The thickness of the negative current collector 30 may be, for example, 5 μm to 30 μm.

[0029] The first negative electrode layer 32a and the second negative electrode layer 32b (hereinafter, the first negative electrode layer 32a and the second negative electrode layer 32b are sometimes collectively referred to as negative electrode layer 32) contain graphite particles. Furthermore, the negative electrode layer 32 preferably contains a binder. Examples of binders include: fluorinated resins, polyacrylonitrile (PAN), polyimide resins, acrylic resins, polyolefin resins, styrene-butadiene rubber (SBR), nitrile rubber (NBR), carboxymethyl cellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts (PAA-Na, PAA-K, etc., and also partially neutralized salts), polyvinyl alcohol (PVA), etc. These can be used alone or in combination of two or more.

[0030] The graphite particles used in this embodiment can include natural graphite, artificial graphite, etc. The interplanar spacing (d) of the (002) plane of the graphite particles used in this embodiment, based on X-ray wide-angle diffraction, is... 002 For example, the crystallite size is 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. Furthermore, the crystallite size (Lc(002)) of the graphite particles used in this embodiment, determined by X-ray diffraction, is 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 crystal size (Lc(002)) meets the above range, compared with the case where the above range is not met, there is a tendency for the capacity of the secondary battery 10 to be larger.

[0031] The graphite particles contained in the first negative electrode binder layer 32a can be produced, for example, as described below. Coke (precursor), which will become the main raw material, is pulverized to a predetermined size, agglomerated with a binder, and then, under pressure and molded into a block shape, calcined at a temperature of 2600°C or higher to graphitize it. By pulverizing and sieving the graphitized block shape, graphite particles of the desired size are obtained. Here, the internal porosity of the graphite particles can be adjusted according to the particle size of the pulverized precursor, the particle size of the agglomerated precursor, etc. For example, the average particle size (median particle size D50 in volume conversion, hereinafter the same) of the pulverized precursor is preferably in the range of 12 μm to 20 μm. Furthermore, the internal porosity of the graphite particles can also be adjusted according to the amount of volatile components added to the block shape. If a portion of the binder added to the coke (precursor) volatilizes during calcination, the binder can be used as a volatile component. Asphalt can be cited as such a binder.

[0032] The graphite particles contained in the second negative electrode mixture layer 32b can be produced, for example, as described below. Coke (precursor), which will become the main raw material, is pulverized to a predetermined size, and while agglomerated using a binder such as pitch, it is calcined at a temperature of 2600°C or higher to graphitize it. After sieving, graphite particles of the desired size can be obtained. Here, the internal porosity of the graphite particles can be adjusted according to the particle size of the pulverized precursor, the particle size of the agglomerated precursor, etc. For example, the average particle size of the pulverized precursor is preferably in the range of 12 μm to 20 μm.

[0033] The ratio (S2 / S1) of the porosity (S2) between graphite particles in the second negative electrode layer 32b to the porosity (S1) between graphite particles in the first negative electrode layer 32a is 1.1 to 2.0, preferably 1.1 to 1.7, and more preferably 1.1 to 1.5. When S2 / S1 is less than 1.1, the electrolyte circulation deteriorates, and the battery capacity decreases due to repeated fast charging. Furthermore, when S2 / S1 exceeds 2.0, it becomes impossible to make the filling density of the second negative electrode layer 32b approximately equal to that of the first negative electrode layer 32a, resulting in a lower battery capacity. Here, the porosity between graphite particles refers to a two-dimensional value calculated as the ratio of the area of ​​the voids between graphite particles to the cross-sectional area of ​​the negative electrode layer 32. S2 / S1 is obtained by calculating the porosity (S1) between graphite particles in the first negative electrode mixture layer 32a and the porosity (S2) between graphite particles in the second negative electrode mixture layer 32b according to the following method.

[0034] <Method for determining the porosity between graphite particles>

[0035] (1) Exposing the cross-section of the negative electrode mixture layer. For example, a method to expose the cross-section is to cut a portion of the negative electrode and process it with an ion milling device (e.g., Hitachi High-Tech Co., Ltd., IM4000PLUS) to expose the cross-section of the negative electrode mixture layer.

[0036] (2) Using a scanning electron microscope, reflective electron images of the exposed cross sections of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b are captured. The magnification of the reflective electron images is, for example, 800x.

[0037] (3) The cross-sectional image obtained above is read into the computer and binarized using image analysis software (e.g., ImageJ, made by the National Institutes of Health, USA) to obtain a binarized image in which the particle cross-sections in the cross-sectional image are converted into black and the gaps in the particle cross-sections are converted into white.

[0038] (4) In the binarized images of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b, the portions other than the voids converted to white, the voids inside the graphite particles (pores not connected to the particle surface), and the pores with a width of less than 3 μm connected to the graphite particle surface are respectively taken as the voids between graphite particles, and the area of ​​the voids between graphite particles is calculated. The porosity between graphite particles can be calculated based on the following formula.

[0039] Porosity between graphite particles = Area of ​​voids between graphite particles / Area of ​​cross-section of negative electrode agent layer × 100

[0040] S1 and S2 are obtained by taking the average of the three measurements mentioned above, and S1 / S2 can be calculated based on their values.

[0041] The ratio (D2 / D1) of the filling density (D2) of the second negative electrode compound layer 32b to the filling density (D1) of the first negative electrode compound layer 32a is 0.9 to 1.1. By ensuring that S2 / S1 is 1.1 to 2.0 and that D2 / D1 is within this range, the reduction in battery capacity can be suppressed. For example, by ensuring that the internal porosity of the graphite particles contained in the first negative electrode compound layer 32a is higher than that of the graphite particles contained in the second negative electrode compound layer 32b, S2 / S1 and D2 / D1 can also meet the above-mentioned ranges.

[0042] The filling density (D1) of the first negative electrode mixture layer 32a and the filling density (D2) of the second negative electrode mixture layer 32b can be set to, for example, 1.3 g / cm³. 3 ~2.0g / cm 3 .

[0043] The filling density of the negative electrode mixture layer 32 refers to the mass per unit volume of the negative electrode mixture layer 32. First, the mass per unit area of ​​the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b is measured using the negative electrode 12. Furthermore, the thickness of the mixture layer of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b is measured using a cross-sectional image obtained when calculating the interparticle porosity. If the mixture layer thickness is unstable, 10 points can be measured in the aforementioned image, and the average value can be taken as the mixture layer thickness. By dividing the mass per unit area of ​​the mixture by the mixture layer thickness, the filling density (D1) of the first negative electrode mixture layer 32a and the filling density (D2) of the second negative electrode mixture layer 32b can be calculated. Based on these values, the ratio (D2 / D1) of the filling density (D2) of the second negative electrode mixture layer 32b to the filling density (D1) of the first negative electrode mixture layer 32a can be obtained.

[0044] Next, the specific methods for forming the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b will be described. For example, firstly, 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 coated on both sides of the negative electrode current collector and dried. Next, the second negative electrode mixture slurry is coated on both sides of the coating film formed by the first negative electrode mixture slurry and dried. Then, the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b are calendered using calendering rollers, thereby forming the negative electrode mixture layer 32.

[0045] Even if calendering is performed simultaneously as described above for the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b, the filling properties of the first graphite particles and the second graphite particles during calendering are not necessarily the same. For example, by changing the particle size distribution of the first graphite particles and the second graphite particles, the filling density of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b can be adjusted. In addition, 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 filling density of the second negative electrode mixture layer 32b. It should be noted that in the above method, the second negative electrode mixture slurry is coated after the first negative electrode mixture slurry is coated and dried, but it is also possible to coat the second negative electrode mixture slurry after coating the first negative electrode mixture slurry and before drying. Alternatively, the second negative electrode mixture slurry can be coated on the first negative electrode mixture layer 32a after coating, drying, and calendering. By changing the calendering conditions of the first negative electrode compound layer 32a and the second negative electrode compound layer 32b, their respective filling densities can be adjusted more freely.

[0046] At least one of the first negative electrode additive layer 32a and the second negative electrode additive layer 32b may contain a Si-based material. The Si-based material is a material capable of reversibly absorbing and releasing lithium ions, functioning as a negative electrode active material. Examples of Si-based materials include Si, Si-containing alloys, and materials composed of SiO2. x (x is 0.8 to 1.6) represents silicon oxide, etc. Si-based materials are negative electrode materials that can improve battery capacity compared with graphite particles. Considering the improvement of battery capacity and the suppression of the decline in fast charge and discharge cycle characteristics, the content of Si-based materials is preferably 1% to 10% by mass relative to the mass of the negative electrode active material, and more preferably 3% to 7% by mass.

[0047] Other materials capable of reversibly absorbing and releasing lithium ions include, in addition to, metals alloyed with lithium such as tin (Sn), or alloys and oxides containing metallic elements such as Sn. The negative electrode active material may contain the aforementioned other materials, and the content of these other materials is preferably 10% by mass or less relative to the mass of the negative electrode active material.

[0048] [positive electrode]

[0049] The positive electrode 11 is, for example, composed of a positive current collector such as a metal foil and a positive electrode flux layer formed on the positive current collector. The positive current collector can be a foil of a metal that is stable within the potential range of the positive electrode, such as aluminum, or a film of the metal disposed on its surface. The positive electrode flux layer may contain, for example, a positive electrode active material, a binder, a conductive agent, etc.

[0050] For example, the positive electrode 11 can be manufactured by coating a positive electrode slurry containing positive electrode active material, binder, conductive agent, etc. onto the positive electrode current collector and drying it to form a positive electrode slurry layer, and then calendering the positive electrode slurry layer.

[0051] Examples of lithium transition metal oxides containing transition metal elements such as Co, Mn, and Ni can be used as positive electrode active materials. For example, Li0.05 is a lithium transition metal oxide. 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 can be used individually or in combination. From the perspective of achieving high capacity in non-aqueous electrolyte secondary batteries, the positive electrode active material preferably contains Li. x NiO2, Li x Co y Ni 1-y O2, Li x Ni 1-y M y O z (At least one of M, 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) and other lithium nickel composite oxides.

[0052] Conductive agents include, for example, carbon black (CB), acetylene black (AB), Ketjen black, graphite, and other carbon-based particles. They can be used alone or in combination of two or more.

[0053] Examples of adhesives include fluorinated resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide resins, acrylic resins, and polyolefin resins. They can be used alone or in combination of two or more.

[0054] [Spacer]

[0055] The spacer 13 can be, for example, a porous sheet with ion permeability and insulation. Specific examples of porous sheets include microporous membranes, woven fabrics, and nonwoven fabrics. The preferred materials for the spacer are olefin resins such as polyethylene and polypropylene, and cellulose. The spacer 13 can be a laminate containing a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin resin. Alternatively, it can be a multilayer spacer containing a polyethylene layer and a polypropylene layer, or a spacer with an aromatic polyamide resin, ceramic, or other material coated on its surface.

[0056] The thickness of the spacer 13 is 10 μm or less. This improves the battery capacity. Furthermore, from a strength perspective, the thickness of the spacer 13 is preferably 6 μm or more.

[0057] The porosity of spacer 13 is 25% to 45%. If it is within this range, even with the aforementioned thin spacer 13, strength can be maintained, ensuring good electrolyte circulation and resulting in a high-capacity battery that suppresses the degradation of fast charge-discharge cycle characteristics. The porosity of spacer 13 can be calculated based on the following formula.

[0058] Porosity of the spacer = [1 - {mass of the spacer / (thickness of the spacer × area of ​​the main surface of the spacer × true density of the material filling the spacer)}] × 100

[0059] [Non-aqueous electrolytes]

[0060] Non-aqueous electrolytes comprise a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Non-aqueous electrolytes are not limited to liquid electrolytes (electrolytes) and can also be solid electrolytes using gel polymers, etc. Non-aqueous solvents can include, for example, esters, ethers, nitrile compounds such as acetonitrile, amides such as dimethylformamide, and mixtures of two or more of these solvents. Non-aqueous solvents can also contain halogen-substituted products in which at least a portion of the hydrogen atoms of these solvents are replaced by halogen atoms such as fluorine.

[0061] Examples of the aforementioned esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butyl 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.

[0062] Examples of the aforementioned ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-epoxybutane, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ethers, cyclic ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, and ethylethylene. Chain ethers such as methyl 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.

[0063] As the halogen substitutes mentioned above, fluorocyclic carbonates such as fluoroethylene carbonate (FEC), fluorochain carbonates, and fluorochain carboxylic acid esters such as methyl fluoropropionate (FMP) are preferred.

[0064] The preferred electrolyte salt is a lithium salt. Examples of lithium salts include LiBF4, LiClO4, LiPF6, LiAsF6, LiSbF6, LiAlCl4, LiSCN, LiCF3SO3, LiCF3CO2, Li(P(C2O4)F4), and LiPF6. 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, lower aliphatic carboxylic acids, Li₂B₄O₇, Li(B(C₂O₄)F₂) and other borates, LiN(SO₂CF₃)₂, LiN(C₁F₂) 2l＋1 SO2)(C m F 2m＋1Imidamine salts such as SO2 (where l and m are integers greater than or equal to 1) are used. Lithium salts can be used individually or in combination. From the perspective of ionic conductivity and electrochemical stability, LiPF6 is preferred. The concentration of lithium salt is preferably set at 0.8–1.8 mol per 1 L.

[0065] Example

[0066] The present invention will be further illustrated below by way of examples, but the present invention is not limited to these examples.

[0067] <Example 1>

[0068] [The production of the positive electrode]

[0069] As the positive electrode active material, lithium nickel cobalt oxide (LiNi) containing aluminum is used. 0.88 Co 0.09 Al 0.03 O2). The above-mentioned positive electrode active material is mixed in the following manner: 100 parts by mass of graphite as a conductive agent, 1 part by mass of polyvinylidene fluoride powder as a binder, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) is added to prepare a positive electrode slurry. The slurry is coated onto both sides of a positive electrode current collector formed from aluminum foil (15 μm thick) using a blade coating method. After the coating is dried, it is calendered using calendering rollers to create a positive electrode with a positive electrode slurry layer formed on both sides of the positive electrode current collector.

[0070] [The Production of Graphite Particle A]

[0071] Asphalt as a binder was added to coke with an average particle size of 17 μm, causing it to agglomerate. Isotropic pressure was applied to the agglomerate to produce a product with a particle size of 1.6 g / cm³. 3 ~1.9g / cm 3 A block-shaped molded body with a density of [missing information]. The block-shaped molded body is graphitized by firing at a temperature of 2800℃, then crushed and sieved to produce graphite particles A with an average particle size of 23μm.

[0072] [The Production of Graphite Particles B]

[0073] Pitch as a binder is added to coke with an average particle size of 13 μm, causing it to agglomerate to an average particle size of 18 μm. This agglomerate is then calcined at 2800 °C to graphitize, and subsequently sieved to produce graphite particles B with an average particle size of 23 μm. In producing graphite particles B, the amount of pitch added to the coke is less than that used in producing graphite particles A, and the average particle size of the agglomerate is adjusted to produce graphite particles B with a smaller internal porosity than graphite particles A.

[0074] [Making the negative electrode]

[0075] Graphite particles A (95 parts by mass) and SiO₂ (5 parts by mass) were mixed to form the negative electrode active material A. The negative electrode active material A, carboxymethyl cellulose (CMC), and styrene-butadiene copolymer rubber (SBR) were mixed in a mass ratio of 100:1:1, and this mixture was kneaded in water to prepare the first negative electrode slurry. Separately, graphite particles B (95 parts by mass) and SiO₂ (5 parts by mass) were mixed to form the negative electrode active material B. The negative electrode active material B, carboxymethyl cellulose (CMC), and styrene-butadiene copolymer rubber (SBR) were mixed in a mass ratio of 100:1:1, and this mixture was kneaded in water to prepare the second negative electrode slurry.

[0076] A first negative electrode slurry is coated onto both sides of a copper foil-formed negative electrode current collector using a blade coating method and then dried to form a first negative electrode slurry layer. Next, a second negative electrode slurry is coated onto the first negative electrode slurry layer and dried to form a second negative electrode slurry layer. At this point, the coating mass ratio of the first to the second negative electrode slurry per unit area is set to 5:5. The first and second negative electrode slurry layers are then calendered using calendering rollers to produce the negative electrode.

[0077] [Preparation of non-aqueous electrolytes]

[0078] In a non-aqueous solvent consisting of 100 parts by mass of a mixture of ethylene carbonate (EC) and dimethyl carbonate in a volume ratio of 1:3, 5 parts by mass of ethylene carbonate (VC) are added to dissolve LiPF6 at a concentration of 1.5 mol / L, which is then used as a non-aqueous electrolyte.

[0079] [Construction of a non-aqueous electrolyte secondary battery]

[0080] (1) As a spacer, a microporous membrane made of polypropylene with a thickness of 6 μm and a porosity of 35% is used.

[0081] (2) After installing a positive lead in the positive current collector and a negative lead in the negative current collector, the electrode body is wound with a spacer between the positive and negative electrodes to make a wound electrode body.

[0082] (3) Insulating plates are placed on the top and bottom of the electrode body respectively. The negative lead is soldered to the outer casing and the positive lead is soldered to the sealing body. The electrode body is then stored in the outer casing.

[0083] (4) After injecting non-aqueous electrolyte into the outer casing by depressurization, seal the opening of the outer casing with a sealing body through the sealing gasket, and use it as a non-aqueous electrolyte secondary battery.

[0084] [Calculation of porosity between graphite particles]

[0085] At an ambient temperature of 25°C, the non-aqueous electrolyte secondary battery was charged to 4.2V at a constant current of 0.2C (920mA), and then charged to C / 50 at a constant voltage of 4.2V. It was then discharged to 2.5V at a constant current of 0.2C. This charge-discharge cycle was considered one cycle, and five cycles were performed. The negative electrode was taken from the non-aqueous electrolyte secondary batteries of each embodiment and comparative example after five cycles, and the porosity between graphite particles was calculated.

[0086] [Determination of capacity retention during rapid charge-discharge cycles]

[0087] The non-aqueous electrolyte secondary batteries of each embodiment and comparative example were charged to 4.2V at a constant current of 1C (4600mA) at an ambient temperature of 25°C, and then charged to 1 / 50C at a constant voltage of 4.2V. They were then discharged to 2.5V at a constant current of 0.5C. This charge-discharge cycle was considered one cycle, and 100 cycles were performed. The capacity retention rate of the non-aqueous electrolyte secondary batteries of each embodiment and comparative example during rapid charge-discharge cycles was then calculated using the following formula.

[0088] Capacity maintenance rate = (Discharge capacity in the 100th cycle / Discharge capacity in the 1st cycle) × 100

[0089] <Example 2>

[0090] The porosity of the spacer was changed to 45%, and a non-aqueous electrolyte secondary battery was fabricated and evaluated in the same manner as in Example 1.

[0091] <Example 3>

[0092] The thickness of the spacer was changed to 10 μm and the porosity was changed to 45%. Otherwise, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1 and evaluated.

[0093] <Comparative Example 1>

[0094] The negative electrode active material A contained in the first negative electrode slurry and the negative electrode active material B contained in the second negative electrode slurry are both mixed in the manner of graphite particles A being 47.5 parts by mass, graphite particles B being 47.5 parts by mass, and SiO being 5 parts by mass. Otherwise, a non-aqueous electrolyte secondary battery is prepared in the same manner as in Example 1 and evaluated.

[0095] <Comparative Example 2>

[0096] The negative electrode active material A contained in the first negative electrode slurry and the negative electrode active material B contained in the second negative electrode slurry are both mixed in the manner that graphite particles A is 47.5 parts by mass, graphite particles B is 47.5 parts by mass, and SiO is 5 parts by mass. Otherwise, a non-aqueous electrolyte secondary battery is prepared in the same manner as in Example 2 and evaluated.

[0097] <Comparative Example 3>

[0098] The negative electrode active material A contained in the first negative electrode slurry and the negative electrode active material B contained in the second negative electrode slurry are both mixed in a manner in which graphite particles A account for 47.5 parts by mass, graphite particles B account for 47.5 parts by mass, and SiO accounts for 5 parts by mass. Otherwise, a non-aqueous electrolyte secondary battery is prepared in the same manner as in Example 3 and evaluated.

[0099] <Comparative Example 4>

[0100] The thickness of the spacer was set to 10 μm and the porosity to 45%. Otherwise, a non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1 and evaluated.

[0101] <Comparative Example 5>

[0102] The negative electrode active material A contained in the first negative electrode slurry and the negative electrode active material B contained in the second negative electrode slurry were both mixed in such a way that graphite particles A accounted for 47.5 parts by mass, graphite particles B accounted for 47.5 parts by mass, and SiO accounted for 5 parts by mass. Otherwise, a non-aqueous electrolyte secondary battery was prepared in the same manner as in Comparative Example 4, and the battery was evaluated.

[0103] Table 1 summarizes the capacity retention and battery capacity results of the non-aqueous electrolyte secondary batteries of the Examples and Comparative Examples during fast charge-discharge cycles. Regarding battery capacity, the thinner the spacer (which does not contribute to charge-discharge), the higher the evaluation. Additionally, Table 1 also shows the porosity, D1, D2, D2 / D1, and S2 / S1 of the spacer. It should be noted that a higher capacity retention value during fast charge-discharge cycles indicates that the reduction in fast charge-discharge cycle characteristics is more suppressed.

[0104] [Table 1]

[0105]

[0106] In this embodiment, a higher capacity retention rate than the comparative example was obtained, achieving both high capacity and excellent fast charge-discharge cycle characteristics. The improved fast charge-discharge cycle characteristics are attributed to the increased interparticle porosity in the second negative electrode compound layer, which enhances the liquid circulation of the electrolyte in the negative electrode. Furthermore, avoiding excessive reduction in the filling density of the second negative electrode compound layer and using spacers with specified thickness and porosity contribute to achieving high capacity in the secondary battery.

[0107] Explanation of reference numerals in the attached figures

[0108] 10 Secondary battery, 11 Positive electrode, 12 Negative electrode, 13 Spacer, 14 Electrode body, 15 Outer body, 16 Sealing body, 17, 18 Insulating plate, 19 Positive electrode lead, 20 Negative electrode lead, 21 Groove, 22 Filter, 23 Lower valve body, 24 Insulating component, 25 Upper valve body, 26 Cover, 26a Opening, 27 Sealing gasket, 30 Negative electrode current collector, 32 Negative electrode compound layer, 32a First negative electrode compound layer, 32b Second negative electrode compound layer.

Claims

1. A non-aqueous electrolyte secondary battery, comprising: An electrode body consisting of a positive electrode and a negative electrode separated by a porous spacer. Non-aqueous electrolytes, and An outer casing for housing the electrode body and the non-aqueous electrolyte. The negative electrode comprises: a negative electrode current collector, a first negative electrode mixture layer disposed on the surface of the negative electrode current collector, and a second negative electrode mixture layer opposite to the positive electrode separated by the spacer. The first negative electrode mixture layer and the second negative electrode mixture layer contain graphite particles with the same average particle size, and the internal porosity of the graphite particles in the first negative electrode mixture layer is higher than that in the second negative electrode mixture layer. The 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 1.1~2.

0. The ratio of the filling density D2 of the second negative electrode mixture layer to the filling density D1 of the first negative electrode mixture layer, D2 / D1, is 0.9~1.

1. The thickness of the spacer is less than 10 μm, and the porosity is 25%~45%.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein, The filling density D1 of the first negative electrode mixture layer and the filling density D2 of the second negative electrode mixture layer are both 1.3 g / cm³. 3 ~2.0g / cm 3 .

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein, At least one of the first negative electrode mixture layer and the second negative electrode mixture layer comprises a Si-based material.

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