Non-aqueous electrolyte secondary battery

Abstract

Provided is a non-aqueous electrolyte secondary battery with which decrease in battery capacity due to repetition of charging and discharging is suppressed. A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is provided with a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode includes a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The negative electrode mixture layer includes a first negative electrode mixture layer opposite to the negative electrode current collector, and a second negative electrode mixture layer laminated on the surface of the first negative electrode mixture layer. The negative electrode mixture layer includes graphite particles and a highly dielectric material having a higher dielectric constant than the graphite particles. The porosity between the graphite particles in the second negative electrode mixture layer is greater than the porosity between the graphite particles in the first negative electrode mixture layer. The content proportion of the highly dielectric material in the first negative electrode mixture layer is greater than the content proportion of the highly dielectric material in the second negative electrode mixture layer.

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 high-energy density secondary batteries. Increasing the packing density of the negative electrode mixture layer to increase the capacity of the battery reduces the voids between the graphite particles, impairing the permeability of the non-aqueous electrolyte into the negative electrode mixture layer and resulting in a decrease in battery capacity with repeated charge and discharge. For example, Patent Document 1 discloses an electrode in which, from the viewpoint of improving the wettability of the electrode surface, a mixed layer containing an active material and an insulating oxide such as alumina or titanium oxide is formed on the surface of the mixture layer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-27879 Summary of the Invention [Problem to be solved by the invention]

[0004] However, simply improving the wettability of the electrode surface does not allow the non-aqueous electrolyte to penetrate deep into the negative electrode mixture layer, and the decrease in battery capacity due to repeated charge and discharge cannot be sufficiently suppressed.

[0005] Therefore, an object of the present disclosure is to provide a nonaqueous electrolyte secondary battery that suppresses a decrease in battery capacity due to repeated charge and discharge. [Means for solving the problem]

[0006] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode includes a negative electrode current collector and a negative electrode mixture layer formed on the surface of the negative electrode current collector. The negative electrode mixture layer includes a first negative electrode mixture layer facing the negative electrode current collector and a second negative electrode mixture layer laminated on the surface of the first negative electrode mixture layer. The negative electrode mixture layer includes graphite particles and a high-dielectric material having a higher dielectric constant than the graphite particles. The porosity between the graphite particles in the second negative electrode mixture layer is higher than the porosity between the graphite particles in the first negative electrode mixture layer, and the content of the high-dielectric material in the first negative electrode mixture layer is higher than the content of the high-dielectric material in the second negative electrode mixture layer. [Effects of the Invention]

[0007] According to one aspect of the present disclosure, it is possible to improve the charge-discharge cycle characteristics of a nonaqueous electrolyte secondary battery. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is an axial cross-sectional view of a cylindrical secondary battery according to an embodiment of the present invention; [Figure 2] FIG. 2 is a cross-sectional view of a negative electrode according to an example embodiment. [Figure 3] FIG. 2 is a schematic diagram showing a cross section of a graphite particle in an example of an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] An example of an embodiment of a cylindrical secondary battery according to the present disclosure will be described in detail below with reference to the drawings. In the following description, specific shapes, materials, numerical values, directions, etc. are examples for facilitating understanding of the present invention and can be appropriately changed according to the specifications of the cylindrical secondary battery. Furthermore, the exterior body is not limited to a cylindrical shape and may be, for example, rectangular. Furthermore, when the following description includes multiple embodiments and modified examples, it is initially assumed that their characteristic features can be appropriately combined and used.

[0010] Fig. 1 is an axial cross-sectional view of a cylindrical secondary battery 10 that is one example of an embodiment. In the secondary battery 10 shown in Fig. 1, an electrode assembly 14 and a non-aqueous electrolyte (not shown) are housed in an exterior body 15. The electrode assembly 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. For ease of explanation, the following description will be given with the sealing body 16 side referred to as "top" and the bottom side of the exterior body 15 referred to as "bottom."

[0011] The open end of the exterior body 15 is sealed with the sealing body 16, thereby sealing the interior of the secondary battery 10. Insulating plates 17 and 18 are provided above and below the electrode body 14. The positive electrode lead 19 extends upward through a through hole in the insulating plate 17 and is welded to the underside of a filter 22, which is the bottom plate of the sealing body 16. In the secondary battery 10, a cap 26, which is the top plate of the sealing body 16 and is electrically connected to the filter 22, serves as the positive electrode terminal. On the other hand, the negative electrode lead 20 extends through a through hole in the insulating plate 18 toward the bottom of the exterior body 15 and is welded to the inner bottom surface of the exterior body 15. In the secondary battery 10, the exterior body 15 serves as the negative electrode terminal. Note that if the negative electrode lead 20 is installed at the terminal end, the negative electrode lead 20 passes outside the insulating plate 18, extends toward the bottom of the exterior body 15, and is welded to the inner bottom surface of the exterior body 15.

[0012] Exterior body 15 is, for example, a cylindrical metal exterior can with a bottom. A gasket 27 is provided between exterior body 15 and sealing body 16, ensuring the airtightness of the interior of secondary battery 10. Exterior body 15 has a grooved portion 21 that supports sealing body 16, formed, for example, by pressing the side surface from the outside. Grooved portion 21 is preferably formed in an annular shape along the circumferential direction of exterior body 15, and supports sealing body 16 on its upper surface via gasket 27.

[0013] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26, which are stacked in this order from the electrode body 14 side. Each component of the sealing body 16 has, for example, a disk or ring shape, and all components except for the insulating member 24 are electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, with the insulating member 24 interposed between their respective peripheral edges. If the internal pressure of the battery increases due to abnormal heat generation, for example, the lower valve body 23 may break, causing the upper valve body 25 to bulge toward the cap 26 and separate from the lower valve body 25, thereby cutting off the electrical connection between them. If the internal pressure continues to increase, the upper valve body 25 may break, and gas may be released from the opening 26a of the cap 26.

[0014] The positive electrode 11, negative electrode 12, separator 13, and non-aqueous electrolyte that constitute the secondary battery 10 will be described in detail below, particularly the negative electrode mixture layer 32 that constitutes the negative electrode 12.

[0015] [Negative electrode] 2 is a cross-sectional view of a negative electrode 12 according to one embodiment. The negative electrode 12 includes a negative electrode current collector 30 and a negative electrode mixture layer 32 formed on the surface of the negative electrode current collector 30. The negative electrode mixture layer 32 includes a first negative electrode mixture layer 32a facing the negative electrode current collector 30 and a second negative electrode mixture layer 32b stacked on the surface of the first negative electrode mixture layer 32a. The first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b may have the same thickness or different thicknesses. The thickness ratio of the first negative electrode mixture layer 32a to the second negative electrode mixture layer 32b is, for example, 3:7 to 7:3, preferably 4:6 to 6:4, and more preferably 5:5 to 6:4.

[0016] A foil of a metal such as copper that is stable in the potential range of the negative electrode, or a film with such a metal disposed on the surface layer, is used for the negative electrode current collector 30. The thickness of the negative electrode current collector 30 is, for example, 5 μm to 30 μm.

[0017] The negative electrode mixture layer 32 contains graphite particles as a negative electrode active material. Examples of graphite particles include natural graphite and artificial graphite. Artificial graphite is preferred in terms of ease of adjusting the internal porosity, which will be described later. The interplanar spacing (d002 ) is preferably, for example, 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. Further, the crystallite size (Lc(002)) of the graphite particles determined by X-ray diffraction method is preferably, for example, 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 ) and the crystallite size (Lc(002)) are within the above ranges, the battery capacity of the non-aqueous electrolyte secondary battery tends to be larger than when they do not satisfy the above ranges.

[0018] FIG. 3 is a schematic diagram showing a cross section of the graphite particles 40. As shown in FIG. 3, the graphite particles 40 have a closed void 42 (hereinafter, internal void 42) that is not connected from the inside of the particle to the particle surface and a void 44 (hereinafter, external void 44) that is connected from the inside of the particle to the particle surface in a cross-sectional view of the graphite particles 40.

[0019] The porosity between the graphite particles in the second negative electrode active material layer 32b is higher than the porosity between the graphite particles in the first negative electrode active material layer 32a. The ratio (S2 / S1) of the porosity (S2) between the graphite particles in the second negative electrode active material layer 32b to the porosity (S1) between the graphite particles in the first negative electrode active material layer 32a is preferably 1 < S2 / S1 ≦ 2, more preferably 1.1 ≦ S2 / S1 ≦ 1.7, and particularly preferably 1.1 ≦ S2 / S1 ≦ 1.5. When S2 / S1 ≦ 1, the permeability of the non-aqueous electrolyte in the second negative electrode active material layer 32b deteriorates, and the battery capacity decreases due to repeated charge and discharge. Further, by setting S2 / S1 ≦ 2, it becomes easy to reduce the difference in packing density between the first negative electrode active material layer 32a and the second negative electrode active material layer 32b. Here, the porosity between the graphite particles is a two-dimensional value obtained from the ratio of the area of the voids between the graphite particles to the cross-sectional area of the negative electrode active material layer 32. S2 / S1 is obtained by calculating the porosity (S1) between the graphite particles in the first negative electrode active material layer 32a and the porosity (S2) between the graphite particles in the second negative electrode active material layer 32b according to the following procedure.

[0020] <Method for measuring void ratio between graphite particles> (1) Exposing the cross section of the negative electrode mixture layer. For example, a method for exposing the cross section includes cutting out a part of the negative electrode and processing it with an ion milling device (e.g., IM4000PLUS manufactured by Hitachi High-Technologies Corporation) to expose the cross section of the negative electrode mixture layer. (2) Using a scanning electron microscope, backscattered electron images of the cross sections of the exposed negative electrode mixture layers are taken for each of the first negative electrode mixture layer 32 a and the second negative electrode mixture layer 32 b. The magnification for taking the backscattered electron images is, for example, 800 times. (3) The cross-sectional image obtained as described above is imported into a computer and binarized using image analysis software (e.g., ImageJ, manufactured by the National Institutes of Health, USA) to obtain a binarized image in which the particle cross-sections in the cross-sectional image are colored black and the voids present in the particle cross-sections are colored white. (4) In the binarized images of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b, the voids converted to white, excluding the internal voids 42 and the external voids 44 with a width of 3 μm or less, are regarded as the voids between the graphite particles, and the area of ​​the voids between the graphite particles is calculated. The void ratio between the graphite particles can be calculated 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 (5) S1 and S2 are each calculated as the average value of the three measurements.

[0021] Examples of means for adjusting the porosity between graphite particles in the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b include a means for adjusting the packing density of the negative electrode mixture layer and a means for adjusting the internal porosity of the graphite particles. The latter means reduces the internal porosity of the graphite particles, thereby increasing the porosity between the graphite particles without reducing the packing density of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b. In other words, by making the packing densities of the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b equal and reducing the internal porosity of the graphite particles B contained in the second negative electrode mixture layer 32b compared to the graphite particles A contained in the first negative electrode mixture layer 32a, the relationship S2 / S1 > 1 can be satisfied.

[0022] The graphite particles A contained in the first negative electrode mixture layer 32a can be produced, for example, as follows. Coke (precursor), the main raw material, is crushed to a predetermined size, agglomerated with a binder, and then press-molded into a block. This block is then fired at a temperature of 2600°C or higher to graphitize it. The graphitized block is crushed and sieved to obtain graphite particles A of a desired size. The internal porosity can be adjusted to be larger than that of graphite particles B, described below, by adjusting the amount of volatile components added to the block. The internal porosity of the graphite particles A is preferably 8% to 20%, more preferably 10% to 18%, and particularly preferably 12% to 16%. When a portion 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.

[0023] The graphite particles B contained in the second negative electrode mixture layer 32b can be produced, for example, as follows. Coke (precursor), which is the main raw material, is crushed to a predetermined size, and the crushed coke is agglomerated with a binder. The crushed coke is then fired at a temperature of 2600°C or higher to graphitize the coke, and the resulting graphite particles are then sieved to obtain graphite particles B of a desired size. The internal porosity of the graphite particles B can be adjusted by adjusting the particle size of the crushed precursor or the particle size of the agglomerated precursor. For example, the internal porosity can be reduced by increasing the particle size of the crushed precursor. The average particle size (volume-equivalent median diameter D50, hereinafter the same) of the crushed precursor may be in the range of 12 μm to 20 μm. The internal porosity of the graphite particles B is preferably 5% or less, more preferably 1% to 5%, and particularly preferably 3% to 5%.

[0024] Next, a 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 A, 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 B, a binder, and a solvent such as water are mixed to prepare a second negative electrode mixture slurry. Then, the first negative electrode mixture slurry is applied to both sides of the negative electrode current collector and dried. After that, the second negative electrode mixture slurry is applied to both sides of the coating of the first negative electrode mixture slurry and dried. Furthermore, the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b are rolled using a rolling roller to form the negative electrode mixture layer 32. In the above method, the first negative electrode mixture slurry is applied and dried, and then the second negative electrode mixture slurry is applied. However, the second negative electrode mixture slurry may be applied after the first negative electrode mixture slurry is applied and before drying. Alternatively, the first negative electrode mixture slurry may be applied, dried, and rolled, and then the second negative electrode mixture slurry may be applied onto the first negative electrode mixture layer 32a. By changing the rolling conditions for the first negative electrode mixture layer 32a and the second negative electrode mixture layer 32b, the packing densities of the respective layers can be more flexibly adjusted.

[0025] The negative electrode mixture layer 32 further includes a high-dielectric material having a higher dielectric constant than the graphite particles. The content of the high-dielectric material in the first negative electrode mixture layer 32a is higher than the content of the high-dielectric material in the second negative electrode mixture layer 32b. By making the content of the high-dielectric material in the first negative electrode mixture layer 32a higher than the content of the high-dielectric material in the second negative electrode mixture layer 32b while maintaining S2 / S1 > 1, the permeability of the non-aqueous electrolyte in the first negative electrode mixture layer 32a facing the negative electrode current collector 30 is improved, and the permeability of the non-aqueous electrolyte in the entire negative electrode mixture layer 32 is improved. This improves the charge-discharge cycle characteristics of the battery. The content of the high-dielectric material is the ratio of the mass of the high-dielectric material to the mass of the negative electrode active material.

[0026] The high-dielectric material is not particularly limited as long as it has a higher dielectric constant than the graphite particles, and examples thereof include titanium oxide (TiO2), lithium titanate (Li2TiO3), and barium oxide (BaO). The high-dielectric material preferably contains at least one of titanium oxide (TiO2) and lithium titanate (Li2TiO3). The graphite particles have a dielectric constant of approximately 12, TiO2 has a dielectric constant of approximately 100, and Li2TiO3 has a dielectric constant of approximately 30. The average particle size (D50) of the high-dielectric material is, for example, 300 nm to 3 μm.

[0027] The content of the high-dielectric material in the first negative electrode mixture layer 32a is preferably 1% by mass to 10% by mass, more preferably 2% by mass to 7% by mass, and particularly preferably 2.5% by mass to 5% by mass, relative to the mass of the negative electrode active material contained in the first negative electrode mixture layer 32a. Within this range, the charge-discharge cycle characteristics can be improved while suppressing a decrease in battery capacity.

[0028] The negative electrode mixture layer 32 may further contain a Si-based material as a negative electrode active 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 battery capacity more than graphite particles. From the viewpoints of improving battery capacity and suppressing deterioration of charge-discharge cycle characteristics, the content of the Si-based material is, for example, preferably 1% by mass to 10% by mass, and more preferably 3% by mass to 7% by mass, relative to the mass of the negative electrode active material.

[0029] Other examples of the other material capable of reversibly absorbing and releasing lithium ions include metals that alloy with lithium, such as tin (Sn), or alloys or oxides containing metal elements such as Sn. The negative electrode active material may contain the other material, and the content of the other material is preferably, for example, 10 mass % or less relative to the mass of the negative electrode active material.

[0030] The negative electrode mixture layer 32 may contain a binder. Examples of binders include fluorine-based resins, PAN, polyimide-based resins, acrylic-based resins, polyolefin-based resins, styrene-butadiene rubber (SBR), nitrile-butadiene rubber (NBR), carboxymethyl cellulose (CMC) or a salt thereof, polyacrylic acid (PAA) or a salt thereof (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.

[0031] [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 can be a foil of a metal such as aluminum that is stable within the potential range of the positive electrode, or a film with such a 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. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing the positive electrode active material, the binder, the conductive agent, etc. to the positive electrode current collector, drying it to form a positive electrode mixture layer, and then rolling this positive electrode mixture layer.

[0032] Examples of the positive electrode active material include lithium transition metal oxides containing transition metal elements such as Co, Mn, and Ni. 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 yO4, LiMPO4, Li2MPO4F (M is 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 high capacity of the non-aqueous electrolyte secondary battery, the cathode active material is Li x NiO2, Li x Co y Ni 1-y O2, Li x Ni 1-y M y O z (M is 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. It is preferable to contain a lithium nickel composite oxide such as

[0033] The conductive agent includes, for example, carbon-based particles such as carbon black (CB), acetylene black (AB), ketjen black, carbon nanotube (CNT), graphene, graphite, etc. These may be used alone or in combination of two or more types.

[0034] The binder includes, for example, fluorine-based resins such as polytetrafluoroethylene (PTFE), 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 types. [[ID=2!5]]

[0035] [Separator] The separator 13 may be, for example, a porous sheet having ion permeability and insulating properties. Specific examples of porous sheets include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator include olefin-based resins such as polyethylene and polypropylene, and cellulose. The separator 13 may be a laminate having a cellulose fiber layer and a thermoplastic resin fiber layer such as an olefin-based resin. Alternatively, the separator 13 may be a multilayer separator including a polyethylene layer and a polypropylene layer, and the surface of the separator 13 may be coated with a material such as an aramid-based resin or ceramic.

[0036] [Non-aqueous electrolyte] The non-aqueous electrolyte is a liquid electrolyte (electrolytic solution) containing a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent that can be used include esters, ethers, nitriles such as acetonitrile, amides such as dimethylformamide, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted product in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine.

[0037] Examples of the esters include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate; chain carbonates such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, and methyl isopropyl carbonate; cyclic carboxylic acid esters such as γ-butyrolactone and γ-valerolactone; and chain carboxylic acid esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate.

[0038] Examples of the above ethers include cyclic ethers such as 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, crown ether, etc., and chain ethers such as 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, etc.

[0039] As the above halogenated derivatives, it is preferable to use fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC), fluorinated chain carbonates, fluorinated chain carboxylic acid esters such as methyl fluoropropionate (FMP), etc.

[0040] 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, lower aliphatic lithium carboxylates, borates such as Li2B4O7, Li(B(C2O4)F2), LiN(SO2CF3)2, LiN(C1F 2l+1 SO2)(C m F 2m+1 Examples of the lithium salt include imide salts such as SiO2) (where l and m are integers of 1 or more). The lithium salt may be used alone or in combination. Among these, LiPF6 is preferred from the viewpoints of ionic conductivity, electrochemical stability, etc. The concentration of the lithium salt is preferably 0.8 to 1.8 mol per 1 L of solvent. [Example]

[0041] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0042] Example 1 [Preparation of positive electrode] The positive electrode active material is aluminum-containing lithium nickel cobalt oxide (LiNi 0.88 Co 0.09 Al 0.03 O2) was used. 100 parts by mass of the above positive electrode active material, 1 part by mass of graphite as a conductive agent, and 0.9 parts by mass of polyvinylidene fluoride powder as a binder were mixed together, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to prepare a positive electrode mixture slurry. This slurry was applied to both sides of a positive electrode current collector made of aluminum foil (thickness 15 μm) by the doctor blade method, and after drying the coating, the coating was rolled with a rolling roller to produce a positive electrode in which a positive electrode mixture layer was formed on both sides of the positive electrode current collector.

[0043] [Preparation of graphite particles A] The coke was crushed to an average particle size (D50) of 17 μm, and pitch was added to the crushed coke as a binder to aggregate the coke. An isotropic pressure of 1.6 g / cm was applied to the aggregate. 3 ~1.9g / cm 3A block-shaped compact having a density of 1000 MPa was produced. This block-shaped compact was graphitized by firing at a temperature of 2800°C, and the graphitized block-shaped compact was then pulverized and sieved using a 250 mesh sieve to obtain graphite particles A having an average particle size (D50) of 23 μm.

[0044] [Preparation of graphite particles B] Coke was pulverized to an average particle size (D50) of 13 μm, and pitch was added as a binder to the pulverized coke, followed by agglomeration to an average particle size (D50) of 18 μm. The agglomerates were graphitized by firing at a temperature of 2800°C. The graphitized block-shaped compact was then pulverized and sieved using a 250-mesh sieve to obtain graphite particles B having an average particle size (D50) of 23 μm.

[0045] [Preparation of negative electrode] Graphite particles A were mixed in an amount of 95 parts by mass and SiO2 in an amount of 5 parts by mass, and this was designated negative electrode active material A. Titanium oxide (TiO2) with an average particle size (D50) of 500 nm was used as the high-dielectric material. These were mixed so that the mass ratio of negative electrode active material A:TiO2:carboxymethyl cellulose (CMC):styrene-butadiene copolymer rubber (SBR) was 100:2.5:1:1, and the mixture was kneaded in water to prepare a first negative electrode mixture slurry. Graphite particles B were mixed in an amount of 95 parts by mass and SiO2 in an amount of 5 parts by mass, and this was designated negative electrode active material B. Negative electrode active material B:carboxymethyl cellulose (CMC):styrene-butadiene copolymer rubber (SBR) in an amount of 100:1:1, and the mixture was kneaded in water to prepare a second negative electrode mixture slurry.

[0046] The first negative electrode mixture slurry was applied to both sides of a copper foil negative electrode current collector by a doctor blade method and dried to form a first negative electrode mixture layer. Furthermore, the second negative electrode mixture slurry was applied to the first negative electrode mixture layer and dried to form a second negative electrode mixture layer. The applied mass ratio per unit area of ​​the first negative electrode mixture slurry to the second negative electrode mixture slurry was 5:5. The first negative electrode mixture layer and the second negative electrode mixture layer were rolled using a rolling roller to produce a negative electrode.

[0047] [Preparation of non-aqueous electrolyte] Five parts by mass of vinylene carbonate (VC) was added to 100 parts by mass of a non-aqueous solvent in which ethylene carbonate (EC) and dimethyl carbonate were mixed in a volume ratio of 1:3, and LiPF6 was dissolved therein at a concentration of 1.5 mol / L to prepare a non-aqueous electrolyte.

[0048] [Fabrication 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 resulting structure was 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.

[0049] <Example 2> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in the fabrication of the negative electrode, the content of TiO2 contained in the first negative electrode mixture slurry was 3 mass% relative to the mass of the negative electrode active material A.

[0050] Example 3 A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in fabricating the negative electrode, the content of TiO2 contained in the first negative electrode mixture slurry was set to 5 mass % relative to the mass of the negative electrode active material A.

[0051] <Comparative Example 1> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in fabricating the negative electrode, TiO2 was not added to the first negative electrode mixture slurry.

[0052] <Comparative Example 2> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Comparative Example 1, except that in fabricating the negative electrode, 2.5 mass % of TiO2 relative to the mass of negative electrode active material B was added to the second negative electrode mixture slurry.

[0053] <Comparative Example 3> A nonaqueous electrolyte secondary battery was fabricated in the same manner as in Example 1, except that in fabricating the negative electrode, the content of TiO contained in the first negative electrode mixture slurry was 1.5 mass % relative to the mass of negative electrode active material A, and 1.5 mass % of TiO was added to the second negative electrode mixture slurry relative to the mass of negative electrode active material B.

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

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

[0056] The evaluation results of the capacity retention rates of the nonaqueous electrolyte secondary batteries of each Example and Comparative Example are summarized in Table 1. Table 1 also shows the graphite particles contained in the first and second negative electrode mixture layers, the TiO2 content in the first and second negative electrode mixture layers, and the S2 / S1 ratio in the negative electrode mixture layers.

[0057] [Table 1]

[0058] The batteries of the examples have improved capacity retention rates compared to the batteries of the comparative examples. In the batteries of the examples, the porosity between the graphite particles in the second negative electrode mixture layer is high, and the first negative electrode mixture layer contains titanium oxide, which has a higher dielectric constant than graphite, which improves the permeability of the electrolyte throughout the negative electrode mixture layer, which is thought to be the reason for the improved capacity retention rates. [Explanation of symbols]

[0059] 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, 40 graphite particles, 42 internal void, 44 external void

Claims

1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, the negative electrode includes a negative electrode current collector and a negative electrode mixture layer formed on a surface of the negative electrode current collector, the negative electrode mixture layer includes a first negative electrode mixture layer facing the negative electrode current collector and a second negative electrode mixture layer stacked on a surface of the first negative electrode mixture layer, the negative electrode mixture layer includes a negative electrode active material containing graphite particles and a high-dielectric material having a dielectric constant higher than that of the graphite particles, the porosity between the graphite particles in the second negative electrode mixture layer is higher than the porosity between the graphite particles in the first negative electrode mixture layer, and when the porosity between the graphite particles in the first negative electrode mixture layer is S1 and the porosity between the graphite particles in the second negative electrode mixture layer is S2, S2 / S1 satisfies 1<S2 / S1≦2; a content of the high dielectric material in the first negative electrode mixture layer is higher than a content of the high dielectric material in the second negative electrode mixture layer, a content of the high dielectric material in the first negative electrode mixture layer is 1% by mass to 10% by mass with respect to the mass of the negative electrode active material contained in the first negative electrode mixture layer.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the high-dielectric material contains at least one of titanium oxide and lithium titanate.

3. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode mixture layer further contains a Si-based material as the negative electrode active material.

4. A method for manufacturing a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, comprising: a first step of forming, on a surface of a negative electrode current collector, a negative electrode mixture layer including a first negative electrode mixture layer facing the negative electrode current collector and a second negative electrode mixture layer laminated on the surface of the first negative electrode mixture layer, the negative electrode mixture layer including a negative electrode active material containing graphite particles and a high dielectric material having a dielectric constant higher than that of the graphite particles, to prepare a negative electrode; a second step of combining the negative electrode with the positive electrode and a non-aqueous electrolyte to form a non-aqueous electrolyte secondary battery; The first step a first anode mixture layer forming step of forming a first anode mixture layer containing the graphite particles and the high-dielectric material on a surface of the anode current collector, and a second anode mixture layer forming step of forming a second anode mixture layer containing the graphite particles on the surface of the first anode mixture layer, a method for manufacturing a nonaqueous electrolyte secondary battery, wherein in the first step, a content of the high dielectric material in the first negative electrode mixture layer is higher than a content of the high dielectric material in the second negative electrode mixture layer, the high dielectric material is mixed in an amount of 1 mass % to 10 mass % with respect to a mass of the negative electrode active material contained in the first negative electrode mixture layer, and the negative electrode mixture layer is formed such that a porosity S2 between graphite particles in the second negative electrode mixture layer, relative to a porosity S1 between graphite particles in the first negative electrode mixture layer, satisfies 1<S2 / S1≦2.

5. In the first negative electrode mixture layer forming step, the graphite particles contained in the first negative electrode mixture layer before rolling have an internal porosity of 8 to 20%, 5. The method for producing a nonaqueous electrolyte secondary battery according to claim 4, wherein in the second negative electrode mixture layer forming step, the graphite particles contained in the second negative electrode mixture layer before being rolled have an internal porosity of 5% or less.

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