Negative electrode for non-aqueous electrolyte secondary battery, and non-aqueous electrolyte secondary battery

Abstract

With respect to a negative electrode for nonaqueous electrolyte secondary batteries according to the present invention, a negative electrode mixture layer comprises a lower layer that is formed on the negative electrode core body side and an upper layer that is formed on the front surface side of the negative electrode mixture layer. According to the present invention, graphite particles contain: first graphite particles which have a circularity of less than 0.92; and second graphite particles which have a higher circularity than the first graphite particles. The ratio of the breaking strength of the first graphite particles to the breaking strength of the second graphite particles is 2 to 5. The content ratio of the first graphite particles relative to the graphite particles in the upper layer is 30% by mass or more, and is higher than the content ratio of the first graphite particles relative to the graphite particles in the lower layer.

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 using the negative electrode. [Background technology]

[0002] Non-aqueous electrolyte secondary batteries such as lithium ion batteries are used in vehicles and for power storage applications. Required performance characteristics of non-aqueous electrolyte secondary batteries for vehicle and power storage applications include high energy density, good charge / discharge cycle characteristics, and rapid charge performance. The negative electrode, a major component of a battery, significantly affects these performance characteristics, and therefore much research has been conducted on the negative electrode. For example, Patent Document 1 discloses a lithium ion battery using a negative electrode active material made of a mixture of at least two types of carbon particles with different particle fracture strengths. Patent Document 2 also discloses a lithium ion battery using a negative electrode active material with a circularity of 0.85 to 0.90. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-151087 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-175842 Summary of the Invention [Problem to be solved by the invention]

[0004] The adhesion of the negative electrode mixture layer to the negative electrode core and the permeability of the electrolyte through the negative electrode mixture layer are important factors in improving battery performance such as the energy density, cycle characteristics, and rapid charging performance of the battery. However, achieving both adhesion and permeability is not easy, and conventional techniques including those described in Patent Documents 1 and 2 still have room for improvement.

[0005] An object of the present disclosure is to provide a negative electrode for a non-aqueous electrolyte secondary battery in which the adhesion of the negative electrode mixture layer to the negative electrode core is good and the electrolyte solution permeability is excellent. [Means for solving the problem]

[0006] The negative electrode for a non-aqueous electrolyte secondary battery according to the present disclosure is a negative electrode for a non-aqueous electrolyte secondary battery comprising a negative electrode core and a negative electrode mixture layer formed on the negative electrode core, wherein the negative electrode mixture layer contains graphite particles as a negative electrode active material and has a lower layer formed on the negative electrode core side and an upper layer formed on the surface side of the negative electrode mixture layer, wherein the graphite particles include first graphite particles having a circularity of less than 0.92 and second graphite particles having a circularity higher than that of the first graphite particles, wherein the ratio of the fracture strength of the first graphite particles to the fracture strength of the second graphite particles is 2 to 5, and the content of the first graphite particles relative to the graphite particles in the upper layer is 30 mass% or more and is higher than the content of the first graphite particles relative to the graphite particles in the lower layer.

[0007] A non-aqueous electrolyte secondary battery according to the present disclosure includes the above-described negative electrode, a positive electrode, and a non-aqueous electrolyte. [Effects of the Invention]

[0008] The negative electrode for a non-aqueous electrolyte secondary battery according to the present disclosure has good adhesion of the negative electrode mixture layer to the negative electrode core and excellent permeability of the electrolyte solution. The non-aqueous electrolyte secondary battery using the negative electrode according to the present disclosure has, for example, a high energy density, good cycle characteristics, and excellent rapid charge performance. [Brief explanation of the drawings]

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

[0010] As described above, improving the adhesion of the negative electrode mixture layer to the negative electrode core and the electrolyte permeability of the negative electrode mixture layer are important challenges in improving battery performance such as the energy density, cycle characteristics, and rapid charging performance of the battery. As a result of extensive research aimed at solving these challenges, the present inventors have found that by forming a negative electrode mixture layer into a layered structure including a lower layer on the negative electrode core side and an upper layer on the surface side, and by disposing a large amount of first graphite particles with a circularity of less than 0.92 in the upper layer, the electrolyte permeability is significantly improved. This is thought to be because the non-spherical first graphite particles provide a moderate roughness to the surface of the negative electrode mixture layer, thereby reducing the contact angle with the electrolyte and facilitating the penetration of the electrolyte into the negative electrode mixture layer.

[0011] Furthermore, by disposing fewer first graphite particles in the lower layer of the negative electrode mixture layer than in the upper layer and disposing more second graphite particles, which are softer and have a higher circularity than the first graphite particles, the contact area between the negative electrode mixture layer and the negative electrode core can be increased, improving the adhesion of the negative electrode mixture layer to the negative electrode core. In other words, there is an optimal balance between the circularity and hardness (breaking strength) of the graphite particles that make up the negative electrode mixture layer with respect to the adhesion of the negative electrode mixture layer and the permeability of the electrolyte. The configuration of the negative electrode mixture layer according to the present disclosure makes it possible to achieve both such adhesion and permeability.

[0012] Hereinafter, an example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure also includes selective combinations of multiple embodiments and modified examples described below.

[0013] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical outer can 16 with a bottom is exemplified, but the outer can of the battery is not limited to a cylindrical outer can and may be, for example, a prismatic outer can (prismatic battery) or a coin-shaped outer can (coin battery), or may be an outer can made of a laminate sheet including a metal layer and a resin layer (laminated battery).The electrode assembly may also be a laminated electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0014] FIG. 1 is a schematic diagram showing a cross section of a nonaqueous electrolyte secondary battery 10 according to an embodiment. As shown in FIG. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte, and an outer can 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte. The electrode assembly 14 has a positive electrode 11, a negative electrode 12, and a separator 13, and has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The outer can 16 is a cylindrical metal container that is open on one axial side and has a bottom, and the opening of the outer can 16 is closed by a sealing member 17. For ease of explanation, the sealing member 17 side of the battery is referred to as the top, and the bottom side of the outer can 16 is referred to as the bottom.

[0015] The non-aqueous electrolyte includes a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. The non-aqueous solvent may contain a halogen-substituted compound in which at least a portion of the hydrogen atoms in these solvents are substituted with a halogen element such as fluorine. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and mixed solvents thereof. The electrolyte salt may be, for example, a lithium salt such as LiPF6. The non-aqueous electrolyte is not limited to a liquid electrolyte, but may also be a solid electrolyte.

[0016] The positive electrode 11, negative electrode 12, and separator 13 that make up the electrode assembly 14 are all long, strip-like bodies that are spirally wound and alternately stacked in the radial direction of the electrode assembly 14. The negative electrode 12 is formed to be slightly larger than the positive electrode 11 in order to prevent lithium deposition. That is, the negative electrode 12 is formed to be longer than the positive electrode 11 in the longitudinal direction and width direction (short direction). The separator 13 is formed to be at least slightly larger than the positive electrode 11, and two separators 13 are arranged to sandwich the positive electrode 11. The electrode assembly 14 has a positive electrode lead 20 connected to the positive electrode 11 by welding or the like, and a negative electrode lead 21 connected to the negative electrode 12 by welding or the like.

[0017] Insulating plates 18 and 19 are disposed above and below the electrode body 14. In the example shown in Fig. 1, the positive electrode lead 20 passes through a through-hole in the insulating plate 18 and extends toward the sealing body 17, and the negative electrode lead 21 passes outside the insulating plate 19 and extends toward the bottom side of the outer can 16. The positive electrode lead 20 is connected to the underside of an internal terminal plate 23 of the sealing body 17 by welding or the like, and a cap 27, which is the top plate of the sealing body 17 and is electrically connected to the internal terminal plate 23, serves as the positive electrode terminal. The negative electrode lead 21 is connected to the inner bottom surface of the outer can 16 by welding or the like, and the outer can 16 serves as the negative electrode terminal.

[0018] As described above, the outer can 16 is a cylindrical metal container with a bottom and an opening on one axial side. A gasket 28 is provided between the outer can 16 and the sealing body 17 to ensure sealing of the battery interior and insulation between the outer can 16 and the sealing body 17. The outer can 16 has a grooved portion 22 that supports the sealing body 17, with part of the side surface protruding inward. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the outer can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the outer can 16 by the grooved portion 22 and the open end of the outer can 16 that is crimped to the sealing body 17.

[0019] The sealing body 17 has a structure in which, in order from the electrode body 14 side, an internal terminal plate 23, a lower valve body 24, an insulating member 25, an upper valve body 26, and a cap 27 are stacked. Each member constituting the sealing body 17 has, for example, a disk or ring shape, and each member except for the insulating member 25 is electrically connected to one another. The lower valve body 24 and the upper valve body 26 are connected at their respective centers, and the insulating member 25 is interposed between their respective peripheral edges. If an abnormality occurs in the battery and the internal pressure increases, the lower valve body 24 deforms and ruptures, pushing the upper valve body 26 toward the cap 27, thereby interrupting the current path between the lower valve body 24 and the upper valve body 26. If the internal pressure further increases, the upper valve body 26 ruptures, and gas is discharged from the opening of the cap 27.

[0020] The positive electrode 11, the negative electrode 12, and the separator 13 that constitute the nonaqueous electrolyte secondary battery 10, and in particular the negative electrode 12, will be described in detail below.

[0021] [Positive electrode] The positive electrode 11 has a positive electrode core and a positive electrode mixture layer formed on the positive electrode core. The positive electrode core can be a foil of a metal, such as aluminum or an aluminum alloy, that is stable within the potential range of the positive electrode 11, or a film with such a metal disposed on its surface. The positive electrode mixture layer contains a positive electrode active material, a conductive agent, and a binder, and is preferably provided on both sides of the positive electrode core except for the exposed portion of the core to which the positive electrode lead is connected. The thickness of the positive electrode mixture layer is, for example, 50 μm to 150 μm on one side of the positive electrode core. The positive electrode 11 can be produced by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, etc. to the surface of the positive electrode core, drying the coating, and then compressing it to form a positive electrode mixture layer on both sides of the positive electrode core.

[0022] The positive electrode active material is composed mainly of a lithium transition metal composite oxide. Elements other than Li contained in the lithium transition metal composite oxide include Ni, Co, Mn, Al, B, Mg, Ti, V, Cr, Fe, Cu, Zn, Ga, Sr, Zr, Nb, In, Sn, Ta, W, Si, and P. An example of a suitable lithium transition metal composite oxide is a composite oxide containing at least one of Ni, Co, and Mn. Specific examples include a lithium transition metal composite oxide containing Ni, Co, and Mn, and a lithium transition metal composite oxide containing Ni, Co, and Al.

[0023] Examples of conductive agents contained in the positive electrode mixture layer include carbon materials such as carbon black, acetylene black, ketjen black, and graphite. Examples of binders contained in the positive electrode mixture layer include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. These resins may be used in combination with cellulose derivatives such as carboxymethyl cellulose (CMC) or its salts, polyethylene oxide (PEO), and the like.

[0024] [Negative electrode] 2 is a diagram showing a portion of a cross section of the negative electrode 12. The negative electrode 12 has a negative electrode core 30 and a negative electrode mixture layer 31 formed on the negative electrode core 30. The negative electrode core 30 can be made of a foil of a metal such as copper or a copper alloy that is stable within the potential range of the negative electrode 12, or a film with such a metal disposed on its surface. The negative electrode mixture layer 31 is preferably provided on both sides of the negative electrode core 30 except for the core exposed portion to which the negative electrode lead is connected. The thickness of the negative electrode mixture layer 31 on one side of the negative electrode core 30 is, for example, 50 μm to 150 μm.

[0025] The negative electrode mixture layer 31 contains graphite particles as a negative electrode active material. The graphite particles include first graphite particles 32A having a circularity of less than 0.92 and second graphite particles 32B having a circularity higher than that of the first graphite particles 32A. As will be described in detail later, the ratio of the fracture strength of the first graphite particles 32A to that of the second graphite particles 32B is 2 to 5. The circularity of the second graphite particles 32B is 0.94 or higher. The negative electrode mixture layer 31 has a lower layer 31B formed on the negative electrode substrate 30 side and an upper layer 31A formed on the lower layer 31B. The content ratio of the first graphite particles 32A to the graphite particles in the upper layer 31A is 30% by mass or higher. Furthermore, the content ratio of the first graphite particles 32A to the graphite particles in the upper layer 31A is higher than that of the second graphite particles 32B to the graphite particles in the lower layer 31B. The negative electrode mixture layer 31 may contain graphite particles other than the first graphite particles 32A and the second graphite particles 32B, or a carbon material other than graphite, as long as the object of the present disclosure is not impaired. In this case, the content of the first graphite particles 32A is expressed as a percentage of the mass of the first graphite particles 32A with respect to the total mass of the first graphite particles 32A and the second graphite particles 32B. The same applies to the second graphite particles 32B.

[0026] The negative electrode mixture layer 31 has a two-layer structure including a lower layer 31B that adheres closely to the negative electrode core 30 and an upper layer 31A that is formed directly on the lower layer 31B and forms the surface of the negative electrode mixture layer 31. As described above, the upper layer 31A and the lower layer 31B differ in the abundance ratio of the first graphite particles 32A and the second graphite particles 32B that constitute the layers. The first graphite particles 32A and the second graphite particles 32B are, for example, artificial graphite, but natural graphite can also be used. The negative electrode mixture layer 31 has such a two-layer structure, which ensures good adhesion of the negative electrode mixture layer 31 to the negative electrode core 30 while improving the permeability of the electrolyte solution through the negative electrode mixture layer 31. Note that, for example, a third layer may be formed between the upper layer 31A and the lower layer 31B, as long as it does not impair the objectives of the present disclosure.

[0027] The negative electrode mixture layer 31 contains a negative electrode active material and a binder. The upper layer 31A and the lower layer 31B may contain, for example, substantially the same amount of the same type of binder. The content of the negative electrode active material is preferably 85 to 99.5 mass %, and more preferably 90 to 99 mass %, relative to the mass of the negative electrode mixture layer 31. The content of the binder is preferably 0.5 to 15 mass %, and more preferably 1 to 10 mass %, relative to the mass of the negative electrode mixture layer 31. The negative electrode mixture layer 31 may contain materials other than the negative electrode active material and the binder, such as a conductive agent and a thickener.

[0028] The negative electrode mixture layer 31 may contain, as the negative electrode active material, an active material containing at least one of an element that alloys with Li, such as Si or Sn, and a compound containing such an element. A suitable example of such an active material is a silicon material. For example, the negative electrode active material may be a combination of a carbon material, such as graphite, and a silicon material. An example of a suitable silicon material is a silicon material in which Si fine particles are dispersed in a silicon oxide phase or a silicate phase, such as lithium silicate. When a carbon material and a silicon material are used in combination as the negative electrode active material, the mixing ratio of the carbon material to the silicon material is preferably 80:20 to 98:2.

[0029] The binder contained in the negative electrode mixture layer 31 can be, as in the case of the positive electrode 11, fluororesin, PAN, polyimide, acrylic resin, polyolefin, etc., but is preferably styrene butadiene rubber (SBR). The negative electrode mixture layer 31 may further contain CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), etc. It is preferable to use a combination of SBR and CMC or a salt thereof as the binder for the negative electrode mixture layer 31.

[0030] As described above, the upper layer 31A contains 30% by mass or more of the first graphite particles 32A. In this embodiment, the upper layer 31A contains substantially only the first graphite particles 32A and the second graphite particles 32B as graphite particles. In the upper layer 31A, the content of the first graphite particles 32A relative to the graphite particles is preferably 50% by mass or more, for example, 50 to 100% by mass. In this case, the permeability of the electrolyte is more effectively improved. In the upper layer 31A, the content of the second graphite particles 32B relative to the graphite particles is 70% by mass or less, preferably equal to or less than the content of the first graphite particles 32A.

[0031] The lower layer 31B preferably contains second graphite particles 32B as a main component. Here, the term "main component" refers to the component with the highest mass ratio among the materials constituting the lower layer 31B. In the lower layer 31B, the content of the second graphite particles 32B relative to the graphite particles may be 100 mass%. The lower layer 31B may contain first graphite particles 32A, but the content of the first graphite particles 32A is preferably 30 mass% or less, and more preferably 20 mass% or less. By arranging a large amount of the second graphite particles 32B in the lower layer 31B that contacts the negative electrode core 30, the adhesion of the negative electrode mixture layer 31 to the negative electrode core 30 is improved.

[0032] The thickness ratio between the upper layer 31A and the lower layer 31B is not particularly limited, but an example of the thickness of the upper layer 31A is 10 to 60% of the thickness of the negative electrode mixture layer 31, and preferably 30 to 55%. In other words, an example of the thickness of the lower layer 31B is preferably 45 to 70% of the thickness of the negative electrode mixture layer 31. The thicknesses of the upper layer 31A and the lower layer 31B may be substantially the same (50:50). If the thickness of the upper layer 31A is within this range, the negative electrode mixture layer 31 will have excellent electrolyte permeability while ensuring good adhesion of the negative electrode mixture layer 31 to the negative electrode core 30, and the rapid charging performance of the battery will be more effectively improved.

[0033] As described above, the first graphite particles 32A are graphite particles with a circularity of less than 0.92. The circularity of a graphite particle is determined in particle image analysis as the ratio of the perimeter of a circle equal to the particle area to the particle perimeter (perimeter of a circle equal to the particle area / particle perimeter). If the particle image shows a perfect circle, i.e., if the particle is spherical, the circularity is 1.0. If the particle shape is angular, flat, acicular, or otherwise different from a sphere, the circularity is lower. The circularity of the graphite particles is measured using an image particle size analyzer (Sysmex, FPIA-3000).

[0034] The circularity of the first graphite particles 32A is preferably 0.87 or more, more preferably 0.88 or more. When the circularity of the first graphite particles 32A is 0.87 or more and less than 0.92, the upper layer 31A Mitsuru This can sufficiently reduce the contact angle of the electrolyte on the surface of the negative electrode mixture layer 31 while suppressing a decrease in packing density. The circularity of the second graphite particles 32B is preferably 0.94 or more, for example, 0.94 to 0.98. As shown in FIG. 2, the first graphite particles 32A are more angular and have sharper corners than the second graphite particles 32B. The second graphite particles 32B are rounder and closer to spherical than the first graphite particles 32A.

[0035] The first graphite particles 32A are 2 to 5 times harder than the second graphite particles 32B. Particle hardness is evaluated by particle fracture strength. Generally, the higher the particle fracture strength, the harder the particle. The fracture strength (StA) of the first graphite particles 32A is, for example, 12 to 100 MPa, more preferably 15 to 40 MPa. The fracture strength (StB) of the second graphite particles 32B is, for example, 2 to 20 MPa, more preferably 5 to 12 MPa. If the fracture strength ratio (StA / StB) is 2 to 5, it becomes easy to maintain a moderately rough surface shape of the upper layer 31A while ensuring good adhesion of the lower layer 31B to the negative electrode substrate 30. Furthermore, the second graphite particles 32B of the lower layer 31B are appropriately crushed and packed at a high density during the production of the negative electrode 12, which also contributes to improving the energy density.

[0036] The breaking strength of the graphite particles is measured by the following method using a microcompression tester (MCT-W201, manufactured by Shimadzu Corporation). (1) In the microcompression tester, the sample is spread on the lower pressure plate (SKS plate). (2) Using an optical microscope, select one particle whose particle size is close to the average particle size. (A single particle was selected between the upper pressure indenter (a 50 μm diameter diamond flat indenter) and the lower pressure plate.) (3) The upper pressure indenter is slowly lowered, and from the point where it comes into contact with the particle (the point where the descent speed changes), a load is applied at a constant acceleration (the amount of deformation of the particle is measured automatically). (4) The point at which the deformation of the particle suddenly changes (the inflection point of the load-deformation profile) is taken as the breaking point, and the breaking strength is calculated from the load and particle diameter at that time using the following formula: The breaking strength was taken as the average of five measured values. St=2.8P / πd 2 St: Breaking strength [N / mm 2 or MPa] P: Load [N] d: Particle diameter [mm]

[0037] The particle sizes of the first graphite particles 32A and the second graphite particles 32B are not particularly limited, but an example of the volume-based median diameter (D50) is 1 μm to 30 μm, preferably 5 μm to 20 μm. D50 refers to the particle size at which the cumulative frequency of particles in the volume-based particle size distribution is 50% from the smallest particle size, and is also called the median diameter. The particle size distribution can be measured using a laser diffraction particle size distribution measuring device (for example, MT3000II manufactured by Microtrac-Bell Corporation) with water as the dispersion medium. The BET specific surface area of ​​the first graphite particles 32A and the second graphite particles 32B is not particularly limited, but an example is 0.5 to 8.0 m 2 The BET specific surface area of ​​the graphite particles is measured in accordance with the BET method (nitrogen adsorption method) described in JIS R1626.

[0038] The first graphite particles 32A and the second graphite particles 32B are produced, for example, using petroleum-based or coal-based coke material as a starting material. The circularity of the graphite particles can be adjusted by the classification rotation speed of a pulverizer (e.g., a jet mill) that pulverizes the coke. For example, increasing the classification rotation speed tends to decrease the particle circularity. Graphite particles are obtained by filling a graphite sheath with the pulverized coke and subjecting it to high-temperature heat treatment using an Acheson furnace. In the production process of the first graphite particles 32A, for example, the heat treatment temperature is set lower than in the production process of the second graphite particles 32B. Artificial graphite may be used for the first graphite particles 32A, and natural graphite may be used for the second graphite particles 32B.

[0039] The negative electrode mixture layer 31 can be formed, for example, using a first negative electrode mixture slurry containing first graphite particles 32A and a binder, and a second negative electrode mixture slurry containing second graphite particles 32B and a binder. For example, the second negative electrode mixture slurry is applied to the surface of the negative electrode core 30, the first coating film is dried, the first negative electrode mixture slurry is applied to this coating film, the second coating film is dried, and the two layers are compressed together with a rolling roller, thereby producing a negative electrode 12 having a two-layer structure including the negative electrode mixture layer 31. It is also possible to compress only the first coating film that will become the lower layer 31B, and then form the second coating film that will become the upper layer 31A.

[0040] [Separator] A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. Suitable materials for the separator 13 include polyolefins such as polyethylene, polypropylene, and copolymers of ethylene and α-olefins, cellulose, polystyrene, polyester, polyphenylene sulfide, polyether ether ketone, and fluororesin. The separator 13 may have either a single-layer structure or a laminated structure. A heat-resistant layer containing inorganic particles, or a heat-resistant layer made of a highly heat-resistant resin such as aramid resin, polyimide, or polyamideimide, may be formed on the surface of the separator 13. [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 first negative electrode mixture slurry] As the negative electrode active material, graphite A1 having a particle breaking strength of 20 MPa, a circularity of 0.90, and a D50 of 18 μm, graphite B1 having a particle breaking strength of 7 MPa, a circularity of 0.95, and a D50 of 18 μm, and SiO x The graphite particles were mixed with a silicon material represented by the formula (1) in a mass ratio of 47:47:6 (the mass ratio of graphite A to graphite B was 50:50). The negative electrode active material, a dispersion of styrene butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC-Na) were mixed in a solids mass ratio of 100:1:1, and an appropriate amount of water was added to prepare the first negative electrode mixture slurry. The particle fracture strength of the graphite particles was measured using a microcompression tester (Shimadzu Corporation, MCT-W201), the circularity was measured using an image particle size distribution analyzer (Sysmex, FPIA-3000), and the D50 was measured using a laser diffraction particle size distribution analyzer (Microtrack Bell, MT3000II).

[0043] [Preparation of second negative electrode mixture slurry] The negative electrode active material was a mixture of graphite B1 and the silicon material in a mass ratio of 94:6. The negative electrode active material, a dispersion of SBR, and CMC-Na were mixed in a solid content mass ratio of 100:1:1, and an appropriate amount of water was added to prepare a second negative electrode mixture slurry.

[0044] [Preparation of negative electrode] Using the doctor blade method, the second negative electrode mixture slurry was applied to one side of a negative electrode core made of 8 μm-thick copper foil. After drying the coating, the first negative electrode mixture slurry was applied to this coating and the coating was dried to form a coating with a two-layer structure consisting of an upper layer and a lower layer. The amount of each mixture slurry applied was adjusted so that the thicknesses of the upper and lower layers were approximately the same. A two-layer coating was also formed on the other side of the negative electrode core using a similar method. The two-layer coating was compressed using a roller, and the negative electrode core was cut to a specified size to produce a negative electrode with a negative electrode mixture layer formed on both sides of the negative electrode core.

[0045] The produced negative electrodes were subjected to an electrolyte permeability test and a peeling test by the following methods, and the evaluation results together with the structure of the negative electrode mixture layer are shown in Table 1 (the same applies to the examples and comparative examples described later).

[0046] [Evaluation of electrolyte permeability] Polyethylene carbonate (PC) was titrated to a thickness of 3 μm onto the surface of the negative electrode mixture layer, and the time it took for the PC to penetrate from the surface of the mixture layer to the interior and disappear (penetration time) was measured. The shorter this penetration time, the better the electrolyte permeability of the negative electrode mixture layer, and generally the better the rapid charging performance of the battery.

[0047] [Evaluation of peel strength] After attaching the negative electrode mixture layer to the double-sided tape attached to the resin plate, the negative electrode was lifted at a constant speed, and the load applied when the negative electrode mixture layer peeled off from the core was measured using a load cell. The higher the peel strength, the better the adhesion of the negative electrode mixture layer to the negative electrode core.

[0048] <Examples 2 to 4> A negative electrode was produced in the same manner as in Example 1, except that in preparing the first negative electrode mixture slurry that forms the upper layer of the negative electrode mixture layer, graphite A1 and graphite B1 were mixed in the mass ratio shown in Table 1, and the above performance evaluation was carried out.

[0049] <Example 5> A negative electrode was produced in the same manner as in Example 1, except that in preparing the first negative electrode mixture slurry that forms the upper layer of the negative electrode mixture layer, graphite B2 having a particle crushing strength of 10 MPa, a circularity of 0.90, and a D50 of 18 μm was used instead of graphite B1, and the performance evaluation was carried out as described above.

[0050] Example 6 A negative electrode was produced in the same manner as in Example 1, except that in preparing the first negative electrode mixture slurry that forms the upper layer of the negative electrode mixture layer, graphite A2 having a particle crushing strength of 35 MPa, a circularity of 0.90, and a D50 of 18 μm was used instead of graphite A1, and the performance evaluation was carried out as described above.

[0051] Example 7 A negative electrode was produced in the same manner as in Example 1, except that in preparing the first negative electrode mixture slurry that forms the upper layer of the negative electrode mixture layer, graphite A3 having a particle crushing strength of 20 MPa, a circularity of 0.91, and a D50 of 18 μm was used instead of graphite A1, and the performance evaluation was carried out as described above.

[0052] Example 8 A negative electrode was produced in the same manner as in Example 1, except that in preparing the second negative electrode mixture slurry that forms the lower layer of the negative electrode mixture layer, graphite A1 was used in place of a portion of graphite B1, and graphite A1 and graphite B1 were mixed in a mass ratio of 20:80, and the performance evaluation was carried out as described above.

[0053] <Comparative Example 1> A negative electrode was produced in the same manner as in Example 1, except that in preparing the first negative electrode mixture slurry that forms the upper layer of the negative electrode mixture layer, graphite A4 having a particle crushing strength of 7 MPa, a circularity of 0.95, and a D50 of 18 μm was used instead of graphite A1, and the performance evaluation was carried out as described above.

[0054] <Comparative Examples 2 to 4> A negative electrode was produced in the same manner as in Example 1, except that in preparing the first negative electrode mixture slurry that forms the upper layer of the negative electrode mixture layer, graphite A5 having a particle crushing strength of 20 MPa, a circularity of 0.95, and a D50 of 18 μm was used instead of graphite A1, and graphite A5 and graphite B1 were mixed in the mass ratio shown in Table 1, and the performance evaluation was carried out.

[0055] <Comparative Example 5> A negative electrode was produced in the same manner as in Example 1, except that in preparing the first negative electrode mixture slurry that forms the upper layer of the negative electrode mixture layer, graphite A1 and graphite B1 were mixed in the mass ratio shown in Table 1, and the above performance evaluation was carried out.

[0056] <Comparative Example 6> A negative electrode was produced in the same manner as in Example 1, except that in preparing the first negative electrode mixture slurry that forms the upper layer of the negative electrode mixture layer, graphite A6 having a particle crushing strength of 45 MPa, a circularity of 0.90, and a D50 of 18 μm was used instead of graphite A1, and the performance evaluation was carried out as described above.

[0057] <Comparative Example 7> A negative electrode was produced in the same manner as in Example 1, except that in preparing the first negative electrode mixture slurry that forms the upper layer of the negative electrode mixture layer, graphite A7 having a particle crushing strength of 45 MPa, a circularity of 0.95, and a D50 of 18 μm was used instead of graphite A1, and the performance evaluation was carried out as described above.

[0058] <Comparative Example 8> A negative electrode was produced in the same manner as in Example 1, except that in preparing the first negative electrode mixture slurry that forms the upper layer of the negative electrode mixture layer, graphite A8 having a particle crushing strength of 7 MPa, a circularity of 0.90, and a D50 of 18 μm was used instead of graphite A1, and the performance evaluation was carried out as described above.

[0059] <Comparative Example 9> In producing the negative electrode, a negative electrode and a battery were produced in the same manner as in Example 1, except that the first negative electrode mixture slurry was used to form the lower layer of the negative electrode mixture layer and the second negative electrode mixture slurry was used to form the upper layer of the negative electrode mixture layer, and the performance evaluation was carried out as described above.

[0060] [Table 1]

[0061] As shown in Table 1, all of the negative electrodes of the examples had high peel strength of the negative electrode mixture layer and short electrolyte penetration times. That is, the negative electrodes of the examples had good adhesion of the negative electrode mixture layer to the negative electrode core and excellent electrolyte penetration. In contrast, the negative electrodes of the comparative examples were unable to achieve both good adhesion of the negative electrode mixture layer and good electrolyte penetration. [Explanation of symbols]

[0062] 10 non-aqueous electrolyte secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 16 outer can, 17 sealing body, 18, 19 insulating plate, 20 positive electrode lead, 21 negative electrode lead, 22 grooved portion, 23 internal terminal plate, 24 lower valve body, 25 insulating member, 26 upper valve body, 27 cap, 28 gasket, 30 negative electrode core, 31 negative electrode mixture layer, 31A upper layer, 31B lower layer, 32A first graphite particles, 32B second graphite particles

Claims

1. A negative electrode for a non-aqueous electrolyte secondary battery, comprising: a negative electrode core; and a negative electrode mixture layer formed on the negative electrode core, the negative electrode mixture layer contains graphite particles as a negative electrode active material, and has a lower layer formed on the negative electrode core side and an upper layer formed on a surface side of the negative electrode mixture layer, the graphite particles include first graphite particles having a circularity of less than 0.92 and second graphite particles having a circularity higher than that of the first graphite particles, a ratio of the fracture strength of the first graphite particles to the fracture strength of the second graphite particles is 2 to 5; a content ratio of the first graphite particles to the graphite particles in the upper layer is 30 mass% or more and is higher than a content ratio of the first graphite particles to the graphite particles in the lower layer; Negative electrode for non-aqueous electrolyte secondary batteries.

2. 2 . The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1 , wherein the lower layer contains the second graphite particles as a main component.

3. 3. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein a content of said first graphite particles relative to said graphite particles in said upper layer is 50 to 100% by mass.

4. 4. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the thickness of said upper layer is 10 to 60% of the thickness of said negative electrode mixture layer.

5. 5. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode mixture layer contains a silicon material as the negative electrode active material.

6. A non-aqueous electrolyte secondary battery comprising the negative electrode for a non-aqueous electrolyte secondary battery according to any one of claims 1 to 4, a positive electrode, and a non-aqueous electrolyte.

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