Nonaqueous electrolyte secondary battery

Abstract

The present invention is characterized in that: a negative electrode (12) includes a negative electrode current collector (30) and a negative electrode mixture layer (32) that is disposed on the surface of the negative electrode current collector (30) and contains a negative electrode active material; the negative electrode mixture layer (32) has a first negative electrode mixture layer (34) that is disposed on the surface side of the negative electrode mixture layer (32), and a second negative electrode mixture layer (36) that is disposed between the first negative electrode mixture layer (34) and the negative electrode current collector (30); the first negative electrode mixture layer (34) contains graphite particles A that have a 20% compressive strength of 20 MPa or more; the second negative electrode mixture layer (36) contains graphite particles B that have a 20% compressive strength of less than 20 MPa; and with respect to the negative electrode mixture layer (32), the ratio of the thickness of the first negative electrode mixture layer (34) to the thickness of the negative electrode mixture layer (32) at the inner end of winding is greater than the ratio of the thickness of the first negative electrode mixture layer (34) to the thickness of the negative electrode mixture layer (32) at the outer end of winding.

Description

Nonaqueous electrolyte secondary battery

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery.

[0002] Non-aqueous electrolyte secondary batteries are widely used as high-energy density secondary batteries. Generally, graphite is used as the negative electrode active material for non-aqueous electrolyte secondary batteries. Patent Document 1 discloses a technique for distributing a large number of graphite particles with low internal porosity on the surface side of a negative electrode mixture layer. This technique can suppress peeling of graphite particles from a negative electrode current collector, thereby improving charge-discharge cycle characteristics.

[0003] International Publication No. 2019 / 239652

[0004] In recent years, with the spread of electric vehicles and other factors, there has been an increasing demand for improved charge-discharge cycle characteristics of batteries. As a result of extensive research, the present inventors have found that in nonaqueous electrolyte secondary batteries equipped with a wound electrode assembly, charge-discharge reactions do not proceed easily on the inner side of the wound negative electrode, which tends to result in poor charge-discharge cycle characteristics. This is presumably because the amount of nonaqueous electrolyte held on the inner side of the wound negative electrode is smaller than the amount of nonaqueous electrolyte held on the outer side of the wound negative electrode. Patent Document 1 does not consider the promotion of charge-discharge reactions on the inner side of the wound negative electrode, and there is still room for improvement.

[0005] A non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is a non-aqueous electrolyte secondary battery including an electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound with a separator interposed therebetween, a non-aqueous electrolyte, and an exterior housing that houses the electrode assembly and the non-aqueous electrolyte, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture layer that is disposed on at least one surface of the negative electrode current collector and contains a negative electrode active material, and the negative electrode mixture layer includes a first negative electrode mixture layer disposed on the surface side of the negative electrode mixture layer and a second negative electrode mixture layer disposed between the first negative electrode mixture layer and the negative electrode current collector. the first anode mixture layer contains graphite particles A having a 20% compressive strength of 20 MPa or more, and the second anode mixture layer contains graphite particles B having a 20% compressive strength of less than 20 MPa, and the anode mixture layer is characterized in that the ratio of the thickness of the first anode mixture layer to the sum of the thicknesses of the first anode mixture layer and the second anode mixture layer at an inner end of the winding is greater than the ratio of the thickness of the first anode mixture layer to the sum of the thicknesses of the first anode mixture layer and the second anode mixture layer at an outer end of the winding.

[0006] According to the nonaqueous electrolyte secondary battery of one aspect of the present disclosure, charge-discharge cycle characteristics can be improved.

[0007] Fig. 1 is an axial cross-sectional view of a nonaqueous electrolyte secondary battery according to an embodiment; Fig. 2 is a perspective view of a wound electrode body included in the nonaqueous electrolyte secondary battery according to an embodiment; Fig. 3 is a view showing a negative electrode included in the nonaqueous electrolyte secondary battery according to an embodiment in a developed state; Fig. 4 is a cross-sectional view of a negative electrode included in the nonaqueous electrolyte secondary battery according to an embodiment; Fig. 5 is a view showing a change in the thickness ratio of a first negative electrode mixture layer in a negative electrode included in the nonaqueous electrolyte secondary battery according to an embodiment;

[0008] 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. 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 nonaqueous electrolyte secondary battery. Furthermore, when the following description includes multiple embodiments and modified examples, it is initially assumed that the characteristic portions thereof will be used in appropriate combination.

[0009] In the following, a cylindrical battery in which a wound electrode assembly 14 is housed in a cylindrical exterior body 16 with a bottom is exemplified as a nonaqueous electrolyte secondary battery, but the exterior body of the battery is not limited to a cylindrical exterior can. The nonaqueous electrolyte secondary battery according to the present disclosure may be, for example, a prismatic battery having a prismatic exterior can, a coin battery having a coin-shaped exterior can, or a pouch-type battery having an exterior body made of a laminate sheet including a metal layer and a resin layer.

[0010] Fig. 1 is a cross-sectional view of an example of a nonaqueous electrolyte secondary battery 10. As shown in Fig. 1, the nonaqueous electrolyte secondary battery 10 includes a wound electrode assembly 14, a nonaqueous electrolyte (not shown), and an exterior body 16 that accommodates the electrode assembly 14 and the nonaqueous electrolyte.

[0011] As will be described in detail later, the electrode body 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 electrode body 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.

[0012] The non-aqueous electrolyte has lithium ion conductivity. 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. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. The electrolyte salt may include, for example, LiPF ６ Lithium salts such as

[0013] The exterior body 16 is a cylindrical metal container with a bottom and an opening at one axial end, and the opening of the exterior body 16 is closed by a sealing body 17. For ease of explanation, the sealing body 17 side of the battery will be referred to as the "top" and the bottom side of the exterior body 16 will be referred to as the "bottom" below.

[0014] 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 exterior body 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 exterior body 16 by welding or the like, and the exterior body 16 serves as the negative electrode terminal.

[0015] A gasket 28 is provided between the exterior body 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior body 16 has a grooved portion 22 formed on its side surface that protrudes inward and supports the sealing body 17. The grooved portion 22 is preferably formed in an annular shape along the circumferential direction of the exterior body 16, and supports the sealing body 17 on its upper surface. The sealing body 17 is fixed to the top of the exterior body 16 by the grooved portion 22 and the open end of the exterior body 16 that is crimped to the sealing body 17.

[0016] 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. When abnormal heat generation causes an increase in the internal pressure of the battery, the lower valve body 24 deforms and breaks, 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. When the internal pressure further increases, the upper valve body 26 breaks, and gas is discharged from the opening of the cap 27.

[0017] Next, the electrode assembly 14 will be described in detail with reference to FIG. 2 . FIG. 2 is a perspective view of the electrode assembly 14. As described above, the electrode assembly 14 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 positive electrode 11, the negative electrode 12, and the separator 13 are all formed in strip shapes and spirally wound around a winding core disposed along a winding axis 29, resulting in an alternating stacked state in the radial direction of the electrode assembly 14. In the electrode assembly 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the width direction of the positive electrode 11 and the negative electrode 12 is the axial direction. The positive electrode lead 20 extends axially from approximately the center in the radial direction between the center and the outermost periphery at the upper end of the electrode assembly 14. The negative electrode lead 21 extends axially from the lower end of the outermost periphery of the electrode assembly 14. The negative electrode lead 21 may extend in the axial direction from the vicinity of the winding axis 29 at the lower end of the electrode body 14 .

[0018] The positive electrode 11 has a positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode current collector 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 the surface layer. The positive electrode mixture layer contains a positive electrode active material, a conductive agent, and a binder. The positive electrode 11 can be produced, for example, by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, etc., onto the positive electrode current collector, drying the coating, and then compressing it to form positive electrode mixture layers on both sides of the positive electrode current collector.

[0019] The positive electrode mixture layer contains a particulate lithium-containing composite oxide as a positive electrode active material. The lithium-containing composite oxide is a composite oxide containing metal elements such as Co, Mn, Ni, and Al in addition to Li. The metal element constituting the lithium-containing composite oxide is, for example, at least one selected from Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, and Bi. Among them, it is preferable to contain at least one selected from Co, Ni, Al, and Mn. Examples of suitable composite oxides include lithium-containing composite oxides containing Ni, Co, and Mn, and lithium-containing composite oxides containing Ni, Co, and Al.

[0020] The lithium-containing composite oxide is, for example, a secondary particle formed by the aggregation of a plurality of primary particles. The volume-based median diameter (D50) of the composite oxide is not particularly limited, but is, for example, 3 μm or more and 30 μm or less, preferably 5 μm or more and 25 μm or less. When the composite oxide is a secondary particle formed by the aggregation of primary particles, the D50 of the composite oxide refers to the D50 of the secondary particles. D50 refers to the particle size at which the cumulative frequency 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 of the composite oxide (as well as that of the negative electrode active material) can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrac-Bell Corporation) using water as a dispersion medium.

[0021] The average particle size of the primary particles constituting the lithium-containing composite oxide is, for example, 0.05 μm or more and 1 μm or less, and is calculated by averaging the diameters of the circumscribed circles of the primary particles extracted by analyzing a scanning electron microscope (SEM) image of the cross section of the secondary particles.

[0022] Examples of conductive agents contained in the positive electrode mixture layer include carbon black such as acetylene black and ketjen black, graphite, carbon nanotubes (CNT), carbon nanofibers, graphene, and other carbon materials. Examples of binders contained in the positive electrode mixture layer include fluorine-containing resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resin, polyolefin, and the like. These resins may also be used in combination with carboxymethyl cellulose (CMC) or a salt thereof, polyethylene oxide (PEO), and the like.

[0023] 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 and polypropylene, and cellulose. The separator 13 may have a single-layer structure or a multi-layer structure. Furthermore, a highly heat-resistant resin layer such as an aramid resin may be formed on the surface of the separator 13.

[0024] A filler layer containing an inorganic filler may be formed at the interface between the separator 13 and at least one of the positive electrode 11 and the negative electrode 12. Examples of inorganic fillers include oxides and phosphate compounds containing metal elements such as Ti, Al, Si, and Mg. The filler layer can be formed by applying a slurry containing the filler to the surface of the positive electrode 11, the negative electrode 12, or the separator 13.

[0025] Next, the negative electrode 12 constituting the electrode assembly 14 will be described in detail with further reference to Figures 3 and 4. Figure 3 is a diagram showing the negative electrode 12 in a developed state, and Figure 4 is a cross-sectional view of the negative electrode 12.

[0026] 3 and 4 , the negative electrode 12 has a negative electrode current collector 30 and a negative electrode mixture layer 32 that contains a negative electrode active material and is disposed on at least one surface of the negative electrode current collector 30. In this embodiment, the negative electrode mixture layer 32 is disposed on both surfaces of the negative electrode current collector 30 except for an exposed portion 38 to which the negative electrode lead 21 is connected. Note that the exposed portion 38 may be provided on the inside of the negative electrode 12 instead of the outside of the negative electrode 12.

[0027] The negative electrode current collector 30 can be a foil of a metal such as copper that is stable within the potential range of the negative electrode 12, or a film having such a metal disposed on its surface. The thickness of the negative electrode current collector 30 is not particularly limited, but from the viewpoint of balancing the strength and weight reduction of the negative electrode 12, it is preferably 1 μm or more and 50 μm or less, and more preferably 5 μm or more and 30 μm or less.

[0028] The negative electrode mixture layer 32 includes a negative electrode active material and includes a first negative electrode mixture layer 34 disposed on the surface side of the negative electrode mixture layer 32, and a second negative electrode mixture layer 36 disposed between the first negative electrode mixture layer 34 and the negative electrode current collector 30. Hereinafter, the first negative electrode mixture layer 34 and the second negative electrode mixture layer 36 may be collectively referred to as the negative electrode mixture layer 32. The thickness of the negative electrode mixture layer 32 is preferably 100 μm or more and 300 μm or less, and more preferably 150 μm or more and 250 μm or less. In this embodiment, the negative electrode mixture layer 32 has a substantially uniform thickness along the longitudinal direction. Furthermore, as will be described in more detail below, the thickness of the first negative electrode mixture layer 34 is greater at the inner winding end 32A than at the outer winding end 32B. In other words, the thickness of the second negative electrode mixture layer 36 is smaller at the inner winding end 32A than at the outer winding end 32B.

[0029] In this embodiment, both the first anode mixture layer 34 and the second anode mixture layer 36 contain graphite particles and a silicon-containing material as the anode active material. The first anode mixture layer 34 and the second anode mixture layer 36 contain different types of graphite particles. The first anode mixture layer 34 or the second anode mixture layer 36 does not necessarily contain a silicon-containing material. The content of the anode active material contained in the anode mixture layer 32 is preferably 85% by mass or more and 99.5% by mass or less, and more preferably 90% by mass or more and 99% by mass or less, relative to the total mass of the anode mixture layer 32. The content of the anode active material in the first anode mixture layer 34 and the second anode mixture layer 36 may be the same or different from each other.

[0030] The first negative electrode mixture layer 34 contains, as the negative electrode active material, graphite particles (hereinafter, sometimes referred to as graphite particles A) having a 20% compressive strength of 20 MPa or more. The 20% compressive strength of graphite particles refers to the pressure required to compress the graphite particles by 20% relative to their particle diameter, and can be measured by the method described below. The first negative electrode mixture layer 34 contains highly hard graphite particles A, which prevents the graphite particles A from cracking during the rolling process in the production of the negative electrode, thereby maintaining voids formed at the grain boundaries of the graphite particles A. Furthermore, as described above, the first negative electrode mixture layer 34 is disposed on the surface side of the negative electrode mixture layer 32. Therefore, the formation of voids within the first negative electrode mixture layer 34 facilitates penetration of the nonaqueous electrolyte through the first negative electrode mixture layer 34 toward the negative electrode current collector 30.

[0031] The 20% compressive strength of the graphite particles A contained in the first negative electrode mixture layer 34 may be 20 MPa or more, preferably 22 MPa or more, and more preferably 24 MPa or more. As the 20% compressive strength of the graphite particles A increases, the graphite particles A are less likely to be compressed during the production of the negative electrode 12, and the voids formed at the grain boundaries of the graphite particles A are more likely to be maintained. Note that, in the rolling step using a roll press machine in the manufacturing process, the 20% compressive strength of the graphite particles A can be controlled using the linear pressure (kgf / cm), which is the load relative to the roll width. In consideration of the compressive strength of the graphite particles A contained in the first negative electrode mixture layer 34 and the density of the first negative electrode mixture layer 34, the roll press pressure required for rolling is preferably 400 kgf / cm or more and 1000 kgf / cm or less.

[0032] The 20% compression strength of graphite particles can be obtained by measuring under the following measurement conditions using a microcompression tester (for example, "MCT-W201" manufactured by Shimadzu Corporation). Specifically, the deformation amount and load of a sample particle are measured when a load is applied to one sample particle at the following loading rate. The 20% compression strength is then calculated by substituting the load (N) when compressed 20% of the particle diameter of the sample particle before deformation (particle diameter measured by a CCD camera) and the particle diameter of the sample particle before deformation into the following formula: 20% compression strength (MPa) = 2.8 × load (N) / {π × (particle diameter (mm)) ２}

[0033] <Measurement conditions for 20% compression strength> Test temperature: room temperature (25°C) Upper pressure indenter: flat indenter with a diameter of 50 μm (material: diamond) Lower pressure plate: SKS flat plate Measurement mode: compression test Test load: minimum 10 mN, maximum 50 mN Load rate: minimum 0.178 mN / sec, maximum 0.221 mN / sec Full scale displacement: 20 μm

[0034] The graphite particles may be any graphite-based material conventionally used as a negative electrode active material in nonaqueous electrolyte secondary batteries. Examples of such graphite particles include natural graphite, such as block graphite and amorphous graphite, and artificial graphite, such as block artificial graphite and graphitized mesophase carbon microbeads. Furthermore, methods for increasing the 20% compressive strength of graphite particles include, for example, mixing graphite particles made of natural graphite or the like with coal-based or petroleum-based pitch, heat-treating the mixture, and coating the surfaces of the graphite particles with amorphous graphite. Coating the surfaces of the graphite particles with amorphous graphite can increase the 20% compressive strength while ensuring the electrical conductivity of the graphite particles. The amount of amorphous graphite coated on the surfaces of the graphite particles is not particularly limited, but is, for example, 2% by mass or more and 6% by mass or less, preferably 3% by mass or more and 5% by mass or less, based on the mass of the graphite particles, taking into consideration the electrical conductivity and the diffusibility of Li ions into the graphite particles.

[0035] In the first negative electrode mixture layer 34, the content of graphite particles A relative to the total mass of the negative electrode active material is preferably 30 mass% or more, more preferably 40 mass% or more, and even more preferably 50 mass% or more. By setting the content of graphite particles A relative to the total mass of the negative electrode active material to 30 mass% or more, voids of appropriate size are formed inside the first negative electrode mixture layer 34, making it easier for the non-aqueous electrolyte to permeate through the first negative electrode mixture layer 34 to the negative electrode current collector 30 side. Furthermore, the content of graphite particles A relative to the total mass of the negative electrode active material is preferably 95 mass% or less, preferably 90 mass% or less, and even more preferably 85 mass% or less. As described above, the graphite particles A are not easily compressed during the rolling process when fabricating the negative electrode. Therefore, if the content of graphite particles A relative to the total mass of the negative electrode active material exceeds 95 mass%, excessive voids are generated in the first negative electrode mixture layer 34, and the thickness of the negative electrode mixture layer 32 is likely to be distributed in the longitudinal direction of the electrode plate. As a result, the charge / discharge reaction becomes non-uniform in the longitudinal direction of the electrode plate, which may actually deteriorate the charge / discharge cycle characteristics. Therefore, the content of graphite particles A relative to the total mass of the negative electrode active material in the first negative electrode mixture layer 34 is preferably 30 mass% to 95 mass%, more preferably 40 mass% to 90 mass%, and even more preferably 50 mass% to 85 mass%. Furthermore, the first negative electrode mixture layer 34 may contain graphite particles having a 20% compressive strength of less than 20 MPa (hereinafter, sometimes referred to as graphite particles B). However, from the viewpoint of forming voids inside the first negative electrode mixture layer 34, it is preferable that the first negative electrode mixture layer 34 contain more graphite particles A than graphite particles B.

[0036] The particle sizes of the graphite particles A and B are not particularly limited, but an example of the volume-based median diameter (D50) is 1 μm or more and 50 μm or less, and preferably 5 μm or more and 40 μm or less. The particle size distribution can be measured using a laser diffraction particle size distribution measuring device (e.g., MT3000II manufactured by Microtrac Bell Co., Ltd.) using water as a dispersion medium. The BET specific surface area of ​​the graphite particles A and B is not particularly limited, but an example is 0.5 m ２ / g or more, 8.0m ２The BET specific surface areas of the graphite particles A and B can be measured according to the BET method (nitrogen adsorption method) described in JIS R1626.

[0037] As described above, the second negative electrode mixture layer 36 contains graphite particles as the negative electrode active material. On the other hand, the second negative electrode mixture layer 36 contains graphite particles B as the graphite particles as a main component, and contains almost no graphite particles A. More specifically, the content of graphite particles A contained in the second negative electrode mixture layer 36 is 10 mass% or less, preferably 5 mass% or less, more preferably 3 mass% or less, and even more preferably 1 mass% or less, relative to the total mass of the negative electrode active material in the second negative electrode mixture layer 36. If the content of graphite particles A contained in the second negative electrode mixture layer 36 exceeds 10 mass%, excessive voids are likely to occur in the second negative electrode mixture layer 36, which may reduce the adhesion between the second negative electrode mixture layer 36 and the negative electrode current collector 30 and, conversely, deteriorate the charge / discharge cycle characteristics of the battery. In the second negative electrode mixture layer 36, the content of the graphite particles B relative to the total mass of the negative electrode active material is, for example, 50 mass % or more and 100 mass % or less.

[0038] In this embodiment, the first negative electrode mixture layer 34 and the second negative electrode mixture layer 36 contain a silicon-containing material as described above. The inclusion of a silicon-containing material facilitates achieving both high capacity and excellent cycle characteristics. Furthermore, the first negative electrode mixture layer 34 and the second negative electrode mixture layer 36 may contain, as the negative electrode active material, a carbon material other than graphite particles or a compound containing an element that alloys with Li, such as Sn, in addition to the graphite particles and the silicon-containing material.

[0039] The silicon-containing material may be any material containing Si, and examples thereof include silicon alloys, silicon compounds, and Si-containing composite materials. Among these, Si-containing composite materials are preferred. The D50 of the composite material is generally smaller than the D50 of graphite particles. The volume-based D50 of the composite material is, for example, 1 μm or more and 15 μm or less. One type of silicon-containing material may be used alone, or two or more types may be used in combination.

[0040] A suitable composite material is a composite particle including an ion-conducting phase, a Si phase dispersed in the ion-conducting phase, and a conductive layer covering the surface of the ion-conducting phase. The ion-conducting phase is, for example, at least one selected from the group consisting of a silicate phase, an amorphous carbon phase, a silicide phase, and a silicon oxide phase. The Si phase is formed by dispersing Si in the form of fine particles. The ion-conducting phase is a continuous phase composed of a collection of particles finer than the Si phase. The conductive layer is composed of a material with higher conductivity than the ion-conducting phase and forms a good conductive path in the negative electrode mixture layer 32.

[0041] An example of a suitable Si-containing composite material has a sea-island structure in which fine Si is dispersed almost uniformly in an amorphous silicon oxide phase, and the overall structure is represented by the general formula SiO ｘ The silicon oxide may be mainly composed of silicon dioxide. The oxygen to silicon content (x) is, for example, 0.5≦x<2.0, preferably 0.8≦x≦1.5.

[0042] Here, the content of the silicon-containing material relative to the total mass of the negative electrode active material in the first negative electrode mixture layer 34 is preferably smaller than the content of the silicon-containing material relative to the total mass of the negative electrode active material in the second negative electrode mixture layer 36. Silicon-containing materials undergo a larger volume change during charge and discharge than carbon materials such as graphite particles. Therefore, by making the content of the silicon-containing material in the first negative electrode mixture layer 34 smaller than the content of the silicon-containing material in the second negative electrode mixture layer 36, voids within the first negative electrode mixture layer 34 are more easily maintained during charge and discharge, and the nonaqueous electrolyte is more easily permeated through the first negative electrode mixture layer 34 to the negative electrode current collector 30. In addition, by making the content of the silicon-containing material in the first negative electrode mixture layer 34 smaller than the content of the silicon-containing material in the second negative electrode mixture layer 36, isolation of the silicon-containing material due to repulsion between the high-hardness graphite and the silicon-containing material during charge and discharge is suppressed, thereby suppressing a deterioration in cycle characteristics. The content of the silicon-containing material relative to the total mass of the negative electrode active material in the first negative electrode mixture layer 34 is, for example, 1 mass% or more and 15 mass% or less. The content of the silicon-containing material relative to the total mass of the negative electrode active material in the second negative electrode mixture layer 36 is, for example, 3 mass% or more and 25 mass% or less.

[0043] The negative electrode mixture layer 32 contains a binder in addition to the negative electrode active material. The content of the binder is preferably 0.5 mass % or more and 15 mass % or less, and more preferably 1 mass % or more and 10 mass % or less, relative to the mass of the negative electrode mixture layer 32. Note that the negative electrode mixture layer 32 may contain materials other than the negative electrode active material and the binder, such as a conductive agent and a thickener.

[0044] As in the case of the positive electrode 11, the binder contained in the negative electrode mixture layer 32 can be a fluororesin, PAN, polyimide, acrylic resin, polyolefin, or the like, but it is preferable to use styrene butadiene rubber (SBR). The negative electrode mixture layer 32 may further contain CMC or a salt thereof, polyacrylic acid (PAA) or a salt thereof, polyvinyl alcohol (PVA), or the like. It is preferable to use a combination of SBR and CMC or a salt thereof as the binder of the negative electrode mixture layer 32.

[0045] Next, the thicknesses of the first negative electrode mixture layer 34 and the second negative electrode mixture layer 36 that constitute the negative electrode mixture layer 32 will be described in detail with further reference to Fig. 5. Fig. 5 is a diagram showing the correlation between the position in the longitudinal direction of the negative electrode 12 and the ratio T of the thickness of the first negative electrode mixture layer 34 to the total thickness of the first negative electrode mixture layer 34 and the second negative electrode mixture layer 36 (the thickness of the negative electrode mixture layer 32). For example, the ratio T is 1.0 when the negative electrode mixture layer 32 is composed of only the first negative electrode mixture layer 34, and is 0 when the negative electrode mixture layer 32 is composed of only the second negative electrode mixture layer 36.

[0046] As shown in Fig. 5, the ratio T of the negative electrode mixture layer 32 at the inner winding end 32A is greater than the ratio T of the outer winding end 32B. When the electrode body 14 is wound, the tortuosity is greater at the inner winding end than at the outer winding end, which increases the stress applied to the electrode plate and reduces the voids inside the negative electrode mixture layer 32. This tends to make the amount of non-aqueous electrolyte held at the inner winding end of the negative electrode 12 smaller than the amount of non-aqueous electrolyte held at the outer winding end of the negative electrode 12. As a result, at the inner winding end of the negative electrode 12, non-uniform charge / discharge reactions occur in the thickness direction of the negative electrode mixture layer 32, and the charge / discharge cycle characteristics of the battery are likely to deteriorate. Therefore, by making the ratio T at the inner end 32A larger than the ratio T at the outer end 32B, i.e., by making the thickness of the first negative electrode mixture layer 34 at the inner end 32A larger than the thickness of the first negative electrode mixture layer 34 at the outer end 32B, the porosity of the surface side of the negative electrode mixture layer 32 at the inner end 32A can be increased. As a result, the amount of nonaqueous electrolyte retained at the inner end 32A can be increased, and charge / discharge reactions on the inner side of the negative electrode 12 tend to proceed uniformly in the thickness direction of the negative electrode mixture layer 32. Note that if the ratio T is increased along the longitudinal direction of the negative electrode mixture layer 32, i.e., if the thickness of the first negative electrode mixture layer 34 is increased along the longitudinal direction of the negative electrode mixture layer 32, it is necessary to increase the pressing pressure required for rolling during the production of the negative electrode 12. This increases the stress remaining in the negative electrode mixture layer 32 due to rolling, and therefore also increases the force generated when the stress is relaxed. As a result, the thickness of the negative electrode mixture layer 32 is likely to become uneven in the longitudinal direction of the electrode plate, causing uneven charge-discharge reactions. In particular, when the thickness of the negative electrode mixture layer 32 is large on the inside of the winding, the gaps through which the non-aqueous electrolyte passes are reduced, which may actually deteriorate the charge-discharge cycle characteristics.

[0047] The ratio T at the inner winding end 32A is preferably 0.3 to 0.8, more preferably 0.35 to 0.75, and even more preferably 0.4 to 0.7. The ratio T at the outer winding end 32B is preferably 0.5 or less, more preferably 0.1 to 0.45, and even more preferably 0.15 to 0.4. This further improves the charge / discharge cycle characteristics of the battery.

[0048] In this embodiment, as shown in FIG. 5( a), the ratio T increases continuously at a constant rate from the outer winding end 32B to the inner winding end 32A. During winding of the electrode assembly 14, the tortuosity increases toward the inner winding end, thereby increasing the stress applied to the electrode plate. Therefore, by continuously increasing the ratio T at a constant rate from the outer winding end 32B to the inner winding end 32A, the thickness of the first negative electrode mixture layer 34 can be increased toward the inner winding end, where the stress applied to the electrode plate increases. As a result, it becomes easier to increase the amount of nonaqueous electrolyte retained in the negative electrode mixture layer 32 while maintaining a constant thickness of the negative electrode mixture layer 32 along the longitudinal direction of the electrode plate, thereby further improving the charge / discharge cycle characteristics of the battery.

[0049] As shown in Fig. 5(b), the slope indicating the rate of increase in ratio T from the outer winding end 32B to the inner winding end 32A does not have to be constant, and the slope may change along the way. As shown in Fig. 5(c), the ratio T may be constant on the outer winding side and increase along the way towards the inner winding end 32A. As shown in Fig. 5(d), the ratio T may be constant on the inner winding side and the outer winding side and change stepwise along the way.

[0050] Next, a method for producing the negative electrode 12 including the negative electrode mixture layer 32 having the first negative electrode mixture layer 34 and the second negative electrode mixture layer 36 will be described. The negative electrode 12 can be produced, for example, through an active material production step of producing a negative electrode active material containing graphite particles and the like, a preparation step of preparing a negative electrode mixture slurry containing the negative electrode active material and a binder, a coating step of applying the negative electrode mixture slurry to the surface of the negative electrode current collector 30 and drying it, and a rolling step of rolling the coating film produced in the coating step. Here, in the preparation step, a first negative electrode mixture slurry containing graphite particles A and a second negative electrode mixture slurry not containing graphite particles A are produced.

[0051] In the coating step, the first and second anode mixture slurries are simultaneously coated onto the surface of the anode current collector 30 using, for example, a coating device having two discharge ports, such that the first and second anode mixture slurries are coated in this order from the surface side of the anode current collector 30. At this time, the discharge rate of the first anode mixture slurry is decreased and the discharge rate of the second anode mixture slurry is increased from the inner side to the outer side of the electrode plate. By varying the discharge rates of the first and second anode mixture slurries, the thickness ratio of the first anode mixture layer 34 and the second anode mixture layer 36 can be changed. Alternatively, in the coating step, a multi-layer die coater may be used to coat the first and second anode mixture slurries while varying their mixing ratio.

[0052] Hereinafter, the present disclosure will be further described with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

[0053] Example 1 [Fabrication of Positive Electrode] Particulate LiCo was used as the positive electrode active material. ０．９７９ Zr ０．００１ Mg ０．０１ Al ０．０１ O ２ A lithium transition metal oxide represented by the formula (I) was used. The above positive electrode active material, acetylene black (AB) as a conductive agent, and polyvinylidene fluoride powder as a binder were mixed in a mass ratio of 95:2.5:2.5, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further added to prepare a positive electrode mixture slurry. The positive electrode mixture slurry was then applied to both sides of a positive electrode current collector made of aluminum foil by a doctor blade method, and the coating film was dried and then 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. In addition, an exposed portion in which the surface of the positive electrode current collector was exposed was provided in a portion of the positive electrode.

[0054] [Fabrication of Negative Electrode] Graphite particles made of natural graphite with a D50 of 18 μm were mixed with pitch and heat-treated to obtain graphite particles a1, in which the surfaces of the graphite particles were coated with amorphous graphite. Graphite particles b1 were also obtained by not carrying out the heat treatment and not coating the graphite particles with amorphous graphite.

[0055] The 20% compressive strength of graphite particles a1 and graphite particles b1 was measured using a microcompression tester ("MCT-W201" manufactured by Shimadzu Corporation). The measurement conditions were as described above. In this measurement, the average value of 10 measurements was taken as the 20% compressive strength. As a result, the 20% compressive strength of graphite particles a1 was 25 MPa, and the 20% compressive strength of graphite particles b1 was 12 MPa.

[0056] Next, graphite particles a1, graphite particles b1, and SiO were mixed in a mass ratio of 40:55:5 to prepare a first negative electrode active material. Then, the first negative electrode active material, carboxymethyl cellulose (CMC) as a thickener, and styrene-butadiene copolymer (SBR) as a binder were mixed in a mass ratio of 98:1:1, and water was added to prepare a first negative electrode mixture slurry.

[0057] Further, graphite particles b1 and SiO were mixed in a mass ratio of 95:5 to prepare a second negative electrode active material. Then, the second negative electrode active material, carboxymethyl cellulose (CMC) as a thickener, and styrene-butadiene copolymer (SBR) as a binder were mixed in a mass ratio of 98:1:1, and water was added to prepare a second negative electrode mixture slurry.

[0058] Next, the first and second negative electrode mixture slurries were simultaneously applied to both sides of a negative electrode current collector made of copper foil. The discharge rate of the first negative electrode mixture slurry was decreased and the discharge rate of the second negative electrode mixture slurry was increased from the inner end of the electrode plate to the outer end of the electrode plate. More specifically, the first and second negative electrode mixture slurries were applied so that the coating mass ratio per unit area was 50:50 at the inner end of the electrode plate and 20:80 at the outer end of the electrode plate. The total coating mass per unit area of ​​the first and second negative electrode mixture slurries was uniform across the length of the electrode plate. The first and second negative electrode mixture slurries were then rolled using a rolling roller to produce a negative electrode. An exposed portion was provided on a portion of the negative electrode, exposing the surface of the negative electrode current collector. After rolling, the ratio T of the thickness of the first negative electrode mixture layer 34 to the thickness of the negative electrode mixture layer 32 at the inner end of the roll was 0.5, and the ratio T at the outer end of the roll was 0.2. Furthermore, as shown in FIG. 5( a ), the ratio T increased continuously at a constant rate from the outer end of the roll to the inner end of the roll.

[0059] [Preparation of non-aqueous electrolyte] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 3:3:4. Lithium hexafluorophosphate (LiPF ６ ) was dissolved in the solution to a concentration of 1.2 mol / L to prepare a non-aqueous electrolyte.

[0060] [Preparation of Test Cell (Non-Aqueous Electrolyte Secondary Battery)] A positive electrode lead was attached to the exposed portion of the positive electrode, and a negative electrode lead was attached to the exposed portion of the negative electrode. A wound electrode assembly was prepared by spirally winding the positive and negative electrodes with a polyolefin separator interposed therebetween. Insulating plates were then placed above and below the electrode assembly, and the negative electrode lead was welded to the case body and the positive electrode lead was welded to a sealing member. The electrode assembly was then housed in the case body. After injecting a non-aqueous electrolyte into the case body under reduced pressure, the opening of the case body was sealed with a sealing member via a gasket to prepare a test cell.

[0061] [Measurement of Liquid Absorption Time] The negative electrode prepared above was dried for 10 hours in a thermostatic chamber heated to 200 ° C. under a nitrogen atmosphere, and then cut into a size of 5 cm x 5 cm to prepare a test piece. Then, 3.0 μL of polypropylene carbonate (PC) was dropped vertically onto the surface of the test piece, and the time until the PC was absorbed into the sample was measured visually. In this measurement, six measurements were performed, and the average value was taken as the liquid absorption time. The shorter the liquid absorption time, the smoother the replacement of non-aqueous electrolyte from the surface of the negative electrode mixture layer to the negative electrode current collector side. Therefore, the non-aqueous electrolyte discharged from the negative electrode mixture layer during discharge is quickly absorbed into the negative electrode mixture layer during charge. Therefore, it is presumed that the shorter the liquid absorption time, the better the charge / discharge cycle characteristics.

[0062] [Evaluation of Cycle Characteristics (Capacity Retention)] The test cell was charged to 4.2 V at a constant current of 0.5 C in a temperature environment of 25°C, and then charged at a constant voltage of 4.2 V until the current value reached 0.05 C. After a 1-hour rest, the cell was discharged to 2.5 V at a constant current of 1.0 C. This constituted one cycle, and the discharge capacity at 1.0 C after 400 cycles was measured. The capacity retention was then calculated using the following formula: Capacity retention [%] = (Discharge capacity after 400 cycles / Discharge capacity at first cycle) × 100

[0063] Comparative Example 1 A test cell was produced and evaluated in the same manner as in Example 1, except that in producing the negative electrode, the coating mass ratio per unit area of ​​the first negative electrode mixture slurry and the second negative electrode mixture slurry was set to 35:65 from the inner end to the outer end of the roll. After rolling, the ratio T of the thickness of the first negative electrode mixture layer 34 to the thickness of the negative electrode mixture layer 32 was constant at 0.35 from the inner end to the outer end of the roll.

[0064] The liquid absorption times and capacity retention rates of Example 1 and Comparative Example 1 are shown in Table 1. The liquid absorption times indicate the liquid absorption times at the inner end and outer end of the winding, respectively. Table 1 also shows the 20% compressive strength of graphite particles a1, the content of graphite particles a1 relative to the total mass of the negative electrode active material in the first negative electrode mixture layer, and the ratio T at the inner end and outer end of the winding. The liquid absorption times of Example 1 and Comparative Example 1 shown in Table 1 are expressed relative to the liquid absorption time of Comparative Example 1, which is set to 100. The smaller the value of the liquid absorption time, the shorter the time.

[0065]

[0066] As shown in Table 1, the test cell of Example 1 had an improved capacity retention rate, i.e., improved cycle characteristics, compared to the test cell of Comparative Example 1. This is presumably because the amount of nonaqueous electrolyte retained on the inner side of the negative electrode was increased due to the shortened liquid absorption time at the inner end of the roll, thereby accelerating the charge-discharge reaction on the inner side of the roll. Therefore, by making the negative electrode mixture layer a two-layer structure consisting of a first negative electrode mixture layer containing graphite particles with a 20% compressive strength of 20 MPa or more and a second negative electrode mixture layer, and by making the ratio T at the inner end of the roll larger than the ratio T at the outer end of the roll, the cycle characteristics of the battery can be improved.

[0067] Example 2 A test cell was prepared and evaluated in the same manner as in Example 1, except that in preparing the negative electrode, graphite particles a1, graphite particles b1, and SiO were mixed in a mass ratio of 60:35:5 to prepare a first negative electrode active material.

[0068] Example 3 A test cell was prepared and evaluated in the same manner as in Example 1, except that in preparing the negative electrode, graphite particles a1, graphite particles b1, and SiO were mixed in a mass ratio of 80:15:5 to prepare a first negative electrode active material.

[0069] <Comparative Example 2> A test cell was produced and evaluated in the same manner as in Example 2, except that in producing the negative electrode, the application mass ratio per unit area of ​​the first negative electrode mixture slurry and the second negative electrode mixture slurry was set to 35:65 from the inner end to the outer end of the winding.

[0070] <Comparative Example 3> A test cell was produced and evaluated in the same manner as in Example 3, except that in producing the negative electrode, the application mass ratio per unit area of ​​the first negative electrode mixture slurry and the second negative electrode mixture slurry was set to 35:65 from the inner end to the outer end of the winding.

[0071] The liquid absorption times and capacity retention rates of Examples 2 and 3 and Comparative Examples 2 and 3 are shown in Tables 2 and 3, respectively. The liquid absorption times are shown for the inner and outer end portions of the roll. Similarly to Table 1, Tables 2 and 3 also show the 20% compressive strength of graphite particles a1, the content of graphite particles a1 relative to the total mass of the negative electrode active material in the first negative electrode mixture layer, and the ratio T at the inner and outer end portions. The liquid absorption times of Example 2 and Comparative Example 2 shown in Table 2 are expressed relative to the liquid absorption time of Comparative Example 2, which is set to 100. The liquid absorption times of Example 3 and Comparative Example 3 shown in Table 3 are expressed relative to the liquid absorption time of Comparative Example 3, which is set to 100.

[0072]

[0073]

[0074] As shown in Tables 2 and 3, the test cells of Examples 2 and 3 have improved capacity retention, i.e., improved cycle characteristics, compared to the test cells of Comparative Examples 2 and 3. Therefore, even when the ratio T at the inner end portion is further increased, the cycle characteristics of the battery can be improved.

[0075] The present disclosure will be further described by the following embodiments. Configuration 1: A non-aqueous electrolyte secondary battery including an electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound with a separator interposed therebetween, a non-aqueous electrolyte, and an exterior housing that houses the electrode assembly and the non-aqueous electrolyte, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture layer that is disposed on at least one surface of the negative electrode current collector and contains a negative electrode active material, and the negative electrode mixture layer includes a first negative electrode mixture layer disposed on a surface side of the negative electrode mixture layer and a second negative electrode mixture layer disposed between the first negative electrode mixture layer and the negative electrode current collector. a first negative electrode mixture layer having a thickness greater than the sum of the thicknesses of the first and second negative electrode mixture layers at an inner end of the winding, the first negative electrode mixture layer containing graphite particles A having a 20% compressive strength of 20 MPa or more, the second negative electrode mixture layer containing graphite particles B having a 20% compressive strength of less than 20 MPa, and the negative electrode mixture layer having a thickness greater than the sum of the thicknesses of the first and second negative electrode mixture layers at an outer end of the winding. Configuration 3: The nonaqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the ratio of the thickness of the first negative electrode mixture layer to the total thickness of the first negative electrode mixture layer and the second negative electrode mixture layer changes stepwise.Configuration 4: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the ratio of the thickness of the first negative electrode mixture layer to the total thickness of the first negative electrode mixture layer and the second negative electrode mixture layer at an inner winding end is 0.3 to 0.8.Configuration 5: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 4, wherein the ratio of the thickness of the first negative electrode mixture layer to the total thickness of the first negative electrode mixture layer and the second negative electrode mixture layer at an outer winding end is 0.5 or less.Configuration 6: The nonaqueous electrolyte secondary battery according to any one of Configurations 1 to 5, wherein the content of the graphite particles A in the first negative electrode mixture layer is 30% by mass or more and 95% by mass or less with respect to the total mass of the negative electrode active material.Aspect 7: The nonaqueous electrolyte secondary battery according to any one of aspects 1 to 6, wherein the content of the graphite particles B relative to the total mass of the negative electrode active material in the second negative electrode mixture layer is 50 mass% or more and 100 mass% or less. Aspect 8: The nonaqueous electrolyte secondary battery according to any one of aspects 1 to 7, wherein the negative electrode mixture layer contains a silicon-containing material. Aspect 9: The nonaqueous electrolyte secondary battery according to aspect 8, wherein the content of the silicon-containing material relative to the total mass of the negative electrode active material in the first negative electrode mixture layer is smaller than the content of the silicon-containing material relative to the total mass of the negative electrode active material in the second negative electrode mixture layer.

[0076] REFERENCE SIGNS LIST 10 nonaqueous electrolyte secondary battery, 11 positive electrode, 12 negative electrode, 13 separator, 14 electrode body, 16 exterior body, 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, 29 winding shaft, 30 negative electrode current collector, 32 negative electrode mixture layer, 32A inner winding end, 32B outer winding end, 34 first negative electrode mixture layer, 36 second negative electrode mixture layer, 38 exposed portion.

Claims

1. A non-aqueous electrolyte secondary battery comprising an electrode assembly in which a strip-shaped positive electrode and a strip-shaped negative electrode are wound with a separator interposed therebetween, a non-aqueous electrolyte, and an exterior housing that contains the electrode assembly and the non-aqueous electrolyte, wherein the negative electrode has a negative electrode current collector and a negative electrode mixture layer that is disposed on at least one surface of the negative electrode current collector and contains a negative electrode active material, the negative electrode mixture layer has a first negative electrode mixture layer that is disposed on the surface side of the negative electrode mixture layer and a second negative electrode mixture layer that is disposed between the first negative electrode mixture layer and the negative electrode current collector, the first negative electrode mixture layer contains graphite particles A having a 20% compressive strength of 20 MPa or more, and the second negative electrode mixture layer contains graphite particles B having a 20% compressive strength of less than 20 MPa, a ratio of a thickness of the first negative electrode mixture layer to a sum of thicknesses of the first negative electrode mixture layer and the second negative electrode mixture layer at an inner end of a winding is greater than a ratio of a thickness of the first negative electrode mixture layer to a sum of thicknesses of the first negative electrode mixture layer and the second negative electrode mixture layer at an outer end of a winding.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode mixture layer has a region in which the ratio of the thickness of the first negative electrode mixture layer to the sum of the thicknesses of the first negative electrode mixture layer and the second negative electrode mixture layer increases continuously from the outer end side of the winding to the inner end side of the winding.

3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode mixture layer has a ratio of the thickness of the first negative electrode mixture layer to the sum of the thicknesses of the first negative electrode mixture layer and the second negative electrode mixture layer that changes stepwise.

4. The nonaqueous electrolyte secondary battery according to claim 1, wherein a ratio of a thickness of the first negative electrode mixture layer to a total thickness of the first negative electrode mixture layer and the second negative electrode mixture layer at an inner end portion of the winding is 0.3 or more and 0.8 or less.

5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the ratio of the thickness of the first negative electrode mixture layer to the sum of the thicknesses of the first negative electrode mixture layer and the second negative electrode mixture layer at the outer end of the winding is 0.5 or less.

6. The nonaqueous electrolyte secondary battery according to claim 1, wherein in the first negative electrode mixture layer, the content of the graphite particles A relative to the total mass of the negative electrode active material is 30 mass % or more and 95 mass % or less.

7. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the graphite particles B in the second negative electrode mixture layer is 50 mass % or more and 100 mass % or less relative to the total mass of the negative electrode active material.

8. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode mixture layer contains a silicon-containing material.

9. The nonaqueous electrolyte secondary battery according to claim 8, wherein a content of the silicon-containing material relative to a total mass of the negative electrode active material in the first negative electrode mixture layer is smaller than a content of the silicon-containing material relative to a total mass of the negative electrode active material in the second negative electrode mixture layer.

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