Negative electrode for nonaqueous electrolyte secondary battery, and nonaqueous electrolyte secondary battery

Abstract

A negative electrode (12) for a nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure comprises a negative electrode core body (30), a first negative electrode mixture layer (31), and a second negative electrode mixture layer (32) disposed between the first negative electrode mixture layer (31) and the negative electrode core body (30), the negative electrode being characterized in that: the first negative electrode mixture layer (31) and the second negative electrode mixture layer (32) contain, as a negative electrode active material, a carbon material and a silicon-containing material; the silicon-containing material has silicon-containing particles and a coating layer that covers at least a portion of the surfaces of the silicon-containing particles; the coating layer contains a sulfonic acid compound and a hydrophobic polymer compound; and the porosity (ε1) of the first negative electrode mixture layer (31) and the porosity (ε2) of the second negative electrode mixture layer (32) satisfy ε1>ε2.

Description

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

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

[0002] It is known that silicon-containing materials can occlude more lithium ions per unit volume than carbon materials such as graphite. Patent Document 1 discloses a nonaqueous electrolyte secondary battery that uses, as a negative electrode active material, a silicon-containing material having silicon-containing particles that include a silicate phase and a silicon phase dispersed within the silicate phase.

[0003] International Publication No. 2019 / 151016

[0004] In non-aqueous electrolyte secondary batteries, it is important to improve the charge-discharge cycle characteristics while increasing the initial battery capacity. Conventional techniques including those described in Patent Document 1 have not been able to adequately address these challenges, and there is still much room for improvement.

[0005] A negative electrode for a non-aqueous electrolyte secondary battery according to one aspect of the present disclosure is a negative electrode for a non-aqueous electrolyte secondary battery including a negative electrode core, a first negative electrode mixture layer, and a second negative electrode mixture layer disposed between the first negative electrode mixture layer and the negative electrode core, wherein the first negative electrode mixture layer and the second negative electrode mixture layer contain a carbon material and a silicon-containing material as negative electrode active materials, the silicon-containing material has silicon-containing particles and a coating layer covering at least a portion of the surface of the silicon-containing particles, the coating layer includes a sulfonic acid compound and a hydrophobic polymer compound, and the porosity (ε1) of the first negative electrode mixture layer and the porosity (ε2) of the second negative electrode mixture layer satisfy the relationship ε1 > ε2.

[0006] According to the negative electrode for a non-aqueous electrolyte secondary battery of one aspect of the present disclosure, a non-aqueous electrolyte secondary battery having high capacity and excellent cycle characteristics can be provided.

[0007] 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery according to an embodiment; FIG. 2 is a cross-sectional view of a negative electrode according to an embodiment; FIG. 3 is a cross-sectional view schematically showing a silicon-containing material according to an embodiment.

[0008] Previously, non-aqueous electrolyte secondary batteries have been known that use a carbon material and a silicon-containing material containing silicon-containing particles as the negative electrode active material in order to achieve high capacity. However, when the negative electrode active material contains silicon-containing particles, the silicon-containing particles tend to be gradually eroded due to a side reaction between the silicon-containing particles and the non-aqueous electrolyte. As this erosion progresses, the silicon-containing material deteriorates, and the cycle characteristics of the non-aqueous electrolyte secondary battery may deteriorate.

[0009] As a result of research by the present inventors, it was found that by providing a coating layer containing a sulfonic acid compound and a hydrophobic polymer compound on the surface of silicon-containing particles, side reactions between the silicon-containing particles and the non-aqueous electrolyte are suppressed, thereby improving the cycle characteristics of non-aqueous electrolyte secondary batteries. However, simply providing a coating layer on the surface of the silicon-containing particles may deteriorate the wettability of the silicon-containing material and reduce the diffusibility of the non-aqueous electrolyte in the negative electrode mixture layer. As a result, there is a risk of a decrease in battery capacity. Therefore, it is not easy to improve the charge / discharge cycle characteristics while improving the initial battery capacity.

[0010] The inventors of the present invention have further studied and succeeded in realizing a non-aqueous electrolyte secondary battery with improved initial battery capacity and charge-discharge cycle characteristics by using a silicon-containing material in which a coating layer is provided on the surface of silicon-containing particles as the negative electrode active material, and by forming a two-layer negative electrode mixture layer in which the porosity of the first negative electrode mixture layer on the surface side is greater than the porosity of the second negative electrode mixture layer on the core side. This is thought to be because, in a negative electrode using a silicon-containing material with a coating layer, the porosity in the first negative electrode mixture layer is increased and the diffusibility of the non-aqueous electrolyte is improved, thereby achieving a uniform distribution of the non-aqueous electrolyte within the negative electrode mixture layer.

[0011] 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 configurations obtained by selectively combining the components of the multiple embodiments and variations described below are included within the scope of the present disclosure. Note that in this specification, "to" refers to a range that includes the upper and lower limits before and after "to."

[0012] 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 as a nonaqueous electrolyte secondary battery, but the battery outer can is not limited to a cylindrical outer can. The nonaqueous electrolyte secondary battery according to the present disclosure may be, for example, a prismatic battery equipped with a prismatic outer can, a coin battery equipped with a coin-shaped outer can, or a pouch-type battery equipped with an outer can made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode assembly is not limited to a wound type, and may be a stacked type electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0013] FIG. 1 is a cross-sectional view 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 with a bottom and an open end in the axial direction, 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 will be referred to as the "top" and the bottom side of the outer can 16 will be referred to as the "bottom."

[0014] 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

[0015] 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 length and width directions. The separator 13 is formed to be at least slightly larger than the positive electrode 11, and, for example, 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.

[0016] 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.

[0017] A gasket 28 is provided between the exterior can 16 and the sealing body 17 to ensure airtightness inside the battery. The exterior can 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 can 16, and supports the sealing body 17 on its top surface. The sealing body 17 is fixed to the top of the exterior can 16 by the grooved portion 22 and the open end of the exterior can 16 that is crimped to the sealing body 17.

[0018] 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 the internal pressure of the battery increases due to abnormal heat generation, 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.

[0019] The positive electrode 11, negative electrode 12, and separator 13 that constitute the electrode assembly 14 are described in detail below. [Positive Electrode] The positive electrode 11 has a positive electrode core and a positive electrode mixture layer disposed on the positive electrode core. The positive electrode core can be made of 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 core, drying the coating, and then compressing it to form positive electrode mixture layers on both sides of the positive electrode core.

[0020] The positive electrode mixture layer contains particulate lithium metal composite oxide as a positive electrode active material. The lithium metal 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 metal 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 metal composite oxides containing Ni, Co, and Mn, and lithium metal composite oxides containing Ni, Co, and Al.

[0021] The lithium-containing composite oxide is, for example, a secondary particle formed by the aggregation of multiple primary particles. The volume-based median diameter (D50) of the composite oxide is not particularly limited, but is, for example, 3 μm to 30 μm, preferably 5 μm to 25 μm. 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.

[0022] 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.

[0023] 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.

[0024] [Negative electrode] Fig. 2 is a cross-sectional view of the negative electrode 12 in one example of the embodiment. As shown in Fig. 2, the negative electrode 12 includes a negative electrode core 30, a first negative electrode mixture layer 31 disposed on the surface of the negative electrode 12, and a second negative electrode mixture layer 32 disposed between the first negative electrode mixture layer 31 and the negative electrode core 30 and having a smaller porosity than the first negative electrode mixture layer 31. Hereinafter, the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 may be collectively referred to as the negative electrode mixture layer.

[0025] The negative electrode core 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 core 30 is not particularly limited, but is preferably 1 to 50 μm, more preferably 5 to 20 μm, from the viewpoint of balancing the strength and weight of the negative electrode. The first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 are preferably provided on both sides of the negative electrode core 30 except for the portion to which the negative electrode lead 21 is connected. The configuration of the negative electrode 12 is not limited to the example shown in FIG. 2 ; for example, a functional layer or the like may be disposed between the second negative electrode mixture layer 32 and the negative electrode core 30.

[0026] As shown in FIG. 2 , if the thickness of the first negative electrode mixture layer is T1 and the thickness of the second negative electrode mixture layer is T2, T1 / (T1+T2) is preferably 0.1 or more, more preferably 0.15 or more, and even more preferably 0.2 or more. In this case, the diffusibility of the nonaqueous electrolyte in the first negative electrode mixture layer 31 is further improved, making it easier to achieve a uniform distribution of the nonaqueous electrolyte in the negative electrode mixture layer. Furthermore, T1 / (T1+T2) is preferably 0.5 or less, more preferably 0.45 or less, and even more preferably 0.4 or less. In this case, it is easier to increase the capacity of the battery. Therefore, an example of a suitable range for T1 / (T1+T2) is 0.1≦T1 / (T1+T2)≦0.5, more preferably 0.15≦T1 / (T1+T2)≦0.45, and even more preferably 0.2≦T1 / (T1+T2)≦0.4. The thickness of each of the first negative electrode mixture layer 31 and the second negative electrode mixture layer 31 is, for example, not less than 10 μm and not more than 300 μm.

[0027] When the porosity of the first negative electrode mixture layer 31 is ε1 and the porosity of the second negative electrode mixture layer 32 is ε2, the relationship ε1 > ε2 is satisfied. When ε1 > ε2 is satisfied, the nonaqueous electrolyte is more easily diffused within the first negative electrode mixture layer, and a uniform distribution of the nonaqueous electrolyte within the negative electrode mixture layer is achieved. As a result, even when a coating layer is provided on the surface of the silicon-containing material, the resistance of the negative electrode is reduced and the battery capacity can be improved.

[0028] ε1 and ε2 preferably satisfy 1 < ε1 / ε2 ≦ 5, and more preferably 1.2 ≦ ε1 / ε2 ≦ 3. In this case, the effects of the present disclosure can be more significantly exhibited. ε1 is preferably 10% or more, and more preferably 20% or more. ε2 is preferably 5% or more, and more preferably 15% or more. The upper limits of ε1 and ε2 are, for example, 50%. The porosity of the negative electrode mixture layer is a two-dimensional value calculated from the ratio of the area of ​​voids to the cross-sectional area of ​​the negative electrode mixture layer in the cross section of the negative electrode mixture layer. More specifically, ε1 and ε2 can be calculated by the following procedure.

[0029] <Method for measuring porosity (ε1, ε2)> (1) Exposing the cross section of the negative electrode mixture layer. For example, a method for exposing the cross section includes cutting out a portion of the negative electrode and processing it with an ion milling device (e.g., IM4000PLUS manufactured by Hitachi High-Technologies Corporation) to expose the cross section of the negative electrode mixture layer. (2) Using a scanning electron microscope, backscattered electron images of the cross sections of the exposed negative electrode mixture layer are taken for each of the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32. The backscattered electron images are taken at a magnification of, for example, 800 times. The following steps (3) and (4) are performed on the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32, and the porosity of each is calculated. (3) The cross-sectional image obtained above is imported into a computer and binarized using image analysis software (e.g., ImageJ, manufactured by the National Institutes of Health, USA). This converts particle cross sections in the cross-sectional image into black and voids present in the particle cross sections into white, resulting in a binarized image. (4) Of the voids converted into white in the binarized image obtained above, voids inside the particles (pores not connected to the particle surface) and pores connected to the particle surface with a width of 3 μm or less are excluded from the voids, and the area of ​​the voids is calculated. The porosity is calculated based on the following formula: Porosity (%) = Area of ​​voids / Area of ​​cross section of negative electrode mixture layer × 100. (5) For each of the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32, the porosity calculations according to (3) and (4) above were performed three times, and the average values ​​were used as the porosity of the first negative electrode mixture layer 31 and the porosity of the second negative electrode mixture layer 32.

[0030] Furthermore, assuming that the tortuosity of the first negative electrode mixture layer 31 is τ1 and the tortuosity of the second negative electrode mixture layer 32 is τ2, it is preferable that τ1 < τ2 be satisfied. Here, the tortuosity of the mixture layer is an index indicating the degree of curvature of the voids (pores) formed in the mixture layer through which the non-aqueous electrolyte passes, and the smaller the tortuosity, the less the curvature of the void path. As described below, the tortuosity is the value obtained by dividing the path length from the start point to the end point of the void in the mixture layer by the linear distance from the start point to the end point of the void in the mixture layer. In other words, when the path length is the same as the linear distance from the start point to the end point of the void in the mixture layer, the tortuosity is 1. Therefore, when τ1 < τ2 is satisfied, the diffusibility of the non-aqueous electrolyte in the first negative electrode mixture layer 31 is improved, and a uniform distribution of the non-aqueous electrolyte in the negative electrode mixture layer is realized. As a result, even when a coating layer is provided on the surface of the silicon-containing material, the resistance of the negative electrode is reduced, and the battery capacity can be improved.

[0031] Preferably, τ1 and τ2 satisfy 0.3≦τ1 / τ2<1, and more preferably 0.4≦τ1 / τ2≦0.8. This allows the effects of the present disclosure to be more pronounced. τ1 is, for example, 1.1 or more and 3.0 or less, and τ2 is, for example, 1.3 or more and 4.0 or less.

[0032] In this specification, the tortuosity of the negative electrode mixture layer is calculated by the following formula: τ is the tortuosity, f is the path length of the medial axis passing through the opposing surfaces in the thickness direction (abbreviated as path length), and s is the length of the straight line connecting the start point and end point of the path of f (abbreviated as straight line distance between start and end points). The sample used to evaluate the tortuosity is evaluated in a fully discharged state. τ = f / s The tortuosity of the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 can be calculated as follows: τ1=f1 / s1 τ2=f2 / s2 (τ1: tortuosity of the first negative electrode mixture layer 31, f1: path length of the first negative electrode mixture layer 31, s1: linear distance between the start and end points of the first negative electrode mixture layer 31) (τ2: tortuosity of the second negative electrode mixture layer 32, f2: path length of the second negative electrode mixture layer 32, s2: linear distance between the start and end points of the second negative electrode mixture layer 32)

[0033] The path length and linear distance are determined by cross-sectional observation and image analysis of the negative electrode mixture layer using a 3D scanning electron microscope (3DSEM, for example, Ethos NX-5000 manufactured by Hitachi High-Tech Corporation). Specific methods for calculating the tortuosity are as follows. (1) Construction of a three-dimensional structure using 3DSEM: The composite is placed on the sample stage of the 3DSEM, and continuous cross-sectional slicing and cross-sectional observation are performed alternately. Observation is performed at an acceleration voltage of 5 kV. The obtained two-dimensional continuous images are binarized using three-dimensional image analysis software (for example, EXFACT VR manufactured by Japan Visual Science Co., Ltd.), and the images are joined together to construct a three-dimensional structure. The three-dimensional structure is preferably 100 μm × 100 μm × 100 μm or more. (2) Determining the Boundary Between the First Negative Electrode Mixture Layer 31 and the Second Negative Electrode Mixture Layer 32 From the three-dimensional structure image obtained in (1), the boundary between the regions where the voids differ in the thickness direction is determined as the boundary line between the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32. (3) Determining f1, f2, s1, and s2 in the First Negative Electrode Mixture Layer 31 and the Second Negative Electrode Mixture Layer 32 The three-dimensional structure image obtained in (1) is divided into the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 at the ratio obtained in (2), and the extracted voids are thinned by binarization to determine the axis passing through the center of the void (medial axis). Medial axes present in the cube that penetrate in a direction perpendicular to the core surface are extracted, and for paths with branches within a single path, the shortest path is determined as the path length (f1 and f2) of the composite. (4) Calculation of tortuosity ratio of the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 Using the average path lengths (f1 and f2) of the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 obtained in (3) and the average linear distances (s1 and s2) connecting the paths, the tortuosity ratio of the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 is calculated from the above formula, and then the ratio thereof, τ1 / τ2, is calculated.

[0034] The porosity and tortuosity of the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 can be controlled by changing the linear pressure in the rolling process of the mixture layer. More specifically, after forming the second negative electrode mixture layer 32 on the surface of the negative electrode core 30 and performing the first rolling process, when forming the first negative electrode mixture layer 31 on the second negative electrode mixture layer 32 and performing the second rolling process, the linear pressure in the second rolling process is set to be smaller than the compressive force in the first rolling process. This allows the porosity (ε1) of the first negative electrode mixture layer 31 to be larger than the porosity (ε2) of the second negative electrode mixture layer, and the tortuosity (τ1) of the first negative electrode mixture layer 31 to be smaller than the tortuosity (τ2) of the second negative electrode mixture layer. The porosity and tortuosity of the first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 can also be controlled by using carbon materials or silicon-containing materials with different hardnesses. By using a carbon material or silicon-containing material with high hardness, the porosity of the negative electrode mixture layer can be increased and the tortuosity can be reduced.

[0035] The first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 contain a carbon material and a silicon-containing material as negative electrode active materials. The inclusion of a silicon-containing material can achieve high capacity. However, because silicon-containing materials experience larger volume changes during charge and discharge than carbon materials, when the negative electrode mixture layer contains a silicon-containing material, non-uniform distribution of the non-aqueous electrolyte is likely to occur within the negative electrode mixture layer. However, as described above, by making the porosity (ε1) of the first negative electrode mixture layer 31 greater than the porosity (ε2) of the second negative electrode mixture layer 32, the non-aqueous electrolyte distribution within the negative electrode mixture layer is made uniform, resulting in a non-aqueous electrolyte secondary battery with high capacity and excellent cycle characteristics.

[0036] The carbon material is, for example, at least one selected from the group consisting of natural graphite, artificial graphite, soft carbon, and hard carbon. Among them, it is preferable to use at least artificial graphite such as massive artificial graphite (MAG) and graphitized mesophase carbon microbeads (MCMB), natural graphite such as flake graphite, massive graphite, and amorphous graphite, or a mixture thereof. The volume-based D50 of the carbon material is, for example, 1 μm or more and 30 μm or less, preferably 5 μm or more and 25 μm or less.

[0037] As shown in FIG. 3 , the silicon-containing material 40 includes silicon-containing particles 41 and a coating layer 42 that covers at least a portion of the surface of the silicon-containing particles 41. The coating layer 42 contains a sulfonic acid compound and a hydrophobic polymer compound. In other words, the coating layer 42 is a mixed layer of the sulfonic acid compound and the hydrophobic polymer compound. By covering the surfaces of the silicon-containing particles 41 with a coating layer containing a sulfonic acid compound, the silicon-containing particles 41 are protected from the non-aqueous electrolyte. As a result, side reactions between the silicon-containing particles 41 and the non-aqueous electrolyte are suppressed, and erosion of the silicon-containing particles 41 associated with the side reactions is suppressed. Furthermore, deterioration of the silicon-containing particles 41 due to the erosion is suppressed, thereby suppressing deterioration in the cycle characteristics of the non-aqueous electrolyte secondary battery.

[0038] By mixing the sulfonic acid compound with the hydrophobic polymer compound in coating layer 42, the retention of the sulfonic acid compound on the surfaces of silicon-containing particles 41 is improved, and the surfaces of silicon-containing particles 41 can be more effectively coated with the sulfonic acid compound, thereby further suppressing side reactions between silicon-containing particles 41 and the non-aqueous electrolyte.

[0039] 3 , the silicon-containing particle 41 preferably includes an ion-conducting phase 43 and a silicon phase 44 dispersed in the ion-conducting phase 43. The ion-conducting phase 43 is a continuous phase composed of an aggregate of particles finer than the silicon phase 44. The ion-conducting phase 43 is, for example, at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. The silicon phase 44 is formed by Si being formed in the form of fine particles and dispersed in the ion-conducting phase 43.

[0040] Furthermore, a conductive layer (not shown) containing a conductive carbon material may be interposed between the silicon-containing particles 41 and the coating layer 42. By interposing the conductive layer, a good conductive path can be formed in the negative electrode mixture layer, and the battery capacity can be further improved.

[0041] Here, assuming that the content of the silicon-containing material 40 in the first negative electrode mixture layer 31 (see FIG. 2 ) is θ1 and the content of the silicon-containing material 40 in the second negative electrode mixture layer 32 (see FIG. 2 ) is θ2, it is preferable that θ1 > θ2. In this case, the charge / discharge reaction in the first negative electrode mixture layer 31 is further promoted, thereby realizing a nonaqueous electrolyte secondary battery with high capacity and excellent cycle characteristics. Furthermore, θ1 and θ2 preferably satisfy 1.1 ≦ θ1 / θ2 ≦ 3, and more preferably 1.2 ≦ θ1 / θ2 ≦ 2. θ1 is, for example, 3 mass% or more and 50 mass% or less of the total mass of the negative electrode active material in the first negative electrode mixture layer 31, and θ2 is, for example, 3 mass% or more and 20 mass% or less of the total mass of the negative electrode active material in the second negative electrode mixture layer 32.

[0042] The first negative electrode mixture layer 31 and the second negative electrode mixture layer 32 may contain a conductive agent and a binder in addition to the negative electrode active material.

[0043] The conductive agent may include particulate carbon such as carbon black, acetylene black, ketjen black, or graphite, but preferably includes fibrous carbon. Examples of fibrous carbon include carbon nanotubes (CNTs) and carbon nanofibers. The CNTs may be single-walled CNTs, double-walled CNTs, multi-walled CNTs, or mixtures thereof. The CNTs may also be vapor-grown carbon fibers known as VGCF (registered trademark). The fibrous carbon has a diameter of 2 nm to 20 μm and a total length of 0.03 μm to 500 μm, for example. Using fibrous carbon as the conductive agent further improves the conductivity of the negative electrode mixture layer and further increases battery capacity.

[0044] Here, assuming that the content of fibrous carbon in the first negative electrode mixture layer 31 is φ1 and the content of fibrous carbon in the second negative electrode mixture layer 32 is φ2, it is preferable to satisfy 1≦φ2 / φ1≦8. In this case, the charge / discharge reaction in the first negative electrode mixture layer 31 is further promoted, and a nonaqueous electrolyte secondary battery with high capacity and excellent cycle characteristics can be realized. It is more preferable to satisfy 1<φ2 / φ1≦8, even more preferable to satisfy 3≦φ2 / φ1≦8, and particularly preferable to satisfy 3≦φ2 / φ1≦5. In this case, a nonaqueous electrolyte secondary battery with even higher capacity and excellent cycle characteristics can be realized.

[0045] Furthermore, in the anode mixture layer where the content (θ1) of the silicon-containing material 40 in the first anode mixture layer 31 and the content (θ2) of the silicon-containing material 40 in the second anode mixture layer 32 satisfy θ1 > θ2, it is more preferable that 1≦φ2 / φ1≦8 be satisfied. In this case, the charge / discharge reaction in the first anode mixture layer 31 is further promoted, resulting in a nonaqueous electrolyte secondary battery with high capacity and excellent cycle characteristics. φ1 is, for example, 0.01% to 5% by mass, preferably 0.01% to 3% by mass, relative to the total mass of the first anode mixture layer 31. φ2 is, for example, 0.005% to 5% by mass, preferably 0.005% to 1% by mass, relative to the total mass of the second anode mixture layer 32.

[0046] Examples of binders include fluorine-containing resins such as styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), polytetrafluoroethylene (PTFE), and polyvinylidene fluoride (PVDF), as well as polyacrylonitrile (PAN), polyimide, acrylic resin, and polyolefin. Among these, SBR and NBR are preferred, with SBR being particularly preferred. These may be used alone or in combination of two or more. The binder contained in the first negative electrode mixture layer 31 and the binder contained in the second negative electrode mixture layer 32 may be different from each other, but are preferably the same.

[0047] The negative electrode mixture layer may further contain a thickener. Examples of thickeners include carboxymethyl cellulose (CMC) or its salts (CMC-Na, etc.), polyacrylic acid (PAA) or its salts (PAA-Na, PAA-K, etc., or a partially neutralized salt), and polyvinyl alcohol (PVA). These may be used alone or in combination of two or more. The thickener contained in the first negative electrode mixture layer 31 and the thickener contained in the second negative electrode mixture layer 32 may be different from each other, but are preferably the same.

[0048] The silicon-containing material contained in the negative electrode active material that constitutes the negative electrode will be described in detail below.

[0049] As described above, the silicon-containing material has silicon-containing particles and a coating layer covering at least a portion of the surface of the silicon-containing particles. The coating layer includes a sulfonic acid compound and a hydrophobic polymer compound.

[0050] [Silicon-Containing Particles] The silicon-containing particles are preferably composite particles containing an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase. This allows the ion-conducting phase to mitigate stress associated with the expansion and contraction of the silicon phase during charging and discharging, thereby suppressing cracking and fracture of the silicon-containing particles. The ion-conducting phase is a continuous phase composed of a collection of particles finer than the silicon phase. The ion-conducting phase is, for example, at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. The ion-conducting phase may be composed of one phase or multiple phases.

[0051] The silicate phase is composed of a compound containing a metal element, Si, and O. The silicide phase is a compound phase consisting of Si and an element more electropositive than Si, such as NiSi, Mg ２ Si, TiSi ２ The silicon oxide phase is composed of a compound of Si and O. The main component of the silicon oxide phase (for example, 95 to 100 mass%) is SiO ２ may be.

[0052] The silicon phase is formed of fine particulate Si and is dispersed within the ion-conducting phase. The average particle size of the silicon phase is, for example, 500 nm or less, preferably 250 nm or less, and more preferably 150 nm or less before charge / discharge. The average particle size of the silicon phase is, for example, preferably 200 nm or less, and more preferably 100 nm or less after charge / discharge. By miniaturizing the silicon phase, the volume change of the silicon-containing material during charge / discharge is reduced, making it easier to suppress the collapse of the electrode structure. The average particle size of the silicon phase is measured by observing the cross section of the silicon-containing particle using a SEM or TEM, and specifically, is determined by averaging the longest diameter of 100 silicon phases.

[0053] The silicon phase is, for example, a particulate phase of simple silicon, and is composed of a single crystallite or multiple crystallites. The crystallite size of the silicon phase is, for example, 5 nm or more and 50 nm or less. When the crystallite size of the silicon phase is 50 nm or less, the volume change due to expansion and contraction of the silicon phase during charge and discharge is small, and cycle characteristics can be further improved. The crystallite size of the silicon phase is calculated using the Scherrer equation from the half-width of the diffraction peak assigned to the Si(111) plane in the X-ray diffraction (XRD) pattern.

[0054] The content of the silicon phase is preferably 20% by mass or more, more preferably 35% by mass or more, relative to the mass of the silicon-containing particles. In this case, it is easier to achieve high capacity. Furthermore, the content of the silicon phase is preferably 95% by mass or less, more preferably 75% by mass or less, relative to the mass of the silicon-containing particles. In this case, the silicon phase can be covered with the ion-conducting phase, which suppresses side reactions between the silicon phase and the non-electrolyte and improves cycle characteristics. Therefore, an example of a suitable range for the content of the silicon phase is 20% by mass or more and 95% by mass or less, more preferably 35% by mass or more and 75% by mass or less, relative to the mass of the silicon-containing particles.

[0055] The volumetric D50 of the silicon-containing particles is generally smaller than the volumetric D50 of the carbon material. The volumetric D50 of the silicon-containing particles is, for example, 1 μm or more and 25 μm or less, and may be 4 μm or more and 15 μm or less. When the volumetric D50 of the silicon-containing particles is within the above range, good battery performance is likely to be obtained.

[0056] An example of the silicon-containing particles is a composite particle having a sea-island structure in which fine Si particles are substantially uniformly dispersed in a carbon phase. Here, the carbon phase is preferably an amorphous carbon phase. The carbon phase may contain a crystalline phase component, but preferably contains a larger amount of amorphous phase component. The amorphous carbon phase is composed of a carbon material having an average interplanar spacing of (002) planes of more than 0.34 nm as measured by X-ray diffraction.

[0057] Another example of the silicon-containing particles is a composite particle containing a silicate phase and a silicon phase dispersed within the silicate phase. The silicate phase may contain at least one element selected from the group consisting of Groups 1 and 2 of the periodic table. The silicate phase may also contain at least one element selected from the group consisting of B, Al, Zr, Nb, Ta, V, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, W, and lanthanides.

[0058] In the silicon-containing particles containing a silicate phase and a silicon phase dispersed in the silicate phase, a preferred silicate phase is a lithium silicate phase containing Li. In this case, lithium ions can easily enter and exit the silicate phase. The lithium silicate phase can be, for example, a compound represented by the general formula Li ２ｚ SiO （２＋ｚ） (0<z<2). The lithium silicate phase contains Li ４ SiO ４ It is preferable that (Z=2) is not included. ４ SiO ４ is an unstable compound and reacts with water to become alkaline, which may cause Si to change and lead to a decrease in charge / discharge capacity. The lithium silicate phase is considered to be a suitable phase for Li, from the viewpoints of stability, productivity, Li ion conductivity, etc. ２ SiO ３(Z=1) or Li ２ Si ２ O ５ It is preferable that (Z=1 / 2) is used as the main component.

[0059] Silicon-containing particles containing a silicate phase and a silicon phase dispersed within the silicate phase are produced, for example, through the following steps (1) to (3). (1) Si and lithium silicate are mixed in a mass ratio of, for example, 20:80 to 95:5 to produce a mixture. (2) Next, the mixture is pulverized and finely divided using a ball mill. It is also possible to produce a mixture after finely dividing each raw material powder. (3) The pulverized mixture is heat-treated, for example, at 600 to 1000°C in an inert atmosphere. In this heat treatment, a sintered body of the mixture may be produced by applying pressure, such as with a hot press. In this case, the sintered body is pulverized to a predetermined particle size. Li ２ｚ SiO （２＋ｚ） Lithium silicate represented by (0<z<2) is stable within the above temperature range and does not react with silicon, so the capacity does not decrease. It is also possible to prepare silicon-containing particles by synthesizing silicon nanoparticles and lithium silicate nanoparticles without using a ball mill, mixing them, and then subjecting them to heat treatment.

[0060] [Coating Layer] As described above, the coating layer contains a sulfonic acid compound and a hydrophobic polymer compound. The thickness of the coating layer is preferably 1 nm or more, more preferably 5 nm or more. In this case, side reactions between the silicon-containing particles and the non-aqueous electrolyte are effectively suppressed. Furthermore, the thickness of the coating layer is preferably 300 nm or less, more preferably 200 nm or less. In this case, a decrease in the conductivity of the silicon-containing material can be suppressed. Therefore, an example of a suitable range for the thickness of the coating layer is 1 nm or more and 300 nm or less, more preferably 3 nm or more and 200 nm or less. The thickness of the coating layer is measured by observing a cross section of the silicon-containing material using an SEM or TEM.

[0061] The sulfonic acid compound contained in the coating layer is preferably a compound represented by the following formula (I): In the formula, A is a Group 1 or Group 2 element, R is a hydrocarbon group, and n is 1 or 2. A is preferably a Group 1 element. Among these, Li or Na is more preferred, and Li is particularly preferred.

[0062] In formula (I), R is preferably an alkyl group. The number of carbon atoms in the alkyl group is preferably 5 or less, more preferably 3 or less. From the viewpoint of reducing the reaction resistance at the negative electrode, an example of a suitable R is an alkyl group having 3 or less carbon atoms, and among these, a methyl group is preferred. In addition, in R, some of the hydrogen atoms bonded to the carbon may be substituted with fluorine. Furthermore, n in formula (I) is preferably 1.

[0063] Specific examples of the sulfonic acid compound include lithium methanesulfonate, lithium ethanesulfonate, lithium propanesulfonate, sodium methanesulfonate, sodium ethanesulfonate, magnesium methanesulfonate, lithium fluoromethanesulfonate, etc. Among these, at least one selected from the group consisting of lithium methanesulfonate, lithium ethanesulfonate, and sodium methanesulfonate is preferred, with lithium methanesulfonate being particularly preferred.

[0064] The amount of sulfonic acid compound covering the surface of the silicon-containing particles is preferably 0.1% by mass or more, more preferably 1% by mass or more, relative to the total mass of the silicon-containing particles. If the amount of sulfonic acid compound is less than 0.1% by mass, the surface of the silicon-containing particles cannot be sufficiently coated with the sulfonic acid compound, and the effect of suppressing side reactions by the sulfonic acid compound may not be obtained. Furthermore, the amount of sulfonic acid compound covering the surface of the silicon-containing particles is preferably 10% by mass or less, more preferably 6% by mass or less, relative to the total mass of the silicon-containing particles. If the amount of sulfonic acid compound exceeds 10% by mass, the coating layer may become too thick, reducing the conductivity of the silicon-containing material and decreasing the battery capacity. Therefore, an example of a suitable range of the amount of sulfonic acid compound covering the surface of the silicon-containing particles is preferably 0.1% by mass or more and 10% by mass or less, more preferably 1% by mass or more and 6% by mass or less, relative to the total mass of the silicon-containing particles.

[0065] The presence of the sulfonic acid compound covering the surface of the silicon-containing particles can be confirmed by Fourier transform infrared spectroscopy (FT-IR). In the infrared absorption spectrum obtained by FT-IR, the silicon-containing material containing lithium methanesulfonate exhibits, for example, a peak at 1238 cm －１ , 1175 cm －１ , 1065 cm －１ , 785 cm －１ It has an absorption peak around 1238 cm －１ , 1175 cm －１ , 1065 cm －１ The peak around 785 cm is due to the SO stretching vibration of lithium methanesulfonate. －１ The peak around this region is a peak due to the C-S stretching vibration derived from lithium methanesulfonate. The presence of silicon-containing materials containing sulfonic acid compounds other than lithium methanesulfonate can also be confirmed from the absorption peaks derived from sulfonic acid compounds in the infrared absorption spectrum.

[0066] The presence of the sulfonic acid compound covering the surface of the silicon-containing particles can also be confirmed by X-ray photoelectron spectroscopy (XPS). In the spectrum obtained by XPS, a peak with a binding energy of approximately 165 to 170 eV and an intensity (c / s) of 200 to 1000 is observed for the silicon-containing material containing lithium methanesulfonate. The presence of the sulfonic acid compound covering the surface of the silicon-containing particles can also be confirmed by ICP, atomic absorption spectroscopy, synchrotron XRD measurement, TOF-SIMS, etc.

[0067] The hydrophobic polymer compound preferably has good binding properties and heat melting properties. In this case, the hydrophobic polymer compound can firmly hold the sulfonic acid compound on the surface of the silicon-containing particles. As a result, side reactions between the silicon-containing particles and the non-aqueous electrolyte are further suppressed, thereby suppressing the erosion of the silicon-containing particles.

[0068] From the viewpoint of stability against non-aqueous electrolytes, the hydrophobic polymer compound preferably contains a fluororesin. Examples of fluororesins include polyvinylidene fluoride (PVDF), polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-ethylene copolymer, chlorotrifluoroethylene-ethylene copolymer, and polychlorotrifluoroethylene. Among these, PVDF is preferred. PVDF has good binding properties and a low melting point, allowing a coating layer to be formed at a low heat treatment temperature.

[0069] The hydrophobic polymer compound may include a polymer containing a vinylidene fluoride unit in addition to polyvinylidene fluoride. Examples of polymers containing vinylidene fluoride units include copolymers of vinylidene fluoride with other monomers. Examples of other monomers include hexafluoropropylene (HFP) and tetrafluoroethylene (TFE). Polymers containing vinylidene fluoride units include polyvinylidene fluoride and its modified products, vinylidene fluoride-hexafluoropropylene copolymers, vinylidene fluoride-chlorotrifluoroethylene copolymers, etc.

[0070] The amount of hydrophobic polymer compound covering the surface of the silicon-containing particles is preferably 0.1% by mass or more, more preferably 1% by mass or more, relative to the total mass of the silicon-containing particles. If the amount of hydrophobic polymer compound is less than 0.1% by mass, the hydrophobic polymer compound may not be able to sufficiently cover the surface of the silicon-containing particles, and the deterioration of the charge-discharge cycle characteristics may not be effectively suppressed. Furthermore, the amount of hydrophobic polymer compound is preferably 10% by mass or less, more preferably 6% by mass or less, relative to the total mass of the silicon-containing particles. If the amount of sulfonic acid compound exceeds 10% by mass, the coating layer may become too thick, reducing the conductivity of the silicon-containing material and decreasing the battery capacity. Therefore, an example of a suitable range of the amount of hydrophobic polymer compound covering the surface of the silicon-containing particles is preferably 0.1% by mass or more and 10% by mass or less, more preferably 1% by mass or more and 6% by mass or less, relative to the total mass of the silicon-containing particles.

[0071] When the coating layer contains a sulfonic acid compound and a fluororesin (e.g., PVDF), the amounts of the sulfonic acid compound and the fluororesin can be determined by washing the silicon-containing material with N-methyl-2-pyrrolidone (NMP), dissolving the fluororesin, and determining the mass of the fluororesin from the difference in mass of NMP before and after dissolution. Alternatively, the remainder that is not dissolved in NMP can be washed with water to dissolve the sulfonic acid compound, and the mass of the sulfonic acid compound dissolved in water can be determined by ICP.

[0072] A method for forming the coating layer includes dry-mixing silicon-containing particles with powders of a sulfonic acid compound and a hydrophobic polymer compound to prepare a mixture, and then heat-treating the mixture at a temperature above the melting point of the hydrophobic polymer compound (e.g., 150°C or higher and 340°C or lower). The heat treatment time is, for example, 1 hour or longer and 3 hours or shorter. Furthermore, the particle sizes of the sulfonic acid compound and hydrophobic polymer compound mixed with the silicon-containing particles are preferably smaller than the particle sizes of the silicon-containing particles. In this case, it becomes easier to uniformly coat the surfaces of the silicon-containing particles with the sulfonic acid compound and hydrophobic polymer compound, thereby significantly enhancing the effects of the present disclosure. The particle sizes of the sulfonic acid compound and hydrophobic polymer compound are, for example, 1 μm or longer and 100 μm or shorter.

[0073] [Conductive Layer] To improve conductivity, a conductive layer containing a conductive carbon material may be interposed between the silicon-containing particles and the coating layer. In other words, the conductive layer covers at least a portion of the surface of the ion-conductive phase of the silicon-containing particles. The conductive layer is made of a material with higher conductivity than the ion-conductive phase and forms a good conductive path in the negative electrode mixture layer. Examples of carbon materials that can be used to form the conductive layer include carbon black, acetylene black, ketjen black, graphite, and mixtures of two or more of these. The conductive layer may also contain a metal or metal compound. Examples of such metals include copper, nickel, and alloys thereof, which are stable within the potential range of the negative electrode. Examples of such metal compounds include copper compounds and nickel compounds.

[0074] Examples of methods for coating the surfaces of silicon-containing particles with carbon include CVD using acetylene, methane, etc., and methods in which coal pitch, petroleum pitch, phenolic resin, etc. is mixed with silicon-containing particles and then heat-treated.

[0075] The conductive layer is preferably provided so as to cover substantially the entire surface of the silicon-containing particles. The thickness of the conductive layer is preferably 1 nm or more and 200 nm or less, and more preferably 5 nm or more and 100 nm or less. If the conductive layer is too thin, it becomes difficult to uniformly coat the silicon-containing particles, and the conductivity of the silicon-containing material decreases. Furthermore, if the conductive layer is too thick, the diffusion of lithium ions into the silicon-containing particles is hindered, and the battery capacity tends to decrease. The thickness of the conductive layer can be measured by observing the cross section of the particles using a SEM or TEM, etc.

[0076] [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 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.

[0077] 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.

[0078] The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0079] Example 1 [Fabrication of Positive Electrode] [Ni ０．８８ Co ０．０９ Al ０．０３ ](OH) ２ The composite hydroxide represented by the formula (I) was calcined at 500°C for 8 hours to obtain an oxide (Ni ０．８８ Co ０．０９ Al ０．０３ O ２Next, LiOH and the composite oxide were mixed so that the molar ratio of Li to the total amount of Ni, Co, and Al was 1.03:1 to obtain a mixture. This mixture was subjected to an oxygen flow (10 cm) with an oxygen concentration of 95%. ３ The mixture was heated from room temperature to 650°C at a rate of 2.0°C / min, and then heated from 650°C to 780°C at a rate of 0.5°C / min to obtain LiNi. ０．８８ Co ０．０９ Al ０．０３ O ２ A lithium-containing composite oxide represented by the following formula was obtained.

[0080] The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of 95:2.5:2.5, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to a positive electrode core made of aluminum foil, the coating was dried and compressed, and then the positive electrode core was cut to a predetermined electrode size to obtain a positive electrode in which a positive electrode mixture layer was disposed on both sides of the positive electrode core. In addition, an exposed portion in which the surface of the positive electrode core was exposed was provided in a part of the positive electrode.

[0081] [Preparation of silicon-containing material] Coal pitch (MCP250, manufactured by JFE Chemical Corporation) as a carbon raw material and raw material silicon (3N, average particle size 10 μm) were mixed in a mass ratio of 50:50, and the mixture was pulverized and micronized using a planetary ball mill (P-5, manufactured by Fritsch). Next, the micronized powder mixture was fired in an inert atmosphere to carbonize the carbon source and obtain a sintered product in which a silicon phase was dispersed within an amorphous carbon phase. Thereafter, the sintered product was pulverized using a jet mill to obtain silicon-containing particles with an average particle size of 10 μm.

[0082] Next, powders of lithium methanesulfonate (MSL) and polyvinylidene fluoride (PVDF) were added to the silicon-containing particles and dry mixed to adhere MSL and PVDF to the surface of the silicon-containing particles, producing an intermediate. At this time, 4 parts by mass of MSL and 2 parts by mass of PVDF were mixed per 100 parts by mass of the silicon-containing particles. The resulting intermediate was then heat-treated in an inert atmosphere at 250 ° C for 2 hours to liquefy the PVDF adhering to the surface of the silicon-containing particles, producing a silicon-containing material having a coating layer on the surface of the silicon-containing particles.

[0083] [Preparation of Negative Electrode] Graphite and the silicon-containing material were mixed in a mass ratio of 90:10 to prepare a negative electrode active material. 100 parts by mass of the negative electrode active material, 1 part by mass of styrene-butadiene rubber (SBR), and 1 part by mass of carboxymethyl cellulose (CMC) were mixed, and an appropriate amount of water was added to prepare a negative electrode mixture slurry.

[0084] Next, a portion of the prepared negative electrode mixture layer slurry was applied to both sides of a negative electrode core made of copper foil, and the coating was dried and compressed to form a second negative electrode mixture layer. Furthermore, the remaining negative electrode mixture slurry was applied to the second negative electrode mixture layer, the coating was dried, and compressed at a linear pressure different from that used when forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio of the slurry used for the first negative electrode mixture layer to the slurry used for the second negative electrode mixture layer was 50:50. The prepared negative electrode had a two-layer structure including a lower layer (second negative electrode mixture layer) and an upper layer (first negative electrode mixture layer) on both sides of the negative electrode core, and the thickness of each negative electrode mixture layer was 100 μm. Furthermore, the value of T1 / (T1+T2) calculated from the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer was 0.5. An exposed portion where the surface of the negative electrode substrate was exposed was provided in a part of the negative electrode.

[0085] The porosity and tortuosity of the prepared negative electrode mixture layer were measured by the above-mentioned method, and the porosity (ε1) of the first negative electrode mixture layer was 30%, the porosity (ε2) of the second negative electrode mixture layer was 20%, and the porosity ratio (ε1 / ε2) was 1.5. The tortuosity (τ1) of the first negative electrode mixture layer was 2.0, the tortuosity (τ2) of the second negative electrode mixture layer was 3.0, and the tortuosity ratio (τ1 / τ2) was 0.7.

[0086] [Preparation of Non-Aqueous Electrolyte (Electrolyte Solution)] LiPF 6 was dissolved in a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 3:3:4 (25° C.). ６ was dissolved in the solution at a concentration of 1.2 mol / L to prepare a non-aqueous electrolyte.

[0087] [Preparation of Test Cell (Non-Aqueous Electrolyte Secondary Battery)] An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to the exposed portion of the negative electrode. The positive and negative electrodes were spirally wound with a polyolefin separator between them to prepare a wound electrode assembly. Insulating plates were placed on the top and bottom of the electrode assembly, and the electrode assembly was housed in an outer can. The negative electrode lead was welded to the bottom of a cylindrical outer can with a bottom, and the positive electrode lead was welded to a sealing member. An electrolyte was poured into the outer can, and the opening of the outer can was sealed with a sealing member via a gasket to prepare a non-aqueous electrolyte secondary battery as a test cell.

[0088] [Evaluation of Initial Discharge Capacity and Charge / Discharge Cycle Characteristics] The test cell was charged at a constant current of 0.3 C in a temperature environment of 25°C until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current value reached 0.02 C. Thereafter, the test cell was discharged at a constant current of 0.5 C until the battery voltage reached 2.5 V, and the discharge capacity at this time was taken as the initial discharge capacity. This charge / discharge cycle was counted as one cycle, and 100 cycles were repeated. The initial discharge capacity and the discharge capacity at the 100th cycle were determined, and the capacity retention rate was calculated using the following formula: Capacity retention rate R (%) = Discharge capacity at the 100th cycle / Initial discharge capacity × 100

[0089] The obtained capacity retention rate R was used to calculate the cycle deterioration rate according to the following formula. In the formula, R0 is the capacity retention rate of the battery of Comparative Example 1. A smaller cycle deterioration rate means better charge / discharge cycle characteristics. Cycle deterioration rate (%) = {(100 - R) / (100 - R0)} x 100

[0090] Example 2 A test cell was prepared and evaluated in the same manner as in Example 1, except that in preparing the silicon-containing material, 2 parts by mass of MSL and 2 parts by mass of PVDF were mixed with 100 parts by mass of silicon-containing particles.

[0091] Example 3 A test cell was produced and evaluated in the same manner as in Example 1, except that single-walled carbon nanotubes (CNTs) were mixed as a conductive agent in the production of the negative electrode. More specifically, in the production of the negative electrode, 100 parts by mass of the negative electrode active material, 0.02 parts by mass of CNTs, 1 part by mass of SBR, and 1 part by mass of CMC were mixed.

[0092] Example 4 A test cell was prepared and evaluated in the same manner as in Example 3, except that in preparing the silicon-containing material, 2 parts by mass of MSL and 2 parts by mass of PVDF were mixed with 100 parts by mass of silicon-containing particles.

[0093] Example 5 In the preparation of the negative electrode, the linear pressure during compression of the first negative electrode mixture layer and the second negative electrode mixture layer was changed, and the porosity (ε1) of the first negative electrode mixture layer was 36%, the porosity (ε2) of the second negative electrode mixture layer was 18%, the tortuosity (τ1) of the surface side of the first negative electrode mixture layer was 1.2, and the tortuosity (τ2) of the core side of the second negative electrode mixture layer was 4.0. Except for this, a test cell was prepared and evaluated in the same manner as in Example 4. That is, the porosity ratio (ε1 / ε2) of the negative electrode mixture layer in Example 5 was 2.0, and the tortuosity ratio (τ1 / τ2) was 0.3. The thickness of the entire negative electrode mixture layer was the same as in Example 4.

[0094] Example 6 In the preparation of the negative electrode, the linear pressure during compression of the first negative electrode mixture layer and the second negative electrode mixture layer was changed, and the porosity (ε1) of the first negative electrode mixture layer was 34%, the porosity (ε2) of the second negative electrode mixture layer was 20%, the tortuosity (τ1) of the surface side of the first negative electrode mixture layer was 1.5, and the tortuosity (τ2) of the core side of the second negative electrode mixture layer was 3.0. Except for this, a test cell was prepared and evaluated in the same manner as in Example 4. That is, the porosity ratio (ε1 / ε2) of the negative electrode mixture layer in Example 5 was 1.7, and the tortuosity ratio (τ1 / τ2) was 0.5. The thickness of the entire negative electrode mixture layer was the same as in Example 4.

[0095] Example 7 In the preparation of a negative electrode, graphite and a silicon-containing material were mixed in a mass ratio of 88:12 to form a first negative electrode active material. Also, graphite and a silicon-containing material were mixed in a mass ratio of 92:8 to form a second negative electrode active material. That is, 100 parts by mass of the first negative electrode active material, 0.02 parts by mass of CNT, 1 part by mass of SBR, and 1 part by mass of CMC were mixed, and an appropriate amount of water was added to prepare a first negative electrode mixture slurry. Also, 100 parts by mass of the second negative electrode active material, 0.02 parts by mass of CNT, 1 part by mass of SBR, and 1 part by mass of CMC were mixed, and an appropriate amount of water was added to prepare a second negative electrode mixture slurry.

[0096] Next, the second negative electrode mixture slurry was applied to both sides of a negative electrode core made of copper foil, and the coating was dried and compressed to form a second negative electrode mixture layer. Furthermore, the first negative electrode mixture slurry was applied to the second negative electrode mixture layer, the coating was dried, and the layer was compressed at a linear pressure different from that used when forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio per unit area of ​​the first negative electrode mixture slurry and the second negative electrode mixture slurry was 50:50. As a result, a negative electrode mixture layer was produced in which the silicon-containing material content (θ1) in the first negative electrode mixture layer was 12 mass%, the silicon-containing material content (θ2) in the second negative electrode mixture layer was 8 mass%, and θ1 / θ2 was 1.5. The thickness of the entire negative electrode mixture layer was the same as in Example 4. Except for this, a test cell was produced and evaluated in the same manner as in Example 4.

[0097] Example 8 In the preparation of the negative electrode, graphite and a silicon-containing material were mixed in a mass ratio of 85:15, and this was used as the first negative electrode active material. Furthermore, graphite and a silicon-containing material were mixed in a mass ratio of 95:5, and this was used as the second negative electrode active material. Except for this, a test cell was prepared and evaluated in the same manner as in Example 7. That is, in the negative electrode of Example 8, the content (θ1) of the silicon-containing material in the first negative electrode mixture layer was 15 mass%, the content (θ2) of the silicon-containing material in the second negative electrode mixture layer was 5 mass%, and θ1 / θ2 was 3.0.

[0098] Example 9 A test cell was prepared and evaluated in the same manner as in Example 4, except that the coating mass ratio of the slurry used for the first negative electrode mixture layer and the slurry used for the second negative electrode mixture layer was changed, the thickness (T1) of the first negative electrode mixture layer was 10 μm, and the thickness (T2) of the second negative electrode mixture layer was 90 μm. That is, the value of T1 / (T1+T2) calculated from T1 and T2 was 0.1. In addition, in the negative electrode mixture layer of Example 9, the porosity (ε1) of the first negative electrode mixture layer was 34%, the porosity (ε2) of the second negative electrode mixture layer on the substrate side was 26%, and the porosity ratio (ε1 / ε2) was 1.3. The tortuosity (τ1) of the first negative electrode mixture layer on the surface side was 2.0, the tortuosity (τ2) of the second negative electrode mixture layer on the core side was 3.0, and the tortuosity ratio (τ1 / τ2) was 0.7.

[0099] Example 10 In the preparation of the negative electrode, the coating mass ratio of the slurry used for the first negative electrode mixture layer to the slurry used for the second negative electrode mixture layer was changed, and the thickness (T1) of the first negative electrode mixture layer was set to 30 μm, and the thickness (T2) of the second negative electrode mixture layer was set to 70 μm. Except for this, a test cell was prepared and evaluated in the same manner as in Example 4. That is, the value of T1 / (T1+T2) calculated from T1 and T2 was 0.3. In addition, in the negative electrode mixture layer of Example 10, the porosity (ε1) of the first negative electrode mixture layer was 32%, the porosity (ε2) of the second negative electrode mixture layer on the core side was 23%, and the porosity ratio (ε1 / ε2) was 1.4. The tortuosity (τ1) of the first negative electrode mixture layer on the surface side was 3.0, the tortuosity (τ2) of the second negative electrode mixture layer on the core side was 3.0, and the tortuosity ratio (τ1 / τ2) was 0.7.

[0100] Example 11 In the preparation of the negative electrode, a test cell was prepared and evaluated in the same manner as in Example 4, except that the coating mass ratio of the slurry used for the first negative electrode mixture layer and the slurry used for the second negative electrode mixture layer was changed, and the thickness (T1) of the first negative electrode mixture layer was 75 μm and the thickness (T2) of the second negative electrode mixture layer was 25 μm. That is, the value of T1 / (T1+T2) calculated from T1 and T2 was 0.75. In addition, in the negative electrode mixture layer of Example 11, the porosity (ε1) of the first negative electrode mixture layer was 27%, the porosity (ε2) of the second negative electrode mixture layer on the core side was 18%, and the porosity ratio (ε1 / ε2) was 1.5. The tortuosity (τ1) of the first negative electrode mixture layer on the surface side was 2.0, the tortuosity (τ2) of the second negative electrode mixture layer on the core side was 3.0, and the tortuosity ratio (τ1 / τ2) was 0.7.

[0101] Example 12 In the preparation of a negative electrode, 100 parts by mass of a negative electrode active material, 0.03 parts by mass of CNT, 1 part by mass of SBR, and 1 part by mass of CMC were mixed together, and an appropriate amount of water was added to prepare a first negative electrode mixture slurry. Also, 100 parts by mass of a negative electrode active material, 0.01 parts by mass of CNT, 1 part by mass of SBR, and 1 part by mass of CMC were mixed together, and an appropriate amount of water was added to prepare a second negative electrode mixture slurry.

[0102] Next, the second negative electrode mixture slurry was applied to both sides of a negative electrode core made of copper foil, and the coating was dried and compressed to form a second negative electrode mixture layer. Furthermore, the first negative electrode mixture slurry was applied to the second negative electrode mixture layer, the coating was dried, and the layer was compressed at a linear pressure different from that used when forming the second negative electrode mixture layer to form a first negative electrode mixture layer. At this time, the coating mass ratio per unit area of ​​the first negative electrode mixture slurry and the second negative electrode mixture slurry was 50:50. As a result, a negative electrode mixture layer was produced in which the CNT content (φ1) in the first negative electrode mixture layer was 0.03 mass%, the CNT content (φ2) in the second negative electrode mixture layer was 0.01 mass%, and the φ1 / φ2 ratio was 3.0. The overall thickness of the negative electrode mixture layer was the same as in Example 4. A test cell was otherwise fabricated and evaluated in the same manner as in Example 4.

[0103] <Example 13> In the preparation of the negative electrode, 100 parts by mass of the negative electrode active material, 0.05 parts by mass of CNT, 1 part by mass of SBR, and 1 part by mass of CMC were mixed, and an appropriate amount of water was added to prepare a first negative electrode mixture slurry. Also, 100 parts by mass of the negative electrode active material, 0.01 parts by mass of CNT, 1 part by mass of SBR, and 1 part by mass of CMC were mixed, and an appropriate amount of water was added to prepare a second negative electrode mixture slurry. Except for this, a test cell was prepared and evaluated in the same manner as in Example 12. That is, in the negative electrode of Example 13, the CNT content (φ1) in the first negative electrode mixture layer was 0.05% by mass, the CNT content (φ2) in the second negative electrode mixture layer was 0.01% by mass, and the φ1 / φ2 ratio was 5.0.

[0104] <Example 14> In the preparation of the negative electrode, 100 parts by mass of the negative electrode active material, 0.08 parts by mass of CNT, 1 part by mass of SBR, and 1 part by mass of CMC were mixed, and an appropriate amount of water was added to prepare a first negative electrode mixture slurry. Also, 100 parts by mass of the negative electrode active material, 0.01 parts by mass of CNT, 1 part by mass of SBR, and 1 part by mass of CMC were mixed, and an appropriate amount of water was added to prepare a second negative electrode mixture slurry. Except for this, a test cell was prepared and evaluated in the same manner as in Example 12. That is, in the negative electrode of Example 14, the CNT content (φ1) in the first negative electrode mixture layer was 0.08% by mass, the CNT content (φ2) in the second negative electrode mixture layer was 0.01% by mass, and the φ1 / φ2 ratio was 8.0.

[0105] Example 15 In the preparation of the negative electrode, graphite and a silicon-containing material were mixed in a mass ratio of 88:12, and this was used as the first negative electrode active material. Also, graphite and a silicon-containing material were mixed in a mass ratio of 92:8, and this was used as the second negative electrode active material. Then, 100 parts by mass of the first negative electrode active material, 0.03 parts by mass of CNT, 1 part by mass of SBR, and 1 part by mass of CMC were mixed, and an appropriate amount of water was added to prepare a first negative electrode mixture slurry. Also, 100 parts by mass of the second negative electrode active material, 0.01 parts by mass of CNT, 1 part by mass of SBR, and 1 part by mass of CMC were mixed, and an appropriate amount of water was added to prepare a second negative electrode mixture slurry. Except for this, a test cell was prepared and evaluated in the same manner as in Example 12. That is, in the negative electrode of Example 14, the content of the silicon-containing material in the first negative electrode mixture layer (θ1) was 12 mass%, the content of the silicon-containing material in the second negative electrode mixture layer (θ2) was 8 mass%, and θ1 / θ2 was 1.5. Also, in the negative electrode of Example 14, the content of the CNT in the first negative electrode mixture layer (φ1) was 0.08 mass%, the content of the CNT in the second negative electrode mixture layer (φ2) was 0.01 mass%, and φ1 / φ2 was 8.0.

[0106] Comparative Example 1 A test cell was prepared and evaluated in the same manner as in Example 1, except that MSL and PVDF were not mixed in the preparation of the silicon-containing material, and that the negative electrode mixture layer had a single-layer structure in the preparation of the negative electrode. That is, the silicon-containing material of Comparative Example 1 did not have a coating layer. The thickness of the negative electrode mixture of Comparative Example 1 was 100 μm, the porosity was 20%, and the tortuosity was 4.0.

[0107] Comparative Example 2 A test cell was prepared in the same manner as in Example 1, except that MSL and PVDF were not mixed in the preparation of the silicon-containing material, and an evaluation was carried out.

[0108] Comparative Example 3 A test cell was produced and evaluated in the same manner as in Example 2, except that the negative electrode mixture layer had a single layer structure in the production of the negative electrode. The porosity of the negative electrode mixture in Comparative Example 3 was 20%, and the tortuosity was 4.0.

[0109] Comparative Example 4 A test cell was produced and evaluated in the same manner as in Example 4, except that the negative electrode mixture layer had a single layer structure in the production of the negative electrode. The porosity of the negative electrode mixture in Comparative Example 4 was 20%, and the tortuosity was 4.0.

[0110] Comparative Example 5 A test cell was prepared and evaluated in the same manner as in Example 7, except that MSL and PVDF were not mixed in the preparation of the silicon-containing material.

[0111] Comparative Example 6 A test cell was prepared and evaluated in the same manner as in Example 8, except that MSL and PVDF were not mixed in the preparation of the silicon-containing material.

[0112] The evaluation results of the test cells of Examples 1 to 15 and Comparative Examples 1 to 6 are shown in Table 1. In Table 1, the initial discharge capacity and cycle deterioration rate of the test cells of Examples 1 to 15 and Comparative Examples 2 to 6 are relative values ​​when the initial discharge capacity of the test cell of Comparative Example 1 is set to 100. A larger value of the initial discharge capacity indicates a better initial discharge capacity, and a smaller value of the cycle deterioration rate indicates a better cycle characteristic.

[0113]

[0114] As shown in Table 1, the test cells of the examples all had improved initial discharge capacity and charge / discharge cycle characteristics compared to the test cell of Comparative Example 1. On the other hand, the test cells of Comparative Examples 3 and 4, in which a coating layer was provided on the surface of the silicon-containing particles of the negative electrode to form a single-layer negative electrode, had improved charge / discharge cycle characteristics but a lower initial discharge capacity compared to the test cell of Comparative Example 1, in which a coating layer was not provided on the surface of the silicon-containing particles of the negative electrode. This is presumably due to the reduced conductivity of the silicon-containing particles caused by the coating layer. In contrast, the test cells of Examples 2 and 4, in which a coating layer was provided on the surface of the silicon-containing particles of the negative electrode to form a two-layer negative electrode, had significantly improved initial discharge capacity compared to the test cells of Comparative Examples 3 and 4. This is presumably due to the improved diffusibility of the nonaqueous electrolyte in the first negative electrode mixture layer, which resulted in a uniform distribution of the nonaqueous electrolyte in the negative electrode mixture layer and reduced resistance of the negative electrode.

[0115] Furthermore, the test cells of Examples 7 and 8, in which the content of silicon-containing material in the first negative electrode mixture layer was made higher than the content of silicon-containing material in the second negative electrode mixture layer, had improved initial discharge capacity and cycle characteristics compared to the test cell of Example 4, in which the content of silicon-containing material in the first negative electrode mixture layer was the same as the content of silicon-containing material in the second negative electrode mixture layer. Furthermore, the test cells of Examples 12 to 14, in which the content of CNT in the first negative electrode mixture layer was made higher than the content of CNT in the second negative electrode mixture layer, had improved initial discharge capacity and cycle characteristics compared to the test cell of Example 4, in which the content of CNT in the first negative electrode mixture layer was the same as the content of CNT in the second negative electrode mixture layer.

[0116] The present disclosure will be further described by the following embodiments. Configuration 1: A negative electrode for a non-aqueous electrolyte secondary battery, comprising a negative electrode core, a first negative electrode mixture layer, and a second negative electrode mixture layer disposed between the first negative electrode mixture layer and the negative electrode core, wherein the first negative electrode mixture layer and the second negative electrode mixture layer contain a carbon material and a silicon-containing material as negative electrode active materials, the silicon-containing material has silicon-containing particles and a coating layer covering at least a portion of the surface of the silicon-containing particles, the coating layer contains a sulfonic acid compound and a hydrophobic polymer compound, and the porosity (ε1) of the first negative electrode mixture layer and the porosity (ε2) of the second negative electrode mixture layer satisfy the relationship ε1 > ε2. Configuration 2: The negative electrode for a non-aqueous electrolyte secondary battery according to Configuration 1, wherein the tortuosity (τ1) of the first negative electrode mixture layer and the tortuosity (τ2) of the second negative electrode mixture layer satisfy τ1 < τ2. Configuration 3: The negative electrode for a non-aqueous electrolyte secondary battery according to Configuration 1 or 2, wherein the tortuosity (τ1) of the first negative electrode mixture layer and the tortuosity (τ2) of the second negative electrode mixture layer satisfy 0.3 ≦ τ1 / τ2 < 1.0. Configuration 4: The negative electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the content (θ1) of the silicon-containing material in the first negative electrode mixture layer and the content (θ2) of the silicon-containing material in the second negative electrode mixture layer satisfy θ1 > θ2. Configuration 5: The negative electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 3, wherein the first negative electrode mixture layer and the second negative electrode mixture layer contain at least fibrous carbon as a conductive agent. Configuration 6: The negative electrode for a non-aqueous electrolyte secondary battery according to Configuration 5, wherein a content (φ1) of the fibrous carbon in the first negative electrode mixture layer and a content (φ2) of the fibrous carbon in the second negative electrode mixture layer satisfy 1≦φ1 / φ2≦8. Configuration 7: The negative electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 6, wherein a thickness (T1) of the first negative electrode mixture layer and a thickness (T2) of the second negative electrode mixture layer satisfy 0.1≦T1 / (T1+T2)≦0.5. Configuration 8: The negative electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 7, wherein the silicon-containing particles include an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase.Aspect 9: The negative electrode for a non-aqueous electrolyte secondary battery according to Aspect 8, wherein the ion-conducting phase is at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. Aspect 10: The negative electrode for a non-aqueous electrolyte secondary battery according to any one of Aspects 1 to 9, wherein the sulfonic acid compound is a compound represented by formula (I), wherein A is a Group 1 or Group 2 element, R is a hydrocarbon group, and n is 1 or 2. Aspect 11: The negative electrode for a non-aqueous electrolyte secondary battery according to Aspect 10, wherein the sulfonic acid compound comprises at least one selected from the group consisting of lithium methanesulfonate, lithium ethanesulfonate, and lithium propanesulfonate. Aspect 12: The negative electrode for a non-aqueous electrolyte secondary battery according to any one of Aspects 1 to 11, wherein the amount of the sulfonic acid compound attached to the surfaces of the silicon-containing particles is 1 mass % or more, based on the total mass of the silicon-containing particles. Aspect 13: The negative electrode for a non-aqueous electrolyte secondary battery according to any one of Aspects 1 to 12, wherein the hydrophobic polymer compound comprises a fluororesin. Aspect 14: The negative electrode for a non-aqueous electrolyte secondary battery according to Aspect 13, wherein the fluororesin comprises at least one selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-ethylene copolymer, chlorotrifluoroethylene-ethylene copolymer, and polychlorotrifluoroethylene. Configuration 15: The negative electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 14, wherein the amount of the hydrophobic polymer compound attached to the surfaces of the silicon-containing particles is 1 mass % or more relative to the total mass of the silicon-containing particles.Configuration 16: The negative electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 15, wherein a peak derived from the sulfonic acid compound is observed in a surface portion of the coating layer when surface analysis is performed by X-ray photoelectron spectroscopy.Configuration 17: The negative electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 16, wherein the silicon-containing material further has a conductive layer containing a conductive carbon material between the silicon-containing particles and the coating layer.Configuration 18: A non-aqueous electrolyte secondary battery comprising the negative electrode for a non-aqueous electrolyte secondary battery according to any one of Configurations 1 to 17, a positive electrode, and a non-aqueous electrolyte.

[0117] REFERENCE SIGNS LIST 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 First negative electrode mixture layer, 32 Second negative electrode mixture layer, 40 Silicon-containing material, 41 Silicon-containing particles, 42 Coating layer, 43 Ion-conducting phase, 44 Silicon phase

Claims

1. A negative electrode for a non-aqueous electrolyte secondary battery comprising a negative electrode core, a first negative electrode mixture layer, and a second negative electrode mixture layer disposed between the first negative electrode mixture layer and the negative electrode core, wherein the first negative electrode mixture layer and the second negative electrode mixture layer contain a carbon material and a silicon-containing material as negative electrode active materials, the silicon-containing material has silicon-containing particles and a coating layer covering at least a portion of a surface of the silicon-containing particles, the coating layer contains a sulfonic acid compound and a hydrophobic polymer compound, and a porosity (ε1) of the first negative electrode mixture layer and a porosity (ε2) of the second negative electrode mixture layer satisfy ε1>ε2.

2. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the tortuosity (τ1) of the first negative electrode mixture layer and the tortuosity (τ2) of the second negative electrode mixture layer satisfy τ1<τ2.

3. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the tortuosity (τ1) of the first negative electrode mixture layer and the tortuosity (τ2) of the second negative electrode mixture layer satisfy 0.3≦τ1 / τ2<1.

0.

4. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein a content (θ1) of the silicon-containing material in the first negative electrode mixture layer and a content (θ2) of the silicon-containing material in the second negative electrode mixture layer satisfy θ1>θ2.

5. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the first negative electrode mixture layer and the second negative electrode mixture layer contain at least fibrous carbon as a conductive agent.

6. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 5, wherein the content (φ1) of the fibrous carbon in the first negative electrode mixture layer and the content (φ2) of the fibrous carbon in the second negative electrode mixture layer satisfy 1≦φ1 / φ2≦8.

7. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the thickness (T1) of the first negative electrode mixture layer and the thickness (T2) of the second negative electrode mixture layer satisfy 0.1≦T1 / (T1+T2)≦0.

5.

8. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the silicon-containing particles include an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase.

9. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 8, wherein the ion-conducting phase is at least one selected from the group consisting of a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase.

10. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the sulfonic acid compound is a compound represented by formula (I), A is a Group 1 or Group 2 element, R is a hydrocarbon group, and n is 1 or 2.

11. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 10, wherein the sulfonic acid compound includes at least one selected from the group consisting of lithium methanesulfonate, lithium ethanesulfonate, and lithium propanesulfonate.

12. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the amount of the sulfonic acid compound attached to the surface of the silicon-containing particle is 1 mass % or more based on the total mass of the silicon-containing particle.

13. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the hydrophobic polymer compound includes a fluororesin.

14. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 13, wherein the fluororesin contains at least one selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-ethylene copolymer, chlorotrifluoroethylene-ethylene copolymer, and polychlorotrifluoroethylene.

15. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the amount of the hydrophobic polymer compound attached to the surface of the silicon-containing particle is 1 mass % or more based on the total mass of the silicon-containing particle.

16. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein, when surface analysis is carried out by X-ray photoelectron spectroscopy, a peak derived from the sulfonic acid compound is observed in the surface portion of the coating layer.

17. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the silicon-containing material further has a conductive layer containing a conductive carbon material between the silicon-containing particle and the coating layer.

18. A nonaqueous electrolyte secondary battery comprising: the negative electrode for a nonaqueous electrolyte secondary battery according to any one of claims 1 to 17; a positive electrode; and a nonaqueous electrolyte.