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

Abstract

To provide a negative electrode for non-aqueous electrolyte secondary battery capable of enhancing cycle characteristics of a non-aqueous electrolyte secondary battery, and the non-aqueous electrolyte secondary battery.SOLUTION: A negative electrode for non-aqueous electrolyte secondary battery comprises a negative electrode mixture including a negative electrode active material capable of electrochemically occluding and emitting lithium ions, carbon nanotubes, and a binder. The negative electrode active material includes a silicon-containing material and the binder includes an acrylic polymer. The polymer includes hydrophilic structural units having a carboxyl group and hydrophilic structural units.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] A non-aqueous electrolyte secondary battery, typified by a lithium ion secondary battery, comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode comprises a negative electrode mixture containing a negative electrode active material capable of electrochemically absorbing and desorbing lithium ions. For example, the negative electrode active material may be a material containing silicon (Si-containing material) that has a large capacity for absorbing lithium ions. Furthermore, in order to increase the conductivity of the negative electrode mixture, it has been investigated to incorporate carbon nanotubes (CNTs) into the negative electrode mixture (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-110876 Summary of the Invention

[0004] When CNTs are included in the negative electrode mixture, a conductive path is formed between the Si-containing material and the surrounding negative electrode active material through the CNTs. However, because CNTs are hydrophobic, sufficient contact between the CNTs and the Si-containing material may not be formed.

[0005] In addition, polyacrylic acid (salt), for example, is used as a binder to improve the binding strength of the negative electrode mixture. However, polyacrylic acid (salt) is hydrophilic and does not fully bind to hydrophobic CNTs. Therefore, the adhesion between the CNTs and the Si-containing material is not sufficiently improved, and the contact points between the CNTs and the Si-containing material may not be fully formed.

[0006] The contact points between the CNT and the Si-containing material may not be formed sufficiently, and the conductive paths may not be formed sufficiently, resulting in insufficient improvement in cycle characteristics.

[0007] In view of the above, one aspect of the present disclosure relates to a negative electrode for a non-aqueous electrolyte secondary battery, comprising a negative electrode mixture including a negative electrode active material capable of electrochemically absorbing and desorbing lithium ions, carbon nanotubes, and a binder, wherein the negative electrode active material includes a silicon-containing material, the binder includes an acrylic polymer, and the polymer includes a hydrophilic structural unit having a carboxyl group and a hydrophobic structural unit.

[0008] Another aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode is the above-described negative electrode.

[0009] According to the present disclosure, the cycle characteristics of a non-aqueous electrolyte secondary battery can be improved. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic perspective view of a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure, with a portion cut away. DETAILED DESCRIPTION OF THE INVENTION

[0011] [Non-aqueous electrolyte secondary battery negative electrode] A negative electrode for a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure includes a negative electrode mixture containing a negative electrode active material capable of electrochemically absorbing and desorbing lithium ions, carbon nanotubes (hereinafter also referred to as CNTs), and a binder. The negative electrode active material contains a silicon-containing material (hereinafter also referred to as Si-containing material), and the binder contains an acrylic polymer (hereinafter also simply referred to as polymer). The acrylic polymer contains a hydrophilic structural unit having a carboxyl group and a hydrophobic structural unit.

[0012] The acrylic polymer contains hydrophilic structural units with carboxyl groups, which facilitates excellent binding to Si-containing materials. Furthermore, the polymer contains hydrophobic structural units, which facilitate excellent binding to CNTs. Therefore, the polymer firmly adheres the Si-containing material to the CNTs, efficiently forming contact points between the CNTs and the Si-containing material. As a result, the CNTs adequately form a conductive path (conductive network) between the Si-containing material and the surrounding negative electrode active material. During expansion and contraction of the Si-containing material due to charging and discharging, the polymer maintains a strong adhesion between the CNTs and the Si-containing material, preventing isolation of the Si-containing material (interruption of the conductive network) due to the expansion and contraction. This results in improved cycle performance.

[0013] Furthermore, the acrylic polymer contains a hydrophilic structural unit having a carboxyl group, which enhances adhesion between the negative electrode mixture layer and the negative electrode current collector. Furthermore, it is easy to obtain a negative electrode with excellent lithium ion conductivity and low resistance. The dispersion stability of the active material and the like in the negative electrode slurry is improved.

[0014] From the viewpoint of improving cycle characteristics, the content of the acrylic polymer in the negative electrode mixture may be 0.02% by mass or more and 1.5% by mass or less, based on the total amount of the negative electrode active material. From the viewpoint of easily reducing the resistance of the negative electrode, ensuring a sufficient amount of the negative electrode active material, and easily achieving high capacity, the content of the polymer may be 1.0% by mass or less, based on the total amount of the negative electrode active material.

[0015] From the viewpoint of further improving cycle characteristics, when the average diameter of the CNTs is 5 nm or less, the content of the acrylic polymer in the negative electrode mixture is preferably 0.05 mass% or more and 1.0 mass% or less, and more preferably 0.05 mass% or more and 0.5 mass% or less, relative to the total negative electrode active material. When the average diameter of the CNTs is greater than 5 nm, the content of the acrylic polymer in the negative electrode mixture may be 0.2 mass% or more and 1.2 mass% or less, or may be 0.3 mass% or more and 1 mass% or less, relative to the total negative electrode active material.

[0016] From the viewpoint of improving cycle characteristics, the mass ratio of the acrylic polymer to the CNTs (hereinafter also referred to as (polymer / CNTs)) in the negative electrode mixture may be 0.2 or more and 600 or less.

[0017] From the viewpoint of further improving cycle characteristics, when the average diameter of the CNTs is 5 nm or less, (acrylic polymer / CNT) is preferably 0.6 to 225, more preferably 2 to 100, and even more preferably 20 to 100. When (polymer / CNT) is within the above range, a good balance is easily achieved between the effect of the polymer in improving the adhesion between the Si-containing material and the CNTs and the effect of the CNTs in forming a conductive path. When the average diameter of the CNTs is greater than 5 nm, (polymer / CNT) may be 0.5 to 3.8, and preferably 1.3 to 2.7.

[0018] (acrylic polymer) The acrylic polymer contains a hydrophilic structural unit having a carboxyl group and a hydrophobic structural unit. The polymer has excellent binding strength, allowing a strongly solidified negative electrode mixture (layer) to be obtained. In the negative electrode mixture (layer), the adhesion between the negative electrode active material containing the Si-containing material and the conductive agent containing CNT is improved, sufficient conductive paths are formed, and the adhesion is maintained during charge and discharge, maintaining the conductive paths sufficiently.

[0019] At least a portion of the carboxyl groups contained in the polymer may form a carboxylate. The carboxylate includes, for example, an alkali metal salt of carboxylic acid. The alkali metal salt includes a sodium salt, a lithium salt, etc. Among them, a lithium salt is preferred because the alkali metal salt exchanges with lithium ions in the electrolyte solution.

[0020] From the viewpoint of dispersion stability of the active material and the like in the negative electrode slurry, the degree of neutralization of the polymer is, for example, 20% to 100%, preferably 50% to 100%, and more preferably 60% to 95%. The degree of neutralization of the polymer refers to the ratio (molar ratio) of carboxyl groups that form carboxylate salts to the total carboxyl groups contained in the structure.

[0021] The degree of neutralization of the structure can be determined by analyzing the polymer by infrared spectroscopy (IR), determining the peak intensity I1 derived from the C=O group of the carboxylic acid and the peak intensity I2 derived from the C=O group of the carboxylate, and then calculating I2 / (I1+I2). Alternatively, the degree of neutralization can be determined by analyzing the amount of carboxylic acid in the polymer by NMR and then analyzing the concentration of alkali metal by ICP emission spectroscopy.

[0022] The hydrophilic structural unit preferably contains a structural unit derived from an ethylenically unsaturated carboxylic acid. In this case, good binding properties are likely to be exhibited for Si-containing materials. Examples of ethylenically unsaturated carboxylic acids include (meth)acrylic acid; (meth)acrylamidoalkylcarboxylic acids such as (meth)acrylamidohexanoic acid and (meth)acrylamidododecanoic acid; succinic acid monohydroxyethyl (meth)acrylate, ω-carboxy-caprolactone mono(meth)acrylate, and β-carboxyethyl (meth)acrylate. One type of ethylenically unsaturated carboxylic acid may be used alone, or two or more types may be used in combination. Among these, ethylenically unsaturated carboxylic acids having an acryloyl group are preferred, and acrylic acid is more preferred, from the viewpoint that a polymer having a long primary chain length can be obtained due to a high polymerization rate and thus excellent binding strength can be obtained. In the case of acrylic acid, a polymer containing many carboxyl groups can be easily obtained.

[0023] In this specification, "(meth)acrylic" means acrylic and / or methacrylic, "(meth)acrylate" means acrylate and / or methacrylate, and "(meth)acrylonitrile" means acrylonitrile and / or methacrylonitrile.

[0024] The hydrophilic structural unit may be formed by (co)polymerizing (meth)acrylic acid ester and then hydrolyzing it, or by polymerizing (meth)acrylamide, (meth)acrylonitrile, etc. and then treating it with a strong alkali. It may also be formed by obtaining a polymer having hydroxyl groups and reacting the hydroxyl groups with a carboxylic acid anhydride.

[0025] The hydrophobic structural unit has the general formula: H2C=CR 1 It is preferable that the compound contains a structural unit derived from a compound represented by -X (hereinafter also referred to as compound A). When the compound contains a structural unit derived from compound A, it can exhibit a strong interaction with CNTs and is likely to exhibit good binding properties to CNTs.

[0026] In the formula, R 1 is a hydrogen atom or a methyl group, and X is COOR 2 or a cyano group (CN), and R 2 is a hydrocarbon group having 1 to 8 carbon atoms. 2 is a hydrocarbon group, the structural unit derived from compound A has hydrophobicity. 2 When X is CN, compound A is (meth)acrylonitrile.

[0027] R 2 is, for example, a linear or branched alkyl group having 1 to 8 carbon atoms. Examples of such an alkyl group include a methyl group, an ethyl group, a propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a tert-butyl group, an n-pentyl group, an n-hexyl group, an n-heptyl group, an n-octyl group, and a 2-ethylhexyl group.

[0028] Also, R 2may be an alkyl or alkenyl group having an alicyclic structure and having 3 to 8 carbon atoms. Examples of the alkyl group having an alicyclic structure and having 3 to 8 carbon atoms include cycloalkyl groups having 3 to 8 carbon atoms, such as cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl; alkyl-substituted cycloalkyl groups having 3 to 8 carbon atoms, such as methylcyclohexyl and ethylcyclohexyl; and cycloalkyl-substituted alkyl groups having 3 to 8 carbon atoms, such as cyclopropylmethyl, cyclopropylethyl, and cyclohexylethyl. Examples of the alkenyl group having an alicyclic structure and having 3 to 8 carbon atoms include cycloalkenyl groups having 3 to 8 carbon atoms, such as cyclopropenyl, cyclobutenyl, cyclopentenyl, cyclohexenyl, cycloheptenyl, and cyclooctenyl.

[0029] Specific examples of compound A include methacrylamidoalkylcarboxylic acids such as methyl methacrylate, methyl acrylate, methacrylamide hexanoic acid, and methacrylamide dodecanoic acid; succinic acid monohydroxyethyl methacrylate, ω-carboxy-caprolactone monomethacrylate, β-carboxyethyl methacrylate, etc. Among these, methyl methacrylate and methyl acrylate are preferred from the viewpoint of less steric hindrance.

[0030] From the viewpoint of facilitating efficient and strong contact between the CNT and the Si-containing material, the polymer preferably contains structural units derived from an ethylenically unsaturated carboxylic acid and structural units derived from compound A. In the acrylic structure, the proportion of structural units derived from compound A in the polymer relative to the total of structural units derived from the ethylenically unsaturated carboxylic acid and structural units derived from compound A is preferably 0.5% or more and 5% or less. When the proportion of structural units derived from compound A is within the above range, the polymer is likely to exhibit excellent binding properties to both the Si-containing material and the CNT. The proportion of structural units derived from compound A is more preferably 1% or more and 3% or less. The proportion of structural units derived from compound A in the polymer is the proportion of the number of structural units derived from compound A relative to the total number of structural units derived from the ethylenically unsaturated carboxylic acid and structural units derived from compound A in the polymer.

[0031] (Method of producing polymer) The method for producing the polymer includes a polymerization step of polymerizing a monomer constituting a hydrophilic structural unit and a monomer constituting a hydrophobic structural unit. In the polymerization step, a known polymerization method such as solution polymerization, precipitation polymerization, suspension polymerization, or reversed-phase emulsion polymerization may be used. The polymerization reaction may be any of known polymerization reactions such as radical polymerization, cationic polymerization, anionic polymerization, or coordination polymerization.

[0032] The polymerization solvent may be appropriately selected from water and various organic solvents, taking into consideration the type of monomer to be used, etc. When a polymer having a long primary chain length is to be obtained, it is preferable to use a solvent having a small chain transfer constant.

[0033] Examples of the polymerization solvent include water-soluble organic solvents such as methanol, tert-butyl alcohol, acetone, acetonitrile, and tetrahydrofuran. Other examples of the polymerization solvent include benzene, ethyl acetate, dichloroethane, n-hexane, cyclohexane, and n-heptane. One polymerization solvent may be used alone, or two or more may be used in combination. A mixed solvent of a water-soluble organic solvent and water may also be used as the polymerization solvent. The water-soluble organic solvent referred to here refers to an organic solvent having a solubility in water of 10 g / 100 ml or more at 20°C.

[0034] A small amount of a highly polar solvent may be added to the polymerization solvent. The addition of a highly polar solvent increases the polymerization rate when polymerizing a highly hydrophilic ethylenically unsaturated carboxylic acid such as acrylic acid, making it easier to obtain a polymer with a long primary chain length. Furthermore, the neutralization reaction can proceed stably and quickly in the neutralization step described below. The content of the highly polar solvent in the polymerization solvent is, for example, 0.05% by mass or more and 10.0% by mass or less. Examples of highly polar solvents include water and methanol. Among these, water is preferred as the highly polar solvent from the viewpoint of increasing the polymerization rate.

[0035] The polymerization initiator used in the polymerization step may be a known polymerization initiator such as an azo compound, an organic peroxide, or an inorganic peroxide. Examples of azo compounds include 2,2'-azobis(2,4-dimethylvaleronitrile). The polymerization initiator may be used alone or in combination with two or more. The conditions for use may be appropriately adjusted to generate an appropriate amount of radicals using a known method such as thermal initiation, redox initiation using a reducing agent, or UV initiation.

[0036] In the polymerization step, for example, an ethylenically unsaturated carboxylic acid monomer and compound A are copolymerized. This results in a polymer containing structural units derived from the ethylenically unsaturated carboxylic acid and structural units derived from compound A. In the above, the proportion (molar ratio) of compound A to the total of the ethylenically unsaturated carboxylic acid and compound A is preferably, for example, 0.5% or more and 5% or less. More preferably, it is 1% or more and 3% or less.

[0037] From the viewpoint of facilitating the production of a polymer with a high polymerization rate, a large molecular weight, and excellent binding properties, the degree of neutralization of the ethylenically unsaturated carboxylic acid is preferably 10% or less, more preferably 5% or less, and even more preferably 0%. The degree of neutralization of the ethylenically unsaturated carboxylic acid refers to the ratio (molar ratio) of carboxyl groups that form a carboxylate salt to the total carboxyl groups contained in the ethylenically unsaturated carboxylic acid.

[0038] The ratio of the monomers (ethylenically unsaturated carboxylic acid and compound A) to the total of the monomers and polymerization solvent is, for example, 2% by mass or more and 30% by mass or less. From the viewpoint of obtaining a polymer with a long primary chain length, a higher monomer concentration within the above range is preferable. The polymerization temperature is, for example, 0°C or more and 100°C or less. The polymerization temperature may be constant or may vary during the polymerization reaction. The polymerization time is, for example, 1 hour or more and 20 hours or less.

[0039] The method for producing a polymer may include a drying step in which the dispersion medium is removed from the polymer dispersion obtained in the polymerization step. The drying step involves, for example, reducing pressure and / or heat treatment. The drying step results in a powdery polymer.

[0040] The method for producing the polymer may include, after the polymerization step and before the drying step, a solid-liquid separation step of separating the polymer particles from unreacted components by centrifugation, filtration, or the like, and a washing step of removing unreacted components adhering to the polymer particles using an organic solvent or a mixed solvent of an organic solvent and water. When the washing step is included, aggregates of the polymer particles are easily broken down, and the remaining unreacted components are removed, improving the binding strength of the polymer and the reliability of the battery characteristics.

[0041] When a monomer with a low neutralization rate (e.g., an ethylenically unsaturated carboxylic acid) is used in the polymerization step, the method for producing the polymer may include a neutralization step in which an alkaline component is added to the dispersion of the polymer obtained in the polymerization step to increase the neutralization rate of the polymer. The neutralization step is performed after the polymerization step and before the drying step (solid-liquid separation step). Alternatively, the neutralization step may be omitted, and after obtaining a powder of a polymer with a low neutralization rate, an alkaline component may be added together with the polymer to the negative electrode slurry during negative electrode production to increase the neutralization rate of the polymer.

[0042] (carbon nanotubes) CNTs are nanometer-sized carbon materials with a cylindrical structure consisting of a sheet of graphene, a six-membered ring network formed by carbon atoms. They have excellent electrical conductivity. When the number of graphene layers making up the cylindrical structure is one, they are called single-walled carbon nanotubes (SWCNTs). When the number of layers is multiple, they are called multi-walled carbon nanotubes (MWCNTs).

[0043] (CNTs with an average diameter of 5 nm or less) From the viewpoint of improving cycle characteristics, the average diameter of the CNTs is preferably 5 nm or less, more preferably 1 nm or more and 5 nm or less, and even more preferably 1 nm or more and 3 nm or less. When the average diameter of the CNTs is 5 nm or less, the CNTs are more likely to be interposed between the Si-containing material and the surrounding negative electrode active material, and the CNTs are more likely to form contact points with the Si-containing material and the surrounding negative electrode active material. When the average diameter of the CNTs is 1 nm or more, the strength of the CNTs is sufficiently ensured, and a conductive path is more likely to be formed between the Si-containing material and the surrounding negative electrode active material, and the conductive path is more likely to be maintained during charge and discharge.

[0044] CNTs with an average diameter of 5 nm or less contain a large proportion of SWCNTs. For example, when the average diameter of CNTs is 5 nm or less, 50% or more of the CNTs are SWCNTs. In other words, the ratio of the number of SWCNTs to the total number of CNTs is 50% or more.

[0045] The proportion of SWCNTs in the CNTs contained in the negative electrode mixture is determined by the following method.

[0046] A scanning electron microscope (SEM) is used to obtain an image of the cross section of the negative electrode mixture layer or the CNTs. Using the SEM image, several CNTs (e.g., approximately 50 to 200) are randomly selected and observed to determine the number of SWCNTs, and the ratio of the number of SWCNTs to the total number of selected CNTs is calculated. Alternatively, the ratio of SWCNTs to the total number of CNTs can also be determined using Raman spectroscopy.

[0047] When the negative electrode mixture contains CNTs with an average diameter of 5 nm or less and a polymer, the CNTs easily form contact points with the Si-containing material and the surrounding negative electrode active material, and even a small amount of CNTs (e.g., a CNT content of 0.1 mass% or less) can efficiently form a conductive path. Therefore, the amount of CNTs can be reduced and the amount of negative electrode active material can be increased, enabling a high capacity. Furthermore, the amount of CNTs contained in the negative electrode slurry during negative electrode preparation can be reduced. When the amount of CNTs in the negative electrode slurry is small, CNT aggregation is suppressed, improving the reliability of the resulting battery (negative electrode) and making it easier to achieve stable cycle characteristics.

[0048] From the viewpoint of improving the reliability and cycle characteristics of the battery, the CNT content in the negative electrode mixture may be 0.1 mass% or less, 0.08 mass% or less, or 0.05 mass% or less, based on the total negative electrode active material. The lower limit of the CNT content may be 0.001 mass% or 0.003 mass%. For example, the CNT content in the negative electrode mixture may be 0.0025 mass% or more and 0.1 mass% or less, or 0.004 mass% or more and 0.08 mass% or less, based on the total negative electrode active material.

[0049] Furthermore, from the viewpoint of further improving the reliability and cycle characteristics of the battery, the content of CNT in the negative electrode mixture is preferably 0.005 mass% or more and 0.05 mass% or less, and more preferably 0.005 mass% or more and 0.02 mass% or less, relative to the total negative electrode active material.

[0050] (CNTs with an average diameter of more than 5 nm) The average diameter of the CNTs contained in the negative electrode mixture may be greater than 5 nm. CNTs with an average diameter greater than 5 nm contain a large amount of MWCNTs. For example, in the case of CNTs with an average diameter greater than 5 nm, more than 50% of the CNTs are MWCNTs. In other words, the ratio of the number of MWCNTs to the total number of CNTs is greater than 50%. The ratio of MWCNTs to the CNTs contained in the negative electrode mixture can be determined by determining the number of MWCNTs using SEM images in the same manner as in determining the ratio of SWCNTs described above, and then calculating the ratio of the number of MWCNTs to the total number of selected CNTs. Alternatively, the ratio of MWCNTs to the CNTs can also be determined by Raman spectroscopy.

[0051] From the viewpoint of improving cycle characteristics, when the average diameter of the CNTs exceeds 5 nm, the content of the CNTs in the negative electrode mixture may be 0.1% by mass or more and 0.5% by mass or less, or may be 0.15% by mass or more and 0.45% by mass or less, based on the total negative electrode active material.

[0052] From the viewpoint of ensuring a conductive path between the Si-containing material and the surrounding negative electrode active material, the average length of the CNTs is preferably 1 μm or more and 100 μm or less, and more preferably 5 μm or more and 20 μm or less.

[0053] The average length and diameter of the CNTs are determined by obtaining an image of the cross section of the negative electrode mixture layer or the CNTs using an SEM, randomly selecting a number of CNTs (e.g., about 50 to 200) using the SEM image, measuring the lengths and diameters of these CNTs, and averaging the measured lengths and diameters. Note that the length of the CNTs refers to the length when they are linear.

[0054] The CNTs can be confirmed, for example, by an SEM image of a cross section of the negative electrode mixture layer. Methods for analyzing the CNTs include, for example, Raman spectroscopy and thermogravimetric analysis.

[0055] (Si-containing material) The Si-containing material preferably includes a composite material in which silicon particles (fine Si phase) are dispersed in a lithium ion conductive phase (matrix). The lithium ion conductive phase preferably includes at least one selected from the group consisting of a SiO2 phase, a silicate phase, and a carbon phase. The lithium ion conductive phase can form an amorphous phase. Since the stress generated due to the expansion and contraction of the silicon particles during charge and discharge is relaxed by the lithium ion conductive phase, the composite material is advantageous for improving cycle characteristics. The Si-containing material may include a composite material in which silicon particles are dispersed in a SiO2 phase, a composite material in which silicon particles are dispersed in a silicate phase, and a composite material in which silicon particles are dispersed in a carbon phase.

[0056] The SiO2 phase is an amorphous phase containing 95% by mass or more of silicon dioxide. The composite material in which silicon particles are dispersed in the SiO2 phase is SiO x represented by, where x is, for example, 0.5 ≤ x < 2, and preferably 0.8 ≤ x ≤ 1.6. SiO x is obtained, for example, by heat-treating silicon monoxide and separating it into a SiO2 phase and a fine Si phase by disproportionation reaction.

[0057] The silicate phase preferably contains at least one of an alkali metal element (Group 1 element other than hydrogen in the long-period type periodic table) and a Group 2 element in the long-period type periodic table. The alkali metal elements include lithium (Li), potassium (K), sodium (Na), etc. The Group 2 elements include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), etc. The lithium silicate phase can have a composition represented by the formula: Li 2y SiO 2+y (0 < y < 2). y may be 1 / 2 or 1. The composite material in which silicon particles are dispersed in the silicate phase can be obtained, for example, by pulverizing a mixture of silicate and raw material silicon while stirring with a ball mill or the like to make fine particles, and then heat-treating the mixture in an inert atmosphere.

[0058] The average particle size of the silicon particles (before the first charge) dispersed within the silicate phase may be 50 nm or more and 500 nm or less, or 100 nm or more and 400 nm or less. The average particle size of the silicon particles is obtained by calculating the average of the maximum diameters of 100 randomly selected silicon particles using an SEM image of the cross section of the composite material. The content of the silicon particles dispersed within the silicate phase may be 30 mass % or more and 95 mass % or less, or 35 mass % or more and 75 mass % or less, based on the total mass of the composite material.

[0059] The carbon phase contains, for example, amorphous carbon with low crystallinity. The amorphous carbon may be, for example, easily graphitized carbon (hard carbon) or hardly graphitized carbon (soft carbon). A composite material in which silicon particles are dispersed within the carbon phase can be obtained, for example, by pulverizing a mixture of a carbon source and raw silicon while stirring it in a ball mill or the like to form fine particles, and then heat-treating the mixture in an inert atmosphere. For example, sugars such as carboxymethyl cellulose (CMC) or water-soluble resins such as polyvinylpyrrolidone are used as the carbon source.

[0060] The composition of the Si-containing material can be determined, for example, by obtaining a backscattered electron image of the cross section of the negative electrode mixture layer using a field emission scanning electron microscope (FE-SEM), observing the Si-containing material particles, and performing elemental analysis on the observed Si-containing material particles. Elemental analysis can be performed using, for example, an electron probe microanalyzer (EPMA). The composition of the lithium ion conductive phase can also be determined by the above analysis.

[0061] The Si-containing material is, for example, a particulate material. The average particle size (D50) of the Si-containing material is, for example, 1 μm or more and 25 μm or less, and preferably 4 μm or more and 15 μm or less. Within the above range, good battery performance is likely to be obtained. In this specification, the average particle size (D50) refers to the particle size (volume-average particle size) at which the volume cumulative value is 50% in the particle size distribution measured by the laser diffraction scattering method. For example, the "LA-750" manufactured by HORIBA Ltd. can be used as a measuring device.

[0062] When the lithium ion conductive phase is an SiO2 phase or a silicate phase, at least a portion of the particle surface of the Si-containing material may be coated with a conductive layer to improve conductivity. The conductive layer contains a conductive material such as conductive carbon. The amount of the conductive layer is, for example, 1 part by mass or more and 10 parts by mass or less per 100 parts by mass of the total of the Si-containing material particles and the conductive layer. The Si-containing material particles having a conductive layer on their surface can be obtained, for example, by mixing coal pitch or the like with the Si-containing material particles and heat-treating the mixture in an inert atmosphere.

[0063] (carbon materials) The negative electrode active material may further include a carbon material that electrochemically absorbs and releases lithium ions. The carbon material expands and contracts less during charge and discharge than the Si-containing material. The combined use of the Si-containing material and the carbon material can maintain better contact between the negative electrode active material particles and between the negative electrode mixture layer and the negative electrode current collector during repeated charge and discharge. This means that the high capacity of the Si-containing material can be imparted to the negative electrode while improving cycle performance. From the viewpoint of increasing capacity and improving cycle performance, the proportion of the carbon material in the total of the Si-containing material and the carbon material is preferably 98% by mass or less, more preferably 70% by mass or more and 98% by mass or less, and even more preferably 75% by mass or more and 95% by mass or less.

[0064] Examples of carbon materials used for the negative electrode active material include graphite, easily graphitizable carbon (soft carbon), non-graphitizable carbon (hard carbon), etc. One type of carbon material may be used alone, or two or more types may be used in combination.

[0065] Among these, graphite is preferred as the carbon material because of its excellent charge / discharge stability and low irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles. The graphite particles may partially contain amorphous carbon, graphitizable carbon, or non-graphitizable carbon.

[0066] Graphite is a carbon material with a developed graphite-type crystal structure. The interplanar spacing d002 of the (002) plane of graphite measured by X-ray diffraction may be, for example, 0.340 nm or less, or 0.3354 nm or more and 0.340 nm or less. The crystallite size Lc(002) of graphite may be, for example, 5 nm or more, or 5 nm or more and 200 nm or less. The crystallite size Lc(002) is measured, for example, by the Scherrer method. When the interplanar spacing d002 of the (002) plane of graphite and the crystallite size Lc(002) are within the above ranges, high capacity is easily obtained.

[0067] [Nonaqueous electrolyte secondary battery] A non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and the negative electrode includes the above-described negative electrode mixture.

[0068] The nonaqueous electrolyte secondary battery will be described in detail below.

[0069] (Negative electrode) The negative electrode may include a negative electrode current collector and a negative electrode mixture layer supported on the surface of the negative electrode current collector. The negative electrode mixture layer can be formed by applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the applied layer. The dried coating may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector.

[0070] The negative electrode mixture contains, as essential components, a negative electrode active material, CNTs, and an acrylic polymer, and may contain, as optional components, a conductive agent other than CNTs, a binder other than the polymer, and the like.

[0071] Examples of conductive agents other than CNT include carbons such as acetylene black, metals such as aluminum, etc. One type of conductive agent may be used alone, or two or more types may be used in combination.

[0072] Examples of binders other than polymers include resin materials other than polymers (acrylic resins). Examples of such resin materials include fluororesins such as polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins such as polyethylene and polypropylene; polyamide resins such as aramid resin; and polyimide resins such as polyimide and polyamideimide. Rubber-like materials such as styrene-butadiene copolymer rubber (SBR) may also be used as binders.

[0073] Examples of binders include carboxymethyl cellulose (CMC) and its modified forms (including salts such as Na salt), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.), etc. One type of binder may be used alone, or two or more types may be used in combination.

[0074] The dispersion medium is not particularly limited, but examples include water, alcohols such as ethanol, ethers such as tetrahydrofuran, amides such as dimethylformamide, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0075] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys. The thickness of the negative electrode current collector is not particularly limited, but may be 1 to 50 μm or 5 to 20 μm.

[0076] (positive electrode) The positive electrode may include a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry, in which the positive electrode mixture is dispersed in a dispersion medium such as NMP, to the surface of the positive electrode current collector and drying the applied layer. The dried coating may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector. The positive electrode mixture includes a positive electrode active material as an essential component and may include a binder, a conductive agent, etc. as optional components.

[0077] As the positive electrode active material, for example, a composite oxide containing lithium and a transition metal is used. Examples of the transition metal include Ni, Co, Mn, etc. Examples of the composite oxide containing lithium and a transition metal include, for example, Li a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b M 1-b O c 、Li a Ni 1-b M b O c 、Li a Mn2O4, Li a Mn 2-b M b O4. Here, a = 0 to 1.2, b = 0 to 0.9, c = 2.0 to 2.3. M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Note that the a value indicating the molar ratio of lithium increases or decreases by charge and discharge.

[0078] Among them, Li a Ni b M 1-b O2 (0 < a ≤ 1.2, 0.3 ≤ b ≤ 1, and M is at least one selected from the group consisting of Mn, Co, and Al.) is preferable. From the viewpoint of increasing the capacity, it is more preferable to satisfy 0.85 ≤ b ≤ 1. From the viewpoint of the stability of the crystal structure, Li a Ni b Co c Al d O2 (0 < a ≤ 1.2, 0.85 ≤ b < 1, 0 < c ≤ 0.15, 0 < d ≤ 0.1, b + c + d = 1) is more preferable.

[0079] The binder may be the resin material exemplified for the negative electrode. The conductive agent may be the same as the material exemplified for the negative electrode. Graphite such as natural graphite or artificial graphite may also be used as the conductive agent.

[0080] The shape and thickness of the positive electrode current collector can be selected from the shape and range corresponding to those of the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0081] (non-aqueous electrolyte) The non-aqueous electrolyte contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the non-aqueous electrolyte is preferably, for example, 0.5 mol / L or more and 2 mol / L or less. By setting the lithium salt concentration within the above range, a non-aqueous electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0082] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0083] Examples of lithium salts include LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, and LiB 10 Cl 10Examples of the lithium salt include lithium phosphates, lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, imide salts, etc. Examples of the borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO2)2), lithium bistrifluoromethanesulfonyl imide (LiN(CF3SO2)2), lithium trifluoromethanesulfonate nonafluorobutanesulfonyl imide (LiN(CF3SO2)(C4F9SO2)), lithium bispentafluoroethanesulfonyl imide (LiN(C2F5SO2)2), etc. One type of lithium salt may be used alone, or two or more types may be used in combination.

[0084] (separator) It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0085] An example of the structure of a nonaqueous electrolyte secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and a nonaqueous electrolyte are housed in an outer casing. Alternatively, instead of a wound electrode group, other types of electrode groups may be used, such as a stacked electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The nonaqueous electrolyte secondary battery may be in any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a laminate shape.

[0086] Hereinafter, the structure of a prismatic nonaqueous electrolyte secondary battery as an example of the nonaqueous electrolyte secondary battery according to the present disclosure will be described with reference to Fig. 1. Fig. 1 is a schematic perspective view of a nonaqueous electrolyte secondary battery according to one embodiment of the present disclosure, with a portion cut away.

[0087] The battery includes a bottomed prismatic battery case 4, an electrode group 1, and a non-aqueous electrolyte housed within the battery case 4. The electrode group 1 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them to prevent direct contact. The electrode group 1 is formed by winding the negative electrode, positive electrode, and separator around a flat plate-shaped winding core and then removing the winding core.

[0088] One end of a negative electrode lead 3 is attached to the negative electrode current collector of the negative electrode by welding or the like. The other end of the negative electrode lead 3 is electrically connected to a negative electrode terminal 6 provided on the sealing plate 5 via a resin insulating plate. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. One end of a positive electrode lead 2 is attached to the positive electrode current collector of the positive electrode by welding or the like. The other end of the positive electrode lead 2 is connected to the back surface of the sealing plate 5 via an insulating plate. In other words, the positive electrode lead 2 is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The insulating plate separates the electrode group 1 from the sealing plate 5 and also separates the negative electrode lead 3 from the battery case 4. The periphery of the sealing plate 5 fits into the open edge of the battery case 4, and the fitting portion is laser-welded. In this way, the opening of the battery case 4 is sealed with the sealing plate 5. The electrolyte injection hole provided in the sealing plate 5 is closed by a sealing plug 8.

[0089] [Example] Examples of the present disclosure will be specifically described below, but the present invention is not limited to the following examples.

[0090] Examples 1 to 12 [Preparation of acrylic polymer] A polymer containing hydrophilic structural units derived from acrylic acid and hydrophobic structural units derived from methyl acrylate was obtained by the following procedure.

[0091] Acrylic acid (Osaka Organic Chemical Industry Co., Ltd.) was used as the hydrophilic monomer, and methyl acrylate (Mitsubishi Rayon Co., Ltd.) was used as the hydrophobic monomer. Both were used as they were distilled under reduced pressure. Lithium persulfate (Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the polymerization initiator. The polymer was produced by radical polymerization. First, a total of 40 g of acrylic acid and methyl acrylate, 350 g of acetonitrile, and 1.8 g of pure water were placed in a stoppered glass ampoule (reaction vessel). The mass ratio of acrylic acid to methyl acrylate added to the ampoule was set to the value (parts by mass) shown in Table 1. After thoroughly replacing the atmosphere with nitrogen, the ampoule was sealed and heated to raise the temperature inside the ampoule to 55°C. 0.04 g of polymerization initiator was then added, and the mixture was stirred end-over-end in a thermostatic chamber for 12 hours to proceed with the polymerization reaction to a degree of polymerization of 90%. Cooling of the reaction solution was then initiated, and after the internal temperature had cooled to 25°C, 20.6 g of lithium hydroxide monohydrate (LiOH·HO, manufactured by Kishida Chemical Co., Ltd.) powder was added to the reaction solution. The reaction solution was then stirred end-over-end at room temperature for 12 hours to obtain a slurry-like polymerization reaction solution. The polymerization reaction solution was centrifuged and the supernatant was removed. The precipitate was then redispersed in the same mass of acetonitrile as the polymerization reaction solution, and the polymer was precipitated by centrifugation and the supernatant was removed three times. The precipitate was then dried under reduced pressure at 80°C for 2 hours to remove the volatiles and obtain the polymer.

[0092] In Example 1, the ratio of hydrophobic structural units to the total of hydrophilic structural units and hydrophobic structural units in the obtained polymer was 2%. In Example 11, the ratio of hydrophobic structural units to the total of hydrophilic structural units and hydrophobic structural units in the obtained polymer was 0.5%. In Example 12, the ratio of hydrophobic structural units to the total of hydrophilic structural units and hydrophobic structural units in the obtained polymer was 5%.

[0093] [Preparation of negative electrode] An appropriate amount of water was added to the negative electrode mixture and mixed to obtain a negative electrode slurry. The negative electrode mixture was a mixture of a negative electrode active material, a binder, and a conductive agent.

[0094] The negative electrode active material was a mixture of Si-containing material and graphite (average particle size (D50) 25 μm). The mass ratio of the Si-containing material to the graphite was 10:90. The Si-containing material contained SiO 2 whose surface was coated with a conductive layer containing conductive carbon. x The conductive layer was coated with SiO particles (x=1, average particle size (D50) 5 μm). x The amount was 5 parts by mass per 100 parts by mass of the total of the particles and the conductive layer.

[0095] The binders used were an acrylic polymer, sodium carboxymethyl cellulose (CMC-Na), and styrene-butadiene rubber (SBR). The acrylic polymer used was the polymer obtained above.

[0096] The conductive material used was CNT (average diameter 1.6 nm, average length 5 μm) containing 90% or more single-walled CNT (SWCNT).

[0097] The CNT and polymer contents (mass ratio to the total negative electrode active material) in the negative electrode mixture were set to the values ​​shown in Table 1. The CMC-Na content in the negative electrode mixture was 1 mass% relative to the total negative electrode active material. The SBR content in the negative electrode mixture was 1.5 mass% relative to the total negative electrode active material.

[0098] Next, the negative electrode slurry was applied to the surface of the copper foil, the coating was dried, and then the copper foil was rolled to form a negative electrode mixture layer (thickness 80 μm, density 1.6 g / cm ) on both sides of the copper foil. 3 ) was formed to obtain a negative electrode.

[0099] [Preparation of positive electrode] Lithium-containing composite oxide (LiNi 0.8 Co 0.18 Al 0.02 2.5 parts by mass of acetylene black, 2.5 parts by mass of polyvinylidene fluoride, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) were added to 95 parts by mass of ethanol (O2) and mixed to obtain a positive electrode slurry. Next, the positive electrode slurry was applied to the surface of an aluminum foil, the coating was dried, and then rolled to form a positive electrode mixture layer (thickness 95 μm, density 3.6 g / cm) on both sides of the aluminum foil. 3) was formed to obtain a positive electrode.

[0100] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte was obtained by dissolving LiPF6 at a concentration of 1.0 mol / L in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio 3:7).

[0101] [Fabrication of non-aqueous electrolyte secondary battery] An Al positive electrode lead was attached to the positive electrode obtained above, and a Ni negative electrode lead was attached to the negative electrode obtained above. The positive electrode and negative electrode were spirally wound with a polyethylene thin film (separator) interposed therebetween in an inert gas atmosphere to produce a wound electrode assembly. The electrode assembly was housed in a bag-shaped exterior body formed of a laminate sheet having an Al layer, and the nonaqueous electrolyte was injected thereinto. The exterior body was then sealed to produce a nonaqueous electrolyte secondary battery. When the electrode assembly was housed in the exterior body, a portion of the positive electrode lead and the negative electrode lead were each exposed to the outside from the exterior body.

[0102] In Table 1, the nonaqueous electrolyte secondary batteries of Examples 1 to 12 are designated A1 to A12, respectively.

[0103] Comparative Example 1 A polymer was produced in the same manner as in Example 1, except that methyl acrylate was not charged into the reaction vessel during the production of the polymer. The obtained polymer did not contain hydrophobic structural units derived from methyl acrylate.

[0104] Battery B1 of Comparative Example 1 was produced in the same manner as Battery A1 of Example 1, except that in producing the negative electrode, the polymer obtained above that did not contain a hydrophobic structural unit was used.

[0105] Comparative Example 2 Battery B2 of Comparative Example 2 was produced in the same manner as battery A1 of Example 1, except that in the production of the negative electrode, CNT was not included in the negative electrode mixture.

[0106] Comparative Example 3 Battery B3 of Comparative Example 3 was produced in the same manner as battery A1 of Example 1, except that in the production of the negative electrode, no polymer was added to the negative electrode mixture.

[0107] Comparative Example 4 Battery B4 of Comparative Example 4 was produced in the same manner as Battery B1 of Comparative Example 1, except that in the production of the negative electrode, CNT was not included in the negative electrode mixture.

[0108] Comparative Example 5 Battery B5 of Comparative Example 5 was produced in the same manner as battery A1 of Example 1, except that in the production of the negative electrode, CNT and polymer were not contained in the negative electrode mixture.

[0109] The batteries obtained above were evaluated as follows.

[0110] [Charge / discharge cycle test] (charging) The battery was charged at a constant current of 0.5 C (180 mA) until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.05 C (18 mA).

[0111] (discharge) A constant current discharge was carried out at a current of 0.7 C (252 mA) until the voltage reached 2.5 V.

[0112] The rest time between charging and discharging was 10 minutes, and charging and discharging were carried out in an environment of 25°C.

[0113] Charge and discharge were repeated under the above conditions. The ratio (percentage) of the discharge capacity at the 100th cycle to the discharge capacity at the 1st cycle was calculated as the capacity retention rate. The evaluation results are shown in Table 1.

[0114] [Table 1]

[0115] Batteries A1 to A12 exhibited higher capacity retention rates than batteries B1 to B5. In particular, batteries A3 to A9, which had a CNT content of 0.004 mass% or more and 0.08 mass% or less and a polymer to CNT mass ratio of 0.6 or more and 225 or less, exhibited higher capacity retention rates.

[0116] Examples 13 to 24 In the preparation of the negative electrode, CNTs (average diameter 10 nm, average length 5 μm) containing 90% or more MWCNTs were used as the conductive agent. The mass ratio of acrylic acid and methyl acrylate added to the reaction vessel during the preparation of the polymer, and the content of CNTs and polymer in the negative electrode mixture (mass ratio to the total negative electrode active material) were set to the values ​​shown in Table 2.

[0117] Except for the above, batteries C1 to C12 of Examples 13 to 24 were produced and evaluated in the same manner as battery A1 of Example 1. The evaluation results are shown in Table 2.

[0118] [Table 2]

[0119] High capacity retention rates were also obtained in batteries C1 to C12. In particular, higher capacity retention rates were obtained in batteries C3 to C8, which had a CNT content of 0.15 mass % or more and 0.45 mass % or less and a polymer to CNT mass ratio of 1.3 to 2.7. [Industrial Applicability]

[0120] The nonaqueous electrolyte secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, and the like. [Explanation of symbols]

[0121] 1 electrode group 2 positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Seal

Claims

1. a negative electrode mixture including a negative electrode active material capable of electrochemically absorbing and desorbing lithium ions, carbon nanotubes, and a binder; the negative electrode active material includes a silicon-containing material, The binder contains an acrylic polymer, The polymer includes a hydrophilic structural unit having a carboxyl group and a hydrophobic structural unit.

2. The polymer is the hydrophilic structural unit derived from an ethylenically unsaturated carboxylic acid; General formula: H ２ C=CR １ -X (wherein, R １ is a hydrogen atom or a methyl group, and X is COOR ２ or a cyano group, and R ２ is a hydrocarbon group having 1 to 8 carbon atoms; The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1 , comprising:

3. 3. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein in the polymer, a ratio of the hydrophobic structural units to a total of the hydrophilic structural units and the hydrophobic structural units is 0.5% or more and 5% or less.

4. 3. The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein in the polymer, a ratio of the hydrophobic structural units to a total of the hydrophilic structural units and the hydrophobic structural units is 1% or more and 3% or less.

5. at least a part of the carboxyl groups forms a carboxylate salt, 5. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the salt includes a lithium salt.

6. 6. The negative electrode for a non-aqueous electrolyte secondary battery in accordance with claim 1, wherein the carbon nanotubes have an average diameter of 5 nm or less.

7. 7. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 6, wherein the content of the carbon nanotubes in the negative electrode mixture is 0.0025% by mass or more and 0.1% by mass or less with respect to the total amount of the negative electrode active material.

8. 7. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 6, wherein the content of the carbon nanotubes in the negative electrode mixture is 0.004% by mass or more and 0.08% by mass or less with respect to the total amount of the negative electrode active material.

9. 7. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 6, wherein a mass ratio of the polymer to the carbon nanotubes in the negative electrode mixture is 0.2 or more and 600 or less.

10. 7. The negative electrode for a non-aqueous electrolyte secondary battery in accordance with claim 6, wherein the mass ratio of the polymer to the carbon nanotubes in the negative electrode mixture is 0.6 or more and 225 or less.

11. 11. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 6, wherein 50% or more of the carbon nanotubes are single-walled carbon nanotubes.

12. 6. The negative electrode for a non-aqueous electrolyte secondary battery in accordance with claim 1, wherein the carbon nanotubes have an average diameter of more than 5 nm.

13. 13. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 12, wherein the content of the carbon nanotubes in the negative electrode mixture is 0.1 mass % or more and 0.5 mass % or less with respect to the total amount of the negative electrode active material.

14. 13. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 12, wherein the content of the carbon nanotubes in the negative electrode mixture is 0.15 mass % or more and 0.45 mass % or less with respect to the total amount of the negative electrode active material.

15. 13. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 12, wherein in the negative electrode mixture, a mass ratio of the polymer to the carbon nanotubes is 0.5 or more and 3.8 or less.

16. 13 . The negative electrode for a non-aqueous electrolyte secondary battery according to claim 12 , wherein in the negative electrode mixture, a mass ratio of the polymer to the carbon nanotubes is 1.3 or more and 2.7 or less.

17. 17. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 12, wherein more than 50% of the carbon nanotubes are multi-walled carbon nanotubes.

18. 18. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the content of the polymer in the negative electrode mixture is 0.02 mass% or more and 1.5 mass% or less with respect to the entire negative electrode active material.

19. A positive electrode, a negative electrode, and a non-aqueous electrolyte, A non-aqueous electrolyte secondary battery, wherein the negative electrode is the negative electrode according to any one of claims 1 to 18.

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