Binder for secondary cell electrode, use thereof, and method for manufacturing binder for secondary cell electrode

Abstract

The present invention provides a binder for a secondary cell electrode, said binder being capable of improving toughness characteristics of a binder coating after electrolyte submersion, electrolyte resistance characteristics of a secondary cell electrode mixture layer, and cycle characteristics of a secondary cell.　Provided is a binder for a secondary cell electrode, said binder containing a carboxyl group-containing polymer or a salt thereof. The carboxyl group-containing polymer includes, with respect to all structural units thereof, 15 mass% to 99.9 mass% inclusive of a structural unit derived from an ethylenically unsaturated carboxylic monomer, and 0.1 mass% to 85 mass% inclusive of a structural unit derived from a keto group-containing ethylenically unsaturated monomer. At least some of the keto groups are functional groups used to form chemical bonds with compounds having reactivity with the keto groups.

Description

Binder for secondary battery electrodes, use thereof, and method for manufacturing binder for secondary battery electrodes

[0001] The present invention relates to a binder for secondary battery electrodes, its use, and a method for producing a binder for secondary battery electrodes.

[0002] Various secondary batteries, such as nickel-metal hydride secondary batteries, lithium-ion secondary batteries, and electric double layer capacitors, have been put to practical use. The electrodes used in these secondary batteries are prepared by applying a composition for forming an electrode mixture layer containing an active material and a binder to a current collector, followed by drying. For example, in lithium-ion secondary batteries, an aqueous binder containing styrene butadiene rubber (SBR) latex and carboxymethyl cellulose (CMC) is used as the binder for the negative electrode mixture layer. On the other hand, a solution of polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) is widely used as the binder for the positive electrode mixture layer.

[0003] In recent years, as the applications of various secondary batteries have expanded, there has been a growing demand for improved energy density, reliability, and durability. For example, in order to increase the electrical capacity of lithium-ion secondary batteries, the use of silicon-based active materials as negative electrode active materials has become more common. However, silicon-based active materials are known to undergo large volume changes during charging and discharging, which can lead to peeling or detachment of the electrode mixture layer with repeated use, resulting in a decrease in battery capacity and deterioration of cycle characteristics (durability). To address these issues, studies have been conducted to improve durability by using a binder to firmly bond the active materials together (adhesion), reducing the size of the active materials to alleviate stress associated with swelling and shrinkage, or by using innovative additives in the electrolyte.

[0004] In this context, it has been reported that acrylic acid polymers are effective as binders that have good cycle characteristics and are effective in improving the durability of negative electrode mixture layers using silicon-based active materials. Patent Document 1 discloses a binder containing a cross-linked acrylic acid polymer in which polyacrylic acid is cross-linked with a specific cross-linking agent, and discloses that even when an active material containing silicon is used, the binder exhibits good cycle characteristics without destroying the electrode structure. While the binder disclosed in Patent Document 1 can impart good cycle characteristics, binders that can achieve even better cycle characteristics are required as the performance of secondary batteries improves.

[0005] As an example of a binder for secondary battery electrodes that can improve the cycle characteristics of secondary batteries, Patent Document 2 discloses a binder for secondary battery electrodes that includes a binder (binding agent) containing a copolymer of an alkali metal neutralized ethylenically unsaturated carboxylic acid and vinyl alcohol, and a crosslinking agent having two or more functional groups reactive with carboxyl groups and / or hydroxyl groups in the binder. Patent Document 3 also discloses a binder for secondary battery electrodes that includes resin fine particles containing 0.5 to 5 mass% of an ethylenically unsaturated carboxylic acid monomer and / or an amide group-containing ethylenically unsaturated monomer, and 0.1 to 10 mass% of structural units derived from a keto group-containing ethylenically unsaturated monomer, and a polyfunctional hydrazide compound having two or more hydrazide groups reactive with the keto groups.

[0006] International Publication No. 2014 / 065407 International Publication No. 2019 / 054348 Japanese Patent Application Laid-Open No. 2011-134618

[0007] In both of the binders for secondary battery electrodes disclosed in Patent Documents 2 and 3, good cycle characteristics can be obtained by the reaction between functional groups in the binder and functional groups in the crosslinking agent, but these are sometimes insufficient. In addition, the toughness of the binder coating film after immersion in an electrolyte solution is insufficient, and the electrolyte resistance of the secondary battery electrode mixture layer is insufficient, which can be problematic.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a binder for secondary battery electrodes that can improve the toughness of a binder coating film after immersion in an electrolyte, the electrolyte resistance of a secondary battery electrode mixture layer, and the cycle characteristics of a secondary battery. The present invention also provides a composition for a secondary battery electrode mixture layer that includes the binder, and a secondary battery electrode and a secondary battery obtained using the composition.

[0009] As a result of intensive research to solve the above-mentioned problems, the present inventors have found that by using a binder for secondary battery electrodes, which contains a carboxyl group-containing polymer or a salt thereof, which contains structural units derived from an ethylenically unsaturated carboxylic acid monomer and a specific amount of a keto group-containing ethylenically unsaturated monomer, and at least a part of the keto groups are functional groups used to form chemical bonds with a compound reactive with the keto group, the toughness of the binder coating film after immersion in an electrolyte, the electrolyte resistance of the secondary battery electrode mixture layer, and the cycle characteristics of the secondary battery are further improved, and have completed the present invention.

[0010] The present invention is as follows: [1] A binder for secondary battery electrodes containing a carboxyl group-containing polymer or a salt thereof, wherein the carboxyl group-containing polymer contains, relative to its total structural units, 15% by mass to 99.9% by mass of structural units derived from ethylenically unsaturated carboxylic acid monomers and 0.1% by mass to 85% by mass of structural units derived from keto group-containing ethylenically unsaturated monomers, and at least some of the keto groups are functional groups used to form chemical bonds with compounds reactive with the keto groups. [2] The binder for secondary battery electrodes according to [1], further containing a compound having two or more functional groups reactive with keto groups (hereinafter referred to as a "polyfunctional crosslinking agent"). [3] The binder for secondary battery electrodes according to [1] or [2], wherein the carboxyl group-containing polymer is a crosslinked polymer. [4] The binder for secondary battery electrodes according to [3], wherein the crosslinked polymer is obtained by polymerizing a monomer composition containing a non-crosslinkable monomer and a crosslinkable monomer (however, different from the polyfunctional crosslinking agent). [5] The binder for secondary battery electrodes according to [4], wherein the amount of the crosslinkable monomer used is 0.1 parts by mass or more and 2.0 parts by mass or less per 100 parts by mass of the total amount of the non-crosslinkable monomers. [6] The binder for secondary battery electrodes according to any one of [3] to [5], wherein the crosslinked polymer, after being neutralized to a degree of neutralization of 80 to 100 mol%, has a volume-based median particle size measured in an aqueous medium of 0.1 μm or more and 10.0 μm or less. [7] The binder for secondary battery electrodes according to any one of [2] to [6], wherein the polyfunctional crosslinking agent includes a polyfunctional crosslinking agent having a hydrazide group. [8] A composition for a secondary battery electrode mix layer, comprising the binder for a secondary battery electrode according to any one of [2] to [7], an active material, and water. [9] The composition for a secondary battery electrode mix layer according to [8], wherein the active material comprises a silicon-based active material.

[10] A secondary battery electrode, comprising a mix layer formed from the composition for a secondary battery electrode mix layer according to [8] or [9] on a surface of a current collector.

[11] A secondary battery, comprising the secondary battery electrode according to

[10] .

[12] A method for producing a binder for a secondary battery electrode containing a carboxyl group-containing polymer or a salt thereof, characterized in that the carboxyl group-containing polymer is obtained by a method comprising a step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer and a monomer component containing a keto group-containing ethylenically unsaturated monomer by precipitation polymerization or dispersion polymerization.

[13] The method according to

[12] , wherein the precipitation polymerization or dispersion polymerization comprises a step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer and a step of adding a monomer component containing a keto group-containing ethylenically unsaturated monomer during the precipitation polymerization or dispersion polymerization.

[14] The method according to

[12] or

[13] , wherein the carboxyl group-containing polymer contains 15% by mass or more and 99.9% by mass or less of the ethylenically unsaturated carboxylic acid monomer and 0.1% by mass or more and 85% by mass or less of the keto group-containing ethylenically unsaturated monomer.

[0011] The binder for secondary battery electrodes of the present invention can improve the toughness of the binder coating film after immersion in an electrolyte, the electrolyte resistance of the secondary battery electrode mixture layer, and the cycle characteristics of the secondary battery.

[0012] The binder for secondary battery electrodes of the present invention (hereinafter also referred to as "the binder") contains a carboxyl group-containing polymer (hereinafter also referred to as "the polymer") or a salt thereof (hereinafter also referred to as "the polymer salt") containing specific amounts of an ethylenically unsaturated carboxylic acid monomer and a keto group-containing ethylenically unsaturated monomer. The binder can be mixed with a compound having two or more functional groups reactive with the keto group (hereinafter also referred to as a "multifunctional crosslinker"), an active material, and water to form a composition for secondary battery electrode mixture layers (hereinafter also referred to as "the composition"). The composition is preferably in the form of an electrode slurry that can be applied to a current collector in order to achieve the effects of the present invention. However, the composition may also be prepared in the form of a wet powder to allow for press processing onto the current collector surface. The secondary battery electrode of the present invention can be obtained by forming a mixture layer from the composition on the surface of a current collector such as copper foil or aluminum foil. Here, the present binder is preferred in that the effects of the present invention are particularly large when used in a secondary battery electrode mixture layer composition containing a silicon-based active material described below as the active material.

[0013] The following describes in detail the polyfunctional crosslinking agent, the present polymer and its production method, the composition for a secondary battery electrode mixture layer obtained using the present binder, the secondary battery electrode, and the secondary battery. In this specification, "(meth)acrylic" means acrylic and / or methacrylic, "(meth)acrylate" means acrylate and / or methacrylate, and "(meth)acryloyl group" means acryloyl group and / or methacryloyl group.

[0014] 1. Polyfunctional Crosslinking Agent The polyfunctional crosslinking agent is a compound having two or more functional groups reactive with the keto group contained in the binder. Examples of functional groups possessed by the polyfunctional crosslinking agent include a hydrazide group, a semicarbazide group, and a hydrazone group. Examples of polyfunctional crosslinking agents having a hydrazide group include aliphatic dicarboxylic acid dihydrazides such as maleic acid dihydrazide, oxalic acid dihydrazide, malonic acid dihydrazide, succinic acid dihydrazide, glutaric acid dihydrazide, adipic acid dihydrazide, and sebacic acid dihydrazide; alicyclic dicarboxylic acid dihydrazides such as trans-1,4-cyclohexanedicarbohydrazide; aromatic dicarboxylic acid dihydrazides such as isophthalic acid dihydrazide, terephthalic acid dihydrazide, and pyromellitic acid dihydrazide; and polycarboxylic acid hydrazides such as pyromellitic acid trihydrazide or tetrahydrazide and polyacrylic acid polyhydrazide. Examples of polyfunctional crosslinking agents having a semicarbazide group include aliphatic bissemicarbazides such as 1,4-tetramethylenebis-N,N-dimethylsemicarbazide and 1,6-hexamethylenebis-N,N-dimethylsemicarbazide, alicyclic bissemicarbazides such as a reaction product of isophorone diisocyanate or a polyisocyanate obtained from isophorone diisocyanate with hydrazine, and aromatic bissemicarbazides such as 1,1,1',1'-tetramethyl-4,4'-(methylene-di-para-phenylene)disemicarbazide. Examples of polyfunctional crosslinking agents having a hydrazone group include aliphatic dihydrazones such as bisacetyldihydrazone. Among the above-mentioned polyfunctional crosslinking agents, it is preferable to use a compound with a relatively low molecular weight (about 300 or less) because it has appropriate hydrophilicity, which makes it easy to disperse it in an aqueous composition and allows a secondary electrode mixture layer with a uniform crosslinked structure to be obtained. In particular, aliphatic dihydrazide compounds having 4 to 12 carbon atoms, such as succinic acid dihydrazide, glutaric acid dihydrazide, adipic acid dihydrazide, and sebacic acid dihydrazide, are preferred.

[0015] 2. The Present Polymer The present polymer has a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as "component (a)") and a structural unit derived from a keto group-containing ethylenically unsaturated monomer (hereinafter also referred to as "component (b)"), and can be introduced into the polymer by precipitation polymerization or dispersion polymerization of a monomer component containing component (a) and component (b). The present polymer may be a crosslinked polymer (hereinafter also referred to as "the present crosslinked polymer") or a non-crosslinked polymer. The present crosslinked polymer and the present non-crosslinked polymer may be used alone or in combination. The present crosslinked polymer or the present non-crosslinked polymer may be used alone or in combination of two or more types.

[0016] <Structural Units Derived from Ethylenically Unsaturated Carboxylic Acid Monomers> The polymer contains structural units derived from component (a) and thus has a carboxyl group, which improves adhesion to a current collector and provides excellent lithium ion desolvation and ionic conductivity, resulting in an electrode with low resistance and excellent high-rate performance. Furthermore, the polymer is imparted with water-swelling properties, which enhances the dispersion stability of the active material and other components in the composition.

[0017] Examples of component (a) include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, and fumaric acid; (meth)acrylamidoalkylcarboxylic acids such as (meth)acrylamidohexanoic acid and (meth)acrylamidododecanoic acid; and carboxyl group-containing ethylenically unsaturated monomers or their (partially) alkali-neutralized products, such as succinic acid monohydroxyethyl (meth)acrylate, ω-carboxy-caprolactone mono(meth)acrylate, and β-carboxyethyl (meth)acrylate. One of these may be used alone, or two or more may be used in combination. Among these, compounds having an acryloyl group as a polymerizable functional group are preferred, with acrylic acid being particularly preferred, because they have a high polymerization rate, resulting in a polymer with a long primary chain length and good binder binding strength. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, a polymer with a high carboxyl group content can be obtained.

[0018] The content of component (a) in the present polymer is 15% by mass or more and 99.9% by mass or less, based on the total structural units of the present polymer. By including component (a) in this range, electrolyte resistance can be improved and the secondary battery electrode mixture layer can be made tough. A lower limit of 20.0% by mass or more is preferable to further improve electrolyte resistance and the toughness of the secondary battery electrode mixture layer. It may be, for example, 30% by mass or more, or, for example, 40.0% by mass or more, or, for example, 50.0% by mass or more. The upper limit is, for example, 96.0% by mass or less, or, for example, 90.0% by mass or less, or, for example, 80.0% by mass or less, or, for example, 70.0% by mass or less. The content range of component (a) can be a range that appropriately combines these lower and upper limits.

[0019] <Structural Units Derived from Keto Group-Containing Ethylenically Unsaturated Monomers> The present polymer has a structural unit derived from component (b), and thus has a keto group. This allows a crosslinked structure to be formed between the present polymers by reaction with a polyfunctional crosslinking agent, improving the toughness of the binder coating film after immersion in an electrolyte solution and making the secondary battery electrode mixture layer even tougher. This is presumably capable of improving the electrolyte resistance of the secondary battery electrode mixture layer and the cycle characteristics of the secondary battery.

[0020] Examples of the (b) component include keto group-containing (meth)acrylamides such as diacetone (meth)acrylamide; keto group-containing vinyl compounds such as N-vinylformamide, vinyl methyl ketone, and vinyl ethyl ketone; keto group-containing (meth)acrylates such as acetoacetoxyethyl (meth)acrylate, acetoacetoxypropyl (meth)acrylate, and acetoacetoxybutyl (meth)acrylate; and unsaturated aldehydes such as acrolein. One of these may be used alone, or two or more may be used in combination. Among the above, keto group-containing (meth)acrylamides and keto group-containing (meth)acrylates are preferred in terms of excellent electrolyte resistance, and diacetone (meth)acrylamide, acetoacetoxyethyl (meth)acrylate, acetoacetoxypropyl (meth)acrylate, and acetoacetoxybutyl (meth)acrylate are more preferred. Furthermore, keto group-containing (meth)acrylamides are even more preferred, and diacetone (meth)acrylamide is particularly preferred because it has even better electrolyte resistance.

[0021] The content of component (b) in the polymer is 0.1% by mass or more and 85% by mass or less, based on the total structural units of the polymer. By including component (b) in this range, the secondary battery electrode mixture layer can be made tougher. A lower limit of 0.5% by mass or more is preferable because it makes the secondary battery electrode mixture layer even tougher. It may be, for example, 2% by mass or more, or 4% by mass or more, or 10% by mass or more. The upper limit is, for example, 70% by mass or less, or 60% by mass or less, or 50% by mass or less, or 40% by mass or less. In particular, 35% by mass or less is preferred because it reduces the electrolyte swelling of the binder coating film and provides sufficient electrolyte resistance. The content range of component (b) can be a range that appropriately combines these lower and upper limits.

[0022] <Other Structural Units> In addition to the components (a) and (b), the present polymer may contain a structural unit derived from another ethylenically unsaturated monomer copolymerizable therewith (hereinafter also referred to as "component (c)"). Examples of component (c) include structural units derived from hydroxyl group-containing ethylenically unsaturated monomers (monomers represented by the following formula (1) and monomers represented by formula (2)), ethylenically unsaturated monomer compounds having anionic groups other than carboxyl groups such as sulfonic acid groups and phosphate groups, or nonionic ethylenically unsaturated monomers. These structural units can be introduced by copolymerizing an ethylenically unsaturated monomer compound having anionic groups other than carboxyl groups such as sulfonic acid groups and phosphate groups, or a monomer containing a nonionic ethylenically unsaturated monomer. CH ２ = C(R １ ) COOR ２ (1) [wherein, R １ represents a hydrogen atom or a methyl group, R ２ represents a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, (R ３ O) ｍ H or R ４ O[CO(CH ２ ) ５ O] ｎ H. Note that R ３ represents an alkylene group having 2 to 4 carbon atoms, R ４ represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15.] CH ２ = C(R ５ ) CONR ６ R ７ (2) [wherein, R ５ represents a hydrogen atom or a methyl group, R ６ represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, R ７ represents a hydrogen atom or a monovalent organic group.

[0023] The proportion of component (c) can be 0.1 mass% or more and 20 mass% or less relative to the total structural units of the polymer. The proportion of component (c) may be 0.5 mass% or more and 17.5 mass% or less, 1.0 mass% or more and 15 mass% or less, 2 mass% or more and 12.5 mass% or less, or 3 mass% or more and 10 mass% or less. Furthermore, when component (c) is contained in an amount of 0.1 mass% or more relative to the total structural units of the polymer, affinity to the electrolyte solution is improved, and therefore, the effect of improving lithium ion conductivity can also be expected.

[0024] Among the above-mentioned, the component (c) is preferably a hydroxyl group-containing ethylenically unsaturated monomer in terms of excellent binding properties of the binder containing the polymer salt. Also, from the viewpoint of obtaining an electrode with good flex resistance, a structural unit derived from a nonionic ethylenically unsaturated monomer is preferred, and examples of the nonionic ethylenically unsaturated monomer include (meth)acrylamide and its derivatives, nitrile group-containing ethylenically unsaturated monomers, and alicyclic structure-containing ethylenically unsaturated monomers.

[0025] The monomer represented by the above formula (1) is a (meth)acrylate compound having a hydroxyl group. ２ When R is a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, the number of the hydroxyl groups may be one or more. The monovalent organic group is not particularly limited, but examples thereof include alkyl groups which may have a linear, branched, or cyclic structure, as well as aryl groups and alkoxyalkyl groups. ２ (R ３ O) ｍ H or R ４ O[CO(CH ２ ) ５ O] ｎ If H, then R ３ or R ４ The alkylene group represented by may be linear or branched.

[0026] Examples of the monomer represented by the formula (1) include hydroxyalkyl (meth)acrylates having a hydroxyalkyl group having 1 to 8 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, hydroxyhexyl (meth)acrylate, and hydroxyoctyl (meth)acrylate; polyalkylene glycol mono(meth)acrylates, such as polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, polybutylene glycol mono(meth)acrylate, and polyethylene glycol-polypropylene glycol mono(meth)acrylate; dihydroxyalkyl (meth)acrylates, such as glycerin mono(meth)acrylate; caprolactone-modified hydroxymethacrylates (manufactured by Daicel Corporation, trade names "PLACCEL FM1," "PLACCEL FM5," etc.), caprolactone-modified hydroxyacrylates (manufactured by Daicel Corporation, trade names "PLACCEL FA1," "PLACCEL FA10L," etc.). The monomer represented by the formula (1) may be used singly or in combination of two or more.

[0027] The monomer represented by the formula (2) is a (meth)acrylamide derivative having a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms. ７ represents a hydrogen atom or a monovalent organic group. The monovalent organic group is not particularly limited, but examples thereof include alkyl groups which may have a linear, branched, or cyclic structure, as well as aryl groups and alkoxyalkyl groups, and is preferably an organic group having 1 to 8 carbon atoms. In addition, R ７ may be a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms.

[0028] Examples of the monomer represented by the formula (2) include hydroxy(meth)acrylamide; (meth)acrylamide derivatives having a hydroxyalkyl group having 1 to 8 carbon atoms, such as N-hydroxyethyl(meth)acrylamide, N-hydroxypropyl(meth)acrylamide, N-hydroxybutyl(meth)acrylamide, N-hydroxyhexyl(meth)acrylamide, and N-hydroxyoctyl(meth)acrylamide, N-methylhydroxyethyl(meth)acrylamide, and N-ethylhydroxyethyl(meth)acrylamide; and N,N-dihydroxyalkyl(meth)acrylamides, such as N,N-dihydroxyethyl(meth)acrylamide and N,N-dihydroxyethyl(meth)acrylamide. The monomer represented by the formula (2) may be used singly or in combination of two or more.

[0029] Examples of the (meth)acrylamide derivative include N-alkyl(meth)acrylamide compounds such as N-isopropyl(meth)acrylamide and N-t-butyl(meth)acrylamide; N-alkoxyalkyl(meth)acrylamide compounds such as N-n-butoxymethyl(meth)acrylamide and N-isobutoxymethyl(meth)acrylamide; and N,N-dialkyl(meth)acrylamide compounds such as N,N-dimethyl(meth)acrylamide and N,N-diethyl(meth)acrylamide. These may be used alone or in combination of two or more.

[0030] Examples of the nitrile group-containing ethylenically unsaturated monomer include (meth)acrylonitrile; (meth)acrylic acid cyanoalkyl ester compounds such as cyanomethyl (meth)acrylate and cyanoethyl (meth)acrylate; cyano group-containing unsaturated aromatic compounds such as 4-cyanostyrene and 4-cyano-α-methylstyrene; vinylidene cyanide; and the like. One of these may be used alone, or two or more may be used in combination. Of the above, acrylonitrile is preferred because of its high nitrile group content.

[0031] Examples of the alicyclic structure-containing ethylenically unsaturated monomer include (meth)acrylic acid cycloalkyl esters which may have an aliphatic substituent, such as cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, cyclodecyl (meth)acrylate, and cyclododecyl (meth)acrylate; isobornyl (meth)acrylate, adamantyl (meth)acrylate, cyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and cycloalkyl polyalcohol mono(meth)acrylates such as cyclohexanedimethanol mono(meth)acrylate and cyclodecanedimethanol mono(meth)acrylate; and the like. These may be used alone or in combination of two or more.

[0032] In view of the excellent binding property of the binder, the present polymer preferably contains structural units derived from the monomer represented by the above formula (1), the monomer represented by the above formula (2), (meth)acrylamide and its derivatives, and nitrile group-containing ethylenically unsaturated monomers, alicyclic structure-containing ethylenically unsaturated monomers, etc. As component (c), in view of the excellent effect of improving the binding property of the present binder, a hydroxyalkyl (meth)acrylate having a hydroxyalkyl group having 1 to 8 carbon atoms is more preferred, and 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate are even more preferred.

[0033] Furthermore, when a structural unit derived from a hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less is introduced as component (c), it can exhibit strong interaction with the electrode material and exhibit good binding properties to the active material. This allows for a firm and well-integrated electrode mixture layer to be obtained, and therefore, an alicyclic structure-containing ethylenically unsaturated monomer is particularly preferred as the "hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less."

[0034] Other nonionic ethylenically unsaturated monomers that may be used include, for example, (meth)acrylic acid esters. Examples of (meth)acrylic acid esters include (meth)acrylic acid alkyl ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; aromatic (meth)acrylic acid ester compounds such as phenyl (meth)acrylate, phenylmethyl (meth)acrylate, phenylethyl (meth)acrylate, and phenoxyethyl (meth)acrylate; and (meth)acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate. These may be used alone or in combination of two or more.

[0035] From the viewpoints of binding strength with the active material and cycle characteristics, aromatic (meth)acrylic acid ester compounds are preferably used. From the viewpoints of further improving lithium ion conductivity and high-rate characteristics, compounds having an ether bond, such as (meth)acrylic acid alkoxyalkyl esters such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate, are preferred, with 2-methoxyethyl (meth)acrylate being more preferred.

[0036] Among nonionic ethylenically unsaturated monomers, compounds having an acryloyl group are preferred because they have a fast polymerization rate, resulting in a polymer with a long primary chain length, and provide good binder binding strength.Furthermore, as the nonionic ethylenically unsaturated monomer, compounds whose homopolymer has a glass transition temperature (Tg) of 0°C or less are preferred because the resulting electrode has good flex resistance.

[0037] The salt of the present polymer is in the form of a salt in which some or all of the carboxyl groups contained in the polymer are neutralized. The type of salt is not particularly limited, but examples include alkali metal salts such as lithium salts, sodium salts, and potassium salts; alkaline earth metal salts such as magnesium salts, calcium salts, and barium salts; other metal salts such as aluminum salts; ammonium salts, and organic amine salts. Among these, alkali metal salts and alkaline earth metal salts are preferred, and alkali metal salts are more preferred, as they are less likely to adversely affect battery characteristics.

[0038] Regarding the present crosslinked polymer: The present polymer is preferably a polymer having a crosslinked structure (the present crosslinked polymer) in that it can achieve both electrolyte resistance and cycle characteristics. The crosslinking method for the present crosslinked polymer is not particularly limited, and examples thereof include the following methods: 1) Copolymerization of a crosslinkable monomer (however, different from the polyfunctional crosslinking agent) 2) Utilization of chain transfer to polymer chains during radical polymerization Because the present crosslinked polymer has a crosslinked structure, a binder containing the present crosslinked polymer salt can have excellent binding strength. Among the above methods, the method of copolymerizing a crosslinkable monomer is preferred because of its simple operation and ease of controlling the degree of crosslinking.

[0039] <Crosslinkable Monomer> Examples of the crosslinkable monomer include a polyfunctional polymerizable monomer having two or more polymerizable unsaturated groups, and a monomer having a self-crosslinkable crosslinkable functional group such as a hydrolyzable silyl group, etc. However, these are different from the polyfunctional crosslinking agent.

[0040] The polyfunctional polymerizable monomer is a compound having two or more polymerizable functional groups such as (meth)acryloyl groups and alkenyl groups in the molecule, and examples thereof include polyfunctional (meth)acryloyl compounds, polyfunctional alkenyl compounds, and compounds having both (meth)acryloyl groups and alkenyl groups. These compounds may be used alone or in combination of two or more. Among these, polyfunctional alkenyl compounds are preferred in that they are easy to obtain a uniform crosslinked structure, and polyfunctional allyl ether compounds having two or more allyl ether groups in the molecule are particularly preferred.

[0041] Examples of polyfunctional (meth)acryloyl compounds include di(meth)acrylates of dihydric alcohols such as ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, polyethylene glycol di(meth)acrylate, and polypropylene glycol di(meth)acrylate; poly(meth)acrylates such as tri(meth)acrylate and tetra(meth)acrylate of trihydric or higher polyhydric alcohols such as trimethylolpropane tri(meth)acrylate, tri(meth)acrylate of ethylene oxide-modified trimethylolpropane, glycerin tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and pentaerythritol tetra(meth)acrylate; and bisamides such as methylenebisacrylamide and hydroxyethylenebisacrylamide.

[0042] Examples of polyfunctional alkenyl compounds include polyfunctional allyl ether compounds such as trimethylolpropane diallyl ether, trimethylolpropane triallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, tetraallyloxyethane, and polyallylsucrose; polyfunctional allyl compounds such as diallyl phthalate; and polyfunctional vinyl compounds such as divinylbenzene.

[0043] Examples of the compound having both a (meth)acryloyl group and an alkenyl group include allyl (meth)acrylate, isopropenyl (meth)acrylate, butenyl (meth)acrylate, pentenyl (meth)acrylate, and 2-(2-vinyloxyethoxy)ethyl (meth)acrylate.

[0044] Specific examples of the monomer having a self-crosslinkable crosslinkable functional group include a hydrolyzable silyl group-containing vinyl monomer, N-methoxyalkyl(meth)acrylamide, etc. These compounds can be used alone or in combination of two or more.

[0045] The hydrolyzable silyl group-containing vinyl monomer is not particularly limited as long as it is a vinyl monomer having at least one hydrolyzable silyl group.For example, it can be mentioned vinyl silanes such as vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane, vinyldimethylmethoxysilane, etc.; silyl group-containing acrylic esters such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, methyldimethoxysilylpropyl acrylate, etc.; silyl group-containing methacrylic esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, dimethylmethoxysilylpropyl methacrylate, etc.; silyl group-containing vinyl ethers such as trimethoxysilylpropyl vinyl ether, etc.; silyl group-containing vinyl esters such as vinyl trimethoxysilylundecanoate, etc.

[0046] When the crosslinked polymer is crosslinked by a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.01 to 5.0 parts by mass, more preferably 0.05 to 3.0 parts by mass, even more preferably 0.1 to 2.0 parts by mass, even more preferably 0.1 to 1.7 parts by mass, and even more preferably 0.5 to 1.5 parts by mass, relative to 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers). The range of the amount of the crosslinkable monomer used can be a range that appropriately combines these lower and upper limits. If the amount of the crosslinkable monomer used is 0.01 parts by mass or more, it is preferable in that the binding properties and sedimentation stability of the electrode slurry are better. If it is 5.0 parts by mass or less, the stability of precipitation polymerization or dispersion polymerization tends to be higher.

[0047] For the same reason, the amount of the crosslinkable monomer used is preferably 0.001 to 2.5 mol %, more preferably 0.01 to 2.0 mol %, even more preferably 0.05 to 1.75 mol %, even more preferably 0.05 to 1.5 mol %, and still more preferably 0.1 to 1.0 mol % relative to the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers). The range of the amount of the crosslinkable monomer used can be a range that appropriately combines these lower and upper limits.

[0048] Regarding the characteristics of the crosslinked polymer salt, the crosslinked polymer is preferably used in the form of a salt in which acid groups such as carboxyl groups derived from the ethylenically unsaturated carboxylic acid monomer are neutralized so that the degree of neutralization in the composition is 20 mol% or more. The degree of neutralization is more preferably 50 mol% or more, even more preferably 70 mol% or more, even more preferably 75 mol% or more, even more preferably 80 mol% or more, and particularly preferably 85 mol% or more. The upper limit of the degree of neutralization is 100 mol%, or may be 98 mol% or 95 mol%. The range of the degree of neutralization can be an appropriate combination of the above lower and upper limits, and may be, for example, 50 mol% or more to 100 mol% or less, 75 mol% or more to 100 mol% or less, or 80 mol% or more to 100 mol% or less. A degree of neutralization of 20 mol% or more is preferred in that it provides good water swelling properties and makes it easier to achieve a dispersion stabilization effect. In this specification, the degree of neutralization can be calculated from the amounts of a monomer having an acid group such as a carboxyl group and a neutralizing agent used for neutralization. The degree of neutralization can be confirmed by measuring the powder of the crosslinked polymer salt after drying it at 80°C under reduced pressure for 3 hours, and then measuring the intensity ratio of the peak derived from the C=O group of the carboxylic acid to the peak derived from the C=O group of the carboxylate salt.

[0049] <Particle size of the present crosslinked polymer salt> In the present composition, it is preferable that the present crosslinked polymer salt is not present as large particle size agglomerates (secondary aggregates) but is well dispersed as water-swellable particles having an appropriate particle size, because this allows a binder containing the crosslinked polymer salt to exhibit good binding performance.

[0050] The crosslinked polymer preferably has a volume-based median particle size (water-swollen particle size) of 0.1 μm or more and 10.0 μm or less when dispersed in water and the degree of neutralization based on the carboxyl groups of the crosslinked polymer is 80 to 100 mol%. A more preferred range for the particle size is 0.15 μm or more and 8.0 μm or less, an even more preferred range is 0.20 μm or more and 6.0 μm or less, an even more preferred range is 0.25 μm or more and 4.0 μm or less, and an even more preferred range is 0.30 μm or more and 2.0 μm or less. When the particle size is in the range of 0.30 μm or more and 2.0 μm or less, the particles are uniformly present in the composition at a suitable size, thereby enabling the composition to exhibit high stability and excellent binding properties. If the particle size exceeds 10.0 μm, there is a risk of insufficient binding properties, as described above. Furthermore, it is difficult to obtain a smooth coating surface, which may result in insufficient coatability. On the other hand, if the particle size is less than 0.1 μm, there is a concern in terms of stable production.

[0051] <Aqueous Solution Viscosity of the Present Crosslinked Polymer Salt> The present crosslinked polymer salt preferably has a viscosity of 100 mPa·s or more in a 2% by mass aqueous solution. When the viscosity of the 2% by mass aqueous solution is 100 mPa·s or more, the storage stability of the composition containing the crosslinked polymer is high and it is possible to exhibit excellent binding properties. The viscosity of the 2% by mass aqueous solution may be 1,000 mPa·s or more, 10,000 mPa·s or more, or 50,000 mPa·s or more. The aqueous solution viscosity can be obtained by uniformly dissolving or dispersing an amount of the present crosslinked polymer salt to achieve a predetermined concentration in water, and then measuring the Brookfield viscosity (25°C) at 12 rpm according to the method described in the Examples.

[0052] This crosslinked polymer salt absorbs water and swells in water. Generally, when a crosslinked polymer has an appropriate degree of crosslinking, the greater the amount of hydrophilic groups contained in the crosslinked polymer, the more easily the crosslinked polymer absorbs water and swells. Furthermore, with regard to the degree of crosslinking, the lower the degree of crosslinking, the more easily the crosslinked polymer swells. However, even if the number of crosslinking points is the same, the greater the molecular weight (primary chain length), the more crosslinking points that contribute to the formation of a three-dimensional network, making the crosslinked polymer less likely to swell. Therefore, the viscosity of the crosslinked polymer aqueous solution can be adjusted by adjusting the amount of hydrophilic groups, the number of crosslinking points, and the primary chain length of the crosslinked polymer. In this case, the number of crosslinking points can be adjusted, for example, by the amount of crosslinkable monomer used, chain transfer reaction to the polymer chain, post-crosslinking reaction, etc. The primary chain length of the polymer can be adjusted by setting conditions related to the amount of radical generation, such as the initiator and polymerization temperature, and by selecting a polymerization solvent taking chain transfer, etc. into consideration.

[0053] <Water Swelling Degree of the Present Crosslinked Polymer Salt> In this specification, the water swelling degree is determined by the dry weight "(W Ａ ) g”, and the amount of water absorbed when the crosslinked polymer salt is swelled to saturation with water “(W Ｂ ) g" based on the following formula: (Water swelling degree) = {(W Ａ ) + (W Ｂ ) / (W Ａ )

[0054] The crosslinked polymer salt preferably has a water swelling degree at pH 8 of 20 or more and 80 or less. When the water swelling degree is within the above range, the crosslinked polymer salt swells appropriately in an aqueous medium, making it possible to ensure a sufficient adhesion area to the active material and current collector when forming an electrode mixture layer, and the binding property tends to be good. The water swelling degree may be, for example, 21 or more, 23 or more, 25 or more, 27 or more, or 30 or more. When the water swelling degree is 20 or more, the crosslinked polymer salt spreads on the surface of the active material or current collector, ensuring a sufficient adhesion area, thereby obtaining good binding property. The upper limit of the water swelling degree at pH 8 may be 75 or less, 70 or less, 65 or less, 60 or less, or 55 or less. When the water swelling degree exceeds 60, the viscosity of the electrode mixture layer composition (electrode slurry) containing the crosslinked polymer salt tends to increase, resulting in a lack of uniformity in the mixture layer, which may result in insufficient binding strength. Furthermore, there is a risk of a decrease in the coatability of the electrode slurry. The range of the water swelling degree at pH 8 can be set by appropriately combining the above upper and lower limits. The water swelling degree at pH 8 can be obtained by measuring the swelling degree of the crosslinked polymer salt in water of pH 8. As the water of pH 8, for example, ion-exchanged water can be used, and the pH value may be adjusted as necessary using an appropriate acid or alkali, or a buffer solution. The pH during measurement is, for example, in the range of 8.0±0.5, preferably in the range of 8.0±0.3, more preferably in the range of 8.0±0.2, and even more preferably in the range of 8.0±0.1. The measurement is performed at 25±5°C.

[0055] Those skilled in the art can adjust the water swelling degree by controlling the composition and structure of the crosslinked polymer salt. For example, the water swelling degree can be increased by introducing an acidic functional group or a highly hydrophilic structural unit into the crosslinked polymer. In addition, the water swelling degree of the crosslinked polymer can usually be increased by decreasing the crosslinking degree.

[0056] 3. Method for Producing the Polymer The polymer can be obtained by a method comprising the steps of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer and a monomer component containing a keto group-containing ethylenically unsaturated monomer by precipitation polymerization or dispersion polymerization. Precipitation polymerization is a method for producing a polymer by carrying out a polymerization reaction in a solvent that dissolves the raw monomers but does not substantially dissolve the resulting polymer. As the polymerization proceeds, the polymer particles grow larger through aggregation and growth, resulting in a dispersion of polymer particles in which primary particles of tens to hundreds of nanometers in size have secondary aggregation to several micrometers to several tens of micrometers. A dispersion stabilizer can be used to control the polymer particle size. Furthermore, the secondary aggregation can be suppressed by selecting the dispersion stabilizer, polymerization solvent, etc. Precipitation polymerization in which secondary aggregation is suppressed is generally also called dispersion polymerization.

[0057] Furthermore, the precipitation polymerization or dispersion polymerization preferably includes a step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer, and a step of adding and polymerizing a monomer component containing a keto group-containing ethylenically unsaturated monomer during the above step. This, particularly when producing a crosslinked polymer, makes it possible to surface-modify the extreme surfaces of the particles with the keto group-containing ethylenically unsaturated monomer. This promotes the reaction between the keto group and the polyfunctional crosslinker, which is presumed to improve the toughness of the binder coating film after immersion in an electrolyte, the electrolyte resistance of the secondary battery electrode mixture layer, and the cycle characteristics of the secondary battery. In the present invention, the above "during" refers to a time point of "0.3T to 0.8T" when T is the time from the start of the step of polymerizing the present monomer to the end of the step, and is preferably 0.4T to 0.8T, more preferably 0.5T to 0.8T, and even more preferably 0.5T to 0.7T, in that a binder containing the present polymer salt can exhibit the toughness of the binder coating film after immersion in an electrolyte, the electrolyte resistance of the secondary battery electrode mixture layer, and the cycle characteristics of the secondary battery. The above lower limit and upper limit can be set in any combination.

[0058] Furthermore, the carboxyl group-containing polymer may contain 15% by mass or more and 99.9% by mass or less of an ethylenically unsaturated carboxylic acid monomer and 0.1% by mass or more and 85% by mass or less of a keto group-containing ethylenically unsaturated monomer, the types and amounts of which are as described above.

[0059] The crosslinking method for the present crosslinked polymer is not particularly limited, and examples thereof include the above-mentioned methods. From the viewpoint of easy control of the degree of crosslinking, a method using copolymerization of a crosslinkable monomer is preferred, and the type and amount of the crosslinkable monomer to be used are as described above.

[0060] 4. Composition for Secondary Battery Electrode Mixture Layer The composition for secondary battery electrode mixture layer of the present invention contains the present binder, a polyfunctional crosslinking agent, an active material, and water. The amount of the present binder used in the composition is, for example, 0.1 parts by mass or more and 20 parts by mass or less, relative to 100 parts by mass of the total amount of active material. The amount used is, for example, 0.2 parts by mass or more and 10 parts by mass or less, such as 0.3 parts by mass or more and 8 parts by mass or less, or for example, 0.4 parts by mass or more and 5 parts by mass or less. When the amount of the binder used is 0.1 parts by mass or more, sufficient binding strength can be obtained. Furthermore, dispersion stability of the active material and the like can be ensured, and a uniform mixture layer can be formed. When the amount of the binder used is 20 parts by mass or less, the present composition does not become highly viscous, and coatability to the current collector can be ensured. As a result, a mixture layer with a uniform and smooth surface can be formed.

[0061] The amount of the polyfunctional crosslinking agent used in the composition is, for example, 0.01 to 45 parts by mass, relative to 100 parts by mass of the total amount of the binder. The amount used may also be, for example, 0.1 to 20 parts by mass, for example, 0.5 to 10 parts by mass, or for example, 1 to 5 parts by mass. In particular, when the amount of the polyfunctional crosslinking agent used is 0.5 parts by mass or more, a crosslinked structure between the polymers is sufficiently formed by the reaction between the keto groups in the polymer and the polyfunctional crosslinking agent, improving the toughness of the binder coating film after immersion in an electrolyte solution, the electrolyte resistance of the secondary battery electrode mixture layer, and the cycle characteristics of the secondary battery. Furthermore, when the amount of the polyfunctional crosslinking agent used is 5 parts by mass or less, the amount of unreacted polyfunctional crosslinking agent is reduced, improving the toughness of the binder coating film after immersion in an electrolyte solution, the electrolyte resistance of the secondary battery electrode mixture layer, and the cycle characteristics of the secondary battery. The range of the amount of the polyfunctional crosslinking agent used may be a range that appropriately combines these lower and upper limits.

[0062] Furthermore, in order to ensure the toughness of the binder coating film after immersion in an electrolyte, the electrolyte resistance of the secondary battery electrode mixture layer, and the cycle characteristics of the secondary battery, the amount (number of moles) of the polyfunctional crosslinking agent used is preferably 0.01 to 10 moles, more preferably 0.05 to 7.5 moles, even more preferably 0.1 to 5 moles, even more preferably 0.25 to 2.5 moles, and even more preferably 0.5 to 1.5 moles, relative to 1.0 mole of keto groups in the binder. The range of the amount of the polyfunctional crosslinking agent used can be a range that appropriately combines these lower and upper limits.

[0063] Among the above active materials, lithium salts of transition metal oxides can be used as the positive electrode active material. For example, layered rock salt type and spinel type lithium-containing metal oxides can be used. Specific compounds of the layered rock salt type positive electrode active material include lithium cobalt oxide, lithium nickel oxide, and ternary NCMs {Li(Ni ｘ , Co ｙ , Mn ｚ ), x+y+z=1} and NCA{Li(Ni １－ａ－ｂ Co ａ Al b)}, etc. Examples of spinel-type positive electrode active materials include lithium manganate, etc. In addition to oxides, phosphates, silicates, sulfur, etc. are also used, and examples of phosphates include olivine-type lithium iron phosphate, etc. As the positive electrode active material, one of the above may be used alone, or two or more may be combined and used as a mixture or composite.

[0064] When a positive electrode active material containing a layered rock salt-type lithium-containing metal oxide is dispersed in water, the lithium ions on the active material surface are exchanged with hydrogen ions in the water, resulting in an alkaline dispersion. This may result in corrosion of aluminum foil (Al), a common positive electrode current collector material. In such cases, it is preferable to neutralize the alkali eluted from the active material by using an unneutralized or partially neutralized present polymer as a binder. Furthermore, it is preferable to use an amount of the unneutralized or partially neutralized present polymer such that the amount of unneutralized carboxyl groups in the present polymer is equivalent to or greater than the amount of alkali eluted from the active material.

[0065] Because all positive electrode active materials have low electrical conductivity, they are generally used with the addition of a conductive additive. Examples of conductive additives include carbon-based materials such as carbon black, carbon nanotubes, carbon fiber, graphite powder, and carbon fiber. Of these, carbon black, carbon nanotubes, and carbon fiber are preferred because they are more likely to provide excellent conductivity. Furthermore, ketjen black and acetylene black are preferred as carbon black. The conductive additives may be used alone or in combination with two or more of the above. The amount of conductive additive used may be, for example, 0.2 to 20 parts by mass, or, for example, 0.2 to 10 parts by mass, per 100 parts by mass of the total active material, from the viewpoint of achieving both electrical conductivity and energy density. Furthermore, the positive electrode active material may be surface-coated with a conductive carbon-based material.

[0066] On the other hand, examples of negative electrode active materials include carbon-based materials, lithium metal, lithium alloys, and metal oxides, and one or more of these can be used in combination. Among these, active materials made of carbon-based materials such as natural graphite, artificial graphite, hard carbon, and soft carbon (hereinafter also referred to as "carbon-based active materials") are preferred, with graphite such as natural graphite and artificial graphite, and hard carbon being more preferred. In the case of graphite, spherical graphite is preferably used from the standpoint of battery performance, and its particle size preferably ranges from 1 to 20 μm, for example, and from 5 to 15 μm. In addition, to increase the energy density, metals or metal oxides capable of absorbing lithium, such as silicon and tin, can also be used as negative electrode active materials. Among these, silicon has a higher capacity than graphite, and active materials made of silicon-based materials such as silicon, silicon alloys, and silicon oxides such as silicon monoxide (SiO) (hereinafter also referred to as "silicon-based active materials") can be used. However, while the silicon-based active material has a high capacity, it undergoes a large volume change during charging and discharging. Therefore, it is preferable to use it in combination with the carbon-based active material. In this case, if the amount of silicon-based active material is large, the electrode material may be destroyed, resulting in a significant decrease in cycle characteristics (durability). From this perspective, when a silicon-based active material is used in combination, the amount of silicon-based active material used is, for example, 60% by mass or less, or, for example, 30% by mass or less, relative to the carbon-based active material.

[0067] Since the carbon-based active material itself has good electrical conductivity, it is not necessarily required to add a conductive additive. When a conductive additive is added for the purpose of further reducing resistance, etc., the amount used is, from the viewpoint of energy density, for example, 10 parts by mass or less, or, for example, 5 parts by mass or less, relative to 100 parts by mass of the total amount of the active material.

[0068] When the composition is in a slurry state, the amount of active material used is, for example, in the range of 10 to 75 mass %, or, for example, in the range of 30 to 65 mass %, based on the total amount of the composition. If the amount of active material used is 10 mass % or more, migration of binders and the like is suppressed, and this is also advantageous in terms of the cost of drying the medium. On the other hand, if the amount is 75 mass % or less, the fluidity and coatability of the composition can be ensured, and a uniform mixture layer can be formed.

[0069] The present composition uses water as a medium. Furthermore, for the purpose of adjusting the properties and drying properties of the present composition, the composition may be mixed with lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, or water-soluble organic solvents such as tetrahydrofuran and N-methyl-2-pyrrolidone. The proportion of water in the mixed medium is, for example, 50% by mass or more, or, for example, 70% by mass or more.

[0070] When the present composition is made into a coatable slurry state, the content of the water-containing medium in the entire present composition can be, for example, in the range of 25 to 60 mass %, and can also be, for example, 35 to 60 mass %, from the viewpoints of the coatability of the slurry, the energy cost required for drying, and productivity.

[0071] The present composition may further contain other binder components, such as styrene butadiene rubber (SBR)-based latex, carboxymethyl cellulose (CMC), acrylic latex, and polyvinylidene fluoride-based latex. When other binder components are used in combination, the amount thereof may be, for example, 0.1 to 5 parts by mass or less, or, for example, 0.1 to 2 parts by mass or less, or, for example, 0.1 to 1 part by mass or less, relative to 100 parts by mass of the total amount of active material. If the amount of other binder components used exceeds 5 parts by mass, resistance increases, and high-rate characteristics may become insufficient. Among the above, SBR-based latex and CMC are preferred in terms of their excellent balance of binding strength and flex resistance, and a combination of SBR-based latex and CMC is more preferred.

[0072] The SBR latex refers to an aqueous dispersion of a copolymer having structural units derived from an aromatic vinyl monomer such as styrene and structural units derived from an aliphatic conjugated diene monomer such as 1,3-butadiene. Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, vinyltoluene, and divinylbenzene, and one or more of these can be used. The structural units derived from the aromatic vinyl monomer in the copolymer can be, for example, in the range of 20 to 70% by mass, or, for example, in the range of 30 to 60% by mass, primarily from the viewpoint of binding properties. Examples of the aliphatic conjugated diene monomer include, for example, 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 2-chloro-1,3-butadiene, and one or more of these can be used. The structural units derived from the aliphatic conjugated diene monomer in the copolymer can be, for example, in the range of 30 to 70% by mass, or, for example, in the range of 40 to 60% by mass, in order to improve the binding properties of the binder and the flexibility of the resulting electrode. In addition to the above-mentioned monomers, the styrene / butadiene latex may also contain other monomers as copolymerization monomers, such as nitrile group-containing monomers such as (meth)acrylonitrile, carboxyl group-containing monomers such as (meth)acrylic acid, itaconic acid, and maleic acid, and ester group-containing monomers such as methyl (meth)acrylate, in order to further improve performance such as binding properties. The structural units derived from the other monomers in the copolymer can be, for example, in the range of 0 to 30% by mass, or, for example, in the range of 0 to 20% by mass.

[0073] The CMC refers to a nonionic cellulose-based semisynthetic polymer compound substituted with a carboxymethyl group and its salt. Examples of the nonionic cellulose-based semisynthetic polymer compound include alkyl celluloses such as methyl cellulose, methyl ethyl cellulose, ethyl cellulose, and microcrystalline cellulose; hydroxyethyl cellulose, hydroxybutyl methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose stearoxy ether, carboxymethyl hydroxyethyl cellulose, alkyl hydroxyethyl cellulose, and nonoxynyl hydroxyethyl cellulose.

[0074] The composition for a secondary battery electrode mixture layer of the present invention contains the above-mentioned binder, polyfunctional crosslinker, active material, and water as essential components, and is obtained by mixing the components using known means. The method for mixing the components is not particularly limited, and known methods can be used. However, a preferred method is to dry-blend powder components such as the active material, conductive additive, and binder, then mix them with a dispersion medium such as water and disperse and knead them. When obtaining the composition in a slurry state, it is preferable to finish the slurry without poor dispersion or aggregation. Known mixers such as planetary mixers, thin film gyratory mixers, and planetary / revolving mixers can be used as mixing means. However, thin film gyratory mixers are preferred because they can achieve a good dispersion state in a short time. When using a thin film gyratory mixer, it is also preferable to pre-disperse the slurry using a stirrer such as a disperser. The pH of the slurry is not particularly limited as long as the effects of the present invention are achieved, but is preferably less than 12.5. For example, when CMC is added, it is more preferable that it be less than 11.5, and even more preferable that it be less than 10.5, in order to reduce the risk of hydrolysis. The viscosity of the slurry is not particularly limited as long as it exhibits the effects of the present invention, but the viscosity may be, for example, in the range of 100 to 6,000 mPa s, or, for example, 500 to 5,000 mPa s, or, for example, 1,000 to 4,000 mPa s, as Brookfield viscosity (25°C) at 20 rpm. If the viscosity of the slurry is within the above range, good coatability can be ensured.

[0075] 5. Secondary Battery Electrode The secondary battery electrode of the present invention comprises a mixture layer formed from the composition for a secondary battery electrode mixture layer of the present invention on the surface of a current collector made of copper, aluminum, or the like. The mixture layer is formed by applying the composition to the surface of the current collector and then drying to remove the medium, such as water. The method for applying the composition is not particularly limited, and known methods such as doctor blade coating, dipping, roll coating, comma coating, curtain coating, gravure coating, and extrusion can be used. Furthermore, the drying can be performed by known methods such as hot air blowing, reduced pressure, (far) infrared radiation, and microwave irradiation. Typically, the mixture layer obtained after drying is subjected to a compression treatment using a mold press, roll press, or the like. Compression brings the active material and binder into close contact, improving the strength of the mixture layer and its adhesion to the current collector. Compression can adjust the thickness of the mixture layer to, for example, approximately 30 to 80% of the thickness before compression, and the thickness of the mixture layer after compression is typically approximately 4 to 200 μm.

[0076] 6. Secondary Battery A secondary battery can be produced by providing the secondary battery electrode of the present invention with a separator and an electrolyte. The electrolyte may be liquid or gel-like. The separator is disposed between the positive and negative electrodes of the battery and serves to prevent short circuits due to contact between the electrodes and to retain the electrolyte to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane that has good ion permeability and mechanical strength. Specific materials that can be used include polyolefins such as polyethylene and polypropylene, and polytetrafluoroethylene.

[0077] The electrolyte may be a known, commonly used one depending on the type of active material. Specific examples of the solvent for lithium ion secondary batteries include cyclic carbonates with high dielectric constants and high electrolyte dissolving ability, such as propylene carbonate and ethylene carbonate, and chain carbonates with low viscosity, such as ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate. These may be used alone or as a mixed solvent. The electrolyte may be prepared by dissolving LiPF in these solvents. ６ , LiSbF６ , LiBF ４ , LiClO ４ , LiAlO ４ In nickel-metal hydride secondary batteries, an aqueous solution of potassium hydroxide can be used as the electrolyte. A secondary battery is obtained by spirally or stacking positive and negative electrode plates separated by a separator and housing them in a case or the like.

[0078] The binder for secondary battery electrodes disclosed herein provides excellent toughness of the binder coating film after immersion in an electrolyte, and the electrode mixture layer for secondary batteries obtained using an electrode slurry containing the binder exhibits electrolyte resistance. Furthermore, secondary batteries equipped with electrodes obtained using the binder can ensure good integrity and exhibit good durability (cycle characteristics) even after repeated charge and discharge, making them suitable for use in automotive secondary batteries, etc.

[0079] The present invention will be specifically described below based on examples. However, the present invention is not limited to these examples. In the following, "parts" and "%" mean parts by mass and % by mass unless otherwise specified. In the following examples, the evaluation of the carboxyl group-containing polymer salt was carried out by the following method.

[0080] <<Production of Carboxyl Group-Containing Polymer Salt>> (Measurement of Particle Size (Water-Swelled Particle Size) in Aqueous Medium) 0.25 g of carboxyl group-containing crosslinked polymer salt powder and 49.75 g of ion-exchanged water were weighed into a 100 cc container and placed in a rotation / revolution mixer (Thinky Corporation, Awatori Rentaro AR-250). The mixture was then stirred (rotation speed 2,000 rpm / revolution speed 800 rpm, 7 minutes) and degassed (rotation speed 2,200 rpm / revolution speed 60 rpm, 1 minute) to produce a hydrogel in which the carboxyl group-containing crosslinked polymer salt was swollen in water. The particle size distribution of the hydrogel was then measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT-3300EXII, Microtrac Bell Corporation) using ion-exchanged water as a dispersion medium. When an excess amount of dispersion medium was circulated relative to the hydrogel, an amount of hydrogel sufficient to obtain an appropriate scattered light intensity was added, and the particle size distribution shape measured stabilized after a few minutes. Once stability was confirmed, the particle size distribution was measured and the volume-based median diameter (D50) was obtained as a representative value of the particle size.

[0081] (Production Example 1: Production of Carboxyl Group-Containing Polymer Salt R-1) For polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser, and a nitrogen inlet tube was used. The reactor was charged with 567 parts of acetonitrile, 2.2 parts of ion-exchanged water, 96 parts of acrylic acid (hereinafter referred to as "AA"), 4 parts of diacetone acrylamide, 0.9 parts of trimethylolpropane diallyl ether (manufactured by Osaka Soda Co., Ltd., trade name "Neoallyl T-20"), and triethylamine equivalent to 1.0 mol % of the AA. After thoroughly replacing the atmosphere inside the reactor with nitrogen, the reactor was heated to raise the internal temperature to 55°C. After confirming that the internal temperature had stabilized at 55°C, 0.040 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name "V-65") was added as a polymerization initiator. The reaction solution became cloudy, and this point was designated as the polymerization initiation point. The monomer concentration was calculated to be 15%. After 12 hours had passed from the start of polymerization, the polymerization reaction solution was started to be cooled, and after the internal temperature had dropped to 25°C, lithium hydroxide monohydrate (hereinafter referred to as "LiOH·H ２After the addition, stirring was continued at room temperature for 12 hours to obtain a polymerization reaction solution in the form of a slurry in which particles of the carboxyl group-containing polymer salt R-1 (lithium salt, degree of neutralization 90 mol%) were dispersed in the medium.

[0082] The resulting polymerization reaction solution was centrifuged to precipitate the polymer, and the supernatant was removed. The precipitate was then redispersed in acetonitrile of the same weight as the polymerization reaction solution, followed by a washing procedure of precipitating the polymer particles by centrifugation and removing the supernatant, which was repeated twice. The precipitate was collected and dried at 80°C for 3 hours under reduced pressure to remove the volatiles, yielding a powder of carboxyl group-containing polymer salt R-1. Because the carboxyl group-containing polymer salt R-1 is hygroscopic, it was stored sealed in a container with water vapor barrier properties. The powder of carboxyl group-containing polymer salt R-1 was subjected to IR analysis, and the degree of neutralization was determined from the intensity ratio of the peak derived from the C=O group of the carboxylic acid to the peak derived from the C=O of the lithium carboxylate. It was 90 mol%, equal to the calculated value from the starting material. The particle size in an aqueous medium was 1.52 μm.

[0083] (Production Example 2: Production of Carboxyl Group-Containing Polymer Salt R-2) The amounts of each raw material charged were as shown in Table 1, and the same operation as in Production Example 1 was carried out, except that diacetone acrylamide was added 7 hours after the start of polymerization (during the polymerization process corresponding to the time point of 0.58 T), to obtain a polymerization reaction liquid containing a carboxyl group-containing polymer salt R-2. Next, the same operation as in Production Example 1 was carried out for each polymerization reaction liquid to obtain a powdery carboxyl group-containing polymer salt R-2. Each carboxyl group-containing polymer salt was stored in a sealed container with water vapor barrier properties. The physical properties of each obtained polymer salt were measured in the same manner as in Production Example 1, and the results are shown in Table 1.

[0084] (Production Examples 3 to 12 and Comparative Production Examples 1 and 2: Production of Carboxyl Group-Containing Polymer Salts R-3 to R-14) The same operation as in Production Example 1 was carried out, except that the types and amounts of each raw material were as shown in Table 1, to obtain polymerization reaction solutions containing carboxyl group-containing polymer salts R-3 to R-14. Next, the same operation as in Production Example 1 was carried out for each polymerization reaction solution to obtain powdery carboxyl group-containing polymer salts R-3 to R-14. Each of the polymer salts was sealed and stored in a container with water vapor barrier properties. The physical properties of each of the obtained polymer salts were measured in the same manner as in Production Example 1, and the results are shown in Table 1.

[0085]

[0086] Details of the compounds used in Table 1 are shown below: AA: Acrylic acid HEAA: N-hydroxyethyl acrylamide DAAM: Diacetone acrylamide AAEM: Acetoacetoxyethyl methacrylate T-20: Trimethylolpropane diallyl ether (manufactured by Osaka Soda Co., Ltd., trade name "Neoallyl T-20") TEA: Triethylamine V-65: 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) LiOH.H ２ O: Lithium hydroxide monohydrate Na ２ CO ３ : Sodium carbonate ・K ２ CO ３ : Potassium carbonate

[0087] Example 1 (Preparation of binder coating film) Carboxyl group-containing polymer salt R-1, styrene / butadiene latex (SBR, sodium carboxymethyl cellulose (CMC), adipic acid dihydrazide, and ion-exchanged water were added to a container in the amounts shown in Table 2 and mixed, followed by pre-dispersion using a disper. After that, main dispersion was carried out for 15 seconds using a thin film rotary mixer (FM-56-30, manufactured by Primix Corporation) at a peripheral speed of 20 m / sec, thereby obtaining an aqueous binder solution. The aqueous binder solution was then poured into a disposable tray, air-dried at room temperature for one week, dried overnight at 40°C, and further vacuum-dried at 80°C for 12 hours. The binder coating film obtained after drying was punched out to a size of 1.0 cm x 6.0 cm to prepare test pieces, and toughness and electrolyte resistance were measured.

[0088] <Toughness of binder coating film> A tensile test was performed on a test piece prepared by punching out the binder coating film using a tensile tester (Tensilon, RTC-1210A manufactured by Orientec Co., Ltd.) at a speed of 10 mm / min to measure the Young's modulus [MPa]. As a result, the Young's modulus was 23.9 MPa. In addition, a tensile test was performed under the same conditions on the test piece used in the electrolyte swelling evaluation to measure the Young's modulus. As a result, the Young's modulus was 14.7 MPa, and the toughness based on the following criteria was evaluated as "A". Note that the higher the Young's modulus of the binder coating film after immersion in the electrolyte solution, the more excellent the toughness of the electrode mixture layer and the more improved the cycle characteristics can be. (Toughness evaluation criteria) A: Young's modulus after immersion in the electrolyte solution is 14.0 MPa or more B: Young's modulus after immersion in the electrolyte solution is 12.0 MPa or more but less than 14.0 MPa C: Young's modulus after immersion in the electrolyte solution is less than 12.0 MPa

[0089] <Electrolyte Resistance of Binder Coating Film> The test piece obtained above was immersed in an electrolytic solution prepared by mixing ethylene carbonate (EC):dimethyl carbonate (DMC) in a mass ratio of 1:3, and left to stand at 40°C for 2 hours. The test piece was then removed from the electrolytic solution, the surface was wiped, and the degree of swelling in the electrolytic solution was measured.

[0090] The method for measuring the swelling degree of the electrolyte solution is described below. The weight of the test piece before and after immersion in the electrolyte solution was measured as [W ０ (g)], [W１ (g)], the swelling degree of the electrolyte solution was calculated by the following formula: Swelling degree of the electrolyte solution (mass%) = (W １ ) / (W ０ ) x 100 According to the above formula, the electrolyte swelling was 115%, and the electrolyte resistance based on the following criteria was evaluated as "A". The lower the electrolyte swelling of the binder coating film, the more difficult it is for the electrode mixture layer to absorb electrolyte components and to swell in the electrolyte. (Criteria for determining electrolyte resistance) A: Electrolyte swelling degree less than 120% B: Electrolyte swelling degree 120% or more but less than 125% C: Electrolyte swelling degree 125% or more

[0091] Examples 2 to 15 and Comparative Examples 1 to 3 Binder coating films were prepared in the same manner as in Example 1, except that the formulations were as shown in Table 2, and the toughness and electrolyte resistance were evaluated. The results are shown in Table 2.

[0092]

[0093] Details of the compounds used in Table 2 are as follows: CMC: sodium carboxymethyl cellulose SBR: styrene butadiene rubber ADH: adipic acid dihydrazide SDH: succinic acid dihydrazide

[0094] Example 1 (Preparation of electrode mixture layer composition) Artificial graphite (manufactured by Showa Denko K.K., product name "SCMG-CF") and SiO (manufactured by Osaka Titanium Technologies Co., Ltd., 5 μm) were used as the active material. A mixture of crosslinked polymer salt R-1, styrene / butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) was used as the binder. Adipic acid dihydrazide (ADH) was used as the multifunctional crosslinking agent. Water was used as a dilution solvent to a planetary mixer (Hibismix 2P-03, manufactured by Primix Corporation) and the mixture was added in a mass ratio of graphite:Si-based active material:R-1:SBR:CMC:ADH=76.8:19.2:1.0:2.0:1.0:0.021 (solids content) so that the solids concentration of the electrode mixture layer composition was 53 mass%, and the mixture was mixed for 1 hour and 30 minutes to prepare a slurry-state electrode mixture layer composition (electrode slurry).

[0095] (Preparation of negative electrode plate) Next, the electrode slurry was applied onto a 16.5 μm thick current collector (copper foil) using a variable applicator, and dried in a forced-air dryer at 80° C. for 15 minutes to form a mixture layer. After that, the thickness of the mixture layer was 50±5 μm and the mixture density was 1.60±0.10 g / cm. ３ After rolling to a size of 1.0 cm x 6.0 cm for the peel strength test and a 3 cm square for battery evaluation were punched out to obtain negative electrode plates.

[0096] (Preparation of Positive Electrode Plate) In N-methylpyrrolidone (NMP) solvent, LiNi was used as a positive electrode active material. ０．５ Co ０．２ Mn ０．３ O ２ A mixture of 100 parts of ethylenediamine fluoride (NCM) and 2 parts of acetylene black was added to the mixture, and 4 parts of polyvinylidene fluoride (PVDF) was added as a positive electrode binder to prepare a positive electrode composite layer composition. The positive electrode composite layer composition was applied to an aluminum current collector (thickness: 20 μm) and dried to form a composite layer. Thereafter, the thickness of the composite layer was 125 μm, and the composite density was 3.0 g / cm. ３ After rolling to a thickness of 1 / 4, the mixture was punched out into a 3 cm square to obtain a positive electrode plate.

[0097] (Preparation of Electrolyte Solution) Vinylene carbonate (VC) was added to a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio of EC:DMC=3:7) so that the concentration of VC was 1 mass % and fluoroethylene carbonate (FEC) was 2 mass %, and LiPF ６ was dissolved in a concentration of 1.2 mol / L to prepare a non-aqueous electrolyte.

[0098] (Preparation of Secondary Battery) The battery was constructed by attaching lead terminals to the positive and negative electrodes, and placing the electrodes facing each other via a separator (made of polyethylene: film thickness 16 μm, porosity 47%), placing the electrode assembly in an aluminum laminate battery exterior, injecting the electrolyte, and sealing it to prepare a test battery. The design capacity of this prototype battery was 50 mAh. The design capacity of the battery was based on a charge cut-off voltage of 4.2 V.

[0099] <Evaluation of cycle characteristics> The lithium ion secondary battery of the laminated cell prepared above was charged and discharged at a charge / discharge rate of 0.1 C under conditions of CC discharge of 2.5 to 4.2 V in an environment of 45°C, and the initial capacity C ０ Furthermore, charging and discharging were repeated in an environment of 25°C at a charge / discharge rate of 0.5C under conditions of CC discharge of 2.5 to 4.2V, and the capacity C ５０ The cycle characteristics (ΔC) were calculated using the following formula: ΔC = C ５０ / C ０ × 100 (%) The ΔC calculated by the above formula was 92.7%, and the cycle characteristics based on the following criteria were evaluated as "A". Note that a higher ΔC value indicates better cycle characteristics. (Criteria for Cycle Characteristics Evaluation) A: Charge / discharge capacity retention rate is 92.0% or more B: Charge / discharge capacity retention rate is 90.0% or more but less than 92.0% C: Charge / discharge capacity retention rate is less than 90.0%

[0100] Examples 2 to 15 and Comparative Examples 1 to 3 Electrode slurries were prepared in the same manner as in Example 1, except that the compositions were as shown in Table 3. The cycle characteristics of the batteries containing negative electrodes obtained using the electrode slurries were evaluated. The results are shown in Table 3.

[0101] <Evaluation Results> As is clear from the results of Examples 1 to 15, the binder for secondary battery electrodes of the present invention provided excellent toughness of the binder coating film after immersion in an electrolyte solution, and a composition for secondary battery electrode mixture layer (electrode slurry) containing the binder for secondary battery electrodes of the present invention provided excellent electrolyte resistance of the secondary battery electrode mixture layer and cycle characteristics of the secondary battery. Among these, when focusing on the presence or absence of a crosslinked structure in the polymer, the toughness of the binder coating film after immersion in an electrolyte solution, the electrolyte resistance of the secondary battery electrode mixture layer, and the cycle characteristics of the secondary battery were all superior to when a crosslinked polymer was used (Example 1) or when a non-crosslinked polymer was used (Example 12). Furthermore, when focusing on the structural units of the polymer, when the content of structural units derived from a keto group-containing ethylenically unsaturated monomer relative to the total structural units was low (Examples 1 and 5), the toughness of the binder coating film after immersion in an electrolyte solution and the electrolyte resistance of the secondary battery electrode mixture layer were both superior to when the content of structural units derived from the monomer was high (Examples 6 and 7). Furthermore, when DAAM was used as the structural unit derived from the keto group-containing ethylenically unsaturated monomer (Example 1), the electrolyte resistance of the secondary battery electrode mixture layer and the cycle characteristics of the secondary battery were even better than when AAEM was used as the monomer (Example 8). Furthermore, when focusing on the method for producing the carboxyl group-containing polymer, when the keto group-containing ethylenically unsaturated monomer was added during the polymerization (Example 2), the toughness of the binder coating film after immersion in the electrolyte, the electrolyte resistance of the secondary battery electrode mixture layer, and the cycle characteristics of the secondary battery were even better than when the keto group-containing ethylenically unsaturated monomer was charged and polymerized at the initial stage (Example 1). This is presumably because the extreme surface of the particles of the crosslinked polymer R-1 was surface-modified with the keto group-containing ethylenically unsaturated monomer, which promoted the reaction between the keto group and the polyfunctional crosslinking agent.

[0102] In contrast, when a carboxyl group-containing polymer salt containing less than 15% by mass of structural units derived from ethylenically unsaturated carboxylic acid monomers was used, the toughness, electrolyte resistance, and cycle characteristics of the binder coating film were all significantly inferior (Comparative Example 1). Furthermore, when the content of structural units derived from carboxyl group-containing ethylenically unsaturated monomers was as low as 30% by mass (when the content of structural units derived from keto group-containing ethylenically unsaturated monomers was as high as 70% by mass) (Example 7 and Comparative Example 2), when a carboxyl group-containing polymer salt not containing a keto group-containing ethylenically unsaturated monomer was used, the toughness of the binder coating film after immersion in electrolyte, the electrolyte resistance of the secondary battery electrode mixture layer, and the cycle characteristics of the secondary battery were all significantly inferior (Comparative Example 2). Furthermore, when a composition for a secondary battery electrode mixture layer (electrode slurry) not containing a polyfunctional crosslinking agent was used, the toughness of the binder coating film after immersion in electrolyte was inferior (Comparative Example 3).

[0103] The binder for secondary battery electrodes disclosed herein provides excellent toughness to the binder coating film after immersion in an electrolyte, and a secondary battery electrode mixture layer obtained using an electrode slurry containing the binder exhibits electrolyte resistance. Furthermore, a secondary battery equipped with an electrode obtained using the binder can ensure good integrity and exhibits good durability (cycling characteristics) even after repeated charge and discharge, and is expected to contribute to the development of high-capacity automotive secondary batteries and the like. The binder for secondary battery electrodes of the present invention can be particularly suitably used for non-aqueous electrolyte secondary battery electrodes, and is particularly useful for non-aqueous electrolyte lithium-ion secondary batteries with high energy density.

Claims

1. A binder for a secondary battery electrode, comprising a carboxyl group-containing polymer or a salt thereof, the carboxyl group-containing polymer contains, relative to all structural units thereof, 15% by mass or more and 99.9% by mass or less of structural units derived from ethylenically unsaturated carboxylic acid monomers and 0.1% by mass or more and 85% by mass or less of structural units derived from keto group-containing ethylenically unsaturated monomers; At least a portion of the keto groups are functional groups that are used to form chemical bonds with compounds reactive with the keto groups. Binder for secondary battery electrodes.

2. 2. The binder for a secondary battery electrode according to claim 1, further comprising a compound having two or more functional groups reactive with a keto group (hereinafter referred to as a "polyfunctional crosslinking agent").

3. 2. The binder for a secondary battery electrode according to claim 1, wherein the carboxyl group-containing polymer is a crosslinked polymer.

4. 4. The binder for a secondary battery electrode according to claim 3, wherein the crosslinked polymer is a crosslinked polymer obtained by polymerizing a monomer composition containing a non-crosslinkable monomer and a crosslinkable monomer (however, different from the polyfunctional crosslinking agent).

5. 5. The binder for a secondary battery electrode according to claim 4, wherein the amount of the crosslinkable monomer used is 0.1 parts by mass or more and 2.0 parts by mass or less per 100 parts by mass of the total amount of the non-crosslinkable monomer.

6. 4. The binder for a secondary battery electrode according to claim 3, wherein the crosslinked polymer has a volume-based median particle size measured in an aqueous medium after being neutralized to a degree of neutralization of 80 to 100 mol % of 0.1 μm or more and 10.0 μm or less.

7. The binder for a secondary battery electrode according to claim 2 , wherein the polyfunctional crosslinking agent includes a polyfunctional crosslinking agent having a hydrazide group.

8. A composition for a secondary battery electrode mixture layer, comprising the binder for a secondary battery electrode according to any one of claims 2 to 7, an active material, and water.

9. The composition for a secondary battery electrode mixture layer according to claim 8 , wherein the active material includes a silicon-based active material.

10. A secondary battery electrode comprising a mixture layer formed from the composition for a secondary battery electrode mixture layer according to claim 8 on a surface of a current collector.

11. A secondary battery comprising the secondary battery electrode according to claim 10.

12. A method for producing a binder for a secondary battery electrode containing a carboxyl group-containing polymer or a salt thereof, comprising: The carboxyl group-containing polymer is obtained by a method comprising a step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer and a monomer component containing a keto group-containing ethylenically unsaturated monomer by precipitation polymerization or dispersion polymerization.

13. The precipitation polymerization or dispersion polymerization includes a step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer; The method according to claim 12, further comprising the step of adding and polymerizing a monomer component containing a keto group-containing ethylenically unsaturated monomer during the step.

14. the carboxyl group-containing polymer contains 15% by mass or more and 99.9% by mass or less of an ethylenically unsaturated carboxylic acid monomer and 0.1% by mass or more and 85% by mass or less of a keto group-containing ethylenically unsaturated monomer, The method according to claim 12 or 13.