Secondary battery electrode binder and use of same

Abstract

A secondary battery electrode binder contains a carboxyl group-containing crosslinked polymer or a salt thereof, wherein: the carboxyl group-containing crosslinked polymer contains 50.0–99.5 mass% of a structural unit that is derived from an ethylenically unsaturated carboxylic acid monomer, has a homopolymer SP value of 7.0–12.5(cal / m3)1 / 2, and contains 0.5–50.0 mass% of a structural unit that is derived from an ethylenically unsaturated monomer that has an ester bond in the molecule; and the degree of neutralization of the carboxyl group-containing crosslinked polymer is 30–85 mol%.

Description

Binders for secondary battery electrodes and their applications 【0001】 This invention relates to a binder for secondary battery electrodes and its use. 【0002】 Various energy storage devices, such as nickel-metal hydride batteries, lithium-ion batteries, and electric double-layer capacitors, have been put into practical use as secondary batteries. The electrodes used in these secondary batteries are manufactured by coating and drying a composition for forming an electrode mixture layer containing an active material and a binder onto a current collector. For example, in lithium-ion batteries, an aqueous binder containing styrene-butadiene rubber (SBR) latex and carboxymethylcellulose (CMC) is used as the binder in the negative electrode mixture layer composition. 【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, to increase the electrical capacity of lithium-ion secondary batteries, there is an increasing trend towards using silicon-based active materials as the negative electrode active material. However, silicon-based active materials are known to undergo large volume changes during charging and discharging, and repeated use can lead to peeling or detachment of the electrode mixture layer, resulting in a decrease in battery capacity and deterioration of cycle characteristics (durability). To suppress such problems, research is being conducted to improve durability by firmly bonding the active materials together with a binder (binding properties), reducing the size of the active materials to alleviate stress associated with swelling and shrinkage, and by devising additives for 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 the negative electrode mixture layer using silicon-based active materials. Patent Document 1 discloses a binder containing a crosslinked acrylic acid polymer, in which polyacrylic acid is crosslinked with a specific crosslinking agent, and discloses that even when a silicon-containing active material is used, the electrode structure is not destroyed and good cycle characteristics are observed. 【0005】 International Publication No. 2014 / 065407 【0006】 The binder disclosed in Patent Document 1 can impart good cycle characteristics to secondary batteries, but it is sometimes insufficient. Furthermore, the secondary battery electrode mixture layer composition (electrode slurry) containing this binder takes a long time to dry, resulting in poor productivity of secondary battery electrodes. 【0007】 The present invention has been made in view of these circumstances, and its object is to provide a binder for secondary battery electrodes that can shorten the drying process of the secondary battery electrode mixture layer composition, thereby improving the productivity of secondary battery electrodes and improving the cycle characteristics of secondary batteries. Furthermore, the present invention also aims to provide a secondary battery electrode mixture layer composition containing the above-mentioned binder, a secondary battery electrode obtained using the composition, and a secondary battery. 【0008】 As a result of diligent research to solve the above problems, the present inventors have found that in a binder for secondary battery electrodes containing a carboxyl group-containing polymer or a salt thereof, by specifying the type and content of structural units of the crosslinked polymer, and further specifying the degree of neutralization of the crosslinked polymer within a specific range, the drying process of the composition for the secondary battery electrode mixture layer can be shortened, thereby improving the productivity of secondary battery electrodes and improving the cycle characteristics of the secondary battery. Thus, the present invention has been completed. 【0009】 The present invention is as follows: [1] A binder for secondary battery electrodes containing a carboxyl group-containing crosslinked polymer or a salt thereof, wherein the carboxyl group-containing crosslinked polymer contains 50.0% by mass or more and 99.5% by mass or less of structural units derived from ethylenically unsaturated carboxylic acid monomers, and the SP value of the homopolymer is 7.0 or more and 12.5 or less (cal / m³). ３ ) １／２A binder for secondary battery electrodes, wherein the polymer contains 0.5% by mass or more and 50.0% by mass or less structural units derived from ethylenically unsaturated monomers having ester bonds in the molecule, and the degree of neutralization of the carboxyl group-containing crosslinked polymer is 30 mol% or more and 85 mol% or less. [2] A composition for secondary battery electrode composite layer, comprising graphite: silicon oxide: the binder for secondary battery electrodes described in claim 1: styrene / butadiene rubber: sodium carboxymethylcellulose in a solid content mass ratio of 76.8:19.2:1.0:2.0:1.0, with a solid content concentration of 53% by mass in water as the solvent, the composition is applied to the surface of the current collector, dried at 80°C for 15 minutes, and then rolled to a thickness of 50 ± 5 μm and a composite density of 1.60 ± 0.10 g / cm³. ３ A binder for secondary battery electrodes according to [1], wherein a secondary battery electrode mixture layer is formed in such a manner, and when the mixture layer is dried under the conditions of -0.1 MPa, 130°C, and 8 hours, the moisture content in the mixture layer is 125 ppm or less. [3] A binder for secondary battery electrodes according to [1] or [2], wherein the carboxyl group-containing crosslinked polymer salt is an alkali metal salt. [4] A binder for secondary battery electrodes according to any one of [1] to [3], wherein the carboxyl group-containing crosslinked polymer or salt thereof is crosslinked with a crosslinkable monomer, and the amount of the crosslinkable monomer used is 0.03 mol% or more and 1.5 mol% or less relative to the total amount of the non-crosslinkable monomer. [5] A composition for a secondary battery electrode mixture layer comprising the binder for secondary battery electrodes according to any one of [1] to [4], an active material, and water. [6] The secondary battery electrode composite layer composition according to [5], wherein the content of the binder for secondary battery electrodes is 0.25 parts by mass or more and 2.0 parts by mass or less per 100 parts by mass of the total amount of the active material. [7] A secondary battery electrode comprising a composite layer formed from the secondary battery electrode composite layer composition according to [5] or [6] on the surface of a current collector. [8] A secondary battery comprising the secondary battery electrode according to [7]. 【0010】 The binder for secondary battery electrodes of the present invention makes it possible to shorten the drying process of the composition for the secondary battery electrode mixture layer, thereby improving the productivity of secondary battery electrodes, and to obtain a secondary battery with excellent cycle characteristics. 【0011】 The binder for secondary battery electrodes of the present invention (hereinafter also referred to as "this binder") contains a carboxyl group-containing crosslinked polymer (hereinafter also referred to as "this crosslinked polymer") or a salt thereof (hereinafter also referred to as "this crosslinked polymer salt"), and can be made into a composition for a secondary battery electrode mixture layer (hereinafter also referred to as "this composition") by mixing with an active material and water. The above composition may be in a slurry state that can be applied to a current collector, or may be prepared in a wet powder state so as to be applicable to pressing on the surface of the current collector. By forming a mixture layer formed from the above composition on the surface of a current collector such as a copper foil or an aluminum foil, the secondary battery electrode of the present invention can be obtained. Here, when this binder is used in a composition for a secondary battery electrode mixture layer containing a silicon-based active material described later as an active material, it is preferable in that the effects exhibited by the present invention are particularly large. 【0012】 Hereinafter, each of the crosslinked polymer, its production method, the composition for a secondary battery electrode mixture layer obtained by using this binder, the secondary battery electrode, and the secondary battery will be described in detail. In this specification, "(meth)acrylic" means acrylic and / or methacrylic, and "(meth)acrylate" means acrylate and / or methacrylate. Also, "(meth)acryloyl group" means acryloyl group and / or methacryloyl group. In the numerical ranges described step by step in this specification, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of another numerically described step-by-step range, and the upper limit value or the lower limit value of that numerical range may be replaced with the value shown in the examples. 【0013】 1. This crosslinked polymer This crosslinked polymer contains 50.0 mass% or more and 99.5 mass% or less of structural units derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as "monomer (a)"), and the SP value of the homopolymer is 7.0 or more and 12.5 or less (cal / m ３ ) １／２The polymer contains 0.5% by mass or more and 50.0% by mass or less of structural units derived from an ethylenically unsaturated monomer having an ester bond in its molecule (hereinafter also referred to as "monomer (b)"), and the monomer components, including monomer (a) and monomer (b), can be introduced into the polymer by precipitation polymerization or dispersion polymerization. 【0014】 <Structural Units Derived from Monomer (a)> This crosslinked polymer is a polymer containing 50.0% to 99.5% by mass of structural units derived from monomer (a) (hereinafter also referred to as "component (a)"). When this crosslinked polymer has carboxyl groups due to the presence of such structural units, adhesion to the current collector is improved, and the desolvation effect of lithium ions and ionic conductivity are excellent, resulting in electrodes with low resistance and excellent high-rate characteristics. Furthermore, water swelling properties are imparted, which can improve the dispersion stability of active materials in this composition. Component (a) can be introduced into the polymer, for example, by polymerizing a monomer containing an ethylenically unsaturated carboxylic acid monomer. Alternatively, it can be obtained by (co)polymerizing (meth)acrylic acid ester monomers and then hydrolyzing them. Alternatively, (meth)acrylamide and (meth)acrylonitrile may be polymerized and then treated with a strong alkali, or an acid anhydride may be reacted with a polymer having hydroxyl groups. 【0015】As the monomer (a), for example, (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid; (meth)acrylamide alkyl carboxylic acids such as (meth)acrylamide hexanoic acid and (meth)acrylamide dodecanoic acid; carboxyl group-containing ethylenically unsaturated monomers such as succinic acid mono-hydroxyethyl (meth)acrylate, ω-carboxy-caprolactone mono (meth)acrylate, β-carboxyethyl (meth)acrylate or their (partial) alkali neutralized products can be mentioned. One of these may be used alone, or two or more kinds may be used in combination. Among the above, a compound having an acryloyl group as a polymerizable functional group is preferable in that a polymer having a long primary chain length can be obtained due to a high polymerization rate and the binding force of the binder becomes good, and acrylic acid is particularly preferable. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, a polymer having a high carboxyl group content can be obtained. 【0016】 The content of the component (a) in the present crosslinked polymer is 50.0% by mass or more and 99.5% by mass or less based on all the structural units of the present crosslinked polymer. By containing the component (a) within such a range, excellent adhesion to the current collector can be easily ensured, and while suppressing the expansion and contraction of the electrode due to charge and discharge of the secondary battery, a high capacity retention rate is exhibited. When the lower limit is 50.0% by mass or more, the dispersion stability of the present composition becomes good and a higher binding force can be obtained, so it is preferable, and it may be 60.0% by mass or more, 70.0% by mass or more, or 80.0% by mass or more. Also, the upper limit is, for example, 99.0% by mass or less, for example 98.0% by mass or less, for example 95.0% by mass or less, for example 90.0% by mass or less, or for example 80.0% by mass or less. 【0017】<Structural Units Derived from Monomer (b)> This crosslinked polymer is a polymer containing 0.5% by mass or more and 50.0% by mass or less of structural units derived from monomer (b) (hereinafter also referred to as "component (b)"). Because this crosslinked polymer has such structural units, it is possible to relax the drying conditions (shorten the drying process) when manufacturing secondary battery electrodes, which can contribute to improving the productivity of secondary battery electrodes, and can also improve the cycle characteristics of secondary batteries equipped with such secondary battery electrodes. 【0018】 As monomer (b), the SP value of the homopolymer is 7.0 to 12.5 (cal / m³). ３ ) １／２Furthermore, among ethylenically unsaturated monomers having an ester bond in their molecule, specific compounds having an acryloyl group include acrylic acid esters such as stearyl acrylate (SP value: 8.9), lauryl acrylate (SP value: 9.2), 2-ethylhexyl acrylate (SP value: 9.2), 2-ethylhexyl acrylate (SP value: 9.2), isobutyl acrylate (SP value: 9.6), n-butyl acrylate (SP value: 9.8), ethoxyethoxyethyl acrylate (SP value: 9.8), ethyl acrylate (SP value: 10.2), 2-methoxyethyl acrylate (SP value: 10.2), methyl acrylate (SP value: 10.6), cyclohexyl acrylate (SP value: 10.8), and phenoxyethyl acrylate (SP value: 11.0) (excluding the perfluoroalkyl acrylates listed below); Examples include perfluoroalkyl acrylates such as 2-(perfluorohexyl)ethyl acrylate (SP value: 7.9) and 2,2,2-trifluoroethyl acrylate (SP value: 8.4); N-alkylacrylamide compounds such as N-isobutoxymethylacrylamide (SP value: 11.3), t-butylacrylamide (SP value: 11.4), N-n-butoxymethylacrylamide (SP value: 11.5), and isopropylacrylamide (SP value: 12.0); and N,N-dialkylacrylamide compounds such as diethylacrylamide (SP value: 11.3) and dimethylacrylamide (SP value: 12.3). Specific compounds having a methacryloyl group include glycidyl methacrylate (SP value: 11.5), methyl methacrylate (SP value: 9.9), and n-butyl methacrylate (SP value: 9.5). One of these may be used alone, or two or more may be used in combination. Among the above, acrylic acid esters are preferred because they exhibit particularly great effects in the present invention, and among them, alkyl acrylates are more preferred. 【0019】Note that the SP values described for each compound can be calculated by the calculation method described in "Polymer Engineering and Science" 14(2), 147 (1974) by R.F. Fedors. Specifically, it is based on the calculation method shown in formula (1). δ: SP value ((cal / cm ３ ) １／２ ) ΔE ｖａｐ : Molar heat of vaporization of each atomic group (cal / mol) V: Molar volume of each atomic group (cm ３ / mol) 【0020】 The SP value (unit: (cal / m ３ )) of the homopolymer of monomer (b) is preferably 7.5 or more and 12.0 or less, more preferably 8.0 or more and 11.5 or less, still more preferably 8.0 or more and 11.0 or less, even more preferably 9.0 or more and 11.0 or less, and still even more preferably 9.5 or more and 11.0 or less, in that the effect of relaxing the drying conditions (shortening the drying process) during the production of the secondary battery electrode is greater and the cycle characteristics are also excellent. 【0021】 For example, in the case of polyacrylic acid in which the carboxyl group-containing crosslinked polymer consists only of the above component (a), the affinity for the electrolyte is not very high. However, the SP value of the homopolymer is 7.0 to 12.5 (cal / cm ３ ) １／２ ​​Since component (b) is close to the SP value of the electrolyte, introducing component (b) into the carboxyl group-containing crosslinked polymer increases its affinity for the electrolyte, and the carboxyl group-containing crosslinked polymer is appropriately plasticized. As a result, the resistance (interfacial resistance) when lithium ions penetrate from the electrolyte into the active material coated with the carboxyl group-containing crosslinked polymer is reduced, improving the high-rate properties. If the content of component (b) is less than 0.5% by mass, the above effect may not be sufficiently obtained. On the other hand, if the content of component (b) exceeds 50% by mass, the carboxyl group-containing crosslinked polymer may be excessively plasticized, resulting in a significant decrease in binding strength and insufficient durability, or the desolvation effect may be insufficient due to a low content of component (a), potentially leading to a decrease in high-rate properties. The main compounds used in electrolytes and their SP values ​​include ethylene carbonate (SP value: 14.7, hereinafter also referred to as "EC"), propylene carbonate (SP value: 13.3, hereinafter also referred to as "PC"), dimethyl carbonate (SP value: 9.9, hereinafter also referred to as "DMC"), diethyl carbonate (SP value: 8.8, hereinafter also referred to as "DEC"), and ethyl methyl carbonate (hereinafter also referred to as "EMC"). The SP values ​​for each compound are reproduced from the values ​​listed in the "Polymer Handbook". The SP value for EMC is not listed in the "Polymer Handbook", but it is estimated to be between the values ​​for DMC and DEC. In actual electrolytes, EC / DEC = 1 / 3 (v / v) and EC / EMC = 1 / 3 (v / v) are used as mixed solvents. 【0022】The content of component (b) in this crosslinked polymer is 0.5% by mass or more and 50.0% by mass or less relative to the total structural units of the crosslinked polymer. By including component (b) within this range, excellent binding properties to the active material can be easily ensured, and the cycle characteristics of the secondary battery can be improved. The lower limit of the content of component (b) is preferably 1.0% by mass or more, more preferably 1.5% by mass or more, even more preferably 3.0% by mass or more, even more preferably 4.0% by mass or more, and particularly preferably 5.0% by mass or more, in terms of good dispersion stability of the composition and obtaining higher binding strength. The upper limit of the content of component (b) is preferably 45.0% by mass or less, more preferably 40.0% by mass or less, even more preferably 35.0% by mass or less, even more preferably 30.0% by mass or less, even more preferably 25.0% by mass or less, and particularly preferably 20.0% by mass or less, in terms of excellent balance between the effect of reducing the amount of water in the electrode mixture layer and the cycle characteristics of the secondary battery. 【0023】 <Other Structural Units> In addition to components (a) and (b), this crosslinked polymer may contain structural units derived from other ethylenically unsaturated monomers copolymerizable with them (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 formula (1) and formula (2) below), 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 monomers containing hydroxyl group-containing ethylenically unsaturated monomers, ethylenically unsaturated monomer compounds having anionic groups other than carboxyl groups, such as sulfonic acid groups and phosphate groups, or nonionic ethylenically unsaturated monomers. CH ２ = C(R １ ) COOR ２ (1) [wherein, R １ R represents a hydrogen atom or a methyl group. ２ (R) is a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group. ３ O) ｍH or R ４ O[CO(CH ２ ) ５ O] ｎ It represents H. Also, R ３ R represents an alkylene group with 2 to 4 carbon atoms. ４ represents an alkylene group with 1 to 8 carbon atoms, m represents an integer from 2 to 15, and n represents an integer from 1 to 15. ] CH ２ = C(R ５ ) CONR ６ R ７ (2) [wherein, R ５ R represents a hydrogen atom or a methyl group. ６ R represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms. ７ [This represents a hydrogen atom or a monovalent organic group.] 【0024】 This crosslinked polymer is a crosslinked polymer having a crosslinked structure. The method of crosslinking in this crosslinked polymer is not particularly limited, and examples of embodiments include the following methods: 1) Copolymerization of crosslinkable monomers 2) Utilization of chain transfer to polymer chains during radical polymerization Because this crosslinked polymer has a crosslinked structure, the binder containing the crosslinked polymer or its salt can have excellent binding strength. Among the above, the method by copolymerization of crosslinkable monomers is preferred because it is easy to operate and the degree of crosslinking can be easily controlled. 【0025】 <Crossable Monomers> Examples of crosslinkable monomers include polyfunctional polymerizable monomers having two or more polymerizable unsaturated groups, and monomers having self-crosslinkable functional groups such as hydrolyzable silyl groups. 【0026】 The above-mentioned polyfunctional polymerizable monomers are compounds having two or more polymerizable functional groups such as (meth)acryloyl groups and alkenyl groups in their molecules, and include polyfunctional (meth)acryloyl compounds, polyfunctional alkenyl compounds, and compounds having both (meth)acryloyl and alkenyl groups. These compounds may be used individually or in combination of two or more. Among these, polyfunctional alkenyl compounds are preferred because they easily yield a uniform crosslinked structure, and polyfunctional allyl ether compounds having two or more allyl ether groups in their molecules are particularly preferred. 【0027】 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; tri(meth)acrylates of trihydric or higher polyhydric alcohols such as trimethylolpropane tri(meth)acrylate, tri(meth)acrylate of trimethylolpropane ethylene oxide modified product, glycerin tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and pentaerythritol tetra(meth)acrylate; and bisamides such as methylenebisacrylamide and hydroxyethylenebisacrylamide. 【0028】 Examples of polyfunctional alkenyl compounds include polyfunctional allyl ether compounds such as trimethylolpropanediallyl ether, trimethylolpropanetriallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, tetraallyloxyethane, and polyallyl saccharose; polyfunctional allyl compounds such as diallyl phthalate; and polyfunctional vinyl compounds such as divinylbenzene. 【0029】 Examples of compounds 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. 【0030】 Specific examples of monomers having self-crosslinkable functional groups include hydrolyzable silyl group-containing vinyl monomers and N-methoxyalkyl(meth)acrylamide. These compounds can be used individually or in combination of two or more. 【0031】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. Examples include vinylsilanes such as vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane, and vinyldimethylmethoxysilane; silyl group-containing acrylic acid esters such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, and methyldimethoxysilylpropyl acrylate; silyl group-containing methacrylic acid esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, and dimethylmethoxysilylpropyl methacrylate; silyl group-containing vinyl ethers such as trimethoxysilylpropyl vinyl ether; and silyl group-containing vinyl esters such as vinyl trimethoxysilylundecanoate. 【0032】 When the crosslinked polymer is crosslinked with a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.1 parts by mass or more and 4.0 parts by mass or less, more preferably 0.3 parts by mass or more and 3.0 parts by mass or less, even more preferably 0.5 parts by mass or more and 2.0 parts by mass or less, even more preferably 0.7 parts by mass or more and 1.5 parts by mass or less, and even more preferably 0.8 parts by mass or more and 1.0 parts by mass or less, based on 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer). If the amount of crosslinkable monomer used is 0.1 parts by mass or more, it is preferable in that, during use over a longer period than conventional methods, the conductive paths between the active materials are well maintained while suppressing expansion and contraction due to charging and discharging, resulting in an excellent charge-discharge capacity retention rate. If it is 4.0 parts by mass or less, the stability of precipitation polymerization or dispersion polymerization tends to be higher. In particular, if it is 2.0 parts by mass or less, the water-swollen particle size in the electrode slurry becomes suitable, and the bonding area to the active material becomes larger, which is preferable in that it is possible to maintain excellent battery performance even during long-term use. 【0033】For similar reasons, the amount of the above-mentioned crosslinkable monomer used is preferably 0.03 mol% to 1.5 mol%, more preferably 0.1 mol% to 1.1 mol%, even more preferably 0.2 mol% to 0.8 mol%, even more preferably 0.25 mol% to 0.6 mol%, and even more preferably 0.3 mol% to 0.4 mol%. 【0034】 This crosslinked polymer salt 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 because they do not adversely affect battery characteristics, and alkali metal salts are more preferred. 【0035】Regarding the properties of this crosslinked polymer salt, the crosslinked polymer is used in salt form after the carboxyl groups derived from the ethylenically unsaturated carboxylic acid monomer are neutralized so that the degree of neutralization is 30 mol% or more and 85 mol% or less. When the degree of neutralization is 30 mol% or more, it is preferable in that it has good water swelling properties and a dispersion stabilization effect is easily obtained. When the degree of neutralization is 85 mol% or less, it is preferable in that the pH is within a suitable range and the storage stability of the electrode slurry is excellent. The above degree of neutralization is more preferably 40 mol% or more and 85 mol%, even more preferably 60 mol% or more and 85 mol%, even more preferably 70 mol% or more and 84 mol%, and even more preferably 80 mol% or more and 83 mol%, in which the electrode slurry has excellent coating properties and exhibits a better charge / discharge capacity retention rate over a longer period of use than conventional products. The reason why coating properties are better with a higher degree of neutralization is presumed to be that the electrostatic repulsion within the particles is reduced, so the spreading of side chains when the polymer swells in water is suppressed and entanglement in the electrode slurry is reduced. The reason why a higher degree of neutralization results in better capacity retention over long-term use is presumed to be that a higher neutralization rate of the polymer raises the glass transition temperature, suppressing fusion of the polymer during the heating and drying process in the electrode manufacturing process, and allowing for the acquisition of a uniform electrode. In this specification, the degree of neutralization can be calculated from the input values ​​of monomers having acidic groups such as carboxyl groups and the neutralizing agent used for neutralization. The degree of neutralization can be confirmed by measuring the intensity ratio of the peak derived from the C=O group of the carboxylic acid and the peak derived from the C=O group of the carboxylate salt after drying the crosslinked polymer salt at 80°C for 3 hours under reduced pressure using IR measurement. 【0036】 <Particle size of the crosslinked polymer salt> In this composition, it is preferable that the crosslinked polymer salt does not exist as large-particle clumps (secondary aggregates) but is well dispersed as water-swellable particles with an appropriate particle size, so that the binder containing the crosslinked polymer salt can exhibit good binding performance. 【0037】Preferably, when the crosslinked polymer is dispersed in water with a neutralization degree based on the carboxyl groups of the crosslinked polymer of 80 to 100 mol%, the particle size (water-swelled particle size) is in the range of 0.1 μm or more and 10.0 μm or less in volume-based median diameter. A more preferred range for the above 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. If the particle size is in the range of 0.30 μm or more and 2.0 μm or less, it will be uniformly present in the composition at a suitable size, thus enabling the composition to have high stability and exhibit excellent binding properties. If the particle size exceeds 10.0 μm, there is a risk that the binding properties will be insufficient as described above. Furthermore, the difficulty in obtaining a smooth coating surface may result in insufficient coating properties. On the other hand, when the particle size is less than 0.1 μm, there are concerns from the standpoint of stable manufacturing. 【0038】 Here, as a composition for a secondary battery electrode composite layer containing the binder, active material, and water, a composition was prepared with a solid content concentration of 53% by mass, using water as the solvent, containing graphite: silicon oxide: the secondary battery electrode binder described in claim 1: styrene / butadiene rubber: carboxymethylcellulose sodium in a solid content mass ratio of 76.8:19.2:1.0:2.0:1.0. The composition was applied to the surface of the current collector, dried at 80°C for 15 minutes, and then rolled to a thickness of 50 ± 5 μm and a composite density of 1.60 ± 0.10 g / cm³. ３When a secondary battery electrode mixture layer is formed in such a manner, and the mixture layer is dried under the conditions of -0.1 MPa, 130°C, and 8 hours, it is preferable that the moisture content in the electrode mixture layer is 125 ppm or less. This makes it possible to relax the drying conditions (shorten the drying process) during the production of secondary battery electrodes, which further contributes to improving the productivity of secondary battery electrodes and improves the cycle characteristics of secondary batteries equipped with such electrodes. The moisture content in the electrode mixture layer is more preferably 115 ppm or less, even more preferably 110 ppm or less, and even more preferably 100 ppm or less. The moisture content in the electrode mixture layer can be measured by the method described in the examples (coulometric titration using a Karl Fischer moisture meter). 【0039】 2. Method for Producing the Crosslinked Polymer The crosslinked polymer can be produced using known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization, but precipitation polymerization and suspension polymerization (reverse-phase suspension polymerization) are preferred in terms of productivity. Heterogeneous polymerization methods such as precipitation polymerization, suspension polymerization, and emulsion polymerization are preferred in terms of obtaining better performance in terms of binding properties, and among these, precipitation polymerization is more preferred. Precipitation polymerization is a method of producing a polymer by carrying out a polymerization reaction in a solvent that dissolves the raw material unsaturated monomers but does not substantially dissolve the polymer to be produced. As polymerization progresses, the polymer particles become larger due to aggregation and growth, and a dispersion of polymer particles is obtained in which primary particles of tens to hundreds of nanometers are secondary aggregated to several micrometers to tens of micrometers. Dispersion stabilizers can also be used to control the particle size of the polymer. Secondary aggregation can also be suppressed by selecting the dispersion stabilizer and polymerization solvent. In general, precipitation polymerization in which secondary aggregation is suppressed is also called dispersion polymerization. 【0040】 In precipitation polymerization, the polymerization solvent can be selected from water and various organic solvents, taking into consideration the type of monomer used. To obtain polymers with longer primary chain lengths, it is preferable to use a solvent with a small chain transfer constant. 【0041】Specific polymerization solvents include water-soluble solvents such as methanol, t-butyl alcohol, acetone, methyl ethyl ketone, acetonitrile, and tetrahydrofuran, as well as benzene, ethyl acetate, dichloroethane, n-hexane, cyclohexane, and n-heptane. These can be used individually or in combination of two or more. Alternatively, they may be used as a mixed solvent with water. In this invention, a water-soluble solvent refers to one whose solubility in water at 20°C is greater than 10 g / 100 ml. Among the above, methyl ethyl ketone and acetonitrile are preferred because they produce fewer coarse particles and adhere less to the reactor, resulting in good polymerization stability; the precipitated polymer fine particles are less prone to secondary aggregation (or even if secondary aggregation occurs, they dissolve easily in the aqueous medium); the chain transfer constant is small, resulting in a polymer with a high degree of polymerization (primary chain length); and the neutralization process described later is easy to handle. 【0042】 The polymerization initiator can be any known polymerization initiator such as azo compounds, organic peroxides, or inorganic peroxides, but is not particularly limited. The usage conditions can be adjusted to achieve an appropriate amount of radical generation using known methods such as thermal initiation, redox initiation with a reducing agent, or UV initiation. In order to obtain a crosslinked polymer with a long primary chain length, it is preferable to set the conditions so that the amount of radical generation is reduced as much as possible within the acceptable range of production time. 【0043】 The preferred amount of polymerization initiator to use is, for example, 0.001 to 2 parts by mass, or for example, 0.005 to 1 part by mass, or for example, 0.01 to 0.1 parts by mass, when the total amount of monomer components used is 100 parts by mass. If the amount of polymerization initiator used is 0.001 parts by mass or more, the polymerization reaction can be carried out stably, and if it is 2 parts by mass or less, it is easy to obtain a polymer with a long primary chain length. 【0044】 The polymerization temperature is preferably 0 to 100°C, and more preferably 20 to 80°C, although this depends on conditions such as the type and concentration of monomers used. The polymerization temperature may be constant or may change during the polymerization reaction. The polymerization time is preferably 1 minute to 20 hours, and more preferably 1 hour to 10 hours. 【0045】 Here, the crosslinked polymer contains 50.0% to 100% by mass of structural units derived from monomer (a) (component (a)) and 0.5% to 50.0% by mass of structural units derived from monomer (b) (component (b)), with a degree of neutralization of 30 mol% to 85 mol%. The preferred types of monomer (a) and monomer (b), and the preferred ranges of content of component (a) and component (b) are as described above. 【0046】 3. Composition for Secondary Battery Electrode Mixture Layer The composition for secondary battery electrode mixture layer of the present invention comprises this binder, an active material, and water. The amount of this binder used in this composition is preferably 0.25 parts by mass or more and 2.0 parts by mass or less, more preferably 0.5 parts by mass or more and 1.8 parts by mass or less, even more preferably 0.7 parts by mass or more and 1.5 parts by mass or less, and most preferably 0.8 parts by mass or more and 1.2 parts by mass or less, per 100 parts by mass of the total amount of the active material. If the amount of this binder used is 0.25 parts by mass or less, sufficient binding properties can be obtained. Furthermore, the dispersion stability of the active material can be ensured, and a uniform mixture layer can be formed. If the amount of this binder used is 2.0 parts by mass or less, the composition will not become highly viscous, and coating properties for the current collector can be ensured. As a result, a mixture layer having a uniform and smooth surface can be formed. This is presumed to be because the entanglement of carboxyl group-containing crosslinked polymer salts and carboxyl group-containing non-crosslinked polymer salts in the electrode slurry is reduced. Furthermore, while suppressing the expansion and contraction of electrodes due to charging and discharging of secondary batteries, it is possible to achieve a superior retention rate of charge and discharge capacity over longer periods of use than conventional batteries. This is presumed to be because the brittleness of this binder has less influence on the electrode properties, and deterioration due to stress from swelling and contraction of the active material during repeated charging and discharging is less likely to occur. 【0047】 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 cobaltate, lithium nickelate, and NCM{Li(Ni)}, which are called ternary systems.ｘ Co ｙ , Mn ｚ ), x+y+z=1} and NCA{Li(Ni １－ａ－ｂ Co ａ Al b Examples include )}. In addition, lithium manganate is an example of a spinel-type positive electrode active material. Besides oxides, phosphates, silicates, and sulfur can also be used, and examples of phosphates include olivine-type lithium iron phosphate. As a 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. 【0048】 Furthermore, when a positive electrode active material containing layered rock salt-type lithium-containing metal oxide is dispersed in water, the dispersion becomes alkaline due to the exchange of lithium ions on the surface of the active material with hydrogen ions in the water. This may cause corrosion of common positive electrode current collector materials such as aluminum foil (Al). In such cases, it is preferable to neutralize the alkali leaching from the active material by using an unneutralized or partially neutralized crosslinked polymer as a binder. Moreover, it is preferable to use an amount of the unneutralized or partially neutralized crosslinked polymer such that the amount of unneutralized carboxyl groups in the crosslinked polymer is equivalent to or greater than the amount of alkali leaching from the active material. 【0049】 Since all positive electrode active materials have low electrical conductivity, a conductive additive may be added. Examples of such conductive additives include carbon-based materials such as carbon black, carbon fiber, graphite powder, and carbon fibers. Of these, carbon black and carbon fiber are preferred because they easily provide excellent conductivity. Ketjenblack and acetylene black are preferred as carbon blacks. One of the conductive additives may be used alone, or two or more may be used in combination. From the viewpoint of balancing conductivity and energy density, the amount of conductive additive used can 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 amount of active material. Furthermore, the positive electrode active material may be surface-coated with a conductive carbon-based material. 【0050】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, spheroidized graphite is preferably used in terms of battery performance, and the preferred range of particle size is, for example, 1 to 20 μm, or for example, 5 to 15 μm. Furthermore, in order to increase the energy density, metals or metal oxides that can absorb 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. The amount of silicon-based active material used is preferably 5.0% by mass or more of the total amount of active material, in order to improve the electrical capacity of the secondary battery. It can also be, for example, 10.0% by mass or more, or for example, 20.0% by mass or more. 【0051】 Since carbon-based active materials possess good electrical conductivity on their own, it is not always necessary to add conductive additives. When conductive additives are added for purposes such as further reducing resistance, the amount used, from the perspective of energy density, should be, for example, 10 parts by mass or less, or for example, 5 parts by mass or less, per 100 parts by mass of the total amount of active material. 【0052】 When the composition is in slurry form, the amount of active material used is, for example, in the range of 10 to 75% by mass, or in the range of 30 to 65% by mass, relative to the total amount of the composition. If the amount of active material used is 10% by mass or more, migration of binders and the like is suppressed, and it is also advantageous in terms of the drying cost of the medium. On the other hand, if it is 75% by mass or less, the fluidity and coating properties of the composition can be ensured, and a uniform mixture layer can be formed. 【0053】This composition uses water as the medium. Furthermore, to adjust the properties and drying properties of this composition, a mixed solvent with lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, tetrahydrofuran, or N-methyl-2-pyrrolidone may be used. The proportion of water in the mixed medium is, for example, 50% by mass or more, and also, for example, 70% by mass or more. 【0054】 When this composition is made into a coatable slurry, the content of the water-containing medium in the whole composition can be, for example, in the range of 25 to 60% by mass, or for example, 35 to 60% by mass, from the viewpoint of the coatability of the slurry, the energy cost required for drying, and productivity. 【0055】 This composition may also contain other binder components such as styrene-butadiene rubber (SBR) latex, carboxymethylcellulose (CMC), acrylic latex, and polyvinylidene fluoride latex. When other binder components are used in combination, the amount used can be, for example, 0.1 to 5 parts by mass or less, or 0.1 to 2 parts by mass or less, or 0.1 to 1 part by mass or less, per 100 parts by mass of the total amount of active material. If the amount of other binder components used exceeds 5 parts by mass, the resistance may increase, and the high-rate properties may become insufficient. Among the above, SBR latex and CMC are preferred in terms of their excellent balance of binding properties and flexural resistance, and the combination of SBR latex and CMC is more preferable. 【0056】The above-mentioned SBR latex refers to an aqueous dispersion of a copolymer having structural units derived from aromatic vinyl monomers such as styrene and structural units derived from aliphatic conjugated diene monomers such as 1,3-butadiene. Examples of aromatic vinyl monomers include styrene, α-methylstyrene, vinyltoluene, and divinylbenzene, and one or more of these can be used. The amount of structural units derived from the aromatic vinyl monomer in the copolymer can be in the range of 20 to 70% by mass, or in the range of 30 to 60% by mass, mainly from the viewpoint of binding properties. Examples of aliphatic conjugated diene monomers include 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 in the range of, for example, 30 to 70% by mass, or 40 to 60% by mass, in that the binder binding properties and the flexibility of the resulting electrode are good. In addition to the above monomers, other monomers such as nitrile group-containing monomers like (meth)acrylonitrile, carboxyl group-containing monomers like (meth)acrylic acid, itaconic acid, maleic acid, and ester group-containing monomers like (meth)acrylate may be used as copolymer monomers to further improve properties such as binding properties. The structural units derived from the above other monomers in the copolymer can be in the range of, for example, 0 to 30% by mass, or 0 to 20% by mass. 【0057】The above-mentioned CMC refers to substituted nonionic cellulosic semi-synthetic polymer compounds obtained by substituting them with carboxymethyl groups, and their salts. Examples of the above-mentioned nonionic cellulosic semi-synthetic polymer compounds include alkylcelluloses such as methylcellulose, methylethylcellulose, ethylcellulose, and microcrystalline cellulose; and hydroxyalkylcelluloses such as hydroxyethylcellulose, hydroxybutylmethylcellulose, hydroxypropylcellulose, hydroxypropylmethylcellulose, hydroxyethylmethylcellulose, hydroxypropylmethylcellulose stearoxy ether, carboxymethylhydroxyethylcellulose, alkylhydroxyethylcellulose, and nonoxynylhydroxyethylcellulose. 【0058】The secondary battery electrode mixture layer composition of the present invention comprises the above-mentioned binder, active material, and water as essential components, and is obtained by mixing each component using known means. The method of mixing each component is not particularly limited, and known methods can be used, but a method of dry-blending powder components such as the active material, conductive additive, and binder, and then mixing them with a dispersion medium such as water, and then dispersing and kneading is preferred. When obtaining the composition in slurry form, it is preferable to produce a slurry that is free from poor dispersion and aggregation. As a mixing means, known mixers such as planetary mixers, thin-film swirling mixers, and orbital mixers can be used, but it is preferable to use a thin-film swirling mixer in that a good dispersion state can be obtained in a short time. Furthermore, when using a thin-film swirling mixer, it is preferable to perform pre-dispersion with a stirrer such as a disperser beforehand. The pH of the slurry is not particularly limited as long as the effects of the present invention are achieved, but it is preferably less than 12.5, and for example, when CMC is included, it is more preferable to have a pH of less than 11.5, and even more preferable to have a pH of less than 10.5, in that there is less concern about hydrolysis. Furthermore, the viscosity of the slurry is not particularly limited as long as it achieves the effects of the present invention, but as a B-type viscosity (25°C) at 20 rpm, it can be, for example, in the range of 100 to 30,000 mPa·s, or in the range of 500 to 20,000 mPa·s, or in the range of 1,000 to 10,000 mPa·s. If the viscosity of the slurry is within the above range, good coating properties can be ensured. 【0059】4. Secondary Battery Electrode The secondary battery electrode of the present invention comprises a composite layer formed from the secondary battery electrode composite layer composition of the present invention on the surface of a current collector such as copper or aluminum. The composite layer is formed by coating the surface of the current collector with the composition and then drying off a medium such as water. The method of coating with the composition is not particularly limited, and known methods such as the doctor blade method, dip method, roll coat method, comma coat method, curtain coat method, gravure coat method, and extrusion method can be used. Furthermore, the drying can be carried out by known methods such as hot air blowing, reduced pressure, (far) infrared radiation, and microwave irradiation. Typically, the composite layer obtained after drying is subjected to compression treatment using a die press and a roll press. Compression can be used to bring the active material and binder into close contact, improving the strength of the composite layer and its adhesion to the current collector. Compression can be used to adjust the thickness of the composite layer to, for example, 30 to 80% of the thickness before compression, and the thickness of the composite layer after compression is generally about 4 to 200 μm. 【0060】 5. Secondary Battery A secondary battery can be manufactured by providing a separator and an electrolyte to the electrodes of the secondary battery of the present invention. The electrolyte may be in liquid or gel form. The separator is placed between the positive and negative electrodes of the battery and plays a role in preventing short circuits caused by contact between the two electrodes and in holding the electrolyte to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane with good ionic permeability and mechanical strength. Specific materials that can be used include polyethylene, polyolefins such as polypropylene, and polytetrafluoroethylene. 【0061】 The electrolyte can be a known one commonly used depending on the type of active material. In lithium-ion secondary batteries, specific solvents include cyclic carbonates with high dielectric constant and high electrolyte solubility, such as propylene carbonate and ethylene carbonate, as well as low-viscosity chain carbonates such as ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate. These can be used individually or as mixed solvents. The electrolyte contains LiPF in these solvents. ６ LiSbF ６LiBF ４ LiClO ４ LiAlO ４ These are used by dissolving lithium salts. In nickel-metal hydride secondary batteries, an aqueous potassium hydroxide solution can be used as the electrolyte. Secondary batteries are obtained by housing positive and negative electrode plates, separated by a separator, in a spiral or stacked structure in a case or the like. 【0062】 The present invention will be described in detail below based on the following examples. However, the present invention is not limited to these examples. In the following, "parts" and "%" mean parts by mass and mass%, respectively, unless otherwise specified. 【0063】 In the following examples, the evaluation of carboxyl group-containing crosslinked polymers or their salts was carried out by the following method. 【0064】 (Measurement of particle size (water-swollen particle size) in an 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 set in a rotation / revolution type stirrer (Sinky Co., Ltd., Awatori Rentaro AR-250). Next, stirring (rotation speed 2,000 rpm / revolution speed 800 rpm, 7 minutes) and defoaming (rotation speed 2,200 rpm / revolution speed 60 rpm, 1 minute) were performed to prepare a hydrogel in which the crosslinked polymer salt was swollen in water. Then, the particle size distribution of the above hydrogel was measured using a laser diffraction / scattering particle size analyzer (Microtrac Bell Co., Ltd., Microtrac MT-3300EZII) with ion-exchanged water as the dispersion medium. When a sufficient amount of hydrogel to obtain an appropriate scattered light intensity was added to a hydrogel circulating in an excess amount of dispersion medium, the particle size distribution shape measured after a few minutes stabilized. Once stability was confirmed, the particle size distribution was measured, and the volume-based median diameter (D50), which is a representative value of the particle size, was obtained. For the cross-linked polymer salts R-12, 13, and 16, the mixture was neutralized with lithium hydroxide monohydrate until the degree of neutralization reached 80 mol%, and hydrogels swollen in water were prepared, and the particle size in the aqueous medium was measured. 【0065】≪Production of Carboxyl Group-Containing Crosslinked Polymer Salts≫ [Production Example 1: Production of Carboxyl Group-Containing Crosslinked Polymer Salt R-1] A reactor equipped with a stirring blade, thermometer, reflux condenser, and nitrogen inlet tube was used for polymerization. 567 parts of acetonitrile, 2.2 parts of deionized water, 80.0 parts of acrylic acid (hereinafter also referred to as "AA"), 20.0 parts of n-butyl acrylate (hereinafter also referred to as "BA"), 0.9 parts of trimethylolpropanediallyl ether (manufactured by Osaka Soda Co., Ltd., trade name "Neoallyl T-20"), and triethylamine (TEA) equivalent to 1.0 mol% of the above AA were charged into the reactor. After thoroughly purging the reactor with nitrogen, the internal temperature was raised 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") were added as a polymerization initiator. A white turbidity was observed in the reaction solution, and this point was designated as the polymerization initiation point. The monomer concentration was calculated to be 15%. The polymerization reaction was continued while maintaining the internal temperature at 50°C by adjusting the external temperature (water bath temperature). Twelve hours after the polymerization initiation point, the reaction solution was cooled. After the internal temperature dropped to 25°C, lithium hydroxide monohydrate (hereinafter referred to as "LiOH·H") was added. ２ 37.3 parts of the powder of (also known as "O") were added. After addition, stirring was continued at room temperature for 12 hours to obtain a slurry-like polymerization reaction solution in which particles of carboxyl group-containing crosslinked polymer salt R-1 were dispersed in the medium. 【0066】The obtained polymerization reaction solution was centrifuged to settle the polymer particles, and the supernatant was removed. Then, the precipitate was redispersed in acetonitrile of the same weight as the polymerization reaction solution, and the washing operation, in which the polymer particles were settled by centrifugation and the supernatant was removed, was repeated twice. The precipitate was collected and dried under reduced pressure at 80°C for 3 hours to remove volatile components, thereby obtaining the cross-linked polymer salt R-1 powder. Since cross-linked polymer salt R-1 is hygroscopic, it was sealed and stored in a container with water vapor barrier properties. Furthermore, the degree of neutralization of the cross-linked polymer salt R-1 powder was determined by IR measurement, using the intensity ratio of the peak derived from the C=O group of the carboxylic acid and the peak derived from the C=O group of the lithium carboxylate. This was equal to the calculated value from the initial stage, which was 80 mol%. The particle size of the carboxyl group-containing cross-linked polymer salt R-1 in an aqueous medium was 1.44 μm. 【0067】 [Production Examples 2-13 and Comparative Production Examples 1-4: Production of Carboxyl Group-Containing Polymer Salts R-2-R-17] The same procedure as in Production Example 1 was performed, 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 crosslinked polymer salts R-2-R-17. Next, the same procedure as in Production Example 1 was performed on each polymerization reaction solution to obtain powdered carboxyl group-containing crosslinked polymer salts R-2-R-17. Each carboxyl group-containing crosslinked polymer salt was sealed and stored in a container with water vapor barrier properties. The physical properties of each obtained crosslinked polymer salt were measured in the same manner as in Production Example 1 and are shown in Table 1. 【0068】 【0069】 The details of the compounds used in Table 1 are shown below. • AA: Acrylic acid • BA: n-butyl acrylate • PEA: Phenoxyethyl acrylate • LA: Lauryl acrylate • SA: Stearyl acrylate • R-1620: 2-(perfluorohexyl)ethyl acrylate • GMA: Glycidyl methacrylate • AAm: Acrylamide • T-20: Trimethylolpropanediallyl 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 ２O: Lithium hydroxide monohydrate, Na ２ CO ３ : Sodium carbonate K ２ CO ３ Potassium carbonate 【0070】 Example 1 (Preparation of Electrode Mixture Layer Composition) Artificial graphite (product name "SCMG-CF" manufactured by Showa Denko Corporation) and SiO (5 μm, manufactured by Osaka Titanium Technologies Co., Ltd.) were used as the active material. A mixture of crosslinked polymer salt R-1, styrene / butadiene rubber (SBR), and sodium carboxymethylcellulose (CMC) was used as the binder. Artificial graphite:SiO:R-1:SBR:CMC were added to a planetary mixer (Hibismix 2P-03, manufactured by Primix Corporation) with water as the diluent so that the solid content concentration of the electrode mixture layer composition was 53% by mass, in a mass ratio of artificial graphite:SiO:R-1:SBR:CMC = 76.8:19.2:1.0:2.0:1.0 (solid content), and the mixture was mixed for 2 hours and 15 minutes to prepare a slurry-like electrode mixture layer composition (electrode slurry). 【0071】 (Preparation of Negative Electrode Plate) Next, the electrode slurry was applied to the current collector (copper foil, thickness: 16.5 μm) using a variable applicator, and a composite layer was formed by drying in a forced-air dryer at 80°C for 15 minutes. After that, the composite layer had a thickness of 50 ± 5 μm and a composite density of 1.60 ± 0.10 g / cm³. ３ After rolling the material to the desired shape, a 3 cm square negative electrode plate was punched out for battery evaluation. Furthermore, the 3 cm square negative electrode plate was sandwiched between two 5 cm x 8 cm glass plates and secured with clips. The plate was then placed vertically on the middle shelf of a vacuum dryer and dried under the conditions of -0.1 MPa, 130°C, and 8 hours. A secondary battery was then fabricated using this negative electrode plate. 【0072】 (Preparation of positive electrode plates) LiNi as the positive electrode active material in N-methylpyrrolidone (NMP) solvent. ０．５ Co ０．２ Mn ０．３ O ２A composition for the positive electrode composite layer was prepared by mixing 100 parts of (NCM) and 2 parts of acetylene black, and adding 4 parts of polyvinylidene fluoride (PVDF) as a positive electrode binder. Next, the composite layer was formed by coating and drying the composition for the positive electrode composite layer onto a current collector (aluminum foil, thickness: 20 μm) using a variable applicator. Subsequently, the thickness of the composite layer was 125 μm ± 1 μm, and the composite density was 3.0 ± 0.10 g / cm³. ３ After rolling the material to the desired shape, a 3 cm square positive electrode plate was punched out for battery evaluation. Furthermore, the 3 cm square positive electrode plate was sandwiched between two 5 cm x 8 cm glass plates and secured with clips. The plate was then placed vertically on the middle shelf of a vacuum dryer and dried under the conditions of -0.1 MPa, 130°C, and 8 hours. A secondary battery was then fabricated using this positive electrode plate. 【0073】 (Preparation of electrolyte) A mixed solvent consisting of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio EC:DMC = 3:7) is to be mixed with vinylene carbonate (VC) to a total of 1% by mass and fluoroethylene carbonate (FEC) to a total of 2% by mass, and LiPF ６ A non-aqueous electrolyte was prepared by dissolving 1.2 mol / liter of [the substance]. 【0074】 (Fabrication of the secondary battery) The battery was constructed by attaching lead terminals to the positive and negative electrodes, and placing the electrode bodies opposite each other with a separator (made of polyethylene: film thickness 16 μm, porosity 47%) in between. The electrodes were then placed in an aluminum laminate battery casing, injected with electrolyte, and sealed to create a test battery. The design capacity of this prototype battery is 50 mAh. The battery's design capacity was based on a charging termination voltage of 4.2 V. 【0075】 <Measurement of moisture content in the negative electrode mixture layer> The rolled negative electrode plate prepared as described above was punched out into a coin-cell shape with a diameter of 16.5 mm, wrapped in aluminum foil, and placed horizontally on the middle shelf of a vacuum dryer. The negative electrode plate was dried under the conditions of -0.1 MPa, 130°C, and 8 hours, and the moisture content in the negative electrode mixture layer was measured using this plate. 【0076】(Measurement conditions for moisture content) The moisture content in the negative electrode mixture layer was measured by coulometric titration using a Karl Fischer moisture meter under the following conditions: Equipment: Vacuum dryer (AVO-200V CR) Karl Fischer moisture meter (MKC-710) manufactured by AS ONE, manufactured by Kyoto Electronics Manufacturing Co., Ltd. Measurement conditions: N ２ Gas flow rate: 200 mL / min Back purge: 180 seconds Cell purge: 1,200 seconds Heating rate: 0.2 °C / second Maximum measurement time: 1,200 seconds Anode liquid: ChemAqua Anode Liquid AGE (manufactured by Kyoto Electronics Manufacturing Co., Ltd.) Cathode liquid: ChemAqua Cathode Liquid CGE (manufactured by Kyoto Electronics Manufacturing Co., Ltd.) Nitrogen was used as the carrier gas, and the temperature was raised from 100 °C to 150 °C, after which measurements were taken at a constant temperature of 150 °C. The end point of the measurement was set when the amount of moisture generated per second became 1 / 20 of the maximum value after 150 °C. Five coin cells were used for each measurement, and the moisture content of only the composite layer of the negative electrode plate was calculated by subtracting the mass of the copper foil from the mass of the coin cells at the time of measurement. Here, the amount of moisture in the negative electrode composite layer was determined using the following formula (2). (1) Moisture content in the negative electrode mixture layer (ppm) = (Amount of electricity required to generate iodine (C) / 10.71 (C)) / (Mass of the negative electrode mixture layer (g) × 1,000) / 100 (2) The moisture content in the negative electrode mixture layer calculated using the above formula (2) is 101 ppm, and the moisture content was evaluated as "A" based on the following criteria. It should be noted that the lower the above moisture content, the easier it is to relax the drying conditions (shorten the drying process) when manufacturing secondary battery electrodes, and the more the productivity of secondary battery electrodes can be improved. (Criteria for determining the drying properties of electrode slurry) S: Moisture content in the negative electrode mixture layer is less than 100 ppm A: Moisture content in the negative electrode mixture layer is 100 ppm or more and less than 110 ppm B: Moisture content in the negative electrode mixture layer is 110 ppm or more and less than 125 ppm C: Moisture content in the negative electrode mixture layer is 125 ppm or more and less than 140 ppm D: Moisture content in the negative electrode mixture layer is 140 ppm or more 【0077】 <Evaluation of Cycle Characteristics> The lithium-ion secondary battery made of laminated cells prepared above was subjected to charge and discharge operations at a charge / discharge rate of 0.1C under conditions of 2.5 to 4.2V in a 45°C environment, and the initial capacity C was evaluated. ０The following was measured. Furthermore, the charge and discharge cycles were repeated under CC discharge conditions of 2.5 to 4.2V in a 25°C environment at a charge and discharge rate of 0.5C, and the capacity C after 100 cycles was measured. １００ The following was measured. Here, the cycle characteristic (ΔC) was calculated using the following formula: ΔC = C １００ / C ０ ×100 (%) The ΔC calculated using the above formula is 84.3%, and the cycle characteristics were evaluated as "B" based on the following criteria. Note that a higher value of ΔC indicates better cycle characteristics. (Cycle characteristics evaluation criteria) A: Charge / discharge capacity retention rate of 85.0% or more B: Charge / discharge capacity retention rate of 83.0% or more and less than 85.0% C: Charge / discharge capacity retention rate of 81.0% or more and less than 83.0% D: Charge / discharge capacity retention rate less than 81.0% 【0078】 ≪Overall Evaluation≫ The overall evaluation was based on the evaluation results of the moisture content in the negative electrode mixture layer and the cycle characteristics, according to the criteria shown in Table 2 below. In this evaluation, A and B ratings are considered passing levels. Since the moisture content in the negative electrode mixture layer of Example 1 was evaluated as "A" and the cycle characteristics were evaluated as "B", the overall evaluation was "A". 【0079】 【0080】 Examples 2 to 15 and Comparative Examples 1 to 4: Electrode slurries were prepared by the same procedure as in Example 1, except that the formulations were as shown in Table 3. The water content in the negative electrode mixture layer obtained using the electrode slurry and the cycle characteristics of the secondary battery were evaluated. The results are shown in Table 3. 【0081】 【0082】 The details of the compounds used in Table 3 are as follows: • SBR: Styrene-butadiene rubber • CMC: Sodium carboxymethylcellulose 【0083】≪Evaluation Results≫ As is clear from the results of Examples 1 to 15, the amount of moisture in the secondary battery electrode mixture layer obtained using the secondary battery electrode binder of the present invention is low, which makes it possible to relax the drying conditions during secondary battery electrode manufacturing (shorten the drying process) and contribute to improving the productivity of secondary battery electrodes. In addition, secondary batteries equipped with the above secondary battery electrodes exhibited excellent cycle characteristics. Among these, the case where the SP value related to component (b) was 8.0 or more and 11.0 or less (Examples 1 to 4) showed a better balance between the effect of reducing the amount of moisture in the electrode mixture layer and the cycle characteristics of the secondary battery than the case where the SP value was less than 8.0 and greater than 11.0 (Examples 5 and 6). 【0084】 Furthermore, focusing on the content of component (b) in the carboxyl group-containing crosslinked polymer, when the content of the component was 30.0 parts by mass or less (Example 1: 20.0 parts by mass, Example 8: 10.0 parts by mass, Example 9: 2.0 parts by mass), the results showed a better balance between the effect of reducing the amount of water in the electrode mixture layer and the cycle characteristics of the secondary battery compared to when the content was greater than 30.0 parts by mass (Example 7: 40.0 parts by mass). In addition, focusing on the neutralized salt of the carboxyl group-containing crosslinked polymer, when the neutralized salt was a lithium salt (Example 1), the results showed superior cycle characteristics compared to when the neutralized salt was a sodium salt (Example 10) or a potassium salt (Example 11). 【0085】 Furthermore, focusing on the content of the carboxyl group-containing crosslinked polymer salt in the secondary battery electrode mixture layer composition, when the content was 1.0 part by mass (Example 1) per 100 parts by mass of the total amount of active material, the result was superior in terms of the balance between the effect of reducing the amount of water in the electrode mixture layer and the cycle characteristics of the secondary battery, compared to the cases of 0.52 parts by mass (Example 14) and 1.6 parts by mass (Example 15). 【0086】 In contrast, when component (b) was not included (Comparative Example 1) or when the SP value of component (b) was greater than 12.5 (Comparative Example 2), the amount of water in the electrode mixture layer increased significantly. Furthermore, when the degree of neutralization of the carboxyl group-containing crosslinked polymer was less than 30% (Comparative Example 3) or greater than 85 mol% (Comparative Example 4), the cycle characteristics deteriorated. 【0087】The secondary battery electrode composite layer composition containing the binder for secondary battery electrodes of the present invention can shorten the drying process and improve the productivity of secondary battery electrodes, and secondary batteries equipped with electrodes obtained using the binder exhibit good durability (cycle characteristics). For this reason, secondary batteries equipped with electrodes obtained using the binder are expected to have good integrity and exhibit good durability (cycle characteristics) even after repeated charging and discharging, and are expected to contribute to increasing the capacity of secondary batteries for automotive use and the like. The binder for secondary battery electrodes of the present invention can be suitably used in non-aqueous electrolyte secondary battery electrodes, and is particularly useful in non-aqueous electrolyte lithium-ion secondary batteries with high energy density.

Claims

1. A binder for secondary battery electrodes containing a carboxyl group-containing crosslinked polymer or a salt thereof, wherein the carboxyl group-containing crosslinked polymer contains 50.0% to 99.5% by mass of structural units derived from ethylenically unsaturated carboxylic acid monomers, and the SP value of the homopolymer is 7.0 to 12.5 (cal / m³). ３ ) １／２ A binder for secondary battery electrodes, wherein the polymer contains 0.5% by mass or more and 50.0% by mass or less of structural units derived from ethylenically unsaturated monomers having ester bonds within the molecule, and the degree of neutralization of the carboxyl group-containing crosslinked polymer is 30 mol% or more and 85 mol% or less.

2. A composition for the secondary battery electrode composite layer is prepared, containing graphite, silicon oxide, the binder for secondary battery electrodes described in claim 1, styrene / butadiene rubber, and sodium carboxymethylcellulose in a solid content mass ratio of 76.8:19.2:1.0:2.0:1.0, with water as the solvent, and having a solid content concentration of 53% by mass. This composition is applied to the surface of the current collector, dried at 80°C for 15 minutes, and then rolled to a thickness of 50 ± 5 μm and a composite density of 1.60 ± 0.10 g / cm³. ３ A binder for secondary battery electrodes according to claim 1, wherein a secondary battery electrode mixture layer is formed in such a manner, and when the mixture layer is dried under the conditions of -0.1 MPa, 130°C, and 8 hours, the moisture content in the mixture layer is 125 ppm or less.

3. The binder for secondary battery electrodes according to claim 1 or 2, wherein the carboxyl group-containing crosslinked polymer salt is an alkali metal salt.

4. The binder for secondary battery electrodes according to claim 1 or 2, wherein the carboxyl group-containing crosslinked polymer or salt thereof is crosslinked with a crosslinkable monomer, and the amount of the crosslinkable monomer used is 0.03 mol% or more and 1.5 mol% or less relative to the total amount of the non-crosslinkable monomer.

5. A composition for a secondary battery electrode mixture layer, comprising the binder for secondary battery electrodes, active material, and water according to claim 1 or 2.

6. The composition for a secondary battery electrode layer according to claim 5, wherein the content of the binder for secondary battery electrodes is 0.25 parts by mass or more and 2.0 parts by mass or less per 100 parts by mass of the total amount of the active material.

7. A secondary battery electrode comprising a composite layer formed from the secondary battery electrode composite layer composition described in claim 5 on the surface of a current collector.

8. A secondary battery comprising the secondary battery electrodes described in claim 7.

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