JP7700786B2 - Carboxyl group-containing crosslinked polymer or its salt and use thereof - Google Patents

Description

The present invention relates to a carboxyl group-containing crosslinked polymer or its salt and its use.

Carboxyl group-containing polymers are used in a variety of applications, such as thickeners and viscosity adjusters for cosmetics, binders for electrodes of non-aqueous electrolyte secondary batteries, anti-settling agents for pigments, and dispersion stabilizers for metal powders.
Among these applications, as a thickener for cosmetics, when the carboxyl group-containing polymer is linear, it has a characteristic that it has a stringiness and feels sticky, but as the degree of crosslinking increases, the stringiness decreases and a fresh feeling is felt. Therefore, in cosmetics that do not require stringiness but require a fresh feeling, a carboxyl group-containing crosslinked polymer is often used because of the advantage that a high thickening effect can be obtained with a small amount of use.

Carboxyl group-containing crosslinked polymers are also often used as binders for electrodes of non-aqueous electrolyte secondary batteries because of the advantages of being able to impart good binding properties and cycle characteristics.
Here, as the applications of various secondary batteries such as nickel-hydrogen secondary batteries, lithium-ion secondary batteries, and electric double-layer capacitors expand, there is a tendency for the demand for improved energy density, reliability, and durability to increase. For example, in order to increase the electric capacity of lithium-ion secondary batteries, specifications using silicon-based active materials as negative electrode active materials are increasing. However, it is known that silicon-based active materials have a large volume change during charging and discharging, and as they are used repeatedly, peeling or falling off of the electrode mixture layer occurs, resulting in a decrease in the capacity of the battery and a deterioration in cycle characteristics (durability). In order to suppress such defects, studies have been conducted to improve durability by firmly binding the active materials together using a binder (binding property), reducing the size of the active materials to relieve stress associated with swelling and shrinkage, and devising additives for the electrolyte.

In this context, it has been reported that acrylic acid-based 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 that by using a polymer in which polyacrylic acid is crosslinked with a specific crosslinking agent as a binder, good cycle characteristics are exhibited without destroying the electrode structure even when an active material containing silicon is used. Patent Document 2 discloses a crosslinked acrylic acid-based polymer having a specific particle size in a 1% NaCl aqueous solution.

International Publication No. 2014/065407 International Publication No. 2017/073589

According to the inventors' investigations, when the slightly cross-linked acrylic acid-based polymer disclosed in Patent Documents 1 and 2 is used as a binder for particles in a slurry (e.g., a binder for active material in a lithium-ion secondary battery electrode slurry), the adhesion between particles in the slurry can be increased by the slightly cross-linking of the acrylic acid-based polymer, but the viscosity increases significantly even when even a small amount is added because the polymer spreads more in water, and there is a limit to how much the slurry viscosity can be reduced, which can cause problems in that it is not possible to achieve both coatability and coating film performance (e.g., the cycle characteristics of a lithium-ion secondary battery).

In addition, the binders for secondary battery electrodes disclosed in Patent Documents 1 and 2 can both impart good cycle characteristics and binding properties, but with the improvement in performance of secondary batteries, there is an increasing demand for binders that can further improve cycle characteristics.
Furthermore, in general, a secondary battery electrode is obtained by applying and drying an electrode mixture layer composition (hereinafter also referred to as "electrode slurry") containing an active material and a binder to the surface of an electrode current collector. In this case, it is advantageous to increase the solid content concentration of the electrode slurry from the viewpoint of increasing the drying efficiency of the electrode slurry and improving the productivity of the electrode. However, it is usually difficult to ensure good coatability as the solid content concentration is increased. As described above, the binders disclosed in Patent Documents 1 and 2 have a large increase in viscosity even with a small amount of addition, making it difficult to increase the solid content concentration.

The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a carboxyl group-containing crosslinked polymer or a salt thereof that can achieve both coatability and coating film performance of a composition containing the crosslinked polymer or a salt thereof.
Furthermore, the present invention aims to provide a binder for a secondary battery electrode, which can obtain a secondary battery exhibiting excellent cycle characteristics while ensuring coatability by reducing the viscosity of the electrode slurry when the solid content concentration of the composition for an electrode mixture layer is higher than that of a conventional composition. In addition, the present invention aims to provide a composition for a secondary battery electrode mixture layer containing the binder, and a secondary battery electrode and a secondary battery obtained by using the composition.

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above problems, and have found that by using a carboxyl group-containing crosslinked polymer or a salt thereof having a heterogeneous network structure size measured by small-angle X-ray scattering that is equal to or smaller than a specific value, it is possible to ensure the coatability of a composition containing the crosslinked polymer or a salt thereof while exhibiting excellent coating film performance, thereby completing the present invention.
Furthermore, the present inventors have found that when the solids concentration of a composition for a secondary battery electrode mixture layer containing a binder for a secondary battery electrode containing a carboxyl group-containing crosslinked polymer or a salt thereof, an active material, and water is high, by containing a carboxyl group-containing crosslinked polymer or a salt thereof having a heterogeneous network structure size obtained by small-angle X-ray scattering measurement of a specific value or less, a secondary battery exhibiting excellent cycle characteristics while ensuring coatability by reducing the viscosity of the electrode slurry can be obtained, and have completed the present invention.

〔２〕前記Ξ１と、小角Ｘ線散乱法（測定温度：２５．０±０．１℃）により、中和度５０～１００モル％に中和された前記架橋重合体の５質量％濃度水溶液を測定することで得られる散乱強度曲線Ｉ（ｑ）に対し、前記式（１）でカーブフィッティングして算出される、前記架橋重合体の不均一網目構造サイズΞ（以下、「Ξ５」ともいう。）の差ΔΞ（Ξ１－Ξ５）が５０以下である、〔１〕に記載のカルボキシル基含有架橋重合体又はその塩。
〔３〕前記架橋重合体は、その全構造単位に対し、エチレン性不飽和カルボン酸単量体に由来する構造単位を５０質量％以上１００質量％以下含む、〔１〕又は〔２〕に記載のカルボキシル基含有架橋重合体又はその塩。
〔４〕前記架橋重合体は、架橋性単量体により架橋されたものであり、当該架橋性単量体の使用量が非架橋性単量体の総量１００質量部に対して０．１質量部以上２．０質量部以下である、〔１〕～〔３〕のいずれか一に記載のカルボキシル基含有架橋重合体又はその塩。
〔５〕前記架橋重合体又はその塩は、中和度８０～１００モル％に中和された後、水媒体中で測定した粒子径が、体積基準メジアン径で０．１μｍ以上５．０μｍ以下である、〔１〕～〔４〕のいずれか一に記載のカルボキシル基含有架橋重合体又はその塩。
〔６〕前記架橋重合体又はその塩は、ｐＨ８における水膨潤度が２０以上８０以下である、〔１〕～〔５〕のいずれか一に記載のカルボキシル基含有架橋重合体又はその塩。
〔７〕〔１〕～〔６〕のいずれか一に記載のカルボキシル基含有架橋重合体又はその塩を含む、二次電池電極用バインダー。
〔８〕〔７〕に記載の二次電池電極用バインダー、活物質及び水を含む二次電池電極合剤層用組成物。
〔９〕集電体表面に、〔８〕に記載の二次電池電極合剤層用組成物から形成される合剤層を備える、二次電池電極。
〔１０〕〔９〕に記載の二次電池電極を備える二次電池。 The present invention is as follows.
[1] A carboxyl group-containing crosslinked polymer or a salt thereof, wherein the crosslinked polymer has a heterogeneous network structure size Ξ (hereinafter also referred to as "Ξ1") of 80 or less, as calculated by curve fitting the following formula (1) to a scattering intensity curve I(q) obtained by measuring a 1 mass % concentration aqueous solution of the crosslinked polymer neutralized to a degree of neutralization of 50 to 100 mol % by a small angle X-ray scattering method (measurement temperature: 25.0±0.1°C).

[2] The carboxyl group-containing crosslinked polymer or a salt thereof according to [1], wherein the difference ΔΞ (Ξ1-Ξ5) between the size of the heterogeneous network structure of the crosslinked polymer, calculated by curve fitting the scattering intensity curve I(q) obtained by measuring a 5% by mass aqueous solution of the crosslinked polymer neutralized to a degree of neutralization of 50 to 100 mol% by a small-angle X-ray scattering method (measurement temperature: 25.0±0.1°C) and the size of the heterogeneous network structure of the crosslinked polymer (hereinafter also referred to as "Ξ5") using the formula (1), is 50 or less.
[3] The carboxyl group-containing crosslinked polymer or its salt according to [1] or [2], wherein the crosslinked polymer contains 50% by mass or more and 100% by mass or less of structural units derived from ethylenically unsaturated carboxylic acid monomers relative to all structural units thereof.
[4] The crosslinked polymer is crosslinked by a crosslinkable monomer, and 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. The carboxyl group-containing crosslinked polymer or its salt according to any one of [1] to [3].
[5] The crosslinked polymer or salt thereof according to any one of [1] to [4], wherein the crosslinked polymer or salt thereof has a volume-based median particle size of 0.1 μm or more and 5.0 μm or less, as measured in an aqueous medium after being neutralized to a degree of neutralization of 80 to 100 mol %.
[6] The carboxyl group-containing crosslinked polymer or its salt according to any one of [1] to [5], wherein the crosslinked polymer or its salt has a water swelling degree at pH 8 of 20 or more and 80 or less.
[7] A binder for secondary battery electrodes, comprising the carboxyl group-containing crosslinked polymer or its salt according to any one of [1] to [6].
[8] A composition for a secondary battery electrode mixture layer, comprising the binder for a secondary battery electrode according to [7], an active material, and water.
[9] A secondary battery electrode comprising a mixture layer formed on a surface of a current collector from the composition for a secondary battery electrode mixture layer according to [8].
[10] A secondary battery comprising the secondary battery electrode according to [9].

According to the carboxyl group-containing crosslinked polymer or a salt thereof of the present invention, it is possible to achieve both excellent coatability and excellent coating film performance for a composition containing the crosslinked polymer or a salt thereof.
Furthermore, according to the binder for secondary battery electrodes containing the carboxyl group-containing crosslinked polymer or a salt thereof of the present invention, even when the solids concentration of the composition for electrode mixture layer is higher than that of the conventional composition, it is possible to obtain a secondary battery that exhibits excellent cycle characteristics while ensuring coatability by reducing the viscosity of the electrode slurry.

FIG. 2 is a schematic diagram showing the size Ξ (dotted line) of the non-uniform network structure of a crosslinked polymer, and the Ξ value in the left diagram is smaller than that in the right diagram. FIG. 1 is a diagram showing a scattering intensity curve I(q) obtained by measuring a carboxyl group-containing crosslinked polymer salt R-2 (aqueous solution with a concentration of 1% by mass and an aqueous solution with a concentration of 5% by mass) by a small-angle X-ray scattering method. FIG. 1 is a diagram showing a scattering intensity curve I(q) obtained by measuring a carboxyl group-containing crosslinked polymer salt R-17 (aqueous solution with a concentration of 1% by mass and an aqueous solution with a concentration of 5% by mass) by a small-angle X-ray scattering method. FIG. 1 is a diagram showing an apparatus used for measuring the water swelling degree of a crosslinked polymer or a salt thereof.

The carboxyl group-containing crosslinked polymer or salt thereof of the present invention (hereinafter also referred to as "the crosslinked polymer") has a heterogeneous network structure size Ξ (Ξ1) of 80 or less, calculated by curve fitting the scattering intensity curve I(q) obtained by measuring a 1% by mass aqueous solution of the crosslinked polymer neutralized to a degree of neutralization of 50 to 100 mol% by small-angle X-ray scattering (measurement temperature: 25.0±0.1°C) with the above formula (1). Note that Ξ will be described in detail in "3. Properties of the Crosslinked Polymer" and "Examples" below.

Furthermore, the binder for secondary battery electrodes containing the present crosslinked polymer or a salt thereof (hereinafter also referred to as "the binder") can be mixed with an active material and water to form a composition for secondary battery electrode mixture layers (hereinafter also referred to as "the composition"). The above composition is in a slurry state that can be applied to a current collector. The secondary battery electrode of the present invention can be obtained by forming a mixture layer formed from the above composition on the surface of a current collector such as copper foil or aluminum foil. Here, the present binder is preferable in that the effect of the present invention is particularly large when used in a composition for secondary battery electrode mixture layers containing a silicon-based active material described later as an active material.

The present crosslinked polymer, the present binder, the secondary battery electrode mixture layer composition obtained using the present binder, the secondary battery electrode, and the secondary battery will be described in detail below.
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.

1. Structural unit of the present crosslinked polymer <Structural unit derived from ethylenically unsaturated carboxylic acid monomer>
The crosslinked polymer has a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as "component (a)"), and can be introduced into the polymer by precipitation polymerization or dispersion polymerization of a monomer component containing an ethylenically unsaturated carboxylic acid monomer. The crosslinked polymer has such a structural unit and thus has a carboxyl group, which improves adhesion to a current collector, and also has excellent desolvation effect and ionic conductivity of lithium ions, resulting in an electrode with low resistance and excellent high-rate characteristics. In addition, water swelling is imparted, which can increase the dispersion stability of the active material in the composition.

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

The content of component (a) in the crosslinked polymer is not particularly limited, but may be, for example, 10% by mass or more and 100% by mass or less based on the total structural units of the crosslinked polymer. By including component (a) in such a range, excellent adhesion to the current collector can be easily ensured. The lower limit is, for example, 20% by mass or more, for example, 30% by mass or more, and for example, 40% by mass or more. When the lower limit is 50% by mass or more, the dispersion stability of the composition becomes good and a higher binding force is obtained, which is preferable, and may be 60% by mass or more, 70% by mass or more, or 80% by mass or more. The upper limit is, for example, 99.9% by mass or less, for example, 99.5% by mass or less, for example, 99% by mass or less, for example, 98% by mass or less, for example, 95% by mass or less, for example, 90% by mass or less, and for example, 80% by mass or less. The range can be an appropriate combination of these lower and upper limits, and can be, for example, 10% by mass or more and 100% by mass or less, for example, 50% by mass or more and 100% by mass or less, for example, 50% by mass or more and 99.9% by mass or less, for example, 50% by mass or more and 99% by mass or less, or for example, 50% by mass or more and 98% by mass or less.

<Other structural units>
In addition to the component (a), the crosslinked polymer may contain a structural unit derived from another ethylenically unsaturated monomer copolymerizable therewith (hereinafter also referred to as "component (b)"). Examples of the component (b) include structural units derived from an ethylenically unsaturated monomer compound having an anionic group other than a carboxyl group, such as a sulfonic acid group and a phosphoric acid group, or a nonionic ethylenically unsaturated monomer. These structural units can be introduced by copolymerizing a monomer containing an ethylenically unsaturated monomer compound having an anionic group other than a carboxyl group, such as a sulfonic acid group and a phosphoric acid group, or a nonionic ethylenically unsaturated monomer.

The proportion of the (b) component can be 0% by mass or more and 90% by mass or less with respect to the total structural units of the crosslinked polymer. The proportion of the (b) component may be 1% by mass or more and 60% by mass or less, 2% by mass or more and 50% by mass or less, 5% by mass or more and 40% by mass or less, or 10% by mass or more and 30% by mass or less. In addition, when the (b) component is contained at 1% by mass or more with respect to the total structural units of the crosslinked polymer, the affinity to the electrolyte is improved, and therefore the effect of improving lithium ion conductivity can also be expected.

As component (b), among the above, structural units derived from nonionic ethylenically unsaturated monomers are preferred from the viewpoint of obtaining an electrode with good bending resistance, and examples of nonionic ethylenically unsaturated monomers include (meth)acrylamide and its derivatives, ethylenically unsaturated monomers containing a nitrile group, and ethylenically unsaturated monomers containing an alicyclic structure.

Examples of (meth)acrylamide derivatives include N-alkyl (meth)acrylamide compounds such as isopropyl (meth)acrylamide and 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 dimethyl (meth)acrylamide and diethyl (meth)acrylamide. One of these may be used alone, or two or more may be used in combination.

Examples of nitrile group-containing ethylenically unsaturated monomers 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. Among the above, acrylonitrile is preferred because of its high nitrile group content.

Examples of alicyclic structure-containing ethylenically unsaturated monomers include cycloalkyl (meth)acrylate 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, dicyclopentenyl (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. One of these may be used alone, or two or more may be used in combination.

In terms of excellent binder binding properties, the present crosslinked polymer or its salt preferably contains structural units derived from (meth)acrylamide and its derivatives, as well as nitrile group-containing ethylenically unsaturated monomers, alicyclic structure-containing ethylenically unsaturated monomers, etc. In addition, 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 (b), it can exhibit strong interaction with the electrode material and exhibit good binding properties to the active material. As a result, a firm and well-integrated electrode mixture layer can be obtained, and therefore, as the above-mentioned "hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g/100 ml or less", an alicyclic structure-containing ethylenically unsaturated monomer is particularly preferred.

As other nonionic ethylenically unsaturated monomers, for example, (meth)acrylic acid esters may be used. Examples of the (meth)acrylic acid esters include (meth)acrylic acid alkyl ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, 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, and phenylethyl (meth)acrylate;
Examples of the alkoxyalkyl (meth)acrylate compounds include 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate. One of these compounds may be used alone, or two or more of them may be used in combination.

From the viewpoint of binding to the active material and cycle characteristics, aromatic (meth)acrylic acid ester compounds can be preferably used. From the viewpoint 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.

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 because they provide good binding strength for the binder. In addition, as nonionic ethylenically unsaturated monomers, compounds whose homopolymer glass transition temperature (Tg) is 0°C or less are preferred because they provide good bending resistance for the resulting electrodes.

The crosslinked polymer may be in the form of a salt in which some or all of the carboxyl groups contained in the polymer have been 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 calcium salts and barium salts; other metal salts such as magnesium salts and aluminum salts; ammonium salts, and organic amine salts. Among these, alkali metal salts and magnesium salts are preferred because they are less likely to adversely affect the battery characteristics, and alkali metal salts are more preferred.

The present crosslinked polymer is a crosslinked polymer having a crosslinked structure. The crosslinking method for the present crosslinked polymer is not particularly limited, and examples thereof include the following methods.
1) Copolymerization of crosslinkable monomers 2) Utilizing chain transfer to polymer chains during radical polymerization 3) After synthesis of a polymer having a reactive functional group, post-crosslinking by adding a crosslinking agent as necessary By having the present crosslinked polymer have a crosslinked structure, a binder containing the crosslinked polymer or a salt thereof can have excellent binding strength. Among the above, the method using copolymerization of crosslinkable monomers is preferred from the viewpoints of simple operation and easy control of the degree of crosslinking.

<Crosslinkable Monomer>
Examples of the crosslinkable monomer include polyfunctional polymerizable monomers having two or more polymerizable unsaturated groups, and monomers having a self-crosslinkable crosslinkable functional group such as a hydrolyzable silyl group.

The above-mentioned 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)acrylate 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.

Examples of polyfunctional (meth)acrylate 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 poly(meth)acrylates such as tetra(meth)acrylate; and bisamides such as methylene bisacrylamide and hydroxyethylene bisacrylamide.

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 polyallyl sucrose; polyfunctional allyl compounds such as diallyl phthalate; and polyfunctional vinyl compounds such as divinylbenzene.

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.

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

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, vinyl silanes such as vinyl trimethoxysilane, vinyl triethoxysilane, vinyl methyl dimethoxysilane, vinyl dimethyl methoxysilane, etc.; silyl group-containing acrylic acid esters such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, methyl dimethoxysilylpropyl acrylate, etc.; silyl group-containing methacrylic acid esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyl dimethoxysilylpropyl methacrylate, dimethyl methoxysilylpropyl methacrylate, etc.; silyl group-containing vinyl ethers such as trimethoxysilylpropyl vinyl ether; silyl group-containing vinyl esters such as vinyl trimethoxysilyl undecanoate, etc. can be mentioned.

When the present crosslinked polymer is crosslinked by a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.05 parts by mass or more and 5.0 parts by mass or less, more preferably 0.1 parts by mass or more and 5.0 parts by mass or less, even more preferably 0.2 parts by mass or more and 4.0 parts by mass or less, and even more preferably 0.3 parts by mass or more and 3.0 parts by mass or less, relative to 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer). If the amount of the crosslinkable monomer used is 0.05 parts by mass or more, it is preferable in that the binding property and the 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 high.
Similarly, the amount of the crosslinkable monomer used is preferably 0.02 to 1.7 mol %, more preferably 0.10 to 1.0 mol %, based on the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers).

2. Method for Producing the Present Crosslinked Polymer The present crosslinked polymer can be obtained by precipitation polymerization or dispersion polymerization of a monomer component containing the above-mentioned ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as the present monomer) in the presence of an exchange chain transfer mechanism type regulator.
Here, precipitation polymerization is a method for producing a polymer by carrying out a polymerization reaction in a solvent that dissolves the raw material monomers but does not substantially dissolve the resulting polymer. As the polymerization proceeds, the polymer particles become larger through aggregation and growth, and a dispersion of polymer particles is obtained in which primary particles of several tens to several hundreds of nm are secondary aggregated to several μm to several tens of μm. A dispersion stabilizer can also be used to control the particle size of the polymer.
The secondary aggregation can be suppressed by selecting a dispersion stabilizer, a polymerization solvent, etc. In general, precipitation polymerization in which secondary aggregation is suppressed is also called dispersion polymerization.

Regarding the exchange chain transfer mechanism type control agent, examples of the exchange chain transfer mechanism type control agent according to the present invention include a control agent in a reversible addition-fragmentation chain transfer polymerization method (RAFT method) (hereinafter, also referred to as a "RAFT agent"), a control agent in an iodine transfer polymerization method, a control agent in a polymerization method using an organotellurium compound (TERP method), a control agent in a polymerization method using an organoantimony compound (SBRP method), and a control agent in a polymerization method using an organobismuth compound (BIRP method). As the exchange chain transfer mechanism type control agent, a polymer having one or more vinyl monomer polymer chains and a living radical polymerization active unit by an exchange chain transfer mechanism (hereinafter, also referred to simply as a "first polymer". Details will be described in paragraphs [0052] to [0078] below), and a control agent other than the polymer can be used. The first polymer and the control agent other than the polymer may be used alone or in combination.
By subjecting the present monomer to precipitation polymerization or dispersion polymerization in the presence of an exchange chain transfer mechanism type regulator, the primary chain length is shortened and the chain length is uniformed to form a uniform crosslinked structure, which makes it possible to increase the water swelling degree of the present crosslinked polymer. As a result, it is presumed that it is possible to achieve both good coatability due to a reduction in the electrode slurry viscosity and excellent cycle characteristics.
Among these, the RAFT agent and the control agent for iodine transfer polymerization are preferred, and the RAFT agent is more preferred, in that they can make the crosslinked structure of the present crosslinked polymer more uniform.

As the RAFT agent, a first polymer (described in detail below) having a living radical polymerization active unit by a reversible addition-fragmentation chain transfer method, and/or a RAFT agent other than the first polymer (such as a dithioester compound, a xanthate compound, a trithiocarbonate compound, or a dithiocarbamate compound) can be used.
Specific examples of the RAFT agent other than the first polymer include 2-cyano-2-propyl benzodithioate, 2-phenyl-2-propyl benzodithioate, trithiocarbonate, 2-cyano-2-propyl dodecyl trithiocarbonate, 2-(dodecylthiocarbonothioylthio)propionic acid, 3-((1-carboxyethylthio)carbonothioylthio))propionic acid, methyl 2-(dodecylthiocarbonothioylthio)-2-methylpropanoate, 1,4-bis(n-dodecylsulfanylthiocarbonylsulfanylmethyl)benzene, dibenzyl trithiocarbonate, distyryl trithiocarbonate, dicumyl trithiocarbonate, and cyanomethyl-N-methyl-N-phenyl dithiocarbamate.
Among RAFT agents, those having a trithiocarbonate group in the molecule are particularly preferred, since they can make the crosslinked structure of the present crosslinked polymer more uniform.

The control agent in the iodine transfer polymerization method may be a first polymer (described in detail below) having a living radical active unit produced by iodine transfer polymerization, and/or a control agent other than the first polymer.
Specific examples of the control agent other than the first polymer include perfluoroalkyl perfluoroalkyl iodides having an alkyl group of 1 to 20 carbon atoms, such as methyl iodide, methylene iodide, iodoform, carbon tetraiodide, 1-phenylethyl iodide, and benzyl iodide, ethyl 2-iodopropionate, methyl 2-iodoisobutyrate, ethyl 2-iodoisobutyrate, ethyl 2-iodo-2-phenylacetate, ethylene glycol bis(2-iodo-2-phenylacetic acid), ethylene glycol bis(2-iodoisobutyric acid), 1,5-diiodo-2,4-dimethylbenzene, and 2-iodopropionitrile.

The chain transfer mechanism control agent may be monofunctional with one active site, or bifunctional or higher with two or more active sites. Chain transfer mechanism control agents with bifunctional or higher functionality extend the polymer chain in two or more directions. From the viewpoint of producing the crosslinked polymer, it may be preferable to use a chain transfer mechanism control agent with bifunctional or trifunctional or higher functionality.

The amount of the exchange chain transfer mechanism control agent used is preferably 0.0001 to 0.50 mol % relative to the total amount of this monomer, from the viewpoint of making the crosslinked structure of the crosslinked polymer more uniform, more preferably 0.0001 to 0.40 mol %, even more preferably 0.0001 to 0.30 mol %, and even more preferably 0.0002 to 0.30 mol %.

The polymerization initiator used together with the exchange chain transfer mechanism type control agent may be a known polymerization initiator such as an azo compound, an organic peroxide, or an inorganic peroxide, but is not particularly limited. The conditions of use can be adjusted so that an appropriate amount of radicals is generated by known methods such as thermal initiation, redox initiation using a reducing agent, UV initiation, etc. In order to obtain the present crosslinked polymer having a long primary chain length, it is preferable to set the conditions so that the amount of radicals generated is as small as possible within the range of the allowable production time.
Among the above polymerization initiators, azo compounds are preferred from the viewpoints of safety, ease of handling, and less occurrence of side reactions during radical polymerization. Specific examples of the azo compounds include 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), dimethyl-2,2'-azobis(2-methylpropionate), 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis[N-(2-propenyl)-2-methylpropionamide], and 2,2'-azobis(N-butyl-2-methylpropionamide). The above radical polymerization initiators may be used alone or in combination of two or more.

A preferred amount of the polymerization initiator used 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 part by mass, when the total amount of the monomer components used is taken as 100 parts by mass. If the amount of the polymerization initiator used is 0.001 part by mass or more, the polymerization reaction can be stably carried out, and if it is 2 parts by mass or less, a polymer having a long primary chain length is easily obtained.
The proportion of the polymerization initiator used is not particularly limited, but from the viewpoint of making the crosslinked structure of the crosslinked polymer uniform, the amount of the polymerization initiator used relative to 1 mol of the exchange chain transfer mechanism type controller is preferably 0.5 mol or less, more preferably 0.2 mol or less. In addition, from the viewpoint of stably carrying out the polymerization reaction, the lower limit of the amount of the polymerization initiator used relative to 1 mol of the exchange chain transfer mechanism type controller is 0.001 mol. Therefore, the amount of the polymerization initiator used relative to 1 mol of the exchange chain transfer mechanism type controller is preferably in the range of 0.001 mol to 0.5 mol, more preferably in the range of 0.005 mol to 0.2 mol.

The polymerization solvent may be selected from water and various organic solvents, taking into consideration the type of monomer to be used, etc. In order to obtain a polymer having a longer primary chain length, it is preferable to use a solvent having a small chain transfer constant.
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, and these can be used alone or in combination of two or more. Alternatively, these can be used as a mixed solvent with water. In the present invention, the water-soluble solvent refers to a solvent having a solubility in water at 20° C. of more than 10 g/100 ml.
Among the above, methyl ethyl ketone and acetonitrile are preferred in that they have good polymerization stability with little generation of coarse particles and little adhesion to the reactor, the precipitated present crosslinked polymer is not prone to secondary aggregation (or even if secondary aggregation does occur, it is easily disintegrated in an aqueous medium), a polymer having a small chain transfer constant and a large degree of polymerization (primary chain length) can be obtained, and the operation is easy during the neutralization step described below.

In addition, in order to stably and rapidly proceed with the neutralization reaction in the neutralization step, it is preferable to add a small amount of a highly polar solvent to the polymerization solvent. Examples of such highly polar solvents include water and methanol. The amount of the highly polar solvent used is preferably 0.05 to 20.0% by mass, more preferably 0.1 to 10.0% by mass, even more preferably 0.1 to 5.0% by mass, and even more preferably 0.1 to 1.0% by mass, based on the total mass of the medium. If the proportion of the highly polar solvent is 0.05% by mass or more, an effect on the neutralization reaction is observed, and if it is 20.0% by mass or less, no adverse effect on the polymerization reaction is observed. In addition, in the polymerization of highly hydrophilic ethylenically unsaturated carboxylic acid monomers such as acrylic acid, the polymerization rate is improved when a highly polar solvent is added, making it easier to obtain a polymer with a long primary chain length. Among the highly polar solvents, water is particularly preferable because it has a large effect of improving the polymerization rate.

The reaction temperature during the polymerization reaction in the presence of an exchange chain transfer mechanism type control agent is preferably 30°C or higher and 120°C or lower, more preferably 40°C or higher and 110°C or lower, and even more preferably 50°C or higher and 100°C or lower. If the reaction temperature is 30°C or higher, the polymerization reaction can proceed smoothly. On the other hand, if the reaction temperature is 120°C or lower, side reactions can be suppressed and restrictions on the initiators and solvents that can be used are relaxed.

The crosslinked polymer dispersion obtained through the polymerization step can be dried under reduced pressure and/or heat treatment to remove the solvent, thereby obtaining the desired crosslinked polymer in a powder state. In this case, it is preferable to provide a solid-liquid separation step such as centrifugation and filtration, and a washing step using an organic solvent or a mixed solvent of an organic solvent and water, following the polymerization step, before the drying step, in order to remove unreacted monomers (and their salts).
When the above-mentioned washing step is provided, even if the crosslinked polymer undergoes secondary aggregation, it is easily disintegrated during use, and further, since the remaining unreacted monomer is removed, good performance is also exhibited in terms of coating film performance.

In the manufacturing method of the present invention, when an unneutralized or partially neutralized salt is used as the ethylenically unsaturated carboxylic acid monomer, an alkali compound may be added to the dispersion of the crosslinked polymer obtained by the polymerization step to neutralize the polymer (hereinafter also referred to as "process neutralization"), and then the solvent may be removed in a drying step. Alternatively, after obtaining a powder of the crosslinked polymer in an unneutralized or partially neutralized salt state, an alkali compound may be added when preparing the slurry composition to neutralize the polymer (hereinafter also referred to as "post-neutralization"). Of the above, process neutralization is preferred because it tends to break up secondary aggregates more easily.

Here, as described above, the exchange chain transfer mechanism type control agent can be a polymer chain (hereinafter, also simply referred to as the "first polymer chain") of one or more vinyl monomers (hereinafter, also simply referred to as the "first monomer") and a polymer (first polymer) having a living radical polymerization active unit by the exchange chain transfer mechanism.

In producing the present crosslinked polymer by polymerizing the present monomer in the presence of the first polymer, the first polymer can be used as a base point for the polymerization of the present monomer and as a dispersion stabilizer in the polymerization solvent for the crosslinked polymer, and the present crosslinked polymer in which a polymer chain having a structural unit derived from the present monomer is bonded to the polymer chain of the first polymer can be obtained as dispersed fine particles. In this way, polymerization stability, i.e., aggregation of the present crosslinked polymer during the polymerization process is suppressed, and the generation of coarse aggregated particles is suppressed, and the present crosslinked polymer can be obtained with a small particle size and a narrow particle size distribution.

In producing the present crosslinked polymer by polymerizing the present monomer in the presence of the first polymer, in order to make the first polymer function as a dispersion stabilizer, for example, the first polymer can be used in an amount of 0.3 parts by mass or more and 50 parts by mass or less per 100 parts by mass of the total mass of the present monomer. By using the first polymer in such a range, the present crosslinked polymer containing the present monomer as the main component can be produced while making the first polymer function as a dispersion stabilizer. In addition, if the first polymer is less than 0.3 parts by mass, it is difficult to obtain a sufficient dispersion stabilizing effect, and the particle size of the present crosslinked polymer is likely to exceed 0.3 μm. Even if it exceeds 50 parts by mass, the functionality as a dispersion stabilizer is difficult to improve, and the effect of reducing the particle size of the present crosslinked polymer is also reduced.

The first polymer can be used in an amount of, for example, 0.5 parts by mass or more, or for example, 1 part by mass or more, per 100 parts by mass of the total mass of the present monomer. The first polymer can be used in an amount of, for example, 40 parts by mass or less, or for example, 30 parts by mass or less, or for example, 20 parts by mass or less. The range of the amount of the first polymer used per 100 parts by mass of the total mass of the present monomer can be set by appropriately combining the above upper and lower limits.

Method for Producing First Polymer By polymerizing a monomer composition containing a first monomer in the presence of a known chain exchange transfer mechanism control agent, it is possible to obtain a first polymer having a first polymer chain having a structural unit derived from the first monomer and a living polymerization active unit by the chain exchange transfer mechanism.

The polymerization conditions for producing the first polymer are well known to those skilled in the art, and examples of the polymerization process include various processes such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. Considering that the polymerization base point in the production of the present crosslinked polymer and the function as a dispersion stabilizer, for example, solution polymerization can be used. In addition, the polymerization conditions such as the type of exchange chain transfer mechanism controller, the type and amount of polymerization initiator used, the polymerization solvent, and the reaction temperature are appropriately selected in accordance with the above paragraphs [0040] to [0043] and [0045] to [0049], and the amount of the exchange chain transfer mechanism controller used is appropriately adjusted depending on the number average molecular weight (Mn) of the target first polymer.
As the exchange chain transfer mechanism control agent, a RAFT agent and a control agent for iodine transfer polymerization are preferable in that they can narrow the molecular weight distribution of the first polymer.
Furthermore, the concentration when producing the first polymer is not particularly limited relative to the total mass of the polymerization solvent, the first monomer, etc., charged, but can be, for example, 10 mass% or more and 80 mass% or less, or, for example, 15 mass% or more and 70 mass% or less, or, for example, 20 mass% or more and 70 mass% or less.

Typically, when a monofunctional chain transfer mechanism type control agent is used, the living polymerization active unit is provided at the end of the first polymer chain, and when a bifunctional or higher chain transfer mechanism type control agent is used, the living polymerization active unit is used as a base point and the first polymer chain is provided in each of the two or more directions. In either case, when another polymer chain is provided, the other polymer chain is directly bonded to the living polymerization active unit, and the first polymer chain is bonded to the distal end of the other polymer chain so that the first polymer chain is provided on the more distal side of the living polymerization active unit.

The first polymer may have two or more kinds of first polymer chains. For example, by carrying out living radical polymerization or the like using one or more kinds of first monomers of a certain composition, and then carrying out living radical polymerization or the like using one or more kinds of first monomers of another composition, it is possible to obtain a first polymer having first polymer chains (blocks) having structural units derived from first monomers of different compositions.

The number average molecular weight (Mn) of the first polymer is not particularly limited, but is, for example, 3,000 or more, for example, 5,000 or more, for example, 7,000 or more, for example, 8,000 or more, or for example, 10,000 or more. Also, the Mn is 50,000 or less, for example, 30,000 or less, for example, 25,000 or less, for example, 20,000 or less, for example, 15,000 or less, for example, 14,000 or less, or for example, 12,000 or less. The range of Mn can be set by appropriately combining the above-mentioned lower and upper limits, and is, for example, 5,000 or more and 25,000 or less, for example, 10,000 or more and 25,000 or less, for example, 10,000 or more and 15,000 or less, and for example, 10,000 or more and 14,000 or less.

The weight average molecular weight (Mw) of the first polymer is not particularly limited, but is, for example, 5,000 or more, for example, 7,000 or more, for example, 9,000 or more, for example, 10,000 or more, for example, 13,000 or more, or for example, 15,000 or more. The Mw is 60,000 or less, for example, 55,000 or less, for example, 50,000 or less, for example, 45,000 or less, for example, 40,000 or less, for example, 36,000 or less, for example, 35,000 or less, for example, 30,000 or less, or for example, 25,000 or less. The range of Mw can be set by appropriately combining the above-mentioned lower and upper limits, and is, for example, 1,000 or more and 40,000 or less, for example, 10,000 or more and 35,000 or less, for example, 10,000 or more and 30,000 or less, and for example, 15,000 or more and 25,000 or less.

The Mw and Mn of the first polymer can be measured by gel permeation chromatography using polystyrene as a standard substance. Details of the chromatography conditions can be those disclosed in the examples below.

The molecular weight distribution (Mw/Mn) of the first polymer is not particularly limited, but may be, for example, 2.5 or less, for example, 2.4 or less, for example, 2.3 or less, for example, 2.0 or less, for example, 1.6 or less, for example, 1.5 or less, for example, 1.4 or less, or for example, 1.3 or less. The molecular weight distribution may be, for example, 1.1 or more, for example, 1.2 or more, for example, 1.3 or more, for example, 1.4 or more, or for example, 1.5 or more. The range of the molecular weight distribution may be set by appropriately combining the above-mentioned lower and upper limits, and may be, for example, 1.1 or more and 2.5 or less, for example, 1.1 or more and 2.4 or less, for example, 1.1 or more and 2.3 or less, or for example, 1.1 or more and 2.0 or less.

The smaller the molecular weight distribution of the first polymer, the smaller the particle size of the resulting crosslinked polymer tends to be. It is preferable that the molecular weight distribution is 2.4 or less, and to obtain a crosslinked polymer with a smaller particle size, it is preferable that the molecular weight distribution is 1.7 or less, more preferably 1.6 or less, and even more preferably 1.4 or less.

<First Monomer>
Examples of the first monomer include styrenes, (meth)acrylonitrile compounds, maleimide compounds, unsaturated acid anhydrides, and unsaturated carboxylic acid compounds. These may be used alone or in combination of two or more.

Styrenes include styrene and its derivatives. Specific examples of compounds include styrene, α-methylstyrene, β-methylstyrene, vinyltoluene, vinylxylene, vinylnaphthalene, o-methylstyrene, m-methylstyrene, p-methylstyrene, o-ethylstyrene, m-ethylstyrene, p-ethylstyrene, p-n-butylstyrene, p-isobutylstyrene, p-t-butylstyrene, o-methoxystyrene, m-methoxystyrene, p-methoxystyrene, o-chloromethylstyrene, p-chloromethylstyrene, o-chlorostyrene, p-chlorostyrene, o-hydroxystyrene, m-hydroxystyrene, p-hydroxystyrene, divinylbenzene, and the like. One or more of these can be used. Among these, from the viewpoint of polymerizability, styrene, o-methoxystyrene, m-methoxystyrene, p-methoxystyrene, o-hydroxystyrene, m-hydroxystyrene, and p-hydroxystyrene are preferred.

(Meth)acrylonitrile compounds include (meth)acrylonitrile, acrylonitrile, α-methylacrylonitrile, etc. For example, acrylonitrile is used.

Maleimide compounds include maleimide and N-substituted maleimide compounds. Specific examples of N-substituted maleimide compounds include N-alkyl-substituted maleimide compounds such as N-methylmaleimide, N-ethylmaleimide, N-n-propylmaleimide, N-isopropylmaleimide, N-n-butylmaleimide, N-isobutylmaleimide, N-tert-butylmaleimide, N-pentylmaleimide, N-hexylmaleimide, N-heptylmaleimide, N-octylmaleimide, N-laurylmaleimide, and N-stearylmaleimide; N-cyclopentylmaleimide, N-cyclohexylmaleimide, and N-cyclohexylmaleimide. Examples of the maleimide include N-cycloalkyl-substituted maleimide compounds such as hexylmaleimide, and N-aryl-substituted maleimide compounds such as N-phenylmaleimide, N-(4-hydroxyphenyl)maleimide, N-(4-acetylphenyl)maleimide, N-(4-methoxyphenyl)maleimide, N-(4-ethoxyphenyl)maleimide, N-(4-chlorophenyl)maleimide, N-(4-bromophenyl)maleimide, and N-benzylmaleimide, and one or more of these may be used. For example, N-phenylmaleimide may be used.

In addition, examples of unsaturated acid anhydrides include maleic anhydride, itaconic anhydride, citraconic anhydride, etc., and one or more of these can be used.

Examples of unsaturated carboxylic acid compounds include unsaturated dicarboxylic acids such as (meth)acrylic acid, maleic acid, fumaric acid, itaconic acid, crotonic acid, citraconic acid, cinnamic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, maleic anhydride, itaconic anhydride, and citraconic anhydride, as well as monoalkyl esters of unsaturated dicarboxylic acids, of which one or more can be used.

Among these, it is preferable that the first monomer contains at least styrenes, for example. This is because styrenes are easy to undergo living polymerization and can impart moderate hydrophobicity and affinity to organic solvents. It is possible to impart hydrophobicity and affinity to organic solvents to the first polymer chain. In this way, for example, when the present crosslinked polymer is produced by a dispersion polymerization method in a polar organic solvent, the first polymer tends to be present in the surface layer of the present crosslinked polymer, improving the dispersion stability of the present crosslinked polymer.

The styrenes are, for example, 20% by mass or more of the total mass of the first monomer. This is because 20% by mass or more facilitates living polymerization and can appropriately impart moderate hydrophobicity and affinity to organic solvents. For example, 30% by mass or more, for example, 35% by mass or more, for example, 40% by mass or more, for example, 50% by mass or more, for example, 60% by mass or more, for example, 65% by mass or more, for example, 70% by mass or more, for example, 75% by mass or more. The styrenes are, for example, 100% by mass or less of the total mass, for example, 95% by mass or less, for example, 90% by mass or less, for example, 85% by mass or less, for example, 80% by mass or less, for example, 75% by mass or less. The range of the styrenes relative to the total mass can be set by appropriately combining the above-mentioned lower and upper limits, and is, for example, 20 mass% or more and 95 mass% or less, for example, 30 mass% or more and 75 mass% or less, and for example, 35 mass% or more and 85 mass% or less.

Each of the (meth)acrylonitrile compound, maleimide compound, acid anhydride and unsaturated carboxylic acid compound can be used alone, and it is preferable to use one or more of these four types in combination with styrenes. This is because all of these four types can maintain, adjust or impart hydrophobicity or organic solvent affinity to the first polymer chain. Among them, one or more of (meth)acrylonitrile compounds such as acrylonitrile, maleimide compounds such as N-phenylmaleimide and acid anhydrides are preferred. Among them, combinations such as styrene and acrylonitrile, and styrene and N-phenylmaleimide are preferred. The unsaturated carboxylic acid compound is preferred in that it can easily change the polarity of the first polymer.

When used in combination with styrenes, the total amount of these one or more first monomers other than styrenes is, for example, 20% by mass or more of the total mass of the first monomer (first monomer unit of the first polymer chain) for polymerizing the first polymer chain. Also, for example, 25% by mass or more, for example, 30% by mass or more, for example, 35% by mass or more, for example, 40% by mass or more, for example, 50% by mass or more, for example, 60% by mass or more. Also, the (meth)acrylonitrile compound is 80% by mass or less, for example, 75% by mass or less, for example, 70% by mass or less, for example, 65% by mass or less, for example, 60% by mass or less, for example, 55% by mass or less, for example, 50% by mass or less of the total mass. The range of the styrenes relative to the total mass can be set by appropriately combining the above-mentioned lower and upper limits, and is, for example, 20 mass% or more and 65 mass% or less, and also, for example, 25 mass% or more and 50 mass% or less.

<First Polymer Chain>
The first polymer chain may be a polymer chain of only the first monomer described above, but if necessary, a vinyl monomer other than the above may be used as the first monomer. For example, known vinyl monomers such as (meth)acrylic acid and (meth)acrylic acid esters such as alkyl (meth)acrylates may be used.
The amount of such other monomers is, for example, 10% by mass or less, or, for example, 5% by mass or less, or, for example, 3% by mass or less, or, for example, 1% by mass or less, or, for example, 0.5% by mass or less, of the total mass of the monomers constituting the first polymer chain.

The first polymer may also have a block (another polymer chain) different from the first polymer chain. Such other polymer chain may be added in a separate synthesis step after the formation of the first polymer chain, for example. In this case, a radical polymerization initiator and another vinyl monomer may be continuously or newly supplied to the first polymer having the first polymer chain to obtain a first polymer having another polymer chain (block) composed of a unit derived from a monomer other than the first monomer having a different composition from the first polymer chain. By being directly linked to the living radical active unit described later and being provided so as to be linked to the first polymer chain, a part of the monomer common to the present monomer used in the present crosslinked polymer can be provided in advance in the first polymer.

<Living radical polymerization active unit>
Since the first polymer has a living radical polymerization active unit by a chain exchange transfer mechanism, various monomers can be selected in the precipitation polymerization or dispersion polymerization of this monomer in consideration of the solubility of the first polymer in the polymerization solvent and its function as a dispersion stabilizer.

Examples of exchange chain transfer mechanisms of the living radical polymerization active units in the first polymer include reversible addition-fragmentation chain transfer polymerization (RAFT), iodine transfer polymerization, polymerization using an organotellurium compound (TERP), polymerization using an organoantimony compound (SBRP), and polymerization using an organobismuth compound (BIRP). Among these, the RAFT and iodine transfer polymerization methods are preferred, with the RAFT method being more preferred, in that the particle size of the crosslinked polymer can be reduced.

ここで、上記式（１）は、架橋の不均一構造を記述できる「ローレンツ２乗モデル」（Ｓｈｉｂａｙａｍａ Ｍ． ｅｔ ａｌ．， Ｍａｃｒｏｍｏｌｅｃｕｌｅｓ ２００９，４２， １３４４－）と、カルボキシル基由来の電解質間の相関を記述できる「ブロードピークモデル」（Ｓｈｉｂａｙａｍａ Ｍ． ｅｔ ａｌ．， Ｊｏｕｒｎａｌ ｏｆ Ｃｈｅｍｉｓｔｒｙ Ｐｈｙｓｉｃｓ ２０１８， １４９， １６３３０１－）を組み合わせた式である。 3. Characteristics of the crosslinked polymer <Heterogeneous network structure size of the crosslinked polymer>
The crosslinked polymer or a salt thereof has a heterogeneous network structure size Ξ (Ξ1) of 80 or less, which is calculated by curve fitting the scattering intensity curve I(q) obtained by measuring a 1 mass % aqueous solution of the crosslinked polymer neutralized to a degree of neutralization of 50 to 100 mol % by small angle X-ray scattering (measurement temperature: 25.0±0.1° C.) with the following formula (1):

Here, the above formula (1) is a combination of the "Lorentz squared model" (Shibayama M. et al., Macromolecules 2009, 42, 1344-), which can describe the non-uniform structure of the crosslink, and the "broad peak model" (Shibayama M. et al., Journal of Chemistry Physics 2018, 149, 163301-), which can describe the correlation between electrolytes derived from carboxyl groups.

From the viewpoint of excellent coatability and coating film performance of a composition containing the present crosslinked polymer or a salt thereof (coatability and cycle characteristics in the case of a composition containing a binder for a secondary battery electrode containing the present crosslinked polymer or a salt thereof, an active material, and water), Ξ1 is 80 or less, preferably 70 or less, more preferably 60 or less, even more preferably 50 or less, and even more preferably 40 or less.
Furthermore, from the above viewpoint, the difference ΔΞ(Ξ1-Ξ5) between the Ξ1 and the size of the heterogeneous network structure Ξ(Ξ5) of the crosslinked polymer, which is calculated by curve fitting the scattering intensity curve I(q) obtained by measuring a 5 mass % aqueous solution of the crosslinked polymer neutralized to a degree of neutralization of 50 to 100 mol % by a small angle X-ray scattering method (measurement temperature: 25.0±0.1° C.) using the above formula (1), is preferably 50 or less, more preferably 30 or less, even more preferably 20 or less, even more preferably 10 or less, and even more preferably 5.0 or less.
Here, Ξ1 and Ξ5 are obtained by the method described in the Examples.

<Viscosity of aqueous solution of the present crosslinked polymer>
The crosslinked polymer or its salt preferably has a viscosity of 100 mPa·s or more in a 2% by weight aqueous solution. When the viscosity of the 2% by weight aqueous solution is 100 mPa·s or more, the composition containing the crosslinked polymer has high storage stability and can exhibit excellent binding properties. The viscosity of the 2% by weight 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 is obtained by uniformly dissolving or dispersing the crosslinked polymer or its salt in water in an amount to give a predetermined concentration, and then measuring the B-type viscosity (25°C) at 12 rpm according to the method described in the Examples.

The crosslinked polymer or its salt absorbs water and swells in water. In general, when a crosslinked polymer has an appropriate degree of crosslinking, the greater the amount of hydrophilic groups that the crosslinked polymer has, the easier it is to absorb water and swell. In addition, in terms of the degree of crosslinking, the lower the degree of crosslinking, the easier it is to swell. 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 it more difficult for the crosslinked polymer 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, the chain transfer reaction to the polymer chain, and the post-crosslinking reaction. In addition, 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 into account chain transfer, etc.

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

The crosslinked polymer preferably has a particle size (water-swollen particle size) in the range of 0.1 μm to 5.0 μm in volumetric median size when the crosslinked polymer has a degree of neutralization based on the carboxyl groups of 80 to 100 mol% and is dispersed in water. A more preferred range of the particle size is 0.1 μm to 4.0 μm, an even more preferred range is 0.1 μm to 3.0 μm, an even more preferred range is 0.2 μm to 3.0 μm, and an even more preferred range is 0.3 μm to 3.0 μm. If the particle size is in the range of 0.1 μm to 5.0 μm, the particles are uniformly present in the composition at a suitable size, and the composition is highly stable and exhibits excellent binding properties. If the particle size exceeds 5.0 μm, there is a risk of insufficient binding properties as described above. In addition, there is a risk of insufficient coatability in that it is difficult to obtain a smooth coating surface. On the other hand, if the particle size is less than 0.1 μm, there is a concern in terms of stable production.

In the present crosslinked polymer, acid groups such as carboxyl groups derived from ethylenically unsaturated carboxylic acid monomers are neutralized so that the degree of neutralization in the present composition is 20 mol% or more, and it is preferable to use it as a salt form. The above-mentioned 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%, and may be 98 mol% or 95 mol%. The range of the degree of neutralization can be appropriately combined with the above lower limit and upper limit, and may be, for example, 50 mol% or more and 100 mol% or less, 75 mol% or more and 100 mol% or less, or 80 mol% or more and 100 mol% or less. When the degree of neutralization is 20 mol% or more, it is preferable in that the water swelling property is good and the dispersion stabilization effect is easily obtained. In this specification, the neutralization degree can be calculated from the charged amount of a monomer having an acid group such as a carboxyl group and a neutralizing agent used for neutralization. The neutralization degree can be confirmed by IR measurement of a powder of a crosslinked polymer or a salt thereof after drying treatment at 80° C. for 3 hours under reduced pressure conditions, and by the intensity ratio of a peak derived from a C═O group of the carboxylic acid to a peak derived from a C═O group of the carboxylate.

<Water swelling degree of the present crosslinked polymer>
In this specification, the water swelling degree is calculated based on the following formula from the dry weight of the crosslinked polymer or a salt thereof, "( _WA ) g," and the amount of water absorbed when the crosslinked polymer or a salt thereof is swelled to saturation with water, "( _WB ) g."
(Water swelling degree)={( _WA )+( _WB )}/( _WA )

The crosslinked polymer or its salt preferably has a water swelling degree at pH 8 of 20 or more and 80 or less. If the water swelling degree is within the above range, the crosslinked polymer or its salt swells moderately in an aqueous medium, so that when forming an electrode mixture layer, it is possible to ensure a sufficient adhesion area to the active material and the current collector, 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 or its salt spreads on the surface of the active material or the current collector, and a sufficient adhesion area can be ensured, so that good binding property can be obtained. 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. If the water swelling degree exceeds 60, the viscosity of the electrode mixture layer composition (electrode slurry) containing the crosslinked polymer or its salt tends to be high, and the uniformity of the mixture layer may be insufficient, resulting in insufficient binding force. In addition, the coatability of the electrode slurry may be deteriorated. The range of the water swelling degree at pH 8 can be set by appropriately combining the above upper and lower limits, and is, for example, 23 to 70, for example, 25 to 65, or for example, 25 to 55.
The degree of swelling in water at pH 8 can be obtained by measuring the degree of swelling of the crosslinked polymer or a salt thereof in water at pH 8. As the water at 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, etc. The pH at the time of 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.

A person skilled in the art can adjust the water swelling degree by controlling the composition and structure of the crosslinked polymer or its 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. Also, the water swelling degree can usually be increased by decreasing the degree of crosslinking of the crosslinked polymer.

4. Composition for secondary battery electrode mixture layer The composition for secondary battery electrode mixture layer of the present invention contains the present binder, an active material, and water.
The amount of the binder used in the composition is, for example, 0.1 parts by mass or more and 20 parts by mass or less, based on 100 parts by mass of the total amount of the active material. The amount is, for example, 0.2 parts by mass or more and 10 parts by mass or less, for example, 0.3 parts by mass or more and 8 parts by mass or less, and for example, 0.4 parts by mass or more and 5 parts by mass or less. If the amount of the binder used is 0.1 parts by mass or more, sufficient binding property can be obtained. In addition, the dispersion stability of the active material and the like can be ensured, and a uniform mixture layer can be formed. If the amount of the binder used is 20 parts by mass or less, the composition does not become highly viscous, and the coatability to the current collector can be ensured. As a result, a mixture layer having a uniform and smooth surface can be formed.

Among the above active materials, lithium salts of transition metal oxides can be used as the positive electrode active material, and 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 NCM {Li (Ni _x , Co _y , Mn _z ), x + y + z = 1} and NCA {Li (Ni _1-a-b Co _a Al _b )}. In addition, lithium manganate can be used as a spinel type positive electrode active material. In addition to oxides, phosphates, silicates, sulfur, and the like are used, and examples of phosphates include olivine type lithium iron phosphate. As the positive electrode active material, one of the above may be used alone, or two or more may be combined to be used as a mixture or composite.

In addition, 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, causing the dispersion to become alkaline. For this reason, there is a risk that aluminum foil (Al), which is a common positive electrode current collector material, may be corroded. In such a case, it is preferable to neutralize the alkali content eluted from the active material by using the unneutralized or partially neutralized present crosslinked polymer as a binder. In addition, it is preferable to use the unneutralized or partially neutralized present crosslinked polymer in an amount such that the amount of unneutralized carboxyl groups in the present crosslinked polymer is equivalent to or greater than the amount of alkali eluted from the active material.

Since all positive electrode active materials have low electrical conductivity, they are generally used with the addition of a conductive assistant. Examples of conductive assistants 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 easy to obtain excellent electrical conductivity. As carbon black, ketjen black and acetylene black are preferred. The conductive assistant may be one of the above alone or two or more may be used in combination. The amount of the conductive assistant 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 amount of the active material, from the viewpoint of achieving both electrical conductivity and energy density. The positive electrode active material may be surface-coated with a carbon-based material having electrical conductivity.

On the other hand, examples of the negative electrode active material 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, and graphite such as natural graphite and artificial graphite, and hard carbon are more preferred. In the case of graphite, spherical graphite is preferably used from the viewpoint of battery performance, and the preferred range of its particle size is, for example, 1 to 20 μm, and also, for example, 5 to 15 μm. In addition, in order to increase the energy density, metals or metal oxides capable of absorbing lithium, such as silicon and tin, can also be used as the negative electrode active material. Among them, 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, the silicon-based active material has a high capacity, but the volume change caused by charging and discharging is large. Therefore, it is preferable to use it in combination with the carbon-based active material. In this case, if the amount of the silicon-based active material is large, the electrode material may collapse, and the cycle characteristics (durability) may be greatly reduced. From this point of view, when the silicon-based active material is used in combination, the amount of the silicon-based active material is, for example, 60% by mass or less, or, for example, 30% by mass or less, relative to the carbon-based active material.

Since carbon-based active materials themselves have good electrical conductivity, it is not necessary 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, per 100 parts by mass of the total amount of active material.

When the composition is in a slurry state, the amount of active material used is, for example, in the range of 10 to 75% by mass, or, for example, in the range of 30 to 65% by mass, based on 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 cost of drying the medium. On the other hand, if it is 75% by mass or less, the fluidity and coatability of the composition can be ensured, and a uniform mixture layer can be formed.

This composition uses water as a medium. In order to adjust the properties and drying properties of this composition, it may be mixed with lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, tetrahydrofuran, N-methylpyrrolidone, and other water-soluble organic solvents. The proportion of water in the mixed medium is, for example, 50% by mass or more, and, for example, 70% by mass or more.

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

The composition may further contain other binder components such as styrene butadiene rubber (SBR) latex, carboxymethyl cellulose (CMC), acrylic latex, and polyvinylidene fluoride latex. When other binder components are used in combination, the amount of the binder components used 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 the active material. If the amount of the other binder components used exceeds 5 parts by mass, the resistance increases and the high-rate characteristics may become insufficient. Among the above, SBR latex and CMC are preferred in terms of the excellent balance between binding properties and bending resistance, and it is more preferable to use SBR latex and CMC in combination.

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 α-methylstyrene, vinyltoluene, divinylbenzene, and the like, in addition to styrene, 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, mainly from the viewpoint of binding properties.
Examples of the aliphatic conjugated diene monomer include 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, etc., in addition to 1,3-butadiene, and one or more of these can be used. The structural unit derived from the aliphatic conjugated diene monomer in the copolymer can be, for example, in the range of 30 to 70 mass %, or, for example, in the range of 40 to 60 mass %, in order to improve the binding property of the binder and the flexibility of the obtained electrode.
In addition to the above-mentioned monomers, the styrene/butadiene-based latex may use other monomers as copolymerization monomers, such as nitrile group-containing monomers such as (meth)acrylonitrile, carboxyl group-containing monomers such as (meth)acrylic acid, itanconic acid, maleic acid, and ester group-containing monomers such as methyl (meth)acrylate, in order to further improve performance such as binding property.
The content of the structural units derived from the other monomers in the copolymer can be, for example, in the range of 0 to 30% by mass, and can be, for example, in the range of 0 to 20% by mass.

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;
Examples of the hydroxyalkyl cellulose include 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.

The composition for secondary battery electrode mixture layer of the present invention is essentially composed of the above active material, water and binder, and is obtained by mixing each component using a known means. The method of mixing each component is not particularly limited, and a known method can be adopted, but a method of dry blending powder components such as active material, conductive assistant and binder, and then mixing with a dispersion medium such as water and dispersing and kneading is preferred. When obtaining the composition in a slurry state, it is preferable to finish the slurry without poor dispersion or aggregation. As a mixing means, known mixers such as planetary mixers, thin film swirl mixers and self-revolving mixers can be used, but it is preferable to use a thin film swirl mixer in order to obtain a good dispersion state in a short time. In addition, when using a thin film swirl mixer, it is preferable to perform preliminary dispersion in advance with a stirrer such as a disperser. The pH of the above slurry is not particularly limited as long as the effect of the present invention is exhibited, but it is preferable that it is less than 12.5, and for example, when CMC is blended, it is more preferable that it is less than 11.5 in terms of the small concern of hydrolysis, and even more preferable that it is less than 10.5. The viscosity of the slurry is not particularly limited as long as the effects of the present invention are achieved, but the viscosity may be, for example, in the range of 100 to 6,000 mPa·s, or, for example, in the range of 500 to 5,000 mPa·s, or, for example, in the range of 1,000 to 4,000 mPa·s, as a B-type viscosity (25°C) at 20 rpm. If the viscosity of the slurry is within the above range, good coatability can be ensured.

5. Secondary Battery Electrode The secondary battery electrode of the present invention comprises a mixture layer formed from the composition for secondary battery electrode mixture layer of the present invention on the surface of a current collector such as copper or aluminum. The mixture layer is formed by applying the composition to the surface of the current collector and then drying and removing the medium such as water. The method for applying the composition is not particularly limited, and known methods such as doctor blade method, dip method, roll coating method, comma coating method, curtain coating method, gravure coating method and extrusion method can be adopted. In addition, the drying can be performed by known methods such as hot air blowing, reduced pressure, (far) infrared radiation, and microwave irradiation.
Usually, the mixture layer obtained after drying is subjected to a compression treatment using a mold press, a roll press, or the like. Compression brings the active material and the binder into close contact with each other, and improves the strength of the mixture layer and its adhesion to the current collector. The thickness of the mixture layer can be adjusted by compression to, for example, about 30 to 80% of the thickness before compression, and the thickness of the mixture layer after compression is generally about 4 to 200 μm.

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 in a liquid state or a gel state.
The separator is disposed between the positive and negative electrodes of the battery, and serves to prevent short circuits caused by contact between the electrodes and to retain the electrolyte to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane having good ion permeability and mechanical strength. Specific examples of the material that can be used include polyolefins such as polyethylene and polypropylene, and polytetrafluoroethylene.

The electrolyte may be a known one that is generally used depending on the type of active material. In the lithium ion secondary battery, specific solvents include cyclic carbonates with high dielectric constant 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, which may be used alone or as a mixed solvent. The electrolyte is used by dissolving lithium salts such as LiPF ₆ , LiSbF ₆ , LiBF ₄ , LiClO ₄ and LiAlO ₄ in these solvents. In the nickel-hydrogen secondary battery, an aqueous potassium hydroxide solution may be used as the electrolyte. The secondary battery is obtained by storing a positive electrode plate and a negative electrode plate separated by a separator in a spiral or stacked structure in a case or the like.

As described above, a secondary battery having an electrode with a mixture layer formed from a composition for a secondary battery electrode mixture layer containing the binder for secondary battery electrodes disclosed in this specification exhibits good durability (cycle characteristics) even after repeated charging and discharging, and is therefore suitable for use as an in-vehicle secondary battery, etc.

The present invention will be specifically described below based on examples. Note that the present invention is not limited to these examples. Note that in the following, "parts" and "%" mean parts by mass and % by mass unless otherwise specified.

<Evaluation of Carboxyl Group-Containing Crosslinked Polymer (Salt)>
(Measurement of the size Ξ of the heterogeneous network structure of crosslinked polymers by small angle X-ray scattering)
1. Preparation of measurement samples
<1> Preparation of 1% by mass aqueous solution 0.5 g of the carboxyl group-containing crosslinked polymer salt powder and 49.5 g of ion-exchanged water were weighed into a 100 cc container and placed in a rotation/revolution type mixer (Thinky Corporation, Awatori Rentaro AR-250). Next, stirring (rotation speed 2000 rpm/revolution speed 800 rpm, 7 minutes) and degassing (rotation speed 2200 rpm/revolution speed 60 rpm, 1 minute) were performed to prepare a 1% by mass aqueous solution in which the carboxyl group-containing crosslinked polymer salt was swollen in water.
<2> Preparation of 5% by mass aqueous solution 2.5 g of the carboxyl group-containing crosslinked polymer salt powder and 47.5 g of ion-exchanged water were weighed into a 100 cc container, and the same procedure as in <1> was carried out to prepare a 5% by mass aqueous solution.

2. Method of Small-Angle X-ray Scattering Measurement <1> Apparatus Small-angle X-ray scattering measurements were performed using SAXSpoint 2.0 manufactured by Anton Paar. The X-ray source used was a Primux 100 micro microfocus X-ray source (Cu Kα ray with a wavelength of 1.542 Å). The detector used was a 2D EIGER2R series HPC detector.
The distance between the measurement sample and the detector is 3.0 m, which provides a measurable q range of 0.015 to 4.0 Å ⁻¹ (the scattering vector q is given by q=4π/λ sin θ, where 2θ is the scattering angle).
The measurements were carried out at a temperature of 25.0° C. and were controlled by a Peltier element with an accuracy of 0.1° C. The raw scattering data were corrected for cell and silver behenate scattering using the relative transmittance.

<2> Measurement method The 1% by mass aqueous solution and the 5% by mass aqueous solution obtained in 1 above were each poured into a cell (cell thickness: 1 mm) sandwiched between polyimide films (Kapton, manufactured by DuPont-Toray Co., Ltd.), and the sample was irradiated with X-rays after maintaining the sample under vacuum to reduce scattering from the air and adjusting the exposure time so as not to damage the detector with strong X-rays, thereby obtaining a two-dimensional scattering image of the sample.
Next, background correction was performed from the two-dimensional scattering image of the sample obtained by the above procedure. Specifically, a two-dimensional scattering image of the background was obtained by performing the same operation as the above procedure without a sample, and the two-dimensional scattering image of the background was subtracted from the two-dimensional scattering image of the sample using image processing software (Igor Pro8) to obtain a two-dimensional scattering image for analysis. Ring-shaped scattering was confirmed in the two-dimensional scattering image for analysis.

<3> Analysis method The two-dimensional scattering image for analysis was converted into a one-dimensional scattering spectrum. Specifically, the two-dimensional scattering image for analysis was loaded into X-ray data processing software (Igor Pro8) and integrated over all azimuth angles to obtain a one-dimensional scattering spectrum (scattering intensity curve I(q)) with the horizontal axis representing the scattering vector q (Å ⁻¹ ) and the vertical axis representing the intensity of the scattered radiation. Next, as a baseline correction, the minimum value of the scattering intensity in the region to be analyzed was found, and the minimum value was subtracted from the entire region to perform baseline correction.
The obtained one-dimensional scattering profile after correction was fitted using the following formula (1) to determine Ξ (Ξ1) in a 1 mass% aqueous solution and Ξ (Ξ5) in a 5 mass% aqueous solution. In the following formula (1), Ξ means the structural size of the non-uniform network of the crosslinked polymer, and when there is a part with a high degree of crosslinking as shown in Figure 1, the size of the part is described by Ξ.
For fitting, waveform separation software (Igor Pro8) was used.

(Water swelling degree at pH 8)
The degree of swelling in water at pH 8 was measured by the following method. The measuring apparatus is shown in FIG.
The measurement device is composed of <1> to <3> in FIG.
<1> Consists of a burette 1 with a branch pipe for venting air, a pinch cock 2, a silicone tube 3, and a polytetrafluoroethylene tube 4.
<2> A support cylinder 8 having a number of holes on its bottom surface is placed on top of the funnel 5, and filter paper 10 for the device is placed on top of that.
<3> A sample 6 (measurement sample) of a crosslinked polymer or a salt thereof is sandwiched between two sheets of sample fixing filter paper 7, and the sample fixing filter paper is fixed with adhesive tape 9. All the filters used are ADVANTEC No. 2, with an inner diameter of 55 mm.
<1> and <2> are connected by a silicon tube 3.
The funnel 5 and the cylindrical support 8 are fixed in height relative to the burette 1, and are set so that the lower end of the polytetrafluoroethylene tube 4 installed inside the burette branch pipe is at the same height as the bottom surface of the cylindrical support 8 (dotted line in FIG. 1).

The measurement method is described below.
The pinch cock 2 in <1> is removed, and ion-exchanged water is poured in from the top of the burette 1 through the silicon tube 3, so that the burette 1 to the filter paper 10 is filled with ion-exchanged water 12. Next, the pinch cock 2 is closed, and air is removed from the polytetrafluoroethylene tube 4 connected to the burette branch pipe with a rubber stopper. In this way, the ion-exchanged water 12 is continuously supplied from the burette 1 to the filter paper 10.
Next, after removing the excess ion-exchanged water 12 that has seeped out from the filter paper 10 for the device, the reading (a) of the scale on the burette 1 is recorded.
Weigh out 0.1 to 0.2 g of the dry powder of the measurement sample and place it evenly in the center of the sample fixing filter paper 7 as shown in <3>. Sandwich the sample with another filter paper and secure the two filter papers with adhesive tape 9 to fix the sample. Place the filter paper with the sample fixed on the instrument filter paper 10 shown in <2>.
Next, the reading (b) of the scale of the burette 1 is recorded 30 minutes after the lid 11 is placed on the filter paper 10 for the device.
The total (c) of the water absorption of the measurement sample and the two sheets of sample fixing filter paper 7 is calculated by (a-b). By the same procedure, the water absorption (d) of only the two sheets of filter paper 7, which do not contain the sample of the crosslinked polymer or its salt, is measured.
After carrying out the above operations, the water swelling degree was calculated from the following formula: The solid content used in the calculation was a value measured by the method described below.
Water swelling degree={dry weight of measurement sample (g)+(c−d)}/{dry weight of measurement sample (g)}
Here, the dry weight (g) of the measurement sample = the weight (g) of the measurement sample × (solid content (%) ÷ 100).

The method for measuring the solid content will be described below.
Approximately 0.5 g of a sample was placed in a weighing bottle whose weight had been measured in advance [weight of weighing bottle = B (g)] and accurately weighed together with the weighing bottle [W ₀ (g)]. The sample together with the weighing bottle was then placed in a windless dryer and dried at 155°C for 45 minutes, at which point the weight of the sample together with the weighing bottle was measured [W ₁ (g)], and the solid content was calculated using the following formula.
Solid content (%) = (W ₁ -B)/(W ₀ -B) x 100

(Measurement of particle size in aqueous medium (water-swollen particle size))
0.25 g of the 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 type mixer (Thinky Corporation, Awatori Rentaro AR-250). Next, the mixture was stirred (rotation speed 2,000 rpm/revolution speed 800 rpm, 7 minutes) and further 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.
Next, the particle size distribution of the above hydrogel was measured using a laser diffraction/scattering particle size distribution analyzer (Microtrack MT-3300EXII, manufactured by Microtrack Bell) using ion-exchanged water as a dispersion medium. When an excess amount of dispersion medium was circulated relative to the hydrogel and an amount of hydrogel was added that provided an appropriate scattered light intensity, the particle size distribution shape measured after a few minutes became stable. 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 diameter.

(Measurement of viscosity of 2% by mass aqueous solution)
2.0 parts of the powder of the carboxyl group-containing crosslinked polymer salt and 98 parts of ion-exchanged water were weighed into a container and set in a rotation/revolution type mixer (Thinky Corporation, Awatori Rentaro AR-250). Then, stirring (rotation speed 2,000 rpm/revolution speed 800 rpm, 7 minutes) and degassing (rotation speed 2,200 rpm/revolution speed 60 rpm, 1 minute) were repeated until no unswollen powdery parts remained, to prepare a hydrogel microparticle dispersion in which the carboxyl group-containing crosslinked polymer salt was swollen in water. After adjusting each of the obtained hydrogel microparticle dispersions to 25°C ± 1°C, the viscosity was measured at a rotor speed of 12 rpm using a B-type viscometer (Toki Sangyo Co., Ltd., TVB-10).

<Synthesis of first polymer>
(Polymer 1)
In a reactor equipped with a stirrer, a thermometer, a reflux condenser and a nitrogen inlet tube, 2.0 parts of a RAFT agent (dibenzyl trithiocarbonate: DBTTC), 0.410 parts of 2,2'-azobis (2-methylbutyronitrile) (manufactured by Japan Finechem Co., Ltd., trade name "ABN-E"), 75 parts of styrene (St), 25 parts of acrylonitrile (AN), and 67 parts of anisole were charged, thoroughly degassed by nitrogen bubbling, and polymerization was started in a thermostatic bath at 80 ° C. After 4 hours, the reaction was stopped by cooling to room temperature. The above polymerization solution was purified by reprecipitation from methanol / water = 90 / 10 (vol%) and vacuum dried to obtain polymer 1. As a result of a test by gas chromatography, the reaction rate of the obtained polymer 1 was 72%. The molecular weight of polymer 1 was Mn 11,900, Mw 15,500, and Mw / Mn was 1.30. Styrene and acrylonitrile correspond to the first monomer.

(Method of measuring molecular weight of first polymer)
The molecular weight of the first polymer was measured by gel permeation chromatography (GPC). That is, the number average molecular weight (Mn) and weight average molecular weight (Mw) were obtained based on polystyrene standards by THF-based GPC. The molecular weight distribution (Mw/Mn) was calculated from the obtained values. GPC was performed under the following conditions.

Column: Tosoh TSKgel SuperMultiporeHZ-M x 4 Solvent: Tetrahydrofuran Temperature: 40°C
Detector: RI
Flow rate: 600μL/min

<Production of Carboxyl Group-Containing Crosslinked Polymer Salt>
(Production Example 1: Production of Carboxyl Group-Containing Crosslinked Polymer Salt R-1)
For the polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen inlet tube was used.
Into the reactor, 567 parts of acetonitrile, 2.20 parts of ion-exchanged water, 100 parts of acrylic acid (hereinafter referred to as "AA"), 0.001 parts of DBTTC, 0.90 parts of trimethylolpropane diallyl ether (manufactured by Daiso Co., Ltd., trade name "Neoallyl T-20"), and triethylamine equivalent to 1.0 mol% relative to the AA were charged. After the inside of the reactor was sufficiently replaced with nitrogen, the inside temperature was raised to 55°C by heating. After confirming that the inside temperature was stable at 55°C, 0.040 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd., trade name "V-65") was added as a polymerization initiator, and the reaction solution became cloudy, so this point was determined to be the polymerization initiation point. The monomer concentration was calculated to be 15.0%. Cooling of the reaction solution was started 12 hours after the polymerization initiation point, and after the internal temperature had dropped to 25° C., 52.4 parts of lithium hydroxide monohydrate (hereinafter referred to as “LiOH.H ₂ O”) powder was added. After 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 (Li salt, neutralization degree 90 mol%) were dispersed in the medium.

The obtained polymerization reaction liquid was centrifuged to settle the polymer particles, and the supernatant was removed. The precipitate was then redispersed in acetonitrile of the same weight as the polymerization reaction liquid, and the washing operation of settling the polymer particles by centrifugation and removing the supernatant was repeated twice. The precipitate was collected and dried at 80°C for 3 hours under reduced pressure conditions to remove the volatile matter, thereby obtaining a powder of carboxyl group-containing polymer salt R-1. Since the carboxyl group-containing polymer salt R-1 is hygroscopic, it was sealed and stored in a container with water vapor barrier properties. The powder of the carboxyl group-containing polymer salt R-1 was subjected to IR measurement, and the degree of neutralization was calculated from the intensity ratio of the peak derived from the C=O group of the carboxylic acid and the peak derived from the C=O of the carboxylic acid Li, which was 90 mol%, equal to the calculated value from the charge. The water swelling degree was 36.4, the particle size in the aqueous medium was 1.72 μm, and the viscosity of a 2% by mass aqueous solution was 9,110 mPa·s.

(Production Examples 2 to 15 and Comparative Production Examples 1 and 2: Production of Carboxyl Group-Containing Crosslinked Polymer Salts R-2 to R-17)
The same operation as in Production Example 1 was carried out except that the amounts of the monomer, crosslinkable monomer, and neutralizing agent charged were as shown in Table 1, thereby obtaining polymerization reaction solutions containing carboxyl group-containing crosslinked polymer salts R-2 to R-17.
Next, the same operation as in Production Example 1 was carried out for each polymerization reaction solution to obtain powdered carboxyl group-containing crosslinked polymer salts R-2 to R-17. Each carboxyl group-containing crosslinked polymer salt was sealed and stored in a container having water vapor barrier properties. The water swelling degree, particle size in an aqueous medium, and viscosity of a 2% by mass aqueous solution of R-2 to R-17 are shown in Table 1. Note that the particle size of R-3 (degree of neutralization 70 mol%) in an aqueous medium was measured after adjusting the degree of neutralization to 90 mol% with LiOH.H ₂ O.

(Small angle X-ray scattering measurement)
Examples of the results of small angle X-ray scattering measurement are shown in FIG. 1 (Production Example 2: R-2, Comparative Production Example 2: R-17).
For all samples, a broad peak was observed in the 1% by mass aqueous solution where q was approximately 0.03 to 0.10 Å ⁻¹ . This peak was due to scattering of charged groups derived from carboxyl groups present inside the hydrogel microparticles dispersed in water.
On the other hand, in the 5% by mass aqueous solution, a peak was observed when q was approximately 0.07 to 0.20 Å ⁻¹ . This is thought to be because the hydrogel microparticles were densely packed in water, approaching an apparently uniform structure, and thus the peak was due to crosslinking.

For the 1% by mass aqueous solution, fitting was performed using equation (1) with q* = 0.053 Å ^-1 and L = 15.4 Å ^-1 for the graph shown in Figure 1, and it was calculated that Ξ1 = 43.3 Å ^-1 for R-2 and Ξ1 = 154.4 ^{Å -1} for R-17. For the 5% by mass aqueous solution, fitting was performed using equation (1) with q* = 0.10 Å ^-1 and L = 10.0 Å ^-1 for the graph shown in Figure 1, and it was calculated that Ξ5 = 41.6 Å ^-1 for R-2 and Ξ5 = 32.8 Å ^-1 for R-17.
In both cases, Ξ1 shows a larger value than Ξ5. This is probably because in the 1% by mass aqueous solution, the hydrogel is not packed in water and is in a saturated swollen state where it can fully absorb water, so that the density of crosslinks becomes clearer and a large non-uniform network structure size is observed.
On the other hand, in the 5% by mass aqueous solution, the hydrogel is packed in water, and the density of the crosslinks is cancelled out, resulting in a structure that appears to be close to a uniform mesh, and as a result, it can be considered that a small non-uniform mesh structure size was observed. Therefore, ΔΞ (Ξ1-Ξ5) can be considered to indirectly indicate the degree of density of the crosslinks. Considering the result that R-2 showed a smaller Ξ in a 1% by mass aqueous solution and a smaller ΔΞ value compared to R-17, it can be considered that R-2 has a more uniform crosslinked structure.
Table 1 shows Ξ1, Ξ5 and ΔΞ calculated by curve fitting the scattering intensity curve I(q) obtained by measuring R-1, R-3 to R-16 by small angle X-ray scattering using the following formula (1).

Details of the compounds used in Table 1 are shown below.
AA: Acrylic acid IBXA: Isobornyl acrylate BM1430: 2-(dodecylthiocarbonothioylthio)propionic acid IBME: Methyl 2-iodoisobutyrate T-20: Trimethylolpropane diallyl ether (manufactured by Daiso Co., Ltd., product name "Neoallyl T-20")
P-30: Pentaerythritol triallyl ether (manufactured by Daiso Co., Ltd., product name "Neoallyl P-30")
TEA: triethylamine AcN: acetonitrile V-65: 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd.)
_LiOH.H2O : Lithium hydroxide _{monohydrate Na2CO3} : Sodium carbonate _K2CO3 : Potassium _carbonate

<Evaluation of Composition Containing Carboxyl Group-Containing Crosslinked Polymer Salt>
(Example 1: Evaluation of a composition containing a carboxyl group-containing crosslinked polymer salt R-1)
<Preparation of Slurry Composition (Composition for Electrode Mixture Layer)>
A SiOx (0.8<x<1.2) surface was coated with 10% carbon by CVD (hereinafter referred to as "Si-based active material"), and a mixture of artificial graphite and the Si-based active material was used as the active material. As the binder, a mixture of the present crosslinked polymer salt R-1, styrene butadiene rubber (SBR)-based latex, and carboxymethyl cellulose (CMC) was used.
In order to make the solid content concentration of the electrode mixture layer composition 50% by mass, water was used as a dilution solvent, and the mixture was mixed for 2 hours using a T.K. Hibismix manufactured by Primix Corporation in a mass ratio of artificial graphite:Si-based active material:R-1:SBR:CMC = 90:10:1.0:1.0:1.0 (solid content), to prepare a slurry composition for the electrode mixture layer (electrode slurry). The viscosity of the electrode slurry was 3,670 mPa s, which was a sufficiently low value.
The obtained electrode slurry was used to prepare an electrode, which was then evaluated. The specific procedure and evaluation method are described below.

<Preparation of negative electrode plate>
The electrode slurry was applied to both sides of a copper foil (thickness: 20 μm) and dried to form a mixture layer. After that, the mixture layer was rolled to a thickness of 27 μm and a packing density of 1.3 g/ ^cm3 , and then punched into a 3 cm square to obtain a negative electrode plate.

(Coatability of electrode slurry)
The coatability of the electrode slurry in the preparation of the negative electrode plate was evaluated based on the following criteria and was evaluated as "A".
<Evaluation criteria>
⊚: No abnormal appearance such as streaks or bumps was observed on the surface.
○: Slight abnormalities in appearance such as streaks or bumps are observed on the surface.
△: Slight abnormalities in appearance such as streaks and bumps are observed on the surface.
×: Significant abnormalities in appearance such as streaks and bumps are observed on the surface.

<Preparation of Positive Electrode Plate>
In N-methylpyrrolidone (NMP) solvent, 100 parts of lithium iron phosphate (LFP) as a positive electrode active material, 0.2 parts of carbon nanotubes as a conductive agent, 2 parts of Ketjen Black, and 0.6 parts of vapor grown carbon fiber (VGCF) were mixed and added, and polyvinylidene fluoride (PVDF) was mixed as a binder for the electrode composition to prepare a positive electrode composition. The positive electrode composition was applied to an aluminum current collector (thickness: 15 μm) and dried to form a mixture layer. Thereafter, the mixture layer was rolled to a thickness of 88 μm and a packing density of 3.1 g / cm ³ , and then punched out into a 3 cm square to obtain a positive electrode plate.

<Preparation of secondary battery>
A laminated type lithium ion secondary battery was produced using the positive electrode plate, the negative electrode plate and the separator. The electrolyte used was a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (DEC) in a volume ratio of 25:75, in which _LiPF6 was dissolved at a concentration of 1.0 mol/L.

(Coating film performance)
In this example, the performance of the coating film obtained from the above electrode slurry was evaluated by measuring the cycle characteristics of a lithium ion secondary battery.
The lithium ion secondary battery of the laminated cell prepared above was charged and discharged at a charge/discharge rate of 0.2 C under conditions of 2.7 to 3.4 V in CC discharge, and the initial capacity C0 was measured. Furthermore, charging and discharging were repeated in an environment of 25°C, and the capacity C50 after 50 cycles was measured. The cycle characteristic (ΔC) calculated by the following formula was 91.8%, and the cycle characteristic based on the following criteria was evaluated as "◯". Note that a higher ΔC value indicates better cycle characteristics.
ΔC=C50/C0×100(%)
<Evaluation criteria>
◎: Charge/discharge capacity retention rate is 95.0% or more. ◯: Charge/discharge capacity retention rate is 90.0% or more and less than 95.0%. △: Charge/discharge capacity retention rate is 85.0% or more and less than 90.0%. ×: Charge/discharge capacity retention rate is less than 85.0%.

(Examples 2 to 15 and Comparative Examples 1 and 2)
Except for using the crosslinked polymer salt as shown in Table 2, the same operation as in Example 1 was carried out to prepare an electrode slurry as a slurry composition, and the slurry viscosity was measured. In addition, the coating property of the electrode slurry and the cycle characteristics of the secondary battery obtained using the electrode slurry were evaluated. The results are shown in Table 2.

<Evaluation Results>
As is clear from the results of Examples 1 to 15, all of the slurry compositions containing the present crosslinked polymer salt of the present invention had good coatability, and the coating film performance of the composition (in this example, the cycle characteristics of a secondary battery equipped with an electrode obtained using the composition) was also excellent.
Among these, when comparing cases where the water swelling degree and the particle size in an aqueous medium are the same (Examples 2, 12, and 13), the cases where a controller for reversible addition-fragmentation chain transfer polymerization was used as the exchange chain transfer mechanism controller (Examples 2 and 12) had smaller values of the heterogeneous network structure size Ξ1 and smaller values of ΔΞ (Ξ1-Ξ5) than the case where an iodine transfer polymerization controller was used (Example 13), and therefore had superior coating film performance (in this example, the charge/discharge capacity retention rate was high and the cycle characteristics were excellent).
In contrast, in the case of a slurry composition containing a crosslinked polymer having a heterogeneous network structure size Ξ1 of more than 80, either the coating film performance (cycle characteristics of secondary batteries) or the coatability was significantly inferior (Comparative Examples 1 and 2).

The composition containing the carboxyl group-containing crosslinked polymer or a salt thereof of the present invention is excellent in both coatability and coating film performance, and is therefore expected to be used in a variety of applications, such as a thickener or viscosity modifier for cosmetics, a binder for electrodes of non-aqueous electrolyte secondary batteries, an anti-settling agent for pigments, and a dispersion stabilizer for metal powders.
Furthermore, a secondary battery having an electrode obtained using a composition for a secondary battery electrode mixture layer containing a binder for a secondary battery electrode containing a carboxyl group-containing crosslinked polymer or a salt thereof of the present invention exhibits good durability (cycle characteristics), and is therefore expected to be applied to a secondary battery mounted on a vehicle. It is also useful for the use of an active material containing silicon, and is expected to contribute to increasing the capacity of the battery. In particular, it is useful for non-aqueous electrolyte lithium ion secondary batteries with high energy density.

Claims (10)

A carboxyl group-containing crosslinked polymer or a salt thereof,
The crosslinked polymer contains 50% by mass or more and 100% by mass or less of structural units derived from an ethylenically unsaturated carboxylic acid monomer based on the total structural units of the crosslinked polymer,
the crosslinked polymer is crosslinked by a crosslinkable monomer, the amount of the crosslinkable monomer used is 0.1 parts by mass or more and 2.0 parts by mass or less based on 100 parts by mass of the total amount of the non-crosslinkable monomers, and the amount of the exchange chain transfer mechanism type regulator used is 0.0001 to 0.50 mol % based on the total amount of the monomers;
A carboxyl group-containing crosslinked polymer or a salt thereof, in which a heterogeneous network structure size Ξ (hereinafter referred to as "Ξ1") of the crosslinked polymer, which is calculated by curve fitting the following formula (1) to a scattering intensity curve I(q) obtained by measuring a 1 mass % concentration aqueous solution of the crosslinked polymer neutralized to a degree of neutralization of 50 to 100 mol % by a small angle X-ray scattering method (measurement temperature: 25.0±0.1° C.), is 41.8 to 80:

The carboxyl group-containing crosslinked polymer or its salt according to claim 1, wherein the difference ΔΞ (Ξ1-Ξ5) between the size of the heterogeneous network structure of the crosslinked polymer (hereinafter referred to as "Ξ5") calculated by curve fitting the scattering intensity curve I(q) obtained by measuring a 5 mass % concentration aqueous solution of the crosslinked polymer neutralized to a degree of neutralization of 50 to 100 mol % by small angle X-ray scattering (measurement temperature: 25.0±0.1°C) with the formula (1) is 50 or less. The carboxyl group-containing crosslinked polymer or its salt according to claim 1 or 2, wherein the Ξ1 is 41.8 or more and 74.5 or less . The carboxyl group -containing crosslinked polymer or its salt according to any one of claims 1 to 3, wherein the crosslinkable monomer comprises a polyfunctional allyl ether compound having two or more allyl ether groups in the molecule . The carboxyl group-containing crosslinked polymer or its salt according to any one of claims 1 to 4, wherein the crosslinked polymer or its salt has a volume-based median particle size of 0.1 μm or more and 5.0 μm or less, measured in an aqueous medium after being neutralized to a degree of neutralization of 80 to 100 mol %. The crosslinked polymer or salt thereof containing a carboxyl group according to any one of claims 1 to 5, wherein the crosslinked polymer or salt thereof has a degree of swelling in water at pH 8 of 20 or more and 80 or less. A binder for secondary battery electrodes comprising the carboxyl group-containing crosslinked polymer or its salt according to any one of claims 1 to 6. A composition for a secondary battery electrode mixture layer comprising the binder for a secondary battery electrode according to claim 7, an active material, and water. A secondary battery electrode having a mixture layer formed from the secondary battery electrode mixture layer composition according to claim 8 on a collector surface. A secondary battery comprising the secondary battery electrode according to claim 9.

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