Carboxyl group-containing crosslinked polymer or its salt and its use

Abstract

The present invention provides a carboxyl group-containing crosslinked polymer or a salt thereof, which can achieve both coatability and coating film performance of a composition containing the crosslinked polymer or the salt thereof.　The carboxyl group-containing crosslinked polymer or the salt thereof is provided such that the non-uniform network structure size Ξ of the crosslinked polymer is at most 80, as calculated by curve fitting, with equation (1), the scattering intensity curve I (q) which is obtained by measuring a 1 mass% aqueous solution of the crosslinked polymer neutralized to a neutralization degree of 50-100 mol% through a small-angle X-ray scattering method (measuring temperature: 25.0±0.1°C). I(q): Intensity of scattered radiation Ξ: Non-uniform network structure size of crosslinked polymer A: Fitting parameter B: Fitting parameter q: Scattering vector q*: Distance between charged groups L: Electrostatic shielding length of polymer chain m: Fitting parameter

Description

Technical Field

[0001] The present invention relates to a carboxyl group-containing crosslinked polymer or a salt thereof and its use.

Background Art

[0002] Carboxyl group-containing polymers are used in various applications such as thickeners and viscosity modifiers for cosmetics, binders for non-aqueous electrolyte secondary battery electrodes, 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 drawability and stickiness, but as the degree of crosslinking increases, the drawability decreases and a refreshing feeling is felt. Therefore, in cosmetics where drawability is not required and a refreshing feeling is desired, carboxyl group-containing crosslinked polymers are often used because high thickening properties can be obtained with a small amount of use.

[0003] Also, as a binder for non-aqueous electrolyte secondary battery electrodes, carboxyl group-containing crosslinked polymers are often used because they can impart good binding properties and cycle characteristics. Here, as the applications of various secondary batteries such as nickel-metal hydride secondary batteries, lithium-ion secondary batteries, and electric double layer capacitors expand, the requirements for improving energy density, reliability, and durability tend to increase. For example, for the purpose of increasing the electric capacity of lithium-ion secondary batteries, the specification of using a silicon-based active material as the negative electrode active material is increasing. However, it is known that silicon-based active materials have a large volume change during charge and discharge, and peeling or dropping of the electrode mixture layer occurs during repeated use, resulting in a decrease in the battery capacity and deterioration of cycle characteristics (durability). In order to suppress such problems, studies have been conducted to firmly bind the active materials with a binder (binding property), reduce the size of the active materials to relieve the stress associated with swelling and shrinkage, and improve durability by devising additives for the electrolyte.

[0004] Under such circumstances, it has been reported that an acrylic acid-based polymer is effective as a binder that has good cycle characteristics and is effective in improving the durability of the negative electrode binder layer using a silicon-based active material. In Patent Document 1, it is disclosed that by using a polymer obtained by cross-linking polyacrylic acid with a specific cross-linking agent as a binder, even when using an active material containing silicon, good cycle characteristics can be exhibited without the electrode structure being destroyed. Patent Document 2 discloses a cross-linked acrylic acid-based polymer having a specific particle size in a 1% NaCl aqueous solution.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] According to the inventors' studies, when using the micro-crosslinked acrylic acid-based polymer disclosed in Patent Documents 1 and 2 as a binder for particles in a slurry (for example, a binder for an active material in a lithium-ion secondary battery electrode slurry), while the micro-crosslinking of the acrylic acid-based polymer can enhance the binding property between particles in the slurry, the spread of the polymer in water increases and the viscosity greatly increases even with a small amount of addition. Therefore, there is a limit to reducing the slurry viscosity, and it has been a problem that coatability and coating film performance (for example, cycle characteristics of a lithium-ion secondary battery) cannot be achieved simultaneously.

[0007] In addition, although the binders for secondary battery electrodes disclosed in Patent Documents 1 and 2 can both impart good cycle characteristics and binding properties, with the improvement of the performance of secondary batteries, the demand for a binder that can further improve cycle characteristics is increasing. Furthermore, generally, a secondary battery electrode is obtained by applying and drying a composition for an electrode binder layer (hereinafter also referred to as "electrode slurry") containing an active material and a binder on the surface of an electrode current collector. At this time, from the viewpoint of enhancing the drying efficiency of the electrode slurry and improving the productivity of the electrode, it is advantageous to increase the solid content concentration of the electrode slurry. However, usually, as the solid content concentration increases, it becomes difficult to ensure good coatability. 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, so it has been difficult to increase the solid content concentration.

[0008] The present invention has been made in view of such circumstances, and an object thereof is to provide a crosslinked polymer containing a carboxyl group or a salt thereof that can achieve both coatability and coating film performance of a composition containing the same. Furthermore, when the solid content concentration of the composition for the electrode composite layer is higher than before, it is possible to obtain a secondary battery that exhibits excellent cycle characteristics while ensuring coatability by reducing the viscosity of the electrode slurry. It is to provide a binder for a secondary battery electrode. In addition, a composition for a secondary battery electrode binder layer containing the above binder, a secondary battery electrode obtained using the composition, and a secondary battery are provided.

Means for Solving the Problems

[0009] As a result of intensive studies to solve the above problems, the present inventors have found that by using a crosslinked polymer containing a carboxyl group or a salt thereof, the size of the inhomogeneous network structure obtained by measurement by the small-angle X-ray scattering method is below a specific value, while ensuring the coatability of the composition containing the crosslinked polymer or a salt thereof, excellent coating film performance can be exhibited, and the present invention has been completed. Furthermore, when the solid content concentration of a composition for a secondary battery electrode binder layer containing a carboxyl group-containing crosslinked polymer or a salt thereof, an active material, and water is high, the composition contains a carboxyl group-containing crosslinked polymer or a salt thereof, and the size of the inhomogeneous network structure obtained by measurement by the small-angle X-ray scattering method is not more than a specific value. Thus, it has been found that a secondary battery can be obtained that exhibits excellent cycle characteristics while ensuring coatability by reducing the viscosity of the electrode slurry, and the present invention has been completed.

[0010] The present invention is as follows. 〔1〕A carboxyl group-containing crosslinked polymer or a salt thereof, wherein the inhomogeneous network structure size Ξ (hereinafter, also referred to as "Ξ1") of the crosslinked polymer 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 neutralization degree of 50 to 100 mol% by the small-angle X-ray scattering method (measurement temperature: 25.0 ± 0.1 °C) with the following formula (1) is 80 or less. A carboxyl group-containing crosslinked polymer or a salt thereof.

Number

[10] A secondary battery including the secondary battery electrode according to [9]. [Effects of the Invention]

[0011] According to the carboxyl group-containing crosslinked polymer or a salt thereof of the present invention, it is possible to achieve both the coatability and the coating film performance of a composition containing the crosslinked polymer or a salt thereof. Furthermore, according to the binder for a secondary battery electrode containing the carboxyl group-containing crosslinked polymer or a salt thereof of the present invention, when the solid content concentration of the composition for the electrode binder layer is higher than before, while ensuring the coatability by reducing the viscosity of the electrode slurry, it is possible to obtain a secondary battery that exhibits excellent cycle characteristics. [Brief Description of the Drawings]

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0013] The carboxyl group-containing crosslinked polymer or a salt thereof of the present invention (hereinafter, also referred to as "the present crosslinked polymer") is obtained by measuring a 1 mass% concentration aqueous solution of the present crosslinked polymer neutralized to a neutralization degree of 50 to 100 mol% by the small-angle X-ray scattering method (measurement temperature: 25.0 ± 0.1 °C). For the scattering intensity curve I(q), the non-uniform network structure size Ξ (Ξ1) of the present crosslinked polymer calculated by curve fitting with the above formula (1) is 80 or less. Note that Ξ will be described in detail in "3. Characteristics of the Present Crosslinked Polymer" and "Examples" described later.

[0014] Furthermore, a binder for a secondary battery electrode containing the present crosslinked polymer or a salt thereof (hereinafter, also referred to as "the present binder") can be made into a composition for a secondary battery electrode binder layer (hereinafter, also referred to as "the present composition") by mixing with an active material and water. The above composition is in a slurry state that can be applied to a current collector. By forming a binder layer formed from the above composition on the surface of a current collector such as a copper foil or an aluminum foil, the secondary battery electrode of the present invention is obtained. Here, the present binder is preferable in that the effects exhibited by the present invention are particularly large when used in a composition for a secondary battery electrode binder layer containing a silicon-based active material described later as an active material.

[0015] Hereinafter, the crosslinked polymer, the binder, the composition for a secondary battery electrode binder layer obtained using the binder, the secondary battery electrode, and the secondary battery will be described in detail. In the present specification, “(meth)acrylic” means acrylic and / or methacrylic, and “(meth)acrylate” means acrylate and / or methacrylate. Further, the “(meth)acryloyl group” means an acryloyl group and / or a methacryloyl group.

[0016] 1. Structural unit of this 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 the ethylenically unsaturated carboxylic acid monomer. Since the crosslinked polymer has such a structural unit and thus has a carboxyl group, the adhesiveness to the current collector is improved, and the desolvation effect of lithium ions and the ionic conductivity are excellent, so that an electrode with low resistance and excellent high-rate characteristics can be obtained. In addition, since water swelling property is imparted, the dispersion stability of the active material and the like in the present composition can be enhanced.

[0017] Examples of the ethylenically unsaturated carboxylic acid monomer include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid; (meth)acrylamide alkyl carboxylic acids such as (meth)acrylamide hexanoic acid and (meth)acrylamide dodecanoic acid; ethylenically unsaturated monomers having a carboxyl group such as succinic acid monohydroxyethyl (meth)acrylate, ω-carboxy-caprolactone mono(meth)acrylate, β-carboxyethyl (meth)acrylate, or (partial) alkali neutralization products thereof. One of these may be used alone, or two or more thereof may be used in combination. Among these, a compound having an acryloyl group as a polymerizable functional group is preferable in that a polymer having a long primary chain length can be obtained due to a high polymerization rate and the binding force of the binder becomes good, and acrylic acid is particularly preferable. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, a polymer having a high carboxyl group content can be obtained.

[0018] The content of component (a) in the present crosslinked polymer is not particularly limited. For example, it can be contained in an amount of 10% by mass or more and 100% by mass or less based on all the structural units of the present crosslinked polymer. By containing component (a) within such a range, excellent adhesiveness to the current collector can be easily ensured. The lower limit is, for example, 20% by mass or more, and also for example, 30% by mass or more, and also for example, 40% by mass or more. When the lower limit is 50% by mass or more, the dispersion stability of the present composition becomes good, and a higher adhesive strength can be obtained, which is preferable. It may be 60% by mass or more, or 70% by mass or more, or 80% by mass or more. Also, the upper limit is, for example, 99.9% by mass or less, and also for example, 99.5% by mass or less, and also for example, 99% by mass or less, and also for example, 98% by mass or less, and also for example, 95% by mass or less, and also for example, 90% by mass or less, and also for example, 80% by mass or less. As the range, a range appropriately combining such lower and upper limits can be adopted. For example, it can be 10% by mass or more and 100% by mass or less, or 50% by mass or more and 100% by mass or less, or 50% by mass or more and 99.9% by mass or less, or 50% by mass or more and 99% by mass or less, or 50% by mass or more and 98% by mass or less, etc.

[0019] <Other structural units> In addition to component (a), the present crosslinked polymer can contain structural units derived from other ethylenically unsaturated monomers copolymerizable therewith (hereinafter also referred to as "component (b)"). Examples of component (b) include structural units derived from ethylenically unsaturated monomer compounds having anionic groups other than carboxyl groups such as sulfonic acid groups and phosphoric acid groups, or nonionic ethylenically unsaturated monomers. These structural units can be introduced by copolymerizing a monomer containing an ethylenically unsaturated monomer compound having anionic groups other than carboxyl groups such as sulfonic acid groups and phosphoric acid groups, or a nonionic ethylenically unsaturated monomer.

[0020] (b) The proportion of the component can be 0% by mass or more and 90% by mass or less based on all the structural units of the present crosslinked polymer. The proportion of the component (b) may be 1% by mass or more and 60% by mass or less, may be 2% by mass or more and 50% by mass or less, may be 5% by mass or more and 40% by mass or less, or may be 10% by mass or more and 30% by mass or less. Further, when the component (b) is contained in an amount of 1% by mass or more based on all the structural units of the present crosslinked polymer, since the affinity for the electrolyte is improved, an effect of improving the lithium ion conductivity can also be expected.

[0021] As the component (b), among those described above, a structural unit derived from a nonionic ethylenically unsaturated monomer is preferable from the viewpoint of obtaining an electrode having good flex resistance. Examples of the nonionic ethylenically unsaturated monomer include (meth)acrylamide and its derivatives, nitrile group-containing ethylenically unsaturated monomers, alicyclic structure-containing ethylenically unsaturated monomers, and the like.

[0022] Examples of the (meth)acrylamide derivative 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 thereof may be used in combination.

[0023] Examples of the nitrile group-containing ethylenically unsaturated monomer include (meth)acrylonitrile; (meth)acrylic acid cyanoalkyl ester compounds such as (meth)acrylic acid cyanomethyl and (meth)acrylic acid cyanoethyl; cyano group-containing unsaturated aromatic compounds such as 4-cyanostyrene and 4-cyano-α-methylstyrene; and vinylidene cyanide. One of these may be used alone, or two or more thereof may be used in combination. Among the above, acrylonitrile is preferable in terms of having a high nitrile group content.

[0024] Examples of the alicyclic structure-containing ethylenically unsaturated monomer include (meth)acrylic acid cycloalkyl esters which may have aliphatic substituents 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, dicyclopentenyl oxyethyl (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 thereof may be used in combination.

[0025] The crosslinked polymer or its salt preferably contains structural units derived from (meth)acrylamide and its derivatives, nitrile group-containing ethylenically unsaturated monomers, alicyclic structure-containing ethylenically unsaturated monomers, etc. in terms of excellent binder binding properties. Further, 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 the component (b), it can exhibit a strong interaction with the electrode material and can exhibit good binding properties to the active material. As a result, a strong and integrally good electrode binder layer can be obtained. 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 preferable.

[0026] In addition, as other nonionic ethylenically unsaturated monomers, for example, (meth)acrylic acid esters may be used. Examples of the (meth)acrylic acid ester 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, phenylethyl (meth)acrylate; Examples include (meth)acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate. One of these may be used alone, or two or more may be used in combination.

[0027] From the viewpoints of binding property to the active material and cycle characteristics, aromatic (meth)acrylic acid ester compounds can be preferably used. From the viewpoints of further improving lithium ion conductivity and high rate characteristics, compounds having an ether bond such as (meth)acrylic acid alkoxyalkyl esters such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate are preferred, and 2-methoxyethyl (meth)acrylate is more preferred.

[0028] Among nonionic ethylenically unsaturated monomers, compounds having an acryloyl group are preferred in that a polymer with a long primary chain length can be obtained due to a high polymerization rate, and the binding force of the binder is good. Further, as the nonionic ethylenically unsaturated monomer, a compound having a glass transition temperature (Tg) of 0°C or lower of the homopolymer is preferred in that the obtained electrode has good flexural resistance.

[0029] This crosslinked polymer may be in the form of a salt in which some or all of the carboxyl groups contained in the polymer are neutralized. The type of the salt is not particularly limited, and examples include alkali metal salts such as lithium salt, sodium salt, and potassium salt; alkaline earth metal salts such as calcium salt and barium salt; other metal salts such as magnesium salt and aluminum salt; ammonium salt and organic amine salt. Among these, alkali metal salts and magnesium salts are preferred, and alkali metal salts are more preferred, in that they are less likely to have an adverse effect on battery characteristics.

[0030] This crosslinked polymer is a crosslinked polymer having a crosslinked structure. The crosslinking method in this crosslinked polymer is not particularly limited, and for example, the following methods are exemplified. 1) Copolymerization of crosslinkable monomers 2) Utilizing chain transfer to the polymer chain during radical polymerization 3) After synthesizing a polymer having reactive functional groups, a crosslinking agent is added for post-crosslinking as necessary Since this crosslinked polymer has a crosslinked structure, a binder containing the crosslinked polymer or a salt thereof can have excellent adhesive strength. Among the above, the method by copolymerization of crosslinkable monomers is preferred because of its simple operation and easy control of the degree of crosslinking.

[0031] <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.

[0032] The above 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 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 terms of easily obtaining a uniform crosslinked structure, and polyfunctional allyl ether compounds having two or more allyl ether groups in the molecule are particularly preferred.

[0033] Examples of the polyfunctional (meth)acrylate compounds include di(meth)acrylates of divalent alcohols such as ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,6 - hexanediol di(meth)acrylate, polyethylene glycol di(meth)acrylate, and polypropylene glycol di(meth)acrylate; poly(meth)acrylates such as trimethylolpropane tri(meth)acrylate, tri(meth)acrylate of ethylene oxide - modified trimethylolpropane, glycerin tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and pentaerythritol tetra(meth)acrylate, which are tri(meth)acrylates and tetra(meth)acrylates of polyhydric alcohols with a valency of 3 or more; bisamides such as methylene bisacrylamide and hydroxyethylene bisacrylamide, etc.

[0034] Examples of the 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; polyfunctional vinyl compounds such as divinylbenzene, etc.

[0035] Examples of the compounds having both a (meth)acryloyl group and an alkenyl group include allyl (meth)acrylate, isopropenyl (meth)acrylate, butenyl (meth)acrylate, pentenyl (meth)acrylate, 2-(2 - vinyloxyethoxy)ethyl (meth)acrylate, etc.

[0036] Specific examples of the monomers having the self - crosslinkable crosslinking functional groups include vinyl monomers containing a hydrolyzable silyl group, N - methylol(meth)acrylamide, N - methoxyalkyl(meth)acrylate, etc. These compounds can be used alone or in combination of two or more.

[0037] 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 vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane, vinyldimethylmethoxysilane; silyl group-containing acrylic esters such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, methyldimethoxysilylpropyl acrylate; silyl group-containing methacrylic esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, dimethylmethoxysilylpropyl methacrylate; silyl group-containing vinyl ethers such as trimethoxysilylpropyl vinyl ether; silyl group-containing vinyl esters such as vinyl trimethoxysilylundecanoate, etc. can be mentioned.

[0038] When the present crosslinked polymer is crosslinked with 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, still 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, based on 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer). When the amount of the crosslinkable monomer used is 0.05 parts by mass or more, it is preferable in terms of better binding properties and stability of the electrode slurry. When 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 monomer).

[0039] 2. Method for producing this crosslinked polymer The present crosslinked polymer is obtained by subjecting a monomer component containing the ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as "the present monomer") to precipitation polymerization or dispersion polymerization in the presence of a chain transfer mechanism type control agent. Here, precipitation polymerization is a method for producing a polymer by performing a polymerization reaction in a solvent that dissolves the monomer as a raw material but does not substantially dissolve the resulting polymer. As the polymerization progresses, the polymer particles grow larger by aggregation and growth, and a dispersion of polymer particles is obtained in which primary particles of several tens of nm to several hundreds of nm are secondarily aggregated into several μm to several tens of μm. A dispersion stabilizer can also be used to control the particle size of the polymer. In addition, the above secondary aggregation can also be suppressed by selecting a dispersion stabilizer, a polymerization solvent, etc. Generally, precipitation polymerization that suppresses secondary aggregation is also called dispersion polymerization.

[0040] Regarding the exchange chain transfer mechanism type controller Examples of the exchange chain transfer mechanism type controller according to the present invention include controllers in reversible addition-fragmentation chain transfer polymerization (RAFT method) (hereinafter, also referred to as "RAFT agent"), controllers in iodine transfer polymerization, controllers in polymerization methods using organic tellurium compounds (TERP method), controllers in polymerization methods using organic antimony compounds (SBRP method), controllers in polymerization methods using organic bismuth compounds (BIRP method), and the like. As the exchange chain transfer mechanism type controller, a polymer having a living radical polymerization active unit by an exchange chain transfer mechanism with a polymerization chain of one or more vinyl monomers (hereinafter, also simply referred to as "first polymer". Details are described in paragraphs

[0052] to

[0078] below), and controllers other than the polymer can be used. The first polymer and the controllers other than the polymer can be used alone or in combination. In the presence of an exchange chain transfer mechanism type controller, by subjecting the monomer to precipitation polymerization or dispersion polymerization, the primary chain length is shortened and the chain lengths are made uniform to form a uniform crosslinked structure, so that it is possible to increase the water swelling degree of the present crosslinked polymer. Along with this, it is presumed that good coatability due to reduction of the electrode slurry viscosity and excellent cycle characteristics can be achieved at the same time. Among these, in terms of being able to make the crosslinked structure of the present crosslinked polymer more uniform, the RAFT agent and the controller in the iodine transfer polymerization method are preferable, and the RAFT agent is more preferable.

[0041] As the RAFT agent, a first polymer having a living radical polymerization active unit by reversible addition-fragmentation chain transfer method (detailed later), and / or a RAFT agent other than the first polymer (dithioester compound, xanthate compound, trithiocarbonate compound, dithiocarbamate compound, etc.) can be used. Specific examples of the RAFT agent other than the first polymer include, for example, 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, cyanomethyl-N-methyl-N-phenyldithiocarbamate, and the like. Among the RAFT agents, those having trithiocarbonate in the molecule are particularly preferred in that they can make the crosslinked structure of the present crosslinked polymer more uniform.

[0042] As the controller in the iodine transfer polymerization method, a first polymer having a living radical polymerization active unit by the iodine transfer polymerization method (detailed later), and / or a controller other than the first polymer can be used. Specific examples of the controller other than the first polymer include, for example, methyl iodide, methylene iodide, iodoform, carbon tetraiodide, 1-phenylethyl iodide, benzyl iodide, perfluoroalkyl perfluoroalkyl iodide having 1 to 20 carbon atoms in the alkyl group, ethyl 2-iodopropionate, methyl 2-iodoisobutyrate, ethyl 2-iodoisobutyrate, ethyl 2-iodo-2-phenylacetate, ethylene glycol bis(2-iodo-2-phenylacetate), ethylene glycol bis(2-iodoisobutyrate), 1,5-diiodo-2,4-dimethylbenzene, 2-iodopropionitrile, and the like.

[0043] The exchange chain transfer mechanism type control agent may be a monofunctional one having one active site, or a polyfunctional one having two or more active sites can also be used. The exchange chain transfer mechanism type control agent having two or more functional groups causes the polymer chain to extend in two or more directions. From the viewpoint of producing the present crosslinked polymer, it may be preferable to use a difunctional or polyfunctional exchange chain transfer mechanism type control agent having three or more functional groups.

[0044] The amount of the exchange chain transfer mechanism type control agent used is preferably 0.0001 to 0.50 mol%, more preferably 0.0001 to 0.40 mol%, still more preferably 0.0001 to 0.30 mol%, and even more preferably 0.0002 to 0.30 mol% with respect to the total amount of the monomers, in that the crosslinked structure of the crosslinked polymer can be made more uniform.

[0045] As the polymerization initiator used together with the exchange chain transfer mechanism type control agent, known polymerization initiators such as azo compounds, organic peroxides, and inorganic peroxides can be used, but it is not particularly limited. The use conditions can be adjusted by known methods such as thermal initiation, redox initiation using a reducing agent in combination, and UV initiation so as to obtain an appropriate amount of radical generation. 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 radical generation is less within the range where the production time is allowed. Among the above polymerization initiators, azo compounds are preferred in terms of easy handling in terms of safety and low likelihood of side reactions during radical polymerization. Specific examples of the above 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], 2,2'-azobis(N-butyl-2-methylpropionamide), and the like. The above radical polymerization initiator may be used alone or in combination of two or more.

[0046] The preferred amount of the polymerization initiator to be 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 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 carried out stably, and if it is 2 parts by mass or less, it is easy to obtain a polymer with a long primary chain length. The usage ratio of the polymerization initiator is not particularly limited, but from the viewpoint of making the crosslinked structure of the present crosslinked polymer uniform, it is preferable that the amount of the polymerization initiator used with respect to 1 mol of the above exchange chain transfer mechanism type regulator is 0.5 mol or less, and more preferably 0.2 mol or less. Also, from the viewpoint of stably carrying out the polymerization reaction, the lower limit of the amount of the polymerization initiator used with respect to 1 mol of the exchange chain transfer mechanism type regulator is 0.001 mol. Therefore, the amount of the polymerization initiator used with respect to 1 mol of the exchange chain transfer mechanism type regulator is preferably in the range of 0.001 mol or more and 0.5 mol or less, and more preferably in the range of 0.005 mol or more and 0.2 mol or less.

[0047] The polymerization solvent can be a solvent selected from water, various organic solvents, etc. in consideration of the type of monomer used, etc. In order to obtain a polymer with a longer primary chain length, it is preferable to use a solvent with a small chain transfer constant. Specific polymerization solvents include, in addition to water-soluble solvents such as methanol, t-butyl alcohol, acetone, methyl ethyl ketone, acetonitrile, and tetrahydrofuran, benzene, ethyl acetate, dichloroethane, n-hexane, cyclohexane, n-heptane, etc. One of these can be used alone or in combination of two or more. Alternatively, it may be used as a mixed solvent with water. In the present invention, the water-soluble solvent refers to one having a solubility in water at 20 °C greater than 10 g / 100 ml. Among the above, methyl ethyl ketone and acetonitrile are preferred in terms of less generation of coarse particles and less adhesion to the reactor, good polymerization stability, the precipitated crosslinked polymer being less likely to undergo secondary aggregation (or easily dissociating in the aqueous medium even if secondary aggregation occurs), a small chain transfer constant and obtaining a polymer with a large degree of polymerization (primary chain length), and easy operation during the neutralization process described later.

[0048] Also, in order to make the neutralization reaction proceed stably and rapidly during the process neutralization, it is preferable to add a small amount of a highly polar solvent to the polymerization solvent. Examples of such highly polar solvents preferably 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, still 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, the effect on the above neutralization reaction is recognized, and if it is 20.0% by mass or less, no adverse effect on the polymerization reaction is observed. Further, in the polymerization of a highly hydrophilic ethylenically unsaturated carboxylic acid monomer such as acrylic acid, when a highly polar solvent is added, the polymerization rate is improved and it becomes easier to obtain a polymer with a long primary chain length. Among the highly polar solvents, water is particularly preferable because of its great effect of improving the above polymerization rate.

[0049] The reaction temperature during the polymerization reaction in the presence of a chain transfer exchange 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 the restrictions on initiators and solvents that can be used are relaxed.

[0050] The dispersion of the crosslinked polymer obtained through the polymerization step can be used to obtain the target crosslinked polymer in a powder state by performing a reduced pressure and / or heat treatment or the like in the drying step to distill off the solvent. At this time, prior to the above drying step, for the purpose of removing unreacted monomers (and their salts), following the polymerization step, it is preferable to include 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 / water. When the above washing step is provided, even when the crosslinked polymer undergoes secondary aggregation, it is easily disaggregated during use, and furthermore, since the remaining unreacted monomers are removed, good performance is also exhibited in terms of coating film performance.

[0051] In the production 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 "in-process neutralization"), and then the solvent may be removed in the drying step. Also, after obtaining the 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"). Among the above, in-process neutralization is preferably used because secondary aggregates tend to be easily disaggregated.

[0052] Here, as the chain transfer exchange mechanism type control agent, as described above, a polymer (first polymer) having a living radical polymerization active unit by a chain transfer exchange mechanism with a polymerization chain of one or more vinyl monomers (hereinafter, also simply referred to as "first monomer") (hereinafter, also simply referred to as "first polymerization chain") can be used.

[0053] 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 of the crosslinked polymer. A 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 dispersion fine particles. By doing so, polymerization stability, that is, aggregation of the present crosslinked polymer during the polymerization process can be suppressed, generation of coarse aggregated particles can be suppressed, and a present crosslinked polymer having a small particle size and a narrow particle size distribution can be obtained.

[0054] In producing the present crosslinked polymer by polymerizing the present monomer in the presence of the first polymer, in order for the first polymer to 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 with respect to 100 parts by mass of the total mass of the present monomer. By using within such a range, while making the first polymer function as a dispersion stabilizer, a present crosslinked polymer mainly containing the present monomer can be produced. Also, when the amount of 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 tends to exceed 0.3 μm. Even when 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 also becomes small.

[0055] The first polymer can be used, for example, in an amount of 0.5 parts by mass or more, and for example, 1 part by mass or more with respect to 100 parts by mass of the total mass of the present monomer. Also, the first polymer can be used, for example, in an amount of 40 parts by mass or less, for example, 30 parts by mass or less, and for example, 20 parts by mass or less. The range of the amount of the first polymer used with respect to 100 parts by mass of the total mass of the present monomer can be set by appropriately combining the above upper and lower limits.

[0056] Method for producing the first polymer In the presence of a known exchange chain transfer mechanism type control agent, by polymerizing a monomer composition containing a first monomer, a first polymer chain having a structural unit derived from the first monomer and a living polymerization active unit by an exchange chain transfer mechanism can be obtained.

[0057] The polymerization conditions for producing the first polymer are well-known to those skilled in the art. Examples of the polymerization process include various processes such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. Considering that it is a polymerization base point in the production of this crosslinked polymer and functions as a dispersion stabilizer, for example, solution polymerization can be used. In addition, the types of exchange chain transfer mechanism control agents, the types and amounts of polymerization initiators used, polymerization solvents, reaction temperatures, and other polymerization conditions are appropriately selected according to the above paragraphs

[0040] to

[0043] and

[0045] to

[0049] . The amount of the exchange chain transfer mechanism control agent used is appropriately adjusted according to the number average molecular weight (Mn) of the target first polymer. As the exchange chain transfer mechanism control agent, RAFT agents and control agents in the iodine transfer polymerization method are preferable in that they can reduce the molecular weight distribution of the first polymer. Furthermore, the concentration when producing the first polymer is not particularly limited with respect to the total mass of the charged amounts such as the polymerization solvent and the first monomer. For example, it can be 10% by mass or more and 80% by mass or less, or for example, 15% by mass or more and 70% by mass or less, or for example, 20% by mass or more and 70% by mass or less.

[0058] Typically, when a monofunctional exchange chain transfer mechanism type control agent is used, the living polymerization active unit is provided at the end of the first polymer chain. When a bifunctional or higher functional exchange chain transfer mechanism type control agent is used, the living polymerization active unit branches in two or more directions and each has a first polymer chain. In any case, when there is another polymer chain, this 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 with respect to the living polymerization active unit.

[0059] The first polymer can also include two or more first polymer chains. For example, after performing living radical polymerization or the like using one or more first monomers of a certain composition, living radical polymerization or the like is performed using one or more first monomers of another composition, whereby a first polymer having first polymer chains (blocks) having structural units derived from first monomers of different compositions can be obtained.

[0060] The number average molecular weight (Mn) of the first polymer is not particularly limited. For example, it is 3,000 or more, and for example, it is 5,000 or more, and for example, it is 7,000 or more, and for example, it is 8,000 or more, and for example, it is 10,000 or more. Also, the same Mn is 50,000 or less, and for example, it is 30,000 or less, and for example, it is 25,000 or less, and for example, it is 20,000 or less, and for example, it is 15,000 or less, and for example, it is 14,000 or less, and for example, it is 12,000 or less. As the range of Mn, the above-described lower limit and upper limit can be appropriately combined and set. For example, it is 5,000 or more and 25,000 or less, and for example, it is 10,000 or more and 25,000 or less, and for example, it is 10,000 or more and 15,000 or less, and for example, it is 10,000 or more and 14,000 or less.

[0061] The weight average molecular weight (Mw) of the first polymer is not particularly limited, but for example, it is 5,000 or more, and for example, 7,000 or more, and for example, 9,000 or more, and for example, 10,000 or more, and for example, 13,000 or more, and for example, 15,000 or more. Also, the same Mw is 60,000 or less, and for example, 55,000 or less, and for example, 50,000 or less, and for example, 45,000 or less, and for example, 40,000 or less, and for example, 36,000 or less, and for example, 35,000 or less, and for example, 30,000 or less, and for example, 25,000 or less. As the range of Mw, the above-mentioned lower limit and upper limit can be set by appropriately combining them. For example, it is 1,000 or more and 40,000 or less, and for example, 10,000 or more and 35,000 or less, and for example, 10,000 or more and 30,000 or less, and for example, 15,000 or more and 25,000 or less.

[0062] Incidentally, both Mw and Mn of the first polymer can be measured by gel permeation chromatography using polystyrene as a standard substance. As the details of the chromatography conditions, the conditions disclosed in the subsequent examples can be adopted.

[0063] The molecular weight distribution (Mw / Mn) of the first polymer is not particularly limited, but for example, it is 2.5 or less, and for example, 2.4 or less, and for example, 2.3 or less, and for example, 2.0 or less, and for example, 1.6 or less, and for example, 1.5 or less, and for example, 1.4 or less, and for example, 1.3 or less. Also, the molecular weight distribution is, for example, 1.1 or more, and for example, 1.2 or more, and for example, 1.3 or more, and for example, 1.4 or more, and for example, 1.5 or more. As the range of the molecular weight distribution, the above-mentioned lower limit and upper limit can be set by appropriately combining them. For example, it can be 1.1 or more and 2.5 or less, and for example, 1.1 or more and 2.4 or less, and for example, 1.1 or more and 2.3 or less, and for example, 1.1 or more and 2.0 or less, etc.

[0064] 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 preferably 2.4 or less, and in order to obtain a crosslinked polymer with a smaller particle size, it is preferably 1.7 or less, more preferably 1.6 or less, and even more preferably 1.4 or less.

[0065] <The first monomer> Examples of the first monomer include styrenes, (meth)acrylonitrile compounds, maleimide compounds, unsaturated acid anhydrides, and unsaturated carboxylic acid compounds. One or more of these can be used in combination.

[0066] Styrenes include styrene and its derivatives. Specific 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, etc. 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.

[0067] Examples of the (meth)acrylonitrile compound include (meth)acrylonitrile, acrylonitrile, α-methylacrylonitrile, etc. For example, acrylonitrile is used.

[0068] Examples of 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, N-stearylmaleimide; N-cycloalkyl-substituted maleimide compounds such as N-cyclopentylmaleimide, N-cyclohexylmaleimide; 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, N-benzylmaleimide, etc. One or more of these can be used. For example, N-phenylmaleimide is used.

[0069] Examples of unsaturated acid anhydrides include maleic anhydride, itaconic anhydride, citraconic anhydride, etc. One or more of these can be used.

[0070] 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, and monoalkyl esters of unsaturated dicarboxylic acids, etc. One or more of these can be used.

[0071] As the first monomer, among these, for example, it is preferably at least styrenes. This is because styrenes are easy to undergo living polymerization and can impart appropriate hydrophobicity and affinity for organic solvents. The first polymer chain can be imparted with hydrophobicity or affinity for organic solvents. By doing so, for example, when producing the present crosslinked polymer by dispersion polymerization in a polar organic solvent, the first polymer tends to be present on the surface layer of the present crosslinked polymer, and the dispersion stability of the present crosslinked polymer is improved.

[0072] Styrenes are, for example, 20% by mass or more of the total mass of the first monomer. This is because when it is 20% by mass or more, living polymerization becomes easy and appropriate hydrophobicity and affinity for organic solvents can be imparted. Also, for example, it is 30% by mass or more, also for example, 35% by mass or more, also for example, 40% by mass or more, also for example, 50% by mass or more, also for example, 60% by mass or more, also for example, 65% by mass or more, also for example, 70% by mass or more, also for example, 75% by mass or more. Further, styrenes are 100% by mass or less of the total mass, also for example, 95% by mass or less, also for example, 90% by mass or less, also for example, 85% by mass or less, also for example, 80% by mass or less, also for example, 75% by mass or less. As the range with respect to the total mass of styrenes, the above-described lower limit and upper limit can be set by appropriately combining them. For example, it is 20% by mass or more and 95% by mass or less, also for example, 30% by mass or more and 75% by mass or less, also for example, 35% by mass or more and 85% by mass or less.

[0073] (Meta)acrylonitrile compounds, maleimide compounds, acid anhydrides, and unsaturated carboxylic acid compounds can each 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 (meta)acrylonitrile compounds such as acrylonitrile, maleimide compounds such as N-phenylmaleimide, and acid anhydrides. Among them, combinations such as styrene and acrylonitrile, styrene and N-phenylmaleimide are preferable. The unsaturated carboxylic acid compound is preferable in that it can easily change the polarity of the first polymer.

[0074] 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 monomers (the first monomer units of the first polymer chain) for polymerizing the first polymer chain. Also, for example, 25% by mass or more, also for example, 30% by mass or more, also for example, 35% by mass or more, also for example, 40% by mass or more, also for example, 50% by mass or more, also for example, 60% by mass or more. Further, the (meta)acrylonitrile compound is 80% by mass or less of the total mass, also for example, 75% by mass or less, also for example, 70% by mass or less, also for example, 65% by mass or less, also for example, 60% by mass or less, also for example, 55% by mass or less, also for example, 50% by mass or less. As the range with respect to the total mass of styrenes, the above-mentioned lower limit and upper limit can be set by appropriately combining them. For example, it is 20% by mass or more and 65% by mass or less, also for example, 25% by mass or more and 50% by mass or less.

[0075] <First polymer chain> The first polymer chain may be a polymer chain consisting only of the above-described first monomer, but if necessary, other vinyl monomers other than the above can be used as the first monomer. For example, known vinyl monomers such as (meth)acrylic acid esters such as (meth)acrylic acid and alkyl (meth)acrylates can be used. In addition, such other monomers are, for example, 10% by mass or less, for example, 5% by mass or less, for example, 3% by mass or less, for example, 1% by mass or less, and for example, 0.5% by mass or less of the total mass of the monomers constituting the first polymer chain.

[0076] Further, the first polymer may include a block (another polymer chain) different from the first polymer chain. Such another polymer chain may be added, for example, in a separate synthesis step after the formation of the first polymer chain. In this case, a radical polymerization initiator and another vinyl monomer are 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 units derived from monomers other than the first monomer having a composition different from that of the first polymer chain. By being provided so as to be directly connected to the living radical polymerization active unit described later and connected to the first polymer chain, a part of the monomer common to the monomer used for the present crosslinked polymer can be provided in the first polymer in advance.

[0077] <Living radical polymerization active unit> Since the first polymer has a living radical polymerization active unit by an exchange chain transfer mechanism, in the precipitation polymerization or dispersion polymerization of the present monomer, various monomers can be selected for the solubility of the first polymer in the polymerization solvent and the function as a dispersion stabilizer.

[0078] As the exchange chain transfer mechanism of the living radical polymerization active unit in the first polymer, there are reversible addition-fragmentation chain transfer polymerization method (RAFT method), iodine transfer polymerization method, polymerization method using an organic tellurium compound (TERP method), polymerization method using an organic antimony compound (SBRP method), polymerization method using an organic bismuth compound (BIRP method), and the like. Among these, the RAFT method and the iodine transfer polymerization method are preferable, and the RAFT method is more preferable, in terms of being able to reduce the particle size of the present crosslinked polymer.

[0079] 3. Characteristics of this crosslinked polymer <Heterogeneous network structure size of the present crosslinked polymer> The present crosslinked polymer or a salt thereof is obtained by curve fitting the scattering intensity curve I(q) obtained by measuring a 1 mass% aqueous solution of the crosslinked polymer neutralized to a neutralization degree of 50 to 100 mol% by the small-angle X-ray scattering method (measurement temperature: 25.0 ± 0.1 °C) with the following formula (1), and the heterogeneous network structure size Ξ (Ξ1) of the present crosslinked polymer is 80 or less.

Equation

[0080] Ξ1 is 80 or less, preferably 70 or less, more preferably 60 or less, further preferably 50 or less, and still more preferably 40 or less, from the viewpoint of excellent coatability and coating film performance of the composition containing the present crosslinked polymer or a salt thereof (in the case of a composition containing a binder for a secondary battery electrode, an active material, and water containing the present crosslinked polymer or a salt thereof, coatability and cycle characteristics). Furthermore, from the above viewpoints, with respect to the scattering intensity curve I(q) obtained by measuring an aqueous solution with a concentration of 5% by mass of the crosslinked polymer neutralized to a neutralization degree of 50 to 100 mol% by the small-angle X-ray scattering method (measurement temperature: 25.0 ± 0.1°C), the difference ΔΞ(Ξ1 - Ξ5) in the inhomogeneous network structure size Ξ(Ξ5) of the crosslinked polymer calculated by curve fitting with the formula (1) is preferably 50 or less, more preferably 30 or less, still 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 according to the method described in the examples.

[0081] <Aqueous solution viscosity of the present crosslinked polymer> It is preferable that the viscosity of an aqueous solution with a concentration of 2% by mass of the present crosslinked polymer or a salt thereof is 100 mPa·s or more. When the viscosity of the aqueous solution with a concentration of 2% by mass is 100 mPa·s or more, the storage stability of the composition containing the crosslinked polymer is high, and it becomes possible to exhibit excellent adhesiveness. The viscosity of the aqueous solution with a concentration of 2% by mass 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 a predetermined amount of the present crosslinked polymer or a salt thereof in water to a predetermined concentration and then measuring the B-type viscosity (25°C) at 12 rpm according to the method described in the examples.

[0082] This crosslinked polymer or a salt thereof absorbs water and swells in water. Generally, when the crosslinked polymer has an appropriate degree of crosslinking, the larger the amount of hydrophilic groups in the crosslinked polymer, the more easily the crosslinked polymer absorbs water and swells. Regarding the degree of crosslinking, the lower the degree of crosslinking, the more easily the crosslinked polymer swells. However, even if the number of crosslinking points is the same, the larger the molecular weight (primary chain length), the more crosslinking points contribute to the formation of a three-dimensional network, so the crosslinked polymer becomes less likely to swell. Therefore, by adjusting the amount of hydrophilic groups, the number of crosslinking points, the primary chain length, etc. of the crosslinked polymer, the viscosity of the crosslinked polymer aqueous solution can be adjusted. At this time, the number of the above crosslinking points can be adjusted, for example, by the amount of crosslinkable monomer used, chain transfer reaction to the polymer chain, post-crosslinking reaction, etc. Also, the primary chain length of the polymer can be adjusted by setting conditions related to the amount of radical generation such as initiator and polymerization temperature, and by selecting a polymerization solvent considering chain transfer, etc.

[0083] <Particle diameter of this crosslinked polymer> In this composition, it is preferable that the crosslinked polymer is not present as large-particle-size lumps (secondary aggregates) but is well-dispersed as water-swellable particles having an appropriate particle diameter, because the binder containing the crosslinked polymer can exhibit good binding performance.

[0084] When the crosslinked polymer having a degree of neutralization based on the carboxyl group of 80 to 100 mol% is dispersed in water, the particle diameter (water-swelled particle diameter) is preferably in the range of 0.1 μm or more and 5.0 μm or less in terms of volume-based median diameter. A more preferable range of the above particle diameter is 0.1 μm or more and 4.0 μm or less, a further preferable range is 0.1 μm or more and 3.0 μm or less, an even more preferable range is 0.2 μm or more and 3.0 μm or less, and a still more preferable range is 0.3 μm or more and 3.0 μm or less. If the particle diameter is in the range of 0.1 μm or more and 5.0 μm or less, it exists uniformly in a suitable size in the present composition, so that the stability of the present composition is high and excellent binding properties can be exhibited. If the particle diameter exceeds 5.0 μm, there is a risk that the binding property becomes insufficient as described above. In addition, there is a risk that the coatability becomes insufficient in that a smooth coated surface is difficult to obtain. On the other hand, when the particle diameter is less than 0.1 μm, concerns arise from the viewpoint of stable manufacturability.

[0085] In the present crosslinked polymer, in the present composition, acid groups such as carboxyl groups derived from ethylenically unsaturated carboxylic acid monomers are preferably neutralized so that the degree of neutralization is 20 mol% or more and used in the form of a salt. The above degree of neutralization is more preferably 50 mol% or more, still more preferably 70 mol% or more, even more preferably 75 mol% or more, yet even more preferably 80 mol% or more, and particularly preferably 85 mol% or more. The upper limit value of the degree of neutralization is 100 mol%, and it may be 98 mol% or 95 mol%. The range of the degree of neutralization can be appropriately combined with the above lower limit value and upper limit value. For example, it may be 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 stabilizing effect is easily obtained. In the present specification, the above degree of neutralization can be calculated from the charged values of the monomer having an acid group such as a carboxyl group and the neutralizing agent used for neutralization. The degree of neutralization can be confirmed 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 group of the carboxylate salt by subjecting the crosslinked polymer or its salt to IR measurement on the powder after drying treatment at 80°C for 3 hours under reduced pressure conditions.

[0086] <Water swelling degree of this crosslinked polymer> In the present specification, the water swelling degree is based on the weight "(W A ) g" of the crosslinked polymer or its salt at the time of drying, and the amount of water "(W B ) g" absorbed when the crosslinked polymer or its salt is saturated and swollen with water, and is calculated based on the following formula. (Water swelling degree) = {(W A ) + (W B )} / (W A )

[0087] The crosslinked polymer or a salt thereof preferably has a degree of water swelling at pH 8 of 20 or more and 80 or less. When the degree of water swelling is within the above range, the crosslinked polymer or a salt thereof swells moderately in an aqueous medium, so that when forming the electrode binder layer, it is possible to secure a sufficient adhesion area to the active material and the current collector, and the binding property tends to be good. The above degree of water swelling may be, for example, 21 or more, 23 or more, 25 or more, 27 or more, or 30 or more. When the degree of water swelling is 20 or more, the crosslinked polymer or a salt thereof spreads on the surface of the active material or the current collector, and a sufficient adhesion area can be secured, so that good binding property can be obtained. The upper limit value of the degree of water swelling at pH 8 may be 75 or less, 70 or less, 65 or less, 60 or less, or 55 or less. When the degree of water swelling exceeds 60, the viscosity of the electrode binder layer composition (electrode slurry) containing the crosslinked polymer or a salt thereof tends to increase, and as a result of insufficient uniformity of the binder layer, sufficient adhesive strength may not be obtained. In addition, there is a risk of deterioration in the coatability of the electrode slurry. The range of the degree of water swelling at pH 8 can be set by appropriately combining the above upper limit value and lower limit value. For example, it is 23 or more and 70 or less, and for another example, it is 25 or more and 65 or less, and for still another example, it is 25 or more and 55 or less. The degree of water swelling 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 if necessary, the pH value may be adjusted using an appropriate acid or alkali, or a buffer solution or the like. The pH during measurement is, for example, in the range of 8.0 ± 0.5, preferably in the range of 8.0 ± 0.3, more preferably in the range of 8.0 ± 0.2, and still more preferably in the range of 8.0 ± 0.1.

[0088] In addition, those skilled in the art can adjust the degree of water swelling by controlling the composition and structure of the crosslinked polymer or a salt thereof. For example, by introducing an acidic functional group or a highly hydrophilic structural unit into the crosslinked polymer, the degree of water swelling can be increased. Also, by lowering the crosslinking degree of the crosslinked polymer, the degree of water swelling usually increases.

[0089] 4. Composition for secondary battery electrode binder layer The composition for a secondary battery electrode binder layer of the present invention contains this binder, an active material, and water. The amount of this binder used in this composition is, for example, 0.1 part by mass or more and 20 parts by mass or less with respect to 100 parts by mass of the total amount of the active material. The above amount used is also, for example, 0.2 part by mass or more and 10 parts by mass or less, and also, for example, 0.3 part by mass or more and 8 parts by mass or less, and also, for example, 0.4 part by mass or more and 5 parts by mass or less. If the amount of the binder used is 0.1 part by mass or more, sufficient binding property can be obtained. Also, the dispersion stability of the active material and the like can be ensured, and a uniform binder layer can be formed. If the amount of the binder used is 20 parts by mass or less, the composition will not have a high viscosity, and the coatability onto the current collector can be ensured. As a result, a binder layer having a uniform and smooth surface can be formed.

[0090] Among the above active materials, as the positive electrode active material, a lithium salt of a transition metal oxide can be used. For example, a lithium-containing metal oxide of a layered rock salt type and a spinel type can be used. Specific compounds of the layered rock salt type positive electrode active material include lithium cobaltate, lithium nickelate, and NCM {Li(Ni x ,Co y ,Mn z ), x + y + z = 1} and NCA {Li(Ni 1-a-b Co a Al b )}, etc. are mentioned. Also, as the spinel type positive electrode active material, lithium manganate and the like are mentioned. In addition to oxides, phosphates, silicates, sulfur, etc. are used, and as the phosphate, olivine type lithium iron phosphate and the like are mentioned. As the positive electrode active material, one of the above may be used alone, or two or more may be combined and used as a mixture or a composite.

[0091] When a positive electrode active material containing a layered rock salt type lithium-containing metal oxide is dispersed in water, lithium ions on the surface of the active material are exchanged with hydrogen ions in water, causing the dispersion to exhibit alkalinity. For this reason, there is a risk of corrosion of aluminum foil (Al) or the like, which is a common current collector material for positive electrodes. In such a case, it is preferable to neutralize the alkali component eluted from the active material by using an unneutralized or partially neutralized crosslinked polymer as a binder. Further, the amount of the unneutralized or partially neutralized crosslinked polymer used is preferably such that the amount of unneutralized carboxyl groups in the crosslinked polymer is equal to or more than the amount of alkali eluted from the active material.

[0092] Since all positive electrode active materials have low electrical conductivity, it is common to use them with the addition of a conductive aid. Examples of the conductive aid include carbon-based materials such as carbon black, carbon nanotubes, carbon fibers, graphite fine powder, and carbon fibers. Among these, carbon black, carbon nanotubes, and carbon fibers are preferable from the viewpoint of easily obtaining excellent electrical conductivity. Further, as the carbon black, ketjen black and acetylene black are preferable. The conductive aid may be used alone or in combination of two or more. The amount of the conductive aid used can be, for example, 0.2 to 20 parts by mass, and also, for example, 0.2 to 10 parts by mass, based on 100 parts by mass of the total amount of the active material, from the viewpoint of achieving both electrical conductivity and energy density. Further, a positive electrode active material surface-coated with a conductive carbon-based material may be used.

[0093] On one hand, examples of the negative electrode active material include carbon-based materials, lithium metal, lithium alloys, and metal oxides, etc., and one or more of these can be used in combination. Among these, an active material composed of a carbon-based material such as natural graphite, artificial graphite, hard carbon, and soft carbon (hereinafter also referred to as "carbon-based active material") is 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 perspective 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. Also, in order to increase the energy density, metals or metal oxides that can occlude 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 an active material composed of a silicon-based material such as silicon, a silicon alloy, and silicon oxides such as silicon monoxide (SiO) (hereinafter also referred to as "silicon-based active material") can be used. However, while the above silicon-based active material has a high capacity, it has a large volume change accompanying charge and discharge. Therefore, it is preferably used in combination with the above carbon-based active material. In this case, if the blending amount of the silicon-based active material is large, it may cause the disintegration of the electrode material and the cycle characteristics (durability) may be greatly reduced. From such a perspective, when the silicon-based active material is used in combination, its usage amount is, for example, 60% by mass or less, and also for example, 30% by mass or less, based on the carbon-based active material.

[0094] Since the carbon-based active material itself has good electrical conductivity, it is not necessarily necessary to add a conductive assistant. When adding a conductive assistant for the purpose of further reducing resistance, etc., from the perspective of energy density, its usage amount is, for example, 10 parts by mass or less, and also for example, 5 parts by mass or less, based on 100 parts by mass of the total amount of the active material.

[0095] When the composition is in a slurry state, the amount of the active material used is, for example, in the range of 10 to 75% by mass, and also, for example, in the range of 30 to 65% by mass, based on the total amount of the composition. If the amount of the active material used is 10% by mass or more, migration of a binder or the like can be suppressed, and it is also advantageous in terms of the drying cost of the medium. On the other hand, if it is 75% by mass or less, the fluidity and coatability of the composition can be ensured, and a uniform agent layer can be formed.

[0096] The composition uses water as the medium. Also, for the purpose of adjusting the properties and drying property of the composition, etc., it may be a mixed solvent with lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, and water-soluble organic solvents such as tetrahydrofuran and N-methylpyrrolidone. The proportion of water in the mixed medium is, for example, 50% by mass or more, and also, for example, 70% by mass or more.

[0097] When the composition is made into a coatable slurry state, the content of the medium containing water in the whole composition can be, from the viewpoints of the coatability of the slurry and the energy cost and productivity required for drying, for example, in the range of 25 to 60% by mass, and also, for example, 35 to 60% by mass.

[0098] The composition may further use other binder components such as styrene-butadiene rubber (SBR) - based latex, carboxymethyl cellulose (CMC), acrylic-based latex, and polyvinylidene fluoride-based latex in combination. When using other binder components in combination, the amount used can be, for example, 0.1 to 5 parts by mass or less, and also, for example, 0.1 to 2 parts by mass or less, and also, for example, 0.1 to 1 part by mass or less, based on 100 parts by mass of the total amount of the active material. If the amount of the other binder component used exceeds 5 parts by mass, the resistance may increase, and the high-rate characteristics may be insufficient in some cases. Among the above, SBR-based latex and CMC are preferable in terms of the excellent balance of binding property and flex resistance, and it is more preferable to use SBR-based latex and CMC in combination.

[0099] The above-mentioned SBR latex refers to an aqueous dispersion of a copolymer having a structural unit derived from an aromatic vinyl monomer such as styrene and a structural unit derived from an aliphatic conjugated diene monomer such as 1,3-butadiene. Examples of the above-mentioned aromatic vinyl monomer include α-methylstyrene, vinyltoluene, divinylbenzene, etc. in addition to styrene, and one or more of these can be used. The structural unit derived from the aromatic vinyl monomer in the above copolymer can be, for example, in the range of 20 to 70% by mass, and can also be, for example, in the range of 30 to 60% by mass, mainly from the viewpoint of binding property. Examples of the above-mentioned 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 above copolymer can be, for example, in the range of 30 to 70% by mass, and can also be, for example, in the range of 40 to 60% by mass, in terms of good binding property of the binder and flexibility of the resulting electrode. In addition to the above monomers, styrene / butadiene latex may use, as other monomers, nitrile group-containing monomers such as (meth)acrylonitrile, carboxyl group-containing monomers such as (meth)acrylic acid, itaconic acid, maleic acid, etc., and ester group-containing monomers such as methyl (meth)acrylate as copolymerization monomers to further improve performance such as binding property. The structural unit derived from the above other monomer in the above copolymer can be, for example, in the range of 0 to 30% by mass, and can also be, for example, in the range of 0 to 20% by mass.

[0100] The above-mentioned CMC refers to a substituted product obtained by substituting a nonionic cellulose-based semi-synthetic polymer compound with a carboxymethyl group and its salts. Examples of the above-mentioned nonionic cellulose-based semi-synthetic polymer compound include alkyl celluloses such as methyl cellulose, methyl ethyl cellulose, ethyl cellulose, microcrystalline cellulose, etc.; Examples of hydroxyalkyl celluloses 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, nonoxinyl hydroxyethyl cellulose, and the like.

[0101] The composition for the secondary battery electrode binder layer of the present invention contains the above active material, water, and binder as essential components, and can be obtained by mixing each component using known means. The mixing method of each component is not particularly limited, and known methods can be adopted. However, a method of dry-blending powder components such as an active material, a conductive auxiliary, and a binder, and then mixing with a dispersion medium such as water and performing dispersion kneading is preferred. When obtaining the present composition in a slurry state, it is preferable to finish it into a slurry without poor dispersion or aggregation. As the mixing means, known mixers such as a planetary mixer, a thin-film swirling mixer, and a self-revolving mixer can be used, but it is preferable to use a thin-film swirling mixer in that a good dispersion state can be obtained in a short time. When using a thin-film swirling mixer, it is preferable to perform preliminary dispersion with a stirrer such as a disperser in advance. The pH of the above slurry is not particularly limited as long as the effects of the present invention are achieved, but it is preferably less than 12.5. For example, when CMC is blended, it is more preferably less than 11.5 and even more preferably less than 10.5 in that there is little concern about its hydrolysis. Also, the viscosity of the above slurry is not particularly limited as long as the effects of the present invention are achieved, but as the B-type viscosity (25°C) at 20 rpm, for example, it can be in the range of 100 to 6,000 mPa·s, and for example, it can be in the range of 500 to 5,000 mPa·s, and for example, it can be in the range of 1,000 to 4,000 mPa·s. If the viscosity of the slurry is within the above range, good coatability can be ensured.

[0102] 5. Secondary battery electrode The secondary battery electrode of the present invention comprises a binder layer formed from the composition for the secondary battery electrode binder layer on the surface of a current collector such as copper or aluminum. The binder layer is formed by applying the present composition on the surface of the current collector and then drying and removing a medium such as water. The method of applying the present composition is not particularly limited, and known methods such as the doctor blade method, dip method, roll coating method, comma coating method, curtain coating method, gravure coating method, and extrusion method can be adopted. Further, the above drying can be performed by known methods such as hot air blowing, reduced pressure, (far) infrared rays, and microwave irradiation. Generally, the obtained binder layer after drying is subjected to a compression treatment such as die pressing and roll pressing. By compression, the active material and the binder can be adhered, and the strength of the binder layer and the adhesion to the current collector can be improved. The thickness of the binder layer can be adjusted to about 30 to 80% of that before compression by compression. The thickness of the binder layer after compression is generally about 4 to 200 μm.

[0103] 6. Secondary battery By providing a separator and an electrolytic solution to the secondary battery electrode of the present invention, a secondary battery can be manufactured. The electrolytic solution may be liquid or gel-like. The separator is disposed between the positive electrode and the negative electrode of the battery and plays a role in preventing short circuit due to contact between the two electrodes and retaining the electrolytic solution to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane having good ion permeability and mechanical strength. As specific materials, polyolefins such as polyethylene and polypropylene, and polytetrafluoroethylene can be used.

[0104] The electrolyte can be a known one generally used according to the type of the active material. In a lithium-ion secondary battery, as specific solvents, cyclic carbonates such as propylene carbonate and ethylene carbonate having a high dielectric constant and a high ability to dissolve electrolytes, and chain carbonates such as ethyl methyl carbonate, dimethyl carbonate and diethyl carbonate having low viscosity, etc. can be mentioned, and these can be used alone or as a mixed solvent. The electrolyte is used by dissolving a lithium salt such as LiPF6, LiSbF6, LiBF4, LiClO4, LiAlO4, etc. in these solvents. In a nickel-metal hydride secondary battery, an aqueous potassium hydroxide solution can be used as the electrolyte. The secondary battery can be obtained by housing a positive electrode plate and a negative electrode plate separated by a separator in a spiral shape or a laminated structure in a case or the like.

[0105] As described above, the secondary battery provided with an electrode having a binder layer formed from the composition for a secondary battery electrode binder layer disclosed in this specification exhibits good durability (cycle characteristics) even when charge and discharge are repeated, and thus is suitable for in-vehicle secondary batteries and the like.

Examples

[0106] Hereinafter, the present invention will be specifically described based on examples. Note that the present invention is not limited by these examples. Hereinafter, "parts" and "%" mean parts by mass and mass % unless otherwise specified.

[0107] ≪Evaluation of carboxyl group-containing crosslinked polymer (salt)≫ (Measurement of the inhomogeneous network structure size Ξ of the crosslinked polymer by small-angle X-ray scattering method) 1. Method for preparing measurement sample <1>Preparation of 1 mass % concentration aqueous solution Weighed 0.5 g of the powder of the carboxyl group-containing crosslinked polymer salt and 49.5 g of ion-exchanged water into a 100 cc container, and set it on a rotation / revolution type stirrer (manufactured by Shinki Co., Ltd., Awatori Rentaro AR-250). Then, carried out stirring (rotation speed 2000 rpm / revolution speed 800 rpm, 7 minutes), and further degassing (rotation speed 2200 rpm / revolution speed 60 rpm, 1 minute) treatment to prepare a 1 mass% concentration aqueous solution in a state where the carboxyl group-containing crosslinked polymer salt was swollen in water. <2> Preparation of 5 mass% concentration aqueous solution Weighed 2.5 g of the powder of the carboxyl group-containing crosslinked polymer salt and 47.5 g of ion-exchanged water into a 100 cc container, and performed the same operation as <1> to prepare a 5 mass% concentration aqueous solution.

[0108] 2. Method for small-angle X-ray scattering measurement <1> Equipment Small-angle X-ray scattering measurement was carried out using SAXSpoint 2.0 manufactured by Anton Paar. The X-ray source used was a Primux100 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 was 3.0 m, which realized a measurable q range of 0.015 - 4.0 Å -1 (The scattering vector q is obtained by q = 4π / λ sinθ, where 2θ is the scattering angle). The measurement was carried out at a temperature of 25.0 °C and controlled by a Peltier element with an accuracy of 0.1 °C. The raw scattering data was corrected for the scattering of the cell and silver behenate using the relative transmittance.

[0109] <2> Measurement method The 1 mass% concentration aqueous solution and 5 mass% concentration aqueous solution obtained in <1> above were respectively injected into a cell (cell thickness: 1 mm) sandwiched between polyimide films (Kapton manufactured by Toray DuPont), maintained under vacuum to reduce scattering from air, and the exposure time was adjusted so that the detector was not damaged by strong X-rays. Then, the sample was irradiated with X-rays to obtain a two-dimensional scattering image of the sample. Next, background correction was performed on 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 in the absence of the 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.

[0110] <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 the full azimuthal angle, so that a one-dimensional scattering spectrum (scattering intensity curve I(q)) with the horizontal axis being the scattering vector q (Å -1 ) and the vertical axis being the intensity of the scattered radiation was obtained. Next, as baseline correction, the minimum value of the scattering intensity in the analysis target region was determined, and baseline correction was performed by subtracting the minimum value over the entire region. For the obtained one-dimensional scattering profile after correction, fitting was performed using the following formula (1) to obtain Ξ (Ξ1) in an aqueous solution with a concentration of 1% by mass and Ξ (Ξ5) in an aqueous solution with a concentration of 5% by mass. In the following formula (1), Ξ represents the structural size of the inhomogeneous network of the crosslinked polymer. As shown in FIG. 1, when there is a part with a dense crosslinking degree, its size is described by Ξ. Note that waveform separation software (Igor Pro8) was used for fitting.

Equation

[0111] (Degree of water swelling at pH 8) The degree of water swelling at pH 8 was measured by the following method. The measuring device is shown in FIG. 1. The measuring device is composed of <1> to <3> in FIG. 1. <1> It consists of a burette 1 with a side tube for air extraction, a pinchcock 2, a silicone tube 3, and a polytetrafluoroethylene tube 4. <2> On top of the funnel 5, there is a support cylinder 8 with numerous holes on its bottom surface, and further on top of that, a filter paper 10 for the device is installed. <3> The sample 6 (measurement sample) of the crosslinked polymer or its salt is sandwiched between two filter papers 7 for fixing the sample, and the filter papers for fixing the sample are fixed by an adhesive tape 9. All the filter papers used are ADVANTEC No. 2 with an inner diameter of 55 mm. <1> and <2> are connected by a silicone tube 3. Also, the height of the funnel 5 and the support cylinder 8 with respect to the burette 1 is fixed, and it is set so that the lower end of the polytetrafluoroethylene tube 4 installed inside the burette branch tube and the bottom surface of the support cylinder 8 are at the same height (dotted line in Fig. 1).

[0112] The measurement method will be described below. Remove the pinchcock 2 in <1>, pour deionized water through the silicone tube 3 from the upper part of the burette 1, and fill it with deionized water 12 from the burette 1 to the filter paper 10 for the device. Then, close the pinchcock 2 and remove the air from the polytetrafluoroethylene tube 4 connected to the burette branch tube with a rubber stopper. In this way, make it a state where deionized water 12 is continuously supplied from the burette 1 to the filter paper 10 for the device. Next, after removing the excess deionized water 12 oozing out from the filter paper 10 for the device, record the reading (a) of the scale of the burette 1. Weigh 0.1 - 0.2 g of the dry powder of the measurement sample and place it uniformly at the center of the filter paper 7 for fixing the sample as in <3>. Sandwich the sample with another filter paper, hold the two filter papers with the adhesive tape 9, and fix the sample. Place the filter paper with the sample fixed on the filter paper 10 for the device shown in <2>. Next, record the reading (b) of the scale of the burette 1 after 30 minutes have elapsed from the time when the lid 11 is placed on the filter paper 10 for the device. The total water absorption amount (c) of the measurement sample and the two filter papers 7 for fixing the sample is obtained by (a - b). By the same operation, measure the water absorption amount (d) of only the two filter papers 7 without including the sample of the crosslinked polymer or its salt. The above operations were performed, and the water swelling degree was calculated from the following formula. The solid content used in the calculation was the value measured by the method described below. Water swelling degree = {dry weight of the measurement sample (g) + (c - d)} / {dry weight of the measurement sample (g)} However, dry weight of the measurement sample (g) = weight of the measurement sample (g) × (solid content (%) ÷ 100)

[0113] Here, the method for measuring the solid content is described below. Approximately 0.5 g of the sample was taken into a weighing bottle whose weight had been measured in advance [weight of the weighing bottle = B (g)], and after accurately weighing the entire weighing bottle [W0 (g)], the sample in the weighing bottle was placed in a non-airflow dryer and dried at 155 °C for 45 minutes, and the weight at that time was measured for the entire weighing bottle [W1 (g)]. The solid content was determined by the following formula. Solid content (%) = (W1 - B) / (W0 - B) × 100

[0114] (Measurement of particle size (water swelling particle size) in an aqueous medium) 0.25 g of the powder of the carboxyl group-containing crosslinked polymer salt and 49.75 g of ion-exchanged water were weighed into a 100 cc container and set in a rotation / revolution type stirrer (manufactured by Shinki Co., Ltd., Awatori Rentaro AR-250). Next, stirring (rotation speed 2,000 rpm / revolution speed 800 rpm, 7 minutes) and further defoaming (rotation speed 2,200 rpm / revolution speed 60 rpm, 1 minute) were performed to prepare a hydrogel in a state where 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 type particle size distribution analyzer (manufactured by Microtrac Bel Co., Ltd., Microtrac MT-3300EXII) with ion-exchanged water as the dispersion medium. When an appropriate amount of the hydrogel that could obtain an appropriate scattered light intensity was introduced into the place where an excessive amount of the dispersion medium was circulating with respect to the hydrogel, the particle size distribution shape measured several minutes later became stable. As soon as the stability was confirmed, the particle size distribution was measured, and the volume-based median diameter (D50) as a representative value of the particle size was obtained.

[0115] (Measurement of viscosity of 2 mass% aqueous solution) 2.0 parts of a carboxyl group-containing crosslinked polymer salt powder and 98 parts of ion-exchanged water were weighed into a container and set in a rotation / revolution type stirrer (manufactured by Shinki Co., Ltd., Awatori Rentaro AR-250). Next, stirring (rotation speed 2,000 rpm / revolution speed 800 rpm, 7 minutes), and further defoaming (rotation speed 2,200 rpm / revolution speed 60 rpm, 1 minute) treatment were repeated until there was no unswollen powder portion, and a hydrogel fine particle dispersion in which the carboxyl group-containing crosslinked polymer salt was swollen in water was prepared. After adjusting each of the obtained hydrogel fine particle dispersions to 25 °C ± 1 °C, the viscosity at a rotor speed of 12 rpm was measured using a B-type viscometer (manufactured by Toki Sangyo Co., Ltd., TVB-10).

[0116] ≪Synthesis of the First Polymer≫ (Polymer 1) Into 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 Nippon Fine Chemical Co., Ltd., trade name "ABN-E"), 75 parts of styrene (St), 25 parts of acrylonitrile (AN), and 67 parts of anisole were charged, sufficiently degassed by nitrogen bubbling, and polymerization was started in a constant temperature bath at 80 °C. After 4 hours, it was cooled to room temperature to stop the reaction. The above polymerization solution was reprecipitated and purified from methanol / water = 90 / 10 (vol%), and vacuum dried to obtain Polymer 1. As a result of testing 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. Note that styrene and acrylonitrile correspond to the first monomers.

[0117] (Method for Measuring the Molecular Weight of the 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) in terms of polystyrene were obtained by THF-based GPC. Also, the molecular weight distribution (Mw / Mn) was calculated from the obtained values. Note that GPC was performed under the following conditions.

[0118] Column: 4 pieces of Tosoh TSKgel SuperMultipore HZ-M Solvent: Tetrahydrofuran Temperature: 40 °C Detector: RI Flow rate: 600 μL / min

[0119] <<Production of crosslinked polymer salt containing carboxyl group>> (Production Example 1: Production of crosslinked polymer salt R-1 containing carboxyl group) 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 part of DBTTC, 0.90 part of trimethylolpropane diallyl ether (manufactured by Daiso Co., Ltd., trade name "Neoallyl T-20"), and triethylamine corresponding to 1.0 mol% based on the above AA were charged. After thoroughly purging the inside of the reactor with nitrogen, it was heated to raise the internal temperature to 55 °C. After confirming that the internal temperature was stable at 55 °C, 0.040 part of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd., trade name "V-65") was added as a polymerization initiator. At this point, turbidity was observed in the reaction solution, so this point was taken as the polymerization start point. The monomer concentration was calculated to be 15.0%. When 12 hours had elapsed from the polymerization start point, cooling of the reaction solution was started. After the internal temperature had dropped to 25 °C, 52.4 parts of lithium hydroxide monohydrate (hereinafter referred to as "LiOH·H2O") powder was added. After the addition, stirring was continued at room temperature for 12 hours to obtain a slurry-like polymerization reaction solution in which particles of crosslinked polymer salt R-1 (Li salt, neutralization degree 90 mol%) were dispersed in the medium.

[0120] The obtained polymerization reaction solution was centrifuged to precipitate polymer particles, and then the supernatant was removed. Thereafter, the precipitate was redispersed in acetonitrile having the same weight as the polymerization reaction solution, and then the washing operation of precipitating 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 volatile components, thereby obtaining a powder of the carboxyl group-containing polymer salt R-1. Since the carboxyl group-containing polymer salt R-1 has hygroscopicity, it was stored sealed in a container having a water vapor barrier property. In addition, the powder of the carboxyl group-containing polymer salt R-1 was measured by IR, and when the neutralization degree was determined 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, it was equal to the calculated value from the charge and was 90 mol%. Further, the water swelling degree was 36.4, the particle diameter in the aqueous medium was 1.72 μm, and the viscosity of the 2 mass% concentration aqueous solution was 9,110 mPa·s.

[0121] (Production Examples 2 to 15 and Comparative Production Examples 1 to 2: Production of Carboxyl Group-Containing Crosslinked Polymer Salts R-2 to R-17) The same operations as in Production Example 1 were carried out except that the charged amounts of the monomer, the crosslinkable monomer, and the neutralizing agent were as shown in Table 1, and a polymerization reaction solution containing carboxyl group-containing crosslinked polymer salts R-2 to R-17 was obtained. Next, the same operations as in Production Example 1 were carried out for each polymerization reaction solution to obtain powdery carboxyl group-containing crosslinked polymer salts R-2 to R-17. Each carboxyl group-containing crosslinked polymer salt was stored sealed in a container having a water vapor barrier property. The water swelling degrees, particle diameters in the aqueous medium, and viscosities of 2 mass% concentration aqueous solutions of R-2 to R-17 are shown in Table 1. Regarding the particle diameter in the aqueous medium of R-3 (neutralization degree 70 mol%), the measurement was carried out after adjusting the neutralization degree to 90 mol% with LiOH·H2O.

[0122] (Small-Angle X-Ray Scattering Measurement) Examples of the small-angle X-ray scattering measurement results are shown in Fig. 1 (Production Example 2: R-2, Comparative Production Example 2: R-17). For all samples, in the 1 mass% concentration aqueous solution, q was 0.03 to 0.10 Å -1A broad peak has been observed nearby. This peak is due to the scattering of charged groups derived from carboxyl groups present inside the particles, with the hydrogel microparticles being dispersed in water. On the other hand, in an aqueous solution with a concentration of 5% by mass, q is 0.07 - 0.20 Å -1 A peak has been observed nearby. This can be considered that since the hydrogel microparticles are densely packed in water, approaching an apparently uniform structure, the peak derived from its crosslinking has been confirmed.

[0123] In an aqueous solution with a concentration of 1% by mass, for the graph shown in Figure 1, with q* = 0.053 Å -1 , L = 15.4 Å -1 By performing fitting using Equation (1) with these values, for R - 2, Ξ1 = 43.3 Å -1 , and for R - 17, Ξ1 = 154.4 Å -1 were calculated. Also, in an aqueous solution with a concentration of 5% by mass, for the graph shown in Figure 1, with q* = 0.10 Å -1 , L = 10.0 Å -1 By performing fitting using Equation (1) with these values, for R - 2, Ξ5 = 41.6 Å -1 , and for R - 17, Ξ5 = 32.8 Å -1 were calculated. In both cases, Ξ1 shows a larger value compared to Ξ5. This can be considered that in an aqueous solution with a concentration of 1% by mass, the hydrogel is not packed in water and is in a saturated swelling state where it can absorb water sufficiently. As a result, the density of crosslinking becomes clearer, and a large non - uniform network structure size is observed. On the other hand, in an aqueous solution with a concentration of 5% by mass, since the hydrogel is packed in water, the density of crosslinking is canceled out, resulting in a structure that appears to be close to a uniform network, and consequently, a small non - uniform network structure size is observed. Therefore, ΔΞ (Ξ1 - Ξ5) can be regarded as indirectly indicating the degree of crosslinking density. Considering the results that Ξ in the 1% by mass aqueous solution for R - 2 is smaller than that for R - 17 and ΔΞ is also smaller, it can be considered that R - 2 has a more uniform crosslinked structure. For the scattering intensity curve I(q) obtained by subjecting R-1, R-3 to R-16 to small-angle X-ray scattering measurement, Ξ1, Ξ5, and ΔΞ calculated by curve fitting with the following formula (1) are shown in Table 1.

[0124]

Table 1

[0125] 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., trade name "Neoallyl T-20") P-30: Pentaerythritol triallyl ether (manufactured by Daiso Co., Ltd., trade 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

[0126] ≪Evaluation of Composition Containing Carboxyl Group-Containing Crosslinked Polymer Salt≫ (Example 1: Evaluation of Composition Containing Carboxyl Group-Containing Crosslinked Polymer Salt R-1) <Preparation of slurry composition (composition for electrode binder layer)> A material obtained by coating 10% carbon on the surface of SiOx (0.8 < x < 1.2) by CVD method (hereinafter referred to as "Si-based active material") was prepared, and a mixture of artificial graphite and Si-based active material was used as the active material. Further, as the binder, a mixture of this crosslinked polymer salt R-1, styrene-butadiene rubber (SBR) latex, and carboxymethyl cellulose (CMC) was used. Using water as a diluting solvent, the composition for the electrode mixture layer was mixed for 2 hours using T.K. Hibiscus Mix manufactured by Primix Corporation at a mass ratio of artificial graphite: Si-based active material: R-1: SBR: CMC = 90:10:1.0:1.0:1.0 (solid content) so that the solid content concentration of the composition for the electrode mixture layer would be 50% by mass, and a slurry composition, i.e., a slurry-like composition for the electrode mixture layer (electrode slurry), was prepared. The viscosity of the electrode slurry was 3,670 mPa·s, which was a sufficiently low value. An electrode was fabricated using the obtained electrode slurry and evaluated. The specific procedures and evaluation methods are shown below.

[0127] <Fabrication of negative electrode plate> The above electrode slurry was applied to both sides of a copper foil (thickness: 20 μm) and dried to form a mixture layer. Thereafter, after rolling so that the thickness of the mixture layer would be 27 μm and the packing density would be 1.3 g / cm 3 ³, it was punched out into a 3 cm square to obtain a negative electrode plate.

[0128] (Coatability of the electrode slurry) The coatability of the electrode slurry in the fabrication of the above negative electrode plate was evaluated based on the following criteria and was evaluated as "◎". <Evaluation criteria> ◎: No appearance abnormalities such as streaks and bumps are observed on the surface at all. 〇: Slight appearance abnormalities such as streaks and bumps are observed on the surface. △: Some appearance abnormalities such as streaks and bumps are observed on the surface. ×: Prominent appearance abnormalities such as streaks and bumps are observed on the surface.

[0129] <Fabrication of positive electrode plate> In an N-methylpyrrolidone (NMP) solvent, 100 parts of lithium iron phosphate (LFP) as a positive electrode active material, 0.2 part of carbon nanotubes as a conductive agent, 2 parts of Ketjen black, and 0.6 part of vapor-grown carbon fiber (VGCF) were mixed and added. Polyvinylidene fluoride (PVDF) was mixed as a binder for the electrode composition to prepare a positive electrode composition. An aluminum current collector (thickness: 15 μm) was coated with the positive electrode composition and dried to form a coating layer. Then, after rolling so that the thickness of the coating layer was 88 μm and the packing density was 3.1 g / cm 3 and punching out into a 3 cm square to obtain a positive electrode plate.

[0130] <Fabrication of secondary battery> Using the above positive electrode plate, the above negative electrode plate, and a separator, a lithium-ion secondary battery of a laminate type cell was fabricated. As the electrolytic solution, a mixed solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (DEC) were in a volume ratio of 25:75 and LiPF6 was dissolved at a concentration of 1.0 mol / liter was used.

[0131] (Coating film performance) Regarding the performance of the coating film obtained from the above electrode slurry, in this example, it was evaluated by measuring the cycle characteristics of the lithium-ion secondary battery. The lithium-ion secondary battery of the laminate type cell fabricated above was charged and discharged under the conditions of 2.7 to 3.4 V by CC discharge at a charge-discharge rate of 0.2 C to measure the initial capacity C0. Further, 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 "〇". Note that the higher the value of ΔC, the better the cycle characteristics. ΔC = C50 / C0 × 100 (%) <Evaluation criteria> ◎: Charge-discharge capacity retention rate is 95.0% or more 〇: 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%

[0132] (Examples 2 to 15 and Comparative Examples 1 to 2) Except that the crosslinked polymer salt was as described in Table 2, the same operations as in Example 1 were carried out to prepare an electrode slurry as a slurry composition, and the viscosity of the slurry was measured. Also, the coatability of the electrode slurry and the cycle characteristics of the secondary battery obtained using the same were evaluated. The results are shown in Table 2.

[0133]

Table 2

[0134] ≪Evaluation Results≫ As is clear from the results of Examples 1 to 15, all of the slurry compositions containing the crosslinked polymer salt of the present invention had good coatability and were also excellent in the coating film performance of the composition (in this example, the cycle characteristics of the secondary battery provided with the electrode obtained using the composition). Among these, when comparing the cases where the water swelling degree and the particle size in the aqueous medium are equivalent (Examples 2, 12, 13), as the exchange chain transfer mechanism type controller, when using the controller in reversible addition fragmentation chain transfer polymerization (Examples 2, 12), the non-uniform network structure size Ξ1 is a smaller value than when using an iodine transfer polymerization controller (Example 13), and since ΔΞ (Ξ1 - Ξ5) is also a smaller value, the coating film performance was excellent (in this example, the charge-discharge capacity retention rate was high and the cycle characteristics were excellent). On the other hand, in the case of a slurry composition containing a crosslinked polymer having a non-uniform network structure size Ξ1 exceeding 80, either the coating film performance (cycle characteristics of the secondary battery) or the coatability was significantly inferior (Comparative Examples 1 and 2).

Industrial Applicability

[0135] 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 thus is expected to be applied to various uses such as a thickener and a viscosity modifier for cosmetics, a binder for non-aqueous electrolyte secondary battery electrodes, an anti-settling agent for pigments, and a dispersion stabilizer for metal powders. Furthermore, a secondary battery provided with an electrode obtained by using a composition for a secondary battery electrode mixture layer containing a binder for a secondary battery electrode containing the carboxyl group-containing crosslinked polymer or a salt thereof of the present invention exhibits good durability (cycle characteristics), and thus is expected to be applied to in-vehicle secondary batteries. It is also useful for the use of an active material containing silicon, and is expected to contribute to an increase in the capacity of the battery. Among them, it is useful for non-aqueous electrolyte lithium ion secondary batteries having a high energy density.

Claims

1. A carboxyl group-containing crosslinked polymer or a salt thereof, wherein 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 with respect to all its structural units, the crosslinked polymer is crosslinked with a crosslinkable monomer, and the amount of the crosslinkable monomer used is 0.1 part by mass or more and 2.0 parts by mass or less with respect to 100 parts by mass of the total amount of non-crosslinkable monomers, and the amount of the exchange chain transfer mechanism type controller used is 0.0001 to 0.50 mol% with respect to the total amount of monomers, a carboxyl group-containing crosslinked polymer or a salt thereof, wherein the heterogeneous network structure size Ξ (hereinafter referred to as "Ξ1") of the crosslinked polymer, which is 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 neutralization degree of 50 to 100 mol% by the small-angle X-ray scattering method (measurement temperature: 25.0 ± 0.1 °C), is 41.8 or more and 80 or less according to the following formula (1). 【Number 1】

2. The carboxyl group-containing crosslinked polymer or a salt thereof according to claim 1, wherein the difference ΔΞ (Ξ1 - Ξ5) between Ξ1 and the heterogeneous network structure size Ξ (hereinafter referred to as "Ξ5") of the crosslinked polymer, which is 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 neutralization degree of 50 to 100 mol% by the small-angle X-ray scattering method (measurement temperature: 25.0 ± 0.1 °C), is 50 or less according to the formula (1).

3. The carboxyl group-containing crosslinked polymer or a salt thereof according to claim 1 or 2, wherein Ξ1 is 41.8 or more and 74.5 or less.

4. The carboxyl group-containing crosslinked polymer or a salt thereof according to any one of claims 1 to 3, wherein the crosslinkable monomer contains a polyfunctional allyl ether compound having two or more allyl ether groups in the molecule.

5. The carboxyl group-containing crosslinked polymer or a salt thereof according to any one of claims 1 to 4, wherein after the crosslinked polymer or a salt thereof is neutralized to a neutralization degree of 80 to 100 mol%, the particle size measured in an aqueous medium is 0.1 μm or more and 5.0 μm or less in terms of volume-based median diameter.

6. The carboxyl group-containing crosslinked polymer or a salt thereof according to any one of claims 1 to 5, wherein the water swelling degree at pH 8 is 20 or more and 80 or less.

7. A binder for a secondary battery electrode, comprising the carboxyl group-containing crosslinked polymer according to any one of claims 1 to 6 or a salt thereof.

8. 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.

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

10. A secondary battery, comprising the secondary battery electrode according to claim 9.

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