Secondary battery electrode binder and use of same

Abstract

The present invention provides a secondary battery electrode binder with which it is possible to improve the initial charge / discharge efficiency and cycle characteristics of a secondary battery. The present invention also provides a secondary battery electrode mixture layer composition containing the aforementioned binder, and a secondary battery electrode and a secondary battery that are obtained using the composition.　Provided is a secondary battery electrode binder that contains a carboxyl-group-containing crosslinked polymer or a salt thereof, and a carboxyl-group-containing non-crosslinked polymer or a salt thereof, wherein the carboxyl-group-containing non-crosslinked polymer has structural units derived from an ethylenically unsaturated carboxylic acid monomer and structural units derived from a nitrile-group-containing ethylenically unsaturated monomer.

Description

Binder for secondary battery electrodes and its use

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

[0002] Various secondary batteries, such as nickel-metal hydride secondary batteries, lithium-ion secondary batteries, and electric double layer capacitors, have been put to practical use. Electrodes used in these secondary batteries are prepared by applying a composition for forming an electrode mixture layer containing an active material and a binder to a current collector, followed by drying. For example, in lithium-ion secondary batteries, an aqueous binder containing styrene butadiene rubber (SBR) latex and carboxymethyl cellulose (CMC) is used as the binder for the negative electrode mixture layer composition. Furthermore, binders containing aqueous solutions or dispersions of acrylic acid-based polymers are known as binders with excellent dispersibility and binding properties. Meanwhile, a solution of polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP) is widely used as the binder for the positive electrode mixture layer.

[0003] In recent years, as the applications of various secondary batteries have expanded, there has been a growing demand for improved energy density, reliability, and durability. For example, in order to increase the electrical capacity of lithium-ion secondary batteries, specifications using silicon-based active materials as negative electrode active materials have become more common. However, silicon-based active materials are known to undergo large volume changes during charge and discharge, and repeated use can lead to peeling or detachment of the electrode mixture layer, resulting in a decrease in battery capacity and deterioration of cycle characteristics (durability). In order to prevent such problems, it is generally effective to increase the binding properties of the binder, and studies on improving the binding properties of binders have been conducted with the aim of improving durability.

[0004] In this context, it has been reported that acrylic acid polymers are effective as binders that have good cycle characteristics and are effective in improving the durability of negative electrode mixture layers that use silicon-based active materials. Patent Document 1 discloses a binder containing a cross-linked acrylic acid polymer in which polyacrylic acid is cross-linked with a specific cross-linking agent, and discloses that even when an active material containing silicon (hereinafter also referred to as a "silicon-based active material") is used, the binder exhibits good cycle characteristics without destroying the electrode structure.

[0005] Patent Document 2 discloses a binder containing a first polymer compound and a second polymer compound having a weight-average molecular weight smaller than that of the first polymer compound, and discloses that at least one of the first polymer compound and the second polymer compound has a carboxyl group, and that hydrogen atoms of some of the carboxyl groups of the polymer compound having the carboxyl group are substituted with lithium ions, thereby improving cycle characteristics.

[0006] International Publication No. 2014 / 065407 Japanese Patent Application Laid-Open No. 2020-61381

[0007] The binders for secondary battery electrodes disclosed in Patent Documents 1 and 2 can follow the expansion and contraction of silicon-based active materials, and therefore can improve cycle characteristics, but they are sometimes insufficient, and furthermore, the initial charge / discharge efficiency is low, which can be a problem.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a binder for a secondary battery electrode that can improve the initial charge-discharge efficiency and cycle characteristics of a secondary battery. It is also an object of the present invention to provide a composition for a secondary battery electrode mixture layer that contains the binder, and a secondary battery electrode and a secondary battery obtained using the composition.

[0009] As a result of intensive research to solve the above problems, the present inventors have found that the initial charge-discharge efficiency and cycle characteristics of a secondary battery can be improved by using a binder for secondary battery electrodes containing a carboxyl group-containing crosslinked polymer or a salt thereof, and a carboxyl group-containing non-crosslinked polymer having a specific structural unit or a salt thereof, and have completed the present invention.

[0010] The present invention is as follows: [1] A binder for secondary battery electrodes containing a carboxyl group-containing crosslinked polymer or a salt thereof, and a carboxyl group-containing non-crosslinked polymer or a salt thereof, wherein the carboxyl group-containing non-crosslinked polymer has structural units derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as "monomer (a)") and structural units derived from a nitrile group-containing ethylenically unsaturated monomer (hereinafter also referred to as "monomer (b)"). [2] The binder for secondary battery electrodes according to [1], wherein the carboxyl group-containing crosslinked polymer contains structural units derived from the monomer (a) in an amount of 50% by mass or more and 100% by mass or less based on the total structural units of the carboxyl group-containing crosslinked polymer. [3] The binder for a secondary battery electrode according to [1] or [2], wherein the carboxyl group-containing non-crosslinked polymer contains, relative to all of its structural units, 40% by mass or more and 99% by mass or less of structural units derived from the monomer (a) and 1% by mass or more and 50% by mass or less of structural units derived from the monomer (b). [4] The binder for a secondary battery electrode according to any one of [1] to [3], wherein the carboxyl group-containing non-crosslinked polymer further contains, relative to all of its structural units, structural units derived from an amide group-containing ethylenically unsaturated monomer (hereinafter also referred to as "monomer (c)"). [5] The binder for secondary battery electrodes according to [4], wherein the carboxyl group-containing non-crosslinked polymer contains, relative to all structural units thereof, 40% by mass or more and 98% by mass or less of structural units derived from the monomer (a), 1% by mass or more and 50% by mass or less of structural units derived from the monomer (b), and 1% by mass or more and 50% by mass or less of structural units derived from the monomer (c). [6] The binder for secondary battery electrodes according to any one of [1] to [5], wherein the carboxyl group-containing non-crosslinked polymer or salt thereof has a weight average molecular weight of 400,000 or more and 2,500,000 or less. [7] The binder for secondary battery electrodes according to any one of [1] to [6], wherein the content of the carboxyl group-containing crosslinked polymer or salt thereof is 10% by mass or more and 80% by mass or less of the total amount of the carboxyl group-containing crosslinked polymer or salt thereof and the carboxyl group-containing non-crosslinked polymer or salt thereof.[8] The binder for a secondary battery electrode according to any one of [1] to [7], further containing styrene butadiene rubber (SBR) or carboxymethyl cellulose (CMC). [9] A composition for a secondary battery electrode mix layer, comprising the binder for a secondary battery electrode according to any one of [1] to [8], an active material, and water.

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

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

[10] .

[0011] According to the binder for secondary battery electrodes of the present invention, a secondary battery having excellent initial charge / discharge efficiency and cycle characteristics can be obtained.

[0012] The binder for secondary battery electrodes of the present invention (hereinafter also referred to as "the binder") contains a carboxyl group-containing crosslinked polymer (hereinafter also referred to as "the crosslinked polymer") or a salt thereof (hereinafter also referred to as "the crosslinked polymer salt"), and a carboxyl group-containing non-crosslinked polymer having structural units derived from monomer (a) and structural units derived from monomer (b) (hereinafter also referred to as "the non-crosslinked polymer") or a salt thereof (hereinafter also referred to as "the non-crosslinked polymer salt"). By mixing this with an active material and water, a secondary battery electrode mixture layer composition (hereinafter also referred to as "the composition") can be obtained. The above composition is preferably in the form of an electrode slurry that can be applied to a current collector in order to achieve the effects of the present invention. However, it may also be prepared in the form of a wet powder so that it can be pressed onto the current collector surface. The secondary battery electrode of the present invention can be obtained by forming a mixture layer formed from the above composition on the surface of a current collector such as copper foil or aluminum foil. Here, the present binder is preferred in that the effects of the present invention are particularly large when used in a secondary battery electrode mixture layer composition containing a silicon-based active material described below as the active material.

[0013] The present crosslinked polymer and its manufacturing method, the present non-crosslinked polymer and its manufacturing method, the present binder, the composition for a secondary battery electrode mixture layer obtained using the present binder, the secondary battery electrode, and the secondary battery are each described in detail below. In this specification, "(meth)acrylic" means acrylic and / or methacrylic, and "(meth)acrylate" means acrylate and / or methacrylate. Furthermore, "(meth)acryloyl group" means acryloyl group and / or methacryloyl group. In the numerical ranges described in this specification in stages, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages, and the upper or lower limit value of that numerical range may be replaced with a value shown in the examples.

[0014] 1. The present crosslinked polymer The present crosslinked polymer can have a structural unit (hereinafter also referred to as "component (a)") derived from an ethylenically unsaturated carboxylic acid monomer (monomer (a)), and can be introduced into the polymer by precipitation polymerization or dispersion polymerization of a monomer component including component (a).

[0015] <Structural Units Derived from Ethylenically Unsaturated Carboxylic Acid Monomers> The crosslinked polymer having a carboxyl group contained in the binder (hereinafter also referred to as the "crosslinked polymer") may have a structural unit (component (a)) derived from an ethylenically unsaturated carboxylic acid monomer. When the crosslinked polymer has such a structural unit and thus a carboxyl group, the adhesion to the current collector is improved, and the lithium ion desolvation effect and ionic conductivity are excellent, resulting in an electrode with low resistance and excellent high-rate characteristics. In addition, water swelling properties are imparted, thereby improving the dispersion stability of active materials and the like in the composition. The above-mentioned component (a) can be introduced into the polymer, for example, by polymerizing a monomer containing an ethylenically unsaturated carboxylic acid monomer. Alternatively, it can be obtained by (co)polymerizing a (meth)acrylic acid ester monomer and then hydrolyzing it. Alternatively, (meth)acrylamide and (meth)acrylonitrile, etc., may be polymerized and then treated with a strong alkali, or a method in which a polymer having a hydroxyl group is reacted with an acid anhydride may be used.

[0016] Examples of the monomer (a) include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, and fumaric acid; (meth)acrylamidoalkylcarboxylic acids such as (meth)acrylamidohexanoic acid and (meth)acrylamidododecanoic acid; and carboxyl group-containing ethylenically unsaturated monomers such as succinic acid monohydroxyethyl (meth)acrylate, ω-carboxy-caprolactone mono(meth)acrylate, and β-carboxyethyl (meth)acrylate, or their (partially) alkali-neutralized products. One of these may be used alone, or two or more may be used in combination. Among these, compounds having an acryloyl group as a polymerizable functional group are preferred, with acrylic acid being particularly preferred, because they have a high polymerization rate, resulting in a polymer with a long primary chain length and good binder binding strength. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, a polymer with a high carboxyl group content can be obtained.

[0017] The content of component (a) in the crosslinked polymer can be 50% by mass or more and 100% by mass or less, based on the total structural units of the crosslinked polymer. By including component (a) in this range, excellent adhesion to the current collector can be easily ensured. A lower limit of 50% by mass or more is preferable because the dispersion stability of the composition is improved and a higher binding strength can be obtained. It may be 60% by mass or more, 70% by mass or more, or even 80% by mass or more. The upper limit is, for example, 99.9% by mass or less, for example, 99.5% by mass or less, for example, 99% by mass or less, for example, 98% by mass or less, for example, 95% by mass or less, for example, 90% by mass or less, or for example, 80% by mass or less.

[0018] <Other Structural Units> In addition to component (a), the present crosslinked polymer may contain a structural unit (hereinafter also referred to as "component (d)") derived from another ethylenically unsaturated monomer copolymerizable therewith (hereinafter also referred to as "monomer (d)"). Examples of monomer (d) include hydroxyl group-containing ethylenically unsaturated monomers (monomers represented by the following formula (1) and formula (2)), ethylenically unsaturated monomer compounds having anionic groups other than carboxyl groups such as sulfonic acid groups and phosphoric acid groups, and nonionic ethylenically unsaturated monomers. The structural unit derived from monomer (d) can be introduced by copolymerizing a hydroxyl group-containing ethylenically unsaturated monomer, an ethylenically unsaturated monomer compound having anionic groups other than carboxyl groups such as sulfonic acid groups and phosphoric acid groups, or a monomer containing a nonionic ethylenically unsaturated monomer. CH ２ = C(R １ ) COOR ２ (1) [wherein, R １ represents a hydrogen atom or a methyl group, R ２ represents a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, (R ３ O) ｍ H or R ４ O[CO(CH ２ ) ５ O] ｎ H. Note that R ３ represents an alkylene group having 2 to 4 carbon atoms, R ４ represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15.] CH ２ = C(R ５ ) CONR ６ R ７ (2) [wherein, R ５ represents a hydrogen atom or a methyl group, R ６ represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, and R ７ represents a hydrogen atom or a monovalent organic group.

[0019] The proportion of the (d) component can be 0% by mass or more and 50% by mass or less relative to the total structural units of the non-crosslinked polymer. The proportion of the (d) component may be 0.1% by mass or more and 40% by mass or less, 0.5% by mass or more and 30% by mass or less, 1.0% by mass or more and 20% by mass or less, 2% by mass or more and 12.5% ​​by mass or less, or 3% by mass or more and 10% by mass or less. Furthermore, when the (d) component is contained in an amount of 0.1% by mass or more relative to the total structural units of the crosslinked polymer, affinity to the electrolyte solution is improved, and therefore, the effect of improving lithium ion conductivity can also be expected.

[0020] Among the above-mentioned components, the (d) component is preferably a structural unit derived from a hydroxyl group-containing ethylenically unsaturated monomer, in view of the excellent binding properties of the binder containing the crosslinked polymer salt. Furthermore, a structural unit derived from a nonionic ethylenically unsaturated monomer is preferred, in view of the obtainment of an electrode with good flex resistance. Examples of nonionic ethylenically unsaturated monomers include (meth)acrylamide and its derivatives, nitrile group-containing ethylenically unsaturated monomers, and alicyclic structure-containing ethylenically unsaturated monomers.

[0021] The monomer represented by the above formula (1) is a (meth)acrylate compound having a hydroxyl group. ２ When R is a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, the number of the hydroxyl groups may be one or more. The monovalent organic group is not particularly limited, but examples thereof include alkyl groups which may have a linear, branched, or cyclic structure, as well as aryl groups and alkoxyalkyl groups. ２ (R ３ O) ｍ H or R ４ O[CO(CH ２ ) ５ O] ｎ If H, then R ３ or R ４ The alkylene group represented by may be linear or branched.

[0022] Examples of the monomer represented by the formula (1) include hydroxyalkyl (meth)acrylates having a hydroxyalkyl group having 1 to 8 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, hydroxyhexyl (meth)acrylate, and hydroxyoctyl (meth)acrylate; polyalkylene glycol mono(meth)acrylates, such as polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, polybutylene glycol mono(meth)acrylate, and polyethylene glycol-polypropylene glycol mono(meth)acrylate; dihydroxyalkyl (meth)acrylates, such as glycerin mono(meth)acrylate; caprolactone-modified hydroxymethacrylates (manufactured by Daicel Corporation, trade names "PLACCEL FM1," "PLACCEL FM5," etc.), caprolactone-modified hydroxyacrylates (manufactured by Daicel Corporation, trade names "PLACCEL FA1," "PLACCEL FA10L," etc.). The monomer represented by the formula (1) may be used singly or in combination of two or more.

[0023] The monomer represented by the above formula (2) is a (meth)acrylamide derivative having a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms. ７ represents a hydrogen atom or a monovalent organic group. The monovalent organic group is not particularly limited, but examples thereof include alkyl groups which may have a linear, branched, or cyclic structure, as well as aryl groups and alkoxyalkyl groups, and is preferably an organic group having 1 to 8 carbon atoms. In addition, R ７ may be a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms.

[0024] Examples of the monomer represented by formula (2) include hydroxy(meth)acrylamide; (meth)acrylamide derivatives having a hydroxyalkyl group having 1 to 8 carbon atoms, such as N-hydroxyethyl(meth)acrylamide, N-(2-hydroxypropyl)(meth)acrylamide, N-hydroxybutyl(meth)acrylamide, N-hydroxyhexyl(meth)acrylamide, and N-hydroxyoctyl(meth)acrylamide, N-methylhydroxyethyl(meth)acrylamide, and N-ethylhydroxyethyl(meth)acrylamide; and N,N-dihydroxyalkyl(meth)acrylamides, such as N,N-dihydroxyethyl(meth)acrylamide and N,N-dihydroxyethyl(meth)acrylamide. The monomer represented by formula (2) may be used singly or in combination of two or more.

[0025] Examples of the (meth)acrylamide derivative include N-alkyl(meth)acrylamide compounds such as N-isopropyl(meth)acrylamide and N-t-butyl(meth)acrylamide; N-alkoxyalkyl(meth)acrylamide compounds such as N-n-butoxymethyl(meth)acrylamide and N-isobutoxymethyl(meth)acrylamide; and N,N-dialkyl(meth)acrylamide compounds such as N,N-dimethyl(meth)acrylamide and N,N-diethyl(meth)acrylamide. These may be used alone or in combination of two or more.

[0026] Examples of the nitrile group-containing ethylenically unsaturated monomer include (meth)acrylonitrile; (meth)acrylic acid cyanoalkyl ester compounds such as cyanomethyl (meth)acrylate and cyanoethyl (meth)acrylate; cyano group-containing unsaturated aromatic compounds such as 4-cyanostyrene and 4-cyano-α-methylstyrene; vinylidene cyanide; and the like. One of these may be used alone, or two or more may be used in combination. Of the above, acrylonitrile is preferred because of its high nitrile group content.

[0027] Examples of the alicyclic structure-containing ethylenically unsaturated monomer include (meth)acrylic acid cycloalkyl esters which may have an aliphatic substituent, such as cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, cyclodecyl (meth)acrylate, and cyclododecyl (meth)acrylate; isobornyl (meth)acrylate, adamantyl (meth)acrylate, cyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and cycloalkyl polyalcohol mono(meth)acrylates such as cyclohexanedimethanol mono(meth)acrylate and cyclodecanedimethanol mono(meth)acrylate; and the like. These may be used alone or in combination of two or more.

[0028] In view of the excellent binding properties of the binder, the present crosslinked polymer preferably contains structural units derived from the monomer represented by the above formula (1), the monomer represented by the above formula (2), (meth)acrylamide and its derivatives, nitrile group-containing ethylenically unsaturated monomers, alicyclic structure-containing ethylenically unsaturated monomers, etc. Among these, as component (b), structural units derived from the monomer represented by the above formula (1) and the monomer represented by the above formula (2) are more preferred in view of the excellent effect of improving the binding properties of the present binder. Among the monomers represented by the above formula (1), hydroxyalkyl (meth)acrylates having a hydroxyalkyl group having 1 to 8 carbon atoms are more preferred, and 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate are even more preferred. Furthermore, among the monomers represented by the above formula (2), (meth)acrylamide derivatives having a hydroxyalkyl group having 1 to 8 carbon atoms are more preferred, and N-hydroxyethyl(meth)acrylamide, N-(2-hydroxypropyl)(meth)acrylamide, and N-hydroxybutyl(meth)acrylamide are even more preferred.

[0029] Furthermore, when a structural unit derived from a hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less is introduced as component (d), it can exhibit strong interaction with the electrode material and exhibit good binding properties to the active material. This allows for a firm and well-integrated electrode mixture layer to be obtained, and therefore, as the above-mentioned "hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less," an alicyclic structure-containing ethylenically unsaturated monomer is particularly preferred.

[0030] Other nonionic ethylenically unsaturated monomers that may be used include, for example, (meth)acrylic acid esters. Examples of (meth)acrylic acid esters include (meth)acrylic acid alkyl ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; aromatic (meth)acrylic acid ester compounds such as phenyl (meth)acrylate, phenylmethyl (meth)acrylate, phenylethyl (meth)acrylate, and phenoxyethyl (meth)acrylate; and (meth)acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate. These may be used alone or in combination of two or more.

[0031] From the viewpoints of binding strength with the active material and cycle characteristics, aromatic (meth)acrylic acid ester compounds are preferably used. From the viewpoints of further improving lithium ion conductivity and high-rate characteristics, compounds having an ether bond, such as (meth)acrylic acid alkoxyalkyl esters such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate, are preferred, with 2-methoxyethyl (meth)acrylate being more preferred.

[0032] Among nonionic ethylenically unsaturated monomers, compounds having an acryloyl group are preferred because they have a fast polymerization rate, resulting in a polymer with a long primary chain length, and provide good binder binding strength.Furthermore, as the nonionic ethylenically unsaturated monomer, compounds whose homopolymer has a glass transition temperature (Tg) of 0°C or less are preferred because the resulting electrode has good flex resistance.

[0033] The present crosslinked polymer is a crosslinked polymer having a crosslinked structure. The crosslinking method for the present crosslinked polymer is not particularly limited, and examples thereof include the following methods: 1) Copolymerization of a crosslinkable monomer 2) Utilization of chain transfer to a polymer chain during radical polymerization Because the present crosslinked polymer has a crosslinked structure, a binder containing the crosslinked polymer or a salt thereof can have excellent binding strength. Among the above, the method of copolymerizing a crosslinkable monomer is preferred because of its simple operation and ease of controlling the degree of crosslinking.

[0034] <Crosslinkable Monomer> Examples of the crosslinkable monomer include a polyfunctional polymerizable monomer having two or more polymerizable unsaturated groups, and a monomer having a self-crosslinkable crosslinkable functional group such as a hydrolyzable silyl group.

[0035] The polyfunctional polymerizable monomer is a compound having two or more polymerizable functional groups such as (meth)acryloyl groups and alkenyl groups in the molecule, and examples thereof include polyfunctional (meth)acryloyl compounds, polyfunctional alkenyl compounds, and compounds having both (meth)acryloyl groups and alkenyl groups. These compounds may be used alone or in combination of two or more. Among these, polyfunctional alkenyl compounds are preferred in that they are easy to obtain a uniform crosslinked structure, and polyfunctional allyl ether compounds having two or more allyl ether groups in the molecule are particularly preferred.

[0036] Examples of polyfunctional (meth)acryloyl compounds include di(meth)acrylates of dihydric alcohols such as ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, polyethylene glycol di(meth)acrylate, and polypropylene glycol di(meth)acrylate; poly(meth)acrylates such as tri(meth)acrylate and tetra(meth)acrylate of trihydric or higher polyhydric alcohols such as trimethylolpropane tri(meth)acrylate, tri(meth)acrylate of ethylene oxide-modified trimethylolpropane, glycerin tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and pentaerythritol tetra(meth)acrylate; and bisamides such as methylenebisacrylamide and hydroxyethylenebisacrylamide.

[0037] Examples of polyfunctional alkenyl compounds include polyfunctional allyl ether compounds such as trimethylolpropane diallyl ether, trimethylolpropane triallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, tetraallyloxyethane, and polyallylsucrose; polyfunctional allyl compounds such as diallyl phthalate; and polyfunctional vinyl compounds such as divinylbenzene.

[0038] Examples of the compound having both a (meth)acryloyl group and an alkenyl group include allyl (meth)acrylate, isopropenyl (meth)acrylate, butenyl (meth)acrylate, pentenyl (meth)acrylate, and 2-(2-vinyloxyethoxy)ethyl (meth)acrylate.

[0039] Specific examples of the monomer having a self-crosslinkable crosslinkable functional group include a hydrolyzable silyl group-containing vinyl monomer, N-methoxyalkyl(meth)acrylamide, etc. These compounds can be used alone or in combination of two or more.

[0040] The hydrolyzable silyl group-containing vinyl monomer is not particularly limited as long as it is a vinyl monomer having at least one hydrolyzable silyl group.For example, it can be mentioned vinyl silanes such as vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane, vinyldimethylmethoxysilane, etc.; silyl group-containing acrylic esters such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, methyldimethoxysilylpropyl acrylate, etc.; silyl group-containing methacrylic esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, dimethylmethoxysilylpropyl methacrylate, etc.; silyl group-containing vinyl ethers such as trimethoxysilylpropyl vinyl ether, etc.; silyl group-containing vinyl esters such as vinyl trimethoxysilylundecanoate, etc.

[0041] When the crosslinked polymer is crosslinked with a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.01 to 5.0 parts by mass, more preferably 0.05 to 3.0 parts by mass, even more preferably 0.1 to 2.0 parts by mass, even more preferably 0.1 to 1.7 parts by mass, and even more preferably 0.5 to 1.5 parts by mass, relative to 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers). When the amount of the crosslinkable monomer used is 0.01 parts by mass or more, the conductive path between the active materials is well maintained while suppressing expansion and contraction due to charge and discharge during long-term use, which is preferable in that excellent charge and discharge capacity retention can be achieved. When the amount is 5.0 parts by mass or less, the stability of precipitation polymerization or dispersion polymerization tends to be improved. In particular, if the amount is 1.0 part by mass or less, the water-swelling particle size in the electrode slurry becomes suitable, and the area of ​​binding to the active material becomes large, which is preferable in that excellent battery performance can be maintained even during long-term use.

[0042] For the same reason, the amount of the crosslinkable monomer used is preferably 0.001 mol % or more and 2.5 mol % or less, more preferably 0.01 mol % or more and 2.0 mol % or less, even more preferably 0.05 mol % or more and 1.75 mol % or less, still more preferably 0.05 mol % or more and 1.5 mol % or less, and even more preferably 0.1 mol % or more and 1.0 mol % or less, based on the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers).

[0043] The crosslinked polymer salt is in the form of a salt in which some or all of the carboxyl groups contained in the polymer have been neutralized. The type of salt is not particularly limited, but examples include alkali metal salts such as lithium salts, sodium salts, and potassium salts; alkaline earth metal salts such as magnesium salts, calcium salts, and barium salts; other metal salts such as aluminum salts; ammonium salts, and organic amine salts. Among these, alkali metal salts and alkaline earth metal salts are preferred in terms of being less likely to adversely affect battery characteristics, alkali metal salts are more preferred, and lithium salts are particularly preferred in terms of providing excellent secondary battery cycle characteristics and excellent low-temperature battery characteristics.

[0044] Regarding the characteristics of the present crosslinked polymer salt, the present crosslinked polymer is preferably used in the form of a salt in which acid groups such as carboxyl groups derived from the ethylenically unsaturated carboxylic acid monomer are neutralized so that the degree of neutralization in the present composition is 20 mol% or more. A degree of neutralization of 20 mol% or more is preferable in that it improves water swelling properties and makes it easier to achieve a dispersion stabilization effect. The degree of neutralization is more preferably 50 mol% or more, even more preferably 70 mol% or more, even more preferably 75 mol% or more, even more preferably 80 mol% or more, and particularly preferably 85 mol% or more, in that it can exhibit excellent charge / discharge capacity retention rates over longer periods of use than conventional methods. The upper limit of the degree of neutralization is 100 mol%, but it may also be 98 mol% or 95 mol%. In this specification, the degree of neutralization can be calculated from the amounts of monomers having acid groups such as carboxyl groups and the neutralizing agent used for neutralization. The degree of neutralization can be confirmed by subjecting the crosslinked polymer salt to IR measurement after drying at 80°C for 3 hours under reduced pressure conditions, and determining the intensity ratio of the peak derived from the C=O group of the carboxylic acid to the peak derived from the C=O group of the carboxylate salt.

[0045] <Particle size of the present crosslinked polymer salt> In the present composition, it is preferable that the present crosslinked polymer salt is not present as large particle size agglomerates (secondary aggregates) but is well dispersed as water-swellable particles having an appropriate particle size, because this allows a binder containing the crosslinked polymer salt to exhibit good binding performance.

[0046] The crosslinked polymer preferably has a volume-based median particle size (water-swollen particle size) of 0.1 μm or more and 10.0 μm or less when dispersed in water and the degree of neutralization based on the carboxyl groups of the crosslinked polymer is 80 to 100 mol%. A more preferred range for the particle size is 0.15 μm or more and 8.0 μm or less, an even more preferred range is 0.20 μm or more and 6.0 μm or less, an even more preferred range is 0.25 μm or more and 4.0 μm or less, and an even more preferred range is 0.30 μm or more and 2.0 μm or less. When the particle size is in the range of 0.30 μm or more and 2.0 μm or less, the particles are uniformly present in the composition at a suitable size, thereby enabling the composition to exhibit high stability and excellent binding properties. If the particle size exceeds 10.0 μm, there is a risk of insufficient binding properties, as described above. Furthermore, it is difficult to obtain a smooth coating surface, which may result in insufficient coatability. On the other hand, if the particle size is less than 0.1 μm, there is a concern in terms of stable production.

[0047] 2. Method for Producing the Crosslinked Polymer: While known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization can be used to produce the crosslinked polymer, precipitation polymerization and suspension polymerization (reverse-phase suspension polymerization) are preferred from the standpoint of productivity. Heterogeneous polymerization methods such as precipitation polymerization, suspension polymerization, and emulsion polymerization are preferred because they provide better performance in terms of binding properties, among which precipitation polymerization is more preferred. Precipitation polymerization is a method for producing a polymer by carrying out a polymerization reaction in a solvent that dissolves the raw material unsaturated monomer but does not substantially dissolve the resulting polymer. As the polymerization proceeds, the polymer particles grow through aggregation and growth, resulting in a dispersion of polymer particles in which primary particles of tens to hundreds of nanometers in size have secondary aggregation to several micrometers to several tens of micrometers. A dispersion stabilizer can also be used to control the polymer particle size. Furthermore, the secondary aggregation can be suppressed by selecting the dispersion stabilizer, polymerization solvent, etc. Precipitation polymerization with suppressed secondary aggregation is generally also called dispersion polymerization.

[0048] In the case of precipitation polymerization, the polymerization solvent can be selected from water and various organic solvents, taking into consideration the type of monomer to be used, etc. In order to obtain a polymer having a longer primary chain length, it is preferable to use a solvent with a small chain transfer constant.

[0049] Specific polymerization solvents include water-soluble solvents such as methanol, t-butyl alcohol, acetone, methyl ethyl ketone, acetonitrile, and tetrahydrofuran, as well as benzene, ethyl acetate, dichloroethane, n-hexane, cyclohexane, and n-heptane. These solvents can be used alone or in combination. Alternatively, they can be used as a mixed solvent with water. In the present invention, the water-soluble solvent refers to a solvent having a solubility in water of greater than 10 g / 100 ml at 20°C. Of the above, methyl ethyl ketone and acetonitrile are preferred in terms of the following: they provide good polymerization stability with minimal generation of coarse particles and adhesion to the reactor; the precipitated polymer fine particles are less likely to undergo secondary aggregation (or, even if secondary aggregation does occur, they are easily disintegrated in an aqueous medium); they produce polymers with a small chain transfer constant and a large degree of polymerization (primary chain length); and they are easy to handle during the neutralization step described below.

[0050] The polymerization initiator may be any known polymerization initiator such as an azo compound, an organic peroxide, or an inorganic peroxide, but is not particularly limited. The conditions for use can be adjusted so as to generate an appropriate amount of radicals by known methods such as thermal initiation, redox initiation using a reducing agent, or UV initiation. To obtain a crosslinked polymer with a long primary chain length, it is preferable to set the conditions so as to generate as little radicals as possible within the allowable production time range.

[0051] A preferred amount of the polymerization initiator used is, for example, 0.001 to 2 parts by mass, or for example, 0.005 to 1 part by mass, or for example, 0.01 to 0.1 parts 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, a polymer having a long primary chain length is likely to be obtained.

[0052] The polymerization temperature varies depending on conditions such as the type and concentration of the monomers used, but is preferably 0 to 100°C, more preferably 20 to 80°C. The polymerization temperature may be constant or may vary over the course of the polymerization reaction. The polymerization time is preferably 1 minute to 20 hours, more preferably 1 hour to 15 hours.

[0053] Here, the present crosslinked polymer may contain 50% by mass or more and 100% by mass or less of structural units derived from an ethylenically unsaturated carboxylic acid monomer (monomer (a)) relative to all structural units thereof, and the preferred range of the content of the structural units is as described above. The type of monomer (a) is as described above.

[0054] 3. Present Non-Crosslinked Polymer The carboxyl group-containing non-crosslinked polymer of the present invention has a structural unit derived from monomer (a) and a structural unit derived from monomer (b), and can be introduced into the present non-crosslinked polymer by polymerizing a monomer component containing monomer (a) and monomer (b).

[0055] <Structural Unit Derived from Ethylenically Unsaturated Carboxylic Acid Monomer> The present non-crosslinked polymer has a structural unit (component (a)) derived from an ethylenically unsaturated carboxylic acid monomer (monomer (a)). When the present non-crosslinked polymer has such a structural unit and thus has a carboxyl group, adhesion to the current collector is improved, and the non-crosslinked polymer has an excellent desolvation effect for lithium ions and an excellent ionic conductivity, thereby obtaining an electrode with low resistance and excellent high-rate characteristics, and improving the dispersion stability of the active material, etc. in the present composition. The above-mentioned component (a) can be introduced into the present non-crosslinked polymer, for example, by polymerizing the monomer (a). Alternatively, it can be obtained by (co)polymerizing a (meth)acrylic acid ester monomer and then hydrolyzing it. Alternatively, (meth)acrylamide, (meth)acrylonitrile, etc. may be polymerized and then treated with a strong alkali, or a method in which a polymer having a hydroxyl group is reacted with an acid anhydride may be used.

[0056] Examples of the monomer (a) include the compounds described in the section on the monomer (a) of the present crosslinked polymer, and one of these may be used alone or two or more may be used in combination. Among these, compounds having an acryloyl group as a polymerizable functional group are preferred, and acrylic acid is particularly preferred, because they have a high polymerization rate, resulting in a polymer with a long primary chain length and good binder binding strength. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, a polymer with a high carboxyl group content can be obtained.

[0057] The content of component (a) in the non-crosslinked polymer can be 40% by mass or more and 99% by mass or less, based on the total structural units of the non-crosslinked polymer. By including component (a) in this range, excellent adhesion to the current collector can be easily ensured, and the initial charge / discharge efficiency and cycle characteristics of the secondary battery can be improved. A lower limit of 40% by mass or more is preferable because the dispersion stability of the composition is improved and a higher binding strength can be obtained. It may be 50% by mass or more, 60% by mass or more, 70% by mass or more, or even 80% by mass or more. The upper limit is, for example, 98% by mass or less, for example, 97% by mass or less, for example, 96% by mass or less, for example, 95% by mass or less, for example, 93% by mass or less, for example, 91% by mass or less, or for example, 90% by mass or less.

[0058] The total amount of monomer (a) and its salts converted into unneutralized form (i.e., all present ethylenically unsaturated carboxylic acid monomers converted into the form of carboxylic acid) relative to the total amount of the non-crosslinked polymer and its salts (hereinafter also referred to as the "residual carboxylic acid monomer (salt) content") can be, for example, 5.0% by mass or less. The residual carboxylic acid monomer (salt) content is, for example, 4.0% by mass or less, for example, 3.0% by mass or less, for example, 2.0% by mass or less, for example, 1.5% by mass or less, for example, 1.0% by mass or less, or for example, 0.5% by mass or less. If the residual carboxylic acid monomer (salt) content is 5.0% by mass or less, high binding strength can be obtained.

[0059] The lower limit of the residual carboxylic acid monomer (salt) content is not particularly limited, but can be, for example, 0.05% by mass or more. It is also, for example, 0.1% by mass or more, for example, 0.2% by mass or more, for example, 0.3% by mass or more, or for example, 0.4% by mass or more. A residual carboxylic acid monomer (salt) content of less than 0.05% by mass may not be advantageous in terms of cleaning costs, etc. The residual carboxylic acid monomer (salt) content is measured by gas chromatography (GC).

[0060] <Structural Unit Derived from Nitrile Group-Containing Ethylenically Unsaturated Monomer> The present non-crosslinked polymer has a structural unit (hereinafter also referred to as "component (b)") derived from a nitrile group-containing ethylenically unsaturated monomer (monomer (b)). The presence of such a structural unit in the present non-crosslinked polymer results in excellent initial charge-discharge efficiency and cycle characteristics of the secondary battery. The component (b) can be introduced into the present non-crosslinked polymer by polymerizing a monomer containing monomer (b).

[0061] Examples of the monomer (b) include (meth)acrylonitrile; (meth)acrylic acid cyanoalkyl ester compounds such as cyanomethyl (meth)acrylate and cyanoethyl (meth)acrylate; cyano group-containing unsaturated aromatic compounds such as 4-cyanostyrene and 4-cyano-α-methylstyrene; vinylidene cyanide; and the like. One of these may be used alone, or two or more may be used in combination. Among the above, acrylonitrile is preferred because it has a high nitrile group content and is more excellent in the initial charge / discharge efficiency and cycle characteristics of the secondary battery.

[0062] The content of component (b) in the non-crosslinked polymer can be 1% by mass or more and 50% by mass or less, based on the total structural units of the non-crosslinked polymer. By including component (b) in this range, excellent adhesion to the current collector can be easily ensured, and the initial charge / discharge efficiency and cycle characteristics of the secondary battery can be improved. A lower limit of 1% by mass or more is preferable because the dispersion stability of the composition is improved and a higher binding strength can be obtained. The lower limit may be 3% by mass or more, 5% by mass or more, 10% by mass or more, 20% by mass or more, or even 25% by mass or more. The upper limit is, for example, 49% by mass or less, 45% by mass or less, or, for example, 40% by mass or less, or, for example, 30% by mass or less.

[0063] The total amount of monomer (b) relative to the total amount of the non-crosslinked polymer and its salt (hereinafter also referred to as the "content of residual monomer (b)") can be, for example, 5.0% by mass or less. The content of residual monomer (b) is, for example, 4.0% by mass or less, for example, 3.0% by mass or less, for example, 2.0% by mass or less, for example, 1.5% by mass or less, for example, 1.0% by mass or less, or for example, 0.5% by mass or less. If the content of residual monomer (b) is 5.0% by mass or less, high binding strength between active materials can be obtained.

[0064] The lower limit of the content of the residual monomer (b) is not particularly limited, but can be, for example, 0.05% by mass or more. It is also, for example, 0.1% by mass or more, for example, 0.2% by mass or more, for example, 0.3% by mass or more, or for example, 0.4% by mass or more. The content of the residual monomer (b) is measured by gas chromatography (GC).

[0065] <Structural Unit Derived from Amide Group-Containing Ethylenically Unsaturated Monomer> The present non-crosslinked polymer preferably further comprises a structural unit (hereinafter also referred to as "component (c)") derived from an amide group-containing ethylenically unsaturated monomer (monomer (c)). When the present non-crosslinked polymer comprises such a structural unit, the secondary battery exhibits excellent cycle characteristics. The component (c) can be introduced into the present non-crosslinked polymer by polymerizing a monomer containing monomer (c).

[0066] Examples of the monomer (c) include (meth)acrylamide, a monomer represented by the following formula (1), and a (meth)acrylamide derivative (a monomer other than the monomer represented by the following formula (1)). Among these, (meth)acrylamide is preferred in that it provides a secondary battery with even better initial charge-discharge efficiency and cycle characteristics.

[0067] CH ２ = C(R ５ ) CONR ６ R ７ (1) [wherein, R ５ represents a hydrogen atom or a methyl group, R ６ represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, and R ７ represents a hydrogen atom or a monovalent organic group.

[0068] The monomer represented by the above formula (1) is a (meth)acrylamide derivative having a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms. ７ represents a hydrogen atom or a monovalent organic group. The monovalent organic group is not particularly limited, but examples thereof include alkyl groups which may have a linear, branched, or cyclic structure, as well as aryl groups and alkoxyalkyl groups, and is preferably an organic group having 1 to 8 carbon atoms. In addition, R ７may be a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms. Examples of the monomer represented by formula (1) include hydroxy(meth)acrylamide; (meth)acrylamide derivatives having a hydroxyalkyl group having 1 to 8 carbon atoms, such as N-hydroxyethyl(meth)acrylamide, N-(2-hydroxypropyl)(meth)acrylamide, N-hydroxybutyl(meth)acrylamide, N-hydroxyhexyl(meth)acrylamide, and N-hydroxyoctyl(meth)acrylamide, N-methylhydroxyethyl(meth)acrylamide, and N-ethylhydroxyethyl(meth)acrylamide; and N,N-dihydroxyalkyl(meth)acrylamides, such as N,N-dihydroxyethyl(meth)acrylamide and N,N-dihydroxyethyl(meth)acrylamide. The monomer represented by formula (1) may be used singly or in combination of two or more. Among the monomers represented by the above formula (1), (meth)acrylamide derivatives having a hydroxyalkyl group having 1 to 8 carbon atoms are more preferred in terms of excellent cycle characteristics of secondary batteries, and N-hydroxyethyl(meth)acrylamide, N-(2-hydroxypropyl)(meth)acrylamide, and N-hydroxybutyl(meth)acrylamide are even more preferred.

[0069] Examples of the (meth)acrylamide derivative include N-alkyl(meth)acrylamide compounds such as N-isopropyl(meth)acrylamide and N-t-butyl(meth)acrylamide; N-alkoxyalkyl(meth)acrylamide compounds such as N-n-butoxymethyl(meth)acrylamide and N-isobutoxymethyl(meth)acrylamide; and N,N-dialkyl(meth)acrylamide compounds such as N,N-dimethyl(meth)acrylamide and N,N-diethyl(meth)acrylamide. These may be used alone or in combination of two or more.

[0070] The content of component (c) in the non-crosslinked polymer can be 1% by mass or more and 50% by mass or less, based on the total structural units of the non-crosslinked polymer. By including component (c) in this range, excellent adhesion to the current collector can be easily ensured, thereby improving the cycle characteristics of the secondary battery. A lower limit of 1% by mass or more is preferable because the dispersion stability of the composition is improved and a higher binding strength can be obtained. It may be 2% by mass or more, 3% by mass or more, or even 5% by mass or more. The upper limit is, for example, 50% by mass or less or 45% by mass or less. In terms of achieving even better initial charge / discharge efficiency of the secondary battery, it is preferably 40% by mass or less, more preferably 30% by mass or less, even more preferably 20% by mass or less, and even more preferably 10% by mass or less.

[0071] The total amount of monomer (c) relative to the total amount of the present non-crosslinked polymer and its salt (hereinafter also referred to as the "content of residual monomer (c)") is 5.0 mass% or less, preferably 4.0 mass% or less, more preferably 3.0 mass% or less, even more preferably 2.0 mass% or less, still more preferably 1.5 mass% or less, still more preferably 1.0 mass% or less, and particularly preferably 0.5 mass% or less, in order to improve the cycle characteristics of the secondary battery.

[0072] The lower limit of the content of the residual monomer (c) is not particularly limited, but can be, for example, 0.05% by mass or more. It is also, for example, 0.1% by mass or more, for example, 0.2% by mass or more, for example, 0.3% by mass or more, or for example, 0.4% by mass or more. The content of the residual monomer (c) is measured by gas chromatography (GC).

[0073] <Other Structural Units> In addition to the components (a) and (b) (and, if necessary, the component (c)), the present non-crosslinked polymer may contain a structural unit (hereinafter also referred to as the "component (e)") derived from another ethylenically unsaturated monomer copolymerizable therewith (hereinafter also referred to as the "monomer (e)"). Examples of the component (e) include a hydroxyl group-containing ethylenically unsaturated monomer (a monomer represented by the following formula (2)), an ethylenically unsaturated monomer compound having an anionic group other than a carboxyl group, such as a sulfonic acid group or a phosphoric acid group, or a nonionic ethylenically unsaturated monomer (excluding monomers classified as nitrogen-containing ethylenically unsaturated monomers). The structural unit derived from the monomer (e) can be introduced by copolymerizing a hydroxyl group-containing ethylenically unsaturated monomer, an ethylenically unsaturated monomer compound having an anionic group other than a carboxyl group, such as a sulfonic acid group or a phosphoric acid group, or a monomer containing a nonionic ethylenically unsaturated monomer. CH ２ = C(R １ ) COOR ２ (2) [wherein, R １ represents a hydrogen atom or a methyl group, R ２ represents a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, (R ３ O) ｍ H or R ４ O[CO(CH ２ ) ５ O] ｎ H. Note that R ３ represents an alkylene group having 2 to 4 carbon atoms, R ４ represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15.

[0074] The proportion of the (e) component can be 0% by mass or more and 50% by mass or less relative to the total structural units of the non-crosslinked polymer. The proportion of the (e) component may be 0.1% by mass or more and 45% by mass or less, 0.5% by mass or more and 40% by mass or less, 1.0% by mass or more and 30% by mass or less, 5.0% by mass or more and 20% by mass or less, or 3% by mass or more and 10% by mass or less. Furthermore, when the (e) component is contained in an amount of 0.1% by mass or more relative to the total structural units of the non-crosslinked polymer, affinity to the electrolyte solution is improved, and therefore, the effect of improving lithium ion conductivity can also be expected.

[0075] Of the above, the component (e) is preferably a structural unit derived from a hydroxyl group-containing ethylenically unsaturated monomer, in that it provides excellent binding properties to a binder containing the present non-crosslinked polymer or a salt thereof.

[0076] The monomer represented by the above formula (2) is a (meth)acrylate compound having a hydroxyl group. ２ When R is a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, the number of the hydroxyl groups may be one or more. The monovalent organic group is not particularly limited, but examples thereof include alkyl groups which may have a linear, branched, or cyclic structure, as well as aryl groups and alkoxyalkyl groups. ２ (R ３ O) ｍ H or R ４ O[CO(CH ２ ) ５ O] ｎ If H, then R ３ or R ４ The alkylene group represented by may be linear or branched.

[0077] Examples of the monomer represented by the formula (2) include the compounds described in the section on the monomer represented by the formula (2) of the present crosslinked polymer, and one of these may be used alone, or two or more may be used in combination.

[0078] Furthermore, from the viewpoint of obtaining an electrode with good flex resistance, a structural unit derived from a nonionic ethylenically unsaturated monomer is preferred. Examples of the nonionic ethylenically unsaturated monomer include an alicyclic structure-containing ethylenically unsaturated monomer. Examples of the alicyclic structure-containing ethylenically unsaturated monomer include the compounds described in the alicyclic structure-containing ethylenically unsaturated monomer of the present crosslinked polymer, and one of these may be used alone, or two or more may be used in combination.

[0079] The present non-crosslinked polymer preferably contains a structural unit derived from a monomer represented by the above formula (2), an alicyclic structure-containing ethylenically unsaturated monomer, etc., in terms of excellent binding properties of the binder. Among these, as component (e), a structural unit derived from a monomer represented by the above formula (2) is more preferred in terms of excellent effect of improving the binding properties of the present binder. Among the monomers represented by the above formula (2), hydroxyalkyl (meth)acrylates having a hydroxyalkyl group having 1 to 8 carbon atoms are more preferred, and 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate are even more preferred.

[0080] Furthermore, when a structural unit derived from a hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less is introduced as component (e), it can exhibit strong interaction with the electrode material and exhibit good binding properties to the active material. This allows for a firm and well-integrated electrode mixture layer to be obtained, and therefore, as the above-mentioned "hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less," an alicyclic structure-containing ethylenically unsaturated monomer is particularly preferred.

[0081] Other nonionic ethylenically unsaturated monomers that may be used include, for example, (meth)acrylic acid esters. Examples of (meth)acrylic acid esters include (meth)acrylic acid alkyl ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; aromatic (meth)acrylic acid ester compounds such as phenyl (meth)acrylate, phenylmethyl (meth)acrylate, phenylethyl (meth)acrylate, and phenoxyethyl (meth)acrylate; and (meth)acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate. These may be used alone or in combination of two or more.

[0082] From the viewpoints of binding strength with the active material and cycle characteristics, aromatic (meth)acrylic acid ester compounds are preferably used. From the viewpoints of further improving lithium ion conductivity and high-rate characteristics, compounds having an ether bond, such as (meth)acrylic acid alkoxyalkyl esters such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate, are preferred, with 2-methoxyethyl (meth)acrylate being more preferred.

[0083] Among nonionic ethylenically unsaturated monomers, compounds having an acryloyl group are preferred because they have a fast polymerization rate, resulting in a polymer with a long primary chain length, and provide good binder binding strength.Furthermore, as the nonionic ethylenically unsaturated monomer, compounds whose homopolymer has a glass transition temperature (Tg) of 0°C or less are preferred because the resulting electrode has good flex resistance.

[0084] The non-crosslinked polymer salt is in the form of a salt in which some or all of the carboxyl groups contained in the polymer have been neutralized. The type of salt is not particularly limited, but examples include alkali metal salts such as lithium salts, sodium salts, and potassium salts; alkaline earth metal salts such as magnesium salts, calcium salts, and barium salts; other metal salts such as aluminum salts; ammonium salts, and organic amine salts. Among these, alkali metal salts and alkaline earth metal salts are preferred because they are less likely to adversely affect battery characteristics, and alkali metal salts are more preferred. Lithium salts are particularly preferred because they provide excellent cycle characteristics of secondary batteries and excellent low-temperature battery characteristics.

[0085] Regarding the characteristics of the non-crosslinked polymer salt, the non-crosslinked polymer is preferably used in the composition as a salt in which acid groups such as carboxyl groups derived from ethylenically unsaturated carboxylic acid monomers are neutralized so that the degree of neutralization is 40 mol% or more. A degree of neutralization of 40 mol% or more is preferred in that it is easier to obtain a dispersion stabilization effect. The degree of neutralization is more preferably 45 mol% or more, even more preferably 60 mol% or more, even more preferably 70 mol% or more, even more preferably 80 mol% or more, and particularly preferably 85 mol% or more, in that it can exhibit superior initial charge / discharge efficiency over longer periods of use than conventional methods. The upper limit of the degree of neutralization is 100 mol%, but it may also be 98 mol% or 95 mol%. In this specification, the degree of neutralization can be calculated from the amounts of monomers having acid groups such as carboxyl groups and the neutralizing agent used for neutralization. The degree of neutralization can be confirmed by dissolving the present non-crosslinked polymer salt in a mixed solvent of ethanol / water = 1 / 1 (mass ratio) and determining the substance amount of the carboxylate by acid titration and the substance amount ratio of the acid group such as a carboxyl group by subsequent base titration.

[0086] Molecular Weight of the Non-Crosslinked Polymer or Its Salt The number average molecular weight (Mn) of the non-crosslinked polymer or its salt is measured as a value equivalent to sodium polyacrylate by gel permeation chromatography (GPC) as described in the Examples. Mn is preferably 4,000 to 400,000 in terms of improving the dispersibility of the active material and achieving excellent charge / discharge capacity retention, and can be adjusted by the monomer concentration and initiator amount. Mn is more preferably 5,000 to 200,000, even more preferably 6,000 to 100,000, and even more preferably 7,500 to 50,000.

[0087] The weight-average molecular weight (Mw) of the non-crosslinked polymer or its salt is measured as a value equivalent to sodium polyacrylate by GPC as described in the Examples. Mw is preferably 400,000 to 2,500,000 in terms of improving the binding strength with the active material and current collector and achieving excellent charge / discharge capacity retention, and can be adjusted by adjusting the monomer concentration and initiator amount. Mw is more preferably 450,000 to 2,250,000, even more preferably 475,000 to 2,000,000, even more preferably 500,000 to 1,600,000, even more preferably 500,000 to 1,400,000, and particularly preferably 500,000 to 1,200,000.

[0088] The molecular weight distribution (Mw / Mn) of the non-crosslinked polymer or its salt is preferably 5 to 300, in terms of improving the binding strength with the active material and current collector, the dispersibility of the active material, and exhibiting an excellent charge / discharge capacity retention rate. This can be adjusted by the monomer concentration and polymerization temperature. Mw / Mn is more preferably 3 to 300, even more preferably 5 to 290, even more preferably 10 to 280, even more preferably 30 to 270, and particularly preferably 50 to 260. It is presumed that by increasing Mw / Mn, high-molecular-weight components that improve the binding strength with the active material and current collector and low-molecular-weight components that improve the dispersibility of the active material coexist, allowing each component to exhibit its respective functions.

[0089] 4. Method for Producing the Non-Crosslinked Polymer The non-crosslinked polymer can be obtained by polymerizing a monomer component containing monomer (a) and monomer (b). The polymerization method can be bulk polymerization without using a solvent, solution polymerization in a solvent, emulsion polymerization in an aqueous solution, mini-emulsion polymerization, or suspension polymerization. Among these, solution polymerization is preferred because it allows the non-crosslinked polymer or a salt thereof to be uniformly dissolved and can be more easily dispersed when added to a slurry-state composition for an electrode mixture layer.

[0090] A specific polymerization solvent is preferably water, since it allows the monomer (a) and the monomer (b) to be uniformly dissolved and polymerized. Other examples include water-soluble solvents such as methanol, t-butyl alcohol, acetone, methyl ethyl ketone, acetonitrile, and tetrahydrofuran, as well as benzene, ethyl acetate, dichloroethane, n-hexane, cyclohexane, and n-heptane. These solvents can be used alone or in combination of two or more. Alternatively, these solvents can be used as a mixed solvent with water. In the present invention, the water-soluble solvent refers to a solvent having a solubility in water at 20°C of greater than 10 g / 100 ml.

[0091] Similarly, in the neutralization step, it is preferable to add a small amount of a highly polar solvent to the polymerization solvent to ensure stable and rapid progress of the neutralization reaction. Examples of such highly polar solvents include water and methanol. The amount of the highly polar solvent used is preferably 0.05 to 20.0 mass% based on the total mass of the medium, more preferably 0.1 to 10.0 mass%, even more preferably 0.1 to 5.0 mass%, and even more preferably 0.1 to 1.0 mass%. If the proportion of the highly polar solvent is 0.05 mass% or more, an effect on the neutralization reaction is observed, while if it is 20.0 mass% or less, no adverse effect on the polymerization reaction is observed. Furthermore, in the polymerization of highly hydrophilic ethylenically unsaturated carboxylic acid monomers such as acrylic acid, the addition of a highly polar solvent increases the polymerization rate, making it easier to obtain polymers with long primary chain lengths. Among highly polar solvents, water is particularly preferable due to its significant effect on improving the polymerization rate.

[0092] The polymerization initiator may be any known polymerization initiator such as an azo compound, an organic peroxide, or an inorganic peroxide, but is not particularly limited. The conditions for use can be adjusted so as to generate an appropriate amount of radicals by known methods such as thermal initiation, redox initiation using a reducing agent, or UV initiation. To obtain a non-crosslinked polymer with a long primary chain length, it is preferable to set the conditions so as to generate as little radicals as possible within the allowable production time range.

[0093] Here, the polymerization initiator in the aqueous solution polymerization is preferably a water-soluble polymerization initiator, and examples thereof include a compound having a hydrophilic group (e.g., a carboxyl group) and / or a salt or hydrate thereof. Among these, 4,4'-azobis(4-cyanovaleric acid), 2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis[2-(imidazolin-2-yl)propane]disulfate dihydrate, 2,2'-azobis(2-methylpropionamidine)dihydrochloride, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]hydrate, and the like are preferred.

[0094] A preferred amount of the polymerization initiator used is, when the total amount of the monomer components used is 100 parts by mass, for example, 0.001 to 3 parts by mass, or for example, 0.005 to 2.5 parts by mass, or for example, 0.01 to 2 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 1.5 parts by mass or less, a polymer having a long primary chain length is likely to be obtained.

[0095] The polymerization temperature varies depending on conditions such as the type and concentration of the monomers used, but is preferably 0 to 100°C, more preferably 20 to 80°C. The polymerization temperature may be constant or may vary over the course of the polymerization reaction. The polymerization time is preferably 1 minute to 20 hours, more preferably 1 hour to 10 hours.

[0096] Here, the present non-crosslinked polymer may contain, relative to its total structural units, 40% by mass or more and 99% by mass or less of structural units derived from an ethylenically unsaturated carboxylic acid monomer (monomer (a)) and 1% by mass or more and 50% by mass or less of structural units derived from a nitrile group-containing ethylenically unsaturated monomer (monomer (b)), with the preferred range of the content of these structural units being as described above. The types of monomer (a) and monomer (b) are as described above. The present non-crosslinked polymer may further contain a structural unit derived from an amide group-containing ethylenically unsaturated monomer (hereinafter referred to as "monomer (c)"), with the preferred range of the content of this structural unit being as described above. The type of monomer (c) is as described above.

[0097] 5. The present binder contains the present crosslinked polymer or a salt thereof and the present non-crosslinked polymer or a salt thereof. From the viewpoint of excellent initial charge / discharge efficiency and cycle characteristics of the secondary battery, the content of the present crosslinked polymer or a salt thereof is preferably 10% by mass or more and 80% by mass or less, more preferably 15% by mass or more and 70% by mass or less, even more preferably 25% by mass or more and 60% by mass or less, and even more preferably 30% by mass or more and 45% by mass or less, based on the total amount of the crosslinked polymer or a salt thereof and the present non-crosslinked polymer or a salt thereof. Furthermore, as described below, from the viewpoint of excellent initial charge / discharge efficiency and cycle characteristics of the secondary battery, the present binder preferably contains styrene-butadiene rubber (SBR) or carboxymethyl cellulose (CMC), and it is more preferable to use SBR and CMC in combination.

[0098] 6. Composition for Secondary Battery Electrode Mixture Layer The composition for secondary battery electrode mixture layer of the present invention contains the present binder, an active material, and water. The amount of the present binder used in the composition is preferably 0.5 parts by mass or more and 7.0 parts by mass or less, relative to 100 parts by mass of the total amount of active material. The amount used is, for example, 0.8 parts by mass or more and 6.0 parts by mass or less, such as 1.0 parts by mass or more and 5.0 parts by mass or less, or for example, 1.2 parts by mass or more and 4.0 parts by mass or less. When the amount of the binder used is 0.5 parts by mass or more, sufficient binding strength can be obtained. Furthermore, dispersion stability of the active material and the like can be ensured, and a uniform mixture layer can be formed. When the amount of the binder used is 4.0 parts by mass or less, the present composition does not become highly viscous, and coatability to the current collector can be ensured. As a result, a mixture layer with a uniform and smooth surface can be formed.

[0099] Among the above active materials, lithium salts of transition metal oxides can be used as the positive electrode active material. For example, layered rock salt type and spinel type lithium-containing metal oxides can be used. Specific compounds of the layered rock salt type positive electrode active material include lithium cobalt oxide, lithium nickel oxide, and ternary NCMs {Li(Ni ｘ , Co ｙ , Mn ｚ ), x+y+z=1} and NCA{Li(Ni １－ａ－ｂ Co ａ Al ｂ )}, etc. Examples of spinel-type positive electrode active materials include lithium manganate, etc. In addition to oxides, phosphates, silicates, sulfur, etc. are also used, and examples of phosphates include olivine-type lithium iron phosphate, etc. As the positive electrode active material, one of the above may be used alone, or two or more may be combined and used as a mixture or composite.

[0100] When a positive electrode active material containing a layered rock salt-type lithium-containing metal oxide is dispersed in water, the lithium ions on the active material surface are exchanged with hydrogen ions in the water, resulting in an alkaline dispersion. This may result in corrosion of aluminum foil (Al), a common positive electrode current collector material. In such cases, it is preferable to neutralize the alkali elution from the active material by using an unneutralized or partially neutralized crosslinked polymer as a binder. Furthermore, it is preferable to use an amount of the unneutralized or partially neutralized crosslinked polymer such that the amount of unneutralized carboxyl groups in the crosslinked polymer is equivalent to or greater than the amount of alkali eluted from the active material.

[0101] Because all positive electrode active materials have low electrical conductivity, a conductive additive other than carbon nanotubes may be added. Examples of such conductive additives include carbon-based materials such as carbon black, carbon fiber, graphite powder, and carbon fiber. Of these, carbon black and carbon fiber are preferred because they are more likely to provide excellent conductivity. Furthermore, ketjen black and acetylene black are preferred as carbon black. The conductive additives may be used alone or in combination with two or more of the above. The amount of conductive additive other than carbon nanotubes used may be, for example, 0.2 to 20 parts by mass, or, for example, 0.2 to 10 parts by mass, per 100 parts by mass of the total active material, from the viewpoint of achieving both electrical conductivity and energy density. Furthermore, the positive electrode active material may be surface-coated with a conductive carbon-based material.

[0102] On the other hand, examples of negative electrode active materials include carbon-based materials, lithium metal, lithium alloys, and metal oxides, and one or more of these can be used in combination. Among these, active materials made of carbon-based materials such as natural graphite, artificial graphite, hard carbon, and soft carbon (hereinafter also referred to as "carbon-based active materials") are preferred, with graphite such as natural graphite and artificial graphite, and hard carbon being more preferred. In the case of graphite, spherical graphite is preferably used from the standpoint of battery performance, and its particle size preferably ranges from 1 to 20 μm, for example, and from 5 to 15 μm. In addition, to increase the energy density, metals or metal oxides capable of absorbing lithium, such as silicon and tin, can also be used as negative electrode active materials. Among these, silicon has a higher capacity than graphite, and active materials made of silicon-based materials such as silicon, silicon alloys, and silicon oxides such as silicon monoxide (SiO) (hereinafter also referred to as "silicon-based active materials") can be used. The amount of silicon-based active material used is 5.0 mass% or more, for example, 10.0 mass% or more, or for example, 20.0 mass% or more, relative to the total amount of active material, in order to improve the electrical capacity of the secondary battery.

[0103] Since the carbon-based active material itself has good electrical conductivity, it is not necessarily required to add a conductive additive. When a conductive additive is added for the purpose of further reducing resistance, etc., the amount used is, from the viewpoint of energy density, for example, 10 parts by mass or less, or, for example, 5 parts by mass or less, relative to 100 parts by mass of the total amount of the active material.

[0104] When the composition is in a slurry state, the amount of active material used is, for example, in the range of 10 to 75 mass %, or, for example, in the range of 30 to 65 mass %, based on the total amount of the composition. If the amount of active material used is 10 mass % or more, migration of binders and the like is suppressed, and this is also advantageous in terms of the cost of drying the medium. On the other hand, if the amount is 75 mass % or less, the fluidity and coatability of the composition can be ensured, and a uniform mixture layer can be formed.

[0105] The present composition uses water as a medium. Furthermore, for the purpose of adjusting the properties and drying properties of the present composition, the composition may be mixed with lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, or water-soluble organic solvents such as tetrahydrofuran and N-methyl-2-pyrrolidone. The proportion of water in the mixed medium is, for example, 50% by mass or more, or, for example, 70% by mass or more.

[0106] When the present composition is made into a coatable slurry state, the content of the water-containing medium in the entire present composition can be, for example, in the range of 25 to 60 mass %, and can also be, for example, 35 to 60 mass %, from the viewpoints of the coatability of the slurry, the energy cost required for drying, and productivity.

[0107] The present composition may further contain other binder components, such as styrene butadiene rubber (SBR)-based latex, carboxymethyl cellulose (CMC), acrylic latex, and polyvinylidene fluoride-based latex. When other binder components are used in combination, the amount thereof may be, for example, 0.1 to 5 parts by mass or less, or, for example, 0.1 to 2 parts by mass or less, or, for example, 0.1 to 1 part by mass or less, relative to 100 parts by mass of the total amount of active material. If the amount of other binder components used exceeds 5 parts by mass, resistance may increase, resulting in insufficient high-rate characteristics. Among the above, SBR-based latex or CMC is preferred, and a combination of SBR-based latex and CMC is more preferred, in terms of excellent initial charge / discharge efficiency and cycle characteristics of the secondary battery.

[0108] The SBR latex refers to an aqueous dispersion of a copolymer having structural units derived from an aromatic vinyl monomer such as styrene and structural units derived from an aliphatic conjugated diene monomer such as 1,3-butadiene. Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, vinyltoluene, and divinylbenzene, and one or more of these can be used. The structural units derived from the aromatic vinyl monomer in the copolymer can be, for example, in the range of 20 to 70% by mass, or, for example, in the range of 30 to 60% by mass, primarily from the viewpoint of binding properties. Examples of the aliphatic conjugated diene monomer include, for example, 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 2-chloro-1,3-butadiene, and one or more of these can be used. The structural units derived from the aliphatic conjugated diene monomer in the copolymer can be, for example, in the range of 30 to 70% by mass, or, for example, in the range of 40 to 60% by mass, in order to improve the binding properties of the binder and the flexibility of the resulting electrode. In addition to the above-mentioned monomers, the styrene / butadiene latex may also contain other monomers as copolymerization monomers, such as nitrile group-containing monomers such as (meth)acrylonitrile, carboxyl group-containing monomers such as (meth)acrylic acid, itaconic acid, and maleic acid, and ester group-containing monomers such as methyl (meth)acrylate, in order to further improve performance such as binding properties. The structural units derived from the other monomers in the copolymer can be, for example, in the range of 0 to 30% by mass, or, for example, in the range of 0 to 20% by mass.

[0109] The CMC refers to a nonionic cellulose-based semisynthetic polymer compound substituted with a carboxymethyl group and its salt. Examples of the nonionic cellulose-based semisynthetic polymer compound include alkyl celluloses such as methyl cellulose, methyl ethyl cellulose, ethyl cellulose, and microcrystalline cellulose; hydroxyethyl cellulose, hydroxybutyl methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose stearoxy ether, carboxymethyl hydroxyethyl cellulose, alkyl hydroxyethyl cellulose, and nonoxynyl hydroxyethyl cellulose.

[0110] The composition for a secondary battery electrode mixture layer of the present invention contains the above-mentioned binder, active material, and water as essential components, and is obtained by mixing the components using known means. The method for mixing the components is not particularly limited, and known methods can be used. However, a preferred method is to dry-blend powder components such as the active material, conductive additive, and binder, then mix them with a dispersion medium such as water and disperse and knead them. When obtaining the composition in a slurry state, it is preferable to finish the slurry without poor dispersion or aggregation. Known mixers such as planetary mixers, thin film gyratory mixers, and planetary / revolving mixers can be used as mixing means. However, a thin film gyratory mixer is preferred because it can achieve a good dispersion state in a short time. Furthermore, when using a thin film gyratory mixer, it is preferable to perform pre-dispersion beforehand using a stirrer such as a disperser. The pH of the slurry is not particularly limited as long as the effects of the present invention are achieved, but it is preferably less than 12.5. For example, when CMC is added, it is more preferable that it be less than 11.5, and even more preferable that it be less than 10.5, because there is little concern about hydrolysis of CMC. The viscosity of the slurry is not particularly limited as long as it exhibits the effects of the present invention, but the viscosity may be, for example, in the range of 100 to 30,000 mPa s, or, for example, 500 to 20,000 mPa s, or, for example, 1,000 to 10,000 mPa s, as Brookfield viscosity (25°C) at 20 rpm. If the viscosity of the slurry is within the above range, good coatability can be ensured.

[0111] 7. Secondary Battery Electrode The secondary battery electrode of the present invention comprises a mixture layer formed from the composition for a secondary battery electrode mixture layer of the present invention on the surface of a current collector made of copper, aluminum, or the like. The mixture layer is formed by applying the composition to the surface of the current collector and then drying to remove the medium, such as water. The method for applying the composition is not particularly limited, and known methods such as doctor blade coating, dipping, roll coating, comma coating, curtain coating, gravure coating, and extrusion can be used. Furthermore, the drying can be performed by known methods such as hot air blowing, reduced pressure, (far) infrared radiation, and microwave irradiation. Typically, the mixture layer obtained after drying is subjected to a compression treatment using a mold press, roll press, or the like. Compression brings the active material and binder into close contact, improving the strength of the mixture layer and its adhesion to the current collector. Compression can adjust the thickness of the mixture layer to, for example, approximately 30 to 80% of the thickness before compression, and the thickness of the mixture layer after compression is typically approximately 4 to 200 μm.

[0112] 8. Secondary Battery A secondary battery can be produced by providing the secondary battery electrode of the present invention with a separator and an electrolyte. The electrolyte may be liquid or gel-like. The separator is disposed between the positive and negative electrodes of the battery and serves to prevent short circuits due to contact between the electrodes and to retain the electrolyte to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane that has good ion permeability and mechanical strength. Specific materials that can be used include polyolefins such as polyethylene and polypropylene, and polytetrafluoroethylene.

[0113] The electrolyte may be a known, commonly used one depending on the type of active material. Specific examples of the solvent for lithium ion secondary batteries include cyclic carbonates with high dielectric constants and high electrolyte dissolving ability, such as propylene carbonate and ethylene carbonate, and chain carbonates with low viscosity, such as ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate. These may be used alone or as a mixed solvent. The electrolyte may be prepared by dissolving LiPF in these solvents. ６ , LiSbF６ , LiBF ４ , LiClO ４ , LiAlO ４ In nickel-metal hydride secondary batteries, an aqueous solution of potassium hydroxide can be used as the electrolyte. A secondary battery is obtained by spirally or stacking positive and negative electrode plates separated by a separator and housing them in a case or the like.

[0114] The present invention will be specifically described below based on examples. However, the present invention is not limited to these examples. In the following, "parts" and "%" mean parts by mass and % by mass unless otherwise specified. In the following examples, the carboxyl group-containing crosslinked polymer salt and the carboxyl group-containing non-crosslinked polymer salt were evaluated by the following methods.

[0115] <<Production of Carboxyl Group-Containing Crosslinked Polymer Salt>> (Measurement of Particle Size (Water-Swelled Particle Size) in Aqueous Medium) 0.25 g of powder of the present crosslinked polymer or its salt and 49.75 g of ion-exchanged water were weighed into a 100 cc container and placed in a rotation / revolution mixer (Thinky Corporation, Awatori Rentaro AR-250). The mixture was then stirred (rotation speed 2,000 rpm / revolution speed 800 rpm, 7 minutes) and further degassed (rotation speed 2,200 rpm / revolution speed 60 rpm, 1 minute) to produce a hydrogel in which the crosslinked polymer or its salt was swollen in water. The particle size distribution of the hydrogel was then measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT-3300EXII, Microtrac Bell Corporation) using ion-exchanged water as a dispersion medium. When an excess amount of dispersion medium was circulated relative to the hydrogel, an amount of hydrogel sufficient to obtain an appropriate scattered light intensity was added, and the particle size distribution shape measured stabilized after a few minutes. Once stability was confirmed, the particle size distribution was measured and the volume-based median diameter (D50) was obtained as a representative value of the particle size.

[0116] (Production Example 1: Production of Carboxyl Group-Containing Crosslinked Polymer Salt R-1) For polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser, and a nitrogen inlet tube was used. The reactor was charged with 567 parts of acetonitrile, 2.2 parts of ion-exchanged water, 100 parts of acrylic acid (hereinafter also referred to as "AA"), 0.9 parts of trimethylolpropane diallyl ether (manufactured by Osaka Soda Co., Ltd., trade name "Neoallyl T-20"), and triethylamine equivalent to 1.0 mol % relative to the AA. After thoroughly replacing the atmosphere inside the reactor with nitrogen, the reactor was heated to raise the internal temperature to 55°C. After confirming that the internal temperature had stabilized at 55°C, 0.040 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name "V-65") was added as a polymerization initiator. The reaction solution became cloudy, and this point was designated as the polymerization initiation point. The monomer concentration was calculated to be 15%. Cooling of the polymerization reaction solution was started 12 hours after the initiation of polymerization. After the internal temperature had dropped to 25°C, lithium hydroxide monohydrate (hereinafter referred to as "LiOH·H ２ To the mixture was added 52.4 parts of a powder of carboxyl group-containing crosslinked polymer salt R-1 (lithium salt, degree of neutralization 90 mol%). After the addition, stirring was continued for 12 hours at room temperature to obtain a polymerization reaction solution in the form of a slurry in which particles of carboxyl group-containing crosslinked polymer salt R-1 (lithium salt, degree of neutralization 90 mol%) were dispersed in the medium.

[0117] The resulting polymerization reaction solution was centrifuged to precipitate the polymer particles, and the supernatant was then removed. The precipitate was then redispersed in the same mass of acetonitrile as the polymerization reaction solution, followed by a washing procedure of precipitating the polymer particles by centrifugation and removing the supernatant, which was repeated twice. The precipitate was collected and dried at 80°C for 3 hours under reduced pressure to remove the volatiles, yielding a powder of carboxyl group-containing crosslinked polymer salt R-1. Since the carboxyl group-containing crosslinked polymer salt R-1 is hygroscopic, it was stored sealed in a container with water vapor barrier properties. The powder of carboxyl group-containing crosslinked polymer salt R-1 was subjected to IR analysis, and the degree of neutralization was determined from the intensity ratio of the peak derived from the C=O group of the carboxylic acid to the peak derived from the C=O of the lithium carboxylate. It was 90 mol%, equal to the calculated value from the starting material. The particle size in an aqueous medium was 1.54 μm.

[0118] (Production Examples 2 to 5: Production of Carboxyl Group-Containing Crosslinked Polymer Salts R-2 to R-5) The same operation as in Production Example 1 was carried out, except that the amounts of each raw material charged were as shown in Table 1, to obtain polymerization reaction solutions containing carboxyl group-containing crosslinked polymer salts R-2 to R-5. Next, the same operation as in Production Example 1 was carried out for each polymerization reaction solution to obtain powdery carboxyl group-containing crosslinked polymer salts R-2 to R-5. Each crosslinked polymer salt was sealed and stored in a container with water vapor barrier properties. The particle size of each obtained crosslinked polymer salt in an aqueous medium was measured in the same manner as in Production Example 1. The results are shown in Table 1.

[0119]

[0120] Details of the compounds used in Table 1 are shown below: AA: acrylic acid HEA: 2-hydroxyethyl acrylate T-20: trimethylolpropane diallyl ether (manufactured by Osaka Soda Co., Ltd., trade name "Neoallyl T-20") TEA: triethylamine V-65: 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) LiOH.H ２ O: Lithium hydroxide monohydrate Na ２ CO ３ : Sodium carbonate ・K ２ CO ３ : Potassium carbonate

[0121] <<Production of Carboxyl Group-Containing Non-Crosslinked Polymer Salt>> (Measurement of Solids Concentration) Approximately 1.0 g of a sample was placed in a weighing bottle whose mass had been measured in advance [mass of weighing bottle = B (g)], and the weighing bottle was accurately weighed [W0 (g)]. The sample together with the weighing bottle was then placed in a windless dryer and dried at 155°C for 45 minutes, at which point the mass together with the weighing bottle was measured [W1 (g)], and the solids concentration was calculated using the following formula: solids concentration (mass%) = (W1 - B) / (W0 - B) x 100

[0122] (Solid-state NMR Measurement) The content of structural units derived from each monomer in the carboxyl group-containing non-crosslinked polymer salt was measured by nuclear magnetic resonance (NMR). The carboxyl group-containing non-crosslinked polymer salt obtained in the Production Examples and Comparative Production Examples was spread on an aluminum cup and vacuum dried at 70°C for 2 hours, after which the dried product was pulverized in a mortar and filled into a zirconia sleeve to obtain a measurement sample. NMR measurement was performed under the conditions described below to obtain the content of structural units derived from each monomer in the carboxyl group-containing non-crosslinked polymer salt.

[0123] <Conditions for solid-state NMR measurement> Apparatus: JNM-ECA400 (manufactured by JEOL) Sample tube: made of zirconia Sample rotation speed: 15 kHz Measurement nuclide: 13C Chemical shift reference: adamantane high field signal was set at 29.5 ppm (external standard) Pulse sequence: CPMAS method

[0124] (Molecular Weight Measurement) The molecular weight of the carboxyl group-containing non-crosslinked polymer salt was measured by gel permeation chromatography (GPC). 0.1 g of each aqueous solution of the carboxyl group-containing non-crosslinked polymer salt obtained in the Production Examples and Comparative Production Examples (0.02 g as the solid content of the polymer salt) was collected and diluted with 40 g of 0.1 M aqueous sodium nitrate solution to obtain a measurement sample. Aqueous GPC measurement was performed on the measurement sample under the conditions described below to obtain the number average molecular weight (Mn) and weight average molecular weight (Mw) calculated in terms of sodium polyacrylate. The molecular weight distribution (Mw / Mn) was also calculated from the obtained values.

[0125] <GPC measurement conditions> Column: Tosoh TSKgel GMPW x 2 Solvent: 0.1 M aqueous sodium nitrate solution Temperature: 40°C Detector: RI Flow rate: 0.5 mL / min

[0126] (Production Example 6: Production of Carboxyl Group-Containing Non-Crosslinked Polymer Salt Y-1) For the polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser, and a nitrogen inlet tube was used. 400.0 parts of ion-exchanged water, 50.0 parts of acrylic acid (AA), and 50.0 parts of acrylonitrile (hereinafter also referred to as "AN") were placed in the reactor, and the atmosphere inside the reactor was thoroughly replaced with nitrogen, and then the reactor was heated to 60°C. 0.60 parts of 2,2'-azobis(2-methylpropionamidine) dihydrochloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name "V-50") as an initiator was added to this solution, and the reaction was carried out for 6 hours. After further reaction at 80°C for 2 hours, cooling of the polymerization reaction solution was started, and after the internal temperature had decreased to 45°C, lithium hydroxide monohydrate (hereinafter "LiOH.H ２ 27.7 parts of a powder of carboxyl group-containing non-crosslinked polymer salt Y-1 was added, and the mixture was kept at 45°C and stirred for 30 minutes to obtain an aqueous solution of carboxyl group-containing non-crosslinked polymer salt Y-1. The solids concentration of the aqueous solution was 22.6% by mass, and solid-state NMR measurement revealed that Y-1 contained 74.0% by mass of structural units derived from AA and 26.0% by mass of structural units derived from AN, based on the total structural units of Y-1. Furthermore, GPC measurement revealed that Y-1 had an Mn of 250,000, an Mw of 1,330,000, and an Mw / Mn of 5.3.

[0127] (Production Examples 7 to 19 and Comparative Production Examples 1 and 2: Production of Carboxyl Group-Containing Non-Crosslinked Polymer Salts Y-2 to Y-16) Aqueous solutions of carboxyl group-containing non-crosslinked polymer salts Y-2 to Y-16 were obtained by the same procedure as in Production Example 6, except that the amounts of each raw material charged were as shown in Table 2. Table 2 shows the solids concentrations of the above aqueous solutions, the AA, AN, and acrylamide (hereinafter also referred to as "AAm") contents of Y-2 to Y-16 measured by solid-state NMR, and Mn, Mw, and Mw / Mn of Y-2 to Y-16 measured by GPC.

[0128]

[0129] The details of the compounds used in Table 2 are as follows: AA: acrylic acid AN: acrylonitrile AAm: acrylamide V-50: 2,2'-azobis(2-methylpropionamidine) dihydrochloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) LiOH.H２ O: Lithium hydroxide monohydrate, NaOH: Sodium hydroxide, KOH: Potassium hydroxide

[0130] Example 1 (Preparation of electrode mixture layer composition) As the active material, artificial graphite (manufactured by Showa Denko K.K., trade name "SCMG-CF") and SiO (5 μm, manufactured by Osaka Titanium Technologies Co., Ltd.) were used. As the binder, a mixture of carboxyl group-containing crosslinked polymer salt R-1, carboxyl group-containing non-crosslinked polymer salt Y-1, and styrene / butadiene rubber (SBR) was used. Water was used as a dilution solvent in a planetary mixer (Hibismix 2P-03 model manufactured by Primix Corporation) at a mass ratio of artificial graphite:SiO:R-1:Y-1:SBR = 76.8:19.2:1.0:2.0:1.0 (solids) so that the solids concentration of the electrode mixture layer composition was 53 mass%, and the mixture was mixed for 1 hour and 30 minutes to prepare a slurry-state electrode mixture layer composition (electrode slurry).

[0131] (Preparation of negative electrode plate) Next, the electrode slurry was applied onto a current collector (copper foil, thickness: 16.5 μm) using a variable applicator, and dried in a ventilated dryer at 80° C. for 15 minutes to form a mixture layer. After that, the thickness of the mixture layer was 50±5 μm and the mixture density was 1.60±0.10 g / cm. ３ After rolling to a thickness of 1 / 4", the mixture was punched out into a 3 cm square to obtain a negative electrode plate for battery evaluation.

[0132] (Preparation of Positive Electrode Plate) In N-methylpyrrolidone (NMP) solvent, LiNi was used as a positive electrode active material. ０．５ Co ０．２ Mn ０．３ O ２ 100 parts of NCM (Natural Carbon Monomer) and 2 parts of acetylene black were mixed and added, and 4 parts of polyvinylidene fluoride (PVDF) was mixed as a positive electrode binder to prepare a positive electrode mixture layer composition. Next, using a variable applicator, the positive electrode mixture layer composition was applied to a current collector (aluminum foil, thickness: 20 μm) and dried to form a mixture layer. Thereafter, the thickness of the mixture layer was 125 μm ± 1 μm, and the mixture density was 3.0 ± 0.10 g / cm ３ After rolling to a thickness of 1 / 4", the mixture was punched out into a 3 cm square to obtain a positive electrode plate for battery evaluation.

[0133] (Preparation of Electrolyte Solution) Vinylene carbonate (VC) was added to a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio of EC:DMC=3:7) so that the concentration of VC was 1 mass % and fluoroethylene carbonate (FEC) was 2 mass %, and LiPF ６ was dissolved in a concentration of 1.2 mol / L to prepare a non-aqueous electrolyte.

[0134] (Preparation of Secondary Battery) The battery was constructed by attaching lead terminals to the positive and negative electrodes, and placing the electrodes facing each other via a separator (made of polyethylene: film thickness 16 μm, porosity 47%), placing them in an aluminum laminate battery exterior, injecting the electrolyte, and sealing them to prepare a test battery. The design capacity of this prototype battery was 50 mAh. The design capacity of the battery was based on a charge cut-off voltage of 4.2 V.

[0135] <Evaluation of initial charge-discharge efficiency> The lithium ion secondary battery of the laminated cell prepared above was charged and discharged at a charge-discharge rate of 0.1 C under conditions of CC discharge of 2.5 to 4.2 V in an environment of 45°C, and the initial charge capacity C ０＊ and initial discharge capacity C ０ Here, the initial charge-discharge efficiency (ΔC ０ ) was calculated. ０ =C ０ / C ０＊ × 100 (%) ΔC calculated by the above formula ０ was 76.2%, and the initial charge-discharge efficiency was evaluated as "A" based on the following criteria. ０ A: Initial charge / discharge efficiency is 76.0% or more. B: Initial charge / discharge efficiency is 75.5% or more and less than 76.0%. C: Initial charge / discharge efficiency is 75.0% or more and less than 75.5%. D: Initial charge / discharge efficiency is less than 75.0%.

[0136] <Evaluation of cycle characteristics> After the initial charge and discharge, the cell was repeatedly charged and discharged at a charge and discharge rate of 0.5 C under CC discharge conditions of 2.5 to 4.2 V in an environment of 45° C., and the capacity C１００ The cycle characteristics (ΔC) were calculated using the following formula: ΔC = C １００ / C ０ × 100 (%) ΔC calculated by the above formula was 83.9%, and the cycle characteristics based on the following criteria were evaluated as "C". Note that a higher ΔC value indicates better cycle characteristics. (Criteria for Cycle Characteristics Evaluation) A: Charge / discharge capacity retention rate is 85.0% or more B: Charge / discharge capacity retention rate is 84.0% or more and less than 85.0% C: Charge / discharge capacity retention rate is 83.0% or more and less than 84.0% D: Charge / discharge capacity retention rate is less than 83.0%

[0137] <<Overall Evaluation>> An overall evaluation was made based on the evaluation results of the initial charge-discharge efficiency and cycle characteristics, and on the criteria shown in Table 3 below. In this evaluation, evaluations A to C are acceptable levels. Example 1 was evaluated as "A" for the initial charge-discharge efficiency and "C" for the cycle characteristics, and therefore the overall evaluation was "B."

[0138]

[0139] Examples 2 to 28 and Comparative Examples 1 to 4 Electrode slurries were prepared by the same procedure as in Example 1, except that the compositions were as shown in Tables 4 and 5. The initial charge-discharge efficiency and cycle characteristics of secondary batteries equipped with negative electrode plates obtained using the electrode slurries were evaluated, as well as the overall evaluation. The results are shown in Tables 4 and 5.

[0140]

[0141]

[0142] Details of the compounds used in Tables 4 and 5 are shown below: SBR: styrene butadiene rubber CMC: sodium carboxymethyl cellulose

[0143] <Evaluation Results> As is clear from the results of Examples 1 to 28, secondary batteries equipped with electrodes obtained using the secondary battery electrode binder of the present invention exhibited excellent initial charge / discharge efficiency and cycle characteristics. Among these, when the content of structural units derived from the nitrile group-containing ethylenically unsaturated monomer (monomer (b)) in the carboxyl group-containing non-crosslinked polymer was 20% by mass or more (Example 5: 45.0% by mass, Example 4: 20.4% by mass > Example 3: 4.0% by mass), the initial charge / discharge efficiency of the secondary battery was excellent. This is presumably because the greater the content of structural units derived from monomer (b) in the carboxyl group-containing non-crosslinked polymer, the easier it was for the binder to adsorb to the active material, allowing it to withstand the expansion and contraction of the active material during the initial charge / discharge.

[0144] Focusing on the structural units derived from the ethylenically unsaturated carboxylic acid monomer (monomer (a)) of the carboxyl group-containing crosslinked polymer, the higher the content of the structural units derived from the monomer (Example 6: 100 mass % > Example 15: 60 mass %), the better the cycle characteristics of the secondary battery.

[0145] Focusing on the content of the carboxyl group-containing non-crosslinked polymer salt in the binder, the higher the content of the carboxyl group-containing non-crosslinked polymer salt relative to the total amount of the carboxyl group-containing crosslinked polymer salt and the carboxyl group-containing non-crosslinked polymer salt (Example 6: 66.7 mass% > Example 20: 50.0 mass% > Example 21: 33.3 mass%), the more excellent the cycle characteristics of the secondary battery.

[0146] Focusing on the degree of neutralization of the carboxyl group-containing non-crosslinked polymer salt, the lower the degree of neutralization of the non-crosslinked polymer salt (Example 6: 45% < Example 7: 60% < Example 8: 95% by mass), the better the cycle characteristics of the secondary battery.

[0147] Focusing on the neutralized salt of the carboxyl group-containing non-crosslinked polymer salt, when a lithium salt was used as the neutralized salt of the non-crosslinked polymer salt (Example 8), the cycle characteristics of the secondary battery were superior to those when a sodium salt (Example 9) or a potassium salt (Example 10) was used as the neutralized salt of the non-crosslinked polymer salt.

[0148] Focusing on the molecular weight of the carboxyl group-containing non-crosslinked polymer salt, when the weight average molecular weight of the non-crosslinked polymer salt was smaller than 1,200,000 (Example 11: 530,000 < Example 12: 1,370,000, Example 13: 1,560,000, Example 14: 1,800,000), the cycle characteristics of the secondary battery were excellent.

[0149] In contrast, when the carboxyl group-containing non-crosslinked polymer salt did not contain a structural unit derived from a nitrile group-containing ethylenically unsaturated monomer, the cycle characteristics were significantly inferior (Comparative Examples 1 and 2).Furthermore, when neither the carboxyl group-containing crosslinked polymer salt nor the carboxyl group-containing non-crosslinked polymer salt was contained, the initial charge / discharge efficiency and cycle characteristics were significantly inferior (Comparative Examples 3 and 4).

[0150] Secondary batteries containing the secondary battery electrode binder of the present invention exhibit good initial charge-discharge efficiency and durability (cycle characteristics). Therefore, secondary batteries equipped with electrodes obtained using the binder are expected to ensure good integrity and exhibit good initial charge-discharge efficiency and durability (cycle characteristics) even after repeated charge-discharge cycles, contributing to the development of high-capacity automotive secondary batteries. The secondary battery electrode binder of the present invention is particularly suitable for use in non-aqueous electrolyte secondary battery electrodes, and is particularly useful for non-aqueous electrolyte lithium-ion secondary batteries with high energy density.

Claims

1. A binder for secondary battery electrodes, comprising a carboxyl group-containing crosslinked polymer or a salt thereof, and a carboxyl group-containing non-crosslinked polymer or a salt thereof, wherein the carboxyl group-containing non-crosslinked polymer has a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter referred to as "monomer (a)") and a structural unit derived from a nitrile group-containing ethylenically unsaturated monomer (hereinafter referred to as "monomer (b)").

2. The binder for secondary battery electrodes according to claim 1, wherein the carboxyl group-containing crosslinked polymer contains 50% by mass or more and 100% by mass or less of structural units derived from the monomer (a) based on the total structural units thereof.

3. The binder for secondary battery electrodes according to claim 1, wherein the carboxyl group-containing non-crosslinked polymer contains 40% by mass or more and 99% by mass or less of structural units derived from the monomer (a) and 1% by mass or more and 50% by mass or less of structural units derived from the monomer (b) relative to all structural units thereof.

4. The binder for secondary battery electrodes according to claim 1 or 3, wherein the carboxyl group-containing non-crosslinked polymer further has a structural unit derived from an amide group-containing ethylenically unsaturated monomer (hereinafter referred to as "monomer (c)") relative to all of its structural units.

5. The binder for secondary battery electrodes according to claim 4, wherein the carboxyl group-containing non-crosslinked polymer contains, relative to all structural units thereof, 40% by mass or more and 98% by mass or less of structural units derived from the monomer (a), 1% by mass or more and 50% by mass or less of structural units derived from the monomer (b), and 1% by mass or more and 50% by mass or less of structural units derived from the monomer (c).

6. The binder for secondary battery electrodes according to claim 1 or 3, wherein the carboxyl-containing non-crosslinked polymer or its salt has a weight average molecular weight of 400,000 or more and 2,500,000 or less.

7. The binder for secondary battery electrodes according to claim 1 or 2, wherein the content of the carboxyl group-containing crosslinked polymer or its salt is 10 mass % or more and 80 mass % or less based on the total amount of the carboxyl group-containing crosslinked polymer or its salt and the carboxyl group-containing non-crosslinked polymer or its salt.

8. The binder for secondary battery electrodes according to claim 1 or 2, further comprising styrene-butadiene rubber (SBR) or carboxymethyl cellulose (CMC).

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

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

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

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