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

Abstract

The present invention provides a binder for a secondary battery electrode capable of improving cycle characteristics of the secondary battery. Also provided are: a composition for a secondary-battery electrode mixture layer, the composition containing the binder; and a secondary-battery electrode and a secondary battery which are obtained using the composition.　The binder for a secondary battery electrode contains a carboxyl group-containing crosslinked polymer or a salt thereof. The carboxyl group-containing crosslinked polymer includes a structural unit derived from a nitrogen-containing ethylenically unsaturated monomer. The carboxyl group-containing crosslinked polymer or the salt thereof has a value (X) calculated by mathematical formula (1) of 5.0 or more and 70.0 or less. X = I1 / I2 (1) I1: A value calculated from formula (2) based on X-ray photoelectron spectroscopy of the carboxyl group-containing crosslinked polymer or the salt thereof. I2: A mass fraction of total nitrogen (based on the nitrogen of the structural unit) contained in the carboxyl group-containing crosslinked polymer or the salt thereof, measured by total nitrogen analysis (oxidative decomposition-chemiluminescence method). I1=AN1 / (AC1+AO1+AN1) (2) AC1: Carbon 1s orbital peak area. AO1: Oxygen 1s orbital peak area. AN1: Nitrogen 1s orbital peak area (based on the nitrogen of the structural unit). (Where AC1+AO1+AN1=1.)

Description

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

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

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

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

[0004] In this context, it has been reported that acrylic acid polymers are effective as binders that have good cycle characteristics and are effective in improving the durability of negative electrode mixture layers using silicon-based active materials. Patent Document 1 discloses a binder containing a crosslinked acrylic acid polymer in which polyacrylic acid is crosslinked with a specific crosslinking 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. While the binder disclosed in Patent Document 1 can impart good binding properties, binders with even stronger binding properties are required as the performance of secondary batteries improves.

[0005] Patent Document 2 discloses a binder containing a copolymer of a salt of acrylic acid or an acrylic acid derivative and acrylonitrile or an acrylonitrile derivative, and describes that the acrylonitrile or acrylonitrile derivative is contained in the copolymer in an amount of 10 to 50 mass % relative to the total mass of the copolymer.

[0006] International Publication No. 2014 / 065407 Japanese Patent Application Laid-Open No. 2015-115109

[0007] The binder for secondary battery electrodes disclosed in Patent Document 2 contains a copolymer with acrylonitrile or an acrylonitrile derivative, and is therefore able to follow the expansion and contraction of the silicon-based active material, thereby improving cycle characteristics. However, this improvement is sometimes insufficient, which can cause problems.

[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 cycle characteristics of a secondary battery. In addition, an object of the present invention is 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-mentioned problems, the present inventors have found that the cycle characteristics of a secondary battery can be improved by using a binder for secondary battery electrodes containing a carboxyl group-containing crosslinked polymer having a structural unit derived from a nitrogen-containing ethylenically unsaturated monomer or a salt thereof, and by setting characteristic values ​​of the crosslinked polymer or the salt thereof based on photoelectron spectroscopy (XPS) and total nitrogen analysis within specific ranges, thereby completing the present invention.

[0010] The present invention is as follows: [1] A binder for a secondary battery electrode comprising a carboxyl group-containing crosslinked polymer or a salt thereof, wherein the carboxyl group-containing crosslinked polymer comprises a structural unit derived from a nitrogen-containing ethylenically unsaturated monomer, and the carboxyl group-containing crosslinked polymer or the salt thereof has a value (X) calculated by the following mathematical formula (1) of 5.0 or more and 70.0 or less: [2] The binder for secondary battery electrodes according to [1], wherein the carboxyl group-containing crosslinked polymer contains structural units derived from ethylenically unsaturated carboxylic acid monomers in an amount of 50.0% by mass or more and 99.9% by mass or less, based on all structural units thereof. [3] The binder for secondary battery electrodes according to [1] or [2], wherein the carboxyl group-containing crosslinked polymer contains structural units derived from nitrogen-containing ethylenically unsaturated monomers in an amount of 0.1% by mass or more and 50.0% by mass or less, based on all structural units thereof. [4] The binder for secondary battery electrodes according to any one of [1] to [3], wherein the nitrogen-containing ethylenically unsaturated monomer includes a nitrile group-containing ethylenically unsaturated monomer or an amide group-containing ethylenically unsaturated monomer. [5] The binder for secondary battery electrodes according to any one of [1] to [4], wherein the carboxyl group-containing crosslinked polymer is crosslinked with a crosslinkable monomer, and the amount of the crosslinkable monomer used is 0.03 mol % or more and 0.8 mol % or less based on the total amount of the non-crosslinkable monomers. [6] A method for producing a binder for secondary battery electrodes containing a carboxyl group-containing crosslinked polymer or a salt thereof, wherein the carboxyl group-containing crosslinked polymer contains a structural unit derived from a nitrogen-containing ethylenically unsaturated monomer, the method comprising the steps of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer by precipitation polymerization or dispersion polymerization, and adding a monomer component containing a nitrogen-containing ethylenically unsaturated monomer during the polymerization. [7] The method for producing the binder for secondary battery electrodes according to [6], wherein the value (X) calculated by the above mathematical formula (1) for the carboxyl group-containing crosslinked polymer or salt thereof is 5.0 or more and 70.0 or less. [8] The method for producing a secondary battery electrode mixture layer according to [6] or [7], wherein the carboxyl group-containing crosslinked polymer contains structural units derived from nitrogen-containing ethylenically unsaturated monomers in an amount of 0.1% by mass or more and 50.0% by mass or less, based on all structural units of the carboxyl group-containing crosslinked polymer. [9] The method for producing a secondary battery electrode mixture layer according to any one of [6] to [8], wherein the nitrogen-containing ethylenically unsaturated monomer contains a nitrile group-containing ethylenically unsaturated monomer or an amide group-containing ethylenically unsaturated monomer.

[10] A composition for a secondary battery electrode mixture layer, comprising the binder for a secondary battery electrode according to any one of [1] to [5], an active material, and water.

[11] A secondary battery electrode comprising a mixture layer formed from the composition for a secondary battery electrode mixture layer according to

[10] on a surface of a current collector.

[12] A secondary battery comprising the secondary battery electrode according to

[11] .

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

[0012] The binder for secondary battery electrodes of the present invention (hereinafter also referred to as "the binder") comprises a carboxyl group-containing crosslinked polymer (hereinafter also referred to as "the crosslinked polymer") having structural units derived from a nitrogen-containing ethylenically unsaturated monomer, or a salt thereof (hereinafter also referred to as "the crosslinked polymer salt"). By mixing the binder with an active material and water, a composition for secondary battery electrode mixture layers (hereinafter also referred to as "the composition") can be prepared. The composition is preferably in the form of an electrode slurry that can be applied to a current collector, from the viewpoint of achieving the effects of the present invention. However, the composition may also be prepared in the form of a wet powder, which allows for press processing onto the current collector surface. The secondary battery electrode of the present invention can be obtained by forming a mixture layer from the composition on the surface of a current collector such as copper foil or aluminum foil. Here, the binder is preferably used in a composition for secondary battery electrode mixture layers containing a silicon-based active material as described below as the active material, since the effects of the present invention are particularly significant.

[0013] The present crosslinked polymer and its manufacturing method, 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] The carboxyl group-containing crosslinked polymer or salt thereof of the present invention (hereinafter also simply referred to as "the present crosslinked polymer (salt)") has a value (X) calculated by the above mathematical formula (1) of 5.0 or more and 70.0 or less, preferably 15.0 or more and 65.0 or less, more preferably 25.0 or more and 60.0 or less, even more preferably 35.0 or more and 55.0 or less, and even more preferably 45.0 or more and 55.0 or less, in order to improve the cycle characteristics of a secondary battery. Here, the above characteristic value (X) is a value based on XPS, and therefore means the proportion of structural units derived from nitrogen-containing ethylenically unsaturated monomers present in the surface layer of the present crosslinked polymer (salt) to a depth of about 20 nm from the surface. 1. The present crosslinked polymer The present crosslinked polymer can have a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as "component (a)") and a structural unit derived from a nitrogen-containing ethylenically unsaturated monomer (hereinafter also referred to as "component (b)"), and can be introduced into the polymer by precipitation polymerization or dispersion polymerization of monomer components including an ethylenically unsaturated carboxylic acid monomer and a nitrogen-containing ethylenically unsaturated monomer.

[0015] <Structural Units Derived from Ethylenically Unsaturated Carboxylic Acid Monomers> The carboxyl group-containing crosslinked polymer (present crosslinked polymer) contained in the present binder may have a structural unit (component (a)) derived from an ethylenically unsaturated carboxylic acid monomer. The present crosslinked polymer has such a structural unit and thus a carboxyl group, which improves adhesion to the current collector and provides excellent lithium ion desolvation and ionic conductivity, resulting in an electrode with low resistance and excellent high-rate characteristics. Furthermore, water swelling properties are imparted, thereby improving the dispersion stability of active materials and the like in the present 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 the copolymer. Alternatively, (meth)acrylamide, (meth)acrylonitrile, or the like 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 ethylenically unsaturated carboxylic acid monomers 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 monohydroxyethyl succinate (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 present crosslinked polymer can be 50.0% by mass or more and 99.9% by mass or less, based on the total structural units of the present crosslinked polymer. By including component (a) 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 50.0% by mass or more is preferable because the dispersion stability of the present composition is improved and a higher binding strength can be obtained. It may also be 60.0% by mass or more, 70.0% by mass or more, or 80.0% by mass or more. The upper limit is, for example, 99.5% by mass or less, for example, 99.0% by mass or less, for example, 98.0% by mass or less, for example, 95.0% by mass or less, for example, 90.0% by mass or less, or for example, 80.0% by mass or less.

[0018] <Structural Unit Derived from Nitrogen-Containing Ethylenically Unsaturated Monomer> The present crosslinked polymer has a structural unit (component (b)) derived from a nitrogen-containing ethylenically unsaturated monomer. The presence of such a structural unit in the present crosslinked polymer results in excellent cycle characteristics of the secondary battery. The above-mentioned component (b) can be introduced into the polymer by polymerizing a monomer including, for example, a nitrile group-containing ethylenically unsaturated monomer, an amide group-containing ethylenically unsaturated monomer, or an amino group-containing ethylenically unsaturated monomer. As the nitrogen-containing ethylenically unsaturated monomer, a nitrile group-containing ethylenically unsaturated monomer or an amide group-containing ethylenically unsaturated monomer is preferred, and a nitrile group-containing ethylenically unsaturated monomer is more preferred, in terms of excellent cycle characteristics of the secondary battery.

[0019] 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 it has a high nitrile group content.

[0020] Examples of the amide group-containing ethylenically unsaturated monomer include a monomer represented by the following formula (1) and a (meth)acrylamide derivative (provided that the monomer is other than the monomer represented by the following formula (1)). 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, R ７ represents a hydrogen atom or a monovalent organic group.

[0021] The monomer represented by the formula (1) is a (meth)acrylamide derivative having a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms. ７represents 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. 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.

[0022] 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.

[0023] The content of component (b) in the present crosslinked polymer can be 0.1% by mass or more and 50.0% by mass or less, based on the total structural units of the present crosslinked polymer. By including component (b) 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 0.1% by mass or more is preferable because the dispersion stability of the present composition is improved and a higher binding strength can be obtained. The lower limit may be 0.5% by mass or more, 1.0% by mass or more, or even 5.0% by mass or more. The upper limit may be, for example, 45.0% by mass or less, for example, 40.0% by mass or less, for example, 30.0% by mass or less, for example, 20.0% by mass or less, for example, 10.0% by mass or less, or for example, 5.0% by mass or less.

[0024] <Other Structural Units> In addition to the components (a) and (b), the present crosslinked polymer may contain a structural unit (hereinafter also referred to as "component (c)") derived from another ethylenically unsaturated monomer copolymerizable therewith. Examples of component (c) include structural units derived from 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). These structural units 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.

[0025] The content of component (c) in the present crosslinked polymer can be 0% by mass or more and 50.0% by mass or less, based on the total structural units of the present crosslinked polymer. The proportion of component (c) may be 0.1% by mass or more and 45.0% by mass or less, 0.5% by mass or more and 40.0% by mass or less, 1.0% by mass or more and 30.0% by mass or less, 5.0% by mass or more and 20.0% by mass or less, or 3% by mass or more and 10% by mass or less. Furthermore, when component (c) is contained in an amount of 0.1% by mass or more based on the total structural units of the present crosslinked polymer, affinity to the electrolyte solution is improved, and therefore, the effect of improving lithium ion conductivity can also be expected.

[0026] Of the above, the component (c) 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 crosslinked polymer salt.

[0027] 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.

[0028] Examples of the monomer represented by the formula (2) 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 (2) may be used singly or in combination of two or more.

[0029] 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 (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.

[0030] The present crosslinked polymer preferably contains a structural unit derived from a monomer represented by the above formula (2), an alicyclic structure-containing ethylenically unsaturated monomer, or the like, in terms of excellent binding properties of the binder. Among these, as component (c), 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.

[0031] Furthermore, when a structural unit derived from a hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less is introduced as component (c), it can exhibit strong interaction with the electrode material and exhibit good binding properties to the active material. This allows for a firm and well-integrated electrode mixture layer to be obtained, and therefore, an alicyclic structure-containing ethylenically unsaturated monomer is particularly preferred as the "hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less."

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] <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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] Specific examples of the monomer having a self-crosslinkable crosslinkable functional group include a hydrolyzable silyl group-containing vinyl monomer, etc. These compounds can be used alone or in combination of two or more.

[0042] 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.

[0043] When the crosslinked polymer is crosslinked with a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.1 to 2.0 parts by mass, more preferably 0.2 to 1.8 parts by mass, even more preferably 0.3 to 1.6 parts by mass, even more preferably 0.5 to 1.4 parts by mass, and even more preferably 0.6 to 1.2 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 0.1 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 2.0 parts by mass or less, the stability of precipitation polymerization or dispersion polymerization tends to be improved. In particular, when the amount is 1.0 part by mass or less, the water-swollen particle size in the electrode slurry becomes favorable, and the binding area to the active material becomes wider, which is preferable in that excellent battery performance can be maintained even during long-term use.

[0044] For the same reason, the amount of the crosslinkable monomer used is preferably 0.03 mol % or more and 0.8 mol % or less, more preferably 0.05 mol % or more and 0.7 mol % or less, even more preferably 0.075 mol % or more and 0.6 mol % or less, still more preferably 0.1 mol % or more and 0.5 mol % or less, and even more preferably 0.2 mol % or more and 0.4 mol % or less, based on the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers).

[0045] 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 because they are less likely to adversely affect battery characteristics, alkali metal salts are more preferred, and lithium salts are particularly preferred because they provide excellent cycle characteristics for secondary batteries.

[0046] 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.

[0047] <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.

[0048] 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.

[0049] 2. Method for Producing the Crosslinked Polymer The crosslinked polymer can be obtained by a method comprising the steps of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer by precipitation polymerization or dispersion polymerization, and adding a monomer component containing a nitrogen-containing ethylenically unsaturated monomer during the polymerization process. In this context, the term "during the polymerization process" refers to a time point between 0.3T and 0.9T, where T is the time from the start of the polymerization process to the end of the process. The nitrogen-containing ethylenically unsaturated monomer can be introduced in a concentrated state on the particle surfaces of the crosslinked polymer, improving the cycle characteristics of secondary batteries. A temperature of 0.4T to 0.9T is preferred, 0.5T to 0.8T is more preferred, and 0.6T to 0.8T is even 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 monomer but does not substantially dissolve the resulting polymer. As the polymerization proceeds, the polymer particles grow larger through aggregation and growth, resulting in a dispersion of polymer particles in which primary particles of several tens to several hundred nanometers have secondary aggregation to several micrometers to several tens of micrometers. A dispersion stabilizer can also be used to control the polymer particle size. Note that the secondary aggregation can also be suppressed by selecting the dispersion stabilizer, polymerization solvent, etc. In general, precipitation polymerization in which secondary aggregation is suppressed is also called dispersion polymerization.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] The crosslinked polymer may contain 50% by mass or more and 99.9% by mass or less of structural units derived from ethylenically unsaturated carboxylic acid monomers and 0.1% by mass or more and 50% by mass or less of structural units derived from nitrogen-containing ethylenically unsaturated monomers, and the preferred ranges of the contents of the structural units are as described above. The types of the ethylenically unsaturated carboxylic acid monomers and nitrogen-containing ethylenically unsaturated monomers are as described above.

[0057] 3. 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 3.0 parts by mass or less, such as 1.0 parts by mass or more and 2.5 parts by mass or less, and for example, 1.2 parts by mass or more and 1.5 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 1.5 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] Since the carbon-based active material itself has good electrical conductivity, it is not necessary to add a conductive additive other than carbon nanotubes. When a conductive additive is added for the purpose of further reducing resistance, etc., from the viewpoint of energy density, the amount used is, 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] The present composition may further contain other binder components, such as styrene butadiene rubber (SBR)-based latex, carboxymethyl cellulose (CMC), acrylic latex, and polyvinylidene fluoride-based latex. When other binder components are used in combination, the amount thereof may be, for example, 0.1 to 5 parts by mass or less, or, for example, 0.1 to 2 parts by mass or less, or, for example, 0.1 to 1 part by mass or less, relative to 100 parts by mass of the total amount of active material. If the amount of other binder components used exceeds 5 parts by mass, resistance increases, and high-rate characteristics may become insufficient. Among the above, SBR-based latex and CMC are preferred in terms of their excellent balance of binding strength and flex resistance, and a combination of SBR-based latex and CMC is more preferred.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 4. 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.

[0071] 5. 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.

[0072] 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.

[0073] The binder for secondary battery electrodes disclosed in this specification provides excellent toughness of the binder coating film after immersion in an electrolyte, and the electrode mixture layer for secondary batteries obtained using an electrode slurry containing the binder exhibits electrolyte resistance. Furthermore, secondary batteries equipped with electrodes obtained using the binder can ensure good integrity and exhibit good durability (cycle characteristics) even after repeated charge and discharge, making them suitable for use in automotive secondary batteries, etc.

[0074] 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 or its salt was evaluated by the following method.

[0075] (Measurement of reaction rate of nitrogen-containing ethylenically unsaturated monomer) After the polymerization reaction solution containing the carboxyl group-containing crosslinked polymer (salt) was centrifuged to precipitate the polymer particles (12,000 rpm x 10 minutes), 0.5 g of the supernatant after centrifugation was added to a 9 mL screw tube, and 1 g of propylene glycol monomethyl ether (2%) was added to the solution as a sample, and gas chromatography (GC) measurement was performed. From the measurement results obtained, the reaction rate (%) of the nitrogen-containing ethylenically unsaturated monomer was calculated.

[0076] <GC measurement conditions> Apparatus: GC-2030 (Shimadzu Corporation) Column: CP-WAX52CB 60 m x 0.32 mm, df = 0.5 μm Column temperature: 40°C (held for 3 minutes) → 250°C (held for 11 minutes) Heating rate: 10°C / min

[0077] (Measurement of the Content of Solvent and Nitrogen-Containing Ethylenically Unsaturated Monomer in Carboxyl Group-Containing Crosslinked Polymer (Salt)) 0.9 g of carboxyl group-containing crosslinked polymer (salt) and 44.1 g of ion-exchanged water were weighed into a 50 mL screw tube and thoroughly mixed to prepare a 2% aqueous solution. 15 g of this was weighed into another 50 mL screw tube, and 15 g of 1-propanol was added and thoroughly mixed to prepare a 1% solution. The above 1% solution was centrifuged (4000 rpm x 10 minutes), and 4 g of the supernatant after centrifugation and 1 g of 2% propylene glycol monomethyl ether were added to a 9 mL screw tube. The supernatant was filtered through a 0.45 μm filter, and the filtered liquid was used as a sample for gas chromatography (GC) measurement. From the obtained measurement results, the contents of solvent and nitrogen-containing ethylenically unsaturated monomer in the carboxyl group-containing crosslinked polymer (salt) were calculated.

[0078] <GC measurement conditions> Apparatus: Agilent 19091N-216 (manufactured by Agilent Technologies) Column: CP-WAX52CB 60 m x 0.32 mm, df = 0.5 μm Column temperature: 80°C (held for 3 minutes) → 250°C (held for 5 minutes) Heating rate: 10°C / min

[0079] (Percentage of peak areas based on X-ray photoelectron spectroscopy analysis I １ Measurement of carboxyl group-containing crosslinked polymer (salt) powder (content of solvent and nitrogen-containing ethylenically unsaturated monomer: less than the lower detection limit) was filled into a powder sample holder to prepare a measurement sample. The measurement sample was measured using an X-ray photoelectron spectrometer, and the ratio I of the peak area derived from the nitrogen 1s orbital of each carboxyl group-containing crosslinked polymer (salt) based on X-ray photoelectron spectroscopic analysis (XPS) was calculated using the following formula: １ was calculated. １ =A Ｎ１ / (A Ｃ１ +A Ｏ１ +A Ｎ１ ) A Ｃ１ : carbon 1s orbital peak area A Ｏ１ : Peak area of ​​oxygen 1s orbital A Ｎ１ : Peak area of ​​nitrogen 1s orbital (where A Ｃ１+A Ｏ１ +A Ｎ１ = 1) In this measurement, when a sample containing solvent and nitrogen-containing ethylenically unsaturated monomers below the detection limit is measured, I １ and I １ A in Ｎ１ are values ​​based on the nitrogen of the structural unit derived from the nitrogen-containing ethylenically unsaturated monomer contained in the carboxyl group-containing crosslinked polymer (salt).

[0080] <XPS measurement conditions> Apparatus: PHI5000 VersaProbeIII manufactured by ULVAC-PHI Inc. X-ray: Al-Kα (1486.6 eV) X-ray incident angle to sample: 0° (angle with respect to the normal to the sample measurement surface) Photoelectron detection angle: 45° (angle with respect to the normal to the sample measurement surface)

[0081] (Mass fraction I of total nitrogen contained in the carboxyl group-containing crosslinked polymer (salt) ２ Measurement of total nitrogen mass fraction I of the total nitrogen contained in the carboxyl group-containing crosslinked polymer (salt) having a structural unit derived from a nitrogen-containing ethylenically unsaturated monomer (solvent and nitrogen-containing ethylenically unsaturated monomer content: less than the lower detection limit) was collected in a quartz boat, and the quartz boat was set in a trace total nitrogen analyzer (oxidative decomposition - chemiluminescence method). The sample was burned and the nitrogen in the combustion gas was detected by chemiluminescence, and the emission peak area was calculated. 10 μL of the standard solution was collected in the quartz port and measured in the same manner. Using a calibration curve created from the concentration and peak area of ​​the standard solution, the mass fraction I of the total nitrogen contained in the carboxyl group-containing crosslinked polymer (salt) having a structural unit derived from a nitrogen-containing ethylenically unsaturated monomer was calculated. ２ In this measurement, when a sample containing a solvent and a nitrogen-containing ethylenically unsaturated monomer less than the lower detection limit was measured, I ２ is a value based on the nitrogen of the structural unit derived from the nitrogen-containing ethylenically unsaturated monomer contained in the carboxyl group-containing crosslinked polymer (salt).

[0082] <Conditions for total nitrogen analysis> Trace total nitrogen analyzer: TN-2100H (manufactured by Mitsubishi Chemical Analytech Co., Ltd.) Combustion conditions Inlet temperature: 800°C Outlet temperature: 900°C Gas conditions Ar flow rate: 300 mL / min x 30 sec O２ Flow rate: 300 mL / min×600 sec Standard solution: prepared by diluting pyridine (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., special grade reagent) with toluene (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., special grade reagent).

[0083] (Calculation of characteristic value (X)) １ and I ２ The characteristic value (X) was calculated from the following formula: X = I １ / I ２

[0084] (Measurement of particle size (water-swollen particle size) in aqueous medium) 0.25 g of carboxyl group-containing crosslinked polymer (salt) powder and 49.75 g of ion-exchanged water were weighed into a 100 cc container and placed in a rotation / revolution mixer (Thinky Corporation, Awatori Rentaro AR-250). The mixture was then stirred (rotation speed 2,000 rpm / revolution speed 800 rpm, 7 minutes) and degassed (rotation speed 2,200 rpm / revolution speed 60 rpm, 1 minute) to produce a hydrogel in which the 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.

[0085] <<Production of the Present Crosslinked Polymer Salt>> (Production Example 1: Production of Carboxyl Group-Containing Crosslinked Polymer Salt R-1) For polymerization, a reactor equipped with a stirring blade, thermometer, reflux condenser, and nitrogen inlet tube was used. The reactor was charged with 567 parts of acetonitrile, 2.2 parts of ion-exchanged water, 85.0 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. Nine hours after the start of polymerization (during the polymerization step corresponding to the time of 0.75T), 15.0 parts of 2-cyanoethyl acrylate (hereinafter also referred to as "CEA") was added all at once. The monomer concentration was calculated to be 15%. 12 hours after the start of polymerization, cooling of the polymerization reaction solution was started, and after the internal temperature had decreased to 25°C, lithium hydroxide monohydrate (hereinafter "LiOH·H ２ 0"). 44.5 parts of a powder of carboxyl group-containing crosslinked polymer salt R-1 (lithium salt, degree of neutralization 90 mol%) was added. 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. Here, the reaction rates of AA and CEA were calculated by GC measurement and were found to be 93.3% and 7.1%, respectively. This indicated that the carboxyl group-containing crosslinked polymer salt R-1 contained 98.7% by mass of structural units derived from AA and 1.3% by mass of structural units derived from CEA, relative to its total structural units.

[0086] The resulting polymerization reaction solution was centrifuged to precipitate the polymer particles, and the supernatant was then removed. The precipitate was then redispersed in acetonitrile of the same weight as the polymerization reaction solution, and the polymer particles were precipitated by centrifugation, followed by removal of the supernatant. This washing procedure was repeated twice. The precipitate was collected and dried at 80°C for 6 hours under reduced pressure to remove volatiles, yielding a powder of carboxyl group-containing crosslinked polymer salt R-1. The solvent content of the carboxyl group-containing crosslinked polymer salt R-1 was calculated by GC measurement, and found to be less than 0.01% by mass (less than the lower detection limit), and the CEA content was calculated, and found to be less than 0.01% by mass (less than the lower detection limit). Because 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 the carboxyl group-containing crosslinked polymer salt R-1 was subjected to IR measurement, and the degree of neutralization was calculated from the intensity ratio of the peak derived from the C═O group of the carboxylic acid to the peak derived from the C═O group of the lithium carboxylate. The result was 90 mol%, which was equal to the calculated value from the amount charged.

[0087] Next, the carboxyl group-containing crosslinked polymer salt R-1 was subjected to XPS measurement. １ The total nitrogen content of the carboxyl group-containing crosslinked polymer salt R-1 was 0.030. ２ The I obtained by the above measurement was 0.0006. １ and I ２ The characteristic value (X) was calculated based on the above formula (1) using the above formula, and was found to be 50.3. The particle size in the aqueous medium was 1.54 μm.

[0088] (Production Example 2: Production of Carboxyl Group-Containing Crosslinked Polymer Salt R-2) The amounts of each raw material charged were as shown in Table 1, and the same operation as in Production Example 1 was carried out, except that CEA was added 7 hours after the start of polymerization (during the polymerization step corresponding to the time point of 0.58 T), to obtain a polymerization reaction solution containing carboxyl group-containing crosslinked polymer salt R-2. Here, the reaction rates of AA and CEA were calculated by GC measurement and were 90.5% and 10.1%, respectively. This indicated that carboxyl group-containing crosslinked polymer salt R-2 contained 98.1% by mass of structural units derived from AA and 1.9% by mass of structural units derived from CEA, relative to its total structural units. Next, the same operation as in Production Example 1 was carried out on the polymerization reaction solution to obtain powdered carboxyl group-containing crosslinked polymer salt R-2. The crosslinked polymer salt R-2 was stored in a sealed container with water vapor barrier properties. Here, the solvent contents of crosslinked polymer salt R-2 were calculated by GC measurement, and were all less than 0.01% by mass (below the lower detection limit), and the CEA content was calculated, and was also less than 0.01% by mass (below the lower detection limit). The characteristic value (X) and particle size in an aqueous medium of the obtained crosslinked polymer salt R-2 were measured in the same manner as in Production Example 1. The results are shown in Table 1.

[0089] (Production Examples 3 to 14: Production of Carboxyl Group-Containing Crosslinked Polymer Salts R-3 to R-14) The same operation as in Production Example 1 was performed, 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-3 to R-14. Table 1 shows the calculation results of the reaction rate of each monomer by GC measurement and the composition of each crosslinked polymer salt (the content of structural units derived from each monomer). Next, the same operation as in Production Example 1 was performed on each polymerization reaction solution to obtain powdered carboxyl group-containing crosslinked polymer salts R-3 to R-14. Each crosslinked polymer salt was sealed and stored in a container with water vapor barrier properties. Here, the solvent content of each crosslinked polymer salt was calculated by GC measurement, and all were less than 0.01% by mass (less than the lower detection limit). The contents of CEA, AN, MAN, and HEAA were also calculated, and all were less than 0.01% by mass (less than the lower detection limit). For each of the obtained crosslinked polymer salts, the characteristic value (X) and the particle size in an aqueous medium were measured in the same manner as in Production Example 1. The results are shown in Table 1.

[0090] Comparative Production Example 1: Production of Carboxyl Group-Containing Crosslinked Polymer Salt R-15 The amounts of each raw material charged were as shown in Table 1, and the same operation as in Production Example 1 was carried out, except that CEA was added at the beginning of the polymerization, to obtain a polymerization reaction solution containing carboxyl group-containing crosslinked polymer salt R-15. Here, the reaction rates of AA and CEA were calculated by GC measurement, and were found to be 90.0% and 20.5%, respectively. This indicated that the carboxyl group-containing crosslinked polymer salt R-15 contained 94.6% by mass of structural units derived from AA and 5.4% by mass of structural units derived from CEA, relative to its total structural units. Next, the same operation as in Production Example 1 was carried out on the polymerization reaction solution to obtain powdered carboxyl group-containing crosslinked polymer salt R-15. The crosslinked polymer salt R-15 was stored in a sealed container with water vapor barrier properties. Here, the solvent contents of the crosslinked polymer salt R-15 were calculated by GC measurement, and were all less than 0.01% by mass (below the lower detection limit), and the CEA content was calculated, and was also less than 0.01% by mass (below the lower detection limit). The characteristic value (X) and particle size in an aqueous medium of the obtained crosslinked polymer salt R-15 were measured in the same manner as in Production Example 1. The results are shown in Table 1.

[0091]

[0092] Details of the compounds used in Table 1 are shown below: AA: acrylic acid HEA: 2-hydroxyethyl acrylate CEA: 2-cyanoethyl acrylate (manufactured by Tokyo Chemical Industry Co., Ltd.) AN: acrylonitrile MAN: methacrylonitrile HEAA: N-(2-hydroxyethyl)acrylamide (manufactured by KJ Chemicals Co., Ltd.) 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

[0093] 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, styrene / butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) 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:SBR:CMC = 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).

[0094] (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.

[0095] (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.

[0096] (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.

[0097] (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.

[0098] <Evaluation of cycle characteristics> The lithium ion secondary battery of the laminated cell prepared above was charged and discharged at a charge / discharge rate of 0.1 C under conditions of CC discharge of 2.5 to 4.2 V in an environment of 45° C., and the initial capacity C ０ Furthermore, charging and discharging were repeated in an environment of 25°C at a charge / discharge rate of 0.5C under conditions of 2.5 to 4.2V by CC discharge, and the capacity C １００ The cycle characteristics (ΔC) were calculated using the following formula: ΔC = C １００ / C ０ × 100 (%) The ΔC calculated by the above formula was 86.7%, and the cycle characteristics based on the following criteria were evaluated as "A". Note that a higher ΔC value indicates better cycle characteristics. (Criteria for Cycle Characteristics Evaluation) A: Charge / discharge capacity retention rate is 86.0% or more B: Charge / discharge capacity retention rate is 85.0% or more and less than 86.0% C: Charge / discharge capacity retention rate is 84.0% or more and less than 85.0% D: Charge / discharge capacity retention rate is less than 84.0%

[0099] Examples 2 to 14 and Comparative Example 1 Electrode slurries were prepared in the same manner as in Example 1, except that the compositions were as shown in Table 2. The cycle characteristics of the batteries containing negative electrodes obtained using the electrode slurries were evaluated. The results are shown in Table 2.

[0100]

[0101] Details of the compounds used in Table 2 are shown below: SBR: styrene butadiene rubber CMC: sodium carboxymethyl cellulose

[0102] <Evaluation Results> As is clear from the results of Examples 1 to 14, secondary batteries equipped with electrodes obtained using the secondary battery electrode binder of the present invention exhibited excellent cycle characteristics. Among these, when comparing Examples 1 and 2, which had the same monomer composition of the carboxyl group-containing crosslinked polymer, the larger the characteristic value X (Example 1: X = 50.3 > Example 2: X = 30.6), the more excellent the cycle characteristics of the secondary battery. This is presumably because the later the timing of addition of the nitrogen-containing ethylenically unsaturated monomer during production of the carboxyl group-containing crosslinked polymer from the start of polymerization (Example 1: 0.75T > Example 2: 0.58T), the more concentrated the monomer was introduced onto the particle surfaces of the carboxyl group-containing crosslinked polymer.

[0103] Furthermore, when the type of structural unit derived from the nitrogen-containing ethylenically unsaturated monomer was considered, the cycle characteristics of the secondary battery were superior when the monomer was a nitrile group-containing ethylenically unsaturated monomer (Example 7) compared to when the monomer was an amide group-containing ethylenically unsaturated monomer (Example 10). When the content of the structural unit derived from the nitrile group-containing ethylenically unsaturated monomer was considered, the cycle characteristics of the secondary battery were superior when the content was 1.3% by mass (Example 1) compared to when the content was 0.4% by mass (Example 5), 4.0% by mass (Example 6), and 10.1% by mass (Example 7). When the type of neutralized salt of the carboxyl group-containing crosslinked polymer was considered, the lithium salt (Example 1) compared to when the neutralized salt was sodium salt (Example 13) and potassium salt (Example 14).

[0104] In contrast, when the characteristic value X was less than 5.0, the cycle characteristics were significantly poor (Comparative Example 1). This is presumably because the timing of adding the nitrogen-containing ethylenically unsaturated monomer during the production of the carboxyl group-containing crosslinked polymer was early in the initiation of polymerization, and therefore the monomer was not concentrated on the particle surfaces of the carboxyl group-containing crosslinked polymer.

[0105] Secondary batteries containing the binder for secondary battery electrodes of the present invention exhibit good durability (cycling characteristics). Therefore, secondary batteries equipped with electrodes obtained using the binder are expected to ensure good integrity and exhibit good durability (cycling characteristics) even after repeated charge and discharge, and are expected to contribute to the development of high-capacity automotive secondary batteries. The binder for secondary battery electrodes of the present invention can be particularly suitably used for non-aqueous electrolyte secondary battery electrodes, and is particularly useful for non-aqueous electrolyte lithium-ion secondary batteries with high energy density.

Claims

1. A binder for secondary battery electrodes comprising a carboxyl group-containing crosslinked polymer or a salt thereof, wherein the carboxyl group-containing crosslinked polymer comprises a structural unit derived from a nitrogen-containing ethylenically unsaturated monomer, and the carboxyl group-containing crosslinked polymer or the salt thereof has a value (X) calculated by the following formula (1) of 5.0 or more and 70.0 or less.

2. The binder for secondary battery electrodes according to claim 1, wherein the carboxyl group-containing crosslinked polymer contains 50.0% by mass or more and 99.9% by mass or less of structural units derived from ethylenically unsaturated carboxylic acid monomers relative to the total structural units thereof.

3. The binder for secondary battery electrodes according to claim 1 or 2, wherein the carboxyl group-containing crosslinked polymer contains 0.1 mass % or more and 50.0 mass % or less of structural units derived from nitrogen-containing ethylenically unsaturated monomers relative to the total structural units of the polymer.

4. The binder for secondary battery electrodes according to claim 1 or 2, wherein the nitrogen-containing ethylenically unsaturated monomer comprises a nitrile group-containing ethylenically unsaturated monomer or an amide group-containing ethylenically unsaturated monomer.

5. The binder for secondary battery electrodes according to claim 1 or 2, wherein the carboxyl group-containing crosslinked polymer is crosslinked with a crosslinkable monomer, and the amount of the crosslinkable monomer used is 0.03 mol % or more and 0.8 mol % or less based on the total amount of the non-crosslinkable monomer.

6. A method for producing a binder for a secondary battery electrode, comprising a carboxyl group-containing crosslinked polymer or a salt thereof, wherein the carboxyl group-containing crosslinked polymer comprises a structural unit derived from a nitrogen-containing ethylenically unsaturated monomer, the method comprising the steps of: polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer by precipitation polymerization or dispersion polymerization; and adding a monomer component containing a nitrogen-containing ethylenically unsaturated monomer during the above step.

7. The method according to claim 6, wherein the carboxyl-containing crosslinked polymer or salt thereof has a value (X) calculated by the following formula (1) of 5.0 or more and 70.0 or less.

8. The method of claim 6 or 7, wherein the carboxyl group-containing crosslinked polymer contains 0.1% by mass or more and 50.0% by mass or less of structural units derived from nitrogen-containing ethylenically unsaturated monomers relative to the total structural units thereof.

9. The method according to claim 6 or 7, wherein the nitrogen-containing ethylenically unsaturated monomer comprises a nitrile group-containing ethylenically unsaturated monomer or an amide group-containing ethylenically unsaturated monomer.

10. 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.

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

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