JP7680209B2 - Binder for secondary battery electrode, composition for secondary battery electrode mix layer, and secondary battery electrode - Google Patents

Description

The present invention relates to a binder for secondary battery electrodes, a composition for secondary battery electrode mixture layers, and a secondary battery electrode.

Various electric storage devices such as nickel-hydrogen secondary batteries, lithium-ion secondary batteries, and electric double-layer capacitors have been put to practical use as secondary batteries. 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, etc., to a current collector and drying it. For example, in lithium-ion secondary batteries, an aqueous binder containing styrene butadiene rubber (SBR) latex and carboxymethyl cellulose (CMC) is used as a binder used in the negative electrode mixture layer composition. In addition, a binder containing an aqueous solution or aqueous dispersion of an acrylic acid-based polymer is known as a binder with excellent dispersibility and binding properties. On the other hand, an N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride (PVDF) is widely used as a binder used in the positive electrode mixture layer.

On the other hand, as the applications of various secondary batteries expand, there is a tendency for the demand for improved energy density, reliability, and durability to increase. For example, in order to increase the electric capacity of lithium-ion secondary batteries, specifications using silicon-based active materials as negative electrode active materials are becoming more common. However, it is known that silicon-based active materials have a large volume change during charging and discharging, and with repeated use, peeling or falling off of the electrode mixture layer occurs, resulting in a decrease in battery capacity and a deterioration in cycle characteristics (durability). In order to suppress such defects, it is generally effective to increase the binding properties of the binder, and studies on improving the binding properties of the binder are being conducted with the aim of improving durability.

For example, Patent Document 1 discloses an acrylic acid polymer crosslinked with polyalkenyl ether as a binder for forming a negative electrode coating film of a lithium ion secondary battery. Patent Document 2 discloses an aqueous electrode binder for secondary batteries that contains a water-soluble polymer having a specific aqueous solution viscosity, including structural units derived from an ethylenically unsaturated carboxylate monomer and structural units derived from an ethylenically unsaturated carboxylate monomer. Patent Document 3 discloses an aqueous dispersion of a specific viscosity that contains a salt of a crosslinked polymer that contains structural units derived from an ethylenically unsaturated carboxylate monomer.

JP 2000-294247 A JP 2015-18776 A International Publication No. 2016/158939

All of the binders disclosed in Patent Documents 1 to 3 are capable of imparting good binding properties, but with improvements in the performance of secondary batteries, there is an increasing demand for binders with stronger binding strength.

In addition, in general, secondary battery electrodes are obtained by applying and drying an electrode mixture layer composition (electrode slurry) containing an active material and a binder to the surface of an electrode current collector. In this case, it is advantageous to increase the active material concentration in the electrode slurry from the viewpoint of increasing the drying efficiency of the electrode slurry and improving the productivity of the electrode. However, since the solid content concentration of the electrode slurry usually increases as the active material concentration increases, it is difficult to ensure good coatability in the case of a high-concentration slurry in which the active material concentration in the electrode slurry exceeds 50 mass%, for example.

The present invention has been made in consideration of these circumstances, and provides a binder for secondary battery electrodes that can exhibit better binding properties than conventional binders and can reduce the electrode slurry viscosity even when the active material concentration in the electrode mixture layer is high. In addition, the present invention also provides a composition for secondary battery electrode mixture layers and a secondary battery electrode obtained using the binder.

As a result of intensive research into solving the above problems, the present inventors have discovered that when a binder containing a crosslinked polymer or a salt thereof that includes a structural unit derived from an ethylenically unsaturated carboxylic acid monomer and a structural unit derived from a specific unsaturated monomer having a hydroxyl group is used, both the viscosity reducing effect and the binding properties of the electrode slurry are excellent. According to the present disclosure, the following means are provided based on this knowledge.

The present invention is as follows.
[1] A binder for a secondary battery electrode containing a crosslinked polymer or a salt thereof,
The crosslinked polymer or salt thereof contains, relative to all structural units thereof, 50% by mass or more and 99.5% by mass or less of a first structural unit derived from an ethylenically unsaturated carboxylic acid monomer, and 0.5% by mass or more and 50% by mass or less of a second structural unit derived from one or more monomers selected from the group consisting of monomers represented by formula (1) and formula (2),
and a particle size, measured in an aqueous medium after being neutralized to a degree of neutralization of 80 to 100 mol %, of 0.1 μm or more and 10 μm or less in volume-based median size.
CH ₂ =C(R ¹ )COOR ² (1)
[In the formula, 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. In addition, 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.]
_CH2 =C( ^R5 ) ^CONR6R7 ( ² )
[In the formula, ^R5 represents a hydrogen atom or a methyl group, ^R6 represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, and ^R7 represents a hydrogen atom or a monovalent organic group.]
[2] A binder for a secondary battery electrode containing a crosslinked polymer or a salt thereof,
The crosslinked polymer or salt thereof contains, relative to all structural units thereof, 50% by mass or more and 99.5% by mass or less of a first structural unit derived from an ethylenically unsaturated carboxylic acid monomer, and 0.5% by mass or more and 50% by mass or less of a second structural unit having a formula weight of 200 or less and derived from a monomer having a (meth)acryloyl group and a hydroxyl group,
and a particle size, measured in an aqueous medium after being neutralized to a degree of neutralization of 80 to 100 mol %, of 0.1 μm or more and 10 μm or less in volume-based median size.
[3] The binder for a secondary battery electrode according to [1] or [2], wherein the second structural unit is a structural unit derived from a hydroxyalkyl (meth)acrylate.
[4] The binder for a secondary battery electrode according to any one of [1] to [3], wherein the crosslinked polymer or the salt thereof has a viscosity of 10,000 mPa·s or less in a 3% by mass aqueous solution.
[5] The binder for a secondary battery electrode according to any one of [1] to [4], wherein the crosslinked polymer or the salt thereof has a water swelling degree at pH 8 of 3.0 or more and 100 or less.
[6] 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.
[7] A secondary battery electrode comprising an electrode mixture layer containing the secondary battery electrode binder according to any one of [1] to [5] on a surface of a current collector.

The binder for secondary battery electrodes of the present invention exhibits excellent binding properties for electrode active materials, etc. Therefore, an electrode mixture layer containing the binder and an electrode provided with the same have excellent binding properties and can maintain their integrity. In addition, an electrode mixture layer composition containing the binder can exhibit low slurry viscosity even under conditions where the active material concentration is high. Therefore, it is possible to reduce the amount of medium such as water that is dried and removed when forming an electrode mixture layer, which can contribute to improving productivity when manufacturing electrodes, etc.

FIG. 1 is a diagram showing an apparatus used for measuring the water swelling degree of a crosslinked polymer or a salt thereof.

The binder for secondary battery electrodes of the present invention contains a crosslinked polymer or a salt thereof, and can be mixed with an active material and water to form a composition for an electrode mixture layer. The composition may be in a slurry state that can be applied to a current collector, or may be prepared in a wet powder state so 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 made of the composition on the surface of a current collector such as copper foil or aluminum foil.

Hereinafter, the binder for a secondary battery electrode of the present invention, the composition for a secondary battery electrode mixture layer obtained by using the binder, and the secondary battery electrode will be described in detail.
In this specification, "(meth)acrylic" means acrylic and/or methacrylic, "(meth)acrylate" means acrylate and/or methacrylate, and "(meth)acryloyl group" means acryloyl group and/or methacryloyl group.

<Binder>
The binder of the present invention includes a crosslinked polymer or a salt thereof. The crosslinked polymer has a first structural unit derived from an ethylenically unsaturated carboxylic acid monomer and a second structural unit derived from a specific monomer having a hydroxyl group.

<Structural Unit of Crosslinked Polymer>
<First structural unit>
The crosslinked polymer may have a first structural unit (hereinafter also referred to as "component (a)") derived from an ethylenically unsaturated carboxylic acid monomer. When the crosslinked polymer has such a structural unit and thus has a carboxyl group, the adhesion to the current collector is improved, and the desolvation effect of lithium ions and ion conductivity are excellent, so that an electrode having low resistance and excellent high-rate characteristics can be obtained. In addition, water swelling is imparted, so that the dispersion stability of the active material and the like in the composition for the electrode mixture layer can be improved.
The above-mentioned (a) component can be introduced into the crosslinked polymer by, for example, polymerizing a monomer containing an ethylenically unsaturated carboxylic acid monomer. In addition, it can also be obtained by (co)polymerizing a (meth)acrylic acid ester monomer and then hydrolyzing it. In addition, it may be a method of polymerizing (meth)acrylamide and (meth)acrylonitrile, etc., and then treating them with a strong alkali, or a method of reacting a polymer having a hydroxyl group with an acid anhydride.

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

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

<Second structural unit>
The crosslinked polymer of the present invention may have a second structural unit (hereinafter also referred to as "component (b)") derived from a specific monomer having a hydroxyl group in addition to component (a). When the crosslinked polymer has component (b), the viscosity of the composition for electrode mixture layer obtained by using a binder containing the crosslinked polymer can be reduced. The component (b) can be introduced into the crosslinked polymer by polymerizing, for example, one or more monomers selected from the group consisting of monomers represented by the following formulas (1) and (2). Alternatively, the component (b) may be introduced into the crosslinked polymer by polymerizing a monomer having a formula weight of 200 or less and having a (meth)acryloyl group and a hydroxyl group.
CH ₂ =C(R ¹ )COOR ² (1)
[In the formula, 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. In addition, 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.]
_CH2 =C( ^R5 ) ^CONR6R7 ( ² )
[In the formula, ^R5 represents a hydrogen atom or a methyl group, ^R6 represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, and ^R7 represents a hydrogen atom or a monovalent organic group.]

The monomer represented by the formula (1) is a (meth)acrylate compound having a hydroxyl group. When R ² is a monovalent organic group having 1 to 8 carbon atoms having a hydroxyl group, the number of the hydroxyl groups may be only one or may be two or more. The monovalent organic group is not particularly limited, and examples thereof include an alkyl group that may have a linear, branched or cyclic structure, an aryl group and an alkoxyalkyl group. When R ² is (R ³ O) _m H or R ⁴ O[CO(CH ₂ ) ₅ O] _n H, the alkylene group represented by R ³ or R ⁴ may be linear or branched.

Examples of monomers represented by the above formula (1) include hydroxyalkyl (meth)acrylates having a hydroxyalkyl group having 1 to 8 carbon atoms, such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, 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, product names "Placcel FM1", "Placcel FM5", etc.), caprolactone-modified hydroxyacrylates (manufactured by Daicel Corporation, product names "Placcel FA1", "Placcel FA10L", etc.), etc. The monomer represented by the above formula (1) may be used alone or in combination of two or more kinds.

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. In formula (2), R ⁷ represents a hydrogen atom or a monovalent organic group. The monovalent organic group is not particularly limited, but examples thereof include an alkyl group that may have a linear, branched or cyclic structure, as well as an aryl group and an alkoxyalkyl group, and is preferably an organic group having 1 to 8 carbon atoms. Alternatively, R ⁷ may be a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms.

Examples of the monomer represented by the above 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-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 the above formula (2) may be used alone or in combination of two or more.

Examples of monomers having a formula weight of 200 or less and having a (meth)acryloyl group and a hydroxyl group include hydroxyalkyl (meth)acrylates such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, hydroxyhexyl (meth)acrylate, and hydroxyoctyl acrylate; dialkylene glycol mono(meth)acrylates such as diethylene glycol mono(meth)acrylate and dipropylene glycol monoacrylate; (meth)acrylamide derivatives having a hydroxyalkyl group having 1 to 8 carbon atoms such as N-hydroxyethyl (meth)acrylamide, N-hydroxypropyl (meth)acrylamide, N-hydroxybutyl (meth)acrylamide, N-hydroxyhexyl (meth)acrylamide, and N-hydroxyoctyl (meth)acrylamide; and N-methylol (meth)acrylamide. One of these may be used alone, or two or more may be used in combination.

As the second structural unit, a hydroxyalkyl (meth)acrylate having a hydroxyalkyl group with 1 to 8 carbon atoms, or a hydroxyalkyl (meth)acrylate having a formula weight of 200 or less is preferred because of its excellent effect of reducing the viscosity of the electrode mixture layer composition. Hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and hydroxybutyl (meth)acrylate are more preferred.

The content of the (b) component in the crosslinked polymer can be 0.5% by mass or more based on the total structural units of the crosslinked polymer. If the content of the (b) component is 0.5% by mass or more, the viscosity of the electrode mixture layer composition (electrode slurry) can be sufficiently reduced, and good coating properties can be ensured. The lower limit may be 1.0% by mass or more, 3.0% by mass or more, 5.0% by mass or more, or 10% by mass or more. In addition, if the content of the (b) component is 50% by mass or less, it is possible to ensure the amount of the (a) component as a result, and the dispersion stability of the electrode mixture layer composition (electrode slurry) can be sufficient. The upper limit may be 40% by mass or less, 30% by mass or less, or 20% by mass or less. The range can be an appropriate combination of these lower and upper limits, and can be, for example, 0.5 mass% or more and 50 mass% or less, or, for example, 1.0 mass% or more and 50 mass% or less, or, for example, 1.0 mass% or more and 30 mass% or less.

<Other structural units>
In addition to the components (a) and (b), the crosslinked polymer may contain a structural unit derived from another ethylenically unsaturated monomer copolymerizable therewith (hereinafter also referred to as "component (c)"). Examples of the component (c) include structural units derived from an ethylenically unsaturated monomer compound having an anionic group other than a carboxyl group, such as a sulfonic acid group and a phosphoric acid group, or a nonionic ethylenically unsaturated monomer other than the component (b). These structural units can be introduced by copolymerizing an ethylenically unsaturated monomer compound having an anionic group other than a carboxyl group, such as a sulfonic acid group and a phosphoric acid group, or a monomer containing a nonionic ethylenically unsaturated monomer other than the component (b). Among these, as the component (c), a structural unit derived from a nonionic ethylenically unsaturated monomer is preferred from the viewpoint of obtaining an electrode with good bending resistance, and (meth)acrylamide and its derivatives, as well as nitrile group-containing ethylenically unsaturated monomers, are preferred from the viewpoint of excellent binder binding properties. In addition, when a structural unit derived from a hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g/100 ml or less is introduced as the component (c), it can exert a strong interaction with the electrode material and exhibit good binding properties to the active material. This is preferable because it is possible to obtain a firm and well-integrated electrode mixture layer. In particular, a structural unit derived from an alicyclic structure-containing ethylenically unsaturated monomer is preferable.

The proportion of the (c) component can be 0% by mass or more and 49.5% by mass or less with respect to the total structural units of the crosslinked polymer. The proportion of the (c) component may be 1% by mass or more and 40% by mass or less, 2% by mass or more and 40% by mass or less, 2% by mass or more and 30% by mass or less, or 5% by mass or more and 30% by mass or less. In addition, when the (c) component is contained at 1% by mass or more with respect to the total structural units of the crosslinked polymer, the affinity to the electrolyte is improved, and therefore the effect of improving lithium ion conductivity can also be expected.

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

Examples of nitrile group-containing ethylenically unsaturated monomers include (meth)acrylonitrile; (meth)acrylic acid cyanoalkyl ester compounds such as cyanomethyl (meth)acrylate and cyanoethyl (meth)acrylate; cyano group-containing unsaturated aromatic compounds such as 4-cyanostyrene and 4-cyano-α-methylstyrene; vinylidene cyanide; and the like. One of these may be used alone, or two or more may be used in combination.

Examples of alicyclic structure-containing ethylenically unsaturated monomers include cycloalkyl (meth)acrylate esters which may have an aliphatic substituent, such as cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, cyclodecyl (meth)acrylate, and cyclododecyl (meth)acrylate; isobornyl (meth)acrylate, adamantyl (meth)acrylate, dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and cycloalkyl polyalcohol mono(meth)acrylates such as cyclohexanedimethanol mono(meth)acrylate and cyclodecanedimethanol mono(meth)acrylate. One of these may be used alone, or two or more may be used in combination. Among the above, compounds having an acryloyl group as a polymerizable functional group are preferred, since they have a high polymerization rate, resulting in a polymer having a long primary chain length, and provide a good binding strength of the binder.

Other nonionic ethylenically unsaturated monomers that may be used include, for example, (meth)acrylic acid esters. Examples of the (meth)acrylic acid esters include (meth)acrylic acid alkyl ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate;
Aromatic (meth)acrylic acid ester compounds such as phenyl (meth)acrylate, phenylmethyl (meth)acrylate, and phenylethyl (meth)acrylate;
(Meth)acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate; and the like. Among these, one type may be used alone, or two or more types may be used in combination.
From the viewpoints of adhesion to the active material and cycle characteristics, aromatic (meth)acrylic acid ester compounds can be preferably used. Also, from the viewpoints of further improving lithium ion conductivity and high rate characteristics, compounds having an ether bond such as (meth)acrylic acid alkoxyalkyl ester compounds are preferred, and 2-methoxyethyl (meth)acrylate is more preferred.

Among nonionic ethylenically unsaturated monomers, compounds having an acryloyl group are preferred because they have a fast polymerization rate, resulting in a polymer with a long primary chain length, and because they provide good binding strength for the binder. In addition, as nonionic ethylenically unsaturated monomers, compounds whose homopolymer glass transition temperature (Tg) is 0°C or less are preferred because they provide good bending resistance for the resulting electrodes.

The crosslinked polymer may be a salt. The type of salt is not particularly limited, but examples include alkali metal salts such as lithium, sodium, and potassium; alkaline earth metal salts such as calcium salts and barium salts; other metal salts such as magnesium salts and aluminum salts; ammonium salts, and organic amine salts. Among these, alkali metal salts and magnesium salts are preferred because they are less likely to adversely affect the battery characteristics, and alkali metal salts are more preferred.

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

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

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

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

Examples of polyfunctional alkenyl compounds include polyfunctional allyl ether compounds such as trimethylolpropane diallyl ether, trimethylolpropane triallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, tetraallyloxyethane, and polyallyl sucrose; polyfunctional allyl compounds such as diallyl phthalate; and polyfunctional vinyl compounds such as divinylbenzene.

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

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

The hydrolyzable silyl group-containing vinyl monomer is not particularly limited as long as it is a vinyl monomer having at least one hydrolyzable silyl group. For example, vinyl silanes such as vinyl trimethoxysilane, vinyl triethoxysilane, vinyl methyl dimethoxysilane, vinyl dimethyl methoxysilane, etc.; silyl group-containing acrylic acid esters such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, methyl dimethoxysilylpropyl acrylate, etc.; silyl group-containing methacrylic acid esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyl dimethoxysilylpropyl methacrylate, dimethyl methoxysilylpropyl methacrylate, etc.; silyl group-containing vinyl ethers such as trimethoxysilylpropyl vinyl ether; silyl group-containing vinyl esters such as vinyl trimethoxysilyl undecanoate, etc. can be mentioned.

When the crosslinked polymer is crosslinked by a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.1 parts by mass or more and 2.0 parts by mass or less, more preferably 0.3 parts by mass or more and 1.5 parts by mass or less, and even more preferably 0.5 parts by mass or more and 1.5 parts by mass or less, relative to 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer). If the amount of the crosslinkable monomer used is 0.1 parts by mass or more, it is preferable in that the binding property and the stability of the electrode slurry are better. If it is 2.0 parts by mass or less, the stability of the crosslinked polymer tends to be high.
Similarly, the amount of the crosslinkable monomer used is preferably 0.02 to 0.7 mol %, more preferably 0.03 to 0.4 mol %, based on the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers).

<Particle size of crosslinked polymer>
In the composition for an electrode mixture layer, when the crosslinked polymer is not present as large lumps (secondary aggregates) but is well dispersed as water-swellable particles having an appropriate particle size, this is preferred because the binder containing the crosslinked polymer can exhibit good binding performance.

The crosslinked polymer or salt thereof of the present invention preferably has a particle size (water-swollen particle size) in the range of 0.1 μm or more and 10.0 μm or less in terms of volume-based median size when the crosslinked polymer has a degree of neutralization based on the carboxyl group of 80 to 100 mol% and is dispersed in water. If the particle size is in the range of 0.1 μm or more and 10.0 μm or less, the particle is uniformly present in a suitable size in the composition for electrode mixture layer, so that the composition for electrode mixture layer is highly stable and can exhibit excellent binding properties. If the particle size exceeds 10.0 μm, there is a risk of insufficient binding properties as described above. In addition, there is a risk of insufficient coatability in that a smooth coating surface is difficult to obtain. On the other hand, if the particle size is less than 0.1 μm, there is a concern from the viewpoint of stable production. The lower limit of the particle size may be 0.2 μm or more, 0.3 μm or more, or 0.5 μm or more. The upper limit of the particle size may be 9.0 μm or less, 8.0 μm or less, 7.0 μm or less, 5.0 μm or less, or 3.0 μm or less. The particle size range can be set by appropriately combining the above lower and upper limits, and may be, for example, 0.1 μm or more and 9.0 μm or less, 0.2 μm or more and 8.0 μm or less, or 0.3 μm or more and 5.0 μm or less.
The water-swollen particle size can be measured by the method described in the Examples of this specification.

If the crosslinked polymer is not neutralized or has a degree of neutralization of less than 80 mol%, it is sufficient to neutralize it to a degree of neutralization of 80 to 100 mol% with an alkali metal hydroxide or the like and measure the particle size when dispersed in water. Generally, crosslinked polymers or their salts exist as aggregated particles in which primary particles are associated and aggregated in the form of powder or solution (dispersion liquid). If the particle size when dispersed in water is within the above range, the crosslinked polymer or its salt has extremely excellent dispersibility, and by neutralizing it to a degree of neutralization of 80 to 100 mol% and dispersing it in water, the aggregated particles are broken down, and even if it is a dispersion of almost primary particles or secondary aggregates, it forms a stable dispersion state in which the particle size is within the range of 0.1 to 10.0 μm.

The particle size distribution, which is the volume average particle size of the water-swollen particle size divided by the number average particle size, is preferably 10 or less, more preferably 5.0 or less, even more preferably 3.0 or less, and even more preferably 1.5 or less, from the viewpoint of binding property and coatability. The lower limit of the particle size distribution is usually 1.0.

In addition, the particle diameter (dry particle diameter) of the crosslinked polymer or salt thereof of the present invention when dried is preferably in the range of 0.03 μm or more and 3 μm or less in volume-based median diameter. A more preferred range of the particle diameter is 0.1 μm or more and 1 μm or less, and an even more preferred range is 0.3 μm or more and 0.8 μm or less.

In the electrode mixture layer composition, the crosslinked polymer or its salt 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 is 20 to 100 mol%. The degree of neutralization is more preferably 50 to 100 mol%, and even more preferably 60 to 95 mol%. When the degree of neutralization is 20 mol% or more, it is preferable in that the water swelling property is good and the dispersion stabilization effect is easily obtained. In this specification, the degree of neutralization can be calculated by calculation from the charge amount of the monomer having an acid group such as a carboxyl group and the neutralizing agent used for neutralization. The degree of neutralization can be confirmed by IR measurement of the powder of the crosslinked polymer or its salt after drying treatment at 80 ° C. for 3 hours under reduced pressure conditions, and the intensity ratio of the peak derived from the C = O group of the carboxylic acid and the peak derived from the C = O group of the carboxylate salt.

<Molecular weight of crosslinked polymer (primary chain length)>
The crosslinked polymer of the present invention has a three-dimensional crosslinked structure and exists as a microgel in a medium such as water. Generally, such a three-dimensional crosslinked polymer is insoluble in a solvent, so that its molecular weight cannot be measured. Similarly, it is usually difficult to measure and quantify the primary chain length of the crosslinked polymer.

<Water swelling degree of crosslinked polymer>
In this specification, the water swelling degree is calculated based on the following formula (3) from the dry weight "(WA) g" of the crosslinked polymer or a salt thereof and the amount of water "(WB) g" absorbed when the crosslinked polymer or a salt thereof is swelled to saturation with water.
(Water swelling degree) = {(WA) + (WB)}/(WA) (3)

The crosslinked polymer or salt thereof of the present invention preferably has a water swelling degree at pH 8 of 3.0 or more and 100 or less. If the water swelling degree is within the above range, the crosslinked polymer or salt thereof swells moderately in an aqueous medium, so that when forming an electrode mixture layer, it is possible to ensure a sufficient adhesion area to the active material and the current collector, and the binding property tends to be good. The water swelling degree may be, for example, 4.0 or more, 5.0 or more, 7.0 or more, 10 or more, or 15 or more. When the water swelling degree is 3.0 or more, the crosslinked polymer or salt thereof spreads on the surface of the active material or the current collector, and a sufficient adhesion area can be ensured, so that good binding property can be obtained. The upper limit of the water swelling degree at pH 8 may be 95 or less, 90 or less, 80 or less, 60 or less, or 50 or less. If the water swelling degree exceeds 100, the viscosity of the electrode mixture layer composition (electrode slurry) containing the crosslinked polymer or its salt tends to be high, and the uniformity of the mixture layer may be insufficient, resulting in insufficient binding strength. In addition, there is a risk of the coating property of the electrode slurry being reduced. The range of the water swelling degree at pH 8 can be set by appropriately combining the above upper and lower limits, and is, for example, 4.0 or more and 100 or less, or, for example, 5.0 or more and 100 or less, or, for example, 5.0 or more and 80 or less.
The degree of swelling in water at pH 8 can be obtained by measuring the degree of swelling of the crosslinked polymer or a salt thereof in water at pH 8. As the water at pH 8, for example, ion-exchanged water can be used, and the pH value may be adjusted as necessary using an appropriate acid or alkali, or a buffer solution, etc. The pH at the time of measurement is, for example, in the range of 8.0±0.5, preferably in the range of 8.0±0.3, more preferably in the range of 8.0±0.2, and even more preferably in the range of 8.0±0.1.

A person skilled in the art can adjust the water swelling degree by controlling the composition and structure of the crosslinked polymer or its salt. For example, the water swelling degree can be increased by introducing an acidic functional group or a highly hydrophilic structural unit into the crosslinked polymer. Also, the water swelling degree can usually be increased by decreasing the degree of crosslinking of the crosslinked polymer.

<Viscosity of aqueous solution of crosslinked polymer>
The crosslinked polymer or salt thereof of the present invention preferably has a viscosity of 10,000 mPa·s or less in a 3% by weight aqueous solution. The viscosity of the 3% by weight aqueous solution is more preferably 7,000 mPa·s or less, even more preferably 5,000 mPa·s or less, and even more preferably 3,000 mPa·s or less. If the viscosity of the 3% by weight aqueous solution is 10,000 mPa·s or less, it is suitable from the viewpoint of ease of handling. The lower limit of the viscosity of the 3% by weight aqueous solution is not particularly limited. The lower limit may be, for example, 10 mPa·s or more, 20 mPa·s or more, 50 mPa·s or more, or 100 mPa·s or more.
In this specification, the viscosity of the 3% by mass aqueous solution is measured at a liquid temperature of 25° C. using a Brookfield viscometer at a rotor speed of 12 rpm.

<Method of producing crosslinked polymer or its salt>
The crosslinked polymer can be prepared by known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization, but precipitation polymerization and suspension polymerization (reverse phase suspension polymerization) are preferred from the viewpoint of productivity. Heterogeneous polymerization methods such as precipitation polymerization, suspension polymerization, and emulsion polymerization are preferred from the viewpoint of obtaining better performance in terms of binding property, etc., and among them, 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 larger through aggregation and growth, and a dispersion of polymer particles is obtained in which primary particles of several tens to hundreds of nm are secondary aggregated to several μm to several tens of μm. A dispersion stabilizer can also be used to control the particle size of the polymer.
The secondary aggregation can be suppressed by selecting a dispersion stabilizer, a polymerization solvent, etc. In general, precipitation polymerization in which secondary aggregation is suppressed is also called dispersion polymerization.

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 used, etc. In order to obtain a polymer with a longer primary chain length, it is preferable to use a solvent with a small chain transfer constant.

Specific polymerization solvents include water-soluble solvents such as methanol, t-butyl alcohol, acetone, methyl ethyl ketone, acetonitrile, and tetrahydrofuran, as well as benzene, ethyl acetate, dichloroethane, n-hexane, cyclohexane, and n-heptane, and these can be used alone or in combination of two or more. Alternatively, these can be used as a mixed solvent with water. In the present invention, the water-soluble solvent refers to a solvent having a solubility in water at 20° C. of more than 10 g/100 ml.
Of the above, methyl ethyl ketone and acetonitrile are preferred because they have good polymerization stability with little generation of coarse particles and little adhesion to the reactor, the precipitated polymer fine particles are not prone to secondary aggregation (or even if secondary aggregation does occur, it is easily disintegrated in an aqueous medium), they give polymers with small chain transfer constants and large degrees of polymerization (primary chain lengths), and they are easy to operate during the neutralization step described below.

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

In the production of the crosslinked polymer or its salt, it is preferable to have a polymerization step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer. For example, it is preferable to have a polymerization step of polymerizing a monomer component containing 50% by mass or more and 99.5% by mass or less of an ethylenically unsaturated carboxylic acid monomer from which the component (a) is derived, 0.5% by mass or more and 50% by mass or less of a specific monomer having a hydroxyl group from which the component (b) is derived, and 0% by mass or more and 49.5% by mass or less of another ethylenically unsaturated monomer from which the component (c) is derived. The specific monomer having a hydroxyl group includes one or more monomers selected from the group consisting of monomers represented by the following formulas (1) and (2), and a monomer having a formula weight of 200 or less and having a (meth)acryloyl group and a hydroxyl group.
CH ₂ =C(R ¹ )COOR ² (1)
[In the formula, 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. In addition, 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.]
_CH2 =C( ^R5 ) ^CONR6R7 ( ² )
[In the formula, ^R5 represents a hydrogen atom or a methyl group, ^R6 represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, and ^R7 represents a hydrogen atom or a monovalent organic group.]

In the above polymerization process, 50% to 99.5% by mass of structural units (component a) derived from ethylenically unsaturated carboxylic acid monomers are introduced into the crosslinked polymer, and 0.5% to 50% by mass of structural units (component b) derived from a specific monomer having a hydroxyl group are introduced into the crosslinked polymer. The amount of the ethylenically unsaturated carboxylic acid monomer used is, for example, 30% to 99.5% by mass, for example, 50% to 99.5% by mass, for example, 50% to 99% by mass, for example, 50% to 99% by mass, for example, 50% to 98% by mass, for example, 50% to 95% by mass. The amount of the specific monomer having a hydroxyl group used is, for example, 1.0% to 50% by mass, for example, 1.0% to 30% by mass.

Examples of the other ethylenically unsaturated monomer include 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 other than component (b). Specific compounds include monomer compounds that can introduce the above-mentioned component (c). The other ethylenically unsaturated monomer may be contained in an amount of 0% by mass or more and 49.5% by mass or less, 1% by mass or more and 40% by mass or less, 2% by mass or more and 40% by mass or less, 2% by mass or more and 30% by mass or less, or 5% by mass or more and 30% by mass or less, based on the total amount of the monomer component.

The monomer components polymerized in the polymerization step may contain a crosslinkable monomer. As described above, 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.
The amount of the crosslinkable monomer used is preferably 0.1 parts by mass or more and 2.0 parts by mass or less, more preferably 0.3 parts by mass or more and 1.5 parts by mass or less, and even more preferably 0.5 parts by mass or more and 1.5 parts by mass or less, relative to 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers).

The monomer concentration during polymerization is preferably high from the viewpoint of obtaining a polymer with a longer primary chain length. However, if the monomer concentration is too high, aggregation of polymer particles is likely to proceed, and the polymerization heat becomes difficult to control, and the polymerization reaction may run away. For this reason, for example, in the case of a precipitation polymerization method, the monomer concentration at the start of polymerization is generally in the range of about 2 to 40 mass%, and preferably in the range of 5 to 40 mass%.
In this specification, the term "monomer concentration" refers to the monomer concentration in the reaction liquid at the time when polymerization is started.

The crosslinked polymer may be produced by carrying out a polymerization reaction in the presence of a basic compound. By carrying out a polymerization reaction in the presence of a basic compound, the polymerization reaction can be carried out stably even under high monomer concentration conditions. The monomer concentration may be 13.0% by mass or more, preferably 15.0% by mass or more, more preferably 17.0% by mass or more, even more preferably 19.0% by mass or more, and even more preferably 20.0% by mass or more. The monomer concentration is still more preferably 22.0% by mass or more, and even more preferably 25.0% by mass or more. In general, the higher the monomer concentration during polymerization, the higher the molecular weight is possible, and a polymer with a long primary chain length can be produced.

The upper limit of the monomer concentration varies depending on the type of monomer and solvent used, as well as the polymerization method and various polymerization conditions, but if it is possible to remove the heat generated by the polymerization reaction, it is approximately 40% in precipitation polymerization, approximately 50% in suspension polymerization, and approximately 70% in emulsion polymerization, as mentioned above.

The basic compound is a so-called alkaline compound, and may be either an inorganic basic compound or an organic basic compound. By carrying out the polymerization reaction in the presence of a basic compound, the polymerization reaction can be stably carried out even under high monomer concentration conditions, such as a monomer concentration exceeding 13.0 mass %. In addition, the polymer obtained by polymerization at such a high monomer concentration has a high molecular weight (long primary chain length) and therefore has excellent binding properties.
Examples of the inorganic base compound include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide, and alkaline earth metal hydroxides such as calcium hydroxide and magnesium hydroxide, and one or more of these can be used.
Examples of the organic base compound include ammonia and organic amine compounds, and one or more of these can be used. Among them, organic amine compounds are preferred from the viewpoints of polymerization stability and binding properties of the binder containing the resulting crosslinked polymer or a salt thereof.

Examples of the organic amine compound include N-alkyl-substituted amines such as monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, triethylamine, monobutylamine, dibutylamine, tributylamine, monohexylamine, dihexylamine, trihexylamine, trioctylamine, and tridodecylamine; (alkyl)alkanolamines such as monoethanolamine, diethanolamine, triethanolamine, propanolamine, dimethylethanolamine, and N,N-dimethylethanolamine; cyclic amines such as pyridine, piperidine, piperazine, 1,8-bis(dimethylamino)naphthalene, morpholine, and diazabicycloundecene (DBU); and diethylenetriamine and N,N-dimethylbenzylamine. One or more of these may be used.
Among these, the use of a hydrophobic amine having a long-chain alkyl group is preferred in that it is easy to ensure polymerization stability even when the monomer concentration is high because larger electrostatic and steric repulsion can be obtained. Specifically, the higher the value (C/N) represented by the ratio of the number of carbon atoms to the number of nitrogen atoms present in the organic amine compound, the higher the polymerization stabilization effect due to the steric repulsion effect. The value of C/N is preferably 3 or more, more preferably 5 or more, even more preferably 10 or more, and even more preferably 20 or more.

The amount of the base compound used is preferably in the range of 0.001 mol% to 4.0 mol% relative to the ethylenically unsaturated carboxylic acid monomer. If the amount of the base compound used is in this range, the polymerization reaction can be carried out smoothly. The amount used may be 0.05 mol% to 4.0 mol%, 0.1 mol% to 4.0 mol%, 0.1 mol% to 3.0 mol%, or 0.1 mol% to 2.0 mol%.
In this specification, the amount of the basic compound used represents the molar concentration of the basic compound used relative to the ethylenically unsaturated carboxylic acid monomer, and does not mean the degree of neutralization. In other words, the valence of the basic compound used is not taken into consideration.

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

Examples of the azo compounds include 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(N-butyl-2-methylpropionamide), 2-(tert-butylazo)-2-cyanopropane, 2,2'-azobis(2,4,4-trimethylpentane), 2,2'-azobis(2-methylpropane), etc., and one or more of these can be used.

The organic peroxides include 2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane (manufactured by NOF Corp., trade name "Pertetra A"), 1,1-di(t-hexylperoxy)cyclohexane (manufactured by NOF Corp., trade name "Perhexa HC"), 1,1-di(t-butylperoxy)cyclohexane (manufactured by NOF Corp., trade name "Perhexa C"), n-butyl-4,4-di(t-butylperoxy)valerate (manufactured by NOF Corp., trade name "Perhexa V"), 2,2-di(t-butylperoxy)butane (manufactured by NOF Corp., trade name "Perhexa 22"), t-butyl hydroperoxide (manufactured by NOF Corp., trade name "Percumyl H"), 1,1,3,3-tetramethylbutyl hydroperoxide (manufactured by NOF Corp., trade name "Perocta H"), t- Butylcumyl peroxide (Perbutyl C), di-t-butyl peroxide (Perbutyl D), di-t-hexyl peroxide (Perhexyl D), di(3,5,5-trimethylhexanoyl) peroxide (Perloyl 355), dilauroyl peroxide (Perloyl L), bis(4-t-butylcyclohexyl) peroxydicarbonate (Perloyl TCP), di-2-ethylhexyl peroxydicarbonate (Perloyl OPP), di-sec-butyl peroxydicarbonate (Perloyl SBP), cumyl peroxyneodecanoate (Percumyl ND), 1,1,3,3-tetramethylbutylperoxyneodecanoate peroxy neodecanoate (Perocta ND), t-hexyl peroxy neodecanoate (Perhexyl ND), t-butyl peroxy neodecanoate (Perbutyl ND), t-butyl peroxy neoheptanoate (Perbutyl NHP), t-hexyl peroxy pivalate (Perhexyl PV), t-butyl peroxy pivalate (Perbutyl PV), 2,5-dimethyl-2,5-di(2-ethylhexanoyl)hexane (Perhexa 250), 1,1,3,3-tetramethylbutyl peroxy-2-ethylhexanoate (Perocta O), t-hexyl peroxy-2-ethylhexanoate (Perhexyl O), t-butyl peroxy-2-ethylhexanoate (Perbutyl NHP), peroxypropyl ether ("Perbutyl O"), t-butylperoxylaurate ("Perbutyl L"), t-butylperoxy-3,5,5-trimethylhexanoate ("Perbutyl 355"), t-hexylperoxyisopropyl monocarbonate ("Perhexyl I"), t-butylperoxyisopropyl monocarbonate ("Perbutyl I"), t-butylperoxy-2-ethylhexyl monocarbonate ("Perbutyl E"), t-butylperoxyacetate ("Perbutyl A"), t-hexylperoxybenzoate ("Perhexyl Z") and t-butylperoxybenzoate ("Perbutyl Z"), and one or more of these can be used.

Examples of the inorganic peroxide include potassium persulfate, sodium persulfate, and ammonium persulfate.
In the case of redox initiation, sodium sulfite, sodium thiosulfate, sodium formaldehyde sulfoxylate, ascorbic acid, sulfurous acid gas (SO ₂ ), ferrous sulfate, and the like can be used as reducing agents.

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 parts by mass or more, the polymerization reaction can be carried out stably, and if it is 2 parts by mass or less, a polymer with a long primary chain length is easily obtained.

The polymerization temperature depends on 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.

The crosslinked polymer dispersion obtained through the polymerization process can be dried under reduced pressure and/or by heating to remove the solvent, thereby obtaining the desired crosslinked polymer in powder form. In this case, it is preferable to provide a solid-liquid separation process such as centrifugation and filtration, and a washing process using water, methanol, or the same solvent as the polymerization solvent, following the polymerization process, in order to remove unreacted monomers (and their salts) and impurities derived from the initiator, prior to the drying process. When the washing process is provided, the crosslinked polymer is easy to disintegrate during use even if it undergoes secondary aggregation, and furthermore, the removal of the remaining unreacted monomers results in good performance in terms of binding and battery characteristics.

In this manufacturing method, a polymerization reaction of a monomer composition containing an ethylenically unsaturated carboxylic acid monomer is carried out in the presence of a base compound, and an alkali compound may be added to the polymer dispersion obtained by the polymerization process to neutralize the polymer (hereinafter also referred to as "process neutralization"), and then the solvent may be removed in a drying process. Alternatively, after obtaining a powder of a crosslinked polymer without carrying out the process neutralization treatment, an alkali compound may be added when preparing an electrode slurry to neutralize the polymer (hereinafter also referred to as "post-neutralization"). Of the above, process neutralization is preferred because it tends to break up secondary aggregates more easily.

<Composition for secondary battery electrode mixture layer>
The composition for a secondary battery electrode mixture layer of the present invention contains a binder containing the above-mentioned crosslinked polymer or a salt thereof, an active material, and water.
The amount of the crosslinked polymer or its salt used in the electrode mixture layer composition of the present invention is, for example, 0.1 parts by mass or more and 20 parts by mass or less, based on 100 parts by mass of the total amount of the active material. The amount used is, for example, 0.2 parts by mass or more and 10 parts by mass or less, for example, 0.3 parts by mass or more and 8 parts by mass or less, and for example, 0.4 parts by mass or more and 5 parts by mass or less. When the amount of the crosslinked polymer and its salt used is less than 0.1 parts by mass, sufficient binding property may not be obtained. In addition, the dispersion stability of the active material may be insufficient, and the uniformity of the mixture layer formed may decrease. On the other hand, when the amount of the crosslinked polymer and its salt used exceeds 20 parts by mass, the viscosity of the electrode mixture layer composition may become high and the coatability to the current collector may decrease. As a result, there is a risk that bumps and unevenness may occur in the obtained mixture layer, which may adversely affect the electrode characteristics.

If the amount of the crosslinked polymer and its salt used is within the above range, a composition with excellent dispersion stability can be obtained, and a mixture layer with extremely high adhesion to the current collector can be obtained, resulting in improved battery durability. Furthermore, the crosslinked polymer and its salt exhibit sufficiently high binding properties even in small amounts (e.g., 5% by mass or less) relative to the active material, and since they have carboxy anions, an electrode with low interfacial resistance and excellent high-rate characteristics can be obtained.

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

In addition, when a positive electrode active material containing a layered rock salt type lithium-containing metal oxide is dispersed in water, the lithium ions on the active material surface are exchanged with hydrogen ions in the water, causing the dispersion to become alkaline. For this reason, there is a risk that aluminum foil (Al), which is a common positive electrode current collector material, may be corroded. In such a case, it is preferable to neutralize the alkali content eluted from the active material by using an unneutralized or partially neutralized crosslinked polymer as a binder. In addition, 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.

Since all positive electrode active materials have low electrical conductivity, they are generally used with the addition of a conductive assistant. Examples of conductive assistants include carbon-based materials such as carbon black, carbon nanotubes, carbon fiber, graphite powder, and carbon fiber. Of these, carbon black, carbon nanotubes, and carbon fiber are preferred because they are easy to obtain excellent electrical conductivity. As carbon black, ketjen black and acetylene black are preferred. The conductive assistant may be one of the above alone or two or more may be used in combination. The amount of the conductive assistant used may be, for example, 0.2 to 20 parts by mass, or, for example, 0.2 to 10 parts by mass, per 100 parts by mass of the total amount of the active material, from the viewpoint of achieving both electrical conductivity and energy density. The positive electrode active material may be surface-coated with a carbon-based material having electrical conductivity.

On the other hand, examples of the negative electrode active material include carbon-based materials, lithium metal, lithium alloys, and metal oxides, and one or more of these can be used in combination. Among these, active materials made of carbon-based materials such as natural graphite, artificial graphite, hard carbon, and soft carbon (hereinafter also referred to as "carbon-based active materials") are preferred, and graphite such as natural graphite and artificial graphite, and hard carbon are more preferred. In the case of graphite, spherical graphite is preferably used from the viewpoint of battery performance, and the preferred range of its particle size is, for example, 1 to 20 μm, and also, for example, 5 to 15 μm. In addition, in order to increase the energy density, metals or metal oxides capable of occluding lithium, such as silicon and tin, can also be used as the negative electrode active material. Among them, silicon has a higher capacity than graphite, and active materials made of silicon-based materials such as silicon, silicon alloys, and silicon oxides such as silicon monoxide (SiO) (hereinafter also referred to as "silicon-based active materials") can be used. However, while the silicon-based active material has a high capacity, it undergoes a large volume change during charging and discharging. For this reason, it is preferable to use the silicon-based active material in combination with the carbon-based active material. In this case, the amount of the silicon-based active material used is preferably 2 to 80% by mass based on the total amount of the carbon-based active material and the silicon-based active material. The amount of the silicon-based active material used may be 5 to 70% by mass, 8 to 60% by mass, or 10 to 50% by mass.

The binder containing the crosslinked polymer of the present invention has a structural unit (component (a)) derived from an ethylenically unsaturated carboxylic acid monomer. Here, component (a) has a high affinity for silicon-based active materials and exhibits good binding properties. Therefore, the binder of the present invention exhibits excellent binding properties even when a high-capacity type active material containing a silicon-based active material is used, and is therefore considered to be effective in improving the durability of the resulting electrode.

The crosslinked polymer of the present invention also has a structural unit (component (b)) derived from a specific monomer having a hydroxyl group. When the crosslinked polymer has component (b), it is possible to suppress or reduce the increase in the slurry viscosity of the electrode mixture layer composition. The reason why such an effect is obtained is not clear, but it is speculated that the crosslinked polymer has a relatively flexible hydroxyl group on the side chain of the polymer, which interacts with the carboxyl group in the polymer, thereby suppressing the swelling of the crosslinked polymer in water. However, the above speculation does not limit the scope of the present invention.

Since carbon-based active materials themselves have good electrical conductivity, it is not necessary to add a conductive additive. If 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 mass% or less, or, for example, 5 weight% or less, relative to the total amount of active material.

When the composition for secondary battery electrode mixture layer is in a slurry state, the amount of active material used is, for example, in the range of 10 to 75 mass% with respect to 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. In addition, since it is advantageous in terms of the cost of drying the medium, the amount of active material used is preferably 30 mass% or more, more preferably 40 mass% or more, and even more preferably 50 mass% or more. On the other hand, if it is 75 mass% or less, the fluidity and coatability of the composition can be ensured, and a uniform mixture layer can be formed.

The composition for the secondary battery electrode mixture layer uses water as a medium. In addition, for the purpose of adjusting the properties and drying property of the composition, it may be a mixed solvent with lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, tetrahydrofuran, N-methylpyrrolidone, and other water-soluble organic solvents. The proportion of water in the mixed medium is, for example, 50% by mass or more, and, for example, 70% by mass or more.

When the electrode mixture layer composition is made into a coatable slurry state, the content of the water-containing medium in the entire composition can be in the range of, for example, 25 to 90 mass %, or can be, for example, 35 to 70 mass %, from the viewpoints of the coatability of the slurry, the energy cost required for drying, and productivity.

The binder of the present invention may consist only of the crosslinked polymer or its salt, but may also be used in combination with other binder components such as styrene/butadiene latex (SBR), acrylic latex, and polyvinylidene fluoride latex. When other binder components are used in combination, the amount of the binder components used may be, for example, 0.1 to 5% by mass or less, or, for example, 0.1 to 2% by mass or less, or, for example, 0.1 to 1% by mass or less, relative to the active material. If the amount of the other binder components used exceeds 5% by mass, the resistance increases and the high-rate characteristics may become insufficient. Among the above, styrene/butadiene latex is preferred because of its excellent balance of binding properties and flex resistance.

The styrene/butadiene 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. In addition to styrene, examples of the aromatic vinyl monomer include 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 60% by mass, or, for example, in the range of 30 to 50% by mass, mainly from the viewpoint of binding properties.

In addition to 1,3-butadiene, examples of the aliphatic conjugated diene monomer include 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, etc., and one or more of these can be used. The structural units derived from the aliphatic conjugated diene monomer in the copolymer can be in the range of, for example, 30 to 70% by mass, or, for example, 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-based latex may use other monomers as copolymerization monomers, such as nitrile group-containing monomers such as (meth)acrylonitrile, and carboxyl group-containing monomers such as (meth)acrylic acid, itanconic acid, and maleic acid, in order to further improve performance such as binding property.
The content of the structural units derived from the other monomers in the copolymer can be, for example, in the range of 0 to 30% by mass, and can be, for example, in the range of 0 to 20% by mass.

The composition for the secondary battery electrode mixture layer of the present invention is essentially composed of the active material, water, and binder, 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, but a method in which the powder components such as the active material, the conductive assistant, and the crosslinked polymer particles serving as the binder are dry-blended, and then mixed with a dispersion medium such as water and kneaded for dispersion is preferred. When the composition for the electrode mixture layer is obtained in a slurry state, it is preferable to finish the slurry without poor dispersion or aggregation. As a mixing means, known mixers such as a planetary mixer, a thin film swirling mixer, and a self-revolving mixer can be used, but it is preferable to use a thin film swirling mixer in order to obtain a good dispersion state in a short time. In addition, when a thin film swirling mixer is used, it is preferable to perform preliminary dispersion in advance using a stirrer such as a disperser. In addition, the viscosity of the above slurry can be, for example, in the range of 500 to 100,000 mPa·s. From the viewpoint of the coatability of the slurry, the upper limit of the viscosity is preferably 20,000 mPa·s or less, more preferably 10,000 mPa·s or less, even more preferably 6,000 mPa·s or less, still more preferably 5,000 mPa·s or less, even more preferably 4,000 mPa·s or less, and still more preferably 3,000 mPa·s or less. The slurry viscosity can be measured at a liquid temperature of 25° C. by the method described in the Examples.

On the other hand, when the composition for the electrode mixture layer is obtained in a wet powder state, it is preferable to knead it to a uniform state without unevenness in concentration using a Henschel mixer, blender, planetary mixer, twin-screw kneader, etc.

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

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

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

As described above, the binder for secondary battery electrodes disclosed in this specification exhibits excellent binding properties with the electrode material and excellent adhesion with the current collector in the mixture layer. Therefore, secondary batteries equipped with electrodes obtained using the above binder are expected to ensure good integrity and exhibit good durability (cycle characteristics) even after repeated charging and discharging, making them suitable for use as in-vehicle secondary batteries, etc.

The present invention will be specifically described below based on examples. Note that the present invention is not limited to these examples. Note that, in the following, "parts" and "%" mean parts by mass and % by mass unless otherwise specified.
In the following examples, the crosslinked polymer (salt) was evaluated by the following methods.

(1) Measurement of particle size in aqueous medium (water-swollen particle size) 0.25 g of the crosslinked polymer salt powder and 49.75 g of ion-exchanged water were weighed into a 100 cc container and placed in a rotation/revolution type mixer (Thinky Corporation, Awatori Rentaro AR-250). Next, the mixture was stirred (rotation speed 2000 rpm/revolution speed 800 rpm, 7 minutes) and further degassed (rotation speed 2200 rpm/revolution speed 60 rpm, 1 minute) to prepare a hydrogel in which the crosslinked polymer salt was swollen in water.
Next, the particle size distribution of the hydrogel was measured using a laser diffraction/scattering particle size distribution meter (Microtrack MT-3300EXII, manufactured by Microtrack Bell) using ion-exchanged water as a dispersion medium. When an excess amount of dispersion medium was circulated in the hydrogel and an amount of hydrogel was added to obtain an appropriate scattered light intensity, the particle size distribution shape measured after a few minutes became stable. As soon as stability was confirmed, the particle size distribution was measured to obtain the volume-based median diameter (D50) as a representative value of the particle size, and the particle size distribution expressed as (volume-based average particle size)/(number-based average particle size).

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

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

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

(3) Viscosity measurement of 3% by mass aqueous solution 3.0 parts of the crosslinked polymer salt powder and 97 parts of ion-exchanged water were weighed into a container and set in a rotation/revolution type mixer (Thinky Corporation, Awatori Rentaro AR-250). Then, stirring (rotation speed 2000 rpm/revolution speed 800 rpm, 7 minutes) and degassing (rotation speed 2200 rpm/revolution speed 60 rpm, 1 minute) were repeated until no unswollen powdery parts remained, to prepare a hydrogel microparticle dispersion in which the crosslinked polymer salt was swollen in water. After adjusting each of the obtained hydrogel microparticle dispersions to 25°C ± 1°C, the viscosity was measured at a rotor speed of 12 rpm using a B-type viscometer (Toki Sangyo Co., Ltd., TVB-10).

<Production of Crosslinked Polymer Salt>
(Production Example 1: Production of Crosslinked Polymer Salt R-1)
For the polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen inlet tube was used.
Into the reactor, 567 parts of acetonitrile, 2.20 parts of ion-exchanged water, 80 parts of acrylic acid (hereinafter referred to as "AA"), 20 parts of 2-hydroxyethyl acrylate (hereinafter referred to as "HEA"), 2.0 parts of trimethylolpropane diallyl ether (manufactured by Daiso Co., Ltd., trade name "Neoallyl T-20"), and triethylamine equivalent to 1.0 mol% relative to the AA were charged. After the inside of the reactor was sufficiently replaced with nitrogen, the inside temperature was raised to 55°C by heating. After confirming that the inside temperature was stable at 55°C, 0.040 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd., trade name "V-65") was added as a polymerization initiator, and the reaction solution became cloudy, so this point was determined as the polymerization initiation point. The monomer concentration was calculated to be 15.0%. The polymerization reaction was continued while maintaining the internal temperature at 55° C. by adjusting the external temperature (water bath temperature), and the internal temperature was raised to 65° C. 6 hours after the polymerization initiation point. The internal temperature was maintained at 65° C., and cooling of the reaction solution was started 12 hours after the polymerization initiation point. After the internal temperature had dropped to 25° C., 41.9 parts of lithium hydroxide monohydrate (hereinafter referred to as "LiOH.H ₂ O") powder was added. After the addition, stirring was continued at room temperature for 12 hours to obtain a slurry-like polymerization reaction solution in which particles of the crosslinked polymer salt R-1 (Li salt, neutralization degree 90 mol%) were dispersed in the medium.

The obtained polymerization reaction solution was centrifuged to settle the polymer particles, and the supernatant was removed. Thereafter, the precipitate was redispersed in acetonitrile of the same weight as the polymerization reaction solution, and the polymer particles were precipitated by centrifugation to remove the supernatant. This washing operation was repeated twice. The precipitate was collected and dried at 80° C. for 3 hours under reduced pressure conditions to remove the volatile matter, thereby obtaining a powder of crosslinked polymer salt R-1. Since the crosslinked polymer salt R-1 is hygroscopic, it was sealed and stored in a container with water vapor barrier properties. The powder of 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 and the peak derived from the C=O of the carboxylic acid Li, which was 90 mol%, equal to the calculated value from the charge.
The particle size (water-swollen particle size) of the crosslinked polymer salt R-1 obtained above in an aqueous medium was measured to be 0.88 μm, and the particle size distribution was calculated to be 1.8. The water swelling degree was 4.1, and the viscosity of a 3% by mass aqueous solution was less than 15 mPa·s.

(Production Examples 2 to 18 and Comparative Production Example 1: Production of Crosslinked Polymer Salts R-2 to R-19)
The same procedure as in Production Example 1 was carried out except that the amounts of the respective raw materials were changed as shown in Tables 1 to 3, thereby obtaining polymerization reaction solutions containing crosslinked polymer salts R-2 to R-19.
Next, each polymerization reaction solution was subjected to the same operation as in Production Example 1 to obtain powdery crosslinked polymer salts R-2 to R-19. Each crosslinked polymer salt was stored in a sealed container having water vapor barrier properties.
For each of the resulting polymer salts, the physical properties were measured in the same manner as in Production Example 1, and the results are shown in Tables 1 to 3.

(Comparative Production Example 2: Production of Crosslinked Polymer Salt R-20)
For the polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen inlet tube was used.
A reactor was charged with 300 parts of methanol, 80 parts of AA, 20 parts of HEA, 0.75 parts of Neoallyl T-20, and 0.20 parts of allyl methacrylate (manufactured by Mitsubishi Gas Chemical Company, hereinafter referred to as "AMA"). Next, with stirring, 25.6 parts of LiOH.H ₂ O powder for initial neutralization and 1.40 parts of ion-exchanged water were slowly added so that the internal temperature was maintained at 40°C or less.
After the inside of the reactor was sufficiently replaced with nitrogen, the inside temperature was raised to 68°C by heating. After confirming that the inside temperature was stable at 68°C, 0.020 parts of 4,4'-azobiscyanovaleric acid (manufactured by Otsuka Chemical Co., Ltd., trade name "ACVA") was added as a polymerization initiator, and the reaction solution became cloudy, so this point was designated as the polymerization initiation point. The polymerization reaction was continued while adjusting the external temperature (water bath temperature) so that the solvent was gently refluxed, and 0.020 parts of ACVA were additionally added at the point 3 hours after the polymerization initiation point, and 0.035 parts of ACVA were additionally added at the point 6 hours after the polymerization initiation point, while continuing to reflux the solvent. When 9 hours had passed from the polymerization initiation point, cooling of the reaction solution was started, and after the inside temperature had dropped to 30°C, 16.3 parts of LiOH.H ₂ O powder was slowly added so that the inside temperature did not exceed 50°C. After the addition of the LiOH.H ₂ O powder, stirring was continued for 3 hours to obtain a polymerization reaction solution in the form of a slurry in which particles of the crosslinked polymer salt R-20 (Li salt, neutralization degree 90 mol %) were dispersed in the medium.

The obtained polymerization reaction solution was centrifuged to settle the polymer particles, and the supernatant was removed. The sediment was then redispersed in methanol of the same weight as the polymerization reaction solution, and the operation of settling the polymer particles by centrifugation and removing the supernatant was repeated twice. The sediment was collected and dried at 80°C for 3 hours under reduced pressure conditions to remove the volatile matter, thereby obtaining a powder of crosslinked polymer salt R-20. Since crosslinked polymer salt R-20 is hygroscopic, it was sealed and stored in a container with water vapor barrier properties. The powder of crosslinked polymer salt R-20 was subjected to IR measurement, and the degree of neutralization was calculated from the intensity ratio of the peak derived from the C=O group of the carboxylic acid and the peak derived from the C=O of the carboxylic acid Li, which was 90 mol%, which was equal to the calculated value from the charge.
The physical properties of the resulting crosslinked polymer salt R-20 were measured in the same manner as in Production Example 1, and the results are shown in Table 3.

Details of the compounds used in Tables 1 to 3 are shown below.
AA: Acrylic acid HEA: 2-hydroxyethyl acrylate 4HBA: 4-hydroxybutyl acrylate HEAAm: Hydroxyethylacrylamide MEA: 2-methoxyethyl acrylate AN: Acrylonitrile T-20: Trimethylolpropane diallyl ether (manufactured by Daiso Co., Ltd., product name "Neoallyl T-20")
AMA: allyl methacrylate TEA: triethylamine AcN: acetonitrile MeOH: methanol V-65: 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd.)
ACVA: 4,4'-azobiscyanovaleric acid (manufactured by Otsuka Chemical Co., Ltd., trade name "ACVA")
LiOH.H ₂ O: Lithium hydroxide monohydrate NaOH.H ₂ O: Sodium hydroxide monohydrate K ₂ CO ₃ : Potassium carbonate

Example
The electrode mixture layer compositions using graphite or graphite and silicon particles as the negative electrode active material as the active material and each crosslinked polymer salt as the binder were used to measure the slurry viscosity and the peel strength between the mixture layer and the current collector (i.e., the binding property of the binder). Natural graphite (manufactured by Nippon Graphite Co., Ltd., product name "CGB-10") was used as the graphite, and Si nanopowder (manufactured by Sigma-Aldrich, particle size <100 nm) was used as the silicon particles.

Example 1
2.4 parts of powdered crosslinked polymer Li salt R-1 was weighed into 100 parts of natural graphite and mixed thoroughly in advance, and then 90 parts of ion-exchanged water was added and pre-dispersed with a disperser. After that, main dispersion was performed for 15 seconds at a peripheral speed of 20 m/sec using a thin film rotary mixer (FM-56-30, manufactured by Primix Corporation), to obtain a slurry-like electrode mixture layer composition (electrode slurry). The active material concentration in the electrode slurry was calculated to be 52.0%, and the solid content concentration of the electrode slurry was calculated to be 53.2%.

<Measurement of Viscosity of Electrode Slurry>
The electrode slurry obtained above was subjected to measurement of the slurry viscosity at 25° C. and a shear rate of 60 ^s using a CP25-5 cone plate (diameter 25 mm, cone angle 5°) using an Anton Paar rheometer (Physica MCR301), and the viscosity was 1,500 mPa·s.

Next, the electrode slurry was applied onto a 20 μm-thick copper foil (manufactured by Nippon Foil Co., Ltd.) using a variable applicator, and dried in a ventilated dryer at 100° C. for 15 minutes to form a mixture layer. The mixture layer was then rolled to a thickness of 50±5 μm and a packing density of 1.70±0.20 g/cm ³ to obtain a negative electrode.

<90° peel strength (bonding strength)>
The negative electrode obtained above was cut into a strip of 25 mm width, and the composite layer surface of the negative electrode was attached to a double-sided tape fixed to a horizontal surface to prepare a peel test sample. The test sample was dried at 60°C under reduced pressure overnight, and then 90° peeling (measurement temperature 23°C) was performed at a tensile speed of 50 mm/min to measure the peel strength between the composite layer and the copper foil. The peel strength was high and good at 11.4 N/m.

Examples 2 to 20 and Comparative Examples 1 to 3
The electrode slurries were prepared by carrying out the same operation as in Example 1, except that the active material, the crosslinked polymer salt used as the binder, and ion-exchanged water were used as shown in Tables 4 to 6. In Examples 3 and 4, natural graphite and silicon particles were stirred at 400 rpm for 1 hour using a planetary ball mill (P-5, manufactured by FRITSCH), and 2.4 parts of powdered crosslinked polymer Li salt R-2 were weighed into the resulting mixture, which was then thoroughly mixed in advance, and the electrode slurries were prepared by carrying out the same operation as in Example 1. The coating properties and 90° peel strength of each electrode slurry were evaluated. The results are shown in Tables 4 to 6.

Each example is an electrode mixture layer composition containing a secondary battery electrode binder belonging to the present invention, and an electrode prepared using the same. Each electrode mixture layer composition (electrode slurry) had a sufficiently low value even under high concentration conditions such as an active material concentration exceeding 50 mass%, and good coatability could be ensured. In addition, the amount of medium (water) removed during drying can be reduced, which can contribute to improving productivity. Furthermore, the peel strength between the mixture layer and the current collector of the obtained electrode was high in all cases, indicating excellent binding properties.
Focusing on the second structural unit derived from a specific monomer having a hydroxyl group, the slurry viscosity (5,200 mPa·s) of Example 16 using crosslinked polymer salt R-14 having a structural unit derived from an acrylamide derivative was 2,200 mPa·s (Example 2) and 2,100 mPa·s (Example 15), respectively, showing a higher slurry viscosity reduction effect.

On the other hand, cross-linked polymer salt R-19, which does not have a second structural unit derived from a specific monomer having a hydroxyl group, had a significantly high slurry viscosity of 18,600 mPa·s (Comparative Example 1), and even when the concentration was diluted in consideration of coating properties, the binding properties were insufficient (Comparative Example 2). Furthermore, cross-linked polymer salt R-20, which exceeds the upper limit of the particle size range specified in the present invention, had a high slurry viscosity and very low peel strength (Comparative Example 3).

The binder for secondary battery electrodes of the present invention exhibits excellent binding properties in the mixture layer. Therefore, a secondary battery equipped with an electrode obtained using the binder is expected to exhibit good durability (cycle characteristics), and is expected to be applied to a secondary battery for vehicle use. It is also useful for the use of active materials containing silicon, and is expected to contribute to increasing the capacity of batteries. Furthermore, even under conditions where the active material concentration is high, the viscosity of the composition for electrode mixture layer (electrode slurry) can be reduced. Therefore, it is advantageous in terms of reducing the drying energy during the formation of the mixture layer and improving productivity.
The binder for secondary battery electrodes of the present invention can be suitably used in particular for electrodes of non-aqueous electrolyte secondary batteries, and is particularly useful for non-aqueous electrolyte lithium ion secondary batteries having high energy density.

Claims (5)

A binder for a secondary battery electrode containing a crosslinked polymer or a salt thereof,
The crosslinked polymer or salt thereof contains, relative to all structural units thereof, 50% by mass or more and 99.5% by mass or less of a first structural unit derived from an ethylenically unsaturated carboxylic acid monomer, and 0.5% by mass or more and 50% by mass or less of a second structural unit derived from a hydroxyalkyl (meth)acrylate,
and after being neutralized to a degree of neutralization of 80 to 100 mol%, the particle size measured in an aqueous medium is 0.1 μm or more and 10 μm or less in volume-based median size,
A binder for a secondary battery electrode, having a water swelling degree at pH 8 of 3.0 or more and 100 or less . A binder for a secondary battery electrode containing a crosslinked polymer or a salt thereof,
The crosslinked polymer or salt thereof contains, relative to all structural units thereof, 50% by mass or more and 99.5% by mass or less of a first structural unit derived from an ethylenically unsaturated carboxylic acid monomer, and 0.5% by mass or more and 50% by mass or less of a second structural unit having a formula weight of 200 or less and derived from a hydroxyalkyl (meth)acrylate;
and after being neutralized to a degree of neutralization of 80 to 100 mol%, the particle size measured in an aqueous medium is 0.1 μm or more and 10 μm or less in terms of volume-based median size;
A binder for a secondary battery electrode, having a water swelling degree at pH 8 of 3.0 or more and 100 or less . The binder for secondary battery electrodes according to claim 1 or 2, wherein the crosslinked polymer or its salt has a viscosity of 10,000 mPa·s or less in a 3% by mass aqueous solution. A composition for a secondary battery electrode mixture layer, comprising the binder for a secondary battery electrode according to any one of claims 1 to 3 , an active material, and water. A secondary battery electrode comprising an electrode mixture layer containing the secondary battery electrode binder according to any one of claims 1 to 3 on a surface of a current collector.

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