JP7767280B2 - Binder for non-aqueous electrolyte secondary battery electrodes and its uses - Google Patents

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority based on Japanese Patent Application No. 2020-91604, filed on May 26, 2020, the entire contents of which are incorporated herein by reference.

This specification relates to a binder for electrodes of non-aqueous electrolyte secondary batteries that can be used in lithium ion secondary batteries, etc., its uses, and a method for producing a cross-linked polymer or its salt used in the binder.

Lithium-ion secondary batteries are well-known examples of non-aqueous electrolyte secondary batteries. Non-aqueous electrolyte secondary battery electrodes are fabricated by applying a composition for forming an electrode mixture layer (hereinafter also referred to as "electrode slurry") containing an active material and a binder to a current collector and drying it. The binder used in the negative electrode mixture layer composition is an aqueous binder containing styrene butadiene rubber (SBR) latex and carboxymethyl cellulose (CMC). Meanwhile, the binder used in the positive electrode mixture layer composition is a solution of polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP). However, with growing environmental awareness in recent years, there is a growing demand for aqueous-based positive electrode mixture layer compositions that do not use organic solvents.

As the applications of lithium-ion secondary batteries expand, there is a growing demand for improved energy density, reliability, and durability. For example, in order to increase the electrical capacity of lithium-ion secondary batteries, an increasing number of specifications are using silicon-based active materials as the negative electrode active material. However, silicon-based active materials are known to undergo large volume changes during charging and discharging, and repeated use can lead to peeling or detachment of the electrode mixture layer, resulting in reduced battery capacity and poor cycle characteristics (durability). To prevent these problems, it is generally effective to use a binder to firmly bond the electrode materials together (adhesion), and research is being conducted to improve binder adhesion in order to improve durability.

Recently, it has been reported that acrylic acid polymers are effective as binders for negative electrode mixture layer compositions using silicon-based active materials. For example, Patent Document 1 discloses an acrylic acid polymer crosslinked with a specific crosslinking agent as a binder for forming a negative electrode coating film in lithium-ion secondary batteries. Patent Document 2 discloses an aqueous electrode binder for secondary batteries containing a water-soluble polymer including structural units derived from an ethylenically unsaturated carboxylate monomer and structural units derived from a vinyl alcohol monomer. Patent Document 3 discloses an aqueous electrode binder for secondary batteries containing a water-soluble polymer including structural units derived from an ethylenically unsaturated carboxylate monomer and structural units derived from a highly hydrophilic ethylenically unsaturated monomer that does not contain carboxylic acid. Patent Document 4 discloses a crosslinked acrylic acid polymer having a specific particle size in a 1% NaCl aqueous solution.

International Publication No. 2014/065407 International Publication No. 2014/207967 International Publication No. 2016/067633 International Publication No. 2017/073589

As the performance of secondary batteries improves, there is a growing demand for binders that can impart better cycle characteristics. Furthermore, when applying and drying an electrode slurry containing a binder, it is advantageous to increase the solids concentration of the electrode slurry in order to improve the drying efficiency of the slurry and increase electrode productivity. However, as the solids concentration increases, it typically becomes more difficult to ensure good coatability. While the binders disclosed in Patent Documents 1 to 4 all impart good adhesion and cycle characteristics, there is room for further improvement in coatability and, in turn, productivity. Thus, to date, no binder has been successfully developed that simultaneously achieves excellent coatability and adhesion, thereby improving both productivity and cycle characteristics.

The disclosure of this specification has been made in light of these circumstances and provides a binder for electrodes of non-aqueous electrolyte secondary batteries that exhibits excellent adhesion and cycle characteristics, and also exhibits excellent coatability even when the solids concentration of the electrode slurry is high, contributing to improved productivity. The present disclosure also provides a composition for electrode mixture layers of non-aqueous electrolyte secondary batteries, non-aqueous electrolyte secondary battery electrodes, and non-aqueous electrolyte secondary batteries obtained using the binder.

As a result of extensive research to solve the above-mentioned problems, the present inventors have discovered that when a binder is used containing a crosslinked polymer or a salt thereof that has a controlled degree of swelling in an aqueous medium (hereinafter also referred to as "water swelling degree") and that contains a specific amount of structural units derived from an ethylenically unsaturated carboxylic acid monomer and structural units derived from a specific unsaturated monomer having a hydroxyl group, the electrode mixture layer slurry exhibits excellent both coatability and cycle characteristics. Based on this discovery, the present disclosure provides the following means.

[1] A binder for a non-aqueous electrolyte 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 first structural units derived from an ethylenically unsaturated carboxylic acid monomer, and 0.5% by mass or more and 50% by mass or less of second structural units derived from one or more monomers selected from the group consisting of monomers represented by formula (1) and formula (2), and
The crosslinked polymer or salt thereof has a degree of swelling in water at pH 8 of 25.0 or more and 40.0 or less.
CH ₂ =C(R ¹ )COOR ² (1)
[In the formula, ^R1 represents a hydrogen atom or a methyl group, ^R2 represents a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, ( ^R3O ) _mH or ^R4O [CO( _CH2 ) _5O ] _nH , ^R3 represents an alkylene group having 2 to 4 carbon atoms, ^R4 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] The binder for a non-aqueous electrolyte secondary battery electrode according to [1], wherein the second structural unit is a structural unit derived from a hydroxyalkyl (meth)acrylate.
[3] The binder for a non-aqueous electrolyte secondary battery electrode according to [1] or [2], wherein the salt of the crosslinked polymer includes a lithium salt.
[4] The binder for a non-aqueous electrolyte secondary battery electrode according to any one of [1] to [3], wherein the crosslinked polymer or the salt thereof has a volume-based median 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.
[5] The binder for a non-aqueous electrolyte secondary battery electrode according to any one of [1] to [4], wherein the crosslinked polymer is crosslinked with a crosslinkable monomer.
[6] The binder for a non-aqueous electrolyte secondary battery electrode according to [5], wherein the amount of the crosslinkable monomer used is 0.1 mol % or more and 1.0 mol % or less based on the total amount of the non-crosslinkable monomer.
[7] The binder for a non-aqueous electrolyte secondary battery electrode according to any one of [1] to [6], wherein the crosslinked polymer or the salt thereof is obtained by precipitation polymerization or dispersion polymerization.
[8] The crosslinked polymer or the salt thereof is produced by the following polymerization step in the precipitation polymerization or dispersion polymerization:
The binder for a non-aqueous electrolyte secondary battery electrode according to [7], which is obtained by carrying out either or both of (A) a polymerization step in which only the ethylenically unsaturated carboxylic acid monomer is polymerized, and (B) a polymerization step in which the monomers represented by the formulas (1) and (2) are continuously or intermittently added to the ethylenically unsaturated carboxylic acid monomer and polymerized.
[9] A composition for a non-aqueous electrolyte secondary battery electrode mixture layer, comprising the binder for a non-aqueous electrolyte secondary battery electrode according to any one of [1] to [8], an active material, and water.
[10] The composition for a non-aqueous electrolyte secondary battery electrode mixture layer according to [9], further comprising a styrene/butadiene-based latex as a binder for a non-aqueous electrolyte secondary battery electrode.
[11] The composition for a non-aqueous electrolyte secondary battery electrode mixture layer according to [9] or [10], which contains a carbon-based material and/or a silicon-based material as a negative electrode active material.
[12] The composition for a non-aqueous electrolyte secondary battery electrode mixture layer according to [9] or [10], which contains a lithium-containing metal oxide as a positive electrode active material.
[13] A non-aqueous electrolyte secondary battery electrode comprising a mixture layer formed on a current collector surface from the composition for a non-aqueous electrolyte secondary battery electrode mixture layer according to any one of [9] to [12].
[14] A non-aqueous electrolyte secondary battery comprising the secondary battery electrode according to [13].

The binder for secondary battery electrodes disclosed in this specification has specific structural units and its degree of swelling in water is controlled, thereby exhibiting excellent coatability and adhesion, contributing to the productivity of secondary battery electrodes, etc., and the cycle characteristics of secondary batteries.

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

The binder for secondary battery electrodes disclosed herein 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 has specific structural units and a controlled degree of swelling in water, thereby exhibiting excellent coatability and adhesion. The composition allows for the application of an electrode mixture layer with excellent coatability, even under conditions of high active material concentration, reducing the amount of water and other media that must be removed by drying. This can contribute to improved productivity when manufacturing electrodes, etc., and ensures excellent cycle capacity retention due to the excellent adhesion of the electrode mixture layer.

The above composition may be in a slurry state that can be applied to a current collector, or may be prepared as a wet powder that can be pressed onto the current collector surface. The secondary battery electrode disclosed in this specification can be obtained by forming a mixture layer made from the above composition on the surface of a current collector such as copper foil or aluminum foil.

Representative, non-limiting examples of the present specification will be described in detail below, with reference to the accompanying drawings as appropriate. This detailed description is intended simply to provide those skilled in the art with details for implementing preferred examples of the present disclosure, and is not intended to limit the scope of the present disclosure. Furthermore, the additional features and disclosures disclosed below can be used separately from or in combination with other features and disclosures of the present specification to provide further improved "binders for electrodes of non-aqueous electrolyte secondary batteries and uses thereof."

Furthermore, the combinations of features and steps disclosed in the following detailed description are not essential to implementing the disclosure of this specification in its broadest sense, but are described solely to specifically illustrate representative embodiments of this specification. Furthermore, the various features of the representative embodiments described above and below, as well as the various features of those described in the independent and dependent claims, do not necessarily have to be combined in the exact embodiments described herein or in the exact order listed to provide additional and useful embodiments of the disclosure of this specification.

All features described in this specification and/or claims are intended to be disclosed individually and independently of one another as limitations on the original disclosure and claimed particulars, apart from any configuration of features described in the examples and/or claims. Furthermore, all numerical ranges and group or aggregate descriptions are intended to disclose intermediate configurations thereof as limitations on the original disclosure and claimed particulars.

Below, we will explain in detail the binder for secondary battery electrodes disclosed in this specification, the composition for secondary battery electrode mixture layers obtained using the binder, and the secondary battery electrodes.

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 disclosed herein may include a crosslinked polymer or a salt thereof. The crosslinked polymer may have 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 a carboxyl group, adhesion to the current collector is improved, and the desolvation effect of lithium ions and ionic conductivity are excellent, resulting in an electrode with low resistance and excellent high-rate characteristics. In addition, water swelling properties are imparted, thereby improving the dispersion stability of the active material and the like in the composition for the electrode mixture layer.

The above-mentioned component (a) can be introduced into the crosslinked polymer by, for example, polymerizing a monomer containing an ethylenically unsaturated carboxylic acid monomer. Alternatively, it can be obtained by (co)polymerizing (meth)acrylic acid ester monomers and then hydrolyzing them. Alternatively, (meth)acrylamide and (meth)acrylonitrile, etc., can be polymerized and then treated with a strong alkali, or a polymer containing hydroxyl groups can be reacted with an acid anhydride.

Examples of ethylenically unsaturated carboxylic acid monomers include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid, (meth)acrylamidohexanoic acid, (meth)acrylamidoalkylcarboxylic acids such as (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 (partially) alkali-neutralized products. These may be used alone or in combination of two or more. Among the above, compounds having an acryloyl group as a polymerizable functional group are preferred, as they have a high polymerization rate, resulting in polymers with long primary chain lengths and good binder adhesion, and acrylic acid is particularly preferred. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, a polymer with a high carboxyl group content can be obtained.

The content of component (a) in the crosslinked polymer is not particularly limited, but can be, for example, 10% by mass or more and 99.5% by mass or less, based on all structural units of the crosslinked polymer. By including component (a) in this range, excellent adhesion to the current collector can be easily ensured. The lower limit is, for example, 20% by mass or more, such as 30% by mass or more, or 40% by mass or more. A lower limit of 50% by mass or more is preferable because it improves the dispersion stability of the electrode mixture layer composition. It may also 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, such as 98% by mass or less, such as 95% by mass or less, such as 90% by mass or less, or such as 80% by mass or less. The range can be a range that appropriately combines these lower and upper limits, 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, for example, 50% by mass or more and 95% by mass or less, for example, 50% by mass or more and 90% by mass or less, for example, 55% by mass or more and 90% by mass or less, for example, 60% by mass or more and 90% by mass or less, for example, 70% by mass or more and 90% by mass or less, or for example, 80% by mass or more and 90% by mass or less.

<Second structural unit>
In addition to component (a), the crosslinked polymer may have a second structural unit (hereinafter also referred to as "component (b)") derived from a specific monomer having a hydroxyl group. When the crosslinked polymer has component (b), the coatability of an electrode mixture layer composition obtained using a binder containing the crosslinked polymer can be improved. 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, 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, ^R1 represents a hydrogen atom or a methyl group, ^R2 represents a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, ( ^R3O ) _mH or ^R4O [CO( _CH2 ) _5O ] _nH , ^R3 represents an alkylene group having 2 to 4 carbon atoms, ^R4 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 above formula (1) is a (meth)acrylate compound having a hydroxyl group. When ^R2 is a monovalent organic group having 1 to 8 carbon atoms and having a hydroxyl group, the number of hydroxyl groups may be one or two or more. The monovalent organic group is not particularly limited, but examples thereof include alkyl groups which may have a linear, branched, or cyclic structure, as well as aryl groups and alkoxyalkyl groups. Furthermore, when ^R2 is ( ^R3O ) _mH or ^R4O [CO( _CH2 ) _5O ] _nH , the alkylene group represented by ^R3 or ^R4 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, trade names "PLACCEL FM1," "PLACCEL FM5," etc.), caprolactone-modified hydroxyacrylates (manufactured by Daicel Corporation, trade names "PLACCEL FA1," "PLACCEL FA10L," etc.), and the like. The monomer represented by the formula (1) may be used singly or in combination of two or more.

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), ^R7 represents a hydrogen atom or a monovalent organic group. The monovalent organic group is not particularly limited, but examples thereof include alkyl groups which may have a linear, branched, or cyclic structure, as well as aryl groups and alkoxyalkyl groups, and an organic group having 1 to 8 carbon atoms is preferred. Alternatively, ^R7 may be a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms.

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

Examples of monomers having a formula weight of 200 or less and containing 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. These may be used alone or in combination of two or more.

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

The content of component (b) in the crosslinked polymer can be 0.5% by mass or more relative to the total structural units of the crosslinked polymer. A content of component (b) of 0.5% by mass or more ensures good coatability of the electrode mixture layer composition. The lower limit may be 1.0% by mass or more, 3.0% by mass or more, 5.0% by mass or more, 10% by mass or more, 15% by mass or more, or 20% by mass or more. Furthermore, a content of component (b) of 50% by mass or less ensures a sufficient amount of component (a), thereby ensuring sufficient dispersion stability of the electrode mixture layer composition (electrode slurry). The upper limit may be 45% by mass or less, 40% by mass or less, 35% by mass or less, 30% by mass or less, 25% by mass or less, or 20% by mass or less. The range can be a range that appropriately combines these lower and upper limits, for example, 0.5% by mass or more and 50% by mass or less, for example, 1.0% by mass or more and 50% by mass or less, for example, 1.0% by mass or more and 45% by mass or less, for example, 1.0% by mass or more and 40% by mass or less, for example, 5.0% by mass or more and 40% by mass or less, for example, 10% by mass or more and 50% by mass or less, for example, 10% by mass or more and 45% by mass or less, or for example, 10% by mass or more and 40% by mass or less.

<Other structural units>
In addition to 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 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 or a phosphate group, or a nonionic ethylenically unsaturated monomer other than 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 or a phosphate group, or a monomer containing a nonionic ethylenically unsaturated monomer other than component (b). Among these, structural units derived from nonionic ethylenically unsaturated monomers are preferred as component (c) from the viewpoint of obtaining an electrode with good flex resistance, and (meth)acrylamide and its derivatives, as well as nitrile-group-containing ethylenically unsaturated monomers, are preferred from the viewpoint of excellent binder adhesion. Furthermore, when a structural unit derived from a hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g/100 ml or less is introduced as component (c), a strong interaction with the electrode material can be achieved, and good adhesion to the active material can be exhibited. This is preferable because it allows for a firm and well-integrated electrode mixture layer to be obtained. In particular, a structural unit derived from an alicyclic structure-containing ethylenically unsaturated monomer is preferable.

The proportion of component (c) can be 0% by mass or more and 49.5% by mass or less, based on the total structural units of the crosslinked polymer. The proportion of component (c) 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. Furthermore, when component (c) is contained at 1% by mass or more based on the total structural units of the crosslinked polymer, affinity with the electrolyte solution is improved, and 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. These may be used alone or in combination of two or more.

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 (meth)acrylic acid cycloalkyl esters that 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. These may be used alone or in combination of two or more. Among the above, compounds having an acryloyl group as a polymerizable functional group are preferred because of their high polymerization rate, which results in a polymer with a long primary chain length and good binder adhesion.

Other nonionic ethylenically unsaturated monomers may include, for example, (meth)acrylic acid esters. Examples of (meth)acrylic acid esters include (meth)acrylic acid alkyl ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, 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; and (meth)acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate. One of these compounds may be used alone, or two or more may be used in combination. From the viewpoints of adhesion to the active material and cycle characteristics, aromatic (meth)acrylic acid ester compounds are preferred. Furthermore, from the viewpoint 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, with 2-methoxyethyl (meth)acrylate being more preferred.

Among nonionic ethylenically unsaturated monomers, compounds containing an acryloyl group are preferred because they have a fast polymerization rate, resulting in polymers with long primary chain lengths and good binder adhesion. Furthermore, as nonionic ethylenically unsaturated monomers, compounds whose homopolymer glass transition temperature (Tg) is 0°C or less are preferred because the resulting electrodes have good flex resistance.

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, as they are less likely to adversely affect battery characteristics, and alkali metal salts are more preferred.

<Embodiments of Crosslinked Polymer>
The crosslinking method for the crosslinked polymer disclosed in this specification is not particularly limited, and examples thereof include the following methods.
1) copolymerization of a crosslinkable monomer; 2) utilizing chain transfer to a polymer chain during radical polymerization; 3) synthesizing a polymer having a reactive functional group, and then adding a crosslinking agent as needed to perform post-crosslinking. By having a crosslinked structure in the polymer, a binder containing the polymer or a salt thereof can have excellent adhesive strength. Among the above methods, the method of copolymerization of a crosslinkable monomer is preferred because it is simple to operate and the degree of crosslinking can be easily controlled.

<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 polyfunctional polymerizable monomer is a compound having two or more polymerizable functional groups, such as (meth)acryloyl groups or alkenyl groups, in the molecule. Examples include polyfunctional (meth)acrylate compounds, polyfunctional alkenyl compounds, and compounds having both (meth)acryloyl groups and alkenyl groups. These compounds may be used alone or in combination of two or more. Among these, polyfunctional alkenyl compounds are preferred because they facilitate the formation of 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; poly(meth)acrylates such as tri(meth)acrylate and tetra(meth)acrylate of trihydric or higher polyhydric alcohols such as trimethylolpropane tri(meth)acrylate, trimethylolpropane ethylene oxide modified tri(meth)acrylate, glycerin tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and pentaerythritol 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 polyallylsucrose; 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 above-mentioned self-crosslinkable monomer having a crosslinkable functional group include hydrolyzable silyl group-containing vinyl monomers, 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. Examples include vinyl silanes such as vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane, and vinyldimethylmethoxysilane; silyl group-containing acrylic esters such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, and methyldimethoxysilylpropyl acrylate; silyl group-containing methacrylic esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, and dimethylmethoxysilylpropyl methacrylate; silyl group-containing vinyl ethers such as trimethoxysilylpropyl vinyl ether; and silyl group-containing vinyl esters such as vinyl trimethoxysilylundecanoate.

When the crosslinked polymer is crosslinked by a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.1 to 2.0 parts by mass, more preferably 0.3 to 1.5 parts by mass, and even more preferably 0.5 to 1.5 parts by mass, relative to 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers). If the amount of the crosslinkable monomer used is 0.1 parts by mass or more, this is preferable in terms of improving adhesion and the stability of the electrode slurry. 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, it is preferable that the crosslinked polymer is not present as large lumps (secondary aggregates) but is well dispersed as water-swellable particles having an appropriate particle size, because the binder containing the crosslinked polymer can exhibit good adhesion performance.

The crosslinked polymer or salt thereof disclosed herein preferably has a particle size (water-swollen particle size) of 0.1 μm or more and 10.0 μm or less, in terms of volume-based median diameter, when dispersed in water, with the degree of neutralization based on the carboxyl groups of the crosslinked polymer being 80 to 100 mol %. A particle size in the range of 0.1 μm or more and 10.0 μm or less allows the particles to be uniformly present at an appropriate size in the electrode mixture layer composition, resulting in high stability of the electrode mixture layer composition and excellent adhesion. A particle size exceeding 10.0 μm may result in insufficient adhesion, as described above. Furthermore, it may be difficult to obtain a smooth coating surface, resulting in insufficient coatability. On the other hand, a particle size of less than 0.1 μm raises concerns about stable production. The lower limit of the particle size may be 0.2 μm or more, 0.3 μm or more, 0.5 μm or more, 0.6 μm or more, 0.7 μm or more, 0.8 μm or more, or 1.0 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, 3.0 μm or less, 2.0 μm or less, 1.8 μm or less, 1.6 μm or less, or 1.5 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, 0.3 μm or more and 5.0 μm or less, 0.5 μm or more and 4.0 μm or less, 0.6 μm or more and 3.0 μm or less, or 0.6 μm or more and 2.0 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 unneutralized or has a degree of neutralization of less than 80 mol%, it can be neutralized to a degree of neutralization of 80-100 mol% with an alkali metal hydroxide or the like, and the particle size can be measured when dispersed in water. Generally, crosslinked polymers or their salts exist as aggregates of primary particles in powder or solution (dispersion) form. If the particle size when dispersed in water is within the above range, the crosslinked polymer or its salt has excellent dispersibility, and neutralizing it to a degree of neutralization of 80-100 mol% and dispersing it in water breaks down the aggregated particles, forming a dispersion of mostly primary particles, or even secondary aggregates, forming a stable dispersion with particle sizes in the range of 0.1-10.0 μm.

The particle size distribution, which is the volume average particle size of water-swollen particles 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 adhesion and coatability. The lower limit of the particle size distribution is usually 1.0.

The particle size (dry particle size) of the crosslinked polymer or salt thereof disclosed herein when dried is preferably in the range of 0.03 μm or more and 3 μm or less in volume-based median size. A more preferred range for the particle size 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.

The crosslinked polymer or its salt is preferably used in the electrode mixture layer composition in the form of a salt, in which acid groups such as carboxyl groups derived from the ethylenically unsaturated carboxylic acid monomer are neutralized to a degree of neutralization of 20 to 100 mol%. The degree of neutralization is more preferably 50 to 100 mol%, and even more preferably 60 to 95 mol%. A degree of neutralization of 20 mol% or higher is preferable in that it provides good water swelling properties and facilitates dispersion stabilization. In this specification, the degree of neutralization can be calculated from the amounts of the monomer containing 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 at 80°C under reduced pressure for 3 hours, and then determining the intensity ratio of the peak derived from the C=O group of the carboxylic acid to the peak derived from the C=O group of the carboxylate salt.

<Molecular weight of crosslinked polymer (primary chain length)>
The crosslinked polymers disclosed herein have a three-dimensional crosslinked structure and exist as microgels in a medium such as water. Generally, such three-dimensional crosslinked polymers are insoluble in solvents, making it impossible to measure their molecular weight. Similarly, it is usually difficult to measure and quantify the primary chain length of crosslinked polymers.

<Water swelling degree of crosslinked polymer>
In this specification, the water swelling degree is calculated based on the following formula (3) from the dry mass "(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)

Furthermore, by focusing on the water swelling degree of the crosslinked polymer and controlling this value, it is possible to simultaneously achieve satisfactory coatability and adhesion. From this perspective, it is preferable that the water swelling degree of the crosslinked polymer at pH 8 be, for example, 25.0 or more and 40.0 or less. If the water swelling degree is less than 25.0, adhesion tends to decrease and cycle characteristics tend to deteriorate, while if the water swelling degree exceeds 40.0, coatability deteriorates.

The lower limit of the water swelling degree at pH 8 is, for example, 26.0, for example, 27.0, for example, 28.0, for example, 29.0, or for example, 30.0. From the standpoint of adhesion, the upper limit of the water swelling degree is, for example, 39.0, 38.0, for example, 37.0, for example, 36.0, for example, 35.0, for example, 34.0, for example, 33.0, or for example, 32.0. The range of the water swelling degree can be selected arbitrarily from the above-mentioned lower and upper limits, but is, for example, 25.0 to 35.0 inclusive, for example, 25.0 to 34.0 inclusive, for example, 25.0 to 33.0 inclusive, or for example, 25.0 to 32.0 inclusive.

The degree of swelling in water at pH 8 can be obtained by measuring the degree of swelling in water of a crosslinked polymer or a salt thereof in water of pH 8. The water of pH 8 can be, for example, ion-exchanged water, and the pH value can be adjusted as needed using an appropriate acid, alkali, or buffer solution. The pH during measurement is, for example, in the range of 8.0±0.5, preferably 8.0±0.3, more preferably 8.0±0.2, and even more preferably 8.0±0.1. Measurements are performed at 25±5°C.

A person skilled in the art would be able to adjust the water swelling degree by controlling the composition and structure of the crosslinked polymer or its salt based on the composition and water swelling degree in the examples herein as well as the common general technical knowledge at the time of filing. For example, the water swelling degree can be adjusted by adjusting the amount of acidic functional groups or highly hydrophilic structural units introduced into the crosslinked polymer, and generally, the introduction of such structural units can improve the water swelling degree. Furthermore, decreasing the degree of crosslinking of the crosslinked polymer usually increases its water swelling degree. Furthermore, when producing a crosslinked polymer by precipitation polymerization or dispersion polymerization, the water swelling degree can also be adjusted by controlling the timing and method of addition of the monomer from which the second structural unit is derived.

<Method of producing a crosslinked polymer or a salt thereof>
Crosslinked polymers can be produced using known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization. However, from the perspective of productivity, precipitation polymerization and suspension polymerization (reverse-phase suspension polymerization) are preferred. Heterogeneous polymerization methods such as precipitation polymerization, suspension polymerization, and emulsion polymerization are preferred because they provide better performance in terms of adhesion, among which precipitation polymerization is more preferred. Precipitation polymerization is a method of producing a polymer by carrying out a polymerization reaction in a solvent that dissolves the raw material unsaturated monomer but does not substantially dissolve the resulting polymer. As the polymerization proceeds, the polymer particles grow through aggregation and growth, resulting in a dispersion of polymer particles in which primary particles of tens to hundreds of nanometers in size have secondary aggregation to several micrometers to several tens of micrometers. A dispersion stabilizer can also be used to control the polymer particle size. Furthermore, the secondary aggregation can be suppressed by selecting the dispersion stabilizer, polymerization solvent, etc. Precipitation polymerization with suppressed secondary aggregation is generally also called dispersion polymerization.

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

Precipitation polymerization or dispersion polymerization is a polymerization method in which polymer chains precipitate from the medium as the polymerization progresses and are layered on the surface of primary particles. Therefore, for example, by adding or feeding constituent monomers during the polymerization reaction, it is possible to control the polymer composition of the particles, and as a result, the degree of swelling in water can be controlled.

To control the composition of polymerized particles in precipitation polymerization or dispersion polymerization, more specifically, the degree of water swelling can be easily controlled by carrying out one or both of the following polymerization steps: (A) a polymerization step in which only ethylenically unsaturated carboxylic acid monomers are polymerized; or (B) a polymerization step in which monomers represented by the above formulas (1) and (2) are continuously or intermittently added to and polymerized with the ethylenically unsaturated carboxylic acid monomers.

The polymerization step (A) may, for example, be a polymerization step in which, in the initial stage of polymerization, the ethylenically unsaturated carboxylic acid monomer from which the first structural unit is derived is added, but the monomers represented by formulas (1) and (2) from which the second structural unit is derived are not added, and only the ethylenically unsaturated carboxylic acid monomer is polymerized first. This polymerization step results in particles having a polymer rich in the first structural unit derived from the ethylenically unsaturated carboxylic acid monomer within the particles, and a polymer rich in the second structural unit derived from the monomers represented by formulas (1) and/or (2) layered on the particle surface, resulting in a high degree of water swelling. In this polymerization step, for example, the entire amount of the ethylenically unsaturated monomer may be added initially, and then the monomers represented by formulas (1) and/or (2) may be added all at once or in portions, or continuously or intermittently over a certain period of time (the latter corresponds to the polymerization step (B) above).

Supplying all at once means supplying the entire required amount of the monomer represented by formula (1) and/or (2) at one time, and supplying in portions means supplying the entire required amount in two or more portions, for example, at intervals of 10 minutes to 1 hour.

The polymerization step (B) may be a polymerization step in which a predetermined amount of the monomer represented by formula (1) and/or (2) is continuously or intermittently supplied in the presence of, for example, 10% by mass or more, for example, 30% by mass or more, for example, 50% by mass or more, for example, 70% by mass or more, for example, 80% by mass or more, or even the entire amount of the ethylenically unsaturated carboxylic acid monomer. "Continuous supply" means supplying without interruption at intervals of less than one minute, preferably while maintaining a constant supply amount. "Intermittent supply" means supplying intermittently at intervals of one minute or more to about five minutes, preferably while maintaining a constant supply amount.

This does not mean that the ethylenically unsaturated carboxylic acid monomer exists only as a monomer, but that it has already been partially polymerized. This means that, at least in the initial stage of supplying the monomers represented by formula (1) and/or (2), the monomers represented by formula (1) and/or (2) are polymerized in the presence of a large excess of the ethylenically unsaturated carboxylic acid monomer. This polymerization step results in particles having a polymer rich in first structural units derived from the ethylenically unsaturated carboxylic acid monomer inside, and a layer of polymer rich in second structural units derived from the monomers represented by formula (1) and/or (2) on the particle surface, resulting in an increased degree of water swelling.

The polymerization step (B) may be carried out for a certain period of time from the beginning of the polymerization reaction, or may be carried out for a certain period of time after the polymerization step (A). The duration of the polymerization step (B) can be freely determined by a person skilled in the art based on the common general technical knowledge at the time of filing, as well as the examples in this specification.

In view of the above, the production method disclosed in this specification may take the following forms, for example.
The polymerization step (A) is carried out from the initial stage of the polymerization reaction, and then the monomers represented by the formula (1) and/or (2) are supplied all at once or in portions.
The polymerization step (A) is carried out from the initial stage of the polymerization reaction, and then the polymerization step (B) is carried out once.
The polymerization step (B) is carried out from the initial stage of the polymerization reaction.

When multiple types of monomers represented by formula (1) and/or (2) are used, the multiple monomers may be supplied all at once continuously or intermittently, or one or more types of monomers may be supplied continuously or intermittently in any order.

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

Similarly, in the neutralization step, it is preferable to add a small amount of a highly polar solvent to the polymerization solvent to ensure stable and rapid progress of the neutralization reaction. Examples of such highly polar solvents include water and methanol. The amount of highly polar solvent used is preferably 0.05 to 20.0 mass% based on the total mass of the medium, more preferably 0.1 to 10.0 mass%, even more preferably 0.1 to 5.0 mass%, and even more preferably 0.1 to 1.0 mass%. A high-polarity solvent content of 0.05 mass% or more is effective in promoting the neutralization reaction, while a high-polarity solvent content of 20.0 mass% or less does not adversely affect the polymerization reaction. Furthermore, in the polymerization of highly hydrophilic ethylenically unsaturated carboxylic acid monomers such as acrylic acid, the addition of a highly polar solvent increases the polymerization rate, making it easier to obtain polymers with long primary chain lengths. Among highly polar solvents, water is particularly preferred due to its significant effect in promoting the polymerization rate.

The production of a crosslinked polymer or a salt thereof preferably includes a polymerization step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer. For example, it preferably includes a polymerization step of polymerizing a monomer component containing 50% by mass to 99.5% by mass of the ethylenically unsaturated carboxylic acid monomer from which component (a) is derived, 0.5% by mass to 50% by mass of a specific monomer having a hydroxyl group from which component (b) is derived, and 0% by mass to 49.5% by mass of another ethylenically unsaturated monomer from which component (c) is derived. Examples of the specific monomer having a hydroxyl group include one or more monomers selected from the group consisting of monomers represented by the following formulas (1) and (2), and monomers 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, ^R1 represents a hydrogen atom or a methyl group, ^R2 represents a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, ( ^R3O ) _mH or ^R4O [CO( _CH2 ) _5O ] _nH , ^R3 represents an alkylene group having 2 to 4 carbon atoms, ^R4 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.]

Through the above polymerization process, the crosslinked polymer contains structural units derived from ethylenically unsaturated carboxylic acid monomers (component (a)) in an amount of 50% by mass to 99.5% by mass, and structural units derived from a specific monomer having a hydroxyl group (component (b)) in an amount of 0.5% by mass to 50% by mass. The amount of ethylenically unsaturated carboxylic acid monomer used may be, for example, 30% by mass to 99.5% by mass, 50% by mass to 99.5% by mass, 50% by mass to 99% by mass, 50% by mass to 98% by mass, or 50% by mass to 95% by mass. It may also be, for example, 50% by mass to 90% by mass, 55% by mass to 90% by mass, or 60% by mass to 90% by mass. The amount of the specific monomer having a hydroxyl group used may be, for example, 1.0% by mass or more and 50% by mass or less, for example, 1.0% by mass or more and 45% by mass or less, for example, 1.0% by mass or more and 40% by mass or less, for example, 5.0% by mass or more and 40% by mass or less, for example, 10% by mass or more and 50% by mass or less, for example, 10% by mass or more and 45% by mass or less, or for example, 10% by mass or more and 40% by mass or less.

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 phosphate groups, as well as nonionic ethylenically unsaturated monomers other than component (b). Specific examples of such compounds include the monomer compounds into which component (c) can be introduced. The other ethylenically unsaturated monomer may be present in an amount of 0% to 49.5% by mass, 1% to 40% by mass, 2% to 40% by mass, 2% to 30% by mass, or 5% to 30% by mass, based on the total amount of the monomer components.

The monomer components polymerized in the polymerization step may contain a crosslinkable monomer. Examples of crosslinkable monomers include, as described above, polyfunctional polymerizable monomers having two or more polymerizable unsaturated groups, and monomers having self-crosslinkable crosslinkable functional groups such as hydrolyzable silyl groups. The amount of crosslinkable monomer used is preferably 0.1 to 2.0 parts by weight, more preferably 0.3 to 1.5 parts by weight, and even more preferably 0.5 to 1.5 parts by weight, per 100 parts by weight of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers).

A higher monomer concentration during polymerization is preferable in order to obtain a polymer with a longer primary chain length. However, if the monomer concentration is too high, the polymer particles are more likely to aggregate, and it may become difficult to control the heat of polymerization, leading to the risk of the polymerization reaction running out of control. For this reason, in the case of precipitation polymerization, for example, the monomer concentration at the start of polymerization is generally in the range of approximately 2 to 40% by mass, and preferably in the range of 5 to 40% by mass. In this specification, "monomer concentration" refers to the concentration of the total mass of the monomers used in polymerization relative to the mass of the entire reaction liquid.

Crosslinked polymers may be produced by carrying out a polymerization reaction in the presence of a basic compound. By carrying out the 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 higher, preferably 15.0% by mass or higher, more preferably 17.0% by mass or higher, even more preferably 19.0% by mass or higher, and even more preferably 20.0% by mass or higher. The monomer concentration is even more preferably 22.0% by mass or higher, and even more preferably 25.0% by mass or higher. In general, the higher the monomer concentration during polymerization, the higher the molecular weight and the longer the primary chain length of the polymer that 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% by mass in precipitation polymerization, approximately 50% by mass in suspension polymerization, and approximately 70% by mass 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 carried out stably even under high monomer concentration conditions, for example, exceeding 13.0 mass%. Furthermore, the polymer obtained by polymerization at such a high monomer concentration has a high molecular weight (long primary chain length) and therefore excellent adhesion.
Examples of inorganic base compounds 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 these, organic amine compounds are preferred from the viewpoints of polymerization stability and adhesion of the resulting binder containing the 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 because it provides greater electrostatic and steric repulsion, making it easier to ensure polymerization stability even at high monomer concentrations. Specifically, the higher the value (C/N) representing 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 C/N value 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. This range of base compound usage allows for smooth polymerization. The usage amount 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%. Note that, in this specification, the amount of base compound used represents the molar concentration of the base compound relative to the ethylenically unsaturated carboxylic acid monomer, and does not refer to the degree of neutralization. In other words, the valence of the base 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 to these. The conditions for 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. 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 low as possible within the allowable production time range.

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 above organic peroxides include 2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane (manufactured by NOF Corporation, trade name "Pertetra A"), 1,1-di(t-hexylperoxy)cyclohexane (manufactured by NOF Corporation, trade name "Perhexa HC"), 1,1-di(t-butylperoxy)cyclohexane (manufactured by NOF Corporation, trade name "Perhexa C"), n-butyl-4,4-di(t-butylperoxy)valerate (manufactured by NOF Corporation, trade name "Perhexa V"), 2,2-di(t-butylperoxy)butane (manufactured by NOF Corporation, trade name "Perhexa 22"), t-butyl hydroperoxide (manufactured by NOF Corporation, trade name "Percumyl H"), 1,1,3,3-tetramethylbutyl hydroperoxide (manufactured by NOF Corporation, 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 peroxyneodecanoate (Perocta ND), t-hexyl peroxyneodecanoate (Perhexyl ND), t-butyl peroxyneodecanoate (Perbutyl ND), t-butyl peroxyneoheptanoate (Perbutyl NHP), t-hexyl peroxypivalate (Perhexyl PV), t-butyl peroxypivalate (Perbutyl PV), 2,5-dimethyl-2,5-di(2-ethylhexanoyl)hexane (Perhexa 250), 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate (Perocta O), t-hexylperoxy-2-ethylhexanoate (Perhexyl O), t-butylperoxy-2-ethylhexanoate peroxyisopropyl tert-butyl ether ("Perbutyl O"), t-butyl peroxylaurate ("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-butyl peroxyacetate ("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, ammonium persulfate, etc. In the case of redox initiation, sodium sulfite, sodium thiosulfate, sodium formaldehyde sulfoxylate, ascorbic acid, sulfurous acid gas (SO ₂ ), ferrous sulfate, etc. can be used as a reducing agent.

A preferred amount of 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, based on 100 parts by mass of the total amount of the monomer components used. If the amount of polymerization initiator used is 0.001 part by mass or more, the polymerization reaction can be carried out stably, and if it is 2 parts by mass or less, it is easier to obtain a polymer with a long primary chain length.

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

The crosslinked polymer dispersion obtained through the polymerization process can be dried under reduced pressure and/or heat to remove the solvent, yielding the desired crosslinked polymer in powder form. In this case, it is preferable to include 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 drying process, in order to remove unreacted monomer (and its salt), impurities derived from the initiator, etc. If the washing process is included, the crosslinked polymer will easily disintegrate during use, even if it undergoes secondary aggregation. Furthermore, the removal of remaining unreacted monomer results in favorable performance in terms of adhesion 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 basic compound. Alternatively, an alkali compound may be added to the polymer dispersion obtained in 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, a crosslinked polymer powder may be obtained without the process neutralization, and then an alkali compound may be added when preparing the electrode slurry to neutralize the polymer (hereinafter also referred to as "post-neutralization"). Of the above methods, 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 disclosed herein includes a binder containing the crosslinked polymer or a salt thereof, an active material, and water. The amount of the crosslinked polymer or salt thereof used in the composition for an electrode mixture layer disclosed herein is, for example, 0.1 parts by mass or more and 20 parts by mass or less, relative to 100 parts by mass of the total amount of active material. The amount used may also be, for example, 0.2 parts by mass or more and 10 parts by mass or less, such as 0.3 parts by mass or more and 8 parts by mass or less, or 0.4 parts by mass or more and 5 parts by mass or less. If the amount of the crosslinked polymer and its salt used is less than 0.1 parts by mass, sufficient adhesion may not be obtained. Furthermore, the dispersion stability of the active material may be insufficient, resulting in reduced uniformity of the mixture layer formed. On the other hand, if the amount of the crosslinked polymer and its salt used exceeds 20 parts by mass, the viscosity of the composition for an electrode mixture layer may increase, reducing its coatability on the current collector. As a result, bumps and irregularities may occur in the resulting mixture layer, adversely affecting electrode characteristics.

When the amount of 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 adhesion to the active material even in small amounts (e.g., 5% by mass or less), and because they contain carboxy anions, they can produce electrodes with low interfacial resistance and excellent high-rate performance.

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

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. This can lead to corrosion of aluminum foil (Al), a common positive electrode current collector material. In such cases, it is preferable to neutralize the alkali leaching from the active material by using an unneutralized or partially neutralized crosslinked polymer as a binder. Furthermore, it is preferable to use an amount of 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 leaching from the active material.

Because all positive electrode active materials have low electrical conductivity, they are generally used with the addition of a conductive additive. Examples of conductive additives 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 more likely to provide excellent conductivity. Furthermore, ketjen black and acetylene black are preferred as carbon black. The conductive additives may be used alone or in combination with two or more of the above. The amount of conductive additive used may be, for example, 0.2 to 20 parts by weight, or, for example, 0.2 to 10 parts by weight, per 100 parts by weight of the total active material, from the perspective of achieving both electrical conductivity and energy density. Furthermore, the positive electrode active material may be surface-coated with a conductive carbon-based material.

On the other hand, examples of negative electrode active materials include carbon-based materials, lithium metal, lithium alloys, and metal oxides, and these can be used alone or in combination. Among these, active materials made from carbon-based materials such as natural graphite, artificial graphite, hard carbon, and soft carbon (hereinafter also referred to as "carbon-based active materials") are preferred, with graphite such as natural graphite and artificial graphite, and hard carbon being more preferred. In the case of graphite, spherical graphite is preferably used from the perspective of battery performance, and its particle size preferably ranges from 1 to 20 μm, for example, and from 5 to 15 μm. Furthermore, to increase energy density, metals or metal oxides capable of absorbing lithium, such as silicon and tin, can also be used as negative electrode active materials. Among these, silicon has a higher capacity than graphite, and active materials made from 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 it in combination with the carbon-based active material. In this case, the amount of silicon 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 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 disclosed herein has structural units (component (a)) derived from an ethylenically unsaturated carboxylic acid monomer. Component (a) has high affinity for silicon-based active materials and exhibits good adhesion. Therefore, the binder disclosed herein exhibits excellent adhesion even when used with high-capacity active materials containing silicon-based active materials, and is therefore believed to be effective in improving the durability of the resulting electrodes.

The bridge polymer disclosed herein also has a structural unit (component (b)) derived from a specific monomer having a hydroxyl group. When the crosslinked polymer contains component (b), the coatability of the electrode mixture layer composition can be improved. While the reason for this effect is unclear, it is believed that the crosslinked polymer has relatively flexible hydroxyl groups on its side chains, which interact with the carboxyl groups in the polymer, thereby suppressing swelling of the crosslinked polymer in water. However, this assumption does not limit the scope of the disclosure herein.

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 purposes such as further reducing resistance, the amount used is, from the standpoint of energy density, for example, 10% by mass or less, or, for example, 5% by mass or less, of the total amount of active material.

When the composition for a 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% by mass, based on the total amount of the composition. If the amount of active material used is 10% by mass or more, migration of binders and the like is suppressed. Furthermore, since this is advantageous in terms of drying costs for the medium, the amount of active material used is preferably 30% by mass or more, more preferably 40% by mass or more, and even more preferably 50% by mass or more. On the other hand, if it is 75% by mass or less, the fluidity and coatability of the composition can be ensured, and a uniform mixture layer can be formed.

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

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

The binder disclosed herein may consist solely of the crosslinked polymer or its salt, but may also contain other binder components, such as styrene/butadiene latex (SBR), acrylic latex, and polyvinylidene fluoride latex. When other binder components are used, the amount 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 other binder components used exceeds 5% by mass, resistance may increase, resulting in insufficient high-rate performance. Among the above, styrene/butadiene latex is preferred due to its excellent balance of adhesion 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. Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, vinyltoluene, and divinylbenzene, and one or more of these can be used. The structural units derived from the aromatic vinyl monomer in the copolymer can be, for example, in the range of 20 to 60% by weight, or, for example, in the range of 30 to 50% by weight, primarily from the standpoint of adhesion.

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, and 2-chloro-1,3-butadiene, and one or more of these can be used. The structural units derived from the aliphatic conjugated diene monomer in the copolymer can be in the range of, for example, 30 to 70% by mass, or, for example, 40 to 60% by mass, in order to improve the adhesion of the binder and the flexibility of the resulting electrode.

In addition to the above-mentioned monomers, styrene/butadiene latex may also contain other copolymerizable 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, to further improve performance such as adhesion. The structural units derived from these other monomers in the copolymer may range, for example, from 0 to 30% by mass, or from 0 to 20% by mass.

The secondary battery electrode mixture layer composition disclosed herein contains the above-described active material, water, and binder as essential components, and is obtained by mixing the components using known means. The method for mixing the components is not particularly limited, and any known method can be used. However, a preferred method involves dry-blending powder components, such as the active material, conductive additive, and binder crosslinked polymer particles, followed by mixing with a dispersion medium such as water and kneading. When obtaining the electrode mixture layer composition in a slurry state, it is preferable to prepare a slurry that is free of poor dispersion and aggregation. Known mixers, such as planetary mixers, thin-film gyratory mixers, and planetary/revolving mixers, can be used as mixing means. However, a thin-film gyratory mixer is preferred because it achieves a good dispersion state in a short time. When using a thin-film gyratory mixer, it is preferable to pre-disperse the components using a stirrer such as a disperser.

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 uneven concentration using a Henschel mixer, blender, planetary mixer, twin-screw kneader, etc.

<Electrode for secondary batteries>
The secondary battery electrode disclosed herein 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 disclosed herein to the surface of the current collector and then drying to remove 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 coating, dipping, roll coating, comma coating, curtain coating, gravure coating, and extrusion can be used. The drying can be performed by known methods such as hot air blowing, reduced pressure, (far) infrared radiation, and microwave irradiation. The mixture layer obtained after drying is usually subjected to a compression treatment using a mold press or roll press. Compression brings the active material and binder into close contact, improving the strength of the mixture layer and its adhesion to the current collector. The thickness of the mixture layer can be adjusted by compression to, for example, approximately 30 to 80% of the thickness before compression, and the thickness of the mixture layer after compression is typically approximately 4 to 200 μm.

A secondary battery can be fabricated by combining the secondary battery electrodes disclosed herein with a separator and an electrolyte solution using an organic solvent. The electrolyte solution may be liquid or gel-like. The separator is disposed between the positive and negative electrodes of the battery and serves to prevent short circuits due to contact between the electrodes and to retain the electrolyte solution to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane with good ion permeability and mechanical strength. Specific materials that can be used include polyolefins such as polyethylene and polypropylene, and polytetrafluoroethylene.

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

As described above, the binder for secondary battery electrodes disclosed herein exhibits excellent adhesion to electrode materials in the mixture layer. Therefore, secondary batteries equipped with electrodes obtained using the binder are expected to ensure good integrity and exhibit good durability (cycling characteristics) even after repeated charge/discharge cycles, making them suitable for automotive secondary batteries, etc. The binder is also useful for the use of active materials containing silicon, which is expected to contribute to increasing the capacity of batteries. Furthermore, it can improve the coatability of the electrode mixture layer composition (electrode slurry) even under conditions with high active material concentrations. This is advantageous in terms of reducing the drying energy required during mixture layer formation and improving productivity. Therefore, the binder for secondary battery electrodes disclosed herein is particularly suitable for use in non-aqueous electrolyte secondary battery electrodes, and is particularly useful for non-aqueous electrolyte lithium-ion secondary batteries with high energy density.

The disclosure of this specification will be explained in more detail below based on examples. However, the disclosure of this specification is not limited to these examples. In the following, "parts" and "%" mean parts by mass and % by mass unless otherwise specified. In the following examples, the crosslinked polymer (salt) was evaluated using the following method.

(1) Measurement of particle size in aqueous medium (water-swollen particle size) 0.25 g of crosslinked polymer salt powder and 49.75 g of ion-exchanged water were weighed into a 100 cc container and placed in a rotation/revolution mixer (Thinky Corporation, Awatori Rentaro AR-250). 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 produce a hydrogel in which the crosslinked polymer salt was swollen in water.

Next, the particle size distribution of the above hydrogel was measured using a laser diffraction/scattering particle size distribution analyzer (Microtrac MT-3300EXII, manufactured by Microtrac Bell) using ion-exchanged water as the dispersant. An excess amount of dispersant was circulated relative to the hydrogel, and an amount of hydrogel sufficient to obtain an appropriate scattered light intensity was added. After a few minutes, the measured particle size distribution shape stabilized. Once stability was confirmed, particle size distribution measurement was performed to obtain the volume-based median diameter (D50) as a representative value of particle size, and the particle size distribution expressed as (volume-based average particle diameter) / (number-based average particle diameter).

(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 Figure 1. The measuring apparatus is composed of <1> to <3> in Figure 1.
<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 with many holes on the bottom 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 of the filter papers 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 Figure 1).

The measurement method is explained below. Unless otherwise specified, the following operations were carried out at 25°C. The pinch cock 2 in <1> was removed, and ion-exchanged water adjusted to pH 8.0 was poured into the top of the burette 1 through the silicone tube 3, until the burette 1 to the filter paper 10 was filled with ion-exchanged water 12. Next, the pinch cock 2 was closed, and air was removed from the polytetrafluoroethylene tube 4 connected to the burette branch pipe with a rubber stopper. In this way, the ion-exchanged water 12 was continuously supplied from the burette 1 to the filter paper 10.
Next, after removing the excess ion-exchanged water 12 that has seeped out of the filter paper 10 for the device, record the reading (a) on the scale of the burette 1. Weigh out 0.1 to 0.2 g of dry powder of the measurement sample and place it evenly in the center of the filter paper 7 for sample fixation, as shown in <3>. The sample is sandwiched between another filter paper, and the two filter papers are secured with adhesive tape 9 to fix the sample. The filter paper with the fixed sample is placed on the filter paper 10 for the device, as shown in <2>. Next, record the reading (b) on the scale of the burette 1 30 minutes after placing the lid 11 on the filter paper 10 for the device. The sum (c) of the water absorption of the measurement sample and the water absorption of the two filter papers 7 for sample fixation is calculated by (a - b). Using the same procedure, measure the water absorption of only the two filter papers 7, without the crosslinked polymer or its salt sample (d).

After carrying out the above procedure, the degree of swelling in water was calculated using the following formula: The solid content used in the calculation was a value measured by the method described below.
Water swelling degree={dry mass of measurement sample (g)+(c−d)}/{dry mass of measurement sample (g)}
Here, dry mass (g) of measurement sample = mass (g) of measurement sample × (solid content % ÷ 100)

The method for measuring the solid content is as follows: 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 and the weighing bottle were 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 solid content (%) was calculated using the following formula:
Solid content (%)=(W ₁ -B)/(W ₀ -B)×100

<Production of Crosslinked Polymer Salt>
(Production Example 1: Production of Crosslinked Polymer Salt R-1)
A reactor equipped with a stirring blade, thermometer, reflux condenser, and nitrogen inlet tube was used for the polymerization. The reactor was charged with 567 parts of acetonitrile, 2.20 parts of ion-exchanged water, 80.0 parts of acrylic acid (hereinafter referred to as "AA"), 0.5 parts of trimethylolpropane diallyl ether (manufactured by Daiso Co., Ltd., trade name "Neoallyl T-20"), and triethylamine equivalent to 1.0 mol% of the AA. The atmosphere inside the reactor was thoroughly purged with nitrogen, and the internal temperature was raised to 55°C. After confirming that the internal temperature had stabilized at 55°C, 0.040 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd., trade name "V-65") was added as a polymerization initiator. The reaction solution became cloudy, and this point was designated as the polymerization initiation point. Two hours after the initiation of polymerization, 20.0 parts of 2-hydroxyethyl acrylate (hereinafter referred to as "HEA") was added all at once during the polymerization. The monomer concentration was calculated to be 15.0%. Cooling of the reaction solution began 12 hours after the initiation of polymerization, and 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 for 12 hours at room temperature, yielding a slurry-like polymerization reaction solution in which particles of crosslinked polymer salt R-1 (Li salt, degree of neutralization 90 mol%) were dispersed in the medium. The reaction rate of AA was 72.4% 2 hours after the initiation of polymerization, and the reaction rates of AA and HEA were calculated to be 97.6% and 98.0%, respectively, 12 hours after the initiation of polymerization. Since HEA was added during the polymerization, it was presumed that particles in which polymer chains containing a large amount of HEA were localized near the surface layer of the particles were obtained.

The resulting polymerization reaction solution was centrifuged to settle the polymer particles, and the supernatant was removed. The precipitate was then redispersed in an equal mass of acetonitrile to the polymerization reaction solution, followed by two washing steps: centrifuging to settle the polymer particles and removing the supernatant. The precipitate was collected and dried under reduced pressure at 80°C for three hours to remove the volatiles, yielding a powder of crosslinked polymer salt R-1. Because crosslinked polymer salt R-1 is hygroscopic, it was stored sealed in a container with water vapor barrier properties. The crosslinked polymer salt R-1 powder was subjected to IR analysis to determine the degree of neutralization based on the intensity ratio of the peak derived from the C=O group of the carboxylic acid to the peak derived from the C=O group of the Li carboxylate. The neutralization degree was 90 mol%, equal to the calculated value calculated from the starting material. The water-swollen particle diameter was 1.4 μm, and the water swelling degree was 38.9.

(Production Examples 2 to 12 and Comparative Production Examples 1 and 2: Production of Crosslinked Polymer Salts R-2 to R-14)
Polymerization reaction solutions containing crosslinked polymer salts R-2 to R-14 were obtained by the same procedure as in Production Example 1, except that the amounts of monomer, crosslinkable monomer, and neutralizing agent charged, and the timing and method of adding the monomers during the course of the polymerization were as shown in Table 1. The "sequential addition" of crosslinked polymer salts R-4 and R-6 involved the polymerization reaction being carried out by continuously adding the entire amount of the monomers added during the course of the polymerization at a constant rate over the specified time period shown in Table 1. The reaction rates of AA and HEA for all polymerization reaction solutions were 95% or higher 12 hours after the start of polymerization. The water-swelling particle sizes and water swelling degrees of R-2 to R-14 are shown in Table 1.

AA: acrylic acid, HEA: 2-hydroxyethyl acrylate, HBA: 4-hydroxybutyl acrylate, HEAAm: hydroxyethyl acrylamide, T-20: trimethylolpropane diallyl ether (manufactured by Daiso Co., Ltd., trade name "Neoallyl T-20")
P-30: Pentaerythritol triallyl ether (manufactured by Daiso Co., Ltd., trade name "Neoallyl P-30")
TEA: Triethylamine AcN: Acetonitrile MeOH: Methanol V-65: 2,2'-Azobis(2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd.)
LiOH.H ₂ O: Lithium hydroxide monohydrate K ₂ CO ₃ : Potassium carbonate

<Example>
Example 1
An electrode was prepared using the crosslinked polymer salt R-1 and evaluated. The specific procedures and evaluation methods are described below.

<Evaluation of electrode slurry coating properties>
(Preparation of negative electrode plate)
A 10% carbon-coated SiO _x (0.8<x<1.2) surface was prepared by CVD, and this was mixed with graphite in a mass ratio of 5:95 to prepare the active material. Furthermore, a mixture of crosslinked polymer salt R-1, styrene/butadiene latex (SBR), and carboxymethyl cellulose (CMC) was used as the binder. Using water as a dilution solvent, the active material: R-1: SBR: CMC = 97.0: 1.0: 1.0: 1.0 (solids) mass ratio was mixed using a T.K. Hibismix manufactured by Primix Corporation to prepare a negative electrode mixture slurry with a solids content of 50%. The negative electrode mixture slurry was applied to both sides of copper foil and dried to form a mixture layer. The mixture layer was then rolled to a thickness of 80 μm per side and a packing density of 1.60 g/cm ³ .

(Coatability evaluation method)
The negative electrode mixture slurry obtained above was applied to a copper foil, dried, and then the appearance of the mixture layer was visually observed to evaluate the coatability. The evaluation criteria were as follows:

A: No abnormal appearance such as streaks or bumps is observed on the surface.
B: Slight abnormalities in appearance such as streaks or bumps are observed on the surface.
C: Significant abnormalities in appearance such as streaks and bumps are observed on the surface.

<Evaluation of 100-cycle capacity retention rate (cycle characteristics)>
Next, a battery including the above-mentioned negative electrode plate using the crosslinked polymer salt R-1 was fabricated, and the 100-cycle capacity retention rate was evaluated. The specific procedure and evaluation method are described below.

(Preparation of positive electrode plate)
In N-methylpyrrolidone (NMP) solvent, _{LiNi0.8Co0.15Al0.05O2} as a positive electrode _{active material} , acetylene _black as a carbon conductive agent, and polyvinylidene fluoride (PVDF) with an average molecular weight of 1.1 million were mixed in a mass ratio of 95:2.5:2.5 using a mixer to prepare a positive electrode mixture slurry with a solid content of 50%. The prepared slurry was applied to both sides of aluminum foil, dried, and rolled so that the thickness of the mixture layer per side was 95 μm and the packing density was 3.60 g/ ^cm3 .

(Preparation of Electrolyte)
A non-aqueous electrolyte was prepared by adding 5 parts by mass of vinylene carbonate (VC) to a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio of EC:DMC = 1:3), and dissolving 1 mol/L of _LiPF6 .

(Battery Construction)
The battery was constructed by attaching lead terminals to the positive and negative electrodes, and then spirally winding the electrodes together with a separator (made of polyethylene: 16 μm thick, 47% porosity) and pressing them to form a flat electrode assembly. The electrode assembly was then placed in an aluminum laminate battery case, and the battery was sealed to form a test battery. The design capacity of this prototype battery was 800 mAh. The design capacity of the battery was based on a charge cut-off voltage of 4.2 V.

(Evaluation of cycle characteristics)
The battery obtained above was subjected to the following charge/discharge test at 25° C. for 100 cycles, and the capacity retention rate was evaluated.

(1) Charging Test The battery was charged at a constant current of 0.3 C (240 mA) up to 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 1/20 C (40 mA).
(2) Discharge Test A constant current discharge was carried out at a current of 0.5 C (400 mA) down to 2.75 V.
(3) Rest The interval between the charge test and the discharge test was 10 minutes.

(1) Charging Conditions The single-electrode battery was charged at a constant current of 0.01 C (0.8 mA) at 25° C. until the V vs. Li reached 0.05.

(Examples 2 to 12, Comparative Examples 1 and 2)
A negative electrode plate and a battery were obtained in the same manner as in Example 1, except that the crosslinked polymer salt was changed as shown in Table 1. The obtained negative electrode plate and battery were evaluated in the same manner as in Example 1, and the results are shown in Table 2.

As shown in Table 1, a comparison of Examples 1 to 12 and Comparative Example 1 indicates that adding all of the monomer components at once at the beginning of the polymerization reaction results in a low degree of water swelling, whereas adding the monomers represented by formula (1) and/or (2) (HEA, HBA, HEAAm in the table) all at once during the polymerization reaction or continuously from the beginning can increase the degree of water swelling. Furthermore, a comparison of Examples 1 to 12 and Comparative Example 2 indicates that the degree of water swelling can be easily controlled by the type and amount of the monomers represented by formula (1) and/or (2), the type and amount of the crosslinkable monomer, the selection of the polymerization solvent, the monomer concentration, etc.

Furthermore, as shown in Tables 1 and 2, it was found that in Examples 1 to 12, which used binders with a water swelling degree of 25.2 to 38.9 at pH 8, both coatability (A to B) and cycle characteristics (80 to 90%) could be achieved. Combining the results of Comparative Examples 1 and 2, it can be seen that a water swelling degree of 25.0 to 40.0 can achieve both coatability and cycle characteristics.

It was also found that good coatability (A) was exhibited in most of the examples (Examples 2 to 4 (Production Examples 2 to 4), Examples 6 to 10 (Production Examples 6 to 10)) in which the water swelling degree was in the range of 25.0 to 32.0. Furthermore, from the perspective of cycle characteristics, it was found that good characteristics (83 to 90%) were exhibited in most of the examples (Examples 1 to 4 (Production Examples 1 to 4), Example 6 (Production Example 6), and Examples 8 to 9 and 11 (Production Examples 8 to 9 and 11)) in which the water swelling degree was in the range of 25.0 to 32.0.

In addition, Example 6 showed the best results by using a binder (Production Example 6) manufactured with the addition of 40 parts of HEA.

From the above, it was found that cross-linked polymers with various degrees of water swelling can be produced, and that by controlling the degree of water swelling within a certain range, it is possible to achieve both coatability and cycle characteristics.

The binder for secondary battery electrodes disclosed herein exhibits excellent adhesion in the mixture layer. Therefore, secondary batteries equipped with electrodes obtained using the binder are expected to exhibit good durability (cycling characteristics) and are expected to be applied to automotive secondary batteries. It is also useful for the use of active materials containing silicon, and is expected to contribute to increasing the capacity of batteries. Furthermore, it can improve the coatability of the electrode mixture layer composition (electrode slurry) even under conditions of high active material concentration. This is advantageous in terms of reducing the drying energy required during mixture layer formation and improving productivity. The binder for secondary battery electrodes disclosed herein can be particularly suitable for use in non-aqueous electrolyte secondary battery electrodes, and is particularly useful for non-aqueous electrolyte lithium-ion secondary batteries with high energy density.

Claims (13)

A binder for a non-aqueous electrolyte 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 first structural units derived from an ethylenically unsaturated carboxylic acid monomer, and 0.5% by mass or more and 50% by mass or less of second structural units derived from one or more monomers selected from the group consisting of monomers represented by formula (1) and formula (2), and
The crosslinked polymer or salt thereof has a degree of swelling in water at pH 8 of 25.0 or more and 40.0 or less.
CH ₂ =C(R ¹ )COOR ² (1)
[In the formula, ^R1 represents a hydrogen atom or a methyl group, ^R2 represents a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, ( ^R3O ) _mH or ^R4O [CO( _CH2 ) _5O ] _nH , ^R3 represents an alkylene group having 2 to 4 carbon atoms, ^R4 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 binder for a non-aqueous electrolyte secondary battery electrode according to claim 1, wherein the second structural unit is a structural unit derived from a hydroxyalkyl (meth)acrylate. The binder for a non-aqueous electrolyte secondary battery electrode according to claim 1 or 2, wherein the salt of the cross-linked polymer includes a lithium salt. 4. The binder for a non-aqueous electrolyte secondary battery electrode according to claim 1, wherein the crosslinked polymer or the salt thereof has a volume-based median 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. The binder for a non-aqueous electrolyte secondary battery electrode according to any one of claims 1 to 4, wherein the cross-linked polymer is cross-linked by a cross-linkable monomer. The binder for a non-aqueous electrolyte secondary battery electrode according to claim 5, wherein the amount of the crosslinkable monomer used is 0.1 mol % or more and 1.0 mol % or less based on the total amount of the non-crosslinkable monomer. The crosslinked polymer or salt thereof according to any one of claims 1 to 6 is polymerized by precipitation polymerization or dispersion polymerization in the following polymerization steps:
(A) a polymerization step in which only the ethylenically unsaturated carboxylic acid monomer is polymerized; and
(B) A production method comprising carrying out one or both of a polymerization step of continuously or intermittently adding the monomers represented by the formula (1) and the formula (2) to the ethylenically unsaturated carboxylic acid monomer and polymerizing the monomers . A composition for a non-aqueous electrolyte secondary battery electrode mixture layer, comprising the binder for a non-aqueous electrolyte secondary battery electrode according to any one of claims 1 to 6 , an active material, and water. The composition for a non-aqueous electrolyte secondary battery electrode mixture layer according to claim 8 , further comprising a styrene/butadiene-based latex as a binder for a non-aqueous electrolyte secondary battery electrode. The composition for a non-aqueous electrolyte secondary battery electrode mixture layer according to claim 8 or 9 , which contains a carbon-based material and/or a silicon-based material as a negative electrode active material. The composition for a non-aqueous electrolyte secondary battery electrode mixture layer according to claim 8 or 9 , which contains a lithium-containing metal oxide as a positive electrode active material. A non-aqueous electrolyte secondary battery electrode comprising a mixture layer formed on a surface of a current collector from the composition for a non-aqueous electrolyte secondary battery electrode mixture layer according to any one of claims 8 to 11 . A non-aqueous electrolyte secondary battery comprising the secondary battery electrode according to claim 12 .