JPWO2020110847A1 - Binder for secondary battery electrode, composition for secondary battery electrode mixture layer and secondary battery electrode - Google Patents

Abstract

The present invention is for a secondary battery electrode capable of exhibiting better binding properties than before and reducing the electrode slurry viscosity even when the concentration of the active material in the electrode binder layer is high. Provide a binder.
A binder for a secondary battery electrode containing a crosslinked polymer or a salt thereof, wherein the crosslinked polymer or a salt thereof is the first derived from an ethylenically unsaturated carboxylic acid monomer with respect to all structural units thereof. It contains 50% by mass or more and 99.5% by mass or less of structural units, and 0.5% by mass or more and 50% by mass or less of a second structural unit derived from a specific monomer having a hydroxyl group, and has a degree of neutralization. After being neutralized to 80 to 100 mol%, the particle size measured in an aqueous medium is adjusted to be 0.1 μm or more and 10 μm or less in terms of volume reference polymer diameter.

Description

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

As the secondary battery, various power storage devices such as a nickel hydrogen secondary battery, a lithium ion secondary battery, and an electric double layer capacitor have been put into practical use. The electrodes used in these secondary batteries are produced by applying, drying, or the like on a current collector a composition for forming an electrode mixture layer containing an active material, a binder, and the like. For example, in a lithium ion secondary battery, an aqueous binder containing styrene-butadiene rubber (SBR) latex and carboxymethyl cellulose (CMC) is used as the binder used in the composition for the negative electrode mixture layer. Further, as a binder having excellent dispersibility and binding property, a binder containing an aqueous acrylic acid polymer solution or an aqueous dispersion is known. On the other hand, as a binder used for the positive electrode mixture layer, an N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride (PVDF) is widely used.

On the other hand, as the applications of various secondary batteries expand, the demand for improving energy density, reliability and durability tends to increase. For example, for the purpose of increasing the electric capacity of a lithium ion secondary battery, specifications for using a silicon-based active material as a negative electrode active material are increasing. However, it is known that the volume of a silicon-based active material changes significantly during charging and discharging, and the electrode mixture layer peels off or falls off as it is used repeatedly, resulting in a decrease in battery capacity and cycle characteristics. There was a problem that (durability) deteriorated. In order to suppress such a defect, it is generally effective to improve the binder's binding property, and for the purpose of improving the durability, studies on improving the binder's binding property have been conducted.

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

Japanese Unexamined Patent Publication No. 2000-294247 Japanese Unexamined Patent Publication No. 2015-1877 International Publication No. 2016/158939

All of the binders disclosed in Patent Documents 1 to 3 can impart good binding property, but as the performance of the secondary battery is improved, there is an increasing demand for a binder having higher binding force. ..

Further, the secondary battery electrode is generally obtained by applying a composition for an electrode mixture layer (electrode slurry) containing an active material and a binder to the surface of an electrode current collector and drying it. At this time, it is advantageous to increase the concentration of the active material in the electrode slurry from the viewpoint of increasing the drying efficiency of the electrode slurry and improving the productivity of the electrode. However, since the solid content concentration of the electrode slurry usually increases as the active material concentration increases, for example, in the case of a high concentration slurry in which the active material concentration in the electrode slurry exceeds 50% by mass, good coating is performed. It is difficult to secure sex.

The present invention has been made in view of such circumstances, and can exhibit better binding properties than before, and even when the concentration of the active material in the electrode mixture layer is high, the present invention can be exhibited. Provided is a binder for a secondary battery electrode capable of reducing the electrode slurry viscosity. In addition, a composition for a secondary battery electrode mixture layer and a secondary battery electrode obtained by using the above binder are also provided.

As a result of diligent studies to solve the above problems, the present inventors include a structural unit derived from an ethylenically unsaturated carboxylic acid monomer and a structural unit derived from a specific unsaturated monomer having a hydroxyl group. It was found that when a crosslinked polymer or a binder containing a salt thereof is used, both the viscosity reducing effect of the electrode slurry and the binding property are excellent. According to the present disclosure, the following means are provided based on such findings.

The present invention is as follows.
[1] A binder for a secondary battery electrode containing a crosslinked polymer or a salt thereof.
The crosslinked polymer or a salt thereof contains 50% by mass or more and 99.5% by mass or less of the first structural unit derived from the ethylenically unsaturated carboxylic acid monomer with respect to all the structural units thereof, and the formula (1). ) And a second structural unit derived from one or more monomers selected from the group consisting of the monomers represented by the formula (2), containing 0.5% by mass or more and 50% by mass or less.
A binder for a secondary battery electrode, wherein the particle size measured in an aqueous medium after being neutralized to a degree of neutralization of 80 to 100 mol% is 0.1 μm or more and 10 μm or less in terms of volume-based median diameter.
CH ₂ = C (R ¹ ) COOR ² (1)
[In the formula, R ¹ represents a hydrogen atom or a methyl group, R ² is a monovalent organic group having a hydroxyl group and having 1 to 8 carbon atoms, (R ³ O) _m H or R ⁴ O [CO (CH _2). ) ₅ O] represents _{n H.} R ³ represents an alkylene group having 2 to ⁴ carbon atoms, R 4 represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15. show. ]
CH ₂ = C (R ⁵ ) CONR ⁶ R ⁷ (2)
[In the formula, R ⁵ represents a hydrogen atom or a methyl group, R ⁶ represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, and R ⁷ represents a hydrogen atom or a monovalent organic group. ]
[2] A binder for a secondary battery electrode containing a crosslinked polymer or a salt thereof.
The crosslinked polymer or a salt thereof contains 50% by mass or more and 99.5% by mass or less of the first structural unit derived from the ethylenically unsaturated carboxylic acid monomer with respect to all the structural units, and the formula amount is 200 or less, containing 0.5% by mass or more and 50% by mass or less of a second structural unit derived from a monomer having a (meth) acryloyl group and a hydroxyl group.
A binder for a secondary battery electrode, wherein the particle size measured in an aqueous medium after being neutralized to a degree of neutralization of 80 to 100 mol% is 0.1 μm or more and 10 μm or less in terms of volume-based median diameter.
[3] The binder for a secondary battery electrode according to [1] or [2], wherein the second structural unit is a structural unit derived from hydroxyalkyl (meth) acrylate.
[4] The binder for a secondary battery electrode according to any one of [1] to [3], wherein the crosslinked polymer or a salt thereof has a viscosity of a 3% by mass aqueous solution of 10,000 mPa · s or less.
[5] The binder for a secondary battery electrode according to any one of [1] to [4], wherein the crosslinked polymer or a salt thereof has a water swelling degree of 3.0 or more and 100 or less at pH 8.
[6] A composition for a secondary battery electrode mixture layer containing the binder for the secondary battery electrode, the active material, and water according to any one of [1] to [5].
[7] A secondary battery electrode comprising an electrode mixture layer containing the binder for the secondary battery electrode according to any one of [1] to [5] on the surface of the current collector.

The binder for a secondary battery electrode of the present invention exhibits excellent binding properties to an electrode active material or the like. Therefore, the electrode mixture layer containing the binder and the electrode provided with the binder are excellent in binding property and can maintain their integrity. Further, the composition for the electrode mixture layer containing the binder can exhibit a low slurry viscosity even under the condition of high active material concentration. Therefore, it is possible to reduce the amount of a medium such as water that is dried and removed when forming the electrode mixture layer, which can contribute to the improvement of productivity when manufacturing the electrode or the like.

It is a figure which shows the apparatus used for measuring the water swelling degree of a crosslinked polymer or a salt thereof.

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

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

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

<Structural unit of crosslinked polymer>
<First structural unit>
The crosslinked polymer can have a first structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter, also referred to as “component (a)”). When the crosslinked polymer has a carboxyl group by having such a structural unit, the adhesiveness to the current collector is improved, and the lithium ion desolvation effect and the ionic conductivity are excellent, so that the resistance is small and the high rate is obtained. An electrode having excellent characteristics can be obtained. Further, since water swelling property is imparted, the dispersion stability of the active material or the like in the composition for the electrode mixture layer can be enhanced.
The component (a) can be introduced into a crosslinked polymer, for example, by polymerizing a monomer containing an ethylenically unsaturated carboxylic acid monomer. Alternatively, it can also be obtained by (co) polymerizing a (meth) acrylic acid ester monomer and then hydrolyzing it. Further, after polymerizing (meth) acrylamide, (meth) acrylonitrile or the like, it may be treated with a strong alkali, or it may be a method of reacting an acid anhydride with a polymer having a hydroxyl group.

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

The content of the component (a) in the crosslinked polymer is not particularly limited, but may be, for example, 10% by mass or more and 99.5% by mass or less with respect to all the structural units of the crosslinked polymer. By containing the component (a) in such a range, excellent adhesiveness to the current collector can be easily ensured. The lower limit is, for example, 20% by mass or more, for example, 30% by mass or more, and for example, 40% by mass or more. When the lower limit is 50% by mass or more, the dispersion stability of the composition for the electrode mixture layer is good, which is preferable, and it may be 60% by mass or more, 70% by mass or more, or 80% by mass. It may be the above. The upper limit is, for example, 99% by mass or less, for example 98% by mass or less, for example 95% by mass or less, for example 90% by mass or less, and for example 80% by mass or less. The range may be a range in which such a lower limit and an upper limit are appropriately combined, and is, for example, 30% by mass or more and 99.5% by mass or less, and for example, 50% by mass or more and 99.5% by mass or less. Yes, 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, and for example, 50% by mass or more and 95% by mass or less.

<Second structural unit>
The crosslinked polymer of the present invention may have a second structural unit (hereinafter, also referred to as “component (b)”) derived from a specific monomer having a hydroxyl group, in addition to the component (a). When the crosslinked polymer has the component (b), the viscosity of the composition for the electrode mixture layer obtained by using the binder containing the crosslinked polymer can be reduced. The component (b) is introduced into the crosslinked polymer, for example, by polymerizing one or more monomers selected from the group consisting of the monomers represented by the following formulas (1) and (2). be able to. Alternatively, it may be introduced into the crosslinked polymer by polymerizing a monomer having a formula amount of 200 or less and having a (meth) acryloyl group and a hydroxyl group.
CH ₂ = C (R ¹ ) COOR ² (1)
[In the formula, R ¹ represents a hydrogen atom or a methyl group, R ² is a monovalent organic group having a hydroxyl group and having 1 to 8 carbon atoms, (R ³ O) _m H or R ⁴ O [CO (CH _2). ) ₅ O] represents _{n H.} R ³ represents an alkylene group having 2 to ⁴ carbon atoms, R 4 represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15. show. ]
CH ₂ = C (R ⁵ ) CONR ⁶ R ⁷ (2)
[In the formula, R ⁵ represents a hydrogen atom or a methyl group, R ⁶ represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, and R ⁷ represents a hydrogen atom or a monovalent organic group. ]

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

Examples of the monomer represented by the above formula (1) include hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, hydroxybutyl (meth) acrylate, hydroxyhexyl (meth) acrylate and hydroxyoctyl (meth). Hydroxyalkyl (meth) acrylate having a hydroxyalkyl group having 1 to 8 carbon atoms such as acrylate; polyethylene glycol mono (meth) acrylate, polypropylene glycol mono (meth) acrylate, polybutylene glycol mono (meth) acrylate and polyethylene glycol- Polyalkylene glycol mono (meth) acrylates such as polypropylene glycol mono (meth) acrylates; Dihydroxyalkyl (meth) acrylates such as glycerin mono (meth) acrylates; Pluxel FM5 "etc.), caprolactone-modified hydroxyacrylate (manufactured by Daicel Co., Ltd., trade names" Pluxel FA1 "," Pluxel FA10L "etc.) and the like. As the monomer represented by the above formula (1), one of these may be used alone, or two or more thereof may be used in combination.

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

Examples of the monomer represented by the above formula (2) include hydroxy (meth) acrylamide; N-hydroxyethyl (meth) acrylamide, N-hydroxypropyl (meth) acrylamide, N-hydroxybutyl (meth) acrylamide, and the like. Hydroxyalkyl groups having 1 to 8 carbon atoms such as N-hydroxyhexyl (meth) acrylamide and N-hydroxyoctyl (meth) acrylamide, N-methylhydroxyethyl (meth) acrylamide and N-ethylhydroxyethyl (meth) acrylamide. (Meta) acrylamide derivatives having (meth) acrylamide; N, N-di-hydroxyalkyl (meth) acrylamide such as N, N-dihydroxyethyl (meth) acrylamide and N, N-dihydroxyethyl (meth) acrylamide and the like can be mentioned. As the monomer represented by the above formula (2), one of these may be used alone, or two or more thereof may be used in combination.

Examples of the monomer having a formula amount of 200 or less and having a (meth) acryloyl group and a hydroxyl group include hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, hydroxybutyl (meth) acrylate, and hydroxyhexyl ( Hydroxyalkyl (meth) acrylates such as meta) acrylates and hydroxyoctyl acrylates; dialkylene glycol mono (meth) acrylates such as diethylene glycol mono (meth) acrylates and dipropylene glycol monoacrylates; N-hydroxyethyl (meth) acrylamide, N- It has a hydroxyalkyl group having 1 to 8 carbon atoms such as hydroxypropyl (meth) acrylamide, N-hydroxybutyl (meth) acrylamide, N-hydroxyhexyl (meth) acrylamide and N-hydroxyoctyl (meth) acrylamide (meth). ) Acrylate derivative; N-methylol (meth) acrylamide and the like can be mentioned, and one of 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 having 1 to 8 carbon atoms or a formula amount of 200 or less is excellent in that the composition for the electrode mixture layer has an excellent viscosity reducing effect. Hydroxyalkyl (meth) acrylates are preferred. More preferably, it is hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate and hydroxybutyl (meth) acrylate.

The content of the component (b) in the crosslinked polymer can be 0.5% by mass or more with respect to all the structural units of the crosslinked polymer. When the content of the component (b) is 0.5% by mass or more, the viscosity of the composition for the electrode mixture layer (electrode slurry) can be sufficiently lowered, and good coatability can be ensured. It becomes. The lower limit may be 1.0% by mass or more, 3.0% by mass or more, 5.0% by mass or more, or 10% by mass or more. Further, when the content of the component (b) is 50% by mass or less, the amount of the component (a) can be secured as a result, and the dispersion stability of the composition for the electrode mixture layer (electrode slurry) can be improved. Can be sufficient. The upper limit may be 40% by mass or less, 30% by mass or less, or 20% by mass or less. The range may be a range in which such a lower limit and an upper limit are appropriately combined, and is, for example, 0.5% by mass or more and 50% by mass or less, and for example, 1.0% by mass or more and 50% by mass or less. Yes, and can be, for example, 1.0% by mass or more, 30% by mass or less, and the like.

<Other structural units>
In addition to the components (a) and (b), the crosslinked polymer is also referred to as a structural unit derived from other ethylenically unsaturated monomers copolymerizable with the component (a) and the component (b) (hereinafter, also referred to as “component (c)”. ) Can be included. As the component (c), for example, an ethylenically unsaturated monomer compound having an anionic group other than the carboxyl group such as a sulfonic acid group and a phosphoric acid group, or a nonionic ethylenically non-component other than the component (b). Examples thereof include structural units derived from saturated monomers and the like. These structural units are ethylenically unsaturated monomer compounds having anionic groups other than carboxyl groups such as sulfonic acid groups and phosphoric acid groups, or nonionic ethylenically unsaturated single amounts other than the component (b). It can be introduced by copolymerizing a monomer containing a body. Among these, as the component (c), a structural unit derived from a nonionic ethylenically unsaturated monomer is preferable from the viewpoint of obtaining an electrode having good bending resistance, and the binder has excellent binding properties. (Meta) acrylamide and its derivatives, nitrile group-containing ethylenically unsaturated monomers and the like are preferable. Further, when a structural unit derived from a hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less is introduced as the component (c), a strong interaction with the electrode material can be achieved. It can exhibit good binding properties to active materials. This is preferable because a solid and well-integrated electrode mixture layer can be obtained. In particular, a structural unit derived from an alicyclic structure-containing ethylenically unsaturated monomer is preferable.

The ratio of the component (c) can be 0% by mass or more and 49.5% by mass or less with respect to all the structural units of the crosslinked polymer. The ratio of the 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, and 2% by mass or more and 30% by mass or less. It may be 5% by mass or more and 30% by mass or less. Further, when the component (c) is contained in an amount of 1% by mass or more with respect to all the structural units of the crosslinked polymer, the affinity for the electrolytic solution is improved, so that the effect of improving the lithium ion conductivity can be expected.

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

Examples of the nitrile group-containing ethylenically unsaturated monomer include (meth) achlorinitrile; (meth) acrylic acid cyanoalkyl ester compounds such as (meth) cyanomethyl acrylate and (meth) cyanoethyl acrylate; 4-cyanostyrene. , 4-Cyano-α-methylstyrene and other unsaturated aromatic compounds containing cyano groups; vinylidene cyanide and the like, and one of these may be used alone or in combination of two or more. You may use it.

Examples of the alicyclic structure-containing ethylenically unsaturated monomer include cyclopentyl (meth) acrylate, cyclohexyl (meth) acrylate, methylcyclohexyl (meth) acrylate, t-butylcyclohexyl (meth) acrylate, and (meth). ) Cyclodecyl acrylate and cyclododecyl (meth) acrylate and other aliphatic substituents may have (meth) cycloalkyl acrylate; isobornyl (meth) acrylate, adamantyl (meth) acrylate, (meth). ) Dicyclopentenyl acrylate, dicyclopentenyloxyethyl (meth) acrylate, dicyclopentanyl (meth) acrylate, and cyclohexanedimethanol mono (meth) acrylate and cyclodecanedimethanol mono (meth) acrylate, etc. Cycloalkyl polyalcohol mono (meth) acrylate and the like can be mentioned, and one of these may be used alone, or two or more thereof may be used in combination. Among the above, a compound having an acryloyl group as a polymerizable functional group is preferable in that a polymer having a long primary chain length can be obtained because the polymerization rate is high and the binding force of the binder is good.

As the other nonionic ethylenically unsaturated monomer, for example, (meth) acrylic acid ester may be used. Examples of the (meth) acrylic acid ester include (meth) methyl acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, isobutyl (meth) acrylate, and 2-ethylhexyl (meth) acrylate. Meta) Acrylic acid alkyl ester compound;
Aromatic (meth) acrylic acid ester compounds such as (meth) phenyl acrylate, (meth) phenylmethyl acrylate, and (meth) phenylethyl acrylate;
Examples thereof include (meth) acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth) acrylic acid and 2-ethoxyethyl (meth) acrylic acid; and one of these may be used alone. However, two or more types may be used in combination.
From the viewpoint of adhesion to the active material and cycle characteristics, an aromatic (meth) acrylic acid ester compound can be preferably used. Further, from the viewpoint of further improving the lithium ion conductivity and the high rate characteristics, a compound having an ether bond such as a (meth) acrylic acid alkoxyalkyl ester compound is preferable, and 2-methoxyethyl (meth) acrylic acid is more preferable.

Among the nonionic ethylenically unsaturated monomers, a compound having an acryloyl group is preferable in that a polymer having a long primary chain length can be obtained because the polymerization rate is high and the binding force of the binder is good. Further, as the nonionic ethylenically unsaturated monomer, a compound having a homopolymer glass transition temperature (Tg) of 0 ° C. or lower is preferable in terms of improving the bending resistance of the obtained electrode.

The crosslinked polymer may be a salt. The type of salt is not particularly limited, but alkali metal salts such as lithium, sodium and potassium; alkaline earth metal salts such as calcium salt and barium salt; other metal salts such as magnesium salt and aluminum salt; ammonium salt and organic Examples include amine salts. Among these, alkali metal salts and magnesium salts are preferable, and alkali metal salts are more preferable, from the viewpoint that adverse effects on battery characteristics are unlikely to occur.

<Aspect of crosslinked polymer>
The cross-linking method in the cross-linked polymer of the present invention is not particularly limited, and examples thereof include the following methods.
1) Copolymerization of crosslinkable monomers 2) Utilizing chain transfer to polymer chains during radical polymerization 3) After synthesizing a polymer having a reactive functional group, post-crosslinking is performed by adding a crosslinking agent as necessary. Since the polymer has a crosslinked structure, the binder containing the polymer or a salt thereof can have an excellent binding force. Among the above, the method by copolymerization of crosslinkable monomers is preferable because the operation is simple and the degree of crosslinking can be easily controlled.

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

The polyfunctional polymerizable monomer is a compound having two or more polymerizable functional groups such as a (meth) acryloyl group and an alkenyl group in the molecule, and is a polyfunctional (meth) acrylate compound, a polyfunctional alkenyl compound, ( Meta) Examples thereof include compounds having both an acryloyl group and an alkenyl group. These compounds may be used alone or in combination of two or more. Among these, a polyfunctional alkenyl compound is preferable because a uniform crosslinked structure can be easily obtained, and a polyfunctional allyl ether compound having two or more allyl ether groups in the molecule is particularly preferable.

Examples of the polyfunctional (meth) acrylate compound include 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. Di (meth) acrylates of dihydric alcohols such as meta) acrylate; trimethylol propantri (meth) acrylate, tri (meth) acrylate of trimethyl propanethylene oxide modified product, glycerin tri (meth) acrylate, pentaerythritol tri (meth) Tri (meth) acrylates of trivalent or higher polyhydric alcohols such as meta) acrylates and pentaerythritol tetra (meth) acrylates, poly (meth) acrylates such as tetra (meth) acrylates; Bisamides and the like can be mentioned.

Examples of the polyfunctional alkenyl compound include polyfunctional allyl ether compounds such as trimethylolpropanediallyl ether, trimethylolpropanetriallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, tetraallyloxyethane, and polyallyl saccharose; diallyl phthalate and the like. Polyfunctional allyl compound; Examples thereof include polyfunctional vinyl compounds such as divinylbenzene.

Compounds having both (meth) acryloyl group and alkenyl group include allyl (meth) acrylate, isopropenyl (meth) acrylate, butenyl (meth) acrylate, pentenyl (meth) acrylate, and (meth) acrylate. 2- (2-Vinyloxyethoxy) ethyl and the like can be mentioned.

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

The hydrolyzable silyl group-containing vinyl monomer is not particularly limited as long as it is a vinyl monomer having at least one hydrolyzable silyl group. For example, vinyl silanes such as vinyl trimethoxysilane, vinyl triethoxysilane, vinyl methyl dimethoxysilane, vinyl dimethyl methoxysilanen; silyl such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, methyldimethoxysilylpropyl acrylate and the like. Group-containing acrylic acid esters; silyl group-containing methacrylate esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, dimethylmethoxysilylpropyl methacrylate; trimethoxysilylpropyl vinyl ether and the like. Cyril group-containing vinyl ethers; Examples thereof include silyl group-containing vinyl esters such as trimethoxysilyl undecanoate vinyl.

When the crosslinked polymer is crosslinked with a crosslinkable monomer, the amount of the crosslinkable monomer used is 100, which is the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer). It is preferably 0.1 parts by mass or more and 2.0 parts by mass or less, more preferably 0.3 parts by mass or more and 1.5 parts by mass or less, and further preferably 0.5 parts by mass or more and 1 part by mass. It is 5.5 parts by mass or less. When the amount of the crosslinkable monomer used is 0.1 parts by mass or more, the binding property and the stability of the electrode slurry are more preferable. 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 may be 0.02 to 0.7 mol% with respect to the total amount of the monomers other than the crosslinkable monomer (non-crosslinkable monomer). It is preferably 0.03 to 0.4 mol%, more preferably 0.03 to 0.4 mol%.

<Particle size of crosslinked polymer>
In the composition for the electrode binder layer, when the crosslinked polymer does not exist as a mass (secondary agglomerate) having a large particle size and is well dispersed as water-swelled particles having an appropriate particle size, the crosslinked polymer is concerned. A binder containing a polymer is preferable because it can exhibit good binding performance.

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

If the crosslinked polymer is unneutralized or has a neutralization degree of less than 80 mol%, neutralize it to a neutralization degree of 80 to 100 mol% with an alkali metal hydroxide or the like, and measure the particle size when dispersed in water. do it. In general, the crosslinked polymer or a salt thereof often exists as agglomerated particles in which primary particles are associated and aggregated in the state of powder or solution (dispersion liquid). When the particle size when dispersed in water is in the above range, the crosslinked polymer or a salt thereof has extremely excellent dispersibility, and is neutralized to a degree of neutralization of 80 to 100 mol% to be water. By dispersing, the agglomerated particles are disintegrated, and even if it is a dispersion of almost primary particles or a secondary agglomerate, a stable dispersed state in which the particle diameter is within the range of 0.1 to 10.0 μm is formed. It is a thing.

The particle size distribution, which is the value obtained by dividing the volume average particle size of the water-swelled particle size by the number average particle size, is preferably 10 or less, more preferably 5.0 or less, from the viewpoint of bondability and coatability. Yes, more preferably 3.0 or less, still more preferably 1.5 or less. The lower limit of the particle size distribution is usually 1.0.

Further, the particle size (dry particle size) of the crosslinked polymer of the present invention or a salt thereof at the time of drying is preferably in the range of 0.03 μm or more and 3 μm or less in terms of volume-based median diameter. A more preferable range of the particle size is 0.1 μm or more and 1 μm or less, and a more preferable range is 0.3 μm or more and 0.8 μm or less.

The crosslinked polymer or a salt thereof contains an acid group such as a carboxyl group derived from an ethylenically unsaturated carboxylic acid monomer so that the neutralization degree is 20 to 100 mol% in the composition for the electrode mixture layer. It is preferably neutralized and used as a salt embodiment. The degree of neutralization is more preferably 50 to 100 mol%, further preferably 60 to 95 mol%. When the degree of neutralization is 20 mol% or more, the water swelling property is good and the dispersion stabilizing effect is easily obtained, which is preferable. In the present specification, the degree of neutralization can be calculated by calculation from the charged values of a monomer having an acid group such as a carboxyl group and a neutralizing agent used for neutralization. The degree of neutralization was determined by measuring the IR of the crosslinked polymer or a salt thereof after drying the crosslinked polymer or a salt thereof at 80 ° C. for 3 hours under reduced pressure, and measuring the peak derived from the C = O group of the carboxylic acid and the C = of the carboxylate. It can be confirmed from the intensity ratio of the peak derived from the O group.

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

<Water swelling degree of crosslinked polymer>
In the present specification, the degree of water swelling is the weight "(WA) g" of the crosslinked polymer or its salt when dried, and the amount of water absorbed when the crosslinked polymer or its salt is saturated and swelled with water. It is calculated from "(WB) g" based on the following formula (3).
(Water swelling degree) = {(WA) + (WB)} / (WA) (3)

The crosslinked polymer of the present invention or a salt thereof preferably has a water swelling degree of 3.0 or more and 100 or less at pH 8. When the degree of water swelling is within the above range, the crosslinked polymer or a salt thereof swells appropriately in an aqueous medium, so that a sufficient adhesive area to the active material and the current collector is provided when forming the electrode mixture layer. It becomes possible to secure it, and the binding property tends to be good. The degree of water swelling may be, for example, 4.0 or more, 5.0 or more, 7.0 or more, 10 or more, or 15 or more. May be good. When the degree of water swelling is 3.0 or more, the crosslinked polymer or a salt thereof spreads on the surface of the active material or the current collector, and a sufficient adhesive area can be secured, so that good binding property can be obtained. The upper limit of the degree of water swelling at pH 8 may be 95 or less, 90 or less, 80 or less, 60 or less, or 50 or less. When the degree of water swelling exceeds 100, the viscosity of the composition for the electrode mixture layer (electrode slurry) containing the crosslinked polymer or a salt thereof tends to increase, resulting in insufficient uniformity of the mixture layer, which is sufficient. Cohesiveness may not be obtained. In addition, the coatability of the electrode slurry may decrease. The range of the degree of water swelling at pH 8 can be set by appropriately combining the above upper limit value and lower limit value, and is, for example, 4.0 or more and 100 or less, and for example, 5.0 or more and 100 or less. Further, for example, it is 5.0 or more and 80 or less.
The degree of water swelling at pH 8 can be obtained by measuring the degree of swelling of the crosslinked polymer or a salt thereof in water at pH 8. As the water having a pH of 8, for example, ion-exchanged water can be used, and the pH value may be adjusted by using an appropriate acid or alkali, a buffer solution or the like, if necessary. The pH at the time of measurement is, for example, in the range of 8.0 ± 0.5, preferably in the range of 8.0 ± 0.3, more preferably in the range of 8.0 ± 0.2, and further. It is preferably in the range of 8.0 ± 0.1.

A person skilled in the art can adjust the degree of water swelling by controlling the composition and structure of the crosslinked polymer or a salt thereof. For example, the degree of water swelling can be increased by introducing an acidic functional group or a highly hydrophilic structural unit into the crosslinked polymer. Further, by lowering the degree of cross-linking of the cross-linked polymer, the degree of water swelling is usually increased.

<Aqueous viscosity of crosslinked polymer>
The crosslinked polymer of the present invention or a salt thereof preferably has a viscosity of a 3% by mass aqueous solution of 10,000 mPa · s or less. The viscosity of the 3% by mass aqueous solution is more preferably 7,000 mPa · s or less, further preferably 5,000 mPa · s or less, and even more preferably 3,000 mPa · s or less. When the viscosity of the 3 mass% concentrated aqueous solution is 10,000 mPa · s or less, it is suitable from the viewpoint of handleability. The lower limit of the viscosity of the 3% by mass aqueous solution is not particularly limited. The lower limit may be, for example, 10 mPa · s or more, 20 mPa · s or more, 50 mPa · s or more, or 100 mPa · s or more.
In the present specification, the viscosity of the above-mentioned 3% by mass aqueous solution can be obtained by measuring the viscosity at a rotor speed of 12 rpm using a B-type viscometer at a liquid temperature of 25 ° C.

<Method for producing crosslinked polymer or salt thereof>
As the crosslinked polymer, known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization can be used, but in terms of productivity, precipitation polymerization and suspension polymerization (reverse phase suspension polymerization) ) Is preferable. Non-homogeneous polymerization methods such as precipitation polymerization, suspension polymerization, and emulsion polymerization are preferable, and the precipitation polymerization method is more preferable, because better performance can be obtained in terms of binding property and the like.
Precipitation polymerization is a method for producing a polymer by carrying out a polymerization reaction in a solvent that dissolves an unsaturated monomer as a raw material but does not substantially dissolve the polymer to be produced. As the polymerization progresses, the polymer particles become larger due to aggregation and growth, and a dispersion liquid of the polymer particles in which the primary particles of several tens of nm to several hundreds nm are secondarily aggregated to several μm to several tens of μm can be obtained. Dispersion stabilizers can also be used to control the particle size of the polymer.
The secondary aggregation can also be suppressed by selecting a dispersion stabilizer, a polymerization solvent, or the like. In general, precipitation polymerization in which secondary agglutination is suppressed is also called dispersion polymerization.

In the case of precipitation polymerization, a solvent selected from water, various organic solvents and the like can be used as the polymerization solvent in consideration of the type of monomer used and the like. In order to obtain a polymer having a longer primary chain length, it is preferable to use a solvent having a small chain transfer constant.

Specific examples of the polymerization solvent 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. , One of these can be used alone or in combination of two or more. Alternatively, it may be used as a mixed solvent of these and water. In the present invention, the water-soluble solvent refers to a solvent having a solubility in water at 20 ° C. of more than 10 g / 100 ml.
Of the above, the formation of coarse particles and adhesion to the reactor are small and the polymerization stability is good, and the precipitated polymer fine particles are difficult to secondary agglomerate (or even if secondary agglomeration occurs, they dissolve in the aqueous medium. Methyl ethyl ketone and acetonitrile are preferable because they are easy to use), a polymer having a small chain transfer constant and a large degree of polymerization (primary chain length) can be obtained, and the operation is easy during the step neutralization described later. ..

Similarly, in order to allow the neutralization reaction to proceed stably and rapidly in the step neutralization, it is preferable to add a small amount of a highly polar solvent to the polymerization solvent. Such highly polar solvents preferably include water and methanol. The amount of the highly polar solvent used is preferably 0.05 to 20.0% by mass, more preferably 0.1 to 10.0% by mass, still more preferably 0.1 to 5 based on the total mass of the medium. It is 0.0% by mass, more preferably 0.1 to 1.0% by mass. When the proportion of the highly polar solvent is 0.05% by mass or more, the effect on the neutralization reaction is recognized, and when it is 20.0% by mass or less, no adverse effect on the polymerization reaction is observed. Further, in the polymerization of highly hydrophilic ethylenically unsaturated carboxylic acid monomer such as acrylic acid, the polymerization rate is improved when a highly polar solvent is added, and it becomes easy to obtain a polymer having a long primary chain length. Among the highly polar solvents, water is particularly preferable because it has a large effect of improving the polymerization rate.

In the production of the crosslinked polymer or a salt thereof, it is preferable to include a polymerization step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer. For example, the ethylenically unsaturated carboxylic acid monomer from which the component (a) is derived is 50% by mass or more and 99.5% by mass or less, and the specific monomer having a hydroxyl group from which the component (b) is derived is 0. The present invention comprises a polymerization step of polymerizing a monomer component containing 5% by mass or more and 50% by mass or less, and 0% by mass or more and 49.5% by mass or less of another ethylenically unsaturated monomer from which the component (c) is derived. Is preferable. As the specific monomer having a hydroxyl group, one or more monomers selected from the group consisting of the monomers represented by the following formulas (1) and (2), and the formula amount of 200 Examples thereof include monomers having a (meth) acryloyl group and a hydroxyl group.
CH ₂ = C (R ¹ ) COOR ² (1)
[In the formula, R ¹ represents a hydrogen atom or a methyl group, R ² is a monovalent organic group having a hydroxyl group and having 1 to 8 carbon atoms, (R ³ O) _m H or R ⁴ O [CO (CH _2). ) ₅ O] represents _{n H.} R ³ represents an alkylene group having 2 to ⁴ carbon atoms, R 4 represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15. show. ]
CH ₂ = C (R ⁵ ) CONR ⁶ R ⁷ (2)
[In the formula, R ⁵ represents a hydrogen atom or a methyl group, R ⁶ represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, and R ⁷ represents a hydrogen atom or a monovalent organic group. ]

By the above polymerization step, 50% by mass or more and 99.5% by mass or less of the structural unit (component a) derived from the ethylenically unsaturated carboxylic acid monomer is introduced into the crosslinked polymer, and a specific single amount having a hydroxyl group is introduced. A structural unit (component b) derived from the body is introduced in an amount of 0.5% by mass or more and 50% by mass or less. The amount of the ethylenically unsaturated carboxylic acid monomer used is, for example, 30% by mass or more and 99.5% by mass or less, and for example, 50% by mass or more and 99.5% by mass or less, and for example. , 50% by mass or more and 99% by mass or less, and for example, 50% by mass or more and 98% by mass or less, and for example, 50% by mass or more and 95% by mass or less. The amount of the specific monomer having a hydroxyl group used is, for example, 1.0% by mass or more and 50% by mass or less, and for example, 1.0% by mass or more and 30% by mass or less.

Examples of the other ethylenically unsaturated monomer include an ethylenically unsaturated monomer compound having an anionic group other than a carboxyl group such as a sulfonic acid group and a phosphoric acid group, and a component other than the component (b). Examples thereof include nonionic ethylenically unsaturated monomers. Specific examples of the compound include a monomeric compound into which the above-mentioned component (c) can be introduced. The other ethylenically unsaturated monomer may contain 0% by mass or more and 49.5% by mass or less with respect to the total amount of the monomer components, and may contain 1% by mass or more and 40% by mass or less. It may be 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.

The monomer component polymerized in the polymerization step may contain a crosslinkable monomer. As the crosslinkable monomer, as described above, it has a polyfunctional polymerizable monomer having two or more polymerizable unsaturated groups and a self-crosslinkable crosslinkable functional group such as a hydrolyzable silyl group. Examples include monomers.
The amount of the crosslinkable monomer used is preferably 0.1 part by mass or more and 2.0 parts by mass or less with respect to 100 parts by mass of the total amount of the monomers other than the crosslinkable monomer (non-crosslinkable monomer). It is more preferably 0.3 parts by mass or more and 1.5 parts by mass or less, and further preferably 0.5 parts by mass or more and 1.5 parts by mass or less.

The monomer concentration at the time of polymerization is preferably high from the viewpoint of obtaining a polymer having a longer primary chain length. However, if the monomer concentration is too high, the agglutination of the polymer particles tends to proceed, and it becomes difficult to control the heat of polymerization, which may cause the polymerization reaction to run away. Therefore, for example, in the case of the precipitation polymerization method, the monomer concentration at the start of polymerization is generally in the range of about 2 to 40% by mass, preferably in the range of 5 to 40% by mass.
In the present specification, the “monomer concentration” indicates the monomer concentration in the reaction solution at the time when the polymerization is started.

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

The upper limit of the monomer concentration differs depending on the type of monomer and solvent used, the polymerization method, various polymerization conditions, etc., but if the heat of the polymerization reaction can be removed, the precipitation polymerization is as described above. It is about 40%, about 50% for suspension polymerization, and about 70% for emulsion polymerization.

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

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

The amount of the base compound used is preferably in the range of 0.001 mol% or more and 4.0 mol% or less with respect to the ethylenically unsaturated carboxylic acid monomer. When the amount of the basic compound used is within this range, the polymerization reaction can be smoothly carried out. The amount used may be 0.05 mol% or more and 4.0 mol% or less, 0.1 mol% or more and 4.0 mol% or less, and 0.1 mol% or more and 3.0 mol. It may be 0.1 mol% or more and 2.0 mol% or less.
In this specification, the amount of the base compound used represents the molar concentration of the base compound used with respect to the ethylenically unsaturated carboxylic acid monomer, and does not mean the degree of neutralization. That is, the valence of the base compound used is not considered.

As the polymerization initiator, known polymerization initiators such as azo compounds, organic peroxides, and inorganic peroxides can be used, but the polymerization initiator is not particularly limited. The conditions of use can be adjusted by known methods such as heat initiation, redox initiation with a reducing agent, and UV initiation so that the amount of radicals generated is appropriate. In order to obtain a crosslinked polymer having a long primary chain length, it is preferable to set the conditions so that the amount of radicals generated is smaller within the allowable range of the production time.

Examples of the azo compound include 2,2'-azobis (2,4-dimethylvaleronitrile), 2,2'-azobis (N-butyl-2-methylpropionamide), and 2- (tert-butylazo) -2. -Cyanopropane, 2,2'-azobis (2,4,4-trimethylpentane), 2,2'-azobis (2-methylpropane) and the like, and one or more of these are used. be able to.

Examples of the organic peroxide include 2,2-bis (4,5-di-t-butylperoxycyclohexyl) propane (manufactured by Nichiyu Co., Ltd., trade name "Pertetra A") and 1,1-di (t-). Hexylperoxy) cyclohexane (same as "perhexa HC"), 1,1-di (t-butylperoxy) cyclohexane (same as "perhexa C"), n-butyl-4,4-di (t-butylperoxy) Valerate (“Perhexa V”), 2,2-di (t-Butyl Peroxy) Butan (“Perhexa 22”), t-Butyl Hydroperoxide (“Per Butyl H”), Kumen Hydroperoxide (Japan) Made by Yuyu Co., Ltd., trade name "Park Mill H"), 1,1,3,3-tetramethylbutylhydroperoxide ("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 (“Perloyl355”), Dilauroyl peroxide (“Parloyl L”), Bis (4-t-butylcyclohexyl) peroxydicarbonate (“Parloyl TCP”), Di-2-ethylhexyl peroxydicarbonate (“Parloyl OPP”), Di-sec-Butylperoxydicarbonate (“Parloyl SBP”), Cumylperoxyneodecanoate (“Parkmill ND”), 1,1,3,3-Tetramethylbutylperoxyneodecanoate (1,1,3,3-tetramethylbutylperoxyneodecanoate) The same "Perocta ND"), t-hexyl peroxyneodecanoate (the same "Perhexyl ND"), t-butylperoxyneodecanoate (the same "Perbutyl ND"), t-butylperoxyneoheptanoate (Same as "Perbutyl NHP"), t-hexyl peroxypivalate (same as "Perhexyl PV"), t-butyl peroxypivalate (same as "Perbutyl PV"), 2,5-dimethyl-2,5-di (same as "Perbutyl PV") 2-Ethylhexanoyl) hexane (“Perhexa 250”), 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate (“Perocta O”), t-hexylperoxy-2 -Ethylhexanoate ("Perhexyl O"), t-Butylperoxy-2-ethylhexanoate ("Perbutyl O"), t-Butyl peroxylaurate ("Perbutyl L"), t- Butyl Luperoxy-3,5,5-trimethylhexanoate (“Perbutyl 355”), t-hexyl peroxyisopropyl monocarbonate (“Perhexyl I”), t-butyl peroxyisopropyl monocarbonate (“Perbutyl I”) ), T-Butylperoxy-2-ethylhexyl monocarbonate (“Perbutyl E”), t-butylperoxyacetate (“Perbutyl A”), t-hexyl peroxybenzoate (“Perhexyl Z”) and t. -Butyl peroxybenzoate (the same "perbutyl Z") and the like can be mentioned, and one or more of these can be used.

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

The preferable amount of the polymerization initiator used is, for example, 0.001 to 2 parts by mass and, for example, 0.005 to 1 part by mass, when the total amount of the monomer components used is 100 parts by mass. Further, for example, it is 0.01 to 0.1 parts by mass. When the amount of the polymerization initiator used is 0.001 part by mass or more, the polymerization reaction can be stably carried out, and when it is 2 parts by mass or less, a polymer having a long primary chain length can be easily obtained.

The polymerization temperature is preferably 0 to 100 ° C., more preferably 20 to 80 ° C., although it depends on conditions such as the type and concentration of the monomer used. The polymerization temperature may be constant or may change during the polymerization reaction. The polymerization time is preferably 1 minute to 20 hours, more preferably 1 hour to 10 hours.

The crosslinked polymer dispersion obtained through the polymerization step can be obtained in a powder state by subjecting the crosslinked polymer dispersion liquid to a reduced pressure and / or heat treatment in the drying step and distilling off the solvent. At this time, prior to the drying step, for the purpose of removing unreacted monomers (and salts thereof), impurities derived from the initiator, etc., following the polymerization step, a solid-liquid separation step such as centrifugation and filtration, and water. , It is preferable to provide a cleaning step using the same solvent as methanol or a polymerization solvent. When the above cleaning step is provided, even when the crosslinked polymer is secondarily agglutinated, it is easily unraveled at the time of use, and the remaining unreacted monomer is removed, which is good in terms of binding property and battery characteristics. Shows excellent performance.

In this production 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, and an alkaline compound is added to the polymer dispersion obtained in the polymerization step. After neutralizing the polymer (hereinafter, also referred to as “step neutralization”), the solvent may be removed in a drying step. Further, after obtaining the powder of the crosslinked polymer without performing the above-mentioned step neutralization treatment, an alkaline compound is added when preparing the electrode slurry to neutralize the polymer (hereinafter, also referred to as “post-neutralization”). You may say). Of the above, process neutralization is preferable because the secondary aggregates tend to be easily disintegrated.

<Composition for secondary battery electrode mixture layer>
The composition for a secondary battery electrode mixture layer of the present invention contains a binder, an active material, and water containing the crosslinked polymer or a salt thereof.
The amount of the crosslinked polymer or a salt thereof used in the composition for the electrode mixture layer of the present invention is, for example, 0.1 part by mass or more and 20 parts by mass or less with respect to 100 parts by mass of the total amount of the active material. The amount used is, for example, 0.2 parts by mass or more and 10 parts by mass or less, for example, 0.3 parts by mass or more and 8 parts by mass or less, and for example, 0.4 parts by mass or more and 5 parts by mass or less. .. If the amount of the crosslinked polymer and its salt used is less than 0.1 parts by mass, sufficient binding properties may not be obtained. In addition, the dispersion stability of the active material or the like may become insufficient, and the uniformity of the formed mixture layer may decrease. On the other hand, when the amount of the crosslinked polymer and its salt used exceeds 20 parts by mass, the composition for the electrode mixture layer becomes highly viscous and the coatability to the current collector may be lowered. As a result, the obtained mixture layer may have lumps or irregularities, which may adversely affect the electrode characteristics.

When the amount of the crosslinked polymer and its salt used is within the above range, a composition having excellent dispersion stability can be obtained, and a mixture layer having extremely high adhesion to the current collector can be obtained. As a result, the durability of the battery is improved. Further, the crosslinked polymer and its salt show sufficiently high binding property to the active material even in a small amount (for example, 5% by mass or less) and have a carboxy anion, so that the interfacial resistance is small and the high rate characteristic is obtained. An excellent electrode can be obtained.

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

When a positive electrode active material containing a layered rock salt type lithium-containing metal oxide is dispersed in water, the dispersion liquid becomes alkaline by exchanging lithium ions on the surface of the active material and hydrogen ions in water. Therefore, there is a risk that aluminum foil (Al) or the like, which is a general current collector material for positive electrodes, will be corroded. In such a case, it is preferable to neutralize the alkali content eluted from the active material by using an unneutralized or partially neutralized crosslinked polymer as the binder. The amount of the unneutralized or partially neutralized crosslinked polymer used should be equal to or greater than the amount of alkali eluted from the active material so that the amount of unneutralized carboxyl groups in the crosslinked polymer is equal to or greater than the amount of alkali eluted from the active material. Is preferable.

Since all positive electrode active materials have low electrical conductivity, they are generally used by adding a conductive auxiliary agent. Examples of the conductive auxiliary agent include carbon-based materials such as carbon black, carbon nanotubes, carbon fibers, graphite fine powder, and carbon fibers. Among these, carbon black, carbon nanotubes, and carbon fibers are easy to obtain excellent conductivity. , Are preferred. Further, as the carbon black, Ketjen black and acetylene black are preferable. As the conductive auxiliary agent, one of the above types may be used alone, or two or more types may be used in combination. The amount of the conductive auxiliary agent used can be, for example, 0.2 to 20 parts by mass with respect to 100 parts by mass of the total amount of the active material from the viewpoint of achieving both conductivity and energy density. It can be 2 to 10 parts by mass. Further, as the positive electrode active material, a material whose surface is coated with a conductive carbon-based material may be used.

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

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

Further, the crosslinked polymer of the present invention has a structural unit (component (b)) derived from a specific monomer having a hydroxyl group. When the crosslinked polymer has the component (b), it is possible to suppress or reduce an increase in the slurry viscosity of the composition for the electrode mixture layer. The reason why such an effect can be obtained is not clear, but since the crosslinked polymer has a relatively flexible hydroxyl group in the side chain of the polymer, as a result of interacting with the carboxyl group in the polymer, the crosslinked polymer in water It is presumed that this was due to the suppression of swelling. However, the above inference does not limit the scope of the present invention.

Since the carbon-based active material itself has good electrical conductivity, it is not always necessary to add a conductive additive. When a conductive additive is added for the purpose of further reducing resistance, the amount used is, for example, 10% by mass or less, and for example, 5% by weight or less, based on the total amount of the active material, from the viewpoint of energy density. Is.

When the composition for the secondary battery electrode mixture layer is in a slurry state, the amount of the active material used is, for example, in the range of 10 to 75% by mass with respect to the total amount of the composition. If the amount of the active material used is 10% by mass or more, migration of the binder or the like can be suppressed. Further, since it is advantageous in terms of the drying cost of the medium, the amount of the active material used is preferably 30% by mass or more, more preferably 40% by mass or more, and further 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 the secondary battery electrode mixture layer uses water as a medium. Further, for the purpose of adjusting the properties and dryness of the composition, lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, and water-soluble organic solvents such as tetrahydrofuran and N-methylpyrrolidone. It may be used as a mixed solvent with. The proportion of water in the mixing medium is, for example, 50% by mass or more, and for example, 70% by mass or more.

When the composition for the electrode mixture layer is put into a coatingable slurry state, the content of the medium containing water in the entire composition is from the viewpoint of the coating property of the slurry, the energy cost required for drying, and the productivity. Therefore, it can be, for example, in the range of 25 to 90% by mass, and can be, for example, 35 to 70% by mass.

The binder of the present invention may consist only of the crosslinked polymer or a salt thereof, but other binders such as styrene / butadiene latex (SBR), acrylic latex and polyvinylidene fluoride latex. Binder components may be used in combination. When other binder components are used in combination, the amount used may be, for example, 0.1 to 5% by mass or less, and for example, 0.1 to 2% by mass or less, based on the active material. And, for example, it can be 0.1 to 1% by mass or less. If the amount of the other binder component used exceeds 5% by mass, the resistance increases and the high rate characteristics may become insufficient. Among the above, styrene / butadiene latex is preferable because it has an excellent balance between binding property and bending resistance.

The styrene / butadiene latex is a copolymer having a structural unit derived from an aromatic vinyl monomer such as styrene and a structural unit derived from an aliphatic conjugated diene monomer such as 1,3-butadiene. Shows an aqueous dispersion. Examples of the aromatic vinyl monomer include α-methylstyrene, vinyltoluene, divinylbenzene and the like in addition to styrene, and one or more of these can be used. The structural unit derived from the aromatic vinyl monomer in the copolymer can be in the range of, for example, 20 to 60% by mass, and for example, 30 to 50, mainly from the viewpoint of binding property. It can be in the range of% by mass.

As the above-mentioned aliphatic conjugated diene-based monomer, in addition to 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3- Butadiene and the like can be mentioned, and one or more of these can be used. The structural unit derived from the aliphatic conjugated diene-based monomer in the copolymer is, for example, 30 to 70% by mass in that the binding property of the binder and the flexibility of the obtained electrode are good. It can be in the range of 40 to 60% by mass, for example.

In addition to the above-mentioned monomers, styrene / butadiene-based monomers include nitrile group-containing monomers such as (meth) acrylonitrile and (meth) as other monomers in order to further improve performance such as binding properties. ) A carboxyl group-containing monomer such as acrylic acid, itanconic acid, and maleic acid may be used as the copolymerization monomer.
The structural unit derived from the other monomer in the copolymer can be, for example, in the range of 0 to 30% by mass, and can be, for example, in the range of 0 to 20% by mass.

The composition for a secondary battery electrode mixture layer of the present invention contains the above-mentioned active material, water and a binder as essential constituents, and can be obtained by mixing the respective components using known means. The mixing method of each component is not particularly limited, and a known method can be adopted. However, after dry-blending powder components such as an active material, a conductive additive, and crosslinked polymer particles as a binder, water is used. A method of mixing with a dispersion medium such as the above and dispersing and kneading is preferable. When the composition for the electrode mixture layer is obtained in a slurry state, it is preferable to finish the composition in a slurry without poor dispersion or aggregation. As the mixing means, a known mixer such as a planetary mixer, a thin film swirl mixer, or a self-revolving mixer can be used, but a thin film swirl mixer is used because a good dispersion state can be obtained in a short time. It is preferable to do this. When using a thin film swirl mixer, it is preferable to pre-disperse in advance with a stirrer such as a disper. The viscosity of the slurry can be, for example, in the range of 500 to 100,000 mPa · s. From the viewpoint of the coatability of the slurry, the upper limit of the viscosity is preferably 20,000 mPa · s or less, more preferably 10,000 mPa · s or less, still more preferably 6,000 mPa · s or less, and further. It is preferably 5,000 mPa · s or less, even more preferably 4,000 mPa · s or less, and even more preferably 3,000 mPa · s or less. The slurry viscosity can be measured by the method described in Examples under the condition of a liquid temperature of 25 ° C.

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

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

A secondary battery can be manufactured by providing the electrode for the secondary battery of the present invention with a separator and an electrolytic solution. The electrolytic solution may be in the form of a liquid or in the form of a gel.
The separator is arranged between the positive electrode and the negative electrode of the battery, and plays a role of preventing a short circuit due to contact between the two electrodes and holding an electrolytic solution to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane having good ion permeability and mechanical strength. As a specific material, polyolefins such as polyethylene and polypropylene, polytetrafluoroethylene and the like can be used.

As the electrolytic solution, a known one that is generally used depending on the type of active material can be used. In a lithium ion secondary battery, specific solvents include cyclic carbonates having a high dielectric constant and a high dissolving ability of an electrolyte such as propylene carbonate and ethylene carbonate, and low-viscosity chains such as ethylmethyl carbonate, dimethyl carbonate and diethyl carbonate. Examples thereof include form carbonates, which can be used alone or as a mixed solvent. The electrolytic solution is used by dissolving lithium salts such as _{LiPF 6} , LiSbF ₆ , LiBF ₄ , LiClO ₄ , and LiAlO _{4 in these solvents.} In the nickel-metal hydride secondary battery, an aqueous potassium hydroxide solution can be used as the electrolytic solution. The secondary battery is obtained by forming a positive electrode plate and a negative electrode plate partitioned by a separator into a spiral or laminated structure and storing them in a case or the like.

As described above, the binder for the secondary battery electrode disclosed in the present specification exhibits excellent adhesion to the electrode material and excellent adhesion to the current collector in the mixture layer. A secondary battery equipped with an electrode obtained by using the above binder is expected to ensure good integrity and exhibit good durability (cycle characteristics) even after repeated charging and discharging, and is expected to exhibit good durability (cycle characteristics) for automobiles. Suitable for batteries and the like.

Hereinafter, the present invention will be specifically described based on Examples. The present invention is not limited to these examples. In the following, "parts" and "%" mean parts by mass and% by mass unless otherwise specified.
In the following examples, the evaluation of the crosslinked polymer (salt) was carried out by the following method.

(1) Measurement of particle size (water-swelling particle size) in an aqueous medium Weigh 0.25 g of crosslinked polymer salt powder and 49.75 g of ion-exchanged water in a 100 cc container, and use a rotating / revolving stirrer (sinky). It was set in Awatori Rentaro AR-250) manufactured by the company. Next, stirring (rotation speed 2000 rpm / revolution speed 800 rpm, 7 minutes) and defoaming (rotation speed 2200 rpm / revolution speed 60 rpm, 1 minute) were performed to prepare a hydrogel in which the crosslinked polymer salt was swollen in water. ..
Next, the particle size distribution of the hydrogel was measured with a laser diffraction / scattering particle size distribution meter (Microtrac MT-3300EXII, manufactured by Microtrac Bell) using ion-exchanged water as a dispersion medium. When an amount of hydrogel capable of obtaining an appropriate scattered light intensity was added to the place where an excessive amount of dispersion medium was circulated with respect to the hydrogel, the particle size distribution shape measured after several minutes became stable. As soon as stability is confirmed, the particle size distribution is measured, and the volume-based median diameter (D50) as a representative value of the particle size and the particle size distribution represented by (volume-based average particle size) / (number-based average particle size). Got

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

The measuring method will be described below.
The pinch cock 2 in <1> is removed, ion-exchanged water is poured from the upper part of the burette 1 through the silicon tube 3, and the burette 1 to the filter paper 10 for the device are filled with the ion-exchanged water 12. Next, the pinch cock 2 is closed, and air is removed from the polytetrafluoroethylene tube 4 connected to the burette branch pipe with a rubber stopper. In this way, the ion-exchanged water 12 is continuously supplied from the burette 1 to the filter paper 10 for the apparatus.
Next, after removing the excess ion-exchanged water 12 oozing from the filter paper 10 for the device, the reading (a) of the scale of the burette 1 is recorded.
Weigh 0.1 to 0.2 g of the dry powder of the measurement sample, and place it evenly on the center of the sample fixing filter paper 7 as shown in <3>. The sample is sandwiched between another filter paper, and the two filter papers are fastened with the adhesive tape 9 to fix the sample. The filter paper on which the sample is fixed is placed on the device filter paper 10 shown in <2>.
Next, the reading (b) of the scale of the burette 1 is recorded 30 minutes after the lid 11 is placed on the filter paper 10 for the device.
The total (c) of the water absorption of the measurement sample and the water absorption of the two sample fixing filter papers 7 is obtained by (ab). By the same operation, the water absorption of only the two filter papers 7 containing no sample of the crosslinked polymer or a salt thereof is measured (d).
The above operation was performed, and the degree of water swelling was calculated from the following formula. As the solid content used in the calculation, the value measured by the method described later was used.
Water swelling degree = {dry weight of measurement sample (g) + (cd)} / {dry weight of measurement sample (g)}
However, the dry weight (g) of the measurement sample = the weight (g) of the measurement sample × (solid content% ÷ 100)

Here, the method for measuring the solid content will be described below.
Approximately 0.5 g of the sample is collected in a weighing bottle [weight of the weighing bottle = B (g)] whose weight has been measured in advance, and the entire weighing bottle is accurately weighed [W ₀ (g)]. The sample was placed in a windless dryer together with the weighing bottle, dried at 155 ° C. for 45 minutes, and the weight at that time was measured together with the weighing bottle [W ₁ (g)], and the solid content% was determined by the following formula. ..
Solid content (%) = (W ₁ −B) / (W ₀ −B) × 100

(3) Viscosity measurement of 3% by mass aqueous solution Weigh 3.0 parts of crosslinked polymer salt powder and 97 parts of ion-exchanged water in a container, and rotate / revolve agitator (Sinky, Awatori Rentaro AR). -250) was set. Then, stirring (rotation speed 2000 rpm / revolution speed 800 rpm, 7 minutes) and defoaming (rotation speed 2200 rpm / revolution speed 60 rpm, 1 minute) were repeated until there were no unswelled powdery parts, and the crosslinked polymer salt swelled in water. A hydrogel fine particle dispersion in the above state was prepared. After adjusting each of the obtained hydrogel fine particle dispersions to 25 ° C. ± 1 ° C., the viscosity at a rotor speed of 12 rpm was measured using a B-type viscometer (TVB-10 manufactured by Toki Sangyo Co., Ltd.).

≪Manufacturing of crosslinked polymer salt≫
(Production Example 1: Production of crosslinked polymer salt R-1)
A reactor equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen introduction tube was used for the polymerization.
In the reactor, 567 parts of acetonitrile, 2.20 parts of ion-exchanged water, 80 parts of acrylic acid (hereinafter referred to as "AA"), 20 parts of 2-hydroxyethyl acrylate (hereinafter referred to as "HEA"), trimethylolpropane diallyl. 2.0 parts of ether (manufactured by Daiso Corporation, trade name "Neoallyl T-20") and triethylamine corresponding to 1.0 mol% with respect to the above AA were charged. After sufficiently replacing the inside of the reactor with nitrogen, the inside temperature was raised to 55 ° C. by heating. After confirming that the internal temperature was stable at 55 ° C., 2,2'-azobis (2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd., trade name "V-65") 0 as a polymerization initiator. When .040 parts were added, white turbidity was observed in the reaction solution, and this point was set as the polymerization initiation point. The monomer concentration was calculated to be 15.0%. The polymerization reaction was continued while maintaining the internal temperature at 55 ° C. by adjusting the external temperature (water bath temperature), and the internal temperature was raised to 65 ° C. when 6 hours had passed from the polymerization initiation point. The internal temperature was maintained at 65 ° C., cooling of the reaction solution was started 12 hours after the polymerization initiation point, and after the internal temperature dropped to 25 ° C., lithium hydroxide monohydrate (hereinafter, “LiOH”). - 41.9 parts powder of _{H 2} O "hereinafter) was added. After the addition, stirring was continued at room temperature for 12 hours to obtain a slurry-like polymerization reaction solution in which particles of the crosslinked polymer salt R-1 (Li salt, neutralization degree 90 mol%) were dispersed in a medium.

The obtained polymerization reaction solution was centrifuged to settle the polymer particles, and then the supernatant was removed. Then, after redispersing the precipitate in acetonitrile having the same weight as the polymerization reaction solution, the washing operation of precipitating the polymer particles by centrifugation to remove the supernatant was repeated twice. The sediment was recovered and dried at 80 ° C. for 3 hours under reduced pressure conditions to remove volatile components to obtain a powder of the crosslinked polymer salt R-1. Since the crosslinked polymer salt R-1 has hygroscopicity, it was stored in a container having a water vapor barrier property. The powder of the crosslinked polymer salt R-1 was measured by IR, and the degree of neutralization was determined from the intensity ratio of the peak derived from the C = O group of the carboxylic acid and the peak derived from the C = O of the carboxylic acid Li. It was 90 mol%, which was equal to the value calculated from.
When the particle size (water-swelled particle size) of the crosslinked polymer salt R-1 obtained above was measured in an aqueous medium, it was 0.88 μm, and the particle size distribution was calculated to be 1.8. The degree of water swelling was 4.1, and the viscosity of the 3% by mass aqueous solution was less than 15 mPa · s.

(Production Examples 2 to 18 and Comparative Production Example 1: Production of crosslinked polymer salts R-2 to R-19)
The same operation as in Production Example 1 was carried out except that the amount of each raw material charged was as shown in Tables 1 to 3, and a polymerization reaction solution containing crosslinked polymer salts R-2 to R-19 was obtained.
Next, the same operations as in Production Example 1 were carried out for each polymerization reaction solution to obtain powdered crosslinked polymer salts R-2 to R-19. Each crosslinked polymer salt was sealed and stored in a container having a water vapor barrier property.
The physical property values of each of the obtained polymer salts were measured in the same manner as in Production Example 1, and the results are shown in Tables 1 to 3.

(Comparative Production Example 2: Production of Crosslinked Polymer Salt R-20)
A reactor equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen introduction tube was used for the polymerization.
300 parts of methanol, 80 parts of AA, 20 parts of HEA, 0.75 part of neoallyl T-20, and 0.20 part of allyl methacrylate (manufactured by Mitsubishi Gas Chemical Company, hereinafter referred to as "AMA") were charged into the reactor. Then, under stirring, LiOH · _{H 2} O powder 25.6 parts for initial neutralization, and the internal temperature of ion exchange water 1.40 parts was added slowly to be maintained at 40 ° C. or less.
After sufficiently replacing the inside of the reactor with nitrogen, the inside temperature was raised to 68 ° C. by heating. After confirming that the internal temperature was stable at 68 ° C., 0.020 parts of 4,4'-azobiscyanovaleric acid (manufactured by Otsuka Chemical Co., Ltd., trade name "ACVA") was added as a polymerization initiator to the reaction solution. Since white turbidity was observed, this point was set as the polymerization initiation point. The polymerization reaction was continued while adjusting the outside temperature (water bath temperature) so that the solvent gently refluxed. When 3 hours had passed from the polymerization start point, 0.020 part of ACVA was used, and when 6 hours had passed from the polymerization start point. An additional 0.035 part of ACVA was added and the reflux of the solvent was continued. To start cooling of the reaction solution where the polymerization initiation points lapse of 9 hours after the internal temperature dropped to 30 ° C., slowly so the internal temperature LiOH · H 2 _O powder 16.3 parts does not exceed 50 ° C. Was added. After addition of LiOH · H 2 _O powder, obtained 3 hours stirring continued crosslinked polymer salt R-20 (Li salt, degree of neutralization 90 mol%) of the slurry polymerization reaction liquid particles are dispersed in the medium of rice field.

The obtained polymerization reaction solution was centrifuged to settle the polymer particles, and then the supernatant was removed. Then, after redispersing the precipitate in methanol having the same weight as the polymerization reaction solution, the operation of precipitating the polymer particles by centrifugation and removing the supernatant was repeated twice. The precipitate was recovered and dried under reduced pressure at 80 ° C. for 3 hours to remove volatile components to obtain a powder of the crosslinked polymer salt R-20. Since the crosslinked polymer salt R-20 has hygroscopicity, it was stored in a container having a water vapor barrier property. The powder of the crosslinked polymer salt R-20 was measured by IR, and the degree of neutralization was determined from the intensity ratio of the peak derived from the C = O group of the carboxylic acid and the peak derived from the C = O of the carboxylic acid Li. It was 90 mol%, which was equal to the value calculated from.
The physical property values of the obtained crosslinked polymer salt R-20 were measured in the same manner as in Production Example 1, and the results are shown in Table 3.

Details of the compounds used in Tables 1 to 3 are shown below.
AA: Acrylic acid HEA: 2-Hydroxyethyl acrylate 4HBA: 4-Hydroxybutyl acrylate HEAAm: Hydroxyethyl acrylamide MEA: 2-methoxyethyl acrylate AN: Acrylonitrile T-20: Trimethylolpropane diallyl ether (manufactured by Daiso) Product name "Neoaryl T-20")
AMA: Allyl methacrylate TEA: Triethylamine AcN: Acetonitrile MeOH: Methanol V-65: 2,2'-azobis (2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd.)
ACVA: 4,4'-azobiscyanovaleric acid (manufactured by Otsuka Chemical Co., Ltd., trade name "ACVA")
LiOH / H ₂ O: Lithium hydroxide / monohydrate NaOH / H ₂ O: Sodium hydroxide / monohydrate K ₂ CO ₃ : Potassium carbonate

<< Example >>
For a composition for an electrode mixture layer using graphite, which is an active material for a negative electrode, or graphite and silicon particles as an active material, and using each crosslinked polymer salt as a binder, the slurry viscosity and the formed mixture layer. / The peeling strength between the current collectors (that is, the binding property of the binder) was measured. Natural graphite (manufactured by Nippon Graphite Co., Ltd., trade name "CGB-10") was used as the graphite, and Si nanopowder (manufactured by Sigma-Aldrich, particle size <100 nm) was used as the silicon particles.

Example 1
2.4 parts of powdered crosslinked polymer Li salt R-1 is weighed in 100 parts of natural graphite, mixed well in advance, 90 parts of ion-exchanged water is added, and predispersion is performed with a disper, and then a thin film swirl type is used. This dispersion was carried out for 15 seconds under the condition of a peripheral speed of 20 m / sec using a mixer (FM-56-30 manufactured by Primix Corporation) to obtain a slurry-like composition for an electrode mixture layer (electrode slurry). The active material concentration in the electrode slurry was calculated to be 52.0%, and the solid content concentration of the electrode slurry was calculated to be 53.2%.

<Viscosity measurement of electrode slurry>
^{Regarding the electrode slurry obtained above, a slurry having a shear rate of 60 s -1} at 25 ° C. on a CP25-5 cone plate (diameter 25 mm, cone angle 5 °) using a rheometer (Physica MCR301) manufactured by Anton Pearl Co., Ltd. When the viscosity was measured, it was 1,500 mPa · s.

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

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

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

Each example is a composition for an electrode mixture layer containing a binder for a secondary battery electrode belonging to the present invention, and an electrode is prepared using the composition. The composition for each electrode mixture layer (electrode slurry) had a sufficiently low value even under high concentration conditions such that the active material concentration exceeded 50% by mass, and good coatability could be ensured. .. In addition, since the amount of medium (water) removed during drying can be reduced, it can contribute to the improvement of productivity. Further, the peel strength between the mixture layer of the obtained electrode and the current collector was high, and the binding property was excellent.
Focusing on the second structural unit derived from a specific monomer having a hydroxyl group, the slurry viscosity of Example 16 using the crosslinked polymer salt R-14 having a structural unit derived from an acrylamide derivative (5,200 mPa. Compared with s), the slurry viscosity when the crosslinked polymer salt R-2 or R-13 having a structural unit derived from hydroxyalkyl (meth) acrylate was used was 2,200 mPa · s (Example 2). ) And 2,100 mPa · s (Example 15), showing a higher effect of reducing slurry viscosity.

On the other hand, in the crosslinked polymer salt R-19 which does not have a second structural unit derived from a specific monomer having a hydroxyl group, the slurry viscosity is remarkably high at 18,600 mPa · s (Comparative Example 1), and the coatability Even when the concentration was diluted in consideration of the above, the result was that the binding property was insufficient (Comparative Example 2). Further, in the crosslinked polymer salt R-20 exceeding the upper limit of the particle size range specified in the present invention, the slurry viscosity was high and the peel strength was also very low (Comparative Example 3).

The binder for a secondary battery electrode of the present invention exhibits excellent binding properties in the mixture layer. Therefore, a secondary battery provided with an electrode obtained by using the above binder has good durability (cycle characteristics). ), And is expected to be applied to in-vehicle 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. Further, the viscosity of the composition for the electrode mixture layer (electrode slurry) can be reduced even under the condition of high concentration of the active material. Therefore, it is advantageous in terms of reducing drying energy and improving productivity when forming a mixture layer.
The binder for a secondary battery electrode of the present invention can be particularly preferably used for a non-aqueous electrolyte secondary battery electrode, and is particularly useful for a non-aqueous electrolyte lithium ion secondary battery having a high energy density.

Claims (7)

A binder for a secondary battery electrode containing a crosslinked polymer or a salt thereof.
The crosslinked polymer or a salt thereof contains 50% by mass or more and 99.5% by mass or less of the first structural unit derived from the ethylenically unsaturated carboxylic acid monomer with respect to all the structural units thereof, and the formula (1). ) And a second structural unit derived from one or more monomers selected from the group consisting of the monomers represented by the formula (2), containing 0.5% by mass or more and 50% by mass or less.
A binder for a secondary battery electrode, wherein the particle size measured in an aqueous medium after being neutralized to a degree of neutralization of 80 to 100 mol% is 0.1 μm or more and 10 μm or less in terms of volume-based median diameter.
CH ₂ = C (R ¹ ) COOR ² (1)
[In the formula, R ¹ represents a hydrogen atom or a methyl group, R ² is a monovalent organic group having a hydroxyl group and having 1 to 8 carbon atoms, (R ³ O) _m H or R ⁴ O [CO (CH _2). ) ₅ O] represents _{n H.} R ³ represents an alkylene group having 2 to ⁴ carbon atoms, R 4 represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15. show. ]
CH ₂ = C (R ⁵ ) CONR ⁶ R ⁷ (2)
[In the formula, R ⁵ represents a hydrogen atom or a methyl group, R ⁶ represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, and R ⁷ represents a hydrogen atom or a monovalent organic group. ] A binder for a secondary battery electrode containing a crosslinked polymer or a salt thereof.
The crosslinked polymer or a salt thereof contains 50% by mass or more and 99.5% by mass or less of the first structural unit derived from the ethylenically unsaturated carboxylic acid monomer with respect to all the structural units, and the formula amount is 200 or less, containing 0.5% by mass or more and 50% by mass or less of a second structural unit derived from a monomer having a (meth) acryloyl group and a hydroxyl group.
A binder for a secondary battery electrode, wherein the particle size measured in an aqueous medium after being neutralized to a degree of neutralization of 80 to 100 mol% is 0.1 μm or more and 10 μm or less in terms of volume-based median diameter. The binder for a secondary battery electrode according to claim 1 or 2, wherein the second structural unit is a structural unit derived from hydroxyalkyl (meth) acrylate. The binder for a secondary battery electrode according to any one of claims 1 to 3, wherein the crosslinked polymer or a salt thereof has a viscosity of a 3% by mass aqueous solution of 10,000 mPa · s or less. The binder for a secondary battery electrode according to any one of claims 1 to 4, wherein the crosslinked polymer or a salt thereof has a water swelling degree of 3.0 or more and 100 or less at pH 8. A composition for a secondary battery electrode mixture layer, which comprises the binder for a secondary battery electrode, an active material, and water according to any one of claims 1 to 5. A secondary battery electrode comprising an electrode mixture layer containing the binder for a secondary battery electrode according to any one of claims 1 to 5 on the surface of a current collector.

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