WO2020110847A1

WO2020110847A1 - Binder for secondary battery electrode, composition for secondary battery electrode mixture layer, and secondary battery electrode

Info

Publication number: WO2020110847A1
Application number: PCT/JP2019/045360
Authority: WO
Inventors: 朋子仲野; 綾乃日笠山; 直彦斎藤
Original assignee: 東亞合成株式会社
Priority date: 2018-11-27
Filing date: 2019-11-20
Publication date: 2020-06-04
Also published as: JPWO2020110847A1

Abstract

The present invention provides a binder that is for a secondary battery electrode and that can exhibit excellent binding properties better than conventional products, and, even when the concentration of an active material in the electrode mixture layer is high, can reduce the viscosity of electrode slurry. This binder for a secondary battery electrode contains a crosslinked polymer or a salt thereof, wherein the crosslinked polymer or a salt thereof contains 50-99.5 mass% of a first structural unit derived from an ethylenically unsaturated carboxylic acid monomer, and 0.5-50 mass% of a second structural unit derived from a specific monomer having a hydroxy group, with respect to all structural units, and the particle size as measured in an aqueous medium after neutralization to a neutralization degree of 80-100 mol% is 0.1-10 μm in terms of volume-based median diameter.

Description

Binder for secondary battery electrode, composition for secondary battery electrode mixture layer, and secondary battery electrode

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

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

On the other hand, as the use of various secondary batteries expands, demands for improving energy density, reliability and durability tend to increase. For example, specifications for using a silicon-based active material as the negative electrode active material are increasing in order to increase the electric capacity of the lithium-ion secondary battery. However, it is known that the volume change of a silicon-based active material during charge/discharge is large, and peeling or dropping of the electrode mixture layer occurs as it is repeatedly used, and as a result, the capacity of the battery decreases and the cycle characteristics There was a problem that (durability) deteriorates. In order to suppress such a problem, it is generally effective to enhance the binding property of the binder, and for the purpose of improving the durability, studies on improving the binding property of the binder have been conducted.

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

Japanese Patent Laid-Open No. 2000-294247 JP, 2015-18776, A International Publication No. 2016/158939

The binders disclosed in Patent Documents 1 to 3 are all capable of imparting good binding properties, but as the performance of secondary batteries is improved, there is an increasing demand for binders with higher binding power. ..

Further, generally, a secondary battery electrode is obtained by applying an electrode mixture layer composition (electrode slurry) containing an active material and a binder onto 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 improving the drying efficiency of the electrode slurry and improving the productivity of the electrode. However, since the solid content concentration of the electrode slurry generally 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 achieved. It is difficult to secure sex.

The present invention has been made in view of such circumstances, while being able to exhibit superior binding properties than conventional, even when the active material concentration in the electrode mixture layer is high, Provided is a binder for a secondary battery electrode, which can reduce the viscosity of an electrode slurry. In addition, a composition for a secondary battery electrode mixture layer and a secondary battery electrode obtained by using the binder are also provided.

The present inventors have conducted extensive studies to solve the above problems, and 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 binder containing a crosslinked polymer or 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 these findings.

The present invention is as follows.
[1] A binder for a secondary battery electrode containing a crosslinked polymer or a salt thereof, comprising:
The crosslinked polymer or a salt thereof has 50% by mass or more and 99.5% by mass or less of the first structural unit derived from an ethylenically unsaturated carboxylic acid monomer, and the formula (1 ) And a second structural unit derived from one or more kinds of monomers selected from the group consisting of the monomers represented by the formula (2) is contained in an amount of 0.5% by mass or more and 50% by mass or less,
A binder for a secondary battery electrode, which has a volume-based median diameter of 0.1 μm or more and 10 μm or less after being neutralized to a degree of neutralization of 80 to 100 mol %.
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] _n H. In addition, R ³ represents an alkylene group having 2 to 4 carbon atoms, R ⁴ represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15. Represent ]
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, comprising:
The cross-linked polymer or a salt thereof has a first structural unit derived from an ethylenically unsaturated carboxylic acid monomer of 50% by mass or more and 99.5% by mass or less, and a formula weight, based on all structural units thereof. 200 or less, containing 0.5% by mass or more and 50% by mass or less of the second structural unit derived from a monomer having a (meth)acryloyl group and a hydroxyl group,
A binder for a secondary battery electrode, which has a volume-based median diameter of 0.1 μm or more and 10 μm or less after being neutralized to a degree of neutralization of 80 to 100 mol %.
[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 at pH 8 of 3.0 or more and 100 or less.
[6] A composition for a secondary battery electrode mixture layer, comprising the binder for a secondary battery electrode according to any one of [1] to [5], an active material and water.
[7] A secondary battery electrode, which comprises an electrode mixture layer containing the binder for a secondary battery electrode according to any one of [1] to [5] on the surface of the current collector.

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

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

The binder for a secondary battery electrode of the present invention contains a crosslinked polymer or a salt thereof, and can be made into a composition for an electrode mixture layer by mixing with an active material and water. The composition may be in a slurry state in which it can be applied to a 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 of the above composition on the surface of a current collector such as copper foil or aluminum foil.

Each of the binder for a secondary battery electrode, the composition for a secondary battery electrode mixture layer obtained by using the binder, and the secondary battery electrode of the present invention will be described in detail below.
In addition, in this specification, "(meth)acryl" means acryl and/or methacryl, 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 cross-linked polymer>
<First structural unit>
The crosslinked polymer may have a first structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter, also referred to as “component (a)”). When the cross-linked polymer has a carboxyl group by having such a structural unit, the adhesion 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. An electrode with excellent characteristics can be obtained. Further, since the water swelling property is imparted, the dispersion stability of the active material and the like in the composition for electrode mixture layer can be enhanced.
The component (a) can be introduced into the crosslinked polymer by polymerizing a monomer containing an ethylenically unsaturated carboxylic acid monomer, for example. Alternatively, it can also be obtained by (co)polymerizing a (meth)acrylic acid ester monomer and then hydrolyzing it. Moreover, after polymerizing (meth)acrylamide, (meth)acrylonitrile, etc., you may process with a strong alkali, and the method of making an acid anhydride react with the polymer which has a hydroxyl group may be sufficient.

Examples of the ethylenically unsaturated carboxylic acid monomer include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid; (meth)acrylamidoalkyl acid such as (meth)acrylamidohexanoic acid and (meth)acrylamidododecanoic acid. Carboxylic acid; ethylenically unsaturated monomers having a carboxyl group such as succinic acid monohydroxyethyl (meth)acrylate, ω-carboxy-caprolactone mono(meth)acrylate, β-carboxyethyl (meth)acrylate or the like (part thereof) ) Alkali-neutralized products are mentioned, and one of them may be used alone, or two or more thereof may be used in combination. Among the above, a polymer having a long primary chain length is obtained because of a high polymerization rate, and a compound having an acryloyl group as a polymerizable functional group is preferable in that the binding force of the binder is good, and acrylic acid is particularly preferable. is there. 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 for example, the content may be 10% by mass or more and 99.5% by mass or less based on the total structural units of the crosslinked polymer. By containing the 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 an electrode mixture layer becomes good, which is preferable, and may be 60% by mass or more, 70% by mass or more, and 80% by mass. It may be more than. Further, 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 the lower limit and the upper limit are appropriately combined, but 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 50 mass% or more and 99 mass% or less, or 50 mass% or more and 98 mass% or less, or, for example, 50 mass% or more and 95 mass% or less.

<Second structural unit>
The crosslinked polymer of the present invention can 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 electrode mixture layer obtained by using the binder containing the crosslinked polymer can be reduced. The component (b) is introduced into the crosslinked polymer by polymerizing, for example, at least one monomer selected from the group consisting of 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 weight of 200 or less and having a (meth)acryloyl group and a hydroxyl group.
CH ₂ =C(R ¹ )COOR ² (1)
[In the formula, R ¹ represents a hydrogen atom or a methyl group, R ² 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] _n H. In addition, R ³ represents an alkylene group having 2 to 4 carbon atoms, R ⁴ represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15. Represent ]
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 group may be only one, or may be two or more. The monovalent organic group is not particularly limited, and examples thereof include an alkyl group which may have a linear, branched or cyclic structure, and an aryl group and an alkoxyalkyl group. Be done. 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)acrylate; Dihydroxyalkyl (meth)acrylates such as glycerin mono(meth)acrylate; Caprolactone modified hydroxymethacrylate (trade name "Placcel FM1" manufactured by Daicel Corp., " Placcel FM5" and the like), caprolactone-modified hydroxyacrylate (manufactured by Daicel, trade names "Plaxel FA1", "Plaxel 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 the 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 and an alkoxyalkyl group. 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, Hydroxyalkyl groups having 1 to 8 carbon atoms such as N-hydroxyhexyl (meth)acrylamide, N-hydroxyoctyl (meth)acrylamide, N-methylhydroxyethyl (meth)acrylamide and N-ethylhydroxyethyl (meth)acrylamide (Meth)acrylamide derivative having: N,N-dihydroxyethyl(meth)acrylamide and N,N-dihydroxyethyl(meth)acrylamide such as N,N-dihydroxyethyl(meth)acrylamide. 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 weight of 200 or less and having a (meth)acryloyl group and a hydroxyl group include, for example, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, hydroxyhexyl ( Hydroxyalkyl (meth)acrylates such as (meth)acrylate and hydroxyoctyl acrylate; Dialkylene glycol mono(meth)acrylates such as diethylene glycol mono(meth)acrylate and dipropylene glycol monoacrylate; N-hydroxyethyl (meth)acrylamide, N- Having 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 (meta) ) Acrylamide derivative; N-methylol (meth)acrylamide and the like can be mentioned, and one of these may be used alone, or two or more thereof may be used in combination.

The second structural unit is a hydroxyalkyl (meth)acrylate having a hydroxyalkyl group having 1 to 8 carbon atoms, or a compound having a formula weight of 200 or less, from the viewpoint of excellent viscosity reducing effect of the composition for electrode mixture layer. Hydroxyalkyl (meth)acrylate is preferred. More preferred are 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 based on the total 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 electrode mixture layer (electrode slurry) can be sufficiently lowered, and good coatability can be secured. Becomes The lower limit may be 1.0 mass% or more, 3.0 mass% or more, 5.0 mass% or more, and 10 mass% or more. Further, when the content of the component (b) is 50% by mass or less, as a result, the amount of the component (a) can be secured, and the dispersion stability of the composition for 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, and 20% by mass or less. The range may be a range in which the lower limit and the upper limit are appropriately combined, and is, for example, 0.5% by mass or more and 50% by mass or less, or, for example, 1.0% by mass or more and 50% by mass or less. It can be set to 1.0% by mass or more and 30% by mass or less.

<Other structural units>
The cross-linked polymer is a structural unit derived from another ethylenically unsaturated monomer copolymerizable with the component (a) and the component (b) (hereinafter, also referred to as “component (c)”). ) Can be included. Examples of the component (c) 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, or a nonionic ethylenic non-ionic compound other than the component (b). Structural units derived from saturated monomers and the like can be mentioned. These structural units include ethylenically unsaturated monomer compounds having anionic groups other than carboxyl groups such as sulfonic acid groups and phosphoric acid groups, or nonionic ethylenically unsaturated monomers other than 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. (Meth)acrylamide and its derivatives, and nitrile group-containing ethylenically unsaturated monomers are preferred. 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), strong interaction with the electrode material can be achieved, Good binding properties can be exhibited for the active material. This is preferable because it is possible to obtain a firm and good electrode mixture layer. A structural unit derived from an alicyclic structure-containing ethylenically unsaturated monomer is particularly preferable.

The ratio of the component (c) can be 0% by mass or more and 49.5% by mass or less based on the total structural units of the crosslinked polymer. The proportion of 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 based on the total structural units of the crosslinked polymer, the affinity for the electrolytic solution is improved, and the effect of improving lithium ion conductivity can also be expected.

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

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

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 ) A cycloalkyl ester of (meth)acrylic acid which may have an aliphatic substituent such as cyclodecyl acrylate and cyclododecyl (meth)acrylate; isomethanyl (meth)acrylate, adamantyl (meth)acrylate, (meth) ) Dicyclopentenyl acrylate, dicyclopentenyloxyethyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and cyclohexanedimethanol mono(meth)acrylate and cyclodecanedimethanol mono(meth)acrylate Examples thereof include cycloalkyl polyalcohol mono(meth)acrylate, 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 of its high polymerization rate and the binding force of the binder is improved.

As the other nonionic ethylenically unsaturated monomer, for example, (meth)acrylic acid ester may be used. Examples of the (meth)acrylic acid ester include methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. (Meth)acrylic acid alkyl ester compound;
Aromatic (meth)acrylate compounds such as phenyl (meth)acrylate, phenylmethyl (meth)acrylate, and phenylethyl (meth)acrylate;
(Meth)acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth)acrylic acid and 2-ethoxyethyl (meth)acrylic acid; and the like, and one of these may be used alone. However, two or more kinds 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 lithium ion conductivity and 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 of its high polymerization rate and the binding force of the binder is good. As the nonionic ethylenically unsaturated monomer, a compound having a homopolymer glass transition temperature (Tg) of 0° C. or lower is preferable in that the obtained electrode has good bending resistance.

The cross-linked 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 salt Examples thereof include amine salts. Among these, alkali metal salts and magnesium salts are preferable, and alkali metal salts are more preferable, because they are unlikely to adversely affect battery characteristics.

<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 method.
1) Copolymerization of crosslinkable monomer 2) Utilizing chain transfer to polymer chain during radical polymerization 3) After synthesizing a polymer having a reactive functional group, postcrosslinking by adding a crosslinking agent if necessary Since the polymer has a crosslinked structure, the binder containing the polymer or a salt thereof can have excellent binding force. Among the above methods, the method of copolymerizing a crosslinkable monomer is preferable because the operation is simple and the degree of crosslinking can be easily controlled.

<Crosslinkable monomer>
Examples of the crosslinkable monomer include polyfunctional polymerizable monomers having two or more polymerizable unsaturated groups, and monomers having self-crosslinkable crosslinkable functional groups such as hydrolyzable silyl groups. 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, ( Examples thereof include compounds having both a (meth)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 in that a uniform crosslinked structure is 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, polypropylene glycol di( Di(meth)acrylates of dihydric alcohols such as (meth)acrylate; trimethylolpropane tri(meth)acrylate, trimethylolpropane ethylene oxide modified tri(meth)acrylate, glycerin tri(meth)acrylate, pentaerythritol tri( Poly(meth)acrylates such as tri(meth)acrylates of trihydric or higher polyhydric alcohols such as (meth)acrylates and pentaerythritol tetra(meth)acrylates; poly(meth)acrylates such as tetra(meth)acrylate; methylenebisacrylamide, hydroxyethylenebisacrylamide, etc. Examples thereof include bisamides.

Examples of the polyfunctional alkenyl compound include trimethylolpropane diallyl ether, trimethylolpropane triallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, tetraallyloxyethane, polyallyl saccharose and the like; diallyl phthalate and the like. Examples of the polyfunctional allyl compound: a polyfunctional vinyl compound such as divinylbenzene.

Examples of the compound having both a (meth)acryloyl group and an alkenyl group include allyl (meth)acrylate, isopropenyl (meth)acrylate, butenyl (meth)acrylate, pentenyl (meth)acrylate, and (meth)acrylic acid. 2-(2-vinyloxyethoxy)ethyl and the like can be mentioned.

Specific examples of the monomer having a crosslinkable functional group capable of self-crosslinking include hydrolyzable silyl group-containing vinyl monomer, N-methylol (meth)acrylamide, N-methoxyalkyl (meth)acrylate and the like. Is mentioned. These compounds may 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, vinylsilanes such as vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane and vinyldimethylmethoxysilane; silyl compounds such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate and methyldimethoxysilylpropyl acrylate. Group-containing acrylic acid ester; silyl group-containing methacrylic acid ester such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, dimethylmethoxysilylpropyl methacrylate; trimethoxysilylpropyl vinyl ether, etc. Examples of the silyl group-containing vinyl ethers include silyl group-containing vinyl esters such as trimethoxysilyl vinyl undecanoate.

When the cross-linked polymer is cross-linked with a cross-linkable monomer, the amount of the cross-linkable monomer used is 100 total amount of monomers other than the cross-linkable monomer (non-cross-linkable 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, further preferably 0.5 parts by mass or more and 1 part by mass with respect to parts by mass. It is 0.5 parts by mass or less. When the amount of the crosslinkable monomer used is 0.1 part by mass or more, the binding property and the stability of the electrode slurry are improved, which is 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 is 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 more preferably 0.03 to 0.4 mol %.

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

The crosslinked polymer or a salt thereof of the present invention has a particle size (water-swelling particle size) when a polymer having a degree of neutralization based on a carboxyl group of the crosslinked polymer 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 electrode mixture layer composition has a high stability because the electrode mixture layer composition is uniformly present in a suitable size. It becomes 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, since it is difficult to obtain a smooth coated surface, the coatability may be insufficient. 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 diameter may be 9.0 μm or less, may be 8.0 μm or less, may be 7.0 μm or less, may be 5.0 μm or less, and may be 3.0 μm or less. It may be. The range of the particle diameter can be set by appropriately combining the lower limit value and the upper limit value described above, and may be, for example, 0.1 μm or more and 9.0 μm or less, or 0.2 μm or more and 8.0 μm or less. It may be present or may be 0.3 μm or more and 5.0 μm or less.
The water-swollen particle size can be measured by the method described in Examples of this specification.

When the crosslinked polymer is unneutralized or has a degree of neutralization of less than 80 mol%, it is neutralized to a degree of neutralization of 80 to 100 mol% with an alkali metal hydroxide and the particle size when dispersed in water is measured. do it. In general, the cross-linked polymer or its salt often exists as aggregated particles in which primary particles are associated and aggregated in a powder or solution (dispersion) state. When the particle size when dispersed in water is within 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% with water. Agglomerates are unraveled by dispersion, and even if it is a dispersion of almost primary particles or a secondary agglomerate, a stable dispersion state is formed in which the particle diameter is within the range of 0.1 to 10.0 μm. It is a thing.

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

The particle size (dry particle size) of the crosslinked polymer of the present invention or a salt thereof when dried 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 diameter is 0.1 μm or more and 1 μm or less, and a further preferable range is 0.3 μm or more and 0.8 μm or less.

The crosslinked polymer or a salt thereof has an acid group such as a carboxyl group derived from an ethylenically unsaturated carboxylic acid monomer so that the degree of neutralization is 20 to 100 mol% in the composition for an electrode mixture layer. It is preferably neutralized and used as the salt form. 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 from the charged values of the monomer having an acid group such as a carboxyl group and the neutralizing agent used for neutralization. The degree of neutralization is determined by IR measurement of the powder obtained by drying the crosslinked polymer or a salt thereof at 80° C. for 3 hours under reduced pressure, and 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 cross-linked polymer (primary chain length)>
The crosslinked polymer of the present invention has a three-dimensional crosslinked structure and exists as a microgel in a medium such as water. Generally, such a three-dimensional crosslinked polymer is insoluble in a solvent, and therefore its molecular weight cannot be measured. Similarly, it is usually difficult to measure and quantify the primary chain length of a crosslinked polymer.

<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 swollen with water. It is calculated from “(WB)g” based on the following equation (3).
(Water swelling degree)={(WA)+(WB)}/(WA) (3)

The crosslinked polymer or salt thereof of the present invention preferably has a water swelling degree at pH 8 of 3.0 or more and 100 or less. When the degree of water swelling is within the above range, the crosslinked polymer or a salt thereof swells appropriately in an aqueous medium, and therefore, when forming the electrode mixture layer, a sufficient adhesion area to the active material and the current collector is provided. It becomes possible to secure the same, and the binding property tends to be good. The water swelling degree may be, for example, 4.0 or more, 5.0 or more, 7.0 or more, 10 or more, 15 or more. 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 is 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 electrode mixture layer (electrode slurry) containing the crosslinked polymer or a salt thereof tends to increase, resulting in insufficient uniformity of the mixture layer, resulting in sufficient The binding strength may not be obtained. In addition, the coatability of the electrode slurry may be reduced. The range of the degree of water swelling at pH 8 can be set by appropriately combining the upper limit value and the lower limit value, and is, for example, 4.0 or more and 100 or less, or, for example, 5.0 or more and 100 or less, Further, for example, it is 5.0 or more and 80 or less.
The water swelling degree at pH 8 can be obtained by measuring the swelling degree of the crosslinked polymer or its salt 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 a suitable 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, and more preferably in the range of 8.0±0.2. The range is preferably 8.0±0.1.

Those skilled in the art can adjust the degree of water swelling by controlling the composition and structure of the crosslinked polymer or its salt. For example, the degree of water swelling can be increased by introducing an acidic functional group or a structural unit having high hydrophilicity into the crosslinked polymer. Further, even if the degree of crosslinking of the crosslinked polymer is lowered, the degree of swelling in water is usually increased.

<Aqueous solution viscosity of cross-linked polymer>
The crosslinked polymer or salt thereof of the present invention 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 further preferably 3,000 mPa·s or less. When the viscosity of the 3% by mass 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 value may be, for example, 10 mPa·s or more, 20 mPa·s or more, 50 mPa·s or more, and 100 mPa·s or more.
In this specification, the viscosity of the 3% by mass aqueous solution is 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>
For the cross-linked polymer, it is possible to use known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization and emulsion polymerization, but from the viewpoint of productivity, precipitation polymerization and suspension polymerization (reverse phase suspension polymerization) ) Is preferred. Heterogeneous polymerization methods such as precipitation polymerization, suspension polymerization, and emulsion polymerization are preferable, and precipitation polymerization method is more preferable, from the viewpoint of obtaining better performance in terms of binding property and the like.
Precipitation polymerization is a method of producing a polymer by carrying out a polymerization reaction in a solvent that dissolves an unsaturated monomer that is a raw material but does not substantially dissolve a produced polymer. As the polymerization progresses, the polymer particles become larger due to aggregation and growth, and a dispersion liquid of polymer particles in which primary particles of several tens nm to several hundreds nm are secondarily aggregated to several μm to several tens μm is obtained. Dispersion stabilizers can also be used to control the particle size of the polymer.
The secondary aggregation can be suppressed by selecting a dispersion stabilizer or a polymerization solvent. Generally, precipitation polymerization in which secondary aggregation is suppressed is also called dispersion polymerization.

In the case of precipitation polymerization, a solvent selected from water, various organic solvents, etc. can be used as the polymerization solvent in consideration of the types of monomers used. 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 polymerization solvents include water-soluble solvents such as methanol, t-butyl alcohol, acetone, methyl ethyl ketone, acetonitrile and tetrahydrofuran, as well as benzene, ethyl acetate, dichloroethane, n-hexane, cyclohexane and n-heptane. These can be used alone or in combination of two or more. Alternatively, they may be used as a mixed solvent of these and water. In the present invention, the water-soluble solvent refers to one having a solubility in water at 20° C. of more than 10 g/100 ml.
Among the above, the formation of coarse particles and the small adhesion to the reactor are good and the polymerization stability is good, and the precipitated polymer particles are difficult to secondary aggregate (or even if secondary aggregation occurs, it is unraveled in an aqueous medium). Methyl ethyl ketone and acetonitrile are preferable in that they are easy), a polymer having a small chain transfer constant and a large degree of polymerization (primary chain length) can be obtained, and that the operation is easy at the time of the step neutralization described later. ..

Similarly, in the process neutralization, it is preferable to add a small amount of a highly polar solvent to the polymerization solvent so that the neutralization reaction proceeds stably and quickly. As such a highly polar solvent, water and methanol are preferably mentioned. 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, and still more preferably 0.1 to 5%, based on the total mass of the medium. It is 0.0% by mass, and 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. In addition, in the polymerization of highly hydrophilic ethylenically unsaturated carboxylic acid monomer such as acrylic acid, when a highly polar solvent is added, the polymerization rate is improved, and a polymer having a long primary chain length is easily obtained. Among the highly polar solvents, water is particularly preferable because it has a large effect of improving the polymerization rate.

The production of the crosslinked polymer or a salt thereof preferably includes a polymerization step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer. For example, 50% by mass or more and 99.5% by mass or less of the ethylenically unsaturated carboxylic acid monomer from which the component (a) is derived, and 0. 5% by mass or more and 50% by mass or less, and a polymerization step of polymerizing a monomer component containing 0% by mass or more and 49.5% by mass or less of another ethylenically unsaturated monomer from which the component (c) is derived Preferably. As the specific monomer having a hydroxyl group, one or more kinds of monomers selected from the group consisting of monomers represented by the following formulas (1) and (2), and a formula weight of 200 The following are 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] _n H. In addition, R ³ represents an alkylene group having 2 to 4 carbon atoms, R ⁴ represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15. Represent ]
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 polymerization step, a structural unit (component a) derived from an ethylenically unsaturated carboxylic acid monomer is introduced into the crosslinked polymer in an amount of 50% by mass or more and 99.5% by mass or less, and a specific unit amount having a hydroxyl group. 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 mass% or more and 99 mass% or less, and, for example, 50 mass% or more and 98 mass% or less, and for example, 50 mass% or more and 95 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, for example, 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 other than the component (b). Examples thereof include nonionic ethylenically unsaturated monomers. Specific examples of the compound include monomer compounds into which the component (c) described above can be introduced. The other ethylenically unsaturated monomer may be contained in an amount of 0% by mass or more and 49.5% by mass or less, or 1% by mass or more and 40% by mass or less, based on the total amount of the monomer components. 2 mass% or more and 40 mass% or less, 2 mass% or more and 30 mass% or less, and 5 mass% or more and 30 mass% or less.

The monomer component polymerized in the polymerization step may contain a crosslinkable monomer. As described above, the crosslinkable monomer has a polyfunctional polymerizable monomer having two or more polymerizable unsaturated groups, and a self-crosslinkable functional group such as a hydrolyzable silyl group. Examples thereof include monomers.
The amount of the crosslinkable monomer used is preferably 0.1 parts by mass or more and 2.0 parts by mass or less based on 100 parts by mass of the total amount of the monomers (non-crosslinking monomers) other than the crosslinking monomer. And 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 during polymerization is preferably higher from the viewpoint of obtaining a polymer having a longer primary chain length. However, if the monomer concentration is too high, aggregation of the polymer particles is likely to proceed, and it is difficult to control the heat of polymerization, and the polymerization reaction may 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 5 to 40% by mass.
In the present specification, the “monomer concentration” refers to the monomer concentration in the reaction liquid at the time of starting the polymerization.

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 base compound, the polymerization reaction can be stably carried out even under high monomer concentration conditions. The monomer concentration may be 13.0 mass% or more, preferably 15.0 mass% or more, more preferably 17.0 mass% or more, and further preferably 19.0 mass% or more. And more preferably 20.0 mass% or more. The monomer concentration is more preferably 22.0% by mass or more, and even more preferably 25.0% by mass or more. Generally, the higher the monomer concentration at the time of polymerization, the higher the molecular weight can be made, and the polymer having a long primary chain length can be produced.

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

The basic compound is a so-called alkaline compound, and either an inorganic basic compound or an organic basic compound may be used. By carrying out the polymerization reaction in the presence of a base compound, the polymerization reaction can be stably carried out even under a high monomer concentration condition of, for example, more than 13.0 mass %. Further, the polymer obtained by polymerizing at such a high monomer concentration has a high molecular weight (because of its long primary chain length) and is therefore excellent in binding property.
Examples of the inorganic base compound include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide and potassium hydroxide, alkaline earth metal hydroxides such as calcium hydroxide and magnesium hydroxide, and the like. 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 them can be used. Among them, the organic amine compound is preferable from the viewpoint of polymerization stability and binding property of the 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. (Alkyl)alkanolamines such as monoethanolamine, diethanolamine, triethanolamine, propanolamine, dimethylethanolamine and N,N-dimethylethanolamine; pyridine, piperidine, piperazine, 1,8- Cyclic amines such as bis(dimethylamino)naphthalene, morpholine and diazabicycloundecene (DBU); diethylenetriamine, N,N-dimethylbenzylamine and the like, and one or more of them can be used. ..
Among these, when a hydrophobic amine having a long-chain alkyl group is used, larger electrostatic repulsion and steric repulsion are obtained, so that the polymerization stability is secured 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 stabilization effect due to the steric repulsion effect. The value of C/N is preferably 3 or more, more preferably 5 or more, further preferably 10 or more, and further preferably 20 or more.

The amount of the base compound used is preferably 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 base 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% or less. % Or less, or 0.1 mol% or more and 2.0 mol% or less.
In the present 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 basic 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 are not particularly limited. The use conditions can be adjusted by a known method such as thermal initiation, redox initiation using a reducing agent in combination, UV initiation, etc., so that an appropriate amount of radicals is generated. In order to obtain a crosslinked polymer having a long primary chain length, it is preferable to set the conditions so that the radical generation amount is smaller within a range where the production time is allowed.

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

Examples of the organic peroxide include 2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane (trade name “Pertetra A” manufactured by NOF CORPORATION), 1,1-di(t- Hexylperoxy)cyclohexane (the same “Perhexa HC”), 1,1-di(t-butylperoxy)cyclohexane (the same “Perhexa C”), n-butyl-4,4-di(t-butylperoxy) Valerate (the same "Perhexa V"), 2,2-di(t-butylperoxy)butane (the same "Perhexa 22"), t-butyl hydroperoxide (the same "Perbutyl H"), cumene hydroperoxide (Japan Oil company, trade name "Parkmill H"), 1,1,3,3-tetramethylbutyl hydroperoxide (the same "Perocta H"), t-butylcumyl peroxide (the same "Perbutyl C"), di- t-butyl peroxide (the same “perbutyl D”), di-t-hexyl peroxide (the same “perhexyl D”), di(3,5,5-trimethylhexanoyl) peroxide (the same “perloyl 355”), Dilauroyl peroxide (the same "Perloyl L"), bis(4-t-butylcyclohexyl) peroxydicarbonate (the same "Perloyl TCP"), di-2-ethylhexyl peroxydicarbonate (the same "Perloyl OPP"), Di-sec-butyl peroxydicarbonate (the same “Perloyl SBP”), cumyl peroxy neodecanoate (the same “Perkyl ND”), 1,1,3,3-tetramethylbutyl peroxy neodecanoate ( "Perocta ND"), t-hexylperoxy neodecanoate ("Perhexyl ND"), t-butylperoxy neodecanoate ("Perbutyl ND"), t-butylperoxy neoheptanoate (The same "perbutyl NHP"), t-hexyl peroxypivalate (the same "perhexyl PV"), t-butyl peroxypivalate (the same "perbutyl PV"), 2,5-dimethyl-2,5-di( 2-Ethylhexanoyl)hexane (the same "Perhexa 250"), 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate (the same "Perocta O"), t-hexylperoxy-2 -Ethyl hexanoate (the same "Perhexyl O"), t-butyl peroxy-2-ethyl hexanoate (the same "Perbutyl O"), t-butyl peroxylaurate (the same "Perbutyl L"), t- Bu Cylperoxy-3,5,5-trimethylhexanoate (the same "Perbutyl 355"), t-hexyl peroxyisopropyl monocarbonate (the same "Perhexyl I"), t-butyl peroxyisopropyl monocarbonate (the same "Perbutyl I") ), t-butylperoxy-2-ethylhexyl monocarbonate (the same “perbutyl E”), t-butyl peroxyacetate (the same “perbutyl A”), t-hexyl peroxybenzoate (the same “perhexyl Z”) and t -Butyl peroxybenzoate (the same "perbutyl Z") and the like, and one or more of them can be used.

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

The preferred 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 part 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 period of the polymerization reaction. The polymerization time is preferably 1 minute to 20 hours, more preferably 1 hour to 10 hours.

The crosslinked polymer dispersion obtained through the polymerization step can be subjected to reduced pressure and/or heat treatment in the drying step to distill off the solvent to obtain the desired crosslinked polymer in a powder state. At this time, prior to the drying step, in order to remove 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, water, etc. It is preferable to include a washing step using methanol, the same solvent as the polymerization solvent, or the like. When the above-mentioned washing step is provided, even if the cross-linked polymer is secondarily aggregated, it is easy to be understood 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 the present production method, a polymerization reaction of a monomer composition containing an ethylenically unsaturated carboxylic acid monomer is performed in the presence of a basic compound, but an alkali compound is added to the polymer dispersion obtained by the polymerization step. After neutralizing the polymer (hereinafter, also referred to as “step neutralization”), the solvent may be removed in the drying step. Further, after obtaining the powder of the crosslinked polymer without performing the treatment of the step neutralization, an alkali 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, the step neutralization is preferable because the secondary aggregate tends to be easily loosened.

<Composition for secondary battery electrode mixture layer>
The composition for a secondary battery electrode mixture layer of the present invention contains a binder containing the above crosslinked polymer or a salt thereof, an active material, and water.
The amount of the crosslinked polymer or salt thereof used in the composition for an electrode mixture layer of the present invention is, for example, 0.1 parts by mass or more and 20 parts by mass or less based on 100 parts by mass of the total amount of the active material. The amount used is, for example, 0.2 parts by mass or more and 10 parts by mass or less, for example 0.3 parts by mass or more and 8 parts by mass or less, and for example, 0.4 parts by mass or more and 5 parts by mass or less. .. When the amount of the crosslinked polymer and its salt used is less than 0.1 part by mass, sufficient binding properties may not be obtained. In addition, the dispersion stability of the active material and the like becomes insufficient, which may reduce the uniformity of the formed mixture layer. On the other hand, when the amount of the crosslinked polymer and its salt used exceeds 20 parts by mass, the composition for electrode mixture layer may have a high viscosity and the coatability on the current collector may be deteriorated. As a result, the obtained mixture layer may have bumps 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 above-mentioned crosslinked polymer and its salt exhibit a sufficiently high binding property with respect to the active material even in a small amount (for example, 5% by mass or less), and since they have a carboxy anion, they have a low interfacial resistance and high rate characteristics. 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, layered rock salt type and spinel type lithium-containing metal oxides can be used. Specific examples of the layered rock salt-type positive electrode active material include lithium cobalt oxide, lithium nickel oxide, and NCM {Li(Ni _x , Co _y , _M n _z ), x+y+z=1} and NCA called ternary system. Examples include {Li(Ni _1-ab Co _a Al _b )}. Examples of the spinel-type positive electrode active material include lithium manganate. In addition to oxides, phosphates, silicates, sulfur, and the like are used. Examples of phosphates include olivine-type lithium iron phosphate. As the positive electrode active material, one of the above may be used alone, or two or more of them may be used in combination as a mixture or composite.

Note that when a positive electrode active material containing a layered rock salt type lithium-containing metal oxide is dispersed in water, the lithium ions on the surface of the active material and hydrogen ions in the water are exchanged, so that the dispersion liquid shows alkaline. Therefore, aluminum foil (Al), which is a general positive electrode current collector material, may be corroded. In such a case, it is preferable to neutralize the alkali content eluted from the active material by using an unneutralized or partially neutralized crosslinked polymer as the binder. Further, the amount of the unneutralized or partially neutralized crosslinked polymer used is such that the amount of unneutralized carboxyl groups of the crosslinked polymer is equivalent to or more than the amount of alkali eluted from the active material. Is preferred.

Since all positive electrode active materials have low electrical conductivity, they are generally used with a conductive additive added. Examples of the conductive aid include carbon-based materials such as carbon black, carbon nanotubes, carbon fibers, graphite fine powder, and carbon fibers. Among them, carbon black, carbon nanotubes, and carbon fibers are easy to obtain excellent conductivity. , Are preferred. Moreover, as the carbon black, Ketjen black and acetylene black are preferable. As the conductive auxiliary agent, one type described above 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 surface-coated carbon-based material having conductivity 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 more of these may be used in combination. Among these, active materials composed 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 Hard carbon is more preferred. In the case of graphite, spheroidized graphite is preferably used from the viewpoint of battery performance, and the preferable particle size range is, for example, 1 to 20 μm and, for example, 5 to 15 μm. Further, in order to increase the energy density, a metal or metal oxide capable of occluding lithium such as silicon or tin can be used as the negative electrode active material. Among them, silicon has a higher capacity than graphite, and active materials made of silicon-based materials such as silicon, silicon alloys and silicon oxides such as silicon monoxide (SiO) (hereinafter, also referred to as “silicon-based active material”). Can be used. However, the above silicon-based active material has a high capacity, but on the other hand, the volume change due to charge and discharge is large. Therefore, it is preferable to use the carbon-based active material together. In this case, the amount of the silicon active material used is preferably 2 to 80 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 the silicon-based active material and exhibits a good binding property. Therefore, the binder of the present invention exhibits excellent binding properties even when a high-capacity type active material containing a silicon-based active material is used, and is therefore effective for improving the durability of the obtained electrode. Thought to be a thing.

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 electrode mixture layer. Although the reason why such an effect is obtained is not clear, since the cross-linked 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 cross-linked polymer in water. It is speculated that the bulge was suppressed. However, the above speculation 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 auxiliary agent. When a conductive auxiliary agent is added for the purpose of further reducing resistance, the amount used is, for example, 10% by mass or less and 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 mass% with respect to the total amount of the composition. When the amount of the active material used is 10% by mass or more, migration of the binder and the like can be suppressed. In addition, 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 because it is advantageous in terms of the cost of drying the medium. .. On the other hand, if it is 75 mass% or less, the fluidity and coatability of the composition can be secured, and a uniform mixture layer can be formed.

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

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

The binder of the present invention may be composed only of the above-mentioned crosslinked polymer or a salt thereof, but in addition to this, other binders such as styrene/butadiene latex (SBR), acrylic latex and polyvinylidene fluoride latex can be used. You may use a binder component together. When the other binder component is used in combination, the amount thereof can 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 can be, for example, 0.1 to 1% by mass or less. If the amount of the other binder component used exceeds 5% by mass, the resistance may increase and the high rate property may become insufficient. Among the above, a styrene/butadiene-based latex is preferable because it has an excellent balance of binding property and flex 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. An aqueous dispersion is shown. 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 mainly from the viewpoint of binding property, and is, for example, 30 to 50% by mass. It can be in the range of mass%.

Examples of the aliphatic conjugated diene-based monomer include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3- Butadiene etc. are mentioned and 1 type or 2 types 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. The range can be, for example, 40 to 60% by mass.

In addition to the above-mentioned monomers, the styrene/butadiene latex is a nitrile group-containing monomer such as (meth)acrylonitrile, (meth), in order to further improve the performance such as binding property. ) A carboxyl group-containing monomer such as acrylic acid, itanconic acid or maleic acid may be used as a 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 in the range of, for example, 0 to 20% by mass.

The composition for a secondary battery electrode mixture layer of the present invention contains the above active material, water and a binder as essential constituent components, and can be obtained by mixing the components using a known means. The mixing method of each component is not particularly limited, and a known method can be adopted, but after dry blending the powder components such as the active material, the conductive auxiliary agent and the crosslinked polymer particles that are the binder, water is added. A method of mixing and dispersing with a dispersion medium such as, for example, is preferable. When the composition for electrode mixture layer is obtained in the form of a slurry, it is preferable to finish the slurry with no dispersion failure or aggregation. As the mixing means, a known mixer such as a planetary mixer, a thin film swirling mixer and a revolving mixer can be used, but a thin film swirling mixer is used in that a good dispersion state can be obtained in a short time. It is preferable to carry out. When a thin film swirling mixer is used, it is preferable to carry out preliminary dispersion with a stirrer such as a disper in advance. The viscosity of the slurry can be set in the range of 500 to 100,000 mPa·s, for example. 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, and further preferably 6,000 mPa·s or less, It is preferably 5,000 mPa·s or less, more preferably 4,000 mPa·s or less, still more preferably 3,000 mPa·s or less. The slurry viscosity can be measured by the method described in Examples under the condition that the liquid temperature is 25°C.

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

<Secondary battery electrode>
The secondary battery electrode of the present invention comprises a mixture layer formed of the above composition for an electrode mixture layer on the surface of a current collector such as copper or aluminum. The mixture layer is formed by coating the surface of the current collector with the composition for an electrode mixture layer of the present invention, and then drying and removing a medium such as water. The method for applying the composition for electrode mixture layer is not particularly limited, and known methods such as a doctor blade method, a dipping method, a roll coating method, a comma coating method, a curtain coating method, a gravure coating method and an extrusion method are used. Can be adopted. Further, the drying can be performed by a known method such as blowing hot air, reducing pressure, (far) infrared rays, or microwave irradiation.
Usually, the mixture layer obtained after drying is subjected to compression treatment by a die press, a roll press or the like. By compressing, the active material and the binder can be brought into close contact, 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.

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

As the electrolytic solution, a commonly used known electrolytic solution can be used depending on the type of active material. In a lithium ion secondary battery, as a specific solvent, a cyclic carbonate having a high dielectric constant and a high electrolyte dissolving ability such as propylene carbonate and ethylene carbonate, and a chain having a low viscosity such as ethyl methyl carbonate, dimethyl carbonate and diethyl carbonate. Carbonates and the like, and these can be used alone or as a mixed solvent. The electrolytic solution is used by dissolving a lithium salt such as LiPF ₆ , LiSbF ₆ , LiBF ₄ , LiClO ₄ , and LiAlO ₄ in these solvents. In the nickel-hydrogen secondary battery, an aqueous solution of potassium hydroxide can be used as the electrolytic solution. The secondary battery is obtained by accommodating a positive electrode plate and a negative electrode plate, which are partitioned by a separator, in a spiral or laminated structure in a case or the like.

As described above, the binder for secondary battery electrodes disclosed in the present specification exhibits excellent binding properties with the electrode material in the mixture layer and excellent adhesion with the current collector. The secondary battery equipped with the electrode obtained by using the above binder is expected to be able to ensure good integrity and to show good durability (cycle characteristics) even after repeated charging and discharging. 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, "part" and "%" mean "part by mass" and "% by mass" unless otherwise specified.
In the following examples, the crosslinked polymer (salt) was evaluated by the following methods.

(1) Measurement of particle size (water swelling particle size) in an aqueous medium 0.25 g of a powder of a cross-linked polymer salt and 49.75 g of ion-exchanged water were weighed into a 100 cc container, and a rotation/revolution agitator (Sinkey) was used. 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 type particle size distribution analyzer (Microtrack MT-3300EXII, manufactured by Microtrack Bell) using ion exchanged water as a dispersion medium. When an excessive amount of the dispersion medium was circulated with respect to the hydrogel, and an amount of the hydrogel capable of obtaining an appropriate scattered light intensity was added, the particle size distribution shape measured after several minutes became stable. As soon as the stability is confirmed, the particle size distribution is measured, and the volume-based median diameter (D50) as a representative value of the particle diameter and the particle diameter distribution represented by (volume-based average particle diameter)/(number-based average particle diameter) Got

(2) Water swelling degree at pH 8 The water swelling degree 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> A buret 1 provided with a branch pipe for venting air, a pinch cock 2, a silicon tube 3 and a polytetrafluoroethylene tube 4.
<2> A support cylinder 8 having a large number of holes on its bottom is provided on the funnel 5, and a filter paper 10 for the apparatus is further provided thereon.
<3> A sample 6 (measurement sample) of a crosslinked polymer or a salt thereof is sandwiched between two pieces of sample fixing filter paper 7, and the sample fixing filter paper is fixed by an adhesive tape 9. All the filter papers used were ADVANTEC No. 2. The inner diameter is 55 mm.
<1> and <2> are connected by the silicon tube 3.
Further, the height of the funnel 5 and the support cylinder 8 is fixed with respect to the buret 1, and the lower end of the polytetrafluoroethylene tube 4 installed inside the buret branch pipe and the bottom surface of the support cylinder 8 are at the same height. It is set as follows (dotted line in FIG. 1).

The measuring method will be described below.
The pinch cock 2 in <1> is removed, ion-exchanged water is introduced from the upper part of the buret 1 through the silicon tube 3, and the buret 1 and the filter paper 10 for the apparatus 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 buret branch pipe with a rubber stopper. Thus, the ion-exchanged water 12 is continuously supplied from the buret 1 to the device filter paper 10.
Next, after removing the excess ion-exchanged water 12 oozing out from the device filter paper 10, the scale reading (a) of the buret 1 is recorded.
0.1 to 0.2 g of the dry powder of the measurement sample is weighed and uniformly placed on the center portion of the sample fixing filter paper 7 as shown in <3>. The sample is sandwiched with another filter paper, and the two filter papers are fixed 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 buret 1 after 30 minutes has elapsed from the time when the lid 11 was placed on the filter paper 10 for an apparatus is recorded.
The sum (c) of the water absorption amount of the measurement sample and the water absorption amount of the two sample fixing filter papers 7 is obtained by (ab). By the same operation, the water absorption amount of only the two filter papers 7 containing no sample of the crosslinked polymer or its salt is measured (d).
The above operation was performed and the water swelling degree was calculated from the following formula. In addition, the solid content used for the calculation used the value measured by the method mentioned later.
Water swelling degree={dry weight of measurement sample (g)+(cd)}/{dry weight of measurement sample (g)}
However, the dry weight of the measurement sample (g) = the weight of the measurement sample (g) x (solid content% / 100)

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

(3) Viscosity measurement of 3 mass% concentration aqueous solution 3.0 parts of powder of cross-linked polymer salt and 97 parts of ion-exchanged water were weighed in a container, and a rotation/revolution stirrer (Awatori Rentaro AR manufactured by Shinky Co., Ltd.) -250). Next, stirring (rotation speed 2000 rpm/revolution speed 800 rpm, 7 minutes) and defoaming (rotation speed 2200 rpm/revolution speed 60 rpm, 1 minute) are repeated until the unswollen powdery parts disappear, and the crosslinked polymer salt swells in water. A hydrogel fine particle dispersion in this 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.).

<<Production of crosslinked polymer salt>>
(Production Example 1: Production of crosslinked polymer salt R-1)
For the polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen introduction tube was used.
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 Co., Ltd., trade name "Neoallyl T-20") and triethylamine corresponding to 1.0 mol% based on the above AA were charged. After sufficiently replacing the inside of the reactor with nitrogen, it was heated to raise the internal temperature to 55°C. 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”) was used as a polymerization initiator. When 0.040 part was added, white turbidity was observed in the reaction solution, and this point was used as the polymerization initiation point. The monomer concentration was calculated to be 15.0%. The polymerization reaction was continued while adjusting the external temperature (water bath temperature) to maintain the internal temperature at 55°C, and the internal temperature was raised to 65°C when 6 hours 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 initiation point of polymerization, and the internal temperature was lowered to 25°C. Then, lithium hydroxide monohydrate (hereinafter, referred to as "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 liquid 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 liquid was centrifuged to precipitate polymer particles, and then the supernatant was removed. Then, a washing operation of redispersing the precipitate in acetonitrile having the same weight as that of the polymerization reaction solution and then allowing the polymer particles to settle 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 matter, thereby obtaining a powder of crosslinked polymer salt R-1. Since the cross-linked polymer salt R-1 has a hygroscopic property, it was sealed and stored in a container having a water vapor barrier property. The powder of the crosslinked polymer salt R-1 was subjected to IR measurement, 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. 90 mol% equal to the value calculated from
The particle diameter (water-swelling particle diameter) of the crosslinked polymer salt R-1 obtained above in an aqueous medium was measured and found to be 0.88 μm, and the particle diameter 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 operations as in Production Example 1 were carried out except that the amounts of the respective raw materials charged were as shown in Tables 1 to 3 to obtain polymerization reaction liquids containing the crosslinked polymer salts R-2 to R-19.
Then, the same operation as in Production Example 1 was carried out for each polymerization reaction liquid to obtain powdery 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 properties 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)
For the polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen introduction tube was used.
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 Co., Inc., hereinafter referred to as "AMA") were charged into the reactor. Then, with stirring, 25.6 parts of LiOH.H ₂ O powder for initial neutralization and 1.40 parts of ion-exchanged water were slowly added so that the internal temperature was maintained at 40°C or lower.
After sufficiently replacing the inside of the reactor with nitrogen, it was heated to raise the internal temperature to 68°C. After confirming that the internal temperature was stable at 68° C., 0.020 part of 4,4′-azobiscyanovaleric acid (trade name “ACVA” manufactured by Otsuka Chemical Co., Ltd.) was added as a polymerization initiator to the reaction solution. Since white turbidity was observed, this point was set as a polymerization initiation point. The polymerization reaction was continued while adjusting the external temperature (water bath temperature) so that the solvent was gently refluxed, and ACVA was 0.020 part after 3 hours from the polymerization start point and 6 hours after the polymerization start point. An additional 0.035 parts of ACVA was added while continuing to reflux the solvent. After 9 hours from the polymerization start point, cooling of the reaction solution was started, and after the internal temperature dropped to 30°C, slowly add 16.3 parts of LiOH·H ₂ O powder so that the internal temperature does not exceed 50°C. Was added. After the addition of the LiOH.H ₂ O powder, stirring was continued for 3 hours to obtain a slurry-like polymerization reaction liquid in which particles of the crosslinked polymer salt R-20 (Li salt, neutralization degree 90 mol%) were dispersed in a medium. It was

The obtained polymerization reaction liquid was centrifuged to precipitate polymer particles, and then the supernatant was removed. After that, the procedure of redispersing the precipitate in the same amount of methanol as the polymerization reaction liquid and then 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 matter, thereby obtaining a powder of crosslinked polymer salt R-20. Since the crosslinked polymer salt R-20 has a hygroscopic property, it was sealed and stored in a container having a water vapor barrier property. The powder of the crosslinked polymer salt R-20 was subjected to IR measurement, and the degree of neutralization was determined from the intensity ratio of the peak derived from the C═O group of carboxylic acid and the peak derived from C═O of Li carboxylic acid. 90 mol% equal to the value calculated from
Physical properties 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 "Neoallyl T-20")
AMA: Allyl Methacrylate TEA: Triethylamine AcN: Acetonitrile MeOH: Methanol V-65: 2,2'-Azobis(2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd.)
ACVA: 4,4'-azobiscyanovaleric acid (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>>
As the active material, graphite which is an active material for a negative electrode, or graphite and silicon particles, and an electrode mixture layer composition using each cross-linked polymer salt as a binder, its slurry viscosity and the formed mixture layer / The peel 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 graphite, and Si nanopowder (manufactured by Sigma-Aldrich, particle diameter <100 nm) was used as silicon particles.

Example 1
2.4 parts of powdered cross-linked polymer Li salt R-1 was weighed in 100 parts of natural graphite, mixed well in advance, 90 parts of ion-exchanged water was added and predispersed with a disper, and then thin film rotation type The main dispersion was performed for 15 seconds at a peripheral speed of 20 m/sec using a mixer (FM-56-30, manufactured by Primix Co., Ltd.) to obtain a slurry composition for electrode mixture layer (electrode slurry). The active material concentration in the electrode slurry was calculated to be 52.0%, and the solid content concentration in the electrode slurry was calculated to be 53.2%.

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

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

<90° peel strength (binding property)>
The negative electrode obtained above was cut into a strip having a width of 25 mm, and 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 peel test. After the test sample was dried at 60° C. under reduced pressure overnight, 90° peeling (measurement temperature: 23° C.) at a pulling speed of 50 mm/min was performed to measure the peel strength between the mixture layer and the copper foil. 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 operations 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 (P-5, manufactured by FRITSCH), and the obtained mixture was mixed with a powdered crosslinked polymer Li salt. 2.4 parts of R-2 was weighed, mixed well in advance, and then the same operation as in Example 1 was performed to prepare an electrode slurry. The coatability and 90° peel strength of each electrode slurry were evaluated. The results are shown in Tables 4 to 6.

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

On the other hand, in the case of the crosslinked polymer salt R-19 having no second structural unit derived from the specific monomer having a hydroxyl group, the slurry viscosity was remarkably high at 18,600 mPa·s (Comparative Example 1), and the coatability was high. Even when the concentration was reduced in consideration of the above, the result was that the binding property was insufficient (Comparative Example 2). Further, with 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 secondary battery electrode binder of the present invention exhibits excellent binding properties in the mixture layer. Therefore, the secondary battery including the electrode obtained by using the binder has good durability (cycle characteristics). ) Is expected to be applied to in-vehicle secondary batteries. It is also useful for using an active material containing silicon, and is expected to contribute to increasing the capacity of the battery. Furthermore, the viscosity of the composition for electrode mixture layer (electrode slurry) can be reduced even under conditions where the active material concentration is high. Therefore, it is advantageous in reducing the drying energy and improving the productivity when forming the mixture layer.
The binder for a secondary battery electrode of the present invention can be suitably used particularly 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

A binder for a secondary battery electrode containing a crosslinked polymer or a salt thereof,
The crosslinked polymer or a salt thereof has 50% by mass or more and 99.5% by mass or less of the first structural unit derived from an ethylenically unsaturated carboxylic acid monomer, and the formula (1 ) And a second structural unit derived from one or more kinds of monomers selected from the group consisting of the monomers represented by the formula (2) is contained in an amount of 0.5% by mass or more and 50% by mass or less,
A binder for a secondary battery electrode, which has a volume-based median diameter of 0.1 μm or more and 10 μm or less after being neutralized to a degree of neutralization of 80 to 100 mol %.
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] n H. In addition, R 3 represents an alkylene group having 2 to 4 carbon atoms, R 4 represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15. Represent ]
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 cross-linked polymer or a salt thereof has a first structural unit derived from an ethylenically unsaturated carboxylic acid monomer of 50% by mass or more and 99.5% by mass or less, and a formula weight, based on all structural units thereof. 200 or less, containing 0.5% by mass or more and 50% by mass or less of the second structural unit derived from a monomer having a (meth)acryloyl group and a hydroxyl group,
A binder for a secondary battery electrode, which has a volume-based median diameter of 0.1 μm or more and 10 μm or less after being neutralized to a degree of neutralization of 80 to 100 mol %.
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 an aqueous 3% by mass 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 at pH 8 of 3.0 or more and 100 or less.
A composition for a secondary battery electrode mixture layer, comprising the secondary battery electrode binder according to any one of claims 1 to 5, an active material and water.
A secondary battery electrode, which comprises 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 the current collector.