WO2020218049A1

WO2020218049A1 - Secondary battery electrode binder and use therefor

Info

Publication number: WO2020218049A1
Application number: PCT/JP2020/016240
Authority: WO
Inventors: 篤史西脇; 朋子仲野; 直彦斎藤; 綾乃日笠山
Original assignee: 東亞合成株式会社
Priority date: 2019-04-26
Filing date: 2020-04-13
Publication date: 2020-10-29
Also published as: JPWO2020218049A1; JP7428181B2

Abstract

The present invention provides a secondary battery electrode binder with which an electrode slurry can be coated uniformly even when the solid component concentration of the electrode slurry is increased to approximately 50 mass%, and high binding properties and cycle characteristics can be exhibited.　The present invention relates to a secondary battery electrode binder containing a cross-linked polymer or a salt thereof, wherein the cross-linked polymer or salt thereof has a sol fraction of less than 5.0 mass%.

Description

Binder for secondary battery electrode and its use

The present invention relates to a binder for a secondary battery electrode and its use. More specifically, the present invention relates to a binder for a secondary battery electrode, a composition for a mixture layer of a secondary battery electrode obtained by using the binder for a secondary battery electrode, a secondary battery electrode, and a secondary battery.

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

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

For example, Patent Document 1 discloses an acrylic acid-based polymer crosslinked with polyalkenyl ether as a binder for forming a negative electrode mixture layer of a lithium ion secondary battery. Patent Document 2 describes lithium containing a copolymer obtained by polymerizing a monomer composition containing an ethylenically unsaturated carboxylic acid compound and a compound having an ethylenically unsaturated bond exhibiting a specific water solubility. Binders for ion secondary battery electrodes are disclosed. Patent Document 3 describes a crosslinked polymer or a salt thereof containing a specific amount of a structural unit derived from an ethylenically unsaturated carboxylic acid monomer, which is in salt water after the crosslinked polymer is neutralized and water-swelled. A binder for a non-aqueous electrolyte secondary battery electrode containing a crosslinked polymer having a particle size of a specific small particle size range is disclosed.

International Publication No. 2014/06547 International Publication No. 2015/186363 International Publication No. 2017/073589

All of the binders disclosed in Patent Documents 1 to 3 can impart good binding properties. However, according to the studies by the present inventors, an electrode using a composition for an electrode mixture layer (hereinafter, also referred to as “electrode slurry”) containing a binder and an electrode active material disclosed in Patent Documents 1 to 3. When the solid content concentration of the electrode slurry is increased to about 50% by mass in order to improve productivity by shortening the drying time of the solvent, the electrode slurry becomes highly viscous and uniform coating becomes difficult, which in turn makes uniform coating difficult. , Affecting cohesiveness and cycle characteristics could be a problem.

The present invention has been made in view of such circumstances, and even when the solid content concentration of the electrode slurry is increased to about 50% by mass, the electrode slurry can be uniformly coated, and the result is high. Provided is a binder for a secondary battery electrode capable of exhibiting wearability and cycle characteristics. In addition, a composition for a secondary battery electrode mixture layer, a secondary battery electrode, and a secondary battery obtained by using the above binder are also provided.

As a result of diligent studies to solve the above problems, the present inventors have made the crosslinked weight even when the solid content concentration of the electrode slurry using the binder containing the crosslinked polymer is increased to about 50% by mass. We have found that by setting the ratio of the sol content contained in the coalescence to a specific value, uniform coating of the electrode slurry is possible, and high binding properties and cycle characteristics can be exhibited. completed.

The present invention is as follows.
[1] A binder for a secondary battery electrode containing a crosslinked polymer or a salt thereof.
The crosslinked polymer or a salt thereof is a binder for a secondary battery electrode having a sol content of less than 5.0% by mass.
[2] The binder for a secondary battery electrode according to the above [1], wherein the crosslinked polymer or a salt thereof contains 50% by mass or more and 100% by mass or less of a structural unit derived from an ethylenically unsaturated carboxylic acid monomer. ..
[3] The crosslinked polymer or a salt thereof is neutralized to a degree of neutralization of 80 to 100 mol%, and then the particle size measured in an aqueous medium is 0.1 μm or more and 10 μm or less in terms of volume-based median diameter. , The binder for the secondary battery electrode according to the above [1] or [2].
[4] The binder for a secondary battery electrode according to any one of [1] to [3] above, wherein the crosslinked polymer or a salt thereof has a water swelling degree of 3.0 or more and 100 or less at pH 8.
[5] The binder for a secondary battery electrode according to any one of [1] to [4] above, wherein the weight average molecular weight of the sol in terms of polyethylene oxide / polyethylene glycol is 300,000 or less.
[6] A composition for a secondary battery electrode mixture layer containing the binder for the secondary battery electrode, the active material, and water according to any one of the above [1] to [5].
[7] A secondary battery electrode comprising a mixture layer formed from the composition for the secondary battery electrode mixture layer according to the above [6] on the surface of a current collector.
[8] A secondary battery including the secondary battery electrode according to the above [7].

According to the binder for the secondary battery electrode of the present invention, it is possible to obtain a secondary battery having a mixture layer having excellent binding properties and having excellent cycle characteristics. Therefore, even when the active material contains a silicon-based active material, it can contribute to improving the durability of the secondary battery.

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

The binder for a secondary battery electrode of the present invention contains a crosslinked polymer or a salt thereof, and can be mixed with an active material and water to form a secondary battery electrode mixture layer composition (hereinafter, simply "the present composition". It can also be called.). It is preferable that the above composition is an electrode slurry in a slurry state that can be applied to the current collector from the viewpoint of achieving the effect of the present invention, but it is prepared in a wet powder state and applied to the surface of the current collector. It may be possible to cope with press working. The secondary battery electrode of the present invention can be obtained by forming a mixture layer formed from the above composition on the surface of a current collector such as a copper foil or an aluminum foil.

Hereinafter, each of the binder for the secondary battery electrode of the present invention, the composition for the mixture layer of the secondary battery electrode obtained by using the binder, the secondary battery electrode, and the secondary battery will be described in detail.
In addition, in this specification, "(meth) acrylic" means acrylic 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.

The binder of the present invention contains a crosslinked polymer or a salt thereof. The crosslinked polymer may have a structural unit derived from an ethylenically unsaturated carboxylic acid.

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

Examples of the ethylenically unsaturated carboxylic acid monomer include (meth) acrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid; and (meth) acrylamide alkyl such as (meth) acrylamide hexane acid and (meth) acrylamide dodecanoic acid. Carboxylic acid; ethylenically unsaturated monomers having a carboxyl group such as monohydroxyethyl succinate (meth) acrylate, ω-carboxy-caprolactone mono (meth) acrylate, β-carboxyethyl (meth) acrylate, or (partial) thereof. ) Alkaline neutralized products may be mentioned, and one of these may be used alone, or two or more thereof may be used in combination. Among the above, a compound having an acryloyl group as a polymerizable functional group is preferable, and acrylic acid is particularly preferable, in that a polymer having a long primary chain length can be obtained because the polymerization rate is high and the binding force of the binder is good. 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 can be, for example, 10% by mass or more and 100% by mass or less with respect to all the structural units of the crosslinked polymer. By containing the component (a) in such a range, excellent adhesiveness to the current collector can be easily ensured. The lower limit is, for example, 20% by mass or more, for example, 30% by mass or more, and for example, 40% by mass or more. When the lower limit is 50% by mass or more, it is preferable in that the dispersion stability, the binding property and the durability as a secondary battery are good. The lower limit may be 60% by mass or more, 70% by mass or more, or 80% by mass or more. The upper limit is, for example, 99% by mass or less, for example 98% by mass or less, for example 95% by mass or less, and for example 90% by mass or less. The range may be a range in which such a lower limit and an upper limit are appropriately combined, and is, for example, 10% by mass or more and 100% by mass or less, and for example, 20% by mass or more and 100% by mass or less, and for example. It can be 30% by mass or more and 100% by mass or less, and can be, for example, 50% by mass or more and 100% by mass or less, and can be, for example, 50% by mass or more and 99% by mass or less.

<Other structural units>
In addition to the component (a), the crosslinked polymer can contain structural units derived from other ethylenically unsaturated monomers copolymerizable with the component (a) (hereinafter, also referred to as “component (b)”). .. The component (b) is a structural unit derived from a monomer having an ethylenically unsaturated group other than the component (a), and for example, an anionic group other than the carboxyl group such as a sulfonic acid group and a phosphoric acid group. Examples thereof include structural units derived from the ethylenically unsaturated monomer compound having, a nonionic ethylenically unsaturated monomer, and the like. These structural units are an ethylenically unsaturated monomer compound having an anionic group other than a carboxyl group such as a sulfonic acid group and a phosphoric acid group, or a monomer containing a nonionic ethylenically unsaturated monomer. Can be introduced by copolymerizing.

The ratio of the component (b) can be 0% by mass or more and 90% by mass or less with respect to all the structural units of the crosslinked polymer. The ratio of the component (b) may be 1% by mass or more and 60% by mass or less, 2% by mass or more and 50% by mass or less, and 5% by mass or more and 40% by mass or less. It may be 10% by mass or more and 30% by mass or less. Further, when the component (b) is contained in an amount of 1% by mass or more with respect to all the structural units of the crosslinked polymer, the affinity for the electrolytic solution is improved, so that the effect of improving the lithium ion conductivity can be expected.

As the component (b), among the above-mentioned components, a structural unit derived from a nonionic ethylenically unsaturated monomer is preferable from the viewpoint of obtaining an electrode having good bending resistance, and a nonionic ethylenically unsaturated monomer is preferable. Examples of the metric include (meth) acrylamide and its derivatives, an alicyclic structure-containing ethylenically unsaturated monomer, and a hydroxyl group-containing ethylenically unsaturated monomer.

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

Examples of the alicyclic structure-containing ethylenically unsaturated monomer include cyclopentyl (meth) acrylate, cyclohexyl (meth) acrylate, methylcyclohexyl (meth) acrylate, t-butylcyclohexyl (meth) acrylate, and (meth). ) Cyclodecyl acrylate and cyclododecyl (meth) acrylate and other aliphatic substituents may have (meth) cycloalkyl acrylate; isobornyl (meth) acrylate, adamantyl (meth) acrylate, (meth). ) Dicyclopentenyl acrylate, dicyclopentenyloxyethyl (meth) acrylate, dicyclopentanyl (meth) acrylate, and cyclohexanedimethanol mono (meth) acrylate and cyclodecanedimethanol mono (meth) acrylate, etc. Cycloalkyl polyalcohol mono (meth) acrylate and the like can be mentioned, and one of these may be used alone, or two or more thereof may be used in combination.

Examples of the hydroxyl group-containing ethylenically unsaturated monomer include hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, and hydroxybutyl (meth) acrylate, and one of them is used alone. It may be used in combination, or two or more kinds may be used in combination.

As the other nonionic ethylenically unsaturated monomer, for example, (meth) acrylic acid ester may be used. Examples of the (meth) acrylic acid ester include (meth) methyl acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, isobutyl (meth) acrylate, and 2-ethylhexyl (meth) acrylate. Meta) Acrylic acid alkyl ester compound;
(Meta) Acrylic acid aralkyl ester compounds such as (meth) phenyl acrylate, (meth) phenylmethyl acrylate, (meth) phenylethyl acrylate;
Examples thereof include (meth) acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth) acrylic acid and 2-ethoxyethyl (meth) acrylic acid, and one of these may be used alone. Two or more types may be used in combination.

The present polymer or a salt thereof preferably contains a structural unit derived from (meth) acrylamide and its derivative, an alicyclic structure-containing ethylenically unsaturated monomer, etc., in that the binder has excellent binding properties. Further, when a structural unit derived from a hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less is introduced as the component (b), a strong interaction with the electrode material can be achieved. , Can exhibit good binding properties to active materials. As a result, a solid and well-integrated electrode mixture layer can be obtained. Therefore, the above-mentioned "hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less" is particularly selected. An alicyclic structure-containing ethylenically unsaturated monomer is preferable.

The crosslinked polymer or salt thereof according to the present invention preferably contains a structural unit derived from a hydroxyl group-containing ethylenically unsaturated monomer because the cycle characteristics of the obtained secondary battery are improved. It is preferably contained in an amount of 0.5% by mass or more and 70% by mass or less, more preferably 0.5% by mass or more and 50% by mass or less, and further preferably 1.0% by mass or more and 50% by mass or less.

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 due to its high polymerization rate and the binder has a good binding force.

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

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

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

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

Examples of the polyfunctional (meth) acrylate compound include ethylene glycol di (meth) acrylate, propylene glycol di (meth) acrylate, 1,6-hexanediol di (meth) acrylate, polyethylene glycol di (meth) acrylate, and polypropylene glycol di (meth) acrylate. Di (meth) acrylates of dihydric alcohols such as meta) acrylate; trimethylol propantri (meth) acrylate, tri (meth) acrylate of trimethyl propanethylene oxide modified product, glycerin tri (meth) acrylate, pentaerythritol tri (meth) Tri (meth) acrylates of trivalent or higher polyhydric alcohols such as meta) acrylates and pentaerythritol tetra (meth) acrylates, poly (meth) acrylates such as tetra (meth) acrylates; methylenebisacrylamide, hydroxyethylenebisacrylamide, etc. Bisamides and the like can be mentioned.

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

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

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

The hydrolyzable silyl group-containing vinyl monomer is not particularly limited as long as it is a vinyl monomer having at least one hydrolyzable silyl group. For example, vinylsilanes such as vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane, vinyldimethylmethoxysilanen; silyls such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, methyldimethoxysilylpropyl acrylate and the like. Group-containing acrylic acid esters; silyl group-containing methacrylate esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, dimethylmethoxysilylpropyl methacrylate; trimethoxysilylpropyl vinyl ether and the like. Cyril group-containing vinyl ethers; silyl group-containing vinyl esters such as trimethoxysilyl undecanoate vinyl and the like can be mentioned.

When the crosslinked polymer is crosslinked with a crosslinkable monomer, the amount of the crosslinkable monomer used is the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer). On the other hand, it is preferably 0.3 to 5 mol%, more preferably 0.7 to 5 mol%, further preferably 0.7 to 3 mol%, and 1.5 to 3 mol%. Is more preferable. When the amount of the crosslinkable monomer used is 0.3 mol% or more, the binding property and the stability of the electrode slurry are more preferable. If it is 5 mol% or less, the stability of the crosslinked polymer tends to be high.

The amount of the crosslinkable monomer used is preferably 0.7 to 20% by mass, more preferably 1.0 to 15% by mass, and further preferably 1.0 to 15% by mass, based on the total constituent monomers of the crosslinked polymer. Is 2.0 to 10% by mass, more preferably 3.0 to 10% by mass.

<Sol fraction of crosslinked polymer>
The sol fraction of the crosslinked polymer or a salt thereof is less than 5.0% by mass. When the sol fraction is the above, an electrode showing excellent binding property can be obtained, and an excellent effect is also exerted on improving the cycle characteristics of the secondary battery including the electrode. The lower limit of the sol fraction may be 0.1% by mass or more, 0.2% by mass or more, 0.5% by mass or more, or 1.0% by mass or more. It may be.

The sol content in the present specification is mainly composed of a polymer having no three-dimensional crosslinked structure, and the sol content contained in the crosslinked polymer or a salt thereof can be adjusted by a known method. it can. That is, the sol fraction can be set in a desired range by adjusting the type of the cross-linking agent, the amount used thereof, the primary chain length of the polymer, and the like. For example, the sol fraction generally decreases by increasing the amount of the cross-linking agent used or increasing the primary chain length.

<Weight average molecular weight of crosslinked polymer sol>
The weight average molecular weight of the sol content of the crosslinked polymer or a salt thereof is preferably 300,000 or less in terms of polyethylene oxide / polyethylene glycol. It is more preferably 200,000 or less, further preferably 150,000 or less, and even more preferably 100,000 or less. When the weight average molecular weight of the sol is as described above, the electrode slurry is excellent in coatability, an electrode showing excellent binding property can be obtained, and the cycle characteristics of the secondary battery including the electrode are also improved. Has an effect. The lower limit of the weight average molecular weight of the sol content may be 1,000 or more, 2,000 or more, 3,000 or more, or 5,000 or more. It may be 10,000 or more.
The weight average molecular weight of the sol content of the crosslinked polymer is a value in terms of polyethylene oxide / polyethylene glycol, and can be measured by the method described in Examples of the present specification.

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

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

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

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

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

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

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

<Water swelling degree of crosslinked polymer>
In this specification, the water swelling degree of the dry crosslinked polymer or a salt thereof by weight "(W _{A) g",} and the cross-linked polymer or a salt thereof of water absorbed when the saturation swelling with water since the amount "(W _{B) g",} it is calculated based on the following equation.
(Water swelling _{_{degree) = {(W A) +}} (W B)} / (W A)

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

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

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

Specific examples of the dispersion stabilizer include a dispersion stabilizer produced by the living radical polymerization method, a macromonomer type dispersion stabilizer, and a nonionic surfactant. The secondary aggregation can also be suppressed by selecting a dispersion stabilizer, a polymerization solvent, or the like. In general, precipitation polymerization in which secondary 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 type of monomer 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 examples of the polymerization solvent include water-soluble solvents such as methanol, t-butyl alcohol, acetone, methyl ethyl ketone, acetonitrile and tetrahydrofuran, as well as benzene, ethyl acetate, dichloroethane, n-hexane, cyclohexane and n-heptane. , One of these can be used alone or in combination of two or more. Alternatively, it may be used as a mixed solvent of these and water. In the present invention, the water-soluble solvent refers to a solvent having a solubility in water at 20 ° C. of more than 10 g / 100 ml.
Of the above, the formation of coarse particles and adhesion to the reactor are small and the polymerization stability is good, and the precipitated polymer fine particles are difficult to secondary agglomerate (or even if secondary agglomeration occurs, they dissolve in the aqueous medium. Methyl ethyl ketone and acetonitrile are preferable because they are easy to use), a polymer having a small chain transfer constant and a large degree of polymerization (primary chain length) can be obtained, and the operation is easy during the step neutralization described later. ..

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

In the production of the crosslinked polymer or a salt thereof, it is preferable to include a polymerization step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer. For example, the ethylenically unsaturated carboxylic acid monomer from which the component (a) is derived is 10% by mass or more and 100% by mass or less, and the other ethylenically unsaturated monomer from which the component (b) is derived is 0% by mass. It is preferable to include a polymerization step of polymerizing a monomer component containing% or more and 90% by mass or less.
By the polymerization step, 10% by mass or more and 100% by mass or less of the structural unit (component (a)) derived from the ethylenically unsaturated carboxylic acid monomer is introduced into the crosslinked polymer, and other ethylenically unsaturated products are introduced. The structural unit (component (b)) derived from the monomer is introduced in an amount of 0% by mass or more and 90% by mass or less. The amount of the ethylenically unsaturated carboxylic acid monomer used is, for example, 20% by mass or more and 100% by mass or less, and for example, 30% by mass or more and 100% by mass or less, and for example, 50% by mass. As mentioned above, it is 100% by mass or less.

Examples of the other ethylenically unsaturated monomer include an ethylenically unsaturated monomer compound having an anionic group other than a carboxyl group such as a sulfonic acid group and a phosphoric acid group, and a nonionic ethylenically. Examples include unsaturated monomers. Specific examples of the compound include monomeric compounds into which the above-mentioned component (b) can be introduced. The other ethylenically unsaturated monomer may contain 0% by mass or more and 90% by mass or less, or 1% by mass or more and 60% by mass or less, based on the total amount of the monomer components. It may be 10% by mass or more and 50% by mass or less, or 10% by mass or more and 30% by mass or less. Further, the above-mentioned crosslinkable monomer may be used in the same manner.

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

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

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

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

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

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

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

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

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

Examples of the organic peroxide include 2,2-bis (4,5-di-t-butylperoxycyclohexyl) propane (manufactured by Nichiyu Co., Ltd., trade name "Pertetra A") and 1,1-di (t-). Hexylperoxy) cyclohexane (“Perhexa HC”), 1,1-di (t-butylperoxy) cyclohexane (“Perhexa C”), n-butyl-4,4-di (t-butylperoxy) Valerate (same as "Perhexa V"), 2,2-di (t-butylperoxy) butane (same as "Perhexa 22"), t-butylhydroperoxide (same as "Perbutyl H"), Kumen Hydroperoxide (same as "Perbutyl H") "Park Mill H"), 1,1,3,3-tetramethylbutylhydroperoxide ("Perocta H"), t-butylcumyl peroxide ("Perbutyl C"), di-t-butyl peroxide ("Peroctyl H") "Perbutyl D"), di-t-hexyl peroxide ("Perhexyl D"), di (3,5,5-trimethylhexanoyl) peroxide ("Perloyl355"), dilauroyl peroxide (same "Perloyl355") "Parloyl L"), bis (4-t-butylcyclohexyl) peroxydicarbonate (same as "parloyl TCP"), di-2-ethylhexyl peroxydicarbonate (same as "parloyl OPP"), di-sec-butylper Oxydicarbonate (“Perloyl SBP”), Kumil Peroxyneodecanoate (“Parkmill ND”), 1,1,3,3-Tetramethylbutylperoxyneodecanoate (“Perocta ND”) , T-Hexyl peroxyneodecanoate (same as "Perhexyl ND"), t-butyl peroxyneodecanoate (same as "Perbutyl ND"), t-butyl peroxyneeoheptanoeate (same as "Perbutyl NHP") ), T-Hexyl peroxypivalate (“Perhexyl PV”), t-Butyl peroxypivalate (“Perbutyl PV”), 2,5-dimethyl-2,5-di (2-ethylhexanoyl) Hexane (same as "Perhexa 250"), 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate (same as "Perocta O"), t-hexylperoxy-2-ethylhexanoate (same as "Perocta O") "Perhexyl O"), t-Butylperoxy-2-ethylhexanoate ("Perbutyl O"), t-Butylperoxylaurate ("Perbutyl L"), t-Butyl Peroxy -3,5,5-trimethylhexanoate (“Perbutyl 355”), t-hexyl peroxyisopropyl monocarbonate (“Perhexyl I”), t-butyl peroxyisopropyl monocarbonate (“Perbutyl I”) , T-Butylperoxy-2-ethylhexyl monocarbonate (“Perbutyl E”), t-Butylperoxyacetate (“Perbutyl A”), t-hexyl peroxybenzoate (“Perhexyl Z”) and t- Butyl peroxybenzoate (the same "perbutyl Z") and the like can be mentioned, and one or more of these can be used.

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

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

The polymerization temperature depends on conditions such as the type and concentration of the monomer used, but is preferably 0 to 100 ° C., more preferably 20 to 80 ° C., the polymerization temperature may be constant, or the polymerization reaction. It may change over time.

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

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

2. Composition for Secondary Battery Electrode Mixing Layer The composition for a secondary battery electrode mixing layer of the present invention contains a binder, an active material and water containing the crosslinked polymer or a salt thereof.
The amount of the crosslinked polymer or its salt used in the present composition is, for example, 0.1 part by mass or more and 20 parts by mass or less with respect to 100 parts by mass of the total amount of the active material. The amount used is, for example, 0.2 parts by mass or more and 10 parts by mass or less, for example, 0.3 parts by mass or more and 8 parts by mass or less, and for example, 0.4 parts by mass or more and 5 parts by mass or less. .. If the amount of the crosslinked polymer and its salt used is less than 0.1 parts by mass, sufficient binding properties may not be obtained. In addition, the dispersion stability of the active material or the like may become insufficient, and the uniformity of the formed mixture layer may decrease. On the other hand, when the amount of the crosslinked polymer and its salt used exceeds 20 parts by mass, the composition may have a high viscosity and the coatability on the current collector may be lowered. 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 crosslinked polymer and its salt show sufficiently high binding property to the active material even in a small amount (for example, 5% by mass or less) and have a carboxy anion, so that the interfacial resistance is small and the high rate characteristic is obtained. An excellent electrode can be obtained.

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

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

Since all positive electrode active materials have low electrical conductivity, they are generally used by adding a conductive auxiliary agent. Examples of the conductive auxiliary agent include carbon-based materials such as carbon black, carbon nanotubes, carbon fibers, graphite fine powder, and carbon fibers. Among these, carbon black, carbon nanotubes, and carbon fibers are easy to obtain excellent conductivity. , Are preferred. Further, as the carbon black, Ketjen black and acetylene black are preferable. As the conductive auxiliary agent, one of the above types may be used alone, or two or more types may be used in combination. The amount of the conductive auxiliary agent used can be, for example, 0.2 to 20 parts by mass with respect to 100 parts by mass of the total amount of the active material from the viewpoint of achieving both conductivity and energy density, and for example, 0. It can be 2 to 10 parts by mass. Further, as the positive electrode active material, a material whose surface is coated with a conductive carbon-based material may be used.

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

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

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

Since the carbon-based active material itself has good electrical conductivity, it is not always necessary to add a conductive additive. When a conductive additive is added for the purpose of further reducing resistance, the amount used is, for example, 10 parts by mass or less with respect to 100 parts by mass of the total amount of the active material, and for example, 5 from the viewpoint of energy density. It is less than a part by mass.

When the composition is in a slurry state, the amount of the active material used is, for example, in the range of 10 to 75% by mass with respect to the total amount of the composition. If the amount of the active material used is 10% by mass or more, migration of the binder or the like can be suppressed. Further, since it is advantageous in terms of the drying cost of the medium, the amount of the active material used is preferably 30% by mass or more, more preferably 40% by mass or more, and further preferably 50% by mass or more. .. On the other hand, if it is 75% by mass or less, the fluidity and coatability of the present composition can be ensured, and a uniform mixture layer can be formed.

This composition uses water as a medium. Further, for the purpose of adjusting the properties and dryness of the composition, lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, and water-soluble organic substances such as tetrahydrofuran and N-methylpyrrolidone. It may be a mixed solvent with a solvent. The proportion of water in the mixing medium is, for example, 50% by mass or more, and for example, 70% by mass or more.

When the present composition is put into a coatingable slurry state, the solid content concentration is not limited to about 50% by mass, and the content of the medium containing water in the entire composition is the coating of the electrode slurry. From the viewpoint of workability, energy cost required for drying, and productivity, it can be, for example, in the range of 25 to 90% by mass, for example, 35 to 70% by mass, and for example, 45. It can be up to 70% by mass.

The binder of the present invention may consist only of the crosslinked polymer or a salt thereof, but other than this, other styrene / butadiene latex (SBR), acrylic latex, polyvinylidene fluoride latex and the like. Binder components may be used in combination. When other binder components are used in combination, the amount used may be, for example, 0.1 to 5 parts by mass or less, and for example, 0.1 to 2 parts by mass, based on 100 parts by mass of the total amount of the active material. It can be less than or equal to parts, and can be, for example, 0.1 to 1 part by mass or less. If the amount of the other binder component used exceeds 5% by mass, the resistance increases and the high rate characteristics may become insufficient. Among the above, styrene / butadiene latex is preferable from the viewpoint of excellent balance between binding property and bending resistance.

The styrene / butadiene latex is a copolymer having a structural unit derived from an aromatic vinyl monomer such as styrene and a structural unit derived from an aliphatic conjugated diene monomer such as 1,3-butadiene. Shows an aqueous dispersion. Examples of the aromatic vinyl monomer include α-methylstyrene, vinyltoluene, divinylbenzene and the like in addition to styrene, and one or more of these can be used. The structural unit derived from the aromatic vinyl monomer in the copolymer can be, for example, in the range of 20 to 70% by mass, and for example, 30 to 60, mainly from the viewpoint of binding property. It can be in the range of% by mass.

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

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

The present composition contains the above-mentioned active material, water and a binder as essential constituents, and can be obtained by mixing the respective components using known means. The mixing method of each component is not particularly limited, and a known method can be adopted. However, after dry-blending powder components such as an active material, a conductive additive, and crosslinked polymer particles as a binder, water is used. A method of mixing with a dispersion medium such as the above and dispersing and kneading is preferable. When the present composition is obtained in a slurry state, it is preferable to finish the electrode slurry without poor dispersion or aggregation. As the mixing means, a known mixer such as a planetary mixer, a thin film swirl mixer, or a self-revolving mixer can be used, but a thin film swirl mixer is used because a good dispersion state can be obtained in a short time. It is preferable to do this. When using a thin film swirl mixer, it is preferable to pre-disperse in advance with a stirrer such as a disper. The viscosity of the electrode slurry can be, for example, in the range of 500 to 10,000 mPa · s. From the viewpoint of coatability of the electrode slurry, the upper limit of the viscosity is preferably 7,000 mPa · s or less, more preferably 6,000 mPa · s or less, and further preferably 5,000 mPa · s or less. It is more preferably 4,000 mPa · s or less, and even more preferably 3,000 mPa · s or less. The slurry viscosity can be measured by the method described in Examples under the condition of a liquid temperature of 25 ° C.

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

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

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

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

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

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

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

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

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

Here, the method for measuring the solid content will be described below.
Approximately 0.5 g of the sample is collected in a weighing bottle [weight of the weighing bottle = B (g)] whose weight has been measured in advance, and the entire weighing bottle is accurately weighed [W ₀ (g)]. The sample was placed in a windless dryer together with the weighing bottle, dried at 155 ° C. for 45 minutes, and the weight at that time was measured together with the weighing bottle [W ₁ (g)], and the solid content was determined by the following formula.
Solid content (%) = (W _1- B) / (W _0- B) x 100

(3) Measurement of sol fraction A crosslinked polymer aqueous dispersion having a concentration of 0.5% by mass was prepared, centrifuged at 4000 rpm for 30 minutes, and then the supernatant was collected. This supernatant was diluted 15-fold with ion-exchanged water to prepare a measurement sample. The Li (or Na, K) concentration in the measurement sample was measured by ICP emission spectrometry (ICAP 7600 / Thermo Fisher Scientific).
By the following equation, Li calculated from the charged values of Table 1 (or Na, K) concentrations Li of the measurement sample for _{[M 1} (ppm)] (or Na, K) concentration _{[M 2} (ppm) ], The sol fraction was calculated.
Sol fraction (%) = M ₂ / M ₁ x 100

(4) Measurement of Weight Average Molecular Weight of Sol A crosslinked polymer aqueous dispersion having a concentration of 0.5% by mass was prepared, centrifuged at 4000 rpm for 30 minutes, and then the supernatant was collected. Under the conditions described below, the supernatant was measured by aqueous gel permeation chromatography (GPC) to obtain a weight average molecular weight (Mw) in terms of polyethylene oxide / polyethylene glycol.

○ Water-based GPC measurement conditions Column: Tosoh TSKgel GMPW x 1 (estimated exclusion limit: 50 million)
Polyethylene Oxide / Polyethylene Glycol Standard Material Molecular Weight:
(Mp: peak top molecular weight, Mw: weight average molecular weight, Mn: number average molecular weight)
Sample 1: Mp = 969000, Mw = 1020000, Mn = 884000
Sample 2: Mp = 450000, Mw = 480000, Mn = 398000
Sample 3: Mp = 222000, Mw = 220,000, Mn = 197,000
Sample 4: Mp = 86200, Mw = 87800, Mn = 75800
Sample 5: Mp = 42700, Mw = 40100, Mn = 30700
Sample 6: Mp = 18600, Mw = 17900, Mn = 14900
Sample 7: Mp = 6690, Mw = 6550, Mn = 6170
Sample 8: Mp = 2100, Mw = 2090, Mn = 2030
Sample 9: Mp = 599, Mw = 601 and Mn = 560
Sample 10: Mp = 238, Mw = 238, Mn = 238
Calibration curve: Prepared by a cubic formula using the Mp value of the above-mentioned polyethylene oxide / polyethylene glycol standard substance molecular weight.
Solvent: 0.1M NaNO ₃ aqueous solution Temperature: 40 ° C
Detector: RI
Flow velocity: 500 μL / min

≪Manufacturing of dispersion stabilizer≫
(Synthesis Example 1: Production of Dispersant A)
2.0 parts of S, S-dibenzyltrithiocarbonate (manufactured by Bоrrоn Mоclecal, trade name "DBTTC"), 2,2'-azobis (2-methyl) as a RAFT agent in a 1 L flask equipped with a stirrer and a thermometer. Butyronitrile) (manufactured by Japan Finechem Company, Inc., trade name "ABN-E") 0.41 part, styrene 75 parts, acrylonitrile 25 parts, and anisole 67 parts were charged, sufficiently degassed by nitrogen bubbling, and kept at a constant temperature of 80 ° C. Living radical polymerization was started in the tank. After 4 hours, the reaction was stopped by cooling to room temperature. The above polymerization solution was reprecipitated and purified from methanol / water = 90/10 (vоl%) and vacuum dried to obtain a polymer (hereinafter referred to as “dispersant A”).
As a result of measurement by THF-based gel permeation chromatography (GPC) under the conditions described below, the polystyrene-equivalent number average molecular weight (Mn) was 11,900, and the weight average molecular weight was (Mn) 15,500. Mw / Mn was 1.30.

○ THF-based GPC measurement conditions Column: Tosoh TSKgel SuperMultipore HZ-M x 4 Solvent: Tetrahydrofuran Temperature: 40 ° C
Detector: RI
Flow velocity: 600 μL / min

≪Manufacturing of crosslinked polymer salt≫
(Production Example 1: Production of crosslinked polymer salt R-1)
A reactor equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen introduction tube was used for the polymerization.
In the reactor, 580 parts of acetonitrile, 2.38 parts of ion-exchanged water, 100 parts of acrylic acid (hereinafter referred to as "AA"), trimethylpropandiallyl ether (manufactured by Daiso, trade name "Neoallyl T-20") 2 0.0 parts and 1.0 mol% of tridodecylamine (manufactured by BASF, trade name "Alamine 304-1") were charged with respect to the above AA. After sufficiently replacing the inside of the reactor with nitrogen, the inside temperature was raised to 30 ° C. by heating. After confirming that the internal temperature was stable at 30 ° C., 2,2'-azobis (4-methoxy-2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd., trade name "V-" was used as a polymerization initiator. 70 ”) When 0.90 parts were added, white turbidity was observed in the reaction solution, and this point was set as the polymerization initiation point. The monomer concentration was calculated to be 15.0%.
The polymerization reaction is continued while adjusting the external temperature (water bath temperature) to maintain the internal temperature at 30 ° C., and when 24 hours have passed from the polymerization initiation point, the reaction solution is cooled and the internal temperature reaches 20 ° C. after reduction, lithium hydroxide monohydrate (hereinafter, "LiOH · _{H 2} O" hereinafter.) was added 52.4 parts of powder. After the addition, stirring was continued at room temperature for 12 hours to obtain a slurry-like polymerization reaction solution in which particles of the crosslinked polymer salt R-1 (Li salt, neutralization degree 90 mol%) were dispersed in a medium.

The obtained polymerization reaction solution was centrifuged to settle the polymer particles, and then the supernatant was removed. Then, after redispersing the precipitate in acetonitrile having the same weight as the polymerization reaction solution, the washing operation of precipitating the polymer particles by centrifugation to remove the supernatant was repeated twice. The sediment was recovered and dried under reduced pressure at 80 ° C. for 3 hours to remove volatile components to obtain a powder of the crosslinked polymer salt R-1. Since the crosslinked polymer salt R-1 has hygroscopicity, it was stored in a container having a water vapor barrier property. The powder of the crosslinked polymer salt R-1 was measured by IR, and the degree of neutralization was determined from the intensity ratio of the peak derived from the C = O group of the carboxylic acid and the peak derived from the C = O of the carboxylic acid Li. It was 90 mol%, which was equal to the value calculated from.
When the particle size (water-swelled particle size) of the crosslinked polymer salt R-1 obtained above was measured in an aqueous medium, it was 2.6 μm, and the particle size distribution was calculated to be 1.3. The degree of water swelling was 83, the sol fraction was 4.7% by mass, and the Mw of the sol was 55,000.

(Production Examples 2 to 16 and Comparative Production Example 1: Production of crosslinked polymer salts R-2 to R-17)
The same operation as in Production Example 1 was carried out except that the amount of each raw material charged was as shown in Table 1, to obtain a polymerization reaction solution containing crosslinked polymer salts R-2 to R-17.
Next, the same operations as in Production Example 1 were carried out for each polymerization reaction solution to obtain powdered crosslinked polymer salts R-2 to R-17. Each crosslinked polymer salt was sealed and stored in a container having a water vapor barrier property.
Physical property values of each of the obtained polymer salts were measured in the same manner as in Production Example 1, and the results are shown in Table 1.

Details of the compounds used in Table 1 are shown below.
AA: CEAA acrylate: β-carboxyethyl acrylate (manufactured by SIGMA-ALDRICH, trade name "2-carboxyethyl acrylate")
IBXA: Isobornyl acrylate HEA: 2-Hydroxyethyl acrylate T-20: Trimethylolpropane diallyl ether (manufactured by Daiso, trade name "Neoallyl T-20")
TMPTMA: Trimethylolpropane Trimethacrylate TDA: Tridodecylamine AcN: Acetonitrile MeOH: Methanol V-70: 2,2'-azobis (4-methoxy-2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd.)
LiOH ・ H ₂ O: Lithium hydroxide ・ Monohydrate Na ₂ CO ₃ : Sodium carbonate K ₂ CO ₃ : Potassium carbonate

Example 1
An electrode using the crosslinked polymer salt R-1 was prepared and evaluated. The specific procedure and evaluation method are shown below.

<Preparation of composition for electrode mixture layer>
Prepare a SiOx (0.8 <x <1.2) surface coated with 10% carbon by the CVD method (hereinafter referred to as "Si-based active material"), and mix graphite and Si-based active material. Was used as an active material. As the binder, a mixture of crosslinked polymer salt R-1, styrene / butadiene latex (SBR) and carboxymethyl cellulose (CMC) was used.
Graphite: Si-based active material: R-1: SBR: CMC = 90: 10: 1.0: 1 using water as a diluting solvent so that the solid content concentration of the composition for the negative electrode mixture layer is 50% by mass. With a mass ratio of 0.0: 1.0 (solid content), T.I. K. The mixture was mixed for 2 hours using a hibis mix to prepare a composition for an electrode mixture layer (electrode slurry) in a slurry state.

(Measurement of viscosity of electrode slurry)
When the slurry viscosity of the electrode slurry obtained above was measured with an E-type viscometer (40 rpm, 25 ° C.), it was 3,910 mPa · s.

<Manufacturing of negative electrode plate>
The electrode slurry was applied to both sides of a copper foil (thickness: 20 μm) and dried to form a mixture layer. Then, after rolling so that the thickness of the mixture layer was 27 μm and the packing density was 1.3 g / cm ³ , the negative electrode plate was obtained by punching 3 cm square.

(Paintability)
The coatability of the electrode slurry in the production of the negative electrode electrode plate was evaluated based on the following criteria, and was evaluated as “◯”.
(Evaluation criteria)
◯: No abnormal appearance such as streaks or bumps is observed on the surface.
Δ: Slight abnormalities in appearance such as streaks and bumps are observed on the surface.
X: Appearance abnormalities such as streaks and bumps are noticeably observed on the surface.

(90 ° peel strength (bonding property))
A mixture layer surface of a 100 mm × 25 mm size negative electrode electrode plate was attached to a 120 mm × 30 mm acrylic plate via a double-sided tape (Nichiban Co., Ltd. Nystack NW-20). Using a small tabletop tester (FGS-TV and FGP-5) manufactured by Nidec-Shimpo Co., Ltd., 90 ° peeling was performed at a measurement temperature of 25 ° C. and a tensile speed of 50 mm / min, and peeling between the mixture layer and the copper foil. The binding property was evaluated by measuring the strength. The peel strength was as high as 24.2 N / m, which was good.

(Evaluation of battery characteristics)
Next, a battery containing the negative electrode plate using the crosslinked polymer salt R-1 was prepared and evaluated. The specific procedure and evaluation method are shown below.

<Manufacturing of positive electrode plate>
In N-methylpyrrolidone (NMP) solvent, 100 parts of lithium iron phosphate (LFP) as the positive electrode active material, 0.2 parts of carbon nanotubes as the conductive agent, 2 parts of Ketjen black, and vapor layer carbon fiber (VGCF). ) 0.6 part was mixed and added, and polyvinylidene fluoride (PVDF) was mixed as a binder for the electrode composition to prepare a composition for a positive electrode. A mixture layer was formed by applying the positive electrode composition to an aluminum current collector (thickness: 15 μm) and drying it. Then, after rolling so that the thickness of the mixture layer was 88 μm and the packing density was 3.1 g / cm ³ , the mixture was punched 3 cm square to obtain a positive electrode plate.

(Battery production)
A laminated lithium ion secondary battery was produced using the above positive electrode, negative electrode and separator. As the electrolytic solution, a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (DEC) at a volume ratio of 25:75 and LiPF ₆ dissolved at a concentration of 1.0 mol / liter was used.

(Evaluation of cycle characteristics)
The lithium-ion secondary battery of the laminated cell created above is charged / discharged at a charge / discharge rate of 0.2 C under the conditions of 2.7 to 3.4 V by CC discharge, and the initial capacity is C _0. Was measured. Furthermore, repeated charging and discharging under 25 ° C. environment was measured capacitance C ₅₀ after 50 cycles. The cycle characteristic (ΔC) calculated by the following formula was 75%, and the cycle characteristic based on the following criteria was evaluated as “B”. The higher the value of ΔC, the better the cycle characteristics.
ΔC = C ₅₀ / C ₀ × 100 (%)
(Evaluation criteria)
A: Charge / discharge capacity retention rate is 80% or more B: Charge / discharge capacity retention rate is 70% or more and less than 80% C: Charge / discharge capacity retention rate is 60% or more and less than 70% D: Charge / discharge capacity retention rate is less than 60%

Examples 2 to 16 and Comparative Example 1
An electrode slurry was prepared by performing the same operation as in Example 1 except that the crosslinked polymer salt was used as shown in Table 2, and the slurry viscosity was measured. In addition, the coatability of the electrode slurry, the peel strength of the negative electrode plate obtained by using the electrode slurry, and the cycle characteristics of the battery were evaluated. The results are shown in Table 2.

In each example, a secondary battery electrode and a secondary battery are manufactured using a binder for a secondary battery electrode belonging to the present invention. As is clear from the results of Examples 1 to 16, even if the solid content concentration of the electrode slurry is 50% by mass, good coatability is exhibited, and all the electrodes have high peel strength and are excellent. It showed a good binding property. Further, it was confirmed that the charge / discharge retention rate of the battery was 60% or more, and the cycle characteristics were excellent. Among these, when focusing on the amount of the crosslinkable monomer used in the crosslinked polymer salts having the same constituent monomer and the same degree of neutralization (R-1 to R-5), the amount of the crosslinked polymer used is large. It was found that the charge / discharge retention rate of the batteries using the salts R-2 to R-5 was very high, and the batteries had excellent durability. Further, the charge / discharge retention rate of the battery using the crosslinked polymer salts R-8 and R-9 containing a specific amount of the structural unit derived from the hydroxyl group-containing ethylenically unsaturated monomer is very high, and excellent durability is obtained. It turned out to have.

On the other hand, in Comparative Example 1 using the crosslinked polymer salt R-17 having a sol fraction of 5% by mass or more, the coatability and the bondability of the electrodes were inferior, and the cycle characteristics were insufficient.

The binder for the secondary battery electrode of the present invention exhibits excellent binding properties in the mixture layer, and the secondary battery provided with the electrodes obtained by using the above binder has good durability (cycle characteristics). showed that. Therefore, it is expected to be applied to in-vehicle secondary batteries. It is also useful for the use of active materials containing silicon, and is expected to contribute to increasing the capacity of batteries.
The binder for a secondary battery electrode of the present invention can be particularly preferably used for a non-aqueous electrolyte secondary battery electrode, and is particularly useful for a non-aqueous electrolyte lithium ion secondary battery having a high energy density.

Claims

A binder for a secondary battery electrode containing a crosslinked polymer or a salt thereof.
The crosslinked polymer or a salt thereof is a binder for a secondary battery electrode having a sol content of less than 5.0% by mass.
The binder for a secondary battery electrode according to claim 1, wherein the crosslinked polymer or a salt thereof contains 50% by mass or more and 100% by mass or less of a structural unit derived from an ethylenically unsaturated carboxylic acid monomer.
The crosslinked polymer or a salt thereof is neutralized to a degree of neutralization of 80 to 100 mol%, and then the particle size measured in an aqueous medium is 0.1 μm or more and 10 μm or less in terms of volume-based median diameter. The binder for a secondary battery electrode according to 1 or 2.
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 water swelling degree of 3.0 or more and 100 or less at pH 8.
The binder for a secondary battery electrode according to any one of claims 1 to 4, wherein the weight average molecular weight of the sol in terms of polyethylene oxide / polyethylene glycol is 300,000 or less.
A composition for a secondary battery electrode mixture layer containing the binder for the secondary battery electrode, the active material, and water according to any one of claims 1 to 5.
A secondary battery electrode comprising a mixture layer formed from the composition for the secondary battery electrode mixture layer according to claim 6 on the surface of a current collector.
A secondary battery comprising the secondary battery electrode according to claim 7.