JP7694393B2 - Aqueous binder for secondary battery electrodes, composition for secondary battery electrode mix layer, secondary battery electrode and secondary battery - Google Patents

Description

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

Various types of secondary batteries, such as nickel-hydrogen secondary batteries, lithium-ion secondary batteries, and electric double-layer capacitors, have been put to practical use. The electrodes used in these secondary batteries are prepared by applying a composition for forming an electrode mixture layer containing an active material and a binder, etc., to a current collector and drying it. For example, in lithium-ion secondary batteries, an aqueous binder containing styrene-butadiene rubber (SBR) latex and carboxymethyl cellulose (CMC) is used as the binder used in the negative electrode mixture layer composition. On the other hand, an N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride (PVDF) is widely used as the binder used in the positive electrode mixture layer.

As the applications of various secondary batteries expand, there is a tendency for the demand for improved energy density, reliability, and durability to grow stronger. For example, in order to increase the electric capacity of lithium-ion secondary batteries, specifications using silicon-based active materials as negative electrode active materials are becoming more common. However, silicon-based active materials are known to have large volume changes during charging and discharging, and as they are used repeatedly, the electrode mixture layer peels off or falls off, resulting in a decrease in battery capacity and a deterioration in cycle characteristics (durability). In order to suppress such defects, studies are being conducted to improve durability by using a binder to firmly bind the active materials together (binding property), reducing the size of the active materials to relieve stress associated with swelling and shrinkage, and devising additives for the electrolyte.

In this context, it has recently been reported that acrylic acid polymers are effective as binders for use in negative electrode mixture layers using silicon-based active materials. For example, Patent Document 1 discloses that by using a polymer in which polyacrylic acid is crosslinked with a specific crosslinking agent as a binder, good cycle characteristics are exhibited without destruction of the electrode structure even when an active material containing silicon is used. Patent Document 2 discloses a crosslinked acrylic acid polymer having a specific particle size in a 1% NaCl aqueous solution, and that it exhibits high binding properties.

International Publication No. 2014/065407 International Publication No. 2017/073589

The binders disclosed in Patent Documents 1 and 2 are both capable of imparting good cycle characteristics or binding properties, but with improvements in the performance of secondary batteries, further improvements in cycle characteristics are being demanded.
Generally, a secondary battery electrode is obtained by applying an electrode mixture layer composition (hereinafter also referred to as "electrode slurry") containing an active material and a binder to the surface of an electrode current collector and drying the applied composition. In this case, it is advantageous to increase the solid content concentration of the electrode mixture layer composition from the viewpoint of increasing the drying efficiency of the electrode slurry and improving the productivity of the electrode, but this makes it difficult to ensure good coatability.
In the electrode mixture layer compositions described in Patent Documents 1 and 2, the acrylic acid polymer used as a binder is micro-crosslinked to increase the spread in water, thereby enhancing the binding property, but even a small amount of the acrylic acid polymer significantly increases the viscosity, making it difficult to reduce the viscosity of the electrode slurry while increasing the solid content of the electrode mixture layer composition.

The present invention has been made in view of the above circumstances, and its purpose is to provide an aqueous binder for secondary battery electrodes that can obtain a secondary battery exhibiting excellent cycle characteristics while ensuring coatability by reducing the viscosity of the electrode slurry when the solid content concentration of the electrode mixture layer composition is higher than that of the conventional composition. In addition, the present invention also provides a composition for secondary battery electrode mixture layers that contains the aqueous binder, and a secondary battery electrode and secondary battery obtained using the composition.

As a result of intensive research conducted by the inventors to solve the above problems, they discovered that when the solids concentration of the composition for the electrode mixture layer is higher than in the past, by using an aqueous binder for secondary battery electrodes that contains a polymer or a salt thereof containing a specific amount of structural units derived from an ethylenically unsaturated carboxylic acid monomer, and a specific alkali metal compound, it is possible to obtain a secondary battery that exhibits excellent cycle characteristics while ensuring coatability by reducing the viscosity of the electrode slurry, and thus completed the present invention.

The present invention is as follows.
[1] An aqueous binder for secondary battery electrodes, comprising a polymer or a salt thereof containing 50% by mass or more and 100% by mass or less of structural units derived from an ethylenically unsaturated carboxylic acid monomer, and an alkali metal hydroxide or an alkali metal salt of a compound having a formula weight of 200 or less and having no ethylenically unsaturated groups.
[2] The aqueous binder for a secondary battery electrode according to [1], wherein the polymer is a crosslinked polymer or a non-crosslinked polymer.
[3] The aqueous binder for a secondary battery electrode according to [2], wherein the crosslinked polymer is obtained by using a crosslinkable monomer, and the amount of the crosslinkable monomer used is 0.05 parts by mass or more and 5.0 parts by mass or less per 100 parts by mass of a total amount of the non-crosslinkable monomers.
[4] The aqueous binder for a secondary battery electrode according to [2], wherein the non-crosslinked polymer contains 50 mass % or less of a structural unit derived from vinyl alcohol.
[5] The aqueous binder for a secondary battery electrode according to any one of [1] to [4], wherein the viscosity of a 2% by mass aqueous solution of the polymer is 10,000 mPa·s or less.
[6] The aqueous binder for a secondary battery electrode according to any one of [1] to [5], wherein the degree of neutralization of the polymer is 70 mol % or more.
[7] The aqueous binder for a secondary battery electrode according to any one of [1] to [6], wherein the amount of the alkali metal salt used is 5.0 parts by mass or more and 175 parts by mass or less per 100 parts by mass of the total amount of the polymer.
[8] The aqueous binder for a secondary battery electrode according to any one of [1] to [7], wherein the alkali metal salt includes at least one selected from the group consisting of a lithium salt, a sodium salt, and a potassium salt.
[9] The aqueous binder for a secondary battery electrode according to [8], wherein the lithium salt is lithium acetate.
[10] The aqueous binder for a secondary battery electrode according to any one of [1] to [9], further comprising a styrene butadiene rubber (SBR)-based latex and/or carboxymethyl cellulose (CMC).
[11] A composition for a secondary battery electrode mixture layer, comprising the aqueous binder for a secondary battery electrode according to any one of [1] to [10], an active material, and water.
[12] The secondary battery electrode mixture layer composition according to [11], wherein the secondary battery electrode mixture layer composition has a pH of less than 12.5.
[13] A secondary battery electrode comprising a mixture layer formed on a surface of a current collector from the composition for a secondary battery electrode mixture layer according to [11] or [12].
[14] A secondary battery comprising the secondary battery electrode according to [13].

According to the aqueous binder for secondary battery electrodes of the present invention, even when the solids concentration of the electrode mixture layer composition is higher than that of the conventional composition, it is possible to obtain a secondary battery that exhibits excellent cycle characteristics while ensuring coatability by reducing the viscosity of the electrode slurry.

The aqueous binder for secondary battery electrodes of the present invention (hereinafter also referred to as "the binder") contains a polymer (hereinafter also referred to as "the polymer") or its salt containing 50% by mass or more and 100% by mass or less of structural units derived from ethylenically unsaturated carboxylic acid monomers, and an alkali metal hydroxide or an alkali metal salt of a compound having a formula weight of 200 or less and not having an ethylenically unsaturated group (hereinafter also referred to as "the alkali metal salt"). By mixing with an active material and water, it can be made into a composition for secondary battery electrode mixture layer (hereinafter also referred to as "the composition"). The above composition is in a slurry state that can be applied to a current collector. The secondary battery electrode of the present invention is obtained by forming a mixture layer formed from the above composition on the surface of a current collector such as copper foil or aluminum foil. Here, when the present binder is used in a composition for secondary battery electrode mixture layer containing a silicon-based active material described later as an active material, the effect of the present invention is particularly large.

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

The binder comprises the polymer or a salt thereof, and an alkali metal hydroxide or an alkali metal salt.
The present polymer may be a crosslinked polymer (hereinafter also referred to as the present crosslinked polymer). The present crosslinked polymer and the present non-crosslinked polymer may be used alone or in combination. The present crosslinked polymer or the present non-crosslinked polymer may be used alone or in combination of two or more kinds.

1. The present crosslinked polymer (structural units derived from ethylenically unsaturated carboxylic acid monomers)
The crosslinked polymer contained in the binder contains 50% by mass or more and 100% by mass or less of a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as "component (a1)"). When the crosslinked polymer has a carboxyl group due to the structural unit, the adhesion to the current collector is improved, and the desolvation effect of lithium ions and ionic conductivity are excellent, so that an electrode with low resistance and excellent high-rate characteristics can be obtained. In addition, water swelling is imparted, so that the dispersion stability of the active material and the like in the composition can be improved.
The (a1) component can be introduced into the polymer by, for example, polymerizing a monomer containing an ethylenically unsaturated carboxylic acid monomer. Alternatively, it can be obtained by (co)polymerizing a (meth)acrylic acid ester monomer and then hydrolyzing it. In addition, it can be obtained by polymerizing (meth)acrylamide and (meth)acrylonitrile, etc., and then treating them with a strong alkali, or by reacting a polymer having a hydroxyl group with an acid anhydride.

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

The content of the (a1) component in this crosslinked polymer can be 50% by mass or more and 100% by mass or less based on the total structural units of this crosslinked polymer. By containing the (a1) component in such a range, excellent adhesion to the current collector can be easily ensured. When the lower limit is 50% by mass or more, the dispersion stability of this composition becomes good and a higher binding force can be obtained, which is preferable, and it may be 60% by mass or more, 70% by mass or more, or 80% by mass or more. In addition, the upper limit is, for example, 99.9% by mass or less, for example, 99.5% by mass or less, for example, 99% by mass or less, for example, 98% by mass or less, for example, 95% by mass or less, for example, 90% by mass or less, or for example, 80% by mass or less. The range can be an appropriate combination of these lower and upper limits, and can be, for example, 50% by mass or more and 100% by mass or less, for example, 50% by mass or more and 99.9% by mass or less, for example, 50% by mass or more and 99% by mass or less, or for example, 50% by mass or more and 98% by mass or less.

<Other structural units>
In addition to the (a1) component, the crosslinked polymer may contain a structural unit derived from another ethylenically unsaturated monomer copolymerizable therewith (hereinafter also referred to as "(b1) component"). Examples of the (b1) component include structural units derived from an ethylenically unsaturated monomer compound having an anionic group other than a carboxyl group, such as a sulfonic acid group and a phosphoric acid group, or a nonionic ethylenically unsaturated monomer. These structural units can be introduced by copolymerizing an ethylenically unsaturated monomer compound having an anionic group other than a carboxyl group, such as a sulfonic acid group and a phosphoric acid group, or a monomer containing a nonionic ethylenically unsaturated monomer.

The proportion of the (b1) component can be 0% by mass or more and 50% by mass or less with respect to the total structural units of the crosslinked polymer. The proportion of the (b1) component may be 1% by mass or more and 50% by mass or less, 2% by mass or more and 50% by mass or less, 5% by mass or more and 50% by mass or less, or 10% by mass or more and 50% by mass or less. In addition, when the (b1) component is contained at 1% by mass or more with respect to the total structural units of the crosslinked polymer, the affinity to the electrolyte is improved, and therefore the effect of improving lithium ion conductivity can also be expected.

As the (b1) component, among those mentioned above, structural units derived from nonionic ethylenically unsaturated monomers are preferred from the viewpoint of obtaining an electrode with good bending resistance, and examples of nonionic ethylenically unsaturated monomers include (meth)acrylamide and its derivatives, nitrile group-containing ethylenically unsaturated monomers, alicyclic structure-containing ethylenically unsaturated monomers, hydroxyl group-containing ethylenically unsaturated monomers, etc.

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

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

Examples of alicyclic structure-containing ethylenically unsaturated monomers include cycloalkyl (meth)acrylate esters which may have an aliphatic substituent, such as cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, cyclodecyl (meth)acrylate, and cyclododecyl (meth)acrylate; isobornyl (meth)acrylate, adamantyl (meth)acrylate, dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and cycloalkyl polyalcohol mono(meth)acrylates such as cyclohexanedimethanol mono(meth)acrylate and cyclodecanedimethanol mono(meth)acrylate. One of these may be used alone, or two or more may be used in combination.

Examples of hydroxyl group-containing ethylenically unsaturated monomers include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and hydroxybutyl (meth)acrylate. One of these may be used alone, or two or more may be used in combination.

In terms of the excellent binding properties of the binder, the crosslinked polymer or its salt preferably contains structural units derived from (meth)acrylamide and its derivatives, as well as nitrile group-containing ethylenically unsaturated monomers, alicyclic structure-containing ethylenically unsaturated monomers, etc. In addition, when a structural unit derived from a hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g/100 ml or less is introduced as component (c), it can exhibit strong interaction with the electrode material and exhibit good binding properties to the active material. As a result, a firm and well-integrated electrode mixture layer can be obtained, and therefore, as the above-mentioned "hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g/100 ml or less", an alicyclic structure-containing ethylenically unsaturated monomer is particularly preferred.

In order to improve the cycle characteristics of the resulting secondary battery, it is preferable that the crosslinked polymer or its salt contains a structural unit derived from a hydroxyl group-containing ethylenically unsaturated monomer, and it is preferable that the crosslinked polymer or its salt contains 0.5% by mass or more and 50% by mass or less of the structural unit, more preferably 2.0% by mass or more and 50% by mass or less, and even more preferably 10.0% by mass or more and 50% by mass or less.

As other nonionic ethylenically unsaturated monomers, for example, (meth)acrylic acid esters may be used. Examples of the (meth)acrylic acid esters include (meth)acrylic acid alkyl ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate;
Aromatic (meth)acrylic acid ester compounds such as phenyl (meth)acrylate, phenylmethyl (meth)acrylate, and phenylethyl (meth)acrylate;
Examples of the alkoxyalkyl (meth)acrylate compounds include 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate. One of these compounds may be used alone, or two or more of them may be used in combination.

From the viewpoint of adhesion to the active material and cycle characteristics, aromatic (meth)acrylic acid ester compounds can be preferably used. From the viewpoint of further improving lithium ion conductivity and high-rate characteristics, compounds having an ether bond, such as (meth)acrylic acid alkoxyalkyl esters such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate, are preferred, with 2-methoxyethyl (meth)acrylate being more preferred.

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

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

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

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

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

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

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

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

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

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

When the present crosslinked polymer is crosslinked by a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.05 parts by mass or more and 5.0 parts by mass or less, more preferably 0.1 parts by mass or more and 5.0 parts by mass or less, even more preferably 0.2 parts by mass or more and 4.0 parts by mass or less, and even more preferably 0.3 parts by mass or more and 3.0 parts by mass or less, relative to 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer). If the amount of the crosslinkable monomer used is 0.05 parts by mass or more, it is preferable in that the binding property and the stability of the electrode slurry are better. If it is 5.0 parts by mass or less, the stability of the polymer tends to be high.
Similarly, the amount of the crosslinkable monomer used is preferably 0.02 to 1.7 mol %, more preferably 0.10 to 1.0 mol %, based on the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers).

<Viscosity of aqueous solution of the present crosslinked polymer>
The crosslinked polymer preferably has a viscosity of 10,000 mPa·s or less in a 2% by weight aqueous solution. When the viscosity of the 2% by weight aqueous solution is 10,000 mPa·s or less, it is possible to provide durability that can follow the volume change of the active material during charging and discharging. The viscosity of the 2% by weight aqueous solution may be 5,000 mPa·s or less, 3,000 mPa·s or less, or 2,000 mPa·s or less.
The aqueous solution viscosity can be obtained by uniformly dissolving or dispersing an amount of the present crosslinked polymer to give a predetermined concentration in water, and then measuring the Brookfield viscosity (25° C.) at 12 rpm according to the method described in the Examples.

The crosslinked polymer or its salt absorbs water and swells in water. In general, when a crosslinked polymer has an appropriate degree of crosslinking, the greater the amount of hydrophilic groups that the crosslinked polymer has, the easier it is to absorb water and swell. In addition, in terms of the degree of crosslinking, the lower the degree of crosslinking, the easier it is to swell. However, even if the number of crosslinking points is the same, the greater the molecular weight (primary chain length), the more crosslinking points that contribute to the formation of a three-dimensional network, making it more difficult for the crosslinked polymer to swell. Therefore, the viscosity of the crosslinked polymer aqueous solution can be adjusted by adjusting the amount of hydrophilic groups, the number of crosslinking points, and the primary chain length of the crosslinked polymer. In this case, the number of crosslinking points can be adjusted, for example, by the amount of crosslinkable monomer used, the chain transfer reaction to the polymer chain, and the post-crosslinking reaction. In addition, the primary chain length of the polymer can be adjusted by setting conditions related to the amount of radical generation, such as the initiator and polymerization temperature, and by selecting a polymerization solvent taking into account chain transfer, etc.

<Particle size of the crosslinked polymer>
In the present composition, it is preferable that the present crosslinked polymer is not present as large particle lumps (secondary aggregates) but is well dispersed as water-swellable particles having an appropriate particle size, because this allows the binder containing the crosslinked polymer to exhibit good binding performance.

The crosslinked polymer preferably has a particle size (water-swollen particle size) of 0.1 μm or more and 10.0 μm or less in volume median size when dispersed in water, the crosslinked polymer having a degree of neutralization based on the carboxyl groups of the crosslinked polymer. A more preferred range of the particle size is 0.1 μm or more and 8.0 μm or less, a further more preferred range is 0.1 μm or more and 7.0 μm or less, a still more preferred range is 0.2 μm or more and 5.0 μm or less, and a still more preferred range is 0.5 μm or more and 3.0 μm or less. If the particle size is in the range of 0.1 μm or more and 10.0 μm or less, the particles are present in the composition at a suitable size and uniformly, so that the composition is highly stable and exhibits excellent binding properties. If the particle size exceeds 10.0 μm, there is a risk of insufficient binding properties as described above. In addition, there is a risk of insufficient coatability in that it is difficult to obtain a smooth coating surface. On the other hand, if the particle size is less than 0.1 μm, there is a concern in terms of stable production.

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

In the present crosslinked polymer, acid groups such as carboxyl groups derived from ethylenically unsaturated carboxylic acid monomers are neutralized so that the degree of neutralization in the present composition is 20 mol% or more, and it is preferable to use it as a salt form. The above degree of neutralization is more preferably 50 mol% or more, even more preferably 70 mol% or more, even more preferably 75 mol% or more, even more preferably 80 mol% or more, and particularly preferably 85 mol% or more. The upper limit of the degree of neutralization is 100 mol%, and may be 98 mol% or 95 mol%. The range of the degree of neutralization can be appropriately combined with the above lower limit and upper limit, and may be, for example, 50 mol% or more and 100 mol% or less, 75 mol% or more and 100 mol% or less, or 80 mol% or more and 100 mol% or less. When the degree of neutralization is 20 mol% or more, it is preferable in that the water swelling property is good and the dispersion stabilization effect is easily obtained. In this specification, the neutralization degree can be calculated from the charged amount of a monomer having an acid group such as a carboxyl group and a neutralizing agent used for neutralization. The neutralization degree can be confirmed by IR measurement of a powder of a crosslinked polymer or a salt thereof after drying treatment at 80° C. for 3 hours under reduced pressure conditions, and by the intensity ratio of a peak derived from a C═O group of the carboxylic acid to a peak derived from a C═O group of the carboxylate.

<Method of producing the present crosslinked polymer>
The present crosslinked polymer can be prepared by known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization, but precipitation polymerization and suspension polymerization (reverse phase suspension polymerization) are preferred from the viewpoint of productivity. Heterogeneous polymerization methods such as precipitation polymerization, suspension polymerization, and emulsion polymerization are preferred from the viewpoint of obtaining better performance in terms of binding property, etc., and among these, precipitation polymerization is more preferred.
Precipitation polymerization is a method for producing a polymer by carrying out a polymerization reaction in a solvent that dissolves the raw material unsaturated monomer but does not substantially dissolve the resulting polymer. As the polymerization proceeds, the polymer particles grow larger through aggregation and growth, and a dispersion of polymer particles is obtained in which primary particles of several tens to hundreds of nm are secondary aggregated to several μm to several tens of μm. A dispersion stabilizer can also be used to control the particle size of the polymer.
The secondary aggregation can be suppressed by selecting a dispersion stabilizer, a polymerization solvent, etc. In general, precipitation polymerization in which secondary aggregation is suppressed is also called dispersion polymerization.

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

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

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

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

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

2. The non-crosslinked polymer contained in the binder contains 50% by mass or more and 100% by mass or less of a structural unit (the (a1) component) derived from an ethylenically unsaturated carboxylic acid monomer. The method of introducing the (a1) component of the non-crosslinked polymer may be the same as the method described in the (a1) component of the crosslinked polymer. In addition, it may be a method of saponifying a polymer containing a structural unit derived from a (meth)acrylic acid alkyl ester compound (the one described above as the (b1) component of the crosslinked polymer), and as the (meth)acrylic acid alkyl ester compound, methyl acrylate and methyl methacrylate are preferred from the viewpoint of the ease of proceeding with the saponification reaction, and one type may be used alone, or two or more types may be used in combination.

The non-crosslinked polymer has a higher viscosity than the crosslinked polymer, which is presumably because the non-crosslinked polymer has extended molecular chains, whereas the crosslinked polymer is particulate, resulting in a smaller apparent molecular weight.
Even though the present non-crosslinked polymer has a higher viscosity than the present crosslinked polymer salt, when an aqueous binder containing the present non-crosslinked polymer or its salt, and an alkali metal hydroxide or the present alkali metal salt is used, the viscosity of the present composition is reduced, thereby improving the coatability, and further improving the cycle characteristics of the secondary battery.

The content of component (a1) in this non-crosslinked polymer is, in terms of solubility in water, 50% by mass or more and 100% by mass or less, preferably 60% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, and even more preferably 80% by mass or more and 100% by mass or less, based on the total structural units of this non-crosslinked polymer.

<Other structural units>
The present non-crosslinked polymer may contain, in addition to the component (a1), a structural unit derived from another ethylenically unsaturated monomer copolymerizable therewith (the above-mentioned component (b1)).
The method of introducing the component (b1) may be the same as the method described for the component (b1) of the present crosslinked polymer. It may also be a method of saponifying a polymer containing a structural unit derived from a vinyl ester compound such as vinyl acetate or vinyl propionate, and as the vinyl ester compound, vinyl acetate is preferred from the viewpoint of ease of availability of raw materials, and one type may be used alone or two or more types may be used in combination.

The proportion of the (b1) component can be 0% by mass or more and 50% by mass or less with respect to the total structural units of the non-crosslinked polymer. The proportion of the (b1) component may be 1% by mass or more and 50% by mass or less, 2% by mass or more and 50% by mass or less, 5% by mass or more and 50% by mass or less, or 10% by mass or more and 50% by mass or less.

The non-crosslinked polymer may be in the form of a salt in which some or all of the carboxyl groups contained in the polymer have been neutralized. The type of salt is not particularly limited, but examples include alkali metal salts such as lithium, sodium, and potassium; magnesium salts; alkaline earth metal salts such as calcium salts and barium salts; other metal salts such as aluminum salts; ammonium salts and organic amine salts. Among these, alkali metal salts and alkaline earth metal salts are preferred because they are less likely to adversely affect battery characteristics, and alkali metal salts are more preferred.

In the present non-crosslinked polymer, acid groups such as carboxyl groups derived from ethylenically unsaturated carboxylic acid monomers are neutralized so that the degree of neutralization in the present composition is 20 mol% or more, and it is preferable to use it as a salt form. The above-mentioned degree of neutralization is more preferably 50 mol% or more, even more preferably 70 mol% or more, even more preferably 75 mol% or more, even more preferably 80 mol% or more, and particularly preferably 85 mol% or more. The upper limit of the degree of neutralization is 100 mol%, and may be 98 mol% or 95 mol%. The range of the degree of neutralization can be appropriately combined with the above-mentioned lower limit and upper limit, and may be, for example, 50 mol% or more and 100 mol% or less, 75 mol% or more and 100 mol% or less, or 80 mol% or more and 100 mol% or less. When the degree of neutralization is 20 mol% or more, it is preferable in that solubility in water is easily ensured. In this specification, the neutralization degree can be calculated from the charged amount of a monomer having an acid group such as a carboxyl group and a neutralizing agent used for neutralization. The neutralization degree can be confirmed by IR measurement of a powder of a crosslinked polymer or a salt thereof after drying treatment at 80° C. for 3 hours under reduced pressure conditions, and by the intensity ratio of a peak derived from a C═O group of the carboxylic acid to a peak derived from a C═O group of the carboxylate.

The weight average molecular weight (Mw) of the non-crosslinked polymer is not particularly limited, but is preferably 5,000 or more, more preferably 10,000 or more, in order to obtain an electrode mixture layer with excellent binding properties. Mw may be 100,000 or more, 500,000 or more, or 1,000,000 or more. The upper limit of Mw is also not particularly limited, but may be, for example, 10,000,000 or less, or 5,000,000 or less, in terms of handling during production.

When the binder contains the crosslinked polymer and the non-crosslinked polymer, the amount of the non-crosslinked polymer used is preferably 7.5 parts by mass or more and 200 parts by mass or less with respect to 100 parts by mass of the total amount of the crosslinked polymer. The amount of the non-crosslinked polymer used may be 15 parts by mass or more, 25 parts by mass or more, 35 parts by mass or more, or 45 parts by mass or more. The upper limit may be 190 parts by mass or less, 180 parts by mass or less, 170 parts by mass or less, or 160 parts by mass or less. The range may be a range that appropriately combines such lower and upper limits, and may be, for example, 15 parts by mass or more and 190 parts by mass or less, for example, 25 parts by mass or more and 180 parts by mass or less, or for example, 35 parts by mass or more and 170 parts by mass or less, or for example, 35 parts by mass or more and 160 parts by mass or less.

In this way, a specific amount of the present non-crosslinked polymer can be used in combination with the present crosslinked polymer, and when the solids concentration of the electrode mixture layer composition is higher than in the past, a secondary battery exhibiting excellent cycle characteristics can be obtained while ensuring coatability by reducing the viscosity of the electrode slurry. If the amount of the present non-crosslinked polymer used is 7.5 parts by mass or more, the effect can be achieved. Furthermore, if the amount of the present non-crosslinked polymer used exceeds 200 parts by mass, sufficient coatability may not be obtained.

<Method of producing the present non-crosslinked polymer>
The present non-crosslinked polymer can be prepared by using a known polymerization method such as solution polymerization, precipitation polymerization, suspension polymerization, or emulsion polymerization, and the polymerization method may be appropriately selected depending on the molecular weight, composition, and the like.

The polymerization initiator may be any known polymerization initiator such as an azo compound, an organic peroxide, an inorganic peroxide, etc., but is not particularly limited. The conditions of use can be adjusted so that an appropriate amount of radicals is generated by a known method such as thermal initiation, redox initiation using a reducing agent, UV initiation, etc.
Furthermore, for the purpose of adjusting the molecular weight, etc., a known chain transfer agent may be used as necessary.

<Viscosity of aqueous solution of the present non-crosslinked polymer>
The present non-crosslinked polymer preferably has a viscosity of 10,000 mPa·s or less in a 2% by weight aqueous solution. When the viscosity of the 2% by weight aqueous solution is 10,000 mPa·s or less, it is possible to provide durability that can follow the volume change of the active material during charging and discharging. The viscosity of the 2% by weight aqueous solution may be 5,000 mPa·s or less, 3,000 mPa·s or less, or 2,000 mPa·s or less.
The aqueous solution viscosity can be obtained by uniformly dissolving or dispersing the present non-crosslinked polymer in water in an amount to give a predetermined concentration, and then measuring the Brookfield viscosity (25°C) at 12 rpm according to the method described in the Examples.

3. Alkali metal hydroxide or present alkali metal salt The present binder contains an alkali metal hydroxide or present alkali metal salt in addition to the present polymer or its salt. When the present binder contains an alkali metal hydroxide, the present crosslinked polymer is a salt neutralized by 100 mol %. The present binder may be obtained by mixing the present crosslinked polymer salt having a neutralization degree of 100 mol % with an alkali metal hydroxide, or by mixing the present polymer salt having a neutralization degree of less than 100 mol % with an amount of alkali metal hydroxide such that the neutralization degree of the polymer salt exceeds 100 mol %.

Specific examples of the alkali metal hydroxide include lithium hydroxide, sodium hydroxide, and potassium hydroxide.
The alkali metal hydroxides may be used alone or in combination of two or more thereof.

The amount of alkali metal hydroxide used is not particularly limited, but is preferably 5 parts by mass or more and 50 parts by mass or less, more preferably 10 parts by mass or more and 45 parts by mass or less, and even more preferably 10 parts by mass or more and 40 parts by mass or less, per 100 parts by mass of the total amount of the polymer having a degree of neutralization of 100 mol%, in order to make the pH of the composition less than 12.5.

Here, it is preferable to include this alkali metal salt, since it suppresses the increase in pH of the electrode slurry and, for example, when carboxymethyl cellulose (CMC) is blended, there is little concern about its hydrolysis. In addition, since this alkali metal salt does not have an ethylenically unsaturated group, polymerization does not proceed during the production of the electrode slurry or during storage of the obtained electrode slurry, so there is little concern about the electrode slurry thickening.

The formula weight of this alkali metal salt is 200 or less, preferably 180 or less, more preferably 160 or less, and even more preferably 140 or less, in that when the solids concentration of the electrode mixture layer composition is higher than in the past, it is possible to obtain a secondary battery that exhibits excellent cycle characteristics while ensuring coatability by reducing the viscosity of the electrode slurry.

The alkali metal salts include organic acid alkali metal salts, alkali metal carbonate compounds, alkali metal hydrogencarbonates, alkali metal nitrite compounds, alkali metal chlorides, alkali metal bromides, etc., and may be in the form of anhydrates or hydrates.
Specific examples of organic acid alkali metal salts include lithium acetate, sodium acetate, potassium acetate, lithium propionate, sodium propionate, potassium propionate, etc.;
Specific examples of alkali metal carbonates include lithium carbonate, sodium carbonate, and potassium carbonate;
Specific examples of alkali metal hydrogen carbonates include lithium hydrogen carbonate, sodium hydrogen carbonate, and potassium hydrogen carbonate;
Specific examples of the alkali metal nitrite compound include lithium nitrite, sodium nitrite, potassium nitrite, etc.;
Specific examples of the alkali metal chloride include lithium chloride, sodium chloride, potassium chloride, etc.;
Specific examples of the alkali metal bromide include lithium bromide, sodium bromide, and potassium bromide.
Among these, organic acid alkali metal salts are preferred in that the effects of the present invention are particularly large, and furthermore, it is preferable to include at least one selected from the group consisting of lithium salts, sodium salts and potassium salts, and lithium acetate is particularly preferred. Here, as the organic acid of the organic acid alkali metal salt, an organic acid having one carboxyl group per molecule is preferred in terms of preventing gelation due to crosslinking of the present polymer having carboxyl groups.
The present alkali metal salt may be used alone or in combination of two or more kinds.

The amount of the alkali metal salt used is not particularly limited, but is preferably 5.0 parts by mass or more and 175 parts by mass or less, more preferably 10 parts by mass or more and 150 parts by mass or less, even more preferably 15 parts by mass or more and 125 parts by mass or less, and even more preferably 20 parts by mass or more and 100 parts by mass or less, relative to 100 parts by mass of the total amount of the polymer, in that when the solids concentration of the electrode mixture layer composition is higher than in the past, it is possible to obtain a secondary battery that exhibits excellent cycle characteristics while ensuring coatability by reducing the viscosity of the electrode slurry.

3. Composition for secondary battery electrode mixture layer The composition for secondary battery electrode mixture layer of the present invention contains the present binder, an active material, and water.
The amount of the binder used in the composition is, for example, 0.1 parts by mass or more and 20 parts by mass or less, based on 100 parts by mass of the total amount of the active material. The amount 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 binder used is 0.1 parts by mass or more, sufficient binding property can be obtained. In addition, the dispersion stability of the active material and the like can be ensured, and a uniform mixture layer can be formed. If the amount of the binder used is 20 parts by mass or less, the composition does not become highly viscous, and the coatability to the current collector can be ensured. As a result, a mixture layer having a uniform and smooth surface can be formed.

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

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

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

On the other hand, examples of the negative electrode active material include carbon-based materials, lithium metal, lithium alloys, and metal oxides, and one or more of these can be used in combination. Among these, active materials made of carbon-based materials such as natural graphite, artificial graphite, hard carbon, and soft carbon (hereinafter also referred to as "carbon-based active materials") are preferred, and graphite such as natural graphite and artificial graphite, and hard carbon are more preferred. In the case of graphite, spherical graphite is preferably used from the viewpoint of battery performance, and the preferred range of its particle size is, for example, 1 to 20 μm, and also, for example, 5 to 15 μm. In addition, in order to increase the energy density, metals or metal oxides capable of absorbing lithium, such as silicon and tin, can also be used as the negative electrode active material. Among them, silicon has a higher capacity than graphite, and active materials made of silicon-based materials such as silicon, silicon alloys, and silicon oxides such as silicon monoxide (SiO) (hereinafter also referred to as "silicon-based active materials") can be used. However, the silicon-based active material has a high capacity, but the volume change caused by charging and discharging is large. Therefore, it is preferable to use it in combination with the carbon-based active material. In this case, if the amount of the silicon-based active material is large, the electrode material may collapse, and the cycle characteristics (durability) may be greatly reduced. From this point of view, when the silicon-based active material is used in combination, the amount of the silicon-based active material is, for example, 60% by mass or less, or, for example, 30% by mass or less, relative to the carbon-based active material.

Since carbon-based active materials themselves have good electrical conductivity, it is not necessary to add a conductive additive. When a conductive additive is added for the purpose of further reducing resistance, etc., the amount used is, from the viewpoint of energy density, for example, 10 parts by mass or less, or, for example, 5 parts by mass or less, per 100 parts by mass of the total amount of active material.

When the composition is in a slurry state, the amount of active material used is, for example, in the range of 10 to 75% by mass, or, for example, in the range of 30 to 65% by mass, based on the total amount of the composition. If the amount of active material used is 10% by mass or more, migration of the binder and the like is suppressed, and it is also advantageous in terms of the drying cost of the medium. On the other hand, if it is 75% by mass or less, the fluidity and coatability of the composition can be ensured, and a uniform mixture layer can be formed.

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

When the present composition is made into a coatable slurry state, the content of the water-containing medium in the entire composition can be, for example, in the range of 25 to 60 mass %, and can be, for example, 35 to 60 mass %, from the viewpoints of the coatability of the slurry, the energy cost required for drying, and productivity.

The composition may further contain other binder components such as styrene butadiene rubber (SBR) latex, carboxymethyl cellulose (CMC), acrylic latex, and polyvinylidene fluoride latex. When other binder components are used in combination, the amount of the binder components used may be, for example, 0.1 to 5 parts by mass or less, or, for example, 0.1 to 2 parts by mass or less, or, for example, 0.1 to 1 part by mass or less, relative to 100 parts by mass of the total amount of the active material. If the amount of the other binder components used exceeds 5 parts by mass, the resistance increases and the high-rate characteristics may become insufficient. Among the above, SBR latex and CMC are preferred in terms of the excellent balance between binding properties and bending resistance, and it is more preferable to use SBR latex and CMC in combination.

The SBR latex refers to an aqueous dispersion of a copolymer having structural units derived from an aromatic vinyl monomer such as styrene and structural units derived from an aliphatic conjugated diene monomer such as 1,3-butadiene. Examples of the aromatic vinyl monomer include α-methylstyrene, vinyltoluene, divinylbenzene, and the like, in addition to styrene, and one or more of these can be used. The structural units derived from the aromatic vinyl monomer in the copolymer can be, for example, in the range of 20 to 70% by mass, or, for example, in the range of 30 to 60% by mass, mainly from the viewpoint of binding properties.
Examples of the aliphatic conjugated diene monomer include 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, etc., in addition to 1,3-butadiene, and one or more of these can be used. The structural unit derived from the aliphatic conjugated diene monomer in the copolymer can be, for example, in the range of 30 to 70 mass %, or, for example, in the range of 40 to 60 mass %, in order to improve the binding property of the binder and the flexibility of the obtained electrode.
In addition to the above-mentioned monomers, the styrene/butadiene-based latex may use other monomers as copolymerization monomers, such as nitrile group-containing monomers such as (meth)acrylonitrile, carboxyl group-containing monomers such as (meth)acrylic acid, itaconic acid, maleic acid, and ester group-containing monomers such as methyl (meth)acrylate, in order to further improve performance such as binding property.
The content of the structural units derived from the other monomers in the copolymer can be, for example, in the range of 0 to 30% by mass, and can be, for example, in the range of 0 to 20% by mass.

The CMC refers to a nonionic cellulose-based semisynthetic polymer compound substituted with a carboxymethyl group and its salt. Examples of the nonionic cellulose-based semisynthetic polymer compound include alkyl celluloses such as methyl cellulose, methyl ethyl cellulose, ethyl cellulose, and microcrystalline cellulose;
Examples of the hydroxyalkyl cellulose include hydroxyethyl cellulose, hydroxybutyl methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose stearoxy ether, carboxymethyl hydroxyethyl cellulose, alkyl hydroxyethyl cellulose, and nonoxynyl hydroxyethyl cellulose.

The composition for secondary battery electrode mixture layer of the present invention is essentially composed of the above active material, water and the binder, and is obtained by mixing the components using known means. The method of mixing the components is not particularly limited, and known methods can be adopted, but a method of dry blending powder components such as the active material, conductive assistant and binder, and then mixing with a dispersion medium such as water and dispersing and kneading is preferred. When obtaining the composition in a slurry state, it is preferable to finish the slurry without poor dispersion or aggregation. As a mixing means, known mixers such as a planetary mixer, a thin film swirling mixer and a self-revolving mixer can be used, but it is preferable to use a thin film swirling mixer in order to obtain a good dispersion state in a short time. In addition, when using a thin film swirling mixer, it is preferable to perform preliminary dispersion in advance with a stirrer such as a disperser. The pH of the above slurry is not particularly limited as long as the effects of the present invention are achieved, but it is preferable that it is less than 12.5, and for example, when CMC is blended, it is more preferable that it is less than 11.5 in terms of the small concern of hydrolysis, and even more preferable that it is less than 10.5. The viscosity of the slurry is not particularly limited as long as the effects of the present invention are achieved, but the viscosity may be, for example, in the range of 100 to 5,000 mPa·s, or, for example, in the range of 500 to 4,500 mPa·s, or, for example, in the range of 1,000 to 3,000 mPa·s, as a B-type viscosity (25°C) at 20 rpm. If the viscosity of the slurry is within the above range, good coatability can be ensured.

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

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

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

As described above, a secondary battery having an electrode with a mixture layer formed from the secondary battery electrode mixture layer composition disclosed in this specification exhibits good durability (cycle characteristics) even after repeated charging and discharging, and is therefore suitable for use as an in-vehicle secondary battery, etc.

The present invention will be specifically described below based on examples. Note that the present invention is not limited to these examples. Note that in the following, "parts" and "%" mean parts by mass and % by mass unless otherwise specified.

(Measurement of Viscosity of 2% by Mass Aqueous Solution of the Present Polymer Salt)
The polymer salt obtained in each of the following Production Examples was dissolved in deionized water to prepare an aqueous solution having a concentration of 2% by mass. The temperature of the aqueous solution was adjusted to 25° C.±1° C., and the viscosity of the aqueous solution having a concentration of 2% by mass was measured using a Brookfield viscometer at 12 rpm.

(pH Measurement of Electrode Slurry)
The electrode slurries obtained in each of the following Examples and Comparative Examples were each added with the same mass of water to adjust the solid content concentration to 25%, and the slurry was adjusted to 25°C ± 1°C, and then the pH of the slurry was measured with a pH meter.

(Measurement of Viscosity of Electrode Slurry)
The electrode slurries obtained in the following respective Examples and Comparative Examples were adjusted to 25° C.±1° C., and then the slurry viscosities were measured at 20 rpm using a Brookfield viscometer.

<Production of the present crosslinked polymer salt>
(Production Example 1: Production of the present crosslinked polymer salt R-1)
For the polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen inlet tube was used.
A reactor was charged with 567 parts of acetonitrile, 2.20 parts of ion-exchanged water, 100.0 parts of acrylic acid (hereinafter referred to as "AA"), 0.90 parts of trimethylolpropane diallyl ether (manufactured by Daiso Co., Ltd., trade name "Neoallyl T-20"), and triethylamine equivalent to 1.0 mol% relative to the AA. After the inside of the reactor was sufficiently replaced with nitrogen, the inside temperature was raised to 55°C by heating. After confirming that the inside temperature was stable at 55°C, 0.040 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name "V-65") was added as a polymerization initiator, and the reaction solution became cloudy, so this point was determined as the polymerization initiation point. The polymerization reaction was continued while maintaining the internal temperature at 55° C. by adjusting the external temperature (water bath temperature), and cooling of the polymerization reaction solution was started 24 hours after the polymerization initiation point, and after the internal temperature had dropped to 25° C., 52.4 parts of lithium hydroxide monohydrate (hereinafter referred to as "LiOH.H ₂ O") powder was added. After the addition, stirring was continued at room temperature for 12 hours to obtain a slurry-like polymerization reaction solution in which particles of the present crosslinked polymer salt R-1 (Li salt, neutralization degree 90 mol%) were dispersed in the medium.

The obtained polymerization reaction solution was centrifuged to settle the polymer particles, and the supernatant was removed. The precipitate was then redispersed in acetonitrile of the same mass as the polymerization reaction solution, and the polymer particles were centrifuged to settle and the supernatant was removed. This washing operation was repeated twice. The precipitate was collected and dried at 80°C for 3 hours under reduced pressure to remove the volatiles, thereby obtaining a powder of the present crosslinked polymer salt R-1. Since the present crosslinked polymer salt R-1 is hygroscopic, it was sealed and stored in a container with water vapor barrier properties. The powder of the present crosslinked polymer salt R-1 was subjected to IR measurement, and the degree of neutralization was calculated from the intensity ratio of the peak derived from the C=O group of the carboxylic acid and the peak derived from the C=O group of the carboxylic acid Li, which was 90 mol%, equal to the calculated value from the charge. The viscosity of a 2% by mass aqueous solution of R-1 is shown in Table 1.

(Production Examples 2 to 6: Production of the present crosslinked polymer salts R-2 to R-6)
The same operation as in Production Example 1 was carried out except that the amounts of the monomer, crosslinkable monomer, ion-exchanged water and neutralizing agent were changed to those shown in Table 1, thereby obtaining polymerization reaction solutions containing the present crosslinked polymer salts R-2 to R-6.
Next, the same operation as in Production Example 1 was carried out for each polymerization reaction solution to obtain powdered crosslinked polymer salts R-2 to R-6. Each of these crosslinked polymer salts was sealed and stored in a container having water vapor barrier properties. The viscosities of 2% by mass aqueous solutions of R-2 to R-6 are shown in Table 1.

(Production Example 7: Production of the Non-Crosslinked Polymer Salt R-7)
For the polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen inlet tube was used.
A monomer solution was prepared by mixing 8 parts of methyl acrylate (hereinafter referred to as "MA") and 12 parts of vinyl acetate (hereinafter referred to as "VAc") and dissolving 0.67 parts of 2,2'-azobis(isobutyrate)dimethyl (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., product name "V-601").
Into a reactor, 410 parts of water, 10 parts of anhydrous sodium sulfate, 1 part of partially saponified polyvinyl alcohol (manufactured by Kuraray Co., Ltd., product name "PVA-217", saponification degree 88%), and 20.67 parts of the monomer solution were charged. After the inside of the reactor was sufficiently replaced with nitrogen, the inside temperature was raised to 60°C by heating. After it was confirmed that the inside temperature was stabilized at 60°C, a mixed solution of 32 parts of MA and 48 parts of VAc was dropped using a dropping funnel over 4 hours, and cooling of the reaction solution was started 1 hour after completion of the dropping, and the reaction was terminated, thereby obtaining a polymerization reaction solution containing a copolymer of MA and VAc.
Here, the amount of remaining monomers was measured by gas chromatography (GC) and the polymerization rate of the monomers was calculated. The polymerization rate of each monomer was 98% for MA and 82% for VAc.
In addition, a part of the obtained polymerization reaction liquid was dissolved in tetrahydrofuran and filtered with a membrane filter (manufactured by ADVANTEC Corporation: pore size 0.45 μm). Then, the copolymer of MA and VAc was subjected to gel permeation chromatography (GPC) measurement under the following conditions to obtain a weight average molecular weight (Mw) in terms of polystyrene of 1,080,000.
Measurement conditions: Column: Tosoh TSKgel SuperMultiporeHZ-M x 4 Solvent: Tetrahydrofuran Temperature: 40°C
Detector: RI
Flow rate: 600μL/min

After the polymerization reaction solution containing the copolymer of MA and VAc was heated to an external temperature of 50° C., the solvent was removed under reduced pressure to remove the remaining monomers. Then, 500 parts of methanol and 38.8 parts of LiOH.H ₂ O were added to 100 parts of the total amount of the copolymerization monomers (MA and VAc), and a saponification reaction was carried out at an external temperature of 50° C. for 3 hours to obtain a reaction solution containing a saponified product of the copolymer of MA and VAc.
The reaction solution containing the saponified product was reprecipitated in acetone, filtered, and then dried at 80° C. for 12 hours to remove volatile matter, thereby obtaining a saponified product of a copolymer of MA and VAc. Based on the polymerization rates of MA and VAc, the saponified product is a lithium salt of a non-crosslinked polymer having "57% by mass of structural units derived from acrylic acid" and "43% by mass of structural units derived from vinyl alcohol."
The lithium salt of the non-crosslinked polymer R-7 is hygroscopic, so it was sealed and stored in a container with water vapor barrier properties. The powder of the non-crosslinked polymer salt R-7 was subjected to IR measurement, and the degree of neutralization was calculated from the intensity ratio of the peak derived from the C=O group of the carboxylic acid and the peak derived from the C=O of the carboxylic acid Li, and was found to be 90 mol%.
The viscosity of a 2% by mass aqueous solution of R-7 was 9,000 mPa·s.

Details of the compounds used in Table 1 are shown below.
AA: acrylic acid HEA: 2-hydroxyethyl acrylate T-20: trimethylolpropane diallyl ether (manufactured by Daiso Co., Ltd., product name "Neoallyl T-20")
TEA: triethylamine AcN: acetonitrile V-65: 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)
MA: methyl acrylate VAc: vinyl acetate PVA: polyvinyl alcohol (manufactured by Kuraray Co., Ltd., product name "PVA-217")
V-601: 2,2'-azobis(isobutyric acid) dimethyl (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)
_LiOH.H2O : Lithium hydroxide monohydrate

Example 1
<Preparation of electrode mixture layer composition>
A 10% carbon coating was applied to the surface of SiOx (0.8<x<1.2) by CVD (hereinafter referred to as "Si-based active material"), and a mixture of graphite and the Si-based active material was used as the active material. As the binder, a mixture of crosslinked polymer salt R-1, lithium acetate, styrene/butadiene latex (SBR) and carboxymethyl cellulose (CMC) was used.
In order to make the solid content concentration of the electrode mixture layer composition 50% by mass, water was used as a dilution solvent, and the graphite:Si-based active material:R-1:lithium acetate:SBR:CMC=90:10:1.0:0.5:1.0:1.0 (solid content) was mixed for 2 hours using T.K. Hibismix manufactured by Primix Corporation to prepare a slurry-state electrode mixture layer composition (electrode slurry). The viscosity of the electrode slurry was 1,670 mPa·s, which was a sufficiently low value.
The obtained electrode slurry was used to prepare an electrode, which was then evaluated. The specific procedure and evaluation method are described below.

<Preparation 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. After that, the mixture layer was rolled to a thickness of 27 μm and a packing density of 1.3 g/ ^cm3 , and then punched into a 3 cm square to obtain a negative electrode plate.

(Coatability)
The coatability of the electrode slurry in the preparation of the negative electrode plate was evaluated based on the following criteria and was evaluated as "◯".
(Evaluation Criteria)
◯: No abnormal appearance such as streaks or bumps was observed on the surface.
△: Slight abnormalities in appearance such as streaks or bumps are observed on the surface.
×: Significant abnormalities in appearance such as streaks and bumps are observed on the surface.

<Preparation of Positive Electrode Plate>
In N-methylpyrrolidone (NMP) solvent, 100 parts of lithium iron phosphate (LFP) as a positive electrode active material, 0.2 parts of carbon nanotubes as a conductive agent, 2 parts of Ketjen Black, and 0.6 parts of vapor grown carbon fiber (VGCF) were mixed and added, and polyvinylidene fluoride (PVDF) was mixed as a binder for the electrode composition to prepare a positive electrode composition. The positive electrode composition was applied to an aluminum current collector (thickness: 15 μm) and dried to form a mixture layer. Thereafter, the mixture layer was rolled to a thickness of 88 μm and a packing density of 3.1 g / cm ³ , and then punched out into a 3 cm square to obtain a positive electrode plate.

<Preparation of secondary battery>
A laminated type lithium ion secondary battery was produced using the positive electrode plate, the negative electrode plate and the separator. The electrolyte used was a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (DEC) in a volume ratio of 25:75, in which _LiPF6 was dissolved at a concentration of 1.0 mol/L.

(Evaluation of cycle characteristics)
The lithium ion secondary battery of the laminated cell prepared above was charged and discharged at a charge/discharge rate of 0.2C under conditions of 2.7 to 3.4V in CC discharge, and the initial capacity _C0 was measured. Furthermore, charging and discharging were repeated in an environment of 25°C, and the capacity _C50 after 50 cycles was measured. The cycle characteristic (ΔC) calculated by the following formula was 93.9%, and the cycle characteristic based on the following criteria was evaluated as "good". Note that a higher ΔC value indicates better cycle characteristics.
ΔC= _C50 / _C0 ×100(%)
(Evaluation Criteria)
◎: Charge/discharge capacity retention rate is 95.0% or more. ◯: Charge/discharge capacity retention rate is 90.0% or more and less than 95.0%. △: Charge/discharge capacity retention rate is 85.0% or more and less than 90.0%. ×: Charge/discharge capacity retention rate is less than 85.0%.

Examples 2 to 19 and Comparative Examples 1 to 3
Except for using the polymer salt and the alkali metal hydroxide or alkali metal salt as shown in Table 2, the same operation as in Example 1 was carried out to prepare an electrode slurry and measure the slurry viscosity. In addition, the coating property of the electrode slurry and the cycle characteristics of the secondary battery obtained using the same were evaluated. The results are shown in Table 2.
In Example 7, the present crosslinked polymer salt R-1 (1.0 part by mass) having a neutralization degree of 90 mol%, lithium hydroxide monohydrate (0.27 parts by mass) as an alkali metal hydroxide, and other components are mixed to obtain an electrode slurry, so that the slurry contains the present crosslinked polymer and lithium hydroxide having a neutralization degree of 100 mol%, and the content of the lithium hydroxide is 21.3 parts by mass as a value calculated by converting lithium hydroxide to "lithium hydroxide monohydrate" relative to a total amount of 100 parts by mass of the present crosslinked polymer having a neutralization degree of 100 mol%.

The formula weights and manufacturers of the alkali metal hydroxides and alkali metal salts used in Table 2 are shown below.
Lithium acetate: formula weight 65.99, manufactured by Tokyo Chemical Industry Co., Ltd. Lithium carbonate: formula weight 73.89, manufactured by Kishida Chemical Co., Ltd. Lithium hydroxide monohydrate: formula weight 41.96, manufactured by Showa Chemical Co., Ltd. Lithium chloride: formula weight 42.39, manufactured by Honjo Chemical Co., Ltd. Lithium bromide monohydrate: formula weight 104.86, manufactured by Fujifilm Wako Pure Chemical Co., Ltd. Lithium nitrite: formula weight 52.94, manufactured by Honjo Chemical Co., Ltd. Sodium acetate: formula weight 82.03, manufactured by Junsei Chemical Co., Ltd. Potassium carbonate: formula weight 138.20, manufactured by Fujifilm Wako Pure Chemical Co., Ltd. Potassium bicarbonate: formula weight 100.12, manufactured by Fujifilm Wako Pure Chemical Co., Ltd.

<Evaluation Results>
As is clear from the results of Examples 1 to 19, all of the compositions for secondary battery electrode mixture layer (electrode slurry) containing the aqueous binder for secondary battery electrodes of the present invention had good coatability, and the secondary battery equipped with the electrode obtained using the composition also had excellent cycle characteristics. Among these, when an aqueous binder containing the present crosslinked polymer having a viscosity of 10,000 mPa·s or less in a 2% by mass aqueous solution was used, the coatability was further improved and the cycle characteristics were also excellent (Examples 1 to 17, 19). In addition, when the amount of lithium acetate used as the present alkali metal salt was 100 parts by mass or less relative to 100 parts by mass of the total amount of the present crosslinked polymer, the greater the amount used, the higher the balance between coatability and cycle characteristics could be achieved (Examples 1, 3, 4).

Although the present non-crosslinked polymer R-7 has a higher viscosity than the present crosslinked polymer salts R-1 to R-5, when used as an aqueous binder containing the present alkali metal salt lithium acetate, the viscosity of the present composition is reduced, resulting in excellent coatability, and the secondary battery also has excellent cycle characteristics (Example 19). The reason why the cycle characteristics when the present crosslinked polymer is used are higher than those when the present non-crosslinked polymer is used is presumably due to the fact that the present crosslinked polymer is a tougher binder.

In contrast, in the case of a composition for a secondary battery electrode mixture layer containing an aqueous binder that does not contain an alkali metal hydroxide or an alkali metal salt, either the cycle characteristics or the coatability were significantly inferior (Comparative Examples 1 to 3).

A secondary battery having an electrode obtained by using the composition for secondary battery electrode mixture layer containing the aqueous binder for secondary battery electrode of the present invention shows good durability (cycle characteristics), and is expected to be applied to a secondary battery for vehicle use. It is also useful for the use of active materials containing silicon, and is expected to contribute to the increase in capacity of batteries.
The aqueous binder for secondary battery electrodes of the present invention can be suitably used in particular for non-aqueous electrolyte secondary battery electrodes, and is particularly useful for non-aqueous electrolyte lithium ion secondary batteries having high energy density.

Claims (15)

A binder for a lithium ion secondary battery electrode, comprising: a polymer or a salt thereof containing 50% by mass or more and 100% by mass or less of structural units derived from an ethylenically unsaturated carboxylic acid monomer; and an alkali metal hydroxide or an alkali metal salt of a compound having a formula weight of 200 or less and having no ethylenically unsaturated groups . 2. The binder for lithium ion secondary battery electrodes according to claim 1, wherein the lithium ion secondary battery electrode is a negative electrode containing a silicon-based negative electrode active material. 3. The binder for a lithium ion secondary battery electrode according to claim 1, wherein the polymer is a crosslinked polymer or a non-crosslinked polymer. 4. The binder for a lithium ion secondary battery electrode according to claim 3, wherein the crosslinked polymer is obtained by using a crosslinkable monomer, and an amount of the crosslinkable monomer used is 0.05 parts by mass or more and 5.0 parts by mass or less per 100 parts by mass of a total amount of the non -crosslinkable monomers. 4. The binder for a lithium ion secondary battery electrode according to claim 3 , wherein the non-crosslinked polymer contains 50 mass % or less of a structural unit derived from vinyl alcohol. The binder for lithium ion secondary battery electrodes according to any one of claims 1 to 5 , wherein a viscosity of a 2% by mass aqueous solution of the polymer is 10,000 mPa·s or less. The binder for a lithium ion secondary battery electrode according to any one of claims 1 to 6 , wherein the degree of neutralization of the polymer is 70 mol % or more. The binder for a lithium ion secondary battery electrode according to any one of claims 1 to 7 , wherein the amount of the alkali metal salt used is 5.0 parts by mass or more and 175 parts by mass or less per 100 parts by mass of the total amount of the polymer. The binder for a lithium ion secondary battery electrode according to any one of claims 1 to 8 , wherein the alkali metal salt includes at least one selected from the group consisting of a lithium salt, a sodium salt, and a potassium salt. 10. The binder for a lithium ion secondary battery electrode according to claim 9 , wherein the lithium salt is lithium acetate. The binder for a lithium ion secondary battery electrode according to any one of claims 1 to 10 , further comprising a styrene butadiene rubber (SBR)-based latex and/or carboxymethyl cellulose (CMC). A composition for a lithium ion secondary battery electrode mixture layer, comprising the lithium ion secondary battery electrode binder according to any one of claims 1 to 11 , an active material, and water. The lithium ion secondary battery electrode mixture layer composition according to claim 12 , wherein the lithium ion secondary battery electrode mixture layer composition has a pH of less than 12.5. A lithium ion secondary battery electrode comprising a mixture layer formed on a surface of a current collector from the composition for a lithium ion secondary battery electrode mixture layer according to claim 12 or 13 . A lithium ion secondary battery comprising the lithium ion secondary battery electrode according to claim 14 .