Secondary battery electrode binder and use thereof, and method for manufacturing secondary battery electrode binder

Abstract

The present invention is a secondary battery electrode binder containing a carboxyl group-containing crosslinked polymer or a salt thereof, wherein the carboxyl group-containing crosslinked polymer contains 0.1-20 mass% of structural units derived from a nitrile group-containing ethylenically unsaturated monomer, and the carboxyl group-containing crosslinked polymer or the salt thereof has a particle diameter measured in an acetonitrile medium with a volume-based median diameter (D50) of 0.20-2.0 μm and a water swelling degree at pH 8 of 30-80.

Description

Binder for secondary battery electrodes and its use, and method for manufacturing a binder for secondary battery electrodes

[0001] This invention relates to a binder for secondary battery electrodes, its use, and a method for manufacturing a binder for secondary battery electrodes.

[0002] Various energy storage devices, such as nickel-metal hydride batteries, lithium-ion batteries, and electric double-layer capacitors, have been put into practical use as secondary batteries. The electrodes used in these secondary batteries are manufactured by coating and drying a composition for forming an electrode mixture layer containing an active material and a binder onto a current collector. For example, in lithium-ion batteries, an aqueous binder containing styrene-butadiene rubber (SBR) latex and carboxymethylcellulose (CMC) is used as the binder for the negative electrode mixture layer composition. On the other hand, organic solvent-based binders, such as a polyvinylidene fluoride (PVDF) N-methyl-2-pyrrolidone (NMP) solution, are widely used as binders for the positive electrode mixture layer.

[0003] In recent years, as the applications of various secondary batteries have expanded, there has been a growing demand for improved energy density, reliability, and durability. For example, to increase the electrical capacity of lithium-ion secondary batteries, there is an increasing trend towards using silicon-based active materials as the negative electrode active material. However, silicon-based active materials are known to undergo large volume changes during charging and discharging, and repeated use can lead to peeling or detachment of the electrode mixture layer, resulting in a decrease in battery capacity and deterioration of cycle characteristics (durability). To suppress such problems, research is being conducted to improve durability by firmly bonding the active materials together with a binder (binding properties), reducing the size of the active materials to alleviate stress associated with swelling and shrinkage, and by devising additives for the electrolyte.

[0004] In this context, acrylic acid polymers have been reported to be effective as binders that possess good cycle characteristics and are effective in improving the durability of anode mixture layers using silicon-based active materials.

[0005] Patent Document 1 discloses a binder containing a copolymer composed of a salt of acrylic acid or an acrylic acid derivative and acrylonitrile or an acrylonitrile derivative. Since it can follow the expansion and contraction of the silicon-based active material, it is described that the cycle characteristics can be improved.

[0006] Japanese Unexamined Patent Application Publication No. 2015-115109

[0007] However, the binder for a secondary battery electrode disclosed in Patent Document 1 contains a copolymer composed of acrylonitrile or an acrylonitrile derivative, and thus can follow the expansion and contraction of the silicon-based active material, and can improve the cycle characteristics. However, it has been a problem that it is impossible to achieve both the storage stability and the coating property of the composition for a secondary battery electrode mixture layer (electrode slurry).

[0008] The present invention has been made in view of such circumstances, and an object thereof is to provide a binder for a secondary battery electrode and a method for producing the same, which are excellent in storage stability in a low-shear region and coating property in a high-shear region, and can improve the cycle characteristics of a secondary battery. In addition, a composition for a secondary battery electrode mixture layer containing the above binder, a secondary battery electrode obtained by using the composition, and a secondary battery are provided.

[0009] As a result of intensive studies to solve the above problems, the present inventors have found that in a binder for a secondary battery electrode containing a carboxyl group-containing polymer or a salt thereof, the content of a structural unit derived from a nitrile group-containing ethylenically unsaturated monomer of the crosslinked polymer is set within a specific range, and the volume-based median diameter (D50) measured in an acetonitrile medium of the crosslinked polymer or a salt thereof and the water swelling degree at pH 8 are set within specific ranges, whereby an electrode slurry excellent in thixotropy can be obtained and the cycle characteristics of a secondary battery can be improved, and the present invention has been completed.

[0010] The present invention is as follows: [1] A binder for secondary battery electrodes containing a carboxyl group-containing crosslinked polymer or a salt thereof, wherein the carboxyl group-containing crosslinked polymer contains 0.1% by mass or more and 20% by mass or less structural units derived from a nitrile group-containing ethylenically unsaturated monomer, and the carboxyl group-containing crosslinked polymer or a salt thereof has a particle size of 0.20 μm or more and 2.0 μm or less in volume-based median diameter (D50) as measured in an acetonitrile medium, and a water swelling degree of 30 or more and 80 or less at pH 8, the binder for secondary battery electrodes. [2] A secondary battery electrode binder according to claim 1, comprising graphite, silicon oxide, styrene / butadiene rubber, and sodium carboxymethylcellulose in a solid content mass ratio of 76.8:19.2:1.0:2.0:1.0, with a solid content concentration of 51% by mass in water as the solvent, wherein the value (TI) calculated by the following formula (1) is 3.0 or greater, as described in [1]. TI = η ２ｒｐｍ / η １００ｒｐｍ (1) η ２ｒｐｍ η: E-type viscosity (mPa·s) at 2 rpm １００ｒｐｍ: E-type viscosity at 100 rpm (mPa·s) [3] The binder for secondary battery electrodes according to [1] or [2], wherein the carboxyl group-containing crosslinked polymer is a polymer having living radical polymerization active units by an exchange chain transfer mechanism. [4] The binder for secondary battery electrodes according to [3], wherein the exchange chain transfer mechanism is a reversible addition-cleavage chain transfer mechanism. [5] The binder for secondary battery electrodes according to any one of [1] to [4], wherein the carboxyl group-containing crosslinked polymer contains 80% by mass or more and 99.9% by mass or less of structural units derived from a carboxyl group-containing ethylenically unsaturated monomer with respect to its total structural units. [6] The binder for secondary battery electrodes according to any one of [1] to [5], wherein the carboxyl group-containing crosslinked polymer is crosslinked with a crosslinkable monomer, and the amount of the crosslinkable monomer used is 0.05 mol% or more and 0.6 mol% or less with respect to the total amount of the non-crosslinkable monomer. [7] The carboxyl group-containing crosslinked polymer is neutralized to a degree of neutralization of 80 to 100 mol%, and the particle size measured in an aqueous medium is 0.1 μm or more and 10.0 μm or less in volume-based median diameter (D50), as described in any one of [1] to [6], for a secondary battery electrode binder. [8] A method for producing a binder for secondary battery electrodes containing a carboxyl group-containing crosslinked polymer or a salt thereof, comprising the steps of: polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer by precipitation polymerization or dispersion polymerization; and adding an exchange chain transfer mechanism type control agent in an amount of 0.0001 mol% or more and 0.50 mol% or less relative to the total amount of the monomer component containing the ethylenically unsaturated carboxylic acid monomer at the beginning or in the middle of the above step, wherein the exchange chain transfer mechanism type control agent contains 20% or more by mass and 99.9% or less by mass of structural units derived from a nitrile group-containing ethylenically unsaturated monomer relative to its total structural units, and is a polymer having living radical polymerization active units by an exchange chain transfer mechanism.[9] The carboxyl group-containing crosslinked polymer contains 0.1% by mass or more and 20% by mass or less of structural units derived from a nitrile group-containing ethylenically unsaturated monomer. The carboxyl group-containing crosslinked polymer or its salt has a particle diameter measured in an acetonitrile medium of 0.20 μm or more and 2.0 μm or less in terms of volume-based median diameter (D50), and a water swelling degree at pH 8 of 30 or more and 80 or less. The production method according to [8].

[10] A composition for a secondary battery electrode mixture layer, comprising the binder for a secondary battery electrode according to any one of [1] to [7], an active material, and water.

[11] A secondary battery electrode comprising a mixture layer formed from the composition for a secondary battery electrode mixture layer according to

[10] on the surface of a current collector.

[12] A secondary battery comprising the secondary battery electrode according to

[11] .

[0011] According to the binder for a secondary battery electrode of the present invention, since the thixotropy of the electrode slurry is excellent, it is possible to improve the storage stability in a low shear region and the coating property in a high shear region. In addition, a secondary battery excellent in cycle characteristics can be obtained.

[0012] It is a diagram showing an apparatus used for measuring the water swelling degree of a crosslinked polymer or its salt.

[0013] The binder for a secondary battery electrode of the present invention (hereinafter, also referred to as "this binder") contains a carboxyl group-containing crosslinked polymer (hereinafter, also referred to as "this crosslinked polymer") or its salt (hereinafter, also referred to as "this crosslinked polymer salt"), and can be made into a composition for a secondary battery electrode mixture layer (hereinafter, also referred to as "this composition") by mixing with an active material and water. The above composition may be in a slurry state that can be coated on a current collector, or may be prepared in a wet powder state so as to be applicable to press processing on the surface of a current collector. 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, the secondary battery electrode of the present invention can be obtained.

[0014] The following describes in detail the crosslinked polymer or its salt, a method for producing the crosslinked polymer or its salt, a composition for a secondary battery electrode mixture layer obtained using the binder, a secondary battery electrode, and a secondary battery. In this specification, "(meth)acrylic" means acrylic and / or methacrylic, and "(meth)acrylate" means acrylate and / or methacrylate. Also, "(meth)acryloyl group" means acryloyl group and / or methacryloyl group. In the numerical ranges described stepwise in this specification, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range described stepwise, and the upper or lower limit of that numerical range may be replaced with the value shown in the example.

[0015] 1. This crosslinked polymer or its salt may have structural units derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as "monomer (a)") and may contain 0.1% by mass or more and 20% by mass or less of structural units derived from a nitrile group-containing ethylenically unsaturated monomer (hereinafter also referred to as "monomer (b)") (hereinafter also referred to as "component (b)"). The monomer components, including monomer (a) and monomer (b), can be introduced into the polymer by precipitation polymerization or dispersion polymerization. First, the particle size and water swelling degree at pH 8 of this crosslinked polymer or its salt will be described, and the structural units of this crosslinked polymer will be described later.

[0016] <Particle size of the crosslinked polymer or its salt in acetonitrile medium> The particle size of the crosslinked polymer or its salt, as measured in acetonitrile medium (hereinafter also simply referred to as "particle size"), is 0.20 μm or more and 2.0 μm or less as the volume-based median diameter (D50). When the particle size is within this range, it exists in a suitable size within the composition, resulting in high stability of the composition and excellent binding properties between active materials, thereby improving the cycle characteristics.

[0017] The preferred range for particle size is 0.25 μm to 1.5 μm as the volume-based median diameter (D50), a more preferred range is 0.3 μm to 1.0 μm, and an even more preferred range is 0.35 μm to 0.6 μm. If the particle size is less than 0.20 μm, the coating properties may be insufficient in that the slurry viscosity of the composition at low shear rates increases. On the other hand, if the particle size is greater than 2.0 μm, the coating properties may be insufficient in that it is difficult to obtain a smooth coated surface of the composition.

[0018] In this specification, the particle size in acetonitrile media refers to the particle size of a crosslinked polymer or its salt in a substantially unswelled state with water. 1.0 g of powdered crosslinked polymer or its salt is mixed with 5.0 g of acetonitrile containing 99.5% by mass or more, and ultrasonic waves are irradiated for 30 seconds at an output of 25 W using an ultrasonic homogenizer (e.g., LUH150, manufactured by Yamato Scientific Co., Ltd., or an equivalent device) to obtain a dispersion. The particle size distribution of this dispersion is measured using a laser diffraction / scattering particle size analyzer (Microtrac MT-3300EXII, manufactured by Microtrac Bell) with the above-mentioned acetonitrile as the dispersion medium. By adding 0.05 mL of the dispersion to a circulating excess amount of dispersion medium, an appropriate scattered light intensity was obtained. After a few minutes, once the stability of the particle size distribution shape is confirmed, the particle size distribution is measured, and the volume-based median diameter (D50) is obtained as a representative value of the particle size.

[0019] <Water swelling degree of the crosslinked polymer or its salt at pH 8> The water swelling degree of the crosslinked polymer or its salt at pH 8 is preferably 30 or more and 80 or less. Within this range, both coating properties to the current collector and adhesion of the binder to the current collector can be satisfied simultaneously. If the water swelling degree is less than 30, the adhesion may decrease and the cycle characteristics may deteriorate, and if the water swelling degree is greater than 80, the coating properties may deteriorate. A more preferred range for the water swelling degree at pH 8 is 32 or more and 70 or less, an even more preferred range is 33 or more and 60 or less, an even more preferred range is 34 or more and 50 or less, and an even more preferred range is 35 or more and 40 or less.

[0020] In this specification, the degree of water swelling is calculated from the dry mass of the crosslinked polymer or its salt "(WA) g" and the amount of water absorbed when the crosslinked polymer or its salt is saturated with water at pH 8 "(WB) g" based on the following formula (2): Degree of water swelling = {(WA) + (WB)} / (WA) (2)

[0021] The degree of water swelling at pH 8 can be obtained by measuring the degree of water swelling of the crosslinked polymer or its salt in water at pH 8. For example, deionized water can be used as the water at pH 8, and the pH value may be adjusted as needed using an appropriate acid, alkali, or buffer solution. The measurement should be performed at 25 ± 5°C.

[0022] Those skilled in the art can adjust the degree of water swelling of the crosslinked polymer salt by controlling its composition and structure. For example, the degree of water swelling can be increased by introducing acidic functional groups or highly hydrophilic structural units into the crosslinked polymer. Furthermore, the degree of water swelling can usually be increased by lowering the degree of crosslinking of the crosslinked polymer.

[0023] Structural Units of This Crosslinked Polymer <Structural Units Derived from Ethylene-Unsaturated Carboxylic Acid Monomers (Monomer (a))> The carboxyl group-containing crosslinked polymer contained in this binder (this crosslinked polymer) may have structural units derived from ethylenically unsaturated carboxylic acid monomers (hereinafter also referred to as "component (a)"). When this crosslinked polymer has carboxyl groups due to the presence of such structural units, adhesion to the current collector is improved, and the desolvation effect of lithium ions and ionic conductivity are excellent, resulting in electrodes with low resistance and excellent high-rate characteristics. In addition, water swelling properties are imparted, which can improve the dispersion stability of active materials, etc., in this composition. Component (a) can be introduced into the polymer, for example, by polymerizing a monomer containing an ethylenically unsaturated carboxylic acid monomer. Alternatively, it can be obtained by (co)polymerizing (meth)acrylic acid ester monomers and then hydrolyzing them. Alternatively, (meth)acrylamide and (meth)acrylonitrile may be polymerized and then treated with a strong alkali, or an acid anhydride may be reacted with a polymer having hydroxyl groups.

[0024] Examples of monomer (a) include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid; (meth)acrylamide alkyl carboxylic acids such as (meth)acrylamidehexanoic acid and (meth)acrylamidedodecanoic acid; carboxyl group-containing ethylenically unsaturated monomers such as monohydroxyethyl (meth)acrylate succinate, ω-carboxy-caprolactone mono(meth)acrylate, β-carboxyethyl (meth)acrylate, or (partially) alkali neutralized products thereof. 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 because they yield polymers with long primary chain lengths due to their high polymerization rate and good binder binding strength, and acrylic acid is particularly preferred. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, polymers with a high carboxyl group content can be obtained.

[0025] The content of component (a) in the crosslinked polymer is preferably 80% by mass or more and 99.9% by mass or less relative to the total structural units of the crosslinked polymer. Including component (a) within this range can further improve the electrolyte resistance of the secondary battery electrode mixture layer and the cycle characteristics of the secondary battery. When the lower limit is 80% by mass or more, it is preferable that the electrolyte resistance of the secondary battery electrode mixture layer and the cycle characteristics of the secondary battery can be further improved, and it may be, for example, 82% by mass or more, or for example, 85% by mass or more, or for example, 90% by mass or more. The upper limit is, for example, 95% by mass or less, or for example, 90% by mass or less, or for example, 85% by mass or less, or for example, 83% by mass or less.

[0026] <Structural units derived from nitrile group-containing ethylenically unsaturated carboxylic acid monomer (monomer (b))> This crosslinked polymer has structural units derived from monomer (b) (hereinafter also referred to as "component (b)"). Due to the presence of these structural units, this crosslinked polymer exhibits excellent thixotropy in compositions for secondary battery electrode mixture layers (electrode slurry) and excellent cycle characteristics in secondary batteries.

[0027] Examples of monomer (b) include (meth)acrylonitrile; cyanoalkyl (meth)acrylate 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; and vinylidene cyanide. 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, and cyanoethyl (meth)acrylate is preferred because of its excellent affinity for the active material.

[0028] The content of the component (b) in the present crosslinked polymer is 0.1% by mass or more and 20% by mass or less based on all the structural units of the present crosslinked polymer. By containing the component (b) within such a range, excellent adhesiveness to the current collector can be easily ensured, and the cycle characteristics of the secondary battery can be improved. Preferably, it is 0.5% by mass or more and 18% by mass or less, more preferably 1.0% by mass or more and 15% by mass or less, and still more preferably 1.5% by mass or more and 10% by mass or less.

[0029] <Other Structural Units> The present crosslinked polymer may contain, in addition to the components (a) and (b), structural units derived from other ethylenically unsaturated monomers copolymerizable with these (hereinafter also referred to as "component (c)"). Examples of the component (c) include hydroxyl group-containing ethylenically unsaturated monomers (monomers represented by the following formula (1), monomers represented by the following formula (2)), ethylenically unsaturated monomer compounds having anionic groups other than carboxyl groups such as sulfonic acid groups and phosphoric acid groups, or nonionic ethylenically unsaturated monomers. The structural units derived from the monomer (d) can be introduced by copolymerizing a monomer containing a hydroxyl group-containing ethylenically unsaturated monomer, an ethylenically unsaturated monomer compound having anionic groups other than carboxyl groups such as sulfonic acid groups and phosphoric acid groups, or a nonionic ethylenically unsaturated monomer. CH ２ =C(R １ )COOR ２ (1) [In the formula, R １ represents a hydrogen atom or a methyl group, R ２ represents a monovalent organic group having 1 to 8 carbon atoms with a hydroxyl group, (R ３ O) ｍ H or R ４ O[CO(CH ２ ) ５ O] ｎ H. Here, R ３ represents an alkylene group having 2 to 4 carbon atoms, R ４ represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15. ] CH ２ =C(R ５ )CONR ６ R ７ (2) [In the formula, R ５R represents a hydrogen atom or a methyl group. ６ R represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms. ７ [This represents a hydrogen atom or a monovalent organic group.]

[0030] The content of component (c) in this crosslinked polymer can be 0% by mass or more and 18% by mass or less relative to the total structural units of the crosslinked polymer. The proportion of component (c) may be 0.1% by mass or more and 15% by mass or less, 0.5% by mass or more and 10% by mass or less, 1.0% by mass or more and 8% by mass or less, 3.0% by mass or more and 7% by mass or less, or 5.0% by mass or more and 6% by mass or less. Furthermore, when component (c) is contained at 0.1% by mass or more relative to the total structural units of the crosslinked polymer, the affinity to the electrolyte is improved, and therefore, an effect of improved lithium ion conductivity can also be expected.

[0031] This crosslinked polymer is a crosslinked polymer having a crosslinked structure. The method of crosslinking in this crosslinked polymer is not particularly limited, and examples of embodiments include the following methods: 1) Copolymerization of crosslinkable monomers 2) Utilization of chain transfer to polymer chains during radical polymerization Because this crosslinked polymer has a crosslinked structure, the binder containing the crosslinked polymer or its salt can have excellent binding strength. Among the above, the method by copolymerization of crosslinkable monomers is preferred because it is easy to operate and the degree of crosslinking can be easily controlled.

[0032] <Crossable Monomers> Examples of crosslinkable monomers include polyfunctional polymerizable monomers having two or more polymerizable unsaturated groups, and monomers having self-crosslinkable functional groups such as hydrolyzable silyl groups.

[0033] The above-mentioned polyfunctional polymerizable monomers are compounds having two or more polymerizable functional groups such as (meth)acryloyl groups and alkenyl groups in their molecules, and include polyfunctional (meth)acryloyl compounds, polyfunctional alkenyl compounds, and compounds having both (meth)acryloyl and alkenyl groups. These compounds may be used individually or in combination of two or more. Among these, polyfunctional alkenyl compounds are preferred because they easily yield a uniform crosslinked structure, and polyfunctional allyl ether compounds having two or more allyl ether groups in their molecules are particularly preferred.

[0034] Examples of polyfunctional (meth)acryloyl 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 bisamides such as methylenebisacrylamide and hydroxyethylenebisacrylamide.

[0035] Examples of polyfunctional alkenyl compounds include polyfunctional allyl ether compounds such as trimethylolpropanediallyl ether, trimethylolpropanetriallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, tetraallyloxyethane, and polyallyl saccharose; polyfunctional allyl compounds such as diallyl phthalate; and polyfunctional vinyl compounds such as divinylbenzene.

[0036] 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.

[0037] Specific examples of monomers having the self-crosslinkable functional group mentioned above include hydrolyzable silyl group-containing vinyl monomers. These compounds can be used individually or in combination of two or more.

[0038] The hydrolyzable silyl group-containing vinyl monomer is not particularly limited as long as it is a vinyl monomer having at least one hydrolyzable silyl group. Examples include vinylsilanes such as vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane, and vinyldimethylmethoxysilane; silyl group-containing acrylic acid esters such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, and methyldimethoxysilylpropyl acrylate; silyl group-containing methacrylic acid esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, and dimethylmethoxysilylpropyl methacrylate; silyl group-containing vinyl ethers such as trimethoxysilylpropyl vinyl ether; and silyl group-containing vinyl esters such as vinyl trimethoxysilylundecanoate.

[0039] When the crosslinked polymer is crosslinked with a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.15 parts by mass or more and 1.7 parts by mass or less, more preferably 0.2 parts by mass or more and 1.6 parts by mass or less, even more preferably 0.3 parts by mass or more and 1.5 parts by mass or less, even more preferably 0.5 parts by mass or more and 1.4 parts by mass or less, and even more preferably 0.6 parts by mass or more and 1.2 parts by mass or less, based on 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer). If the amount of crosslinkable monomer used is 0.15 parts by mass or more, it is preferable because, over a longer period of use than conventional methods, the conductive paths between the active materials are well maintained while suppressing expansion and contraction due to charging and discharging, resulting in an excellent retention rate of charge and discharge capacity. If it is 1.7 parts by mass or less, the stability of precipitation polymerization or dispersion polymerization tends to be higher. In particular, if the amount is 1.0 part by mass or less, the water-swollen particle size in the electrode slurry becomes suitable, and the bonding surface area to the active material increases, which is preferable because it allows the battery performance to be maintained even during excellent long-term use.

[0040] For similar reasons, the amount of the above-mentioned crosslinkable monomer used is preferably 0.05 mol% to 0.6 mol%, more preferably 0.07 mol% to 0.55 mol%, even more preferably 0.1 mol% to 0.5 mol%, even more preferably 0.15 mol% to 0.45 mol%, and even more preferably 0.2 mol% to 0.4 mol%.

[0041] This crosslinked polymer salt is in the form of a salt in which some or all of the carboxyl groups contained in the polymer are 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 do not adversely affect battery characteristics, alkali metal salts are more preferred, and lithium salts are particularly preferred because they have excellent cycle characteristics for secondary batteries.

[0042] Regarding the properties of this crosslinked polymer salt, it is preferable that the crosslinked polymer be used in the composition in the form of a salt, with the acidic groups such as carboxyl groups derived from the ethylenically unsaturated carboxylic acid monomer neutralized to such an extent that the degree of neutralization is 20 mol% or more. A degree of neutralization of 20 mol% or more is preferable because it results in good water swelling and makes it easier to obtain a dispersion stabilization effect. 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, in order to exhibit a superior charge / discharge capacity retention rate over a longer period of use than conventional methods. The upper limit of the degree of neutralization is 100 mol%, but it may also be 98 mol% or 95 mol%. In this specification, the above degree of neutralization can be calculated from the charge values ​​of the monomer having an acidic group such as a carboxyl group and the neutralizing agent used for neutralization. The degree of neutralization can be determined by measuring 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 carboxylate salt after drying the cross-linked polymer salt powder under reduced pressure at 80°C for 3 hours using IR measurement.

[0043] <Particle size of the crosslinked polymer salt> In this composition, it is preferable that the crosslinked polymer salt does not exist as large-particle clumps (secondary aggregates) but is well dispersed as water-swellable particles with an appropriate particle size, so that the binder containing the crosslinked polymer salt can exhibit good binding performance.

[0044] Preferably, when the crosslinked polymer has a degree of neutralization based on the carboxyl groups of 80 to 100 mol% and is dispersed in water, the particle size (water-swelled particle size) is in the range of 0.1 μm or more and 10.0 μm or less in terms of volume-based median diameter (D50). A more preferred range for the above particle size is 0.15 μm or more and 8.0 μm or less, an even more preferred range is 0.20 μm or more and 6.0 μm or less, an even more preferred range is 0.25 μm or more and 4.0 μm or less, and an even more preferred range is 0.30 μm or more and 2.0 μm or less. If the particle size is in the range of 0.30 μm or more and 2.0 μm or less, it will be uniformly present in the composition at a suitable size, thus enabling the composition to have high stability and exhibit excellent binding properties. If the particle size exceeds 10.0 μm, there is a risk that the binding properties will be insufficient as described above. Furthermore, the difficulty in obtaining a smooth coating surface may result in insufficient coating properties. On the other hand, when the particle size is less than 0.1 μm, there are concerns from the standpoint of stable manufacturing.

[0045] <Thixotropy when used as a composition for secondary battery electrode composite layer> Here, as a composition for secondary battery electrode composite layer containing the binder, active material and water, it is preferable that the value (TI) calculated by the above formula (1) is 3.0 or higher for a composition for secondary battery electrode composite layer containing graphite:silicon oxide:secondary battery electrode binder described in claim 1:styrene / butadiene rubber:carboxymethylcellulose sodium in a solid content mass ratio of 76.8:19.2:1.0:2.0:1.0, with water as the solvent and a solid content concentration of 51% by mass, the value (TI) calculated by the above formula (1) is 3.0 or higher. By doing so, excellent storage stability in the low shear region and coating properties in the high shear region can be obtained, and the cycle characteristics of the secondary battery can be improved. The TI value is more preferably 3.2 or higher, even more preferably 3.5 or higher, and even more preferably 4.0 or higher. The upper limit of the TI value is preferably 30.0 or lower, may be 25.0 or lower, or 20.0 or lower.

[0046] 2. Method for producing the crosslinked polymer The crosslinked polymer is obtained by a production method comprising: a step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as "the monomer") by precipitation polymerization or dispersion polymerization; and a step of adding an exchange chain transfer mechanism type control agent in an amount of 0.0001 mol% or more and 0.50 mol% or less relative to the total amount of the monomer component containing the ethylenically unsaturated carboxylic acid monomer at the beginning or in the middle of the said step. Here, in the present invention, the above "beginning" means the point in time "0T" when the time from the start of the process of polymerizing the monomer to the end of the said process is defined as T. The above "in the middle" means the point in time "greater than 0T and 1.0T or less" when the time from the start of the process of polymerizing the monomer to the end of the said process is defined as T. In particular, when the concentration is "0.15T or more and 0.8T or less", it is preferable that the nitrile group-containing ethylenically unsaturated monomer can be introduced in a concentrated state on the particle surface of the crosslinked polymer while maintaining the water swelling degree of the crosslinked polymer at a suitable value, thereby improving the cycle characteristics of the secondary battery. Therefore, it is preferable that the concentration is 0.2T or more and 0.7T or less, even more preferable that it is 0.3T or more and 0.6T or less, and even more preferable that it is 0.3T or more and 0.5T or less.

[0047] Precipitation polymerization is a method of producing polymers by carrying out a polymerization reaction in a solvent that dissolves the monomer raw materials but does not substantially dissolve the resulting polymer. As polymerization progresses, the polymer particles grow larger through aggregation and growth, and a dispersion of polymer particles is obtained in which primary particles of tens to hundreds of nanometers are secondary aggregated to several micrometers to tens of micrometers. Dispersion stabilizers can also be used to control the particle size of the polymer. Furthermore, secondary aggregation can be suppressed by selecting the appropriate dispersion stabilizers and polymerization solvents. Generally, precipitation polymerization in which secondary aggregation is suppressed is also called dispersion polymerization.

[0048] Regarding the exchange chain transfer mechanism type control agent, the exchange chain transfer mechanism type control agent according to the present invention (hereinafter also simply referred to as "polymer (A)") contains 20% by mass or more and 99.9% by mass or less of structural units derived from a nitrile group-containing ethylenically unsaturated monomer (monomer (b)) with respect to its total structural units, and is a polymer having living radical polymerization active units by an exchange chain transfer mechanism. Examples of polymer (A) include a control agent in the reversible addition-cleavage chain transfer polymerization method (RAFT method) (hereinafter also referred to as "RAFT agent"), a control agent in the iodine transfer polymerization method, a control agent in the polymerization method using organic tellurium compounds (TERP method), a control agent in the polymerization method using organic antimony compounds (SBRP method), and a control agent in the polymerization method using organic bismuth compounds (BIRP method). It is preferable to use a polymer having a polymerization chain of a monomer composition containing monomer (b) and living radical polymerization active units by an exchange chain transfer mechanism. Polymer (A) may be used alone or in combination of two or more types.

[0049] Examples of monomer (b) include (meth)acrylonitrile; cyanoalkyl (meth)acrylate 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; and vinylidene cyanide. 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, and cyanoethyl (meth)acrylate is preferred because of its excellent affinity for the active material.

[0050] By adding polymer (A) at the beginning or during the polymerization process of monomer components containing ethylenically unsaturated carboxylic acid monomers by precipitation polymerization or dispersion polymerization, it becomes possible to surface-modify the polar surface of the particles with structural units derived from nitrile group-containing ethylenically unsaturated monomers. This is expected to enable both excellent binding properties and sedimentation stability. Among these, RAFT agents and control agents in iodine transfer polymerization are preferred, with RAFT agents being more preferred, as they can make the crosslinked structure of this crosslinked polymer more uniform.

[0051] As the RAFT agent, a polymer (A) having living radical polymerization active units by a reversible addition-cleavage chain transfer method can be used. Among the RAFT agents, those having trithiocarbonate in the molecule are particularly preferred because they can make the crosslinking structure of this crosslinked polymer even more uniform.

[0052] In the iodine transfer polymerization method, the control agent can be a polymer (A) having living radical active units obtained by the iodine transfer polymerization method.

[0053] Polymer (A) may be monofunctional, possessing one active site, or it may be bifunctional or more functional, possessing two or more active sites. A bifunctional or more exchange chain transfer mechanism type control agent allows the polymerization chain to extend in two or more directions. From the viewpoint of producing this crosslinked polymer, it may be preferable to use a bifunctional or trifunctional or more exchange chain transfer mechanism type control agent.

[0054] The amount of polymer (A) used is preferably 0.0001 mol% to 0.50 mol%, more preferably 0.0005 mol% to 0.40 mol%, more preferably 0.001 mol% to 0.30 mol%, and even more preferably 0.005 mol% to 0.20 mol%, relative to the total amount of the monomer, in order to make the crosslinked structure of the crosslinked polymer more uniform.

[0055] As the polymerization initiator used with polymer (A), known polymerization initiators such as azo compounds, organic peroxides, and inorganic peroxides can be used, but are not particularly limited. The usage conditions can be adjusted to achieve an appropriate amount of radical generation using known methods such as thermal initiation, redox initiation with a reducing agent, and UV initiation. In order to obtain this crosslinked polymer with a long primary chain length, it is preferable to set the conditions so that the amount of radical generation is reduced as much as possible within the range of acceptable manufacturing time. Among the above polymerization initiators, azo compounds are preferred because they are easy to handle safely and are less likely to cause side reactions during radical polymerization. Specific examples of the above azo compounds include, for example, 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), dimethyl-2,2'-azobis(2-methylpropionate), 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane-1-carbonitride), 2,2'-azobis[N-(2-propenyl)-2-methylpropionamide], and 2,2'-azobis(N-butyl-2-methylpropionamide). The above radical polymerization initiators may be used individually or in combination of two or more.

[0056] The preferred amount of polymerization initiator to use is, for example, 0.001 parts by mass or more and 2 parts by mass or more, or for example, 0.005 parts by mass or more and 1 part by mass or more, or for example, 0.01 parts by mass or more and 0.1 parts by mass or less, when the total amount of monomer components used is 100 parts by mass. If the amount of 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, it is easy to obtain a polymer with a long primary chain length. The ratio of polymerization initiator to be used is not particularly limited, but from the viewpoint of making the crosslinked structure of this crosslinked polymer uniform, it is preferable to use 0.5 mol or less of the polymerization initiator per 1 mol of the exchange chain transfer mechanism type control agent, and more preferably 0.2 mol or less. Furthermore, from the viewpoint of carrying out the polymerization reaction stably, the lower limit of the amount of polymerization initiator used per 1 mol of the exchange chain transfer mechanism type control agent is 0.001 mol. Therefore, the amount of polymerization initiator used per mol of exchange chain transfer mechanism type control agent is preferably in the range of 0.001 mol to 0.5 mol, and more preferably in the range of 0.005 mol to 0.2 mol.

[0057] The polymerization solvent can be selected from water and various organic solvents, taking into consideration the type of monomer used. To obtain a polymer with a longer primary chain length, it is preferable to use a solvent with a small chain transfer constant. Examples of 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. One of these can be used alone or in combination of two or more. Alternatively, they may be used as a mixed solvent with water. In this invention, a water-soluble solvent refers to one whose solubility in water at 20°C is greater than 10 g / 100 ml. Among the above, methyl ethyl ketone and acetonitrile are preferred because they produce fewer coarse particles and adhere less to the reactor, resulting in good polymerization stability; the precipitated crosslinked polymer is less prone to secondary aggregation (or even if secondary aggregation occurs, it dissolves easily in the aqueous medium); a small chain transfer constant is obtained, resulting in a polymer with a high degree of polymerization (primary chain length); and the neutralization process described later is easy to handle.

[0058] Furthermore, in order to ensure that the neutralization reaction proceeds stably and rapidly during the neutralization process, it is preferable to add a small amount of a highly polar solvent to the polymerization solvent. Examples of such highly polar solvents include water and methanol. The amount of highly polar solvent used is preferably 0.05% to 20.0% by mass, more preferably 0.1% to 10.0% by mass, even more preferably 0.1% to 5.0% by mass, and even more preferably 0.1% to 1.0% by mass, based on the total mass of the medium. If the proportion of the highly polar solvent is 0.05% by mass or more, an effect on the neutralization reaction is observed, and if it is 20.0% by mass or less, no adverse effect on the polymerization reaction is observed. In addition, in the polymerization of highly hydrophilic ethylenically unsaturated carboxylic acid monomers such as acrylic acid, the addition of a highly polar solvent improves the polymerization rate, making it easier to obtain polymers with longer primary chain lengths. Among highly polar solvents, water is particularly preferred due to its significant effect in improving the polymerization rate.

[0059] The reaction temperature during the polymerization reaction in the presence of polymer (A) is preferably 30°C to 120°C, more preferably 40°C to 110°C, and even more preferably 50°C to 100°C. If the reaction temperature is 30°C or higher, the polymerization reaction can proceed smoothly. On the other hand, if the reaction temperature is 120°C or lower, side reactions can be suppressed, and the restrictions on the initiators and solvents that can be used are relaxed.

[0060] Here, as polymer (A), as described above, a polymer (polymer (A)) can be used that has a polymerization chain (hereinafter simply referred to as "first polymerization chain") of a monomer composition (hereinafter simply referred to as "first monomer") containing a nitrile group-containing ethylenically unsaturated monomer (monomer (b)) as an essential component and living radical polymerization active units via an exchange chain transfer mechanism.

[0061] In producing the crosslinked polymer by polymerizing the monomer in the presence of polymer (A), polymer (A) can be used as a starting point for polymerization of the monomer and as a dispersion stabilizer in the polymerization solvent of the crosslinked polymer. This allows for the acquisition of the crosslinked polymer as dispersed fine particles, in which polymerization chains having structural units derived from the monomer are bonded to the polymerization chains of polymer (A). This improves polymerization stability, that is, suppresses aggregation of the crosslinked polymer during the polymerization process, thereby suppressing the generation of coarse aggregated particles and resulting in a crosslinked polymer with small particle size and a narrow particle size distribution.

[0062] When polymerizing the monomer in the presence of polymer (A) to produce the crosslinked polymer, polymer (A) can be used in an amount of 0.1 parts by mass to 20 parts by mass per 100 parts by mass of the total mass of the monomer, for example, in order to make polymer (A) function as a dispersion stabilizer. By using it within this range, it is possible to produce the crosslinked polymer mainly containing the monomer while polymer (A) functions as a dispersion stabilizer. This is because if polymer (A) is less than 0.1 parts by mass, it is difficult to obtain a sufficient dispersion stabilization effect, and if it exceeds 20 parts by mass, it is difficult to improve its functionality as a dispersion stabilizer.

[0063] Polymer (A) can be used in amounts of, for example, 0.5 parts by mass or more, or for example, 1 part by mass or more, per 100 parts by mass of the total mass of the monomer. Polymer (A) can also be used in amounts of, for example, 15 parts by mass or less, for example, 10 parts by mass or less, or for example, 5 parts by mass or less.

[0064] A polymer (A) can be obtained by polymerizing a monomer composition containing a first monomer in the presence of a known exchange chain transfer mechanism type control agent, thereby obtaining a polymer (A) having a first polymerization chain having structural units derived from the first monomer and living polymerization active units via an exchange chain transfer mechanism.

[0065] The polymerization conditions for producing polymer (A) are well known to those skilled in the art, and various polymerization processes can be cited, such as bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization. However, considering that the polymer serves as the polymerization starting point and functions as a dispersion stabilizer in the production of this crosslinked polymer, solution polymerization can be used, for example. Furthermore, polymerization conditions such as the type of exchange chain transfer mechanism control agent, the type and amount of polymerization initiator used, the polymerization solvent, and the reaction temperature are appropriately selected in accordance with the description in [Regarding Exchange Chain Transfer Mechanism Control Agents] above, and the amount of exchange chain transfer mechanism control agent used is appropriately adjusted according to the number average molecular weight (Mn) of the target polymer (A). As exchange chain transfer mechanism control agents, RAFT agents and control agents in iodine transfer polymerization are preferred because they can narrow the molecular weight distribution of polymer (A). Furthermore, the concentration of polymer (A) when producing polymer (A) is not particularly limited in relation to the total mass of the polymerization solvent and the first monomer charged, but can be, for example, 10% by mass or more and 80% by mass or less, for example, 15% by mass or more and 70% by mass or less, or for example, 20% by mass or more and 70% by mass or less.

[0066] Typically, when a monofunctional exchange chain transfer mechanism type control agent is used, the living polymerization active unit is provided at the end of the first polymerization chain. When a bifunctional or more exchange chain transfer mechanism type control agent is used, the living polymerization active unit is used as a base point, and branches are formed in two or more directions, each containing the first polymerization chain. In either embodiment, if another polymerization chain is provided, this other polymerization chain is directly bonded to the living polymerization active unit, and the first polymerization chain is bonded to the distal end of the other polymerization chain so that the first polymerization chain is provided more distally to the living polymerization active unit.

[0067] Polymer (A) may also comprise two or more first polymerization chains. For example, by performing living radical polymerization using a first monomer of a certain composition, and then performing living radical polymerization using the first monomer of a different composition, polymer (A) can be obtained that comprises a first polymerization chain (block) having structural units derived from the first monomer of different compositions.

[0068] The number-average molecular weight (Mn) of polymer (A) is not particularly limited, but is 1,000 or more, for example, 3,000 or more, for example, 5,000 or more, for example, 7,000 or more, for example, 8,000 or more, for example, 10,000 or more. Also, the same Mn is, for example, 150,000 or less, for example, 100,000 or less, for example, 80,000 or less, for example, 50,000 or less, for example, 40,000 or less, for example, 30,000 or less, for example, 25,000 or less, for example, 20,000 or less, for example, 15,000 or less, for example, 12,000 or less. If the Mn value is less than 3,000, the binding properties of the crosslinked polymer salt will be insufficient, and if it is greater than 150,000, it will be difficult to dissolve in the polymerization solvent for dispersion polymerization, making it difficult to obtain the crosslinked polymer. In particular, the Mn value is preferably 1,000 to 40,000, more preferably 1,000 to 30,000, and even more preferably 1,000 to 20,000, in terms of excellent thixotometry properties of the electrode slurry.

[0069] The weight-average molecular weight (Mw) of polymer (A) is not particularly limited, but for example it may be 5,000 or more, 7,000 or more, 9,000 or more, 10,000 or more, 13,000 or more, or 15,000 or more. Furthermore, Mw is, for example, 200,000 or less, 150,000 or less, 100,000 or less, 80,000 or less, 60,000 or less, 55,000 or less, 50,000 or less, 45,000 or less, 40,000 or less, 36,000 or less, 35,000 or less, 30,000 or less, and 25,000 or less. If Mw is less than 5,000, the binding properties of the crosslinked polymer salt are insufficient, and if it is greater than 200,000, it becomes difficult to dissolve in the polymerization solvent for dispersion polymerization, making it difficult to obtain the crosslinked polymer.

[0070] The Mw and Mn of polymer (A) can both be measured by gel permeation chromatography using polystyrene as a standard substance. The detailed chromatography conditions can be those disclosed in the examples below.

[0071] The molecular weight distribution (Mw / Mn) of polymer (A) is not particularly limited, but for example it is 2.5 or less, 2.4 or less, 2.3 or less, 2.0 or less, 1.6 or less, 1.5 or less, 1.4 or less, and 1.3 or less. The molecular weight distribution is, for example, 1.1 or more, 1.2 or more, 1.3 or more, 1.4 or more, and 1.5 or more.

[0072] The narrower the molecular weight distribution of polymer (A), the smaller the particle size of the resulting crosslinked polymer tends to be. A molecular weight distribution of 2.4 or less is preferable, 1.7 or less is preferable to improve polymerization stability, 1.6 or less is preferable, and 1.4 or less is even preferable.

[0073] The SP value of polymer (A) is not particularly limited, but in terms of producing this crosslinked polymer with excellent settling stability and binding properties, for example, 17 to 27 (MPa) 1 / 2 It is preferable that the SP value of polymer (A) is, for example, 27 (MPa). 1 / 2 ) or less, and also, for example, 26 ((MPa) 1 / 2 ) or less, and for example, 25 ((MPa) 1 / 2 ) or less. Also, the SP value of polymer (A) is, for example, 17 (MPa) 1 / 2 ) or higher, and for example, 18 (MPa) 1 / 2 ) or higher, and for example, 19 (MPa) 1 / 2 That's all.

[0074] The above SP value can be calculated using the calculation method described in "Polymer Engineering and Science" 14(2), 147 (1974) by R. F. Fedors. Specifically, the calculation method is as shown in formula (3). δ: SP value ((MPa) １／２ ) ΔE ｖａｐ : Molar heat of vaporization of each atomic group (cal / mol) V: Molar volume of each atomic group (cm³) ３ / mol)

[0075] <First Monomer> The first monomer contains a nitrile group-containing ethylenically unsaturated monomer as an essential component, and may also contain (meth)acrylic acid esters, styrenes, maleimide compounds, unsaturated acid anhydrides and unsaturated carboxylic acid compounds, hydroxyl group-containing ethylenically unsaturated monomers, etc. One or more of these can be used in combination.

[0076] Examples of (meth)acrylic acid esters include alkyl (meth)acrylic acid ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, n-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, phenylethyl (meth)acrylate, and phenoxyethyl (meth)acrylate; and alkoxyalkyl (meth)acrylic acid ester compounds such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate. One or more of these can be used. Among these, methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate are preferred from the viewpoint of polymerizability.

[0077] Styrene compounds include styrene and its derivatives. Examples of styrene derivatives include α-methylstyrene, β-methylstyrene, vinylxylene, vinylnaphthalene, o-methylstyrene, m-methylstyrene, p-methylstyrene, o-ethylstyrene, m-ethylstyrene, p-ethylstyrene, p-n-butylstyrene, p-isobutylstyrene, p-t-butylstyrene, o-methoxystyrene, m-methoxystyrene, p-methoxystyrene, o-chloromethylstyrene, p-chloromethylstyrene, o-chlorostyrene, p-chlorostyrene, o-hydroxystyrene, m-hydroxystyrene, p-hydroxystyrene, divinylbenzene, etc., and one or more of these can be used. Among these, styrene, o-methoxystyrene, m-methoxystyrene, p-methoxystyrene, o-hydroxystyrene, m-hydroxystyrene, and p-hydroxystyrene are preferred from the viewpoint of polymerizability.

[0078] Maleimide compounds include maleimides and N-substituted maleimide compounds. Examples of N-substituted maleimide compounds include N-methylmaleimide, N-ethylmaleimide, N-n-propylmaleimide, N-isopropylmaleimide, N-n-butylmaleimide, N-isobutylmaleimide, N-tert-butylmaleimide, N-pentylmaleimide, N-hexylmaleimide, N-heptylmaleimide, N-octylmaleimide, N-laurylmaleimide, N-stearylmaleimide, and other N-alkyl-substituted maleimide compounds; N-cyclopentylmaleimide, N-cyclo Examples include N-cycloalkyl-substituted maleimide compounds such as hexylmaleimide; and N-aryl-substituted maleimide compounds such as N-phenylmaleimide, N-(4-hydroxyphenyl)maleimide, N-(4-acetylphenyl)maleimide, N-(4-methoxyphenyl)maleimide, N-(4-ethoxyphenyl)maleimide, N-(4-chlorophenyl)maleimide, N-(4-bromophenyl)maleimide, and N-benzylmaleimide. One or more of these can be used. For example, N-phenylmaleimide is used.

[0079] Examples of unsaturated anhydrides include maleic anhydride, itaconic anhydride, and citraconic anhydride, and one or more of these can be used.

[0080] Examples of unsaturated carboxylic acid compounds include (meth)acrylic acid, cinnamic acid, crotonic acid, and unsaturated dicarboxylic acids such as maleic acid, fumaric acid, itaconic acid, and citraconic acid, as well as monoalkyl esters of unsaturated dicarboxylic acids. One or more of these can be used.

[0081] The first monomer preferably includes, among these, at least (meth)acrylic acid esters, styrenes, or hydroxyl group-containing ethylenically unsaturated monomers (monomers represented by formula (1) and formula (2) above). This is because (meth)acrylic acid esters and styrenes are easily subjected to living polymerization and can impart appropriate hydrophobicity and affinity to organic solvents. This can impart hydrophobicity or affinity to organic solvents to the first polymerization chain. In this way, for example, when the crosslinked polymer is produced by dispersion polymerization in a polar organic solvent, polymer (A) tends to be present on the surface of the crosslinked polymer, thereby improving the dispersion stability of the crosslinked polymer. This is because hydroxyl group-containing ethylenically unsaturated monomers can impart water dispersibility and affinity to the active material of the binder containing the crosslinked polymer salt. This can improve binding properties.

[0082] The (meth)acrylic acid esters are, for example, 20% by mass or more of the total mass of the first monomer. This is because a concentration of 20% by mass or more facilitates living polymerization and appropriately imparts moderate hydrophobicity and affinity to organic solvents. Alternatively, the concentration may be 30% by mass or more, 35% by mass or more, 40% by mass or more, 50% by mass or more, 60% by mass or more, 65% by mass or more, 70% by mass or more, or 75% by mass or more. Furthermore, the acrylic acid esters are 80% by mass or less of the total mass, for example, 75% by mass or less, 70% by mass or less, 65% by mass or less, 60% by mass or less, or 55% by mass or less.

[0083] The amount of styrene is, for example, 20% by mass or more of the total mass of the first monomer. This is because living polymerization is facilitated when the amount is 20% by mass or more, and appropriate hydrophobicity and affinity to organic solvents can be appropriately imparted. Alternatively, the amount may be 30% by mass or more, 35% by mass or more, 40% by mass or more, 50% by mass or more, 60% by mass or more, 65% by mass or more, 70% by mass or more, or 75% by mass or more. Alternatively, the amount of styrene may be 80% by mass or less of the total mass, 75% by mass or less, 70% by mass or less, 65% by mass or less, 60% by mass or less, or 55% by mass or less.

[0084] Maleimide compounds, acid anhydrides, and unsaturated carboxylic acid compounds can each be used individually, but it is preferable to use one or more of these four types in combination with styrenes. This is because all four types can maintain, adjust, or impart the hydrophobicity or organic solvent affinity of the first polymerization chain. In particular, one or more of (meth)acrylonitrile compounds such as acrylonitrile, maleimide compounds such as N-phenylmaleimide, and acid anhydrides are preferred. Unsaturated carboxylic acid compounds are preferred because they can easily change the polarity of polymer (A), among other things.

[0085] When used in combination with styrenes, the total amount of these first monomers other than styrenes is, for example, 20% by mass or more of the total mass of the first monomers (first monomer units of the first polymerization chain) used to polymerize the first polymerization chain. Alternatively, for example, it may be 25% by mass or more, 30% by mass or more, 35% by mass or more, 40% by mass or more, 50% by mass or more, or 60% by mass or more.

[0086] The hydroxyl group-containing ethylenically unsaturated monomer is, for example, 20% by mass or more of the total mass of the first monomer. This is because a concentration of 20% by mass or more can impart sedimentation stability to the electrode slurry containing the crosslinked polymer salt. Alternatively, it may be 30% by mass or more, 35% by mass or more, 40% by mass or more, 45% by mass or more, 50% by mass or more, 60% by mass or more, 65% by mass or more, or 70% by mass or more. Furthermore, the hydroxyl group-containing ethylenically unsaturated monomer may be 80% by mass or less of the total mass, 75% by mass or less, 70% by mass or less, 65% by mass or less, 60% by mass or less, or 55% by mass or less.

[0087] <First Polymerization Chain> The first polymerization chain may consist only of the first monomer described above, but other vinyl monomers may be used as the first monomer as needed. For example, known vinyl monomers such as (meth)acrylic acid and (meth)acrylic acid esters such as alkyl (meth)acrylates can be used. The amount of such other monomers is, for example, 10% by mass or less, 5% by mass or less, 3% by mass or less, 1% by mass or less, or 0.5% by mass or less, based on the total mass of the monomers constituting the first polymerization chain.

[0088] Furthermore, polymer (A) may also comprise blocks (other polymerization chains) different from the first polymerization chain. Such other polymerization chains may be added, for example, in a separate synthesis step after the formation of the first polymerization chain. In this case, polymer (A) comprising other polymerization chains (blocks) consisting of units derived from monomers other than the first monomer and having a different composition from the first polymerization chain can be obtained by continuously or newly supplying a radical polymerization initiator and other vinyl monomers to polymer (A) comprising the first polymerization chain. By being provided so as to be directly linked to the living radical active units described later and linked to the first polymerization chain, a portion of the monomers common to the monomer used in this crosslinked polymer can be provided in polymer (A) beforehand.

[0089] <Living Radical Polymerization Active Units> Polymer (A) possesses living radical polymerization active units via an exchange chain transfer mechanism. Therefore, when performing precipitation polymerization or dispersion polymerization of this monomer, various monomers can be selected for their solubility in the polymerization solvent and their function as dispersion stabilizers.

[0090] Examples of exchange chain transfer mechanisms for living radical polymerization active units in polymer (A) include reversible addition-cleavage chain transfer polymerization (RAFT method), iodine transfer polymerization, polymerization using organotellurium compounds (TERP method), polymerization using organoantimony compounds (SBRP method), and polymerization using organobismuth compounds (BIRP method). Among these, the RAFT method and iodine transfer polymerization method are preferred, with the RAFT method being more preferred, because they can reduce the particle size of the crosslinked polymer.

[0091] 3. Composition for Secondary Battery Electrode Mixture Layer The composition for secondary battery electrode mixture layer of the present invention comprises this binder, an active material, and water. Preferably, the amount of this binder used in this composition is 0.5 parts by mass or more and 7.0 parts by mass or less per 100 parts by mass of the total amount of the active material. The above amount may also be, for example, 0.8 parts by mass or more and 3.0 parts by mass or less, for example, 1.0 part by mass or more and 2.5 parts by mass or less, or for example, 1.2 parts by mass or more and 1.5 parts by mass or less. If the amount of binder used is 0.5 parts by mass or more, sufficient binding properties can be obtained. Furthermore, the dispersion stability of the active material can be ensured, and a uniform mixture layer can be formed. If the amount of binder used is 1.5 parts by mass or less, the composition will not become highly viscous, and coating properties for the current collector can be ensured. As a result, a mixture layer having a uniform and smooth surface can be formed.

[0092] Among the above active materials, lithium salts of transition metal oxides can be used as the positive electrode active material. For example, layered rock salt 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 cobaltate, lithium nickelate, and NCM{Li(Ni)}, which are called ternary systems. ｘ Co ｙ , Mn ｚ ), x+y+z=1} and NCA{Li(Ni １－ａ－ｂ Co ａ Al b Examples include )}. In addition, lithium manganate is an example of a spinel-type positive electrode active material. Besides oxides, phosphates, silicates, and sulfur can also be used, and examples of phosphates include olivine-type lithium iron phosphate. As a positive electrode active material, one of the above may be used alone, or two or more may be combined and used as a mixture or composite.

[0093] Furthermore, when a positive electrode active material containing layered rock salt-type lithium-containing metal oxide is dispersed in water, the dispersion becomes alkaline due to the exchange of lithium ions on the surface of the active material with hydrogen ions in the water. This may cause corrosion of common positive electrode current collector materials such as aluminum foil (Al). In such cases, it is preferable to neutralize the alkali leaching from the active material by using an unneutralized or partially neutralized crosslinked polymer as a binder. Moreover, it is preferable to use an amount of the unneutralized or partially neutralized crosslinked polymer such that the amount of unneutralized carboxyl groups in the crosslinked polymer is equivalent to or greater than the amount of alkali leaching from the active material.

[0094] Since all positive electrode active materials have low electrical conductivity, a conductive additive may be added. Examples of such conductive additives include carbon-based materials such as carbon black, carbon nanotubes, carbon fibers, graphite powder, and carbon fibers. Among these, carbon black, carbon nanotubes, and carbon fibers are preferred because they easily provide excellent conductivity. Ketjenblack and acetylene black are preferred as carbon blacks. One of the conductive additives may be used alone, or two or more may be used in combination. From the viewpoint of balancing conductivity and energy density, the amount of conductive additive used can 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 active material. Furthermore, the positive electrode active material may be surface-coated with a conductive carbon-based material.

[0095] On the other hand, examples of negative electrode active materials 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, with graphite such as natural graphite and artificial graphite, and hard carbon being more preferred. In the case of graphite, spheroidized graphite is preferably used in terms of battery performance, and the preferred range of particle size is, for example, 1 to 20 μm, or for example, 5 to 15 μm. Furthermore, in order to increase the energy density, metals or metal oxides that can absorb lithium, such as silicon and tin, can also be used as negative electrode active materials. Among these, silicon has a higher capacity than graphite, and active materials made 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. The amount of silicon-based active material used is 5.0% by mass or more of the total amount of active material, in order to improve the electrical capacity of the secondary battery. It can also be, for example, 10.0% by mass or more, or for example, 20.0% by mass or more.

[0096] Since carbon-based active materials possess good electrical conductivity on their own, it is not always necessary to add conductive additives. When conductive additives are added for purposes such as further reducing resistance, the amount used, from the perspective of energy density, should be, 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.

[0097] When the composition is in slurry form, the amount of active material used is, for example, in the range of 10 to 75% by mass, or in the range of 30 to 65% by mass, relative to the total amount of the composition. If the amount of active material used is 10% by mass or more, migration of binders and the like is suppressed, 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 coating properties of the composition can be ensured, and a uniform mixture layer can be formed.

[0098] This composition uses water as the medium. Furthermore, to adjust the properties and drying properties of this composition, a mixed solvent with lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, tetrahydrofuran, or N-methyl-2-pyrrolidone may be used. The proportion of water in the mixed medium is, for example, 50% by mass or more, and also, for example, 70% by mass or more.

[0099] When this composition is made into a coatable slurry, the content of the water-containing medium in the whole composition can be, for example, in the range of 25 to 60% by mass, or for example, 35 to 60% by mass, from the viewpoint of the coatability of the slurry, the energy cost required for drying, and productivity.

[0100] This composition may also contain other binder components such as styrene-butadiene rubber (SBR) latex, carboxymethylcellulose (CMC), acrylic latex, and polyvinylidene fluoride latex. When other binder components are used in combination, the amount used can be, for example, 0.1 to 5 parts by mass or less, or 0.1 to 2 parts by mass or less, or 0.1 to 1 part by mass or less, per 100 parts by mass of the total amount of active material. If the amount of other binder components used exceeds 5 parts by mass, the resistance may increase, and the high-rate properties may become insufficient. Among the above, SBR latex and CMC are preferred in terms of their excellent balance of binding properties and flexural resistance, and the combination of SBR latex and CMC is more preferable.

[0101] The above-mentioned SBR latex refers to an aqueous dispersion of a copolymer having structural units derived from aromatic vinyl monomers such as styrene and structural units derived from aliphatic conjugated diene monomers such as 1,3-butadiene. Examples of aromatic vinyl monomers include styrene, α-methylstyrene, vinyltoluene, and divinylbenzene, and one or more of these can be used. The amount of structural units derived from the aromatic vinyl monomer in the copolymer can be in the range of 20 to 70% by mass, or in the range of 30 to 60% by mass, mainly from the viewpoint of binding properties. Examples of aliphatic conjugated diene monomers include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 2-chloro-1,3-butadiene, and one or more of these can be used. The structural units derived from the aliphatic conjugated diene monomer in the copolymer can be in the range of, for example, 30 to 70% by mass, or 40 to 60% by mass, in that the binder binding properties and the flexibility of the resulting electrode are good. In addition to the above monomers, other monomers such as nitrile group-containing monomers like (meth)acrylonitrile, carboxyl group-containing monomers like (meth)acrylic acid, itaconic acid, maleic acid, and ester group-containing monomers like (meth)acrylate may be used as copolymer monomers to further improve properties such as binding properties. The structural units derived from the above other monomers in the copolymer can be in the range of, for example, 0 to 30% by mass, or 0 to 20% by mass.

[0102] The above-mentioned CMC refers to substituted nonionic cellulosic semi-synthetic polymer compounds obtained by substituting them with carboxymethyl groups, and their salts. Examples of the above-mentioned nonionic cellulosic semi-synthetic polymer compounds include alkylcelluloses such as methylcellulose, methylethylcellulose, ethylcellulose, and microcrystalline cellulose; and hydroxyalkylcelluloses such as hydroxyethylcellulose, hydroxybutylmethylcellulose, hydroxypropylcellulose, hydroxypropylmethylcellulose, hydroxyethylmethylcellulose, hydroxypropylmethylcellulose stearoxy ether, carboxymethylhydroxyethylcellulose, alkylhydroxyethylcellulose, and nonoxynylhydroxyethylcellulose.

[0103] The secondary battery electrode mixture layer composition of the present invention comprises the above-mentioned binder, active material, and water as essential components, and is obtained by mixing each component using known means. The method of mixing each component is not particularly limited, and known methods can be used, but a method of dry-blending powder components such as the active material, conductive additive, and binder, and then mixing them with a dispersion medium such as water, and then dispersing and kneading is preferred. When obtaining the composition in slurry form, it is preferable to produce a slurry that is free from poor dispersion and aggregation. As a mixing means, known mixers such as planetary mixers, thin-film swirling mixers, and orbital mixers can be used, but it is preferable to use a thin-film swirling mixer in that a good dispersion state can be obtained in a short time. Furthermore, when using a thin-film swirling mixer, it is preferable to perform pre-dispersion with a stirrer such as a disperser beforehand. The pH of the slurry is not particularly limited as long as the effects of the present invention are achieved, but it is preferably less than 12.5, and for example, when CMC is included, it is more preferable to have a pH of less than 11.5, and even more preferable to have a pH of less than 10.5, in that there is less concern about hydrolysis. Furthermore, the viscosity of the slurry is not particularly limited as long as it achieves the effects of the present invention, but as a B-type viscosity (25°C) at 20 rpm, it can be, for example, in the range of 100 to 30,000 mPa·s, or in the range of 500 to 20,000 mPa·s, or in the range of 1,000 to 10,000 mPa·s. If the viscosity of the slurry is within the above range, good coating properties can be ensured.

[0104] 4. Secondary Battery Electrode The secondary battery electrode of the present invention comprises a composite layer formed from the secondary battery electrode composite layer composition of the present invention on the surface of a current collector such as copper or aluminum. The composite layer is formed by coating the surface of the current collector with the composition and then drying off a medium such as water. The method of coating with the composition is not particularly limited, and known methods such as the doctor blade method, dip method, roll coat method, comma coat method, curtain coat method, gravure coat method, and extrusion method can be used. Furthermore, the drying can be carried out by known methods such as hot air blowing, reduced pressure, (far) infrared radiation, and microwave irradiation. Typically, the composite layer obtained after drying is subjected to compression treatment using a die press and a roll press. Compression can be used to bring the active material and binder into close contact, improving the strength of the composite layer and its adhesion to the current collector. Compression can be used to adjust the thickness of the composite layer to, for example, 30 to 80% of the thickness before compression, and the thickness of the composite layer after compression is generally about 4 to 200 μm.

[0105] 5. Secondary Battery A secondary battery can be manufactured by providing a separator and an electrolyte to the electrodes of the secondary battery of the present invention. The electrolyte may be in liquid or gel form. The separator is placed between the positive and negative electrodes of the battery and plays a role in preventing short circuits caused by contact between the two electrodes and in holding the electrolyte to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane with good ionic permeability and mechanical strength. Specific materials that can be used include polyethylene, polyolefins such as polypropylene, and polytetrafluoroethylene.

[0106] The electrolyte can be a known one commonly used depending on the type of active material. In lithium-ion secondary batteries, specific solvents include cyclic carbonates with high dielectric constant and high electrolyte solubility, such as propylene carbonate and ethylene carbonate, as well as low-viscosity chain carbonates such as ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate. These can be used individually or as mixed solvents. The electrolyte contains LiPF in these solvents. ６ LiSbF ６LiBF ４ LiClO ４ LiAlO ４ Lithium salts such as these are dissolved and used. In nickel-metal hydride secondary batteries, an aqueous potassium hydroxide solution can be used as the electrolyte. Secondary batteries are obtained by housing positive and negative electrode plates, separated by a separator, in a spiral or stacked structure in a case or the like.

[0107] The present invention will be described in detail below based on examples. However, the present invention is not limited to these examples. In the following, "parts" and "%" mean parts by mass and mass%, respectively, unless otherwise specified. In the following examples, the evaluation of the metal salt of the carboxyl group-containing crosslinked polymer was carried out by the following method.

[0108] <<Synthesis of Polymer (A)>> (Method for Measuring the Molecular Weight of Polymer (A)) For polymer (A) obtained as described below, gel permeation chromatography (GPC) was performed under the conditions described below to obtain the number-average molecular weight (Mn) and weight-average molecular weight (Mw) in terms of polystyrene. The molecular weight distribution (Mw / Mn) was also calculated from the obtained values.

[0109] The GPC was performed under the following conditions: Column: TSKgel SuperMultiporeHZ-M (Tosoh Corporation) x 4 Solvent: Tetrahydrofuran Temperature: 40°C Detector: RI Flow rate: 600 μL / min

[0110] (Synthesis Example 1: Synthesis of Polymer 1) In a 1 L flask equipped with a stirrer and thermometer, 2.5 parts of RAFT agent (2-{[(2-carboxyethyl)sulfanylthiocarbonyl]sulfanyl}propanoic acid: hereinafter also referred to as "BM1429"), 1.2 parts of 2,2'-azobis (2,4-dimethylvaleronitrile (manufactured by Fujifilm Wako Pure Chemical Industries, trade name "V-65": hereinafter also referred to as "V-65"), 100 parts of 2-cyanoethyl acrylate (hereinafter also referred to as "2-CEA"), and 400 parts of acetonitrile were charged, and nitrogen b The mixture was thoroughly degassed by bruning, and polymerization was started in a constant temperature bath at 60°C. After 4 hours, the reaction was stopped by cooling to room temperature. Polymer 1 was obtained by reprecipitation and purification of the polymerization solution from methanol / water = 90 / 10 (vol%), followed by vacuum drying. Gas chromatography measurements showed that the reaction rate of the obtained polymer 1 was 89%, and the composition ratio of polymer 1 calculated based on this reaction rate was 96.0% by mass. The molecular weight of polymer 1 was Mn 9,800, Mw 11,000, and Mw / Mn 1.12.

[0111] (Synthesis Examples 2-5: Synthesis of Polymers 2-5) Polymers 2-5 were obtained by performing the same procedure as in the synthesis of Polymer 1, except that the amount of each raw material used was as shown in Table 1. The physical properties of each obtained polymer were measured in the same manner as in Polymer 1, and the results are shown in Table 1.

[0112]

[0113] The details of the compounds used in Table 1 are shown below: • 2-CEA: 2-cyanoethyl acrylate • AN: acrylonitrile • MA: methyl acrylate • BA: n-butyl acrylate • BM1429: 2-{[(2-carboxyethyl)sulfanylthiocarbonyl]sulfanyl}propanoic acid • V-65: 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.)

[0114] (Measurement of particle size (pre-swelling particle size) in acetonitrile medium) 1.0 g of powder of a carboxyl group-containing crosslinked polymer or its salt, and 5.0 g of acetonitrile with an output of 99.5% by mass or more were weighed into a 20 cc container, and an ultrasonic homogenizer (Yamato Scientific Co., Ltd., LUH150) was irradiated at an output of 25 W for 30 seconds to obtain a dispersion. Next, the particle size distribution of the above dispersion was measured using a laser diffraction / scattering particle size analyzer (Microtrac MT-3300EXII, Microtrac Bell Corporation) with acetonitrile as the dispersion medium. By adding 0.05 mL of the dispersion to a circulating mixture of excess dispersion medium, an appropriate scattered light intensity was obtained. After a few minutes, once the stability of the particle size distribution shape was confirmed, the particle size distribution was measured to obtain the volume-based median diameter (D50) as a representative value of the particle size.

[0115] [Water swelling degree at pH 8] The water swelling degree at pH 8 is expressed as the ratio of the water-swelled mass of the sample to the dry mass of the sample. The water swelling degree was measured by the following method. The measuring apparatus is shown in Figure 1. The measuring apparatus consists of <Element 1> to <Element 3> in Figure 1. <Element 1> consists of a burette 1 with a branch tube for air venting, a pinchcock 2, a silicone tube 3, and a polytetrafluoroethylene tube 4. <Element 2> consists of a support cylinder 8 with many holes on its bottom surface on top of a funnel 5, and a filter paper 10 for the apparatus is placed on top of that. <Element 3> A sample 6 (measurement sample) of a crosslinked polymer or its salt is sandwiched between two sample-fixing filter papers 7, and the sample-fixing filter papers 7 are fixed with adhesive tape 9. All filter papers used are ADVANTEC No. 2, with an inner diameter of 55 mm. <Element 1> and <Element 2> are connected by a silicone tube 3. Furthermore, the height of the funnel 5 and the support cylinder 8 is fixed relative to the burette 1, and they are set so that the lower end of the polytetrafluoroethylene tube 4 installed inside the burette branch tube is at the same height as the bottom surface of the support cylinder 8 (dotted line in Figure 1).

[0116] Next, the measurement method will be explained. (1) Remove the pinchcock 2 in <Element 1>, and pour deionized water (pH 8.0 ± 1.0) through the silicone tube 3 from the top of the burette 1, so that the burette 1 is filled with deionized water 12 up to the filter paper 10 for the apparatus. Next, close the pinchcock 2 and remove the air from the polytetrafluoroethylene tube 4 connected to the burette branch tube with a rubber stopper. In this way, the deionized water 12 is continuously supplied from the burette 1 to the filter paper 10 for the apparatus. (2) Next, remove the excess deionized water 12 that has seeped out from the filter paper 10 for the apparatus, and then record the reading (a) on the scale of the burette 1. (3) Weigh 0.1 to 0.2 g of the sample powder and place it evenly in the center of the sample fixing filter paper 7 as shown in <Element 3>. Sandwich the sample with another sheet of filter paper and secure the two sheets of filter paper with adhesive tape 9 to fix the sample. The filter paper on which the sample is fixed is placed on the filter paper 10 for the apparatus shown in <Element 2>. (4) Next, the reading of the scale of the burette 1 (b) after 30 minutes have elapsed since the lid 11 was placed on the filter paper 10 for the apparatus is recorded. (5) The sum of the water absorption of the sample to be measured and the water absorption of the two sample-fixing filter papers 7 (c) is calculated by (a-b). By the same procedure, the water absorption of only the two filter papers 7, which do not contain the crosslinked polymer or its salt sample (d) is measured. (6) After performing the above procedure, the degree of water swelling was calculated using the following formula. The solid content used in the calculation was the value measured by the method described later. This formula is equivalent to the calculation formula (2) described above. Water swelling degree = {Dry weight of the sample (g) + (c - d)} / {Dry weight of the sample (g)} where Dry weight of the sample (g) = Weight of the sample (g) × (Solid content (%) ÷ 100)

[0117] The method for measuring the solid content is described below. Approximately 0.5 g of the sample is placed in a weighing bottle whose weight has been measured in advance [Weighing bottle weight = B (g)], and the entire weighing bottle is accurately weighed [W ０ (g) Place the sample in the weighing bottle into an airless drying oven and dry at 155°C for 45 minutes, then measure the weight of the weighing bottle at that time [W １ (g)], the solid content was calculated using the following formula: Solid content (%) = (W １ -B) / (W０ -B) × 100

[0118] (Measurement of particle size (water-swollen particle size) in an aqueous medium) 0.25 g of cross-linked polymer salt powder and 49.75 g of deionized water were weighed into a 100 cc container and placed in a rotation / revolution type agitator (Sinky Co., Ltd., Awatori Rentaro AR-250). Next, stirring (rotation speed 2,000 rpm / revolution speed 800 rpm, 7 minutes) and defoaming (rotation speed 2,200 rpm / revolution speed 60 rpm, 1 minute) were performed to prepare a hydrogel in which the cross-linked polymer salt was swollen in water. Then, the particle size distribution of the above hydrogel was measured using a laser diffraction / scattering particle size analyzer (Microtrac MT-3300EXII, Microtrac Bell Co., Ltd.) with deionized water as the dispersion medium. When a sufficient amount of hydrogel to obtain an appropriate scattered light intensity was added to a hydrogel circulating in an excess amount of dispersion medium, the particle size distribution shape measured after a few minutes stabilized. Once stability was confirmed, the particle size distribution was measured, and the volume-based median diameter (D50), which is a representative value of the particle size, was obtained. For the cross-linked polymer salt R-13, it was neutralized with lithium hydroxide monohydrate until the degree of neutralization reached 80 mol%, and a hydrogel in a swollen state in water was prepared, and the particle size in the aqueous medium was measured.

[0119] (Production Example 1: Production of Carboxyl Group-Containing Crosslinked Polymer Salt R-1) For polymerization, a reactor equipped with a stirring blade, thermometer, reflux condenser, and nitrogen inlet tube was used. 567 parts of acetonitrile, 2.2 parts of deionized water, 98.0 parts of acrylic acid (hereinafter also referred to as "AA"), 0.9 parts of trimethylolpropanediallyl ether (manufactured by Osaka Soda Co., Ltd., trade name "Neoallyl T-20"), and triethylamine equivalent to 1.0 mol% of the above AA were charged into the reactor. After thoroughly purging the reactor with nitrogen, the internal temperature was raised to 55°C. After confirming that the internal temperature had stabilized at 55°C, 0.040 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name "V-65") were added as a polymerization initiator. When turbidity was observed in the reaction solution, this point was taken as the polymerization initiation point. Four hours after the start of polymerization (corresponding to the point of 0.33 T during the polymerization process), 2.0 parts of polymer 1 were added all at once. The monomer concentration was calculated to be 15%. Twelve hours after the start of polymerization, the polymerization reaction solution was cooled, and after the internal temperature dropped to 25°C, lithium hydroxide monohydrate (hereinafter referred to as "LiOH·H") was added. ２ 51.4 parts of the powder of (also known as "O") were added. After addition, stirring was continued at room temperature for 12 hours to obtain a slurry-like polymerization reaction solution in which particles of carboxyl group-containing crosslinked polymer salt R-1 (lithium salt, degree of neutralization 90 mol%) were dispersed in the medium.

[0120] The obtained polymerization reaction solution was centrifuged to settle the polymer particles, and the supernatant was removed. Then, the precipitate was redispersed in acetonitrile of the same weight as the polymerization reaction solution, and the washing operation, in which the polymer particles were settled by centrifugation and the supernatant was removed, was repeated twice. The precipitate was recovered and dried under reduced pressure at 80°C for 3 hours to remove volatile components, thereby obtaining a powder of carboxyl group-containing crosslinked polymer salt R-1. Since carboxyl group-containing crosslinked polymer salt R-1 is hygroscopic, it was sealed and stored in a container with water vapor barrier properties. Furthermore, the powder of carboxyl group-containing crosslinked polymer salt R-1 was subjected to IR measurement, and the degree of neutralization was determined from the intensity ratio of the peak derived from the C=O group of the carboxylic acid and the peak derived from the C=O group of lithium carboxylate, which was 90 mol%, equal to the calculated value from the initial stage. The particle size in acetonitrile medium was 0.53 μm, the degree of water swelling at pH 8 was 36, and the water-swollen particle size was 1.75 μm.

[0121] (Production Examples 2-16 and Comparative Production Examples 1-5: Production of Carboxyl Group-Containing Crosslinked Polymer Salts R-2-R-21) The same procedure as in Production Example 1 was performed, except that the amount of each raw material used was as shown in Table 2, to obtain polymerization reaction solutions containing carboxyl group-containing crosslinked polymer salts R-2-R-21. Next, the same procedure as in Production Example 1 was performed on each polymerization reaction solution to obtain powdered carboxyl group-containing crosslinked polymer salts R-2-R-21. Each carboxyl group-containing crosslinked polymer salt was sealed and stored in a container with water vapor barrier properties. The physical properties of each obtained crosslinked polymer salt were measured in the same manner as in Production Example 1, and the results are shown in Table 2.

[0122]

[0123] The details of the compounds used in Table 2 are shown below. • AA: Acrylic acid • ACMO: Acryloylmorpholine • T-20: Trimethylolpropanediallyl ether (manufactured by Osaka Soda Co., Ltd., trade name "Neoallyl T-20") • TEA: Triethylamine • TOA: Trioctylamine • V-65: 2,2'-Azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) • LiOH • H ２ O: Lithium hydroxide monohydrate, Na ２ CO ３: Sodium carbonate K ２ CO ３ Potassium carbonate

[0124] Example 1 (Preparation of Electrode Mixture Layer Composition) Artificial graphite (product name "SCMG-CF" manufactured by Showa Denko Corporation) and SiO (5 μm, manufactured by Osaka Titanium Technologies Co., Ltd.) were used as active materials. A mixture of crosslinked polymer salt R-1, styrene / butadiene rubber (SBR), and sodium carboxymethylcellulose (CMC) was used as the binder. Artificial graphite:SiO:R-1:SBR:CMC were added to a planetary mixer (Hibismix 2P-03, manufactured by Primix Corporation) in a mass ratio of 76.8:19.2:1.0:2.0:1.0 (solids) with water as the diluent so that the solids content of the electrode mixture layer composition was 51% by mass. The mixture was mixed for 1 hour and 30 minutes to prepare an electrode mixture layer composition (electrode slurry) in a slurry state with a solids content of 51% by mass using water as the solvent.

[0125] <Evaluation of Thixotropy of Electrode Slurry> For each of the following examples and comparative examples, the electrode slurries obtained were adjusted to 25°C ± 1°C, and the viscosity was measured using an E-type viscometer at 2 rpm and 100 rpm. The thixotropy (TI value) of the slurry was then determined using the following formula (1): TI = η ２ｒｐｍ / η １００ｒｐｍ (1) η ２ｒｐｍ η: E-type viscosity (mPa·s) at 2 rpm １００ｒｐｍ : E-type viscosity (mPa·s) at 100 rpm of the electrode slurry of Example 1 ２ｒｐｍ It is 12,000 mPa·s, and η １００ｒｐｍ Since the pressure was 3,400 mPa·s, the TI value calculated using the above formula was 3.5, and the thixotropy of the slurry was evaluated as "A" based on the following criteria. Note that a higher TI value indicates better storage stability and coating properties of the electrode slurry. (Criteria for determining thixotropy) A: TI value of 3.5 or higher B: TI value of 3.0 or higher but less than 3.5 C: TI value less than 3.0

[0126] (Preparation of Negative Electrode Plate) Next, the electrode slurry was applied to the current collector (copper foil, thickness: 16.5 μm) using a variable applicator, and a composite layer was formed by drying in a forced-air dryer at 80°C for 15 minutes. After that, the composite layer had a thickness of 50 ± 5 μm and a composite density of 1.60 ± 0.10 g / cm³. ３ After rolling to the desired shape, the negative electrode plate was punched out in a 3 cm square for battery evaluation.

[0127] (Preparation of positive electrode plates) LiNi as the positive electrode active material in N-methylpyrrolidone (NMP) solvent. ０．５ Co ０．２ Mn ０．３ O ２ A composition for the positive electrode composite layer was prepared by mixing 100 parts of (NCM) and 2 parts of acetylene black, and adding 4 parts of polyvinylidene fluoride (PVDF) as a positive electrode binder. Next, the composite layer was formed by coating and drying the composition for the positive electrode composite layer onto a current collector (aluminum foil, thickness: 20 μm) using a variable applicator. Subsequently, the thickness of the composite layer was 125 μm ± 1 μm, and the composite density was 3.0 ± 0.10 g / cm³. ３ After rolling to achieve the desired shape, the positive electrode plate was punched out in a 3 cm square for battery evaluation.

[0128] (Preparation of electrolyte) A mixed solvent consisting of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio EC:DMC = 3:7) is to be mixed with vinylene carbonate (VC) to a total of 1% by mass and fluoroethylene carbonate (FEC) to a total of 2% by mass, and LiPF ６ A non-aqueous electrolyte was prepared by dissolving 1.2 mol / liter of [the substance].

[0129] (Fabrication of the secondary battery) The battery was constructed by attaching lead terminals to the positive and negative electrodes, and placing the electrode bodies opposite each other with a separator (made of polyethylene: film thickness 16 μm, porosity 47%) in between. The electrodes were then placed in an aluminum laminate battery casing, injected with electrolyte, and sealed to create a test battery. The design capacity of this prototype battery is 50 mAh. The battery's design capacity was based on a charging termination voltage of 4.2 V.

[0130] <Evaluation of Cycle Characteristics> The lithium-ion secondary battery made of laminated cells prepared above was subjected to charge and discharge operations at a charge / discharge rate of 0.1C under conditions of 2.5 to 4.2V in a 45°C environment, and the initial capacity C was evaluated. ０ The following was measured. Furthermore, the charge and discharge cycles were repeated under CC discharge conditions of 2.5 to 4.2V in a 25°C environment at a charge and discharge rate of 0.5C, and the capacity C after 100 cycles was measured. １００ The following was measured. Here, the cycle characteristic (ΔC) was calculated using the following formula: ΔC = C １００ / C ０ ×100 (%) The ΔC calculated using the above formula is 86.9%, and the cycle characteristics were evaluated as "A" based on the following criteria. Note that a higher value of ΔC indicates better cycle characteristics. (Cycle characteristics evaluation criteria) A: Charge / discharge capacity retention rate of 86.0% or more B: Charge / discharge capacity retention rate of 85.0% or more and less than 86.0% C: Charge / discharge capacity retention rate of 84.0% or more and less than 85.0% D: Charge / discharge capacity retention rate less than 84.0%

[0131] ≪Overall Evaluation≫ The overall evaluation was based on the thixotropy and cycle characteristics evaluation results of the electrode slurry described above, according to the criteria shown in Table 3 below. In this evaluation, A to C are considered passing levels. The thixotropy evaluation of the electrode slurry of Example 1 was "A", and the cycle characteristics evaluation was also "A", so the overall evaluation was "A".

[0132]

[0133] Examples 2 to 16 and Comparative Examples 1 to 5: Electrode slurries were prepared by the same procedure as in Example 1, except that the formulations were as shown in Table 4. The thixotropy of the electrode slurry and the cycle characteristics of the secondary battery obtained using it were evaluated. The results are shown in Table 4.

[0134]

[0135] The details of the compounds used in Table 4 are as follows: • SBR: Styrene-butadiene rubber • CMC: Sodium carboxymethylcellulose

[0136] ≪Evaluation Results≫ As is clear from the results of Examples 1 to 16, the electrode slurry obtained using the carboxyl group-containing crosslinked polymer salt of the present invention exhibited excellent storage stability in the low-shear region and coating properties in the high-shear region, and the secondary battery obtained using the electrode slurry exhibited excellent cycle characteristics. Among these, focusing on the content of structural units derived from nitrile group-containing ethylenically unsaturated monomers in the carboxyl group-containing crosslinked polymer salt, the cycle characteristics of the secondary battery were superior when the content was 10% by mass or less (Example 1: 1.9% by mass, Example 2: 1.0% by mass, Example 3: 4.8% by mass) compared to when the content was greater than 10% by mass (Example 4: 19.2% by mass). This is presumed to be because when the content is 10% by mass or less, swelling in the electrolyte becomes less likely, improving the binding properties, and it was found that there is an optimal range for this value. Furthermore, when the content of structural units derived from nitrile group-containing ethylenically unsaturated monomers in the carboxyl group-containing crosslinked polymer salt was 1.0% by mass or more and 20% by mass or less (Examples 1, 2, 3, and 4), the electrode slurry exhibited excellent thixot properties.

[0137] Furthermore, the smaller the particle size in acetonitrile (Example 1: 0.53 μm < Example 7: 0.92 μm < Example 8: 1.89 μm), the better the cycle characteristics. In addition, focusing on the degree of neutralization of the carboxyl group-containing crosslinked polymer, when the degree of neutralization was 90 mol% or higher (Example 1), the thixotropy and cycle characteristics of the electrode slurry were better than when the degree of neutralization was 70 mol% (Example 13). Furthermore, focusing on the type of carboxyl group-containing crosslinked polymer salt, when it was a lithium salt (Example 1), the cycle characteristics of the secondary battery were better than when it was a sodium salt (Example 14) or a potassium salt (Example 15).

[0138] Here, focusing on the timing of the addition of the exchange chain transfer mechanism type control agent (polymer (A)) as a method for producing carboxyl group-containing crosslinked polymer salts, the results showed that when the addition timing was 0.33T (Example 1), the cycle characteristics of the secondary battery were superior to those when the addition timing was 0T (Example 5) or 0.67T (Example 6). Furthermore, focusing on the molecular weight of the exchange chain transfer mechanism type control agent (polymer (A)), the results showed that when the number average molecular weight was 20,000 or less (Example 1: 9,800, Example 9: 1,900), the thixotropy of the electrode slurry was superior to when the number average molecular weight was greater than 20,000 (Example 10: 49,000).

[0139] In contrast, when structural units derived from nitrile group-containing ethylenically unsaturated monomers were not included, the thixotropy of the electrode slurry and the cycle characteristics of the secondary battery were inferior (Comparative Example 1). Furthermore, when the degree of water swelling was less than 30 (Comparative Example 2) or greater than 80 (Comparative Example 3), the thixotropy and cycle characteristics of the electrode slurry were significantly inferior. Moreover, when the particle size in acetonitrile was less than 0.20 μm (Comparative Example 4) or greater than 2.0 μm (Comparative Example 5), the cycle characteristics were significantly inferior.

[0140] The secondary battery electrode mixture layer composition (electrode slurry) containing the binder for secondary battery electrodes of the present invention exhibits good thixotropy, and has excellent storage stability in the low shear region and coating properties in the high shear region. Furthermore, secondary batteries equipped with electrodes obtained using the above binder exhibit good durability (cycle characteristics). Therefore, good integrity can be ensured, and it is expected that good durability (cycle characteristics) will be observed even after repeated charging and discharging, and it is expected to contribute to increasing the capacity of secondary batteries for automotive use and the like. The binder for secondary battery electrodes of the present invention can be suitably used in non-aqueous electrolyte secondary battery electrodes, and is particularly useful in non-aqueous electrolyte lithium-ion secondary batteries with high energy density.

Claims

1. A binder for secondary battery electrodes containing a carboxyl group-containing crosslinked polymer or a salt thereof, wherein the carboxyl group-containing crosslinked polymer contains 0.1% by mass or more and 20% by mass or less structural units derived from a nitrile group-containing ethylenically unsaturated monomer, and the carboxyl group-containing crosslinked polymer or a salt thereof has a particle size of 0.20 μm or more and 2.0 μm or less in volume-based median diameter (D50) as measured in an acetonitrile medium, and a water swelling degree of 30 or more and 80 or less at pH 8.

2. A secondary battery electrode binder according to claim 1, comprising graphite, silicon oxide, the binder for secondary battery electrodes described in claim 1, styrene / butadiene rubber, and sodium carboxymethylcellulose in a solid content mass ratio of 76.8:19.2:1.0:2.0:1.0, with a solid content concentration of 51% by mass in water as the solvent, wherein the value (TI) calculated by the following formula (1) is 3.0 or higher. TI = η ２ｒｐｍ / η １００ｒｐｍ (1) η ２ｒｐｍ η: E-type viscosity (mPa·s) at 2 rpm １００ｒｐｍ : E-type viscosity at 100 rpm (mPa·s) 3. The binder for secondary battery electrodes according to claim 1 or 2, wherein the carboxyl group-containing crosslinked polymer is a polymer having living radical polymerization active units by an exchange chain transfer mechanism.

4. The binder for secondary battery electrodes according to claim 3, wherein the exchange chain transfer mechanism is a reversible addition-breaking chain transfer mechanism.

5. The binder for secondary battery electrodes according to claim 1 or 2, wherein the carboxyl group-containing crosslinked polymer contains 80% by mass or more and 99.9% by mass or less of structural units derived from a carboxyl group-containing ethylenically unsaturated monomer, relative to its total structural units.

6. The binder for secondary battery electrodes according to claim 1 or 2, wherein the carboxyl group-containing crosslinked polymer is crosslinked with a crosslinkable monomer, and the amount of the crosslinkable monomer used is 0.05 mol% or more and 0.6 mol% or less relative to the total amount of the non-crosslinkable monomer.

7. The binder for secondary battery electrodes according to claim 1 or 2, wherein the carboxyl group-containing crosslinked polymer is neutralized to a degree of neutralization of 80 to 100 mol%, and the particle size measured in an aqueous medium is 0.1 μm or more and 10.0 μm or less in volume-based median diameter (D50).

8. A method for producing a binder for secondary battery electrodes containing a carboxyl group-containing crosslinked polymer or a salt thereof, comprising: a step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer by precipitation polymerization or dispersion polymerization; and a step of adding an exchange chain transfer mechanism type control agent in an amount of 0.0001 mol% or more and 0.50 mol% or less relative to the total amount of the monomer component containing the ethylenically unsaturated carboxylic acid monomer at the beginning or in the middle of the above step, wherein the exchange chain transfer mechanism type control agent contains 20% or more by mass and 99.9% by mass or less of structural units derived from a nitrile group-containing ethylenically unsaturated monomer relative to its total structural units, and is a polymer having living radical polymerization active units by an exchange chain transfer mechanism.

9. The method for producing the carboxyl group-containing crosslinked polymer according to claim 8, wherein the carboxyl group-containing crosslinked polymer contains 0.1% by mass to 20% by mass of structural units derived from a nitrile group-containing ethylenically unsaturated monomer, and the carboxyl group-containing crosslinked polymer or a salt thereof has a particle size of 0.20 μm to 2.0 μm in volume-based median diameter (D50) as measured in an acetonitrile medium, and a water swelling degree of 30 to 80 at pH 8.

10. A composition for a secondary battery electrode mixture layer, comprising the binder for secondary battery electrodes, active material, and water according to claim 1 or 2.

11. A secondary battery electrode comprising a composite layer formed from the secondary battery electrode composite layer composition described in claim 10 on the surface of a current collector.

12. A secondary battery comprising the secondary battery electrode described in claim 11.

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