Aqueous binder for secondary battery electrodes, composition for secondary battery electrode mixture layer, secondary battery electrode, and secondary battery

Abstract

The present invention provides an aqueous binder for secondary battery electrodes, said aqueous binder enabling the achievement of a secondary battery that exhibits excellent cycle characteristics, while ensuring good coatability by reducing the electrode slurry viscosity in cases where the solid content concentration of a composition for electrode mixture layers is higher than that in the prior art. The present invention also provides: a composition for secondary battery electrode mixture layers, said composition containing the above-described aqueous binder; a secondary battery electrode which is obtained using this composition; and a secondary battery.　The present invention relates to an aqueous binder for secondary battery electrodes, said aqueous binder containing: a polymer which contains from 50% by mass to 100% by mass of a structural unit that is derived from an ethylenically unsaturated carboxylic acid monomer, or a salt of the polymer; and an alkali metal hydroxide or an alkali metal salt of a compound that does not have an ethylenically unsaturated group, while having a formula weight of 200 or less.

Description

Technical Field

[0001] The present invention relates to an aqueous binder for a secondary battery electrode, a composition for a secondary battery electrode mixture layer, a secondary battery electrode, and a secondary battery.

Background Art

[0002] As secondary batteries, various power storage devices such as nickel-metal hydride secondary batteries, lithium-ion secondary batteries, and electric double layer capacitors have been put into practical use. Electrodes used in these secondary batteries are produced by applying and drying a composition for forming an electrode mixture layer containing an active material, a binder, etc. on a current collector. For example, in a lithium-ion secondary battery, as a binder used in the composition for the negative electrode mixture layer, an aqueous binder containing styrene-butadiene rubber (SBR) latex and carboxymethyl cellulose (CMC) is used. On the other hand, as a binder used in the positive electrode mixture layer, an N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride (PVDF) is widely used.

[0003] As the applications of various secondary batteries expand, there is a tendency for the requirements for improving energy density, reliability, and durability to increase. For example, for the purpose of increasing the electric capacity of a lithium-ion secondary battery, the specification of using a silicon-based active material as the negative electrode active material is increasing. However, it is known that the silicon-based active material has a large volume change during charge and discharge, and peeling or dropping of the electrode mixture layer occurs with repeated use. As a result, there has been a problem that the capacity of the battery decreases and the cycle characteristics (durability) deteriorate. In order to suppress such problems, studies have been conducted to improve durability by firmly binding the active materials with a binder (binding property), reducing the size of the active materials to relieve the stress associated with swelling and shrinkage, or devising additives for the electrolytic solution.

[0004] Under such circumstances, it has recently been reported that an acrylic acid-based polymer is effective as a binder for a negative electrode binder layer using a silicon-based active material. For example, in Patent Document 1, it is disclosed that by using a polymer obtained by crosslinking polyacrylic acid with a specific crosslinking agent as a binder, even when an active material containing silicon is used, good cycle characteristics can be exhibited without the electrode structure being destroyed. Patent Document 2 discloses a crosslinked acrylic acid polymer having a specific particle size in a 1% NaCl aqueous solution, and it is disclosed that the polymer exhibits high binding properties.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] Although the binders disclosed in Patent Documents 1 and 2 can both impart good cycle characteristics or binding properties, with the improvement of the performance of secondary batteries, further improvement of cycle characteristics is required. In general, a secondary battery electrode is obtained by applying and drying a composition for an electrode binder layer (hereinafter, also referred to as "electrode slurry") containing an active material and a binder on the surface of an electrode current collector. At this time, from the viewpoint of increasing the drying efficiency of the electrode slurry and improving the productivity of the electrode, it is advantageous to increase the solid content concentration of the composition for the electrode binder layer, but it becomes difficult to ensure good coatability. Under such circumstances, the compositions for the electrode binder layer described in Patent Documents 1 and 2 increase the binding properties by greatly increasing the spread in water by micro-crosslinking of the acrylic acid-based polymer used as a binder, but the viscosity greatly increases even with a small addition. Therefore, it has been difficult to reduce the viscosity of the electrode slurry in a state where the solid content concentration of the composition for the electrode binder layer is increased.

[0007] The present invention has been made in view of the above circumstances, and an object thereof is to obtain a secondary battery that can ensure coating properties by reducing the viscosity of an electrode slurry and exhibit excellent cycle characteristics when the solid content concentration of the composition for the electrode binder layer is higher than that of the conventional one. It is to provide an aqueous binder for a secondary battery electrode. Further, together with this, it is to provide a composition for a secondary battery electrode binder layer containing the above aqueous binder, a secondary battery electrode obtained using the composition, and a secondary battery.

Means for Solving the Problems

[0008] As a result of intensive studies to solve the above problems, the present inventors have found that when the solid content concentration of the composition for the electrode binder layer is higher than that of the conventional one, a polymer containing a specific amount of a structural unit derived from an ethylenically unsaturated carboxylic acid monomer or a salt thereof, and By using an aqueous binder for a secondary battery electrode containing a specific alkali metal compound, it is possible to obtain a secondary battery that can ensure coating properties by reducing the viscosity of the electrode slurry and exhibit excellent cycle characteristics, and the present invention has been completed.

[0009] The present invention is as follows. 〔1〕An aqueous binder for a secondary battery electrode, which contains a polymer or a salt thereof containing 50% by mass or more and 100% by mass or less of a structural unit derived from an ethylenically unsaturated carboxylic acid monomer, and an alkali metal hydroxide or an alkali metal salt of a compound having a formula weight of 200 or less and having no ethylenically unsaturated group. 〔2〕The aqueous binder for a secondary battery electrode according to 〔1〕, wherein the polymer is a crosslinked polymer or a non-crosslinked polymer. 〔3〕The crosslinked polymer is obtained by using a crosslinkable monomer, and the amount of the crosslinkable monomer used is 0.05 parts by mass or more and 5.0 parts by mass or less with respect to 100 parts by mass of the total amount of non-crosslinkable monomers. The aqueous binder for a secondary battery electrode according to 〔2〕. 〔4〕The non-crosslinked polymer contains 50% by mass or less of a structural unit derived from vinyl alcohol. The aqueous binder for a secondary battery electrode according to 〔2〕. 〔5〕The viscosity of the 2 mass% aqueous solution of the polymer is 10,000 mPa·s or less. The aqueous binder for a secondary battery electrode according to any one of 〔1〕to 〔4〕. 〔6〕The degree of neutralization of the polymer is 70 mol% or more. The aqueous binder for a secondary battery electrode according to any one of 〔1〕to 〔5〕. 〔7〕The amount of the alkali metal salt used is 5.0 parts by mass or more and 175 parts by mass or less with respect to 100 parts by mass of the total amount of the polymer. The aqueous binder for a secondary battery electrode according to any one of 〔1〕to 〔6〕. 〔8〕The alkali metal salt contains at least one selected from the group consisting of lithium salts, sodium salts, and potassium salts. The aqueous binder for a secondary battery electrode according to any one of 〔1〕to 〔7〕. 〔9〕The lithium salt is lithium acetate. The aqueous binder for a secondary battery electrode according to 〔8〕. 〔10〕Furthermore, it contains styrene-butadiene rubber (SBR) latex and / or carboxymethyl cellulose (CMC). The aqueous binder for a secondary battery electrode according to any one of 〔1〕to 〔9〕. 〔11〕A composition for a secondary battery electrode mixture layer containing the aqueous binder for a secondary battery electrode according to any one of 〔1〕to 〔10〕, an active material, and water. 〔12〕The pH of the composition for a secondary battery electrode mixture layer is less than 12.5. The composition for a secondary battery electrode mixture layer according to 〔11〕. 〔13〕A secondary battery electrode having a mixture layer formed from the composition for a secondary battery electrode mixture layer according to 〔11〕or 〔12〕on the surface of a current collector. 〔14〕A secondary battery including the secondary battery electrode according to 〔13〕.

Advantages of the Invention

[0010] According to the aqueous binder for a secondary battery electrode of the present invention, when the solid content concentration of the composition for a secondary battery electrode mixture layer is higher than before, it is possible to obtain a secondary battery that exhibits excellent cycle characteristics while ensuring coatability by reducing the viscosity of the electrode slurry.

Embodiments for Carrying Out the Invention

[0011] The aqueous binder for a secondary battery electrode of the present invention (hereinafter, also referred to as "this binder") contains a polymer (hereinafter, also referred to as "this polymer") containing 50% by mass or more and 100% by mass or less of a structural unit derived from an ethylenically unsaturated carboxylic acid monomer or a salt thereof, and an alkali metal hydroxide or an alkali metal salt of a compound having a formula weight of 200 or less and having no ethylenically unsaturated group (hereinafter, also referred to as "this alkali metal salt"). By mixing it with an active material and water, a composition for a secondary battery electrode binder layer (hereinafter, also referred to as "this composition") can be obtained. The above composition is in a slurry state that can be applied to a current collector. By forming a binder 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. Here, this binder is preferable in that the effects exhibited by the present invention are particularly large when used in a composition for a secondary battery electrode binder layer containing a silicon-based active material described later as an active material.

[0012] Hereinafter, each of the aqueous binder for a secondary battery electrode of the present invention, the composition for a secondary battery electrode binder layer obtained using the binder, the secondary battery electrode, and the secondary battery will be described in detail. In addition, in this specification, "(meth)acryl" means acrylic and / or methacrylic, and "(meth)acrylate" means acrylate and / or methacrylate. Further, "(meth)acryloyl group" means acryloyl group and / or methacryloyl group.

[0013] This binder contains this polymer or a salt thereof, and an alkali metal hydroxide or this alkali metal salt. This polymer may be a crosslinked polymer (hereinafter, also referred to as "this crosslinked polymer") or a non-crosslinked polymer (hereinafter, also referred to as "this non-crosslinked polymer"). This crosslinked polymer and this non-crosslinked polymer may be used alone or in combination. Further, this crosslinked polymer or this non-crosslinked polymer may be used alone or in combination of two or more.

[0014] 1. This crosslinked polymer <Structural unit derived from ethylenically unsaturated carboxylic acid monomer> The crosslinked polymer contained in this binder contains 50% by mass or more and 100% by mass or less of a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as “component (a1)”). When the crosslinked polymer has such a structural unit and has a carboxyl group, the adhesiveness to the current collector is improved, and the desolvation effect of lithium ions and the ionic conductivity are excellent, so that the resistance is small and an electrode excellent in high rate characteristics can be obtained. In addition, since water swelling property is imparted, the dispersion stability of the active material and the like in this composition can be enhanced. The above-mentioned component (a1) can be introduced into the polymer by polymerizing, for example, a monomer containing an ethylenically unsaturated carboxylic acid monomer. In addition, it can also be obtained by hydrolyzing after (co)polymerizing a (meth)acrylate monomer. Further, after polymerizing (meth)acrylamide, (meth)acrylonitrile, etc., it may be treated with a strong alkali, or a method of reacting an acid anhydride with a polymer having a hydroxyl group may also be used.

[0015] Examples of the ethylenically unsaturated carboxylic acid monomer include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid; (meth)acrylamide alkyl carboxylic acids such as (meth)acrylamide hexanoic acid and (meth)acrylamide dodecanoic acid; ethylenically unsaturated monomers having a carboxyl group such as succinic acid monohydroxyethyl (meth)acrylate, ω-carboxy-caprolactone mono (meth)acrylate, β-carboxyethyl (meth)acrylate, or their (partial) alkali neutralized products. One of these may be used alone, or two or more thereof may be used in combination. Among the above, a compound having an acryloyl group as a polymerizable functional group is preferable in that a polymer having a long primary chain length can be obtained due to a high polymerization rate, and the binding force of this binder becomes good. Particularly preferable is acrylic acid. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, a polymer having a high carboxyl group content can be obtained.

[0016] The content of the component (a1) in the present crosslinked polymer can be 50% by mass or more and 100% by mass or less based on all the structural units of the present crosslinked polymer. By containing the component (a1) within such a range, excellent adhesiveness to the current collector can be easily ensured. When the lower limit is 50% by mass or more, the dispersion stability of the present composition becomes good, and a higher adhesive strength can be obtained, which is preferable. It may be 60% by mass or more, may be 70% by mass or more, or may be 80% by mass or more. Further, the upper limit is, for example, 99.9% by mass or less, also for example 99.5% by mass or less, also for example 99% by mass or less, also for example 98% by mass or less, also for example 95% by mass or less, also for example 90% by mass or less, or also for example 80% by mass or less. As the range, a range obtained by appropriately combining such lower and upper limits can be adopted. For example, it can be 50% by mass or more and 100% by mass or less, also for example 50% by mass or more and 99.9% by mass or less, also for example 50% by mass or more and 99% by mass or less, or also for example 50% by mass or more and 98% by mass or less, etc.

[0017] <Other structural units> In addition to the component (a1), the present crosslinked polymer can contain structural units derived from other ethylenically unsaturated monomers copolymerizable therewith (hereinafter, also referred to as "component (b1)"). Examples of the component (b1) include structural units derived from 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. These structural units can be introduced by copolymerizing a monomer containing 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.

[0018] (b1) The proportion of the component can be 0% by mass or more and 50% by mass or less based on all the structural units of the present crosslinked polymer. The proportion of the (b1) component may be 1% by mass or more and 50% by mass or less, may be 2% by mass or more and 50% by mass or less, may be 5% by mass or more and 50% by mass or less, or may be 10% by mass or more and 50% by mass or less. Further, when the (b1) component is contained in an amount of 1% by mass or more based on all the structural units of the present crosslinked polymer, the affinity for the electrolyte solution is improved, and thus the effect of improving the lithium ion conductivity can also be expected.

[0019] As the (b1) component, among those described above, a structural unit derived from a nonionic ethylenically unsaturated monomer is preferable from the viewpoint of obtaining an electrode with good flex resistance. Examples of the nonionic ethylenically unsaturated monomer include (meth)acrylamide and its derivatives, nitrile group-containing ethylenically unsaturated monomers, alicyclic structure-containing ethylenically unsaturated monomers, hydroxyl group-containing ethylenically unsaturated monomers, and the like.

[0020] Examples of the (meth)acrylamide derivative include N-alkyl (meth)acrylamide compounds such as isopropyl (meth)acrylamide and t-butyl (meth)acrylamide; N-alkoxyalkyl (meth)acrylamide compounds such as N-n-butoxymethyl (meth)acrylamide and N-isobutoxymethyl (meth)acrylamide; and N,N-dialkyl (meth)acrylamide compounds such as dimethyl (meth)acrylamide and diethyl (meth)acrylamide. One of these may be used alone, or two or more thereof may be used in combination.

[0021] Examples of the nitrile group-containing ethylenically unsaturated monomer include (meth)acrylonitrile; (meth)acrylic acid cyanoalkyl ester compounds such as cyanomethyl (meth)acrylate and cyanoethyl (meth)acrylate; cyano group-containing unsaturated aromatic compounds such as 4-cyanostyrene and 4-cyano-α-methylstyrene; and vinylidene cyanide. Among these, one of them may be used alone, or two or more of them may be used in combination. Among the above, acrylonitrile is preferable in terms of having a high nitrile group content.

[0022] Examples of the alicyclic structure-containing ethylenically unsaturated monomer include (meth)acrylic acid cycloalkyl esters which may have an aliphatic substituent such as (meth)acrylic acid cyclopentyl, (meth)acrylic acid cyclohexyl, (meth)acrylic acid methylcyclohexyl, (meth)acrylic acid t-butylcyclohexyl, (meth)acrylic acid cyclodecyl, and (meth)acrylic acid cyclododecyl; (meth)acrylic acid isobornyl, (meth)acrylic acid adamantyl, (meth)acrylic acid dicyclopentenyl, (meth)acrylic acid dicyclopentenyl oxyethyl, (meth)acrylic acid dicyclopentanyl; and cycloalkyl polyalcohol mono(meth)acrylates such as cyclohexanedimethanol mono(meth)acrylate and cyclodecanedimethanol mono(meth)acrylate. Among these, one of them may be used alone, or two or more of them may be used in combination.

[0023] Examples of the hydroxyl group-containing ethylenically unsaturated monomer include (meth)acrylic acid hydroxyethyl, (meth)acrylic acid hydroxypropyl, and (meth)acrylic acid hydroxybutyl. Among these, one of them may be used alone, or two or more of them may be used in combination.

[0024] This crosslinked polymer or a salt thereof preferably contains structural units derived from (meth)acrylamide and its derivatives, nitrile group-containing ethylenically unsaturated monomers, alicyclic structure-containing ethylenically unsaturated monomers, etc., in that the binder has excellent binding properties. Further, when a structural unit derived from a hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less is introduced as the component (c), it can exhibit a strong interaction with the electrode material and can exhibit good binding properties to the active material. As a result, a strong and well-integrated electrode binder layer can be obtained. Therefore, as the above-mentioned "hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less", an alicyclic structure-containing ethylenically unsaturated monomer is particularly preferable.

[0025] This crosslinked polymer or a salt thereof preferably contains a structural unit derived from a hydroxyl group-containing ethylenically unsaturated monomer in terms of improving the cycle characteristics of the secondary battery obtained, and preferably contains 0.5% by mass or more and 50% by mass or less of the structural unit, more preferably 2.0% by mass or more and 50% by mass or less, and still more preferably 10.0% by mass or more and 50% by mass or less.

[0026] Also, as other nonionic ethylenically unsaturated monomers, for example, (meth)acrylate esters may be used. Examples of the (meth)acrylate esters include alkyl (meth)acrylate compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; aromatic (meth)acrylate compounds such as phenyl (meth)acrylate, phenylmethyl (meth)acrylate, and phenylethyl (meth)acrylate; (meth)acrylate alkoxyalkyl ester compounds such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate, etc. One of these may be used alone, or two or more thereof may be used in combination.

[0027] From the viewpoints of adhesion to the active material and cycle characteristics, an aromatic (meth)acrylate compound can be preferably used. From the viewpoint of further improving lithium ion conductivity and high rate characteristics, compounds having an ether bond, such as (meth)acrylic acid alkoxyalkyl esters such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate, are preferred, and 2-methoxyethyl (meth)acrylate is more preferred.

[0028] Among nonionic ethylenically unsaturated monomers, a compound having an acryloyl group is preferred in that a polymer having a long primary chain length can be obtained due to its high polymerization rate, and the binding force of this binder is good. Further, as the nonionic ethylenically unsaturated monomer, a compound having a glass transition temperature (Tg) of 0 °C or lower of the homopolymer is preferred in that the obtained electrode has good bending resistance.

[0029] This crosslinked polymer may be in the form of a salt in which some or all of the carboxyl groups contained in the polymer are neutralized. The type of the salt is not particularly limited, and 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, and alkali metal salts are more preferred, because they are less likely to have an adverse effect on battery characteristics.

[0030] This polymer is preferably a polymer having a crosslinked structure (this crosslinked polymer). The crosslinking method in this crosslinked polymer is not particularly limited, and for example, the following methods are exemplified. 1) Copolymerization of a crosslinkable monomer 2) Utilizing chain transfer to the polymer chain during radical polymerization 3) After synthesizing a polymer having a reactive functional group, adding a crosslinking agent as necessary for post-crosslinking Since the polymer has a crosslinked structure, the binder containing the polymer or a salt thereof can have excellent adhesion. Among the above, the method by copolymerization of a crosslinkable monomer is preferred because of its simple operation and easy control of the degree of crosslinking.

[0031] <Crosslinkable monomer> Examples of the crosslinkable monomer include polyfunctional polymerizable monomers having two or more polymerizable unsaturated groups, and monomers having a self-crosslinkable crosslinkable functional group such as a hydrolyzable silyl group.

[0032] The above polyfunctional polymerizable monomer is a compound having two or more polymerizable functional groups such as (meth)acryloyl group and alkenyl group in the molecule, and examples include polyfunctional (meth)acrylate compounds, polyfunctional alkenyl compounds, and compounds having both (meth)acryloyl group and alkenyl group. These compounds may be used alone or in combination of two or more. Among these, polyfunctional alkenyl compounds are preferred in terms of easily obtaining a uniform crosslinked structure, and polyfunctional allyl ether compounds having two or more allyl ether groups in the molecule are particularly preferred.

[0033] Examples of the polyfunctional (meth)acrylate compound include di(meth)acrylates of divalent 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 and tetra(meth)acrylates of polyvalent alcohols having a trivalent or higher valence such as trimethylolpropane tri(meth)acrylate, tri(meth)acrylate of ethylene oxide - modified trimethylolpropane, glycerin tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and pentaerythritol tetra(meth)acrylate, i.e., poly(meth)acrylates; bisamides such as methylenebisacrylamide and hydroxyethylenebisacrylamide.

[0034] Examples of the polyfunctional alkenyl compound include polyfunctional allyl ether compounds such as trimethylolpropane diallyl ether, trimethylolpropane triallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, tetraallyloxyethane, and polyallyl sucrose; polyfunctional allyl compounds such as diallyl phthalate; and polyfunctional vinyl compounds such as divinylbenzene.

[0035] Examples of the compound having both a (meth)acryloyl group and an alkenyl group include allyl (meth)acrylate, isopropenyl (meth)acrylate, butenyl (meth)acrylate, pentenyl (meth)acrylate, 2-(2-vinyloxyethoxy)ethyl (meth)acrylate, and the like.

[0036] Specific examples of the monomer having the self-crosslinkable crosslinkable functional group include hydrolyzable silyl group-containing vinyl monomers, N-methylol (meth)acrylamide, N-methoxyalkyl acrylamide, and the like. These compounds can be used alone or in combination of two or more.

[0037] The hydrolyzable silyl group-containing vinyl monomer is not particularly limited as long as it is a vinyl monomer having at least one hydrolyzable silyl group. For example, vinyl silanes such as vinyl trimethoxysilane, vinyl triethoxysilane, vinyl methyldimethoxysilane, and vinyldimethylmethoxysilane; silyl group-containing acrylic esters such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, and methyldimethoxysilylpropyl acrylate; silyl group-containing methacrylic 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.

[0038] When the crosslinked polymer is crosslinked with a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.05 parts by mass or more and 5.0 parts by mass or less, more preferably 0.1 parts by mass or more and 5.0 parts by mass or less, still more preferably 0.2 parts by mass or more and 4.0 parts by mass or less, and even more preferably 0.3 parts by mass or more and 3.0 parts by mass or less, based on 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer). If the amount of the crosslinkable monomer used is 0.05 parts by mass or more, it is preferable in terms of better binding properties and stability of the electrode slurry. If it is 5.0 parts by mass or less, the stability of the polymer tends to be higher. Similarly, the amount of the crosslinkable monomer used is preferably 0.02 to 1.7 mol%, more preferably 0.10 to 1.0 mol%, based on the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer).

[0039] <Aqueous solution viscosity of the crosslinked polymer> The crosslinked polymer preferably has a viscosity of 10,000 mPa·s or less in a 2% by mass aqueous solution. When the viscosity of the 2% by mass aqueous solution is 10,000 mPa·s or less, it becomes possible to have durability that can follow the volume change of the active material during charge and discharge. The viscosity of the 2% by mass aqueous solution may be 5,000 mPa·s or less, 3,000 mPa·s or less, or 2,000 mPa·s or less. The aqueous solution viscosity is obtained by uniformly dissolving or dispersing an amount of the crosslinked polymer that results in a predetermined concentration in water and then measuring the B-type viscosity (25 °C) at 12 rpm according to the method described in the examples.

[0040] This crosslinked polymer or a salt thereof absorbs water and swells in water. Generally, when the crosslinked polymer has an appropriate degree of crosslinking, the larger the amount of hydrophilic groups in the crosslinked polymer, the easier it is for the crosslinked polymer to absorb water and swell. Also, regarding the degree of crosslinking, the lower the degree of crosslinking, the easier it is for the crosslinked polymer to swell. However, even when the number of crosslinking points is the same, the larger the molecular weight (primary chain length), the more crosslinking points contribute to the formation of the three-dimensional network, so the crosslinked polymer becomes less likely to swell. Therefore, by adjusting the amount of hydrophilic groups, the number of crosslinking points, the primary chain length, etc. of the crosslinked polymer, the viscosity of the aqueous solution of the crosslinked polymer can be adjusted. At this time, the number of the above crosslinking points can be adjusted, for example, by the amount of crosslinkable monomer used, the chain transfer reaction to the polymer chain, and the post-crosslinking reaction, etc. Also, the primary chain length of the polymer can be adjusted by setting conditions related to the amount of radical generation such as the initiator and the polymerization temperature, and by selecting a polymerization solvent considering chain transfer, etc.

[0041] <Particle diameter of this crosslinked polymer> In this composition, it is preferable that the crosslinked polymer is not present as large-particle aggregates (secondary aggregates) but is well-dispersed as water-swellable particles having an appropriate particle diameter, because the binder containing the crosslinked polymer can exhibit good binding performance.

[0042] When the crosslinked polymer having a degree of neutralization based on the carboxyl groups of 70 to 100 mol% is dispersed in water, the particle diameter (water-swelled particle diameter) is preferably in the range of 0.1 μm or more and 10.0 μm or less in terms of volume-based median diameter. A more preferable range of the above particle diameter is 0.1 μm or more and 8.0 μm or less, a further more preferable range is 0.1 μm or more and 7.0 μm or less, an even more preferable range is 0.2 μm or more and 5.0 μm or less, and an even more preferable range is 0.5 μm or more and 3.0 μm or less. If the particle diameter is in the range of 0.1 μm or more and 10.0 μm or less, it exists uniformly in the composition in a suitable size, so the stability of the composition is high and it is possible to exhibit excellent binding properties. If the particle diameter exceeds 10.0 μm, there is a risk that the binding property becomes insufficient as described above. Also, there is a risk that the coatability becomes insufficient in that it is difficult to obtain a smooth coated surface. On the other hand, when the particle diameter is less than 0.1 μm, there are concerns from the viewpoint of stable manufacturability.

[0043] Also, the particle diameter (dry particle diameter) of the crosslinked polymer when dry is preferably in the range of 0.03 μm or more and 3 μm or less in terms of volume-based median diameter. A more preferable range of the above particle diameter is 0.1 μm or more and 1 μm or less, and a further more preferable range is 0.3 μm or more and 0.8 μm or less.

[0044] In the present crosslinked polymer, in the present composition, acid groups such as carboxyl groups derived from ethylenically unsaturated carboxylic acid monomers are neutralized so that the degree of neutralization is 20 mol% or more, and it is preferably used in the form of a salt. The above degree of neutralization is more preferably 50 mol% or more, still more preferably 70 mol% or more, even more preferably 75 mol% or more, even more preferably 80 mol% or more, and particularly preferably 85 mol% or more. The upper limit value of the degree of neutralization is 100 mol%, and it may be 98 mol% or 95 mol%. The range of the degree of neutralization can be appropriately combined with the above lower limit value and upper limit value. For example, it may be 50 mol% or more and 100 mol% or less, 75 mol% or more and 100 mol% or less, or 80 mol% or more and 100 mol% or less. When the degree of neutralization is 20 mol% or more, it is preferable in that the water swelling property is good and the dispersion stabilizing effect is easily obtained. In this specification, the above degree of neutralization can be calculated from the charged values of the monomer having an acid group such as a carboxyl group and the neutralizing agent used for neutralization. The degree of neutralization can be confirmed from the intensity ratio of the peak derived from the C=O group of the carboxylic acid and the peak derived from the C=O group of the carboxylate by IR measurement of the powder after drying the crosslinked polymer or its salt at 80 °C for 3 hours under reduced pressure conditions.

[0045] <Method for Producing the Present Crosslinked Polymer> The present crosslinked polymer can be produced using known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization. However, precipitation polymerization and suspension polymerization (inverse phase suspension polymerization) are preferable from the viewpoint of productivity. Heterogeneous polymerization methods such as precipitation polymerization, suspension polymerization, and emulsion polymerization are preferable in that better performance can be obtained with respect to binding properties and the like. Among them, the precipitation polymerization method is more preferable. Precipitation polymerization is a method for producing a polymer by performing a polymerization reaction in a solvent that dissolves the unsaturated monomer as a raw material but does not substantially dissolve the produced polymer. As the polymerization proceeds, the polymer particles grow larger by aggregation and secondary aggregation of primary particles of several tens of nm to several hundreds of nm into polymer particles of several μm to several tens of μm, and a dispersion of polymer particles is obtained. A dispersion stabilizer can also be used to control the particle size of the polymer. Secondary aggregation can also be suppressed by selecting a dispersion stabilizer, a polymerization solvent, or the like. Generally, precipitation polymerization with suppressed secondary aggregation is also called dispersion polymerization.

[0046] In the case of precipitation polymerization, the polymerization solvent can be a solvent selected from water and various organic solvents, etc., considering the type of monomer to be used. In order to obtain a polymer with a longer primary chain length, it is preferable to use a solvent with a small chain transfer constant.

[0047] Specific polymerization solvents include water-soluble solvents such as methanol, t-butyl alcohol, acetone, methyl ethyl ketone, acetonitrile, and tetrahydrofuran, as well as benzene, ethyl acetate, dichloroethane, n-hexane, cyclohexane, and n-heptane. These can be used alone or in combination of two or more. Alternatively, they may be used as a mixed solvent with water. In the present invention, the water-soluble solvent refers to a substance with a solubility in water at 20 °C greater than 10 g / 100 ml. Among the above, in terms of less generation of coarse particles and adhesion to the reactor, good polymerization stability, the precipitated polymer fine particles being less likely to undergo secondary aggregation (or easily dissociating in the aqueous medium even if secondary aggregation occurs), a small chain transfer constant and obtaining a polymer with a high degree of polymerization (long primary chain length), and easy operation during the neutralization process described later, methyl ethyl ketone and acetonitrile are preferable.

[0048] As the polymerization initiator, known polymerization initiators such as azo compounds, organic peroxides, and inorganic peroxides can be used, but it is not particularly limited. The use conditions can be adjusted by known methods such as thermal initiation, redox initiation using a reducing agent in combination, and UV initiation, so as to obtain an appropriate amount of radical generation. In order to obtain a cross-linked polymer with a long primary chain length, it is preferable to set the conditions so that the amount of radical generation is less within the range allowed by the production time.

[0049] The preferable usage amount of the polymerization initiator is, for example, 0.001 to 2 parts by mass, for example, 0.005 to 1 part by mass, and for example, 0.01 to 0.1 part by mass when the total amount of the monomer components used is 100 parts by mass. If the usage amount of the polymerization initiator is 0.001 part 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.

[0050] The polymerization temperature depends on conditions such as the type and concentration of the monomer used, but is preferably 0 to 100 °C, more preferably 20 to 80 °C. The polymerization temperature may be constant or may change during the polymerization reaction period. Also, the polymerization time is preferably 1 minute to 20 hours, more preferably 1 hour to 10 hours.

[0051] 2. This non-crosslinked polymer The non-crosslinked polymer contained in this binder contains 50% by mass or more and 100% by mass or less of structural units (the component (a1)) derived from ethylenically unsaturated carboxylic acid monomers. The method for introducing the (a1) component of the non-crosslinked polymer may be the same as the method described for the (a1) component of the crosslinked polymer. Also, it may be a method by saponification of a polymer containing structural units derived from an (alkyl (meth)acrylate compound (the one described above as the (b1) component of the crosslinked polymer)), and as the (alkyl (meth)acrylate compound), methyl acrylate and methyl methacrylate are preferable from the viewpoint of easy progress of the saponification reaction, etc., and one kind may be used alone or two or more kinds may be used in combination.

[0052] The non-crosslinked polymer has a higher viscosity than the crosslinked polymer. This is presumably because the molecular chains of the non-crosslinked polymer are spread out, while the crosslinked polymer is in a particulate state, resulting in a small apparent molecular weight. Even if the non-crosslinked polymer has a higher viscosity than the crosslinked polymer salt, when using an aqueous binder containing the non-crosslinked polymer or its salt, and an alkali metal hydroxide or the alkali metal salt, the viscosity of the composition is reduced, improving the coatability, and further improving the cycle characteristics of the secondary battery.

[0053] The content of the component (a1) in the non-crosslinked polymer is 50% by mass or more and 100% by mass or less, preferably 60% by mass or more and 100% by mass or less, more preferably 70% by mass or more and 100% by mass or less, and still more preferably 80% by mass or more and 100% by mass or less, based on all the structural units of the non-crosslinked polymer, from the viewpoint of solubility in water.

[0054] <Other structural units> In addition to the component (a1), the non-crosslinked polymer may contain structural units (the component (b1)) derived from other ethylenically unsaturated monomers copolymerizable therewith. The method of introducing the component (b1) may be the same as the method described for the component (b1) in the crosslinked polymer. Further, it may be a method of saponifying a polymer containing structural units derived from vinyl ester compounds such as vinyl acetate and vinyl propionate. As the vinyl ester compound, vinyl acetate is preferable from the viewpoint of easy availability of raw materials, etc., and it may be used alone or in combination of two or more.

[0055] The proportion of the component (b1) can be 0% by mass or more and 50% by mass or less based on all the structural units of the non-crosslinked polymer. The proportion of the component (b1) may be 1% by mass or more and 50% by mass or less, 2% by mass or more and 50% by mass or less, 5% by mass or more and 50% by mass or less, or 10% by mass or more and 50% by mass or less.

[0056] The non-crosslinked polymer may be in the form of a salt in which part or all of the carboxyl groups contained in the polymer are neutralized. The type of the salt is not particularly limited, and examples include alkali metal salts such as lithium, sodium, and potassium; 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 preferable, and alkali metal salts are more preferable, from the viewpoint of being less likely to have an adverse effect on battery characteristics.

[0057] In the present composition, it is preferable that the non-crosslinked polymer is used in the form of a salt in which acid groups such as carboxyl groups derived from ethylenically unsaturated carboxylic acid monomers are neutralized so that the degree of neutralization is 20 mol% or more. The degree of neutralization is more preferably 50 mol% or more, still more preferably 70 mol% or more, even more preferably 75 mol% or more, yet even more preferably 80 mol% or more, and particularly preferably 85 mol% or more. The upper limit value of the degree of neutralization is 100 mol%, and it may be 98 mol% or 95 mol%. The range of the degree of neutralization can be appropriately combined with the above lower limit value and upper limit value. For example, it may be 50 mol% or more and 100 mol% or less, 75 mol% or more and 100 mol% or less, or 80 mol% or more and 100 mol% or less. When the degree of neutralization is 20 mol% or more, it is preferable in terms of easily ensuring solubility in water. In the present specification, the degree of neutralization can be calculated from the charged values of monomers having acid groups such as carboxyl groups and neutralizing agents used for neutralization. The degree of neutralization can be confirmed from the intensity ratio of the peak derived from the C=O group of the carboxylic acid and the peak derived from the C=O group of the carboxylate by subjecting the crosslinked polymer or its salt to IR measurement on the powder after drying treatment at 80°C for 3 hours under reduced pressure conditions.

[0058] The weight average molecular weight (Mw) of the present non-crosslinked polymer is not particularly limited, but it is preferably 5,000 or more, more preferably 10,000 or more, in terms of obtaining an electrode binder layer having excellent binding properties. Mw may be 100,000 or more, 500,000 or more, or 1,000,000 or more. The upper limit value of Mw is not particularly limited either, but from the viewpoint of handling in production, for example, it may be 10,000,000 or less, or 5,000,000 or less.

[0059] When this binder contains the present crosslinked polymer and the present non-crosslinked polymer, the amount of the present non-crosslinked polymer used is preferably 7.5 parts by mass or more and 200 parts by mass or less with respect to 100 parts by mass of the total amount of the present crosslinked polymer. The amount of the present non-crosslinked polymer used may be 15 parts by mass or more, may be 25 parts by mass or more, may be 35 parts by mass or more, or may be 45 parts by mass or more. The upper limit may be 190 parts by mass or less, may be 180 parts by mass or less, may be 170 parts by mass or less, or may be 160 parts by mass or less. As the range, a range obtained by appropriately combining such lower and upper limits can be adopted. For example, it is 15 parts by mass or more and 190 parts by mass or less, for example, 25 parts by mass or more and 180 parts by mass or less, and also for example 35 parts by mass or more and 170 parts by mass or less, and also for example 35 parts by mass or more and 160 parts by mass or less, etc.

[0060] Thus, a specific amount of the present non-crosslinked polymer can also be used in combination with the present crosslinked polymer, and when the solid content concentration of the electrode binder layer composition is higher than before, a secondary battery can be obtained that exhibits excellent cycle characteristics while ensuring coatability by reducing the viscosity of the electrode slurry. If the amount of the non-crosslinkable polymer used is 7.5 parts by mass or more, such an effect can be exhibited. Further, when the amount of the non-crosslinkable polymer used exceeds 200 parts by mass, sufficient coatability may not be obtained.

[0061] <Method for Producing the Present Non-Crosslinked Polymer> For the present non-crosslinked polymer, known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization can be used, and it can be appropriately selected according to the molecular weight, composition, etc.

[0062] As the polymerization initiator, known polymerization initiators such as azo compounds, organic peroxides, and inorganic peroxides can be used, but it is not particularly limited. The use conditions can be adjusted by known methods such as thermal initiation, redox initiation using a reducing agent in combination, and UV initiation so as to generate an appropriate amount of radicals. Further, for the purpose of adjusting the molecular weight, etc., a known chain transfer agent may be used as necessary.

[0063] <Aqueous solution viscosity of the non-crosslinked polymer> The non-crosslinked polymer preferably has a viscosity of an aqueous solution at a concentration of 2% by mass of 10,000 mPa·s or less. When the viscosity of the aqueous solution at a concentration of 2% by mass is 10,000 mPa·s or less, it becomes possible to have durability that can follow the volume change of the active material during charge and discharge. The viscosity of the aqueous solution at a concentration of 2% by mass may be 5,000 mPa·s or less, may be 3,000 mPa·s or less, or may be 2,000 mPa·s or less. The aqueous solution viscosity is obtained by uniformly dissolving or dispersing an amount of the non-crosslinked polymer that results in a predetermined concentration in water, and then measuring the B-type viscosity (25 °C) at 12 rpm according to the method described in the examples.

[0064] 3. Alkali metal hydroxide or this alkali metal salt In addition to the polymer or its salt, the binder contains an alkali metal hydroxide or the alkali metal salt. When the binder contains an alkali metal hydroxide, the crosslinked polymer is a salt neutralized to 100 mol%. A binder may be obtained by mixing a crosslinked polymer salt with a neutralization degree of 100 mol% and an alkali metal hydroxide, or a binder may be obtained by mixing a polymer salt with a neutralization degree of less than 100 mol% and an amount of an alkali metal hydroxide such that the neutralization degree of the polymer salt exceeds 100 mol%.

[0065] Specific examples of the alkali metal hydroxide include lithium hydroxide, sodium hydroxide, potassium hydroxide, and the like. The alkali metal hydroxide may be used alone or in combination of two or more.

[0066] The amount of the alkali metal hydroxide used is not particularly limited, but in terms of being able to make the pH of the composition less than 12.5, it is preferably 5 parts by mass or more and 50 parts by mass or less, more preferably 10 parts by mass or more and 45 parts by mass or less, and still more preferably 10 parts by mass or more and 40 parts by mass or less with respect to 100 parts by mass of the total amount of the polymer with a neutralization degree of 100 mol%.

[0067] Here, when suppressing the increase in the pH of the electrode slurry, for example, when carboxymethyl cellulose (CMC) is blended, it is preferable to contain this alkali metal salt because there is little concern about its hydrolysis. Further, since this alkali metal salt does not have an ethylenically unsaturated group, polymerization does not proceed during the production of the electrode slurry or during the storage of the obtained electrode slurry, so there is little concern about the thickening of the electrode slurry.

[0068] The formula weight of this alkali metal salt is 200 or less, preferably 180 or less, more preferably 160 or less, and still more preferably 140 or less, in that a secondary battery having excellent cycle characteristics can be obtained while ensuring coatability by reducing the viscosity of the electrode slurry when the solid content concentration of the electrode binder layer composition is higher than before.

[0069] Examples of this alkali metal salt include alkali metal organic acid salts, alkali metal carbonate compounds, alkali metal hydrogen carbonates, alkali metal nitrite compounds, alkali metal chlorides, alkali metal bromides, etc., and these may be anhydrous or hydrated. Specific examples of the alkali metal organic acid salt include lithium acetate, sodium acetate, potassium acetate, lithium propionate, sodium propionate, potassium propionate, etc.; Specific examples of the alkali metal carbonate include lithium carbonate, sodium carbonate, potassium carbonate, etc.; Specific examples of the alkali metal hydrogen carbonate include lithium hydrogen carbonate, sodium hydrogen carbonate, potassium hydrogen carbonate, etc.; Specific examples of the alkali metal nitrite compound include lithium nitrite, sodium nitrite, potassium nitrite, etc.; Specific examples of the alkali metal chloride include lithium chloride, sodium chloride, potassium chloride, etc.; Specific examples of the alkali metal bromide include lithium bromide, sodium bromide, potassium bromide, etc. Among these, from the viewpoint that the effects exhibited by the present invention are particularly great, an alkali metal salt of an organic acid is preferable, and further, it preferably contains at least one selected from the group consisting of lithium salts, sodium salts, and potassium salts, and in particular, lithium acetate is preferable. Here, as the organic acid of the alkali metal salt of an organic acid, from the viewpoint of preventing gelation due to crosslinking of the present polymers having a carboxyl group, an organic acid having one carboxyl group in one molecule is preferable. This alkali metal salt may be used alone or in combination of two or more.

[0070] The amount of the alkali metal salt used is not particularly limited, but when the solid content concentration of the composition for the electrode binder layer is higher than that in the past, while ensuring the coatability by reducing the viscosity of the electrode slurry, in terms of being able to obtain a secondary battery that exhibits excellent cycle characteristics, it is preferably 5.0 parts by mass or more and 175 parts by mass or less, more preferably 10 parts by mass or more and 150 parts by mass or less, still more preferably 15 parts by mass or more and 125 parts by mass or less, and even more preferably 20 parts by mass or more and 100 parts by mass or less with respect to 100 parts by mass of the total amount of the present polymer.

[0071] 3. Composition for secondary battery electrode binder layer The composition for the secondary battery electrode binder layer of the present invention contains this binder, an active material, and water. The amount of this binder used in the present composition is, for example, 0.1 part by mass or more and 20 parts by mass or less with respect to 100 parts by mass of the total amount of the active material. The above usage amounts are also, for example, 0.2 part by mass or more and 10 parts by mass or less, and also, for example, 0.3 part by mass or more and 8 parts by mass or less, and also, for example, 0.4 part by mass or more and 5 parts by mass or less. If the amount of the binder used is 0.1 part by mass or more, sufficient binding property can be obtained. Also, the dispersion stability of the active material and the like can be ensured, and a uniform binder layer can be formed. If the amount of the binder used is 20 parts by mass or less, the present composition will not have a high viscosity, and the coatability onto the current collector can be ensured. As a result, a binder layer having a uniform and smooth surface can be formed.

[0072] Among the above active materials, as the positive electrode active material, a lithium salt of a transition metal oxide can be used. 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 x ,Co y ,Mn z ), x + y + z = 1} and NCA {Li(Ni 1-a-b Co a Al b )}, etc. Examples of the spinel type positive electrode active material include lithium manganate. In addition to oxides, phosphates, silicates, sulfur, etc. are used. Examples of the phosphate include olivine type lithium iron phosphate. As the positive electrode active material, one of the above may be used alone, or two or more may be combined and used as a mixture or composite.

[0073] In addition, when a positive electrode active material containing a layered rock salt type lithium-containing metal oxide is dispersed in water, lithium ions on the surface of the active material are exchanged with hydrogen ions in water, so that the dispersion shows alkalinity. For this reason, there is a risk that a general current collector material for a positive electrode, such as aluminum foil (Al), will be corroded. In such a case, it is preferable to neutralize the alkali component eluted from the active material by using the unneutralized or partially neutralized polymer as this binder. In addition, the amount of the unneutralized or partially neutralized polymer used is preferably such that the amount of unneutralized carboxyl groups in the polymer is equal to or more than the amount of alkali eluted from the active material.

[0074] Since all cathode active materials have low electrical conductivity, it is common to use them with the addition of a conductive additive. Examples of conductive additives include carbon-based materials such as carbon black, carbon nanotubes, carbon fibers, graphite fine powder, and carbon fibers. Among these, carbon black, carbon nanotubes, and carbon fibers are preferred because they are likely to provide excellent electrical conductivity. Also, as carbon black, Ketjen black and acetylene black are preferred. The conductive additive may be used alone or in combination of two or more. From the viewpoint of achieving both electrical conductivity and energy density, the amount of the conductive additive used can be, for example, 0.2 to 20 parts by mass, or for example, 0.2 to 10 parts by mass, based on 100 parts by mass of the total amount of the active material. Further, a cathode active material surface-coated with a conductive carbon-based material may be used.

[0075] On one hand, examples of the negative electrode active material include carbon-based materials, lithium metal, lithium alloys, and metal oxides, etc., and one or more of these can be used in combination. Among these, an active material composed of a carbon-based material such as natural graphite, artificial graphite, hard carbon, and soft carbon (hereinafter also referred to as "carbon-based active material") is preferred, and graphite such as natural graphite and artificial graphite, as well as hard carbon, are more preferred. In the case of graphite, spherical graphite is preferably used from the perspective of battery performance, and the preferred range of its particle size is, for example, 1 to 20 μm, and also for example, 5 to 15 μm. Also, in order to increase the energy density, a metal or metal oxide that can occlude lithium such as silicon or tin can also be used as the negative electrode active material. Among them, silicon has a higher capacity than graphite, and an active material composed of a silicon-based material such as silicon, a silicon alloy, and silicon oxides such as silicon monoxide (SiO) (hereinafter also referred to as "silicon-based active material") can be used. However, while the above silicon-based active material has a high capacity, it has a large volume change during charge and discharge. Therefore, it is preferably used in combination with the above carbon-based active material. In this case, if the blending amount of the silicon-based active material is large, it may cause the disintegration of the electrode material and the cycle characteristics (durability) may be significantly reduced. From such a perspective, when the silicon-based active material is used in combination, its usage amount is, for example, 60 mass% or less, and also for example, 30 mass% or less, based on the carbon-based active material.

[0076] Since the carbon-based active material itself has good electrical conductivity, it is not necessarily necessary to add a conductive assistant. When adding a conductive assistant for the purpose of further reducing resistance, etc., from the perspective of energy density, its usage amount is, for example, 10 parts by mass or less, and also for example, 5 parts by mass or less, based on 100 parts by mass of the total amount of the active material.

[0077] When the composition is in a slurry state, the amount of the active material used is in the range of, for example, 10 to 75% by mass, and also in the range of, for example, 30 to 65% by mass, based on the total amount of the composition. If the amount of the active material used is 10% by mass or more, migration of the binder and the like can be suppressed, and it is also advantageous in terms of the drying cost of the medium. On the other hand, if it is 75% by mass or less, the fluidity and coatability of the composition can be ensured, and a uniform agent layer can be formed.

[0078] The composition uses water as the medium. Also, for the purpose of adjusting the properties and drying properties of the composition, etc., it may be a mixed solvent with lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, and water-soluble organic solvents such as tetrahydrofuran and N-methylpyrrolidone. 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.

[0079] When the composition is made into a coatable slurry state, the content of the medium containing water in the whole composition can be in the range of, for example, 25 to 60% by mass, and also, for example, 35 to 60% by mass, from the viewpoints of the coatability of the slurry, the energy cost required for drying, and productivity.

[0080] The composition may further use other binder components such as styrene-butadiene rubber (SBR) - based latex, carboxymethyl cellulose (CMC), acrylic - based latex, and polyvinylidene fluoride - based latex in combination. When using other binder components in combination, the amount used can be, for example, 0.1 to 5 parts by mass or less, and also, for example, 0.1 to 2 parts by mass or less, and also, for example, 0.1 to 1 part by mass or less, based on 100 parts by mass of the total amount of the active material. If the amount of other binder components used exceeds 5 parts by mass, the resistance may increase, and the high-rate characteristics may be insufficient in some cases. Among the above, SBR - based latex and CMC are preferable in terms of the excellent balance of binding property and flex resistance, and it is more preferable to use SBR - based latex and CMC in combination.

[0081] The above-mentioned SBR latex refers to an aqueous dispersion of a copolymer having a structural unit derived from an aromatic vinyl monomer such as styrene and a structural unit derived from an aliphatic conjugated diene monomer such as 1,3-butadiene. Examples of the above-mentioned aromatic vinyl monomer include α-methylstyrene, vinyltoluene, divinylbenzene, etc. in addition to styrene, and one or more of these can be used. The structural unit derived from the aromatic vinyl monomer in the above copolymer can be, for example, in the range of 20 to 70% by mass, and can also be, for example, in the range of 30 to 60% by mass, mainly from the viewpoint of binding property. Examples of the above-mentioned aliphatic conjugated diene monomer include 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, etc. in addition to 1,3-butadiene, and one or more of these can be used. The structural unit derived from the aliphatic conjugated diene monomer in the above copolymer can be, for example, in the range of 30 to 70% by mass, and can also be, for example, in the range of 40 to 60% by mass, in terms of good binding property of the binder and flexibility of the resulting electrode. In addition to the above monomers, styrene / butadiene latex may use, as other monomers, nitrile group-containing monomers such as (meth)acrylonitrile, carboxyl group-containing monomers such as (meth)acrylic acid, itaconic acid, maleic acid, etc., and ester group-containing monomers such as methyl (meth)acrylate as copolymerization monomers to further improve performance such as binding property. The structural unit derived from the above other monomer in the above copolymer can be, for example, in the range of 0 to 30% by mass, and can also be, for example, in the range of 0 to 20% by mass.

[0082] The above-mentioned CMC refers to a substituted product obtained by substituting a nonionic cellulose-based semi-synthetic polymer compound with a carboxymethyl group and its salt. Examples of the above-mentioned nonionic cellulose-based semi-synthetic polymer compound include alkyl celluloses such as methyl cellulose, methyl ethyl cellulose, ethyl cellulose, microcrystalline cellulose, etc.; Examples of hydroxyalkyl celluloses include hydroxyethyl cellulose, hydroxybutyl methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose stearoxy ether, carboxymethyl hydroxyethyl cellulose, alkyl hydroxyethyl cellulose, nonoxinyl hydroxyethyl cellulose, and the like.

[0083] The composition for the secondary battery electrode binder layer of the present invention contains the above active material, water, and this binder as essential components, and is obtained by mixing each component using known means. The mixing method of each component is not particularly limited, and known methods can be adopted. However, after dry-blending powder components such as the active material, conductive auxiliary, and binder, it is preferably mixed with a dispersion medium such as water and then dispersion-kneaded. When obtaining this composition in a slurry state, it is preferably finished into a slurry without poor dispersion or aggregation. As the mixing means, known mixers such as a planetary mixer, a thin-film swing mixer, and a self-revolving mixer can be used. However, it is preferably carried out using a thin-film swing mixer in terms of obtaining a good dispersion state in a short time. Further, when using a thin-film swing mixer, it is preferably pre-dispersed with a stirrer such as a disper in advance. The pH of the above slurry is not particularly limited as long as the effects of the present invention are achieved, but it is preferably less than 12.5. For example, when CMC is blended, it is more preferably less than 11.5 and even more preferably less than 10.5 in terms of having less concern about its hydrolysis. Also, the viscosity of the above slurry is not particularly limited as long as the effects of the present invention are achieved, but as the B-type viscosity (25 ° C) at 20 rpm, for example, it can be in the range of 100 to 5,000 mPa·s, and for example, it can be in the range of 500 to 4,500 mPa·s, and for example, it can be in the range of 1,000 to 3,000 mPa·s. If the viscosity of the slurry is within the above range, good coatability can be ensured.

[0084] 4. Secondary battery electrode The secondary battery electrode of the present invention comprises a binder layer formed from the composition for the secondary battery electrode binder layer on the surface of a current collector such as copper or aluminum. The binder layer is formed by applying the present composition to the surface of the current collector and then drying and removing a medium such as water. The method of applying the present composition is not particularly limited, and known methods such as the doctor blade method, dip method, roll coating method, comma coating method, curtain coating method, gravure coating method, and extrusion method can be adopted. Further, the above drying can be carried out by known methods such as hot air blowing, reduced pressure, (far) infrared rays, and microwave irradiation. Generally, the obtained binder layer after drying is subjected to a compression treatment by a die press, a roll press, or the like. By compression, the active material and the binder can be adhered, and the strength of the binder layer and the adhesion to the current collector can be improved. The thickness of the binder layer can be adjusted to about 30 to 80% of that before compression by compression. The thickness of the binder layer after compression is generally about 4 to 200 μm.

[0085] 5. Secondary battery By providing a separator and an electrolytic solution to the secondary battery electrode of the present invention, a secondary battery can be manufactured. The electrolytic solution may be liquid or gel-like. The separator is disposed between the positive electrode and the negative electrode of the battery and plays a role of preventing short circuit due to contact between both electrodes and holding the electrolytic solution to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane having good ion permeability and mechanical strength. As specific materials, polyolefins such as polyethylene and polypropylene, polytetrafluoroethylene, etc. can be used.

[0086] The electrolyte can be a known one generally used according to the type of the active material. In a lithium ion secondary battery, as specific solvents, cyclic carbonates having a high dielectric constant and a high electrolyte dissolving ability such as propylene carbonate and ethylene carbonate, and chain carbonates having a low viscosity such as ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate can be mentioned, and these can be used alone or as a mixed solvent. The electrolyte is used by dissolving a lithium salt such as LiPF6, LiSbF6, LiBF4, LiClO4, or LiAlO4 in these solvents. In a nickel hydrogen secondary battery, an aqueous potassium hydroxide solution can be used as the electrolyte. The secondary battery is obtained by housing a positive electrode plate and a negative electrode plate partitioned by a separator in a spiral shape or a laminated structure in a case or the like.

[0087] As described above, the secondary battery including the electrode provided with the binder layer formed from the composition for the secondary battery electrode binder layer disclosed in this specification exhibits good durability (cycle characteristics) even when charge and discharge are repeated, and thus is suitable for in-vehicle secondary batteries and the like.

Examples

[0088] Hereinafter, the present invention will be specifically described based on examples. Note that the present invention is not limited by these examples. In the following, "parts" and "%" mean parts by mass and mass% unless otherwise specified.

[0089] (Measurement of viscosity of 2 mass% aqueous solution of this polymer salt) The polymer salt obtained in each of the following production examples was dissolved in deionized water to prepare an aqueous solution having a concentration of 2 mass%. After adjusting the temperature of the above aqueous solution to 25°C ± 1°C, the viscosity of the 2 mass% aqueous solution was measured with a B-type viscometer at 12 rpm.

[0090] (Measurement of pH of electrode slurry) To the electrode slurries obtained in the following examples and comparative examples, water of the same mass was added and adjusted to a solid content concentration of 25%. After adjusting the temperature to 25 °C ± 1 °C, the pH of the slurry was measured using a pH meter.

[0091] (Measurement of the viscosity of the electrode slurry) Regarding the electrode slurries obtained in the following examples and comparative examples, after adjusting the temperature to 25 °C ± 1 °C, the slurry viscosity was measured using a B-type viscometer at 20 rpm.

[0092] ≪Production of the crosslinked polymer salt≫ (Production Example 1: Production of the crosslinked polymer salt R-1) For the polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser, and a nitrogen inlet tube was used. Into the reactor, 567 parts of acetonitrile, 2.20 parts of ion-exchanged water, 100.0 parts of acrylic acid (hereinafter referred to as "AA"), 0.90 part of trimethylolpropane diallyl ether (manufactured by Daiso Co., Ltd., trade name "Neoallyl T-20"), and triethylamine corresponding to 1.0 mol% with respect to the above AA were charged. After thoroughly purging the inside of the reactor with nitrogen, it was heated to raise the internal temperature to 55 °C. After confirming that the internal temperature was stable at 55 °C, 0.040 part of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Corporation, trade name "V-65") was added as a polymerization initiator. At this point, turbidity was observed in the reaction solution, so this point was taken as the polymerization start point. While maintaining the internal temperature at 55 °C by adjusting the external temperature (water bath temperature), the polymerization reaction was continued. When 24 hours had elapsed since the polymerization start point, the cooling of the polymerization reaction solution was started. After the internal temperature had dropped to 25 °C, 52.4 parts of lithium hydroxide monohydrate (hereinafter referred to as "LiOH·H2O") powder was added. After the addition, stirring was continued at room temperature for 12 hours to obtain a slurry-like polymerization reaction solution in which particles of the crosslinked polymer salt R-1 (Li salt, neutralization degree 90 mol%) were dispersed in the medium.

[0093] The obtained polymerization reaction solution was centrifuged to precipitate polymer particles, and then the supernatant was removed. Thereafter, the precipitate was redispersed in acetonitrile having the same mass as the polymerization reaction solution, and then a washing operation of precipitating the polymer particles by centrifugation and removing the supernatant was repeated twice. The precipitate was collected and dried at 80 °C for 3 hours under reduced pressure to remove volatile components, thereby obtaining a powder of the present crosslinked polymer salt R-1. Since the present crosslinked polymer salt R-1 has hygroscopicity, it was stored in a hermetically sealed container having a water vapor barrier property. In addition, the powder of the present crosslinked polymer salt R-1 was measured by IR, and the degree of neutralization was determined from the intensity ratio of the peak derived from the C=O group of the carboxylic acid and the peak derived from the C=O of the carboxylic acid Li. As a result, it was equal to the calculated value from the preparation and was 90 mol%. The viscosity of a 2 mass% aqueous solution of R-1 is shown in Table 1.

[0094] (Production Examples 2 to 6: Production of the present crosslinked polymer salts R-2 to R-6) The same operations as in Production Example 1 were carried out except that the charged amounts of the monomer, crosslinkable monomer, ion-exchanged water and neutralizing agent were as described in Table 1, to obtain a polymerization reaction solution containing the present crosslinked polymer salts R-2 to R-6. Next, the same operations as in Production Example 1 were carried out for each polymerization reaction solution to obtain powdery crosslinked polymer salts R-2 to R-6. Each of the present crosslinked polymer salts was stored in a hermetically sealed container having a water vapor barrier property. The viscosities of 2 mass% aqueous solutions of R-2 to R-6 are shown in Table 1.

[0095] (Production Example 7: Production of the present non-crosslinked polymer salt R-7) For the polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen introduction tube was used. 8 parts of methyl acrylate (hereinafter referred to as "MA") and 12 parts of vinyl acetate (hereinafter referred to as "VAc") were mixed, and 0.67 part of 2,2'-azobis(isobutyric acid)dimethyl (manufactured by Fujifilm Wako Pure Chemical Corporation, trade name "V-601") was dissolved to prepare a monomer solution. Into the reactor, 410 parts of water, 10 parts of anhydrous sodium sulfate, 1 part of partially saponified polyvinyl alcohol (manufactured by Kuraray Co., Ltd., trade name "PVA-217", saponification degree 88%), and 20.67 parts of the monomer solution were charged. After thoroughly purging the inside of the reactor with nitrogen, it was heated to raise the internal temperature to 60°C. After confirming that the internal temperature had stabilized at 60°C, a mixed solution of 32 parts of MA and 48 parts of VAc was added dropwise over 4 hours using a dropping funnel. When 1 hour had elapsed since the completion of the dropwise addition, cooling of the reaction solution was started to terminate the reaction, and a polymerization reaction solution containing a copolymer of MA and VAc was obtained. Here, when the amount of residual monomers was measured by gas chromatography (GC) measurement and the polymerization rate of the monomers was calculated, the polymerization rate of each monomer was 98% for MA and 82% for VAc. Also, after dissolving a part of the obtained polymerization reaction solution in tetrahydrofuran and then filtering it through a membrane filter (manufactured by ADVANTEC: pore size 0.45 μm), gel permeation chromatography (GPC) measurement was performed on the copolymer of MA and VAc under the following conditions, and as a result of obtaining the weight average molecular weight (Mw) in terms of polystyrene, it was 1.08 million. ○ Measurement conditions Column: 4 columns of TSKgel SuperMultipore HZ-M manufactured by Tosoh Corporation Solvent: Tetrahydrofuran Temperature: 40°C Detector: RI Flow rate: 600 μL / min

[0096] After raising the temperature of the polymerization reaction solution containing the copolymer of MA and VAc to an external temperature of 50°C, the solvent was removed under reduced pressure conditions to remove the residual monomers. Then, 500 parts of methanol and 38.8 parts of LiOH·H2O were charged with respect to a total of 100 parts of the charged amount of the copolymer monomers (MA and VAc), and a saponification reaction was carried out at an external temperature of 50°C for 3 hours to obtain a reaction solution containing a saponified product of the copolymer of MA and VAc. The reaction solution containing the saponified product in acetone was reprecipitated and filtered, and then dried at 80 °C for 12 hours to remove volatile components, thereby obtaining a saponified product of a copolymer of MA and VAc. Here, based on the polymerization rates of the above-mentioned MA and VAc, the saponified product is a lithium salt of a non-crosslinked polymer having "57% by mass of structural units derived from acrylic acid" and "43% by mass of structural units derived from vinyl alcohol". Since the lithium salt R-7 of the non-crosslinked polymer has hygroscopicity, it was stored sealed in a container having a water vapor barrier property. In addition, when the powder of the non-crosslinked polymer salt R-7 was measured by IR and the neutralization degree was determined from the intensity ratio of the peak derived from the C=O group of the carboxylic acid and the peak derived from the C=O of Li carboxylic acid, it was 90 mol%. Moreover, the viscosity of a 2% by mass aqueous solution of R-7 was 9,000 mPa·s.

[0097]

Table 1

[0098] Details of the compounds used in Table 1 are shown below. AA: Acrylic acid HEA: 2-Hydroxyethyl acrylate T-20: Trimethylolpropane diallyl ether (manufactured by Daiso Co., Ltd., trade name "Neoallyl T-20") TEA: Triethylamine AcN: Acetonitrile V-65: 2,2'-Azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Corporation) MA: Methyl acrylate VAc: Vinyl acetate PVA: Polyvinyl alcohol (manufactured by Kuraray Co., Ltd., trade name "PVA-217") V-601: 2,2'-Azobis(isobutyric acid) dimethyl (manufactured by Fujifilm Wako Pure Chemical Corporation) LiOH·H2O: Lithium hydroxide monohydrate

[0099] Example 1 <Preparation of Composition for Electrode Binder Layer> Carbon was coated at 10% on the surface of SiOx (0.8 < x < 1.2) by CVD method (hereinafter referred to as "Si-based active material"), and a mixture of graphite and Si-based active material was used as the active material. Also, as this binder, a mixture of crosslinked polymer salt R-1, lithium acetate, styrene / butadiene latex (SBR), and carboxymethyl cellulose (CMC) was used. Using water as a diluting solvent, T.K. High Bismix manufactured by Primix Corporation was mixed for 2 hours at a mass ratio of graphite:Si-based active material:R-1:lithium acetate:SBR:CMC = 90:10:1.0:0.5:1.0:1.0 (solid content) so that the solid content concentration of the electrode binder layer composition became 50% by mass, and a slurry-like electrode binder layer composition (electrode slurry) was prepared. The viscosity of the electrode slurry was 1,670 mPa·s, which was a sufficiently low value. An electrode was fabricated using the obtained electrode slurry and evaluated. The specific procedures and evaluation methods are shown below.

[0100] <Fabrication of Negative Electrode Plate> The above electrode slurry was applied to both sides of a copper foil (thickness: 20 μm) and dried to form a binder layer. Then, after rolling so that the thickness of the binder layer became 27 μm and the packing density became 1.3 g / cm 3 it was punched out into a 3 cm square to obtain a negative electrode plate.

[0101] (Coatability) The coatability of the electrode slurry in the production of the above negative electrode plate was evaluated based on the following criteria and was evaluated as "○". (Evaluation Criteria) ○: No appearance abnormalities such as streaks and bumps are observed on the surface at all. △: Slight appearance abnormalities such as streaks and bumps are observed on the surface. ×: Prominent appearance abnormalities such as streaks and bumps are observed on the surface.

[0102] <Fabrication of Positive Electrode Plate> In an N-methylpyrrolidone (NMP) solvent, 100 parts of lithium iron phosphate (LFP) as a positive electrode active material, 0.2 parts of carbon nanotubes as a conductive agent, 2 parts of Ketjen black, and 0.6 parts of vapor-grown carbon fiber (VGCF) were mixed and added. Polyvinylidene fluoride (PVDF) was mixed as a binder for the electrode composition to prepare a positive electrode composition. A paste layer was formed by applying and drying the positive electrode composition on an aluminum current collector (thickness: 15 μm). Then, after rolling so that the thickness of the paste layer was 88 μm and the packing density was 3.1 g / cm 3 3 , it was punched into a 3 cm square to obtain a positive electrode plate.

[0103] <Fabrication of Secondary Battery> Using the above positive electrode plate, the above negative electrode plate, and a separator, a lithium-ion secondary battery of a laminate type cell was fabricated. As the electrolytic solution, a mixed solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (DEC) were in a volume ratio of 25:75 and LiPF6 was dissolved at a concentration of 1.0 mol / liter was used.

[0104] (Evaluation of cycle characteristics) The lithium-ion secondary battery of the laminate type cell fabricated above was subjected to charge and discharge operations at a charge and discharge rate of 0.2C under the conditions of 2.7 to 3.4V by CC discharge, and the initial capacity C0 was measured. Further, charge and discharge were repeated in an environment at 25°C, and the capacity C 50 after 50 cycles was measured. The cycle characteristics (ΔC) calculated by the following formula were 93.9%, and the cycle characteristics based on the following criteria were evaluated as "〇". Note that the higher the value of ΔC, the better the cycle characteristics. ΔC = C 50 / C0 × 100 (%) (Evaluation criteria) ◎: Charge and discharge capacity retention rate is 95.0% or more 〇: Charge and discharge capacity retention rate is 90.0% or more and less than 95.0% △: Charge and discharge capacity retention rate is 85.0% or more and less than 90.0% ×: Charge and discharge capacity retention rate is less than 85.0%

[0105] Examples 2 to 19 and Comparative Examples 1 to 3 The electrode slurry was prepared by performing the same operations as in Example 1, except that the polymer salt and the alkali metal hydroxide or the alkali metal salt were as shown in Table 2, and the viscosity of the slurry was measured. Further, the coatability of the electrode slurry and the cycle characteristics of the secondary battery obtained using the slurry were evaluated. The results are shown in Table 2. In Example 7, since the crosslinked polymer salt R-1 (1.0 part by mass) with a neutralization degree of 90 mol%, lithium hydroxide monohydrate (0.27 part by mass) as the alkali metal hydroxide, and other components were mixed to obtain an electrode slurry, the slurry contains the crosslinked polymer with a neutralization degree of 100 mol% and lithium hydroxide. The content of the lithium hydroxide is 21.3 parts by mass as the value obtained by converting lithium hydroxide to "lithium hydroxide monohydrate" with respect to 100 parts by mass of the total amount of the crosslinked polymer with a neutralization degree of 100 mol%.

[0106]

Table 2

[0107] The formula weights and manufacturers of the alkali metal hydroxides and the alkali metal salts used in Table 2 are shown below. Lithium acetate: formula weight 65.99, manufactured by Tokyo Chemical Industry Co., Ltd. Lithium carbonate: formula weight 73.89, manufactured by Kishida Chemical Co., Ltd. Lithium hydroxide monohydrate: formula weight 41.96, manufactured by Showa Chemical Co., Ltd. Lithium chloride: formula weight 42.39, manufactured by Honjo Chemical Co., Ltd. Lithium bromide monohydrate: formula weight 104.86, manufactured by Fujifilm Wako Pure Chemical Corporation. Lithium nitrite: formula weight 52.94, manufactured by Honjo Chemical Co., Ltd. Sodium acetate: formula weight 82.03, manufactured by Junsei Chemical Co., Ltd. Potassium carbonate: formula weight 138.20, manufactured by Fujifilm Wako Pure Chemical Corporation. Potassium hydrogen carbonate: formula weight 100.12, manufactured by Fujifilm Wako Pure Chemical Corporation.

[0108] ≪Evaluation Results≫ As is clear from the results of Examples 1 to 19, the composition for a secondary battery electrode binder layer (electrode slurry) containing the aqueous binder for a secondary battery electrode of the present invention all had good coatability and excellent cycle characteristics of the secondary battery equipped with the electrode obtained using the composition. Among these, when using an aqueous binder containing this crosslinked polymer with a viscosity of the 2 mass% concentration aqueous solution of 10,000 mPa·s or less, it was even more excellent in coatability and also excellent in cycle characteristics (Examples 1 to 17, 19). Further, when the usage amount of lithium acetate used as this alkali metal salt was 100 parts by mass or less with respect to 100 parts by mass of the total amount of this crosslinked polymer, the higher the usage amount, the more the compatibility of coatability and cycle characteristics could be achieved at a high level (Examples 1, 3, 4).

[0109] Even if this non-crosslinked polymer R-7 had a higher viscosity than the crosslinked polymer salts R-1 to R-5 of the present invention, when used as an aqueous binder containing lithium acetate which is this alkali metal salt, the viscosity of this composition was reduced, resulting in excellent coatability and further excellent cycle characteristics of the secondary battery (Example 19). It is presumed that the reason why the cycle characteristics when using this crosslinked polymer are higher than those when using this non-crosslinked polymer is that the crosslinked polymer is a tougher binder.

[0110] On the other hand, in the case of a composition for a secondary battery electrode binder layer containing an aqueous binder not containing an alkali metal hydroxide or this alkali metal salt, either the cycle characteristics or the coatability was significantly inferior (Comparative Examples 1 to 3).

Industrial Applicability

[0111] The secondary battery equipped with the electrode obtained using the composition for a secondary battery electrode binder layer containing the aqueous binder for a secondary battery electrode of the present invention exhibits good durability (cycle characteristics), so it is expected to be applied to in-vehicle secondary batteries. It is also useful for the use of an active material containing silicon and is expected to contribute to increasing the capacity of the battery. The aqueous binder for secondary battery electrodes of the present invention can be particularly preferably used for non-aqueous electrolyte secondary battery electrodes, and among them, it is useful for non-aqueous electrolyte lithium ion secondary batteries having a high energy density.

Claims

1. A binder for a lithium-ion secondary battery electrode, comprising a polymer or a salt thereof containing 50% by mass or more and 100% by mass or less of a structural unit derived from an ethylenically unsaturated carboxylic acid monomer, and an alkali metal hydroxide or an alkali metal salt of a compound having a formula weight of 200 or less and having no ethylenically unsaturated group.

2. The binder for a lithium-ion secondary battery electrode according to Claim 1, wherein the lithium-ion secondary battery electrode is a negative electrode containing a silicon-based negative electrode active material.

3. The binder for a lithium-ion secondary battery electrode according to Claim 1 or 2, wherein the polymer is a crosslinked polymer or a non-crosslinked polymer.

4.

3. The binder for a lithium-ion secondary battery electrode according to Claim 3, wherein the crosslinked polymer is obtained using a crosslinkable monomer, and the amount of the crosslinkable monomer used is 0.05 parts by mass or more and 5.0 parts by mass or less with respect to 100 parts by mass of the total amount of non-crosslinkable monomers.

5.

3. The binder for a lithium-ion secondary battery electrode according to Claim 3, wherein the non-crosslinked polymer contains 50% by mass or less of a structural unit derived from vinyl alcohol.

6. 【Claim 1 - 5】 The binder for a lithium-ion secondary battery electrode according to any one of Claims 1 to 5, wherein the viscosity of a 2% by mass aqueous solution of the polymer is 10,000 mPa·s or less.

7. 【Claim 1 - 6】 The binder for a lithium-ion secondary battery electrode according to any one of Claims 1 to 6, wherein the degree of neutralization of the polymer is 70 mol% or more.

8. 【Claim 1 - 7】 The binder for a lithium-ion secondary battery electrode according to any one of Claims 1 to 7, wherein the amount of the alkali metal salt used is 5.0 parts by mass or more and 175 parts by mass or less with respect to 100 parts by mass of the total amount of the polymer.

9. The binder for a lithium-ion secondary battery electrode according to any one of claims 1 to 8, wherein the alkali metal salt contains at least one selected from the group consisting of a lithium salt, a sodium salt, and a potassium salt.

10. The binder for a lithium-ion secondary battery electrode according to claim 9, wherein the lithium salt is lithium acetate.

11. Furthermore, the binder for a lithium-ion secondary battery electrode according to any one of claims 1 to 10, which contains styrene-butadiene rubber (SBR) latex and / or carboxymethyl cellulose (CMC).

12. A composition for a lithium-ion secondary battery electrode mixture layer, comprising the binder for a lithium-ion secondary battery electrode according to any one of claims 1 to 11, an active material, and water.

13. The composition for a lithium-ion secondary battery electrode mixture layer according to claim 12, wherein the pH of the composition for a lithium-ion secondary battery electrode mixture layer is less than 12.

5.

14. A lithium-ion secondary battery electrode, comprising a mixture layer formed from the composition for a lithium-ion secondary battery electrode mixture layer according to claim 12 or 13 on the surface of a current collector.

15. A lithium-ion secondary battery, comprising the lithium-ion secondary battery electrode according to claim 14.

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