Binder for lithium-sulfur secondary battery electrodes and its use

Abstract

The present invention provides a binder for a lithium-sulfur secondary battery electrode that has excellent coatability in a composition for an electrode mixture layer (electrode slurry) even when the solids fraction concentration of the electrode slurry is high, is capable of improving the productivity of secondary battery electrodes by increasing the drying efficiency thereof, and is capable of greatly increasing the sedimentation stability of the electrode slurry.　A binder for a lithium-sulfur secondary battery electrode containing a carboxyl-group-containing polymer or a salt thereof, wherein the carboxyl-group-containing polymer includes a structural unit derived from an ethylenic unsaturated carboxylic acid monomer (A) and a structural unit derived from an ethylenic unsaturated monomer (B) (excluding monomers classified as (A)), and the ethylenic unsaturated monomer (B) has a solubility of 10 g or less in 100 g of 20°C water.

Description

[Technical Field]

[0001] The present invention relates to a binder for a lithium-sulfur secondary battery electrode, a composition for a lithium-sulfur secondary battery electrode mix layer, and a lithium-sulfur secondary battery electrode. [Background technology]

[0002] Various secondary batteries, such as nickel-metal hydride batteries, lithium-ion batteries, and electric double-layer capacitors, have been put to practical use. Among them, lithium-ion batteries are widely used due to their high energy density and battery capacity. Recently, lithium-sulfur secondary batteries, which use sulfur-based active materials as the positive electrode active material instead of the transition metal oxides, such as lithium cobalt oxide, used in lithium-ion secondary batteries, have attracted attention.

[0003] Lithium-sulfur secondary batteries, like lithium-ion batteries, basically have a positive electrode, a negative electrode, and an electrolyte, and charge and discharge is performed by transferring lithium ions between the electrodes via the electrolyte. Sulfur, used as the positive electrode active material in lithium-sulfur secondary batteries, has an extremely high theoretical capacity density of 1672 mAh / g, and lithium-sulfur secondary batteries are expected to be high-capacity batteries.

[0004] On the other hand, in lithium-sulfur secondary batteries, sulfur is converted by a stepwise reduction reaction during discharge, and the resulting lithium polysulfide (LiSx) is easily eluted into the electrolyte. This leads to problems with lithium-sulfur secondary batteries, such as poor cycle characteristics and a short lifespan. Another factor contributing to the short lifespan of lithium-sulfur secondary batteries is the large volume change of sulfur during charge and discharge, which can lead to peeling and falling off of the electrode mixture layer with repeated use, resulting in a decrease in battery capacity.

[0005] In recent years, attempts have been made to solve these problems by using binders. Patent Document 1 discloses an acrylic binder for a positive electrode of a lithium-sulfur secondary battery, which contains polymerized units of a polymerizable monomer having a polar functional group (one or more selected from a nitrogen-containing functional group, an alkylene oxide group, a hydroxy group, and an alkoxysilyl group) that interacts with a positive electrode active material. Patent Document 2 discloses an acrylic binder for a positive electrode of a lithium-sulfur secondary battery, which contains polymerized units of a first polymerizable monomer having a polar functional group (one or more selected from the group consisting of an amide group, a nitrile group, and an alkylene oxide group) that interacts with a positive electrode active material, and polymerized units of a second polymerizable monomer having a crosslinkable functional group (one or more selected from the group consisting of an amide group, a nitrile group, and an alkylene oxide group). Patent Document 3 discloses a binder for producing a positive electrode of a lithium-sulfur secondary battery, which contains an acrylic polymer containing 30% by weight or more of acrylic monomer polymerized units, as well as non-acrylic monomer polymerized units and redox monomer polymerized units. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2017 / 074004 [Patent Document 2] International Publication No. 2018 / 056782 [Patent Document 3] International Publication No. 2019 / 022359 Summary of the Invention [Problem to be solved by the invention]

[0007] The positive electrode of a lithium-sulfur secondary battery is generally prepared by applying a composition for forming an electrode mixture layer containing a sulfur active material, a binder, a medium, etc. (hereinafter also referred to as "electrode slurry") to the surface of a current collector and then removing the medium. Water is preferably used as the medium for the electrode slurry from the viewpoint of reducing environmental impact. According to the inventors' investigations, when water is used as a medium, sulfur is difficult to disperse in the electrode slurry due to its hydrophobicity, and when sulfur is present in the electrode slurry as aggregates, the coating film becomes rough and pinholes occur, causing problems in terms of coatability.

[0008] Furthermore, when carboxymethyl cellulose (CMC), which is often used as a thickener for lithium-ion secondary batteries, is used as a binder, it disperses the sulfur active material well and produces a coating film without roughness or pinholes. However, to improve the coatability of the electrode slurry, the dispersibility of sulfur is insufficient, so the solids concentration of the electrode slurry must be low. As a result, in the production of secondary battery electrodes, a large amount of water evaporates during drying, making it difficult to dry efficiently, which has led to productivity problems.

[0009] The binders disclosed in Patent Documents 1 to 3 can also impart good cycle characteristics, but the problems of coatability and productivity described above have not been thoroughly investigated, and improvements are needed. Furthermore, the binders disclosed in Patent Documents 1 to 3 tend to cause the sulfur active material to settle when the electrode slurry is stored for a long period of time, and therefore, improvement is also required in terms of settling stability.

[0010] The present invention has been made in view of the above circumstances, and its object is to provide a binder for lithium-sulfur secondary battery electrodes that can improve the productivity of secondary battery electrodes by improving the drying efficiency even when the electrode mixture layer composition (electrode slurry) has a high solids concentration, and can also provide a lithium-sulfur secondary battery electrode composition and a lithium-sulfur secondary battery electrode obtained using the binder. [Means for solving the problem]

[0011] As a result of intensive research to solve the above-mentioned problems, the present inventors have found that when a carboxyl group-containing polymer contains a structural unit derived from an ethylenically unsaturated monomer having a water solubility of a specific value or less, the coating properties of the composition for electrode mixture layer (electrode slurry) are good even when the solids concentration of the composition is high, and the productivity of secondary battery electrodes can be improved by increasing the drying efficiency, and the sedimentation stability of the electrode slurry can be significantly improved, thereby completing the present invention.

[0012] The present invention is as follows. [1] A binder for a lithium-sulfur secondary battery electrode containing a carboxyl group-containing polymer or a salt thereof, The carboxyl group-containing polymer contains a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (A) and a structural unit derived from an ethylenically unsaturated monomer (B) (excluding monomers classified as (A)), The binder for a lithium-sulfur secondary battery electrode, wherein the ethylenically unsaturated monomer (B) has a solubility of 10 g or less in 100 g of water at 20°C. [2] The binder for a lithium-sulfur secondary battery electrode according to [1], wherein the carboxyl group-containing polymer contains structural units derived from the ethylenically unsaturated monomer (B) in an amount of 1.0 mass% or more and 50 mass% or less based on the total structural units of the carboxyl group-containing polymer. [3] The binder for a lithium-sulfur secondary battery electrode according to [1] or [2], wherein the carboxyl group-containing polymer contains 50% by mass or more and 99.9% by mass or less of structural units derived from the ethylenically unsaturated carboxylic acid monomer (A) relative to all structural units thereof. [4] The binder for a lithium-sulfur secondary battery electrode according to any one of [1] to [3], wherein the carboxyl group-containing polymer is a crosslinked polymer. [5] The binder for a lithium-sulfur secondary battery electrode according to [4], wherein the crosslinked polymer is a crosslinked polymer obtained by polymerizing a monomer composition containing a non-crosslinkable monomer and a crosslinkable monomer. [6] The binder for a lithium-sulfur secondary battery electrode according to [5], wherein the amount of the crosslinkable monomer used is 0.1 mol % or more and 2.0 mol % or less based on the total amount of the non-crosslinkable monomer. [7] The binder for a lithium-sulfur secondary battery electrode according to [5] or [6], wherein the crosslinkable monomer includes a compound having two or more allyl ether groups in the molecule. [8] The binder for secondary battery electrodes according to any one of [4] to [7], wherein the crosslinked polymer or the salt thereof has a volume-based median particle size measured in an aqueous medium after being neutralized to a degree of neutralization of 80 to 100 mol % of 0.1 μm or more and 7.0 μm or less. [9] The binder for a lithium-sulfur secondary battery electrode according to any one of [1] to [8], which is used for producing a positive electrode of a lithium-sulfur secondary battery.

[10] A composition for a lithium-sulfur secondary battery electrode mixture layer, comprising the binder for a lithium-sulfur secondary battery electrode according to any one of [1] to [9], an active material, and water.

[11] The composition for an electrode mixture layer of a lithium-sulfur secondary battery according to

[10] , wherein the active material contains elemental sulfur or a sulfur-based compound.

[12] A lithium-sulfur secondary battery electrode comprising a mixture layer formed on a surface of a current collector from the composition for a secondary battery electrode mixture layer according to

[10] or

[11] . [Effects of the Invention]

[0013] According to the binder for lithium-sulfur secondary battery electrodes of the present invention, even when the solids concentration of the electrode mixture layer composition (electrode slurry) is high, it is possible to increase the drying efficiency of the electrode slurry while ensuring coatability, thereby improving productivity, and also to significantly increase the sedimentation stability of the electrode slurry, thereby making it possible to obtain a lithium-sulfur secondary battery that exhibits excellent cycle characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0014] The binder for a lithium-sulfur secondary battery electrode of the present invention contains a carboxyl group-containing polymer or a salt thereof, and can be mixed with an active material and water to form a composition for a lithium-sulfur secondary battery electrode mixture layer. The composition is preferably in the form of an electrode slurry that can be applied to a current collector surface, in order to achieve the effects of the present invention. However, the composition may be prepared in the form of a wet powder that can be pressed onto the current collector surface. The lithium-sulfur secondary battery electrode of the present invention can be obtained by forming an electrode mixture layer from the composition on the surface of a current collector such as copper foil or aluminum foil.

[0015] The binder for a lithium-sulfur secondary battery electrode, the composition for a lithium-sulfur secondary battery electrode mixture layer obtained using the binder, the lithium-sulfur secondary battery electrode, and the lithium-sulfur secondary battery will be described in detail below. In this specification, "(meth)acrylic" means acrylic and / or methacrylic, "(meth)acrylate" means acrylate and / or methacrylate, and "(meth)acryloyl group" means acryloyl group and / or methacryloyl group.

[0016] 1. Binder The binder of the present invention comprises a carboxyl group-containing polymer (hereinafter also referred to as "the polymer") or a salt thereof, and the carboxyl group-containing polymer comprises a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (A) and a structural unit derived from an ethylenically unsaturated monomer (B) (excluding monomers classified as (A)) having a solubility of 10 g or less in 100 g of water at 20°C.

[0017] 1-1. Structural units of carboxyl group-containing polymers <Structural Unit Derived from Ethylenically Unsaturated Carboxylic Acid Monomer (A)> This polymer has a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (A) (hereinafter also referred to as "component (a)"), and can be introduced into the polymer by precipitation polymerization or dispersion polymerization of a monomer component containing the ethylenically unsaturated carboxylic acid monomer. The polymer contains such a structural unit and thus has a carboxyl group, which improves adhesion to the current collector and provides excellent lithium ion desolvation and ionic conductivity, resulting in an electrode with low resistance and excellent high-rate performance. Furthermore, when this polymer is a crosslinked polymer, it is imparted with water swelling properties, which can improve the sedimentation stability of the active material and other components in the composition. The component (a) can be introduced into the polymer by, for example, polymerizing a monomer containing an ethylenically unsaturated carboxylic acid monomer (A). Alternatively, it can be obtained by (co)polymerizing a (meth)acrylic acid ester monomer and then hydrolyzing it. Alternatively, it can be obtained by polymerizing (meth)acrylamide and (meth)acrylonitrile, etc., followed by treatment with a strong alkali, or by reacting a polymer having a hydroxyl group with an acid anhydride.

[0018] Examples of ethylenically unsaturated carboxylic acid monomers include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, and fumaric acid; (meth)acrylamidoalkylcarboxylic acids such as (meth)acrylamidohexanoic acid and (meth)acrylamidododecanoic acid; and carboxyl-containing ethylenically unsaturated monomers such as succinic acid monohydroxyethyl (meth)acrylate, ω-carboxy-caprolactone mono(meth)acrylate, and β-carboxyethyl (meth)acrylate, or their (partially) alkali-neutralized products. These may be used alone or in combination of two or more. Among these, compounds having an acryloyl group as a polymerizable functional group are preferred, with acrylic acid being particularly preferred, because they have a high polymerization rate, resulting in polymers with long primary chain lengths and good binder binding strength. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, polymers with a high carboxyl group content can be obtained.

[0019] The content of component (a) in the polymer is not particularly limited, but can be, for example, 50% by mass or more and 99.0% by mass or less, based on the total structural units of the polymer. By including component (a) in this range, excellent adhesion to the current collector can be easily ensured. The lower limit is, for example, 55% by mass or more, or, for example, 60% by mass or more, or, for example, 65% by mass or more. A lower limit of 50% by mass or more is preferable because the sedimentation stability of the composition is improved and a higher binding strength is obtained. It may also be 60% by mass or more, 70% by mass or more, or even 75% by mass or more. The upper limit is, for example, 99.0% by mass or less, or, for example, 98% by mass or less, or, for example, 96% by mass or less, or, for example, 94% by mass or less, or, for example, 92% by mass or less, or, for example, 90% by mass or less, or, for example, 85% by mass or less. The content range of component (a) can be a range that appropriately combines these lower and upper limits.

[0020] <Structural units derived from ethylenically unsaturated monomer (B)> The present polymer has a structural unit (hereinafter also referred to as "component (b)") derived from an ethylenically unsaturated monomer (B) (excluding monomers classified as (A)) whose solubility in 100 g of water at 20°C (hereinafter also referred to simply as "water solubility") is 10 g or less. The polymer containing component (b) exhibits strong interaction with the electrode material and good binding ability to the active material, which improves the sedimentation stability of the electrode slurry and allows for the formation of a firm, well-integrated electrode mixture layer. Here, the water solubility is preferably 8 g or less, more preferably 6 g or less, even more preferably 4 g or less, still more preferably 2 g or less, even more preferably 1 g or less, and even more preferably 0.5 g or less, in terms of excellent sedimentation stability of the electrode slurry.

[0021] Examples of the ethylenically unsaturated monomer (B) include alkyl (meth)acrylates, aromatic (meth)acrylates, styrenes, and aliphatic conjugated diene monomers. Among these, alkyl (meth)acrylates and aromatic (meth)acrylates are preferred in terms of excellent sedimentation stability of the electrode slurry, alkyl (meth)acrylates are particularly preferred, and among alkyl (meth)acrylates, alkyl (meth)acrylates having an alkyl group with 4 or more carbon atoms are preferred.

[0022] The alkyl (meth)acrylates include aliphatic alkyl (meth)acrylates and alicyclic alkyl (meth)acrylates. Examples of aliphatic alkyl (meth)acrylates include methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. Examples of alicyclic alkyl (meth)acrylates include cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, cyclodecyl (meth)acrylate, cyclododecyl (meth)acrylate, isobornyl (meth)acrylate, adamantyl (meth)acrylate, dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, and dicyclopentanyl (meth)acrylate. These may be used alone or in combination of two or more.

[0023] Examples of aromatic (meth)acrylates include phenyl (meth)acrylate, phenylmethyl (meth)acrylate, phenylethyl (meth)acrylate, and phenoxyethyl (meth)acrylate. One of these may be used alone, or two or more may be used in combination.

[0024] Examples of styrenes include styrene, α-methylstyrene, β-methylstyrene, vinylxylene, vinylnaphthalene, o-methylstyrene, m-methylstyrene, p-methylstyrene, o-ethylstyrene, m-ethylstyrene, p-ethylstyrene, p-n-butylstyrene, p-isobutylstyrene, pt-butylstyrene, o-methoxystyrene, m-methoxystyrene, p-methoxystyrene, o-chloromethylstyrene, p-chloromethylstyrene, o-chlorostyrene, p-chlorostyrene, o-hydroxystyrene, m-hydroxystyrene, p-hydroxystyrene, and divinylbenzene. One of these may be used alone, or two or more may be used in combination.

[0025] Examples of the aliphatic conjugated diene monomer include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, etc., and one of these may be used alone or two or more may be used in combination.

[0026] The content of component (b) in the present polymer is not particularly limited, but can be, for example, 1% by mass or more and 50% by mass or less, based on all structural units of the present polymer. By including component (b) in this range, the coatability and sedimentation stability of the electrode slurry can be improved. The lower limit is, for example, 1% by mass or more, or, for example, 3% by mass or more, or, for example, 5% by mass or more, or, for example, 10% by mass or more. A lower limit of 1% by mass or more is preferable in that the sedimentation stability of the electrode slurry is improved. The upper limit is, for example, 50% by mass or less, or, for example, 40% by mass or less, or, for example, 30% by mass or less, or, for example, 25% by mass or less. The range of the content of component (b) can be a range that appropriately combines these lower and upper limits.

[0027] <Other structural units> In addition to components (a) and (b), the present polymer may contain structural units (hereinafter also referred to as "component (c)") derived from other ethylenically unsaturated monomers copolymerizable with these components (excluding monomers classified as (A) and (B)). Component (c) is a structural unit derived from a monomer having an ethylenically unsaturated group other than components (a) and (b), and examples thereof include structural units derived from ethylenically unsaturated monomers having an anionic group other than a carboxyl group, such as a sulfonic acid group or a phosphoric acid group, or nonionic ethylenically unsaturated monomers. These structural units can be introduced by copolymerizing an ethylenically unsaturated monomer having an anionic group other than a carboxyl group, such as a sulfonic acid group or a phosphoric acid group, or a monomer containing a nonionic ethylenically unsaturated monomer.

[0028] The proportion of component (c) can be 0% by mass or more and 50% by mass or less, based on all structural units of the polymer. The proportion of component (c) may be 1% by mass or more and 40% by mass or less, 3% by mass or more and 30% by mass or less, 5% by mass or more and 20% by mass or less, or 10% by mass or more and 15% by mass or less. The content range of component (c) can be a range that appropriately combines these lower and upper limits. Here, when component (c) is contained in an amount of 1% by mass or more based on all structural units of the polymer, affinity for the electrolyte solution is improved, and therefore, the effect of improving lithium ion conductivity can also be expected.

[0029] As the component (c), among those mentioned above, a structural unit derived from a nonionic ethylenically unsaturated monomer is preferred from the viewpoint of obtaining an electrode with good flex resistance, and examples of the nonionic ethylenically unsaturated monomer include (meth)acrylamide and derivatives thereof, and hydroxyl group-containing ethylenically unsaturated monomers.

[0030] Examples of (meth)acrylamide derivatives include N-alkyl(meth)acrylamide compounds such as isopropyl(meth)acrylamide and t-butyl(meth)acrylamide; N-alkoxyalkyl(meth)acrylamide compounds such as Nn-butoxymethyl(meth)acrylamide and N-isobutoxymethyl(meth)acrylamide; and N,N-dialkyl(meth)acrylamide compounds such as dimethyl(meth)acrylamide and diethyl(meth)acrylamide. These may be used singly or in combination of two or more.

[0031] Examples of hydroxyl group-containing ethylenically unsaturated monomers include 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate, and one of these may be used alone or two or more may be used in combination.

[0032] Other nonionic ethylenically unsaturated monomers include, for example, alkoxyalkyl (meth)acrylates such as 2-methoxyethyl acrylate and 2-ethoxyethyl acrylate, and one of these may be used alone or two or more may be used in combination.

[0033] In order to improve the cycle characteristics of the resulting lithium-sulfur secondary battery, the polymer or its salt preferably contains structural units derived from a hydroxyl group-containing ethylenically unsaturated monomer, and preferably contains the structural units in an amount of 1% by mass or more and 30% by mass or less, more preferably 3% by mass or more and 20% by mass or less, and even more preferably 5% by mass or more and 15% by mass or less. The range can be any range that combines these lower and upper limits as appropriate.

[0034] Among nonionic ethylenically unsaturated monomers, compounds having an acryloyl group are preferred because they have a fast polymerization rate, which results in a polymer with a long primary chain length, and they also provide good binding strength for the binder.

[0035] The polymer may be a salt. The type of salt is not particularly limited, but examples include alkali metal salts such as lithium salts, sodium salts, and potassium salts; alkaline earth metal salts such as magnesium salts, calcium salts, and barium salts; other metal salts such as aluminum salts; ammonium salts, and organic amine salts. Among these, alkali metal salts and alkaline earth metal salts are preferred from the viewpoint of less adverse effects on battery characteristics, and alkali metal salts are more preferred. Furthermore, lithium salts are particularly preferred from the viewpoint of obtaining a battery with low resistance.

[0036] 1-2. This crosslinked polymer <Preferred Embodiments of the Present Polymer> The carboxyl group-containing polymer of the present invention is preferably a polymer having a crosslinked structure (hereinafter also simply referred to as "the present crosslinked polymer"), because an electrode mixture layer composition containing a binder including the polymer ensures good electrode slurry coatability even at a high solids concentration, has excellent sedimentation stability of the electrode slurry, and can further exhibit good binding performance. The crosslinking method for the crosslinked polymer is not particularly limited, and examples thereof include the following methods. 1) Copolymerization of crosslinkable monomers 2) Utilizing chain transfer to polymer chains during radical polymerization 3) After synthesizing a polymer having a reactive functional group, a crosslinking agent is added as needed to perform post-crosslinking. The polymer has a crosslinked structure, and thus a binder containing the polymer or a salt thereof can have excellent binding strength. Among the above methods, the method of copolymerizing a crosslinkable monomer is preferred because it is simple to operate and the degree of crosslinking can be easily controlled. <Crosslinking 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.

[0037] The polyfunctional polymerizable monomer is a compound having two or more polymerizable functional groups, such as (meth)acryloyl groups or alkenyl groups, in the molecule, and examples thereof include polyfunctional (meth)acryloyl compounds, polyfunctional alkenyl compounds, and compounds having both (meth)acryloyl groups and alkenyl groups. These compounds may be used alone or in combination of two or more. Among these, polyfunctional alkenyl compounds are preferred because they facilitate the formation of a uniform crosslinked structure, and polyfunctional allyl ether compounds having two or more allyl ether groups in the molecule are particularly preferred.

[0038] Examples of polyfunctional (meth)acryloyl compounds include di(meth)acrylates of dihydric alcohols such as ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, polyethylene glycol di(meth)acrylate, and polypropylene glycol di(meth)acrylate; poly(meth)acrylates such as tri(meth)acrylate and tetra(meth)acrylate of trihydric or higher polyhydric alcohols such as trimethylolpropane tri(meth)acrylate, tri(meth)acrylate of trimethylolpropane ethylene oxide modified product, glycerin tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and pentaerythritol tetra(meth)acrylate; and bisamides such as methylenebisacrylamide and hydroxyethylenebisacrylamide.

[0039] Examples of polyfunctional alkenyl compounds include polyfunctional allyl ether compounds such as trimethylolpropane diallyl ether, trimethylolpropane triallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, tetraallyloxyethane, and polyallylsucrose; polyfunctional allyl compounds such as diallyl phthalate; and polyfunctional vinyl compounds such as divinylbenzene.

[0040] Examples of compounds having both a (meth)acryloyl group and an alkenyl group include allyl (meth)acrylate, isopropenyl (meth)acrylate, butenyl (meth)acrylate, pentenyl (meth)acrylate, and 2-(2-vinyloxyethoxy)ethyl (meth)acrylate.

[0041] Specific examples of the monomer having a self-crosslinkable crosslinkable functional group include a hydrolyzable silyl group-containing vinyl monomer, N-methylol(meth)acrylamide, N-methoxyalkyl(meth)acrylamide, etc. These compounds can be used alone or in combination of two or more.

[0042] 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 vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane, vinyldimethylmethoxysilane, etc.; silyl group-containing acrylic esters such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, methyldimethoxysilylpropyl acrylate, etc.; silyl group-containing methacrylic esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, dimethylmethoxysilylpropyl methacrylate, etc.; silyl group-containing vinyl ethers such as trimethoxysilylpropyl vinyl ether; silyl group-containing vinyl esters such as vinyl trimethoxysilylundecanoate, etc.

[0043] When the crosslinked polymer is crosslinked by a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.01 to 5 mol %, more preferably 0.05 to 2.0 mol %, even more preferably 0.1 to 2.0 mol %, even more preferably 0.1 to 1.0 mol %, and even more preferably 0.2 to 0.6 mol % relative to the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers). The range can be any range that appropriately combines these lower and upper limits. A crosslinkable monomer amount of 0.1 mol % or more is preferred in terms of improved binding strength and sedimentation stability of the electrode slurry. A crosslinkable monomer amount of 2.0 mol % or less is preferred in terms of improved binding strength.

[0044] <Particle size of cross-linked polymer> In the composition for an electrode mixture layer, it is preferable that the crosslinked polymer is not present as large lumps (secondary aggregates) but is well dispersed as water-swellable particles having an appropriate particle size, since the binder containing the crosslinked polymer can exhibit good binding performance.

[0045] The crosslinked polymer or salt thereof of the present invention preferably has a particle size (water-swollen particle size) in the volume-based median range of 0.1 μm to 7.0 μm when dispersed in water, the crosslinked polymer having a degree of neutralization based on the carboxyl groups of the crosslinked polymer. If the particle size is in the range of 0.1 μm to 7.0 μm, the particles are uniformly present in the composition for an electrode mixture layer at a suitable size, thereby enabling the composition for an electrode mixture layer to exhibit high stability and excellent binding properties. If the particle size exceeds 7.0 μm, there is a risk of insufficient binding properties, as described above. Furthermore, there is a risk of insufficient coatability, as it is difficult to obtain a smooth coating surface. On the other hand, if the particle size is less than 0.1 μm, there are concerns about stable production. The lower limit of the particle size may be 0.2 μm or more, 0.3 μm or more, 0.4 μm or more, 0.5 μm or more, 0.6 μm or more, 0.7 μm or more, or 0.8 μm or more. The upper limit of the particle size may be 6.0 μm or less, 5.0 μm or less, 4.0 μm or less, 3.0 μm or less, 2.5 μm or less, or 2.0 μm or less. The particle size range can be set by appropriately combining the above lower and upper limits. The water-swollen particle size can be measured by the method described in the Examples of this specification.

[0046] If the crosslinked polymer is unneutralized or has a degree of neutralization of less than 80 mol%, it can be neutralized with an alkali metal hydroxide or the like to a degree of neutralization of 80 to 100 mol% and dispersed in water, and the particle size can be measured. Generally, crosslinked polymers or salts thereof exist in the form of aggregated particles in which primary particles associate and aggregate when in powder or solution (dispersion). If the particle size when dispersed in water is within the above range, the crosslinked polymer or salt thereof has extremely excellent dispersibility, and by neutralizing to a degree of neutralization of 80 to 100 mol% and dispersing in water, the aggregated particles are broken down, forming a dispersion of mostly primary particles, or even secondary aggregates, forming a stable dispersion state with particle sizes in the range of 0.1 to 7.0 μm.

[0047] The particle size distribution, which is the volume average particle size of water-swollen particles divided by the number average particle size, is preferably 2.0 or less, more preferably 1.5 or less, even more preferably 1.4 or less, and even more preferably 1.3 or less, from the viewpoint of binding property and coatability. The lower limit of the particle size distribution is usually 1.0.

[0048] The particle size (dry particle size) of the crosslinked polymer or salt thereof of the present invention when dried is preferably in the range of 0.1 μm to 2.0 μm in volumetric median size, more preferably 0.2 μm to 1.0 μm, and even more preferably 0.3 μm to 0.7 μm.

[0049] The crosslinked polymer or its salt is preferably used in the form of a salt in which acid groups such as carboxyl groups derived from the ethylenically unsaturated carboxylic acid monomer are neutralized to a degree of neutralization of 20 to 100 mol% in the composition for an electrode mixture layer. The degree of neutralization is more preferably 50 to 100 mol%, and even more preferably 60 to 95 mol%. A degree of neutralization of 20 mol% or higher is preferable because it provides good water swelling properties and facilitates dispersion stabilization. In this specification, the degree of neutralization can be calculated from the amounts 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 by IR analysis of the powder obtained by drying the crosslinked polymer or its salt under reduced pressure at 80°C for 3 hours, and then determining the intensity ratio of the peak derived from the C=O group of the carboxylic acid to the peak derived from the C=O group of the carboxylate salt.

[0050] <Molecular weight of cross-linked polymer (primary chain length)> Crosslinked polymers have a three-dimensional crosslinked structure and exist as microgels in a medium such as water. Generally, such three-dimensional crosslinked polymers are insoluble in solvents, making it impossible to measure their molecular weight. Similarly, it is usually difficult to measure and quantify the primary chain length of crosslinked polymers.

[0051] 1-3. Method for producing the present polymer or its salt The present polymer can be produced by known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization, but precipitation polymerization and suspension polymerization (reverse-phase suspension polymerization) are preferred in terms of productivity. Heterogeneous polymerization methods such as precipitation polymerization, suspension polymerization, and emulsion polymerization are preferred in terms of obtaining better performance in terms of binding properties, etc., and among these, precipitation polymerization is more preferred. Precipitation polymerization is a method for producing polymers by carrying out a polymerization reaction in a solvent that dissolves the raw material unsaturated monomers but does not substantially dissolve the resulting polymer. As the polymerization proceeds, the polymer particles grow larger through aggregation and growth, resulting in a dispersion of polymer particles in which primary particles of tens to hundreds of nanometers in size are secondary aggregates of several micrometers to several tens of micrometers. A dispersion stabilizer can also be used to control the polymer particle size. Specific examples of the dispersion stabilizer include macromonomer-type dispersion stabilizers and nonionic surfactants. The secondary aggregation can be suppressed by selecting the dispersion stabilizer, polymerization solvent, etc. In general, precipitation polymerization in which secondary aggregation is suppressed is also called dispersion polymerization.

[0052] In the case of precipitation polymerization, the polymerization solvent can be selected from water and various organic solvents, taking into consideration the type of monomer to be used, etc. In order to obtain a polymer having a longer primary chain length, it is preferable to use a solvent with a small chain transfer constant.

[0053] Examples of polymerization solvents include water-soluble solvents such as methanol, t-butyl alcohol, acetone, methyl ethyl ketone, acetonitrile, and tetrahydrofuran, as well as benzene, ethyl acetate, dichloroethane, n-hexane, cyclohexane, and n-heptane. These solvents can be used alone or in combination. Alternatively, they can be used as a mixed solvent with water. In the present invention, the water-soluble solvent refers to a solvent whose solubility in water at 20°C is greater than 10 g / 100 ml. Of the above, methyl ethyl ketone and acetonitrile are preferred because they have good polymerization stability with little generation of coarse particles and little adhesion to the reactor, the precipitated polymer fine particles are not prone to secondary aggregation (or even if secondary aggregation does occur, it is easily dissolved in an aqueous medium), a polymer with a small chain transfer constant and a large degree of polymerization (primary chain length) can be obtained, and the operation is easy during the neutralization step described below.

[0054] Similarly, in the neutralization step, it is preferable to add a small amount of a highly polar solvent to the polymerization solvent to ensure stable and rapid progress of the neutralization reaction. Examples of such highly polar solvents include water and methanol. The amount of the highly polar solvent used is preferably 0.05 to 20.0 mass% based on the total mass of the medium, more preferably 0.1 to 10.0 mass%, even more preferably 0.1 to 5.0 mass%, and even more preferably 0.1 to 1.0 mass%. When the proportion of the highly polar solvent is 0.05 mass% or more, it is effective in the neutralization reaction, while when it is 20.0 mass% or less, no adverse effect on the polymerization reaction is observed. Furthermore, in the polymerization of highly hydrophilic ethylenically unsaturated carboxylic acid monomers such as acrylic acid, the addition of a highly polar solvent increases the polymerization rate, making it easier to obtain a polymer with a long primary chain length. Among highly polar solvents, water is particularly preferred due to its significant effect in improving the polymerization rate.

[0055] The production of the present polymer or a salt thereof preferably includes a polymerization step of polymerizing a monomer component containing a structural unit derived from the ethylenically unsaturated carboxylic acid monomer (A) and an ethylenically unsaturated monomer (B). For example, it preferably includes a polymerization step of polymerizing a monomer component containing 50% by mass or more and 99.0% by mass or less of the ethylenically unsaturated carboxylic acid monomer (A) from which the component (a) is derived and 1.0% by mass or more and 50% by mass or less of the ethylenically unsaturated monomer (B) from which the component (b) is derived. By the above polymerization step, the polymer contains 50% by mass or more and 99.0% by mass or less of structural units (component (a)) derived from the ethylenically unsaturated carboxylic acid monomer (A), and 1.0% by mass or more and 50% by mass or less of structural units (component (b)) derived from the ethylenically unsaturated monomer (B). The amount of the ethylenically unsaturated carboxylic acid monomer (A) used is, for example, 50% by mass or more and 99.0% by mass or less, for example, 60% by mass or more and 96% by mass or less, for example, 65% by mass or more and 93% by mass or less, or for example, 70% by mass or more and 90% by mass or less. The amount of the ethylenically unsaturated monomer (B) used is, for example, 1.0% by mass or more and 50% by mass or less, for example, 3% by mass or more and 40% by mass or less, for example, 5% by mass or more and 35% by mass or less, for example, 8% by mass or more and 30% by mass or less, or for example, 10% by mass or more and 30% by mass or less.

[0056] In addition to components (a) and (b), the present polymer may contain structural units (component (c)) derived from other ethylenically unsaturated monomers copolymerizable therewith. Examples of other ethylenically unsaturated monomers from which component (c) is derived include ethylenically unsaturated monomer compounds having anionic groups other than carboxyl groups, such as sulfonic acid groups and phosphoric acid groups, and nonionic ethylenically unsaturated monomers. Specific examples of such compounds include the above-mentioned monomer compounds into which component (c) can be introduced. The other ethylenically unsaturated monomer may be present in an amount of 0% to 50% by mass, 1% to 40% by mass, 3% to 30% by mass, 5% to 20% by mass, or 10% to 15% by mass, based on the total amount of the monomer components.

[0057] The monomer components polymerized in the polymerization step may contain a crosslinkable monomer. As described above, examples of the crosslinkable monomer include a polyfunctional polymerizable monomer having two or more polymerizable unsaturated groups and a monomer having a self-crosslinkable crosslinkable functional group such as a hydrolyzable silyl group. The amount of the crosslinkable monomer used is as described above.

[0058] A higher monomer concentration during polymerization is preferable from the viewpoint of obtaining a polymer with a longer primary chain length. However, if the monomer concentration is too high, aggregation of polymer particles is likely to proceed, and control of the polymerization heat becomes difficult, which may cause the polymerization reaction to run away. Therefore, for example, in the case of precipitation polymerization, the monomer concentration at the start of polymerization is generally in the range of about 2 to 40 mass%, and preferably in the range of 5 to 40 mass%. In this specification, the term "monomer concentration" refers to the concentration of the monomer in the reaction liquid at the time when polymerization is initiated.

[0059] The present polymer may be produced by carrying out a polymerization reaction in the presence of a basic compound. By carrying out the polymerization reaction in the presence of a basic compound, the polymerization reaction can be carried out stably even under high monomer concentration conditions. The monomer concentration may be 13.0% by mass or more, preferably 15.0% by mass or more, more preferably 17.0% by mass or more, even more preferably 19.0% by mass or more, and even more preferably 20.0% by mass or more. The monomer concentration is still more preferably 22.0% by mass or more, and even more preferably 25.0% by mass or more. In general, the higher the monomer concentration during polymerization, the higher the molecular weight that can be achieved, and the longer the primary chain length of the polymer can be produced. Furthermore, polymers with long primary chain lengths tend to be incorporated into three-dimensional crosslinked structures, which tends to reduce the sol fraction.

[0060] The upper limit of the monomer concentration varies depending on the types of monomer and solvent used, the polymerization method, various polymerization conditions, and the like. However, if the heat generated by the polymerization reaction can be removed, the upper limit is, as described above, approximately 40% in precipitation polymerization, approximately 50% in suspension polymerization, and approximately 70% in emulsion polymerization.

[0061] The basic compound is a so-called alkaline compound, and may be either an inorganic basic compound or an organic basic compound. By carrying out the polymerization reaction in the presence of a basic compound, the polymerization reaction can be carried out stably even under conditions of a high monomer concentration, for example, exceeding 13.0% by mass. Furthermore, the polymer obtained by polymerization at such a high monomer concentration has a high molecular weight (long primary chain length) and therefore excellent binding properties. Examples of inorganic base compounds include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide; alkaline earth metal hydroxides such as calcium hydroxide and magnesium hydroxide; and alkali metal carbonates such as sodium carbonate and potassium carbonate. One or more of these compounds can be used. Examples of the organic base compound include ammonia and organic amine compounds, and one or more of these can be used. Among them, organic amine compounds are preferred from the viewpoints of polymerization stability and the binding properties of the binder containing the resulting crosslinked polymer or a salt thereof.

[0062] Examples of the organic amine compound include N-alkyl-substituted amines such as monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, triethylamine, monobutylamine, dibutylamine, tributylamine, monohexylamine, dihexylamine, trihexylamine, trioctylamine, and tridodecylamine; (alkyl)alkanolamines such as monoethanolamine, diethanolamine, triethanolamine, propanolamine, dimethylethanolamine, and N,N-dimethylethanolamine; cyclic amines such as pyridine, piperidine, piperazine, 1,8-bis(dimethylamino)naphthalene, morpholine, and diazabicycloundecene (DBU); and diethylenetriamine and N,N-dimethylbenzylamine. One or more of these may be used. Among these, the use of hydrophobic amines having long-chain alkyl groups is preferred because they provide greater electrostatic and steric repulsion, making it easier to ensure polymerization stability even at high monomer concentrations. Specifically, the higher the value (C / N), which represents the ratio of the number of carbon atoms to the number of nitrogen atoms present in the organic amine compound, the greater the polymerization stabilization effect due to the steric repulsion effect. The C / N value is preferably 3 or more, more preferably 5 or more, even more preferably 10 or more, and even more preferably 20 or more.

[0063] The amount of the basic compound used is preferably in the range of 0.001 mol% to 4.0 mol% relative to the ethylenically unsaturated carboxylic acid monomer. When the amount of the basic compound used is within this range, the polymerization reaction can be carried out smoothly. The amount used may be 0.05 mol% to 4.0 mol%, 0.1 mol% to 4.0 mol%, 0.1 mol% to 3.0 mol%, or 0.1 mol% to 2.0 mol%. In this specification, the amount of the basic compound used represents the molar concentration of the basic compound used relative to the ethylenically unsaturated carboxylic acid monomer, and does not mean the degree of neutralization, i.e., the valence of the basic compound used is not taken into consideration.

[0064] The polymerization initiator may be any known polymerization initiator, such as an azo compound, organic peroxide, or inorganic peroxide, but is not particularly limited. The conditions for use can be adjusted to generate an appropriate amount of radicals using known methods such as thermal initiation, redox initiation in combination with a reducing agent, or UV initiation. To obtain a crosslinked polymer with a long primary chain length, it is preferable to set the conditions so that the amount of radicals generated is as small as possible within the allowable production time range.

[0065] Examples of the azo compounds include 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(N-butyl-2-methylpropionamide), 2-(tert-butylazo)-2-cyanopropane, 2,2'-azobis(2,4,4-trimethylpentane), and 2,2'-azobis(2-methylpropane), and one or more of these can be used.

[0066] Examples of the organic peroxides include 2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane (manufactured by NOF Corporation, trade name "Pertetra A"), 1,1-di(t-hexylperoxy)cyclohexane (manufactured by NOF Corporation, trade name "Perhexa HC"), 1,1-di(t-butylperoxy)cyclohexane (manufactured by NOF Corporation, trade name "Perhexa C"), n-butyl-4,4-di(t-butylperoxy)valerate (manufactured by NOF Corporation, trade name "Perhexa V"), 2,2-di(t-butylperoxy)butane (manufactured by NOF Corporation, trade name "Perhexa 22"), t-butyl hydroperoxide ("Perbutyl H"), cumene hydroperoxide ("Percumyl H"), 1,1,3,3-tetramethylbutyl hydroperoxide ("Perocta H"), t-butylcumyl peroxide ("Perbutyl C"), di-t-butyl peroxide ("Perbutyl D"), di-t-hexyl peroxide ("Perhexyl D"), di(3,5,5-trimethylhexanoyl) peroxide ("Perro"). Peroyl 355), dilauroyl peroxide (Peroyl L), bis(4-t-butylcyclohexyl) peroxydicarbonate (Peroyl TCP), di-2-ethylhexyl peroxydicarbonate (Peroyl OPP), di-sec-butyl peroxydicarbonate (Peroyl SBP), cumyl peroxyneodecanoate (Percumyl ND), 1,1,3,3-tetramethylbutyl peroxyneodecanoate (Peroyl Octa ND), t-hexyl peroxyneodecanoate (Perhexyl ND), t-butyl peroxyneodecanoate (Perbutyl ND), t-butyl peroxyneoheptanoate (Perbutyl NHP), t-hexyl peroxypivalate (Perhexyl PV), t-butyl peroxypivalate (Perbutyl PV), 2,5-dimethyl-2,5-di(2-ethylhexanoyl)hexane (Perhexa 250), 1,1,3,3-Tetramethylbutylperoxy-2-ethylhexanoate (Perocta O), t-hexylperoxy-2-ethylhexanoate (Perhexyl O), t-butylperoxy-2-ethylhexanoate (Perbutyl O), t-butylperoxylaurate (Perbutyl L), t-butylperoxy-3,5,5-trimethylhexanoate (Perbutyl 355), t-hexylperoxyisopropyl monocarbonate Examples include peroxyisopropyl tert-butyl peroxycarbonate ("Perhexyl I"), t-butylperoxyisopropyl monocarbonate ("Perbutyl I"), t-butylperoxy-2-ethylhexyl monocarbonate ("Perbutyl E"), t-butylperoxyacetate ("Perbutyl A"), t-hexylperoxybenzoate ("Perhexyl Z"), and t-butylperoxybenzoate ("Perbutyl Z"), and one or more of these can be used.

[0067] Examples of the inorganic peroxide include potassium persulfate, sodium persulfate, and ammonium persulfate. In the case of redox initiation, sodium sulfite, sodium thiosulfate, sodium formaldehyde sulfoxylate, ascorbic acid, sulfurous acid gas (SO2), ferrous sulfate, etc. can be used as reducing agents.

[0068] A preferred amount of the polymerization initiator used is, for example, 0.001 to 2 parts by mass, or for example, 0.005 to 1 part by mass, or for example, 0.01 to 0.1 part by mass, when the total amount of the monomer components used is 100 parts by mass. If the amount of the polymerization initiator used is 0.001 part by mass or more, the polymerization reaction can be carried out stably, and if it is 2 parts by mass or less, a polymer with a long primary chain length can be easily obtained.

[0069] The polymerization temperature varies depending on conditions such as the type and concentration of the monomers used, but is preferably 0 to 100°C, more preferably 20 to 80°C, and the polymerization temperature may be constant or may change during the polymerization reaction.

[0070] The polymer dispersion obtained through the polymerization step can be dried under reduced pressure and / or by heating to remove the solvent, thereby obtaining the desired polymer in powder form. In this case, it is preferable to include a solid-liquid separation step such as centrifugation and filtration, and a washing step using water, methanol, or the same solvent as the polymerization solvent, following the polymerization step, before the drying step, in order to remove unreacted monomers (and their salts) and impurities derived from the initiator. When the washing step is included, the polymer is easily disintegrated during use, even if it undergoes secondary aggregation. Furthermore, the removal of the remaining unreacted monomers results in good performance in terms of binding strength and battery characteristics.

[0071] In this production method, a polymerization reaction of monomer components containing structural units derived from an ethylenically unsaturated carboxylic acid monomer (A) and an ethylenically unsaturated monomer (B) (excluding monomers classified as (A)) is carried out in the presence of a basic compound. Alternatively, an alkali compound may be added to the polymer dispersion obtained in the polymerization step to neutralize the polymer (hereinafter also referred to as "in-process neutralization"), and then the solvent may be removed in a drying step. Alternatively, a powder of this polymer may be obtained without the in-process neutralization treatment, and then an alkali compound may be added when preparing an electrode slurry to neutralize the polymer (hereinafter also referred to as "post-neutralization"). Among the above methods, in-process neutralization is preferred because it tends to break up secondary aggregates more easily.

[0072] 2. Composition for electrode mixture layer of lithium-sulfur secondary battery The composition for an electrode mixture layer of a lithium-sulfur secondary battery of the present invention contains a binder containing the present polymer or a salt thereof, an active material, and water. The active materials include sulfur element or a sulfur-based compound as a positive electrode active material, and lithium metal or a lithium alloy as a negative electrode active material. The binder according to the present invention exhibits the effects of the present invention particularly when used for producing a positive electrode, but may also be used for producing a negative electrode.

[0073] The sulfur element or sulfur-based compound may be used alone or in combination of two or more. The sulfur-based compound may be Li2Sn (n≧1), an organic sulfur compound, a carbon-sulfur polymer (C2S x ) n , x=2.5 to 50, n≧2).

[0074] The lithium metal or lithium alloy used as the negative electrode active material is a material that can reversibly absorb or release lithium ions, or a material that can reversibly form a lithium-containing compound by reacting with lithium ions. The material capable of reversibly absorbing and desorbing lithium ions includes crystalline carbon, amorphous carbon, and mixtures thereof. Examples of the substance capable of reacting with lithium ions to reversibly form a lithium-containing compound include tin oxide and silicone. The lithium alloy may be, for example, an alloy of lithium and a metal selected from the group consisting of sodium, potassium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, radium, aluminum, and tin.

[0075] The amount of the present polymer or its salt used in the electrode mixture layer composition of the present invention is, for example, 0.1% by mass to 20% by mass, based on the total amount of active material. The amount may also be, for example, 0.2% by mass to 10% by mass, for example, 0.3% by mass to 8% by mass, or for example, 0.4% by mass to 5% by mass. If the amount of the present polymer and its salt used is less than 0.1% by mass, sufficient binding strength may not be obtained. Furthermore, the dispersion stability of the active material may be insufficient, resulting in reduced uniformity of the resulting mixture layer. On the other hand, if the amount of the present polymer and its salt used exceeds 20% by mass, the viscosity of the electrode mixture layer composition may increase, reducing its coatability onto the current collector. As a result, bumps and irregularities may occur in the resulting mixture layer, adversely affecting electrode characteristics.

[0076] When the amount of the present polymer and its salt is within the above range, a composition having excellent sedimentation stability can be obtained, and a mixture layer having extremely high adhesion to the current collector can be obtained, resulting in improved battery durability. Furthermore, the present polymer and its salt exhibit sufficiently high binding ability to the active material even in a small amount (for example, 5 mass % or less), and because they contain carboxy anions, an electrode having low interfacial resistance and excellent high-rate characteristics can be obtained.

[0077] Because elemental sulfur or sulfur-based compounds have low electrical conductivity, they are generally used with the addition of a conductive aid. Examples of conductive aids include carbon-based materials such as carbon black, carbon nanotubes, carbon fiber, graphite powder, and carbon fiber. Among these, carbon black, carbon nanotubes, and carbon fiber are preferred because they are more likely to provide excellent conductivity. Furthermore, ketjen black and acetylene black are preferred as carbon black. The conductive aids may be used alone or in combination of two or more. The amount of conductive aid used may be, for example, 0.2 to 20 parts by mass, or, for example, 0.2 to 10 parts by mass, per 100 parts by mass of the total active material, from the viewpoint of achieving both electrical conductivity and energy density. Furthermore, the positive electrode active material may be surface-coated with a conductive carbon-based material.

[0078] When the composition for a lithium-sulfur secondary battery electrode mixture layer is in a slurry state, the amount of active material used is, for example, in the range of 10 to 75 mass % relative to the total amount of the composition. If the amount of active material used is 10 mass % or more, migration of binders and the like can be suppressed. In addition, since this is advantageous in terms of the cost of drying the medium, the amount of active material used is preferably 30 mass % or more, more preferably 40 mass % or more, and even more preferably 50 mass % or more. On the other hand, if it is 75 mass % or less, the fluidity and coatability of the composition can be ensured, and a uniform mixture layer can be formed.

[0079] The composition for a lithium-sulfur secondary battery electrode mixture layer uses water as a medium. Furthermore, to adjust the properties and drying properties of the composition, the composition may be mixed with lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, or water-soluble organic solvents such as tetrahydrofuran and N-methyl-2-pyrrolidone. The proportion of water in the mixed medium is, for example, 50% by mass or more, or, for example, 70% by mass or more.

[0080] When the composition for a lithium-sulfur secondary battery electrode mixture layer is made into a coatable slurry state, the solids concentration is not limited to about 50 mass %, and the content of the water-containing medium in the entire composition can be, for example, in the range of 25 to 90 mass %, or can be, for example, 35 to 70 mass %, or can be, for example, 45 to 70 mass %, from the viewpoints of the coatability of the electrode slurry, the energy cost required for drying, and productivity.

[0081] The binder of the present invention may consist solely of the present polymer or its salt, or may contain other binder components, such as styrene / butadiene latex (SBR), acrylic latex, and polyvinylidene fluoride latex. When other binder components are used, the amount used may be, for example, 0.1 to 5 parts by mass or less, or, for example, 0.1 to 2 parts by mass or less, or, for example, 0.1 to 1 part by mass or less, per 100 parts by mass of the total amount of active material. If the amount of other binder components used exceeds 5 parts by mass, the resistance increases, and high-rate characteristics may become insufficient. Among the above, styrene / butadiene latex is preferred because of its excellent balance of binding strength and flex resistance.

[0082] The styrene / butadiene latex refers to an aqueous dispersion of a copolymer having structural units derived from an aromatic vinyl monomer such as styrene and structural units derived from an aliphatic conjugated diene monomer such as 1,3-butadiene. Examples of the aromatic vinyl monomer include α-methylstyrene, vinyltoluene, and divinylbenzene, in addition to styrene, and one or more of these can be used. The structural units derived from the aromatic vinyl monomer in the copolymer can be, for example, in the range of 20 to 60% by mass, or, for example, in the range of 30 to 50% by mass, mainly from the viewpoint of binding properties.

[0083] Examples of the aliphatic conjugated diene monomer include 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, etc., in addition to 1,3-butadiene, and one or more of these can be used. The structural units derived from the aliphatic conjugated diene monomer in the copolymer can be in the range of, for example, 30 to 70 mass %, or in the range of, for example, 40 to 60 mass %, in order to improve the binding properties of the binder and the flexibility of the resulting electrode.

[0084] In addition to the above-mentioned monomers, the styrene / butadiene latex may use other monomers as copolymerization monomers, such as nitrile group-containing monomers such as (meth)acrylonitrile, carboxyl group-containing monomers such as (meth)acrylic acid, itanconic acid, and maleic acid, and ester group-containing monomers such as methyl (meth)acrylate, in order to further improve performance such as binding property. The structural units derived from the other monomers in the copolymer can be in the range of, for example, 0 to 30% by mass, and can also be in the range of, for example, 0 to 20% by mass.

[0085] The electrode mixture layer composition for lithium-sulfur secondary batteries of the present invention contains the above-described active material, water, and binder as essential components and can be obtained by mixing the components using known means. The method for mixing the components is not particularly limited, and known methods can be used. However, a preferred method involves dry-blending powder components such as the active material, conductive additive, and binder polymer particles, followed by mixing with a dispersion medium such as water and dispersing and kneading. When obtaining the electrode mixture layer composition in a slurry state, it is preferable to prepare an electrode slurry that is free of poor dispersion or aggregation. Known mixers such as planetary mixers, thin-film gyratory mixers, and planetary / revolving mixers can be used as mixing means. However, thin-film gyratory mixers are preferred because they can achieve a good dispersion state in a short time. When using a thin-film gyratory mixer, it is preferable to perform pre-dispersion using a stirrer such as a disperser. The viscosity of the slurry can be, for example, in the range of 500 to 10,000 mPa·s. From the viewpoint of the coatability of the electrode slurry, the upper limit of the viscosity is preferably 7,000 mPa s or less, more preferably 6,000 mPa s or less, even more preferably 5,000 mPa s or less, still more preferably 4,000 mPa s or less, and even more preferably 3,000 mPa s or less. The slurry viscosity can be measured at a liquid temperature of 25°C by the method described in the Examples.

[0086] On the other hand, when the composition for a lithium-sulfur secondary battery electrode mixture layer is obtained in a wet powder state, it is preferable to knead it to a uniform state without unevenness in concentration using a Henschel mixer, blender, planetary mixer, twin-screw kneader, or the like.

[0087] 3. Electrodes for lithium-sulfur secondary batteries and lithium-sulfur secondary batteries The electrode for a lithium-sulfur secondary battery of the present invention comprises a mixture layer formed from the electrode mixture layer composition on the surface of a current collector such as copper or aluminum. The mixture layer is formed by applying the electrode mixture layer composition of the present invention to the surface of the current collector and then drying to remove the medium such as water. The method for applying the electrode mixture layer composition is not particularly limited, and known methods such as doctor blade, dipping, roll coating, comma coating, curtain coating, gravure coating, and extrusion can be used. The drying can be performed by known methods such as hot air blowing, reduced pressure, (far) infrared radiation, and microwave irradiation. Typically, the mixture layer obtained after drying is subjected to a compression treatment using a mold press, a roll press, or the like. Compression brings the active material and the binder into close contact, improving the strength of the mixture layer and its adhesion to the current collector. Compression can adjust the thickness of the mixture layer to, for example, about 30 to 80% of the thickness before compression, and the thickness of the mixture layer after compression is generally about 4 to 200 μm.

[0088] A lithium-sulfur secondary battery can be fabricated by providing the electrode for a lithium-sulfur secondary battery of the present invention with a separator and an electrolytic solution. The electrolytic solution may be liquid, gel, or a solid electrolyte such as a polymer electrolyte. The separator is placed between the positive and negative electrodes of the battery and serves to prevent short circuits caused by contact between the electrodes and to retain the electrolyte to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane with good ion permeability and mechanical strength. Specific materials that can be used include polyolefins such as polyethylene and polypropylene, and polytetrafluoroethylene.

[0089] The electrolyte may be a known, commonly used one depending on the type of active material. Among the electrolytes, it is more preferable to use a non-aqueous electrolyte. As the non-aqueous electrolyte, an organic electrolyte used in conventional electrochemical devices or an ionic liquid electrolyte may be used. Also, known polymer electrolytes such as polyethylene oxide, polyacrylonitrile, and polymethyl methacrylate may be used.

[0090] The organic electrolyte solution contains an electrolyte salt that serves as an ion carrier, and is composed of the electrolyte salt and an organic solvent that dissolves the electrolyte salt.

[0091] Examples of the electrolyte salt include metal salts of Group 1 elements and metal salts of Group 2 elements. Representative Group 1 element metal salts include, for example, lithium salts, sodium salts, and potassium salts, and Representative Group 2 element metal salts include, for example, magnesium salts, calcium salts, and the like. The anion of the electrolyte salt is, for example, BF4 - , NO3 - , PF6 - , SbF6 - , CH3CH2OSO3 - , CH3CO2 - , or CF3CO2 - , CF3SO3 - , (CF3SO2)2N - [Bis(trifluoromethylsulfonyl)imide (TFSI)], (FSO2)2N - [Bis(fluorosulfonyl)imide (FSI)], (CF3SO2)3C - and the like. Specific examples of the electrolyte salt include lithium salts such as LiClO4, LiAsF6, LiPF6, LiPF4, LiBF4, LiB(C6H5)4, LiCl, LiBr, CH3SO3Li, LiFSI, LiTFSI, CF3SO3Li, etc. Among these, LiFSI is more preferred.

[0092] Examples of the organic solvent include ethers, ketones, lactones, nitriles, amines, amides, sulfur compounds, chlorinated hydrocarbons, esters, carbonates, phosphate ester compounds, sulfolane compounds, and nitro compounds. Specific examples of the organic solvent include ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, anisole, and 1,2-dimethoxyethane (DME), ketones such as 4-methyl-2-pentanone, lactones such as γ-butyrolactone, nitriles such as acetonitrile, propionitrile, butyronitrile, valeronitrile, and benzonitrile, chlorinated hydrocarbons such as 1,2-dichloroethane, esters such as methyl formate, carbonates such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate, and diethyl carbonate (DEM), amides such as dimethylformamide and dimethylthioformamide, phosphate ester compounds such as trimethyl phosphate and triethyl phosphate, and sulfolane compounds such as dimethyl sulfoxide sulfolane and 3-methyl-sulfolane. These may be used alone or as a mixed solvent.

[0093] The following electrolytes can also be used as the organic electrolyte. Specifically, for example, a mixed solvent consisting of DME or DEM as the main solvent and polar solvents such as 1,3-dioxolane (DOL), EC, PC, or ethyl methyl sulfone (EMS) as the secondary solvent, or a solvent represented by the chemical formula "R1(CH2CH2O)" such as tetraethylene glycol dimethyl ether (TEGDME) n R2 (n=2 to 10, R1 is an alkyl group or an alkoxy group, and R2 is an alkyl group. In particular, when R1 is an alkoxy group, it is called a "glyme") alone or as a main solvent, and a mixed solvent obtained by combining DOL or the like as a sub-solvent.

[0094] The "ionic liquid" in the above ionic liquid electrolyte means a salt that exists in a liquid state at 100°C or below. Examples of the cation of the ionic liquid include imidazolium, pyridinium, pyrrolidinium, piperidinium, tetraalkylammonium, pyrazolium, and tetraalkylphosphonium.

[0095] A lithium-sulfur secondary battery is obtained by spirally or stacking positive and negative electrode plates separated by a separator and housing them in a case or the like.

[0096] As described above, an electrode slurry containing the binder for lithium-sulfur secondary battery electrodes disclosed in this specification is expected to exhibit excellent coating properties and sedimentation stability, and therefore, excellent binding properties with the electrode material and excellent adhesion properties with the current collector in the mixture layer. Therefore, a lithium-sulfur secondary battery including an electrode obtained using the binder is expected to ensure good integrity and exhibit good durability (cycling characteristics) even after repeated charge and discharge, making it suitable for use in automotive secondary batteries, etc. [Example]

[0097] The present invention will be specifically described below based on examples. However, the present invention is not limited to these examples. In the following, "parts" and "%" mean parts by mass and % by mass unless otherwise specified. In the following examples, the carboxyl group-containing polymer (salt) was evaluated by the following methods.

[0098] <Measurement of particle size in aqueous medium (water-swollen particle size)> 0.25 g of carboxyl group-containing polymer salt powder and 49.75 g of ion-exchanged water were weighed into a 100 cc container and placed in a rotation / revolution mixer (Thinky Corporation, Awatori Rentaro AR-250). Next, the mixture was stirred (rotation speed 2000 rpm / revolution speed 800 rpm, 7 minutes) and further degassed (rotation speed 2200 rpm / revolution speed 60 rpm, 1 minute) to produce a hydrogel in which the cross-linked polymer salt was swollen in water. Next, the particle size distribution of the above hydrogel was measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT-3300EXII, manufactured by Microtrac Bell) using ion-exchanged water as a dispersion medium. An excess amount of dispersion medium was circulated around the hydrogel, and an amount of hydrogel sufficient to obtain an appropriate scattered light intensity was added. After several minutes, the measured particle size distribution shape stabilized. Once stability was confirmed, particle size distribution measurement was performed to obtain the volume-based median diameter (D50) as a representative value of particle size, and the particle size distribution expressed as (volume-based average particle diameter) / (number-based average particle diameter).

[0099] <Production of Carboxyl Group-Containing Polymer Salt> (Production Example 1: Production of Carboxyl Group-Containing Polymer Salt R-1) For the polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen inlet tube was used. A reactor was charged with 567 parts acetonitrile, 80.0 parts acrylic acid (hereinafter also referred to as "AA"), 20.0 parts methyl acrylate (water solubility: 6 g / 100 g water, hereinafter also referred to as "MA"), 0.9 parts trimethylolpropane diallyl ether (manufactured by Osaka Soda Co., Ltd., trade name "Neoallyl T-20"), and triethylamine equivalent to 1.0 mol% of the AA. The atmosphere inside the reactor was thoroughly purged with nitrogen, and the internal temperature was raised to 55°C. After confirming that the internal temperature had stabilized at 55°C, 0.040 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name "V-65") was added as a polymerization initiator. The reaction solution became cloudy, and this point was designated as the polymerization initiation point. The monomer concentration was calculated to be 15.0%. Twelve hours after the start of polymerization, cooling of the reaction solution began. After the internal temperature had dropped to 25°C, 41.9 parts of lithium hydroxide monohydrate (hereinafter also referred to as "LiOH·HO") powder was added. After the addition, stirring was continued at room temperature for 12 hours, resulting in a slurry-like polymerization reaction solution in which particles of crosslinked polymer salt R-1 (Li salt, neutralization degree 90 mol%) were dispersed in the medium. The conversion rates of AA and MA were calculated to be 97.6% and 96.9%, respectively, 12 hours after the start of polymerization.

[0100] The resulting polymerization reaction solution was centrifuged to settle the polymer particles, and the supernatant was removed. The precipitate was then redispersed in an equal weight of acetonitrile, followed by centrifuging to settle the polymer particles and removing the supernatant. This washing procedure was repeated twice. The precipitate was collected and dried at 80°C under reduced pressure for 3 hours to remove the volatiles, yielding a powder of crosslinked polymer salt R-1. Because crosslinked polymer salt R-1 is hygroscopic, it was stored sealed in a container with water vapor barrier properties. The crosslinked polymer salt R-1 powder was subjected to IR analysis to determine the degree of neutralization from the intensity ratio of the peak derived from the C=O group of the carboxylic acid to the peak derived from the C=O group of the Li carboxylate. The neutralization degree was 90 mol%, equal to the calculated value from the starting material. The particle size in aqueous medium was 1.4 μm.

[0101] (Production Examples 2 to 12 and Comparative Production Examples 1 to 3: Production of Carboxyl Group-Containing Polymer Salts R-2 to R-15) Polymerization reaction solutions containing carboxyl group-containing polymer salts R-2 to R-15 were obtained by the same procedure as in Production Example 1, except that the amounts of each raw material were as shown in Table 1. In all polymerization reaction solutions, the reaction rates of AA, ethylenically unsaturated monomer (B), and other monomers were 90% or higher 12 hours after the initiation of polymerization. The water solubilities of the ethylenically unsaturated monomer (B) and other monomers are shown in Table 1. Next, each polymerization reaction solution was subjected to the same operation as in Production Example 1 to obtain powdery carboxyl group-containing polymer salts R-2 to R-15. Each carboxyl group-containing polymer salt was stored in a sealed container with water vapor barrier properties. The physical properties of each of the obtained polymer salts were measured in the same manner as in Production Example 1. The results are shown in Table 1.

[0102] [Table 1]

[0103] Details of the compounds used in Table 1 are shown below. AA: Acrylic acid MA: Methyl acrylate EA: Ethyl acrylate BA: n-butyl acrylate PEA: Phenoxyethyl acrylate (Osaka Organic Chemical Industry Co., Ltd., product name "Viscoat #192") HEA: 2-hydroxyethyl acrylate DMAAm: N,N-dimethylacrylamide T-20: Trimethylolpropane diallyl ether (manufactured by Osaka Soda Co., Ltd., product name "Neoallyl T-20") TEA: Triethylamine AcN: Acetonitrile V-65: 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name "V-65") LiOH H2O: Lithium hydroxide monohydrate ·Na2CO3: Sodium carbonate K2CO3: Potassium carbonate

[0104] Example 1 An electrode was prepared using the carboxyl group-containing polymer salt R-1 and evaluated. The specific procedures and evaluation methods are described below.

[0105] (Preparation of electrode mixture layer composition (electrode slurry)) Sulfur (colloidal sulfur powder, manufactured by Sigma Aldrich) was used as the positive electrode active material, and acetylene black (DENKA BLACK Li-400, manufactured by Denka Co., Ltd.) was used as the conductive additive. To obtain an electrode slurry with a solids concentration suitable for coating, the mixture was thoroughly mixed in advance using water as a dilution solvent in a mass ratio of sulfur:acetylene black:R-1 = 100:5:3.2 (solids), and then ion-exchanged water was added and pre-dispersed using a disperser. After that, main dispersion was carried out for 15 seconds using a thin film rotary mixer (FM-56-30, manufactured by Primix Corporation) at a peripheral speed of 20 m / s, thereby preparing an electrode slurry for the positive electrode. Here, the amount of water added as a dilution solvent is determined so that the viscosity of the electrode slurry reaches a shear rate of 60 s -1The viscosity was adjusted appropriately to about 1,000 to 10,000 mPa·s before addition. The slurry viscosity was measured for the electrode slurries using each carboxyl group-containing polymer salt as a binder.

[0106] <Measurement of electrode slurry viscosity> The positive electrode mixture slurry obtained above was subjected to shear rate of 60 s at 25°C using a CP25-5 cone plate (diameter 25 mm, cone angle 5°) using an Anton Paar rheometer (Physica MCR301). -1 The viscosity of the slurry was measured and found to be 3,600 mPa·s.

[0107] <Evaluation of electrode slurry coating properties> Next, the electrode slurry was applied to a 20 μm thick aluminum foil using a variable applicator and dried overnight at 70°C in a ventilated dryer to form a mixture layer. After that, the mixture layer had a thickness of 80±5 μm and a packing density of 1.10±0.10 g / cm. 3 The plate was rolled to obtain a positive electrode plate.

[0108] (Criteria for determining coatability) The electrode slurry obtained above was applied to aluminum foil, dried, and then the appearance of the mixture layer was visually observed to evaluate the coatability according to the following criteria (passing level: B grade or higher). As a result, the coating was graded B.

[0109] A: No abnormalities in appearance such as streaks or bumps are found on the surface. B: Slight abnormalities in appearance such as streaks or bumps are observed on the surface. C: Significant abnormalities in appearance such as streaks and bumps are observed on the surface.

[0110] <Sedimentation stability of electrode slurry> The solid content concentration of the supernatant of the electrode slurry obtained above immediately after preparation and the solid content concentration of the supernatant of the electrode slurry after leaving the electrode slurry to stand at 25° C. for one week were measured. Here, the method for measuring the solid content concentration will be described below.

[0111] Approximately 0.5 g of the supernatant immediately after the preparation of the electrode slurry and the supernatant after the electrode slurry was left to stand at 25°C for one week were each collected in a weighing bottle whose weight had been measured in advance [weight of weighing bottle = B (g)], and after accurately weighing each of the weighing bottles [W0 (g)], the samples were placed in a windless dryer together with the weighing bottle and dried at 155°C for 45 minutes, and the weight of each of the weighing bottles at that time was measured [W1 (g)], and the solids concentration was calculated using the following formula. Solid content concentration (mass%)=(W1-B) / (W0-B)×100

[0112] The rate of change in the supernatant solid concentration was calculated using the following formula, and the sedimentation stability was evaluated according to the following criteria (pass level: grade B or higher). The rate of change (%) in the supernatant solid concentration was 14.3%, earning a grade of B. Change rate of supernatant solid concentration (%) = 100 - (supernatant solid concentration after leaving it for 1 week) / (supernatant solid concentration immediately after preparation) × 100

[0113] (Criteria for determining sedimentation stability) A: The change in the supernatant solids concentration is less than 10% B: The change rate of the supernatant solids concentration is 10% or more but less than 20% C: The change rate of the supernatant solid concentration is 20% or more. When the active material in the electrode slurry settles, the active material concentration in the supernatant solids decreases, and the rate of change in the supernatant solids concentration increases.

[0114] (Examples 2 to 12 and Comparative Examples 1 to 3) An electrode slurry was prepared and a positive electrode plate was obtained in the same manner as in Example 1, except that the carboxyl group-containing polymer salt used as the binder was changed as shown in Table 2. The slurry viscosity, coatability, and sedimentation stability were also evaluated. The results are shown in Table 2.

[0115] [Table 2]

[0116] As is clear from the results of Examples 1 to 12, the compositions for lithium-sulfur secondary battery electrode mixture layers (electrode slurries) containing the binders for lithium-sulfur secondary battery electrodes of the present invention were excellent in coatability and sedimentation stability. Among these, focusing on the solubility of the monomer (B) in 100 g of water at 20°C, the results showed that the sedimentation stability of the electrode slurries was even better when the solubility was 2 g or less (Examples 2 to 4). Furthermore, focusing on the presence or absence of crosslinking in the polymer, the results showed that the sedimentation stability of the electrode slurry containing the crosslinked polymer (Example 3) was superior to that of the non-crosslinked polymer (Example 12). In contrast, when a polymer salt not containing a structural unit derived from the ethylenically unsaturated monomer (B) was used (Comparative Examples 1 to 3), the coating properties and sedimentation stability of the electrode slurry containing the polymer salt were significantly inferior compared to the Examples. [Industrial Applicability]

[0117] The electrode slurry containing the binder for lithium-sulfur secondary battery electrodes of the present invention is expected to exhibit excellent coating properties and sedimentation stability, and therefore to exhibit excellent binding properties with the electrode material and excellent adhesion with the current collector in the electrode mixture layer. Therefore, lithium-sulfur secondary batteries equipped with electrodes obtained using the binder are expected to ensure good integrity and exhibit good durability (cycling characteristics) even after repeated charge and discharge, and are expected to contribute to the development of high-capacity automotive secondary batteries and the like.

Claims

1. A binder for a lithium-sulfur secondary battery electrode containing a carboxyl group-containing polymer or a salt thereof, The carboxyl group-containing polymer contains a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (A) and a structural unit derived from an ethylenically unsaturated monomer (B) (excluding monomers classified as (A)), The binder for a lithium-sulfur secondary battery electrode, wherein the ethylenically unsaturated monomer (B) has a solubility of 10 g or less in 100 g of water at 20°C.

2. 2. The binder for a lithium-sulfur secondary battery electrode according to claim 1, wherein the carboxyl group-containing polymer contains structural units derived from the ethylenically unsaturated monomer (B) in an amount of 1.0 mass% or more and 50 mass% or less relative to all structural units of the carboxyl group-containing polymer.

3. 3. The binder for a lithium-sulfur secondary battery electrode according to claim 1, wherein the carboxyl group-containing polymer contains structural units derived from the ethylenically unsaturated carboxylic acid monomer (A) in an amount of 50% by mass or more and 99.9% by mass or less, based on all structural units of the carboxyl group-containing polymer.

4. The binder for a lithium-sulfur secondary battery electrode according to any one of claims 1 to 3, wherein the carboxyl group-containing polymer is a crosslinked polymer.

5. 5. The binder for a lithium-sulfur secondary battery electrode according to claim 4, wherein the crosslinked polymer is obtained by polymerizing a monomer composition containing a non-crosslinkable monomer and a crosslinkable monomer.

6. 6. The binder for a lithium-sulfur secondary battery electrode according to claim 5, wherein the amount of the crosslinkable monomer used is 0.1 mol % or more and 2.0 mol % or less with respect to the total amount of the non-crosslinkable monomer.

7. 7. The binder for a lithium-sulfur secondary battery electrode according to claim 5, wherein the crosslinkable monomer includes a compound having two or more allyl ether groups in the molecule.

8. The binder for a secondary battery electrode according to any one of claims 4 to 7, wherein the crosslinked polymer or the salt thereof has a volume-based median particle size measured in an aqueous medium after being neutralized to a degree of neutralization of 80 to 100 mol % of 0.1 µm or more and 7.0 µm or less.

9. The binder for lithium-sulfur secondary battery electrodes according to any one of claims 1 to 8, which is used for producing a positive electrode of a lithium-sulfur secondary battery.

10. A composition for a lithium-sulfur secondary battery electrode mix layer, comprising the binder for a lithium-sulfur secondary battery electrode according to any one of claims 1 to 9, an active material, and water.

11. The composition for an electrode mixture layer of a lithium-sulfur secondary battery according to claim 10 , wherein the active material contains elemental sulfur or a sulfur-based compound.

12. A lithium-sulfur secondary battery electrode comprising a mixture layer formed on a surface of a current collector from the composition for a secondary battery electrode mixture layer according to claim 10 or 11.

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