Binder for non-aqueous electrolyte secondary battery electrodes and its uses

Abstract

The present invention pertains to a non-aqueous electrolyte secondary battery electrode binder. This binder contains a cross-linked polymer or a salt thereof. The cross-linked polymer or the salt thereof contains: 50%–99.5% by mass of a first structural unit derived from an ethylenically unsaturated carboxylic acid monomer, relative to the total number of structural units; and 0.5%–50% by mass of a second structural unit derived from a specified monomer having a hydroxyl group. The cross-linked polymer or the salt thereof has a cold-water swelling rate at pH 8 of 25.0–40.0.

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims priority based on Japanese Patent Application No. 2020-91604, filed on May 26, 2020, the entire contents of which are hereby incorporated by reference as part of this specification.

[0002] The present specification relates to a binder for electrodes of non-aqueous electrolyte secondary batteries that can be used in lithium ion secondary batteries and the like, its uses, and a method for producing a crosslinked polymer or a salt thereof used in the binder. [Background technology]

[0003] Lithium-ion secondary batteries are well known examples of nonaqueous electrolyte secondary batteries. Nonaqueous electrolyte secondary battery electrodes are produced by applying a composition for forming an electrode mixture layer (hereinafter also referred to as "electrode slurry") containing an active material, a binder, and the like onto a current collector and drying the composition. Aqueous binders containing styrene butadiene rubber (SBR) latex and carboxymethyl cellulose (CMC) are used as binders for negative electrode mixture layer compositions. Meanwhile, N-methyl-2-pyrrolidone (NMP) solutions of polyvinylidene fluoride (PVDF) are widely used as binders for positive electrode mixture layer compositions. However, with the recent rise in environmental awareness, there is a growing demand for aqueous positive electrode mixture layer compositions that do not use organic solvents.

[0004] As the applications of lithium-ion secondary batteries expand, there is a growing demand for improved energy density, reliability, and durability. For example, in order to increase the electrical capacity of lithium-ion secondary batteries, specifications using silicon-based active materials as the negative electrode active material are becoming more common. However, silicon-based active materials are known to undergo large volume changes during charge and discharge, which can lead to peeling or detachment of the electrode mixture layer with repeated use, resulting in a decrease in battery capacity and deterioration of cycle characteristics (durability). To prevent such problems, it is generally effective to use a binder to firmly bond (adhesion) the electrode materials together. Therefore, research into improving the adhesion of binders has been conducted in order to improve durability.

[0005] In this context, it has recently been reported that acrylic acid polymers are effective as binders for use in negative electrode mixture layer compositions using silicon-based active materials. For example, Patent Document 1 discloses an acrylic acid polymer crosslinked with a specific crosslinking agent as a binder for forming a negative electrode coating film in lithium-ion secondary batteries. Patent Document 2 discloses an aqueous electrode binder for secondary batteries containing a water-soluble polymer including structural units derived from an ethylenically unsaturated carboxylate monomer and structural units derived from a vinyl alcohol monomer. Patent Document 3 discloses an aqueous electrode binder for secondary batteries containing a water-soluble polymer including structural units derived from an ethylenically unsaturated carboxylate monomer and structural units derived from a highly hydrophilic ethylenically unsaturated monomer that does not contain carboxylic acid. Patent Document 4 discloses a crosslinked acrylic acid polymer having a specific particle size in a 1% NaCl aqueous solution. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2014 / 065407 [Patent Document 2] International Publication No. 2014 / 207967 [Patent Document 3] International Publication No. 2016 / 067633 [Patent Document 4] International Publication No. 2017 / 073589 Summary of the Invention

[0007] As the performance of secondary batteries improves, there is an increasing demand for binders that can impart better cycle characteristics. Furthermore, when applying and drying an electrode slurry containing a binder, it is advantageous to increase the solids concentration of the electrode slurry in order to improve the drying efficiency of the slurry and improve electrode productivity. However, as the solids concentration increases, it usually becomes more difficult to ensure good coatability. While the binders disclosed in Patent Documents 1 to 4 all impart good adhesion and cycle characteristics, there is still room for improvement in coatability and, in turn, productivity. Thus, to date, no binder has been successfully developed that simultaneously achieves excellent coatability and adhesion, thereby improving both productivity and cycle characteristics.

[0008] The present disclosure has been made in view of the above circumstances, and provides a binder for a non-aqueous electrolyte secondary battery electrode that exhibits excellent adhesion and can exhibit excellent cycle characteristics, and that exhibits excellent coatability even when the solids concentration of the electrode slurry is high, thereby contributing to improved productivity. The present disclosure also provides a composition for a non-aqueous electrolyte secondary battery electrode mixture layer, a non-aqueous electrolyte secondary battery electrode, and a non-aqueous electrolyte secondary battery obtained using the binder. [Means for solving the problem]

[0009] As a result of intensive research to solve the above-mentioned problems, the present inventors have found that when a binder containing a crosslinked polymer or a salt thereof that has a controlled swelling degree in an aqueous medium (hereinafter also referred to as "water swelling degree") and that contains a specific amount of structural units derived from an ethylenically unsaturated carboxylic acid monomer and structural units derived from a specific unsaturated monomer having a hydroxyl group is used, the electrode mixture layer slurry has excellent both coatability and cycle characteristics. According to the present disclosure, the following means are provided based on this finding.

[0010] [1] A binder for a non-aqueous electrolyte secondary battery electrode containing a crosslinked polymer or a salt thereof, The crosslinked polymer or salt thereof contains, relative to all structural units thereof, 50% by mass or more and 99.5% by mass or less of first structural units derived from an ethylenically unsaturated carboxylic acid monomer, and 0.5% by mass or more and 50% by mass or less of second structural units derived from one or more monomers selected from the group consisting of monomers represented by formula (1) and formula (2), and The crosslinked polymer or salt thereof has a water swelling degree at pH 8 of 25.0 or more and 40.0 or less, as a binder for electrodes of non-aqueous electrolyte secondary batteries. CH2=C(R 1 )COOR 2 (1) [In the formula, R 1 represents a hydrogen atom or a methyl group, and R 2 is a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, (R 3 O) m H or R 4 O[CO(CH2)5O] n H. Note that R 3 represents an alkylene group having 2 to 4 carbon atoms, and R 4 represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15.] CH2=C(R 5 )CONR 6 R 7 (2) [In the formula, R 5 represents a hydrogen atom or a methyl group, and R 6 represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, and R 7 represents a hydrogen atom or a monovalent organic group. [2] The binder for a non-aqueous electrolyte secondary battery electrode according to [1], wherein the second structural unit is a structural unit derived from a hydroxyalkyl (meth)acrylate. [3] The binder for a non-aqueous electrolyte secondary battery electrode according to [1] or [2], wherein the salt of the crosslinked polymer contains a lithium salt. [4] The binder for electrodes of non-aqueous electrolyte secondary batteries according to any one of [1] to [3], 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 10 μm or less. [5] The binder for a non-aqueous electrolyte secondary battery electrode according to any one of [1] to [4], wherein the crosslinked polymer is crosslinked with a crosslinkable monomer. [6] The binder for a non-aqueous electrolyte secondary battery electrode according to [5], wherein the amount of the crosslinkable monomer used is 0.1 mol % or more and 1.0 mol % or less based on the total amount of the non-crosslinkable monomer. [7] The binder for a non-aqueous electrolyte secondary battery electrode according to any one of [1] to [6], wherein the crosslinked polymer or the salt thereof is obtained by precipitation polymerization or dispersion polymerization. [8] The crosslinked polymer or the salt thereof is produced by the following polymerization step in the precipitation polymerization or dispersion polymerization: (A) a polymerization step in which only the ethylenically unsaturated carboxylic acid monomer is polymerized; and (B) The binder for a non-aqueous electrolyte secondary battery electrode according to [7], obtained by carrying out one or both of a polymerization step of continuously or intermittently adding the monomers represented by the formulas (1) and (2) to the ethylenically unsaturated carboxylic acid monomer and polymerizing them. [9] A composition for a non-aqueous electrolyte secondary battery electrode mixture layer, comprising the binder for a non-aqueous electrolyte secondary battery electrode according to any one of [1] to [8], an active material, and water.

[10] The composition for a non-aqueous electrolyte secondary battery electrode mixture layer according to [9], further comprising a styrene / butadiene-based latex as a binder for a non-aqueous electrolyte secondary battery electrode.

[11] The composition for a non-aqueous electrolyte secondary battery electrode mixture layer according to [9] or

[10] , which contains a carbon-based material and / or a silicon-based material as a negative electrode active material.

[12] The composition for a non-aqueous electrolyte secondary battery electrode mixture layer according to [9] or

[10] , which contains a lithium-containing metal oxide as a positive electrode active material.

[13] A non-aqueous electrolyte secondary battery electrode comprising a mixture layer formed on a surface of a current collector from the composition for a non-aqueous electrolyte secondary battery electrode mixture layer according to any one of [9] to

[12] .

[14] A non-aqueous electrolyte secondary battery comprising the secondary battery electrode according to

[13] . [Effects of the Invention]

[0011] The binder for secondary battery electrodes disclosed in the present specification has a specific structural unit and a controlled degree of swelling in water, and therefore exhibits excellent coatability and adhesion, which can contribute to the productivity of secondary battery electrodes and the like and the cycle characteristics of secondary batteries. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram showing an apparatus used to measure the water swelling degree of a crosslinked polymer or a salt thereof. DETAILED DESCRIPTION OF THE INVENTION

[0013] The binder for secondary battery electrodes disclosed herein contains a crosslinked polymer or a salt thereof, and can be mixed with an active material and water to form a composition for an electrode mixture layer. The composition has specific structural units and a controlled degree of swelling in water, thereby exhibiting excellent coatability and adhesion. The composition allows the electrode mixture layer to be applied with excellent coatability even under conditions of high active material concentration, reducing the amount of water and other media that must be dried and removed. This can contribute to improving productivity when manufacturing electrodes, etc., and ensures excellent cycle capacity retention due to the excellent adhesion of the electrode mixture layer.

[0014] The composition may be in the form of a slurry that can be applied to a current collector, or may be prepared as a wet powder that can be pressed onto the surface of the current collector. A mixture layer made from the composition is formed on the surface of a current collector such as copper foil or aluminum foil, thereby obtaining the secondary battery electrode disclosed in this specification.

[0015] Representative but non-limiting specific examples of the present specification will be described in detail below, with reference to the accompanying drawings as appropriate. This detailed description is intended simply to provide those skilled in the art with details for implementing preferred examples of the present disclosure, and is not intended to limit the scope of the present disclosure. Furthermore, the additional features and disclosures disclosed below can be used separately or together with other features and disclosures of the present specification to provide a further improved "binder for non-aqueous electrolyte secondary battery electrode and use thereof."

[0016] Furthermore, the combinations of features and steps disclosed in the following detailed description are not essential to implementing the disclosure in its broadest sense, but are described solely to specifically illustrate representative embodiments of the present disclosure. Furthermore, the various features of the representative embodiments described above and below, and the various features of those described in the independent and dependent claims, do not necessarily have to be combined in the exact embodiments described herein, or in the exact order listed, to provide additional and useful embodiments of the disclosure.

[0017] All features described in this specification and / or claims are intended to be disclosed individually and independently of one another as limitations on the original disclosure and claimed particulars, apart from any configuration of features described in the examples and / or claims. Furthermore, all numerical ranges and group or aggregate descriptions are intended to disclose intermediate configurations thereof as limitations on the original disclosure and claimed particulars.

[0018] The binder for a secondary battery electrode disclosed in this specification, the composition for a secondary battery electrode mixture layer obtained using the binder, and the secondary battery electrode will be described in detail below.

[0019] 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.

[0020] <Binder> The binder disclosed herein may include a crosslinked polymer or a salt thereof. The crosslinked polymer may have a first structural unit derived from an ethylenically unsaturated carboxylic acid monomer and a second structural unit derived from a specific monomer having a hydroxyl group.

[0021] <Structural unit of cross-linked polymer> <First structural unit> The crosslinked polymer may have a first structural unit (hereinafter also referred to as "component (a)") derived from an ethylenically unsaturated carboxylic acid monomer. When the crosslinked polymer has such a structural unit and thus a carboxyl group, adhesion to the current collector is improved, and the desolvation effect of lithium ions and ionic conductivity are excellent, resulting in an electrode with low resistance and excellent high-rate characteristics. In addition, water swelling properties are imparted, which can improve the dispersion stability of the active material and the like in the composition for the electrode mixture layer.

[0022] The component (a) can be introduced into the crosslinked polymer by, for example, polymerizing a monomer containing an ethylenically unsaturated carboxylic acid monomer. Alternatively, it can be obtained by (co)polymerizing a (meth)acrylic acid ester monomer and then hydrolyzing it. Alternatively, it can be obtained by polymerizing (meth)acrylamide and (meth)acrylonitrile, etc., and then treating them with a strong alkali, or by reacting a polymer having a hydroxyl group with an acid anhydride.

[0023] Examples of ethylenically unsaturated carboxylic acid monomers include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid, (meth)acrylamidohexanoic acid, (meth)acrylamidoalkylcarboxylic acids such as (meth)acrylamidododecanoic acid, carboxyl group-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 adhesion. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, a polymer with a high carboxyl group content can be obtained.

[0024] The content of component (a) in the crosslinked polymer is not particularly limited, but can be, for example, 10% by mass or more and 99.5% by mass or less, based on all structural units of the crosslinked polymer. By including component (a) in such a range, excellent adhesion to the current collector can be easily ensured. The lower limit is, for example, 20% by mass or more, such as 30% by mass or more, or for example, 40% by mass or more. A lower limit of 50% by mass or more is preferable because it improves the dispersion stability of the composition for electrode mixture layer. It may also be 60% by mass or more, 70% by mass or more, or 80% by mass or more. The upper limit is, for example, 99% by mass or less, such as 98% by mass or less, such as 95% by mass or less, such as 90% by mass or less, or for example, 80% by mass or less. The range can be a range that appropriately combines these lower and upper limits, for example, 30% by mass or more and 99.5% by mass or less, for example, 50% by mass or more and 99.5% by mass or less, for example, 50% by mass or more and 99% by mass or less, for example, 50% by mass or more and 98% by mass or less, for example, 50% by mass or more and 95% by mass or less, for example, 50% by mass or more and 90% by mass or less, for example, 55% by mass or more and 90% by mass or less, for example, 60% by mass or more and 90% by mass or less, for example, 70% by mass or more and 90% by mass or less, or for example, 80% by mass or more and 90% by mass or less.

[0025] <Second structural unit> In addition to component (a), the crosslinked polymer may have a second structural unit (hereinafter also referred to as "component (b)") derived from a specific monomer having a hydroxyl group. When the crosslinked polymer has component (b), the coatability of an electrode mixture layer composition obtained using a binder containing the crosslinked polymer can be improved. Component (b) can be introduced into the crosslinked polymer by polymerizing one or more monomers selected from the group consisting of monomers represented by the following formulas (1) and (2). Alternatively, component (b) may be introduced into the crosslinked polymer by polymerizing a monomer having a formula weight of 200 or less and having a (meth)acryloyl group and a hydroxyl group.

[0026] CH2=C(R 1 )COOR 2 (1) [In the formula, R 1 represents a hydrogen atom or a methyl group, and R 2 is a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, (R 3 O) m H or R 4 O[CO(CH2)5O] n H. Note that R 3 represents an alkylene group having 2 to 4 carbon atoms, and R 4 represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15.]

[0027] CH2=C(R 5 )CONR 6 R 7 (2) [In the formula, R 5 represents a hydrogen atom or a methyl group, and R 6 represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, and R 7 represents a hydrogen atom or a monovalent organic group.

[0028] The monomer represented by the above formula (1) is a (meth)acrylate compound having a hydroxyl group. 2 When R is a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, the number of the hydroxyl group may be one or more. The monovalent organic group is not particularly limited, but examples thereof include alkyl groups which may have a linear, branched, or cyclic structure, as well as aryl groups and alkoxyalkyl groups. 2 (R 3 O) m H or R 4 O[CO(CH2)5O] n If H, then R 3 or R 4 The alkylene group represented by may be linear or branched.

[0029] Examples of the monomer represented by the formula (1) include hydroxyalkyl (meth)acrylates having a hydroxyalkyl group having 1 to 8 carbon atoms, such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, hydroxyhexyl (meth)acrylate, and hydroxyoctyl (meth)acrylate; polyalkylene glycol mono(meth)acrylates, such as polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, polybutylene glycol mono(meth)acrylate, and polyethylene glycol-polypropylene glycol mono(meth)acrylate; dihydroxyalkyl (meth)acrylates, such as glycerin mono(meth)acrylate; caprolactone-modified hydroxymethacrylates (manufactured by Daicel Corporation, trade names "PLACCEL FM1," "PLACCEL FM5," etc.), and caprolactone-modified hydroxyacrylates (manufactured by Daicel Corporation, trade names "PLACCEL FA1," "PLACCEL FA10L," etc.). The monomer represented by the above formula (1) may be used singly or in combination of two or more.

[0030] The monomer represented by the above formula (2) is a (meth)acrylamide derivative having a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms. 7 represents a hydrogen atom or a monovalent organic group. The monovalent organic group is not particularly limited, but examples thereof include alkyl groups which may have a linear, branched, or cyclic structure, as well as aryl groups and alkoxyalkyl groups, and is preferably an organic group having 1 to 8 carbon atoms. In addition, R 7 may be a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms.

[0031] Examples of the monomer represented by the formula (2) include hydroxy(meth)acrylamide; (meth)acrylamide derivatives having a hydroxyalkyl group having 1 to 8 carbon atoms, such as N-hydroxyethyl(meth)acrylamide, N-hydroxypropyl(meth)acrylamide, N-hydroxybutyl(meth)acrylamide, N-hydroxyhexyl(meth)acrylamide, and N-hydroxyoctyl(meth)acrylamide, N-methylhydroxyethyl(meth)acrylamide, and N-ethylhydroxyethyl(meth)acrylamide; and N,N-dihydroxyalkyl(meth)acrylamides, such as N,N-dihydroxyethyl(meth)acrylamide and N,N-dihydroxyethyl(meth)acrylamide. The monomer represented by the formula (2) may be used singly or in combination of two or more.

[0032] Examples of the monomer having a formula weight of 200 or less and having a (meth)acryloyl group and a hydroxyl group include hydroxyalkyl (meth)acrylates such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, hydroxyhexyl (meth)acrylate, and hydroxyoctyl acrylate; dialkylene glycol mono(meth)acrylates such as diethylene glycol mono(meth)acrylate and dipropylene glycol monoacrylate; (meth)acrylamide derivatives having a hydroxyalkyl group having 1 to 8 carbon atoms such as N-hydroxyethyl (meth)acrylamide, N-hydroxypropyl (meth)acrylamide, N-hydroxybutyl (meth)acrylamide, N-hydroxyhexyl (meth)acrylamide, and N-hydroxyoctyl (meth)acrylamide; and N-methylol (meth)acrylamide. These may be used singly or in combination of two or more.

[0033] As the second structural unit, a hydroxyalkyl(meth)acrylate having a hydroxyalkyl group with 1 to 8 carbon atoms or a hydroxyalkyl(meth)acrylate having a formula weight of 200 or less is preferred, as they are excellent in improving the coatability of the composition for an electrode mixture layer. Hydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate, and hydroxybutyl(meth)acrylate are more preferred.

[0034] The content of component (b) in the crosslinked polymer can be 0.5% by mass or more relative to all structural units of the crosslinked polymer. A content of component (b) of 0.5% by mass or more ensures good coatability of the electrode mixture layer composition. The lower limit may be 1.0% by mass or more, 3.0% by mass or more, 5.0% by mass or more, 10% by mass or more, 15% by mass or more, or 20% by mass or more. Furthermore, a content of component (b) of 50% by mass or less ensures the amount of component (a) and ensures sufficient dispersion stability of the electrode mixture layer composition (electrode slurry). The upper limit may be 45% by mass or less, 40% by mass or less, 35% by mass or less, 30% by mass or less, 25% by mass or less, or 20% by mass or less. The range can be a range that appropriately combines these lower and upper limits, for example, 0.5% by mass or more and 50% by mass or less, for example, 1.0% by mass or more and 50% by mass or less, for example, 1.0% by mass or more and 45% by mass or less, for example, 1.0% by mass or more and 40% by mass or less, for example, 5.0% by mass or more and 40% by mass or less, for example, 10% by mass or more and 50% by mass or less, for example, 10% by mass or more and 45% by mass or less, or for example, 10% by mass or more and 40% by mass or less.

[0035] <Other structural units> In addition to components (a) and (b), the crosslinked polymer may contain structural units derived from other ethylenically unsaturated monomers copolymerizable with these components (hereinafter also referred to as "component (c)"). Examples of component (c) include structural units derived from ethylenically unsaturated monomer compounds having anionic groups other than carboxyl groups, such as sulfonic acid groups and phosphate groups, or nonionic ethylenically unsaturated monomers other than component (b). These structural units can be introduced by copolymerizing ethylenically unsaturated monomer compounds having anionic groups other than carboxyl groups, such as sulfonic acid groups and phosphate groups, or monomers containing nonionic ethylenically unsaturated monomers other than component (b). Among these, structural units derived from nonionic ethylenically unsaturated monomers are preferred as component (c) from the viewpoint of obtaining electrodes with good flex resistance, and (meth)acrylamide and its derivatives, as well as nitrile-group-containing ethylenically unsaturated monomers, are preferred from the viewpoint of excellent binder adhesion. Furthermore, when a structural unit derived from a hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less is introduced as component (c), it can exhibit strong interaction with the electrode material and exhibit good adhesion to the active material. This is preferable because it allows for a firm and well-integrated electrode mixture layer to be obtained. In particular, a structural unit derived from an alicyclic structure-containing ethylenically unsaturated monomer is preferable.

[0036] The proportion of component (c) can be 0% by mass or more and 49.5% by mass or less, based on the total structural units of the crosslinked polymer. The proportion of component (c) may be 1% by mass or more and 40% by mass or less, 2% by mass or more and 40% by mass or less, 2% by mass or more and 30% by mass or less, or 5% by mass or more and 30% by mass or less. Furthermore, when component (c) is contained at 1% by mass or more based on the total structural units of the crosslinked polymer, affinity for the electrolyte solution is improved, and therefore improved lithium ion conductivity can also be expected.

[0037] 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.

[0038] Examples of nitrile group-containing ethylenically unsaturated monomers include (meth)acrylonitrile; (meth)acrylic acid cyanoalkyl ester compounds such as cyanomethyl (meth)acrylate and cyanoethyl (meth)acrylate; cyano group-containing unsaturated aromatic compounds such as 4-cyanostyrene and 4-cyano-α-methylstyrene; and vinylidene cyanide. One of these may be used alone, or two or more may be used in combination.

[0039] Examples of the alicyclic structure-containing ethylenically unsaturated monomer include (meth)acrylic acid cycloalkyl esters which may have an aliphatic substituent, such as cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, cyclodecyl (meth)acrylate, and cyclododecyl (meth)acrylate; isobornyl (meth)acrylate, adamantyl (meth)acrylate, dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and cycloalkyl polyalcohol mono(meth)acrylates such as cyclohexanedimethanol mono(meth)acrylate and cyclodecanedimethanol mono(meth)acrylate. One of these may be used alone, or two or more may be used in combination. Among the above, compounds having an acryloyl group as a polymerizable functional group are preferred, since they have a high polymerization rate, resulting in a polymer with a long primary chain length and good binder adhesion.

[0040] Other nonionic ethylenically unsaturated monomers may include, for example, (meth)acrylic acid esters. Examples of (meth)acrylic acid esters include (meth)acrylic acid alkyl ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; aromatic (meth)acrylic acid ester compounds such as phenyl (meth)acrylate, phenylmethyl (meth)acrylate, and phenylethyl (meth)acrylate; and (meth)acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate. One of these compounds may be used alone, or two or more may be used in combination. From the viewpoints of adhesion to the active material and cycle characteristics, aromatic (meth)acrylic acid ester compounds are preferred. Furthermore, from the viewpoints of further improving lithium ion conductivity and high-rate characteristics, compounds having an ether bond, such as (meth)acrylic acid alkoxyalkyl ester compounds, are preferred, with 2-methoxyethyl (meth)acrylate being more preferred.

[0041] Among nonionic ethylenically unsaturated monomers, compounds having an acryloyl group are preferred because they have a fast polymerization rate, resulting in a polymer with a long primary chain length, and provide good binder adhesion.Furthermore, as nonionic ethylenically unsaturated monomers, compounds whose homopolymer glass transition temperature (Tg) is 0°C or less are preferred because the resulting electrode has good flex resistance.

[0042] The crosslinked polymer may be a salt. The type of salt is not particularly limited, but examples include alkali metal salts such as lithium, sodium, and potassium; alkaline earth metal salts such as calcium salts and barium salts; other metal salts such as magnesium salts and aluminum salts; ammonium salts, and organic amine salts. Among these, alkali metal salts and magnesium salts are preferred, and alkali metal salts are more preferred, as they are less likely to adversely affect battery characteristics.

[0043] <Embodiments of Crosslinked Polymer> The crosslinking method for the crosslinked polymer disclosed in this specification 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 having a crosslinked structure allows a binder containing the polymer or a salt thereof to have excellent adhesive 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.

[0044] <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.

[0045] 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)acrylate compounds, polyfunctional alkenyl compounds, and compounds having both (meth)acryloyl groups and alkenyl groups. These compounds may be used alone or in combination of two or more. Among these, polyfunctional alkenyl compounds are preferred because they are easy to obtain a uniform crosslinked structure, and polyfunctional allyl ether compounds having two or more allyl ether groups in the molecule are particularly preferred.

[0046] Examples of the polyfunctional (meth)acrylate compound 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 methylene bisacrylamide and hydroxyethylene bisacrylamide.

[0047] 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.

[0048] 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.

[0049] 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)acrylate, etc. These compounds can be used alone or in combination of two or more.

[0050] 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.

[0051] When the crosslinked polymer is crosslinked by a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.1 to 2.0 parts by mass, more preferably 0.3 to 1.5 parts by mass, and even more preferably 0.5 to 1.5 parts by mass, relative to 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers). An amount of the crosslinkable monomer used of 0.1 parts by mass or more is preferred in terms of improving adhesion and the stability of the electrode slurry. An amount of 2.0 parts by mass or less tends to increase the stability of the crosslinked polymer. Similarly, the amount of the crosslinkable monomer used is preferably 0.02 to 0.7 mol %, more preferably 0.03 to 0.4 mol %, based on the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers).

[0052] <Particle size of cross-linked polymer> In the composition for the 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, because the binder containing the crosslinked polymer can exhibit good adhesion performance.

[0053] The crosslinked polymer or salt thereof disclosed herein preferably has a particle size (water-swollen particle size) of 0.1 μm or more and 10.0 μm or less in volumetric median diameter when dispersed in water, the crosslinked polymer having a degree of neutralization based on the carboxyl groups of the crosslinked polymer. When the particle size is in the range of 0.1 μm or more and 10.0 μm or less, the particles are uniformly present in the composition for an electrode mixture layer at an appropriate size, thereby enabling the composition for an electrode mixture layer to exhibit high stability and excellent adhesion. If the particle size exceeds 10.0 μm, there is a risk of insufficient adhesion 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, 0.5 μm or more, 0.6 μm or more, 0.7 μm or more, 0.8 μm or more, or 1.0 μm or more. The upper limit of the particle size may be 9.0 μm or less, 8.0 μm or less, 7.0 μm or less, 5.0 μm or less, 3.0 μm or less, 2.0 μm or less, 1.8 μm or less, 1.6 μm or less, or 1.5 μm or less. The particle size range can be set by appropriately combining the above lower and upper limits, and may be, for example, 0.1 μm to 9.0 μm, 0.2 μm to 8.0 μm, 0.3 μm to 5.0 μm, 0.5 μm to 4.0 μm, 0.6 μm to 3.0 μm, or 0.6 μm to 2.0 μm. The water-swollen particle size can be measured by the method described in the Examples of this specification.

[0054] 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 agglomerates, in which primary particles associate and aggregate, when in the form of a powder or solution (dispersion). If the particle size upon dispersion in water is within the above range, the crosslinked polymer or salt thereof has extremely excellent dispersibility, and neutralization to a degree of neutralization of 80 to 100 mol% and dispersion in water breaks down the aggregated particles, 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 10.0 μm.

[0055] The particle size distribution, which is the volume average particle size of water-swollen particles divided by the number average particle size, is preferably 10 or less, more preferably 5.0 or less, even more preferably 3.0 or less, and even more preferably 1.5 or less, from the viewpoint of adhesion and coatability. The lower limit of the particle size distribution is usually 1.0.

[0056] The particle size (dry particle size) of the crosslinked polymer or salt thereof disclosed herein when dried is preferably in the range of 0.03 μm to 3 μm in volumetric median size, more preferably 0.1 μm to 1 μm, and even more preferably 0.3 μm to 0.8 μm.

[0057] 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.

[0058] <Molecular weight of cross-linked polymer (primary chain length)> The crosslinked polymers disclosed herein 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.

[0059] <Water swelling degree of crosslinked polymer> In this specification, the water swelling degree is calculated based on the following formula (3) from the dry mass "(WA) g" of the crosslinked polymer or a salt thereof and the amount of water "(WB) g" absorbed when the crosslinked polymer or a salt thereof is swelled to saturation with water. (Water swelling degree)={(WA)+(WB)} / (WA) (3)

[0060] Furthermore, by focusing on the water swelling degree of the crosslinked polymer and controlling the value, it is possible to simultaneously satisfy both coatability and adhesion. From this viewpoint, it is preferable that the water swelling degree of the crosslinked polymer at pH 8 is, for example, 25.0 or more and 40.0 or less. If the water swelling degree is less than 25.0, adhesion tends to decrease and cycle characteristics tend to deteriorate, while if the water swelling degree is more than 40.0, coatability deteriorates.

[0061] The lower limit of the water swelling degree at pH 8 is, for example, 26.0, for example, 27.0, for example, 28.0, for example, 29.0, or for example, 30.0. The upper limit of the water swelling degree is, for example, from the viewpoint of adhesion, for example, 39.0, 38.0, for example, 37.0, for example, 36.0, for example, 35.0, for example, 34.0, for example, 33.0, or for example, 32.0. The range of the water swelling degree can be arbitrarily selected from the above-mentioned lower and upper limits, for example, 25.0 or more and 35.0 or less, for example, 25.0 or more and 34.0 or less, for example, 25.0 or more and 33.0 or less, or for example, 25.0 or more and 32.0 or less.

[0062] The degree of swelling in water at pH 8 can be obtained by measuring the degree of swelling in water of a crosslinked polymer or a salt thereof in water of pH 8. As the water of pH 8, for example, ion-exchanged water can be used, and the pH value may be adjusted as necessary using an appropriate acid or alkali, or a buffer solution. The pH during measurement is, for example, in the range of 8.0±0.5, preferably in the range of 8.0±0.3, more preferably in the range of 8.0±0.2, and even more preferably in the range of 8.0±0.1. The measurement is performed at 25±5°C.

[0063] Furthermore, a person skilled in the art can adjust the water swelling degree by controlling the composition and structure of the crosslinked polymer or its salt based on the composition and water swelling degree in the examples of this specification as well as the common general technical knowledge at the time of filing. For example, the water swelling degree can be adjusted by adjusting the amount of acidic functional groups or highly hydrophilic structural units introduced into the crosslinked polymer, and generally, the water swelling degree can be improved by introducing such structural units. In addition, the water swelling degree can usually be increased by reducing the crosslinking degree of the crosslinked polymer. Furthermore, when producing a crosslinked polymer by precipitation polymerization or dispersion polymerization, the water swelling degree can also be adjusted by controlling the timing and method of addition of the monomer from which the second structural unit is derived.

[0064] <Method of producing a crosslinked polymer or a salt thereof> Crosslinked polymers can be produced using known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization. However, precipitation polymerization and suspension polymerization (reverse-phase suspension polymerization) are preferred from the standpoint of productivity. Heterogeneous polymerization methods such as precipitation polymerization, suspension polymerization, and emulsion polymerization are preferred because they provide better performance in terms of adhesion, among which 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 monomer 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 aggregated to several micrometers to several tens of micrometers. A dispersion stabilizer can also be used to control the polymer particle size. Furthermore, the secondary aggregation can be suppressed by selecting the dispersion stabilizer, polymerization solvent, etc. Precipitation polymerization in which secondary aggregation is suppressed is generally also called dispersion polymerization.

[0065] 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.

[0066] Precipitation polymerization or dispersion polymerization is a polymerization method in which polymer chains precipitated from the medium as the polymerization proceeds are stacked on the surface of primary particles. Therefore, for example, in the polymerization reaction, it is possible to control the polymer composition of the particles by adding or feeding constituent monomers during the polymerization reaction, and as a result, it is possible to control the water swelling degree.

[0067] More specifically, in order to control the composition of polymerized particles in precipitation polymerization or dispersion polymerization, the water swelling degree can be easily controlled by carrying out either or both of the following polymerization steps: (A) a polymerization step in which only an ethylenically unsaturated carboxylic acid monomer is polymerized; or (B) a polymerization step in which the monomers represented by the formulas (1) and (2) are continuously or intermittently added to the ethylenically unsaturated carboxylic acid monomer to polymerize it.

[0068] The polymerization step (A) may be exemplified by a polymerization step in which, at the initial stage of polymerization, an ethylenically unsaturated carboxylic acid monomer from which the first structural unit is derived is introduced, but the monomers represented by formulas (1) and (2) from which the second structural unit is derived are not introduced, and only the ethylenically unsaturated carboxylic acid monomer is polymerized first. This polymerization step results in particles having a polymer rich in the first structural unit derived from the ethylenically unsaturated carboxylic acid monomer inside, and a polymer rich in the second structural unit derived from the monomers represented by formulas (1) and / or (2) stacked on the particle surface, resulting in a high degree of water swelling. In this polymerization step, for example, the entire amount of the ethylenically unsaturated monomer may be introduced initially, and then the monomers represented by formulas (1) and / or (2) may be introduced all at once or in portions, or continuously or intermittently over a certain period of time (the latter corresponds to the polymerization step (B) above).

[0069] Supplying all at once means supplying the entire predetermined amount of the monomer represented by formula (1) and / or (2) at one time, and supplying in portions means supplying the entire predetermined amount in two or more portions at intervals of, for example, 10 minutes to 1 hour.

[0070] The polymerization step (B) may be a polymerization step in which a predetermined amount of the monomer represented by formula (1) and / or (2) is continuously or intermittently supplied in the presence of, for example, 10% by mass or more, for example, 30% by mass or more, for example, 50% by mass or more, for example, 70% by mass or more, or for example, 80% by mass or more, or for example, the entire amount of the ethylenically unsaturated carboxylic acid monomer. Supplying continuously means supplying without interruption at intervals of less than one minute, preferably while maintaining a constant supply amount, and supplying intermittently means supplying intermittently at intervals of one to five minutes, preferably while maintaining a constant supply amount.

[0071] The ethylenically unsaturated carboxylic acid monomer does not only exist as a monomer, but also exists as a partially polymerized monomer. In this way, at least in the initial stage of supplying the monomers represented by formula (1) and / or (2), the monomers represented by formula (1) and / or (2) are polymerized in the presence of a large excess of the ethylenically unsaturated carboxylic acid monomer. Through this polymerization step, the particles have a polymer containing a large amount of the first structural unit derived from the ethylenically unsaturated carboxylic acid monomer inside, and a polymer containing a large amount of the second structural unit derived from the monomers represented by formula (1) and / or (2) is layered on the particle surface, resulting in an increased degree of swelling in water.

[0072] The polymerization step (B) may be carried out for a certain period of time from the beginning of the polymerization reaction, or may be carried out for a certain period of time after the polymerization step (A). Note that the time for the polymerization step (B) can be arbitrarily set by a person skilled in the art based on the examples in this specification as well as the common general technical knowledge at the time of filing the present application.

[0073] In view of the above, the production method disclosed in this specification may take the following forms, for example. The polymerization step (A) is carried out from the initial stage of the polymerization reaction, and then the monomers represented by formula (1) and / or (2) are supplied all at once or in portions. The polymerization step (A) is carried out from the beginning of the polymerization reaction, and then the polymerization step (B) is carried out once. The polymerization step (B) is carried out from the beginning of the polymerization reaction.

[0074] When multiple types of monomers represented by formula (1) and / or (2) are used, the multiple monomers may be supplied all at once continuously or intermittently, or one or more types of monomers may be supplied continuously or intermittently in any order.

[0075] Specific polymerization solvents that can be used in precipitation polymerization and dispersion polymerization 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 this specification, the term "water-soluble solvent" refers to a solvent having a solubility in water of greater than 10 g / 100 ml at 20°C. Among these, methyl ethyl ketone and acetonitrile are preferred because they provide good polymerization stability with minimal generation of coarse particles and adhesion to the reactor, are less likely to cause secondary aggregation of the precipitated polymer particles (or, even if secondary aggregation does occur, are easily disintegrated in an aqueous medium), produce polymers with a small chain transfer constant and a large degree of polymerization (primary chain length), and are easy to handle during the neutralization process described below.

[0076] 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.

[0077] The production of a crosslinked polymer or a salt thereof preferably includes a polymerization step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer. For example, the method preferably includes a polymerization step of polymerizing a monomer component containing 50% to 99.5% by mass of the ethylenically unsaturated carboxylic acid monomer from which component (a) is derived, 0.5% to 50% by mass of a specific monomer having a hydroxyl group from which component (b) is derived, and 0% to 49.5% by mass of another ethylenically unsaturated monomer from which component (c) is derived. Examples of the specific monomer having a hydroxyl group include one or more monomers selected from the group consisting of monomers represented by the following formulas (1) and (2), and a monomer having a formula weight of 200 or less and containing a (meth)acryloyl group and a hydroxyl group. CH2=C(R 1 )COOR 2 (1) [In the formula, R 1 represents a hydrogen atom or a methyl group, and R 2 is a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, (R 3 O) m H or R 4 O[CO(CH2)5O] n H. Note that R 3represents an alkylene group having 2 to 4 carbon atoms, and R 4 represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15.]

[0078] CH2=C(R 5 )CONR 6 R 7 (2) [In the formula, R 5 represents a hydrogen atom or a methyl group, and R 6 represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, and R 7 represents a hydrogen atom or a monovalent organic group.

[0079] Through the polymerization step, the crosslinked polymer contains 50% to 99.5% by mass of structural units (component (a)) derived from an ethylenically unsaturated carboxylic acid monomer, and 0.5% to 50% by mass of structural units (component (b)) derived from a specific monomer having a hydroxyl group. The amount of the ethylenically unsaturated carboxylic acid monomer used is, for example, 30% to 99.5% by mass, for example, 50% to 99.5% by mass, for example, 50% to 99% by mass, for example, 50% to 98% by mass, or for example, 50% to 95% by mass. It can also be, for example, 50% to 90% by mass, for example, 55% to 90% by mass, or for example, 60% to 90% by mass. The amount of the specific monomer having a hydroxyl group used is, for example, 1.0% by mass or more and 50% by mass or less, for example, 1.0% by mass or more and 45% by mass or less, for example, 1.0% by mass or more and 40% by mass or less, for example, 5.0% by mass or more and 40% by mass or less, for example, 10% by mass or more and 50% by mass or less, for example, 10% by mass or more and 45% by mass or less, or for example, 10% by mass or more and 40% by mass or less.

[0080] Examples of the other ethylenically unsaturated monomer 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 other than component (b). Specific examples of such compounds include the monomer compounds into which component (c) can be introduced. The other ethylenically unsaturated monomer may be contained in an amount of 0% by mass or more and 49.5% by mass or less, 1% by mass or more and 40% by mass or less, 2% by mass or more and 40% by mass or less, 2% by mass or more and 30% by mass or less, or 5% by mass or more and 30% by mass or less, based on the total amount of the monomer components.

[0081] 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 preferably 0.1 to 2.0 parts by mass, more preferably 0.3 to 1.5 parts by mass, and even more preferably 0.5 to 1.5 parts by mass, relative to 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomers).

[0082] 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. For this reason, 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% by mass, and preferably in the range of 5 to 40% by mass. In this specification, the term "monomer concentration" refers to the concentration of the total mass of the monomers used in polymerization relative to the mass of the entire reaction solution.

[0083] The crosslinked polymer may be produced by carrying out a polymerization reaction in the presence of a basic compound. By carrying out the polymerization reaction in the presence of a basic compound, the polymerization reaction can be 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 that can be produced.

[0084] 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 of the polymerization reaction can be removed, the upper limit is approximately 40% by mass in precipitation polymerization, approximately 50% by mass in suspension polymerization, and approximately 70% by mass in emulsion polymerization, as described above.

[0085] 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 high monomer concentration conditions, 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 adhesion. Examples of inorganic base compounds include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide, and alkaline earth metal hydroxides such as calcium hydroxide and magnesium hydroxide, and one or more of these can be used. Examples of the organic base compound include ammonia and organic amine compounds, and one or more of these can be used. Among them, organic amine compounds are preferred from the viewpoints of polymerization stability and adhesion of the resulting binder containing the crosslinked polymer or a salt thereof.

[0086] 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.

[0087] 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 refers to the molar concentration of the basic compound used relative to the ethylenically unsaturated carboxylic acid monomer, and does not refer to the degree of neutralization. In other words, the valence of the basic compound used is not taken into consideration.

[0088] 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.

[0089] 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.

[0090] 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 (manufactured by NOF Corporation, trade name "Percumyl H"), cumene hydroperoxide (manufactured by NOF Corporation, trade name "Percumyl H"), 1,1,3,3-tetramethylbutyl hydroperoxide (manufactured by NOF Corporation, trade name "Perocta H"), t-butylcumyl peroxide (manufactured by NOF Corporation, trade name "Perbutyl C"), di-t-butyl peroxide (manufactured by NOF Corporation, trade name "Perbutyl D"), di-t-hexyl peroxide (manufactured by NOF Corporation, trade name "Perhexyl D"), di(3,5,5-trimethylhexanoyl) peroxide (manufactured by NOF Corporation, trade name "Percumyl H"). 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 (Percolumn ND), 1,1,3,3-tetramethylbutyl peroxyneodecanoate (Percolumn 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.

[0091] Examples of the inorganic peroxides include potassium persulfate, sodium persulfate, ammonium persulfate, etc. In the case of redox initiation, sodium sulfite, sodium thiosulfate, sodium formaldehyde sulfoxylate, ascorbic acid, sulfurous acid gas (SO), ferrous sulfate, etc. can be used as reducing agents.

[0092] 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.

[0093] 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. The polymerization temperature may be constant or may vary over the course of the polymerization reaction. The polymerization time is preferably 1 minute to 20 hours, more preferably 1 hour to 10 hours.

[0094] The crosslinked 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 crosslinked 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 crosslinked polymer easily disintegrates during use, even if it undergoes secondary aggregation. Furthermore, the removal of the remaining unreacted monomers results in good performance in terms of adhesion and battery characteristics.

[0095] In this production method, a polymerization reaction of a monomer composition containing an ethylenically unsaturated carboxylic acid monomer 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 "process neutralization"), and then the solvent may be removed in a drying step. Alternatively, a crosslinked polymer powder may be obtained without the process neutralization, and then an alkali compound may be added when preparing an electrode slurry to neutralize the polymer (hereinafter also referred to as "post-neutralization"). Of the above, process neutralization is preferred because it tends to break up secondary aggregates more easily.

[0096] <Composition for secondary battery electrode mixture layer> The composition for a secondary battery electrode mixture layer disclosed herein includes a binder containing the crosslinked polymer or its salt, an active material, and water. The amount of the crosslinked polymer or its salt used in the composition for an electrode mixture layer disclosed herein is, for example, 0.1 to 20 parts by mass per 100 parts by mass of the total amount of active material. The amount used may also be, for example, 0.2 to 10 parts by mass, such as 0.3 to 8 parts by mass, or 0.4 to 5 parts by mass. If the amount of the crosslinked polymer and its salt used is less than 0.1 parts by mass, sufficient adhesion may not be achieved. Furthermore, the dispersion stability of the active material may be insufficient, resulting in reduced uniformity of the mixture layer formed. On the other hand, if the amount of the crosslinked polymer and its salt used exceeds 20 parts by mass, the viscosity of the composition for an electrode mixture layer may increase, reducing its coatability on the current collector. As a result, bumps and irregularities may occur in the resulting mixture layer, adversely affecting electrode characteristics.

[0097] When the amount of the crosslinked polymer and its salt is within the above range, a composition having excellent dispersion stability can be obtained, and a mixture layer having extremely high adhesion to the current collector can be obtained, resulting in improved battery durability. Furthermore, the crosslinked polymer and its salt exhibit sufficiently high adhesion to the active material even in a small amount (for example, 5% by mass or less), and because they contain carboxy anions, an electrode having low interfacial resistance and excellent high-rate characteristics can be obtained.

[0098] Among the above active materials, lithium salts of transition metal oxides can be used as the positive electrode active material. For example, layered rock salt type and spinel type lithium-containing metal oxides can be used. Specific compounds of the layered rock salt type positive electrode active material include lithium cobalt oxide, lithium nickel oxide, and ternary NCM{Li(Ni x ,Co y ,Mn z ), x+y+z=1} and NCA{Li(Ni 1-a-b Co a Al b)}, etc. Examples of spinel-type positive electrode active materials include lithium manganate, etc. In addition to oxides, phosphates, silicates, sulfur, etc. are also used, and examples of phosphates include olivine-type lithium iron phosphate, etc. 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.

[0099] When a positive electrode active material containing a layered rock salt-type lithium-containing metal oxide is dispersed in water, the lithium ions on the active material surface are exchanged with hydrogen ions in the water, resulting in an alkaline dispersion. This can potentially corrode aluminum foil (Al), a common positive electrode current collector material. In such cases, it is preferable to neutralize the alkali elution from the active material by using an unneutralized or partially neutralized crosslinked polymer as a binder. Furthermore, it is preferable to use an amount of unneutralized or partially neutralized crosslinked polymer such that the amount of unneutralized carboxyl groups in the crosslinked polymer is equivalent to or greater than the amount of alkali eluted from the active material.

[0100] Because all positive electrode active materials have low electrical conductivity, they are generally used with the addition of a conductive additive. Examples of conductive additives include carbon-based materials such as carbon black, carbon nanotubes, carbon fiber, graphite powder, and carbon fiber. Of these, carbon black, carbon nanotubes, and carbon fiber are preferred because they are more likely to provide excellent conductivity. Furthermore, ketjen black and acetylene black are preferred as carbon black. The conductive additives may be used alone or in combination of two or more. The amount of conductive additive 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.

[0101] On the other hand, examples of negative electrode active materials include carbon-based materials, lithium metal, lithium alloys, and metal oxides, and these can be used alone or in combination. Among these, active materials made of carbon-based materials such as natural graphite, artificial graphite, hard carbon, and soft carbon (hereinafter also referred to as "carbon-based active materials") are preferred, with graphite such as natural graphite and artificial graphite, and hard carbon being more preferred. In the case of graphite, spherical graphite is preferably used from the viewpoint of battery performance, and its particle size preferably ranges from 1 to 20 μm, for example, and from 5 to 15 μm. In addition, to increase the energy density, metals or metal oxides capable of absorbing lithium, such as silicon and tin, can also be used as negative electrode active materials. Among these, silicon has a higher capacity than graphite, and active materials made of silicon-based materials such as silicon, silicon alloys, and silicon oxides such as silicon monoxide (SiO) (hereinafter also referred to as "silicon-based active materials") can be used. However, while the silicon-based active material has a high capacity, it undergoes a large volume change during charging and discharging. For this reason, it is preferable to use it in combination with the carbon-based active material. In this case, the amount of silicon active material used is preferably 2 to 80 mass% based on the total amount of the carbon-based active material and the silicon-based active material. The amount of silicon-based active material used may be 5 to 70 mass%, 8 to 60 mass%, or 10 to 50 mass%.

[0102] The binder containing the crosslinked polymer disclosed herein has a structural unit (component (a)) derived from an ethylenically unsaturated carboxylic acid monomer. Component (a) has a high affinity for silicon-based active materials and exhibits good adhesion. Therefore, the binder disclosed herein exhibits excellent adhesion even when used with a high-capacity active material containing a silicon-based active material, and is therefore believed to be effective in improving the durability of the resulting electrode.

[0103] Furthermore, the crosslinked polymer disclosed herein has a structural unit (component (b)) derived from a specific monomer having a hydroxyl group. When the crosslinked polymer contains component (b), the coatability of the electrode mixture layer composition can be improved. While the reason for this effect is unclear, it is believed that the crosslinked polymer has relatively flexible hydroxyl groups on its side chains, which interact with carboxyl groups in the polymer, thereby suppressing swelling of the crosslinked polymer in water. However, this speculation does not limit the scope of the disclosure of this specification.

[0104] Since the carbon-based active material itself has good electrical conductivity, it is not necessary to add a conductive additive. When a conductive additive is added for the purpose of further reducing resistance, the amount used is, for example, 10% by mass or less, or, for example, 5% by mass or less, based on the total amount of the active material, from the viewpoint of energy density.

[0105] When the composition for a 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.

[0106] The composition for a secondary battery electrode mixture layer uses water as a medium. Furthermore, in order 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-methylpyrrolidone. The proportion of water in the mixed medium is, for example, 50% by mass or more, or, for example, 70% by mass or more.

[0107] When the composition for an electrode mixture layer is made into a coatable slurry state, the content of the water-containing medium in the entire composition can be, for example, in the range of 25 to 90 mass %, or can be, for example, 35 to 70 mass %, from the viewpoints of the coatability of the slurry, the energy cost required for drying, and productivity.

[0108] The binder disclosed herein may consist solely of the crosslinked polymer or its salt, or may be used in combination with other binder components such as styrene / butadiene latex (SBR), acrylic latex, and polyvinylidene fluoride latex. When other binder components are used in combination, the amount used may be, for example, 0.1 to 5% by mass or less, or, for example, 0.1 to 2% by mass or less, or, for example, 0.1 to 1% by mass or less, relative to the active material. If the amount of other binder components used exceeds 5% by mass, the resistance increases, and high-rate characteristics may become insufficient. Among the above, styrene / butadiene latex is preferred due to its excellent balance of adhesion and flex resistance.

[0109] 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 adhesion.

[0110] 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 adhesion of the binder and the flexibility of the resulting electrode.

[0111] In addition to the above-mentioned monomers, the styrene / butadiene latex may contain other monomers as copolymerization monomers, such as nitrile group-containing monomers such as (meth)acrylonitrile, and carboxyl group-containing monomers such as (meth)acrylic acid, itanconic acid, and maleic acid, in order to further improve performance such as adhesion. The structural units derived from the other monomers in the copolymer may be in the range of 0 to 30% by mass, for example, or in the range of 0 to 20% by mass.

[0112] The composition for a secondary battery electrode mixture layer disclosed herein contains the above-described active material, water, and binder as essential components, and is 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 crosslinked polymer particles, followed by mixing with a dispersion medium such as water and dispersing and kneading. When obtaining the composition for an electrode mixture layer in a slurry state, it is preferable to prepare a slurry that is free of poor dispersion and aggregation. Known mixers such as planetary mixers, thin film gyratory mixers, and planetary / revolving mixers can be used as mixing means. However, a thin film gyratory mixer is preferred because it allows for a good dispersion state to be obtained 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.

[0113] On the other hand, when the electrode mixture layer composition 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.

[0114] <Electrode for secondary batteries> The secondary battery electrode disclosed herein comprises a mixture layer formed from the electrode mixture layer composition on the surface of a current collector made of copper, aluminum, or the like. The mixture layer is formed by applying the electrode mixture layer composition disclosed herein 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 coating, 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. The mixture layer obtained after drying is usually subjected to a compression treatment using a mold press or roll press. Compression brings the active material and binder into close contact, improving the strength of the mixture layer and its adhesion to the current collector. The thickness of the mixture layer can be adjusted by compression to, for example, approximately 30 to 80% of the thickness before compression, and the thickness of the mixture layer after compression is generally approximately 4 to 200 μm.

[0115] A secondary battery can be fabricated by providing the secondary battery electrode disclosed herein with a separator and an electrolyte solution using an organic solvent. The electrolyte solution may be liquid or gel-like. The separator is disposed between the positive and negative electrodes of the battery and serves to prevent short circuits due to contact between the electrodes and to retain the electrolyte solution to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane that has good ion permeability and mechanical strength. Specific materials that can be used include polyolefins such as polyethylene and polypropylene, and polytetrafluoroethylene.

[0116] The electrolyte may be a commonly used known solvent depending on the type of active material. For lithium-ion secondary batteries, specific solvents include cyclic carbonates with high dielectric constants and high electrolyte dissolving capabilities, such as propylene carbonate and ethylene carbonate, and chain carbonates with low viscosity, such as ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate. These solvents can be used alone or as a mixed solvent. The electrolyte is prepared by dissolving lithium salts, such as LiPF6, LiSbF6, LiBF4, LiClO4, and LiAlO4, in these solvents. For nickel-metal hydride secondary batteries, an aqueous potassium hydroxide solution can be used. A secondary battery can be obtained by spirally or stacking positive and negative electrode plates separated by a separator and encasing them in a case.

[0117] As described above, the binder for secondary battery electrodes disclosed herein exhibits excellent adhesion to electrode materials in the mixture layer. Therefore, 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 / discharge cycles, making them suitable for automotive secondary batteries, etc. The binder is also useful for the use of active materials containing silicon, which is expected to contribute to increasing the capacity of batteries. Furthermore, the binder can improve the coatability of the electrode mixture layer composition (electrode slurry) even under conditions of high active material concentration. This is advantageous in terms of reducing the drying energy required during the formation of the mixture layer and improving productivity. Therefore, the binder for secondary battery electrodes disclosed herein is particularly suitable for use in non-aqueous electrolyte secondary battery electrodes, and is particularly useful for non-aqueous electrolyte lithium-ion secondary batteries with high energy density. [Example]

[0118] The disclosure of this specification will be specifically explained below based on examples. However, the disclosure of this specification 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 crosslinked polymer (salt) was evaluated by the following method.

[0119] (1) Measurement of particle size in aqueous medium (water-swollen particle size) 0.25 g of the cross-linked 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). The mixture was then stirred (rotation speed 2000 rpm / revolution speed 800 rpm, 7 minutes) and 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.

[0120] 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).

[0121] (2) Water swelling at pH 8 The degree of swelling in water at pH 8 was measured by the following method. The measuring device is shown in Figure 1. <1> ~ <3> It consists of: <1> It consists of a burette 1 with a branch pipe for venting air, a pinch cock 2, a silicone tube 3, and a polytetrafluoroethylene tube 4. <2> A support cylinder 8 with many holes on the bottom is placed on top of the funnel 5, and filter paper 10 for the device is placed on top of that. <3> A sample 6 (measurement sample) of a crosslinked polymer or its salt is sandwiched between two sheets of sample-fixing filter paper 7, which is fixed with adhesive tape 9. All of the filter papers used are ADVANTEC No. 2, with an inner diameter of 55 mm. <1> and <2> are connected by a silicone tube 3. The funnel 5 and the support cylinder 8 are fixed in height relative to the burette 1, and are set so that the bottom end of the polytetrafluoroethylene tube 4 installed inside the burette branch pipe is at the same height as the bottom surface of the support cylinder 8 (dotted line in Figure 1).

[0122] The measurement method is explained below. The following operations were carried out at 25°C unless otherwise specified. <1> The pinch cock 2 at the top is removed, and ion-exchanged water adjusted to pH 8.0 is poured into the burette 1 through the silicone tube 3 from the top of the burette, filling the burette 1 to the apparatus filter paper 10 with ion-exchanged water 12. Next, the pinch cock 2 is closed, and air is removed from the polytetrafluoroethylene tube 4 connected to the burette branch pipe with a rubber stopper. In this way, ion-exchanged water 12 is continuously supplied from the burette 1 to the apparatus filter paper 10. Next, after removing the excess ion-exchanged water 12 that has seeped out from the filter paper 10 for the device, the reading (a) on the scale of the burette 1 is recorded. 0.1 to 0.2 g of the dry powder of the measurement sample is weighed out. <3> As shown in the figure, place the sample evenly in the center of the filter paper 7 for fixing the sample. Then, sandwich the sample with another filter paper, and fasten the two filter papers with adhesive tape 9 to fix the sample. <2> The lid 11 is placed on the filter paper 10 for the apparatus shown in FIG. 2. Next, the reading (b) on the scale of the burette 1 is recorded 30 minutes after the lid 11 is placed on the filter paper 10 for the apparatus. The sum (c) of the water absorption of the measurement sample and the water absorption of the two filter papers 7 for fixing the sample is calculated from (ab). Using the same procedure, the water absorption of only the two filter papers 7, which do not contain the crosslinked polymer or its salt sample, is measured (d).

[0123] After carrying out the above procedure, the degree of swelling in water was calculated using the following formula: The solid content used in the calculation was a value measured by the method described below. Water swelling index = {dry mass of measurement sample (g) + (cd)} / {dry mass of measurement sample (g)} However, dry mass of the measurement sample (g) = mass of the measurement sample (g) × (solid content % ÷ 100)

[0124] The method for measuring the solid content is as follows: Approximately 0.5 g of sample was placed in a weighing bottle whose weight had been measured in advance [weight of weighing bottle = B(g)], and the weighing bottle was accurately weighed [W0(g)]. The sample and the weighing bottle were then placed in a windless dryer and dried at 155°C for 45 minutes, at which point the weight of the weighing bottle was measured [W1(g)], and the solid content (%) was calculated using the following formula: Solid content (%)=(W1-B) / (W0-B)×100

[0125] <Production of Crosslinked Polymer Salt> (Production Example 1: Production of Crosslinked Polymer Salt R-1) A reactor equipped with a stirring blade, thermometer, reflux condenser, and nitrogen inlet tube was used for the polymerization. The reactor was charged with 567 parts of acetonitrile, 2.20 parts of ion-exchanged water, 80.0 parts of acrylic acid (AA), 0.5 parts of trimethylolpropane diallyl ether (manufactured by Daiso Co., Ltd., trade name "Neoallyl T-20"), and 1.0 mol% triethylamine based on the AA. After thoroughly replacing the atmosphere with nitrogen, the reactor was heated to an internal temperature of 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 Wako Pure Chemical Industries, Ltd., trade name "V-65") was added as a polymerization initiator. The reaction mixture became cloudy, and this point was designated as the polymerization initiation point. Two hours after the initiation of polymerization, 20.0 parts of 2-hydroxyethyl acrylate (HEA) was added in one portion. The monomer concentration was calculated to be 15.0%. Twelve hours after the initiation 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 referred to as "LiOH·HO") powder was added. Stirring was continued at room temperature for 12 hours after the addition, 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 rate of AA was 72.4% two hours after the initiation of polymerization, and the conversion rates of AA and HEA were calculated to be 97.6% and 98.0%, respectively, 12 hours after the initiation of polymerization. Because HEA was added during the polymerization, it is believed that particles with HEA-rich polymer chains localized near the particle surface were obtained.

[0126] 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 mass of acetonitrile to the polymerization reaction solution, 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 calculated from the starting material. The water-swollen particle size was 1.4 μm, and the water swelling index was 38.9.

[0127] (Production Examples 2 to 12 and Comparative Production Examples 1 and 2: Production of Crosslinked Polymer Salts R-2 to R-14) Polymerization reaction solutions containing crosslinked polymer salts R-2 to R-14 were obtained by the same procedure as in Production Example 1, except that the amounts of monomer, crosslinkable monomer, and neutralizing agent charged, and the timing and method of adding the monomers during the course of the polymerization were as shown in Table 1. The "sequential addition" of crosslinked polymer salts R-4 and R-6 involved the continuous dropwise addition of the entire amount of monomer added during the specified time period shown in Table 1 to the polymerization reaction solution at a constant rate. In all polymerization reaction solutions, the reaction rates of AA and HEA were 95% or higher 12 hours after the start of polymerization. The water-swelling particle sizes and water swelling degrees of R-2 to R-14 are shown in Table 1.

[0128] [Table 1]

[0129] AA: acrylic acid HEA: 2-hydroxyethyl acrylate HBA: 4-hydroxybutyl acrylate HEAAm: Hydroxyethylacrylamide T-20: Trimethylolpropane diallyl ether (manufactured by Daiso Co., Ltd., trade name "Neoallyl T-20") P-30: Pentaerythritol triallyl ether (manufactured by Daiso Co., Ltd., trade name "Neoallyl P-30") TEA: Triethylamine AcN: Acetonitrile MeOH: Methanol V-65: 2,2'-Azobis(2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd.) LiOH·H2O: Lithium hydroxide monohydrate K2CO3: Potassium carbonate

[0130] <Example> (Example 1) An electrode using the crosslinked polymer salt R-1 was fabricated and evaluated. The specific procedures and evaluation methods are shown below.

[0131] <Evaluation of the coating property of the electrode slurry> (Fabrication of the negative electrode plate) SiO x (0.8 < x < 1.2) with 10% carbon coated on its surface by CVD method was prepared, and a mixture of this and graphite with a mass ratio of 5:95 was used as the active material. Also, as the binder, a mixture of the crosslinked polymer salt R-1, styrene / butadiene-based latex (SBR), and carboxymethyl cellulose (CMC) was used. Using water as the diluent solvent, Primix Corporation's T.K. High Bismix was used to mix at a mass ratio of active material:R-1:SBR:CMC = 97.0:1.0:1.0:1.0 (solid content) to prepare a negative electrode mixture slurry with a solid content of 50%. The above negative electrode mixture slurry was coated on both sides of a copper foil and dried to form an electrode mixture layer. Then, it was rolled so that the thickness of the electrode mixture layer per side was 80 μm and the packing density was 1.60 g / cm 3 as required.

[0132] (Coating property evaluation method) Regarding the negative electrode mixture slurry obtained above, after coating on a copper foil and drying, the appearance of the electrode mixture layer was visually observed to evaluate the coating property. The evaluation criteria were as follows.

[0133] 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.

[0134] <Evaluation of 100-cycle capacity retention rate (cycle characteristics)> Next, a battery including the above-mentioned negative electrode plate using the crosslinked polymer salt R-1 was fabricated, and the 100-cycle capacity retention rate was evaluated. The specific procedure and evaluation method are described below.

[0135] (Preparation of positive electrode plate) In N-methylpyrrolidone (NMP) solvent, LiNi as the positive electrode active material 0.8 Co 0.15 Al 0.05 O2, acetylene black (a carbon conductive agent), and polyvinylidene fluoride (PVDF) with an average molecular weight of 1.1 million were mixed in a mass ratio of 95:2.5:2.5 using a mixer to prepare a positive electrode mixture slurry with a solid content of 50%. The prepared slurry was applied to both sides of aluminum foil, and after drying, the mixture layer thickness per side was 95 μm and the packing density was 3.60 g / cm. 3 It was rolled to become.

[0136] (Preparation of Electrolyte) A non-aqueous electrolyte was prepared by adding 5 parts by mass of vinylene carbonate (VC) to a mixed solvent consisting of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio of EC:DMC=1:3) and dissolving 1 mol / L of LiPF6.

[0137] (Battery construction) The battery is constructed by attaching lead terminals to the positive and negative electrodes, and then wrapping them in a spiral with a separator (made of polyethylene: film thickness 16 μm, porosity 47%) between them. This is then pressed and flattened into an electrode assembly, which is then placed in an aluminum laminate battery exterior, after which the electrolyte is poured in and the battery is sealed to form a test battery. The design capacity of this prototype battery is 800 mAh. The design capacity of the battery was based on a charge cut-off voltage of 4.2 V.

[0138] (Evaluation of cycle characteristics) The battery obtained above was subjected to the following charge / discharge test at 25° C. for 100 cycles, and the capacity retention rate was evaluated.

[0139] (1) Charging test The battery was charged at a constant current of 0.3 C (240 mA) up to 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 1 / 20 C (40 mA). (2) Discharge test A constant current discharge was performed at a current of 0.5 C (400 mA) down to 2.75 V. (3) Suspension The interval between the charge and discharge tests was 10 minutes.

[0140] (1)Charging conditions The single-electrode battery was subjected to constant current charging at 25° C. at 0.01 C (0.8 mA) until the VvsLi reached 0.05.

[0141] (Examples 2 to 12, Comparative Examples 1 and 2) A negative electrode plate and a battery were obtained in the same manner as in Example 1, except that the crosslinked polymer salt was changed as shown in Table 1. The obtained negative electrode plate and battery were evaluated in the same manner as in Example 1, and the results are shown in Table 2.

[0142] [Table 2]

[0143] As shown in Table 1, a comparison between Examples 1 to 12 and Comparative Example 1 indicates that adding all of the monomer components at once at the beginning of the polymerization reaction results in a low degree of water swelling, whereas adding the monomers represented by formula (1) and / or (2) (HEA, HBA, HEAAm in the table) all at once during the polymerization reaction or continuously from the beginning can increase the degree of water swelling. Furthermore, a comparison between Examples 1 to 12 and Comparative Example 2 indicates that the degree of water swelling can be easily controlled by the type and amount of the monomers represented by formula (1) and / or (2), the type and amount of the crosslinkable monomer, the selection of the polymerization solvent, the monomer concentration, etc.

[0144] Furthermore, as shown in Tables 1 and 2, it was found that in Examples 1 to 12, which used binders with a water swelling degree of 25.2 to 38.9 at pH 8, both coatability (A to B) and cycle characteristics (80 to 90%) could be achieved. Combining the results of Comparative Examples 1 and 2, it can be seen that a water swelling degree of 25.0 to 40.0 can achieve both coatability and cycle characteristics.

[0145] It was also found that good coatability (A) could be exhibited in most of the Examples (Examples 2 to 4 (Production Examples 2 to 4), Examples 6 to 10 (Production Examples 6 to 10)) in which the water swelling degree was in the range of 25.0 to 32.0. In terms of cycle characteristics, it was also found that good characteristics (83 to 90%) were exhibited in most of the Examples (Examples 1 to 4 (Production Examples 1 to 4), Example 6 (Production Example 6), and Examples 8 to 9 and 11 (Production Examples 8 to 9 and 11)) in which the water swelling degree was in the range of 25.0 to 32.0.

[0146] In addition, Example 6 showed the best results by using a binder (Production Example 6) produced by adding 40 parts of HEA.

[0147] From the above, it was found that crosslinked polymers having various degrees of water swelling can be produced, and that by controlling the degree of water swelling within a certain range, both coatability and cycle characteristics can be achieved. [Industrial Applicability]

[0148] The binder for secondary battery electrodes disclosed herein exhibits excellent adhesion in the mixture layer. Therefore, secondary batteries equipped with electrodes obtained using the binder are expected to exhibit good durability (cycling characteristics) and are expected to be applied to automotive secondary batteries. It is also useful for the use of active materials containing silicon, and is expected to contribute to increasing the capacity of batteries. Furthermore, it can improve the coatability of the electrode mixture layer composition (electrode slurry) even under conditions of high active material concentration. This is advantageous in terms of reducing the drying energy required during mixture layer formation and improving productivity. The binder for secondary battery electrodes disclosed herein can be particularly suitably used for non-aqueous electrolyte secondary battery electrodes, and is particularly useful for non-aqueous electrolyte lithium-ion secondary batteries with high energy density.

Claims

1. A binder for a non-aqueous electrolyte secondary battery electrode containing a crosslinked polymer or a salt thereof, The crosslinked polymer or salt thereof contains, relative to all structural units thereof, 50% by mass or more and 99.5% by mass or less of first structural units derived from an ethylenically unsaturated carboxylic acid monomer, and 0.5% by mass or more and 50% by mass or less of second structural units derived from one or more monomers selected from the group consisting of monomers represented by formula (1) and formula (2), and The crosslinked polymer or salt thereof has a degree of swelling in water at pH 8 of 25.0 or more and 40.0 or less. CH ２ =C(R １ )COOR ２ (1) [In the formula, R １ represents a hydrogen atom or a methyl group, R ２ represents a monovalent organic group having 1 to 8 carbon atoms and a hydroxyl group, (R ３ O) ｍ H or R ４ O[CO(CH ２ ) ５ O] ｎ H. Note that R ３ represents an alkylene group having 2 to 4 carbon atoms, R ４ represents an alkylene group having 1 to 8 carbon atoms, m represents an integer of 2 to 15, and n represents an integer of 1 to 15. CH ２ =C(R ５ )CONR ６ R ７ (2) [In the formula, R ５ represents a hydrogen atom or a methyl group, R ６ represents a hydroxyl group or a hydroxyalkyl group having 1 to 8 carbon atoms, R ７ represents a hydrogen atom or a monovalent organic group.

2. 2. The binder for a non-aqueous electrolyte secondary battery electrode according to claim 1, wherein the second structural unit is a structural unit derived from a hydroxyalkyl (meth)acrylate.

3. 3. The binder for a non-aqueous electrolyte secondary battery electrode according to claim 1, wherein the salt of the crosslinked polymer includes a lithium salt.

4. 4. The binder for a non-aqueous electrolyte secondary battery electrode according to claim 1, 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 10 μm or less.

5. 5. The binder for a non-aqueous electrolyte secondary battery electrode according to claim 1, wherein the crosslinked polymer is crosslinked with a crosslinkable monomer.

6. 6. The binder for a non-aqueous electrolyte secondary battery electrode according to claim 5, wherein the amount of the crosslinkable monomer used is 0.1 mol % or more and 1.0 mol % or less based on the total amount of the non-crosslinkable monomer.

7. The crosslinked polymer or salt thereof according to claim 1, wherein the crosslinked polymer or salt thereof is polymerized by precipitation polymerization or dispersion polymerization using the following polymerization steps: (A) a polymerization step in which only the ethylenically unsaturated carboxylic acid monomer is polymerized; and (B) A production method comprising carrying out one or both of a polymerization step of continuously or intermittently adding the monomers represented by the formula (1) and the formula (2) to the ethylenically unsaturated carboxylic acid monomer and polymerizing the monomers.

8. A composition for a non-aqueous electrolyte secondary battery electrode mixture layer, comprising the binder for a non-aqueous electrolyte secondary battery electrode according to any one of claims 1 to 6, an active material, and water.

9. The composition for a non-aqueous electrolyte secondary battery electrode mixture layer according to claim 8 , further comprising a styrene / butadiene-based latex as a binder for a non-aqueous electrolyte secondary battery electrode.

10. The composition for a non-aqueous electrolyte secondary battery electrode mixture layer according to claim 8 or 9, which contains a carbon-based material and / or a silicon-based material as a negative electrode active material.

11. The composition for a non-aqueous electrolyte secondary battery electrode mixture layer according to claim 8 or 9, which contains a lithium-containing metal oxide as a positive electrode active material.

12. A non-aqueous electrolyte secondary battery electrode comprising a mixture layer formed on a surface of a current collector from the composition for a non-aqueous electrolyte secondary battery electrode mixture layer according to any one of claims 8 to 11.

13. A non-aqueous electrolyte secondary battery comprising the secondary battery electrode according to claim 12.

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