Binder for secondary battery electrode and use thereof, and method for producing binder for secondary battery electrode

Abstract

The present invention provides a binder that is for a secondary battery electrode, that exhibits excellent binding properties, and that is thus capable of improving the cycle characteristics of a secondary battery. Additionally, the present invention provides: a composition which is for a secondary battery electrode mixture layer and which comprises the binder; a secondary battery electrode which is obtained using the composition; and a secondary battery.　Provided is a binder for a secondary battery electrode, said binder comprising a carboxyl group-containing non-crosslinked polymer or a salt thereof, wherein: the carboxyl group-containing non-crosslinked polymer includes a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter referred to as "monomer (a)") and a structural unit derived from a nitrile group-containing ethylenically unsaturated monomer (hereinafter referred to as "monomer (b)"); and a value (X) of the carboxyl group-containing non-crosslinked polymer or salt thereof, as calculated using formula (1), is 0.050-0.500.

Description

Binder for secondary battery electrodes, use thereof, and method for manufacturing binder for secondary battery electrodes

[0001] The present invention relates to a binder for secondary battery electrodes, its use, and a method for producing a secondary battery electrode binder.

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

[0003] In recent years, as the applications of various secondary batteries have expanded, there has been a growing demand for improved energy density, reliability, and durability. For example, in order to increase the electrical capacity of lithium-ion secondary batteries, the use of silicon-based active materials as negative electrode active materials has become more common. However, silicon-based active materials are known to undergo large volume changes during charging and discharging, 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 address these issues, studies have been conducted to improve durability by using binders to firmly bond the current collector and active material and between active materials themselves (binding properties), reducing the size of active materials to alleviate stress associated with swelling and shrinkage, and improving electrolyte additives.

[0004] In this context, it has been reported that acrylic acid polymers are effective as binders that have good cycle characteristics and are effective in improving the durability of negative electrode mixture layers that use silicon-based active materials. Patent Document 1 discloses a binder containing a cross-linked acrylic acid polymer in which polyacrylic acid is cross-linked with a specific cross-linking agent, and discloses that even when an active material containing silicon (hereinafter also referred to as a "silicon-based active material") is used, the binder exhibits good cycle characteristics without destroying the electrode structure.

[0005] Furthermore, Patent Document 2 discloses a binder containing a non-crosslinked acrylic acid-based polymer composed of a salt of acrylic acid or an acrylic acid derivative and acrylonitrile or an acrylonitrile derivative, and discloses that the binder can follow the expansion and contraction of the silicon-based active material, thereby improving cycle characteristics.

[0006] International Publication No. 2014 / 065407 Japanese Patent Application Laid-Open No. 2015-115109

[0007] However, although the binders for secondary battery electrodes disclosed in Patent Documents 1 and 2 can impart good binding properties, when the ratio of silicon-based active material is increased to improve the performance of the secondary battery, the cycle characteristics become insufficient, which can be a problem.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a binder for secondary battery electrodes that exhibits excellent binding properties and can improve the cycle characteristics of secondary batteries. It is also an object of the present invention to provide a composition for secondary battery electrode mixture layers that contains the binder, and a secondary battery electrode and a secondary battery obtained using the composition.

[0009] As a result of intensive research to solve the above-mentioned problems, the present inventors have found that in a binder for secondary battery electrodes containing a carboxyl group-containing non-crosslinked polymer or a salt thereof, the non-crosslinked polymer has structural units derived from two specific monomers, and by setting a characteristic value related to the trimer of the specific monomer, detected by pyrolysis GC / MS of the non-crosslinked polymer or a salt thereof, within a specific range, excellent binding properties can be exhibited and the cycle characteristics of the secondary battery can be improved, thereby completing the present invention.

[0010] The present invention is as follows: [1] A binder for secondary battery electrodes containing a carboxyl group-containing non-crosslinked polymer or a salt thereof, wherein the carboxyl group-containing non-crosslinked polymer contains a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter referred to as "monomer (a)") and a structural unit derived from a nitrile group-containing ethylenically unsaturated monomer (hereinafter referred to as "monomer (b)"), and the carboxyl group-containing non-crosslinked polymer or salt thereof has a value (X) calculated by the following mathematical formula (1) of 0.050 or more and 0.500 or less: The detection of a trimer of the monomer (b) by the pyrolysis GC / MS method means that a "site where three or more structural units derived from the monomer (b) are linked" is present in the polymer. ２ The Mw and Mn of the polyacrylonitrile used in the measurement are values ​​measured in terms of polymethyl methacrylate according to N,N-dimethylformamide gel permeation chromatography described in the Examples.

[0011] [2] The binder for secondary battery electrodes according to [1], wherein the carboxyl group-containing non-crosslinked polymer contains structural units derived from the monomer (b) in an amount of 1% by mass to 50% by mass, based on all structural units thereof. [3] The binder for secondary battery electrodes according to [1] or [2], wherein the carboxyl group-containing non-crosslinked polymer further contains structural units derived from an amide group-containing ethylenically unsaturated monomer (hereinafter referred to as "monomer (c)"). [4] The binder for secondary battery electrodes according to [3], wherein the carboxyl group-containing non-crosslinked polymer contains structural units derived from the monomer (a) in an amount of 40% by mass to 98% by mass, structural units derived from the monomer (b) in an amount of 1% by mass to 50% by mass, and structural units derived from the monomer (c) in an amount of 1% by mass to 50% by mass, based on all structural units thereof. [5] The binder for secondary battery electrodes according to any one of [1] to [4], wherein the carboxyl group-containing non-crosslinked polymer or salt thereof has a number average molecular weight of 4,000 to 400,000. [6] The binder for secondary battery electrodes according to any one of [1] to [5], wherein the salt of the carboxyl group-containing non-crosslinked polymer is a salt in which 40 mol % or more of the carboxyl groups in the non-crosslinked polymer have been neutralized. [7] A method for producing a binder for a secondary battery electrode containing a carboxyl group-containing non-crosslinked polymer or a salt thereof, wherein the carboxyl group-containing non-crosslinked polymer contains a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter referred to as "monomer (a)") and a structural unit derived from a nitrile group-containing ethylenically unsaturated monomer (hereinafter referred to as "monomer (b)"), the method comprising a polymerization step of polymerizing a monomer component containing the monomer (a) and the monomer (b), wherein the polymerization is carried out while continuously or intermittently supplying at least a portion of the monomer (a). [8] The method according to [7], wherein the carboxyl group-containing non-crosslinked polymer or salt thereof has a value (X) calculated by the above mathematical formula (1) of 0.050 or more and 0.500 or less. [9] The method according to [7] or [8], wherein the carboxyl group-containing non-crosslinked polymer contains structural units derived from the monomer (b) in an amount of 1% by mass or more and 50% by mass or less relative to all structural units thereof.

[10] A composition for a secondary battery electrode mixture layer, comprising the binder for a secondary battery electrode according to any one of [1] to [6], an active material, and water.

[11] A secondary battery electrode, comprising a mixture layer formed on a surface of a current collector from the composition for a secondary battery electrode mixture layer according to

[10] .

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

[11] .

[0012] The binder for secondary battery electrodes of the present invention exhibits excellent binding properties, making it possible to obtain a secondary battery having excellent cycle characteristics.

[0013] The binder for secondary battery electrodes of the present invention (hereinafter also referred to as "the binder") contains a carboxyl group-containing non-crosslinked polymer (hereinafter also referred to as "the non-crosslinked polymer") or a salt thereof (hereinafter also referred to as "the non-crosslinked polymer salt") having structural units derived from monomer (a) and structural units derived from monomer (b). By mixing the binder with an active material and water, a composition for secondary battery electrode mixture layer (hereinafter also referred to as "the composition") can be prepared. The composition is preferably in the form of an electrode slurry that can be applied to a current collector, from the viewpoint of achieving the effects of the present invention. However, the composition may also be prepared in the form of a wet powder to allow for press processing onto the current collector surface. The secondary battery electrode of the present invention can be obtained by forming a mixture layer from the composition on the surface of a current collector such as copper foil or aluminum foil. Here, the binder is preferably used in a composition for secondary battery electrode mixture layer containing a silicon-based active material as described below as the active material, since the effects of the present invention are particularly significant.

[0014] The present non-crosslinked polymer or its salt and its manufacturing method, the composition for a secondary battery electrode mixture layer obtained using the present binder, the secondary battery electrode, and the secondary battery are each described in detail below. In this specification, "(meth)acrylic" means acrylic and / or methacrylic, and "(meth)acrylate" means acrylate and / or methacrylate. Furthermore, "(meth)acryloyl group" means acryloyl group and / or methacryloyl group. In the numerical ranges described in this specification in stages, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages, and the upper or lower limit value of that numerical range may be replaced with a value shown in the examples.

[0015] The carboxyl group-containing non-crosslinked polymer or salt thereof of the present invention (hereinafter also referred to simply as "the present non-crosslinked polymer (salt)") has a value calculated by the above formula (1) (hereinafter also referred to simply as "characteristic value X") of 0.050 or more and 0.500 or less, preferably 0.050 or more and 0.400 or less, more preferably 0.060 or more and 0.300 or less, even more preferably 0.070 or more and 0.200 or less, and even more preferably 0.080 or more and 0.150 or less, in order to improve binding properties and cycle characteristics of secondary batteries. Here, the characteristic value X means the relative abundance ratio of "sites in which three or more structural units derived from monomer (b) are linked" in the present non-crosslinked polymer (salt). 1. The present non-crosslinked polymer or salt thereof

[0016] The carboxyl group-containing non-crosslinked polymer of the present invention has a structural unit derived from the monomer (a) and a structural unit derived from the monomer (b), and can be introduced into the non-crosslinked polymer by polymerizing a monomer component containing the monomer (a) and the monomer (b).

[0017] <Structural Unit Derived from Ethylenically Unsaturated Carboxylic Acid Monomer> The present non-crosslinked polymer has a structural unit (hereinafter also referred to as "component (a)") derived from an ethylenically unsaturated carboxylic acid monomer (monomer (a)). When the present non-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, and the dispersion stability of the active material and the like in the present composition can be improved. The above-mentioned component (a) can be introduced into the present non-crosslinked polymer, for example, by polymerizing monomer (a). Alternatively, it can be obtained by (co)polymerizing a (meth)acrylic acid ester monomer and then hydrolyzing it. Alternatively, (meth)acrylamide, (meth)acrylonitrile, or the like can be polymerized and then treated with a strong alkali, or a method in which a polymer having a hydroxyl group is reacted with an acid anhydride can be used.

[0018] Examples of the monomer (a) include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, and fumaric acid; (meth)acrylamidoalkylcarboxylic acids such as (meth)acrylamidohexanoic acid and (meth)acrylamidododecanoic acid; and carboxyl 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. One of these may be used alone, or two or more may be used in combination. 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 a polymer with a long primary chain length and good binder binding strength. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, a polymer with a high carboxyl group content can be obtained.

[0019] The content of component (a) in the non-crosslinked polymer can be 40% by mass or more and 98% by mass or less, based on the total structural units of the non-crosslinked polymer. By including component (a) in this range, excellent adhesion to the current collector can be easily ensured, thereby improving the cycle characteristics of the secondary battery. A lower limit of 40% by mass or more is preferable because the dispersion stability of the composition is improved and a higher binding strength can be obtained. The lower limit may be 50% by mass or more, 60% by mass or more, 70% by mass or more, or even 80% by mass or more. The upper limit may be, for example, 97% by mass or less, for example, 96% by mass or less, for example, 95% by mass or less, for example, 93% by mass or less, for example, 91% by mass or less, or for example, 90% by mass or less.

[0020] The total amount of monomer (a) and its salts converted into unneutralized form (i.e., all present ethylenically unsaturated carboxylic acid monomers converted into the form of carboxylic acid) relative to the total amount of the non-crosslinked polymer and its salts (hereinafter also referred to as the "residual carboxylic acid monomer (salt) content") can be, for example, 5.0% by mass or less. The residual carboxylic acid monomer (salt) content is, for example, 4.0% by mass or less, for example, 3.0% by mass or less, for example, 2.0% by mass or less, for example, 1.5% by mass or less, for example, 1.0% by mass or less, or for example, 0.5% by mass or less. If the residual carboxylic acid monomer (salt) content is 5.0% by mass or less, high binding strength can be obtained.

[0021] The lower limit of the residual carboxylic acid monomer (salt) content is not particularly limited, but can be, for example, 0.05% by mass or more. It is also, for example, 0.1% by mass or more, for example, 0.2% by mass or more, for example, 0.3% by mass or more, or for example, 0.4% by mass or more. A residual carboxylic acid monomer (salt) content of less than 0.05% by mass may not be advantageous in terms of cleaning costs, etc. The residual carboxylic acid monomer (salt) content is measured by gas chromatography (GC).

[0022] <Structural Unit Derived from Nitrile Group-Containing Ethylenically Unsaturated Monomer> The present non-crosslinked polymer has a structural unit (hereinafter also referred to as "component (b)") derived from a nitrile group-containing ethylenically unsaturated monomer (monomer (b)). The presence of such a structural unit in the present non-crosslinked polymer results in excellent initial charge-discharge efficiency and cycle characteristics of the secondary battery. The component (b) can be introduced into the present non-crosslinked polymer by polymerizing a monomer containing monomer (b).

[0023] Examples of the monomer (b) include (meth)acrylonitrile; (meth)acrylic acid cyanoalkyl ester compounds such as cyanomethyl (meth)acrylate and cyanoethyl (meth)acrylate; cyano group-containing unsaturated aromatic compounds such as 4-cyanostyrene and 4-cyano-α-methylstyrene; vinylidene cyanide; and the like. One of these may be used alone, or two or more may be used in combination. Among the above, acrylonitrile is preferred because it has a large nitrile group content and is more excellent in the initial charge / discharge efficiency and cycle characteristics of the secondary battery.

[0024] The content of component (b) in the non-crosslinked polymer can be 1% by mass or more and 50% by mass or less, based on the total structural units of the non-crosslinked polymer. By including component (b) in this range, excellent adhesion to the current collector can be easily ensured, and the initial charge / discharge efficiency and cycle characteristics of the secondary battery can be improved. A lower limit of 1% by mass or more is preferable because the dispersion stability of the composition is improved and a higher binding strength can be obtained. The lower limit may be 3% by mass or more, 5% by mass or more, 10% by mass or more, 20% by mass or more, or even 25% by mass or more. The upper limit is, for example, 49% by mass or less, 45% by mass or less, or, for example, 40% by mass or less, or, for example, 30% by mass or less.

[0025] The total amount of monomer (b) relative to the total amount of the non-crosslinked polymer and its salt (hereinafter also referred to as the "content of residual monomer (b)") can be, for example, 5.0% by mass or less. The content of residual monomer (b) is, for example, 4.0% by mass or less, for example, 3.0% by mass or less, for example, 2.0% by mass or less, for example, 1.5% by mass or less, for example, 1.0% by mass or less, or for example, 0.5% by mass or less. If the content of residual monomer (b) is 5.0% by mass or less, high binding strength between active materials can be obtained.

[0026] The lower limit of the content of the residual monomer (b) is not particularly limited, but can be, for example, 0.05% by mass or more. It is also, for example, 0.1% by mass or more, for example, 0.2% by mass or more, for example, 0.3% by mass or more, or for example, 0.4% by mass or more. The content of the residual monomer (b) is measured by gas chromatography (GC).

[0027] <Structural Unit Derived from Amide Group-Containing Ethylenically Unsaturated Monomer> The present non-crosslinked polymer preferably further comprises a structural unit (hereinafter also referred to as "component (c)") derived from an amide group-containing ethylenically unsaturated monomer (monomer (c)). When the present non-crosslinked polymer comprises such a structural unit, the secondary battery exhibits excellent cycle characteristics. The component (c) can be introduced into the present non-crosslinked polymer by polymerizing a monomer containing monomer (c).

[0028] Examples of the monomer (c) include (meth)acrylamide, a monomer represented by the following formula (1), and a (meth)acrylamide derivative (a monomer other than the monomer represented by the following formula (1)). Among these, (meth)acrylamide is preferred in that it provides a secondary battery with even better initial charge-discharge efficiency and cycle characteristics.

[0029] CH ２ = C(R ５ ) CONR ６ R ７ (1) [wherein, 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, R７ represents a hydrogen atom or a monovalent organic group.

[0030] The monomer represented by the formula (1) 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. Examples of the monomer represented by formula (1) include hydroxy(meth)acrylamide; (meth)acrylamide derivatives having a hydroxyalkyl group having 1 to 8 carbon atoms, such as N-hydroxyethyl(meth)acrylamide, N-(2-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 formula (1) may be used singly or in combination of two or more. Among the monomers represented by the above formula (1), (meth)acrylamide derivatives having a hydroxyalkyl group having 1 to 8 carbon atoms are more preferred in terms of excellent cycle characteristics of secondary batteries, and N-hydroxyethyl(meth)acrylamide, N-(2-hydroxypropyl)(meth)acrylamide, and N-hydroxybutyl(meth)acrylamide are even more preferred.

[0031] Examples of the (meth)acrylamide derivative include N-alkyl(meth)acrylamide compounds such as N-isopropyl(meth)acrylamide and N-t-butyl(meth)acrylamide; N-alkoxyalkyl(meth)acrylamide compounds such as N-n-butoxymethyl(meth)acrylamide and N-isobutoxymethyl(meth)acrylamide; and N,N-dialkyl(meth)acrylamide compounds such as N,N-dimethyl(meth)acrylamide and N,N-diethyl(meth)acrylamide. These may be used alone or in combination of two or more.

[0032] The content of component (c) in the non-crosslinked polymer can be 1% by mass or more and 50% by mass or less, based on the total structural units of the non-crosslinked polymer. By including component (c) in this range, excellent adhesion to the current collector can be easily ensured, thereby improving the cycle characteristics of the secondary battery. A lower limit of 1% by mass or more is preferable because the dispersion stability of the composition is improved and a higher binding strength can be obtained. It may be 2% by mass or more, 3% by mass or more, or even 5% by mass or more. The upper limit is, for example, 50% by mass or less or 45% by mass or less. In terms of achieving even better initial charge / discharge efficiency of the secondary battery, it is preferably 40% by mass or less, more preferably 30% by mass or less, even more preferably 20% by mass or less, and even more preferably 10% by mass or less.

[0033] The total amount of monomer (c) relative to the total amount of the present non-crosslinked polymer and its salt (hereinafter also referred to as the "content of residual monomer (c)") is 5.0 mass% or less, preferably 4.0 mass% or less, more preferably 3.0 mass% or less, even more preferably 2.0 mass% or less, still more preferably 1.5 mass% or less, still more preferably 1.0 mass% or less, and particularly preferably 0.5 mass% or less, in order to improve the cycle characteristics of the secondary battery.

[0034] The lower limit of the content of the residual monomer (c) is not particularly limited, but can be, for example, 0.05% by mass or more. It is also, for example, 0.1% by mass or more, for example, 0.2% by mass or more, for example, 0.3% by mass or more, or for example, 0.4% by mass or more. The content of the residual monomer (c) is measured by gas chromatography (GC).

[0035] <Other Structural Units> In addition to the components (a) and (b) (and, if necessary, the component (c)), the present non-crosslinked polymer may contain a structural unit (hereinafter also referred to as the "component (d)") derived from another ethylenically unsaturated monomer copolymerizable therewith (hereinafter also referred to as the "monomer (d)"). Examples of the component (d) include a hydroxyl group-containing ethylenically unsaturated monomer (a monomer represented by the following formula (2)), an ethylenically unsaturated monomer compound having an anionic group other than a carboxyl group, such as a sulfonic acid group or a phosphoric acid group, or a nonionic ethylenically unsaturated monomer (excluding monomers classified as nitrogen-containing ethylenically unsaturated monomers). The structural unit derived from the monomer (d) can be introduced by copolymerizing a hydroxyl group-containing ethylenically unsaturated monomer, an ethylenically unsaturated monomer compound having an anionic group other than a carboxyl group, such as a sulfonic acid group or a phosphoric acid group, or a monomer containing a nonionic ethylenically unsaturated monomer. CH ２ = C(R １ ) COOR ２ (2) [wherein, 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.

[0036] The proportion of the (d) component can be 0% by mass or more and 50% by mass or less relative to the total structural units of the non-crosslinked polymer. The proportion of the (d) component may be 0.1% by mass or more and 45% by mass or less, 0.5% by mass or more and 40% by mass or less, 1.0% by mass or more and 30% by mass or less, 5.0% by mass or more and 20% by mass or less, or 3% by mass or more and 10% by mass or less. Furthermore, when the (d) component is contained in an amount of 0.1% by mass or more relative to the total structural units of the non-crosslinked polymer, affinity to the electrolyte solution is improved, and therefore, the effect of improving lithium ion conductivity can also be expected.

[0037] Of the above, the component (d) is preferably a structural unit derived from a hydroxyl group-containing ethylenically unsaturated monomer, in that it provides excellent binding properties to a binder containing the present non-crosslinked polymer or a salt thereof.

[0038] The monomer represented by the above formula (2) 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 groups 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(CH ２ ) ５ O] ｎ If H, then R ３ or R ４ The alkylene group represented by may be linear or branched.

[0039] Examples of the monomer represented by the formula (2) include hydroxyalkyl (meth)acrylates having a hydroxyalkyl group having 1 to 8 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, hydroxyhexyl (meth)acrylate, and hydroxyoctyl (meth)acrylate; polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, polybutylene glycol mono(meth)acrylate, and polyethylene glycol mono(meth)acrylate. Examples of the polyalkylene glycol mono(meth)acrylate include polyalkylene glycol mono(meth)acrylates such as ethanol-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.), caprolactone-modified hydroxyacrylates (manufactured by Daicel Corporation, trade names "PLACCEL FA1", "PLACCEL FA10L", etc.), and any of these may be used alone or in combination of two or more.

[0040] Furthermore, from the viewpoint of obtaining an electrode with good flex resistance, a structural unit derived from a nonionic ethylenically unsaturated monomer is preferred. Examples of the nonionic ethylenically unsaturated monomer include an alicyclic structure-containing ethylenically unsaturated monomer. 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, cyclopentenyl (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; and the like. These may be used alone or in combination of two or more.

[0041] The present non-crosslinked polymer preferably contains a structural unit derived from a monomer represented by the above formula (2), an alicyclic structure-containing ethylenically unsaturated monomer, or the like, in terms of excellent binding properties of the binder. Among these, as component (d), a structural unit derived from a monomer represented by the above formula (2) is more preferred in terms of excellent effect of improving the binding properties of the present binder. Among the monomers represented by the above formula (2), hydroxyalkyl (meth)acrylates having a hydroxyalkyl group having 1 to 8 carbon atoms are more preferred, and 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate are even more preferred.

[0042] 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 (d), it can exhibit strong interaction with the electrode material and exhibit good binding properties to the active material. This allows for a firm and well-integrated electrode mixture layer to be obtained, and therefore, as the above-mentioned "hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less," an alicyclic structure-containing ethylenically unsaturated monomer is particularly preferred.

[0043] Other nonionic ethylenically unsaturated monomers that may be used 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, n-butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; aromatic (meth)acrylic acid ester compounds such as phenyl (meth)acrylate, phenylmethyl (meth)acrylate, phenylethyl (meth)acrylate, and phenoxyethyl (meth)acrylate; and (meth)acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate. These may be used alone or in combination of two or more.

[0044] From the viewpoints of binding strength with the active material and cycle characteristics, aromatic (meth)acrylic acid ester compounds are preferably used. From the viewpoints of further improving lithium ion conductivity and high-rate characteristics, compounds having an ether bond, such as (meth)acrylic acid alkoxyalkyl esters such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate, are preferred, with 2-methoxyethyl (meth)acrylate being more preferred.

[0045] 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 binding strength.Furthermore, as the nonionic ethylenically unsaturated monomer, compounds whose homopolymer has a glass transition temperature (Tg) of 0°C or less are preferred because the resulting electrode has good flex resistance.

[0046] The non-crosslinked polymer salt is in the form of a salt in which some or all of the carboxyl groups contained in the polymer have been neutralized. The type of salt is not particularly limited, but examples include alkali metal salts such as lithium salts, sodium salts, and potassium salts; alkaline earth metal salts such as magnesium salts, calcium salts, and barium salts; other metal salts such as aluminum salts; ammonium salts, and organic amine salts. Among these, alkali metal salts and alkaline earth metal salts are preferred because they are less likely to adversely affect battery characteristics, and alkali metal salts are more preferred. Lithium salts are particularly preferred because they provide excellent cycle characteristics of secondary batteries and excellent low-temperature battery characteristics.

[0047] Regarding the characteristics of the non-crosslinked polymer salt, the non-crosslinked polymer is preferably used in the composition as a salt in which acid groups such as carboxyl groups derived from ethylenically unsaturated carboxylic acid monomers are neutralized so that the degree of neutralization is 40 mol% or more. A degree of neutralization of 40 mol% or more is preferred in that it is easier to obtain a dispersion stabilization effect. The degree of neutralization is more preferably 45 mol% or more, even more preferably 60 mol% or more, even more preferably 70 mol% or more, even more preferably 80 mol% or more, and particularly preferably 85 mol% or more, in that it can exhibit superior initial charge / discharge efficiency over longer periods of use than conventional methods. The upper limit of the degree of neutralization is 100 mol%, but it may also be 98 mol% or 95 mol%. In this specification, the degree of neutralization can be calculated from the amounts of monomers having acid groups such as carboxyl groups and the neutralizing agent used for neutralization. The degree of neutralization can be confirmed by dissolving the present non-crosslinked polymer salt in a mixed solvent of ethanol / water = 1 / 1 (mass ratio) and determining the substance amount of the carboxylate by acid titration and the substance amount ratio of the acid group such as a carboxyl group by subsequent base titration.

[0048] Molecular Weight of the Non-Crosslinked Polymer or Its Salt The number average molecular weight (Mn) of the non-crosslinked polymer or its salt is measured as a value equivalent to sodium polyacrylate by aqueous gel permeation chromatography (GPC) as described in the Examples. Mn is preferably 4,000 to 400,000 in terms of improving the dispersibility of the active material and achieving excellent charge / discharge capacity retention, and can be adjusted by the monomer concentration and initiator amount. Mn is more preferably 4,500 to 200,000, even more preferably 5,000 to 100,000, and even more preferably 6,000 to 50,000.

[0049] The weight-average molecular weight (Mw) of the non-crosslinked polymer or its salt is measured as a value equivalent to sodium polyacrylate by aqueous GPC as described in the Examples. Mw is preferably 400,000 to 4,000,000 in order to improve the binding strength with the active material and current collector and to exhibit excellent charge / discharge capacity retention, and can be adjusted by adjusting the monomer concentration and initiator amount. Mw is more preferably 450,000 to 3,000,000, even more preferably 475,000 to 2,000,000, even more preferably 500,000 to 1,600,000, even more preferably 500,000 to 1,400,000, and particularly preferably 500,000 to 1,300,000.

[0050] The molecular weight distribution (Mw / Mn) of the non-crosslinked polymer or its salt is preferably 5 to 300, in terms of improving the binding strength with the active material and current collector, the dispersibility of the active material, and exhibiting an excellent charge / discharge capacity retention rate. This can be adjusted by the monomer concentration and polymerization temperature. Mw / Mn is more preferably 3 to 300, even more preferably 5 to 290, even more preferably 10 to 280, even more preferably 30 to 270, and particularly preferably 50 to 260. It is presumed that by increasing Mw / Mn, high-molecular-weight components that improve the binding strength with the active material and current collector and low-molecular-weight components that improve the dispersibility of the active material coexist, allowing each component to exhibit its respective functions.

[0051] 2. Method for Producing the Non-Crosslinked Polymer The non-crosslinked polymer can be obtained by a "production method comprising a step of polymerizing monomer components including an ethylenically unsaturated carboxylic acid monomer (monomer (a)) and a nitrile group-containing ethylenically unsaturated monomer (monomer (b)), wherein the polymerization step is carried out while continuously or intermittently supplying at least a portion of the monomer (a)." Here, by carrying out the polymerization while continuously or intermittently supplying at least a portion of the monomer (a), the relative abundance ratio of "sites in which three or more structural units derived from monomer (b) are linked" in the non-crosslinked polymer can be controlled, and the characteristic value X can be set to 0.050 or more and 0.500 or less. More specifically, the amount of the monomer (a) to be continuously or intermittently supplied is preferably 50% by mass or less (more preferably 30% by mass or less) of the total amount of the monomer (a), thereby making the characteristic value X approach 0.050, while the amount is preferably more than 50% by mass (more preferably 70% by mass or more) of the total amount of the monomer (a), thereby making the characteristic value X approach 0.500. The preferred range of the characteristic value X is as described above.

[0052] The polymerization method may be bulk polymerization without using a solvent, solution polymerization in a solvent system, emulsion polymerization in an aqueous system, mini-emulsion polymerization, suspension polymerization, etc. Among these, solution polymerization is preferred because it allows the present non-crosslinked polymer or a salt thereof to be uniformly dissolved and can be more easily dispersed when added to a slurry-state composition for an electrode mixture layer.

[0053] A specific polymerization solvent is preferably water, since it allows the monomer (a) and the monomer (b) to be uniformly dissolved and polymerized. Other examples 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 of two or more. Alternatively, these solvents can be used as a mixed solvent with water. In the present invention, the water-soluble solvent refers to a solvent having a solubility in water at 20°C of greater than 10 g / 100 ml.

[0054] Similarly, in the neutralization step, it is preferable to add a small amount of a highly polar solvent to the polymerization solvent to ensure stable and rapid progress of the neutralization reaction. Examples of such highly polar solvents include water and methanol. The amount of the highly polar solvent used is preferably 0.05 to 20.0 mass% based on the total mass of the medium, more preferably 0.1 to 10.0 mass%, even more preferably 0.1 to 5.0 mass%, and even more preferably 0.1 to 1.0 mass%. If the proportion of the highly polar solvent is 0.05 mass% or more, an effect on the neutralization reaction is observed, while if 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 polymers with long primary chain lengths. Among highly polar solvents, water is particularly preferable due to its significant effect on improving the polymerization rate.

[0055] The polymerization initiator may be any known polymerization initiator such as an azo compound, an organic peroxide, or an inorganic peroxide, but is not particularly limited. The conditions for use can be adjusted so as to generate an appropriate amount of radicals by known methods such as thermal initiation, redox initiation using a reducing agent, or UV initiation. To obtain a non-crosslinked polymer with a long primary chain length, it is preferable to set the conditions so as to generate as little radicals as possible within the allowable production time range.

[0056] Here, the polymerization initiator in the aqueous solution polymerization is preferably a water-soluble polymerization initiator, and examples thereof include a compound having a hydrophilic group (e.g., a carboxyl group) and / or a salt or hydrate thereof. Among these, 4,4'-azobis(4-cyanovaleric acid), 2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis[2-(imidazolin-2-yl)propane]disulfate dihydrate, 2,2'-azobis(2-methylpropionamidine)dihydrochloride, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]hydrate, and the like are preferred.

[0057] A preferred amount of the polymerization initiator used is, when the total amount of the monomer components used is 100 parts by mass, for example, 0.001 to 3 parts by mass, or for example, 0.005 to 2.5 parts by mass, or for example, 0.01 to 2 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 1.5 parts by mass or less, a polymer having a long primary chain length is likely to be obtained.

[0058] 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.

[0059] Here, the present non-crosslinked polymer may contain, relative to its total structural units, 40% by mass or more and 99% by mass or less of structural units derived from an ethylenically unsaturated carboxylic acid monomer (monomer (a)) and 1% by mass or more and 50% by mass or less of structural units derived from a nitrile group-containing ethylenically unsaturated monomer (monomer (b)), with the preferred range of the content of these structural units being as described above. The types of monomer (a) and monomer (b) are as described above. The present non-crosslinked polymer may further contain a structural unit derived from an amide group-containing ethylenically unsaturated monomer (hereinafter referred to as "monomer (c)"), with the preferred range of the content of this structural unit being as described above. The type of monomer (c) is as described above.

[0060] 3. Composition for Secondary Battery Electrode Mixture Layer The composition for secondary battery electrode mixture layer of the present invention contains the present binder, an active material, and water. The amount of the present binder used in the composition is preferably 0.5 parts by mass or more and 7.0 parts by mass or less, relative to 100 parts by mass of the total amount of active material. The amount used is, for example, 0.8 parts by mass or more and 3.0 parts by mass or less, such as 1.0 parts by mass or more and 2.5 parts by mass or less, and for example, 1.2 parts by mass or more and 1.5 parts by mass or less. When the amount of the binder used is 0.5 parts by mass or more, sufficient binding strength can be obtained. Furthermore, dispersion stability of the active material and the like can be ensured, and a uniform mixture layer can be formed. When the amount of the binder used is 1.5 parts by mass or less, the present composition does not become highly viscous, and coatability to the current collector can be ensured. As a result, a mixture layer with a uniform and smooth surface can be formed.

[0061] 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 NCMs {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.

[0062] 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 may result in corrosion of aluminum foil (Al), a common positive electrode current collector material. In such cases, it is preferable to neutralize the alkali eluted from the active material by using the unneutralized or partially neutralized non-crosslinked polymer as a binder. Furthermore, it is preferable to use the unneutralized or partially neutralized non-crosslinked polymer in an amount such that the amount of unneutralized carboxyl groups in the non-crosslinked polymer is equivalent to or greater than the amount of alkali eluted from the active material.

[0063] Because all positive electrode active materials have low electrical conductivity, a conductive additive other than carbon nanotubes may be added. Examples of such conductive additives include carbon-based materials such as carbon black, carbon fiber, graphite powder, and carbon fiber. Of these, carbon black 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 with two or more of the above. The amount of conductive additive other than carbon nanotubes 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.

[0064] On the other hand, examples of negative electrode active materials include carbon-based materials, lithium metal, lithium alloys, and metal oxides, and one or more of these can be used in combination. Among these, active materials made of carbon-based materials such as natural graphite, artificial graphite, hard carbon, and soft carbon (hereinafter also referred to as "carbon-based active materials") are preferred, with graphite such as natural graphite and artificial graphite, and hard carbon being more preferred. In the case of graphite, spherical graphite is preferably used from the standpoint 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. The amount of silicon-based active material used is 5.0 mass% or more, for example, 10.0 mass% or more, or for example, 20.0 mass% or more, relative to the total amount of active material, in order to improve the electrical capacity of the secondary battery.

[0065] Since the carbon-based active material itself has good electrical conductivity, it is not necessary to add a conductive additive other than carbon nanotubes. When a conductive additive is added for the purpose of further reducing resistance, etc., from the viewpoint of energy density, the amount used is, for example, 10 parts by mass or less, or, for example, 5 parts by mass or less, relative to 100 parts by mass of the total amount of the active material.

[0066] When the composition is in a slurry state, the amount of active material used is, for example, in the range of 10 to 75 mass %, or, for example, in the range of 30 to 65 mass %, based on the total amount of the composition. If the amount of active material used is 10 mass % or more, migration of binders and the like is suppressed, and this is also advantageous in terms of the cost of drying the medium. On the other hand, if the amount is 75 mass % or less, the fluidity and coatability of the composition can be ensured, and a uniform mixture layer can be formed.

[0067] The present composition uses water as a medium. Furthermore, for the purpose of adjusting the properties and drying properties of the present composition, the composition may be mixed with lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, or water-soluble organic solvents such as tetrahydrofuran and N-methyl-2-pyrrolidone. The proportion of water in the mixed medium is, for example, 50% by mass or more, or, for example, 70% by mass or more.

[0068] When the present composition is made into a coatable slurry state, the content of the water-containing medium in the entire present composition can be, for example, in the range of 25 to 60 mass %, and can also be, for example, 35 to 60 mass %, from the viewpoints of the coatability of the slurry, the energy cost required for drying, and productivity.

[0069] The present composition may further contain other binder components, such as styrene butadiene rubber (SBR)-based latex, carboxymethyl cellulose (CMC), acrylic latex, and polyvinylidene fluoride-based latex. When other binder components are used in combination, the amount thereof may be, for example, 0.1 to 5 parts by mass or less, or, for example, 0.1 to 2 parts by mass or less, or, for example, 0.1 to 1 part by mass or less, relative to 100 parts by mass of the total amount of active material. If the amount of other binder components used exceeds 5 parts by mass, resistance increases, and high-rate characteristics may become insufficient. Among the above, SBR-based latex and CMC are preferred in terms of their excellent balance of binding strength and flex resistance, and a combination of SBR-based latex and CMC is more preferred.

[0070] The SBR latex refers to an aqueous dispersion of a copolymer having structural units derived from an aromatic vinyl monomer such as styrene and structural units derived from an aliphatic conjugated diene monomer such as 1,3-butadiene. Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, vinyltoluene, and divinylbenzene, and one or more of these can be used. The structural units derived from the aromatic vinyl monomer in the copolymer can be, for example, in the range of 20 to 70% by mass, or, for example, in the range of 30 to 60% by mass, primarily from the viewpoint of binding properties. Examples of the aliphatic conjugated diene monomer include, for example, 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 2-chloro-1,3-butadiene, and one or more of these can be used. The structural units derived from the aliphatic conjugated diene monomer in the copolymer can be, for example, in the range of 30 to 70% by mass, or, for example, in the range of 40 to 60% by mass, in order to improve the binding properties of the binder and the flexibility of the resulting electrode. In addition to the above-mentioned monomers, the styrene / butadiene latex may also contain other monomers as copolymerization monomers, such as nitrile group-containing monomers such as (meth)acrylonitrile, carboxyl group-containing monomers such as (meth)acrylic acid, itaconic acid, and maleic acid, and ester group-containing monomers such as methyl (meth)acrylate, in order to further improve performance such as binding properties. The structural units derived from the other monomers in the copolymer can be, for example, in the range of 0 to 30% by mass, or, for example, in the range of 0 to 20% by mass.

[0071] The CMC refers to a nonionic cellulose-based semisynthetic polymer compound substituted with a carboxymethyl group and its salt. Examples of the nonionic cellulose-based semisynthetic polymer compound include alkyl celluloses such as methyl cellulose, methyl ethyl cellulose, ethyl cellulose, and microcrystalline cellulose; hydroxyethyl cellulose, hydroxybutyl methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose stearoxy ether, carboxymethyl hydroxyethyl cellulose, alkyl hydroxyethyl cellulose, and nonoxynyl hydroxyethyl cellulose.

[0072] The composition for a secondary battery electrode mixture layer of the present invention contains the above-mentioned binder, active material, and water 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 is to dry-blend powder components such as the active material, conductive additive, and binder, then mix them with a dispersion medium such as water and disperse and knead them. When obtaining the composition in a slurry state, it is preferable to finish the slurry without poor dispersion or aggregation. Known mixers such as planetary mixers, thin film gyratory mixers, and planetary / revolving mixers can be used as mixing means. However, a thin film gyratory mixer is preferred because it can achieve a good dispersion state in a short time. Furthermore, when using a thin film gyratory mixer, it is preferable to perform pre-dispersion beforehand using a stirrer such as a disperser. The pH of the slurry is not particularly limited as long as the effects of the present invention are achieved, but it is preferably less than 12.5. For example, when CMC is added, it is more preferable that it be less than 11.5, and even more preferable that it be less than 10.5, because there is little concern about hydrolysis of CMC. The viscosity of the slurry is not particularly limited as long as it exhibits the effects of the present invention, but the viscosity may be, for example, in the range of 100 to 30,000 mPa s, or, for example, 500 to 20,000 mPa s, or, for example, 1,000 to 10,000 mPa s, as Brookfield viscosity (25°C) at 20 rpm. If the viscosity of the slurry is within the above range, good coatability can be ensured.

[0073] 4. Secondary Battery Electrode The secondary battery electrode of the present invention comprises a mixture layer formed from the composition for a secondary battery electrode mixture layer of the present invention on the surface of a current collector made of copper, aluminum, or the like. The mixture layer is formed by applying the composition to the surface of the current collector and then drying to remove the medium, such as water. The method for applying the 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. Furthermore, the drying can be performed by known methods such as hot air blowing, reduced pressure, (far) infrared radiation, and microwave irradiation. Typically, the mixture layer obtained after drying is subjected to a compression treatment using a mold press, roll press, or the like. Compression brings the active material and binder into close contact, improving the strength of the mixture layer and its adhesion to the current collector. Compression can adjust the thickness of the mixture layer to, for example, approximately 30 to 80% of the thickness before compression, and the thickness of the mixture layer after compression is typically approximately 4 to 200 μm.

[0074] 5. Secondary Battery A secondary battery can be produced by providing the secondary battery electrode of the present invention with a separator and an electrolyte. The electrolyte may be 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 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.

[0075] The electrolyte may be a known, commonly used one depending on the type of active material. Specific examples of the solvent for lithium ion secondary batteries include cyclic carbonates with high dielectric constants and high electrolyte dissolving ability, such as propylene carbonate and ethylene carbonate, and chain carbonates with low viscosity, such as ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate. These may be used alone or as a mixed solvent. The electrolyte may be prepared by dissolving LiPF in these solvents. ６ , LiSbF６ , LiBF ４ , LiClO ４ , LiAlO ４ In nickel-metal hydride secondary batteries, an aqueous solution of potassium hydroxide can be used as the electrolyte. A secondary battery is obtained by spirally or stacking positive and negative electrode plates separated by a separator and housing them in a case or the like.

[0076] The present invention will be specifically described below based on examples. However, the present invention is not limited to these examples. In the following, "parts" and "%" mean parts by mass and % by mass unless otherwise specified. In the following examples, the carboxyl group-containing non-crosslinked polymer or its salt was evaluated by the following method.

[0077] (Measurement of solid content concentration) Approximately 1.0 g of a 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 was then placed in a windless dryer and dried at 155°C for 45 minutes, and the weight of the weighing bottle at that time was measured [W1 (g)], and the solid content concentration was calculated using the following formula: solid content concentration (mass%) = (W1 - B) / (W0 - B) x 100

[0078] (Measurement of the content of each structural unit in the carboxyl group-containing non-crosslinked polymer salt) The content of the structural units derived from each monomer in the carboxyl group-containing non-crosslinked polymer salt was measured using a nuclear magnetic resonance (NMR) spectrometer. The carboxyl group-containing non-crosslinked polymer salt obtained in the Production Examples and Comparative Production Examples was spread on an aluminum cup and vacuum dried at 70°C for 2 hours to obtain a dried product. The dried product was pulverized using a mortar and filled into a zirconia sleeve to obtain a measurement sample. Solid-state NMR measurement was carried out under the conditions described below to determine the content (mass%) of the structural units derived from each monomer in the carboxyl group-containing non-crosslinked polymer salt. Here, the mass fraction of the structural units derived from monomer (b) in the carboxyl group-containing non-crosslinked polymer salt was defined as W １ It was decided.

[0079] <Conditions for solid-state NMR measurement> Apparatus: JNM-ECA400 (manufactured by JEOL) Sample tube: Zirconia sleeve Sample rotation speed: 15 kHz Measurement nuclide: 13C Chemical shift reference: Adamantane high field signal was set at 29.5 ppm (external standard) Pulse sequence: CPMAS method

[0080] (Molecular Weight Measurement) The molecular weight of the carboxyl group-containing non-crosslinked polymer salt was measured using gel permeation chromatography (GPC). 0.1 g of each aqueous solution of the carboxyl group-containing non-crosslinked polymer salt obtained in the Production Examples and Comparative Production Examples (0.02 g as the solid content of the polymer salt) was collected and diluted with 40 g of 0.1 M aqueous sodium nitrate solution to obtain a measurement sample. Aqueous GPC measurement was performed on the measurement sample under the conditions described below to obtain the weight average molecular weight (Mw) and number average molecular weight (Mn) calculated in terms of sodium polyacrylate. The molecular weight distribution (Mw / Mn) was also calculated from the obtained values.

[0081] <Aqueous GPC measurement conditions> Column: Tosoh TSKgel GMPW x 2 Solvent: 0.1 M aqueous sodium nitrate solution Temperature: 40°C Detector: RI Flow rate: 0.5 mL / min

[0082] The molecular weight of polyacrylonitrile used as a standard substance for the characteristic value X was measured using gel permeation chromatography (GPC). 0.005 g of polyacrylonitrile was collected and diluted with 5 g of N,N-dimethylformamide (0.01 M lithium bromide mixed solution) to prepare a measurement sample. N,N-dimethylformamide (DMF) GPC measurement was performed on the measurement sample under the conditions described below, and the weight average molecular weight (Mw) and number average molecular weight (Mn) calculated in terms of polymethyl methacrylate were obtained.

[0083] <DMF-based GPC measurement conditions> Column: Tosoh TSKgel Super HM-M × 3 Solvent: N,N-dimethylformamide (0.01 M lithium bromide mixed solution) Temperature: 40°C Detector: RI Flow rate: 0.3 mL / min

[0084] (I １Measurement of I) 0.5 mg of an aqueous solution (0.1 mg as solid content) of each carboxyl group-containing non-crosslinked polymer salt was used as a measurement sample, and the following pyrolysis gas chromatography / mass spectrometry (pyrolysis GC / MS) measurement was carried out. The GC peak area of ​​the "trimer of nitrile group-containing ethylenically unsaturated monomer (monomer (b))" detected by this measurement was converted per unit mass to obtain I １ asked for.

[0085] (I ２ Measurement of characteristic value X) 0.01 mg of polyacrylonitrile (Mw 430,000, Mn 110,000; solid content 100% by mass) used as a standard substance for characteristic value X was used as a measurement sample, and the following pyrolysis GC / MS measurement was carried out. The GC peak area of ​​the "trimer of monomer (b)" detected by this measurement was converted per unit mass to obtain I ２ asked for.

[0086] <Pyrolysis GC / MS Measurement Conditions> (Pyrolysis) Apparatus: PY-3030D Double Shot Pyrolyzer manufactured by FRONTIER LAB Pyrolysis temperature: 550°C (GC) Apparatus: 8890A GC manufactured by Agilent Column: CP-sil5CB (30m x 0.32mm ID, 3.0µm df) Column heating conditions: 60°C (3min) - 10°C / min - 300°C (5min) Vaporization chamber / GCIF / ion source: 280°C / 250°C / 250°C Carrier gas (SP ratio): He 1.5ml / min (SP ratio 1:20) (MS) Apparatus: JEOL Jms-T2000GC MS Mass analysis range: m / z = 29 to 800 Ionization method: EI+ (EI ion source), FI+ (EI / FI ion source) Ion current: 300 μA (EI), 200 μA (FI) Ionization energy: 70 eV Reservoir temperature: 45° C. Detector voltage: 2400 V

[0087] <<Production of the Present Non-Crosslinked Polymer Salt>> (Production Example 1: Production of Carboxyl Group-Containing Non-Crosslinked Polymer Salt R-1) For polymerization, a reactor equipped with a stirring blade, thermometer, reflux condenser, and nitrogen inlet tube was used. Under a nitrogen atmosphere, 450.0 parts of ion-exchanged water, 26.5 parts of acrylic acid (hereinafter also referred to as "AA"), 20.0 parts of acrylonitrile (hereinafter also referred to as "AN"), and 13.5 parts of acrylamide (hereinafter also referred to as "AAm") were placed in the reactor and heated to 60°C. To this solution, 1.20 parts of the initiator 2,2'-azobis(2-methylpropionamidine) dihydrochloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name "V-50", hereinafter also referred to as "V-50") was added to initiate polymerization. From the start of polymerization, 26.5 parts of AA and 13.5 parts of AAm were continuously fed over 2 hours, and the reaction was allowed to proceed for 6 hours. After the reaction was continued for 2 hours at 80°C, the polymerization reaction solution was cooled. After the internal temperature had dropped to 45°C, lithium hydroxide monohydrate (hereinafter referred to as "LiOH·H ２ 0". 18.5 parts of a powder of carboxyl group-containing non-crosslinked polymer salt R-1 (lithium salt, degree of neutralization 60 mol%) was added, and the mixture was kept at 45°C and stirred for 30 minutes to obtain an aqueous solution of carboxyl group-containing non-crosslinked polymer salt R-1 (lithium salt, degree of neutralization 60 mol%). The solids concentration of the aqueous solution was 18.9% by mass. Solid-state NMR measurement revealed that R-1 contained 59% by mass of structural units derived from AA, 11% by mass of structural units derived from AN, and 30% by mass of structural units derived from AAm, based on the total structural units of R-1. Aqueous GPC measurement revealed that Mn of R-1 was 5,000, Mw was 1,270,000, and Mw / Mn was 254. I of R-1 based on pyrolysis GC / MS method １ The value is 7.12 x 10 ８ , I ２ The value is 5.14 x 10 １２ , W １ The value was 0.11, and the characteristic value X was calculated to be 0.126.

[0088] (Production Examples 2 to 22: Production of Carboxyl Group-Containing Non-Crosslinked Polymer Salts R-2 to R-22) Aqueous solutions of carboxyl group-containing non-crosslinked polymer salts R-2 to R-22 were obtained by the same procedure as in Production Example 1, except that the amounts of each raw material charged were as shown in Tables 1 and 2. Tables 1 and 2 show the solids concentrations of the above aqueous solutions, the compositions of R-2 to R-22 determined by solid-state NMR measurement, Mn, Mw, and Mw / Mn of R-2 to R-22 determined by aqueous GPC measurement, and characteristic value X of R-2 to R-22 determined by pyrolysis GC / MS.

[0089] Comparative Production Example 1: Production of Carboxyl Group-Containing Non-Crosslinked Polymer Salt R-23 An aqueous solution of carboxyl group-containing non-crosslinked polymer salt R-23 was obtained by the same procedure as in Production Example 1, using the amounts of each raw material as shown in Table 2. The solids concentration of the aqueous solution, the composition of R-23 measured by solid-state NMR, and Mn, Mw, and Mw / Mn of R-23 measured by aqueous GPC are shown in Table 2. The I of R-23 based on pyrolysis GC / MS method １ The value is 1.02 x 10 ８ , I ２ The value is 5.14 x 10 １２ , W １ The value was 0.11, and the characteristic value X was calculated to be 0.018.

[0090]

[0091]

[0092] The details of the compounds used in Tables 1 and 2 are as follows: AA: acrylic acid AAm: acrylamide AN: acrylonitrile V-50: 2,2'-azobis(2-methylpropionamidine) dihydrochloride (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) LiOH.H ２ O: Lithium hydroxide monohydrate, NaOH: Sodium hydroxide, KOH: Potassium hydroxide

[0093] Example 1 (Preparation of electrode mixture layer composition) As the active material, artificial graphite (manufactured by Showa Denko K.K., trade name "SCMG-CF") and SiO (5 μm, manufactured by Osaka Titanium Technologies Co., Ltd.) were used. As the binder, an aqueous solution of carboxyl group-containing non-crosslinked polymer salt R-1, a mixture of styrene / butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC) was used. Water was used as a dilution solvent in a planetary mixer (Hibismix 2P-03 manufactured by Primix Corporation) at a mass ratio of artificial graphite:SiO:R-1:SBR = 76.8:19.2:3.0:1.0: (solids) so that the solids concentration of the electrode mixture layer composition was 50 mass%, and the mixture was mixed for 1 hour and 30 minutes to prepare a slurry-state electrode mixture layer composition (electrode slurry).

[0094] (Preparation of negative electrode plate) Next, the electrode slurry was applied onto a current collector (copper foil, thickness: 16.5 μm) using a variable applicator, and dried in a ventilated dryer at 80° C. for 15 minutes to form a mixture layer. After that, the thickness of the mixture layer was 50±5 μm and the mixture density was 1.60±0.10 g / cm. ３ After rolling to a thickness of 1.0 cm × 6.0 cm, a negative electrode plate was obtained by punching out a 1.0 cm × 6.0 cm piece for a peel strength test and a 3 cm square piece for battery evaluation. The 90° peel strength between the mixture layer and the current collector (i.e., the binder binding property) of the negative electrode plate was measured.

[0095] (Preparation of Positive Electrode Plate) In N-methylpyrrolidone (NMP) solvent, LiNi was used as a positive electrode active material. ０．５ Co ０．２ Mn ０．３ O ２ 100 parts of NCM (Natural Carbon Monomer) and 2 parts of acetylene black were mixed and added, and 4 parts of polyvinylidene fluoride (PVDF) was mixed as a positive electrode binder to prepare a positive electrode composite layer composition. Next, using a variable applicator, the positive electrode composite layer composition was applied to a current collector (aluminum foil, thickness: 20 μm) and dried to form a composite layer. Thereafter, the thickness of the composite layer was 125 μm ± 1 μm, and the composite density was 3.0 ± 0.10 g / cm ３ After rolling to a thickness of 1 / 4", the mixture was punched out into a 3 cm square to obtain a positive electrode plate for battery evaluation.

[0096] (Preparation of Electrolyte Solution) Vinylene carbonate (VC) was added to a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio of EC:DMC=3:7) so that the concentration of VC was 1 mass % and fluoroethylene carbonate (FEC) was 2 mass %, and LiPF ６ was dissolved in a concentration of 1.2 mol / L to prepare a non-aqueous electrolyte.

[0097] (Preparation of Secondary Battery) The battery was constructed by attaching lead terminals to the positive and negative electrodes, and placing the electrodes facing each other via a separator (made of polyethylene: film thickness 16 μm, porosity 47%), placing them in an aluminum laminate battery exterior, injecting the electrolyte, and sealing them to prepare a test battery. The design capacity of this prototype battery was 50 mAh. The design capacity of the battery was based on a charge cut-off voltage of 4.2 V.

[0098] <Evaluation of 90° Peel Strength (Adhesion)> The mixture layer surface of the negative electrode plate of 100 mm x 25 mm size was attached to a 120 mm x 30 mm acrylic plate via double-sided tape (Nichiban Co., Ltd., Nicetack NW-20) to prepare a peel test sample. After drying at 60 ° C. under reduced pressure overnight, a tensile tester (ORIENTEC Co., Ltd., Tensilon Universal Testing Machine RTE-1210) was used to perform 90 ° peeling at a measurement temperature of 25 ° C. and a tensile speed of 50 mm / min. The adhesion was evaluated by measuring the peel strength between the mixture layer and the copper foil. The peel strength was 29.2 N / m, and the adhesion based on the following criteria was rated as "A". [Adhesion evaluation criteria] A: Peel strength is 29.0 N / m or more B: Peel strength is 25.0 N / m or more and less than 29.0 N / m C: Peel strength is 20.0 N / m or more and less than 25.0 N / m D: Peel strength is less than 20.0 N / m

[0099] <Evaluation of cycle characteristics> Furthermore, charging and discharging were repeated in an environment of 25° C. under CC discharge conditions of 2.5 to 4.2 V at a charge / discharge rate of 0.5 C. After 100 cycles, the capacity C １００ The cycle characteristics (ΔC) were calculated using the following formula: ΔC = C １００ / C ０× 100 (%) The ΔC calculated by the above formula was 86.1%, and the cycle characteristics based on the following criteria were evaluated as "A". Note that a higher ΔC value indicates better cycle characteristics. (Criteria for Cycle Characteristics Evaluation) A: Charge / discharge capacity retention rate is 86.0% or more B: Charge / discharge capacity retention rate is 84.0% or more and less than 86.0% C: Charge / discharge capacity retention rate is 82.0% or more and less than 84.0% D: Charge / discharge capacity retention rate is less than 82.0%

[0100] <<Overall Evaluation>> An overall evaluation was made based on the evaluation results of the binding ability and cycle characteristics, and on the criteria shown in Table 3 below. In this evaluation, evaluations A to C are acceptable levels. The carboxyl group-containing non-crosslinked polymer salt R-1 was evaluated as "A" for binding ability and "A" for cycle characteristics, and therefore the overall evaluation was given as "A."

[0101]

[0102] Examples 2 to 23 and Comparative Example 1 Electrode slurries were prepared in the same manner as in Example 1, except that the compositions were as shown in Tables 4 and 5. The cycle characteristics of the batteries containing negative electrode plates obtained using the electrode slurries were evaluated. The results are shown in Tables 4 and 5.

[0103]

[0104]

[0105] Details of the compounds used in Tables 4 and 5 are shown below: SBR: styrene butadiene rubber CMC: sodium carboxymethyl cellulose

[0106] <Evaluation Results> As is clear from the results of Examples 1 to 23, the secondary batteries using the carboxyl group-containing non-crosslinked polymer salt of the present invention were excellent in binding properties and cycle characteristics of the secondary batteries. This is presumably because the carboxyl group-containing non-crosslinked polymer contains a nitrile group-containing ethylenically unsaturated monomer (monomer (b)), and the presence of a specific amount of moieties in which three or more structural units derived from monomer (b) are linked improves the adsorption to the active material.

[0107] Furthermore, the larger the characteristic value X, which indicates the relative abundance ratio of "sites in which three or more structural units derived from monomer (b) are linked" in the present non-crosslinked polymer salt (Example 1: 0.126 > Example 2: 0.090 > Example 3: 0.054), the better the binding properties and cycle characteristics of the secondary battery. This is presumably because the large amount of sites in which three or more structural units derived from monomer (b) are linked in the present non-crosslinked polymer improves the adsorption to the active material. When the characteristic value X was even larger (Example 1: 0.126 < Example 4: 0.189 < Example 5: 0.450), the binding properties were excellent, but the cycle characteristics of the secondary battery deteriorated. This is presumably because, when the amount of sites in which three or more structural units derived from monomer (b) in the present non-crosslinked polymer are linked further increases, the adsorption to the active material improves, resulting in excellent binding properties, whereas, when the amount of such sites, which are highly hydrophobic, increases, the solubility in water decreases, making it difficult for the present non-crosslinked polymer salt to disperse in the electrode slurry, leading to a deterioration in the cycle characteristics of the secondary battery.

[0108] Furthermore, when focusing on the structural units derived from the amide group-containing ethylenically unsaturated monomer (monomer (c)), the case in which the structural units derived from monomer (c) were contained (Example 14) resulted in better binding strength and secondary battery cycle characteristics than the case in which the structural units derived from monomer (c) were not contained (Example 16). This is presumably because the presence of the structural units derived from monomer (c) further improved the adsorption to the active material.

[0109] Furthermore, when the content of the structural unit derived from the monomer (b) was considered, when the content was 15% by mass or 29% by mass (Examples 17 and 18), the binding property was superior to that when the content was 2% by mass (Example 20). This is presumably because the higher the proportion of nitrile groups present in the non-crosslinked polymer, the better the adsorption property to the active material.

[0110] In addition, when focusing on Mn, when Mn was 100,000 or less (Examples 8 and 9), the cycle characteristics of the secondary battery were more excellent than when Mn was more than 100,000 (Examples 10 and 11). When focusing on Mw / Mn, when the distribution was 10 or more and 280 or less (Examples 1 to 3, 8, and 9: in the range of 17.1 to 254), the cycle characteristics of the secondary battery were more excellent than when it was less than 10 (Example 10: 9.2, Example 11: 5.1). This is presumably because increasing Mw / Mn allows the coexistence of a high-molecular-weight component that improves the binding strength with the active material and current collector and a low-molecular-weight component that improves the dispersibility of the active material, allowing each component to exert its respective functions.

[0111] In contrast to these, when the characteristic value X of the carboxyl group-containing non-crosslinked polymer salt was less than 0.050 (Comparative Example 1), the binding property and cycle characteristics were significantly deteriorated.

[0112] Secondary batteries containing the binder for secondary battery electrodes of the present invention exhibit good durability (cycling characteristics). 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 and discharge, and are expected to contribute to the development of high-capacity automotive secondary batteries. The binder for secondary battery electrodes of the present invention 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 secondary battery electrodes containing a carboxyl group-containing non-crosslinked polymer or a salt thereof, wherein the carboxyl group-containing non-crosslinked polymer contains a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter referred to as "monomer (a)") and a structural unit derived from a nitrile group-containing ethylenically unsaturated monomer (hereinafter referred to as "monomer (b)"), and the carboxyl group-containing non-crosslinked polymer or salt thereof has a value (X) calculated by the following mathematical formula (1) of 0.050 or more and 0.500 or less.

2. The binder for secondary battery electrodes according to claim 1, wherein the carboxyl group-containing non-crosslinked polymer contains structural units derived from the monomer (b) in an amount of 1% by mass or more and 50% by mass or less relative to the total structural units of the polymer.

3. The binder for secondary battery electrodes according to claim 1 or 2, wherein the carboxyl group-containing non-crosslinked polymer further contains a structural unit derived from an amide group-containing ethylenically unsaturated monomer (hereinafter referred to as "monomer (c)").

4. The binder for secondary battery electrodes according to claim 3, wherein the carboxyl group-containing non-crosslinked polymer contains, relative to its total structural units, 40% by mass or more and 98% by mass or less of structural units derived from the monomer (a), 1% by mass or more and 50% by mass or less of structural units derived from the monomer (b), and 1% by mass or more and 50% by mass or less of structural units derived from the monomer (c).

5. The binder for secondary battery electrodes according to claim 1 or 2, wherein the carboxyl group-containing non-crosslinked polymer or its salt has a number average molecular weight of 4,000 or more and 400,000 or less.

6. A binder for secondary battery electrodes according to claim 1 or 2, wherein the salt of the carboxyl group-containing non-crosslinked polymer is a salt in which 40 mol % or more of the carboxyl groups possessed by the non-crosslinked polymer have been neutralized.

7. A method for producing a binder for secondary battery electrodes containing a carboxyl group-containing non-crosslinked polymer or a salt thereof, wherein the carboxyl group-containing non-crosslinked polymer contains a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter referred to as "monomer (a)") and a structural unit derived from a nitrile group-containing ethylenically unsaturated monomer (hereinafter referred to as "monomer (b)"), and the method comprises a polymerization step of polymerizing monomer components containing the monomer (a) and the monomer (b), wherein in the polymerization step, polymerization is carried out while continuously or intermittently supplying at least a portion of the monomer (a).

8. The method of claim 7, wherein the carboxyl group-containing non-crosslinked polymer or its salt has a value (X) calculated by the following formula (1) of 0.050 or more and 0.500 or less.

9. The manufacturing method described in claim 7 or 8, wherein the carboxyl group-containing non-crosslinked polymer contains structural units derived from the monomer (b) in an amount of 1% by mass or more and 50% by mass or less relative to the total structural units of the polymer.

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

11. A secondary battery electrode comprising a current collector having a mixture layer formed from the composition for a secondary battery electrode mixture layer according to claim 10 on the surface thereof.

12. A secondary battery comprising the secondary battery electrode according to claim 11.

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