Salt of carboxyl group-containing crosslinked polymer and use of same

Abstract

Disclosed is a salt of a carboxyl group-containing crosslinked polymer, the salt containing an amine salt of a carboxyl group-containing crosslinked polymer, wherein an amine that constitutes the amine salt has a nitrile group. The salt of a carboxyl group-containing crosslinked polymer has a value (Z) calculated by mathematical formula (1) of 0.5 to 100 inclusive.　(1): Z = N1 / N2 × 100 N1: am amine salt of a carboxyl group-containing crosslinked polymer (mol%) N2: an amine salt of a carboxyl group-containing crosslinked polymer and an alkali metal salt of a carboxyl group-containing crosslinked polymer (mol%)

Description

Salts of carboxyl group-containing crosslinked polymers and their uses

[0001] This invention relates to salts of carboxyl group-containing crosslinked polymers and their uses.

[0002] Salts of carboxyl group-containing polymers are used in a variety of applications, including thickeners and viscosity modifiers for cosmetics, binders for non-aqueous electrolyte secondary battery electrodes, anti-settling agents for pigments, and dispersion stabilizers for metal powders. Among these applications, as a thickener for cosmetics, carboxyl group-containing polymer salts exhibit stringiness and stickiness when they are linear, but as the degree of crosslinking increases, the stringiness decreases and a refreshing feeling emerges. Therefore, in cosmetics where stringiness is not desired and a refreshing feel is required, salts of carboxyl group-containing crosslinked polymers are often used because they offer the advantage of providing high viscosity with only a small amount.

[0003] Furthermore, carboxyl group-containing crosslinked polymer salts are often used as binders for non-aqueous electrolyte secondary battery electrodes due to their advantages in providing good binding properties and cycle characteristics. As the applications of various secondary batteries, such as nickel-metal hydride secondary batteries, lithium-ion secondary batteries, and electric double-layer capacitors, expand, there is a growing demand for improved energy density, reliability, and durability. For example, to increase the electrical capacity of lithium-ion secondary batteries, there is an increasing trend to use silicon-based active materials as the negative electrode active material. However, silicon-based active materials are known to undergo large volume changes during charging and discharging, and repeated use can lead to peeling or detachment of the electrode mixture layer, resulting in a decrease in battery capacity and deterioration of cycle characteristics (durability). To suppress such problems, studies are being conducted to improve durability by firmly binding the active materials together with a binder (binding properties), reducing the size of the active materials to alleviate stress associated with swelling and shrinkage, and by devising additives for the electrolyte.

[0004] In this context, it has been reported that salts of crosslinked acrylic acid polymers are effective as binders that have good cycle characteristics and are effective in improving the durability of the negative electrode mixture layer using silicon-based active materials. For example, Patent Document 1 discloses a binder containing a sodium salt of a crosslinked acrylic acid polymer, in which polyacrylic acid is crosslinked with a specific crosslinking agent, and discloses that even when an active material containing silicon is used, the electrode structure is not destroyed and good cycle characteristics are observed.

[0005] International Publication No. 2014 / 065407

[0006] According to the inventors' studies, when using a micro-crosslinked acrylic acid polymer disclosed in Patent Document 1 as a binder for particles in a slurry (for example, a binder for the active material in a lithium-ion secondary battery electrode slurry), while micro-crosslinking of the acrylic acid polymer can improve the binding properties between particles in the slurry, it also increases the spread of the polymer in water, causing a significant increase in viscosity even with small amounts of addition. This limits the reduction of slurry viscosity, making it difficult to achieve both coating properties and coating film performance (for example, the cycle characteristics of a lithium-ion secondary battery), which can be problematic.

[0007] Furthermore, while the binders for secondary battery electrodes disclosed in Patent Document 1 can all provide good cycle characteristics and binding properties, the demand for binders that can further improve cycle characteristics is increasing as the performance of secondary batteries improves. Moreover, generally, secondary battery electrodes are obtained by coating and drying an electrode mixture layer composition (hereinafter also referred to as "electrode slurry") containing an active material and a binder onto the surface of an electrode current collector. In this case, it is advantageous to increase the solid content concentration of the electrode slurry from the viewpoint of increasing the drying efficiency of the electrode slurry and improving the productivity of electrodes. However, as is usually the case, it becomes difficult to ensure good coating properties as the solid content concentration increases. As mentioned above, the binder disclosed in Patent Document 1 has a large increase in viscosity even with the addition of a small amount, making it difficult to increase the solid content concentration.

[0008] The present invention has been made in view of these circumstances, and its objective is to provide a salt of a carboxyl group-containing crosslinked polymer that can achieve both coating properties and coating film performance in a composition containing the salt of the said crosslinked polymer. Furthermore, the present invention aims to provide a binder for secondary battery electrodes that can obtain a secondary battery exhibiting excellent cycle characteristics while ensuring coating properties by reducing the viscosity of the electrode slurry, even when the solid content concentration of the electrode composite layer composition is higher than conventional methods. In addition, the present invention aims to provide a secondary battery electrode composite layer composition containing the above binder, a secondary battery electrode obtained using the said composition, and a secondary battery.

[0009] As a result of diligent research to solve the above problems, the present inventors have found that in a salt of a carboxyl group-containing crosslinked polymer containing an amine salt of the carboxyl group-containing crosslinked polymer, the amine constituting the amine salt has a nitrile group, and by setting the characteristic value related to the content of the amine salt within a specific range, it is possible to achieve both coating properties and coating film performance in a composition containing the salt of the crosslinked polymer, thus completing the present invention.

[0010] The present invention is as follows: [1] A salt of a carboxyl group-containing crosslinked polymer comprising an amine salt of the carboxyl group-containing crosslinked polymer, wherein the amine constituting the amine salt has a nitrile group, and the salt of the carboxyl group-containing crosslinked polymer has a value (Z) calculated by the following formula (1) of 0.5 or more and 100 or less. Z = N １ / N ２ ×100 (1) N １ : Amine salt of carboxyl group-containing crosslinked polymer (mol%) N ２: Amine salts of carboxyl group-containing crosslinked polymers and alkali metal salts (mol%) of carboxyl group-containing crosslinked polymers [2] A salt of the carboxyl group-containing crosslinked polymer according to [1], wherein the carboxyl group-containing crosslinked polymer contains 50% by mass or more and 100% by mass of structural units derived from ethylenically unsaturated carboxylic acid monomers with respect to its total structural units. [3] A salt of the carboxyl group-containing crosslinked polymer according to [1] or [2], wherein 60 mol% or more of the carboxyl groups of the carboxyl group-containing crosslinked polymer are neutralized. [4] A salt of the carboxyl group-containing crosslinked polymer according to any one of [1] to [3], wherein the amine constituting the amine salt is a monofunctional amine. [5] A salt of the carboxyl group-containing crosslinked polymer according to any one of [1] to [4], wherein the carboxyl group-containing crosslinked polymer is a crosslinked polymer obtained by polymerizing a monomer composition containing a non-crosslinkable monomer and a crosslinkable monomer. [6] A salt of the carboxyl group-containing crosslinked polymer according to any one of [1] to [5], wherein the carboxyl group-containing crosslinked polymer is crosslinked with a crosslinkable monomer, and the amount of the crosslinkable monomer used is 0.03 mol% or more and 0.8 mol% or less relative to the total amount of the non-crosslinkable monomer. [7] A binder for secondary battery electrodes containing a salt of the carboxyl group-containing crosslinked polymer according to any one of [1] to [6]. [8] A composition for a secondary battery electrode mixture layer comprising the binder for secondary battery electrodes according to [7], an active material, and water. [9] A secondary battery electrode having a mixture layer formed from the composition for a secondary battery electrode mixture layer according to [8] on the surface of a current collector.

[10] A secondary battery comprising the secondary battery electrode according to [9].

[0011] The present invention provides a salt of a carboxyl group-containing crosslinked polymer that enables both good coating properties and good coating film performance in a composition containing the salt of the crosslinked polymer. Furthermore, the present invention provides a binder for secondary battery electrodes that contains a salt of a carboxyl group-containing crosslinked polymer that enables the creation of a secondary battery exhibiting excellent cycle characteristics while ensuring coating properties by reducing the viscosity of the electrode slurry, even when the solid content concentration of the electrode mixture layer composition is higher than conventional methods.

[0012] The salt of the carboxyl group-containing crosslinked polymer of the present invention (hereinafter also referred to as "the crosslinked polymer salt") contains an amine salt of the carboxyl group-containing crosslinked polymer, the amine constituting the amine salt has a nitrile group, and the value (Z) calculated by the above formula (1) is 0.5 or more and 100 or less.

[0013] Furthermore, the binder for secondary battery electrodes containing the crosslinked polymer salt (hereinafter also referred to as "the binder") can be mixed with an active material and water to form a composition for a secondary battery electrode composite layer (hereinafter also referred to as "the composition"). The above composition is in a slurry state that can be applied to a current collector. By forming a composite layer formed from the above composition on the surface of a current collector such as copper foil or aluminum foil, the secondary battery electrode of the present invention can be obtained. Here, the binder is particularly preferred when used in a composition for a secondary battery electrode composite layer containing a silicon-based active material described later as the active material, as this is because the effects of the present invention are especially significant.

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

[0015] 1. This Crosslinked Polymer <Structural Units Derived from Ethylene-Unsaturated Carboxylic Acid Monomers> This crosslinked polymer may have structural units (hereinafter also referred to as "component (a)") derived from ethylenically unsaturated carboxylic acid monomers (hereinafter also referred to as "monomer (a)"), and monomer components containing monomer (a) can be introduced into the polymer by precipitation polymerization or dispersion polymerization. Because this crosslinked polymer has carboxyl groups due to the presence of such structural units, its adhesion to the current collector is improved, and it exhibits excellent lithium ion desolvation effect and ionic conductivity, resulting in electrodes with low resistance and excellent high-rate characteristics. Furthermore, because it is given water-swellable properties, the dispersion stability of active materials, etc., in this composition can be improved.

[0016] Examples of monomer (a) include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid; (meth)acrylamide alkyl carboxylic acids such as (meth)acrylamidehexanoic acid and (meth)acrylamidedodecanoic acid; ethylenically unsaturated monomers having a carboxyl group such as monohydroxyethyl (meth)acrylate succinate, ω-carboxy-caprolactone mono(meth)acrylate, 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 the above, compounds having an acryloyl group as a polymerizable functional group are preferred because they yield polymers with a long primary chain length due to their high polymerization rate and good binder binding strength, and acrylic acid is particularly preferred. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, polymers with a high carboxyl group content can be obtained.

[0017] The content of component (a) in the present crosslinked polymer is not particularly limited. For example, it can be contained in an amount of 10% by mass or more and 100% by mass or less based on all the structural units of the present crosslinked polymer. By containing component (a) within such a range, excellent adhesiveness to the current collector can be easily ensured. The lower limit is, for example, 20% by mass or more, and for example, 30% by mass or more, and for example, 40% by mass or more. When the lower limit is 50% by mass or more, the dispersion stability of the present composition becomes good, and a higher adhesive force can be obtained, which is preferable. It may be 60% by mass or more, 70% by mass or more, or 80% by mass or more. The upper limit is, for example, 99.9% by mass or less, and for example, 99.5% by mass or less, and for example, 99% by mass or less, and for example, 98% by mass or less, and for example, 95% by mass or less, and for example, 90% by mass or less, and for example, 80% by mass or less.

[0018] <Other structural units> The present crosslinked polymer can contain, in addition to component (a), structural units (hereinafter also referred to as "component (b)") derived from other ethylenically unsaturated monomers copolymerizable with these (hereinafter also referred to as "monomer (b)"). Examples of monomer (b) include ethylenically unsaturated monomer compounds having anionic groups other than carboxyl groups such as sulfonic acid groups and phosphoric acid groups, or nonionic ethylenically unsaturated monomers. Component (b) can be introduced by copolymerizing a monomer containing an ethylenically unsaturated monomer compound having anionic groups other than carboxyl groups such as sulfonic acid groups and phosphoric acid groups, or a nonionic ethylenically unsaturated monomer.

[0019] The proportion of component (b) can be 0% by mass or more and 90% by mass or less based on all the structural units of the present crosslinked polymer. The proportion of component (b) may be 1% by mass or more and 60% by mass or less, 2% by mass or more and 50% by mass or less, 5% by mass or more and 40% by mass or less, or 10% by mass or more and 30% by mass or less. When component (b) is contained in an amount of 1% by mass or more based on all the structural units of the present crosslinked polymer, the affinity for the electrolyte is improved, and thus an effect of improving lithium ion conductivity can also be expected.

[0020] (b) Among the above, structural units derived from nonionic ethylenically unsaturated monomers are preferred as components, from the viewpoint of obtaining electrodes with good flexibility. Examples of nonionic ethylenically unsaturated monomers include (meth)acrylamide and its derivatives, nitrile group-containing ethylenically unsaturated monomers, and alicyclic structure-containing ethylenically unsaturated monomers.

[0021] 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 N-n-butoxymethyl (meth)acrylamide and N-isobutoxymethyl (meth)acrylamide; and N,N-dialkyl (meth)acrylamide compounds such as dimethyl (meth)acrylamide and diethyl (meth)acrylamide. One of these may be used alone, or two or more may be used in combination.

[0022] Examples of nitrile group-containing ethylenically unsaturated monomers include (meth)acrylonitrile; cyanoalkyl (meth)acrylate compounds such as cyanomethyl (meth)acrylate and cyanoethyl (meth)acrylate; cyano group-containing unsaturated aromatic compounds such as 4-cyanostyrene and 4-cyano-α-methylstyrene; and vinylidene cyanide. One of these may be used alone, or two or more may be used in combination. Among the above, acrylonitrile is preferred because of its high nitrile group content.

[0023] Examples of the alicyclic structure-containing ethylenically unsaturated monomer include (meth)acrylic acid cycloalkyl esters which may have aliphatic substituents 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, dicyclopentenyl oxyethyl (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 thereof may be used in combination.

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

[0025] In addition, other nonionic ethylenically unsaturated monomers may be used, for example, (meth)acrylic acid esters. Examples of (meth)acrylic acid esters include alkyl (meth)acrylic acid 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 alkoxyalkyl (meth)acrylic acid ester compounds such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate. One of these may be used alone, or two or more may be used in combination.

[0026] From the viewpoint of binding to the active material and cycle characteristics, aromatic (meth)acrylic acid ester compounds can be preferably used. From the viewpoint of further improving lithium ion conductivity and high-rate characteristics, compounds having an ether bond, such as alkoxyalkyl (meth)acrylate esters like 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate, are preferred, and 2-methoxyethyl (meth)acrylate is more preferred.

[0027] Among nonionic ethylenically unsaturated monomers, compounds having an acryloyl group are preferred because they have a fast polymerization rate, resulting in polymers with long primary chain lengths and good binder binding strength. Furthermore, among nonionic ethylenically unsaturated monomers, compounds with a homopolymer glass transition temperature (Tg) of 0°C or lower are preferred because they result in good flexural resistance of the resulting electrodes.

[0028] This crosslinked polymer is a crosslinked polymer having a crosslinked structure. The method of crosslinking in this crosslinked polymer is not particularly limited, and examples include the following methods: 1) Copolymerization of crosslinkable monomers 2) Utilization of chain transfer to polymer chains during radical polymerization 3) After synthesizing a polymer having reactive functional groups, a crosslinking agent is added as needed to crosslink the crosslinked polymer. Because this crosslinked polymer has a crosslinked structure, the crosslinked polymer or a binder containing a salt of the crosslinked polymer can have excellent binding strength. Among the above, the method by copolymerization of crosslinkable monomers is preferred because it is easy to operate and the degree of crosslinking can be easily controlled.

[0029] <Crossable Monomers> Examples of crosslinkable monomers include polyfunctional polymerizable monomers having two or more polymerizable unsaturated groups, and monomers having self-crosslinkable functional groups such as hydrolyzable silyl groups.

[0030] The above-mentioned polyfunctional polymerizable monomers are compounds having two or more polymerizable functional groups such as (meth)acryloyl groups and alkenyl groups in their molecules, and include polyfunctional (meth)acrylate compounds, polyfunctional alkenyl compounds, and compounds having both (meth)acryloyl and alkenyl groups. These compounds may be used individually or in combination of two or more. Among these, polyfunctional alkenyl compounds are preferred because they easily yield a uniform crosslinked structure, and polyfunctional allyl ether compounds having two or more allyl ether groups in their molecules are particularly preferred.

[0031] Examples of polyfunctional (meth)acrylate compounds include di(meth)acrylates of dihydric alcohols such as ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, polyethylene glycol di(meth)acrylate, and polypropylene glycol di(meth)acrylate; tri(meth)acrylates of trihydric or higher polyhydric alcohols such as trimethylolpropane tri(meth)acrylate, tri(meth)acrylate of trimethylolpropane ethylene oxide modified product, glycerin tri(meth)acrylate, pentaerythritol tri(meth)acrylate, and pentaerythritol tetra(meth)acrylate; and bisamides such as methylenebisacrylamide and hydroxyethylenebisacrylamide.

[0032] Examples of polyfunctional alkenyl compounds include polyfunctional allyl ether compounds such as trimethylolpropanediallyl ether, trimethylolpropanetrialyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, tetraallyloxyethane, and polyallyl saccharose; polyfunctional allyl compounds such as diallyl phthalate; and polyfunctional vinyl compounds such as divinylbenzene.

[0033] 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.

[0034] Specific examples of monomers having self-crosslinkable functional groups include hydrolyzable silyl group-containing vinyl monomers, N-methylol(meth)acrylamide, and N-methoxyalkyl(meth)acrylate. These compounds can be used individually or in combination of two or more.

[0035] The hydrolyzable silyl group-containing vinyl monomer is not particularly limited as long as it is a vinyl monomer having at least one hydrolyzable silyl group. Examples include vinylsilanes such as vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane, and vinyldimethylmethoxysilane; silyl group-containing acrylic acid esters such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, and methyldimethoxysilylpropyl acrylate; silyl group-containing methacrylic acid esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, and dimethylmethoxysilylpropyl methacrylate; silyl group-containing vinyl ethers such as trimethoxysilylpropyl vinyl ether; and silyl group-containing vinyl esters such as vinyl trimethoxysilylundecanoate.

[0036] When the crosslinked polymer is crosslinked with a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.1 parts by mass or more and 2.0 parts by mass or less, more preferably 0.2 parts by mass or more and 1.8 parts by mass or less, even more preferably 0.3 parts by mass or more and 1.6 parts by mass or less, even more preferably 0.5 parts by mass or more and 1.4 parts by mass or less, and even more preferably 0.6 parts by mass or more and 1.2 parts by mass or less, based on 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer). If the amount of crosslinkable monomer used is 0.1 parts by mass or more, it is preferable in that, during use over a longer period than conventional methods, expansion and contraction due to charging and discharging are suppressed, and the conductive paths between the active materials are well maintained, resulting in an excellent charge-discharge capacity retention rate. If it is 2.0 parts by mass or less, the stability of precipitation polymerization or dispersion polymerization tends to be higher. In particular, if it is 1.0 part by mass or less, the water-swollen particle size in the electrode slurry becomes suitable, and the bonding area to the active material becomes larger, which is preferable in that excellent battery performance can be maintained even during long-term use.

[0037] For similar reasons, the amount of the above-mentioned crosslinkable monomer used is preferably 0.03 mol% to 0.8 mol%, more preferably 0.05 mol% to 0.7 mol%, even more preferably 0.075 mol% to 0.6 mol%, even more preferably 0.1 mol% to 0.5 mol%, and even more preferably 0.2 mol% to 0.4 mol% based on the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer).

[0038] <Method for Producing This Crosslinked Polymer> This crosslinked polymer can be produced using known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization, but precipitation polymerization and suspension polymerization (reverse-phase suspension polymerization) are preferred in terms of productivity. Heterogeneous polymerization methods such as precipitation polymerization, suspension polymerization, and emulsion polymerization are preferred in terms of obtaining better performance in terms of binding properties, and among these, precipitation polymerization is more preferred. Precipitation polymerization is a method of producing a polymer by carrying out a polymerization reaction in a solvent that dissolves the raw material unsaturated monomers but does not substantially dissolve the polymer to be produced. As polymerization progresses, the polymer particles become larger due to aggregation and growth, and a dispersion of polymer particles is obtained in which primary particles of tens to hundreds of nanometers are secondary aggregated to several micrometers to tens of micrometers. Dispersion stabilizers can also be used to control the particle size of the polymer. Furthermore, secondary aggregation can be suppressed by selecting the dispersion stabilizer and polymerization solvent. Generally, precipitation polymerization in which secondary aggregation is suppressed is also called dispersion polymerization.

[0039] This crosslinked polymer may contain 50% to 100% by mass of structural units derived from ethylenically unsaturated carboxylic acid monomers relative to its total structural units, and the preferred range and type of such structural units are as described above.

[0040] In precipitation polymerization, the polymerization solvent can be selected from water and various organic solvents, taking into consideration the type of monomer used. To obtain polymers with longer primary chain lengths, it is preferable to use a solvent with a small chain transfer constant.

[0041] Specific polymerization solvents include water-soluble solvents such as methanol, t-butyl alcohol, acetone, methyl ethyl ketone, acetonitrile, and tetrahydrofuran, as well as benzene, ethyl acetate, dichloroethane, n-hexane, cyclohexane, and n-heptane. These can be used individually or in combination of two or more. Alternatively, they may be used as a mixed solvent with water. In this invention, a water-soluble solvent refers to one whose solubility in water at 20°C is greater than 10 g / 100 ml. Among the above, methyl ethyl ketone and acetonitrile are preferred because they produce fewer coarse particles and adhere less to the reactor, resulting in good polymerization stability; the precipitated polymer fine particles are less prone to secondary aggregation (or even if secondary aggregation occurs, they dissolve easily in the aqueous medium); the chain transfer constant is small, resulting in a polymer with a high degree of polymerization (primary chain length); and the neutralization process described later is easy to handle.

[0042] The polymerization initiator can be any known polymerization initiator such as azo compounds, organic peroxides, or inorganic peroxides, but is not particularly limited. The usage conditions can be adjusted to achieve an appropriate amount of radical generation using known methods such as thermal initiation, redox initiation with a reducing agent, or UV initiation. In order to obtain a crosslinked polymer with a long primary chain length, it is preferable to set the conditions so that the amount of radical generation is reduced as much as possible within the acceptable range of production time.

[0043] The preferred amount of polymerization initiator to use 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 parts by mass, when the total amount of monomer components used is 100 parts by mass. If the amount of polymerization initiator used is 0.001 parts by mass or more, the polymerization reaction can be carried out stably, and if it is 2 parts by mass or less, it is easy to obtain a polymer with a long primary chain length.

[0044] The polymerization temperature is preferably 0 to 100°C, and more preferably 20 to 80°C, although this depends on conditions such as the type and concentration of monomers used. The polymerization temperature may be constant or may change during the polymerization reaction. The polymerization time is preferably 1 minute to 20 hours, and more preferably 1 hour to 10 hours.

[0045] 2. This crosslinked polymer salt This crosslinked polymer salt contains an amine salt in which some or all of the carboxyl groups contained in the crosslinked polymer are neutralized. The amine constituting the amine salt has a nitrile group, and the value (Z) calculated by the following formula (1) is 0.5 or more and 100 or less. Z = N １ / N ２ ×100 (1) N １ : Amine salt of carboxyl group-containing crosslinked polymer (mol%) N ２ : Amine salt of carboxyl group-containing crosslinked polymer and alkali metal salt of carboxyl group-containing crosslinked polymer (mol%)

[0046] As the value of Z above, in terms of excellent coating properties and cycle characteristics of the electrode slurry, it is preferably 0.6 or more and 80.0 or less, more preferably 0.7 or more and 60.0 or less, still more preferably 5.0 or more and 30.0 or less, and even more preferably 7.0 or more and 9.0 or less. The degree of neutralization (N １ ) of the nitrile group-containing amine salt of the crosslinked polymer is calculated according to the method described in the examples. Also, the degree of neutralization (N ２ ) of the nitrile group-containing amine salt and alkali metal salt of the crosslinked polymer is also calculated according to the method described in the examples.

[0047] The "nitrile group-containing amines" that constitute the amine salt of this crosslinked polymer are classified into "monofunctional amines" having one amino group per molecule and "polyfunctional amines" having two or more amino groups per molecule. Monofunctional amines are preferred because they offer excellent coating properties for electrode slurries and excellent cycle characteristics for secondary batteries. Specific examples of monofunctional amines include 3-aminopropionitrile, 3-methylaminopropionitrile, 3-(N-nitrosomethylamino)propionitrile, tris(2-cyanoethyl)amine, and cyanoethylcyclohexylamine. Specific examples of polyfunctional amines include cyanoethylmethylenediamine, cyanoethylethylenediamine, cyanoethylpropylenediamine, and cyanoethylbutylenediamine. Among these, 3-aminopropionitrile and 3-methylaminopropionitrile are preferred, and 3-aminopropionitrile is more preferred, because they allow for a high concentration of nitrile groups per unit mass of the amine salt of this crosslinked polymer.

[0048] Examples of alkali metal salts for this crosslinked polymer include alkali metal salts such as lithium salt, sodium salt, and potassium salt; alkaline earth metal salts such as calcium salt and barium salt; and other metal salts such as magnesium salt and aluminum salt. Among these, alkali metal salts and magnesium salts are preferred because they do not adversely affect battery characteristics, alkali metal salts are more preferred, and lithium salts are particularly preferred because they have excellent cycle characteristics for secondary batteries.

[0049] In this composition, the crosslinked polymer is preferably used in the form of a salt, with the acidic groups such as carboxyl groups derived from the ethylenically unsaturated carboxylic acid monomer neutralized to such an extent that the degree of neutralization is 20 mol% or more. The degree of neutralization is more preferably 50 mol% or more, even more preferably 70 mol% or more, even more preferably 75 mol% or more, even more preferably 80 mol% or more, and particularly preferably 85 mol% or more, from the viewpoint of excellent coating properties of the electrode slurry and cycle characteristics of the secondary battery. 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 is calculated according to the method described in the examples.

[0050] In this process, in order to ensure that the neutralization reaction proceeds stably and rapidly, it is preferable to add a small amount of a highly polar solvent to the polymerization solvent. Preferred highly polar solvents include water and methanol. The amount of highly polar solvent used is preferably 0.05 to 5.0% by mass, more preferably 0.1 to 2.5% by mass, even more preferably 0.1 to 1.0% by mass, and even more preferably 0.1 to 0.5% by mass, based on the total mass of the medium. If the proportion of the highly polar solvent is 0.05% by mass or more, an effect on the neutralization reaction is observed, and if it is 20.0% by mass or less, no adverse effect on the polymerization reaction is observed. Furthermore, in the polymerization of highly hydrophilic ethylenically unsaturated carboxylic acid monomers such as acrylic acid, the addition of a highly polar solvent improves the polymerization rate, making it easier to obtain polymers with longer primary chain lengths. Among highly polar solvents, water is particularly preferred due to its significant effect in improving the polymerization rate.

[0051] <Particle size of the crosslinked polymer salt> In this composition, it is preferable that the crosslinked polymer salt does not exist as large-particle clumps (secondary aggregates) but is well dispersed as water-swellable particles with an appropriate particle size, so that the binder containing the crosslinked polymer salt can exhibit good binding performance.

[0052] Preferably, when the crosslinked polymer is dispersed in water with a neutralization degree based on the carboxyl groups of the crosslinked polymer of 80 to 100 mol%, the particle size (water-swelled particle size) is in the range of 0.1 μm or more and 10.0 μm or less in volume-based median diameter. A more preferred range for the above particle size is 0.15 μm or more and 8.0 μm or less, an even more preferred range is 0.20 μm or more and 6.0 μm or less, an even more preferred range is 0.25 μm or more and 4.0 μm or less, and an even more preferred range is 0.30 μm or more and 2.0 μm or less. If the particle size is in the range of 0.30 μm or more and 2.0 μm or less, it will be uniformly present in the composition at a suitable size, thus enabling the composition to have high stability and exhibit excellent binding properties. If the particle size exceeds 10.0 μm, there is a risk that the binding properties will be insufficient as described above. Furthermore, there is a risk of insufficient coating properties due to the difficulty in obtaining a smooth coated surface. On the other hand, if the particle size is less than 0.1 μm, there are concerns from the viewpoint of stable manufacturing. The water-swollen particle size of this cross-linked polymer salt is measured according to the method described in the examples.

[0053] <Water swelling degree of this crosslinked polymer salt at pH 8> The water swelling degree of this crosslinked polymer salt at pH 8 is preferably 20 to 50. Within this range, both coating properties for current collectors and adhesion of the binder to the current collectors can be simultaneously satisfied. If the water swelling degree is less than 20, the adhesion may decrease and the cycle characteristics may deteriorate, and if the water swelling degree exceeds 50, the coating properties may deteriorate. A more preferable range for the water swelling degree at pH 8 is 21 to 48, an even more preferable range is 22 to 45, an even more preferable range is 23 to 35, and an even more preferable range is 24 to 30.

[0054] In this specification, the degree of water swelling is calculated from the dry mass of the crosslinked polymer salt "(WA) g" and the amount of water absorbed when the crosslinked polymer salt is saturated with water at pH 8 "(WB) g" based on the following formula (2): Degree of water swelling = {(WA) + (WB)} / (WA) (2)

[0055] The degree of water swelling at pH 8 can be obtained by measuring the degree of water swelling of the cross-linked polymer salt in water at pH 8. For example, deionized water can be used as the water at pH 8, and the pH value may be adjusted as needed using an appropriate acid, alkali, or buffer solution. The measurement should be performed at 25 ± 5°C.

[0056] Those skilled in the art can adjust the degree of water swelling of the crosslinked polymer salt by controlling its composition and structure. For example, the degree of water swelling can be increased by introducing acidic functional groups or highly hydrophilic structural units into the crosslinked polymer. Furthermore, the degree of water swelling can usually be increased by lowering the degree of crosslinking of the crosslinked polymer.

[0057] 3. Composition for Secondary Battery Electrode Mixture Layer The composition for secondary battery electrode mixture layer of the present invention comprises this binder, an active material, and water. The amount of this binder used in this composition is, for example, 0.1 parts by mass or more and 20 parts by mass or less, per 100 parts by mass of the total amount of active material. The above amount can also be, for example, 0.2 parts by mass or more and 10 parts by mass or more, for example, 0.3 parts by mass or more and 8 parts by mass or more, or for example, 0.4 parts by mass or more and 5 parts by mass or less. If the amount of binder used is 0.1 parts by mass or more, sufficient binding properties can be obtained. Furthermore, the dispersion stability of the active material can be ensured, and a uniform mixture layer can be formed. If the amount of binder used is 20 parts by mass or less, this composition will not become highly viscous, and coating properties for the current collector can be ensured. As a result, a mixture layer having a uniform and smooth surface can be formed.

[0058] Among the above active materials, lithium salts of transition metal oxides can be used as the positive electrode active material. For example, layered rock salt type and spinel type lithium-containing metal oxides can be used. Specific compounds of the layered rock salt type positive electrode active material include lithium cobaltate, lithium nickelate, and NCM{Li(Ni)}, which are called ternary systems. ｘ Co ｙ , Mn ｚ ), x+y+z=1} and NCA{Li(Ni １－ａ－ｂ Co ａ Al bExamples include )}. In addition, lithium manganate is an example of a spinel-type positive electrode active material. Besides oxides, phosphates, silicates, and sulfur can also be used, and examples of phosphates include olivine-type lithium iron phosphate. As a positive electrode active material, one of the above may be used alone, or two or more may be combined and used as a mixture or composite.

[0059] Furthermore, when a positive electrode active material containing layered rock salt-type lithium-containing metal oxide is dispersed in water, the dispersion becomes alkaline due to the exchange of lithium ions on the surface of the active material with hydrogen ions in the water. This may cause corrosion of common positive electrode current collector materials such as aluminum foil (Al). In such cases, it is preferable to neutralize the alkali leaching from the active material by using an unneutralized or partially neutralized crosslinked polymer as a binder. Moreover, it is preferable to use an amount of the unneutralized or partially neutralized crosslinked polymer such that the amount of unneutralized carboxyl groups in the crosslinked polymer is equivalent to or greater than the amount of alkali leaching from the active material.

[0060] Since 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 fibers, graphite powder, and carbon fibers. Among these, carbon black, carbon nanotubes, and carbon fibers are preferred because they easily provide excellent conductivity. Ketjenblack and acetylene black are preferred as carbon blacks. One of the above conductive additives may be used alone, or two or more may be used in combination. From the viewpoint of balancing conductivity and energy density, the amount of conductive additive used can be, for example, 0.2 to 20 parts by mass, or for example, 0.2 to 10 parts by mass, per 100 parts by mass of the total amount of active material. In addition, the positive electrode active material may be surface-coated with a conductive carbon-based material.

[0061] On the other hand, examples of negative electrode active materials include carbon-based materials, lithium metal, lithium alloys, and metal oxides, and one or more of these can be used in combination. Among these, active materials made of carbon-based materials such as natural graphite, artificial graphite, hard carbon, and soft carbon (hereinafter also referred to as "carbon-based active materials") are preferred, with graphite such as natural graphite and artificial graphite, and hard carbon being more preferred. In the case of graphite, spheroidized graphite is preferably used in terms of battery performance, and the preferred range of particle size is, for example, 1 to 20 μm, or for example, 5 to 15 μm. Furthermore, in order to increase the energy density, metals or metal oxides that can absorb lithium, such as silicon and tin, can also be used as negative electrode active materials. Among these, silicon has a higher capacity than graphite, and active materials made of silicon-based materials such as silicon, silicon alloys, and silicon oxides such as silicon monoxide (SiO) (hereinafter also referred to as "silicon-based active materials") can be used. The amount of silicon-based active material used is 5.0% by mass or more of the total amount of active material, in order to improve the electrical capacity of the secondary battery. It can also be, for example, 10.0% by mass or more, or for example, 20.0% by mass or more.

[0062] Since carbon-based active materials possess good electrical conductivity on their own, it is not always necessary to add conductive additives. When conductive additives are added for purposes such as further reducing resistance, the amount used, from the perspective of energy density, should be, for example, 10 parts by mass or less, or for example, 5 parts by mass or less, per 100 parts by mass of the total amount of active material.

[0063] When the composition is in slurry form, the amount of active material used is, for example, in the range of 10 to 75% by mass, or in the range of 30 to 65% by mass, relative to the total amount of the composition. If the amount of active material used is 10% by mass or more, migration of binders and the like is suppressed, and it is also advantageous in terms of the drying cost of the medium. On the other hand, if it is 75% by mass or less, the fluidity and coating properties of the composition can be ensured, and a uniform mixture layer can be formed.

[0064] This composition uses water as the medium. Furthermore, to adjust the properties and drying properties of this composition, a mixed solvent with lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, tetrahydrofuran, or N-methyl-2-pyrrolidone may be used. The proportion of water in the mixed medium is, for example, 50% by mass or more, and also, for example, 70% by mass or more.

[0065] When this composition is made into a coatable slurry, the content of the water-containing medium in the whole composition can be, for example, in the range of 25 to 60% by mass, or for example, 35 to 60% by mass, from the viewpoint of the coatability of the slurry, the energy cost required for drying, and productivity.

[0066] This composition may also contain other binder components such as styrene-butadiene rubber (SBR) latex, carboxymethylcellulose (CMC), acrylic latex, and polyvinylidene fluoride latex. When other binder components are used in combination, the amount used can be, for example, 0.1 to 5 parts by mass or less, or 0.1 to 2 parts by mass or less, or 0.1 to 1 part by mass or less, per 100 parts by mass of the total amount of active material. If the amount of other binder components used exceeds 5 parts by mass, the resistance may increase, and the high-rate properties may become insufficient. Among the above, SBR latex and CMC are preferred in terms of their excellent balance of binding properties and flexural resistance, and the combination of SBR latex and CMC is more preferable.

[0067] The above-mentioned SBR latex refers to an aqueous dispersion of a copolymer having structural units derived from aromatic vinyl monomers such as styrene and structural units derived from aliphatic conjugated diene monomers such as 1,3-butadiene. Examples of aromatic vinyl monomers include styrene, α-methylstyrene, vinyltoluene, and divinylbenzene, and one or more of these can be used. The amount of structural units derived from the aromatic vinyl monomer in the copolymer can be in the range of 20 to 70% by mass, or in the range of 30 to 60% by mass, mainly from the viewpoint of binding properties. Examples of aliphatic conjugated diene monomers include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 2-chloro-1,3-butadiene, and one or more of these can be used. The structural units derived from the aliphatic conjugated diene monomer in the copolymer can be in the range of, for example, 30 to 70% by mass, or 40 to 60% by mass, in that the binder binding properties and the flexibility of the resulting electrode are good. In addition to the above monomers, other monomers such as nitrile group-containing monomers like (meth)acrylonitrile, carboxyl group-containing monomers like (meth)acrylic acid, itaconic acid, maleic acid, and ester group-containing monomers like (meth)acrylate may be used as copolymer monomers to further improve properties such as binding properties. The structural units derived from the above other monomers in the copolymer can be in the range of, for example, 0 to 30% by mass, or 0 to 20% by mass.

[0068] The above-mentioned CMC refers to substituted nonionic cellulosic semi-synthetic polymer compounds obtained by substituting them with carboxymethyl groups, and their salts. Examples of the above-mentioned nonionic cellulosic semi-synthetic polymer compounds include alkylcelluloses such as methylcellulose, methylethylcellulose, ethylcellulose, and microcrystalline cellulose; and hydroxyalkylcelluloses such as hydroxyethylcellulose, hydroxybutylmethylcellulose, hydroxypropylcellulose, hydroxypropylmethylcellulose, hydroxyethylmethylcellulose, hydroxypropylmethylcellulose stearoxy ether, carboxymethylhydroxyethylcellulose, alkylhydroxyethylcellulose, and nonoxynylhydroxyethylcellulose.

[0069] The secondary battery electrode mixture layer composition of the present invention comprises the above-mentioned binder, active material, and water as essential components, and is obtained by mixing each component using known means. The method of mixing each component is not particularly limited, and known methods can be used, but a method of dry-blending powder components such as the active material, conductive additive, and binder, and then mixing them with a dispersion medium such as water, and then dispersing and kneading is preferred. When obtaining the composition in slurry form, it is preferable to produce a slurry that is free from poor dispersion and aggregation. As a mixing means, known mixers such as planetary mixers, thin-film swirling mixers, and orbital mixers can be used, but it is preferable to use a thin-film swirling mixer in that a good dispersion state can be obtained in a short time. Furthermore, when using a thin-film swirling mixer, it is preferable to perform pre-dispersion with a stirrer such as a disperser beforehand. The pH of the slurry is not particularly limited as long as the effects of the present invention are achieved, but it is preferably less than 12.5, and for example, when CMC is included, it is more preferable to have a pH of less than 11.5, and even more preferable to have a pH of less than 10.5, in that there is less concern about hydrolysis. Furthermore, the viscosity of the slurry is not particularly limited as long as it achieves the effects of the present invention, but as a B-type viscosity (25°C) at 20 rpm, it can be, for example, in the range of 100 to less than 8,500 mPa·s, or for example, in the range of 500 to 7,000 mPa·s, or for example, in the range of 1,000 to less than 6,500 mPa·s. If the viscosity of the slurry is within the above range, good coating properties can be ensured.

[0070] 4. Secondary Battery Electrode The secondary battery electrode of the present invention comprises a composite layer formed from the secondary battery electrode composite layer composition of the present invention on the surface of a current collector such as copper or aluminum. The composite layer is formed by coating the surface of the current collector with the composition and then drying off a medium such as water. The method of coating with the composition is not particularly limited, and known methods such as the doctor blade method, dip method, roll coat method, comma coat method, curtain coat method, gravure coat method, and extrusion method can be used. Furthermore, the drying can be carried out by known methods such as hot air blowing, reduced pressure, (far) infrared radiation, and microwave irradiation. Typically, the composite layer obtained after drying is subjected to compression treatment using a die press and a roll press. Compression can be used to bring the active material and binder into close contact, improving the strength of the composite layer and its adhesion to the current collector. Compression can be used to adjust the thickness of the composite layer to, for example, 30 to 80% of the thickness before compression, and the thickness of the composite layer after compression is generally about 4 to 200 μm.

[0071] 5. Secondary Battery A secondary battery can be manufactured by providing a separator and an electrolyte to the electrodes of the secondary battery of the present invention. The electrolyte may be in liquid or gel form. The separator is placed between the positive and negative electrodes of the battery and plays a role in preventing short circuits caused by contact between the two electrodes and in holding the electrolyte to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane with good ionic permeability and mechanical strength. Specific materials that can be used include polyethylene, polyolefins such as polypropylene, and polytetrafluoroethylene.

[0072] The electrolyte can be a known one commonly used depending on the type of active material. In lithium-ion secondary batteries, specific solvents include cyclic carbonates with high dielectric constant and high electrolyte solubility, such as propylene carbonate and ethylene carbonate, as well as low-viscosity chain carbonates such as ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate. These can be used individually or as mixed solvents. The electrolyte contains LiPF in these solvents. ６ LiSbF ６LiBF ４ LiClO ４ LiAlO ４ These are used by dissolving lithium salts. In nickel-metal hydride secondary batteries, an aqueous potassium hydroxide solution can be used as the electrolyte. Secondary batteries are obtained by housing positive and negative electrode plates, separated by a separator, in a spiral or stacked structure in a case or the like.

[0073] As described above, a secondary battery equipped with an electrode having a composite layer formed from a secondary battery electrode composite layer composition containing the secondary battery electrode binder disclosed herein exhibits good durability (cycle characteristics) even after repeated charging and discharging, and is therefore suitable for automotive secondary batteries and the like.

[0074] The present invention will be described in detail below based on the following examples. However, the present invention is not limited to these examples. In the following, "parts" and "%" mean parts by mass and mass%, respectively, unless otherwise specified.

[0075] (Measurement of particle size (water-swollen particle size) in an aqueous medium) 0.25 g of the powder of this cross-linked polymer salt and 49.75 g of ion-exchanged water were weighed into a 100 cc container and set in a rotation / revolution type agitator (Sinky Co., Ltd., Awatori Rentaro AR-250). Next, stirring (rotation speed 2,000 rpm / revolution speed 800 rpm, 7 minutes) and defoaming (rotation speed 2,200 rpm / revolution speed 60 rpm, 1 minute) were performed to prepare a hydrogel in which the cross-linked polymer salt was swollen in water. Then, the particle size distribution of the above hydrogel was measured using a laser diffraction / scattering particle size analyzer (Microtrac-Bell Co., Ltd., Microtrac MT-3300EZII) with ion-exchanged water as the dispersion medium. When a sufficient amount of hydrogel to obtain an appropriate scattered light intensity was added to a hydrogel circulating in an excess amount of dispersion medium, the particle size distribution shape measured after a few minutes stabilized. Once stability was confirmed, the particle size distribution was measured, and the volume-based median diameter (D50), which is a representative value of the particle size, was obtained. For the crosslinked polymer salts R-4, 5, 6, 11, and 12, the hydrogels were prepared in a swollen state in water by neutralization with lithium hydroxide monohydrate until the degree of neutralization reached 80 mol%, and the particle size in the aqueous medium was measured.

[0076] (Calculation of the degree of neutralization of nitrile group-containing amine salts of carboxyl group-containing crosslinked polymers) Powder of the salt of a carboxyl group-containing crosslinked polymer (solvent content: below the detection limit) was collected in a quartz boat, and the quartz boat was set in a trace total nitrogen analyzer (oxidative decomposition - chemiluminescence method). The sample was burned, and nitrogen in the combustion gas was detected by chemiluminescence, and the area of ​​the emission peak was determined. 10 μL of standard solution was collected in a quartz port and measured similarly, and the total amount of nitrogen contained in the carboxyl group-containing crosslinked polymer salt was determined using a calibration curve created from the concentration of the standard solution and the peak area. In this measurement, when a sample with a solvent content below the detection limit was measured, the total amount of nitrogen is based on the nitrogen of the structural units derived from the nitrile group-containing amines in the crosslinked polymer salt. From the total amount of nitrogen obtained above, the degree of neutralization N of the nitrile group-containing amine salt of the carboxyl group-containing crosslinked polymer was calculated. １ The result was calculated.

[0077] <Total Nitrogen Analysis Conditions> Trace Total Nitrogen Analyzer: TN-2100H (Mitsubishi Chemical Analytech Co., Ltd.) Combustion Conditions Inlet Temperature: 800℃ Outlet Temperature: 900℃ Gas Conditions Ar Flow Rate: 300 mL / min × 30 sec O ２ Flow rate: 300 mL / min × 600 sec. Standard solution: Prepared by diluting pyridine (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent grade) with toluene (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., reagent grade).

[0078] (Degree of neutralization of nitrile group-containing amine salts and alkali metal salts of carboxyl group-containing crosslinked polymer salts N ２ (Calculation of) The powder of this crosslinked polymer salt is placed in a sample holder and IR measurement is performed. The degree of neutralization of the nitrile group-containing amine salt and alkali metal salt of this crosslinked polymer is calculated from the intensity ratio of the peak derived from the C=O group of the carboxylic acid and the peak derived from the C=O group of the carboxylic acid Li salt and carboxylic acid amine salt. ２ They sought it.

[0079] <IR Measurement Conditions> Equipment: Thermo Fisher Scientific, Nicolet iS50 FT-IR Measurement Method: Diamond-ATR (45°) method

[0080] (Calculation of characteristic value (Z)) The degree of neutralization N obtained above １ and degree of neutralization N ２ Using the following formula, the characteristic value (Z) was calculated: Z = N １ / N ２ ×100

[0081] ≪Production of this Crosslinked Polymer Salt≫ (Production Example 1: Production of R-1 salt of carboxyl group-containing crosslinked polymer) For polymerization, a reactor equipped with a stirring blade, thermometer, reflux condenser, and nitrogen inlet tube was used. 567 parts of acetonitrile, 2.2 parts of deionized water, 100.0 parts of acrylic acid (hereinafter referred to as "AA"), 0.9 parts of trimethylolpropanediallyl ether (manufactured by Osaka Soda Co., Ltd., trade name "Neoallyl T-20"), and triethylamine equivalent to 1.0 mol% of the above AA were charged into the reactor. After thoroughly purging the reactor with nitrogen, the internal temperature was raised to 55°C. After confirming that the internal temperature had stabilized at 55°C, 0.040 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name "V-65") were added as a polymerization initiator. When turbidity was observed in the reaction solution, this point was taken as the polymerization initiation point. The polymerization reaction was continued while maintaining the internal temperature at 55°C by adjusting the external temperature (water bath temperature). After 24 hours from the start of polymerization, the reaction solution was cooled, and after the internal temperature dropped to 25°C, 1.0 part of 3-aminopropionitrile (hereinafter referred to as "3-APN") and lithium hydroxide monohydrate (hereinafter referred to as "LiOH·H") were added. ２ 51.8 parts of the powder of "O" were added. After the addition, stirring was continued at room temperature for 12 hours to obtain a slurry-like polymerization reaction solution in which particles of the carboxyl group-containing crosslinked polymer salt R-1 (degree of neutralization 90 mol%) were dispersed in the medium.

[0082] The resulting polymerization reaction solution was centrifuged to settle the polymer particles, and the supernatant was removed. Then, the precipitate was redispersed in an equal weight of acetonitrile, and the washing operation, in which the polymer particles were settled by centrifugation and the supernatant was removed, was repeated twice. The precipitate was collected and dried under reduced pressure at 80°C for 3 hours to remove volatile components, thereby obtaining a powder of carboxyl group-containing polymer salt R-1. Since crosslinked polymer salt R-1 is hygroscopic, it was stored in a sealed container with water vapor barrier properties.

[0083] Next, the powder of the carboxyl group-containing crosslinked polymer salt R-1 was analyzed for total nitrogen to determine the degree of neutralization of the nitrile group-containing amine salt N １ The result was found to be 0.6 mol%. Furthermore, by IR measurement of the cross-linked polymer salt, the degree of neutralization N of alkali metal salts and nitrile group-containing amine salts was determined from the intensity ratio of the peak originating from the C=O group of the carboxylic acid and the peak originating from the C=O group of the carboxylic acid Li salt and carboxylic acid amine salt. ２ The result was 89.6 mol%, which was approximately equal to the calculated value from the initial preparation. The degree of neutralization obtained from the above measurement N １ and degree of neutralization N ２ Using the above formula (1), the characteristic value (Z) of R-1 was calculated to be 0.7. Furthermore, the water-swollen particle size of R-1 was 1.56 μm.

[0084] (Production Examples 2-14 and Comparative Production Example 1: Production of Carboxyl Group-Containing Crosslinked Polymer Salts R-2 to R-15) The same procedure as in Production Example 1 was carried out, except that the amounts of monomer, crosslinkable monomer, and neutralizing agent used were as shown in Table 1, to obtain polymerization reaction solutions containing carboxyl group-containing crosslinked polymer salts R-2 to R-15. Next, the same procedure as in Production Example 1 was carried out for each polymerization reaction solution to obtain powdered carboxyl group-containing crosslinked polymer salts R-2 to R-15. R-2 to R-15 were sealed and stored in containers with water vapor barrier properties. Table 1 shows the characteristic values ​​(Z) and water-swelled particle sizes of R-2 to R-15.

[0085]

[0086] The details of the compounds used in Table 1 are shown below. • AA: Acrylic acid • HEA: 2-Hydroxyethyl acrylate • T-20: Trimethylolpropanediallyl ether (manufactured by Osaka Soda Co., Ltd., trade name "Neoallyl T-20") • TEA: Triethylamine • V-65: 2,2'-Azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name "V-65") • 3-APN: 3-Aminopropionitrile (manufactured by Tokyo Chemical Industries, Ltd.) • 3-MAPN: 3-Methylaminopropionitrile (manufactured by Tokyo Chemical Industries, Ltd.) • 3-NMAPN: 3-(N-Nitrosomethylamino)propionitrile (manufactured by Tokyo Chemical Industries, Ltd.) • LiOH • H ２ O: Lithium hydroxide monohydrate, Na ２ CO ３ : Sodium carbonate K ２ CO ３ Potassium carbonate

[0087] Example 1 (Preparation of Electrode Mixture Layer Composition) Artificial graphite (product name "SCMG-CF" manufactured by Showa Denko Corporation) and SiO (5 μm, manufactured by Osaka Titanium Technologies Co., Ltd.) were used as the active material. A mixture of crosslinked polymer salt R-1, styrene / butadiene rubber (SBR), and sodium carboxymethylcellulose (CMC) was used as the binder. Artificial graphite:SiO:R-1:SBR:CMC were added to a planetary mixer (Hibismix 2P-03, manufactured by Primix Corporation) with water as the diluent so that the solid content concentration of the electrode mixture layer composition was 53% by mass, in a mass ratio of artificial graphite:SiO:R-1:SBR:CMC = 76.8:19.2:1.0:2.0:1.0 (solid content), and mixed for 1 hour and 30 minutes to prepare a slurry-like electrode mixture layer composition (electrode slurry). The viscosity of the above electrode slurry was 7,800 mPa·s, and its coating properties were evaluated as "B" based on the following criteria.

[0088] <Evaluation of Coatability of Electrode Slurry> For each of the following examples and comparative examples, the electrode slurries obtained were adjusted to 25°C ± 1°C, and their viscosity was measured using a B-type viscometer at 20 rpm. Coatability was then evaluated based on the following criteria: (Criteria for determining coatability) A: Less than 6,500 mPa·s B: 6,500 mPa·s or more and less than 8,500 mPa·s C: 8,500 mPa·s or more

[0089] (Preparation of Negative Electrode Plate) Next, the electrode slurry was applied to the current collector (copper foil, thickness: 16.5 μm) using a variable applicator, and a composite layer was formed by drying in a forced-air dryer at 80°C for 15 minutes. After that, the composite layer had a thickness of 50 ± 5 μm and a composite density of 1.60 ± 0.10 g / cm³. ３ After rolling to the desired shape, the negative electrode plate was punched out in a 3 cm square for battery evaluation.

[0090] (Preparation of positive electrode plates) LiNi as the positive electrode active material in N-methylpyrrolidone (NMP) solvent. ０．５ Co ０．２ Mn ０．３ O ２ A composition for the positive electrode composite layer was prepared by mixing 100 parts of (NCM) and 2 parts of acetylene black, and adding 4 parts of polyvinylidene fluoride (PVDF) as a positive electrode binder. Next, the composite layer was formed by coating and drying the composition for the positive electrode composite layer onto a current collector (aluminum foil, thickness: 20 μm) using a variable applicator. Subsequently, the thickness of the composite layer was 125 μm ± 1 μm, and the composite density was 3.0 ± 0.10 g / cm³. ３ After rolling to achieve the desired shape, the positive electrode plate was punched out in a 3 cm square for battery evaluation.

[0091] (Preparation of electrolyte) A mixed solvent consisting of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio EC:DMC = 3:7) is to be mixed with vinylene carbonate (VC) to a total of 1% by mass and fluoroethylene carbonate (FEC) to a total of 2% by mass, and LiPF ６ A non-aqueous electrolyte was prepared by dissolving 1.2 mol / liter of [the substance].

[0092] (Fabrication of the secondary battery) The battery was constructed by attaching lead terminals to the positive and negative electrodes, and placing the electrode bodies opposite each other with a separator (made of polyethylene: film thickness 16 μm, porosity 47%) in between. The electrodes were then placed in an aluminum laminate battery casing, injected with electrolyte, and sealed to create a test battery. The design capacity of this prototype battery is 50 mAh. The battery's design capacity was based on a charging termination voltage of 4.2 V.

[0093] <Evaluation of Cycle Characteristics> The lithium-ion secondary battery made of laminated cells prepared as described above was subjected to charge and discharge operations at a charge / discharge rate of 0.1C under conditions of 2.5 to 4.2V in a 45°C environment, and the initial capacity C was evaluated. ０ The following was measured. Furthermore, the charge and discharge cycles were repeated under CC discharge conditions of 2.5 to 4.2V in a 25°C environment at a charge and discharge rate of 0.5C, and the capacity C after 100 cycles was measured. １００ The following was measured. Here, the cycle characteristic (ΔC) was calculated using the following formula: ΔC = C １００ / C ０ ×100 (%) The ΔC calculated using the above formula is 85.3%, and the cycle characteristics were evaluated as "B" based on the following criteria. Note that a higher value of ΔC indicates better cycle characteristics. (Cycle characteristics evaluation criteria) A: Charge / discharge capacity retention rate of 86.0% or more B: Charge / discharge capacity retention rate of 85.0% or more and less than 86.0% C: Charge / discharge capacity retention rate of 84.0% or more and less than 85.0% D: Charge / discharge capacity retention rate less than 84.0%

[0094] ≪Overall Evaluation≫ The overall evaluation was based on the coating properties and cycle characteristics evaluation results of the electrode slurry described above, according to the criteria shown in Table 2 below. In this evaluation, A to C are considered passing levels. The coating properties and cycle characteristics evaluation of the electrode slurry of Example 1 were both "B", so the overall evaluation was "B".

[0095]

[0096] Examples 2 to 14 and Comparative Example 1: Electrode slurries were prepared by the same procedure as in Example 1, except that the formulations were as shown in Table 3, and the viscosity of the slurries was measured. The coating properties of the electrode slurries and the cycle characteristics of the secondary batteries obtained using them were also evaluated. The results are shown in Table 3.

[0097]

[0098] The details of the compounds used in Table 3 are shown below. SBR: Styrene-butadiene rubber CMC: Sodium carboxymethylcellulose

[0099] ≪Evaluation Results≫ As is clear from the results of Examples 1 to 14, the electrode slurry obtained using the salt of the carboxyl group-containing crosslinked polymer of the present invention exhibited excellent coating properties, and the secondary battery obtained using the electrode slurry exhibited excellent cycle characteristics. Among these, when comparing by the characteristic value Z of the salt of the carboxyl group-containing crosslinked polymer, the case where the value was 7.4 (Example 3) showed superior cycle characteristics of the secondary battery compared to cases where the value was 0.7, 1.4, 44.5, and 100.0 (Examples 1, 2, 4, and 5, respectively). This is presumed to be because if the value is too low, the binding to the active material decreases, and if the value is too high, the water-swollen particle size of the polymer decreases, reducing the binding area to the active material. It was found that there is an optimal range for this value. When the characteristic value Z of the crosslinked polymer salt was 1.4 or higher (Examples 2, 3, 4, and 5), the electrode slurry showed excellent coating properties. This is thought to be because the higher the value, the easier it is to adsorb to the active material, and the amount of free crosslinked polymer salt decreases, resulting in a decrease in slurry viscosity.

[0100] Furthermore, focusing on the content of structural units derived from ethylenically unsaturated carboxylic acid monomers, the case of 100% by mass (Example 3) resulted in superior cycle characteristics of the secondary battery compared to the cases of 80% by mass (Example 9) and 60% by mass (Example 10). Moreover, focusing on the degree of neutralization of the carboxyl group-containing crosslinked polymer, when the degree of neutralization was 60 mol% or higher (Example 3: 86.4 mol%, Example 12: 66.4 mol%), the coating performance of the electrode slurry was superior to the case of 45.0 mol% (Example 6). Furthermore, focusing on the type of metal salt contained in the salt of the carboxyl group-containing crosslinked polymer, the case of lithium metal salt (Example 3) resulted in superior cycle characteristics of the secondary battery compared to the cases of sodium metal salt (Example 13) and potassium metal salt (Example 14).

[0101] In contrast, when the characteristic value Z was 0 (the degree of neutralization of the nitrile group-containing amine salt of the carboxyl group-containing crosslinked polymer was 0 mol%), the coating properties of the electrode slurry and the cycle characteristics of the secondary battery were significantly inferior (Comparative Example 1). This is presumed to be due to a decrease in the amount of adsorption to the active material.

[0102] The composition containing a salt of the carboxyl group-containing crosslinked polymer of the present invention exhibits excellent coating properties and film performance, and is therefore expected to be applied to a variety of uses, such as thickeners and viscosity modifiers for cosmetics, binders for non-aqueous electrolyte secondary battery electrodes, anti-sedimentation agents for pigments, and dispersion stabilizers for metal powders. Furthermore, secondary batteries equipped with electrodes obtained using a secondary battery electrode mixture layer composition containing a binder for secondary battery electrodes containing a salt of the carboxyl group-containing crosslinked polymer of the present invention exhibit good durability (cycle characteristics), and are therefore expected to be applied to automotive secondary batteries. It is also useful for the use of silicon-containing active materials and is expected to contribute to increasing the capacity of batteries. In particular, it is useful for non-aqueous electrolyte lithium-ion secondary batteries with high energy density.

Claims

1. A salt of a carboxyl group-containing crosslinked polymer, comprising an amine salt of the carboxyl group-containing crosslinked polymer, wherein the amine constituting the amine salt has a nitrile group, and the salt of the carboxyl group-containing crosslinked polymer has a value (Z) calculated by the following formula (1) that is 0.5 or more and 100 or less. Z = N １ / N ２ ×100 (1) N １ : Amine salt of carboxyl group-containing crosslinked polymer (mol%) N ２ : Amine salts of carboxyl group-containing crosslinked polymers and alkali metal salts of carboxyl group-containing crosslinked polymers (mol%) 2. The salt of the carboxyl group-containing crosslinked polymer according to claim 1, wherein the carboxyl group-containing crosslinked polymer contains 50% by mass or more and 100% by mass or less of structural units derived from ethylenically unsaturated carboxylic acid monomers with respect to its total structural units.

3. A salt of a carboxyl group-containing crosslinked polymer according to claim 1 or 2, wherein 60 mol% or more of the carboxyl groups of the carboxyl group-containing crosslinked polymer are neutralized.

4. The salt of the carboxyl group-containing crosslinked polymer according to claim 1 or 2, wherein the amine constituting the amine salt is a monofunctional amine.

5. The salt of the carboxyl group-containing crosslinked polymer according to claim 1 or 2, wherein the carboxyl group-containing crosslinked polymer is a crosslinked polymer obtained by polymerizing a monomer composition containing a non-crosslinkable monomer and a crosslinkable monomer.

6. The carboxyl group-containing crosslinked polymer is crosslinked with a crosslinkable monomer, and the amount of the crosslinkable monomer used is 0.03 mol% or more and 0.8 mol% or less relative to the total amount of the non-crosslinkable monomer, wherein the salt of the carboxyl group-containing crosslinked polymer according to claim 1 or 2.

7. A binder for secondary battery electrodes, comprising a salt of the carboxyl group-containing crosslinked polymer described in claim 1 or 2.

8. A composition for a secondary battery electrode mixture layer, comprising the binder for secondary battery electrodes, active material, and water as described in claim 7.

9. A secondary battery electrode comprising a composite layer formed from the secondary battery electrode composite layer composition described in claim 8 on the surface of a current collector.

10. A secondary battery comprising the secondary battery electrodes described in claim 9.

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