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

Abstract

A secondary battery electrode binder containing a polymer emulsion, wherein: the polymer emulsion comprises a carboxyl group-containing non-crosslinked polymer (hereinafter referred to as "polymer (A)") and a polymer different from polymer (A) (hereinafter referred to as "polymer (B)"); polymer (A) has 50 mass% or more of structural units derived from an ethylenically unsaturated carboxylic acid monomer, and at least some of the structural units are neutralized; and polymer (B) has 50 mass% or more of structural units derived from an ethylenically unsaturated monomer for which the energy level of the highest occupied molecular orbital (HOMO) of a compound obtained by adding a methyl group to both terminals of the ethylenically unsaturated monomer is -9.5 eV or less.

Description

Binder for secondary battery electrodes and its use, and method for manufacturing a binder for secondary battery electrodes 【0001】 This invention relates to a binder for secondary battery electrodes, its use, and a method for manufacturing a binder for secondary battery electrodes. 【0002】 Various energy storage devices, such as nickel-metal hydride batteries, lithium-ion batteries, and electric double-layer capacitors, have been put into practical use as secondary batteries. The electrodes used in these secondary batteries are manufactured by coating and drying a composition for forming an electrode mixture layer containing an active material and a binder onto a current collector. For example, in lithium-ion batteries, an aqueous binder containing styrene-butadiene rubber (SBR) latex and carboxymethylcellulose (CMC) is used as the binder for the negative electrode mixture layer composition. In addition, aqueous binders containing aqueous solutions or aqueous dispersions of acrylic acid polymers are known to have excellent dispersibility and binding properties. On the other hand, organic solvent-based binders, such as an N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride (PVDF), are widely used as binders for the positive electrode mixture layer composition. 【0003】 In recent years, as the applications of various secondary batteries that achieve carbon neutrality have expanded, there has been a growing demand for reducing the environmental impact of battery manufacturing processes. For this reason, in addition to using lithium iron phosphate (LFP) and lithium manganese iron phosphate (LMFP), which have high stability to water, as cathode active materials, water-based cathode mixture compositions that do not use organic solvents are being investigated by using water-based binders. 【0004】 SBR can be used as an aqueous binder in compositions for the positive electrode mixture layer. However, SBR has insufficient oxidation resistance, which is problematic because it degrades due to oxidation under high potential conditions of the positive electrode. Therefore, aqueous acrylic binders with excellent oxidation resistance have been developed (for example, Patent Document 1). 【0005】Patent Document 1 discloses a binder composition for the positive electrode of a lithium-ion secondary battery, comprising an emulsion in which polymer particles (A) derived from an ethylenically unsaturated monomer are dispersed and stabilized in a polyvinyl alcohol-based resin (B), characterized in that the content ratio (A / B) of polymer particles (A) to polyvinyl alcohol-based resin (B) is 1 / 99 to 40 / 60 in terms of solid weight ratio. It is stated that this binder composition has excellent oxidation resistance and, when a lithium-ion secondary battery is formed, exhibits good charge-discharge cycle characteristics at high temperatures. 【0006】 Japanese Patent Publication No. 2018-018728 【0007】 However, when the binder disclosed in Patent Document 1 is used as the positive electrode of a secondary battery, the inclusion of polyvinyl alcohol as a dispersant in the emulsion can reduce its dispersibility with respect to the positive electrode active material and conductive additive, which can lead to problems such as a decrease in the capacity retention rate of the secondary battery and an increase in DC resistance. 【0008】 This invention has been made in view of the above circumstances, and aims to provide a binder for secondary battery electrodes that exhibits dispersibility with active materials and conductive additives, and can produce a secondary battery that shows a high capacity retention rate and low DC resistance. Furthermore, it also aims to provide a composition for a secondary battery electrode mixture layer containing the above binder, a secondary battery electrode obtained using the 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 binder for secondary battery electrodes containing a polymer emulsion comprising a carboxyl group-containing non-crosslinked polymer (polymer (A)) and a polymer different from the carboxyl group-containing non-crosslinked polymer (polymer (B)), by setting the content of structural units derived from ethylenically unsaturated carboxylic acid monomers in polymer (A) to a specific amount, and the content of structural units derived from monomers whose highest occupied orbital (HOMO) energy level is below a specific value in polymer (B) to a specific amount, it is possible to obtain a secondary battery that exhibits dispersibility with respect to the active material and conductive additive, and has a high capacity retention rate and low DC resistance, thus completing the present invention. 【0010】The present invention is as follows: [1] A binder for secondary battery electrodes containing a polymer emulsion, wherein the polymer emulsion contains a carboxyl group-containing non-crosslinked polymer (hereinafter referred to as "polymer (A)") and a polymer different from polymer (A) (hereinafter referred to as "polymer (B)"), wherein polymer (A) has 50% by mass or more of structural units derived from an ethylenically unsaturated carboxylic acid monomer, and at least a portion of said structural units is neutralized, and polymer (B) has 50% by mass or more of structural units derived from an ethylenically unsaturated monomer (hereinafter referred to as "monomer (b)") in which methyl groups are added to both ends of the ethylenically unsaturated monomer, and the energy level of the highest occupied orbital (HOMO) of the compound is -9.5 eV or less. [2] The binder for secondary battery electrodes according to [1], wherein the degree of neutralization of polymer (A) is 10 mol% or more and 95 mol% or less. [3] The binder for secondary battery electrodes according to [1] or [2], wherein the content of polymer (A) in the polymer emulsion is 5% by mass or more and 70% by mass or less when the total amount of polymer (A) and polymer (B) is 100% by mass. [4] The binder for secondary battery electrodes according to any one of [1] to [3], wherein monomer (b) is an alkyl (meth)acrylate. [5] The binder for secondary battery electrodes according to any one of [1] to [4], wherein the glass transition temperature of polymer (B) is 50°C or less. [6] The binder for secondary battery electrodes according to any one of [1] to [5], wherein the particle size of the polymer emulsion is 100 to 950 nm as measured by laser diffraction / scattering. [7] A composition for a secondary battery electrode mixture layer comprising the binder for secondary battery electrodes according to any one of [1] to [6], an active material, and water. [8] A secondary battery electrode comprising a composite layer formed from the composite composition for secondary battery electrode composite layer described in [7] on the surface of a current collector. [9] A secondary battery comprising the secondary battery electrode described in [8].

[10] A method for producing a binder for secondary battery electrodes containing a polymer emulsion, comprising a carboxyl group-containing non-crosslinked polymer (hereinafter referred to as "polymer (A)").A method for producing a polymer emulsion comprising the step of polymerizing a monomer component containing 50% by mass or more of an ethylenically unsaturated monomer (hereinafter referred to as "monomer (b)"), wherein the ethylenically unsaturated monomer has methyl groups added to both ends thereof, and the energy level of the highest occupied orbital (HOMO) of the compound is -9.5 eV or lower, in the presence of a polymer emulsion, wherein the polymer (A) has 50% by mass or more of structural units derived from the ethylenically unsaturated carboxylic acid monomer, and at least a portion of the structural units is neutralized.

[11] The method for producing the polymer according to

[10] , wherein the degree of neutralization of the polymer (A) is 10 mol% or more and 95 mol% or less.

[12] The method for producing the polymer according to

[10] or

[11] , wherein the amount of polymer (A) used is 5% by mass or more and 70% by mass or less, when the total amount of polymer (A) and the monomer component is 100% by mass.

[13] The manufacturing method according to any one of

[10] to

[12] , wherein the monomer (b) is an alkyl (meth)acrylate. 【0011】 According to the binder for secondary battery electrodes of the present invention, it is possible to obtain a secondary battery that exhibits excellent dispersibility with active material and conductive additive, high capacity retention rate (cycle characteristics), and low DC resistance. 【0012】 The binder for secondary battery electrodes of the present invention (hereinafter also referred to as "this binder") contains a polymer emulsion (hereinafter also referred to as "this polymer emulsion") containing polymer (A) and polymer (B), and can be mixed with an active material and water to form a composition for a secondary battery electrode mixture layer (hereinafter also referred to as "this composition"). The above composition may be in a slurry state that can be coated onto a current collector, or it may be prepared in a wet powder state to accommodate press processing on the surface of a current collector. By forming a mixture 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. 【0013】The polymer (A), polymer (B), the polymer emulsion, the composition for the secondary battery electrode mixture layer obtained using the binder, the secondary battery electrode, and the secondary battery will be described in detail below. 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. 【0014】 1. Polymer (A) Polymer (A) is a carboxyl group-containing non-crosslinked polymer having 50% by mass or more of structural units (hereinafter also referred to as "component (a1)") derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as "monomer (a)"), and at least a portion of said structural units being neutralized. Polymer (A) does not form a block copolymer with polymer (B) described later. 【0015】 The above component (a1) can be introduced into a polymer by polymerizing a monomer containing monomer (a). Alternatively, it can be obtained by (co)polymerizing (meth)acrylic acid ester monomers and then hydrolyzing them. Furthermore, (meth)acrylamide and (meth)acrylonitrile may be polymerized and then treated with a strong alkali, or an acid anhydride may be reacted with a polymer having hydroxyl groups. 【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, 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 the above, (meth)acrylic acid is preferred in that it has good dispersibility with the active material and conductive additive. Furthermore, compounds having an acryloyl group as a polymerizable functional group are preferred, particularly acrylic acid, in that they yield polymers with a long primary chain length due to their high polymerization rate and good binder binding strength. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, polymers with a high carboxyl group content can be obtained. 【0017】 The content of component (a1) in polymer (A) is 50% by mass or more, preferably 50% by mass or more and 100% by mass or less, more preferably 60% by mass or more and 100% by mass or less, even more preferably 70% by mass or more and 100% by mass or less, and even more preferably 80% by mass or more and 100% by mass or less, relative to the total structural units of polymer (A), in terms of the dispersibility of particles in the polymer emulsion. 【0018】<Other Structural Units> In addition to component (a1), polymer (A) may contain structural units derived from other ethylenically unsaturated monomers copolymerizable with these (hereinafter also referred to as "monomer (a2)") (hereinafter also referred to as "component (a2)"). Component (a2) can be introduced by copolymerizing a monomer containing monomer (a2). Examples of monomer (a2) include aromatic vinyl monomers, maleimide compounds, (meth)acrylic acid ester monomers, (meth)acrylamide and its derivatives, nitrile group-containing ethylenically unsaturated monomers, etc. The amount of each of the above monomers used is, for example, 50% by mass or less, for example, 40% by mass or less, for example, 30% by mass or less, for example, 20% by mass or less, and for example, 10% by mass or less, relative to the total amount of monomers constituting polymer (A). 【0019】 Examples of aromatic vinyl monomers include styrene, α-methylstyrene, vinylnaphthalene, and isopropenylnaphthalene. One of these may be used alone, or two or more may be used in combination. 【0020】Maleimide compounds include maleimides and N-substituted maleimide compounds. Examples of N-substituted maleimide compounds include N-methylmaleimide, N-ethylmaleimide, N-n-propylmaleimide, N-isopropylmaleimide, N-n-butylmaleimide, N-isobutylmaleimide, N-tert-butylmaleimide, N-pentylmaleimide, N-hexylmaleimide, N-heptylmaleimide, N-octylmaleimide, N-laurylmaleimide, N-stearylmaleimide, and other N-alkyl-substituted maleimide compounds; N-cyclopentylmaleimide, N-cyclo Examples include N-cycloalkyl-substituted maleimide compounds such as hexylmaleimide; and N-aryl-substituted maleimide compounds such as N-phenylmaleimide, N-(4-hydroxyphenyl)maleimide, N-(4-acetylphenyl)maleimide, N-(4-methoxyphenyl)maleimide, N-(4-ethoxyphenyl)maleimide, N-(4-chlorophenyl)maleimide, N-(4-bromophenyl)maleimide, and N-benzylmaleimide. One or more of these can be used. 【0021】 Examples of (meth)acrylate monomers include alkyl (meth)acrylate ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl acrylate, isobornyl acrylate, and 1,6-hexanediol diacrylate; aromatic (meth)acrylate ester compounds such as phenyl (meth)acrylate, phenylmethyl (meth)acrylate, phenylethyl (meth)acrylate, and phenoxyethyl (meth)acrylate; alkoxyalkyl (meth)acrylate ester compounds such as 2-methoxyethyl (meth)acrylate and ethoxyethyl (meth)acrylate; and hydroxyalkyl (meth)acrylate ester compounds such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and hydroxybutyl (meth)acrylate. One of these may be used alone, or two or more may be used in combination. 【0022】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; N,N-dialkyl (meth)acrylamide compounds such as dimethyl (meth)acrylamide and diethyl (meth)acrylamide; and cyclic (meth)acrylamide compounds such as 4-acryloylmorpholine. One of these may be used alone, or two or more may be used in combination. 【0023】 Examples of nitrile group-containing ethylenically unsaturated monomers include (meth)acrolinitrile; 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. 【0024】 The content of component (a2) can be 0% by mass or more and 50% by mass or less relative to the total structural units of polymer (A). The content of component (a2) may be 1% by mass or more and 40% by mass or less, 2% by mass or more and 30% by mass or less, 5% by mass or more and 20% by mass or less, or 10% by mass or more and 30% by mass or less. Furthermore, when component (a2) is contained at 1% by mass or more relative to the total structural units of polymer (A), the affinity to the electrolyte is improved, and therefore, an effect of improved lithium ion conductivity can also be expected. 【0025】Polymer (A) is in the form of a salt in which some or all of the carboxyl groups contained in the polymer are 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 calcium salts and barium salts; other metal salts such as magnesium salts and aluminum salts; ammonium salts and organic amine salts. Among these, alkaline earth metal salts are preferred because they do not adversely affect battery characteristics, alkali metal salts are more preferred, and lithium salts, sodium salts, and potassium salts are even more preferred. 【0026】 In this binder, polymer (A) is preferably used in the form of a salt, such as when the acidic groups, such as carboxyl groups, derived from the ethylenically unsaturated carboxylic acid monomer are neutralized to a degree of neutralization of 10 mol% or more, in order to ensure solubility in water. The lower limit of the degree of neutralization is more preferably 15 mol% or more, and even more preferably 20 mol% or more. The upper limit of the degree of neutralization is 100 mol%, and is preferably 95 mol% or less, more preferably 90 mol% or less, even more preferably 80 mol% or less, even more preferably 70 mol% or less, even more preferably 60 mol% or less, and particularly preferably 50 mol% or less, in order to improve particle stability by improving the affinity between polymer (A) and polymer (B). In this specification, the degree of neutralization can be calculated from the charge values ​​of the monomer having acidic groups such as carboxyl groups and the neutralizing agent used for neutralization. The degree of neutralization can be determined by measuring the intensity ratio of the peak derived from the C=O group of the carboxylic acid and the peak derived from the C=O group of the carboxylate salt after drying the crosslinked polymer or its salt at 80°C for 3 hours under reduced pressure using IR measurement. 【0027】The weight average molecular weight (Mw) of the polymer (A) is not particularly limited, but from the viewpoint of obtaining an electrode binder layer with excellent binding properties, it is preferably 5,000 or more, more preferably 10,000 or more. Mw may be 100,000 or more, may be 200,000 or more, or may be 1,000,000 or more. The upper limit value of Mw is not particularly limited either, but from the viewpoint of handling during production, for example, it may be 10,000,000 or less, or may be 5,000,000 or less. 【0028】 As the polymerization form, known polymerization methods can be adopted. From the point that the monomer component containing the monomer (a) can be uniformly dissolved and polymerized, aqueous solution polymerization using water as a solvent is preferred. 【0029】 As the polymerization initiator, known polymerization initiators such as azo compounds, organic peroxides, and inorganic peroxides can be used, but it is not particularly limited. The use conditions can be adjusted by known methods such as thermal initiation, redox initiation using a reducing agent in combination, and UV initiation so that an appropriate amount of radical generation is obtained. In order to obtain a non-crosslinked polymer with a long primary chain length, it is preferable to set the conditions so that the amount of radical generation is less within the range where the production time is acceptable. 【0030】 Here, as the polymerization initiator in aqueous solution polymerization, a water-soluble polymerization initiator is preferred. For example, compounds having a hydrophilic group (for example, a carboxyl group) and / or their salts or hydrates are mentioned. Among the above, 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, etc. are preferred. 【0031】The preferred amount of the polymerization initiator to be used is, 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, when the total amount of the monomer components used is 100 parts by mass. If the amount of the polymerization initiator used is 0.001 part by mass or more, the polymerization reaction can be carried out stably, and if it is 3 parts by mass or less, it is easy to obtain a polymer having a long primary chain length. 【0032】 The polymerization temperature depends on conditions such as the type and concentration of the monomer used, but is preferably 0 to 100°C, more preferably 20 to 80°C. The polymerization temperature may be constant or may change during the polymerization reaction period. Also, the polymerization time is preferably 1 minute to 20 hours, more preferably 1 hour to 10 hours. 【0033】 2. Polymer (B) The polymer (B) is a polymer different from the polymer (A), and is a polymer having 50% by mass or more of a structural unit (hereinafter also referred to as "(b) component") derived from an ethylenically unsaturated monomer in which the energy level of the highest occupied molecular orbital (HOMO) of a compound having methyl groups added to both ends of the ethylenically unsaturated monomer is -9.5 eV or less. (Hereinafter, the ethylenically unsaturated monomer is also referred to as "monomer (b)".) The energy level of the HOMO being -9.5 eV or less is preferable in that the electrochemical stability (particularly, oxidation resistance) of the binder is improved and the reaction and deterioration of the binder during charge and discharge are suppressed. In the present specification, the energy level of the HOMO is a calculated value obtained by a method (PM6 method) of performing structural optimization on a compound having methyl groups added to both ends of an ethylenically unsaturated monomer by the Hartree-Fock method (semi-empirical molecular orbital method) and calculating the energy level of the HOMO for the obtained optimal structure. 【0034】The HOMO energy level of monomer (b) is preferably -9.5 eV or lower, more preferably -9.6 eV or lower, even more preferably -9.7 eV or lower, and even more preferably -9.8 eV or lower, in terms of improving oxidation resistance. The lower limit of the HOMO energy level of monomer (b) is not particularly limited, but may be, for example, -12.0 eV, -12.5 eV, -13.0 eV, or -13.5 eV. Examples of monomer (b) include (meth)acrylic acid ester monomers, nitrile group-containing ethylenically unsaturated monomers, (meth)acrylamide derivatives, etc. (meth)acrylic acid esters are preferred, and alkyl (meth)acrylic acid esters are more preferred, in terms of improving oxidation resistance and polymer particle stability. Specific examples of these monomers include, for example, those exemplified as specific examples of monomer (a2) described above. 【0035】 (b) The content of component is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and even more preferably 90% by mass or more, in terms of improving oxidation resistance. 【0036】 The glass transition temperature (hereinafter also simply referred to as "Tg") of polymer (B) is preferably 50°C or lower, more preferably 40°C or lower, and may also be 30°C or lower, 20°C or lower, or 10°C or lower, in terms of improving binding properties. The lower limit of Tg is -100°C, may also be -80°C or higher, -65°C or higher, -50°C or higher, -40°C or higher, or -30°C or higher, due to limitations on the usable constituent monomer units. In this specification, Tg can be measured by the differential scanning calorimeter (DSC) described in the examples. 【0037】Polymer (B) may be crosslinked with a crosslinkable monomer. 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. The above polyfunctional polymerizable monomers are compounds having two or more polymerizable functional groups such as (meth)acryloyl groups and alkenyl groups in the molecule, and examples include polyfunctional (meth)acryloyl 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 (meth)acryloyl compounds are preferred because they easily provide a strong crosslinked structure, and polyfunctional (meth)acrylate compounds having two or more (meth)acryloyl groups in the molecule are particularly preferred. 【0038】 When polymer (B) is crosslinked with a crosslinkable monomer, the amount of the crosslinkable monomer used is preferably 0.01 parts by mass or more and 1.0 parts by mass or less, more preferably 0.02 parts by mass or more and 0.8 parts by mass or less, even more preferably 0.03 parts by mass or more and 0.6 parts by mass or less, even more preferably 0.04 parts by mass or more and even more preferably 0.05 parts by mass or more and 0.3 parts by mass or less, based on 100 parts by mass of the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer). If the amount of the crosslinkable monomer used is 0.01 parts by mass or more, it is preferable in that excessive swelling of the electrolyte is suppressed during use over a longer period than conventional methods, resulting in an excellent retention rate of charge and discharge capacity. If it is 1.0 part by mass or less, it is preferable in that it exhibits appropriate flexibility, improving binding properties and maintaining battery performance even during long-term use. 【0039】3. This polymer emulsion is a polymer emulsion containing polymer (A) and polymer (B). The content of polymer (A) in this polymer emulsion is preferably 5% by mass or more and 70% by mass or less, more preferably 10% by mass or more and 50% by mass or less, and even more preferably 15% by mass or more and 30% by mass or less, when the total amount of polymer (A) and polymer (B) is 100% by mass, in terms of excellent dispersibility with the active material and conductive additive. When the content of polymer (A) is 5% by mass or more, the stability of the polymer emulsion and its dispersibility with the active material and conductive additive are improved, and when it is 70% by mass or less, it has a positive effect on battery performance. 【0040】 Here, a method for producing this polymer emulsion is to polymerize a monomer component containing 50% by mass or more of monomer (b) by emulsion polymerization in the presence of polymer (A). A surfactant may also be used in this emulsion polymerization. The emulsion polymerization of the monomer component containing 50% by mass or more of monomer (b) is stabilized by the neutralization of at least a portion of the structural units derived from the ethylenically unsaturated carboxylic acid monomers of polymer (A). 【0041】 In this emulsion polymerization, the amount of polymer (A) used is preferably 5% to 70% by mass, more preferably 10% to 50% by mass, and even more preferably 15% to 30% by mass, when the total amount of polymer (A) and monomer components is 100% by mass, in terms of improving the stability of the polymer particles. Examples of monomer (b) include alkyl (meth)acrylate monomers, nitrile group-containing ethylenically unsaturated monomers, and (meth)acrylamide derivatives, and alkyl (meth)acrylate is preferred in terms of improving the stability of the polymer particles. 【0042】The particle size of the polymer emulsion is preferably 100 to 950 nm, as measured by laser diffraction / scattering, in terms of improving the stability, binding properties, and cycle characteristics of the polymer particles. Furthermore, it is more preferably 150 to 800 nm, and even more preferably 250 to 700 nm, in terms of excellent dispersibility with the active material and conductive additive. In this specification, the particle size can be measured by the laser diffraction / scattering method described in the examples. 【0043】 Methods for emulsion polymerization include, for example, a batch reaction in which monomers, a carboxyl group-containing non-crosslinked polymer (polymer (A)), a surfactant, and water are all charged into a reaction vessel and reacted, and a dropwise reaction in which monomers are gradually added dropwise to the reaction vessel and reacted. Although not particularly limited, a batch reaction is preferred because the ratio of polymer (A) to monomers during emulsion polymerization can be kept constant throughout the process. 【0044】 For emulsion polymerization, it is preferable to use a radical polymerization initiator (hereinafter also referred to as "polymerization initiator"). The polymerization initiator can be a known oil-soluble polymerization initiator or a water-soluble polymerization initiator. Examples of oil-soluble initiators include organic peroxides such as benzoyl peroxide, tert-butyloxybenzoate, tert-butyl hydroperoxide, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-3,5,5,trimethylhexanoate, ditert-butyl peroxide, cumene hydroperoxide, and p-menthane hydroperoxide, as well as azobis compounds such as 2,2'-azobisisobutyronitrile, 2,2'-azobis-2,4-dimethylvaleronitrile, 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), and 1,1'-azobis-cyclohexane-1-carbonitride. Examples of water-soluble polymerization initiators include ammonium persulfate, sodium persulfate, potassium persulfate, hydrogen peroxide, and 2,2'-azobis(2-methylpropionamidine) dihydrochloride. 【0045】In emulsion polymerization, a reducing agent can be used in combination with a polymerization initiator. This can accelerate the polymerization reaction. Examples of such reducing agents include reducing organic compounds such as ascorbic acid, erythorbic acid, tartaric acid, citric acid, glucose, and metal salts such as formaldehyde sulfoxylate; reducing inorganic compounds such as sodium sulfite, sodium bisulfite, sodium metabisulfite (SMBS), and sodium hyposulfite; and ferrous chloride, longalite, and thiourea dioxide. 【0046】 For emulsion polymerization, it is preferable to use a water-soluble polymerization initiator. The polymerization initiator is preferably used in an amount of 0.05 to 5% by mass per 100 parts by mass of the monomer mixture. The reducing agent is preferably used in an amount of 0.01 to 2.5% by mass per 100 parts by mass of the monomer mixture. 【0047】 During emulsion polymerization, buffers, chain transfer agents, basic compounds, etc., can be used as needed. Examples of buffers include sodium acetate, sodium citrate, and sodium bicarbonate. Examples of chain transfer agents include 2-mercaptoethanol, octyl mercaptan, tertial decyl mercaptan, lauryl mercaptan, stearyl mercaptan, 2-ethylhexyl mercaptoacetate, octyl mercaptoacetate, 2-ethylhexyl mercaptopropionate, and octyl mercaptopropionate. 【0048】4. Composition for Secondary Battery Electrode Binder Layer This binder contains this polymer emulsion, and the composition for the secondary battery electrode binder layer of the present invention includes this binder, an active material, and water. The amount of the binder used in this composition is, for example, 0.1 parts by mass or more and 20 parts by mass or less with respect to 100 parts by mass of the total amount of the active material. The above usage amount is also, for example, 0.2 parts by mass or more and 10 parts by mass or less, and also, for example, 0.3 parts by mass or more and 8 parts by mass or less, and also, for example, 0.4 parts by mass or more and 5 parts by mass or less. If the amount of the binder used is 0.1 parts by mass or more, sufficient binding properties can be obtained. Also, the dispersion stability of the active material and the like can be ensured, and a uniform binder layer can be formed. If the amount of the binder used is 20 parts by mass or less, the composition will not have a high viscosity, and the coating property on the current collector can be ensured. As a result, a binder layer having a uniform and smooth surface can be formed. 【0049】 Among the above active materials, as the positive electrode active material, a lithium salt of a transition metal oxide can be used. For example, a layered rock salt type and a spinel type lithium-containing metal oxide can be used. Specific compounds of the layered rock salt type positive electrode active material include lithium cobaltate, lithium nickelate, and NCM {Li(Ni ｘ , Co ｙ , Mn ｚ ), x + y + z = 1} and NCA {Li(Ni １－ａ－ｂ Co ａ Al b )}, etc. Also, as the spinel type positive electrode active material, lithium manganate and the like can be mentioned. In addition to oxides, phosphates, silicates, sulfur, etc. are also used. As the phosphate, an olivine type lithium-containing compound can be mentioned. Specific examples include LiFePO ４ , LiCoPO ４ , LiMnPO ４ , Li ０．９０ Ti ０．０５ Nb ０．０５ Fe ０．３０ Co ０．３０ Mn ０．３０ PO ４ , etc. Among these, especially LiFePO ４Lithium iron phosphate is preferred because the iron compounds used as raw materials are readily available and inexpensive, and furthermore, it is preferred because it exhibits particularly strong effects in the present invention. 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. 【0050】 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 can lead to corrosion of common positive electrode current collector materials such as aluminum foil (Al). In such cases, it is preferable to use a current collector material with a carbon coating on the aluminum foil surface to prevent corrosion. 【0051】 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. As for carbon black, Ketjenblack, acetylene black, and furnace black are preferred. 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. 【0052】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. 【0053】 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. 【0054】 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. 【0055】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. 【0056】 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. 【0057】 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, 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 is 5 parts by mass or less, an increase in resistance can be avoided. Among the above, CMC is preferred because it has an excellent balance of binding properties and flexural resistance. 【0058】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. 【0059】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. 【0060】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 in the range of 500 to 7,000 mPa·s. If the viscosity of the slurry is within the above range, good coating properties can be ensured. 【0061】5. 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. 【0062】 6. 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. 【0063】 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 Lithium salts such as these are dissolved and used. 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. 【0064】 The present invention will be described in detail below based on 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. 【0065】 In the production example and comparative production example, the molecular weight of the carboxyl group-containing non-crosslinked polymer (hereinafter also referred to as "polymer 1"), the particle size of the polymer emulsion, and the glass transition temperature of the polymer having structural units derived from monomer (b) in the polymer emulsion (hereinafter also referred to as "polymer 2") were evaluated as follows. 【0066】 <Molecular Weight Measurement> The molecular weight of the carboxyl group-containing non-crosslinked polymer (polymer 1) was determined by gel permeation chromatography (GPC). Specifically, the weight-average molecular weight (Mw) in terms of sodium polyacrylate was obtained by aqueous GPC. 0.1 g (0.02 g as polymer solids) of an aqueous solution containing polymer 1 obtained in Production Examples 1-1 to 1-5 and Comparative Production Example 1-1 was taken and diluted with 40 g of 0.1 M sodium nitrate aqueous solution to prepare the measurement sample. Gel permeation chromatography (GPC) was performed on the measurement sample under the conditions described below, and the weight-average molecular weight (Mw) in terms of sodium polyacrylate was calculated. 【0067】 (GPC measurement conditions) Column: 2 x TSKgel GMPW columns manufactured by Tosoh Solvent: 0.1 M sodium nitrate aqueous solution Temperature: 40°C Detector: RI Flow rate: 0.5 mL / min 【0068】<Measurement of Particle Size of Polymer Emulsion> The particle size distribution of the polymer emulsion was measured using a laser diffraction / scattering particle size analyzer (Microtrac MT-3300EXII, Microtrac Bell Co., Ltd.) with deionized water as the dispersion medium. When the emulsion was added in an amount that provided an appropriate scattered light intensity to a dispersion liquid in which an excess amount of dispersion medium was circulated, 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) was obtained as a representative value of the particle size. 【0069】 <Method for measuring the glass transition temperature of polymer 2 in polymer emulsion> When the temperature of the dried polymer emulsion film and the standard material alumina was raised from -50°C to 150°C at a rate of 10°C / min using a differential scanning calorimeter (NETZSCH, DSC 214 Polyma), the heat change point on the lower temperature side was defined as the glass transition temperature of polymer 2 in the polymer emulsion. 【0070】 <<Production of Carboxyl Group-Containing Non-Crosslinked Polymers>> (Production Example 1-1: Production of Polymer A-1) A reactor equipped with a stirring blade, thermometer, reflux condenser, and nitrogen inlet tube was used for polymerization. Under a nitrogen atmosphere, 400.0 parts of deionized water, 100.0 parts of acrylic acid, and 0.11 parts of 2-(2-carboxyethylsulfanylthiocarbonylsulfanyl)propionic acid (manufactured by BORON MOLECULAR, trade name "BM-1429"), which is a RAFT agent, were added to the reactor and heated to 60°C. 0.015 parts of 2,2'-azobis(2-methylpropionamidine) dihydrochloride (manufactured by Fujifilm Wako Pure Chemical Industries, trade name "V-50", hereinafter also referred to as "V-50"), which is an initiator, were added to this solution and the reaction was carried out until the polymerization conversion rate reached 95%. Next, 0.39 parts of N-ethylpiperidine hypophosphite (manufactured by Sigma-Aldrich) and 0.23 parts of V-50 were added to this solution, and the mixture was reacted at 60°C for 2 hours to obtain an aqueous solution containing carboxyl group-containing non-crosslinked polymer A-1. The weight-average molecular weight (Mw) of polymer A-1 was 255,000. 【0071】(Production Examples 1-2 to 1-5 and Comparative Production Example 1-1: Production of Polymers A-2 to A-6) The same procedure as in Production Example 1-1 was carried out, except that the amount of each raw material used was as shown in Table 1, to obtain polymerization reaction solutions containing carboxyl group-containing non-crosslinked polymers A-2 to A-6. The Mw of polymers A-2 to A-6 was measured in the same manner as for polymer A-1, and the results are shown in Table 1. 【0072】 【0073】 The details of the compounds used in Table 1 are shown below: AA: Acrylic acid, MAA: Methacrylic acid, AAm: Acrylamide, MEA: 2-Methoxyethyl acrylate, BA: n-Butyl acrylate, BM-1429: 2-(2-Carboxyethylsulfanylthiocarbonylsulfanyl)propionic acid, V-50: 2,2'-Azobis(2-Methylpropionamidine) dihydrochloride 【0074】 (Production Example 2-1: Production of Polymer Emulsion E-1) For polymerization, a reactor equipped with a stirring blade, thermometer, reflux condenser, and nitrogen inlet tube was used. Under a nitrogen atmosphere, 400.0 parts of water and 17.0 parts of AC-10H (by solid content) as polymer (A) were added to the reactor, and then 2.0 parts of lithium hydroxide monohydrate were added to neutralize 20 mol% of the carboxyl group content. 83.0 parts of cyclohexyl acrylate were added to this solution and mixed while heating to 60°C. Further, 0.040 parts of V-50 (by solid content) were added to this solution and the reaction was carried out until the polymerization conversion rate exceeded 98% to obtain polymer emulsion E-1 containing AC-10H (polymer (A)) and a polymer having structural units derived from cyclohexyl acrylate (polymer (B)). The Tg of polymer (B) was 28°C, and the particle size of polymer emulsion E-1 was 570 nm. 【0075】(Production Examples 2-2 to 2-28 and Comparative Production Examples 2-1 to 2-3: Production of Polymer Emulsions E-2 to E-31) Polymer emulsions E-2 to E-31 were obtained by performing the same procedure as in Production Example 2-1, except that the amount of each raw material used was as shown in Tables 2 and 3. The Tg of each polymer 2 and the particle size of polymer emulsions E-2 to E-31 are shown in Tables 2 and 3. Note that some particle aggregation was observed in polymer emulsion E-29 (Comparative Production Example 2-1), and the accurate particle size could not be measured. 【0076】 【0077】 【0078】 Details of the compounds used in Tables 2 and 3 are shown below. • AC-10H: Polyacrylic acid, Mw = approximately 800,000 (manufactured by Toagosei Co., Ltd.) • AC-10P: Polyacrylic acid, Mw = approximately 8,000 (manufactured by Toagosei Co., Ltd.) • AC-10L: Polyacrylic acid, Mw = approximately 30,000 (manufactured by Toagosei Co., Ltd.) • AC-10LH: Polyacrylic acid, Mw = approximately 1,500,000 (manufactured by Toagosei Co., Ltd.) • LiOH・H ２ O: Lithium hydroxide monohydrate, NaOH: Sodium hydroxide, CHA: Cyclohexyl acrylate, BA: n-butyl acrylate, 2-EHA: 2-ethylhexyl acrylate, IBXA: Isobornyl acrylate, MA: Methyl acrylate, MMA: Methyl methacrylate, HEMA: 2-hydroxyethyl methacrylate, AN: Acrylonitrile, HDDA: 1,6-Hexanediol diacrylate, St: Styrene, V-50: 2,2'-Azobis(2-methylpropionamidine) dihydrochloride 【0079】Example 1 (Preparation of composition for positive electrode mixture layer (positive electrode slurry)) Lithium iron phosphate (manufactured by BTR, hereinafter also referred to as "LFP") was used as the active material. Furnace black (product name "SuperP" manufactured by Imerys, hereinafter also referred to as "FB") was used as the conductive additive. Polymer emulsion E-1 was used as the binder. The composition for the positive electrode mixture layer was added to a rotating / revolving mixer (Awatori Rentaro, manufactured by Thinky Co., Ltd.) with water as the diluent, in a mass ratio of LFP:FB:polymer emulsion E-1 = 100:5.0:3.0 (solids) so that the solids content of the composition for the positive electrode mixture layer was 40% by mass, and mixed to prepare a slurry-like composition for the positive electrode mixture layer (positive electrode slurry). 【0080】 <Conditions for Evaluating the Dispersibility of the Cathode Slurry> The dispersibility of the cathode slurry was evaluated based on the results of a grind gauge test. A BYK-Gardner grind gauge (product name "Grindometer 0-50 μm") was used, and the point where the slurry stopped was recorded as the measurement result. The measurement result was 15 μm, and the dispersibility was evaluated as "A" based on the following criteria. Note that a smaller measurement result indicates better dispersibility. (Criteria for determining dispersibility) A: Measurement result is less than 25 μm B: Measurement result is 25 μm or more and less than 50 μm C: Measurement result is 50 μm or more 【0081】 (Preparation of positive electrode plate) Next, the positive electrode slurry was applied to a current collector (aluminum foil) with a thickness of 20.0 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 45 ± 5 μm and a composite density of 1.90 ± 0.10 g / cm³. ３ After rolling to achieve the desired shape, the positive electrode plate was punched out in a 3 cm square. 【0082】(Preparation of composition for negative electrode mixture layer (negative electrode slurry)) Graphite (manufactured by Resonaq, trade name "MAGE3") was used as the active material. Polymer emulsion E-1 was used as the binder. The composition for the negative electrode mixture layer was mixed in a rotating / revolving mixer (Awatori Rentaro, manufactured by Thinky Co., Ltd.) with water as the diluent, in a mass ratio of graphite:polymer emulsion E-1 = 100:3.0 (solids) so that the solids content of the composition for the negative electrode mixture layer was 50% by mass, and the mixture was prepared as a slurry. 【0083】 <Conditions for Evaluating the Dispersibility of the Negative Electrode Slurry> The dispersibility of the negative electrode slurry was evaluated based on the results of a grind gauge test. A BYK-Gardner grind gauge (product name "Grindometer 0-100 μm") was used, and the point where the slurry stopped was recorded as the measurement result. The measurement result was 50 μm, and the dispersibility was evaluated as "A" based on the following criteria. Note that a smaller measurement result indicates better dispersibility. (Criteria for Determining Dispersibility) A: Measurement result is less than 60 μm B: Measurement result is 60 μm or more and less than 100 μm C: Measurement result is 100 μm or more 【0084】 (Preparation of Negative Electrode Plate) Next, the negative electrode slurry was applied to a current collector (copper foil) with a thickness of 16.0 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 37 ± 5 μm and a composite density of 1.40 ± 0.10 g / cm³. ３ After rolling to achieve the desired shape, the negative electrode plate was punched out in a 3 cm square. 【0085】 (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 1.0 mol / liter of [the substance]. 【0086】(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 via a separator (polyethylene: film thickness 20 μm, porosity 48%). 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 15 mAh. The battery's design capacity was based on a charging termination voltage of 4.0 V. 【0087】 <Evaluation of Cycle Characteristics> The lithium-ion secondary battery made of laminated cells prepared above was subjected to charge and discharge operations at a charge / discharge rate of 1 / 3C under conditions of 2.0 to 4.0V in a 25°C environment, and the initial capacity C was evaluated. ０ The following was measured. Furthermore, the capacity C was measured after 300 cycles of repeated charge-discharge (CC) operation at a charge-discharge rate of 1 / 2C under conditions of 2.0 to 4.0V in a 25°C environment. ３００ The following was measured. Here, the cycle characteristic (ΔC) was calculated using the following formula: ΔC = C ３００ / C ０ ×100 (%) The ΔC calculated using the above formula is 95.0%, and the cycle characteristics were evaluated as "A" 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 95% or more B: Charge / discharge capacity retention rate of 85% or more and less than 95% C: Charge / discharge capacity retention rate less than 85% 【0088】 <Evaluation of DC Resistance> The laminate-type lithium-ion secondary battery prepared above was adjusted to SOC = 50% at a charge / discharge rate of 1 / 3C in an environment of 25°C. After this, the DC resistance R was determined by calculating the slope of the current-voltage from the measured voltage during 10s discharge at currents of 1 / 3C → 1 / 2C → 1C → 2C. R was 3.6Ω, and the cycle characteristics were evaluated as "A" based on the following criteria. Note that a smaller value of DC resistance indicates better resistance characteristics of the secondary battery. A: DC resistance less than 4.0Ω B: DC resistance 4.0Ω or more and less than 4.5Ω C: DC resistance 4.5Ω or more 【0089】Examples 2-29 and Comparative Examples 1-3: The same procedure as in Example 1 was followed, except that the formulations were as shown in Tables 4 and 5, to prepare positive electrode slurries and negative electrode slurries. The dispersibility of each slurry, the cycle characteristics of the secondary battery obtained using the slurry, and the DC resistance were evaluated, and the results are shown in Tables 4 and 5. 【0090】 【0091】 【0092】 The details of the compounds used in Table 4 are shown below. • CMC: Sodium carboxymethylcellulose 【0093】 <<Evaluation Results>> As is clear from the results of Examples 1 to 29, the binder for secondary battery electrodes of the present invention exhibited excellent dispersibility with respect to the active material and conductive additive. In addition, the resulting secondary battery exhibited excellent cycle characteristics and suppressed DC resistance. 【0094】 Among these, focusing on the content of polymer (A) and polymer (B) in the polymer emulsion, particularly good dispersibility with respect to the active material and conductive additive was observed when the content of polymer (A) was 10% by mass or more (Examples 1, 14-16). This suggests that when the content of polymer (A) in the polymer emulsion is 10% by mass or more, the water dispersibility of the polymer emulsion itself improves, contributing to the dispersion of the active material and conductive additive to which the polymer emulsion is bound. On the other hand, in terms of cycle characteristics, particularly good cycle characteristics were observed when the content of polymer (A) was in the range of 5% by mass to 50% by mass (Examples 1, 13-15). This suggests that when the content of polymer (A) in the polymer emulsion is 5% by mass or more, the electrolyte swelling can be kept low, and sufficient active material binding is maintained even under electrolyte conditions. Furthermore, when the content of polymer (A) in the polymer emulsion is 50% by mass or less, the flexibility of the binder improves, and it is thought that it has the toughness to withstand the expansion and contraction of the active material. 【0095】Furthermore, when focusing on the Tg of polymer (B) in the polymer emulsion, particularly good cycle characteristics were observed in the range where Tg is 50°C or below (Examples 1, 20-22, 24-27). This is thought to be because having Tg within an appropriate range provided sufficient flexibility to the electrode mixture layer, suppressing cracking during drying of the electrode mixture layer, and thus providing toughness that can withstand the expansion and contraction of the active material, as well as good bonding between the active materials. 【0096】 In addition, focusing on the particle size of the polymer emulsion, when the particle size was in the range of 100 to 950 nm, the cycle characteristics were improved compared to cases where the particle size was less than 100 nm (Example 16) or greater than 950 nm (Example 12). This is thought to be because, when the particle size is 100 nm or greater, non-uniformity of the binder due to aggregation is less likely to occur, and the binding ability to the active material is fully exhibited, resulting in improved cycle characteristics. Furthermore, when the particle size is 950 nm or less, the number of particles increases, and the number of binding sites increases, which is thought to improve the binding ability of the binder to the active material and result in good cycle characteristics. 【0097】 In contrast, when a polymer containing carboxyl groups and not neutralized structural units derived from ethylenically unsaturated carboxylic acid monomers was used as the carboxyl group-containing non-crosslinked polymer (Comparative Example 1), the active material and conductive additive were not well dispersed in the electrode slurry, resulting in a deterioration of battery performance (cycle characteristics, DC resistance). Furthermore, when a polymer containing less than 50% by mass of structural units derived from ethylenically unsaturated carboxylic acid monomers was used as the carboxyl group-containing non-crosslinked polymer (Comparative Example 2), the active material and conductive additive were not sufficiently dispersed in the electrode slurry, resulting in an increase in DC resistance. The performance degradation in Comparative Examples 1 and 2 is thought to be due to the weakening of electrostatic repulsion by the carboxylate salt of the carboxyl group-containing non-crosslinked polymer, which makes the emulsion more prone to aggregation and thus more likely to cause non-uniformity of the binder. In addition, when a polymer containing less than 50% by mass of structural units derived from monomer (b) was used (Comparative Example 3), the cycle characteristics of the secondary battery deteriorated. 【0098】The secondary battery electrode mixture layer composition (electrode slurry) containing the binder for secondary battery electrodes of the present invention allows for the manufacture of electrodes in an aqueous system, and a state in which the active material and conductive additive are well dispersed is obtained. Furthermore, secondary batteries equipped with electrodes obtained using the above binder exhibit good durability (cycle characteristics) and resistance characteristics, and are therefore expected to be applied to automotive secondary batteries. In addition, secondary battery electrodes containing the binder for secondary battery electrodes of the present invention have excellent oxidation resistance, which is expected to extend the lifespan of secondary batteries. The binder for secondary battery electrodes of the present invention can be suitably used in non-aqueous electrolyte secondary battery electrodes, and is particularly useful in non-aqueous electrolyte lithium-ion secondary batteries with high energy density.

Claims

1. A binder for secondary battery electrodes containing a polymer emulsion, wherein the polymer emulsion comprises a carboxyl group-containing non-crosslinked polymer (hereinafter referred to as "polymer (A)") and a polymer different from polymer (A) (hereinafter referred to as "polymer (B)"), wherein polymer (A) has 50% by mass or more of structural units derived from an ethylenically unsaturated carboxylic acid monomer, and at least a portion of said structural units is neutralized, and polymer (B) has 50% by mass or more of structural units derived from an ethylenically unsaturated monomer (hereinafter referred to as "monomer (b)") in which methyl groups are added to both ends of the ethylenically unsaturated monomer, and the energy level of the highest occupied orbital (HOMO) of the compound is -9.5 eV or lower.

2. The binder for secondary battery electrodes according to claim 1, wherein the degree of neutralization of the polymer (A) is 10 mol% or more and 95 mol% or less.

3. The binder for secondary battery electrodes according to claim 1 or 2, wherein the content of polymer (A) in the polymer emulsion is 5% by mass or more and 70% by mass or less, when the total amount of polymer (A) and polymer (B) is 100% by mass.

4. The binder for secondary battery electrodes according to claim 1 or 2, wherein monomer (b) is an alkyl (meth)acrylate.

5. The binder for secondary battery electrodes according to claim 1 or 2, wherein the glass transition temperature of the polymer (B) is 50°C or lower.

6. The binder for secondary battery electrodes according to claim 1 or 2, wherein the particle size of the polymer emulsion is 100 to 950 nm as measured by laser diffraction / scattering.

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

8. A secondary battery electrode comprising a composite layer formed from the secondary battery electrode composite layer composition described in claim 7 on the surface of a current collector.

9. A secondary battery comprising the secondary battery electrodes described in claim 8.

10. A method for producing a binder for secondary battery electrodes containing a polymer emulsion, comprising the step of polymerizing a monomer component containing 50% by mass or more of an ethylenically unsaturated monomer (hereinafter referred to as "monomer (b)"), in the presence of a carboxyl group-containing non-crosslinked polymer (hereinafter referred to as "polymer (A)"), by emulsion polymerization, wherein the ethylenically unsaturated monomer has methyl groups added to both ends thereof, and the energy level of the highest occupied orbital (HOMO) of the ethylenically unsaturated monomer is -9.5 eV or lower, and the polymer (A) has 50% by mass or more of structural units derived from an ethylenically unsaturated carboxylic acid monomer, and at least a portion of the structural units is neutralized.

11. The manufacturing method according to claim 10, wherein the degree of neutralization of the polymer (A) is 10 mol% or more and 95 mol% or less.

12. The manufacturing method according to claim 10 or 11, wherein the amount of polymer (A) used is 5% by mass or more and 70% by mass or less, when the total amount of polymer (A) and the monomer components is 100% by mass.

13. The manufacturing method according to claim 10 or 11, wherein monomer (b) is an alkyl (meth)acrylate.

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