Binder for secondary battery positive electrode, uses of same, and method for manufacturing binder for secondary battery positive electrode

Abstract

The present invention provides a binder for a secondary battery positive electrode, the binder exhibiting dispersibility with respect to a positive electrode active material and a conductive assistant, and enabling the achievement of excellent capacity retention rate (cycle characteristics) of the secondary battery positive electrode, and suppression of deterioration under high potential conditions (maintenance of residual capacity after a float test and reduction of DC resistance increase rate). The present invention also provides: a composition for a secondary battery positive electrode mixture layer, the composition containing the binder; a secondary battery positive electrode which is obtained using the composition; and a secondary battery.　The present invention specifically provides a binder for a secondary battery positive electrode, the binder containing an emulsion which contains a block polymer that has a polymer block (A) and a polymer block (B), wherein: the polymer block (A) includes a structural unit that is derived from an ethylenically unsaturated carboxylic acid monomer; and the polymer block (B) includes a structural unit that is derived from a (meth)acrylic acid ester monomer (hereinafter referred to as "monomer (b)") which has a solubility of less than 1 g with respect to 100 g of water at 20°C.

Description

Binder for secondary battery positive electrode, use thereof, and method for manufacturing binder for secondary battery positive electrode

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

[0002] Various secondary batteries, such as nickel-metal hydride secondary batteries, lithium-ion secondary batteries, and electric double layer capacitors, have been put into practical use. Electrodes used in these secondary batteries are prepared by applying a composition for forming an electrode mixture layer containing an active material and a binder to a current collector, followed by drying. For example, in lithium-ion secondary batteries, an aqueous binder containing styrene butadiene rubber (SBR) latex and carboxymethyl cellulose (CMC) is used as the binder for the negative electrode mixture layer composition. Furthermore, aqueous binders containing an aqueous acrylic acid polymer solution or dispersion are known as binders with excellent dispersibility and binding properties. Meanwhile, organic solvent-based binders, such as a solution of polyvinylidene fluoride (PVDF) in N-methyl-2-pyrrolidone (NMP), 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) or lithium manganese iron phosphate (LMFP), which are highly stable in water, as a positive electrode active material, aqueous positive electrode mixture layer compositions that do not use organic solvents by using an aqueous binder have been investigated.

[0004] As the aqueous binder used in the composition for the positive electrode mixture layer, SBR latex can be used. However, since SBR has insufficient oxidation resistance, degradation due to oxidation occurs under high potential of the positive electrode, which is a problem. Therefore, aqueous acrylic binders with excellent oxidation resistance have been developed (for example, Patent Document 1).

[0005] Patent Document 1 discloses a binder for battery electrodes, which is characterized by containing a polymer including a structural unit derived from a (meth)acrylate monomer having a hydroxyl group, a structural unit derived from a polyfunctional (meth)acrylate monomer, and a structural unit derived from a reactive surfactant. The binder is described as an aqueous binder with a low environmental impact, which has excellent electrode slurry application properties, high binding properties, and does not cause oxidative degradation in electrode environments (particularly positive electrode environments).

[0006] International Publication No. 2015 / 133492

[0007] However, when the binder disclosed in Patent Document 1 is used in a secondary battery positive electrode, although it has oxidation resistance and can suppress deterioration under high potential, there are cases where it has problems such as poor dispersibility in the positive electrode active material and conductive additive and low capacity retention rate.

[0008] The present invention has been made in view of the above circumstances, and aims to provide a binder for a secondary battery positive electrode that exhibits dispersibility for a positive electrode active material and a conductive additive, and that exhibits excellent capacity retention (cycle characteristics) for a secondary battery positive electrode and suppresses deterioration under high potential conditions (maintenance of remaining capacity after a float test and a reduction in the rate of increase in DC resistance). Another aim of the present invention is to provide a composition for a secondary battery positive electrode mixture layer that contains the binder, and a secondary battery positive electrode and secondary battery obtained using the composition.

[0009] As a result of intensive research to solve the above-mentioned problems, the present inventors have found that a composition for a positive electrode mixture layer obtained from a binder for a secondary battery positive electrode containing an emulsion containing a block polymer having polymer block (A) containing structural units derived from an ethylenically unsaturated carboxylic acid monomer and polymer block (B) containing structural units derived from a (meth)acrylic acid ester monomer (hereinafter also referred to as "monomer (b)") having a solubility of less than 1 g in 100 g of water at 20°C exhibits dispersibility for a positive electrode active material and a conductive additive, and that a secondary battery positive electrode obtained using this composition exhibits a high capacity retention rate and suppression of deterioration under high potential conditions, thereby completing the present invention.

[0010] The present invention is as follows: [1] A binder for a secondary battery positive electrode, comprising an emulsion containing a block polymer having polymer block (A) and polymer block (B), wherein the polymer block (A) contains structural units derived from an ethylenically unsaturated carboxylic acid monomer, and the polymer block (B) contains structural units derived from a (meth)acrylic acid ester monomer (hereinafter referred to as "monomer (b)") having a solubility of less than 1 g in 100 g of water at 20°C. [2] The binder for a secondary battery positive electrode according to [1], wherein a proportion of the polymer block (A) in the block polymer is 1% by mass or more and 50% by mass or less. [3] The binder for a secondary battery positive electrode according to [1] or [2], wherein the polymer block (A) contains structural units derived from an ethylenically unsaturated carboxylic acid monomer in an amount of 50% by mass or more relative to the total structural units of the polymer block (A). [4] The binder for a secondary battery positive electrode according to any one of [1] to [3], wherein the polymer block (B) contains structural units derived from the monomer (b) in an amount of 50 mass% or more relative to the total structural units of the polymer block (B). [5] The binder for a secondary battery positive electrode according to any one of [1] to [4], wherein the block polymer does not contain structural units derived from a crosslinkable monomer. [6] The binder for a secondary battery positive electrode according to any one of [1] to [5], wherein the block polymer is a salt in which 40 mol% or more of the carboxyl groups in the block polymer have been neutralized. [7] The binder for a secondary battery positive electrode according to any one of [1] to [6], wherein the particle size of the emulsion is 150 to 950 nm as measured by a laser diffraction / scattering method. [8] A composition for a secondary battery positive electrode mixture layer, comprising the binder for a secondary battery positive electrode according to any one of [1] to [7], a positive electrode active material, and water. [9] The composition for a secondary battery positive electrode mixture layer according to [8], wherein the positive electrode active material contains an olivine-type lithium-containing compound.

[10] A secondary battery positive electrode comprising, on a current collector surface, a mixture layer formed from the composition for a secondary battery positive electrode mixture layer according to [8] or [9].

[11] A secondary battery comprising the secondary battery positive electrode according to

[10] .

[12] A method for producing a binder for a secondary battery positive electrode containing an emulsion containing a block polymer, wherein the block polymer has a polymer block (A) and a polymer block (B), the method comprising the steps of: polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer by a living radical polymerization method to produce the polymer block (A); and emulsion polymerizing a monomer component containing a (meth)acrylic acid ester monomer having a solubility of less than 1 g in 100 g of water at 20° C. in the presence of the polymer block (A) to produce the polymer block (B).

[13] The method for producing a binder for a secondary battery positive electrode according to

[12] , wherein the living radical polymerization method is reversible addition-fragmentation chain transfer polymerization (RAFT).

[14] The method for producing a binder for a secondary battery positive electrode according to

[12] or

[13] , wherein the emulsion polymerization is soap-free polymerization.

[0011] The binder for secondary battery positive electrodes of the present invention has excellent dispersibility in the positive electrode active material and the conductive additive, and can provide a secondary battery that exhibits a high capacity retention rate (cycle characteristics) and suppressed deterioration under high potential conditions (maintenance of remaining capacity after a float test and a reduced rate of increase in DC resistance).

[0012] The binder for a secondary battery positive electrode of the present invention contains an emulsion containing a block polymer, and can be mixed with a positive electrode active material and water to form a composition for a secondary battery positive electrode mixture layer (hereinafter also referred to as "the composition"). The composition may be in a slurry state that can be applied to a current collector, or it may be prepared as a wet powder state so that it can be pressed onto the current collector surface. The secondary battery positive electrode of the present invention can be obtained by forming a mixture layer formed from the composition on the surface of a current collector such as aluminum foil.

[0013] The binder for a secondary battery positive electrode of the present invention, the composition for a secondary battery positive electrode mixture layer obtained using the binder, the secondary battery positive electrode, and the secondary battery are each described in detail below. In this specification, "(meth)acrylic" means acrylic and / or methacrylic, and "(meth)acrylate" means acrylate and / or methacrylate. Furthermore, "(meth)acryloyl group" means acryloyl group and / or methacryloyl group. In the numerical ranges described in this specification in stages, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages, and the upper or lower limit value of that numerical range may be replaced with a value shown in the examples.

[0014] 1. Binder The binder for secondary battery positive electrodes of the present invention is an emulsion containing a block polymer (hereinafter also referred to as the "block polymer"). The block polymer has a polymer block (A) containing structural units derived from an ethylenically unsaturated carboxylic acid monomer and a polymer block (B) containing structural units derived from the monomer (b). From the viewpoints of improving the flexibility of the binder, bonding the positive electrode active material and the binder over a wider area, and improving the bonding strength, it is preferable that the block polymer does not contain structural units derived from a crosslinkable monomer. Here, the crosslinkable monomer includes a polyfunctional polymerizable monomer having two or more polymerizable unsaturated groups, and a monomer having a self-crosslinkable crosslinkable functional group such as N-methylol(meth)acrylamide or a hydrolyzable silyl group. The block polymer is described in detail below.

[0015] <Polymer Block (A)> The polymer block (A) is a polymer block containing a structural unit derived from an ethylenically unsaturated carboxylic acid monomer, and can be obtained, for example, by polymerizing a monomer composition containing an ethylenically unsaturated carboxylic acid monomer. The block polymer has such a structural unit and thus a carboxyl group, which improves adhesion to a current collector and provides excellent desolvation effect and ionic conductivity for lithium ions, thereby providing a positive electrode with low resistance and excellent high-rate performance.

[0016] Examples of ethylenically unsaturated carboxylic acid monomers include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, and fumaric acid; (meth)acrylamidoalkylcarboxylic acids such as (meth)acrylamidohexanoic acid and (meth)acrylamidododecanoic acid; and ethylenically unsaturated monomers having a carboxyl group, such as monohydroxyethyl succinate (meth)acrylate, ω-carboxy-caprolactone mono(meth)acrylate, and β-carboxyethyl (meth)acrylate, or (partially) alkali-neutralized products thereof. One of these may be used alone, or two or more may be used in combination. Among these, compounds having an acryloyl group as a polymerizable functional group are preferred, with acrylic acid being particularly preferred, because they have a high polymerization rate, resulting in a polymer with a long primary chain length and good binder binding strength. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, a polymer with a high carboxyl group content can be obtained.

[0017] The content of structural units derived from ethylenically unsaturated carboxylic acid monomers in the polymer block (A) based on all structural units of the polymer block (A) is not particularly limited, but may be, for example, 1% by mass or more and 100% by mass or less, for example, 30% by mass or more and 100% by mass or less, for example, 50% by mass or more and 100% by mass or less, for example, 60% by mass or more and 100% by mass or less, or for example, 70% by mass or more and 100% by mass or less. By containing structural units derived from ethylenically unsaturated carboxylic acid monomers in such a range, excellent adhesion to the current collector can be easily ensured. It is more preferable for the lower limit of the structural units derived from (meth)acrylic acid to be 50% by mass or more, because this improves dispersibility in the active material and conductive additive.

[0018] Furthermore, polymer block (A) may contain a structural unit derived from a monomer other than an ethylenically unsaturated carboxylic acid monomer (hereinafter also referred to as "monomer (a)"), and is not particularly limited. Examples of monomer (a) include aromatic vinyl monomers, maleimide compounds, (meth)acrylic acid ester monomers, (meth)acrylamide and its derivatives, and nitrile group-containing ethylenically unsaturated monomers. 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, or for example, 10% by mass or less, based on the total amount of monomers constituting polymer block (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] The maleimide compounds include maleimide and N-substituted maleimide compounds. Examples of the N-substituted maleimide compounds include N-alkyl-substituted maleimide compounds such as 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, and N-stearylmaleimide; N-cyclopentylmaleimide, N-cyclopentylmaleimide, and N-cyclopentylmaleimide; 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, and one or more of these may be used.

[0021] Examples of the (meth)acrylic acid ester monomer include (meth)acrylic acid alkyl ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; aromatic (meth)acrylic acid ester compounds such as phenyl (meth)acrylate, phenylmethyl (meth)acrylate, phenylethyl (meth)acrylate, and phenoxyethyl (meth)acrylate; (meth)acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth)acrylate and ethoxyethyl (meth)acrylate; and (meth)acrylic acid hydroxyalkyl ester compounds such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and hydroxybutyl (meth)acrylate. These may be used alone or in combination of two or more.

[0022] Examples of the (meth)acrylamide derivative 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. These may be used singly or in combination of two or more.

[0023] Examples of the nitrile group-containing ethylenically unsaturated monomer include (meth)acrylonitrile; (meth)acrylic acid cyanoalkyl ester compounds such as cyanomethyl (meth)acrylate and cyanoethyl (meth)acrylate; cyano group-containing unsaturated aromatic compounds such as 4-cyanostyrene and 4-cyano-α-methylstyrene; and vinylidene cyanide. These may be used singly or in combination of two or more.

[0024] The number-average molecular weight (Mn) of the polymer block (A) is not particularly limited, but is preferably in the range of 1,000 to 1,000,000. A number-average molecular weight of 1,000 or more can exhibit high cohesive strength as a binder and contribute to improving the binding strength of the positive electrode. Furthermore, a number-average molecular weight of 1,000,000 or less is preferable in that good fluidity can be ensured, making handling during production, etc. easy. The number-average molecular weight of the block polymer (A) is preferably 5,000 or more, more preferably 8,000 or more, even more preferably 10,000 or more, and even more preferably 20,000 or more. The upper limit is preferably 800,000 or less, more preferably 750,000 or less, even more preferably 500,000 or less, and even more preferably 350,000 or less.

[0025] The molecular weight distribution (Mw / Mn) obtained by dividing the weight average molecular weight (Mw) of the polymer block (A) by the number average molecular weight (Mn) is not particularly limited, but from the viewpoint of smoothly carrying out the second polymerization step, it is preferably 3.0 or less, more preferably 2.5 or less, even more preferably 2.0 or less, still more preferably 1.5 or less, and even more preferably 1.3 or less. The lower limit of the molecular weight distribution (Mw / Mn) is usually 1.0.

[0026] The Mw and Mn of the polymer block (A) can be measured by gel permeation chromatography (GPC) using sodium polyacrylate as a standard substance. The detailed GPC measurement conditions can be those disclosed in the Examples section below.

[0027] <Polymer Block (B)> The polymer block (B) is a polymer block different from the polymer block (A) and is a polymer block containing a structural unit derived from a monomer (b) (a (meth)acrylic acid ester monomer having a solubility of less than 1 g in 100 g of water at 20° C.) The polymer block (B) can be obtained, for example, by polymerizing a monomer composition containing the monomer (b).

[0028] Examples of the monomer (b) include (meth)acrylic acid ester monomers such as (meth)acrylic acid alkyl ester compounds and aromatic (meth)acrylic acid ester compounds. Among these, (meth)acrylic acid alkyl ester compounds and aromatic (meth)acrylic acid ester compounds are preferred in terms of excellent binder flexibility, and (meth)acrylic acid alkyl ester compounds are more preferred in terms of suppressing deterioration under high potential conditions (particularly oxidation resistance).

[0029] Examples of the (meth)acrylic acid alkyl ester compound include aliphatic alkyl (meth)acrylates and alicyclic alkyl (meth)acrylates. Examples of the aliphatic alkyl (meth)acrylates include n-butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. Examples of the alicyclic alkyl (meth)acrylates include cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, cyclodecyl (meth)acrylate, cyclododecyl (meth)acrylate, isobornyl (meth)acrylate, adamantyl (meth)acrylate, dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, and dicyclopentanyl (meth)acrylate. These compounds may be used alone or in combination of two or more.

[0030] Examples of aromatic (meth)acrylic acid ester compounds include phenyl (meth)acrylate, phenylmethyl (meth)acrylate, phenylethyl (meth)acrylate, and phenoxyethyl (meth)acrylate. One of these compounds may be used alone, or two or more of them may be used in combination.

[0031] Here, block (B) may contain structural units derived from a monomer other than monomer (b), as long as the effects of the present invention are not impaired. Examples of monomers other than monomer (b) include (meth)acrylic acid ester monomers (hereinafter also referred to as "monomer (b1)"), styrenes, and aliphatic conjugated diene monomers, each of which has a solubility of 1 g or more in 100 g of water at 20°C. Examples of monomer (b1) include methyl acrylate, ethyl acrylate, and 2-methoxyethyl acrylate.

[0032] Examples of styrenes include styrene, α-methylstyrene, β-methylstyrene, vinylxylene, vinylnaphthalene, o-methylstyrene, m-methylstyrene, p-methylstyrene, o-ethylstyrene, m-ethylstyrene, p-ethylstyrene, p-n-butylstyrene, p-isobutylstyrene, p-t-butylstyrene, o-methoxystyrene, m-methoxystyrene, p-methoxystyrene, o-chloromethylstyrene, p-chloromethylstyrene, o-chlorostyrene, p-chlorostyrene, o-hydroxystyrene, m-hydroxystyrene, p-hydroxystyrene, and divinylbenzene. One of these may be used alone, or two or more may be used in combination.

[0033] Examples of the aliphatic conjugated diene monomer include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, and the like. One of these may be used alone, or two or more may be used in combination.

[0034] As the monomer (b), a compound having an acryloyl group is preferred, since it has a fast polymerization rate, which results in a polymer having a long primary chain length, and the binding strength of the binder is good.

[0035] The content of the structural units derived from the monomer (b) relative to the total structural units of the polymer block (B) is preferably 20% by mass or more and 100% by mass or less, more preferably 30% by mass or more and 100% by mass or less, even more preferably 40% by mass or more and 100% by mass or less, even more preferably 50% by mass or more and 100% by mass or less, and may be 60% by mass or more and 100% by mass or less, 70% by mass or more and 100% by mass or less, 80% by mass or more and 100% by mass or less, or even 90% by mass or more and 100% by mass or less. By containing the monomer (b) in such a range, the stability of the emulsion particles in water is improved, and it can be uniformly adsorbed on the surface of, for example, an olivine-type lithium-containing compound. As a result, it is possible to firmly bind the positive electrode active material, and a secondary battery positive electrode with excellent binding properties can be obtained. Here, in terms of the great effect of the present invention, the water solubility is preferably 0.8 g or less, more preferably 0.7 g or less, even more preferably 0.6 g or less, still more preferably 0.5 g or less, even more preferably 0.4 g or less, particularly preferably 0.35 g or less, and may be 0.3 g or less, 0.25 g or less, 0.2 g or less, or 0.15 g or less.

[0036] The number average molecular weight (Mn) of the polymer block (B) is not particularly limited, but is preferably 1,000 or more. When the number average molecular weight is 1,000 or more, the polymer block (B) can be adsorbed to the positive electrode active material, etc., and can contribute to improving the binding property of the positive electrode.

[0037] The glass transition temperature (Tg) of the polymer block (B) is preferably 100°C or lower from the viewpoint of suppressing cracking during drying of the positive electrode mixture layer. Tg may be, for example, 80°C or lower, 60°C or lower, or 40°C or lower. Due to limitations on the constituent monomer units that can be used, the lower limit of Tg is -100°C. Tg may be, for example, -80°C or higher, or -60°C or higher.

[0038] In this specification, the Tg of the polymer block (B) can be obtained by analyzing the block polymer using differential scanning calorimetry (DSC). Here, the Tg of the polymer block (B) is a value based on the structural units derived from the monomer components of the polymer block (B), including the monomer (b). Furthermore, when DSC measurement is not possible, the Tg can also be calculated using the FOX formula from the Tg of the homopolymer of each monomer constituting the polymer block.

[0039] <Block Polymer> The block copolymer may have at least one polymer block (A) and at least one polymer block (B). Examples include an (AB) diblock copolymer consisting of polymer block (A) and polymer block (B), an (ABA) triblock copolymer consisting of polymer block (A) / polymer block (B) / polymer block (A), or a (BAB) triblock copolymer. The block copolymer may also have a structure such as (ABC) or (ABCA), which includes a polymer block (C) other than polymer block (A) and polymer block (B). Among these, the block copolymer preferably has an AB structure. Such a structure is advantageous in that it facilitates self-emulsification during polymerization and facilitates the production of an emulsion. The AB structure may be present in all or part of the copolymer, and may be, for example, a copolymer having an ABC structure. The block polymer may also be a mixture of two or more block polymers belonging to the diblock copolymer and triblock copolymer. In addition to the block polymer, the block copolymer may also contain a polymer consisting only of polymer block (A) or a polymer consisting only of polymer block (B).

[0040] The proportion of the polymer block (A) in the present block copolymer can be 0.1% by mass or more and 90% by mass or less from the viewpoints of dispersibility in the active material and conductive additive and suppression of cracking during drying of the positive electrode mixture layer. The proportion of the polymer block (A) may be 0.1% by mass or more, 0.5% by mass or more, 1% by mass or more, 3% by mass or more, 5% by mass or more, or 10% by mass or more. The proportion of the polymer block (A) may be 90% by mass or less, 80% by mass or less, 70% by mass or less, 60% by mass or less, 50% by mass or less, or 40% by mass or less. On the other hand, the proportion of the polymer block (B) in the present block polymer can be 10% by mass or more and 99% by mass or less. Within these ranges, cracking during drying of the positive electrode mixture layer can be suppressed. The proportion of the polymer block (B) may be 10% by mass or more, 30% by mass or more, 50% by mass or more, 60% by mass or more, or 70% by mass or more. The proportion of the polymer block (B) may be 99% by mass or less, 95% by mass or less, 90% by mass or less, or 85% by mass or less.

[0041] In the present block polymer, the proportion of the total amount of polymer block (A) and polymer block (B) is preferably 60% by mass or more. Within this range, particle stability is improved and particle sedimentation is suppressed, thereby improving storage stability. The proportion of the total amount of polymer block (A) and polymer block (B) is more preferably 70% by mass or more, even more preferably 80% by mass or more, and even more preferably 90% by mass or more. The proportion of the total amount of polymer block (A) and polymer block (B) may be 100% by mass.

[0042] The mass ratio of polymer block (A) to polymer block (B) in the present block copolymer is not particularly limited, but can be, for example, 0.1 to 80 / 20 to 99.9. Within this range, good binding properties can be exhibited. The mass ratio may be, for example, 0.5 to 70 / 30 to 99.5, 1.0 to 50 / 50 to 99, 5.0 to 30 / 70 to 95, or 10 to 30 / 70 to 90.

[0043] In the present composition, the block polymer is preferably used in the form of a salt in which acid groups such as carboxyl groups derived from the ethylenically unsaturated carboxylic acid monomer are neutralized so that the degree of neutralization is 40 mol% or more. The degree of neutralization is more preferably 50 mol% or more, even more preferably 60 mol% or more, even more preferably 70 mol% or more, even more preferably 80 mol% or more, and particularly preferably 90 mol% or more. The upper limit of the degree of neutralization is 100 mol%, and may be 98 mol% or less or 95 mol% or less. A degree of neutralization of 40 mol% or more is preferred in terms of improving dispersion stability in water. Furthermore, a degree of neutralization of 40 mol% or more is preferred in terms of improving dispersibility in active materials and conductive additives due to increased electrostatic repulsion between particles. In this specification, the degree of neutralization can be calculated from the amounts of monomers having acid groups such as carboxyl groups and the neutralizing agent used for neutralization. The degree of neutralization can be confirmed by subjecting the polymer or its salt to IR measurement of the binder coating film after drying treatment at 80°C for 12 hours under reduced pressure conditions, and determining the intensity ratio of the peak derived from the C=O group of the carboxylic acid to the peak derived from the C=O group of the carboxylate salt.

[0044] The type of salt of the present block polymer is not particularly limited, and examples thereof include alkali metal salts such as lithium, sodium, potassium, etc.; alkaline earth metal salts such as magnesium salt, calcium salt, barium salt, etc.; other metal salts such as aluminum salt; ammonium salt, organic amine salt, etc. Among these, alkali metal salts and alkaline earth metal salts are preferred, alkali metal salts are more preferred, and lithium salt, sodium salt, and potassium salt are even more preferred, in terms of being less likely to adversely affect battery characteristics.

[0045] <Emulsion Containing the Present Block Polymer> The emulsion according to the present invention contains the present block copolymer, and the particle size of the emulsion in water is preferably 150 to 950 nm as measured by a laser diffraction / scattering method in order to achieve an excellent capacity retention rate, and more preferably 150 to 800 nm, and even more preferably 250 to 700 nm in order to achieve excellent dispersibility in the positive electrode active material and the conductive additive.

[0046] <Method for Producing an Emulsion Containing the Present Block Polymer> A method for producing a block polymer having the above-mentioned polymer block (A) and the above-mentioned polymer block (B) includes the steps of: polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer by a living radical polymerization method to produce the polymer block (A); and emulsion polymerizing a monomer component containing monomer (b) (a (meth)acrylic acid ester monomer having a solubility of less than 1 g in 100 g of water at 20°C) in the presence of the polymer block (A) to produce the polymer block (B). While a surfactant may be used in the emulsion polymerization, polymerizing a monomer component containing monomer (b) in the presence of the polymer block (A) results in so-called "soap-free polymerization," which is preferred in that it allows for the production of a binder that does not contain a surfactant that adversely affects battery performance. Furthermore, the present production method employs the living radical polymerization method, as described above, from the viewpoints of simplicity of operation and applicability to a wide range of monomers. The method for producing the block copolymer is not particularly limited, and any known production method can be employed as long as it uses living radical polymerization. For example, a method of coupling polymers having functional groups together can be mentioned. Another method includes copolymerizing a macromonomer having polymer block (A) with a monomer constituting polymer block (B) to obtain a polymer having a structural unit consisting of polymer block (A) / polymer block (B) / polymer block (A) in the molecule.

[0047] The living radical polymerization may be any of a batch process, a semi-batch process, a tubular continuous polymerization process, a continuous stirred tank process (CSTR), etc. The polymerization method may be applied to various modes, such as bulk polymerization without using a solvent, solvent-based solution polymerization, aqueous emulsion polymerization, mini-emulsion polymerization, or suspension polymerization. Among these, emulsion polymerization is preferred because it allows for easy control of the polymerization, is easy to carry out, and produces a block polymer with excellent binding properties.

[0048] There are no particular limitations on the type of living radical polymerization method, and various polymerization methods can be used, such as reversible addition-fragmentation chain transfer polymerization (RAFT method), nitroxy radical method (NMP method), atom transfer radical polymerization (ATRP method), polymerization method using an organotellurium compound (TERP method), polymerization method using an organoantimony compound (SBRP method), polymerization method using an organobismuth compound (BIRP method), and iodine transfer polymerization. Among these, the RAFT method is preferred because of its controllability and ease of implementation, and because it can produce a block polymer with excellent binding properties.

[0049] In the RAFT method, polymerization proceeds via a reversible chain transfer reaction in the presence of a polymerization control agent (RAFT agent) and a free radical polymerization initiator. Examples of RAFT agents include a dithioester compound represented by formula (1) or a salt thereof, a trithiocarbonate compound represented by formula (2) or a salt thereof, a dithiocarbamate compound represented by formula (3) or a salt thereof, and a xanthate compound represented by formula (4) or a salt thereof. The RAFT agent may be monofunctional, having only one active site, or may be bifunctional or higher. The amount of the RAFT agent used is adjusted appropriately depending on the type of monomer and RAFT agent used, etc.

[0050] (In the formula, R １ ~R ９ represents an optionally substituted alkyl group, an optionally substituted aryl group, an optionally substituted heteroaryl group, or an optionally substituted aralkyl group, and R ６ and R ７may be bonded to each other to form a ring together with the adjacent nitrogen atom, and the ring may have a substituent.

[0051] R １ ~R ９ Examples of the "alkyl group" in the alkyl group represented by the following formula (I) which may have a substituent include linear or branched alkyl groups having 1 to 16 carbon atoms (preferably 1 to 12 carbon atoms, more preferably 1 to 6 carbon atoms, and particularly preferably 1 to 4 carbon atoms). Specific examples include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, etc. When the alkyl group has a substituent, examples of the substituent include a carboxyl group, an ester group (an alkoxycarbonyl group, etc.), a cyano group, a hydroxyl group, an alkoxy group, etc. The alkyl group may have 1 to 4 substituents selected from these substituents.

[0052] R １ ~R ９ Examples of the "aryl group" of the aryl group represented by the following formula (I) which may have a substituent include monocyclic or bicyclic aryl groups. Specific examples include a phenyl group, a toluyl group, a xylyl group, a naphthyl group, etc. When the aryl group has a substituent, examples of the substituent include a carboxyl group, an ester group (an alkoxycarbonyl group, etc.), a cyano group, a hydroxyl group, an alkoxy group, a halogen atom, etc. The aryl group may have 1 to 5 substituents selected from these substituents.

[0053] R １ ~R ９ Examples of the "heteroaryl group" of the heteroaryl group represented by the following formula (I) which may have a substituent include monocyclic or bicyclic heteroaryl groups containing at least one heteroatom selected from the group consisting of oxygen, nitrogen, and sulfur atoms as a ring-constituting atom. Specific examples include a pyridyl group, a pyrimidinyl group, and a pyrazinyl group. When the heteroaryl group has a substituent, examples of the substituent include a carboxyl group, an ester group (such as an alkoxycarbonyl group), a cyano group, a hydroxyl group, an alkoxy group, and a halogen atom. The heteroaryl group may have 1 to 4 substituents selected from these substituents.

[0054] R １ ~R ９ The "aralkyl group" of the optionally substituted aralkyl group represented by the following formula (I) means an alkyl group substituted with an aryl group, and examples thereof include a benzyl group and a phenethyl group. When the aralkyl group has a substituent, examples of the substituent include a carboxyl group, an ester group (an alkoxycarbonyl group, etc.), a cyano group, a hydroxyl group, an alkoxy group, a halogen atom, etc. The aryl group in the aralkyl group may have 1 to 5 substituents selected from these substituents.

[0055] Furthermore, R represented by formula (3) ６ and R ７ may be bonded to each other to form a ring together with the adjacent nitrogen atom, and the ring may have a substituent. Examples of such rings include a pyrrolidine ring, a piperidine ring, and a morpholine ring. When the ring has a substituent, examples of the substituent include an alkyl group and an oxo group (═O). The ring may have 1 to 3 substituents selected from these substituents.

[0056] Here, the RAFT agent in aqueous solution polymerization is preferably a water-soluble RAFT agent, and examples thereof include a compound having a thiocarbonylthio group (—CS—S—) and a hydrophilic group (for example, a carboxyl group) in the molecule and / or a salt thereof.

[0057] Examples of salts of the compounds represented by formulas (1) to (4) include, when the compound has an acidic group, alkali metal salts (sodium salt, potassium salt, etc.) and ammonium salts, and when the compound has a basic group, inorganic acid salts such as hydrochloric acid and sulfuric acid; and organic acid salts of carboxylates (acetate, etc.) and sulfonates (p-toluenesulfonate, etc.).

[0058] Among the above RAFT agents, a compound represented by formula (2) or a salt thereof is preferred. ３ and R ４are the same or different and are preferably alkyl groups which may have a substituent, more preferably alkyl groups having 1 to 12 carbon atoms (more preferably 1 to 6 carbon atoms) which may have at least one (particularly 1 to 3) substituent selected from the group consisting of a carboxyl group, an ester group (such as an alkoxycarbonyl group), a cyano group, a hydroxyl group, and an alkoxy group, and particularly preferably an alkyl group having 1 to 4 carbon atoms which may have one carboxyl group.

[0059] The compound represented by formula (2) includes, for example, a compound represented by formula (2A): (wherein R ３Ａ and R ４Ａ are the same or different and represent an alkylene group having 1 to 6 carbon atoms (particularly 1 to 4 carbon atoms) which may have at least one (particularly 1 to 3) substituent selected from the group consisting of an ester group (such as an alkoxycarbonyl group), a cyano group, a hydroxyl group, and an alkoxy group. ３Ａ and R ４Ａ is preferably an alkylene group having 1 to 3 carbon atoms, and examples thereof include 2-{[(2-carboxyethyl)sulfanylthiocarbonyl]sulfanyl}propanoic acid, 4-[(2-carboxyethylsulfanylthiocarbonyl)sulfanyl]-4-cyanopentanoic acid, etc.

[0060]

[0061] As the polymerization initiator used in polymerization by the RAFT method, known radical polymerization initiators such as azo compounds, organic peroxides, and persulfates can be used, but azo compounds are preferred because they are easy to handle safely and are less likely to cause side reactions during radical polymerization. Examples of the azo compounds include 2,2'-azobisisobutyronitrile, 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), dimethyl-2,2'-azobis(2-methylpropionate), 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis[N-(2-propenyl)-2-methylpropionamide], 2,2'-azobis(N-butyl ...1,1'-azobis(cyclohexane-1-carbonitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis[2-(2-imidazolin-2-yl)propane], 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. The radical polymerization initiators may be used alone or in combination of two or more.

[0062] Here, the polymerization initiator in the aqueous solution polymerization is preferably a water-soluble polymerization initiator, and examples thereof include a compound having a hydrophilic group (e.g., a carboxyl group) and / or a salt or hydrate thereof. Among these, 4,4'-azobis(4-cyanovaleric acid), 2,2'-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 2,2'-azobis[2-(imidazolin-2-yl)propane]disulfate dihydrate, 2,2'-azobis(2-methylpropionamidine)dihydrochloride, 2,2'-azobis[N-(2-carboxyethyl)-2-methylpropionamidine]hydrate, and the like are preferred.

[0063] The proportion of the radical polymerization initiator used is not particularly limited, but from the viewpoint of obtaining a polymer with a narrower molecular weight distribution, the amount of the radical polymerization initiator used per 1 mol of the RAFT agent is preferably 0.5 mol or less, and more preferably 0.2 mol or less. Furthermore, from the viewpoint of stably carrying out the polymerization reaction, the lower limit of the amount of the radical polymerization initiator used per 1 mol of the RAFT agent is 0.001 mol. Therefore, the amount of the radical polymerization initiator used per 1 mol of the RAFT agent is preferably in the range of 0.001 mol or more and 0.5 mol or less, and more preferably in the range of 0.005 mol or more and 0.2 mol or less.

[0064] The reaction temperature during the polymerization reaction by the RAFT method is preferably 30° C. or higher and 120° C. or lower, more preferably 40° C. or higher and 110° C. or lower, and even more preferably 50° C. or higher and 100° C. or lower. If the reaction temperature is 30° C. or higher, the polymerization reaction can proceed smoothly. On the other hand, if the reaction temperature is 120° C. or lower, side reactions can be suppressed and restrictions on usable initiators and solvents can be alleviated.

[0065] The TERP polymerization method is a method of polymerizing a water-soluble vinyl monomer in the presence of an organotellurium compound (see, for example, Chemical Review, 2009, 109, pp. 5051-5068).

[0066] The SBPR polymerization method is a method of polymerizing a water-soluble vinyl monomer in the presence of an organoantimony compound (see, for example, Chemical Review, 2009, 109, pp. 5051-5068).

[0067] The BIRP polymerization method is a method of polymerizing a water-soluble vinyl monomer in the presence of an organic bismuth compound (see, for example, Chemical Review, 2009, 109, pp. 5051-5068).

[0068] For the iodine transfer polymerization method, see, for example, Chemical Review, 2006, 106, pp. 3936-3962.

[0069] In the present disclosure, known polymerization solvents can be used in living radical polymerization. Specific examples include aromatic compounds such as benzene, toluene, xylene, and anisole; ester compounds such as methyl acetate, ethyl acetate, propyl acetate, and butyl acetate; ketone compounds such as acetone and methyl ethyl ketone; dimethylformamide, acetonitrile, dimethyl sulfoxide, alcohol, and water. The above polymerization solvents may be used alone or in combination of two or more. Among these, water or methanol is preferred from the viewpoints of controllability of polymerization, ease of implementation, and the ability to obtain a block polymer with excellent binding properties.

[0070] Here, from the viewpoint of simplifying the manufacturing process of the positive electrode slurry, when the polymerization solvent contains a solvent other than water, it is preferable to remove the solvent from the dispersion liquid after polymerization and replace it with water.

[0071] The polymer block (A) may be produced by carrying out a polymerization reaction in the presence of a basic compound. From the viewpoint of improving productivity, the monomer concentration may be 13.0% by mass or more, preferably 15.0% by mass or more, more preferably 17.0% by mass or more, even more preferably 19.0% by mass or more, and still more preferably 20.0% by mass or more.

[0072] The basic compound is a so-called alkaline compound, and either an inorganic basic compound or an organic basic compound may be used. Examples of inorganic basic compounds include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide, and alkaline earth metal hydroxides such as calcium hydroxide and magnesium hydroxide, and one or more of these may be used. Examples of organic basic compounds include ammonia and organic amine compounds such as monoethylamine, diethylamine, and triethylamine, and one or more of these may be used. Among these, alkali metal hydroxides are preferred from the viewpoint of the binding ability of a binder containing a polymer or a salt thereof.

[0073] From the viewpoint of improving productivity, it is desirable that the polymer block (B) has a monomer concentration as high as possible. The monomer concentration may be 13.0% by mass or more, preferably 15.0% by mass or more, more preferably 17.0% by mass or more, and even more preferably 20.0% by mass or more.

[0074] In the present production method, an alkali compound may be added to a dispersion containing the present block polymer obtained after the polymerization reaction to neutralize the present block polymer (hereinafter also referred to as "in-process neutralization"). Alternatively, a dispersion of the present polymer may be obtained without the in-process neutralization treatment, and then an alkali compound may be added when preparing a positive electrode slurry to neutralize the polymer (hereinafter also referred to as "post-neutralization"). Of the above, in-process neutralization is preferred from the viewpoint of imparting dispersion stability to the emulsion.

[0075] 2. Composition for Secondary Battery Positive Electrode Mixture Layer The composition for secondary battery positive electrode mix layer of the present invention comprises a binder containing an emulsion containing the present block polymer, a positive electrode active material, and water. The amount of the present block polymer used in the composition is, for example, 0.1 parts by mass or more and 20 parts by mass or less, relative to 100 parts by mass of the total amount of the positive electrode active material. The amount used is, for example, 0.2 parts by mass or more and 10 parts by mass or less, such as 0.3 parts by mass or more and 8 parts by mass or less, or for example, 0.4 parts by mass or more and 5 parts by mass or less. If the amount of the present block polymer used is less than 0.1 parts by mass, sufficient binding strength may not be obtained. Furthermore, the dispersion stability of the positive electrode active material may be insufficient, resulting in reduced uniformity of the resulting mix layer. On the other hand, if the amount of the present block polymer used exceeds 20 parts by mass, the viscosity of the composition may increase, reducing its coatability on the current collector. As a result, bumps and irregularities may occur in the resulting mix layer, adversely affecting the positive electrode characteristics.

[0076] When the amount of the block polymer used is within the above range, a composition having excellent dispersion stability of the positive electrode active material can be obtained, and a mixture layer having extremely high adhesion to the current collector can be obtained, resulting in improved battery durability. Furthermore, the block polymer exhibits sufficiently high binding ability to the positive electrode active material even in a small amount (e.g., 5% by mass or less), and because it contains carboxy anions, a positive electrode having low interfacial resistance and excellent high-rate performance can be obtained.

[0077] As the positive electrode active material, lithium salts of transition metal oxides can be used, for example, layered rock salt type and spinel type lithium-containing metal oxides can be used. Specific compounds of the layered rock salt type positive electrode active material include lithium cobalt oxide, lithium nickel oxide, and ternary NCMs {Li(Ni ｘ , Co ｙ , Mn ｚ ), x+y+z=1} and NCA{Li(Ni １－ａ－ｂ Co ａ Al b )}. Examples of spinel-type positive electrode active materials include lithium manganate. In addition to oxides, phosphates, silicates, sulfur, and the like are also used. Examples of phosphates include olivine-type lithium-containing compounds, and a specific example is LiFePO ４ (lithium iron phosphate), LiMn ｘ Fe １-ｘ P.O. ４ (lithium manganese iron phosphate, 0<x<1), LiCoPO ４ , LiMnPO ４ , Li ０．９０ Ti ０．０５ Nb ０．０５ Fe ０．３０ Co ０．３０ Mn ０．３０ P.O. ４ , LiMn ｘ Fe １-ｘ P.O. ４ Among these, LiFePO ４ and LiMn ｘ Fe １-ｘ P.O. ４ is preferred in that the effects of the present invention are large. ４is particularly preferred in that it has high stability and can improve the cycle characteristics and safety of the battery, and LiMn ｘ Fe １-ｘ P.O. ４ is particularly preferred in that it has a high potential and can improve the energy density of the battery. 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.

[0078] When a positive electrode active material containing a layered rock salt-type lithium-containing metal oxide is dispersed in water, lithium ions on the surface of the positive electrode active material are exchanged with hydrogen ions in the water, resulting in an alkaline dispersion. This may result in corrosion of aluminum foil (Al), a common positive electrode current collector material. In such cases, it is preferable to neutralize the alkali content leaching from the positive electrode active material by using an unneutralized or partially neutralized block polymer as a binder. Furthermore, it is preferable to use an amount of the unneutralized or partially neutralized block polymer such that the amount of unneutralized carboxyl groups in the block polymer is equivalent to or greater than the amount of alkali leaching from the positive electrode active material.

[0079] Because all positive electrode active materials have low electrical conductivity, they are generally used with the addition of a conductive additive. Examples of conductive additives include carbon-based materials such as carbon black, carbon nanotubes, carbon fiber, graphite powder, and carbon fiber. Of these, carbon black, carbon nanotubes, and carbon fiber are preferred because they are more likely to provide excellent conductivity. Furnace black, ketjen black, and acetylene black are also preferred as carbon black. The conductive additives may be used alone or in combination with two or more of the above. The amount of conductive additive used may be, for example, 0.2 to 20 parts by mass, or, for example, 0.2 to 10 parts by mass, per 100 parts by mass of the total positive electrode active material, from the perspective of achieving both electrical conductivity and energy density. Furthermore, the positive electrode active material may be surface-coated with a conductive carbon-based material.

[0080] The binder containing this block polymer has structural units derived from an ethylenically unsaturated carboxylic acid monomer and exhibits good dispersion stability in water. Therefore, the binder of the present invention can exhibit good dispersibility in active materials and conductive additives. Furthermore, because the binder of the present invention is in the form of particles, it exhibits uniform and excellent "point binding" even when a positive electrode active material such as olivine-type lithium iron phosphate is used, and is therefore believed to be effective in improving the durability of the resulting positive electrode.

[0081] When the composition is in a slurry state, the amount of the positive electrode active material used is, for example, in the range of 10 to 75 mass %, or, for example, in the range of 30 to 65 mass %, based on the total amount of the composition. If the amount of the active material used is 10 mass % or more, migration of the binder 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 mass % or less, the fluidity and coatability of the composition can be ensured, and a uniform mixture layer can be formed.

[0082] When the composition is prepared in a wet powder state, the amount of the positive electrode active material used is, for example, in the range of 60 to 97 mass %, or, for example, in the range of 70 to 90 mass %, based on the total amount of the composition. From the viewpoint of energy density, the amount of non-volatile components other than the positive electrode active material, such as binders and conductive additives, should be as small as possible within the range that ensures the necessary binding properties and conductivity.

[0083] The present composition uses water as a medium. Furthermore, for the purpose of adjusting the properties and drying properties of the composition, the composition may be mixed with lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, or water-soluble organic solvents such as tetrahydrofuran and N-methylpyrrolidone. The proportion of water in the mixed medium is, for example, 40% by mass or more, or, for example, 70% by mass or more.

[0084] When the composition is made into a coatable slurry state, the content of the water-containing medium relative to the entire composition can be, for example, in the range of 25 to 90 mass %, or for example, 35 to 70 mass %, from the viewpoints of the coatability of the slurry, the energy cost required for drying, and productivity. When the composition is made into a pressable wet powder state, the content of the medium can be, for example, in the range of 3 to 40 mass %, or for example, 10 to 30 mass %, from the viewpoint of the uniformity of the mixture layer after pressing.

[0085] The binder of the present invention may consist solely of the present block polymer, or may contain other binder components, such as styrene / butadiene latex (SBR), acrylic latex, polyacrylic acid (PAA) or a salt thereof (however, different from the present block polymer), carboxymethyl cellulose (CMC), and polyvinylidene fluoride latex. When other binder components are used in combination, the amount used may be, for example, 0.1 to 5 parts by mass or less, or, for example, 0.1 to 2 parts by mass or less, or, for example, 0.1 to 1 part by mass or less, relative to 100 parts by mass of the total amount of the positive electrode active material. If the amount of other binder components used exceeds 5 parts by mass, the resistance may increase, resulting in insufficient high-rate characteristics. Among the above, PAA and CMC are preferred in that they do not inhibit the dispersion of the active material and conductive additive.

[0086] The styrene / butadiene latex refers to an aqueous dispersion of a copolymer having structural units derived from an aromatic vinyl monomer such as styrene and structural units derived from an aliphatic conjugated diene monomer such as 1,3-butadiene. Examples of the aromatic vinyl monomer include α-methylstyrene, vinyltoluene, and divinylbenzene, in addition to styrene, and one or more of these can be used. The structural units derived from the aromatic vinyl monomer in the copolymer can be, for example, in the range of 20 to 70% by mass, or, for example, in the range of 30 to 60% by mass, mainly from the viewpoint of binding properties.

[0087] Examples of the aliphatic conjugated diene monomer include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, etc., and one or more of these can be used. The structural unit derived from the aliphatic conjugated diene monomer in the copolymer can be, for example, in the range of 30 to 70 mass %, or, for example, in the range of 40 to 60 mass %, in order to improve the binding properties of the binder and the flexibility of the resulting positive electrode.

[0088] In addition to the above-mentioned monomers, the styrene / butadiene latex may use other monomers as copolymerization monomers, such as nitrile group-containing monomers such as (meth)acrylonitrile, carboxyl group-containing monomers such as (meth)acrylic acid, itanconic acid, and maleic acid, and ester group-containing monomers such as methyl (meth)acrylate, in order to further improve performance such as binding ability. The structural units derived from the other monomers in the copolymer may be in the range of 0 to 30% by mass, for example, or in the range of 0 to 20% by mass.

[0089] The CMC refers to a nonionic cellulose-based semisynthetic polymer compound substituted with a carboxymethyl group and its salt. Examples of the nonionic cellulose-based semisynthetic polymer compound include alkyl celluloses such as methyl cellulose, methyl ethyl cellulose, ethyl cellulose, and microcrystalline cellulose; hydroxyethyl cellulose, hydroxybutyl methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose stearoxy ether, carboxymethyl hydroxyethyl cellulose, alkyl hydroxyethyl cellulose, and nonoxynyl hydroxyethyl cellulose.

[0090] The composition contains the above-described positive electrode active material, water, and the binder as essential components, and is obtained by mixing the components using known means. The method for mixing the components is not particularly limited, and known methods can be used. However, a preferred method involves dry-blending powder components such as the positive electrode active material and conductive additive, followed by mixing with the binder and a dispersion medium such as water, followed by dispersion and kneading. When obtaining the composition for the positive electrode mixture layer in a slurry state, it is preferable to finish the slurry without poor dispersion or aggregation. Known mixers such as planetary mixers, thin film gyratory mixers, and planetary / revolving mixers can be used as mixing means. However, thin film gyratory mixers are preferred because they can achieve a good dispersion state in a short time. When using a thin film gyratory mixer, it is also preferable to perform pre-dispersion beforehand using a stirrer such as a disperser. The viscosity of the slurry can be, for example, in the range of 100 to 10,000 mPa·s, or, for example, 1,000 to 5,000 mPa·s, as a B-type viscosity at 20 rpm.

[0091] On the other hand, when the present composition is obtained in the form of a wet powder, it is preferable to knead it to a uniform state without unevenness in concentration using a Henschel mixer, blender, planetary mixer, twin-screw kneader or the like.

[0092] 3. Secondary Battery Positive Electrode The secondary battery positive electrode of the present invention comprises a mixture layer formed from the present composition on the surface of a current collector such as aluminum. The mixture layer is formed by applying the present composition to the surface of the current collector and then drying to remove the medium, such as water. The method for applying the present composition is not particularly limited, and known methods such as doctor blade coating, dipping, roll coating, comma coating, curtain coating, gravure coating, and extrusion can be used. The drying can be performed by known methods such as hot air blowing, reduced pressure, (far) infrared radiation, and microwave irradiation. The mixture layer obtained after drying is usually subjected to a compression treatment using a mold press or roll press. Compression brings the positive electrode active material and binder into close contact, improving the strength of the mixture layer and its adhesion to the current collector. Compression can adjust the thickness of the mixture layer to, for example, approximately 30 to 80% of the thickness before compression, and the thickness of the mixture layer after compression is typically approximately 4 to 200 μm.

[0093] 4. Secondary Battery A secondary battery can be fabricated by providing the secondary battery positive electrode of the present invention with a secondary battery negative electrode, a separator, and an electrolyte. The electrolyte may be liquid or gel-like. The separator is disposed between the positive and negative electrodes of the battery and serves to prevent short circuits due to contact between the two electrodes and to retain the electrolyte to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane that has good ion permeability and mechanical strength. Specific materials that can be used include polyolefins such as polyethylene and polypropylene, and polytetrafluoroethylene.

[0094] Examples of negative electrode active materials used in the secondary battery negative electrode include carbon-based materials, lithium metal, lithium alloys, and metal oxides. One or more of these materials can be used in combination. Among these, negative electrode active materials (hereinafter also referred to as "carbon-based negative electrode active materials") made of carbon-based materials such as natural graphite, artificial graphite, hard carbon, and soft carbon are preferred, with graphite such as natural graphite and artificial graphite, and hard carbon being more preferred. In the case of graphite, spherical graphite is preferably used from the perspective of battery performance, and its particle size preferably ranges from 1 to 20 μm, for example, to 5 to 15 μm. Furthermore, to increase energy density, metals or metal oxides capable of absorbing lithium, such as silicon and tin, can also be used as negative electrode active materials. Among these, silicon has a higher capacity than graphite, and negative electrode active materials (hereinafter also referred to as "silicon-based negative electrode active materials") made of silicon-based materials such as silicon, silicon alloys, and silicon oxides such as silicon monoxide (SiO) can be used. However, while the silicon-based negative electrode active material has a high capacity, it undergoes a large volume change during charging and discharging. Therefore, it is preferable to use it in combination with the carbon-based negative electrode active material. In this case, a large amount of silicon-based negative electrode active material may cause the electrode material to collapse, significantly reducing the cycle characteristics (durability). From this perspective, when a silicon-based negative electrode active material is used in combination, the amount of silicon-based negative electrode active material used is, for example, 60% by mass or less, or, for example, 30% by mass or less, relative to the carbon-based negative electrode active material.

[0095] Since the carbon-based negative electrode active material itself has good electrical conductivity, it is not necessarily required to add a conductive additive. When a conductive additive is added for the purpose of further reducing resistance, etc., the amount used is, from the viewpoint of energy density, for example, 10 mass % or less, or, for example, 5 mass % or less, relative to the total amount of the negative electrode active material.

[0096] The electrolyte may be a known, commonly used one depending on the type of active material. Specific examples of the solvent for lithium ion secondary batteries include cyclic carbonates with high dielectric constants and high electrolyte dissolving ability, such as propylene carbonate and ethylene carbonate, and chain carbonates with low viscosity, such as ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate. These may be used alone or as a mixed solvent. The electrolyte may be prepared by dissolving LiPF in these solvents. ６ , LiSbF ６ , LiBF ４ , LiClO ４ , LiAlO ４ In nickel-metal hydride secondary batteries, an aqueous solution of potassium hydroxide can be used as the electrolyte. A secondary battery is obtained by spirally or stacking positive and negative electrode plates separated by a separator and housing them in a case or the like.

[0097] The present invention will be specifically described below based on examples. However, the present invention is not limited to these examples. In the following, "parts" and "%" mean parts by mass and % by mass unless otherwise specified.

[0098] In the Production Examples and Comparative Production Examples, the number average molecular weight of the polymer block (A), the glass transition temperature (Tg) of the polymer block (B), and the particle size of the emulsion were evaluated as follows.

[0099] <Molecular Weight Measurement> The molecular weight of the block polymer (A) was measured by gel permeation chromatography (GPC). That is, the number average molecular weight (Mn) in terms of sodium polyacrylate was obtained by aqueous GPC. The GPC was performed under the following conditions. 0.1 g of an aqueous solution containing the polymer obtained in each Production Example (0.02 g as the solid content of the polymer) was collected and diluted with 40 g of a 0.1 M aqueous sodium nitrate solution to prepare a measurement sample. The measurement sample was subjected to gel permeation chromatography (GPC) measurement under the conditions described below, and the number average molecular weight (Mn) in terms of sodium polyacrylate was calculated.

[0100] (GPC measurement conditions) Column: Tosoh TSKgel GMPW x 2 Solvent: 0.1 M aqueous sodium nitrate solution Temperature: 40°C Detector: RI Flow rate: 0.5 mL / min

[0101] <Measurement of Glass Transition Temperature (Tg) of Polymer Block (B)> The glass transition temperature (Tg) of the polymer block (B) of the block polymer contained in the obtained emulsion was determined from the intersection of the baseline and the tangent at the inflection point of a heat flux curve obtained using a differential scanning calorimeter (DSC). The heat flux curve was obtained under the following conditions: approximately 5 mg of a sample was cooled to -50°C, held for 3 minutes, then heated to 150°C at 10°C / min, subsequently cooled to -50°C, held for 3 minutes, and then heated to 150°C at 10°C / min. The Tg obtained under these measurement conditions is a value based on the "structural units derived from the monomer component including monomer (b)" contained in the polymer block (B).

[0102] (DSC measurement conditions) Measurement device: DSC 214 Polymer manufactured by NETZSCH Measurement atmosphere: Nitrogen atmosphere

[0103] <Particle size measurement> The particle size distribution of an emulsion containing a block polymer was measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT-3300EXII, manufactured by Microtrac Bell) using ion-exchanged water as the dispersion medium. An excess amount of dispersion medium was circulated in the dispersion, and the above emulsion was added in an amount sufficient to obtain an appropriate scattered light intensity. After several minutes, the shape of the measured particle size distribution 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.

[0104] <<Production of Polymer>> (Production Example 1-1: Production of Polymer F-1) For the polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser, and a nitrogen inlet tube was used. Under a nitrogen atmosphere, 400.0 parts of ion-exchanged 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") as a polymerization inhibitor (RAFT agent) were placed in the reactor and heated to 60°C. To this solution, 0.015 parts of 2,2'-azobis(2-methylpropionamidine) dihydrochloride (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd., trade name "V-50", hereinafter also referred to as "V-50") as a polymerization initiator were added, and the reaction was continued until the polymerization conversion rate reached 92%, yielding an aqueous solution of Polymer F-1. The number average molecular weight (Mn) was 225,000.

[0105] (Production Examples 1-2 to 1-9: Production of F-2 to F-9) Polymerization reaction solutions containing polymers F-2 to F-9 were obtained in the same manner as in Production Example 1-1, except that the amounts of each raw material charged were as shown in Table 1. The Mn of polymers F-2 to F-9 was measured in the same manner as for polymer F-1, and the results are shown in Table 1.

[0106]

[0107] The details of the compounds used in Table 1 are as follows: AA: acrylic acid AAm: acrylamide MAA: methacrylic acid HEMA: 2-hydroxyethyl methacrylate 4-HBA: 4-hydroxybutyl acrylate BM-1429: 2-(2-carboxyethylsulfanylthiocarbonylsulfanyl)propionic acid V-50: 2,2'-azobis(2-methylpropionamidine) dihydrochloride

[0108] (Production Example 2-1: Production of Block Polymer S-1) For polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser, and a nitrogen inlet tube was used. Under a nitrogen atmosphere, 160.0 parts of ion-exchanged water, 240.0 parts of methanol, 17.0 parts of Polymer F-1 (solids equivalent), and 83.0 parts of cyclohexyl acrylate were added to the reactor and mixed while heating to 55°C. 0.020 parts of V-50 was added to this solution, and the reaction was continued until the polymerization conversion rate exceeded 98%, yielding a polymerization reaction liquid.

[0109] (Desolvation and Neutralization) The obtained polymerization reaction solution was desolvated using an evaporator to remove methanol from the polymerization reaction solution. 8.9 parts by solids of lithium hydroxide monohydrate was added to the emulsion after desolvation, and 90 mol % of the carboxylic acid content was neutralized to obtain an emulsion of block polymer S-1. The particle size of the emulsion of S-1 was 600 nm.

[0110] (Production Examples 2-2 to 2-23 and Comparative Production Example 2-1: Production of Block Polymers S-2 to S-24) Block polymers S-2 to S-24 were obtained by the same procedure as in Production Example 2-1, except that the amounts of each raw material charged were as shown in Tables 2 and 3. Block polymers S-2 to S-23 were emulsions, and S-24 was a water-soluble polymer. The particle sizes of the emulsions of S-2 to S-23 are shown in Tables 2 and 3.

[0111]

[0112]

[0113] Details of the compounds used in Tables 2 and 3 are shown below: CHA: cyclohexyl acrylate BA: n-butyl acrylate 2-EHA: 2-ethylhexyl acrylate IBXA: isobornyl acrylate MA: methyl acrylate MEA: 2-methoxyethyl acrylate

[0114] Synthesis Example 1: Synthesis of Carboxyl Group-Containing Crosslinked Polymer Salt For polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser, and a nitrogen inlet tube was used. The reactor was charged with 567 parts of acetonitrile, 2.20 parts of ion-exchanged water, 100.0 parts of acrylic acid (hereinafter referred to as "AA"), 0.90 parts of trimethylolpropane diallyl ether (manufactured by Osaka Soda Co., Ltd., trade name "Neoallyl T-20"), and triethylamine equivalent to 1.0 mol% relative to the AA. After thoroughly replacing the atmosphere inside the reactor with nitrogen, the reactor was heated to raise the internal temperature to 55°C. After confirming that the internal temperature had stabilized at 55°C, 0.040 parts of 2,2'-azobis(2,4-dimethylvaleronitrile) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., trade name "V-65") was added as a polymerization initiator. The reaction solution became cloudy, and this point was designated as the polymerization initiation point. The polymerization reaction was continued while maintaining the internal temperature at 55°C by adjusting the external temperature (water bath temperature). 24 hours after the start of polymerization, cooling of the reaction solution was started. After the internal temperature had dropped to 25°C, lithium hydroxide monohydrate (hereinafter referred to as "LiOH·H ２ To the resulting mixture, 52.4 parts of a powder of carboxyl group-containing crosslinked polymer salt R-1 (Li salt, degree of neutralization 90 mol%) was added. After the addition, stirring was continued for 12 hours at room temperature, yielding a polymerization reaction solution in the form of a slurry in which particles of carboxyl group-containing crosslinked polymer salt R-1 (Li salt, degree of neutralization 90 mol%) were dispersed in the medium. The resulting polymerization reaction solution was centrifuged to precipitate the polymer particles, and the supernatant was removed. Thereafter, the precipitate was redispersed in the same weight of acetonitrile as the polymerization reaction solution, and then a washing procedure in which the polymer particles were precipitated by centrifugation and the supernatant was removed was repeated twice. The precipitate was recovered and dried at 80°C under reduced pressure for 3 hours to remove the volatiles, yielding a powder of carboxyl group-containing polymer salt R-1. Since the crosslinked polymer salt R-1 is hygroscopic, it was stored sealed in a container with water vapor barrier properties. The powder of crosslinked polymer salt R-1 was subjected to IR measurement, and the degree of neutralization was calculated from the intensity ratio of the peak derived from the C═O group of the carboxylic acid to the peak derived from the C═O group of the carboxylic acid Li. The result was 90 mol%, which was equal to the calculated value from the amount charged.

[0115] (Synthesis Example 2: Synthesis of SBR emulsion) A reactor was charged with 220 parts of ion-exchanged water, 22 parts of a monomer mixture consisting of 14 parts of 1,3-butadiene, 6 parts of styrene, and an acrylic acid portion, 1 part of sodium alkyldiphenyletherdisulfonate as an emulsifier, and 0.2 parts of potassium persulfate as a polymerization initiator, and the mixture was polymerized at 60°C for 18 hours with stirring to obtain an SBR emulsion.

[0116] Example 1 (Preparation of Positive Electrode Mixture Layer Composition (Positive Electrode Slurry)) Lithium iron phosphate (manufactured by BTR, hereinafter also referred to as "LFP") was used as the positive electrode active material. Acetylene black (manufactured by Denka Company, trade name "Li-400", hereinafter also referred to as "AB") was used as the conductive additive. Block polymer S-1 was used as the binder. Ion-exchanged water was added as a dilution solvent to a rotation-revolution mixer (Thinky Corporation, Awatori Rentaro) in a mass ratio of LFP:AB:block polymer S-1 = 100:9.0:3.0 (solids) so that the solids concentration of the positive electrode mix layer composition was 40 mass %, and the mixture was mixed to prepare a slurry-state positive electrode mix layer composition (positive electrode slurry).

[0117] <Evaluation of dispersibility of positive electrode slurry> Dispersibility was evaluated from the results of a grind gauge test of the positive electrode slurry. A grind gauge manufactured by BYK-Gardner (product name "Grindometer 0-50 μm") was used, and the point where the slurry stopped was taken as the measurement result. The measurement result was 14 μm, and the dispersibility was evaluated as "A" based on the following criteria. The smaller the measurement result value, the better the dispersibility. (Dispersibility evaluation criteria) A: Measurement result is less than 15 μm B: Measurement result is 15 μm or more and less than 30 μm C: Measurement result is 30 μm or more and less than 50 μm D: Measurement result is 50 μm or more

[0118] (Preparation of Positive Electrode Plate) Next, the positive electrode slurry was applied onto a 20.0 μm-thick current collector (aluminum foil) using a variable applicator, and dried in a forced-air dryer at 80° C. for 15 minutes to form a mixture layer. After that, the thickness of the mixture layer was 45±5 μm, and the mixture density was 1.90±0.10 g / cm. ３ After rolling to a thickness of 1 / 4", the mixture was punched out into a 3 cm square to obtain a positive electrode plate.

[0119] (Preparation of Negative Electrode Plate) 100 parts of graphite was added as a negative electrode active material to an ion-exchanged water solvent, and 1.5 parts (solid content) of the SBR emulsion obtained in Synthesis Example 2 and 1.1 parts of carboxymethyl cellulose sodium (hereinafter referred to as "CMC") were mixed as a negative electrode binder to prepare a composition for a negative electrode mixture layer. The composition for a negative electrode mixture layer was applied to copper foil (thickness: 16 μm) and dried to form a mixture layer. Thereafter, the thickness of the mixture layer was 37 μm, and the mixture density was 1.4 g / cm. ３ After rolling to a thickness of 1 / 4, the mixture was punched out into a 3 cm square to obtain a negative electrode plate.

[0120] (Preparation of Electrolyte Solution) Vinylene carbonate (VC) was added to a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio of EC:DMC=3:7) so that the concentration of VC was 1 mass % and fluoroethylene carbonate (FEC) was 2 mass %, and LiPF ６ was dissolved in a concentration of 1.0 mol / L to prepare a non-aqueous electrolyte.

[0121] (Preparation of Secondary Battery) The battery was constructed by attaching lead terminals to the positive and negative electrodes, and placing the electrodes facing each other via a separator (made of polyethylene: film thickness 20 μm, porosity 48%) in an aluminum laminate battery exterior, injecting the electrolyte, and sealing it to prepare a test battery. The design capacity of this prototype battery was 15 mAh. The design capacity of the battery was based on a charge cut-off voltage of 4.0 V.

[0122] <Evaluation of cycle characteristics> The lithium ion secondary battery of the laminated cell prepared above was charged and discharged at a charge / discharge rate of 1 / 3 C under conditions of CC discharge of 2.0 to 4.0 V in an environment of 25°C, and the initial capacity C ０ Furthermore, charging and discharging were repeated in an environment of 25°C at a charge / discharge rate of 1 / 2C under conditions of 2.0 to 4.0V by CC discharge, and the capacity C ３００ The cycle characteristics (ΔC) were calculated using the following formula, and the results are shown in Table 4. ΔC = C ３００ / C ０× 100 (%) The ΔC calculated by the above formula was 96%, and the cycle characteristics based on the following criteria were evaluated as "A". Note that a higher ΔC value indicates better cycle characteristics. (Criteria for Cycle Characteristics Evaluation) A: Charge / discharge capacity retention rate is 95% or more B: Charge / discharge capacity retention rate is 90% or more and less than 95% C: Charge / discharge capacity retention rate is 85% or more and less than 90% D: Charge / discharge capacity retention rate is less than 85%

[0123] <Evaluation of remaining capacity after float test> The lithium ion secondary battery of the laminated cell prepared above was CCCV charged at a charge rate of 1 / 3C with a 4.0V cutoff in a 60°C environment, and CCCV charging was continued at 4.0V for one week. After this, it was CC discharged at a discharge rate of 1 / 3C with a 2.0V cutoff in a 25°C environment, and the discharge capacity was taken as the remaining capacity, which is shown in Table 4. The discharge capacity was 98%, and the remaining capacity after the float test was evaluated as "A" based on the following criteria. Note that a higher remaining capacity indicates better suppression of positive electrode degradation at high potentials. (Criteria for determining remaining capacity after float test) A: Remaining capacity is 95% or more B: Remaining capacity is 92.5% or more but less than 95% C: Remaining capacity is 90% or more but less than 92.5% D: Remaining capacity is less than 90%

[0124] <Evaluation of DC resistance increase rate after float test> The lithium ion secondary battery of the laminated cell prepared above was adjusted to SOC = 50% at a charge / discharge rate of 1 / 3 C in an environment of 25°C. After this, the battery was discharged for 10 seconds at currents of 1 / 3 C → 1 / 2 C → 1 C → 2 C, and the slope of the current-voltage curve was calculated from the measured voltage to determine the DC resistance R before the float test. １ The current-voltage slope was calculated from the measured voltage when the battery was discharged for 10 seconds at a current of 1 / 3C, 1C, and 2C. ２ The DC resistance increase rate (ΔR) after the float test was calculated using the following formula: ΔR = R ２ / R １The ΔR calculated by the above formula was 1.15 times, and the DC resistance increase rate after the float test was evaluated as "A" based on the following criteria. The smaller the DC resistance increase rate, the more the deterioration of the positive electrode at high potential is suppressed. (Criteria for determining the DC resistance increase rate after the float test) A: DC resistance increase rate is less than 1.3 times B: DC resistance increase rate is 1.3 times or more but less than 1.5 times C: DC resistance increase rate is 1.5 times or more

[0125] Examples 2 to 27 and Comparative Examples 1 to 3 Positive electrode slurries were prepared in the same manner as in Example 1, except that the formulations were as shown in Tables 4 and 5. The dispersibility of each positive electrode slurry, as well as the cycle characteristics of the battery of the positive electrode plate obtained using each positive electrode slurry, the remaining capacity after a float test, and the rate of increase in DC resistance were evaluated, and the results are shown in Tables 4 and 5.

[0126]

[0127]

[0128] Details of the compounds used in Tables 4 and 5 are shown below: CMC: sodium carboxymethyl cellulose R-1: carboxyl group-containing crosslinked polymer salt R-1 obtained in Synthesis Example 1 SBR: styrene butadiene rubber (SBR emulsion obtained in Synthesis Example 2)

[0129] <Evaluation Results> As is clear from the results of Examples 1 to 27, the binder for secondary battery positive electrodes of the present invention exhibited excellent dispersibility in the positive electrode active material and conductive additive. In addition, the resulting secondary battery positive electrodes exhibited excellent capacity retention (cycle characteristics), high remaining capacity after a float test, and a reduced DC resistance increase rate after the test, thereby suppressing degradation under high potential conditions. Furthermore, when focusing on the Tg of the polymer block (B) of the block polymer, particularly good results were observed when the Tg was in the range of less than 50°C (Examples 1, 12 to 16, 18, and 19). This is thought to be because the Tg in the appropriate range imparted sufficient flexibility to the positive electrode mixture layer, suppressing cracking during drying of the positive electrode mixture layer, thereby providing the toughness to withstand the expansion and contraction of the positive electrode active material and the adhesiveness between the positive electrode active materials.

[0130] Furthermore, when focusing on the ratio of polymer block (A) to polymer block (B) in the block polymer, particularly good slurry dispersibility was observed when the ratio of polymer block (A) was 10% or more (Examples 1, 9 to 11). This is presumably because, when the ratio of polymer block (A) in the block polymer is 10% or more, the water dispersibility of the block polymer itself is improved, contributing to the dispersion of the positive electrode active material and conductive additive bound to the block polymer.

[0131] Furthermore, when the degree of neutralization of the carboxylic acid was taken into consideration, the 40% neutralized (Example 4) or 90% neutralized (Examples 1 and 6) particles exhibited better slurry dispersibility than the unneutralized (Example 5). This is presumably because when the neutralization rate of the carboxylic acid is low, the electrical repulsion between the block polymer particles decreases, resulting in a decrease in dispersibility of the positive electrode active material and the conductive additive.

[0132] In addition, focusing on the particle size of the emulsion containing the block polymer, the cycle characteristics improved when the particle size was in the range of 210 nm (Example 21) to 750 nm (Example 22). This is thought to be because when the particle size was 750 nm or less, the number of particles increased, increasing the number of binding points, thereby improving the binding strength of the binder with the positive electrode active material and resulting in good cycle characteristics. Furthermore, when the particle size was 210 nm or more, binder non-uniformity due to aggregation was less likely to occur, and sufficient binding strength to the positive electrode active material was exhibited, which is thought to have improved the cycle characteristics.

[0133] In contrast, when the block polymer S-24, which does not form an emulsion, was used as the binder (Comparative Example 1), the cycle characteristics were reduced. This is presumably because the block polymer S-24 swelled in the electrolyte solution and was unable to maintain its strength. When a polymer not containing the polymer block (B) was used as the binder (Comparative Example 2), the positive electrode mixture layer cracked during drying, and a positive electrode suitable for battery evaluation was not obtained. Furthermore, when only SBR was used as the binder (Comparative Example 3), dispersion of the positive electrode active material and the conductive additive was poor, and therefore a coatable positive electrode slurry was not obtained.

[0134] A composition for a secondary battery positive electrode mixture layer (positive electrode slurry) containing the binder for a secondary battery positive electrode of the present invention can be used to prepare a positive electrode in an aqueous system, and the positive electrode active material and conductive additive can be well dispersed. Therefore, a secondary battery equipped with a positive electrode obtained using the binder is expected to exhibit good resistance characteristics and durability (cycling characteristics), and is expected to be applied to an automotive secondary battery. Furthermore, a secondary battery positive electrode containing the binder for a secondary battery positive electrode of the present invention has excellent oxidation resistance, and is therefore expected to have a long lifespan. The binder for a secondary battery positive electrode of the present invention can be particularly suitably used for the positive electrode of a non-aqueous electrolyte secondary battery, and is particularly useful for non-aqueous electrolyte lithium-ion secondary batteries with high energy density.

Claims

1. A binder for a secondary battery positive electrode, comprising an emulsion containing a block polymer having a polymer block (A) and a polymer block (B), wherein the polymer block (A) contains structural units derived from an ethylenically unsaturated carboxylic acid monomer, and the polymer block (B) contains structural units derived from a (meth)acrylic acid ester monomer (hereinafter referred to as "monomer (b)") having a solubility of less than 1 g in 100 g of water at 20°C.

2. The binder for a secondary battery positive electrode according to claim 1, wherein the proportion of the polymer block (A) in the block polymer is 1% by mass or more and 50% by mass or less.

3. The binder for a secondary battery positive electrode according to claim 1 or 2, wherein the polymer block (A) contains structural units derived from an ethylenically unsaturated carboxylic acid monomer in an amount of 50 mass% or more based on the total structural units of the polymer block (A).

4. The binder for a secondary battery positive electrode according to claim 1 or 2, wherein the polymer block (B) contains structural units derived from the monomer (b) in an amount of 50 mass % or more based on the total structural units of the polymer block (B).

5. The binder for a secondary battery positive electrode according to claim 1 or 2, wherein the block polymer does not contain a structural unit derived from a crosslinkable monomer.

6. The binder for a secondary battery positive electrode according to claim 1 or 2, wherein the block polymer is a salt in which 40 mol % or more of the carboxyl groups of the block polymer have been neutralized.

7. The binder for secondary battery positive electrodes according to claim 1 or 2, wherein the particle size of the emulsion is 150 to 950 nm as measured by a laser diffraction / scattering method.

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

9. The composition for a secondary battery positive electrode mixture layer according to claim 8, wherein the positive electrode active material includes an olivine-type lithium-containing compound.

10. A secondary battery positive electrode comprising a current collector having a mixture layer formed from the composition for a secondary battery positive electrode mixture layer according to claim 8 on the surface thereof.

11. A secondary battery comprising the secondary battery positive electrode according to claim 10.

12. A method for producing a binder for a secondary battery positive electrode, which contains an emulsion containing a block polymer, wherein the block polymer has a polymer block (A) and a polymer block (B), the method comprising: a step of producing the polymer block (A) by polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer by a living radical polymerization method; and a step of producing the polymer block (B) by emulsion polymerizing, in the presence of the polymer block (A), a monomer component containing a (meth)acrylic acid ester monomer having a solubility of less than 1 g in 100 g of water at 20°C.

13. The method for producing a binder for a secondary battery positive electrode according to claim 12, wherein the living radical polymerization method is a reversible addition-fragmentation chain transfer polymerization method (RAFT method).

14. The method for producing a binder for a secondary battery positive electrode according to claim 12 or 13, wherein the emulsion polymerization is soap-free polymerization.

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