Aqueous binder for secondary-battery positive electrode and use thereof

Abstract

This aqueous binder for a secondary-battery positive electrode contains polymer particles and a polyvinyl-alcohol-based resin, wherein: the polymer particles have structural units derived from an ethylenically unsaturated monomer; the glass transition temperature of the polymer particles is 30°C or lower; the particle diameter of the polymer particles is greater than 200 nm and no greater than 600 nm as a scattering intensity reference median diameter (D50) measured by using a dynamic light scattering method; and the polyvinyl-alcohol-based resin substantially does not have any 1,2-diol structures in a side chain.

Description

Aqueous binders for the positive electrode of secondary batteries and their applications

[0001] This invention relates to an aqueous binder for the positive electrode of a secondary battery and its use.

[0002] Various energy storage devices, such as nickel-metal hydride batteries, lithium-ion batteries, and electric double-layer capacitors, have been put into practical use as secondary batteries. The electrodes used in these secondary batteries are manufactured by coating and drying a composition for forming an electrode mixture layer containing an active material and a binder onto a current collector. For example, in lithium-ion batteries, an aqueous binder containing styrene-butadiene rubber (SBR) latex and carboxymethylcellulose (CMC) is used as the binder for the negative electrode mixture layer composition. In addition, aqueous binders containing aqueous solutions or aqueous dispersions of acrylic acid polymers are known to have excellent dispersibility and binding properties. On the other hand, organic solvent-based binders, such as an N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride (PVDF), are widely used as binders for the positive electrode mixture layer composition.

[0003] In recent years, as the applications of various secondary batteries that achieve carbon neutrality have expanded, there has been a growing demand for reducing the environmental impact of battery manufacturing processes. For this reason, in addition to using lithium iron phosphate (LFP) and lithium manganese iron phosphate (LMFP), which have high stability to water, as cathode active materials, water-based cathode mixture compositions that do not use organic solvents are being investigated by using water-based binders.

[0004] SBR latex can be used as an aqueous binder in the cathode composite layer composition. However, SBR has insufficient oxidation resistance, which is problematic as it degrades due to oxidation under high cathode potentials. Therefore, aqueous acrylic binders with superior oxidation resistance have been developed.

[0005] Patent Document 1 discloses a binder for a secondary battery positive electrode, comprising a polymer containing at least one of carboxylic acid group-containing monomer units and sulfonic acid group-containing monomer units in an amount of 3% to 40% by mass, and 55% to 90% by mass of hydroxyl group-containing acrylic acid ester monomer units. It is stated that a slurry composition containing this binder exhibits excellent dispersibility and can provide excellent adhesion and water-removal properties to the positive electrode composite layer.

[0006] International Publication No. 2024 / 024410

[0007] However, while the binder for the positive electrode of a secondary battery disclosed in Patent Document 1 has excellent dispersibility with respect to the positive electrode active material and conductive additive, and excellent bonding properties for the positive electrode mixture layer, the flexibility of the positive electrode mixture layer is low, which sometimes leads to cracking problems.

[0008] The present invention has been made in view of these circumstances, and aims to provide an aqueous binder for secondary battery positive electrodes that exhibits excellent dispersibility with positive electrode active material and conductive additive, as well as excellent binding properties and flexibility of the positive electrode mixture layer. Furthermore, the present invention aims to provide a composition for secondary battery positive electrode mixture layer containing the above binder, a secondary battery positive electrode obtained using the composition, and a secondary battery.

[0009] As a result of diligent research to solve the above problems, the present inventors have found that in an aqueous binder for the positive electrode of a secondary battery containing polymer particles of a specific structure and a polyvinyl alcohol-based resin of a specific structure, by setting the glass transition temperature and particle size of the polymer particles within a specific range, the binder exhibits excellent dispersibility with respect to the positive electrode active material and conductive additive, as well as excellent binding properties and flexibility of the positive electrode mixture layer, thereby completing the present invention.

[0010] The present invention is as follows: [1] An aqueous binder for a secondary battery positive electrode containing polymer particles and a polyvinyl alcohol resin, wherein the polymer particles have structural units derived from ethylenically unsaturated monomers, the glass transition temperature of the polymer particles is 30°C or less, the particle size of the polymer particles is greater than 200 nm and less than or equal to 600 nm as the scattering intensity reference median diameter (D50) measured by dynamic light scattering, and the polyvinyl alcohol resin substantially does not have 1,2-diol structures in its side chains. [2] The aqueous binder for a secondary battery positive electrode according to [1], wherein the ethylenically unsaturated monomer includes an ethylenically unsaturated carboxylic acid ester monomer. [3] The aqueous binder for a secondary battery positive electrode according to [1] or [2], wherein the polymer particles have 30% by mass or less of structural units derived from ethylenically unsaturated carboxylic acid monomers relative to their total structural units. [4] The aqueous binder for the positive electrode of a secondary battery according to [3], wherein the polymer particles have 35% by mass or less of structural units derived from a hydroxyl group-containing ethylenically unsaturated monomer relative to their total structural units. [5] The aqueous binder for the positive electrode of a secondary battery according to any one of [1] to [4], wherein the polymer particles substantially do not have a structure grafted with polyvinyl alcohol. [6] The aqueous binder for the positive electrode of a secondary battery according to any one of [1] to [5], wherein the degree of polymerization of the polyvinyl alcohol resin is 200 to 2,000. [7] The aqueous binder for the positive electrode of a secondary battery according to any one of [1] to [6], wherein the solid content mass ratio (polymer particles / polyvinyl alcohol resin) of the polymer particles to the polyvinyl alcohol resin is 99 / 1 to 40 / 60 when the total amount of the polymer particles and the polyvinyl alcohol resin is 100. [8] A composition for a secondary battery positive electrode composite layer, comprising a water-based binder for a secondary battery positive electrode, a positive electrode active material, and water, as described in any one of [1] to [7]. [9] The composition for a secondary battery positive electrode composite layer, as described in [8], wherein the positive electrode active material comprises an olivine-type lithium-containing compound.

[10] A secondary battery positive electrode comprising a composite layer formed on the surface of a current collector from the composition for a secondary battery positive electrode composite layer, as described in [8] or [9].

[11] A secondary battery comprising the secondary battery positive electrode as described in

[10] .

[0011] According to the aqueous binder for the positive electrode of a secondary battery of the present invention, it is possible to obtain a composition for the positive electrode mixture layer of a secondary battery that has excellent dispersibility with respect to the positive electrode active material and conductive additive, and to obtain a secondary battery that has excellent binding properties and flexibility of the positive electrode mixture layer.

[0012] The aqueous binder for the positive electrode of a secondary battery of the present invention (hereinafter also referred to as "this binder") contains the above polymer particles (hereinafter also referred to as "this polymer particles") and the above polyvinyl alcohol-based resin (hereinafter also referred to as "this PVA"), and can be mixed with a positive electrode active material and water to form a composition for the negative electrode mixture layer of a secondary battery (hereinafter also referred to as "this composition"). The above composition is preferably in the form of an electrode slurry that can be coated onto a current collector, in order to achieve the effects of the present invention, but it may also be prepared in the form of a wet powder to accommodate press processing on the surface of a current collector. The positive electrode of a secondary battery of the present invention can be obtained by forming a mixture layer formed from the above composition on the surface of a current collector such as aluminum foil.

[0013] The polymer particles, PVA, binder, composition for secondary battery positive electrode mixture layer, secondary battery positive electrode, and secondary battery will be described in detail below. In this specification, "(meth)acrylic" means acrylic and / or methacrylic, and "(meth)acrylate" means acrylate and / or methacrylate. Also, "(meth)acryloyl group" means acryloyl group and / or methacryloyl group. In the numerical ranges described stepwise in this specification, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range described stepwise, and the upper or lower limit of that numerical range may be replaced with the value shown in the example.

[0014] 1. This polymer particle binder, by containing this polymer particle together with this PVA, exhibits excellent dispersibility with the positive electrode active material and conductive additive, as well as excellent binding and flexibility of the positive electrode mixture layer. <Glass transition temperature of this polymer particle> The glass transition temperature of this polymer particle (hereinafter also simply referred to as "Tg") is 30°C or lower, preferably 20°C or lower, in terms of excellent flexibility of the positive electrode mixture layer, and more preferably 0°C or lower, in terms of excellent flexibility of the positive electrode mixture layer. Furthermore, it is even more preferable to have a temperature of -10°C or lower, and even more preferable to have a temperature of -15°C or lower, in terms of excellent dispersibility with the positive electrode active material and conductive additive, as well as excellent binding and flexibility of the positive electrode mixture layer. The lower limit of Tg of this polymer particle is not particularly limited, but may be, for example, -60°C, -50°C, or -40°C. In this specification, Tg can be measured by the differential scanning calorimeter (DSC) described in the examples.

[0015] <Particle Size of the Polymer Particles> Regarding the particle size of the polymer particles, in terms of excellent flexibility and binding properties of the positive electrode mixture layer, the scattering intensity group median diameter (D50) measured by dynamic light scattering is preferably between 200 nm and 600 nm, more preferably between 220 nm and 550 nm, more preferably between 240 nm and 520 nm, and even more preferably between 260 nm and 500 nm. If the particle size is 200 nm or less, the flexibility of the positive electrode mixture layer decreases. This is presumed to be because the required elongation per polymer particle increases with respect to bending of the positive electrode mixture layer, making it more prone to cracking. On the other hand, if the particle size is greater than 600 nm, the binding properties of the positive electrode mixture layer decrease. This is presumed to be because the number of polymer particles binding to the positive electrode active material is small, and the amount of polymer particles contributing to the binding of the positive electrode active materials is small. In this specification, the particle size can be measured by the dynamic light scattering method described in the examples.

[0016] Furthermore, the particle size of the polymer particles is preferably larger than the pore size of the positive electrode active material, as this provides excellent binding properties and flexibility to the positive electrode mixture layer. This is presumed to be because the PVA preferentially adsorbs onto the positive electrode active material, suppressing the penetration of the polymer particles into the pores of the positive electrode active material and allowing the positive electrode active material and the polymer particles to bind strongly together.

[0017] <Structural Units of the Polymer Particles> The monomers constituting the polymer particles are not particularly limited, but it is preferable that they have structural units derived from ethylenically unsaturated monomers. The content of such structural units is preferably 50% by mass or more and 100% by mass or less, more preferably 60% by mass or more and 100% by mass or less, even more preferably 70% by mass or more and 100% by mass or less, and even more preferably 80% by mass or more and 100% by mass or less, relative to the total structural units of the polymer particles. Examples of ethylenically unsaturated monomers include ethylenically unsaturated carboxylic acid ester monomers (hereinafter also referred to as "monomer (a1)"), ethylenically unsaturated carboxylic acid monomers (hereinafter also referred to as "monomer (a2)"), hydroxyl group-containing ethylenically unsaturated monomers (hereinafter also referred to as "monomer (a3)"), nitrile group-containing ethylenically unsaturated monomers (hereinafter also referred to as "monomer (a4)"), (meth)acrylamide and its derivatives, maleimide compounds, etc. Among these, it is preferable that the polymer particles have structural units derived from monomer (a1) in order to have excellent oxidation resistance.

[0018] Furthermore, it is preferable that the polymer particles substantially do not have a structure grafted with polyvinyl alcohol. Here, in this specification, "not having a structure grafted with polyvinyl alcohol" means that the polymer particles do not exhibit properties derived from such a structure. However, it is permissible for the polymer particles to have a trace amount of a structure grafted with polyvinyl alcohol to an extent that does not hinder the effects of the present invention. Specifically, the proportion of the structure grafted with polyvinyl alcohol in the polymer particles is typically 2% by mass or less, preferably 1% by mass or less, more preferably 0.5% by mass or less, and even more preferably 0.1% by mass or less, and it is particularly preferable that the polymer particles do not have a structure grafted with polyvinyl alcohol.

[0019] Furthermore, it is preferable that the polymer particles substantially do not contain structural units derived from aromatic vinyl monomers. Here, in this specification, "structural units derived from aromatic vinyl monomers" means that the polymer particles do not exhibit properties derived from such structures. However, it is permissible for the polymer particles to have trace amounts of structural units derived from aromatic vinyl monomers, to the extent that they do not hinder the effects of the present invention. Specifically, the proportion of structural units derived from aromatic vinyl monomers in the polymer particles is typically 2% by mass or less, preferably 1% by mass or less, more preferably 0.5% by mass or less, and even more preferably 0.1% by mass or less, and it is particularly preferable that they do not contain structural units derived from aromatic vinyl monomers.

[0020] <Structural units derived from monomer (a1)> The monomer (a1) is preferably a (meth)acrylate monomer, for example, alkyl (meth)acrylate ester compounds such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; aromatic (meth)acrylate ester compounds such as phenyl (meth)acrylate, phenylmethyl (meth)acrylate, phenylethyl (meth)acrylate, and phenoxyethyl (meth)acrylate; alkoxyalkyl (meth)acrylate ester compounds such as 2-methoxyethyl (meth)acrylate and 2-ethoxyethyl (meth)acrylate, and one of these may be used alone or two or more may be used in combination.

[0021] The content of structural units derived from monomer (a1) in the polymer particles is not particularly limited, but for example, it can be 50% by mass or more relative to the total structural units of the polymer particles. Including structural units derived from (a1) within this range can improve the oxidation resistance of the polymer particles, with 60% by mass or more being preferred, 65% by mass or more being more preferred, 70% by mass or more being even more preferred, 75% by mass or more being even more preferred, and 80% by mass or more being even more preferred.

[0022] <Structural units derived from monomer (a2)> Examples of monomer (a2) include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid; (meth)acrylamide alkyl carboxylic acids such as (meth)acrylamidehexanoic acid and (meth)acrylamidedodecanoic acid; monohydroxyethyl (meth)acrylate succinate, ω-carboxy-caprolactone mono(meth)acrylate, β-carboxyethyl (meth)acrylate, etc. One of these may be used alone, or two or more may be used in combination.

[0023] The content of structural units derived from monomer (a2) in the polymer particles is not particularly limited, but for example, it may be 30% by mass or less relative to the total structural units of the polymer particles. Including structural units derived from (a2) within this range can improve the ability to suppress dissolution in the electrolyte, and is preferably 25% by mass or less, more preferably 20% by mass or less, even more preferably 15% by mass or less, even more preferably 10% by mass or less, and even more preferably 5% by mass or less. The lower limit of the content of structural units derived from monomer (a2) in the polymer particles is not particularly limited, but for example, it may be 0.1% by mass, 0.5% by mass, or 1% by mass relative to the total structural units of the polymer particles.

[0024] <Structural units derived from monomer (a3)> Examples of monomer (a3) ​​include hydroxyalkyl ester compounds of (meth)acrylate such as 2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate. One of these may be used alone, or two or more may be used in combination.

[0025] The content of structural units derived from monomer (a3) ​​in the polymer particles is not particularly limited, but for example, it may be 35% by mass or less relative to the total structural units of the polymer particles. Including structural units derived from (a3) ​​within this range can improve the ability to suppress dissolution in the electrolyte, and is preferably 30% by mass or less, more preferably 25% by mass or less, even more preferably 20% by mass or less, even more preferably 15% by mass or less, and even more preferably 10% by mass or less. The lower limit of the content of structural units derived from monomer (a3) ​​in the polymer particles is not particularly limited, but for example, it may be 0.1% by mass, 0.5% by mass, 1% by mass, or 2% by mass relative to the total structural units of the polymer particles. In particular, it is preferable that the polymer particles have structural units derived from monomer (a3) ​​in addition to structural units derived from monomer (a2), as this can further improve the ability to suppress dissolution in the electrolyte. This is presumed to be due to the reaction of carboxylic acid and hydroxyl group during the drying of the composition for the positive electrode layer of secondary batteries.

[0026] <Other Structural Units> As other structural units in the polymer particles, structural units derived from nitrile group-containing ethylenically unsaturated monomers (monomer (a4)), structural units derived from (meth)acrylamide and its derivatives, structural units derived from maleimide compounds, etc. can be mentioned.

[0027] As the nitrile group-containing ethylenically unsaturated monomer, for example, (meth)acrylonitrile; (meth)acrylic acid cyanoalkyl ester compounds such as cyanoethyl (meth)acrylate and cyanomethyl (meth)acrylate; cyano group-containing unsaturated aromatic compounds such as 4-cyanostyrene and 4-cyano-α-methylstyrene; vinylidene cyanide, etc. can be mentioned. One of these can be used alone, or two or more of them can be used in combination.

[0028] As the (meth)acrylamide derivative, for example, N-alkyl (meth)acrylamide compounds such as N-isopropyl (meth)acrylamide and N-t-butyl (meth)acrylamide; N-alkoxyalkyl (meth)acrylamide compounds such as N-n-butoxymethyl (meth)acrylamide and N-isobutoxymethyl (meth)acrylamide; N,N-dialkyl (meth)acrylamide compounds such as N,N-dimethyl (meth)acrylamide and N,N-diethyl (meth)acrylamide, cyclic (meth)acrylamide compounds such as 4-acryloylmorpholine, etc. can be mentioned. One of these can be used alone, or two or more of them can be used in combination.

[0029] As the maleimide compound, maleimide and N-substituted maleimide compounds are included. 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, N-stearylmaleimide, etc.; N-cycloalkyl-substituted maleimide compounds such as N-cyclopentylmaleimide, N-cyclohexylmaleimide, etc.; 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, N-benzylmaleimide, etc. One of these may be used alone, or two or more of them may be used in combination.

[0030] Among these, in terms of excellent binding property of the positive electrode active material layer, it is preferable that the polymer particles have a structural unit derived from the monomer (a4). The content of the other structural units in the polymer particles is not particularly limited. For example, it is 25% by mass or less, for example 20% by mass or less, for example 15% by mass or less, for example 10% by mass or less, for example 5% by mass or less, based on all the structural units of the polymer particles. The lower limit of the content of the other structural units in the polymer particles is not particularly limited, but it may be 0.1% by mass, 0.5% by mass, 1% by mass, or 5% by mass, based on all the structural units of the polymer particles.

[0031] <Method for Producing the Polymer Particles> The polymer particles can be produced using known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization, which can be appropriately selected based on molecular weight or composition. While known polymerization initiators such as azo compounds, organic peroxides, and inorganic peroxides can be used, they are not particularly limited. The conditions for use can be adjusted to achieve an appropriate radical generation amount using known methods such as thermal initiation, redox initiation with a reducing agent, and UV initiation. Furthermore, known chain transfer agents may be used as needed for purposes such as adjusting molecular weight.

[0032] Here, among polymerization methods, emulsion polymerization is preferred because it can obtain polymer particles of the above particle size and greatly enhances the effects of the present invention. Examples of emulsion polymerization methods include a batch reaction in which monomers, surfactant, and water are all charged into a reaction vessel and reacted, and a dropwise reaction in which monomers are gradually added dropwise to the reaction vessel and reacted. Among these, the dropwise reaction is preferred because it is easier to control the heat generated during the polymerization reaction. Furthermore, in order to further improve polymerization stability, in the dropwise reaction, it is preferable to mix and stir monomers, water, and surfactant to form a monomer preemulsification before adding the monomers dropwise. In this case, a solvent mainly consisting of water is present in the reaction vessel. It is preferable to mix the surfactant into the solvent. Furthermore, it is preferable to preheat the solvent, and the heating conditions are, for example, 50 to 120°C and 70 to 100°C.

[0033] Emulsion polymerization is preferably carried out in the presence of at least one of a surfactant and a protective colloid. The surfactant is preferably anionic, cationic, or nonionic, with anionic and nonionic being more preferred. Polymerizable surfactants having ethylenically unsaturated double bonds can also be used.

[0034] Anionic surfactants are surfactants that can become ions in aqueous solutions, with the hydrophilic portion being an anion. Nonionic surfactants are surfactants that exhibit surface activity without dissociating into ions in aqueous solutions. Cationic surfactants are surfactants that can become ions in aqueous solutions, with the hydrophilic portion being a cationic. Polymerizable surfactants are anionic or nonionic surfactants that have one or more unsaturated double bonds in their molecule that are capable of radical polymerization.

[0035] The surfactant can be used alone or in combination of two or more types. There are no particular restrictions on the amount of surfactant used, but it is preferable to include 0.1 to 20 parts by mass per 100 parts by mass of monomer mixture (in this specification, "monomer mixture" means a mixture containing monomers and a chain transfer agent). Using an appropriate amount of surfactant further improves the mechanical stability of the resin particles, and using an appropriate amount of polymerizable surfactant further improves mechanical stability. If the amount used is less than 0.1 parts by mass, it becomes difficult to ensure emulsification stability. Also, if it exceeds 20 parts by mass, the water resistance decreases significantly.

[0036] For emulsion polymerization, it is preferable to use a radical polymerization initiator (hereinafter also referred to as "polymerization initiator"). The polymerization initiator can be a known oil-soluble polymerization initiator or a water-soluble polymerization initiator. Examples of oil-soluble initiators include organic peroxides such as benzoyl peroxide, tert-butyloxybenzoate, tert-butyl hydroperoxide, tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxy-3,5,5,trimethylhexanoate, ditert-butyl peroxide, cumene hydroperoxide, and p-menthane hydroperoxide, as well as azobis compounds such as 2,2'-azobisisobutyronitrile, 2,2'-azobis-2,4-dimethylvaleronitrile, 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), and 1,1'-azobis-cyclohexane-1-carbonitride. Examples of water-soluble polymerization initiators include ammonium persulfate, sodium persulfate, potassium persulfate, hydrogen peroxide, and 2,2'-azobis(2-methylpropionamidine) dihydrochloride.

[0037] In emulsion polymerization, a reducing agent can be used in combination with a polymerization initiator. This can accelerate the polymerization reaction. Examples of such reducing agents include reducing organic compounds such as ascorbic acid, erythorbic acid, tartaric acid, citric acid, glucose, and metal salts such as formaldehyde sulfoxylate; reducing inorganic compounds such as sodium sulfite, sodium bisulfite, sodium metabisulfite (SMBS), and sodium hyposulfite; and ferrous chloride, longalite, and thiourea dioxide.

[0038] For emulsion polymerization, it is preferable to use a water-soluble polymerization initiator. It is preferable to use 0.05 to 5% by mass of the polymerization initiator per 100 parts by mass of the monomer mixture. It is preferable to use 0.01 to 2.5% by mass of the reducing agent per 100 parts by mass of the monomer mixture.

[0039] During emulsion polymerization, buffers, chain transfer agents, basic compounds, etc., can be used as needed. Examples of buffers include sodium acetate, sodium citrate, and sodium bicarbonate. Examples of chain transfer agents include 2-mercaptoethanol, octyl mercaptan, tertial decyl mercaptan, lauryl mercaptan, stearyl mercaptan, 2-ethylhexyl mercaptoacetate, octyl mercaptoacetate, 2-ethylhexyl mercaptopropionate, and octyl mercaptopropionate.

[0040] 2. The PVA This binder, by containing the PVA together with the polymer particles, exhibits excellent binding properties and flexibility in the positive electrode mixture layer. This is presumed to be because the PVA preferentially adsorbs to the positive electrode active material, suppressing the penetration of the polymer particles into the pores of the positive electrode active material and allowing the positive electrode active material and the polymer particles to bind strongly. The PVA contains vinyl alcohol-based polymers and their derivatives, and substantially does not have structural units having a 1,2-diol structure in its side chain. Here, in this specification, "substantially does not have a 1,2-diol structure" means that the PVA does not exhibit properties derived from a 1,2-diol structure. However, it is permissible for the PVA to have a trace amount of 1,2-diol structure to an extent that does not hinder the effects of the present invention. Specifically, the proportion of 1,2-diol structures in this PVA is typically 2% by mass or less, preferably 1% by mass or less, more preferably 0.5% by mass or less, and even more preferably 0.1% by mass or less, and it is particularly preferable that it does not contain 1,2-diol structures.

[0041] One method for producing this PVA is to saponify a polymer obtained by using a known polymerization method (solution polymerization, precipitation polymerization, suspension polymerization, emulsion polymerization, etc.) on monomer components containing vinyl ester compounds such as vinyl acetate and vinyl propionate. Vinyl acetate is preferred as the vinyl ester compound because the raw materials are readily available and the saponification reaction proceeds easily. One vinyl ester compound may be used alone, or two or more may be used in combination. Furthermore, this PVA may be "unmodified PVA" consisting only of vinyl alcohol units, or it may be "modified PVA" further containing units derived from monomers other than vinyl alcohol units as a modifying component.

[0042] The degree of polymerization of this PVA is preferably 200 to 2,000, more preferably 300 to 2,000, in terms of excellent binding properties to the positive electrode mixture layer, and even more preferably 500 to 1,500, in terms of excellent dispersibility with the positive electrode active material and conductive additive.

[0043] The lower limit of the degree of saponification of this PVA (molar fraction of hydroxyl groups to the total of hydroxyl groups and ester bonds in the PVA) is preferably 70 mol% or more, more preferably 75 mol% or more, even more preferably 80 mol% or more, and even more preferably 85 mol% or more, in terms of excellent dispersibility with the positive electrode active material and conductive additive. The upper limit of the degree of saponification of this PVA may be 100 mol%, but is preferably 99.99 mol% or less, and more preferably 99 mol% or less. The degree of saponification can be measured in accordance with JIS-K6726:1994.

[0044] 3. The Binder The binder contains the polymer particles and the PVA, and can be manufactured by mixing them using conventional methods. In the binder, the solid content mass ratio of the polymer particles to the PVA (polymer particles / PVA) is preferably 99 / 1 to 40 / 60, when the total amount of the polymer particles and the PVA is set to 100. When the solid content mass ratio of the polymer particles to the PVA is within the above range, a positive electrode mixture layer with excellent binding properties and flexibility can be obtained. The solid content mass ratio of the polymer particles to the PVA is more preferably 95 / 5 to 50 / 50, even more preferably 95 / 5 to 60 / 40, even more preferably 95 / 5 to 70 / 30, and even more preferably 95 / 5 to 80 / 20.

[0045] 4. Composition for the Positive Electrode Layer of a Secondary Battery The composition for the positive electrode layer of a secondary battery of the present invention comprises this binder, a positive electrode active material, and water. The amount of this binder used in this composition is, for example, 0.1 parts by mass or more and 20 parts by mass or less, per 100 parts by mass of the total amount of positive electrode active material. The above amount may also be, for example, 0.2 parts by mass or more and 10 parts by mass or less, for example, 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 this binder used is less than 0.1 parts by mass, sufficient binding may not be obtained. In addition, the dispersion stability of the positive electrode active material, etc. may become insufficient, and the uniformity of the formed mixture layer may decrease. On the other hand, if the amount of this binder used exceeds 20 parts by mass, this composition may become highly viscous, and its coating properties to the current collector may decrease. As a result, bumps and irregularities may occur in the resulting mixture layer, which may adversely affect the positive electrode characteristics.

[0046] If the amount of this binder used is within the above range, a composition with excellent dispersion stability of the positive electrode active material can be obtained, as well as a composite layer with extremely high adhesion to the current collector, resulting in improved battery durability. Furthermore, this binder exhibits sufficiently high binding properties even in small amounts (e.g., 5% by mass or less) relative to the positive electrode active material, and because it contains carboxyanions, a positive electrode with low interfacial resistance and excellent high-rate characteristics can be obtained.

[0047] As the positive electrode active material, a lithium salt of a transition metal oxide 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 cobaltate, lithium nickelate, and NCM {Li(Ni ｘ , Co ｙ , Mn ｚ ), x + y + z = 1} and NCA {Li(Ni １－ａ－ｂ Co ａ Al b ), etc. Examples of the spinel type positive electrode active material include lithium manganate. In addition to oxides, phosphates, silicates, sulfur, etc. are also used. As the phosphate, olivine type lithium-containing compounds are included. Specific examples include LiFePO ４ (lithium iron phosphate), LiMn ｘ Fe １-ｘ PO 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 PO ４ , LiMn ｘ Fe １-ｘ ]]PO ４ , etc. Among these, LiFePO ４ and LiMn ｘ Fe １-ｘ PO ４ are preferable in terms of the large effects achieved by the present invention. Further, LiFePO ４ is particularly preferable in that its high stability can improve the cycle characteristics and safety of the battery, and LiMn ｘ Fe １-ｘ PO ４ is particularly preferable in that its high potential 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.

[0048] Furthermore, when a positive electrode active material containing layered rock salt-type lithium-containing metal oxide is dispersed in water, the dispersion becomes alkaline due to the exchange of lithium ions on the surface of the positive electrode active material with hydrogen ions in the water. This may cause corrosion of common positive electrode current collector materials such as aluminum foil (Al). In such cases, it is preferable to neutralize the alkali leaching from the positive electrode active material by using an unneutralized or partially neutralized binder. It is also preferable to use an amount of the unneutralized or partially neutralized binder such that the amount of unneutralized carboxyl groups in the binder is equivalent to or greater than the amount of alkali leaching from the positive electrode active material.

[0049] Since all positive electrode active materials have low electrical conductivity, they are generally used with the addition of a conductive additive. Examples of conductive additives include carbon-based materials such as carbon black, carbon nanotubes, carbon fibers, graphite powder, and carbon fibers. Of these, carbon black, carbon nanotubes, and carbon fibers are preferred because they easily provide excellent conductivity. Among carbon blacks, furnace black, Ketjen black, and acetylene black are preferred. One of the above conductive additives may be used alone, or two or more may be used in combination. From the viewpoint of balancing conductivity and energy density, the amount of conductive additive used can be, for example, 0.05 to 20 parts by mass, or for example, 0.05 to 10 parts by mass, per 100 parts by mass of the total amount of positive electrode active material. The positive electrode active material may also be a carbon-based material with a conductive surface coating.

[0050] When the composition is in a slurry state, the amount of positive electrode active material used is, for example, in the range of 10 to 75% by mass, or in the range of 30 to 65% by mass, relative to the total amount of the composition. If the amount of active material used is 10% by mass or more, migration of binders and the like is suppressed, and it is also advantageous in terms of the drying cost of the medium. On the other hand, if it is 75% by mass or less, the fluidity and coating properties of the composition can be ensured, and a uniform mixture layer can be formed.

[0051] Furthermore, when preparing this composition in a wet powder state, the amount of positive electrode active material used is, for example, in the range of 60 to 97% by mass, or in the range of 70 to 90% by mass, relative to the total amount of this composition. Also, from the viewpoint of energy density, it is preferable to use as little nonvolatile components other than the positive electrode active material as possible, within the range where the necessary binding properties and conductivity are ensured.

[0052] This composition uses water as the medium. Furthermore, to adjust the properties and drying properties of the composition, a mixed solvent with lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, tetrahydrofuran, or N-methylpyrrolidone may be used. The proportion of water in the mixed medium is, for example, 40% by mass or more, and also, for example, 70% by mass or more.

[0053] When this composition is in a coatable slurry state, the content of the water-containing medium in the whole composition can be, for example, in the range of 25 to 90% by mass, or in the range of 35 to 70% by mass, from the viewpoint of the coatability of the slurry, the energy cost required for drying, and productivity. Furthermore, when the composition is in a pressable wet powder state, the content of the above medium can be, for example, in the range of 3 to 40% by mass, or in the range of 10 to 30% by mass, from the viewpoint of the uniformity of the mixture layer after pressing.

[0054] The binder of the present invention may consist solely of this block polymer, but other binder components such as styrene / butadiene latex (SBR), acrylic latex, carboxymethylcellulose (CMC), and polyvinylidene fluoride latex may also be used in combination. When other binder components are used in combination, the amount used can be, for example, 0.1 to 5 parts by mass or less, or 0.1 to 2 parts by mass or less, or 0.1 to 1 part by mass or less, based on 100 parts by mass of the total amount of positive electrode active material. If the amount of other binder components used exceeds 5 parts by mass, the resistance may increase, and the high-rate characteristics may become insufficient. Among the above, CMC is preferred because it does not hinder the dispersion of the active material and the conductive additive.

[0055] The styrene / butadiene latex mentioned above 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 aromatic vinyl monomers include styrene, α-methylstyrene, vinyltoluene, and divinylbenzene, and one or more of these can be used. The amount of structural units derived from the aromatic vinyl monomer in the copolymer can be in the range of, for example, 20 to 70% by mass, or in the range of, for example, 30 to 60% by mass, mainly from the viewpoint of binding properties.

[0056] Examples of the above-mentioned aliphatic conjugated diene monomers 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 units derived from the above-mentioned aliphatic conjugated diene monomers in the copolymer can be in the range of, for example, 30 to 70% by mass, or in the range of 40 to 60% by mass, in order to ensure good binder binding and flexibility of the resulting cathode.

[0057] In addition to the monomers mentioned above, styrene / butadiene latex may also use other monomers as copolymer monomers to further improve properties such as binding properties, including 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 (meth)acrylate. The structural units derived from the other monomers in the copolymer can be in the range of, for example, 0 to 30% by mass, or in the range of, for example, 0 to 20% by mass.

[0058] The above-mentioned CMC refers to substituted nonionic cellulosic semi-synthetic polymer compounds obtained by substituting them with carboxymethyl groups, and their salts. Examples of the above-mentioned nonionic cellulosic semi-synthetic polymer compounds include alkylcelluloses such as methylcellulose, methylethylcellulose, ethylcellulose, and microcrystalline cellulose; and hydroxyalkylcelluloses such as hydroxyethylcellulose, hydroxybutylmethylcellulose, hydroxypropylcellulose, hydroxypropylmethylcellulose, hydroxyethylmethylcellulose, hydroxypropylmethylcellulose stearoxy ether, carboxymethylhydroxyethylcellulose, alkylhydroxyethylcellulose, and nonoxynylhydroxyethylcellulose.

[0059] This composition comprises the above-mentioned positive electrode active material, water, and this binder as essential components, and is obtained by mixing each component using known means. The method of mixing each component is not particularly limited, and known methods can be used, but it is preferable to dry blend the powder components such as the positive electrode active material and conductive additive, and then mix them with this binder and a dispersion medium such as water, followed by dispersion kneading. When obtaining the composition for the positive electrode mixture layer in slurry form, it is preferable to produce a slurry that is free from poor dispersion and aggregation. As a mixing means, known mixers such as planetary mixers, thin-film swirling mixers, and orbital mixers can be used, but it is preferable to use a thin-film swirling mixer because a good dispersion state can be obtained in a short time. Furthermore, when using a thin-film swirling mixer, it is preferable to perform pre-dispersion with an agitator such as a disper beforehand. The viscosity of the slurry can be, for example, in the range of 100 to 10,000 mPa·s as a B-type viscosity at 20 rpm, or in the range of 1,000 to 5,000 mPa·s.

[0060] On the other hand, when obtaining this composition in a wet powder state, it is preferable to knead it using a Henschel mixer, blender, planetary mixer, twin-shaft kneader, etc., until it reaches a uniform state without uneven concentration.

[0061] 5. Secondary Battery Positive Electrode The positive electrode of the secondary battery of the present invention comprises a composite layer formed from this composition on the surface of a current collector such as aluminum. The composite layer is formed by coating the surface of the current collector with this composition and then drying and removing a medium such as water. The method of coating with this composition is not particularly limited, and known methods such as the doctor blade method, dip method, roll coating method, comma coating method, curtain coating method, gravure coating method, and extrusion method can be used. Furthermore, the drying can be carried out by known methods such as hot air blowing, reduced pressure, (far) infrared radiation, and microwave irradiation. Typically, the composite layer obtained after drying is subjected to compression treatment using a die press and a roll press. Compression can be used to bring the positive electrode active material and binder into close contact, improving the strength of the composite layer and its adhesion to the current collector. Compression can be used to adjust the thickness of the composite layer to, for example, 30 to 80% of the thickness before compression, and the thickness of the composite layer after compression is generally about 4 to 200 μm.

[0062] 6. Secondary Battery A secondary battery can be manufactured by providing the positive electrode of the secondary battery of the present invention with a negative electrode, a separator, and an electrolyte. The electrolyte may be liquid or gel. The separator is placed between the positive and negative electrodes of the battery and plays a role in preventing short circuits caused by contact between the two electrodes and in holding the electrolyte to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane with good ionic permeability and mechanical strength. Specific materials that can be used include polyethylene, polyolefins such as polypropylene, and polytetrafluoroethylene.

[0063] Examples of negative electrode active materials used in the negative electrode of the secondary battery include carbon-based materials, lithium metal, lithium alloys, and metal oxides, and one or more of these can be used in combination. Among these, negative electrode active materials made of carbon-based materials such as natural graphite, artificial graphite, hard carbon, and soft carbon (hereinafter also referred to as "carbon-based negative electrode active materials") are preferred, with graphite such as natural graphite and artificial graphite, and hard carbon being more preferred. In the case of graphite, spheroidized graphite is preferably used from the viewpoint of battery performance, and the preferred range of particle size is, for example, 1 to 20 μm, or for example, 5 to 15 μm. Furthermore, in order to increase the energy density, metals or metal oxides that can absorb lithium, such as silicon and tin, can also be used as negative electrode active materials. Among these, silicon has a higher capacity than graphite, and negative electrode active materials made of silicon-based materials such as silicon, silicon alloys, and silicon oxides such as silicon monoxide (SiO) (hereinafter also referred to as "silicon-based negative electrode active materials") can be used. However, while the silicon-based anode active material has high capacity, it undergoes large volume changes during charging and discharging. For this reason, it is preferable to use it in combination with the carbon-based anode active material. In this case, if the amount of silicon-based anode active material is too high, it can lead to the breakdown of the electrode material, which can significantly reduce the cycle characteristics (durability). From this perspective, when using silicon-based anode active material in combination, the amount used should be, for example, 60% by mass or less, or for example, 30% by mass or less, relative to the carbon-based anode active material.

[0064] Since carbon-based anode active materials possess good electrical conductivity on their own, it is not always necessary to add conductive additives. When conductive additives are added for purposes such as further reducing resistance, the amount used should be, for example, 10% by mass or less, or 5% by mass or less, relative to the total amount of anode active material, from the viewpoint of energy density.

[0065] The electrolyte can be a known one commonly used depending on the type of active material. In lithium-ion secondary batteries, specific solvents include cyclic carbonates with high dielectric constant and high electrolyte solubility, such as propylene carbonate and ethylene carbonate, as well as low-viscosity chain carbonates such as ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate. These can be used individually or as mixed solvents. The electrolyte contains LiPF in these solvents. ６ LiSbF ６ LiBF ４ LiClO ４ LiAlO ４ Lithium salts such as these are dissolved and used. In nickel-metal hydride secondary batteries, an aqueous potassium hydroxide solution can be used as the electrolyte. Secondary batteries are obtained by housing positive and negative electrode plates, separated by a separator, in a spiral or stacked structure in a case or the like.

[0066] The present invention will be described in detail below based on examples. However, the present invention is not limited to these examples. In the following, "parts" and "%" mean parts by mass and mass%, respectively, unless otherwise specified.

[0067] In the production example and comparative production example, the glass transition temperature (Tg) and particle size of the polymer particles were evaluated as follows.

[0068] <Measurement of Glass Transition Temperature (Tg) of Polymer Particles> The glass transition temperature (Tg) of polymer particles was determined from the intersection of the baseline and the tangent at the inflection point of the heat flux curve obtained using a differential scanning calorimeter (DSC). The heat flux curve was obtained by cooling approximately 5 mg of the sample to -80°C, holding for 3 minutes, then raising the temperature to 100°C at a rate of 10°C / min, continuing to cool to -80°C, holding for 3 minutes, and then raising the temperature to 100°C at a rate of 10°C / min.

[0069] (DSC measurement conditions) Measurement device: NETZSCH DSC 214 Polyma Measurement atmosphere: Nitrogen atmosphere

[0070] <Measurement of Polymer Particle Size> 0.04 g of an aqueous dispersion containing polymer particles was diluted approximately 500 times with 20 g of pure water. Then, the particle size distribution was measured using a particle size analyzer (Otsuka Electronics, nanoSAQLA) that uses dynamic light scattering, and the scattering intensity reference median diameter (D50) was obtained as a representative value of the particle size.

[0071] ≪Production of Polymers≫ (Production Example 1: Production of Polymer Particles A-1) For polymerization, a reactor equipped with a stirring blade, thermometer, reflux condenser, and nitrogen inlet tube was used. Under a nitrogen atmosphere, 47 parts of ion-exchanged water and 0.003 parts of a 16% aqueous solution of sodium dodecylbenzenesulfonate (manufactured by Kao Corporation, trade name "Neoperex G-15", hereinafter also referred to as "G-15") in solid content equivalent were charged into the reactor and heated to 75°C (hereinafter this process is also referred to as "initial charging"). Next, 28 parts of ion-exchanged water, 0.7 parts of G-15 in solid content equivalent as surfactant, 73 parts of n-butyl acrylate, 16 parts of methyl methacrylate, 2 parts of methacrylic acid, and 9 parts of 2-hydroxyethyl methacrylate as monomers were added to prepare a solution (hereinafter also referred to as "pre-emulsion") in which the monomers were emulsified. Furthermore, 1.8 parts of the pre-emulsion and 1.0 part of deionized water were added to the reactor, and after 10 minutes, 0.2 parts of ammonium persulfate (hereinafter also referred to as "APS"), which is a polymerization initiator, were added and the mixture was allowed to react for 15 minutes. Subsequently, the remaining pre-emulsion was added to the reactor at a constant rate over 5 hours, while simultaneously adding a mixture of 0.2 parts of APS, 0.01 parts of 25% aqueous ammonia, and 8 parts of deionized water to the reaction solution at a constant rate over 5 hours to obtain an aqueous dispersion of polymer particles A-1. The Tg of polymer particles A-1 was -17°C, and the particle size was 290 nm.

[0072] (Production Examples 2-5 and Comparative Production Examples 1 and 2: Production of Polymer Particles A-2 to A-7) The same procedure as in Production Example 1 was followed, except that the amount of each raw material added in the initial preparation, and the types and amounts of each raw material added in the pre-emulsion were as shown in Table 1, to obtain aqueous dispersions of polymer particles A-2 to A-7. The measurement results of Tg and particle size of polymer particles A-2 to A-7 are shown in Table 1.

[0073]

[0074] The details of the compounds used in Table 1 are as follows: • BA: n-butyl acrylate • MMA: methyl methacrylate • HEMA: 2-hydroxyethyl methacrylate • MAA: methacrylic acid • AN: acrylonitrile • G-15: 16% aqueous solution of sodium dodecylbenzenesulfonate (manufactured by Kao Corporation, product name "Neoperex G-15")

[0075] Example 1 (Preparation of aqueous binder dispersion for positive electrode) Polyvinyl alcohol PVA-1 (Kuraray Co., Ltd., Kuraray Poval 5-88 (saponification degree 88 mol%, polymerization degree 500)) was mixed with deionized water to prepare an 8% aqueous solution. Next, in a separate container, the aqueous dispersion of polymer particles A-1 and the above PVA-1 aqueous solution were added so that the mass ratio of polymer particles A-1:PVA-1 = 90:10 (solids), and the mixture was then mixed with a mixing rotor at 80 rpm for 1 hour to obtain an aqueous binder dispersion for positive electrode.

[0076] (Preparation of composition for positive electrode mixture layer (positive electrode slurry)) Lithium iron phosphate (manufactured by BTR Corporation, hereinafter also referred to as "LFP") was used as the positive electrode active material. Single-walled carbon nanotubes (manufactured by Kusumoto Chemical Co., Ltd., trade name "TUBALL BATT H") were used as the conductive additive. ２ A 0.4% solution of LFP (hereinafter also referred to as SW-CNT) was used. As a dispersant, sodium carboxymethylcellulose (product name "7A" manufactured by Daiichi Kogyo Seiyaku Co., Ltd., hereinafter also referred to as CMC) was used. In a rotating / revolving mixer (Awatori Rentaro manufactured by Thinky Co., Ltd.), the composition for the positive electrode mixture layer was added and mixed in a mass ratio of LFP:SW-CNT:CMC:positive electrode binder = 97:0.1:1.0:3.0 (solids) using deionized water as a diluent so that the solid content concentration of the composition for the positive electrode mixture layer was 62% by mass, and a slurry-like composition for the positive electrode mixture layer (positive electrode slurry) was prepared.

[0077] <Evaluation of the dispersibility of the positive electrode slurry> The dispersibility of the positive electrode slurry was evaluated based on the results of a grind gauge test. A BYK-Gardner grind gauge (product name "Grindometer 0-50 μm") was used, and the point where the slurry stopped was recorded as the measurement result. The measurement result was 2 μm, and the dispersibility was evaluated as "A" based on the following criteria. Note that a smaller measurement result indicates better dispersibility. (Criteria for determining dispersibility) A: Measurement result is less than 5 μm B: Measurement result is 5 μm or more and less than 10 μm C: Measurement result is 10 μm or more

[0078] (Preparation of positive electrode plate) Next, the positive electrode slurry was applied to a current collector (carbon-coated aluminum foil) with a thickness of 22.0 μm using a variable applicator, and a composite layer was formed by drying in a forced-air dryer at 50°C for 20 minutes. After that, the composite layer had a thickness of 60 ± 5 μm and a composite density of 2.00 ± 0.10 g / cm³. ３ The material was rolled to obtain a positive electrode plate. After punching out the positive electrode plate to a size of 1.0 cm × 6.0 cm, the 180° peel strength between the composite layer and the current collector (i.e., the binder's binding properties) and the flexibility of the positive electrode composite layer were measured.

[0079] <Evaluation of 180° Peel Strength (Bonding Properties) of Positive Electrode Plate> A sample for peel test was prepared by attaching the composite layer surface of the above positive electrode plate, measuring 1.0 cm x 6.0 cm, to a 2.5 cm x 9.0 cm aluminum plate using double-sided tape (Nichiban Nicetack NW-25). Subsequently, using a peel test machine (IMADA Force Gauge DSV-5N, measuring stand MX-500N), 180° peeling was performed at a measurement temperature of 25°C and a tensile speed of 300 mm / min, and the bonding properties were evaluated by measuring the peel strength between the composite layer and the aluminum foil. The peel strength was 6.6 N / m, and the bonding properties were evaluated as "A" based on the following criteria. Note that a higher peel strength indicates better bonding properties of the positive electrode composite layer. (Criteria for determining binding properties) A: Peel strength of 6.0 N / m or more B: Peel strength of 2.0 N / m or more but less than 6.0 N / m C: Peel strength less than 2.0 N / m

[0080] <Evaluation of the flexibility of the positive electrode mixture layer> The positive electrode plate was wrapped around a polyvinyl chloride rod of a different diameter on the aluminum foil side of the positive electrode to evaluate whether the positive electrode mixture layer cracked. The evaluation result was that it did not crack at Φ5 mm, and the flexibility was evaluated as "A" based on the following criteria. Note that the smaller the diameter of the rod that the positive electrode mixture layer does not crack, the better the flexibility of the positive electrode mixture layer. (Flexibility evaluation criteria) A: Does not crack at Φ5 mm B: Cracks at Φ5 mm, but does not crack at Φ15 mm C: Cracks at Φ15 mm

[0081] Examples 2-11 and Comparative Examples 1-4: Cathode slurries were prepared by the same procedure as in Example 1, except that the formulations were as shown in Table 2. The dispersibility of each cathode slurry, as well as the binding and flexibility of the cathode mixture layers obtained using each cathode slurry, were evaluated, and the results are shown in Table 2.

[0082]

[0083] The details of the compounds used in Table 2 are as follows: • CMC: Sodium carboxymethylcellulose • PVA-1: Polyvinyl alcohol (Kuraray Co., Ltd., Kuraray Poval 5-88 (saponification degree 88 mol%, polymerization degree 500)) • PVA-2: Polyvinyl alcohol (Kuraray Co., Ltd., Kuraray Poval 5-98 (saponification degree 98 mol%, polymerization degree 500)) • PVA-3: Polyvinyl alcohol (Kuraray Co., Ltd., Kuraray Poval 22-88 (saponification degree 88 mol%, polymerization degree 1,700)) • PVA-4: Polyvinyl alcohol (Kuraray Co., Ltd., Kuraray Poval 3-88 (saponification degree 88 mol%, polymerization degree 300))

[0084] ≪Evaluation Results≫ As is clear from the results of Examples 1 to 11, the aqueous binder for the positive electrode of the secondary battery of the present invention exhibits excellent dispersibility with respect to the positive electrode active material and conductive additive, as well as excellent bonding and flexibility of the positive electrode mixture layer. Among these, focusing on the glass transition temperature (Tg) of the polymer particles, when Tg was -10°C or lower (Example 1: -17°C), the results were even better in terms of dispersibility with respect to the positive electrode active material and conductive additive, as well as bonding and flexibility of the positive electrode mixture layer, compared to when Tg was above -10°C (Example 5: -1°C, Example 6: 20°C). This is thought to be because sufficient flexibility was imparted to the positive electrode mixture layer, suppressing cracking during drying of the positive electrode mixture layer and enabling bonding between the positive electrode active materials.

[0085] Furthermore, focusing on the degree of polymerization of the polyvinyl alcohol-based resin, the cases with a degree of polymerization of 500 or higher (Examples 1-3) showed particularly good binding properties compared to the case with a degree of polymerization of 300 (Example 4). In addition, the cases with a degree of polymerization of 1,500 or lower (Examples 1, 2, and 4) showed particularly excellent dispersibility with the positive electrode active material and conductive additive compared to the case with a degree of polymerization of 1,700 (Example 3).

[0086] Furthermore, focusing on the solid content mass ratio of polymer particles to polyvinyl alcohol-based resin, the cases with a solid content mass ratio (polymer particles / polyvinyl alcohol-based resin) of 95 / 5 to 80 / 20 (Examples 1, 7, and 8) and 60 / 40 (Example 9) showed particularly good flexibility in the cathode mixture layer.

[0087] In contrast, when a binder containing only polymer particles A-1 and no polyvinyl alcohol-based resin was used (Comparative Example 1), the binding properties and flexibility of the positive electrode mixture layer decreased. This is presumed to be because the polymer particles penetrated the pores of the positive electrode active material, resulting in a small amount of A-1 contributing to the binding of the positive electrode active material. Furthermore, when a binder containing polymer particles A-6 with a particle diameter of 200 nm or less (Comparative Example 2) or polymer particles A-7 with a particle diameter exceeding 600 nm (Comparative Example 3) was used, the binding properties or flexibility of the positive electrode mixture layer decreased. This is presumed to be because polymer particles A-6 have a smaller particle diameter than the original polymer particles, increasing the required elongation per polymer particle when bending the positive electrode mixture layer, making the positive electrode mixture layer more prone to cracking, and thus reducing its flexibility. On the other hand, since polymer particles A-7 have a larger particle size than the polymer particles in question, fewer polymer particles bind to the positive electrode active material, and therefore the amount of polymer particles contributing to the binding of the positive electrode active material is reduced, which is presumed to have resulted in a decrease in the binding properties of the positive electrode mixture layer. Furthermore, when a binder containing only polyvinyl alcohol-based resin and no polymer particles was used (Comparative Example 4), the positive electrode mixture layer lacked flexibility and cracked during drying. This is presumed to be due to the high Tg of the polyvinyl alcohol-based resin.

Claims

1. A water-based binder for a secondary battery positive electrode, comprising polymer particles and a polyvinyl alcohol-based resin, wherein the polymer particles have structural units derived from ethylenically unsaturated monomers, the glass transition temperature of the polymer particles is 30°C or less, the particle size of the polymer particles is greater than 200 nm and less than or equal to 600 nm as the scattering intensity reference median diameter (D50) measured by dynamic light scattering, and the polyvinyl alcohol-based resin substantially does not have 1,2-diol structures in its side chains.

2. The aqueous binder for the positive electrode of a secondary battery according to claim 1, wherein the ethylenically unsaturated monomer comprises an ethylenically unsaturated carboxylic acid ester monomer.

3. The aqueous binder for the positive electrode of a secondary battery according to claim 1 or 2, wherein the polymer particles have 30% by mass or less of structural units derived from ethylenically unsaturated carboxylic acid monomers with respect to their total structural units.

4. The aqueous binder for the positive electrode of a secondary battery according to claim 3, wherein the polymer particles have 35% by mass or less of structural units derived from a hydroxyl group-containing ethylenically unsaturated monomer with respect to their total structural units.

5. The aqueous binder for the positive electrode of a secondary battery according to claim 1 or 2, wherein the polymer particles substantially do not have a structure grafted with polyvinyl alcohol.

6. The aqueous binder for the positive electrode of a secondary battery according to claim 1 or 2, wherein the degree of polymerization of the polyvinyl alcohol-based resin is 200 to 2,000.

7. The aqueous binder for the positive electrode of a secondary battery according to claim 1 or 2, wherein the solid content mass ratio (polymer particles / polyvinyl alcohol resin) of the polymer particles to the polyvinyl alcohol resin is 99 / 1 to 40 / 60 when the total amount of the polymer particles and the polyvinyl alcohol resin is set to 100.

8. A composition for a secondary battery positive electrode mixture layer, comprising the aqueous binder for the positive electrode of a secondary battery, a positive electrode active material, and water, as described in claim 1.

9. The composition for a secondary battery cathode composite layer according to claim 8, wherein the cathode active material comprises an olivine-type lithium-containing compound.

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

11. A secondary battery comprising the positive electrode of a secondary battery as described in claim 10.

Images