JP5547504B2 - Secondary battery electrode binder - Google Patents

Description

  The present invention relates to a binder for a secondary battery electrode.

  Secondary batteries typified by lithium ion batteries are lightweight and have a high energy density, and are therefore being used as power sources for automobiles and houses, including small electronic devices. In the production of an electrode of a lithium ion battery, polyvinylidene fluoride or butadiene rubber is usually used as a binder, and it is blended with an active material (positive electrode active material and negative electrode constituent material) to prepare an electrode composition. The active material is bound to the current collector by coating and drying the electrode composition on the current collector.

  Recently, as a binder, a fluorine-based polymer such as polyvinylidene fluoride that needs to be dissolved in an organic solvent has shifted to an aqueous butadiene-based rubber latex that does not require recovery of the organic solvent.

  Like polyvinylidene fluoride, butadiene rubber latex can be used to produce electrodes with high binding power (possible in small quantities) and low surface electrical resistance that can be adapted to high performance and downsizing of secondary batteries. Is required. Therefore, research on the composition and structure constituting the butadiene rubber latex has been advanced and various proposals have been made.

  For example, in Japanese Patent No. 3101775 (JP-A-5-74461; Patent Document 1), a styrene-butadiene rubber latex having a butadiene of 40 to 95% and a gel content of 75 to 100% is disclosed in JP-A-8-250123. In the gazette (Patent Document 2), a styrene-butadiene rubber latex having a butadiene of 40 to 98% and a gel content of 20 to 74% is disclosed in Japanese Patent No. 3567618 (Japanese Patent Laid-Open No. 9-320604: Patent Document 3). Japanese Patent Laid-Open No. 10-302797 (Patent Document 4) discloses a carboxy-modified styrene-butadiene rubber latex having a butadiene content of 30 to 98% and a gel content of 20 to 95%. Butadiene rubber latex is disclosed in Japanese Patent No. 3721727 (Japanese Patent Laid-Open No. 11-25899: Patent In item 5), a butadiene rubber latex consisting of 30 to 80% aromatic vinyl, less than 30% butadiene, 10 to 40% ester and 0.1 to 10% carboxylic acid, and having a glass transition temperature of 15 to 150 ° C. In Japanese Patent No. 3755544 (JP-A-9-87571: Patent Document 6), aromatic vinyl 25-60%, butadiene 15-50%, ester 0-40% and carboxylic acid 0-40%, A butadiene rubber latex having a total of 15 to 40% of ester and carboxylic acid has been proposed.

Japanese Patent No. 3101775 (JP-A-5-74461)

JP-A-8-250123

Japanese Patent No. 3567618 (Japanese Patent Laid-Open No. 9-320604)

Japanese Patent Laid-Open No. 10-302797

Japanese Patent No. 3721727 (Japanese Patent Laid-Open No. 11-25899)

Japanese Patent No. 3755544 (Japanese Patent Laid-Open No. 9-87571)

However, the butadiene rubber latex proposed in the publicly known literatures including the above-mentioned patent documents 1 to 6 has a binding force with a current collector and an active material and an electrode coating layer required for a secondary battery that will be further miniaturized in the future. In terms of surface electrical resistivity, it is insufficient as a binder.
An object of the present invention is to provide an excellent binder for a secondary battery electrode capable of forming an electrode coating layer having excellent binding power to a current collector and an active material and having a low surface electrical resistivity of the electrode coating layer. There is to do.

In order to solve the above-mentioned problems, the present inventors have studied the composition of a butadiene rubber latex as an aqueous binder for a lithium ion secondary battery electrode, which is used in an aqueous battery electrode composition. The inventors have found that a polymer having a specific composition range can form an electrode coating layer having excellent binding power to a current collector and an active material and having a low surface electrical resistivity of the electrode coating layer. The specific polymer is a polymer having the following five components as essential components, and the composition range thereof is 0.5 to 10% by weight of an ethylenically unsaturated carboxylic acid monomer, a vinyl cyanide monomer. 5 to 20% by weight, ethylenically unsaturated carboxylic acid ester monomer 13 to 25% by weight, aromatic vinyl monomer 35 to 55% by weight, aliphatic conjugated diene monomer 18 to 28% by weight It is a copolymer latex obtained by emulsion polymerization of the monomer to be constituted.

  By using the binder for a secondary battery electrode of the present invention, the electrode coating layer has a lower surface electrical resistivity than the conventionally proposed binder, which has superior binding power to the current collector and active material. A work layer can be formed, and a higher performance secondary battery can be provided.

  The binder for secondary battery electrodes of the present invention includes 1) ethylenically unsaturated carboxylic acid monomer 0.5 to 10% by weight, 2) vinyl cyanide monomer 5 to 20% by weight, 3) ethylenically unsaturated Saturated carboxylic acid ester monomer 13 to 25% by weight 4) Aromatic vinyl monomer 35 to 55% by weight 5) Aliphatic conjugated diene monomer 18 to 28% by weight (monomer total 100) A binder for a secondary battery electrode comprising a copolymer latex obtained by emulsion polymerization of a monomer comprising Preferably, 1) ethylenically unsaturated carboxylic acid monomer 1 to 8% by weight, 2) vinyl cyanide monomer 8 to 15% by weight, 3) ethylenically unsaturated carboxylic acid ester monomer 15 to 20% by weight, 4) 40-50% by weight of aromatic vinyl monomer, and 5) 20-25% by weight of aliphatic conjugated diene monomer.

1) Examples of the ethylenically unsaturated carboxylic acid monomer include mono- or dicarboxylic acids (anhydrides) such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, and itaconic acid. Or 2 or more types can be used. Preferably, acrylic acid, fumaric acid and itaconic acid are used.

  2) Examples of the vinyl cyanide monomer include acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, α-ethylacrylonitrile and the like, and one or more can be used. Preferably, acrylonitrile and methacrylonitrile are used.

3) Examples of the ethylenically unsaturated carboxylic acid ester monomer include, for example, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, etc., “acrylic acid alkyl ester having 1 to 8 carbon atoms”, For example, “methacrylic acid alkyl ester having an alkyl group having 1 to 4 carbon atoms” such as methyl methacrylate and ethyl methacrylate, for example, “maleic acid having an alkyl group having 1 to 4 carbon atoms” such as dimethyl maleate and diethyl maleate "Alkyl esters" such as dimethyl itaconate "Itaconic acid alkyl esters having an alkyl group having 1 to 4 carbon atoms" such as monomethyl fumarate, monoethyl fumarate, dimethyl fumarate, diethyl fumarate, etc. Number 1 Alkyl ester of fumaric acid having 4 alkyl groups ", 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, 3-chloro-2-hydroxypropyl Methacrylate, di- (ethylene glycol) maleate, di- (ethylene glycol) itaconate, 2-hydroxyethyl maleate, bis (2-hydroxyethyl) maleate, 2-hydroxyethyl methyl fumarate, etc. “hydroxyl group-containing unsaturated carboxylic acid Alkyl ester monomers "and the like, and one or more of these can be used. Preferably, methacrylic acid alkyl ester having an alkyl group having 1 to 4 carbon atoms, more preferably methyl methacrylate.

  4) Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, methyl α-methylstyrene, vinyltoluene, and divinylbenzene, and one or more types can be used. Preferably, styrene is used.

  5) Examples of the aliphatic conjugated diene monomer include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3 butadiene, and 2-chloro-1,3- Examples thereof include butadiene, substituted linear conjugated pentadienes, substituted and side chain conjugated hexadienes, and one or more kinds can be used. Preferably, 1,3-butadiene is used.

Furthermore, in addition to the above five types of monomers, for example, unsaturated carboxylic acid amide monomers such as acrylamide, methacrylamide, N-methylol acrylamide, N-methylol methacrylamide, N, N-dimethylacrylamide and the like. They can be used alone or in combination of two or more. Preferably, acrylamide and methacrylamide are used.

  The monomer is 1) ethylenically unsaturated carboxylic acid monomer 0.5 to 10% by weight, 2) vinyl cyanide monomer 5 to 20% by weight, 3) ethylenically unsaturated carboxylic acid ester 13 to 25% by weight of monomer, 4) 35 to 55% by weight of aromatic vinyl monomer, and 5) 18 to 28% by weight of aliphatic conjugated diene monomer.

Even if the kind of monomer is the same, if the composition ratio deviates from the above range, the effect of the present invention cannot be obtained.
Preferably, 1) ethylenically unsaturated carboxylic acid monomer 1 to 8% by weight, 2) vinyl cyanide monomer 8 to 15% by weight, 3) ethylenically unsaturated carboxylic acid ester monomer 15 to 20% by weight, 4) 40 to 50% by weight of an aromatic vinyl monomer, and 5) 20 to 25% by weight of an aliphatic conjugated diene monomer.

And a copolymer latex is obtained by emulsion-polymerizing a monomer in water.
The monomer is polymerized in the presence of various chemical substances used in known emulsion polymerization including an emulsifier, a polymerization initiator, a reducing agent, and a chain transfer agent.
Examples of the emulsifier include anionic anions such as sulfate esters of higher alcohols, alkylbenzene sulfonates, alkyl diphenyl ether disulfonates, aliphatic sulfonates, aliphatic carboxylates, and sulfate esters of nonionic surfactants. Surfactants, for example, nonionic surfactants such as polyethylene glycol alkyl ester type, alkylphenyl ether type, alkyl ether type, etc., can be mentioned, and one or more are used. Preferably, an anionic surfactant is mentioned, More preferably, alkylbenzene sulfonate and alkyl diphenyl ether disulfonate are mentioned.
The emulsifier is used in a proportion of, for example, 0.05 to 5 parts by weight, preferably 0.1 to 3 parts by weight, with respect to 100 parts by weight of monomers (total).

  The polymerization initiator is a radical polymerization initiator, for example, a water-soluble polymerization initiator such as potassium persulfate, sodium persulfate, ammonium persulfate, such as cumene hydroperoxide, benzoyl peroxide, t-butyl hydroperoxide. And oil-soluble polymerization initiators such as acetyl peroxide, diisopropylbenzene hydroperoxide, and 1,1,3,3-tetramethylbutyl hydroperoxide. Preferably, the water-soluble polymerization initiator includes potassium persulfate, sodium persulfate, and ammonium persulfate, and the oil-soluble polymerization initiator includes cumene hydroperoxide.

  Examples of the reducing agent include ferrous sulfate, sulfite, bisulfite, pyrosulfite, nitrite, nithionate, thiosulfate, formaldehyde sulfonate, benzaldehyde sulfonate, such as L- Examples thereof include carboxylic acids such as ascorbic acid, erythorbic acid, tartaric acid, citric acid, and salts thereof, for example, reducing sugars such as dextrose and saccharose, and amines such as dimethylaniline and triethanolamine. Preferably, ferrous sulfate, carboxylic acids and salts thereof are used, and more preferably, ferrous sulfate and erythorbic acid are used.

Examples of the chain transfer agent include alkyl having 6 to 18 carbon atoms such as n-hexyl mercaptan, n-octyl mercaptan, t-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, and n-stearyl mercaptan. Mercaptans, e.g. xanthogen compounds such as dimethylxanthogen disulfide, diisopropylxanthogen disulfide, e.g. terpinolene, e.g. thiuram compounds such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetramethylthiuram monosulfide, e.g. 2,6-di -Phenol compounds such as t-butyl-4-methylphenol and styrenated phenol, for example, allyl compounds such as allyl alcohol, such as dichloro Halogenated hydrocarbon compounds such as methane, dibromomethane, carbon tetrabromide, for example, vinyl ethers such as α-benzyloxystyrene, α-benzyloxyacrylonitrile, α-benzyloxyacrylamide, such as triphenylethane, pentaphenylethane, Acrolein, methacrolein, thioglycolic acid, thiomalic acid, 2-ethylhexyl thioglycolate, α-methylstyrene dimer and the like can be mentioned, and one or more can be used. Preferred are α-methylstyrene dimer and alkyl mercaptan, and more preferred are α-methylstyrene dimer and t-dodecyl mercaptan.
A chain transfer agent is used in the ratio of 0-5 weight part with respect to 100 weight part of monomers (total), for example, Preferably, 0.05-3 weight part.

  In the emulsion polymerization, an unsaturated hydrocarbon can be used if necessary. Examples of the unsaturated hydrocarbon include pentene, hexene, heptene, cyclopentene, cyclohexene, cycloheptene, 4-methylcyclohexene, 1-methylcyclohexene, and preferably cyclohexene. Cyclohexene has a low boiling point and can be easily recovered and reused by, for example, steam distillation after polymerization, and is suitable from the viewpoint of environmental burden.

As other polymerization aids, for example, anti-aging agents, preservatives, dispersants, thickeners and the like can be added as necessary.
Moreover, it does not specifically limit as a polymerization method, Batch polymerization, semibatch polymerization, seed polymerization, etc. can be used. Moreover, the addition method of various components is not particularly limited, and a batch addition method, a divided addition method, a continuous addition method, a power feed method, or the like can be used.

Thereby, a copolymer latex in which the monomers are emulsion-polymerized and the obtained copolymer is dispersed in water can be obtained.
Although there is no restriction | limiting in particular in solid content of the obtained copolymer latex, For example, 40-55 weight%, Preferably, it is 47-52 weight% in consideration of a transport form and subsequent handling.

The glass transition temperature (Tg) of the copolymer in the obtained copolymer latex and the insoluble content (gel content) with respect to toluene are not limited, but are, for example, Tg-20 to 90 ° C, preferably -15. ~ 70 ° C. The insoluble content (gel content) with respect to toluene is 50 to 100% by weight, and preferably 60 to 99% by weight. When the Tg and gel contents are outside the above ranges, the binding force of the electrode coating layer tends to decrease.
Further, the number average particle diameter of the copolymer particles in the obtained copolymer latex is not particularly limited, but in terms of the binding force of the electrode coating layer, it is preferably a number average particle diameter, preferably 50 to 300 nm, more preferably 70 to 250 nm.

The binder for a secondary battery electrode of the present invention is used to form an electrode of a secondary battery such as a lithium ion secondary battery, a nickel hydride battery, or a nickel cadmium battery. The particles and the negative electrode constituent material or the positive electrode active material and the current collector are bound together.
Specifically, the composition for battery electrodes is prepared by blending the binder for secondary battery electrodes into the negative electrode constituent material or the positive electrode active material. That is, the composition for negative electrodes used for the negative electrode of a secondary battery is prepared by mix | blending the binder for secondary battery electrodes with a negative electrode constituent material. Moreover, the composition for positive electrodes used for the positive electrode of a secondary battery is prepared by mix | blending the binder for secondary battery electrodes with a positive electrode active material.

  The negative electrode constituent material is not particularly limited, but in the case of a non-aqueous electrolyte secondary battery, for example, carbon fluoride, graphite, carbon fiber, resin-fired carbon, linear graphite hybrid, coke, pyrolytic gas-layer-grown carbon Conductive carbonaceous materials such as calcined furfuryl alcohol resin, mesocarbon microbeads, mesophase pitch carbon, graphite whiskers, pseudo-isotropic carbon, fired natural materials, and pulverized products thereof, such as polyacene Examples thereof include conductive polymers such as organic semiconductors, polyacetylene, and poly-p-phenylene, and one kind or two or more kinds can be used.

  The positive electrode active material is not particularly limited, but includes, for example, transition metal oxides such as MnO2, MoO3, V2O5, V6O13, Fe2O3, and Fe3O4, composite oxides including lithium such as LiCoO2, LiMnO2, LiNiO2, and LiXCoYSnZO2, LiFePO4, etc. Examples thereof include composite metal oxides containing lithium, for example, transition metal sulfides such as TiS2, TiS3, MoS3, and FeS2, for example, metal fluorides such as CuF2 and NiF2, and one kind or two or more kinds can be used.

When preparing the composition for battery electrodes, the solid content of the copolymer latex is, for example, 0.1 to 7 with respect to 100 parts by weight of the negative electrode constituent material or the positive electrode active material. It mix | blends so that it may become a weight part, Preferably 0.5-4 weight part.
If the solid content of the copolymer latex with respect to 100 parts by weight of the negative electrode constituent material or the positive electrode active material is less than 0.1 parts by weight, there is a tendency that good adhesion to a current collector or the like cannot be obtained, and it exceeds 7 parts by weight. When the battery is assembled as a secondary battery, the overvoltage is remarkably increased and the battery characteristics tend to be deteriorated.

  Moreover, various additives, such as a water-soluble thickener, a dispersing agent, and a stabilizer, can be added to the battery electrode composition as necessary. Examples of the water-soluble thickener include carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid (salt), oxidized starch, phosphorylated starch, and casein. Examples of the dispersant include Examples thereof include sodium hexametaphosphate, sodium tripolyphosphate, sodium pyrophosphate, and sodium polyacrylate, and examples of the stabilizer include nonionic and anionic surfactants.

When a water-soluble thickener is added to the battery electrode composition, for example, the water-soluble thickener is, for example, 0.1% in terms of solid content with respect to 100 parts by weight of the negative electrode constituent material or the positive electrode active material. -10 parts by weight, preferably 0.5 to 5 parts by weight.
The battery electrode composition is applied to a current collector and dried to form an electrode coating layer on the current collector to obtain an electrode sheet. Such an electrode sheet is used as a positive electrode plate or a negative electrode plate of a lithium ion secondary battery.

Examples of the current collector include a metal foil such as copper and nickel as the current collector for negative electrode, and examples include a metal foil such as aluminum as the current collector for positive electrode.
As a method for applying the battery electrode composition to the current collector, a known method such as a reverse roll method, a comma bar method, a gravure method, an air knife method can be used. A warm air dryer, an infrared heater, a far infrared heater, or the like is used. The drying temperature is usually 50 ° C. or higher.

  According to the binder for a secondary battery electrode of the present invention, it is possible to form an electrode coating layer that is excellent in binding force with a current collector or an active material and has a low surface electrical resistivity.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not limited to these Examples. In the examples, parts and% indicating the blending ratio are based on weight.

1. Synthesis of copolymer latex (1) Examples 1-4 and Comparative Examples 1-6
In a pressure-resistant polymerization reactor, 100 parts of pure water, 0.6 part of sodium dodecylbenzenesulfonate, and 1 part of potassium persulfate were charged and sufficiently stirred. Next, the total amount of carboxylic acid shown in Table 1, 10% of each monomer other than carboxylic acid, 0.3 part of t-dodecyl mercaptan, and 4 parts of cyclohexene were charged into the polymerization reactor. While stirring, the internal temperature was raised to 70 ° C., and after confirming the exotherm due to the start of polymerization, the remainder of each monomer (90%), pure 10 parts of water and 0.3 part of sodium alkyldiphenyl ether disulfonate were continuously added.
From 480 minutes to 780 minutes, the polymerization was continued while maintaining the internal temperature at 75 ° C. After confirming that the polymerization conversion rate exceeded 97%, the internal temperature was cooled to 35 ° C. or lower.
After adjusting the pH to about 8 using an aqueous potassium hydroxide solution, unreacted monomers and the like were removed by steam distillation to obtain various copolymer latexes.

(2) Method for measuring gel content of copolymer latex.
A latex film is prepared in an atmosphere at a temperature of 80 ° C. and a humidity of 85%. Thereafter, about 1 g of the latex film is weighed, put in 400 ml of toluene, and expanded and dissolved for 48 hours. Thereafter, this was filtered through a 300-mesh wire mesh, and the toluene-insoluble portion captured by the wire mesh was dried and weighed.

(3) A method for measuring the number average particle size of the copolymer latex.
The number average particle size of the copolymer latex was measured by a dynamic light scattering method. In the measurement, FPAR-1000 (manufactured by Otsuka Electronics) was used.

(4) Method for measuring glass transition point (Tg) of copolymer latex.
The copolymer latex was cast on a glass plate and dried at 70 ° C. for 4 hours to produce a film. This film was packed in an aluminum pan and set in a differential scanning calorimeter (DSC6200: manufactured by Seiko Instruments Inc.).
The apparatus was cooled to a temperature about 50 ° C. lower than the expected Tg in advance, and then heated at a heating rate of 10 ° C./min to draw a DSC curve.
The glass transition temperature (Tg) was defined as the temperature at which the straight line equidistant in the vertical axis direction from the extended straight line of each baseline intersects with the curve of the stepwise change portion of the glass transition.

2. Preparation of electrode sheet (1) Preparation of electrode composition Natural graphite having an average particle diameter of 15 μm is used as the conductive carbonaceous material, and carboxymethylcellulose aqueous solution (solid content) as a thickener with respect to 100 parts by weight of natural graphite. ) 2 parts by weight and 3 parts by weight of the copolymer latex (solid content) obtained in each Example and each Comparative Example were added with an appropriate amount of water so that the solid content of the electrode composition was 40%. And kneaded to prepare various electrode compositions of Examples and Comparative Examples.

(2) Preparation of electrode sheet The electrode composition of each example and each comparative example was applied to both sides of a 20 μm thick copper foil serving as a current collector, dried at 100 ° C. for 25 minutes, and then pressed at room temperature. Thus, an electrode sheet having a coating layer thickness of 80 μm (per one side) was obtained.

3. Electrode sheet performance test (1) Measurement of binding force of electrode coating layer Using a knife on the surface of the electrode sheet of each example and each comparative example, cut from the coating layer to the depth reaching the current collector. Grids having 25 (5 × 5) squares were formed by placing 6 in each direction at 2 mm intervals. Adhesive tape was applied to this grid.
Binding force-1: The adhesive tape was immediately peeled off, and the degree of graphite falling off was visually evaluated.
Binding force-2: 30 ° C., 2 kg / cm 2 load, left for 24 hours, then peeled off the adhesive tape, and visually evaluated the degree of graphite falling off. The results are shown in Table 1.
A: No peeling.
○: 1 to 3 squares peeled off.
Δ: 4-10 squares peeled off.
X: 11 or more squares peeled off.

(2) Measurement of surface electrical resistance of electrode coating layer The electrode composition of each Example and each Comparative Example was applied to a commercially available polyester film and dried at 130 ° C. for 5 minutes. Further, by rolling with a roll press, a coating layer sample having a coating layer thickness of about 60 μm and a coating layer density of about 1.3 g / cm 3 was obtained.
The surface resistivity of the coating layer was measured with Loresta GP manufactured by Mitsubishi Chemical Analytech Co., Ltd. The results are shown in Table 1.

  The binder for a secondary battery electrode of the present invention is a secondary battery electrode binder for binding active materials (positive electrode active material and negative electrode constituent material) to each other and an active material in a secondary battery electrode. Used.

Claims (2)

Ethylenically unsaturated carboxylic acid monomer 0.5 to 10% by weight, vinyl cyanide monomer 5 to 20% by weight, ethylenically unsaturated carboxylic acid ester monomer 13 to 25% by weight, aromatic vinyl systems monomer 35 to 55 wt%, the aliphatic conjugated diene Ri Do a copolymer latex obtained by emulsion polymerization of a configured monomer from monomer 18 to 28 wt%, the battery electrode of the aqueous A water-based binder for lithium ion secondary battery electrodes , characterized by being used in a composition for a battery. Ethylenically unsaturated carboxylic acid monomer 1 to 8% by weight, vinyl cyanide monomer 8 to 15% by weight, ethylenically unsaturated carboxylic acid ester monomer 15 to 20% by weight, aromatic vinyl monomer mer 40-50 wt%, Ri Do a copolymer latex obtained by emulsion polymerization of a configured monomer from aliphatic conjugated diene monomer 20-25 wt%, the composition for a battery electrode of the aqueous An aqueous binder for lithium ion secondary battery electrodes , characterized by being used in a product .