HK1184275A - Binder for secondary cell electrode, slurry for secondary cell electrode and secondary cell electrode - Google Patents

Description

Binder for secondary battery electrode, slurry for secondary battery electrode, and secondary battery electrode

Technical Field

The invention relates to a binder for a secondary battery electrode, a slurry for a secondary battery electrode, and a secondary battery electrode.

Background

In recent years, reusable secondary batteries have become increasingly popular. Among them, lithium ion secondary batteries are lightweight and have a high energy density, and therefore, they are suitable for small electronic devices, and in recent years, they have been studied as automobile or house batteries. In the production of an electrode for a lithium ion secondary battery, a polymer binder is generally used as a binder, and an active material (a negative electrode constituent material and a positive electrode active material) is incorporated into the polymer binder to prepare an electrode composition, and the electrode composition is applied to a current collector and dried to bind the active material to the current collector. The polymer binder is required to have adhesiveness to an active material, adhesiveness to a current collector, resistance to a polar solvent as an electrolyte, and stability in an electrochemical environment.

Conventionally, fluorine-based polymers such as polyvinylidene fluoride have been used as such polymer binders. However, the fluorine-based polymer must be dissolved in an organic solvent, and there is a problem that the organic solvent volatilizes when the electrode composition is applied to a current collector and dried. In addition, since the adhesive force is poor, it is necessary to incorporate a large amount of polymer binder to obtain sufficient adhesive force, and thus there is a problem in that the conductivity of the secondary battery electrode is suppressed.

In order to improve these problems, various techniques have been proposed for using an aqueous dispersion of a non-fluorine-based polymer as a polymer binder.

For example, japanese patent laying-open No. 8-287915 (patent document 1) proposes a technique for obtaining a secondary battery having excellent recyclability, capacity, and manufacturing suitability by using a (meth) acrylate-acrylonitrile-carboxylic acid latex as a polymer binder.

Further, Japanese patent laid-open No. 2000-195521 (patent document 2) proposes a technique for obtaining a secondary battery having good charge/discharge cycle characteristics at high temperatures by using a (meth) acrylate-carboxylic acid latex having a specific composition as a polymer binder.

In addition, Japanese patent laid-open No. 2001-332265 (patent document 3) proposes a technique for obtaining a secondary battery having good charge/discharge cycle characteristics at high temperatures by using a (meth) acrylate-acrylonitrile latex having a specific composition as a polymer binder.

Documents of the prior art

Patent document

Patent document 1 Japanese patent laid-open No. Hei 8-287915

Patent document 2 Japanese patent laid-open No. 2000-195521

Patent document 3 Japanese patent laid-open No. 2001-332265

Disclosure of Invention

Technical problem to be solved by the invention

However, the polymer binders described in patent documents 1 and 3 have improved adhesion to the active material and adhesion to the current collector to some extent as compared with the fluorine-based binders, but the conductivity of the electrode using the binder is not at a sufficient level, and improvement is required.

Although the polymer binder described in patent document 2 improves the conductivity of the electrode to a certain extent, the adhesiveness to the active material and the adhesiveness to the current collector are not sufficient to satisfy further improvements required by the market.

Accordingly, an object of the present invention is to provide a binder for secondary battery electrodes, which solves the problem of volatilization of an organic solvent during electrode production and has good adhesion to a current collector and an active material, and good crack resistance, flexibility, and conductivity.

Technical scheme for solving technical problem

In order to solve the above problems, the binder for a secondary battery electrode of the present invention is characterized by comprising a copolymer latex obtained by emulsion polymerization of a monomer composition not including a vinyl cyanide monomer, wherein the monomer composition includes 60 to 94.8 wt% of an unsaturated carboxylic acid alkyl ester monomer, 0.1 to 10 wt% of an ethylenically unsaturated carboxylic acid monomer, 5 to 20 wt% of at least one monomer selected from an aliphatic conjugated diene monomer and an alkenyl aromatic monomer, and 0.1 to 10 wt% of a monomer copolymerizable with these monomers.

ADVANTAGEOUS EFFECTS OF INVENTION

The binder for secondary battery electrodes of the present invention comprises an aqueous dispersion of a copolymer obtained by emulsion polymerization of a monomer composition comprising an unsaturated carboxylic acid alkyl ester monomer, an ethylenically unsaturated carboxylic acid monomer, at least one monomer selected from aliphatic conjugated diene monomers and alkenyl aromatic monomers, and a monomer copolymerizable with these monomers in a specific ratio.

Therefore, the electrode can be produced without volatilization of the organic solvent, and the electrode coating layer can have good adhesion to the current collector and the active material, crack resistance, flexibility and conductivity. As a result, the secondary battery using the electrode of the present invention has good battery characteristics such as charge-discharge cycle characteristics.

Modes for carrying out the invention

The binder for secondary battery electrodes of the present invention comprises a copolymer latex obtained by emulsion-polymerizing a monomer composition comprising an unsaturated carboxylic acid alkyl ester monomer, an ethylenically unsaturated carboxylic acid monomer, at least one monomer selected from the group consisting of an aliphatic conjugated diene monomer and an alkenyl aromatic monomer, and a monomer copolymerizable with these monomers in a specific ratio, wherein the monomer copolymerizable with these monomers does not comprise a cyano group-containing vinyl monomer.

Examples of the unsaturated carboxylic acid alkyl ester monomer include acrylic acid alkyl esters having an alkyl group having 1 to 8 carbon atoms such as methyl acrylate, ethyl acrylate, butyl acrylate and 2-ethylhexyl acrylate, methacrylic acid alkyl esters having an alkyl group having 1 to 4 carbon atoms such as methyl methacrylate and ethyl methacrylate, maleic acid alkyl esters having an alkyl group having 1 to 4 carbon atoms such as dimethyl maleate and diethyl maleate, itaconic acid alkyl esters having an alkyl group having 1 to 4 carbon atoms such as dimethyl itaconate, and fumaric acid alkyl esters having an alkyl group having 1 to 4 carbon atoms such as monomethyl fumarate, monoethyl fumarate, dimethyl fumarate and diethyl fumarate; these monomers may be used in 1 or 2 or more. Preferable examples thereof include ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate and methyl methacrylate.

Examples of the ethylenically unsaturated carboxylic acid monomer include monocarboxylic acid and dicarboxylic acid such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, and itaconic acid, and anhydrides thereof, and 1 or 2 or more species thereof may be used. Acrylic acid, methacrylic acid, fumaric acid, and itaconic acid are preferred.

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

Examples of the alkenyl aromatic monomer include styrene, α -methylstyrene, methyl- α -methylstyrene and vinyltoluene, and 1 or 2 or more species can be used. Styrene is preferably used.

Examples of the monomer copolymerizable with these monomers, i.e., the above-mentioned unsaturated carboxylic acid alkyl ester monomer, ethylenically unsaturated carboxylic acid monomer, and at least one monomer selected from the group consisting of aliphatic conjugated diene monomers and alkenyl aromatic monomers, include a hydroxyalkyl group-containing unsaturated monomer, an unsaturated carboxylic acid amide monomer, and a polyfunctional ethylenically unsaturated monomer having 2 or more unsaturated double bonds, and 1 or 2 or more species thereof may be used.

Examples of the hydroxyalkyl group-containing unsaturated monomer include 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, 3-chloro-2-hydroxypropyl methacrylate, bis (ethylene glycol) maleate, bis (ethylene glycol) itaconate, 2-hydroxyethyl maleate, bis (2-hydroxyethyl maleate), and 2-hydroxyethyl methyl fumarate, and 1 or 2 or more kinds thereof can be used. 2-hydroxyethyl acrylate is preferably used.

Examples of the unsaturated carboxylic acid amide monomer include acrylamide, methamphetamide, N-methylolacrylamide, N-methylolmethacrylamide, and N, N-dimethylacrylamide, and 1 or 2 or more species can be used. Acrylamide, methacrylamide and N-methylolacrylamide are preferred.

Examples of the polyfunctional ethylenically unsaturated monomer having 2 or more unsaturated double bonds include polyethylene glycol di (meth) acrylates such as allyl methacrylate, ethylene glycol di (meth) acrylate, diethylene glycol di (meth) acrylate and triethylene glycol di (meth) acrylate, and divinyl compounds such as divinylbenzene, and 1 or 2 or more species can be used. Allyl methacrylate, ethylene glycol dimethacrylate and divinylbenzene are preferable.

The monomer composition of the copolymer latex used for the secondary battery electrode adhesive is characterized by comprising 60-94.8 wt% of unsaturated carboxylic acid alkyl ester monomer, 0.1-10 wt% of ethylene unsaturated carboxylic acid monomer, 5-20 wt% of at least one monomer selected from aliphatic conjugated diene monomer and alkenyl aromatic monomer, and 0.1-10 wt% of monomer which can be copolymerized with the monomers and does not comprise vinyl monomer containing cyano group.

If the content of the unsaturated carboxylic acid alkyl ester monomer is less than 60% by weight, the crack resistance of the electrode coating layer is low; if it exceeds 94.8 wt%, the adhesion to the current collector is low, and therefore the adhesion of the electrode coating layer is low. Preferably 65 to 88% by weight.

If the content of the ethylenically unsaturated carboxylic acid monomer is less than 0.1% by weight, the adhesion of the electrode coating layer is low, and the stability of the electrode composition is low, resulting in the formation of aggregates. If the amount exceeds 10% by weight, the flexibility of the electrode coating layer is low, the viscosity of the copolymer latex increases, and the workability of the copolymer latex is low. Preferably 0.5 to 8% by weight.

If the content of at least one monomer selected from the group consisting of aliphatic conjugated diene monomers and alkenyl aromatic monomers is less than 5% by weight, the adhesion of the electrode coating layer is low; if it exceeds 20% by weight, the conductivity of the electrode coating layer is low.

If the content of the monomer copolymerizable with these monomers exceeds the range of 0.1 to 10% by weight, it is difficult to achieve both the adhesion of the electrode coating layer and the conductivity of the electrode coating layer.

The copolymer of the present invention does not include a cyano group-containing vinyl monomer such as acrylonitrile, methacrylonitrile, α -chloroacrylonitrile, α -ethylacrylonitrile, or the like. Copolymers containing these monomers have poor electrolyte resistance and the adhesion of the electrode coating layer is low.

Subsequently, the monomer composition was emulsion-polymerized in water to obtain a copolymer latex.

To emulsion polymerize the monomer composition, an emulsifier and a polymerization initiator are added to the monomer composition.

Examples of the emulsifier include anionic surfactants such as higher alcohol sulfate, alkylbenzene sulfonate, alkyldiphenyloxide disulfonate, aliphatic sulfonate, aliphatic carboxylate and nonionic surfactant sulfate, and nonionic surfactants such as polyethylene glycol alkyl ester type, alkylphenyl ether type and alkyl ether type, and 1 or 2 or more kinds of the emulsifiers can be used. The surfactant may preferably be an anionic surfactant, and more preferably an alkylbenzene sulfonate, an alkyldiphenyloxide disulfonate, or a polyoxyethylene alkyl ether.

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, based on 100 parts by weight of the monomer composition.

The polymerization initiator is a radical polymerization initiator, and examples thereof include water-soluble polymerization initiators such as potassium persulfate, sodium persulfate and ammonium persulfate, and oil-soluble polymerization initiators such as cumene hydroperoxide, benzoyl peroxide, tert-butyl hydroperoxide, acetyl peroxide, diisopropylbenzene hydroperoxide and 1,1,3, 3-tetramethylbutylhydroperoxide. The water-soluble polymerization initiator may preferably be potassium persulfate, sodium persulfate or ammonium persulfate, and the oil-soluble polymerization initiator may preferably be cumene hydroperoxide.

The polymerization initiator is added, for example, in a proportion of 0 to 3 parts by weight, preferably 0.01 to 2 parts by weight, based on 100 parts by weight of the monomer composition.

In addition, when the monomer composition is emulsion polymerized, a reducing agent or a chain transfer agent may be added as necessary.

Examples of the reducing agent include ferrous sulfate, sulfite, bisulfite, pyrosulfite, dithionite, dithionate, thiosulfate, formaldehyde sulfonate, benzaldehyde sulfonate, carboxylic acids such as isoascorbic acid, L-ascorbic acid, tartaric acid, and citric acid, and salts thereof, reducing sugars such as glucose and sucrose, and amines such as dimethylaniline and triethanolamine. Preferable examples thereof include ferrous sulfate, carboxylic acids and salts thereof, and more preferable examples thereof include ferrous sulfate and erythorbic acid.

The reducing agent is added, for example, in a proportion of 0 to 3 parts by weight, preferably 0.005 to 1 part by weight, based on 100 parts by weight of the monomer composition.

Examples of the chain transfer agent include alkyl mercaptans having an alkyl group having 6 to 18 carbon atoms such as n-hexyl mercaptan, n-octyl mercaptan, tert-octyl mercaptan, n-dodecyl mercaptan, tert-dodecyl mercaptan and n-octadecyl mercaptan, xanthic compounds such as dimethyl xanthogen disulfide and diisopropyl xanthogen disulfide, terpinolene, thiuram compounds such as tetramethyl thiuram disulfide, tetraethyl thiuram disulfide and tetramethyl thiuram monosulfide, phenol compounds such as 2, 6-di-tert-butyl-4-methylphenol and styrenated phenol, allyl compounds such as allyl alcohol, halogenated hydrocarbon compounds such as methylene chloride, methylene bromide and tetrabrominated carbon, vinyl ethers such as alpha-benzyloxystyrene, alpha-benzyloxyacrylonitrile and alpha-benzyloxyacrylamide, for example, triphenylethane, pentaphenylethane, acrolein, methacrolein, thioglycolic acid, thiomalic acid, 2-ethylhexyl thioglycolate, α -methylstyrene dimer and the like may be used, and 1 or 2 or more species may be used. Examples of the alkyl mercaptan include preferably alpha-methylstyrene dimer and alkyl mercaptan, and more preferably alpha-methylstyrene dimer and t-dodecyl mercaptan.

The chain transfer agent is added, for example, in a proportion of 0 to 3 parts by weight, preferably 0.01 to 1 part by weight, based on 100 parts by weight of the monomer composition.

In addition, an unsaturated hydrocarbon may be added as needed in the emulsion polymerization. Examples of the unsaturated hydrocarbon include pentene, hexene, heptene, cyclopentene, cyclohexene, cycloheptene, 4-methylcyclohexene, and 1-methylcyclohexene, and cyclohexene is preferable. Cyclohexene has a low boiling point and is therefore easily recovered and reused by steam distillation or the like after completion of polymerization, and is preferable from the viewpoint of environmental load.

As other additives, for example, chelating agents, antioxidants, preservatives, dispersants, thickeners and the like may be added as necessary.

The polymerization method is not particularly limited, and batch polymerization, semi-continuous polymerization, seed polymerization, and the like can be used. The method of adding the various components is not particularly limited, and a one-shot addition method, a batch addition method, a continuous addition method, a step-feed method, and the like can be used.

Thus, the monomer composition is emulsion-polymerized to obtain a copolymer latex in which the copolymer obtained is dispersed in water.

The solid content of the copolymer latex to be obtained is, for example, 35 to 55% by weight, preferably 40 to 50% by weight.

The glass transition temperature (Tg) of the copolymer in the copolymer latex to be obtained is not particularly limited, and is, for example, -60 to 70 ℃ and preferably-50 to 50 ℃.

The insoluble content (gel content) of the copolymer latex to toluene is not particularly limited, and is, for example, 50 to 100% by weight, preferably 80 to 100% by weight. If the gel content is less than 50 wt%, the adhesion of the electrode coating layer tends to be low and the conductivity of the electrode coating layer tends to be low.

The number average particle diameter of the copolymer in the copolymer latex to be obtained is not particularly limited, and is, for example, 50 to 350nm, preferably 70 to 300 nm.

The binder for secondary battery electrodes of the present invention is used for forming an electrode of a secondary battery such as a lithium ion battery, a nickel hydride battery, and a nickel cadmium battery, and binds particles of a negative electrode constituent material or a positive electrode active material and particles of a negative electrode constituent material or a positive electrode active material to a current collector. Hereinafter, an example of the lithium ion secondary battery will be specifically described.

The slurry for an electrode of an aqueous dispersion battery of the present invention is prepared by incorporating an active material (negative electrode constituent material or positive electrode active material) into the binder for an electrode of a secondary battery of the present invention. That is, a binder for a secondary battery electrode is mixed into a negative electrode constituent material to prepare a negative electrode slurry for a negative electrode of a secondary battery. Further, a binder for a secondary battery electrode is mixed with a positive electrode active material to prepare a positive electrode slurry for a positive electrode of a secondary battery.

The negative electrode constituting material is not particularly limited, and in the case of a nonaqueous electrolyte secondary battery, for example, there may be used 1 or 2 or more kinds of conductive carbonaceous materials such as carbon fluoride, graphite, carbon fiber, resin-fired carbon, linear graphite mixture, coke, pyrolytic vapor-grown carbon, furfuryl alcohol resin-fired carbon, mesophase carbon microbeads, mesophase pitch-based carbon, graphite whiskers, quasi-isotropic carbon, fired products of natural materials and pulverized products thereof, for example, polyacene-based organic semiconductors, polyacetylene, polyparaphenylene-based conductive polymers, Sn-based or Si-based alloy-based materials, and the like.

The positive electrode active material is not particularly limited, and may, for example, be MnO₂、MoO₃、V₂O₅、V₆O₁₃、Fe₂O₃、Fe₃O₄Isotransition metal oxides, LiCoO₂、LiMnO₂、LiNiO₂、Li_XCo_YSnZO₂Etc. lithium-containing composite oxide, LiFePO₄Lithium-containing composite metal oxides, e.g. TiS₂、TiS₃、MoS₃、FeS₂Isotransition metal sulfides, e.g. CuF₂、NiF₂As the metal fluoride, 1 or 2 or more kinds can be used.

When preparing the slurry for an electrode of an aqueous dispersion battery of the present invention, the binder for an electrode of a secondary battery of the present invention is incorporated in an amount of, for example, 0.1 to 7 parts by weight, preferably 0.5 to 4 parts by weight in terms of solid content, based on 100 parts by weight of the negative electrode constituent material or the positive electrode active material.

When the amount of the binder for a secondary battery electrode to be incorporated is less than 0.1 part by weight based on 100 parts by weight of the negative electrode constituent material or the positive electrode active material, good adhesion to the current collector or the like tends not to be obtained; if the amount exceeds 7 parts by weight, overvoltage will be significantly increased when assembling the secondary battery, and battery characteristics will tend to be degraded.

In the slurry for an aqueous dispersion battery electrode of the present invention, various additives such as a water-soluble thickener, a dispersant, and a stabilizer may be added 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, casein, and the like; examples of the dispersant include sodium hexametaphosphate, sodium tripolyphosphate, sodium pyrophosphate, sodium polyacrylate, and the like; examples of the stabilizer include nonionic surfactants and anionic surfactants.

When the water-soluble thickener is added to the slurry for an electrode of an aqueous dispersion battery of the present invention, the water-soluble thickener is incorporated, for example, in an amount of 0.1 to 10 parts by weight, preferably 0.5 to 5 parts by weight, in terms of solid content, based on 100 parts by weight of the negative electrode constituent material or the positive electrode active material.

The slurry for an aqueous dispersion battery electrode of the present invention is applied to a current collector and dried to form an electrode coating layer on the current collector, thereby obtaining a secondary battery electrode of the present invention having excellent fracture resistance, flexibility, and electrical conductivity of the electrode coating layer. Such an electrode can be used as a positive or negative electrode plate of a lithium ion secondary battery.

As the current collector, for example, a metal foil of copper, nickel or the like may be cited as the current collector for the negative electrode, and for example, a metal foil of aluminum or the like may be cited as the current collector for the positive electrode.

As a method for applying the slurry for an aqueous dispersion battery electrode of the present invention to a current collector, a known method such as a reverse roll method, a comma roll method, a gravure printing method, or an air knife method can be used, and a standing dryer, an air dryer, a warm air dryer, an infrared heater, a far infrared heater, or the like can be used for drying. The drying temperature is usually above 50 ℃.

When the binder for a secondary battery electrode of the present invention is used, the binder has good adhesion to a current collector and an active material, and a secondary battery electrode having good crack resistance, flexibility, and electrical conductivity of an electrode coating layer can be obtained. The secondary battery using the secondary battery electrode of the present invention has good battery characteristics such as charge-discharge cycle characteristics.

Examples

The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to these examples. In the examples, parts and% indicating compounding ratios are on a weight basis.

1. Synthesis of copolymer latex

(1) Preparation of copolymer latex (a)

Into a pressure-resistant polymerization reactor were charged 140 parts of polymerization water, 0.2 part of sodium dodecylbenzenesulfonate as an emulsifier, and 0.8 part of potassium persulfate, and after sufficiently stirring, each monomer and other compounds of stage 1 shown in Table 1 were added to start polymerization at 65 ℃. After 1 hour, the monomers and other compounds of stage 2 were added continuously at 70 ℃ for 5 hours, and the polymerization was terminated when the polymerization conversion exceeded 97%. Subsequently, the copolymer latex was adjusted to pH7 with an aqueous solution of sodium hydroxide, 500ppm of an antifoaming agent was added, and unreacted monomers and other low boiling compounds were removed by steam distillation to obtain a copolymer latex (a).

(2) Preparation of copolymer latex (b)

Polymerization was carried out in the same manner as in the copolymer latex (a) except that the monomers and other compounds used were as described in Table 1 and the polymerization temperature in stage 1 was set to 60 ℃. Subsequently, the copolymer latex was adjusted to pH7 with an aqueous lithium hydroxide solution, 500ppm of an antifoaming agent was added, and then unreacted monomers were removed by steam distillation to obtain a copolymer latex (b).

(3) Preparation of copolymer latex (c)

160 parts of polymerization water, 0.1 part of sodium dodecylbenzenesulfonate as an emulsifier, 0.1 part of sodium alkyldiphenylether disulfonate, 1.0 part of polyoxyethylene lauryl ether, and 0.3 part of cumene hydroperoxide were charged into a pressure-resistant polymerization reactor, and after sufficient stirring, the monomers and other compounds in stage 1 shown in Table 1 were added and polymerization was started at 35 ℃. After 8 hours, the monomers and other compounds of stage 2 were added continuously at 45 ℃ for 8 hours, and the polymerization was terminated when the polymerization conversion exceeded 97%. Subsequently, the copolymer latex was adjusted to pH7 with an aqueous ammonia solution, 1000ppm of an antifoaming agent was added, and then unreacted monomers were removed by steam distillation to obtain a copolymer latex (c).

(4) Preparation of copolymer latex (d)

250 parts of polymerization water, 1.5 parts of sodium dodecylbenzenesulfonate as an emulsifier, and 0.3 part of potassium persulfate were charged into a pressure-resistant polymerization reactor, and after sufficiently stirring, each monomer and other compounds of stage 1 shown in Table 1 were added and polymerized at 65 ℃ for 8 hours. The polymerization was terminated when the polymerization conversion exceeded 97%. Subsequently, the copolymer latex was adjusted to pH7 with an aqueous potassium hydroxide solution, 700ppm of an antifoaming agent was added, and then unreacted monomers were removed by steam distillation to obtain a copolymer latex (d).

(5) Preparation of copolymer latices (e), (f), (h), (i), (j)

Polymerization was carried out in the same manner as in the copolymer latex (a) except that the monomers and other compounds used were as described in tables 1 and 2. Then, these copolymer latexes were adjusted to pH7 with an aqueous sodium hydroxide solution, 500ppm of an antifoaming agent was added, and unreacted monomers and other low boiling compounds were removed by steam distillation to obtain copolymer latexes (e), (f), (h), (i) and (j).

(6) Preparation of copolymer latices (g), (k), (l), (m)

250 parts of polymerization water, 2.0 parts of sodium dodecylbenzenesulfonate as an emulsifier, and 0.3 part of potassium persulfate were charged into a pressure-resistant polymerization reactor, and after sufficiently stirring, each monomer of stage 1 shown in Table 2 was added and polymerized at a polymerization temperature shown in Table 2 for 8 hours. The polymerization was terminated when the polymerization conversion exceeded 97%. Then, these copolymer latexes were adjusted to pH7 with an aqueous sodium hydroxide solution, 700ppm of an antifoaming agent was added, and unreacted monomers were removed by steam distillation to obtain copolymer latexes (g), (k), (l), and (m).

2.Measurement of number average particle diameter of copolymer latex

For 1000 latex particles photographed by a transmission electron microscope, the area circle equivalent diameter was measured by an image analyzer, and the number average particle diameter was calculated. The results are shown in tables 1 and 2.

3.Determination of the glass transition temperature of the copolymer latex

The copolymer latex was cast on a glass plate, dried at 70 ℃ for 4 hours to prepare a film, and measured with a differential scanning calorimeter (manufactured by Seiko technologies Co., Ltd. (セイコーインスツルメンツ Co., Ltd., DSC 6200)) at a temperature rise rate of 10 ℃/min. The measurement results are shown in tables 1 and 2.

4.Determination of the gel content of the copolymer latex

Latex films were prepared in an atmosphere of 40 ℃ and 85% humidity using the copolymer latexes obtained in each synthesis example and each comparative synthesis example. About 1g of the prepared latex film was weighed and added to 400ml of toluene to swell and dissolve it for 48 hours. Then, the residue was filtered through a 300-mesh wire gauze, and the toluene-insoluble matter trapped on the wire gauze was dried and weighed. Next, the dry weight percentage of the insoluble component relative to the weight of the latex film was calculated. The measurement results are shown in tables 1 and 2.

[ Table 1]

[ Table 2]

5.Preparation of slurry for aqueous dispersion electrode

(1) Preparation of slurry for negative electrode

The negative electrode composition of each example and each comparative example was prepared by using natural graphite having an average particle size of 20 μm as a negative electrode constituent material, adding 2 parts by solid content of an aqueous solution of carboxymethyl cellulose as a thickener and 3 parts by weight of each copolymer latex to 100 parts of natural graphite, and kneading the mixture with an appropriate amount of water under conditions such that the solid content of the electrode composition was 40%.

(2) Preparation of slurry for positive electrode

LiCoO as a positive electrode active material in an amount of 100 parts₂Each of 5 parts of acetylene black as a conductive agent, 1 part of an aqueous solution of carboxymethyl cellulose as a thickener in terms of solid content, and 4 parts of each copolymer latex as a binder was kneaded after adding an appropriate amount of water under a condition that the total solid content was 40%, to prepare a composition for a positive electrode of each example and each comparative example.

6.Preparation of electrode sheet

(1) Preparation of negative plate

The compositions for negative electrodes of examples and comparative examples were applied to both surfaces of a copper foil having a thickness of 20 μm as a current collector, dried at 120 ℃ for 5 minutes, and then pressed at room temperature to obtain negative electrodes having a coating layer thickness of 80 μm (per surface).

(2) Preparation of positive plate

The positive electrode compositions of examples and comparative examples were applied to both surfaces of an aluminum foil having a thickness of 20 μm as a current collector, dried at 120 ℃ for 5 minutes, and then pressed at room temperature to obtain negative electrodes having a coating layer thickness of 80 μm (on each surface).

7.Performance test of electrode sheet

(1) Measurement of adhesive force of electrode coating layer

On the surface of the electrode sheet of each example and each comparative example, 6 cuts were made at intervals of 2mm in length and width from the coating layer to the depth reaching the current collector to form a checkerboard having 25 (5 × 5) squares. The coated layer was peeled off immediately after the adhesive tape was attached to the checkerboard, and the degree of peeling of the coated layer was evaluated by visual observation. The results are shown in tables 3 and 4.

Excellent performance, no peeling.

Peeling 1-3 squares.

And peeling off 4-10 squares.

And (x) stripping more than 11 squares.

(2) Measurement of crack resistance of electrode coating layer

The electrode sheets of the examples and comparative examples were cut into a 10cm × 5cm rectangular shape, and folded in half at 180 °, to prepare a 5cm square test piece. The test piece was pressed with a heat sealer at a pressure of 0.05MPa for 2 seconds. The inside and outside crease portions of the test piece taken out were observed by an optical microscope. Based on the observation results, evaluation was performed as follows. The results are shown in tables 3 and 4.

Excellent performance, no fracture.

Fine cracks were observed on the surface of the electrode sheet, but no exposure of the current collector was observed.

And a crack was observed on the surface of the electrode sheet, and a minute exposure of the current collector was observed.

Fracture was observed on the surface of the electrode sheet, and exposure of the current collector was observed at the fractured portions at multiple locations.

(3) Measurement of flexibility of electrode coating layer

The electrode sheet of each example and each comparative example was cut into a rectangular shape of 8cm × 2cm, and the bending resistance of the electrode sheet was measured with a slit width of 5mm using a softness tester manufactured by Toyo Seiki Seisaku-Sho Ltd. The results are shown in tables 3 and 4.

Very good resistance is lower than 60 g.

The resistance is 60g or more and less than 75 g.

And a resistance value of 75g or more and less than 90 g.

The resistance is more than 90 g.

(4) Measurement of conductivity of electrode coating layer

The electrode compositions of examples and comparative examples were applied to a commercially available polyester film and dried at 130 ℃ for 5 minutes. Rolling to obtain the coatingThe cloth layer has a thickness of about 60 μm and the density of the coating layer is about 1.3g/cm³The coated layer sample of (1).

Mitsubishi chemical analysis technology corporation (Mitsubishi chemical アナリテ)_ック corporation) was measured for the surface resistivity of the coating layer. The results are shown in tables 3 and 4.

The above invention is provided as an exemplary embodiment of the present invention, which is merely an example and should not be construed as limiting. Modifications of the present invention that are obvious to those skilled in the art are intended to be included within the scope of the following claims.

[ Table 3]

[ Table 4]

Possibility of industrial utilization

The binder for a secondary battery electrode of the present invention is used as a binder for a secondary battery electrode for binding an active material (a negative electrode constituent material and a positive electrode active material) to a current collector in an electrode of a secondary battery. The secondary battery electrode of the present invention is excellent in crack resistance, flexibility and conductivity of the electrode coating layer, and a secondary battery using the secondary battery electrode is excellent in battery characteristics such as charge-discharge cycle characteristics, and is very useful.

Claims (3)

1. The adhesive for secondary battery electrodes is characterized by comprising a copolymer latex obtained by emulsion polymerization of a monomer composition, wherein the monomer composition comprises 60-94.8 wt% of unsaturated carboxylic acid alkyl ester monomer, 0.1-10 wt% of ethylene unsaturated carboxylic acid monomer, 5-20 wt% of at least one monomer selected from aliphatic conjugated diene monomer and alkenyl aromatic monomer, and 0.1-10 wt% of other monomer copolymerizable with the monomers, and the other monomer does not comprise cyano-containing vinyl monomer.

2. A slurry for an electrode of a water-dispersed secondary battery,

comprising a binder for secondary battery electrodes and an active material,

the adhesive for secondary battery electrodes comprises a copolymer latex obtained by emulsion polymerization of a monomer composition, wherein the monomer composition comprises 60 to 94.8 wt% of an unsaturated carboxylic acid alkyl ester monomer, 0.1 to 10 wt% of an ethylene unsaturated carboxylic acid monomer, 5 to 20 wt% of at least one monomer selected from an aliphatic conjugated diene monomer and an alkenyl aromatic monomer, and 0.1 to 10 wt% of another monomer copolymerizable with these monomers, and the other monomer does not comprise a cyano group-containing vinyl monomer.

3. A secondary battery electrode produced using a slurry for a secondary battery electrode in an aqueous dispersion, characterized in that,

the slurry for a secondary battery electrode in aqueous dispersion contains a binder for a secondary battery electrode and an active material,

the adhesive for secondary battery electrodes comprises a copolymer latex obtained by emulsion polymerization of a monomer composition, wherein the monomer composition comprises 60 to 94.8 wt% of an unsaturated carboxylic acid alkyl ester monomer, 0.1 to 10 wt% of an ethylene unsaturated carboxylic acid monomer, 5 to 20 wt% of at least one monomer selected from an aliphatic conjugated diene monomer and an alkenyl aromatic monomer, and 0.1 to 10 wt% of another monomer copolymerizable with these monomers, and the other monomer does not comprise a cyano group-containing vinyl monomer.