HK1159317B - Binder for secondary battery electrodes - Google Patents

Description

Binder for secondary battery electrode

Technical Field

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

Background

Lithium ion secondary batteries are lightweight and have a high energy density, and therefore, application thereof as a power source for small electronic devices, automobiles, and houses is being studied. 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 positive electrode active material and a negative electrode constituent material) is mixed with 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 adhesion to an active material, adhesion to a current collector, resistance to a polar solvent as an electrolyte, and stability in an electrochemical environment.

Heretofore, fluorine-based polymers such as polyvinylidene fluoride have been used as such polymer binders. However, the fluorine-based polymer needs to be dissolved in an organic solvent, and when the electrode composition is applied to a current collector and dried, the organic solvent may be volatilized. Further, since the adhesive force is poor, it is necessary to blend a large amount of polymer binder in order to obtain a sufficient adhesive force, and there is a problem that the conductivity of the secondary battery is hindered.

In order to improve the above-mentioned problems, various proposals have been made to use an aqueous dispersion of a non-fluorine-containing polymer as a polymer binder.

For example, Japanese patent application laid-open No. 5-74461 (patent document 1) proposes a secondary battery having excellent recyclability, storage characteristics, and safety, which is obtained by using a styrene-butadiene latex having a specific composition and gel content as a polymer binder.

In addition, Japanese patent application laid-open No. 11-25989 (patent document 2) proposes a secondary battery having a high capacity, excellent discharge characteristics, excellent charge/discharge cycle characteristics, and excellent safety, which is obtained by using an aqueous dispersion of a copolymer having a specific composition and a glass transition temperature as a polymer binder.

In addition, Japanese patent application laid-open No. 8-250122 (patent document 3) proposes a method of obtaining a battery electrode excellent in recyclability, storage characteristics and safety by using a styrene-butadiene latex in a specific range as a binder and drying the latex at 50 ℃ or higher.

(Prior art document)

(patent document)

Patent document 1: japanese unexamined patent publication Hei 5-74461

Patent document 2: japanese unexamined patent publication No. Hei 11-25989

Patent document 3: japanese unexamined patent publication Hei 8-250122

Disclosure of Invention

(problems to be solved by the invention)

The polymer binders described in patent documents 1 and 2 are prepared by using the electrode compositions as aqueous dispersions, and therefore, although the problems of volatilization of organic solvents during the production of electrodes can be reduced, the polymer binders are not sufficient in terms of adhesion to the current collector or active material, folding resistance of the electrode coating, and flexibility.

Accordingly, an object of the present invention is to provide a binder for secondary battery electrodes, which is capable of forming an electrode coating having low adhesion, excellent fracture resistance and flexibility, and excellent in adhesion to a current collector or an active material.

(means for solving the problems)

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 having an insoluble content of 50 to 100% by weight with respect to toluene, which is obtained by emulsion-polymerizing a monomer composition containing 12.0 to 39.5% by weight of an aliphatic conjugated diene monomer, 1.5 to 8.5% by weight of an unsaturated carboxylic acid alkyl ester monomer, 0.1 to 10.0% by weight of an ethylenically unsaturated carboxylic acid monomer, and 42.0 to 86.4% by weight of a monomer copolymerizable with these monomers.

(effect of the invention)

The binder for secondary battery electrodes of the present invention comprises a copolymer latex having an insoluble content of toluene of 50 to 100% by weight, which is obtained by emulsion polymerization of a monomer composition containing, in a specific ratio, an aliphatic conjugated diene monomer, an unsaturated carboxylic acid alkyl ester monomer, an ethylenically unsaturated carboxylic acid monomer, and a monomer copolymerizable with these monomers.

Therefore, the present invention can form an electrode coating layer having excellent adhesion to a current collector or an active material, less adhesion, good workability, and excellent folding crack resistance and flexibility.

Detailed Description

The binder for secondary batteries of the present invention comprises a copolymer latex obtained by emulsion polymerization of a monomer composition containing an aliphatic conjugated diene monomer, an unsaturated carboxylic acid alkyl ester monomer, an ethylenically unsaturated carboxylic acid monomer, and a monomer copolymerizable with these monomers.

Examples of the aliphatic conjugated diene monomer include: for example, 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 the like may be used alone or in combination of two or more. 1, 3-butadiene is preferred.

As the unsaturated carboxylic acid alkyl ester, there may be mentioned: for example, one or two or more of alkyl acrylates having an alkyl group having 1 to 8 carbon atoms such as methyl acrylate, ethyl acrylate, butyl acrylate and 2-ethylhexyl acrylate, alkyl methacrylates 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 can be used. Preferred examples thereof include alkyl methacrylates having an alkyl group having 1 to 4 carbon atoms, and more preferred is methyl methacrylate.

As the ethylenically unsaturated carboxylic acid monomer, there may be mentioned: for example, monocarboxylic acids such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, and itaconic acid, dicarboxylic acids (anhydrides), and the like may be used alone or in combination. Acrylic acid, fumaric acid, itaconic acid are preferred.

Examples of the monomer copolymerizable with the aliphatic conjugated diene monomer, the unsaturated carboxylic acid alkyl ester monomer, and the ethylenically unsaturated carboxylic acid monomer include: for example, one or two or more of an alkenyl aromatic monomer, an acrylonitrile monomer, an unsaturated monomer containing a hydroxyalkyl group, an unsaturated carboxylic acid amide monomer, and the like can be used.

As the alkenyl aromatic monomer, there may be mentioned: for example, styrene, α -methylstyrene, methyl- α -methylstyrene, vinyltoluene, vinylbenzene, etc., and one kind or two or more kinds thereof may be used. Styrene is preferred.

As the acrylonitrile monomer, there can be mentioned: for example, acrylonitrile, methacrylonitrile, α -chloroacrylonitrile, α -ethylacrylonitrile, etc., may be used alone or in combination of two or more. Preferred are acrylonitrile and methacrylonitrile.

As the unsaturated monomer containing a hydroxyalkyl group, there may be mentioned: for example, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, 2-hydroxypropyl 3-chloro-2-methacrylate, di (ethylene glycol) maleate, di (ethylene glycol) itaconate, 2-hydroxyethyl maleate, bis (2-hydroxyethyl) maleate, 2-hydroxyethyl methylfumarate, and the like, and one kind or two or more kinds thereof may be used. 2-hydroxyethyl acrylate is preferred.

As the unsaturated carboxylic acid amide monomer, there may be mentioned: for example, acrylamide, methacrylamide, N-methylolacrylamide, N-methylolmethacrylamide, N-dimethylacrylamide and the like, and one kind or two or more kinds thereof may be used. Acrylamide and methacrylamide are preferred.

In addition to the above monomers, for example, radically polymerizable monomers such as ethylene, propylene, vinyl acetate, vinyl propionate, vinyl chloride, and vinylidene chloride can be used.

The monomer composition contains 12.0 to 39.5 wt%, preferably 17 to 39 wt% of an aliphatic conjugated diene monomer; containing 1.5 to 8.5% by weight, preferably 2 to 8% by weight, of an unsaturated carboxylic acid alkyl ester monomer; 0.1 to 10.0% by weight, preferably 0.5 to 5% by weight, of an ethylenically unsaturated carboxylic acid monomer; and as residues, monomers copolymerizable with these monomers, for example, 42.0 to 86.4% by weight, preferably 48 to 80.5% by weight.

If the content of the aliphatic conjugated diene monomer is less than 12.0 wt%, the adhesion to the current collector decreases, and therefore the adhesion of the electrode coating decreases, and if the content exceeds 39.5 wt%, the adhesion of the electrode coating increases, and the workability decreases.

When the content of the unsaturated carboxylic acid alkyl ester monomer is less than 1.5% by weight, flexibility of the electrode coating is reduced, and when it exceeds 8.5% by weight, folding crack resistance of the electrode coating is reduced.

When the content of the ethylenically unsaturated carboxylic acid monomer is less than 0.1% by weight, the stability of the composition for an electrode and the adhesive force of an electrode coating layer are lowered, and when it exceeds 10.0% by weight, the viscosity of the copolymer latex is increased and the workability of the copolymer latex is lowered.

When the content of the monomer exceeds the range of 42.0 to 86.4 wt%, it becomes difficult to achieve both adhesion and adherence of the electrode coating.

Then, the monomer is emulsion-polymerized in water to obtain a copolymer latex.

When emulsion polymerization is performed on the monomer composition, an emulsifier and a polymerization initiator are added to the monomer composition.

As the emulsifier, there can be mentioned: for example, anionic surfactants such as higher alcohol sulfate, alkylbenzene sulfonate, alkyldiphenyloxide disulfonate, aliphatic sulfonate, aliphatic carboxylate, and sulfate of nonionic surfactant, for example, nonionic surfactants such as polyvinyl alcohol alkyl ester type, alkylbenzene ether type, and alkyl ether type, may be used alone or in combination of two or more. The surfactant is preferably an anionic surfactant, and alkylbenzenesulfonate and alkyldiphenyloxide disulfonate are more preferably listed.

The emulsifier is blended 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 there may be mentioned: examples of the water-soluble polymerization initiator include potassium persulfate, sodium persulfate, and ammonium persulfate, and examples thereof include oil-soluble polymerization initiators such as cumene hydroperoxide, benzoyl peroxide, tert-butyl hydroperoxide, acetylated hydroperoxide, diisopropylbenzene hydroperoxide, and 1, 1, 3, 3-tetramethylbutyl hydroperoxide. Preferably, potassium persulfate, sodium persulfate, or ammonium persulfate is used as the water-soluble polymerization initiator, and cumene hydroperoxide is used as the oil-soluble polymerization initiator.

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

As the reducing agent, there may be mentioned: examples of the inorganic acid include ferrous sulfate, sulfite, bisulfite, pyrosulfite, hyposulfite, thiosulfate, formaldehyde sulfonate, and benzaldehyde sulfonate, carboxylic acids such as L-ascorbic acid, isoascorbic acid, tartaric acid, and citric acid, and salts thereof, reducing sugars such as glucose and sucrose, and amines such as dimethylaniline and triethanolamine. Preferred are ferrous sulfate, carboxylic acids and salts thereof, and more preferred are ferrous sulfate and erythorbic acid.

As the chain transfer agent, there may be mentioned: for example, alkyl mercaptans having an alkyl group 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-octadecyl mercaptan, xanthate compounds such as dimethyl xanthogen disulfide and diisopropyl xanthogen disulfide, terpinolene, thiuram compounds such as tetramethyl thiuram disulfide, tetraethyl thiuram disulfide and tetramethyl thiuram sulfide, phenol compounds such as 2, 6-di-t-butyl-4-methylphenol and styrenated phenol, allyl compounds such as allyl alcohol, halogenated hydrocarbon compounds such as methylene chloride, methylene bromide and carbon tetrabromide, vinyl ethers such as α -benzyloxystyrene, α -benzyloxyacrylonitrile and α -benzyloxyacrylamide, for example, triphenylethane, pentaphenylethane, acrolein, methacrolein, thioglycolic acid, thiomalic acid, 2-ethylhexyl acetic acid, α -methylstyrene dimer acid, and the like may be used, and one or two or more of these may be used, and α -methylstyrene dimer acid and alkyl mercaptan are preferable, and α -methylstyrene dimer acid and t-dodecyl mercaptan are more preferable.

The chain transfer agent is added in a proportion of, for example, 0 to 5 parts by weight, preferably 0.05 to 3 parts by weight, based on 100 parts by weight of the monomer composition.

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

Further, as other additives, an antioxidant, a preservative, a dispersant, a thickener, and the like may be added as necessary.

The polymerization method is not particularly limited, and batch polymerization, semi-batch polymerization, addition polymerization, or the like can be used. The method of adding the various components is not particularly limited, and a one-time addition method, a batch addition method, a continuous addition method, an automatic addition method, and the like can be used.

Thus, a copolymer latex in which a copolymer obtained by emulsion polymerization of a monomer composition is dispersed in water can be obtained.

The solid content in the resulting copolymer latex is, for example, 40 to 55% by weight, preferably 47 to 52% by weight.

Furthermore, the glass transition temperature (Tg) of the copolymer in the resulting copolymer latex is, for example, -20 to 90 ℃, preferably-15 to 70 ℃.

The insoluble content (gel content) of toluene is 50 to 100% by weight, preferably 60 to 99% by weight, based on the copolymer latex obtained. If the gel content is less than 50 wt%, the adhesion of the electrode coating tends to be lowered, or the adhesion of the electrode coating tends to be increased, and workability tends to be lowered.

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 300nm, preferably 70 to 250 nm.

The binder for a secondary battery electrode of the present invention is used for forming an electrode of a secondary battery such as a lithium ion secondary battery, a nickel-metal hydride battery, or a nickel-cadmium battery, and binds particles of a negative electrode constituent material or a positive electrode active material to each other, and binds the negative electrode constituent material or the positive electrode active material to a current collector.

Specifically, a binder for a secondary battery electrode is mixed with a negative electrode constituent material or a positive electrode active material to prepare a battery electrode composition. That is, a binder for a secondary battery electrode is mixed with a negative electrode constituent material to prepare a negative electrode composition for a secondary battery negative electrode. Further, a positive electrode composition for a positive electrode of a secondary battery is prepared by blending a binder for a secondary battery electrode with a positive electrode active material.

The negative electrode constituent material is not particularly limited. In the case of a nonaqueous electrolyte secondary battery, there may be mentioned: for example, one or two or more of conductive carbonaceous materials such as carbon fluoride, graphite, carbon fiber, resin-fired carbon, a linear graphite mixture, coke, thermally decomposed gas-phase grown carbon, furfuryl alcohol resin-fired carbon, mesophase carbon microbeads, mesophase pitch-based carbon, graphite whiskers, pseudo isotropic carbon, a sintered body of a natural material, and a pulverized product of these materials, for example, a polyacene-based organic semiconductor, a conductive polymer such as polyacetylene, and polyphenylene, can be used.

The positive electrode active material is not particularly limited. Mention may be made of: for example, MnO₂、MoO₃、V₂O₅、V₆O₁₃、Fe₂O₃、Fe₃O₄Isotransition metal oxides, LiCoO₂、LiMnO₂、LiNiO₂、Li_xCo_ySn_zO₂Are composed ofComposite oxides with lithium, LiFePO₄And the like lithium-containing composite metal oxides, e.g. TiS₂、TiS₃、MoS₃、FeS₂Isotransition metal sulfides, e.g. CuF₂、NiF₂And the like, and one kind or two or more kinds thereof may be used.

In the case of preparing the composition for a battery electrode, the binder for a secondary battery electrode is compounded so that the solid content of the copolymer latex is, for example, 0.1 to 7 parts by weight, preferably 0.5 to 4 parts by weight, based on 100 parts by weight of the negative electrode constituent material or the positive electrode active material.

If the solid content of the copolymer latex 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 a current collector or the like tends not to be obtained, and if the solid content exceeds 7 parts by weight, overvoltage tends to increase significantly during assembly into a secondary battery, and battery characteristics tend to decrease.

In addition, various additives such as a water-soluble thickener, a dispersant, and a stabilizer may be added to the battery electrode composition as necessary. Examples of the water-soluble thickener include: for example, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, polyacrylic acid (salt), oxidized starch, phosphorylated starch, casein, etc., as the dispersant, for example, sodium hexametaphosphate, sodium tripolyphosphate, sodium pyrophosphate, sodium polyacrylate, etc., as the stabilizer, there are exemplified: for example, nonionic, anionic surfactants and the like.

When a water-soluble thickener is added to the battery electrode composition, the water-soluble thickener is added, for example, in a proportion 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 battery electrode composition is applied to a current collector and dried to form an electrode coating layer on the current collector, thereby obtaining an electrode sheet. Such an electrode sheet is suitable for a positive electrode plate or a negative electrode plate of a lithium ion secondary battery.

Examples of the current collector for the negative electrode include: for example, a metal foil such as copper or nickel may be used as the positive electrode current collector: for example, a metal foil such as aluminum.

As a method for applying the composition for a battery electrode to the current collector, known methods such as a reverse roll coating method, a comma roll coating method, a gravure roll coating method, an air knife coating method, and the like can be used, and drying may be performed using a standing dryer, an air dryer, a warm air dryer, an infrared heater, a far infrared heater, and the like. The drying temperature is usually 50 ℃ or higher.

The binder for a secondary battery electrode of the present invention can form an electrode coating layer having excellent adhesion to a current collector or an active material, good workability due to less adhesion, and excellent folding endurance, cracking endurance, and flexibility.

(examples)

The present invention will be described more specifically with reference to examples. However, the present invention is not limited to these examples. In addition, the parts and percentages in the examples indicating compounding ratios are values obtained on a weight basis.

1. Synthesis of copolymer latex

(1) Synthesis example 1

In a polymerization reactor having pressure resistance, 120 parts of pure water, 1 part of sodium dodecylbenzenesulfonate and 1 part of potassium persulfate were charged and sufficiently stirred.

Next, each monomer in the first row among the monomer feeding rows shown in table 1, and 8 parts of tert-dodecyl mercaptan and cyclohexene were fed into the polymerization reactor.

In another method, the monomers shown in Table 1 were charged into the second row among the rows and mixed to prepare a monomer mixture.

Then, the polymerization reactor was stirred and its internal temperature was increased to 70 ℃ to confirm heat generation by the initiation of polymerization.

Thereafter, from initiation of polymerization to 480 minutes, while maintaining the internal temperature at 70 ℃, the monomer mixture, 10 parts of pure water, and 0.3 part of sodium alkyldiphenyloxide disulfonate were continuously added. The polymerization was continued from 480 minutes to 780 minutes while maintaining the internal temperature at 75 ℃.

After 780 minutes from initiation of polymerization, it was confirmed that the polymerization conversion rate exceeded 97%, and then the internal temperature was cooled to 35 ℃ or lower.

After the pH was adjusted to 8 using an aqueous potassium hydroxide solution, unreacted monomers and the like were removed by steam distillation to obtain a copolymer latex (a).

(2) Synthesis example 2

In a pressure-resistant polymerization reactor, 90 parts of pure water, 0.5 part of sodium dodecylbenzenesulfonate and 1 part of potassium persulfate were charged and sufficiently stirred.

Next, 4 parts of each monomer in the first row among the monomer input rows shown in table 1 and cyclohexene were charged into the polymerization reactor.

In another method, each monomer in the second column among the monomer input columns shown in table 1 and tert-dodecyl mercaptan were mixed to prepare a monomer mixture.

Then, the polymerization reactor was stirred and its internal temperature was increased to 65 ℃ to confirm heat generation by the initiation of polymerization.

Thereafter, from initiation of polymerization to 480 minutes, while maintaining the internal temperature at 70 ℃, the monomer mixture, 10 parts of pure water, and 1.0 part of fumaric acid were continuously added. From 480 minutes to 540 minutes, each monomer in the third column among the monomer charge columns shown in table 1 and tert-dodecyl mercaptan were continuously added. The polymerization was continued by maintaining the internal temperature at 70 ℃ from 540 minutes to 780 minutes.

After 780 minutes from initiation of polymerization, it was confirmed that the polymerization conversion rate exceeded 97%, a polymerization stopper was added, and the internal temperature was cooled to 35 ℃ or lower.

After the pH was adjusted to 7 using an aqueous lithium hydroxide solution, unreacted monomers and the like were removed by steam distillation to obtain a copolymer latex (b).

(3) Synthesis example 3

In a polymerization reactor having pressure resistance, 110 parts of pure water, 0.15 part of sodium dodecylbenzenesulfonate and 0.45 part of potassium persulfate were charged and sufficiently stirred.

Then, 2 parts of cyclohexene and 0.1 part of α -methylstyrene dimer were charged into the polymerization reactor.

In another method, each monomer in the first row among the monomer input rows shown in table 1 and tert-dodecyl mercaptan were mixed to prepare a monomer mixture.

Then, the polymerization reactor was stirred and its internal temperature was increased to 60 ℃ to confirm heat generation by the initiation of polymerization.

Thereafter, from initiation of polymerization to 540 minutes, while maintaining the internal temperature at 60 ℃, the monomer mixture, 10 parts of pure water, and 0.1 part of sodium dodecylbenzenesulfonate were continuously added. The polymerization was continued by maintaining the internal temperature at 80 ℃ from 540 to 720 minutes.

After 720 minutes from initiation of the polymerization, it was confirmed that the polymerization conversion rate exceeded 97%, a polymerization stopper was added, and the internal temperature was cooled to 35 ℃ or lower.

After the pH was adjusted to 6 using an aqueous solution of sodium hydroxide, unreacted monomers and the like were removed by steam distillation to obtain a copolymer latex (c).

(4) Synthesis example 4

130 parts of pure water, 0.4 part of sodium alkyldiphenylether disulfonate, 1 part of polyoxyethylene lauryl ether (エマルゲン 109P, manufactured by Kao corporation), 0.001 part of ferrous sulfate, 0.08 part of isoascorbic acid, and 0.01 part of tetrasodium ethylenediaminetetraacetate were put into a polymerization reactor having pressure resistance and sufficiently stirred.

Next, each monomer in the first row among the monomer input rows shown in table 1 and tertiary dodecyl mercaptan were charged into the polymerization reactor.

In another method, each monomer in the second column among the monomer input columns shown in table 1 and tert-dodecyl mercaptan were mixed to prepare a monomer mixture.

Next, 0.06 part of cumene hydroperoxide was added to raise the internal temperature to 35 ℃ to confirm heat generation by the initiation of polymerization.

Thereafter, from initiation of polymerization to 300 minutes, after keeping the internal temperature at 35 ℃ and between 300 minutes and 360 minutes, the internal temperature was raised by 60 ℃. From 360 to 600 minutes, a mixture of 15 parts of pure water, 0.4 part of sodium alkyldiphenyloxide disulfonate, and 0.3 part of potassium persulfate was continuously added while maintaining the internal temperature at 60 ℃. After maintaining the internal temperature at 60 ℃ for 600 to 750 minutes, the internal temperature was raised to 70 ℃ for 750 to 990 minutes, and maintained at 70 ℃ to continue the polymerization.

After 990 minutes from initiation of the polymerization, it was confirmed that the polymerization conversion rate exceeded 97%, a polymerization stopper was added, and the in-tank temperature was cooled to 35 ℃ or lower.

After the pH was adjusted to about 7.5 using aqueous ammonia, unreacted monomers and the like were removed by steam distillation to obtain a copolymer latex (d).

(5) Synthesis example 5

Copolymer latex (e) was obtained in the same manner as in (synthesis example 4) except that the kinds and amounts of the respective monomers were changed as shown in table 1.

(6) Comparative Synthesis examples

Comparative Synthesis example 1

Copolymer latex (f) was obtained in the same manner as in (synthesis example 4) except that the kinds and amounts of the respective monomers were changed as shown in table 2.

Comparative Synthesis example 2

Copolymer latexes (g) were obtained in the same manner as in (synthesis example 3) except that the kinds and amounts of the respective monomers were changed as shown in table 2.

Comparative Synthesis example 3

Copolymer latex (h) was obtained in the same manner as in (synthesis example 1) except that the kinds and amounts of the respective monomers were changed as shown in table 2.

Comparative Synthesis example 4

Copolymer latex (i) was obtained in the same manner as in (synthesis example 2) except that the kinds and amounts of the respective monomers were changed as shown in table 2.

Comparative Synthesis example 5

Copolymer latex (j) was obtained in the same manner as in (synthesis example 3) except that the kinds and amounts of the monomers were changed as shown in table 2.

Comparative Synthesis example 6

Copolymer latex (k) was obtained in the same manner as in (synthesis example 4) except that the kinds and amounts of the respective monomers were changed as shown in table 2.

2. Measurement of toluene-insoluble content (gel content) of copolymer latex

Latex films were produced in an environment of 40 ℃ and 85% humidity using the copolymer latexes obtained in the respective synthetic examples and comparative examples. The prepared latex film weighed about 1g, and was put into 400ml of toluene to swell and dissolve for 48 hours. Thereafter, the residue was filtered through a 300-mesh wire gauze, and the toluene-insoluble matter trapped on the wire gauze was dried and weighed. Then, the percentage of the dry weight of the toluene-insoluble matter relative to the weight of the latex film was calculated. The results are shown in tables 1 and 2.

3. Production of electrode sheet

(1) Production of composition for electrode

The electrode compositions of examples and comparative examples were prepared by using natural graphite having an average particle size of 20 μm as a conductive carbonaceous material, adding 2 parts by weight of an aqueous solution of carboxymethylcellulose as a thickener in terms of solid content, 3 parts by weight of copolymer latex obtained in each synthetic example and each comparative synthetic example, and an appropriate amount of water to 100 parts by weight of natural graphite, and mixing them so that the solid content of the electrode composition became 40%.

(2) Production of electrode sheet

The electrode compositions 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 20 minutes, and then pressed at room temperature to obtain an electrode sheet having a coating thickness of 80 μm (on each surface).

4. Performance testing of electrode sheets

(1) Determination of the adhesion of electrode coatings

Six cutting lines each having a cutting depth of 2mm from the coating layer to the current collector were formed on the surface of each electrode sheet of each example and each comparative example, using a cutter, and a checkerboard pattern having 25 (5 × 5) square holes was formed. The adhesive tape was attached to the checkerboard, and immediately peeled off, and the degree of exfoliation of graphite was visually evaluated. The results are shown in tables 3 and 4.

Very good: no peeling.

O: peeling off 1-3 square eyes.

And (delta): peeling off 4-10 square eyes.

X: peeling more than 11 square eyes.

(2) Determination of the adhesion of electrode coatings

Two sheets of the electrode sheets of each example and each comparative example were stacked, pressed at 50 ℃ for 5 minutes by a bench press, and then peeled off by hand, and evaluated according to the following criteria. The results are shown in tables 3 and 4.

Very good: easy to peel off.

O: the film peeled off smoothly with little resistance.

And (delta): there is considerable resistance and the sound is produced when peeling.

X: the adhesive state makes peeling difficult.

(3) Determination of the resistance to fracture of the electrode coating

The electrode sheets of the examples and comparative examples were cut into a rectangular shape of 10cm × 5cm at 180. Folded in half to prepare a test piece of 5cm square. The test piece was pressed with a heat sealer under a pressure of 0.05MPa for 2 seconds. The folded portions of the inner and outer sides of the test piece after the removal were observed with a microscope. The evaluation was performed as follows based on the results. The results are shown in tables 3 and 4.

Very good: no fracture and crack.

O: although slight fracture cracking was observed on the surface of the electrode sheet, no exposure of the current collector was observed.

And (delta): fracture cracks were observed on the surface of the electrode sheet, and slight exposure of the current collector was observed.

X: fracture cracks were observed on the surface of the electrode sheet, and the current collector was exposed in most of the fracture cracks.

(4) Measurement of flexibility of electrode coating

The electrode sheets of the examples and comparative examples were cut into a rectangular shape of 8cm × 2cm, and the bending resistance of the electrode sheet was measured using a textile texture measuring instrument manufactured by toyoyo seiki corporation, with a crack width of 5 mm. The results are shown in tables 3 and 4.

Very good: the resistance was less than 60 g.

O: the resistance is more than 60g and less than 75 g.

And (delta): the resistance is more than 75g and less than 90 g.

X: the resistance is more than 90 g.

(Table 3)

TABLE 3

(Table 4)

TABLE 4

The present invention is also provided in the illustrative embodiments thereof. This is by way of example only and is not to be construed as limiting. Modifications of the present invention that are obvious to those skilled in the art are also included in the scope of the claims set forth above.

(Industrial Applicability)

The binder for a secondary battery electrode of the present invention is used for a secondary battery electrode in which an active material (a positive electrode active material and a negative electrode constituent material) is bonded to a current collector in an electrode of a secondary battery.

Claims (1)

1. A binder for a secondary battery electrode, characterized by comprising a binder containing:

12.0 to 39.5 weight percent of aliphatic conjugated diene monomer,

1.5-8.5 wt% of unsaturated carboxylic acid alkyl ester monomer,

0.1 to 10.0% by weight of an ethylenically unsaturated carboxylic acid monomer, and

a copolymer latex having an insoluble content of 60 to 99 wt% with respect to toluene, which is obtained by emulsion polymerization of a monomer composition containing 42.0 to 86.4 wt% of a monomer copolymerizable with these monomers.