JP2012238489A - Electrode binder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve such a problem that, in conventional electrode binders, adhesive properties and electrolyte-swellable properties of an electrode including the electrode binders to a collector are not always satisfactorily enough.SOLUTION: There are provided: an electrode binder obtained by polymerizing 20-69 pts. wt. of a (meth)acrlylic acid ester, 1-50 pts. wt. of acrylonitrile, and 30-50 pts. wt. of a vinyl monomer containing an acid group; an emulsion for an electrode containing the electrode binder and a dispersion medium; an electrode containing the electrode binder; and a lithium ion secondary battery including the electrode.

Description

  The present invention relates to an electrode binder, an electrode emulsion, an electrode, a lithium ion secondary battery, and the like.

Patent Document 1 describes polyvinylidene fluoride as a binder for lithium ion secondary battery electrodes.
Patent Document 2 describes a copolymer of butyl acrylate, methyl methacrylate, and methacrylic acid as a binder for a lithium ion secondary battery electrode.
Furthermore, Patent Document 3 describes a copolymer obtained by polymerizing (meth) acrylic acid ester, acrylonitrile and an acidic group-containing vinyl monomer as a binder for a lithium ion secondary battery electrode. The amount of the structural unit derived from each component of the coalescence is 50 mol% or more and 95% or less of the structural unit derived from (meth) acrylic ester, 3 mol% or more and 40 mol% or less of the structural unit derived from acrylonitrile, acidic It is described that the structural unit derived from the group-containing vinyl monomer is 1 mol% or more and 25 mol% or less.

JP 2009-193932 A Special table 2003-1000029 gazette JP-A-8-287915

  In the conventional electrode binder, the adhesion of the electrode containing the electrode binder to the current collector and the electrolyte swellability are not always satisfactory.

The present invention includes the inventions described in [1] to [7] below.
[1] A binder for electrodes obtained by polymerizing 20 to 69 parts by weight of (meth) acrylic acid ester, 1 to 50 parts by weight of acrylonitrile and 30 to 50 parts by weight of an acidic group-containing vinyl monomer.
[2] The binder for electrodes according to [1], wherein the (meth) acrylic acid ester is butyl (meth) acrylate.
[3] The binder for electrodes according to [1] or [2], wherein the acidic group-containing vinyl monomer is (meth) acrylic acid.
[4] Any one of [1] to [3] obtained by emulsion polymerization of 20 to 69 parts by weight of (meth) acrylic acid ester, 1 to 50 parts by weight of acrylonitrile and 30 to 50 parts by weight of an acidic group-containing vinyl monomer. Electrode binder.
[5] An electrode emulsion comprising the electrode binder according to any one of [1] to [4] and a dispersion medium.
[6] An electrode containing the electrode binder according to any one of [1] to [4].
[7] A lithium ion secondary battery having the electrode according to [6].

  According to the binder for electrodes of the present invention, it is possible to obtain an electrode having excellent adhesiveness to a current collector and suppressing electrolyte swellability.

[Binder for electrode]
The electrode binder of the present invention (hereinafter sometimes referred to as “binder”) is composed of (meth) acrylic acid ester 20 to 69 parts by weight, acrylonitrile 1 to 50 parts by weight, and acidic group-containing vinyl monomer 30 to 50 parts by weight. Obtained by polymerization.

((Meth) acrylic acid ester)
In this specification, “(meth) acrylic acid ester” is a general term for acrylic acid esters and methacrylic acid esters. Here, as the (meth) acrylic acid ester, for example, alkyl having 1 to 16 carbon atoms such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, hexyl acrylate, octyl acrylate, lauryl acrylate, etc. Acrylic acid alkyl ester having a group: Meta having an alkyl group having 1 to 16 carbon atoms such as ethyl methacrylate, propyl methacrylate, butyl methacrylate, hexyl methacrylate, octyl methacrylate, lauryl methacrylate, etc. An alkyl acrylate ester is mentioned, Preferably the (meth) acrylic acid ester which has a C1-C6 alkyl group is mentioned, More preferably, a butyl (meth) acrylate is mentioned.

The amount of (meth) acrylic acid ester used in the binder of the present invention is, for example, 20 to 69 parts by weight, preferably 30 to 69 parts by weight, more preferably 40 to 69 parts per 100 parts by weight of the total monomers. Parts by weight.
In the present specification, “total monomer amount” refers to the total amount of (meth) acrylic acid ester, acrylonitrile, acidic group-containing vinyl monomer and other monomers used to obtain the binder of the present invention.

(Acrylonitrile)
The amount of acrylonitrile used in the binder of the present invention is, for example, 1 to 50 parts by weight, preferably 10 to 50 parts by weight, and more preferably 30 to 50 parts by weight with respect to 100 parts by weight of the total monomers.

(Acid group-containing vinyl monomer)
Examples of the acidic group include -COOH.
Examples of the acidic group-containing vinyl monomer include acrylic acid, methacrylic acid, 2-acryloyloxyethyl succinate, 2-methacryloyloxyethyl succinate, carboxyethyl acrylate, carboxyethyl methacrylate, and preferably acrylic. An acid and methacrylic acid are mentioned, More preferably, acrylic acid is mentioned.
The usage-amount of the acidic group containing vinyl monomer in the binder of this invention is 30-50 weight part with respect to 100 weight part of total monomers, for example.

(Other monomers)
The binder of the present invention may be obtained by polymerizing (meth) acrylic acid ester, acrylonitrile, acidic group-containing vinyl monomer and other monomers. Other monomers are monomers different from (meth) acrylic acid esters, acrylonitrile and acidic group-containing vinyl monomers, such as dimethyl maleate, diethyl maleate, dibutyl maleate, dioctyl maleate, dilauryl maleate, etc. Maleic acid dialkyl ester having a C1-16 alkyl group; Dialkyl fumarate having a C1-16 alkyl group such as dimethyl fumarate, diethyl fumarate, dibutyl fumarate, dioctyl fumarate, dilauryl fumarate, etc. An itaconic acid dialkyl ester having an alkyl group having 1 to 16 carbon atoms such as diethyl itaconate, dibutyl itaconate, dihexyl itaconate, dioctyl itaconate, and dilauryl itaconate;
The usage-amount of the other monomer in the binder of this invention is 0-50 weight part with respect to 100 weight part of total monomers, for example, Preferably it is 0-40 weight part, More preferably, it is 0 weight part.

[Method for producing binder of the present invention]
The binder of the present invention is obtained by solution polymerization or emulsion polymerization of (meth) acrylic acid ester, acrylonitrile and an acidic group-containing vinyl monomer, and is preferably obtained by emulsion polymerization.

(Solution polymerization)
Solution polymerization is carried out by polymerizing (meth) acrylic acid ester, acrylonitrile, an acidic group-containing vinyl monomer, and if necessary, other monomers in the presence of a solvent. In that case, it is preferable to use a polymerization initiator. Their mixing order is not particularly limited. Polymerization is carried out while adding a mixture containing (meth) acrylic acid ester, acrylonitrile, acidic group-containing vinyl monomer, and other monomers if necessary, and a polymerization initiator in a solvent under polymerization temperature conditions. It is preferable.
The solvent is preferably a non-aqueous solvent. Nonaqueous solvents include ketone solvents such as methyl ethyl ketone and methyl isobutyl ketone; ester solvents such as ethyl acetate and butyl acetate; nitrogen-containing polar solvents such as N-methyl-2-pyrrolidone and N, N-dimethylacetamide. . A nitrogen-containing polar solvent is preferable, and N-methyl-2-pyrrolidone is more preferable. You may use these individually or in combination of 2 or more types.
As the polymerization initiator, examples of the polymerization initiator include ammonium persulfate, potassium persulfate, sodium persulfate, 2,2′-azobis (4-methoxy-2,4-dimethylvaleronitrile), 2,2′-. Azobis (2,4-dimethylvaleronitrile), 2,2′-azobis (2-methylpropionitrile), 2,2′-azobis (2-methylbutyronitrile), and the like, preferably potassium persulfate Is mentioned.
The polymerization temperature is preferably in the range of 30 to 120 ° C.
The obtained mixture is subjected to post-treatment such as filtration treatment as necessary to obtain a solution containing the electrode binder of the present invention. It is also possible to remove the solvent from the solution and take out the electrode binder of the present invention.

(Emulsion polymerization)
Emulsion polymerization is carried out by polymerizing (meth) acrylic acid ester, acrylonitrile, an acidic group-containing vinyl monomer, and, if necessary, other monomers in the presence of water. In that case, it is preferable to use an emulsifier and a polymerization initiator. Their mixing order is not particularly limited. In a mixture of emulsifier and water, (meth) acrylic acid ester, acrylonitrile, acidic group-containing vinyl monomer, and other monomers if necessary, and a polymerization initiator are mixed, and the resulting mixture is stirred. The temperature is preferably increased while polymerization.
Examples of the emulsifier include anionic surfactants such as sodium alkylbenzene sulfonate and alkyl sulfosuccinate; polyvinyl alcohol, polyoxyethylene alkyl phenyl ether, polyoxyethylene alkyl ether, polyoxyethylene which may have a sulfo group, and the like. Nonionic surfactants such as oxyethylene fatty acid esters, polyoxyethylene sorbitan fatty acid esters and polyoxyethylene-polyoxypropylene block copolymers; water-soluble polymers such as hydroxyethyl cellulose; and the like.
As the polymerization initiator, examples of the polymerization initiator include ammonium persulfate, potassium persulfate, sodium persulfate, 2,2′-azobis (4-methoxy-2,4-dimethylvaleronitrile), 2,2′-. Azobis (2,4-dimethylvaleronitrile), 2,2′-azobis (2-methylpropionitrile), 2,2′-azobis (2-methylbutyronitrile), and the like, preferably potassium persulfate Is mentioned.
The polymerization temperature is preferably in the range of 20 to 95 ° C.
The obtained mixture is subjected to post-treatment such as filtration treatment as necessary to obtain an emulsion containing the electrode binder of the present invention. The emulsion can be directly used as the electrode emulsion of the present invention described below, or water can be removed from the emulsion to extract the electrode binder of the present invention.

[Electrode emulsion]
The electrode emulsion of the present invention (hereinafter sometimes referred to as “emulsion”) contains the binder of the present invention and a dispersion medium. In the emulsion of the present invention, the binder of the present invention is dispersed in a dispersion medium.
The emulsion of the present invention preferably further contains one or more selected from the group consisting of an emulsifier and a polymerization initiator.
In addition to the emulsion obtained by the above emulsion polymerization, the emulsion of the present invention is an emulsion obtained by mixing the electrode binder of the present invention and a dispersion medium in the presence of an emulsifier or a polymerization initiator as necessary. Can be mentioned.

(Dispersion medium)
The dispersion medium may be any solvent that can emulsify the binder of the present invention. Such a dispersant is preferably water.
The solid content of the emulsion of the present invention is, for example, 5 to 50, preferably 5 to 35. In the present specification, the “solid content” is the amount (% by weight) of the component obtained by removing the dispersion medium from the emulsion of the present invention relative to the emulsion of the present invention.

(emulsifier)
Examples of the emulsifier include anionic surfactants such as sodium alkylbenzene sulfonate and alkyl sulfosuccinate; polyvinyl alcohol, polyoxyethylene alkyl phenyl ether, polyoxyethylene alkyl ether, polyoxyethylene which may have a sulfo group, and the like. Nonionic surfactants such as oxyethylene fatty acid esters, polyoxyethylene sorbitan fatty acid esters and polyoxyethylene-polyoxypropylene block copolymers; water-soluble polymers such as hydroxyethyl cellulose;
The content of the emulsifier in the emulsion of the present invention is, for example, 0.5 to 30% by weight, preferably 1 to 15% by weight.

(Polymerization initiator)
Examples of the polymerization initiator include ammonium persulfate, potassium persulfate, sodium persulfate, 2,2′-azobis (4-methoxy-2,4-dimethylvaleronitrile), 2,2′-azobis (2,4- Dimethylvaleronitrile), 2,2′-azobis (2-methylpropionitrile), 2,2′-azobis (2-methylbutyronitrile), and the like, preferably potassium persulfate.
The content of the polymerization initiator in the emulsion of the present invention is, for example, 0.01 to 1% by weight, preferably 0.05 to 0.5% by weight.

[electrode]
The electrode of the present invention (hereinafter sometimes referred to as “electrode”) contains the binder of the present invention. The electrode can be used for positive electrodes and negative electrodes of, for example, dry batteries, piezoelectric element sensors, electric double layer capacitors, lithium ion capacitors, lithium ion secondary batteries, sodium ion secondary batteries, and fuel cells.

[Electrode for lithium ion secondary battery]
Examples of the electrode for a lithium ion secondary battery of the present invention include a positive electrode for a lithium ion secondary battery and a negative electrode for a lithium ion secondary battery.
The electrode for a lithium ion secondary battery of the present invention is obtained by applying or dipping the composition for a lithium ion secondary battery electrode of the present invention (hereinafter sometimes referred to as “composition”) onto a current collector by a doctor blade method or the like. And a method of drying, pressing and heat-treating a sheet obtained by kneading, molding and drying the composition of the present invention to the surface of the current collector via a conductive adhesive, etc. After a mixture comprising a binder and an electrode active material to be described later is molded on a current collector, the obtained sheet-like molded product can be obtained by a method of stretching in a uniaxial or multiaxial direction.
When making an electrode into a sheet form, the thickness is about 5-1000 micrometers normally.

[Composition for lithium ion secondary battery electrode]
The composition of the present invention contains the binder and electrode active material of the present invention, and a dispersion medium and / or solvent.
The composition of the present invention preferably further contains a conductive material.

(Electrode active material)
As the electrode active material, an active material capable of occluding and releasing lithium ions is preferably used. The electrode active material includes a positive electrode active material and a negative electrode active material.

Examples of the positive electrode active material include metal composite oxides, particularly metal composite oxides containing at least one metal selected from lithium and iron, cobalt, nickel, and manganese. Preferably, Li _x MO ₂ (however, M represents one or more transition metals, preferably at least one of Co, Mn or Ni, and 1.10>x> 0.05, or Li _x M ₂ O ₄ (wherein M is 1 An active material containing a transition metal of more than one species, preferably Mn, and 1.10>x> 0.05. For example, LiCoO ₂ , LiNiO ₂ , Li _x Ni _y Co _{(1-y )} O ₂ (wherein 1.10>x> 0.05 and 1>y> 0), complex oxides represented by LiMn ₂ O ₄ and the like.

Examples of the negative electrode active material include various silicon oxides (SiO _{2 and the} like), carbonaceous materials, metal composite oxides, etc., preferably amorphous carbon, graphite, natural graphite, MCMB, pitch-based carbon fiber, polyacene, and the like. A _x M _y O _Z (wherein A is Li, M is at least one selected from Co, Ni, Al, Sn and Mn, O represents an oxygen atom, x, y and z are The numbers are in the range of 1.10 ≧ x ≧ 0.05, 4.00 ≧ y ≧ 0.85, and 5.00 ≧ z ≧ 1.5.) An oxide etc. are mentioned.

(Dispersion medium)
Examples of the dispersion medium include the same ones as described above.
The amount of the dispersion medium in the composition of the present invention is not particularly limited, and may be an amount that can provide a property that allows easy application to the current collector. It is blended so as to be preferably 1 to 1000 times, more preferably 2 to 500 times, still more preferably 3 to 300 times, still more preferably 5 to 200 times based on the weight of the binder of the present invention.

(solvent)
Examples of the solvent include a solvent that can dissolve the binder of the present invention.
Solvents include propyl ether, isopropyl ether, butyl ether, isobutyl ether, n-amyl ether, isoamyl ether, methyl butyl ether, methyl isobutyl ether, methyl n-amyl ether, methyl isoamyl ether, ethyl propyl ether, ethyl isopropyl ether, ethyl butyl ether Saturated aliphatic ether compounds such as ethyl isobutyl ether, ethyl n-amyl ether and ethyl isoamyl ether; unsaturated aliphatic ether compounds such as allyl ether and ethyl allyl ether; aromatics such as anisole, phenetole, phenyl ether and benzyl ether Ether compounds; cyclic ether compounds such as tetrahydrofuran, tetrahydropyran, dioxane, etc .; ethylene glycol monomethyl Ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, and other ethylene glycol ether compounds; formic acid, acetic acid, acetic anhydride, acrylic acid, citric acid, propionic acid, butyric acid, etc. Monocarboxylic acid compounds: butyl formate, amyl formate, propyl acetate, isopropyl acetate, butyl acetate, butyl acetate, amyl acetate, isoamyl acetate, 2-ethylhexyl acetate, cyclohexyl acetate, butyl cyclohexyl acetate, ethyl propionate, propionic acid Butyl, amyl propionate, butyl butyrate, diethyl carbonate, diethyl oxalate, methyl lactate, ethyl lactate, butyl lactate, triethyl phosphate Organic acid ester compounds such as acetone; ketone compounds such as acetone, ethyl ketone, propyl ketone, butyl ketone, methyl isopropyl ketone, methyl isobutyl ketone, diisobutyl ketone, acetylacetone, diacetone alcohol, cyclohexanone, cyclopentanone, methylcyclohexanone, cycloheptanone; Dicarboxylic acid compounds such as succinic acid, glutaric acid, adipic acid, undecanedioic acid, pyruvic acid, citraconic acid; 1,4-dioxane, furfural, N-methylpyrrolidone and the like can be mentioned. Of these, N-methylpyrrolidone is preferred.
The amount of the solvent in the composition of the present invention is not particularly limited, and may be an amount that can provide a property that allows easy application to the current collector. It is blended so as to be preferably 1 to 1000 times, more preferably 2 to 500 times, still more preferably 3 to 300 times, still more preferably 5 to 200 times based on the weight of the binder of the present invention.

  The ratio of the content of the electrode active material and the binder of the present invention in the composition of the present invention is, for example, 100: 0.1 to 100: 10, preferably 100: 0.5 to 100: 5, by weight. More preferably, it is 100: 1 to 100: 3.

(Conductive material)
Examples of the conductive material include conductive carbon such as graphite, carbon black, acetylene black, ketjen black and activated carbon; graphite-based conductive material such as natural graphite, thermally expanded graphite, scale-like graphite, and expanded graphite; vapor-grown carbon fiber Carbon fiber such as aluminum, nickel, copper, silver, gold, platinum, etc .; conductive metal oxide such as ruthenium oxide or titanium oxide; conductivity such as polyaniline, polypyrrole, polythiophene, polyacetylene, polyacene Examples include polymers.
Carbon black, acetylene black and ketjen black are preferred in that the conductivity is effectively improved with a small amount.
The content of the conductive material is, for example, about 0 to 50 parts by weight, and preferably about 0 to 30 parts by weight with respect to 100 parts by weight of the electrode active material.

[Current collector]
Examples of the current collector material include metals such as nickel, aluminum, titanium, copper, gold, silver, platinum, aluminum alloys, and stainless steel, such as carbon materials or activated carbon fibers, nickel, aluminum, zinc, copper, and tin. Formed by plasma spraying or arc spraying of lead or an alloy thereof, for example, a conductive film in which a conductive material is dispersed in a resin such as rubber or styrene-ethylene-butylene-styrene copolymer (SEBS) Is mentioned.
Examples of the shape of the current collector include a foil, a flat plate, a mesh, a net, a lath, a punching or an emboss, or a combination thereof (for example, a mesh flat). .
Concavities and convexities may be formed on the surface of the current collector by etching.

[Lithium ion secondary battery]
The lithium ion secondary battery having the lithium ion secondary battery electrode of the present invention will be described below. The lithium ion secondary battery includes, for example, a positive electrode, a negative electrode, an electrolytic solution, a separator, and the like, and lithium is oxidized and reduced at both the positive electrode and the negative electrode to store and release electric energy.

(Electrolyte)
Examples of the electrolytic solution used in the lithium ion battery of the present invention include a non-aqueous electrolytic solution in which a lithium salt is dissolved in an organic solvent. Lithium salts include LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , Li ₂ B ₁₀ Cl ₁₀ , One or a mixture of two or more of lower aliphatic carboxylic acid lithium salts, LiAlCl _{4 and the} like can be mentioned.
The lithium salt is selected from the group consisting of LiPF ₆ containing fluorine, LiAsF ₆ , LiSbF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , and LiC (CF ₃ SO ₂ ) ₃ among these. It is preferable to use one containing at least one selected from the above.

  Examples of the organic solvent used in the electrolytic solution of the present invention include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, 1,2-di Carbonates such as (methoxycarbonyloxy) ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, tetrahydrofuran, 2- Ethers such as methyltetrahydrofuran; Esters such as methyl formate, methyl acetate and γ-butyrolactone; Nitriles such as acetonitrile and butyronitrile; N, N-dimethylformamide, N, N-dimethylacetate Amides such as amide; Carbamates such as 3-methyl-2-oxazolidone; Sulfur-containing compounds such as sulfolane, dimethyl sulfoxide and 1,3-propane sultone, or those obtained by introducing a fluorine substituent into the above organic solvent Usually, a mixture of two or more of these is used.

(Separator)
The separator plays a role of separating the positive electrode and the negative electrode and holding the electrolytic solution, and an insulating film having high ion permeability and mechanical strength is used.
Examples of the separator include, for example, paper making such as viscose rayon or natural cellulose, mixed paper obtained by making fibers such as cellulose and polyester, electrolytic paper, kraft paper, manila paper, polyethylene non-woven fabric, polypropylene non-woven fabric, polyester non-woven fabric, glass Fiber, porous polyethylene, porous polypropylene, porous polyester, aramid fiber, polybutylene terephthalate nonwoven fabric, para-type wholly aromatic polyamide, vinylidene fluoride, tetrafluoroethylene, copolymer of vinylidene fluoride and propylene hexafluoride , Non-woven fabrics such as fluorine-containing resins such as fluoro rubber, or porous membranes.
The separator may be a molded product made of ceramic powder particles such as silica and the binder. The molded article is usually formed integrally with the working electrode and the counter electrode. Moreover, about the separator using polyethylene, a polypropylene, etc., in order to improve hydrophilicity, you may mix surfactant and a silica particle. Furthermore, the separator may contain an organic solvent such as acetone and a plasticizer such as dibutyl phthalate (DBP).

As the separator, a proton conductive polymer may be used.
As separators, in particular, electrolytic paper, viscose rayon or natural cellulose paper, kraft paper, manila paper, mixed paper obtained by making cellulose or polyester fibers, polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyester nonwoven fabric, manila hemp sheet, glass A fiber sheet or the like is preferable.
The pore diameter of the separator is usually about 0.01 to 10 μm. The thickness of the separator is usually about 1 to 300 μm, preferably about 5 to 30 μm.
The separator may be a laminate of separators having different porosity. In particular, a separator composed of a polyolefin porous membrane and a polyester resin porous membrane is suitable.

Hereinafter, the present invention will be described in more detail with reference to examples. Parts and% in the examples mean weight basis unless otherwise specified.
The solid content was measured by a measuring method according to JIS K-6828.

Example 1 (Emulsion production example)
In a glass polymerization vessel equipped with a reflux condenser, a thermometer, a stirring blade, and a nitrogen blowing port, 230 parts of ion exchange water and 5 parts of an emulsifier “AQUALON (registered trademark) HS-10” (Daiichi Kogyo Seiyaku Co., Ltd.) And charged. While stirring the obtained solution, a monomer mixed solution composed of 68 parts of butyl acrylate, 2 parts of acrylonitrile and 30 parts of acrylic acid was charged therein. After the monomer was charged, temperature increase and nitrogen substitution were started. Furthermore, 0.2 part of potassium persulfate was added, the temperature was raised to 75 ° C., and polymerization was carried out for 5 hours. After completion of the reaction, the mixture was cooled and filtered through a polyester mesh corresponding to 150 μm to obtain an emulsion.

Example 2 (Example of electrode production)
92 parts of lithium manganate having an average particle size of 12 μm as an electrode active material (Horai Co., Ltd., HLB: 0711216), 5 parts of carbon black (Denka Black HS-100, manufactured by Denki Kagaku Kogyo Co., Ltd.) and thickening As an agent, 1 part of carboxymethylcellulose was mixed, and 42 parts of water was further added and mixed. To the resulting mixture, 2 parts of the emulsion obtained in Example 1 (butyl acrylate: acrylonitrile: acrylic acid = 68: 2: 30) was added and further mixed. The obtained mixture was applied onto an aluminum current collector having a thickness of 20 μm by the doctor blade method and dried at 80 ° C. for 2 hours. Subsequently, it cut | disconnected in the strip shape of width 3cm, and obtained the electrode for peeling strength measurement.

(Peel strength evaluation)
The electrode side of the electrode cut into strips and the glass epoxy resin plate are attached with a double-sided adhesive tape, fixed to a peel strength measuring instrument, and the thickness is measured using a small desktop testing machine (Shimadzu, EZ Test EZ-L) Peeled in the direction. The value obtained by dividing the magnitude of the force by the width (2.5 cm) of the double-sided adhesive tape was taken as the peel strength (N / m).
A sample having a peel strength of 10 N / m or more was evaluated as ◯, and a sample having a peel strength of less than 10 N / m was evaluated as ×.

(Swellability evaluation)
As a swelling evaluation, a binder (a binder formed by removing water when using an emulsion) was dissolved in N-methyl-2-pyrrolidone, applied to an aluminum cup, and vacuum-dried at 120 ° C. for 8 hours. The obtained film-like binder was cut into 2 cm square (50 mg to 500 mg), and the weight W ¹ of the sample was measured. This sample was immersed in a mixed solution of ethylene carbonate: dimethyl carbonate: ethyl methyl carbonate (= 30: 35: 35). After 24 hours, a sample was taken out, the weight W ² was measured, and the degree of swelling (%) was calculated from the following formula.
Swelling degree (%) = 100 × (W ² −W ¹ ) / W ¹
Wherein, ^{W 1} represents the weight before immersion.
W ² represents the weight after 24 hours was immersed. ]
A sample having a degree of swelling of less than 70% was evaluated as ◯, and a sample having a degree of swelling of 70% or more as ×.

Comparative Example 1
(Example of electrode production)
An electrode was produced in the same manner as in Example 2 except that polytetrafluoroethylene (Mitsui / DuPont Fluorochemical Co., Ltd., 31-JR) was used as the electrode binder. Peel strength evaluation and swelling evaluation were performed using electrodes.

Comparative Example 2
(Emulsion production example)
An emulsion was obtained in the same manner as in Example 1 except that 69 parts of butyl acrylate, 23 parts of methyl methacrylate and 8 parts of acrylic acid were used as the monomer mixture.
(Example of electrode production)
An electrode was produced in the same manner as in Example 2 except that butyl acrylate: methyl methacrylate: methacrylic acid (= 69: 23: 8) was used as the electrode binder. Peel strength evaluation and swelling evaluation were performed using electrodes.

Comparative Example 3
(Emulsion production example)
An emulsion was obtained in the same manner as in Example 1 except that 68 parts of butyl acrylate, 24 parts of acrylonitrile and 8 parts of acrylic acid were used as the monomer mixture.
(Example of electrode production)
An electrode was produced in the same manner as in Example 2 except that butyl acrylate: acrylonitrile: methacrylic acid (= 68: 24: 8) was used as the electrode binder. Peel strength evaluation and swelling evaluation were performed using electrodes.

  According to the binder for electrodes of the present invention, it is possible to obtain an electrode having excellent adhesiveness to a current collector and suppressing electrolyte swellability.

Claims (7)

  A binder for electrodes obtained by polymerizing 20 to 69 parts by weight of (meth) acrylic acid ester, 1 to 50 parts by weight of acrylonitrile and 30 to 50 parts by weight of an acidic group-containing vinyl monomer.   The binder for electrodes according to claim 1, wherein the (meth) acrylic acid ester is butyl (meth) acrylate.   The binder for electrodes according to claim 1 or 2, wherein the acidic group-containing vinyl monomer is (meth) acrylic acid.   The binder for electrodes according to any one of claims 1 to 3, obtained by emulsion polymerization of 20 to 69 parts by weight of (meth) acrylic acid ester, 1 to 50 parts by weight of acrylonitrile and 30 to 50 parts by weight of an acidic group-containing vinyl monomer.   The electrode emulsion containing the binder for electrodes in any one of Claims 1-4, and a dispersion medium.   The electrode containing the binder for electrodes in any one of Claims 1-4.   A lithium ion secondary battery comprising the electrode according to claim 6.