TW201721945A - Binder for negative electrodes of lithium ion secondary batteries, slurry composition for negative electrodes, negative electrode, and lithium ion secondary battery - Google Patents

Abstract

The objective of the present invention is to provide a binder for negative electrodes, which serves as a material for lithium ion secondary batteries having excellent rate characteristics. The present invention is a binder for negative electrodes of lithium ion secondary batteries, which contains a dispersion liquid that is obtained by dispersing core-corona polymer fine particles in an aqueous dispersion medium, and wherein the core-corona polymer fine particles have a structure wherein a hydrophilic corona part which is composed of a structural unit derived from a carboxyl group-containing hydrophilic macromonomer covers the periphery of a hydrophobic core part which is composed of a structural unit derived from a hydrophobic monomer.

Description

Adhesive for negative electrode of lithium ion secondary battery, slurry composition for negative electrode and negative electrode, and lithium ion secondary battery

The present invention relates to a binder for a negative electrode of a lithium ion secondary battery, a slurry composition for a negative electrode, a negative electrode, and a lithium ion secondary battery.

Lithium-ion secondary batteries are used as power sources for electronic devices such as mobile phones and notebook computers because of their high weight and high energy density and high durability against repeated charge and discharge. Moreover, it is also used as an electric vehicle that can discharge and charge, and is used in an electric vehicle such as an electric vehicle.

A lithium ion secondary battery generally has a structure in which a positive electrode and a negative electrode are connected to each other via an electrolyte layer, and a positive electrode active material layer containing a positive electrode active material is formed on both sides of a positive electrode current collector, and the negative electrode is a negative electrode active material. The negative electrode active material layer is formed on both surfaces of the negative electrode current collector.

The negative electrode of the lithium ion secondary battery is formed by applying a mixed slurry of a graphite powder which is a negative electrode active material and a binder for a negative electrode to a surface of a current collector and drying it. The function of the binder for the negative electrode is to bond the graphite powders belonging to the negative electrode active material to each other and to bond the metal foil such as copper foil of the current collector to the graphite powder. Since the binder for a negative electrode having such a function functions as an internal resistance at the negative electrode, it is necessary to find a material which exhibits a high adhesion force in a small amount. Further, since the type of the binder affects the battery output characteristics (rate characteristics), it is necessary to find a binder for a negative electrode having a good rate characteristic.

As the resin which is a main component of the binder for a negative electrode, polyvinylidene fluoride (PVDF) is conventionally used similarly to the positive electrode. Since PVDF is insufficient in adhesion, styrene-butadiene latex (SBR) or the like has recently been used (for example, Patent Documents 1 and 2). However, the binder for the negative electrode containing SBR has insufficient rate characteristics.

Since the requirements for the rate-of-rate characteristics of the lithium ion secondary battery are increasing, it is desired to use a binder for a negative electrode having excellent rate characteristics. Advanced technical literature

[Patent Document 1] Japanese Laid-Open Patent Publication No. Hei 04-074459 (Patent Document 2)

In the circumstances, the main object of the present invention is to provide a binder for a negative electrode, which can be used as a material for a lithium ion secondary battery having excellent rate characteristics. Means to solve the problem

In order to develop a binder for a negative electrode which can be used as a material for a lithium ion secondary battery having excellent rate characteristics, the present invention has been specifically studied to find a dispersion containing a core-corona type polymer fine particle. a binder, wherein the core-halo type polymer microparticle has a structure surrounding a hydrophobic core portion with a hydrophilic halo portion, and the hydrophilic halo portion is formed by a constituent unit derived from a carboxyl group-containing hydrophilic macromonomer. The hydrophobic core portion is formed of a constituent unit derived from a hydrophobic monomer, and it has been found that the above problem can be solved by using such a binder. The present invention is based on this finding, and the results of the study are further repeated and completed.

In other words, the present invention relates to a lithium ion secondary battery negative electrode binder, a lithium ion secondary battery negative electrode slurry composition, a lithium ion secondary battery negative electrode, and a lithium ion secondary battery. . Item 1. A binder for a negative electrode for a lithium ion secondary battery, comprising a dispersion in which a core-half type polymer fine particle has been dispersed in an aqueous dispersion medium, wherein the core-halo type polymer particle has a hydrophilic halo Surrounding a structure surrounding the hydrophobic core portion, the hydrophilic halo portion is formed by a constituent unit derived from a carboxyl group-containing hydrophilic macromonomer, which is formed of a constituent unit derived from a hydrophobic monomer . The binder according to Item 1, wherein the core-half type polymer fine particles are polymer fine particles obtained by radically polymerizing the carboxyl group-containing hydrophilic macromonomer and the hydrophobic monomer, and the above-mentioned The carboxyl hydrophilic macromonomer is a macromonomer obtained by reacting a carboxyl group-containing polymer compound with a compound having a polymerizable reactive group in the molecule and a functional group capable of reacting with a carboxyl group to form a covalent bond. The bonding agent according to Item 2, wherein the compound having a polymerizable reactive group in the molecule and a functional group reactive with a carboxyl group to form a covalent bond is selected from the following formula (1) to (3) at least one compound of the group consisting of the compounds shown:

a compound of the formula (1): [Chemical Formula 1]

[wherein, R ₁ represents a hydrogen atom or a methyl group, and Q ₁ represents an oxygen atom or -NH-;] a compound represented by the formula (2):

[Chemical Formula 2]

[wherein, R ₂ represents a hydrogen atom or a methyl group, Q ₂ represents an oxygen atom or -NH-, and n represents an integer of 1 to 4;] and a compound represented by the formula (3):

[Chemical Formula 3]

[wherein R ₃ represents a hydrogen atom or a methyl group, R ₄ represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms or a halogen atom, and X ₁ represents a halogen atom]. Item 4. A slurry composition for a negative electrode of a lithium ion secondary battery according to any one of the above items 1 to 3, and an active material. Item 5. A lithium ion secondary battery negative electrode obtained by applying the slurry composition for a lithium ion secondary battery negative electrode according to the above item 4 to a current collector and drying it. Item 6. A lithium ion secondary battery comprising the lithium ion secondary battery negative electrode according to Item 5 above. Effect of the invention

The binder for a negative electrode of a lithium ion secondary battery of the present invention is a point of the surface of the negative electrode active material and the surface of the current collector by a core-half type polymer compound containing a fine particle shape, so that the negative electrode active materials and the negative electrode can be made. The bonding between the active material and the current collector does not hinder the contact between the negative electrode active material, the contact between the negative electrode active material and the current collector, and the negative electrode activity can be improved by the hydrophilic halo present on the surface of the polymer fine particles. The adhesion between the substance and the negative electrode active material and the current collector can reduce the electrode resistance. In addition, since the hydrophobic core portion can follow the volume change caused by the swelling and contraction of the negative electrode active material during charge and discharge, it is possible to prevent the contact portion between the negative electrode active material and the negative electrode active material and the current collector from disappearing and being rendered unconductive. Part (electrical insulation of the negative active material). Therefore, by using such a binder for a negative electrode of a lithium ion secondary battery, a lithium ion secondary battery excellent in rate characteristics can be obtained.

Hereinafter, the lithium ion secondary battery negative electrode binder, the lithium ion secondary battery negative electrode slurry composition, the lithium ion secondary battery negative electrode, and the lithium ion secondary battery will be specifically described.

1. A binder for a negative electrode of a lithium ion secondary battery of the present invention, a binder for a negative electrode of a lithium ion secondary battery (hereinafter also referred to as a "gluing agent"), a dispersion containing a core-halo polymer fine particle dispersed in an aqueous dispersion medium .

In the binder of the present invention, the core-half type polymer compound dispersed in the dispersion is in the form of fine particles, and the core feature is that the core-halo type polymer particles have a structure in which a hydrophilic halo surrounds the hydrophobic core portion, and the hydrophilicity The halo portion is formed of a constituent unit derived from a carboxyl group-containing hydrophilic macromonomer, and the hydrophobic core portion is formed of a constituent unit derived from a hydrophobic monomer.

When the core-half type polymer compound has a fine particle shape, the surface of the negative electrode active material and the surface of the current collector are in point, and the contact between the negative electrode active material and the contact between the negative electrode active material and the current collector are not hindered. Since the negative electrode active materials and the negative electrode active material and the current collector are bonded to each other, the electrode resistance can be reduced. This is because the hydrophilic halo present on the surface of the polymer microparticles has a plurality of carboxyl groups, and the carboxyl group has a good affinity with the negative electrode active material. Therefore, the binder of the present invention can be uniformly dispersed in the negative electrode active material layer to improve the active material between the negative electrode active materials. And the adhesion of the negative electrode active material to the current collector. Further, since the hydrophobic core portion is formed of a hydrophobic monomer which can constitute a polymer compound having a low glass transition temperature (Tg), it is possible to follow a volume change caused by swelling and contraction of the negative electrode active material during charge and discharge. It is possible to prevent the contact portion between the negative electrode active material and the negative electrode active material and the current collector from disappearing and to cause a portion that cannot be electrically connected (electrical insulation of the negative electrode active material). By forming the electrode by using the binder, the negative electrode active material peeling due to the charge and discharge cycle can be reduced. Therefore, by using the lithium ion secondary battery negative electrode binder of the present invention, lithium ion having excellent rate characteristics can be obtained. Secondary battery.

The core-half type polymer fine particles are polymer fine particles obtained by radically polymerizing a carboxyl group-containing hydrophilic macromonomer and a hydrophobic monomer.

Hereinafter, the carboxyl group-containing hydrophilic macromonomer will be described.

The carboxyl group-containing hydrophilic macromonomer is obtained by reacting a carboxyl group-containing polymer compound with a compound having a polymerizable reactive group in the molecule and a functional group capable of reacting with a carboxyl group to form a covalent bond.

The carboxyl group-containing polymer compound is not particularly limited as long as it contains a carboxyl group in a repeating unit of the polymer compound, and examples thereof include a polymer containing a carboxyl group and a carboxyl group-containing polysaccharide.

The carboxyl group-containing monomer may, for example, be a monomer containing a carbon-carbon unsaturated double bond and a carboxyl group and/or a salt thereof as essential components. Examples of such a monomer include unsaturated monocarboxylic acids such as (meth)acrylic acid, crotonic acid, α-hydroxyacrylic acid, and α-hydroxymethyl acrylate, and the like; and itaconic acid, fumaric acid, and maleic acid; And an unsaturated dicarboxylic acid such as 2-methylene glutaric acid, and the like. These monomers may be used alone or in combination of two or more. In the present specification, "(meth)acrylic acid" means acrylic acid and/or methacrylic acid.

The carboxyl group-containing polysaccharide may, for example, be a carboxyalkyl cellulose such as carboxymethyl cellulose or carboxyethyl cellulose; carboxymethyl starch, carboxymethyl amylose, hyaluronic acid, alginic acid, pectin, and the like. Salt and so on. These polysaccharides may be used alone or in combination of two or more.

The salt may, for example, be a metal salt, an ammonium salt or an organic amine salt. The metal salt may, for example, be an alkali metal salt such as a lithium salt, a sodium salt or a potassium salt; an alkaline earth metal salt such as a magnesium salt or a calcium salt; an aluminum salt or an iron salt. The organic amine salt may, for example, be an alkanolamine salt such as a monoethanolamine salt, a diethanolamine salt or a triethanolamine salt; an alkylamine salt such as a monoethylamine salt, a diethylamine salt or a triethylamine salt; or an ethylenediamine salt. Polyamines such as triethylenediamine. These salts may be used alone or in combination of two or more.

In the raw material monomer of the polymer of the carboxyl group-containing monomer, in addition to the carboxyl group-containing monomer, a nonionic monomer copolymerizable with the monomer may be further combined. Examples of such a nonionic monomer include acrylamide, methacrylamide, n-isopropylacrylamide, methyl acrylate, methyl methacrylate, and the like.

A homopolymer or copolymer of a carboxyl group-containing monomer or a copolymer in which a carboxyl group-containing monomer and a nonionic monomer are combined can be produced by a conventional polymerization method. The above-mentioned carboxyl group-containing polymer compound may be used singly or in combination of two or more. The above-mentioned carboxyl group-containing polymer compound is preferably polyacrylic acid or a salt thereof, and sodium polyacrylate is more preferable.

The ratio of the carboxyl group-containing constituent unit in the carboxyl group-containing polymer compound is preferably 60 mol% or more of the total constituent unit of the polymer compound, more preferably 80 mol% or more, and the entire constituent unit is preferably a carboxyl group-containing unit. .

The molecular weight of the carboxyl group-containing polymer compound is not particularly limited as long as it is a number average molecular weight obtained by a known production method, and is preferably about 500 to 500,000.

The compound having a polymerizable reactive group in the molecule and a functional group capable of reacting with a carboxyl group to form a covalent bond (hereinafter also referred to as "functional group-containing compound") may, for example, be a compound represented by the following formula (1) ( The following is also called "compound (1)"):

[Chemical Formula 4]

[wherein R ₁ represents a hydrogen atom or a methyl group. Q ₁ represents an oxygen atom or -NH-. And a compound represented by the following formula (2) (hereinafter also referred to as "compound (2)"):

[Chemical Formula 5]

[wherein R ₂ represents a hydrogen atom or a methyl group. Q ₂ represents an oxygen atom or -NH-. n represents an integer from 1 to 4. And a compound represented by the following formula (3) (hereinafter also referred to as "compound (3)"):

[Chemical Formula 6]

[wherein R ₃ represents a hydrogen atom or a methyl group. R ₄ represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms or a halogen atom. X ₁ represents a halogen atom] and the like.

As the functional group-containing compound, at least one compound selected from the group consisting of the compound (1), the compound (2), and the compound (3) can be used.

In the above formula (1), R ₁ represents a hydrogen atom or a methyl group, and Q ₁ represents an oxygen atom or -NH-.

The compound (1) may, for example, be a propylene-containing vinyl monomer such as glycidyl acrylate, glycidyl methacrylate, epoxy propyl decylamine or propylene methacrylamide. . These compounds may be used alone or in combination of two or more.

In the above formula (2), R ₂ represents a hydrogen atom or a methyl group. Q ₂ represents an oxygen atom or -NH-. n represents an integer from 1 to 4, preferably 2.

The compound (2) may, for example, contain isocyanate groups such as ethyl isocyanate, ethyl isocyanate, ethyl acetoacetate or ethyl methacrylate. Vinyl monomer. These compounds may be used alone or in combination of two or more.

In the above formula (3), R ₃ represents a hydrogen atom or a methyl group. R ₄ represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms or a halogen atom. X ₁ represents a halogen atom.

The alkyl group having 1 to 4 carbon atoms represented by R ₄ may, for example, be a methyl group, an ethyl group, a n-propyl group, an isopropyl group or a n-butyl group, and the halogen atom may be a fluorine atom, a chlorine atom, a bromine atom or an iodine atom. .

The halogen atom represented by X ₁ may, for example, be a fluorine atom, a chlorine atom, a bromine atom or an iodine atom.

In the above formula (3), R ₃ is preferably a hydrogen atom, R ₄ is preferably a hydrogen atom, and X ₁ is preferably a chlorine atom or a bromine atom.

The compound (3) may, for example, be chloromethylstyrene, bromomethylstyrene or iodomethylstyrene. These compounds may be used alone or in combination of two or more.

The synthesis of the carboxyl group-containing hydrophilic macromonomer formed by reacting the carboxyl group-containing polymer compound with the functional group-containing compound can be usually carried out in an aqueous medium at a temperature of 10 to 80 ° C, preferably 20 to 60 ° C. The reaction is carried out by reacting the carboxyl group-containing polymer compound with the functional group-containing compound.

As the aqueous medium, water; alcohol such as methanol, ethanol or propanol; ketone such as acetone or methyl ethyl ketone; dimethylformamide or the like can be used. As the aqueous medium, water alone, a mixed solvent of an alcohol and water, a mixed solvent of a ketone and water, or the like may be used, and water alone is preferably used.

The ratio of the carboxyl group-containing polymer compound to the functional group-containing compound is not particularly limited, and the functional group-containing compound is preferably used in an amount of from 40 to 1 mol per 100 parts of the carboxyl group-containing constituent unit of the polymer compound. . The preferred range of the above functional group-containing compound is from 35 to 1 mol, and particularly preferably from 10 to 1 mol.

Here, in the case where the functional group-containing compound is any one of the compound (1), the compound (2) or the compound (3), the ratio of the functional group-containing compound is a ratio of the compound alone. When the functional group-containing compound contains two or more kinds of the compound (1), the compound (2) or the compound (3), the ratio of the total compound contained is in the above range.

The reaction of the carboxyl group-containing polymer compound with the compound (1) is a nucleophilic reaction of a carboxyl group to a glycidyl group. The reaction of the carboxyl group-containing polymer compound with the compound (2) is a nucleophilic reaction of a carboxyl group to an isocyanate group. The reaction of the carboxyl group-containing polymer compound with the compound (3) is a nucleophilic reaction of a carboxyl group to a benzyl group.

The reaction time varies depending on the reaction temperature and the like, and is usually about 30 minutes to 10 hours.

In the synthesis of a carboxyl group-containing hydrophilic macromonomer, polyacrylic acid or polyacrylic acid salt is preferably used as the carboxyl group-containing polymer compound, and is selected from the group consisting of the above compound (1), compound (2) and compound (3). It is preferred to use the compound (1) for at least one compound of the group. In the compound (1), it is preferred to use glycidyl methacrylate.

Therefore, in the above-mentioned carboxyl group-containing hydrophilic macromonomer, polyacrylic acid or polyacrylic acid salt which combines the above-mentioned carboxyl group-containing polymer compound, and methacrylic acid epoxy which is the aforementioned compound (1) as the functional group-containing compound The macromonomer prepared by propyl ester is preferred. For example, when the carboxyl group-containing polymer compound is a polyacrylate, the structure of the macromonomer is exemplified by the structure represented by the following formula (4a):

[Chemical Formula 7]

(wherein M represents an alkali metal ion, an ammonium ion or an organic ammonium ion, and l and m are integers representing the number of repeating units). In the above structure, the structure represented by the following formula (4b) when M is a sodium ion is preferable:

[Chemical Formula 8]

(where l and m are the same as above).

The carboxyl group-containing hydrophilic macromonomer thus obtained is copolymerized with a hydrophobic monomer in an aqueous medium in the presence of a polymerization initiator, whereby a core-halo type polymer fine particle can be obtained.

A known hydrophobic monomer having an ethylenically unsaturated bonding group can be widely used as the hydrophobic monomer, and examples thereof include a styrene monomer, a (meth) acrylate, a vinyl monomer, and a hydrocarbon conjugated diene. Monomers, etc. Examples of the styrene-based monomer include styrene, methyl styrene, dimethyl styrene, chlorostyrene, dichlorostyrene, chloromethylstyrene, 4-methoxystyrene, and 4-ethyl hydrazine. Oxystyrene and the like. Examples of the (meth) acrylate include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, and (meth)acrylic acid. Ester, n-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylate, n-dodecyl (meth)acrylate, octadecyl (meth)acrylate Benzyl (meth)acrylate, benzyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, and the like. Examples of the vinyl monomer include vinyl acetate, ethylene propionate, ethylene benzoate, N-butyl acrylamide, acrylonitrile, and vinyl chloride. Examples of the hydrocarbon conjugated diene monomer include 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. 3-hexadiene, 1,3-heptadiene, 2-phenyl-1,3-butadiene, 3-methyl-1,3-pentadiene, and the like. These monomers may be used alone or in combination of two or more. Among the above monomers, from the viewpoint of the glass transition temperature of the polymer compound obtained, a (meth) acrylate is preferred, and 2-ethylhexyl (meth) acrylate is preferred.

A crosslinked structure may also be introduced into the hydrophobic core portion of the polymer microparticles. For this purpose, a bifunctional hydrophobic monomer such as divinylbenzene or ethylene glycol dimethacrylate can be used. The bifunctional hydrophobic monomer can be used in the range of about 0.01 to 20% by weight, preferably about 0.1 to 10% by weight, based on the total radical polymerizable monomer constituting the polymer fine particle core.

As the aqueous medium, water; alcohol such as methanol, ethanol or propanol; ketone such as acetone or methyl ethyl ketone; dimethylformamide or the like can be used. As the aqueous medium, water alone, a mixed solvent of an alcohol and water, a mixed solvent of a ketone and water, or the like may be used, and water alone is preferably used. Further, the aqueous medium used herein is preferably the same as the aqueous medium used in the synthesis of the above-mentioned carboxyl group-containing hydrophilic macromonomer.

As the polymerization initiator, a known polymerization initiator can be widely used, and examples thereof include ammonium persulfate, potassium persulfate, hydrogen peroxide, benzammonium peroxide, tertiary butyl peroxide, azobisisobutyronitrile, and azo. Bis(2-aminodipropane) hydrochloride or the like. The polymerization initiator may be used singly or in combination of two or more. The polymerization initiator is preferably used in an amount of about 0.1 to 10 moles per 100 moles of the hydrophobic monomer. A known molecular weight modifier may also be added as needed.

The polymerization temperature is preferably 50 to 100 ° C.

The polymerization time varies depending on the type and amount of the polymerization initiator, the polymerization temperature, and the like, and is usually about 30 minutes to 10 hours. The polymerization is preferably carried out until the hydrophobic monomer and the hydrophilic macromonomer which contribute to the formation of the core portion of the fine particles are consumed by polymerization.

The ratio of the hydrophilic macromonomer to the hydrophobic monomer in the synthesis of the polymer microparticles is not particularly limited. The ratio of the repeating unit to the hydrophobic monomer of the hydrophilic macromonomer is preferably in the range of 1:100 to 0.01, preferably in the range of 1:10 to 0.05, and particularly preferably in the range of 1:5 to 0.1.

Hereinafter, the mechanism by which the core-halo type polymer fine particles are produced will be described with reference to Fig. 1 . Fig. 1 is a schematic diagram showing a typical mechanism for producing polymer microparticles when a carboxyl group-containing hydrophilic macromonomer is copolymerized with a hydrophobic monomer. The carboxyl group-containing hydrophilic macromonomer 1 is composed of an acrylic unit 1a and a vinyl group-containing side chain 1b. First, the carboxyl group-containing hydrophilic macromonomer 1 and the hydrophobic monomer 2 are mixed (step A) to polymerize the hydrophobic monomer, and at the same time as the polymerization of the hydrophobic monomer (step B), and the vinyl group The copolymerization of the side chains 1b also occurs synchronously. As a result of the copolymerization, a polymer compound having a structure in which a carboxyl group-containing hydrophilic macromonomer is grafted onto a hydrophobic monomer polymer can be obtained. Since the reaction proceeds in an aqueous medium, the hydrophobic monomer polymer selectively aggregates on the inner side, and the carboxyl group-containing hydrophilic macromonomer 1 selectively aggregates on the outer side (step C). By performing the polymerization to be completed in this manner, the polymer microparticles 5 are obtained, and the hydrophilic halos 4 formed from the constituent units of the carboxyl group-containing hydrophilic macromonomer are located at the constituent units derived from the hydrophobic monomer. The surface of the hydrophobic core portion 3 (step D).

The binder for a negative electrode of a lithium ion secondary battery of the present invention contains a dispersion in which the above-described core-half type polymer fine particles are dispersed in an aqueous dispersion medium.

As the aqueous dispersion medium, water; alcohol such as methanol, ethanol or propanol; ketone such as acetone or methyl ethyl ketone; dimethylformamide or the like can be used. As the aqueous dispersion medium, water alone, a mixed solvent of an alcohol and water, a mixed solvent of a ketone and water, or the like may be used, and water alone is preferably used.

The amount of the polymer fine particles contained in the binder for a negative electrode of a lithium ion secondary battery of the present invention is usually 10 to 60% by weight, preferably 15 to 50% by weight, based on the solid content of the dispersion.

The presence of the polymer microparticles can be easily confirmed by, for example, transmission electron microscopy, optical microscopy, or the like. The volume average particle diameter of the polymer microparticles is from 1 nm to 1000 nm, preferably from 10 nm to 500 nm. The volume average particle diameter can be measured by, for example, a dynamic light scattering device Zetasizer®, a particle size analyzer Microtrac®, or the like.

The method of obtaining the above dispersion liquid is not particularly limited. For example, according to the above method, a method in which a polymer microparticle has been dispersed in an emulsion of an aqueous medium, and the obtained latex is directly used as a dispersion; and a method in which an aqueous medium of the obtained latex is substituted into another aqueous medium is visually observed. Good use of manufacturing efficiency, etc., to be used appropriately. In the case of the method of substituting the dispersion medium, in the case where the polymer fine particles are produced in an alcohol and the alcohol is substituted with an aqueous medium other than the above-mentioned alcohol, for example, an aqueous medium other than the alcohol is added to the latex, and then a distillation method is used. A method of removing an alcohol component in a dispersion medium, etc. by a filtration method, a dispersion medium phase transformation method, or the like.

The resulting dispersion can be used directly as a binder for a negative electrode. Alternatively, an additive such as a viscosity modifier or a fluidizer for improving the coating property may be appropriately added in addition to the dispersion. These additives may, for example, be cellulose compounds such as carboxymethylcellulose, methylcellulose or hydroxypropylcellulose; and such ammonium salts and alkali metal salts; polyacrylates such as sodium poly(meth)acrylate; Copolymer of vinyl alcohol, polyvinyloxy, polyvinylpyrrolidone, (meth)acrylic acid or (meth)acrylate and vinyl alcohol, maleic anhydride or maleic acid or copolymer of fumaric acid and vinyl alcohol, modified A water-soluble polymer such as polyvinyl alcohol, modified poly(meth)acrylic acid, polyethylene glycol, polycarboxylic acid, ethylene-vinyl alcohol copolymer, or vinyl acetate polymer. The use ratio of these additives can be appropriately selected as needed.

Further, the binder of the present invention may contain a polymer or polymer particles other than the above polymer fine particles. As the polymer or polymer particles, a known polymer which is generally used as a binder for battery electrodes can be widely used. The amount thereof to be used is preferably 1 part by weight or less based on 1 part by weight of the polymer fine particles.

The binder of the present invention is a dispersion containing core-half type polymer microparticles produced by the above method. The solvent-soluble PVDF which is conventionally used as a binder resin is disposed so as to coat the surface of the negative electrode active material, thereby preventing contact between the negative electrode active material and contact between the negative electrode active material and the current collector, resulting in electrode resistance. When the binder of the present invention is enlarged, the binder of the present invention is abbreviated to the surface of the negative electrode active material and the surface of the current collector by a micro-half type polymer compound having a fine particle shape, so that the negative electrode active material and the negative electrode active material can be made. Bonding with the current collector does not hinder the contact between the negative electrode active material and the contact between the negative electrode active material and the current collector, so that the electrode resistance can be reduced.

Further, the core-half type polymer fine particles are different from the SBR which is conventionally used as a binder resin, and have a hydrophilic halo having high affinity with the negative electrode active material and the current collector on the surface of the polymer fine particles, so that even In the expansion and contraction of the negative electrode active material caused by the reverse charge and discharge, the adhesion between the negative electrode active material and the negative electrode active material and the current collector can be maintained, and the contact between the negative electrode active material and the negative electrode active material and the current collector can be maintained. The contact portion is maintained to ensure that the electrode resistance becomes small.

Further, since the hydrophobic core portion of the core-halo type polymer fine particles can follow the volume change caused by the swelling and contraction of the negative electrode active material during charge and discharge, it is possible to prevent the negative electrode active material from expanding and contracting due to absorption and release of lithium ions. In the process, the contact portion between the negative electrode active material and the negative electrode active material and the current collector disappears to cause a portion that is not conductive (electrical insulation of the negative electrode active material).

According to this, by using such a binder for a negative electrode of a lithium ion secondary battery, a lithium ion secondary battery excellent in rate characteristics can be obtained.

2. Slurry composition for negative electrode of lithium ion secondary battery The slurry composition for negative electrode of lithium ion secondary battery of the present invention contains the above-mentioned binder and active material of the present invention. The slurry composition for a negative electrode of a lithium ion secondary battery of the present invention can be prepared by mixing the above-mentioned binder and active material of the present invention.

Any active material can be used as long as it is generally used for producing an electrode for a lithium ion secondary battery. Examples of the negative electrode active material include graphite-based carbon materials (graphite) such as natural graphite, artificial graphite, and expanded graphite, carbon black, activated carbon, carbon fiber, coke, soft carbon, and hard carbon. Carbonaceous materials such as pitch-based carbon fibers; conductive polymer compounds such as polyacene; composite metal oxides and other metal oxides. Among them, a carbonaceous material is preferred, and graphite such as natural graphite, artificial graphite or expanded graphite is preferred. The content of the active material in the slurry composition is not particularly limited, but is usually 10 to 95% by weight, preferably 20 to 80% by weight, and preferably 35 to 65% by weight.

The average particle diameter of the active material is not particularly limited, but is preferably 1 to 100 μm, more preferably 3 to 50 μm, and still more preferably 5 to 25 μm. Further, the average particle diameter of the active material is measured by a laser diffraction type particle size distribution measurement (laser diffraction-scattering method).

The content ratio of the active material to the binder in the slurry composition for the negative electrode is preferably 0.1 to 10 parts by weight, preferably 0.5 to 5 parts by weight, based on 100 parts by weight of the active material, based on the solid content of the binder. 1 to 4 parts by weight is particularly preferred. If the content of the binder for the negative electrode is too high, the internal resistance will increase. On the other hand, if the amount is too small, the desired bonding strength cannot be obtained, and the negative electrode becomes unstable, and the charge/discharge cycle characteristics tend to be lowered.

The slurry composition for a negative electrode may further contain other additives in addition to the above-mentioned active material and a binder for a negative electrode. Examples of other additives include a conductive auxiliary agent, a carrier salt (lithium salt), and the like. The blending ratio of the components can be adjusted by appropriately referring to the blending ratio in the general range or the known knowledge of the lithium ion secondary battery.

The conductive auxiliary agent refers to a blend which is blended in order to improve conductivity. The conductive additive may be a carbon powder such as graphite, a carbon fiber such as a vapor-grown carbon fiber reinforced (VGCF), or a carbon micropowder having a particle diameter of several nm to several tens of nm such as acetylene carbon or Ketjenblack. . The blending amount of the conductive auxiliary agent is preferably from 1 to 10% by weight based on the total mass of the active material layer.

Further, in consideration of workability at the time of production of the negative electrode, etc., it is possible to prepare a slurry composition for a negative electrode by adding a solvent in accordance with the purpose of viscosity adjustment, solid content adjustment of the binder, and the like. As the solvent, for example, an amide-based solvent such as N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide or methylformamide, or an alcohol such as methanol, ethanol or higher alcohol can be used. Solvent.

A mixer, a defoaming machine, a bead mill, a high pressure homogenizer, or the like can be used for the binder for the negative electrode, the active material, and the additive or solvent to be used as needed. Further, it is preferred that the slurry composition for the negative electrode is prepared under reduced pressure. Thereby, generation of bubbles in the produced active material layer can be prevented.

3. Negative Electrode for Lithium Ion Secondary Battery The lithium ion secondary battery negative electrode can be produced by applying the slurry composition for a negative electrode prepared as described above to a current collector and drying it. If necessary, after coating and drying, it is recommended to pressurize to increase the density.

The current collector for the negative electrode can be used as a current collector of a negative electrode of a lithium ion secondary battery. Specifically, in view of the metal which is electrochemically inert in the potential range in which the function of the negative electrode (carbon electrode) is exhibited, a metal foil such as copper or nickel, an etched metal foil, a porous metal or the like can be used.

On such a current collector, a negative electrode layer can be formed by applying and drying a slurry composition for a negative electrode. The method of applying the slurry composition for a negative electrode to a current collector may be a doctor blade method, a reverse roll method, a comma-bar method, a gravure printing method, an air knife method, or the like. Further, the drying treatment conditions of the coating film of the slurry composition for a negative electrode are usually 20 to 250 ° C, preferably 50 to 150 ° C. Moreover, the processing time is usually 1 to 120 minutes, preferably 5 to 60 minutes.

The thickness of the active material layer (thickness of one side of the coating layer) is usually 20 to 500 μm, preferably 25 to 300 μm, and preferably 30 to 150 μm.

4. Lithium ion secondary battery A lithium ion secondary battery having the negative electrode produced as described above will be described.

The positive electrode is not particularly limited, and a known general positive electrode can be combined.

As the positive electrode active material, for example, olivine-type lithium iron phosphate, lithium cobaltate, lithium manganate, lithium nickelate, ternary nickel-cobalt-manganate, lithium nickel cobalt aluminate (NCA) composite oxide, or the like can be used.

The current collector of the positive electrode may be a metal material such as aluminum, copper, nickel, tantalum, stainless steel or titanium, and may be appropriately selected depending on the type of the storage element to be used.

A known aprotic polar solvent capable of dissolving a lithium salt can be widely used as the electrolyte. For example, a cyclic carbonate having a high dielectric constant or a high boiling point such as ethyl carbonate or propylene carbonate may contain dimethyl carbonate, ethyl methyl carbonate or diethyl carbonate in a low viscosity solvent. A lower chain carbonate such as an ester is used. Specific examples thereof include ethyl carbonate, ethyl chlorocarbonate, propyl trifluorocarbonate, butyl carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, and carbonic acid. Methyl isopropyl ester, ethyl propyl carbonate, ethyl isopropyl carbonate, methylbutyl carbonate, ethyl butyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, γ-butyrolactone, tetrahydrofuran , 2-methyltetrahydrofuran, cyclobutyl hydrazine, 3-methylcyclobutyl hydrazine, 2,4-dimethylcyclobutyl hydrazine, 1,3-dioxopentacycloalkane, methyl acetate, ethyl acetate, formic acid Ester, ethyl formate, and the like. These are preferably mixed.

The lithium salt of the electrolyte uses inorganic salts such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCl, LiBr; LiCF ₃ SO ₃ , LiN(SO ₂ CF ₃ )) ₂ , LiN(SO ₂ C ₂ F ₅ ) ₂ , An organic salt such as LiC(SO ₂ CF ₃ ) ₃ or LiN(SO ₃ CF ₃ )) ₂ may be used as a non-aqueous electrolyte electrolyte. Among these, it is preferred to use LiPF ₆ , LiBF ₄ or LiClO ₄ .

The separator is not particularly limited, and a polyolefin nonwoven fabric, a porous film, or the like can be used.

The structure of the secondary battery is not particularly limited, and may be a laminate type (flat type) battery or a winding type (cylindrical type) battery, and may be applied to any form and structure known from the prior art. Further, in the electrical connection form (electrode structure) in the lithium ion secondary battery, either an internal parallel type battery or an internal series type bipolar type battery can be applied.

The lithium ion secondary battery produced as described above has a high initial discharge capacity and stable output characteristics based on the use of the binder for a negative electrode of the present invention. Example

Hereinafter, the present invention will be described in further detail by way of examples and comparative examples. Further, the present invention is not limited to the following embodiments.

(Synthesis Example 1) 1-1. Synthesis of sodium acrylate macromonomer (carboxyl-containing hydrophilic macromonomer) In a reaction vessel equipped with a stirring device, an air-cooling tube and a thermometer, water was sequentially fed with 18.00 g of polyacrylic acid. A 40% aqueous solution of 53.20 g (295 mmol of an acrylic acid unit, a number average molecular weight of 5,300), 24.60 g of a 48% aqueous solution of sodium hydroxide, and 4.20 g (29.5 mmol) of glycidyl methacrylate were heated and the temperature was raised to 40 °C. The temperature of the reaction liquid was maintained at 40 ° C for 6 hours to obtain a sodium polyacrylate macromonomer solution having a methacryl oxime group (solid content: 32% by weight). After the reaction was completed, the polyacrylic macromonomer was purified by reprecipitation several times with acetone. The number average molecular weight of the obtained polyacrylic acid macromonomer obtained by GPC (Liquid Chromatography) was 5,600.

1-2. Copolymerization of sodium acrylate macromonomer with 2-ethylhexyl methacrylate (preparation of core-halo-type polymer microparticles) Next, in a reaction vessel equipped with a stirring device, a reflux condenser, and a thermometer, The feed of the above polyacrylic acid macromonomer solution is 22.16 g [65 mmol based on the repeating unit of the acrylic acid unit (according to the feed forward)], 2-ethylhexyl methacrylate 12.87 g (65 mmol), and water. 63.45g and heated to 70 °C. To the mixture was added 1.50 g (1.31 mmol) of a 20% by weight aqueous solution of ammonium persulfate, and copolymerization was carried out for 6 hours to obtain a milky white dispersion (solid content: 20% by weight). The obtained polymer microparticles having a hydrophilic carboxyl group containing a sodium carboxylate had an average particle diameter of 190 nm.

(Synthesis Example 2) Copolymerization of sodium acrylate macromonomer with 2-ethylhexyl methacrylate (Preparation of core-halo type polymer microparticles) The sodium polyacrylate macromonomer solution was changed to 46.67 g [by acrylic acid In the same manner as in Synthesis Example 1-2, a milky white dispersion was obtained, except that the unit repeating unit was changed to 130 mmol], 2-ethylhexyl methacrylate was changed to 6.44 g (33 mmol), and water was changed to 45.39 g. Liquid (solids concentration 22% by weight). The obtained polymer microparticles having a hydrophilic carboxyl group containing a sodium carboxylate had an average particle diameter of 250 nm.

(Synthesis Example 3) 3-1. Synthesis of sodium acrylate macromonomer (carboxyl-containing hydrophilic macromonomer) The same procedure as in Synthesis Example 1-2 was carried out except that a polyacrylic acid 40% aqueous solution having a number average molecular weight of 16,500 was used. A sodium polyacrylate macromonomer solution having a methacryl oxime group (solids concentration: 32% by weight) was obtained. The number average molecular weight of the obtained polyacrylic acid macromonomer obtained by GPC (Liquid Chromatography) was 17,000.

3-2. Copolymerization of sodium acrylate macromonomer with 2-ethylhexyl methacrylate (preparation of core-halo type polymer microparticles) The sodium polyacrylate macromonomer solution synthesized in Synthesis Example 3-1 was used. In the same manner as in Synthesis Example 1-2, a milky white dispersion (solid content: 20% by weight) was obtained. The polymer microparticles having a hydrophilic halo chain containing sodium carboxylate obtained had an average particle diameter of 320 nm.

(Synthesis Example 4) 4-1. Synthesis of sodium carboxymethylcellulose (hereinafter referred to as CMCNa) macromonomer (carboxyl-containing hydrophilic macromonomer) in a reaction vessel equipped with a stirring device, an air cooling tube, and a thermometer, 89.08 g of water, 10.00 g of powder CMC (30 mmol of carboxyl group, number average molecular weight of 15,600), 0.50 g of sodium hydroxide 48% aqueous solution, and 0.42 g (3.0 mmol) of glycidyl methacrylate were sequentially fed, and the temperature was raised by heating. 40 ° C. The temperature of the reaction liquid was maintained at 40 ° C for 6 hours to obtain a CMCNa macromonomer solution having a methacrylonitrile group (solid content: 10% by weight). After the reaction was completed, the CMCNa macromonomer was purified by reprecipitation several times with acetone. The number average molecular weight of the obtained CMCNa macromonomer obtained by GPC (Liquid Chromatography) was 16,000.

4-2. Copolymerization of CMCNa macromonomer with 2-ethylhexyl methacrylate (preparation of core-halo polymer microparticles) Next, in a reaction vessel equipped with a stirring device, a reflux condenser and a thermometer, in sequence Feeding 50.00 g of the above CMCNa macromonomer solution [22 mmol (based on feed forward) in the unit of glucose unit repeat], 4.37 g (22 mmol) of 2-ethylhexyl methacrylate, and 44.13 g of water and The temperature was raised to 70 °C. To the mixture was added 1.50 g (1.31 mmol) of a 20% by weight aqueous solution of ammonium persulfate, and copolymerization was carried out for 6 hours to obtain a milky white dispersion (solid content: 10% by weight). The polymer microparticles having the CMCNa halo obtained had an average particle diameter of 200 nm.

(Comparative Synthesis Example 1) In a reaction vessel equipped with a stirring device, a reflux condenser, and a thermometer, 21.94 g (70 mmol) of a 30% by weight aqueous solution of sodium acrylate and 10% by weight of an aqueous solution of sodium dodecylbenzenesulfonate were sequentially fed. g, 2-ethylhexyl methacrylate 13.86 g (70 mmol), and 52.70 g of water, and the temperature was raised to 70 °C. To the mixture was added 1.50 g (1.31 mmol) of a 20% by weight aqueous solution of ammonium persulfate, and copolymerization was carried out for 6 hours to obtain a milky white dispersion (solid content: 22% by weight). The resulting emulsion had an average particle diameter of 230 nm.

Example 1 A lithium ion secondary battery evaluation half-cell of the present invention was prepared by the following method using the binder for a negative electrode of Synthesis Example 1. In addition, the materials for the half-cells used for evaluation were the following materials. Negative electrode active material: natural graphite powder counter electrode: lithium metal foil conductive additive: carbon micropowder tackifier: sodium carboxymethyl cellulose (CMCNa) electrolyte: 1mol / L lithium hexafluorophosphate (LiPF ₆ ) / carbonic acid A mixture of ethyl ester (EC) and diethyl carbonate (DEC) (EC: DEC (volume ratio) = 1:1) Separator: Cellulose separator Collector: Copper foil

2% by weight of the CMCNa aqueous solution was prepared, and the natural graphite powder, the carbon fine powder, the CMCNa and the negative electrode binder of Synthesis Example 1 were mixed at a ratio of the electrode composition ratio (weight ratio) after drying to 95:3:1:1. A slurry is prepared. After the slurry was appropriately added with pure water to obtain a uniform slurry, it was applied to a copper foil by a doctor blade, and dried at 100 ° C for 20 minutes. A 16 mm circular electrode was cut out and dried in a temperature of 80 ° C under a reduced pressure of 10 ^-1 Pa for 24 hours to obtain a carbon negative electrode of the intended product. A carbon negative electrode and a reverse polarity lithium metal foil were disposed, and a separator was placed between the electrodes, and an electrolyte solution was injected to prepare a two-pole evaluation half-cell.

Example 2 A two-electrode evaluation half-cell was prepared in the same manner as in Example 1 except that the binder for the negative electrode was changed to the binder of Synthesis Example 2.

(Example 3) A two-electrode evaluation half-cell was prepared in the same manner as in Example 1 except that the binder for the negative electrode was changed to the binder of Synthesis Example 3.

(Example 4) A two-electrode evaluation half-cell was prepared in the same manner as in Example 1 except that the negative electrode binder was changed to the binder of Synthesis Example 4.

Example 5 The binder of the synthesis example 4 was changed to the binder of the synthesis example 4 without using a tackifier (CMCNa), and the composition ratio (weight ratio) of the natural graphite powder, the carbon fine powder, and the binder of the synthesis example 4 was dried. A slurry was prepared by mixing natural graphite powder, carbon fine powder, and a binder of Synthesis Example 5 in a ratio of 95:3:2. Otherwise in the same manner as in Example 1, a two-electrode half-cell for evaluation was prepared.

Comparative Example 1 A two-electrode half-cell for evaluation was prepared in the same manner as in Example 1 except that the binder for the negative electrode was changed to SBR (milk liquid, solid concentration: 40% by weight).

Comparative Example 2 A two-electrode evaluation half-cell was prepared in the same manner as in Example 1 except that the binder for the negative electrode was changed to the emulsion of Comparative Synthesis Example 1.

Comparative Example 3 A two-electrode evaluation half-cell was prepared in the same manner as in Example 1 except that the binder for the negative electrode was changed to a sodium polyacrylate aqueous solution (solid content: 40% by weight, number average molecular weight: 16,500).

[Measurement Example 1: Rate characteristic of lithium ion battery] The evaluation half-cells of Examples 1 to 5 and Comparative Examples 1 to 3 were subjected to one cycle after an operating voltage range of 0.05 to 1.2 V and a current density of 0.1 C. The charge and discharge were performed for 5 cycles at current densities of 1C, 3C, 5C, 8C, and 10C, and finally returned to the 1C rate for 5 cycles of charge and discharge, and the discharge capacity at each current density was investigated. The discharge capacity of the fifth cycle of each current density was divided by the initial (0.1 C rate) discharge capacity, and the percentage (%) thereof was taken as the volume retention ratio. The results are shown in Table 1 and Figure 2. The higher the capacity retention rate, the higher the rate characteristic.

[Table 1]

The components in Table 1 are as follows. SBR: Styrene-butadiene latex EHMA: 2-ethylhexyl methacrylate AAcNa: sodium acrylate PAAcNa: sodium polyacrylate CMCNa: sodium carboxymethylcellulose

As can be seen from Table 1 and Fig. 2, the two-stage evaluation half-cell of the embodiment can maintain a high capacity at a high rate compared with the half-cell of the comparative example.

[Measurement Example 2: Impedance of Lithium Ion Battery] The half-cells of Example 1 and Comparative Example 1 used in Measurement Example 1 were further subjected to an AC impedance with an open circuit voltage at a potential amplitude of 10 mV and an AC frequency range of 100 kHz to 10 mHz. Determination. The Nyquist plot (Cole-Cole plot) with the real axis impedance (Z') for the X axis and the imaginary impedance (Z" for the Y axis is shown in Figure 3.

As can be seen from Fig. 3, the battery resistance value of Example 1 was lower than that of the battery of Comparative Example 1.

1‧‧‧Carboxyl-containing hydrophilic macromonomer
1a‧‧‧Acrylic unit
1b‧‧‧ vinyl side chain
2‧‧‧hydrophobic monomer
3‧‧‧Digital nuclear division
4‧‧‧Hydrophilic halo
5‧‧‧ Polymer microparticles

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view showing the mechanism by which nuclear-halo type polymer microparticles can be produced. 2 is a graph showing the rate characteristics of the evaluation half-cells of Examples 1, 4, and 5, and Comparative Example 1. 3 is a graph showing the results of impedance measurement of the evaluation half-cells of Example 1 and Comparative Example 1.

no

Claims (6)

A bonding agent for a negative electrode for a lithium ion secondary battery containing a dispersion, wherein the dispersion-core-half-type polymer microparticles are dispersed in an aqueous dispersion medium, and the nuclear-halo-type polymer microparticles have a hydrophilic halo a portion surrounding the structure around the hydrophobic core portion, the hydrophilic halo portion being formed by a constituent unit derived from a carboxyl group-containing hydrophilic macromonomer, which is composed of a constituent unit derived from a hydrophobic monomer form. The bonding agent according to claim 1, wherein the core-half type polymer fine particles are polymer microparticles obtained by radically polymerizing the carboxyl group-containing hydrophilic macromonomer and the hydrophobic monomer, and the carboxyl group-containing hydrophilicity The macromonomer is a macromonomer obtained by reacting a carboxyl group-containing polymer compound with a compound having a polymerizable reactive group in the molecule and a functional group capable of reacting with a carboxyl group to form a covalent bond. The binder according to claim 2, wherein the compound having both a polymerizable reactive group and a functional group reactive with a carboxyl group to form a covalent bond is selected from the following general formulae (1) to (3) At least one compound of the group consisting of the compounds shown: a compound of the formula (1): [Chemical Formula 1] [wherein, R ₁ represents a hydrogen atom or a methyl group, and Q ₁ represents an oxygen atom or -NH-;] a compound represented by the formula (2): [Chemical Formula 2] Wherein R ₂ represents a hydrogen atom or a methyl group, Q ₂ represents an oxygen atom or -NH-, and n represents an integer of 1 to 4; and a compound represented by the formula (3): [Chemical Formula 3] [wherein R ₃ represents a hydrogen atom or a methyl group, R ₄ represents a hydrogen atom, an alkyl group having 1 to 4 carbon atoms or a halogen atom, and X ₁ represents a halogen atom]. A slurry composition for a negative electrode of a lithium ion secondary battery, comprising the binder for a negative electrode of a lithium ion secondary battery according to any one of claims 1 to 3, and an active material. A lithium ion secondary battery negative electrode is formed by applying a slurry composition for a lithium ion secondary battery negative electrode of claim 4 onto a current collector and drying it. A lithium ion secondary battery comprising the lithium ion secondary battery negative electrode of claim 5.