JP6737045B2 - Binder composition for lithium-ion secondary battery positive electrode, lithium-ion secondary battery positive electrode, and lithium-ion secondary battery - Google Patents

Description

The present invention relates to a binder composition for a lithium-ion secondary battery positive electrode, a lithium-ion secondary battery positive electrode, and a lithium-ion secondary battery. Specifically, it is used together with an active material to form a positive electrode of a lithium-ion secondary battery, has excellent oxidation resistance, and has good charge-discharge cycle characteristics at high temperatures when a lithium-ion secondary battery is formed. And a lithium ion secondary battery positive electrode obtained by using the binder composition, and a lithium ion secondary battery having the lithium ion secondary battery positive electrode.

BACKGROUND OF THE INVENTION Lithium ion secondary batteries are used as power sources for electronic devices such as mobile phones and notebook computers because they are lightweight, have high energy density, and have high durability against repeated charge and discharge. It is also used in electric vehicles such as electric vehicles as a power supply device capable of discharging and charging. In particular, as batteries have become larger in size in recent years, batteries with higher performance, such as high capacity and rapid charging/discharging capability, are required.

In a lithium ion secondary battery, generally, a positive electrode active material layer containing a positive electrode active material is formed on both sides of a positive electrode current collector, and a negative electrode active material layer containing a negative electrode active material is formed on both sides of a negative electrode current collector. The formed negative electrode is connected via the electrolyte layer and housed in the battery case. Such an electrode is formed by applying a mixed slurry of an active material and a binder for an electrode on the surface of a current collector and drying it.

Here, the electrode binder binds the active materials to each other and also binds the metal foil, which is the current collector, to the active material. If the binder cannot bind a sufficient amount of the active material to the current collector or the active materials cannot bind to each other, a battery having a large capacity cannot be obtained. Further, by repeating charge and discharge, the binder is gradually oxidized, the binding force of the active material to the current collector is reduced, and the active material may fall off from the current collector, which may reduce the capacity of the battery.
Further, since the manufacturing process of the electrode includes a roll-to-roll forming process, the manufactured electrode is required to have high flexibility and flexibility. If the electrode does not have sufficient flexibility or flexibility, the mixture layer may be cracked and the active material may be peeled off or dropped off, resulting in a decrease in battery capacity. Therefore, a binder having both binding force and flexibility is required.

As a method of solving this problem, a method of using a flexible fluororesin such as polyvinylidene fluoride (PVDF) as a binder is known. However, since PVDF has a weak binding force, there is a risk that the binding force will decrease unless a large amount of binder is used.

Further, as a binder composition for a lithium ion secondary battery electrode, it has been proposed to use a water-soluble binder such as a polyvinyl alcohol-based resin in the case of a negative electrode. For example, in order to obtain an electrode having high charge-discharge capacity and high flexibility, polymer particles derived from an ethylenically unsaturated monomer are an aqueous solution of a polyvinyl alcohol-based resin having a viscosity average degree of polymerization of 400 to 3000. A binder composition for a lithium ion secondary battery electrode containing an emulsion dispersed therein has been proposed (see, for example, Patent Document 1).

JP, 2016-66601, A

However, although the above-mentioned Patent Document 1 describes the binder composition for the negative electrode in the Examples, it does not describe the binder composition for the positive electrode up to a specific evaluation.

It is important that the binder composition for a lithium ion secondary battery positive electrode is excellent in oxidation resistance because it is exposed to severe oxidation conditions. For example, a lithium ion secondary battery manufactured using the binder composition has It is desired that the charge/discharge cycle characteristics at high temperature be good, but the binder composition disclosed in Patent Document 1 is not yet satisfactory in terms of such oxidation resistance at high temperature. It was

Under the circumstances, the present invention provides a binder composition for a positive electrode of a lithium ion secondary battery, which has excellent oxidation resistance and has good charge/discharge cycle characteristics at high temperatures when a lithium ion secondary battery is formed. It is an object of the present invention to provide a lithium ion secondary battery positive electrode obtained using the binder composition, and a lithium ion secondary battery having the lithium ion secondary battery positive electrode.

However, as a result of intensive studies conducted by the present inventors in view of such circumstances, an emulsion in which polymer particles (A) derived from an ethylenically unsaturated monomer are dispersed and stabilized with a polyvinyl alcohol resin (B). When [I] is used in the binder composition for a lithium ion secondary battery positive electrode, by increasing the content ratio of the polyvinyl alcohol resin (B) to the polymer particles (A), the oxidation resistance is excellent and The present invention has been completed by finding that when an ion secondary battery is formed, the charge/discharge cycle characteristics at high temperature are improved.

That is, the gist of the present invention is to provide a lithium ion secondary battery positive electrode containing an emulsion [I] in which polymer particles (A) derived from an ethylenically unsaturated monomer are dispersed and stabilized with a polyvinyl alcohol resin (B). a use binder composition, the content ratio of the polymer particles (a) and the polyvinyl alcohol-based resin (B) (a / B) is at a weight ratio of solids, Ri 1/99 to 40/60 der relates polymer particles (a) (meth) lithium ion secondary battery positive electrode binder composition characterized Rukoto such as the main component an acrylic resin.

Further, the gist of the present invention is to provide a lithium ion secondary battery positive electrode obtained by using the binder composition for a lithium ion secondary battery positive electrode of the present invention, and a lithium ion secondary battery having the lithium ion secondary battery positive electrode of the present invention. It also relates to batteries.

In the following, the “binder composition for positive electrode of lithium ion secondary battery” may be simply referred to as “binder composition for positive electrode”. In addition, "polyvinyl alcohol-based resin" may be referred to as "PVA-based resin".

The lithium ion secondary battery positive electrode binder composition of the present invention comprises an emulsion [I] in which polymer particles (A) derived from an ethylenically unsaturated monomer are dispersed and stabilized with a PVA-based resin (B). Since the PVA-based resin (B) contains more than the polymer particles (A), the positive electrode obtained using the binder composition is excellent in oxidation resistance and has a charge/discharge cycle characteristic at high temperature. A good lithium-ion secondary battery can be obtained.

In the present invention, the reason why the oxidation resistance of the binder composition for a positive electrode is excellent is not clear, but due to the large amount of highly hydrophilic PVA-based resin (B) being contained, an electrolyte solution (organic solvent) is added. The positive electrode is less likely to swell, the binding property between the active material and the current collector can be maintained, the side reaction (oxidation reaction) on the binder composition is less likely to occur, and the oxidation of the polymer particles (A) is suppressed. It is supposed to be because of this.

Hereinafter, the constitution of the present invention will be described in detail, but these show examples of desirable embodiments, and the present invention is not limited to these contents.
In the present specification, when acrylic and methacrylic are not particularly distinguished, they are collectively referred to as (meth)acrylic, and when acrylate and methacrylate are not specifically distinguished, they are collectively referred to as (meth)acrylate.
In the present invention, the solid content means that obtained by subjecting the object to a loss on drying method at 105° C. for 3 hours.

<Binder composition for positive electrode of lithium ion secondary battery>
The lithium ion secondary battery positive electrode binder composition of the present invention comprises an emulsion [I] in which polymer particles (A) derived from an ethylenically unsaturated monomer are dispersed and stabilized with a PVA-based resin (B). It is a waste.

The polymer particles (A) can be obtained by emulsion-polymerizing an ethylenically unsaturated monomer in a water dispersion medium using the PVA-based resin (B) as a dispersant. The details will be described below.

[Description of polymer particles: ethylenically unsaturated monomer]
The polymer in the polymer particles (A) is a polymer of ethylenically unsaturated monomer. Examples of the ethylenically unsaturated monomer include the following (a) to (m). These may be used alone or in combination of two or more.
(A) Alkyl ester of (meth)acrylic acid.
(B) A hydroxyl group-containing ethylenically unsaturated monomer.
(C) A carboxyl group-containing ethylenically unsaturated monomer.
(D) Epoxy group-containing ethylenically unsaturated monomer.
(E) A methylol group-containing ethylenically unsaturated monomer.
(F) Alkoxyalkyl group-containing ethylenically unsaturated monomer.
(G) A cyano group-containing ethylenically unsaturated monomer.
(H) An ethylenically unsaturated monomer having two or more radically polymerizable double bonds.
(I) An ethylenically unsaturated monomer having an amino group.
(J) An ethylenically unsaturated monomer having a sulfonic acid group.
(K) An ethylenically unsaturated monomer having a phosphoric acid group.
(L) Aromatic ethylenically unsaturated monomer.
(M) Fatty acid ester unsaturated monomer.

In addition to the above (a) to (m), monomers such as vinylpyrrolidone, methyl vinyl ketone, butadiene, ethylene, propylene, vinyl chloride and vinylidene chloride can be appropriately used as desired.

Next, the monomers exemplified in the above (a) to (m) will be described in detail.
Examples of the (meth)acrylic acid alkyl ester (a) include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth). Aliphatic (meth)acrylates such as acrylate, n-hexyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and lauryl (meth)acrylate are mentioned, and preferably the alkyl group has 1 carbon atom. To 20 aliphatic (meth)acrylates, aromatic (meth)acrylates such as benzyl (meth)acrylate and phenyl (meth)acrylate. These may be used alone or in combination of two or more.
Among these, particularly preferable are aliphatic (meth) having an alkyl group having 1 to 10 carbon atoms such as butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, methyl (meth)acrylate, and cyclohexyl (meth)acrylate. It is an acrylate.

Examples of the hydroxyl group-containing ethylenically unsaturated monomer (b) include hydroxyethyl (meth)acrylates such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and polyethylenglycol (meth)acrylates. Examples include alkylene glycol (meth)acrylate and the like. These may be used alone or in combination of two or more.
Among them, hydroxyalkyl (meth)acrylate having a hydroxyalkyl group having 2 to 4 carbon atoms and polyalkylene glycol (meth)acrylate having an alkylene group having 2 to 4 carbon atoms are preferable, and hydroxyethyl ( (Meth)acrylate.

Examples of the carboxyl group-containing ethylenically unsaturated monomer (c) include monocarboxylic acid monomers such as (meth)acrylic acid, crotonic acid and undecylenic acid, dicarboxylic acids such as maleic acid, fumaric acid and itaconic acid, and The anhydride monomer etc. are mentioned. (Meth)acrylic acid is preferred. These may be used alone or in combination of two or more.
Among these, (meth)acrylic acid and itaconic acid are particularly preferable. Note that dicarboxylic acids such as maleic acid, fumaric acid, and itaconic acid include these monoesters and monoamides.

Examples of the epoxy group-containing ethylenically unsaturated monomer (d) include glycidyl (meth)acrylate, allyl glycidyl ether, and methyl glycidyl (meth)acrylate. These may be used alone or in combination of two or more. Of these, glycidyl (meth)acrylate is preferable.

Examples of the methylol group-containing ethylenically unsaturated monomer (e) include N-methylol (meth)acrylamide and dimethylol (meth)acrylamide. These may be used alone or in combination of two or more.

Examples of the alkoxyalkyl group-containing ethylenically unsaturated monomer (f) include N-methoxymethyl (meth)acrylamide, N-butoxymethyl (meth)acrylamide, methoxyethyl (meth)acrylate, methoxypropyl (meth). Examples thereof include acrylate, ethoxyethyl (meth)acrylate, alkoxyalkyl (meth)acrylate such as ethoxypropyl (meth)acrylate, and polyalkylene glycol monoalkoxy (meth)acrylate such as polyethylene glycol monomethoxy (meth)acrylate. These may be used alone or in combination of two or more.

As the cyano group-containing ethylenically unsaturated monomer (g), for example, an α,β-unsaturated nitrile compound is used. Specifically, acrylonitrile-based monomers such as acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, and α-ethylacrylonitrile; vinyl group-substituted vinyl monomers such as vinylidene cyanide; methylcyanoacrylate, ethylcyanoacrylate, butylcyanoacrylate, etc. Unsaturated group-containing cyanoacrylates, tetracyanoquinodimethane, 2,2-diarylmalononitrile, and the like. Among these, acrylonitrile-based monomers are preferable, (meth)acrylonitrile is particularly preferable, and acrylonitrile is more preferable. These may be used alone or in combination of two or more.

Examples of the ethylenically unsaturated monomer (h) having two or more radical-polymerizable double bonds are di(meth)acrylate, tri(meth)acrylate, tetra(meth)acrylate, conjugated Examples include dienes.
Examples of the di(meth)acrylate include divinylbenzene, polyoxyethylene di(meth)acrylate, polyoxypropylene di(meth)acrylate, neopentyl glycol di(meth)acrylate, butanediol di(meth)acrylate and the like. To be
Examples of tri(meth)acrylates include trimethylolpropane tri(meth)acrylate, and examples of tetra(meth)acrylates include pentaerythritol tetra(meth)acrylate.
Examples of the conjugated diene-based monomer include 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene, 1,3-heptadiene, 2 , 3-Dimethylbutadiene, 2-phenyl-1,3-butadiene, 3-methyl-1,3-pentadiene, 1,3-hexadiene, and other hydrocarbon conjugated diene-based monomers; 2-chloro-1,3-butadiene, etc. And halogen-containing conjugated diene-based monomers; substituted linear conjugated pentadienes; substituted and side chain conjugated hexadienes and the like. Among these, a conjugated diene-based monomer having 4 to 6 carbon atoms is preferable, and 1,3-butadiene is particularly preferable.
These ethylenically unsaturated monomers (h) having two or more radically polymerizable double bonds can be used alone or in combination of two or more.

Examples of the ethylenically unsaturated monomer (i) having an amino group include N, such as (meth)acrylamide, N,N-dimethylaminoethyl (meth)acrylate and N,N-diethylaminoethyl (meth)acrylate. , N-dialkylaminoalkyl(meth)acrylate and the like. These may be used alone or in combination of two or more. Of these, (meth)acrylamide is preferable.

Examples of the ethylenically unsaturated monomer (j) having a sulfonic acid group include vinyl sulfonic acid, vinyl styrene sulfonic acid (salt) and the like. These may be used alone or in combination of two or more.

Examples of the ethylenically unsaturated monomer (k) having a phosphoric acid group include vinylphosphonic acid, vinyl phosphate, acid phosphoxyethyl (meth)acrylate, acid phosphoxypropyl (meth)acrylate, bis[(meth ) Acryloyloxyethyl]phosphate, diphenyl-2-(meth)acryloyloxyethyl phosphate, dibutyl-2-(meth)acryloyloxyethyl phosphate, dioctyl-2(meth)acryloyloxyethyl phosphate and the like. These may be used alone or in combination of two or more.

Examples of the aromatic ethylenically unsaturated monomer (l) include styrene, vinyltoluene, α-methylstyrene and the like, and these may be used alone or in combination of two or more. Is also good. Among them, styrene is preferable.

Examples of the fatty acid ester unsaturated monomer (m) include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl caprate, vinyl laurate, Examples thereof include vinyl stearate, vinyl benzoate and vinyl versatate. These may be used alone or in combination of two or more.

The polymer particles (A) include a structural unit derived from a monomer other than the above (hereinafter sometimes simply referred to as “other monomer”) within a range not impairing the object of the present invention. May be. The content of the other monomer is usually 30% by weight or less, preferably 20% by weight or less, particularly preferably 10% by weight or less, based on the polymer particles. The content of the structural unit derived from the other monomer is proportional to the amount of the other monomer charged in the production of the emulsion of the present invention.

In the present invention, characterized in that the polymer particles (A) is a main component (meth) acrylic resin.
The (meth)acrylic resin is obtained by polymerizing a monomer component containing a (meth)acrylic monomer, and includes, for example, (a) to (m) unsaturated monomers ( It is obtained by polymerizing a monomer component containing one or more monomers having a (meth)acrylic structure.
Of the monomer components constituting the (meth)acrylic resin, the content ratio of the monomer having a (meth)acrylic structure is preferably 10 to 100% by weight, particularly preferably 20 to 95% by weight, The content of the other monomer is preferably 0 to 90% by weight, particularly preferably 5 to 80% by weight.

[Polyvinyl alcohol-based resin (B)]
Examples of the PVA-based resin (B) for stabilizing the dispersion of the polymer particles (A) include publicly known general water-soluble PVA-based resins.
The PVA-based resin (B) is obtained, for example, by polymerizing a vinyl ester-based monomer and saponifying it.

Examples of the vinyl ester-based monomer include vinyl acetate. Further, instead of vinyl acetate, for example, vinyl propionate, vinyl butyrate, vinyl caproate, vinyl caprylate, vinyl caprate, vinyl laurate, vinyl myristate, vinyl palmitate, vinyl stearate, vinyl cyclohexanecarboxylate, piperin. Examples include vinyl acrylate, vinyl octylate, vinyl monochloroacetate, vinyl adipate, vinyl methacrylate, vinyl crotonate, vinyl sorbate, vinyl benzoate, vinyl cinnamate, vinyl trifluoroacetate, etc. From the viewpoint of, vinyl acetate is preferably used.

The vinyl ester-based monomer can be polymerized by any known polymerization method, for example, solution polymerization, suspension polymerization, emulsion polymerization, or the like. Above all, it is preferable to carry out the solution polymerization under reflux so that the heat of reaction can be efficiently removed. As a solvent for solution polymerization, an alcohol is usually used, and a lower alcohol having 1 to 3 carbon atoms is preferably used.

For the saponification of the obtained copolymer, a known saponification method that has been carried out with a conventional PVA-based resin can be adopted. That is, saponification can be performed using an alkali catalyst or an acid catalyst in a state where the polymer is dissolved in alcohol or water/alcohol solvent.
As the alkali catalyst, hydroxides or alcoholates of alkali metals such as potassium hydroxide, sodium hydroxide, sodium methylate, sodium ethylate, potassium methylate and lithium methylate can be used.
Usually, a transesterification reaction using an alkali catalyst in an anhydrous alcohol solvent is preferably used in terms of reaction rate and reduction of impurities such as fatty acid salts.

The reaction temperature of the saponification reaction is usually 20°C to 60°C. If the reaction temperature is too low, the reaction rate tends to be low and the reaction efficiency tends to be low, and if it is too high, it may be higher than the boiling point of the reaction solvent, and the safety in production tends to be lowered. When saponifying under high pressure using a tower-type continuous saponification tower having high pressure resistance, it is possible to saponify at a higher temperature, for example, 80 to 150°C, and a small amount of saponification catalyst is used. However, a high degree of saponification can be obtained in a short time.

The degree of saponification of PVA-based resin (B) (measured in accordance with JIS K6726) is preferably 80 to 100 mol %, particularly preferably 85 to 99.9 mol %, and further preferably 90 to 99 mol %. It is 0.5 mol %. If the saponification degree is too low, the protective colloidal property of the PVA-based resin (B) will be too high, and the emulsion viscosity will be too high, or the cloud point will appear in the PVA-based resin, for example during emulsion polymerization. Polymerization stability of 1 is extremely decreased, and it tends to be difficult to obtain an intended emulsion. On the other hand, if the saponification degree is too low, it tends to swell in the electrolytic solution, and as a result, the binding property between the positive electrode members tends to decrease. A PVA-based resin having a high degree of saponification, particularly a completely saponified product, tends to be industrially difficult to produce.

The viscosity average degree of polymerization (measured according to JIS K6726) is preferably 300 to 3000, particularly preferably 350 to 2800, and further preferably 400 to 2500. If the viscosity average degree of polymerization is too low, the protective colloidal property to the acrylic monomer and the like tends to be reduced, and the flexibility of the film formed from the prepared emulsion is reduced, thereby increasing the flexibility of the positive electrode. Tends to decline. On the other hand, if it is too high, the viscosity of the polymerization reaction solution becomes too high, and it becomes difficult to stir during the polymerization, which tends to make the polymerization difficult. In addition, since the protective colloidal property becomes too high, for example, the dropping monomer during emulsion polymerization becomes difficult to be absorbed in the polymer particles, and the number of new particles derived from the dropping monomer increases, resulting in coarse particles in the emulsion. The amount tends to increase.

The PVA-based resin (B) has a structural unit derived from a monomer other than the vinyl ester-based monomer in a range that does not impair the effects of the present invention (for example, less than 10 mol%, preferably 5 mol% or less). May have.
For example, monomers having a vinyl group and an epoxy group such as glycidyl (meth)acrylate, glycidyl (meth)allyl ether, 3,4-epoxycyclohexyl (meth)acrylate, and allyl glycidyl ether; triallyloxyethylene, diallyl maleate, and triaryloxyethylene. Monomers having two or more allyl groups such as allyl cyanurate, triallyl isocyanurate, tetraallyloxyethane and diallyl phthalate; allyl ester type monomers such as allyl acetate, acetoacetic vinyl ester, acetoacetic allyl ester and diacetoacetic allyl ester. Acetoacetoxyalkyl (meth)acrylates such as acetoacetoxyethyl (meth)acrylate and acetoacetoxypropyl (meth)acrylate; acetoacetoxyalkyl crotonates such as acetoacetoxyethyl crotonate and acetoacetoxypropyl crotonate; 2-cyanoacetoacetoxy Ethyl (meth)acrylate; divinylbenzene; ethylene glycol di(meth)acrylate, 1,2-propylene glycol di(meth)acrylate, 1,3-propylene glycol di(meth)acrylate, 1,4-butanediol di(meth ) Acrylate, 1,6-hexanediol di(meth)acrylate, alkylene glycol (meth)acrylate such as neopentyl glycol di(meth)acrylate; trimethylolpropane tri(meth)acrylate; allyl (meth)acrylate; 2-hydroxy Hydroxyalkyl (meth)acrylates such as ethyl (meth)acrylate, hydroxypropyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate (wherein the alkyl portion is a C1 to C10 alkyl group, preferably a C1 to C6 alkyl group); Nitrile monomers such as (meth)acrylonitrile; styrene monomers such as styrene and α-methylstyrene; olefins such as ethylene, propylene, 1-butene, isobutene; vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride, etc. Halogenated olefins; olefinic monomers such as ethylene sulfonic acid; dienes such as butadiene-1,3,2-methylbutadiene, 1,3 or 2,3-dimethylbutadiene-1,3, 2-chlorobutadiene-1,3 -Based monomers: 3-buten-1-ol, 4-penten-1-ol, 5-hexene-1,2-diol, glycerin monoallyl ether Derivatives of hydroxy group-containing α-olefins such as ter and their acyl compounds; 1,3-diacetoxy-2-methylenepropane, 1,3-dipropionyloxy-2-methylenepropane, 1,3-dibutyronyl Hydroxymethylvinylidene diacetates such as oxy-2-methylenepropane; unsaturated acids such as itaconic acid, maleic acid, acrylic acid, salts thereof or mono- or dialkyl esters; nitriles such as acrylonitrile, methacrylamide, diacetone acrylamide, etc. Compounds such as amides, ethylene sulfonic acid, allyl sulfonic acid, methallyl sulfonic acid, olefin sulfonic acid such as AMPS or salts thereof, vinyltriethoxysilane, vinyltrimethoxysilane, vinyltripropoxysilane, vinyltributoxysilane, Vinylalkyldialkoxysilanes such as vinylmethyldimethoxysilane and vinylmethyldiethoxysilane; γ-(meth)acryloxy such as γ-(meth)acryloxypropyltrimethoxysilane and γ-(meth)acryloxypropyltriethoxysilane. Propyltrialkoxysilane; γ-(meth)acryloxypropylmethyldimethoxysilane, γ-(meth)acryloxypropylmethyldiethoxysilane, and other γ-(meth)acryloxypropylalkyldialkoxysilanes; vinyltris(β-methoxyethoxy) ) Silane and hydroxymethyl vinylidene diacetate. Specific examples of hydroxymethylvinylidene diacetate include 1,3-diacetoxy-2-methylenepropane, 1,3-dipropionyloxy-2-methylenepropane, 1,3-dibutyronyloxy-2-methylenepropane. Etc. These monomers may be used alone or in combination of two or more.

In addition, the PVA-based resin (B) of the present invention is a PVA-based resin modified within a range that does not impair the effects of the present invention (usually 15 mol% or less, preferably 10 mol% or less), for example, PVA-based resin. Formal compound, acetal compound, acetoacetyl compound, butyral compound, urethane compound, ester compound with sulfonic acid or carboxylic acid, etc. may be contained.

In the present invention, it is preferable to use a derivative-modified PVA-based resin such as a hydroxyl group-containing α-olefin and its acylated product as the PVA-based resin (B) contained in the emulsion [I]. For example, as the PVA-based resin (B), it is preferable to use a modified polyvinyl alcohol-based resin (b) containing a structural unit having a primary hydroxyl group in its side chain. The number of primary hydroxyl groups in such a structural unit is usually 1 to 5, preferably 1 to 2, and particularly preferably 1. Further, it is preferable to have a secondary hydroxyl group in addition to the primary hydroxyl group.
Examples of the PVA-based resin containing a structural unit having a primary hydroxyl group in its side chain include, for example, a PVA-based resin having a 1,2-diol structural unit in its side chain and a PVA having a hydroxyalkyl group structural unit in its side chain. Examples include resin based resins. Above all, it is particularly preferable to use a PVA-based resin containing a structural unit having a 1,2-diol structure in a side chain, which is represented by the following general formula (1) (hereinafter, referred to as “side chain 1,2-diol structure”). It may be referred to as a "unit-containing PVA-based resin").

（上記一般式（１）において、Ｒ^１〜Ｒ^６はそれぞれ独立して水素原子又は有機基を表し、Ｘは単結合又は結合鎖を表す。）

(In the general formula (1), R ^{1 to} R ⁶ each independently represent a hydrogen atom or an organic group, and X represents a single bond or a bonded chain.)

In the general formula (1), R ^{1 to} R ⁶ each independently represent a hydrogen atom or an organic group. It is desirable that R ^{1 to} R ⁶ are all hydrogen atoms, but they may be organic groups as long as they do not significantly impair the resin properties. The organic group is not particularly limited, and examples thereof include an alkyl group having 1 to 4 carbon atoms such as a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, and a tert-butyl group. Preferably, it may have a substituent such as a halogen group, a hydroxyl group, an ester group, a carboxylic acid group, or a sulfonic acid group, if necessary.

In the above general formula (1), X is a single bond or a binding chain, and from the viewpoint of electrolytic solution resistance (that is, a property in which swelling by an electrolytic solution is unlikely to occur) due to a reduction in free volume (intermolecular void) in an amorphous part. It is preferably a single bond. The bonding chain is not particularly limited, and examples thereof include hydrocarbons such as alkylene, alkenylene, alkynylene, phenylene, and naphthylene (these hydrocarbons may be substituted with halogen such as fluorine, chlorine, or bromine). _{other, -O -, - (CH 2} O) m -, - (OCH 2) m -, - (CH 2 O) mCH 2 -, - CO -, - COCO -, - CO (CH 2) mCO- _{, -CO (C 6 H 4)} CO -, - S -, - CS -, - SO -, - SO 2 -, - NR -, - CONR -, - NRCO -, - CSNR -, - NRCS -, - _{NRNR -, - HPO 4 -,} - Si (OR) 2 -, - OSi (OR) 2 -, - OSi (OR) 2 O -, - Ti (OR) 2 -, - OTi (OR) 2 -, - Examples thereof include OTi(OR) ₂ O-, -Al(OR)-, -OAl(OR)-, and -OAl(OR)O-. Each R is independently an arbitrary substituent, preferably a hydrogen atom or an alkyl group, and m is a natural number. Of these, an alkylene group having 6 or less carbon atoms, particularly a methylene group, or —CH ₂ OCH ₂ — is preferable from the viewpoint of viscosity stability during production, heat resistance, and the like.

Particularly preferred structure of the 1,2-diol structural unit represented by the general formula (1) is that R ^{1 to} R ⁶ are all hydrogen atoms, and X is a single bond. That is, the structural unit represented by the following structural formula (1a) is particularly preferable.

The side chain 1,2-diol structural unit-containing PVA-based resin can be manufactured by a known manufacturing method. For example, it can be manufactured by the methods described in JP-A-2002-284818, JP-A-2004-285143, and JP-A-2006-95825. That is, (i) a method of saponifying a copolymer of a vinyl ester-based monomer and a compound represented by the following general formula (2), (ii) a vinyl ester-based monomer and vinyl ethylene represented by the following general formula (3) Method for saponifying and decarboxylating copolymer with carbonate, (iii) Copolymerization of vinyl ester monomer and 2,2-dialkyl-4-vinyl-1,3-dioxolane represented by the following general formula (4) It can be produced by a method of saponifying and deketalizing the polymer.

In the general formulas (2), (3) and (4), R ^{1 to} R ⁶ are the same as in the general formula (1). R ⁷ and R ⁸ are each independently hydrogen or R ⁹ —CO— (in the formula, R ⁹ is an alkyl group having 1 to 4 carbon atoms), and R ¹⁰ and R ¹¹ are each independently. And a hydrogen atom or an organic group, and the organic group is the same as in the general formula (1).
Among the above methods, the method (i) is preferable because it is excellent in copolymerization reactivity and industrial handling. Particularly, R ^{1 to} R ⁶ are hydrogen, X is a single bond, and R ⁷ and R ⁸ are R ^9. is -CO-, ^{R 9} preferably is 3,4-diacyloxy-1-butene with an alkyl group, in particular ^{R 9} is preferably used is 3,4-diacetoxy-1-butene with a methyl group therein.

The side chain 1,2-diol structural unit-containing PVA-based resin obtained by the method (ii) or (iii) has a low saponification degree, or when decarboxylation or deacetalization is insufficient. May leave a carbonate ring or an acetal ring in the side chain. When such a PVA-based resin is used as a dispersant, the polymer particles obtained tend to have an increased proportion of coarse particles. For this reason also, the PVA-based resin obtained by the method (i) is particularly suitable for this application.

When the PVA-based resin (B) contains the above-mentioned 1,2-diol structural unit, the content thereof is such that the PVA-based resin is grafted onto the ethylenically unsaturated polymer particles as the dispersoid during emulsion polymerization or the electrolytic solution From the viewpoints of properties, stability of leaving emulsion, etc., it is usually 0.5 to 15 mol %, preferably 1 to 10 mol %, more preferably 1 to 8 mol %. If the content is too small, the grafting ratio of the PVA-based resin onto the ethylenically unsaturated polymer particles that are dispersoids tends to decrease, and the electrolytic solution resistance and emulsion leaving stability tend to decrease. If the content is too large, the internal resistance during charging/discharging tends to increase.
The content of 1,2-diol structural units in the PVA-based resin (B) is ¹ H-NMR spectrum (solvent: DMSO-d6, internal standard: tetramethyl) of the PVA-based resin having a saponification degree of 100 mol %. Silane). Specifically, it can be calculated from the peak areas derived from the hydroxyl group proton, methine proton, and methylene proton in the 1,2-diol structural unit, the methylene proton of the main chain, the proton of the hydroxyl group linked to the main chain, and the like.

The solvent in the emulsion [I] is usually water, and is miscible with water as long as it does not hinder the dissolution of the PVA-based resin (B) (for example, less than 20% by weight, preferably less than 10% by weight of the solvent). It may contain an organic solvent.
Examples of the organic solvent include amide solvents such as N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, and methylformamide, sulfoxides such as dimethylsulfoxide, and lower C1-C3 solvents such as methanol and ethanol. Examples thereof include alcohols and alcohol solvents such as 1,1,1,3,3,3-hexafluoro-2-propanol.

[Synthesis of Emulsion [I] by Emulsion Polymerization Using PVA Resin (B) as Emulsifier]
In this emulsion [I], the PVA resin (B) as the dispersant and the polymer particles (A) derived from the ethylenically unsaturated monomer as the dispersoid are dispersed in an aqueous dispersion medium. There is something. Such an emulsion [I] can be obtained, for example, by emulsion-polymerizing the above-mentioned acrylic monomer (and optionally a monomer other than the acrylic monomer) in the presence of a dispersant.

As a method for carrying out the emulsion polymerization, a) a raw material of a dispersoid, for example, an acrylic monomer (and optionally other monomer) is added in the presence of water, a PVA resin (B) as a dispersant and a polymerization catalyst. A method of emulsion-polymerizing by temporarily or continuously mixing and heating and stirring; b) a vinyl-based monomer, for example, an acrylic-based monomer (and optionally a monomer other than the acrylic-based monomer), a PVA-based resin (B). To prepare a dispersion liquid which is mixed and dispersed in an aqueous solution of, and the prepared dispersion liquid is temporarily or continuously blended in a system in which water, PVA-based resin (B) and a polymerization catalyst are blended, and heated, Examples thereof include a method of stirring and performing emulsion polymerization. The method using the dispersion liquid thus prepared in advance is particularly called a pre-emulsion method. Such a method is preferable because it is possible to carry out emulsion polymerization while maintaining productivity even if the monomer composition to be polymerized is complicated.

The dispersion medium in the reaction solution used for the emulsion polymerization is usually water. If desired, the water-miscible organic solvents mentioned above as the solvent can be used in combination with water. However, from the viewpoint of dispersibility of the monomer used for emulsion polymerization, water alone is preferable.

In particular, in the present invention, in order to make the average particle diameter of the polymer particles (A) within a specific small range, (1) an emulsion dispersion of a surfactant or a water-soluble polymer protective colloid agent added during the preparation of emulsion. The method of controlling the composition and amount of the agent, (2) the method of controlling the addition conditions of the monomer and the polymerization catalyst, (3) the method of controlling the designed non-volatile content of the emulsion and the like can be adopted.
In addition, a method of controlling the size of the mixing and stirring blade of the polymerization apparatus, the stirring speed, the stirring time, and the like can be adopted. Furthermore, a film emulsification method in which a particle diameter is controlled by passing a monomer through a porous film, an ultrasonic emulsification method in which ultrasonic waves are used as a stirring method, or the like can be adopted.

When the PVA-based resin (B) is used as a dispersant during emulsion polymerization, the amount of the PVA-based resin (B) to be blended may vary depending on the type of the PVA-based resin (B) used, the concentration of the emulsion to be synthesized, and the like. It is usually 0.1 to 80% by weight, preferably 10 to 70% by weight, particularly preferably 20 to 60% by weight, based on the whole.
If the amount of PVA-based resin is too small, the emulsified state of the ethylenically unsaturated monomer becomes unstable and the polymerization reactivity decreases, and the emulsion state stability of the particles in the emulsion obtained by polymerization is stable. Tends to decrease. On the other hand, if the content of the PVA-based resin is too large, the viscosity of the reaction solution increases too much, the uniform dispersibility deteriorates, the polymerization rate cannot be increased, or the viscosity of the obtained emulsion becomes too high, resulting in the production. Yield tends to decrease.

As the polymerization catalyst, a polymerization catalyst usually used in the field of emulsion polymerization can be used. For example, potassium persulfate, ammonium persulfate, potassium bromate, sodium acid sulfite, hydrogen peroxide-tartaric acid, hydrogen peroxide-iron salt, hydrogen peroxide-ascorbic acid-iron salt, hydrogen peroxide-Rongalit, hydrogen peroxide- Examples include water-soluble redox-based polymerization catalysts such as Rongalit-iron salt. These may be used alone or in combination of two or more. Specifically, a catalyst composed of an organic peroxide such as "Kayabutyl B" manufactured by Kayaku Akzo Co., Ltd. and "Kayabutyl A-50C" manufactured by the same company and a redox system can be used.

The amount of the polymerization catalyst used is usually 0.01 to 10 parts by weight, preferably 0.05 to 5 parts by weight, particularly preferably 0.1 to 3 parts by weight, based on 100 parts by weight of the monomer used for the polymerization. Is. If the amount of such a polymerization initiator used is too small, the polymerization rate tends to decrease, and conversely, if it is too large, the polymerization stability tends to decrease.
The method of blending the polymerization initiator is not particularly limited, and may be blended in the reaction solution all at once in the initial stage, or may be continuously added as the polymerization progresses.

The emulsion polymerization may be carried out in one step or in a plurality of steps of two or more steps. In particular, in the case of carrying out in two steps, the glass transition point (Tg) of the inner layer formed in the first step and the glass layer (Tg) formed in the second step is changed by changing the charged amount (charge ratio) of the monomer in the first step and the second step. It is possible to change it. Specifically, the following two-stage polymerization can be mentioned.

(1) First-stage polymerization step A reaction vessel containing a dispersion medium and a dispersant is charged with a part of the monomer to be polymerized, and the first-stage emulsion polymerization is carried out. The amount of the monomer charged in the first stage is not particularly limited, but is usually about 1 to 50% by weight, preferably 5 to 30% by weight of the monomer used for the polymerization. The conditions of the first stage emulsion polymerization step can be appropriately determined depending on the type and composition of the monomer used, the amount of the polymerization initiator used, and the like.
The temperature of the emulsion polymerization reaction is usually 30 to 90°C, preferably 40 to 80°C, and the polymerization time is preferably 1 to 4 hours. In the first-stage emulsion polymerization step, the polymerization conversion rate is preferably 30% or more, and particularly preferably 60% or more.

(2) Second-stage polymerization step The second-stage emulsion polymerization is performed by charging the remaining monomers into the reaction vessel in which the first-stage polymerization has been completed. It is preferable that the charging is performed while dropping. A polymerization catalyst may be added during the second-stage polymerization. The second stage emulsion polymerization is carried out under the conditions of a polymerization temperature of 40 to 80° C. and a polymerization time of 1 to 6 hours.
It is also possible to use a power feed polymerization method in which the monomer composition ratio to be dropped is continuously changed and dropped. Further, the polymerization may be carried out while dropping a dispersion liquid prepared by mixing and dispersing the monomers in the presence of a PVA-based resin as a dispersant.
If necessary, it is also possible to carry out the follow-up polymerization usually for 1 to 6 hours after such a step. A polymerization catalyst may be added during such polymerization.

In the emulsion polymerization as described above, a molecular weight modifier may be used if necessary. Specific examples of the molecular weight modifier include alkyl mercaptans such as n-hexyl mercaptan, n-octyl mercaptan, t-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, and n-stearyl mercaptan; dimethylxanthogen disulfide, Xanthogen compounds such as diisopropylxanthogen disulfide; thiuram compounds such as terpinolene, tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetramethylthiuram monosulfide; 2,6-di-t-butyl-4-methylphenol, styrenated phenol, etc. Phenol compounds; allyl compounds such as allyl alcohol; halogenated hydrocarbon compounds such as dichloromethane, dibromomethane, carbon tetrabromide and carbon tetrachloride; α-benzyloxystyrene, α-benzyloxyacrylonitrile, α-benzyloxyacrylamide Vinyl ethers such as triphenylethane, pentaphenylethane, acrolein, methacrolein, thioglycolic acid, thiomalic acid, 2-ethylhexylthioglycolate, α-methylstyrene dimer, carbon tetrachloride and the like. In addition, in an emulsion polymerization process, these molecular weight regulators can be used individually by 1 type or in combination of 2 or more types.

In addition, in the above-mentioned polymerization step, a surfactant such as a nonionic surfactant or anionic surfactant is added in addition to the dispersant contained in advance, as long as the dispersion stabilizing effect of the PVA-based resin (2) is not impaired. They may coexist in the system. The content of such a surfactant is usually 10% by weight or less, preferably 5% by weight or less, based on the whole emulsion polymerization reaction system.

Examples of the nonionic surfactant include polyoxyethylene-alkyl ether type, polyoxyethylene-alkylphenol type, polyoxyethylene-polyhydric alcohol ester type, esters of polyhydric alcohol and fatty acid, oxyethylene/oxypropylene. Block polymers and the like can be mentioned.

Examples of the anionic surfactant include higher alcohol sulfates, higher fatty acid alkali salts, polyoxyethylene alkylphenol ether sulfates, alkylbenzene sulfonates, naphthalene sulfonate formalin condensates, alkyldiphenyl ether sulfonates, dialkylsulfosuccinates. Examples thereof include salts and higher alcohol phosphate ester salts.
Further, a plasticizer such as a phthalic acid ester or a phosphoric acid ester, a pH adjusting agent such as sodium carbonate, sodium acetate, or sodium phosphate may be used in combination.

[Emulsion]
By performing emulsion polymerization as described above, an emulsion [I] in which polymer particles (A) derived from an ethylenically unsaturated monomer are dispersion-stabilized with a polyvinyl alcohol resin (B) can be obtained.

The solid content of the obtained emulsion [I] is usually 10 to 60% by weight, preferably 20 to 58% by weight, particularly preferably 30 to 55% by weight, and further preferably 35 to 53% by weight. Is. As the solid content of the emulsion, a value obtained by measuring the residue obtained by heating and drying at 105° C. for 3 hours in a dryer is adopted.

The viscosity of the obtained emulsion is usually 100 to 20,000 mPa·s, preferably 300 to 10,000 mPa·s, and particularly preferably 450 to 8,000 mPa·s. For the viscosity of the emulsion, the value measured by a B-type viscometer is used.

The emulsion as described above may be directly used for the production of the binder composition for a positive electrode of the present invention, or a water-soluble polymer other than the PVA-based resin (B) in order to adjust the solid content and viscosity of the emulsion. Etc. may be added as appropriate.

Examples of the water-soluble polymer other than the PVA-based resin (B) include methyl cellulose, ethyl cellulose, hydroxymethyl cellulose, hydroxypropyl methyl cellulose, hydroxybutyl methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, aminomethyl hydroxypropyl cellulose, aminoethyl hydroxypropyl cellulose and the like. Derivatives of cellulose; starch, tragacanth, pectin, glue, alginic acid or salts thereof; gelatin; polyvinylpyrrolidone; polyacrylic acid or salts thereof, polymethacrylic acid or salts thereof; acrylamides such as polyacrylamide and polymethacrylamide; vinyl acetate And an unsaturated acid such as maleic acid, maleic anhydride, acrylic acid, acrylic acid, methacrylic acid, itaconic acid, fumaric acid, crotonic acid; a copolymer of styrene and the above unsaturated acid; vinyl ether And a copolymer of the above unsaturated acid; and a salt or ester of the above unsaturated acid and each copolymer, carrageenan, xanthan gum, sodium hyaluronate, locust bean gum, tara gum, guar gum, tamarind seed gum, etc. Examples include sugars.

[Polymer particles (A)]
The average particle diameter of the polymer particles (A) in the present invention is preferably 50 nm or more and 600 nm or less, and particularly preferably 150 to 300 nm. As the average particle diameter of the particles, a value measured by a zeta potential measuring device is adopted.

[Binder composition for positive electrode of lithium ion secondary battery]
The binder composition for a positive electrode of a lithium ion secondary battery of the present invention contains the above emulsion.

As described above, in the binder composition for a positive electrode of the present invention, it is preferable to add a PVA-based resin in addition to the PVA-based resin (B) as a dispersant in the emulsion contained in the binder composition. .. Since the modified PVA-based resin containing a structural unit having a primary hydroxyl group in its side chain has low crystallinity, a modification containing a structural unit having a primary hydroxyl group in its side chain is separately included in the PVA-based resin aqueous solution of the emulsion. The inclusion of the PVA-based resin makes it possible to impart viscosity stability to the water paste, which is the binder composition for the positive electrode, and improve work efficiency.

In the binder composition of the present invention, as a method of setting the amount of the PVA-based resin dissolved in the dispersion medium to a specific range, (i) the PVA-based resin as the dispersant in the emulsion (for convenience, the first PVA-based resin is used). A resin) may be used in a larger amount than usual, or (ii) a PVA-based resin (may be referred to as a second PVA-based resin for convenience) as another component together with the emulsion. , (Iii) a method of using the above (i) and (ii) together, and the like. In the case of the method (ii), the PVA-based resin may be a solid or a solution dissolved in a solvent having an affinity for the dispersion medium of the binder composition, but a solution is preferable.
In any of the methods, the PVA-based resin content is included in the solid content in the binder composition of the present invention.

In the above method (i), the amount of the PVA-based resin used as a dispersant in the emulsion polymerization of the above-mentioned emulsion is usually more than 10% by weight and preferably 80% by weight or less based on the solid content of the emulsion, preferably It can be 20 to 70% by weight, particularly preferably 30 to 60% by weight.

In the above method (ii), as the PVA-based resin (second PVA-based resin) to be added later to the prepared emulsion composition, a known PVA-based resin is used in the same manner as the above-mentioned PVA-based resin as the dispersant. It can be used. Further, the modified PVA-based resin can be used in the second PVA-based resin as well as the PVA-based resin as the dispersant as long as the object of the present invention is not impaired.

In the method (ii), different types of PVA-based resins can be used in addition to the PVA-based resin used for preparing the emulsion.

In the method (ii), a PVA-based resin of a type different from the first PVA-based resin can be used as the second PVA-based resin.
When different types of PVA-based resins are used, the first PVA-based resin and the second PVA-based resin, which are the dispersants of the emulsion used in the present invention, are completely compatible with each other and form a uniform phase.
In addition, particularly in the method (ii), it is preferable that the saponification degree of the first PVA-based resin, which is the dispersant for the emulsion used in the present invention, is similar to the saponification degree of the second PVA-based resin. ..
In such a case, it is considered that the first PVA-based resin can form a stable matrix in the continuous phase of the polymer obtained from the binder composition, so that the strength of the continuous layer is maintained well.

The blending amount of the second PVA-based resin may be the same as the blending amount of the first PVA-based resin in the above method (i), or may be a different blending amount. It is usually 0.1 to 30% by weight, preferably 0.5 to 20% by weight, particularly preferably 1 to 10% by weight.

The second PVA-based resin may be blended when the emulsion is used as a binder composition, or may be blended in the emulsion in advance. In particular, it is preferable from the viewpoint of the dispersion stability of the dispersoid of the emulsion to mix it in advance with the emulsion to prepare an emulsion composition.

The content ratio (A/B) of the polymer particles (A) and the PVA-based resin (B) in the binder composition for a lithium ion secondary battery positive electrode of the present invention is 1/99 to 40 in terms of solid content weight ratio. /60, preferably 5/95 to 40/60, particularly preferably 10/90 to 35/65, and further preferably 15/85 to 30/70.
The content of the polymer particles (A) in the positive electrode binder composition of the present invention is preferably 1 to 40% by weight, more preferably 5 to 40% by weight, and particularly preferably 5 to 35% by solid content. %, more preferably 10 to 35% by weight, particularly preferably 15 to 30% by weight. If the content of the polymer particles (A) is too small, the internal resistance as a binder tends to increase. When the content of the polymer particles (A) is too large, generally the swelling ratio of the electrolytic solution tends to be high, and the heat resistance and the film strength tend to be lowered.
Furthermore, the content of the PVA-based resin (B) in the emulsion [I] of the binder composition for a positive electrode of the present invention is, in terms of solid content, preferably 60 to 99% by weight, particularly preferably 60 to 95% by weight, It is more preferably 65 to 90% by weight, and particularly preferably 70 to 85% by weight.
If the content of the PVA-based resin (B) is too small, heat resistance and oxidation resistance tend to be lowered. On the other hand, if the content of the PVA-based resin (B) is too large, the internal resistance as a binder increases, and the charge/discharge capacity tends to decrease.

<Other ingredients>
The binder composition of the present invention can be blended with a coating material usually used for a coating film and a compounding agent used for a molding resin. For example, light stabilizers, UV absorbers, thickeners, leveling agents, thixotropic agents, defoamers, freeze stabilizers, matting agents, crosslinking reaction catalysts, pigments, curing catalysts, crosslinking agents {boric acid, methylolation Melamine, zirconium carbonate, diisopropoxytitanium bistriethanolaminate, etc.}, anti-skinning agent, dispersant, wetting agent, antioxidant, ultraviolet absorber, rheology control agent, film forming aid, rust preventive agent, dye, Examples include plasticizers, lubricants, reducing agents, antiseptics, antifungal agents, deodorants, anti-yellowing agents, antistatic agents, and charge control agents. It can be selected or combined according to the purpose of each. When the binder composition contains these compounding agents, the organic content of the compounding agent contained is included in the solid content of the binder composition.
The compounding amount of the compounding agent is usually less than 10 parts by weight, preferably less than 5 parts by weight, based on 100 parts by weight of the solid content of the emulsion in the binder composition.

[Preparation of slurry for positive electrode: manufacture of positive electrode]
The positive electrode binder composition of the present invention and the active material can be mixed to prepare a lithium ion secondary battery positive electrode slurry.

As such an active material, it is possible to use a well-known general active material used for a positive electrode of a lithium ion secondary battery.
As the positive electrode active material, for example, olivine-type lithium iron phosphate, lithium cobalt oxide, lithium manganate, lithium nickel oxide, ternary nickel lithium cobalt manganese oxide, lithium nickel cobalt aluminum composite oxide, or the like can be used. ..
The content of the active material in the slurry is 10 to 95% by weight, preferably 20 to 80% by weight, and particularly preferably 35 to 65% by weight.

The average particle diameter of the active material is usually 1 to 100 μm, preferably 1 to 50 μm, and particularly preferably 1 to 25 μm. As the average particle diameter of the active material, a value measured by a laser diffraction particle size distribution measurement (laser diffraction scattering method) is adopted.

The content ratio of the active material and the binder composition in the positive electrode slurry is usually 0.1 to 10 parts by weight based on 100 parts by weight of the active material, based on the solid content of the polymer particles (A) contained in the binder composition. Parts, preferably 0.1 to 5 parts by weight, particularly preferably 0.1 to 4 parts by weight. When the content of the binder composition for the positive electrode is too large, the internal resistance tends to increase. On the other hand, if the amount is too small, the desired binding force between the active materials and the adhesive force to the current collector cannot be obtained, the positive electrode becomes unstable, and the charge/discharge cycle characteristics tend to deteriorate.

The positive electrode slurry may contain other substances in addition to the above active material and the positive electrode binder composition. For example, a conductive aid, a supporting salt (lithium salt), etc. may be included. The compounding ratio of these components is within a known general range. The compounding ratio can also be adjusted by appropriately referring to known knowledge about lithium-ion secondary batteries.

The conductive additive refers to a compounded compound to improve conductivity. Examples of the conductive aid include carbon powder such as graphite, various carbon fibers such as vapor grown carbon fiber (VGCF (registered trademark)) and super gloss nanotube. When it is necessary to further enhance the conductivity of the binder as a result of various blending in the production of the lithium ion secondary battery positive electrode of the present invention, it is preferable to blend a conductive aid, and VGCF (registered trademark) is used as the conductive aid. When (1) is used, the active material is effectively used, and the decrease in charge/discharge capacity due to the use of a large amount of the binder can be suppressed. At this time, the blending amount of VGCF (registered trademark) is preferably 1 to 10% by weight based on the total mass of the active material layer.

Further, in consideration of workability during production of the positive electrode, a solvent may be added to prepare a positive electrode slurry for the purpose of adjusting the viscosity, adjusting the solid content of the binder composition, and the like. As such a solvent, the same organic solvent as described above can be used.

To the positive electrode slurry, a thickener is added separately from the binder composition for the purpose of improving the dispersibility of the active material or the positive electrode binder composition and the conductive additive, or improving the leveling property during coating. Good. The type of the thickener is not particularly limited, but mainly a water-soluble polymer is preferable because of its miscibility with PVA-based resin and the fact that water is preferably used as the dispersion medium of the emulsion composition. Used for.
Examples of the water-soluble polymer include cellulose derivatives such as methyl cellulose, ethyl cellulose, hydroxymethyl cellulose, hydroxypropyl methyl cellulose, hydroxybutyl methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, aminomethyl hydroxypropyl cellulose, aminoethyl hydroxypropyl cellulose; starch, tragacanth. , Pectin, glue, alginic acid or a salt thereof; gelatin; polyvinylpyrrolidone; polyacrylic acid or a salt thereof, polymethacrylic acid or a salt thereof; polyacrylamide, acrylamides of polymethacrylamide sugars; vinyl acetate and maleic acid, maleic anhydride, Copolymers of unsaturated acids such as acrylic acid, acrylic acid, methacrylic acid, itaconic acid, fumaric acid, crotonic acid; copolymers of styrene and the above unsaturated acids; copolymerization of vinyl ether and the above unsaturated acids And a salt or ester of the above-mentioned unsaturated acid and each copolymer, carrageenan, xanthan gum, sodium hyaluronate, locust bean gum, tara gum, guar gum, tamarind seed gum and other natural polysaccharides.
The amount of the thickener used in the positive electrode slurry is usually 0.01 to 5% by weight, preferably 0.1 to 3% by weight, and particularly preferably 0.5 to 2 based on the solid content of the positive electrode slurry. % By weight. If the amount used in the slurry is too small, the dispersion stability of the active material, the binder for the positive electrode, the conductive additive, and the like becomes poor, and the positive electrode tends to be non-uniform and stable charge/discharge cannot be obtained. On the other hand, if the amount is too large, the viscosity of the positive electrode slurry tends to be too high, which tends to make it difficult to uniformly coat the current collector when the positive electrode is prepared. The internal resistance of the battery tends to increase and the charge/discharge capacity tends to decrease.

The binder composition for the positive electrode, the active material, and the compounding agent and the solvent used as necessary can be mixed with a stirrer, a defoaming machine, a bead mill, a high-pressure homogenizer, or the like. In addition, the preparation of the positive electrode slurry is preferably performed under reduced pressure. This can prevent bubbles from being generated in the obtained active material layer.

The lithium ion secondary battery positive electrode of the present invention (hereinafter sometimes abbreviated as “the positive electrode of the present invention”) by applying and drying the positive electrode slurry prepared as described above on a current collector. Can be manufactured. It is preferable to increase the density by pressing after application, if necessary.

As the current collector used for the positive electrode of the present invention, the current collector used for the positive electrode of the lithium ion secondary battery can be used. Specific examples thereof include metal materials such as aluminum, copper, nickel, tantalum, stainless steel, and titanium, which can be appropriately selected and used according to the type of the intended electricity storage device.

A positive electrode layer can be formed by applying and drying a positive electrode slurry on such a current collector. Examples of the method for applying the positive electrode slurry to the current collector include a doctor blade method, a reverse roll method, a comma bar method, a gravure method, and an air knife method. The conditions for drying the coating film of the positive electrode slurry are usually 20 to 250°C, and preferably 50 to 150°C. The treatment time is usually 1 to 120 minutes, preferably 5 to 60 minutes.

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

The electrolytic solution swelling rate of the positive electrode binder composition of the present invention in the obtained positive electrode is usually 10% or less, preferably 7% or less, particularly preferably, although it depends on the kind and composition of the ethylenically unsaturated monomer. Is 0.1 to 5%.
When the electrolytic solution swelling rate is in the above range, the positive electrode binder composition of the present invention swells moderately with respect to the electrolytic solution, effectively lowers the internal resistance, and tends to realize better charge/discharge characteristics. There is. Further, by keeping the swelling of the binder composition for a positive electrode of the present invention in the electrolytic solution within a certain range, it becomes possible to suppress an increase in resistance during long-term charging/discharging. It is presumed that this is because by suppressing contact between the positive electrode active material and the electrolytic solution, it is possible to suppress an increase in battery resistance caused by oxidative decomposition of the electrolytic solution.

The electrolytic solution swelling ratio refers to a value measured as follows, for example.
The acrylic emulsion prepared as the binder composition for the positive electrode was cast on a PET film using a 500 μm applicator and then dried by heating in a dryer at 105° C. for 3 hours to obtain a film. The obtained film is cut into a predetermined size and the weight thereof is measured (W ₀ (g)). This film is immersed in 10 g of propylene carbonate (PC) or a 3/7 mixed solution (volume ratio) of ethylene carbonate/dimethyl carbonate (EC/DMC) and heated at 60° C. for 3 hours. After cooling to room temperature, the film was taken out, the electrolytic solution adhering to the film surface was wiped off, and the swelling ratio of the electrolytic solution was calculated from the weight after immersion (W ₁ (g)) after the test according to the following formula.
Electrolyte swelling rate (%)=((W ₁ −W ₀ )/W ₀ )×100

Since the positive electrode obtained by using the binder composition for a lithium ion secondary battery positive electrode of the present invention has improved flexibility and oxidation resistance as compared with the conventional positive electrode, the mixture layer is cracked, and the active material is peeled or dropped. The effect that oxidative decomposition of the electrolyte solution is less likely to occur is obtained.

[Lithium-ion secondary battery]
A lithium ion secondary battery having a positive electrode prepared by using the binder composition for a lithium ion secondary battery positive electrode of the present invention will be described.
The lithium ion secondary battery has at least a positive electrode, a negative electrode, an electrolytic solution, and a separator.

The negative electrode can be produced in the same manner as the above-described positive electrode, and for example, a negative electrode slurry containing a general binder composition, a negative electrode active material, and a compounding agent used if necessary is a current collector. It can be formed by coating on top and drying.
A carbon material is preferable as the negative electrode active material. Examples of the carbon 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. Other than that, it is preferable to use lithium metal, lithium-containing metal composite oxide, carbon powder, silicon powder, tin powder, or a mixture thereof.
As the current collector of the negative electrode, for example, a metal foil such as copper or nickel, an etching metal foil, or an expanded metal is used.

An aprotic polar solvent that dissolves a lithium salt is used as the electrolytic solution. Although not particularly limited, a cyclic carbonate ester-based high-dielectric constant/high-boiling point solvent such as ethylene carbonate or propylene carbonate containing a lower chain carbonate such as dimethyl carbonate, ethylmethyl carbonate or diethyl carbonate, which is a low-viscosity solvent. Used. Specifically, ethylene carbonate, chloroethylene carbonate, trifluoropropylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, isopropyl methyl carbonate, ethyl propyl carbonate, isopropyl ethyl carbonate, butyl methyl carbonate, Butylethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 1,3-dioxolane, methyl acetate, acetic acid Examples thereof include ethyl, methyl formate and ethyl formate, and it is preferable to use them in a mixture.

Examples of the lithium salt of the electrolyte include inorganic salts such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCl and LiBr, LiCF ₃ SO ₃ , LiN(SO ₂ CF ₃ ) ₂ and LiN(SO ₂ C ₂ F _5). ) ₂ , LiC(SO ₂ CF ₃ ) ₃ , LiN(SO ₃ CF ₃ ) _{2 and} other organic salts that are commonly used as the electrolyte of the non-aqueous electrolyte may be used. Among these, it is preferable to use LiPF ₆ , LiBF _4, or LiClO ₄ .

The separator is not particularly limited, and a nonwoven fabric of polyolefin or a porous film, a glass filter, a film made of polyaramid, or a nonwoven fabric made of PVA-based resin can be used.

The structure of the secondary battery is not particularly limited, and may be applied to any conventionally known form and structure such as a laminated (flat) battery and a wound (cylindrical) battery. The electrical connection form (electrode structure) in the lithium-ion secondary battery can be applied to both (internal parallel connection type) battery and bipolar (internal series connection type) battery.

The lithium ion secondary battery obtained as described above is improved in flexibility of the electrode based on the use of the binder composition for an electrode of the present invention, cracking of the mixture layer, peeling/falling of the active material. Less likely to occur. In addition, since the oxidation resistance is also improved, the initial discharge capacity is high, and a high charge/discharge capacity can be stably obtained.

Further, in order to suppress the internal resistance of the battery, a method of controlling the particle size of the dispersoid in the emulsion may be used. The binder composition for a lithium-ion secondary battery electrode of the present invention comprises an emulsion in which polymer particles derived from an ethylenically unsaturated monomer are dispersed in a PVA-based resin aqueous solution, thereby making the binder composition In the obtained electrode, the surface area of the PVA-based resin matrix in the binder is large and the thickness of the PVA layer is thin. As a result, the internal resistance of the battery is reduced and the safety and durability of the battery are improved while maintaining the electrolytic solution resistance and oxidation resistance peculiar to the PVA-based resin. The reason why these effects are obtained is that the number of graft-forming products formed during emulsion polymerization at the interface between the polymer particles derived from an ethylenically unsaturated monomer and the PVA-based matrix increases It is considered that this is because the dispersed particle size of the polymer particles derived from the unsaturated monomer is further reduced, the particle size distribution is made uniform, and the internal resistance can be reduced.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded. In the examples, “part” means weight basis. In addition, “Vol” in the examples means a volume ratio.

[Analysis method]
The PVA-based resins produced in the following examples and comparative examples were analyzed by the following method.

(1) Degree of saponification (mol%)
The residual vinyl acetate and the consumption of alkali required for the hydrolysis of 3,4-diacetoxy-1-butene were analyzed.

(2) Viscosity average degree of polymerization Measured according to JIS K 6726.

(3) Content of side chain 1,2-diol structural unit (modification amount) (mol %)
It was measured by ¹ H-NMR (400 MHz, proton NMR, solvent: heavy aqueous solution, temperature: 50° C.) by using AVANCEIIIHD 400 manufactured by BRUKER, and calculated from the integrated value based on the obtained NMR chart.

[Measurement evaluation method]
The battery characteristics of the binders prepared in the following Examples and Comparative Examples were evaluated by the following methods.

(1) Battery characteristics The produced cell was left at 25° C. for 50 hours and then subjected to a charge/discharge test.
A constant current charge/discharge test was performed in a potential range of 2.7 to 4.2 V with a current density of 30 mA/g, and the initial discharge capacity (mAh/g) and Coulombic efficiency (%) were measured. After that, the current density was 75 mA/g, the potential range was 2.7 to 4.2 V, the temperature at the time of measurement was 50° C., the constant current charge/discharge test was continuously performed, and the capacity retention rate after the high temperature long-term cycle was measured.

(2-1) Initial discharge capacity (mAh/g)
A value obtained by dividing the product of the constant current value (control current value (mA)) at the time of the first discharge and the time (h) until reaching the set potential by the weight (g) of the electrode active material (positive electrode active material). (MAh/g).
When the absolute value of the initial discharge capacity is 140 (mAh/g) or more, excellent (⊚), 130 (mAh/g) or more, and less than 140 (mAh/g) is good (◯), less than 130 (mAh/g). It was evaluated as poor (x).

(2-2) First coulombic efficiency (%)
The initial coulombic efficiency was defined as the percentage (%) obtained by dividing the initial discharge capacity (mAh/g) by the initial charge capacity (mAh/g).
The initial coulombic efficiency was evaluated as excellent (⊚) when it was 90% or more, good (∘) when it was 80% or more and less than 90%, and poor (x) when it was less than 80%.

(2-3) High temperature long-term cycle characteristics Current density of 75 mA/g, discharge capacity (mAh/g) after the first time at 50° C. cycle divided by initial discharge capacity (mAh/g) was taken as the capacity retention rate. The higher the capacity retention rate, the better the cycle characteristics.
The absolute value of the capacity retention rate should be closer to 100%.
A capacity retention rate of 40% or more was evaluated as excellent (⊚) 20% or more, and less than 40% was evaluated as good (◯), and less than 20% was evaluated as poor (x).

[Polymerization Example 1: Polymerization of base emulsion]
In a separable flask equipped with a reflux condenser, a stirrer, a dropping funnel, and a thermometer, 969.3 parts of water as a dispersion medium and a side chain 1,2-diol structure having a structure represented by the above structural formula (1a) as a dispersant. PVA-based resin containing units (saponification degree: 99.1 mol%, viscosity average degree of polymerization: 470, 1,2-diol structural unit content shown in the structural formula (1a): 6 mol%) 172.8 Parts and 0.57 parts of sodium acetate were poured in, and the mixture was dissolved at 95°C for 2 hours with stirring, and then the temperature in the flask was cooled to 75°C.
In this warm bath, 27.0 parts of a mixed monomer (mixing weight ratio: BA/MMA=70/30) of butyl acrylate (BA) and methyl methacrylate (MMA) was polymerized as a first-stage emulsion polymerization monomer. As an initiator, 5.40 parts of an aqueous solution of sodium hydrogen sulfite (5% by weight) and 17.82 parts of an aqueous solution of ammonium persulfate (1% by weight) were added to start emulsion polymerization of the first stage. Polymerization was carried out for 1 hour while maintaining the reaction temperature at 75°C to 80°C.

Next, the second stage emulsion polymerization was performed. While maintaining the temperature of the reaction system in which the first-stage emulsion polymerization was carried out in the range of 75 to 80° C., 243.0 parts of a mixed monomer having the same composition as the above was used as the second-stage emulsion polymerization monomer for 3 hours and a half. Was dropped. During this dropping, 35.64 parts of an aqueous solution of ammonium persulfate (1% by weight) was divided into 14 and blended every 15 minutes. Then, the polymerization was continued for 90 minutes while maintaining the temperature at 75°C. During this period, 5.94 parts of an aqueous solution of ammonium persulfate (1% by weight) was divided into two and blended every 45 minutes.
After the second-stage emulsion polymerization, the reaction temperature was lowered to 50° C. and the polymerization was carried out for 1 hour. During this forced-in polymerization, 2.7 parts of an aqueous t-butylhydroperoxy solution (10% by weight) and 3.2 parts of an aqueous L-ascorbic acid solution (10% by weight) were each divided into two and blended every 30 minutes. Then, the mixture was cooled to room temperature, and a mixed composition of butyl acrylate (BA) and methyl methacrylate (MMA) was added to an aqueous PVA-based resin solution containing a side chain 1,2-diol structural unit having a structure represented by the structural formula (1a). An emulsion in which the polymer particles of 1 were dispersed was obtained. The solid content in this emulsion was 29.7% by weight.

[Polymerization Example 2: Polymerization of Base Emulsion]
In a separable flask equipped with a reflux condenser, a stirrer, a dropping funnel, and a thermometer, 1016.0 parts of water as a dispersion medium, and a side chain 1,2-diol structure having a structure represented by the above structural formula (1a) as a dispersant. 256.0 PVA-based resin containing units (saponification degree: 99.1 mol%, viscosity average polymerization degree: 1200, 1,2-diol structural unit content shown in the structural formula (1a): 6 mol%) Parts and 0.42 parts of sodium acetate were poured in, and the mixture was dissolved at 95°C for 2 hours with stirring, and then the temperature in the flask was cooled to 75°C.
In this warm bath, 20.0 parts of a mixed monomer (mixing weight ratio: BA/MMA=70/30) of butyl acrylate (BA) and methyl methacrylate (MMA) was polymerized as a first-stage emulsion polymerization monomer. As an initiator, 4.00 parts of an aqueous solution of sodium hydrogen sulfite (5% by weight) and 13.20 parts of an aqueous solution of ammonium persulfate (1% by weight) were added to start the first stage emulsion polymerization. Polymerization was carried out for 1 hour while maintaining the reaction temperature at 75°C to 80°C.

Next, the second stage emulsion polymerization was performed. While maintaining the temperature of the reaction system in which the first-stage emulsion polymerization was carried out in the range of 75 to 80° C., 180.0 parts of a mixed monomer having the same composition as the above was used as the second-stage emulsion polymerization monomer for 3 hours and a half. Was dropped. 26.40 parts of an aqueous solution of ammonium persulfate (1% by weight) was divided into 14 portions and compounded every 15 minutes during the dropping. Then, the polymerization was continued for 90 minutes while maintaining the temperature at 75°C. During this period, 4.40 parts of an aqueous solution of ammonium persulfate (1% by weight) was divided into two and blended every 45 minutes.
After the second-stage emulsion polymerization, the reaction temperature was lowered to 50° C. and the polymerization was carried out for 1 hour. During such forced-in polymerization, 2.0 parts of an aqueous t-butylhydroperoxy solution (10% by weight) and 2.4 parts of an aqueous L-ascorbic acid solution (10% by weight) were each divided into two and blended every 30 minutes. Then, the mixture is cooled to room temperature, and a mixed composition of butyl acrylate (BA) and methyl methacrylate (MMA) is added to an aqueous PVA-based resin solution containing a side chain 1,2-diol structural unit having a structure represented by the structural formula (1a). An emulsion in which the polymer particles of 1 were dispersed was obtained. The solid content in this emulsion was 30.6% by weight.

[Polymerization Example 3: Polymerization of Base Emulsion]
In a separable flask equipped with a reflux condenser, a stirrer, a dropping funnel, and a thermometer, 969.3 parts of water as a dispersion medium and a side chain 1,2-diol structure having a structure represented by the above structural formula (1a) as a dispersant. PVA-based resin containing units (saponification degree: 99.1 mol%, viscosity average degree of polymerization: 300, 1,2-diol structural unit content rate shown in the structural formula (1a): 8 mol%) 172.8 Parts and 0.57 parts of sodium acetate were poured in, and the mixture was dissolved at 95°C for 2 hours with stirring, and then the temperature in the flask was cooled to 75°C.
In this warm bath, 27.0 parts of a mixed monomer (mixing weight ratio: BA/MMA=70/30) of butyl acrylate (BA) and methyl methacrylate (MMA) was polymerized as a first-stage emulsion polymerization monomer. As an initiator, 5.40 parts of an aqueous solution of sodium hydrogen sulfite (5% by weight) and 17.82 parts of an aqueous solution of ammonium persulfate (1% by weight) were added to start emulsion polymerization of the first stage. Polymerization was carried out for 1 hour while maintaining the reaction temperature at 75°C to 80°C.

Next, the second stage emulsion polymerization was performed. While maintaining the temperature of the reaction system in which the first-stage emulsion polymerization was carried out in the range of 75 to 80° C., 243.0 parts of a mixed monomer having the same composition as the above was used as the second-stage emulsion polymerization monomer for 3 hours and a half. Was dropped. During this dropping, 35.64 parts of an aqueous solution of ammonium persulfate (1% by weight) was divided into 14 and blended every 15 minutes. Then, the polymerization was continued for 90 minutes while maintaining the temperature at 75°C. During this period, 5.94 parts of an aqueous solution of ammonium persulfate (1% by weight) was divided into two and blended every 45 minutes.
After the second-stage emulsion polymerization, the reaction temperature was lowered to 50° C. and the polymerization was carried out for 1 hour. During this forced-in polymerization, 2.7 parts of an aqueous t-butylhydroperoxy solution (10% by weight) and 3.2 parts of an aqueous L-ascorbic acid solution (10% by weight) were each divided into two and blended every 30 minutes. Then, the mixture was cooled to room temperature, and a mixed composition of butyl acrylate (BA) and methyl methacrylate (MMA) was added to an aqueous PVA-based resin solution containing a side chain 1,2-diol structural unit having a structure represented by the structural formula (1a). An emulsion in which the polymer particles of 1 were dispersed was obtained. The solid content in this emulsion was 29.7% by weight.

[Production Example 1: Production of binder solution]
The base emulsion prepared in Polymerization Example 1 was collected, purified water for dilution, and a PVA-based resin containing a side chain 1,2-diol structural unit having a structure represented by the structural formula (1a) (saponification). Degree: 99.1 mol %, viscosity average degree of polymerization: 470, and 10 wt% aqueous solution of 1,2-diol structural unit content shown in the structural formula (1a): 6 mol %). A wt% binder solution was prepared. At this time, the content ratio (A/B) of the polymer particles (A) and the PVA-based resin (B) in the solid content of the binder was adjusted so that the weight ratio of the solid content was 20/80.

[Production Example 2: Production of binder solution]
The base emulsion prepared in the above Polymerization Example 2 was separated and purified water for dilution, and a PVA-based resin (saponified resin) containing a side chain 1,2-diol structural unit having a structure represented by the above structural formula (1a). Degree: 99.1 mol %, viscosity average degree of polymerization: 1200, and 1,2-diol structural unit content shown in the above structural formula (1a): 6 mol%) in an amount of 10 wt% aqueous solution. A wt% binder solution was prepared. At this time, the content ratio (A/B) of the polymer particles (A) and the PVA-based resin (B) in the solid content of the binder was adjusted so that the weight ratio of the solid content was 20/80.

[Production Example 3: Production of binder solution]
The base emulsion prepared in Polymerization Example 3 was collected and purified water for dilution, and a PVA-based resin (saponified resin) containing a side chain 1,2-diol structural unit having a structure represented by the above structural formula (1a). Degree: 99.1 mol %, viscosity average degree of polymerization: 300, and 1,2 diol structural unit content shown in the structural formula (1a): 8 mol%) added to a 10 wt% aqueous solution to give 10 A wt% binder solution was prepared. At this time, the content ratio (A/B) of the polymer particles (A) and the PVA-based resin (B) in the solid content of the binder is two types of 88/12 and 44/56 in the weight ratio of the solid content. I adjusted it to be.

[Battery preparation]
Using the positive electrode binder prepared in the above Production Examples 1 to 3, a positive electrode slurry liquid was prepared as described below to prepare a lithium ion secondary battery positive electrode and then a lithium ion secondary battery. The battery characteristics of the manufactured battery were measured and evaluated according to the above-described evaluation method. The results are shown in Table 1.

[Example 1]
[Preparation of positive electrode for lithium-ion secondary battery]
<Production of battery positive electrode using positive electrode active material>
94 parts of LiNiMnCoO ₂ (manufactured by Nippon Kagaku Kogyo Co., Ltd., average particle size: 10 μm) as an active material, 3 parts of acetylene black (“Denka Black” manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conduction aid, and 1. 1.5 parts by weight of carboxymethylcellulose #2200 (manufactured by Daicel FineChem Co., Ltd.) prepared in a 5% by weight aqueous solution was added to the solid content, and purified water was added at the appropriate time. Rentaro”) was mixed to obtain a paste having a solid content concentration of 66.3% by weight (after mixing at 2000 rpm for 8 minutes, defoaming was further performed at 2200 rpm for 0.5 minutes).
In the obtained paste, 1.5 parts of the binder solution (25% by weight) prepared in Production Example 1 as a binder for the positive electrode was added in solid content conversion, and purified water was added at appropriate times, and then a planetary kneader was further added. By using and mixing under the same conditions, an active material paste having a solid content concentration of 55.9% by weight was obtained.
Next, using a 180 μm applicator and a coating machine (“Control coater (coating machine)” manufactured by Imoto Machinery Co., Ltd.) on the surface of a rolled aluminum foil (foil made by corporation, thickness 18 μm) as a current collector. Then, the active material paste was applied at a coating speed of 10 mm/sec. This was dried at 60° C. for 10 minutes to obtain a battery positive electrode.

(Charge/discharge test)
A 2032 type coin cell was used as the outer layer of the battery for evaluation. The obtained battery positive electrode was punched into a size of 11 mm in diameter, further vacuum dried at 80° C. for 24 hours or more, and then charged into a glove box. The prepared battery positive electrode was used as a working electrode, a metal lithium negative electrode (diameter 13 mm) was used as a counter electrode, and a separator (diameter 18 mm) made of a polypropylene porous film having a thickness of 16 μm was interposed so that the electrodes were arranged to face each other. It was A solvent in which ethylene carbonate and diethylene carbonate were mixed at a volume ratio of 3:7 was used as an electrolytic solution, and LiPF ₆ dissolved in a concentration of 1 mol/liter was used as an electrolyte. The outer layer container was covered with a stainless steel cap via a polypropylene packing and fixed, and the battery can was sealed to form a half cell, and a battery for evaluation was prepared.

[Example 2]
A positive electrode and a battery were prepared in the same manner as in Example 1 except that the binder solution prepared in Production Example 2 was used as the positive electrode binder, and the battery performance was evaluated.

[Comparative Example 1]
In the binder solution prepared in Production Example 3 as the binder for the positive electrode, a binder solution in which the content ratio (A/B) of the polymer particles (A) and the PVA-based resin (B) in the solid content is 44/56 is used. A positive electrode and a battery were manufactured in the same manner as in Example 1 except that the battery performance was evaluated.

[Comparative Example 2]
Among the binder solutions prepared in Production Example 3 as the binder for the positive electrode, a binder solution in which the content ratio (A/B) of the polymer particles (A) and the PVA-based resin (B) in the solid content is 12/88 is used. A positive electrode and a battery were manufactured in the same manner as in Example 1 except that the battery performance was evaluated.

From the results shown in Table 1, the present invention in which the content ratio (A/B) of the polymer particles (A) and the polyvinyl alcohol-based resin (B) is 5/95 to 40/60 in terms of solid content weight ratio. It can be seen that in the battery using the binder composition for a positive electrode, the initial discharge capacity is high and the charge/discharge cycle characteristics at a high temperature of 50° C. are good.

Claims (6)

A binder composition for a lithium ion secondary battery positive electrode, comprising an emulsion [I] in which polymer particles (A) derived from an ethylenically unsaturated monomer are dispersion-stabilized with a polyvinyl alcohol resin (B).
Content of the polymer particles (A) and the polyvinyl alcohol-based resin (B) (A / B) is at a weight ratio of solids, Ri 1/99 to 40/60 der,
Polymer particles (A) (meth) lithium ion secondary battery positive electrode binder composition characterized Rukoto such as the main component an acrylic resin. The binder composition for a positive electrode of a lithium secondary battery according to claim 1, wherein the polyvinyl alcohol resin (B) has a viscosity average degree of polymerization of 300 to 3,000. The binder for a lithium secondary battery positive electrode according to claim 1 or 2, wherein the polyvinyl alcohol-based resin (B) is a modified polyvinyl alcohol-based resin (b) containing a structural unit having a primary hydroxyl group in a side chain. Composition. The binder composition for a lithium secondary battery positive electrode according to claim 3 , wherein the structural unit having a primary hydroxyl group in the side chain is a structural unit having a 1,2-diol structure in the side chain. Claim 1-4 lithium ion secondary battery positive electrode characterized in that it is obtained by using the lithium secondary battery positive electrode binder composition of any one. A lithium ion secondary battery having the lithium ion secondary battery positive electrode according to claim 5 .