JP6733474B2 - 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, a binder that is used together with an active material to form a positive electrode of a lithium-ion secondary battery and that has good charge-discharge cycle characteristics at room temperature and at low temperature when a lithium-ion secondary battery is formed. The present invention relates to a composition, 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.

On the other hand, a lithium-ion secondary battery having a high energy density is used as a drive power source for a vehicle such as an electric vehicle or a hybrid vehicle, and thus requires high output characteristics. In particular, automobiles are required to have a high load input/output in a short time even in a cold region, and therefore high input/output characteristics are required not only at room temperature but also at low temperature.
However, the non-aqueous electrolyte solution used in the lithium ion secondary battery may cause a significant decrease in ionic conductivity due to an increase in viscosity at low temperatures. Therefore, improving the input/output characteristics of the lithium-ion secondary battery at low temperatures is an important issue when it is used as a drive power source for automobiles such as electric vehicles and hybrid vehicles.

Therefore, the present invention, under such a background, when the lithium ion secondary battery is formed, the binder composition for a lithium ion secondary battery positive electrode, which has good cycle characteristics of charge and discharge at room temperature and at a low temperature, It is an object of the present invention to provide a lithium ion secondary battery positive electrode obtained by using a 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, a polyvinyl alcohol-based resin (A) and a styrene-based thermoplastic elastomer (B) having a predetermined average saponification degree were used as a lithium ion secondary battery positive electrode. The latex [I] containing the polyvinyl alcohol resin (A) and the styrene thermoplastic elastomer (B) is preferably used for the binder composition for a lithium ion secondary battery positive electrode by using the same for the binder composition for a lithium ion secondary battery. According to the above, it was found that, when a lithium ion secondary battery is formed, the charge/discharge cycle characteristics at room temperature and at low temperature are improved, and the present invention has been completed.

That is, the gist of the present invention, Ri greens contain an average degree of saponification 60-90 mol% of a polyvinyl alcohol-based resin (A) and styrene-based thermoplastic elastomer (B), and polyvinyl alcohol-based resin (A) styrene content of the thermoplastic elastomer (B) (a / B) is a lithium ion secondary battery positive electrode binder composition, characterized in 90 / 10-60 / 40 der Rukoto in weight ratio, preferably Ri Na contain an average degree of saponification 60-90 mol% of a polyvinyl alcohol-based resin (a) and styrene-based thermoplastic elastomer (B), a polyvinyl alcohol-based resin (a) and the styrene-based thermoplastic elastomer (B) the content of (a / B) is, to a lithium ion secondary battery positive electrode binder composition comprising a 90 / 10-60 / 40 der Ru latex at a weight ratio of [I].

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".

INDUSTRIAL APPLICABILITY The binder composition for a positive electrode of a lithium ion secondary battery of the present invention is a PVA-based one while ensuring the binding property between active materials or between an active material and a current collector when a lithium ion secondary battery is formed. Since the resin exhibits a moderate affinity with the electrolytic solution, particularly with the organic electrolytic solution, it is possible to suppress the decrease in ionic conductivity at room temperature and at low temperature, and therefore charge and discharge at room temperature and at low temperature. It is possible to obtain a lithium-ion secondary battery having excellent cycle characteristics of.

In the present invention, the binder is present on the interface between the active material and the electrolytic solution and has a great influence on the battery resistance in the lithium ion battery. This has a more pronounced effect at low temperatures when the viscosity of the electrolytic solution increases and the Li ion conductivity significantly decreases. In the binder composition containing the PVA-based resin (A) and the styrene-based thermoplastic elastomer (B) used in the present invention, since the PVA-based resin (A) has an appropriate affinity with the organic electrolytic solution, It does not hinder the movement of Li ions even at low temperatures. Therefore, it becomes possible to exhibit good charge/discharge cycle characteristics under normal temperature and low temperature.

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 binder composition for a lithium ion secondary battery positive electrode of the present invention comprises a PVA-based resin (A) having an average saponification degree of 60 to 90 mol% and a styrene-based thermoplastic elastomer (B), Preferably, the latex [I] containing the PVA resin (A) and the styrene thermoplastic elastomer (B) is contained.
Examples of the method for obtaining the latex [I] include (1) a method in which a melt-kneaded mixture of the PVA-based resin (A) and the styrene-based thermoplastic elastomer (B) is dispersed in water to obtain a latex, (2 ) A method of emulsion-polymerizing a styrene-based monomer in the presence of the PVA-based resin (A) to obtain a latex of the styrene-based thermoplastic elastomer (B), and the like.

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

A typical example of the vinyl ester-based monomer is 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.
Examples of the alkali catalyst that can be used include alkali metal hydroxides and alcoholates such as potassium hydroxide, sodium hydroxide, sodium methylate, sodium ethylate, potassium methylate, and lithium methylate.
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 average degree of saponification of PVA-based resin (A) (measured in accordance with JIS K6726) is 60 to 90 mol%, preferably 75 to 90 mol%, and more preferably 80 to 90 mol%. ..
If the average saponification degree is too low, it tends to swell with respect to the electrolytic solution, and as a result, the binding property between the positive electrode members tends to decrease, and if too high, the affinity with the electrolytic solution decreases, Battery resistance tends to increase at low temperatures.

Further, in the case of melt-kneading the PVA-based resin (A) and the styrene-based thermoplastic elastomer (B), if the average saponification degree is too low, the melt viscosity is significantly lowered, and the melt moldability tends to be lowered. However, if the average degree of saponification is too high, the melting point tends to increase, and the production stability during melt kneading tends to decrease.
On the other hand, in the case of emulsion-polymerizing a styrene-based monomer in the presence of the PVA-based resin (A) to obtain a latex of the styrene-based thermoplastic elastomer (B), if the average saponification degree is too low, the PVA-based resin (A ) Is too high, the viscosity at the time of polymerization tends to be too high, the cloud point is likely to be exhibited, the polymerization stability tends to be lowered, and the average degree of saponification is too high. And the emulsifying power of the PVA-based resin (A) becomes too low, the polymerization stability tends to decrease, and the productivity tends to decrease.

The viscosity average degree of polymerization (measured in accordance with JIS K6726) of the PVA-based resin (A) is preferably 350 to 3,000, particularly preferably 350 to 2,800, further preferably 400 to 2,500. Is. If the viscosity average degree of polymerization is too low, the flexibility of the formed film is reduced, and thus the flexibility of the positive electrode tends to be reduced. On the contrary, if it is too high, the viscosity of the latex tends to be high, and it is difficult to mix it with the electrode active material, and the electrode tends to be non-uniform.

In the present invention, when the PVA resin (A) and the styrene thermoplastic elastomer (B) are melt-kneaded, the viscosity average degree of polymerization (measured in accordance with JIS K6726) is 300 to 3,000. Is particularly preferable, 350 to 2,000 is particularly preferable, and 400 to 1,200 is more preferable.
If the viscosity average degree of polymerization is too low, the moldability tends to decrease. If the viscosity average degree of polymerization is too high, the melt viscosity tends to be too high, and the production by melt kneading tends to be difficult.

In the present invention, when a styrene-based monomer is emulsion-polymerized in the presence of a PVA-based resin (A) to obtain a latex of a styrene-based thermoplastic elastomer (B), a viscosity average polymerization degree (measured according to JIS K6726) ) Is preferably 250 to 3,000, particularly preferably 300 to 2,800, and further preferably 400 to 2,500.
If the viscosity average degree of polymerization is too low, the binding property of the binder composition tends to decrease. If the viscosity average degree of polymerization is too high, the viscosity of the polymerization reaction solution becomes too high, which makes stirring difficult during the polymerization, which tends to make the polymerization reaction difficult.

The PVA-based resin (A) 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 (A) 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, 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 (A). For example, as the PVA-based resin (A), it is preferable to use a modified polyvinyl alcohol-based resin (a) 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 latex dispersant, the proportion of coarse particles tends to increase. For this reason also, the PVA-based resin obtained by the method (i) is particularly suitable for this application.

When the PVA-based resin (A) 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 which are dispersoids during emulsion polymerization, or the electrolytic solution is resistant. From the viewpoints of properties, stability of leaving latex, and the like, 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 rate of the PVA-based resin onto the ethylenically unsaturated polymer particles as the dispersoid tends to decrease, and the electrolytic solution resistance, the leaving stability of the latex and the like 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 (A) 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.

In the present invention, among the PVA-based resins (A), one kind may be used, or two kinds may be used in combination.

[Styrene-based thermoplastic elastomer (B)]
The styrene-based thermoplastic elastomer (B) used in the present invention will be described.
The elastomer (B) used in the present invention has a polymer block of an aromatic vinyl compound represented by styrene as a hard segment and a polymer block of a conjugated diene compound as a soft segment, or a double block remaining in such a polymer block. Some or all of the bonds have a hydrogenated block or a polymer block of isobutylene.
In particular, in the present invention, such an elastomer having a carboxylic acid group or a derivative group in its side chain is preferably used because it significantly improves the flexibility of the positive electrode and the production stability of the battery electrode. To be The reason why such an effect is obtained is not clear, but the presence of the carboxylic acid group or its derivative group in the side chain of the styrene-based thermoplastic elastomer (B) results in the styrene-based thermoplastic elastomer (B). Presumably acts as a reactive compatibilizer and enables fine dispersion in a PVA-based resin, particularly a PVA-based resin containing a structural unit having a primary hydroxyl group in its side chain.

The structure of each block in the elastomer (B) is such that, when the hard segment is represented by X and the soft segment is represented by Y, a diblock copolymer represented by XY, XYX or Y. Examples thereof include a triblock copolymer represented by —X—Y, and a polyblock copolymer in which X and Y are alternately connected, and the structure thereof may be linear, branched or star-shaped. be able to. Among them, a linear triblock copolymer represented by XYX is preferable in terms of mechanical properties.

Examples of the monomer used for forming the polymer block of the aromatic vinyl compound which is the hard segment include styrene; α-methylstyrene, β-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, Alkyl styrenes such as t-butyl styrene, 2,4-dimethyl styrene and 2,4,6-trimethyl styrene; halogenated styrenes such as monofluoro styrene, difluoro styrene, monochloro styrene, dichloro styrene and methoxy styrene; vinyl naphthalene and vinyl Examples thereof include vinyl compounds having an aromatic ring other than the benzene ring, such as anthracene, indene, and acetonaphthylene, and their derivatives. The polymer block of the aromatic vinyl compound may be a homopolymer block of the above-mentioned monomer or a copolymer block of a plurality of monomers, but a styrene homopolymer block is preferably used.

The aromatic vinyl compound polymer block may be a copolymer of a small amount of a monomer other than the aromatic vinyl compound, as long as the effect of the present invention is not impaired. Examples of such a monomer include olefins such as butene, pentene, and hexene; diene compounds such as butadiene and isoprene; vinyl ether compounds such as methyl vinyl ether and allyl ether compounds; and the copolymerization ratio thereof is usually It is 10 mol% or less based on the whole polymer block.

The weight average molecular weight of the polymer block of the aromatic vinyl compound in the elastomer (B) is usually 10,000 to 300,000, particularly 20,000 to 200,000, more preferably 50,000 to 100,000. Those are preferably used.

Examples of the monomer used for forming the polymer block that is the soft segment include 1,3-butadiene, isoprene (2-methyl-1,3-butadiene), and 2,3-dimethyl-1,3-butadiene. , A conjugated diene compound such as 1,3-pentadiene, and isobutylene, and these may be used alone or in combination of two or more. Of these, homopolymerized blocks or copolymerized blocks of isoprene, butadiene, and isobutylene are preferable, and homopolymerized blocks of butadiene or isobutylene are particularly preferably used.

In the case of such a polymer block of a conjugated diene compound, there are cases in which a plurality of bond types are formed by polymerization. For example, in butadiene, a butadiene unit (—CH ₂ —CH (CH═CH ₂ ) with 1,2-bonds is used. -) and a butadiene unit by 1,4 bonds _{(-CH 2 -CH = CH-CH} 2 -) is generated. The production ratios of these differ depending on the type of the conjugated diene compound, and therefore cannot be generally stated, but in the case of butadiene, the production ratio of 1,2-bonds is usually in the range of 20 to 80 mol %.

In the polymer block made of such a conjugated diene compound, the heat resistance and weather resistance of the styrene-based thermoplastic elastomer can be improved by hydrogenating a part or all of the remaining double bonds. The hydrogenation rate at that time is preferably 50 mol% or more, and particularly preferably 70 mol% or more.
Note that, by such a hydrogenation, for example butadiene unit Butadiene 1,2 bonds, oxybutylene units _{(-CH 2 -CH (CH 2 -CH} 3) -) , and the butadiene units produced by 1,4-bond two consecutive ethylene units _{(-CH 2 -CH 2 -CH 2 -CH} 2 -) a, but it is usually produced in preference former.

The polymer block which is such a soft segment may be a copolymer of a small amount of a monomer other than the above-mentioned monomers, as long as the effect of the present invention is not impaired. Examples of such monomers include aromatic vinyl compounds such as styrene, olefins such as butene, pentene and hexene, vinyl ether compounds such as methyl vinyl ether and allyl ether compounds, and the copolymerization ratio thereof is usually a polymer. It is 10 mol% or less of the whole block.

The weight average molecular weight of the conjugated diene compound in the elastomer (B) or the polymer block derived from isobutylene is usually 10,000 to 300,000, particularly 20,000 to 200,000, and further 50, Those of 000 to 100,000 are preferably used.

As described above, in the elastomer (B) used in the present invention, the hard segment is a polymer block of an aromatic vinyl compound, and the soft segment is a polymer block of a conjugated diene compound, or a part of the residual double bond or All of them are composed of a hydrogenated polymer block, an isobutylene polymer block, and the like, and typical examples thereof include a styrene/butadiene block copolymer (SBS) made from styrene and butadiene, and a butadiene structure of SBS. Styrene/butadiene/butylene block copolymer (SBBS) in which side chain double bonds in the unit are hydrogenated, and styrene/ethylene/butylene block copolymer (SEBS) in which main chain double bonds are hydrogenated, styrene And styrene/isoprene block copolymers (SIPS) made from styrene and isoprene as raw materials, and styrene/isobutylene block copolymers (SIBS) made from styrene and isobutylene as raw materials, among which heat stability and weather resistance are excellent. SEBS and SIBS are preferably used in this respect.

The content ratio of the polymer block of the aromatic vinyl compound as the hard segment and the polymer block of the soft segment in the elastomer (B) is usually 10/90 to 70/30 in terms of weight ratio, and particularly, , Those in the range of 20/80 to 50/50 are preferable. If the content ratio of the polymer block of the aromatic vinyl compound is too large or too small, the balance of flexibility and rubber elasticity of the elastomer (B) tends to be lost, and as a result, the composition of the present invention, especially latex. The characteristics of the dried film obtained from the above tend to be insufficient.

Such an elastomer (B) is obtained by obtaining a block copolymer having a polymer block of an aromatic vinyl compound and a polymer block of a conjugated diene compound or isobutylene, and further, if necessary, in a polymer block of the conjugated diene compound. It can be obtained by hydrogenating the double bond.
First, as a method for producing a block copolymer having a polymer block of an aromatic vinyl compound and a conjugated diene compound, or a polymer block of isobutylene, a known method can be used, for example, an alkyllithium compound or the like. Examples of the initiator include a method of sequentially polymerizing an aromatic vinyl compound and a conjugated diene compound or isobutylene in an inert organic solvent.
Next, as a method for hydrogenating the block copolymer having the polymer block of the aromatic vinyl compound and the polymer block of the conjugated diene compound, a known method can be used, for example, a borohydride compound or the like. And a hydrogen reduction using a metal catalyst such as platinum, palladium or Raney nickel.

Further, the elastomer (B) used in the present invention preferably has a carboxylic acid group or its derivative group in the side chain, and by using a styrene-based thermoplastic elastomer having such a carboxylic acid (derivative) group It becomes possible to obtain a composition having particularly excellent stability, particularly a latex.
That is, the elastomer (B) having a carboxylic acid group or its derivative group in the side chain acts as a reactive compatibilizer by performing a graft reaction with the PVA-based resin (A) during melt kneading. By the thermoplastic elastomer (B) acting as a reactive compatibilizer, the dispersibility of the thermoplastic elastomer (B) in the PVA-based resin (A) is improved and fine dispersion is possible. Further, since it forms a strong interface with the PVA-based resin (A), the dispersion stability when the binder composition is dispersed in water is improved.

The content of the carboxylic acid (derivative) group in the elastomer (B) is such that the acid value measured by a titration method is usually 0.5 to 20 mgCH ₃ ONa/g, and particularly 1 to 15 mgCH ₃ ONa/g, Those having 2 to 10 mg CH ₃ ONa/g are preferably used.
If the acid value is too low, the effect of introducing a carboxylic acid (derivative) group tends to be insufficient, and if it is too high, a gel tends to be generated during melt kneading with the PVA-based resin (A). is there.

As a method for introducing such a carboxylic acid group or a derivative group thereof into the elastomer, a known method can be used. For example, during production of the elastomer, that is, during copolymerization, α,β-unsaturated carboxylic acid or the A method of copolymerizing a derivative or a method of adding an α,β-unsaturated carboxylic acid or a derivative thereof after producing an elastomer is preferably used. Examples of such an addition method include a method of radical reaction in a solution in the presence or absence of a radical initiator, and a method of melt-kneading in an extruder.

Examples of the α,β-unsaturated carboxylic acid or its derivative used for introducing the carboxyl group (derivative group) include, for example, α,β-unsaturated monocarboxylic acids such as acrylic acid and methacrylic acid; maleic acid, succinic acid, Α,β-Unsaturated dicarboxylic acids such as itaconic acid and phthalic acid; α,β-unsaturated monocarboxylic acid esters such as glycidyl acrylate, glycidyl methacrylate, hydroxyethyl acrylate and hydroxymethyl methacrylate; maleic anhydride, succinic anhydride, Examples thereof include α,β-unsaturated dicarboxylic acid anhydrides such as itaconic anhydride and phthalic anhydride.

The weight average molecular weight of the elastomer (B) used in the present invention is usually 50,000 to 500,000, particularly preferably 120,000 to 450,000, and more preferably 150,000 to 400,000. ..
The melt viscosity of the elastomer (B) at 220° C. and a shear rate of 122 sec ⁻¹ is usually 100 to 3000 mPa·s, particularly 300 to 2000 mPa·s, and more preferably 800 to 1500 mPa·s.
If the weight average molecular weight is too large or the melt viscosity is too high, the workability during melt kneading with the PVA-based resin (A) and the dispersibility in the PVA-based resin (A) tend to decrease, and If the weight average molecular weight is too small or the melt viscosity is too low, the mechanical strength of the dry film tends to be insufficient.
The weight average molecular weight of the elastomer (B) is a value obtained by using GPC and using polystyrene as a standard.

Further, in the present invention, as the elastomer (B), one kind may be used, but a plurality of kinds may be appropriately mixed and used for the purpose of obtaining desired characteristics. In that case, those having a carboxylic acid (derivative) group and those not having a carboxylic acid (derivative) group may be used in combination.

Examples of commercially available styrene-based thermoplastic elastomer (B) having such a carboxylic acid (derivative) group include "Tuftec M series" manufactured by Asahi Kasei Co., Ltd. which is a carboxyl group-modified SBS product, and "f- Examples thereof include “Dynaron” and “Clayton FG” manufactured by Shell Japan.

Further, examples of commercially available styrene-based thermoplastic elastomer (B) having no carboxylic acid (derivative) group are SBS's “Toughprene”, “Asaprene T”, “Asaflex” and SBBS manufactured by Asahi Kasei Corporation. Asahi Kasei's “Tough Tech P Series”, SEBS' Asahi Kasei's “Tough Tech H Series” and the like can be mentioned.
Other commercially available products include, for example, "Clayton G", "Clayton D" and "Califlex TR" manufactured by Shell Japan Co., "Septon", "Hypler" manufactured by Kuraray Co., Ltd., "Dynaron" manufactured by JSR, and "JSR-". Examples thereof include “TR”, “JSR-SIS”, “Quintac” manufactured by Nippon Zeon Co., Ltd., and “Electrified STR” manufactured by Denki Kagaku.

[Binder composition for positive electrode of lithium ion secondary battery]
The binder composition for a lithium ion secondary battery positive electrode of the present invention comprises the PVA-based resin (A) and the styrene-based thermoplastic elastomer (B) having an average saponification degree of 60 to 90 mol%. Preferably, (1) a resin containing the PVA-based resin (A) and the styrene-based thermoplastic elastomer (B) melt-kneaded and dispersed in water to form a latex, and (2) a PVA-based resin. Examples thereof include those obtained by emulsion-polymerizing (A) and the styrene-based thermoplastic elastomer (B) under high pressure conditions to form a latex. Among them, a latex [I] containing a PVA-based resin (A) and a styrene-based thermoplastic elastomer (B) from the viewpoint of having a simple manufacturing process for battery electrodes and suppressing an increase in resistance during low-temperature operation. It is preferable that it contains.
The latex [I] contained in the positive electrode binder composition of the present invention will be described below.

The latex [I] used in the present invention is usually obtained by melt-kneading the above-mentioned PVA-based resin (A) and styrene-based thermoplastic elastomer (B), putting the obtained melt-kneaded product in water, and melt-kneading. The product PVA resin (A) is dissolved in water and the elastomer (B) is dispersed in water.

Latex used in the present invention [I] is, PVA-based content ratio of the resin (A) and styrene-based thermoplastic elastomer (B) (A / B) is, Ri 90 / 10-60 / 40 Der in weight ratio , is preferably used particularly from 80 / 20-70 / 30. If the content ratio (A/B) is too small, the elastomer (B) will not be sufficiently dispersed in the melt-kneaded product, and it will be difficult to obtain a uniformly finely dispersed latex. On the other hand, if it is too large, the concentration of the elastomer in the latex [I] obtained will be low, and the proportion of the PVA-based resin in the dried product obtained from the latex will be high, so that the water resistance tends to decrease.
The melt kneading of the PVA-based resin (A) and the elastomer (B) can be carried out by using a known kneading device such as a kneader ruder, an extruder, a mixing roll, a Banbury mixer, a blast mill, and the like. The method using a machine is industrially preferable.
Examples of such an extruder include a single-screw extruder and a twin-screw extruder. Among them, a twin-screw extruder in which the screws rotate in the same direction is more preferable because sufficient kneading can be obtained by appropriate shearing. The L/D of such an extruder is usually 10 to 80, preferably 15 to 70, and more preferably 15 to 60. If the L/D is too small, melt kneading may be insufficient, and the dispersibility of the elastomer (B) in the melt kneaded product may be insufficient. On the contrary, if it is too large, excessive shearing is preferable, which is preferable. Not prone to shear heat generation.

The screw rotation speed of the extruder is usually 10 to 400 rpm, particularly preferably 30 to 300 rpm, and more preferably 50 to 250 rpm. If the number of revolutions is too low, the ejection tends to be unstable, and if it is too high, undesired shear heat generation may cause deterioration of the resin.

The temperature of the resin in the extruder is usually 80 to 260°C, preferably 130 to 240°C, and more preferably 180 to 230°C. If the resin temperature is too high, the PVA-based resin (A) or the elastomer (B) tends to be thermally decomposed. On the contrary, if the resin temperature is too low, melt kneading is insufficient and the elastomer (B) in the melt kneaded product is Dispersibility tends to decrease.
The method of adjusting the resin temperature is not particularly limited, but usually a method of appropriately setting the temperature of the cylinder in the extruder and the rotation speed is used.

The melt-kneaded product discharged from the extruder is preferably pelletized in terms of transfer to the next step, and easy handling, and the pelletizing method is not particularly limited, but the resin composition is diced. A method is used in which the product is extruded in a strand form from the product, cooled, and then cut into an appropriate length. The cooling method is not particularly limited, but a method of contacting with a liquid held at a temperature lower than the temperature of the extruded resin and an air cooling method of blowing cold air are preferably used. The above-mentioned liquid needs to be an organic solvent that does not dissolve the PVA-based resin (A), and examples thereof include an alcohol-based solvent. From the environmental point of view, the air-cooling method is preferably used.
The shape of such pellets is usually cylindrical, and the size thereof is preferably small considering that the PVA-based resin (A) therein is dissolved and removed later by contacting this with water and, for example, a die. It is preferable that the caliber has a diameter of 2 to 6 mm and the strand cut length is about 1 to 6 mm. A method in which the melt-kneaded product discharged from the extruder is cut in the air or in an organic solvent while the melt-kneaded product is still in a molten state can be used. In that case, pellets having a nearly spherical shape can be obtained. As the size in that case, a pellet having a diameter of 2 to 5 mm is preferably used.

Next, the step of producing the latex [I] from the melt-kneaded product of the PVA-based resin (A) and the styrene-based thermoplastic elastomer (B) obtained in the above step will be described.
In this step, the PVA-based resin (A) contained in the melt-kneaded product of the PVA-based resin (A) and the elastomer (B) thus obtained is dissolved, and the method is not particularly limited. Usually, the latex [I] can be obtained by charging the pellets of the melt-kneaded product obtained by the above-mentioned method into water, stirring and heating if necessary.

The solid content concentration of the latex [I] obtained by such a method is usually 10 to 50% by weight, and particularly preferably 10 to 40% by weight. When the solid content concentration is too low, it takes a lot of energy and time to remove the water, and when the solid content concentration is too high, the fluidity tends to be lowered and the workability tends to be impaired.

The particle size of the styrene-based thermoplastic elastomer (B) in the latex [I] obtained by such a method is usually 50 to 2000 nm, particularly preferably 100 to 1000 nm, more preferably 150 to 800 nm μm.

<Other ingredients>
The binder composition for a positive electrode of the present invention can be compounded with a compounding agent or the like which is usually used for coating materials used for coating films and molding resins. 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. In addition, when the binder composition for positive electrodes contains these compounding agents, the organic content of the compounding agent contained is contained in the solid content of the binder composition for positive electrodes.
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 latex [I] in the binder composition for a positive electrode.

[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% in terms of the solid content of the styrene-based thermoplastic elastomer (B) contained in the positive electrode binder composition with respect to 100 parts by weight of the active material. It is 1 to 10 parts by weight, preferably 0.1 to 5 parts by weight, and 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, it becomes difficult to obtain desired binding force between the active materials and adhesive force to the current collector, the positive electrode becomes unstable, and 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 at the time of producing the positive electrode and the like, a solvent may be added to prepare a positive electrode slurry for the purpose of adjusting the viscosity, adjusting the solid content of the positive electrode 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 positive electrode binder composition for the purpose of improving the dispersibility of the active material, the positive electrode binder composition and the conductive auxiliary agent, or the leveling property during coating. You may. The type of the thickener is not particularly limited, but mainly a water-soluble polymer is preferably used because of its miscibility with PVA-based resin and the fact that water is preferably used as a dispersion medium for the latex. To be
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 with respect to the slurry is too small, the dispersion stability of the active material, the binder for the positive electrode, the conductive additive, and the like tends to be low, or the positive electrode tends to be nonuniform 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 positive electrode slurry prepared as described above is applied onto a current collector and dried to obtain a lithium ion secondary battery positive electrode of the present invention (hereinafter, may be abbreviated as “the positive electrode of the present invention”). 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 binder composition for a positive electrode of the present invention in the obtained positive electrode is usually 10% or less, preferably 7% or less, particularly preferably 0, although it depends on the kind and composition of the styrene-based thermoplastic elastomer. 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 latex prepared as the binder composition for the positive electrode was cast on a PET film using an applicator having a thickness of 500 μm, and then heat-dried 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 (W0 (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 is taken out, the electrolytic solution adhering to the film surface is wiped off, and the electrolytic solution swelling ratio is calculated from the weight after immersion (W1 (g)) after the test according to the following formula.
Electrolyte swelling rate (%)=((W1−W0)/W0)×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, for example, ethylene carbonate, chloroethylene carbonate, trifluoropropylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, isopropylmethyl carbonate, ethylpropyl carbonate, isopropylethyl carbonate, butylmethyl. Carbonate, butylethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, sulfolane, 3-methylsulfolane, 2,4-dimethylsulfolane, 1,3-dioxolane, methyl acetate , Ethyl acetate, methyl formate, ethyl formate, etc., and it is preferable to use them in combination.

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 2, LiN(SO ₂ C). _{2 F 5) 2, LiC (} SO 2 CF 3) 3, LiN ( such as SO 3 _{CF 3) 2} organic salts such as may be used those conventionally used as an electrolyte for non-aqueous electrolyte. Among these, it is preferable to use LiPF ₆ , LiBF _4, or LiClO ₄ .

As the separator, for example, 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 can be applied to any conventionally known form and structure such as a stacked (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 latex may be used. The binder composition for a lithium ion secondary battery positive electrode of the present invention contains a latex in which a styrene-based thermoplastic elastomer is dispersed in a PVA-based resin aqueous solution, so that the binder composition is used in an electrode obtained by using the binder composition. The surface area of the PVA-based resin matrix therein is large, and the thickness of the PVA-based resin 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 not clear, but the styrene-based thermoplastic elastomer and the PVA-based resin matrix increase the number of graft-forming products formed during the production of the binder composition at the interface between the styrene-based thermoplastic elastomer and the PVA-based resin matrix. It is considered that the dispersed particle diameter of the thermoplastic elastomer is further reduced, the particle diameter 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.

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

(1) Average 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.

(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).
The absolute value of the initial discharge capacity is 140 (mAh/g) or more as “excellent” (⊚), and 120 (mAh/g) or more and less than 140 (mAh/g) is “good” (∘) or 120 (mAh/g). Less than 1 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 of 90% or more was evaluated as “excellent” (⊚), 80% or more and less than 90% was evaluated as “good” (◯), and less than 80% was evaluated as “poor” (x).

(2-3) Room Temperature Resistance Characteristics After 5 cycles at a current density of 30 mA/g and 25° C., the resistance value was measured at 25° C. impedance in a charged state of 50%. In this impedance measurement, the frequency range was 0.1 m to 100 MHz, and the voltage amplitude was ±10 mV. It is shown that the lower the resistance value at the frequency of 10 Hz, the better. The absolute value of the resistance value should be as close to 0 as possible.
A resistance value (Ω) at 10 Hz of 40 or less was evaluated as “excellent” (⊚), a value of more than 40 and 80 or less was evaluated as “good” (◯), and a value of more than 80 was evaluated as “poor” (x).

(2-4) Low temperature resistance characteristics After 5 cycles at a current density of 30 mA/g and 25°C, the resistance value was measured by the 0°C impedance in a charged state of 50%. In this impedance measurement, the frequency range was 0.1 m to 100 MHz, and the voltage amplitude was ±10 mV. It is shown that the lower the resistance value at the frequency of 10 Hz, the better. The absolute value of the resistance value should be as close to 0 as possible.
A resistance value (Ω) at 10 Hz of 40 or less was evaluated as “excellent” (⊚), a value of more than 40 and 80 or less was evaluated as “good” (◯), and a value of more than 80 was evaluated as “poor” (x).

The following items were prepared.
[PVA-based resin (A1)]
A reaction vessel equipped with a reflux condenser, a dropping funnel and a stirrer was charged with 72.1 parts of vinyl acetate, 21.5 parts of methanol and 4.1 parts of 3,4-diacetoxy-1-butene, and azobisisobutyronitrile. Was charged in an amount of 0.3 mol% (to the charged vinyl acetate), and the temperature was raised under a nitrogen stream while stirring to start polymerization. When the polymerization rate of vinyl acetate reached 90%, m-dinitrobenzene was added to terminate the polymerization, and then unreacted vinyl acetate monomer was removed outside the system by a method of blowing methanol vapor. The combined solution was used as a methanol solution.

Then, the above methanol solution was further diluted with methanol, adjusted to a concentration of 45% and charged in a kneader. While maintaining the solution temperature at 35° C., a 2% methanol solution of sodium hydroxide was added to the vinyl acetate structural unit in the copolymer. And saponification was performed by adding 3,4-diacetoxy-1-butene structural units in a ratio of 3 mmol per 1 mol of the total amount. As the saponification progressed and the saponified product was deposited and when it became particles, it was filtered, washed well with methanol and dried in a hot air drier to produce the desired PVA-based resin (A1).

The degree of saponification of the obtained PVA-based resin (A1) was 88 mol% when analyzed by the amount of alkali consumption required for hydrolysis of residual vinyl acetate and 3,4-diacetoxy-1-butene. Further, the average degree of polymerization was 340 when analyzed according to JIS K 6726. The content of the 1,2-diol structural unit represented by the general formula (1) is ¹ H-NMR (300 MHz proton NMR, d6-DMSO solution, internal standard substance; tetramethylsilane, 50° C.). It was 3 mol% when calculated from the measured integral value.
The MFR (210° C., load 2160 g) was 28.2 g/10 minutes, and the melt viscosity (220° C., shear rate 122 sec ⁻¹ ) was 324 Pa·s.

[PVA-based resin (A2)]
A reaction vessel equipped with a reflux condenser, a dropping funnel and a stirrer was charged with 68.0 parts of vinyl acetate, 23.8 parts of methanol and 8.2 parts of 3,4-diacetoxy-1-butene, and azobisisobutyronitrile. Was charged in an amount of 0.3 mol% (to the charged vinyl acetate), and the temperature was raised under a nitrogen stream while stirring to start polymerization. When the polymerization rate of vinyl acetate reached 90%, m-dinitrobenzene was added to terminate the polymerization, and then unreacted vinyl acetate monomer was removed outside the system by a method of blowing methanol vapor. The combined solution was used as a methanol solution.

Then, the above methanol solution was further diluted with methanol, adjusted to a concentration of 45% and charged in a kneader. While maintaining the solution temperature at 35° C., a 2% methanol solution of sodium hydroxide was added to the vinyl acetate structural unit in the copolymer. And saponification was performed by adding 3,4-diacetoxy-1-butene structural units in a ratio of 3 mmol per 1 mol of the total amount. As the saponification progressed and the saponified product was deposited and when it became particulate, it was filtered off, washed well with methanol and dried in a hot air drier to produce the desired PVA-based resin (A2).

The degree of saponification of the obtained PVA-based resin (A2) was 88 mol% when analyzed by the amount of alkali consumption required for hydrolysis of residual vinyl acetate and 3,4-diacetoxy-1-butene. The average degree of polymerization was 450 when analyzed according to JIS K 6726. The content of the 1,2-diol structural unit represented by the general formula (1) is ¹ H-NMR (300 MHz proton NMR, d6-DMSO solution, internal standard substance; tetramethylsilane, 50° C.). It was 6 mol% when calculated from the measured integral value.
The MFR (210° C., load 2160 g) was 6.1 g/10 minutes, and the melt viscosity (220° C., shear rate 122 sec ⁻¹ ) was 858 Pa·s.

[PVA-based resin (A'1)]
A reaction vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 68.5 parts of vinyl acetate, 20.5 parts of methanol, and 11 parts of 3,4-diacetoxy-1-butene, and 0 of azobisisobutyronitrile was charged. Then, 0.3 mol% (vs. charged vinyl acetate) was added, and the temperature was raised under a nitrogen stream while stirring to start polymerization. When the polymerization rate of vinyl acetate reached 90%, m-dinitrobenzene was added to terminate the polymerization, and then unreacted vinyl acetate monomer was removed outside the system by a method of blowing methanol vapor. The combined solution was used as a methanol solution.

Then, the above methanol solution was further diluted with methanol, adjusted to a concentration of 45% and charged in a kneader. While maintaining the solution temperature at 35° C., a 2% methanol solution of sodium hydroxide was added to the vinyl acetate structural unit in the copolymer. Then, saponification was carried out by adding 3,4-diacetoxy-1-butene structural units in a ratio of 10.5 mmol to 1 mol of the total amount. As the saponification proceeds, the saponified product precipitates, and when it becomes particulate, it is filtered, washed well with methanol, and dried in a hot-air dryer to produce the desired PVA-based resin (A'1). did.

The saponification degree of the obtained PVA-based resin (A′1) was 99 mol% when analyzed by the amount of alkali consumption required for hydrolysis of residual vinyl acetate and 3,4-diacetoxy-1-butene. .. The average degree of polymerization was 300 when analyzed according to JIS K 6726. The content of the 1,2-diol structural unit represented by the general formula (1) is ¹ H-NMR (300 MHz proton NMR, d6-DMSO solution, internal standard substance; tetramethylsilane, 50° C.). It was 8 mol% as calculated from the measured integral value.
The MFR (210° C., load 2160 g) was 28 g/10 minutes, and the melt viscosity (220° C., shear rate 122 sec ⁻¹ ) was 274 Pa·s.

[PVA-based resin (A'2)]
A reaction vessel equipped with a reflux condenser, a dropping funnel and a stirrer was charged with 68.0 parts of vinyl acetate, 23.8 parts of methanol and 8.2 parts of 3,4-diacetoxy-1-butene, and azobisisobutyronitrile. Was charged in an amount of 0.3 mol% (to the charged vinyl acetate), and the temperature was raised under a nitrogen stream while stirring to start polymerization. When the polymerization rate of vinyl acetate reached 90%, m-dinitrobenzene was added to terminate the polymerization, and then unreacted vinyl acetate monomer was removed outside the system by a method of blowing methanol vapor. The combined solution was used as a methanol solution.

Then, the above methanol solution was further diluted with methanol, adjusted to a concentration of 45% and charged in a kneader. While maintaining the solution temperature at 35° C., a 2% methanol solution of sodium hydroxide was added to the vinyl acetate structural unit in the copolymer. And saponification was carried out by adding 3,4-diacetoxy-1-butene structural units in a ratio of 10.5 mmol to the total amount of 1 mol. As the saponification proceeds, the saponified product precipitates, and when it becomes particulate, it is filtered, washed well with methanol, and dried in a hot air drier to produce the desired PVA-based resin (A'2). did.

The saponification degree of the obtained PVA-based resin (A′2) was 99 mol% when analyzed by the amount of alkali consumption required for hydrolysis of residual vinyl acetate and 3,4-diacetoxy-1-butene. .. The average degree of polymerization was 450 when analyzed according to JIS K 6726. The content of the 1,2-diol structural unit represented by the general formula (1) is ¹ H-NMR (300 MHz proton NMR, d6-DMSO solution, internal standard substance; tetramethylsilane, 50° C.). It was 6 mol% when calculated from the measured integral value.
The MFR (210° C., load 2160 g) was 5.5 g/10 minutes, and the melt viscosity (220° C., shear rate 122 sec ⁻¹ ) was 1040 Pa·s.

[Styrene-based thermoplastic elastomer (B1)]
Styrene/ethylene/butylene block copolymer (SEBS) having a carboxyl group (“Tuftec M1911” manufactured by Asahi Kasei (oxidation 10 mg CH ₃ ONa/g, melt viscosity 1100 mPa·s (220° C., shear rate 122 sec ⁻¹ ))

[Example 1]
(Production of resin composition)
49 parts of PVA-based resin (A1), 21 parts of PVA-based resin (A2) and 30 parts of styrene-based thermoplastic elastomer (B1) were dry-blended, and then melt-kneaded using a twin-screw extruder under the following conditions to form a strand. The mixture was extruded and cut with a pelletizer to obtain melt-kneaded product pellets (diameter about 1 mm, length about 2 mm).
Diameter (D) 15 mm
L/D=60
Screw rotation speed: 200 rpm
Set temperature: C1/C2/C3/C4/C5/C6/D=90/205/210/210/210/215/220/220/220°C
Discharge rate: 1.5kg/hr

(Production of Binder Composition Solution)
30 parts of the obtained pellets was added to 70 parts of room temperature water, the temperature was raised with stirring, and the mixture was stirred at 80° C. for 90 minutes to obtain latex [I].
The obtained latex [I] was fractionated and mixed with purified water for dilution to obtain a 20% positive electrode binder composition solution.

(Manufacture of batteries)
Using the positive electrode binder composition solution obtained above, a positive electrode slurry was prepared in the following manner 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.

[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).
1.5 parts by weight of the binder composition solution (20% by weight) obtained above as a binder for the positive electrode in solid content in the obtained paste, and purified water was added at appropriate time, and then the planetary kneader Was mixed under the same conditions to obtain an active material paste having a solid content concentration of 55.9% by weight.
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. A positive electrode for a battery prepared as a working electrode, a lithium metal negative electrode (diameter 13 mm) as a counter electrode, and a separator (diameter 18 mm) made of a polypropylene porous film having a thickness of 16 μm were interposed, and the electrodes were arranged so as to face each other. 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 prepare a half cell, and a battery for evaluation was prepared.

[Comparative Example 1]
A battery was prepared in the same manner as in Example 1 except that the positive electrode binder composition solution prepared in the same manner as the resin composition below was used, and the same evaluation as in Example 1 was performed.

(Production of resin composition)
42 parts of PVA-based resin (A'1), 28 parts of PVA-based resin (A'2) and 30 parts of styrene-based thermoplastic elastomer (B1) are dry-blended, and then melt-kneaded under the following conditions using a twin-screw extruder. Then, the mixture was extruded into a strand and cut with a pelletizer to obtain a melt-kneaded product pellet (diameter: about 1 mm, length: about 2 mm).
Diameter (D) 15 mm
L/D=60
Screw rotation speed: 200 rpm
Set temperature: C1/C2/C3/C4/C5/C6/D=90/205/210/210/210/215/220/220/220°C
Discharge rate: 1.5kg/hr

From the results shown in Table 1, in Example 1 using the PVA-based resin (A) having a low average saponification degree, the initial discharge capacity was high, and the battery resistance characteristics at a low temperature of 0° C. were good. I understand. On the other hand, in Comparative Example 1 in which the PVA-based resin having a high average saponification degree was used, the battery resistance value was particularly high at a low temperature, and the object of the present invention was not satisfied.

Claims (7)

Ri Na contain an average degree of saponification 60-90 mol% of a polyvinyl alcohol-based resin (A) and styrene-based thermoplastic elastomer (B), a polyvinyl alcohol-based resin (A) and the styrene-based thermoplastic elastomer (B) the content of (a / B) is a lithium ion secondary battery positive electrode binder composition, characterized in 90 / 10-60 / 40 der Rukoto in weight. Ri Na contain an average degree of saponification 60-90 mol% of a polyvinyl alcohol-based resin (A) and styrene-based thermoplastic elastomer (B), a polyvinyl alcohol-based resin (A) and the styrene-based thermoplastic elastomer (B) the content of (a / B) is, the weight ratio at 90 / 10-60 / 40 der Ru latex [I] a lithium ion secondary battery positive electrode binder composition of claim 1, wherein the containing .. The binder composition for a lithium ion secondary battery positive electrode according to claim 1 or 2, wherein the polyvinyl alcohol resin (A) has a viscosity average degree of polymerization of 350 to 3,000. Polyvinyl alcohol resin (A) is, according to claim 1 to 3 lithium ion secondary battery positive electrode according to any one, which is a polyvinyl alcohol-based resin containing a structural unit having a primary hydroxyl group in a side chain (a) Binder composition. Styrene-based thermoplastic elastomer (B) is, according to claim 1 for 4 or lithium ion secondary battery positive electrode according which is a styrene-based thermoplastic elastomer having a carboxylic acid group or its derivative group in the side chain Binder composition. Lithium ion secondary battery positive electrode characterized by comprising using the claim 1-5 lithium ion secondary battery positive electrode binder composition of any one. A lithium-ion secondary battery comprising the positive electrode of the lithium-ion secondary battery according to claim 6 .