JP2018060603A - Binder composition for lithium ion secondary battery positive electrode, lithium ion secondary battery positive electrode, and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a binder composition for a lithium ion secondary battery positive electrode, which has good cycle characteristics of charge/discharge at room temperature and at low temperature when a lithium ion secondary battery is formed. A binder composition for a positive electrode of a lithium ion secondary battery, comprising a polyvinyl alcohol resin (A) having an average degree of saponification of 60 to 95 mol% and a styrene thermoplastic elastomer (B). Stuff. [Selection diagram] None

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 has good charge / discharge cycle characteristics at room temperature and 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 using the binder composition, and a lithium ion secondary battery having the lithium ion secondary battery positive electrode.

  Lithium ion secondary batteries are light and have high energy density and high durability against repeated charge and discharge, and are therefore used as power sources for electronic devices such as mobile phones and laptop computers. Moreover, it is utilized as a power supply device that can be discharged and charged in an electric vehicle such as an electric vehicle. In particular, with the recent increase in size of batteries, there is a demand for batteries having higher performance such as high capacity and capable of rapid charge / discharge.

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

Here, the binder for electrodes serves to bind the active materials to each other and bind the metal foil, which is a current collector, to the active material. If the binder cannot bind a sufficient amount of the active material to the current collector, or if the binder cannot bind the active materials, 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 and the battery capacity may be reduced.
Moreover, 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 and flexibility, the mixture layer may be cracked and the active material may be peeled or dropped, resulting in a decrease in battery capacity. Therefore, a binder having both binding power and flexibility is required.

  As a method for solving this problem, a method is known in which a flexible fluorine-based resin such as polyvinylidene fluoride (PVDF) is used as a binder. However, since PVDF has a weak binding force, there is a possibility that the binding force may decrease unless a large amount of binder is used.

  In addition, 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 resin in the case of a negative electrode. For example, an aqueous solution of a polyvinyl alcohol-based resin having a viscosity average degree of polymerization of 400 to 3000, in which polymer particles derived from an ethylenically unsaturated monomer exhibit a high charge / discharge capacity and a highly flexible electrode. A binder composition for lithium ion secondary battery electrodes containing an emulsion dispersed therein has been proposed (for example, see Patent Document 1).

JP, 2006-66601, A

  However, although the said patent document 1 has described the binder composition for negative electrodes in the Example, it does not describe until the specific evaluation about the binder composition for positive electrodes.

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

  Therefore, the present invention, in such a background, when a lithium ion secondary battery is formed, the binder composition for a lithium ion secondary battery positive electrode, in which the cycle characteristics of charge and discharge at room temperature and low temperature are good, An object of the present invention is 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 in view of such circumstances, the present inventors have determined that a polyvinyl alcohol resin (A) and a styrene thermoplastic elastomer (B) having a predetermined average saponification degree are lithium ion secondary battery positive electrodes. 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. Thus, when a lithium ion secondary battery is formed, the charge / discharge cycle characteristics at room temperature and low temperature are improved, and the present invention has been completed.

  That is, the gist of the present invention is a lithium ion secondary battery comprising a polyvinyl alcohol resin (A) having an average saponification degree of 60 to 95 mol% and a styrene thermoplastic elastomer (B). A positive electrode binder composition, preferably comprising a latex [I] containing a polyvinyl alcohol resin (A) having an average degree of saponification of 60 to 95 mol% and a styrene thermoplastic elastomer (B). The present invention relates to a lithium ion secondary battery positive electrode binder composition.

  Further, the gist of the present invention is a lithium ion secondary battery positive electrode obtained using the binder composition for a lithium ion secondary battery positive electrode of the present invention, and further 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, “a binder composition for a positive electrode of a lithium ion secondary battery” may be simply abbreviated as “a binder composition for a positive electrode”. In addition, “polyvinyl alcohol resin” may be referred to as “PVA resin”.

  The binder composition for a lithium ion secondary battery positive electrode of the present invention, when forming a lithium ion secondary battery, ensures the binding between the active materials or between the active material and the current collector, while being a PVA-based material. Since the resin exhibits moderate affinity with electrolytes, especially organic electrolytes, it is possible to suppress a decrease in ionic conductivity at normal and low temperatures, and therefore charge and discharge at normal and low temperatures. A lithium ion secondary battery with good cycle characteristics can be obtained.

  In the present invention, the binder exists on the interface between the active material and the electrolytic solution, and greatly affects the battery resistance in the lithium ion battery. This influences more clearly at low temperatures when the viscosity of the electrolyte increases and the Li ion conductivity decreases significantly. In the binder composition comprising the PVA resin (A) and the styrene thermoplastic elastomer (B) used in the present invention, the PVA resin (A) has an appropriate affinity with the organic electrolyte solution. It does not inhibit the movement of Li ions even at low temperatures. Therefore, it is possible to exhibit good charge / discharge cycle characteristics at room temperature and low temperature.

Hereinafter, although it demonstrates in detail about the structure of this invention, these show an example of a desirable embodiment, and this invention is not specified by these content.
In this specification, acrylic and methacryl are collectively referred to as (meth) acryl when not particularly distinguished, and acrylate and methacrylate are collectively referred to as (meth) acrylate when not particularly distinguished.
In the present invention, the solid content means that obtained by subjecting an object to a drying loss 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 resin (A) having an average saponification degree of 60 to 95 mol% and a styrene thermoplastic elastomer (B), Preferably, a latex [I] containing a PVA resin (A) and a styrene thermoplastic elastomer (B) is included.
As a method for obtaining such a latex [I], for example, (1) a method of obtaining a latex by dispersing a melt-kneaded mixture of such a PVA resin (A) and a styrene thermoplastic elastomer (B) in water, (2 And a method of emulsion polymerization of a styrene monomer in the presence of a PVA resin (A) to obtain a latex of a styrene thermoplastic elastomer (B).

[Polyvinyl alcohol resin (A)]
As a PVA-type resin (A) used by this invention, a well-known general water-soluble PVA-type resin is mentioned, for example.
Such a PVA resin (A) can be obtained, for example, by polymerizing and saponifying a vinyl ester monomer.

  A typical example of the vinyl ester monomer is vinyl acetate. Also, 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, piperine Examples include vinyl acid, vinyl octylate, vinyl monochloroacetate, vinyl adipate, vinyl methacrylate, vinyl crotonate, vinyl sorbate, vinyl benzoate, vinyl cinnamate, and vinyl trifluoroacetate. In view of the above, vinyl acetate is preferably used.

  The polymerization of the vinyl ester monomer can be performed by any known polymerization method such as solution polymerization, suspension polymerization, emulsion polymerization and the like. Especially, it is preferable to perform the solution polymerization which can remove reaction heat efficiently under recirculation | reflux. As the solvent for the solution polymerization, alcohol is usually used, and preferably a lower alcohol having 1 to 3 carbon atoms is used.

Also for the saponification of the obtained copolymer, a known saponification method performed with a conventional PVA-based resin can be employed. 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 include alkali metal hydroxides and alcoholates such as potassium hydroxide, sodium hydroxide, sodium methylate, sodium ethylate, potassium methylate, and lithium methylate.
Usually, an ester exchange reaction using an alkali catalyst in an anhydrous alcohol solvent is suitably 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 decrease and the reaction efficiency tends to decrease. If it is too high, the reaction solvent may have a boiling point or higher, and the safety in production tends to decrease. In addition, 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. However, it is possible to obtain a highly saponified product for a short time.

The average saponification degree (measured according to JIS K6726) of the PVA-based resin (A) is 60 to 95 mol%, preferably 75 to 95 mol%, more preferably 80 to 90 mol%. .
If the average degree of saponification is too low, it tends to swell with respect to the electrolyte solution. As a result, the binding property between the positive electrode members tends to decrease, and if too high, the affinity with the electrolyte solution decreases. Battery resistance at low temperatures tends to increase.

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

  The viscosity average polymerization degree (measured in accordance with JIS K6726) of the PVA resin (A) is preferably 350 to 3,000, particularly preferably 350 to 2,800, and more preferably 400 to 2,500. It is. When the viscosity average degree of polymerization is too low, the flexibility of the positive electrode tends to decrease due to the decrease in flexibility of the film formed. On the other hand, when the viscosity is too high, the viscosity of the latex tends to increase, and when mixed with the electrode active material, it is difficult to mix and the electrode tends to become 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 preferable, particularly preferably 350 to 2,000, and still more preferably 400 to 1,200.
If the viscosity average degree of polymerization is too low, moldability tends to decrease. When the viscosity average degree of polymerization is too high, the melt viscosity becomes too large and production by melt kneading tends to be difficult.

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

The PVA-based resin (A) has a structural unit derived from another monomer other than the vinyl ester monomer within a range not inhibiting the effects of the present invention (for example, less than 10 mol%, preferably 5 mol% or less). You 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, allyl glycidyl ether; triallyloxyethylene, diallyl maleate, tri Monomers having two or more allyl groups such as allyl cyanurate, triallyl isocyanurate, tetraallyloxyethane, diallyl phthalate; allyl ester monomers such as allyl acetate, vinyl acetoacetate, allyl acetoacetate, allyl diacetate Acetoacetoxyethyl (meth) acrylate, acetoacetoxyalkyl (meth) acrylates such as acetoacetoxypropyl (meth) acrylate, acetoacetoxyethyl crotonate, Acetoacetoxyalkyl crotonates such as cetoacetoxypropyl crotonate; 2-cyanoacetoacetoxyethyl (meth) acrylate; divinylbenzene; ethylene glycol di (meth) acrylate, 1,2-propylene glycol di (meth) acrylate, 1,3 -Alkylene glycol (meth) acrylates such as propylene glycol di (meth) acrylate, 1,4-butanediol di (meth) acrylate, 1,6-hexanediol di (meth) acrylate, neopentyl glycol di (meth) acrylate; Trimethylolpropane tri (meth) acrylate; allyl (meth) acrylate; 2-hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) Hydroxyalkyl (meth) acrylates such as acrylate (the alkyl moiety is a C1 to C10 alkyl group, preferably a C1 to C6 alkyl group); Nitrile monomers such as (meth) acrylonitrile; Styrenes such as styrene and α-methylstyrene Monomers; Olefins such as ethylene, propylene, 1-butene and isobutene; Halogenated olefins such as vinyl chloride, vinylidene chloride, vinyl fluoride and vinylidene fluoride; Olefin monomers such as ethylene sulfonic acid; Butadiene-1,3,2 -Diene monomers such as methylbutadiene, 1,3 or 2,3-dimethylbutadiene-1,3, 2-chlorobutadiene-1,3; 3-buten-1-ol, 4-penten-1-ol, 5 -Hexene-1,2-diol, glycerol monoallylamine Hydroxyl group-containing α-olefins such as ruthenium and derivatives thereof such as acylated products thereof; 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 and acrylic acid, salts or mono- or dialkyl esters thereof; Nitriles such as acrylonitrile, methacrylamide, diacetone acrylamide, etc. Amides, ethylene sulfonic acid, allyl sulfonic acid, methallyl sulfonic acid, olefin sulfonic acid such as AMPS or salts thereof, vinyltriethoxysilane, vinyltrimethoxysilane, vinyltripropoxysilane, vinyltributoxy Vinyl, alkyldialkoxysilanes such as lan, vinylmethyldimethoxysilane, vinylmethyldiethoxysilane; γ- (meth) such as γ- (meth) acryloxypropyltrimethoxysilane, γ- (meth) acryloxypropyltriethoxysilane Acryloxypropyltrialkoxysilane; γ- (meth) acryloxypropylalkyldialkoxysilane such as γ- (meth) acryloxypropylmethyldimethoxysilane, γ- (meth) acryloxypropylmethyldiethoxysilane; vinyl tris (β- Methoxyethoxy) silane, hydroxymethylvinylidene 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 resin (A) of the present invention includes a PVA resin modified within a range that does not inhibit the effects of the present invention (usually 15 mol% or less, preferably 10 mol% or less), such as a PVA resin. Formalized products, acetalized products, acetoacetylated products, butyralized products, urethanized products, esterified products with sulfonic acids, carboxylic acids, and the like may be included.

In the present invention, as the PVA resin (A), it is preferable to use a derivative-modified PVA resin such as a hydroxyl group-containing α-olefin and an acylated product thereof. For example, a modified polyvinyl alcohol resin (a) containing a structural unit having a primary hydroxyl group in the side chain is preferably used as the PVA resin (A). The number of primary hydroxyl groups in such a structural unit is usually 1 to 5, preferably 1 to 2, and particularly preferably 1. In addition to the primary hydroxyl group, it preferably has a secondary hydroxyl group.
Examples of the PVA resin containing a structural unit having a primary hydroxyl group in the side chain include, for example, a PVA resin having a 1,2-diol structural unit in the side chain and a PVA having a hydroxyalkyl group structural unit in the side chain. Based resins and the like. Among these, it is particularly preferable to use a PVA resin containing a structural unit having a 1,2-diol structure in the side chain represented by the following general formula (1) (hereinafter referred to as “side chain 1,2-diol structure”). Sometimes referred to as “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 bond chain.)

In the general formula (1), R ^{1 to} R ⁶ each independently represent a hydrogen atom or an organic group. R ^{1 to} R ⁶ are preferably all hydrogen atoms, but may be organic groups as long as the resin properties are not significantly impaired. Although it does not specifically limit as this organic group, For example, C1-C4 alkyl groups, such as a methyl group, an ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, tert-butyl group, are mentioned. 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 as necessary.

In the above general formula (1), X is a single bond or a bonded chain, and is from the viewpoint of the resistance to electrolytic solution (that is, the property that swelling by the electrolytic solution hardly occurs) due to the reduction of free volume (intermolecular void) in the amorphous part. A single bond is preferred. 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, and 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 -, - OTi (OR) ₂ O—, —Al (OR) —, —OAl (OR) -, -OAl (OR) O- and the like. R is each independently an arbitrary substituent, preferably a hydrogen atom or an alkyl group, and m is a natural number. Among 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.

In the particularly preferred structure of the 1,2-diol structural unit represented by the general formula (1), 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.

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

In the general formulas (2), (3), and (4), R ^{1 to} R ⁶ are the same as those in the general formula (1). R ⁷ and R ⁸ are each independently hydrogen or R ⁹ —CO— (wherein R ⁹ is an alkyl group having 1 to 4 carbon atoms), and R ¹⁰ and R ¹¹ are each independently A hydrogen atom or an organic group, and the organic group is the same as in the case of the general formula (1).
Among the above methods, the method (i) is preferable in that it is excellent in copolymerization reactivity and industrial handling. Particularly, R ^{1 to} R ⁶ are hydrogen, X is a single bond, R ⁷ and R ⁸ are R ^9. 3,4-diasiloxy-1-butene, which is —CO— and R ⁹ is an alkyl group, is preferred, and 3,4-diacetoxy-1-butene in which R ⁹ is a methyl group is particularly preferred.

  The side chain 1,2-diol structural unit-containing PVA resin obtained by the method (ii) or (iii) is used when the degree of saponification is low, or when decarboxylation or deacetalization is insufficient. In some cases, a carbonate ring or an acetal ring may remain in the side chain. When such a PVA resin is used as a latex dispersant, the ratio of coarse particles tends to increase. For this reason, the PVA resin obtained by the method (i) is particularly suitable for this application.

When the PVA-based resin (A) contains the 1,2-diol structural unit, the content thereof is grafting of the PVA-based resin onto the ethylenically unsaturated polymer particles as a dispersoid during emulsion polymerization or an electrolytic solution. From the standpoints of the properties, stability of latex standing, 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 be lowered, and the resistance to electrolytic solution, the standing stability of latex, etc. tend to be lowered. When there is too much content, there exists a tendency for the internal resistance at the time of charging / discharging to become large.
The content of the 1,2-diol structural unit in the PVA resin (A) is ¹ H-NMR spectrum (solvent: DMSO-d6, internal standard: tetramethyl) of the PVA resin having a saponification degree of 100 mol%. Silane). Specifically, it can be calculated from the peak area derived from the hydroxyl proton, methine proton, and methylene proton in the 1,2-diol structural unit, the methylene proton in the main chain, the proton in the hydroxyl group linked to the main chain, and the like.

  In the present invention, one type of PVA resin (A) may be used, or two types may be used in combination.

[Styrenic thermoplastic elastomer (B)]
The styrenic 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 typified by styrene as a hard segment, a polymer block of a conjugated diene compound as a soft segment, and a double block remaining in the polymer block. Some or all of the bonds have hydrogenated blocks or isobutylene polymer blocks.
In particular, in the present invention, such an elastomer having a carboxylic acid group or its derivative group in the side chain is preferably used from the viewpoint of greatly improving the flexibility of the positive electrode and improving the production stability of the battery electrode. It is done. The reason why such an effect is obtained is not clear, but the presence of a carboxylic acid group or a derivative group thereof in the side chain of the styrene thermoplastic elastomer (B) results in the styrene thermoplastic elastomer (B). It is presumed that this acts as a reactive compatibilizing agent and can be finely dispersed in a PVA resin, particularly a PVA resin containing a structural unit having a primary hydroxyl group in the side chain.

  The constitution of each block in the elastomer (B) is as follows: 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 include a triblock copolymer represented by -XY, and a polyblock copolymer in which X and Y are alternately connected, and the structure includes linear, branched, and star shapes. be able to. Among these, a linear triblock copolymer represented by XYX is preferable in terms of mechanical properties.

  As a monomer used for forming a polymer block of an aromatic vinyl compound that is a hard segment, for example, styrene; α-methylstyrene, β-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, alkyl styrene such as t-butylstyrene, 2,4-dimethylstyrene, 2,4,6-trimethylstyrene; halogenated styrene such as monofluorostyrene, difluorostyrene, monochlorostyrene, dichlorostyrene, methoxystyrene; vinyl naphthalene, vinyl Examples thereof include vinyl compounds having an aromatic ring other than a benzene ring, such as anthracene, indene, and acetonaphthylene, and derivatives thereof. The polymer block of the aromatic vinyl compound may be a homopolymer block of the above-described monomer or a copolymer block of a plurality of monomers, but a styrene homopolymer block is preferably used.

  In addition, the polymer block of the aromatic vinyl compound may be obtained by copolymerizing a small amount of monomers other than the aromatic vinyl compound as long as the effects of the present invention are not impaired. Examples of such monomers 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. It is 10 mol% or less of 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.

  Moreover, as a monomer used for formation of the polymer block which is a soft segment, for example, 1,3-butadiene, isoprene (2-methyl-1,3-butadiene), 2,3-dimethyl-1,3-butadiene Conjugated diene compounds such as 1,3-pentadiene, and isobutylene, and these may be used alone or in combination. Of these, a homopolymer block or copolymer block of isoprene, butadiene, and isobutylene is preferable, and a homopolymer block of butadiene or isobutylene is particularly preferably used.

In the case of such a polymer block of a conjugated diene compound, a plurality of bonding forms may be taken by polymerization. For example, in butadiene, a butadiene unit (—CH ₂ —CH (CH═CH ₂ ) by 1,2-bonding is used. -) And 1,4-bonded butadiene units (—CH ₂ —CH═CH—CH ₂ —) are formed. Since these production ratios vary depending on the type of conjugated diene compound, it cannot be generally stated, but in the case of butadiene, the ratio of 1,2-bond production is usually in the range of 20 to 80 mol%.

The polymer block of such a conjugated diene compound can improve the heat resistance and weather resistance of the styrenic thermoplastic elastomer by hydrogenating 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.
By this hydrogenation, for example, the butadiene unit of 1,2-bond of butadiene becomes a butylene unit (—CH ₂ —CH (CH ₂ —CH ₃ ) —), and the butadiene unit generated by the 1,4-bond is Two continuous ethylene units (—CH ₂ —CH ₂ —CH ₂ —CH ₂ —) are formed, but usually the former is preferred.

  In addition, the polymer block which is such a soft segment may be obtained by copolymerizing a small amount of monomers other than the above-mentioned monomers within a range not inhibiting the effects of the present invention. 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. 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 polymer block derived from the conjugated diene compound or isobutylene in the elastomer (B) 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, the soft segment is a polymer block of a conjugated diene compound, or a part of the remaining double bond or It is composed of a polymer block that is entirely hydrogenated, a polymer block of isobutylene, etc., and typical examples thereof include a styrene / butadiene block copolymer (SBS) using styrene and butadiene as raw materials, 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 Styrene / isoprene block co-polymer made from styrene and isoprene Body (SIPS), styrene / isobutylene block copolymer of styrene and isobutylene as a raw material (SIBS) and the like can be illustrated, among others thermal stability, SEBS or SIBS is preferably used in terms of excellent weather resistance.

  The content ratio of the polymer block of the aromatic vinyl compound that is a hard segment and the polymer block that is a soft segment in the elastomer (B) is usually 10/90 to 70/30 in weight ratio, A range of 20/80 to 50/50 is preferred. If the content ratio of the polymer block of the aromatic vinyl compound is too large or too small, the balance between the flexibility of the elastomer (B) and the rubber elasticity tends to be lost, and as a result, the composition of the present invention, especially the latex There is a tendency that properties such as a dry film obtained from the above are insufficient.

The elastomer (B) obtains a block copolymer having a polymer block of an aromatic vinyl compound and a polymer block of a conjugated diene compound or isobutylene, and if necessary, in the polymer block of the conjugated diene compound. It can be obtained by hydrogenating double bonds.
First, as a method for producing a block copolymer having a polymer block of an aromatic vinyl compound and a conjugated diene compound or an isobutylene polymer block, a known method can be used, for example, an alkyl lithium compound or the like. As an initiator, a method of sequentially polymerizing an aromatic vinyl compound and a conjugated diene compound or isobutylene in an inert organic solvent can be exemplified.
Next, as a method of 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, etc. And a method using a reducing agent, hydrogen reduction using a metal catalyst such as platinum, palladium, Raney nickel, and the like.

The elastomer (B) used in the present invention preferably has a carboxylic acid group or a derivative group thereof in the side chain. By using a styrenic thermoplastic elastomer having such a carboxylic acid (derivative) group. In particular, it becomes possible to obtain a composition excellent in stability, particularly a latex.
That is, the elastomer (B) having a carboxylic acid group or a derivative group thereof in the side chain acts as a reactive compatibilizing agent by graft reaction with the PVA resin (A) during melt kneading. When the thermoplastic elastomer (B) acts as a reactive compatibilizing agent, the dispersibility of the thermoplastic elastomer (B) in the PVA resin (A) is improved, and fine dispersion becomes possible. Moreover, since a firm interface is formed with the PVA 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 mg CH ₃ ONa / g, particularly 1 to 15 mg CH ₃ ONa / g, those 2~10mgCH ₃ ONa / g is preferably used.
If the acid value is too low, the effect of introducing a carboxylic acid (derivative) group tends to be difficult to obtain, and if it is too high, a gel tends to be generated during melt-kneading with the PVA 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, an α, β-unsaturated carboxylic acid or its carboxylic acid group or its A method of copolymerizing a derivative or a method of adding an α, β-unsaturated carboxylic acid or a derivative thereof to the elastomer after the production of the elastomer is preferably used. Examples of such an addition method include a method using a 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 derivative thereof used for introducing the carboxyl group (derivative group) include α, β-unsaturated monocarboxylic acids such as acrylic acid and methacrylic acid; maleic acid, succinic acid, Α, β-unsaturated dicarboxylic acid such as itaconic acid and phthalic acid; α, β-unsaturated monocarboxylic acid ester such as glycidyl acrylate, glycidyl methacrylate, hydroxyethyl acrylate, hydroxymethyl methacrylate; maleic anhydride, succinic anhydride, Examples 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 from 50,000 to 500,000, particularly preferably from 120,000 to 450,000, more preferably from 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 resin (A) and the dispersibility in the PVA resin (A) tend to be reduced. If the weight average molecular weight is too small or the melt viscosity is too low, the mechanical strength of the dried 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.

  In the present invention, as the above-mentioned elastomer (B), one type may be used, but a plurality of types 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 having no carboxylic acid (derivative) group may be used in combination.

  Examples of commercially available styrenic thermoplastic elastomers (B) having such a carboxylic acid (derivative) group include “Tuftec M Series” manufactured by Asahi Kasei Co., Ltd., which is a carboxyl group-modified product of SBS, and “f−” manufactured by JSR. Dynalon ”,“ Clayton FG ”manufactured by Shell Japan, and the like can be mentioned.

Examples of commercially available styrenic thermoplastic elastomers (B) having no carboxylic acid (derivative) group include “Tufprene”, “Asaprene T”, “Asaflex”, and SBBS manufactured by Asahi Kasei Corporation. Asahi Kasei's “Tough Tech P Series” and SEBS's “Tough Tech H Series” can be listed.
Other commercially available products include, for example, “Clayton G”, “Clayton D”, “Califlex TR” manufactured by Shell Japan, “Septon”, “Hypler” manufactured by Kuraray, “Dynalon” manufactured by JSR, “JSR-” TR ”,“ JSR-SIS ”,“ Quintac ”manufactured by Zeon Corporation,“ Electrified STR ”manufactured by Electrochemical Co., Ltd., and the like.

[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 resin (A) having an average saponification degree of 60 to 95 mol% and a styrene thermoplastic elastomer (B). Yes, preferably (1) containing a PVA resin (A) and a styrene thermoplastic elastomer (B) melt-kneaded and dispersed in water to form a latex; (2) PVA resin Examples thereof include those obtained by subjecting (A) and a styrene-based thermoplastic elastomer (B) to emulsion polymerization under high pressure conditions to form a latex. Among them, latex [I] containing PVA-based resin (A) and styrene-based thermoplastic elastomer (B) in that it has simplicity in battery electrode production and suppresses an increase in resistance during low-temperature operation. It is preferable that it contains.
Below, latex [I] contained in the binder composition for positive electrodes of this invention is demonstrated.

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

In the latex [I] used in the present invention, the content ratio (A / B) of the PVA resin (A) and the styrene thermoplastic elastomer (B) is 90/10 to 60/40 in weight ratio. In particular, the range of 80/20 to 70/30 is preferably used. When the content ratio (A / B) is too small, the elastomer (B) is not sufficiently dispersed in the melt-kneaded product, and it is difficult to obtain a uniformly finely dispersed latex. On the other hand, if it is too large, the elastomer concentration in the resulting latex [I] will be low, and the ratio of the PVA 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 resin (A) and the elastomer (B) can be performed using a known kneading apparatus such as a kneader ruder, an extruder, a mixing roll, a Banbury mixer, or a blast mill. A method using a machine is industrially preferred.
Examples of such an extruder include a single screw extruder and a twin screw extruder. Among these, a twin screw extruder having the same screw rotation 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, and those having 15 to 70, more preferably 15 to 60 are preferably used. If the L / D is too small, melt kneading is insufficient, and the dispersibility of the elastomer (B) in the melt kneaded product may be insufficient. Conversely, if too large, it is preferable to give excessive shear. There is no tendency to cause shearing fever.

  The screw rotation speed of the extruder is usually from 10 to 400 rpm, particularly preferably from 30 to 300 rpm, more preferably from 50 to 250 rpm. If the rotational speed is too small, the ejection tends to be unstable, and if it is too large, the resin may be deteriorated due to undesirable shearing heat generation.

The resin temperature in the extruder is usually 80 to 260 ° C., and particularly preferably 130 to 240 ° C., more preferably 180 to 230 ° C. If the resin temperature is too high, the PVA resin (A) or the elastomer (B) tends to be thermally decomposed. On the other hand, if the resin temperature is too low, the melt kneading is insufficient and the elastomer (B) in the melt kneaded product is insufficient. Dispersibility tends to decrease.
The method for 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 number of rotations is used.

The melt-kneaded product discharged from the extruder is preferably made into pellets from the viewpoint of transfer to the next step and ease of handling, and the pelletizing method is not particularly limited. A method is used in which the material is extruded into strands, 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 or an air cooling method of blowing cool air is preferably used. The liquid needs to be an organic solvent that does not dissolve the PVA resin (A). For example, an alcohol solvent may be used, but an air cooling method is preferably used in terms of the environment.
The shape of such a pellet is usually cylindrical, and the size thereof is preferably smaller considering that it is later brought into contact with water and the PVA resin (A) therein is dissolved and removed. The diameter is preferably 2 to 6 mm, and the strand cut length is preferably about 1 to 6 mm. A method of cutting in the air or in an organic solvent while the melt-kneaded material discharged from the extruder is still in a molten state can be used, and in that case, a nearly spherical pellet is obtained. As the size in that case, pellets having a diameter in the range of 2 to 5 mm are preferably used.

Next, the process for producing the latex [I] from the melt-kneaded product of the PVA resin (A) and the styrene thermoplastic elastomer (B) obtained in the above process will be described.
This process involves dissolving the PVA resin (A) contained therein from the melt-kneaded product of the obtained PVA resin (A) and elastomer (B), and the method is not particularly limited. Usually, however, the latex [I] can be obtained by putting the pellets of the melt-kneaded product obtained by the above-mentioned method into water and stirring and heating as necessary.

  The solid 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. If the solid content concentration is too small, it takes a lot of energy and time to remove the moisture. If the solid content concentration is too large, the fluidity tends to decrease and workability tends to be hindered.

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

<Other ingredients>
In the positive electrode binder composition of the present invention, a coating agent used for a coating film, a compounding agent used for a molding resin, or the like can be blended. For example, light stabilizer, ultraviolet absorber, thickener, leveling agent, thixotropic agent, antifoaming agent, freezing stabilizer, matting agent, crosslinking reaction catalyst, pigment, curing catalyst, crosslinking agent {boric acid, methylolation Melamine, zirconium carbonate, diisopropoxytitanium bistriethanolaminate, etc.}, anti-skinning agents, dispersants, wetting agents, antioxidants, UV absorbers, rheology control agents, film forming aids, rust inhibitors, dyes, Examples include plasticizers, lubricants, reducing agents, antiseptics, antifungal agents, deodorants, anti-yellowing agents, antistatic agents, and charge control agents. They can be selected or combined according to their purpose. In addition, when the binder composition for positive electrodes contains these compounding agents, the organic content of the compounding agent to contain is contained in 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 with respect to 100 parts by weight of the solid content of the latex [I] in the positive electrode 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 slurry for a lithium ion secondary battery positive electrode.

As such an active material, a known general active material used for a positive electrode of a lithium ion secondary battery can be used.
As the positive electrode active material, for example, olivine-type lithium iron phosphate, lithium cobaltate, lithium manganate, lithium nickelate, ternary nickel cobalt lithium manganate, 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, 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. In addition, the value measured by the laser diffraction type particle size distribution measurement (laser diffraction scattering method) shall be employ | adopted for the average particle diameter of an active material.

  The content ratio of the active material and the binder composition in the positive electrode slurry is generally 0. 0% by solid content of the styrene thermoplastic elastomer (B) contained in the positive electrode binder composition with respect to 100 parts by weight of the active material. 1 to 10 parts by weight, preferably 0.1 to 5 parts by weight, particularly preferably 0.1 to 4 parts by weight. When the content of the positive electrode binder composition is too large, the internal resistance tends to increase. On the other hand, if the amount is too small, it is difficult to obtain a desired binding force between the active materials and an adhesive force to the current collector, the positive electrode becomes unstable, and the charge / discharge cycle characteristics tend to deteriorate.

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

  A conductive assistant means the compounded in order to improve electroconductivity. Examples of the conductive assistant include carbon powder such as graphite, and various carbon fibers such as vapor grown carbon fiber (VGCF (registered trademark)) and super gloss nanotube. In the production of the positive electrode of the lithium ion secondary battery of the present invention, when it is necessary to further increase the conductivity of the binder as a result of various blending, it is preferable to blend a conductive assistant, and VGCF (registered trademark) as the conductive assistant. ), The active material is effectively utilized, 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 with respect to the total mass of the active material layer.

  Furthermore, in consideration of workability at the time of producing the positive electrode, a positive electrode slurry may be prepared by adding a solvent for the purpose of adjusting the viscosity and adjusting the solid content of the positive electrode binder composition. 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, positive electrode binder composition and conductive additive, or improving the leveling properties during coating. May be. The type of thickener is not particularly limited, but mainly water-soluble polymers are preferably used because of miscibility with PVA-based resins and the use of water as a latex dispersion medium. It is done.
Examples of water-soluble polymers 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 salt thereof; gelatin; polyvinyl pyrrolidone; polyacrylic acid or salt thereof, polymethacrylic acid or salt thereof; polyacrylamide, acrylamide of polymethacrylamide sugar; vinyl acetate and maleic acid, maleic anhydride, Copolymerization with unsaturated acids such as acrylic acid, acrylic acid, methacrylic acid, itaconic acid, fumaric acid and crotonic acid A copolymer of styrene and the unsaturated acid; a copolymer of vinyl ether and the unsaturated acid; and salts or esters of the unsaturated acid and each copolymer, carrageenan, xanthan gum, sodium hyaluronate, Examples include natural polysaccharides such as locust bean gum, tara gum, guar gum, tamarind seed gum and the like.
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, particularly preferably 0.5 to 2%, based on the solid content of the positive electrode slurry. % By weight. If the amount used for the slurry is too small, the dispersion stability of the active material, the positive electrode binder and the conductive auxiliary agent tends to be reduced, or the positive electrode becomes non-uniform and stable charge / discharge tends not to be obtained. . On the other hand, if the amount is too large, the viscosity of the positive electrode slurry becomes too high, and it tends to be difficult to uniformly coat the current collector when preparing the positive electrode. There is a tendency that the internal resistance increases and the charge / discharge capacity decreases.

  For the mixing of the positive electrode binder composition, the active material, and the compounding agent and solvent used as necessary, a stirrer, a defoamer, a bead mill, a high-pressure homogenizer, or the like can be used. The positive electrode slurry is preferably prepared under reduced pressure. Thereby, it can prevent that a bubble arises in the active material layer obtained.

  The positive electrode slurry prepared as described above is applied onto a current collector and dried, whereby the lithium ion secondary battery positive electrode of the present invention (hereinafter sometimes abbreviated as “positive electrode of the present invention”). Can be manufactured. If necessary, it is preferable to increase the density by pressing after coating.

  As the current collector used in the positive electrode of the present invention, those used as the current collector of the positive electrode of the lithium ion secondary battery can be used. Specifically, for example, metal materials such as aluminum, copper, nickel, tantalum, stainless steel, titanium, and the like can be given, and can be appropriately selected and used according to the type of the target power 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. Moreover, as conditions for the drying process of the coating film of the slurry for positive electrodes, process temperature is 20-250 degreeC normally, Preferably it is 50-150 degreeC. Moreover, processing time is 1 to 120 minutes normally, Preferably it is 5 to 60 minutes.

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

The electrolyte swelling ratio 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 0, although it depends on the type and composition of the styrene-based thermoplastic elastomer. .1-5%.
When the electrolytic solution swelling ratio is in the above range, the positive electrode binder composition of the present invention tends to swell appropriately with respect to the electrolytic solution, effectively reducing internal resistance, and realizing better charge / discharge characteristics. There is. Moreover, it becomes possible to suppress the increase in resistance at the time of long-term charging / discharging by restrict | swelling with respect to the electrolyte solution of the binder composition for positive electrodes of this invention to a fixed range. This is presumably because the increase in battery resistance caused by the oxidative decomposition of the electrolytic solution can be suppressed by suppressing the contact between the positive electrode active material and the electrolytic solution.

Such an electrolyte swelling rate refers to a value measured as follows, for example.
The latex prepared as the positive electrode binder composition was cast on a PET film using a 500 μm applicator, and then heat-dried with a dryer at 105 ° C. for 3 hours to obtain a film. The resulting film is cut into a predetermined size and the weight is measured (W0 (g)). This film is immersed in 10 g of propylene carbonate (PC) or 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, and the electrolyte solution adhered to the film surface is wiped off. Then, the electrolyte swelling rate is calculated from the weight after immersion after the test (W1 (g)) according to the following formula.
Electrolytic solution swelling rate (%) = ((W1-W0) / W0) × 100

  Since the positive electrode obtained using the binder composition for a lithium ion secondary battery positive electrode of the present invention has improved flexibility and oxidation resistance compared to the conventional positive electrode, the mixture layer is cracked, the active material is peeled off and dropped off. In addition, an effect that the oxidative decomposition of the electrolyte solution hardly occurs can be obtained.

[Lithium ion secondary battery]
A lithium ion secondary battery having a positive electrode produced using the binder composition for a positive electrode of the lithium ion secondary battery 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 by the same method as the above-described positive electrode. For example, a negative electrode slurry containing a general binder composition, a negative electrode active material, and a compounding agent used as necessary is collected. It can be formed by coating 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. In addition, 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 for the negative electrode, for example, a metal foil such as copper or nickel, an etching metal foil, an expanded metal, or the like is used.

  As the electrolytic solution, an aprotic polar solvent that dissolves a lithium salt is used. Although not particularly limited, low-chain solvents such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, which are low-viscosity solvents, are contained in cyclic carbonate high dielectric constant / high boiling point solvents such as ethylene carbonate and propylene carbonate. Used. Specifically, for example, 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, butyl ethyl 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 these are preferably used as a mixture. Arbitrariness.

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 What is commonly used as an electrolyte of a nonaqueous electrolytic solution such as an organic salt such as ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , or LiN (SO ₃ CF ₃ ) ₂ may be used. Of these, LiPF ₆ , LiBF ₄ or LiClO ₄ is preferably used.

  As the separator, for example, a non-woven fabric of polyolefin or a porous film, a glass filter, a polyaramid film, a non-woven fabric or the like can be used from PVA resin.

  The structure of the secondary battery can be applied to any conventionally known form / structure such as a stacked (flat) battery or a wound (cylindrical) battery. In addition, the electrical connection form (electrode structure) in the lithium ion secondary battery can be applied to both (internal parallel connection type) batteries and bipolar (internal series connection type) batteries.

  The lithium ion secondary battery obtained as described above has improved flexibility of the electrode based on the use of the electrode binder composition of the present invention, cracking of the mixture layer, peeling and dropping of the active material. It becomes difficult to occur. Moreover, since the oxidation resistance is improved, the initial discharge capacity is high, and a high charge / discharge capacity can be stably obtained.

  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 positive electrode of a lithium ion secondary battery of the present invention includes a latex in which a styrene-based thermoplastic elastomer is dispersed in a PVA-based resin aqueous solution. The surface area of the PVA resin matrix inside is large, and the thickness of the PVA resin layer is reduced. As a result, the internal resistance of the battery is reduced while maintaining the electrolytic solution resistance and oxidation resistance unique to the PVA resin, and the safety and durability of the battery are improved. The reason why these effects are obtained is not clear, but the graft formation formed during the production of the binder composition at the interface between the styrene-based thermoplastic elastomer and the PVA-based resin matrix increases. This is presumably because the dispersed particle size of the thermoplastic elastomer is further reduced, the particle size distribution is made uniform, and the internal resistance can be reduced.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not limited to a following example, unless the summary is exceeded. In the examples, “part” means a weight basis.

[Analysis method]
The PVA resin produced in the following examples and comparative examples was analyzed by the following method.

(1) Average saponification degree (mol%)
The analysis was performed based on the alkali consumption required for hydrolysis of residual vinyl acetate and 3,4-diacetoxy-1-butene.

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

(3) Content of side chain 1,2-diol structural unit (modified amount) (mol%)
Using an AVANCE III HD 400 manufactured by BRUKER, ¹ H-NMR (400 MHz, proton NMR, solvent: heavy aqueous solution, temperature: 50 ° C.) was calculated from the integrated value based on the obtained NMR chart.

[Measurement evaluation method]
Binders prepared in the following examples and comparative examples were evaluated for battery characteristics by the following methods.

(1) Battery characteristics The prepared cell was left at 25 ° C. for 50 hours and then subjected to a charge / discharge test.
A constant current charge / discharge test was conducted at a current density of 30 mA / g and a potential range of 2.7-4.2 V, and the initial discharge capacity (mAh / g) and Coulomb efficiency (%) were measured.

(2-1) Initial discharge capacity (mAh / g)
The value obtained by dividing the product of the constant current value (control current value (mA)) at 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” (◎), 120 (mAh / g) or more and less than 140 (mAh / g) as “good” (◯), 120 (mAh / g) Less than was evaluated as “bad” (×).

(2-2) First-time coulomb efficiency (%)
The percentage (%) obtained by dividing the initial discharge capacity (mAh / g) by the initial charge capacity (mAh / g) was defined as the initial coulomb efficiency.
The initial coulombic efficiency was evaluated as “excellent” (）) when 90% or more, “good” (◯) when 80% or more and less than 90%, and “bad” (×) when less than 80%.

(2-3) Normal temperature resistance characteristics After 5 cycles at a current density of 30 mA / g and 25 ° C., the resistance value was measured by impedance at 25 ° C. 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. The lower the resistance value at a frequency of 10 Hz, the better. The absolute value of the resistance value is preferably closer to 0.
A resistance value (Ω) at 10 Hz of 40 or less was evaluated as “excellent” (◎), 40 to 80 or less evaluated as “good” (◯), and 80 or more evaluated as “bad” (×).

(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 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. The lower the resistance value at a frequency of 10 Hz, the better. The absolute value of the resistance value is preferably closer to 0.
A resistance value (Ω) at 10 Hz of 40 or less was evaluated as “excellent” (◎), 40 to 80 or less evaluated as “good” (◯), and 80 or more evaluated as “bad” (×).

The following were prepared.
[PVA 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 added in an amount of 0.3 mol% (compared with vinyl acetate), and the temperature was raised under a nitrogen stream while stirring to initiate polymerization. When the polymerization rate of vinyl acetate reaches 90%, m-dinitrobenzene is added to complete the polymerization, and then unreacted vinyl acetate monomer is removed out of the system by blowing methanol vapor. A combined methanol solution was obtained.

  Next, the methanol solution was further diluted with methanol, adjusted to a concentration of 45%, charged into a kneader, and a 2% methanol solution of sodium hydroxide was added to the vinyl acetate structural unit in the copolymer while maintaining the solution temperature at 35 ° C. Further, saponification was carried out by adding 3 mmol per 1 mol of the total amount of 3,4-diacetoxy-1-butene structural units. As saponification progressed, when saponified substances were precipitated and formed into particles, they were separated by filtration, washed well with methanol, and dried in a hot air drier to prepare the intended PVA resin (A1).

The saponification degree of the obtained PVA resin (A1) was 88 mol% when analyzed by the alkali consumption required for hydrolysis of residual vinyl acetate and 3,4-diacetoxy-1-butene. 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 computed 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 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 added in an amount of 0.3 mol% (compared with vinyl acetate), and the temperature was raised under a nitrogen stream while stirring to initiate polymerization. When the polymerization rate of vinyl acetate reaches 90%, m-dinitrobenzene is added to complete the polymerization, and then unreacted vinyl acetate monomer is removed out of the system by blowing methanol vapor. A combined methanol solution was obtained.

  Next, the methanol solution was further diluted with methanol, adjusted to a concentration of 45%, charged into a kneader, and a 2% methanol solution of sodium hydroxide was added to the vinyl acetate structural unit in the copolymer while maintaining the solution temperature at 35 ° C. Further, saponification was carried out by adding 3 mmol per 1 mol of the total amount of 3,4-diacetoxy-1-butene structural units. As saponification progressed, when saponified substances were precipitated and formed into particles, they were separated by filtration, washed well with methanol, and dried in a hot air dryer to prepare the intended PVA resin (A2).

The saponification degree of the obtained PVA-based resin (A2) was 88 mol% when analyzed by the 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 computed from the measured integral value.
The MFR (210 ° C., load 2160 g) was 6.1 g / 10 min, and the melt viscosity (220 ° C., shear rate 122 sec ⁻¹ ) was 858 Pa · s.

[PVA 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 added. .3 mol% (vs. vinyl acetate charged) was added, and the temperature was raised under a nitrogen stream while stirring to initiate polymerization. When the polymerization rate of vinyl acetate reaches 90%, m-dinitrobenzene is added to complete the polymerization, and then unreacted vinyl acetate monomer is removed out of the system by blowing methanol vapor. A combined methanol solution was obtained.

  Next, the methanol solution was further diluted with methanol, adjusted to a concentration of 45%, charged into a kneader, and a 2% methanol solution of sodium hydroxide was added to the vinyl acetate structural unit in the copolymer while maintaining the solution temperature at 35 ° C. And saponification was performed by adding 10.5 mmol with respect to 1 mol of the total amount of 3,4-diacetoxy-1-butene structural units. When saponification progresses and saponification precipitates and becomes particulate, it is filtered, washed well with methanol and dried in a hot air dryer to produce the desired PVA resin (A'1) did.

The saponification degree of the obtained PVA-based resin (A′1) was 99 mol% when analyzed by the alkali consumption required for hydrolysis of residual vinyl acetate and 3,4-diacetoxy-1-butene. . Moreover, 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% when computed from the measured integral value.
The MFR (210 ° C., load 2160 g) was 28 g / 10 min, and the melt viscosity (220 ° C., shear rate 122 sec ⁻¹ ) was 274 Pa · s.

[PVA 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 added in an amount of 0.3 mol% (compared with vinyl acetate), and the temperature was raised under a nitrogen stream while stirring to initiate polymerization. When the polymerization rate of vinyl acetate reaches 90%, m-dinitrobenzene is added to complete the polymerization, and then unreacted vinyl acetate monomer is removed out of the system by blowing methanol vapor. A combined methanol solution was obtained.

  Next, the methanol solution was further diluted with methanol, adjusted to a concentration of 45%, charged into a kneader, and a 2% methanol solution of sodium hydroxide was added to the vinyl acetate structural unit in the copolymer while maintaining the solution temperature at 35 ° C. And saponification was performed by adding 10.5 mmol with respect to 1 mol of the total amount of 3,4-diacetoxy-1-butene structural units. When saponification progresses and saponification precipitates and forms particles, it is filtered, washed well with methanol and dried in a hot air dryer to produce the desired PVA resin (A'2) did.

The saponification degree of the obtained PVA-based resin (A′2) was 99 mol% when analyzed by the 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 computed 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.

[Styrenic thermoplastic elastomer (B1)]
Styrene / ethylene / butylene block copolymer having a carboxyl group (SEBS) (manufactured by Asahi Kasei Corporation, "Tuftec M1911" (oxidation 10mgCH ₃ ONa / g, the melt viscosity of 1100 mPa · s (220 ° C. shear rate ^{122 sec} -1))

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

(Manufacture of binder composition solution)
30 parts of the obtained pellets were put into 70 parts of water at room temperature, heated while stirring, and 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 as follows, and a lithium ion secondary battery positive electrode and then a lithium ion secondary battery were prepared. The battery characteristics were measured and evaluated according to the evaluation method described above. The results are shown in Table 1.

[Preparation of positive electrode for lithium ion secondary battery]
<Production of positive electrode for battery using positive electrode active material>
94 parts of LiNiMnCoO ₂ (manufactured by Nippon Chemical Industry Co., Ltd., average particle size 10 μm) as an active material, 3 parts of acetylene black (“DENKA BLACK” produced by Denki Kagaku Kogyo Co., Ltd.) as a conductive additive, and 1. After adding 1.5 parts of carboxymethylcellulose # 2200 (manufactured by Daicel Finechem Co., Ltd.) prepared in a 5% by weight aqueous solution in terms of solid content and adding purified water in a timely manner, a planetary kneader (“Foam Removal” manufactured by Shinky Co., Ltd.) Rintaro ") was used to obtain a paste having a solid 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 composition solution (20% by weight) obtained above as a positive electrode binder in terms of solid content is added, and purified water is added in a timely manner, and then a planetary kneader. Was used under the same conditions to obtain an active material paste having a solid content concentration of 55.9% by weight.
Next, a 180 μm applicator and a coating machine (“Control Coater (coating machine)” manufactured by Imoto Seisakusho Co., Ltd.) are used on the surface of a rolled aluminum foil (made by Foil, thickness 18 μm) as a current collector. Then, the active material paste was applied at a coating speed of 10 mm / second. This was dried at 60 ° C. for 10 minutes to obtain a positive electrode for a battery.

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

[Comparative Example 1]
In Example 1, it carried out similarly except having used the binder composition solution for positive electrodes prepared similarly using the following resin composition, the battery was produced, and evaluation similar to Example 1 was performed.

(Manufacture of resin composition)
After dry blending 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), it was melt kneaded under the following conditions using a twin screw extruder. Then, it was extruded into a strand shape and cut with a pelletizer to obtain a melt-kneaded pellet (diameter: about 1 mm, length: about 2 mm).
Diameter (D) 15mm
L / D = 60
Screw rotation speed: 200rpm
Setting 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 resin (A) having a low average saponification degree, the initial discharge capacity is high, and the battery resistance characteristics at a low temperature of 0 ° C. are good. I understand. On the other hand, in Comparative Example 1 using a PVA-based resin having a high average degree of saponification, the battery resistance value was particularly high at a low temperature, and the object of the present invention was not satisfied.

Claims (8)

  A binder composition for a positive electrode of a lithium ion secondary battery, comprising a polyvinyl alcohol resin (A) having an average saponification degree of 60 to 95 mol% and a styrene thermoplastic elastomer (B).   The lithium ion according to claim 1, comprising a latex [I] comprising a polyvinyl alcohol resin (A) having an average saponification degree of 60 to 95 mol% and a styrene thermoplastic elastomer (B). Binder composition for secondary battery positive electrode.   The content ratio (A / B) of the polyvinyl alcohol-based resin (A) and the styrene-based thermoplastic elastomer (B) is 90/10 to 60/40 in weight ratio. The lithium ion secondary battery positive electrode binder composition.   The binder composition for a lithium ion secondary battery positive electrode according to any one of claims 1 to 3, wherein the polyvinyl alcohol resin (A) has a viscosity average polymerization degree of 350 to 3000.   5. The lithium ion secondary battery positive electrode according to claim 1, wherein the polyvinyl alcohol resin (A) is a polyvinyl alcohol resin (a) containing a structural unit having a primary hydroxyl group in the side chain. Binder composition.   6. The lithium ion secondary battery positive electrode according to claim 1, wherein the styrenic thermoplastic elastomer (B) is a styrenic thermoplastic elastomer having a carboxylic acid group or a derivative group thereof in the side chain. Binder composition.   A lithium ion secondary battery positive electrode comprising the binder composition for a lithium ion secondary battery positive electrode according to any one of claims 1 to 6.   A lithium ion secondary battery comprising the lithium ion secondary battery positive electrode according to claim 7.