JP6907505B2 - Binder composition for secondary battery negative electrode, secondary battery negative electrode and secondary battery - Google Patents

Description

The present invention relates to a binder composition for a negative electrode of a secondary battery, a negative electrode of a secondary battery, and a secondary battery. Specifically, a binder composition used together with a negative electrode active material to form a negative electrode of a secondary battery, a secondary battery negative electrode containing the binder composition, and a secondary battery having the secondary battery negative electrode. Regarding.

In a 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 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 generally formed by applying a mixed slurry of an active material and an electrode binder to the surface of a current collector and drying it.

Here, the binder for the electrode has a function of binding the active materials to each other and also binding the metal foil which is a current collector and the active material. If the binder cannot bind a sufficient amount of active material to the current collector or bind the active materials to each other, a battery having a large capacity cannot be obtained. Further, if the binding force of the binder is reduced due to the volume fluctuation of the active material due to repeated charging and discharging, the active material may fall off from the current collector and the capacity of the battery may be reduced.

As for secondary batteries, there is a tendency that a large capacity is required for in-vehicle use and the like, and the problem is that the active material of the battery electrode falls off due to the deformation of the active material layer during charging and discharging. Therefore, there is a demand for a high-performance binder for a secondary battery electrode that sufficiently binds the active material and the current collector with a small amount of binder.

On the other hand, in recent years, as an active material for a battery negative electrode capable of increasing the capacity, instead of the conventional graphite-based active material, a semi-metal, a metal or an active material of these alloys (hereinafter, collectively referred to as "alloy-based active material"). The use of "substances") is being considered. For example, in Patent Document 1, polyvinyl alcohol is used as an electrode binder for a secondary battery for an alloy-based active material, which has a larger volume expansion than a graphite-based active material and is less likely to form a uniform slurry than a graphite-based active material. It is disclosed that it is possible.
However, in the examples of the same document, a solvent-based polyacrylic nitrile is used as a binder. In addition to being pointed out as having problems with environmental compatibility, polyacrylic nitrile has room for further improvement in binding properties and dispersibility of alloy-based active materials.

Further, in Patent Document 2, a binder composition containing an emulsion using a polyvinyl alcohol-based resin having a 1,2-diol structural unit in the side chain as a dispersant is used for an alloy-based active material such as silicon powder. Is disclosed.
However, the binder composition of the same document also has room for further improvement in the binding property and the dispersibility of the alloy-based active material.

Special Table 2012-519948 JP-A-2015-144124

The present invention has been made in view of such circumstances, and an object of the present invention is for a secondary battery negative electrode having a high active material binding property that sufficiently binds an alloy-based active material and a current collector. The present invention provides a binder composition, a secondary battery negative electrode containing the binder composition, and a secondary battery having the secondary battery negative electrode.

As a result of diligent studies, the present inventors have found that a binder composition containing a polyvinyl alcohol-based resin having a relatively high proportion of structural units having a primary hydroxyl group in the side chain is merely additive in the combination. However, it has been clarified that it has a higher binding property to active substances, and the present invention has been completed.

That is, the gist of the present invention is a binder composition for producing a negative electrode of a secondary battery containing an element capable of forming an alloy with lithium as an active material, and has 7 structural units having a primary hydroxyl group in the lower chain. It exists in a binder composition for a negative electrode of a secondary battery containing a polyvinyl alcohol-based resin having ~ 15 mol%.

Hereinafter, the “secondary battery negative electrode binder composition” may be simply abbreviated as the “negative electrode binder composition”.

Further, the gist of the present invention also exists in the secondary battery negative electrode containing the binder composition for the secondary battery negative electrode of the present invention and the secondary battery having the secondary battery negative electrode of the present invention.

Since the binder composition for the negative electrode of the secondary battery of the present invention contains a polyvinyl alcohol-based resin having a relatively high proportion of structural units having a primary hydroxyl group in the side chain, binding of the alloy-based active material and the current collector Excellent in sex. Therefore, peeling / falling off of the alloy-based active material is less likely to occur, and a secondary battery using a secondary battery negative electrode containing the binder composition can stably obtain a high charge / discharge capacity. Become.
The reason why the active material binding property of the secondary battery negative electrode containing the binder composition for the secondary battery negative electrode of the present invention is good is not clear, but the following reasons are presumed.
First, since the structural unit having a primary hydroxyl group in the side chain of the polyvinyl alcohol-based resin contains an active hydrogen group, it has good water dispersibility and also has a function of suppressing the formation of crystal nuclei (microjunctions) in an aqueous solution. can get. As a result, the flow characteristics of the slurry obtained by mixing the binder and the alloy-based active material are improved, and a uniform electrode layer tends to be obtained.
Generally, in order to improve the flow characteristics of the slurry, it is conceivable to lower the solid content concentration, but in that case, the dispersion stability is lowered and excessive energy is required at the time of drying.
However, by using a polyvinyl alcohol-based resin having a relatively high proportion of structural units having a primary hydroxyl group in the side chain of the present invention as a binder, the dispersion stability and good flow of the slurry are maintained even if the solid content concentration is kept high. It becomes possible to guarantee the characteristics. In particular, alloy-based active materials tend to be more likely to be secondarily agglutinated than graphite-based active materials due to the interaction between particles on the surface of the active material. Therefore, as the binder for the alloy-based negative electrode, a binder having a higher dispersibility than the binder for the graphite-based negative electrode is required, and the binder has a structural unit having a primary hydroxyl group in the side chain at a relatively high ratio of 7 to 15 mol%. By using a binder containing a polyvinyl alcohol-based resin, the problem can be preferably solved. In the electrode made of the slurry thus obtained, the active material and the binder are likely to be uniformly distributed, and it is considered that the active material binding force is less likely to decrease due to the uneven distribution of the binder.

It is a figure which shows the actual measurement result in the peeling test of the negative electrode obtained in Example 1. FIG. It is a figure which shows the actual measurement result in the peeling test of the negative electrode obtained in the comparative example 1. FIG. It is a figure which shows the actual measurement result in the peeling test of the negative electrode obtained in the comparative example 2. It is a figure which shows the actual measurement result in the peeling test of the negative electrode obtained in the comparative example 3. FIG.

The binder composition for the negative electrode of the secondary battery of the present invention contains a polyvinyl alcohol-based resin having 7 to 15 mol% of structural units having a primary hydroxyl group in the side chain (hereinafter, the polyvinyl alcohol-based resin is also referred to as PVA-based resin). do. Hereinafter, the PVA-based resin will be described.

[PVA-based resin]
The PVA-based resin in the present invention has at least a structural unit having a primary hydroxyl group in the side chain and a structural unit derived from a vinyl ester monomer, and constitutes, for example, a vinyl ester-based monomer and a structural unit having a primary hydroxyl group in the side chain. It is obtained by polymerizing and saponifying a monomer to be produced, and in some cases, another monomer.

(Vinyl ester monomer)
Typical examples of the vinyl ester-based monomer include vinyl acetate. Instead of vinyl acetate, for example, vinyl propionate, vinyl butyrate, vinyl caproate, vinyl caprilate, vinyl caprate, vinyl laurate, vinyl myristate, vinyl palmitate, vinyl stearate, vinyl cyclohexanecarboxylate, piperin Vinyl acetate, vinyl octylate, vinyl monochloroacetate, vinyl adipate, vinyl methacrylate, vinyl crotonic acid, vinyl sorbate, vinyl benzoate, vinyl katsura, vinyl trifluoroacetate, etc. can be exemplified, but the price and availability are easy. From this point of view, vinyl acetate is preferably used.

(Structural unit having a primary hydroxyl group in the side chain and the monomers constituting it)
In the structural unit having a primary hydroxyl group in the side chain, the number of primary hydroxyl groups 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 such a structural unit having a primary hydroxyl group in the side chain include a structural unit having a 1,2-diol structure in the side chain, a structural unit having a hydroxyalkyl group in the side chain, and the like. Among them, a structural unit having a 1,2-diol structure in the side chain, which is represented by the following general formula (1), is particularly preferable.
Hereinafter, the structural unit having a 1,2-diol structure is referred to as "1,2-diol structural unit", and the structural unit having a 1,2-diol structure in the side chain is referred to as "side chain 1,2-diol structural unit". Also called.

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

In the above formula (1), R ¹ , R ² and R ³ are independently hydrogen atoms or organic groups, and similarly, R ⁴ , R ⁵ and R ⁶ are independently hydrogen atoms or organic groups, respectively. be. It is desirable that all of R ^{1 to} R ⁶ are hydrogen atoms, but they may be organic groups as long as the amount does not significantly impair the resin properties. The organic group is not particularly limited, and for example, 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 can be used. 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 bonded chain, and is resistant to electrolytic solution by reducing free volume (intermolecular voids) in the amorphous portion (that is, a property that swelling by the electrolytic solution is unlikely to occur). , It is preferable that it is a single bond. The binding chain is not particularly limited, and for example, hydrocarbons such as alkylene, alkenylene, alkynylene, phenylene, and naphthylene (these hydrocarbons may be substituted with halogens such as fluorine, chlorine, and bromine). In addition, -O-,-(CH ₂ O) _m -,-(OCH ₂ ) _m -,-(CH ₂ O) _m CH _2- , -CO-, -COCO-, -CO (CH ₂ ) _{m CO -, - 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 independently a hydrogen atom or an arbitrary substituent, a hydrogen atom or an alkyl group (especially C ₁ -C ₄ alkyl group). Further, m is a natural number, preferably 1 to 10.
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.

A particularly preferable structure in 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.

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

In the above formulas (2), (3) and (4), R ^{1 to} R ⁶ are all the same as in the case of the general formula (1). R ⁷ and R ⁸ are 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 independent, respectively. It is a hydrogen atom or an organic group, and the organic group is the same as in the case of the formula (1).
Of the above methods, the method (i) is preferable in that it is excellent in copolymerization reactivity and industrial handling, and in particular, R ^{1 to} R ⁶ are hydrogen, X is a single bond, and R ⁷ and R ⁸ are R ^9. 3,4-Diacyloxy-1-butene which is −CO− and R ⁹ is an alkyl group is preferable, and among them, 3,4-diacetoxy-1-butene in which ^{R 9 is a methyl group is particularly preferably used.}

The PVA-based resin containing the side chain 1,2-diol structural unit obtained by the methods (ii) and (iii) has a low degree of saponification or is insufficiently decarboxylated or deacetalized. In some cases, a carbonate ring or an acetal ring may remain in the side chain. When such a PVA-based resin is used, it tends to swell in the electrolytic solution and the binding force tends to decrease. For this reason as well, the PVA-based resin obtained by the method (i) is particularly suitable for this application.

(Other monomers)
The PVA-based resin contains vinyl ester-based monomers and structural units other than those represented by the general formula (1) within a range that does not impair the effects of the present invention (for example, 10 mol% or less, preferably 5 mol% or less). It may have a structural unit derived from another monomer.
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. A monomer having two or more allyl groups such as allyl cyanurate, triallyl isocyanurate, tetraallyloxyethane, and diallyl phthalate; allyl ester-based monomers such as allyl acetate, acetoacetic acid vinyl ester, acetoacetic acid allyl ester, and diacetacetate allyl ester. Acetoacetoxyalkyl (meth) acrylates such as acetoacetoxyethyl (meth) acrylate and acetoacetoxypropyl (meth) acrylate; 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) ) Alkylene glycol (meth) acrylates such as acrylate, 1,6-hexanediol di (meth) acrylate, neopentyl glycol di (meth) acrylate; allyl (meth) acrylate; 2-hydroxy Hydroxyalkyl (meth) acrylates such as ethyl (meth) acrylate, hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate (the alkyl moiety is a C1-C10 alkyl group, preferably a C1-C6 alkyl group); (Meta) nitrile-based monomers such as acrylonitrile; styrene-based monomers such as styrene and α-methylstyrene; olefins such as ethylene, propylene, 1-butene and isobutene; vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride and the like. Halled olefins; olefin-based 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 System monomers: 3-butene-1-ol, 4-pentene-1-ol, 5-hexene-1,2-diol, glycerin monoallylae Hydroxy group-containing α-olefins such as tel and derivatives 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 thereof or mono or dialkyl esters; nitriles such as acrylonitrile, methacrylamide, diacetoneacrylamide, etc. Examples thereof include compounds such as amides, ethylene sulfonic acid, allyl sulfonic acid, metaallyl sulfonic acid, olefin sulfonic acid such as AMPS, and salts thereof. These monomers may be used alone or in combination of two or more.

The PVA-based resin contained in the binder composition for the negative electrode of the secondary battery of the present invention is, for example, a vinyl ester-based monomer, a monomer having a primary hydroxyl group in the side chain, and particularly a monomer constituting a 1,2-diol structural unit. , In some cases, it is obtained by polymerizing other monomers and saponifying them.
In addition, each unit composed of each monomer may exist at random or may exist in a block.

The polymerization of these monomers can be carried out by any known polymerization method, for example, solution polymerization, suspension polymerization, emulsion polymerization and the like. Above all, it is preferable to carry out solution polymerization under reflux, which can efficiently remove the heat of reaction. Alcohol is usually used as the solvent for solution polymerization, and lower alcohol having 1 to 3 carbon atoms is preferably used.

As for the saponification of the obtained polymer, a known saponification method used in conventional PVA-based resins can be adopted. That is, saponification can be carried out using an alkali catalyst or an acid catalyst in a state where the polymer is dissolved in alcohol or water / alcohol solvent.
As the alkali catalyst, hydroxides or alcoholates of alkali metals such as potassium hydroxide, sodium hydroxide, sodium methylate, sodium ethylate, potassium methylate, and lithium methylate can be used.
Usually, a transesterification reaction using an alkali catalyst under 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 decrease and the reaction efficiency tends to decrease, and if it is too high, the boiling point of the reaction solvent may be exceeded, which tends to reduce the safety in terms of production. When saponification is performed under high pressure using a tower-type continuous saponification tower having high pressure resistance, saponification can be performed at a higher temperature, for example, 80 to 150 ° C., and a small amount of saponification catalyst can be used. However, it is possible to obtain a product with a high degree of saponification in a short time.
The resin concentration at the start of the saponification reaction is 10 to 70% by weight, preferably 25 to 60% by weight, and particularly preferably 30 to 58% by weight. If the resin concentration is too low, the bulk specific gravity of the saponified product becomes small, and it tends to be difficult to granulate the binder composition. Further, if the resin concentration is too high, the dispersion of the alkali catalyst used as the saponification catalyst tends to be poor, and the saponification degree distribution tends to widen.

In the present invention, the content (modification amount) of the structural unit having a primary hydroxyl group in the side chain in the PVA resin is 7 to 15 mol%, preferably 8 to 13 mol%, and particularly preferably 8 to 11 mol. %. If the content is too low, the dispersibility to the alloy-based active material will be lowered, and the binding property to the current collector will be lowered. If the content is too high, the inside of the battery will be charged and discharged when the battery is manufactured. The resistance will increase.
The content of the structural unit having a primary hydroxyl group in the side chain in the PVA resin is ¹ H-NMR spectrum (solvent: DMSO-d6, internal standard: tetramethylsilane) of the PVA resin having a saponification degree of 100 mol%. ) Can be obtained. For example, specifically, in the case of a side chain 1,2-diol structural unit, it is linked to a hydroxyl group proton, a methine proton, and a methylene proton in the side chain 1,2-diol structural unit, a methylene proton in the main chain, and a main chain. It can be calculated from the peak area derived from the proton of the hydroxyl group.

In the present invention, the saponification degree of the PVA-based resin (measured according to JIS K6726) is usually 85 to 100 mol%, preferably 90 to 99.9 mol%. If the degree of saponification is too low, it tends to swell with respect to the electrolytic solution, and as a result, the binding property between the electrode members tends to decrease. On the other hand, a PVA-based resin having a high degree of saponification, particularly a completely saponified PVA resin, tends to be industrially difficult to produce, and the preparation of an aqueous solution and the stability of standing are lowered.

The viscosity average degree of polymerization (measured according to JIS K6726) is usually 400 to 4000, preferably 500 to 2800, and particularly preferably 600 to 2500. If the viscosity average degree of polymerization is too low, the obtained film tends to be brittle and the electrode stability tends to decrease. On the contrary, if it is too high, the solvent / monomer ratio at the time of polymerization is reduced and the product is produced at a low polymerization rate. As a result, productivity tends to decrease significantly. If a low temperature catalyst is used to increase the productivity, the polymerization rate becomes slow and it is difficult to obtain practical productivity.

[Binder composition for secondary battery negative electrode]
The binder composition for the negative electrode of the secondary battery of the present invention contains at least a PVA-based resin having a structural unit having a primary hydroxyl group in the side chain.
The form of the binder composition of the present invention is not particularly limited, and may be, for example, a solid such as powder or flakes, or a liquid such as a paste, solution or suspension.

Various compounding agents can be blended in the binder composition of the present invention. For example, light stabilizers, UV absorbers, thickeners, leveling agents, tincture agents, antifoaming agents, freeze stabilizers, matting agents, cross-linking reaction catalysts, pigments, curing catalysts, cross-linking agents {rheology, methylolization. Melamine, zirconium carbonate, diisopropoxytitanium bistriethanolaminate, etc.}, anti-skinning agent, dispersant, wetting agent, antioxidant, UV absorber, rheology control agent, film-forming aid, rust preventive, dye, Examples thereof include plasticizers, lubricants, reducing agents, preservatives, fungicides, deodorants, anti-yellowing agents, anti-static agents and anti-static agents. It can be selected or combined according to each purpose.
The blending amount of the above-mentioned compounding agent is usually 10 parts by weight or less, preferably 5 parts by weight or less, based on 100 parts by weight of the solid content in the binder composition.

[Secondary battery negative electrode]
The negative electrode of the secondary battery of the present invention contains at least the binder composition for the negative electrode of the secondary battery of the present invention and the alloy-based active material for the negative electrode. The secondary battery negative electrode of the present invention is usually produced by mixing a binder composition and an alloy-based active material to prepare a slurry for a secondary battery negative electrode, applying this slurry on a current collector, and drying the slurry. be able to.
The secondary battery negative electrode of the present invention can be applied to various secondary batteries, for example, lithium ion secondary battery, lithium ion polymer secondary battery, lead storage battery, nickel / hydrogen storage battery, nickel / cadmium storage battery, nickel. -Can be applied to iron storage batteries, nickel / zinc storage batteries, silver oxide / zinc storage batteries, sodium batteries, air aluminum batteries, etc. In the following, a lithium ion secondary battery will be described as an example.

The active material used for the negative electrode of the secondary battery is not particularly limited as long as it contains an element capable of forming an alloy with lithium, and specifically, tin, aluminum, silicon, bismuth, zinc, arsenic, antimony, etc. And at least one or more elements selected from the group consisting of lead can be used. The negative electrode active material in the present invention can be used in a powder state, but it may be composed of a single element, or may be an oxide or an alloy thereof. Among these, silicon, tin, and oxides and alloys containing these are preferable, and oxides and alloys containing silicon are particularly preferable, because they can be alloyed with a large amount of lithium per unit weight. Examples of alloys containing silicon or tin include silicon-titanium alloys, silicon-nickel alloys, tin-iron alloys, tin-nickel alloys, tin-copper alloys, tin-zinc alloys, tin-titanium alloys and the like.

The average particle size of the active material is usually 5 nm to 100 μm, preferably 20 nm to 50 μm, and particularly preferably 50 nm to 25 μm. The average particle size of the active material shall be the value measured by the laser diffraction type particle size distribution measurement (laser diffraction / scattering method).

The content ratio of the active material to the binder composition in the negative electrode slurry is usually 0.1 to 20 parts by weight, preferably 0, in terms of solid content of the binder composition with respect to 100 parts by weight of the active material. .1 to 15 parts by weight, particularly preferably 0.1 to 10 parts by weight. If the content of the binder composition for the negative electrode is too large, the internal resistance tends to increase. On the other hand, if the amount is too small, the desired binding force between the active materials and the adhesive force to the current collector cannot be obtained, the electrodes become unstable, and the charge / discharge cycle characteristics tend to deteriorate.

The negative electrode slurry may contain other substances in addition to the alloy-based active material and the negative electrode binder composition. Other substances may include, for example, a conductive additive, a supporting salt (lithium salt), an ionic conductive polymer and the like. When an ionic conductive polymer is included, a polymerization initiator for polymerizing the polymer may be included. The compounding ratio of these components is in a known general range. The compounding ratio can also be adjusted by appropriately referring to known knowledge about the lithium ion secondary battery.

The conductive auxiliary agent refers to a compound compounded to improve conductivity. Examples of the conductive auxiliary agent include carbon powder such as graphite, various carbon fibers such as vapor-grown carbon fiber (VGCF (registered trademark)), and super-gloss nanotubes. When it is necessary to further increase the conductivity of the binder as a result of various blending in the production of the negative electrode of the secondary battery of the present invention, it is preferable to blend a conductive auxiliary agent, and VGCF (registered trademark) is used as the conductive auxiliary agent. When used, the active material can be 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.

Further, in consideration of workability at the time of producing the negative electrode, a solvent may be added to prepare a slurry for the negative electrode for the purpose of adjusting the viscosity, adjusting the solid content of the binder composition, and the like. As such a solvent, the same solvent as the above-mentioned organic solvent can be used.

The negative electrode slurry may contain a dispersant for the purpose of improving the dispersibility of the active material, the negative electrode binder composition, the conductive auxiliary agent, and the like. The type of dispersant is not particularly limited, but a water-soluble polymer is mainly preferably used because of its miscibility with PVA-based resins. The water-soluble polymer in the present invention excludes not only PVA-based resins having a structural unit having a primary hydroxyl group in the side chain, but also other PVA-based resins.
Examples of water-soluble polymers other than PVA-based resins include water-soluble polymers such as methyl cellulose, ethyl cellulose, hydroxymethyl cellulose, hydroxypropyl methyl cellulose, hydroxybutyl methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, aminomethyl hydroxypropyl cellulose, and aminoethyl hydroxypropyl cellulose. Cellulose; starch, tragant, pectin, glue, alginic acid or salts thereof; gelatin; polyvinylpyrrolidone; polyacrylic acid or salts thereof, polymethacrylic acid or salts thereof; polyacrylamides, acrylamides of polymethacrylicamide sugars; vinyl acetate and maleic acid , Polymeric anhydride, acrylic acid, acrylic acid, methacrylic acid, itaconic acid, fumaric acid, crotonic acid and other unsaturated acids; polymer of styrene and the unsaturated acid; vinyl ether and the unsaturated acid. Polymers with acids; and natural polysaccharides such as salts or esters of the unsaturated acids and each copolymer, carrageenan, xanthan gum, sodium hyaluronate, locust bean gum, tara gum, guar gum, tamarind seed gum, etc. , Preferably water-soluble cellulose.

The amount of the dispersant used in the negative electrode slurry is usually 0.01 to 5% by weight, preferably 0.1 to 4% by weight, particularly preferably 0.5 to 4% by weight, based on the solid content of the negative electrode slurry. %. If the amount used for the slurry is too small, the dispersion stability of the active material, the binder for the negative electrode, the conductive auxiliary agent, etc. becomes low, and the electrodes tend to be non-uniform, making it difficult to obtain stable charge / discharge. .. On the other hand, if the amount is too large, the viscosity of the slurry for the negative electrode becomes too high, and it tends to be difficult to uniformly coat the current collector when manufacturing the negative electrode. The internal resistance of the battery tends to increase and the charge / discharge capacity tends to decrease.

A stirrer, a defoamer, a bead mill, a high-pressure homogenizer, or the like can be used for mixing the binder composition for the negative electrode, the active material, and the compounding agent and the solvent used as needed.

The secondary battery negative electrode of the present invention (hereinafter, may be abbreviated as "negative electrode of the present invention") is manufactured by applying the negative electrode slurry prepared as described above onto a current collector and drying it. can do. If necessary, the density can be adjusted by pressing after application.

As the current collector used for the negative electrode of the present invention, the current collector used as the negative electrode of the lithium ion secondary battery can be used. Specifically, metal foils such as copper, SUS, and nickel, etched metal foils, expanded metals, and the like are used, and can be appropriately selected and used according to the type of the target power storage device.

A negative electrode layer can be formed by applying a negative electrode slurry on such a current collector and drying it. Examples of the method of applying the negative 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. Further, as a condition for drying the coating film of the negative electrode slurry, the treatment temperature is usually 20 to 180 ° C, 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 side of the coating layer) is usually 1 to 300 μm, preferably 2 to 100 μm, and particularly preferably 2 to 30 μm.

The negative electrode obtained by using the binder composition for the negative electrode of the secondary battery of the present invention is excellent in the dispersibility of the alloy-based active material and the binding property between the active material and the current collector, so that the active material can be peeled off or dropped off. The effect of being less likely to occur can be obtained.

[Secondary battery]
As an example of the secondary battery of the present invention, a lithium ion secondary battery having a negative electrode manufactured by using the binder composition for the negative electrode of the 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.

As the electrolytic solution, an aprotic polar solvent that dissolves a lithium salt is used. Although not particularly limited, a cyclic carbonate-based high dielectric constant / high boiling point solvent such as ethylene carbonate or propylene carbonate is contained with a low viscosity solvent such as dimethyl carbonate, ethyl methyl carbonate or diethyl carbonate or the like. Used for. Specifically, ethylene carbonate, chloroethylene carbonate, trifluoropropylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, isopropylmethyl carbonate, ethylpropyl carbonate, isopropylethyl carbonate, butylmethyl carbonate, Butyl ethyl carbonate, dipropyl carbonate, 1,2-dimethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyl tetrahydrofuran, sulfolane, 3-methylsulfone, 2,4-dimethylsulfolane, 1,3-dioxolane, methyl acetate, acetate Ethyl, methyl formate, ethyl formate and the like can be mentioned, and these are preferably used 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 , and LiN (SO ₂ C ₂ F _5). ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , LiN (SO ₃ CF ₃ ) _{2, and} other organic salts that are commonly used as electrolytes for non-aqueous electrolytes may be used. Of these, LiPF ₆ , LiBF ₄ or LiClO ₄ is preferably used.

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

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

The lithium ion secondary battery obtained as described above is based on the use of the binder composition for the negative electrode of the present invention, and has dispersibility of the alloy-based active material and binding property between the alloy-based active material and the current collector. It is possible to stably obtain a high initial discharge capacity and a high charge / discharge capacity because the active material is less likely to peel off or fall off.

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 example, "part" means a weight standard unless otherwise specified.

Analytical Method The PVA-based resins produced in the following Examples and Comparative Examples were analyzed by the following methods.

(1) Saponification degree (mol%)
Alkali consumption required for hydrolysis of residual vinyl acetate and 3,4-diacetoxy-1-butene was analyzed.

(2) 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: deuterated aqueous solution, temperature: 50 ° C.) using AVANCE III HD 400 manufactured by BRUKER, and calculated from the integrated value based on the obtained NMR chart.

[Production Example 1: Production of PVA-based resin 1]
Azobisisobutyro was charged with 70 parts of vinyl acetate, 7 parts of methanol, and 10 mol% of 3,4-diacetoxy-1-butene (against vinyl acetate) in a reaction vessel equipped with a reflux condenser, a dropping funnel, and a stirrer. 0.058 mol% (against charged vinyl acetate) of nitrile was added, and the temperature was raised under a nitrogen stream with stirring to initiate polymerization. When the polymerization rate of vinyl acetate reached 70%, a predetermined amount of m-dinitrobenzene was added to terminate the polymerization, and then the unreacted vinyl acetate monomer was distilled out of the system to obtain the copolymer methanol. A solution was obtained.
Next, the methanol solution was adjusted to a resin concentration of 8% by weight and charged into a container, and while maintaining the solution temperature at 40 ° C., a 2% (sodium equivalent) methanol solution of sodium hydroxide was added to the vinyl acetate structure in the copolymer. The unit and the total amount of 3,4-diacetoxy-1-butene structural unit were added in a ratio of 10 mmol to 1 mol to carry out saponification. When the saponification progresses and the saponified product precipitates and becomes particulate, it is filtered off, a methanol acetate solution for neutralization (20% by weight) is added, the filter is separated, and the mixture is thoroughly washed with methanol and hot air. It was dried in a dryer to obtain a PVA-based resin 1.
With respect to the obtained PVA-based resin 1, the degree of saponification, the average degree of polymerization, and the content of side chain 1,2-diol structural units were measured based on the above measurement method. The degree of saponification is 99.2 mol%, the average degree of polymerization is 730, and the content of the side chain 1,2-diol structural unit is the actual modification amount (the amount introduced into the molecular chain) with respect to the charged amount of 10 mol%. Was 8.9 mol%.

[Example 1]
<Manufacture of negative electrode for batteries using silicon active material>
75 parts of silicon powder (sold by AlfaAesar, average particle size: 50 nm) as an active material, 15 parts of acetylene black (“Denka Black” manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive aid, and 1.72 as a dispersant. After adding 3 parts of carboxymethyl cell roll # 2260 (manufactured by Daicel FineChem Co., Ltd.) prepared in a weight% aqueous solution in terms of solid content and purified water in a timely manner, a planetary kneader (manufactured by Shinky Co., Ltd. Taro ”) was used for mixing to obtain a paste having a solid content concentration of 34.8% (mixed at 2000 rpm for 4.5 minutes, and then defoamed at 2200 rpm for 0.5 minutes).
In the obtained paste, 7 parts of a 7% aqueous solution of PVA-based resin 1 prepared in Production Example 1 as a binder for the negative electrode was added in terms of solid content, and purified water was added in a timely manner, and then a planetary kneader was used. By mixing under the same conditions, an active material paste having a solid content concentration of 17.1% was obtained.
Next, as a current collector, a 25 μm applicator and a coating machine (“Control Coater (coating machine)” manufactured by Imoto Seisakusho Co., Ltd.) were applied to the surface of rolled copper foil (UACJ foil, thickness 18 μm). The active material paste was applied at a coating speed of 10 mm / sec. This was dried at 80 ° C. for 2 hours, and then vacuum dried at 120 ° C. for 4 hours. The obtained electrode was further heat-treated at 150 ° C. for 30 minutes to obtain a negative electrode for a battery.

[Comparative Example 1]
In Example 1, as a binder for the negative electrode, Gosenol NM-11 (manufactured by Nippon Synthetic Chemical Industry Co., Ltd., average degree of polymerization 1100, degree of saponification 99.5 mol%), which is a commercially available product of a polyvinyl alcohol-based resin having no modifying group. ) Was used, and a negative electrode for a battery was produced under the same conditions as in Example 1.

[Comparative Example 2]
In Example 1, the PVA-based resin 2 (content of side chain 1,2-diol structural unit content 3 mol%, average degree of polymerization 1200, saponification degree 99) produced as a negative electrode binder by a method similar to that of Production Example 1. A negative electrode for a battery was prepared under the same conditions as in Example 1 except that 0.0 mol%) was used.

[Comparative Example 3]
In Example 1, the PVA-based resin 3 (content of side chain 1,2-diol structural unit content 6 mol%, average degree of polymerization 1200, saponification degree 99) produced as a negative electrode binder by a method similar to that of Production Example 1. A negative electrode for a battery was prepared under the same conditions as in Example 1 except that 0.1 mol%) was used.

(Evaluation of electrode binding)
The negative electrode for a battery produced above was cut into a strip of 2.5 cm × 12.5 cm. This strip-shaped negative electrode for a battery was laminated so as to overlap with a double-sided tape (manufactured by Nichiban Co., Ltd., recycled paper double-sided tape "Nystack NW-25") bonded on a SUS plate. Further, the bonded surfaces were pressure-bonded with a 2 kg press roller to obtain a test piece for electrode bondability evaluation. The obtained test piece was stored overnight in a thermostatic chamber at 23 ° C. in a desiccator equipped with phosphorus pentoxide before use.
Regarding the evaluation test piece, the active material layer was peeled off from the current collector (copper foil) by 2.5 cm from the end of the strip-shaped battery electrode, and fixed to the pulling band. Using a precision universal testing machine (manufactured by Shimadzu Corporation, Autograph AG-IS, load cell 50N), the binding force (excluding spikes) between the current collector and the active material layer is measured by the 180-degree peel test method. By doing so, the binding property of the electrode was evaluated. The peeling test speed was 100 mm / min.

(Evaluation of electrode uniformity)
The actual measurement results of the peeling test in the above electrode binding property evaluation are shown in FIGS. 1 and 2. The evaluation was performed with n = 3, and “◯” was shown for the negative electrode whose peeling behavior was constant, and “x” was shown for the negative electrode where the spike peak was unevenly generated.

From the comparison between Example 1 and Comparative Examples 1 to 3 shown in Table 1, it can be seen that when a PVA-based resin having many side chain 1,2-diol structural units is used as the binder, the binding force of the negative electrode is improved. Understand.
Further, from the comparison between FIGS. 1 and 2 to 4, when the alloy-based active material and the PVA-based resin in which the side chain 1,2-diol structural unit is less than the lower limit of the specific range are combined, the electrode peeling test is performed. As a result, the peeling force was not uniform and spikes were observed in some places, so it is clear that the binder distribution in the negative electrode layer is non-uniform.
On the other hand, in the negative electrode produced in Example 1, it can be seen that the distribution of the binder is uniform because the uniform peeling behavior is obtained.

A secondary battery having a negative electrode obtained by using the binder composition for a negative electrode of a secondary battery of the present invention can stably obtain a high charge / discharge capacity, and is particularly formed in an electrode using an alloy-based active material. It has excellent effects of improving the adhesive force and improving the uniformity during the production of the negative electrode. Therefore, the binder composition for a negative electrode of the present invention is useful as a binder composition for a negative electrode for improving the performance of a secondary battery such as a lithium ion secondary battery.

Claims (4)

A binder composition for producing a negative electrode of a secondary battery containing an element capable of forming an alloy with lithium as an active material.
A binder composition for a negative electrode of a secondary battery, which comprises a polyvinyl alcohol-based resin having 7 to 15 mol% of structural units having a primary hydroxyl group in a side chain. A secondary battery negative electrode comprising the binder composition for a secondary battery negative electrode according to claim 1. The negative electrode of a secondary battery according to claim 2, which contains water-soluble cellulose as a dispersant for an active material. A secondary battery comprising the secondary battery negative electrode according to claim 2 or 3.

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