JP2018206658A - Binder for power storage device negative electrode - Google Patents

Abstract

To provide: a binder for a power storage device negative electrode, which is superior in negative electrode active material's dispersibility and adhesiveness and high in durability against an electrolyte solution, and which enables the manufacturing of a high-capacity power storage device; a binder for a power storage device negative electrode, which is high in electrolyte solution wettability and which enables the efficient manufacturing of negative electrodes; a composition for a power storage device negative electrode, which is arranged by use of the binder for a power storage device negative electrode; a power storage device electrode; and a power storage device.SOLUTION: A binder for a power storage device negative electrode is to be used for a negative electrode of a power storage device. The binder comprises: particles of a polyvinyl acetal-based resin; and a polyvinyl alcohol-based resin.SELECTED DRAWING: None

Description

The present invention relates to a binder for an electricity storage device negative electrode that is excellent in dispersibility and adhesiveness of a negative electrode active material, has high durability against an electrolytic solution, and can produce a high-capacity electricity storage device. Further, the present invention relates to a binder for an electricity storage device negative electrode in which the wettability of an electrolytic solution is high and a negative electrode can be efficiently manufactured. Furthermore, it is related with the composition for negative electrode of an electrical storage device, the electrical storage device electrode, and the electrical storage device using the binder for electrical storage devices negative electrodes.

In recent years, with the widespread use of portable electronic devices such as portable video cameras and portable personal computers, the demand for secondary batteries as mobile power sources has increased rapidly. In addition, there are very high demands for such secondary batteries to be reduced in size, weight, and energy density.
Thus, as a secondary battery that can be repeatedly charged and discharged, a lead battery, a nickel-cadmium battery, and the like have been mainly used. However, although these batteries are excellent in charge / discharge characteristics, it cannot be said that they have characteristics that are sufficiently satisfactory as a mobile power supply for portable electronic devices in terms of battery weight and energy density.

Therefore, research and development of lithium secondary batteries using lithium or a lithium alloy as a negative electrode as a secondary battery are actively performed. This lithium secondary battery has excellent characteristics such as high energy density, low self-discharge, and light weight.
The negative electrode of a lithium secondary battery is usually prepared by kneading a negative electrode active material and a binder together with a solvent to disperse the negative electrode active material into a slurry, and then applying the slurry on a current collector by a doctor blade method or the like and drying. It is formed by thinning.

At present, graphite-based materials are mainly used as a negative electrode active material for lithium secondary batteries. However, the graphite-based negative electrode active material has a problem that it has a small battery capacity per weight and cannot meet the demand for an increase in battery capacity.
Therefore, the use of a silicon-based negative electrode active material such as silicon or silicon oxide, which has a battery capacity per weight much higher than that of graphite, has been studied as a negative electrode active material different from graphite.
However, since the silicon-based negative electrode active material expands and contracts violently with charge and discharge, the binding force is insufficient when a fluorine-based resin such as polyvinylidene fluoride (PVDF), which is most widely used as a binder, is used. As a result, the negative electrode active material is peeled off due to expansion and contraction, and the battery characteristics are deteriorated.

On the other hand, Patent Document 1 proposes to use a polyvinyl alcohol-based resin as a binder.
However, although such a binder is excellent in binding force, it covers the entire negative electrode active material, so that contact between the electrolyte and the negative electrode active material is cut off, and a large current is added when used in a lithium battery. In this case, there is a problem that the conduction of lithium ions becomes insufficient and the battery capacity is reduced.

JP 2009-238681 A

An object of the present invention is to provide a binder for an electricity storage device negative electrode that is excellent in dispersibility and adhesiveness of a negative electrode active material, has high durability against an electrolytic solution, and can produce a high-capacity electricity storage device. Another object of the present invention is to provide a binder for an electricity storage device negative electrode in which the wettability of the electrolytic solution is high and the negative electrode can be produced efficiently. Furthermore, it aims at providing the composition for electrical storage device negative electrodes using this binder for electrical storage device negative electrodes, an electrical storage device electrode, and an electrical storage device.

The present invention is a binder used for a negative electrode of an electricity storage device, and is a binder for an electricity storage device negative electrode containing a fine particle composed of a polyvinyl acetal resin and a polyvinyl alcohol resin.

ADVANTAGE OF THE INVENTION According to this invention, the negative electrode active material is excellent in the dispersibility and adhesiveness, the durability with respect to electrolyte solution is high, and the electrical storage device negative electrode binder which can produce a high capacity | capacitance electrical storage device is producible. A binder for an electricity storage device negative electrode can be provided. In addition, it is possible to provide a binder for an electricity storage device negative electrode in which the wettability of the electrolytic solution is high and the negative electrode can be efficiently manufactured. Furthermore, the composition for negative electrode of an electrical storage device using the binder for negative electrode of an electrical storage device, the electrical storage device electrode, and the electrical storage device can be provided.

The present invention is described in detail below.

As a result of intensive studies, the inventors of the present invention have excellent dispersibility and binding properties of the negative electrode active material by using a combination of fine particles of a polyvinyl acetal resin and a polyvinyl alcohol resin as a binder for an electricity storage device negative electrode. The inventors have found that an electricity storage device having high durability against an electrolytic solution and a high capacity can be produced, and the present invention has been completed.

The binder for an electricity storage device negative electrode of the present invention contains fine particles comprising a polyvinyl acetal resin and a polyvinyl alcohol resin.
In the present invention, the binder resin does not cover the entire surface of the negative electrode active material and the conductive auxiliary agent by using a combination of the fine particles comprising the polyvinyl acetal resin and the polyvinyl alcohol resin, so that the electrolyte solution, the negative electrode active material, It is possible to maintain good contactability. Also, by covering the periphery of the fine particles made of polyvinyl acetal resin with the polyvinyl alcohol resin, the adhesion is improved, and even when the negative electrode active material expands and contracts violently, the peeling between the negative electrode active materials is suppressed. , Battery capacity reduction can be suppressed. For this reason, even when using a silicon-based negative electrode active material that expands and contracts violently during charging and discharging, it is possible to effectively suppress separation between the negative electrode active materials, and even when repeated charging and discharging, the discharge capacity is reduced. The decrease can be suppressed.

The binder for an electricity storage device negative electrode of the present invention contains fine particles made of a polyvinyl acetal resin.
In the present invention, by using a polyvinyl acetal resin as the resin component of the binder, an attractive interaction works between the fine particles of the polyvinyl acetal resin and the negative electrode active material, and the negative electrode active material is fixed with a small amount of the binder. Can do.
Further, the fine particles exert an attractive interaction with the conductive auxiliary agent, and the distance between the negative electrode active material and the conductive auxiliary agent can be kept within a certain range. Thus, the dispersibility of a negative electrode active material is improved significantly by making the distance of a negative electrode active material and a conductive support agent moderate.
Furthermore, compared with the case of using a resin such as SBR or PVDF, the adhesiveness with the current collector can be significantly improved. In addition, compared with the case of using carboxymethylcellulose, the negative electrode active material is excellent in dispersibility and adhesiveness, and a sufficient effect can be exhibited even when the amount of the binder added is small.
Furthermore, by making the binder into a fine particle shape, it is possible to partially adhere (point contact) without covering the entire surface of the negative electrode active material and the conductive additive. As a result, the contact between the electrolytic solution and the negative electrode active material is improved, and even when a large current is applied when used in a lithium battery, the lithium ion conduction is sufficiently maintained and the decrease in battery capacity is suppressed. The advantage that it can be obtained.

The polyvinyl acetal resin is a structural unit having a hydroxyl group represented by the following formula (1-1), a structural unit having an acetyl group represented by the following formula (1-2), or a formula (1-3) below. It is preferable that it has a structural unit which has an acetal group containing the structural unit which has an acetal group represented, and the ionic functional group represented by following formula (1-4).
As a result, the dispersibility of the fine particles comprising the polyvinyl acetal resin, the dispersibility of the negative electrode active material and the conductive additive can be made particularly excellent, and the adhesive strength to the current collector and the resistance to the electrolyte solution are also particularly excellent. Therefore, a reduction in the discharge capacity of the lithium secondary battery can be particularly suppressed.

In the above formula (1-3), R ¹ represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms, and in formula (1-4), R ² represents an alkylene group or aromatic ring having 1 to 20 carbon atoms. , X represents an ionic functional group.

Examples of the alkyl group having 1 to 20 carbon atoms include methyl group, ethyl group, propyl group, butyl group, pentyl group, hexyl group, heptyl group, octyl group, nonyl group, decyl group, undecyl group, dodecyl group, Examples include tridecyl group, tetradecyl group, pentadecyl group, hexadecyl group, heptadecyl group, octadecyl group, nonadecyl group, and eicosyl group. Of these, a methyl group, an ethyl group, and a propyl group are preferable.

Examples of the alkylene group having 1 to 20 carbon atoms include linear alkylene groups such as a methylene group, an ethylene group, an n-propylene group, a tetramethylene group, a pentamethylene group, a hexamethylene group, an octamethylene group, and a decamethylene group. Branched alkylene groups such as methylmethylene group, methylethylene group, 1-methylpentylene group and 1,4-dimethylbutylene group, and cyclic alkylene groups such as cyclopropylene group, cyclobutylene group and cyclohexylene group. . Of these, a methylene group, an ethylene group, and an n-propylene group are preferable, and a methylene group and an ethylene group are more preferable.

The content of the structural unit having a hydroxyl group in the polyvinyl acetal resin constituting the fine particles composed of the polyvinyl acetal resin (hereinafter also referred to as a hydroxyl group amount) is preferably 30 mol%, and preferably 65 mol%. .
When the amount of the hydroxyl group is 30 mol% or more, the resistance to the electrolytic solution can be sufficiently improved, and the resin component can be prevented from being eluted into the electrolytic solution when the negative electrode is immersed in the electrolytic solution. When the amount of the hydroxyl group is 65 mol% or less, the stability of the fine particles comprising the polyvinyl acetal resin in the aqueous medium is improved, and the occurrence of coalescence between the particles is suppressed, and the dispersibility of the negative electrode active material Can be improved.
The lower limit of the amount of the hydroxyl group is more preferably 35 mol%, and the upper limit is more preferably 55 mol%.

When the content of the structural unit having an acetal group in the polyvinyl acetal resin constituting the fine particles of the polyvinyl acetal resin (hereinafter also referred to as acetal group amount) is used when any acetalization of a single aldehyde or a mixed aldehyde is used. However, the preferred lower limit is 40 mol% and the preferred upper limit is 70 mol%, based on the total amount of acetal groups.
When the total acetal group amount is 40 mol% or more, the flexibility of the resin can be sufficiently improved, and the adhesive force with the current collector can be improved. When the degree of acetalization is 70 mol% or less, the resistance to the electrolytic solution can be sufficiently improved, and the resin component can be prevented from being eluted into the electrolytic solution when the negative electrode is immersed in the electrolytic solution. .
The amount of the acetal group is more preferably a lower limit of 45 mol% and a more preferable upper limit of 65 mol%.

The content of the structural unit having an acetyl group in the polyvinyl acetal resin constituting the fine particles composed of the polyvinyl acetal resin (hereinafter also referred to as acetyl group amount) is preferably 0.2 mol%, and preferably 20%. Mol%.
When the amount of acetyl groups in the polyvinyl acetal resin is 0.2 mol% or more, the flexibility can be sufficiently improved and the adhesion to the metal foil can be sufficiently improved. When the amount of the acetyl group is 20 mol% or less, it is possible to prevent the resin component from being eluted into the electrolytic solution when the negative electrode is immersed in the electrolytic solution with sufficient resistance to the electrolytic solution. .
The more preferable lower limit of the acetyl group amount is 1 mol%, and the more preferable upper limit is 15 mol%.

The preferable lower limit of the degree of polymerization of the polyvinyl acetal resin constituting the fine particles comprising the polyvinyl acetal resin is 250, and the preferable upper limit is 4000.
When the degree of polymerization is 250 or more, resistance to the electrolytic solution is sufficient, and elution of the negative electrode into the electrolytic solution can be suppressed, thereby preventing a short circuit. When the polymerization degree is 4000 or less, the adhesive force with the negative electrode active material can be sufficiently improved, and a reduction in the discharge capacity of the electricity storage device can be suppressed.
The minimum with said more preferable polymerization degree is 280, and a more preferable upper limit is 3500.

As for the glass transition temperature of the polyvinyl acetal resin constituting the fine particles comprising the polyvinyl acetal resin, a preferable lower limit is 50 ° C., a more preferable lower limit is 65 ° C., a preferable upper limit is 90 ° C., and a more preferable upper limit is 87 ° C.

The method for acetalization is not particularly limited, and a conventionally known method can be used. Examples thereof include a method of adding various aldehydes to an aqueous solution of polyvinyl alcohol in the presence of an acid catalyst such as hydrochloric acid.

The aldehyde used for the acetalization is not particularly limited. For example, formaldehyde (including paraformaldehyde), acetaldehyde (including paraacetaldehyde), propionaldehyde, butyraldehyde, amylaldehyde, hexylaldehyde, heptylaldehyde, 2-ethylhexylaldehyde, Examples include cyclohexyl aldehyde, furfural, glyoxal, glutaraldehyde, benzaldehyde, 2-methylbenzaldehyde, 3-methylbenzaldehyde, 4-methylbenzaldehyde, p-hydroxybenzaldehyde, m-hydroxybenzaldehyde, phenylacetaldehyde, β-phenylpropionaldehyde and the like. Of these, acetaldehyde or butyraldehyde is preferable in terms of productivity and property balance. These aldehydes may be used alone or in combination of two or more.

The polyvinyl alcohol may be a saponified copolymer obtained by copolymerizing a vinyl ester and an α-olefin. Furthermore, it is good also as polyvinyl alcohol which copolymerizes an ethylenically unsaturated monomer and contains the component originating in an ethylenically unsaturated monomer. Also used is a terminal polyvinyl alcohol obtained by copolymerizing a vinyl ester monomer such as vinyl acetate with an α-olefin in the presence of a thiol compound such as thiol acetic acid or mercaptopropionic acid, and saponifying it. be able to. The α-olefin is not particularly limited, and examples thereof include methylene, ethylene, propylene, isopropylene, butylene, isobutylene, pentylene, hexylene, cyclohexylene, cyclohexylethylene, and cyclohexylpropylene.

The polyvinyl acetal resin may have a crosslinked structure.
When the polyvinyl acetal-based resin has a cross-linked structure, it is possible to suppress the aggregation of the binder resin, to uniformly bond the negative electrode active material and the current collector interface, and to suppress peeling, and to charge and discharge. Even if it repeats, the fall of battery capacity can be suppressed.

Examples of the method of crosslinking the polyvinyl acetal resin include a method of acetalizing polyvinyl alcohol with a polyfunctional aldehyde, a method of adding a crosslinking agent to a polyvinyl acetal resin obtained by acetalizing polyvinyl alcohol, and the like. .
Especially, since the polyvinyl acetal type resin which has sufficient crosslinking degree is obtained, the method of adding a crosslinking agent to the polyvinyl acetal type resin obtained by acetalizing polyvinyl alcohol is preferable. Further, by using a method of adding a crosslinking agent to a polyvinyl acetal resin, uncrosslinked polyvinyl acetal resins having a fine particle shape can be crosslinked with each other, and a polyvinyl acetal resin having fine particles crosslinked can be obtained. .
In the method of acetalizing polyvinyl alcohol with a polyfunctional aldehyde, the degree of crosslinking of the obtained polyvinyl acetal resin becomes insufficient, and the aggregation suppressing effect may not be sufficiently exhibited. Furthermore, when the resulting polyvinyl acetal resin forms a cross-linked structure with a polyfunctional aldehyde, the polymer chain of the polyvinyl acetal resin becomes a continuous state, and the predetermined physical properties can be obtained by increasing the viscosity. There may not be.

Examples of the crosslinking agent include polyfunctional aldehydes such as malondialdehyde, succinaldehyde, glutaraldehyde, terephthalaldehyde, isophthalaldehyde, orthophthalaldehyde; sorbitol polyglycidyl ether, polyglycerol polyglycidyl ether, diglycerol polyglycidyl Multifunctional epoxides such as ether, glycerol polyglycidyl ether, polyethylene glycol diglycidyl ether; melamine resins; urethane resins; many such as 2,4-diisocyanate toluene, 1,6-diisocyanate hexane, isoformyl diisocyanate, diphenylmethane diisocyanate Functional isocyanate; phenol-formaldehyde resin and the like. Of these, a polyfunctional aldehyde is preferable and glutaraldehyde is particularly preferable from the viewpoint of high crosslinking reactivity with respect to the resin.

The polyvinyl acetal resin preferably has a degree of crosslinking at 25 ° C. of 5% by weight or more.
When the degree of crosslinking is 5% by weight or more, the polyvinyl acetal resin is sufficiently crosslinked, and the dispersibility of the binder resin can be improved.
The degree of crosslinking is more preferably 10% by weight or more. The degree of crosslinking is preferably 75% by weight or less, and more preferably 70% by weight or less.
The said crosslinking degree means the weight ratio (weight%) of the insoluble component with respect to the weight (before immersion) of a polyvinyl acetal type resin when immersed in a specific solvent.
Moreover, the degree of crosslinking at 25 ° C. means the degree of crosslinking when immersed in a solvent at 25 ° C.

The polyvinyl acetal resin preferably contains a component derived from a crosslinking agent.
The components derived from the cross-linking agent include polyfunctional aldehydes such as malondialdehyde, succinaldehyde, glutaraldehyde, terephthalaldehyde, isophthalaldehyde, orthophthalaldehyde; sorbitol polyglycidyl ether, polyglycerol polyglycidyl ether, diglycerol Polyfunctional epoxides such as polyglycidyl ether, glycerol polyglycidyl ether, polyethylene glycol diglycidyl ether; melamine resin; urethane resin; toluene 2,4-diisocyanate, 1,6-diisocyanate hexane, isoformyl diisocyanate, diphenylmethane diisocyanate, etc. A component derived from a crosslinking agent such as a phenol-formaldehyde resin.

The content of the component derived from the crosslinking agent in the polyvinyl acetal-based resin is preferably 0.015 mol% in terms of the substance amount, and preferably 10 mol% in terms of the preferable upper limit. When the content of the component derived from the crosslinking agent is 0.015 mol% or more, the polyvinyl acetal resin is sufficiently crosslinked, and the dispersibility of the binder resin can be improved. The softness | flexibility of binder resin can fully be improved as content of the component originating in the said crosslinking agent is 10 mol% or less. As for content of the component originating in the said crosslinking agent, a more preferable minimum is 0.025 mol%, and a more preferable upper limit is 5 mol%.
The content of the component derived from the crosslinking agent can be measured by, for example, NMR.

As for the ratio of the content of the component derived from the crosslinking agent and the hydroxyl group content (content of the component derived from the crosslinking agent / hydroxyl content), the preferred lower limit is 0.03 and the preferred upper limit is 33. When the ratio is 0.03 or more, the polyvinyl acetal resin is sufficiently crosslinked, and the dispersibility of the binder resin can be improved. When the ratio is 33 or less, the flexibility of the binder resin can be sufficiently improved. The ratio between the content of the component derived from the crosslinking agent and the hydroxyl group content is more preferably a lower limit of 0.05 and a more preferable upper limit of 15.

The content of the ionic functional group in the polyvinyl acetal resin is preferably 0.01 to 1 mmol / g. When the content of the ionic functional group is 0.01 mmol / g or more, the dispersibility of the fine particles in the composition for a negative electrode of a lithium secondary battery and the dispersibility of the negative electrode active material and the conductive additive when the negative electrode is formed Can be made good. When the content of the ionic functional group is 1 mmol / g or less, it is possible to improve the discharge capacity of the lithium secondary battery by making the binder durable when the battery is used. The content of the ionic functional group in the polyvinyl acetal resin has a more preferable lower limit of 0.02 mmol / g and a more preferable upper limit of 0.5 mmol / g. The content of the ionic functional group can be measured by using NMR.

The presence form of the ionic functional group may be directly present in the polyvinyl acetal resin structure, or may be present in the graft chain of a polyvinyl acetal resin containing a graft chain (hereinafter also simply referred to as a graft copolymer). You may do it. Especially, since the dispersibility of the negative electrode active material at the time of setting it as an electrolyte solution and a negative electrode and a conductive support agent can be made excellent, it is preferable to exist directly in a polyvinyl acetal type resin structure.
When the ionic functional group is directly present in the polyvinyl acetal resin structure, a chain molecular structure in which the ionic functional group is bonded to carbon constituting the main chain of the polyvinyl acetal resin, or through an acetal bond. It is preferable that the molecular structure has an ionic functional group bonded thereto. Further, a molecular structure in which an ionic functional group is bonded through an acetal bond is particularly preferable.
The presence of the ionic functional group in the above structure improves the dispersibility of the fine particles comprising the polyvinyl acetal resin in the composition for the negative electrode of the lithium secondary battery, and particularly improves the dispersibility of the negative electrode active material and the conductive additive. It can be excellent. In addition, since the deterioration of the binder when the battery is made is suppressed, a decrease in the discharge capacity of the lithium secondary battery can be suppressed.

The method for producing the polyvinyl acetal resin having the ionic functional group directly in the polyvinyl acetal resin structure is not particularly limited. For example, the modified polyvinyl alcohol raw material having the ionic functional group is reacted with an aldehyde. A method for acetalization is mentioned. Moreover, after producing a polyvinyl acetal type resin, the method etc. with which it reacts with the compound with another functional group and ionic functional group which have reactivity with respect to the functional group of this polyvinyl acetal type resin, etc. are mentioned.

When the polyvinyl acetal resin has an ionic functional group via an acetal bond, the acetal bond and the ionic functional group are preferably connected by a chain or cyclic alkyl group or an aromatic ring. . Among these, it is preferably connected by an alkylene group having 1 or more carbon atoms, a cyclic alkylene group having 5 or more carbon atoms, an aryl group having 6 or more carbon atoms, or the like, and particularly by an alkylene group having 1 or more carbon atoms or an aromatic ring. It is preferable that they are connected.
As a result, the resistance to the electrolytic solution and the dispersibility of the negative electrode active material and the conductive additive when used as a negative electrode can be improved, and the deterioration of the binder when used as a battery is suppressed. A reduction in the discharge capacity of the secondary battery can be suppressed.
Examples of the aromatic ring include aromatic rings such as a benzene ring and a pyridine ring, and condensed polycyclic aromatic groups such as a naphthalene ring and an anthracene ring.

The content of the acetal bond having an ionic functional group in the polyvinyl acetal resin is preferably adjusted so that the content of the ionic functional group in the polyvinyl acetal resin is within the appropriate range. In order to make the content of the ionic functional group in the polyvinyl acetal resin within the above suitable range, for example, when one ionic functional group is introduced by one acetal bond, it has an ionic functional group The content of the acetal bond is preferably about 0.1 to 10 mol%. Moreover, when two ionic functional groups are introduced by one acetal bond, the content of the acetal bond having the ionic functional group is preferably about 0.05 to 5 mol%. In addition, in order to increase both the dispersibility of the fine particles comprising the polyvinyl acetal resin, the flexibility of the resin, and the adhesion to the current collector, the acetal bond having an ionic functional group in the polyvinyl acetal resin is used. The content is preferably 0.5 to 20 mol% of all acetal bonds.
By making the content of the ionic functional group in the polyvinyl acetal resin within the above range, the dispersibility of the fine particles composed of the polyvinyl acetal resin in the composition for the negative electrode of the lithium secondary battery is improved, and with respect to the electrolytic solution The dispersibility of the negative electrode active material and the conductive additive when used as a negative electrode or a negative electrode can be improved. Furthermore, since the deterioration of the binder when used as a battery is suppressed, a decrease in the discharge capacity of the lithium secondary battery can be suppressed.

The method for producing a polyvinyl acetal resin having an ionic functional group through an acetal bond in the polyvinyl acetal resin structure is not particularly limited. For example, the method of acetalizing after making the aldehyde which has the said ionic functional group react beforehand with a polyvinyl alcohol raw material is mentioned. Moreover, when acetalizing polyvinyl alcohol, the aldehyde raw material is mixed with the aldehyde which has the said ionic functional group, and the method of acetalizing is mentioned. Furthermore, the method of making the aldehyde which has the said ionic functional group after making a polyvinyl acetal type-resin is mentioned.

When the polyvinyl acetal resin has a chain molecular structure in which an ionic functional group is bonded to carbon constituting the main chain of the polymer, it preferably has a structural unit represented by the following general formula (2). When the polyvinyl acetal resin has a structural unit represented by the following general formula (2), the dispersibility of the fine particles comprising the polyvinyl acetal resin in the composition for a negative electrode of a lithium secondary battery, and the durability of the binder when used as a battery The property can be made particularly excellent.

In formula (2), C represents a carbon atom of the polymer main chain, R ³ represents a hydrogen atom or a methyl group, R ⁴ represents an alkylene group having 1 or more carbon atoms, and R ⁵ represents an ionic functional group. .

R ³ is particularly preferably a hydrogen atom.
Examples of R ⁴ include a methylene group, an ethylene group, a propylene group, an isopropylene group, a butylene group, an isobutylene group, a sec-butylene group, and a tert-butylene group. Of these, R ⁴ is preferably a methylene group.
R ⁵ may be a structure substituted with a substituent having a hetero atom. Examples of the substituent include an ester group, an ether group, a sulfide group, an amide group, an amine group, a sulfoxide group, a ketone group, and a hydroxyl group.

The method for producing a polyvinyl acetal resin in which an ionic functional group is directly present in the polyvinyl acetal resin structure is not particularly limited. For example, a method of reacting an aldehyde with the above-mentioned modified polyvinyl alcohol raw material having an ionic functional group to acetalize, after producing a polyvinyl acetal resin, another functional group having reactivity to the functional group of the polyvinyl acetal resin And a method of reacting with a compound having a group and an ionic functional group.

As a method for producing the modified polyvinyl alcohol having an ionic functional group, for example, after copolymerizing a vinyl ester monomer such as vinyl acetate and a monomer having a structure represented by the following general formula (3), The method of saponifying the ester site | part of the obtained copolymer resin with an alkali or an acid is mentioned.

In formula (3), R ⁶ represents a hydrogen atom or a methyl group, R ⁷ represents an alkylene group having 1 or more carbon atoms, and R ⁸ represents an ionic functional group.

It does not specifically limit as a monomer which has a structure shown in the said General formula (3), For example, carboxyl groups and polymeric functional groups, such as 3-butenoic acid, 4-pentenoic acid, 5-hexenoic acid, and 9-decenoic acid, are included. Sulfonic acid groups such as allylsulfonic acid, 2-methyl-2-propene-1-sulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, 3- (methacryloyloxy) propanesulfonic acid and polymerizable functional groups , Those having an amino group and a polymerizable functional group such as N, N-diethylallylamine, and salts thereof.
In particular, when allyl sulfonic acid and a salt thereof are used, the dispersibility of the fine particles comprising the polyvinyl acetal resin in the composition for the negative electrode of the lithium secondary battery is improved, and the negative electrode when used as a resistance to the electrolytic solution or as a negative electrode. The dispersibility of the active material and the conductive additive can be made excellent. Furthermore, since the deterioration of the binder in the battery is suppressed, it is preferable because the reduction in the discharge capacity of the lithium secondary battery can be suppressed. In particular, it is preferable to use sodium allyl sulfonate.
These monomers may be used independently and may use 2 or more types together.

R ⁶ is particularly preferably a hydrogen atom.
Examples of R ⁷ include a methylene group, an ethylene group, a propylene group, an isopropylene group, a butylene group, an isobutylene group, a sec-butylene group, and a tert-butylene group. Of these, R ⁷ is preferably a methylene group.
R ⁸ may be a structure substituted with a substituent having a hetero atom. Examples of the substituent include an ester group, an ether group, a sulfide group, an amide group, an amine group, a sulfoxide group, a ketone group, and a hydroxyl group.

The content of the structural unit represented by the general formula (2) in the polyvinyl acetal resin is preferably adjusted so that the content of the ionic functional group in the polyvinyl acetal resin is within the appropriate range. In order to make the content of the ionic functional group in the polyvinyl acetal resin within the appropriate range, for example, when one ionic functional group is introduced by the general formula (3), the general formula ( The content of the structural unit shown in 2) is preferably about 0.05 to 5 mol%. In addition, when two ionic functional groups are introduced by the general formula (3), the content of the structural unit represented by the general formula (2) should be about 0.025 to 2.5 mol%. Is preferred.
By making the content of the ionic functional group in the polyvinyl acetal resin within the above range, the dispersibility of the fine particles composed of the polyvinyl acetal resin in the composition for the negative electrode of the lithium secondary battery is improved, and with respect to the electrolytic solution The dispersibility of the negative electrode active material and the conductive additive when used as a negative electrode or a negative electrode can be improved. Furthermore, since the deterioration of the binder when used as a battery is suppressed, a decrease in the discharge capacity of the lithium secondary battery can be suppressed.

In the binder for an electricity storage device negative electrode of the present invention, the content of the fine particles comprising the polyvinyl acetal resin is preferably 30% by weight in the solid content of the binder for the electricity storage device negative electrode of the present invention, and preferably 70% by weight in the solid content. is there.
When the content of the fine particles made of the polyvinyl acetal resin is 30% by weight or more, the adhesive force between the current collector interface and the active material can be sufficiently improved. When the content of the fine particles made of the polyvinyl acetal resin is 70% by weight or less, the resin component can be prevented from covering the entire negative electrode active material particles, and an increase in resistance of the negative electrode can be suppressed.
The more preferable lower limit of the content of fine particles made of the polyvinyl acetal resin is 35% by weight, and the more preferable upper limit is 65% by weight.
The solid content is a non-volatile component and does not volatilize during molding or heating.

A preferable lower limit of the volume average particle diameter of the fine particles made of the polyvinyl acetal resin is 50 nm, and a preferable upper limit is 700 nm.
When the volume average particle diameter is 50 nm or more, the binder does not completely cover the surfaces of the negative electrode active material and the conductive additive, and the electrolyte solution and the negative electrode active material can be sufficiently brought into contact with each other. When the volume average particle size is 700 nm or less, the dispersibility of the negative electrode active material and the conductive additive when the negative electrode is formed can be sufficiently improved, and the discharge capacity of the electricity storage device can be sufficiently increased.
A more preferable lower limit of the volume average particle diameter of the fine particles made of the polyvinyl acetal resin is 200 nm, and a more preferable upper limit is 400 nm.
The volume average particle size of the fine particles comprising the polyvinyl acetal resin can be measured using a laser diffraction / scattering particle size distribution measuring device, a transmission electron microscope, a scanning electron microscope, or the like.

The upper limit of the CV value is preferably 40% for the volume average particle diameter of the fine particles comprising the polyvinyl acetal resin. When the CV value is 40% or less, fine particles having a large particle size do not exist, and sedimentation of the fine particles can be suppressed to obtain a stable binder for an electricity storage device negative electrode.
A more preferable upper limit of the CV value is 35%, a further preferable upper limit is 32%, and a particularly preferable upper limit is 30%. The CV value is a percentage (%) of a value obtained by measuring the particle diameter of 50 fine particles made of polyvinyl acetal resin, obtaining the standard deviation from the obtained particle diameter measurement result, and dividing by the volume average particle diameter. The numerical value shown.

The binder for an electricity storage device negative electrode of the present invention contains a polyvinyl alcohol-based resin.
In the present invention, by using a polyvinyl alcohol resin as the resin component of the binder, the binding force can be improved, and even when a negative electrode active material that expands and contracts severely is used, contact between the negative electrode active materials is cut off. Therefore, a reduction in discharge capacity can be suppressed.
Further, by using a combination of fine particles made of polyvinyl acetal resin and polyvinyl alcohol resin, the negative electrode active materials are in point contact with fine particles made of polyvinyl acetal resin, so that the electrolyte solution and the negative electrode active material Good contact can be achieved. Moreover, adhesion of the negative electrode active materials can be suppressed by reinforcing the adhesive force by covering the periphery of the fine particles made of the polyvinyl acetal resin with the polyvinyl alcohol resin.
Moreover, the softness | flexibility of a negative electrode can be improved by combining the microparticles | fine-particles which consist of polyvinyl acetal resin, and polyvinyl alcohol-type resin, and electrode peeling at the time of electrode bending at the time of negative electrode preparation and winding can be suppressed.

As for the saponification degree of the said polyvinyl alcohol-type resin, a preferable minimum is 90 mol% and a preferable upper limit is 99.9 mol%.
When the saponification degree is not less than the above preferable lower limit and not more than the above preferable upper limit, the adhesive force can be sufficiently improved, and the effect of suppressing peeling between the negative electrode active materials can be further improved.
As for the saponification degree of the said polyvinyl alcohol-type resin, a more preferable minimum is 98 mol% and a more preferable upper limit is 99.5 mol%.
When the saponification degree is not less than the more preferable lower limit and not more than the more preferable upper limit, the supporting salt in the electrolytic solution is hardly decomposed, and the cycle characteristics of the battery can be improved.
The saponification degree can be measured by a method based on JIS K6726.

As for the polymerization degree of the said polyvinyl alcohol-type resin, a preferable minimum is 400 and a preferable upper limit is 3000.
When the degree of polymerization is not less than the above preferable lower limit and not more than the above preferable upper limit, the electrolyte solution is absorbed and swelled, and the periphery of the fine particles made of polyvinyl acetal resin is effectively covered, thereby reinforcing the adhesive force. Can be further improved.
A more preferable lower limit of the degree of polymerization of the polyvinyl alcohol resin is 1000, and a more preferable upper limit is 2500.

The glass transition temperature of the polyvinyl alcohol-based resin has a preferable lower limit of 50 ° C., a more preferable lower limit of 85 ° C., a preferable upper limit of 95 ° C., and a more preferable upper limit of 90 ° C.

The polyvinyl alcohol-based resin can be obtained by polymerizing a vinyl ester according to a conventionally known method to obtain a polymer, and then saponifying, ie, hydrolyzing the polymer. For saponification, an alkali or an acid is generally used. It is preferable to use an alkali for the saponification.

Examples of the vinyl ester include vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl versatate, vinyl laurate, vinyl stearate and vinyl benzoate.

The method for polymerizing the vinyl ester is not particularly limited. Examples of the polymerization method include a solution polymerization method, a bulk polymerization method, and a suspension polymerization method.

Examples of the polymerization catalyst used when polymerizing the vinyl ester include 2-ethylhexyl peroxydicarbonate (“TrigonoxEHP” manufactured by Tianjin McEIT), 2,2′-azobisisobutyronitrile (AIBN), t-butyl. Peroxyneodecanoate, bis (4-t-butylcyclohexyl) peroxydicarbonate, di-n-propylperoxydicarbonate, di-n-butylperoxydicarbonate, di-cetylperoxydicarbonate and di-s-butylperoxy Examples include dicarbonate. As for the said polymerization catalyst, only 1 type may be used and 2 or more types may be used together.

Since the degree of saponification is easily controlled within a suitable range, the polymer obtained by polymerizing the vinyl ester is preferably a polyvinyl ester. The polymer obtained by polymerizing the vinyl ester may be a copolymer of the vinyl ester and another monomer. That is, the polyvinyl alcohol may be formed using a copolymer of vinyl ester and another monomer. Examples of the other monomers, that is, comonomers to be copolymerized, include olefins, (meth) acrylic acid and salts thereof, (meth) acrylic acid esters, (meth) acrylamide derivatives, N-vinylamides, vinyl ethers, and nitriles. , Vinyl halides, allyl compounds, maleic acid and salts thereof, maleic acid esters, itaconic acid and salts thereof, itaconic acid esters, vinylsilyl compounds, and isopropenyl acetate. As for the other monomer, only one kind may be used, or two or more kinds may be used in combination.

Examples of the olefins include ethylene, propylene, 1-butene and isobutene. Examples of the (meth) acrylic acid esters include methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, i-propyl (meth) acrylate, and (meth) acrylic acid n-. Examples include butyl and 2-ethylhexyl (meth) acrylate. Examples of the (meth) acrylamide derivative include acrylamide, n-methylacrylamide, N-ethylacrylamide, N, N-dimethylacrylamide, and (meth) acrylamidepropanesulfonic acid and salts thereof. Examples of the N-vinyl amides include N-vinyl pyrrolidone. Examples of the vinyl ethers include methyl vinyl ether, ethyl vinyl ether, n-propyl vinyl ether, i-propyl vinyl ether, and n-butyl vinyl ether. Examples of the nitriles include (meth) acrylonitrile. Examples of the vinyl halides include vinyl chloride and vinylidene chloride. Examples of the allyl compound include allyl acetate and allyl chloride. Examples of the vinylsilyl compound include vinyltrimethoxysilane.

The form of the polyvinyl alcohol-based resin is not particularly limited, and may be liquid or particulate.

In the binder for an electricity storage device negative electrode of the present invention, the content of the polyvinyl alcohol-based resin has a preferable lower limit of 30 wt% and a preferable upper limit of 70 wt% in the solid content of the binder for an electric storage device negative electrode of the present invention.
When the content of the polyvinyl alcohol-based resin is 30% by weight or more, adhesion between particles such as a negative electrode active material and a conductive additive in the negative electrode can be sufficiently improved. When the content of the polyvinyl alcohol resin is 70% by weight or less, an increase in electrode resistance can be suppressed without completely covering the particle surface such as the negative electrode active material and the conductive auxiliary agent.
As for content of the said polyvinyl alcohol-type resin, a more preferable minimum is 35 weight% and a more preferable upper limit is 65 weight%.

In the binder for an electricity storage device negative electrode of the present invention, the content of the fine particles comprising the polyvinyl acetal resin with respect to 100 parts by weight of the polyvinyl alcohol resin is preferably 20 parts by weight, and preferably 100 parts by weight.
When the content of the fine particles made of the polyvinyl acetal resin is 20 parts by weight or more, flexibility can be imparted to the negative electrode, and peeling of the electrode can be suppressed when the electrode is bent or wound.

The binder for an electricity storage device negative electrode of the present invention may further contain an aqueous medium as a dispersion medium.
By using the aqueous medium, the solvent remaining in the negative electrode can be reduced as much as possible, and a lithium secondary battery can be manufactured.
In the binder for an electricity storage device negative electrode of the present invention, the aqueous medium may be water alone, or a solvent other than water may be added in addition to the water.
As the solvent other than water, a solvent having solubility in water and high volatility is preferable, and examples thereof include alcohols such as isopropyl alcohol, normal propyl alcohol, ethanol, and methanol. The said solvent may be used independently and may use 2 or more types together. A preferable upper limit of the amount of the solvent other than water is 30 parts by weight with respect to 100 parts by weight of water, and a more preferable upper limit is 20 parts by weight.

Although content of the said aqueous medium in the binder for electrical storage device negative electrodes of this invention is not specifically limited, A preferable minimum is 40 weight% and a preferable upper limit is 98 weight%. When the content of the aqueous medium is 40% by weight or more, the stability of the binder resin in the aqueous medium is improved, the dispersibility of the negative electrode active material is improved, and an electricity storage device such as a lithium secondary battery The discharge capacity can be improved. When the content of the aqueous medium is 98% by weight or less, the binder is mixed with the negative electrode active material to obtain a sufficient amount of the binder resin for the negative electrode active material when the negative electrode active material is used. Adhesive strength to the electric body can be made suitable. The content of the aqueous medium has a more preferable lower limit of 50% by weight and a more preferable upper limit of 95% by weight.

Although the total content of the fine particles comprising the polyvinyl acetal resin and the polyvinyl alcohol resin in the binder for an electricity storage device negative electrode of the present invention is not particularly limited, the preferred lower limit is 2% by weight and the preferred upper limit is 60% by weight. . When the total content is 2% by weight or more, the binder is mixed with the negative electrode active material to obtain a composition for an electricity storage device negative electrode, and the amount of the binder resin relative to the negative electrode active material is sufficient. Adhesive strength to can be made suitable. When the total content is 60% by weight or less, the stability of the binder resin in an aqueous medium is improved, the dispersibility of the negative electrode active material is improved, and the discharge of an electricity storage device such as a lithium secondary battery is performed. Capacity can be improved. A more preferable lower limit of the total content is 5% by weight, and a more preferable upper limit is 50% by weight.

The binder for an electricity storage device negative electrode of the present invention is a binder used for the negative electrode of an electricity storage device.
Examples of the electricity storage device include a lithium secondary battery, an electric double layer capacitor, and a lithium ion capacitor. Especially, it can be used especially suitably for a lithium secondary battery and a lithium ion capacitor.

The method for producing the electrical storage device electrode binder of the present invention is not particularly limited. For example, after performing the step of producing a polyvinyl acetal resin, the polyvinyl acetal resin is dissolved in an organic solvent in which the polyvinyl acetal resin dissolves, and then a poor solvent such as water is added little by little. Alternatively, a method of preparing fine particles by precipitating a polyvinyl acetal resin by removing the organic solvent under reduced pressure can be used. Moreover, after adding the solution which the said polyvinyl acetal type resin melt | dissolved in a large amount of water, the method of heating and / or pressure-reducing as needed, removing an organic solvent, and precipitating a polyvinyl acetal type resin, and producing microparticles | fine-particles. Can be mentioned. Further, there may be mentioned a method in which a polyvinyl acetal resin is heated at a temperature equal to or higher than the glass transition temperature of the polyvinyl acetal resin and kneaded with a kneader or the like, and water is added little by little under heat and pressure. Examples of the organic solvent include tetrahydrofuran, acetone, toluene, methyl ethyl ketone, ethyl acetate, methanol, ethanol, butanol, isopropyl alcohol, and the like.
Among them, since the volume-average particle diameter of the fine particles made of the obtained polyvinyl acetal resin is small and fine particles with a narrow particle size distribution can be obtained, the polyvinyl acetal resin is dissolved after dissolving the polyvinyl acetal resin in an organic solvent. A method of preparing fine particles by precipitation and further adding a polyvinyl alcohol-based resin is preferable.
Moreover, the volume average particle diameter of the fine particles made of the polyvinyl acetal resin can be adjusted by appropriately setting the conditions for removing the organic solvent that dissolves the polyvinyl acetal resin.
In the above production method, fine particles comprising a polyvinyl acetal resin may be prepared and dried, and then dispersed in an aqueous medium. The solvent used in the production of the fine particles comprising a polyvinyl acetal resin is used as an aqueous medium as it is. May be.

It can be set as the composition for electrical storage device negative electrodes by adding a negative electrode active material to the binder for electrical storage devices negative electrodes of this invention. The composition for negative electrode of an electrical storage device containing the binder for negative electrode of an electrical storage device of this invention and the negative electrode active material is also one of this invention.

The content of the fine particles comprising the polyvinyl acetal resin in the composition for negative electrode of the electricity storage device of the present invention is not particularly limited, but the preferred lower limit is 0.5 parts by weight and the preferred upper limit is 100 parts by weight of the negative electrode active material. 20 parts by weight.
When the content of the fine particles comprising the polyvinyl acetal resin is 0.5 parts by weight or more, the adhesive force to the current collector can be sufficiently improved. When the content of the fine particles made of the polyvinyl acetal resin is 20 parts by weight or less, a decrease in the discharge capacity of the lithium secondary battery can be sufficiently suppressed.
The content of the fine particles made of the polyvinyl acetal resin is more preferably 1 part by weight and more preferably 5 parts by weight.

The content of the polyvinyl alcohol-based resin in the composition for negative electrode of an electricity storage device of the present invention is preferably 0.5 parts by weight and preferably 20 parts by weight with respect to 100 parts by weight of the negative electrode active material.
When the content of the polyvinyl alcohol resin is 0.5 parts by weight or more, the effect of improving the adhesive force can be sufficiently exhibited. When the content of the polyvinyl alcohol resin is 20 parts by weight or less, the contact between the electrolytic solution and the negative electrode active material can be kept good.
As for content of the said polyvinyl alcohol-type resin, a more preferable minimum is 1 weight part and a more preferable upper limit is 10 weight part.

The composition for an electricity storage device negative electrode of the present invention contains a negative electrode active material.
Examples of the negative electrode active material include carbon materials such as graphite, natural graphite, graphite carbon, and amorphous carbon, lithium transition metal oxides, lithium transition metal nitrides, silicon compounds such as silicon and silicon oxide, and the like.

The composition for an electricity storage device negative electrode of the present invention preferably contains a conductive additive.
The conductive auxiliary agent is used to increase the output of the electricity storage device.
Examples of the conductive aid include acetylene black, carbon black, ketjen black, and vapor grown carbon fiber. Of these, acetylene black is preferable.

In addition to the above-described negative electrode active material, conductive auxiliary agent, fine particles comprising polyvinyl acetal resin, polyvinyl alcohol resin, and aqueous medium, the composition for negative electrode of an electricity storage device of the present invention includes a flame retardant auxiliary as necessary. Additives such as thickeners, antifoaming agents, leveling agents, and adhesion-imparting agents may be added.

The method for producing the negative electrode composition for an electricity storage device of the present invention is not particularly limited. For example, the negative electrode active material, the conductive additive, the fine particles comprising the polyvinyl acetal resin, the polyvinyl alcohol resin, the aqueous medium, and the necessity There is a method of mixing various additives to be added according to various mixers such as a ball mill, a blender mill, and a three roll.

For example, the composition for an electricity storage device negative electrode of the present invention is applied on a conductive substrate and dried to form an electricity storage device electrode. An electricity storage device electrode obtained using such a composition for an electricity storage device negative electrode and an electricity storage device are also one aspect of the present invention.
As an application method for applying the composition for negative electrode of an electricity storage device of the present invention to a conductive substrate, various application methods such as an extrusion coater, a reverse roller, a doctor blade, and an applicator can be employed.
Moreover, it is preferable to perform a pressing step so as to reach an appropriate electrode density after coating on the conductive substrate. It is preferable to use a roll press for the pressing step.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.

(Preparation of dispersion of polyvinyl acetal resin fine particles 1)
In a reaction vessel equipped with a thermometer, a stirrer, and a cooling tube, a polyvinyl acetal resin (polymerization degree 1000, butyralization degree 55.0 mol%, hydroxyl group amount 43.6 mol%, acetyl group amount 1.4 mol%) 20 parts by weight was dissolved in 80 parts by weight of isopropanol to obtain a solution. 1 part by weight of sodium 2-formylbenzenesulfonate and 0.05 part by weight of 12M concentrated hydrochloric acid were added to the resulting solution and reacted at 75 ° C. for 4 hours. The reaction solution was cooled, and 200 parts by weight of water was added dropwise. Next, after maintaining the liquid temperature at 30 ° C. and evaporating isopropanol and water by stirring while reducing the pressure, the solution is concentrated until the solid content becomes 20% by weight, and the fine particles 1 (hereinafter referred to as “polyvinyl”) comprising a polyvinyl acetal resin. A dispersion (content of fine particles 1 made of polyvinyl acetal resin: 20% by weight) in which acetal resin fine particles 1 are dispersed was prepared.
In addition, when the obtained polyvinyl acetal type resin was measured by NMR, the butyralization degree was 54.0 mol%, the amount of hydroxyl groups was 42.0 mol%, and the amount of acetyl groups was 1.1 mol%. The amount of ionic functional group contained in the polyvinyl acetal resin is 0.2 mmol / g, and an acetal bond having an ionic functional group (in formula (1-4), R ² is a benzene ring, X is sodium sulfonate. ) Content was 5.0 mol%. Furthermore, when the volume average particle diameter of the obtained polyvinyl acetal resin fine particles 1 was measured with a transmission electron microscope, it was 300 nm and the CV value was 20%.

(Preparation of dispersion of polyvinyl acetal resin fine particles 2)
In a reaction vessel equipped with a thermometer, a stirrer, and a cooling tube, a polyvinyl acetal resin (polymerization degree 1000, butyralization degree 55.5 mol%, hydroxyl group content 40.5 mol%, acetyl group content 4.0 mol%) 20 parts by weight was dissolved in 80 parts by weight of isopropanol to obtain a solution. 1 part by weight of sodium 2-formylbenzenesulfonate and 0.05 part by weight of 12M concentrated hydrochloric acid were added to the resulting solution and reacted at 75 ° C. for 4 hours. The reaction solution was cooled, and 200 parts by weight of water was added dropwise. Next, after maintaining the liquid temperature at 30 ° C. and volatilizing isopropanol and water by stirring under reduced pressure, the solution is concentrated until the solid content becomes 20% by weight, and the fine particles 2 made of polyvinyl acetal resin (polyvinyl acetal system) A dispersion liquid (also referred to as resin fine particles 2) in which the content of the fine particles 2 made of a polyvinyl acetal resin was 20% by weight was prepared.
When the obtained polyvinyl acetal resin was measured by NMR, the degree of butyralization was 55.0 mol%, the amount of hydroxyl groups was 40.0 mol%, and the amount of acetyl groups was 2.0 mol%. The amount of ionic functional group contained in the polyvinyl acetal resin is 0.2 mmol / g, and an acetal bond having an ionic functional group (in formula (1-4), R ² is a benzene ring, X is sodium sulfonate. ) Content was 3.0 mol%. Furthermore, when the volume average particle diameter of the obtained polyvinyl acetal resin fine particles 2 was measured with a transmission electron microscope, it was 340 nm and the CV value was 22%.

(Preparation of dispersion of polyvinyl acetal resin fine particles 3)
In a reaction vessel equipped with a thermometer, a stirrer, and a cooling tube, a polyvinyl acetal resin (degree of polymerization 1000, degree of butyralization 46.0 mol%, hydroxyl amount 51.9 mol%, acetyl group amount 2.1 mol%) 20 parts by weight was dissolved in 80 parts by weight of isopropanol to obtain a solution. 2 parts by weight of sodium 2-formylbenzenesulfonate and 0.05 parts by weight of 12M concentrated hydrochloric acid were added to the resulting solution and reacted at 75 ° C. for 4 hours. The reaction solution was cooled, and 200 parts by weight of water was added dropwise. Next, after maintaining the liquid temperature at 30 ° C. and evaporating isopropanol and water by stirring while reducing the pressure, the solution is concentrated until the solid content becomes 20% by weight, and the fine particles 3 (polyvinyl acetal type) comprising a polyvinyl acetal type resin. A dispersion liquid (also referred to as resin fine particles 3) in which the content of the fine particles 3 made of polyvinyl acetal resin was 20% by weight was prepared.
When the obtained polyvinyl acetal resin was measured by NMR, the degree of butyralization was 45.2 mol%, the amount of hydroxyl groups was 42.0 mol%, and the amount of acetyl groups was 1.8 mol%. The amount of ionic functional groups contained in the polyvinyl acetal resin is 1.0 mmol / g, an acetal bond having an ionic functional group (in formula (1-4), R ² is a benzene ring, X is sodium sulfonate. ) Content was 11.0 mol%. Furthermore, when the volume average particle diameter of the obtained polyvinyl acetal resin fine particles 3 was measured with a transmission electron microscope, it was 400 nm and the CV value was 18%.

(Preparation of polyvinyl acetal resin fine particles 4)
100 parts by weight of a dispersion liquid in which polyvinyl acetal resin fine particles 3 are dispersed (content of polyvinyl acetal resin fine particles 3: 20% by weight), 0.05 parts by weight of 12M concentrated hydrochloric acid, and 25% aqueous glutaraldehyde solution as a crosslinking agent 0.2 part by weight was added. Then, it was made to react at 70 degreeC on the conditions for 4 hours, and the polyvinyl acetal type-resin 4 (henceforth the polyvinyl acetal type resin fine particle 4) which has a crosslinked structure was produced.
When the obtained polyvinyl acetal resin was measured by NMR, the degree of butyralization was 45.2 mol%, the amount of hydroxyl groups was 42.0 mol%, and the amount of acetyl groups was 1.8 mol%. The amount of ionic functional groups contained in the polyvinyl acetal resin is 1.0 mmol / g, an acetal bond having an ionic functional group (in formula (1-4), R ² is a benzene ring, X is sodium sulfonate. ) Content was 11.0 mol%. Furthermore, when the volume average particle diameter of the obtained polyvinyl acetal resin fine particles 4 was measured with a transmission electron microscope, it was 400 nm and the CV value was 18%.

Example 1
Dispersion of 100 parts by weight of a dispersion of the obtained polyvinyl acetal resin fine particles 1 (content of polyvinyl acetal resin fine particles 1: 20% by weight) and a polyvinyl alcohol resin (polymerization degree 1700, saponification degree 98 mol%) 200 parts by weight of a liquid (polyvinyl alcohol-based resin content: 10% by weight) was mixed to prepare a binder for an electricity storage device negative electrode. The content of the polyvinyl acetal resin fine particles 1 in the solid content of the obtained binder for the electricity storage device negative electrode was 50% by weight, and the content of the polyvinyl alcohol resin was 50% by weight.
A negative active material mixture was prepared by mixing 80 parts by weight of spherical natural graphite and 20 parts by weight of a silicon compound (SiO).
94 parts by weight of the negative electrode active material mixture and acetylene black (Denka Black, Denki Kagaku Kogyo Co., Ltd.) 2 as the conductive auxiliary agent with respect to the obtained binder for negative electrode of the electricity storage device (solid content: 4 parts by weight) The weight part was mixed and the electrical storage device negative electrode composition was obtained.

(Example 2)
A power storage device negative electrode binder and a power storage device negative electrode composition were obtained in the same manner as in Example 1 except that the obtained dispersion liquid of polyvinyl acetal resin fine particles 2 was used.

Example 3
A power storage device negative electrode binder and a power storage device negative electrode composition were obtained in the same manner as in Example 1 except that the obtained dispersion liquid of polyvinyl acetal resin fine particles 3 was used.

(Example 4)
100 parts by weight of a dispersion of polyvinyl acetal resin fine particles 4 dispersed in water (content of polyvinyl acetal resin fine particles 4: 20% by weight) and polyvinyl alcohol resin (polymerization degree 1700, saponification degree 98 mol%) The dispersion liquid (polyvinyl alcohol-based resin content: 10% by weight) was mixed with 200 parts by weight to prepare a binder for an electricity storage device negative electrode. The content of the polyvinyl acetal resin fine particles 4 in the solid content of the obtained binder for the electricity storage device negative electrode was 50% by weight, and the content of the polyvinyl alcohol resin was 50% by weight.
Except having used the obtained binder for electrical storage device negative electrodes, it carried out similarly to Example 1, and obtained the composition for electrical storage device negative electrodes.

(Comparative Example 1)
Using styrene-butadiene rubber (SBR) as a binder, 94 parts by weight of the negative electrode active material mixture obtained in Example 1, 3 parts by weight of binder, and 2 weights of acetylene black (Denka Black, manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive auxiliary. Part and 1 part by weight of carboxymethyl cellulose (manufactured by Aldrich) as a thickener were mixed to obtain a composition for an electricity storage device negative electrode.

(Comparative Example 2)
A composition for an electricity storage device negative electrode was obtained in the same manner as in Comparative Example 1 except that polyvinylidene fluoride (PVDF) was used as a binder.

<Evaluation>
The following evaluation was performed about the composition for electrical storage device negative electrodes obtained by the Example and the comparative example. The results are shown in Table 2.

(1) Adhesiveness About the composition for electrical storage device negative electrodes obtained in Examples 1-4 and Comparative Examples 1 and 2, the adhesiveness with respect to copper foil was evaluated.

(Adhesiveness to copper foil)
On the copper foil (thickness 15 μm), the composition for an electricity storage device negative electrode was applied and dried so that the film thickness after drying was 40 μm, and a test piece in which the electrode was formed in a sheet shape on the aluminum foil was obtained. It was.
This sample was cut into a length of 10 cm and a width of 5 cm, and the aluminum foil side was attached to a 2 mm thick acrylic plate with a double-sided tape. A tape with a width of 18 mm (trade name: Cellotape (registered trademark) No. 252 (manufactured by Sekisui Chemical Co., Ltd.) (JIS Z1522 regulations)) is applied to the electrode surface of the test piece, and the tape is taped at a speed of 300 mm / min in the 90 ° direction. The peeling force (N) when peeling was measured using AUTOGRAPH (manufactured by Shimadzu Corporation, “AGS-J”).

(2) Electrolytic solution resistance (preparation of binder resin sheet)
On the polyethylene terephthalate (PET) film subjected to the mold release treatment, the binders used in Examples and Comparative Examples were coated and dried so that the film thickness after drying was 50 μm, to prepare a binder resin sheet.
The obtained binder resin sheet was cut out into 30x50 mm, and the test piece was produced.

(Elution / swelling evaluation)
The weight (a) of the film was measured by weighing the film obtained by drying the obtained test piece at 110 ° C. for 2 hours.
Next, diethyl carbonate was used as the electrolytic solution, and the obtained film was immersed in the electrolytic solution at 25 ° C. for 3 days. Thereafter, the film was taken out and immediately the surface electrolyte solution was wiped off and then weighed to measure the weight (b).
Thereafter, the film was immersed in 500 g of pure water for 2 days to completely remove the electrolyte inside the film, dried at 110 ° C. for 2 hours, and weighed to measure the weight (c).
From each weight, the dissolution rate and swelling rate of the binder were calculated by the following equations.
Dissolution rate (%) = [(ac) / a] × 100
Swelling rate (%) = (b−c / c) × 100
In addition, it means that resin is easy to melt | dissolve in electrolyte solution, so that the value of a dissolution rate is high, and it means that resin is easy to swell with electrolyte solution, so that a swelling rate is high.

(3) Battery performance evaluation (a) Coin battery production The composition for negative electrode of electricity storage device obtained in Examples 1 to 4 and Comparative Examples 1 and 2 was uniformly applied to a copper foil having a thickness of 15 μm, dried, and pressed. Thereafter, this was punched out to φ16 mm to obtain a negative electrode layer.
Moreover, lithium metal foil punched out to φ14 mm as a counter electrode, a porous PP film having a thickness of 25 μm and a glass filter (Advantech GA-100) as a separator, and ethylene carbonate and diethyl carbonate containing LiPF ₆ (1M) as an electrolytic solution Using a mixed solvent (volume ratio 1: 1), a coin cell (CR2032 type) was produced.

(B) Discharge capacity evaluation and charge / discharge cycle evaluation About the obtained coin battery, discharge capacity evaluation and charge / discharge cycle evaluation were performed using the charge / discharge test apparatus (made by Toyo System Co., Ltd.).
This discharge capacity evaluation and charge / discharge cycle evaluation were performed at a voltage range of 0.1 to 1.5 V and an evaluation temperature of 25 ° C. The capacity maintenance rate in the charge / discharge cycle evaluation was defined as the capacity maintenance rate at the 100th cycle, where the discharge capacity at the 5th cycle was 100% and the discharge capacity at the 100th cycle divided by the discharge capacity at the 5th cycle.

(C) Rate characteristic evaluation About the obtained coin battery, the electric current amount at the time of discharge was set to 0.2C, 1C, and 3C, and it charged / discharged 5 cycles at each rate. All charging was performed at 0.2C. Calculate the ratio of the discharge capacity at the 5th cycle when the discharge current amount is 1C and 3C when the discharge capacity at the 5th cycle is 100 when the discharge current value is 0.2C, and evaluate the rate characteristics did. “1C” is the amount of current required to discharge the capacity calculated from the amount of active material in 1 hour. Similarly, “0.2C” is 5 hours, and “3C” is 20 minutes. , The amount of current required to discharge each.

(4) On the copper foil (thickness: 15 μm), the composition for an electricity storage device negative electrode is applied and dried so that the film thickness after drying is 40 μm, and the electrode is formed in a sheet shape on the copper foil. A test piece was obtained.
This sample was cut into a length of 50 cm and a width of 2 cm, wound around a glass rod having a diameter of 2 mm and left for one day, and then the sample was unwound and the occurrence of cracks or cracks in the electrode was evaluated according to the following criteria.
◎: No cracks or cracks were observed. ○: Slight cracks or cracks were confirmed, but no peeling of the active material was confirmed. △: Cracks or cracks were confirmed, partial peeling of the active material. ×: Cracks and cracks were confirmed on the entire surface, and most of the active material was also confirmed to be peeled off

In Examples 1-4, it turns out that the high adhesiveness is exhibited compared with the comparative example 1 which uses SBR, and the comparative example 2 which uses PVDF. Moreover, it turns out that electrolyte solution is hard to swell or it is hard to melt | dissolve in electrolyte solution, and it is excellent in electrolyte solution resistance. Furthermore, it turns out that the battery obtained using the composition for negative electrodes obtained in Examples 1-4 is excellent in a capacity | capacitance maintenance factor and a rate characteristic, and is exhibiting the high battery characteristic. Moreover, it turns out that the electrode obtained using the composition for negative electrodes obtained in Examples 1-4 is hard to produce a crack, a crack, and peeling, and is excellent in bending resistance.

ADVANTAGE OF THE INVENTION According to this invention, the binder for electrical storage device negative electrodes which is excellent in the dispersibility and adhesiveness of a negative electrode active material, has high durability with respect to electrolyte solution, and can produce a high capacity | capacitance electrical storage device can be provided. In addition, it is possible to provide a binder for an electricity storage device negative electrode in which the wettability of the electrolytic solution is high and the negative electrode can be efficiently manufactured. Furthermore, the composition for negative electrode of an electrical storage device using the binder for negative electrode of an electrical storage device, the electrical storage device electrode, and the electrical storage device can be provided.

Claims (9)

A binder used for a negative electrode of an electricity storage device,
A binder for an electricity storage device negative electrode comprising fine particles comprising a polyvinyl acetal resin and a polyvinyl alcohol resin. The electrical storage device negative electrode binder according to claim 1, wherein the polyvinyl acetal resin has a hydroxyl group content of 30 to 65 mol%. The electrical storage device negative electrode binder according to claim 1, wherein the polyvinyl acetal resin has an ionic functional group. 4. The binder for an electricity storage device negative electrode according to claim 1, comprising 20 to 100 parts by weight of fine particles comprising a polyvinyl acetal resin with respect to 100 parts by weight of the polyvinyl alcohol resin. 5. The binder for an electricity storage device negative electrode according to claim 1, wherein the polyvinyl alcohol resin has a saponification degree of 98 to 99.5 mol%. Furthermore, an aqueous medium is contained as a dispersion medium, The binder for electrical storage device negative electrodes of Claim 1, 2, 3, 4 or 5 characterized by the above-mentioned. A binder for a power storage device negative electrode according to claim 1, 2, 3, 4, 5 or 6, and a composition for a power storage device negative electrode comprising a negative electrode active material,
A composition for negative electrode of an electricity storage device, comprising 0.5 to 20 parts by weight of fine particles comprising a polyvinyl acetal resin with respect to 100 parts by weight of the negative electrode active material. An electrical storage device electrode comprising the electrical storage device negative electrode composition according to claim 7. An electricity storage device comprising the electricity storage device electrode according to claim 8.