JP6876478B2 - Binder for power storage device electrodes - Google Patents

Description

The present invention relates to a binder for a power storage device electrode, which has adhesiveness and adhesiveness and can impart toughness and flexibility to the obtained electrode. Further, the present invention relates to a composition for a power storage device electrode including the binder for the power storage device electrode, a power storage device electrode, and a power storage device.

In recent years, with the spread of portable electronic devices such as portable video cameras and portable personal computers, the demand for secondary batteries as a mobile power source has been rapidly increasing. In addition, there is a great demand for miniaturization, weight reduction, and high energy density for such secondary batteries.
As described above, as a secondary battery capable of repeated charging and discharging, a lead battery, a nickel-cadmium battery and the like have been the mainstream. However, although these batteries have excellent charge / discharge characteristics, they cannot be said to have sufficiently satisfactory characteristics as a mobile power source for portable electronic devices in terms of battery weight and energy density.

Therefore, as a secondary battery, research and development of a lithium secondary battery using lithium or a lithium alloy as a negative electrode is being actively carried out. This lithium secondary battery has excellent features such as high energy density, low self-discharge, and light weight.
As the structure of the charge / discharge portion of the lithium secondary battery, a laminated battery in which a positive electrode and a negative electrode are laminated with a separator sandwiched between them, and a revolving battery such as a cylindrical battery are used. The retractable battery can have a large effective electrode area for a small volume, and can have a high capacity and a high output.

Currently, in particular, the most widely used binder for electrodes (negative electrodes) of lithium secondary batteries is a fluororesin typified by polyvinylidene fluoride (PVDF).
However, polyvinylidene fluoride has a problem that the solubility in a solvent is poor and the production efficiency is remarkably lowered when producing a slurry for an electrode.
On the other hand, N-methylpyrrolidone is generally used as a solvent for a slurry that dissolves vinylidene fluoride, but since N-methylpyrrolidone has a high boiling point, a large amount of heat energy is required in the slurry drying process. Not only that, there is a problem that N-methylpyrrolidone that has not been completely dried remains in the electrode, which deteriorates the performance of the battery.

On the other hand, carboxymethyl cellulose or the like is used as the water-based binder, but when carboxymethyl cellulose is used, the flexibility of the resin becomes insufficient, so that the effect of binding the active material becomes insufficient, or the effect of binding the active material becomes insufficient. There is a problem that the adhesive force to the current collector is significantly reduced.
Further, in Patent Document 1, low crystalline carbon having a graphite interlayer distance (d002) of 0.345 to 0.370 nm is used as a negative electrode active material, styrene-butadiene copolymer (SBR) is used as a binder, and carboxymethyl cellulose is used as a thickener. It has been shown that a good negative electrode can be obtained by using the battery, and a battery having excellent output characteristics can be obtained.
However, since SBR has low elasticity and low resilience, it has a low followability to expansion and contraction of the active material, and there is a problem that the binder is peeled off particularly when the active material contracts. As a result, there is a problem that conduction failure occurs between the active materials and at the current collector interface, and the capacity decreases due to repeated charging and discharging.

A method using a binder resin having high elasticity and high resilience has also been proposed, but in a wound battery, a non-uniform stress distribution is generated in the wound body due to expansion and contraction of the active material during charging and discharging. There is a problem that the capacity is reduced due to the occurrence of pressure on the electrodes, peeling between the active materials and the interface of the current collector, cracking of the electrodes, and curling of the electrode ends.

Japanese Unexamined Patent Publication No. 2009-158099

An object of the present invention is to provide a binder for a power storage device electrode which has adhesiveness and adhesiveness and can impart toughness and flexibility to the obtained electrode. A further object of the present invention is to provide a composition for a power storage device electrode including a binder for the power storage device electrode, a power storage device electrode, and a power storage device.

The present invention is a binder used for an electrode of a power storage device, ^{which is a binder (A) having a storage elasticity of 1 × 10 8} to 1 × 10 ¹⁰ Pa at 50 ° C. and 10 Hz, and storage at 50 ° C. and 10 Hz. It contains a binder (B) having an elasticity of 1 × 10 ⁵ to 1 × 10 ⁷ Pa, the binder (A) is a polyvinyl acetal-based resin (A), and the binder (B) is a polyvinyl acetal-based resin. The binder (B) contains a resin (B) and a plasticizing agent, and the binder (A) is 85-99.9% by weight based on 100% by weight of the total of the binder (A) and the binder (B). Is a binder for a power storage device electrode containing 0.1 to 15% by weight.
The present invention will be described in detail below.

As a result of diligent studies, the present inventors have made a binder using a polyvinyl acetal resin (A) having high elasticity and high resilience as a main component as a binder used in the composition for a power storage device electrode, and have low elasticity and low resilience. It has been found that the binder itself can be imparted with adhesiveness by adding a predetermined amount of the binder. Further, in particular, when used in a revolving battery such as a cylindrical battery, it is possible to suppress cracking of the electrode and curling of the electrode end portion, and it is possible to suppress a decrease in capacity due to repeated charging and discharging. The heading has led to the completion of the present invention.

The binder for the power storage device electrode of the present invention contains the binder (A).
The binder (A) has a lower limit of 1 × 10 ⁸ Pa and an upper limit of 1 × 10 ¹⁰ Pa at a storage elastic modulus at 50 ° C. and 10 Hz.
When the storage elastic modulus of the binder (A) is 1 × 10 ⁸ Pa or more, the binder resin does not become too soft, and even during repeated charging and discharging, the deformation of the binder resin is reduced and the active material is used. Peeling can be suppressed. When the storage elastic modulus of the binder (A) is 1 × 10 ¹⁰ Pa or less, the binder resin does not become too hard, and the adhesiveness between the active materials and the interface between the current collectors can be improved.
Regarding the storage elastic modulus of the binder (A), a preferable lower limit is 2.0 × 10 ⁸ Pa, a more preferable lower limit is 3.0 × 10 ⁸ Pa, a preferable upper limit is 2.5 × 10 ⁹ Pa, and a more preferable upper limit is 1. It is .5 × 10 ⁹ Pa.
The storage elastic modulus at 50 ° C. and 10 Hz can be measured by using a dynamic viscoelasticity measuring device.

The weight average molecular weight of the binder (A) is preferably 10000 to 10000000, and more preferably 20000 to 5000000.

The glass transition temperature of the binder (A) has a preferable lower limit of 55 ° C., a more preferable lower limit of 60 ° C., a preferable upper limit of 250 ° C., and a more preferable upper limit of 190 ° C.

The binder (A) is a polyvinyl acetal resin (A).
In the present invention, by using the polyvinyl acetal resin (A) as the binder (A), an attractive interaction acts on the active material, and the active material can be immobilized with a small amount of the binder. In addition, it also exerts an attractive interaction with the conductive auxiliary agent, and the distance between the active material and the conductive auxiliary agent can be kept within a certain range. By adjusting the distance between the active material and the conductive additive in this way, the dispersibility of the active material is significantly improved.
Further, as compared with the case of using a resin such as PVDF, the adhesiveness with the current collector can be remarkably improved. In addition, as compared with the case of using carboxymethyl cellulose, the dispersibility and adhesiveness of the active material are excellent, and a sufficient effect can be exhibited even when the amount of the binder added is small.

The preferable lower limit of the amount of hydroxyl groups in the polyvinyl acetal resin (A) is 30 mol%, and the preferable upper limit is 60 mol%. When the amount of the hydroxyl groups is 30 mol% or more, the resistance to the electrolytic solution is made sufficient, and when the electrode is immersed in the electrolytic solution, the resin component can be suppressed from being eluted into the electrolytic solution. When the amount of the hydroxyl groups is 60 mol% or less, the stability of the polyvinyl acetal resin (A) in the aqueous medium is improved, the aggregation of the binder resin is suppressed, and the active material is sufficiently dispersed. Can be done.
The more preferable lower limit of the amount of hydroxyl groups is 35 mol%, and the more preferable upper limit is 55 mol%.

Regarding the degree of acetalization of the polyvinyl acetal resin (A), the preferable lower limit is 40 mol% and the preferable upper limit is 70 mol% in terms of the total acetalization degree regardless of whether acetalization of a single aldehyde or a mixed aldehyde is used. When the degree of acetalization is 40 mol% or more, the flexibility of the resin is made good, and sufficient adhesive force to the current collector can be exhibited. When the degree of acetalization is 70 mol% or less, the resistance to the electrolytic solution is sufficient, and the elution of the resin component into the electrolytic solution when the electrode is immersed in the electrolytic solution can be suppressed. The more preferable lower limit of the degree of acetalization is 45 mol%, and the more preferable upper limit is 65 mol%.

The preferable lower limit of the acetyl group amount of the polyvinyl acetal resin (A) is 0.2 mol%, and the preferable upper limit is 20 mol%. When the amount of the acetyl group is 0.2 mol% or more, the flexibility of the resin can be improved. When the amount of the acetyl group is 20 mol% or less, the resistance to the electrolytic solution is sufficient, and the elution of the resin component into the electrolytic solution when the electrode is immersed in the electrolytic solution can be suppressed. The more preferable lower limit of the amount of the acetyl group is 1 mol%.

The degree of polymerization of the polyvinyl acetal resin (A) has a preferable lower limit of 250 and a preferable upper limit of 4000. When the degree of polymerization is 250 or more, the resistance to the electrolytic solution is sufficient, and the elution of the electrode into the electrolytic solution can be suppressed. When the degree of polymerization is 4000 or less, the adhesive force with the active material is sufficient, and the decrease in the discharge capacity of the lithium secondary battery can be suppressed. The more preferable lower limit of the degree of polymerization is 280, and the more preferable upper limit is 3500.

Further, the polyvinyl acetal resin (A) preferably has an ionic functional group. The ionic functional group is at least one selected from the group consisting of a carboxyl group, a sulfonic acid group, a sulfinic acid group, a sulfinic acid group, a phosphoric acid group, a phosphonic acid group, an amino group, and salts thereof. Functional groups are preferred. Of these, a carboxyl group, a sulfonic acid group, and a salt thereof are more preferable, and a sulfonic acid group and a salt thereof are particularly preferable. Since the polyvinyl acetal resin (A) has an ionic functional group, the dispersibility of the polyvinyl acetal resin (A) in the composition for the lithium secondary battery electrode is improved, and the active material and the conductive auxiliary agent Dispersibility can be particularly excellent.
Examples of the salt include sodium salt, potassium salt and the like.

The content of the ionic functional group in the polyvinyl acetal resin (A) has a preferable lower limit of 0.01 mmol / g and a preferable upper limit of 1 mmol / g. When the content of the ionic functional group is 0.01 mmol / g or more, the dispersibility of the polyvinyl acetal resin (A) in the composition for the lithium secondary battery electrode and the active material when used as the electrode And the dispersibility of the conductive auxiliary agent can be sufficiently improved. When the content of the ionic functional group is 1 mmol / g or less, the durability of the binder when used as a battery can be improved, and the discharge capacity of the lithium secondary battery can be improved. The content of the ionic functional group in the polyvinyl acetal resin (A) 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 present form of the ionic functional group may be directly present in the polyvinyl acetal resin (A) structure, and the polyvinyl acetal resin (A) containing a graft chain (hereinafter, also simply referred to as a graft copolymer). ) May be present in the graft chain. In particular, it is directly present in the polyvinyl acetal resin (A) structure because it can be excellent in resistance to the electrolytic solution and dispersibility of the active material and the conductive auxiliary agent when used as an electrode. preferable.
When the ionic functional group is directly present in the structure of the polyvinyl acetal-based resin (A), the chain molecular structure in which the ionic functional group is bonded to the carbon constituting the main chain of the polyvinyl acetal-based resin (A). Or, it is preferably a molecular structure in which an ionic functional group is bonded via an acetal bond. Further, it is particularly preferable that the molecular structure has an ionic functional group bonded via an acetal bond.
The presence of the ionic functional group in the above structure improves the dispersibility of the polyvinyl acetal resin (A) in the composition for the lithium secondary battery electrode, and the active material and the conductive auxiliary agent used as the electrode. The dispersibility can be made particularly excellent, and the deterioration of the binder when the battery is used is suppressed, so that the decrease in the discharge capacity of the lithium secondary battery can be suppressed.

The method for producing the polyvinyl acetal-based resin (A) having the ionic functional group directly in the polyvinyl acetal-based resin (A) structure is not particularly limited. For example, a method of reacting an aldehyde with a modified polyvinyl alcohol raw material having an ionic functional group to acetalize the polyvinyl alcohol-based resin (A), and then reacting with the functional group of the polyvinyl acetal-based resin (A). Examples thereof include a method of reacting with another functional group having a property and a compound having an ionic functional group.

When the polyvinyl acetal-based resin (A) has an ionic functional group via an acetal bond, the acetal bond and the ionic functional group are connected by a chain or cyclic alkyl group or an aromatic ring. Is preferable. Among them, it is preferable that they are 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, and the like, and in particular, an alkylene group having 1 or more carbon atoms and an aromatic ring. It is preferable that they are connected by.
As a result, the resistance to the electrolytic solution and the dispersibility of the active material and the conductive auxiliary agent when used as an electrode can be improved, and the deterioration of the binder when used as a battery is suppressed, so that the lithium secondary is used. It is possible to suppress a decrease in the discharge capacity of the battery.
Examples of the aromatic substituent 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.

When the polyvinyl acetal-based resin (A) has an ionic functional group via an acetal bond, the polyvinyl acetal-based resin (A) has a hydroxyl group represented by the following formula (1-1). A structural unit, a structural unit having an acetyl group represented by the following formula (1-2), a structural unit having an acetal group represented by the following formula (1-3), and a structural unit represented by the following formula (1-4). It is preferable that it has a structural unit having an acetal group containing an ionic functional group.
As a result, the dispersibility of the polyvinyl acetal resin (A) and the dispersibility of the active material and the conductive auxiliary agent can be made particularly excellent, and the adhesive force to the current collector and the resistance to the electrolytic solution are also particularly excellent. Therefore, it is possible to particularly suppress a decrease in the discharge capacity of the lithium secondary battery.

In 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 an aromatic ring having 1 to 20 carbon atoms. X represents an ionic functional group.

The content of the acetal bond having an ionic functional group in the polyvinyl acetal resin (A) is adjusted so that the content of the ionic functional group in the polyvinyl acetal resin (A) is within the appropriate range. Is preferable. In order to keep the content of the ionic functional group in the polyvinyl acetal resin (A) within the above-mentioned appropriate range, for example, when one ionic functional group is introduced by one acetal bond, the ionic functional group is introduced. The content of the acetal bond having a group is preferably about 0.1 to 10 mol%. When two ionic functional groups are introduced by one acetal bond, the content of the acetal bond having an ionic functional group is preferably about 0.05 to 5 mol%. Further, it has an ionic functional group in the polyvinyl acetal resin (A) in order to enhance both the dispersibility of the polyvinyl acetal resin (A) and the flexibility of the resin and the adhesive force to the current collector. The content of the acetal bond is preferably 0.5 to 20 mol% of the total acetal bond.
By setting the content of the ionic functional group in the polyvinyl acetal resin (A) within the above range, the dispersibility of the polyvinyl acetal resin (A) in the composition for the lithium secondary battery electrode is improved, and the dispersibility of the polyvinyl acetal resin (A) is improved. The resistance to the electrolytic solution and the dispersibility of the active material and the conductive auxiliary agent when used as an electrode can be improved. Further, since deterioration of the binder when used as a battery is suppressed, it is possible to suppress a decrease in the discharge capacity of the lithium secondary battery.

The method for producing the polyvinyl acetal-based resin (A) having an ionic functional group in the structure of the polyvinyl acetal-based resin (A) via an acetal bond is not particularly limited. For example, a method of acetalizing after reacting a polyvinyl alcohol raw material with an aldehyde having an ionic functional group in advance can be mentioned. Further, when acetalizing polyvinyl alcohol, a method of mixing an aldehyde having the above ionic functional group with an aldehyde raw material to acetalize, a method of producing the polyvinyl acetal resin (A) and then aldehyde having the above ionic functional group. Examples include a method of reacting.

Examples of the aldehyde having an ionic functional group include an aldehyde having a sulfonic acid group, an aldehyde having an amino group, an aldehyde having a phosphoric acid group, and an aldehyde having a carboxyl group. Specifically, for example, disodium 4-formylbenzene-1,3-disulfonate, sodium 4-formylbenzenesulfonate, sodium 2-formylbenzenesulfonate, 3-pyridinecarbaldehyde hydrochloride, 4-diethylaminobenzaldehyde hydrochloride. , 4-Dimethylaminobenzaldehyde hydrochloride, betainealdehyde chloride, (2-hydroxy-3-oxopropoxy) phosphate, 5-phosphate pyridoxal, terephthalaldehyde acid, isophthalaldehyde acid and the like.

The polyvinyl acetal-based resin (A) has an ionic functional group via an acetal bond, the ionic functional group is a sulfonic acid group or a salt thereof, and the acetal bond and the ionic functional group are formed by a benzene ring. It is particularly preferable that they are connected. Since the polyvinyl acetal-based resin (A) has such a molecular structure, the dispersibility of the polyvinyl acetal-based resin (A) in the composition for a lithium secondary battery electrode, the active material when used as an electrode, and the conductivity aid The dispersibility of the agent and the durability of the binder when used as a battery can be made particularly excellent.

When the polyvinyl acetal-based resin (A) has a chain-like 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 (A) has a structural unit represented by the following general formula (2), the dispersibility of the polyvinyl acetal resin (A) in the composition for a lithium secondary battery electrode can be obtained as a battery. The durability of the binder can be made particularly excellent.

In the 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. ..

Particularly preferably a hydrogen atom as the R ^3.
As the ^{R 4,} for example, methylene group, ethylene group, propylene group, isopropylene group, butylene group, isobutylene group, sec- butylene, tert- butylene group. Among them, ^{it is preferable that R 4} is a methylene group.
The R ⁴ may have 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, a hydroxyl group and the like.

The method for producing the polyvinyl acetal-based resin (A) in which the ionic functional group is directly present in the structure of the polyvinyl acetal-based resin (A) is not particularly limited. For example, a method of reacting an aldehyde with a modified polyvinyl alcohol raw material having an ionic functional group to acetalize the polyvinyl alcohol-based resin (A), and then reacting with the functional group of the polyvinyl acetal-based resin (A). Examples thereof include a method of reacting with another functional group having a property and a compound having an ionic functional group.

As a method for producing the modified polyvinyl alcohol raw material having the above ionic functional group, for example, a vinyl ester monomer such as vinyl acetate is copolymerized with a monomer having a structure represented by the following general formula (3), and then the monomer is copolymerized. Examples thereof include a method of saponifying the ester moiety of the obtained copolymer resin with an alkali or an acid.

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.

The monomer having the structure represented by the general formula (3) is not particularly limited, and for example, a carboxyl group such as 3-butenoic acid, 4-pentenoic acid, 5-hexenoic acid, or 9-decenoic acid and a polymerizable functional group can be used. Sulfonic acid groups such as allylsulfonic acid, 2-methyl-2-propen-1-sulfonic acid, 2-acrylamide-2-methylpropanesulfonic acid, 3- (methacryloyloxy) propanesulfonic acid and polymerizable functional groups Examples thereof include those having an amino group such as N, N-diethylallylamine and a polymerizable functional group, and salts thereof.
In particular, when allyl sulfonic acid and a salt thereof are used, the dispersibility of the polyvinyl acetal-based resin (A) in the composition for the lithium secondary battery electrode is improved, and the resistance to the electrolytic solution and the activity when used as the electrode are improved. The dispersibility of the substance and the conductive auxiliary agent can be made excellent. Further, since deterioration of the binder when used as a battery is suppressed, it is possible to suppress a decrease in the discharge capacity of the lithium secondary battery, which is preferable. In particular, it is preferable to use sodium allylsulfonate.
These monomers may be used alone or in combination of two or more.

Particularly preferably a hydrogen atom as the R ^6.
Examples of the 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, a tert-butylene group and the like. Among them, the above R ⁷ is preferably a methylene group.
The R ⁷ may have 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, a hydroxyl group and the like.

The content of the structural unit represented by the general formula (2) in the polyvinyl acetal resin (A) is such that the content of the ionic functional group in the polyvinyl acetal resin (A) is within the appropriate range. It is preferable to adjust. In order to keep the content of the ionic functional group in the polyvinyl acetal resin (A) within the above-mentioned appropriate range, for example, when one ionic functional group is introduced by the above general formula (3), the above The content of the structural unit represented by the general formula (2) is preferably about 0.05 to 5 mol%. When two ionic functional groups are introduced according to 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 preferable.
By setting the content of the ionic functional group in the polyvinyl acetal resin (A) within the above range, the dispersibility of the polyvinyl acetal resin (A) in the composition for the lithium secondary battery electrode is improved, and the dispersibility of the polyvinyl acetal resin (A) is improved. The resistance to the electrolytic solution and the dispersibility of the active material and the conductive auxiliary agent when used as an electrode can be improved. Further, since deterioration of the binder when used as a battery is suppressed, it is possible to suppress a decrease in the discharge capacity of the lithium secondary battery.

The polyvinyl acetal resin (A) preferably has a structural unit represented by the following formula (4). In formula (4), R ⁹ represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms.
By having the structural unit represented by the following formula (4), the resin component is swelled by the electrolytic solution, and the resin component is contained in the electrolytic solution, because the binding property is excellent and the resistance to the electrolytic solution is good. It is possible to suppress elution. As a result, the electrode density of the obtained electrode can be improved.

In the above formula (4), ^{the alkyl group having 1 to 20 carbon atoms represented by R 9} is not particularly limited, and for example, a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, and a heptyl group. Examples thereof include a group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, an octadecyl group, a nonadecil group and an eicosyl group.
The ^{viewpoint of R 9} is that the binding property between the active materials and the active material and the current collector can be made more excellent, and the swelling resistance to the electrolytic solution can be made higher. A propyl group is preferred.

The preferable lower limit of the content of the structural unit represented by the above formula (4) in the polyvinyl acetal resin (A) is 0.3 mol%. When the content of the structural unit represented by the above formula (4) is 0.3 mol% or more, the effect of improving the resistance to the electrolytic solution can be sufficiently exhibited. In addition, the flexibility of the resin is improved, and the occurrence of cracks and cracks can be suppressed.
The more preferable lower limit of the content of the structural unit represented by the above formula (4) is 0.4 mol%, the more preferable lower limit is 0.5 mol%, the preferable upper limit is 5 mol%, and the more preferable upper limit is 3 mol%. A more preferred upper limit is 2 mol%.

The content of the structural unit represented by the above formula (4) can be calculated by the following method.
Specifically, the polyvinyl acetal-based resin (A) is dissolved in deuterated dimethyl sulfoxide so that the concentration is 1% by weight, proton NMR is measured at a measurement temperature of 150 ° C., and a peak appearing near 4.8 ppm (a peak appearing at around 4.8 ppm). The following equation is used using the integrated values of a), the peak (b) appearing near 4.2 ppm, the peak (c) appearing near 1.0 to 1.8 ppm, and the peak (d) appearing near 0.9 ppm. Can be calculated by
Content (mol%) of the structural unit represented by the formula (4) = {(ab / 2) / [(c-4d / 3) / 2]} × 100

The content of the structural unit represented by the above formula (4) in the polyvinyl acetal resin (A) is preferably set high when the amount of hydroxyl groups in the polyvinyl acetal resin (A) is high. When the amount of hydroxyl groups of the polyvinyl acetal resin (A) is high, the binder tends to become hard due to the hydrogen bonds between the molecules, so that cracks and cracks are likely to occur. However, the structural unit represented by the above formula (4). By increasing the content of the resin, the flexibility of the resin is improved, and the occurrence of cracks and cracks can be suppressed.
On the other hand, when the amount of hydroxyl groups of the polyvinyl acetal resin (A) is low, the content of the structural unit represented by the above formula (4) in the polyvinyl acetal resin (A) is preferably set low. ..
When the amount of hydroxyl groups of the polyvinyl acetal resin (A) is low, the flexibility of the resin is sufficiently exhibited even in the range where the content of the structural unit represented by the above formula (4) is low, and cracks and cracks occur. Can be suppressed, and the resistance to the electrolytic solution can be made high.

As a method for producing the polyvinyl acetal-based resin (A) having the structural unit represented by the above formula (4) in the polyvinyl acetal-based resin (A), for example, the configuration represented by the above formula (4). Examples thereof include a method of reacting an aldehyde with a modified polyvinyl alcohol raw material having a unit to acetalize it. Further, when the polyvinyl acetal resin (A) is produced, a compound having reactivity with the functional group of the polyvinyl alcohol raw material is allowed to act to retain the structural unit represented by the formula (4) in the molecule. The method can be mentioned. Further, after producing the polyvinyl acetal-based resin (A), a compound having reactivity with the functional group of the polyvinyl acetal-based resin (A) is reacted to form a structural unit represented by the formula (4) as a molecule. There is a method of holding it inside. In particular, from the viewpoint of productivity and the ease of adjusting the content of the structural unit represented by the formula (4), the polyvinyl acetal-based resin (A) is prepared and then the polyvinyl acetal-based resin (A) is prepared. A method in which a compound having reactivity with a functional group is reacted to retain the structural unit represented by the formula (4) in the molecule is preferable.

As a method of reacting a compound having reactivity with the functional group of the polyvinyl acetal-based resin (A), two hydroxyl groups are added to one carbon atom with respect to one hydroxyl group of the polyvinyl acetal-based resin (A). Examples thereof include a method of dehydrating and condensing the possessed geminal diol compound, a method of adding an aldehyde compound to one hydroxyl group of the polyvinyl acetal-based resin (A), and the like. Among them, the method of adding an aldehyde compound to one hydroxyl group of the polyvinyl acetal resin (A) is preferable because it is easy to adjust the productivity and the content of the structural unit represented by the formula (4). is there.

As a method of adding an aldehyde compound to one hydroxyl group of the polyvinyl acetal-based resin (A), for example, the polyvinyl acetal-based resin (A) is dissolved in an acidic isopropyl alcohol and then 70 to 80. Examples thereof include a method of reacting an aldehyde under a high temperature condition of about ° C. Further, in order to adjust the content of the structural unit represented by the above formula (4) in the above polyvinyl acetal resin (A) so as to be within the above appropriate range, the above reaction time and acid concentration should be adjusted. Is preferable. When the content of the structural unit represented by the above formula (4) in the polyvinyl acetal resin (A) is set low, it is preferable to lengthen the reaction time and increase the acid concentration. .. When the content of the structural unit represented by the above formula (4) in the polyvinyl acetal resin (A) is set high, it is preferable to shorten the reaction time and lower the acid concentration. .. The preferred reaction time is 0.1 to 10 hours and the preferred acid concentration is 0.5 mM to 0.3 M.

The binder (A) is preferably in the form of fine particles. Since the binder (A) has a fine particle shape, it is possible to partially adhere (point contact) the binder (A) without covering the entire surface of the active material and the conductive auxiliary agent. As a result, the contact between the electrolytic solution and the active material is improved, and even if a large current is applied when a lithium battery is used, the conduction of lithium ions is sufficiently maintained, and a decrease in battery capacity can be suppressed. The advantage is obtained.
The volume average particle size of the binder (A) is preferably 10 to 1000 nm. When the volume average particle diameter is 10 nm or more, the dispersibility of the active material and the conductive auxiliary agent when used as an electrode can be improved, and the discharge capacity of the lithium secondary battery can be improved. Further, when it is 1000 nm or less, the binder does not completely cover the surfaces of the active material and the conductive auxiliary agent, and the contact property between the electrolytic solution and the active material can be improved. Therefore, the lithium battery was used at a large current. At that time, the conduction of lithium ions becomes sufficient, and the battery capacity can be improved. The more preferable volume average particle size of the polyvinyl acetal-based resin (A) having a fine particle shape is 15 to 800 nm, and a more preferable volume average particle size is 15 to 700 nm.
The volume average particle diameter 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 volume average particle size of the binder (A) preferably has an upper limit of CV value of 40%. When the CV value is 40% or less, fine particles having a large particle size do not exist, and the decrease in stability of the composition for the lithium secondary battery electrode due to the precipitation of the large particle size particles is suppressed. Can be done.
The preferable upper limit of the CV value is 35%, the more preferable upper limit is 32%, and the further preferable upper limit is 30%. The CV value is a numerical value indicated by a percentage (%) of the value obtained by dividing the standard deviation by the volume average particle diameter.

The method for producing the polyvinyl acetal resin (A) is not particularly limited. For example, after performing the step of producing the polyvinyl acetal-based resin (A), the polyvinyl acetal-based resin (A) is subjected to polyvinyl such as tetrahydrofuran, acetone, toluene, methyl ethyl ketone, ethyl acetate, methanol, ethanol, butanol, isopropyl alcohol and the like. The polyvinyl acetal-based resin (A) is dissolved in an organic solvent in which the acetal-based resin (A) is dissolved, and then a poor solvent such as water is added little by little, and the polyvinyl acetal-based resin (A) is removed by heating and / or reducing the pressure. Examples thereof include a method of producing fine particles by precipitation. Further, after adding the solution in which the polyvinyl acetal resin (A) is dissolved in a large amount of water, the organic solvent is removed by heating and / or reducing the pressure as necessary, and the polyvinyl acetal resin is precipitated to prepare fine particles. The method can be mentioned. Further, there is a method in which the polyvinyl acetal resin (A) is heated at a temperature equal to or higher than the glass transition temperature of the polyvinyl acetal resin (A) and kneaded with a kneader or the like, and water is added little by little under heating and pressurization to knead the resin. Can be mentioned.
Among them, since it is possible to obtain fine particles having a small volume average particle size and a narrow particle size distribution of the obtained polyvinyl acetal-based resin (A), the polyvinyl acetal-based resin (A) is dissolved in an organic solvent. Later, a method of precipitating the polyvinyl acetal resin (A) to produce fine particles is preferable.
In the above production method, fine particles made of the polyvinyl acetal resin (A) may be prepared, dried and then dispersed in an aqueous medium, and the solvent used when preparing the fine particles made of the polyvinyl acetal resin (A) may be used. It may be used as it is as an aqueous medium.

The binder for a power storage device electrode of the present invention contains a binder (B).
The binder (B) has a lower limit of 1 × 10 ⁵ Pa and an upper limit of 1 × 10 ⁷ Pa at a storage elastic modulus at 50 ° C. and 10 Hz.
When the storage modulus of the binder (B) is at 1 × 10 5 ^Pa or more, it is possible to suppress the elution of the binder resin. When the storage modulus of the binder (B) is less than 1 × 10 7 ^Pa, it is possible to improve the stress distribution of the binder resin, to prevent cracking of the electrode.
Regarding the storage elastic modulus of the binder (B), a preferable lower limit is 5.0 × 10 ⁵ Pa, a more preferable lower limit is 7.0 × 10 ⁵ Pa, a preferable upper limit is 9.0 × 10 ⁶ Pa, and a more preferable upper limit is 6. It is 0.0 × 10 ⁶ Pa.
The storage elastic modulus at 50 ° C. and 10 Hz can be measured by using a dynamic viscoelasticity measuring device.

The weight average molecular weight of the binder (B) is preferably 10,000,000 to 10000000.

The glass transition temperature of the binder (B) is preferably −30 ° C., a more preferable lower limit is −10 ° C., a preferred upper limit is 53 ° C., and a more preferable upper limit is 49 ° C.

The binder (B) contains a polyvinyl acetal resin (B) and a plasticizer.
When the binder (B) contains a polyvinyl acetal resin (B) and a plasticizer, the binder for a power storage device electrode of the present invention can improve stress dispersibility and prevent cracks in the electrodes. it can.

Examples of the polyvinyl acetal-based resin (B) include those similar to those exemplified as the polyvinyl acetal-based resin (A).
In the present invention, the polyvinyl acetal-based resin (B) may be the same resin as the polyvinyl acetal-based resin (A) or may be a different resin. Since the affinity between the binder (A) and the binder (B) can be improved, the polyvinyl acetal-based resin (B) is preferably the same resin as the polyvinyl acetal-based resin (A).

Examples of the plasticizer include organic ester-based plasticizers such as monobasic organic acid esters and polybasic organic acid esters, and organic phosphoric acid plasticizers. Of these, organic ester plasticizers are preferable.

Examples of the monobasic organic acid ester include glycol esters obtained by reacting glycol with a monobasic organic acid. Examples of the glycol include triethylene glycol, tetraethylene glycol, tripropylene glycol and the like. Examples of the monobasic organic acid include butyric acid, isobutyric acid, caproic acid, 2-ethylbutyric acid, heptyl acid, n-octyl acid, 2-ethylhexic acid, n-nonyl acid and decyl acid.

Examples of the polybasic organic acid ester include an ester compound of a multibasic organic acid and an alcohol having a linear or branched structure having 4 to 8 carbon atoms. Examples of the polybasic organic acid include adipic acid, sebacic acid, azelaic acid and the like.

Specific examples of the organic ester-based plasticizer include triethylene glycol di-2-ethylpropanoate, triethylene glycol di-2-ethylbutyrate, triethylene glycol di-2-ethylhexanoate, and tri. Ethylene Glycol Dicaprylate, Triethylene Glycol Di-n-Octanoate, Triethylene Glycol Di-n-Heptanoate, Tetraethylene Glycol Di-n-Heptanoate, Dibutyl Sevacate, Dioctyl Azelate, Dibutyl Carbitol Adipate, Ethylene Glycol Di- 2-Ethylene Butyrate, 1,3-propylene Glycol Di-2-Ethyl Butyrate, 1,4-Butylene Glycol Di-2-Ethyl Butyrate, Diethylene Glycol Di-2-Ethyl Butyrate, Diethylene Glycol Di-2-Ethlhexa Noate, dipropylene glycol di-2-ethylbutyrate, triethylene glycol di-2-ethylpentanoate, tetraethylene glycol di-2-ethylbutyrate, diethylene glycol dicaprylate, dihexyl adipate, dioctyl adipate, Hexylcyclohexyl adipate, a mixture of heptyl adipate and nonyl adipate, diisononyl adipate, diisodecyl adipate, heptylnonyl adipate, dibutyl sebacate, oil-modified sebacic acid alkyd, and a mixture of phosphate ester and adipate ester, etc. Can be mentioned. Of these, triethylene glycol di-2-ethylhexanoate, triethylene glycol di-2-ethylbutyrate or triethylene glycol di-2-ethylpropanoate is preferable, and triethylene glycol di-2-ethylpropanoate is preferable. It is more preferably ethylhexanoate or triethylene glycol di-2-ethylbutyrate, and even more preferably triethylene glycol di-2-ethylhexanoate.

Examples of the organophosphate plasticizer include tributoxyethyl phosphate, isodecylphenyl phosphate, triisopropyl phosphate and the like.

The content of the plasticizer in the binder (B) is not particularly limited, but the preferable lower limit is 6 parts by weight and the preferable upper limit is 60 parts by weight with respect to 100 parts by weight of the polyvinyl acetal resin (B).
When the content of the plasticizer is 6 parts by weight or more, the stress dispersibility can be particularly improved by setting the storage elastic modulus of the binder (B) in a suitable range. When the content of the plasticizer is 60 parts by weight or less, the elution of the binder resin can be suppressed and the decrease in the discharge capacity can be suppressed.
The content of the plasticizer has a more preferable lower limit of 10 parts by weight and a more preferable upper limit of 45 parts by weight.

The shape of the binder (B) is not particularly limited, but is preferably a fine particle shape.
When the binder (B) has a fine particle shape, the volume average particle diameter is preferably 10 to 1000 nm, more preferably 15 to 700 nm.

In the binder for the power storage device electrode of the present invention, the content of the binder (A) is 85 to 99.9% by weight with respect to the total of 100% by weight of the binder (A) and the binder (B).
When the content of the binder (A) is 85% by weight or more, the followability of the active material to expansion and contraction can be improved, and peeling between the active materials and the interface between the current collectors can be suppressed. When the content of the binder (A) is 99.9% by weight or less, the stress dispersibility of the binder resin can be improved, and cracking of the electrode and curling of the electrode end can be suppressed.
The content of the binder (A) has a more preferable lower limit of 90% by weight and a more preferable upper limit of 95% by weight.

In the binder for the power storage device electrode of the present invention, the content of the binder (B) is 0.1 to 15% by weight with respect to the total of 100% by weight of the binder (A) and the binder (B).
When the content of the binder (B) is 0.1% by weight or more, the stress dispersibility of the binder resin can be improved, and cracking of the electrode and curling of the electrode end can be suppressed. When the content of the binder (B) is 15% by weight or less, the followability of the active material to expansion and contraction can be improved, and peeling between the active materials and the interface between the current collectors can be suppressed.
The content of the binder (B) has a more preferable lower limit of 3% by weight and a more preferable upper limit of 10% by weight.

The ratio of the storage elastic modulus of the binder (A) to the storage elastic modulus of the binder (B) (storage elastic modulus of the binder (A) / storage elastic modulus of the binder (B)) has a preferable lower limit of 400, more preferably. The lower limit is 430, the preferred upper limit is 280, and the more preferred upper limit is 250.

The binder for the power storage device electrode of the present invention may further contain an aqueous medium.
By containing the above-mentioned aqueous medium, the amount of solvent remaining on the electrode can be reduced.
The aqueous medium may be water alone, or a solvent other than water may be added in addition to the water.
Examples of the solvent other than water preferably have a solubility in water and high volatility, and examples thereof include alcohols such as isopropyl alcohol, normal propyl alcohol, ethanol, and methanol. The above solvent may be used alone or in combination of two or more. The preferable upper limit of the amount of the solvent other than water added is 30 parts by weight with respect to 100 parts by weight of water, and the more preferable upper limit is 20 parts by weight.

The total content of the binder (A) and the binder (B) in the binder for the power storage device electrode of the present invention is not particularly limited, but the preferable lower limit is 2% by weight and the preferable upper limit is 60% by weight. When the total content of the binder (A) and the binder (B) is 2% by weight or more, the amount of the binder resin with respect to the active material when the binder is mixed with the active material to form a composition for a current collecting device electrode. It is possible to improve the adhesive force to the current collector by making the above sufficient. When the total content of the binder (A) and the binder (B) is 60% by weight or less, the stability of the binder resin in the dispersion medium can be improved and the dispersibility of the active material can be improved. , It is possible to suppress a decrease in the discharge capacity of a power storage device such as a lithium secondary battery. The total content of the binder (A) and the binder (B) is more preferably 5 to 50% by weight.

The binder for a power storage device electrode of the present invention is a binder used for an electrode of a power storage device.
Examples of the power storage device include a lithium secondary battery, an electric double layer capacitor, and a lithium ion capacitor. Among them, it can be particularly preferably used for a lithium secondary battery and a lithium ion capacitor.

A composition for a power storage device electrode can be obtained by adding an active material to the binder for a power storage device electrode of the present invention. Such a binder for a power storage device electrode of the present invention and a composition for a power storage device electrode containing an active material are also one of the present inventions.

The total content of the binder (A) and the binder (B) in the composition for the power storage device electrode of the present invention is not particularly limited, but the preferable lower limit is 0.1 part by weight with respect to 100 parts by weight of the active material. The preferred upper limit is 12 parts by weight. When the total content is 0.1 parts by weight or more, the adhesive force to the current collector can be sufficiently improved, and when it is 12 parts by weight or less, the discharge capacity of the lithium secondary battery is suppressed from decreasing. can do. More preferably, it is 0.5 to 5 parts by weight.

The composition for a power storage device electrode of the present invention contains an active material.
The composition for a power storage device electrode of the present invention may be used for either a positive electrode or a negative electrode, or may be used for both a positive electrode and a negative electrode. Therefore, as the active material, there are a positive electrode active material and a negative electrode active material.

Examples of the positive electrode active material include lithium-containing cobalt oxide (LiCoO ₂ ), lithium-containing nickel oxide (LiNiO ₂ ), Co-Ni-Mn lithium composite oxide, and Ni-Mn-Al lithium composite oxide. , Ni—Co—Al lithium composite oxide, lithium manganate (LiMn ₂ O ₄ ), lithium phosphate compound (LiXMPO ₄ (in the formula, M is Mn, Fe, Co, Ni, Cu, Mg, Zn, At least one selected from V, Ca, Sr, Ba, Ti, Al, Si, B and Mo, 0 ≦ X ≦ 2)) and the like can be mentioned. Further, the iron-based oxide having poor electrical conductivity may be used as an electrode active material covered with a carbon material by allowing a carbon source material to be present at the time of reduction firing. Further, these compounds may be partially elementally substituted.
These may be used alone or in combination of two or more.

As the negative electrode active material, for example, a material conventionally used as a negative electrode active material for a lithium secondary battery can be used, and for example, natural graphite (graphite), artificial graphite, amorphous carbon, carbon black, or Examples thereof include those obtained by adding a different element to these components. Of these, graphite is preferable, and spherical natural graphite is particularly preferable.
Further, silicon (Si, SiO, Si alloy) having a high lithium ion occlusion capacity may be used as the negative electrode active material.

The composition for a power storage device electrode of the present invention preferably contains a conductive auxiliary agent.
The above-mentioned conductive auxiliary agent is used to increase the output of the power storage device, and an appropriate one can be used depending on whether it is used for a positive electrode or a negative electrode.
Examples of the conductive auxiliary agent include graphite, acetylene black, carbon black, Ketjen black, vapor-grown carbon fiber and the like. Of these, acetylene black is preferable.

In addition to the above-mentioned active material, conductive auxiliary agent, polyvinyl acetal-based resin, and aqueous medium, the composition for a power storage device electrode of the present invention includes, if necessary, a flame retardant aid, a thickener, a defoaming agent, and the like. Additives such as leveling agents and adhesion imparting agents may be added. Among them, it is preferable to add a thickener because the coating film can be made uniform when the composition for the power storage device electrode is applied to the current collector.

The method for producing the composition for a power storage device electrode of the present invention is not particularly limited, and for example, the above-mentioned active material, conductive auxiliary agent, polyvinyl acetal-based resin, aqueous medium, and various additives to be added as needed are ball milled. , Blender mill, a method of mixing using various mixers such as three rolls.

The energy storage device electrode composition of the present invention is formed, for example, by applying it on a conductive substrate and drying it. A power storage device obtained by using such a composition for a power storage device electrode is also one of the present inventions.
As a coating method for applying the composition for a power storage device electrode of the present invention to a conductive substrate, for example, various coating methods such as an extrusion coater, a reverse roller, a doctor blade, and an applicator can be adopted.

The present invention can provide a binder for a power storage device electrode which has adhesiveness and adhesiveness and can impart toughness and flexibility to the obtained electrode. Further, a composition for a power storage device electrode including the binder for the power storage device electrode, a power storage device electrode, and a power storage device can be provided.

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

(Preparation of dispersion of polyvinyl acetal-based resin fine particles 1)
20 weights of polyvinyl acetal resin (degree of polymerization 1100, degree of butyralization 52 mol%, hydroxyl group amount 47.8 mol%, acetyl group amount 0.2 mol%) in a reaction vessel equipped with a thermometer, a stirrer and a cooling tube. The portion was dissolved in 80 parts by weight of isopropanol. Then, 1 part by weight of sodium 2-formylbenzenesulfonate and 0.05 part by weight of 12M concentrated hydrochloric acid were added to the solution, and the mixture was reacted at 70 ° C. for 4 hours. Next, 0.2 parts by weight of butyraldehyde and 1 part by weight of 12M concentrated hydrochloric acid were added, and the reaction was carried out at 82 ° C. for 6 hours, the reaction solution was cooled, and the resin was purified by the reprecipitation method. Finally, it was dried to recover the resin. Next, the obtained resin was dissolved again in 80 parts by weight of isopropanol, and 200 parts by weight of water was added dropwise. Next, the liquid temperature was kept at 30 ° C., and the isopropanol and water were volatilized by stirring while reducing the pressure, and then concentrated until the solid content became 20% by weight, and the fine particles 1 made of a polyvinyl acetal resin (hereinafter, hereinafter, A dispersion liquid (content of polyvinyl acetal-based resin fine particles 1: 20% by weight) in which polyvinyl acetal-based resin fine particles 1 were dispersed was prepared.
When the obtained polyvinyl acetal-based resin was measured by NMR, ^{the content of the structural unit represented by the formula (4) (R 9} is a propyl group in the formula (4)) was 1 mol%, and the degree of butyralization was 51 mol%. , 45 mol% of hydroxyl groups, 0.2 mol% of acetyl groups, 0.2 mmol / g of ionic functional groups contained in polyvinyl acetal-based resin, acetal bonds having ionic functional groups (formula (1-4)). Among them, R ² was a benzene ring and X was sodium sulfonate), and the content was 2.8 mol%. The volume average particle size of the obtained polyvinyl acetal-based resin fine particles 1 was measured with a transmission electron microscope and found to be 90 nm and a Tg (glass transition temperature) of 83 ° C.

(Preparation of dispersion of polyvinyl acetal-based resin fine particles 2)
20 weights of polyvinyl acetal resin (degree of polymerization 1700, degree of butyralization 37.2 mol%, hydroxyl group amount 61 mol%, acetyl group amount 1.8 mol%) in a reaction vessel equipped with a thermometer, a stirrer and a cooling tube. The portion was dissolved in 80 parts by weight of isopropanol. Then, 6 parts by weight of disodium 4-formylbenzene-1,3-disulfonate and 0.05 parts by weight of 12M concentrated hydrochloric acid were added to the solution, and the mixture was reacted at 70 ° C. for 4 hours. Next, 1 part by weight of butyraldehyde and 0.5 part by weight of 12 M concentrated hydrochloric acid were added, and the reaction was carried out at 75 ° C. for 4 hours, the reaction solution was cooled, and the resin was purified by the reprecipitation method. Finally, it was dried to recover the resin. Next, the obtained resin was dissolved again in 80 parts by weight of isopropanol, and 200 parts by weight of water was added dropwise. Next, the liquid temperature was kept at 30 ° C., and the isopropanol and water were volatilized by stirring while reducing the pressure, and then concentrated until the solid content became 20% by weight, and the fine particles 2 made of a polyvinyl acetal resin (hereinafter, hereinafter, A dispersion liquid (content of polyvinyl acetal-based resin fine particles 2: 20% by weight) in which polyvinyl acetal-based resin fine particles 2 were dispersed was prepared.
When the obtained polyvinyl acetal-based resin was measured by NMR, ^{the content of the structural unit represented by the formula (4) (R 9} is a propyl group in the formula (4)) was 1 mol%, and the degree of butyralization was 38.5. Mol%, hydroxyl group amount 45 mol%, acetyl group amount 1.5 mol%, amount of ionic functional group contained in polyvinyl acetal-based resin is 1.8 mmol / g, acetal bond having ionic functional group (formula (1) In -4), R ² was a benzene ring and X was sodium sulfonate), and the content was 14 mol%. The volume average particle diameter of the obtained polyvinyl acetal-based resin fine particles 2 was measured with a transmission electron microscope and found to be 9 nm and a Tg of 70 ° C.

(Preparation of dispersion of polyvinyl acetal-based resin fine particles 3)
Polyvinyl acetal resin (degree of polymerization 800, degree of butyralization 50.5 mol%, amount of hydroxyl groups 47.7 mol%, amount of acetyl groups 1.8 mol%) in a reaction vessel equipped with a thermometer, a stirrer, and a cooling tube. 20 parts by weight was dissolved in 80 parts by weight of isopropanol. Then, 0.25 parts by weight of sodium 2-formylbenzenesulfonate and 0.05 parts by weight of 12M concentrated hydrochloric acid were added to the solution, and the mixture was reacted at 70 ° C. for 4 hours. Next, 1 part by weight of butyraldehyde and 0.5 part by weight of 12 M concentrated hydrochloric acid were added, and the reaction was carried out at 75 ° C. for 4 hours, the reaction solution was cooled, and the resin was purified by the reprecipitation method. Finally, it was dried to recover the resin. Next, the obtained resin was dissolved again in 80 parts by weight of isopropanol, and 200 parts by weight of water was added dropwise. Next, the liquid temperature was kept at 30 ° C., and the isopropanol and water were volatilized by stirring while reducing the pressure, and then concentrated until the solid content became 20% by weight, and the fine particles 3 made of a polyvinyl acetal resin (hereinafter referred to as “the following)). A dispersion liquid (content of polyvinyl acetal-based resin fine particles 3: 20% by weight) in which polyvinyl acetal-based resin fine particles 3 were dispersed was prepared.
When the obtained polyvinyl acetal-based resin was measured by NMR, ^{the content of the structural unit represented by the formula (4) (R 9} is a propyl group in the formula (4)) was 1 mol%, and the degree of butyralization was 51.8. Mol%, hydroxyl group amount 45 mol%, acetyl group amount 1.5 mol%, amount of ionic functional group contained in polyvinyl acetal-based resin is 0.05 mmol / g, acetal bond having ionic functional group (formula (1) In -4), R ² was a benzene ring and X was sodium sulfonate), and the content was 0.7 mol%. The volume average particle diameter of the obtained polyvinyl acetal-based resin fine particles 3 was measured with a transmission electron microscope and found to be 700 nm and a Tg of 75 ° C.

(Preparation of polyvinyl acetal resin 4)
20 weights of polyvinyl acetal resin (degree of polymerization 1000, degree of butyralization 47.8 mol%, hydroxyl group amount 51 mol%, acetyl group amount 1.2 mol%) in a reaction vessel equipped with a thermometer, a stirrer and a cooling tube. The portion was dissolved in 80 parts by weight of isopropanol. Then, 1 part by weight of sodium 2-formylbenzenesulfonate and 0.05 part by weight of 12M concentrated hydrochloric acid were added to the solution, and the mixture was reacted at 70 ° C. for 4 hours. Next, 0.3 parts by weight of 12 M concentrated hydrochloric acid was added, and the reaction was carried out at 74 ° C. for 3 hours. Then, the reaction solution was cooled, the resin was purified by the reprecipitation method, and finally dried and made of polyvinyl. An acetal-based resin 4 was produced.
When the obtained polyvinyl acetal-based resin was measured by NMR, the degree of butyralization was 47.4 mol%, the amount of hydroxyl groups was 48.6 mol%, the amount of acetyl groups was 1 mol%, and the ionic functionality contained in the polyvinyl acetal-based resin. The amount of the group is 0.2 mmol / g, the content of the acetal bond having an ionic functional group (in formula (1-4), R ² is a benzene ring and X is sodium sulfonate) is 3 mol%, and Tg. Was 76 ° C.

(Preparation of plasticizer-mixed polyvinyl acetal-based resin fine particles 1)
100 parts by weight of the polyvinyl acetal-based resin fine particles 1 were weighed and 5 parts by weight of triethylene glycol-2-ethylhexanoate was added to prepare a plasticizer-mixed polyvinyl acetal-based resin fine particles 1 having a low elastic modulus. The volume average particle diameter of the obtained plasticizer-mixed polyvinyl acetal-based resin fine particles 1 was measured with a transmission electron microscope and found to be 90 nm and a Tg (glass transition temperature) of 78 ° C.

(Preparation of plasticizer-mixed polyvinyl acetal-based resin fine particles 2)
100 parts by weight of the polyvinyl acetal-based resin fine particles 1 were weighed and 10 parts by weight of triethylene glycol-2-ethylhexanoate was added to prepare a plasticizer-mixed polyvinyl acetal-based resin fine particles 2 having a low elastic modulus. The volume average particle diameter of the obtained plasticizer-mixed polyvinyl acetal-based resin fine particles 2 was measured with a transmission electron microscope and found to be 90 nm and a Tg of 66 ° C.

(Preparation of plasticizer-mixed polyvinyl acetal-based resin fine particles 3)
100 parts by weight of the polyvinyl acetal-based resin fine particles 1 were weighed and 15 parts by weight of triethylene glycol-2-ethylhexanoate was added to prepare a plasticizer-mixed polyvinyl acetal-based resin fine particles 3 having a low elastic modulus. The volume average particle diameter of the obtained plasticizer-mixed polyvinyl acetal-based resin fine particles 3 was measured with a transmission electron microscope and found to be 90 nm and a Tg of 58 ° C.

(Preparation of plasticizer-mixed polyvinyl acetal-based resin fine particles 4)
100 parts by weight of the polyvinyl acetal-based resin fine particles 1 were weighed and 50 parts by weight of triethylene glycol-2-ethylhexanoate was added to prepare a plasticizer-mixed polyvinyl acetal-based resin fine particles 4 having a low elastic modulus. The volume average particle diameter of the obtained plasticizer-mixed polyvinyl acetal-based resin fine particles 4 was measured with a transmission electron microscope and found to be 90 nm and a Tg of 43 ° C.

(Examples 1 to 7)
(Preparation of composition for positive electrode of lithium secondary battery)
The polyvinyl acetal-based resin and the plasticizer-mixed polyvinyl acetal-based resin fine particles shown in Table 2 were mixed at the ratio shown in Table 2, and 18 parts by weight of water was further added as a solvent to prepare a binder for a power storage device electrode.
50 parts by weight of lithium cobalt oxide (manufactured by Nippon Kagaku Kogyo Co., Ltd., Celseed C-5) as a positive electrode active material and acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive auxiliary agent with respect to 20 parts by weight of the obtained binder for power storage device electrodes , Denka Black) and 1 part by weight of carboxymethyl cellulose (CMC Daicel # 2200, manufactured by Daicel FineChem) as a thickener to obtain a composition for the positive electrode of a lithium secondary battery.

(Examples 8 to 14)
(Preparation of composition for negative electrode of lithium secondary battery)
A binder for a power storage device electrode was prepared in the same manner as in Example 1 except that the polyvinyl acetal-based resin and the plasticizer-mixed polyvinyl acetal-based resin fine particles shown in Table 2 were mixed at the ratio shown in Table 2.
With respect to 20 parts by weight of the obtained binder for power storage device electrodes, 50 parts by weight of spherical natural graphite (manufactured by Nippon Graphite Industry Co., Ltd., CGB-20) as a negative electrode active material, and acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive auxiliary agent. 1 part by weight of Denka Black) and 1 part by weight of carboxymethyl cellulose (CMC Daicel # 2200, manufactured by Daicel FineChem) as a thickener were added and mixed to obtain a composition for a negative electrode of a lithium secondary battery.

(Examples 15 to 21)
(Preparation of composition for negative electrode of lithium secondary battery)
A binder for a power storage device electrode was prepared in the same manner as in Example 1 except that the polyvinyl acetal-based resin and the plasticizer-mixed polyvinyl acetal-based resin fine particles shown in Table 2 were mixed at the ratio shown in Table 2.
With respect to 20 parts by weight of the obtained binder for power storage device electrodes, 45 parts by weight of spherical natural graphite (manufactured by Nippon Graphite Industry Co., Ltd., CGB-20) and 5 parts by weight of silicon (SiO, manufactured by Osaka Titanium Technologies) as negative electrode active materials. , 1 part by weight of acetylene black (Denka Black, manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive auxiliary agent, and 1 part by weight of carboxymethyl cellulose (CMC Daicel # 2200, manufactured by Daicel FineChem) as a thickener are added and mixed, and lithium secondary is added. A composition for the negative electrode of the battery was obtained.

(Comparative Examples 1 to 4)
A binder for a power storage device electrode was prepared in the same manner as in Example 1 except that the polyvinyl acetal-based resin and the plasticizer-mixed polyvinyl acetal-based resin fine particles shown in Table 3 were mixed at the ratio shown in Table 3.
A composition for a lithium secondary battery electrode was obtained in the same manner as in Example 1 except that the obtained binder for a power storage device electrode was used.

(Comparative Example 5)
A composition for a positive electrode of a lithium secondary battery was obtained in the same manner as in Example 1 except that a styrene-butadiene copolymer (SBR) latex (volume average particle diameter 180 nm, solid content 50% by weight) was used as a binder. ..

(Comparative Examples 6-9)
A binder for a power storage device electrode was prepared in the same manner as in Example 1 except that the polyvinyl acetal-based resin and the plasticizer-mixed polyvinyl acetal-based resin fine particles shown in Table 3 were mixed at the ratio shown in Table 3.
A composition for the negative electrode of the lithium secondary battery was obtained in the same manner as in Example 8 except that the obtained binder for the electrode of the power storage device was used.

(Comparative Example 10)
A composition for a negative electrode of a lithium secondary battery was obtained in the same manner as in Example 8 except that a styrene-butadiene copolymer (SBR) latex (volume average particle diameter 180 nm, solid content 50% by weight) was used as a binder. ..

(Comparative Example 11)
A binder for a power storage device electrode was prepared in the same manner as in Example 1 except that the polyvinyl acetal-based resin fine particles mixed with the plasticizer and the styrene-butadiene copolymer were mixed at the ratios shown in Table 3.
A composition for the negative electrode of the lithium secondary battery was obtained in the same manner as in Example 8 except that the obtained binder for the electrode of the power storage device was used.

(Comparative Example 12)
A binder for a power storage device electrode was prepared in the same manner as in Example 1 except that the polyvinyl acetal-based resin and the plasticizer-mixed polyvinyl acetal-based resin fine particles shown in Table 3 were mixed at the ratio shown in Table 3.
A composition for the negative electrode of the lithium secondary battery was obtained in the same manner as in Example 8 except that the obtained binder for the electrode of the power storage device was used.

(Comparative Examples 13 to 16)
A binder for a power storage device electrode was prepared in the same manner as in Example 1 except that the polyvinyl acetal-based resin and the plasticizer-mixed polyvinyl acetal-based resin fine particles shown in Table 3 were mixed at the ratio shown in Table 3.
A composition for the negative electrode of the lithium secondary battery was obtained in the same manner as in Example 15 except that the obtained binder for the electrode of the power storage device was used.

(Comparative Example 17)
A composition for a negative electrode of a lithium secondary battery was obtained in the same manner as in Example 15 except that a styrene-butadiene copolymer (SBR) latex (volume average particle diameter 180 nm, solid content 50% by weight) was used as a binder. ..

(Comparative Example 18)
A binder for a power storage device electrode was prepared in the same manner as in Example 1 except that the polyvinyl acetal-based resin and the plasticizer-mixed polyvinyl acetal-based resin fine particles shown in Table 3 were mixed at the ratio shown in Table 3.
A composition for the negative electrode of the lithium secondary battery was obtained in the same manner as in Example 15 except that the obtained binder for the electrode of the power storage device was used.

<Evaluation>
Polyvinyl acetal resin fine particles used in Examples and Comparative Examples, plasticizer-mixed polyvinyl acetal resin fine particles, styrene-butadiene copolymer latex, and secondary battery electrode compositions obtained in Examples and Comparative Examples (for positive electrodes). , For negative electrode) The following evaluations were made. The results are shown in Tables 2 and 3.

(1) Storage Elastic Modulus Regarding the polyvinyl acetal-based resin fine particles and the plasticizer-mixed polyvinyl acetal-based resin fine particles used in Examples and Comparative Examples, DVA-200 (manufactured by IT Measurement Control Co., Ltd.) was used, and the frequency was 10 Hz, 5 ° C./ The storage elastic modulus was measured at 50 ° C. and 10 Hz by heating at a heating rate of min. For the styrene-butadiene copolymer latex, the resin component was separated by removing water, and the storage elastic modulus was measured in the same manner.

(2) Dispersibility (average dispersion diameter)
After mixing and diluting 10 parts by weight of the composition for lithium secondary battery electrodes and 90 parts by weight of water obtained in Examples and Comparative Examples, 10 by an ultrasonic disperser (manufactured by SND Co., Ltd., “US-303”). Stir for minutes. After that, the particle size distribution was measured using a laser diffraction type particle size distribution meter (LA-910, manufactured by HORIBA, Ltd.), and the average dispersion diameter was measured.

(3) Adhesiveness (peeling force)
The compositions for the positive electrode of the lithium secondary battery obtained in Examples 1 to 7 and Comparative Examples 1 to 5 were evaluated for their adhesiveness to the aluminum foil, and were obtained in Examples 8 to 21 and Comparative Examples 6 to 18. Regarding the composition for the negative electrode of the lithium secondary battery, the adhesiveness to the copper foil was evaluated.

(3-1) Adhesion to Aluminum Foil A lithium secondary battery positive electrode composition is applied onto an aluminum foil (thickness 15 μm) so that the thickness after drying is 40 μm, dried, and placed on the aluminum foil. A test piece having a sheet-like electrode was obtained.
This sample is cut into 10 cm in length and 5 cm in width, and the electrode sheet is pulled up while fixing the test piece using AUTOGRAPH (manufactured by Shimadzu Corporation, "AGS-J") until the electrode sheet is completely peeled off from the aluminum foil. The required peeling force (N) was measured.

(3-2) Adhesiveness to Copper Foil In the above "(3-1) Adhesiveness to Aluminum Foil", the peeling force was measured by exactly the same method except that the aluminum foil was changed to a copper foil (thickness 15 μm).

(4) Electrolyte resistance (preparation of binder resin sheet)
The polyvinyl acetal-based resin solution used in Examples and Comparative Examples was applied onto the release-treated polyethylene terephthalate (PET) film so that the film thickness after drying was 50 μm, and dried to obtain a binder resin sheet. Made.
The obtained binder resin sheet was cut out to a size of 30 × 50 mm to prepare a test piece.

The weight (a) of the film was measured by drying the obtained test piece at 110 ° C. for 2 hours and weighing the obtained film.
Next, a mixed solution of ethylene carbonate and diethyl carbonate (volume ratio 1: 1) was used as the electrolytic solution, and the obtained film was immersed in the electrolytic solution at 25 ° C. for 3 days. Then, the film was taken out, and the electrolytic solution on the surface was immediately wiped off and then weighed to measure the weight (b).
Then, the film was immersed in 500 g of pure water for 2 days to completely remove the electrolytic solution inside the film, dried at 110 ° C. for 2 hours, and then weighed to measure the weight (c).
From each weight, the dissolution rate and swelling rate of the binder were calculated by the following formulas.
Dissolution rate (%) = [(ac) / a] x 100
Swelling rate (%) = (b / c) x 100
The higher the dissolution rate, the easier it is for the resin to dissolve in the electrolytic solution, and the higher the swelling rate, the easier it is for the resin to swell with the electrolytic solution.

(5) Flexibility With respect to the compositions for the positive electrode of the lithium secondary battery obtained in Examples 1 to 7 and Comparative Examples 1 to 5, the flexibility of the electrode produced by using the aluminum foil was evaluated, and Examples 8 to 8 to 21. With respect to the compositions for the negative electrode of the lithium secondary battery obtained in Comparative Examples 6 to 18, the flexibility of the electrodes produced by using the copper foil was evaluated.
(5-1) A composition for a lithium secondary battery electrode is applied onto a flexible aluminum foil (thickness 15 μm) using an aluminum foil so that the film thickness after drying is 40 μm, dried, and the aluminum foil is dried. A test piece having a sheet-like electrode formed on it 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 to stand for one day, and then the sample was unwound and the occurrence of cracks and cracks in the electrodes was evaluated according to the following criteria.
⊚: No cracks or cracks were confirmed ○: Slight cracks or cracks were confirmed, but no peeling of the active material was confirmed △: Cracks or cracks were confirmed, and partial peeling of the active material was confirmed. Also confirmed ×: Cracks and cracks were confirmed on the entire surface, and peeling of most of the active material was also confirmed. (5-2) Flexibility using copper foil The above "(5-1) Aluminum foil was used. In "flexibility", the flexibility was evaluated by exactly the same method except that the aluminum foil was changed to copper foil (thickness 15 μm).

(6) Battery Performance Evaluation (6-1) Examples 1-7, Comparative Examples 1-5
(A) Preparation of Coin Battery The lithium secondary battery positive electrode composition obtained in Examples 1 to 7 and Comparative Examples 1 to 5 was uniformly applied to an aluminum foil having a thickness of 15 μm, dried, and punched to φ16 mm. Obtained a positive electrode layer.
_{A mixed solvent (volume ratio 1: 1) of ethylene carbonate and diethyl carbonate containing LiPF 6} (1M) is used as the electrolytic solution, the positive electrode layer is impregnated with the electrolytic solution, and then this positive electrode layer is used as a positive electrode current collector. A porous PP film (separator) having a thickness of 25 mm impregnated with an electrolytic solution was placed on the surface.
Further, a lithium metal plate to be a negative electrode layer was placed on this, and a negative electrode current collector coated with an insulating packing was superposed on the lithium metal plate. Pressure was applied to this laminate by a caulking machine to obtain a sealed coin battery.

(B) Discharge Capacity Evaluation and Charge / Discharge Cycle Evaluation The obtained coin battery was subjected to discharge capacity evaluation and charge / discharge cycle evaluation using (Charge / Discharge Test Device manufactured by Hosen Co., Ltd.).
The discharge capacity evaluation and charge / discharge cycle evaluation were performed in a voltage range of 2.7 to 4.2 V and evaluation temperatures of 25 ° C. and 50 ° C. The charge / discharge cycle evaluation was calculated from the ratio of the discharge capacity at the 30th cycle to the initial discharge capacity.
In Comparative Examples 2, 4 and 5, when the electrolytic solution was injected into the cell, it was confirmed that the active material was peeled off from the electrode due to the elution of the resin component. In this state, the secondary battery was manufactured and the discharge capacity was evaluated, but it was judged as "unmeasurable" because charging and discharging could not be performed.

(6-2) Examples 8 to 21, Comparative Examples 6 to 18
The lithium secondary battery negative electrode compositions obtained in Examples 8 to 21 and Comparative Examples 6 to 18 were uniformly applied to a copper foil having a thickness of 15 μm, dried, and punched to φ16 mm to obtain a negative electrode layer.
After obtaining a sealed coin battery by the same method as in (6-1) except that the obtained negative electrode layer was used, discharge capacity evaluation and charge / discharge cycle evaluation were performed. The charge / discharge cycle evaluation was calculated from the ratio of the discharge capacity at the 30th cycle to the initial discharge capacity.
In Comparative Examples 7, 9, 10, 14, 16 and 17, when the electrolytic solution was injected into the cell, it was confirmed that the active material was peeled off from the electrode due to the elution of the resin component. In this state, the secondary battery was manufactured and the discharge capacity was evaluated, but it was judged as "unmeasurable" because charging and discharging could not be performed.

(7) Electrode curlability The test piece obtained in the above "(3) Adhesiveness (peeling force)" is wound around a round bar of φ1.5 mm, and the curl of the electrode end when rubbed 100 times continuously is visually observed. Evaluated according to the following criteria.
◎: No curl in the bent part ○: There is a curl in the bent part, but the base (aluminum foil, copper foil) cannot be confirmed ×: There is a curl in the bent part, and the base (aluminum foil, copper foil) is Can be confirmed

The present invention can provide a binder for a power storage device electrode which has adhesiveness and adhesiveness and can impart toughness and flexibility to the obtained electrode. Further, a composition for a power storage device electrode including the binder for the power storage device electrode, a power storage device electrode, and a power storage device can be provided.

Claims (8)

A binder used for the electrodes of power storage devices.
Binder (A) having a storage elastic modulus of 1 × 10 ⁸ to 1 × 10 ¹⁰ Pa at 50 ° C. and 10 Hz, and a binder having a storage elastic modulus of 1 × 10 ⁵ to 1 × 10 ⁷ Pa at 50 ° C. and 10 Hz. Contains (B)
The binder (A) is a polyvinyl acetal resin (A).
The binder (B) contains a polyvinyl acetal resin (B) and a plasticizer, and contains.
The binder (A) is contained in an amount of 85 to 99.9% by weight and the binder (B) is contained in an amount of 0.1 to 15% by weight based on 100% by weight of the total of the binder (A) and the binder (B). Binder for power storage device electrodes. The binder for a power storage device electrode according to claim 1, further comprising a binder (A), a binder (B), and an aqueous medium. Binder (A), the microparticles shape der is, the volume average particle diameter is characterized 10~1000nm der Rukoto claim 1 or 2, wherein the electric storage device electrode binder. The binder (A) has a storage elastic modulus of 1 × 10 at 50 ° C. and 10 Hz. ⁸ ~ 1.5 × 10 ⁹ The binder for a power storage device electrode according to claim 1, 2 or 3. Binder (B) is particulate form der is, the volume average particle diameter is characterized 10~1000nm der Rukoto claim 1, 2, 3 or 4, wherein the electric storage device electrode binder. The binder (A) for a power storage device electrode according to claim 1, 2, 3, 4 or 5 , and the composition for a power storage device electrode containing an active material, based on 100 parts by weight of the active material. A composition for a power storage device electrode, wherein the total content of the binder (B) and the binder (B) is 0.1 to 12 parts by weight. A power storage device electrode comprising the composition for a power storage device electrode according to claim 6. A power storage device comprising the power storage device electrode according to claim 7.