JPWO2019202985A1 - Binder for secondary battery electrode and its use - Google Patents

Abstract

The present invention provides an aqueous binder for a secondary battery capable of imparting excellent binding properties and good slurry stability, a composition for a mixture layer of a secondary battery electrode obtained by using the binder, and a secondary battery electrode. To do.
A binder for a secondary battery electrode containing a carboxyl group-containing polymer or a salt thereof, wherein the carboxyl group-containing polymer is a structural unit derived from an ethylenically unsaturated carboxylic acid monomer with respect to all structural units thereof. Is contained in an amount of 50% by mass or more and 99.9% by mass or less, and 0.1% by mass or more and 4.9% by mass or less of structural units derived from the nitrile group-containing ethylenically unsaturated monomer.

Description

The present invention relates to a binder for a secondary battery electrode and its use.

As the secondary battery, various power storage devices such as a nickel hydrogen secondary battery, a lithium ion secondary battery, and an electric double layer capacitor have been put into practical use. The electrodes used in these secondary batteries are produced by applying, drying, or the like on a current collector a composition for forming an electrode mixture layer containing an active material, a binder, and the like. For example, in a lithium ion secondary battery, an aqueous binder containing styrene-butadiene rubber (SBR) latex and carboxymethyl cellulose (CMC) is used as the binder used in the negative electrode mixture layer composition. Further, as a binder having excellent dispersibility and binding property, a binder containing an aqueous acrylic acid polymer solution or an aqueous dispersion is known. On the other hand, as a binder used for the positive electrode mixture layer, an N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride (PVDF) is widely used.

On the other hand, as the applications of various secondary batteries expand, the demand for improving energy density, reliability and durability tends to increase. For example, for the purpose of increasing the electric capacity of a lithium ion secondary battery, specifications for using a silicon-based active material as a negative electrode active material are increasing. However, it is known that the volume of a silicon-based active material changes significantly during charging and discharging, and the electrode mixture layer peels off or falls off as it is used repeatedly, resulting in a decrease in battery capacity and cycle characteristics. There was a problem that (durability) deteriorated. In order to suppress such a defect, it is generally effective to improve the binder's binding property, and for the purpose of improving the durability, studies on improving the binder's binding property have been conducted.

An acrylic acid-based polymer copolymerized with a nitrile-based monomer has been proposed as a binder having good binding properties and exerting an effect on durability.
Patent Document 1 discloses a slurry for a negative electrode of a lithium ion secondary battery containing a salt of acrylic acid or an acrylic acid derivative and a copolymer of acrylonitrile or an acrylonitrile derivative. Patent Document 2 describes a binder for a lithium battery, which comprises a monomer unit derived from acrylic acid and (meth) acrylonitrile as constituent components and is composed of a polymer crosslinked by a specific crosslinking agent. Patent Document 3 contains a polymer obtained by polymerizing a lithium salt of an unsaturated acid, an unsaturated acid, and a monomer composition containing α, β-unsaturated nitrile in a specific ratio, and a dispersion medium. A binder composition for a lithium ion secondary battery electrode is disclosed.

Japanese Unexamined Patent Publication No. 2015-115109 International Publication No. 2015/163302 International Publication No. 2015/151518

All of the binders disclosed in Patent Documents 1 to 3 can impart good binding properties. On the other hand, according to the studies by the present inventors, when a binder made of a copolymer obtained by copolymerizing a nitrile-based monomer is used, the long-term stability of the electrode slurry containing the electrode active material may be insufficient. In some cases, it was found that the slurry had a problem such as partial sedimentation.

The present disclosure has been made in view of such circumstances, and is an aqueous binder for a secondary battery electrode capable of imparting excellent binding properties and good slurry stability, and a polymer used for the binder or a polymer thereof. A method for producing a salt is provided. The present disclosure also provides a composition for a secondary battery electrode mixture layer and a secondary battery electrode obtained by using the above binder.

As a result of diligent studies to solve the above problems, the present inventors have obtained a polymer obtained by copolymerizing an ethylenically unsaturated carboxylic acid monomer as a main component and a small amount of a nitrile group-containing ethylenically unsaturated monomer as a main component. It was found that when a binder containing the salt was used, both the stability and the binding property of the electrode mixture layer slurry were excellent. According to the present disclosure, the following means are provided based on such findings.

The present invention is as follows.
[1] A binder for a secondary battery electrode containing a carboxyl group-containing polymer or a salt thereof.
The carboxyl group-containing polymer contains 50% by mass or more and 99.9% by mass or less of structural units derived from an ethylenically unsaturated carboxylic acid monomer with respect to all the structural units, and a nitrile group-containing ethylenically unsaturated monomer. A binder for a secondary battery electrode containing 0.1% by mass or more and 4.9% by mass or less of a structural unit derived from a polymer.
[2] The binder for a secondary battery electrode according to [1], wherein the carboxyl group-containing polymer has a degree of neutralization of 70 mol% or more.
[3] The binder for a secondary battery electrode according to [1] or [2], wherein the carboxyl group-containing polymer is a crosslinked polymer having a crosslinked structure.
[4] A method for producing a carboxyl group-containing polymer or a salt thereof used in a binder for a secondary battery electrode.
Monomer component containing 50% by mass or more and 99.9% by mass or less of an ethylenically unsaturated carboxylic acid monomer and 0.1% by mass or more and 4.9% by mass or less of an ethylenically unsaturated monomer containing a nitrile group. A method comprising a polymerization step of polymerizing by a precipitation polymerization method.
[5] A composition for a secondary battery electrode mixture layer containing the binder, active material and water according to any one of [1] to [3].
[6] The composition for a secondary battery electrode mixture layer according to [5], which contains a carbon-based material or a silicon-based material as a negative electrode active material.
[7] A secondary battery electrode provided with a mixture layer formed from the composition for the secondary battery electrode mixture layer according to [5] or [6] on the surface of a current collector.

The binder for a secondary battery electrode of the present invention exhibits excellent binding properties to an electrode active material or the like. In addition, the binder can exhibit good adhesiveness to the current collector. Therefore, the electrode mixture layer containing the binder and the electrode provided with the binder are excellent in binding property and can maintain their integrity. Therefore, deterioration of the electrode mixture layer due to volume change and shape change of the active material due to charge / discharge is suppressed, and a secondary battery having high durability (cycle characteristics) can be obtained. Further, since the mixture layer slurry containing the binder for the secondary battery electrode of the present invention has excellent stability, the change in slurry viscosity is suppressed or reduced even after long-term storage, and the uniformity is excellent. The electrode mixture layer can be stably obtained.

The binder for a secondary battery electrode of the present invention contains a carboxyl group-containing polymer or a salt thereof, and can be mixed with an active material and water to form an electrode mixture layer composition. The above composition may be in a slurry state that can be applied to the current collector, or may be prepared in a wet powder state so that it can be pressed on the surface of the current collector. The secondary battery electrode of the present invention can be obtained by forming a mixture layer formed from the above composition on the surface of a current collector such as a copper foil or an aluminum foil.

Hereinafter, each of the binder for the secondary battery electrode of the present invention, the composition for the mixture layer of the secondary battery electrode obtained by using the binder, and the secondary battery electrode will be described in detail.
In the present specification, "(meth) acrylic" means acrylic and / or methacrylic, and "(meth) acrylate" means acrylate and / or methacrylate. Further, the “(meth) acryloyl group” means an acryloyl group and / or a methacryloyl group.

<Binder>
The binder of the present invention contains a carboxyl group-containing polymer or a salt thereof. The carboxyl group-containing polymer has a structural unit derived from an ethylenically unsaturated carboxylic acid and a nitrile group-containing ethylenically unsaturated monomer.

<Structural unit of carboxyl group-containing polymer>
<Structural unit derived from ethylenically unsaturated carboxylic acid monomer>
The carboxyl group-containing polymer (hereinafter, also referred to as “the present polymer”) can have a structural unit (hereinafter, also referred to as “component (a)”) derived from the ethylenically unsaturated carboxylic acid monomer. When the present polymer has a carboxyl group by having such a structural unit, the adhesiveness to the current collector is improved, and the lithium ion desolvation effect and the ionic conductivity are excellent, so that the resistance is small and the high rate is obtained. An electrode having excellent characteristics can be obtained. Further, since water swelling property is imparted, the dispersion stability of the active material or the like in the mixture layer composition can be enhanced.
The component (a) can be introduced into a polymer, for example, by polymerizing a monomer containing an ethylenically unsaturated carboxylic acid monomer. Alternatively, it can also be obtained by (co) polymerizing a (meth) acrylic acid ester monomer and then hydrolyzing it. Further, after polymerizing (meth) acrylamide, (meth) acrylonitrile or the like, it may be treated with a strong alkali, or it may be a method of reacting an acid anhydride with a polymer having a hydroxyl group.

Examples of the ethylenically unsaturated carboxylic acid monomer include (meth) acrylic acid, itaconic acid, crotonic acid, maleic acid, and fumaric acid; and (meth) acrylamide alkyl such as (meth) acrylamide hexane acid and (meth) acrylamide dodecanoic acid. Carboxylic acid; ethylenically unsaturated monomers having carboxyl groups such as monohydroxyethyl succinate (meth) acrylate, ω-carboxy-caprolactone mono (meth) acrylate, β-carboxyethyl (meth) acrylate, or (partial) thereof. ) Alkaline neutralized products may be mentioned, and one of these may be used alone, or two or more thereof may be used in combination. Among the above, a compound having an acryloyl group as a polymerizable functional group is preferable, and acrylic acid is particularly preferable, in that a polymer having a long primary chain length can be obtained due to a high polymerization rate and the binder has a good binding force. is there. When acrylic acid is used as the ethylenically unsaturated carboxylic acid monomer, a polymer having a high carboxyl group content can be obtained.

The content of the component (a) in the present polymer is not particularly limited, but may be, for example, 10% by mass or more and 99.9% by mass or less with respect to all the structural units of the present polymer. By containing the component (a) in such a range, excellent adhesiveness to the current collector can be easily ensured. The lower limit is, for example, 20% by mass or more, for example, 30% by mass or more, and for example, 40% by mass or more. When the lower limit is 50% by mass or more, the dispersion stability of the composition for the electrode mixture layer becomes good and a higher binding force can be obtained, which is preferable. It may be 60% by mass or more, and may be 70% by mass or more. It may be 80% by mass or more. Further, the upper limit is, for example, 99.5% by mass or less, for example 99% by mass or less, for example 98% by mass or less, for example 95% by mass or less, and for example 90% by mass or less. Yes, and for example, 80% by mass or less. The range may be a range in which such a lower limit and an upper limit are appropriately combined, and is, for example, 10% by mass or more and 99.9% by mass or less, and for example, 50% by mass or more and 99.9% by mass or less. Yes, for example, 50% by mass or more and 99% by mass or less, for example, 50% by mass or more and 98% by mass or less, and for example, 50% by mass or more and 95% by mass or less.

<Structural unit derived from nitrile group-containing ethylenically unsaturated monomer>
In addition to the component (a), the present polymer may have a structural unit derived from a nitrile group-containing ethylenically unsaturated monomer (hereinafter, also referred to as “component (b)”). The component (b) can exert a strong interaction with the electrode material and can exhibit good binding property to the active material. As a result, a solid and well-integrated electrode mixture layer can be obtained. The component (b) can be introduced into the present polymer, for example, by polymerizing a nitrile group-containing ethylenically unsaturated monomer.

Examples of the nitrile group-containing ethylenically unsaturated monomer include (meth) achlorinitrile; (meth) cyanomethyl acrylate, (meth) cyanoethyl acrylate and other (meth) acrylate cyanoalkyl ester compounds; 4-cyanostyrene. , 4-Cyano-α-methylstyrene and other unsaturated aromatic compounds containing cyano groups; vinylidene cyanide and the like, and one of these may be used alone or in combination of two or more. You may use it. Among the above, acrylonitrile is preferable because it has a high nitrile group content.

The content of the component (b) in the present polymer is 4.9% by mass or less with respect to the total structural units of the present polymer. If the content of the component (b) exceeds 4.9% by mass, the dispersion stability of the present polymer in the mixture layer composition (slurry) becomes insufficient, and the stability and binding property of the slurry deteriorate. There is a risk. The upper limit may be 4.5% by mass or less, 4.0% by mass or less, or 3.0% by mass or less. The lower limit is not particularly limited, but may include, for example, 0.1% by mass or more with respect to all the structural units of the present polymer. When the present polymer contains 0.1% by mass of the component (b), an electrode mixture layer having excellent binding properties can be obtained. The lower limit is preferably 0.2% by mass or more, more preferably 0.3% by mass or more, and further preferably 0.5% by mass or more. The range may be a range in which such a lower limit and an upper limit are appropriately combined, and is, for example, 0.1% by mass or more and 4.9% by mass or less, and for example, 0.1% by mass or more and 4.5. It may be 0% by mass or less, and may be, for example, 0.2% by mass or more and 4.0% by mass or less.

<Other structural units>
This polymer is a structural unit derived from other ethylenically unsaturated monomers copolymerizable with the component (a) and the component (b) (hereinafter, also referred to as “component (c)”). Can be included. As the component (c), for example, an ethylenically unsaturated monomer compound having an anionic group other than the carboxyl group such as a sulfonic acid group and a phosphoric acid group, or a nonionic ethylenically non-component other than the component (b). Examples thereof include structural units derived from saturated monomers and the like. These structural units are an ethylenically unsaturated monomer compound having an anionic group other than a carboxyl group such as a sulfonic acid group and a phosphoric acid group, or a nonionic ethylenically unsaturated monomer other than the component (b). It can be introduced by copolymerizing a monomer containing a body. Among these, as the component (c), a structural unit derived from a nonionic ethylenically unsaturated monomer is preferable from the viewpoint of obtaining an electrode having good bending resistance, and the binder has excellent binding properties. (Meta) acrylamide and its derivatives are preferable. Further, when a structural unit derived from a hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less is introduced as the component (c), a strong interaction with the electrode material can be achieved. It can exhibit good binding properties to active materials. This is preferable because a solid and well-integrated electrode mixture layer can be obtained. In particular, a structural unit derived from an alicyclic structure-containing ethylenically unsaturated monomer is preferable.

The ratio of the component (c) can be 0% by mass or more and 49.9% by mass or less with respect to the total structural units of the present polymer. The ratio of the component (c) may be 1% by mass or more and 40% by mass or less, 2% by mass or more and 40% by mass or less, and 2% by mass or more and 30% by mass or less. It may be 5% by mass or more and 30% by mass or less. Further, when the component (c) is contained in an amount of 1% by mass or more with respect to all the structural units of the present polymer, the affinity for the electrolytic solution is improved, so that the effect of improving the lithium ion conductivity can be expected.

Examples of the (meth) acrylamide derivative include N-alkyl (meth) acrylamide compounds such as isopropyl (meth) acrylamide and t-butyl (meth) acrylamide; Nn-butoxymethyl (meth) acrylamide and N-isobutoxymethyl. N-alkoxyalkyl (meth) acrylamide compounds such as (meth) acrylamide; N, N-dialkyl (meth) acrylamide compounds such as dimethyl (meth) acrylamide and diethyl (meth) acrylamide include one of them. It may be used alone or in combination of two or more.

Examples of the alicyclic structure-containing ethylenically unsaturated monomer include cyclopentyl (meth) acrylate, cyclohexyl (meth) acrylate, methylcyclohexyl (meth) acrylate, t-butylcyclohexyl (meth) acrylate, and (meth). ) Cyclodecyl acrylate and cyclododecyl (meth) acrylate and other aliphatic substituents may have (meth) cycloalkyl acrylate; isobornyl (meth) acrylate, adamantyl (meth) acrylate, (meth). ) Dicyclopentenyl acrylate, dicyclopentenyloxyethyl (meth) acrylate, dicyclopentanyl (meth) acrylate, and cyclohexanedimethanol mono (meth) acrylate and cyclodecanedimethanol mono (meth) acrylate, etc. Cycloalkyl polyalcohol mono (meth) acrylate and the like can be mentioned, and one of these may be used alone, or two or more thereof may be used in combination. Among the above, a compound having an acryloyl group as a polymerizable functional group is preferable in that a polymer having a long primary chain length can be obtained because the polymerization rate is high and the binding force of the binder is good.

As the other nonionic ethylenically unsaturated monomer, for example, (meth) acrylic acid ester may be used. Examples of the (meth) acrylic acid ester include (meth) methyl acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, isobutyl (meth) acrylate, and 2-ethylhexyl (meth) acrylate. Meta) Acrylic acid alkyl ester compound;
Aromatic (meth) acrylic acid ester compounds such as (meth) phenyl acrylate, (meth) phenylmethyl acrylate, and (meth) phenylethyl acrylate;
(Meta) acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth) acrylate and ethoxyethyl (meth) acrylate;
Examples thereof include (meth) hydroxyalkyl ester compounds such as hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate and hydroxybutyl (meth) acrylate, and one of them is used alone. It may be used in combination of 2 or more types. From the viewpoint of adhesion to the active material and cycle characteristics, an aromatic (meth) acrylic acid ester compound can be preferably used.

From the viewpoint of further improving lithium ion conductivity and high-rate characteristics, compounds having an ether bond such as (meth) acrylic acid alkoxyalkyl ester such as 2-methoxyethyl (meth) acrylate and ethoxyethyl (meth) acrylate are preferable. , 2-Methoxyethyl (meth) acrylate is more preferred.

Among the nonionic ethylenically unsaturated monomers, a compound having an acryloyl group is preferable in that a polymer having a long primary chain length can be obtained due to its high polymerization rate and the binder has a good binding force. Further, as the nonionic ethylenically unsaturated monomer, a compound having a homopolymer glass transition temperature (Tg) of 0 ° C. or lower is preferable in terms of improving the bending resistance of the obtained electrode.

The present polymer may be a salt. The type of salt is not particularly limited, but alkali metal salts such as lithium, sodium and potassium; alkaline earth metal salts such as calcium salt and barium salt; other metal salts such as magnesium salt and aluminum salt; ammonium salt and organic. Examples include amine salts. Among these, alkali metal salts and magnesium salts are preferable, and alkali metal salts are more preferable, from the viewpoint that adverse effects on battery characteristics are unlikely to occur.

The present polymer may be a crosslinked polymer having a crosslinked structure. The cross-linking method in the cross-linked polymer is not particularly limited, and examples thereof include the following methods.
1) Copolymerization of crosslinkable monomers 2) Utilizing chain transfer to polymer chains during radical polymerization 3) After synthesizing a polymer having a reactive functional group, post-crosslinking is performed by adding a crosslinking agent as necessary. Since the present polymer has a crosslinked structure, the binder containing the polymer or a salt thereof can have an excellent binding force. Among the above, the method by copolymerizing the crosslinkable monomer is preferable from the viewpoint that the operation is simple and the degree of crosslinking can be easily controlled.

<Crosslinkable monomer>
Examples of the crosslinkable monomer include a polyfunctional polymerizable monomer having two or more polymerizable unsaturated groups, a monomer having a self-crosslinkable crosslinkable functional group such as a hydrolyzable silyl group, and the like. Can be mentioned.

The polyfunctional polymerizable monomer is a compound having two or more polymerizable functional groups such as a (meth) acryloyl group and an alkenyl group in the molecule, and is a polyfunctional (meth) acrylate compound, a polyfunctional alkenyl compound, ( Meta) Examples thereof include compounds having both an acryloyl group and an alkenyl group. These compounds may be used alone or in combination of two or more. Among these, a polyfunctional alkenyl compound is preferable because a uniform crosslinked structure can be easily obtained, and a polyfunctional allyl ether compound having a plurality of allyl ether groups in the molecule is particularly preferable.

Examples of the polyfunctional (meth) acrylate compound include ethylene glycol di (meth) acrylate, propylene glycol di (meth) acrylate, 1,6-hexanediol di (meth) acrylate, polyethylene glycol di (meth) acrylate, and polypropylene glycol di (meth) acrylate. Di (meth) acrylates of dihydric alcohols such as meta) acrylate; trimethylol propantri (meth) acrylate, tri (meth) acrylate of trimethyl propanethylene oxide modified product, glycerin tri (meth) acrylate, pentaerythritol tri (meth) Tri (meth) acrylates of trivalent or higher polyhydric alcohols such as meta) acrylates and pentaerythritol tetra (meth) acrylates, poly (meth) acrylates such as tetra (meth) acrylates; Bisamides and the like can be mentioned.

Examples of the polyfunctional alkenyl compound include polyfunctional allyl ether compounds such as trimethylolpropanediallyl ether, trimethylolpropanetriallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, tetraallyloxyethane, and polyallyl saccharose; diallyl phthalate and the like. Polyfunctional allyl compound; Polyfunctional vinyl compounds such as divinylbenzene and the like can be mentioned.

Compounds having both (meth) acryloyl group and alkenyl group include allyl (meth) acrylate, isopropenyl (meth) acrylate, butenyl (meth) acrylate, pentenyl (meth) acrylate, and (meth) acrylate. 2- (2-Vinyloxyethoxy) ethyl and the like can be mentioned.

Specific examples of the monomer having a self-crosslinkable crosslinkable functional group include a hydrolyzable silyl group-containing vinyl monomer, N-methylol (meth) acrylamide, N-methoxyalkyl (meth) acrylate and the like. Can be mentioned. These compounds can be used alone or in combination of two or more.

The hydrolyzable silyl group-containing vinyl monomer is not particularly limited as long as it is a vinyl monomer having at least one hydrolyzable silyl group. For example, vinyl silanes such as vinyl trimethoxysilane, vinyl triethoxysilane, vinyl methyl dimethoxysilane, vinyl dimethyl methoxysilane; silyl groups such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, methyldimethoxysilylpropyl acrylate and the like. Containing acrylic acid esters; silyl group-containing methacrylate esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, dimethylmethoxysilylpropyl methacrylate; trimethoxysilylpropyl vinyl ether and the like. Cyril group-containing vinyl ethers; silyl group-containing vinyl esters such as trimethoxysilyl undecanoate vinyl and the like can be mentioned.

When the present polymer is crosslinked with a crosslinkable monomer, the amount of the crosslinkable monomer used is 100, which is the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer). It is preferably 0.1 parts by mass or more and 2.0 parts by mass or less, more preferably 0.3 parts by mass or more and 1.5 parts by mass or less, and further preferably 0.5 parts by mass or more and 1 part by mass. It is 5.5 parts by mass or less. When the amount of the crosslinkable monomer used is 0.1 parts by mass or more, it is preferable in that the binding property and the stability of the mixture layer slurry are improved. If it is 2.0 parts by mass or less, the stability of the polymer tends to be high.
Similarly, the amount of the crosslinkable monomer used may be 0.02 to 0.7 mol% with respect to the total amount of the monomers other than the crosslinkable monomer (non-crosslinkable monomer). It is preferably 0.03 to 0.4 mol%, more preferably 0.03 to 0.4 mol%.

<Particle size of crosslinked polymer>
When the present polymer is a crosslinked polymer, in the mixture layer composition, the crosslinked polymer does not exist as a mass (secondary agglomerate) having a large particle size, but as water-swelling particles having an appropriate particle size. Good dispersion is preferable because the binder containing the crosslinked polymer can exhibit good binding performance.

The crosslinked polymer or a salt thereof has a volume of a particle size (water-swelling particle size) when a crosslinked polymer having a degree of neutralization based on a carboxyl group of 80 to 100 mol% is dispersed in water. The reference median diameter is preferably in the range of 0.1 μm or more and 10.0 μm or less. The more preferable range of the particle size is 0.1 μm or more and 8.0 μm or less, the more preferable range is 0.1 μm or more and 7.0 μm or less, and the more preferable range is 0.2 μm or more and 5.0 μm or less. Yes, and even more preferable ranges are 0.5 μm or more and 3.0 μm or less. When the particle size is in the range of 0.1 μm or more and 10.0 μm or less, the mixture layer composition has a suitable size and is uniformly present in the mixture layer composition. Therefore, the stability of the mixture layer composition is high and excellent results are obtained. It is possible to demonstrate the wearability. If the particle size exceeds 10.0 μm, the binding property may be insufficient as described above. In addition, there is a risk that the coatability will be insufficient because it is difficult to obtain a smooth coated surface. On the other hand, when the particle size is less than 0.1 μm, there is concern from the viewpoint of stable manufacturability.

If the crosslinked polymer is unneutralized or has a neutralization degree of less than 80 mol%, neutralize it to a neutralization degree of 80 to 100 mol% with an alkali metal hydroxide or the like, and measure the particle size when dispersed in water. do it. In general, the crosslinked polymer or a salt thereof often exists as agglomerated particles in which primary particles are associated and aggregated in the state of powder or solution (dispersion liquid). When the particle size when dispersed in water is in the above range, the crosslinked polymer or a salt thereof has extremely excellent dispersibility, and is neutralized to a degree of neutralization of 80 to 100 mol% to be water. By dispersing, the agglomerated particles are disintegrated, and even if it is a dispersion of almost primary particles or a secondary agglomerate, a stable dispersed state in which the particle diameter is within the range of 0.1 to 10.0 μm is formed. It is a thing.

The particle size distribution, which is the value obtained by dividing the volume-based median diameter of the water-swelled particle size by the number-based median diameter, is preferably 10 or less, more preferably 5.0 or less, from the viewpoint of bondability and coatability. Yes, more preferably 3.0 or less, still more preferably 1.5 or less. The lower limit of the particle size distribution is usually 1.0.

The particle size (dry particle size) of the crosslinked polymer or a salt thereof at the time of drying is preferably in the range of 0.03 μm or more and 3 μm or less in terms of volume-based median diameter. A more preferable range of the particle size is 0.1 μm or more and 1 μm or less, and a more preferable range is 0.3 μm or more and 0.8 μm or less.

In this polymer or a salt thereof, acid groups such as a carboxyl group derived from an ethylenically unsaturated carboxylic acid monomer are neutralized so that the degree of neutralization is 20 mol% or more in the mixture layer composition. , It is preferable to use it as an embodiment of salt. The degree of neutralization is more preferably 50 mol%, further preferably 60 mol% or more, further preferably 70 mol% or more, still more preferably 80 mol% or more, and particularly preferably 85. More than mol%. The upper limit of the degree of neutralization is 100 mol%, and may be 98 mol% or 95 mol%. The range of the degree of neutralization may be appropriately combined with the above lower limit value and upper limit value, and may be, for example, 50 mol% or more and 100 mol% or less, or 70 mol% or more and 100 mol% or less. , 80 mol% or more and 100 mol% or less. When the degree of neutralization is 20 mol% or more, the water swelling property is good and the dispersion stabilizing effect is easily obtained, which is preferable. In the present specification, the degree of neutralization can be calculated by calculation from the charged values of a monomer having an acid group such as a carboxyl group and a neutralizing agent used for neutralization. The degree of neutralization was determined by measuring the IR of the crosslinked polymer or a salt thereof after drying the crosslinked polymer or a salt thereof at 80 ° C. for 3 hours under reduced pressure. It can be confirmed from the intensity ratio of the peak derived from the O group.

<Method for producing this polymer or its salt>
Known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization can be used for this polymer, but precipitation polymerization and suspension polymerization (reverse phase suspension polymerization) can be used in terms of productivity. ) Is preferable. Non-uniform polymerization methods such as precipitation polymerization, suspension polymerization, and emulsion polymerization are preferable, and the precipitation polymerization method is more preferable, because better performance can be obtained in terms of binding property and the like.
Precipitation polymerization is a method for producing a polymer by carrying out a polymerization reaction in a solvent that dissolves an unsaturated monomer as a raw material but does not substantially dissolve the polymer to be produced. As the polymerization progresses, the polymer particles become larger due to aggregation and growth, and a dispersion of polymer particles in which primary particles of several tens of nm to several hundreds nm are secondarily aggregated to several μm to several tens of μm can be obtained. Dispersion stabilizers can also be used to control the particle size of the polymer.
The secondary aggregation can also be suppressed by selecting a dispersion stabilizer, a polymerization solvent, or the like. In general, precipitation polymerization in which secondary aggregation is suppressed is also called dispersion polymerization.

In the case of precipitation polymerization, a solvent selected from water, various organic solvents and the like can be used as the polymerization solvent in consideration of the type of monomer used and the like. In order to obtain a polymer having a longer primary chain length, it is preferable to use a solvent having a small chain transfer constant.

Specific examples of the polymerization solvent include water-soluble solvents such as methanol, t-butyl alcohol, acetone, methyl ethyl ketone, acetonitrile and tetrahydrofuran, as well as benzene, ethyl acetate, dichloroethane, n-hexane, cyclohexane and n-heptane. , One of these can be used alone or in combination of two or more. Alternatively, it may be used as a mixed solvent of these and water. In the present invention, the water-soluble solvent refers to a solvent having a solubility in water at 20 ° C. of more than 10 g / 100 ml.
Of the above, the formation of coarse particles and adhesion to the reactor are small and the polymerization stability is good, and the precipitated polymer fine particles are difficult to secondary agglomerate (or even if secondary agglomeration occurs, they dissolve in the aqueous medium. Methyl ethyl ketone and acetonitrile are preferable because they are easy to use), a polymer having a small chain transfer constant and a large degree of polymerization (primary chain length) can be obtained, and the operation is easy during the step neutralization described later. ..

Similarly, in order to allow the neutralization reaction to proceed stably and rapidly in the step neutralization, it is preferable to add a small amount of a highly polar solvent to the polymerization solvent. Such highly polar solvents preferably include water and methanol. The amount of the highly polar solvent used is preferably 0.05 to 20.0% by mass, more preferably 0.1 to 10.0% by mass, still more preferably 0.1 to 5 based on the total mass of the medium. It is 0.0% by mass, more preferably 0.1 to 1.0% by mass. When the proportion of the highly polar solvent is 0.05% by mass or more, the effect on the neutralization reaction is recognized, and when it is 20.0% by mass or less, no adverse effect on the polymerization reaction is observed. Further, in the polymerization of a highly hydrophilic ethylenically unsaturated carboxylic acid monomer such as acrylic acid, the polymerization rate is improved when a highly polar solvent is added, and it becomes easy to obtain a polymer having a long primary chain length. Among the highly polar solvents, water is particularly preferable because it has a large effect of improving the polymerization rate.

In the production of the present polymer or a salt thereof, it is preferable to include a polymerization step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer and a nitrile group-containing ethylenically unsaturated monomer. For example, the ethylenically unsaturated carboxylic acid monomer from which the component (a) is derived is 50% by mass or more and 99.9% by mass or less, and the nitrile group-containing ethylenically unsaturated monomer from which the component (b) is derived is used. Polymerize a monomer component containing 0.1% by mass or more and 4.9% by mass or less, and 0% by mass or more and 49.9% by mass or less of another ethylenically unsaturated monomer from which the component (c) is derived. It is preferable to include a polymerization step.
Through the above polymerization step, 50% by mass or more and 99.9% by mass or less of the structural unit (component a) derived from the ethylenically unsaturated carboxylic acid monomer is introduced into the present polymer, and the nitrile group-containing ethylenically unsaturated material is introduced. A structural unit (component b) derived from the monomer is introduced in an amount of 0.1% by mass or more and 4.9% by mass or less. The amount of the ethylenically unsaturated carboxylic acid monomer used is, for example, 50% by mass or more and 99% by mass or less, and for example, 50% by mass or more and 98% by mass or less, and for example, 50% by mass. As mentioned above, it is 95% by mass or less. The amount of the nitrile group-containing ethylenically unsaturated monomer used is, for example, 0.1% by mass or more and 4.5% by mass or less, and for example, 0.2% by mass or more and 4.0% by mass or less. Is.

Examples of the other ethylenically unsaturated monomer include an ethylenically unsaturated monomer compound having an anionic group other than a carboxyl group such as a sulfonic acid group and a phosphoric acid group, and a component other than the component (b). Examples thereof include nonionic ethylenically unsaturated monomers. Specific examples of the compound include a monomeric compound into which the above-mentioned component (c) can be introduced. The other ethylenically unsaturated monomer may contain 0% by mass or more and 49.9% by mass or less with respect to the total amount of the monomer components, and may contain 1% by mass or more and 40% by mass or less. It may be 2% by mass or more and 40% by mass or less, 2% by mass or more and 30% by mass or less, or 5% by mass or more and 30% by mass or less. Further, the above-mentioned crosslinkable monomer may be used in the same manner.

The monomer concentration at the time of polymerization is preferably high from the viewpoint of obtaining a polymer having a longer primary chain length. However, if the monomer concentration is too high, the agglutination of the polymer particles tends to proceed, and it becomes difficult to control the heat of polymerization, which may cause the polymerization reaction to run away. Therefore, for example, in the case of the precipitation polymerization method, the monomer concentration at the start of polymerization is generally in the range of about 2 to 40% by mass, preferably in the range of 5 to 40% by mass.
In the present specification, the “monomer concentration” indicates the monomer concentration in the reaction solution at the time when the polymerization is started.

The present polymer may be produced by carrying out a polymerization reaction in the presence of a basic compound. By carrying out the polymerization reaction in the presence of a basic compound, the polymerization reaction can be stably carried out even under high monomer concentration conditions. The monomer concentration may be 13.0% by mass or more, preferably 15.0% by mass or more, more preferably 17.0% by mass or more, and further preferably 19.0% by mass or more. It is more preferably 20.0% by mass or more. The monomer concentration is still preferably 22.0% by mass or more, and even more preferably 25.0% by mass or more. In general, the higher the monomer concentration during polymerization, the higher the molecular weight, and when the present polymer is a crosslinked polymer, a polymer having a long primary chain length can be produced.
In the present specification, the “monomer concentration” indicates the monomer concentration in the reaction solution at the time when the polymerization is started.

The upper limit of the monomer concentration differs depending on the type of monomer and solvent used, the polymerization method, various polymerization conditions, etc., but if the heat of the polymerization reaction can be removed, the precipitation polymerization is as described above. It is about 40%, about 50% for suspension polymerization, and about 70% for emulsion polymerization.

The above-mentioned base compound is a so-called alkaline compound, and either an inorganic base compound or an organic base compound may be used. By carrying out the polymerization reaction in the presence of a basic compound, the polymerization reaction can be stably carried out even under high monomer concentration conditions such as exceeding 13.0% by mass. Further, the polymer obtained by polymerizing at such a high monomer concentration has a high molecular weight (because the primary chain length is long), and therefore has excellent binding properties.
Examples of the inorganic base compound include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide and potassium hydroxide, and alkaline earth metal hydroxides such as calcium hydroxide and magnesium hydroxide. One kind or two or more kinds can be used.
Examples of the organic base compound include ammonia and organic amine compounds, and one or more of these can be used. Of these, an organic amine compound is preferable from the viewpoint of polymerization stability and binding property of a binder containing the obtained polymer or a salt thereof.

Examples of the organic amine compound include monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, triethylamine, monobutylamine, dibutylamine, tributylamine, monohexylamine, dihexylamine, trihexylamine, trioctylamine and tridodecylamine. N-alkyl substituted amines such as: monoethanolamine, diethanolamine, triethanolamine, propanolamine, dimethylethanolamine and (alkyl) alkanolamines such as N, N-dimethylethanolamine; pyridine, piperidine, piperazine, 1,8-. Cyclic amines such as bis (dimethylamino) naphthalene, morpholin and diazabicycloundecene (DBU); diethylenetriamine, N, N-dimethylbenzylamine, and one or more of these can be used. ..
Among these, when a hydrophobic amine having a long-chain alkyl group is used, larger electrostatic repulsion and steric repulsion can be obtained, so that polymerization stability is ensured even when the monomer concentration is high. It is preferable because it is easy. Specifically, the higher the value (C / N) represented by the ratio of the number of carbon atoms to the number of nitrogen atoms present in the organic amine compound, the higher the polymerization stabilizing effect due to the steric repulsion effect. The C / N value is preferably 3 or more, more preferably 5 or more, still more preferably 10 or more, and even more preferably 20 or more.

The amount of the base compound used is preferably in the range of 0.001 mol% or more and 4.0 mol% or less with respect to the ethylenically unsaturated carboxylic acid monomer. When the amount of the basic compound used is within this range, the polymerization reaction can be smoothly carried out. The amount used may be 0.05 mol% or more and 4.0 mol% or less, 0.1 mol% or more and 4.0 mol% or less, and 0.1 mol% or more and 3.0 mol. It may be 0.1 mol% or more and 2.0 mol% or less.
In this specification, the amount of the base compound used represents the molar concentration of the base compound used with respect to the ethylenically unsaturated carboxylic acid monomer, and does not mean the degree of neutralization. That is, the valence of the base compound used is not considered.

As the polymerization initiator, known polymerization initiators such as azo compounds, organic peroxides, and inorganic peroxides can be used, but the polymerization initiator is not particularly limited. The conditions of use can be adjusted by known methods such as heat initiation, redox initiation with a reducing agent, and UV initiation so that the amount of radicals generated is appropriate. In order to obtain a crosslinked polymer having a long primary chain length, it is preferable to set the conditions so that the amount of radicals generated is smaller within the allowable range of the production time.

Examples of the azo compound include 2,2'-azobis (2,4-dimethylvaleronitrile), 2,2'-azobis (N-butyl-2-methylpropionamide), and 2- (tert-butylazo) -2. -Cyanopropane, 2,2'-azobis (2,4,4-trimethylpentane), 2,2'-azobis (2-methylpropane) and the like, and one or more of these are used. be able to.

Examples of the organic peroxide include 2,2-bis (4,5-di-t-butylperoxycyclohexyl) propane (manufactured by Nichiyu Co., Ltd., trade name "Pertetra A") and 1,1-di (t-). Hexylperoxy) cyclohexane (same as "perhexa HC"), 1,1-di (t-butylperoxy) cyclohexane (same as "perhexa C"), n-butyl-4,4-di (t-butylperoxy) Valerate (“Perhexa V”), 2,2-di (t-Butyl Peroxy) Butan (“Perhexa 22”), t-Butyl Hydroperoxide (“Per Butyl H”), Kumen Hydroperoxide (Japan) Made by Yuyu Co., Ltd., trade name "Park Mill H"), 1,1,3,3-tetramethylbutylhydroperoxide ("Perocta H"), t-butylcumyl peroxide ("Perbutyl C"), di- t-Butyl peroxide (“Perbutyl D”), di-t-hexyl peroxide (“Perhexyl D”), di (3,5,5-trimethylhexanoyl) peroxide (“Perloyl355”), Dilauroyl peroxide (“Parloyl L”), Bis (4-t-butylcyclohexyl) peroxydicarbonate (“Parloyl TCP”), Di-2-ethylhexyl peroxydicarbonate (“Parloyl OPP”), Di-sec-Butylperoxydicarbonate (“Parloyl SBP”), Cumylperoxyneodecanoate (“Parkmill ND”), 1,1,3,3-Tetramethylbutylperoxyneodecanoate (1,1,3,3-tetramethylbutylperoxyneodecanoate) The same "Perocta ND"), t-hexyl peroxyneodecanoate (the same "Perhexyl ND"), t-butylperoxyneodecanoate (the same "Perbutyl ND"), t-butylperoxyneoheptanoate (Same as "Perbutyl NHP"), t-hexyl peroxypivalate (same as "Perhexyl PV"), t-butyl peroxypivalate (same as "Perbutyl PV"), 2,5-dimethyl-2,5-di (same as "Perbutyl PV") 2-Ethylhexanoyl) hexane (“Perhexa 250”), 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate (“Perocta O”), t-hexylperoxy-2 -Ethylhexanoate (“Perhexyl O”), t-Butylperoxy-2-ethylhexanoate (“Perbutyl O”), t-Butyl peroxylaurate (“Perbutyl L”), t- Butyl Luperoxy-3,5,5-trimethylhexanoate (“Perbutyl 355”), t-hexyl peroxyisopropyl monocarbonate (“Perhexyl I”), t-butyl peroxyisopropyl monocarbonate (“Perbutyl I”) ), T-Butylperoxy-2-ethylhexyl monocarbonate (“Perbutyl E”), t-butylperoxyacetate (“Perbutyl A”), t-hexyl peroxybenzoate (“Perhexyl Z”) and t. -Butyl peroxybenzoate (the same "perbutyl Z") and the like can be mentioned, and one or more of these can be used.

Examples of the inorganic peroxide include potassium persulfate, sodium persulfate, ammonium persulfate and the like.
When redox is started, sodium sulfite, sodium thiosulfate, sodium formaldehyde sulfoxylate, ascorbic acid, sulfurous acid gas (SO ₂ ), ferrous sulfate and the like can be used as the reducing agent.

The preferable amount of the polymerization initiator used is, for example, 0.001 to 2 parts by mass and, for example, 0.005 to 1 part by mass, when the total amount of the monomer components used is 100 parts by mass. Further, for example, it is 0.01 to 0.1 parts by mass. When the amount of the polymerization initiator used is 0.001 part by mass or more, the polymerization reaction can be stably carried out, and when it is 2 parts by mass or less, a polymer having a long primary chain length can be easily obtained.

The polymerization temperature is preferably 0 to 100 ° C., more preferably 20 to 80 ° C., although it depends on conditions such as the type and concentration of the monomer used. The polymerization temperature may be constant or may change during the polymerization reaction. The polymerization time is preferably 1 minute to 20 hours, more preferably 1 hour to 10 hours.

The present polymer dispersion obtained through the polymerization step can be obtained in a powder state by subjecting the polymer dispersion to a reduced pressure and / or heat treatment in the drying step and distilling off the solvent. At this time, prior to the drying step, for the purpose of removing unreacted monomers (and salts thereof), impurities derived from the initiator, etc., following the polymerization step, a solid-liquid separation step such as centrifugation and filtration, and water. , It is preferable to provide a cleaning step using the same solvent as methanol or a polymerization solvent. When the above cleaning step is provided, even when the present polymer is secondarily agglutinated, it is easily unraveled at the time of use, and the remaining unreacted monomer is removed, which is good in terms of binding property and battery characteristics. Shows excellent performance.

In this production method, even if an alkaline compound is added to the polymer dispersion obtained in the polymerization step to neutralize the polymer (hereinafter, also referred to as “step neutralization”), and then the solvent is removed in the drying step. Good. Further, after obtaining the powder of the present polymer without performing the above-mentioned step neutralization treatment, an alkaline compound is added when preparing the electrode mixture layer slurry to neutralize the polymer (hereinafter, "after-internal"). It may also be called "sum"). Of the above, process neutralization is preferable because the secondary aggregates tend to be easily disintegrated.

<Composition for secondary battery electrode mixture layer>
The composition for the secondary battery electrode mixture layer of the present invention contains a binder, an active material and water containing the present polymer or a salt thereof.
The amount of the present polymer or a salt thereof used in the electrode mixture layer composition of the present invention is, for example, 0.1% by mass or more and 20% by mass or less with respect to the total amount of the active material. The amount used is, for example, 0.2% by mass or more and 10% by mass or less, for example, 0.3% by mass or more and 8% by mass or less, and for example, 0.4% by mass or more and 5% by mass or less. .. If the amount of the polymer and its salt used is less than 0.1% by mass, sufficient binding properties may not be obtained. In addition, the dispersion stability of the active material or the like may become insufficient, and the uniformity of the formed mixture layer may decrease. On the other hand, when the amount of the frame polymer and its salt used exceeds 20% by mass, the electrode mixture layer composition may have a high viscosity and the coatability to the current collector may be lowered. As a result, the obtained mixture layer may have lumps or irregularities, which may adversely affect the electrode characteristics.

When the amount of the present polymer and its salt used is within the above range, a composition having excellent dispersion stability can be obtained, and a mixture layer having extremely high adhesion to the current collector can be obtained. As a result, the durability of the battery is improved. Furthermore, the Motohashi polymer and its salt show sufficiently high binding properties to the active material even in a small amount (for example, 5% by mass or less) and have a carboxy anion, so that the interfacial resistance is small and the high rate characteristics are excellent. The electrode is obtained.

Among the above active materials, a lithium salt of a transition metal oxide can be used as the positive electrode active material, and for example, layered rock salt type and spinel type lithium-containing metal oxides can be used. Specific compounds of the layered rock salt type positive electrode active material include lithium cobalt oxide, lithium nickel oxide, and NCM {Li (Ni _x , Co _y , Mn _z ), x + y + z = 1} and NCA, which are called ternary systems. {Li (Ni _1-ab Co _a Al _b )} and the like can be mentioned. Moreover, as a spinel type positive electrode active material, lithium manganate and the like can be mentioned. In addition to oxides, phosphates, silicates, sulfur and the like are used, and examples of phosphates include olivine-type lithium iron phosphate and the like. As the positive electrode active material, one of the above may be used alone, or two or more thereof may be combined and used as a mixture or a composite.

When a positive electrode active material containing a layered rock salt type lithium-containing metal oxide is dispersed in water, the dispersion liquid becomes alkaline by exchanging lithium ions on the surface of the active material and hydrogen ions in water. Therefore, there is a risk that aluminum foil (Al) or the like, which is a general current collector material for positive electrodes, will be corroded. In such a case, it is preferable to neutralize the alkali content eluted from the active material by using the present polymer which has not been neutralized or partially neutralized as the binder. The amount of the unneutralized or partially neutralized present polymer used should be equal to or greater than the amount of alkali eluted from the active material so that the amount of unneutralized carboxyl groups in the present polymer is equal to or greater than the amount of alkali eluted from the active material. Is preferable.

Since all positive electrode active materials have low electrical conductivity, they are generally used by adding a conductive auxiliary agent. Examples of the conductive auxiliary agent include carbon-based materials such as carbon black, carbon nanotubes, carbon fibers, graphite fine powder, and carbon fibers. Among these, carbon black, carbon nanotubes, and carbon fibers are easy to obtain excellent conductivity. , Are preferred. Further, as the carbon black, Ketjen black and acetylene black are preferable. As the conductive auxiliary agent, one of the above types may be used alone, or two or more types may be used in combination. The amount of the conductive auxiliary agent used can be, for example, 0.2 to 20% by mass, and for example, 0.2 to 10% by mass, based on the total amount of the active material from the viewpoint of achieving both conductivity and energy density. It can be mass%. Further, as the positive electrode active material, a material whose surface is coated with a conductive carbon-based material may be used.

On the other hand, examples of the negative electrode active material include carbon-based materials, lithium metals, lithium alloys, metal oxides, and the like, and one or a combination of two or more of these can be used. Among these, active materials made of carbon-based materials such as natural graphite, artificial graphite, hard carbon and soft carbon (hereinafter, also referred to as "carbon-based active material") are preferable, graphite such as natural graphite and artificial graphite, and graphite. Hard carbon is more preferred. Further, in the case of graphite, spherical graphite is preferably used from the viewpoint of battery performance, and the preferable range of the particle size thereof is, for example, 1 to 20 μm, and for example, 5 to 15 μm. Further, in order to increase the energy density, a metal or a metal oxide capable of occluding lithium such as silicon or tin can be used as the negative electrode active material. Among them, silicon has a higher capacity than graphite, and is an active material made of a silicon-based material such as silicon, a silicon alloy, and a silicon oxide such as silicon monoxide (SiO) (hereinafter, also referred to as "silicon-based active material"). ) Can be used. However, while the silicon-based active material has a high capacity, the volume change due to charging and discharging is large. Therefore, it is preferable to use it in combination with the above carbon-based active material. In this case, if the amount of the silicon-based active material is large, the electrode material may be disintegrated and the cycle characteristics (durability) may be significantly deteriorated. From such a viewpoint, when a silicon-based active material is used in combination, the amount used is, for example, 60% by mass or less, and for example, 30% by mass or less, based on the carbon-based active material.

The binder containing the present polymer has a structural unit (component (a)) in which the polymer is derived from an ethylenically unsaturated carboxylic acid monomer. Here, the component (a) has a high affinity for a silicon-based active material and exhibits good binding properties. Therefore, since the binder of the present invention exhibits excellent binding properties even when a high-capacity type active material containing a silicon-based active material is used, it is also effective for improving the durability of the obtained electrode. It is considered to be.

In addition, the present polymer has a structural unit (component (b)) derived from a nitrile group-containing ethylenically unsaturated monomer. The component (b) can exert a strong interaction with the electrode material and can exhibit good binding property to the active material. Therefore, according to the binder of the present invention, it is possible to obtain a solid and well-integrated electrode mixture layer.

Since the carbon-based active material itself has good electrical conductivity, it is not always necessary to add a conductive additive. When a conductive additive is added for the purpose of further reducing resistance, the amount used is, for example, 10% by mass or less, and for example, 5% by weight or less, based on the total amount of active material from the viewpoint of energy density. Is.

When the composition for the secondary battery electrode mixture layer is in a slurry state, the amount of the active material used is, for example, in the range of 10 to 75% by mass, and for example, 30 to 65% by mass, based on the total amount of the composition. Is the range of. If the amount of the active material used is 10% by mass or more, migration of the binder or the like can be suppressed, and the drying cost of the medium is also advantageous. On the other hand, if it is 75% by mass or less, the fluidity and coatability of the composition can be ensured, and a uniform mixture layer can be formed.

When preparing a composition for an electrode mixture layer in a wet powder state, the amount of the active material used is, for example, in the range of 60 to 97% by mass, and for example, 70 to 90, based on the total amount of the composition. It is in the range of mass%. Further, from the viewpoint of energy density, it is preferable that the amount of non-volatile components other than the active material such as the binder and the conductive auxiliary agent is as small as possible within the range in which the necessary binding property and conductivity are guaranteed.

The composition for the secondary battery electrode mixture layer uses water as a medium. Further, for the purpose of adjusting the properties and dryness of the composition, lower alcohols such as methanol and ethanol, carbonates such as ethylene carbonate, ketones such as acetone, and water-soluble organic solvents such as tetrahydrofuran and N-methylpyrrolidone. It may be used as a mixed solvent with. The proportion of water in the mixing medium is, for example, 50% by mass or more, and for example, 70% by mass or more.

When the composition for the electrode mixture layer is put into a coatingable slurry state, the content of the medium containing water in the entire composition is from the viewpoint of the coating property of the slurry, the energy cost required for drying, and the productivity. Therefore, it can be, for example, in the range of 25 to 90% by mass, and can be, for example, 35 to 70% by mass. Further, in the case of a pressable wet powder state, the content of the medium can be, for example, in the range of 3 to 40% by mass from the viewpoint of the uniformity of the mixture layer after pressing, and for example, 10 It can be in the range of ~ 30% by mass.

The binder of the present invention may consist only of the present polymer or a salt thereof, but other binders such as styrene / butadiene latex (SBR), acrylic latex and polyvinylidene fluoride latex. Ingredients may be used together. When other binder components are used in combination, the amount used may be, for example, 0.1 to 5% by mass or less, and for example, 0.1 to 2% by mass or less, based on the active material. And, for example, it can be 0.1 to 1% by mass or less. If the amount of the other binder component used exceeds 5% by mass, the resistance increases and the high rate characteristics may become insufficient. Among the above, styrene / butadiene latex is preferable because it has an excellent balance between binding property and bending resistance.

The styrene / butadiene latex is a copolymer having a structural unit derived from an aromatic vinyl monomer such as styrene and a structural unit derived from an aliphatic conjugated diene monomer such as 1,3-butadiene. Shows an aqueous dispersion. Examples of the aromatic vinyl monomer include α-methylstyrene, vinyltoluene, divinylbenzene and the like in addition to styrene, and one or more of these can be used. The structural unit derived from the aromatic vinyl monomer in the copolymer can be in the range of, for example, 20 to 60% by mass, and for example, 30 to 50, mainly from the viewpoint of binding property. It can be in the range of% by mass.

As the above-mentioned aliphatic conjugated diene-based monomer, in addition to 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3- Butadiene and the like can be mentioned, and one or more of these can be used. The structural unit derived from the aliphatic conjugated diene-based monomer in the copolymer is, for example, 30 to 70% by mass in that the binding property of the binder and the flexibility of the obtained electrode are good. It can be in the range of 40 to 60% by mass, for example.

In addition to the above-mentioned monomers, styrene / butadiene-based monomers include nitrile group-containing monomers such as (meth) acrylonitrile and (meth) as other monomers in order to further improve performance such as binding properties. ) A carboxyl group-containing monomer such as acrylic acid, itanconic acid, and maleic acid may be used as the copolymerization monomer.
The structural unit derived from the other monomer in the copolymer can be, for example, in the range of 0 to 30% by mass, and can be, for example, in the range of 0 to 20% by mass.

The composition for a secondary battery electrode mixture layer of the present invention contains the above-mentioned active material, water and a binder as essential constituents, and can be obtained by mixing the respective components using known means. The mixing method of each component is not particularly limited, and a known method can be adopted, but after dry blending the powder components such as the active material, the conductive additive, and the carboxyl group-containing polymer particles which are binders. , A method of mixing with a dispersion medium such as water and dispersing and kneading is preferable. When the composition for the electrode mixture layer is obtained in a slurry state, it is preferable to finish the composition in a slurry without poor dispersion or aggregation. As the mixing means, a known mixer such as a planetary mixer, a thin film swirl mixer, or a self-revolving mixer can be used, but a thin film swirl mixer is used because a good dispersion state can be obtained in a short time. It is preferable to do this. When using a thin film swirl mixer, it is preferable to pre-disperse in advance with a stirrer such as a disper. The viscosity of the slurry can be, for example, in the range of 500 to 100,000 mPa · s, or in the range of 1,000 to 50,000 mPa · s, for example, as the B-type viscosity at 60 rpm. it can.

On the other hand, when the composition for the electrode mixture layer is obtained in a wet powder state, it is preferable to knead the composition to a uniform state without uneven concentration by using a Henschel mixer, a blender, a planetary mixer, a twin-screw kneader or the like.

<Electrodes for secondary batteries>
The electrode for a secondary battery of the present invention is provided with a mixture layer formed from the above-mentioned electrode mixture layer composition on the surface of a current collector such as copper or aluminum. The mixture layer is formed by applying the composition for an electrode mixture layer of the present invention to the surface of a current collector and then drying and removing a medium such as water. The method for applying the mixture layer composition is not particularly limited, and known methods such as a doctor blade method, a dip method, a roll coating method, a comma coating method, a curtain coating method, a gravure coating method and an extrusion method are adopted. be able to. Further, the drying can be performed by a known method such as blowing warm air, reducing the pressure, (far) infrared rays, and irradiating microwaves.
Usually, the mixture layer obtained after drying is subjected to a compression treatment by a mold press, a roll press or the like. By compressing, the active material and the binder can be brought into close contact with each other, and the strength of the mixture layer and the adhesion to the current collector can be improved. The thickness of the mixture layer can be adjusted to, for example, about 30 to 80% before compression by compression, and the thickness of the mixture layer after compression is generally about 4 to 200 μm.

A secondary battery can be manufactured by providing the electrode for the secondary battery of the present invention with a separator and an electrolytic solution. The electrolytic solution may be in the form of a liquid or a gel.
The separator is arranged between the positive electrode and the negative electrode of the battery, and plays a role of preventing a short circuit due to contact between the two electrodes and holding an electrolytic solution to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane having good ion permeability and mechanical strength. As a specific material, polyolefins such as polyethylene and polypropylene, polytetrafluoroethylene and the like can be used.

As the electrolytic solution, a known one that is generally used depending on the type of active material can be used. In the lithium ion secondary battery, specific solvents include cyclic carbonates having a high dielectric constant and high solubility of electrolytes such as propylene carbonate and ethylene carbonate, and low-viscosity chains such as ethylmethyl carbonate, dimethyl carbonate and diethyl carbonate. Examples thereof include form carbonates, which can be used alone or as a mixed solvent. The electrolytic solution is used by dissolving lithium salts such as _{LiPF 6} , LiSbF ₆ , LiBF ₄ , LiClO ₄ , and LiAlO _{4 in these solvents.} In the nickel-metal hydride secondary battery, an aqueous potassium hydroxide solution can be used as the electrolytic solution. The secondary battery is obtained by forming a positive electrode plate and a negative electrode plate partitioned by a separator into a spiral or laminated structure and storing them in a case or the like.

As described above, the binder for the secondary battery electrode disclosed in the present specification exhibits excellent adhesion to the electrode material and excellent adhesion to the current collector in the mixture layer. A secondary battery equipped with an electrode obtained by using the above binder is expected to ensure good integrity and exhibit good durability (cycle characteristics) even after repeated charging and discharging, and is expected to exhibit good durability (cycle characteristics) for automobiles. Suitable for batteries and the like.

Hereinafter, the present invention will be specifically described based on Examples. The present invention is not limited to these examples. In the following, "parts" and "%" mean parts by mass and% by mass unless otherwise specified.

≪Manufacturing of this polymer salt≫
(Production Example 1: Production of Carboxylic Acid Group-Containing Polymer Salt R-1)
A reactor equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen introduction tube was used for the polymerization.
In the reactor, 567 parts of acetonitrile, 2.20 parts of ion-exchanged water, 99.8 parts of acrylic acid (hereinafter referred to as "AA"), 0.2 part of acrylonitrile (hereinafter referred to as "AN"), trimethylolpropane diallyl ether. (Manufactured by Daiso Co., Ltd., trade name "Neoallyl T-20") 0.90 parts and 1.0 mol% of triethylamine with respect to the above AA were charged. After sufficiently replacing the inside of the reactor with nitrogen, the inside temperature was raised to 55 ° C. by heating. After confirming that the internal temperature was stable at 55 ° C., 2,2'-azobis (2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd., trade name "V-65") 0 as a polymerization initiator. When .040 parts were added, white turbidity was observed in the reaction solution, and this point was set as the polymerization initiation point. The monomer concentration was calculated to be 15.0%. The polymerization reaction was continued while maintaining the internal temperature at 55 ° C. by adjusting the external temperature (water bath temperature), and the internal temperature was raised to 65 ° C. when 6 hours had passed from the polymerization initiation point. The internal temperature was maintained at 65 ° C., cooling of the reaction solution was started 12 hours after the polymerization initiation point, and after the internal temperature dropped to 25 ° C., lithium hydroxide monohydrate (hereinafter, “LiOH”). - 52.4 parts powder of _{H 2} O "hereinafter) was added. After the addition, stirring was continued at room temperature for 12 hours to obtain a slurry-like polymerization reaction solution in which particles of the carboxyl group-containing polymer salt R-1 (Li salt, neutralization degree 90 mol%) were dispersed in a medium.

The obtained polymerization reaction solution was centrifuged to settle the polymer particles, and then the supernatant was removed. Then, after redispersing the precipitate in acetonitrile having the same weight as the polymerization reaction solution, the washing operation of precipitating the polymer particles by centrifugation to remove the supernatant was repeated twice. The precipitate was recovered and dried under reduced pressure at 80 ° C. for 3 hours to remove volatile components to obtain a powder of the carboxyl group-containing polymer salt R-1. Since the carboxyl group-containing polymer salt R-1 has hygroscopicity, it was stored in a container having a water vapor barrier property. The powder of the carboxyl group-containing polymer salt R-1 was measured by IR, and the degree of neutralization was determined from the intensity ratio of the peak derived from the C = O group of the carboxylic acid and the peak derived from the C = O of the carboxylic acid Li. , 90 mol% equal to the calculated value from the preparation.

(Production Examples 2 to 16: Production of Carboxylic Group-Containing Polymer Salts R-2 to R-16)
The same operation as in Production Example 1 was performed except that the amounts of the monomer, the crosslinkable monomer, the polymerization solvent, the polymerization initiator and the neutralizing agent (process neutralization) were as shown in Tables 1 and 2. This was carried out to obtain a polymerization reaction solution containing the carboxyl group-containing polymer salts R-2 to R-16.
Next, the same operations as in Production Example 1 were carried out for each polymerization reaction solution to obtain powdery carboxyl group-containing polymer salts R-2 to R-16. Each carboxyl group-containing polymer salt was sealed and stored in a container having a water vapor barrier property.
In the Production Example 10, by using NaOH instead of powder LiOH · _{H 2} O, to give a carboxyl group-containing polymer Na salt (degree of neutralization 90 mol%).

Details of the compounds used in Tables 1 and 2 are shown below.
AA: Acrylic acid AN: Acrylonitrile MAN: Methacrylonitrile ACNE: Cyanoethyl acrylate DMAA: Dimethylacrylamide T-20: Trimethylolpropane diallyl ether (manufactured by Daiso, trade name "Neoallyl T-20")
P-30: Pentaerythritol triallyl ether (manufactured by Daiso, trade name "Neoallyl P-30")
AcN: Acetonitrile MEK: Methyl ethyl ketone V-65: 2,2'-azobis (2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd.)

(Evaluation of electrodes)
As the active material, graphite, which is an active material for negative electrodes, or silicon particles and graphite are used, and a composition for a mixture layer using a carboxyl group-containing polymer salt as a binder is stable and formed. The peeling strength between the current collectors (that is, the binding property of the binder) was measured. Natural graphite (manufactured by Nippon Graphite Co., Ltd., trade name "CGB-10") was used as the graphite, and (Sigma-Aldrich, Si nanopowder, particle size <100 nm) was used as the silicon particles.

Example 1
3.2 parts of powdered carboxyl group-containing polymer Li salt R-1 is weighed in 100 parts of natural graphite, mixed well in advance, 160 parts of ion-exchanged water is added, predispersed with a disperser, and then a thin film is formed. This dispersion was carried out for 15 seconds under the condition of a peripheral speed of 20 m / sec using a swirling mixer (FM-56-30 manufactured by Primix Corporation) to obtain a slurry-like composition for a negative electrode mixture layer. The slurry concentration (solid content) was calculated to be 39.2%.

<Viscosity stability of electrode slurry>
The viscosity of the composition (slurry) for the negative electrode mixture layer obtained above was measured immediately after preparation and after standing at 25 ° C. for 4 weeks. The viscosity of the slurry was measured using a rheometer (Physica MCR301) manufactured by Anton Pearl Co., Ltd. (plate: CP25-5, shear rate: 60s ^-1 ). The composition for the battery electrode after 4 weeks of standing at room temperature was measured with a spatula at a speed such that bubbles did not enter, after sufficiently stirring until the slurry viscosity was visually constant. The viscosity change rate was calculated from the following formula, and the viscosity stability was determined according to the following criteria. The results are shown in Table 3.

Viscosity change rate (%) = (slurry viscosity after standing for 4 weeks) / (slurry viscosity immediately after production) x 100
(Criteria)
A: Viscosity change rate less than 110% B: Viscosity change rate 110% or more, less than 130% C: Viscosity change rate 130% or more

<90 ° peel strength (bonding property)>
Using a variable applicator, apply the composition for the mixture layer on a copper foil (manufactured by Nippon Foil Co., Ltd.) with a thickness of 20 μm, and dry it in a ventilation dryer for 100 ° C for 15 minutes to mix the mixture. A layer was formed. Then, the mixture layer was rolled so that the thickness was 50 ± 5 μm and the packing density was 1.70 ± 0.20 g / cm ³ , and a negative electrode was prepared.

The negative electrode obtained above was cut into strips having a width of 25 mm, and then the mixture layer surface of the sample was attached to a double-sided tape fixed on a horizontal surface to prepare a sample for a peeling test. After the test sample was dried under reduced pressure at 60 ° C. overnight, 90 ° peeling (measurement temperature 23 ° C.) was performed at a tensile speed of 50 mm / min, and the peeling strength between the mixture layer and the copper foil was measured. The peel strength was as high as 15.2 N / m, which was good.

Examples 2 to 15 and Comparative Examples 1 to 4
A mixture layer composition was prepared by carrying out the same operation as in Example 1 except that the carboxyl group-containing polymer salt used as the active material and the binder was used as shown in Tables 3 and 4. In Examples 5 and 6 and Comparative Example 2, natural graphite and silicon particles were stirred at 400 rpm for 1 hour using a planetary ball mill (manufactured by FRITSCH, P-5), and a powdery carboxyl was added to the obtained mixture. 3.2 parts of the group-containing polymer Li salt R-4 or R-14 was weighed, mixed well in advance, and then the same operation as in Example 1 was carried out to prepare a mixture layer composition. Viscosity stability and 90 ° peel strength were evaluated for each mixture layer composition. The results are shown in Tables 3 and 4.

In each example, an electrode mixture layer composition containing a binder for a secondary battery electrode belonging to the present invention and an electrode prepared using the same. The viscosity stability of each mixture layer composition (slurry) is good, and the peel strength between the mixture layer of the obtained electrode and the current collector is high, and the binding property is excellent. Was shown.
Focusing on the amount of the cyano group-containing ethylenically unsaturated monomer used, the results showed higher peel strength as the amount of copolymerization increased (Examples 1 to 4 and 7).

On the other hand, Comparative Examples 1 and 2 using polymer salts that do not use cyano group-containing ethylenically unsaturated monomers, and comparisons using polymer salts derived from ethylenically unsaturated carboxylic acid monomers and having few structural units. The peel strength of Example 4 was lower than that of Examples. Further, Comparative Example 3 uses a polymer salt in which the amount of the cyano group-containing ethylenically unsaturated monomer used exceeds the range specified in the present invention, but the stability of the slurry viscosity is greatly reduced. Results were obtained.

The binder for a secondary battery electrode of the present invention exhibits excellent binding properties in the mixture layer. Therefore, a secondary battery provided with an electrode obtained by using the above binder has good durability (cycle characteristics). ), And is expected to be applied to in-vehicle secondary batteries. It is also useful for the use of active materials containing silicon, and is expected to contribute to increasing the capacity of batteries.
The binder for a secondary battery electrode of the present invention can be particularly preferably used for a non-aqueous electrolyte secondary battery electrode, and is particularly useful for a non-aqueous electrolyte lithium ion secondary battery having a high energy density.

Claims (7)

A binder for a secondary battery electrode containing a carboxyl group-containing polymer or a salt thereof.
The carboxyl group-containing polymer contains 50% by mass or more and 99.9% by mass or less of structural units derived from an ethylenically unsaturated carboxylic acid monomer with respect to all the structural units, and a nitrile group-containing ethylenically unsaturated monomer. A binder for a secondary battery electrode containing 0.1% by mass or more and 4.9% by mass or less of a structural unit derived from a polymer. The binder for a secondary battery electrode according to claim 1, wherein the carboxyl group-containing polymer has a neutralization degree of 70 mol% or more. The binder for a secondary battery electrode according to claim 1 or 2, wherein the carboxyl group-containing polymer is a crosslinked polymer having a crosslinked structure. A method for producing a carboxyl group-containing polymer or a salt thereof used in a binder for a secondary battery electrode.
Monomer component containing 50% by mass or more and 99.9% by mass or less of an ethylenically unsaturated carboxylic acid monomer and 0.1% by mass or more and 4.9% by mass or less of an ethylenically unsaturated monomer containing a nitrile group. A method comprising a polymerization step of polymerizing by a precipitation polymerization method. A composition for a secondary battery electrode mixture layer containing the binder, active material, and water according to any one of claims 1 to 3. The composition for a secondary battery electrode mixture layer according to claim 5, which comprises a carbon-based material or a silicon-based material as the negative electrode active material. A secondary battery electrode comprising a mixture layer formed from the composition for the secondary battery electrode mixture layer according to claim 5 or 6 on the surface of a current collector.