JP7211418B2 - Binder for secondary battery electrode and its use - Google Patents

Description

TECHNICAL FIELD The present invention relates to a binder for secondary battery electrodes and its use.

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

On the other hand, as the use of various secondary batteries expands, the demand for improvement in energy density, reliability and durability tends to increase. For example, for the purpose of increasing the electrical capacity of lithium ion secondary batteries, specifications using silicon-based active materials as negative electrode active materials are increasing. However, silicon-based active materials are known to undergo large volume changes during charging and discharging, and as they are used repeatedly, the electrode mixture layer may peel or come off. (durability) deteriorated. In order to suppress such problems, it is generally effective to increase the binding property of the binder, and studies have been made on improving the binding property of the binder for the purpose of improving the durability.

Acrylic acid-based polymers obtained by copolymerizing nitrile-based monomers have been proposed as binders that have good binding properties and exert an effect on durability.
Patent Document 1 discloses a slurry for a lithium ion secondary battery negative electrode containing a salt of acrylic acid or an acrylic acid derivative and acrylonitrile or a copolymer of an acrylonitrile derivative. Patent Literature 2 describes a binder for a lithium battery comprising a polymer containing acrylic acid-derived monomer units and (meth)acrylonitrile as constituent components and crosslinked with a specific crosslinking agent. Patent Document 3 discloses a polymer obtained by polymerizing a monomer composition containing a lithium salt of an unsaturated acid, an unsaturated acid, and an α,β-unsaturated nitrile in a specific ratio, and contains a dispersion medium. A binder composition for a lithium ion secondary battery electrode is disclosed.

JP 2015-115109 A WO2015/163302 WO2015/151518

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

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

As a result of intensive studies by the present inventors to solve the above problems, the main component is an ethylenically unsaturated carboxylic acid monomer, and a small amount of a nitrile group-containing ethylenically unsaturated monomer is copolymerized or The inventors have found that when a binder containing the salt is used, both the stability and the binding property of the slurry for the electrode mixture layer are excellent. According to the present disclosure, the following means are provided based on these findings.

The present invention is as follows.
[1] A secondary battery electrode binder 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, and a nitrile group-containing ethylenically unsaturated monomer, based on all structural units. 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 secondary battery electrodes,
A 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 a nitrile group-containing ethylenically unsaturated monomer by a precipitation polymerization method.
[5] A composition for a secondary battery electrode mixture layer comprising the binder according to any one of [1] to [3], an active material and water.
[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 comprising, on the surface of a current collector, a mixture layer formed from the composition for a secondary battery electrode mixture layer according to [5] or [6].

The binder for secondary battery electrodes of the present invention exhibits excellent binding properties to electrode active materials and the like. In addition, the binder can exhibit good adhesion to the current collector. Therefore, the electrode mixture layer containing the binder and the electrode provided therewith have excellent binding properties and can maintain their integrity. Therefore, deterioration of the electrode mixture layer due to changes in the volume and shape of the active material due to charging and discharging is suppressed, and a secondary battery with high durability (cycle characteristics) can be obtained. Furthermore, the mixture layer slurry containing the binder for secondary battery electrodes of the present invention has excellent stability, so that the change in slurry viscosity is suppressed or reduced even after long-term storage, and the uniformity is excellent. It is possible to stably obtain an electrode mixture layer.

The secondary battery electrode binder 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 onto the surface of the current collector. A 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 copper foil or aluminum foil.

Below, each of the binder for secondary battery electrodes of this invention, the composition for secondary battery electrode mixture layers obtained using the said binder, and a secondary battery electrode is demonstrated in detail.
In this specification, "(meth)acryl" means acryl and/or methacryl, and "(meth)acrylate" means acrylate and/or methacrylate. A "(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 structural units 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 "this polymer") can have a structural unit (hereinafter also referred to as "component (a)") derived from an ethylenically unsaturated carboxylic acid monomer. When the present polymer has a carboxyl group due to having such a structural unit, the adhesion to the current collector is improved, and the lithium ion desolvation effect and ionic conductivity are excellent, so the resistance is small and the rate is high. An electrode with excellent properties can be obtained. In addition, since water-swellability is imparted, the dispersion stability of the active material and the like in the mixture layer composition can be enhanced.
The component (a) can be introduced into the polymer, for example, by polymerizing a monomer containing an ethylenically unsaturated carboxylic acid monomer. Alternatively, it can be obtained by (co)polymerizing a (meth)acrylic acid ester monomer and then hydrolyzing it. Moreover, after polymerizing (meth)acrylamide, (meth)acrylonitrile, etc., it may be treated with a strong alkali, or a method of reacting a polymer having a hydroxyl group with an acid anhydride may be used.

Ethylenically unsaturated carboxylic acid monomers include (meth)acrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid; Carboxylic acid; ethylenically unsaturated monomers having a carboxyl group such as succinic acid monohydroxyethyl (meth)acrylate, ω-carboxy-caprolactone mono(meth)acrylate, β-carboxyethyl (meth)acrylate, or their (parts ) alkali-neutralized products, and one of them 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 because a polymer having a long primary chain length can be obtained due to its high polymerization rate, and the binding force of the binder is good, and acrylic acid is particularly preferable. be. 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 component (a) in the present polymer is not particularly limited, but can be, for example, 10% by mass or more and 99.9% by mass or less based on the total structural units of the present polymer. By containing the component (a) in such a range, it is possible to easily ensure excellent adhesion to the current collector. The lower limit is, for example, 20% by mass or more, or, for example, 30% by mass or more, or, 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 is improved and a higher binding force is obtained, which is preferable. may be 80% by mass or more. Further, the upper limit is, for example, 99.5% by mass or less, or, for example, 99% by mass or less, or, for example, 98% by mass or less, or, for example, 95% by mass or less, or, for example, 90% by mass or less. There is, and it is 80 mass % or less, for example. The range can be a range in which such a lower limit and an upper limit are appropriately combined. Also, it can be, for example, 50% by mass or more and 99% by mass or less, or for example, 50% by mass or more and 98% by mass or less, or for example, 50% by mass or more and 95% by mass or less.

<Structural unit derived from nitrile group-containing ethylenically unsaturated monomer>
The present polymer can have a structural unit derived from a nitrile group-containing ethylenically unsaturated monomer (hereinafter also referred to as "(b) component") in addition to the (a) component. The component (b) can have a strong interaction with the electrode material and exhibit good binding properties to the active material. This makes it possible to obtain an electrode mixture layer that is firm and has good integrity. Component (b) can be introduced into the present polymer, for example, by polymerizing a nitrile group-containing ethylenically unsaturated monomer.

Examples of nitrile group-containing ethylenically unsaturated monomers include (meth)acrylonitrile; cyanoalkyl (meth)acrylate compounds such as cyanomethyl (meth)acrylate and cyanoethyl (meth)acrylate; 4-cyanostyrene , cyano group-containing unsaturated aromatic compounds such as 4-cyano-α-methylstyrene; may be used. Among the above, acrylonitrile is preferable because of its high nitrile group content.

The content of 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 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 properties of the slurry decrease. There is fear. The upper limit may be 4.5% by mass or less, 4.0% by mass or less, or 3.0% by mass or less. Moreover, the lower limit is not particularly limited, but for example, it can be contained in an amount of 0.1% by mass or more based on the total structural units of the present polymer. When the present polymer contains 0.1% by mass of the component (b), an electrode mixture layer with 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 still more preferably 0.5% by mass or more. The range can be a range in which such lower and upper limits are appropriately combined. % by mass or less, and can be, for example, 0.2% by mass or more and 4.0% by mass or less.

<Other structural units>
In addition to components (a) and (b), the present polymer contains structural units derived from other ethylenically unsaturated monomers copolymerizable therewith (hereinafter also referred to as "(c) component"). can include Component (c) may be, for example, 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 compound other than component (b). Structural units derived from saturated monomers and the like are included. These structural units are ethylenically unsaturated monomer compounds having anionic groups other than carboxyl groups such as sulfonic acid groups and phosphoric acid groups, or nonionic ethylenically unsaturated monomers other than component (b). can be introduced by copolymerizing monomers containing Among these, the component (c) is preferably a structural unit derived from a nonionic ethylenically unsaturated monomer from the viewpoint of obtaining an electrode with good bending resistance, and excellent binder binding properties. (Meth)acrylamide and derivatives thereof are preferred. 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. Good binding properties can be exhibited with respect to the active material. This is preferable because it is possible to obtain an electrode mixture layer that is firm and has good integrity. Structural units derived from ethylenically unsaturated monomers containing an alicyclic structure are particularly preferred.

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

(Meth)acrylamide derivatives include, for example, N-alkyl(meth)acrylamide compounds such as isopropyl(meth)acrylamide and t-butyl(meth)acrylamide; Nn-butoxymethyl(meth)acrylamide, 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, and one of these It may be used alone or in combination of two or more.

The alicyclic structure-containing ethylenically unsaturated monomers include, for example, cyclopentyl (meth)acrylate, cyclohexyl (meth)acrylate, methylcyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, (meth) ) Cyclodecyl acrylate and cyclododecyl (meth) acrylate which may have an aliphatic substituent (meth) acrylic acid cycloalkyl ester; (meth) isobornyl acrylate, (meth) adamantyl acrylate, (meth) ) dicyclopentenyl acrylate, dicyclopentenyloxyethyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and cyclohexanedimethanol mono(meth)acrylate and cyclodecanedimethanol mono(meth)acrylate, etc. Cycloalkylpolyalcohol mono(meth)acrylates, etc., 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 in that a polymer having a long primary chain length can be obtained due to a high polymerization rate and the binding force of the binder is good.

As other nonionic ethylenically unsaturated monomers, for example, (meth)acrylic acid esters may be used. (Meth)acrylic acid esters, for example, methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, isobutyl (meth)acrylate and 2-ethylhexyl (meth)acrylate ( meth) acrylic acid alkyl ester compound;
Aromatic (meth)acrylic acid ester compounds such as phenyl (meth)acrylate, phenylmethyl (meth)acrylate, and phenylethyl (meth)acrylate;
(meth)acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth)acrylate and ethoxyethyl (meth)acrylate;
hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate and hydroxybutyl (meth)acrylate, and the like, hydroxyalkyl ester compounds of (meth)acrylic acid, and the like, and one of these is used alone. may be used, or two or more may be used in combination. From the viewpoint of adhesion to the active material and cycle characteristics, aromatic (meth)acrylic acid ester compounds 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 esters such as 2-methoxyethyl (meth)acrylate and ethoxyethyl (meth)acrylate, are preferred. , 2-methoxyethyl (meth)acrylate is more preferred.

Among the nonionic ethylenically unsaturated monomers, compounds having an acryloyl group are preferable because they have a high polymerization rate, so that a polymer having a long primary chain length can be obtained, and the binding force of the binder is good. Further, as the nonionic ethylenically unsaturated monomer, a compound having a homopolymer glass transition temperature (Tg) of 0° C. or less is preferable because the obtained electrode has good bending resistance.

The 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 salts and barium salts; other metal salts such as magnesium salts and aluminum salts; amine salts and the like. Among these, alkali metal salts and magnesium salts are preferred, and alkali metal salts are more preferred, since they are less likely to adversely affect battery characteristics.

The polymer may be a crosslinked polymer having a crosslinked structure. The method of cross-linking the cross-linked polymer is not particularly limited, and examples thereof include the following methods.
1) Copolymerization of crosslinkable monomers 2) Utilization of chain transfer to polymer chains during radical polymerization 3) After synthesizing a polymer having a reactive functional group, postcrosslinking by adding a crosslinking agent as necessary By having the crosslinked structure of the present polymer, the binder containing the polymer or salt thereof can have excellent binding power. Among the above methods, the method by copolymerization of a crosslinkable monomer is preferable because the operation is simple and the degree of crosslinking can be easily controlled.

<Crosslinkable monomer>
Examples of crosslinkable monomers include polyfunctional polymerizable monomers having two or more polymerizable unsaturated groups, and monomers having self-crosslinkable functional groups such as hydrolyzable silyl groups. mentioned.

The polyfunctional polymerizable monomer is a compound having two or more polymerizable functional groups such as (meth)acryloyl groups and alkenyl groups in the molecule, and includes polyfunctional (meth)acrylate compounds, polyfunctional alkenyl compounds, ( Examples include compounds having both a meth)acryloyl group and an alkenyl group. These compounds may be used individually by 1 type, and may be used in combination of 2 or more type. Among these, a polyfunctional alkenyl compound is preferable because it is easy to obtain a uniform crosslinked structure, and a polyfunctional allyl ether compound having a plurality of allyl ether groups in the molecule is particularly preferable.

Examples of polyfunctional (meth)acrylate compounds include ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di( Di(meth)acrylates of dihydric alcohols such as meth)acrylates; Poly(meth)acrylates such as tri(meth)acrylates and tetra(meth)acrylates of trihydric or higher polyhydric alcohols such as meth)acrylates and pentaerythritol tetra(meth)acrylates; Bisamides and the like can be mentioned.

Polyfunctional alkenyl compounds include polyfunctional allyl ether compounds such as trimethylolpropane diallyl ether, trimethylolpropane triallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, tetraallyloxyethane, and polyallyl saccharose; and polyfunctional vinyl compounds such as divinylbenzene.

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

Specific examples of the monomer having a self-crosslinkable crosslinkable functional group include hydrolyzable silyl group-containing vinyl monomers, N-methylol(meth)acrylamide, N-methoxyalkyl(meth)acrylate, and the like. is mentioned. These compounds can be used individually by 1 type or in combination of 2 or more types.

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, vinylsilanes such as vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane, and vinyldimethylmethoxysilane; silyl groups such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, and methyldimethoxysilylpropyl acrylate; Containing acrylic acid esters; Silyl group-containing methacrylic acid esters such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, and dimethylmethoxysilylpropyl methacrylate; silyl group-containing vinyl ethers; and silyl group-containing vinyl esters such as vinyl trimethoxysilylundecanoate.

When the present polymer is crosslinked with a crosslinkable monomer, the amount of the crosslinkable monomer used is the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer) 100 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 still more preferably 0.5 parts by mass or more and 1 .5 parts by mass or less. When the amount of the crosslinkable monomer used is 0.1 part 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 is 0.02 to 0.7 mol% with respect to the total amount of monomers other than crosslinkable monomers (non-crosslinkable monomers). It is preferably 0.03 to 0.4 mol %, and 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 large particle size aggregates (secondary aggregates), and forms water-swollen 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 salt thereof has a particle diameter (water-swollen particle diameter) when dispersed in water having a degree of neutralization based on the carboxyl groups of the crosslinked polymer of 80 to 100 mol%. The reference median diameter is preferably in the range of 0.1 μm or more and 10.0 μm or less. A more preferable range of the particle size is 0.1 μm or more and 8.0 μm or less, a further preferable range is 0.1 μm or more and 7.0 μm or less, and a still more preferable range is 0.2 μm or more and 5.0 μm or less. A more preferable range is 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 particles are uniformly present in a suitable size in the mixture layer composition, so that the mixture layer composition has high stability and excellent bonding. It becomes possible to demonstrate the adhesion. If the particle size exceeds 10.0 μm, the binding properties may be insufficient as described above. In addition, since it is difficult to obtain a smooth coating surface, the coatability may be insufficient. On the other hand, when the particle size is less than 0.1 μm, there is a concern in terms of stable production.

If the crosslinked polymer is not neutralized or has a degree of neutralization of less than 80 mol%, neutralize it with an alkali metal hydroxide or the like to a degree of neutralization of 80 to 100 mol%, and measure the particle size when dispersed in water. do it. In general, a crosslinked polymer or a salt thereof often exists as aggregated particles in which primary particles are associated and aggregated in a powder or solution (dispersion) state. When the particle size when dispersed in water is within the above range, the crosslinked polymer or salt thereof has extremely excellent dispersibility, and is neutralized to a degree of neutralization of 80 to 100 mol%. By dispersing, aggregated particles are loosened, and a stable dispersed state is formed in which the particle diameter is in the range of 0.1 to 10.0 μm even if the particles are mostly a dispersion of primary particles or secondary aggregates. It is.

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

Moreover, the particle size (dry particle size) of the crosslinked polymer or salt thereof when dried is preferably in the range of 0.03 μm or more and 3 μm or less as a 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 further preferable range is 0.3 μm or more and 0.8 μm or less.

In the present polymer or a salt thereof, acid groups such as carboxyl groups derived from ethylenically unsaturated carboxylic acid monomers are neutralized so that the degree of neutralization in the mixture layer composition is 20 mol % or more. , is preferably used in the form of a salt. The degree of neutralization is more preferably 50 mol%, still more preferably 60 mol% or more, still more preferably 70 mol% or more, still more preferably 80 mol% or more, particularly preferably 85 mol% or more. 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 can be an appropriate combination of the above lower and upper limits. , 80 mol % or more and 100 mol % or less. When the degree of neutralization is 20 mol % or more, it is preferable in that the water swelling property is improved and the dispersion stabilizing effect is easily obtained. In the present specification, the degree of neutralization can be calculated from the charged values of the monomer having an acid group such as a carboxyl group and the neutralizing agent used for neutralization. The degree of neutralization is obtained by IR measurement of the powder after drying the crosslinked polymer or its salt at 80 ° C. for 3 hours under reduced pressure conditions, and measuring the peak derived from the C=O group of the carboxylic acid and the C= of the carboxylate. It can be confirmed from the intensity ratio of the peak derived from the O group.

<Method for producing the present polymer or salt thereof>
Known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization can be used for the present polymer. ) is preferred. Heterogeneous polymerization methods such as precipitation polymerization, suspension polymerization, and emulsion polymerization are preferred, and precipitation polymerization is more preferred among them, in that better performance can be obtained with respect to binding properties and the like.
Precipitation polymerization is a method of producing a polymer by carrying out a polymerization reaction in a solvent that dissolves the unsaturated monomer as the starting material but does not substantially dissolve the resulting polymer. As the polymerization progresses, the polymer particles become larger due to aggregation and growth, and a dispersion liquid of polymer particles is obtained in which primary particles of several tens of nm to several hundreds of nm are secondary aggregated to several μm to several tens of μm. A dispersion stabilizer can also be used to control the particle size of the polymer.
The secondary aggregation can be suppressed by selecting a dispersion stabilizer, a polymerization solvent, and 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 and various organic solvents can be used as the polymerization solvent in consideration of the type of monomers to be used. In order to obtain a polymer having a longer primary chain length, it is preferable to use a solvent with a small chain transfer constant.

Specific polymerization solvents 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. , these can be used singly or in combination of two or more. Alternatively, a mixed solvent of these and water may be used. A water-soluble solvent in the present invention refers to a solvent having a solubility in water at 20° C. of greater than 10 g/100 ml.
Among the above, the formation of coarse particles and adhesion to the reactor are small, and the polymerization stability is good. methyl ethyl ketone and acetonitrile are preferable in that they are easy to use), that a polymer with a small chain transfer constant and a large degree of polymerization (primary chain length) can be obtained, and that the operation is easy during the neutralization process described later. .

In addition, it is preferable to add a small amount of a highly polar solvent to the polymerization solvent in order to allow the neutralization reaction to proceed stably and rapidly in the process neutralization. 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% by mass, based on the total mass of the medium. 0 mass %, more preferably 0.1 to 1.0 mass %. When the proportion of the highly polar solvent is 0.05% by mass or more, the effect on the neutralization reaction is observed, and when it is 20.0% by mass or less, no adverse effect on the polymerization reaction is observed. In addition, in the polymerization of highly hydrophilic ethylenically unsaturated carboxylic acid monomers such as acrylic acid, the addition of a highly polar solvent improves the polymerization rate, making it easier 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.

The production of the present polymer or a salt thereof preferably includes 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 derived from the component (a) is 50% by mass or more and 99.9% by mass or less, and the nitrile group-containing ethylenically unsaturated monomer derived from the component (b) is Polymerizing 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 other ethylenically unsaturated monomers derived from component (c) It is preferable to include a polymerization step.
By the above polymerization step, 50% by mass or more and 99.9% by mass or less of a structural unit (component a) derived from an ethylenically unsaturated carboxylic acid monomer is introduced into the polymer, and the nitrile group-containing ethylenically unsaturated A structural unit (component b) derived from a 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 is, for example, 50% by mass or more and 98% by mass or less, and is, for example, 50% by mass. Above, it is below 95 mass %. 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, or for example, 0.2% by mass or more and 4.0% by mass or less. is.

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

The monomer concentration during polymerization is preferably higher from the viewpoint of obtaining a polymer having a longer primary chain length. However, if the concentration of the monomer is too high, aggregation of the polymer particles is likely to proceed, and control of the heat of polymerization becomes difficult, which may cause the polymerization reaction to go out of control. For this reason, for example, in the case of precipitation polymerization, 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.
As used herein, the term "monomer concentration" indicates the monomer concentration in the reaction solution at the time of initiation of polymerization.

The present polymer may be produced by conducting a polymerization reaction in the presence of a basic compound. By conducting the polymerization reaction in the presence of the 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 still more preferably 19.0% by mass or more. and more preferably 20.0% by mass or more. The monomer concentration is still preferably 22.0% by mass or more, and still 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, it is possible to produce a polymer having a long primary chain length.
As used herein, the term "monomer concentration" indicates the monomer concentration in the reaction solution at the time of initiation of polymerization.

The upper limit of the monomer concentration varies depending on the type of monomer and solvent used, as well as the polymerization method and various polymerization conditions. It is about 40% in general, about 50% in suspension polymerization, and about 70% in emulsion polymerization.

The base compound is a so-called alkaline compound, and may be either an inorganic base compound or an organic base compound. By conducting the polymerization reaction in the presence of the basic compound, the polymerization reaction can be stably carried out even under conditions of a high monomer concentration exceeding, for example, 13.0% by mass. In addition, the polymer obtained by polymerization at such a high monomer concentration has a high molecular weight (due to a long primary chain length) 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. 1 type(s) or 2 or more types can be used.
Organic base compounds include ammonia and organic amine compounds, and one or more of these can be used. Among them, an organic amine compound is preferable from the viewpoint of the binding property of the binder containing the polymerization stability and the resulting polymer or salt thereof.

Examples of organic amine compounds include monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, triethylamine, monobutylamine, dibutylamine, tributylamine, monohexylamine, dihexylamine, trihexylamine, trioctylamine and tridodecylamine. (alkyl)alkanolamines such as monoethanolamine, diethanolamine, triethanolamine, propanolamine, dimethylethanolamine and N,N-dimethylethanolamine; pyridine, piperidine, piperazine, 1,8- Cyclic amines such as bis(dimethylamino)naphthalene, morpholine and diazabicycloundecene (DBU); diethylenetriamine, N,N-dimethylbenzylamine, one or more of which can be used .
Among these, when a hydrophobic amine having a long-chain alkyl group is used, greater electrostatic repulsion and steric repulsion can be obtained, so polymerization stability can be 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 stabilization effect due to the steric repulsion effect. The C/N value is preferably 3 or more, more preferably 5 or more, even more preferably 10 or more, and still more preferably 20 or more.

The amount of the basic 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. If the amount of the basic compound used is within this range, the polymerization reaction can be carried out smoothly. The amount used may be 0.05 mol % or more and 4.0 mol % or less, may be 0.1 mol % or more and 4.0 mol % or less, or may be 0.1 mol % or more and 3.0 mol % % or less, or 0.1 mol % or more and 2.0 mol % or less.
In this specification, the amount of the basic compound used represents the molar concentration of the basic 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 basic 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. Using known methods such as thermal initiation, redox initiation using a reducing agent, and UV initiation can be used to adjust the conditions of use so that an appropriate amount of radicals is generated. 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 radical generation is less within the allowable production time.

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

Examples of the organic peroxide include 2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane (manufactured by NOF Corporation, trade name "Pertetra A"), 1,1-di(t- Hexylperoxy)cyclohexane (“Perhexa HC” in the same), 1,1-di(t-butylperoxy)cyclohexane (“Perhexa C” in the same), n-butyl-4,4-di(t-butylperoxy) Valerate ("Perhexa V"), 2,2-di(t-butylperoxy)butane ("Perhexa 22"), t-butyl hydroperoxide ("Perbutyl H"), cumene hydroperoxide (Japanese Yusha, trade name "Perocta H"), 1,1,3,3-tetramethylbutyl hydroperoxide ("Perocta H"), t-butyl cumyl peroxide ("Perbutyl C"), di- t-butyl peroxide (same as "Perbutyl D"), di-t-hexyl peroxide (same as "Perhexyl D"), di(3,5,5-trimethylhexanoyl) peroxide (same as "Perloyl 355"), dilauroyl peroxide (“Perloyl L”), bis(4-t-butylcyclohexyl) peroxydicarbonate (“Perloyl TCP”), di-2-ethylhexyl peroxydicarbonate (“Perloyl OPP”), Di-sec-butyl peroxydicarbonate (“Perloyl SBP”), cumyl peroxyneodecanoate (“Percumyl ND”), 1,1,3,3-tetramethylbutyl peroxyneodecanoate ( t-hexyl peroxyneodecanoate (Perocta ND), t-butyl peroxyneodecanoate (Perbutyl ND), t-butyl peroxyneoheptanoate (“Perbutyl NHP” in the same), t-hexyl peroxypivalate (“Perbutyl PV” in the same), t-butyl peroxypivalate (“Perbutyl PV” in the same), 2,5-dimethyl-2,5-di( 2-ethylhexanoyl)hexane (“Perhexa 250” in the same), 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate (“Perocta O” in the same), t-hexylperoxy-2 -Ethylhexanoate (“Perhexyl O” in the same), t-butyl peroxy-2-ethylhexanoate (“Perbutyl O” in the same), t-butyl peroxylaurate (“Perbutyl L” in the same), t- Spot Ruperoxy-3,5,5-trimethylhexanoate (“PERBUTYL 355”), t-hexylperoxyisopropyl monocarbonate (“PERBUTYL I”), t-butylperoxyisopropyl monocarbonate (“PERBUTYL I”) ), t-butyl peroxy-2-ethylhexyl monocarbonate (“Perbutyl E” in the same), t-butyl peroxyacetate (“Perbutyl A” in the same), t-hexyl peroxybenzoate (“Perhexyl Z” in the same) and t -Butyl peroxybenzoate (same as "perbutyl Z") and the like, and one or more of these can be used.

Examples of the inorganic peroxides include potassium persulfate, sodium persulfate and ammonium persulfate.
In the case of redox initiation, sodium sulfite, sodium thiosulfate, sodium formaldehyde sulfoxylate, ascorbic acid, sulfurous acid gas (SO ₂ ), ferrous sulfate, etc. can be used as reducing agents.

A preferred amount of the polymerization initiator to be 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 parts by mass or more, the polymerization reaction can be stably carried out, and when it is 2 parts by mass or less, it is easy to obtain a polymer having a long primary chain length.

The polymerization temperature is preferably 0 to 100.degree. C., more preferably 20 to 80.degree. C., although it depends on the type and concentration of the monomers used. The polymerization temperature may be constant or may vary 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 liquid obtained through the polymerization step is subjected to depressurization and/or heat treatment in the drying step to evaporate the solvent, whereby the desired present polymer can be obtained in the form of powder. At this time, before 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, water , methanol, or a washing step using the same solvent as the polymerization solvent. When the above-mentioned washing step is provided, even if the polymer is secondaryly aggregated, it is easily disintegrated during use, and the remaining unreacted monomer is removed, so that the binding property and battery characteristics are good. performance.

In the present production method, an alkali compound is added to the polymer dispersion obtained in the polymerization step to neutralize the polymer (hereinafter also referred to as "process neutralization"), and then the solvent is removed in the drying step. good. In addition, after obtaining the powder of the present polymer without performing the neutralization treatment in the above process, an alkali compound is added to neutralize the polymer when preparing the electrode mixture layer slurry (hereinafter referred to as "post-in-process (also called "wa"). Among the above, process neutralization is preferable because the secondary agglomerates tend to be easily disintegrated.

<Composition for secondary battery electrode mixture layer>
The composition for a secondary battery electrode mixture layer of the present invention contains a binder containing the present polymer or a salt thereof, an active material and water.
The amount of the polymer or 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, or is, for example, 0.3% by mass or more and 8% by mass or less, or is, for example, 0.4% by mass or more and 5% by mass or less. . When the amount of the present 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 and the like may become insufficient, and the uniformity of the mixture layer formed may deteriorate. On the other hand, when the amount of the crosslinked polymer and its salt exceeds 20% by mass, the viscosity of the electrode mixture layer composition becomes high, and the coatability to the current collector may deteriorate. As a result, bumps and irregularities may occur in the mixture layer obtained, which may adversely affect the electrode characteristics.

When the amount of the present polymer and its salt 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, battery durability is improved. In addition, the bridge polymer and its salt show sufficiently high binding properties even in a small amount (for example, 5% by mass or less) with respect to the active material, and have a carboxy anion, so the interfacial resistance is small and the high rate characteristics are excellent. A good electrode is obtained.

Among the above active materials, lithium salts of transition metal oxides can be used as positive electrode active materials, 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 cobaltate, lithium nickelate, and NCM {Li (Nix, Coy, Mnz), _x + _y + _z =1} and NCA called ternary system. {Li(Ni _1-ab Co _a Al _b )} and the like. Moreover, lithium manganate etc. are mentioned as a spinel type positive electrode active material. In addition to oxides, phosphates, silicates, sulfur, and the like are used, and examples of phosphates include olivine-type lithium iron phosphate. As the positive electrode active material, one of the above materials may be used alone, or two or more of them may be used in combination as a mixture or composite.

When the positive electrode active material containing the layered rock salt type lithium-containing metal oxide is dispersed in water, the lithium ions on the surface of the active material are exchanged with the hydrogen ions in the water, so that the dispersion becomes alkaline. For this reason, aluminum foil (Al) or the like, which is a general positive electrode current collector material, may be corroded. In such a case, it is preferable to neutralize the alkali content eluted from the active material by using the unneutralized or partially neutralized present polymer as a binder. In addition, the amount of the unneutralized or partially neutralized polymer used should be such that the amount of unneutralized carboxyl groups of the polymer is equal to or greater than the amount of alkali eluted from the active material. is preferred.

Since all positive electrode active materials have low electrical conductivity, they are generally used by adding a conductive aid. Examples of conductive aids include carbon-based materials such as carbon black, carbon nanotubes, carbon fibers, graphite fine powder, and carbon fibers. , is preferred. As carbon black, ketjen black and acetylene black are preferable. The conductive aid may be used alone or in combination of two or more. The amount of the conductive aid used can be, for example, 0.2 to 20% by mass relative to the total amount of the active material from the viewpoint of achieving both conductivity and energy density, and for example, 0.2 to 10%. % by mass. Also, the positive electrode active material may be surface-coated with a conductive carbonaceous material.

On the other hand, examples of negative electrode active materials include carbonaceous materials, lithium metals, lithium alloys, and metal oxides, and one or more of these can be used in combination. 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 materials") are preferable, and graphite such as natural graphite and artificial graphite, and Hard carbon is more preferred. In the case of graphite, spherical graphite is preferably used from the standpoint of battery performance, and the preferred range of particle size is, for example, 1 to 20 μm, and for example, 5 to 15 μm. In addition, in order to increase the energy density, a metal such as silicon or tin, or a metal oxide that can occlude lithium can be used as the negative electrode active material. Among them, silicon has a higher capacity than graphite, and active materials made of silicon-based materials such as silicon, silicon alloys, and silicon oxides such as silicon monoxide (SiO) (hereinafter also referred to as "silicon-based active materials") ) can be used. However, while the silicon-based active material has a high capacity, it undergoes a large change in volume during charging and discharging. Therefore, it is preferable to use it together with the carbon-based active material. In this case, if the silicon-based active material is contained in a large amount, the electrode material may collapse and the cycle characteristics (durability) may be greatly reduced. From this point of view, when the silicon-based active material is used together, the amount used is, for example, 60% by mass or less, and for example, 30% by mass or less, relative to the carbon-based active material.

The binder containing the polymer has a structural unit (component (a)) derived from an ethylenically unsaturated carboxylic acid monomer. Here, the component (a) has a high affinity for the silicon-based active material and exhibits good binding properties. Therefore, 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, and is therefore effective in improving the durability of the resulting electrode. It is considered to be a thing.

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

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

When the secondary battery electrode mixture layer composition 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, relative to the total amount of the composition. is in the range of When the amount of the active material used is 10% by mass or more, the migration of the binder and the like can be suppressed, and it is advantageous in terms of the drying cost of the medium. 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 material mixture layer can be formed.

Further, when preparing the electrode mixture layer composition in a wet powder state, the amount of the active material used is, for example, in the range of 60 to 97% by mass, or, for example, 70 to 90% with respect to the total amount of the composition. It is in the range of % by mass. Moreover, from the viewpoint of energy density, nonvolatile components other than the active material, such as binders and conductive aids, should be as small as possible within the range in which necessary binding properties and conductivity are ensured.

The composition for secondary battery electrode mixture layer uses water as a medium. For the purpose of adjusting the properties and drying properties 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 may be added. may be used as a mixed solvent with The proportion of water in the mixed medium is, for example, 50% by mass or more, and is, for example, 70% by mass or more.

When the composition for the electrode mixture layer is in a coatable slurry state, the content of the medium containing water in the entire composition is determined from the viewpoint of the coatability of the slurry, the energy cost required for drying, and the productivity. from, for example, 25 to 90% by mass, or, for example, 35 to 70% by mass. In addition, in the case of wet powder state that can be pressed, the content of the medium can be in the range of, for example, 3 to 40% by mass, from the viewpoint of uniformity of the mixture layer after pressing, and, for example, 10%. It can be in the range of up to 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 in combination. When other binder components are used in combination, the amount used can 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. or, for example, 0.1 to 1% by mass or less. If the amount of the other binder component used exceeds 5% by mass, the resistance may increase, resulting in insufficient high-rate characteristics. Among the above, styrene/butadiene-based latexes are preferable from the viewpoint of excellent balance between binding properties 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. An aqueous dispersion is shown. Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, vinyltoluene, divinylbenzene and the like, and one or more of these can be used. The structural unit derived from the aromatic vinyl monomer in the copolymer may be in the range of, for example, 20 to 60% by mass, mainly from the viewpoint of binding properties, and may be, for example, 30 to 50% by mass. It can be in the range of % by mass.

Examples of the aliphatic conjugated diene-based monomer include 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 resulting electrode are good. and, for example, 40 to 60% by mass.

In addition to the above monomers, the styrene/butadiene latex contains nitrile group-containing monomers such as (meth)acrylonitrile, ) Carboxyl group-containing monomers such as acrylic acid, itanconic acid and maleic acid may be used as comonomers.
The structural units derived from the other monomers in the copolymer can be in the range of, for example, 0 to 30% by mass, and can be in the range of, for example, 0 to 20% by mass.

The composition for a secondary battery electrode mixture layer of the present invention comprises the above active material, water and binder as essential components, and is obtained by mixing each component using a known means. The mixing method of each component is not particularly limited, and a known method can be adopted. , and a method of mixing with a dispersion medium such as water and dispersing and kneading. When the electrode mixture layer composition is obtained in a slurry state, it is preferable to finish the slurry without poor dispersion or aggregation. As a mixing means, known mixers such as a planetary mixer, a thin-film swirling mixer, and a rotation-revolution mixer can be used. It is preferable to Moreover, when using a thin-film gyration mixer, it is preferable to pre-disperse in advance with a stirrer such as a disper. In addition, the viscosity of the slurry can be, for example, in the range of 500 to 100,000 mPa s as B-type viscosity at 60 rpm, and can be in the range of 1,000 to 50,000 mPa s. can.

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

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

A secondary battery can be produced by providing the secondary battery electrode of the present invention with a separator and an electrolytic solution. The electrolytic solution may be liquid or gel.
A 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 retaining an electrolytic solution to ensure ionic conductivity. The separator is preferably a film-like insulating microporous membrane having good ion permeability and mechanical strength. Specific materials that can be used include polyolefins such as polyethylene and polypropylene, and polytetrafluoroethylene.

As the electrolytic solution, a commonly used known one can be used depending on the type of active material. In lithium-ion secondary batteries, specific solvents include cyclic carbonates such as propylene carbonate and ethylene carbonate, which have a high dielectric constant and high ability to dissolve the electrolyte, and low-viscosity chains such as ethylmethyl carbonate, dimethyl carbonate, and diethyl carbonate. carbonates, etc., and these 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. A potassium hydroxide aqueous solution can be used as an electrolytic solution in a nickel-metal hydride secondary battery. A secondary battery is obtained by housing a positive electrode plate and a negative electrode plate partitioned by a separator in a spiral or laminated structure in a case or the like.

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

EXAMPLES The present invention will be specifically described below based on examples. It should be noted that the present invention is not limited by these examples. In the following, "parts" and "%" mean parts by weight and % by weight unless otherwise specified.

≪Production of the present polymer salt≫
(Production Example 1: Production of carboxyl group-containing polymer salt R-1)
A reactor equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen inlet tube was used for the polymerization.
567 parts of acetonitrile, 2.20 parts of ion-exchanged water, 99.8 parts of acrylic acid (hereinafter referred to as "AA"), 0.2 parts of acrylonitrile (hereinafter referred to as "AN"), trimethylolpropane diallyl ether were placed in the reactor. 0.90 parts (manufactured by Daiso Co., Ltd., trade name "Neoallyl T-20") and triethylamine corresponding to 1.0 mol % with respect to the above AA were charged. After the interior of the reactor was sufficiently replaced with nitrogen, the interior temperature was raised to 55°C by heating. After confirming that the internal temperature has stabilized at 55° C., 2,2′-azobis(2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd., trade name “V-65”) is added as a polymerization initiator. When 0.040 part was added, cloudiness was observed in the reaction solution, and this point was taken 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). The internal temperature was maintained at 65 ° C., and when 12 hours had passed since the polymerization initiation point, cooling of the reaction solution was started, and after the internal temperature decreased to 25 ° C., lithium hydroxide monohydrate (hereinafter, “LiOH * _H2O ") powder was added. After the addition, stirring was continued for 12 hours at room temperature to obtain a slurry-like polymerization reaction liquid in which particles of the carboxyl group-containing polymer salt R-1 (Li salt, degree of neutralization: 90 mol %) were dispersed in the medium.

After centrifuging the resulting polymerization reaction solution to precipitate the polymer particles, the supernatant was removed. After that, the sediment was redispersed in acetonitrile of the same weight as the polymerization reaction solution, and then the polymer particles were sedimented by centrifugation, and the washing operation of removing the supernatant was repeated twice. The sediment was collected and dried at 80° C. for 3 hours under reduced pressure to remove volatile matter to obtain a powder of carboxyl group-containing polymer salt R-1. Since the carboxyl group-containing polymer salt R-1 is hygroscopic, it was sealed and stored in a container having water vapor barrier properties. The powder of the carboxyl group-containing polymer salt R-1 was subjected to IR measurement, and the degree of neutralization was obtained 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 group of the carboxylic acid Li. , equal to 90 mol % calculated from the charge.

(Production Examples 2 to 16: Production of carboxyl group-containing polymer salts R-2 to R-16)
The same operation as in Production Example 1 was performed except that the charged amounts of the monomer, crosslinkable monomer, polymerization solvent, polymerization initiator and neutralizing agent (process neutralization) were as shown in Tables 1 and 2. to obtain polymerization reaction solutions containing carboxyl group-containing polymer salts R-2 to R-16.
Then, each polymerization reaction solution was subjected to the same operation as in Production Example 1 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 water vapor barrier properties.
In Production Example 10, a carboxyl group-containing polymer Na salt (neutralization degree of 90 mol %) was obtained by using NaOH instead of the LiOH.H ₂ O powder.

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)
Regarding the mixture layer composition using graphite, which is an active material for negative electrodes, or silicon particles and graphite as an active material, and using a carboxyl group-containing polymer salt as a binder, the stability and the formed mixture layer / The peel strength between 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 graphite, and (Sigma-Aldrich, Si nanopowder, particle size <100 nm) was used as silicon particles.

Example 1
3.2 parts of powdery carboxyl group-containing polymer Li salt R-1 was weighed into 100 parts of natural graphite, mixed well in advance, added with 160 parts of ion-exchanged water, pre-dispersed with a disper, and then thin film. A rotary mixer (FM-56-30, manufactured by Primix Co., Ltd.) was used to carry out the main dispersion for 15 seconds at a peripheral speed of 20 m/sec to obtain a slurry composition for a negative electrode mixture layer. The slurry concentration (solids content) was calculated to be 39.2%.

<Viscosity stability of electrode slurry>
The viscosity of the negative electrode mixture layer composition (slurry) 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 Paar (plate: CP25-5, shear rate: 60 s ^-1 ). In addition, the composition for a battery electrode after standing at room temperature for 4 weeks was sufficiently stirred with a spatula at such a speed that bubbles did not enter, and the viscosity of the slurry was visually observed and then measured. The viscosity change rate was calculated from the following formula, and the viscosity stability was determined according to the following criteria. Table 3 shows the results.

Viscosity change rate (%) = (slurry viscosity after standing for 4 weeks) / (slurry viscosity immediately after preparation) x 100
(criterion)
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, the composition for the mixture layer is applied onto a copper foil (manufactured by Nippon Foil Co., Ltd.) having a thickness of 20 μm, and dried in a ventilation dryer at 100° C. for 15 minutes to form a mixture. formed a layer. Then, it was rolled so that the mixture layer had a thickness of 50±5 μm and a packing density of 1.70±0.20 g/cm ³ to prepare a negative electrode.

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

Examples 2-15 and Comparative Examples 1-4
A mixture layer composition was prepared in the same manner as in Example 1 except that the carboxyl group-containing polymer salt used as the active material and 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 powdered carboxyl was added to the resulting mixture. After weighing 3.2 parts of the group-containing polymer Li salt R-4 or R-14 and mixing well in advance, the same operation as in Example 1 was performed to prepare a mixture layer composition. Each mixture layer composition was evaluated for viscosity stability and 90° peel strength. Tables 3 and 4 show the results.

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

On the other hand, Comparative Examples 1 and 2 with polymer salts that do not use a cyano group-containing ethylenically unsaturated monomer, and comparison with polymer salts with less structural units derived from ethylenically unsaturated carboxylic acid monomers The peel strength of Example 4 was lower than that of Examples. In Comparative Example 3, the amount of the cyano group-containing ethylenically unsaturated monomer used was a polymer salt exceeding the range specified in the present invention, but the stability of the slurry viscosity was greatly reduced. The results were obtained.

The secondary battery electrode binder of the present invention exhibits excellent binding properties in the mixture layer. ), and is expected to be applied to automotive secondary batteries. It is also useful for the use of silicon-containing active materials, and is expected to contribute to increasing the capacity of batteries.
The binder for secondary battery electrodes of the present invention can be suitably used particularly for non-aqueous electrolyte secondary battery electrodes, and is particularly useful for non-aqueous electrolyte lithium ion secondary batteries with high energy density.

Claims (6)

A secondary battery electrode binder containing a carboxyl group-containing polymer or a salt thereof,
The carboxyl group-containing polymer contains 60 % by mass or more and 99.9% by mass or less of structural units derived from an ethylenically unsaturated carboxylic acid monomer, and 0% by mass of structural units derived from acrylonitrile , based on the total structural units. .1% by mass or more and 4.9% by mass or less ,
A secondary battery electrode binder having a degree of neutralization of 80 mol % or more . The binder for a secondary battery electrode according to claim 1, 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 secondary battery electrodes,
A polymerization step of polymerizing a monomer component containing 60 % 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 acrylonitrile by a precipitation polymerization method. When,
and adjusting the degree of neutralization of the carboxyl group-containing polymer obtained in the polymerization step to 80 mol % or more . A composition for a secondary battery electrode mixture layer, comprising the binder according to claim 1 or 2 , an active material and water. 5. The composition for a secondary battery electrode mixture layer according to claim 4 , which contains a carbon-based material or a silicon-based material as a negative electrode active material. A secondary battery electrode comprising a mixture layer formed from the composition for a secondary battery electrode mixture layer according to claim 4 or 5 on a surface of a current collector.