JP6638747B2 - Binder for secondary battery electrode and its use - Google Patents

Description

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

  As a secondary battery, various power storage devices such as a nickel hydride secondary battery, a lithium ion secondary battery, and an electric double layer capacitor have been put to practical use. The electrodes used in these secondary batteries are produced by applying 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, an aqueous binder containing styrene butadiene rubber (SBR) latex and carboxymethyl cellulose (CMC) is used as a binder used in the negative electrode mixture layer composition. Further, as an excellent binder having excellent dispersibility and binding properties, a binder containing an aqueous solution of an acrylic acid-based polymer 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 types of secondary batteries are expanded, there is a tendency that demands for improvements in energy density, reliability, and durability increase. For example, specifications for using a silicon-based active material as a negative electrode active material for the purpose of increasing the electric capacity of a lithium ion secondary battery are increasing. However, it is known that the silicon-based active material has a large volume change during charge and discharge, and as it is repeatedly used, the electrode mixture layer peels or falls off, resulting in a decrease in battery capacity and cycle characteristics. (Durability) is deteriorated. In order to suppress such a problem, it is generally effective to increase the binding property of the binder. For the purpose of improving the durability, studies have been made on improving the binding property of the binder.

  For example, Patent Literature 1 discloses an acrylic acid polymer crosslinked with polyalkenyl ether as a binder for forming a negative electrode coating film of a lithium ion secondary battery. Patent Document 2 contains a water-soluble polymer having a specific aqueous solution viscosity, containing a structural unit derived from an ethylenically unsaturated carboxylate monomer and a structural unit derived from an ethylenically unsaturated carboxylic acid ester monomer. An aqueous electrode binder for a secondary battery is disclosed. Patent Literature 3 discloses an aqueous dispersion having a specific viscosity containing a salt of a crosslinked polymer containing a structural unit derived from an ethylenically unsaturated carboxylate monomer.

JP 2000-294247 A JP 2015-18776 A International Publication No. WO 2016/158939

The binders disclosed in Patent Literatures 1 to 3 can impart good binding properties, but with the improvement of the performance of secondary batteries, demands for binders having higher binding power are increasing. .
Generally, in order to enhance the binding property, it is effective to increase the molecular weight of the polymer as the binder. As a method for increasing the molecular weight of the binder polymer, a method of extending the primary chain length of the polymer and a method of extending the primary chain length by performing fine crosslinking are known. However, when the molecular weight is increased by these methods, the viscosity of the electrode mixture layer slurry containing the binder polymer increases, which results in deterioration of coating properties. Although the viscosity of the slurry can be reduced by lowering the concentration of the active material and the binder in the slurry, it is not preferable in terms of productivity.

  The present disclosure has been made in view of such circumstances, and while having good coatability, an aqueous binder for a secondary battery electrode having better binding properties than before, and used for the binder. Provided is a method for producing a crosslinked polymer or a salt thereof. The present disclosure also provides a composition for a secondary battery electrode mixture layer and a secondary battery electrode obtained by using the binder.

  The present inventors have conducted intensive studies in order to solve the above problems, and include a structural unit derived from an ethylenically unsaturated carboxylic acid monomer and a structural unit derived from a nitrile group-containing ethylenically unsaturated monomer. It has been found that when a binder containing a crosslinked polymer or a salt thereof is used, both the coating property and the binding property of the electrode mixture layer slurry are excellent. According to the present disclosure, the following means is provided based on such knowledge.

The present invention is as follows.
[1] A binder for a secondary battery electrode containing a crosslinked polymer or a salt thereof,
The crosslinked polymer has a structural unit derived from the ethylenically unsaturated carboxylic acid monomer of 50% by mass or more and 99.9% by mass or less, and a nitrile group-containing ethylenically unsaturated monomer, based on all the structural units. Containing 0.1% by mass or more and 20% by mass or less of structural units derived from
A binder for a secondary battery electrode, wherein the particle diameter measured in an aqueous medium after neutralization to a neutralization degree of 80 to 100 mol% is 0.1 μm or more and 10 μm or less as a volume-based median diameter.
[2] The cross-linked polymer is cross-linked by a cross-linkable monomer, and the amount of the cross-linkable monomer used is 0.1 mass part per 100 mass parts of the total amount of the non-cross-linkable monomer. The binder for a secondary battery electrode according to [1], which is not less than 2.0 parts by mass and not more than 2.0 parts by mass.
[3] The secondary battery electrode according to [1] or [2], wherein the crosslinked polymer is obtained by polymerizing a monomer component constituting the crosslinked polymer in the presence of an organic amine compound. For binders.
[4] A method for producing a crosslinked polymer or a salt thereof used for a binder for a secondary battery electrode,
In the presence of the organic amine compound, the ethylenically unsaturated carboxylic acid monomer is contained in an amount of 50% by mass to 99.9% by mass, and the nitrile group-containing ethylenically unsaturated monomer is contained in an amount of 0.1% by mass to 20% by mass. A method comprising a polymerization step of polymerizing a monomer component by a precipitation polymerization method.
[5] A composition for a secondary battery electrode mixture layer comprising the binder, the active material, and water according to any one of [1] to [3].
[6] The composition for an electrode mixture layer for a secondary battery according to [5], comprising 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 a secondary battery electrode of the present invention exhibits excellent binding properties to an electrode active material and the like. In addition, the binder can exhibit good adhesiveness with the current collector. For this reason, the electrode mixture layer containing the binder and the electrode provided with the same have excellent binding properties and can maintain their integrity. Therefore, deterioration of the electrode mixture layer due to change in volume and shape of the active material due to charge and discharge is suppressed, and a secondary battery with high durability (cycle characteristics) can be obtained. Furthermore, since the mixture layer slurry containing the binder for a secondary battery electrode of the present invention has low viscosity, it has good coatability.

  The binder for a secondary battery electrode of the present invention contains a crosslinked polymer or a salt thereof, and can be made into an electrode mixture layer composition by mixing with an active material and water. The above-mentioned composition may be in a slurry state that can be applied to the current collector, or may be prepared as a wet powder state so as to be able to cope with press working 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, the binder for a secondary battery electrode, the composition for a secondary battery electrode mixture layer, and the secondary battery electrode obtained using the binder will be described in detail.
In the present specification, “(meth) acryl” means acryl and / or methacryl, and “(meth) acrylate” means acrylate and / or methacrylate. Further, “(meth) acryloyl group” means an acryloyl group and / or a methacryloyl group.

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

<Structural unit of crosslinked polymer>
<Structural unit derived from ethylenically unsaturated carboxylic acid monomer>
The crosslinked polymer can have a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter, also referred to as “component (a)”). When the crosslinked 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 ion conductivity are excellent, so that the resistance is small and the high rate An electrode having excellent characteristics 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 a crosslinked polymer by, for example, polymerizing a monomer containing an ethylenically unsaturated carboxylic acid monomer. In addition, it can also be obtained by (co) polymerizing a (meth) acrylate monomer and then hydrolyzing it. Moreover, after polymerizing (meth) acrylamide and (meth) acrylonitrile, etc., it may be treated with a strong alkali, or a method of reacting an acid anhydride with a polymer having a hydroxyl group may be used.

  Examples of the ethylenically unsaturated carboxylic acid monomer include (meth) acrylic acid, itaconic acid, crotonic acid, maleic acid, and fumaric acid; and (meth) acrylamidoalkyl such as (meth) acrylamidohexanoic acid and (meth) acrylamidododecanoic acid. Carboxylic acid; ethylenically unsaturated monomers having a carboxyl group such as monohydroxyethyl (meth) acrylate succinate, ω-carboxy-caprolactone mono (meth) acrylate, β-carboxyethyl (meth) acrylate, and (parts thereof) ) Alkaline neutralized products; one of these may be used alone, or two or more 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 is obtained because of a high polymerization rate, and the binding force of a binder is improved, and acrylic acid is particularly preferable. 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 crosslinked polymer is not particularly limited, but may be, for example, 10% by mass or more and 99.9% by mass or less based on all structural units of the crosslinked polymer. By containing the component (a) in such a range, excellent adhesiveness to the current collector can be easily secured. 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 an electrode mixture layer is preferably good, and may be 60% by mass or more, 70% by mass or more, or 80% by mass. It may be the above. 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. And it is, for example, 80% by mass or less. The range can be a range in which the lower limit and the upper limit are appropriately combined. For example, the range is 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>
The crosslinked polymer of the present invention may have, in addition to the component (a), 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 properties to the active material. This makes it possible to obtain a solid electrode mixture layer having good integration. The component (b) can be introduced into the crosslinked polymer by, for example, polymerizing a nitrile group-containing ethylenically unsaturated monomer.

  Examples of the nitrile group-containing ethylenically unsaturated monomer include (meth) acrylonitrile; cyanoalkyl (meth) acrylate compounds such as cyanomethyl (meth) acrylate and cyanoethyl (meth) acrylate; 4-cyanostyrene , 4-cyano-α-methylstyrene and other cyano group-containing unsaturated aromatic compounds; vinylidene cyanide and the like; one of these may be used alone, or two or more thereof may be used in combination. May be used. Among these, acrylonitrile is preferred because of its high nitrile group content.

  The content of the component (b) in the crosslinked polymer is 20% by mass or less based on all structural units of the crosslinked polymer. When the content of the component (b) exceeds 20% by mass, the production stability and the dispersion stability of the crosslinked polymer in the mixture layer composition (slurry) become insufficient, and the coating property and the binding property decrease. There is a risk of doing so. The upper limit may be 18% by mass or less, or 15% by mass or less. The lower limit is not particularly limited, but may be, for example, 0.1% by mass or more based on all structural units of the crosslinked polymer. When the crosslinked 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 0.3% by mass or more, more preferably 0.5% by mass or more, further preferably 1.0% by mass or more, and further preferably 3.0% by mass or more. The range can be a range in which the lower limit and the upper limit are appropriately combined. For example, the range is 0.1% by mass or more and 20% by mass or less, and for example, 0.3% by mass or more and 20% by mass or less. Yes, for example, 0.5% by mass or more and 20% by mass or less, and for example, 0.5% by mass or more and 18% by mass or less.

<Other structural units>
The crosslinked polymer is a structural unit derived from another ethylenically unsaturated monomer copolymerizable therewith, in addition to 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 a carboxyl group such as a sulfonic acid group and a phosphoric acid group, or a nonionic ethylenically unsaturated compound other than the component (b) Examples include a structural unit derived from a saturated monomer or the like. These structural units 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, or a nonionic ethylenically unsaturated monomer other than the component (b). It can be introduced by copolymerizing a monomer containing a compound. 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 flex resistance, and the binder has excellent binding properties. And (meth) acrylamide and its derivatives are preferred. 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 a component (c), a strong interaction with an electrode material can be achieved, Good binding properties to the active material can be exhibited. This is preferable because a solid electrode mixture layer with good integration can be obtained. In particular, a structural unit derived from an alicyclic structure-containing ethylenically unsaturated monomer is preferable.

  The proportion of the component (c) can be 0% by mass or more and 49.9% by mass or less based on all structural units of the crosslinked polymer. The proportion 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, or 2% by mass or more and 30% by mass or less. Or 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 crosslinked polymer, the affinity for the electrolytic solution is improved, and the effect of improving the lithium ion conductivity can be expected.

  Examples of (meth) acrylamide derivatives include 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; They 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, (meth) ) Cycloalkyl (meth) acrylates which may have an aliphatic substituent such as cyclodecyl acrylate and cyclododecyl (meth) 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 Cycloalkylpo Alcohol mono (meth) acrylate and the like, may be used one of these alone or may be used in combination of two or more. 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 is obtained due to a high polymerization rate, and the binding power of a binder is improved.

As other nonionic ethylenically unsaturated monomers, for example, (meth) acrylic acid esters may be used. Examples of the (meth) acrylate include, for example, methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, isobutyl (meth) acrylate and 2-ethylhexyl (meth) acrylate. (Meth) alkyl acrylate compounds;
Aromatic (meth) acrylate compounds such as phenyl (meth) acrylate, phenylmethyl (meth) acrylate, and phenylethyl (meth) acrylate;
Alkoxyalkyl (meth) acrylate compounds such as 2-methoxyethyl (meth) acrylate and ethoxyethyl (meth) acrylate;
Examples include hydroxyalkyl (meth) acrylate compounds such as hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate and hydroxybutyl (meth) acrylate, and one of these may be used alone. Or two or more of them may be used in combination. From the viewpoint of adhesion to the active material and cycle characteristics, an aromatic (meth) acrylate 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 alkoxyalkyl (meth) acrylates such as 2-methoxyethyl (meth) acrylate and ethoxyethyl (meth) acrylate are preferable. And 2-methoxyethyl (meth) acrylate are 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 is obtained because the polymerization rate is high, and the binding power of the binder is improved. Further, as the nonionic ethylenically unsaturated monomer, a compound having a glass transition temperature (Tg) of a homopolymer of 0 ° C. or lower is preferable from the viewpoint that the obtained electrode has good bending resistance.

  The crosslinked polymer may be a salt. Although the kind of the salt is not particularly limited, 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 salt Amine salts and the like. Among these, alkali metal salts and magnesium salts are preferred because they do not easily adversely affect battery characteristics, and alkali metal salts are more preferred.

<Embodiment of crosslinked polymer>
The cross-linking method in the cross-linked polymer of the present invention is not particularly limited, and for example, the following embodiments are exemplified.
1) Copolymerization of crosslinkable monomer 2) Utilization of chain transfer to polymer chain at the time of radical polymerization 3) After synthesizing a polymer having a reactive functional group, if necessary, adding a crosslinking agent and postcrosslinking When the polymer has a crosslinked structure, the binder containing the polymer or a salt thereof can have excellent binding power. Among the above, the method by copolymerization of a crosslinkable monomer is preferable because the operation is simple and the degree of crosslinking is easily controlled.

<Crosslinkable monomer>
Examples of the crosslinkable monomer include a polyfunctional polymerizable monomer having two or more polymerizable unsaturated groups, and a monomer having a self-crosslinkable crosslinkable functional group such as a hydrolyzable silyl group. No.

  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 a molecule, and includes a polyfunctional (meth) acrylate compound, a polyfunctional alkenyl compound, Compounds having both a (meth) acryloyl group and an alkenyl group are exemplified. One of these compounds may be used alone, or two or more thereof may be used in combination. Among these, a polyfunctional alkenyl compound is preferable in that a uniform crosslinked structure is easily obtained, and a polyfunctional allyl ether compound having a plurality of allyl ether groups in a 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 ( Di (meth) acrylates of dihydric alcohols such as (meth) acrylate; trimethylolpropane tri (meth) acrylate, trimethylolpropane ethylene oxide modified tri (meth) acrylate, glycerin tri (meth) acrylate, pentaerythritol tri ( Poly (meth) acrylates such as tri (meth) acrylate and tetra (meth) acrylate of trihydric or higher polyhydric alcohols such as meth) acrylate and pentaerythritol tetra (meth) acrylate. Rate; methylenebisacrylamide, it can be mentioned bisamides such as hydroxyethylene bisacrylamide.

  Examples of the polyfunctional alkenyl compound include polyfunctional allyl ether compounds such as trimethylolpropane diallyl ether, trimethylolpropane triallyl ether, pentaerythritol diallyl ether, pentaerythritol triallyl ether, tetraallyloxyethane, and polyallyl saccharose; diallyl phthalate and the like And polyfunctional vinyl compounds such as divinylbenzene.

  Examples of the compound having both the (meth) acryloyl group and the 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 a hydrolyzable silyl group-containing vinyl monomer, N-methylol (meth) acrylamide, N-methoxyalkyl (meth) acrylate, and the like. Is 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, vinylsilanes such as vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane and vinyldimethylmethoxysilane; silyl such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate and methyldimethoxysilylpropyl acrylate Group-containing acrylates; silyl group-containing methacrylates such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, and dimethylmethoxysilylpropyl methacrylate; trimethoxysilylpropyl vinyl ether And silyl group-containing vinyl esters such as vinyl trimethoxysilylundecanoate.

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

<Particle size of crosslinked polymer>
In the mixture layer composition, when the crosslinked polymer is not present as a large-diameter lump (secondary aggregate) but is well dispersed as water-swelled particles having an appropriate particle size, the crosslinked polymer is Is preferred because it can exhibit good binding performance.

The crosslinked polymer of the present invention or a salt thereof has a particle diameter (water swelling particle diameter) when a substance having a degree of neutralization based on a carboxyl group of the crosslinked polymer of 80 to 100 mol% is dispersed in water. Preferably, the volume-based median diameter is in the range of 0.1 μm or more and 10.0 μm or less. The more preferable range of the particle diameter is 0.1 μm or more and 8.0 μm or less, the further 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, an even more preferable range is 0.5 μm or more and 3.0 μm or less. When the particle diameter is in the range of 0.1 μm or more and 10.0 μm or less, the mixture layer composition is uniformly present in a suitable size in the mixture layer composition. It is possible to exhibit the wearability. If the particle size exceeds 10.0 μm or less, the binding property may be insufficient as described above. In addition, there is a possibility that the coatability becomes insufficient because a smooth coated surface is hardly obtained. On the other hand, when the particle diameter is less than 0.1 μm, there is a concern from the viewpoint of stable production.
The water swelling particle diameter can be measured by the method described in Examples of the present specification.

  When the cross-linked polymer is unneutralized or has a degree of neutralization of less than 80 mol%, neutralize to a degree of neutralization 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 aggregated and aggregated primary particles in the form of a powder or a solution (dispersion). 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% and water By dispersing, the agglomerated particles are unraveled, and a stable dispersion state in which the particle diameter is in the range of 0.1 to 10.0 μm is formed even if the dispersion is substantially a primary particle or a secondary aggregate. Things.

  The particle diameter distribution, which is a value obtained by dividing the volume-based median diameter of the water-swelled particle diameter by the number-based median diameter, 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 even more preferably 1.5 or less. The lower limit of the particle size distribution is usually 1.0.

  Further, the particle size (dry particle size) of the crosslinked polymer of the present invention 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. The more preferable range of the particle diameter is 0.1 μm or more and 1 μm or less, and the more preferable range is 0.3 μm or more and 0.8 μm or less.

  In general, the longer the length of the polymer chain (primary chain length) of the crosslinked polymer, the higher the toughness, the higher the binding ability, and the higher the viscosity of the aqueous dispersion. A crosslinked polymer (salt) obtained by subjecting a polymer having a long primary chain length to a relatively small amount of crosslinking exists in water as a microgel body swollen in water. In the composition for an electrode mixture layer of the present invention, the interaction between the microgels exhibits a thickening effect and a dispersion stabilizing effect. The interaction of the microgel varies with the degree of water swelling of the microgel and the strength of the microgel, which are affected by the degree of crosslinking of the crosslinked polymer. If the degree of crosslinking is too low, the strength of the microgel may be insufficient, and the dispersion stabilizing effect and the binding property may be insufficient. On the other hand, if the degree of crosslinking is too high, the swelling degree of the microgel may be insufficient, and the dispersion stabilizing effect and the binding property may be insufficient. That is, the crosslinked polymer is desirably a finely crosslinked polymer obtained by subjecting a polymer having a sufficiently long primary chain length to appropriate crosslinking.

  The crosslinked polymer or a salt thereof is neutralized with an acid group such as a carboxyl group derived from an ethylenically unsaturated carboxylic acid monomer so that the degree of neutralization is 20 to 100 mol% in the mixture layer composition. And it is preferable to use it as an embodiment of a salt. The degree of neutralization is more preferably from 50 to 100 mol%, even more preferably from 60 to 95 mol%. 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 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 was determined by IR measurement of the powder after drying the crosslinked polymer or a salt thereof at 80 ° C. for 3 hours under reduced pressure. The peak derived from the CＣO group of the carboxylic acid and the C ＝ It can be confirmed from the intensity ratio of the peak derived from the O group.

<Method for producing crosslinked polymer or salt thereof>
For the crosslinked polymer, known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization can be used, but precipitation polymerization and suspension polymerization (reverse phase suspension polymerization) are possible in terms of productivity. Is preferred. A heterogeneous polymerization method such as precipitation polymerization, suspension polymerization, or emulsion polymerization is preferred, and a precipitation polymerization method is more preferred, in terms of obtaining better performance with respect to binding properties and the like.
Precipitation polymerization is a method for producing a polymer by performing a polymerization reaction in a solvent that dissolves the unsaturated monomer as a raw material but does not substantially dissolve the produced polymer. As the polymerization proceeds, the polymer particles become larger due to aggregation and growth, and a dispersion of polymer particles in which primary particles of several tens nm to several hundreds of nm are secondarily aggregated to several μm to several tens μm is obtained. Dispersion stabilizers 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. Generally, 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 the monomer to be 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 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 may be used alone or in combination of two or more. Alternatively, they 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.
Among the above, the generation of coarse particles and adhesion to the reactor are small and the polymerization stability is good, and the precipitated polymer fine particles are hardly secondary-agglomerated (or dissolve in an aqueous medium even if secondary aggregation occurs). Methyl ethyl ketone and acetonitrile are preferred in that a polymer having a small chain transfer constant and a large degree of polymerization (primary chain length) is obtained, and that the operation is easy during the neutralization of the process described below. .

  Similarly, in order to allow the neutralization reaction to proceed stably and quickly in the process 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, and still more preferably 0.1 to 5% by mass based on the total mass of the medium. 0.0% by mass, and 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 the ratio 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, when a highly polar solvent is added, the polymerization rate is improved, and a polymer having a long primary chain length is easily obtained. Among the highly polar solvents, water is particularly preferred because it has a large effect of improving the polymerization rate.

In the production of a crosslinked 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. For example, 50% by mass or more and 99.9% by mass or less of the ethylenically unsaturated carboxylic acid monomer derived from the component (a) and the nitrile group-containing ethylenically unsaturated monomer derived from the component (b) may be used. A polymerization step of polymerizing a monomer component containing 0.1% by mass or more and 20% by mass or less, and 0% by mass or more and 49.9% by mass or less of another ethylenically unsaturated monomer as a component (c); It is preferable to provide
By the above polymerization step, a structural unit (a component) derived from an ethylenically unsaturated carboxylic acid monomer is introduced into the crosslinked polymer in an amount of 50% by mass or more and 99.9% by mass or less, 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 20% 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, for example, 50% by mass or more and 98% by mass or less, for example, 50% by mass or less. At least 95% by mass. The used amount of the nitrile group-containing ethylenically unsaturated monomer is, for example, 0.3% by mass or more and 20% by mass or less, and for example, 0.5% by mass or more and 20% by mass or less, For example, the content is 0.5% by mass or more and 18% by mass or less.

  Examples of the other ethylenically unsaturated monomer include, 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, and a compound other than the component (b). Nonionic ethylenically unsaturated monomers and the like can be mentioned. Specific examples of the compound include a monomer compound into which the component (c) described above 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 based on the total amount of the monomer components. 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. Moreover, you may use the said crosslinking monomer similarly.

The monomer concentration at the time of polymerization is preferably higher from the viewpoint of obtaining a polymer having a longer primary chain length. However, if the monomer concentration is too high, the aggregation of the polymer particles tends to proceed, and it is difficult to control the heat of polymerization, so that the polymerization reaction may run away. Therefore, for example, in the case of the precipitation polymerization method, the monomer concentration at the start of the polymerization is generally in the range of about 2 to 40% by mass, and 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 of starting the polymerization.

The crosslinked polymer may be produced by performing a polymerization reaction in the presence of a basic compound. By performing the polymerization reaction in the presence of a base compound, the polymerization reaction can be stably performed 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 at least 20.0% by mass. The monomer concentration is still more preferably 22.0% by mass or more, and even more preferably 25.0% by mass or more. In general, the higher the monomer concentration at the time of polymerization, the higher the molecular weight can be obtained, and a polymer having a long primary chain length can be produced. Since the crosslinked polymer of the present invention is a finely crosslinked polymer obtained by subjecting a polymer having a sufficiently long primary chain length to appropriate crosslinking, it is analytically difficult to directly measure the primary chain length. . In general, it is known that the primary chain length of a polymer is correlated with the solution viscosity. However, in the case of a crosslinked polymer, the solution viscosity varies depending on the degree of crosslinking.
Therefore, it is very difficult to define the crosslinked polymer obtained by the above method by the structure or properties of the polymer.
In the present specification, the “monomer concentration” indicates the monomer concentration in the reaction solution at the time of starting the polymerization.

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

The basic compound is a so-called alkaline compound, and any of an inorganic basic compound and an organic basic compound may be used. By performing the polymerization reaction in the presence of a base compound, the polymerization reaction can be stably performed even under a high monomer concentration condition exceeding, for example, 13.0% by mass. Further, a polymer obtained by polymerizing at such a high monomer concentration has a high molecular weight (because of a long primary chain length) and thus has excellent binding properties.
Examples of the inorganic base compound include lithium hydroxide, sodium hydroxide, alkali metal hydroxides such as potassium hydroxide, calcium hydroxide, alkaline earth metal hydroxides such as magnesium hydroxide, and the like. One or more kinds can be used.
Examples of the organic base compound include ammonia and an organic amine compound, and one or more of these can be used. Among them, an organic amine compound is preferable from the viewpoint of polymerization stability and binding property of a binder containing the obtained crosslinked polymer or a salt thereof.

Examples of the organic amine compound include, for example, 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, morpholine and diazabicycloundecene (DBU); diethylenetriamine, N, N- Methylbenzylamine, and the like, may be used alone or two or more of these.
Among these, when a hydrophobic amine having a long-chain alkyl group is used, greater 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 value of C / N is preferably 3 or more, more preferably 5 or more, further preferably 10 or more, and further preferably 20 or more.

The amount of the basic compound used is preferably in the range of 0.001 mol% to 4.0 mol% based on the ethylenically unsaturated carboxylic acid monomer. When the amount of the basic compound used is within this range, the polymerization reaction can be smoothly performed. The used amount may be 0.05 mol% or more and 4.0 mol% or less, 0.1 mol% or more and 4.0 mol% or less, or 0.1 mol% or more and 3.0 mol%. % Or less, and may be 0.1 mol% or more and 2.0 mol% or less.
In this specification, the amount of the basic compound used indicates 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, a known polymerization initiator such as an azo compound, an organic peroxide, or an inorganic peroxide can be used, but is not particularly limited. Use conditions can be adjusted by a known method such as heat initiation, redox initiation using a reducing agent in combination, UV initiation, etc., 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 conditions so that the amount of generated radicals is reduced within a range where the production time is allowed.

  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 (4,4-di-t-butylperoxycyclohexyl) propane (trade name "Pertetra A" manufactured by NOF Corporation), 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-butylperoxy) butane (perhexa22), t-butyl hydroperoxide (perbutyl H), cumene hydroperoxide (JP) Oil Company, trade name "Parkmill H"), 1,1,3,3-tetramethylbutyl hydroperoxide ("Perocta H"), t-butylcumyl peroxide ("" -Butyl C "), di-t-butyl peroxide (same as" perbutyl D "), di-t-hexyl peroxide (same as" perhexyl D "), di (3,5,5-trimethylhexanoyl) peroxide ( "Perloyl 355"), dilauroyl peroxide ("Perloyl L"), bis (4-t-butylcyclohexyl) peroxydicarbonate ("Perloyl TCP"), di-2-ethylhexylperoxydicarbonate ( "Perloyl OPP"), di-sec-butyl peroxydicarbonate ("Perloyl SBP"), cumyl peroxy neodecanoate ("Parkmill ND"), 1,1,3,3-tetramethylbutyl Peroxyneodecanoate (same as “perocta ND”), t-hexylperoxyneodecanoate (Perhexyl ND), t-butylperoxy neodecanoate (perbutyl ND), t-butyl peroxyneoheptanoate (perbutyl NHP), t-hexylperoxypi Valate ("perhexyl PV"), t-butyl peroxypivalate ("perbutyl PV"), 2,5-dimethyl-2,5-di (2-ethylhexanoyl) hexane ("perhexa 250") 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate (the same as “peroctyl O”), t-hexylperoxy-2-ethylhexanoate (the same as “perhexyl O”), t -Butylperoxy-2-ethylhexanoate (perbutyl O); t-butylperoxylaurate (perbutyl L); t-butyl Luperoxy-3,5,5-trimethylhexanoate ("Perbutyl 355"), t-hexylperoxyisopropyl monocarbonate ("Perhexyl I"), t-butyl peroxyisopropyl monocarbonate ("Perbutyl I") ), T-butyl peroxy-2-ethylhexyl monocarbonate ("Perbutyl E"), t-butyl peroxyacetate ("Perbutyl A"), t-hexyl peroxybenzoate ("Perhexyl Z") and t -Butyl peroxybenzoate (the same as "perbutyl Z"), and one or more of these can be used.

Examples of the inorganic peroxide include potassium persulfate, sodium persulfate, and ammonium persulfate.
In the case of redox initiation, sodium sulfite, sodium thiosulfate, sodium formaldehyde sulfoxylate, ascorbic acid, sulfur dioxide (SO ₂ ), ferrous sulfate and the like can be used as a reducing agent.

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

  The polymerization temperature is preferably from 0 to 100 ° C, more preferably from 20 to 80 ° C, depending 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 crosslinked polymer dispersion obtained through the polymerization step is subjected to reduced pressure and / or heat treatment or the like in the drying step to distill off the solvent, whereby the desired crosslinked polymer can be obtained in a powder state. At this time, prior to the drying step, for the purpose of removing unreacted monomers (and salts thereof), impurities derived from the initiator, and the like, the polymerization step is followed by a solid-liquid separation step such as centrifugation and filtration, and water. , Methanol or the same solvent as the polymerization solvent. When the above washing step is provided, even when the crosslinked polymer is secondary aggregated, it is easy to be unraveled at the time of use, and the remaining unreacted monomer is removed, so that it is also good in terms of binding properties and battery characteristics. High performance.

  In the present production method, a polymerization reaction of a monomer composition containing an ethylenically unsaturated carboxylic acid monomer is performed in the presence of a base compound, and an alkali compound is added to a polymer dispersion obtained in the polymerization step. After neutralizing the polymer (hereinafter, also referred to as “step neutralization”), the solvent may be removed in a drying step. Also, after obtaining the crosslinked polymer powder without performing the above-described neutralization treatment, an alkali compound is added when preparing the electrode mixture layer slurry to neutralize the polymer (hereinafter, referred to as " Sum). Among the above, the step neutralization is preferable because the secondary aggregates tend to be easily unraveled.

<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 crosslinked polymer or a salt thereof, an active material, and water.
The amount of the crosslinked polymer or its salt 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 based on the total amount of the active material. The used amount 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 crosslinked polymer and its salt is less than 0.1% by mass, sufficient binding properties may not be obtained. Further, the dispersion stability of the active material or the like may be insufficient, and the uniformity of the formed mixture layer may decrease. On the other hand, when the use amount of the crosslinked polymer and its salt exceeds 20% by mass, the composition of the electrode mixture layer becomes high in viscosity, and the coatability to the current collector may be reduced. As a result, bumps and irregularities may occur in the obtained mixture layer, which may adversely affect the electrode characteristics.

  If the amount of the crosslinked 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, the durability of the battery is improved. Furthermore, the crosslinked polymer and its salt show a sufficiently high binding property to the active material even in a small amount (for example, 5% by mass or less) and have a carboxy anion. Excellent electrodes are obtained.

Among the above active materials, a lithium salt of a transition metal oxide can be used as the positive electrode active material. For example, a layered rock salt type and a spinel type lithium-containing metal oxide can be used. Specific compounds of the positive electrode active material of layered rock-salt, lithium cobaltate, lithium nickelate, _{and, NCM {Li (Ni x,} Co y, Mn z), x + y + z = 1} called ternary and NCA {Li (Ni _1-ab Co _a Al _b )} and the like. In addition, examples of the spinel-type positive electrode active material include lithium manganate. In addition to oxides, phosphates, silicates, sulfur, and the like are used, and examples of the phosphates include olivine-type lithium iron phosphate. As the positive electrode active material, one of the above may be used alone, or two or more may be used in combination 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, lithium ions on the surface of the active material are exchanged with hydrogen ions in water, so that the dispersion exhibits alkalinity. For this reason, there is a possibility that aluminum foil (Al), which is a general positive electrode current collector material, may be corroded. In such a case, it is preferable to use an unneutralized or partially neutralized crosslinked polymer as a binder to neutralize alkalis eluted from the active material. The amount of the unneutralized or partially neutralized crosslinked polymer used should be such that the amount of unneutralized carboxyl groups of the crosslinked polymer is at least equivalent to the amount of alkali eluted from the active material. Is preferred.

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

  On the other hand, examples of the negative electrode active material include a carbon-based material, a lithium metal, a lithium alloy, and a metal oxide, and one or more of these can be used in combination. Among these, natural graphite, artificial graphite, active materials composed of carbon-based materials such as hard carbon and soft carbon (hereinafter, also referred to as "carbon-based active material") are preferred, graphite such as natural graphite and artificial graphite, and Hard carbon is more preferred. In the case of graphite, spheroidized graphite is suitably used from the viewpoint of battery performance, and a preferred range of the particle size 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 such as silicon or tin which can store lithium can be used as the negative electrode active material. Among them, silicon has a higher capacity than graphite, and an active material composed of silicon-based material such as silicon, silicon alloy, and silicon oxide such as silicon monoxide (SiO) (hereinafter also referred to as “silicon-based active material”). ) Can be used. However, the silicon-based active material has a high capacity, but has a large volume change due to charge and discharge. For this reason, it is preferable to use together with the carbon-based active material. In this case, if the blending amount of the silicon-based active material is large, the electrode material may be broken, and the cycle characteristics (durability) may be significantly reduced. From such a viewpoint, when the silicon-based active material is used in combination, the amount of use is, for example, 60% by mass or less, and for example, 30% by mass or less based on the carbon-based active material.

  In the binder containing the crosslinked polymer of the present invention, the crosslinked 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. For this reason, 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 in improving the durability of the obtained electrode. It is considered something.

  Further, the crosslinked polymer of the present invention 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 properties to the active material. For this reason, according to the binder of the present invention, it is possible to obtain a solid electrode mixture layer having good integration.

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

  When the composition for a 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. Range. When the amount of the active material used is 10% by mass or more, migration of a binder or the like is suppressed, and the cost of drying the medium is also advantageous. On the other hand, when the content is 75% by mass or less, the fluidity and coatability of the composition can be secured, and a uniform mixture layer can be formed.

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

  The composition for a secondary battery electrode mixture layer uses water as a medium. Further, 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, tetrahydrofuran, and water-soluble organic solvents such as N-methylpyrrolidone. May be used as a mixed solvent. The ratio of water in the mixed medium is, for example, 50% by mass or more, and for example, 70% by mass or more.

  When the composition for an electrode mixture layer is made into a slurry state that can be applied, the content of the medium containing water in the entire composition is determined based on the applicability of the slurry, the energy cost required for drying, and the viewpoint of 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. In the case of a wet powder state capable of being pressed, the content of the medium can be, for example, in the range of 3 to 40% by mass from the viewpoint of uniformity of the mixture layer after pressing, and, for example, 10%. -30% by mass.

  The binder of the present invention may be composed of only the above-mentioned crosslinked polymer or a salt thereof, but other than this, other binders such as styrene / butadiene latex (SBR), acrylic latex and polyvinylidene fluoride latex may be used. You may use together a binder component. When other binder components are used in combination, the amount of use 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, 0.1 to 1% by mass or less. If the use amount of the other binder component exceeds 5% by mass, the resistance increases, and the high-rate characteristics may become insufficient. Among them, styrene / butadiene-based latex is preferable in terms of excellent balance between binding property and flex resistance.

  The styrene / butadiene-based 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-based monomer such as 1,3-butadiene. 1 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 may be, for example, in a range of 20 to 60% by mass mainly from the viewpoint of binding properties, and may be, for example, 30 to 50% by mass. % By mass.

  Examples of the aliphatic conjugated diene monomer include 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, and 2-chloro-1,3-in addition to 1,3-butadiene. 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 improved. And, for example, in the range of 40 to 60% by mass.

The styrene / butadiene-based latex may be a monomer containing a nitrile group such as (meth) acrylonitrile or the like, other than the above-mentioned monomers, in order to further improve performance such as binding property. ) A carboxyl group-containing monomer such as acrylic acid, itaconic acid, or maleic acid may be used as a copolymer monomer.
The structural unit derived from the other monomer in the copolymer may be, for example, in a range of 0 to 30% by mass, and may be, for example, in a 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 components, and can be 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.However, after dry-blending powder components such as crosslinked polymer particles as an active material, a conductive auxiliary agent and a binder, water is added. A method of mixing with a dispersing medium such as described above and kneading with dispersion is preferred. When the composition for an electrode mixture layer is obtained in a slurry state, it is preferable to finish the slurry without poor dispersion or aggregation. As the mixing means, known mixers such as a planetary mixer, a thin-film whirl mixer and a self-revolving mixer can be used, but a thin-film whirl mixer is used because a good dispersion state can be obtained in a short time. It is preferable to carry out. When a thin-film swirling mixer is used, it is preferable to perform preliminary dispersion using a stirrer such as a disperser in advance. The viscosity of the slurry may be, for example, in the range of 500 to 100,000 mPa · s as B-type viscosity at 60 rpm, and may be in the range of 1,000 to 50,000 mPa · s, for example. it can.

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

<Electrode for secondary battery>
The electrode for a secondary battery of the present invention is provided with a mixture layer formed from the composition for an electrode mixture layer 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 of applying the mixture layer composition is not particularly limited, and a known method such as a doctor blade method, a dip method, a roll coat method, a comma coat method, a curtain coat method, a gravure coat method, and an extrusion method is employed. be able to. Further, the drying can be performed by a known method such as hot air blowing, reduced pressure, (far) infrared ray, microwave irradiation, or the like.
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 are brought into close contact, 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 a separator and an electrolytic solution in the electrode for a secondary battery of the present invention. The electrolyte may be liquid or gel.
The separator is disposed 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 secure ionic conductivity. The separator is preferably a film-shaped insulating microporous film having good ion permeability and mechanical strength. As specific materials, polyolefins such as polyethylene and polypropylene, polytetrafluoroethylene, and the like can be used.

As the electrolyte, a known electrolyte generally used depending on the type of the active material can be used. In a lithium ion secondary battery, as a specific solvent, a cyclic carbonate having a high dielectric constant and a high solubility for an electrolyte such as propylene carbonate and ethylene carbonate, and a low viscosity chain such as ethyl methyl carbonate, dimethyl carbonate and diethyl carbonate are used. Carbonates and the like, and these can be used alone or as a mixed solvent. The electrolyte is used by dissolving lithium salts such as LiPF ₆ , LiSbF ₆ , LiBF ₄ , LiClO ₄ , and LiAlO ₄ in these solvents. In a nickel-metal hydride secondary battery, an aqueous solution of potassium hydroxide can be used as an electrolytic solution. A secondary battery is obtained by storing a positive electrode plate and a negative electrode plate separated by a separator in a spiral shape or a laminated structure in a case or the like.

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

Hereinafter, the present invention will be specifically described based on examples. Note that the present invention is not limited by these examples. In the following, "parts" and "%" mean parts by mass and% by mass, respectively, unless otherwise specified.
In the following examples, the evaluation of the crosslinked polymer (salt) was performed by the following method.

(1) Measurement of average particle diameter before swelling The polymerization reaction solution containing the crosslinked polymer salt was measured with a laser diffraction / scattering particle size distribution meter (Microtrac Bell Corp., Microtrac MT-3300EXII) using acetonitrile as a dispersion medium. The particle diameter was measured to determine a volume-based median diameter.

(2) Average particle size measurement in aqueous medium (water swelling particle size)
0.25 g of the crosslinked polymer salt powder and 49.75 g of ion-exchanged water were weighed into a 100 cc container, and set in a rotation / revolution type stirrer (Shinky Co., Awatori Rentaro AR-250). Next, stirring (rotation speed: 2,000 rpm / revolution speed: 800 rpm, 7 minutes) and defoaming (revolution speed: 2,200 rpm / revolution speed: 60 rpm, 1 minute) were performed to prepare a hydrogel in which the crosslinked polymer salt was swollen in water. .
Next, the particle size distribution of the hydrogel was measured using a laser diffraction / scattering type particle size distribution meter (Microtrack Bell Co., Ltd., Microtrack MT-3300EXII) using ion exchanged water as a dispersion medium. When an amount of the hydrogel at which an appropriate intensity of scattered light was obtained was introduced into a place where an excessive amount of the dispersion medium was circulated with respect to the hydrogel, the particle size distribution shape measured after several minutes was stabilized. As soon as the stability was confirmed, a volume-based particle size distribution was measured to obtain a median diameter (D50) as an average particle diameter and a particle size distribution represented by (volume-based median diameter) / (number-based median diameter). The evaluation of the monodispersity was determined based on the following criteria.
≪Monodispersity≫
◎: The value of the particle size distribution is less than 1.5 径: The value of the particle size distribution is 1.5 or more and less than 3.0 △: The value of the particle size distribution is 3.0 or more and less than 7.0 ×: The value of the particle size distribution Value is 7.0 or more

(3) Solid content The measuring method is described below.
About 0.5 g of a sample is collected in a weighing bottle (weight of the weighing bottle = B (g)) whose weight has been measured in advance, and accurately weighed together with the weighing bottle [W ₀ (g)]. The sample was placed in a windless drier together with the weighing bottle and dried at 155 ° C. for 45 minutes, and the weight at that time was measured for the weighing bottle [W ₁ (g)], and the solid content% was determined by the following equation. .
Solid content (NV) (%) = [(W ₀ −B) − (W ₁ −B)] × 100

≫Production of crosslinked polymer salt 塩
(Production Example 1: Production of crosslinked polymer salt R-1)
For the polymerization, a reactor equipped with a stirring blade, a thermometer, a reflux condenser, and a nitrogen inlet tube was used.
567 parts of acetonitrile, 2.20 parts of ion-exchanged water, 99.5 parts of acrylic acid (hereinafter, referred to as “AA”), 0.5 part of acrylonitrile (hereinafter, referred to as “AN”), and trimethylolpropane diallyl ether in a reactor 0.90 part (manufactured by Daiso Co., Ltd., trade name "Neoallyl T-20") and triethylamine corresponding to 1.0 mol% based on the AA were charged. After sufficiently replacing the inside of the reactor with nitrogen, the reactor was heated to raise the internal temperature to 55 ° C. After confirming that the internal temperature was stabilized at 55 ° C., 2,2′-azobis (2,4-dimethylvaleronitrile) (trade name “V-65” manufactured by Wako Pure Chemical Industries, Ltd.) was used as a polymerization initiator. When 0.040 parts was added, cloudiness was observed in the reaction solution, and this point was regarded as the polymerization initiation point. The monomer concentration was calculated to be 15.0%. The polymerization reaction was continued while adjusting the external temperature (water bath temperature) to maintain the internal temperature at 55 ° C, and the internal temperature was raised to 65 ° C after 6 hours from the polymerization start point. The internal temperature was maintained at 65 ° C., and after 12 hours from the start of polymerization, cooling of the reaction solution was started. After the internal temperature was lowered to 25 ° C., lithium hydroxide monohydrate (hereinafter, “LiOH - 52.5 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 a crosslinked polymer salt R-1 (Li salt, neutralization degree: 90 mol%) were dispersed in a medium.

The resulting polymerization reaction solution was centrifuged to precipitate polymer particles, and then the supernatant was removed. Thereafter, a washing operation of re-dispersing the sediment in acetonitrile of the same weight as the polymerization reaction liquid and then sedimenting the polymer particles by centrifugation to remove the supernatant was repeated twice. The precipitate was recovered, dried under reduced pressure at 80 ° C. for 3 hours, and volatile matter was removed to obtain a crosslinked polymer salt R-1 powder. Since the crosslinked polymer salt R-1 has a hygroscopic property, it was sealed and stored in a container having a water vapor barrier property. The powder of the crosslinked polymer salt R-1 was subjected to IR measurement, 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 crosslinked polymer salt R-1 was sealed and stored in a container having a water vapor barrier property.
The particle size of the crosslinked polymer salt R-1 obtained above before swelling was measured and found to be 0.52 μm. Further, the average particle size (water swelling particle size) in an aqueous medium was measured to be 1.7 μm, the particle size distribution was calculated as 1.4, and the monodispersity was evaluated as “「 ”.

(Production Examples 2 to 14: Production of Crosslinked Polymer Salts R-2 to R-14)
The same operation as in Production Example 1 was carried out except that the charged amounts of the respective raw materials were as shown in Table 1, to obtain polymerization reaction solutions containing the crosslinked polymer salts R-2 to R-14.
Next, the same operation as in Production Example 1 was performed for each polymerization reaction solution to obtain powdery crosslinked polymer salts R-2 to R-14. Each crosslinked polymer salt was sealed and stored in a container having a water vapor barrier property.
For each of the obtained polymer salts, the particle size before swelling and the average particle size in an aqueous medium were measured in the same manner as in Production Example 1, and the results are shown in Table 1.
In Production Example 11, a crosslinked polymer Na salt (neutralization degree: 90 mol%) was obtained by using NaOH instead of 52.5 parts of LiOH.H ₂ O powder. 1

The details of the compounds used in Table 1 are shown below.
AA: Acrylic acid AN: Acrylonitrile MAN: Methacrylonitrile ACE: Cyanoethyl acrylate T-20: Trimethylolpropane diallyl ether (manufactured by Daiso Co., trade name "Neoallyl T-20")
P-30: Pentaerythritol triallyl ether (manufactured by Daiso Corporation, trade name "Neoallyl P-30")
TEA: triethylamine TDA: tridodecylamine TMA: trimethylamine AcN: acetonitrile V-65: 2,2′-azobis (2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd.)

(Evaluation of electrode)
As the active material, a negative electrode active material, graphite, or silicon particles and graphite was used, and for a mixture layer composition using each crosslinked polymer salt as a binder, its coatability and formed mixture layer / The peel 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 graphite, and silicon particles (Sigma-Aldrich, Si nanopowder, particle diameter <100 nm) were used.

Example 1
To 100 parts of natural graphite, 3.2 parts of a powdered crosslinked polymer Li salt R-1 was weighed, mixed well in advance, added with 160 parts of ion-exchanged water, pre-dispersed with a disper, and then subjected to thin film swirling. This dispersion was carried out for 15 seconds at a peripheral speed of 20 m / sec using a mixer (manufactured by Primix, FM-56-30) to obtain a slurry-type composition for a negative electrode mixture layer.
Using a variable-type applicator, the composition for a mixture layer is applied on a copper foil (manufactured by Nippon Foil Co., Ltd.) having a thickness of 20 μm and dried at 100 ° C. for 15 minutes in a ventilation dryer. A layer was formed. Thereafter, the mixture was rolled so that the thickness of the mixture layer became 50 ± 5 μm and the packing density became 1.70 ± 0.20 g / cm ³ .

The external appearance of the resulting mixture layer was visually observed, and the coatability was evaluated based on the following criteria.
<Coating property judgment criteria>
：: No appearance abnormality such as stripe unevenness or spots was observed on the surface.
Δ: Slight irregularities such as stripe unevenness and spots are observed on the surface.
X: Appearance abnormalities such as stripe unevenness and spots are remarkably observed on the surface.

<90 ° peel strength (binding property)>
After the obtained negative electrode was cut into a strip having a width of 25 mm, the mixture layer surface of the sample was stuck to a double-sided tape fixed to a horizontal surface to prepare a sample for a peel test. After the test sample was dried under reduced pressure at 60 ° C. overnight, 90 ° peeling was performed at a tensile speed of 50 mm / min, and the peel strength between the mixture layer and the copper foil was measured. The peel strength was as high as 15.3 N / m, which was good.

Examples 2 to 14 and Comparative Examples 1 to 3
A mixture layer composition was prepared by performing the same operation as in Example 1 except that the crosslinked polymer salt used as the active material and the binder was used as shown in Tables 2 and 3. In Examples 4 and 5 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 the resulting mixture was powder-like cross-linked. After weighing 3.2 parts of the polymer Li salt R-3 or R-13 and mixing them well in advance, the same operation as in Example 1 was performed to prepare a mixture layer composition. The coatability and 90 ° peel strength of each mixture layer composition were evaluated. The results are shown in Tables 2 and 3.

In each of the examples, an electrode mixture layer composition containing a binder for a secondary battery electrode belonging to the present invention and an electrode were produced using the same. The coatability 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, and excellent binding properties were obtained. It was shown.
Focusing on the amount of the cyano group-containing ethylenically unsaturated monomer used, high binding properties and good coating properties were exhibited in the range of 0.5 to 20 parts by mass. Above all, when the amount used was up to 10 parts by mass, the result that the peel strength (binding property) improved as the amount used increased was obtained (Examples 1 to 3 and Examples 6 to 8).

  On the other hand, Comparative Examples 1 and 2 using a crosslinked polymer salt not using a cyano group-containing ethylenically unsaturated monomer did not show satisfactory peel strength. In Comparative Example 3, since the amount of the cyano group-containing ethylenically unsaturated monomer used exceeded the range specified in the present invention, the result that the peel strength was greatly reduced was obtained.

Since the binder for a secondary battery electrode of the present invention exhibits excellent binding properties in a mixture layer, a secondary battery provided with an electrode obtained using the above binder has good durability (cycle characteristics). ), And is expected to be applied to secondary batteries for vehicles. It is also useful for using 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 suitably used particularly for a nonaqueous electrolyte secondary battery electrode, and is particularly useful for a nonaqueous electrolyte lithium ion secondary battery having a high energy density.

Claims (7)

A binder for a secondary battery electrode containing a crosslinked polymer or a salt thereof,
The crosslinked polymer has a structural unit derived from an ethylenically unsaturated carboxylic acid monomer of 80% by mass or more and 99.5% by mass or less, and a nitrile group-containing ethylenically unsaturated monomer, based on all the structural units. Containing 0.5% by mass or more and 20% by mass or less of a structural unit derived from
And, after being neutralized to a degree of neutralization of 80 to 100 mol%, the particle diameter measured in an aqueous medium is 1.1 μm or more and 6.0 μm or less as a volume-based median diameter,
Further, a binder for a secondary battery electrode, wherein the particle diameter distribution in a water-swelled state is 1.8 or less.   The cross-linked polymer is cross-linked by a cross-linkable monomer, and the amount of the cross-linkable monomer used is at least 0.1 part by mass with respect to 100 parts by mass of the total amount of the non-cross-linkable monomer. The binder for a secondary battery electrode according to claim 1, wherein the binder is not more than 0.0 parts by mass.   3. The binder for a secondary battery electrode according to claim 1, wherein the crosslinked polymer is obtained by polymerizing a monomer component constituting the crosslinked polymer in the presence of an organic amine compound. 4. A method for producing a crosslinked polymer or a salt thereof used for a binder for a secondary battery electrode,
In the presence of an organic amine compound, an ethylenically unsaturated carboxylic acid monomer is contained in an amount of from 80% by mass to 99.5% by mass, and a nitrile group-containing ethylenically unsaturated monomer is contained in an amount of from 0.5% by mass to 20% by mass. A polymerization step for polymerizing the monomer component by a precipitation polymerization method is provided,
After the crosslinked polymer has been neutralized to a degree of neutralization of 80 to 100 mol%, the particle diameter measured in an aqueous medium is 1.1 μm or more and 6.0 μm or less as a volume-based median diameter,
And a method wherein the particle size distribution in a water-swelled state is 1.8 or less.   A composition for a secondary battery electrode mixture layer, comprising the binder according to claim 1, an active material, and water.   The composition for a secondary battery electrode mixture layer according to claim 5, wherein the negative electrode active material contains a carbon-based material or a silicon-based material.   A secondary battery electrode comprising, on a surface of a current collector, a mixture layer formed from the composition for a secondary battery electrode mixture layer according to claim 5.