JP2019139893A - Binder for secondary battery electrode and use thereof - Google Patents

Abstract

To provide: an aqueous binder for a secondary battery, which has a binding property superior to conventional one while having good coatability; and a composition for a secondary battery electrode mixture layer and a secondary battery electrode which are obtained by use of the binder.SOLUTION: A binder for a secondary battery electrode comprises a cross-linked polymer or a salt thereof. The cross-linked polymer includes the following to all of its structural units: 50 mass% or more and 99.9 mass% or less of a structural unit originating from an ethylenic unsaturated carboxylic acid monomer; and 0.1 mass% or more and 20 mass% or less of a structural unit originating from a nitrile group-containing ethylene unsaturated monomer. The cross-linked polymer has a volume-based median diameter of 0.1 μm or larger and 10 μm or smaller as a particle diameter measured in aqueous medium after neutralization to a neutralization degree of 80-100 mol%.SELECTED DRAWING: None

Description

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

  Various power storage devices such as nickel metal hydride secondary batteries, lithium ion secondary batteries, and electric double layer capacitors have been put to practical use as secondary batteries. 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 and a binder 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 a binder excellent in dispersibility and binding property, a binder containing an acrylic acid polymer aqueous solution or an aqueous dispersion is known. On the other hand, as a binder used for the positive electrode mixture layer, an N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride (PVDF) is widely used.

  On the other hand, as the use of various secondary batteries expands, demands for improving energy density, reliability, and durability tend to increase. For example, in order to increase the electric capacity of a lithium ion secondary battery, specifications using a silicon-based active material as an active material for a negative electrode are increasing. However, silicon-based active materials are known to have a large volume change during charge and discharge, and peeling or dropping of the electrode mixture layer occurs as they are repeatedly used, resulting in a decrease in battery capacity and cycle characteristics. There was a problem that (durability) deteriorated. In order to suppress such inconveniences, it is generally effective to increase the binding property of the binder, and studies have been conducted on improving the binding property of the binder for the purpose of improving durability.

  For example, Patent Document 1 discloses an acrylic acid polymer crosslinked with a polyalkenyl ether as a binder for forming a negative electrode coating film of a lithium ion secondary battery. Patent Document 2 includes a structural unit derived from an ethylenically unsaturated carboxylate monomer and a structural unit derived from an ethylenically unsaturated carboxylic acid ester monomer, and contains a water-soluble polymer having a specific aqueous solution viscosity. An aqueous electrode binder for a secondary battery is disclosed. Patent Document 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 2015-188776 A International Publication No. 2016/158939

The binders disclosed in Patent Documents 1 to 3 are all capable of imparting good binding properties, but as the performance of the secondary battery is improved, there is an increasing demand for binders with higher binding power. .
In general, in order to increase the binding property, it is effective to increase the molecular weight of the polymer serving as the binder. As a method for increasing the molecular weight of the binder polymer, a method for extending the primary chain length of the polymer and a method for extending the primary chain length by micro-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 is increased, leading to deterioration in coating properties. Although it is possible to reduce the viscosity of the slurry by lowering the concentration of the active material and binder in the slurry, it is not preferable from the viewpoint of productivity.

  The present disclosure has been made in view of such circumstances, and is used for an aqueous binder for a secondary battery electrode that has better coating properties while having good coating properties, and the binder. A method for producing a crosslinked polymer or a salt thereof is provided. Moreover, this indication also provides the composition for secondary battery electrode mixture layers obtained using the said binder, and a secondary battery electrode.

  As a result of intensive studies to solve the above problems, the present inventors 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. When the binder containing a crosslinked polymer or its salt was used, the knowledge that it was excellent in both the applicability | paintability and binding property of an electrode mixture layer slurry was acquired. According to the present disclosure, the following means are provided based on such findings.

The present invention is as follows.
[1] A secondary battery electrode binder containing a crosslinked polymer or a salt thereof,
The cross-linked polymer comprises 50% by mass or more and 99.9% by mass or less of a structural unit derived from an ethylenically unsaturated carboxylic acid monomer and a nitrile group-containing ethylenically unsaturated monomer with respect to the total structural unit. Containing 0.1% by mass or more and 20% by mass or less of structural units derived from
And the binder for secondary battery electrodes whose particle diameter measured in the aqueous medium after neutralizing to 80-100 mol% of a neutralization volume is 0.1 micrometer or more and 10 micrometers or less by volume reference | standard median diameter.
[2] The cross-linked polymer is cross-linked with a cross-linkable monomer, and the amount of the cross-linkable monomer used is 0.1 mass with respect to 100 parts by mass of the total amount of non-cross-linkable monomers. The binder for secondary battery electrodes according to [1], which is not less than 2.0 parts and not more than 2.0 parts by mass.
[3] The secondary battery electrode according to [1] or [2], wherein the cross-linked polymer is obtained by polymerizing monomer components constituting the cross-linked polymer in the presence of an organic amine compound. Binder.
[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, it contains 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 20% by mass or less of a nitrile group-containing ethylenically unsaturated monomer. 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, active material and water according to any one of [1] to [3].
[6] The composition for a secondary battery electrode mixture layer according to [5], which contains a carbon-based material or a silicon-based material as a negative electrode active material.
[7] A secondary battery electrode comprising a mixture layer formed from the composition for a secondary battery electrode mixture layer according to [5] or [6] on a current collector surface.

  The binder for secondary battery electrodes of the present invention exhibits excellent binding properties to electrode active materials and the like. Moreover, the said binder can exhibit favorable adhesiveness with a collector. For this reason, the electrode mixture layer containing the binder and the electrode provided with the binder have excellent binding properties and can maintain the integrity thereof. For this reason, the deterioration of the electrode mixture layer due to the volume change and shape change of the active material accompanying charge / discharge is suppressed, and a secondary battery having high durability (cycle characteristics) can be obtained. Furthermore, since the mixture layer slurry containing the binder for secondary battery electrodes of the present invention has a low viscosity, it has good coatability.

  The binder for secondary battery electrodes 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 composition described above 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 applied to pressing on the surface of the current collector. The secondary battery electrode of the present invention can be obtained by forming a mixture layer formed of the above composition on the surface of a current collector such as a copper foil or an 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 the present specification, “(meth) acryl” means acryl and / or methacryl, and “(meth) acrylate” means acrylate and / or methacrylate. The “(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.

<Crosslinked polymer structural unit>
<Structural unit derived from ethylenically unsaturated carboxylic acid monomer>
The crosslinked polymer may 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 adhesion to the current collector is improved, and since the lithium ion desolvation effect and ion conductivity are excellent, the resistance is small, and the high rate An electrode having excellent characteristics can be obtained. Moreover, since water swelling property is provided, dispersion stability of the active material etc. in a mixture layer composition can be improved.
The said (a) component can be introduce | transduced into a crosslinked polymer by superposing | polymerizing the monomer containing an ethylenically unsaturated carboxylic acid monomer, for example. In addition, it can also be obtained by (co) polymerizing (meth) acrylic acid ester monomers and then hydrolyzing them. Moreover, after polymerizing (meth) acrylamide, (meth) acrylonitrile, etc., you may process with a strong alkali and the method of making an acid anhydride react with the polymer which has a hydroxyl group may be sufficient.

  Examples of ethylenically unsaturated carboxylic acid monomers include (meth) acrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid; (meth) acrylamide alkyl such as (meth) acrylamide hexanoic acid and (meth) acrylamide dodecanoic 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 the like (parts thereof) ) Alkali neutralized products are exemplified, and one of these may be used alone, or two or more may be used in combination. Among them, 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 a binder has a good binding force, and particularly preferably acrylic acid. 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.

  Although content of (a) component in a crosslinked polymer is not specifically limited, For example, 10 mass% or more and 99.9 mass% or less can be included with respect to all the structural units of a crosslinked polymer. By containing the component (a) within such a range, excellent adhesiveness to the current collector can be easily ensured. The lower limit is, for example, 20% by mass or more, for example, 30% by mass or more, and for example, 40% by mass or more. When the lower limit is 50% by mass or more, it is preferable because the dispersion stability of the composition for the electrode mixture layer becomes good, and may be 60% by mass or more, 70% by mass or more, and 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. For example, it is 80 mass% or less. As a range, it can be set as the range which combined these lower limits and upper limits suitably, For example, they are 10 mass% or more and 99.9 mass% or less, for example, are 50 mass% or more and 99.9 mass% or less. For example, it is 50 mass% or more and 99 mass% or less, for example, it is 50 mass% or more and 98 mass% or less, for example, can be 50 mass% or more and 95 mass% or less.

<Structural unit derived from nitrile group-containing ethylenically unsaturated monomer>
The crosslinked polymer of the present invention can have a structural unit derived from a nitrile group-containing ethylenically unsaturated monomer (hereinafter also referred to as “component (b)”) in addition to the component (a). The component (b) can exert a strong interaction with the electrode material and can exhibit a good binding property to the active material. As a result, it is possible to obtain a solid electrode mixture layer having good integrity. The component (b) can be introduced into the crosslinked polymer by polymerizing, for example, a nitrile group-containing ethylenically unsaturated monomer.

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

  Content of (b) component in a crosslinked polymer is 20 mass% or less with respect to all the structural units of a 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 are lowered. There is a risk of doing. The upper limit may be 18% by mass or less, or 15% by mass or less. Moreover, although a minimum is not specifically limited, For example, 0.1 mass% or more can be included with respect to all the structural units of a crosslinked polymer. When the crosslinked polymer contains 0.1% by mass of component (b), an electrode mixture layer having excellent binding properties can be obtained. A lower limit becomes like this. Preferably it is 0.3 mass% or more, More preferably, it is 0.5 mass% or more, More preferably, it is 1.0 mass% or more, More preferably, it is 3.0 mass% or more. As a range, it can be set as the range which combined these lower limits and upper limits suitably, For example, they are 0.1 mass% or more and 20 mass% or less, for example, 0.3 mass% or more and 20 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>
In addition to the component (a) and the component (b), the present crosslinked polymer is also referred to as a structural unit derived from another ethylenically unsaturated monomer copolymerizable therewith (hereinafter referred to as “component (c)”). ) Can be included. Examples of the component (c) 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 ethylenic compound other than the component (b). Examples include structural units derived from saturated monomers. These structural units are composed of 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 introduce | transduce by copolymerizing the monomer containing a body. Among these, as the component (c), a structural unit derived from a nonionic ethylenically unsaturated monomer is preferable from the viewpoint of obtaining an electrode having good bending resistance, and the binder has excellent binding properties. (Meth) acrylamide and its derivatives are preferred. In addition, 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), it can exert a strong interaction with the electrode material, Good binding property to the active material can be exhibited. This is preferable because an electrode mixture layer that is firm and has good integrity can be obtained. In particular, a structural unit derived from an alicyclic structure-containing ethylenically unsaturated monomer is preferred.

  The proportion of the component (c) can be 0% by mass or more and 49.9% by mass or less with respect to all the 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. 5 mass% or more and 30 mass% or less may be sufficient. In addition, 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 to the electrolytic solution is improved, so that the effect of improving 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 are mentioned, and one of these is You may use individually and may be used in combination of 2 or more type.

  Examples of the alicyclic structure-containing ethylenically unsaturated monomer include cyclopentyl (meth) acrylate, cyclohexyl (meth) acrylate, methyl cyclohexyl (meth) acrylate, t-butylcyclohexyl (meth) acrylate, (meth ) (Meth) acrylic acid cycloalkyl ester optionally having 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 cyclodecane dimethanol mono (meth) acrylate Cycloalkyl 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 these, 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 binder has a good binding force.

As other nonionic ethylenically unsaturated monomers, for example, (meth) acrylic acid esters may be used. Examples of (meth) acrylic acid esters include (meth) methyl acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, isobutyl (meth) acrylate and 2-ethylhexyl (meth) acrylate ( (Meth) acrylic acid alkyl ester compounds;
Aromatic (meth) acrylic ester compounds such as phenyl (meth) acrylate, phenylmethyl (meth) acrylate, phenylethyl (meth) acrylate;
(Meth) acrylic acid alkoxyalkyl ester compounds such as (meth) acrylic acid 2-methoxyethyl and (meth) acrylic acid ethoxyethyl;
(Meth) acrylic acid hydroxyalkyl, (meth) acrylic acid hydroxypropyl and (meth) acrylic acid hydroxyalkyl ester compounds such as hydroxybutyl, etc. are used, and one of these is used alone. You may use it in combination of 2 or more types. An aromatic (meth) acrylic acid ester compound can be preferably used from the viewpoints of adhesion with the active material and cycle characteristics.

  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. More preferred is 2-methoxyethyl (meth) acrylate.

  Among 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 of a high polymerization rate, and the binding power of the binder is good. In addition, as the nonionic ethylenically unsaturated monomer, a compound having a glass transition temperature (Tg) of a homopolymer of 0 ° C. or less is preferable in that the obtained electrode has good bending resistance.

  The crosslinked polymer may be a salt. Although it does not specifically limit as a kind of salt, Alkali metal salts, such as lithium, sodium, and potassium; Alkaline earth metal salts, such as calcium salt and barium salt; Other metal salts, such as magnesium salt and aluminum salt; Ammonium salt and organic Examples include amine salts. Of these, alkali metal salts and magnesium salts are preferred, and alkali metal salts are more preferred because they are less likely to adversely affect battery characteristics.

<Aspect of cross-linked polymer>
The cross-linking method in the cross-linked polymer of the present invention is not particularly limited, and examples include the following method.
1) Copolymerization of crosslinkable monomer 2) Utilization of chain transfer to polymer chain during radical polymerization 3) Synthesis of a polymer having a reactive functional group, followed by postcrosslinking by adding a crosslinking agent as required When the polymer has a crosslinked structure, the binder containing the polymer or a salt thereof can have an excellent binding force. Among these, a method based on 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 functional group such as a hydrolyzable silyl group. Can be mentioned.

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

  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) 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) acrylates and tetra (meth) acrylates of trihydric or higher polyhydric alcohols such as (meth) acrylates and pentaerythritol tetra (meth) acrylates Rate; methylenebisacrylamide, can be mentioned bisamides such as hydroxyethylene bisacrylamide.

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

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

  Specific examples of the monomer having a crosslinkable functional group capable of self-crosslinking 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 alone or in combination of two or more.

  The hydrolyzable silyl group-containing vinyl monomer is not particularly limited as long as it is a vinyl monomer having at least one hydrolyzable silyl group. For example, vinyl silanes such as vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane, and vinyldimethylmethoxysilane; silyl such as trimethoxysilylpropyl acrylate, triethoxysilylpropyl acrylate, and methyldimethoxysilylpropyl acrylate Group-containing acrylic acid esters; silyl group-containing methacrylates such as trimethoxysilylpropyl methacrylate, triethoxysilylpropyl methacrylate, methyldimethoxysilylpropyl methacrylate, dimethylmethoxysilylpropyl methacrylate; trimethoxysilylpropyl vinyl ether, etc. Silyl group-containing vinyl ethers; and silyl group-containing vinyl esters such as vinyl trimethoxysilylundecanoate.

When the cross-linked polymer is cross-linked with a cross-linkable monomer, the amount of the cross-linkable monomer used is 100 total monomers other than the cross-linkable monomer (non-cross-linkable monomer). Preferably they are 0.1 mass part or more and 2.0 mass parts or less with respect to a mass part, More preferably, they are 0.3 mass part or more and 1.5 mass parts or less, More preferably, they are 0.5 mass part or more and 1 or less. .5 parts by mass or less. If the use amount of the crosslinkable monomer is 0.1 parts by mass or more, the binding property and the stability of the mixture layer slurry are preferable. If it is 2.0 mass parts or less, there exists a tendency for stability of a crosslinked polymer to become high.
Similarly, the amount of the crosslinkable monomer used may be 0.02 to 0.7 mol% with respect to the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer). Preferably, it is 0.03-0.4 mol%.

<Particle diameter of crosslinked polymer>
In the mixture layer composition, when the cross-linked polymer does not exist as a lump (secondary aggregate) having a large particle size and is well dispersed as water-swelled particles having an appropriate particle size, the cross-linked polymer A binder containing is preferable because it can exhibit good binding performance.

The crosslinked polymer of the present invention or a salt thereof has a particle diameter (water-swelled particle diameter) when dispersed in water having a neutralization degree of 80 to 100 mol% based on the carboxyl group of the crosslinked polymer. The volume-based 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 diameter 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 more preferable range is 0.2 μm or more and 5.0 μm or less. There is an even more preferable range of 0.5 μm or more and 3.0 μm or less. If the particle size 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, so that the mixture layer composition has high stability and excellent binding. It becomes possible to demonstrate wearability. If the particle diameter exceeds 10.0 μm or less, the binding property may be insufficient as described above. Moreover, there exists a possibility that coating property may become inadequate at the point which cannot obtain a smooth coating surface. On the other hand, when the particle diameter is less than 0.1 μm, there is a concern from the viewpoint of stable productivity.
In addition, the said water swelling particle diameter can be measured by the method as described in an Example of this specification.

  When the cross-linked polymer is unneutralized or has a neutralization degree of less than 80 mol%, the particle diameter is measured when neutralized with an alkali metal hydroxide to a neutralization degree of 80 to 100 mol% and 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 diameter 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% and water. By dispersing, the aggregated particles are released, and even if it is a primary particle dispersion or secondary aggregate, a stable dispersion state is formed in which the particle diameter is in the range of 0.1 to 10.0 μm. Is.

  The particle size distribution, which is a value obtained by dividing the volume-based median diameter of the water-swelling particle diameter by the number-based median diameter, is preferably 10 or less, more preferably 5.0 or less, from the viewpoints 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.

  The particle diameter (dry particle diameter) of the crosslinked polymer of the present invention or a salt thereof when dried is preferably in the range of 0.03 μm to 3 μm in terms of volume-based median diameter. A more preferable range of the particle diameter 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 general, as the length of the polymer chain (primary chain length) of the crosslinked polymer increases, the toughness increases, it becomes possible to obtain high binding properties, and the viscosity of the aqueous dispersion increases. Further, a crosslinked polymer (salt) obtained by subjecting a polymer having a long primary chain length to a relatively small amount of crosslinking exists as a microgel body swollen in water in water. In the composition for electrode mixture layers of the present invention, a thickening effect and a dispersion stabilizing effect are expressed by the interaction of the microgel bodies. The interaction of the microgel body varies depending on the water swelling degree of the microgel body and the strength of the microgel body, and these are affected by the degree of crosslinking of the crosslinked polymer. When the degree of crosslinking is too low, the strength of the microgel is 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 degree of swelling of the microgel may be insufficient and the dispersion stabilizing effect and binding properties may be insufficient. That is, the cross-linked polymer is desirably a micro-crosslinked polymer obtained by appropriately crosslinking a polymer having a sufficiently long primary chain length.

  In the mixture layer composition, the cross-linked polymer or 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%. And preferably used as a salt embodiment. The neutralization degree is more preferably 50 to 100 mol%, and further preferably 60 to 95 mol%. When the degree of neutralization is 20 mol% or more, water swellability is good and a dispersion stabilizing effect is easily obtained. In this specification, the said neutralization degree can be computed by calculation from the preparation value of the monomer which has acid groups, such as a carboxyl group, and the neutralizing agent used for neutralization. The degree of neutralization is measured by IR measurement of the crosslinked polymer or salt thereof, and the powder after drying treatment at 80 ° C. for 3 hours under reduced pressure, and the peak derived from the C═O group of the carboxylic acid and the C═ of the carboxylate. This 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, emulsion polymerization and the like can be used, but precipitation polymerization and suspension polymerization (reverse phase suspension polymerization) are considered in terms of productivity. ) Is preferred. A heterogeneous polymerization method such as precipitation polymerization, suspension polymerization, and emulsion polymerization is preferable in that better performance can be obtained with respect to binding properties and the like, and precipitation polymerization is more preferable.
Precipitation polymerization is a method for producing a polymer by carrying out a polymerization reaction in a solvent that dissolves an unsaturated monomer as a raw material but does not substantially dissolve the 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 nm are secondarily aggregated to several μm to several tens μm is obtained. A dispersion stabilizer can also be used to control the particle size of the polymer.
In addition, 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, the polymerization solvent may be a solvent selected from water and various organic solvents in consideration of the type of monomer used. 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 can be used alone or in combination of two or more. Or you may use 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, the polymerization stability is good, and the precipitated polymer fine particles are difficult to agglomerate (or even if secondary agglomeration occurs, they can be dissolved in an aqueous medium). Methyl ethyl ketone and acetonitrile are preferred from the standpoints that a polymer having a small chain transfer constant and a high degree of polymerization (primary chain length) can be obtained, and that the operation can be easily performed during the process neutralization described later. .

  Similarly, a small amount of a highly polar solvent is preferably added to the polymerization solvent in order to allow the neutralization reaction to proceed stably and quickly in the process neutralization. Such a highly polar solvent preferably includes water and methanol. The amount of the highly polar solvent used is preferably 0.05 to 20.0% by mass based on the total mass of the medium, more preferably 0.1 to 10.0% by mass, and even more preferably 0.1 to 5%. It is 0.0 mass%, More preferably, it is 0.1-1.0 mass%. If the ratio of the highly polar solvent is 0.05% by mass or more, the effect on the neutralization reaction is recognized, and if 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, when a highly polar solvent is added, the polymerization rate is improved, and a polymer having a long primary chain is easily obtained. Among the highly polar solvents, water is particularly preferable because it has the effect of improving the polymerization rate.

The production of the crosslinked polymer or a salt thereof preferably includes a polymerization step of polymerizing a monomer component containing an ethylenically unsaturated carboxylic acid monomer. For example, the ethylenically unsaturated carboxylic acid monomer that is 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 that is derived from the component (b) 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 from which component (c) is derived. It is preferable to provide.
Through the polymerization step, a structural unit (component a) 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 ethylenic unsaturated The structural unit (component b) derived from the monomer is introduced in an amount of 0.1% by mass to 20% by mass. 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, and for example, 50% by mass. Above, it is 95 mass% or less. 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, for example, 0.5% by mass or more and 20% by mass or less, For example, it is 0.5 mass% or more and 18 mass% or less.

  Examples of the other ethylenically unsaturated monomers include ethylenically unsaturated monomer compounds having an anionic group other than a carboxyl group such as a sulfonic acid group and a phosphoric acid group, and other than the component (b). Nonionic ethylenically unsaturated monomers are exemplified. As a specific compound, the monomer compound which can introduce | transduce the component (c) mentioned above is mentioned. The other ethylenically unsaturated monomer may be contained in an amount of 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. 2 mass% or more and 40 mass% or less may be sufficient, 2 mass% or more and 30 mass% or less may be sufficient, and 5 mass% or more and 30 mass% or less may be sufficient. Similarly, the crosslinkable monomer may be used.

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 the control of the polymerization heat becomes difficult and the polymerization reaction may run away. For this reason, 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” refers to 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 base compound. By carrying out the polymerization reaction in the presence of the base compound, the polymerization reaction can be carried out stably even under high monomer concentration conditions. The monomer concentration may be 13.0% by mass or more, preferably 15.0% by mass or more, more preferably 17.0% by mass or more, and further preferably 19.0% by mass or more. And more preferably 20.0% by mass or more. The monomer concentration is still preferably 22.0% by mass or more, and more preferably 25.0% by mass or more. In general, the higher the monomer concentration during polymerization, the higher the molecular weight, and it is possible to produce a polymer having a long primary chain length. Since the crosslinked polymer of the present invention is a finely crosslinked polymer obtained by appropriately crosslinking a polymer having a sufficiently long primary chain length, it is analytically difficult to directly measure the primary chain length. . In general, it is known that the primary chain length of a polymer correlates with the solution viscosity. 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 characteristics of the polymer.
In the present specification, the “monomer concentration” refers to 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 monomer and solvent used, the polymerization method and various polymerization conditions, etc., but if the heat of polymerization reaction can be removed, precipitation polymerization is as described above. 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 carrying out the polymerization reaction in the presence of the base compound, the polymerization reaction can be carried out stably even under high monomer concentration conditions, for example, exceeding 13.0% by mass. In addition, a polymer obtained by polymerization at such a high monomer concentration has a high molecular weight (because of a long primary chain length) and is excellent in 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. Among these, 1 type (s) or 2 or more types can be used.
Examples of the organic base compound include ammonia and organic amine compounds, and one or more of them can be used. Among these, an organic amine compound is preferable from the viewpoints of polymerization stability and binding property of a binder containing the obtained crosslinked polymer or a salt thereof.

Examples of organic amine compounds include monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, triethylamine, monobutylamine, dibutylamine, tributylamine, monohexylamine, dihexylamine, trihexylamine, trioctylamine and tridodecylamine. N-alkyl-substituted amines such as monoethanolamine, diethanolamine, triethanolamine, propanolamine, dimethylethanolamine and (alkyl) alkanolamines such as N, N-dimethylethanolamine; pyridine, piperidine, piperazine, 1,8- Cyclic amines such as bis (dimethylamino) naphthalene, 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, larger electrostatic repulsion and steric repulsion can be obtained, so that polymerization stability is ensured even when the monomer concentration is high. It is preferable in terms of 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, still more preferably 10 or more, and still more preferably 20 or more.

The amount of the base compound used is preferably in the range of 0.001 mol% to 4.0 mol% with respect to the ethylenically unsaturated carboxylic acid monomer. If the usage-amount of a basic compound is this range, a polymerization reaction can be performed smoothly. The amount used may be 0.05 mol% or more and 4.0 mol% or less, 0.1 mol% or more and 4.0 mol% or less, or 0.1 mol% or more and 3.0 mol% or less. % Or less, or 0.1 mol% or more and 2.0 mol% or less.
In addition, in this specification, the usage-amount of a base compound represents the molar concentration of the base compound used with respect to the ethylenically unsaturated carboxylic acid monomer, and does not mean the degree of neutralization. That is, the valence of the base compound used is not taken into consideration.

  The polymerization initiator may be a known polymerization initiator such as an azo compound, an organic peroxide, or an inorganic peroxide, but is not particularly limited. The use conditions can be adjusted by a known method such as thermal initiation, redox initiation using a reducing agent in combination, UV initiation, or the like so as to obtain an appropriate radical generation amount. In order to obtain a crosslinked polymer having a long primary chain length, it is preferable to set conditions so that the amount of radicals generated is reduced within a range in which 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 (manufactured by NOF Corporation, trade name “Pertetra A”), 1,1-di (t- Hexylperoxy) cyclohexane (same as “Perhexa HC”), 1,1-di (t-butylperoxy) cyclohexane (same as “PerhexaC”), 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 (Japan) Manufactured by Oil Co., Ltd., trade name “Park Mill H”), 1,1,3,3-tetramethylbutyl hydroperoxide (“Per Octa H”), t-butyl cumyl peroxide (“ -Butyl C "), di-t-butyl peroxide (same" perbutyl D "), di-t-hexyl peroxide (same" perhexyl D "), di (3,5,5-trimethylhexanoyl) peroxide ( "Parroyl 355"), dilauroyl peroxide ("Parroyl L"), bis (4-t-butylcyclohexyl) peroxydicarbonate ("Parroyl TCP"), di-2-ethylhexyl peroxydicarbonate ( "Parroyl OPP"), di-sec-butyl peroxydicarbonate ("Parroyl SBP"), cumyl peroxyneodecanoate ("Parcumyl ND"), 1,1,3,3-tetramethylbutyl Peroxyneodecanoate ("Perocta ND"), t-hexylperoxyneodecano (Perhexyl ND), t-butyl peroxyneodecanoate (perbutyl ND), t-butyl peroxyneoheptanoate (perbutyl NHP), t-hexyl peroxypi Valate (same “Perhexyl PV”), t-butyl peroxypivalate (same “Perbutyl PV”), 2,5-dimethyl-2,5-di (2-ethylhexanoyl) hexane (“Perhexa 250”) 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate ("PeroctaO"), t-hexylperoxy-2-ethylhexanoate ("PerhexylO"), t -Butylperoxy-2-ethylhexanoate (same "Perbutyl O"), t-butyl peroxylaurate (same "Perbutyl L"), t-Butyl Luperoxy-3,5,5-trimethylhexanoate (same as “Perbutyl 355”), t-hexyl peroxyisopropyl monocarbonate (same as “Perhexyl I”), t-butyl peroxyisopropyl monocarbonate (same as “Perbutyl I”) ), T-butyl peroxy-2-ethylhexyl monocarbonate (same as “perbutyl E”), t-butyl peroxyacetate (same as “perbutyl A”), t-hexyl peroxybenzoate (same as “perhexyl Z”) and t -Butyl peroxybenzoate (the same "perbutyl Z") etc. are mentioned, The 1 type (s) or 2 or more types 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, sulfurous acid gas (SO ₂ ), ferrous sulfate and the like can be used as a reducing agent.

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

  Although superposition | polymerization temperature is based also on conditions, such as a kind and density | concentration of a monomer to be used, 0-100 degreeC is preferable and 20-80 degreeC is more preferable. The polymerization temperature may be constant or may change during the polymerization reaction. The polymerization time is preferably 1 minute to 20 hours, and more preferably 1 hour to 10 hours.

  The cross-linked polymer dispersion obtained through the polymerization step is subjected to reduced pressure and / or heat treatment in the drying step, and the solvent is distilled off to obtain the desired cross-linked polymer 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, etc., following the polymerization step, a solid-liquid separation step such as centrifugation and filtration, water, It is preferable to provide a washing step using methanol or the same solvent as the polymerization solvent. When equipped with the above washing step, even when the crosslinked polymer is secondary agglomerated, it is easy to unravel at the time of use, and furthermore, the remaining unreacted monomer is removed, and the binding property and battery characteristics are also good. Performance.

  In this production method, a monomer composition containing an ethylenically unsaturated carboxylic acid monomer is polymerized in the presence of a base compound, and an alkali compound is added to the polymer dispersion obtained in the polymerization step. After neutralizing the polymer (hereinafter also referred to as “process neutralization”), the solvent may be removed in a drying process. In addition, after obtaining the powder of the crosslinked polymer without performing the above-described process neutralization, an alkali compound is added when preparing the electrode mixture layer slurry to neutralize the polymer (hereinafter, May also be referred to as “sum”. Among the above, the process neutralization is preferable because the secondary aggregate tends to be easily broken.

<Composition for secondary battery electrode mixture layer>
The composition for a secondary battery electrode mixture layer of the present invention contains a binder, an active material, and water containing the crosslinked polymer or a salt thereof.
The usage-amount of the crosslinked polymer in the electrode mixture layer composition of this invention or its salt is 0.1 to 20 mass% with respect to the whole quantity of an active material, for example. The amount used is, for example, 0.2% by mass or more and 10% by mass or less, for example, 0.3% by mass or more and 8% by mass or less, and for example, 0.4% by mass or more and 5% by mass or less. . When the amount of the crosslinked polymer and its salt used 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 becomes insufficient, and the uniformity of the formed mixture layer may be lowered. On the other hand, when the usage-amount of a crosslinked polymer and its salt exceeds 20 mass%, an electrode mixture layer composition may become high viscosity and the coating property to a collector may fall. As a result, bumps and irregularities are generated in the obtained mixture layer, which may adversely affect the electrode characteristics.

  If the use 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. Further, the crosslinked polymer and the salt thereof have a sufficiently high binding property even in a small amount (for example, 5% by mass or less) with respect to the active material, and have a carboxy anion, so that the interface resistance is small, and the high rate property An excellent electrode can be obtained.

Among the 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. Examples of the spinel positive electrode active material include lithium manganate. In addition to oxides, phosphates, silicates, sulfur and the like are used, and examples of the phosphate 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, the dispersion exhibits alkalinity by exchanging lithium ions on the surface of the active material and hydrogen ions in water. For this reason, there exists a possibility that the aluminum foil (Al) etc. which are general collector materials for positive electrodes may be corroded. In such a case, it is preferable to neutralize the alkali content eluted from the active material by using an unneutralized or partially neutralized crosslinked polymer as a binder. The amount of unneutralized or partially neutralized crosslinked polymer used should be such that the amount of unneutralized carboxyl groups in the crosslinked polymer is equal to or greater than the amount of alkali eluted from the active material. Is preferred.

  Since any positive electrode active material has low electrical conductivity, it is common to add a conductive auxiliary agent. Examples of the conductive assistant include carbon-based materials such as carbon black, carbon nanotube, carbon fiber, graphite fine powder, and carbon fiber. Among these, carbon black, carbon nanotube, and carbon fiber are easy to obtain excellent conductivity. Are preferred. Moreover, as carbon black, ketjen black and acetylene black are preferable. As the conductive assistant, one of the above may be used alone, or two or more may be used in combination. The usage-amount of a conductive support agent can be 0.2-20 mass% with respect to the whole quantity of an active material from a viewpoint of making electroconductivity and energy density compatible, for example, 0.2-10 It can be made into the mass%. The positive electrode active material may be a surface coated with a conductive carbon-based material.

  On the other hand, examples of the negative electrode active material include carbon materials, lithium metals, lithium alloys, metal oxides, and the like, and one or more of them can be used in combination. Among these, active materials composed of carbon-based materials such as natural graphite, artificial graphite, hard carbon, and soft carbon (hereinafter, also referred to as “carbon-based active material”) are preferable, graphite such as natural graphite and artificial graphite, and Hard carbon is more preferable. In the case of graphite, spheroidized graphite is suitably used from the viewpoint of battery performance, and the preferable range of the particle size is, for example, 1 to 20 μm, and for example, 5 to 15 μm. In order to increase the energy density, a metal or metal oxide that can occlude lithium such as silicon or tin can be used as the negative electrode active material. Among them, silicon has a higher capacity than graphite, and an active material composed of silicon-based materials such as silicon, silicon alloys and silicon oxides such as silicon monoxide (SiO) (hereinafter 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 / discharge. For this reason, it is preferable to use together with the carbon-based active material. In this case, if the compounding amount of the silicon-based active material is large, the electrode material may be collapsed and the cycle characteristics (durability) may be greatly reduced. From such a point of view, when a silicon-based active material is used in combination, the amount used is, for example, 60% by mass or less, for example, 30% by mass or less with respect to the carbon-based active material.

  The binder containing the crosslinked polymer of the present invention has a structural unit (component (a)) derived from the 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 effective for improving the durability of the obtained electrode. It is considered a thing.

  Moreover, the crosslinked polymer of this invention has the structural unit ((b) component) derived from a nitrile group containing ethylenically unsaturated monomer. The component (b) can exert a strong interaction with the electrode material and can exhibit a good binding property 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 integrity.

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

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

  Moreover, when preparing the composition for electrode mixture layers in a wet powder state, the usage-amount of an active material is the range of 60-97 mass% with respect to the composition whole quantity, for example, is 70-90, for example. It is the range of mass%. Further, from the viewpoint of energy density, it is preferable that the non-volatile components other than the active material such as the binder and the conductive assistant are as small as possible within a range in which necessary binding properties and conductivity are ensured.

  The composition for a 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, water-soluble organic solvents such as tetrahydrofuran and N-methylpyrrolidone It is good also 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 electrode mixture layer composition is in a slurry state that can be applied, the content of the medium containing water in the entire composition is determined from the viewpoints of slurry coating properties, energy costs required for drying, and productivity. For example, it can be set as the range of 25-90 mass%, and can be set, for example as 35-70 mass%. Moreover, when it is set as the damp powder state which can be pressed, content of the said medium can be made into the range of 3-40 mass% from a viewpoint of the uniformity of the mixture layer after a press, for example, for example, 10 It can be made into the range of -30 mass%.

  The binder of the present invention may be composed only of the above-mentioned crosslinked polymer or a salt thereof, but in addition to this, other binders such as styrene / butadiene latex (SBR), acrylic latex, and polyvinylidene fluoride latex are used. A binder component may be used in combination. When other binder components are used in combination, the amount used can be, for example, 0.1-5% by mass or less, for example, 0.1-2% by mass or less, based on the active material. For example, it can be 0.1-1 mass% or less. When the amount of other binder components used exceeds 5% by mass, the resistance increases and the high rate characteristics may be insufficient. Among these, styrene / butadiene latex is preferable in terms 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 α-methylstyrene, vinyltoluene, divinylbenzene and the like in addition to styrene, and one or more of these can be used. The structural unit derived from the aromatic vinyl monomer in the copolymer can be, for example, in the range of 20 to 60% by mass mainly from the viewpoint of binding properties, and for example, 30 to 50%. It can be made into the range of the mass%.

  Examples of the aliphatic conjugated diene monomer include 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene 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 monomer in the copolymer is, for example, 30 to 70% by mass in that the binding property of the binder and the flexibility of the obtained electrode are good. For example, it may be in the range of 40 to 60% by mass.

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

  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. However, after dry blending the powder components such as the active material, the conductive auxiliary agent and the crosslinked polymer particles as a binder, A method of mixing and dispersing and kneading with a dispersion medium such as the above is preferable. When the composition for an electrode mixture layer is obtained in a slurry state, it is preferable to finish the slurry without any poor dispersion or aggregation. As a mixing means, known mixers such as a planetary mixer, a thin film swirl mixer, and a self-revolving mixer can be used, but a thin film swirl mixer is used because a good dispersion state can be obtained in a short time. It is preferable to carry out. Moreover, when using a thin film swirling mixer, it is preferable to perform preliminary dispersion with a stirrer such as a disper in advance. Moreover, the viscosity of the said slurry can be made into the range of 500-100,000 mPa * s, for example as B-type viscosity in 60 rpm, for example, may be set to the range of 1,000-50,000 mPa * s. it 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 density unevenness using a Henschel mixer, a blender, a planetary mixer, a biaxial kneader, or the like.

<Electrode for secondary battery>
The electrode for secondary batteries of the present invention comprises a mixture layer formed from the composition for electrode mixture layer on the surface of a current collector such as copper or aluminum. The mixture layer is formed by coating the surface of the current collector with the composition for electrode mixture layer of the present invention, and then drying and removing a medium such as water. The method for 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 adopted. be able to. Moreover, the said drying can be performed by well-known methods, such as hot air spraying, pressure reduction, (far) infrared rays, and microwave irradiation.
Usually, the mixture layer obtained after drying is subjected to a compression treatment by a die press, a roll press or the like. By compressing the active material and the binder, the strength of the mixture layer and the adhesiveness to the current collector can be improved. The thickness of the mixture layer can be adjusted, for example, to about 30 to 80% before compression, and the thickness of the mixture layer after compression is generally about 4 to 200 μm.

By providing the secondary battery electrode of the present invention with a separator and an electrolytic solution, a secondary battery can be produced. The electrolytic solution may be liquid or gelled.
The separator is disposed between the positive electrode and the negative electrode of the battery, and plays a role of ensuring ionic conductivity by preventing a short circuit due to contact between both electrodes and holding an electrolytic solution. The separator is preferably a film-like 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 electrolytic solution, a known one that is generally used according to the type of the active material can be used. In the lithium ion secondary battery, as specific solvents, cyclic carbonates having high dielectric constant and high electrolyte dissolving ability such as propylene carbonate and ethylene carbonate, and low viscosity chains such as ethyl methyl carbonate, dimethyl carbonate and diethyl carbonate are used. And the like, 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. In a nickel metal hydride secondary battery, an aqueous potassium hydroxide solution can be used as the electrolytic solution. A secondary battery is obtained by making a positive electrode plate and a negative electrode plate partitioned by a separator into a spiral or laminated structure and storing them in a case or the like.

  As described above, the secondary battery electrode binder disclosed in the present specification exhibits excellent binding properties with the electrode material and excellent adhesion with the current collector in the mixture layer. Secondary batteries equipped with electrodes obtained using the above binders are able to ensure good integrity and are expected to show good durability (cycle characteristics) even after repeated charge and discharge. Suitable for batteries and the like.

Hereinafter, the present invention will be specifically described based on examples. In addition, this invention is not limited by these Examples. In the following, “parts” and “%” mean mass parts and mass% unless otherwise specified.
In the following examples, evaluation of the crosslinked polymer (salt) was carried out 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 MT-3300EXII, manufactured by Microtrac Bell) using acetonitrile as a dispersion medium. The particle diameter was measured to determine the volume-based median diameter.

(2) Average particle size measurement in aqueous medium (water swollen 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 stirrer (Shinky Corp., Awatori Rentaro AR-250). Next, stirring (spinning speed 2000 rpm / revolution speed 800 rpm, 7 minutes) and defoaming (spinning speed 2200 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 with a laser diffraction / scattering particle size distribution meter (Microtrac MT-3300EXII, manufactured by Microtrac Bell) using ion-exchanged water as a dispersion medium. When an excessive amount of dispersion medium was circulated with respect to the hydrogel, an amount of the hydrogel capable of obtaining an appropriate scattered light intensity was added, and the particle size distribution shape measured after a few minutes was stabilized. As soon as stability was confirmed, volume-based particle size distribution measurement was performed 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). Moreover, the evaluation of monodispersibility was determined based on the following criteria.
≪Monodispersibility≫
◎: Particle size distribution value is less than 1.5 ○: Particle size distribution value is 1.5 or more and less than 3.0 Δ: Particle size distribution value is 3.0 or more and less than 7.0 ×: Particle size distribution The value is 7.0 or more

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

≪Production of crosslinked polymer salt≫
(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 introduction tube was used.
In the reactor, 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”), trimethylolpropane diallyl ether (Product name “Neoallyl T-20”, manufactured by Daiso Corporation) 0.90 parts and triethylamine corresponding to 1.0 mol% with respect to AA were charged. The reactor was sufficiently purged with nitrogen and then 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.) 0 was used as a polymerization initiator. When .040 parts was added, white turbidity 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 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 had elapsed from the polymerization start point. The internal temperature was maintained at 65 ° C., and cooling of the reaction liquid was started when 12 hours had elapsed from the polymerization start point. After the internal temperature dropped to 25 ° C., lithium hydroxide monohydrate (hereinafter referred to as “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 liquid in which particles of the crosslinked polymer salt R-1 (Li salt, neutralization degree 90 mol%) were dispersed in the medium.

The resulting polymerization reaction liquid was centrifuged to precipitate polymer particles, and then the supernatant was removed. Thereafter, after the sediment was redispersed in acetonitrile having the same weight as the polymerization reaction solution, the washing operation of sedimenting the polymer particles by centrifugation and removing the supernatant was repeated twice. The precipitate was collected, dried under reduced pressure conditions at 80 ° C. for 3 hours, and volatile components were removed to obtain a crosslinked polymer salt R-1 powder. Since the crosslinked polymer salt R-1 has hygroscopicity, it was hermetically 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. Was 90 mol% equal to the calculated value from Crosslinked polymer salt R-1 was hermetically stored in a container having a water vapor barrier property.
It was 0.52 micrometer when the particle diameter before swelling was measured about crosslinked polymer salt R-1 obtained above. Further, when the average particle size (water-swelling particle size) in the aqueous medium was measured, it was 1.7 μm, the particle size distribution was calculated to be 1.4, and the monodispersity was evaluated as “◎”.

(Production Examples 2 to 14: Production of crosslinked polymer salts R-2 to R-14)
Except having changed the preparation amount of each raw material as described in Table 1, operation similar to manufacture example 1 was performed, and the polymerization reaction liquid containing bridge | crosslinking polymer salt R-2 to R-14 was obtained.
Subsequently, operation similar to manufacture example 1 was performed about each polymerization reaction liquid, and powdery crosslinked polymer salt R-2 to R-14 was obtained. Each crosslinked polymer salt was hermetically stored in a container having a water vapor barrier property.
About each obtained polymer salt, the particle diameter before swelling and the average particle diameter in an aqueous medium were measured similarly to manufacture example 1, and the result was 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

Details of the compounds used in Table 1 are shown below.
AA: Acrylic acid AN: Acrylonitrile MAN: Methacrylonitrile ANCE: Cyanoethyl acrylate T-20: Trimethylolpropane diallyl ether (trade name “Neoallyl T-20” manufactured by Daiso Corporation)
P-30: Pentaerythritol triallyl ether (manufactured by Daiso, 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, graphite, which is an active material for the negative electrode, or silicon particles and graphite, and the composition for a mixture layer using each cross-linked polymer salt as a binder, its coating properties 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 the graphite, and (Sigma-Aldrich, Si nanopowder, particle diameter <100 nm) was used as the silicon particles.

Example 1
After weighing 3.2 parts of powdered crosslinked polymer Li salt R-1 into 100 parts of natural graphite and mixing well in advance, 160 parts of ion-exchanged water was added and preliminary dispersion was performed with a disper, and then a thin film swirl type By carrying out this dispersion for 15 seconds under the condition of a peripheral speed of 20 m / sec using a mixer (manufactured by Primics, FM-56-30), a slurry-like composition for a negative electrode mixture layer was obtained.
Using a variable applicator, the mixture layer composition is applied onto a 20 μm thick copper foil (manufactured by Nippon Foil Co., Ltd.) and dried in an air dryer at 100 ° C. for 15 minutes. A layer was formed. Thereafter, the mixture layer was rolled to have a thickness of 50 ± 5 μm and a packing density of 1.70 ± 0.20 g / cm ³ .

As a result of visually observing the appearance of the obtained mixture layer and evaluating the coatability based on the following criteria, it was judged as “◯”.
<Criteria for coating properties>
○: No abnormal appearance of streaks, irregularities, etc. on the surface.
Δ: Slight irregularities and irregularities such as irregularities are slightly observed on the surface.
X: Appearance abnormalities such as uneven stripes and irregularities are remarkably observed on the surface.

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

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 powdered cross-linked. After weighing 3.2 parts of polymer Li salt R-3 or R-13 and mixing well in advance, a mixture layer composition was prepared by performing the same operation as in Example 1. The coating property and 90 ° peel strength were evaluated for each mixture layer composition. The results are shown in Tables 2 and 3.

In each example, an electrode mixture layer composition containing a binder for a secondary battery electrode belonging to the present invention and an electrode produced using the same. The coating property of each mixture layer composition (slurry) is good, and the peel strength between the mixture layer of the obtained electrode and the current collector is high, and excellent binding properties are obtained. Was shown.
When attention was paid to the amount of the cyano group-containing ethylenically unsaturated monomer used, high binding properties and good coatability were exhibited in the range of 0.5 to 20 parts by mass. In particular, when the amount used was up to 10 parts by mass, results in which the peel strength (binding property) was improved as the amount used was increased (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. Moreover, since the usage-amount of the cyano group containing ethylenically unsaturated monomer exceeded the range prescribed | regulated by this invention in the comparative example 3, the result which peel strength falls significantly was obtained.

Since the binder for secondary battery electrodes of the present invention exhibits excellent binding properties in the mixture layer, the secondary battery equipped with the electrode obtained using the above binder has good durability (cycle characteristics). ) And is expected to be applied to in-vehicle secondary batteries. It is also useful for the use of active materials containing silicon, and is expected to contribute to higher battery capacity.
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 cross-linked polymer comprises 50% by mass or more and 99.9% by mass or less of a structural unit derived from an ethylenically unsaturated carboxylic acid monomer and a nitrile group-containing ethylenically unsaturated monomer with respect to the total structural unit. Containing 0.1% by mass or more and 20% by mass or less of structural units derived from
And the binder for secondary battery electrodes whose particle diameter measured in the aqueous medium after neutralizing to 80-100 mol% of a neutralization volume is 0.1 micrometer or more and 10 micrometers or less by volume reference | standard median diameter.   The cross-linked polymer is cross-linked with a cross-linkable monomer, and the amount of the cross-linkable monomer used is 0.1 to 2 parts by mass with respect to 100 parts by mass of the total amount of non-cross-linkable monomers. The binder for a secondary battery electrode according to claim 1, wherein the binder is 0.0 part by mass or less.   The binder for a secondary battery electrode according to claim 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. A method for producing a crosslinked polymer or a salt thereof used as a binder for a secondary battery electrode,
In the presence of an organic amine compound, it contains 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 20% by mass or less of a nitrile group-containing ethylenically unsaturated monomer. A method comprising a polymerization step of polymerizing a monomer component by a precipitation polymerization method.   The composition for secondary battery electrode mixture layers containing the binder of any one of Claims 1-3, an active material, and water.   The composition for a secondary battery electrode mixture layer according to claim 5, comprising a carbon-based material or a silicon-based material as the negative electrode active material.   The secondary battery electrode provided with the mixture layer formed from the composition for secondary battery electrode mixture layers of Claim 5 or 6 on the collector surface.