WO2021100582A1

WO2021100582A1 - Secondary battery electrode compound layer composition, secondary battery electrode, and secondary battery

Info

Publication number: WO2021100582A1
Application number: PCT/JP2020/042171
Authority: WO
Inventors: 綾乃日笠山; 朋子仲野; 直彦斎藤
Original assignee: 東亞合成株式会社
Priority date: 2019-11-20
Filing date: 2020-11-12
Publication date: 2021-05-27
Also published as: JPWO2021100582A1

Abstract

The present invention provides a secondary battery electrode compound layer composition that is capable of yielding a secondary battery that exhibits superior cycle properties while ensuring ease of application through reduced electrode slurry viscosity at a higher solids concentration on the part of the electrode compound layer composition than in the prior art. The present invention also provides a secondary battery electrode and a secondary battery obtained by using the composition.　The present invention pertains to a secondary battery electrode compound layer composition containing a binder, an active material, and water, the solids concentration of the composition being 40% by mass or higher, wherein: the binder contains a cross-linked polymer comprising a carboxyl group, and a non-cross-linked polymer comprising a structural unit derived from an ethylenically unsaturated carboxylic acid monomer; and the amount used of the non-cross-linked polymer is 7.5 parts by mass to 200 parts by mass, inclusive, per 100 total parts by mass of the cross-linked polymer.

Description

Composition for secondary battery electrode mixture layer, secondary battery electrode and secondary battery

The present invention relates to a composition for a secondary battery electrode mixture layer, a secondary battery electrode, and a secondary battery.

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

As the applications of various secondary batteries have expanded, the demand for improved energy density, reliability and durability tends to increase. For example, for the purpose of increasing the electric capacity of a lithium ion secondary battery, specifications for using a silicon-based active material as a negative electrode active material are increasing. However, it is known that the volume of a silicon-based active material changes significantly during charging and discharging, and the electrode mixture layer peels off or falls off as it is used repeatedly, resulting in a decrease in battery capacity and cycle characteristics. There was a problem that (durability) deteriorated. In order to suppress such a defect, it is generally effective to firmly bond the active materials with a binder (bonding property), and for the purpose of improving durability, the binding property of the binder is effective. Consideration is being made on improvement.

Under such circumstances, it has recently been reported that an acrylic acid-based polymer is effective as a binder used in a negative electrode mixture layer using a silicon-based active material. For example, in Patent Document 1, by using a polymer obtained by cross-linking polyacrylic acid with a specific cross-linking agent as a binder, the electrode structure is not destroyed even when an active material containing silicon is used. It is disclosed that it can be provided. Patent Document 2 discloses a binder containing a crosslinked polycarboxylic acid salt and a linear polycarboxylic acid, whereby an active material having high durability against internal stress during volume expansion and contraction during charge and discharge. It is disclosed that layers are obtained.

International Publication No. 2014/060407 International Publication No. 2016/0518111

The binders disclosed in Patent Documents 1 and 2 can impart good binding properties, but as the performance of the secondary battery is improved, an electrode mixture layer having high cycle characteristics is required. It has become.
Further, the secondary battery electrode is generally obtained by applying a composition for an electrode mixture layer containing an active material and a binder (hereinafter, also referred to as “electrode slurry”) to the surface of an electrode current collector and drying it. At this time, from the viewpoint of increasing the drying efficiency of the electrode slurry and improving the productivity of the electrode, it is advantageous to increase the solid content concentration of the composition for the electrode mixture layer, but to ensure good coatability. It becomes difficult.
Under such circumstances, the composition for the electrode mixture layer described in Patent Document 1 enhances the binding property by increasing the spread in water by microcrosslinking the acrylic acid-based polymer used as a binder. However, even a small amount of addition greatly increases the viscosity. Therefore, it has been difficult to reduce the viscosity of the electrode slurry in a state where the solid content concentration of the composition for the electrode mixture layer is increased.
Further, the composition for an electrode mixture layer described in Patent Document 2 is an electrode obtained by using a crosslinked polycarboxylic acid salt and an unneutralized linear polycarboxylic acid in combination as a binder. Although the cycle characteristics (durability) of the secondary battery provided with the above are improved, the amount of the binder added to the active material is large (Example: 100 parts by mass of the total amount of the active material, the total amount of the binder 22 Mass part). Therefore, in order to reduce the viscosity of the electrode slurry, it is necessary to increase the blending amount of water, and the solid content concentration (Example: 10% by mass) of the composition for the electrode mixture layer is low and the coatability is good. However, the productivity of the electrodes was low.

The present invention has been made in view of the above circumstances, and an object of the present invention is to secure coatability by reducing the viscosity of the electrode slurry when the solid content concentration of the composition for the electrode mixture layer is higher than before. At the same time, it is an object of the present invention to provide a composition for a secondary battery electrode mixture layer capable of obtaining a secondary battery exhibiting excellent cycle characteristics. In addition, a secondary battery electrode and a secondary battery obtained by using the above composition are also provided.

As a result of diligent studies to solve the above problems, the present inventors have identified the binder when the solid content concentration of the composition for the secondary battery electrode mixture layer containing the binder, the active material and water is high. By containing a crosslinked polymer and a specific non-crosslinked polymer and setting the amount of the non-crosslinked polymer used within a specific range with respect to 100 parts by mass of the total amount of the crosslinked polymer, the electrode slurry is coated by reducing the viscosity. We have found that it is possible to obtain a secondary battery that exhibits excellent cycle characteristics while ensuring workability, and completed the present invention.

The present invention is as follows.
[1] A composition for a secondary battery electrode mixture layer containing a binder, an active material, and water.
The solid content concentration of the composition is 40% by mass or more, and the solid content concentration is 40% by mass or more.
The binder contains a crosslinked polymer having a carboxyl group and a non-crosslinked polymer having a structural unit derived from an ethylenically unsaturated carboxylic acid monomer.
The amount of the non-crosslinked polymer used is 7.5 parts by mass or more and 200 parts by mass or less with respect to 100 parts by mass of the total amount of the crosslinked polymer, which is a composition for a secondary battery electrode mixture layer.
[2] The crosslinked polymer has 50% by mass or more and 100% by mass or less of structural units derived from an ethylenically unsaturated carboxylic acid monomer with respect to all the structural units.
The secondary battery electrode combination according to [1], wherein the non-crosslinked polymer has 50% by mass or more and 100% by mass or less of structural units derived from an ethylenically unsaturated carboxylic acid monomer with respect to all the structural units. Composition for agent layer.
[3] The composition for a secondary battery electrode mixture layer according to [1] or [2], wherein the degree of neutralization of the crosslinked polymer is 70 mol% or more.
[4] The crosslinked polymer was obtained by using a crosslinkable monomer, and the amount of the crosslinked monomer used was 0 with respect to 100 parts by mass of the total amount of the non-crosslinkable monomer. The composition for a secondary battery electrode mixture layer according to any one of [1] to [3], which is 0.05 parts by mass or more and 5.0 parts by mass or less.
[5] The composition for a secondary battery electrode mixture layer according to any one of [1] to [4], wherein the degree of neutralization of the non-crosslinked polymer is 70 mol% or more.
[6] The composition for a secondary battery electrode mixture layer according to any one of [1] to [5], wherein the non-crosslinked polymer has a weight average molecular weight of 5,000 or more.
[7] The composition for a secondary electrode mixture layer according to any one of [1] to [6], wherein the binder further contains a styrene-butadiene rubber (SBR) -based latex and / or carboxymethyl cellulose (CMC). Stuff.
[8] A secondary battery electrode provided with a mixture layer formed from the composition for the secondary battery electrode mixture layer according to any one of [1] to [7] on the surface of a current collector.
[9] A secondary battery comprising the secondary battery electrode according to [8].

According to the composition for the secondary battery electrode mixture layer of the present invention, when the solid content concentration of the composition for the electrode mixture layer is higher than before, the coatability is ensured by reducing the viscosity of the electrode slurry. It is possible to obtain a secondary battery that exhibits excellent cycle characteristics.

The composition for the secondary battery electrode mixture layer of the present invention (hereinafter, also referred to as "this composition") is in a slurry state that can be applied to a 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 present composition will be described together with the elements constituting the composition. Further, the secondary battery electrode obtained by using this composition will be described in detail.
In the present specification, "(meth) acrylic" means acrylic and / or methacrylic, and "(meth) acrylate" means acrylate and / or methacrylate. Further, the “(meth) acryloyl group” means an acryloyl group and / or a methacryloyl group.

This composition contains a binder, an active material and water, and the solid content concentration of the composition is 40% by mass or more. The binder also contains a crosslinked polymer having a carboxyl group and a non-crosslinked polymer having a structural unit derived from an ethylenically unsaturated carboxylic acid monomer.

1. 1. Crosslinked polymer with carboxyl group <Structural unit derived from ethylenically unsaturated carboxylic acid monomer>
The crosslinked polymer having a carboxyl group contained in the binder (hereinafter, also referred to as “the present crosslinked polymer”) is a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter, “component (a1)”). Also referred to as). 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 ionic conductivity are excellent, so that the resistance is small. An electrode having excellent high-rate characteristics can be obtained. Further, since water swelling property is imparted, the dispersion stability of the active material or the like in the composition for the electrode mixture layer can be enhanced.
The above component (a1) can be introduced into a polymer, for example, by polymerizing a monomer containing an ethylenically unsaturated carboxylic acid monomer. Alternatively, it can also be obtained by (co) polymerizing a (meth) acrylic acid ester monomer and then hydrolyzing it. Further, after polymerizing (meth) acrylamide, (meth) acrylonitrile or the like, it may be treated with a strong alkali, or it may be a method of reacting an acid anhydride with a polymer having a hydroxyl group.

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

The content of the component (a1) in the present crosslinked polymer is not particularly limited, but may be, for example, 10% by mass or more and 100% by mass or less with respect to all the structural units of the present crosslinked polymer. By containing the component (a1) in such a range, excellent adhesiveness to the current collector can be easily ensured. The lower limit is, for example, 20% by mass or more, for example, 30% by mass or more, and for example, 40% by mass or more. When the lower limit is 50% by mass or more, the dispersion stability of the present composition becomes good and a higher binding force can be obtained, which is preferable, and it may be 60% by mass or more, or 70% by mass or more. It may be 80% by mass or more. Further, the upper limit is, for example, 99.9% by mass or less, for example, 99.5% by mass or less, for example, 99% by mass or less, for example, 98% by mass or less, and for example, 95% by mass. It is less than or equal to, for example, 90% by mass or less, and for example, 80% by mass or less. The range may be a range in which such a lower limit and an upper limit are appropriately combined, and is, for example, 10% by mass or more and 100% by mass or less, and for example, 50% by mass or more and 100% by mass or less, and for example. It can be 50% by mass or more and 99.9% by mass or less, and can be, for example, 50% by mass or more and 99% by mass or less, and can be, for example, 50% by mass or more and 98% by mass or less.

<Other structural units>
The crosslinked polymer may contain, in addition to the component (a1), a structural unit derived from another ethylenically unsaturated monomer copolymerizable with the component (hereinafter, also referred to as “component (b1)”). it can. Examples of the component (b1) 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, a nonionic ethylenically unsaturated monomer, and the like. The structural unit from which it is derived can be mentioned. These structural units are an ethylenically unsaturated monomer compound having an anionic group other than a carboxyl group such as a sulfonic acid group and a phosphoric acid group, or a monomer containing a nonionic ethylenically unsaturated monomer. Can be introduced by copolymerizing.

The ratio of the component (b1) can be 0% by mass or more and 90% by mass or less with respect to all the structural units of the present crosslinked polymer. The ratio of the component (b1) may be 1% by mass or more and 60% by mass or less, 2% by mass or more and 50% by mass or less, and 5% by mass or more and 40% by mass or less. It may be 10% by mass or more and 30% by mass or less. Further, when the component (b1) 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, so that the effect of improving the lithium ion conductivity can be expected.

As the component (b1), among the above-mentioned components, a structural unit derived from a nonionic ethylenically unsaturated monomer is preferable from the viewpoint of obtaining an electrode having good bending resistance, and a nonionic ethylenically unsaturated monomer is preferable. Examples of the monomer include (meth) acrylamide and its derivatives, a nitrile group-containing ethylenically unsaturated monomer, an alicyclic structure-containing ethylenically unsaturated monomer, a hydroxyl group-containing ethylenically unsaturated monomer, and the like. ..

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

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

Examples of the alicyclic structure-containing ethylenically unsaturated monomer include cyclopentyl (meth) acrylate, cyclohexyl (meth) acrylate, methylcyclohexyl (meth) acrylate, t-butylcyclohexyl (meth) acrylate, and (meth). ) Cyclodecyl acrylate and cyclododecyl (meth) acrylate and other aliphatic substituents may have (meth) cycloalkyl acrylate; isobornyl (meth) acrylate, adamantyl (meth) acrylate, (meth). ) Dicyclopentenyl acrylate, dicyclopentenyloxyethyl (meth) acrylate, dicyclopentanyl (meth) acrylate, and cyclohexanedimethanol mono (meth) acrylate and cyclodecanedimethanol mono (meth) acrylate, etc. Cycloalkyl polyalcohol mono (meth) acrylate and the like can be mentioned, and one of these may be used alone, or two or more thereof may be used in combination.

Examples of the hydroxyl group-containing ethylenically unsaturated monomer include hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, and hydroxybutyl (meth) acrylate, and one of these is used alone. It may be used in combination, or two or more kinds may be used in combination.

The crosslinked polymer or a salt thereof has excellent binder binding properties, and is an amount of (meth) acrylamide and its derivatives, a nitrile group-containing ethylenically unsaturated monomer, and an alicyclic structure-containing ethylenically unsaturated monomer. It is preferable to include a structural unit derived from a body or the like. Further, when a structural unit derived from a hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less is introduced as the component (c), a strong interaction with the electrode material can be achieved. , Can exhibit good binding properties to active materials. As a result, a solid and well-integrated electrode mixture layer can be obtained. Therefore, the above-mentioned "hydrophobic ethylenically unsaturated monomer having a solubility in water of 1 g / 100 ml or less" is particularly selected. An alicyclic structure-containing ethylenically unsaturated monomer is preferable.

The crosslinked polymer or a salt thereof preferably contains a structural unit derived from a hydroxyl group-containing ethylenically unsaturated monomer from the viewpoint of improving the cycle characteristics of the obtained secondary battery, and the structural unit is 0.5. It is preferably contained in an amount of mass% or more and 70% by mass or less, more preferably 2.0% by mass or more and 50% by mass or less, and further preferably 10.0% by mass or more and 50% by mass or less.

Further, as the other nonionic ethylenically unsaturated monomer, for example, (meth) acrylic acid ester may be used. Examples of the (meth) acrylic acid ester include (meth) methyl acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, isobutyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, and the like. Meta) Acrylic acid alkyl ester compound;
Aromatic (meth) acrylic acid ester compounds such as (meth) phenyl acrylate, (meth) phenylmethyl acrylate, and (meth) phenylethyl acrylate;
Examples thereof include (meth) acrylic acid alkoxyalkyl ester compounds such as 2-methoxyethyl (meth) acrylic acid and 2-ethoxyethyl (meth) acrylic acid, and one of these may be used alone. Two or more types may be used in combination.

From the viewpoint of adhesion to the active material and cycle characteristics, an aromatic (meth) acrylic acid ester compound can be preferably used. From the viewpoint of further improving lithium ion conductivity and high-rate characteristics, compounds having an ether bond such as (meth) acrylic acid alkoxyalkyl ester such as 2-methoxyethyl (meth) acrylate and ethoxyethyl (meth) acrylate are preferable. , 2-Methoxyethyl (meth) acrylate is more preferred.

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

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

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

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

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

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

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

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

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

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

When the present crosslinked polymer is crosslinked with a crosslinkable monomer, the amount of the crosslinkable monomer used is the total amount of monomers other than the crosslinkable monomer (non-crosslinkable monomer). It is preferably 0.05 parts by mass or more and 5.0 parts by mass or less, more preferably 0.1 parts by mass or more and 5.0 parts by mass or less, and further preferably 0.2 parts by mass or more with respect to 100 parts by mass. It is 4.0 parts by mass or less, more preferably 0.3 parts by mass or more and 3.0 parts by mass or less. When the amount of the crosslinkable monomer used is 0.05 parts by mass or more, it is preferable in that the binding property and the stability of the mixture layer slurry are improved. If it is 5.0 parts by mass or less, the stability of the polymer tends to be high.
Similarly, the amount of the crosslinkable monomer used may be 0.02 to 1.7 mol% with respect to the total amount of the monomers other than the crosslinkable monomer (non-crosslinkable monomer). It is preferably 0.10 to 1.0 mol%, more preferably 0.10 to 1.0 mol%.

<Particle size of this crosslinked polymer>
In the present composition, the crosslinked polymer does not exist as a mass (secondary agglomerate) having a large particle size, but is well dispersed as water-swelled particles having an appropriate particle size. A binder containing the above is preferable because it can exhibit good binding performance.

In this crosslinked polymer, the particle size (water-swelling particle size) when a crosslinked polymer having a degree of neutralization based on a carboxyl group of 70 to 100 mol% is dispersed in water is a volume-based median diameter. It is preferably in the range of 0.1 μm or more and 10.0 μm or less. The more preferable range of the particle size is 0.1 μm or more and 8.0 μm or less, the more preferable range is 0.1 μm or more and 7.0 μm or less, and the more preferable range is 0.2 μm or more and 5.0 μm or less. Yes, and even more preferable ranges are 0.5 μm or more and 3.0 μm or less. When the particle size is in the range of 0.1 μm or more and 10.0 μm or less, the composition is uniformly present in a suitable size in the present composition, so that the present composition is highly stable and exhibits excellent binding properties. It becomes possible to do. If the particle size exceeds 10.0 μm, the binding property may be insufficient as described above. In addition, there is a risk that the coatability will be insufficient because it is difficult to obtain a smooth coated surface. On the other hand, when the particle size is less than 0.1 μm, there is concern from the viewpoint of stable manufacturability.

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

In the present crosslinked polymer, acid groups such as a carboxyl group derived from an ethylenically unsaturated carboxylic acid monomer are neutralized so that the degree of neutralization is 20 mol% or more in the present composition, and the mode of the salt is It is preferable to use as. The degree of neutralization is more preferably 50 mol% or more, further preferably 70 mol% or more, still more preferably 75 mol% or more, still more preferably 80 mol% or more, and particularly preferably. It is 85 mol% or more. The upper limit of the degree of neutralization is 100 mol%, and may be 98 mol% or 95 mol%. The range of the degree of neutralization may be appropriately combined with the above lower limit value and upper limit value, and may be, for example, 50 mol% or more and 100 mol% or less, or 75 mol% or more and 100 mol% or less. , 80 mol% or more and 100 mol% or less. When the degree of neutralization is 20 mol% or more, the water swelling property is good and the dispersion stabilizing effect is easily obtained, which is preferable. In the present specification, the degree of neutralization can be calculated by calculation from the charged values of a monomer having an acid group such as a carboxyl group and a neutralizing agent used for neutralization. The degree of neutralization was determined by measuring the IR of the crosslinked polymer or a salt thereof after drying the crosslinked polymer or a salt thereof at 80 ° C. for 3 hours under reduced pressure, and measuring the peak derived from the C = O group of the carboxylic acid and the C = of the carboxylate. It can be confirmed from the intensity ratio of the peak derived from the O group.

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

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

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

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

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

2. Non-crosslinked polymer having a structural unit derived from an ethylenically unsaturated carboxylic acid monomer This composition is derived from an ethylenically unsaturated carboxylic acid monomer as a binder in addition to a crosslinked polymer having a carboxyl group. Includes a non-crosslinked polymer having a structural unit (hereinafter, also referred to as “the present non-crosslinked polymer”). The present non-crosslinked polymer has a structural unit derived from an ethylenically unsaturated carboxylic acid monomer (hereinafter, also referred to as “component (a2)”), and the component (a2) and the method for introducing the component (a2) are the present crosslinked. It is the same as the content described in the component (a1) of the polymer.

The content of the component (a2) in the non-crosslinked polymer is not particularly limited, but is preferably 50% by mass with respect to the total structural units of the non-crosslinked polymer in terms of solubility in water. The above is 100% by mass or less, more preferably 60% by mass or more and 100% by mass or less, further preferably 70% by mass or more and 100% by mass or less, still more preferably 80% by mass or more and 100% by mass or less. It is as follows.

<Other structural units>
The non-crosslinked polymer contains, in addition to the component (a2), a structural unit derived from another ethylenically unsaturated monomer copolymerizable with the component (hereinafter, also referred to as “component (b2)”). Can be done. The component (b2) and the method for introducing the component (b2) are the same as those described in the component (b1) of the present crosslinked polymer.
The ratio of the component (b2) can be 0% by mass or more and 50% by mass or less with respect to all the structural units of the non-crosslinked polymer. The ratio of the component (b2) may be 1% by mass or more and 40% by mass or less, 2% by mass or more and 30% by mass or less, and 5% by mass or more and 20% by mass or less. May be good.

The non-crosslinked polymer may be in the form of a salt in which some or all of the carboxyl groups contained in the polymer are neutralized. The type of salt is not particularly limited, but alkali metal salts such as lithium, sodium and potassium; alkaline earth metal salts such as calcium salt and barium salt; other metal salts such as magnesium salt and aluminum salt; ammonium salt and organic. Examples include amine salts. Among these, alkali metal salts and magnesium salts are preferable, and alkali metal salts are more preferable, from the viewpoint that adverse effects on battery characteristics are unlikely to occur.

In the present non-crosslinked polymer, acid groups such as a carboxyl group derived from an ethylenically unsaturated carboxylic acid monomer are neutralized so that the degree of neutralization is 20 mol% or more in the present composition, and the salt is a salt. It is preferable to use it as an embodiment. The degree of neutralization is more preferably 50 mol% or more, further preferably 70 mol% or more, still more preferably 75 mol% or more, still more preferably 80 mol% or more, and particularly preferably. It is 85 mol% or more. The upper limit of the degree of neutralization is 100 mol%, and may be 98 mol% or 95 mol%. The range of the degree of neutralization may be appropriately combined with the above lower limit value and upper limit value, and may be, for example, 50 mol% or more and 100 mol% or less, or 75 mol% or more and 100 mol% or less. , 80 mol% or more and 100 mol% or less. When the degree of neutralization is 20 mol% or more, it is preferable because the solubility in water can be easily ensured. In the present specification, the degree of neutralization can be calculated by calculation from the charged values of a monomer having an acid group such as a carboxyl group and a neutralizing agent used for neutralization. The degree of neutralization was determined by measuring the IR of the crosslinked polymer or a salt thereof after drying the crosslinked polymer or a salt thereof at 80 ° C. for 3 hours under reduced pressure, and measuring the peak derived from the C = O group of the carboxylic acid and the C = of the carboxylate. It can be confirmed from the intensity ratio of the peak derived from the O group.

The weight average molecular weight (Mw) of the non-crosslinked polymer is not particularly limited, but is preferably 5,000 or more, more preferably 10 in terms of obtaining an electrode mixture layer having excellent binding properties. It is over 000. Mw may be 100,000 or more, 500,000 or more, or 1,000,000 or more. The upper limit of Mw is not particularly limited, but from the viewpoint of manufacturing handling, it may be, for example, 1,000,000 or less, and may be 5,000,000 or less.

The binder used in this composition contains the present crosslinked polymer and the present non-crosslinked polymer. The amount of the non-crosslinked polymer used is 7.5 parts by mass or more and 200 parts by mass or less with respect to 100 parts by mass of the total amount of the crosslinked polymer. The amount of the non-crosslinked polymer used may be 15 parts by mass or more, 25 parts by mass or more, 35 parts by mass or more, or 45 parts by mass or more. The upper limit may be 190 parts by mass or less, 180 parts by mass or less, 170 parts by mass or less, or 160 parts by mass or less. The range may be a range in which such a lower limit and an upper limit are appropriately combined, and is, for example, 15 parts by mass or more and 190 parts by mass or less, for example, 25 parts by mass or more and 180 parts by mass or less, and for example. It may be 35 parts by mass or more and 170 parts by mass or less, and may be, for example, 35 parts by mass or more and 160 parts by mass or less.

As described above, by using the present non-crosslinked polymer in combination with the present crosslinked polymer, the viscosity of the electrode slurry is higher than that in the conventional case when the solid content concentration of the composition for the electrode mixture layer is higher than before. It is possible to obtain a secondary battery that exhibits excellent cycle characteristics while ensuring coatability by reducing the amount. When the amount of the non-crosslinkable polymer used is 7.5 parts by mass or more, such an effect can be exhibited. Further, if the amount of the non-crosslinkable polymer used exceeds 200 parts by mass, sufficient coatability may not be obtained.

<Method for producing this non-crosslinked polymer>
As this non-crosslinked polymer, known polymerization methods such as solution polymerization, precipitation polymerization, suspension polymerization, and emulsion polymerization can be used, and may be appropriately selected depending on the molecular weight, composition, and the like.

As the polymerization initiator, known polymerization initiators such as azo compounds, organic peroxides, and inorganic peroxides can be used, but the polymerization initiator is not particularly limited. The conditions of use can be adjusted by known methods such as heat initiation, redox initiation with a reducing agent, and UV initiation so that the amount of radicals generated is appropriate.
Further, a known chain transfer agent may be used if necessary for the purpose of adjusting the molecular weight or the like.

3. 3. 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.
The amount of the binder used in the present composition is, for example, 0.1 part by mass or more and 20 parts by mass or less with respect to 100 parts by mass of the total amount of the active material. The amount used is, for example, 0.2 parts by mass or more and 10 parts by mass or less, for example, 0.3 parts by mass or more and 8 parts by mass or less, and for example, 0.4 parts by mass or more and 5 parts by mass or less. .. When the amount of the binder used is 0.1 parts by mass or more, sufficient binding property can be obtained. Further, the dispersion stability of the active material or the like can be ensured, and a uniform mixture layer can be formed. When the amount of the binder used is 20 parts by mass or less, the composition does not have a high viscosity, and the coatability to the current collector can be ensured. As a result, a mixture layer having a uniform and smooth surface can be formed.

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

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

Since all positive electrode active materials have low electrical conductivity, they are generally used with a conductive additive added. Examples of the conductive auxiliary agent include carbon-based materials such as carbon black, carbon nanotubes, carbon fibers, graphite fine powder, and carbon fibers. Among these, carbon black, carbon nanotubes, and carbon fibers are easy to obtain excellent conductivity. , Are preferred. Further, as the carbon black, Ketjen black and acetylene black are preferable. As the conductive auxiliary agent, one of the above types may be used alone, or two or more types may be used in combination. The amount of the conductive auxiliary agent used can be, for example, 0.2 to 20 parts by mass with respect to 100 parts by mass of the total amount of the active material from the viewpoint of achieving both conductivity and energy density, and for example, 0. It can be 2 to 10 parts by mass. Further, as the positive electrode active material, a material whose surface is coated with a conductive carbon-based material may be used.

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

Since the carbon-based active material itself has good electrical conductivity, it is not always necessary to add a conductive additive. When a conductive additive is added for the purpose of further reducing resistance, the amount used is, for example, 10 parts by mass or less with respect to 100 parts by mass of the total amount of the active material, and for example, 5 from the viewpoint of energy density. It is less than a part by mass.

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

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

When the composition is in a coatable slurry state, the content of the medium containing water in the entire composition is, for example, from the viewpoint of the coatability of the slurry, the energy cost required for drying, and the productivity. , 25-60% by mass, and can be, for example, 35-60% by mass.

The present composition may further contain other binder components such as styrene-butadiene rubber (SBR) -based latex, carboxymethyl cellulose (CMC), acrylic-based latex, and polyvinylidene fluoride-based latex. When other binder components are used in combination, the amount used may be, for example, 0.1 to 5 parts by mass or less, and for example, 0.1 to 2 parts by mass, based on 100 parts by mass of the total amount of the active material. It can be less than or equal to parts, and can be, for example, 0.1 to 1 part by mass or less. If the amount of the other binder component used exceeds 5% by mass, the resistance increases and the high rate characteristics may become insufficient. Among the above, SBR-based latex and CMC are preferable, and SBR-based latex and CMC are more preferable in combination because they are excellent in the balance between binding property and bending resistance.

The SBR latex is an aqueous dispersion of 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. Show the body. 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 70% by mass, and for example, 30 to 60, mainly from the viewpoint of binding property. It can be in the range of% by mass.
As the above-mentioned aliphatic conjugated diene-based monomer, in addition to 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3- Butadiene and the like can be mentioned, and one or more of these can be used. The structural unit derived from the aliphatic conjugated diene-based monomer in the copolymer is, for example, 30 to 70% by mass in that the binding property of the binder and the flexibility of the obtained electrode are good. It can be in the range of 40 to 60% by mass, for example.
In addition to the above-mentioned monomers, styrene / butadiene-based monomers include nitrile group-containing monomers such as (meth) acrylonitrile and (meth) as other monomers in order to further improve performance such as binding properties. ) A carboxyl group-containing monomer such as acrylic acid, itanconic acid, and maleic acid, and an ester group-containing monomer such as methyl (meth) acrylic acid may be used as the copolymerization monomer.
The structural unit derived from the other monomer in the copolymer can be, for example, in the range of 0 to 30% by mass, or can be, for example, in the range of 0 to 20% by mass.

The CMC refers to a substitute obtained by substituting a nonionic cellulosic semi-synthetic polymer compound with a carboxymethyl group and a salt thereof. Examples of the nonionic cellulose-based semi-synthetic polymer compound include alkyl celluloses such as methyl cellulose, methyl ethyl cellulose, ethyl cellulose, and microcrystallin cellulose;
Examples thereof include hydroxyethyl cellulose, hydroxybutyl methyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, hydroxypropyl methyl cellulose stearoxy ether, carboxymethyl hydroxyethyl cellulose, alkyl hydroxyethyl cellulose, hydroxyalkyl cellulose such as nonoxynyl hydroxyethyl cellulose and the like.

The composition for the secondary battery electrode mixture layer of the present invention contains the above-mentioned active material, water and a binder as essential constituents, and can be obtained by mixing the respective components using known means. The mixing method of each component is not particularly limited, and a known method can be adopted. However, powder components such as an active material, a conductive additive and a binder are dry-blended and then mixed with a dispersion medium such as water. However, the method of dispersion kneading is preferable. When the present composition is obtained in a slurry state, it is preferable to finish the composition into a slurry having no poor dispersion or aggregation. As the mixing means, a known mixer such as a planetary mixer, a thin film swirl mixer, or a self-revolving mixer can be used, but a thin film swirl mixer is used because a good dispersion state can be obtained in a short time. It is preferable to do this. When using a thin film swirl mixer, it is preferable to pre-disperse in advance with a stirrer such as a disper. The viscosity of the slurry is not particularly limited as long as the effect of the present invention is exhibited, but the B-type viscosity (25 ° C.) at 20 rpm can be, for example, in the range of 100 to 5,000 mPa · s, and for example. , 500 to 4,500 mPa · s, or, for example, the range of 1,000 to 3,000 mPa · s. When the viscosity of the slurry is within the above range, good coatability can be ensured.

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

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

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

As described above, the secondary battery provided with the electrode having the mixture layer formed from the composition for the secondary battery electrode mixture layer disclosed in the present specification is good even if charging and discharging are repeated. Since it exhibits durability (cycle characteristics), it is suitable for in-vehicle secondary batteries and the like.

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

(Measurement of viscosity of 2% by mass aqueous solution of this crosslinked polymer)
The crosslinked polymer having a carboxyl group obtained in each of the following production examples was dissolved in deionized water to prepare an aqueous solution having a concentration of 2% by mass. After adjusting the liquid temperature to 25 ° C. ± 1 ° C., the viscosity of the 2% by mass aqueous solution was measured with a B-type viscometer at 12 rpm.

(Measurement of viscosity of electrode slurry)
The electrode slurries obtained in each of the following Examples and Comparative Examples were adjusted to 25 ° C. ± 1 ° C., and then the slurry viscosity was measured with a B-type viscometer at 20 rpm.

≪Manufacturing of this crosslinked polymer salt≫
(Production Example 1: Production of Carboxylic Acid Group-Containing Crosslinked Polymer Salt R-1)
A reactor equipped with a stirring blade, a thermometer, a reflux condenser and a nitrogen introduction tube was used for the polymerization.
567 parts of acetonitrile, 2.20 parts of ion-exchanged water, 100.0 parts of acrylic acid (hereinafter referred to as "AA"), trimethylolpropane diallyl ether (manufactured by Daiso, trade name "Neoallyl T-20") in the reactor. Triethylamine corresponding to 0.90 parts and 1.0 mol% with respect to the above AA was charged. After sufficiently replacing the inside of the reactor with nitrogen, the inside temperature was raised to 55 ° C. by heating. After confirming that the internal temperature was stable at 55 ° C., 2,2'-azobis (2,4-dimethylvaleronitrile) (manufactured by Wako Pure Chemical Industries, Ltd., trade name "V-65") 0 as a polymerization initiator. When .040 parts were added, white turbidity was observed in the reaction solution, and this point was set as the polymerization initiation point. The polymerization reaction is continued while adjusting the external temperature (water bath temperature) to maintain the internal temperature at 55 ° C., and when 24 hours have passed from the polymerization initiation point, the reaction solution is cooled and the internal temperature reaches 25 ° C. after reduction, lithium hydroxide monohydrate (hereinafter, referred to as "LiOH · _{H 2} O") were added 52.4 parts of powder. After the addition, stirring was continued at room temperature for 12 hours to obtain a slurry-like polymerization reaction solution in which particles of the carboxyl group-containing crosslinked polymer salt R-1 (Li salt, neutralization degree 90 mol%) were dispersed in the medium.

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

(Production Examples 2 to 5: Production of carboxyl group-containing crosslinked polymer salts R-2 to R-5)
The same operation as in Production Example 1 was carried out except that the amounts of the monomer, the crosslinkable monomer, the ion-exchanged water and the neutralizing agent were as shown in Table 1, and the carboxyl group-containing crosslinked polymer salt R- A polymerization reaction solution containing 2 to R-5 was obtained.
Next, the same operations as in Production Example 1 were carried out for each polymerization reaction solution to obtain powdery carboxyl group-containing crosslinked polymer salts R-2 to R-5. Each carboxyl group-containing crosslinked polymer salt was sealed and stored in a container having a water vapor barrier property.

Details of the compounds used in Table 1 are shown below.
AA: Acrylic acid HEA: 2-Hydroxyethyl acrylate T-20: Trimethylolpropane diallyl ether (manufactured by Daiso, trade name "Neoallyl T-20")
TEA: Triethylamine AcN: Acetonitrile

<< Production of this non-crosslinked polymer salt >>
(Production Examples 6 to 9: Production of the present non-crosslinked polymer salts R-6 to R-9)
Details of the non-crosslinked polymers R-6 to R-9 used in Examples and Comparative Examples are shown below.
R-6 ~ R-8 is in the non-crosslinked polyacrylic acid following unneutralized, respectively, lithium hydroxide monohydrate (hereinafter, referred to as "LiOH · _{H 2} O") powder 100 mol% The mixture was added so as to be neutralized, and after the addition, stirring was continued for 12 hours at room temperature to prepare the mixture. Also, R-9, except that the addition of LiOH · _{H 2} O to neutralize 75 mol% was prepared in the same manner as the R-6 ~ R-8.
-R-6: Non-crosslinked polyacrylic acid Li neutralized salt. As the unneutralized non-crosslinked polyacrylic acid, the trade name "Julimer AC-10LHP" (manufactured by Toagosei Co., Ltd., Mw 1,500,000) was used.
-R-7: Non-crosslinked polyacrylic acid Li neutralized salt. As the unneutralized non-crosslinked polyacrylic acid, the trade name "Aron A-6801" (manufactured by Toagosei Co., Ltd., Mw100,000) was used.
-R-8: Non-crosslinked polyacrylic acid Li neutralized salt. As the unneutralized non-crosslinked polyacrylic acid, the trade name "Aron A-10SL" (manufactured by Toagosei Co., Ltd., Mw 6,000) was used.
-R-9: Non-crosslinked polyacrylic acid Li partially neutralized salt. As the unneutralized non-crosslinked polyacrylic acid, the trade name "Julimer AC-10LHP" (manufactured by Toagosei Co., Ltd., Mw 1,500,000) was used.
"Julimer" and "Aron" are registered trademarks of Toagosei Co., Ltd.

Example 1
<Preparation of composition for electrode mixture layer>
Prepare a SiOx (0.8 <x <1.2) surface coated with 10% carbon by the CVD method (hereinafter referred to as "Si-based active material"), and mix graphite and Si-based active material. Was used as an active material. As the binder, a mixture of the present crosslinked polymer salt R-1, the present non-crosslinked polymer salt R-6, styrene-butadiene rubber (SBR) -based latex, and carboxymethyl cellulose (CMC) was used.
Graphite: Si-based active material: R-1: R-6: SBR: CMC = 90: 10: 1 using water as a diluting solvent so that the solid content concentration of the composition for the electrode mixture layer is 50% by mass. .0: 0.5: 1.0: 1.0 (solid content) with a mass ratio of T.I. K. The mixture was mixed for 2 hours using a hibis mix to prepare a composition for an electrode mixture layer (electrode slurry) in a slurry state. The viscosity of the electrode slurry was 3,140 mPa · s, which was a sufficiently low value.
An electrode was prepared using the obtained electrode slurry and evaluated. The specific procedure and evaluation method are shown below.

<Manufacturing of negative electrode plate>
The electrode slurry was applied to both sides of a copper foil (thickness: 20 μm) and dried to form a mixture layer. Then, after rolling so that the thickness of the mixture layer was 27 μm and the packing density was 1.3 g / cm ³ , the negative electrode electrode plate was obtained by punching 3 cm square.

(Paintability)
The coatability of the electrode slurry in the production of the negative electrode electrode plate was evaluated based on the following criteria, and was evaluated as “◯”.
(Evaluation criteria)
◯: No abnormal appearance such as streaks or bumps is observed on the surface.
Δ: Slight abnormalities in appearance such as streaks and bumps are observed on the surface.
X: Appearance abnormalities such as streaks and bumps are noticeably observed on the surface.

<Manufacturing of positive electrode plate>
In N-methylpyrrolidone (NMP) solvent, 100 parts of lithium iron phosphate (LFP) as the positive electrode active material, 0.2 parts of carbon nanotubes as the conductive agent, 2 parts of Ketjen black, and vapor layer carbon fiber (VGCF). ) 0.6 part was mixed and added, and polyvinylidene fluoride (PVDF) was mixed as a binder for the electrode composition to prepare a composition for a positive electrode. A mixture layer was formed by applying the positive electrode composition to an aluminum current collector (thickness: 15 μm) and drying it. Then, after rolling so that the thickness of the mixture layer was 88 μm and the packing density was 3.1 g / cm ³ , the mixture was punched 3 cm square to obtain a positive electrode plate.

<Making secondary batteries>
Using the positive electrode plate, the negative electrode plate, and the separator, a lithium ion secondary battery of a laminated cell was produced. _{As the electrolytic solution, one in which LiPF 6} was dissolved at a concentration of 1.0 mol / liter in a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (DEC) at a volume ratio of 25:75 was used.

(Evaluation of cycle characteristics)
The lithium-ion secondary battery of the laminated cell produced above is charged / discharged at a charge / discharge rate of 0.2 C under the conditions of 2.7 to 3.4 V by CC discharge, and the initial capacity is C _0. Was measured. Furthermore, repeated charging and discharging under 25 ° C. environment was measured capacitance C ₅₀ after 50 cycles. The cycle characteristic (ΔC) calculated by the following formula was 93.0%, and the cycle characteristic based on the following criteria was evaluated as “◯”. The higher the value of ΔC, the better the cycle characteristics.
ΔC = C ₅₀ / C ₀ × 100 (%)
(Evaluation criteria)
⊚: Charge / discharge capacity retention rate is 95.0% or more 〇: Charge / discharge capacity retention rate is 90.0% or more and less than 95.0% Δ: Charge / discharge capacity retention rate is 85.0% or more and less than 90.0% × : Charge / discharge capacity retention rate is less than 85.0%

Examples 2 to 10 and Comparative Examples 1 to 3
An electrode slurry was prepared by performing the same operation as in Example 1 except that the present crosslinked polymer salt and the present non-crosslinked polymer salt were as shown in Table 2, and the slurry viscosity was measured. In addition, the coatability of the electrode slurry and the cycle characteristics of the secondary battery obtained by using the electrode slurry were evaluated. The results are shown in Table 2.

≪Evaluation result≫
As is clear from the results of Examples 1 to 10, all of the compositions for the secondary battery electrode mixture layer (electrode slurry) of the present invention have good coatability and use the composition. The cycle characteristics of the secondary battery equipped with the obtained electrodes were also excellent.
On the other hand, when the non-crosslinked polymer having a structural unit derived from the ethylenically unsaturated carboxylic acid monomer is not contained (Comparative Example 1), the amount of the non-crosslinked polymer used is the amount of the crosslinked polymer. When the total amount is less than 7.5 parts by mass with respect to 100 parts by mass (Comparative Example 2), the cycle characteristics are significantly inferior, and the amount of the non-crosslinked polymer used is 200 with respect to 100 parts by mass of the total amount of the crosslinked polymer. In the case of more than parts by mass (Comparative Example 3), the coatability was significantly inferior.

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

Claims

A composition for a secondary battery electrode mixture layer containing a binder, an active material, and water.
The solid content concentration of the composition is 40% by mass or more, and the solid content concentration is 40% by mass or more.
The binder contains a crosslinked polymer having a carboxyl group and a non-crosslinked polymer having a structural unit derived from an ethylenically unsaturated carboxylic acid monomer.
The amount of the non-crosslinked polymer used is 7.5 parts by mass or more and 200 parts by mass or less with respect to 100 parts by mass of the total amount of the crosslinked polymer, which is a composition for a secondary battery electrode mixture layer.
The crosslinked polymer has 50% by mass or more and 100% by mass or less of structural units derived from an ethylenically unsaturated carboxylic acid monomer with respect to all the structural units.
The secondary battery electrode combination according to claim 1, wherein the non-crosslinked polymer has 50% by mass or more and 100% by mass or less of structural units derived from an ethylenically unsaturated carboxylic acid monomer with respect to all the structural units. Composition for agent layer.
The composition for a secondary battery electrode mixture layer according to claim 1 or 2, wherein the degree of neutralization of the crosslinked polymer is 70 mol% or more.
The crosslinked polymer was obtained by using a crosslinkable monomer, and the amount of the crosslinked monomer used was 0.05 mass by mass with respect to 100 parts by mass of the total amount of the non-crosslinkable monomer. The composition for a secondary battery electrode mixture layer according to any one of claims 1 to 3, which is not more than parts and not more than 5.0 parts by mass.
The composition for a secondary battery electrode mixture layer according to any one of claims 1 to 4, wherein the degree of neutralization of the non-crosslinked polymer is 70 mol% or more.
The composition for a secondary battery electrode mixture layer according to any one of claims 1 to 5, wherein the non-crosslinked polymer has a weight average molecular weight of 5,000 or more.
The composition for a secondary electrode mixture layer according to any one of claims 1 to 6, wherein the binder further contains a styrene-butadiene rubber (SBR) -based latex and / or carboxymethyl cellulose (CMC).
A secondary battery electrode provided with a mixture layer formed from the composition for the secondary battery electrode mixture layer according to any one of claims 1 to 7 on the surface of the current collector.
A secondary battery comprising the secondary battery electrode according to claim 8.