JP7151938B2 - Aqueous Resin Composition for Lithium Ion Secondary Battery Separator Heat-Resistant Layer Binder - Google Patents

Description

TECHNICAL FIELD The present invention relates to an aqueous resin composition for lithium ion secondary battery separator heat-resistant layer binder.

As a separator used for manufacturing a lithium ion secondary battery, a porous body obtained by using a polyolefin resin or the like is generally used in many cases. Lithium ion secondary batteries normally function as batteries when ions in the electrolyte move through the pores that form the separator.

On the other hand, as the output of the lithium ion secondary battery increases, there is a concern that the lithium ion secondary battery may cause ignition due to abnormal heat generation.

As a method for preventing the ignition, for example, when the lithium ion secondary battery generates heat, a method is known in which the microporosity of the separator can be rendered non-porous by the influence of heat. This is expected to stop the conduction of ions in the electrolytic solution and prevent further heat generation and ignition.

However, the separator shrinks significantly under the influence of heat, and as a result, the ion conduction in the electrolytic solution cannot be stopped, which may cause a short circuit in the lithium ion secondary battery. .

As a separator that does not easily cause heat shrinkage, there is known a separator in which a porous heat-resistant layer is provided on the surface of a porous body obtained using polyolefin resin or the like. A multilayer porous film having a porous layer containing inorganic particles and a resin binder on at least one side, wherein the resin binder is one or more monomers selected from (meth)acrylic acid ester monomers. , an unsaturated carboxylic acid monomer, and a crosslinkable monomer as raw material units.

However, although this material exhibits a certain effect as a heat-resistant layer, it has insufficient heat resistance in a high-temperature environment such as 180°C, so there is a possibility of short-circuiting the battery when abnormal heat is generated. .

Japanese Unexamined Patent Application Publication No. 2011-000832

The problem to be solved by the present invention is to provide an aqueous resin composition for a lithium ion secondary battery separator heat-resistant layer binder which is excellent in heat shrinkage resistance.

The inventors of the present invention have conducted intensive studies to solve the above problems, and found that the above problems can be solved by using an aqueous resin composition containing a specific radical polymer and an aqueous medium. completed.

That is, the present invention provides at least one monomer (a2 ), an unsaturated monomer having a carboxyl group (a3), and acrylonitrile (a4) as essential raw materials, a radical polymer (A), and an aqueous medium (B) Lithium ion secondary battery separator heat resistant Lithium ion secondary battery separator heat resistant, which is an aqueous resin composition for a layer binder, wherein the acrylonitrile (a4) in the monomer raw material of the radical polymer (A) is 5 to 30% by mass. The present invention relates to an aqueous resin composition for a layer binder.

The aqueous resin composition for a lithium ion secondary battery separator heat-resistant layer binder of the present invention can be suitably used as a binder for a lithium ion secondary battery separator heat-resistant layer because a separator having excellent heat shrinkage resistance can be obtained.

The aqueous resin composition for a lithium ion secondary battery separator heat-resistant layer binder of the present invention comprises an acrylic monomer (a1) having an alkyl group having 4 to 18 carbon atoms, diacetone (meth)acrylamide and N-methylol (meth). A radical polymer (A) comprising as essential raw materials at least one or more monomers (a2) selected from acrylamide, an unsaturated monomer (a3) having a carboxyl group, and acrylonitrile (a4), and an aqueous medium ( B), wherein the acrylonitrile (a4) in the monomer raw material of the radical polymer (A) is 5 to 30% by mass. There is something.

First, the radical polymer (A) will be described. The acrylic monomer (a1) is an acrylic monomer having an alkyl group having 4 to 18 carbon atoms, such as n-butyl (meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate, n-hexyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate and the like. . These acrylic monomers (a1) can be used alone or in combination of two or more. Among these, butyl (meth)acrylate and 2-ethylhexyl (meth)acrylate are preferably used.

In the present invention, "(meth)acrylate" refers to one or both of acrylate and methacrylate, "(meth)acrylamide" refers to one or both of acrylamide and methacrylamide, and "(meth)acrylamide" "Acid" refers to one or both of acrylic acid and methacrylic acid, and "(meth)acryloyl" refers to one or both of acryloyl and methacryloyl.

The monomer (a2) is at least one acrylic monomer selected from diacetone (meth)acrylamide and N-methylol (meth)acrylamide.

The unsaturated monomer (a3) is an unsaturated monomer having a carboxyl group, and examples thereof include unsaturated monocarboxylic acids such as (meth)acrylic acid and crotonic acid, maleic anhydride, maleic Unsaturated dicarboxylic acids such as itaconic acid, itaconic acid, and fumaric acid are included, but (meth)acrylic acid is preferred because it further improves heat shrinkage resistance. These monomers (a3) can be used alone or in combination of two or more.

The radical polymer (A) contains the acrylic monomer (a1), the monomer (a2), the unsaturated monomer (a3), and acrylonitrile (a4) as essential raw materials, but other than these You may use the other monomer (a5) as a monomer raw material.

Examples of the monomer (a5) include alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate and propyl (meth)acrylate; 2-hydroxyethyl (meth)acrylate and 3-hydroxypropyl (meth) acrylate, 4-hydroxy-n-butyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 2-hydroxy-n-butyl (meth) acrylate, 3-hydroxy-n-butyl (meth) acrylate, 1,4-cyclohexanedimethanol mono(meth)acrylate, N-(2-hydroxyethyl)(meth)acrylamide, glycerin mono(meth)acrylate, polyethylene glycol mono(meth)acrylate, polypropylene glycol mono(meth)acrylate, 2 -Hydroxy-3-phenoxypropyl (meth)acrylate, 2-(meth)acryloyloxyethyl-2-hydroxyethyl phthalate, monomers having hydroxyl groups such as lactone-modified (meth)acrylates having terminal hydroxyl groups; N, N -Dimethylaminoethyl (meth) acrylate, N,N-diethylaminoethyl (meth) acrylate, N,N-dimethylaminopropyl (meth) acrylate, N,N-diethylaminopropyl (meth) having an amino group (meth) ) Monomers having a nitrogen atom such as monomers having an N-hydroxymethylamide group such as acrylates and (meth)acrylamide, and monomers having an N-alkoxymethylamide group such as N-butoxymethylacrylamide; glycidyl (Meth)acrylates having a glycidyl group such as (meth)acrylate; vinyltrimethoxysilane, vinyltriethoxysilane, vinylmethyldimethoxysilane, 3-(meth)acryloyloxypropyltrimethoxysilane, 3-(meth)acryloyloxypropyl Monomers having an alkoxysilyl group such as triethoxysilane, 3-(meth)acryloyloxypropylmethyldimethoxysilane; polyethylene glycol (meth)acrylate, methoxypolyethyleneglycol (meth)acrylate, polypropyleneglycol (meth)acrylate, methoxypolypropylene Polyalkylene glycol (meth)acrylate such as glycol (meth)acrylate, polybutylene glycol (meth)acrylate, methoxypolybutylene glycol (meth)acrylate, etc. ) acrylate; vinyl monomers such as styrene, α-methylstyrene, paramethylstyrene, chloromethylstyrene, and vinyl acetate; tetrahydrofurfuryl (meth)acrylate, benzyl (meth)acrylate; ethylene glycol di(meth)acrylate, propylene and di(meth)acrylates such as glycol di(meth)acrylate. These monomers (a5) can be used alone or in combination of two or more.

The amount of the monomer (a1) in the monomer raw material of the radical polymer (A) is preferably 50 to 80% by mass, more preferably 55 to 75% by mass, because the substrate adhesion is further improved. .

The monomer (a2) in the monomer raw material of the radical polymer (A) is preferably 1 to 20% by mass, more preferably 3 to 20% by mass, because the heat shrinkage resistance is further improved. 5 to 15% by mass is more preferable.

The monomer (a3) in the monomer raw material of the radical polymer (A) is preferably 0.5 to 5% by mass, more preferably 0.5 to 3% by mass, because the heat shrinkage resistance is further improved. is more preferred.

The acrylonitrile (a4) in the monomer raw material of the radical polymer (A) is 5 to 35% by mass, but is preferably 10 to 35% by mass, more preferably 20 to 35%, because the heat shrinkage resistance is further improved. % by mass is more preferred.

As a method for producing the radical polymer (A), there are various methods, but an underwater polymerization method or an emulsion polymerization method is preferable because the radical polymer (A) can be obtained easily.

As a method of obtaining the radical polymer (A) by an emulsion polymerization method, for example, the monomer (a1), which is a raw material of the radical polymer (A), is added in an aqueous medium with an emulsifier and a polymerization initiator. A method of radical polymerization at a temperature of 50 to 100° C. in the presence of an agent may be mentioned.

Examples of the emulsifier include sulfuric acid esters of higher alcohols and salts thereof, alkylbenzene sulfonates, polyoxyethylene alkylphenyl sulfonates, polyoxyethylene alkyldiphenyl ether sulfonates, sulfuric acid half ester salts of polyoxyethylene alkyl ethers, Anionic emulsifiers such as alkyldiphenyl ether disulfonates and dialkyl succinate sulfonates; polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene diphenyl ethers, polyoxyethylene-polyoxypropylene block copolymers, nonionic emulsifiers such as acetylene diol; cationic emulsifiers such as alkylammonium salts; and amphoteric emulsifiers such as alkyl(amide)betaines and alkyldimethylamine oxides. These emulsifiers can be used alone or in combination of two or more. The monomer (a1) can also be used as an emulsifier.

Examples of the polymerization initiator include 2,2′-azobis(isobutyronitrile), 2,2′-azobis(2-methylbutyronitrile), azo compounds such as azobiscyanovaleric acid; tert-butyl peroxy organic peroxides such as pivalate, tert-butyl peroxybenzoate, tert-butyl peroxy-2-ethylhexanoate, di-tert-butyl peroxide, cumene hydroperoxide, benzoyl peroxide, tert-butyl hydroperoxide; Oxides; inorganic peroxides such as hydrogen peroxide, ammonium persulfate, potassium persulfate and sodium persulfate; These polymer initiators can be used alone or in combination of two or more. These polymerization initiators are preferably used within a range of 0.1 to 10% by weight with respect to the total amount of monomers that are raw materials for the polymer.

Since the dispersion stability of the radical polymer (A) is further improved, it is preferable to adjust the pH with a basic compound and/or an acidic compound. Examples of the basic compound include methylamine and dimethylamine. , trimethylamine, ethylamine, diethylamine, triethylamine, 2-aminoethanol, 2-dimethylaminoethanol and other organic amines; ammonia (water), sodium hydroxide, inorganic basic compounds such as potassium hydroxide; tetramethylammonium hydroxide, tetra quaternary ammonium hydroxides such as n-butylammonium hydroxide and trimethylbenzylammonium hydroxide; These basic compounds can be used alone or in combination of two or more.

Examples of the acidic compound include carboxylic acid compounds such as formic acid, acetic acid, propionic acid and lactic acid; phosphoric acid monoesters and diesters such as phosphoric acid monomethyl ester and phosphoric acid dimethyl ester; methanesulfonic acid, benzenesulfonic acid and dodecylbenzenesulfonic acid. organic sulfonic acid compounds such as; inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid; Among these, carboxylic acid compounds are preferred. These acidic compounds can be used alone or in combination of two or more.

Examples of the aqueous medium (B) include water, organic solvents miscible with water, and mixtures thereof. Examples of water-miscible organic solvents include alcohols such as methanol, ethanol, n-propanol and isopropanol; ketones such as acetone and methyl ethyl ketone; polyalkylene glycols such as ethylene glycol, diethylene glycol and propylene glycol; and alkyl ethers of polyalkylene glycols. and lactams such as N-methyl-2-pyrrolidone. In the present invention, only water may be used, a mixture of water and an organic solvent miscible with water may be used, or only an organic solvent miscible with water may be used. From the viewpoint of safety and environmental load, water alone or a mixture of water and an organic solvent miscible with water is preferred, and water alone is particularly preferred.

For the aqueous medium (B), it is convenient and preferable to use the aqueous medium as it is that is used when the polymer (A) is produced by an aqueous polymerization method or an emulsion polymerization method.

The aqueous resin composition of the present invention contains the radical polymer (A) and the aqueous medium (B). It is preferably dispersed in the medium (B).

Moreover, the organic solvent amount in the resin composition of this invention can be reduced by passing through a solvent removal process as needed.

The water-based resin composition of the present invention obtained by the above method has a further improved coating workability. is preferably contained, more preferably in the range of 10 to 50% by mass.

Further, since the aqueous resin composition of the present invention further improves the coating workability, those containing the aqueous medium (B) in the range of 95 to 40% by mass with respect to the total amount of the aqueous resin composition More preferably, it is contained in the range of 90 to 50% by mass.

The aqueous resin composition of the present invention may optionally contain a curing agent, a curing catalyst, a lubricant, a filler, a thixotropic agent, a tackifier, a wax, a heat stabilizer, a light stabilizer, a fluorescent brightener, and a foaming agent. additives such as agents, pH adjusters, leveling agents, anti-gelling agents, dispersion stabilizers, antioxidants, radical scavengers, heat resistance imparting agents, inorganic fillers, organic fillers, plasticizers, reinforcing agents, catalysts , antibacterial agents, antifungal agents, rust inhibitors, thermoplastic resins, thermosetting resins, pigments, dyes, conductivity imparting agents, antistatic agents, moisture permeability improvers, water repellent agents, oil repellent agents, hollow foams, Crystalline water-containing compound, flame retardant, water absorbing agent, moisture absorbent, deodorant, foam stabilizer, antifoaming agent, antifungal agent, antiseptic agent, anti-algae agent, pigment dispersant, anti-blocking agent, anti-hydrolysis agent, A pigment can be used in combination.

When using diacetone (meth)acrylamide as the monomer (a2), it is preferable to use a dihydrazide compound together.

The aqueous resin composition of the present invention can be suitably used as a binder for a heat-resistant layer of a lithium-ion secondary battery, since a separator having a small heat shrinkage can be obtained.

By adding an inorganic filler to the aqueous resin composition of the present invention, a heat-resistant layer having excellent heat resistance can be obtained.

Examples of the inorganic filler include oxides such as silica, alumina, titania, zirconia, magnesia, zinc oxide and iron oxide, nitrides such as silicon nitride, titanium nitride and boron nitride, silicon carbide, calcium carbonate, and magnesium sulfate. , aluminum sulfate, aluminum hydroxide, aluminum hydroxide oxide, potassium titanate, talc, kaolinite, dakite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, silicic acid Calcium, magnesium silicate, diatomaceous earth, silica sand, calcined kaolin, and the like can be used. Among these, it is preferable to use calcined kaolin or alumina in order to form a heat-resistant layer with even more excellent heat resistance.

A separator having communicating pores capable of conducting ions can be used in a range where the solid content mass ratio between the inorganic filler and the water-based resin composition of the present invention is 1/1,000 to 1/5. and forming a separator excellent in heat resistance.

The present invention will be described in more detail below with specific examples.

(Example 1: Synthesis and evaluation of aqueous resin composition (1) for lithium ion secondary battery separator heat-resistant layer binder)
Into a 1.0 L reaction vessel equipped with a stirrer, a thermometer, a cooler, and a nitrogen blower, 112.0 parts by mass of ion-exchanged water was charged and heated to 60° C., followed by adding 1.0 parts by mass of acrylic acid, A mixture of 5.0 parts by mass of N-methylolacrylamide, 60.0 parts by mass of n-butyl acrylate, and 34.0 parts by mass of acrylonitrile was added to an emulsifier (Daiichi Kogyo Seiyaku "Hitenol N-08", an anionic emulsifier). 5 parts by mass of the mixture was emulsified with an emulsifier aqueous solution prepared by dissolving 5 parts by mass of the mixture in 35.5 parts by mass of deionized water to obtain a monomer premix. An aqueous solution prepared by dissolving this monomer premix and 0.4 parts by mass of ammonium persulfate in 35 parts by mass of ion-exchanged water was added dropwise for 3 hours to carry out a reaction. After completion of the reaction, the mixture was held at the same temperature for 2 hours and then cooled.
After cooling, ion-exchanged water and 12.5% by mass ammonia aqueous solution were used to adjust the non-volatile content to 39.8% by mass and pH to 3.8. The viscosity at this time was 185 mPa·s. The viscosity is a value measured with a BM type viscometer (25° C., φ No. 2, 60 rpm).

[Preparation of heat-resistant layer slurry]
100 parts by mass of alumina ("AKP-50" by Sumitomo Chemical Co., Ltd.) was added to 100 parts by mass of 1% by mass carboxymethyl cellulose ("DN-800H" by Daicel Chemical Co., Ltd.) while stirring at 5,000 rpm with a homodisper. Gradually added and dispersed. After these are uniformly mixed, 20.0 parts by mass of the aqueous resin composition (1) synthesized above and 305.0 parts by mass of ion-exchanged water are added and uniformly mixed to prepare heat-resistant layer slurry (1). did.

[Preparation of separator having heat-resistant layer]
The heat-resistant layer slurry (1) obtained above was coated on both sides of a polyethylene separator base material having a thickness of 12 μm with a bar coater so that the film thickness after drying was 4 μm to obtain a separator (1). The drying temperature was 60° C. and the drying time was 5 minutes.

[Evaluation of heat shrinkage resistance]
The separator having the heat-resistant layer prepared above was cut into 5 cm squares, sandwiched between cardboard sheets, and allowed to stand in a hot air dryer at 180° C. for 1 hour to conduct a heat resistance test. The vertical and horizontal lengths of the separator after the test are measured, and the shrinkage rate in the vertical direction (MD shrinkage rate (%)) and the shrinkage rate in the horizontal direction (TD shrinkage rate (%)) are calculated. evaluated.
Shrinkage rate (%) = length of separator after test/length of separator before test x 100 (%)

(Example 2: Synthesis and evaluation of aqueous resin composition (2) for lithium ion secondary battery separator heat-resistant layer binder)
Into a 1.0 L reaction vessel equipped with a stirrer, a thermometer, a cooler, and a nitrogen blower, 112.0 parts by mass of ion-exchanged water was charged and heated to 60° C., followed by adding 3.0 parts by mass of acrylic acid, A mixture of 10.0 parts by mass of N-methylolacrylamide, 60.0 parts by mass of n-butyl acrylate, and 27.0 parts by mass of acrylonitrile was added to an emulsifier (Daiichi Kogyo Seiyaku "Hitenol N-08", an anionic emulsifier). 5 parts by mass of the mixture was emulsified with an emulsifier aqueous solution prepared by dissolving 5 parts by mass of the mixture in 35.5 parts by mass of deionized water to obtain a monomer premix. An aqueous solution prepared by dissolving this monomer premix and 0.4 parts by mass of ammonium persulfate in 35 parts by mass of ion-exchanged water was added dropwise for 3 hours to carry out a reaction. After completion of the reaction, the mixture was held at the same temperature for 2 hours and then cooled.
After cooling, ion-exchanged water and 12.5% by mass ammonia aqueous solution were used to adjust the non-volatile content to 39.9% by mass and pH to 3.9. The viscosity at this time was 315 mPa·s.

Example 1 except that the aqueous resin composition for lithium ion secondary battery separator heat-resistant layer binder (1) used in Example 1 was changed to lithium ion secondary battery separator heat-resistant layer binder aqueous resin composition (2) A heat-resistant layer slurry (2) and a separator (2) were prepared by operating in the same manner as in , and the heat shrinkage resistance was evaluated.

(Example 3: Synthesis and evaluation of aqueous resin composition (3) for lithium ion secondary battery separator heat-resistant layer binder)
Into a 1.0 L reaction vessel equipped with a stirrer, a thermometer, a cooler, and a nitrogen blower, 112.0 parts by mass of ion-exchanged water was charged and heated to 60° C., followed by adding 3.0 parts by mass of acrylic acid, A mixture of 10.0 parts by mass of diacetone acrylamide, 60.0 parts by mass of n-butyl acrylate, and 27.0 parts by mass of acrylonitrile was mixed with an emulsifier (Daiichi Kogyo Seiyaku "Hitenol N-08", an anionic emulsifier) 1.5. A part by mass of the mixture was dissolved in 35.5 parts by mass of ion-exchanged water and emulsified with an emulsifier aqueous solution to obtain a monomer premix. An aqueous solution prepared by dissolving this monomer premix and 0.4 parts by mass of ammonium persulfate in 35 parts by mass of ion-exchanged water was added dropwise for 3 hours to carry out a reaction. After completion of the reaction, the mixture was held at the same temperature for 2 hours and then cooled.
After cooling, 5.0 parts by mass of adipic acid dihydrazide was added and dissolved, and the non-volatile content was adjusted to 40.1% by mass and pH to 4.1 using deionized water and 12.5% by mass aqueous ammonia solution. The viscosity at this time was 128 mPa·s.

Example 1 except that the aqueous resin composition for lithium ion secondary battery separator heat-resistant layer binder (1) used in Example 1 was changed to lithium ion secondary battery separator heat-resistant layer binder aqueous resin composition (3) A heat-resistant layer slurry (3) and a separator (3) were prepared by operating in the same manner as in , and the heat shrinkage resistance was evaluated.

(Comparative Example 1: Synthesis and evaluation of aqueous resin composition (R1) for lithium ion secondary battery separator heat-resistant layer binder)
Into a 1.0 L reactor equipped with a stirrer, a thermometer, a cooler, and a nitrogen blower, 112.0 parts by mass of ion-exchanged water was charged and heated to 60° C., then 2.0 parts by mass of methacrylic acid, A mixture of 1.6 parts by mass of acrylamide, 92.8 parts by mass of n-butyl acrylate, and 2.0 parts by mass of acrylonitrile was added to 1.5 parts by mass of an emulsifier (Daiichi Kogyo Seiyaku "Hitenol N-08", an anionic emulsifier). was dissolved in 35.5 parts by mass of ion-exchanged water and emulsified with an aqueous emulsifier solution to obtain a monomer premix. An aqueous solution prepared by dissolving this monomer premix and 0.4 parts by mass of ammonium persulfate in 35 parts by mass of ion-exchanged water was added dropwise for 3 hours to carry out a reaction. After completion of the reaction, the mixture was held at the same temperature for 2 hours and then cooled.
After cooling, ion-exchanged water and a 5% by mass sodium hydroxide aqueous solution were used to adjust the non-volatile content to 39.9% by mass and pH to 6.0. The viscosity at this time was 35 mPa·s.

Example 1 except that the aqueous resin composition (1) for the lithium ion secondary battery separator heat-resistant layer binder used in Example 1 was changed to the lithium ion secondary battery separator heat-resistant layer binder aqueous resin composition (R1). A heat-resistant layer slurry (R1) and a separator (R1) were prepared by operating in the same manner as in , and heat shrinkage resistance was evaluated.

Table 1 shows the evaluation results of Examples 1 to 3 and Comparative Example 1 described above.

It was confirmed that the heat-resistant layers obtained from the water-based resin compositions of Examples 1 to 3 are excellent in heat shrinkage resistance.

On the other hand, Comparative Example 1 is an example in which the acrylonitrile (a4) in the monomer raw material is less than 5% by mass, which is the lower limit of the present invention, but it was confirmed that the heat shrinkage resistance of the obtained heat-resistant layer was inferior. rice field.

Claims (2)

acrylic monomer (a1) having an alkyl group having 4 to 18 carbon atoms, at least one monomer (a2) selected from diacetone (meth)acrylamide and N-methylol (meth)acrylamide, having a carboxyl group An aqueous resin composition for a lithium ion secondary battery separator heat-resistant layer binder containing a radical polymer (A) comprising an unsaturated monomer (a3) and acrylonitrile (a4) as essential raw materials, and an aqueous medium (B) wherein the monomer (a2) in the monomer raw material of the radical polymer (A) is 3 to 20 % by mass, and the acrylonitrile (a4) is 5 to 35% by mass. An aqueous resin composition for lithium ion secondary battery separator heat-resistant layer binder. In the radical polymer (A), the acrylic monomer (a1)-derived component is 50 to 85% by mass, and the unsaturated monomer (a3)-derived component is 0.5 to 5% by mass. 2. The water-based resin composition for a lithium ion secondary battery separator heat-resistant layer binder according to claim 1.