JP7777606B2 - Coating material for secondary battery separator, secondary battery separator, and secondary battery - Google Patents

Description

The present invention relates to a coating material for secondary battery separators, a secondary battery separator, and a secondary battery. More specifically, the present invention relates to a coating material for secondary battery separators, a secondary battery separator having a coating film of the coating material for secondary battery separators, and a secondary battery having the secondary battery separator.

Traditionally, secondary batteries are equipped with a separator that separates the positive and negative electrodes and allows ions in the electrolyte to pass through.

An example of such a separator is a polyolefin porous membrane.

On the other hand, if the separator changes shape due to heat shrinkage, there is a possibility of a short circuit between the positive and negative electrodes. Therefore, separators are required to be heat resistant. To impart heat resistance to the separator, a heat-resistant coating layer may be provided on the surface of the separator. Such a heat-resistant coating layer is formed, for example, by applying a coating material for secondary battery separators to the surface of the separator and drying it.

As such a coating material for secondary battery separators, for example, a coating material for secondary battery separators containing an inorganic filler, a dispersant, and a coating material raw material for secondary battery separators containing a water-soluble polymer has been proposed (see, for example, Patent Document 1).

Japanese Patent Application Laid-Open No. 2021-103676

On the other hand, the separator of a secondary battery needs to allow ions to pass through in order to generate electricity. This means that the heat-resistant coating layer is required to further improve the separator's air permeability.

The present invention provides a coating material for secondary battery separators for producing separators with excellent gas permeability, a secondary battery separator having a coating film of the coating material for secondary battery separators, and a secondary battery having the secondary battery separator.

The present invention [1] is a coating material for secondary battery separators, comprising inorganic particles, a binder resin, and an associative thickener.

The present invention [2] includes the coating material for secondary battery separators described in [1] above, in which the associative thickener is an amphipathic compound having a hydrophobic group and a hydrophilic group, and the hydrophobic groups are located at both ends.

The present invention [3] includes a coating material for secondary battery separators described in [1] or [2] above, in which the binder resin includes a water-soluble polymer having a repeating unit derived from methacrylamide and a repeating unit derived from a carboxyl group-containing vinyl monomer.

The present invention [4] includes a coating material for a secondary battery separator described in any one of [1] to [3] above, in which the hydrophilic group includes a polyether.

The present invention [5] includes a coating material for a secondary battery separator described in any one of [1] to [4] above, in which the associative thickener has a urethane bond.

The present invention [6] includes a secondary battery separator comprising a porous membrane and a coating film of the coating material for secondary battery separators described in any one of [1] to [5] above, which is disposed on at least one side of the porous membrane.

The present invention [7] includes a secondary battery comprising a positive electrode, a negative electrode, and the secondary battery separator described in [6] above, disposed between the positive electrode and the negative electrode.

The coating material for secondary battery separators of the present invention contains an associative thickener. Therefore, when this coating material for secondary battery separators is applied to the surface of a porous membrane to form a coating film, the associative thickener associates in the coating film, forming a network structure as a whole. This network structure then captures the binder resin, preventing the binder resin from penetrating the pores of the porous membrane. As a result, a separator with excellent gas permeability can be manufactured.

The secondary battery separator of the present invention is provided with a coating film of the coating material for secondary battery separators of the present invention. Therefore, it has excellent air permeability.

The secondary battery of the present invention is equipped with the secondary battery separator of the present invention. Therefore, it has excellent gas permeability. As a result, it has excellent power generation efficiency.

1A and 1B are schematic diagrams of a linear associative thickener. Fig. 1A is an explanatory diagram illustrating the structure of a linear associative thickener. Fig. 1B is an explanatory diagram illustrating that linear associative thickeners associate to form a network structure as a whole. 2A and 2B are schematic diagrams of a branched associative thickener. Fig. 2A is an explanatory diagram illustrating the structure of a branched associative thickener. Fig. 2B is an explanatory diagram illustrating that the branched associative thickeners associate to form a network structure as a whole. 3A to 3C are explanatory diagrams illustrating air permeability. FIG. 3A is an explanatory diagram illustrating a decrease in the air permeability of a secondary battery separator. FIG. 3B is an explanatory diagram illustrating the air permeability of a secondary battery separator obtained using a coating material for a secondary battery separator containing a non-associative thickener. FIG. 3C is an explanatory diagram illustrating the air permeability of a secondary battery separator obtained using a coating material for a secondary battery separator containing an associative thickener.

The coating material for secondary battery separators contains inorganic particles, a binder resin, and an associative thickener.

<Inorganic particles>
Inorganic particles include, for example, oxides, nitrides, carbides, sulfates, hydroxides, and potassium titanate. Oxides include, for example, alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide. Nitrides include, for example, silicon nitride, titanium nitride, and boron nitride. Carbides include, for example, silicon carbide and calcium carbonate. Sulfates include, for example, magnesium sulfate and aluminum sulfate. Hydroxides include, for example, aluminum hydroxide and aluminum oxide hydroxide. Silicates include, for example, talc, kaolinite, dickite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amesite, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, silica sand, and glass.

Preferably, inorganic particles include oxides and hydroxides, more preferably alumina and aluminum oxide hydroxide.

The average particle diameter of the inorganic particles is, for example, 0.1 μm or more, preferably 0.5 μm or more, and, for example, 5 μm or less, preferably 1 μm or less.

The average particle size of inorganic particles can be measured by creating a particle size distribution curve using a laser diffraction/scattering particle size distribution measuring device and calculating the particle size equivalent to 50% by mass.

The blending ratio of inorganic particles is, for example, 50 parts by mass or more, preferably 70 parts by mass or more, more preferably 80 parts by mass or more, and even more preferably 85 parts by mass or more, and, for example, 95 parts by mass or less, per 100 parts by mass of the coating material for secondary battery separators.

Inorganic particles can be used alone or in combination of two or more types.

<Binder resin>
The binder resin includes a water-soluble polymer.

[Water-soluble polymer]
The water-soluble polymer is a polymer obtained by polymerizing a water-soluble polymer raw material.

A water-soluble polymer is defined as a polymer that has been dried, and when 1 g of the polymer is dissolved in 100 ml of water by stirring for 24 hours, and then filtered through a 300 mesh wire mesh, the residual solid content is 0.1% or less.

The water-soluble polymer raw materials include methacrylamide and a carboxyl group-containing vinyl monomer.

(methacrylamide)
Methacrylamide may be used in combination with acrylamide, or methacrylamide may be used alone without being used in combination with acrylamide.

Methacrylamide is preferably used alone, rather than in combination with acrylamide, in order to produce a separator with excellent heat resistance (described below).

In the water-soluble polymer raw material, the methacrylamide content is, for example, 60 parts by mass or more, preferably 70 parts by mass or more, more preferably 75 parts by mass or more, and for example, 98 parts by mass or less, preferably 96 parts by mass or less, more preferably 95 parts by mass or less, per 100 parts by mass of the total amount of the water-soluble polymer raw material.

(Carboxy group-containing vinyl monomer)
The carboxy group-containing vinyl monomer is a copolymerizable monomer that is copolymerizable with methacrylamide and contains a carboxy group.

Carboxy group-containing vinyl monomers include, for example, monocarboxylic acids, dicarboxylic acids, and salts thereof. Monocarboxylic acids include, for example, (meth)acrylic acid. Dicarboxylic acids include, for example, itaconic acid, maleic acid, fumaric acid, itaconic anhydride, maleic anhydride, and fumaric anhydride.

(Meth)acrylic includes acrylic and methacrylic (the same applies below).

If the water-soluble polymer raw material contains a carboxyl group-containing vinyl monomer, a separator (described below) with excellent air permeability can be produced.

Preferably, the carboxyl group-containing vinyl monomer is a monocarboxylic acid, more preferably (meth)acrylic acid, and even more preferably, methacrylic acid, from the viewpoint of producing a separator (described below) with even better air permeability.

In the water-soluble polymer raw material, the content of the carboxyl group-containing vinyl monomer is, for example, 1 part by mass or more, preferably 2 parts by mass or more, more preferably 3 parts by mass or more, and for example, 30 parts by mass or less, preferably 20 parts by mass or less, more preferably 15 parts by mass or less, per 100 parts by mass of the total amount of the water-soluble polymer raw material.

Carboxy group-containing vinyl monomers can be used alone or in combination of two or more types.

(Water-soluble copolymerizable monomer)
The water-soluble polymer raw material may also contain a copolymerizable monomer (hereinafter referred to as a water-soluble copolymerizable monomer) that is copolymerizable with methacrylamide and/or a carboxy group-containing vinyl monomer.

Examples of water-soluble copolymerizable monomers include (meth)acrylic acid alkyl esters, functional group-containing vinyl monomers (excluding carboxy group-containing vinyl monomers), vinyl esters, aromatic vinyl monomers, N-substituted unsaturated carboxylic acid amides, heterocyclic vinyl compounds, vinylidene halide compounds, α-olefins, dienes, and crosslinkable vinyl monomers.

Examples of (meth)acrylic acid alkyl esters include alkyl (meth)acrylates having an alkyl moiety with 1 to 12 carbon atoms. Examples of alkyl (meth)acrylates having an alkyl moiety with 1 to 12 carbon atoms include alkyl (meth)acrylates having an alkyl moiety with 1 to 4 carbon atoms and alkyl (meth)acrylates having an alkyl moiety with 5 to 12 carbon atoms. Examples of alkyl (meth)acrylates having an alkyl moiety with 1 to 4 carbon atoms include methyl (meth)acrylate, ethyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, and t-butyl (meth)acrylate. Examples of alkyl(meth)acrylates having an alkyl moiety with 5 to 12 carbon atoms include n-amyl(meth)acrylate, isoamyl(meth)acrylate, n-hexyl(meth)acrylate, 2-ethylhexyl(meth)acrylate, octyl(meth)acrylate, decyl(meth)acrylate, dodecyl(meth)acrylate, and octadecyl(meth)acrylate.

Examples of functional group-containing vinyl monomers (excluding carboxy group-containing vinyl monomers) include hydroxy group-containing vinyl monomers, amino group-containing vinyl monomers, glycidyl group-containing vinyl monomers, cyano group-containing vinyl monomers, sulfonic acid group-containing vinyl monomers and their salts, acetoacetoxy group-containing vinyl monomers, and phosphate group-containing compounds.

Examples of hydroxyl group-containing vinyl monomers include 2-hydroxyethyl (meth)acrylate and 2-hydroxypropyl (meth)acrylate.

Examples of amino group-containing vinyl monomers include 2-aminoethyl (meth)acrylate, 2-(N-methylamino)ethyl (meth)acrylate, and 2-(N,N-dimethylamino)ethyl (meth)acrylate.

Examples of glycidyl group-containing vinyl monomers include glycidyl (meth)acrylate.

Examples of cyano group-containing vinyl monomers include (meth)acrylonitrile.

Examples of sulfonic acid group-containing vinyl monomers include allyl sulfonic acid, methallyl sulfonic acid, and acrylamido t-butyl sulfonic acid. Furthermore, examples of salts of the sulfonic acid group-containing vinyl monomers include alkali metal salts (e.g., sodium salts, potassium salts), and ammonium salts. Specific examples include sodium allyl sulfonate, sodium methallyl sulfonate, and ammonium methallyl sulfonate.

Examples of acetoacetoxy group-containing vinyl monomers include acetoacetoxyethyl (meth)acrylate.

Examples of phosphate group-containing compounds include 2-methacryloyloxyethyl acid phosphate.

Examples of vinyl esters include vinyl acetate and vinyl propionate.

Aromatic vinyl monomers include, for example, styrene, α-methylstyrene, p-methylstyrene, vinyltoluene, and chlorostyrene.

An example of an N-substituted unsaturated carboxylic acid amide is N-methylol (meth)acrylamide.

Examples of heterocyclic vinyl compounds include vinylpyrrolidone.

Examples of vinylidene halide compounds include vinylidene chloride and vinylidene fluoride.

Examples of α-olefins include ethylene and propylene.

Examples of dienes include butadiene.

Examples of crosslinkable vinyl monomers include vinyl monomers containing two or more vinyl groups. Examples of vinyl monomers containing two or more vinyl groups include methylene bis(meth)acrylamide, divinylbenzene, polyethylene glycol chain-containing di(meth)acrylate, trimethylolpropane tetraacrylate, pentaerythritol triacrylate, and pentaerythritol tetraacrylate.

In the water-soluble polymer raw material, the content of the water-soluble copolymerizable monomer is, for example, 20 parts by mass or less, preferably 15 parts by mass or less, and, for example, 0 parts by mass or more, particularly preferably 0 parts by mass, per 100 parts by mass of the total amount of the water-soluble polymer raw material.

Water-soluble copolymerizable monomers can be used alone or in combination of two or more types.

The water-soluble polymer raw material preferably does not contain a water-soluble copolymerizable monomer, but contains methacrylamide and a carboxy group-containing vinyl monomer. The water-soluble polymer raw material more preferably consists of methacrylamide and a carboxy group-containing vinyl monomer.

(Preparation of Water-Soluble Polymer)
The water-soluble polymer can be obtained by polymerizing a water-soluble polymer raw material by a known method.

Specifically, for example, water-soluble polymer raw materials and a polymerization initiator are mixed with water, the water-soluble polymer raw materials are polymerized, and then the mixture is aged as necessary.

The polymerization initiator is not particularly limited, but examples include water-soluble initiators, oil-soluble initiators, and redox initiators. Examples of water-soluble initiators include persulfates (e.g., ammonium persulfate, potassium persulfate), hydrogen peroxide, organic hydroperoxides, and 4,4'-azobis(4-cyanovaleric) acid. Examples of oil-soluble initiators include benzoyl peroxide and azobisisobutyronitrile.

Preferably, the polymerization initiator is a water-soluble initiator, more preferably a persulfate, and even more preferably ammonium persulfate or potassium persulfate.

The proportion of the polymerization initiator is, for example, 0.01 parts by mass or more, preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, even more preferably 0.5 parts by mass or more, and for example, 3 parts by mass or less, preferably 1 part by mass or less, per 100 parts by mass of the water-soluble polymer raw material.

Polymerization initiators can be used alone or in combination of two or more types.

The polymerization temperature under atmospheric pressure is, for example, 30°C or higher, preferably 50°C or higher, and for example, 95°C or lower, preferably 85°C or lower. The polymerization time is, for example, 0.5 hours or higher, preferably 1.5 hours or higher, and for example, 20 hours or lower, preferably 10 hours or lower.

The maturation time is, for example, 0.5 hours or more, preferably 1.5 hours or more, and, for example, 6 hours or less, preferably 4 hours or less.

In addition, in the above polymerization, known additives such as pH adjusters, metal ion sequestering agents (e.g., ethylenediaminetetraacetic acid and its salts), and molecular weight regulators (chain transfer agents) (e.g., mercaptans, low-molecular-weight halogen compounds) can be blended in appropriate proportions in order to improve production stability.

In addition, a neutralizing agent such as ammonia can be added before or after the above polymerization to adjust the pH to a range of 6 or more and 11 or less.

This results in a water-soluble polymer (aqueous solution containing the water-soluble polymer) being obtained as a polymer of the water-soluble polymer raw material.

Water-soluble polymers are relatively hydrophilic compared to the water-insoluble polymers described below.

In addition, since the water-soluble polymer is a polymer made from water-soluble polymer raw materials containing methacrylamide and a carboxyl group-containing vinyl monomer, it has repeating units derived from methacrylamide and repeating units derived from the carboxyl group-containing vinyl monomer. When the water-soluble polymer has repeating units derived from methacrylamide and repeating units derived from the carboxyl group-containing vinyl monomer, it is possible to produce a separator (described below) with excellent heat resistance and air permeability.

The content of repeating units derived from methacrylamide in the water-soluble polymer is the same as the content of methacrylamide in the water-soluble polymer raw material.

In addition, the content of repeating units derived from carboxyl group-containing vinyl monomers in the water-soluble polymer is the same as the content of carboxyl group-containing vinyl monomers in the water-soluble polymer raw material.

Also, if the water-soluble polymer raw material contains a water-soluble copolymerizable monomer, the water-soluble polymer contains repeating units derived from the water-soluble copolymerizable monomer.

The content of repeating units derived from water-soluble copolymerizable monomers in the water-soluble polymer is the same as the content of water-soluble copolymerizable monomers in the water-soluble polymer raw material.

The weight average molecular weight of the water-soluble polymer is, from the viewpoint of suppressing the penetration of the binder resin into the pores of the porous membrane (described below) and producing a separator (described below) with excellent gas permeability, for example, 20,000 or more, preferably 30,000 or more, and more preferably 40,000 or more. From the viewpoint of producing a separator (described below) with excellent heat resistance, for example, 200,000 or less, preferably 180,000 or less, and more preferably 150,000 or less.

The weight average molecular weight is the PEG (polyethylene glycol)/PEO (polyethylene oxide) equivalent molecular weight obtained from a gel permeation chromatogram.

Furthermore, in the aqueous solution of the water-soluble polymer, the content (solids concentration) of the water-soluble polymer is, for example, 3% by mass or more, preferably 5% by mass or more, more preferably 8% by mass or more, and, for example, 50% by mass or less, preferably 30% by mass or less, more preferably 20% by mass or less.

[Water-insoluble polymer]
The binder resin may contain a water-insoluble polymer in addition to the water-soluble polymer described above.

A water-insoluble polymer is a polymer obtained by polymerizing water-insoluble polymer raw materials.

A water-insoluble polymer is defined as a polymer that has been dried, and when 1 g of the polymer is dissolved in 100 ml of water by stirring for 24 hours, and then filtered through a 300 mesh wire mesh, the remaining solid content is 90% or more.

Water-insoluble polymer raw materials include, for example, (meth)acrylic acid alkyl esters.

Examples of (meth)acrylic acid alkyl esters include the (meth)acrylic acid alkyl esters described above.

In the water-insoluble polymer raw material, the content ratio of (meth)acrylic acid alkyl ester is set appropriately depending on the purpose and application.

(Meth)acrylic acid alkyl esters can be used alone or in combination of two or more types.

In addition, the water-insoluble polymer raw material may contain a copolymerizable monomer (hereinafter referred to as a water-insoluble copolymerizable monomer) that can be copolymerized with the (meth)acrylic acid alkyl ester.

Examples of water-insoluble copolymerizable monomers include the above-mentioned carboxy group-containing vinyl monomers, the above-mentioned functional group-containing vinyl monomers, the above-mentioned vinyl esters, the above-mentioned aromatic vinyl monomers, the above-mentioned N-substituted unsaturated carboxylic acid amides, the above-mentioned heterocyclic vinyl compounds, the above-mentioned vinylidene halide compounds, the above-mentioned α-olefins, the above-mentioned dienes, and the above-mentioned crosslinkable vinyl monomers.

In the water-insoluble polymer raw material, the content ratio of water-insoluble copolymerizable monomer is set appropriately depending on the purpose and application.

Non-water-soluble copolymerizable monomers can be used alone or in combination of two or more types.

The water-insoluble polymer (dispersion containing the water-insoluble polymer) is obtained by polymerizing the water-insoluble polymer raw material using a known method.

The water-insoluble polymer is relatively hydrophobic compared to the water-soluble polymer.

The binder resin preferably does not contain a water-insoluble polymer (a dispersion containing a water-insoluble polymer) but contains a water-soluble polymer (an aqueous solution containing a water-soluble polymer). The binder resin more preferably consists of a water-soluble polymer (an aqueous solution containing a water-soluble polymer).

The blending ratio of the binder resin is, for example, 1 part by mass or more, preferably 3 parts by mass or more, and, for example, 15 parts by mass or less, preferably 10 parts by mass or less, per 100 parts by mass of the coating material for secondary battery separators.

The binder resin content is, for example, 2 parts by mass or more, preferably 4 parts by mass or more, and, for example, 20 parts by mass or less, preferably 10 parts by mass or less, per 100 parts by mass of inorganic particles.

<Associative thickener>
Associative thickeners are compounds that exert their thickening effect by association.

Specifically, associative thickeners are amphiphilic compounds that contain hydrophobic and hydrophilic groups in their molecules. Examples of such associative thickeners include linear associative thickeners and branched associative thickeners.

As shown in Figure 1A, the linear associative thickener 1 has hydrophobic groups 3 at both ends of the hydrophilic polymer 2 (main chain).

In the linear associative thickener 1, as shown in Figure 1B, the hydrophobic groups 3 bond together through associative forces (the area surrounded by the dashed line in Figure 1B), forming a network structure and exerting a thickening effect.

As shown in Figure 2A, the branched associative thickener 4 has a hydrophilic polymer 2 as the main chain and a hydrophobic group unit 5 as the side chain.

The hydrophobic group unit 5 is preferably composed of a plurality of hydrophobic groups 3, as shown in the enlarged view of Figure 2A. Furthermore, the hydrophobic group unit 5 is more preferably composed of a plurality of hydrophobic groups 3 so as to form a comb shape.

In the branched associative thickener 4, as shown in Figure 2B, the hydrophobic group units 5 bond together through associative forces (the area surrounded by the dashed line in Figure 2B) to form a network structure, thereby exerting a thickening effect.

In addition, associative thickeners (the linear associative thickeners and branched associative thickeners described above) can be produced, for example, by reacting a polyether monool having an alkyl chain with 12 to 23 carbon atoms with a diisocyanate, or by reacting a polyether diol with a diisocyanate and then reacting the resulting mixture with a monoalcohol having an alkyl chain with 12 to 23 carbon atoms.

In other words, the associative thickeners (the linear associative thickeners and the branched associative thickeners described above) are preferably urethane associative thickeners having urethane bonds in the molecule. Furthermore, such urethane associative thickeners contain polyether (polyether chains, preferably polyoxyethylene groups) as the hydrophilic group.

If the associative thickener is the urethane associative thickener described above, the binder resin can be further prevented from penetrating into the pores of the porous membrane. As a result, a separator (described below) with even better air permeability can be produced.

Commercially available urethane associative thickeners can also be used. Examples of commercially available urethane associative thickeners include the ADEKA NOL series (e.g., UH-420, UH-756VF, UH450VF, UH-530, linear associative thickeners, manufactured by ADEKA Corporation), the Thixostar series (branched associative thickeners, manufactured by Japan Materials Technology Co., Ltd.), and the SN Thickener series (e.g., SN Thickener 660T, linear associative thickener, manufactured by San Nopco Ltd.).

From the perspective of further improving air permeability, the associative thickener is preferably a linear associative thickener. In other words, the associative thickener is preferably an amphiphilic compound having a hydrophobic group and a hydrophilic group, with the hydrophobic groups located at both ends. Furthermore, a more preferred associative thickener is a linear urethane associative thickener.

The viscosity (25°C) of the associative thickener is, for example, from the viewpoint of coatability, 1,000 mPa·s or more, preferably 2,000 mPa·s or more, and more preferably 3,000 mPa·s or more; and, for example, from the viewpoint of improving air permeability, 100,000 mPa·s or less, preferably 45,000 mPa·s or less, and more preferably 30,000 mPa·s or less.

The blending ratio of the associative thickener is, for example, 1 part by mass or more, preferably 2 parts by mass or more, and, for example, 10 parts by mass or less, preferably 5 parts by mass or less, per 100 parts by mass of the coating material for secondary battery separators.

The blending ratio of the associative thickener is, for example, 20 parts by mass or more, preferably 40 parts by mass or more, and, for example, 80 parts by mass or less, preferably 65 parts by mass or less, per 100 parts by mass of binder resin.

Associative thickeners can be used alone or in combination of two or more types.

<Production of coating material for secondary battery separator>
To produce a coating material for a secondary battery separator, first, inorganic particles and, if necessary, a dispersant are mixed with water to prepare an inorganic particle dispersion. When a dispersant is mixed, the coating material for a secondary battery separator contains the dispersant.

Examples of dispersants include ammonium polycarboxylate and sodium polycarboxylate. Preferred dispersants include ammonium polycarboxylate.

The proportion of dispersant used is, for example, 1 part by mass or more, preferably 2 parts by mass or more, and, for example, 10 parts by mass or less, preferably 5 parts by mass or less, per 100 parts by mass of inorganic particles.

Dispersants can be used alone or in combination of two or more types.

Next, the binder resin and associative thickener are added to the inorganic particle dispersion in the above proportions and stirred.

The stirring method is not particularly limited, and examples include a ball mill, bead mill, planetary ball mill, vibrating ball mill, sand mill, colloid mill, attritor, roll mill, high-speed impeller dispersion, disperser, homogenizer, high-speed impact mill, ultrasonic dispersion, and stirring blade.

Additionally, additives such as hydrophilic resins, wetting agents, antifoaming agents, and pH adjusters can be blended into the coating material for secondary battery separators in appropriate proportions as needed. In other words, the coating material for secondary battery separators contains additives as needed.

These additives can be used alone or in combination of two or more types.

This results in a coating material for secondary battery separators. Furthermore, such coating material for secondary battery separators is obtained as a dispersion in water.

The solids concentration of the dispersion of the coating material for secondary battery separators is, for example, 10% by mass or more, preferably 20% by mass or more, more preferably 30% by mass or more, and, for example, 50% by mass or less.

The coating material for secondary battery separators contains an associative thickener. This allows for the production of separators with excellent gas permeability (described in more detail below).

The following describes in detail the secondary battery separator obtained using this coating material for secondary battery separators.

<Secondary battery separator>
The secondary battery separator includes a porous membrane and a coating film of a coating material for a secondary battery separator that is disposed on at least one surface of the porous membrane.

[Porous membrane]
Examples of the porous membrane include a polyolefin porous membrane and an aromatic polyamide porous membrane. Examples of the polyolefin porous membrane include a polyethylene porous membrane and a polypropylene porous membrane. A preferred example of the porous membrane is a polyolefin porous membrane. The porous membrane may be surface-treated as needed. Examples of the surface treatment include corona treatment and plasma treatment.

The thickness of the porous membrane is, for example, 1 μm or more, preferably 5 μm or more, and, for example, 40 μm or less, preferably 20 μm or less.

[Coating film]
The coating film is a heat-resistant layer for imparting heat resistance to the porous film, and is made of a coating material for a secondary battery separator.

The thickness of the coating film is, for example, 1 μm or more, preferably 3 μm or more, and, for example, 10 μm or less, preferably 8 μm or less.

[Method for manufacturing secondary battery separator]
The method for manufacturing a secondary battery separator includes a first step of preparing a porous membrane, and a second step of applying a separator coating material to at least one surface of the porous membrane.

(1st step)
In the first step, a porous membrane is prepared.

In the second step, a coating material for a secondary battery separator (a dispersion of the coating material for the separator) is applied to at least one side of the porous membrane, and then dried, if necessary, to obtain a coating film.

There are no particular restrictions on the application method, and examples include gravure coater method, small diameter gravure coater method, reverse roll coater method, transfer roll coater method, kiss coater method, dip coater method, microgravure coat method, knife coater method, air doctor coater method, blade coater method, rod coater method, squeeze coater method, cast coater method, die coater method, screen printing method, and spray application method.

The drying temperature is, for example, 40°C or higher and, for example, 80°C or lower.

This produces a secondary battery separator comprising a porous membrane and a coating film of the above-mentioned secondary battery separator coating material placed on at least one side of the porous membrane.

In the above explanation, a coating film of a coating material for a secondary battery separator is placed on at least one side of the porous membrane, but the above coating film can also be placed on both sides of the porous membrane.

This secondary battery separator has a coating film of the above-mentioned secondary battery separator coating material. Therefore, it has excellent air permeability. Therefore, the secondary battery separator can be suitably used in the manufacture of secondary batteries.

<Secondary battery>
The secondary battery comprises a positive electrode, a negative electrode, the above-mentioned secondary battery separator disposed between the positive electrode and the negative electrode, and an electrolyte impregnated in the positive electrode, the negative electrode, and the above-mentioned secondary battery separator.

As the positive electrode, for example, a known electrode comprising a positive electrode current collector and a positive electrode active material laminated on the positive electrode current collector is used.

Examples of conductive materials for the positive electrode current collector include aluminum, titanium, stainless steel, nickel, baked carbon, conductive polymers, and conductive glass.

The positive electrode active material is not particularly limited, but examples include known positive electrode active materials such as lithium-containing transition metal oxides, lithium-containing phosphates, and lithium-containing sulfates.

These positive electrode active materials can be used alone or in combination of two or more types.

As the negative electrode, for example, a known electrode comprising a negative electrode current collector and a negative electrode active material laminated on the negative electrode current collector is used.

Examples of negative electrode current collectors include conductive materials such as copper and nickel.

The negative electrode active material is not particularly limited, but may be a carbon active material. Examples of carbon active materials include graphite, soft carbon, and hard carbon.

These negative electrode active materials can be used alone or in combination of two or more types.

When a lithium ion battery is used as the secondary battery, the electrolyte may be, for example, a solution in which lithium salt is dissolved in a carbonate compound such as ethylene carbonate (EC), propylene carbonate (PC), or ethyl methyl carbonate (EMC).

To manufacture a secondary battery, for example, a separator for the secondary battery is sandwiched between a positive electrode and a negative electrode, and these are housed in a battery casing (cell), and an electrolyte is injected into the battery casing.
In this way, a secondary battery can be obtained.

The above secondary battery has excellent gas permeability because it is equipped with the above secondary battery separator. As a result, it has excellent power generation efficiency.

<Action and effect>
The coating material for a secondary battery separator contains an associative thickener, which allows the production of a separator with excellent gas permeability.

Specifically, as shown in Figure 3A, in the coating material for secondary battery separators, multiple inorganic particles 10 are bonded together by binder resin 11.

To manufacture a secondary battery separator using this coating material for a secondary battery separator, as shown in Figure 3B, the coating material for a secondary battery separator (a dispersion of the coating material for the separator) is applied to at least one side of the porous membrane 12, and then dried if necessary, thereby obtaining a coated film (see the second step described above).

At this time, the binder resin 11' that does not contribute to bonding the inorganic particles 10 permeates the porous membrane 12 and eventually penetrates the pores of the porous membrane 12. This reduces the air permeability.

In addition, in order to suppress the above-mentioned penetration, it has been considered to incorporate a thickener (more specifically, a non-associating thickener (e.g., hydroxypropyl methylcellulose)) into the coating material for secondary battery separators. However, while incorporating a thickener can slow the above-mentioned penetration, the penetration progresses over time, and as a result, the penetration cannot be sufficiently suppressed. Furthermore, the penetration of a binder resin 11' with a higher viscosity may further reduce air permeability.

In contrast, this secondary battery separator coating material contains an associative thickener. Therefore, in this secondary battery separator coating material, as described above, the associative thickener associates to form a network structure as a whole, as shown in Figures 1B and 2B. Then, as shown in the enlarged view of Figure 3C, binder resin 11' that does not contribute to bonding of inorganic particles 10 is captured in the network structure. This suppresses the penetration described above, thereby improving air permeability.

Next, the present invention will be described based on examples and comparative examples, but the present invention is not limited to these examples. Note that "parts" and "%" are by mass unless otherwise specified. Furthermore, specific numerical values for blending ratios (content ratios), physical properties, parameters, etc. used in the following description can be substituted with the corresponding upper limit (a numerical value defined as "equal to or less than") or lower limit (a numerical value defined as "equal to or greater than") of the blending ratios (content ratios), physical properties, parameters, etc. described in the "Description of the Invention" above.

<Synthesis of binder resin>
Synthesis Example 1
A separable flask equipped with a stirrer and reflux condenser was charged with 392.0 parts of distilled water, and the atmosphere was replaced with nitrogen gas. The temperature was then raised to 80°C. Next, 0.6 parts of ammonium persulfate was added, and the water-soluble polymer raw material described below was added continuously over 3 hours. The mixture was then maintained for another 3 hours, and the pH was adjusted to 9.0 with aqueous ammonia to complete the polymerization. An appropriate amount of water was added to obtain an aqueous solution of the water-soluble polymer with a solids content of 15.0%. This produced a binder resin.
Methacrylamide 95.0 parts Methacrylic acid 5.0 parts 25% aqueous ammonia 5.0 parts Distilled water 150.0 parts

<Production of Coating Material for Secondary Battery Separator and Secondary Battery Separator>
Example 1
[Production of coating material for secondary battery separator]
An inorganic particle dispersion was prepared by uniformly dispersing 100 parts by mass of aluminum hydroxide oxide (manufactured by Taimei Chemical Industry Co., Ltd., boehmite Grade C06, particle size: 0.7 μm) as inorganic particles and 3.0 parts by mass (solid content equivalent) of an aqueous solution of ammonium polycarboxylate (manufactured by San Nopco Ltd., SN Dispersant 5468) as a dispersant in 110 parts by mass of water.

Next, the binder resin from Synthesis Example 1 was added to this inorganic particle dispersion to a solids content of 5 parts by mass, and 2.5 parts by mass of ADEKA NOL UH-420 (a linear urethane associative thickener containing polyether (polyether chain) as the hydrophilic group, viscosity: 20,000 mPa·s (25°C), manufactured by ADEKA Corporation) was added as an associative thickener. Water was then added to bring the solids content to 40%, and the mixture was stirred for 15 minutes. This resulted in the preparation of a coating material for secondary battery separators (separator coating material dispersion).

[Manufacturing of secondary battery separator]
(1st step)
As the porous membrane, a corona-treated polyolefin resin porous membrane was prepared. More specifically, as the corona treatment method, a polyolefin resin porous membrane, product number SW509C+ (film thickness 9.6 μm, porosity 40.6%, air permeability 158 g/100 ml, surface density 5.5 g/m ² , Changzhou Xingyuan New Energy Materials Co., Ltd.) was prepared. Next, the surface of the polyolefin resin porous membrane was cut to A4 size, and then the surface of the polyolefin resin porous membrane was corona-treated using a switchback automatic traveling corona surface treatment device (manufactured by Wedge Co., Ltd.) under the conditions of output 0.15 kW, conveying speed 3.0 m/s × 2 times, and corona discharge distance 9 mm.

(Second process)
A coating material for a secondary battery separator (a dispersion of a separator coating material) was applied to the surface (one side) of the corona-treated polyolefin resin porous membrane using a wire bar. The coating material was then dried at 50°C. A coating film (thickness: 5 μm) was thus formed on the surface (one side) of the polyolefin resin porous membrane. A secondary battery separator was thus produced.

Examples 2 to 4 and Comparative Examples 1 to 4
Following the same procedure as in Example 1, a coating material for a secondary battery separator and a secondary battery separator were manufactured. However, the formulation of each component was changed according to Table 1. In Table 1, the numerical values for each component are parts by mass of solid content. In Example 3, ADEKA NOL UH-450VF (a linear urethane associative thickener containing a polyether (polyether chain) as the hydrophilic group, viscosity: 50,000 mPa·s (25°C), manufactured by ADEKA Corporation) was used as the associative thickener. In Example 4, ADEKA NOL UH-530 (a linear urethane associative thickener containing a polyether (polyether chain) as the hydrophilic group, viscosity: 4,500 mPa·s (25°C), manufactured by ADEKA Corporation) was used as the associative thickener. The thickener (hydroxypropyl methylcellulose) used in Comparative Examples 1 and 3 is a non-associative thickener.

<Evaluation>
[Air permeability]
The air permeability of the secondary battery separators of each example and each comparative example was measured in accordance with JIS-P-8117 using an Oken-type air permeability and smoothness tester manufactured by Asahi Seiko Co., Ltd. The smaller the air permeability, the better the air permeability can be evaluated. The results are shown in Table 1.

The above invention is provided as an exemplary embodiment of the present invention, but this is merely an example and should not be interpreted as limiting. Modifications of the present invention that are obvious to those skilled in the art are intended to be included within the scope of the following claims.

The coating material for secondary battery separators and secondary battery separators of the present invention are suitable for use, for example, in the manufacture of secondary batteries. The secondary batteries of the present invention are suitable for use, for example, in various devices such as automobiles.

Claims (6)

The composition contains inorganic particles, a binder resin, and an associative thickener,
The binder resin comprises a water-soluble polymer having a repeating unit derived from methacrylamide and a repeating unit derived from a carboxy group-containing vinyl monomer . the associative thickener is an amphiphilic compound having a hydrophobic group and a hydrophilic group,
The coating material for a secondary battery separator according to claim 1 , wherein the hydrophobic groups are arranged at both ends. The coating material for a secondary battery separator according to claim 2 , wherein the hydrophilic group includes a polyether. The coating material for secondary battery separators according to claim 1, wherein the associative thickener has a urethane bond. A porous membrane;
A secondary battery separator comprising: a coating film of the coating material for a secondary battery separator according to claim 1 disposed on at least one surface of the porous membrane. A secondary battery comprising: a positive electrode; a negative electrode; and the secondary battery separator according to claim 5 disposed between the positive electrode and the negative electrode.