Coating material for secondary battery separator, secondary battery separator, and secondary battery

Abstract

This coating material for a secondary battery separator comprises inorganic particles, a binder resin, and an associative thickener.

Description

[Technical Field]

[0001] The present invention relates to a coating material for a secondary battery separator, a secondary battery separator, and a secondary battery. More specifically, the present invention relates to a coating material for a secondary battery separator, a secondary battery separator having a coating film of the coating material for a secondary battery separator, and a secondary battery having the secondary battery separator. [Background technology]

[0002] Conventionally, a separator is provided in a secondary battery to separate the positive electrode from the negative electrode and to allow ions in the electrolyte to pass through.

[0003] As such a separator, for example, a polyolefin porous film is known.

[0004] On the other hand, if the separator changes shape due to heat shrinkage, there is a possibility of short-circuiting between the positive electrode and the negative electrode. Therefore, the separator is required to have heat resistance. 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 a secondary battery separator to the surface of the separator and drying it.

[0005] 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). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2021-103676 Summary of the Invention [Problem to be solved by the invention]

[0007] On the other hand, the separator of a secondary battery must be able to pass ions in order to generate electricity, which means that the heat-resistant coating layer must further improve the gas permeability of the separator.

[0008] The present invention provides a coating material for a secondary battery separator for producing a separator with excellent gas permeability, a secondary battery separator having a coating film of the coating material for a secondary battery separator, and a secondary battery having the secondary battery separator. [Means for solving the problem]

[0009] The present invention [1] is a coating material for a secondary battery separator, which contains inorganic particles, a binder resin, and an associative thickener.

[0010] The present invention [2] includes the coating material for secondary battery separators according to the above [1], wherein the associative thickener is an amphipathic compound having a hydrophobic group and a hydrophilic group, and the hydrophobic groups are located at both ends.

[0011] The present invention [3] includes the coating material for secondary battery separators according to the above [1] or [2], wherein the binder resin contains a water-soluble polymer having a repeating unit derived from methacrylamide and a repeating unit derived from a carboxy group-containing vinyl monomer.

[0012] The present invention [4] includes the coating material for a secondary battery separator according to any one of the above [1] to [3], wherein the hydrophilic group includes a polyether.

[0013] The present invention [5] includes the coating material for a secondary battery separator according to any one of the above [1] to [4], wherein the associative thickener has a urethane bond.

[0014] The present invention [6] includes a secondary battery separator comprising a porous membrane and a coating film of the coating material for secondary battery separator according to any one of the above [1] to [5], which is disposed on at least one side of the porous membrane.

[0015] The present invention [7] includes a secondary battery comprising a positive electrode, a negative electrode, and the secondary battery separator described in the above [6] disposed between the positive electrode and the negative electrode. [Effects of the Invention]

[0016] The coating material for a secondary battery separator of the present invention contains an associative thickener. Therefore, when this coating material for a secondary battery separator is applied to the surface of a porous membrane to form a coating film, the associative thickener associates in the coating film to form a network structure as a whole. This network structure then captures the binder resin, thereby preventing the binder resin from penetrating into the pores of the porous membrane. As a result, a separator with excellent gas permeability can be produced.

[0017] The secondary battery separator of the present invention has a coating film of the coating material for a secondary battery separator of the present invention, and therefore has excellent air permeability.

[0018] The secondary battery of the present invention includes the secondary battery separator of the present invention, and therefore has excellent gas permeability, resulting in excellent power generation efficiency. [Brief explanation of the drawings]

[0019] [Figure 1] 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. [Figure 2] Figures 2A and 2B show schematic diagrams of a branched associative thickener. Figure 2A is an explanatory diagram illustrating the structure of a branched associative thickener. Figure 2B is an explanatory diagram illustrating that the branched associative thickeners associate to form a network structure as a whole. [Figure 3] 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. DETAILED DESCRIPTION OF THE INVENTION

[0020] The coating material for a secondary battery separator contains inorganic particles, a binder resin, and an associative thickener.

[0021] <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.

[0022] The inorganic particles are preferably oxides or hydroxides, more preferably alumina or aluminum oxide hydroxide.

[0023] The inorganic particles have an average particle size of, 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.

[0024] The average particle size of the inorganic particles can be determined by creating a particle size distribution curve using a laser diffraction / scattering particle size distribution analyzer and calculating the particle size equivalent to 50% by mass.

[0025] The blending ratio of the 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.

[0026] The inorganic particles can be used alone or in combination of two or more kinds.

[0027] <Binder resin> The binder resin includes a water-soluble polymer.

[0028] [Water-soluble polymer] The water-soluble polymer is a polymer obtained by polymerizing a water-soluble polymer raw material.

[0029] 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.

[0030] The water-soluble polymer raw material includes methacrylamide and a carboxy group-containing vinyl monomer.

[0031] (methacrylamide) Methacrylamide may be used in combination with acrylamide, or methacrylamide may be used alone without being used in combination with acrylamide.

[0032] From the viewpoint of producing a separator (described later) having excellent heat resistance, methacrylamide is preferably used alone without being used in combination with acrylamide.

[0033] 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.

[0034] (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.

[0035] Examples of the carboxyl group-containing vinyl monomer include monocarboxylic acids, dicarboxylic acids, and salts thereof. Examples of the monocarboxylic acid include (meth)acrylic acid. Examples of the dicarboxylic acid include itaconic acid, maleic acid, fumaric acid, itaconic anhydride, maleic anhydride, and fumaric anhydride.

[0036] It should be noted that (meth)acrylic includes acrylic and methacrylic (the same applies hereinafter).

[0037] If the water-soluble polymer raw material contains a carboxyl group-containing vinyl monomer, a separator (described below) with excellent gas permeability can be produced.

[0038] As the carboxy group-containing vinyl monomer, preferably, a monocarboxylic acid is used, more preferably, (meth)acrylic acid is used, and further preferably, from the viewpoint of producing a separator (described later) having even more excellent air permeability, methacrylic acid is used.

[0039] In the water-soluble polymer raw material, the content ratio of the carboxy 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, relative to 100 parts by mass of the total amount of the water-soluble polymer raw material.

[0040] The carboxyl group-containing vinyl monomers can be used alone or in combination of two or more kinds.

[0041] (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.

[0042] 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.

[0043] Examples of alkyl (meth)acrylates 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.

[0044] 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 salts thereof, acetoacetoxy group-containing vinyl monomers, and phosphoric acid group-containing compounds.

[0045] Examples of hydroxyl group-containing vinyl monomers include 2-hydroxyethyl (meth)acrylate and 2-hydroxypropyl (meth)acrylate.

[0046] 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.

[0047] An example of the glycidyl group-containing vinyl monomer is glycidyl (meth)acrylate.

[0048] An example of the cyano group-containing vinyl monomer is (meth)acrylonitrile.

[0049] Examples of sulfonic acid group-containing vinyl monomers include allyl sulfonic acid, methallyl sulfonic acid, and acrylamido t-butyl sulfonic acid. 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.

[0050] An example of an acetoacetoxy group-containing vinyl monomer is acetoacetoxyethyl (meth)acrylate.

[0051] An example of the phosphate group-containing compound is 2-methacryloyloxyethyl acid phosphate.

[0052] Examples of vinyl esters include vinyl acetate and vinyl propionate.

[0053] Examples of aromatic vinyl monomers include styrene, α-methylstyrene, p-methylstyrene, vinyltoluene, and chlorostyrene.

[0054] An example of the N-substituted unsaturated carboxylic acid amide is N-methylol (meth)acrylamide.

[0055] An example of the heterocyclic vinyl compound is vinylpyrrolidone.

[0056] Examples of vinylidene halide compounds include vinylidene chloride and vinylidene fluoride.

[0057] Examples of α-olefins include ethylene and propylene.

[0058] An example of the dienes is butadiene.

[0059] Examples of crosslinkable vinyl monomers include vinyl monomers containing two or more vinyl groups, such as methylenebis(meth)acrylamide, divinylbenzene, polyethylene glycol chain-containing di(meth)acrylate, trimethylolpropane tetraacrylate, pentaerythritol triacrylate, and pentaerythritol tetraacrylate.

[0060] 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.

[0061] The water-soluble copolymerizable monomers can be used alone or in combination of two or more kinds.

[0062] The water-soluble polymer material preferably does not contain a water-soluble copolymerizable monomer and contains methacrylamide and a carboxy group-containing vinyl monomer, and more preferably consists of methacrylamide and a carboxy group-containing vinyl monomer.

[0063] (Preparation of Water-Soluble Polymers) The water-soluble polymer can be obtained by polymerizing a water-soluble polymer raw material by a known method.

[0064] Specifically, for example, a water-soluble polymer raw material and a polymerization initiator are mixed with water, the water-soluble polymer raw material is polymerized, and then the mixture is aged as necessary.

[0065] The polymerization initiator is not particularly limited, and examples thereof 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.

[0066] The polymerization initiator is preferably a water-soluble initiator, more preferably a persulfate, and even more preferably ammonium persulfate or potassium persulfate.

[0067] The blending ratio 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.

[0068] The polymerization initiators can be used alone or in combination of two or more kinds.

[0069] The polymerization temperature under normal pressure is, for example, 30° C. or more, preferably 50° C. or more, and for example, 95° C. or less, preferably 85° C. or less. The polymerization time is, for example, 0.5 hours or more, preferably 1.5 hours or more, and for example, 20 hours or less, preferably 10 hours or less.

[0070] The aging 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.

[0071] In the above polymerization, known additives such as a pH adjuster, a metal ion sequestering agent (e.g., ethylenediaminetetraacetic acid and its salts), and a molecular weight regulator (chain transfer agent) (e.g., mercaptans, low-molecular-weight halogen compounds) can be blended in an appropriate ratio from the viewpoint of improving production stability.

[0072] Before or after the polymerization, a neutralizing agent such as ammonia may be added to adjust the pH to within the range of 6 to 11.

[0073] As a result, a water-soluble polymer (aqueous solution containing the water-soluble polymer) is obtained as a polymer of the water-soluble polymer raw material.

[0074] The water-soluble polymer is relatively hydrophilic compared to the water-insoluble polymer described below.

[0075] Furthermore, since the water-soluble polymer is a polymer of a water-soluble polymer raw material containing methacrylamide and a carboxyl group-containing vinyl monomer, it has a repeating unit derived from methacrylamide and a repeating unit derived from a carboxyl group-containing vinyl monomer. When the water-soluble polymer has a repeating unit derived from methacrylamide and a repeating unit derived from a carboxyl group-containing vinyl monomer, a separator (described below) having excellent heat resistance and gas permeability can be produced.

[0076] 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.

[0077] The content of the repeating unit derived from the carboxyl group-containing vinyl monomer in the water-soluble polymer is the same as the content of the carboxyl group-containing vinyl monomer in the raw material of the water-soluble polymer.

[0078] Furthermore, when 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.

[0079] The content of the repeating units derived from the water-soluble copolymerizable monomer in the water-soluble polymer is the same as the content of the water-soluble copolymerizable monomer in the water-soluble polymer raw material.

[0080] 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 later) and producing a separator (described later) having excellent gas permeability, for example, 20,000 or more, preferably 30,000 or more, more preferably 40,000 or more, and from the viewpoint of producing a separator (described later) having excellent heat resistance, for example, 200,000 or less, preferably 180,000 or less, more preferably 150,000 or less.

[0081] The weight average molecular weight is a molecular weight calculated as PEG (polyethylene glycol) / PEO (polyethylene oxide) by gel permeation chromatogram.

[0082] In addition, 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.

[0083] [Water-insoluble polymer] The binder resin may contain a water-insoluble polymer in addition to the water-soluble polymer described above.

[0084] The water-insoluble polymer is a polymer obtained by polymerizing a water-insoluble polymer raw material.

[0085] 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.

[0086] The water-insoluble polymeric material includes, for example, an alkyl (meth)acrylate ester.

[0087] Examples of the (meth)acrylic acid alkyl ester include the above-mentioned (meth)acrylic acid alkyl esters.

[0088] In the water-insoluble polymer raw material, the content of the (meth)acrylic acid alkyl ester is appropriately set depending on the purpose and application.

[0089] The (meth)acrylic acid alkyl esters can be used alone or in combination of two or more kinds.

[0090] The water-insoluble polymer raw material may also contain a copolymerizable monomer copolymerizable with the (meth)acrylic acid alkyl ester (hereinafter referred to as a water-insoluble copolymerizable monomer).

[0091] 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.

[0092] The content of the water-insoluble copolymerizable monomer in the water-insoluble polymer raw material is appropriately set depending on the purpose and application.

[0093] The water-insoluble copolymerizable monomers can be used alone or in combination of two or more kinds.

[0094] The water-insoluble polymer (dispersion containing the water-insoluble polymer) can be obtained by polymerizing a water-insoluble polymer raw material by a known method.

[0095] The water-insoluble polymer is relatively hydrophobic compared to the water-soluble polymer.

[0096] 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).

[0097] The mixing ratio of the binder resin relative to 100 parts by mass of the coating material for a secondary battery separator 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.

[0098] The mixing ratio of the binder resin relative to 100 parts by mass of the inorganic particles 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.

[0099] <Associative thickener> Associative thickeners are compounds that exert their thickening effect by association.

[0100] Specifically, the associative thickener is, for example, an amphipathic compound having a hydrophobic group and a hydrophilic group in the molecule. Examples of such associative thickeners include linear associative thickeners and branched associative thickeners.

[0101] As shown in FIG. 1A, the linear associative thickener 1 has hydrophobic groups 3 at both ends of a hydrophilic polymer 2 (main chain).

[0102] In the linear associative thickener 1, as shown in FIG. 1B, the hydrophobic groups 3 bond to each other through an associative force (the area surrounded by the dashed line in FIG. 1B), thereby forming a network structure and exerting a thickening effect.

[0103] As shown in FIG. 2A, the branched associative thickener 4 has a hydrophilic polymer 2 as a main chain and a hydrophobic group unit 5 as a side chain.

[0104] As shown in the enlarged view of Fig. 2A, the hydrophobic group unit 5 is preferably composed of a plurality of hydrophobic groups 3. More preferably, the hydrophobic group unit 5 is composed of a plurality of hydrophobic groups 3 in a comb shape.

[0105] In the branched associative thickener 4, as shown in FIG. 2B, the hydrophobic group units 5 bond to each other through an associative force (the area surrounded by the dashed line in FIG. 2B), thereby constructing a network structure and exerting a thickening effect.

[0106] Furthermore, the associative thickeners (the linear associative thickeners and the branched associative thickeners) can be produced, for example, by a method of reacting a polyether monool having an alkyl chain with 12 to 23 carbon atoms with a diisocyanate, or by a method of 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.

[0107] That is, the associative thickener (the linear associative thickener and the branched associative thickener) is preferably a urethane associative thickener having a urethane bond in the molecule. Furthermore, such a urethane associative thickener contains a polyether (a polyether chain, preferably a polyoxyethylene group) as a hydrophilic group.

[0108] If the associative thickener is the urethane associative thickener, the binder resin can be further prevented from penetrating into the pores of the porous membrane, and as a result, a separator (described later) having even more excellent air permeability can be produced.

[0109] 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.).

[0110] As the associative thickener, from the viewpoint of further improving air permeability, a linear associative thickener is preferably used. That is, the associative thickener is preferably an amphipathic compound having a hydrophobic group and a hydrophilic group, and the hydrophobic groups are located at both ends. Furthermore, as the associative thickener, a linear urethane associative thickener is more preferably used.

[0111] The viscosity (25°C) of the associative thickener is, for example, from the viewpoint of coatability, 1000 mPa·s or more, preferably 2000 mPa·s or more, more preferably 3000 mPa·s or more, and, for example, from the viewpoint of improving air permeability, is 100,000 mPa·s or less, preferably 45,000 mPa·s or less, more preferably 30,000 mPa·s or less.

[0112] The mixing 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.

[0113] The mixing ratio of the associative thickener relative to 100 parts by mass of the binder resin 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.

[0114] The associative thickeners can be used alone or in combination of two or more kinds.

[0115] <Manufacturing coating materials for secondary battery separators> 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.

[0116] Examples of the dispersant include ammonium polycarboxylate and sodium polycarboxylate, and preferably ammonium polycarboxylate.

[0117] The mixing ratio of the dispersant relative to 100 parts by mass of the inorganic particles 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.

[0118] The dispersants can be used alone or in combination of two or more kinds.

[0119] Next, the binder resin and the associative thickener are mixed in the inorganic particle dispersion in the above-mentioned ratio and stirred.

[0120] The stirring method is not particularly limited, and examples thereof include a ball mill, a bead mill, a planetary ball mill, a vibrating ball mill, a sand mill, a colloid mill, an attritor, a roll mill, a high-speed impeller dispersion, a disperser, a homogenizer, a high-speed impact mill, ultrasonic dispersion, and a stirring blade.

[0121] Furthermore, 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.

[0122] These additives can be used alone or in combination of two or more kinds.

[0123] This allows the production of a coating material for a secondary battery separator. The coating material for a secondary battery separator is obtained as a dispersion in water.

[0124] The solid content concentration of the dispersion of the coating material for a secondary battery separator is, for example, 10 mass % or more, preferably 20 mass % or more, more preferably 30 mass % or more, and for example, 50 mass % or less.

[0125] The coating material for secondary battery separators contains an associative thickener, which allows the production of separators (described later) with excellent gas permeability, as will be described in detail later.

[0126] Hereinafter, a secondary battery separator obtained by using this coating material for a secondary battery separator will be described in detail.

[0127] <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.

[0128] [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.

[0129] 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.

[0130] [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.

[0131] 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.

[0132] [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.

[0133] (1st step) In the first step, a porous membrane is prepared.

[0134] In the second step, a coating material for a secondary battery separator (a dispersion of a coating material for a separator) is applied to at least one surface of the porous membrane, and then dried as necessary to obtain a coating film.

[0135] The application method is not particularly limited, and examples thereof 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.

[0136] The drying temperature is, for example, 40°C or higher and, for example, 80°C or lower.

[0137] In this way, a secondary battery separator is manufactured, which includes a porous membrane and a coating film of the above-mentioned coating material for a secondary battery separator that is disposed on at least one surface of the porous membrane.

[0138] In the above description, the coating film of the coating material for a secondary battery separator is disposed on at least one side of the porous membrane, but the coating film can also be disposed on both sides of the porous membrane.

[0139] This secondary battery separator has a coating film of the above-mentioned coating material for secondary battery separators, and therefore has excellent gas permeability, making it suitable for use in the manufacture of secondary batteries.

[0140] <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.

[0141] As the positive electrode, for example, a known electrode including a positive electrode current collector and a positive electrode active material laminated on the positive electrode current collector is used.

[0142] Examples of the positive electrode current collector include conductive materials such as aluminum, titanium, stainless steel, nickel, baked carbon, conductive polymers, and conductive glass.

[0143] The positive electrode active material is not particularly limited, but examples thereof include known positive electrode active materials such as lithium-containing transition metal oxides, lithium-containing phosphates, and lithium-containing sulfates.

[0144] These positive electrode active materials can be used alone or in combination of two or more.

[0145] As the negative electrode, for example, a known electrode including a negative electrode current collector and a negative electrode active material laminated on the negative electrode current collector is used.

[0146] The negative electrode current collector may be made of a conductive material such as copper or nickel.

[0147] The negative electrode active material is not particularly limited, but may be a carbon active material, such as graphite, soft carbon, or hard carbon.

[0148] These negative electrode active materials can be used alone or in combination of two or more.

[0149] When a lithium ion battery is used as the secondary battery, the electrolyte may be, for example, a solution in which a lithium salt is dissolved in a carbonate compound such as ethylene carbonate (EC), propylene carbonate (PC), or ethyl methyl carbonate (EMC).

[0150] 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.

[0151] The secondary battery has excellent gas permeability due to the inclusion of the secondary battery separator, resulting in excellent power generation efficiency.

[0152] <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.

[0153] Specifically, as shown in FIG. 3A, in the coating material for a secondary battery separator, a plurality of inorganic particles 10 are bound by a binder resin 11.

[0154] To manufacture a secondary battery separator using this coating material for a secondary battery separator, as shown in FIG. 3B, the coating material for a secondary battery separator (a dispersion of the coating material for a separator) is applied to at least one side of the porous membrane 12, and then dried as necessary to obtain a coating film (see the second step described above).

[0155] At this time, the binder resin 11' that does not contribute to binding the inorganic particles 10 permeates into the porous membrane 12 and eventually permeates into the pores of the porous membrane 12. This reduces the air permeability.

[0156] In order to suppress the penetration, it has been considered to add a thickener (more specifically, a non-associating thickener (e.g., hydroxypropyl methylcellulose)) to the coating material for secondary battery separators. However, although adding a thickener can slow down the 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 the air permeability.

[0157] 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 aggregates to form a network structure as a whole, as shown in FIGS. 1B and 2B. Then, as shown in the enlarged view of FIG. 3C, binder resin 11' that does not contribute to bonding of inorganic particles 10 is trapped in the network structure. This suppresses the permeation described above, thereby improving air permeability. [Example]

[0158] Next, the present invention will be described based on examples and comparative examples, but the present invention is not limited to the following examples. Note that "parts" and "%" are by mass unless otherwise specified. Furthermore, specific numerical values ​​such as blending ratios (content ratios), physical property values, and parameters used in the following description can be substituted with the corresponding upper limit values ​​(numeric values ​​defined as "equal to or less than") or lower limit values ​​(numeric values ​​defined as "equal to or more than" or "exceeding") of the blending ratios (content ratios), physical property values, parameters, etc. described in the above "Form for Carrying Out the Invention."

[0159] <Synthesis of binder resin> Synthesis Example 1 392.0 parts of distilled water were placed in a separable flask equipped with a stirrer and reflux condenser, 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% ammonia water 5.0 parts Distilled water 150.0 parts

[0160] <Production of coating material for secondary battery separator and secondary battery separator> Example 1 [Manufacturing coating materials for secondary battery separators] An inorganic particle dispersion was prepared by uniformly dispersing 100 parts by mass of aluminum hydroxide oxide (manufactured by Taimei Chemical 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.

[0161] Next, the binder resin of Synthesis Example 1 was added to this inorganic particle dispersion to a solid 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, and water was added to a solid content of 40%, followed by stirring for 15 minutes. This produced a coating material for a secondary battery separator (a dispersion of a coating material for a separator).

[0162] [Manufacturing secondary battery separators] (1st step) As a porous membrane, a corona-treated polyolefin resin porous membrane was prepared. More specifically, as a 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) was used. 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 an output of 0.15 kW, a conveying speed of 3.0 m / s × 2 times, and a corona discharge distance of 9 mm.

[0163] (2nd process) A coating material for a secondary battery separator (a dispersion of a coating material for a separator) was applied to the surface (one side) of the corona-treated polyolefin resin porous membrane using a wire bar. Then, it was dried at 50°C. As a result, a coating film (thickness 5 μm) was formed on the surface (one side) of the polyolefin resin porous membrane. In this way, a secondary battery separator was produced.

[0164] Examples 2 to 4 and Comparative Examples 1 to 4 A coating material for a secondary battery separator and a secondary battery separator were produced according to the same procedure as in Example 1. 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 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 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.

[0165] <Evaluation> [Air permeability] The air permeability of the secondary battery separators of each example and 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 lower the air permeability, the better the air permeability can be evaluated. The results are shown in Table 1.

[0166] [Table 1]

[0167] 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 in the scope of the following claims. [Industrial Applicability]

[0168] The coating material for a secondary battery separator and the secondary battery separator of the present invention are suitable for use in, for example, the production of secondary batteries. The secondary batteries of the present invention are suitable for use in various devices such as automobiles.

Claims

1. 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.

2. 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.

3. The coating material for a secondary battery separator according to claim 2 , wherein the hydrophilic group includes a polyether.

4. The coating material for a secondary battery separator according to claim 1 , wherein the associative thickener has a urethane bond.

5. A porous membrane; 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 separator comprising:

6. 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.

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