Raw material for coating material for secondary battery separator, coating material for secondary battery separator, secondary battery separator, and secondary battery

Abstract

A raw material for a coating material for a secondary battery separator comprises composite particles that contain polyvinyl alcohol and a metal neutralized salt of a water-soluble polymer that does not contain constituent units derived from polyvinyl alcohol and does contain constituent units derived from a polycarboxylic acid. The metals in the metal neutralized salt are an alkali metal and a divalent or higher valent metal.

Description

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

[0001] The present invention relates to a coating material raw material for a secondary battery separator, a coating material for a secondary battery separator, a secondary battery separator, and a secondary battery. Specifically, it relates to a coating material raw material for a secondary battery separator, a coating material for a secondary battery separator containing the coating material raw material for a secondary battery separator, a secondary battery separator provided with a coating film of the coating material for a secondary battery separator, and a secondary battery provided with the secondary battery separator.

[0002] Conventionally, a separator for separating a positive electrode and a negative electrode and allowing ions in an electrolytic solution to pass through is provided in a secondary battery.

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

[0004] On the other hand, a coating layer may be provided on the surface of the separator to impart various physical properties. Such a coating layer is formed, for example, by applying a coating material for a secondary battery separator (binder) and inorganic particles on the surface of the separator and drying it.

[0005] As such a coating material for a secondary battery separator, a binder for a non-aqueous electrolyte battery obtained by neutralizing a copolymer of polyvinyl alcohol and acrylic acid (polyacrylic acid) with magnesium acetate and lithium hydroxide has been proposed (see, for example, Example 1 of Patent Document 1).

[0006] International Publication No. 2019 / 117056 pamphlet

[0007] On the other hand, when the secondary battery generates heat, the separator may be deformed by the heat. Then, a short circuit occurs between the positive electrode and the negative electrode. Therefore, the coating layer (specifically, the coating material for a secondary battery separator for forming the coating material) is required to have heat resistance.

[0008] Furthermore, a high moisture content in the coating layer reduces the discharge rate of the secondary battery. Therefore, it is required to reduce the moisture content of the coating layer (specifically, the coating material for the secondary battery separator used to form the coating material).

[0009] The present invention aims to provide a coating material raw material for secondary battery separators that has excellent heat resistance and can reduce moisture content, a coating material for secondary battery separators containing the coating material raw material, a secondary battery separator having a coating film of the coating material, and a secondary battery equipped with the secondary battery separator.

[0010] The present invention [1] is a coating material raw material for secondary battery separators, comprising composite particles containing a metal neutralization salt of a water-soluble polymer that does not contain constituent units derived from polyvinyl alcohol but contains constituent units derived from polycarboxylic acid, and polyvinyl alcohol, wherein the metal in the metal neutralization salt is an alkali metal and a metal with a valency of 2 or higher.

[0011] The present invention [2] includes the raw material for a secondary battery separator coating material described in [1] above, wherein in the composite particles, the content ratio of the metal neutralization salt of the water-soluble polymer is 70 parts by mass or more and 97 parts by mass or less with respect to 100 parts by mass of the total amount of the metal neutralization salt of the water-soluble polymer and the polyvinyl alcohol, and in the composite particles, the content ratio of the polyvinyl alcohol is 3 parts by mass or more and 30 parts by mass or less with respect to 100 parts by mass of the total amount of the metal neutralization salt of the water-soluble polymer and the polyvinyl alcohol.

[0012] The present invention [3] includes a coating material for secondary battery separators as described in [1] or [2] above, wherein the degree of polymerization of the polyvinyl alcohol is 100 or more and 2000 or less, and the degree of saponification of the polyvinyl alcohol is 90 mol% or less.

[0013] The present invention [4] includes a coating material for secondary battery separators according to any one of the above claims [1] to [3], wherein the metal neutralization salt of the water-soluble polymer has a degree of neutralization by the divalent or higher metal of 15% or more.

[0014] The present invention [5] includes a coating material for secondary battery separators according to any one of the above claims [1] to [4], wherein the divalent or higher metal is at least one selected from the group consisting of calcium, magnesium, and zinc.

[0015] The present invention [6] includes a coating material for a secondary battery separator, comprising the coating material raw material for a secondary battery separator described in any one of the above items [1] to [5], and inorganic particles.

[0016] The present invention [7] includes a secondary battery separator comprising a porous membrane and a coating film of the secondary battery separator coating material described in [6] above, which is disposed on at least one side of the porous membrane.

[0017] The present invention [8] includes a secondary battery comprising a positive electrode, a negative electrode, and a secondary battery separator as described in [7] above, disposed between the positive electrode and the negative electrode.

[0018] The raw material for the secondary battery separator coating of the present invention contains composite particles comprising a metal neutralization salt of a water-soluble polymer that does not contain constituent units derived from polyvinyl alcohol but contains constituent units derived from polycarboxylic acid, and polyvinyl alcohol, wherein the metal in the metal neutralization salt is an alkali metal and a divalent or higher metal. Therefore, it has excellent heat resistance and can reduce moisture content.

[0019] The coating material for secondary battery separators of the present invention contains the raw materials for the coating material for secondary battery separators of the present invention. Therefore, it has excellent heat resistance and can reduce moisture content.

[0020] The secondary battery separator of the present invention comprises a coating film of the coating material for secondary battery separators of the present invention. Therefore, it has excellent heat resistance and can reduce moisture content.

[0021] The secondary battery of the present invention is equipped with the secondary battery separator of the present invention. Therefore, it has excellent heat resistance and can reduce moisture content.

[0022] 1. Raw materials for coating materials for secondary battery separators: The raw materials for coating materials for secondary battery separators contain composite particles and water.

[0023] The composite particles contain a metal neutralization salt of a water-soluble polymer that does not contain structural units derived from polyvinyl alcohol but does contain structural units derived from polycarboxylic acid, and polyvinyl alcohol. The hydroxyl groups of the polyvinyl alcohol and the carboxyl groups of the metal neutralization salt of the water-soluble polymer containing structural units derived from polycarboxylic acid undergo hydrophobic association via hydrogen bonding to form a particulate composite.

[0024] The total content ratio of the metal neutralization salt of the water-soluble polymer and polyvinyl alcohol is, for example, 80% by mass or more, preferably 90% by mass or more, and more preferably 100% by mass, relative to the composite particles. In other words, the composite particles more preferably consist of the metal neutralization salt of the water-soluble polymer and polyvinyl alcohol.

[0025] <Metal neutralization salts of water-soluble polymers> (Water-soluble polymers) Water-soluble polymers do not contain constituent units derived from polyvinyl alcohol, but contain constituent units derived from polycarboxylic acid.

[0026] Water-soluble polymers are polymers that do not contain polyvinyl alcohol and contain carboxyl group-containing vinyl monomers as polymerization components. In other words, water-soluble polymers do not contain structural units derived from polyvinyl alcohol and have structural units obtained by radical polymerization of carboxyl group-containing vinyl monomers (structural units derived from polycarboxylic acid). That is to say, water-soluble polymers are distinguished from copolymers of polyvinyl alcohol and carboxyl group-containing vinyl monomers.

[0027] Furthermore, a water-soluble polymer is defined as a polymer in which, after being dried, 1 g of the polymer is dissolved in 100 ml of water, stirred at 25°C for 24 hours, and then filtered through a 300-mesh wire mesh, the residual solid content is 0.1% or less.

[0028] Examples of carboxyl group-containing vinyl monomers include monocarboxylic acids and dicarboxylic acids.

[0029] Examples of monocarboxylic acids include (meth)acrylic acid. (Meth)acrylic acid is also known as methacrylic acid and / or acrylic acid.

[0030] Examples of dicarboxylic acids include itaconic acid, maleic acid, fumaric acid, itaconic anhydride, maleic anhydride, and fumaric anhydride.

[0031] Preferably, monocarboxylic acids are used as carboxyl group-containing vinyl monomers. More preferably, (meth)acrylic acid is used as a carboxyl group-containing vinyl monomer. In other words, more preferably, the structural units obtained by radical polymerization of carboxyl group-containing vinyl monomers (structural units derived from polycarboxylic acid) are structural units derived from poly(meth)acrylic acid. To put it another way, more preferably, the structural units derived from polycarboxylic acid are structural units derived from a homopolymer of methacrylic acid, structural units derived from a homopolymer of acrylic acid, or structural units derived from a copolymer of methacrylic acid and acrylic acid.

[0032] More preferably, acrylic acid is used as the carboxyl group-containing vinyl monomer. In other words, more preferably, the constituent units derived from polycarboxylic acid are constituent units derived from polyacrylic acid.

[0033] Carboxyl group-containing vinyl monomers can be used alone or in combination of two or more types.

[0034] The content of the carboxyl group-containing vinyl monomer is, for example, 80% by mass or more, preferably 90% by mass or more, and more preferably 100% by mass, relative to the polymerization component.

[0035] The polymerization components may also include (meth)acrylamide, if necessary.

[0036] The content of (meth)acrylamide is, for example, 20% by mass or less, preferably 10% by mass or less, and more preferably 0% by mass, relative to the polymerization component. In other words, more preferably, the polymerization component does not contain (meth)acrylamide and consists of a carboxyl group-containing vinyl monomer.

[0037] (Metal neutral salts) In the metal neutral salts of water-soluble polymers, the metal is an alkali metal and a metal with a valence of 2 or more. That is, the metal neutral salt of a water-soluble polymer is a neutralization reaction product in which the carboxyl groups derived from carboxyl group-containing vinyl monomers in the water-soluble polymer are neutralized by an alkali metal and a metal with a valence of 2 or more.

[0038] Examples of the alkali metal include sodium, potassium, and lithium. From the viewpoint of heat resistance, sodium is preferably mentioned as the alkali metal.

[0039] Examples of the metal with a valence of 2 or more include alkaline earth metals and other metals with a valence of 2 or more.

[0040] Examples of the alkaline earth metal include calcium, magnesium, and barium. From the viewpoint of reducing the moisture content, calcium is preferably mentioned as the alkaline earth metal. From the viewpoint of heat resistance, magnesium is preferably mentioned as the alkaline earth metal.

[0041] Examples of other metals with a valence of 2 or more include zinc, copper (II), and iron (II). Zinc is preferably mentioned as other metals with a valence of 2 or more.

[0042] From the viewpoints of improving heat resistance and reducing the moisture content, the metal with a valence of 2 or more is preferably at least one selected from the group consisting of calcium, magnesium, and zinc.

[0043] In the metal neutral salt of a water-soluble polymer, the degree of neutralization by an alkali metal is, for example, 50% to 90%, preferably 60% to 85%, more preferably 65% to 80%, still more preferably 67% to 78%, and particularly preferably 68% to 72%.

[0044] Specifically, in the metal neutral salt of the water-soluble polymer, the degree of neutralization by an alkali metal is, from the viewpoint of heat resistance, for example, 50% or more, preferably 60% or more, more preferably 65% or more, still more preferably 67% or more, particularly preferably 68% or more, and from the viewpoint of heat resistance, for example, 90% or less, preferably 85% or less, more preferably 80% or less, still more preferably 78% or less, particularly preferably 72% or less.

[0045] In the metal neutral salt of the water-soluble polymer, the degree of neutralization by a divalent or higher metal is, for example, 10% to 40%, preferably 15% to 30%, more preferably 16% to 25%, still more preferably 17% to 22%.

[0046] Specifically, in the metal neutral salt of the water-soluble polymer, the degree of neutralization by a divalent or higher metal is, from the viewpoint of reducing the water content, for example, 10% or more, preferably 15% or more, more preferably 16% or more, still more preferably 17% or more, and from the viewpoint of reducing the water content, 40% or less, preferably 30% or less, more preferably 25% or less, still more preferably 22% or less.

[0047] The degree of neutralization of the water-soluble polymer (the sum of the degree of neutralization by an alkali metal and the degree of neutralization by a divalent or higher metal) is, for example, 60% to 99%, preferably 70% to 95%, more preferably 80% to 92%, still more preferably 85% to 90%.

[0048] Specifically, the degree of neutralization of the water-soluble polymer (the sum of the degree of neutralization by an alkali metal and the degree of neutralization by a divalent or higher metal) is, from the viewpoints of improving heat resistance and reducing the water content, for example, 60% or more, preferably 70% or more, more preferably 80% or more, still more preferably 85% or more, and from the viewpoints of improving heat resistance and reducing the water content, for example, 99% or less, preferably 95% or less, more preferably 92% or less, still more preferably 90% or less.

[0049] The ratio of the degree of neutralization by alkali metals to the degree of neutralization by divalent or higher metals (degree of neutralization by alkali metals / degree of neutralization by divalent or higher metals) is, for example, greater than 1, preferably 2 or more, and for example, 8 or less, preferably 6 or less, more preferably 4 or less.

[0050] The degree of neutralization can be calculated based on the number of moles of carboxyl groups in the carboxyl group-containing vinyl monomer used in the preparation and the valence of the metal ions in the neutralizing agent (described later).

[0051] (Content ratio) In the composite particles, the content ratio of the metal neutralization salt of the water-soluble polymer is, for example, 70 to 97 parts by mass, preferably 80 to 95 parts by mass, more preferably 85 to 93 parts by mass, and even more preferably 88 to 92 parts by mass, based on 100 parts by mass of the total amount of the metal neutralization salt of the water-soluble polymer and polyvinyl alcohol.

[0052] More specifically, in the composite particles, the content ratio of the metal neutralization salt of the water-soluble polymer is, from the viewpoint of heat resistance, for example, 70 parts by mass or more, preferably 80 parts by mass or more, more preferably 85 parts by mass or more, and even more preferably 88 parts by mass or more, relative to 100 parts by mass of the total amount of the metal neutralization salt of the water-soluble polymer and polyvinyl alcohol. Furthermore, from the viewpoint of reducing the moisture content, for example, 97 parts by mass or less, preferably 95 parts by mass or less, more preferably 93 parts by mass or less, and even more preferably 92 parts by mass or less.

[0053] <Polyvinyl Alcohol> Polyvinyl alcohol provides wettability to the coating material raw material for secondary battery separators and improves adhesion with porous films (described later).

[0054] Examples of polyvinyl alcohol include unmodified polyvinyl alcohol and modified polyvinyl alcohol.

[0055] Examples of modified polyvinyl alcohols include anionic group-modified polyvinyl alcohol (e.g., carboxyl group-modified polyvinyl alcohol, sulfo group-modified polyvinyl alcohol), and hydrophobic group-modified polyvinyl alcohol.

[0056] Preferably, the polyvinyl alcohol is unmodified polyvinyl alcohol.

[0057] The degree of polymerization of polyvinyl alcohol is, for example, 100 to 2000, preferably 150 to 1000, more preferably 200 to 500, and even more preferably 250 to 400.

[0058] More specifically, the degree of polymerization of polyvinyl alcohol is, from the viewpoint of heat resistance, for example, 100 or more, preferably 150 or more, more preferably 200 or more, and even more preferably 250 or more. Furthermore, from the viewpoint of reducing the moisture content, it is, for example, 2000 or less, preferably 1000 or less, more preferably 500 or less, and even more preferably 400 or less.

[0059] The degree of polymerization of polyvinyl alcohol can be determined in accordance with JIS K6726 (1994).

[0060] The degree of saponification of polyvinyl alcohol is, for example, 60 mol% or more, preferably 70 mol% or more, more preferably 80 mol% or more, and even more preferably 85 mol% or more. Also, for example, from the viewpoint of heat resistance, it is 100 mol% or less, preferably 95 mol% or less, and more preferably 90 mol% or less.

[0061] The degree of saponification of polyvinyl alcohol can be determined in accordance with JIS K6726 (1994).

[0062] Polyvinyl alcohol can be used alone or in combination with two or more other types.

[0063] In the composite particles, the content of polyvinyl alcohol is, for example, 3 to 30 parts by mass, preferably 5 to 20 parts by mass, more preferably 7 to 15 parts by mass, and even more preferably 8 to 12 parts by mass, based on 100 parts by mass of the total amount of the metal neutralization salt of the water-soluble polymer and polyvinyl alcohol.

[0064] More specifically, in the composite particles, the polyvinyl alcohol content is, from the viewpoint of reducing the water content, for example, 3 parts by mass or more, preferably 5 parts by mass or more, more preferably 7 parts by mass or more, and even more preferably 8 parts by mass or more, relative to 100 parts by mass of the total amount of the metal neutralization salt of the water-soluble polymer and polyvinyl alcohol. Furthermore, from the viewpoint of heat resistance, for example, 30 parts by mass or less, preferably 20 parts by mass or less, more preferably 15 parts by mass or less, and even more preferably 12 parts by mass or less.

[0065] <Method for producing composite particles> Composite particles are obtained by polymerizing neutralized polymer components in the presence of polyvinyl alcohol.

[0066] Specifically, first, polyvinyl alcohol is mixed with water and heated to dissolve it, thereby preparing an aqueous solution of polyvinyl alcohol.

[0067] The heating conditions include a heating temperature of, for example, 70°C to 100°C, and a heating time of, for example, 60 minutes to 240 minutes.

[0068] Next, a polymerization component, a polymerization initiator, and a neutralizing agent are added to an aqueous solution of polyvinyl alcohol.

[0069] Examples of polymerization initiators include persulfates and 4,4'-azobis(4-cyanovaleric acid). Examples of persulfates include ammonium persulfate and potassium persulfate. Ammonium persulfate is preferably used as the polymerization initiator.

[0070] The mixing ratio of the polymerization initiator is, for example, 1 to 10 parts by mass per 100 parts by mass of polyvinyl alcohol.

[0071] As neutralizing agents, neutralizing agents containing the above-mentioned alkali metals and neutralizing agents containing the above-mentioned divalent or higher metals are selected.

[0072] Examples of neutralizing agents containing the above-mentioned alkali metals include alkali metal hydroxides.

[0073] Examples of alkali metal hydroxides include sodium hydroxide, potassium hydroxide, and lithium hydroxide. From the viewpoint of heat resistance, sodium hydroxide is preferred as the alkali metal hydroxide.

[0074] Examples of neutralizing agents containing the above-mentioned divalent or higher metals include hydroxides of divalent or higher metals.

[0075] Examples of hydroxides of metals with a valency of two or more include hydroxides of alkaline earth metals and hydroxides of other metals with a valency of two or more.

[0076] Examples of alkaline earth metal hydroxides include calcium hydroxide, magnesium hydroxide, and barium hydroxide. From the viewpoint of reducing water content, calcium hydroxide is preferred as the alkaline earth metal hydroxide. Furthermore, from the viewpoint of heat resistance, magnesium hydroxide is preferred as the alkaline earth metal hydroxide.

[0077] Other examples of hydroxides of metals with a valency of two or more include zinc hydroxide, copper(II) hydroxide, and iron(II) hydroxide. Zinc hydroxide is a preferred example of a hydroxide of a metal with a valency of two or more.

[0078] From the viewpoint of improving heat resistance and reducing moisture content, the hydroxide of a metal with a valency of 2 or higher is preferably at least one selected from the group consisting of calcium hydroxide, magnesium hydroxide, and zinc hydroxide.

[0079] The neutralizing agent is formulated such that the degree of neutralization by the alkali metal and the degree of neutralization by the metal with a valence of 2 or higher fall within the specified range.

[0080] Neutralizing agents can be used alone or in combination of two or more types.

[0081] At this time, the carboxyl groups derived from the carboxyl group-containing vinyl monomer in the polymerization component are neutralized by a neutralizing agent (alkali metal and divalent or higher metal).

[0082] Next, the neutralized polymerization component is polymerized in the presence of polyvinyl alcohol.

[0083] The polymerization conditions include a polymerization temperature of, for example, 40°C to 120°C, preferably 60°C to 100°C. The polymerization time is, for example, 0.5 hours to 12 hours.

[0084] This yields a metal neutralization salt of the water-soluble polymer, which is the polymer of the polymerization component.

[0085] Then, the metal neutralization salt of the water-soluble polymer and the polyvinyl alcohol undergo hydrophobic association, separating from water and forming composite particles dispersed in water. Specifically, the carboxyl groups of the metal neutralization salt of the water-soluble polymer and the hydroxyl groups of the polyvinyl alcohol undergo hydrophobic association, separating from water and forming composite particles dispersed in water (at a solid content concentration of 10% by mass or more). This yields an aqueous dispersion containing the composite particles (composite particles and water).

[0086] More specifically, the above-mentioned composite particles can be produced by polymerizing the polymerization components in the presence of polyvinyl alcohol. In other words, for example, even if an aqueous solution of polyvinyl alcohol and an aqueous solution of the metal neutralization salt of a water-soluble polymer are prepared separately and mixed, the above-mentioned composite particles cannot be produced (in such cases, an aqueous solution in which polyvinyl alcohol and the metal neutralization salt of a water-soluble polymer are miscible in water is produced). Furthermore, at concentrations above a certain level, precipitates are generated over time due to the hydrophobic association described above, but these precipitates settle without dispersing in water, and therefore a uniform dispersion cannot be obtained.

[0087] In an aqueous dispersion containing composite particles, the solid content concentration of the composite particles is, for example, 10% to 30% by mass.

[0088] Based on the above, composite particles and water are used to produce raw materials for coating materials for secondary battery separators.

[0089] The proportion of composite particles is, for example, 80% by mass or more, preferably 90% by mass or more, and more preferably 100% by mass, relative to the solid content of the coating material raw material for secondary battery separators. In other words, the coating material raw material for secondary battery separators contains only composite particles as solid content.

[0090] In the above explanation, a metal neutralization salt of a water-soluble polymer is produced by mixing a polymerization component, a polymerization initiator, and a neutralizing agent with an aqueous solution of polyvinyl alcohol. However, it is also possible to first obtain a water-soluble polymer by blending a polymerization component and a polymerization initiator with an aqueous solution of polyvinyl alcohol, and then produce a metal neutralization salt of the water-soluble polymer by blending in a neutralizing agent.

[0091] Furthermore, the raw materials for the coating material of secondary battery separators may contain additives such as wetting agents, dispersants, hydrophilic resins, humectants, defoamers, and pH adjusters in appropriate proportions as needed. In other words, the raw materials for the coating material of secondary battery separators may contain additives as needed.

[0092] The raw material for the coating material used in secondary battery separators contains the above-mentioned composite particles. Therefore, it has excellent heat resistance and reduced moisture content.

[0093] Furthermore, such raw materials for coating materials for secondary battery separators can be particularly suitable for use as raw materials for coating materials for secondary battery separators.

[0094] The following describes in detail the coating material for secondary battery separators obtained using this raw material for secondary battery separator coatings.

[0095] 2. Coating material for secondary battery separators The coating material for secondary battery separators comprises the above-mentioned raw materials for secondary battery separator coating materials and inorganic particles.

[0096] The content ratio of the raw materials for the coating material for secondary battery separators is, for example, 1% to 10% by mass, preferably 3% to 7% by mass, in the coating material for secondary battery separators.

[0097] Examples of inorganic particles include oxides, nitrides, carbides, sulfates, hydroxides, and potassium titanate. Examples of oxides include alumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide, and iron oxide. Examples of nitrides include silicon nitride, titanium nitride, and boron nitride. Examples of carbides include silicon carbide and calcium carbonate. Examples of sulfates include magnesium sulfate and aluminum sulfate. Examples of hydroxides include aluminum hydroxide and aluminum hydroxide oxide. Examples of silicates include talc, kaolinite, decite, nacrite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amethyst, bentonite, asbestos, zeolite, calcium silicate, magnesium silicate, diatomaceous earth, silica sand, and glass.

[0098] Preferably, the inorganic particles are hydroxides. More preferably, the inorganic particles are aluminum hydroxide oxide.

[0099] The average median diameter D50 of the inorganic particles is, for example, 0.1 μm to 5 μm, preferably 0.5 μm to 1 μm.

[0100] Inorganic particles can be used alone or in combination of two or more types.

[0101] The inorganic particle content in the coating material for secondary battery separators is, for example, 30% to 60% by mass, preferably 40% to 50% by mass.

[0102] To manufacture the coating material for secondary battery separators, first, an aqueous dispersion of inorganic particles is prepared by mixing inorganic particles and, if necessary, a dispersant with water. If a dispersant is included, the coating material for secondary battery separators will contain the dispersant.

[0103] Examples of dispersants include ammonium polycarboxylate and sodium polycarboxylate. Ammonium polycarboxylate is preferred as the dispersant.

[0104] The proportion of the dispersant (solid content) is, for example, 0.1 to 10 parts by mass, preferably 0.5 to 5 parts by mass, per 100 parts by mass of inorganic particles.

[0105] Dispersants can be used alone or in combination of two or more types.

[0106] Next, the raw materials for the coating material of secondary battery separators are added to the aqueous dispersion of inorganic particles and stirred.

[0107] The stirring method is not particularly limited and includes, for example, ball mills, bead mills, planetary ball mills, vibrating ball mills, sand mills, colloid mills, attritors, roll mills, high-speed impeller dispersion, stirrers, dispersers, homogenizers, high-speed impact mills, ultrasonic dispersion, and stirring blades.

[0108] Furthermore, the coating material for secondary battery separators may contain the above-mentioned additives in appropriate proportions as needed. In other words, the coating material for secondary battery separators may contain the above-mentioned additives as needed.

[0109] These additives can be used individually or in combination of two or more types.

[0110] This yields a coating material for secondary battery separators. Furthermore, such a coating material for secondary battery separators can be obtained as an aqueous dispersion in water.

[0111] The solid content concentration of the aqueous dispersion of the coating material for secondary battery separators is, for example, 10% to 60% by mass, preferably 30% to 50% by mass.

[0112] The coating material for secondary battery separators contains the above-mentioned raw materials for secondary battery separator coating materials. Therefore, secondary battery separators equipped with a coating film obtained using this coating material have excellent heat resistance and reduced moisture content.

[0113] The secondary battery separator obtained using this secondary battery separator coating material will be described in detail below.

[0114] 3. Secondary Battery Separator The secondary battery separator comprises a porous membrane and a coating film of a secondary battery separator coating material disposed on at least one side of the porous membrane.

[0115] [Porous membranes] Examples of porous membranes include polyolefin porous membranes and aromatic polyamide porous membranes. Examples of polyolefin porous membranes include polyethylene porous membranes and polypropylene porous membranes. Preferably, a polyolefin porous membrane is used as the porous membrane.

[0116] The thickness of the porous film is, for example, 1 μm to 40 μm, preferably 5 μm to 20 μm.

[0117] [Coated Film] The coated film imparts heat resistance to the porous film. The coated film consists of a coating material for secondary battery separators.

[0118] The thickness of the coating film is, for example, 1 μm to 10 μm, preferably 2 μm to 8 μm or less.

[0119] [Method for manufacturing a secondary battery separator] The method for manufacturing a secondary battery separator comprises a first step of preparing a porous membrane, and a second step of applying a separator coating material to at least one side of the porous membrane.

[0120] (Step 1) In Step 1, a porous membrane is prepared.

[0121] (Second step) In the second step, a coating material for secondary battery separators is applied to at least one side of the porous film, and then dried as necessary. This obtains a coated film.

[0122] To apply a coating material for secondary battery separators to at least one side of a porous film, first, if necessary, a surface treatment is applied to one side of the porous film to form a surface treatment layer on that side.

[0123] In other words, in such cases, the secondary battery separator comprises a porous film, a surface treatment layer, and a coating film of a coating material for secondary battery separators.

[0124] Examples of surface treatments include corona discharge treatment, glow discharge treatment, plasma treatment, and ozone treatment. In this manufacturing method, surface treatment is preferably omitted from the viewpoint of improving ion permeability. In other words, the secondary battery separator preferably does not have a surface treatment layer.

[0125] Furthermore, there are no particular limitations on the coating method for applying the coating material for secondary battery separators. Examples include the wire bar method, gravure coater method, small-diameter gravure coater method, reverse roll coater method, transfer roll coater method, kiss coater method, dip coater method, microgravure coating 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 coating method. Preferably, the wire bar method is used as the coating method.

[0126] The drying temperature is, for example, 40°C to 80°C.

[0127] This allows for the manufacture of a secondary battery separator comprising a porous membrane and a coating film of the aforementioned secondary battery separator coating material disposed on at least one side of the porous membrane.

[0128] In the above explanation, the coating film for secondary battery separators was placed on at least one side of the porous film, but it is also possible to place the above coating film on both sides of the porous film.

[0129] This secondary battery separator is coated with the aforementioned secondary battery separator coating material. Therefore, the secondary battery separator has excellent heat resistance and reduced moisture content. For this reason, this secondary battery separator can be suitably used in the manufacture of secondary batteries.

[0130] 4. Secondary Battery A secondary battery comprises a positive electrode, a negative electrode, a 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 secondary battery separator.

[0131] 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.

[0132] Examples of current collectors for the positive electrode include conductive materials such as aluminum, titanium, stainless steel, nickel, calcined carbon, conductive polymers, and conductive glass.

[0133] The positive electrode active material is not particularly limited, but known positive electrode active materials include lithium-containing transition metal oxides, lithium-containing phosphates, and lithium-containing sulfates.

[0134] These positive electrode active materials can be used individually or in combination of two or more types.

[0135] As the negative electrode, for example, a known electrode comprising a negative electrode current collector and a negative electrode active material stacked on the negative electrode current collector can be used.

[0136] Examples of current collectors for the negative electrode include conductive materials such as copper and nickel.

[0137] The negative electrode active material is not particularly limited, but carbon active materials are examples. Examples of carbon active materials include graphite, soft carbon, and hard carbon.

[0138] These negative electrode active materials can be used individually or in combination of two or more types.

[0139] When lithium-ion batteries are used as the electrolyte and secondary battery, examples include solutions in which lithium salts are dissolved in carbonate compounds such as ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC).

[0140] To manufacture a rechargeable battery, for example, a separator for the rechargeable battery is placed between the positive electrode and the negative electrode, these are then housed in a battery casing (cell), and an electrolyte is injected into the battery casing. This allows a rechargeable battery to be obtained.

[0141] The above-mentioned secondary battery is equipped with the above-mentioned secondary battery separator. Therefore, it has excellent heat resistance and reduced moisture content.

[0142] <Effects> The coating material raw material for secondary battery separators contains composite particles containing a metal neutralization salt of a water-soluble polymer and polyvinyl alcohol. The metal in the metal neutralization salt is an alkali metal and a metal with a valency of 2 or higher. Therefore, it has excellent heat resistance and can reduce moisture content.

[0143] More specifically, because the metals in the metal neutralization salt are alkali metals and metals with a valency of 2 or higher, the dispersibility of the composite particles is improved, and because the glass transition temperature of the composite particles is high, the heat resistance can be improved.

[0144] Furthermore, because the metal in the metal neutralization salt is a metal with a valency of 2 or higher, it forms a cross-linked structure, and the hydrophilicity of the water-soluble polymer in the metal neutralization salt decreases, thus reducing the water content.

[0145] Furthermore, as described above, the composite particles are particles formed by the hydrophobic association of a metal neutralized salt of a water-soluble polymer and polyvinyl alcohol, and are distinct from aqueous solutions in which polyvinyl alcohol and a metal neutralized salt of a water-soluble polymer are miscible in water.

[0146] The raw material for the coating material for secondary battery separators contains the above-mentioned composite particles, and therefore exhibits excellent coating properties from the viewpoint of dispersibility. On the other hand, the aqueous solution in which polyvinyl alcohol and the metal neutralization salt of a water-soluble polymer are miscible in water deteriorates in coating properties over time due to the above-mentioned precipitates.

[0147] Furthermore, as described above, in the composite particles, the metal neutralization salt of the water-soluble polymer and polyvinyl alcohol are not copolymerized. In other words, it is distinct from the metal neutralization salt of the copolymer obtained by neutralizing the copolymer of polyvinyl alcohol and acrylic acid (polyacrylic acid) with magnesium acetate and lithium hydroxide in the binder for non-aqueous electrolyte batteries described in Patent Document 1.

[0148] The specific numerical values ​​such as blending ratios (content percentages), physical properties, and parameters used in the following description may be replaced with the corresponding upper limits (numbers defined as "less than or equal to" or "less than") or lower limits (numbers defined as "greater than or equal to" or "greater than") of the blending ratios (content percentages), physical properties, and parameters described in the "Modes for Carrying Out the Invention" above. Furthermore, unless otherwise specified in the following description, "parts" and "%" refer to mass.

[0149] <Details of Ingredients> The product names and abbreviations of the ingredients used in each example and each comparative example are described in detail below. 3-88: Unmodified polyvinyl alcohol, product name "Kuraray Poval 3-88", degree of saponification 87 mol% to 89 mol%, degree of polymerization 300, manufactured by Kuraray Co., Ltd. 5-74: Unmodified polyvinyl alcohol, product name "Kuraray Poval 5-74", degree of saponification 73 mol% to 75 mol%, degree of polymerization 500, manufactured by Kuraray Co., Ltd. 30-88: Unmodified polyvinyl alcohol, product name "Kuraray Poval 30-88", degree of saponification 87 mol% to 89 mol%, degree of polymerization 2000, manufactured by Kuraray Co., Ltd. NaOH: 40% by mass sodium hydroxide aqueous solution KOH: 40% by mass potassium hydroxide aqueous solution Ca(OH) ２ : 20% by mass calcium hydroxide emulsion Zn(OH) ２ : 20% by mass zinc hydroxide aqueous solution Mg(OH) ２ : 20% by mass magnesium hydroxide aqueous solution

[0150] <Manufacturing of coating material raw materials for secondary battery separators, coating material for secondary battery separators, and secondary battery separators> Example 1 (Manufacturing of coating material raw materials for secondary battery separators) 566.2 parts by mass of distilled water were charged into a separable flask equipped with a stirrer and reflux cooling, and 34 parts by mass of polyvinyl alcohol were added while stirring. The temperature was raised to 95°C and stirring was continued for 3 hours to prepare an aqueous solution of polyvinyl alcohol. After that, the inside of the flask was replaced with nitrogen gas and the temperature was lowered to 80°C.

[0151] Next, 1.56 parts by mass of ammonium persulfate was added as a polymerization initiator. Then, polymerization components (214.8 parts by mass of acrylic acid), neutralizing agents (223.6 parts by mass of 40% sodium hydroxide aqueous solution and 57.0 parts by mass of 20% calcium hydroxide emulsion), and 650.0 parts by mass of distilled water were continuously added over 3 hours. The mixture was then held for another 3 hours, after which an appropriate amount of water was added. This polymerized the polymerization components to obtain a water-soluble polymer, which was then neutralized to obtain a metal neutralization salt of the water-soluble polymer. Thus, a raw material for a secondary battery separator coating (solid content concentration 16.0% by mass) containing a water dispersion of composite particles containing a metal neutralization salt of the water-soluble polymer and polyvinyl alcohol was produced.

[0152] (Manufacturing of coating material for secondary battery separators) 1 part by mass of dispersant (ammonium polycarboxylate) was added to 123 parts by mass of water. Then, while stirring with a disperser (1000 rpm), 100 parts by mass of boehmite (aluminum hydroxide oxide, manufactured by Navaltec, trade name "Apiral AOH60", average median diameter D50: 0.9 μm) was gradually added as inorganic particles. After addition, the mixture was further stirred with a homogenizer (5000 rpm). This yielded an aqueous dispersion of inorganic particles (solid content concentration of inorganic particles 45% by mass).

[0153] Next, 5 parts by mass of the coating material raw materials for secondary battery separators of each example and each comparative example were added to an aqueous dispersion of inorganic particles, water was added as needed, and the mixture was stirred.

[0154] Subsequently, this was filtered through a 300-mesh filter (filtration particle size 48 μm). This produced a coating material for secondary battery separators (a dispersion of the coating material for secondary battery separators). The solid content concentration of the dispersion of the coating material for secondary battery separators was 40% by mass.

[0155] (Manufacturing of secondary battery separator) [Step 1] A polyolefin porous membrane (without surface treatment (corona treatment)) was prepared as the porous membrane.

[0156] [Second Step] Using a wire bar, the above-mentioned coating material for secondary battery separators (dispersion of the coating material for secondary battery separators) was applied to one side of the polyolefin porous membrane, and then dried at 50°C. This formed coated films of the coating material for secondary battery separators (thickness 2 μm, thickness 2.5 μm, thickness 3.5 μm) on one side of the polyolefin porous membrane. This completed the production of the secondary battery separator.

[0157] Examples 2 to 9 and Comparative Examples 1 to 3: Based on the same procedure as in Example 1, raw materials for secondary battery separator coatings, coatings for secondary battery separators, and coatings for secondary battery separators were manufactured. However, the amounts of polymerization components and neutralizing agents were changed so that the content ratio of the metal neutralization salt of the water-soluble polymer, the content ratio of polyvinyl alcohol, and the degree of neutralization by the neutralizing agent were as shown in Table 1. In Example 9, 50 parts by mass of acrylic acid and 50 parts by mass of methacrylic acid were used as polymerization components.

[0158] (Manufacturing of coating material for secondary battery separators and secondary battery separators) A coating material for secondary battery separators and a coating material for secondary battery separators were manufactured based on the same procedure as in Example 1.

[0159] <Evaluation> [Heat Resistance] The secondary battery separators (coating film thickness 2.5 μm and 3.5 μm) of each example and comparative example were cut into 5 cm x 5 cm pieces to serve as test specimens. These test specimens were left in an oven at 150°C for 1 hour. The length of each side of the test specimen was measured before and after the oven was opened. The shrinkage rate was calculated from the lengths of each side before and after shrinkage using the following formula (1). The results are shown in Table 1. A smaller shrinkage rate indicates better heat resistance. Shrinkage rate (%) = {Average length of one side before shrinkage (cm) - Average length of one side after shrinkage (cm)} / Average length of one side before shrinkage (cm) × 100 (1)

[0160] [Moisture Content] For each example and comparative example, the moisture content of the secondary battery separator (coated film thickness 2 μm) was determined using the moisture vaporization method with a Karl Fischer moisture meter 852 Titrando. The results are shown in Table 1.

[0161] <Discussion> Examples 1 to 9 contain composite particles comprising a metal neutralization salt of a water-soluble polymer containing constituent units derived from polycarboxylic acid and polyvinyl alcohol, and the metal in the metal neutralization salt is an alkali metal and a metal with a valency of 2 or higher. Therefore, it can be seen that it has excellent heat resistance and can reduce the moisture content.

[0162] In Comparative Example 1, the carboxyl groups derived from acrylic acid in the water-soluble polymer are neutralized by ammonia. Therefore, it can be seen that the heat resistance is reduced.

[0163] In Comparative Examples 2 and 3, the metal in the metal neutralization salt is solely an alkali metal. Therefore, it is clear that the water content cannot be reduced.

[0164]

[0165] The above invention is provided as an illustrative embodiment of the present invention, but this is merely illustrative and should not be interpreted restrictively. Modifications of the present invention that are obvious to those skilled in the art are included in the claims described below.

[0166] The raw materials for coating materials for secondary battery separators, the coating material for secondary battery separators, and the secondary battery separator of the present invention are suitably used, for example, in the manufacture of secondary batteries. The secondary battery of the present invention is suitably used, for example, in vehicles that require heat resistance.

Claims

1. A raw material for a coating material for secondary battery separators, comprising composite particles containing a metal neutralization salt of a water-soluble polymer that does not contain constituent units derived from polyvinyl alcohol but contains constituent units derived from polycarboxylic acid, and polyvinyl alcohol, wherein the metal in the metal neutralization salt is an alkali metal and a metal with a valency of 2 or higher.

2. The raw material for a coating material for a secondary battery separator according to claim 1, wherein in the composite particles, the content ratio of the metal neutralization salt of the water-soluble polymer is 70 parts by mass or more and 97 parts by mass or less with respect to 100 parts by mass of the total amount of the metal neutralization salt of the water-soluble polymer and the polyvinyl alcohol, and in the composite particles, the content ratio of the polyvinyl alcohol is 3 parts by mass or more and 30 parts by mass or less with respect to 100 parts by mass of the total amount of the metal neutralization salt of the water-soluble polymer and the polyvinyl alcohol.

3. The raw material for a coating material for a secondary battery separator according to claim 1, wherein the degree of polymerization of the polyvinyl alcohol is 100 or more and 2000 or less, and the degree of saponification of the polyvinyl alcohol is 90 mol% or less.

4. The raw material for a coating material for a secondary battery separator according to claim 1, wherein the degree of neutralization by the divalent or higher metal in the metal neutralization salt of the water-soluble polymer is 15% or more.

5. The raw material for a coating material for a secondary battery separator according to claim 1, wherein the divalent or higher metal is at least one selected from the group consisting of calcium, magnesium, and zinc.

6. A coating material for a secondary battery separator, comprising the raw material for a secondary battery separator coating material according to any one of claims 1 to 5, and inorganic particles.

7. A secondary battery separator comprising a porous membrane and a coating film of the secondary battery separator coating material according to claim 6, disposed on at least one side of the porous membrane.

8. A secondary battery comprising a positive electrode, a negative electrode, and a secondary battery separator as described in claim 7, disposed between the positive electrode and the negative electrode.

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