Insulating paste and insulating layer for lithium-ion secondary battery current collector

Abstract

To provide an insulating paste which can achieve satisfactory shelf stability and coating workability, and an insulator layer which is satisfactory in finishing quality after coating, especially adhesion to a current collector.SOLUTION: An insulating paste for a lithium ion secondary battery current collector comprises an inorganic filler (A), a binder (B), a dispersant (C) and a solvent (D). An insulator layer obtained by coating a current collector with the insulating paste has an adhesive force of 2.5 N / m or larger.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an insulating paste to be applied onto a current collector of a positive electrode and / or a negative electrode of a lithium ion secondary battery, and to an insulating layer obtained by applying the paste. [Background technology]

[0002] Lithium-ion secondary batteries include those in which positive and negative electrodes are stacked or wound. These positive and negative electrodes are manufactured by applying a mixture layer to both sides of a metal foil (current collector), drying, and pressing. Many positive and negative electrodes have exposed portions at the ends of the metal foil as a current path. Techniques have been disclosed to prevent short circuits (insulate) these exposed portions.

[0003] For example, Patent Document 1 discloses a lithium-ion secondary battery including an insulating layer. This insulating layer prevents short circuits between the positive and negative electrode plates, but the layer has poor storage stability and coating workability, making it difficult to achieve a satisfactory finish. Furthermore, there is a risk that the insulating layer may fall off when subjected to physical load.

[0004] In particular, if high loads (such as bending, cutting, pressure, and scratching) are applied during manufacturing processes such as pressing, the insulating layer may peel or fall off from the current collector, preventing the battery from achieving its intended performance. Therefore, the adhesion between the current collector and the insulating layer is extremely important, as it has a significant impact on battery performance and safety. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-232425 Summary of the Invention [Problem to be solved by the invention]

[0006] The problem to be solved by the present invention is to provide an insulating paste having good storage properties (pigment settling property, viscosity) and coating workability, and an insulating layer having good finish after coating, particularly good adhesion to a current collector. [Means for solving the problem]

[0007] As a result of intensive research into solving the above-mentioned problems, the inventors have found that the above-mentioned problems can be solved by an insulating paste for a current collector of a lithium ion secondary battery, which contains an inorganic filler (A), a binder (B), a dispersant (C), and a solvent (D), and the insulating paste is applied to a current collector to obtain an insulating layer having an adhesion strength of 2.5 N / m or more, and have thus completed the present invention.

[0008] That is, the present invention provides the following insulating paste and insulating layer. Item 1. An insulating paste for a lithium ion secondary battery current collector, comprising an inorganic filler (A), a binder (B), a dispersant (C), and a solvent (D), wherein the adhesive strength of an insulating layer obtained by applying the insulating paste onto a current collector is 2.5 N / m or more. Item 2. The insulating paste for a lithium ion secondary battery current collector according to Item 1, wherein the inorganic filler (A) has a volume average particle size (D50) of 0.5 to 7 μm and a particle size distribution standard deviation of 1.4 μm or less. Item 3. The insulating paste for a lithium ion secondary battery current collector according to Item 1 or 2, wherein the dispersant (C) contains a polar group-containing acrylic resin. Item 4. The insulating paste for a lithium ion secondary battery current collector according to Item 3, wherein the polar group of the polar group-containing acrylic resin is a phosphate group. Item 5. An insulating paste for a lithium ion secondary battery current collector according to Item 3 or 4, wherein the polar group-containing acrylic resin is a polymer of a raw material monomer including a polymerizable unsaturated monomer (c2) having a hydrocarbon group having 4 or more carbon atoms. Item 6. The insulating paste for a lithium ion secondary battery current collector according to any one of Items 3 to 5, wherein the polar group-containing acrylic resin has a weight average molecular weight in the range of 1,000 to 100,000. Item 7. An insulating paste for a lithium ion secondary battery current collector, wherein the insulating layer according to any one of items 1 to 6 is non-porous. Item 8. The insulating paste for a lithium ion secondary battery current collector according to any one of Items 1 to 7, wherein the inorganic filler (A) is at least one selected from the group consisting of alumina, silica, TiO2, BaTiO3, ZrO2, boehmite, zeolite, apatite, and kaolin. Item 9. The insulating paste for a lithium ion secondary battery current collector according to any one of Items 1 to 8, wherein the binder (B) is modified or unmodified polyvinylidene fluoride. Item 10. The insulating paste for a lithium ion secondary battery current collector according to any one of Items 1 to 9, wherein the solvent (D) is N-methyl-2-pyrrolidone. Item 11. An insulating paste for a current collector of a lithium ion secondary battery, characterized in that the insulating paste according to any one of items 1 to 10 does not substantially contain an electrode active material. Item 12. A method for producing an insulating layer for a lithium ion secondary battery current collector, comprising applying the insulating paste according to any one of Items 1 to 11 to a part or all of a current collector, and then heating and drying the paste to form an insulating layer. Item 13. A method for producing an insulating layer for a current collector of a lithium ion secondary battery, comprising applying the insulating paste according to any one of Items 1 to 11 to a part or all of a current collector, and then heating and drying the applied paste to form an insulating layer, wherein the sodium content of the insulating layer is 15 ppm or more. Item 14. The method for producing an insulating layer for a current collector of a lithium ion secondary battery according to Item 12 or 13, wherein the current collector is made of aluminum or a composite metal thereof. [Effects of the Invention]

[0009] The insulating paste of the present invention has good storage properties (pigment settling property, viscosity) and coating workability, and the insulating layer obtained has good finish and adhesion. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention will be described in detail below.

[0011] In this specification, unless otherwise specified, the phrase "a raw material for a resin contains a monomer X" means that the resin is a (co)polymer of raw material monomers containing the monomer X. In addition, in this specification, a (co)polymer means a polymer or a copolymer.

[0012] In this specification, "(meth)acrylate" means acrylate and / or methacrylate, "(meth)acrylic acid" means acrylic acid and / or methacrylic acid, "(meth)acryloyl" means acryloyl and / or methacryloyl, and "(meth)acrylamide" means acrylamide and / or methacrylamide.

[0013] Furthermore, in the present specification, the term "insulating layer" may be alternatively referred to as an "insulating film," a "coating film," or a "film."

[0014] Insulating paste The insulating paste for a lithium ion secondary battery current collector of the present invention is an insulating paste containing an inorganic filler (A), a binder (B), a dispersant (C), and a solvent (D).

[0015] The adhesive strength of the insulating layer obtained by applying the insulating paste to the current collector is usually 2.5 N / m or more, preferably 4.5 N / m or more, more preferably 6.5 N / m or more, and even more preferably 10 N / m or more. If the adhesive strength is 2.5 N / m or more, the insulating layer can maintain a good condition even when subjected to loads such as pressing, bending, and impact.

[0016] The adhesive strength of the insulating layer to the current collector can be measured by the following test method.

[0017] <Method for measuring adhesive strength of insulating layer> The insulating paste was applied to an aluminum current collector using an applicator and then dried at 120°C for 30 minutes to form a coating film (dry film thickness 15 μm). Next, a 300 μm thick PET film was attached to the coating film of a test plate consisting of a laminate of the current collector and coating film using double-sided tape for reinforcement. The resulting laminate was cut into 10 cm long, 1.5 cm wide strips with a cutter, measuring the thickness from the PET to the coating film. Furthermore, double-sided tape was attached to the current collector side of the horizontally placed sample, and the sample was adhesively fixed to a tin plate. A 180° peel test was performed using an "Ez-Test" (trade name, manufactured by Shimadzu Corporation) at a tensile speed of 10 cm / min.

[0018] The sodium content in the insulating paste is generally adjusted to 450 ppm or less, preferably 10 to 350 ppm, more preferably 20 to 300 ppm, and even more preferably 30 to 200 ppm, from the viewpoints of storage stability (suppression of a decrease in the viscosity of the insulating paste during storage (hereinafter referred to as storage viscosity thinning)) and adhesive strength. If the sodium content exceeds 450 ppm, viscosity thinning occurs during storage at high temperatures, which may result in reduced pigment sedimentation and finish quality (including sagging). Furthermore, if the sodium content is below 10 ppm (particularly 5 ppm), the adhesive strength of the insulating layer to the current collector may decrease.

[0019] The insulating paste contains sodium ions due to contamination from various raw materials (especially the inorganic filler described below) and during the manufacturing process.

[0020] The cause of the above-mentioned storage viscosity reduction is not fully understood, but one possible reason is that, for example, the viscosity of the paste is normally maintained by interactions between the inorganic filler (A), binder (B), and dispersion resin (C) through hydrogen bonds. However, when the paste contains more than a certain amount of sodium ions, this interaction is gradually broken down, causing the viscosity of the insulating paste to decrease.

[0021] The relationship between the adhesive strength and the sodium content is not fully understood, but it is believed that, for example, if the insulating layer contains a polar component in a certain amount or more, the adhesive strength with the current collector increases.

[0022] The sodium content in the insulating paste can be measured, for example, using an ICP optical emission spectrometer. Specifically, the sample (insulating paste) is dissolved in a nitric acid / sulfuric acid mixture (mixing ratio: 1 / 1), and the sodium content can be measured using an ICP optical emission spectrometer (Shimadzu Corporation, "ICPS-8100").

[0023] Inorganic filler (A) The inorganic filler (A) that can be used in the insulating paste of the present invention can be any non-conductive inorganic filler, and examples thereof include alumina, silica, TiO2, BaTiO3, ZrO2, boehmite, zeolite, apatite, and kaolin. One type can be used alone, or two or more types can be used in combination. Among these, alumina and / or boehmite are preferred. Alumina is aluminum oxide represented by Al2O3, and boehmite is alumina monohydrate represented by the composition γ-AlO(OH).

[0024] The volume average particle size (D50) of the inorganic filler (A) in the insulating paste of the present invention is preferably 0.5 to 7 μm, more preferably 0.8 to 5.5 μm, and even more preferably 1.2 to 3.5 μm.

[0025] If the particle size is small, the surface area of ​​the particles will increase, which will increase the viscosity of the paste and deteriorate the ease of application and the finish, while if the particle size is large, the finish of the surface of the insulating layer will deteriorate. Also, although the details are not known, it is thought that if the inorganic filler has a particle size above a certain level, the cohesive force of the insulating layer to the current collector will improve, improving adhesion.

[0026] The particle size distribution is preferably as narrow as possible, and the standard deviation of the particle size distribution is preferably 1.4 μm or less, more preferably 1.0 μm or less.

[0027] In the present invention, the volume average particle size (D50) and the standard deviation of particle size distribution of the inorganic filler present in the insulating paste were measured by measuring the insulating paste with a particle size distribution measuring device (manufactured by Microtrac-Bell, product name: Microtrac MT3000).

[0028] The volume average particle size (D50) of the inorganic filler (A) itself, which is the primary particle size, is preferably from 0.01 to 6 μm, more preferably from 0.1 to 5 μm, and even more preferably from 0.7 to 3.0 μm.

[0029] The shape may be spherical, elliptical, plate-like, cube-like, scale-like, needle-like, rod-like, or the like, and any of these may be suitably used, with those with an aspect ratio of 1.1 or more being preferred.

[0030] When using boehmite as the inorganic filler (A), particular attention must be paid to the sodium content in the insulating paste. Since sodium hydroxide is used in the manufacturing process of aluminum hydroxide, which is the raw material for boehmite, the boehmite also contains a certain amount of sodium ions. The sodium ion content can be adjusted by washing or the like.

[0031] Specifically, the sodium content of the boehmite is usually 40 to 2000 ppm or less, preferably 40 to 1500 ppm, and more preferably 100 to 1200 ppm, based on the solid content of the boehmite.

[0032] Binder (B) The binder (B) that can be used in the insulating paste of the present invention is a copolymer having a polymerizable unsaturated group-containing monomer represented by the following formula (1) as a constituent component, and can be synthesized by copolymerizing a monomer containing the polymerizable unsaturated group-containing monomer. (R1-)(R2-)C=C(-R3)(-R4) ···(1) (wherein R1 to R4 are atoms selected from hydrogen, fluorine and chlorine, or linear, branched and / or cyclic organic groups.) The polymerizable unsaturated group-containing monomer can be used without any particular limitation as long as it has the structure of the above formula (1), and examples thereof include vinyl chloride, vinylidene chloride, vinyl fluoride, vinylidene fluoride, fatty acid vinyl ester, vinyl ether, vinylpyrrolidone, styrene, (meth)acryloyl group-containing monomers, and (meth)acrylamide group-containing monomers, and these can be used alone or in combination of two or more.

[0033] The polymerization method for the polymerizable unsaturated group-containing monomer may be a polymerization method known per se, for example, solution polymerization of a monomer containing the polymerizable unsaturated group-containing monomer in an organic solvent, but is not limited thereto, and may also be bulk polymerization, emulsion polymerization, suspension polymerization, etc. When solution polymerization is carried out, continuous polymerization or batch polymerization may be used, and the monomer may be charged all at once or in portions, or may be added continuously or intermittently. Various modifications can also be made after polymerization (for example, hydrolysis after polymerization, acetalization, reaction with other resins for grafting, etc.). The polymerization initiator used in the above polymerization is not particularly limited, and known radical polymerization initiators such as peroxide initiators, azo initiators, redox initiators, and organic halide initiators can be used.

[0034] As the solvent used in the above polymerization, any known solvent can be used without any particular limitation, and the organic solvents mentioned below in relation to the dispersing resin (C) can be suitably used.

[0035] The polymerization reaction temperature is not particularly limited, but can usually be set in the range of about 30 to 200°C.

[0036] Examples of the binder (B) include polyvinylidene fluoride, polyvinyl alcohol, polyvinyl acetal, acrylic resin, polyvinyl acetate, polyvinyl chloride, polystyrene, polyvinyl ether, and polyvinylpyrrolidone, which may be modified with various functional groups, and polar functional groups such as acid groups and bases can be suitably used as the functional groups. These may be used alone or in combination of two or more, and modified or unmodified polyvinylidene fluoride is preferred from the viewpoints of insulation properties and coating film strength (i.e., cohesive force).

[0037] The weight average molecular weight of the binder (B) is preferably greater than 100,000, more preferably within the range of 110,000 to 5,000,000, and even more preferably within the range of 200,000 to 2,000,000.

[0038] Dispersion resin (C) The dispersing resin (C) that can be used in the present invention preferably contains an acrylic resin, and in particular, it preferably contains a polar group-containing acrylic resin (c) that is a copolymer of raw material monomers including at least one polymerizable unsaturated monomer having a polar group.

[0039] If the dispersing resin (C) contains polar groups such as acid groups, it improves adhesion to the substrate and pigment dispersibility, but if there are too many, the polarity increases too much, resulting in poor finish. The presence of low-polarity monomers with 4 or more carbon atoms increases compatibility with the low-polarity fluororesin, improving the finish. Dispersibility also improves slightly.

[0040] For this reason, it is more preferable that the polar group-containing acrylic resin is a copolymer of raw material monomers containing a polymerizable unsaturated monomer (c1) having a polar functional group and a polymerizable unsaturated monomer (c2) having a hydrocarbon group having 4 or more carbon atoms, and that the weight-average molecular weight of the copolymer is 1,000 to 100,000.Polyvinyl alcohol (PVA) has high polarity, and therefore has poorer finish properties than such a polar group-containing acrylic resin (c).

[0041] <Polar group-containing acrylic resin (c)> The polar group-containing acrylic resin (c) contains a polymerizable unsaturated monomer (c1) having a polar functional group and a polymerizable unsaturated monomer (c2) having a hydrocarbon group having 4 or more carbon atoms, thereby achieving both dispersibility of the inorganic filler (A), compatibility with the binder (B), and adhesion to the current collector.

[0042] <Polymerizable unsaturated monomer having a polar group (c1)> The polymerizable unsaturated monomer (c1) having a polar group can be any polymerizable unsaturated monomer having a polar group without any particular limitation. Examples of the polar group include a carboxyl group, a phosphate group, a sulfonic acid group, an amino group, a quaternary base, a hydroxyl group, and a polyalkylene glycol group, and the polymerizable unsaturated monomer may have multiple polar groups.

[0043] Specific examples of the polymerizable unsaturated monomer (c1) having a polar group include carboxyl group-containing polymerizable unsaturated monomers such as (meth)acrylic acid, maleic acid, crotonic acid, and β-carboxyethyl acrylate; phosphoric acid group-containing polymerizable unsaturated monomers such as 2-(meth)acryloyloxyethyl acid phosphate and 2-(meth)acryloyloxypropyl acid phosphate; sulfonic acid group-containing polymerizable unsaturated monomers such as 2-acrylamido-2-methylpropanesulfonic acid, 2-sulfoethyl (meth)acrylate, allylsulfonic acid, and 4-styrenesulfonic acid, as well as sodium and ammonium salts of these sulfonic acids; amino group-containing polymerizable unsaturated monomers such as N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-dimethylaminopropyl (meth)acrylamide, and adducts of glycidyl (meth)acrylate and amines; 2-(methacryloyloxy)ethyl trimethylsilyl acrylate; Examples of suitable polymerizable unsaturated monomers include quaternary base-containing polymerizable unsaturated monomers such as methylammonium chloride; monoesters of (meth)acrylic acid with dihydric alcohols having 2 to 8 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate; ε-caprolactone-modified monoesters of (meth)acrylic acid with dihydric alcohols having 2 to 8 carbon atoms; N-hydroxymethyl (meth)acrylamide; allyl alcohol; and hydroxyl group-containing polymerizable unsaturated monomers such as (meth)acrylates having a polyoxyalkylene chain with a hydroxyl group at the molecular terminal; and polyalkylene glycol group-containing polymerizable unsaturated monomers such as polyethylene glycol (meth)acrylate and polypropylene glycol (meth)acrylate. Among these, preferred are polymerizable unsaturated monomers having an acid group, and more preferred are polymerizable unsaturated monomers having a phosphoric acid group. These may be used alone or in combination of two or more.

[0044] In the raw material monomers, the content of the polymerizable unsaturated monomer (c1) having a polar group is preferably 1 to 80 mass %, more preferably 5 to 70 mass %, and even more preferably 20 to 60 mass %.

[0045] When the content of the polymerizable unsaturated monomer (c1) having a polar group in the raw material monomers is within the above range, compatibility with the binder, pigment dispersibility, and adhesion are improved.

[0046] Furthermore, when the polar group-containing acrylic resin (c) contains acid groups, the acid value is preferably 200 mgKOH / g or less, more preferably within the range of 5 to 150 mgKOH / g; when it contains amino groups, the amine value is preferably 200 mgKOH / g or less, more preferably within the range of 5 to 150 mgKOH / g; and when it contains hydroxyl groups, the hydroxyl value is preferably 200 mgKOH / g or less, more preferably within the range of 5 to 150 mgKOH / g.

[0047] The acid value of the polar group-containing acrylic resin (c) can be measured according to JIS K-5601-2-1 (1999), and the amine value of the polar group-containing acrylic resin (a) can be measured according to JIS K7237 (1995).

[0048] <Polymerizable unsaturated monomer (c2) having an alkyl group having 4 or more carbon atoms> From the viewpoint of compatibility with the binder (B), the raw material monomer of the polar group-containing acrylic resin (c) preferably contains a polymerizable unsaturated monomer (c2) having an alkyl group with 4 or more carbon atoms. In particular, when the binder (B) is polyvinylidene fluoride, which has a relatively low polarity, a polymerizable unsaturated monomer (c2) having an alkyl group with 4 or more carbon atoms can be suitably used.

[0049] The polymerizable unsaturated monomer (c2) having an alkyl group with 4 or more carbon atoms can be a straight-chain, branched, or cyclic alkyl group, etc., and is not particularly limited as long as it is a polymerizable unsaturated monomer having an alkyl group with 4 or more carbon atoms. Specific examples include straight-chain, branched, or cyclic alkyl group-containing (meth)acrylates such as styrene, naphthyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, isostearyl acrylate, cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, and tridecyl (meth)acrylate, and these can be used alone or in combination of two or more.

[0050] The polymerizable unsaturated monomer (c2) having an alkyl group with 4 or more carbon atoms preferably has 4 to 24 carbon atoms, more preferably 8 to 20 carbon atoms, and particularly preferably 10 to 17 carbon atoms. In addition, the polymerizable unsaturated monomer having a linear or branched alkyl group structure is preferred.

[0051] In the raw material monomers, the polymerizable unsaturated monomer (c2) having an alkyl group with 4 or more carbon atoms is contained in an amount of preferably 1 to 95 mass %, more preferably 10 to 80 mass %, and even more preferably 20 to 60 mass %.

[0052] When the content of the polymerizable unsaturated monomer (c2) having an alkyl group with 4 or more carbon atoms in the raw material monomer falls within the above range, the dispersibility and storage stability are improved.

[0053] (Other polymerizable unsaturated monomers) As raw material monomers for obtaining the polar group-containing acrylic resin (c), other polymerizable unsaturated monomers (c2) other than the above-mentioned polymerizable unsaturated monomers (c1) having a polar group and polymerizable unsaturated monomers having an alkyl group with 4 or more carbon atoms can also be suitably used. Examples of other polymerizable unsaturated monomers include polymerizable unsaturated monomers having an alkyl group with 3 or less carbon atoms, such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, and isopropyl (meth)acrylate; and polymerizable unsaturated monomers having two or more polymerizable unsaturated groups in one molecule.

[0054] The polar group-containing acrylic resin (c) can be produced by a conventionally known polymerization method. For example, it can be produced by solution polymerization of a polymerizable unsaturated monomer (raw material monomer) in an organic solvent, but this is not limiting. For example, bulk polymerization, emulsion polymerization, suspension polymerization, etc. may also be used. When solution polymerization is carried out, continuous polymerization or batch polymerization may be used, and the polymerizable unsaturated monomer may be charged all at once or in portions, or may be added continuously or intermittently.

[0055] The radical polymerization initiator used in the polymerization can be a conventionally known method. For example, cyclohexanone peroxide, 3,3,5-trimethylcyclohexanone peroxide, methylcyclohexanone peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 1,1-bis(t-butylperoxy)cyclohexane, n-butyl-4,4-bis(t-butylperoxy)valerate, cumene hydroperoxide, 2,5-dimethylhexane-2,5-dihydroperoxide, 1,3-bis(t-butylperoxy-m-isopropyl)benzene, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, diisopropylbenzene peroxide, t-butylcumyl peroxide, decanoyl peroxide, lauroyl peroxide, benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, di-t-amyl peroxide, bis(methyl methyl cyclohexane), ... Peroxide polymerization initiators such as tert-butylcyclohexyl peroxydicarbonate, tert-butyl peroxybenzoate, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, and tert-butylperoxy-2-ethylhexanoate; 2,2'-azobis(isobutyronitrile), 1,1-azobis(cyclohexane-1-carbonitrile), azocumene, and 2,2'-azobis(2-methylbutyronitrile) Examples of azo polymerization initiators include 2,2'-azobisdimethylvaleronitrile, 4,4'-azobis(4-cyanovaleric acid), 2-(t-butylazo)-2-cyanopropane, 2,2'-azobis(2,4,4-trimethylpentane), 2,2'-azobis(2-methylpropane), and dimethyl 2,2'-azobis(2-methylpropionate), and these can be used alone or in combination of two or more.

[0056] The solvent used for the polymerization or dilution is not particularly limited, and examples thereof include water, organic solvents, and mixtures thereof. Examples of organic solvents include hydrocarbon solvents such as n-butane, n-hexane, n-heptane, n-octane, cyclopentane, cyclohexane, and cyclobutane; aromatic solvents such as toluene and xylene; ketone solvents such as methyl isobutyl ketone; ether solvents such as n-butyl ether, dioxane, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, and diethylene glycol; ethyl acetate, n-butyl acetate, isobutyl acetate, butyl butyrate, ethylene glycol monomethyl ether acetate, Examples of known solvents include ester solvents such as butyl carbitol acetate; ketone solvents such as methyl ethyl ketone, methyl isobutyl ketone, and diisobutyl ketone; alcohol solvents such as ethanol, isopropanol, n-butanol, s-butanol, and isobutanol; and amide solvents such as Equamide (trade name, manufactured by Idemitsu Kosan Co., Ltd.), N,N-dimethylformamide, N,N-dimethylacetamide, N-methylformamide, N-methylacetamide, N-methylpropioamide, and N-methyl-2-pyrrolidone. These can be used alone or in combination of two or more. However, if the solvent used in polymerizing and / or diluting the polar group-containing acrylic resin (c) is not removed by a solvent removal step, it will be carried over into the insulating paste of the present invention. Therefore, it is preferable to use the solvent so that it falls within the solubility parameter range specified for the solvent (D) described below.

[0057] In solution polymerization in an organic solvent, a method is used in which a polymerization initiator, a polymerizable unsaturated monomer component, and a solvent are mixed and heated while stirring, or a method is used in which a solvent is charged into a reaction vessel to suppress a temperature rise in the system due to the heat of reaction, and the polymerizable unsaturated monomer component and the polymerization initiator are mixed or separately added dropwise over a predetermined time while stirring at a temperature of 60°C to 200°C and blowing in an inert gas such as nitrogen or argon as necessary.

[0058] The polymerization can generally be carried out for about 1 to 10 hours. After each polymerization stage, an additional catalyst step may be performed, in which the reaction vessel is heated while a polymerization initiator is added dropwise, as needed.

[0059] The weight average molecular weight (Mw) of the polar group-containing acrylic resin (c) obtained as described above is preferably within the range of 1,000 to 100,000, more preferably 2,000 to 100,000, even more preferably 3,000 to 100,000, and particularly preferably 5,000 to 80,000.

[0060] When the weight average molecular weight of the polar group-containing acrylic resin (c) is within the above range, the dispersibility and storage stability are improved.

[0061] In this specification, the weight-average molecular weight is a value obtained by converting the retention time (retention volume) measured using a gel permeation chromatograph (GPC) into the molecular weight of polystyrene using the retention time (retention volume) of a standard polystyrene of known molecular weight measured under the same conditions. Specifically, the measurement can be performed using a gel permeation chromatograph "HLC8120GPC" (trade name, manufactured by Tosoh Corporation) and four columns "TSKgel G-4000HXL," "TSKgel G-3000HXL," "TSKgel G-2500HXL," and "TSKgel G-2000HXL" (trade names, all manufactured by Tosoh Corporation) under the conditions of a mobile phase of tetrahydrofuran, a measurement temperature of 40°C, a flow rate of 1 mL / min, and a detector RI.

[0062] <Other resins> In the present invention, the dispersing resin (C) can be any known resin that can be used together with the acrylic resin as needed, without any particular limitation, and specific examples include polyester resins, epoxy resins, urethane resins, epoxy resins, polyether resins, fluororesins, silicone resins, polycarbonate resins, melamine resins, chlorine-based resins, fluorine-based resins, cellulose-based resins, polybutadiene rubber, and modified or composite resins thereof. These resins can be used alone or in combination with the acrylic resin.

[0063] Solvent (D) As the solvent (D) that can be used in the insulating paste of the present invention, conventionally known solvents can be used without any particular limitation. Specifically, for example, hydrocarbon solvents such as n-butane, n-hexane, n-heptane, n-octane, cyclopentane, cyclohexane, cyclobutane, etc.; aromatic solvents such as toluene and xylene, etc.; ketone solvents such as methyl isobutyl ketone, etc.; ether solvents such as n-butyl ether, dioxane, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol, etc.; ethyl acetate, n-butyl acetate, isobutyl acetate, ethylene glycol mono, Examples of suitable solvents include ester-based solvents such as methyl ether acetate and butyl carbitol acetate; ketone-based solvents such as methyl ethyl ketone, methyl isobutyl ketone and diisobutyl ketone; alcohol-based solvents such as ethanol, isopropanol, n-butanol, sec-butanol and isobutanol; and amide-based solvents such as Equamide (trade name: manufactured by Idemitsu Kosan Co., Ltd., an amide-based solvent), N,N-dimethylformamide, N,N-dimethylacetamide, N-methylformamide, N-methylacetamide, N-methylpropioamide and N-methyl-2-pyrrolidone. These solvents can be used alone or in combination of two or more.

[0064] Among these, the solvent (D) that can be used in the insulating paste of the present invention is preferably a non-hydrocarbon solvent from the viewpoints of the solubility of the dispersion resin (C) and the dispersion stability of the insulating paste. For example, the solvent preferably contains a solvent having an ester bond, an ether bond, a ketone group, an amide bond, a lactam bond, or a hydroxy group, and N-methylpyrrolidone is particularly preferred.

[0065] Furthermore, from the viewpoint of dispersibility of the insulating paste and preventing deterioration or hydrolysis of the resin, it is preferable that the insulating paste is substantially free of water. Here, "substantially free of water" generally means that the water content is 1 mass % or less, based on the total amount of the insulating paste.

[0066] In the present invention, the moisture content of the insulating paste can be measured by Karl Fischer coulometric titration. Specifically, the moisture content can be measured using a Karl Fischer moisture meter (manufactured by Kyoto Electronics Manufacturing Co., Ltd., product name: MKC-610) with the temperature of the moisture vaporizer (manufactured by Kyoto Electronics Manufacturing Co., Ltd., product name: ADP-611) attached to the meter set to 130°C.

[0067] Insulating paste manufacturing The insulating paste of the present invention may contain, as necessary, other components such as pigments, resins, additives, etc., in addition to the inorganic filler (A), binder (B), dispersion resin (C), and solvent (D). It is preferable that the insulating paste of the present invention does not substantially contain an electrode active material.

[0068] Examples of the additives include neutralizing agents, pigment dispersants, binders, antifoaming agents, preservatives, rust inhibitors, plasticizers, and antistatic agents.

[0069] The content of the inorganic filler (A) solid content in the insulating paste solid content is usually 50 to 99 mass %, preferably 70 to 80 mass %, which is suitable from the viewpoints of insulation properties, coating workability, pigment sedimentation and dispersibility.

[0070] In this specification, the term "solid content" refers to the residue remaining after removing volatile components, and the residue may be solid or liquid at room temperature. The solid content mass can be calculated by multiplying the sample mass before drying by the solid content percentage, which is the ratio of the amount of material remaining after drying to the mass before drying. The drying conditions for determining the solid content can be, for example, 105°C for 3 hours.

[0071] The content of the binder (B) solid content in the insulating paste is usually 2 to 10 mass %, preferably 4 to 7 mass %, which is suitable from the viewpoints of coating workability, adhesion, and the like.

[0072] The content of the binder (B) solid content in the insulating paste solid content is usually 5 to 40 mass %, preferably 10 to 28 mass %, which is suitable from the viewpoints of insulation properties, coating workability, adhesion, dispersibility, etc.

[0073] The content of the dispersed resin (C) solid content in the insulating paste is usually 0.1 to 5 mass %, preferably 0.2 to 2 mass %, which is suitable from the viewpoints of dispersibility, coating workability, adhesion, etc.

[0074] The content of the dispersed resin (C) solid content in the insulating paste solid content is usually 0.05 to 4 mass %, preferably 0.1 to 3.0 mass %, which is suitable from the viewpoints of dispersibility, coating workability, adhesion, etc.

[0075] The ratio of the solid content of the dispersed resin (C) to the solid content of the inorganic filler (A) in the insulating paste is: The mass ratio of inorganic filler (A) / dispersion resin (C) is usually 100 / 0.5 to 100 / 15, preferably 100 / 1.0 to 100 / 6, which is suitable from the viewpoints of dispersibility, coating workability, adhesion, and the like.

[0076] The insulating paste can be prepared by uniformly mixing and dispersing the above-mentioned components using a conventionally known dispersing machine, such as a disperser, a paint shaker, a sand mill, a ball mill, a pebble mill, an LMZ mill, a DCP pearl mill, a planetary ball mill, a homogenizer, a twin-screw kneader, or a thin film rotary high-speed mixer.

[0077] Insulation layer The insulating paste is applied (coated) to a current collector to form an insulating layer (insulating film, coated film). In the present invention, insulation refers to a material having a volume resistivity of 1.0×10 6 This means that the resistance is Ω·cm or more.

[0078] In the present invention, an insulating layer (insulating film, coating film) refers to a solid film formed by applying a liquid insulating paste to a substrate (charger) and then heating and drying it. The insulating layer (insulating film, coating film) can be peeled off from the substrate to obtain an insulating film, or it can be applied to both sides of a plate-shaped substrate (current collector) to obtain an insulating layer (insulating film, coating film). Furthermore, the insulating layer (insulating film, coating film) of the present invention is preferably non-porous.

[0079] The current collector to be coated is not particularly limited as long as it is made of metal, but is preferably aluminum or a composite metal thereof, and may be degreased or surface-treated.

[0080] The method for applying the insulating paste is not particularly limited as long as it can be applied within a certain film thickness range, and examples include roller coating, brush coating, atomization coating, dipping coating, applicator coating, shower coat coating, roll coater coating, and die coater coating.

[0081] The thickness of the coated film is preferably 1 to 50 μm, more preferably 2 to 20 μm, in terms of dry thickness. The temperature for drying the coated film is preferably 60 to 300° C., more preferably 80 to 200° C.

[0082] By heating and drying, it is preferable that 90% or more of the solvent contained in the insulating paste is removed, more preferably 95% or more, and particularly preferably 99% or more.

[0083] Furthermore, the sodium content in the insulating layer is generally adjusted to 15 ppm or more, preferably 35 to 1600 ppm, more preferably 35 to 1300 ppm, even more preferably 70 to 1000 ppm, and particularly preferably 100 to 700 ppm, from the viewpoint of the storage stability of the insulating paste and the adhesive strength of the insulating layer.

[0084] The sodium content in the insulating layer can be calculated from the sodium content and solid content of the insulating paste. [Example]

[0085] The present invention will be further described below with reference to examples and comparative examples.

[0086] The synthesis methods of various resins, the manufacturing methods of insulating pastes, insulating layers and secondary batteries, and the evaluation and testing methods are all methods known in the art.

[0087] However, the present invention is not limited to this, and various modifications and variations are possible within the technical spirit of the present invention and the scope of equivalents of the claims.

[0088] In each example, "parts" indicates parts by mass, and "%" indicates % by mass.

[0089] <Production of acrylic resin> Manufacturing Example 1 A reaction vessel equipped with a stirring heater and a cooling tube was charged with 40 parts of N-methyl-2-pyrrolidone, and after replacing the atmosphere with nitrogen, the temperature was maintained at 115° C. The following monomer mixture was added dropwise to the reaction vessel over a period of 4 hours. <Monomer mixture> 30 parts styrene n-Butyl acrylate 20 parts Lauryl methacrylate 15 parts Methyl methacrylate 35 parts t-Butyl peroxy-2-ethylhexanoate (polymerization initiator) 3 parts One hour after the completion of the dropwise addition, a solution of 0.5 parts of t-butylperoxy-2-ethylhexanoate in 10 parts of N-methyl-2-pyrrolidone was added dropwise over one hour. After the dropwise addition, the mixture was maintained at 115°C for an additional hour. N-methyl-2-pyrrolidone was then added to the mixture to a solids content of 50%, yielding an acrylic resin (C-1) solution with a solids content of 50%. The acrylic resin (C-1) had a weight-average molecular weight (Mw) of 18,000.

[0090] Manufacturing Examples 2-17 Solutions of acrylic resins (C-2) to (C-17) were produced in the same manner as in Production Example 1, except that the monomer compositions were as shown in Table 1 below.

[0091] The weight average molecular weight (Mw) of each resin is shown in Table 1 below.

[0092] [Table 1]

[0093] <Insulating paste manufacturing> Example 1 80 parts of boehmite (A-1), 20 parts of polyvinylidene fluoride (PVDF) (weight average molecular weight 900,000, unmodified), 4.8 parts of acrylic resin (C-1) solution (resin solid content 2.4), and 250 parts of N-methyl-2-pyrrolidone (NMP) were placed in a container and thoroughly dispersed using a planetary mixer to obtain insulating paste (X-1). This process was carried out at room temperature of about 20°C.

[0094] Examples 2 to 27 and Comparative Examples 1 to 2 Insulating pastes (X-2) to (X-29) were produced in the same manner as in Example 1, except that the raw material compositions were as shown in Table 2 below.

[0095] The adhesive strength (N / m), volume average particle size (μm), and particle size distribution standard deviation (μm) measured for the obtained insulating paste are shown in Table 2. The adhesive strength (N / m), volume average particle size (μm), and particle size distribution standard deviation (μm) were measured by the methods described in the present specification.

[0096] The results of the evaluation of dispersibility, pigment sedimentation, finish, and bendability, which will be described later, are also recorded. If even one of the items is marked "x," the insulating paste is deemed to have failed.

[0097] [Table 2]

[0098] The amounts of filler, binder and dispersing resin in the table are values ​​of solid content. (Note 1) In Example 18, polyvinyl alcohol (saponification degree 99%, weight average molecular weight 20,000, solid content 100%) was used as the dispersing resin.

[0099] The fillers in the table are as follows: The sodium content of the boehmite was calculated based on the solid content of the boehmite. Boehmite (A-1): Volume average particle size (D50) 1.3 μm, sodium content 500 ppm Boehmite (A-2): Volume average particle size (D50) 0.5 μm, sodium content 500 ppm Boehmite (A-3): Volume average particle size (D50) 0.8 μm, sodium content 500 ppm Boehmite (A-4): Volume average particle size (D50) 1.9 μm, sodium content 500 ppm Alumina (A-5): Volume average particle size (D50) 1.2 μm, sodium content 200 ppm The moisture content of the insulating pastes prepared in the above examples and comparative examples was all less than 0.8 mass %.

[0100] Examples 28 to 37 Insulating pastes (X-30) to (X-39) were produced in the same manner as in Example 1, except that the raw material compositions were as shown in Table 3 below. Table 3 below also shows the insulating paste (X-4) of Example 4.

[0101] Table 3 below shows the sodium content (ppm) in the insulating paste and the evaluation results of storage stability (viscosity reduction rate) described below.

[0102] The sodium content of the insulating paste is also shown in the table. The sodium content was measured by the method described in this specification. The sodium content in the insulating layer can be calculated from the sodium content and solid content of the insulating paste.

[0103] The results of evaluation of storage stability (viscosity reduction rate) as determined by the method described below are also shown.

[0104] [Table 3]

[0105] The amounts of inorganic filler, binder and dispersing resin in the table are values ​​of solid content.

[0106] The sodium content of the insulating pastes (X-30) to (X-32) was adjusted by adding sodium hydroxide to the paste.

[0107] The inorganic fillers in the table are as follows: The sodium content of the boehmite was calculated based on the solid content of the boehmite. Boehmite (A-1): Volume Average particle size (D50) 1.3 μm, sodium content 500 ppm Boehmite (A-5): Volume average particle size (D50) 1.3 μm, sodium content 1000 ppm Boehmite (A-6): Volume average particle size (D50) 1.3 μm, sodium content 2000 ppm Boehmite (A-7): Volume average particle size (D50) 1.3 μm, sodium content 200 ppm Boehmite (A-8): Volume average particle size (D50) 1.3 μm, sodium content 100 ppm Boehmite (A-9): Volume average particle size (D50) 1.3 μm, sodium content 50 ppm Boehmite (A-10): Volume average particle size (D50) 1.3 μm, sodium content 25 ppm

[0108] <Evaluation test> <Dispersibility> Dispersibility (degree of dispersion) was evaluated by the particle gauge method in accordance with JIS K5600-2-5. Specifically, the paste was dropped onto a particle gauge table, thinly spread into the gauge groove with a scraper, and the particle size of the largest particle observed on the gauge was measured. The measurement was performed three times, and the average value was taken as the measured value.

[0109] The dispersibility of the obtained insulating paste was evaluated according to the following criteria. ⊚: Pigment is dispersed at less than 15 μm. Dispersibility is very good. ◯: The pigment is dispersed at a size of 15 μm or more and less than 20 μm. Dispersibility is good. Good / Good: The pigment is dispersed at a size of 20 μm or more and less than 25 μm, but no aggregates can be visually confirmed. Dispersibility is standard. △: The pigment is dispersed at 25 μm or more, but no aggregates are visible to the naked eye. Dispersibility is somewhat poor. ×: 50 μm aggregates were observed. Dispersibility was very poor.

[0110] <Pigment settling> The obtained insulating paste was stored at 40°C for 60 days, and the pigment sedimentation was checked. The state after 60 days of storage was evaluated according to the following criteria. ◎: No change. ◯: Very slight settling of inorganic filler was observed, but when the paste was stirred by hand, it immediately returned to the state before storage, so there was no problem. ◯△: Slight settling of inorganic filler was observed, but the paste returned to its pre-storage state when vigorously stirred manually. △: Sedimentation of inorganic filler was observed, and the paste did not return to the state before storage even when stirred by hand, but when the paste was stirred with a disperser, it returned to the state before storage. ×: Significant settling of the inorganic filler was observed, and the paste did not return to the state before storage even when stirred with a disperser.

[0111] <Finishing quality> The obtained insulating paste was applied to an aluminum current collector using an applicator and then dried at 80°C for 60 minutes to form a coating film (dry film thickness 15 μm). The gloss, unevenness, and smoothness of the coating film of each test plate were visually observed. ⊚: There is some loss of gloss, but the finish is very good. ◯: There is some loss of gloss, but unevenness and smoothness are good and there is no problem in practical use. ◯△: Some loss of gloss is observed, but unevenness and smoothness are good and at a level that does not pose a problem in practical use. △: Loss of gloss and unevenness are observed, but there is smoothness and it is at a level that does not cause any problems in practical use. ×: Loss of gloss and unevenness are observed, and the smoothness is poor, and there is clearly a problem.

[0112] <Bending ability> The obtained insulating paste was applied to a 1 mm thick aluminum current collector using an applicator and then dried at 80°C for 60 minutes to form a coating film (dry film thickness: 20 μm). Next, the coated plate was bent 180 degrees (with the coating film facing outward), and the condition of the bent coating film was visually observed. Evaluation was based on the following criteria. ⊚: The condition of the coating film is excellent and there is no abnormality. ○: There are cracks of less than 2 mm in the coating film, but the substrate is not exposed. ◯△: There are cracks in the coating film that are 2 mm or more and less than 10 mm in size, but the substrate is not exposed. △: There is a crack of 10 mm or more in the coating film, and the substrate is slightly exposed. ×: The coating film peeled off along with cracks, exposing the substrate.

[0113] <Storage stability (viscosity reduction rate)> The obtained insulating paste was stored at 40° C. for 30 days. The viscosity was checked before and after storage, and the storage stability (viscosity reduction rate) was evaluated according to the following criteria. A to C are acceptable, and D is unacceptable. The viscosity was measured using a cone and plate type viscometer "Mars2" (product name, manufactured by HAAKE) at a shear rate of 1 s -1 is the value. Viscosity reduction rate (%) = 100 - (viscosity after storage) / (viscosity before storage) x 100 A: The viscosity reduction rate is less than 7% (including cases where the viscosity after storage is equal to or greater than the viscosity before storage). B: The viscosity reduction rate after storage is 7% or more and less than 30%. C: The viscosity reduction rate after storage is 30% or more and less than 50%. D: The viscosity reduction rate after storage is 50% or more.

Claims

1. An insulating paste for a lithium ion secondary battery current collector, comprising: an inorganic filler (A), a binder (B), a dispersion resin (C), and a solvent (D), The dispersion resin (C) contains a phosphoric acid group-containing acrylic resin, The insulating paste is applied to a current collector made of aluminum to form an insulating layer having a dry thickness of 15 μm, and when the insulating layer is measured by a 180-degree peel test at a tensile speed of 10 cm / min, the adhesive strength of the insulating layer is 2.5 N / m or more. Insulating paste for current collectors in lithium-ion secondary batteries.

2. An insulating paste for a current collector of a lithium ion secondary battery, comprising an inorganic filler (A), a binder (B), a dispersion resin (C), and a solvent (D), The insulating paste is applied to a current collector made of aluminum to form an insulating layer having a dry thickness of 15 μm, and when the insulating layer is measured by a 180-degree peel test at a tensile speed of 10 cm / min, the adhesive strength of the insulating layer is 2.5 N / m or more; The insulating layer is non-porous. Insulating paste for current collectors in lithium-ion secondary batteries.

3. An insulating paste for a current collector of a lithium ion secondary battery, comprising an inorganic filler (A), a binder (B), a dispersion resin (C), and a solvent (D), The insulating paste is applied to a current collector made of aluminum to form an insulating layer having a dry thickness of 15 μm, and when the insulating layer is measured by a 180-degree peel test at a tensile speed of 10 cm / min, the adhesive strength of the insulating layer is 2.5 N / m or more; The sodium content of the insulating layer is 15 ppm or more. Insulating paste for current collectors in lithium-ion secondary batteries.

4. The insulating paste for a lithium ion secondary battery current collector according to any one of claims 1 to 3, characterized in that the inorganic filler (A) has a volume average particle size (D50) of 0.5 to 7 µm and a particle size distribution standard deviation of 1.4 µm or less.

5. 5. The insulating paste for a lithium ion secondary battery current collector according to claim 2, wherein the dispersed resin (C) contains a polar group-containing acrylic resin.

6. 6. The insulating paste for a current collector of a lithium ion secondary battery according to claim 5, wherein the polar group of the polar group-containing acrylic resin is a phosphate group.

7. 7. The insulating paste for a lithium ion secondary battery current collector according to claim 5, wherein the polar group-containing acrylic resin is a polymer of raw material monomers including a polymerizable unsaturated monomer (c2) having a hydrocarbon group having 4 or more carbon atoms.

8. 8. The insulating paste for a lithium ion secondary battery current collector according to claim 5, wherein the polar group-containing acrylic resin has a weight average molecular weight in the range of 1,000 to 100,000.

9. 9. An insulating paste for a current collector of a lithium ion secondary battery, wherein the insulating layer according to any one of claims 1 and 3 to 8 is non-porous.

10. The inorganic filler (A) is alumina, silica, TiO ２ , BaTiO ３ , ZrO ２ 10. The insulating paste for a lithium ion secondary battery current collector according to claim 1, wherein the insulating paste is at least one selected from the group consisting of quartz, boehmite, zeolite, apatite, and kaolin.

11. 11. The insulating paste for a lithium ion secondary battery current collector according to claim 1, wherein the binder (B) is modified or unmodified polyvinylidene fluoride.

12. 12. The insulating paste for a lithium ion secondary battery current collector according to claim 1, wherein the solvent (D) is N-methyl-2-pyrrolidone.

13. 13. An insulating paste for a current collector of a lithium ion secondary battery, comprising the insulating paste according to any one of claims 1 to 12, and substantially no active material for an electrode.

14. A method for producing an insulating layer for a lithium ion secondary battery current collector, comprising applying the insulating paste according to any one of claims 1 to 13 to a part or all of a current collector, and then heating and drying the paste to form an insulating layer.

15. A method for producing an insulating layer for a current collector of a lithium ion secondary battery, comprising applying the insulating paste according to any one of claims 1 to 13 to a part or all of a current collector, and then heating and drying the applied paste to form an insulating layer, wherein the sodium content of the insulating layer is 15 ppm or more.

16. 16. The method for producing an insulating layer for a current collector of a lithium ion secondary battery according to claim 14 or 15, wherein the current collector is made of aluminum or a composite metal thereof.

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