JP7751978B2 - Insulating paste for current collectors in lithium-ion secondary batteries - Google Patents

Description

The present invention relates to an insulating paste to be applied to the current collectors of the positive and/or negative electrodes of lithium-ion secondary batteries, and a method for producing an insulating layer obtained by applying the paste.

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.
For example, Patent Document 1 discloses a lithium-ion secondary battery including an insulating layer. While this insulating layer prevents short circuits between the positive and negative electrode plates, it has been found that the battery has poor storage stability and coating workability, making it difficult to obtain a satisfactory finish. Furthermore, when a physical load is applied during pressing, the insulating layer may fall off, making it difficult to ensure stable insulation.

JP 2013-232425 A

The problem that this invention aims to solve is to provide an insulating paste that has good storage properties (pigment settling, viscosity), dispersibility, and coating workability, and that provides an insulating layer that has good finish after coating and adhesion to the current collector.

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 lithium ion secondary battery current collector, which contains an inorganic filler (A), a binder (B), a dispersion resin (C), and a solvent (D), wherein the viscosity (shear rate 1 s ⁻¹ ) of the paste is 2000 mPa·s or more and the TI value (the ratio of the viscosity at 1 s ⁻¹ to the viscosity at a shear rate of 1000 s ⁻¹ ) is greater than 1, and have thus completed the present invention.

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 dispersion resin (C), and a solvent (D), wherein the paste has a viscosity (shear rate 1 ^s-1 ) of 2000 mPa·s or more and a TI value (ratio of the viscosity at 1 s ^-1 to the viscosity at a shear rate of 1000 s ^-1 ) of greater than 1.
Item 2. An insulating paste for a lithium ion secondary battery current collector according to Item 1, wherein the dispersed resin (C) is a copolymer of raw material monomers including 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 the copolymer contains a polar group-containing acrylic resin (c) having a weight average molecular weight of 1,000 to 100,000.
Item 3. The insulating paste for a lithium ion secondary battery current collector according to Item 1 or 2, wherein the inorganic filler (A) is at least one selected from the group consisting of alumina, silica, TiO ₂ , BaTiO ₃ , ZrO ₂ , boehmite, zeolite, apatite, and kaolin.
Item 4. The insulating paste for a lithium ion secondary battery current collector according to any one of Items 1 to 3, wherein the binder (B) is modified or unmodified polyvinylidene fluoride.
Item 5. The insulating paste for a lithium ion secondary battery current collector according to any one of Items 1 to 4, wherein the solvent (D) is N-methyl-2-pyrrolidone.
Item 6. 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 5 does not substantially contain an electrode active material.
Item 7. 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 6 to a part or all of a current collector, and then heating and drying the applied paste to form an insulating layer.
Item 8. A method for producing an insulating layer for a current collector of a lithium ion secondary battery, wherein the insulating layer according to Item 7 is non-porous.
Item 9. The method for producing an insulating layer for a current collector of a lithium ion secondary battery according to Item 7 or 8, wherein the current collector is made of aluminum or a composite metal thereof.

The insulating paste of the present invention has good storage properties (pigment settling, viscosity), dispersibility, and coating workability, and the resulting insulating layer has good finish and adhesion.

The present invention is described in detail below.

In this specification, unless otherwise specified, the phrase "a resin contains monomer X, which is its raw material" means that the resin is a (co)polymer of raw material monomers, including monomer X. Also, in this specification, "(co)polymer" means a polymer or copolymer.

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.

Also, throughout the specification, "insulating layer" may be referred to as "insulating film," "coating film," or "film."

Insulating Paste The insulating paste of the present invention is an insulating paste for a current collector of a lithium ion secondary battery, which contains an inorganic filler (A), a binder (B), a dispersion resin (C), and a solvent (D).

The viscosity (shear rate 1 s ^-1 ) of the insulating paste is usually 2000 mPa s or more, preferably 2000 to 7000 mPa s, and more preferably 2500 to 5000 mPa s, from the viewpoints of storage stability and finish. If the viscosity is 7000 mPa s or more, coating workability and finish quality will decrease, and if it is 2000 mPa s or less, storage stability (pigment sedimentation), finish quality, and sagging will decrease.
The TI value (ratio of viscosity at a shear rate of 1 s ⁻¹ to viscosity at a shear rate of 1000 s ⁻¹ ) is usually greater than 1, preferably 2 to 10, and more preferably 3 to 6.
For example, when the TI value is 1 or more, the viscosity decreases during coating (shear rate is about 1000 ^s-1 ) and the fluidity (coating workability) is good, and the viscosity of the insulating layer (coating film) is high after coating (shear rate is about 1 s ^- 1) and the insulating layer does not flow, resulting in good finish quality.

The above viscosity can be measured, for example, using a cone and plate viscometer "Mars2" (product name, manufactured by HAAKE Corporation).

The sodium content in the insulating paste is typically adjusted to 450 ppm or less, preferably 10 to 350 ppm, more preferably 10 to 300 ppm, and even more preferably 10 to 200 ppm, from the perspective of storage stability (suppressing a decrease in the insulating paste's viscosity during storage (hereinafter referred to as storage viscosity thinning)). If the sodium content exceeds 450 ppm, viscosity thinning occurs during high-temperature storage, which may result in reduced pigment sedimentation and finish quality (including sagging). Furthermore, insulating pastes may contain sodium ions due to carryover from various raw materials (especially inorganic fillers, described below) or contamination during the manufacturing process, and completely removing these ions would be uneconomical.

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 a certain amount of sodium ions is contained, this interaction is gradually broken down, causing the insulating paste to reduce in viscosity.

The sodium content in the insulating paste can be measured, for example, using an ICP optical emission spectrometer. Specifically, the sample (insulating paste) can be 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").

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 without limitation, and examples thereof include alumina, silica, TiO ₂ , BaTiO ₃ , ZrO ₂ , boehmite, zeolite, apatite, and kaolin. One type can be used alone, or two or more types can be used in combination. Of these, alumina and/or boehmite are preferred. Alumina is aluminum oxide represented by Al ₂ O ₃ , and boehmite is alumina monohydrate represented by the composition γ-AlO(OH).

The volume average particle size of the inorganic filler (A) in the insulating paste of the present invention is preferably 0.5 to 7 μm.

Particle size measurements can be performed, for example, using a particle size distribution measuring device (manufactured by Microtrac Bell, product name: Microtrac MT3000).

When boehmite is used as the inorganic filler (A), particular attention must be paid to the sodium content in the insulating paste.
Since sodium hydroxide is generally used in the manufacturing process of aluminum hydroxide, which is the raw material for boehmite, boehmite also contains a certain amount of sodium ions. Although it is possible to remove sodium ions by washing, etc., it is economically difficult to completely remove them.

Specifically, the sodium content of the boehmite is typically 2000 ppm or less, preferably 40 to 1500 ppm, and more preferably 200 to 1200 ppm, based on the solid content of the boehmite.

Binder (B)
The binder (B) that can be used in the insulating paste of the present invention is a copolymer having as a constituent component a polymerizable unsaturated group-containing monomer represented by the following formula (1), and can be synthesized by copolymerizing a monomer containing the polymerizable unsaturated group-containing monomer.

(wherein R ₁ to R ₄ 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, a (meth)acryloyl group-containing monomer, and a (meth)acrylamide group-containing monomer, and these can be used alone or in combination of two or more.

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.

As the solvent used in the 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.

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

Examples of the binder (B) include polyvinylidene fluoride, polyvinyl alcohol, polyvinyl acetal, acrylic resin, polyvinyl acetate, polyvinyl chloride, polystyrene, polyvinyl ether, and polyvinylpyrrolidone, and these can be used alone or in combination of two or more. Among these, polyvinylidene fluoride is preferred due to its insulating properties and coating film strength (i.e., cohesive force).

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

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

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

For this reason, it is more preferable that the polar group-containing acrylic resin is a copolymer of raw material monomers including a polymerizable unsaturated monomer (c1) having a polar functional group and a polymerizable unsaturated monomer (c2) having a hydrocarbon group with 4 or more carbon atoms, and that the weight-average molecular weight of the copolymer is 1,000 to 100,000. Polyvinyl alcohol (PVA) is highly polar, so its finish is inferior to that of such polar group-containing acrylic resin (c).

<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.

<Polymerizable Unsaturated Monomer Having a Polar Group (c1)>
The polymerizable unsaturated monomer 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. The polymerizable unsaturated monomer may have multiple polar groups.

Specific examples of the polymerizable unsaturated monomer having the polar group include, for example, 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)ethyltrimethyl Examples of suitable polymerizable unsaturated monomers include quaternary base-containing polymerizable unsaturated monomers such as ammonium chloride; monoesters of (meth)acrylic acid and 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 and 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. Of these, polymerizable unsaturated monomers having an acid group are preferred, and polymerizable unsaturated monomers having a phosphate group are more preferred. These may be used alone or in combination of two or more.

The raw material monomers preferably contain 1 to 80% by mass of polymerizable unsaturated monomers having polar groups, more preferably 5 to 70% by mass, and even more preferably 20 to 60% by mass.

When the content of polymerizable unsaturated monomers having polar groups in the raw material monomers is within the above range, compatibility with the binder, pigment dispersibility, and adhesion are improved.

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

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

<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) includes a polymerizable unsaturated monomer having an alkyl group having 4 or more carbon atoms, which can be suitably used particularly when the binder (B) is polyvinylidene fluoride, which has a relatively low polarity.

The polymerizable unsaturated monomer having an alkyl group of 4 or more carbon atoms can be a straight-chain, branched, or cyclic alkyl group, and can be used without particular limitation, as long as it is a polymerizable unsaturated monomer having an alkyl group of 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. These can be used alone or in combination of two or more.

Polymerizable unsaturated monomers having an alkyl group with 4 or more carbon atoms preferably have 4 to 24 carbon atoms, more preferably 8 to 20 carbon atoms, and particularly preferably 10 to 17 carbon atoms. Furthermore, polymerizable unsaturated monomers having a linear or branched alkyl group structure are preferred.

The raw material monomers preferably contain 1 to 95% by mass of polymerizable unsaturated monomers having an alkyl group with 4 or more carbon atoms, more preferably 10 to 80% by mass, and even more preferably 20 to 60% by mass.

When the content of polymerizable unsaturated monomers having an alkyl group with 4 or more carbon atoms in the raw material monomers is within the above range, dispersibility and storage properties are improved.

<Other polymerizable unsaturated monomers>
As the raw material monomer for obtaining the polar group-containing acrylic resin (c), other polymerizable unsaturated monomers besides the above-mentioned polymerizable unsaturated monomers 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.

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. are also possible. When solution polymerization is performed, it may be continuous or batch polymerization, and the polymerizable unsaturated monomer may be charged all at once, charged in portions, or added continuously or intermittently.

Conventional known methods can be used as the radical polymerization initiator used in the polymerization. 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-butylperoxybenzoate, 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.

The solvent used for the polymerization or dilution is not particularly limited, and examples thereof include water, organic solvents, and mixtures thereof. Organic solvents are particularly preferred. 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, and ethylene glycol monomethyl ether acetate. Examples of conventional solvents include ester-based solvents such as 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, s-butanol, and isobutanol; and amide-based 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 desolvation process, it will be carried over into the insulating paste of the present invention. Therefore, it is preferable to use the solvent so that its solubility parameter falls within the range specified for the solvent (D) described below.

Solution polymerization in an organic solvent can be carried out in a variety of ways, including mixing a polymerization initiator, polymerizable unsaturated monomer components, and solvent and heating them while stirring, or by charging a solvent into a reaction vessel to prevent the system from heating up due to the heat of reaction, stirring at a temperature between 60°C and 200°C, and adding the polymerizable unsaturated monomer components and polymerization initiator dropwise or mixed over a specified period of time while blowing in an inert gas such as nitrogen or argon as necessary.

Polymerization can generally be carried out for approximately 1 to 10 hours. If necessary, an additional catalytic step can be carried out after each polymerization stage, in which the reaction vessel is heated while a polymerization initiator is added dropwise.

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 95,000, even more preferably 3,000 to 90,000, and particularly preferably 5,000 to 80,000.

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

In this specification, the weight-average molecular weight is a value calculated 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 gel permeation chromatograph is "HLC8120GPC" (trade name, manufactured by Tosoh Corporation) and four columns are used: "TSKgel G-4000HXL," "TSKgel G-3000HXL," "TSKgel G-2500HXL," and "TSKgel G-2000HXL" (trade names, all manufactured by Tosoh Corporation). Measurements can be performed under the following conditions: a mobile phase of tetrahydrofuran, a measurement temperature of 40°C, a flow rate of 1 mL/min, and an RI detector.

<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.

Solvent (D)
As the solvent (D) that can be used in the insulating paste of the present invention, any conventionally known solvent can be used without any particular limitation. Specific examples 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, 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.

Of these, from the viewpoints of the solubility of the dispersion resin (C) and the dispersion stability of the insulating paste, the solvent (D) that can be used in the insulating paste of the present invention preferably contains a solvent having a polar functional group such as a hydroxyl group, carboxyl group, amide group, amino group, or ether group, and N-methyl-2-pyrrolidone is particularly preferred.

Furthermore, from the viewpoint of dispersibility of the insulating paste and preventing deterioration or hydrolysis of the resin, it is preferable that the paste contains substantially no water. Here, "substantially no water" means that the water content is typically 1% by mass or less, based on the total amount of the insulating paste.

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

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

Additives include neutralizing agents, pigment dispersants, binders, defoamers, preservatives, rust inhibitors, plasticizers, and antistatic agents.

The solid content of inorganic filler (A) in the insulating paste is typically 5 to 40% by mass, preferably 10 to 30% by mass, and more preferably 18 to 26% by mass, which is suitable from the standpoints of insulation properties, paintability, pigment sedimentation, dispersibility, etc.

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

The solid content of inorganic filler (A) in the insulating paste solids is typically 50 to 99% by mass, preferably 70 to 80% by mass, which is suitable from the standpoints of insulation properties, paintability, pigment sedimentation, dispersibility, etc.

The solid content of binder (B) in the insulating paste is typically 2 to 10 mass%, preferably 4 to 7 mass%, which is suitable from the standpoints of coating workability, adhesion, etc.

The binder (B) solid content in the insulating paste solids is typically 5 to 40 mass %, preferably 10 to 28 mass %, which is suitable from the standpoints of insulation properties, paintability, adhesion, dispersibility, etc.

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

The content of dispersed resin (C) solids in the insulating paste solids is typically 0.05 to 4 mass%, preferably 0.1 to 3.0 mass%, which is suitable from the standpoints of dispersibility, coating workability, adhesion, etc.

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 ratio of inorganic filler (A) to dispersing 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, etc.

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

Insulating Layer : The insulating paste is applied (coated) onto a current collector to form an insulating layer (insulating film, coated film). Note that, in the present invention, "insulating" means that the volume resistivity is 1.0×10 ⁶ Ω·cm or more.

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 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 material.
The insulating layer (insulating film, coating film) of the present invention is preferably non-porous.

The current collector of the substrate is not particularly limited as long as it is made of metal, but aluminum or a composite metal thereof is preferred, and it may be degreased or surface-treated.

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.

The dry film thickness is preferably 1 to 50 μm, more preferably 2 to 20 μm. The drying temperature is preferably 60 to 300°C, more preferably 80 to 200°C.

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 most preferably 99% or more.

The present invention will be further explained below with reference to examples and comparative examples.

The methods for synthesizing various resins, manufacturing insulating pastes, insulating layers, and secondary batteries, and evaluation and testing methods are all well-known in the relevant technical field.

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.

In each example, "parts" means parts by mass and "%" means % by mass.

<Production of acrylic resin>
Production Example 1
A reaction vessel equipped with a stirring heater and a condenser 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>
Styrene 30 parts n-butyl acrylate 20 parts lauryl methacrylate 15 parts methyl methacrylate 35 parts t-butylperoxy-2-ethylhexanoate (polymerization initiator) 3 parts. One hour after the completion of the dropwise addition, a solution of 0.5 parts t-butylperoxy-2-ethylhexanoate dissolved in 10 parts N-methyl-2-pyrrolidone was added dropwise to the mixture over one hour. After the completion of the dropwise addition, the mixture was maintained at 115°C for an additional hour. Next, N-methyl-2-pyrrolidone was added so that the solids content was 50%, to obtain 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.

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

The weight average molecular weight (Mw) of each resin is listed in Table 1 below.

<Production of insulating paste>
Example 1
80 parts of boehmite, 20 parts of polyvinylidene fluoride (PVDF) (weight average molecular weight 500,000, unmodified), 4.8 parts of acrylic resin (C-1) solution (resin solid content 2.4 parts), 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.

Examples 2 to 28 and Comparative Examples 1 to 3
Insulating pastes (X-2) to (X-31) were produced in the same manner as in Example 1, except that the raw material compositions were as shown in Table 2 below.

Table 2 below lists the viscosity (shear rate 1 s ^-1 , viscosity unit mPa·s) of the obtained insulating paste measured using a cone and plate type viscometer "Mars2" (trade name, manufactured by HAAKE Corporation) and the TI value (ratio of viscosity at a shear rate of 1 s- ¹ to the viscosity at a shear rate of 1000 s- ¹ ).

In addition, an insulating layer (coating film) was prepared by the method described below, and evaluation tests were conducted on the insulating paste and the insulating layer (coating film). The evaluation results for adhesion, dispersibility, finish (surface), and pigment sedimentation are shown in Table 2 below. If even one of the items was not satisfactory, the insulating paste was deemed to have failed.
In addition, since the dispersibility of Comparative Example 1 was unacceptable, the pigment settling property was not evaluated.

The amounts of filler, binder and dispersing resin in the table are values of solid content.
*1 Polyvinyl alcohol (saponification degree 99%, weight average molecular weight 20,000) was used as the dispersing resin.

The moisture content of the insulating pastes prepared in the above examples and comparative examples was less than 0.8% by mass.

Example 29
80 parts of boehmite (1-1), 20 parts of polyvinylidene fluoride (PVDF) (weight average molecular weight 500,000, unmodified), 4.8 parts of acrylic resin (C-3) solution (resin solid content 2.4 parts), and 250 parts of N-methyl-2-pyrrolidone (NMP) were placed in a container and thoroughly dispersed using a planetary mixer to obtain an insulating paste (X-29).

Examples 30 to 36
Insulating pastes (X-30) to (X-36) were produced in the same manner as in Example 1, except that the raw material compositions were as shown in Table 3 below.

The viscosity of the obtained insulating paste was measured using a cone and plate type viscometer "Mars2" (product name, manufactured by HAAKE Corporation), and the measured viscosity (shear rate 1 s ^-1 , viscosity unit is mPa·s) and TI value (ratio of viscosity at shear rate 1 ^s-1 to viscosity at shear rate 1000 s ^-1 ) are shown in Table 3 below.

The sodium content of the insulating paste is also shown in the table. The sodium content was measured using the method described in this specification.

The results of the evaluation of storage stability (viscosity reduction rate) using the method described below are also listed.

Note that the amounts of inorganic filler, binder, and dispersing resin listed in the table are solid content values.

The sodium content of insulating pastes (X-31) to (X-33) was adjusted by adding sodium hydroxide to the paste.

The moisture content of the insulating pastes prepared in the above examples was less than 0.8% by mass.

The inorganic fillers in the table are as follows:
Boehmite (1-1): Volume average particle size (D50) 1.3 μm, sodium content 500 ppm
Boehmite (1-2): Volume average particle size (D50) 1.3 μm, sodium content 1000 ppm
Boehmite (1-3): Volume average particle size (D50) 1.3 μm, sodium content 2000 ppm

<Evaluation test>
<Adhesion>
The resulting 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 (20 μm thick after drying). 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 with double-sided tape for reinforcement. The resulting laminate was cut into 10 cm long, 1 cm wide strips with a cutter, spanning the thickness from the PET to the coating film. Furthermore, double-sided tape was applied to the surface of the horizontally placed current collector, and the sample was adhesively fixed to a tin plate. A 180° peel test was performed at a tensile speed of 10 cm/min using an "Ez-Test" (trade name, manufactured by Shimadzu Corporation). The coating film was peeled from the current collector by clamping one of the two shorter sides of the sample. The adhesion between the current collector and the coating film (insulating layer) was evaluated according to the following criteria: A-C indicates pass, and D indicates fail.
A: 10 N/m or more, very good.
B: 3 N/m or more and less than 10 N/m, which is good.
C: 1 N/m or more and less than 3 N/m, which is at a level that does not pose a problem in practical use.
D: Less than 1 N/m, not practical.

<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.

The dispersibility of the obtained insulating paste was evaluated according to the following criteria: A to D are acceptable, and E is unacceptable.
A: The pigment is dispersed at a particle size of less than 15 μm. Dispersibility is very good.
B: The pigment is dispersed at a particle size of 15 μm or more and less than 20 μm. Dispersibility is good.
C: The pigment is dispersed to a size of 20 μm or more and less than 25 μm, but no aggregates are visible. Dispersibility is standard.
D: The pigment is dispersed at a particle size of 25 μm or more, but no aggregates are visible. Dispersibility is somewhat poor.
E: Aggregates of 50 μm were observed. Dispersibility was very poor.

<Finish (surface)>
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). Each test plate was visually inspected for gloss (luster), unevenness, and smoothness. A-D were passed, and E was failed.
A: There is some loss of gloss, but the finish is very good.
B: There is some loss of gloss, but unevenness and smoothness are good and there is no practical problem.
C: Although gloss loss is observed, unevenness and smoothness are good and at a level that does not pose a problem in practical use.
D: 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.
E: Loss of gloss and unevenness are observed, and the smoothness is poor, and there is clearly a problem.

<Pigment settling>
The obtained insulating paste was stored at 40°C for 60 days, and the pigment sedimentation was confirmed. The state after 60 days of storage was evaluated according to the following criteria. A to C are acceptable, and D is unacceptable.
A: No change.
B: Very slight settling of filler was observed, but when the paste was stirred by hand, it immediately returned to the state before storage, and there was no problem.
C: Settling of the filler was observed, and the paste did not return to the state before storage even when stirred by hand, but the paste returned to the state before storage when stirred with a disperser.
D: Significant settling of the filler was observed, and the paste did not return to the state before storage even when stirred with a disperser.

<Storage stability (viscosity reduction rate)>
The obtained insulating paste was stored for 30 days at 40° C. 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 is a value measured at a shear rate of 1 s ⁻¹ using a cone and plate type viscometer "Mars2" (trade name, manufactured by HAAKE).
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 higher 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 (10)

An insulating paste for a lithium ion secondary battery current collector, comprising an inorganic filler (A), a binder (B), a dispersion resin (C) having a phosphate group, and a solvent (D), wherein the paste has a viscosity (at a shear rate of 1 s ^-1 ) of 2000 mPa·s or more and a TI value (the ratio of the viscosity at a shear rate of 1 s- ¹ to the viscosity at a shear rate of 1000 s ^-1 ) of greater than 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 polar group-containing acrylic resin (c) which is a copolymer of raw material monomers including 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 the weight average molecular weight of the copolymer is 1,000 to 100,000;
The viscosity of the paste (shear rate 1 s-1) is 2000 mPa·s or more, and the TI value (ratio of the viscosity at 1 s-1 to the viscosity at a shear rate of 1000 s-1) is greater than 1. An insulating paste for a lithium ion secondary battery current collector, comprising: an inorganic filler (A); a binder (B); a dispersion resin (C) having a phosphate group; and a solvent (D),
The sodium content of the paste is 10 to 350 ppm;
The viscosity of the paste (shear rate 1 s-1) is 2000 mPa·s or more, and the TI value (ratio of the viscosity at 1 s-1 to the viscosity at a shear rate of 1000 s-1) is greater than 1. _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) is at least one selected from the group consisting of alumina, silica, TiO ₂ , BaTiO ₃ , ZrO 2 , boehmite, zeolite, apatite and kaolin. An insulating paste for a lithium-ion secondary battery current collector according to any one of claims 1 to 4, characterized in that the binder (B) is modified or unmodified polyvinylidene fluoride. An insulating paste for a lithium-ion secondary battery current collector according to any one of claims 1 to 5, characterized in that the solvent (D) is N-methyl-2-pyrrolidone. An insulating paste for a lithium-ion secondary battery current collector, characterized in that the insulating paste according to any one of claims 1 to 6 does not substantially contain an electrode active material. A method for manufacturing an insulating layer for a current collector of a lithium ion secondary battery, comprising applying the insulating paste described in any one of claims 1 to 7 to part or all of a current collector, and then heating and drying the paste to form an insulating layer. A method for producing an insulating layer for a lithium-ion secondary battery current collector, wherein the insulating layer according to claim 8 is non-porous. The method for manufacturing an insulating layer for a lithium-ion secondary battery current collector according to claim 8 or 9, characterized in that the current collector is made of aluminum or a composite metal thereof.