CN107492625B - Battery separator and method for manufacturing same - Google Patents

Battery separator and method for manufacturing same Download PDF

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Publication number
CN107492625B
CN107492625B CN201710203782.2A CN201710203782A CN107492625B CN 107492625 B CN107492625 B CN 107492625B CN 201710203782 A CN201710203782 A CN 201710203782A CN 107492625 B CN107492625 B CN 107492625B
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Prior art keywords
vinylidene fluoride
copolymer
acrylic resin
battery separator
battery
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CN107492625A (en
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辻本润
水野直树
梶田笃史
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Toray Industries Inc
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Toray Industries Inc
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • H01M50/411—Organic material
    • H01M50/414—Synthetic resins, e.g. thermoplastics or thermosetting resins
    • H01M50/426—Fluorocarbon polymers
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J5/00—Manufacture of articles or shaped materials containing macromolecular substances
    • C08J5/20—Manufacture of shaped structures of ion-exchange resins
    • C08J5/22—Films, membranes or diaphragms
    • C08J5/2206—Films, membranes or diaphragms based on organic and/or inorganic macromolecular compounds
    • C08J5/2218—Synthetic macromolecular compounds
    • C08J5/2231—Synthetic macromolecular compounds based on macromolecular compounds obtained by reactions involving unsaturated carbon-to-carbon bonds
    • C08J5/2237—Synthetic macromolecular compounds based on macromolecular compounds obtained by reactions involving unsaturated carbon-to-carbon bonds containing fluorine
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/403—Manufacturing processes of separators, membranes or diaphragms
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • H01M50/411—Organic material
    • H01M50/414—Synthetic resins, e.g. thermoplastics or thermosetting resins
    • H01M50/417—Polyolefins
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • H01M50/411—Organic material
    • H01M50/414—Synthetic resins, e.g. thermoplastics or thermosetting resins
    • H01M50/42—Acrylic resins
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • H01M50/431—Inorganic material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • H01M50/443—Particulate material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • H01M50/446—Composite material consisting of a mixture of organic and inorganic materials
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/409—Separators, membranes or diaphragms characterised by the material
    • H01M50/449—Separators, membranes or diaphragms characterised by the material having a layered structure
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/463—Separators, membranes or diaphragms characterised by their shape
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H01M50/489—Separators, membranes, diaphragms or spacing elements inside the cells, characterised by their physical properties, e.g. swelling degree, hydrophilicity or shut down properties
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2323/00—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers
    • C08J2323/26—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers modified by chemical after-treatment
    • C08J2323/28—Characterised by the use of homopolymers or copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond; Derivatives of such polymers modified by chemical after-treatment by reaction with halogens or halogen-containing compounds
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2327/00—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by a halogen; Derivatives of such polymers
    • C08J2327/02—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by a halogen; Derivatives of such polymers not modified by chemical after-treatment
    • C08J2327/12—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by a halogen; Derivatives of such polymers not modified by chemical after-treatment containing fluorine atoms
    • C08J2327/16—Homopolymers or copolymers of vinylidene fluoride
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

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  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Electrochemistry (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Inorganic Chemistry (AREA)
  • Materials Engineering (AREA)
  • Health & Medical Sciences (AREA)
  • Medicinal Chemistry (AREA)
  • Polymers & Plastics (AREA)
  • Organic Chemistry (AREA)
  • Composite Materials (AREA)
  • Cell Separators (AREA)

Abstract

The present inventors have aimed to provide a battery separator which is prepared for the widespread increase in size of a battery (particularly, a laminate-type battery) which is likely to be developed in the future and which can satisfy the dry bending strength, the dry peeling force, and the wet bending strength as problems at the same time, and a method for manufacturing the same. A battery separator comprising a microporous membrane and a porous layer provided on at least one surface of the microporous membrane, wherein the porous layer comprises a vinylidene fluoride-hexafluoropropylene copolymer (A), a vinylidene fluoride unit-containing polymer (B), and an acrylic resin, the vinylidene fluoride-hexafluoropropylene copolymer (A) contains a hydrophilic group and 0.3 to 3 mol% of hexafluoropropylene unit, the melting point of the vinylidene fluoride unit-containing polymer (B) is 60 to 145 ℃, and the weight average molecular weight is 10 to 75 ten thousand.

Description

Battery separator and method for manufacturing same
Technical Field
The present invention relates to a battery separator and a method for manufacturing the same.
Background
Nonaqueous electrolyte secondary batteries, particularly lithium ion secondary batteries, are widely used in small electronic devices such as mobile phones and mobile information terminals, and cylindrical batteries, prismatic batteries, laminate batteries, and the like have been developed. In general, these batteries have a structure in which an electrode body (laminated electrode body) in which a positive electrode and a negative electrode are laminated with a separator interposed therebetween, an electrode body (wound electrode body) wound in a spiral shape, and a nonaqueous electrolytic solution are housed in an exterior body.
A conventional separator for a nonaqueous electrolyte secondary battery mainly uses a microporous membrane made of a polyolefin resin, and blocks pores of the separator when the battery abnormally generates heat to suppress current flow and prevent ignition or the like.
In recent years, attempts have been made to improve battery characteristics by providing a porous layer on one or both sides of a microporous membrane. For example, there is a separator provided with a porous layer containing a fluororesin and an acrylic resin for imparting a function such as electrode adhesiveness (patent documents 1 to 8 of prior art documents). Further, if inorganic particles are added to the porous layer, even if the battery is pierced by a sharp metal, suddenly short-circuited and scalded when an accident or the like occurs, it is possible to prevent the melting shrinkage of the separator and suppress the enlargement of the short-circuited portion between the electrodes.
Patent document 1 describes an electrode body having a positive electrode, a negative electrode, a three-layer separator made of polypropylene/polyethylene/polypropylene, and a pressure-sensitive adhesive resin layer made of polyvinylidene fluoride and alumina powder disposed between the electrodes and the separator.
Example 1 of patent document 2 describes an organic separator with a porous film, which is obtained by: after a NMP (N-methylpyrrolidone) solution containing a first polymer (polyvinylidene fluoride homopolymer) and a NMP solution containing a second polymer (a polymer containing an acrylonitrile monomer, a monomer derived from 1, 3-butadiene, a methacrylic acid monomer, and a butyl acrylate monomer) were stirred by a primary stirrer (primary mixer), a binder NMP solution was prepared, and a slurry prepared by mixing and dispersing the prepared NMP solution and alumina particles was applied to a polypropylene separator.
In the examples of patent document 3, there is described an electrode body obtained by thermocompression bonding a positive electrode and a negative electrode with a sheet containing inorganic fine particles (insulating adhesive layer) interposed therebetween, wherein the sheet containing inorganic fine particles (insulating adhesive layer) is obtained by: an NMP solution in which a complex material composed of vinylidene fluoride-hexafluoropropylene copolymer (VdF-HFP copolymer) and polyethyl methacrylate was dissolved was added to an NMP solution in which spherical alumina powder was dispersed, and after mixing by a ball mill, the prepared slurry was applied onto a base PET (polyethylene terephthalate) film and dried.
In example 1 of patent document 4, there is described a separator obtained by: VdF-HFP copolymer and cyanoethylpullan are added into acetone, barium titanate powder is then added, and slurry obtained after dispersion by a ball mill is coated on a polyethylene porous film to obtain the composite material.
In example 1 of patent document 5, there is described a separator in which VdF-HFP copolymer (HFP unit 0.6 mol% (mol%)) and VdF-HFP copolymer (weight average molecular weight 47 ten thousand, HFP unit 4.8 mol%) are dissolved in a dimethylacetamide and tripropylene glycol solution, and then applied to a polyethylene microporous membrane to form a porous layer.
In example 1 of patent document 6, a separator is described in which a porous layer is formed by dissolving PvdF (weight average molecular weight 50 ten thousand) and VdF-HFP copolymer (weight average molecular weight 40 ten thousand, 5 mol% of HFP unit) in a solution of dimethylacetamide and tripropylene glycol and then applying the resulting solution to a microporous polyethylene membrane.
In example 1 of patent document 7, there is described a separator in which a porous layer is formed by dissolving PvdF (weight average molecular weight 70 ten thousand) and VdF-HFP copolymer (weight average molecular weight 47 ten thousand, HFP unit 4.8 mol%) in a solution of dimethylacetamide and tripropylene glycol and then applying the resulting solution to a microporous polyethylene membrane.
In example 1 of patent document 8, a separator is described in which a porous layer is formed by dissolving PvdF (weight average molecular weight 35 ten thousand) and VdF-HFP polymer (weight average molecular weight 27 ten thousand, HFP polymer 4.8 mol%) in a solution of dimethylacetamide and tripropylene glycol and then applying the resulting solution to a microporous polyethylene membrane.
The separators disclosed in patent documents 1 to 8 and the layers disposed between the electrodes and the separators each contain a polyvinylidene fluoride-based resin.
[ Prior art documents ]
[ patent document ]
Patent document 1: japanese patent application laid-open No. 1999-036981.
Patent document 2: japanese patent laid-open No. 2013-206846.
Patent document 3: japanese patent laid-open No. 2013-122009.
Patent document 4: japanese patent laid-open publication No. 2013-519206.
Patent document 5: japanese patent No. 5282179.
Patent document 6: japanese patent No. 5282180.
Patent document 7: japanese patent No. 5282181.
Patent document 8: japanese patent No. 5342088.
Disclosure of Invention
[ problems to be solved by the invention ]
In recent years, nonaqueous electrolyte secondary batteries are expected to be applied to large-sized applications such as large-sized flat equipment, lawn mowers, electric bicycles, electric vehicles, hybrid vehicles, and small ships, and it is expected that large-sized batteries will be spread.
The wound electrode body is manufactured by winding a positive electrode and a negative electrode with a separator interposed therebetween while applying tension to the respective members. At this time, the positive electrode and the negative electrode applied to the metal collector hardly expand or contract against the tensile force, and the separator is wound while being stretched in the mechanical direction to some extent. After the wound body was left to stand for a certain period of time, the separator portion gradually contracted to the original length. As a result, a force in a parallel direction is generated at the interface between the electrode and the separator, and the wound electrode assembly (particularly, an electrode assembly wound in a flat shape) is likely to be bent or distorted. Further, as the battery is increased in size, the width and length of the separator become wider and these problems become more pronounced, and there is a concern that the yield in production may decrease. It is expected that the separator and the electrode need to be more strongly adhered than now in order to suppress the occurrence of deflection and distortion of the wound electrode body. In the present specification, the adhesiveness is measured by the dry bending strength obtained by the measurement method described later.
Further, when the electrode body is conveyed, if the members are not in a sufficiently bonded state, the electrodes and the separators are peeled off, and the electrode body cannot be conveyed with high yield. The problem of adhesiveness during transportation is more pronounced due to the increase in size of the battery, and there is a concern that the yield may decrease. Therefore, it is expected that the separator needs to have a high peeling force at drying which is difficult to peel from the electrode.
In particular, in the laminate type battery, it is difficult to apply pressure as compared with a prismatic or cylindrical battery in which pressure is applied through an outer package, and the electrode expands and contracts with charge and discharge, so that the interface between the separator and the electrode is easily partially dissociated. As a result, the battery expands, the resistance inside the battery increases, and the cycle performance decreases. Therefore, the separator is required to have a certain adhesion to the electrode in the battery after the electrolyte is injected. In the present specification, the adhesiveness is indicated by the wet bending strength obtained by the measurement method described later. When the strength is large, it is expected to improve the characteristics of the battery, for example, to suppress the swelling of the battery after repeated charge and discharge.
In the conventional techniques, there is a trade-off relationship among the bending strength at the time of drying, the peeling force at the time of drying, and the bending strength at the time of wetting, and it is extremely difficult to satisfy all the physical properties. The purpose of the present invention is to provide a battery separator that satisfies the conditions of dry bending strength, dry peeling force, and wet bending strength in preparation for the widespread use of larger batteries (particularly, laminate batteries) that will be expected to grow in the future.
The wet bending strength described in the present specification indicates the adhesion between the separator and the electrode in a state where the separator contains an electrolyte. The bending strength at drying and the peeling force at drying indicate the adhesiveness to the interface between the separator and the electrode in a state where the separator does not substantially contain the electrolyte. The term "not substantially contained" means that the electrolyte solution in the separator is 500ppm or less.
[ means for solving problems ]
In order to solve the above problems, a battery separator and a method for manufacturing the same according to the present invention have the following configurations.
(1) A battery separator comprising a microporous membrane and a porous layer provided on at least one surface of the microporous membrane, wherein the porous layer comprises a vinylidene fluoride-hexafluoropropylene copolymer (A), a vinylidene fluoride unit-containing polymer (B), and an acrylic resin, the vinylidene fluoride-hexafluoropropylene copolymer (A) comprises a hydrophilic group and 0.3 to 3 mol% of a hexafluoropropylene unit, the vinylidene fluoride unit-containing polymer (B) has a melting point of 60 ℃ to 145 ℃ inclusive, and a weight average molecular weight of 10 to 75 ten thousand inclusive.
(2) In the battery separator of the present invention, the weight average molecular weight of the vinylidene fluoride-hexafluoropropylene copolymer (a) is preferably more than 75 ten thousand and 200 ten thousand or less.
(3) In the battery separator of the present invention, the porous layer preferably contains particles.
(4) In the battery separator of the present invention, the content of the vinylidene fluoride-hexafluoropropylene copolymer (a) is preferably 15 to 85 weight percent based on the total weight of the vinylidene fluoride-hexafluoropropylene copolymer (a) and the vinylidene fluoride unit-containing polymer (B), and the content of the acrylic resin is preferably 4 to 40 weight percent based on the total weight of the vinylidene fluoride-hexafluoropropylene copolymer (a), the vinylidene fluoride unit-containing polymer (B), and the acrylic resin.
(5) In the battery separator of the present invention, the acrylic resin is preferably a copolymer of a (meth) acrylate and a monomer having a cyano group.
(6) In the battery separator of the present invention, the acrylic resin is preferably a copolymer containing butyl acrylate.
(7) In the battery separator of the present invention, the acrylic resin is preferably a copolymer of butyl acrylate and acrylonitrile.
(8) In the battery separator of the present invention, the content of butyl acrylate in the acrylic resin is preferably 50 mol% to 75 mol%.
(9) In the battery separator of the present invention, the content of the hydrophilic group of the vinylidene fluoride-hexafluoropropylene copolymer (a) is preferably 0.1 mol% to 5 mol%.
(10) In the battery separator of the present invention, the bending strength is preferably 4N or more when wet, 5N or more when dry, and the peeling force is preferably 8N/m when dry.
(11) In the battery separator of the present invention, the content of the particles is preferably 50 weight percent or more and 90 weight percent or less based on the total weight of the porous layer.
(12) In the battery separator of the present invention, the particles preferably contain at least one selected from the group consisting of alumina, titania, and boehmite.
(13) In the battery separator of the present invention, the porous layer preferably has a thickness of 0.5 μm to 3 μm on one surface.
(14) In the battery separator of the present invention, the microporous membrane is preferably a polyolefin microporous membrane.
In order to solve the above problems, the method for producing a polyolefin microporous membrane of the present invention has the following structure.
(15) A method for producing a battery separator described in any one of (1) to (14), comprising the following steps in this order.
(1) And a step of dissolving the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride unit-containing polymer (B) in a solvent to obtain a fluororesin solution.
(2) And a step of adding an acrylic resin solution obtained by dissolving an acrylic resin in a solvent to the fluorine resin solution, and mixing the acrylic resin solution and the fluorine resin solution to obtain a coating liquid.
(3) And a step of applying a coating liquid to the microporous membrane, immersing the microporous membrane in a coagulating liquid, and washing and drying the microporous membrane, wherein the vinylidene fluoride-hexafluoropropylene copolymer (A) contains a hydrophilic group and 0.3 to 3 mol% of a hexafluoropropylene unit, the vinylidene fluoride unit-containing polymer (B) has a melting point of 60 ℃ to 145 ℃ and a weight average molecular weight of 10 to 75 ten thousand, and the acrylic resin contains butyl acrylate.
[ Effect of the invention ]
According to the present invention, it is possible to provide a battery separator that can satisfy the conditions of dry bending strength, dry peeling force, and wet bending strength in preparation for the spread of a large-sized battery that may be developed in the future.
Drawings
FIG. 1 is a front cross-sectional view schematically showing a bending strength test in a wet state.
Fig. 2 is a front cross-sectional view schematically showing a bending strength test at the time of drying.
Detailed Description
The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.
1. Microporous membrane
First, the microporous membrane of the present invention will be described.
In the present invention, the microporous membrane refers to a membrane having interconnected voids therein. The microporous membrane is not particularly limited, and a nonwoven fabric and a microporous membrane can be used. Hereinafter, a case where the resin constituting the microporous membrane is a polyolefin resin will be described in detail, but the present invention is not limited thereto.
[1] Polyolefin resin
The polyolefin resin constituting the polyolefin microporous membrane contains a polyethylene resin and a polypropylene resin as main components. The content of the polyethylene resin is preferably 70 mass% or more, more preferably 90 mass% or more, and still more preferably 100 mass% when the total mass of the polyolefin resin is 100 mass%.
Examples of the polyolefin resin include homopolymers, 2-stage polymers, copolymers, and mixtures thereof obtained by polymerizing ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and the like. Various additives such as an antioxidant and an inorganic filler may be added to the polyolefin resin as necessary within a range not to impair the effects of the present invention.
[2] Process for producing polyolefin microporous membrane
The method for producing the polyolefin microporous membrane is not particularly limited as long as it can produce a polyolefin microporous membrane having desired properties, and conventionally known methods can be used, and for example, the methods described in japanese patent No. 2132327 and japanese patent No. 3347835, international publication No. 2006/137540, and the like can be used. Specifically, the method preferably includes the following steps (1) to (5).
(1) And a step of melt-kneading the polyolefin resin and the film-forming solvent to prepare a polyolefin solution.
(2) And extruding the polyolefin solution, cooling the extruded polyolefin solution, and forming a gel sheet.
(3) And a 1 st stretching step of stretching the gel sheet.
(4) And removing the film-forming solvent from the stretched gel sheet.
(5) And drying the sheet from which the film-forming solvent has been removed.
Hereinafter, each step will be described.
(1) Process for producing polyolefin solution
After adding an appropriate film-forming solvent to each of the polyolefin resins, the resulting mixture is melt-kneaded to prepare a polyolefin solution. As the melt kneading method, for example, a method using a twin-screw extruder described in japanese patent No. 2132327 and japanese patent No. 3347835 can be used. Since the melt kneading method is well known, the description thereof will be omitted.
The blending ratio of the polyolefin resin and the film-forming solvent in the polyolefin solution is not particularly limited, but is preferably from 70 to 80 parts by mass with respect to 20 to 30 parts by mass of the polyolefin resin. When the proportion of the polyolefin resin is within the above range, expansion and retraction at the die outlet can be prevented when the polyolefin solution is extruded, and the moldability and self-supporting property of the extruded molded article (gel-like molded article) are good.
(2) Step of Forming gel sheet
And conveying the polyolefin solution from the extruder to a die, and extruding into sheets. From the extruder, the polyolefin solutions of the same or different composition are fed to 1 die, where they are layered or extruded in sheet form.
The extrusion method may be any of a flat die method and a blown film method. The extrusion temperature is preferably from 140 ℃ to 250 ℃ and the extrusion speed is preferably from 0.2 m/min to 15 m/min. The film thickness can be adjusted by adjusting the respective extrusion amounts of the polyolefin solutions. As the extrusion method, for example, methods disclosed in japanese patent No. 2132327 and japanese patent No. 3347835 can be used.
The obtained extrusion-molded article was cooled to form a gel sheet. As a method for forming the gel sheet, for example, methods disclosed in japanese patent No. 2132327 and japanese patent No. 3347835 can be used. Preferably at a rate of 50 deg.c/min or more to at least the gelling temperature. The cooling is preferably carried out at a temperature below 25 ℃. Upon cooling, the microphase of the polyolefin separated from the film-forming solvent can be fixed. When the cooling rate is within the above range, the degree of crystallization can be maintained within an appropriate range, and a gel sheet suitable for stretching can be formed. As a cooling method, a method of contacting with a refrigerant such as cold air or cooling water, a method of contacting with a cooling roller, or the like can be used, and it is preferable to cool the roller by contacting with a roller cooled by a refrigerant.
(3) 1 st drawing step
Subsequently, the obtained gel sheet is stretched in at least a uniaxial direction. Since the gel sheet contains a film-forming solvent, it can be uniformly stretched. After heating, the gel sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, a film blowing method, or a combination of these methods. The stretching may be uniaxial stretching or biaxial stretching, but biaxial stretching is preferable. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching, and multi-stage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used.
The stretch ratio (area stretch ratio) in this step is preferably 9 times or more, more preferably 16 times or more, and particularly preferably 25 times or more. The stretch ratios in the Machine Direction (MD) and the width direction (TD) may be the same or different from each other. The stretching ratio in this step is an area stretching ratio of the microporous membrane before the next step based on the microporous membrane before this step.
The stretching temperature in this step is preferably in the range of from the crystal dispersion temperature (Tcd) to Tcd +30 ℃ of the polyolefin resin, more preferably in the range of from the crystal dispersion temperature (Tcd) +5 ℃ to the crystal dispersion temperature (Tcd) +28 ℃, and particularly preferably in the range of from Tcd +10 ℃ to Tcd +26 ℃. For example, polyethylene, the stretching temperature is preferably 90 ℃ to 140 ℃, more preferably 100 ℃ to 130 ℃. The crystal dispersion temperature (Tcd) was determined by measuring the temperature characteristics of dynamic viscoelasticity of ASTM D4065 (American Society for Testing Materials; American Society for Testing Materials Standard).
The stretching causes cracks between the polyethylene sheets, and the polyethylene phase is refined to form numerous fibrils. The fibrils form a three-dimensional irregularly connected network structure. By stretching, the mechanical strength can be improved, and the pores can be enlarged, and when stretching is performed under appropriate conditions, the through-hole diameter can be controlled, and a high porosity can be obtained even in a thin film thickness.
The microporous membrane may be stretched by setting a temperature distribution in the thickness direction according to the desired physical properties, thereby obtaining a microporous membrane having further excellent mechanical strength. The details of this method are described in japanese patent No. 3347854.
(4) Removal of film-forming solvent
The film-forming solvent is removed (washed) using a washing solvent. Since the polyolefin phase is separated from the film-forming solvent phase, the film-forming solvent is removed, and thus a porous film having three-dimensionally irregularly connected pores (voids) and composed of fibrils forming a fine three-dimensional network structure can be obtained. The cleaning solvent and the method of removing the film-forming solvent using the cleaning solvent are well known, and therefore, the description thereof is omitted. For example, the methods disclosed in the specification of Japanese patent No. 2132327 and Japanese patent laid-open No. 2002-256099 can be used.
(5) Drying
The microporous membrane from which the solvent for film formation has been removed is dried by a heat drying method or an air drying method. The drying temperature is preferably not higher than the crystal dispersion temperature (Tcd) of the polyolefin resin, and particularly preferably not lower than Tcd by 5 ℃. When the microporous membrane is 100 mass% (dry weight), it is preferably dried until the residual cleaning solvent is 5 mass% or less, and more preferably dried until 3 mass% or less. When the residual cleaning solvent is within the above range, the porosity of the microporous membrane can be maintained and deterioration of the permeability can be suppressed when the subsequent stretching step and heat treatment step of the microporous membrane are performed.
2. Porous layer
In the present invention, the porous layer contains a vinylidene fluoride-hexafluoropropylene (VdF-HFP) copolymer (a), a vinylidene fluoride unit-containing polymer (B), and an acrylic resin. The respective resins are explained below.
[1] Vinylidene fluoride-hexafluoropropylene (VdF-HFP) copolymer (A)
The vinylidene fluoride-hexafluoropropylene (VdF-HFP) copolymer (a) used in the present invention contains a hydrophilic group and 0.3 to 3 mol% of hexafluoropropylene. The copolymer (a) has high affinity for a nonaqueous electrolytic solution, high chemical and physical stability, and high bending strength when wet, and therefore can sufficiently maintain affinity for an electrolytic solution even when used at high temperatures.
The vinylidene fluoride-hexafluoropropylene copolymer (a) has a hydrophilic group, and thus can be strongly bonded to an active material present on the surface of an electrode and a binder component in the electrode. It is presumed that such adhesion is caused by hydrogen bonding. Examples of the hydrophilic group include a hydroxyl group, a carboxylic acid group, a sulfonic acid group, and salts thereof. Carboxylic acid groups and carboxylic acid esters are particularly preferred.
When a hydrophilic group is introduced into vinylidene fluoride, for example, the following methods can be mentioned: in the synthesis of the vinylidene fluoride-hexafluoropropylene copolymer (a), a method of introducing a monomer having a hydrophilic group such as maleic anhydride, maleic acid ester, and monomethyl maleate into the main chain by copolymerization, and a method of introducing a monomer as a side chain by grafting are used. The hydrophilic group modification rate can be measured by FT-IR (Fouier Transform Infrared Spectroscopy; Fourier Transform Infrared Spectroscopy), NMR (nuclear magnetic resonance), quantitative titration, or the like. For example, in the case of a carboxylic acid group, the absorption intensity ratio of C — H stretching vibration and C ═ O stretching vibration of the carboxylic acid group can be calculated from a homopolymer as a reference by using FT-IR.
The lower limit of the content of the hydrophilic group in the vinylidene fluoride-hexafluoropropylene copolymer (a) is preferably 0.1 mol% or more, more preferably 0.3 mol% or more, and the upper limit is preferably 5 mol% or less, more preferably 4 mol% or less. When the content of the hydrophilic group exceeds 5 mol%, the crystallinity of the polymer is too low, the swelling degree with respect to the electrolyte solution becomes high, and the bending strength in wet becomes poor. When the porous layer contains particles, the content of the hydrophilic group is set within the above preferable range, whereby the particles can be prevented from falling off.
The lower limit of the hexafluoropropylene content in the vinylidene fluoride-hexafluoropropylene copolymer (a) is preferably 0.3 mol% or more, more preferably 0.5 mol% or more, and the upper limit is preferably 3 mol% or less, more preferably 2.5 mol% or less. If the content of hexafluoropropylene is less than 0.3 mole%, the crystallinity of the polymer becomes high, and the swelling degree with respect to the electrolyte becomes low, so that it is difficult to sufficiently obtain the wet bending strength. When the amount exceeds 3 mol%, swelling of the electrolyte solution becomes excessive, and the bending strength in wet state is lowered.
The content of the vinylidene fluoride-hexafluoropropylene copolymer (a) is preferably 15 weight percent or more in the lower limit, more preferably 25 weight percent or more in the upper limit, and preferably 85 weight percent or less, more preferably 25 weight percent or less in the upper limit, relative to the total weight of the copolymer (a) and the polymer (B).
The weight average molecular weight of the vinylidene fluoride-hexafluoropropylene copolymer (a) has a lower limit of more than 75 ten thousand, preferably 90 ten thousand or more, and an upper limit of preferably 200 ten thousand or less, more preferably 150 ten thousand or less. By setting the weight average molecular weight of the copolymer (a) within the above-mentioned preferable range, the time for dissolving the copolymer (a) in the solvent does not become extremely long, and the production efficiency can be improved. Further, the gel strength can be maintained at an appropriate level when the gel swells in the electrolyte solution, and the bending strength when the gel is wet can be improved. In addition, the weight average molecular weight described in the present invention is a polystyrene equivalent value calculated by gel permeation chromatography.
The vinylidene fluoride-hexafluoropropylene copolymer (a) can be obtained by a well-known polymerization method. As a well-known polymerization method, for example, a method exemplified in Japanese patent laid-open No. Hei 11-130821 can be cited. That is, ion-exchanged water, monomethyl maleate, vinylidene fluoride, and hexafluoropropylene were put into an autoclave and subjected to suspension polymerization, and then a polymer slurry was dehydrated, washed, and dried to obtain a polymer powder. In this case, methylcellulose may be suitably used as a suspending agent, and diisopropyl peroxydicarbonate or the like may be used as a radical initiator.
The vinylidene fluoride-hexafluoropropylene copolymer (a) may be a copolymer obtained by further polymerizing a monomer other than the monomer having the hydrophilic group, as long as the properties are not impaired. Examples of the monomer other than the monomer having a hydrophilic group include monomers such as tetrafluoroethylene, chlorotrifluoroethylene, trichloroethylene, and fluorinated ethylene.
[2] Vinylidene fluoride unit-containing polymer (B)
The vinylidene fluoride unit-containing polymer (B) used in the present invention has a melting point of 60 to 145 ℃ inclusive, a weight average molecular weight of 10 to 75 ten thousand inclusive, a high affinity for a nonaqueous electrolytic solution, high chemical and physical stability, and can obtain a bending strength at drying and a peeling force at drying. The mechanism of this is not clear, but the inventors speculate that the polymer (B) has fluidity under heating and pressurizing conditions under which the bending strength at drying and the peeling force at drying can be found, and enters the porous layer of the electrode to become a binder, whereby the porous layer and the electrode have strong adhesiveness. The polymer (B) contributes to the realization of bending strength at the time of drying and peeling force at the time of drying, and contributes to the prevention of deflection and distortion of the wound electrode body and the laminated electrode body, and improves the handling property. The vinylidene fluoride unit-containing polymer (B) is a resin different from the vinylidene fluoride-hexafluoropropylene copolymer (a).
The lower limit of the melting point of the vinylidene fluoride unit-containing polymer (B) is preferably 60 ℃ or higher, more preferably 80 ℃ or higher, and the upper limit thereof is preferably 145 ℃ or lower, more preferably 140 ℃ or lower. The melting point as used herein refers to the peak temperature of the endothermic peak at the time of temperature rise as measured by Differential Scanning Calorimetry (DSC).
The vinylidene fluoride unit-containing polymer (B) is a resin composed of a copolymer having a polyvinylidene fluoride or vinylidene fluoride unit. The polymer (B) can be obtained by the same suspension polymerization method as the copolymer (a) or the like. The melting point of the polymer (B) can be adjusted by controlling the crystallinity of the site composed of vinylidene fluoride units. For example, when a monomer other than vinylidene fluoride units is contained in the polymer (B), the melting point can be adjusted by controlling the proportion of vinylidene fluoride units. The monomer other than the vinylidene fluoride unit may have 1 or 2 or more kindsThe following substances above, namely: tetrafluoroethylene, chlorotrifluoroethylene, trichloroethylene, hexafluoropropylene, ethylene chloride maleic anhydride, maleic acid esters, monomethyl maleate, and the like. Examples thereof include a method of introducing the above-mentioned monomer into the main chain by copolymerization when the polymer (B) is polymerized, and a method of introducing the monomer as a side chain by grafting. Furthermore, it is also possible to control the Head-to-Head bond (-CH) of the vinylidene fluoride unit2-CF2-CF2-CH2-) to adjust the melting point.
The lower limit of the weight average molecular weight of the vinylidene fluoride unit-containing polymer (B) is preferably 10 ten thousand or more, more preferably 15 ten thousand or more, and the upper limit is preferably 75 ten thousand or less, more preferably 70 ten thousand or less.
When the melting point and the weight average molecular weight of the vinylidene fluoride unit-containing polymer (B) are set within the above preferred ranges, the polymer (B) flows easily under heating and pressurizing conditions, and sufficient bending strength at drying and peeling force at drying can be obtained. When the melting point of the polymer (B) exceeds the upper limit of the above preferable range, the pressing temperature in the production process of the roll body needs to be increased in order to obtain the bending strength at the time of drying and the peeling force at the time of drying. Thus, the microporous membrane mainly composed of polyolefin may shrink. When the weight average molecular weight of the polymer (B) exceeds the upper limit of the above preferable range, the amount of linkage of the molecular chain increases, and the polymer may not flow sufficiently under pressurized conditions. When the weight average molecular weight of the polymer (B) is less than the lower limit of the above preferable range, the linking amount of the molecular chains is too small, the resin strength is weakened, and cohesive failure of the porous layer is likely to occur.
[3] Acrylic resin
Further, the porous layer contains an acrylic resin, whereby the bending strength at the time of drying and the peeling force at the time of drying can be improved. When the vinylidene fluoride-hexafluoropropylene (VdF-HFP) copolymer (a) and the vinylidene fluoride unit-containing polymer (B) are contained alone, a separator satisfying the dry bending strength, wet bending strength and dry peeling strength cannot be obtained.
The acrylic resin is preferably a (meth) acrylate polymer or a copolymer thereof. In the present invention, the (meth) acrylate means acrylate (acrylic acid) and methacrylate (methacrylic acid). Examples of the (meth) acrylate include methyl acrylate, ethyl acrylate, n-butyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, and 2-ethylhexyl methacrylate. Butyl acrylate is particularly preferably contained. Butyl acrylate improves the flexibility of the coating film and is expected to have an effect of suppressing particle shedding.
From the viewpoint of adhesiveness to an electrode, it is more preferable that the acrylic resin is a copolymer of a (meth) acrylate and a monomer having a cyano group. The monomer having a cyano group includes an α, β -ethylenically unsaturated monomer having a cyano group, and is preferably acrylonitrile or methacrylonitrile, for example. Further, it is particularly preferable that the acrylic resin is a copolymer of butyl acrylate and acrylonitrile, and the degree of swelling with respect to the electrolyte is adjusted by controlling the molar ratio, and the resin can be provided with appropriate flexibility and the bending strength in wet condition can be improved. The lower limit of the content of the butyl acrylate unit in the acrylic resin is preferably 50 mol% or more, more preferably 55 mol% or more, and the upper limit is preferably 75 mol% or less, more preferably 70 mol% or less. When the lower limit of the content of the butyl acrylate unit in the acrylic resin is set within the above preferable range, the porous layer can have appropriate flexibility and the falling-off of the porous film can be suppressed. Further, by setting the content of the butyl acrylate unit in the acrylic resin within the above preferable range, the balance of the bending strength at the time of drying, the bending strength at the time of wetting, and the peeling force at the time of drying can be made favorable.
The acrylic resin can be obtained by a well-known polymerization method, for example, a method exemplified in Japanese patent laid-open No. 2013-206846. Examples thereof include the following methods: ion exchange water, N-butyl acrylate, and acrylonitrile were placed in an autoclave equipped with a stirrer, and water of a dispersion liquid obtained by dispersing polymer particles obtained by emulsion polymerization in water was replaced with N-methyl-2-pyrrolidone to obtain an acrylic resin solution. In the polymerization reaction, potassium persulfate may be suitably used as a radical polymerization initiator, and tert-dodecyl mercaptan or the like may be used as a molecular weight modifier.
The lower limit of the content of the acrylic resin is preferably 4 weight percent or more, more preferably 5 weight percent or more, and the upper limit is preferably 40 weight percent or less, more preferably 30 weight percent or less, and still more preferably 20 weight percent or less, based on the total weight of the vinylidene fluoride-hexafluoropropylene copolymer (a), the vinylidene fluoride unit-containing polymer (B), and the acrylic resin. By setting the content of the acrylic resin within the above preferable range, the total content of the copolymer (a) and the polymer (B) can be made a certain value or more, and the oxidation resistance of the porous layer can be maintained.
By setting the content of the vinylidene fluoride-hexafluoropropylene copolymer (a) and the content of the acrylic resin within the above preferred ranges, the porous layer can obtain the bending strength at dry time, the bending strength at wet time, and the peeling force at dry time.
[4] Particles
The porous layer in the present invention may contain particles. By containing the particles in the porous layer, the probability of short-circuiting between the positive electrode and the negative electrode can be reduced, and improvement in safety can be expected. Examples of the particles include inorganic particles and organic particles.
Examples of the inorganic particles include calcium carbonate, calcium phosphate, amorphous silica, crystalline glass particles, kaolin, talc, titanium dioxide, alumina, silica-alumina composite oxide particles, barium sulfate, calcium fluoride, lithium fluoride, zeolite, molybdenum sulfide, mica, boehmite, and magnesium oxide. In particular, titanium dioxide, alumina, boehmite, and barium sulfate are preferable from the viewpoint of crystal growth, cost, and ease of use of the vinylidene fluoride-hexafluoropropylene copolymer.
Examples of the organic particles include crosslinked polystyrene particles, crosslinked acrylic resin particles, and crosslinked methyl methacrylate particles.
The upper limit of the content of the particles contained in the porous layer is preferably 90 weight percent or less, more preferably 85 weight percent or less, and the lower limit is preferably 50 weight percent or more, more preferably 60 weight percent or more, and further preferably 65 weight percent or more, based on the total weight of the porous layer. By setting the content of the particles within the above preferable range, a good balance of the gas resistance can be easily achieved.
When the porous layer contains non-adhesive particles, the wet bending strength, the dry bending strength, and the dry peeling force tend to be reduced. However, even if the porous layer obtained from the resin component of the present invention contains particles within the above preferred range, the balance of the wet bending strength, the dry bending strength, and the dry peeling force of the electrode is good.
From the viewpoint of particle shedding, the average particle diameter of the particles is preferably 1.5 times or more and 50 times or less, and more preferably 2.0 times or more and 20 times or less, the average flow pore diameter of the microporous membrane. The mean flow pore diameter is measured in accordance with JIS K3832 and ASTM F316-86, for example, by using a pore diameter tester (manufactured by PMI Co., Ltd., CFP-1500A) in the order of Dry-up and Wet-up. In the case of Wet-up, a pressure is applied to the sufficiently impregnated microporous membrane by Galwick (trade name) manufactured by PMI company, which is known as a surface tension, and the pore diameter converted from the pressure at which air starts to penetrate is set as the maximum pore diameter. The average flow pore diameter was converted from the pressure at the point where the curve representing the 1/2 inclination of the pressure/flow rate curve in the Dry-up measurement intersects the curve for the Wet-up measurement. The following equation was used to convert the pressure and the pore diameter.
d ═ C · γ/P (in the above formula, "d (μm)" represents the pore diameter of the microporous membrane, "γ (mN/m)" represents the surface tension of the liquid, "P (pa)" represents the pressure, "C" represents a constant)
The average particle diameter of the particles is preferably 0.3 to 1.8 μm, more preferably 0.5 to 1.5 μm, and further preferably 0.9 to 1.3 μm from the viewpoint of slidability with the winding core and particle shedding at the time of winding the battery. The average particle diameter of the particles can be measured using a measuring apparatus of a laser diffraction method or a dynamic light scattering method. For example, particles dispersed in an aqueous solution containing a surfactant are measured by a particle size distribution measuring apparatus (Microtrac HRA, manufactured by japan ltd.) using an ultrasonic probe, and a value of a particle diameter (D50) when accumulated at 50% on a volume basis from a small particle side is preferably used as an average particle diameter. The shape of the particles is not particularly limited, and examples thereof include spherical, substantially spherical, plate-like and needle-like.
[5] Physical properties of porous layer
The single-sided film thickness of the porous layer is preferably 0.5 μm to 3 μm, more preferably 1 μm to 2.5 μm, and further preferably 1 μm to 2 μm. When the thickness of one surface is 0.5 μm or more, the wet bending strength, the dry bending strength and the dry peeling force can be secured. If the thickness of one surface is 3 μm or less, the winding volume can be suppressed, and this is suitable for increasing the capacity of a battery which will be developed in the future.
The porosity of the porous layer is preferably 30% to 90%, more preferably 40% to 70%. By setting the porosity of the porous layer within the above preferable range, the resistance of the separator can be prevented from increasing, a large current can be passed, and the film strength can be maintained.
[6] Method for manufacturing battery separator
The method for producing a battery separator of the present invention includes the following steps (1) to (3) in this order.
(1) And a step of dissolving the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride unit-containing polymer (B) in a solvent to obtain a fluororesin solution.
(2) And a step of adding an acrylic resin solution to the fluorine-based resin solution and mixing the resulting mixture to obtain a coating liquid.
(3) And a step of applying the coating liquid to the microporous membrane, immersing the microporous membrane in a coagulating liquid, and washing and drying the microporous membrane.
(1) Process for obtaining fluorine-based resin solution
The solvent is not particularly limited as long as it can dissolve the vinylidene fluoride-hexafluoropropylene copolymer (a) and the vinylidene fluoride unit-containing polymer (B), dissolve or disperse the acrylic resin, and can be mixed with the coagulating liquid. From the viewpoint of solubility and low volatility, the solvent is preferably N-methyl-2-pyrrolidone.
When the porous layer containing the particles is provided, it is important to prepare a fluorine resin solution (also referred to as a dispersion) in which the particles are dispersed in advance. A fluororesin solution in which particle aggregation is reduced is obtained by dissolving a vinylidene fluoride-hexafluoropropylene copolymer (a) and a vinylidene fluoride unit-containing polymer (B) in a solvent, adding particles while stirring, predispersing the mixture by stirring for a certain period of time (for example, about 1 hour) with a Disperser (DISPER) or the like, and then dispersing the particles by a step (dispersing step) of dispersing the particles using a sand mill or a paint stirrer.
(2) Step of obtaining coating liquid
An acrylic resin solution is added to a fluororesin solution containing a vinylidene fluoride-hexafluoropropylene copolymer (a) and a vinylidene fluoride unit-containing polymer (B), and mixed by, for example, a Three-One Motor with stirring vanes to prepare a coating liquid.
The acrylic resin solution is a solution obtained by dissolving or dispersing an acrylic resin in a solvent, and the solvent used here is preferably the same solvent as in the step (1). In particular, N-methyl-2-pyrrolidone is preferable from the viewpoint of solubility and low volatility. From the viewpoint of handling, it is preferable that after the acrylic resin is polymerized, N-methyl-2-pyrrolidone is added and distilled, etc., to replace the solvent, thereby obtaining an acrylic resin solution.
When the porous layer containing the particles is provided, it is important to add a fluorine resin solution (dispersion liquid) in which the particles are dispersed to an acrylic resin solution. That is, it is important not to add the acrylic resin in the dispersing step. When the vinylidene fluoride-hexafluoropropylene copolymer (a), the vinylidene fluoride unit-containing polymer (B), the acrylic resin, and the particles are simultaneously added to the solvent to prepare the coating liquid, it is presumed that the coating liquid gradually starts to gel due to the heat and the cutting at the time of dispersion of the hydrophilic group and the acrylic resin (particularly, when butyl acrylate is contained) contained in the copolymer (a), and thus the coating liquid is not suitable for industrial use. Further, when the thickness of the porous layer on one side is 3 μm or less due to the effect of thickening, it is difficult to perform thin film coating. According to steps (1) and (2) in the production method of the present invention, gelation of the coating liquid can be suppressed, and film coating can be performed, and the storage stability of the coating liquid can be improved.
(3) Applying the coating liquid to the microporous membrane, immersing the microporous membrane in a coagulating liquid, washing the microporous membrane, and drying the microporous membrane
The coating liquid is applied to a microporous membrane, the coated microporous membrane is immersed in a coagulating liquid, a vinylidene fluoride-hexafluoropropylene copolymer (a), a vinylidene fluoride unit-containing polymer (B), and an acrylic resin are subjected to phase separation, coagulated in a state having a three-dimensional mesh structure, and then washed and dried. Thus, a battery separator having a microporous membrane and a porous layer on the surface of the microporous membrane can be obtained.
As a method for applying the coating liquid to the microporous membrane, a known method may be used, and examples thereof include a dip coating method, a reverse roll coating method, a gravure coating method, a kiss coating method, a roll brush coating method, a spray coating method, an air knife coating method, a meyer bar coating method, a pipe blade coating method (pipe blade method), a blade coating method, and a die coating method (impregnating), and these methods may be carried out alone or in combination.
The coagulating liquid is preferably water, preferably an aqueous solution containing 1 to 20 weight percent of a good solvent for the vinylidene fluoride-hexafluoropropylene copolymer (a), the vinylidene fluoride unit-containing polymer (B), and the acrylic resin, and more preferably an aqueous solution containing 5 to 15 weight percent. Examples of the good solvent include N-methyl-2-pyrrolidone, N-dimethylformamide, and N, N-dimethylacetamide. The time for immersing in the coagulation liquid is preferably 3 seconds or more. The upper limit is not limited, but 10 seconds is sufficient.
Water may be used for cleaning. In the drying, for example, hot air at 100 ℃ or lower may be used for drying.
The battery separator of the present invention can be used for battery separators for secondary batteries such as nickel-metal hydride batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, lithium ion secondary batteries, lithium polymer secondary batteries, and lithium sulfur batteries. Particularly preferably used as a separator for a lithium ion secondary battery.
[7] Physical properties of battery separator
The bending strength of the battery separator when wet is preferably 4N or more. The upper limit of the wet bending strength is not particularly limited, but is sufficient to be 15N. By setting the content within the above preferable range, the separation of the interface between the separator and the electrode can be suppressed, and the increase in the internal resistance of the battery and the decrease in the battery characteristics can be suppressed.
The bending strength of the battery separator when dried is preferably 5N or more. The upper limit of the bending strength in drying is not particularly limited, but it is sufficient to be 25N. By setting the amount within the above preferable range, it is expected that the deflection and distortion of the wound electrode assembly are suppressed.
The peeling force of the battery separator during drying is preferably 8N/m or more. The upper limit of the peeling force during drying is not particularly limited, but it is sufficient to be 40N/m. By setting the thickness within the above preferable range, it is expected that the wound electrode body or the laminated electrode body can be conveyed without scattering the electrode body.
Specifically, the battery separator satisfies the conditions of a wet flexural strength of 4N or more, a dry flexural strength of 5N or more, and a dry peel force of 8N/m or more, which are measured by the following measurement methods.
The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. The measurement values in the examples are values measured by the following methods.
1. Bending strength in wet state
Generally, a binder of a fluororesin is used for the positive electrode, and when a porous layer of a fluororesin is provided on the separator, the binder is easily secured by interdiffusion between the fluororesins. On the other hand, since the negative electrode uses a binder other than the fluororesin, the fluororesin is less likely to diffuse, and therefore, the negative electrode is less likely to have adhesion to the separator than the positive electrode. Therefore, in the present measurement, the adhesion between the separator and the negative electrode was evaluated using the following flexural strength as an index.
(1) Production of negative electrode
Adding 1.5 parts by mass of aqueous solution containing carboxymethyl cellulose to 96.5 parts by mass of artificial graphite, mixing, and making into solidAnd 2 parts by mass of styrene-butadiene latex is added and mixed to form slurry containing the negative electrode composite additive. The slurry containing the negative electrode composite additive was uniformly applied to both surfaces of a negative electrode current collector made of a copper foil having a thickness of 8 μm, dried to form a negative electrode layer, and then compression-molded by a roll press so that the density of the negative electrode layer excluding the current collector was 1.5g/cm3Thereby, a negative electrode was produced.
(2) Production of wound body for testing
The negative electrode (161 mm in the machine direction × 30mm in the width direction) produced above and the separator (160 mm in the machine direction × 34mm in the width direction) produced in the examples and comparative examples were stacked, the separator and the negative electrode were wound with the separator on the inner side with a metal plate (300 mm in length, 25mm in width, 1mm in thickness) as a winding core, and the metal plate was pulled out to obtain a wound body for a test. The length of the test coil was about 34mm and the width was about 28 mm.
(3) Method for measuring bending strength in wet state
2 sheets of a laminated film (length 70mm, width 65mm, thickness 0.07mm) made of polypropylene were stacked, and the test roll was placed in a bag-like laminated film in which 3 sides were welded out of 4 sides. Make LiPF6The electrolyte solution was dissolved at a ratio of 1mol/L in a solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 3:7 to obtain 500. mu.L of the electrolyte solution, and the electrolyte solution was injected from the opening of the laminate film in a glove box, immersed in the test jelly roll, and one side of the opening was sealed with a vacuum sealer.
Then, the test roll with the laminated film sealed therein was sandwiched between 2 spacers (thickness: 1mm, 5 cm. times.5 cm), pressed at 98 ℃ and 0.6MPa for 2 minutes by a precision heating and pressing apparatus (CYPT-10, manufactured by Xindong industries, Ltd.), and cooled at room temperature. The test wound body after pressing was subjected to bending strength measurement under wet conditions using a universal testing machine (AGS-J, manufactured by Shimadzu corporation) while maintaining the laminated film sealed therein. The details are described below.
2 pieces of aluminum L-shaped angle iron (length 1mm, 10 mm. times.10 mm, length 5cm) were arranged in parallel with the 90 ° portion facing upward and the ends aligned, and were fixed with the 90 ° portion as a fulcrum so that the distance between the fulcrums was 15 mm. The test roll was configured by: the middle of the distance between the supporting points of 2 pieces of aluminum L-shaped angle iron, that is, 7.5mm, was aligned with the midpoint of the side (about 28mm) in the width direction of the test roll and did not exceed the side in the length direction of the L-shaped angle iron.
Then, as an indenter, the side in the longitudinal direction of the test roll (about 34mm) was made not to exceed the side in the longitudinal direction of the aluminum L-shaped angle (thickness 1mm, 10mm × 10mm, length 4cm) and the two sides were made parallel, the midpoint of the side in the width direction of the test roll was aligned with the 90 ° portion of the aluminum L-shaped angle, and the aluminum L-shaped angle was fixed to the load cell (load cell capacity 50N) of the universal tester so that the 90 ° portion was oriented. The average value of the maximum test force obtained by measuring 3 test wound bodies at a load speed of 0.5mm/min was defined as the wet bending strength.
2. Bending strength on drying
(1) Production of negative electrode
The negative electrode having the same bending strength when wet as in the above 1.
(2) Production of wound body for testing
The test wound body having the same bending strength under wet condition as in the above 1.
(3) Method for measuring bending strength during drying
The prepared test wound body was sandwiched between 2 spacers (thickness: 1mm, 5 cm. times.5 cm), pressed at 70 ℃ and 0.6MPa for 2 minutes by a precision heating and pressing apparatus (CYPT-10, manufactured by Xindong industries, Ltd.), and cooled at room temperature. The test wound body was arranged in the same manner as the method for measuring wet flexural strength described in the above 1, and the average value of the maximum test forces obtained after measuring 3 test wound bodies under the following conditions was used as the dry flexural strength using a universal testing machine (AGS-J, manufactured by shimadzu corporation).
Distance between fulcrums: 15 mm.
Load cell capacity: 50N.
Load speed: 0.5 mm/min.
3. Peel force on drying
(1) Production of negative electrode
The negative electrode having the same bending strength when wet as in the above 1.
(2) Production of peeling test piece
The negative electrode (70 mm. times.15 mm) prepared above and the separator (90 mm. times.20 mm in the machine direction) prepared in examples and comparative examples were stacked, sandwiched by 2 spacers (0.5 mm in thickness, 95 mm. times.27 mm in thickness), pressed by a precision heat and pressure apparatus (CYPT-10, manufactured by Xindong industries, Ltd.) at 90 ℃ and 8MPa for 2 minutes, and cooled at room temperature. A double-sided tape having a width of 1cm was attached to the negative electrode side of the laminate of the negative electrode and the separator, and the other side of the double-sided tape was attached to an SUS plate (thickness: 3mm, length: 150 mm. times. width: 50mm) so that the mechanical direction of the separator was parallel to the longitudinal direction of the SUS plate. This was used as a peel test piece.
(3) Method for measuring peeling force during drying
The 180-degree peel test was performed at a test speed of 300 mm/min by clamping the separator between load cell-side chucks using a universal tester (AGS-J, manufactured by Shimadzu corporation). The measured values from the stroke 20mm to 70mm in the peel test were averaged, and the averaged value was used as the peel force of the peel test piece. A total of 3 peel test pieces were measured, and the average value of the peel force was converted into the width to obtain a dry peel force (N/m).
4. Melting point determination
Using a differential scanning calorimetry analyzer (DSC, manufactured by Perkin Elmer, Ltd.), 7mg of a resin was placed in a measuring crucible as a sample for measurement, and the measurement was carried out under the following conditions. After the first temperature rise and cooling, the peak of the endothermic peak at the 2 nd temperature rise was defined as the melting point.
Temperature rise and cooling rate: +/-10 deg.C/min.
Measurement temperature range: 30 ℃ to 230 ℃.
5. Film thickness
The film thickness was measured at 20 spots with a super hard spherical stylus φ 9.5mm under a condition of a weight of 0.01N using a contact type film thickness meter ("Lighting apparatus" (registered trademark) series318, manufactured by Mitutoyo corporation), and the average value of the obtained measurement values was determined as the film thickness.
[ examples ]
Example 1
[ copolymer (a) ]
As the copolymer (a), the copolymer (a) was synthesized in the following manner. Vinylidene fluoride-hexafluoropropylene copolymer (a) was synthesized by suspension polymerization using vinylidene fluoride, hexafluoropropylene and monomethyl maleate as starting materials. It was confirmed by NMR measurement that the weight average molecular weight of the obtained copolymer (a) was 150 ten thousand and the molar ratio of vinylidene fluoride/hexafluoropropylene/monomethyl maleate was 98.0/1.5/0.5.
[ copolymer (b1) ]
As the copolymer (B), a copolymer (B1) was synthesized in the following manner. Vinylidene fluoride-hexafluoropropylene copolymer (b1) was synthesized by suspension polymerization using vinylidene fluoride and hexafluoropropylene as starting materials. It was confirmed by NMR measurement that the weight average molecular weight of the obtained copolymer (b1) was 30 ten thousand and the molar ratio of vinylidene fluoride/hexafluoropropylene was 93/7.
[ acrylic resin ]
Acrylonitrile and N-butyl acrylate were used as starting materials, an acrylic resin was synthesized by an emulsion polymerization method, and then water was replaced into N-methyl-2-pyrrolidone to obtain an acrylic resin solution having a solid content concentration of 5 mass%. The Tg of the acrylic resin thus obtained was-5 ℃ and the molar ratio of acrylonitrile units/acrylate units was 38/62, as determined by NMR measurement.
[ production of separator for Battery ]
7.1 parts by mass of the copolymer (a), 21.4 parts by mass of the copolymer (b1) and 359.3 parts by mass of NMP were mixed, and 70 parts by mass of alumina particles (average particle diameter 1.1 μm) were added while stirring with a Disperser (DISPER), followed by preliminary stirring with a Disperser (DISPER) at 2000rpm for 1 hour. Next, the mixture was treated 3 times with a DYNO-MILL (DYNO-MILL Multi Lab manufactured by SHINMAU ENTERPRISES) (1.46L vessel, filling rate 80%, diameter 0.5mm alumina beads) at a flow rate of 11kg/hr and a peripheral speed of 10m/s to obtain a dispersion. Mixing the acrylic resin solution in the dispersion, stirring the mixture for 30 minutes at 500rpm by using a Three-One Motor with a stirring blade, and filtering the mixture to obtain a solid content with the concentration of 20.5 mass percent and alumina particles: copolymer (a): copolymer (b 1): the weight ratio of the acrylic resin is 70:7.1:21.4: 1.5. The coating liquid was applied to both sides of a 7 μm thick microporous polyethylene membrane by dip coating, immersed in an aqueous solution, washed with purified water, and dried at 50 ℃ to obtain an 11 μm thick separator for a battery.
Example 2
Except that the alumina particles having a solid content concentration of 18.0 mass percent were used: copolymer (a): copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating solution having the weight ratio of acrylic resin of 70:14.3:14.2: 1.5.
Example 3
Except that the alumina particles having a solid content concentration of 15.5 mass% were used: copolymer (a): copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating solution having the weight ratio of acrylic resin of 70:21.4:7.1: 1.5.
Example 4
Except that the alumina particles having a solid content concentration of 20.5 mass% were used: copolymer (a): copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating solution having the weight ratio of acrylic resin of 70:6.8:20.2: 3.0.
Example 5
Except that the alumina particles having a solid content concentration of 18.0 mass percent were used: copolymer (a): copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating solution having the weight ratio of acrylic resin of 70:13.5:13.5: 3.0.
Example 6
Except that the alumina particles having a solid content concentration of 15.5 mass% were used: copolymer (a): copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating solution having the acrylic resin weight ratio of 70:20.2:6.8: 3.0.
Example 7
Except that the alumina particles having a solid content concentration of 20.5 mass% were used: copolymer (a): copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating solution having the weight ratio of acrylic resin of 70:6.4:19.1: 4.5.
Example 8
Except that the alumina particles having a solid content concentration of 18.0 mass percent were used: copolymer (a): copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating solution having the weight ratio of acrylic resin of 70:12.8:12.7: 4.5.
Example 9
Except that the alumina particles having a solid content concentration of 19.5 mass% were used: copolymer (a): copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating solution having the weight ratio of acrylic resin of 70:19.1:6.4: 4.5.
Example 10
Except that the alumina particles having a solid content concentration of 20.5 mass% were used: copolymer (a): copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating solution having the acrylic resin weight ratio of 70:6.0:18.0: 6.0.
Example 11
Except that the alumina particles having a solid content concentration of 18.0 mass percent were used: copolymer (a): copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating solution having the acrylic resin weight ratio of 70:12.0:12.0: 6.0.
Example 12
Except that the alumina particles having a solid content concentration of 15.5 mass% were used: copolymer (a): copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating solution having the acrylic resin weight ratio of 70:18.0:6.0: 6.0.
Example 13
Except that the alumina particles having a solid content concentration of 21.0 mass percent were used: copolymer (a): copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating solution having the weight ratio of the acrylic resin of 78.8:9.0:9.0: 3.2.
Example 14
Except that the alumina particles having a solid content concentration of 25.0 mass% were used: copolymer (a): copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating solution having the weight ratio of the acrylic resin of 85.2:6.3:6.3: 2.2.
Example 15
[ copolymer (b2) ]
As the copolymer (B), a copolymer (B2) was synthesized in the following manner. Vinylidene fluoride-tetrafluoroethylene copolymer (b2) was synthesized by suspension polymerization using vinylidene fluoride and tetrafluoroethylene as starting materials. It was confirmed by NMR measurement that the weight average molecular weight of the obtained vinylidene fluoride-tetrafluoroethylene copolymer (b2) was 28 ten thousand, and the molar ratio of vinylidene fluoride/tetrafluoroethylene was 90/10.
[ production of separator for Battery ]
Except that the copolymer (b2) was used in place of the copolymer (b1), alumina particles at a solid content concentration of 18.0 mass percent: copolymer (a): copolymer (b 2): a battery separator was obtained in the same manner as in example 1, except for preparing a coating solution under the conditions that the weight ratio of the acrylic resin was 70:13.5:13.5: 3.0.
Example 16
45 parts by mass of copolymer (a), 45 parts by mass of copolymer (b1), and 1329 parts by mass of NMP were mixed and dissolved. The acrylic resin solution was mixed with the liquid, and the mixture was stirred at 500rpm for 30 minutes by a Three-One Motor with a stirring blade, and then filtered to obtain a copolymer (a) having a solid content of 6.6 mass percent: copolymer (b 1): the weight ratio of the acrylic resin is 45.0:45.0: 10.0. The coating liquid was applied to both sides of a 7 μm thick microporous polyethylene membrane by dip coating, immersed in an aqueous solution, washed with purified water, and dried at 50 ℃ to obtain an 11 μm thick separator for a battery.
Comparative example 1
30.0 parts by mass of copolymer (b1) and 334.8 parts by mass of NMP were mixed, and 70 parts by mass of alumina particles (average particle diameter 1.1 μm) were added while stirring with a Disperser (DISPER), followed by preliminary stirring with a Disperser (DISPER) at 2000rpm for 1 hour. Next, the mixture was treated 3 times with a DYNO-MILL (DYNO-MILL Multi Lab manufactured by SHINMAU ENTERPRISES) (1.46L vessel, filling rate 80%, diameter 0.5mm alumina beads) at a flow rate of 11kg/hr and a peripheral speed of 10m/s to obtain a dispersion. Filtering the mixture to obtain alumina particles with the solid content concentration of 23.0 mass percent: a coating solution in which the weight ratio of the copolymer (b1) was 70: 30.0. The coating liquid was applied to both sides of a 7 μm thick microporous polyethylene membrane by dip coating, immersed in an aqueous solution, washed with purified water, and dried at 50 ℃ to obtain an 11 μm thick separator for a battery.
Comparative example 2
Preparation and filtration were carried out in the same manner as in comparative example 1 except that 7.5 parts by mass of the copolymer (a), 22.5 parts by mass of the copolymer (b1) and 387.8 parts by mass of NMP were mixed to obtain alumina particles at a solid content concentration of 20.5 mass%: copolymer (a): a coating liquid prepared under the condition that the weight ratio of the copolymer (b1) was 70:7.5: 22.5. After the coating liquid was coated in the same manner as in comparative example 1, a battery separator was obtained.
Comparative example 3
Except that alumina particles at a solid content concentration of 18.0 mass% were used: copolymer (a): a battery separator was obtained in the same manner as in comparative example 2, except for the coating liquid prepared under the condition that the weight ratio of the copolymer (b1) was 70:15.0: 15.0.
Comparative example 4
Except that alumina particles having a solid content concentration of 15.5 mass% were used: copolymer (a): a battery separator was obtained in the same manner as in comparative example 2, except for the coating liquid prepared under the conditions that the weight ratio of the copolymer (b1) was 70:22.5: 7.5.
Comparative example 5
In the same manner as in comparative example 1 except for mixing 30 parts by mass of the copolymer (a) and 669.2 parts by mass of NMP, after preparation and filtration, alumina particles were obtained at a solid content concentration of 13.0 mass%: a coating liquid prepared under the condition that the weight ratio of the copolymer (a) is 70: 30.0. After the coating liquid was coated in the same manner as in comparative example 1, a battery separator was obtained.
Comparative example 6
Except that alumina particles at a solid content concentration of 23.0 mass% were used: copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating liquid prepared under the condition that the weight ratio of the acrylic resin was 70:28.5: 1.5.
Comparative example 7
Except that alumina particles at a solid content concentration of 13.0 mass% were used: copolymer (a): a battery separator was obtained in the same manner as in example 1, except for the coating liquid prepared under the condition that the weight ratio of the acrylic resin was 70:28.5: 1.5.
Comparative example 8
Except that alumina particles at a solid content concentration of 23.0 mass% were used: copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating liquid prepared under the condition that the weight ratio of the acrylic resin was 70:27.0: 3.0.
Comparative example 9
Except that alumina particles at a solid content concentration of 13.0 mass% were used: copolymer (a): a battery separator was obtained in the same manner as in example 1, except for the coating liquid prepared under the condition that the weight ratio of the acrylic resin was 70:27.0: 3.0.
Comparative example 10
Except that alumina particles at a solid content concentration of 23.0 mass% were used: copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating liquid prepared under the conditions that the weight ratio of the acrylic resin was 70:25.5: 4.5.
Comparative example 11
Except that alumina particles at a solid content concentration of 23.0 mass% were used: copolymer (b 1): a battery separator was obtained in the same manner as in example 1, except for the coating liquid prepared in such a manner that the weight ratio of the acrylic resin was 70:24.0: 6.0.
Comparative example 12
[ copolymer (b3) ]
Vinylidene fluoride and tetrafluoroethylene were used as starting materials, and a vinylidene fluoride-tetrafluoroethylene copolymer was synthesized by a suspension polymerization method. It was confirmed by NMR measurement that the weight average molecular weight of the obtained vinylidene fluoride-tetrafluoroethylene copolymer was 95 ten thousand, and the molar ratio of vinylidene fluoride/tetrafluoroethylene was 95/5.
[ production of separator for Battery ]
Except that the copolymer (b3) was used in place of the copolymer (b1), alumina particles at a solid content concentration of 18.0 mass percent: copolymer (a): copolymer (b 3): a battery separator was obtained in the same manner as in example 1, except for preparing a coating solution under the conditions that the weight ratio of the acrylic resin was 70:13.5:13.5: 3.0.
The characteristics of the battery separators obtained in examples 1 to 16 and comparative examples 1 to 12 are shown in table 1.
[ Table 1]
Figure GDA0001381388150000361
Figure GDA0001381388150000371
Content of copolymer (A (%)*: represents the weight percentage of copolymer (A) relative to the total weight of copolymer (A) and polymer (B).
Content of acrylic resin (%)**: represents the weight percentage of the acrylic resin relative to the total weight of the copolymer (A), the polymer (B) and the acrylic resin.
In the figure:
1 negative electrode
2 separator for battery
3 laminated film
4 aluminium L-shaped angle iron
5 aluminum L-shaped angle iron for pressing head

Claims (13)

1. A battery separator comprising a microporous membrane and a porous layer provided on at least one surface of the microporous membrane, the porous layer contains a vinylidene fluoride-hexafluoropropylene copolymer (A), a vinylidene fluoride unit-containing polymer (B), an acrylic resin, and particles, the vinylidene fluoride-hexafluoropropylene copolymer (A) contains a hydrophilic group and 0.3 to 3 mol% of a hexafluoropropylene unit, the content of the hydrophilic group of the vinylidene fluoride-hexafluoropropylene copolymer (A) is 0.1 to 5 mol%, the vinylidene fluoride unit-containing polymer (B) has a melting point of 60 to 145 ℃ and a weight-average molecular weight of 10 to 75 ten thousand, the content of the particles is 50 weight percent or more and 90 weight percent or less based on the total weight of the porous layer.
2. The battery separator according to claim 1, wherein the weight average molecular weight of the vinylidene fluoride-hexafluoropropylene copolymer (A) is greater than 75 ten thousand and 200 ten thousand or less.
3. The battery separator according to claim 1 or 2, wherein the content of the vinylidene fluoride-hexafluoropropylene copolymer (a) is 15 weight percent or more and 85 weight percent or less based on the total weight of the vinylidene fluoride-hexafluoropropylene copolymer (a) and the vinylidene fluoride unit-containing polymer (B), and the content of the acrylic resin is 4 weight percent or more and 40 weight percent or less based on the total weight of the vinylidene fluoride-hexafluoropropylene copolymer (a), the vinylidene fluoride unit-containing polymer (B), and the acrylic resin.
4. The battery separator according to claim 1 or 2, wherein the acrylic resin is a copolymer of a (meth) acrylate and a monomer having a cyano group.
5. The battery separator according to claim 1 or 2, wherein the acrylic resin is a copolymer containing butyl acrylate.
6. The battery separator according to claim 1 or 2, wherein the acrylic resin is a copolymer of butyl acrylate and acrylonitrile.
7. The battery separator according to claim 5, wherein the content of butyl acrylate in the acrylic resin is 50 to 75 mol%.
8. The battery separator according to claim 6, wherein the content of butyl acrylate in the acrylic resin is 50 to 75 mol%.
9. The battery separator according to claim 1 or 2, wherein the bending strength in wet state is 4N or more, the bending strength in dry state is 5N or more, and the peeling force in dry state is 8N/m.
10. The separator for a battery according to claim 1, wherein the particles contain at least 1 selected from the group consisting of alumina, titania, boehmite, and barium sulfate.
11. The separator for a battery according to claim 1 or 2, wherein the thickness of one surface of the porous layer is 0.5 μm to 3 μm.
12. The battery separator according to claim 1 or 2, wherein the microporous film is a polyolefin microporous film.
13. A method for producing a battery separator according to any one of claims 1 to 12, comprising the steps of:
(1) a step of dissolving a vinylidene fluoride-hexafluoropropylene copolymer (a) and a vinylidene fluoride unit-containing polymer (B) in a solvent to obtain a fluorine-based resin solution in which particles are dispersed;
(2) a step of adding an acrylic resin solution obtained by dissolving an acrylic resin in a solvent to a fluorine resin solution, and mixing the acrylic resin solution and the fluorine resin solution to obtain a coating liquid;
(3) and a step of applying a coating liquid to the microporous membrane, immersing the microporous membrane in the coating liquid, washing the microporous membrane, and drying the microporous membrane, wherein the vinylidene fluoride-hexafluoropropylene copolymer (A) contains a hydrophilic group and 0.3 to 3 mol% of a hexafluoropropylene unit, the content of the hydrophilic group in the vinylidene fluoride-hexafluoropropylene copolymer (A) is 0.1 to 5 mol%, the melting point of the vinylidene fluoride unit-containing polymer (B) is 60 to 145 ℃, the weight average molecular weight is 10 to 75 ten thousand, the acrylic resin contains a butyl acrylate unit, and the content of the particles is 50 to 90 wt% of the total weight of the porous layer.
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JP2017183212A (en) 2017-10-05
KR102209887B1 (en) 2021-02-01

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