CN107925036B

CN107925036B - Battery separator

Info

Publication number: CN107925036B
Application number: CN201680045387.XA
Authority: CN
Inventors: 辻本润; 水野直树
Original assignee: Toray Industries Inc
Current assignee: Toray Industries Inc
Priority date: 2015-08-11
Filing date: 2016-08-09
Publication date: 2020-12-22
Anticipated expiration: 2036-08-09
Also published as: KR20180034393A; WO2017026485A1; JP6863283B2; JPWO2017026485A1; KR102190595B1; CN107925036A

Abstract

The present inventors have aimed to provide a battery separator which does not deteriorate the gas resistance and has excellent adhesion properties when dry and when wet, which are new problems caused by the increasing size of batteries that have been developed in the future. The battery separator has a microporous membrane and a porous layer provided on at least one surface of the microporous membrane, wherein the porous layer contains a vinylidene fluoride-hexafluoropropylene copolymer and an acrylic resin, the vinylidene fluoride-hexafluoropropylene copolymer contains a monomer unit having a hydrophilic group and contains a hexafluoropropylene monomer unit in an amount of 0.3 mol% or more and 3 mol% or less, and the acrylic resin contains a butyl acrylate monomer unit.

Description

Battery separator

Technical Field

The present invention relates to a battery separator.

Background

The battery separator is required to have mechanical strength, heat resistance, ion permeability, pore blocking property (shutdown property), melt rupture property (meltdown property), and the like. Therefore, studies have been made on the use of a battery separator provided with a porous film and a porous layer on the surface thereof. In addition, in recent years, there are the following problems: partial dissociation at the interface between the separator and the electrode (caused by irregularities on the electrode surface or expansion and contraction of the electrode during charge and discharge) leads to an increase in the internal resistance of the battery and a decrease in the battery cycle characteristics. Therefore, the separator is required to have adhesiveness to an electrode in a battery (that is, in the presence of a nonaqueous electrolyte) (hereinafter referred to as adhesiveness in the wet state), and in order to impart adhesiveness in the wet state, for example, a battery separator provided with a porous layer containing a fluororesin that swells in an electrolyte solution has been studied.

Patent document 1 describes an electrode body including a positive electrode, a negative electrode, a three-layer separator including polypropylene/polyethylene/polypropylene, and an adhesive resin layer including polyvinylidene fluoride and alumina powder provided between the electrodes and the separator.

In example 1 of patent document 2, there is described an organic separator with a porous membrane, which is obtained by stirring an NMP solution containing a first polymer (polyvinylidene fluoride homopolymer) and an NMP solution containing a second polymer (a polymer containing an acrylonitrile monomer, a 1, 3-butadiene-derived monomer, a methacrylic acid monomer, and a butyl (meth) acrylate monomer) with a primary mixer (primary mixer) to prepare an NMP solution of a binder, mixing and dispersing the prepared NMP solution with alumina particles, and applying the prepared slurry to a polypropylene separator.

In the examples of patent document 3, there is described an electrode body in which a positive electrode and a negative electrode are thermocompression bonded via an inorganic fine particle-containing sheet (insulating adhesive layer) obtained by: to an NMP solution in which spherical alumina powder was dispersed, an NMP solution in which a compounding material including a vinylidene fluoride-hexafluoropropylene copolymer (VdF-HFP copolymer) and polyethylmethacrylate was dissolved was added, and the mixture was mixed using a ball mill, and the prepared slurry was applied onto a base PET film and dried.

In example 1 of patent document 4, there is described a separator obtained by adding VdF-HFP copolymer and cyanoethylpullan to acetone, then adding barium titanate powder, dispersing the mixture by a ball mill, and applying the obtained slurry to a polyethylene porous film.

Documents of the prior art

Patent document

Patent document 1: japanese re-publication No. 1999-036981

Patent document 2: japanese patent laid-open publication No. 2013-206846

Patent document 3: japanese patent laid-open publication No. 2013-122009

Patent document 4: japanese Kohyo publication No. 2013-519206

Disclosure of Invention

Problems to be solved by the invention

In recent years, nonaqueous electrolyte secondary batteries, particularly lithium ion secondary batteries, are not limited to small electronic devices such as mobile phones and portable information terminals, but are expected to be developed for large-sized applications such as large-sized tablet personal computers, lawn mowers, electric bicycles, electric vehicles, hybrid vehicles, and small ships, and the increase in size of batteries has been considered in association with the development.

Examples of such batteries include: a cylindrical battery using an electrode body obtained by laminating a positive electrode and a negative electrode with a separator interposed therebetween, or an electrode body (wound electrode body) obtained by winding; a pouch battery obtained by pressure-molding the wound electrode body and covering the same with a laminate outer package; a square battery obtained by inserting the battery into a square outer packaging can; and so on.

When the adhesion between the active material surface of the electrode and the separator is insufficient in the manufacturing process of the electrode body due to the increase in size of the battery, the following problems are expected: the wound electrode assembly is bent or deformed due to the occurrence of the gap, and cannot be stored in a predetermined volume. This may cause a trouble in the transportation of the electrode body, or may make it difficult to fit the electrode body into the exterior body, which may significantly reduce productivity. Even after the electrolyte is injected, the gap is maintained, and the adhesion between the electrode and the separator becomes uneven, which causes a reduction in the cycle characteristics of the battery. This tendency is expected to be more pronounced as the battery becomes larger.

Therefore, in order to prevent bending or deformation of the electrode body and improve productivity or battery performance, adhesiveness (adhesiveness when dried) to the electrode when the electrolyte is not wet in the production process of the electrode body is increasingly required for the separator. If an excessive amount of adhesive component is added to ensure adhesiveness during drying or thermocompression bonding is performed under excessive conditions, the air permeability of the separator deteriorates. Moreover, the adhesive function for maintaining the adhesion between the electrodes when wet is also impaired. This makes it extremely difficult to achieve both wet adhesion and dry adhesion.

The present inventors have aimed to provide a battery separator which does not deteriorate the gas resistance and has excellent adhesion properties when dry and when wet, which are new problems caused by the increasing size of batteries that have been developed in the future. In the present specification, the term "adhesiveness when wet" refers to the adhesiveness between a separator and an electrode in a state where the separator contains an electrolyte, and is expressed by the bending strength when wet, which is obtained by the measurement method described later. The dry adhesion means adhesion between the separator and the electrode in a state where the separator does not substantially contain an electrolyte, and is expressed by dry bending strength obtained by a measurement method described later. The term "substantially not contained" means that the electrolyte solution in the separator is 500ppm or less.

Means for solving the 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. That is to say that the first and second electrodes,

(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 and an acrylic resin,

the vinylidene fluoride-hexafluoropropylene copolymer contains a monomer unit having a hydrophilic group and a hexafluoropropylene monomer unit in an amount of 0.3 mol% or more and 3 mol% or less,

the acrylic resin contains butyl acrylate monomer units.

(2) In the battery separator of the present invention, the porous layer preferably contains particles.

(3) In the battery separator of the present invention, the vinylidene fluoride-hexafluoropropylene copolymer preferably contains 0.1 mol% or more and 5 mol% or less of a monomer unit having a hydrophilic group.

(4) In the battery separator of the present invention, the content of the acrylic resin is preferably 5 mass% or more and less than 40 mass% with respect to the total amount of the vinylidene fluoride-hexafluoropropylene copolymer and the acrylic resin.

(5) In the battery separator of the present invention, the acrylic resin is preferably an acrylic copolymer containing a butyl acrylate unit and an acrylonitrile unit.

(6) In the battery separator of the present invention, the mass average molecular weight of the vinylidene fluoride-hexafluoropropylene copolymer is preferably 50 to 200 ten thousand.

(7) In the battery separator of the present invention, the content of the butyl acrylate unit in the acrylic resin is preferably 50 mol% or more and 75 mol% or less.

(8) The battery separator of the present invention preferably has a wet bending strength of 14N or more and a dry bending strength of 7N or more.

(9) In the battery separator of the present invention, the content of the particles is preferably 50 mass% or more and 85 mass% or less with respect to the total amount of the vinylidene fluoride-hexafluoropropylene copolymer, the acrylic resin, and the particles.

(10) In the battery separator of the present invention, the thickness of the microporous membrane per one-side porous layer is preferably 0.5 μm or more and 3 μm or less.

(11) 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.

(12) In the battery separator of the present invention, the average particle diameter of the particles is preferably 0.3 μm or more and 3.0 μm or less.

(13) In the battery separator of the present invention, the microporous membrane is preferably a polyolefin microporous membrane.

In order to solve the above problems, a method for manufacturing a battery separator according to the present invention has the following configuration.

That is to say that the first and second electrodes,

(14) a method for producing a battery separator, comprising the following steps (a) to (c) in this order.

A step (a) of dissolving a vinylidene fluoride-hexafluoropropylene copolymer in a solvent to obtain a fluororesin solution,

a step (b) of adding an acrylic resin solution to the fluororesin solution and mixing the resulting mixture to obtain a coating solution,

and (c) applying the coating liquid to the microporous membrane, immersing the microporous membrane in a coagulation bath, and then washing and drying the microporous membrane.

Effects of the invention

According to the present invention, it is possible to provide a battery separator, particularly a battery separator suitable for a large-sized wound battery, which achieves adhesion when dry and adhesion when wet without deteriorating gas resistance.

Drawings

FIG. 1 is a front cross-sectional view schematically showing a test of bending strength in wet condition.

FIG. 2 is a front cross-sectional view schematically showing a test of bending strength at the time of drying.

Detailed Description

The outline of the microporous membrane and the porous layer in the battery separator of the present invention will be described, but the present invention is not limited to these typical examples.

1. Microporous membrane

In the present invention, the microporous membrane refers to a membrane having interconnected voids therein. The microporous membrane is not particularly limited, and a microporous membrane containing a polyolefin resin 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 as a main component. The content of the polyethylene resin is preferably 70% by mass or more, more preferably 90% by mass or more, and still more preferably 100% by mass, with the total mass of the polyolefin resin being 100% by mass.

Examples of the polyolefin resin include homopolymers, two-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, japanese patent No. 3347835, international publication No. 2006/137540, and the like can be used. Specifically, the following steps (1) to (5) are preferably included, and the following steps (6) to (8) may be further included.

A step (1) for producing a polyolefin solution by melt-kneading the polyolefin resin and a film-forming solvent;

a step (2) of extruding and cooling the polyolefin solution to form a gel-like sheet;

a first stretching step of stretching the gel-like sheet;

a step (4) of removing the film-forming solvent from the stretched gel-like sheet;

a step (5) of drying the sheet from which the film-forming solvent has been removed;

a step (6) of stretching the dried sheet;

a step (7) of heat-treating the dried sheet;

and (8) subjecting the sheet after the stretching step to a crosslinking treatment and/or a hydrophilization treatment.

Hereinafter, each step will be described.

(1) Process for producing polyolefin solution

After adding a suitable film-forming solvent to the polyolefin resin, the mixture is melt-kneaded to prepare a polyolefin solution. As a melt kneading method, for example, a method using a twin-screw extruder as described in japanese patent No. 2132327 and japanese patent No. 3347835 can be used. Since the melt kneading method is known, the description thereof is omitted.

The mixing ratio of the polyolefin resin and the film-forming solvent in the polyolefin solution is not particularly limited, and the film-forming solvent is preferably 70 to 80 parts by mass relative to 20 to 30 parts by mass of the polyolefin resin. When the proportion of the polyolefin resin is within the above range, swelling 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) can be improved.

(2) Process for Forming gel-like sheet

The polyolefin solution was supplied from the extruder to a die and extruded into a sheet shape. It is also possible to feed a plurality of polyolefin solutions of the same or different compositions from an extruder to a die where they are layered and extruded into a sheet.

The extrusion method may be either a flat die method or an inflation method. The extrusion temperature is preferably 140-250 ℃, and the extrusion speed is preferably 0.2-15 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-like sheet. As a method for forming a gel-like 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 gelation temperature. Preferably to below 25 ℃. Upon cooling, the microphase of the polyolefin separated by the film-forming solvent can be immobilized. When the cooling rate is within the above range, the crystallinity is maintained in an appropriate range, and a gel-like sheet suitable for stretching is obtained. 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 the roller cooled by the refrigerant.

(3) First drawing step

Next, the obtained gel-like sheet is stretched at least in a uniaxial direction. Since the gel-like sheet contains a film-forming solvent, it can be uniformly stretched. After heating, the gel-like sheet is preferably stretched at a predetermined magnification by a tenter method, a roll method, an inflation 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 immediately before the next step, based on the microporous membrane immediately before this step.

The stretching temperature in this step is preferably within a range from the crystal dispersion temperature (Tcd) of the polyolefin resin to Tcd +30 ℃, more preferably within a range from the crystal dispersion temperature (Tcd) +5 ℃ to the crystal dispersion temperature (Tcd) +28 ℃, and particularly preferably within a range from Tcd +10 ℃ to Tcd +26 ℃. For example, in the case of 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 according to ASTM D4065.

The drawing causes cracks between the polyethylene sheet layers, and the polyethylene phase is refined to form a large amount of fibrils (fibrils). The fibrils form a three-dimensional irregularly linked network. The mechanical strength is improved and the pores are enlarged by drawing, but if drawing is performed under appropriate conditions, the through-hole diameter can be controlled, and high porosity can be obtained even when the film is made thinner.

Depending on the desired physical properties, the microporous membrane can be stretched by setting a temperature distribution in the thickness direction, thereby obtaining a microporous membrane having excellent mechanical strength. This method is described in detail in japanese patent No. 3347854.

(4) Removal of film-forming solvent

The solvent for film formation is removed (washed) using a washing solvent. Since the polyolefin phase and the film-forming solvent phase are phase-separated, when the film-forming solvent is removed, a porous film having three-dimensionally irregularly connected pores (voids) formed from fibrils which form a fine three-dimensional network structure can be obtained. Since a cleaning solvent and a method for removing a film-forming solvent using the cleaning solvent are known, the description thereof is omitted. For example, the methods disclosed in Japanese patent No. 2132327 and Japanese patent application 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 more than the crystal dispersion temperature (Tcd) of the polyolefin resin, and particularly preferably not less than 5 ℃ lower than Tcd. The microporous membrane is dried to 100 mass% (dry mass), preferably until the residual cleaning solvent is 5 mass% or less, more preferably until the residual cleaning solvent is 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 microporous membrane is subjected to the subsequent stretching step and heat treatment step.

(6) Second drawing step

The dried microporous membrane is preferably stretched at least in a uniaxial direction. The microporous membrane may be stretched by a tenter method or the like while heating. The stretching may be uniaxial stretching or biaxial stretching. In the case of biaxial stretching, simultaneous biaxial stretching and sequential stretching may be used. The stretching temperature in this step is not particularly limited, but is preferably 90 to 135 ℃ in general, and more preferably 95 to 130 ℃.

The stretching ratio (area stretching ratio) at which the microporous membrane is stretched in the uniaxial direction in this step is 1.0 to 2.0 times in the mechanical direction or the width direction in the case of uniaxial stretching. In the case of biaxial stretching, the lower limit of the area stretching magnification is preferably 1.0 times or more, more preferably 1.1 times or more, and further preferably 1.2 times or more. The upper limit value is preferably 3.5 times or less. The stretching ratios in the machine direction and the width direction may be the same or different from each other, and the stretching ratios in the machine direction and the width direction may be 1.0 to 2.0 times. The stretching ratio in this step is a stretching ratio of the microporous membrane immediately before the next step, based on the microporous membrane immediately before this step.

(7) Thermal treatment

Further, the dried microporous membrane may be subjected to heat treatment. By the heat treatment, the crystal is stabilized and the sheet layer is homogenized. As the heat treatment method, a heat setting treatment and/or a heat relaxation treatment may be used. The heat setting treatment is a heat treatment in which heating is performed while keeping the film size constant. The thermal relaxation treatment is a heat treatment for thermally shrinking the film in the machine direction or the width direction during heating. The heat-setting treatment is preferably performed by a tenter system or a roll system. For example, as a thermal relaxation treatment method, a method disclosed in japanese patent application laid-open No. 2002-256099 is given. The heat treatment temperature is preferably within the range of Tcd to Tm of the polyolefin resin, more preferably within the range of ± 5 ℃ of the stretching temperature of the microporous membrane, and particularly preferably within the range of ± 3 ℃ of the second stretching temperature of the microporous membrane.

(8) Crosslinking treatment and hydrophilization treatment

The microporous membrane after bonding or stretching may be further subjected to a crosslinking treatment and a hydrophilization treatment. For example, the microporous membrane is irradiated with ionizing radiation such as α rays, β rays, γ rays, and electron beams to perform crosslinking treatment. When the electron beam is irradiated, the amount of the electron beam is preferably 0.1 to 100Mrad, and the acceleration voltage is preferably 100 to 300 kV. The crosslinking treatment increases the melting temperature of the microporous membrane. In addition, hydrophilization treatment can be performed by monomer grafting, surfactant treatment, corona discharge, or the like. The monomer grafting is preferably carried out after the crosslinking treatment.

2. Porous layer

The porous layer of the battery separator of the present invention contains a vinylidene fluoride-hexafluoropropylene copolymer and an acrylic resin. This makes it possible to achieve both dry adhesion and wet adhesion.

[1] Vinylidene fluoride-hexafluoropropylene (VdF-HFP) copolymer

The vinylidene fluoride-hexafluoropropylene copolymer used in the present invention has high affinity with a nonaqueous electrolytic solution, and has high chemical stability and physical stability to the nonaqueous electrolytic solution. Therefore, the porous layer containing the copolymer exhibits adhesiveness in the wet state, and can sufficiently maintain affinity with the electrolyte even when used at high temperatures.

The vinylidene fluoride-hexafluoropropylene copolymer comprises a monomer unit having a hydrophilic group. This allows the electrode to interact with an active material present on the surface of the electrode and a binder component in the electrode, thereby enabling firm adhesion.

Examples of the hydrophilic group include a hydroxyl group, a carboxyl group, a carboxylate group, a sulfonic acid group, and salts thereof. Particularly preferred are carboxyl groups and carboxylate groups.

For introducing a hydrophilic group into the vinylidene fluoride-hexafluoropropylene copolymer, for example, the following methods can be mentioned: a method in which a monomer having a hydrophilic group such as maleic anhydride, maleic acid ester, and monomethyl maleate is introduced into the main chain by copolymerization in the synthesis of a vinylidene fluoride-hexafluoropropylene copolymer; a method of introducing a monomer as a side chain by grafting.

The lower limit of the content of the monomer unit having a hydrophilic group in the vinylidene fluoride-hexafluoropropylene copolymer is preferably 0.1 mol%, more preferably 0.3 mol%, and the upper limit is preferably 5 mol%, more preferably 4 mol%. When the content of the monomer unit having a hydrophilic group is within the above-mentioned preferable range, the hydrophilic group interacts with the hydrophilic group on the surface of the active material in the electrode or the hydrophilic site of the binder component in the electrode, and sufficient adhesiveness in wet can be obtained. When the content of the monomer unit having a hydrophilic group is 5 mol% or less, sufficient crystallinity of the polymer can be secured, and thus the swelling degree in the electrolyte can be suppressed to a low level, and high adhesiveness in wet state can be obtained. In addition, when the porous layer contains particles, the particles can be prevented from falling off by setting the content of the monomer unit having a hydrophilic group within the above-described preferable range. The content of the monomer unit having a hydrophilic group in the vinylidene fluoride-hexafluoropropylene copolymer can be measured by FT-IR, NMR, quantitative titration, or the like. For example, in the case of a carboxyl group, it can be determined from the absorption intensity ratio of C — H stretching vibration to C ═ O stretching vibration of the carboxyl group based on a homopolymer using FT-IR.

The lower limit of the content of the hexafluoropropylene monomer unit in the vinylidene fluoride-hexafluoropropylene copolymer is preferably 0.3 mol%, more preferably 0.5 mol%, and the upper limit is preferably 3 mol%, more preferably 2.5 mol%. If the content of the hexafluoropropylene monomer unit is less than 0.3 mol%, the crystallinity of the polymer is high, and the swelling degree with respect to the electrolyte is lowered, whereby sufficient adhesiveness in wet state cannot be obtained. If the hexafluoropropylene content is more than 3 mol%, the polymer excessively swells in the electrolyte solution, and the adhesiveness is lowered when the polymer is wet.

The vinylidene fluoride-hexafluoropropylene copolymer can be obtained by known polymerization methods. As a known polymerization method, for example, a method exemplified in Japanese patent laid-open No. 11-130821 can be cited. The method comprises the following steps: ion exchange water, monomethyl maleate, vinylidene fluoride, and hexafluoropropylene were put into an autoclave, suspension polymerization was performed, and then the polymer slurry was dehydrated, washed with water, and dried to obtain polymer powder. In this case, methylcellulose may be suitably used as the suspending agent, and diisopropyl peroxydicarbonate or the like may be suitably used as the radical initiator.

The vinylidene fluoride-hexafluoropropylene copolymer can be obtained by further polymerizing monomer units other than the monomer unit having the hydrophilic group within a range not impairing the characteristics. Examples of the monomer other than the monomer unit having a hydrophilic group include monomer units such as tetrafluoroethylene, trifluoroethylene, trichloroethylene, and fluorinated ethylene.

The lower limit of the weight average molecular weight of the vinylidene fluoride-hexafluoropropylene copolymer is preferably 50 ten thousand, more preferably 90 ten thousand, and the upper limit thereof is preferably 200 ten thousand, more preferably 150 ten thousand. When the weight average molecular weight of the vinylidene fluoride-hexafluoropropylene copolymer is within the above preferable range, the time for dissolving the copolymer in the solvent does not become extremely long, and the copolymer can be used without lowering the productivity. In addition, when swollen in the electrolyte, an appropriate gel strength can be maintained. The weight average molecular weight is a value obtained by gel permeation chromatography in terms of polystyrene.

[2] Acrylic resin

The acrylic resin is a copolymer containing butyl acrylate units. The porous layer containing an acrylic resin can exhibit adhesiveness when dried. When the porous layer contains particles, the flexibility of the coating film is improved by butyl acrylate, and an effect of suppressing particle shedding can be expected.

From the viewpoint of electrode adhesiveness, the acrylic resin is preferably a copolymer of butyl acrylate and acrylonitrile. By controlling the molar ratio of butyl acrylate to acrylonitrile, the degree of swelling with respect to the electrolyte can be adjusted, and the resin can be made to have appropriate flexibility. This also improves the adhesiveness when wet.

The lower limit of the content of the butyl acrylate unit in the acrylic resin is preferably 50 mol%, more preferably 55 mol%, and the upper limit is preferably 75 mol%, more preferably 70 mol%. When the lower limit of the content of the butyl acrylate unit in the acrylic resin is within the above preferable range, the porous layer can be provided with appropriate flexibility, and the peeling of the coating film can be suppressed. When the content of the butyl acrylate unit in the acrylic resin is within the above-described preferable range, a good balance between the adhesiveness at the time of drying and the adhesiveness at the time of wetting can be easily obtained.

The acrylic resin can be obtained by a known polymerization method such as the method exemplified in Japanese patent laid-open publication No. 2013-206846. Examples of the method include the following: an acrylic resin was obtained by charging ion-exchanged water, N-butyl acrylate and acrylonitrile into an autoclave equipped with a stirrer, carrying out emulsion polymerization to obtain an aqueous polymer particle dispersion, and replacing the water in the system with N-methyl-2-pyrrolidone. In the polymerization, potassium persulfate may be suitably used as a radical polymerization initiator, and tert-dodecyl mercaptan may be suitably used as a molecular weight modifier.

The lower limit of the content of the acrylic resin is preferably 5% by mass, and the upper limit is preferably 40% by mass, and more preferably 20% by mass, relative to the total amount of the vinylidene fluoride-hexafluoropropylene copolymer and the acrylic resin. In particular, the upper limit value is more preferably less than 10% by mass. When the content is within the above preferable range, sufficient dry adhesiveness and wet adhesiveness can be obtained. By setting the content of the acrylic resin to 5% by mass or more, both the wet adhesiveness and the dry adhesiveness can be more sufficiently achieved. By setting the content of the acrylic resin to 40 mass% or less, the effect of adhesiveness in the wet state by the vinylidene fluoride-hexafluoropropylene copolymer can be easily obtained.

[3] Particles

The porous layer of the battery separator of 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. The particles may be inorganic particles or 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, and boehmite. In particular, from the viewpoint of crystal growth, cost, and availability of the vinylidene fluoride-hexafluoropropylene copolymer, titanium dioxide, alumina, and boehmite are preferable.

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 85 mass%, more preferably 80 mass%, and still more preferably 75 mass%, and the lower limit is preferably 50 mass%, more preferably 60 mass%, and still more preferably 65 mass% with respect to the total amount of the vinylidene fluoride-hexafluoropropylene copolymer, the acrylic resin, and the particles. When the content of the particles is within the above preferable range, a good balance of the air resistance can be easily obtained.

From the viewpoint of suppressing 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 pore diameter of the microporous membrane. The average flow pore diameter is measured in accordance with JIS K3832 and ASTMF316-86, for example, by using Perm.Porometer (manufactured by PMI Co., Ltd., CFP-1500A) in the order of Dry (Dry-up) and Wet (Wet-up). In the wet process, a microporous membrane sufficiently impregnated with Galwick (trade name) manufactured by PMI corporation whose surface tension is known is pressurized, and the pore diameter obtained by converting the pressure at which air starts to penetrate is defined as the maximum pore diameter. The average flow pore diameter was converted from the pressure at the intersection of the curve representing the slope of 1/2 in the pressure/flow rate curve in the dry measurement and the curve in the wet measurement. The following equation is used for conversion of 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.

From the viewpoint of the sliding property with the winding core and the particle shedding when the battery (cell) is wound, the average particle diameter of the particles is preferably 0.3 μm or more and 1.8 μm or less, more preferably 0.5 μm or more and 1.5 μm or less, and still more preferably 1.0 μm or more and 3.0 μm or less. The average particle diameter of the particles can be measured using a measuring device of a laser diffraction method or a dynamic light scattering method. For example, particles dispersed in an aqueous solution to which a surfactant is added are measured by a particle size distribution measuring apparatus (available from Nikkiso K.K., microtrac HRA) 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 the average particle diameter. The particle shape includes, but is not particularly limited to, a spherical shape, a substantially spherical shape, a plate shape, and a needle shape.

[4] Physical Properties of porous layer

The thickness of the microporous membrane per one-side porous layer is preferably 0.5 μm or more and 3 μm or less, more preferably 1 μm or less and 2.5 μm or more, and still more preferably 1 μm or more and 2 μm or less. When the film thickness is 0.5 μm or more per one surface, adhesiveness in wet and adhesiveness in dry can be secured. If the film thickness is 3 μm or less per one surface, the winding volume can be suppressed, and the method 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% or more and 90% or less, and more preferably 40% or more and 70% or less. When the porosity of the porous layer is within the above preferable range, the increase in the membrane resistance can be prevented, a large current can be applied, and the membrane strength can be maintained.

[5] Method for manufacturing battery separator

A method for producing a battery separator according to one embodiment of the present invention includes the following steps (a) to (c) in this order.

A step (a) in which a vinylidene fluoride-hexafluoropropylene copolymer is dissolved in a solvent to obtain a fluororesin solution;

a step (b) of adding an acrylic resin solution to the fluororesin solution and mixing the resulting mixture to obtain a coating liquid;

(a) Step of obtaining fluororesin solution

The solvent is not particularly limited as long as it can dissolve the vinylidene fluoride-hexafluoropropylene copolymer, dissolve or disperse the acrylic resin, and mix with the coagulation liquid. From the viewpoint of solubility and low volatility, the solvent is preferably N-methyl-2-pyrrolidone.

In the case of providing a porous layer containing particles, it is important to prepare a fluororesin solution (also referred to as a dispersion) in which the particles are dispersed in advance. The vinylidene fluoride-hexafluoropropylene copolymer is dissolved in a solvent, particles are added thereto while stirring, and the mixture is stirred for a certain time (for example, about 1 hour) by a dispersing machine or the like to perform preliminary dispersion. Further, a fluororesin solution with less particle aggregation can be obtained through a step of dispersing particles by a sand mill or a paint shaker (dispersing step).

(b) Step of obtaining coating liquid

This step is a step of adding an acrylic resin solution to a fluororesin solution and mixing them with, for example, a three-one motor with a stirring paddle to prepare a coating liquid.

The acrylic resin solution used in this step is a solution obtained by dissolving or dispersing an acrylic resin in a solvent. The solvent used here is preferably the same solvent as in the step (a). From the viewpoint of solubility and low volatility, N-methyl-2-pyrrolidone is particularly preferable. From the viewpoint of workability, the acrylic resin solution is preferably obtained in the following manner: after polymerization of the acrylic resin, the solvent is replaced by adding N-methyl-2-pyrrolidone, followed by distillation or the like.

In the case of providing a porous layer containing particles, it is important to add (post-add) an acrylic resin solution after dispersing the particles in a fluororesin solution. That is, it is important that the acrylic resin is not added in the dispersing step. When the vinylidene fluoride-hexafluoropropylene copolymer, the acrylic resin, and the particles are simultaneously added to the solvent, it is presumed that: the hydrophilic group contained in the vinylidene fluoride-hexafluoropropylene copolymer and butyl acrylate contained in the acrylic resin start to gel gradually due to heat and shear during dispersion, and thus the coating liquid is not suitable for industrial use. Further, due to the effect of the increase in viscosity, it is difficult to perform thin film coating in which the thickness of the porous layer is 3 μm or less. In the steps (a) and (b) of the production method of the present invention, gelation of the coating liquid is suppressed, film coating can be performed, and the storage stability of the coating liquid is improved.

(c) A step of applying the coating liquid to a microporous membrane, immersing the microporous membrane in a coagulation bath, and then washing and drying the microporous membrane

The process comprises the following steps: the coating liquid is applied to a microporous membrane, the coated microporous membrane is immersed in a coagulating liquid to separate a vinylidene fluoride-hexafluoropropylene copolymer and an acrylic resin phase, the microporous membrane has a three-dimensional network structure, and the microporous membrane is coagulated in this state, washed and dried. Thus, a battery separator having a microporous membrane and a porous layer on the surface of the microporous membrane was obtained.

The method for applying the coating liquid to the microporous membrane may be a known method, and examples thereof include a dip coating method, a reverse roll coating method, a gravure coating method, a kiss coating method, a roll brush method, a spray coating method, an air knife coating method, a meyer bar coating method, a tube blade coating method, a blade coating method, and a die coating method, and these methods may be used alone or in combination.

The coagulating liquid is preferably water, and is preferably an aqueous solution containing a good solvent (which is a good solvent for the vinylidene fluoride-hexafluoropropylene copolymer and the acrylic resin) in an amount of 1 mass% to 20 mass%, and more preferably an aqueous solution containing the good solvent in an amount of 5 mass% to 15 mass%. Examples of the good solvent include N-methyl-2-pyrrolidone, N-dimethylformamide, and N, N-dimethylacetamide. The immersion time in the coagulation bath is preferably 3 seconds or more. The upper limit is not limited, and is sufficient when the time is 10 seconds.

Water may be used for the cleaning. For drying, hot air at 100 ℃ or lower, for example, can be used.

The battery separator of the present invention can be used as a battery separator for a secondary battery such as a nickel-hydrogen battery, a nickel-cadmium battery, a nickel-zinc battery, a silver-zinc battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium-sulfur battery. Particularly, it is preferably used as a separator of a lithium ion secondary battery.

[6] Physical Properties of Battery separator

The adhesiveness of the battery separator when wet can be evaluated by the bending strength when wet, and the bending strength when wet is 14N or more. The upper limit of the wet bending strength is not particularly limited, but is sufficient if it is 30N. When the bending strength in wet state is within the above preferable range, the partial separation at 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 adhesiveness of the battery separator during drying can be evaluated by the bending strength during drying, and the lower limit of the bending strength during drying is preferably 7N or more, and more preferably 9N or more. The upper limit of the bending strength at the time of drying is not particularly limited, and is sufficient when the bending strength is 30N. When the bending strength during drying is within the above-described preferable range, the deflection and deformation of the wound electrode body can be easily suppressed.

From the viewpoint of the balance between the adhesiveness when dry and the adhesiveness when wet, the battery separator preferably has a wet flexural strength of 14N or more and a dry flexural strength of 7N or more.

The following examples are given for illustrative purposes, but the present invention is not limited to these examples. The measured values in the examples are values measured by the following methods.

1. Bending strength in wet state

In general, when a binder of a fluororesin is used for the positive electrode and a porous layer containing a fluororesin is provided on the separator, adhesiveness is easily ensured by mutual diffusion of the fluororesins. On the other hand, since a binder other than the fluororesin is used for the negative electrode, diffusion of the fluororesin is less likely to occur, 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 bending strength as an index as described below.

(1) Production of negative electrode

An aqueous solution containing 1.5 parts by mass of carboxymethyl cellulose was added to 96.5 parts by mass of artificial graphite, followed by mixing, and 2 parts by mass of styrene butadiene latex as a solid component was further added and mixed to prepare a slurry containing a negative electrode mixture. The slurry containing the negative electrode mixture was uniformly coated on both sides of a negative electrode current collector formed 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 produced in the above manner (161 mm in the machine direction × 30mm in the width direction) was stacked on the separator produced in examples and comparative examples (160 mm in the machine direction × 34mm in the width direction), and a metal sheet (300 mm in length, 25mm in width, and 1mm in thickness) was wound around a core so that the separator was located inside, and the separator and the negative electrode were wound, and the metal sheet was pulled out to obtain a wound body for testing. The test roll had a length of about 34mm and a width of about 28 mm.

(3) Method for measuring bending strength in wet state

A test roll was placed on a laminated film (length: 110mm, width: 65mm, thickness: 0.12mm) formed of aluminum and polypropylene, the laminated film was folded in half in the longitudinal direction, and both sides of the laminated film were welded to make a bag shape with one side open. In a solvent obtained by mixing ethylene carbonate and ethyl methyl carbonate at a volume ratio of 3: 7, 1mol/L was dissolvedLiPF₆Thus, an electrolyte solution was obtained, and 500 μ L of the electrolyte solution was injected through the opening in a glove box to impregnate the wound body for a test, and then one side of the opening was sealed with a vacuum sealer.

Subsequently, the test roll sealed in the laminated film was sandwiched between 2 spacers (thickness: 1mm, 5 cm. times.5 cm), pressed at 98 ℃ and 0.6MPa for 2 minutes using a precision heating and pressing apparatus (CYPT-10, manufactured by Xindong industries, Ltd.), and allowed to stand at room temperature for cooling. The test wound body sealed in the laminated film after pressurization was measured for the bending strength when wet as shown in the schematic diagram of fig. 1 by using a universal testing machine (AGS-J, manufactured by shimadzu corporation). This will be described in detail below.

2 aluminum L-shaped angle members (angle)4 (thickness 1mm, 10mm × 10mm, length 5cm) were arranged in parallel with the end portions aligned so that the 90 ° portions faced upward, and were fixed with the 90 ° portions as fulcrums and with the distance between the fulcrums being 15 mm. The middle point of the side (about 28mm) in the width direction of the test roll was overlapped with 7.5mm, which is the middle of the distance between the supporting points of 2 aluminum L-shaped angle members, and the test roll was disposed so as not to be exposed from the side in the longitudinal direction of the L-shaped angle member.

Next, the aluminum L-shaped angle member 3 was fixed to a load cell of a universal testing machine (the capacity of the load cell was 50N) such that the 90 ° portion of the aluminum L-shaped angle member 3 was overlapped with the midpoint of the side in the width direction of the test winding body and the 90 ° portion was directed downward, and the side in the length direction of the test winding body (approximately 34mm) was disposed in parallel so as not to be exposed from the side in the length direction of the aluminum L-shaped angle member 3 (thickness 1mm, 10mm × 10mm, length 4cm) as an indenter. The average value of 3 test coils was defined as the wet bending strength of the measured value at a stroke (stroke) of 0.5mm when the test load became 0.05N at a load rate of 0.5 mm/min.

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 same test wound body as that of the above 1. bending strength under wet condition was used.

(3) Method for measuring bending strength during drying

The prepared test roll was held between 2 spacers (thickness: 1mm, 5 cm. times.5 cm), pressed at 90 ℃ and 0.6MPa for 2 minutes using a precision heat and pressure apparatus (CYPT-10, manufactured by Xindong industries, Ltd.), and allowed to stand at room temperature for cooling. The test wound bodies after pressurization were arranged in the same manner as the method for measuring wet flexural strength described above 1, and 3 test wound bodies were measured under the following conditions using a universal tester (AGS-J, manufactured by shimadzu corporation), and the average value of the maximum test force was defined as dry flexural strength, as shown in fig. 2.

Distance between fulcrums: 15mm

Battery capacity: 50N

Load speed: 0.5mm/min

3. Evaluation of powder falling

The diaphragm was fixed to the bottom surface (bottom area 5.5cm × 6cm) of the handled spindle (1143g) flat without wrinkles, with the porous layer as a surface. The amount of transfer of the porous layer to the drawing paper was confirmed after moving the weight on the drawing paper (manufactured by Kao paper Co., Ltd., C-55, black) by a distance of 20cm 10 times in a reciprocating manner. The number of coating film flakes having a thickness of 150 μm or more was measured by an optical microscope with the range of 5mm × 5mm selected arbitrarily at 10 spots, and the powder drop was evaluated as follows based on the number of flakes.

Good: the total number of coating film-detached matters in 10 is 50 or less

Poor: the total number of the coating film-detached matters in 10 is 51 or more

4. Film thickness

A contact type film thickness meter ("Litimatic" (registered trademark) series318, Mitsutoyo, K.K.) was used, and a super hard sphere gauge was used

The film thickness was determined as the average of the measured values obtained by measuring 20 points under a load of 0.01N。

Examples

Example 1

[ vinylidene fluoride-hexafluoropropylene (VdF-HFP) copolymer ]

Vinylidene fluoride-hexafluoropropylene copolymer (a) was synthesized by suspension polymerization using vinylidene fluoride, hexafluoropropylene and monomethyl maleate as starting materials. The weight average molecular weight of the resulting vinylidene fluoride-hexafluoropropylene copolymer was 150 ten thousand, and the molar ratio of vinylidene fluoride monomer units/hexafluoropropylene monomer units/monomethyl maleate monomer units was 98.5/1.0/0.5, as determined by NMR measurement.

[ acrylic resin ]

Acrylonitrile and N-butyl acrylate were used as starting materials, a butyl acrylate-acrylonitrile copolymer was synthesized as an acrylic resin by an emulsion polymerization method, and then water was replaced with N-methyl-2-pyrrolidone (NMP) to obtain an acrylic resin solution having a solid content concentration of 5 mass%. The acrylic resin thus obtained was confirmed to have a Tg of-5 ℃ and a molar ratio of acrylonitrile monomer units/n-butyl acrylate monomer units of 38/62 by NMR measurement.

[ production of separator for Battery ]

28.5 parts by mass of vinylidene fluoride-hexafluoropropylene copolymer (a) and 641 parts by mass of NMP were mixed, and then 70 parts by mass of alumina particles (average particle diameter 1.1 μm) as inorganic particles were added while stirring with a dispersing machine, and preliminary stirring was carried out with a dispersing machine at 2000rpm for 1 hour. Next, Dyno-Mill (Dyno-Mill MultiLab manufactured by SHINMAU ENTERPRISES CORPORATION) (1.46L container, filling rate 80%,

alumina beads)), were treated 3 times at a flow rate of 11kg/h and a peripheral speed of 10m/s to obtain a dispersion. The acrylic resin solution was mixed with the dispersion, and the mixture was stirred at 500rpm for 30 minutes using a Three-One Motor with a stirring paddle, and then filtered to obtain a coating liquid having a solid content concentration of 13 mass% and a mass ratio of alumina particles to the copolymer (a) to the acrylic resin of 70: 28.5: 1.5. Coating a coating solution on both sides of a 7 μm thick microporous polyethylene membrane by dip coating, and drying the coating solutionThe membrane was immersed in an aqueous solution, washed with pure water, and then dried at 50 ℃ to obtain a battery separator having a thickness of 11 μm.

Example 2

A battery separator was obtained in the same manner as in example 1 except that a coating liquid prepared so that the solid content concentration was 13 mass% and the mass ratio of the alumina particles, the vinylidene fluoride-hexafluoropropylene copolymer (a) and the acrylic resin was 70: 27.2: 2.8 was used. The obtained battery separator was evaluated for powder falling, and the results were good.

Example 3

A battery separator was obtained in the same manner as in example 1 except that a coating liquid prepared so that the solid content concentration was 13 mass% and the mass ratio of the alumina particles, the vinylidene fluoride-hexafluoropropylene copolymer (a) and the acrylic resin was 70: 25.2: 4.8 was used.

Example 4

A battery separator was obtained in the same manner as in example 1 except that a coating liquid prepared so that the solid content concentration was 13 mass% and the mass ratio of the alumina particles, the vinylidene fluoride-hexafluoropropylene copolymer (a) and the acrylic resin was 70: 22.5: 7.5 was used.

Example 5

A battery separator was obtained in the same manner as in example 1 except that a coating liquid prepared so that the solid content concentration was 13 mass% and the mass ratio of the alumina particles, the vinylidene fluoride-hexafluoropropylene copolymer (a), and the acrylic resin was 70: 18: 12 was used.

Example 6

A battery separator was obtained in the same manner as in example 1 except that a coating liquid prepared so that the solid content concentration was 12 mass% and the mass ratio of the alumina particles, the vinylidene fluoride-hexafluoropropylene copolymer (a) and the acrylic resin was 65: 31.7: 3.3 was used.

Example 7

A battery separator was obtained in the same manner as in example 1 except that a coating liquid prepared so that the solid content concentration was 18 mass% and the mass ratio of the alumina particles, the vinylidene fluoride-hexafluoropropylene copolymer (a) and the acrylic resin was 85: 12.4: 2.6 was used.

Example 8

A battery separator was obtained in the same manner as in example 2, except that a coating solution prepared using boehmite (average particle size of 2.3 μm) as inorganic particles was used.

Example 9

A battery separator was obtained in the same manner as in example 2, except that a coating solution prepared using titanium dioxide (average particle size of 1 μm) as inorganic particles was used.

Example 10

A battery separator was obtained in the same manner as in example 2, except that the thickness of the battery separator was changed to 10 μm.

Comparative example 1

Alumina particles were used in a solid content concentration of 13 mass%: a battery separator was obtained in the same manner as in example 1 except that the coating solution was prepared so that the mass ratio of the vinylidene fluoride-hexafluoropropylene copolymer (a) was 70: 30. The obtained battery separator was evaluated for powder falling, and found to be defective.

Comparative example 2

A battery separator was obtained in the same manner as in example 2 except that a coating solution was prepared using a PVdF homopolymer (KF #7300 (molecular weight: 100 ten thousand or more), manufactured by Kureha, co., ltd.) instead of the vinylidene fluoride-hexafluoropropylene copolymer, and the obtained coating solution was used.

Comparative example 3

A battery separator was obtained in the same manner as in example 2 except that a coating solution was prepared using a vinylidene fluoride-hexafluoropropylene copolymer (b) having a hexafluoropropylene monomer content of 4.5 mol% (manufactured by Arkema co., ltd., kynar2801(VdF/HFP molar ratio 95.5/4.5, molecular weight less than 50 ten thousand)) instead of the vinylidene fluoride-hexafluoropropylene copolymer (a), and the obtained coating solution was used.

Comparative example 4

[ Synthesis of acrylic resin ]

Acrylonitrile and ethyl acrylate were used as starting materials, an ethyl acrylate-acrylonitrile copolymer was synthesized as an acrylic resin by an emulsion polymerization method, and then water was replaced with N-methyl-2-pyrrolidone to obtain an acrylic resin solution having a solid content concentration of 5 mass%. The acrylic resin obtained was confirmed to have a Tg of 10 ℃ and a molar ratio of acrylonitrile monomer units/ethyl acrylate monomer units of 37/63 by NMR measurement. A battery separator was obtained in the same manner as in example 2, except that a coating solution was prepared using the acrylic resin and the obtained coating solution was used. The obtained battery separator was evaluated for powder falling, and the results were good.

Comparative example 5

A battery separator was obtained in the same manner as in example 2 except that a coating solution was prepared using a solution of CRV (cyanoethyl PVA, manufactured by shin-Etsu chemical Co., Ltd.) having a solid content concentration of 5 mass% and N-methyl-2-pyrrolidone instead of the acrylic resin solution of example 2, and the obtained coating solution was used.

Comparative example 6

A battery separator was obtained in the same manner as in example 2 except that a coating liquid was prepared so that the solid content concentration was 25 mass% and the mass ratio of the alumina particles, the vinylidene fluoride-hexafluoropropylene copolymer (a), and the acrylic resin was 90: 9.1: 0.9.

Comparative example 7

Inorganic particles, vinylidene fluoride-hexafluoropropylene copolymer (a), acrylic resin, and N-methyl-2-pyrrolidone were mixed together at the same time so that the solid content concentration became 13 mass% and the mass ratio of alumina particles, vinylidene fluoride-hexafluoropropylene copolymer (a) and acrylic resin became 70: 27.2: 2.8, and dispersion was carried out to prepare a coating liquid, but the viscosity of the coating liquid increased and the coating liquid could not be applied to a microporous polyethylene membrane.

Comparative example 8

A battery separator was obtained in the same manner as in example 2, except that the thickness of the battery separator was set to 9 μm.

Comparative example 9

A battery separator was obtained in the same manner as in example 1 except that a coating liquid was prepared using N-methyl-2-pyrrolidone so that the solid content concentration of the vinylidene fluoride-hexafluoropropylene copolymer was 5 mass%, and the obtained coating liquid was used to set the thickness of the battery separator to 9.5 μm.

The properties of the battery separators obtained in examples 1 to 10 and comparative examples 1 to 9 are shown in table 1.

The content (% by mass) of the acrylic resin represents a mass% of the acrylic resin with respect to the total mass of the fluororesin and the acrylic resin. "post-addition" in the preparation of the coating material means that an acrylic resin solution is added to a fluororesin solution in which particles are dispersed. "simultaneously adding" means that the fluorine resin solution, the acrylic resin solution and the particles are simultaneously added to perform the dispersion treatment.

Description of the reference numerals

1: negative electrode

2: diaphragm

3: aluminium L-shaped angle section bar for indenter

4: aluminum L-shaped angle section bar

5: laminated film

Claims

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 particles, a vinylidene fluoride-hexafluoropropylene copolymer, and an acrylic resin,

the vinylidene fluoride-hexafluoropropylene copolymer contains 0.1 mol% or more and 5 mol% or less of monomer units having a hydrophilic group, and 0.3 mol% or more and 3 mol% or less of hexafluoropropylene monomer units,

the acrylic resin comprises butyl acrylate monomer units.

2. The battery separator according to claim 1, wherein the content of the acrylic resin is 5% by mass or more and less than 40% by mass relative to the total amount of the vinylidene fluoride-hexafluoropropylene copolymer and the acrylic resin.

3. The battery separator according to claim 1 or 2, wherein the acrylic resin is an acrylic copolymer comprising butyl acrylate units and acrylonitrile units.

4. The battery separator according to claim 1 or 2, wherein the weight average molecular weight of the vinylidene fluoride-hexafluoropropylene copolymer is 50 ten thousand or more and 200 ten thousand or less.

5. The battery separator according to claim 1 or 2, wherein the content of the butyl acrylate unit in the acrylic resin is 50 mol% or more and 75 mol% or less.

6. The battery separator according to claim 1 or 2, wherein the bending strength in wet state is 14N or more and the bending strength in dry state is 7N or more.

7. The battery separator according to claim 1 or 2, wherein a content of the particles is 50 mass% or more and 85 mass% or less with respect to a total weight of the vinylidene fluoride-hexafluoropropylene copolymer, the acrylic resin, and the particles.

8. The battery separator according to claim 1 or 2, wherein the thickness of the porous layer on each side of the microporous membrane is 0.5 μm or more and 3 μm or less.

9. The battery separator according to claim 1 or 2, wherein the particles comprise at least one selected from the group consisting of alumina, titania, and boehmite.

10. The battery separator according to claim 1 or 2, wherein the average particle diameter of the particles is 0.3 μm or more and 3.0 μm or less.

11. The battery separator according to claim 1 or 2, wherein the microporous film is a polyolefin microporous film.

12. The method for producing a battery separator according to claim 1 or 2, which comprises the following steps (a) to (c) in this order:

a step (a) in which a vinylidene fluoride-hexafluoropropylene copolymer containing 0.1 to 5 mol% of monomer units having a hydrophilic group is dissolved in a solvent to obtain a fluororesin solution in which particles are dispersed;