JP6863283B2 - Battery separator - Google Patents

Description

The present invention relates to a battery separator.

Battery separators are required to have mechanical strength, heat resistance, ion permeability, pore closing characteristics (shutdown characteristics), melt rupture characteristics (meltdown characteristics), and the like. Therefore, the use of a porous membrane and a battery separator provided with a porous layer on the surface thereof has been studied so far. Furthermore, in recent years, the unevenness of the electrode surface and the partial release of the interface between the separator and the electrode due to the expansion and contraction of the electrode due to charge and discharge lead to an increase in the internal resistance of the battery and a decrease in the cycle characteristics of the battery, which has become a problem. ing. Therefore, the separator is required to have adhesiveness to the electrode in the battery (that is, in the presence of a non-aqueous electrolyte) (hereinafter referred to as wet adhesiveness), and in order to impart wet adhesiveness, for example, A battery separator provided with a porous layer containing a fluororesin that swells in an electrolytic solution has been studied.

Patent Document 1 describes an electrode provided with a three-layer separator made of a positive electrode, a negative electrode, polypropylene, polyethylene, and polypropylene, and an adhesive resin layer made of polyvinylidene fluoride and alumina powder arranged between these electrodes and the separator. The body is listed.

In Example 1 of Patent Document 2, an NMP solution containing the first polymer (polyvinylidene fluoride homopolymer) and a second polymer (acrylonitrile monomer and a monomer derived from 1,3-butadiene) are described. And an NMP solution containing (a polymer containing a methacrylic acid monomer and a (meth) butyl acrylate monomer) are stirred with a primary mixer to prepare an NMP solution of a binder, and then the prepared NMP solution. And an organic separator with a porous film obtained by applying a slurry prepared by mixing and dispersing alumina particles to a polypropylene separator is described.

In the examples of Patent Document 3, a compounding material composed of a vinylidene fluoride-hexafluoropropylene copolymer (VdF-HFP copolymer) and ethyl polymethacrylate was dissolved in an NMP solution in which spherical alumina powder was dispersed. The slurry prepared by adding the NMP solution and mixing with a ball mill was applied to the base material PET film, and the positive electrode and the negative electrode were thermally pressure-bonded via the inorganic fine particle-containing sheet (insulating adhesive layer) obtained by drying. The electrode body is described.

In Example 1 of Patent Document 4, a VdF-HFP copolymer and cyanoethyl pullulan were added to acetone, then barium titanate powder was added, and the slurry obtained by dispersing with a ball mill was applied to a polyethylene porous membrane. The separator obtained in the above is described.

Re-table 1999-036981 Japanese Unexamined Patent Publication No. 2013-206846 Japanese Unexamined Patent Publication No. 2013-122009 Japanese Patent Application Laid-Open No. 2013-519206

In recent years, non-aqueous electrolyte secondary batteries, especially lithium ion secondary batteries, are not limited to small electronic devices such as mobile phones and mobile information terminals, but also large tablets, mowers, electric motorcycles, electric vehicles, hybrid vehicles, small ships, etc. It is expected to be developed for large-scale applications, and it is expected that the size of batteries will increase accordingly.

These batteries include an electrode body in which a positive electrode and a negative electrode are laminated via a separator, a cylindrical battery using a wound electrode body (wound electrode body), and a wound electrode body that is press-molded and laminated. Examples thereof include a pouch battery covered with an outer body, a square battery inserted in a square outer can, and the like.

Due to the increase in size of the battery, if the active material surface of the electrode and the separator are not sufficiently adhered in the manufacturing process of the electrode body, a gap is generated and the wound electrode body is bent or distorted and does not fit in the predetermined volume. Such a problem is assumed. This may hinder the transport of the electrode body or make it difficult to insert the electrode body into the exterior body, resulting in a significant decrease in productivity. Further, even after the electrolytic solution is injected, the gap is maintained and the adhesion between the electrode and the separator becomes non-uniform, resulting in a decrease in the cycle characteristics of the battery. It is predicted that this tendency will become more pronounced as the size of the battery increases.

Therefore, in order to prevent bending and distortion of the electrode body and improve productivity and battery performance, the separator has adhesiveness (adhesion during drying) to the electrode when the electrolytic solution in the manufacturing process of the electrode body is not wet. Gender) has come to be required. If an excessive adhesive component is added or thermocompression bonding is performed under excessive conditions in order to ensure the adhesiveness at the time of drying, the air permeability of the separator is deteriorated. Not only that, the adhesive function for maintaining the adhesion between the electrodes when wet is also impaired. Therefore, it is extremely difficult to achieve both wet adhesiveness and dry adhesiveness.

The present inventors provide a battery separator having excellent dry adhesiveness and wet adhesiveness, which is a new issue for preparing for the future increase in battery size without deteriorating the air permeability resistance. It is aimed at. In the present specification, the wet adhesiveness means the adhesiveness between the separator and the electrode in a state where the separator contains an electrolytic solution, and is represented by the flexural strength when wet obtained by the measurement method described later. Further, the adhesiveness at the time of drying means the adhesiveness between the separator and the electrode in a state where the separator does not substantially contain the electrolytic solution, and is represented by the bending strength at the time of drying obtained by the measurement method described later. In addition, substantially not contained means that the electrolytic solution in the separator is 500 ppm or less.

In order to solve the above problems, the battery separator and the manufacturing method thereof according to the present invention have the following configurations. That is,
(1) A microporous film and a porous layer provided on at least one surface of the microporous film are provided, and the porous layer contains a vinylidene fluoride-hexafluoropropylene copolymer and an acrylic resin, and the foot. The vinylidene-hexafluoropropylene copolymer contains a monomer unit having a hydrophilic group, contains a hexafluoropropylene monomer unit of 0.3 mol% or more and 3 mol% or less, and the acrylic resin is a butyl acrylate simple substance. A battery separator, including a polymer unit.
(2) In the battery separator according to the present invention, it is preferable that the porous layer contains particles.
(3) In the battery separator according to 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) The battery separator according to the present invention has an acrylic resin content of 5% by mass or more and less than 40% by mass with respect to the total amount of the vinylidene fluoride-hexafluoropropylene copolymer and the acrylic resin. preferable.
(5) The battery separator according to the present invention is preferably an acrylic copolymer in which the acrylic resin contains a butyl acrylate unit and an acrylonitrile unit.
(6) The battery separator according to the present invention preferably has a vinylidene fluoride-hexafluoropropylene copolymer having a mass average molecular weight of 500,000 or more and 2 million or less.
(7) The battery separator according to the present invention preferably has a butyl acrylate unit content of 50 mol% or more and 75 mol% or less in the acrylic resin.
(8) The battery separator according to the present invention preferably has a flexural strength of 14 N or more when wet and a flexural strength of 7 N or more when dry.
(9) The battery separator according to the present invention has a particle content of 50% by mass or more and 85% by mass or less with respect to the total amount of the vinylidene fluoride-hexafluoropropylene copolymer, the acrylic resin and the particles. It is preferable to have.
(10) In the battery separator according to the present invention, the thickness of the porous layer is preferably 0.5 μm or more and 3 μm or less per side.
(11) The battery separator according to the present invention preferably contains at least one type of particles selected from the group consisting of alumina, titania, and boehmite.
(12) The battery separator according to the present invention preferably has an average particle size of 0.3 μm or more and 3.0 μm or less.
(13) In the battery separator according to the present invention, the microporous membrane is preferably a polyolefin microporous membrane.
In order to solve the above problems, the method for manufacturing a battery separator of the present invention has the following configuration.
That is,
(14) A method for manufacturing a battery separator, which sequentially includes the following steps (a) to (c).
(A) Step of obtaining a fluororesin solution in which a vinylidene fluoride-hexafluoropropylene copolymer is dissolved in a solvent (b) Step of adding an acrylic resin solution to the fluororesin solution and mixing to obtain a coating solution (c) A process in which a coating solution is applied to a microporous film, immersed in a coagulation bath, washed, and dried.

According to the present invention, it is possible to provide a battery separator having both dry adhesiveness and wet adhesiveness, particularly a battery separator suitable for a large wound battery, without deteriorating the air permeation resistance.

It is a front sectional view which shows typically the test of bending strength at the time of wetting. It is a front sectional view which shows typically the test of bending strength at the time of drying.

The outline of the microporous membrane and the porous layer in the battery separator of the present invention will be described, but of course, the present invention is not limited to this representative example.
1. 1. Microporous Membrane In the present invention, the microporous membrane means a membrane having voids connected to the inside. The microporous membrane is not particularly limited, and a microporous membrane containing a polyolefin resin can be used. Hereinafter, the case where the resin constituting the microporous film 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 film contains 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, still more preferably 100% by mass, assuming that the total mass of the polyolefin resin is 100% by mass.

Examples of the polyolefin resin include homopolymers obtained by polymerizing ethylene, propylene, 1-butene, 4-methyl1-pentene, 1-hexene and the like, two-step polymers, copolymers and mixtures thereof. If necessary, various additives such as antioxidants and inorganic fillers may be added to the polyolefin resin as long as the effects of the present invention are not impaired.

[2] Method for Producing Polyolefin Microporous Membrane The method for producing a polyolefin microporous membrane is not particularly limited as long as a polyolefin microporous membrane having desired characteristics can be produced, and a conventionally known method can be used, for example. The methods described in Japanese Patent No. 2132327, Japanese Patent No. 3347835, International Publication No. 2006/137540, etc. can be used. Specifically, it is preferable to include the following steps (1) to (5), and further, the following steps (6) to (8) can be included.
(1) A step of melt-kneading the polyolefin resin and a solvent for film formation to prepare a polyolefin solution (2) A step of extruding the polyolefin solution and cooling to form a gel-like sheet (3) Stretching the gel-like sheet First stretching step (4) Step of removing the film-forming solvent from the stretched gel-like sheet (5) Step of drying the sheet after removing the film-forming solvent (6) The dried sheet Second stretching step of stretching (7) Step of heat-treating the dried sheet (8) Step of cross-linking and / or hydrophilizing the sheet after the stretching step

Hereinafter, each step will be described.
(1) Preparation Step of Polyolefin Solution After adding an appropriate film-forming solvent to each of the polyolefin resins, melt-knead the polyolefin resin 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 known, the description thereof will be omitted.

The mixing ratio of the polyolefin resin and the film-forming solvent in the polyolefin solution is not particularly limited, but is preferably 70 to 80 parts by mass of the film-forming solvent 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, swells and neck-ins can be prevented at the die outlet when the polyolefin solution is extruded, and the moldability and self-supporting property of the extruded molded product (gel-shaped molded product) are improved.

(2) Gel-like sheet forming step The polyolefin solution is fed from an extruder to a die and extruded into a sheet. A plurality of polyolefin solutions having the same or different composition may be fed from an extruder to one die, where the layers may be laminated 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 to 250 ° C., and the extrusion speed is preferably 0.2 to 15 m / min. The film thickness can be adjusted by adjusting each extrusion amount of the polyolefin solution. As the extrusion method, for example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used.

A gel-like sheet is formed by cooling the obtained extruded molded product. As a method for forming the gel-like sheet, for example, the methods disclosed in Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Cooling is preferably performed at a rate of 50 ° C./min or higher, at least up to the gelation temperature. Cooling is preferably performed up to 25 ° C. or lower. By 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 roll, or the like can be used, but it is preferable to contact with a roll cooled with the refrigerant for cooling.

(3) First Stretching Step Next, the obtained gel-like sheet is stretched at least in the uniaxial direction. Since the gel-like sheet contains a solvent for film formation, it can be uniformly stretched. After heating, the gel-like sheet is preferably stretched at a predetermined ratio by a tenter method, a roll method, an inflation method, or a combination thereof. 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 stretching ratio (area stretching ratio) in this step is preferably 9 times or more, more preferably 16 times or more, and particularly preferably 25 times or more. Further, the draw ratios in the mechanical direction (MD) and the width direction (TD) may be the same or different from each other. The draw ratio in this step refers to the area stretch ratio of the microporous membrane immediately before being subjected to the next step, based on the microporous membrane immediately before this step.

The stretching temperature in this step is preferably in the range of the crystal dispersion temperature (Tcd) to Tcd + 30 ° C. of the polyolefin resin, and is in the range of the crystal dispersion temperature (Tcd) + 5 ° C. to the crystal dispersion temperature (Tcd) + 28 ° C. Is more preferable, and it is particularly preferable that the temperature is in the range of Tcd + 10 ° C. to Tcd + 26 ° C. For example, in the case of polyethylene, the stretching temperature is preferably 90 to 140 ° C, more preferably 100 to 130 ° C. The crystal dispersion temperature (Tcd) is determined by measuring the temperature characteristics of dynamic viscoelasticity with ASTM D4065.

Due to the above stretching, cleavage occurs between the polyethylene lamellae, the polyethylene phase becomes finer, and a large number of fibrils are formed. Fibrils form a three-dimensionally irregularly connected network structure. The mechanical strength is improved and the pores are expanded by stretching, but if stretching is performed under appropriate conditions, it is possible to control the through-hole diameter and to have a high pore ratio even with a thinner film thickness.

Depending on the desired physical properties, a temperature distribution may be provided in the film thickness direction for stretching, whereby a microporous film having excellent mechanical strength can be obtained. Details of the method are described in Japanese Patent No. 3347854.

(4) Removal of film-forming solvent The film-forming solvent is removed (cleaned) using a cleaning solvent. Since the polyolefin phase is phase-separated from the film-forming solvent phase, when the film-forming solvent is removed, it is composed of fibrils that form a fine three-dimensional network structure, and pores (voids) that communicate irregularly in three dimensions. A porous membrane having the above is obtained. Since the cleaning solvent and the method for removing the film-forming solvent using the cleaning solvent are known, the description thereof will be omitted. For example, the method disclosed in Japanese Patent No. 2132327 and Japanese Patent Application Laid-Open No. 2002-256099 can be used.

(5) Drying The microporous film from which the film-forming solvent has been removed is dried by a heat-drying method or an air-drying method. The drying temperature is preferably not less than the crystal dispersion temperature (Tcd) of the polyolefin resin, and particularly preferably 5 ° C. or more lower than Tcd. Drying is preferably carried out with the microporous membrane as 100% by mass (dry mass) until the residual cleaning solvent is 5% by mass or less, and more preferably 3% by mass or less. When the residual cleaning solvent is within the above range, the porosity of the microporous membrane is maintained when the subsequent stretching step and heat treatment step of the microporous membrane are performed, and deterioration of permeability is suppressed.

(6) Second Stretching Step It is preferable to stretch the microporous membrane after drying at least in the uniaxial direction. The microporous membrane can be stretched by the tenter method or the like in the same manner as described above while heating. The stretching may be uniaxial stretching or biaxial stretching. In the case of biaxial stretching, either simultaneous biaxial stretching or sequential stretching may be used. The stretching temperature in this step is not particularly limited, but is usually preferably 90 to 135 ° C, more preferably 95 to 130 ° C.

In the case of uniaxial stretching, the stretching ratio (area stretching ratio) of the stretching of the microporous membrane in this step is 1.0 to 2.0 times in the mechanical direction or the width direction. In the case of biaxial stretching, the lower limit of the area stretching ratio 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 is preferably 3.5 times or less. It may be 1.0 to 2.0 times in the machine direction and the width direction, respectively, and the draw ratios in the machine direction and the width direction may be the same or different from each other. The draw ratio in this step refers to the draw ratio of the microporous membrane immediately before being subjected to the next step, based on the microporous membrane immediately before this step.

(7) Heat treatment Further, the microporous membrane after drying can be heat-treated. The heat treatment stabilizes the crystals and homogenizes the lamella. As the heat treatment method, a heat fixing treatment and / or a heat relaxation treatment can be used. The heat fixing treatment is a heat treatment in which the film is heated while being held so that the dimensions of the film do not change. The heat relaxation treatment is a heat treatment in which the membrane is heat-shrinked in the mechanical direction and the width direction during heating. The heat fixing treatment is preferably performed by a tenter method or a roll method. For example, as a heat relaxation treatment method, the method disclosed in JP-A-2002-256099 can be mentioned. The heat treatment temperature is preferably in the range of Tcd to Tm of the polyolefin resin, more preferably in the range of the stretching temperature of the microporous membrane ± 5 ° C., and particularly preferably in the range of the second stretching temperature of the microporous membrane ± 3 ° C.

(8) Crosslinking Treatment and Hydrophilization Treatment Further, the microporous membrane after bonding or stretching can be further subjected to a crosslinking treatment and a hydrophilic treatment. For example, the microporous membrane is crosslinked by irradiating it with ionizing radiation such as α-rays, β-rays, γ-rays, and electron beams. In the case of electron beam irradiation, an electron dose of 0.1 to 100 Mrad is preferable, and an acceleration voltage of 100 to 300 kV is preferable. The cross-linking treatment raises the meltdown temperature of the microporous membrane. Further, the hydrophilization treatment can be performed by a monomer graft, a surfactant treatment, a corona discharge or the like. The monomer graft is preferably carried out after the cross-linking treatment.

2. Porous layer The porous layer contained in the battery separator according to the present invention contains a vinylidene fluoride-hexafluoropropylene copolymer and an acrylic resin. This makes it possible to achieve both dry adhesiveness and wet adhesiveness.
[1] Vinylidene Fluoride-Hexafluoropropylene (VdF-HFP) Copolymer The vinylidene fluoride-hexafluoropropylene copolymer used in the present invention has a high affinity with a non-aqueous electrolytic solution and is compatible with a non-aqueous electrolytic solution. High chemical and physical stability. Therefore, the porous layer containing this copolymer exhibits adhesiveness when wet, and can sufficiently maintain the affinity with the electrolytic solution even when used at a high temperature.

The vinylidene fluoride-hexafluoropropylene copolymer contains a monomer unit having a hydrophilic group. This makes it possible to interact with the active material existing on the electrode surface and the binder component in the electrode to firmly bond them.

Examples of the hydrophilic group include a hydroxyl group, a carboxylic acid group, a carboxylic acid ester group, a sulfonic acid group, and salts thereof. In particular, a carboxylic acid group and a carboxylic acid ester group are preferable.

To introduce a hydrophilic group into the vinylidene fluoride-hexafluoropropylene copolymer, for example, in the synthesis of the vinylidene fluoride-hexafluoropropylene copolymer, maleic anhydride, maleic acid, maleic acid ester, maleic acid monomethyl ester Examples thereof include a method of introducing a monomer having a hydrophilic group such as the above into the main chain by copolymerization and a method of introducing the 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 5 mol. % Is preferable, and more preferably 4 mol%. By setting the content of the monomer unit having a hydrophilic group within the above preferable range, the interaction between this hydrophilic group and the hydrophilic site hydrophilic group of the active material surface in the electrode or the binder component in the electrode can be caused. It works and can have sufficient adhesion when wet. When the content of the monomer unit having a hydrophilic group is 5 mol% or less, sufficient polymer crystallinity can be ensured, so that the degree of swelling with respect to the electrolytic solution can be suppressed low, and high adhesion when wet can be obtained. Further, when the porous layer contains particles, it is possible to prevent the particles from falling off by setting the content of the monomer unit having a hydrophilic group within the above 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 and the like. For example, in the case of a carboxylic acid group, it can be obtained from the absorption intensity ratio of CH stretching vibration and C = O stretching vibration of a carboxyl group with reference to a homopolymer using FT-IR.

The lower limit of the content of hexafluoropropylene monomer unit in the vinylidene fluoride-hexafluoropropylene copolymer is preferably 0.3 mol%, more preferably 0.5 mol%, and the upper limit is 3 mol%. Is preferable, and more preferably 2.5 mol%. If the content of the hexafluoropropylene monomer unit is less than 0.3 mol%, the polymer crystallinity becomes high and the degree of swelling with respect to the electrolytic solution becomes low, so that sufficient adhesion when wet cannot be obtained. Further, if the content of hexafluoropropylene exceeds 3 mol%, it swells too much with respect to the electrolytic solution, and the adhesiveness at the time of wetting deteriorates.

The vinylidene fluoride-hexafluoropropylene copolymer can be obtained by a known polymerization method. Examples of known polymerization methods include the methods exemplified in JP-A-11-130821. This is a method in which ion-exchanged water, maleic anhydride, vinylidene fluoride and hexafluoropropylene are placed in an autoclave, suspension polymerization is carried out, and then the polymer slurry is dehydrated, washed with water and dried to obtain a polymer powder. .. At this time, methyl cellulose as a suspending agent and diisopropylperoxydicarbonate as a radical initiator can be appropriately used.

The vinylidene fluoride-hexafluoropropylene copolymer may be obtained by further polymerizing a monomer unit other than the monomer unit having a hydrophilic group as long as the properties are not impaired. Examples of the monomer other than the monomer unit having a hydrophilic group include a monomer unit such as tetrafluoroethylene, trifluoroethylene, trichlorethylene, and vinyl fluoride.

The lower limit of the weight average molecular weight of the vinylidene fluoride-hexafluoropropylene copolymer is preferably 500,000, more preferably 900,000, and the upper limit is preferably 2 million, more preferably 1.5 million. By setting the weight average molecular weight of the vinylidene fluoride-hexafluoropropylene copolymer within the above-mentioned preferable range, the time for dissolving the copolymer in the solvent is not extremely long, and the copolymer can be used without reducing the productivity. In addition, an appropriate gel strength can be maintained when the gel is swollen in the electrolytic solution. The weight average molecular weight is a polystyrene-equivalent value obtained by gel permeation chromatography.

[2] Acrylic resin Acrylic resin is a copolymer containing a butyl acrylate unit. The porous layer containing the acrylic resin can exhibit adhesiveness when dried. Further, when the porous layer contains particles, the butyl acrylate increases the flexibility of the coating film and can be expected to have the effect of suppressing the particles from falling off.

The acrylic resin is preferably a copolymer of butyl acrylate and acrylonitrile from the viewpoint of electrode adhesion. By controlling the molar ratio of butyl acrylate and acrylonitrile, the degree of swelling with respect to the electrolytic solution can be adjusted, and the resin can be given moderate flexibility. This makes it possible to improve 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%. By setting the lower limit of the content of the butyl acrylate unit in the acrylic resin within the above-mentioned preferable range, the porous layer can be provided with appropriate flexibility, and the coating film can be suppressed from falling off. By setting the content of the butyl acrylate unit in the acrylic resin within the above preferable range, it is easy to obtain a good balance between the adhesiveness when dried and the adhesiveness when wet.

The acrylic resin can be obtained by a known polymerization method, for example, the method exemplified in JP2013-206846A. Ion-exchanged water, n-butyl acrylate, and acrylonitrile were charged into an autoclave equipped with a stirrer and emulsion-polymerized to obtain a polymer particle aqueous dispersion, and the water in the system was replaced with N-methyl-2-pyrrolidone to obtain an acrylic resin. There is a method of obtaining. At the time of polymerization, potassium persulfate as a radical polymerization initiator, t-dodecyl mercaptan as a molecular weight modifier, or the like may be appropriately used.

Regarding the content of the acrylic resin, the lower limit is preferably 5% by mass, the upper limit is preferably 40% by mass, and more preferably 20% by mass with respect to the total amount of the vinylidene fluoride-hexafluoropropylene copolymer and the acrylic resin. Is. In particular, the upper limit is more preferably less than 10% by mass. Within the above preferable range, sufficient adhesiveness when dried and adhesive when wet can be obtained. By setting the content of the acrylic resin to 5% by mass or more, it is possible to more sufficiently achieve both wet adhesiveness and dry adhesiveness. By setting the content of the acrylic resin to 40% by mass or less, the effect of adhesiveness when wet by the vinylidene fluoride-hexafluoropropylene copolymer can be easily obtained.

[3] Particles The porous layer of the battery separator according to the present invention may contain particles. By including particles in the porous layer, the probability of a short circuit 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.

Inorganic particles include calcium carbonate, calcium phosphate, amorphous silica, crystalline glass particles, kaolin, talc, titania, alumina, silica-alumina composite oxide particles, barium sulfate, calcium fluoride, lithium fluoride, zeolite, etc. Examples include molybdenum sulfide, mica, and zeolite. In particular, titania, alumina, and boehmite are preferable because of the crystal growth property, cost, and availability of the vinylidene fluoride-hexafluoropropylene copolymer.

Examples of the organic particles include crosslinked polystyrene particles, crosslinked acrylic resin particles, and crosslinked methyl methacrylate-based particles.

The content of the particles contained in the porous layer is preferably 85% by mass, more preferably 80% by mass, and more preferably 80% by mass with respect to the total amount of the vinylidene fluoride-hexafluoropropylene copolymer, the acrylic resin, and the particles. It is preferably 75% by mass, and the lower limit is preferably 50% by mass, more preferably 60% by mass, and further preferably 65% by mass. By keeping the particle content within the above-mentioned preferable range, a good balance of air permeation resistance can be easily obtained.

The average particle size of the particles is preferably 1.5 times or more and 50 times or less, more preferably 2.0 times or more and 20 times the average pore diameter of the microporous membrane from the viewpoint of suppressing the falling off of the particles. It is as follows. The average flow rate pore diameter was measured according to JIS K3832 or ASTMF316-86, and was measured in the order of Dry-up and Wet-up using, for example, a palm polo meter (CFP-1500A manufactured by PMI). For Wet-up, pressure was applied to a microporous membrane sufficiently immersed with Galwick (trade name) manufactured by PMI, which has a known surface tension, and the pore diameter converted from the pressure at which air began to penetrate was defined as the maximum pore diameter. Regarding the average flow rate pore diameter, the pore size was converted from the pressure at the intersection of the pressure and the slope of 1/2 of the flow rate curve in the Dry-up measurement and the pressure at the intersection of the Wet-up measurement curve. The following formula was used to convert the pressure and pore diameter.
d = C · γ / P
In the above formula, "d (μm)" is the pore size of the microporous membrane, "γ (mN / m)" is the surface tension of the liquid, "P (Pa)" is the pressure, and "C" is a constant.

The average particle size 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, from the viewpoint of slipperiness with the winding core during cell winding and particle dropout. More preferably, it is 1.0 μm or more and 3.0 μm or less. The average particle size of the particles can be measured using a laser diffraction method or a dynamic light scattering method measuring device. For example, particles dispersed in an aqueous solution containing a surfactant using an ultrasonic probe were measured with a particle size distribution measuring device (Microtrac HRA manufactured by Nikkiso Co., Ltd.), and 50% of the particles were accumulated from the small particle side in terms of volume. It is preferable that the value of the particle size (D50) at the time is the average particle size. The shape of the particles includes a spherical shape, a substantially spherical shape, a plate shape, and a needle shape, but is not particularly limited.

[4] Physical characteristics of the porous layer The film thickness of the porous layer is preferably 0.5 μm or more and 3 μm or less, more preferably 1 μm or less, 2.5 μm or more, and further preferably 1 μm or more and 2 μm or less per side surface. When the film thickness per side is 0.5 μm or more, the adhesiveness when wet and the adhesiveness when dry can be ensured. If the film thickness per side is 3 μm or less, the winding volume can be suppressed, which is suitable for increasing the capacity of batteries, which will be advanced in the future.

The porosity of the porous layer is preferably 30% or more and 90% or less, more preferably 40% or more and 70% or less. By setting the porosity of the porous layer within the above-mentioned preferable range, it is possible to prevent an increase in the electrical resistance of the film, allow a large current to flow, and maintain the film strength.

[5] Method for Manufacturing Battery Separator The method for manufacturing a battery separator according to one aspect of the present invention sequentially includes the following steps (a) to (c).
(A) Step of obtaining a fluororesin solution in which a vinylidene fluoride-hexafluoropropylene copolymer is dissolved in a solvent (b) Step of adding an acrylic resin solution to the fluororesin solution and mixing to obtain a coating solution (c) A process in which a coating solution is applied to a microporous film, immersed in a coagulation bath, washed, and dried.

(A) Step of obtaining a fluororesin solution The solvent is not particularly limited as long as it dissolves a vinylidene fluoride-hexafluoropropylene copolymer, dissolves or disperses an acrylic resin, and is miscible with a coagulating solution. From the viewpoint of solubility and low volatility, the solvent is preferably N-methyl-2-pyrrolidone.

When providing a porous layer containing particles, it is important to prepare a fluororesin solution (also referred to as a dispersion liquid) in which the particles are dispersed in advance. The vinylidene fluoride-hexafluoropropylene copolymer is dissolved in a solvent, particles are added thereto with stirring, and the mixture is pre-dispersed by stirring with a disper or the like for a certain period of time (for example, about 1 hour). Further, by going through a step of dispersing the particles (dispersion step) using a bead mill or a paint shaker, a fluororesin solution with less agglutination of the particles can be obtained.

(B) Step of obtaining coating liquid This step is a step of adding an acrylic resin solution to a fluororesin solution and mixing it with, for example, a three-one motor equipped with a stirring blade to prepare a coating liquid.

The acrylic resin solution used in this step is a solution in which acrylic resin is dissolved or dispersed in a solvent. The solvent used here is preferably the same solvent as in step (a). In particular, N-methyl-2-pyrrolidone is preferable from the viewpoint of solubility and low volatility. From the viewpoint of operability, it is preferable to obtain the acrylic resin solution by substituting the solvent by polymerizing the acrylic resin and then adding N-methyl-2-pyrrolidone and distilling.

When providing a porous layer containing particles, it is important to disperse the particles in a fluororesin solution and then add (post-insert) the acrylic resin solution. That is, it is important that the acrylic resin does not enter in the dispersion process. When vinylidene fluoride-hexafluoropropylene copolymer, acrylic resin, and particles are added to the solvent at the same time, the hydrophilic groups contained in the vinylidene fluoride-hexafluoropropylene copolymer and the butyl acrylate contained in the acrylic resin are present. It is industrially unsuitable because it is presumed that the coating liquid gradually begins to gel due to heat and shear during dispersion. Further, thin film coating with a thickness of 3 μm or less of the porous layer becomes difficult due to the influence of thickening. By the steps (a) and (b) in the production method of the present invention, gelation of the coating liquid is suppressed, thin film coating becomes possible, and the storage stability of the coating liquid is also improved.

(C) Step of applying the coating liquid to the microporous membrane, immersing it in the coagulation bath, washing and drying In this step, the coating liquid is applied to the microporous membrane and the applied microporous membrane is immersed in the coagulation liquid. Then, the vinylidene fluoride-hexafluoropropylene copolymer and the acrylic resin are phase-separated, solidified in a state having a three-dimensional network structure, washed, and dried. As a result, a microporous membrane and a battery separator having a porous layer on the surface thereof can be obtained.

The method of applying the coating liquid to the microporous film may be a known method, for example, 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, etc. Examples include the air knife coating method, the Meyer bar coating method, the pipe doctor method, the blade coating method and the die coating method, and these methods can be used alone or in combination.

The coagulation liquid is preferably water, preferably an aqueous solution containing 1% by mass or more and 20% by mass or less of a good solvent for the vinylidene fluoride-hexafluoropropylene copolymer and the acrylic resin, and more preferably 5% by mass. As mentioned above, it is an aqueous solution containing 15% by mass or less. Examples of good solvents include N-methyl-2-pyrrolidone, N, N-dimethylformamide, and N, N-dimethylacetamide. The immersion time in the coagulation bath is preferably 3 seconds or longer. The upper limit is not limited, but 10 seconds is sufficient.

Water can be used for cleaning. For drying, for example, hot air of 100 ° C. or lower can be used.

The battery separator according to the present invention is 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. It can be used as a battery separator for the above. In particular, it is preferably used as a separator for a lithium ion secondary battery.

[6] Physical Properties of Battery Separator The wet adhesiveness of the battery separator can be evaluated by the wet bending strength, and the wet bending strength is 14 N or more. The upper limit of the flexural strength when wet is not particularly specified, but 30 N is sufficient. By setting the flexural strength during wetting within the above preferable range, it is possible to suppress partial release at the interface between the separator and the electrode, and suppress an increase in battery internal resistance and a decrease in battery characteristics.

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 7 N or more, more preferably 9 N or more. The upper limit of the bending strength during drying is not particularly set, but 30 N is sufficient. By setting the flexural strength during drying within the above-mentioned preferable range, it becomes easy to suppress the deflection and distortion of the wound electrode body.

From the viewpoint of the balance between the adhesiveness during drying and the adhesiveness during wetness, the battery separator preferably has a flexural strength during wetness of 14 N or more and a flexural strength during dryness of 7 N or more.

Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited to these Examples. The measured values in the examples are values measured by the following methods.

1. 1. Flexural strength when wet Generally, when a fluororesin binder is used for the positive electrode and a porous layer containing the fluororesin is provided on the separator, the adhesiveness is easily ensured by mutual diffusion between the fluororesins. On the other hand, a binder other than the fluororesin is used for the negative electrode, and the fluororesin is less likely to diffuse, so that the negative electrode is less likely to have adhesiveness to the separator than the positive electrode. Therefore, in this measurement, the adhesiveness between the separator and the negative electrode was evaluated using the flexural strength described below as an index.
(1) Preparation 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 and mixed, and then 2 parts by mass of styrene-butadiene latex as a solid content was added and mixed to combine the negative electrodes. The slurry was prepared as an agent-containing slurry. This negative electrode mixture-containing slurry is uniformly applied to both sides of a negative electrode current collector made of copper foil having a thickness of 8 μm and dried to form a negative electrode layer, and then compression-molded by a roll press to collect current. A negative electrode was prepared by setting the density of the negative electrode layer excluding the body to 1.5 g / cm ^3.
(2) Preparation of test winding body The negative electrode (machine direction 161 mm × width direction 30 mm) prepared above and the separator (machine direction 160 mm × width direction 34 mm) prepared in Examples and Comparative Examples are overlapped with each other to form a metal plate. The separator and the negative electrode were wound around the winding core (length 300 mm, width 25 mm, thickness 1 mm) so that the separator was on the inside, and the metal plate was pulled out to obtain a test wound body. The test wound body had a length of about 34 mm and a width of about 28 mm.
(3) Method for measuring bending strength when wet A test wound body is placed on a laminated film (length 110 mm, width 65 mm, thickness 0.12 mm) made of aluminum and polypropylene, and the laminated film is placed in the length direction. It was bent in half and welded on two sides of the laminated film to form a bag with one side open. _{500 μL of an electrolytic solution prepared by dissolving LiPF 6} at a ratio of 1 mol / L in a solvent in which ethylene carbonate and ethyl methyl carbonate are mixed at a volume ratio of 3: 7 is injected through an opening in a glove box and impregnated into a test wound body. Then, one side of the opening was sealed with a vacuum sealer.
Next, the test wound body enclosed in the laminated film is sandwiched between two gaskets (thickness 1 mm, 5 cm x 5 cm), and 98 with a precision heating and pressurizing device (CYPT-10 manufactured by Shinto Kogyo Co., Ltd.). The film was pressurized at 0.6 MPa for 2 minutes and allowed to cool at room temperature. The flexural strength of the test wound body enclosed in the pressurized laminated film was measured using a universal testing machine (manufactured by Shimadzu Corporation, AGS-J) as shown in the schematic diagram of FIG. .. Details will be described below.
Two aluminum L-shaped angles 4 (thickness 1 mm, 10 mm × 10 mm, length 5 cm) are arranged in parallel with the 90 ° portion facing up, and the 90 ° portion is used as the fulcrum between the fulcrums. It was fixed so that the distance was 15 mm. Align the midpoint of the width direction side (about 28 mm) of the test winding body with the 7.5 mm point, which is the middle of the distance between the fulcrums of the two aluminum L-shaped angles, in the length direction of the L-shaped angle. The test winding body was arranged so as not to protrude from the side.
Next, as an indenter, the side (about 34 mm) in the length direction of the test winding body does not protrude from the side in the length direction of the aluminum L-shaped angle 3 (thickness 1 mm, 10 mm × 10 mm, length 4 cm). Align the 90 ° portion of the aluminum L-shaped angle 3 with the midpoint of the widthwise side of the test winding body, and align the 90 ° portion of the aluminum L-shaped angle 3 so that the 90 ° portion is on the bottom. It was fixed to the load cell (load cell capacity 50N) of the universal testing machine. For the measured values at the stroke 0.5 mm point after the test load became 0.05 N at a load speed of 0.5 mm / min, the average value of the three test winding bodies was taken as the bending strength when wet.

2. Flexural strength during drying (1) Preparation of negative electrode 1. A negative electrode having the same bending strength as when wet was used.
(2) Preparation of test winding body 1. A test wound body having the same bending strength as when wet was used.
(3) Method for measuring flexural strength during drying The prepared test winding body is sandwiched between two gaskets (thickness 1 mm, 5 cm x 5 cm), and a precision heating and pressurizing device (manufactured by Shinto Kogyo Co., Ltd., CYPT-). The pressure was increased at 90 ° C. and 0.6 MPa for 2 minutes at 10), and the mixture was allowed to cool at room temperature. Regarding the test wound body after pressurization, as shown in FIG. 2, the above 1. Arranged in the same manner as the method for measuring flexural strength when wet, and using a universal testing machine (manufactured by Shimadzu Corporation, AGS-J), three test winding bodies were measured under the following conditions, and the maximum test force was obtained. The average value of was taken as the bending strength at the time of drying.
Distance between fulcrums: 15 mm
Cell capacity: 50N
Load speed: 0.5 mm / min

3. 3. Evaluation of powder removal A separator was fixed flatly on the bottom surface (bottom area 5.5 cm x 6 cm) of a weight (1143 g) with a handle so that the porous layer was the surface without wrinkles. After moving the weight 10 times back and forth over a distance of 20 cm on drawing paper (manufactured by Daio Paper Corporation, C-55, Kuro), the amount of the porous layer transferred to the drawing paper was confirmed. Ten arbitrary 5 mm × 5 mm ranges were selected, and the number of coating film shed objects of 150 μm or more was measured using an optical microscope, and powder shedding was evaluated by the number of shed objects as follows.
Good: Total number of paint film shed items in 10 places is 50 or less Defective: Total number of paint film shed items in 10 places is 51 or more

4. Film thickness Using a contact-type film thickness meter (“Lightmatic” (registered trademark) series 318 manufactured by Mitutoyo Co., Ltd.), 20 points were measured under the condition of a weight of 0.01 N using a super hard spherical probe φ9.5 mm. The average value of the obtained measured values was taken as the film thickness.

Example 1
[Vinylidene Fluoride-Hexafluoropropylene (VdF-HFP) Copolymer]
A vinylidene fluoride-hexafluoropropylene copolymer (a) was synthesized by a suspension polymerization method using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. The obtained vinylidene fluoride-hexafluoropropylene copolymer has a weight average molecular weight of 1.5 million and a molar ratio of vinylidene fluoride monomer unit / hexafluoropropylene monomer unit / maleic acid monomethyl ester monomer unit of 98. It was confirmed by NMR measurement that it was .5 / 1.0 / 0.5.
[acrylic resin]
A butyl acrylate-acrylonitrile copolymer was synthesized as an acrylic resin by an emulsion polymerization method using acrylonitrile and n-butyl acrylate as starting materials, and then water was replaced with N-methyl-2-pyrrolidone (NMP) to obtain a solid content concentration. Obtained a 5% by mass acrylic resin solution. It was confirmed by NMR measurement that the obtained acrylic resin had a Tg of −5 ° C. and a molar ratio of acrylonitrile monomer unit / n-butyl acrylate monomer unit of 38/62.
[Making a battery separator]
28.5 parts by mass of vinylidene fluoride-hexafluoropropylene copolymer (a) and 641 parts by mass of NMP are mixed, and then 70 parts by mass of alumina particles (average particle size 1.1 μm) are added as inorganic particles while stirring with a disper. Then, the mixture was pre-stirred at 2000 rpm for 1 hour with a disper. Next, using a dyno mill (Dyno Mill Multilab (1.46 L container, filling rate 80%, φ0.5 mm alumina beads) manufactured by Simmal Enterprises), the flow rate is 11 kg / h and the peripheral speed is 10 m / s. The treatment was performed 3 times to obtain a dispersion liquid. Acrylic resin solution is mixed with the dispersion, stirred with a three-one motor equipped with stirring blades at 500 rpm for 30 minutes, and filtered to have a solid content concentration of 13% by mass. Alumina particles: copolymer (a): mass of acrylic resin A coating solution having a ratio of 70: 28.5: 1.5 was obtained. A coating liquid is applied to both sides of a polyethylene microporous membrane having a thickness of 7 μm by a dip coating method, immersed in an aqueous solution, washed with pure water, and then dried at 50 ° C. to obtain a battery separator having a thickness of 11 μm. Obtained.

Example 2
A coating solution prepared so that the solid content concentration is 13% by mass and the mass ratio of alumina particles: vinylidene fluoride-hexafluoropropylene copolymer (a): acrylic resin is 70: 27.2: 2.8. A battery separator was obtained in the same manner as in Example 1 except that it was used. The obtained battery separator was evaluated for powder removal and was good.

Example 3
A coating solution prepared so that the solid content concentration is 13% by mass and the mass ratio of alumina particles: vinylidene fluoride-hexafluoropropylene copolymer (a): acrylic resin is 70: 25.2: 4.8. A battery separator was obtained in the same manner as in Example 1 except that it was used.

Example 4
A coating solution prepared so that the solid content concentration is 13% by mass and the mass ratio of alumina particles: vinylidene fluoride-hexafluoropropylene copolymer (a): acrylic resin is 70: 22.5: 7.5. A battery separator was obtained in the same manner as in Example 1 except that it was used.

Example 5
Except for using a coating liquid prepared so that the solid content concentration was 13% by mass and the mass ratio of alumina particles: vinylidene fluoride-hexafluoropropylene copolymer (a): acrylic resin was 70:18:12. A battery separator was obtained in the same manner as in Example 1.

Example 6
A coating solution prepared so that the solid content concentration is 12% by mass and the mass ratio of alumina particles: vinylidene fluoride-hexafluoropropylene copolymer (a): acrylic resin is 65: 31.7: 3.3. A battery separator was obtained in the same manner as in Example 1 except that it was used.

Example 7
A coating solution prepared so that the solid content concentration is 18% by mass and the mass ratio of alumina particles: vinylidene fluoride-hexafluoropropylene copolymer (a): acrylic resin is 85: 12.4: 2.6. A battery separator was obtained in the same manner as in Example 1 except that it was used.

Example 8
A battery separator was obtained in the same manner as in Example 2 except that a coating liquid prepared using boehmite (average particle size 2.3 μm) was used as the inorganic particles.

Example 9
A battery separator was obtained in the same manner as in Example 2 except that a coating liquid prepared using titania (average particle size 1 μm) was used as the inorganic particles.

Example 10
A battery separator was obtained in the same manner as in Example 2 except that the thickness of the battery separator was 10 μm.

Comparative Example 1
Same as in Example 1 except that a coating liquid prepared so that the solid content concentration is 13% by mass and the mass ratio of alumina particles: vinylidene fluoride-hexafluoropropylene copolymer (a) is 70:30 is used. To obtain a separator for batteries. The obtained battery separator was evaluated as being defective when powder was removed.

Comparative Example 2
Same as Example 2 except that a coating solution prepared using PVdF homopolymer (Kureha Corporation, KF # 7300 (molecular weight of 1 million or more)) was used instead of the vinylidene fluoride-hexafluoropropylene copolymer. To obtain a battery separator.

Comparative Example 3
Vinylidene fluoride-hexafluoropropylene copolymer (b) containing 4.5 mol% of hexafluoropropylene monomer instead of vinylidene fluoride-hexafluoropropylene copolymer (a) (manufactured by Alchema Co., Ltd.) , Kynar2801 (VdF / HFP molar ratio 95.5 / 4.5, molecular weight less than 500,000)) was used to obtain a battery separator in the same manner as in Example 2 except that a coating solution was used. ..

Comparative Example 4
[Synthesis of acrylic resin]
An ethyl acrylate-acrylonitrile copolymer was synthesized as an acrylic resin by an emulsion polymerization method using acrylonitrile and ethyl acrylate as starting materials, and then water was replaced with N-methyl-2-pyrrolidone, and the solid content concentration was 5% by mass. An acrylic resin solution was obtained. It was confirmed by NMR measurement that the obtained acrylic resin had a Tg of 10 ° C. and a molar ratio of acrylonitrile monomer unit / ethyl acrylate monomer unit was 37/63. A battery separator was obtained in the same manner as in Example 2 except that the coating liquid prepared using this acrylic resin was used. The obtained battery separator was evaluated for powder removal and was good.

Comparative Example 5
Instead of the acrylic resin solution of Example 2, a coating solution prepared by using a solution of CRV (cyanoethyl PVA manufactured by Shin-Etsu Chemical Co., Ltd.) having a solid content concentration of 5% by mass and N-methyl-2-pyrrolidone was used. A battery separator was obtained in the same manner as in Example 2 except for the above.

Comparative Example 6
The coating liquid was prepared so that the solid content concentration was 25% by mass and the mass ratio of alumina particles: vinylidene fluoride-hexafluoropropylene copolymer (a): acrylic resin was 90: 9.1: 0.9. A separator for a battery was obtained in the same manner as in Example 2 except for the above.

Comparative Example 7
Inorganic particles and fluoride so that the solid content concentration is 13% by mass and the mass ratio of alumina particles: vinylidene fluoride-hexafluoropropylene copolymer (a): acrylic resin is 70: 27.2: 2.8. Vinylidene-hexafluoropropylene copolymer (a), acrylic resin, and N-methyl-2-pyrrolidone were mixed and dispersed at the same time to prepare a coating liquid, but the coating liquid was thickened and polyethylene microporous. The film could not be coated.

Example 11 (Comparative Example 8 is read as Example 11 in the following)
A battery separator was obtained in the same manner as in Example 2 except that the thickness of the battery separator was 9 μm.

Comparative Example 9
A coating solution prepared with N-methyl-2-pyrrolidone so that the solid content concentration of the vinylidene fluoride-hexafluoropropylene copolymer was 5% by mass was used, and the thickness of the battery separator was 9.5 μm. A battery separator was obtained in the same manner as in Example 1.

Table 1 shows the characteristics of the battery separators obtained in Examples 1 to 10 and Comparative Examples 1 to 9.

アクリル樹脂の含有量（質量％）とは、フッ素樹脂とアクリル樹脂との総質量に対するアクリル樹脂の質量％を表す。塗材調製の「後入」とは、粒子を分散させたフッ素樹脂溶液にアクリル樹脂溶液を加えることを表す。「同時入」とは、フッ素樹脂溶液、アクリル樹脂溶液、粒子を同時に加えて分散処理することを表す。

The acrylic resin content (mass%) represents the mass% of the acrylic resin with respect to the total mass of the fluororesin and the acrylic resin. "Post-injection" of coating material preparation means adding an acrylic resin solution to a fluororesin solution in which particles are dispersed. "Simultaneous injection" means that a fluororesin solution, an acrylic resin solution, and particles are added at the same time for dispersion treatment.

1: Negative electrode 2: Separator 3: Aluminum L-shaped angle for indenter 4: Aluminum L-shaped angle 5: Laminated film

Claims (11)

Microporous membrane and
A porous layer provided on at least one side of the microporous membrane is provided.
The porous layer contains a vinylidene fluoride-hexafluoropropylene copolymer and an acrylic resin and particles .
The vinylidene fluoride-hexafluoropropylene copolymer contains a monomer unit having a hydrophilic group, and contains a hexafluoropropylene monomer unit of 0.3 mol% or more and 3 mol% or less.
The acrylic resin is seen containing a butyl acrylate monomer unit,
The content of the particles is 50% by mass or more and 85% by mass or less with respect to the total weight of the vinylidene fluoride-hexafluoropropylene copolymer, the acrylic resin, and the particles, and the porous material. A battery separator having a layer thickness of 0.5 μm or more and 3 μm or less per side. The battery separator according to claim 1, wherein the vinylidene fluoride-hexafluoropropylene copolymer contains 0.1 mol% or more and 5 mol% or less of a monomer unit having a hydrophilic group. Any one of claims 1 and 2, wherein the content of the acrylic resin is 5% by mass or more and less than 40% by mass with respect to the total amount of the vinylidene fluoride-hexafluoropropylene copolymer and the acrylic resin. The battery separator described in the section. The battery separator according to any one of claims 1 to 3, wherein the acrylic resin is an acrylic copolymer containing a butyl acrylate unit and an acrylonitrile unit. The separator according to any one of claims 1 to 4, wherein the vinylidene fluoride-hexafluoropropylene copolymer has a weight average molecular weight of 500,000 or more and 2 million or less. The battery separator according to any one of claims 1 to 5, wherein the content of the butyl acrylate unit in the acrylic resin is 50 mol% or more and 75 mol% or less. The battery separator according to any one of claims 1 to 6, wherein the flexural strength when wet is 14 N or more and the flexural strength when dry is 7 N or more. The battery separator according to any one of claims 1 to 7, wherein the particles contain at least one selected from the group consisting of alumina, titania, and boehmite. The battery separator according to any one of claims 1 to 8, wherein the average particle size of the particles is 0.3 μm or more and 3.0 μm or less. The battery separator according to any one of claims 1 to 9, wherein the microporous membrane is a polyolefin microporous membrane. The method for manufacturing a battery separator according to claim 1 to 10, further comprising the following steps (a) to (c) in sequence.
(A) Step of dissolving vinylidene fluoride-hexafluoropropylene copolymer in a solvent to obtain a fluororesin solution in which particles are dispersed (b) Add an acrylic resin solution to the fluororesin solution, mix and coat the solution. Step (c) A step of applying a coating solution to a microporous film, immersing it in a coagulation bath, washing and drying it.

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