JPWO2017026485A1 - Battery separator - Google Patents

Abstract

The present inventors provide a battery separator having excellent dry adhesiveness and wet adhesiveness, which is a new problem in preparation for the future enlargement of batteries without deteriorating the air resistance. Is aimed at. A microporous membrane, and a porous layer provided on at least one side of the microporous membrane, the porous layer including a vinylidene fluoride-hexafluoropropylene copolymer and an acrylic resin, and the vinylidene fluoride-hexafluoropropylene The 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 contains a butyl acrylate monomer unit. Battery separator. [Selection] Figure 1

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 film breaking characteristics (meltdown characteristics), and the like. For this reason, the use of a separator for a battery in which a porous film and a porous layer are provided on the surface has been studied. Further, in recent years, unevenness of the electrode surface and partial separation of the separator-electrode interface due to electrode expansion / contraction caused by charging / discharging has led to an increase in battery internal resistance and a decrease in battery cycle characteristics. ing. Therefore, the separator is required to have adhesiveness with the electrode in the battery (that is, in the presence of a non-aqueous electrolyte) (hereinafter referred to as wet adhesiveness). A battery separator provided with a porous layer containing a fluorine resin that swells in an electrolytic solution has been studied.

  Patent Document 1 discloses an electrode including a positive electrode, a negative electrode, a three-layer separator made of polypropylene / polyethylene / polypropylene, and an adhesive resin layer made of polyvinylidene fluoride and alumina powder disposed between the electrode and the separator. The body is listed.

  Example 1 of Patent Document 2 includes an NMP solution containing a first polymer (polyvinylidene fluoride homopolymer), a second polymer (acrylonitrile monomer, and a monomer derived from 1,3-butadiene. And an NMP solution containing a methacrylic acid monomer and a (meth) butyl acrylate monomer) 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.

  In the example of Patent Document 3, a compound material composed of vinylidene fluoride-hexafluoropropylene copolymer (VdF-HFP copolymer) and polyethyl methacrylate was dissolved in an NMP solution in which spherical alumina powder was dispersed. A slurry prepared by adding an NMP solution and mixing with a ball mill was applied to a base PET film and dried, and the positive electrode and the negative electrode were thermocompression bonded through an inorganic fine particle-containing sheet (insulating adhesive layer) obtained by drying. An electrode body is described.

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

No. 1999-036981 JP 2013-206846 A JP2013-122009A Special table 2013-519206 gazette

  In recent years, non-aqueous electrolyte secondary batteries, in particular lithium ion secondary batteries, are not limited to small electronic devices such as mobile phones and personal digital assistants, but also include large tablets, mowers, electric motorcycles, electric cars, hybrid cars, small ships, etc. Expansion for large-scale applications is expected, and with this increase in size of batteries is expected.

  As these batteries, 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 (rolled electrode body), and the wound electrode body is press-molded and laminated. Examples thereof include a pouch battery covered with an exterior body and a square battery inserted into a rectangular exterior can.

  Due to the increase in size of the battery, if the active material surface of the electrode and the separator are not sufficiently bonded in the manufacturing process of the electrode body, a gap will be generated and the wound electrode body will bend or distort, and will not fit within the prescribed volume. Such a problem is assumed. As a result, the conveyance of the electrode body may be hindered or the insertion into the exterior body may be difficult, and the productivity may be significantly reduced. Furthermore, the gap is maintained even after the electrolyte solution is injected, and the adhesion between the electrode and the separator becomes non-uniform, resulting in deterioration of the cycle characteristics of the battery. This tendency is expected to appear more prominently as the battery becomes larger.

  Therefore, in order to prevent deflection and distortion of the electrode body and improve productivity and battery performance, the separator has an adhesive property with the electrode when the electrolyte in the electrode body manufacturing process is not wet (adhesion when dry) Sex) has been required. In order to ensure the adhesiveness at the time of drying, if an excessive adhesive component is provided or thermocompression bonding is performed under excessive conditions, the air permeability of the separator is deteriorated. In addition, the adhesion function for maintaining the adhesion between the electrodes when wet is impaired. For this reason, 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 problem in preparation for the future enlargement of batteries without deteriorating the air resistance. Is aimed at. In the present specification, the wet adhesion means the adhesion between the separator and the electrode in a state in which the separator contains an electrolytic solution, and is expressed by the wet bending strength obtained by a measurement method described later. Moreover, 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 electrolyte, and is expressed by the bending strength at the time of drying obtained by a measuring method described later. In addition, it does not contain substantially means that the electrolyte solution in a separator is 500 ppm or less.

In order to solve the above problems, a battery separator and a manufacturing method thereof according to the present invention have the following configurations. That is,
(1) A microporous membrane and a porous layer provided on at least one surface of the microporous membrane, the porous layer including a vinylidene fluoride-hexafluoropropylene copolymer and an acrylic resin, The vinylidene chloride-hexafluoropropylene copolymer contains monomer units having a hydrophilic group, contains hexafluoropropylene monomer units in an amount of 0.3 mol% to 3 mol%, and the acrylic resin is a butyl acrylate monomer. It is a battery separator containing a monomer unit.
(2) In the battery separator according to the present invention, the porous layer preferably 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) In the battery separator according to the present invention, 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. preferable.
(5) In the battery separator according to the present invention, the acrylic resin is preferably an acrylic copolymer containing 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 to 2,000,000.
(7) In the battery separator according to the present invention, the content of butyl acrylate units in the acrylic resin is preferably 50 mol% or more and 75 mol% or less.
(8) The battery separator according to the present invention preferably has a wet bending strength of 14 N or more and a dry bending strength of 7 N or more.
(9) In the battery separator according to the present invention, the content of the particles is 50% by mass to 85% by mass with respect to the total amount of the vinylidene fluoride-hexafluoropropylene copolymer, the acrylic resin, and the particles. Preferably there is.
(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 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-described problems, the battery separator manufacturing method of the present invention has the following configuration.
That is,
(14) A battery separator manufacturing method comprising the following steps (a) to (c) in sequence.
(A) A step of obtaining a fluororesin solution in which a vinylidene fluoride-hexafluoropropylene copolymer is dissolved in a solvent (b) A step of adding an acrylic resin solution to the fluororesin solution and mixing to obtain a coating solution (c) The process of applying the coating liquid to the microporous membrane, immersing it in a coagulation bath, washing and drying

  According to the present invention, it is possible to provide a battery separator that is compatible with dry adhesiveness and wet adhesiveness, in particular, a battery separator suitable for a wound large battery, without deteriorating the air resistance.

It is front sectional drawing which shows typically the test of bending strength at the time of wetness. It is front sectional drawing 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 below, but it is naturally not limited to this representative example.
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 membrane has 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 further preferably 100% by mass, where the total mass of the polyolefin resin is 100% by mass.

  Examples of the polyolefin resin include a homopolymer obtained by polymerizing ethylene, propylene, 1-butene, 4-methyl 1-pentene, 1-hexene and the like, a two-stage polymer, a copolymer, or a mixture thereof. You may add various additives, such as antioxidant and an inorganic filler, to the polyolefin resin in the range which does not impair the effect of this invention as needed.

[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 conventionally known methods can be used. 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.
(1) Step of melt-kneading the polyolefin resin and a film forming solvent to prepare a polyolefin solution (2) Step of extruding and cooling the polyolefin solution to form a gel sheet (3) Stretching the gel sheet (4) The process of removing the film-forming solvent from the stretched gel sheet (5) The process of drying the sheet after removing the film-forming solvent (6) The sheet after drying Second stretching step for stretching (7) Step for heat-treating the dried sheet (8) Step for crosslinking 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 the polyolefin resin, it is melt-kneaded to prepare a polyolefin solution. As a melt-kneading method, for example, a method using a twin-screw extruder described in the specifications of Japanese Patent No. 2132327 and Japanese Patent No. 3347835 can be used. Since the melt-kneading method is well-known, description is abbreviate | omitted.

  The blending ratio of the polyolefin resin and the film forming solvent in the polyolefin solution is not particularly limited, but it is preferably 70 to 80 parts by weight of the film forming solvent with respect to 20 to 30 parts by weight of the polyolefin resin. When the ratio of the polyolefin resin is within the above range, swell and neck-in can be prevented at the die exit when extruding the polyolefin solution, and the moldability and self-supporting property of the extruded molded body (gel-shaped molded body) are improved.

(2) Gel-like sheet forming step A 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 compositions may be fed from an extruder to a single die, where they are laminated in layers and extruded into sheets.

  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 an extrusion method, for example, 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 extrusion-molded body. As a method for forming the gel-like sheet, for example, 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 more at least up to the gelation temperature. Cooling is preferably performed to 25 ° C. or lower. By cooling, the polyolefin microphase separated by the film-forming solvent can be immobilized. When the cooling rate is within the above range, the crystallization degree 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 cooling medium 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 that the cooling is performed by contacting with a roll cooled with a cooling medium.

(3) 1st extending | stretching process Next, the obtained gel-like sheet | seat is extended | stretched to a uniaxial direction at least. Since the gel-like sheet contains a film-forming solvent, it can be stretched uniformly. The gel-like sheet is preferably stretched at a predetermined ratio after heating 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 preferred. In the case of biaxial stretching, any of simultaneous biaxial stretching, sequential stretching, and multistage stretching (for example, a combination of simultaneous biaxial stretching and sequential stretching) may be used.

  The draw ratio (area draw ratio) in this step is preferably 9 times or more, more preferably 16 times or more, and particularly preferably 25 times or more. Moreover, the draw ratios in the machine direction (MD) and the width direction (TD) may be the same or different. In addition, the draw ratio in this process means the area draw ratio of the microporous film immediately before being used for the next process on the basis of the microporous film immediately before this process.

  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 in the range of crystal dispersion temperature (Tcd) + 5 ° C. to crystal dispersion temperature (Tcd) + 28 ° C. Is more preferable, and the range of Tcd + 10 ° C. to Tcd + 26 ° C. is particularly preferable. 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 characteristic of dynamic viscoelasticity according to ASTM D4065.

  By stretching as described above, cleavage occurs between polyethylene lamellae, the polyethylene phase becomes finer, and a large number of fibrils are formed. Fibrils form a three-dimensional irregularly connected network structure. Stretching improves the mechanical strength and enlarges the pores. However, when stretching is performed under appropriate conditions, the through-hole diameter can be controlled, and a high porosity can be achieved even with a thinner film thickness.

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

(4) Removal of film-forming solvent The film-forming solvent is removed (washed) using a cleaning solvent. Since the polyolefin phase is phase-separated from the film-forming solvent phase, removing the film-forming solvent consists of fibrils that form a fine three-dimensional network structure, and pores (voids) that communicate irregularly in three dimensions. A porous membrane having the following is obtained. Since the cleaning solvent and the method for removing the film-forming solvent using the same are known, the description thereof is 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 higher than the crystal dispersion temperature (Tcd) of the polyolefin resin, and particularly preferably 5 ° C. or more lower than Tcd. Drying is preferably performed until the residual cleaning solvent is 5% by mass or less, more preferably 3% by mass or less, with the microporous membrane being 100% by mass (dry mass). When the residual cleaning solvent is within the above range, the porosity of the microporous membrane is maintained when the subsequent microporous membrane stretching step and heat treatment step are performed, and deterioration of permeability is suppressed.

(6) Second stretching step It is preferable to stretch the dried microporous membrane in at least a 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. Although the extending | stretching temperature in this process is not specifically limited, Usually, 90-135 degreeC is preferable, More preferably, it is 95-130 degreeC.

  In this step, the stretching ratio (area stretching ratio) in the uniaxial direction of stretching of the microporous membrane is 1.0 to 2.0 times in the machine direction or the width direction in the case of uniaxial stretching. 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 still more preferably 1.2 times or more. The upper limit is preferably 3.5 times or less. The stretching ratio in the machine direction and the width direction may be the same or different from each other in the machine direction and the width direction. In addition, the draw ratio in this process means the draw ratio of the microporous film just before being provided to the next process on the basis of the microporous film immediately before this process.

(7) Heat treatment Moreover, the microporous film after drying can be heat-treated. The crystal is stabilized by heat treatment, and the lamella is made uniform. As the heat treatment method, heat setting treatment and / or heat relaxation treatment can be used. The heat setting treatment is a heat treatment in which heating is performed while keeping the dimensions of the film unchanged. The thermal relaxation treatment is a heat treatment that heat-shrinks the film in the machine direction or the width direction during heating. The heat setting treatment is preferably performed by a tenter method or a roll method. For example, as a thermal relaxation treatment method, a method disclosed in Japanese Patent Application Laid-Open No. 2002-256099 can be given. The heat treatment temperature is preferably within the range of Tcd to Tm of the polyolefin resin, more preferably within the range of the stretching temperature of the microporous membrane ± 5 ° C, and particularly preferably within the range of the second stretching temperature ± 3 ° C of the microporous membrane.

(8) Crosslinking treatment and hydrophilization treatment Further, a crosslinking treatment and a hydrophilization treatment can also be performed on the microporous membrane after bonding or stretching. For example, the crosslinking treatment is performed by irradiating the microporous film 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 meltdown temperature of the microporous membrane is increased by the crosslinking treatment. The hydrophilic treatment can be performed by monomer grafting, surfactant treatment, corona discharge, or the like. Monomer grafting is preferably performed after the crosslinking treatment.

2. Porous layer The porous layer which the battery separator according to the present invention has includes a vinylidene fluoride-hexafluoropropylene copolymer and an acrylic resin. This makes it possible to achieve both adhesiveness when dry and adhesiveness when wet.
[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 electrolyte and is suitable for a non-aqueous electrolyte. High chemical and physical stability. For this reason, the porous layer containing this copolymer exhibits adhesiveness when wet, and can sufficiently maintain affinity with the electrolyte even when used at high temperatures.

  The vinylidene fluoride-hexafluoropropylene copolymer contains a monomer unit having a hydrophilic group. As a result, the active material existing on the electrode surface and the binder component in the electrode can interact and be firmly bonded.

  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.

  In order to introduce a hydrophilic group into vinylidene fluoride-hexafluoropropylene copolymer, for example, in the synthesis of 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 a main chain by copolymerization and 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 5 mol. % Is preferable, and 4 mol% is more preferable. By setting the content of the monomer unit having a hydrophilic group within the above-mentioned preferable range, there is an interaction between the hydrophilic group and the hydrophilic part hydrophilic group of the active material surface in the electrode or the binder component in the electrode. It can work and have sufficient wet adhesion. If 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 electrolyte can be kept low, and high wet adhesion can be obtained. Moreover, when particles are included in the porous layer, dropping of the particles can be prevented 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 or the like. For example, in the case of a carboxylic acid group, it can be determined from the absorption intensity ratio of C—H stretching vibration and carboxyl group C═O stretching vibration using FT-IR with reference to a homopolymer.

  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 3 mol%. Is more preferable, and 2.5 mol% is more preferable. When the content of the hexafluoropropylene monomer unit is less than 0.3 mol%, the polymer crystallinity increases and the degree of swelling with respect to the electrolytic solution decreases, so that sufficient wet adhesion cannot be obtained. On the other hand, if the content of hexafluoropropylene exceeds 3 mol%, it will swell excessively with respect to the electrolyte solution, and the wet adhesiveness will decrease.

  The vinylidene fluoride-hexafluoropropylene copolymer can be obtained by a known polymerization method. As a known polymerization method, for example, a method exemplified in JP-A-11-130821 can be mentioned. In this method, ion-exchanged water, maleic acid monomethyl ester, vinylidene fluoride and hexafluoropropylene are placed in an autoclave to perform suspension polymerization, and then the polymer slurry is dehydrated, washed with water and dried to obtain a polymer powder. . At this time, methylcellulose as a suspending agent and diisopropyl peroxydicarbonate as a radical initiator can be used as appropriate.

  The vinylidene fluoride-hexafluoropropylene copolymer may be obtained by further polymerizing another monomer unit other than the monomer unit having a hydrophilic group as long as the characteristics are not impaired. Examples of the monomer other than the monomer unit having a hydrophilic group include monomer units such as tetrafluoroethylene, trifluoroethylene, trichloroethylene, 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 2,000,000, more preferably 1,500,000. By setting the weight average molecular weight of the vinylidene fluoride-hexafluoropropylene copolymer within the above preferred range, the time for dissolving the copolymer in the solvent is not extremely long and can be used without reducing the productivity. Moreover, moderate gel strength can be maintained when it swells in electrolyte solution. In addition, the said weight average molecular weight is a polystyrene conversion value by a gel permeation chromatography.

[2] Acrylic resin An acrylic resin is a copolymer containing butyl acrylate units. The porous layer containing an acrylic resin can exhibit adhesiveness when dried. Moreover, when a particle | grain is contained in a porous layer, the softness | flexibility of a coating film increases with butyl acrylate and the effect which suppresses drop-off | omission of particle | grains can also be anticipated.

  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 adequate flexibility. As a result, the wet adhesion can be improved.

  The lower limit of the content of butyl acrylate units 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 preferable range, the porous layer can be provided with appropriate flexibility, and the coating film can be prevented from falling off. By setting the content of the butyl acrylate unit in the acrylic resin within the above preferred range, a good balance between the adhesiveness during drying and the adhesiveness during wetness can be easily obtained.

  The acrylic resin can be obtained by a known polymerization method, for example, a method exemplified in JP2013-206946A. Ion exchange water, n-butyl acrylate, and acrylonitrile are charged into an autoclave equipped with a stirrer and emulsion polymerization is performed to obtain an aqueous dispersion of polymer particles. The water in the system is replaced with N-methyl-2-pyrrolidone, and an acrylic resin is obtained. And the like. At the time of polymerization, potassium persulfate as a radical polymerization initiator and t-dodecyl mercaptan as a molecular weight regulator may be appropriately used.

  As for the content of the acrylic resin, the lower limit is preferably 5% by mass, and the upper limit is preferably 40% by mass, more preferably 20% by mass with respect to the total amount of the vinylidene fluoride-hexafluoropropylene copolymer and the acrylic resin. It is. In particular, the upper limit value is more preferably less than 10% by mass. By setting it within the above preferred range, sufficient adhesiveness during drying and adhesiveness during wetness can be obtained. By setting the content of the acrylic resin to 5% by mass or more, the wet adhesiveness and the dry adhesiveness can be more fully compatible. When the content of the acrylic resin is 40% by mass or less, the effect of adhesiveness when wet by the vinylidene fluoride-hexafluoropropylene copolymer is easily obtained.

[3] Particles The porous layer of the battery separator according to the present invention may include particles. By including particles in the porous layer, the probability of a short circuit between the positive electrode and the negative electrode can be lowered, and an 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, Examples include molybdenum sulfide, mica, and boehmite. 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 particles.

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

The average particle diameter 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 dropout of the particles. It is as follows. The average flow pore size was measured according to JISK3832 and ASTM F316-86, and for example, measured using a palm porometer (PMI, CFP-1500A) in the order of Dry-up and Wet-up. For the wet-up, pressure was applied to a microporous membrane sufficiently immersed with Galwick (trade name) manufactured by PMI, whose surface tension is known, and the maximum pore size was defined as the pore size converted from the pressure at which air began to penetrate. As for the average flow pore size, the pore size was converted from the pressure at the point where the curve showing the slope of 1/2 of the pressure and flow curve in Dry-up measurement and the curve of Wet-up measurement intersected. The following formula was used for conversion of pressure and pore diameter.
d = C · γ / P
In the above formula, “d (μm)” is the pore diameter 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 dropping. More preferably, it is 1.0 μm or more and 3.0 μm or less. The average particle diameter of the particles can be measured using a laser diffraction method or 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 apparatus (manufactured by Nikkiso Co., Ltd., Microtrac HRA) and accumulated 50% from the small particle side in terms of volume. The value of the particle size (D50) at the time is preferably the average particle size. Examples of the shape of the particles include a true spherical shape, a substantially spherical shape, a plate shape, and a needle shape, but are not particularly limited.

[4] Physical properties of 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. If the film thickness per side is 0.5 μm or more, the adhesiveness when wet and the adhesiveness when dry can be secured. 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 that will be developed 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 preferred range, an increase in electrical resistance of the film can be prevented, a large current can be passed, and the film strength can be maintained.

[5] Method for Producing 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 order.
(A) A step of obtaining a fluororesin solution in which a vinylidene fluoride-hexafluoropropylene copolymer is dissolved in a solvent (b) A step of adding an acrylic resin solution to the fluororesin solution and mixing to obtain a coating solution (c) The process of applying the coating liquid to the microporous membrane, immersing it in a coagulation bath, washing and drying

(A) Step of obtaining fluororesin solution The solvent is not particularly limited as long as it dissolves vinylidene fluoride-hexafluoropropylene copolymer, dissolves or disperses acrylic resin, and is miscible 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 particles are dispersed in advance. A vinylidene fluoride-hexafluoropropylene copolymer is dissolved in a solvent, particles are added thereto while stirring, and the mixture is preliminarily dispersed by stirring with a disper for a certain time (for example, about 1 hour). Furthermore, a fluororesin solution with less aggregation of particles can be obtained through a step of dispersing particles (dispersion step) using a bead mill or a paint shaker.

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

  The acrylic resin solution used in this step is a solution in which an 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. It is preferable from the viewpoint of operability that the acrylic resin solution is obtained by polymerizing the acrylic resin and then replacing the solvent by adding N-methyl-2-pyrrolidone and distilling.

  When providing a porous layer containing particles, it is important to add (after-insert) an acrylic resin solution after dispersing the particles in the fluororesin solution. That is, it is important that acrylic resin does not enter in the dispersion step. When vinylidene fluoride-hexafluoropropylene copolymer, acrylic resin, and particles are added to the solvent at the same time, the hydrophilic group contained in the vinylidene fluoride-hexafluoropropylene copolymer and butyl acrylate contained in the acrylic resin are Since it is presumed that the coating solution gradually begins to gel due to heat and shear during dispersion, it is industrially unsuitable. Furthermore, thin film coating with a porous layer thickness of 3 μm or less becomes difficult due to the effect 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 is possible, and the storage stability of the coating liquid is also improved.

(C) The step of applying the coating solution to the microporous membrane, immersing it in a coagulation bath, washing and drying This step applies the coating solution to the microporous membrane and immersing the applied microporous membrane in the coagulation solution 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. Thereby, the separator for batteries provided with the microporous membrane and the porous layer on the surface is obtained.

  The method of applying the coating liquid to the microporous film may be a known method, for example, dip coating method, reverse roll coating method, gravure coating method, kiss coating method, roll brush method, spray coating method, Examples thereof include an air knife coating method, a Mayer bar coating method, a pipe doctor method, a blade coating method, and a die coating method, and these methods can be used alone or in combination.

  The coagulation liquid is preferably water, and is preferably an aqueous solution containing 1% by mass to 20% by mass of a good solvent for the vinylidene fluoride-hexafluoropropylene copolymer and the acrylic resin, more preferably 5% by mass. As mentioned above, it is the aqueous solution containing 15 mass% or less. Examples of the good solvent include N-methyl-2-pyrrolidone, N, 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, but 10 seconds is sufficient.

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

  Battery separators according to the present invention include secondary batteries such as nickel-hydrogen batteries, nickel-cadmium batteries, nickel-zinc batteries, silver-zinc batteries, lithium ion secondary batteries, lithium polymer secondary batteries, and lithium-sulfur batteries. It can be used as a battery separator. 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 bending strength when wet is not particularly limited, but 30 N is sufficient. By setting the bending strength when wet within the above preferable range, partial release at the interface between the separator and the electrode can be suppressed, and increase in battery internal resistance and battery characteristic deterioration can be suppressed.

  The adhesiveness during drying of the battery separator can be evaluated by the bending strength during drying, and the lower limit of the bending strength during drying is preferably 7N or more, more preferably 9N or more. The upper limit of the bending strength during drying is not particularly defined, but 30N is sufficient. By setting the bending strength at the time of drying within the above preferable range, it becomes easy to suppress the deflection and distortion of the wound electrode body.

  The battery separator preferably has a wet bending strength of 14 N or more and a dry bending strength of 7 N or more from the viewpoint of the balance between the dry adhesive property and the wet adhesive property.

  Hereinafter, although an example is shown and explained concretely, the present invention is not restrict | limited at all by these examples. In addition, the measured value in an Example is a value measured with the following method.

1. Bending strength when wet Generally, when a binder of a fluororesin is used for the positive electrode and a porous layer containing the fluororesin is provided on the separator, the adhesion is easily secured by mutual diffusion of the fluororesins. On the other hand, a binder other than a fluororesin is used for the negative electrode, and the diffusion of the fluororesin is less likely to occur. Therefore, the negative electrode is less likely to have adhesion with the separator than the positive electrode. Therefore, in this measurement, the adhesion between the separator and the negative electrode was evaluated using the bending strength described below as an index.
(1) Production of negative electrode An aqueous solution containing 1.5 parts by mass of carboxymethylcellulose was added to 96.5 parts by mass of artificial graphite and mixed, and further 2 parts by mass of styrene butadiene latex was added and mixed as a solid content to mix the negative electrode. An agent-containing slurry was obtained. This negative electrode mixture-containing slurry is uniformly applied to both surfaces of a negative electrode current collector made of a copper foil having a thickness of 8 μm and dried to form a negative electrode layer. The density of the negative electrode layer excluding the body was 1.5 g / cm ³ to produce a negative electrode.
(2) Production of test winding body The negative electrode (machine direction: 161 mm × width direction: 30 mm) created above and the separator (machine direction: 160 mm × width direction: 34 mm) created in Examples and Comparative Examples were stacked to form a metal plate The separator and the negative electrode were wound with the length (300 mm, width 25 mm, thickness 1 mm) as the winding core so that the separator was inside, and the metal plate was pulled out to obtain a test winding. The test wound body had a length of about 34 mm and a width of about 28 mm.
(3) Measuring method of bending strength when wet A test roll is placed on a laminate film (length 110 mm, width 65 mm, thickness 0.12 mm) made of aluminum and polypropylene, and the laminate film is moved in the length direction. Folded in half and welded the two sides of the laminate film into a bag shape with one side open. 500 μL of electrolytic solution in which LiPF ₆ is dissolved 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 from the opening in the glove box and impregnated in the test winding body. Then, one side of the opening was sealed with a vacuum sealer.
Next, the test winding body enclosed in the laminate film was sandwiched between two gaskets (thickness 1 mm, 5 cm × 5 cm), and 98 with a precision heating and pressing apparatus (CYPT-10, manufactured by Shinto Kogyo Co., Ltd.). The mixture was pressurized at 0.6 ° C. for 2 minutes and allowed to cool at room temperature. About the winding body for a test enclosed in the laminated film after pressurization, the bending strength at the time of wetness was measured using a universal testing machine (manufactured by Shimadzu Corporation, AGS-J) as shown in the schematic diagram of FIG. . Details are described below.
Two aluminum L-shaped angles 4 (thickness 1 mm, 10 mm x 10 mm, length 5 cm) are arranged in parallel so that the 90 ° part is on top, and the 90 ° part is used as a fulcrum. The distance was fixed to 15 mm. Align the midpoint of the side in the width direction of the test winding body (about 28 mm) with the 7.5 mm point, which is the middle distance between the fulcrums of the two aluminum L-shaped angles, and adjust the length direction of the L-shaped angle. The test winding body was arranged so as not to protrude from the side.
Next, the length direction side (about 34 mm) of the test winding body does not protrude from the length direction side of the aluminum L-shaped angle 3 (thickness 1 mm, 10 mm × 10 mm, length 4 cm) as an indenter. The 90 ° portion of the aluminum L-shaped angle 3 is aligned with the midpoint of the side in the width direction of the test winding body, and the aluminum L-shaped angle 3 is placed so that the 90 ° portion is down. It was fixed to the load cell (load cell capacity 50N) of the universal testing machine. Regarding the measured value at the point of 0.5 mm stroke after the test load became 0.05 N at a load speed of 0.5 mm / min, the average value of the three test windings was defined as the bending strength when wet.

2. Bending strength during drying (1) Production of negative electrode A negative electrode having the same bending strength when wet was used.
(2) Preparation of test winding body A test wound body having the same bending strength when wet was used.
(3) Measuring method of bending strength at the time of drying The prepared test winding body is sandwiched between two gaskets (thickness 1 mm, 5 cm × 5 cm), and a precision heating and pressing apparatus (CYPT-, manufactured by Shinto Kogyo Co., Ltd.). In 10), pressure was applied at 90 ° C. and 0.6 MPa for 2 minutes, and the mixture was allowed to cool at room temperature. About the test winding body after pressurization, as shown in FIG. Using a universal testing machine (manufactured by Shimadzu Corp., AGS-J) in the same way as the measurement method of bending strength when wet, measure three test rolls under the following conditions to obtain the maximum test force. Was the bending strength when dry.
Distance between fulcrums: 15mm
Cell capacity: 50N
Load speed: 0.5mm / min

3. Evaluation of Powder Falling A separator was fixed flat on the bottom surface (bottom area 5.5 cm × 6 cm) of a weight (1143 g) with a handle so that the porous layer became the surface without wrinkles. On the drawing paper (C-55, made by Daio Paper Co., Ltd.), the distance of 20 cm was moved 10 times and the weight was moved, and then the amount of the porous layer transferred to the drawing paper was confirmed. An arbitrary range of 5 mm × 5 mm was selected at 10 locations, and the number of the fallen coating films having a thickness of 150 μm or more was counted using an optical microscope.
Good: The total of film dropouts in 10 places is 50 or less. Bad: The total of film dropouts 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 Corporation), 20 points were measured under the condition of a weight of 0.01 N using a carbide spherical measuring element 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 suspension polymerization using vinylidene fluoride, hexafluoropropylene and maleic acid monomethyl ester as starting materials. The resulting vinylidene fluoride-hexafluoropropylene copolymer has a weight average molecular weight of 1,500,000 and a molar ratio of vinylidene fluoride monomer unit / hexafluoropropylene monomer unit / monomethyl monomethyl ester monomer unit of 98. It was confirmed by NMR measurement that the ratio was 0.5 / 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). A 5% by mass acrylic resin solution was obtained. 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.
[Preparation of battery separator]
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 size 1.1 μm) were added as inorganic particles while stirring with a disper. The mixture was pre-stirred with a disper for 1 hour at 2000 rpm. Next, using a dyno mill (Dyno mill multilab made by Shinmaru Enterprises (1.46 L container, filling rate 80%, φ0.5 mm alumina beads)) under the conditions of a flow rate of 11 kg / h and a peripheral speed of 10 m / s. Three times of treatment was performed to obtain a dispersion. The dispersion is mixed with an acrylic resin solution, stirred with a three-one motor with a stirring blade at 500 rpm for 30 minutes, and filtered to obtain 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 solution is applied to both sides of a polyethylene microporous film having a thickness of 7 μm by a dip coating method, immersed in an aqueous solution, washed with pure water, dried at 50 ° C., and a battery separator having a thickness of 11 μm is obtained. 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. When the obtained battery separator was evaluated for powder falling, it was good.

Example 3
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): 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 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 70:18:12. A battery separator was obtained in the same manner as in Example 1.

Example 6
A coating liquid 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 liquid prepared such 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) as inorganic particles was used.

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) 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 10 μm.

Comparative Example 1
The same as in Example 1 except that the 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) was 70:30 was used. Thus, a battery separator was obtained. When the obtained battery separator was evaluated for powder omission, it was defective.

Comparative Example 2
The same as in Example 2 except that a coating solution prepared using PVdF homopolymer (manufactured by Kureha Co., Ltd., KF # 7300 (molecular weight of 1 million or more)) was used in place of the vinylidene fluoride-hexafluoropropylene copolymer. Thus, a battery separator was obtained.

Comparative Example 3
Instead of vinylidene fluoride-hexafluoropropylene copolymer (a), vinylidene fluoride-hexafluoropropylene copolymer (b) having a hexafluoropropylene monomer content of 4.5 mol% (manufactured by Arkema Co., Ltd.) A battery separator was obtained in the same manner as in Example 2 except that a coating liquid prepared using kynar2801 (Molar ratio of VdF / HFP was 95.5 / 4.5, molecular weight less than 500,000) was used. .

Comparative Example 4
[Synthesis of acrylic resin]
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. The obtained acrylic resin was confirmed by NMR measurement to have a Tg of 10 ° C. and a molar ratio of acrylonitrile monomer unit / ethyl acrylate monomer unit of 37/63. A battery separator was obtained in the same manner as in Example 2 except that the coating solution prepared using this acrylic resin was used. When the powder separator was evaluated for the obtained battery separator, it was good.

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

Comparative Example 6
The coating solution 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 battery separator was obtained in the same manner as Example 2 except for the above.

Comparative Example 7
Inorganic particles and fluoride such 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 vinylidene-hexafluoropropylene copolymer (a), an acrylic resin, and N-methyl-2-pyrrolidone were simultaneously mixed and dispersed to prepare a coating liquid. The film could not be applied.

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 9 μm.

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

  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 relative to the total mass of the fluororesin and the acrylic resin. “After-loading” in preparing the coating material means adding an acrylic resin solution to a fluororesin solution in which particles are dispersed. “Simultaneous input” means that a fluororesin solution, an acrylic resin solution, and particles are simultaneously added and dispersed.

1: Negative electrode 2: Separator 3: Aluminum L-shaped angle 4: Aluminum L-shaped angle 5: Laminate film

Claims (14)

A microporous membrane,
A porous layer provided on at least one side of the microporous membrane,
The porous layer includes 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,
The battery separator, wherein the acrylic resin includes a butyl acrylate monomer unit. The battery separator according to claim 1, wherein the porous layer contains particles. The battery separator according to claim 1 or 2, wherein the vinylidene fluoride-hexafluoropropylene copolymer contains 0.1 mol% to 5 mol% of a monomer unit having a hydrophilic group. 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 1. The battery separator according to any one of claims 1 to 4, wherein the acrylic resin is an acrylic copolymer containing a butyl acrylate unit and an acrylonitrile unit. The battery separator according to any one of claims 1 to 5, wherein the vinylidene fluoride-hexafluoropropylene copolymer has a weight average molecular weight of 500,000 to 2,000,000. The battery separator according to any one of claims 1 to 6, wherein a content of a 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 7, wherein the wet bending strength is 14 N or more and the dry bending strength is 7 N or more. The content of the particles is 50% by mass or more and 85% by mass or less based on the total weight of the vinylidene fluoride-hexafluoropropylene copolymer, the acrylic resin, and the particles. 9. The battery separator according to any one of 8 above. The battery separator according to claim 1, wherein the porous layer has a thickness of 0.5 μm or more and 3 μm or less per side. The battery separator according to any one of claims 2 to 10, wherein the particles include at least one selected from the group consisting of alumina, titania, and boehmite. The battery separator according to any one of claims 2 to 11, wherein an 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 12, wherein the microporous membrane is a polyolefin microporous membrane. The manufacturing method of the battery separator of Claims 1-13 including the following processes (a)-(c) sequentially.
(A) A step of obtaining a fluororesin solution in which a vinylidene fluoride-hexafluoropropylene copolymer is dissolved in a solvent (b) A step of adding an acrylic resin solution to the fluororesin solution and mixing to obtain a coating solution (c) The process of applying the coating liquid to the microporous membrane, immersing it in a coagulation bath, washing and drying

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