JP6766411B2 - Battery separator and its manufacturing method - Google Patents

Description

The present invention relates to a battery separator and a method for manufacturing the same.

Non-aqueous electrolyte secondary batteries, especially lithium ion secondary batteries, are widely used in small electronic devices such as mobile phones and mobile information terminals, and cylindrical batteries, square batteries, laminated batteries, etc. have been developed. There is. Generally, these batteries are composed of an electrode body (laminated electrode body) in which a positive electrode and a negative electrode are laminated via a separator, an electrode body (wound electrode body) wound in a spiral shape, and a non-aqueous electrolyte solution. Has a configuration housed in.

Conventional separators for non-aqueous electrolyte secondary batteries mainly use a microporous membrane made of polyolefin resin, and when the battery generates abnormal heat, the pores of the separator are blocked, which suppresses the flow of current and causes ignition. I'm preventing it.

In recent years, attempts have been made to improve battery characteristics by providing a porous layer on one side or both sides of a microporous membrane. For example, there are separators provided with a porous layer containing a fluororesin or an acrylic resin in order to impart functions such as electrode adhesiveness (Prior Documents 1 to 8). In addition, when inorganic particles are added to the porous layer, sharp metal penetrates the battery due to an accident or the like, causing a sudden short circuit and preventing heat generation from melting and shrinking of the separator and suppressing the expansion of the short circuit between the electrodes. Can be done.

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, monomer derived from 1,3-butadiene, The NMP solution containing the methacrylic acid monomer and the polymer containing the butyl acrylate monomer) is stirred with a primary mixer to prepare the NMP solution of the binder, and then the adjusted NMP solution and the alumina particles are mixed. Described is an organic separator with a porous film obtained by applying a dispersed and prepared slurry to a polypropylene separator.

In the example 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 film. The separator obtained in the above is described.

In Example 1 of Patent Document 5, a VdF-HFP copolymer (HFP unit 0.6 mol%) and a VdF-HFP copolymer (weight average molecular weight 470,000, HFP unit 4.8 mol%) are dimethylacetamide. And a separator which is dissolved in a tripropylene glycol solution and coated on a polyethylene microporous film to form a porous layer are described.

In Example 1 of Patent Document 6, PVdF (weight average molecular weight 500,000) and VdF-HFP copolymer (weight average molecular weight 400,000, HFP unit 5 mol%) were dissolved in a solution of dimethylacetamide and tripropylene glycol. A separator in which a porous layer is formed by applying this to a polyethylene microporous film is described.

In Example 1 of Patent Document 7, PVdF (weight average molecular weight 700,000) and VdF-HFP copolymer (weight average molecular weight 470,000, HFP unit 4.8 mol%) are dissolved in a solution of dimethylacetamide and tripropylene glycol. A separator in which a porous layer is formed by applying this to a polyethylene microporous film is described.

In Example 1 of Patent Document 8, PVdF (weight average molecular weight 350,000) and VdF-HFP polymer (weight average molecular weight 270,000, HFP copolymer 4.8 mol%) are dissolved in a solution of dimethylacetamide and tripropylene glycol. A separator in which a porous layer is formed by applying this to a polyethylene microporous film is described.

The separators disclosed in Patent Documents 1 to 8 and the layers arranged between the electrodes and the separators all contain a polyvinylidene fluoride resin.

Re-table 1999-036981 Japanese Unexamined Patent Publication No. 2013-206846 Japanese Unexamined Patent Publication No. 2013-122009 Special Table 2013-519206 Patent No. 5282179 Patent No. 5282180 Patent No. 5282181 Patent No. 534208

In recent years, non-aqueous electrolyte secondary batteries are expected to be developed for large-scale applications such as large tablets, mowers, electric motorcycles, electric vehicles, hybrid vehicles, and small vessels, and large batteries are expected to become widespread accordingly.

The wound electrode body is manufactured by winding a positive electrode and a negative electrode while applying tension to each member via a separator. At this time, the positive electrode and the negative electrode coated on the metal current collector hardly expand or contract with respect to the tension, but the separator is wound while extending to some extent in the mechanical direction. If this wound body is left for a while, the separator part will slowly shrink and try to return to its original length. As a result, a force in the parallel direction is generated at the boundary surface between the electrode and the separator, and the wound electrode body (particularly the flatly wound electrode body) is liable to be bent or distorted. Further, these problems become apparent due to the widening and lengthening of the separator due to the increase in size of the battery, and there is a concern that the yield at the time of production may deteriorate. It is expected that the separator is required to have more adhesiveness to the electrode than ever in order to suppress the occurrence of bending and distortion of the wound electrode body. In the present specification, the bending strength at the time of drying obtained by the measuring method described later is used as an index for this adhesiveness.

Further, when the electrode body is conveyed, the electrode and the separator are peeled off unless the members are sufficiently adhered to each other, and the electrode body cannot be conveyed with good yield. The problem of adhesiveness during transportation becomes apparent due to the increase in size of the battery, and there is a concern that the yield may deteriorate. Therefore, it is expected that the separator is required to have a high peeling force at the time of drying, which is difficult to peel off from the electrode.

Furthermore, especially in a laminated battery, it is harder to apply pressure than a square or cylindrical battery to which pressure is applied by the exterior body, and due to the expansion and contraction of the electrode due to charging and discharging, the interface between the separator and the electrode Partial release is likely to occur. As a result, the battery swells, the resistance inside the battery increases, and the cycle performance deteriorates. Therefore, the separator is required to have adhesiveness to the electrode in the battery after the electrolytic solution is injected. In the present specification, the bending strength at the time of wetting obtained by the measuring method described later is used as an index for this adhesiveness. If this strength is large, improvement of battery characteristics such as suppression of battery swelling after repeated charging and discharging is expected.

In the prior art, there is a trade-off relationship between the bending strength during drying, the peeling force during drying, and the bending strength during wetting, and it is extremely difficult to satisfy all the physical properties. The present invention aims to provide a battery separator that satisfies flexural strength during drying, peeling strength during drying, and bending strength during wetness in preparation for the widespread use of large-sized batteries (particularly laminated batteries) that will advance in the future. Is.

The bending strength when wet as used herein refers to the adhesiveness between the separator and the electrode when the separator contains an electrolytic solution. The bending strength during drying and the peeling force during drying represent the adhesiveness to the interface between the separator and the electrode when the separator contains substantially no electrolytic solution. It should be noted that 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 of the present invention and the method for manufacturing the same have the following configurations. That is,
(1) A microporous film and a porous layer provided on at least one side of the microporous film are provided, and the porous layer contains a vinylidene fluoride-hexafluoropropylene copolymer (A) and a vinylidene fluoride unit. The polymer (B) containing the polymer (B) and an acrylic resin are contained, and the vinylidene fluoride-hexafluoropropylene copolymer (A) contains a hydrophilic group and a hexafluoropropylene unit in an amount of 0.3 mol% to 3 mol%. The polymer (B) containing a vinylidene fluoride unit is a separator for a battery having a melting point of 60 ° C. or higher and 145 ° C. or lower and a weight average molecular weight of 100,000 or higher and 750,000 or lower.
(2) In the battery separator of the present invention, it is preferable that the vinylidene fluoride-hexafluoropropylene copolymer (A) has a weight average molecular weight of more than 750,000 and 2 million or less.
(3) In the battery separator of the present invention, it is preferable that the porous layer contains particles.
(4) The battery separator of the present invention has a weight in which the content of the vinylidene fluoride-hexafluoropropylene copolymer (A) contains the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride unit. A polymer containing 15% by weight or more and 85% by weight or less based on the total weight of the coalescence (B), and the content of the acrylic resin is the vinylidene fluoride-hexafluoropropylene copolymer (A) and the vinylidene fluoride unit. It is preferably 4% by weight or more and 40% by weight or less with respect to the total weight of (B) and the acrylic resin.
(5) In the battery separator of the present invention, the acrylic resin is preferably a copolymer of a (meth) acrylic acid ester and a monomer having a cyano group.
(6) The battery separator of the present invention is preferably a copolymer in which the acrylic resin contains butyl acrylate.
(7) In the battery separator of the present invention, the acrylic resin is preferably a copolymer of butyl acrylate and acrylonitrile.
(8) The battery separator of the present invention preferably has a butyl acrylate content of 50 mol% to 75 mol% in the acrylic resin.
(9) The battery separator of the present invention preferably has a hydrophilic group content of vinylidene fluoride-hexafluoropropylene copolymer (A) of 0.1 mol% to 5 mol%.
(10) The battery separator of the present invention preferably has a flexural strength of 4 N or more when wet, a flexural strength of 5 N or more when dry, and a peeling force of 8 N / m when dry.
(11) The battery separator of the present invention preferably has a particle content of 50% by weight or more and 90% by weight or less with respect to the total weight of the porous layer.
(12) The battery separator of the present invention preferably contains at least one type of particles selected from the group consisting of alumina, titania, and boehmite.
(13) In the battery separator of the present invention, the thickness of the porous layer is preferably 0.5 to 3 μm per side.
(14) In the battery separator of the present invention, the microporous membrane is preferably a polyolefin microporous membrane.
In order to solve the above problems, the method for producing a microporous polyolefin membrane of the present invention has the following constitution. That is,
(15) The battery separator of the present invention is
(1) A step of obtaining a fluororesin solution in which a vinylidene fluoride-hexafluoropropylene copolymer (A) and a polymer (B) containing a vinylidene fluoride unit are dissolved in a solvent.
(2) A step of adding an acrylic resin solution in which an acrylic resin is dissolved in a solvent to a fluororesin solution and mixing them to obtain a coating liquid.
(3) The step of applying the coating liquid to the microporous film, immersing it in the coagulating liquid, washing and drying is sequentially included, and the vinylidene fluoride-hexafluoropropylene copolymer (A) has a hydrophilic group and hexa. The polymer (B) containing 0.3 mol% to 3 mol% of fluoropropylene units and containing the vinylidene fluoride unit has a melting point of 60 ° C. or higher and 145 ° C. or lower, and a weight average molecular weight of 100,000 or more and 750,000 or lower. The method for producing a battery separator according to any one of claims 1 to 14, wherein the acrylic resin contains butyl acrylate.

According to the present invention, it is possible to provide a battery separator that satisfies the flexural strength during drying, the peeling force during drying, and the bending strength during wetness in preparation for the widespread use of larger batteries that will progress in the future.

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

The outline of the battery separator having at least the microporous membrane and the porous layer of the present invention will be described, but of course, the present invention is not limited to this representative example.
1. 1. Microporous Membrane First, the microporous membrane of the present invention will be described.
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 non-woven fabric or a microporous membrane 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 is mainly composed of polyethylene resin or polypropylene resin. 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 Film The method for producing a polyolefin microporous film is not particularly limited as long as a polyolefin microporous film 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).
(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 it 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 sheet (5) Step of drying the sheet after removing the film-forming solvent.

Hereinafter, each step will be described.
(1) Preparation Step of Polyolefin Solution After adding an appropriate solvent for film formation to each polyolefin resin, melt-knead 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 blending 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 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 sheet is preferably stretched at a predetermined magnification 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. The drying is preferably carried out with the microporous membrane as 100% by mass (dry weight) 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.

2. Porous layer In the present invention, the porous layer contains a vinylidene fluoride-hexafluoropropylene (VdF-HFP) copolymer (A), a polymer (B) containing a vinylidene fluoride unit, and an acrylic resin. Each resin will be described below.
[1] Vinylidene Fluoride-Hexafluoropropylene (VdF-HFP) Copolymer (A)
The vinylidene fluoride-hexafluoropropylene copolymer (A) used in the present invention contains a hydrophilic group and contains 0.3 mol% to 3 mol% of hexafluoropropylene. The copolymer (A) has a high affinity for a non-aqueous electrolytic solution, has high chemical and physical stability, exhibits bending strength when wet, and can be used at high temperatures with the electrolytic solution. Affinity can be sufficiently maintained.

Since the vinylidene fluoride-hexafluoropropylene copolymer (A) has a hydrophilic group, it can be firmly adhered to the active material existing on the electrode surface and the binder component in the electrode. It is presumed that such adhesive force is due to hydrogen bonds. Examples of the hydrophilic group include a hydroxyl group, a carboxylic acid group, a sulfonic acid group, and salts thereof. In particular, carboxylic acid groups and carboxylic acid esters are preferable.

When a hydrophilic group is introduced into vinylidene fluoride, for example, in the synthesis of vinylidene fluoride-hexafluoropropylene copolymer (A), hydrophilicity such as maleic anhydride, maleic acid, maleic acid ester, and maleic acid monomethyl ester is used. Examples thereof include a method of introducing a monomer having a group into a main chain by copolymerizing it and a method of introducing it as a side chain by grafting. The hydrophilic group denaturation rate 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 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 hydrophilic groups in the vinylidene fluoride-hexafluoropropylene copolymer (A) is preferably 0.1 mol% or more, more preferably 0.3 mol% or more, and the upper limit is preferably 5 mol% or less. , More preferably 4 mol% or less. If the content of the hydrophilic group exceeds 5 mol%, the polymer crystallinity becomes too low, the degree of swelling with respect to the electrolytic solution becomes high, and the bending strength at the time of wetting deteriorates. Further, when the porous layer contains particles, it is possible to suppress the particles from falling off by setting the content of the hydrophilic groups within the above preferable range.

The lower limit of the hexafluoropropylene content in the vinylidene fluoride-hexafluoropropylene copolymer (A) is preferably 0.3 mol% or more, more preferably 0.5 mol% or more, and the upper limit is 3 mol% or less. It is preferably, more preferably 2.5 mol% or less. If the content of hexafluoropropylene 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 it is difficult to obtain sufficient bending strength when wet. On the other hand, if it exceeds 3 mol%, it swells too much with respect to the electrolytic solution, and the bending strength at the time of wetting decreases.

The lower limit of the content of the vinylidene fluoride-hexafluoropropylene copolymer (A) is preferably 15% by weight or more, more preferably 15% by weight or more, based on the total weight of the copolymer (A) and the polymer (B). It is 25% by weight or more, and the upper limit value is preferably 85% by weight or less, more preferably 25% by weight or less.

The lower limit of the weight average molecular weight of the vinylidene fluoride-hexafluoropropylene copolymer (A) is larger than 750,000, preferably 900,000 or more, and the upper limit is preferably 2 million or less, more preferably 1.5 million or less. is there. By setting the weight average molecular weight of the copolymer (A) within the above-mentioned preferable range, the time for dissolving the copolymer (A) in the solvent is not extremely long, and the production efficiency can be improved. In addition, an appropriate gel strength can be maintained when the gel is swollen in the electrolytic solution, and the bending strength when wet is improved. The weight average molecular weight referred to in the present invention is a polystyrene-equivalent value obtained by gel permeation chromatography.

The vinylidene fluoride-hexafluoropropylene copolymer (A) can be obtained by a known polymerization method. As a known polymerization method, for example, the method exemplified in JP-A-11-130821 can be mentioned. 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 then 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 (A) may be a copolymer obtained by further polymerizing a monomer other than the monomer having a hydrophilic group as long as the characteristics are not impaired. Examples of the monomer other than the monomer having a hydrophilic group include a monomer such as tetrafluoroethylene, trifluoroethylene, trichlorethylene, and vinyl fluoride.

[2] Polymer containing vinylidene fluoride unit (B)
The polymer (B) containing a vinylidene fluoride unit used in the present invention has a melting point of 60 ° C. or higher and 145 ° C. or lower, a weight average molecular weight of 100,000 or more and 750,000 or less, and has an affinity for a non-aqueous electrolytic solution. It is highly stable, chemically and physically stable, and provides bending strength during drying and peeling force during drying. Although the mechanism is not clear about this, the polymer (B) becomes fluid under heating and pressurizing conditions that develop bending strength during drying and peeling force during drying, and anchors by entering the porous layer of the electrode. The inventors speculate that this is because the porous layer and the electrode have strong adhesiveness. The polymer (B) contributes to the bending strength during drying and the peeling force during drying, and can contribute to the prevention of bending and distortion of the wound electrode body and the laminated electrode body and the improvement of transportability. The polymer (B) containing a vinylidene fluoride unit is a resin different from the vinylidene fluoride-hexafluoropropylene copolymer (A).

The lower limit of the melting point of the polymer (B) containing vinylidene fluoride units is preferably 60 ° C. or higher, more preferably 80 ° C. or higher, and the upper limit is preferably 145 ° C. or lower, more preferably 140 ° C. or lower. The melting point referred to here is the temperature of the peak top of the endothermic peak at the time of temperature rise measured by the differential scanning calorimetry (DSC) method.

The polymer (B) containing a vinylidene fluoride unit is a resin made of polyvinylidene fluoride or a copolymer having a vinylidene fluoride unit. The polymer (B) can be obtained by a suspension polymerization method similar to that of the copolymer (A). The melting point of the polymer (B) can be adjusted by controlling the crystallinity of the moiety composed of vinylidene fluoride units. For example, when the polymer (B) contains a monomer other than the vinylidene fluoride unit, the melting point can be adjusted by controlling the ratio of the vinylidene fluoride unit. As the monomer other than the vinylidene fluoride unit, one or two kinds of tetrafluoroethylene, trifluoroethylene, trichlorethylene, hexafluoropropylene, vinyl fluoride maleic anhydride, maleic acid, maleic acid ester, maleic acid monomethyl ester and the like are used. You may have more than that. Examples thereof include a method in which the above-mentioned monomer is added when the polymer (B) is polymerized and introduced into the main chain by copolymerization, and a method in which the polymer (B) is introduced as a side chain by grafting. Further, the melting point may be adjusted by controlling the ratio of the head-to-Head bond (-CH _2- CF _2- CF _2- CH _2- ) in units of vinylidene fluoride.

The lower limit of the weight average molecular weight of the polymer (B) containing vinylidene fluoride unit is preferably 100,000 or more, more preferably 150,000 or more, and the upper limit is preferably 750,000 or less, more preferably 700,000 or less. is there.

By keeping the melting point and weight average molecular weight of the polymer (B) containing vinylidene fluoride units within the above-mentioned preferable ranges, the polymer (B) becomes easy to flow under heating and pressurizing conditions, and is sufficiently bent during drying. Strength and peeling power when dried can be obtained. When the melting point of the polymer (B) exceeds the upper limit of the above preferable range, it is necessary to raise the press temperature in the process of manufacturing the wound product in order to obtain bending strength during drying and peeling force during drying. Then, the microporous membrane containing polyolefin as a main component may shrink. Further, when the weight average molecular weight of the polymer (B) exceeds the upper limit of the above preferable range, the amount of entanglement of the molecular chains increases, and there is a possibility that the polymer (B) cannot sufficiently flow under the pressing conditions. When the weight average molecular weight of the polymer (B) is less than the lower limit of the above-mentioned preferable range, the amount of entanglement of the molecular chains is too small, the resin strength is weakened, and the porous layer is likely to be aggregated and broken.

[3] Acrylic Resin Further, by containing the acrylic resin in the porous layer, the bending strength at the time of drying and the peeling power at the time of drying can be improved. The vinylidene fluoride-hexafluoropropylene (VdF-HFP) copolymer (A) and the polymer (B) containing the vinylidene fluoride unit alone satisfy the flexural strength during drying, the bending strength during wetness, and the peeling force during drying. I can't get a separator.

The acrylic resin is preferably a (meth) acrylic acid ester polymer or a copolymer thereof. In the present invention, the (meth) acrylic acid ester represents an acrylic acid ester (acrylate) and a methacrylic acid ester (methacrylate). Examples of the (meth) acrylic acid ester include methyl acrylate, ethyl acrylate, n-butyl acrylate, t-butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, and 2-ethylhexyl methacrylate. And so on. In particular, it preferably contains butyl acrylate. Butyl acrylate can be expected to have the effect of increasing the flexibility of the coating film and suppressing the shedding of particles.

From the viewpoint of adhesiveness to the electrode, the acrylic resin is more preferably a copolymer of a (meth) acrylic acid ester and a monomer having a cyano group. Examples of the monomer having a cyano group include α, β-ethylenically unsaturated monomers having a cyano group, and for example, acrylonitrile or methacrylonitrile is preferable. Further, the acrylic resin is particularly preferably a copolymer of butyl acrylate and acrylonitrile, and the degree of swelling with respect to the electrolytic solution can be adjusted by controlling the molar ratio, and the resin can be given moderate flexibility when wet. Bending strength can also be improved. The lower limit of the content of the butyl acrylate unit in the acrylic resin is preferably 50 mol% or more, more preferably 55 mol% or more, and the upper limit is preferably 75 mol% or less, more preferably 70 mol% or less. By setting the lower limit of the content of the butyl acrylate unit in the acrylic resin to the above-mentioned preferable range, the porous layer can be provided with appropriate flexibility, and the loss of the porous film can be suppressed. Further, by setting the content of the butyl acrylate unit in the acrylic resin within the above preferable range, the balance between the bending strength during drying, the bending strength during wetting, and the peeling force during drying becomes good.

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 the water in the dispersion in which the polymer particles obtained by emulsion polymerization were dispersed in water was replaced with N-methyl-2-pyrrolidone to replace the acrylic resin. Examples include a method of obtaining a solution. In the polymerization reaction, potassium persulfate as a radical polymerization initiator, t-dodecyl mercaptan or the like as a molecular weight modifier may be appropriately used.

The lower limit of the acrylic resin content is 4% by weight or more with respect to the total weight of the vinylidene fluoride-hexafluoropropylene copolymer (A), the polymer (B) containing the vinylidene fluoride unit, and the acrylic resin. It is preferable, more preferably 5% by weight or more, and the upper limit value is preferably 40% by weight or less, more preferably 30% by weight or less, still more preferably 20% by weight or less. By setting the content of the acrylic resin within the above preferable range, the total content of the copolymer (A) and the polymer (B) is kept above a certain level, and the oxidation resistance of the porous layer is maintained. be able to.

By setting the content of the vinylidene fluoride-hexafluoropropylene copolymer (A) and the content of the acrylic resin within the above preferable ranges, the porous layer has bending strength during drying, bending strength during wetness and peeling during drying. Power is gained.

[4] Particles The porous layer in 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. Examples of the particles include inorganic particles and organic particles.

Inorganic particles include calcium carbonate, calcium phosphate, amorphous silica, crystalline glass particles, kaolin, talc, titanium dioxide, alumina, silica-alumina composite oxide particles, barium sulfate, calcium fluoride, lithium fluoride, and zeolite. , Molybdenum sulfide, mica, zeolite, magnesium oxide and the like. In particular, titanium dioxide, alumina, boehmite, and barium sulfate 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.

Regarding the content of particles contained in the porous layer, the upper limit is preferably 90% by weight or less, more preferably 85% by weight or less, and the lower limit is 50% by weight or more with respect to the total weight of the porous layer. It is preferable, more preferably 60% by weight or more, still more preferably 65% by weight or more. By keeping the content of the particles within the above preferable range, a good balance of air permeation resistance can be easily obtained.

When the porous layer contains particles having no adhesiveness, the bending strength when wet, the bending strength when drying, and the peeling force when drying tend to decrease. However, even if the porous layer obtained by the resin composition of the present invention contains particles in the above-mentioned preferable range, the balance between the bending strength when wet, the bending strength when drying and the peeling force when drying is good with respect to the electrode.

From the viewpoint of particle shedding, 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 or less the average flow rate pore diameter of the microporous membrane. .. The average flow rate pore diameter was measured according to JIS K3832 or ASTM F316-86, 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 to 1.8 μm, more preferably 0.5 μm to 1.5 μm, and further preferably 0, from the viewpoint of slipperiness with the winding core during cell winding and particle dropout. It is 9.9 μm to 1.3 μm. 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.

[5] Physical properties of the porous layer The film thickness of the porous layer is preferably 0.5 to 3 μm, more preferably 1 to 2.5 μm, and further preferably 1 to 2 μm per side. When the film thickness per side is 0.5 μm or more, the bending strength when wet, the bending strength when drying, and the peeling force when drying 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, which will be advanced in the future.

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

[6] Method for Manufacturing Battery Separator The method for manufacturing a battery separator of the present invention sequentially includes the following steps (1) to (3).
(1) Step of obtaining a fluororesin solution in which a vinylidene fluoride-hexafluoropropylene copolymer (A) and a polymer (B) containing a vinylidene fluoride unit are dissolved in a solvent (2) Acrylic resin solution in a fluororesin solution Step of adding and mixing to obtain a coating liquid (3) Step of applying the coating liquid to a microporous film, immersing it in a coagulating liquid, washing and drying

(1) Step of obtaining a fluororesin solution The solvent can dissolve the vinylidene fluoride-hexafluoropropylene copolymer (A) and the polymer (B) containing the vinylidene fluoride unit, and can dissolve or disperse the acrylic resin. It is not particularly limited as long as it can be produced and can be mixed with the coagulating liquid. From the viewpoint of solubility and low volatility, the solvent is preferably N-methyl-2-pyrrolidone.

When providing a porous layer containing particles, it is important to prepare a fluororesin solution (also referred to as a dispersion liquid) in which particles are dispersed in advance. The vinylidene fluoride-hexafluoropropylene copolymer (A) and the polymer (B) containing the vinylidene fluoride unit are dissolved in a solvent, and particles are added thereto with stirring for a certain period of time (for example, about 1 hour). ) Pre-disperse by stirring with a disper or the like, and further disperse the particles using a bead mill or a paint shaker (dispersion step) to obtain a fluororesin solution with reduced aggregation of the particles.

(2) Step of obtaining a coating liquid An acrylic resin solution is added to a fluororesin solution containing a vinylidene fluoride-hexafluoropropylene copolymer (A) and a polymer (B) containing a vinylidene fluoride unit, and for example, stirring is performed. A coating liquid is prepared by mixing with a three-one motor with blades.

The acrylic resin solution is a solution in which an acrylic resin is dissolved or dispersed in a solvent, and the solvent used here is preferably the same solvent as in step (1). 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 add an acrylic resin solution to the fluororesin solution (dispersion solution) in which the particles are dispersed. That is, it is important that the acrylic resin does not enter in the dispersion step. When vinylidene fluoride-hexafluoropropylene copolymer (A), polymer containing vinylidene fluoride unit (B), acrylic resin and particles are added to the solvent at the same time to prepare a coating liquid, the copolymer (A) ) And the acrylic resin (particularly when butyl acrylate is contained) are presumed to gradually start gelling due to heat and shear during dispersion, which is industrially unsuitable. Further, due to the effect of thickening, it becomes difficult to apply a thin film so that the thickness of the porous layer is 3 μm or less per side. By the steps (1) and (2) 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.

(3) Step of applying the coating liquid to the microporous membrane, immersing it in the coagulating liquid, washing and drying the coating liquid is applied to the microporous membrane, and the applied microporous membrane is immersed in the coagulating liquid to form fluoride. The vinylidene-hexafluoropropylene copolymer (A), the polymer (B) containing a vinylidene fluoride unit, and the acrylic resin are phase-separated, coagulated 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 of the microporous membrane 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, and is an aqueous solution containing 1 to 20% by weight of a vinylidene fluoride-hexafluoropropylene copolymer (A), a polymer (B) containing a vinylidene fluoride unit, and a good solvent for an acrylic resin. Is preferable, and more preferably, it is an aqueous solution containing 5 to 15% by weight. Examples of good solvents include N-methyl-2-pyrrolidone, N, N-dimethylformamide, and N, N-dimethylacetamide. The immersion time in the coagulating liquid is preferably 3 seconds or longer. The upper limit is not limited, but 10 seconds is sufficient.

Water can be used for cleaning. Drying can be performed using, for example, hot air at 100 ° C. or lower.

The battery separator of the present invention includes 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.

[7] Physical Properties of Battery Separator The bending strength of the battery separator when wet is preferably 4N or more. The upper limit of the flexural strength when wet is not particularly set, but 15N is sufficient. By setting the content within the above preferable range, partial release at the interface between the separator and the electrode can be suppressed, an increase in battery internal resistance and a decrease in battery characteristics can be suppressed.

The flexural strength of the battery separator during drying is preferably 5N or more. The upper limit of the flexural strength during drying is not particularly set, but 25N is sufficient. By keeping the temperature within the above preferable range, it can be expected that the bending and distortion of the wound electrode body can be suppressed.

The peeling force of the battery separator during drying is preferably 8 N / m or more. The upper limit of the peeling force during drying is not particularly set, but 40 N / m is sufficient. Within the above preferable range, it is expected that the wound electrode body or the laminated electrode body can be conveyed without the electrode body being separated.

The battery separator has both a wet bending strength, a dry bending strength, and a dry peeling force. Specifically, the wet bending strength is 4 N or more and the dry bending strength is 5 N or more by the measurement method shown below. Moreover, the peeling force at the time of drying satisfies 8 N / m 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 diffusion of the fluororesin is unlikely to occur, so that the negative electrode is less likely to obtain 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 A negative electrode mixture is prepared by adding an aqueous solution containing 1.5 parts by mass of carboxymethyl cellulose to 96.5 parts by mass of artificial graphite and mixing, and further adding 2 parts by mass of styrene-butadiene latex as a solid content and mixing. It was used as a contained 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 and metal. The separator and the negative electrode were wound around a plate (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 winding body had a length of about 34 mm and a width of about 28 mm.
(3) Method for measuring flexural strength when wet Inside a bag-shaped laminated film in which two polypropylene laminated films (length 70 mm, width 65 mm, thickness 0.07 mm) are stacked and three of the four sides are welded together. The test winding body was put in. 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 the opening of the laminate film in a glove box, and the test winding is performed. The body was impregnated and 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 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 bending strength when wet was measured using a universal testing machine (AGS-J, manufactured by Shimadzu Corporation) for the test wound body after pressurization while being sealed in the laminate film. Details will be described below.
Two aluminum L-shaped angles (thickness 1 mm, 10 mm x 10 mm, length 5 cm) are arranged in parallel with the 90 ° part facing up, with the ends aligned, and between the fulcrums with the 90 ° part as the fulcrum. 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, and the side in the length direction of the L-shaped angle. The test winding body was arranged so as not to protrude from the outside.
Next, as an indenter, the side (about 34 mm) in the length direction of the test winding body should not protrude from the side in the length direction of the aluminum L-shaped angle (thickness 1 mm, 10 mm × 10 mm, length 4 cm). Align the 90 ° part of the aluminum L-shaped angle with the midpoint of the width direction side of the test winding body, and make the aluminum L-shaped angle a universal tester so that the 90 ° part is on the bottom. It was fixed to the load cell (load cell capacity 50N). The average value of the maximum test forces obtained by measuring the three test winding bodies at a load speed of 0.5 mm / min 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 70 ° C. and 0.6 MPa for 2 minutes at 10), and the mixture was allowed to cool at room temperature. Regarding the test winding body, the above 1. Obtained by measuring three test windings under the following conditions using a universal testing machine (manufactured by Shimadzu Corporation, AGS-J) arranged in the same manner as the method for measuring flexural strength when wet. The average value of the maximum test force was taken as the bending strength during drying.
Distance between fulcrums: 15 mm
Load cell capacity: 50N
Load speed: 0.5 mm / min

3. 3. Peeling force during drying (1) Preparation of negative electrode 1. A negative electrode having the same bending strength as when wet was used.
(2) Preparation of peeling test piece The negative electrode (70 mm × 15 mm) prepared above and the separator (machine direction 90 mm × width direction 20 mm) prepared in Examples and Comparative Examples are overlapped with each other, and two gaskets (20 mm in width direction) are laminated. It was sandwiched between (thickness 0.5 mm, 95 mm × 27 mm), pressurized with a precision heating and pressurizing device (CYPT-10 manufactured by Shinto Kogyo Co., Ltd.) at 90 ° C. and 8 MPa for 2 minutes, and allowed to cool at room temperature. A double-sided tape having a width of 1 cm is attached to the negative electrode side of the laminate of the negative electrode and the separator, and the other side of the double-sided tape is placed on a SUS plate (thickness 3 mm, length 150 mm × width 50 mm) in the mechanical direction of the separator. And the SUS plate were attached so that the length direction was parallel. This was used as a peeling test piece.
(3) Method for measuring peeling force during drying A separator was sandwiched between load cell side chucks using a universal testing machine (AGS-J manufactured by Shimadzu Corporation), and a 180-degree peeling test was carried out at a test speed of 300 mm / min. The value obtained by averaging the measured values of the strokes from 20 mm to 70 mm during the peeling test was taken as the peeling force of the peeling test piece. A total of three peeling test pieces were measured, and the value obtained by converting the average value of the peeling force into a width was defined as the peeling force during drying (N / m).

4. Melting point measurement With a differential scanning calorimetry device (DSC manufactured by PerkinElmer Co., Ltd.), 7 mg of resin was placed in a measuring pan to prepare a sample for measurement, and measurement was performed under the following conditions. After the temperature was raised and cooled first, the peak top of the endothermic peak at the time of the second temperature rise was taken as the melting point.
Temperature rise and cooling rate: ± 10 ° C / min
Measurement temperature range: 30-230 ° C

5. 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
[Copolymer (a)]
As the copolymer (A), the copolymer (a) was synthesized as follows. 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 copolymer (a) had a weight average molecular weight of 1.5 million, and the molar ratio of vinylidene fluoride / hexafluoropropylene / maleic acid monomethyl ester was 98.0 / 1.5 / 0.5 by NMR measurement. I confirmed it in.
[Copolymer (b1)]
As the polymer (B), the copolymer (b1) was synthesized as follows. A vinylidene fluoride-hexafluoropropylene copolymer (b1) was synthesized by a suspension polymerization method using vinylidene fluoride and hexafluoropropylene as starting materials. It was confirmed by NMR measurement that the obtained copolymer (b1) had a weight average molecular weight of 300,000 and a molar ratio of vinylidene fluoride / hexafluoropropylene of 93/7.
[acrylic resin]
An acrylic resin was synthesized by an emulsion polymerization method using acrylonitrile and n-butyl acrylate as starting materials, and then water was replaced with N-methyl-2-pyrrolidone to obtain an acrylic resin solution having a solid content concentration of 5% by mass. .. It was confirmed by NMR measurement that the obtained acrylic resin had a Tg of −5 ° C. and a molar ratio of acrylonitrile unit / acrylic acid ester unit of 38/62.
[Manufacturing of battery separator]
7.1 parts by mass of the copolymer (a), 21.4 parts by mass of the copolymer (b1), and 359.3 parts by mass of NMP are mixed, and then alumina particles (average particle size 1.1 μm) are stirred with a disper. Was added in an amount of 70 parts by mass, and the mixture was further pre-stirred with a disper for 1 hour at 2000 rpm. Next, using a Dynomill (Dynomill Multilab (1.46 L container, filling rate 80%, φ0.5 mm alumina beads) manufactured by Simmal Enterprises), under the conditions of a flow rate of 11 kg / hr and a peripheral speed of 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, filtered, and solid content concentration is 20.5% by mass. Alumina particles: copolymer (a): copolymer weight. A coating solution having a weight ratio of coalescing (b1): acrylic resin of 70: 7.1: 21.4: 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. It was.

Example 2
Coating liquid having a solid content concentration of 18.0% by mass and a weight ratio of alumina particles: copolymer (a): copolymer (b1): acrylic resin 70: 14.3: 14.2: 1.5 A battery separator was obtained in the same manner as in Example 1 except that the above was used.

Example 3
Coating liquid having a solid content concentration of 15.5% by mass and a weight ratio of alumina particles: copolymer (a): copolymer (b1): acrylic resin 70: 21.4: 7.1: 1.5 A battery separator was obtained in the same manner as in Example 1 except that the above was used.

Example 4
A coating liquid having a solid content concentration of 20.5% by mass and a weight ratio of alumina particles: copolymer (a): copolymer (b1): acrylic resin of 70: 6.8: 20.2: 3.0. A battery separator was obtained in the same manner as in Example 1 except that the above was used.

Example 5
Coating liquid with solid content concentration of 18.0% by mass, alumina particles: copolymer (a): copolymer (b1): acrylic resin with a weight ratio of 70: 13.5: 13.5: 3.0 A battery separator was obtained in the same manner as in Example 1 except that the above was used.

Example 6
Coating liquid having a solid content concentration of 15.5% by mass and a weight ratio of alumina particles: copolymer (a): copolymer (b1): acrylic resin of 70: 20.2: 6.8: 3.0. A battery separator was obtained in the same manner as in Example 1 except that the above was used.

Example 7
Coating liquid having a solid content concentration of 20.5% by mass and a weight ratio of alumina particles: copolymer (a): copolymer (b1): acrylic resin of 70: 6.4: 19.1: 4.5. A battery separator was obtained in the same manner as in Example 1 except that the above was used.

Example 8
Coating liquid having a solid content concentration of 18.0% by mass and a weight ratio of alumina particles: copolymer (a): copolymer (b1): acrylic resin of 70: 12.8: 12.7: 4.5. A battery separator was obtained in the same manner as in Example 1 except that the above was used.

Example 9
A coating liquid having a solid content concentration of 19.5% by mass and a weight ratio of alumina particles: copolymer (a): copolymer (b1): acrylic resin of 70: 19.1: 6.4: 4.5. A battery separator was obtained in the same manner as in Example 1 except that the above was used.

Example 10
Coating liquid having a solid content concentration of 20.5% by mass and a weight ratio of alumina particles: copolymer (a): copolymer (b1): acrylic resin of 70: 6.0: 18.0: 6.0. A battery separator was obtained in the same manner as in Example 1 except that the above was used.

Example 11
Coating liquid having a solid content concentration of 18.0% by mass and a weight ratio of alumina particles: copolymer (a): copolymer (b1): acrylic resin of 70: 12.0: 12.0: 6.0. A battery separator was obtained in the same manner as in Example 1 except that the above was used.

Example 12
Coating liquid having a solid content concentration of 15.5% by mass and a weight ratio of alumina particles: copolymer (a): copolymer (b1): acrylic resin of 70: 18.0: 6.0: 6.0. A battery separator was obtained in the same manner as in Example 1 except that the above was used.

Example 13
Coating with a solid content concentration of 21.0% by mass, alumina particles: copolymer (a): copolymer (b1): acrylic resin with a weight ratio of 78.8: 9.0: 9.0: 3.2 A battery separator was obtained in the same manner as in Example 1 except that the working solution was used.

Example 14
Coating with a solid content concentration of 25.0% by mass and a weight ratio of alumina particles: copolymer (a): copolymer (b1): acrylic resin of 85.2: 6.3: 6.3: 2.2. A battery separator was obtained in the same manner as in Example 1 except that the working solution was used.

Example 15
[Copolymer (b2)]
As the polymer (B), the copolymer (b2) was synthesized as follows. A vinylidene fluoride-tetrafluoroethylene copolymer (b2) was synthesized by a suspension polymerization method using vinylidene fluoride and tetrafluoroethylene as starting materials. It was confirmed by NMR measurement that the obtained vinylidene fluoride-tetrafluoroethylene copolymer (b2) had a weight average molecular weight of 280,000 and a molar ratio of vinylidene fluoride / tetrafluoroethylene of 90/10.
[Manufacturing of battery separator]
Using the copolymer (b2) instead of the copolymer (b1), the solid content concentration is 18.0% by mass, alumina particles: copolymer (a): copolymer (b2): weight of acrylic resin. A battery separator was obtained in the same manner as in Example 1 except that a coating liquid prepared so that the ratio was 70: 13.5: 13.5: 3.0 was used.

Example 16
45 parts by mass of the copolymer (a), 45 parts by mass of the copolymer (b1), and 1329 parts by mass of NMP were mixed and dissolved. Acrylic resin solution is mixed with this solution, stirred with a three-one motor equipped with a stirring blade at 500 rpm for 30 minutes, filtered to have a solid content concentration of 6.6% by mass, and the copolymer (a): copolymer (b1). ): A coating solution having a weight ratio of acrylic resin of 45.0: 45.0: 10.0 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. It was.

Comparative Example 1
30.0 parts by mass of the copolymer (b1) and 334.8 parts by mass of NMP are mixed, and then 70 parts by mass of alumina particles (average particle size 1.1 μm) are added while stirring with a disper, and further, 1 with a disper. Pre-stirring at 2000 rpm for hours. Next, using a Dynomill (Dynomill Multilab (1.46 L container, filling rate 80%, φ0.5 mm alumina beads) manufactured by Simmal Enterprises), under the conditions of a flow rate of 11 kg / hr and a peripheral speed of 10 m / s. The treatment was performed 3 times to obtain a dispersion liquid. This was filtered to obtain a coating liquid having a solid content concentration of 23.0% by mass and an alumina particle: copolymer (b1) weight ratio of 70: 30.0. 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. It was.

Comparative Example 2
Prepared and filtered in the same manner as in Comparative Example 1 except that 7.5 parts by mass of the copolymer (a), 22.5 parts by mass of the copolymer (b1), and 387.8 parts by mass of NMP were mixed. Was 20.5% by mass, and a coating liquid prepared so that the weight ratio of alumina particles: copolymer (a): copolymer (b1) was 70: 7.5: 22.5 was obtained. This was applied in the same manner as in Comparative Example 1 to obtain a battery separator.

Comparative Example 3
A coating liquid prepared so that the solid content concentration is 18.0% by mass and the weight ratio of alumina particles: copolymer (a): copolymer (b1) is 70: 15.0: 15.0 is used. A battery separator was obtained in the same manner as in Comparative Example 2 except that the particles were present.

Comparative Example 4
A coating liquid prepared so that the solid content concentration is 15.5% by mass and the weight ratio of alumina particles: copolymer (a): copolymer (b1) is 70: 22.5: 7.5 is used. A battery separator was obtained in the same manner as in Comparative Example 2 except that the particles were present.

Comparative Example 5
The copolymer (a) was prepared in the same manner as in Comparative Example 1 except that 30 parts by mass and NMP669.2 parts by mass were mixed, and the solid content concentration was 13.0% by mass. Alumina particles: copolymer (copolymer (a) A coating liquid prepared so that the weight ratio of a) was 70: 30.0 was obtained. This was applied in the same manner as in Comparative Example 1 to obtain a battery separator.

Comparative Example 6
Except for using a coating liquid prepared so that the solid content concentration is 23.0% by mass and the weight ratio of alumina particles: copolymer (b1): acrylic resin is 70: 28.5: 1.5. A battery separator was obtained in the same manner as in Example 1.

Comparative Example 7
Except for using a coating liquid prepared so that the solid content concentration is 13.0% by mass and the weight ratio of alumina particles: copolymer (a): acrylic resin is 70: 28.5: 1.5. A battery separator was obtained in the same manner as in Example 1.

Comparative Example 8
Except for using a coating liquid prepared so that the solid content concentration is 23.0% by mass and the weight ratio of alumina particles: copolymer (b1): acrylic resin is 70: 27.0: 3.0. A battery separator was obtained in the same manner as in Example 1.

Comparative Example 9
Except for using a coating liquid prepared so that the solid content concentration is 13.0% by mass and the weight ratio of alumina particles: copolymer (a): acrylic resin is 70: 27.0: 3.0. A battery separator was obtained in the same manner as in Example 1.

Comparative Example 10
Except for using a coating liquid prepared so that the solid content concentration is 23.0% by mass and the weight ratio of alumina particles: copolymer (b1): acrylic resin is 70: 25.5: 4.5. A battery separator was obtained in the same manner as in Example 1.

Comparative Example 11
Except for using a coating liquid prepared so that the solid content concentration is 23.0% by mass and the weight ratio of alumina particles: copolymer (b1): acrylic resin is 70: 24.0: 6.0. A battery separator was obtained in the same manner as in Example 1.

Comparative Example 12
[Copolymer (b3)]
A vinylidene fluoride-tetrafluoroethylene copolymer was synthesized by a suspension polymerization method using vinylidene fluoride and tetrafluoroethylene as starting materials. It was confirmed by NMR measurement that the obtained vinylidene fluoride-tetrafluoroethylene copolymer had a weight average molecular weight of 950,000 and a molar ratio of vinylidene fluoride / tetrafluoroethylene was 95/5.
[Manufacturing of battery separator]
Using the copolymer (b3) instead of the copolymer (b1), the solid content concentration is 18.0% by mass, alumina particles: copolymer (a): copolymer (b3): weight of acrylic resin. A battery separator was obtained in the same manner as in Example 1 except that a coating liquid prepared so that the ratio was 70: 13.5: 13.5: 3.0 was used.

Table 1 shows the characteristics of the battery separators obtained in Examples 1 to 16 and Comparative Examples 1 to 12.

共重合体（Ａ）の含有量（％）^＊：共重合体（Ａ）と重合体（Ｂ）の総重量に対する共重合体（Ａ）の重量％を表す。
アクリル樹脂の含有量（％）^＊＊：共重合体（Ａ）、重合体（Ｂ）及びアクリル樹脂の総重量に対するアクリル樹脂の重量％を表す。

Content of copolymer (A) (%) ^* : Represents the weight% of the copolymer (A) with respect to the total weight of the copolymer (A) and the polymer (B).
Acrylic resin content (%) ^** : Represents the weight% of the acrylic resin with respect to the total weight of the copolymer (A), the polymer (B) and the acrylic resin.

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

Claims (13)

A microporous film and a porous layer provided on at least one side of the microporous film are provided, and the porous layer is a polymer containing a vinylidene fluoride-hexafluoropropylene copolymer (A) and a vinylidene fluoride unit. The vinylidene fluoride-hexafluoropropylene copolymer (A) containing (B) and an acrylic resin has a hydrophilic group content of 0.1 mol% to 5 mol%, and has a hexafluoropropylene unit of 0. The polymer (B) containing 3 mol% to 3 mol%, having a weight average molecular weight of more than 750,000 and 2 million or less, and containing the vinylidene fluoride unit has a melting point of 60 ° C. or higher and 145 ° C. or lower, and a weight average molecular weight of 100,000. Separator for batteries that is 750,000 or less. The battery separator according to claim 1, wherein the porous layer contains particles. The content of the vinylidene fluoride-hexafluoropropylene copolymer (A) is 15 relative to the total weight of the vinylidene fluoride-hexafluoropropylene copolymer (A) and the polymer (B) containing the vinylidene fluoride unit. By weight% or more and 85% by weight or less, the content of the acrylic resin is based on the total weight of the vinylidene fluoride-hexafluoropropylene copolymer (A), the polymer (B) containing the vinylidene fluoride unit, and the acrylic resin. The battery separator according to claim 1 or 2 , wherein the content is 4% by weight or more and 40% by weight or less. The battery separator according to any one of claims 1 to 3 , wherein the acrylic resin is a copolymer of a (meth) acrylic acid ester and a monomer having a cyano group. The battery separator according to any one of claims 1 to 3 , wherein the acrylic resin is a copolymer containing butyl acrylate. The battery separator according to any one of claims 1 to 3 , wherein the acrylic resin is a copolymer of butyl acrylate and acrylonitrile. The battery separator according to claim 5 or 6 , wherein the content of butyl acrylate in the acrylic resin is 50 mol% to 75 mol%. The battery separator according to any one of claims 1 to 7, wherein the flexural strength when wet is 4 N or more, the flexural strength when dry is 5 N or more, and the peeling force when dry is 8 N / m. The battery separator according to any one of claims 2 to 8 , wherein the content of the particles is 50% by weight or more and 90% by weight or less with respect to the total weight of the porous layer. The battery separator according to any one of claims 2 to 9 , wherein the particles include at least one selected from the group consisting of alumina, titania, boehmite, and barium sulfate. The battery separator according to any one of claims 1 to 10 , wherein the thickness of the porous layer is 0.5 to 3 μm per side. The battery separator according to any one of claims 1 to 11, wherein the microporous membrane is a polyolefin microporous membrane. (1) A step of obtaining a fluororesin solution in which a vinylidene fluoride-hexafluoropropylene copolymer (A) and a polymer (B) containing a vinylidene fluoride unit are dissolved in a solvent.
(2) A step of adding an acrylic resin solution in which an acrylic resin is dissolved in a solvent to a fluororesin solution and mixing them to obtain a coating liquid.
(3) The step of applying the coating liquid to the microporous film, immersing it in the coagulating liquid, washing and drying is sequentially included, and the vinylidene fluoride-hexafluoropropylene copolymer (A) has a hydrophilic group content. Is 0.1 mol% to 5 mol%, contains 0.3 mol% to 3 mol% of hexafluoropropylene units, has a weight average molecular weight of more than 750,000 and is 2 million or less, and contains the vinylidene fluoride unit. B) has a melting point of 60 ° C. or higher and 145 ° C. or lower, a weight average molecular weight of 100,000 or more and 750,000 or less, and the acrylic resin contains a butyl acrylate unit for a battery according to any one of claims 1 to 12 . Method of manufacturing separator.

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