JP7361484B2 - Aqueous composition for coating - Google Patents

Description

The present invention relates to aqueous coating compositions, particularly coating aqueous compositions for lithium ion battery separator films.

In recent years, there has been a demand for storage batteries with high energy density, high output, durability, and safety that are suitable for vehicles and mobile devices.Lithium-ion batteries are widely used due to their characteristics, and research and development is active. It is being done.

In a lithium ion battery, a battery separator film (hereinafter simply referred to as a separator) made of a microporous polyolefin membrane and a sheet electrode are stacked or rotated inside the battery body with an electrolyte layer interposed therebetween. There are structures to be placed. The separator has the function of preventing direct contact between the positive and negative electrodes and allowing ions to pass through the electrolytic solution held in the porous base material. The ion permeability of a separator is a property directly related to its charging and discharging ability.

When the temperature inside the battery rises, ion permeability (ion resistance) may deteriorate at a temperature lower than the melting temperature of the polyolefin that shuts down, so separators have traditionally been desired to have excellent heat resistance. Ta.

In recent years, in order to further improve the heat resistance of batteries, development of multilayer heat-resistant separator materials in which a separator is coated with an inorganic particle layer has been progressing. A multilayer heat-resistant separator material can be manufactured by applying a heat-resistant layer material containing inorganic particles and a binder to a base film such as a microporous polyolefin membrane and drying the film.

Patent Document 1 discloses that at least one side of a porous polyolefin resin membrane is provided with a porous layer containing an inorganic filler and a resin binder, and the polyolefin resin porous membrane has a maximum heat shrinkage stress of 10 g or less, thereby improving heat resistance. There is a description that a multilayer porous membrane with excellent permeability and permeability can be produced.

Further, Patent Documents 2 and 3 describe that the mechanical stability of the battery is improved by using at least one selected from acrylic polymers, fluororesins, and polyamides as a resin binder. In Patent Documents 2 and 3, resin latex (emulsion type) is preferred as the form of the resin binder. However, emulsion-type resins mainly consist of particle binding, and have a problem in that their adhesion is lower than in a state in which molecular chains are entangled.

Japanese Patent Application Publication No. 2009-026733 Japanese Patent Application Publication No. 2016-139490 Japanese Patent Application Publication No. 2016-139489

In view of the above issues, we have developed an aqueous coating composition that is less likely to increase ionic resistance than a single-layer porous membrane and has high coating film adhesion, especially an aqueous composition for coating a lithium ion battery separator film. The purpose is to provide.

The present inventors have conducted extensive research to solve the above problems, and as a result, have arrived at the present invention. That is, the present invention includes the following.
[1] An aqueous composition for coating a porous resin film comprising a binder containing 50% by weight or more of a water-soluble acrylic resin and a filler having a D90 of 1 μm or less, wherein the water-soluble acrylic resin contains a hydroxyl group. An aqueous composition for coating a porous resin film containing 40 to 45% by weight of monomer units and 8 to 12% by weight of an acetyl group-containing monomer.
[2] The aqueous coating composition according to [1], wherein 80% or more of particles of the filler have a particle size of 50 to 700 nm.
[3] The aqueous coating composition according to [1] or [2], wherein the filler is alumina.
[4] [1] to [3] containing 2.5 to 5.0% by weight of the binder and 92 to 97% by weight of the filler based on the total solid content of the aqueous coating composition. The aqueous coating composition according to any one of the above.
[5] The aqueous coating composition according to any one of [1] to [4], wherein the binder does not substantially contain a water-insoluble resin.
[6] The water-soluble acrylic resin has a pH of 6 to 9 in a 1% by weight aqueous solution, and a viscosity of 4 to 8 Pa·s at 25° C. in a 15% by weight aqueous solution, [1] to [5] The aqueous coating composition according to any one of the above.
[7] The aqueous coating composition according to any one of [1] to [6], wherein the resin porous film is a separator film for an electricity storage device.
[8] The aqueous coating composition according to any one of [1] to [7], wherein the porous resin film is a separator film for lithium ion batteries.

According to the present invention, there is provided an aqueous coating composition that does not easily increase the membrane resistance after coating and has high adhesion of the coating film to the porous resin film.

FIG. 1 is a SEM image of the surface of the coating film of Example 15. FIG. 2 is a SEM image of the surface of the coating film of Comparative Example 2. FIG. 3 is a SEM image of the surface of the coating film of Comparative Example 3. FIG. 4 is a schematic diagram of a method for measuring tape peel strength.

The present invention will be described below, but the present invention is not limited to the specific examples and examples described in the detailed description, unless it departs from the spirit of the invention.

<Aqueous composition for coating>
The aqueous coating composition of the present invention contains a binder containing 50% by weight or more of a water-soluble acrylic resin, and a filler.

(binder)
The binder contained in the aqueous coating composition of the present invention is capable of binding a filler, and the water-soluble acrylic resin contains 40 to 45% by weight of a hydroxyl group-containing monomer unit and an acetyl group-containing monomer unit. It contains a water-soluble acrylic resin containing 8 to 12% by weight of polymer. The water-soluble acrylic resin is characterized by being an acrylic resin that is soluble in water. By using a water-soluble acrylic resin in the aqueous coating composition, the adhesion of the coating film composed of the solid content of the aqueous coating composition to the substrate is improved.

The water-soluble acrylic resin that can be used in the present invention preferably has a pH of 6 to 9 in a 1% by weight aqueous solution. Within the above range, the dispersibility of the filler in the solvent will improve.

The water-soluble acrylic resin preferably has a viscosity of 4 to 8 Pa·s in a 15% by weight aqueous solution at 25°C. Within the above range, the aqueous coating composition can be stably stored.

The water-soluble acrylic resin contains 40 to 45% by weight of hydroxyl group-containing monomer units. Within the above range, the adhesion of the coating film composed of the solid content of the aqueous coating composition to the substrate will be improved.

The water-soluble acrylic resin contains 8 to 12% by weight of an acetyl group-containing monomer. Within the above range, the adhesion of the coating film composed of the solid content of the aqueous coating composition to the substrate will be improved.

After being polymerized with the above monomer, the water-soluble acrylic resin may be subjected to a neutralization treatment in order to adjust its solubility in water.

The binder contains 50% by weight or more of a water-soluble acrylic resin based on the entire resin constituting the binder. By containing 50% by weight or more, the effect of improving adhesion is more likely to be exhibited. The water-soluble acrylic resin preferably accounts for 60% by weight or more based on the entire resin constituting the binder. There is no particular upper limit to the content of the water-soluble acrylic resin with respect to the entire resin constituting the binder, but it is preferably 90% by weight or less.

The binder can contain resins other than water-soluble acrylic resins. Examples of the resin that can be included include PVDF, PMMA, and emulsion type acrylic resin.

Conventionally, emulsion type acrylic resins have been used as coating films, particularly as coating films used in separator films for power storage devices. Since emulsion type acrylic resin is a resin that is insoluble in water when used alone, it can be dispersed in water by using a surfactant. For the binder used in the aqueous coating composition of the present invention, from the viewpoint of adhesion to the substrate, it is preferable to reduce the amount of water-insoluble resin, such as an emulsion type acrylic resin. The water-insoluble resin contained in the binder is preferably 20% by weight or less, more preferably 10% by weight or less, still more preferably 5% by weight or less, and particularly preferably 0% by weight. That is, it is preferable that the binder contained in the aqueous coating composition of the present invention does not substantially contain a water-insoluble resin. Here, "substantially free of water-insoluble resins" means that water-insoluble resins are not actively included, and that the resin is present at an unavoidable impurity level (total level of the resin constituting the binder). On the other hand, for example, it is permitted that the resin is contained in a content of 0.1% by weight or less.

<Filler>
The aqueous coating composition of the present invention is used to improve the heat resistance of a substrate to be coated. In order to solve the above problems, a filler is used in the aqueous coating composition. The filler is not particularly limited as long as it can achieve the above purpose.

Fillers that can be used in the aqueous coating composition include oxide ceramics such as silica, alumina, titania, zirconia, magnesia, barium titanate, hydroxide ceramics such as aluminum hydroxide and magnesium hydroxide, and silicon nitride. , nitride ceramics such as titanium nitride and boron nitride, or minerals such as boehmite, talc, kaolin, zeolite, apatite, halloysite, pyrophyllite, montmorillonite, sericite, mica, amethyte, bentonite, calcium silicate, magnesium silicate, etc. Examples include resource-derived substances. Alumina is preferred from the viewpoint of heat resistance and chemical stability.

The filler used in the aqueous coating composition preferably has a smaller particle size than fillers conventionally used in coatings. Specifically, the D90 of the filler is preferably 1 μm or less, more preferably 0.6 μm or less, and even more preferably 0.4 μm or less. Furthermore, it is preferable that 80% or more of the particles of the filler have a particle size within the range of 50 to 700 nm.

More specifically, the average particle diameter of the particles constituting the filler is preferably 50 to 500 nm, more preferably 100 to 400 nm, and even more preferably 150 to 250 nm. In addition, by setting the D90 of the filler, the particle size distribution, and/or the average particle size of the filler within the above range, it is possible to suppress clogging of the separator and maintain air permeability, resulting in excellent adhesion with the separator. A coating film can be obtained.

<Other ingredients>
The aqueous coating composition of the present invention can contain a dispersant. By including a dispersant, the dispersibility of the filler in the solvent can be improved, the aqueous coating composition can be stably stored, and a uniform coating film can be formed without bias.

The dispersant that can be used in the aqueous coating composition is not particularly limited as long as it can achieve the effects of the dispersant described above, but specific examples of the dispersant include polyalkyl ether, ammonium polyacrylate, and carboxymethyl cellulose. Examples include.

The aqueous coating composition of the present invention can contain water from the viewpoint of making it easier to apply to a substrate. Although it does not prevent other components from being mixed into the water used in the present invention, the amount of other components contained in the water is preferably 10% by weight or less, more preferably 3% by weight or less, More preferably it is 1% by weight or less, particularly preferably 0.1% by weight or less.

The aqueous coating composition of the present invention may contain other components depending on the application. Components that can be included in the aqueous coating composition include antibacterial agents, antifungal agents, wetting agents, thickeners, wetting agents, pH adjusters, and the like.

<Component ratio>
The coating aqueous composition of the present invention includes a binder and a filler, and the component ratios thereof will be explained.
The lower limit of the binder is preferably 2.5% by weight or more, more preferably 2.5% by weight or more, based on the entire solid content of the aqueous coating composition (assuming the entire solid content of the aqueous coating composition is 100%). It contains at least .7% by weight, more preferably at least 3.0% by weight.
The upper limit of the binder is preferably 5.0% by weight or less, more preferably 4.5% by weight or less, and even more preferably 4.0% by weight or less, based on the entire solid content of the aqueous coating composition. include.

The lower limit of the filler content is preferably 92% by weight or more, more preferably 93% by weight or more, still more preferably 94% by weight or more, based on the total solid content of the aqueous coating composition.
Further, the filler is preferably included in an upper limit of 97% by weight or less, more preferably 96% by weight or less, still more preferably 95% by weight or less, based on the total solid content of the aqueous coating composition.

When the aqueous coating composition of the present invention contains a dispersant, the lower limit is preferably 1.0% by weight or more, more preferably 1.5% by weight or more based on the entire solid content of the aqueous coating composition. , more preferably 2.0% by weight or more. Further, the upper limit is preferably 4.0% by weight or less, more preferably 3.0% by weight or less, still more preferably 2.5% by weight or less.
By setting the content (composition) of the binder, filler, and dispersant in the solid content of the aqueous coating composition within the above range, the adhesion between the coating film and the separator and the air permeability of the coating film can be balanced. A detached coating film is obtained. Note that the solid content refers to the components of the aqueous coating composition excluding solvents such as water.

The solid content of the aqueous coating composition of the present invention is preferably 20 to 50% by weight, more preferably 25 to 35% by weight based on the entire aqueous coating composition.

When the aqueous composition for coating of the present invention contains other components such as antifoaming agents, antibacterial agents, and antifungal agents, the upper limit is preferably 1.0% by weight or less, more preferably 1.0% by weight or less based on the total solid content. is 0.5% by weight or less, more preferably 0% by weight.

<Viscosity>
The aqueous coating composition of the present invention preferably has a viscosity of 0.01 to 0.1 Pa·s at 25°C.

<Method for producing aqueous composition for coating>
The aqueous coating composition of the present invention can be prepared by dispersing a binder containing 50% by weight or more of a water-soluble acrylic resin, a filler, and other components such as a dispersant and water. Dispersion can be carried out by conventional methods, and examples of dispersion devices include planetary stirrers, propeller stirrers, homodispers, bead mills, ultrasonic dispersion devices, and the like.

From the viewpoint of uniformity of the aqueous coating composition, it is preferable to further perform a secondary dispersion treatment after dispersing the aqueous coating composition. Examples of devices for performing dispersion treatment include wet jet mill devices and the like.

<Separator>
(Coating film)
The aqueous coating composition of the present invention can be applied to form a coating film. Examples of the coating method include a gravure coater method, a microgravure coater method, a die coater method, a knife coater method, a screen printing method, a Meyer bar method, a reverse roll coater method, an inkjet method, a spray method, and a roll coater method. Note that moisture can be removed by drying after application. The drying temperature is preferably 50°C or higher, more preferably 60°C or higher, even more preferably 70°C or higher.

The thickness of the coating film is not particularly limited as long as it can exhibit the characteristics of the coating film, and can be, for example, 0.5 to 10 μm, preferably 1.5 to 5 μm. can do.

When the coating film is used as a separator film for a power storage device, the basis weight can be set to 0.1 to 50 g/m ² from the viewpoint of power storage capacity, preferably 0.1 to 30 g/m ² , and more preferably. can be between 0.1 and 10 g/m ² .

The air permeability per unit thickness of the coating film can be 1 to 100 sec/μm, preferably 1 to 70 sec/μm, more preferably 1 to 70 sec/μm, from the viewpoint of permeation of ions when used as a separator film for an electricity storage device. can be set to 1 to 30 sec/μm.

(Base material)
A coating film formed from the aqueous coating composition of the present invention can be placed on a substrate and used as a separator. The base material is not particularly limited, but when used as a separator, it is necessary to allow ions to pass therethrough, so a porous resin film (microporous resin membrane) is preferable. As the material for the resin porous film, polyolefin is preferred.

Materials for the polyolefin base material (or polyolefin porous film) include, but are not limited to, polyethylene, polypropylene, polybutene, and the like.

The polyolefin base material can be surface-treated if necessary. Examples of the surface treatment include corona treatment, plasma treatment, electron beam treatment, radiation treatment, fluorine treatment, ultraviolet ray method, crosslinking treatment method, acid treatment method, and the like. By carrying out the above treatment, uniform penetration of the aqueous coating composition can be promoted and the adhesion between the coating film and the separator can be improved.

The method for producing a polyolefin base material for a separator is not particularly limited, but for example, when the base material is polypropylene, it is produced through the following steps 1 to 6. That is, first, raw material polypropylene is melt-kneaded (step 1). Next, the material is extruded from a T-die using a single-screw extruder to produce a raw film having a predetermined thickness (Step 2). Next, the raw film is heat treated at a predetermined temperature (100 to 170°C) (Step 3). The raw film that has undergone step 3 is cold stretched in the length direction at a predetermined temperature (15 to 30° C.) (step 4). The stretched film that has undergone step 4 is warm stretched in the length direction at a predetermined temperature (100 to 170° C.) (step 5). The stretched film is relaxed at a predetermined temperature (100 to 150° C.) so that the length of the stretched film after step 5 becomes a predetermined length (step 6). In this way, a polypropylene porous film having a predetermined final thickness can be obtained.

(Separator film for power storage devices (separator film for lithium ion batteries))
The coating film formed from the aqueous coating composition of the present invention can be used for various purposes, and typical examples include coating films for separator films of electricity storage devices, and specifically, separator films for lithium ion batteries. It can be used as a coating film for films.

One of the characteristics of the separator film for electricity storage devices (separator film for lithium ion batteries) comprising the coating film formed from the aqueous coating composition of the present invention is that the film resistivity is low. The membrane resistance increase rate (Rr) after coating is preferably 1.5 or less, more preferably 1.4 or less, and particularly preferably 1.3 or less.
Membrane resistance increase rate (Rr) = (membrane resistance after coating) / (membrane resistance before coating)

One of the features of the separator for electricity storage devices (separator for lithium ion batteries) comprising a coating film formed from the aqueous coating composition of the present invention is that the coating film has high adhesion to the base material. It will be done. Adhesion can be measured, for example, as tape peel strength. The tape peel strength was determined by applying a double-sided adhesive tape (manufactured by 3M Japan Co., Ltd., Scotch PREMIER GOLD (product name)) on the coating film on one side of a substrate cut into 2 cm (TD) x 7 cm (MD). )), then paste kraft paper cut to 2 cm wide x 7 cm long on one side of the double-sided adhesive tape, sandwich the coating film and the edges of the kraft paper with chucks, and pull the coating film at a pulling speed of 500 mm/min. The force when peeling off the interface with the base material can be measured as the interfacial peel strength (gf) between the base material and the coating film formed on the base material.
The value measured using a tensile tester is preferably 1000 gf or more, more preferably 1500 gf or more, and even more preferably 2000 gf or more.

EXAMPLES Hereinafter, the present invention will be explained in detail using Examples, but the present invention is not limited to these Examples.

(Type of filler)
Table 1 shows the particle characteristics of fillers 1, 2, and 3 (all alumina) contained in the aqueous coating compositions produced in the examples.

(Type of binder)
Binder 1 listed in the table is a water-soluble acrylic resin manufactured by Matsumoto Yushi Pharmaceutical Co., Ltd., and contains 40 to 45% by weight of hydroxyl group-containing monomer units and 8 to 12% by weight of acetyl group-containing monomers. It was something. (Binder 1 is used as a binder for Marpolyb AQ-2C manufactured by Matsumoto Yushi Pharmaceutical Co., Ltd.)
Binder 2 listed in the table was an acrylic emulsion type binder, a water-insoluble acrylic resin, and had an average particle diameter of 1 μm or less.

(Examples 1 to 15, Comparative Examples 1 to 3)
(1) Preparation of aqueous composition for coating In a beaker of a predetermined amount, each solid component (binder, filler, dispersant (polyalkyl ether)) is added. ) and water were added so that the solid content was about 30% by weight based on the entire aqueous coating composition, and the mixture was stirred with a stirrer. When the appearance was approximately uniform, the stirred raw materials were mixed using a planetary stirrer Mazerstar (trade name, manufactured by Kurabo Industries, Ltd., rotation: 1340 rpm, revolution: 1340 rpm, stirring time: 10 minutes). Next, the obtained mixture was subjected to a dispersion treatment using a wet jet mill device Nano Veita (trade name, manufactured by Yoshida Kikai Kogyo Co., Ltd., treatment pressure 100 MPa, number of treatment times 3). The resulting mixtures were used as coating aqueous compositions (Examples 1 to 15, Comparative Examples 1 to 3).

(2) Production of polypropylene porous film The raw material polypropylene has an MFR (measured according to JIS K6758 (230°C, 21.18N)) of 0.50 g/10 minutes, an Mw/Mn of 8.1, and a melting point of 163. ℃ propylene polymer was used. The raw material polypropylene was melt-kneaded and extruded from a T-die using a single-screw extruder to produce a raw film with a thickness of 17 μm. Next, the raw film was heat-treated at 145°C, and the raw film was cold-stretched to 1.02 times in the length direction at 25°C. Then, the stretched film was warm-stretched to 3.31 times in the length direction at 150°C, and then relaxed at 150°C so that the length of the stretched film became 0.75 times, so that the final thickness was 14 μm. A porous film was obtained.

(3) Production of separator film for lithium ion batteries The aqueous coating composition obtained in (1) above was applied to the polypropylene porous film of (2) using a coating device μ coater (Yasui Seiki Co., Ltd.). A coating film was formed by applying the aqueous coating composition to one side under the conditions of a line speed of 2 m/min and a drying temperature of 80°C. The properties of the obtained lithium ion separator film are shown in Tables 2 to 7. Note that Reference Examples 1, 2, and 3 in Table 6 are the measured values of Examples 12, 13, and 14, respectively, in a state where no coating film was formed.

(4) Evaluation conditions The properties of the aqueous coating composition, polypropylene porous film, and lithium ion separator film were evaluated under the following conditions.

A. Particle size distribution (average particle size, D10, D50, D90)
The particle size distribution of the dispersed filler was measured by diluting the aqueous coating composition 1000 times with pure water using LA-950V2 manufactured by HORIBA.

B. Film Thickness The obtained coating film and separator film for lithium ion batteries were cut into a circle with a diameter of 72 mm, and the thickness of the coating film was The thickness was measured at 15 arbitrary locations, and the average value of the 15 locations was taken as the film thickness.

C. Fabric Weight The obtained coating film and lithium ion battery separator film were cut into a circular shape with a diameter of 72 mm to obtain a sample. The basis weight value (g/m ² ) was obtained by weighing the sample using an electronic balance.

D. Air Permeability The time (sec) for 100 mL of air to pass through the obtained lithium ion battery separator film was measured using a Gurley densometer (manufactured by Toyo Seiki Seisakusho Co., Ltd.) in accordance with JIS P 8117. Further, the difference between the air permeability after coating and the air permeability of the film before coating was calculated as the coating film air permeability. Further, the air permeability per unit thickness of the coating film (sec/μm) was calculated by dividing the air permeability of the coating film by the thickness of the coating film.

E. Membrane Resistance An electrolytic solution was prepared by dissolving LiPF ₆ at a ratio of 1 mol/L in a mixed solvent of equal volumes of ethylene carbonate and diethyl carbonate. Place the aluminum positive and negative electrodes, the obtained coating film and lithium ion battery separator film between the positive and negative electrodes in this electrolyte, and measure the Cole-Cole plot using the complex impedance method using an LCR meter. Then, the real part of the impedance at 100,000 Hz was determined and used as an index of membrane resistance. The measurements were performed at 30° C. in a constant temperature bath. In the table, the rate of increase in membrane resistance was calculated from the membrane resistance obtained by the above measurement method, and is listed therein.

F. Heat Shrinkage Marks were placed at 5 cm intervals in the MD direction of the obtained separator film for lithium ion batteries, and the film was heated at 150° C. for 2 hours using a constant temperature bath (oven) without applying any load. The length between the marks after heating was measured and calculated as the shrinkage rate. In the table, it is expressed as heat shrinkage.

G. Tape peel strength This will be explained with reference to FIG. 4. A resin porous film 1 (obtained lithium ion battery separator film) provided with a coating film 2 was cut into 2 cm (TD) x 7 cm (MD), and a double-sided adhesive tape 3 (3M Japan Co., Ltd.) was also cut into a length of 2 cm. Scotch PREMIER GOLD (trade name) manufactured by Co., Ltd. was pasted on top of the coating film 2. Thereafter, a piece of kraft paper 4 cut to a width of 2 cm and a length of 7 cm is pasted on one side of the double-sided adhesive tape 3, and the ends of the coating film 2 and the paper 4 are sandwiched with chucks, and the coating layer 2 is pulled at a pulling speed of 500 mm/min. The interfacial peel strength (gf) of the resin porous film 1 was measured. In the table, it is expressed as tape peel strength.

[Explanation of symbols]
1. Resin porous film 2. Coating film 3. Double-sided adhesive tape 4. kraft paper

Claims (8)

An aqueous composition for coating a porous resin film, comprising a binder containing 50% by weight or more of a water-soluble acrylic resin, and a filler having a D90 of 1 μm or less as measured by a laser diffraction scattering particle size distribution analyzer, comprising: The water-soluble acrylic resin contains 40 to 45% by weight of hydroxyl group-containing monomer units and 8 to 12% by weight of acetyl group-containing monomer units ,
An aqueous coating composition for coating a porous resin film, comprising 1.6 to 6.4% by weight of the binder and 90.6 to 97% by weight of the filler based on the total solid content of the coating aqueous composition. Composition. The aqueous coating composition according to claim 1, wherein 80% or more of the filler particles on a volume basis have a size of 50 to 700 nm. The aqueous coating composition according to claim 1 or 2, wherein the filler is alumina. 2.5 to 5.0% by weight of the binder based on the total solid content of the aqueous coating composition;
The aqueous coating composition according to any one of claims 1 to 3, comprising 92 to 97% by weight of the filler. The aqueous coating composition according to any one of claims 1 to 4, wherein the binder is substantially free of water-insoluble resin. Any one of claims 1 to 5, wherein the water-soluble acrylic resin has a pH of 6 to 9 in a 1% by weight aqueous solution, and a viscosity of 4 to 8 Pa·s at 25°C in a 15% by weight aqueous solution. The aqueous coating composition described in . The aqueous coating composition according to any one of claims 1 to 6, wherein the resin porous film is a separator film for an electricity storage device. The aqueous coating composition according to any one of claims 1 to 7, wherein the resin porous film is a separator film for lithium ion batteries.