JP2011210680A - Separator for battery - Google Patents

Abstract

An object of the present invention is to provide a battery separator that improves the liquid absorption rate of a porous film, particularly a polyolefin porous film that has been made porous by a dry method, and has high safety.
A battery separator comprising a porous film and a nonwoven fabric laminated on at least one surface of the porous film, the porous film being made of a thermoplastic polymer having a melting point of 150 ° C. or higher. The nonwoven fabric has a weight per unit area of 5 g / m ² or more and 30 g / m ² or less, and a maximum fiber diameter of the main constituent fiber is 3 μm or more and 20 μm or less. A battery separator, wherein the film thickness ratio a / b is 1 or more and 10 or less when the average film thickness is a μm and the average film thickness of the nonwoven fabric is b μm.
[Selection figure] None

Description

  The present invention relates to a composite separator of a polyolefin porous film and a nonwoven fabric useful as a battery separator or an electrolytic capacitor diaphragm.

  Conventionally, polyolefin porous films have been used as battery separators, diaphragms for electrolytic capacitors, and the like. Taking a battery as an example, a non-aqueous electrolyte battery such as a lithium battery with high energy density, high electromotive force, and low self-discharge, particularly a lithium ion secondary battery, has been developed and put into practical use. The role of the separator, which is a constituent material of the lithium secondary battery, is to hold the non-aqueous electrolyte while preventing a short circuit between the positive and negative electrodes. Currently, various polyolefin porous films as described below have been proposed.

  Porous methods for polyolefin porous film are roughly classified into dry methods (stretching methods) and wet methods (extraction methods). Wet methods involve extruding a resin composition containing a filler and a plasticizer in a thermoplastic resin. This is a method of manufacturing a film, and then extracting the filler and plasticizer from the film to make it porous to obtain a porous film. This method requires the blending and extraction of the filler and plasticizer, and is fine and uniform. In order to obtain a porous film having an appropriate pore size, not only the operation process is complicated, but also there are problems such as processing of the extract. On the other hand, the dry method is manufactured by a method in which a thermoplastic resin is extruded and then stretched and made porous, so that the process is simple and does not require an extraction liquid treatment.

  Various types of battery separators using the porous film obtained by the above method have been proposed. In recent years, separators for large-sized lithium ion secondary batteries represented by electric vehicles and stationary batteries for power storage are proposed. As the required characteristics, it is important to improve the rate of electrolyte absorption and to ensure safety when an internal short circuit occurs.

  The polyolefin porous film as shown in Patent Document 1 has a shutdown function for ensuring safety when an internal short circuit occurs or overcharges, but because the polyolefin resin itself is hydrophobic, the electrolyte absorption rate However, the electrolyte injection process takes a long time. In particular, in recent years, with the increase in size of batteries, there is an increasing demand for improvement in liquid absorption speed. For this reason, the electrolyte solution absorption speed | rate of a battery separator is improved and the shortening of a liquid injection process is calculated | required.

  As a method for hydrophilizing a battery separator, Patent Document 2 discloses a technique for chemically hydrophilizing, but a neutralization / washing / drying process is necessary, which leads to a large cost increase. Patent Document 3 describes a technique for hydrophilization by corona discharge treatment or the like, but if the polyolefin porous film is immediately subjected to corona discharge treatment, the film surface is hydrophilized. It is known that the hydrophilization effect decreases with the progress of.

  Patent Document 4 describes a separator in which a nonwoven fabric formed from a melted liquid crystalline polyester fiber and an olefinic microporous film are bonded together, but the electrolyte solution absorption speed is not considered. There is a need for further improvements.

JP-A-7-307146 JP 2001-11764 A JP 2000-299096 A JP 2009-76350 A

  The present invention has been made in view of the above situation, and improves the liquid absorption speed of a porous film, particularly a polyolefin porous film porous by a dry method, and has a high safety. The purpose is to provide.

  As a result of diligent research, the present inventors have determined that a nonwoven fabric having a specific weight per unit weight and a nonwoven fabric in which the maximum fiber diameter of fibers mainly composed of the nonwoven fabric is in a specific range and a porous film are constant. It has been found that a battery separator having a further excellent safety and an excellent liquid absorption speed can be obtained by stacking and integrating with a film thickness of a ratio.

The present invention is a battery separator having a porous film and a nonwoven fabric laminated on at least one surface of the porous film, wherein the porous film is made of a thermoplastic polymer having a melting point of 150 ° C. or higher. The nonwoven fabric has a weight per unit area of 5 g / m ² or more and 30 g / m ² or less, and a maximum fiber diameter of the main constituent fiber is 3 μm or more and 20 μm or less. The present invention relates to a battery separator, wherein the average film thickness is a μm and the average film thickness of the nonwoven fabric is b μm. Here, the fiber constituting the main body means a fiber contained in the nonwoven fabric in an amount exceeding 50% by weight.

  The battery separator of the present invention has the effect of improving the electrolyte absorption rate and heat resistance by laminating a porous film and a weight per unit area and a nonwoven fabric that controls the maximum fiber diameter of the main constituent fiber. It is done. It is considered that the improvement of the liquid absorption speed is manifested by a capillary phenomenon between the porous film layer and the nonwoven fabric layer. In addition, since the porous film has a thermoplastic polymer layer having a melting point of 150 ° C. or higher, thermal shrinkage of the porous film is suppressed to the melting temperature, and after melting, the polymer closes the voids of the nonwoven fabric. Furthermore, heat resistance is improved by maintaining the shape at a higher temperature.

It is a figure which shows the temperature characteristic of the non-porous behavior of a composite separator and a polyolefin porous film.

  The separator of this invention has a porous film and the nonwoven fabric laminated | stacked on the at least one surface of this porous film as above-mentioned.

  If the separator's non-porous temperature is too high, it will be difficult to ensure safety when an internal short circuit occurs, and if it is too low, it may become non-porous in the temperature range of the normal use range, thus impairing the convenience of the battery. . For this reason, although it sets according to the characteristic and use environment of a battery, it is preferable to set so that a non-porous temperature may become 130-140 degreeC in a specific use. In addition, the non-porous maintenance temperature of the separator of the present invention exceeds the temperature of the separator made of only a conventional porous film, but in order to maintain non-porous up to a high temperature, the porous film alone is 170 ° C. or more. It is preferable to have a non-porous maintenance temperature.

  In order to satisfy such characteristics, the porous film of the present invention has a thermoplastic polymer layer having a melting point of 150 ° C. or higher, preferably a laminated porous film, and preferably has a melting point of 150 ° C. or higher. And a thermoplastic polymer layer having a melting point in the range of 120 ° C to 140 ° C.

  The porous film is preferably made of a polyolefin-based material. The thermoplastic polymer layer having a melting point of 150 ° C. or higher may be formed of polypropylene (PP), and the thermoplastic polymer layer having a melting point in the range of 120 ° C. to 140 ° C. may be formed of polyethylene (PE). preferable. Most preferably, it is a porous film laminated in the order of PP / PE / PP.

  Although the film thickness of a porous film is based also on the kind of battery used, generally 10-100 micrometers is preferable, More preferably, it is 15-60 micrometers.

  Further, the porous film needs to have an appropriate air permeability (gas permeation rate) although it varies somewhat depending on the production conditions, and usually has a Gurley value of 100 to 1000 seconds / 100 cc, preferably 150 to 600 seconds / Preferably it has a range of 100 cc. When the Gurley value is too high, the function when used as a battery separator is not sufficient, and the production rate of a film having a low Gurley value is lowered.

  Reinforcing the porous film with resin additives such as colorants, plasticizers, lubricants, flame retardants, anti-aging agents, antioxidants, etc. An agent may be included.

  The method for producing the porous film used in the present invention is not particularly limited, but in the case of a polyolefin laminated porous film, it is particularly preferably produced by a dry stretching method. It can be produced by a known method such as Japanese Patent No. 181651.

As the nonwoven fabric used in the present invention, the basis weight is preferably 5 g / m ² or more. When the weight per unit area is out of this range, when the battery generates heat and the polyolefin porous film is melted, there is a high possibility that a short circuit will occur after the shutdown function is exhibited. The weight per unit area is usually 30 g / m ² or less.

  Moreover, it is preferable that the maximum fiber diameter of the fiber mainly composed of the nonwoven fabric is 20 μm or less. If the maximum fiber diameter exceeds 20 μm, the thickness of the non-woven fabric becomes too thick and too thick as a battery separator. Here, “to constitute as a main body” means a fiber contained in the nonwoven fiber in an amount exceeding 50% by weight as described above. Further, the maximum fiber diameter of the fibers constituting the main body is preferably 3 μm or more. When the maximum fiber diameter is less than 3 μm, the film thickness and strength of the nonwoven fabric are lowered, handling becomes difficult in the paper making process and the lamination process, and it is difficult to stably produce the battery separator, which is not preferable.

  In addition, from the viewpoint of ensuring safety when an internal short circuit occurs in a battery, the nonwoven fabric is made of a group in which the fibers constituting the nonwoven fabric are composed of fibers having a melting point of 210 ° C. or higher and fibers that do not undergo thermal decomposition at 210 ° C. or lower. It is preferable to be selected. Examples of fibers that satisfy this condition include cellulose, polyethylene terephthalate (PET), polyamide, polyimide, polyamideimide, polyacrylonitrile, wholly aromatic polyester, and glass. In particular, it is preferable to contain cellulose fibers, and it is particularly preferable that the fibers constituting the main body are cellulose fibers. Nonwoven fabrics composed of cellulose fibers and PET fibers are preferred from the standpoint of nonwoven fabric productivity, film thickness uniformity, and material cost.

  The nonwoven fabric in the present invention may use either short fibers or long fibers as the fibers constituting the web, and is formed by a general-purpose manufacturing method. As a manufacturing method, for example, a carding machine or the like is used to defibrate synthetic resin or the like to form a sheet, an air lay to make the defibrated fiber into a sheet by air flow, or the fiber in water. A paper-making method that evenly disperses and flows this on a wire to form a sheet, and then dehydrates and dries. In addition, the spunbond method that spreads and accumulates the spun continuous yarn to form a web, and the spinning machine Examples thereof include a melt blowing method in which continuous yarn is blown off with hot air and then collected on a conveyor to form a web. Further, any method such as a method combining the above methods may be used, and a nonwoven fabric produced by a plurality of production methods may be combined. Furthermore, as a method of bonding or entanglement of the web, a thermal bonding method such as a chemical bonding method, a calendar method, an air-through heating method, etc., in which an adhesive or a heat-bonding fiber is mixed to bond the fibers in the web, Any method such as a mechanical bonding method such as a needle punch method, a hydroentanglement method, or a stitch bond method may be used.

  In addition, the nonwoven fabric fiber of the present invention has various other commonly used additives, for example, a crystal nucleating agent, an anti-coloring agent, an antioxidant, a release agent, a plasticizer, as long as the object of the present invention is not impaired. Additives such as a lubricant, a flame retardant, a colorant, an ultraviolet absorber, a light stabilizer, an antistatic agent, or an inorganic powder can be added. Furthermore, the cross section of the non-woven fabric fiber may be circular, elliptical, polygonal, such as triangular or square, or an irregular sectional shape, such as flat or hollow.

  The papermaking method will be described as an example of the method for producing the nonwoven fabric used in the present invention. First, the fiber is dispersed in water. In order to uniformly disperse, a dispersing device such as a pulper or an agitator or an ultrasonic dispersing device is used. Next, it beats using a general beating machine such as a ball mill, beater, lampel mill, PFI mill, SDR (single disc refiner), DDR (double disc refiner), high pressure homogenizer, homomixer, or other refiner. The obtained fiber dispersion is made by applying a wet paper machine such as a long-mesh type, a short-mesh type, a circular net type, or an inclined type, and dehydrated in a continuous wire mesh dehydration part. Furthermore, after overlapping the sheets, the nonwoven fabric of the present invention can be obtained by passing through a drying part such as a multi-cylinder type or a Yankee type dryer. The papermaking method is suitable for the production of a nonwoven fabric containing cellulose fibers, and is also suitable in that the distribution of fibers tends to be uniform when two or more types of fibers are included.

  When the average film thickness of the porous film is a μm and the average film thickness of the nonwoven fabric is b μm, the film thickness ratio a / b is preferably 1 ≦ a / b ≦ 10, particularly preferably 1.5 ≦. a / b <5. This is because the nonwoven fabric in the present invention has more voids than the porous film, and thus the nonwoven fabric itself has a large amount of electrolyte solution. That is, if the value of the film thickness ratio a / b is smaller than 1, the nonwoven fabric becomes relatively thick, and the total film thickness as the battery separator becomes too thick. As a result, the amount of injection of the electrolyte increases, which contributes to an increase in battery manufacturing cost. Further, when the amount of liquid injection is insufficient, the battery performance is lowered, which is not preferable. Further, if the value of the film thickness ratio a / b is larger than 10, it becomes difficult to make the nonwoven fabric thin, so that the film thickness of the porous film is relatively increased and the film thickness is not suitable as a battery separator. .

  The nonwoven fabric is preferably laminated in contact with a layer of a thermoplastic polymer having a melting point of 150 ° C. or higher of the porous film. For example, in the case of a laminated porous film of PP and PE, a nonwoven fabric is laminated in contact with PP, and if a porous film having a three-layer structure of PP / PE / PP, the nonwoven fabric is naturally laminated in contact with PP. Is done.

  The separator of the present invention may be formed by laminating at least one porous film and one non-woven fabric, but depending on the application, the structure in which the non-woven fabric is laminated on both sides of the porous film, porous on both sides of the non-woven fabric. A structure in which films are laminated may be used.

  The method for laminating the porous film and the nonwoven fabric is not particularly limited, and can be appropriately selected according to the film and fiber materials used. When laminating a polyolefin porous film and a nonwoven fabric mainly composed of cellulose fibers, for example, an adhesive can be used for lamination.

  In the present invention, the average film thickness, Gurley value, maximum fiber diameter of nonwoven fabric, basis weight, adhesive coating amount, liquid absorption rate, liquid retention amount, non-porous temperature, non-porous maintenance temperature are evaluated by the following methods. went.

(1) Gurley value Measured according to JIS P8117. A B-type Gurley Densometer (manufactured by Toyo Seiki Co., Ltd.) was used as a measuring device. The sample piece is clamped in a circular hole having a diameter of 28.6 mm and an area of 645 mm ² . With the inner cylinder weight of 567 g, the air in the cylinder is allowed to pass out of the cylinder from the test circular hole. The air permeability (Gurley value) was measured by measuring the time for 100 cc of air to pass.

(2) An electronic micrometer Millitron 1202D (manufactured by mahr) was used as an average film thickness measuring instrument. The test piece film thickness was measured at 20 points, and the average value of 20 points was defined as the average film thickness.

(3) Maximum fiber diameter of non-woven fabric The maximum fiber diameter [μm] of the non-woven fabric is observed at any five locations with a field emission scanning electron microscope (FE-SEM), and the diameter of the thickest fiber constituting the main body is determined. taking measurement. Similarly, measurement was performed at five different fields of view, and the fiber diameter of the thickest fiber was taken as the maximum fiber diameter of the nonwoven fabric.

(4) Weight per unit of weight About each of the nonwoven fabric, the porous film, and the prepared composite separator, a test piece was cut into a square having a side of 200 mm, and the weight of the test piece was measured using a balance. Then, the basis weight of each test piece was calculated by the following formula 1.

(Formula 1)
[Weight weight g / m ² ] = [Test piece weight g] / [Test piece area 0.04 m ² ]

(5) Adhesive application amount Adhesive application amount [g / m ² ] is a value obtained by subtracting the weight per unit area of the nonwoven fabric and the porous film used in the preparation of the composite separator from the basis weight of the composite separator calculated by the method described above. It was.

(6) Liquid absorption speed A sample piece for measuring the liquid absorption speed is cut into a width of 30 mm and a length of 60 mm, and 5 mm is immersed in the test liquid from one end in the longitudinal direction of the sample piece. As a test solution, propylene carbonate (purity 98% or more manufactured by Tokyo Chemical Industry Co., Ltd.) was used. The test piece 2 minutes after the immersion was taken out, and the wetted length of the liquid soaked from one end of the immersed side was defined as the liquid absorption height h. The liquid absorption speed was calculated by the following equation 2.

(Formula 2)
[Liquid absorption speed mm / min] = [Liquid absorption height h mm] / [Liquid absorption time 2 min]

(7) Liquid retention amount measurement A test piece for measuring the liquid retention amount was cut into a width of 50 mm and a length of 50 mm and immersed in the test solution for 10 minutes. Ten minutes later, the test piece was pulled up, and excess test solution on the test piece was wiped off with waste cloth. From the change in weight before and after the immersion, the amount of liquid retained per unit area was calculated from Equation 3. Liquid paraffin having a density of 0.87 g / cm ³ at 25 ° C. was used as a test solution.

(Formula 3)
[Liquid retention amount g / m ² ] = [weight after immersion−weight before immersion g] / [area of test piece m ² ]

(8) Measurement of non-porous temperature and non-porous maintaining temperature As a non-porous property evaluation, the temperature dependence of electrical resistance was measured using an electrical resistance measuring cell. The electric resistance measurement cell was a nickel electrode having a pair of electrode areas of 2.0 cm ² . A separator sample piece wetted with an electrolytic solution was sandwiched between measurement cells, and the temperature was raised at 10 ° C./min using a ceramic heater. The interelectrode resistance was measured under the condition of a measurement frequency of 1 kHz using a resistance measuring device: LCR HiTester (manufactured by Hioki Electric Co., Ltd.). The electrolyte used for the measurement is a mixed solvent of propylene carbonate (PC) and diethyl carbonate (DEC) in a volume ratio of PC / DEC = 3/7, and has a concentration of 1 mol / L with respect to the mixed solvent. Thus, lithium hexafluorophosphate was dissolved to obtain an electrolytic solution. At this time, the temperature at which the electrical resistance reached 1000Ω was defined as the non-porous temperature. Further, the temperature was continuously raised to 200 ° C. even after the non-porous temperature to check whether or not a re-short circuit occurred, and the temperature at which the re-short circuit occurred was defined as a non-porous maintaining temperature.

<Example 1>
Polyolefin film made of polypropylene (PP) having a melting point of 161 ° C. and polyethylene (PE) having a melting point of 132 ° C. are laminated in the order of PP / PE / PP and made porous by a dry stretching method. did. This porous film had an average film thickness of 16.3 μm and a Gurley value measured according to JIS P8117 of 310 seconds / 100 cc.

As a nonwoven fabric, the cellulose fiber consists of 70 parts by weight and 30 parts by weight of PET fiber, the average film thickness is 10.5 μm, the basis weight is 5.8 g / m ² , and the maximum fiber diameter range of the cellulose fiber constituting the main body is 15.5 μm. A non-woven fabric was used.

A polyolefin porous film and a non-woven fabric were adhered and laminated with an SBR emulsion. The polyolefin porous film and nonwoven fabric laminated in a drying oven at 90 ° C. were thermally dried for 30 minutes to obtain a composite separator in which the polyolefin porous film and nonwoven fabric were laminated and integrated. As the adhesive, an SBR emulsion having a resin component concentration of 48.5%, a hydrogen ion index pH of 7.3, and a viscosity at 25 ° C. measured using a B-type rotary viscometer of 51 mPa · s is purified water. Was used after diluting to a resin component concentration of 7%. The adhesive was applied so that the adhesive application amount after heat drying was about 1.5 g / m ² .

  When the average thickness of the composite separator is 30.7 μm, the Gurley value is 335 seconds / 100 cc, the average thickness of the polyolefin porous film is a μm, and the average thickness of the nonwoven fabric is b μm, the value of the thickness ratio a / b Was 1.55. Using this composite separator, the liquid absorption speed, the liquid retention amount, the non-porous temperature and the non-porous maintaining temperature were measured. The results are shown in Tables 1 and 2. Moreover, the temperature characteristics of the non-porous behavior are shown in FIG.

<Comparative Example 1>
Without laminating the nonwoven fabric, the polyolefin porous film described in Example 1 alone was tested using a polyethylene porous film having a melting point of 133 ° C. The results are shown in Tables 1 and 2 and FIG.

<Comparative example 2>
The nonwoven fabric is composed of 70 parts by weight of cellulose fibers and 30 parts by weight of PET fibers, the average film thickness is 21.0 μm, the basis weight is 11.7 g / m ² , and the maximum fiber diameter of the main constituent fiber is 16.5 μm. A composite separator was obtained in the same manner as in Example 1 except that a non-woven fabric was used. This composite separator had an average film thickness of 43.2 μm, a Gurley value of 323 seconds / 100 cc, and a film thickness ratio a / b of 0.78. The test results are shown in Tables 1 and 2.

<Comparative Example 3>
As the polyolefin porous film, a PE monolayer film made porous by a wet method having an average film thickness of 25.2 μm and a Gurley value of 592 seconds / 100 cc was used. A composite separator was obtained in the same manner as in Example 1 except that the PE single layer film was used. This composite separator had an average film thickness of 40.4 μm, a Gurley value of 780 seconds / 100 cc, and a film thickness ratio a / b of 2.40. The test results are shown in Tables 1 and 2 and FIG.

<Comparative example 4>
Nonwoven fabric comprising 70 parts by weight of cellulose fibers and 30 parts by weight of PET fibers, an average film thickness of 10.0 μm, a basis weight of 4.7 g / m ² , and the maximum fiber diameter of the main constituent fiber is 14.5 μm A composite separator was obtained in the same manner as in Example 1 except that was used. This composite separator had an average film thickness of 30.8 μm, a Gurley value of 400 seconds / 100 cc, and a film thickness ratio a / b of 1.63. The test results are shown in Tables 1 and 2 and FIG.

  As is clear from the results of Tables 1 and 2 and FIG. 1, the separator of the present invention shown in Example 1 was confirmed to have a significant improvement in the liquid absorption speed without impairing the air permeability. Moreover, the improvement of the non-porous maintenance temperature was confirmed without impairing the non-porous characteristics.

  From the results shown in Comparative Example 2, if the value of the film thickness ratio a / b is smaller than 1, the liquid absorption speed is excellent, but the liquid retention amount increases excessively, which may cause an increase in cost. Further, since the non-porous property is equivalent to that of Example 1, the comparative example 2 does not show the property superior to that of Example 1.

  From the result shown in Comparative Example 3, when a PE single layer product made porous by a wet method is used as the polyolefin porous film, the nonporous maintenance temperature is lower than that of the three-layer separator made of PP / PE / PP. Therefore, it is judged that the safety as a battery separator is low.

From the result shown by the comparative example 4, when the fabric weight of 5 g / m < ² > or less was laminated | stacked as a nonwoven fabric, the non-porous maintenance temperature fell. Although the cause is not clear, it is considered that when the weight per unit area of the nonwoven fabric decreases, the number of voids increases, and the melted polyolefin porous film cannot completely block the voids of the nonwoven fabric.

  The separator of the present invention is high in safety and has an improved liquid absorption rate. For this reason, it is advantageously used as a separator for a battery, for example, a lithium ion secondary battery.

Claims (9)

A battery separator having a porous film and a nonwoven fabric laminated on at least one surface of the porous film,
The porous film has a layer of a thermoplastic polymer having a melting point of 150 ° C. or higher,
The nonwoven fabric has a weight per unit area of 5 g / m ² or more and 30 g / m ² or less, and the maximum fiber diameter of the main constituent fiber is 3 μm or more and 20 μm or less,
A battery separator, wherein the average film thickness of the porous film is a μm, and the average film thickness of the nonwoven fabric is b μm, the value of the film thickness ratio a / b is 1 or more and 10 or less.   The porous film is a laminated porous film having a thermoplastic polymer layer having a melting point of 150 ° C. or higher and a thermoplastic polymer layer having a melting point in the range of 120 ° C. to 140 ° C. The battery separator according to claim 1.   The battery separator according to claim 1 or 2, wherein the porous film is manufactured by a dry stretching method.   The thermoplastic polymer layer having a melting point of 150 ° C. or higher is a polypropylene layer, the thermoplastic polymer layer having a melting point in the range of 120 ° C. to 140 ° C. is a polyethylene layer, and the polypropylene layer is the nonwoven fabric. The battery separator according to claim 2, wherein the battery separator is laminated in contact therewith.   2. The battery separator according to claim 1, wherein the fibers constituting the nonwoven fabric are selected from the group consisting of fibers having a melting point of 210 [deg.] C. or higher and fibers that do not undergo thermal decomposition at 210 [deg.] C. or lower.   The fiber constituting the nonwoven fabric is one or more kinds of fibers selected from the group consisting of cellulose, polyethylene terephthalate (PET), polyamide, polyimide, polyamideimide, polyacrylonitrile, wholly aromatic polyester, and glass. The battery separator according to claim 5.   6. The battery separator according to claim 5, wherein the fibers mainly composed of the nonwoven fabric are cellulose fibers.   6. The battery separator according to claim 5, wherein the nonwoven fabric is composed of cellulose fibers and polyethylene terephthalate fibers.   The battery separator according to claim 8, wherein the fiber mainly composed of the nonwoven fabric is a cellulose fiber.

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