JPH11115084A - Laminated porous film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laminated porous film, the strength of which in pore- vanishing temperature region is large, which is excellent in the shape retention at melting or the like and can realize the perfect pore-losing in the pore- vanishing temperature region and substantially can shrink in the pore-vanishing temperature region under the condition being used as a cell separator and surely losses pores when an abnormality occurs. SOLUTION: This film is a laminated porous film having three or more layers, which consists of a high melting point porous polyolefin and a low melting point porous polyolefin, the difference between the melting point of which is 20 deg.C or more, and the surface layer part made of porous polyethylene under the condition that the gulley value of the total layers is 100-700 sec/100 cc and the elastic modulus of porous polyethylene surface layer part in a pore- vanishing temperature region is 10<4> dyne/cm<2> or higher.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laminated porous film useful as a separator for a battery or a diaphragm for an electrolytic capacitor.

[0002]

2. Description of the Related Art Hitherto, polyolefin porous films have been used as separators for batteries, diaphragms for electrolytic capacitors, and the like. In particular, with the advancement of technology in recent years, high-precision, high-performance separators have been required in lithium batteries and the like, and separators of laminated porous films have been attracting attention instead of conventional single-layer porous films. It has become.

[0003] Taking a battery as an example, batteries have both positive and negative
A separator is interposed to prevent short-circuiting of the pole.
Annual energy density, high electromotive force, low self-discharge
Non-aqueous electrolyte batteries such as lithium batteries, especially lithium secondary batteries
Ponds have been developed and put into practical use. Used as a negative electrode for lithium batteries
For example, the combination of lithium metal and lithium with other metals
Adsorb lithium ions such as gold, carbon and graphite
Ability to absorb or occlude by intercalation
Carbon material with lithium ion doping
Conductive polymer materials are known, and examples of positive electrodes
For example, (CF_x)_nFluorinated graphite, MnO_Two, V
_TwoO_Five, CuO, Ag_TwoCrO_Four, TiO _Two, LiCo
O_Four, LiMn_TwoO_FourMetal oxides, sulfides, chlorides, etc.
It has been known.

As a non-aqueous electrolyte, ethylene carbonate is used.
, Propylene carbonate, γ-butyrolactone,
Acetonitrile, 1,2-dimethoxyethane, tetrahi
LiPF for organic solvents such as drofuran₆, LiBF_Four, L
iCLO_Four, LiCF_ThreeSO _ThreeDissolved electrolytes such as
Is used. But lithium is particularly reactive
If an abnormal current flows due to an external short circuit or incorrect connection,
If the battery temperature rises significantly, a fire or other accident may occur.
Or cause thermal damage to equipment incorporating it.
I have a mind. To avoid such dangers,
Various porous films such as the following can be used as parators.
Has been proposed.

A single-layer porous film of a thermoplastic resin such as polyethylene or polypropylene (Japanese Patent Publication No. 46-401)
No. 19, Japanese Patent Publication No. 55-32531, Japanese Patent Publication No. 5
9-37292, JP-A-60-23954, JP-A-2-75151, U.S. Pat.
No. 538). Porous films made of a mixture of polyethylene having different molecular weights or a mixture of polyethylene and polypropylene (JP-A-2-21559, JP-A-5-33130)
No. 6). A porous film using a thermoplastic resin or a non-woven fabric as a support (JP-A-3-245457, JP-A-1-25883)
58 gazette). Laminated porous films in which a plurality of porous films of thermoplastic resins of different materials are laminated (JP-A-62-10857, JP-A-63-308866, JP-A-2-77)
No. 108, Japanese Patent Application Laid-Open No. 5-13062, Japanese Patent Publication No. 3
JP-A-65776, JP-A-6-55629, JP-A-6-20671, JP-A-7-307146, and the like. The porous film is generally prepared by stretching a non-stretched film by stretching, or extracting a filler, a plasticizer, etc. with a solvent from an unstretched film containing an extractable filler and a plasticizer. It is manufactured by an extraction method.

[0006] The basic concept of using a single-layer or laminated porous film as a separator is to prevent short-circuiting between both electrodes.
In addition to maintaining the battery voltage, etc., when the internal temperature of the battery rises above a predetermined temperature due to abnormal current or the like, the porous film is made nonporous, in other words, the pores are closed, and ions flow between the two electrodes. An object of the present invention is to increase the electrical resistance so as not to cause the battery function to stop, prevent a danger such as ignition due to an excessive rise in temperature, and ensure safety. The function of preventing danger due to excessive temperature rise is a very important function as a battery separator, and is generally called non-porous or shutdown (abbreviated as SD).

[0007] In a battery separator, if the non-porous temperature is too low, the flow of ions is prevented by a slight rise in temperature, which poses a problem in terms of practicality. There is a problem in terms of safety because there is a risk of causing ignition. Generally, it has been recognized that the non-porous temperature is preferably 110 to 160 ° C, preferably 120 to 150 ° C. In a battery using a porous film as a separator, if the temperature inside the battery rises above the heat-resistant temperature of the porous film, the film is melted and broken, the non-porous state is lost, and ions are regenerated. It starts to flow and causes a further rise in temperature. Therefore, it is required that the separator for a battery has an appropriate non-porous temperature, has a high heat resistance temperature, and is reliably non-porous in a non-porous temperature range. Further, as a battery separator, in addition to the properties related to non-porous, low electrical resistance, tensile modulus,
It is required that mechanical strength such as piercing strength is high, and that variations such as uneven thickness and electric resistance are small.

[0008]

Various types of porous films have been proposed as described above. However, as a battery separator, a single-layer porous film of polyolefin, for example, polypropylene, has a non-porous temperature of 170. There is a drawback that it is close to the melting point of lithium, which is about ℃ or higher, and the single-layer porous film of polyethylene has an appropriate nonporous temperature of about 135 ° C, but in addition to the heat resistance temperature of about 145 ° C, Since the piercing strength is low, holes are easily formed during the production process and use of the battery, and there is room for further improvement in the safety and strength of a porous film of a polyolefin single layer as a battery separator.

A porous film obtained by making a mixture of polyethylene having different molecular weights into a porous film has a heat-resistant temperature of about 150 ° C., which is slightly higher than the above-mentioned single-layer porous film of polyethylene. A sea-island structure porous film obtained by stretching a mixture of polyethylene and polypropylene to make it porous is improved in the SD function at a heat-resistant temperature of about 180 ° C compared to the polyethylene mixture, but the mechanical properties and the like are still insufficient. In addition, the formation of a sea-island structure that is made porous by stretching the mixture is likely to cause variations in quality, and there is a problem in its reproducibility. Further, a porous film using a non-woven fabric or the like as a support has not only a disadvantage in safety due to the non-woven fabric or the like but also a high temperature as in the case of the porous film such as the above polyethylene and polypropylene in terms of safety and the like. It is necessary to improve the reliability of the system.

With respect to a laminated porous film in which a plurality of porous films of thermoplastic resins of different materials are laminated and laminated, the film is previously made porous by a stretching method, an extraction method, or the like, so that two types of materials and different melting points are used. After the porous film is manufactured, it is laminated and stretched, compressed, manufactured by bonding with an adhesive, or manufactured by a method such as stretching a laminated film obtained by thermocompression bonding of polyolefins having different melting points. Have been.

These laminated porous films are obtained by combining two types of porous films having different melting points.
Provided is a battery separator having a non-porous temperature of about 135 ° C. and a heat-resistant temperature of about 180 ° C. and having excellent safety. Also, by combining two types of porous films having different mechanical strengths, it is possible to provide a battery separator having high mechanical strength such as piercing strength, and basically having excellent characteristics as a battery separator. I have.

[0012] These laminated porous films can be roughly classified into a stretching method (dry method) and an extraction method (wet method). The wet method is a method in which a filler is added to a thermoplastic resin having a different material or melting point. It is a method of producing a laminated film by co-extruding a resin composition containing a plasticizer and a plasticizer, then extracting a filler or a plasticizer from the film to make it porous, and obtaining a laminated porous film. In order to form a laminated porous film having a fine and uniform pore size, not only the operation steps are complicated, but also there are problems such as treatment of the extract and the like. On the other hand, in the stretching method, thermoplastic resins having different melting points are co-extruded or separately extruded and then laminated to be stretched and porous, or thermoplastic resins having different melting points are separately extruded and stretched. It is manufactured by a method of laminating after making it porous. Since these stretching methods are dry processes that do not use any solvent at all, they are extremely simple, have excellent safety, and are low-cost processes.In addition, a porous membrane having a fine and uniform pore size can be obtained. It is superior to the wet method as a production method. However,
Even in the case of the stretching method, there is a demand for the development of a separator which has a relatively narrow non-porous temperature range and can surely be non-porous.

In particular, a laminated porous film in which a plurality of porous films of thermoplastic resins of different materials are laminated is preferably used as a material having a plurality of functions such as SD characteristics and heat resistance for a battery separator and the like. ing. Above all, those using porous polyethylene for the intermediate layer and porous polypropylene for both outer layers have been put to practical use by making good use of the difference in their melting points. However, it has been pointed out that a laminated porous film whose surface layer (outer layer) is made of porous polypropylene is inferior in wettability and permeability of an electrolytic solution or the like.
An object of the present invention is to solve the above problems and to provide a laminated porous film having improved wettability to an electrolytic solution while maintaining heat resistance.

[0014]

According to the present invention, the melting point is 20 ° C.
A laminated porous film composed of the above-mentioned different high-melting-point porous polyolefin and low-melting-point porous polyolefin, wherein the surface layer portion is a porous polyethylene and three or more layers are laminated, and the Gurley value of all the layers is 100 to 700 sec / 100.
The present invention relates to a laminated porous film characterized in that the porous polyethylene film has a modulus of elasticity of 10 ⁴ dyne / cm ² or more in a nonporous temperature range of the surface polyethylene.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a high-melting-point porous polyolefin and a low-melting-point porous polyolefin have a small melting point difference. Those having different melting points of 20 ° C. or more, preferably 30 ° C. or more are used. In the present specification, the melting point of the polyolefin is about 1 sample using a differential scanning calorimeter (DSC-50, manufactured by Shimadzu Corporation).
It means the endothermic peak temperature accompanying melting when 0 mg is measured from a room temperature at a heating rate of 10 ° C./min under a nitrogen stream. As the high-melting-point porous polyolefin, for example, polypropylene, poly4-methylpentene-1, poly-3-methylbutene-1 and the like are used, and as the low-melting-point polyolefin, polyethylene, polybutene, ethylene-propylene copolymer and the like are used. . Preferably, polypropylene is used as the high-melting-point porous polyolefin, and polyethylene is used as the low-melting-point porous polyolefin. Polypropylene is preferably high in stereoregularity, and polyethylene is preferably high density polyethylene, but may be medium density polyethylene. The high-melting-point porous polyolefin and the low-melting-point porous polyolefin may appropriately contain additives such as a surfactant, an antioxidant, a plasticizer, a flame retardant, and a coloring agent.

In the present invention, the porous polyethylene of the surface layer has an elastic modulus of 10 ⁴ d in a nonporous temperature range.
yne / cm ² or more is used.

The method of measuring the elastic modulus in the present invention is carried out by the method described below.

If the elastic modulus of the porous polyethylene is excessively small, the shape retention during melting and the film strength are deteriorated. Therefore, a porous polyethylene having an elastic modulus of 10 ⁴ dyne / cm ² or more is used. . If the elastic modulus is too high, the film becomes brittle. Therefore, those having an elastic modulus of 10 ¹¹ dyne / cm ² or less in a non-porous temperature range are preferably used.

In general, when a polymer material is heated to a temperature close to its melting point, even if the crystal part is melted, it does not suddenly show a fluidized state like a low molecular substance, but has a region called a so-called rubbery state. Therefore, in the non-porous temperature range generally near the melting point, the porous polymer film does not easily flow and it is difficult to achieve complete non-porous.

In the present invention, when a porous polyethylene film having an elastic modulus of 10 ⁴ dyne / cm ² or more in a non-porous temperature range is used as the porous polymer film, the film strength is large and the viscosity is relatively high. It is excellent in shape retention and the like at the time of melting, and can achieve complete nonporosity by substantial shrinkage in a nonporous temperature range. In particular, the action of the shrinking force has the effect of shrinking the porous polymer film that is not easily flowable, and complete nonporosity can be achieved.

Therefore, when the porous polymer film of the present invention is used as a battery separator, the porous polymer film is reliably made nonporous when an abnormality occurs.

It is preferable that the shrinkage required for nonporosity is usually several percent or more, preferably 5% or more at the nonporous temperature. A larger shrinkage ratio is preferred in that the larger the shrinkage force, the greater the shrinkage force. However, if the shrinkage ratio is too large, the dimensional change will be too large. The non-porous temperature in the present invention refers to a resistance of a porous polymer film in an electrolyte at room temperature of 10%.
The temperature is 0 times or more, and the measuring method and the resistance measuring device are as follows.

Measurement method: Electrolyte solution An equal volume mixture of propylene carbonate and dimethoxyethane in which 1 mol of lithium perchlorate is dissolved is used. Electrode area 1 cm ² Resistance measuring device: LCR HiTester (manufactured by Hioki Electric Co., Ltd.) Measurement frequency 1 kHz A porous polymer film sample is immersed in an electrolytic solution for 5 minutes, set between electrodes, and set in an oven at a rate of 2 ° C./min. The resistance value was measured while raising the temperature at.

The wettability to the electrolytic solution was evaluated by the following measuring method. Measurement conditions: Temperature 23 ° C, Humidity 50% Electrolyte Use an equal volume mixture of propylene carbonate and dimethoxyethane in which 1 mol of lithium perchlorate is dissolved. Measuring device: Image processing type contact angle meter CA-X type Kyowa Interface Chemical Co., Ltd.

The smaller the contact angle, the better the separator wettability with the electrolyte. If the wettability is poor, the permeability of the electrolytic solution to the separator is inferior, the time required for the permeation becomes longer, or the electrolytic solution is easily depleted, which is not preferable.

The Gurley value of the laminated porous film is 100
It is preferably ~ 700 sec / 100 cc. If the Gurley value is excessively larger than this, not only the wettability is reduced, but also the movement speed of ions in the electrolyte is undesirably reduced. On the other hand, if the Gurley value is too small, the mechanical strength decreases, which is not preferable. By setting the Gurley value to the above range and making the laminated porous film in which the surface layer portion is always made of porous polyethylene, the wettability to the electrolyte is improved, and the permeability of the electrolyte to the separator is improved. .

The porosity of the porous polyethylene in the surface layer is 4
When the content is 0 to 60%, the wettability is further improved, which is preferable.
If the porosity is too large, the mechanical strength decreases, which is not preferable. If the porosity is too small, not only the wettability will decrease but also the ion moving speed will be undesirably low.

The film breaking temperature was determined by the following measuring method. Measuring method: sample width 25 mm, sample length 100 mm The temperature is raised with both ends in the length direction fixed. From room temperature to 1
Raise the temperature every 0 ° C. Hold at each temperature for about 10 minutes. The rupture state is visually observed. The temperature at the time of film rupture is defined as the film rupture temperature.

The elastic modulus is 10 ⁴ dy in the non-porous temperature range.
By laminating a porous polyethylene film having a ne / cm ² or more in a surface layer portion, a porous polyethylene film can be laminated with a high-melting-point porous polyolefin even at a temperature higher than a temperature range in which the porous polyethylene is nonporous. Accordingly, heat resistance equal to or higher than the melting point of the high-melting-point porous polyolefin can be maintained without causing film breakage.

The coefficient of static friction was determined by the following measuring method. Measuring method: Measured in accordance with JIS K7125 Dynamic / Static Friction Coefficient Measuring Device Rigaku Kogyo Co., Ltd. Sample size 63 mm square, load 200 g, moving speed 100 mm / min Counterpart material Chrome-plated stainless steel

A material having a small coefficient of static friction is preferred because it has excellent pin detachability when the electrode and the separator are wound together during battery assembly. When the porous polyethylene is disposed on the surface layer, the coefficient of static friction can be reduced, and the characteristics at the time of battery assembly are improved.

The present invention will be described in more detail below with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

Example 1 Using a T-die having a discharge width of 1000 mm and a discharge lip opening of 2 mm, a number average molecular weight of 70,000 and a weight average molecular weight of 48 were used.
0000, a melt index of 3, and a melting point of 166 ° C. polypropylene (Ube Polypro F10 manufactured by Ube Industries, Ltd.)
3EA) was melt extruded at 200 ° C. The discharge film was guided to a cooling roll at 90 ° C., cooled by blowing cool air at 25 ° C., and then taken out at 40 m / min.
The film thickness of the obtained unstretched polypropylene film is 11.
It was 5 μm. The unstretched polypropylene film is placed in a 3 inch diameter paper tube at 350 mm for heat treatment.
After being wound in a state of being wound by 0 m, it was placed in a hot-air circulation oven (PS-222, manufactured by Tabai Seisakusho) maintained at 120 ° C., allowed to stand for 24 hours, taken out of the oven, and cooled to room temperature.

Discharge width 1000 mm, discharge lip opening 2 m
Using a T die having a density of 0.964, a melt index of 0.33, and a melting point of 132 ° C., high-density polyethylene (HIZEX5202B manufactured by Mitsui Petrochemical Co., Ltd.)
It was melt extruded at 173 ° C. The discharge film was guided to a cooling roll at 115 ° C., cooled by blowing cool air at 25 ° C., and then taken out at 20 m / min. The film thickness of the obtained unstretched polyethylene film was 10.5 μm. This unstretched polyethylene film was placed in a hot-air circulation oven (type PS-222, manufactured by Taibai Seisakusho) held at 95 ° C. in a state of being wound around a 3-inch diameter paper tube at 3,500 m for 24 hours in order to heat-treat it. After standing, it was taken out of the oven and cooled to room temperature.

Next, a three-layer laminated film having a sandwich structure in which both outer layers were polyethylene and the inner layer was polypropylene was manufactured as follows. The heat-treated unstretched polypropylene film and unstretched polyethylene film were unwound at a winding speed of 4.0 m / min from three sets of roll roll tunds.
Thermocompression bonding was performed at 28 ° C. and a linear pressure of 1.5 kg / cm, and then guided to a cooling roll at 50 ° C. at the same speed and wound. The speed at this time was 4.0 m / min, and the unwinding tension was 3 kg for the polypropylene film and 0.9 kg for the polyethylene film.
kg.

This three-layer laminated film was stretched at a low temperature of 30% of the original size between nip rolls maintained at 35 ° C. At this time, the distance between the rolls was 350 mm, and the roll speed on the supply side was 1.6 m / min. Continue at 120 ° C
After being drawn into a hot air circulation oven heated to a temperature of 200% and stretched at a high temperature to a total stretching amount of 200% between the rollers using a difference in roll peripheral speed, the roll heated to 120 ° C. is 25%
The film was relaxed and heat-set for 25 seconds to continuously obtain a laminated porous film.

Table 1 shows the measurement results of the Gurley value of all layers, the porosity (%) of the surface layer, the contact angle (degree), the rupture temperature (° C.), and the coefficient of static friction of the obtained laminated porous film.

In the obtained laminated porous polymer film, the porous polyethylene film has a modulus of elasticity of 6.8 × 10 ⁸ dyne / cm ² at a nonporous temperature range of 133 ° C.
Met. The measurement of the contact angle (degree), the rupture temperature (° C.), and the coefficient of static friction were performed by the above methods. The porosity (%) of the surface layer and the Gurley value of the entire layer were measured by the following methods.

Porosity and maximum pore diameter The porosity and the maximum pore diameter were determined from the maximum value of a pore distribution curve measured with a mercury porosimeter (manufactured by Yuasa Ionic). Specifically, a sample piece having a MD of 30 mm and a TD of 300 mm was collected and placed in a cell, and the porosity and the maximum pore diameter were determined from the amount of mercury and the pressure relative to the pore diameter. Gurley value It was 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 from the test hole to the outside of the cylinder. The time required for 100 cc of air to pass was measured and defined as the air permeability (Gurley value).

[0040]

[Table 1]

Example 2 A laminated porous film was produced in the same manner as in Example 1, except that the stretching ratio was set to 250% stretching and 20% relaxation. Table 1 shows the measurement results of the Gurley value of all layers, the porosity (%) of the surface layer, the contact angle (degree), the rupture temperature (° C.), and the coefficient of static friction of the obtained laminated porous film. In the obtained laminated porous film, the porous polyethylene film had a modulus of elasticity of 6.8 × at a nonporous temperature range of 133 ° C.
It was 10 ⁸ dyne / cm ² . The above evaluation method was performed in the same manner as in Example 1.

Example 3 Except that the thickness of the raw polypropylene was 14.3 μm, the thickness of the raw polyethylene was 9.5 μm, and the stretching ratio was set to 270% stretching and 20% relaxation. A laminated porous film was manufactured in the same manner as in Example 1. Table 1 shows the measurement results of the Gurley value of all layers, the porosity (%) of the surface layer, the contact angle (degree), the rupture temperature (° C.), and the coefficient of static friction of the obtained laminated porous film. Further, in the obtained laminated porous film, the porous polyethylene film is:
Elasticity 6.8 × 10 ⁸ dyne in non-porous temperature range 133 ° C
/ Cm ² .

Comparative Example 1 As a laminated polymer film, a three-layer laminated film having a sandwich structure in which both outer layers were polypropylene and the inner layer was polyethylene was produced as follows. The heat-treated unstretched polypropylene film and unstretched polyethylene film were unwound at a winding speed of 4.0 m / min from three sets of roll roll tunds, respectively, and guided to a heating roll at a temperature of 134 ° C. and a linear pressure of 1.8 kg / min. cm, and then led to a 50 ° C. cooling roll at the same speed and wound up.
The speed at this time was 4.0 m / min, and the unwinding tension was 3 kg for the polypropylene film and 0.9 kg for the polyethylene film.

This three-layer laminated film was stretched at a low temperature of 20% between nip rolls maintained at 35 ° C. At this time, the distance between the rolls was 350 mm, and the roll speed on the supply side was 1.
It was 6 m / min. Subsequently, it is guided into a hot air circulating oven heated to 126 ° C., stretched at a high temperature to a total stretching amount of 180% between rollers using a difference in roll peripheral speed, and then 17% by a roll heated to 126 ° C. Relax, 2
By heat setting for 5 seconds, a laminated porous film was continuously obtained. Further, in the obtained laminated porous film, the elastic modulus of the porous polyethylene was 6.degree.
It was 7 × 10 ⁸ dyne / cm ² .

Table 1 shows the measurement results of the Gurley value of all layers, the porosity (%) of the surface layer, the contact angle (degree), the rupture temperature (° C.), and the coefficient of static friction of the obtained laminated porous film.

Comparative Example 2 Using a T-die having a discharge width of 1000 mm and a discharge lip opening of 2 mm, a density of 0.964 and a melt index of 0.3 were used.
3. A high-density polyethylene having a melting point of 132 ° C. (Hizex 5202B manufactured by Mitsui Petrochemical Co., Ltd.) was melt-extruded at 163 ° C. The discharge film is guided to a cooling roll of 125 ° C., and is cooled by blowing cool air of 25 ° C.
It was picked up at 10 m / min.

The unstretched polyethylene film is placed in a hot-air circulation oven (Model PS-222, manufactured by Taibai Seisakusho) which is kept at 125 ° C. while being wound around a 3-inch diameter paper tube at 3,500 m for heat treatment. For 150 seconds under 10% strain. Next, the heat-treated film was stretched at a low temperature by 50% between nip rolls maintained at 35 ° C. At this time, the distance between the rolls was 350 mm, and the roll speed on the supply side was 1.2 m / min. Continue 80
After being drawn into a hot-air circulation oven heated to 80 ° C. and stretched between the rollers at a high temperature up to a total stretching amount of 200% by reducing the peripheral speed of the rolls, the roll is heated to 108 ° C. and relaxed by 25%. It was heat-fixed for 28 seconds to continuously obtain a polyethylene single-layer porous film.

Table 1 shows the measurement results of the Gurley value of all layers, the porosity (%) of the surface layer, the contact angle (degree), the rupture temperature (° C.), and the coefficient of static friction of the obtained laminated porous film.

[0049]

The porous polymer film of the present invention has high film strength, relatively high viscosity, excellent shape retention during melting, and complete nonporosity in a nonporous temperature range. Can be achieved. Therefore, when the porous polymer film is used as a battery separator, it can be substantially contracted in a non-porous temperature range, and when an abnormality occurs, the porous polymer film is reliably non-porous. Further, the porous polymer film of the present invention has excellent wettability to an electrolytic solution,
Excellent workability during battery manufacturing.

──────────────────────────────────────────────────の Continued on front page (51) Int.Cl. ⁶ Identification code FI H01M 2/16 H01M 2/16 P

Claims (3)

[Claims] 1. A laminated porous film comprising a high-melting-point porous polyolefin and a low-melting-point porous polyolefin having melting points different from each other by 20 ° C. or more, and a surface layer portion of which is a laminated layer of three or more layers of porous polyethylene. Wherein the Gurley value of the laminated porous film is 100 to 700 sec / 100 cc, and the elastic modulus in the nonporous temperature range of the surface polyethylene is at least 10 ⁴ dyne / cm ² . 2. The porosity of the porous polyethylene of the surface layer is 4
The laminated porous film according to claim 1, wherein the content is 0 to 60%. 3. The laminated porous film according to claim 1, wherein the high-melting-point porous polyolefin is porous polypropylene.