JPH10279718A - Porous film, separator for battery and battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prepare a porous film capable of preventing the temperature increase of a lithium battery owing to the reaction of metallic lithium, which deposits on a negative pole when the lithium battery becomes an overcharge state, with the electrolyte, and suitable as a separator. SOLUTION: This porous film contains polyethylene and polypropylene as essential components, and it has a multilayered structure consisting of a low- content layer, whose polyethylene content is 0-20 wt.%, and a high-content layer, whose polyethylene content is 61-100 wt.%, and which contains >=0.5 wt.% a polyethylene having a melt index of >=3. When the porous film is used in a lithium battery, the polyethylene which forms the high-content layer covers the surface of metallic lithium which has molten and deposited on a negative pole to prevent the contact of the metallic lithium with the electrolyte, and thus the temperature elevation of the battery is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multilayer porous film containing polyethylene (hereinafter, referred to as "PE") and polypropylene (hereinafter, referred to as "PP") as essential components, a battery separator comprising the porous film, and a battery separator. The present invention relates to a battery incorporating a separator.

[0002]

2. Description of the Related Art Various types of batteries have been put to practical use.
Lithium batteries that are lightweight, provide high electromotive force and high energy, and have low self-discharge have attracted attention. For example, cylindrical lithium ion secondary batteries are being mass-produced for mobile phones, notebook computers, and the like.

As the negative electrode material of this lithium battery, there can be mentioned an intercalation compound such as lithium metal, a lithium alloy and a carbon material capable of occluding and releasing lithium ions, and the positive electrode material is MeO ₂ , LiMeO _2
2 (Me is a transition metal such as Co, Ni, Mn, and Fe). Further, as the electrolytic solution, ethylene carbonate, propylene carbonate, acetonitrile, γ-butyrolactone, 1,2-dimethoxyethane,
LiPF ₆ , LiC
It is known that F ₃ SO ₃ , LiClO ₄ , LiBF ₄ or the like is dissolved as an electrolyte.

When an abnormal current flows due to an external short circuit or erroneous connection of the positive and negative electrodes, the temperature of the lithium battery made of the above-mentioned materials significantly increases, and the temperature of the battery becomes extremely high. There is a concern of giving.

Therefore, when the temperature rises due to the abnormal current, the battery reaction is cut off by increasing the electric resistance of the separator incorporated to prevent a short circuit between the positive electrode and the negative electrode, thereby preventing the temperature from rising excessively. I have to.

[0006] In this way, when the temperature of the battery rises, the function of interrupting the battery reaction by increasing the electric resistance and ensuring safety by preventing the temperature from excessively rising is generally called a shutdown (Shut-down) characteristic. This is an important property for a lithium battery separator.

The applicant of the present invention has proposed a porous film containing polyethylene and polypropylene as essential components and having a polyethylene content varying in the thickness direction of the film, as such a battery separator having excellent SD characteristics. (Japanese Patent Application Laid-Open No. 7-216118).

[0008]

[0005] External short-circuits and erroneous connections are one of the factors that impair the safety of lithium batteries.
The problem of "precipitation of metallic lithium" can be cited.

[0009] The term "precipitation of metallic lithium" means that when the battery is overcharged, the balance between the amount of the positive electrode active material and the amount of the negative electrode active material in the battery is lost. This refers to a phenomenon in which ions are deposited on the negative electrode surface as crystals of metallic lithium. When metallic lithium precipitates on the surface of the negative electrode, it reacts with the electrolytic solution and generates heat, which causes an increase in battery temperature. This increase in battery temperature due to the deposition of metallic lithium is, of course, undesirable, but at present no effective means has been proposed to prevent it.

[0010]

DISCLOSURE OF THE INVENTION The present invention has been made as a result of intensive studies in view of the above situation. When lithium metal is deposited, the reaction between the lithium metal and the electrolytic solution is interrupted at an early stage, and the battery is used. Provided is a porous film suitable as a separator having a function of effectively preventing inconvenience such as reaching a dangerous zone.

That is, the porous film according to the present invention comprises P
A porous film containing E and PP as essential components, wherein a PE low content layer having a PE content of 0 to 20% by weight;
PE having an E content of 61 to 100% by weight and a PE having a melt index of 3 or more and 0.5% by weight or more
A high-content layer.

In the present invention, the pore diameter of the high PE content layer is preferably 0.04 to 0.15 μm.

The porous film of the present invention has a three-layer structure in which a high PE content layer is provided on both sides of a low PE content layer, or a high PE content layer is provided on one surface of a low PE content layer. It is preferable to have a two-layer structure.

Further, in the porous film of the present invention, the thickness of the PE-rich layer is preferably 10 to 50% of the total thickness of the film.

[0015] The porous film of the present invention is suitably used as a battery separator.

The present invention also includes a positive electrode, a negative electrode, a separator and an electrolyte interposed between these two electrodes, wherein the separator is the above-mentioned porous film, and the PE-rich layer of the porous film is provided. Is also provided as a battery in which the battery is incorporated in contact with the negative electrode.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The porous film of the present invention contains PE and PP as essential components as described above,
As the PP which is one of the essential components, isotactic PP having good stereoregularity is preferable from the viewpoint of easy porosity, but other PP can also be used. Although not particularly limited, PP having a weight average molecular weight of 500,000 or more is preferably used in consideration of the strength of the porous film, and more preferably 800,000 or more. In addition, PE, which is another essential component, is not particularly limited, but high-density PE or medium-density PE is preferable from the viewpoint of easy porosity.

The porous film of the present invention contains PE and PP as essential components as described above.
It is necessary to have two layers having different contents. One is a low PE layer having a PE content of 0 to 20% by weight, and the other is a PE having a PE content of 61 to 100% by weight and a melt index (hereinafter referred to as "MI"). ) Is a high PE layer containing 0.5% by weight or more of 3 or more PEs.

As described above, the porous film of the present invention is made of PE
Having a low content layer and a high PE content layer inevitably results in a multilayer structure. In addition, these PE low content layer and PE high content layer are both porous layers.

The low PE content layer is a layer formed of only PP or a mixture of PE and PP, and the ratio of the PE weight to the total weight of PE and PP (PE content), which is a component of this layer, is: 0 to 20%.

The low PE content layer has the function of maintaining the mechanical strength of the porous film and maintaining the shape of the film. When the PE content exceeds 20%, these functions tend to decrease, so that it is preferable. Absent. For example, a porous film having a PE content of more than 20% in this layer has a low mechanical strength, and a lithium battery incorporating this as a separator has a relatively low overall temperature when the internal temperature rises for some reason. Is in a molten state, and it is difficult to maintain the shape. As a result, the function of preventing short-circuiting of the positive and negative electrodes may be lost at an early stage.

On the other hand, the PE-rich layer is composed of only PE or P
It is a layer formed from a mixture of E and PP, and the ratio of the PE weight to the total weight of PE and PP (PE content) is 61 to 100% (preferably 80%).
１００100%). What is important in this high PE layer is that the PE content is set high and the MI content is high.
Is that the use ratio of PE of 3 or more is 0.5% by weight or more.

This PE-rich layer serves, for example, in a lithium battery incorporating a porous film as a separator, to shut off the exothermic reaction accompanying the deposition of metallic lithium. Therefore, the PE content is set to 61% or more. It is doing. If the PE content in this layer is less than 61%, the function of blocking the exothermic reaction is not sufficient, and the temperature tends to be excessively increased, which is not preferable.

In the present invention, as the PE which is a component forming the high PE content layer, the MI is 3 or more (preferably 3 to
30), the function of blocking the exothermic reaction is further ensured. In addition, as PE which is a forming component of this layer,
Only those having an MI of 3 or more may be used, but may be used in combination with a PE having an MI of less than 3. However, the ratio of the weight of the PE whose MI is 3 or more to the total weight of the PE and PP forming this layer is 0.5% or more (preferably 10 to 10%).
0%).

In the porous film of the present invention, due to the presence of the high PE content layer, the reaction between the lithium metal deposited on the negative electrode surface due to overcharging and the electrolytic solution is promptly interrupted and the battery temperature rises excessively. Can be prevented and safety can be ensured.

The security mechanism is considered as follows. In a lithium battery, the separator is in contact with the surfaces of the positive electrode and the negative electrode. However, the electrode surface is not necessarily smooth, and there are countless minute irregularities. Exists. Then, when the battery is overcharged, metallic lithium precipitates on the surface of the negative electrode, which reacts with the electrolytic solution penetrating into the minute gap to generate heat, which is a factor of safety impairment. However,
When the porous film of the present invention is used as a separator, when the metal lithium precipitates and reacts with the electrolytic solution and the temperature starts to rise, the PE in the PE-rich layer is softened and melted early, and the softened melt is Since it flows into the minute gap on the negative electrode surface and fills it to cover the metal lithium surface, the contact between the metal lithium and the electrolyte is cut off and the reaction is interrupted.
Excessive temperature rise is prevented. Further, in the porous film of the present invention, since PE having good fluidity at the time of melting is used as a component having a high PE content of 3 or more, and the amount of the PE used is 0.5% by weight or more. In addition, it is easy to fill the minute gaps on the negative electrode surface with the molten component and cover the metallic lithium surface, and the reaction between the deposited metallic lithium and the electrolytic solution is surely interrupted, thereby further improving safety.

In order to incorporate the porous film of the present invention into a lithium battery as a separator in order to exert such a mechanism for ensuring safety, the PE-rich layer of the porous film is brought into contact with the negative electrode. However, this does not apply to the case where such a battery is not necessarily required to ensure safety.

The porous film of the present invention has a low PE content layer and a high PE content layer as described above, and is a multilayer film in which these layers are laminated. Examples thereof include a three-layer type in which a high PE content layer is provided on both surfaces of the content layer and a two-layer type in which a high PE content layer is provided on one surface of a low PE content layer. However, if an increase in the thickness is allowed, a four or more layer structure can be adopted. When four or more porous films are used as a lithium battery separator, at least one surface layer is a PE-rich layer.

The low PE and / or high PE layers may optionally contain other synthetic resin, nucleating agent, processing aid, surfactant, antioxidant, plasticizer, flame retardant, coloring, etc. An appropriate amount of an additive such as an agent can be blended.

The porous film of the present invention is made of, for example, PE
It can be produced by a method of forming a laminated film having a layer composed of a component forming a low content layer and a layer composed of a component forming a high PE content, and then making the film porous by stretching. Hereinafter, this manufacturing method will be described.

In this method, first, a laminated film having a layer composed of a component forming a low PE layer and a layer composed of a component forming a high PE layer is formed. Molding of the laminated film comprises a composition comprising a PE low-containing layer forming component,
A method comprising preparing a composition comprising a PE-rich layer-forming component and simultaneously extruding them with a multilayer extruder, extruding a composition comprising a PE-poor layer-forming component (or a composition comprising a PE-rich layer-forming component) After forming the film,
A method of extruding a composition comprising a PE-rich layer-forming component (or a composition comprising a PE-poor layer-forming component) onto the film, or a composition comprising a PE-poor layer-forming component and a PE-rich layer-forming component , A film is formed, and then these are heat-sealed.

The thus obtained laminated film can be subjected to a heat treatment. This heat treatment is performed for the purpose of improving the crystallinity of the laminated film and the like. The method of heat treatment may be any method, for example, a method of contacting a heated roll or a metal plate, a method of heating a laminated film in air or an inert gas, winding the laminated film on a roll on a core body. Then, a method of heating this in a gas phase or a medium can be adopted. In the case where the laminated film is brought into contact with a heated roll or a metal plate, or when the laminated film is heated in air or an inert gas, both sides of the laminated film may be sandwiched between carrier films. When the laminated film is wound into a roll on a core body and heated in a gas phase or a medium, a release sheet may be superimposed on the laminated film and wound to prevent blocking. it can.

The temperature and time of the heat treatment are set according to the method of the heat treatment and the like.
C., for a time in the range of about 2 seconds to 50 hours. By performing such a heat treatment, the crystallinity of the laminated film is increased, and the formation of fine pores by subsequent stretching is facilitated, so that a porous film having a higher porosity can be obtained.

After the heat treatment, the laminated film is made porous by stretching it. As the stretching method, it is preferable to apply a two-stage stretching method in which the film is stretched at a low temperature and then stretched at a high temperature.

That is, first, the laminated film is uniaxially stretched at a low temperature. The temperature at this time is usually -20.
C. to 60C. If the temperature is lower than −20 ° C., the film is easily broken during stretching, and if it is higher than 60 ° C., it is difficult to make the film porous. This stretching method can be performed by a conventionally known roll stretching, tenter stretching or the like.

The stretching ratio at this time is usually about 20 to 40.
0%, preferably 30 to 200%. The stretching ratio (E ₁ ) can be determined by the following _{equation 1} using the dimension (L ₀ ) before low-temperature stretching and the dimension (L ₁ ) after low-temperature stretching.

[0037]

(Equation 1)

Following the low-temperature stretching, high-temperature stretching is performed.
The high-temperature stretching is performed in the same direction as the low-temperature stretching of the porous film after the low-temperature stretching, usually at a temperature of 60 ° C. to the melting point of PE or lower. The stretching ratio at the time of high temperature stretching is usually about 10 to 50
0%. The stretching ratio (E2) is obtained by using the dimension before low-temperature stretching (L ₀ ), the dimension after low-temperature stretching (L ₁ = dimension before high-temperature stretching), and the dimension after high-temperature stretching (L ₂ ). You can ask.

[0039]

(Equation 2)

The porous film thus obtained retains stress applied during low-temperature stretching and high-temperature stretching, and tends to shrink in the stretching direction to cause dimensional change. By doing so, dimensional stability can be improved. The degree of contraction may be arbitrary,
Usually, the size is reduced to about 10 to 40%.

The so-called “heat set” in which the dimensions of the porous film in the stretching direction are not changed and the heating is performed at a predetermined temperature (for example, a stretching temperature or higher) is also performed. The dimensional stability can be improved in the same manner as described above. As a method of this heat setting, a method of contacting a porous film after hot stretching with a heated roll, a method of heating a porous film in air or an inert gas, and a method of rolling a porous film on a core body And a method of heating it in a gaseous phase or a medium. Of course, both shrinkage and heat setting may be performed to improve dimensional stability.

According to the above method, the porous film of the present invention can be easily obtained. This porous film is P
The low E content layer and the high PE content layer have innumerable micropores formed therein, and the pore size of the micropores varies depending on the composition and the elongation ratio of each layer. ~
0.06 μm, and the pore diameter of the PE-rich layer was 0.02 to
0.20 μm.

In the porous film used as a separator for a lithium battery, the pore diameter of the PE-rich layer is preferably 0.04 to 0.15 μm (more preferably 0.05 to 0.1 μm). It is known. P
When the pore size of the E-rich layer is set in this manner, the effect of preventing clogging by metallic lithium deposited on the negative electrode surface is particularly excellent.

Similarly, when this porous film is used as a separator for a lithium battery, the thickness of the PE-rich layer is preferably set to 10 to 50% of the total thickness of the film. As described above, in a lithium battery incorporating this porous film as a separator, when metallic lithium precipitates on the negative electrode surface and reacts with the electrolytic solution, PE in the P-rich layer melts and flows, and It fills the minute gaps between the separators (this covers the metallic lithium surface) and blocks the reaction. Therefore, the amount of PE that melts and flows needs to be sufficient to fill the minute gap, and in order to secure this amount, P
It is preferred that the thickness of the E-rich layer be 10 to 50% of the total thickness of the film. When the thickness of the PE-rich layer relative to the total thickness of the porous film is within this range, this amount can be sufficiently ensured and the film strength can be satisfied.

Further, it has been found that this porous film is preferable as a battery separator when the Gurley value is 1500 sec / 100 cc or less.

When this porous film is used as a battery separator, its electric resistance at a specific temperature within a range of about 120 to 130 ° C. is several tens to several thousand times or more the resistance at room temperature. On the other hand, it has been confirmed that the shape of the film is maintained at a temperature at least about 25 ° C. higher than the specific temperature and exhibits excellent SD characteristics.

The porous film of the present invention can be applied to a wide variety of applications such as a separator, an air-permeable film for construction, an air-permeable film for clothing, and the like by utilizing its characteristics in addition to the battery separator.

[0048]

The present invention will be described in more detail with reference to the following examples.

Example 1 An isotactic PP (a component forming a low PE content layer) having a weight average molecular weight of 1,000,000 and an MI of 0.5 was prepared. On the other hand, separately from this, high-density PE72 having the same 10% by weight of PP, a density of 0.964 g / cm ³ and an MI of 0.3
A mixture consisting of 18% by weight of high-density PE having a weight percentage of 0.961 g / cm ³ and an MI of 7.3 is prepared (PE-rich layer forming component).

Using a T-die extruder, a three-layer simultaneous extrusion method at a die temperature of 230.degree. C. and a cooling roll temperature of 80.degree.
A long laminated film on which a mixture layer of 2 μm of PE and PP is formed.

The heat treatment is performed by heating this laminated film in the air at 125 ° C. for 40 hours. Then, using a roll stretching machine, the film is stretched at a temperature of 25 ° C. in the machine direction at a low temperature so as to have a stretching ratio of 40%, and then at a temperature of 120 ° C. and stretched at a high temperature so that the stretching ratio is 200% in the same direction. Was performed. After the stretching, the three-layer structure in which the PE-rich layer is formed on both sides of the PE-poor layer by shrinking the dimension in the stretching direction by 10% (based on the dimension after the high-temperature stretching) at 115 ° C. Was obtained (total thickness: 24 μm).

The weight average molecular weight, density and MI of PE and PP used are values measured by the following methods.

(Weight-Average Molecular Weight) Measured at 135 ° C. by gel permeation chromatography (GPC-150C, manufactured by Waters Corporation) using O-dichlorobenzene as a solvent. The column used was Shodex-80M (manufactured by Showa Denko KK), and a data processing system manufactured by TRC was used for data processing. The molecular weight was calculated based on polystyrene.

(Density) The density was measured according to ASTM D1505. The unit is “g / cm ³ ”.

(MI) The measurement was carried out according to ASTM D1238. The unit is “g / 10 min”.

Example 2 An isotactic PP (a component forming a low PE-containing layer) having a weight average molecular weight of 960,000 and an MI of 0.4 was prepared. on the other hand,
Apart from this, the same PP 20% by weight and density of 0.
964 g / cm ³ , 50% by weight of high density PE with MI of 0.3, and density of 0.961 g / cm ³ , MI of 8.0
Of a high-density PE of 30% by weight (PE-rich layer forming component) is prepared.

Using a T-die extruder, a three-layer simultaneous extrusion method at a die temperature of 220.degree. C. and a cooling roll temperature of 80.degree.
A long laminated film having a mixed layer of PE and PP having a thickness of μm is formed.

The heat treatment is performed by heating this laminated film in air at 127 ° C. for 48 hours. Next, using a roll stretching machine, the film is stretched at a temperature of 25 ° C. at a low temperature in the machine direction at a stretch ratio of 40%, and then at a temperature of 110 ° C., stretched at a high temperature in the same direction at a stretch ratio of 200%. Done. After the stretching, the dimension in the stretching direction is 1 at 115 ° C.
By shrinking by 0%, both sides of the PE low content layer
A white porous film (thickness: 25 μm) having a three-layer structure in which a high-content layer was formed was obtained.

Example 3 An isotactic PP (low PE layer forming component) having a weight average molecular weight of 960,000 and an MI of 0.4 is prepared. on the other hand,
Separately, the same 10% by weight of PP and a density of 0.
966 g / cm ³ , 54% by weight of high density PE with MI of 0.3, and density of 0.961 g / cm ³ , MI of 4.0
(PE-rich layer forming component) comprising 36% by weight of high-density PE.

The same operation as in Example 2 was carried out except that the above-mentioned materials were used and the shrinkage after high-temperature stretching was set to 20%. White porous film having a layer structure (total thickness 25
μm).

Comparative Example 1 An isotactic PP (for forming a low PE content layer) having a weight average molecular weight of 1,000,000 and an MI of 0.5 was prepared. on the other hand,
Separately, the same high-density PE65 as above with 35% by weight of PP, a density of 0.964 g / cm ³ and an MI of 0.3.
A mixture consisting of% by weight (for forming a high PE content layer) is prepared.

The same operation as in Example 2 was carried out except that the above-mentioned materials were used and that the shrinkage after high-temperature stretching was set to 20%. White porous film having a layer structure (total thickness 25
μm).

Comparative Example 2 Isotactic PP (a component forming a low PE content layer) having a weight average molecular weight of 1,000,000 and an MI of 0.5 was prepared. On the other hand, separately from the above, 10% by weight of the same PP as above, and a high-density P having a density of 0.964 g / cm ³ and an MI of 0.3
Mixture consisting of 90% by weight of E (PE-rich layer forming component)
Prepare

Using a T-die extruder, a three-layer simultaneous extrusion method at a die temperature of 260 ° C. and a cooling roll temperature of 80 ° C. was applied to both sides of a 21 μm-thick PE-rich layer to form a 7 μm-thick PE low-layer. A long laminated film on which the containing layer was formed was formed.

The heat treatment is performed by heating this laminated film in air at 130 ° C. for 48 hours. Next, using a roll stretching machine, after performing low-temperature stretching at a temperature of 50 ° C. so as to have a stretching ratio of 40% in the machine direction, performing high-temperature stretching at a temperature of 125 ° C. so as to have a stretching ratio of 200% in the same direction. Done. After the stretching, the dimension in the stretching direction is set to 1 at a temperature of 125 ° C.
By shrinking by 0%, both sides of the PE-rich layer
A white porous film (total thickness: 27 μm) having a three-layer structure in which each of the low content layers was formed was obtained.

Table 1 shows the results of measurement of the pore diameter, adhesion and Gurley value of the porous films obtained in the above Examples and Comparative Examples. In addition, the measurement of these characteristics was based on the following method.

(Pore Size) Measured by the BET method (adsorption method). The unit is “μm”.

(Adhesion) A nonwoven fabric, a negative electrode, a porous film previously impregnated with an electrolytic solution and a nonwoven fabric are superimposed in this order, and a 3.5 mm-thick iron flat plate is disposed on each side of the superimposed body. 50 kg at 135 ° C
After heating and pressurizing under the condition of f / cm ² for 5 minutes, the flat plate and the nonwoven fabric are removed. Then, the negative electrode and the porous film were peeled by hand, and the situation at that time was observed, and "◎", "「 "," △ "
And “x” are determined in four stages. "◎" indicates that the negative electrode was hardly peeled off and traces of carbon powder (components of the negative electrode) were formed on the porous film surface, and "「 "indicates that the negative electrode was slightly hardly peeled off and carbon powder was attached. “△” indicates that the negative electrode was slightly peeled off and some carbon powder was attached, and “×” indicates that the negative electrode was easily peeled off and no carbon powder was attached.

This “adhesion” refers to the case where, in a lithium battery in which a porous film is used as a separator and the low PE content layer is incorporated so as to be in contact with the negative electrode, the metal lithium and the electrolytic solution are deposited when lithium metal is deposited on the negative electrode surface. This indicates the quality of the reaction blocking function. Good “adhesion” means that the fluidity of the PE molten component (molten PE in the low PE layer) due to the temperature rise when the precipitated metallic lithium reacts with the electrolytic solution is high, and the molten component causes a gap between the negative electrode and the separator. This indicates that the minute gap existing in the sample is efficiently filled and the electrolytic solution is reliably eliminated (reaction is reliably shut off).

[0070]

[Table 1]

[0071]

When the porous film of the present invention is used as a battery separator, it is possible to effectively prevent an excessive rise in temperature caused by deposited metal lithium. Therefore, a battery incorporating this porous film as a separator has the advantage of high safety. Further, since this porous film has air permeability, it can be used for various uses other than the battery separator.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takashi Wano 1-1-2 Shimohozumi, Ibaraki-shi, Osaka Nitto Denko Corporation (72) Inventor Kiichiro Matsushita 1-1-2 Shimohozumi, Ibaraki-shi, Osaka No. Nitto Denko Corporation (72) Inventor Kazunari Yamamoto 1-2-1, Shimohozumi, Ibaraki-shi, Osaka Nitto Denko Corporation

Claims (7)

[Claims] 1. A porous film containing polyethylene and polypropylene as essential components, a polyethylene low content layer having a polyethylene content of 0 to 20% by weight, a polyethylene content of 61 to 100% by weight, and ,
A porous film, comprising: a polyethylene-rich layer containing 0.5% by weight or more of polyethylene having a melt index of 3 or more. 2. The high polyethylene content layer has a pore size of 0.04.
The porous film according to claim 1, which has a thickness of from 0.1 to 0.15 µm. 3. The porous film according to claim 1, wherein a polyethylene-rich layer is provided on both sides of the polyethylene-poor layer. 4. The porous film according to claim 1, wherein a polyethylene-rich layer is provided on one side of the polyethylene-poor layer. 5. The porous film according to claim 1, wherein the thickness of the polyethylene-rich layer is 10 to 50% of the total thickness of the film. 6. A battery separator comprising the porous film according to claim 1. 7. A positive electrode, a negative electrode, a separator interposed between these two electrodes, and an electrolytic solution, wherein the separator is the porous film according to any one of claims 1 to 5, and A battery, wherein a polyethylene-rich layer of a film is incorporated so as to be in contact with a negative electrode.