KR20000020908A - Process for preparing microporous separating film - Google Patents

Abstract

PURPOSE: A separating film is prepared which has an improved adhesiveness and thermal property. CONSTITUTION: Ethylene-propylene copolymer having 0.1-1 micrometer of average diameter and 30-70 % of porosity is laminated in the polyethylene film to give the separating film having 1,600-1,720 kg per square cm of tensile strength and 25-50 micrometer of total thickness. Thus, ethylene gas and propylene in equivalent ratio of 25: 75 are polymerized at 10 deg.C for 1 hour, followed by adding 40 wt % of paraffin wax to give first resin. 60 wt % of polyethylene resin containing 15 mole % of low density polyethylene, 80 mole % of high density polyethylene and 5 mole % of super high density, and 40 wt % of paraffin wax are compounded to give second resin. The ethylene-propylene copolymer film and polyethylene film are immersed for 30 seconds in methylene chloride to extract paraffin wax, and laminated to give the separating film.

Description

Microporous separator and its manufacturing method

The present invention relates to a microporous separator and a method for manufacturing the same, and more particularly, to a microporous separator and a method of manufacturing the improved adhesiveness and thermal properties.

In lithium secondary batteries, the shutdown characteristic is directly related to the stability of the battery. In more detail, when the electrolyte is partially decomposed by an external short circuit, and the temperature inside the battery is rapidly increased by the heat generated in the decomposition reaction, the separator is partially melted. As a result, as the microporous region of the separator is melted down and blocked, the electrical resistance rapidly increases and current flow is interrupted, thereby maintaining the overall shape of the separator as it is, thereby maintaining battery stability.

The mechanical strength of the separator is also closely related to the stability of the separator. Particularly, the puncture strength is very important among the mechanical strengths, since the separator may be partially damaged by the minute protrusions on the surface of the electrode plate, which may cause a fatal defect in the battery. And the mechanical strength of the separator is directly related to the productivity of the battery. In other words, if the mechanical strength of the separator is excellent, the assembly speed can be increased during battery assembly, thereby improving productivity.

As a conventional separator for lithium secondary batteries, a separator having a two-layer structure composed of a polyethylene film and a polypropylene film or a one-layer separator made of a mixture of polyethylene and polypropylene is used. However, such a separator has a shutdown start temperature of 130 ° C. or higher, which is close to 150 ° C. which is a sublimation point of lithium ions. Therefore, in order to ensure the stability of the battery, the melting temperature of the separator is preferably about 115 to 120 ℃.

In addition, as a separator for another lithium ion secondary battery, a microporous separator made of polyethylene is disclosed as disclosed in Japanese Patent Laid-Open Nos. 3-64334 and 6-212006. However, this separator has a problem in that the separator is easily deformed when the current rapidly increases due to a short circuit and the internal temperature of the battery suddenly rises.

As an improvement to solve the above problems, Japanese Patent Laid-Open Nos. 7-60084 and 6-96753 disclose methods for producing a microporous separator using a composition in which polyethylene is mixed with polymethylpentene or polypropylene. According to this method, since the resin is not sufficiently mixed in the dry mixing method, a separate compounding step must be performed.

In addition, Japanese Patent Laid-Open No. 6-234181 discloses a separator produced by laminating a nonwoven fabric with heat in a conventional polyethylene film in order to improve the heat resistance of the separator. By the way, this separation membrane is poor compatibility between the polyethylene film and the nonwoven fabric, and must undergo a thermal laminating process in which an adhesive or the like is coated in the manufacturing process.

However, even if the process is repeatedly performed, not only the adhesiveness between the polyethylene film and the nonwoven fabric is sufficient, but also the inherent properties of the polyethylene film, which is the base film, are degraded by repeated thermal laminating.

The technical problem to be achieved by the present invention is to solve the above problems to provide a microporous separator and a method of manufacturing the improved adhesiveness and thermal properties.

In order to achieve the above technical problem, the present invention provides a microporous separator, characterized in that it comprises an ethylene-propylene copolymer film with micropores and a polyethylene film with micropores.

The diameter of the micropores is preferably an average diameter of 0.1 to 1㎛, porosity of 30 to 70%.

Another technical problem of the present invention is to prepare a first resin and a second resin by (a) adding a pore-forming agent to the ethylene-propylene copolymer and polyethylene resin, respectively;

(b) melting and co-extruding the first and second resins to produce an unstretched sheet;

(c) stretching the unstretched sheet in the longitudinal and transverse directions, respectively; And

(d) removing the pore-forming agent from the stretched result; it is made by the method of manufacturing a microporous separator comprising the.

In the case of forming (b) the unstretched ethylene-copolymer film and the unstretched polyethylene film, respectively, after (d), laminating the ethylene-copolymer film and the polyethylene film formed from (c) Do more steps.

On the other hand, when forming a laminated film including the unstretched ethylene-propylene copolymer film and the polyethylene film in step (b), the laminating step as described above is unnecessary.

The present invention is to prepare a separator using a microporous polyethylene film and a microporous ethylene-propylene copolymer film having excellent compatibility, and various structural modifications are possible in preparing the separator. For example, a two-layer structure composed of a polyethylene film and an ethylene-propylene copolymer film, a polyethylene film as an intermediate layer, and a three-layer structure in which ethylene-propylene copolymer films are formed on both sides of the intermediate layer may be used.

Hereinafter, a method for preparing an ethylene-propylene copolymer used to prepare a microporous separator according to the present invention will be described.

First, aluminum halide and silicon compound are reacted, and then magnesium compound is added thereto to react. Thereafter, the reaction product is added with a titanium halide compound as a catalyst to react, and suspended in an inert solvent such as toluene, n-heptene to obtain a catalyst slurry.

The catalyst slurry is mixed with an alkylaluminum or an alkylaluminum halide, an organosilicon compound, propylene, and ethylene gas, which are organoaluminum compounds, in a predetermined mixing ratio, and prepolymerized at a low temperature of about 10 ° C. Subsequently, polymerization is performed at 50 to 60 ° C., followed by drying under reduced pressure to prepare an ethylene-propylene copolymer.

The content of ethylene in the ethylene-propylene copolymer prepared according to the above method is preferably 5 to 50 mol%. Here, when the content of ethylene is less than 5 mol%, the effect of lowering the melting temperature of the ethylene-propylene copolymer is effective. Insignificant, when the ethylene content exceeds 50 mol%, there is a problem that the mechanical strength is sharply lowered. The melt index of the ethylene-propylene copolymer is preferably 1 to 25 gr / min, and the melting temperature is 117 to 125 ° C. If the melt index is less than 1 gr / min, there is a problem of discharging during melt extrusion of the ethylene-propylene copolymer, and if the melt index is more than 25 gr / min, there is a problem of breakage during the film forming process.

As a specific example of the aluminum halide, aluminum trichloride is most preferred. As the silicon compound, tetramethoxysilane, tetraethoxysilane, tetrabutoxysilane, methyltriethoxysilane, dimethyldiethoxysilane and the like are used. Among them, tetramethoxysilane and methyltriethoxysilane are most preferred.

As the magnesium compound, at least one selected from the group consisting of ethyl magnesium chloride, propyl magnesium chloride, butyl magnesium chloride, hexyl magnesium chloride, octyl magnesium chloride, and ethyl magnesium bromide is used.

Titanium tetrachloride, tetrabromotitanium, or the like is used as the titanium halide compound, and at least one selected from trimethylaluminum, triethylaluminum, and triisobutylaluminum is used as the organoaluminum compound. Dipiperidino dimethoxy silane, dipyrrolidino dimethoxy silane, isopropyl piperidino dimethoxy silane and the like are used as the organosilicon compound.

On the other hand, the polyethylene resin used in the preparation of the microporous membrane according to the present invention, 10 to 35 mol% linear low density polyethylene (LLDPE) having a viscosity average molecular weight (Mv) of 250,000, high density polyethylene (Mv 390,000) It is prepared by compounding 60 to 80 mol% of High Density PolyEthylene (HDPE) and 5 to 10 mol% of Ultra High Molecular weight PolyEthylen (UHMwPE) having Mv of 4.5 million. The melt index of the polyethylene resin thus prepared is 1-25 gr / min, and the melting temperature is 130-145 ° C. If the melt index of the polyethylene resin is less than 1 gr / min, there is a problem of discharging during melt extrusion of the polyethylene resin, and if the melt index exceeds 25 gr / min, there is a problem of breaking during the film forming process.

As described above, the content of the LLDPE is preferably 10 to 35 mol%, if the content is less than 10 mol% there is a problem that the shutdown start temperature rises to 125 ℃ or more when the melting temperature of the polyethylene resin rises above 145 ℃ If the content exceeds 35 mol%, the mechanical strength, particularly the tensile strength, of the separator falls below 1600 kg / cm 2, which is undesirable. And when the HDPE content is less than 60 mol% or the UHMwPE content is less than 5 mol%, the mechanical strength of the separator, in particular the tensile strength is less than 1600kg / ㎠, the HDPE content is more than 80 mol% or the UHMwPE content is more than 10 mol% In this case, the thermal shutdown characteristics of the separator are not preferable because the shutdown start temperature rapidly rises to 125 ° C or higher.

Hereinafter, a process of preparing a microporous separator using the above-described ethylene-propylene copolymer and polyethylene resin will be described.

First, 30 to 70% by weight, preferably 40 to 50% by weight of pore formers are respectively added to the ethylene-propylene copolymer and the polyethylene resin, and then the mixture is compounded to form each blending chip. Manufacture.

As the pore-forming agent, at least one selected from ethylene bissteamide wax, liquid paraffin, paraffin wax, process oil, stearyl alcohol, dioctyl phthalate and dibutyl phthalate is used.

Thereafter, an unstretched sheet is manufactured by using the blending chip. There are two methods of manufacturing the unstretched sheet.

The first method is a method of melting and extruding the blending chip according to a conventional film production method and then cooling to prepare each unstretched sheet. When the separation membrane is manufactured using the thus obtained unstretched sheet, the lamination process to be described later must be performed.

The second method is a method using an extruder and a feed block equipped with a feed block in which the thickness ratio of the ethylene-propylene copolymer film and the polyethylene film is in a predetermined range, and the blending layer is melted and extruded by a conventional manufacturing method. It is a method of manufacturing unstretched sheet | seat by cooling. When the separation membrane is manufactured using the thus obtained unstretched sheet, a separate laminating process is unnecessary, unlike the first method.

Thereafter, the unstretched sheets formed according to the first and second methods are biaxially stretched in the longitudinal and transverse directions to prepare laminated films of ethylene-propylene film and polyethylene film or ethylene-propylene film and polyethylene film. At this time, the stretching ratio in the longitudinal direction and the transverse direction is preferably 3 to 7 times, more preferably 4 to 4.5 times. If the stretching ratio in the longitudinal and transverse directions is less than three times, the tensile strength drops to less than 1600 kg / cm 2, and if it exceeds seven times, fracture occurs frequently during the film forming process. The tensile strength in the longitudinal and transverse directions of the ethylene-propylene film and the polyethylene film is preferably 1600 to 1720 kg / cm 2, and the puncture strength is 800 to 850 Gf. If the longitudinal and lateral tensile strengths are less than 1600 kg / cm 2, the film breaks during battery assembly, and the productivity is lowered. If the tensile strength is more than 1720 kg / cm 2, the shutdown characteristic is lowered. If the pore strength is less than 800 Gf, the separator is damaged by the fine protrusions on the surface of the electrode plate, thereby degrading the stability of the battery.

Thereafter, the biaxially stretched ethylene-propylene film and polyethylene film or the laminated film of the biaxially stretched ethylene-propylene film and polyethylene film is immersed in a volatile solvent for 10 to 50 seconds to extract and remove the pore-forming agent in the film. . In this process, micropores are formed in the film to obtain a laminated film of the microporous ethylene-propylene copolymer film and the polyethylene film or the microporous ethylene-propylene film and the polyethylene film. The average diameter of the micropores is 0.1 to 1㎛. If the pore diameter is less than 0.1 mu m, smooth movement of lithium ions becomes difficult. If the pore diameter exceeds 1 mu m, the stability of the battery is deteriorated due to a short circuit caused by contact of the cathode in the battery.

Further, in the laminated film of the microporous ethylene-propylene copolymer film and the polyethylene film or the microporous ethylene-propylene copolymer film and the polyethylene film, the porosity is preferably 30 to 70%. If the porosity is less than 30%, lithium ions in the lithium battery may not move smoothly, and the efficiency of the battery may be lowered. If the porosity is greater than 70%, the strength of the separator may be rapidly decreased.

As the volatile solvent used in the present invention, it is preferable to use differently depending on the pore-forming agent to be extracted and removed. Specific examples use one or more selected from isopropyl alcohol, dichloroethylene, trichloroethylene, acetone, 1,1,1-trichloroethane, methanol, ethanol, methylethylketone, heptane and hexane.

On the other hand, the laminated | multilayer film of the microporous ethylene propylene film and polyethylene film obtained by the said method is. It can be used as a separator by itself.

On the other hand, the microporous ethylene-propylene film and the microporous polyethylene film are thermally laminated in a two-layer or three-layer structure to complete the microporous separator according to the present invention. Here, in the three-layer separator, the ethylene-propylene copolymer film is laminated on both sides of the polyethylene film.

Looking at the thermal laminating conditions, the temperature is 110 ~ 140 ℃, preferably 130 ℃, operating speed is 30 ~ 70m / min, preferably 50m / min. When the thermal laminating temperature is less than 110 ° C, the adhesive force of each film constituting the separator rapidly drops, and when the thermal laminating temperature exceeds 140 ° C, the physical properties of the film decrease, which is not preferable. And when the operating speed exceeds 70m / min, the adhesive force of each film is sharply lowered, if less than 30m / min, there is a problem that the overall physical properties of the film due to excessive thermal contact.

The microporous membrane obtained according to the above-described method improves melt bonding and mechanical strength characteristics by starting shutdown at a much lower temperature than the conventional polyethylene resin membrane or other membranes.

In the separator according to the present invention, the total thickness of the separator is preferably 25 to 50 μm. If the thickness of the separator exceeds 50㎛ result in lowering the capacity of the lithium battery during assembly, less than 25㎛ undesirably decrease the tensile strength to less than 1600kg / ㎠.

In addition, the thickness ratio of the ethylene-propylene copolymer film in the separator should be 40 to 60% of the total thickness of the separator. If the thickness ratio of the ethylene-propylene copolymer film is more than 60%, the melt-integrity characteristic is sharply lowered, and if it is less than 40%, the shutdown start temperature is rapidly increased, which is not preferable.

The use of the microporous separator of the present invention is not particularly limited, but is very useful as a separator of a lithium secondary battery such as a lithium ion battery, a lithium polymer battery, or the like.

Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited only to the following Examples.

Example 1

A 1: 1 equivalent ratio of aluminum trichloride and tetramethoxysilane were mixed, followed by addition of 2 equivalents of ethyl magnesium chloride based on the aluminum trichloride equivalent. Titanium tetrachloride was added to the reaction product, and n-heptane was added thereto to prepare a catalyst slurry.

The catalyst slurry was pre-polymerized for 1 hour by adding ethylene gas and propylene in a 25:75 equivalent ratio at 10 DEG C while adding and reacting a 1: 1 equivalent ratio of trimethylaluminum and dipiperidino dimethoxysilane. Subsequently, polymerization was carried out at 60 ° C. for 1 hour to prepare an ethylene-propylene copolymer (melt index: 4.5, ethylene content: 25 mol%).

To 60% by weight of the ethylene-propylene copolymer, 40% by weight of paradine wax was added and compounded to obtain a first resin. This first resin was vacuum dried at 80 deg. C for 12 hours.

Separately, polyethylene resin consisting of 15 mol% of linear low density polyethylene (viscosity average molecular weight: 250,000), 80 mol% of high density polyethylene (viscosity average molecular weight: 390,000) and 5 mol% of ultra high density polyethylene (viscosity average molecular weight: 4.5 million) To 60% by weight, 40% by weight of paraffin wax was added and compounded to obtain a second resin. This second resin was vacuum dried at 80 캜 for 12 hours.

Thereafter, the first resin was melted and coextruded using a T-die equipped with an extruder and a feed block, and then cooled closely to a roll adjusted to 60 ° C to obtain an unstretched sheet. Subsequently, this unoriented sheet was biaxially stretched 6.3 times in the longitudinal direction and 6.3 times in the transverse direction at 105 ° C. to obtain an ethylene-propylene copolymer film having a total thickness of 12 μm.

Subsequently, the second resin was melted and coextruded using a T-die equipped with an extruder and a feed block, and then closely cooled to a roll adjusted to 60 ° C to obtain an unstretched sheet. Subsequently, this unstretched sheet was simultaneously stretched 5.5 times in the longitudinal direction and 5.5 times in the transverse direction at 105 ° C. to obtain a polyethylene film having a total thickness of 18 μm.

Thereafter, the ethylene-propylene copolymer film and the polyethylene film were immersed in methylene chloride for 30 seconds to extract and remove paraffin wax from the sheet, thereby preparing a microporous ethylene-propylene copolymer film and a microporous polyethylene film. At this time, the porosity of the mid-porous ethylene-propylene copolymer film and the microporous polyethylene film was 40%, and the average diameter of the pores was 0.1 μm.

Subsequently, the ethylene-propylene copolymer film obtained by using the thermal laminating method and the polyethylene film were bonded together to form a two-layer microporous separator (total thickness: 30 µm). The thickness ratio of the ethylene-propylene copolymer film and the polyethylene film is 40:60, laminating temperature is 130 ℃. The speed was 50 m / min.

Example 2-3

The total thickness of the microporous membrane was kept constant at 30 μm, except that the thickness ratio of the ethylene-propylene copolymer film and the polyethylene film was adjusted to be 50:50. A layer microporous separator was prepared.

Example 4-6

In preparing a microporous separator, a polyethylene film is used as an intermediate layer, and first and second ethylene-propylene copolymer films are formed on both sides of the intermediate layer, and the first ethylene-propylene copolymer film, polyethylene film, and second ethylene- A three-layer microporous separator was prepared in the same manner as in Example 1 except that the thickness ratios of the propylene copolymer films were 20:60:20, 25:50:25, and 30:40:30.

Example 7

A 1: 1 equivalent ratio of aluminum trichloride and tetramethoxysilane were mixed, followed by addition of 2 equivalents of ethyl magnesium chloride based on the aluminum trichloride equivalent. Titanium tetrachloride was added to the reaction product, and n-heptane was added thereto to prepare a catalyst slurry.

The catalyst slurry was pre-polymerized for 1 hour by adding ethylene gas and propylene in a 25:75 equivalent ratio at 10 DEG C while adding and reacting a 1: 1 equivalent ratio of trimethylaluminum and dipiperidino dimethoxysilane. Subsequently, polymerization was carried out at 60 DEG C for 1 hour to obtain an ethylene-propylene copolymer having a melt index of 4.5 and an ethylene content of 25 mol%.

The first resin was obtained by adding 40 wt% of paradine wax to 60 wt% of the ethylene-propylene copolymer. This first resin was vacuum dried at 80 deg. C for 12 hours.

Separately, polyethylene resin consisting of 15 mol% of linear low density polyethylene (viscosity average molecular weight: 250,000), 80 mol% of high density polyethylene (viscosity average molecular weight: 390,000) and 5 mol% of ultra high density polyethylene (viscosity average molecular weight: 4.5 million) To 60% by weight, 40% by weight of paraffin wax was added and compounded to obtain a second resin. This second resin was vacuum dried at 80 캜 for 12 hours.

Thereafter, the first resin and the second resin were melted and coextruded using an extruder and a feed block equipped with a die block, in which the thickness ratio of the ethylene-propylene copolymer film and the polyethylene film was 40:60, respectively. , And closely cooled to a roll adjusted to 60 ° C to obtain an undrawn sheet. Next, this unoriented sheet was biaxially stretched 5 times in the longitudinal direction and 5 times in the transverse direction at 105 ° C. to obtain a two-layer film having a total thickness of 30 μm.

The bilayer film was immersed in methylene chloride for 30 seconds to extract paraffin wax from the film to complete the microporous separator. At this time, the porosity of the MIDI pore separation membrane was 40%, the average diameter of the pores was 0.1㎛.

Example 8-9

A two-layer microporous separator was prepared in the same manner as in Example 1 except that the thickness ratio of the ethylene-propylene copolymer film and the polyethylene film was 50:50 and 60:40, respectively.

Example 10-12

The layer structure of the microporous separator is a polyethylene film as an intermediate layer, and first and second ethylene-propylene copolymer films are formed on both sides of the intermediate layer, and the first ethylene-propylene copolymer film, polyethylene film and second ethylene A three-layer microporous separator was prepared in the same manner as in Example 7, except that the thickness ratios of the propylene copolymer films were 20:60:20, 25:50:25, and 30:40:30.

Comparative Example 1-2

Except that the thickness ratio of the ethylene-propylene copolymer film and the polyethylene film is 30:70 and 70:30, respectively, was carried out in the same manner as in Example 1 to prepare a two-layer microporous separator.

Comparative Example 3-4

Except for the thickness ratio of the first ethylene-propylene copolymer film, polyethylene film and second ethylene-propylene copolymer film is 15:70:15 and 35:30:35, was carried out in the same manner as in Example 1. To prepare a three-layer microporous separator.

Comparative Example 5-6

A two-layer microporous separator was prepared in the same manner as in Example 7, except that the thickness ratio of the ethylene-propylene copolymer film and the polyethylene film was 30:70 and 70:30, respectively.

Comparative Example 7-8

It carried out according to the same method as Example 7, except that the thickness ratio of the first ethylene-propylene copolymer film, the polyethylene film and the second ethylene-propylene copolymer film is 15:70:15 and 35:30:35. To prepare a three-layer microporous separator.

The properties of the microporous membranes prepared according to Examples 1-12 and Comparative Examples 1-8 were evaluated according to the following methods.

1) film thickness

The cross section of the film was observed using an electron scanning microscope manufactured by Jeol of Japan, and then measured through an image analyzer.

2) sting strength

Measurements are made using a universal tensile machine (UTM) from Instron, USA, with a needle diameter of 1 mm.

3) tensile strength

It is measured according to ASTM-D-882 method using UTM of Instron, USA.

4) Shutdown start temperature

Shutdown initiation temperature was measured by measuring impedance in the shutdown region.

5) melt integrity temperature

A differential thermal analyzer (DSC-7, Perkin Elmer, Inc.) is used, and the sample is heated at a temperature increase rate of 5 ° C./minute to measure the temperature at the time of the highest heat of fusion.

The properties according to Examples 1-12 and Comparative Examples 1-8 are shown in Table 1 below.

Tensile Strength (kg / ㎠) (Longitudinal / Horizontal) Prick strength (Gf) Shutdown start temperature (℃) Melt Bonding Temperature (℃) Example 1 1650/1630 815 120 145 Example 2 1680/1650 800 119 143 Example 3 1710/1700 825 118 142 Example 4 1650/1640 840 120 145 Example 5 1680/1670 830 118 144 Example 6 1720/1700 830 117 144 Example 7 1660/1635 818 119 145 Example 8 1695/1655 805 119 144 Example 9 1720/1705 828 118 144 Example 10 1680/1670 843 120 145 Example 11 1710/1685 835 118 144 Example 12 1720/1710 835 117 144 Comparative Example 1 1610/1580 795 132 145 Comparative Example 2 1715/1690 800 117 133 Comparative Example 3 1610/1595 797 131 145 Comparative Example 4 1720/1695 815 117 132 Comparative Example 5 1605/1580 803 133 145 Comparative Example 6 1710/1650 800 117 135 Comparative Example 7 1615/1598 798 134 145 Comparative Example 8 1720/1700 817 117 137

As can be seen from Table 1, the separator according to Example 1-12 was found to be excellent in all of the shutdown characteristics, melt bonding temperature characteristics and mechanical strength characteristics. In particular, the three-layer separators according to Examples 4-6 and 10-12 had better puncture strength characteristics than the two-layer separators of Examples 1-3 and 7-9 corresponding thereto.

On the other hand, the separation membrane according to Comparative Example 1-8 was found to be inferior in thermal characteristics such as shutdown start temperature, melt bonding temperature characteristics compared to the case of the corresponding example.

The microporous membrane of the present invention is excellent in thermal properties such as shutdown start temperature, melt bonding temperature, and mechanical properties such as sticking strength. Therefore, when such a microporous separator is used for a separator of a lithium secondary battery such as a lithium ion battery, a lithium polymer battery, etc., not only the productivity during battery assembly but also the stability of the battery is improved.

Claims (15)

A microporous separator comprising a ethylene-propylene copolymer film having micropores and a polyethylene film having micropores. The microporous separator according to claim 1, wherein ethylene-propylene copolymer films are formed on both sides of the polyethylene film. The microporous membrane of claim 1, wherein the pore has an average diameter of 0.1 to 1 μm and a porosity of 30 to 70%. The microporous separator according to claim 1, wherein the tensile strength is 1600 to 1720 kg / cm 2 and the puncture strength is 800 to 850 Gf. The microporous separator according to claim 1, wherein the total thickness is 25 to 50 µm. The microporous separator according to claim 1, wherein the ethylene-propylene copolymer film has a thickness ratio of 40 to 60% of the total thickness. The microporous separator according to claim 1, which is used as a separator for a lithium secondary battery. (a) preparing a first resin and a second resin by adding a pore-forming agent to the ethylene-propylene copolymer and polyethylene resin, respectively; (b) melting and co-extruding the first and second resins to produce an unstretched sheet; (c) stretching the unstretched sheet in the longitudinal and transverse directions, respectively; And (d) removing the pore-forming agent from the stretched product. The method according to claim 8, wherein in the case of forming the unstretched ethylene-copolymer film and the unstretched polyethylene film respectively in step (b), after step (d), the ethylene-copolymer film formed from step (c) And laminating the polyethylene film. The ethylene-propylene copolymer resin of claim 8, wherein the ethylene-propylene copolymer resin of step (a) has a melting temperature of 117 to 125 ° C, an ethylene resin content of 5 to 50 mol%, and a melting index of 1 to 25 gr / min. Method for producing a microporous separation membrane. The method of claim 8, wherein the polyethylene resin of step (a) has a melting temperature of 130 to 145 ° C and a melt index of 1 to 25 gr / min. The method of claim 8, wherein the pore-forming agent is 30 to 70 parts by weight based on 100 parts by weight of the ethylene-propylene copolymer or polyethylene resin. The method of claim 8, wherein the pore-forming agent is at least one selected from the group consisting of ethylene bissteamide wax, liquid paradine, paraffin wax, process oil, stearyl alcohol, dibutyl phthalate and dioctyl phthalate Method for producing a microporous separator. The method of claim 8, wherein the average diameter of the pores formed in the separation membrane is 0.1 to 1㎛, porosity is 30 to 70%. The method of claim 8, wherein the removing of the pore-forming agent of step (d) is isopropyl alcohol, dichloroethylene, trichloroethylene, acetone, 1,1,1-trichloroethane, methanol, ethanol, methylethyl A method for producing a microporous separator, characterized in that by depositing at least one selected from the group consisting of ketone, heptane and hexane.