WO2012062011A1

WO2012062011A1 - Method for preparing microporous polyolefin membrane and application thereof

Info

Publication number: WO2012062011A1
Application number: PCT/CN2010/080314
Authority: WO
Inventors: 陈秀峰; 陈良; 雷彩红; 高东波; 陈勇
Original assignee: 深圳市星源材质科技股份有限公司
Priority date: 2010-11-12
Filing date: 2010-12-27
Publication date: 2012-05-18
Also published as: KR101805241B1; KR20130132841A; CN102153771B; CN102153771A

Abstract

Disclosed is a method for preparing microporous polyolefin membrane, which comprises the following steps: melt extruding polyolefin at 180-260℃ and casting into film to obtain casted polyolefin base membrane; stretching the casted polyolefin base membrane before performing thermoforming treatment, so as to obtain a preformed microporous polyolefin membrane; soaking the preformed microporous polyolefin membrane in the organic solvent which has a surface tension of lower than 30mN/m, and then removing the organic solvent to obtain the microporous polyolefin membrane. The application of the method for preparing microporous polyolefin membrane in preparing separators or lithium ion battery is also provided. In the method, after being stretched and thermoforming treated, the preformed microporous membrane is soaked in an organic solvent which has a surface tension of lower than 30mN/m to obtain the final microporous polyolefin membrane with round and bigger pores and evenly distributed gas permeability along the transverse direction of the microporous membran, so that the ventilation property of the microporous polyolefin membrane is improved.

Description

Preparation method of polyolefin microporous membrane and application thereof

Technical field

The invention belongs to the technical field of polyolefin thin films, and in particular relates to a preparation method of a polyolefin microporous membrane and an application thereof.

Background technique

At present, polyolefin thin film materials are getting more and more applications and developments. The most widely used application is as a separator material for lithium ion batteries, and it is one of the key inner layer components of lithium ion batteries. The membrane material has a crucial influence on the performance of the actual battery. The membrane itself is not a good conductor of electrons, but it also allows electrolyte ions to pass through. In addition, the separator material must have good chemical, electrochemical stability and mechanical properties as well as maintain high wettability of the electrolyte during repeated charge and discharge. The diaphragm is generally made of polypropylene or polyethylene plastic material, which can isolate the positive and negative electrodes of the battery to prevent short circuit. It can also block the current conduction in the battery through the closed hole function when the battery is overheated. The performance of the diaphragm determines the interface structure and internal resistance of the battery, which directly affects the battery capacity, cycle performance and safety performance. The separator with excellent performance plays an important role in improving the overall performance of the battery.

Lithium-ion battery separators are mainly polyolefin microporous membranes. Currently, industrial preparation methods mainly include wet biaxial stretching and dry uniaxial stretching. Wet method, also known as phase separation method or thermal phase separation method, is to add high-boiling small molecules as porogen to polyolefin, heat and melt into a homogeneous system, then phase-separate after cooling, and extract with organic solvent after stretching. Small molecules can be used to prepare microporous membrane materials that are interpenetrating. Representative foreign companies using this method include Japan's Asahi Kasei, Toho, and Entek of the United States, and are currently mainly used to produce single-layer polyethylene (PE) separators. The separator produced by the wet biaxial stretching method has high longitudinal and transverse strength due to biaxial stretching, but it requires certain organic problems and production safety due to the need to extract with an organic solvent in the production process. consider.

The basic principle of the preparation of the microporous membrane by the dry melt drawing method is that the polymer melt is extruded and crystallized under a high tensile stress field to form a platelet crystal structure which is parallel to the extrusion direction and arranged in parallel, and then subjected to heat treatment. A hard elastic material is obtained. After the polymer film having hard elasticity is stretched, the platelets are separated to form a large number of microporous structures, and then the microporous film is prepared by heat setting. Japan UBE and the United States Celgard Company used this method to prepare microporous membranes of PE and polypropylene (PP). The separator formed by the method has disadvantages such as difficulty in controlling the pore diameter and porosity, and the transverse strength of the film is low, which brings a series of problems to the production and use process. For example, the low transverse strength causes the separator to be easily torn in the longitudinal direction, resulting in a lateral direction. It is easy to stretch and deform, resulting in dimensional changes, making it difficult to flatten the film.

As mentioned above, the performance of the diaphragm directly affects the overall performance of the battery. In the performance index of the diaphragm, the permeability is a key indicator, the permeability is too low, the lithium ion penetration ability is too small; the gas permeability is too high, the internal branch of the battery When the crystal is formed, the battery is easily short-circuited. Due to current process limitations, it is difficult to achieve better gas permeability.

technical problem

In view of the above, a method for preparing a polyolefin microporous membrane having improved gas permeability and uniform distribution of transverse gas permeability is provided.

And, the use of the above polyolefin microporous membrane preparation method for preparing a separator or a lithium ion battery is provided.

Technical solution

A method for preparing a polyolefin microporous membrane, comprising the steps of:

The polyolefin is melt extruded at 180 to 260 ° C, and cast into a film to obtain a polyolefin cast base film;

Stretching the polyolefin cast base film, and then performing heat setting treatment to obtain a preformed polyolefin microporous film;

The preformed polyolefin microporous membrane is immersed in an organic solvent having a surface tension of less than 30 mN/m, and the organic solvent is removed after immersion to obtain the polyolefin microporous membrane.

And the use of the above polyolefin microporous membrane preparation method in the preparation of a separator or a lithium ion battery.

Beneficial effect

In the above polyolefin microporous membrane preparation method, on the basis of stretching and heat setting to form a preformed polyolefin microporous membrane, and then immersing in an organic solvent having a surface tension of less than 30 mN/m, so that the polyolefin micropores The pores of the membrane are more rounded, and a polyolefin microporous membrane having improved gas permeability and uniform distribution of lateral gas permeability along the microporous membrane and having reduced appearance defects is obtained. The method can be widely applied to the preparation of a separator or a lithium ion battery, and improves the overall performance of the separator and the lithium ion battery.

DRAWINGS

The present invention will be further described below in conjunction with the accompanying drawings and embodiments, in which:

1 is a flow chart of a method for preparing a polyolefin microporous membrane according to an embodiment of the present invention;

2 is a scanning electron micrograph of a polyolefin microporous membrane obtained by a method for preparing a polyolefin microporous membrane according to an embodiment of the present invention (hereinafter referred to as an SEM image);

Figure 3 is an SEM image of a conventional polyolefin microporous film formed without immersion;

4 is a graph showing a pore size distribution curve before and after immersion treatment with an organic solvent in a method for preparing a polyolefin microporous membrane according to an embodiment of the present invention;

5 is a graph showing the lateral distribution of gas permeability along a microporous membrane before and after immersion treatment with an organic solvent in a method for preparing a polyolefin microporous membrane according to an embodiment of the present invention;

6 is a graph showing changes in transmittance of a separator before and after immersion treatment of a polyolefin microporous membrane according to an embodiment of the present invention.

Embodiments of the invention

The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It is understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

Referring to FIG. 1 , a flow chart of a method for preparing a polyolefin microporous membrane according to an embodiment of the present invention includes the following steps:

S01, melt extrusion, casting film formation: the polyolefin is melt extruded at 180 to 260 ° C, and cast into a film to obtain a polyolefin cast base film;

S02, stretching and setting: stretching the polyolefin cast base film, and then performing heat setting treatment to obtain a preformed polyolefin microporous film;

S03, immersion treatment: the preformed polyolefin microporous membrane is immersed in an organic solvent having a surface tension of less than 30 mN/m, and the organic solvent is removed after immersion to obtain the polyolefin microporous membrane.

In step S01, the polyolefin may be selected from, but not limited to, polypropylene, polyethylene, polytetramethylpentene or a mixture thereof. The polypropylene used may be a homopolypropylene or a copolymerized polypropylene, of which homopolypropylene is preferred. The polyethylene used may be high density polyethylene, low density polyethylene, linear low density polyethylene or ultra high molecular weight polyethylene, of which high density polyethylene is preferred. The polyolefin used has a melt index of from 0.5 to 15.0 g/10 min, preferably from 0.8 to 3.0 g/10 min, for example, 1.0 g/10 min, 1.2 g/10 min, 1.5 g/10 min, 1.8 g/10 min, 2.0 g/10 min, 2.5 g/10 min, 2.8 g/10 min, and the like. When a blend is used, such as polypropylene and polyethylene, some blends may be pre-added to the polypropylene and then melt extruded. For example, the polyethylene blend can be high density polyethylene, low density polyethylene, linear low density polyethylene or ultra high molecular weight polyethylene. In polypropylene and polyethylene blends, the mass ratio of polypropylene to polyethylene can be, for example, 1:0.2-1. In addition, some additives such as antioxidants, slip agents, etc., which do not adversely affect the main properties of the polyolefin material, may be added to the above blend, and the antioxidant can enhance the oxidation resistance of the polyolefin material. Performance, slip agents are used to reduce friction between the film and the film and between the film and the processing equipment.

The specific melt extrusion process includes: extruding the polyolefin (or together with other blends) by an extruder at 180 to 260 ° C, casting the film through a casting die or a ring-shaped blow molding die, After cooling (such as forced cooling), it is drawn at a certain speed to obtain a polyolefin cast base film having a uniform thickness. Specifically, the melt extrusion may be performed by a single screw or twin screw extruder, and the extrusion temperature is not lower than 180 ° C in consideration of molding processing and plasticizing properties of the material. The roll temperature for casting film formation is 30 to 100 ° C, the line speed is 10 to 80 m/min, and the molding thickness is 10 to 40 μm.

After casting the film, the obtained polyolefin cast base film may be further heat treated to further improve the crystallinity of the base film and the integrity of the wafer, the heat treatment temperature is 110 to 150 ° C, and the heat treatment time is 10 minutes to 15 hours. .

Step S02 is specifically that the polyolefin cast base film is stretched twice in the longitudinal direction, and the total stretching temperature ranges from 10 to 150 °C. The stretching temperature of the first stretching process (also referred to as cold drawing) is 10 to 40 ° C, and the stretching ratio is 10 to 50%, that is, 1.1 to 1.5 times that of the casting film before stretching. The stretching temperature of the second stretching process (also referred to as hot drawing) is 110 to 150 ° C, and the stretching ratio is 100 to 500%. After two longitudinal stretching in step S02, the polyolefin initially forms a microporous film, that is, a preformed polyolefin microporous film. The microstructure of the micropores obtained at this time is shown in Fig. 3. As can be seen from the figure, the micropores are long in the stretching direction, are elongated and long holes, have no round hole shape, and have poor gas permeability. The median pore diameter of the micropores in the preformed polyolefin microporous membrane after longitudinal stretching is 70 to 120 nm. During the heat setting process, the heat setting treatment temperature is 110 to 150 ° C, and the heat setting treatment time is 10 seconds to 3 minutes.

In the step S03 immersion treatment, the immersion time is preferably 2 minutes to 60 minutes, and the specific immersion time varies according to the film thickness, and the thick film needs a long immersion time. The organic solvent is preferably a solvent having a boiling point of less than 100 ° C, such as, but not limited to, at least one of a ketone, an alcohol, an alkane, a halogen-containing alkane or a heterocyclic ring. The ketone may be, but not limited to, acetone or methyl ethyl ketone or the like, and the alcohol may be, but not limited to, methanol, ethanol or isopropanol, or the like, and the alkane may be, but not limited to, pentane, heptane, n-hexane. Or cyclohexane, or their analogs, etc., the halogen-containing substituent alkane may be, but not limited to, dichloromethane, chloroform, tetrachloromethane or dichloroethane, or the like, and the heterocyclic ring may be It is, but not limited to, furan, 2-methylfuran, tetrahydrofuran, 2-methyltetrahydrofuran, or the like. The organic solvent has a surface tension of less than 30 mN/m, preferably 10 to 30 mN/m, further preferably 20 to 30 mN/m, for example, 15 mN/m, 20 mN/m, 22 mN/m, 24 mN/m, 26 mN/m, 28 mN/m, and the like.

After the predetermined time of immersion, the organic solvent is removed to obtain the polyolefin microporous film. The organic solvent may be removed by natural volatilization at room temperature or normal temperature (e.g., 20-30 ° C). For example, after the microporous membrane leaves the infusion solution, the organic solvent is volatilized and dried, and the drying process may be carried out at a normal temperature or at a temperature higher than the boiling point of the organic solvent without causing deformation of the microporous membrane. When a solvent having a boiling point lower than 100 ° C is used, it can be removed by natural volatilization to save process cost. The organic solvent volatilization process can be carried out by industrial absorption, freeze condensation or adsorption by means of adsorption of activated carbon and activated carbon fiber solids. After drying, the microporous film is wound up into a film. In addition, the immersion is an in-line method in which the preformed polyolefin microporous membrane is treated by in-line soaking with an organic solvent. First, the organic solvent is placed in a sealed box in which a roller is installed in the middle, and then the pre-formed microporous film subjected to the above-mentioned stretching and setting treatment is passed through a roller, and is naturally volatilized and dried after leaving the sealed box, and then wound up to form a film. The on-line processing time can be adjusted by changing the film travel speed or by changing the roll space arrangement in the sealed box. Similarly, the organic solvent volatilization process can be carried out by industrial absorption, freeze condensation or adsorption with activated carbon and activated carbon fiber solids.

The above preparation method can be widely applied to preparing a separator or a lithium ion battery, and improving the overall performance of the lithium ion battery. When applied to the preparation of the separator, the polyolefin microporous membrane is first produced by the above method, and the resulting polyolefin microporous membrane can be directly used as a separator after being cut. When the separator is applied to a lithium ion battery, for example, a separator is mounted on an electrode material, and other components are mounted, positioned, and packaged to form a lithium ion battery. By adopting the above polyolefin microporous membrane, the electrochemical performance of the lithium ion battery is improved by utilizing its excellent gas permeability and uniformity of lateral gas permeability distribution along the microporous membrane.

The preparation method of the polyolefin microporous membrane and its properties and the like are exemplified below by various examples.

Example 1

The film was extruded into a thick film using a homopolymer polypropylene single-screw extruder having a melt index of 3.0 g/10 min. The extrusion temperature was 180 ° C, the casting roll temperature was 30 ° C, and the thickness of the cast film was 40 μm. It was then heat treated in an oven at 110 ° C for 10 hours. The cold drawing temperature is normal temperature, the cold drawing rate is 20%, the hot drawing temperature is 110 ° C, and the hot drawing rate is 120%. The heat setting treatment was further carried out, and the heat setting temperature was 110 ° C, and the heat setting time was 20 s. Then, the film was immersed in acetone on the line for 5 minutes, the infusion solution was left, and the film was dried at room temperature to obtain a final PP microporous film.

Example 2

The film was extruded into a thick film using a homopolymer polypropylene single-screw extruder having a melt index of 2.6 g/10 min. The extrusion temperature was 200 ° C, the casting roll temperature was 60 ° C, and the thickness of the cast film was 40 μm. It was then heat treated in an oven at 150 ° C for 10 hours. The cold drawing temperature is normal temperature, the cold drawing ratio is 20%, the hot drawing temperature is 150 ° C, and the hot drawing ratio is 120%. The heat setting treatment was further carried out, and the heat setting temperature was 150 ° C, and the heat setting time was 180 s. Then, the film was immersed in acetone on the line for 5 min, the infusion solution was removed, and the film was dried at 60 ° C to obtain a final PP microporous film.

Example 3:

The film was extruded into a thick film using a homopolymer polypropylene single-screw extruder having a melt index of 3.0 g/10 min. The extrusion temperature was 260 ° C, the casting roll temperature was 100 ° C, and the thickness of the cast film was 40 μm. It was then heat treated in an oven at 150 ° C for 10 hours. The cold drawing temperature is normal temperature, the cold drawing ratio is 20%, the hot drawing temperature is 140 ° C, and the hot drawing ratio is 120%. The heat setting treatment was further carried out, and the heat setting temperature was 140 ° C, and the heat setting time was 20 s. Then, the film was immersed in acetone in the line for 30 minutes, the infusion solution was left, and the film was dried at room temperature to obtain a final PP microporous film.

Example 4:

The film was extruded into a thick film using a homopolymer polypropylene single-screw extruder having a melt index of 3.0 g/10 min. The extrusion temperature was 220 ° C, the casting roll temperature was 60 ° C, and the thickness of the cast film was 40 μm. It was then heat treated in an oven at 150 ° C for 10 hours. The cold drawing temperature is normal temperature, the cold drawing ratio is 20%, the hot drawing temperature is 140 ° C, and the hot drawing ratio is 120%. The heat setting treatment was further carried out, and the heat setting temperature was 140 ° C, and the heat setting time was 20 s. Then, the film was immersed in cyclohexane on-line, soaked for 5 min, left the soaking solution, and the film was dried at room temperature to obtain a final PP microporous film.

Example 5:

The film was extruded into a thick film using a homopolymer polypropylene single-screw extruder having a melt index of 3.0 g/10 min. The extrusion temperature was 220 ° C, the casting roll temperature was 60 ° C, and the thickness of the cast film was 40 μm. It was then heat treated in an oven at 150 ° C for 10 hours. The cold drawing temperature is normal temperature, the cold drawing ratio is 20%, the hot drawing temperature is 140 ° C, and the hot drawing ratio is 120%. The heat setting treatment was further carried out, and the heat setting temperature was 140 ° C, and the heat setting time was 20 s. Then, the film was immersed in dichloroethane on-line, soaked for 5 min, left the soaking solution, and the film was dried at room temperature to obtain a final PP microporous film.

Example 6

The film was extruded into a thick film using a homopolymer polypropylene single-screw extruder having a melt index of 3.0 g/10 min. The extrusion temperature was 220 ° C, the casting roll temperature was 60 ° C, and the thickness of the cast film was 40 μm. It was then heat treated in an oven at 150 ° C for 10 hours. The cold drawing temperature is normal temperature, the cold drawing ratio is 20%, the hot drawing temperature is 140 ° C, and the hot drawing ratio is 120%. The heat setting treatment was further carried out, and the heat setting temperature was 140 ° C, and the heat setting time was 20 s. Then, the film was immersed in isopropyl alcohol on-line, soaked for 5 min, left the soaking solution, and the film was dried at room temperature to obtain a final PP microporous film.

Example 7

The film was extruded into a thick film using a homopolymer polypropylene single-screw extruder having a melt index of 3.0 g/10 min. The extrusion temperature was 220 ° C, the casting roll temperature was 60 ° C, and the thickness of the cast film was 40 μm. It was then heat treated in an oven at 150 ° C for 10 hours. The cold drawing temperature is normal temperature, the cold drawing ratio is 20%, the hot drawing temperature is 140 ° C, and the hot drawing ratio is 120%. The heat setting treatment was further carried out, and the heat setting temperature was 140 ° C, and the heat setting time was 20 s. Then, the film was immersed in tetrahydrofuran on-line, soaked for 5 min, left the soaking solution, and the film was dried at room temperature to obtain a final PP microporous film.

Example 8

The film was extruded into a thick film using a homopolymer polypropylene single-screw extruder having a melt index of 3.0 g/10 min. The extrusion temperature was 220 ° C, the casting roll temperature was 60 ° C, and the thickness of the cast film was 40 μm. It was then heat treated in an oven at 150 ° C for 10 hours. The cold drawing temperature is normal temperature, the cold drawing ratio is 20%, the hot drawing temperature is 140 ° C, and the hot drawing ratio is 120%. The heat setting treatment was further carried out, and the heat setting temperature was 140 ° C, and the heat setting time was 20 s. Then, the film was immersed in methanol on the line for 5 minutes, the infusion solution was left, and the film was dried at room temperature to obtain a final PP microporous film.

Example 9

The film was extruded into a thick film using a high-density polyethylene single-screw extruder having a melt index of 0.8 g/10 min, an extrusion temperature of 220 ° C, a casting roll temperature of 60 ° C, and a cast film thickness of 40 μm. It was then heat treated in an oven at 120 ° C for 10 hours. The cold drawing temperature is normal temperature, the cold drawing ratio is 20%, the hot drawing temperature is 120 ° C, and the hot drawing ratio is 120%. The heat setting treatment was carried out, and the heat setting temperature was 125 ° C, and the heat setting time was 20 s. Then, the film was immersed in isopropyl alcohol on-line, soaked for 5 min, left the soaking solution, and the film was dried at room temperature to obtain a final HDPE microporous film.

Comparative example 1

The film was extruded into a thick film using a polypropylene single-screw extruder having a melt index of 3.0 g/10 min, an extrusion temperature of 220 ° C, a casting roll temperature of 60 ° C, and a cast film thickness of 40 μm. It was then heat treated in an oven at 150 ° C for 10 hours. The cold drawing temperature is normal temperature, the cold drawing ratio is 20%, the hot drawing temperature is 140 ° C, and the hot drawing ratio is 120%. The heat setting treatment was further carried out, the heat setting temperature was 140 ° C, and the heat setting time was 20 s to obtain a final PP microporous film.

Comparative example 2

The film was extruded into a thick film using a high-density polyethylene single-screw extruder having a melt index of 0.8 g/10 min, an extrusion temperature of 220 ° C, a casting roll temperature of 60 ° C, and a cast film thickness of 40 μm. It was then heat treated in an oven at 120 ° C for 10 hours. The cold drawing temperature is normal temperature, the cold drawing ratio is 20%, the hot drawing temperature is 120 ° C, and the hot drawing ratio is 120%. The heat setting treatment was further carried out, the heat setting temperature was 125 ° C, and the heat setting time was 20 s to obtain a final HDPE microporous film.

Microporous membrane performance test:

Breathability: Characterize the gas permeability of microporous membranes with Gurley values, according to ASTM The D726 standard, the Gurley value is the time required for 50 ml of gas to pass through the membrane at a certain pressure, and the low Gurley value indicates that the membrane has good gas permeability.

Diaphragm microstructure: The sample was sprayed with gold by S3400 scanning electron microscope of Shimadzu Corporation of Japan, and the test voltage was 5KV, and the magnification was 20,000 times.

Pore size and distribution: tested with AutoPore IV Model 9510 mercury intrusion meter.

Table 1 Comparison of examples and comparative examples

	Example 1	Example 2	Example 3	Example 4	Example 5	Example 6	Example 7	Example 8	Example 9	Comparative example 1	Comparative example 2
Thickness μm	32	32	32	32	32	32	32	32	32	32	32
Air permeability s	524	510	520	535	480	518	530	505	500	613	550

Table 1 The thickness in the middle refers to the thickness of the final formed microporous film. As can be seen from the data in Table 1, the Gurley values of Examples 1-9 are in the range of 480-535, which is significantly lower than Comparative Example 1 in the case where the thickness of the microporous film is the same. And 2 results.

In the above preparation method, on the basis of stretching and heat setting to form a preformed polyolefin microporous film, immersing in an organic solvent having a surface tension of less than 30 mN/m, so that the pores of the polyolefin microporous film are more rounded. Large, polyolefin microporous film with improved air permeability and uniform distribution of lateral gas permeability along the microporous film and reduced appearance defects. Specifically, as shown in FIG. 2 and FIG. 3, the microstructures of the separator before and after the solvent treatment are respectively shown. By comparison, the diameter of the pores of the membrane becomes larger after the treatment with the above organic solvent, and the pore walls are smooth, and the pores become more round, no longer Narrow holes before treatment. Figure 4 shows the pore size distribution curve before and after the organic solvent treatment. It can be seen from the figure that the pore size distribution after treatment is in accordance with the normal distribution and the distribution is uniform, and most of the pore size is increased, specifically, the final shape after transverse stretching. The microporous membrane of the polyolefin microporous membrane has a median pore diameter of 110 to 140 nm. Figure 5 shows the transverse distribution curve of gas permeability along the microporous membrane before and after treatment with an organic solvent. It can be seen from the figure that after treatment with an organic solvent, the gas permeability changes less along the transverse direction of the microporous membrane, and the gas permeability distribution in the transverse direction of the microporous membrane is shown. More even. Figure 6 is the change of the transmittance of the separator before and after treatment with different organic solvents. The curves of the organic solvent treatment film in the figure are from Example 1 to Example 6 from top to bottom, respectively. It can be seen from the figure that acetone, dichloroethane and isopropyl are used. The light transmission performance of the microporous membrane was significantly reduced after the alcohol treatment, which further showed that the diameter of the micropore became larger after the treatment with the organic solvent. These figures show that the direct effect of treatment with an organic solvent is an improvement in the gas permeability of the membrane.

The above is only the preferred embodiment of the present invention, and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection of the present invention. Within the scope.

Claims

A method for preparing a polyolefin microporous membrane, comprising the steps of:

The polyolefin is melt extruded at 180 to 260 ° C, and cast into a film to obtain a polyolefin cast base film;

Stretching the polyolefin cast base film, and then performing heat setting treatment to obtain a preformed polyolefin microporous film;

The preformed polyolefin microporous membrane is immersed in an organic solvent having a surface tension of less than 30 mN/m, and the organic solvent is removed after immersion to obtain the polyolefin microporous membrane.
The method for producing a polyolefin microporous membrane according to claim 1, wherein the organic solvent has a boiling point of less than 100 °C.
The method for producing a polyolefin microporous membrane according to claim 1 or 2, wherein the organic solvent is at least one of a ketone, an alcohol, an alkane, a halogen-containing alkane or a heterocyclic ring.
The method for preparing a polyolefin microporous membrane according to claim 1 or 2, wherein the organic solvent is acetone, methyl ethyl ketone, methanol, ethanol, isopropanol, pentane, heptane, n-hexane or cyclohexane. At least one of alkane, dichloromethane, chloroform, tetrachloromethane, dichloroethane, furan, 2-methylfuran, tetrahydrofuran, and 2-methyltetrahydrofuran.
The method for preparing a polyolefin microporous membrane according to claim 1, wherein the soaking time is from 2 minutes to 60 minutes.
The method for preparing a polyolefin microporous film according to claim 1, wherein the stretched polyolefin cast base film comprises two stretching processes, and the stretching temperature of the first stretching process is 10 to 40. °C, the stretching ratio is 10 to 50%; the stretching temperature in the second stretching process is 110 to 150 ° C, and the stretching ratio is 100 to 500%.
The method for preparing a polyolefin microporous membrane according to claim 1, wherein after the casting film formation, the obtained polyolefin cast base film is subjected to heat treatment, and the heat treatment temperature is 110 to 150 ° C, and the heat treatment time is It is 10 minutes to 15 hours.
The method for preparing a polyolefin microporous membrane according to claim 1, wherein the median pore diameter of the micropores in the preformed polyolefin microporous membrane after longitudinal stretching is 70 to 120 nm, and is immersed in an organic solvent. The pores of the finally formed polyolefin microporous membrane have a median pore diameter of 110 to 140 nm.
The method for preparing a polyolefin microporous membrane according to claim 1, wherein the heat setting treatment temperature is 110 to 150 ° C. The heat setting treatment time is from 10 seconds to 3 minutes.
Use of the polyolefin microporous membrane preparation method according to any one of claims 1 to 9 for the preparation of a separator or a lithium ion battery.