CN110561800B

CN110561800B - Polypropylene microporous isolation membrane and preparation method and application thereof

Info

Publication number: CN110561800B
Application number: CN201910753668.6A
Authority: CN
Inventors: 赵艳; 彭文磊; 谢嘉宜; 雷彩红; 徐睿杰
Original assignee: Guangdong University of Technology
Current assignee: Guangdong University of Technology
Priority date: 2019-08-15
Filing date: 2019-08-15
Publication date: 2022-05-10
Anticipated expiration: 2039-08-15
Also published as: CN110561800A

Abstract

The invention belongs to the technical field of battery isolation membrane preparation, and particularly relates to a polypropylene microporous isolation membrane and a preparation method and application thereof. According to the invention, a grafting reaction principle is adopted, and a polypropylene grafting material with active groups is grafted on a polypropylene molecular chain in a melt grafting manner, so as to obtain polypropylene pre-used granules with active groups; and then, carrying out melt extrusion, tape casting and blow molding on the pre-used granules by a single-screw extruder through a melt stretching method to obtain an initial tape casting film, and then sequentially carrying out heat treatment, hot stretching, heat setting and re-setting in hot water for 3-7 hours to obtain the polypropylene microporous isolating film. The isolating membrane overcomes the problems of poor high-temperature thermal dimensional stability, low mechanical property and air permeability of the existing isolating membrane, and greatly avoids potential safety hazards in the operation of batteries.

Description

Polypropylene microporous isolation membrane and preparation method and application thereof

Technical Field

The invention belongs to the technical field of battery isolation membrane preparation, and particularly relates to a polypropylene microporous isolation membrane and a preparation method and application thereof.

Background

The polyolefin microporous membrane is widely applied to the lithium ion battery as an isolating membrane, and the isolating membrane is a core component of the lithium ion battery and approximately accounts for 18-30% of the cost of the whole lithium battery. The performance of the lithium battery plays a crucial role in the overall performance of the lithium battery, and is one of the key technologies for restricting the development of the lithium battery. With the development of electronic products and the expansion of application fields, people have higher and higher requirements on the performance of lithium batteries. In order to meet the development requirements of lithium batteries, microporous separators should have high mechanical strength, excellent thermal stability, good micropore distribution, low manufacturing cost, and the like.

At present, the lithium ion battery isolating membrane is mainly made of crystalline polyolefin materials such as Polyethylene (PE), polypropylene (PP) and the like, but the two polyolefin membranes have defects and shortcomings. The size stability of the separator at high temperature is also very important, if the shape of the separator cannot be maintained at high temperature, the positive electrode and the negative electrode are in direct contact, the battery is in a dangerous state due to short circuit, and polypropylene and polyethylene have higher potential safety hazards because the polypropylene and polyethylene have lower melting points and the size stability of the microporous membrane cannot be maintained at high temperature.

The wet film preparation process is complex, uses a large amount of organic solvent, has high cost and does not accord with the environmental protection concept.

The polyolefin microporous membrane is prepared by a melt-stretching method (dry method), and the polyolefin microporous membrane can be prepared without using an organic solvent. The microporous membrane obtained by the invented patent has lower closed pore temperature, poor high-temperature thermal dimensional stability, no improvement in mechanical property and puncture strength, and is easy to increase potential safety hazard in the battery operation process due to the limitation of low melting point of the polyolefin material.

Disclosure of Invention

In order to overcome the defects and shortcomings of the prior art, the primary object of the present invention is to provide a method for preparing a microporous polypropylene isolation membrane, which has simple steps and low pollution level.

Another object of the present invention is to provide a microporous polypropylene separator prepared by the above preparation method, which has excellent mechanical properties, such as: high-temperature thermal dimensional stability, high mechanical strength, high puncture strength and the like.

The invention also aims to provide application of the polypropylene microporous isolating membrane.

The purpose of the invention is realized by the following technical scheme:

a preparation method of a polypropylene microporous isolation membrane comprises the following steps:

(1) dissolving an initiator, an antioxidant and an active reactant vinyl tri (beta-methoxyethoxy) silane in a solvent to obtain a reaction active solution; then adding polypropylene, and uniformly mixing to obtain a polypropylene grafting material solution with active groups;

(2) mixing the polypropylene grafting material solution with the active group prepared in the step (1), nano silicon dioxide and polypropylene to obtain a blend;

(3) extruding and granulating the blend prepared in the step (2) by a double-screw extruder to obtain polypropylene pre-used granules with active groups;

(4) carrying out melt extrusion, tape casting and blow molding on the polypropylene pre-used granules with the active groups prepared in the step (3) by a single-screw extruder to obtain an initial tape casting film;

(5) sequentially carrying out heat treatment, hot stretching and heat setting on the initial casting film prepared in the step (4) to obtain a polypropylene microporous film grafted with reactive groups;

(6) flattening, clamping and fixing the polypropylene microporous membrane grafted with the reactive groups prepared in the step (5) to apply uniform tension; then, shaping in hot water at the temperature of 50-80 ℃ for 3-7 hours to obtain a polypropylene microporous isolating membrane;

the initiator in step (1) is preferably at least one of BPO (dibenzoyl peroxide) and ABVN (azobisisoheptonitrile);

the dosage of the initiator in the step (1) is preferably 0.1-0.5% of the mass of the polypropylene grafting material (the total mass of the initiator, the antioxidant, the active reactant vinyl tri (beta-methoxyethoxy) silane and the polypropylene);

the antioxidant in the step (1) is preferably 1010 (tetra [ beta- (3, 5-di-tert-butyl-4-hydroxyphenyl) propionic acid ] pentaerythritol ester);

the dosage of the antioxidant in the step (1) is preferably 0.2-1% of the mass of the polypropylene grafting material (the total mass of the initiator, the antioxidant, the active reactant vinyl tri (beta-methoxyethoxy) silane and the polypropylene);

the dosage of the active reactant vinyl tri (beta-methoxyethoxy) silane in the step (1) is preferably 0.01-1% of the mass of the polypropylene grafting material (the total mass of the initiator, the antioxidant, the active reactant vinyl tri (beta-methoxyethoxy) silane and the polypropylene);

the solvent in the step (1) is preferably acetone;

the mixing in step (1) is preferably mixed uniformly in a high-speed mixer;

the preferable melt index of the polypropylene in the step (2) is 2-8 g/10min, and the polypropylene has good fluidity;

the dosage of the polypropylene graft material with the active group in the step (2) is preferably 2-10% of the total mass of the polypropylene graft material with the active group and the polypropylene;

the dosage of the nano silicon dioxide in the step (2) is preferably 0.02-0.2% of the total mass of the polypropylene graft material with active groups and the polypropylene;

the mixing in the step (2) is preferably pre-mixed uniformly in a high-speed mixer;

the conditions for the melt extrusion, the tape casting and the blow molding to form the film in the step (4) are preferably that the temperature of a die head is 210-240 ℃, the temperature of a tape casting roller is 80-100 ℃, and the drafting ratio is 70-150;

the heat treatment in step (5) is preferably carried out in an oven;

the heat treatment in the step (5) is preferably carried out at 145 ℃ for 30 min;

the hot stretching in the step (5) is preferably one-step hot stretching;

the specific operation of the one-step hot stretching is preferably as follows: directly heating the film to 130-140 ℃ after heat treatment and carrying out hot stretching;

the heat setting condition in the step (5) is preferably 140-145 ℃ for 10-20 min;

the heat shrinkage rate of the polypropylene microporous isolation membrane prepared in the step (6) is 3-6%;

a polypropylene microporous barrier film, which is prepared by the preparation method;

the polypropylene microporous isolating membrane is applied to the field of battery preparation;

the mechanism of the invention is as follows:

according to the invention, a grafting reaction principle is adopted, and a polypropylene grafting material with active groups is grafted on a polypropylene molecular chain in a melt grafting manner, so as to obtain polypropylene pre-used granules with active groups; then, by a melt stretching method, the pre-used granules are melt extruded, cast and blown into a film by a single screw extruder to obtain an initial cast film, wherein the initial cast film firstly forms a parallel-arranged lamellar structure vertical to the extrusion direction under the tensile stress field of an extrusion cast melt, the lamellar and the lamellar can be separated to generate a porous structure under the specific stretching condition of the film with the lamellar structure, and then the polypropylene microporous film grafted with reactive groups with smaller shrinkage and better thermal stability is obtained by heat treatment, thermal stretching and heat setting under certain conditions; and finally, shaping in hot water for 3-7 hours, and giving a certain tension to the diaphragm to perform a crosslinking reaction to form a net structure and further release internal stress, thereby finally obtaining the stable polypropylene microporous membrane with the crosslinking structure.

Compared with the prior art, the invention has the following advantages and effects:

(1) compared with the wet method for preparing the polypropylene microporous membrane, the method for preparing the polypropylene microporous isolating membrane by adopting the melt-stretching method does not need to use a large amount of solvent, is more environment-friendly in preparation, and can reduce the environmental pollution; the preparation of polypropylene microporous membrane by wet method needs a large amount of organic solvent, which causes high environmental pollution and high preparation cost.

(2) According to the invention, the silane coupling agent (namely an active reactant) is added into the system, so that the excellent performance of the original diaphragm is not reduced, and the defect problems of the existing microporous membrane (namely, the thermal shrinkage of the industrial microporous membrane is large, and the potential safety hazard of the battery is caused by low mechanical strength of the diaphragm) are solved, and the potential safety hazard problems existing in the use process of the battery are obviously improved and enhanced, wherein the potential safety hazard problems in the use process of the battery are greatly improved by the reduction of the thermal shrinkage and the enhancement of the puncture strength.

(3) The polypropylene microporous isolating membrane provided by the invention has excellent mechanical properties, such as: high-temperature thermal dimensional stability, high puncture strength and the like, and can be used as an isolating membrane of a battery. The microporous isolation film overcomes the technical defects that the microporous isolation film in the prior art has lower closed pore temperature, low mechanical strength, poor high-temperature thermal dimensional stability, easy operation of the battery and potential safety hazard.

Drawings

FIG. 1 is an infrared spectrum of a polypropylene preliminary pellet having a reactive group prepared in example 2.

FIG. 2 is a thermal analysis spectrum of the polypropylene pre-pellets having active groups prepared in example 3.

FIG. 3 is SEM images of the polypropylene microporous separator 2 prepared in the comparative example (setting for 5h) and the polypropylene microporous separator prepared in the example 2 at different soaking hot water crosslinking times; wherein, A: comparative example polypropylene microporous separator 2 (set 5h), B: polypropylene microporous separator film 1 (set 3h) from example 2, C: polypropylene microporous separator film 2 from example 2 (set 5h), D: the microporous polypropylene separator 3 from example 2 was prepared (set for 7 h).

Detailed Description

The present invention will be described in further detail with reference to examples and drawings, but the present invention is not limited thereto.

All the materials of the following examples are commercially available. The polypropylene is F401 series of Huajin petrochemical company, and the MFI of the polypropylene is 2.02 g/min.

Example 1

(1) Dissolving an initiator BPO, an antioxidant 1010 and vinyl tri (beta-methoxyethoxy) silane in 100mL of acetone to obtain a reactive solution; then adding pure polypropylene, and uniformly mixing in a high-speed mixer to obtain a polypropylene grafting material solution with active groups; wherein the mass ratio of the initiator, the antioxidant and the active reactant of vinyl tri (beta-methoxyethoxy) silane to the polypropylene is 0.3: 0.5: 1: 98.2 of the total weight of the mixture;

(2) uniformly mixing the polypropylene graft material solution with the active groups, the nano-silica and the pure polypropylene material prepared in the step (1) in a high-speed mixer to obtain a blend with different contents of the polypropylene graft materials with the active groups, wherein the mass ratio of the polypropylene graft materials with the active groups to the nano-silica to the pure polypropylene material is (2, 5, 8 and 10): 0.1: (98, 95, 92, 90);

(4) carrying out melt extrusion, tape casting and blow molding on the polypropylene pre-used granules with the active groups prepared in the step (3) in a single-screw extruder to obtain an initial tape casting film; wherein the die head temperature is 220 ℃, the casting roller temperature is 95 ℃, and the draw ratio is 80;

(5) carrying out heat treatment on the initial casting film prepared in the step (4) at 145 ℃ for 30min to obtain a heat treatment film with a further improved structure; stretching the heat-treated film at a temperature of 130 ℃ at a speed of 50mm/min to form a hole; finally, heat setting is carried out for 10min at 145 ℃ to obtain the polypropylene microporous membrane grafted with the reactive groups, and the microporous membrane has uniform pore size distribution and good microporous structure;

(6) flattening, clamping and fixing the polypropylene microporous membrane grafted with the reactive groups prepared in the step (5) to apply uniform tension; and then shaping for 5 hours in hot water at 50 ℃ to obtain a polypropylene microporous isolation membrane 1 (a polypropylene grafting material 2 with an active group), a polypropylene microporous isolation membrane 2 (a polypropylene grafting material 5 with an active group), a polypropylene microporous isolation membrane 3 (a polypropylene grafting material 8% with an active group) and a polypropylene microporous isolation membrane 4 (a polypropylene grafting material 10% with an active group), wherein the isolation membranes have a net-shaped cross-linked structure.

Example 2

(1) Dissolving an initiator BPO, an antioxidant 1010 and vinyl tri (beta-methoxyethoxy) silane in 100mL of acetone to obtain a reaction active solution; then adding pure polypropylene, and uniformly mixing in a high-speed mixer to obtain a polypropylene grafting material solution with active groups; wherein the mass ratio of the initiator, the antioxidant and the active reactant of vinyl tri (beta-methoxyethoxy) silane to the polypropylene is 0.3: 0.5: 1: 98.2 of the total weight of the mixture;

(2) uniformly mixing the polypropylene grafting material solution with the active groups prepared in the step (1), nano-silica and pure polypropylene materials in a high-speed mixer to obtain a blend, wherein the mass ratio of the polypropylene grafting material with the active groups to the nano-silica to the pure polypropylene materials is 8: 0.1: 92;

(3) extruding and granulating the blend prepared in the step (2) by a double-screw extruder to obtain polypropylene pre-used granules with active groups; as can be seen from fig. 1 and 2, the silane groups are grafted to the polypropylene;

(6) flattening, clamping and fixing the polypropylene microporous membrane grafted with the reactive groups prepared in the step (5) to apply uniform tension; and then shaping in hot water at 50 ℃ for 3, 5 and 7 hours to obtain a polypropylene microporous isolation membrane 1 (shaping for 3 hours), a polypropylene microporous isolation membrane 2 (shaping for 5 hours) and a polypropylene microporous isolation membrane 3 (shaping for 7 hours), wherein the isolation membranes all have a reticular cross-linking structure.

Example 3

(1) Dissolving an initiator BPO, an antioxidant 1010 and vinyl tri (beta-methoxyethoxy) silane in 100mL of acetone to obtain a reaction active solution; then adding pure polypropylene, and uniformly mixing in a high-speed mixer to obtain a polypropylene grafting material solution with active groups; wherein the mass ratio of the initiator, the antioxidant and the active reactant of vinyl tri (beta-methoxyethoxy) silane to the polypropylene is 0.1: 0.2: 0.1: 99.6;

(2) uniformly mixing the polypropylene grafting material solution with the active groups prepared in the step (1), nano-silica and pure polypropylene materials in a high-speed mixer to obtain a blend, wherein the mass ratio of the polypropylene grafting material with the active groups to the nano-silica to the pure polypropylene materials is 10: 0.02: 90, respectively;

(4) carrying out melt extrusion, tape casting and blow molding on the polypropylene pre-used granules with the active groups prepared in the step (3) in a single-screw extruder to obtain an initial tape casting film; wherein the die head temperature is 210 ℃, the casting roller temperature is 80 ℃, and the draw ratio is 70;

(5) carrying out heat treatment on the initial casting film prepared in the step (4) at 145 ℃ for 30min to obtain a heat treatment film with a further improved structure; stretching the heat-treated film at a speed of 50mm/min at a temperature of 135 ℃ to form a hole; finally, heat setting is carried out for 20min at 140 ℃ to obtain the polypropylene microporous membrane grafted with the reactive groups, and the microporous membrane has uniform pore size distribution and good microporous structure;

(6) flattening, clamping and fixing the polypropylene microporous membrane grafted with the reactive groups prepared in the step (5) to apply uniform tension; and then shaping for 5 hours in hot water at 60 ℃ to obtain the polypropylene microporous isolating membrane, wherein the isolating membrane has a reticular cross-linked structure.

Example 4

(1) Dissolving an initiator BPO, an antioxidant 1010 and vinyl tri (beta-methoxyethoxy) silane in 100mL of acetone to obtain a reaction active solution; then adding pure polypropylene, and uniformly mixing in a high-speed mixer to obtain a polypropylene grafting material solution with active groups; wherein the mass ratio of the initiator, the antioxidant and the active reactant of vinyl tri (beta-methoxyethoxy) silane to the polypropylene is 0.5: 1: 1: 97.5;

(2) uniformly mixing the polypropylene grafting material solution with the active groups prepared in the step (1), nano-silica and pure polypropylene materials in a high-speed mixer to obtain a blend, wherein the mass ratio of the polypropylene grafting material with the active groups to the nano-silica to the pure polypropylene materials is 8: 0.2: 92;

(4) carrying out melt extrusion, tape casting and blow molding on the polypropylene pre-used granules with the active groups prepared in the step (3) in a single-screw extruder to obtain an initial tape casting film; wherein the die head temperature is 240 ℃, the casting roller temperature is 100 ℃, and the draw ratio is 150;

(5) carrying out heat treatment on the initial casting film prepared in the step (4) at 145 ℃ for 30min to obtain a heat treatment film with a further improved structure; stretching the heat-treated film at 140 ℃ at a speed of 50mm/min to form holes; finally, heat setting is carried out for 15min at 140 ℃ to obtain the polypropylene microporous membrane grafted with the reactive groups, and the microporous membrane has uniform pore size distribution and good microporous structure;

(6) flattening, clamping and fixing the polypropylene microporous membrane grafted with the reactive groups prepared in the step (5) to apply uniform tension; and then shaping in hot water of 80 ℃ for 3 hours to obtain the polypropylene microporous isolating membrane, wherein the isolating membrane has a reticular cross-linked structure.

Comparative examples

(1) Extruding pure polypropylene into an initial casting film in a single-screw extruder, wherein the die head temperature is 220 ℃, the casting roll temperature is 95 ℃, and the drafting ratio is 80;

(2) carrying out heat treatment on the initial casting film prepared in the step (1) at 145 ℃ for 30min to obtain a heat-treated film; stretching the heat-treated film at a temperature of 130 ℃ at a speed of 50mm/min to form a hole; finally, heat setting is carried out for 10min at 145 ℃ to obtain a polypropylene microporous membrane;

(2) flattening, clamping and fixing the polypropylene microporous membrane prepared in the step (2) to apply uniform tension; and then shaping in hot water at 50 ℃ for 3, 5 and 7 hours to obtain a polypropylene microporous isolating membrane 1 (shaping for 3 hours), a polypropylene microporous isolating membrane 2 (shaping for 5 hours) and a polypropylene microporous isolating membrane 3 (shaping for 7 hours).

Effects of the embodiment

Example 1 under the premise of ensuring the same other parameters, in the process of preparing the blend, polypropylene graft materials (2%, 5%, 8%, 10%) with different contents of active groups are respectively added, and after the blend is shaped in hot water at 50 ℃ for 5 hours, a polypropylene microporous isolation membrane 1 (polypropylene graft material 2% with active groups), a polypropylene microporous isolation membrane 2 (polypropylene graft material 5% with active groups), a polypropylene microporous isolation membrane 3 (polypropylene graft material 8% with active groups) and a polypropylene microporous isolation membrane 4 (polypropylene graft material 10% with active groups) are obtained, and the properties of the membrane such as MD/TD breaking strength, MD/TD breaking elongation, puncture strength, thermal shrinkage, air permeability and ionic conductivity are respectively detected, and the polypropylene microporous isolation membrane 2 (containing no polypropylene graft material with active groups and nano-silica) obtained by shaping in hot water at 50 ℃ for 5 hours in the comparative example is used as a counter pair And (6) irradiating. The specific method comprises the following steps:

and (3) testing mechanical properties: cutting the sample into sample strips with the width of 15mm and the length of 50mm in the MD direction, and stretching the sample strips on a universal test stretcher at the speed of 50mm/min until the sample strips are broken; cutting the TD direction microporous film into sample strips with the width of 15mm and the length of 30mm, and stretching the sample strips on a universal test stretcher at the speed of 50mm/min until the sample strips are broken; three replicates were averaged for each variable.

Puncture strength: the test was also carried out on a stretcher, using a compression mode, setting the compression speed at

50mm/min, the needle used for puncturing is a customized needle with the diameter of 1cm, and the needle is pressed down at a constant speed until the recording force of the rupture of the microporous membrane is maximum.

Heat shrinkage ratio: is an important index for representing the thermal stability of the battery diaphragm, the diaphragm is cut into a square of 5cm multiplied by 5cm, the size change of the diaphragm before and after heating is measured after heat treatment for 1h at 90 ℃, and the area of the diaphragm before the test is 25cm²The area of the diaphragm after the test is S, and the calculation formula of the thermal shrinkage rate is as follows:

air permeability value: the time required for the air to fall when the air passes through 100ml using the air permeability test instrument is the air permeability value in s.

Ionic conductivity: the test was carried out in a glove box filled with argon, with the diaphragm sandwiched between two steel sheets, assembled into a steel sheet system. The ionic conductivity of the cells was tested using an electrochemical workstation (IM6, Zzhner, Germany) with a disturbance voltage amplitude of 5mV and a frequency of 100mHz-100 KHz. The ionic conductivity is calculated as follows:

where δ represents the ionic conductivity, L represents the thickness of the diaphragm, R represents the diaphragm resistance, and S represents the area of the steel sheet (the area of the steel sheet is used because the area of the diaphragm is larger than the area of the steel sheet).

In addition, the initial cast films and the heat-treated films obtained in example 1 and comparative example were examined for their spring rates: and (3) adopting a universal tensile testing machine, wherein the gauge length of a clamp is 50mm, stretching to 100mm, staying for 180s, returning to 50mm, and circularly stretching for 2 times. The rebound resilience is calculated as follows:

wherein E represents the rebound resilience, L represents 100mm, L₀Indicating a gauge length of 50mm and L' indicating the length after recovery from the second cycle of stretch recovery.

The results are shown in tables 1 and 2.

It can be seen from tables 1 and 2 that, under the condition that the performance of the original separator is not affected, the elongation at break, the breaking strength, the puncture strength and the ionic conductivity of the separator after the silane coupling agent added with the nano-silica is crosslinked are improved, and the thermal shrinkage problem of the microporous membrane is also improved, which shows that the mechanical strength of the separator is improved after the nano-silica and the silane active substance are added for crosslinking, and the problem of poor mechanical performance in the battery is improved.

TABLE 1 influence of the content of different polypropylene grafts having active groups on the rebound of the film

TABLE 2 Effect of different Polypropylene graft content with reactive groups on the film Properties

(2) Example 2 on the premise of ensuring the same other parameters, the polypropylene microporous barrier film 1, the polypropylene microporous barrier film 2 and the polypropylene microporous barrier film 3 are obtained by respectively sizing in 50 ℃ hot water for 3h, 5h and 7h, and the thermal shrinkage rate, the air permeability value and the puncture strength of the polypropylene microporous barrier film 1, the polypropylene microporous barrier film 2 and the polypropylene microporous barrier film 3 prepared in the comparative example are respectively detected, and the specific method is the same as the step (1) with the polypropylene microporous barrier film 1 (sizing for 3h), the polypropylene microporous barrier film 2 (sizing for 5h) and the polypropylene microporous barrier film 3 prepared in the comparative example as references.

The results are shown in Table 3.

TABLE 3 Effect of different setting times on film Properties

Fig. 3 is SEM images of the polypropylene microporous separator 2 prepared in the comparative example (set 5h) and the polypropylene microporous separator prepared in example 2 at different soaking hot water crosslinking times. It can be seen from the figure that the holes of the pure polypropylene microporous membrane are long and narrow micropores, while the micropores of the microporous membrane after the silane coupling agent is added gradually become round holes, and the bridge of the micropores formed before gradually becomes thick, which may be one of the reasons for improving the mechanical properties of the microporous membrane, and the reason for greatly improving the TD strength and puncture strength of the test because the mechanical strength in the TD direction is improved when the micropores are round.

The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such changes, modifications, substitutions, combinations, and simplifications are intended to be included in the scope of the present invention.

Claims

1. A preparation method of a polypropylene microporous isolation membrane is characterized by comprising the following steps:

(6) flattening, clamping and fixing the polypropylene microporous membrane grafted with the reactive groups prepared in the step (5) to apply uniform tension; and then shaping in hot water at the temperature of 50-80 ℃ for 3-7 hours to obtain the polypropylene microporous isolating membrane.

2. The method for preparing microporous polypropylene separator film according to claim 1, wherein:

the initiator in the step (1) is at least one of BPO and ABVN;

the amount of the initiator in the step (1) is 0.1-0.5% of the mass of the polypropylene grafting material.

3. The method for preparing microporous polypropylene separator film according to claim 1, wherein:

the antioxidant in the step (1) is 1010;

the dosage of the antioxidant in the step (1) is 0.2-1% of the mass of the polypropylene graft material.

4. The method for preparing microporous polypropylene separator film according to claim 1, wherein:

the dosage of the active reactant vinyl tri (beta-methoxyethoxy) silane in the step (1) is 0.01-1% of the mass of the polypropylene graft material.

5. The method for preparing microporous polypropylene separator film according to claim 1, wherein:

the melt index of the polypropylene in the step (2) is 2-8 g/10 min.

6. The method for preparing microporous polypropylene separator film according to claim 1, wherein:

the dosage of the polypropylene graft material with the active group in the step (2) is 2-10% of the total mass of the polypropylene graft material with the active group and the polypropylene.

7. The method for preparing microporous polypropylene separator film according to claim 1, wherein:

the dosage of the nano silicon dioxide in the step (2) is 0.02-0.2% of the total mass of the polypropylene graft material with the active groups and the polypropylene.

8. The method for preparing microporous polypropylene separator film according to claim 1, wherein:

the conditions of the melt extrusion, the tape casting and the blow molding of the film in the step (4) are that the temperature of a die head is 210-240 ℃, the temperature of a tape casting roller is 80-100 ℃, and the drafting ratio is 70-150;

the heat treatment condition in the step (5) is that the heat treatment is carried out for 30min at 145 ℃;

the hot stretching in the step (5) is one-step hot stretching;

the one-step hot stretching method comprises the following specific operations: directly heating the film to 130-140 ℃ after heat treatment and carrying out hot stretching;

the heat setting condition in the step (5) is heat setting at 140-145 ℃ for 10-20 min.

9. A microporous polypropylene separator prepared by the method of any one of claims 1 to 8.

10. Use of the microporous polypropylene separator of claim 9 in the field of battery production.