High-strength ultrathin three-layer co-extrusion diaphragm as well as preparation method and application thereof

Abstract

The invention discloses a high-strength ultra-thin three-layer co-extrusion diaphragm and a preparation method and application thereof, and relates to the technical field of battery diaphragm materials, the prepared three-layer co-extrusion diaphragm belongs to an ultra-thin diaphragm, the thickness of the diaphragm can reach 5 microns, and on the premise of ensuring safety, the ultra-thin diaphragm can not only reduce the internal resistance of a battery, but also improve the reliability of the battery. More space is provided for the electrode material, and the possible dislocation problem in the winding process of the electrode plate can be reduced; the three-layer co-extrusion diaphragm prepared by the invention has the characteristics of high mechanical strength and good puncture resistance and thermal stability, can bear larger pressure and stress, ensures that the battery is not easy to damage in the use process, can keep a complete structure even when being impacted by a sharp object or in a high-temperature environment, prevents short circuit in the battery, and improves the service life of the battery. And the safety of the battery is improved.

Description

Technical Field

[0001] This invention relates to the field of battery separator material technology, specifically to a high-strength ultrathin three-layer co-extruded separator, its preparation method, and its application. Background Technology

[0002] The separator is a key component of lithium-ion batteries. Its basic function is to isolate the positive and negative electrodes to prevent short circuits, while allowing lithium ions to pass freely to ensure normal charging and discharging. Currently, commercially available lithium-ion battery separator materials are mainly polyolefins, including polypropylene (PP) films, polyethylene (PE) films, and PP / PE / PP composite films. Among them, PP films have high mechanical strength and good chemical stability, but low porosity, resulting in relatively high internal resistance. PE films have a low melting point and low pore-closing temperature, improving battery safety; and high porosity, resulting in good lithium-ion transport efficiency; however, their mechanical strength is relatively weak. PP / PE / PP composite films combine the advantages of PP and PE films, possessing the advantages of low pore-closing temperature and high melting temperature. At higher temperatures, the separator self-closes its pores without melting, and the outer PP film has antioxidant properties, which can improve the cycle performance and safety performance of the separator.

[0003] Ultra-thin and ultra-reinforced separators are a future development trend. A thinner separator means more electrode material can be placed inside the battery, thus increasing its energy density and capacity. For wound batteries, a thinner separator results in lower internal resistance, which helps improve charge and discharge efficiency. A thinner separator also allows more space for electrode material, making the battery design more compact and improving overall energy density.

[0004] The present invention aims to provide a three-layer co-extruded separator with a thickness of up to 5 μm, which breaks through the technical bottleneck that the thickness of existing separators is generally above 10 μm. Furthermore, the separator has high strength, which can improve battery safety and ensure battery cycle performance. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a high-strength ultra-thin three-layer co-extruded separator and its preparation method, and to apply the obtained high-strength ultra-thin three-layer co-extruded separator to lithium-ion batteries, which significantly improves the cycle performance and safety performance of lithium-ion batteries and extends the service life of lithium-ion batteries.

[0006] The technical problem to be solved by this invention is achieved by the following technical solution:

[0007] The first objective of this invention is to provide a method for preparing a high-strength, ultra-thin, three-layer co-extruded separator, comprising the following steps:

[0008] (1) After melting and plasticizing the outer and inner layers respectively, the three layers are co-extruded, drawn, and cooled to obtain a cast sheet;

[0009] (2) The casting sheet is annealed, stretched, and heat-set to obtain a three-layer co-extruded diaphragm.

[0010] Further, the outer layer material comprises polypropylene, dimethyl(dimethylamino)vinylsilane and an initiator; the mass ratio of polypropylene to dimethyl(dimethylamino)vinylsilane is (85-90):(10-15), and the amount of initiator is 5-10% of the mass of dimethyl(dimethylamino)vinylsilane.

[0011] This invention uses dimethyl (dimethylamino)vinylsilane to graft copolymerize polypropylene, with the aim of optimizing the mechanical strength and thermal stability of the PP / PE / PP three-layer co-extruded separator.

[0012] Furthermore, the inner layer material comprises polyethylene.

[0013] Furthermore, the outer and inner layers also include additives, which are selected from one or more of plasticizers, antioxidants, stabilizers, and slip agents. Commonly used plasticizers, antioxidants, stabilizers, and slip agents in the art can be selected. The addition of additives can improve the processing performance and performance of the three-layer co-extruded separator.

[0014] Furthermore, the melt index of the polypropylene at a temperature of 230°C and a load of 2.16 kg is less than 1 g / 10 min. Melt index refers to the weight of molten plastic passing through a standard die capillary in 10 minutes under specific temperature and pressure conditions.

[0015] Furthermore, the melt index of the polyethylene at a temperature of 230°C and a load of 2.16 kg is less than 1 g / 10 min.

[0016] This invention selects low melt index polypropylene and polyethylene as raw materials. The low melt index separator can quickly close the pores when the battery overheats, blocking ion conduction and effectively preventing short circuits. In addition, it has good dimensional stability at high temperatures and can maintain stable performance over a wide temperature range, further improving battery safety.

[0017] Furthermore, the traction speed is 50–100 m / min.

[0018] Furthermore, the annealing temperature is 110–140°C, and the annealing time is 8–24 hours. Annealing can eliminate stress within the film, enhance its stability, and improve its mechanical properties.

[0019] Furthermore, the stretching includes cold stretching and hot stretching. The cold stretching temperature is 30–60°C, and the stretching ratio is 1.2–1.5. The hot stretching temperature is 120–150°C, and the stretching ratio is 1.5–3. Stretching peels away the crystal interface, forming a porous structure.

[0020] Furthermore, the heat setting temperature is 120–150°C, and the heat setting time is 1–5 minutes. Heat setting can improve the dimensional stability of the film, eliminate internal stress, improve surface smoothness, enhance heat resistance, and improve mechanical properties.

[0021] The second objective of this invention is to provide a high-strength, ultra-thin, three-layer co-extruded separator prepared by the aforementioned preparation method.

[0022] A third objective of this invention is to provide the application of the high-strength, ultra-thin, three-layer co-extruded separator in lithium-ion batteries.

[0023] The beneficial effects of this invention are as follows: The three-layer co-extruded separator prepared by this invention is an ultra-thin separator with a thickness of up to 5 μm. Under the premise of ensuring safety, the ultra-thin separator can not only reduce the internal resistance of the battery and provide more space for electrode materials, but also reduce the misalignment problem that may occur during the electrode winding process. Furthermore, the three-layer co-extruded separator prepared by this invention has the characteristics of high mechanical strength, good puncture resistance and thermal stability. It can withstand greater pressure and stress, ensuring that it is not easily damaged during battery use. Even when subjected to impact from sharp objects or in high-temperature environments, it can maintain structural integrity, prevent internal short circuits in the battery, and improve battery safety. Detailed Implementation

[0024] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments.

[0025] The following descriptions of the raw materials used in the examples and comparative examples are as follows:

[0026] Polypropylene: CNOOC Shell 2420D, with a melt index of 0.25 g / 10 min at a temperature of 230℃ and a load of 2.16 kg.

[0027] Polyethylene: Yanshan Petrochemical 2F0.4A-1, with a melt index of 0.4 g / 10 min under the conditions of temperature 230℃ and load 2.16 kg.

[0028] Example 1

[0029] (1) The outer and inner layers are melted and plasticized separately, then co-extruded in three layers, drawn, and cooled to obtain a cast sheet. The outer layer material includes polypropylene, dimethyl(dimethylamino)vinylsilane, and dicumyl peroxide; the mass ratio of polypropylene to dimethyl(dimethylamino)vinylsilane is 85:15, and the amount of dicumyl peroxide is 10% of the mass of dimethyl(dimethylamino)vinylsilane; the inner layer material is polyethylene; the melt plasticizing temperature of the outer layer material is 140-200℃, and the screw speed is 100 rpm; the melt plasticizing temperature of the inner layer material is 150-210℃, and the screw speed is 100 rpm; the die temperature of the three-layer co-extrusion is 190-210℃, the traction speed is 80 m / min, and the cooling roller temperature is 50℃.

[0030] (2) The casting prepared in step (1) is annealed at a temperature of 110°C for 12 hours. Then, longitudinal cold stretching and hot stretching are performed. The cold stretching temperature is 50°C and the stretching ratio is 1.2. The hot stretching temperature is 130°C and the stretching ratio is 2. Finally, heat setting is performed at a temperature of 120°C for 3 minutes to obtain a three-layer co-extruded diaphragm.

[0031] Example 2

[0032] (1) The outer and inner layers are melted and plasticized separately, then co-extruded in three layers, drawn, and cooled to obtain a cast sheet. The outer layer material includes polypropylene, dimethyl(dimethylamino)vinylsilane, and dicumyl peroxide; the mass ratio of polypropylene to dimethyl(dimethylamino)vinylsilane is 90:10, and the amount of dicumyl peroxide is 10% of the mass of dimethyl(dimethylamino)vinylsilane; the inner layer material is polyethylene; the melt plasticizing temperature of the outer layer material is 140-200℃, and the screw speed is 50 rpm; the melt plasticizing temperature of the inner layer material is 150-210℃, and the screw speed is 50 rpm; the die temperature of the three-layer co-extrusion is 190-210℃, the traction speed is 50 m / min, and the cooling roller temperature is 40℃.

[0033] (2) Anneal the casting prepared in step (1) at a temperature of 120°C for 10 hours. Then, perform longitudinal cold stretching and hot stretching at a temperature of 60°C and a stretching ratio of 1.5. Perform hot stretching at a temperature of 150°C and a stretching ratio of 3. Finally, perform heat setting at a temperature of 150°C for 1 minute to obtain a three-layer co-extruded diaphragm.

[0034] Example 3

[0035] (1) The outer and inner layers are melted and plasticized separately, then co-extruded in three layers, drawn, and cooled to obtain a cast sheet. The outer layer material includes polypropylene, dimethyl(dimethylamino)vinylsilane, and dicumyl peroxide; the mass ratio of polypropylene to dimethyl(dimethylamino)vinylsilane is 88:12, and the amount of dicumyl peroxide is 5% of the mass of dimethyl(dimethylamino)vinylsilane; the inner layer material is polyethylene; the melt plasticizing temperature of the outer layer material is 140-200℃, and the screw speed is 150 rpm; the melt plasticizing temperature of the inner layer material is 150-210℃, and the screw speed is 150 rpm; the die temperature of the three-layer co-extrusion is 190-210℃, the traction speed is 100 m / min, and the cooling roller temperature is 50℃.

[0036] (2) The casting prepared in step (1) is annealed at a temperature of 130°C for 8 hours. Then, longitudinal cold stretching and hot stretching are performed. The cold stretching temperature is 40°C and the stretching ratio is 1.2. The hot stretching temperature is 120°C and the stretching ratio is 1.5. Finally, heat setting is performed at a temperature of 140°C for 2 minutes to obtain a three-layer co-extruded diaphragm.

[0037] Example 4

[0038] (1) The outer and inner layers are melted and plasticized separately, then co-extruded in three layers, drawn, and cooled to obtain a cast sheet. The outer layer material includes polypropylene, dimethyl(dimethylamino)vinylsilane, and dicumyl peroxide; the mass ratio of polypropylene to dimethyl(dimethylamino)vinylsilane is 85:15, and the amount of dicumyl peroxide is 8% of the mass of dimethyl(dimethylamino)vinylsilane; the inner layer material is polyethylene; the melt plasticizing temperature of the outer layer material is 140-200℃, and the screw speed is 80 rpm; the melt plasticizing temperature of the inner layer material is 150-210℃, and the screw speed is 80 rpm; the die temperature of the three-layer co-extrusion is 190-210℃, the traction speed is 50 m / min, and the cooling roller temperature is 35℃.

[0039] (2) The casting prepared in step (1) is annealed at a temperature of 140°C for 8 hours. Then, longitudinal cold stretching and hot stretching are performed. The cold stretching temperature is 30°C and the stretching ratio is 1.2. The hot stretching temperature is 140°C and the stretching ratio is 2. Finally, heat setting is performed at a temperature of 130°C for 3 minutes to obtain a three-layer co-extruded diaphragm.

[0040] Example 5

[0041] (1) The outer and inner layers are melted and plasticized separately, then co-extruded in three layers, drawn, and cooled to obtain a cast sheet. The outer layer material includes polypropylene, dimethyl(dimethylamino)vinylsilane, and dicumyl peroxide; the mass ratio of polypropylene to dimethyl(dimethylamino)vinylsilane is 85:15, and the amount of dicumyl peroxide is 10% of the mass of dimethyl(dimethylamino)vinylsilane; the inner layer material is polyethylene; the melt plasticizing temperature of the outer layer material is 140-200℃, and the screw speed is 100 rpm; the melt plasticizing temperature of the inner layer material is 150-210℃, and the screw speed is 100 rpm; the die temperature of the three-layer co-extrusion is 190-210℃, the traction speed is 100 m / min, and the cooling roller temperature is 30℃.

[0042] (2) The casting sheet prepared in step (1) is annealed at a temperature of 110°C for 8 hours. Then, longitudinal cold stretching and hot stretching are performed. The cold stretching temperature is 55°C and the stretching ratio is 1.3. The hot stretching temperature is 150°C and the stretching ratio is 2.5. Finally, heat setting is performed at a temperature of 140°C for 2 minutes to obtain a three-layer co-extruded diaphragm.

[0043] Example 6

[0044] The method is the same as in Example 1, except that calcium stearate and antioxidant 1010 are added as adjuvants in Example 6.

[0045] (1) The outer and inner layers are melted and plasticized separately, then co-extruded in three layers, drawn, and cooled to obtain a cast sheet. The outer layer material includes polypropylene, dimethyl(dimethylamino)vinylsilane, dicumyl peroxide, calcium stearate, and antioxidant 1010; the mass ratio of polypropylene to dimethyl(dimethylamino)vinylsilane, calcium stearate, and antioxidant 1010 is 85:15:0.05:0.1, and the amount of dicumyl peroxide is 10% of the mass of dimethyl(dimethylamino)vinylsilane; the inner layer material is polyethylene; the melt plasticizing temperature of the outer layer material is 140-200℃, and the screw speed is 100 rpm; the melt plasticizing temperature of the inner layer material is 150-210℃, and the screw speed is 100 rpm; the die temperature of the three-layer co-extrusion is 190-210℃, the traction speed is 80 m / min, and the cooling roller temperature is 50℃.

[0046] (2) The casting prepared in step (1) is annealed at a temperature of 110°C for 12 hours. Then, longitudinal cold stretching and hot stretching are performed. The cold stretching temperature is 50°C and the stretching ratio is 1.2. The hot stretching temperature is 130°C and the stretching ratio is 2. Finally, heat setting is performed at a temperature of 120°C for 3 minutes to obtain a three-layer co-extruded diaphragm.

[0047] Example 7

[0048] The method of Example 1 is followed, except that in Example 7, dioctyl phthalate and oleamide are added as additives.

[0049] (1) After melting and plasticizing the outer and inner layers respectively, the three layers are co-extruded, drawn, and cooled to obtain a cast sheet. The outer layer material comprises polypropylene, dimethyl(dimethylamino)vinylsilane, dicumyl peroxide, dioctyl phthalate, and oleamide; the mass ratio of polypropylene to dimethyl(dimethylamino)vinylsilane, dioctyl phthalate, and oleamide is 85:15:5:0.1, and the amount of dicumyl peroxide is 10% of the mass of dimethyl(dimethylamino)vinylsilane; the inner layer material comprises polyethylene, dioctyl phthalate, and oleamide; the mass ratio of polyethylene to dioctyl phthalate and oleamide is 100:5:0.05; the melt plasticizing temperature of the outer layer material is 140–200℃, and the screw speed is 100 rpm; the melt plasticizing temperature of the inner layer material is 150–210℃, and the screw speed is 100 rpm; the die temperature for the three-layer co-extrusion is 190–210℃, the traction speed is 80 m / min, and the cooling roller temperature is 50℃.

[0050] (2) The casting prepared in step (1) is annealed at a temperature of 110°C for 12 hours. Then, longitudinal cold stretching and hot stretching are performed. The cold stretching temperature is 50°C and the stretching ratio is 1.2. The hot stretching temperature is 130°C and the stretching ratio is 2. Finally, heat setting is performed at a temperature of 120°C for 3 minutes to obtain a three-layer co-extruded diaphragm.

[0051] Example 8

[0052] The method of Example 1 is followed, except that in Example 8, dioctyl phthalate, oleamide, calcium stearate and antioxidant 1010 are added as additives.

[0053] (1) The outer and inner layers are melted and plasticized separately, then co-extruded in three layers, drawn, and cooled to obtain a cast sheet. The outer layer includes polypropylene, dimethyl(dimethylamino)vinylsilane, dicumyl peroxide, dioctyl phthalate, oleamide, calcium stearate, and antioxidant 1010; the mass ratio of polypropylene to dimethyl(dimethylamino)vinylsilane, dioctyl phthalate, oleamide, calcium stearate, and antioxidant 1010 is 85:15:5:0.1:0.05:0.1, and the amount of dicumyl peroxide is 10% of the mass of dimethyl(dimethylamino)vinylsilane; the inner layer includes polyethylene, dioctyl phthalate, dimethyl(dimethylamino)vinylsilane, dioctyl phthalate, dioctyl peroxide ... The mass ratio of polyethylene to dioctyl phthalate, oleamide, calcium stearate, and antioxidant 1010 is 100:5:0.1:0.05:0.1. The melt plasticizing temperature of the outer layer material is 140–200℃, and the screw speed is 100 rpm. The melt plasticizing temperature of the inner layer material is 150–210℃, and the screw speed is 100 rpm. The die temperature of the three-layer co-extrusion is 190–210℃, the traction speed is 80 m / min, and the cooling roller temperature is 50℃.

[0054] (2) The casting prepared in step (1) is annealed at a temperature of 110°C for 12 hours. Then, longitudinal cold stretching and hot stretching are performed. The cold stretching temperature is 50°C and the stretching ratio is 1.2. The hot stretching temperature is 130°C and the stretching ratio is 2. Finally, heat setting is performed at a temperature of 120°C for 3 minutes to obtain a three-layer co-extruded diaphragm.

[0055] Comparative Example 1

[0056] The method of Example 1 is followed, except that in Comparative Example 1, allylamine is used instead of dimethyl(dimethylamino)vinylsilane to graft modify polypropylene.

[0057] (1) The outer and inner layers are melted and plasticized separately, then co-extruded in three layers, drawn, and cooled to obtain a cast sheet. The outer layer material includes polypropylene, allylamine, and diisopropylbenzene peroxide; the mass ratio of polypropylene to allylamine is 85:15, and the amount of diisopropylbenzene peroxide is 10% of the mass of allylamine; the inner layer material is polyethylene; the melt plasticizing temperature of the outer layer material is 140-200℃, and the screw speed is 100 rpm; the melt plasticizing temperature of the inner layer material is 150-210℃, and the screw speed is 100 rpm; the die temperature of the three-layer co-extrusion is 190-210℃, the traction speed is 80 m / min, and the cooling roller temperature is 50℃.

[0058] (2) The casting prepared in step (1) is annealed at a temperature of 110°C for 12 hours. Then, longitudinal cold stretching and hot stretching are performed. The cold stretching temperature is 50°C and the stretching ratio is 1.2. The hot stretching temperature is 130°C and the stretching ratio is 2. Finally, heat setting is performed at a temperature of 120°C for 3 minutes to obtain a three-layer co-extruded diaphragm.

[0059] Comparative Example 2

[0060] The method of Example 1 is followed, except that in Comparative Example 2, vinyltriethoxysilane is used instead of dimethyl(dimethylamino)vinylsilane to graft modify polypropylene.

[0061] (1) The outer and inner layers are melted and plasticized separately, then co-extruded in three layers, drawn, and cooled to obtain a cast sheet. The outer layer material includes polypropylene, vinyltriethoxysilane, and dicumyl peroxide; the mass ratio of polypropylene to vinyltriethoxysilane is 85:15, and the amount of dicumyl peroxide is 10% of the mass of vinyltriethoxysilane; the inner layer material is polyethylene; the melt plasticizing temperature of the outer layer material is 140-200℃, and the screw speed is 100 rpm; the melt plasticizing temperature of the inner layer material is 150-210℃, and the screw speed is 100 rpm; the die temperature of the three-layer co-extrusion is 190-210℃, the traction speed is 80 m / min, and the cooling roller temperature is 50℃.

[0062] (2) The casting prepared in step (1) is annealed at a temperature of 110°C for 12 hours. Then, longitudinal cold stretching and hot stretching are performed. The cold stretching temperature is 50°C and the stretching ratio is 1.2. The hot stretching temperature is 130°C and the stretching ratio is 2. Finally, heat setting is performed at a temperature of 120°C for 3 minutes to obtain a three-layer co-extruded diaphragm.

[0063] Comparative Example 3

[0064] The method was the same as in Example 1, except that in Comparative Example 3, no monomer was used to graft polypropylene.

[0065] (1) The outer and inner layers are melted and plasticized separately, and then co-extruded in three layers, drawn, and cooled to obtain a cast sheet. The outer layer is polypropylene; the inner layer is polyethylene; the melting and plasticizing temperature of the outer layer is 140-200℃, and the screw speed is 100 rpm; the melting and plasticizing temperature of the inner layer is 150-210℃, and the screw speed is 100 rpm; the die temperature of the three-layer co-extrusion is 190-210℃, the traction speed is 80 m / min, and the cooling roller temperature is 50℃.

[0066] (2) The casting prepared in step (1) is annealed at a temperature of 110°C for 12 hours. Then, longitudinal cold stretching and hot stretching are performed. The cold stretching temperature is 50°C and the stretching ratio is 1.2. The hot stretching temperature is 130°C and the stretching ratio is 2. Finally, heat setting is performed at a temperature of 120°C for 3 minutes to obtain a three-layer co-extruded diaphragm.

[0067] The performance of the three-layer co-extruded membranes (with a thickness of 5 μm) prepared in Examples 1 to 8 and Comparative Examples 1 to 2 was tested, and the test results are shown in Table 1.

[0068] The longitudinal tensile strength, longitudinal elongation at break and longitudinal heat shrinkage of the diaphragm were tested according to standard GB / T 36363-2018 (105℃, 1h).

[0069] Table 1 Performance test results of the diaphragm

[0070] Tensile strength (MPa) Elongation at break (%) Heat shrinkage rate (%) Example 1 223 158.4 1.27 Example 2 208 156.9 1.52 Example 3 212 153.7 1.63 Example 4 217 162.8 1.59 Example 5 220 164.5 1.68 Example 6 221 152.3 1.34 Example 7 209 166.9 1.75 Example 8 203 168.7 1.32 Comparative Example 1 194 113.4 5.80 Comparative Example 2 168 125.8 3.96 Comparative Example 3 172 102.3 6.54

[0071] As can be seen from Table 1, compared with grafting polypropylene using other monomers or without using monomers, the grafting modification of polypropylene using dimethyl(dimethylamino)vinylsilane in this invention can give the prepared three-layer co-extruded membrane excellent mechanical properties and thermal stability.

[0072] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a high-strength, ultra-thin, three-layer co-extruded separator, characterized in that, Includes the following steps: (1) After melting and plasticizing the outer and inner layers respectively, the three layers are co-extruded, drawn, and cooled to obtain a cast sheet; (2) The casting sheet is annealed, stretched, and heat-set to obtain a three-layer co-extruded diaphragm.

2. The preparation method according to claim 1, characterized in that: The outer layer material comprises polypropylene, dimethyl(dimethylamino)vinylsilane and an initiator, wherein the mass ratio of polypropylene to dimethyl(dimethylamino)vinylsilane is (85-90):(10-15), and the amount of initiator is 5-10% of the mass of dimethyl(dimethylamino)vinylsilane.

3. The preparation method according to claim 1, characterized in that: The inner layer material includes polyethylene.

4. The preparation method according to claim 1, characterized in that: The polypropylene has a melt flow index of less than 1 g / 10 min at a temperature of 230°C and a load of 2.16 kg. The polyethylene has a melt index of less than 1 g / 10 min under the conditions of temperature 230°C and load 2.16 kg.

5. The preparation method according to claim 1, characterized in that: The traction speed is 50–100 m / min.

6. The preparation method according to claim 1, characterized in that: The annealing temperature is 110–140°C, and the annealing time is 8–24 hours.

7. The preparation method according to claim 1, characterized in that: The stretching includes cold stretching and hot stretching. The cold stretching temperature is 30-60℃ and the stretching ratio is 1.2-1.

5. The hot stretching temperature is 120-150℃ and the stretching ratio is 1.5-3.

8. The preparation method according to claim 1, characterized in that: The heat setting temperature is 120–150°C, and the heat setting time is 1–5 min.

9. A high-strength, ultra-thin, three-layer co-extruded diaphragm prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the high-strength ultra-thin three-layer co-extruded separator as described in claim 9 in lithium-ion batteries.