Low-temperature closed-pore lithium battery diaphragm and preparation method thereof

Abstract

The invention discloses a low-temperature closed-pore lithium battery diaphragm and a preparation method thereof, and belongs to the technical field of battery diaphragms.The low-temperature closed-pore lithium battery diaphragm is prepared from, by weight, 5-15 parts of ethylene-vinyl acetate, 5-6 parts of an amphiphilic metal organic framework and 85-95 parts of ultra-high molecular weight polyethylene resin. The preparation method comprises the following steps: 1, banburying ethylene-vinyl acetate, a lubricant and an amphiphilic metal organic framework, and extruding and granulating to obtain premixed granules; and 2, mixing the premixed granules, ultra-high molecular weight polyethylene resin and a dispersing aid, performing melt extrusion, film casting, two-way stretching and dichloromethane ultrasonic extraction, and drying to obtain the low-temperature closed-cell lithium battery diaphragm. Low-melting-point ethylene-vinyl acetate and ultra-high molecular weight polyethylene resin are used as main raw materials, and the low-melting-point ethylene-vinyl acetate is used for reducing the hole closing temperature of the diaphragm.

Description

Technical Field

[0001] This invention belongs to the field of battery separator technology, specifically relating to a low-temperature closed-pore lithium battery separator and its preparation method. Background Technology

[0002] The thermally closed-cell characteristic of lithium-ion battery separators is a crucial function of polyolefin materials, providing essential protection for the safe operation of lithium-ion batteries. When an abnormal operating condition such as an internal short circuit occurs, the battery system temperature rises sharply. If the electrochemical reaction continues, heat will accumulate. Since effective heat dissipation is impossible in a short time, this thermal runaway can lead to safety accidents such as combustion and explosion. Therefore, an effective thermal protection mechanism is needed to terminate the battery reaction promptly.

[0003] Under high-temperature conditions, polyolefin-based separators achieve pore closure through material melting, thereby blocking lithium-ion transport channels and terminating the electrochemical reaction. However, the melting temperatures of polyethylene and polypropylene are approximately 130°C and 160°C, respectively, meaning that pore closure protection is only triggered when the battery temperature significantly exceeds the normal operating range. Furthermore, the material melting process requires a time gradient, during which the battery temperature may continue to rise to the separator's rupture critical point, leading to direct contact between the positive and negative electrodes and causing more serious safety hazards.

[0004] To optimize thermal response characteristics, ethylene-vinyl acetate copolymer (EVA) is often used as a modifying material in practical applications. This material has a melting temperature range of 80-120℃, and its thermodynamic properties are closely related to the vinyl acetate (VA) content: as the VA component ratio increases, the material's flexibility and light transmittance improve, but the melting temperature shows a slight decreasing trend. Studies show that EVA materials with low VA content can form a more compatible system with the polyethylene matrix, which is beneficial for maintaining the basic performance of the membrane.

[0005] This characteristic, however, leads to a contradiction in material selection: while low-VA content EVA ensures good compatibility with polyethylene, its melt temperature adjustment range is limited, making it difficult to achieve the ideal low-temperature closed-cell effect. This contradiction between material properties and functional requirements urgently needs to be reconciled through innovation in material modification technology. Summary of the Invention

[0006] The purpose of this invention is to provide a low-temperature closed-pore lithium battery separator and its preparation method, so as to solve the problem of poor basic performance of low-temperature closed-pore lithium battery separators.

[0007] The objective of this invention can be achieved through the following technical solutions: A low-temperature closed-cell lithium battery separator, by weight, comprises the following raw materials: 5-15 parts ethylene-vinyl acetate, 5-6 parts amphiphilic metal-organic framework, and 85-95 parts ultra-high molecular weight polyethylene resin.

[0008] Furthermore, the amphiphilic metal-organic framework is a metal-organic framework of γ-cyclodextrin modified with long-chain hydrocarbon succinic anhydride.

[0009] Furthermore, the vinyl acetate content in ethylene-vinyl acetate is 9% to 18% by mass.

[0010] Furthermore, the thickness of the low-temperature closed-pore lithium battery separator is ≤25μm.

[0011] A method for preparing a low-temperature closed-pore lithium battery separator includes the following steps: Step 1: Add ethylene-vinyl acetate, lubricant and amphiphilic metal-organic framework into a mixer and mix at 130-140℃ for 15-20 minutes. After extrusion granulation, the premixed granules are obtained. The second step involves mixing the premixed granules, ultra-high molecular weight polyethylene resin, and dispersant, followed by melt extrusion at a temperature of 200-210℃. The mixture is then cast into a film and subjected to biaxial stretching at 115-120℃ using a static biaxial stretching tester. After stretching, the film is ultrasonically extracted with dichloromethane and then dried to obtain a low-temperature closed-cell lithium battery separator.

[0012] Furthermore, the amphiphilic metal-organic framework is prepared by the following steps: γ-Cyclodextrin was added to water, and the pH was adjusted to 8-9 with sodium hydroxide. Long-chain hydrocarbon succinic anhydride was added, and the reaction was stirred. After the pH of the system stabilized, stirring was stopped, and the pH was adjusted to 6.5-7. After washing with a mixed solution of n-hexane and isopropanol in a volume ratio of 2-3:1 and freeze-drying, long-chain hydrocarbon succinic anhydride modified γ-cyclodextrin was obtained. Long-chain hydrocarbon succinic anhydride-modified γ-cyclodextrin and potassium hydroxide were added to water, followed by methanol. The mixture was incubated at 50-55℃ for 30-40 min, then methanol and hexadecyltrimethylammonium bromide were added. The mixture was incubated at 25-30℃ for 100-120 min. After the incubation, the mixture was washed with isopropanol and centrifuged to obtain the amphiphilic metal-organic framework.

[0013] Furthermore, the mass ratio of γ-cyclodextrin to long-chain hydrocarbon succinic anhydride is 100:6-9; the mass ratio of long-chain hydrocarbon succinic anhydride-modified γ-cyclodextrin to potassium hydroxide is 2:0.08-0.12.

[0014] Furthermore, the long-chain hydrocarbon succinic anhydride is one of dodecyl succinic anhydride, hexadecyl succinic anhydride, octadecenyl succinic acid, and octenyl succinic anhydride.

[0015] Furthermore, in the first step, the amount of lubricant added is 20% to 25% of the amount of ethylene-vinyl acetate, and in the second step, the amount of dispersant added is 25% to 30% of the amount of ultra-high molecular weight polyethylene resin.

[0016] The beneficial effects of this invention are: This invention provides a low-temperature closed-cell lithium battery separator, using low-melting-point ethylene-vinyl acetate and ultra-high molecular weight polyethylene resin as the main raw materials. The low-melting-point ethylene-vinyl acetate lowers the pore-closing temperature of the separator, which is beneficial to improving the high-temperature safety of the battery. This invention also adds an amphiphilic metal-organic framework to further ensure the overall compatibility of the raw materials, improve the mechanical properties of the separator, ensure the basic performance of the separator, and prevent the overall performance of the separator from declining due to excessive addition of ethylene-vinyl acetate or excessively high vinyl acetate content in ethylene-vinyl acetate. Detailed Implementation

[0017] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0018] The following is a detailed description of a low-temperature closed-pore lithium battery separator and its preparation method according to an embodiment of this application.

[0019] A low-temperature closed-cell lithium battery separator, by weight, comprises 5-15 parts ethylene-vinyl acetate, 5-6 parts amphiphilic metal-organic framework, and 85-95 parts ultra-high molecular weight polyethylene resin. It is known that the closed-cell temperature of unmodified or untreated ultra-high molecular weight polyethylene resin separators is >130°C. The closed-cell temperature of the separator is lowered by adding low-melting-point ethylene-vinyl acetate. Simultaneously, to improve the compatibility of ethylene-vinyl acetate and ultra-high molecular weight polyethylene resin and enhance the overall performance of the battery separator, this invention adds an amphiphilic metal-organic framework. The open metal sites in the metal-organic framework pores of the amphiphilic metal-organic framework can chelate with anions in the electrolyte and are commonly used as additives to increase the migration rate of cations. In this invention, by modifying it, it can not only be added to the separator in a mixed form but also improve the mechanical properties of the separator.

[0020] In some possible embodiments, the amphiphilic metal-organic framework is a metal-organic framework of γ-cyclodextrin modified with long-chain hydrocarbon succinic anhydride.

[0021] In some possible embodiments, the vinyl acetate content (VA content) in ethylene-vinyl acetate is 9% to 18%.

[0022] In some possible embodiments, the viscosity-average molecular weight of the ultra-high molecular weight polyethylene resin is ≥800,000.

[0023] In some possible embodiments, the thickness of the low-temperature closed-pore lithium battery separator is ≤25μm.

[0024] A method for preparing a low-temperature closed-pore lithium battery separator includes the following steps: Step 1: Add ethylene-vinyl acetate, white oil (lubricant), and amphiphilic metal-organic framework to a mixer and mix at 130-140℃ for 15-20 minutes. After extrusion granulation, a premixed granule is obtained. The amount of white oil (lubricant) added is 20% to 25% of the amount of ethylene-vinyl acetate. The second step involves mixing the premixed granules with ultra-high molecular weight polyethylene resin (UHMWPE, viscosity-average molecular weight 1.7 million) and white oil (dispersant), followed by melt extrusion at a temperature of 200-210℃. The mixture is then cast into a film and subjected to biaxial stretching at 115-120℃ using a static biaxial tensile testing machine. After stretching, the film is subjected to ultrasonic extraction with dichloromethane and then dried to obtain a low-temperature closed-cell lithium battery separator. The amount of white oil (dispersant) added is 25% to 30% of the amount of ultra-high molecular weight polyethylene resin.

[0025] In some specific embodiments, the amphiphilic metal-organic framework is prepared by the following steps: γ-Cyclodextrin was added to water, and the pH was adjusted to 8-9 with sodium hydroxide. Long-chain hydrocarbon succinic anhydride was added, and the reaction was stirred. After the pH of the system stabilized, stirring was stopped, and the pH was adjusted to 6.5-7. After washing with a mixed solution of n-hexane and isopropanol in a volume ratio of 3:1 and freeze-drying, long-chain hydrocarbon succinic anhydride modified γ-cyclodextrin was obtained. According to the ratio, 2g of long-chain hydrocarbon succinic anhydride-modified γ-cyclodextrin and 0.08-0.12g of potassium hydroxide were added to 100mL of water, followed by 50mL of methanol. The mixture was incubated at 50-55℃ for 30-40min, then 25mL of methanol and 0.1g of hexadecyltrimethylammonium bromide were added. The mixture was incubated at 25-30℃ for 100-120min. After the incubation, the mixture was washed with isopropanol and centrifuged to obtain the amphiphilic metal-organic framework.

[0026] In some specific embodiments, the mass ratio of γ-cyclodextrin to long-chain hydrocarbon succinic anhydride is 100:6-9; the mass ratio of long-chain hydrocarbon succinic anhydride-modified γ-cyclodextrin to potassium hydroxide is 2:0.08-0.12.

[0027] In some specific embodiments, the long-chain hydrocarbon succinic anhydride is one of dodecyl succinic anhydride, hexadecyl succinic anhydride, octadecenyl succinic acid, and octenyl succinic anhydride.

[0028] The following is a detailed description with reference to specific examples.

[0029] Example 1

[0030] This embodiment provides a low-temperature closed-pore lithium battery separator, comprising, by weight, 10 parts ethylene-vinyl acetate, 5 parts amphiphilic metal-organic framework, and 90 parts ultra-high molecular weight polyethylene resin. The vinyl acetate content (VA content) in the ethylene-vinyl acetate is 16%. The amphiphilic metal-organic framework is prepared through the following steps: γ-Cyclodextrin was added to water, and the pH was adjusted to 8 with sodium hydroxide. Long-chain hydrocarbon succinic anhydride was then added, and the mixture was stirred until the pH stabilized. Stirring was then stopped, and the pH was adjusted to 6.5. The mixture was washed with a 3:1 (v / v) mixture of hexane and isopropanol, and then freeze-dried to obtain γ-cyclodextrin modified with long-chain hydrocarbon succinic anhydride. The mass ratio of γ-cyclodextrin to long-chain hydrocarbon succinic anhydride was 100:6; the long-chain hydrocarbon succinic anhydride was octenyl succinic anhydride. According to the ratio, 2g of long-chain hydrocarbon succinic anhydride modified γ-cyclodextrin and 0.1g of potassium hydroxide were added to 100mL of water, 50mL of methanol were added, and the mixture was incubated at 50℃ for 35min. Then, 25mL of methanol and 0.1g of hexadecyltrimethylammonium bromide were added, and the mixture was incubated at 25℃ for 100min. After the incubation, the mixture was washed with isopropanol and centrifuged to obtain the amphiphilic metal-organic framework.

[0031] The preparation steps for a low-temperature closed-pore lithium battery separator are as follows: Step 1: Add ethylene-vinyl acetate, white oil, and amphiphilic metal-organic framework to a mixer and mix at 130°C for 15 minutes. After extrusion granulation, a premixed granule is obtained. The amount of white oil added is 20% of the amount of ethylene-vinyl acetate. The second step involves mixing the premixed granules with ultra-high molecular weight polyethylene resin (UHMWPE, viscosity-average molecular weight 1.7 million) and white oil, followed by melt extrusion at a temperature of 200°C. The mixture is then cast into a film and subjected to biaxial stretching at 115°C using a static biaxial stretching tester. After stretching, the film is subjected to ultrasonic extraction with dichloromethane and then dried to obtain a low-temperature closed-cell lithium battery separator. The amount of white oil added is 30% of the amount of ultra-high molecular weight polyethylene resin used.

[0032] Example 2

[0033] This embodiment provides a low-temperature closed-cell lithium battery separator, comprising, by weight, 5 parts ethylene-vinyl acetate, 5 parts amphiphilic metal-organic framework, and 95 parts ultra-high molecular weight polyethylene resin. The vinyl acetate content (VA content) in the ethylene-vinyl acetate is 16%. The amphiphilic metal-organic framework is the same as in Example 1.

[0034] The preparation steps for a low-temperature closed-pore lithium battery separator are as follows: Step 1: Add ethylene-vinyl acetate, white oil, and amphiphilic metal-organic framework to a mixer and mix at 130°C for 15 minutes. After extrusion granulation, a premixed granule is obtained. The amount of white oil added is 20% of the amount of ethylene-vinyl acetate. The second step involves mixing the premixed granules with ultra-high molecular weight polyethylene resin (UHMWPE, viscosity-average molecular weight 1.7 million) and white oil, followed by melt extrusion at a temperature of 200°C. The mixture is then cast into a film and subjected to biaxial stretching at 115°C using a static biaxial stretching tester. After stretching, the film is ultrasonically extracted with dichloromethane and then dried to obtain a low-temperature closed-cell lithium battery separator. The amount of white oil added is 30% of the amount of ultra-high molecular weight polyethylene resin used.

[0035] Example 3

[0036] This embodiment provides a low-temperature closed-cell lithium battery separator, comprising, by weight, 15 parts ethylene-vinyl acetate, 5 parts amphiphilic metal-organic framework, and 85 parts ultra-high molecular weight polyethylene resin. The vinyl acetate content (VA content) in the ethylene-vinyl acetate is 16%. The amphiphilic metal-organic framework is the same as in Example 1.

[0037] The preparation steps for a low-temperature closed-pore lithium battery separator are as follows: Step 1: Add ethylene-vinyl acetate, white oil, and amphiphilic metal-organic framework to a mixer and mix at 130°C for 15 minutes. After extrusion granulation, a premixed granule is obtained. The amount of white oil added is 20% of the amount of ethylene-vinyl acetate. The second step involves mixing the premixed granules with ultra-high molecular weight polyethylene resin (UHMWPE, viscosity-average molecular weight 1.7 million) and white oil, followed by melt extrusion at a temperature of 200°C. The mixture is then cast into a film and subjected to biaxial stretching at 115°C using a static biaxial stretching tester. After stretching, the film is ultrasonically extracted with dichloromethane and then dried to obtain a low-temperature closed-cell lithium battery separator. The amount of white oil added is 30% of the amount of ultra-high molecular weight polyethylene resin used.

[0038] Example 4

[0039] This embodiment provides a low-temperature closed-cell lithium battery separator, comprising, by weight, 10 parts ethylene-vinyl acetate, 6 parts amphiphilic metal-organic framework, and 90 parts ultra-high molecular weight polyethylene resin. The vinyl acetate content (VA content) in the ethylene-vinyl acetate is 16%. The amphiphilic metal-organic framework is the same as in Example 1.

[0040] The preparation steps for a low-temperature closed-pore lithium battery separator are as follows: Step 1: Add ethylene-vinyl acetate, white oil, and amphiphilic metal-organic framework to a mixer and mix at 130°C for 15 minutes. After extrusion granulation, a premixed granule is obtained. The amount of white oil added is 20% of the amount of ethylene-vinyl acetate. The second step involves mixing the premixed granules with ultra-high molecular weight polyethylene resin (UHMWPE, viscosity-average molecular weight 1.7 million) and white oil, followed by melt extrusion at a temperature of 200°C. The mixture is then cast into a film and subjected to biaxial stretching at 115°C using a static biaxial stretching tester. After stretching, the film is ultrasonically extracted with dichloromethane and then dried to obtain a low-temperature closed-cell lithium battery separator. The amount of white oil added is 30% of the amount of ultra-high molecular weight polyethylene resin used.

[0041] Example 5

[0042] This embodiment provides a low-temperature closed-cell lithium battery separator, comprising, by weight, 10 parts ethylene-vinyl acetate, 5 parts amphiphilic metal-organic framework, and 90 parts ultra-high molecular weight polyethylene resin. The vinyl acetate content (VA content) in the ethylene-vinyl acetate is 10%. The amphiphilic metal-organic framework is the same as in Example 1.

[0043] The preparation steps for a low-temperature closed-pore lithium battery separator are as follows: Step 1: Add ethylene-vinyl acetate, white oil, and amphiphilic metal-organic framework to a mixer and mix at 130°C for 15 minutes. After extrusion granulation, a premixed granule is obtained. The amount of white oil added is 20% of the amount of ethylene-vinyl acetate. The second step involves mixing the premixed granules with ultra-high molecular weight polyethylene resin (UHMWPE, viscosity-average molecular weight 1.7 million) and white oil, followed by melt extrusion at a temperature of 200°C. The mixture is then cast into a film and subjected to biaxial stretching at 115°C using a static biaxial stretching tester. After stretching, the film is ultrasonically extracted with dichloromethane and then dried to obtain a low-temperature closed-cell lithium battery separator. The amount of white oil added is 30% of the amount of ultra-high molecular weight polyethylene resin used.

[0044] Example 6

[0045] This embodiment provides a low-temperature closed-pore lithium battery separator, comprising, by weight, 10 parts ethylene-vinyl acetate, 5 parts amphiphilic metal-organic framework, and 90 parts ultra-high molecular weight polyethylene resin. The vinyl acetate content (VA content) in the ethylene-vinyl acetate is 16%. The amphiphilic metal-organic framework is prepared through the following steps: γ-Cyclodextrin was added to water, and the pH was adjusted to 8 with sodium hydroxide. Long-chain hydrocarbon succinic anhydride was then added, and the mixture was stirred until the pH stabilized. Stirring was then stopped, and the pH was adjusted to 6.5. The mixture was washed with a 3:1 volume ratio of hexane and isopropanol, and then freeze-dried to obtain γ-cyclodextrin modified with long-chain hydrocarbon succinic anhydride. The mass ratio of γ-cyclodextrin to long-chain hydrocarbon succinic anhydride was 100:6. The long-chain hydrocarbon succinic anhydride was octadecenylsuccinic acid. According to the ratio, 2g of long-chain hydrocarbon succinic anhydride modified γ-cyclodextrin and 0.1g of potassium hydroxide were added to 100mL of water, 50mL of methanol were added, and the mixture was incubated at 50℃ for 35min. Then, 25mL of methanol and 0.1g of hexadecyltrimethylammonium bromide were added, and the mixture was incubated at 25℃ for 100min. After the incubation, the mixture was washed with isopropanol and centrifuged to obtain the amphiphilic metal-organic framework.

[0046] The preparation steps for a low-temperature closed-pore lithium battery separator are as follows: Step 1: Add ethylene-vinyl acetate, white oil, and amphiphilic metal-organic framework to a mixer and mix at 130°C for 15 minutes. After extrusion granulation, a premixed granule is obtained. The amount of white oil added is 20% of the amount of ethylene-vinyl acetate. The second step involves mixing the premixed granules with ultra-high molecular weight polyethylene resin (UHMWPE, viscosity-average molecular weight 1.7 million) and white oil, followed by melt extrusion at a temperature of 200°C. The mixture is then cast into a film and subjected to biaxial stretching at 115°C using a static biaxial stretching tester. After stretching, the film is subjected to ultrasonic extraction with dichloromethane and then dried to obtain a low-temperature closed-cell lithium battery separator. The amount of white oil added is 30% of the amount of ultra-high molecular weight polyethylene resin used.

[0047] Comparative Example 1

[0048] The difference between this comparative example and Example 1 is that no amphiphilic metal-organic framework is added, while the other raw materials and preparation process remain the same as in Example 1.

[0049] Comparative Example 2

[0050] The difference between this comparative example and Example 5 is that no amphiphilic metal-organic framework is added, while the other raw materials and preparation process remain the same as in Example 5.

[0051] Comparative Example 3

[0052] The difference between this comparative example and Example 1 is that a cyclodextrin metal-organic framework is used instead of an amphiphilic metal-organic framework, while the other raw materials and preparation process remain the same as in Example 1.

[0053] Cyclodextrin metal-organic frameworks are prepared via the following steps: Add 2g of γ-cyclodextrin and 0.1g of potassium hydroxide to 100mL of water, add 50mL of methanol, and incubate at 50℃ for 35min. Then add 25mL of methanol and 0.1g of cetyltrimethylammonium bromide, and incubate at 25℃ for 100min. After the incubation, wash with isopropanol and centrifuge to obtain the cyclodextrin metal-organic framework.

[0054] Comparative Example 4

[0055] The difference between this comparative example and Comparative Example 3 lies in the different preparation steps of the low-temperature closed-pore lithium battery separator: Step 1: Add ethylene-vinyl acetate, white oil, and amphiphilic metal-organic framework to a mixer and mix at 130°C for 15 minutes. After extrusion granulation, a premixed granule is obtained. The amount of white oil added is 30% of the amount of ethylene-vinyl acetate. The second step involves mixing the premixed granules with ultra-high molecular weight polyethylene resin (UHMWPE, viscosity-average molecular weight 1.7 million) and white oil, followed by melt extrusion at a temperature of 200°C. The mixture is then cast into a film and subjected to biaxial stretching at 115°C using a static biaxial stretching tester. After stretching, the film is subjected to ultrasonic extraction with dichloromethane and then dried to obtain a low-temperature closed-cell lithium battery separator. The amount of white oil added is 50% of the amount of ultra-high molecular weight polyethylene resin used.

[0056] Performance tests were conducted on Examples 1-6 and Comparative Examples 1-4.

[0057] Test procedure for liquid absorption rate: Cut the diaphragm sample into 25mm×100mm pieces with a thickness of approximately 20μm. Weigh the original weight of the diaphragm and record it as W0. Immerse the weighed sample completely in the electrolyte (1mol / L LiPF6 is used as the electrolyte, and the solvent is a mixture of ethylene carbonate, dimethyl carbonate, and methyl ethyl carbonate in a mass ratio of 3:3:2) for 60min. Use filter paper to absorb excess electrolyte on the surface until there are no obvious droplets. Weigh the sample and record the weight. Calculate the liquid absorption rate A = (W0-W1) / W0×100%.

[0058] Mechanical property testing: The diaphragm sample was cut into 25mm×100mm pieces, and the tensile strength and elongation at break of the diaphragm were tested using a tensile testing machine.

[0059] The results are shown in Table 1: Table 1

[0060] As shown in Table 1, based on the test results of Examples 1-6, under the conditions of this invention, within a certain range, the liquid absorption rate increases with the increase of ethylene-vinyl acetate. Increasing the content of polar groups (e.g., ester groups) in the membrane is beneficial to improving the liquid absorption rate. A comparison of Example 1 and Comparative Examples 1-4 shows that the addition of amphiphilic metal-organic frameworks not only improves the liquid absorption rate of the membrane but also enhances the compatibility between ethylene-vinyl acetate and ultra-high molecular weight polyethylene resin, thus improving the mechanical properties of the membrane. In Comparative Examples 1 and 2, without the addition of amphiphilic metal-organic frameworks, relying solely on lubricants and dispersants is insufficient to significantly improve the mechanical properties of the material. In Comparative Example 3, an unmodified cyclodextrin metal-organic framework was used. In Comparative Example 4, adding lubricants and dispersants based on Comparative Example 3 also failed to improve the membrane performance.

[0061] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0062] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A low-temperature closed-pore lithium battery separator, characterized in that, By weight, it includes the following raw materials: 5-15 parts ethylene-vinyl acetate, 5-6 parts amphiphilic metal-organic framework and 85-95 parts ultra-high molecular weight polyethylene resin.

2. The low-temperature closed-pore lithium battery separator according to claim 1, characterized in that, The amphiphilic metal-organic framework is a metal-organic framework of γ-cyclodextrin modified with long-chain hydrocarbon succinic anhydride.

3. The low-temperature closed-pore lithium battery separator according to claim 1, characterized in that, The vinyl acetate content in ethylene-vinyl acetate is 9% to 18% by mass.

4. The low-temperature closed-pore lithium battery separator according to claim 1, characterized in that, The thickness of the low-temperature closed-pore lithium battery separator is ≤25μm.

5. A method for preparing a low-temperature closed-cell lithium battery separator, used to prepare the low-temperature closed-cell lithium battery separator according to any one of claims 1-4, characterized in that, Includes the following steps: Step 1: Add ethylene-vinyl acetate, lubricant and amphiphilic metal-organic framework into a mixer and mix at 130-140℃ for 15-20 minutes. After extrusion granulation, the premixed granules are obtained. The second step involves mixing the premixed granules with ultra-high molecular weight polyethylene resin and a dispersant, followed by melt extrusion at a temperature of 200-210℃. The mixture is then cast into a film, biaxially stretched, and finally ultrasonically extracted with dichloromethane before drying to obtain a low-temperature closed-cell lithium battery separator.

6. The method for preparing a low-temperature closed-pore lithium battery separator according to claim 5, characterized in that, The amphiphilic metal-organic framework is prepared by the following steps: γ-Cyclodextrin was added to water, and the pH was adjusted to 8-9 with sodium hydroxide. Long-chain hydrocarbon succinic anhydride was added, and the reaction was stirred. The pH was adjusted to 6.5-7, and the product was washed and freeze-dried to obtain γ-cyclodextrin modified with long-chain hydrocarbon succinic anhydride. Long-chain hydrocarbon succinic anhydride-modified γ-cyclodextrin and potassium hydroxide were added to water, followed by methanol. The mixture was incubated at 50-55℃ for 30-40 min, then methanol and hexadecyltrimethylammonium bromide were added. The mixture was incubated at 25-30℃ for 100-120 min. After the incubation, the mixture was washed with isopropanol and centrifuged to obtain the amphiphilic metal-organic framework.

7. The method for preparing a low-temperature closed-pore lithium battery separator according to claim 6, characterized in that, The mass ratio of γ-cyclodextrin to long-chain hydrocarbon succinic anhydride is 100:6-9.

8. The method for preparing a low-temperature closed-pore lithium battery separator according to claim 6, characterized in that, The mass ratio of long-chain hydrocarbon succinic anhydride-modified γ-cyclodextrin to potassium hydroxide is 2:0.08-0.

12.

9. The method for preparing a low-temperature closed-pore lithium battery separator according to claim 6, characterized in that, The long-chain hydrocarbon succinic anhydride is one of the following: n-dodecyl succinic anhydride, n-hexadecyl succinic anhydride, octadecenyl succinic acid, and octenyl succinic anhydride.

10. The method for preparing a low-temperature closed-pore lithium battery separator according to claim 5, characterized in that, In the first step, the amount of lubricant added is 20% to 25% of the amount of ethylene-vinyl acetate, and in the second step, the amount of dispersant added is 25% to 30% of the amount of ultra-high molecular weight polyethylene resin.

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