Polyethylene microporous diaphragm as well as preparation process and application thereof

Abstract

The invention discloses a polyethylene microporous diaphragm as well as a preparation process and application thereof. The diaphragm is composed of an HDPE base film and a functional coating layer. The coating liquid comprises a polyurethane prepolymer, a synergistic fluorine-containing sulfonic acid polymer solution (8-12%), a boehmite nanofiller (2.5-3.5%), a trifunctional aziridine cross-linking agent (0.8-1.2%), an azodicarbonamide foaming agent (0.5-0.8%) and a dibutyltin dilaurate catalyst (0.08-0.12%) according to a specific ratio, wherein the polyurethane prepolymer is generated by reacting heteroaromatic polyester glycol (15-18%) with aromatic isocyanate (4-6%), and a solvent is DMF (Dimethyl Formamide). The diaphragm has high ionic conductivity, excellent electrolyte corrosion resistance, low thermal shrinkage rate and enhanced mechanical strength, and is particularly suitable for a lithium ion battery with high safety and long cycle life.

Description

Technical Field

[0001] The present invention relates to the technical field of functional polymer films, and in particular to a polyethylene microporous diaphragm and a preparation process and application thereof. Background Art

[0002] As a core component of new energy storage, lithium-ion batteries face a key challenge in terms of safety and performance. As a key component within the battery, the diaphragm plays the important role of isolating the positive and negative electrodes, preventing short circuits, and providing ion channels. Currently, commercial polyethylene (PE) diaphragms are widely used due to their excellent mechanical properties and electrochemical stability, but their inherent defects are becoming increasingly prominent. On the one hand, traditional PE diaphragms lack thermal stability and are prone to severe shrinkage above 130°C, leading to battery short circuits. On the other hand, the existing dry uniaxial stretching process suffers from problems such as poor transverse strength and high solvent contamination in the wet process, which restricts the development of high-performance batteries.

[0003] To improve the performance of diaphragms, the industry generally adopts surface coating modification technology. Ceramic-coated diaphragms enhance thermal stability through the use of inorganic particles such as boehmite. However, the interfacial bonding between the coating and the base membrane is weak, making it easy to peel off during long-term cycles. Excessive filling leads to a decrease in porosity and ionic conductivity to 0.7–0.9 mS / cm. Although organic polymer coating can improve interfacial compatibility, it is difficult to achieve a balance between high-temperature resistance and ion transmission efficiency. In existing technologies, both the complex process control of dry biaxial stretching and the solvent recovery problems of wet processes have led to high production costs.

[0004] Current separator technology faces three bottlenecks: First, insufficient thermomechanical performance and shrinkage at high temperatures prevent it from meeting the safety requirements of high-nickel ternary batteries. Second, the production process is energy-intensive, with the wet process consuming large amounts of organic solvents such as DMF, increasing environmental costs. Third, the functional coating design is limited, making it difficult for traditional coating solutions to simultaneously achieve high ionic conductivity, low interfacial impedance, and ultra-low thermal shrinkage. These shortcomings severely limit the potential for achieving breakthroughs in energy density and safety performance in power batteries. Summary of the Invention

[0005] The purpose of the present invention is to provide a polyethylene microporous membrane and its preparation process and application, which solves the technical problems of traditional polyethylene battery membranes such as large high-temperature shrinkage, poor electrolyte wettability, and difficulty in balancing mechanical strength and ionic conductivity.

[0006] The present invention achieves the above-mentioned purpose through the following technical solutions: A polyethylene microporous diaphragm is composed of a base film and a coating liquid. The coating liquid raw materials include, by mass percentage: Heteroaromatic polyester diol: 15-18%; Aromatic isocyanate: 4-6%; Inorganic nanofiller: 2.5-3.5%; Fluorinated sulfonic acid polymer solution: 8-12%; Cross-linking agent: 0.8-1.2%; Foaming agent: 0.5-0.8%; Catalyst: 0.08-0.12%; Solvent: balance.

[0007] In the present invention, the terminal hydroxyl group (-OH) of the aromatic heterocyclic polyester diol and the isocyanate group (-NCO) of the aromatic isocyanate are polycondensed at 60° C. under the catalysis of dibutyltin dilaurate to generate a terminal -NCO group polyurethane prepolymer.

[0008] According to a preferred embodiment of the present invention, the aromatic heterocyclic polyester diol is purchased from Jining Huakai Resin Co., Ltd.

[0009] According to a preferred embodiment of the present invention, the aromatic isocyanate is purchased from Wanhua Chemical Group as WANNATE® TL-75E.

[0010] According to a preferred embodiment of the present invention, the base film is HDPE resin, the density of the HDPE resin is 0.945-0.955 g / cm³, and the molecular weight distribution is ≤4.0.

[0011] According to a preferred embodiment of the present invention, the HDPE resin is selected from one or more of Borealis HE3490-LS, ExxonMobil 7960 and Shanghai Petrochemical MH602.

[0012] According to a preferred embodiment of the present invention, the inorganic nanofiller is boehmite, and the median particle size d 50 It is 70-80nm.

[0013] According to a preferred embodiment of the present invention, the boehmite is purchased from Anhui Yishitong Materials Technology Co., Ltd.

[0014] In the present invention, boehmite (d 50 =70-80nm) are anchored to the polyurethane network through Al-O-Si covalent bonds. The nano-rigid particles and the rigid molecular chains of aromatic heterocyclic polyester diols (containing benzene rings / heterocyclic structures) synergistically inhibit high-temperature chain segment movement, making the coating's thermal shrinkage rate at 200°C less than 5% while improving puncture strength.

[0015] According to a preferred embodiment of the present invention, the preparation method of the fluorinated sulfonic acid polymer solution comprises: in a nitrogen-protected high-pressure reactor, adding 200 parts of deionized water, 1.5 parts of perfluorohexanoic acid sulfonate emulsifier, and 35 parts of perfluoro(4-methyl-3,6-dioxa-7-octene)sulfonyl fluoride in sequence, stirring and emulsifying at 40°C; heating to 70°C, injecting 65 parts of tetrafluoroethylene to a pressure of 1.5 MPa, adding 0.5 parts of ammonium persulfate aqueous solution to initiate polymerization, and cooling and releasing the pressure after reacting for 8 hours; the emulsion is coagulated, washed, and vacuum-dried to obtain a perfluorosulfonic acid copolymer resin; and finally, the resin is dissolved in N,N-dimethylformamide.

[0016] According to a preferred embodiment of the present invention, the perfluorohexane sulfonate emulsifier is purchased from Shandong Kunxiang Chemical Technology Co., Ltd.

[0017] According to a preferred embodiment of the present invention, the perfluoro(4-methyl-3,6-dioxa-7-octene)sulfonyl fluoride is purchased from Baoji Funuokang Industrial Co., Ltd.

[0018] According to a preferred embodiment of the present invention, the aqueous solution of ammonium persulfate was purchased from Shanghai Ruji Biotechnology Development Co., Ltd. According to a preferred embodiment of the present invention, the tetrafluoroethylene is purchased from Dongyue Fluorosilicon Technology Group Co., Ltd.

[0019] According to a preferred embodiment of the present invention, the N,N-dimethylformamide is purchased from Jinan Chuangshi Chemical Co., Ltd.

[0020] The -SO3⁻ group of the fluorinated sulfonic acid polymer (sulfonic acid equivalent 800-850g / mol, intrinsic viscosity 0.8dL / g) in the present invention forms a continuous ion channel (containing 1.18-1.25 ionic groups per molecule), which, combined with the ultra-low surface energy of the perfluorocarbon chain, makes the electrolyte wetting angle <10° and the ionic conductivity reach 1.5mS / cm; the submicron pores (0.2-0.8μm) generated by the foaming agent form a gradient with the 42% pores (pore size 0.1-1μm) of the HDPE base membrane. The trifunctional aziridine simultaneously connects the polyurethane chain, FSP sulfonic acid group and boehmite surface amino group through covalent bonds, making the coating / base film adhesion greater than 1.5N / cm; the HDPE base film (density 0.945-0.955g / cm³, molecular weight distribution ≤4.0) melts at 122℃ and blocks the current. The sulfonic acid group mobility of FSP doubles at >100℃ (ionic conductivity >80% at 150℃), and boehmite dehydrates and converts to α-Al2O3 at 300℃.

[0021] According to a preferred embodiment of the present invention, the emulsification is stirred at 800 rpm at 40°C for 30 minutes and vacuum dried at 80°C for 24 hours. The intrinsic viscosity of the perfluorosulfonic acid copolymer resin is 0.8 dL / g and the sulfonic acid equivalent is 800-850 g / mol.

[0022] According to a preferred embodiment of the present invention, the cross-linking agent is trifunctional aziridine; the foaming agent is azodicarbonamide; the catalyst is dibutyltin dilaurate; and the solvent is N,N-dimethylformamide.

[0023] According to a preferred embodiment of the present invention, the trifunctional aziridine is purchased from Shanghai Holden Materials Co., Ltd.

[0024] According to a preferred embodiment of the present invention, the azodicarbonamide is purchased from Shanghai Zhitong Chemical Co., Ltd.

[0025] According to a preferred embodiment of the present invention, the dibutyltin dilaurate is purchased from Wuhan Kemik Biopharmaceutical Technology Co., Ltd.

[0026] According to a preferred embodiment of the present invention, the N,N-dimethylformamide is purchased from Jinan Chuangshi Chemical Co., Ltd.

[0027] The trifunctional aziridine crosslinker in the present invention simultaneously bonds the -NCO group of the prepolymer (forming an R-NH-CN(CH2)3 structure) and the sulfonic acid group (-SO3H, forming an R-SO2-NH-(CH2)3 bond) of the fluorinated sulfonic acid polymer through the opening of the aziridine ring to construct a three-dimensional interpenetrating network.

[0028] In the present invention, the azodicarbonamide foaming agent decomposes at 115°C to release N2 / CO / NH3 gas (NH2-C(O)-N=NC(O)-NH2→ N2↑+ 2CO↑+ 2NH3↑), forming submicron closed pores in the cross-linked network; the sulfonyl fluoride group (-SO2F) in the fluorinated sulfonic acid polymer is hydrolyzed to a sulfonic acid group (-SO3H), which dissociates in the electrolyte to provide lithium ion channels.

[0029] The present invention also provides a process for preparing the polyethylene microporous membrane, comprising the following steps: S1, melt-extrude the HDPE resin at 210°C, pass through a cooling roller at 40°C to form a 200 μm cast base film, first longitudinally stretch to 1.25 times at 30°C, then stretch to a total ratio of 1.85 times, and finally heat-set at 122°C to obtain the base film; S2, adding aromatic heterocyclic polyester diol, aromatic isocyanate, and catalyst in sequence to a nitrogen-protected reactor, reacting at 60°C to form a prepolymer, adding DMF solvent to dilute to a solid content of 30%, then adding boehmite nanoparticles treated with silane coupling agent KH-550, FSP solution, crosslinking agent, and foaming agent, and maintaining stirring at 25°C; S3, the coating liquid is applied to the base film with a wet film thickness of 12 μm using double-sided micro-gravure coating. The coating film continuously passes through a 70°C preheating zone and then enters a 115°C curing zone for curing.

[0030] In the present invention, after the boehmite nanoparticles are hydrolyzed by the silane coupling agent KH-550 (CH3CH2O-Si-(CH2)3NH2→ HO-Si-(CH2)3NH2), the hydroxyl groups (Al-OH) on their surface condense with the silanol groups (Al-OH + HO-Si- → Al-O-Si-), and the amino groups (-NH2) at the terminals react with the -NCO groups of the polyurethane prepolymer (-NCO + H2N-Si- → -NH-C(O)-NH-Si-) to achieve chemical coupling.

[0031] According to a preferred embodiment of the present invention, in step S1, the flow rate of the molten HDPE resin is 0.35 g / 10 min; the heat setting time is 30 min; the thickness of the base film is 16 μm and the porosity is 42%; in step S2, the prepolymer is generated by reacting at 60° C. for 1 h; the stirring speed is 1000 rpm and the time is 2 h.

[0032] According to a preferred embodiment of the present invention, in step S3, the coating film continuously passes through a 70°C preheating zone at a speed of 20 m / min for 2 minutes, and then enters a 115°C curing zone for 5 minutes.

[0033] The present invention also provides an application of the polyethylene microporous membrane or the polyethylene microporous membrane prepared by the preparation process in a lithium ion battery.

[0034] The beneficial effects of the present invention are: The present invention forms a polyurethane backbone by reacting a specific ratio of aromatic heterocyclic polyester diols with aromatic isocyanates. This is then combined with a fluorinated sulfonic acid polymer solution and surface-treated boehmite nanofillers to synergistically construct a composite coating with excellent mechanical strength, high ionic conductivity, and outstanding electrolyte wettability. The aromatic heterocyclic structure imparts excellent resistance to electrolyte swelling and thermal stability to the coating, while the uniformly dispersed nanoboehmite effectively enhances the coating's rigidity and resistance to thermal shrinkage. The fluorinated sulfonic acid polymer significantly optimizes the ion transport pathway, ultimately enabling the diaphragm to maintain high porosity while possessing excellent dimensional stability and low interfacial impedance.

[0035] The use of a prepolymer synthesis and step-by-step mixing process ensures uniform dispersion and interfacial bonding of boehmite nanoparticles within the fluorinated sulfonic acid polymer-polyurethane system. A specially formulated coating solution (containing precisely measured amounts of crosslinker, blowing agent, and catalyst) combined with an optimized double-sided micro-gravure coating and a stepped heat treatment process creates a structurally uniform, highly adhesive functional layer with hierarchical micro-nanopores on the base membrane surface. The precise introduction of the blowing agent further modulates the microporous structure within the coating, while a 122°C heat setting process stabilizes the crystal morphology and pore structure of the HDPE base membrane, ensuring the uniformity and mechanical integrity of the overall structure of the separator.

[0036] This separator combines the inherent closed-pore properties of a high-density polyethylene (HDPE) base film with the thermally responsive enhancement mechanism of a composite coating, providing dual thermal protection against abnormal battery temperature increases. The boehmite filler and fluorinated sulfonic acid resin synergistically enhance the coating's high-temperature resistance and effectively suppress thermal shrinkage. Simultaneously, the optimized pore structure and ion channels significantly reduce battery polarization, improving rate capability and cycle stability. This separator is particularly suitable for high-energy-density lithium-ion batteries, significantly enhancing battery safety and lifespan while maintaining electrochemical performance. DETAILED DESCRIPTION

[0037] The following specific implementation methods are only used to further illustrate the present application and should not be understood as limiting the scope of protection of the present application. Technicians in this field may make some non-essential improvements and adjustments to the present application based on the above application content.

[0038] 1. Implementation Example 1 Take HDPE resin with a melt flow rate of 0.35g / 10min (Borealis HE3490-LS, density 0.950g / cm 3, molecular weight distribution 3.8) 100 kg, added with 0.2 kg calcium stearate and 0.1 kg antioxidant 1010, melt extruded at 210 ° C and cooled at 40 ° C to form a 200 μm cast film; at 30 ° C, it was first longitudinally stretched to 1.25 times at a rate of 520% / min, and then stretched to a total ratio of 1.85 times at a rate of 40% / min, and heat-set at 122 ° C for 30 minutes to obtain a base film with a thickness of 16 μm and a porosity of 42%. Preparation of fluorinated sulfonic acid polymer solution: 200g deionized water, 1.5g perfluorohexanoic acid sulfonate emulsifier, and 35g perfluoro(4-methyl-3,6-dioxa-7-octene)sulfonyl fluoride were added to a nitrogen-protected autoclave, and emulsified at 800rpm for 30min at 40°C. The temperature was raised to 70°C, 65g tetrafluoroethylene was injected to a pressure of 1.5MPa, and 0.5g ammonium persulfate (dissolved in 10g water) was added to initiate polymerization. After reacting for 8h, the temperature was lowered and the pressure was released. The emulsion was coagulated, washed, and vacuum-dried at 80°C for 24h to obtain a perfluorosulfonic acid resin with an intrinsic viscosity of 0.8dL / g and a sulfonic acid equivalent of 810g / mol. 20g of this resin was dissolved in 80g DMF to prepare a 20wt% FSP solution for later use. Coating solution preparation: 165g pyridine ring polyester diol, 50g toluene diisocyanate, KH-550 treated boehmite (d 50 =75nm), 30g of polyester diol (polyester diol = 75nm), 100g of 20wt% FSP solution, 10g of trimethylolpropane-triaziridine, 6g of azodicarbonamide, 1g of dibutyltin dilaurate, and 638g of DMF. Under nitrogen protection, the polyester diol was reacted with TDI at 60°C for 1 hour. After dilution with DMF, boehmite, FSP solution, crosslinker, and foaming agent were added. The mixture was stirred at 1000 rpm for 2 hours at 25°C. Double-sided micro-gravure coating (180-mesh anilox roller, wet film thickness 12μm per side) was performed at a line speed of 20m / min. After preheating at 70°C for 2 minutes, the membrane was cured at 115°C for 5 minutes to produce a 20μm thick separator.

[0039] Example 2 A 16μm base film (38% porosity) was prepared using 100kg of HDPE resin (ExxonMobil 7960, melt index 0.50g / 10min, density 0.945g / cm³). The coating solution consisted of 180g of a pyridine-containing polyester diol, 40g of TDI, 25g of boehmite (d50 = 70nm), 120g of a 20wt% FSP solution, 10g of a trifunctional aziridine, 6g of azodicarbonamide, 1g of dibutyltin dilaurate, and 618g of DMF.

[0040] Example 3 100 kg of HDPE resin (Shanghai Petrochemical MH602, melt index 0.30 g / 10 min, density 0.955 g / cm³) was used to prepare a 16 μm base film (porosity 45%). The coating solution contained 150 g of pyridine ring polyester diol, 60 g of TDI, and boehmite (d 50=80nm) 35g, 20wt% FSP solution 80g, trifunctional aziridine 10g, azodicarbonamide 6g, dibutyltin dilaurate 1g, DMF 658g.

[0041] Comparative Example 1 The base film is the same as that in Example 1. Coating liquid: containing 165 g of pyridine ring polyester diol, 50 g of TDI, 30 g of boehmite, 10 g of trifunctional aziridine, 6 g of azodicarbonamide, 1 g of dibutyltin dilaurate, and 738 g of DMF (without adding FSP solution).

[0042] Comparative Example 2 The components of the base film and the coating liquid were the same as those in Example 1, except that the cross-linking agent was replaced with 10 g of isocyanate trimer (Tolonate HDT-LV).

[0043] Comparative Example 3 The HDPE resin was the same as in Example 1, except that the two-stage stretching was omitted and a single-stage stretching was performed at 30° C. (to a total stretching ratio of 1.85).

[0044] 2. Performance Testing The materials prepared in Examples 1-3 and Comparative Examples 1-3 were tested for performance according to the following method: 1. Thickness test: According to the national standard GB / T 36363-2018, use a Mitutoyo micrometer (accuracy 0.1μm) to evenly select 5 measurement points on the diaphragm surface, record the values ​​and calculate the average.

[0045] 2. Porosity test: Using the n-butanol immersion method, cut a 5cm x 5cm square sample and immerse it in n-butanol (density 0.81g / cm³) for 30 minutes. Remove and blot the surface liquid with filter paper, and immediately weigh the wet weight (W_w). Dry the sample in an 80°C vacuum oven for 4 hours, and then weigh the dry weight (W_d). Porosity calculation formula: Porosity (%) = [(W_w - W_d) / 0.81] / [(W_w - W_d) / 0.81 + W_d / 0.95] × 100% (Note: 0.95g / cm³ is the density of polyethylene).

[0046] 3. Thermal Shrinkage Test: According to GB / T 13519-2016, cut a 10.0 cm x 10.0 cm sample and place it horizontally in a 150°C forced air oven for 1 hour. After cooling to room temperature, measure the longitudinal and transverse lengths (L_1). Thermal shrinkage is calculated as: Thermal Shrinkage (%) = (10.0 - L_1) / 10.0 × 100%.

[0047] 4. Ionic Conductivity Testing: A symmetrical stainless steel / diaphragm / stainless steel cell (electrode area 1.766 cm²) was assembled and filled with a 1M LiPF6 / EC:DMC (1:1) electrolyte. AC impedance measurements were performed using an electrochemical workstation (Solartron 1260) with a frequency range of 0.1 Hz to 1 MHz and an amplitude of 10 mV. The bulk resistance (R_b) was obtained from the intercept of the low-frequency region of the Nyquist plot. Ionic conductivity was calculated as follows: ionic conductivity (mS / cm) = [thickness (mm) × 10] / (R_b × 1.766).

[0048] 5. Peel Strength Test: According to ASTM D903, a 25 mm wide diaphragm sample was mounted on an Instron 3343 tensile testing machine. The coating was peeled off at a speed of 50 mm / min in a 180° direction. The average force during the stable peeling phase was recorded (unit: N / cm).

[0049] 6. Puncture strength test: Refer to GB / T 21302-2007. Use a 1.0 mm diameter spherical needle to vertically puncture the septum at a speed of 100 mm / min. Record the maximum puncture force (unit: gf).

[0050] 7. Diaphragm performance test results: Table 1: Test results of membrane performance of various embodiments and comparative examples

[0051] As can be seen from Table 1, the present invention systematically solves the three major technical problems of traditional polyethylene diaphragms through the three technological innovations of composite coating structure design, two-stage cold stretching process and FSP solution modification: Large high-temperature shrinkage: The transverse heat shrinkage (150℃ / 1h) of Examples 1-3 is controlled at 1.5-2.3%, which is more than 90% lower than that of traditional PE films (>20%). The core lies in the synergistic formation of a three-dimensional open-pore network (porosity 51.8-57.9%) by the aziridine crosslinker and the foaming agent, which effectively inhibits the thermal deformation of the base film; the shrinkage of Comparative Example 2 soared to 8.5% due to the use of an isocyanate crosslinker, which reversely verifies the necessity of aziridine. Poor electrolyte wettability: The sulfonic acid group (-SO3H) of the FSP solution reacts with Li + The formation of coordination channels makes the ionic conductivity of the embodiment reach 1.78-1.92mS / cm, which is 40% higher than that of comparative example 1 (without FSP), and the 5C rate performance is greater than 89.7% (traditional ceramic coating film is less than 85%). At the same time, the open-cell foam structure improves the electrolyte retention rate to more than 95%, solving the problem of high interfacial impedance caused by insufficient infiltration of traditional PE membrane (Example 1 is only 12Ω·cm 2 vs. traditional >25Ω·cm 2). Conflict between mechanical strength and ionic conductivity: The two-stage cold stretching process achieves a puncture strength of >398 gf at a base membrane porosity of 42±2% (the lowest single-stage stretching in Example 3 is only 315 gf). The FSP-modified coating maintains high ionic conductivity (1.85 mS / cm) while achieving a peel strength of 2.9-3.3 N / cm (compared to only 0.9 N / cm in Example 2), breaking through the limitation of traditional technology that "high porosity leads to reduced strength."

[0052] The above-described embodiments merely illustrate several implementations of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, and all such variations and improvements fall within the scope of protection of the present invention.

Claims

1. A polyethylene microporous diaphragm, consisting of a base film and a coating liquid, characterized in that: In terms of mass percentage, the coating liquid raw materials include: Heteroaromatic polyester diol: 15-18%; Aromatic isocyanate: 4-6%; Inorganic nanofiller: 2.5-3.5%; Fluorinated sulfonic acid polymer solution: 8-12%; Cross-linking agent: 0.8-1.2%; Foaming agent: 0.5-0.8%; Catalyst: 0.08-0.12%; Solvent: balance.

2. The polyethylene microporous membrane according to claim 1, characterized in that The base film is HDPE resin, the density of the HDPE resin is 0.945-0.955 g / cm³, and the molecular weight distribution is ≤4.

0.

3. The polyethylene microporous membrane according to claim 1, characterized in that The inorganic nanofiller is boehmite, and the median particle size d of the boehmite is 50 It is 70-80nm.

4. The polyethylene microporous membrane according to claim 1, characterized in that The preparation method of the fluorinated sulfonic acid polymer solution comprises: adding 200 parts of deionized water, 1.5 parts of perfluorohexanoic acid sulfonate emulsifier, and 35 parts of perfluoro(4-methyl-3,6-dioxa-7-octene)sulfonyl fluoride in sequence into a nitrogen-protected high-pressure reactor, stirring and emulsifying at 40° C.; heating to 70° C., injecting 65 parts of tetrafluoroethylene to a pressure of 1.5 MPa, adding 0.5 parts of an ammonium persulfate aqueous solution to initiate polymerization, reacting for 8 hours, then cooling and releasing the pressure; coagulating, washing, and vacuum drying the emulsion to obtain a perfluorosulfonic acid copolymer resin; and finally dissolving the resin in N,N-dimethylformamide.

5. The polyethylene microporous membrane according to claim 4, characterized in that: Emulsification was carried out at 800 rpm and stirring at 40° C. for 30 minutes; and vacuum drying was carried out at 80° C. for 24 hours. The intrinsic viscosity of the perfluorosulfonic acid copolymer resin was 0.8 dL / g, and the sulfonic acid equivalent was 800-850 g / mol.

6. The polyethylene microporous membrane according to claim 1, characterized in that The cross-linking agent is trifunctional aziridine; the foaming agent is azodicarbonamide; the catalyst is dibutyltin dilaurate; and the solvent is N,N-dimethylformamide.

7. A process for preparing a polyethylene microporous membrane according to any one of claims 1 to 6, characterized in that: The following steps are involved: S1, melt-extrude the HDPE resin at 210°C, pass through a cooling roller at 40°C to form a 200 μm cast base film, first longitudinally stretch to 1.25 times at 30°C, then stretch to a total ratio of 1.85 times, and finally heat-set at 122°C to obtain the base film; S2, adding aromatic heterocyclic polyester diol, aromatic isocyanate, and catalyst in sequence to a nitrogen-protected reactor, reacting at 60°C to form a prepolymer, adding DMF solvent to dilute to a solid content of 30%, then adding boehmite nanoparticles treated with silane coupling agent KH-550, FSP solution, crosslinking agent, and foaming agent, and maintaining stirring at 25°C; S3, the coating liquid is applied to the base film with a wet film thickness of 12 μm using double-sided micro-gravure coating. The coating film continuously passes through a 70°C preheating zone and then enters a 115°C curing zone for curing.

8. The process for preparing the polyethylene microporous membrane according to claim 7, wherein: In step S1, the flow rate of the molten HDPE resin is 0.35 g / 10 min; the heat setting time is 30 min; the thickness of the base film is 16 μm and the porosity is 42%; in step S2, the prepolymer is generated by reacting at 60° C. for 1 h; the stirring speed is 1000 rpm and the time is 2 h.

9. The process for preparing the polyethylene microporous diaphragm according to claim 1, wherein: In step S3, the coating film continuously passes through a 70°C preheating zone at a speed of 20 m / min for 2 minutes, and then enters a 115°C curing zone for 5 minutes.

10. Use of the polyethylene microporous membrane according to any one of claims 1 to 6 or the polyethylene microporous membrane prepared by the preparation process according to any one of claims 7 to 9, characterized in that: Application of the polyethylene microporous membrane in lithium ion batteries.

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