Ultrathin high-strength lithium battery diaphragm and preparation method thereof

Abstract

The invention discloses an ultrathin high-strength lithium ion battery diaphragm and a preparation method thereof, and belongs to the technical field of lithium ion battery diaphragms. The preparation method of the ultrathin high-strength lithium battery diaphragm comprises the following steps: carrying out pre-swelling treatment on mixed low-molecular-weight PE (Polyethylene) and a pore-foaming agent, and then melting and plasticizing to obtain a melt 1; carrying out pre-swelling treatment on the mixed high-molecular-weight PE and a pore-foaming agent, and then carrying out melting plasticization to obtain a melt 2; carrying out tape casting treatment on a uniform melt formed by mixing the melt 1 and the melt 2 to obtain a cast sheet; and carrying out multi-stage longitudinal stretching on the obtained casting piece, and then carrying out synchronous two-way stretching, extraction and transverse stretching to obtain a finished product diaphragm. According to the invention, PE raw materials with different high molecular weights are adopted, and are respectively plasticized and processed, and then are subjected to mixing, curtain coating, multi-stage stretching and other operations, so that the diaphragm can be thinned, and meanwhile, the strength and the thermal stability of the diaphragm can be effectively balanced.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery separator technology, specifically relating to an ultra-thin, high-strength lithium-ion battery separator and its preparation method. Background Technology

[0002] Lithium-ion batteries are widely used in electronic products, new energy vehicles, and wind power energy storage as the main energy storage devices in modern society. As a key component of lithium-ion batteries, the separator plays a crucial role in separating the positive and negative electrodes, preventing short circuits, and allowing the electrolyte solution to pass through. Its performance directly affects the battery's capacity, cycle performance, and safety performance.

[0003] Currently, polyolefin materials are widely used in the preparation of lithium-ion battery separators due to their excellent chemical stability, corrosion resistance, high strength, and good processability. However, traditional polyolefin separators have some problems, such as the conflict between thickness and energy density, poor thermal stability, and weak affinity with polar dielectrics. Meanwhile, with the continuous increase in lithium battery energy density and the acceleration of charging speed, the heat released by the battery and the energy from external explosions are gradually increasing, placing higher demands on the strength and safety of the separator.

[0004] Currently, methods to improve the strength of polyethylene lithium-ion battery separators mainly include using high-molecular-weight ultra-high-molecular-weight polyethylene, high-rate stretching, and adding elastomer materials and crosslinking agents. However, these methods suffer from problems such as high processing difficulty, severe molecular weight degradation, impurity crystal point formation, and tensile cracking. Furthermore, as the separator thickness decreases, the needle penetration strength decreases, thermal shrinkage increases, the risk of battery short circuits rises, and safety performance cannot be guaranteed. Therefore, how to improve the strength, heat resistance, and needle penetration strength of the separator while simultaneously reducing its thickness to meet the demands of high energy density, fast charging performance, and safety is a pressing technical problem that needs to be solved in this field. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing an ultra-thin, high-strength lithium battery separator, thereby solving the problem that existing polyethylene separator materials often fail to maintain the ultra-thin characteristics of the separator's basic physical properties while ensuring strength, thus failing to meet the dual requirements of ultra-high energy density lithium batteries for separator thickness and strength. This invention utilizes different high-molecular-weight PE raw materials, which are then plasticized separately before being mixed, cast, and stretched. This allows for the effective balance between separator thinning and its strength, thermal stability, and needle-punch strength. The present invention also provides a lithium battery separator prepared by the above method, which can effectively balance the requirements of separator thickness and separator strength. The present invention also provides a lithium battery comprising the above-described lithium battery separator.

[0006] To achieve the above objectives, the technical solution provided by the present invention is as follows: The first aspect of this invention provides a method for preparing an ultrathin, high-strength lithium battery separator, wherein the thickness of the lithium battery separator is 1.0~7.0 μm, and the preparation method includes: The mixed low molecular weight PE and pore-forming agent are pre-swelled and then melt-plasticized to obtain melt 1; the molecular weight of the low molecular weight PE is 700,000 to 1,000,000. The mixed high molecular weight PE and pore-forming agent are pre-swelled and then melt-plasticized to obtain melt 2; the molecular weight of the high molecular weight PE is 1.5 million to 2 million. The homogeneous melt formed by mixing melt 1 and melt 2 is subjected to casting process to obtain a cast sheet; The obtained castings are subjected to multi-stage longitudinal stretching to obtain pre-stretched castings. The pre-stretched castings were simultaneously biaxially stretched and then extracted to obtain a microporous membrane free of pore-forming agents.

[0007] To address the technical problem of effectively balancing the thickness, strength, and safety performance of lithium battery separators in existing technologies, this invention uses low molecular weight PE and high molecular weight PE as the main raw materials for the separator substrate. The two different PE raw materials are pre-swelled and plasticized separately, and then mixed together for subsequent casting, stretching, and other operations. This can reduce the separator thickness and achieve a thinner separator while effectively improving the strength and thermal stability of the resulting separator, thereby ensuring the safety performance of the separator.

[0008] Specifically, by mixing high-molecular-weight and low-molecular-weight PE raw materials and controlling the molecular weight range of the two types of PE, the requirements of the membrane for mechanical strength and pore structure can be effectively balanced. While improving the mechanical strength of the resulting membrane, the porosity and pore size distribution of the membrane are optimized, and the uniformity of the micropore size on the membrane is improved.

[0009] Among them, the high molecular weight PE molecules have long and densely entangled chains, making them less prone to slippage during stretching and more easily oriented along the stretching direction (high orientation degree). The oriented long chains can form a "rigid network," providing core mechanical strength for the membrane. On the other hand, low molecular weight PE molecules have shorter chains and better flowability, allowing the molecular arrangement to be adjusted synchronously with stretching, making the channels more regularly connected along the stretching direction. Therefore, this is beneficial for producing membranes with more uniform pore size.

[0010] In addition, by pre-treating and plasticizing two PE raw materials with different molecular weights, the present invention can reduce the interaction forces between molecular chains, improve the fluidity of the melt, reduce the processing difficulty of the extruder plasticizing process, and reduce the risk of thermal degradation of PE molecules.

[0011] According to any of the technical solutions described in the first aspect of the present invention, the lithium battery separator has a needle penetration strength > 440 gf, a tensile strength (MD) and a tensile strength (TD) both greater than 2000 kgf, an elongation at break (MD) > 90%, and an elongation at break (TD) > 150%. More preferably, the tensile strength (MD) and tensile strength (TD) of the separator are both greater than 2200 kgf.

[0012] Furthermore, the lithium battery separator has a pore size of 0.030~0.035μm and a distribution width of ≤0.008μm. The small pore size and good pore size distribution uniformity are beneficial to ensuring the energy density and cycle life of the separator.

[0013] According to any of the technical solutions described in the first aspect of the present invention, the temperature of the pre-swelling treatment is 80~110℃ and the treatment time is 4~8h; the mixing mass ratio of melt 1 to melt 2 is 1:2~2:1. By controlling the mixing ratio of the two melts, it is beneficial to achieve a better match between the mechanical strength and safety performance of the obtained diaphragm and its pore size distribution structure.

[0014] According to any of the technical solutions described in the first aspect of the present invention, the mixed low molecular weight PE and porogen are pre-swelled and then melt-plasticized at a temperature of 160~190°C; the mixed high molecular weight PE and porogen are pre-swelled and then melt-plasticized at a temperature of 180~210°C.

[0015] Based on the molecular weight of the two PE raw materials, the process parameters for their melt plasticization were optimized and designed. They were then subjected to melt plasticization at different temperatures, which helped to further improve the fluidity of the mixed melt, reduce processing difficulty, and decrease the risk of thermal degradation of PE molecules.

[0016] According to any of the technical solutions described in the first aspect of the present invention, the multi-stage longitudinal stretching can employ at least two stages of longitudinal stretching process, for example, it can be a two-stage longitudinal stretching process, or a three-stage, four-stage or five-stage longitudinal stretching process.

[0017] A further preferred method is to employ a two-stage longitudinal stretching process, which specifically includes: First, preheat the casting sheet at a temperature of 80-100℃ for 5-20 seconds. After preheating, perform the first longitudinal stretching at a ratio of 1.1-1.3 and a temperature of 90-110℃. The casting sheet after the first longitudinal stretching is cooled to 15~25℃, and then preheated again before undergoing a second longitudinal stretching. The preheating temperature is controlled at 100~120℃ and the preheating time is 5~20 s. The stretching ratio of the second longitudinal stretching is 1.3~2.0 times, and the stretching temperature is controlled at 110~130℃.

[0018] The present invention further improves the mechanical strength and puncture resistance of the resulting diaphragm by performing multi-stage step-by-step pre-stretching on the casting sheet, thereby inducing the orientation of the branched chains through staged stretching.

[0019] The multi-stage stepwise pre-stretching preferably employs a two-stage longitudinal stretching process, with optimized design of the specific process and parameters for the two-stage longitudinal stretching process. This allows the molecular chains to align longitudinally through initial pre-stretching, forming a uniform orientation framework. The second pre-stretching further strengthens the orientation and reduces the relaxation of lateral branch chains. Therefore, during subsequent synchronous bidirectional stretching, the longitudinal orientation framework acts as a support, enabling more uniform expansion of pores. This overcomes the bottleneck of ultra-thin separators, ensuring that the battery separator achieves both thinness and increased strength, thereby improving the battery's energy density and effectively enhancing the safety performance of the lithium battery.

[0020] Compared with the traditional single-stage pre-stretching process, the multi-stage pre-stretching process of this invention can achieve the following advantages: ① Reduce stress concentration in single-step stretching and reduce the risk of film breakage; ② Improve the uniformity of molecular chain orientation and optimize the synergy of subsequent biaxial stretching; ③ Control the ratio of crystalline and amorphous regions to balance the mechanical properties and tensile plasticity of the membrane, especially effectively improving the TD tensile strength and flexibility of the obtained membrane.

[0021] According to any of the technical solutions described in the first aspect of the present invention, the stretching ratio of the synchronous biaxial stretching is 7 to 9 times, and the stretching temperature is controlled at 115 to 125°C.

[0022] According to any of the technical solutions described in the first aspect of the present invention, it further includes: The microporous membrane, after being extracted to remove the pore-forming agent, is dried and then subjected to transverse stretching heat setting treatment. The transverse stretching ratio is 1.5 to 1.8 times, and the transverse stretching temperature is controlled at 125 to 130°C. The pore-forming agent includes white oil. The microporous membrane after transverse stretching and heat setting is subjected to high-temperature heat treatment, with the heat treatment temperature controlled at 130~135℃ and the heat treatment time at 5~10 s.

[0023] The second aspect of the present invention also provides an ultra-thin high-strength lithium battery separator, wherein the separator has a thickness of 1.0~7 μm, a needle penetration strength >440 gf, and both MD tensile strength and TD tensile strength are greater than 2000 kgf; the separator pore size is 0.030~0.035 μm, and the distribution width is ≤0.008 μm.

[0024] Furthermore, the thickness of the diaphragm is 1.0~7 μm, for example, it can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 5.5 μm, 6 μm, 7 μm, preferably 5~5.5 μm.

[0025] Furthermore, the diaphragm has an MD elongation at break > 90% and a TD elongation at break > 150%.

[0026] The third aspect of the present invention also provides an ultra-thin, high-strength lithium battery separator, which is prepared by any of the preparation methods described in the first aspect of the present invention.

[0027] The fourth aspect of the present invention also provides a lithium battery, wherein the lithium battery uses a separator prepared by any of the preparation methods described in the first aspect of the present invention, or uses a separator described in the second or third aspect of the present invention.

[0028] Compared with the prior art, the present invention can achieve the following technical effects: (1) This invention uses two PE raw materials with different molecular weight ranges and plasticizes high molecular weight and low molecular weight PE separately. This not only improves the mechanical strength and safety performance of the ultrathin diaphragm, but also improves the micropore distribution in the diaphragm. At the same time, it can better match the processing conditions of PE with different molecular weights, effectively reduce the processing difficulty of the extruder plasticizing process, reduce the degradation of high molecular weight PE in the process, and thus help ensure the strength and processing performance of the film.

[0029] (2) The present invention further optimizes the mixing mass ratio of high molecular weight PE and low molecular weight PE, as well as the process parameters during plasticizing, so as to achieve the best coordination between the mechanical strength, safety performance, micropore distribution and processing performance of the diaphragm.

[0030] (3) The present invention pre-stretches the casting in a graded longitudinal direction before performing synchronous bidirectional stretching, which is beneficial to the longitudinal "pre-orientation" arrangement of molecular chains, forming a uniform orientation skeleton and reducing the relaxation of transverse branch chains. This ensures that the pores can be expanded more uniformly during subsequent bidirectional synchronous stretching, and ensures that the prepared diaphragm can still achieve the comprehensive performance of "high strength and uniform pore structure" even at a thin thickness. Attached Figure Description

[0031] Figure 1This is a flowchart of the preparation method according to an embodiment of the present invention. Detailed Implementation

[0032] To provide a more detailed description of the present invention, some embodiments are listed below, but these embodiments do not limit the present invention in any way. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in this technical field. The essential features and significant effects of the present invention can be seen from the following embodiments. The described embodiments are some, but not all, embodiments of the present invention, and therefore do not limit the present invention in any way. Any non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are within the protection scope of the present invention.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0034] It should also be understood that, for clarity, certain features of this disclosure may be described herein in the context of individual embodiments, but may also be provided in combination with each other in individual embodiments. That is, unless obviously incompatible or specifically excluded, each individual embodiment is considered to be combinable with any other embodiment, and such combination is considered to represent another different embodiment. Conversely, for brevity, various features of this disclosure described in the context of individual embodiments may also be provided individually or in any sub-combination. Finally, while a particular embodiment may be described as part of a series of steps or part of a more general structure, each step or substructure may also be considered an independent embodiment in itself.

[0035] Furthermore, unless shown or otherwise indicated in the operational embodiments, all figures used to represent the amounts of components, physicochemical properties, etc., in the specification and claims should be understood to be adjusted by the term "about" in all cases. Therefore, unless stated to the contrary, the numerical parameters listed in the foregoing specification and appended claims are approximations, and those skilled in the art can appropriately modify these approximations to obtain the desired characteristics by utilizing the teachings disclosed herein. The use of numerical ranges indicated by endpoints includes all numbers within that range and any range within that range.

[0036] Example 1 Combination Figure 1 As shown, this embodiment provides a method for preparing an ultrathin, high-strength membrane, specifically including the following steps: Step 1: Mix PE with a molecular weight of 1 million at a mass ratio of 1:5 with white oil, stir at 110°C for 4 hours for pre-swelling, and obtain a turbid liquid; transfer the turbid liquid to a twin-screw extruder and plasticize and melt at 190°C to obtain melt 1; Step 2: Mix PE with a molecular weight of 2 million at a mass ratio of 1:5 with white oil, stir at 110°C for 4 hours for pre-swelling to obtain a turbid liquid; transfer the turbid liquid to a twin-screw extruder and plasticize and melt at 210°C to obtain melt 2; Step 3: Mix melt 1 and melt 2 in a mass ratio of 1:1 using a gear pump to deliver them to a static mixer for uniform mixing to obtain a homogeneous melt; Step 4: The mixed melt is extruded through a mold and cooled at 15°C by cooling rollers to form an initial casting sheet, called sheet 1. Sheet 1 is then fed into a pre-stretching device for multi-stage longitudinal stretching: First, it is preheated at 90°C for 5 seconds by preheating rollers; then, it undergoes a first longitudinal stretch at 105°C with a stretching ratio of 1.2 times; after cooling at 25°C by cooling rollers, it becomes casting sheet 2; then, casting sheet 2 is preheated at 110°C for 10 seconds and undergoes a second longitudinal stretch at 120°C with a stretching ratio of 1.4 times, finally yielding the pre-stretched casting sheet. Step 5: The pre-stretched casting is continuously clamped and fixed by two sets of chain clamps and fed into a synchronous bidirectional stretching machine. First, it is preheated at 115℃ for 15 seconds, and then subjected to horizontal / longitudinal synchronous bidirectional stretching at 120℃ under the clamping of the left and right chain clamps. The stretching ratio is 7.5 times. After stretching, it is cooled to 25℃ to obtain an oil film with a certain thickness. In steps 6 and 5, the oil film prepared in the oil film is continuously extracted and washed in an extraction tank containing dichloromethane under the transfer of rollers. The extraction and washing time is 60 s. After extraction, the film is dried by hot rollers at 40 ℃ to obtain a microporous membrane without white oil. Step 7: The film obtained after extraction is conveyed to the transverse stretching device through rollers and stretched transversely at 130°C with a stretching ratio of 1.6 times. The stretched film is then subjected to high-temperature heat treatment at 135°C for 10 seconds to improve crystallization. Step 8: Use a winding device to wind the membrane, and finally obtain an ultra-thin, high-strength finished membrane with uniform pore size distribution.

[0037] Example 2 This embodiment provides a method for preparing an ultrathin, high-strength membrane, specifically including the following steps: Step 1: Mix PE with a molecular weight of 800,000 at a mass ratio of 1:5 with white oil, stir at 80°C for 8 hours to pre-swell, and obtain a turbid liquid; transport the turbid liquid to a twin-screw extruder and plasticize and melt at 170°C to obtain melt 1; Step 2: Mix PE with a molecular weight of 1.8 million at a mass ratio of 1:5 with white oil, stir at 80°C for 8 hours to pre-swell, and obtain a turbid liquid; transport the turbid liquid to a twin-screw extruder and plasticize and melt at 195°C to obtain melt 2; Step 3: Mix melt 1 and melt 2 in a mass ratio of 2:1 using a gear pump to deliver them to a static mixer for uniform mixing to obtain a homogeneous melt; Step 4: The mixed melt is extruded through a mold and cooled by a cooling roller at 25°C to form an initial casting sheet, called sheet 1. Sheet 1 is then fed into a pre-stretching device for multi-stage longitudinal stretching: First, it is preheated at 100°C for 10 seconds by a preheating roller; then, it undergoes a first longitudinal stretch at 110°C with a stretching ratio of 1.3 times; after cooling by a cooling roller at 15°C, it becomes casting sheet 2; then, casting sheet 2 is preheated at 120°C for 5 seconds and then undergoes a second longitudinal stretch at 130°C with a stretching ratio of 1.3 times, finally yielding the pre-stretched casting sheet. Step 5: The pre-stretched casting is continuously clamped and fixed by two sets of chain clamps and fed into a synchronous bidirectional stretching machine. First, it is preheated at 120°C for 10 seconds, and then subjected to horizontal / longitudinal synchronous bidirectional stretching at 125°C under the clamping of the left and right chain clamps. The stretching ratio is 9 times. After stretching, it is cooled to 25°C to obtain an oil film with a certain thickness. In steps 6 and 5, the oil film prepared in the oil film is continuously extracted and washed in an extraction tank containing dichloromethane under the transfer of rollers. The extraction and washing time is 60 s. After extraction, the film is dried by hot rollers at 40 ℃ to obtain a microporous membrane without white oil. Step 7: The film obtained after extraction is conveyed to the transverse stretching device through rollers and stretched transversely at 128°C with a stretching ratio of 1.5 times. The stretched film is then subjected to high-temperature heat treatment at 132°C for 5 seconds to improve crystallization. Step 8: Use a winding device to wind up the membrane to obtain an ultra-thin, high-strength finished membrane with uniform pore size distribution.

[0038] Example 3 This embodiment provides a method for preparing an ultrathin, high-strength membrane, specifically including the following steps: Step 1: Mix PE with a molecular weight of 700,000 at a mass ratio of 1:5 with white oil, stir at 110°C for 4 hours for pre-swelling to obtain a turbid liquid; transfer the turbid liquid to a twin-screw extruder and plasticize and melt at 160°C to obtain melt 1. Step 2: Mix PE with a molecular weight of 1.5 million at a mass ratio of 1:5 with white oil, stir at 110°C for 4 hours for pre-swelling, and obtain a turbid liquid; transfer the turbid liquid to a twin-screw extruder and plasticize and melt at 180°C to obtain melt 2; Step 3: Mix melt 1 and melt 2 in a mass ratio of 1:2 using a gear pump to deliver them to a static mixer for uniform mixing to obtain a homogeneous melt; Step 4: The mixed melt is extruded through a mold and cooled at 25°C by cooling rollers to form an initial casting sheet, called sheet 1. Sheet 1 is then fed into a pre-stretching device for multi-stage longitudinal stretching: First, it is preheated at 80°C for 20 seconds by preheating rollers; then, it undergoes a first longitudinal stretch at 90°C with a stretching ratio of 1.1 times; after cooling at 15°C by cooling rollers, it becomes casting sheet 2; then, casting sheet 2 is preheated at 100°C for 20 seconds and undergoes a second longitudinal stretch at 110°C with a stretching ratio of 2.0 times, finally yielding the pre-stretched casting sheet. Step 5: The pre-stretched casting is continuously clamped and fixed by two sets of chain clamps and fed into a synchronous bidirectional stretching machine. First, it is preheated at 115℃ for 5 seconds, and then subjected to horizontal / longitudinal synchronous bidirectional stretching at 120℃ under the clamping of the left and right chain clamps. The stretching ratio is 7 times. After stretching, it is cooled to 25℃ to obtain an oil film with a certain thickness. In steps 6 and 5, the oil film prepared in the oil film is continuously extracted and washed in an extraction tank containing dichloromethane under the transfer of rollers. The extraction and washing time is 60 s. After extraction, the film is dried by hot rollers at 40 ℃ to obtain a microporous membrane without white oil. Step 7: The film obtained after extraction is conveyed to the transverse stretching device through rollers and stretched transversely at 125°C with a stretching ratio of 1.8 times. The stretched film is then subjected to high-temperature heat treatment at 130°C for 10 seconds to improve crystallization. Step 8: Use a winding device to wind up the membrane to obtain an ultra-thin, high-strength finished membrane with uniform pore size distribution.

[0039] Comparative Example 1 The preparation method of the ultrathin lithium battery separator in this comparative example is basically the same as that in Example 1. The main difference is that in step 4 of this comparative example, the homogeneous melt is extruded from the die head, cooled and shaped into an initial casting by the cooling roller, and then the initial casting is pre-stretched by 1.8 times using a single pre-stretching machine to obtain the pre-stretched casting.

[0040] Comparative Example 2 The preparation method of the ultrathin lithium battery separator in this comparative example is basically the same as that in Example 1, except that only PE raw material with a molecular weight of 1 million is used in this comparative example.

[0041] Comparative Example 3 The preparation method of the ultrathin lithium battery separator in this comparative example is basically the same as that in Example 1, except that only PE raw material with a molecular weight of 2 million is used in this comparative example.

[0042] Comparative Example 4 The preparation method of the ultrathin lithium battery separator in this comparative example is basically the same as that in Example 1. The main difference is that in this comparative example, PE with a molecular weight of 1 million and PE with a molecular weight of 2 million are mixed together with white oil, and the mixture is obtained by pre-swelling treatment and melt extrusion.

[0043] Comparative Example 5 The preparation method of the ultrathin lithium battery separator in this comparative example is basically the same as that in Example 1, except that the mixing mass ratio of melt 1 to melt 2 in this comparative example is 5:2.

[0044] Comparative Example 6 The preparation method of the ultrathin lithium battery separator in this comparative example is basically the same as that in Example 1, except that the mixing mass ratio of melt 1 to melt 2 in this comparative example is 1:3.

[0045] Test method: Diaphragm thickness: The test was conducted in accordance with the requirements of GB / T 36363-2018 "Polyolefin Separators for Lithium-ion Batteries".

[0046] Five square diaphragm samples were cut along the TD direction using a 10cm×10cm mold and tested. If the TD direction sample was less than 10cm, a 10cm sample was cut along the MD direction; in this case, the sample was not square. The four corners and the center point of each sample were measured using a Mahr thickness gauge (C1202). The average value of these five points was taken as the thickness of a single sample. The average value of the five samples was taken as the thickness of the diaphragm.

[0047] Puncture intensity: The test was conducted in accordance with the requirements of GB / T 36363-2018 "Polyolefin Separators for Lithium-ion Batteries".

[0048] Cut a diaphragm with a size of 500*100 mm along the TD direction. Fix the diaphragm on the sample holder of the puncture test machine (model: NDG5, KES KATO). Use a steel needle with a diameter of 1.0 mm to puncture at a speed of 300 mm / min. Read the maximum load of the steel needle penetrating the diaphragm. Perform the test more than 5 times and take the arithmetic mean.

[0049] Tensile strength or elongation at break: The test was conducted according to GB / T 1040.3-2006. A 2.5cm × 20cm specimen was cut and marked with the MD / TD direction of the diaphragm. The specimen was tested using a tensile testing machine (High-speed Rail Testing Instruments (Dongguan) Co., Ltd., AL-300-U). The specimen was fixed between the upper and lower clamps of the tensile testing machine (the distance between the clamps was 100±5 mm). The specimen was ensured to be flat and wrinkle-free, and vertical and not skewed. The tensile speed was 250mm / min. The tensile testing machine output the tensile strength value based on the width and thickness of the specimen. The test was performed 3 times and the average value was taken as the tensile strength (MPa) and elongation at break (%) in the MD / TD direction.

[0050] Mean aperture: Reference: ASTM F316-17 "Standard Test Method for Determining Pore SizeDistribution of Porous Materials by the Gas Permeation Method".

[0051] Remove the clamps and cut a circular sample with a diameter of 2.5 cm. From bottom to top, place the metal gasket, circular A4 paper, circular diaphragm, and rubber ring flat in the lower sample clamp. Place the lower sample clamp, upper sample clamp, seal, and sample chamber shell into the sample chamber. Connect the gas tubing and use a gas permeation pore size analyzer (PMI, ICFP-1500AE) for testing (high pressure gauge: output pressure 3.5-3.8 MPa, low pressure gauge: output pressure 0.7-0.8 MPa). Set the surface tension to 15.9 dynes / cm and perform a dry test. Remove the gas tubing and clamps, and place the metal gasket, circular A4 paper, circular diaphragm, and rubber ring flat in the lower sample clamp. Add Galwet wetting solution to the diaphragm sample and analyze the output average pore size.

[0052] The membranes prepared in Examples 1-3 and Comparative Examples 1-6 were subjected to physical property tests, and the test results are shown in Table 1 below: Table 1. Comparison of physical properties of the membranes prepared in Examples 1-3 and Comparative Examples 1-6

[0053] As shown in Table 1, the high-strength ultrathin lithium battery separators prepared in Examples 1-3 of this invention have a needle penetration strength > 440 gf, and both MD and TD tensile strengths are greater than 2000 kgf, with a relatively uniform pore size distribution. In Comparative Example 2, because only a single low molecular weight PE was used as the raw material, the pore size distribution of the prepared separator was relatively concentrated, but the overall strength was low. In Comparative Example 3, because only a single high molecular weight PE was used, the strength was improved, but the pore size distribution was wider (0.010 μm), and the MD elongation at break deteriorated significantly. In Comparative Example 1, because only a single pre-stretching was performed, the MD orientation of the obtained separator was relatively concentrated, resulting in a lower TD tensile strength, and the pore size distribution was also wider than in the examples (the pore size distribution width in Comparative Example 1 was 0.009 μm). In Comparative Example 4, due to the use of a single extrusion processing technology, the processing parameters of the high molecular weight and low molecular weight PE were difficult to match, resulting in severe degradation of PE molecules and a significant deterioration in strength. As can be seen from Comparative Examples 5 and 6, when the mixing mass ratio of melt 1 to melt 2 exceeds the range of 2:1 to 1:2, the overall strength of the diaphragm is low, or the MD orientation of the polymer PE is high during the pre-stretching process, making it difficult to balance the subsequent stretching orientation in the TD direction. Therefore, the elongation at break in the MD direction is low, and the pore size distribution is wide.

[0054] As can be seen from the above, the technical solution of this invention, especially the use of two PE raw materials with different molecular weights, and the pre-swelling and plasticizing of low molecular weight PE and high molecular weight PE respectively, combined with multi-stage pre-stretching before synchronous stretching, can better balance the overall performance of the separator. Specifically, low molecular weight PE ensures a narrower pore size distribution and molecular chain flexibility during stretching; the long-chain structure of high molecular weight PE acts as a framework to improve membrane surface strength. The step-by-step pre-stretching can better optimize molecular chain orientation, improve the uniformity of strength in all directions of the separator, effectively avoid poor stability due to heat release and reduced strength due to thinner separators during actual use of lithium batteries, thereby reducing the risk of short circuits and further improving the safety performance of lithium batteries.

Claims

1. A method for preparing an ultrathin, high-strength lithium battery separator, characterized in that, The thickness of the lithium battery separator is 1.0~7.0 μm, and its preparation method includes: The mixed low molecular weight PE and pore-forming agent are pre-swelled and then melt-plasticized to obtain melt 1; the molecular weight of the low molecular weight PE is 700,000 to 1,000,000. The mixed high molecular weight PE and pore-forming agent are pre-swelled and then melt-plasticized to obtain melt 2; the molecular weight of the high molecular weight PE is 1.5 million to 2 million. The homogeneous melt formed by mixing melt 1 and melt 2 is subjected to casting process to obtain a cast sheet; The obtained castings are subjected to multi-stage longitudinal stretching to obtain pre-stretched castings. The pre-stretched castings were simultaneously biaxially stretched and then extracted to obtain a microporous membrane free of pore-forming agents.

2. The preparation method according to claim 1, characterized in that, The lithium battery separator has a needle penetration strength > 440 gf, and both its MD tensile strength and TD tensile strength are greater than 2000 kgf; the lithium battery separator has a pore size of 0.030~0.035 μm, a distribution width ≤ 0.008 μm, and its MD elongation at break is > 90% and TD elongation at break is > 150%.

3. The preparation method according to claim 1 or 2, characterized in that, The pre-swelling treatment is performed at a temperature of 80~110℃ for 4~8h; the mixing mass ratio of melt 1 to melt 2 is 2:1~1:

2.

4. The preparation method according to claim 1 or 2, characterized in that, The mixture of low molecular weight PE and pore-forming agent is pre-swelled and then melt-plasticized at a temperature of 160~190℃. The mixture of high molecular weight PE and pore-forming agent is pre-swelled and then melt-plasticized at a temperature of 180~210℃.

5. The preparation method according to claim 4, characterized in that, The multi-stage longitudinal stretching process employs at least two stages of longitudinal stretching. When a two-stage longitudinal stretching process is used, it specifically includes: First, preheat the casting sheet at a temperature of 80-100℃ for 5-20 seconds. After preheating, perform the first longitudinal stretching at a ratio of 1.1-1.3 and a temperature of 90-110℃. The casting sheet after the first longitudinal stretching is cooled to 15~25℃, and then preheated again before undergoing a second longitudinal stretching. The preheating temperature is controlled at 100~120℃ and the preheating time is 5~20 s. The stretching ratio of the second longitudinal stretching is 1.3~2.0 times, and the stretching temperature is controlled at 110~130℃.

6. The preparation method according to claim 1 or 2, characterized in that, The synchronous biaxial stretching has a stretching ratio of 7 to 9 times, and the stretching temperature is controlled at 115 to 125°C.

7. The preparation method according to claim 1 or 2, characterized in that, Also includes: The microporous membrane, after being extracted to remove the pore-forming agent, is dried and then subjected to transverse stretching heat setting treatment. The transverse stretching ratio is 1.5 to 1.8 times, and the transverse stretching temperature is controlled at 125 to 130°C. The pore-forming agent includes white oil. The microporous membrane after transverse stretching and heat setting is subjected to high-temperature heat treatment, with the heat treatment temperature controlled at 130~135℃ and the heat treatment time at 5~10 s.

8. An ultra-thin, high-strength lithium battery separator, characterized in that, The diaphragm has a thickness of 1.0~7.0 μm, a needle penetration strength of >440 gf, and both MD tensile strength and TD tensile strength are greater than 2000 kgf; the diaphragm pore size is 0.030~0.035 μm, and the distribution width is ≤0.008 μm.

9. An ultra-thin, high-strength lithium battery separator, characterized in that, The diaphragm is prepared by the method described in any one of claims 1-7.

10. A lithium battery, characterized in that, The lithium battery uses a separator prepared according to any one of claims 1-7, or a separator as described in claim 8 or 9.

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