Ultrathin high-strength lithium battery microporous diaphragm produced by wet method and preparation method thereof

Abstract

The invention discloses an ultrathin high-strength lithium battery microporous diaphragm produced by a wet method and a preparation method thereof, and relates to the technical field of lithium ion battery diaphragms, the microporous diaphragm is prepared from the following components by weight: 15-35% of a high-molecular polymer, 65-85% of paraffin oil, and 0.1-1.0% of an antioxidant; according to the invention, high molecular weight polyethylene, paraffin oil and additives are adopted, and a synchronous two-way stretching process is added between a longitudinal stretching process and a one-time transverse stretching process in a conventional wet asynchronous production line, so that the diaphragm can be subjected to two-way stretching before transverse stretching, the total stretching multiple of the diaphragm is increased, and the transverse consistency of the diaphragm is improved; the requirements of high energy density, safety and the like of the battery cell are met.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery separator technology, and in particular to a wet process for producing ultra-thin, high-strength microporous lithium battery separators and its preparation method. Background Technology

[0002] Lithium-ion batteries use microporous separators, which are polymer materials with micropores, to isolate the positive and negative electrodes while allowing ions to pass freely. Currently, the mainstream technologies for lithium-ion battery microporous separators are polyethylene wet molding and polypropylene dry molding. Wet molding is further divided into wet asynchronous stretching and wet synchronous stretching based on the stretching method.

[0003] Wet-processed separators offer significant advantages over dry-processed separators in terms of pore structure uniformity, physicochemical properties, and mechanical properties. The traditional wet-process production process for microporous separators in asynchronous lithium-ion batteries primarily uses polyethylene as the base material, paraffin oil as a pore-forming agent and plasticizer, and adds trace amounts of antioxidants and other additives. The process includes raw material melt extrusion, cold roll casting, longitudinal stretching, primary transverse stretching, extraction, secondary transverse stretching, heat setting, winding, and slitting. Specifically, paraffin oil and polyethylene raw materials are simultaneously fed into a twin-screw extruder and mixed above the polyethylene melting temperature to ensure thorough mixing with the polyethylene base material. The uniformly mixed melt is extruded through a die and adhered to a chiller roll to complete the casting. Then, guided by guide rollers, the separator is initially formed through a longitudinal stretching machine and a primary transverse stretching machine. After initial forming, the paraffin oil on the separator needs to be thoroughly washed away using a dichloromethane extraction process. Subsequently, recrystallization and shaping are completed through a secondary transverse stretching and heat setting process, followed by winding to obtain the base film master roll. The base film large master roll is processed into base film small master roll products through a large slitting process.

[0004] Ultra-thin, high-strength microporous separators are an inevitable direction in the development of lithium-ion battery separators, aligning with the increasing demands for high energy density and safety in batteries. To improve the puncture strength of the separator, the mainstream methods include the following three approaches:

[0005] (1) Ultra-high molecular weight polyethylene (viscosity-average molecular weight ≥ 1.8 million) is used as the membrane substrate for processing and production. Processing ultra-high molecular weight polyethylene requires the extruder to have stronger processing capabilities, but the high temperature and high shear processing environment will cause polyethylene molecules to oxidize and degrade, forming small molecule by-products, as well as product membrane appearance defects such as "black spots" and "crystal spots" formed after high temperature carbonization.

[0006] (2) High-ratio stretching is employed. After the extruded melt undergoes a casting process, the diaphragm is subjected to bidirectional high-ratio stretching in a highly elastic state. This maximizes the stretching ratio during the processing of the diaphragm into the separator product, resulting in a higher degree of orientation in the separator product and thus improving its strength. This method requires the separator production line to be capable of high-ratio stretching. However, a higher stretching ratio will reduce the separator elongation and transverse consistency. Furthermore, if the internal stress of the separator is not fully released during the stretching process, the resulting product will exhibit increased wavy edges after aging.

[0007] (3) Antioxidants, crosslinking agents, and other additives are added during melt extrusion. This method can improve the membrane strength by reducing the oxidative degradation of polyethylene or increasing the intermolecular interactions of polyethylene. However, too many types of additives will increase the difficulty of process control in extrusion, and trace amounts of additives will still remain in the membrane, posing a great threat to the safety of the battery cell. In view of this, this study aims to explore a wet process for producing ultra-thin, high-strength microporous membranes for lithium batteries and its preparation method. Summary of the Invention

[0008] This invention proposes a wet process for producing ultrathin, high-strength microporous separators for lithium batteries and its preparation method, in order to solve the technical problems existing in the background art.

[0009] To solve the above problems, the technical solution of the present invention is:

[0010] The first aspect of this invention provides a wet process for producing ultra-thin, high-strength microporous separators for lithium batteries and a method thereof, comprising the following steps:

[0011] Step 1: Mix the polymer, paraffin oil, and antioxidants, and add them into a twin-screw extruder. Melt and plasticize the mixture at 160-230℃ with a screw speed of 40-180 rpm to obtain the melt.

[0012] Step 2: The melt is extruded from the T-die and attached to a chilling roller at 5-20℃ to obtain a casting sheet with a thickness of 0.8-3.5mm;

[0013] Step 3: Feed the casting into the longitudinal stretching machine for longitudinal stretching treatment of 1-8 times. The longitudinal stretching machine is set up with a preheating zone, a stretching zone and a setting zone in sequence. The preheating zone is equipped with 8 rollers and the temperature is 80-110℃. The stretching zone is equipped with 6 rollers and the temperature is 100-125℃. The setting zone is equipped with 4 rollers and the temperature is 60-100℃.

[0014] Step Four:

[0015] The longitudinally stretched film is placed in a synchronous biaxial stretching oven for biaxial stretching treatment. The longitudinal stretching ratio is 1-5 times, and the transverse stretching ratio is synchronously controlled at 1-5 times. The synchronous biaxial stretching oven is set with a preheating zone, a stretching zone and a setting zone in sequence. The temperature of the preheating zone is 90-120℃, the temperature of the stretching zone is 100-135℃, and the temperature of the setting zone is 85-125℃.

[0016] Step 5: The biaxially stretched film is fed into a transverse stretching machine for a single transverse stretch of 5-15 times. The transverse stretching machine is equipped with a preheating zone, a stretching zone and a setting zone in sequence. The temperature of the preheating zone is 100-130℃. The stretching zone is equipped with 8 pairs of transverse clamps with a temperature of 100-135℃. The temperature of the setting zone is 100-130℃.

[0017] Step 6: After the membrane has been stretched laterally once, it is fed into a three-stage countercurrent extraction tank and paraffin oil is removed at room temperature using dichloromethane as the extractant.

[0018] Step Seven:

[0019] After extraction and drying, the membrane is sent to a transverse stretching oven for a second transverse stretch of 1-2.5 times. The transverse stretching oven is set with a preheating zone, a stretching zone and a setting zone in sequence. The temperature of the preheating zone is 105-115℃, the temperature of the stretching zone is 110-140℃, and the temperature of the setting zone is 100-130℃.

[0020] Step 8: Heat-set the film after the second transverse stretching at 25-100℃, cool it to room temperature, then roll it up and cut it to obtain the base film master roll;

[0021] The microporous membrane is made from the following components by weight percentage: 15-35% polymer, 65-85% paraffin oil, and 0.1-1.0% antioxidant.

[0022] Preferably, the polymer is one of high-density polyethylene or a polyethylene-polypropylene mixture, wherein the molecular weight of high-density polyethylene is 800,000 to 1,300,000, and the molecular weight of polypropylene is 800,000 to 1,300,000.

[0023] Preferably, the paraffin oil is grade 50#-70#.

[0024] Preferably, the antioxidant is one or more of antioxidants 1076, 1010, or 168.

[0025] Preferably, in step six, the specific extraction steps are as follows:

[0026] S1. The membrane enters from the first-stage extraction unit and flows counter-currently to dichloromethane;

[0027] S2. Fresh dichloromethane first enters the third-stage extraction unit and comes into contact with the membrane, which has undergone the first two stages of extraction and has a low oil content, to initially extract the paraffin oil from the membrane surface and shallow micropores.

[0028] S3 and dichloromethane flow countercurrently to the second and first stage extraction units, respectively, to fully contact the membrane with higher oil content and achieve deep extraction. The flow ratio of dichloromethane in each extraction unit is 3:2:1. An ultrasonic oscillation device with a frequency of 20-40kHz is installed in the extraction tank.

[0029] S4. Monitor the extractant concentration in real time using an online concentration detector. When the concentration of the third-stage extraction unit...

[0030] Automatic replenishment of fresh dichloromethane when concentration is <99.5%;

[0031] S5. After extraction, the membrane enters the cleaning unit, is quickly rinsed with a small amount of fresh dichloromethane, and then dried with hot air at 40-60℃.

[0032] A second aspect of the present invention provides a lithium battery microporous separator prepared according to the above preparation method.

[0033] The above-described technical solution of the present invention has the following beneficial technical effects:

[0034] The lithium battery microporous separator prepared by this invention uses a high molecular weight polyethylene or polyethylene-polypropylene blend system, which reduces the difficulty of melt extrusion processing when using ultra-high molecular weight polyethylene. Furthermore, the synchronous biaxial stretching oven is used to simultaneously stretch the longitudinally stretched membrane, which can effectively increase the total stretching rate of the separator, maintain the transverse consistency of the separator product, improve the strength of the separator, and avoid the aggravation of the wavy edge after aging. Attached Figure Description

[0035] Figure 1 This is a scanning electron microscope image of the microporous membrane of the lithium battery in Example 1 of the present invention.

[0036] Figure 2 This is a scanning electron microscope image of the microporous membrane of the lithium battery in Example 2 of the present invention.

[0037] Figure 3 This is a scanning electron microscope image of the microporous membrane of the lithium battery in Example 3 of the present invention. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that these descriptions are merely exemplary and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concepts of the invention.

[0039] Example 1

[0040] A wet-process method for preparing ultrathin, high-strength microporous separators for lithium batteries, such as... Figure 1 As shown, it includes the following steps:

[0041] Step 1: Mix polyethylene with a viscosity-average molecular weight of 900,000, antioxidant 1010, and 50# white oil, with the polymer weight percentage being 25%, antioxidant 1010 weight percentage being 0.5%, and 50# white oil weight percentage being 74.5%. Add the mixture to a twin-screw extruder and melt and plasticize it at 195°C with a screw speed of 130 rpm to obtain the melt.

[0042] Step 2: The melt is extruded from the T-die and attached to a 10°C quenching roller to obtain a casting sheet with a thickness of 2.8 mm.

[0043] Step 3: Feed the casting into the longitudinal stretching machine for a 4.5 times longitudinal stretching treatment. The longitudinal stretching machine has 8 rollers in the preheating zone at a temperature of 100℃; 6 rollers in the stretching zone at a temperature of 107℃; and 4 rollers in the setting zone at a temperature of 80℃.

[0044] Step 4: The longitudinally stretched film is placed into a synchronous biaxial stretching oven. The longitudinal stretching ratio is 2.5 times, and the transverse stretching ratio is synchronously controlled at 3.5 times. The synchronous biaxial stretching oven is set with a preheating zone, a stretching zone, and a setting zone in sequence. The temperature of the preheating zone is 110℃. There are 6 pairs of synchronous stretching clamps in the stretching zone, and the temperature is 115℃. The temperature of the setting zone is 110℃.

[0045] Step 5: Feed the biaxially stretched film into the transverse stretching machine for a single transverse stretch of 7.1 times. The transverse stretching machine is set with a preheating zone, a stretching zone and a setting zone in sequence. The temperature of the preheating zone is 120℃. The stretching zone is equipped with 8 pairs of transverse clamps and the temperature is 120℃. The temperature of the setting zone is 120℃.

[0046] Step Six: The membrane, after one transverse stretching, is fed into a three-stage countercurrent extraction tank. At room temperature, dichloromethane is used as the extractant to remove paraffin oil. Specifically, the membrane enters from the first-stage extraction unit, flowing counter-currently with the dichloromethane. Fresh dichloromethane first enters the third-stage extraction unit. Dichloromethane then flows counter-currently to the second and first-stage extraction units, with a flow ratio of 3:2:1 for each extraction unit. An ultrasonic oscillation device with a frequency of 25kHz is installed in the extraction tank. The concentration of the extractant is monitored in real time by an online concentration detector. When the concentration in the third-stage extraction unit is <99.5%, fresh dichloromethane is automatically replenished. After extraction, the membrane enters the cleaning unit for rapid rinsing with a small amount of fresh dichloromethane, followed by hot air drying at 40-60℃ to ensure that the oil content of the membrane is ≤0.3%.

[0047] Step 7: The extracted and dried membrane is sent into a transverse stretching oven for a second transverse stretch of 1.65 times. The temperature of the preheating zone of the transverse stretching oven is 105℃, the temperature of the stretching zone is 135℃, and the temperature of the setting zone is 120℃.

[0048] Step 8: Heat-set the film after secondary transverse stretching at 70°C, cool it to room temperature, then roll it up and cut it to obtain the base film master roll.

[0049] Example 2

[0050] A wet-process method for preparing ultrathin, high-strength microporous separators for lithium batteries, such as... Figure 2 As shown, it includes the following steps:

[0051] Step 1: Mix polyethylene with a viscosity-average molecular weight of 1.3 million with 50# white oil, with the polymer weight percentage being 28%, antioxidant 1010 weight percentage being 0.4%, and 50# white oil weight percentage being 71.6%. Add the mixture to a twin-screw extruder and melt and plasticize it at 203°C with a screw speed of 100 rpm to obtain the melt.

[0052] Step 2: The melt is extruded from the T-die and attached to the 10℃ chilling roller to obtain a casting sheet with a thickness of 2.5mm;

[0053] Step 3: Feed the casting into the longitudinal stretching machine for 4.5 times longitudinal stretching. The longitudinal stretching machine has 8 rollers in the preheating zone and a temperature of 108℃; 6 rollers in the stretching zone and a temperature of 108℃; and 4 rollers in the setting zone and a temperature of 85℃.

[0054] Step 4: The longitudinally stretched film is placed into a synchronous biaxial stretching oven. The longitudinal stretching ratio is 2.5 times, and the transverse stretching ratio is controlled synchronously at 2.5 times. The synchronous biaxial stretching oven is set with a preheating zone, a stretching zone, and a setting zone in sequence. The temperature of the preheating zone is 90℃. There are 6 pairs of synchronous stretching clamps in the stretching zone, and the temperature is 120℃. The temperature of the setting zone is 85℃.

[0055] Step 5: Feed the biaxially stretched film into the transverse stretching machine for a single transverse stretch of 8.0 times. The transverse stretching machine is set with a preheating zone, a stretching zone and a setting zone in sequence. The temperature of the preheating zone is 100℃. The stretching zone is equipped with 8 pairs of transverse clamps and the temperature is 120℃. The temperature of the setting zone is 110℃.

[0056] Step 6: After one transverse stretching, the membrane is subjected to three-stage countercurrent extraction with a dichloromethane flow ratio of 3:2:1 and an ultrasonic frequency of 30kHz. The remaining extraction steps are the same as in Example 1, so that the oil content of the membrane is ≤0.3%.

[0057] Step 7: Send the extracted and dried membrane into the transverse stretching oven for a second transverse stretch of 1.5 times. The temperature of the preheating zone of the transverse stretching oven is 110℃, the temperature of the stretching zone is 137℃, and the temperature of the setting zone is 125℃.

[0058] Step 8: Heat-set the film after secondary transverse stretching at 70°C, cool it to room temperature, then roll it up and cut it to obtain the base film master roll.

[0059] Example 3

[0060] A wet-process method for preparing ultrathin, high-strength microporous separators for lithium batteries, such as... Figure 3 As shown, it includes the following steps:

[0061] Step 1: Blend polyethylene with a viscosity-average molecular weight of 1.2 million and polypropylene with a viscosity-average molecular weight of 1.2 million in a 7:3 ratio, and mix with antioxidant 1010 and 50# white oil. The polymer weight percentage is 27%, antioxidant 1010 weight percentage is 0.3%, and 50# white oil weight percentage is 72.7%. Add the mixture to a twin-screw extruder and melt and plasticize it at 203°C with a screw speed of 100 rpm to obtain the melt.

[0062] Step 2: The melt is extruded from the T-die and attached to a 10°C quenching roller to obtain a 2.5mm thick casting sheet;

[0063] Step 3: Feed the casting into the longitudinal stretching machine for a 4.5 times longitudinal stretching treatment. The longitudinal stretching machine has 8 rollers in the preheating zone and a temperature of 110℃; 6 rollers in the stretching zone and a temperature of 110℃; and 4 rollers in the setting zone and a temperature of 90℃.

[0064] Step 4: The longitudinally stretched film is placed into a synchronous biaxial stretching oven. The longitudinal stretching ratio is 2.5 times, and the transverse stretching ratio is controlled synchronously at 2.5 times. The synchronous biaxial stretching oven is set with a preheating zone, a stretching zone, and a setting zone in sequence. The temperature of the preheating zone is 115℃. The stretching zone is equipped with 6 pairs of synchronous stretching clamps at a temperature of 122℃. The temperature of the setting zone is 115℃.

[0065] Step 5: Feed the biaxially stretched film into the transverse stretching machine for a single transverse stretch of 8.0 times. The transverse stretching machine is set with a preheating zone, a stretching zone and a setting zone in sequence. The temperature of the preheating zone is 110℃, the stretching zone is equipped with 8 pairs of transverse clamps and the temperature is 123℃, and the temperature of the setting zone is 120℃.

[0066] Step 6: After one transverse stretching, the membrane is processed according to the three-stage countercurrent extraction process. The ultrasonic frequency in the extraction tank is 30kHz, and the rest is the same as in Example 1, to ensure that the oil content of the membrane is ≤0.3%.

[0067] Step 7: Send the extracted and dried membrane into the transverse stretching oven for a second transverse stretch of 1.5 times. The temperature of the preheating zone of the transverse stretching oven is 110℃, the temperature of the stretching zone is 135℃, and the temperature of the setting zone is 120℃.

[0068] Step 8: Heat-set the film after secondary transverse stretching at 70°C, cool it to room temperature, then roll it up and cut it to obtain the base film master roll.

[0069] Example 4

[0070] A wet-process method for preparing ultrathin, high-strength lithium battery microporous separators includes the following steps:

[0071] Step 1: Mix polyethylene with a viscosity-average molecular weight of 1 million with 50# white oil, etc., with the polymer weight percentage being 20%, antioxidant 1010 weight percentage being 0.6%, and 50# white oil weight percentage being 79.4%. Add the mixture to a twin-screw extruder and melt and plasticize it at 190°C with the screw speed at 100 rpm to obtain the melt.

[0072] Step 2: The melt is extruded from the T-die and attached to a 15°C quenching roller to obtain a 2.5mm thick casting sheet;

[0073] Step 3: Feed the casting into the longitudinal stretching machine for a 4-fold longitudinal stretching process. The longitudinal stretching machine has 8 rollers in the preheating zone at a temperature of 105℃; 6 rollers in the stretching zone at a temperature of 105℃; and 4 rollers in the setting zone at a temperature of 80℃.

[0074] Step 4: The longitudinally stretched film is put into the synchronous biaxial stretching oven. The longitudinal stretching ratio is 3 times, and the transverse stretching ratio is controlled synchronously at 3 times. The synchronous biaxial stretching oven is set with a preheating zone, a stretching zone and a setting zone in sequence. The temperature of the preheating zone is 120℃. There are 6 pairs of synchronous stretching clamps in the stretching zone, and the temperature is 120℃. The temperature of the setting zone is 115℃.

[0075] Step 5: Feed the biaxially stretched film into the transverse stretching machine for a 5-fold transverse stretch. The transverse stretching machine is set with a preheating zone, a stretching zone and a setting zone in sequence. The temperature of the preheating zone is 100℃, the stretching zone is equipped with 8 pairs of transverse clamps and the temperature is 120℃, and the temperature of the setting zone is 110℃.

[0076] Step 6: After one transverse stretching, the membrane is subjected to three-stage countercurrent extraction with an ultrasonic frequency of 28kHz. The remaining extraction operations are the same as in Example 1, so that the oil content of the membrane is ≤0.3%.

[0077] Step 7: Send the extracted and dried membrane into the transverse stretching oven for a second transverse stretch of 1.5 times. The temperature of the preheating zone of the transverse stretching oven is 105℃, the temperature of the stretching zone is 135℃, and the temperature of the setting zone is 120℃.

[0078] Step 8: Heat-set the film after secondary transverse stretching at 70°C, cool it to room temperature, then roll it up and cut it to obtain the base film master roll.

[0079] Example 5

[0080] A wet-process method for preparing ultrathin, high-strength lithium battery microporous separators includes the following steps:

[0081] Step 1: Mix polyethylene with a viscosity-average molecular weight of 1.1 million with 50# white oil, etc., with the polymer weight percentage being 30%, antioxidant 1010 weight percentage being 0.2%, and 50# white oil weight percentage being 69.8%. Add the mixture to a twin-screw extruder and melt and plasticize it at 210℃ with a screw speed of 130 rpm to obtain the melt.

[0082] Step 2: The melt is extruded from the T-die and attached to a 10°C quenching roller to obtain a casting sheet with a thickness of 2.8 mm.

[0083] Step 3: Feed the casting into the longitudinal stretching machine for a 5-fold longitudinal stretching process. The longitudinal stretching machine has 8 rollers in the preheating zone at 110°C, 6 rollers in the stretching zone at 110°C, and 4 rollers in the setting zone at 90°C.

[0084] Step 4: The longitudinally stretched film is put into the synchronous biaxial stretching oven. The longitudinal stretching ratio is 2 times, and the transverse stretching ratio is controlled synchronously at 3 times. The synchronous biaxial stretching oven is set with a preheating zone, a stretching zone and a setting zone in sequence. The temperature of the preheating zone is 110℃. Six pairs of synchronous stretching clamping rollers are set in the stretching zone and the temperature is 120℃. The temperature of the setting zone is 110℃.

[0085] Step 5: Feed the biaxially stretched film into the transverse stretching machine for a single transverse stretch of 7 times; the transverse stretching machine is set with a preheating zone, a stretching zone and a setting zone in sequence. The temperature of the preheating zone is 105℃; the stretching zone is equipped with 8 pairs of transverse clamps at a temperature of 125℃, and the temperature of the setting zone is 115℃.

[0086] Step 6: After one transverse stretching, the membrane is subjected to three-stage countercurrent extraction with an ultrasonic frequency of 32kHz. The rest is the same as in Example 1, ensuring that the oil content of the membrane is ≤0.3%.

[0087] Step 7: Send the extracted and dried membrane into the transverse stretching oven for a second transverse stretching of 2 times. The temperature of the preheating zone of the transverse stretching oven is 110℃, the temperature of the stretching zone is 140℃, and the temperature of the setting zone is 125℃.

[0088] Step 8: Heat-set the film after secondary transverse stretching at 70°C, cool it to room temperature, then roll it up and cut it to obtain the base film master roll.

[0089] Comparative Example 1

[0090] A wet-process ultrathin high-strength lithium battery microporous separator was prepared, omitting step four, the synchronous biaxial stretching process, while other steps were the same as in Example 1.

[0091] Comparative Example 2

[0092] A wet-process method is used to prepare an ultrathin, high-strength microporous membrane for lithium batteries. The preparation steps are as follows:

[0093] Step 1: Mix polyethylene with a viscosity-average molecular weight of 1.8 million, antioxidant 1010, and 50# white oil. The polymer weight percentage is 25%, antioxidant 1010 weight percentage is 0.5%, and 50# white oil weight percentage is 74.5%. Add the mixture to a twin-screw extruder and melt and plasticize it at 230°C with a screw speed of 100 rpm to obtain the melt. During the process, the screw torque fluctuates by 2%, and "crystal point" defects appear in the cast sheet.

[0094] Step 2: The melt is extruded from the T-die and attached to a 10°C quenching roller to obtain a 2.5mm thick casting sheet;

[0095] Step 3: Feed the casting into the longitudinal stretching machine for a 4.5 times longitudinal stretching treatment, with the temperature settings for each zone the same as in Example 2;

[0096] Step 4: Place the longitudinally stretched film into a synchronous biaxial stretching oven. The longitudinal stretching ratio is 2.5 times, and the transverse stretching ratio is synchronously controlled at 2.5 times. The temperature settings for each zone are the same as in Example 2.

[0097] Step 5: Feed the biaxially stretched film into the transverse stretching machine for a single transverse stretch of 7 times. The settings of each area of ​​the transverse stretching machine are the same as in Example 2.

[0098] Step 6: The membrane after one transverse stretching is fed into a three-stage countercurrent extraction tank for extraction treatment, the same as in Example 2;

[0099] Step 7: Send the extracted and dried membrane into a transverse stretching oven for a second transverse stretch of 1.5 times. The temperature settings for each zone are the same as in Example 2.

[0100] Step 8: Heat-set the film after secondary transverse stretching at 70°C, cool it to room temperature, then roll it up and cut it to obtain the base film master roll.

[0101] Comparative Example 3

[0102] A wet-process method for producing ultrathin, high-strength lithium battery microporous membranes was developed. Steps one, two, three, four, five, seven, and eight were the same as in Example 1. The difference in step six was that the membrane after one transverse stretching was fed into a three-stage countercurrent extraction tank, and dichloromethane was used as the extractant at room temperature. The specific process was the same as in Example 1, but an ultrasonic oscillation device was not set up. After extraction, the membrane entered the cleaning unit, was quickly rinsed with a small amount of fresh dichloromethane, and then dried with hot air at 40-60°C.

[0103] Performance testing

[0104] The lithium battery microporous membranes prepared in Examples 1-5 and Comparative Examples 1-3 were subjected to performance tests. The prepared lubricants were labeled as Example 1, Example 2, Example 3, Example 4, Example 5, Comparative Example 1, Comparative Example 2, and Comparative Example 3, respectively. The performance of Examples 1-5 and Comparative Examples 1-3 was tested, and the test data are recorded in the table below.

[0105]

[0106]

[0107] The data in the table shows that this invention uses high molecular weight polyethylene, paraffin oil, and additives to add a synchronous bidirectional stretching process between the longitudinal stretching process and the first transverse stretching process in a conventional wet asynchronous production line. This allows the separator to be bidirectionally stretched before transverse stretching, increasing the total stretching ratio of the separator, improving its transverse consistency, and thus enhancing its strength to meet the requirements of high energy density and safety in battery cells. This method is simple, has controllable conditions, good pore size uniformity, and does not require the use of ultra-high molecular weight polyethylene, thus having broad application prospects. The above examples illustrate that the wet process for producing ultra-thin, high-strength lithium battery microporous separators and its preparation method provided in this invention have better market application prospects.

[0108] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing an ultra-thin, high-strength microporous separator for lithium batteries using a wet process, characterized in that, Includes the following steps: Step 1: Mix the polymer, paraffin oil, and antioxidants, and add them into a twin-screw extruder. Melt and plasticize the mixture at 160-230℃ with a screw speed of 40-180 rpm to obtain the melt. Step 2: The melt is extruded from the T-die and attached to a chilling roller at 5-20℃ to obtain a casting sheet with a thickness of 0.8-3.5mm; Step 3: Feed the casting into the longitudinal stretching machine for longitudinal stretching treatment of 1-8 times. The longitudinal stretching machine is set up with a preheating zone, a stretching zone and a setting zone in sequence. The preheating zone is equipped with 8 rollers and the temperature is 80-110℃. The stretching zone is equipped with 6 rollers and the temperature is 100-125℃. The setting zone is equipped with 4 rollers and the temperature is 60-100℃. Step 4: The longitudinally stretched film is placed in a synchronous biaxial stretching oven for biaxial stretching treatment. The longitudinal stretching ratio is 1-5 times, and the transverse stretching ratio is controlled synchronously at 1-5 times. The synchronous biaxial stretching oven is set with a preheating zone, a stretching zone and a setting zone in sequence. The temperature of the preheating zone is 90-120℃, the temperature of the stretching zone is 100-135℃, and the temperature of the setting zone is 85-125℃. Step 5: The biaxially stretched film is fed into a transverse stretching machine for a single transverse stretch of 5-15 times. The transverse stretching machine is equipped with a preheating zone, a stretching zone and a setting zone in sequence. The temperature of the preheating zone is 100-130℃. The stretching zone is equipped with 8 pairs of transverse clamps with a temperature of 100-135℃. The temperature of the setting zone is 100-130℃. Step 6: After the membrane has been stretched laterally once, it is fed into a three-stage countercurrent extraction tank and paraffin oil is removed at room temperature using dichloromethane as the extractant. Step 7: Send the extracted and dried membrane into a transverse stretching oven for a second transverse stretch of 1-2.5 times. The transverse stretching oven is set with a preheating zone, a stretching zone and a setting zone in sequence. The temperature of the preheating zone is 105-115℃, the temperature of the stretching zone is 110-140℃, and the temperature of the setting zone is 100-130℃. Step 8: Heat-set the film after the second transverse stretching at 25-100℃, cool it to room temperature, then roll it up and cut it to obtain the base film master roll; The microporous membrane is made from the following components by weight percentage: 15-35% polymer, 65-85% paraffin oil, and 0.1-1.0% antioxidant.

2. The method for preparing an ultra-thin, high-strength lithium battery microporous separator using a wet process according to claim 1, characterized in that, The polymer is either high-density polyethylene or a polyethylene-polypropylene mixture, wherein the molecular weight of high-density polyethylene is 800,000 to 1,300,000, and the molecular weight of polypropylene is 800,000 to 1,300,000.

3. The method for preparing an ultrathin, high-strength lithium battery microporous separator using a wet process according to claim 1, characterized in that, The paraffin oil is graded 50#-70#.

4. The method for preparing an ultra-thin, high-strength lithium battery microporous separator using a wet process according to claim 1, characterized in that, The antioxidant is one or more of antioxidants 1076, 1010, or 168.

5. The method for preparing an ultrathin, high-strength lithium battery microporous separator using a wet process according to claim 1, characterized in that, In step six, the specific extraction steps are as follows: S1. The membrane enters from the first-stage extraction unit and flows counter-currently to dichloromethane; S2. Fresh dichloromethane first enters the third-stage extraction unit and comes into contact with the membrane, which has undergone the first two stages of extraction and has a low oil content, to initially extract the paraffin oil from the membrane surface and shallow micropores. S3 and dichloromethane flow countercurrently to the second and first stage extraction units, respectively, to fully contact the membrane with higher oil content and achieve deep extraction. The flow ratio of dichloromethane in each extraction unit is 3:2:

1. An ultrasonic oscillation device with a frequency of 20-40kHz is installed in the extraction tank. S4. The concentration of the extractant is monitored in real time by an online concentration detector, and fresh dichloromethane is automatically added when the concentration of the third-stage extraction unit is <99.5%. S5. After extraction, the membrane enters the cleaning unit, is quickly rinsed with a small amount of fresh dichloromethane, and then dried with hot air at 40-60℃.

6. The lithium battery microporous membrane prepared by the wet process for producing an ultra-thin, high-strength lithium battery microporous membrane according to any one of claims 1-5.

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