A preparation method for reducing large transparent defects on the surface of a lithium ion battery separator

Abstract

The purpose of this invention is to provide a preparation method for reducing large transparency defects on the surface of lithium-ion battery separators. This invention belongs to the field of lithium-ion battery separator production and preparation technology. This invention eliminates large transparency defects by changing the speed ratio of the extraction lifting tank roller. The extraction lifting tank roller reduces the amount of liquid on the membrane surface by setting a high speed ratio, making the separator dry more uniformly. When passing through the drying roller, the contact state between the entire membrane surface and the drying roller is optimized, eliminating large bright spot defects.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery separator production and preparation technology, specifically relating to a preparation method for reducing large transparency defects on the surface of lithium-ion battery separators. Background Technology

[0002] As one of the important internal components of lithium batteries, the lithium battery separator plays a crucial role in the battery's operation. It isolates the positive and negative electrodes, prevents internal short circuits, improves the permeability of ions in the electrolyte, and ensures that the battery does not short-circuit and catch fire after being squeezed and deformed. However, defects on the outer surface of the lithium-ion battery separator directly affect the battery's safety performance and lifespan. In particular, "large transparency defects" on the separator surface can seriously affect the quality of the separator and reduce the yield of separator production.

[0003] Currently, the wet-process lithium-ion battery separators produced using high molecular weight polyolefin materials are still undergoing continuous optimization, even excluding inherent equipment instability. This results in varying membrane appearance defects during the production of porous lithium-ion battery separators due to differences in process conditions and raw materials. "Large transparency defects" are unacceptable in normal production; their presence leads to direct downgrading and reduced product yield. Summary of the Invention

[0004] The purpose of this invention is to provide a method for reducing large transparency defects on the surface of lithium-ion battery separators. This method can minimize the generation of large transparency defects while keeping other physical properties of the separator stable, thereby improving the safety performance and product qualification rate of the separator.

[0005] The present invention adopts the following technical solution:

[0006] A method for reducing large transparency defects on the surface of a lithium-ion battery separator includes the following steps:

[0007] The first step is to mix the raw materials in the extruder.

[0008] The extruder system uses high-temperature heating while utilizing the stirring and propulsion action of the screw to heat and mix solid PE powder and white oil at high temperature, so that the raw materials can be fully contacted.

[0009] The second step is the casting process.

[0010] The high-temperature fluid generated in the first step is produced into a sheet through a T-shaped die head, and then rapidly cooled and bonded by a casting roller. The casting roller is 2cm away from the die head outlet. A high-frequency range hood is installed at the die head to remove white oil vapor. The casting roller is set to a speed of 6-8m / min to perform the initial stretching of the sheet.

[0011] The third step is the longitudinal stretching process.

[0012] The sheet material that has undergone the first stretching in the second step is fed into the longitudinal stretching machine through guide rollers for stretching. This process is divided into a preheating zone, a stretching zone, and a shaping zone, with a stretching ratio of 6-8 times.

[0013] Step 4, First lateral stretch

[0014] The diaphragm obtained by longitudinal stretching in the third step is fed into the guide rail chain clamp through the guide roller and enters the oven for the first transverse stretching. The oven is divided into a preheating zone, a stretching zone and a shaping zone, and the stretching ratio is 10 times.

[0015] Step 5, Extraction Stage

[0016] The diaphragm, after its first lateral stretching in step four, is fed into the extraction tank via guide rollers. The white oil in the diaphragm pores is extracted using a 99.95% concentration dichloromethane liquid. The dichloromethane liquid feed rate is 5-6 ml. 3 The membrane after extraction enters the extraction-drying tank at a speed of / h. The roller speed ratios in the lifting tank within the extraction-drying tank are 99.85-99.95%, 99.85-100%, 99.9-100.1%, 100-100.2%, and 100.05-100.3%, respectively.

[0017] Step 6, Second Lateral Stretching Stage

[0018] The diaphragm extracted in step 5 is conveyed by guide rollers into the second transverse stretching oven. The oven is divided into three zones: preheating, stretching, and setting. The stretching ratio is 1.2-1.5.

[0019] Step 7, Heat setting stage

[0020] After the diaphragm that has undergone the second transverse stretching in step 6 is trimmed, it is pulled through the heat setting roller by the guide roller.

[0021] Step 8, Rolling up

[0022] The diaphragm that has been heat-set and stretched in step seven is wound up with a winding tension of 105-120N, so that the diaphragm is wound up flat on the winding roller.

[0023] Step 9, Slicing Stage

[0024] Cut the diaphragm that was rolled up in step 8 into small mother rolls for later use.

[0025] Furthermore, in the first step, the oil-to-PE powder ratio is set to 28% solid content, with oil pump one at 46% and oil pump two at 26%.

[0026] Furthermore, the temperature setting of the extruder in the first step is higher at the front and lower at the back, with the front temperature being 80-100℃ and the back temperature being 185-220℃.

[0027] Furthermore, the temperature of the casting cooling roller in the second step is 15-25°C.

[0028] Furthermore, in the third step, the temperature of the preheating zone is 95-100℃, the temperature of the stretching zone is 100-110℃, and the temperature of the setting zone is 40-60℃.

[0029] Furthermore, in step four, the temperature of the preheating zone is 120-130℃, the temperature of the stretching zone and the shaping zone is 110-120℃, and the frequency of the fan inside the oven is 35-55Hz.

[0030] Furthermore, the temperature of the three zones described in step six—preheating, stretching, and shaping—is 130-135℃.

[0031] Furthermore, in step seven, the heat setting roller is set with different temperature gradients, with the high-temperature zone in the front section set at 70-80℃ and the low-temperature zone in the rear section set at 20-30℃.

[0032] The beneficial effects of this invention are as follows:

[0033] The extraction process uses dichloromethane liquid to precipitate the white oil inside the membrane, releasing its internal pore structure. However, dichloromethane liquid residue remains on the membrane surface. If the membrane surface is not sufficiently dried by scraping, the dichloromethane liquid adheres to the membrane surface. After passing through the extraction lifting roller, uneven drying causes friction between the membrane surface and the roller surface, resulting in large transparency defects on the membrane surface.

[0034] This invention eliminates large transparency defects by changing the speed ratio of the extraction lifting tank rollers. The extraction lifting tank rollers reduce the amount of liquid on the membrane surface by setting a high speed ratio, making the diaphragm drying more uniform. When passing through the drying roller, the contact state between the entire membrane surface and the drying roller is optimized, eliminating large bright spot defects. Detailed Implementation

[0035] The extraction process uses dichloromethane liquid to precipitate the white oil inside the membrane, releasing its internal pore structure. However, dichloromethane liquid residue remains on the membrane surface. If the membrane surface is not sufficiently dried by scraping, the dichloromethane liquid adheres to the membrane surface. After passing through the extraction lifting roller, uneven drying causes friction between the membrane surface and the roller surface, resulting in large transparency defects on the membrane surface.

[0036] This invention eliminates large transparency defects by changing the speed ratio of the extraction lifting tank rollers. The extraction lifting tank rollers, by setting a high speed ratio, reduce the amount of liquid on the membrane surface, making the diaphragm drying more uniform. When passing through the drying roller, the contact state between the entire membrane surface and the drying roller is optimized, eliminating large bright spots. However, the extraction-drying roller speed ratio cannot be set too high; if the drying roller speed ratio is set too high, defects such as scratches may appear on the membrane surface.

[0037] The process of preparing a separator to reduce large transparency defects on the surface of a lithium-ion battery separator involved in this invention is as follows:

[0038] (1) Extruder raw material mixing

[0039] The extruder system uses high-temperature heating and the stirring and propulsion action of the screw to mix a certain amount of solid PE powder and white oil at high temperature, so that the raw materials can be fully contacted.

[0040] The oil-to-solids ratio is set to 28%, with oil pump 1 at 46% and oil pump 2 at 26%. Under the high-temperature environment inside the extruder barrel, the raw materials, under the propulsion and shearing action of the screw, are thoroughly mixed with the white oil and PE powder before extrusion.

[0041] The extruder temperature is set higher at the front and lower at the back. The front section temperature is set between 80℃ and 100℃, and the middle and rear sections of the extruder are set as high-temperature zones with temperatures between 185℃ and 220℃.

[0042] (2) Casting process

[0043] The high-temperature fluid generated in process (1) passes through the sheet produced by the T-shaped die head and is rapidly cooled and bonded by the casting roller. The casting roller is 2cm away from the die head outlet to reduce the shrinkage phenomenon generated during the sheet bonding process.

[0044] The temperature of the casting sheet cooling roller is set between 15℃ and 25℃. The white oil vapor at the die head is sucked away and recycled by a high-frequency oil fume extractor for reuse. At the same time, the abnormal phenomenon of oil droplets generated by the condensation of oil fumes at the die head is reduced. The casting sheet roller performs the initial stretching of the sheet by setting a speed of 6m / min-8m / min.

[0045] (3) Longitudinal stretching process

[0046] The sheet material that has undergone the first stretching in (2) is fed into the longitudinal stretching machine through guide rollers for stretching. The rollers of the longitudinal stretching machine are stretched by setting a speed difference. This process can be divided into three areas: the preheating zone, the stretching zone, and the shaping zone.

[0047] Preheating zone: The sheet is preheated to soften it and make it easier to stretch. The temperature of the preheating zone is set to 95℃-100℃.

[0048] Stretching zone: The main stage of longitudinal stretching of the diaphragm, with the temperature set at 100℃-110℃, and multi-stage stretching is carried out;

[0049] Shaping zone: The temperature is set to 40℃-60℃ to shape the stretched diaphragm.

[0050] The stretching ratio in the longitudinal stretching process is set to 6-8 times. By increasing the speed ratio of the stretching zone rollers, multi-stage stretching is performed to achieve the purpose of longitudinal stretching to enlarge the hole structure.

[0051] (4) First transverse stretching stage

[0052] The diaphragm obtained by longitudinal stretching in (3) is conveyed to the guide rail chain clamp by guide rollers and enters the oven for the first transverse stretching. The oven is divided into three areas: preheating zone, stretching zone, and shaping zone.

[0053] By changing the width of the guide rail chain clamp, the width of the diaphragm heated at high temperature inside the oven is changed. After being heated again, the internal pore structure of the diaphragm is released, and the microstructure of the diaphragm is changed.

[0054] The oven temperature is set to gradient heating, with the preheating zone temperature set at 120℃-130℃, the stretching and shaping zone temperatures set at 110℃-120℃, the oven fan frequency set at 35Hz-55Hz, and the stretching ratio inside the oven at 10 times.

[0055] The stretched diaphragm becomes wider and has more internal pores, thus increasing its porosity and making the internal pore structure of the diaphragm more uniform.

[0056] (5) Extraction stage

[0057] The diaphragm, after its first transverse stretching in step (4), is fed into the extraction tank via guide rollers. The white oil in the pores of the diaphragm is extracted using a high-concentration (99.95%) dichloromethane liquid, releasing the pore space within the diaphragm. The dichloromethane liquid feed rate is set to 5-6 ml. 3 / h, the roller speed ratio is set in the extraction tank to allow the diaphragm to spread out flat and fully pass through the dichloromethane liquid.

[0058] After dichloromethane liquid extraction, the diaphragm enters the extraction drying tank. The drying roller in the drying tank is set with a gradient temperature, with the temperature set between 30℃ and 60℃. The temperature gradually increases, and at the same time, the auxiliary drying blowing device is turned on with a frequency set to 30-35Hz to accelerate the evaporation of residual dichloromethane liquid on the membrane surface. The air pressure in the drying tank is maintained between negative pressure 10pa and 15pa.

[0059] After the extraction process, the membrane width will shrink to a certain extent, which changes the internal pore structure of the membrane.

[0060] (6) Second transverse stretching stage

[0061] The diaphragm extracted in (5) is conveyed into the second transverse stretching oven through the guide roller. The oven is divided into three areas: preheating, stretching and shaping. The diaphragm is stretched transversely again by high temperature heating, and the pore structure is changed at high temperature.

[0062] The temperature of the three zones is set between 130℃ and 135℃, the frequency of the fan inside the oven is set between 35Hz and 40Hz, the stretching ratio of the membrane surface inside the oven is between 1.2 and 1.5 times, and the width of the chain clamp is set as a gradient width of "narrow in the preheating zone - wide in the stretching zone - narrow in the shaping zone", so that the diaphragm can be fully stretched and retracted in the oven, and the internal pore structure of the diaphragm becomes uniform.

[0063] (7) Heat setting stage

[0064] After the diaphragm that has undergone the second transverse stretching in (6) is trimmed, it is pulled by the guide roller through the heat setting roller. The heat setting roller is set with different gradient temperatures. By setting the speed difference, the diaphragm is stretched and retracted again, changing the internal pore structure of the diaphragm.

[0065] The high-temperature zone at the front of the heat-setting roller is set at 70℃-80℃, and the low-temperature zone at the rear is set at 20℃-30℃. The internal structure of the pores is stabilized by adjusting the amount of diaphragm retraction.

[0066] (8) Roll-up stage

[0067] The diaphragm that has been heat-set and stretched in (7) is wound up, and a certain winding tension is set, ranging from 105N to 120N, so that the diaphragm is wound up flat on the winding roller. After winding up a certain number of meters according to the specifications, it is cut.

[0068] (9) Slicing stage

[0069] Cut the diaphragm that has been rolled up in (8) into small master rolls of a certain size for later use.

[0070] Example 1 shows the extraction lifting roller speed ratio and the number of large transparency defects. Examples 2, 3, 4, and 5 show the extraction lifting tank speed ratio being changed to eliminate large transparency defects, as detailed below:

[0071] Example 1: The speed ratios of the lifting rollers are set sequentially to 99.8%, 99.8%, 99.85%, 99.9%, and 100%, with a maximum of 5 large transparent defects.

[0072] Example 2: The speed ratios of the lifting groove rollers are set sequentially to 99.85%, 99.85%, 99.9%, 100%, and 100.05%, with a maximum of 3 large transparent defects.

[0073] Example 3: The speed ratios of the raised groove rollers are set sequentially to 99.9%, 99.9%, 99.95%, 100.1%, and 100.1%, with one large transparent defect.

[0074] Example 4: The speed ratios of the lifting trough rollers are set sequentially to 99.9%, 99.95%, 100%, 100.2%, and 100.3%, with 0 large transparent defects.

[0075] Example 5: The speed ratios of the raised groove rollers were set sequentially to 99.95%, 100%, 100.1%, 100.2%, and 100.3%, with 0 large transparent defects.

Claims

1. A method for preparing a lithium-ion battery separator with reduced large transparency defects on its surface, characterized in that: Includes the following steps: The first step is to mix the raw materials in the extruder. The extruder system uses high-temperature heating while simultaneously utilizing the stirring and propulsion action of the screw to mix solid PE powder and white oil at high temperature, ensuring full contact between the raw materials. The oil-to-PE powder ratio is set to 28% solid content, with oil injection pump one at 46% and oil injection pump two at 26%. The second step is the casting process. The high-temperature fluid generated in the first step is produced into a sheet through a T-shaped die head, and then rapidly cooled and bonded by a casting roller. The casting roller is 2cm away from the die head outlet. A high-frequency range hood is installed at the die head to remove white oil vapor. The casting roller is set to a speed of 6-8m / min to perform the initial stretching of the sheet. The third step is the longitudinal stretching process. The sheet material that has undergone the first stretching in the second step is fed into the longitudinal stretching machine through guide rollers for stretching. This process is divided into a preheating zone, a stretching zone, and a shaping zone, with a stretching ratio of 6-8 times. Step 4, First lateral stretch The diaphragm obtained by longitudinal stretching in the third step is fed into the guide rail chain clamp through the guide roller and enters the oven for the first transverse stretching. The oven is divided into a preheating zone, a stretching zone and a shaping zone, and the stretching ratio is 10 times. Step 5, Extraction Stage The diaphragm, after its first lateral stretching in step four, is fed into the extraction tank via guide rollers. The white oil in the diaphragm pores is extracted using a 99.95% concentration dichloromethane liquid. The dichloromethane liquid feed rate is 5-6 ml. 3 The membrane after extraction enters the extraction-drying tank at a speed of / h. The roller speed ratios in the lifting tank within the extraction-drying tank are 99.85-99.95%, 99.85-100%, 99.9-100.1%, 100-100.2%, and 100.05-100.3%, respectively. Step 6, Second Lateral Stretching Stage The diaphragm extracted in step 5 is conveyed by guide rollers into the second transverse stretching oven. The oven is divided into three zones: preheating, stretching, and setting. The stretching ratio is 1.2-1.

5. Step 7, Heat setting stage After the diaphragm that has undergone the second transverse stretching in step 6 is trimmed, it is pulled through the heat setting roller by the guide roller. Step 8, Rolling up The diaphragm that has been heat-set and stretched in step seven is wound up with a winding tension of 105-120N, so that the diaphragm is wound up flat on the winding roller. Step 9, Slicing Stage Cut the diaphragm that was rolled up in step 8 into small mother rolls for later use.

2. The preparation method for reducing large transparency defects on the surface of a lithium-ion battery separator according to claim 1, characterized in that: The temperature setting of the extruder in the first step is higher at the front and lower at the back, with the front temperature being 80-100℃ and the back temperature being 185-220℃.

3. The preparation method for reducing large transparency defects on the surface of a lithium-ion battery separator according to claim 1, characterized in that: The temperature of the casting rollers in the second step is 15-25℃.

4. The preparation method for reducing large transparency defects on the surface of a lithium-ion battery separator according to claim 1, characterized in that: The temperature of the preheating zone in the third step is 95-100℃, the temperature of the stretching zone is 100-110℃, and the temperature of the setting zone is 40-60℃.

5. The preparation method for reducing large transparency defects on the surface of a lithium-ion battery separator according to claim 1, characterized in that: The temperature of the preheating zone in step four is 120-130℃, the temperature of the stretching zone and the shaping zone is 110-120℃, and the frequency of the fan in the oven is 35-55Hz.

6. The preparation method for reducing large transparency defects on the surface of a lithium-ion battery separator according to claim 1, characterized in that: The temperature of the three zones described in step six—preheating, stretching, and setting—is 130-135℃.

7. The preparation method for reducing large transparency defects on the surface of a lithium-ion battery separator according to claim 1, characterized in that: In step seven, the heat setting rollers are set with different temperature gradients: the high-temperature zone at the front is set at 70-80℃, and the low-temperature zone at the back is set at 20-30℃.