A method for preparing a lithium ion battery wet separator

Abstract

The present application relates to a kind of lithium ion battery wet method separator preparation method, successively include batching and melt extrusion casting piece, longitudinal stretching, transverse stretching, extraction, drying, heat setting and winding process;The drying process uses pre-positioned liquid scraper roll pre-liquid removal cooperation segmented gradient drying synergistic process, including: after the separator is extracted out, first by liquid scraper roll physical scraping its surface free extraction liquid, then sequentially through the low-temperature liquid removal zone of front section, the medium-temperature drying zone of middle section and the low-temperature stress relief zone of rear section;Wherein, the low-temperature liquid removal zone removes residual free liquid by low-temperature drying roller, to avoid a large amount of extraction agent volatilization;The medium-temperature drying zone removes trace liquid in the inside of drying film by medium-temperature drying roller and air knife, to ensure uniform drying;The low-temperature stress relief zone eliminates stress in the film by low-temperature drying roller, to improve the flatness and dimensional stability of separator.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery separator production, and in particular to a method for preparing a lithium-ion battery separator, specifically a wet process method for preparing a lithium-ion battery separator. Background Technology

[0002] Traditional wet-process separators for lithium-ion batteries use polyolefin resin as the main raw material and are prepared through processes such as melt extrusion casting, longitudinal stretching, transverse stretching, extraction, drying, heat setting, and winding. In current wet-process separator production, the drying process generally has significant shortcomings.

[0003] In existing technologies, most air knives in drying ovens are conventional structures with upper and lower equal-width air boxes and air outlets. The cross-section of the air cavity is consistent, and the air output parameters are fixed throughout the process, making segmented adjustment impossible. For example, patent CN214555858U discloses a dual-channel air knife that divides the air cavity into two equal-width air channels through a flow divider to achieve uniform air output. This may lead to the following situations: the diaphragm carries a large amount of extractant out of the extraction tank; traditional drying ovens use high-temperature, high-volume, strong blowing at the front, resulting in a large amount of extractant volatilization, high losses, and high costs.

[0004] The diaphragm has thick edges on both sides, which retain more liquid, resulting in poor diaphragm adhesion and uneven heating. Conventional air knives cannot adjust the lateral air velocity and temperature in different areas, easily leading to uneven drying of the thick edges, membrane shrinkage, defects, and poor dimensional stability. In contrast, the air outlet parameters of an integrated air knife are fixed throughout the process, leaving little room for process adjustment and making it difficult to match the drying requirements of diaphragms of different thicknesses and operating speeds.

[0005] Therefore, improvements are urgently needed to better meet production demands. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by providing a wet process method for preparing lithium-ion battery separators, which can effectively solve technical problems such as high extractant loss, uneven drying, and poor process adjustment flexibility in the existing wet process separator drying process.

[0007] The technical solution of this invention is: A method for preparing a wet-process separator for lithium-ion batteries includes, in sequence, batching, melt extrusion to form a cast sheet, longitudinal stretching, transverse stretching, extraction, drying, heat setting, and winding; the drying process employs a pre-draining process using a front-mounted scraper roller combined with a segmented gradient drying process, including: After extraction, the membrane first undergoes physical scraping of the free extract on its surface by a scraper roller, and then sequentially passes through a low-temperature descaling zone, a medium-temperature drying zone, and a low-temperature stress relief zone. In the low-temperature descaling zone, residual free liquid is removed by a low-temperature drying roller to prevent excessive evaporation of the extractant. In the medium-temperature drying zone, a medium-temperature drying roller and an air knife stabilize the trace amounts of liquid inside the membrane, ensuring uniform drying. In the low-temperature stress relief zone, a low-temperature drying roller eliminates internal membrane stress, improving the membrane's flatness and dimensional stability.

[0008] Furthermore, the batching and melt extrusion casting process includes: mixing polyolefin resin and pore-forming agent in proportion, stirring evenly in a high-speed mixer and then adding to a twin-screw extruder, where the material is melted and plasticized at high temperature, filtered through a filter screen and extruded through a die head, and rapidly cooled and shaped on a cooling roller to form a thick cast film, with the temperature of the cooling roller controlled at 8℃–20℃. The longitudinal stretching process includes: introducing the cast film thick film into the longitudinal stretching unit, passing through the preheating section, stretching section and setting section in sequence. The preheating temperature is 80℃–120℃, the stretching temperature is 80℃–100℃, the longitudinal stretching ratio is 8–12 times, and the setting temperature is 30℃–60℃. The transverse stretching process is as follows: the longitudinally stretched film is introduced into the transverse stretching machine, and the film edge is clamped by clamps and enters the preheating, transverse stretching and heat setting area. The preheating temperature is 115℃–125℃, the stretching temperature is 110℃–120℃, the transverse stretching ratio is 6–9 times, and the setting temperature is 90℃–110℃. The extraction process includes: introducing the transversely stretched membrane into the extraction tank, and using an extractant to fully extract and remove the pore-forming agent inside the membrane to form a continuous and interconnected microporous structure. The heat setting and winding process includes: the dried separator enters the heat setting zone for stress relaxation and dimensional stabilization treatment, and the heat setting temperature is 120℃–140℃; after the setting is completed, it is cooled, thickness measured, and defect detected, and finally wound up by the winding machine to obtain the finished lithium-ion battery wet separator.

[0009] Furthermore, the scraper roller is a temperature-controlled scraper roller with a hollow structure and an internal circulating temperature-controlled flow channel. The outer surface of the roller is mirror-polished, and the external pipeline of the scraper roller is connected to an industrial temperature-controlled mold temperature controller. Two sets of scraper rollers are symmetrically arranged at the diaphragm outlet of the drying oven water tank to press and adhere the front and back sides of the diaphragm, and a U-shaped liquid receiving tray is set below the scraper roller. The lower sides of the liquid receiving tray are provided with guide grooves that communicate with the extraction tank to recover the scraped extractant for recycling.

[0010] Furthermore, the process parameters of the scraper roller are set as follows: the scraper roller temperature is 25℃–45℃; the scraper roller and the diaphragm are bonded by interference fit, and the adjustment reference is: the scraper roller advances 2mm–10mm after just contacting the membrane surface; the scraper roller rotates at low speed in the same direction as the diaphragm.

[0011] Furthermore, the air knife is installed inside the drying oven, above the diaphragm material feeding path, and its interior is divided into a left section air cavity, a middle section air cavity, and a right section air cavity by a partition along the diaphragm material feeding direction; the left section air cavity, the middle section air cavity, and the right section air cavity are independent of each other and do not cross-flow with each other, and each is equipped with a separate air inlet; the air inlet is equipped with an air volume regulating valve.

[0012] Furthermore, the air inlet is equipped with a temperature testing probe, which can be linked with the air volume regulating valve and the mold temperature controller.

[0013] Furthermore, the transverse partitioning process of the segmented air knife is as follows: the middle section air cavity is set according to standard process parameters; the temperature of the left and right sections air cavities is increased by 5℃–15℃ compared to the middle section air cavity, and the air velocity is increased by 2–8m / s compared to the middle section air cavity, so as to enhance the drying capacity of the thick edge parts on both sides of the membrane.

[0014] Furthermore, the temperature of the drying roller in the front low-temperature dehydration zone is 18℃–35℃ to remove residual free liquid and prevent excessive volatilization of the extractant.

[0015] Furthermore, the temperature of the drying roller in the intermediate temperature drying zone is 50℃–70℃, the temperature of the air cavity in the intermediate section of the air knife is 40℃–70℃, the angle of the air knife is 5°–25° offset forward from the vertical roller surface, and the air velocity in the intermediate section of the air knife is 35–65m / s, in order to stabilize the trace liquid inside the drying film and ensure uniform drying.

[0016] Furthermore, the temperature of the drying roller in the subsequent low-temperature stress relief zone is 35℃–55℃ to eliminate intra-film stress at low temperature and improve flatness and dimensional stability.

[0017] The beneficial effects of this invention are: First, it significantly reduces the cost of extractant consumption: by physically pre-removing the extractant through a pre-low temperature scraper roller, the original high temperature and high air volume evaporation and dehydration process is replaced, and the loss of extractant volatilization is greatly reduced.

[0018] Second, the uniformity of diaphragm drying is greatly improved: the independent air control in the left, middle and right sections accurately matches the differences in liquid content at different positions and drying stages of the diaphragm, completely solving the problems of uneven drying at thick edges and localized incomplete drying. The rate of diaphragm warping and shrinkage deformation defects is reduced, and the product yield is greatly improved.

[0019] Third, the quality and performance of the separator product are improved: the gradient low-temperature drying process avoids the internal stress caused by the rapid drying of the separator at high temperature, and the dimensional stability and flatness of the separator are significantly improved, meeting the production standards of high-end power lithium battery separators.

[0020] Fourth, the equipment has lower energy consumption and more flexible process adjustment: the parameters of each section of the air knife can be adjusted independently, the process debugging range is large, it can adapt to the mass production switching of diaphragms of different thicknesses and specifications, the equipment operation and maintenance is simple, the overall energy consumption of the drying oven is reduced, and the production adaptability is stronger.

[0021] Fifth, the process adjustment is efficient and quantifiable: It has a clear adjustable range and adjustment logic for temperature and air speed, which allows operators to quickly locate problems and accurately correct parameters, greatly shortening changeover and debugging time and improving production line efficiency. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the drying process structure of the present invention.

[0023] Figure 2 This is a schematic diagram of the air knife structure.

[0024] Wherein: 1-Drying furnace water tank; 2-U-shaped liquid receiving tray; 3-Scraper roller; 4-Diaphragm; 5-Air knife; 51-Left section air chamber; 52-Middle section air chamber; 53-Right section air chamber; 54-Air inlet; 55-Air volume regulating valve. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0026] like Figure 1 and 2 As shown.

[0027] This invention discloses a wet-process method for preparing a lithium-ion battery separator, comprising: ingredient preparation, melt extrusion to form a cast sheet, longitudinal stretching, transverse stretching, extraction, drying, heat setting, and winding, specifically:

[0028] 1) Batching, melt extrusion, and casting process: Polyolefin resin and pore-forming agent are mixed in a certain proportion and thoroughly stirred in a high-speed mixer before being fed into a twin-screw extruder. The material is melted and plasticized at high temperature inside the extruder. After impurities are removed by filtering through a filter screen, it is uniformly extruded through a die. The extruded material is rapidly cooled and shaped on cooling rollers to form a uniformly thick cast film. The temperature of the cooling rollers is strictly controlled within a low-temperature range of 8℃–20℃. This low-temperature cooling condition ensures that an extractable phase separation structure suitable for subsequent extraction is formed inside the cast film.

[0029] 2) Longitudinal stretching process: The thick film cast sheet is fed into the longitudinal stretching unit, passing sequentially through a preheating section, a stretching section, and a setting section. The preheating section temperature is controlled at 80℃–120℃ to bring the cast sheet to a suitable stretching state. The stretching section temperature is controlled at 80℃–100℃, achieving longitudinal stretching through the speed difference between the front and rear rollers, with a longitudinal stretching ratio set at 8–12 times to create longitudinal molecular orientation within the cast sheet. The setting section temperature is controlled at 30℃–60℃, stabilizing the longitudinal dimensions through appropriate cooling. The longitudinal stretching ratio is set according to the target thickness and porosity requirements of the product, providing the film with the structural foundation required for subsequent transverse stretching.

[0030] 3) Transverse stretching process: The longitudinally stretched membrane is fed into the transverse stretching machine, where clamps hold the membrane edges as it sequentially enters the preheating zone, transverse stretching zone, and heat-setting zone. The preheating temperature is controlled at 115℃–125℃ to uniformly heat and soften the membrane, creating conditions for transverse stretching. The stretching temperature is controlled at 110℃–120℃, and transverse stretching is achieved through guide rail expansion, with a stretching ratio controlled at 6–9 times, forming a microporous structure within the membrane. The heat-setting temperature is controlled at 90℃–110℃ to stabilize the micropore morphology and size. The transverse stretching ratio and temperature parameters are matched to ensure that the membrane porosity, air permeability, and mechanical strength meet the target requirements.

[0031] 4) Extraction process: The laterally stretched membrane is introduced into the extraction tank, where an extractant is used to fully extract and remove the pore-forming agent inside the membrane, forming a continuous and interconnected microporous structure within the membrane. The extraction tank employs a multi-stage design to ensure clean extraction without residue. The extractant flow rate is matched to the membrane thickness and production speed, and controlled by monitoring the extractant concentration to avoid insufficient extraction or excessive membrane swelling.

[0032] 5) Drying process: After the diaphragm 4 exits the extraction tank, it undergoes physical pretreatment by being scraped off by low-temperature scraping rollers. At the diaphragm outlet of the drying oven water tank 1, two sets of scraping rollers 3 are symmetrically positioned on both sides of the diaphragm 4, with the two rollers pressing against each other to adhere to the surfaces of the diaphragm. The scraping roller 3 has a hollow structure with an internal circulating temperature-controlled flow channel and an external pipeline independently connected to an industrial temperature-controlled mold temperature controller. The mold temperature controller precisely controls the surface temperature of the scraping roller to maintain a low-temperature constant state of 25℃–45℃. Simultaneously, the outer surface of the scraping roller 3 is mirror-polished and it is pressed against the diaphragm using an interference fit. The adjustment benchmark is that the scraping roller advances 2mm–10mm after just contacting the membrane surface, causing it to rotate slowly in the same direction as the diaphragm's flow direction. After being physically squeezed by the scraping rollers 3, most of the free extractant liquid on the surface of the diaphragm 4 is scraped off, removing more than 70% of the surface free liquid in advance. Furthermore, a U-shaped liquid receiving tray 2 is provided below the scraping roller 3, and guide grooves are provided on both sides below the U-shaped liquid receiving tray 2 so that the scraped extractant liquid can be recovered into the extraction tank through the guide grooves, thereby realizing the recycling of the extractant.

[0033] After pretreatment by scraping, the diaphragm enters the drying area, which is equipped with an air knife 5. This air knife 5 is installed inside the drying oven, above the diaphragm's material path. Internally, multiple vertical sealing partitions divide the original integrated, continuous air chamber along the diaphragm's material path into three independent, non-cross-flowing, sealed segmented air chambers: a left segment 51, a middle segment 52, and a right segment 53. The left and right segments each occupy 1 / 5 of the total air chamber, while the middle segment occupies 3 / 5. Each segmented air chamber has an independent air inlet 54, and an airflow regulating valve 55 is installed on each inlet, allowing for individual adjustment of the outlet air velocity in each segment without interference. Simultaneously, a temperature probe is installed on each air inlet, which can be linked to the mold temperature controller and the airflow regulating valve to achieve semi-automatic parameter control.

[0034] The drying process is divided into three gradient zones along the diaphragm feeding direction: The front section is a low-temperature dehydration zone, which includes several drying rollers. The temperature of these drying rollers is controlled between 18℃ and 35℃, primarily using low-temperature drying to remove residual free liquid and prevent excessive evaporation of the extractant. Since the pre-mounted scraper rollers have already removed most of the surface free liquid, there is no need to set high-temperature, high-volume process parameters in the front section.

[0035] The middle section is a medium-temperature drying zone, containing a drying roller. The temperature of this drying roller is controlled at 50℃–70℃, while the temperature of the air cavity in the middle section of the air knife is controlled at 40℃–70℃. The air knife angle is offset forward from the roller surface by 5°–25°, and the air velocity in the middle section of the air knife is controlled at 35–65 m / s to stabilize the trace liquid inside the drying film and ensure uniform drying. Simultaneously, the middle section of the air knife corresponds to the substrate area in the middle of the diaphragm, and is set according to standard process parameters with moderate airflow and temperature. The left and right sections of the air knife correspond to the thicker edges of the film on both sides, with temperatures 5℃–15℃ higher and air velocities 2–8 m / s higher than the middle section, to enhance the drying capacity of the thicker edges and solve the problems of incomplete drying and edge shrinkage.

[0036] The latter section is a low-temperature stress relief zone, which contains several drying rollers. The temperature of these drying rollers is controlled between 35℃ and 55℃. This low-temperature treatment eliminates internal stress in the membrane, improving the flatness and dimensional stability of the diaphragm.

[0037] 6) Heat setting and winding process: After drying, the separator enters the heat-setting zone, where the heat-setting temperature is controlled at 120℃–140℃ for stress relaxation and dimensional stabilization treatment, further reducing internal stress and improving high-temperature dimensional stability. After heat setting, it undergoes cooling, thickness measurement, and defect detection, and is finally wound up by a winding machine to obtain the finished wet-process separator for lithium-ion batteries.

[0038] This invention physically pre-removes the extractant using a front-mounted low-temperature scraper roller, replacing the original high-temperature, high-volume evaporation process, significantly reducing extractant evaporation loss. Independent airflow control in left, middle, and right segments precisely matches the liquid levels at different locations and drying stages of the diaphragm, completely resolving issues of uneven drying at thick edges and localized incomplete drying. A gradient low-temperature drying process avoids internal stress caused by rapid high-temperature drying of the diaphragm, significantly improving diaphragm dimensional stability and flatness. The parameters of each section of the air knife are independently adjustable, allowing for a wide range of process adjustments and adaptability to mass production switching of diaphragms of different thicknesses and specifications. Overall energy consumption of the drying oven is reduced, resulting in greater production adaptability.

[0039] All parts not covered in this invention are the same as or can be implemented using existing technologies.

Claims

1. A method for preparing a wet-process separator for lithium-ion batteries, comprising the steps of batching, melt extrusion into a cast sheet, longitudinal stretching, transverse stretching, extraction, drying, heat setting, and winding, characterized in that, The drying process employs a combined process of pre-drying with a front-mounted scraper roller and segmented gradient drying, including: After extraction, the membrane first undergoes physical scraping of the free extract on its surface by a scraper roller, and then sequentially passes through a low-temperature descaling zone, a medium-temperature drying zone, and a low-temperature stress relief zone. In the low-temperature descaling zone, residual free liquid is removed by a low-temperature drying roller to prevent excessive evaporation of the extractant. In the medium-temperature drying zone, a medium-temperature drying roller and an air knife stabilize the trace amounts of liquid inside the membrane, ensuring uniform drying. In the low-temperature stress relief zone, a low-temperature drying roller eliminates internal membrane stress, improving the membrane's flatness and dimensional stability.

2. The method for preparing a wet-process separator for lithium-ion batteries according to claim 1, characterized in that, The batching and melt extrusion casting process includes: mixing polyolefin resin and pore-forming agent in proportion, stirring evenly with a high-speed mixer and then adding to a twin-screw extruder, where the material is melted and plasticized at high temperature, filtered through a screen and extruded through a die head, and rapidly cooled and shaped on a cooling roller to form a thick cast film, with the temperature of the cooling roller controlled at 8℃–20℃. The longitudinal stretching process includes: introducing the cast film thick film into the longitudinal stretching unit, passing through the preheating section, stretching section and setting section in sequence. The preheating temperature is 80℃–120℃, the stretching temperature is 80℃–100℃, the longitudinal stretching ratio is 8–12 times, and the setting temperature is 30℃–60℃. The transverse stretching process is as follows: the longitudinally stretched film is introduced into the transverse stretching machine, and the film edge is clamped by clamps and enters the preheating, transverse stretching and heat setting area. The preheating temperature is 115℃–125℃, the stretching temperature is 110℃–120℃, the transverse stretching ratio is 6–9 times, and the setting temperature is 90℃–110℃. The extraction process includes: introducing the transversely stretched membrane into the extraction tank, and using an extractant to fully extract and remove the pore-forming agent inside the membrane to form a continuous and interconnected microporous structure. The heat setting and winding process includes: the dried separator enters the heat setting zone for stress relaxation and dimensional stabilization treatment, and the heat setting temperature is 120℃–140℃; after the setting is completed, it is cooled, thickness measured, and defect detected, and finally wound up by the winding machine to obtain the finished lithium-ion battery wet separator.

3. The method for preparing a wet-process separator for lithium-ion batteries according to claim 1, characterized in that, The scraper roller is a temperature-controlled scraper roller with a hollow structure and an internal circulating temperature-controlled flow channel. The outer surface of the roller is mirror-polished, and the external pipeline of the scraper roller is connected to an industrial temperature-controlled mold temperature controller. Two sets of scraper rollers are symmetrically arranged at the diaphragm outlet of the drying oven water tank to press and adhere the front and back sides of the diaphragm, and a U-shaped liquid receiving tray is set below the scraper roller. The lower sides of the liquid receiving tray are provided with guide grooves connected to the extraction tank to recover the scraped extractant for recycling.

4. The method for preparing a wet-process separator for lithium-ion batteries according to claim 1, characterized in that, The process parameters of the scraper roller are set as follows: the scraper roller temperature is 25℃–45℃; the scraper roller and the diaphragm are bonded by interference fit, and the adjustment reference is: the scraper roller advances 2mm–10mm after just contacting the membrane surface; the scraper roller rotates at low speed in the same direction as the diaphragm.

5. The method for preparing a wet-process separator for a lithium-ion battery according to claim 1, characterized in that, The air knife is installed inside the drying oven, above the diaphragm material feeding path. Its interior is divided into a left section air cavity, a middle section air cavity, and a right section air cavity by a partition along the diaphragm material feeding direction. The left section air cavity, the middle section air cavity, and the right section air cavity are independent of each other and do not cross-flow with each other. Each air cavity is equipped with a separate air inlet. The air inlet is equipped with an air volume regulating valve.

6. The method for preparing a wet-process separator for a lithium-ion battery according to claim 5, characterized in that, The air inlet is equipped with a temperature test probe, which can be linked with the air volume regulating valve and the mold temperature controller.

7. The method for preparing a wet-process separator for a lithium-ion battery according to claim 5, characterized in that, The transverse partitioning process of the segmented air knife is as follows: the middle section air cavity is set according to standard process parameters; the temperature of the left and right sections air cavities is increased by 5℃–15℃ and the air velocity is increased by 2–8m / s compared with the middle section air cavity, so as to enhance the drying capacity of the thick edge parts on both sides of the membrane.

8. The method for preparing a wet-process separator for a lithium-ion battery according to claim 1, characterized in that, The temperature of the drying roller in the front low-temperature dehydration zone is 18℃–35℃ to remove residual free liquid and prevent excessive volatilization of the extractant.

9. The method for preparing a wet-process separator for a lithium-ion battery according to claim 1, characterized in that, The temperature of the drying roller in the intermediate temperature drying zone is 50℃–70℃, the temperature of the air cavity in the middle section of the air knife is 40℃–70℃, the angle of the air knife is 5°–25° offset forward from the vertical roller surface, and the air velocity in the air cavity in the middle section of the air knife is 35–65m / s, in order to stabilize the trace liquid inside the drying film and ensure uniform drying.

10. The method for preparing a wet-process separator for a lithium-ion battery according to claim 1, characterized in that, The temperature of the drying roller in the low-temperature stress relief zone is 35℃–55℃, which eliminates internal stress in the film at low temperature and improves flatness and dimensional stability.

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