Method and device for preparing a high-elongation diaphragm

Abstract

This invention discloses a high-elongation membrane preparation apparatus and method. First, a low-temperature stretching device is used to rapidly cool the cast sheet, facilitating the formation of a more uniform microfiber network and thus improving the tensile and puncture strength of the membrane. Then, at least two heating-stretching-heat setting-cooling cycles are employed to achieve stable gradient stretching, better controlling the stretching rate and achieving uniform stretching over small spans. After extraction, gradient temperature-controlled drying is used, with each independent drying module performing drying at different temperatures. Each drying module is set with a gradient from low to high temperature, allowing the membrane containing a high concentration of dichloromethane liquid to evaporate slowly at low temperatures, achieving uniform evaporation, controlling capillary force, and preventing pore structure collapse caused by excessively rapid evaporation of the extractant in the membrane corresponding to the earlier drying module. Therefore, the micropore structure of the membrane after gradient drying does not change significantly, the membrane shrinkage is small, and it maintains good lateral performance consistency.

Description

Technical Field

[0001] This invention relates to a method and apparatus for preparing a high elongation separator, belonging to the field of wet-process lithium-ion battery separator production technology. Background Technology

[0002] The synchronous stretching process for wet-process lithium-ion battery separators involves adding PE and paraffin oil (white oil) to a twin-screw extruder. After plasticization in the extruder, the mixture is fed to a die for extrusion and cooling. The oil-containing film undergoes biaxial stretching inside a synchronous biaxial stretching machine, followed by extraction, transverse stretching, and heat treatment. Finally, it is wound up to complete the wet-process lithium-ion battery separator processing. In the asynchronous stretching process for wet-process lithium-ion battery separators, the film is stretched asynchronously and sequentially. This stretching requires the oil-containing film, after plasticization and cooling in the extruder, to first undergo longitudinal stretching, then transverse stretching, followed by extraction, transverse stretching, and heat treatment. Finally, it is wound up to complete the wet-process lithium-ion separator processing. In the wet-process lithium-ion battery separator production process, high elongation and high porosity separators can be prepared by stretching ultra-high molecular weight polyethylene at high rates and high temperatures.

[0003] To obtain diaphragms with high porosity and high elongation, a simultaneous double-stretching process with a high stretching ratio is used in the oil film stretching stage. This process, involving simultaneous double-stretching at a high ratio, improves porosity, strength, and elongation to some extent at high temperatures and high stretching ratios. However, at excessively high ratios, elongation is inversely proportional to tensile strength, limiting further increases in elongation. In the wet asynchronous stretching process, a high MD ratio stretching followed by a high TD ratio stretching is used to obtain diaphragms with high porosity and high elongation. However, after MD stretching, the diaphragm forms narrow, elongated pores, increasing the difficulty of TD stretching and making it difficult to form uniformly high-porosity, high-elongation diaphragms. Diaphragms containing dichloromethane undergo drying and cooling in an extraction-drying unit. During drying, the diaphragm experiences significant shrinkage, resulting in prominent issues with lateral performance consistency. Changes in the microstructure of the pores during the extraction-drying process are crucial to the pore structure and elongation. During the drying process, the capillary force at the liquid-gas interface within the pores is the key factor causing the shrinkage phenomenon: the radius of curvature (R) of the meniscus is directly related to the capillary pressure (∆P), described by the Yang-Laplace equation: ∆P = σ(1 / R1 + 1 / R2). Assuming the pore diameter is (r), and the principal curvature of the meniscus R = r / cosα, then the capillary pressure is: ∆P = 2σcosα / r. Direction: The ∆P generated by the concave meniscus points towards the gas side (i.e., "pulling" the liquid into the pore). This capillary force leads to the collapse of the pore structure; the diaphragm is composed of numerous micropores, and the local collapse of each pore macroscopically manifests as overall shrinkage (reduction in thickness or area).

[0004] High-elongation membranes with good performance possess a uniform microporous structure and high porosity, maintaining high ion permeability while preventing pore blockage due to deformation, thus supporting high-rate charge and discharge. Furthermore, high-elongation membranes with good performance can undergo significant deformation without rupture under extreme conditions (such as needle penetration and compression), effectively preventing internal short circuits. However, existing high-elongation membrane technologies struggle to simultaneously guarantee high elongation, high porosity, tensile strength, and puncture strength, making it difficult to balance various physical properties. Summary of the Invention

[0005] The purpose of this invention is to provide a high elongation membrane preparation apparatus and preparation method, so as to solve the problem that the high elongation membranes of the prior art cannot have the characteristics of uniform microporous structure and high porosity at the same time as high elongation.

[0006] The high elongation membrane preparation method of the present invention adopts the following technical solution: A high elongation membrane preparation method, comprising the following steps: S1, feeding extrusion: paraffin oil and polyethylene raw materials are premixed and fed into an extruder, and the extruder die extrudes a polyethylene-paraffin oil mixed melt; S2, casting sheet quenching and low-temperature stretching: the melt obtained in step S1 enters the casting sheet quenching and low-stretching process, the casting sheet is subjected to segmented cooling and shaping, the cooling temperature of each segment is gradually increased, the speed of each segment is independently controlled, and a speed difference is set between segments to control stretching, and the speed of each segment gradually increases; S3, gradient synchronous stretching: the casting sheet after casting sheet quenching and low stretching enters the gradient synchronous stretching process, and bidirectional synchronous stretching is performed to obtain The oil film undergoes a bidirectional synchronous stretching process consisting of heating, stretching, heat setting, and cooling. This bidirectional synchronous stretching is repeated at least twice, with the stretching ratio gradually increasing each time. S4, Extraction and Gradient Temperature Controlled Drying: The oil film after gradient synchronous stretching enters an extraction tank containing an extractant. The extractant is used to extract the paraffin oil from the film. The extracted oil film then enters a gradient temperature controlled drying process for gradient temperature increase drying, completing the extraction process. S5, Transverse Stretching and Setting: The dried film is heat-set in a transverse stretching and setting machine to obtain a diaphragm with highly consistent transverse properties. S6, Winding: The diaphragm obtained in the previous step is wound up using either contact or intermittent winding methods.

[0007] In step S2, the casting is cooled and shaped in three sections, with the temperatures of each section being 3℃~7℃, 8℃~12℃, and 13℃~17℃ respectively, preferably 5℃, 10℃, and 15℃; the running linear speed of each section is 1~20m / min.

[0008] The bidirectional synchronous stretching is repeated twice. The process temperature of the first stage of heating-stretching-heat setting-cooling is set sequentially as (112℃~117℃)-(118℃~120℃)-(121℃~125℃)-(10℃~18℃), more preferably 115℃-120℃-122℃-15℃, with a synchronous stretching ratio of 2~5, more preferably 3. The process temperature of the second stage of heating-stretching-heat setting-cooling is set sequentially as (115℃~120℃)-(122℃~125℃)-(126℃~128℃)-(10℃~18℃), more preferably 118℃-125℃-126℃-12℃, with a synchronous stretching ratio of 3~7, more preferably 5.

[0009] The gradient heating drying temperature is set sequentially as (28℃~32℃)-(33℃~37℃)-(38℃~42℃)-(43℃~47℃)-(48℃~52℃)-(53℃~57℃), and more preferably as 30℃-35℃-40℃-45℃-50℃-55℃.

[0010] The high elongation membrane preparation apparatus of the present invention adopts the following technical solution: A high elongation membrane preparation apparatus includes a casting sheet quenching and low stretching device, a gradient synchronous stretching device, an extraction tank, and a gradient temperature-controlled drying device, all disposed after an extruder. The casting sheet quenching and low stretching device includes two or more casting sheet modules. Each casting sheet module includes a casting sheet front drive roller, a casting sheet rear drive roller, and a casting sheet conveyor belt connecting the casting sheet front drive roller and the casting sheet rear drive roller. The casting sheet conveyor belt is provided with a transfer cooling body with an internal cavity and an arc-shaped structure. The transfer cooling body supports the casting sheet conveyor belt to form an arc-shaped structure. The outer side of the casting sheet conveyor belt supports and guides the casting sheet. The centers of the transfer cooling bodies of each casting sheet module coincide, so that each conveyor belt is on the same arc surface. Each transfer cooling body is filled with a cooling medium. Along the casting sheet forward direction, the temperature of the cooling medium inside each transfer cooling body gradually increases. The gradient synchronous stretching device includes at least two stages. The stretching zone of each stage includes a preheating zone, a synchronous stretching zone, a shaping zone, and a cooling zone arranged sequentially. The gradient temperature-controlled drying device includes two or more drying modules. Each drying module includes a pre-drying drive roller, a post-drying drive roller, and a drying conveyor belt connected between the pre-drying drive roller and the post-drying drive roller. The drying conveyor belt is used to support the diaphragm surface. A heating element with an internal cavity is provided inside the drying conveyor belt. The upper and lower surfaces of the heating element are in contact with the inner surface of the drying conveyor belt. A drying air inlet static pressure box and an active drive roller are provided on the side of the drying conveyor belt supporting the diaphragm. The active drive roller is in contact with the diaphragm. The drying air inlet static pressure box is spaced apart from the diaphragm to provide hot air to the diaphragm. Drying air outlet static pressure boxes are respectively provided between the drying air inlet static pressure box and the two ends of the drying conveyor belt. Along the diaphragm transmission direction, the inlet air temperature of the drying air inlet static pressure box of each drying module increases sequentially. A transverse stretching and shaping device and a winding device are arranged sequentially after the gradient temperature-controlled drying device.

[0011] The distance between two adjacent casting modules is 50-100mm. A first detection roller is provided between the two adjacent casting modules. The first detection roller is used to detect the speed and tension of the casting. The first detection roller and the casting conveyor belt are located on both sides of the casting.

[0012] Each casting module is equipped with an oil removal roller located inside the casting conveyor belt. The oil removal roller is used to remove oil from the outer surface of the casting conveyor belt.

[0013] The casting sheet rapid cooling low stretching device and the gradient synchronous stretching device are sequentially provided with a first cooling roller, a second cooling roller and a guide roller.

[0014] In each drying module, the drying conveyor belt is horizontally arranged; the active drive roller includes an upper active drive roller and a lower active drive roller located above and below the drying conveyor belt, respectively.

[0015] The drying module consists of six sets, which are divided into three layers arranged vertically. Each layer of the drying module has two sets, and there are two guide rollers and a second detection roller between the upper and lower drying modules. The second detection roller is located between the two guide rollers.

[0016] The beneficial effects of this invention are as follows: Firstly, a low-temperature stretching device for casting sheets is used to perform low-temperature stretching. Different linear velocities are set for each casting sheet module to control the stretching rate. A cooling medium is introduced into the transfer cooling body of the casting sheet module to achieve rapid cooling of the casting sheet. This low-temperature stretching, to a certain extent, facilitates the formation of a more uniform microfiber network, improving upon the traditional high-temperature method which causes excessive deentanglement of polymer chains, leading to excessive molecular chain slippage during stretching and reducing the stability of the orientation structure. This results in more uniform stress transmission during stretching, forming a denser microfiber network structure, thereby improving the tensile strength and puncture strength of the diaphragm. Then, at least two heating-stretching-heat setting-cooling cycles are used to achieve gradient stable stretching, better control the stretching rate, and achieve uniform stretching with small spans. Following extraction, a gradient temperature-controlled drying process is employed. Each independent drying module operates at different temperatures, with a gradient from low to high temperatures. This allows the membrane, initially containing a high concentration of dichloromethane liquid, to evaporate slowly at low temperatures, achieving uniform evaporation. This process controls capillary force and prevents pore structure collapse caused by excessively rapid evaporation of the extractant in the membranes of the earlier drying modules. Therefore, the microstructure of the membrane after gradient drying remains largely unchanged, resulting in low shrinkage and maintaining good lateral performance consistency. This invention, through a novel stretching process and extraction-drying apparatus, controls the drying process and thus the stability of the pore structure, producing membranes with high porosity and elongation, as well as highly uniform wet-process lithium-ion battery membranes. This approach offers greater advantages in controlling the processing stability of ultrathin films. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the casting sheet rapid cooling and low-temperature stretching device in a high elongation membrane preparation apparatus according to an embodiment of the present invention. Figure 2 This is a schematic diagram of the gradient synchronous stretching device in a high elongation membrane preparation apparatus according to an embodiment of the present invention; Figure 3 This is a schematic diagram of a gradient temperature-controlled drying device in a high elongation membrane preparation apparatus according to an embodiment of the present invention; Figure 4 This is a table of performance parameters of a diaphragm prepared according to an example of the present invention.

[0018] In the diagram: 1-Casting module, 1.1-Casting front drive roller, 1.2-Casting rear drive roller, 1.3-Transfer cooling body, 1.4-Casting conveyor belt, 1.5-First detection roller, 1.6-Degreasing roller, 2-Gradient synchronous stretching device, 2.1-Heating zone, 2.2-Stretching zone, 2.3-Heat setting zone, 2.4-Cooling zone, 3-Extraction tank, 4-Drying module, 4.1-Drying front drive roller, 4.2-Drying rear drive roller, 4.3-Drying conveyor belt, 4.4-Drying inlet static pressure box, 4.5-Drying exhaust static pressure box, 4.6-Active drive roller, 4.7-Heating body, 4.8-Guide roller, 4.9-Second detection roller, 5-Casting, 6-First cooling roller, 7-Second cooling roller, 8-Guide roller, 9-Transverse stretching and setting device, 10-Diaphragm, 11-Extruder die head, 12-Rewinding device. Detailed Implementation

[0019] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0020] like Figures 1 to 3 As shown, an embodiment of the high elongation membrane preparation apparatus of the present invention includes a casting sheet quenching and low stretching device 2, a gradient synchronous stretching device 2, an extraction tank 3, and a gradient temperature-controlled drying device 4, all disposed after the extruder. The casting sheet rapid cooling low-strength device includes two or more casting sheet modules 1. Each casting sheet module 1 includes a front driving roller 1.1, a rear driving roller 1.2, and a casting sheet conveyor belt 1.4 connecting the front driving roller 1.1 and the rear driving roller 1.2. The casting sheet conveyor belt 1.4 has an arc-shaped transfer cooling body 1.3 with an internal cavity. The transfer cooling body 1.3 supports the casting sheet conveyor belt 1.4, forming an arc-shaped structure. The outer side of the casting sheet conveyor belt 1.4 supports and guides the casting sheets. The centers of the transfer cooling bodies 1.3 in group 1 coincide, so that each casting sheet conveyor belt 1.4 is on the same arc surface. Each transfer cooling body 1.3 is filled with cooling medium. Along the forward direction of the casting sheet 5, the temperature of the cooling medium inside each transfer cooling body 1.3 gradually increases. The distance between two adjacent casting sheet modules 1 is 50-100mm. A first detection roller 1.5 is provided between two adjacent casting sheet modules 1. The first detection roller 1.5 is used to detect the speed and tension of the casting sheet 5. The first detection roller 1.5 and the casting sheet conveyor belt 1.4 are located on both sides of the casting sheet 5. The cooling body is made of stainless steel or aluminum / aluminum alloy with an internal cavity, preferably stainless steel. The cavity inside serves as a refrigerant flow channel, with a temperature control of 1-12℃. The coaxiality accuracy of the casting modules is controlled to ≤0.05mm, the cylindricity accuracy is controlled to ≤0.05mm, the linear speed of the casting modules is 1-20m / min, the distance between the casting modules is controlled to 50-100mm, and the annular diameter formed by the casting modules is between 1200mm and 2000mm.

[0021] Each casting module 1 is equipped with an oil removal roller 1.6 located inside the casting conveyor belt 1.4. The oil removal roller 1.6 is used to remove oil from the outer surface of the casting conveyor belt 1.4. A first cooling roller 6, a second cooling roller 7, and a guide roller 8 are sequentially arranged between the casting rapid cooling low stretching device and the gradient synchronous stretching device 2.

[0022] The gradient synchronous stretching device 2 includes at least two stretching zones. Each stretching zone includes a preheating zone 2.1, a synchronous stretching zone 2.2, a shaping zone 2.3, and a cooling zone 2.4 arranged sequentially, thereby realizing two or more cycles of heating-stretching-heat shaping-cooling, thus achieving gradient stable stretching, better control of the stretching rate, and achieving uniform stretching of small spans. The first-stage synchronous stretching can achieve 1-5 times stretching, and the second-stage synchronous stretching can achieve 5-10 times stretching.

[0023] The gradient temperature-controlled drying device includes two or more drying modules 4. Each drying module 4 includes a pre-drying drive roller 4.1, a post-drying drive roller 4.2, and a drying conveyor belt 4.3 connecting the pre-drying drive roller 4.1 and the post-drying drive roller 4.2. The drying conveyor belt 4.3 supports the diaphragm surface. A drying air inlet static pressure box 4.4 and an active drive roller 4.6 are provided on the side of the drying conveyor belt 4.3 supporting the diaphragm 10. The active drive roller 4.6 contacts the diaphragm 10. The drying air inlet static pressure box 4.4 is spaced apart from the diaphragm 10 to provide hot air to the diaphragm 10. Drying exhaust static pressure boxes 4.5 are respectively provided between the two ends of the drying inlet static pressure box 4.4 and the drying conveyor belt 4.3. Along the transmission direction of the diaphragm 10, the inlet air temperature of the drying inlet static pressure box 4.4 of each drying module 4 increases sequentially. In each drying module 4, the drying conveyor belt 4.3 is horizontally arranged, and a heating element 4.7 with an internal cavity is provided inside the drying conveyor belt 4.3. The upper and lower surfaces of the heating element 4.7 are in contact with the inner surface of the drying conveyor belt 4.3. The active drive roller 4.6 includes an upper active drive roller and a lower active drive roller located above and below the drying conveyor belt 4.3, respectively. There are a total of six drying modules 4. The drying modules are divided into three layers arranged vertically. Each layer of drying modules has two sets. Two guide rollers 4.8 and a second detection roller 4.9 are provided between the upper and lower drying modules. The second detection roller 4.9 is located between the two guide rollers 4.8.

[0024] Each drying module is independently set up, adopting a serial layered arrangement structure. The linear speed can be set according to the drying rate, and the number of serial arrangements can be increased. The drying body inside the drying module is made of stainless steel or aluminum / aluminum alloy with an internal cavity, preferably stainless steel. The internal cavity is a temperature-controlled media circulation channel with a temperature control range of 15-80℃. The active drive rollers set at the top and bottom of the drying conveyor belt have the function of pressing and drying the film containing dichloromethane. The active drive rollers have a roller temperature control function with a temperature control range of 15-80℃. The drying air inlet static pressure box and the drying air exhaust static pressure box set at the top or bottom of the active drive roller are respectively set up. The drying air inlet static pressure box dries the film surface with a temperature control range of 15-80℃. The exhaust static pressure boxes set at the front and rear have the function of isolating the air field between each independent drying module to allow drying at different temperatures between each independent drying module. The guide roller adopts a temperature-controlled roller structure, which controls the temperature from 15 to 80°C. The detection roller is made of carbon fiber, which is manufactured with low rotational inertia. It has tension detection and speed detection functions. Based on the tension and film linear speed detected by the detection roller, the speed of each independent drying module is adjusted in order to control the shrinkage rate and film surface flatness during film drying. The speed ratio (the ratio of the rear linear velocity to the front linear velocity) of the drying modules arranged along the diaphragm movement direction is set independently, preferably 1:0.98:0.99:1:1.05:1; the temperature of each drying module is set in a gradient heating mode, sequentially set to 30℃-35℃-40℃-45℃-50℃-55℃; the temperature of the inlet static pressure box is set to be equal to the temperature of the corresponding drying module; the wind speed control range of the inlet and outlet static pressure boxes is 1-25m / s, preferably 15m / s; the tension detection range of the detection roller is 50~300KG, preferably controlled at 100~150KG; the drying time is 10~35s, preferably 10~20s.

[0025] An extraction tank 3 is installed before the gradient temperature control drying device, and a transverse stretching and shaping device 9 and a winding device 12 are installed sequentially after the gradient temperature control drying device.

[0026] The method for preparing a high elongation diaphragm using the high elongation diaphragm preparation apparatus of the above embodiments includes the following process steps: feeding extrusion → casting sheet rapid cooling and low stretching → gradient synchronous stretching → extraction and gradient temperature controlled drying → transverse stretching and shaping → winding, specifically: S1. Feeding and extrusion: Paraffin oil and polyethylene raw materials are premixed and fed into the extruder. The extruder die extrudes the polyethylene-paraffin oil mixed melt. S2, Rapid Cooling and Low-Temperature Stretching of Castings: The melt obtained in step S1 enters the rapid cooling and low-tensile stretching process of castings, and the castings are cooled and shaped in stages. The cooling temperature of each stage increases gradually, and the speed of each stage is controlled independently. A speed difference is set between stages to control the stretching, and the speed of each stage gradually increases. In step S2, the castings are cooled and shaped in three stages, with the temperatures of each stage being 5℃, 10℃, and 15℃ respectively. The castings are made at a slow and controllable rate, and the running linear speed of each stage is 1 to 20 m / min.

[0027] In this step, the molten sheet extruded through the extruder die is placed on an annular cylindrical surface composed of three casting dies. The stretching rate of the casting is controlled by the different linear velocities of each casting die. After being cooled by the three annular casting dies, the casting enters the first and second cooling rollers for further cooling and shaping, and then enters the gradient synchronous stretching process via guide rollers.

[0028] The casting conveyor belt of the casting module is driven by front and rear drive rollers. The transmission cooling body is made of stainless steel, with an internal cavity serving as a refrigerant flow channel, maintaining a temperature of 1–12℃. The transmission cooling body contacts the casting conveyor belt to cool the castings. An oil removal roller is installed inside the annular casting module to remove oil from the outer surface of the conveyor belt. The coaxiality accuracy of each casting module is controlled to ≤0.05mm, and the cylindricity accuracy of the cylindrical surface formed by the casting modules is controlled to ≤0.05mm.

[0029] S3. Gradient Synchronous Stretching: After rapid cooling and low stretching, the cast sheet enters the gradient synchronous stretching process to obtain an oil film through bidirectional synchronous stretching. The bidirectional synchronous stretching steps are heating-stretching-heat setting-cooling, repeated more than twice, with the stretching ratio gradually increasing each time. In this embodiment, the bidirectional synchronous stretching is repeated twice, with two cycles of heating-stretching-heat setting-cooling, achieving stable gradient stretching, better controlling the stretching rate, and achieving small-span uniform stretching. One-stage synchronous stretching can achieve 1-5 times stretching, and two-stage synchronous stretching can achieve 5-10 times stretching. The process temperatures for the first stage of heating-stretching-heat setting-cooling are set sequentially to 115℃-120℃-122℃-15℃, with a synchronous stretching ratio of 3; the process temperatures for the second stage of heating-stretching-heat setting-cooling are set sequentially to 118℃-125℃-126℃-12℃, with a synchronous stretching ratio of 5.

[0030] S4. Extraction and Gradient Temperature Controlled Drying: The oil film after gradient synchronous stretching enters the extraction tank containing the extractant. The extractant is used to extract the paraffin oil from the film. The extracted oil film then enters the gradient temperature controlled drying process for gradient temperature increase drying, thus completing the extraction process.

[0031] The gradient temperature-controlled drying process is as follows: The dichloromethane-containing membranes, after extraction treatment, sequentially enter each drying module. Each drying module's inlet static pressure box provides hot air for drying the membranes. Two exhaust static pressure boxes discharge the dried hot air from the membranes. The inlet static pressure box dries the membrane surface at a temperature range of 15–80℃. The exhaust static pressure boxes isolate the airflow between the independent drying modules, allowing for different drying temperatures within each module. The inlet and exhaust static pressure boxes enable segmented drying of the membranes. The inlet and exhaust static pressure boxes are positioned according to the membrane's location, and are located on the same side as the membrane. The dichloromethane-containing membranes undergo gradient drying sequentially through six identically configured drying modules. The gradient temperature for each module is set sequentially as follows: 30℃-35℃-40℃-45℃-50℃-55℃. Each drying module is equipped with a gradient stage from low temperature to high temperature, so that the diaphragm containing high levels of dichloromethane liquid evaporates slowly at low temperature. The ultimate goal is to achieve uniform evaporation, control capillary force, and prevent the pore structure from collapsing due to excessively rapid evaporation of the extractant in the diaphragm of the front drying module, thereby avoiding uneven thickness and pore size caused by diaphragm shrinkage.

[0032] A heating element is installed inside the drying conveyor belt, which contacts the inner side of the belt for heat transfer. The heating element is made of stainless steel, and its internal cavity serves as a temperature-controlled medium flow channel, maintaining a temperature range of 15–80℃. Active drive rollers are positioned on the upper and lower sides of the drying conveyor belt, acting as a pressure plate to press against the diaphragm, allowing the heat from the heating medium inside the heating element to be transferred to the diaphragm. The active drive rollers also have a roller temperature control function, with a temperature range of 15–80℃.

[0033] The six drying modules are arranged in three layers and operate in series. Each layer has cross-arranged rollers at the membrane outlet. Two guide rollers between adjacent drying modules guide the membrane. These guide rollers employ a temperature-controlled roller structure, maintaining a temperature between 15 and 80°C. The second detection roller is a carbon fiber roller with low rotational inertia, serving both tension and speed detection functions. Based on the tension and membrane linear velocity detected by the detection rollers, the speed of each independent drying module is adjusted to control the shrinkage rate and membrane surface flatness during drying.

[0034] S5. Lateral stretching and setting: The dried film is heat-set in a lateral stretching and setting machine to obtain a diaphragm with highly consistent lateral properties.

[0035] S6. Winding: Wind up the diaphragm obtained in the previous step. The winding method can be contact winding or gap winding.

[0036] The following is a specific application example of a high elongation membrane preparation method: (1) Feeding extrusion: Paraffin oil and polyethylene raw materials are premixed, and the mass ratio of polyethylene to paraffin oil is 1.9:8.1. The oil injection port of the extruder is set with a 3-stage oil injection, and the oil injection ratios are 45%:25%:30%. After being mixed, melted and plasticized by the extruder, it is extruded through a slit die to obtain a polyethylene-paraffin oil mixed melt with a uniform phase. The mixing-plasticizing-extrusion homogenization temperature of the extruder is set to 225℃-215℃-205℃.

[0037] (2) Rapid cooling and low-temperature stretching of the cast sheet: The melt obtained in the above steps is rapidly cooled under rapid cooling and low stretching of the cast sheet to obtain an oil film on the cast sheet. Along the direction of the cast sheet's advance, the temperatures of the three casting sheet modules are set to 5℃, 10℃, and 15℃ respectively, and the linear speeds of the casting sheet modules are set to 10m / min, 10.15m / min, and 10.25m / min respectively. The temperatures of the two first detection rollers are set to 5℃ and 10℃ respectively, and the linear speeds of the two first detection rollers are 10.15m / min and 10.25m / min respectively. The speed of the degreasing roller is the same as that of the corresponding casting sheet module. The temperature of the first cooling roller and the second cooling roller is 20℃, and the guide roller is a non-powered aluminum guide roller.

[0038] (3) Gradient synchronous stretching: The process temperature of the first stage heating-stretching-heat setting-cooling is set to 115℃-120℃-122℃-15℃ in sequence, and the synchronous stretching ratio is 3; the process temperature of the second stage heating-stretching-heat setting-cooling is set to 118℃-125℃-126℃-12℃ in sequence, and the synchronous stretching ratio is 5.

[0039] (4) Extraction: The paraffin oil in the porous oil film obtained above is extracted using dichloromethane extractant. The extraction temperature is 10-35℃, preferably 20℃, and the extraction time is 1-3 min, preferably 1.5 min.

[0040] Gradient temperature-controlled drying: The extracted diaphragm is dried within a specific temperature range to ensure complete evaporation of the extractant. To control the diaphragm shrinkage rate during drying and reduce capillary forces that could cause pore structure collapse, segmented drying and tension detection are used to control the uniformity of drying. The speed ratio (the ratio of the rear linear velocity to the front linear velocity) of the six drying modules arranged along the film movement direction is set sequentially to 1:0.98:0.99:1:1.05:1. The temperature of the drying modules is controlled by the heat medium inside the heating element. The temperature of the six drying modules is set sequentially in a gradient heating mode, set to 30℃-35℃-40℃-45℃-50℃-55℃. The inlet static pressure box corresponding to each drying module is set to the same temperature, and the air velocity in the inlet static pressure box is controlled within the range of 1-25 m / s, with a preferred air velocity of 15 m / s. The air velocity in the exhaust static pressure box is 15 m / s. The tension detection range of the testing roller is 50-300KG, preferably controlled at 100-150KG, and the drying time is 10-35s, preferably 10-20s.

[0041] (5) Lateral stretching and setting: The film obtained in the above steps is heat-set in a lateral stretching and setting machine to obtain a diaphragm with highly consistent lateral properties. Preferably, the temperature of the lateral stretching preheating-stretching-heat setting-cooling zone is set to 130℃-131℃-132℃-20℃.

[0042] (6) Winding: The winding tension is controlled within the range of 5 to 10 N / m. Contact winding or gap winding is preferred, with small gap winding being the preferred method.

[0043] The performance parameters of the diaphragm prepared in this example are as follows: Figure 4 As shown in the table, the average elongation in the MD direction is 208% (201~211%), the average elongation in the TD direction is 159% (145~177%), and the average areal density is 4.4 g / m³. 2 (4.27~4.49 g / m 2 The material has an average thickness of 6.99 μm (6.86~7.04 μm), a porosity of 33.7% (33.3~34.5%), an air permeability of 170 s / 100cc (162~180 s / 100cc), and an average tensile strength in the MD direction of 3216 kgf / cm². 2 (3168~3309 kgf / cm) 2 The average tensile strength in the TD direction is 3257 kgf / cm. 2 (3200~3401 kgf / cm) 2 ).

[0044] The method for testing areal density is as follows: According to GB / T 451.2-2002 (Paper Surface Density Test), five square separator samples were cut along the transverse (TD) direction using a 10cm×10cm mold for testing. If the TD direction was less than 10cm, a 10cm sample was cut along the longitudinal (MD) direction; in this case, the sample was not square. The mass of the sample was weighed using an analytical balance (Mettler-Toledo Instruments (Shanghai) Co., Ltd., ME204E / 02) with a measurement accuracy of 0.0001g. The separator surface density = mass of separator sample / (length of separator sample × width of separator sample). The average value of the five samples was taken as the surface density of the multi-separator. The longitudinal direction refers to the length of the battery separator, and the transverse direction refers to the width of the battery separator.

[0045] Thickness testing method: 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] Porosity testing methods: Cut a 10cm × 10cm sample and measure its thickness (Mahr thickness gauge, C1202) and mass (electronic balance, Mettler Toledo Instruments (Shanghai) Co., Ltd., ME204E / 02). Calculate the surface density (unit: g / cm³) 2 ), where m represents the mass of the sample (in g), L represents the length of the sample (in cm), and b represents the width of the sample (in cm). According to Calculate the porosity, where p is the porosity of the sample (in %) and d is the thickness of the sample (in cm).

[0048] Air permeability test method: The test was conducted in accordance with the requirements of GB / T 36363—2018 "Determination of air permeability of polyolefin separators for lithium-ion batteries". A 600mm×100mm separator sample was cut, and an air permeability meter (ASAHI Corporation, EG01-55-1MR) was used. The test time was 3s. The air permeability of the separator was measured at any position at 100mm intervals along the 600mm TD direction. The average value of the above 5 test points was recorded as the air permeability of the separator.

[0049] Tensile strength and elongation test methods: 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 (%) in the MD / TD direction.

Claims

1. A method for preparing a high elongation separator, characterized by, It includes the following steps: S1. Feeding and extrusion: Paraffin oil and polyethylene raw materials are premixed and fed into the extruder. The extruder die extrudes the polyethylene-paraffin oil mixed melt. S2. Rapid Cooling and Low-Temperature Stretching of Castings: The melt obtained in step S1 enters the rapid cooling and low-temperature stretching process for castings, where the castings are cooled and shaped in segments. The cooling temperature of each segment increases gradually, and the speed of each segment is controlled independently. A speed difference is set between segments to control the stretching, and the speed of each segment gradually increases. In step S2, the castings are cooled and shaped in three segments, with temperatures of 3℃~7℃, 8℃~12℃, and 13℃~17℃ respectively. The running linear speed of each segment is 1~20m / min. S3. Gradient Synchronous Stretching: After rapid cooling and low-temperature stretching, the cast sheet enters the gradient synchronous stretching process to obtain an oil film through bidirectional synchronous stretching. The bidirectional synchronous stretching steps are heating-stretching-heat setting-cooling, and the bidirectional synchronous stretching is repeated more than twice, with the stretching ratio gradually increasing each time. The bidirectional synchronous stretching is repeated twice. The process temperatures for the first stage of heating-stretching-heat setting-cooling are set sequentially as (112℃~117℃)-(118℃~120℃)-(121℃~125℃)-(10℃~18℃), with a synchronous stretching ratio of 2~5. The process temperatures for the second stage of heating-stretching-heat setting-cooling are set sequentially as (115℃~120℃)-(122℃~125℃)-(126℃~128℃)-(10℃~18℃), with a synchronous stretching ratio of 3~7. S4. Extraction and Gradient Temperature Controlled Drying: The oil film after gradient synchronous stretching enters an extraction tank containing an extractant. The extractant is used to extract the paraffin oil from the film. The extracted oil film then enters a gradient temperature controlled drying process for gradient temperature increase drying to complete the extraction process. The gradient temperature increase drying temperature is set sequentially as (28℃~32℃)-(33℃~37℃)-(38℃~42℃)-(43℃~47℃)-(48℃~52℃)-(53℃~57℃). S5. Lateral stretching and setting: The dried film is heat-set in a lateral stretching and setting machine to obtain a diaphragm with highly consistent lateral properties. S6. Winding: Wind up the diaphragm obtained in the previous step. The winding method can be contact winding or gap winding.

2. A high elongation separator production apparatus characterized by: The high elongation membrane preparation apparatus is the method for preparing a high elongation membrane according to claim 1. The high elongation membrane preparation apparatus includes a cast sheet rapid cooling and low-temperature stretching device, a gradient synchronous stretching device, an extraction tank, and a gradient temperature-controlled drying device, all located after the extruder. The rapid cooling and low-temperature stretching device for cast sheets includes two or more cast sheet modules. Each cast sheet module includes a front drive roller, a rear drive roller, and a cast sheet conveyor belt connected between the front and rear drive rollers. The cast sheet conveyor belt is equipped with an arc-shaped transfer cooling body with an internal cavity. The transfer cooling body supports the cast sheet conveyor belt, making it form an arc-shaped structure. The outer side of the cast sheet conveyor belt supports and guides the cast sheet. The centers of the transfer cooling bodies of each cast sheet module coincide, so that each conveyor belt is on the same arc surface. Each transfer cooling body is filled with a cooling medium. Along the direction of cast sheet movement, the temperature of the cooling medium inside each transfer cooling body gradually increases. The gradient synchronous stretching device includes at least two stretching zones, each of which includes a preheating zone, a synchronous stretching zone, a shaping zone, and a cooling zone arranged sequentially. The gradient temperature-controlled drying device includes two or more drying modules. Each drying module includes a pre-drying drive roller, a post-drying drive roller, and a drying conveyor belt connecting the pre-drying drive roller and the post-drying drive roller. The drying conveyor belt supports the diaphragm surface. A heating element with an internal cavity is provided inside the drying conveyor belt. The upper and lower surfaces of the heating element are in contact with the inner surface of the drying conveyor belt. A drying air inlet static pressure box and an active drive roller are provided on the side of the drying conveyor belt that supports the diaphragm. The active drive roller is in contact with the diaphragm. The drying air inlet static pressure box is spaced apart from the diaphragm to provide hot air to the diaphragm. Drying air exhaust static pressure boxes are provided between the drying air inlet static pressure box and the two ends of the drying conveyor belt. Along the diaphragm transmission direction, the inlet air temperature of the drying air inlet static pressure box of each drying module increases sequentially. Following the gradient temperature-controlled drying device are a transverse stretching and shaping device and a winding device.

3. The high elongation separator manufacturing apparatus of claim 2, wherein: The distance between two adjacent casting modules is 50-100mm. A first detection roller is provided between the two adjacent casting modules. The first detection roller is used to detect the speed and tension of the casting. The first detection roller and the casting conveyor belt are located on both sides of the casting.

4. The high elongation separator manufacturing apparatus of claim 2, wherein: Each casting module is equipped with an oil removal roller located inside the casting conveyor belt. The oil removal roller is used to remove oil from the outer surface of the casting conveyor belt.

5. The high elongation membrane preparation apparatus according to claim 2, characterized in that: The casting sheet rapid cooling low-temperature stretching device and the gradient synchronous stretching device are sequentially provided with a first cooling roller, a second cooling roller and a guide roller.

6. The high elongation membrane preparation apparatus according to claim 2, characterized in that: In each drying module, the drying conveyor belt is horizontally arranged; the active drive roller includes an upper active drive roller and a lower active drive roller located above and below the drying conveyor belt, respectively.

7. The high elongation membrane preparation apparatus according to claim 2, characterized in that: The drying module consists of six sets, which are arranged in three layers, one above the other. Each layer of the drying module has two sets, and there are two guide rollers and a second detection roller between the upper and lower drying modules. The second detection roller is located between the two guide rollers.

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