Method for solving precipitate on membrane surface of wet-process diaphragm

Abstract

The invention discloses a method for solving a membrane surface precipitate of a wet-process diaphragm, and belongs to the technical field of battery diaphragms. The method comprises the following steps: S1, stirring and mixing a diaphragm base material, a functional additive and a high-boiling-point plasticizer, adding a composite solvent and a dispersing agent, uniformly mixing, melting, extruding, and casting to obtain a sheet; s2, the sheet in the S1 is subjected to three-stage cooling for cooling and shaping; s3, biaxially stretching the sheet treated in the S2 to obtain a thin film; and S4, carrying out solvent extraction on the film obtained in S3 through a four-temperature-zone counter-current extraction device, drying, and rolling to obtain the battery diaphragm. The composite solvent is obtained by mixing hydrogenated naphthenic oil and isoparaffin according to a mass ratio of (50-70): (30-50); polyoxyethylene sorbitan monooleate is adopted as the dispersing agent; the battery diaphragm prepared by the method for solving the precipitates on the membrane surface of the wet-process diaphragm can effectively inhibit and remove the precipitates.

Description

Technical Field

[0001] This invention relates to a method for solving the problem of precipitates on the surface of wet-process diaphragm membranes. Background Technology

[0002] With the rapid development of the new energy industry, lithium-ion batteries, as key energy storage devices, are receiving increasing attention for their performance and safety. The separator, as one of the core components of lithium-ion batteries, plays a crucial role in isolating the positive and negative electrodes, preventing short circuits, and allowing lithium ions to move freely. Currently, approximately 90% of wet-process lithium-ion battery separators on the market use polyethylene (PE) as the base material, which is widely adopted due to its excellent mechanical properties, chemical stability, and cost advantages.

[0003] The wet process is the mainstream method for preparing high-performance polyethylene separators. Its typical process flow includes: melt-blending a polyethylene substrate with a high-boiling-point plasticizer (such as paraffin oil, white oil, or phthalates) and functional additives (such as antioxidants and nucleating agents), followed by extrusion into sheets; forming a uniform microporous structure through biaxial stretching (MD) in the longitudinal direction and TD in the transverse direction; finally, removing the plasticizer using a low-boiling-point solvent such as dichloromethane, and drying to form a porous separator. This process can effectively control the porosity, thickness, and mechanical properties of the separator, meeting the requirements of high-energy-density batteries.

[0004] However, several key technical challenges remain in the existing wet-process separator manufacturing process, leading to the appearance of membrane surface precipitates during subsequent use, which seriously affects battery performance and safety. Specifically: • Plasticizer residue issue: During the extraction process, due to limitations in process conditions or insufficient solvent diffusion efficiency, some high-boiling-point plasticizers may not be completely removed from the inside or surface of the membrane micropores. These residual plasticizers will slowly migrate to the surface during the storage or slitting of the membrane, forming oily or solid precipitates, which will affect subsequent coating processes.

[0005] • Migration and precipitation of functional additives: Low molecular weight additives such as antioxidants and lubricants in the formulation are prone to migration, aggregation or even thermal decomposition during high-temperature processing or cooling, generating insoluble particles or oily substances that adhere to the membrane surface or embed in the pores, affecting the membrane porosity and ion transport performance.

[0006] • Polymer degradation product precipitation: During high-temperature processing, polyethylene molecular chains may undergo chain breakage or oxidation reactions, generating low-molecular-weight degradation products. These substances can migrate to the membrane surface or pores under specific temperature or humidity conditions, forming precipitates that further deteriorate membrane performance.

[0007] The presence of precipitates on the membrane surface will lead to a series of serious consequences. First, the precipitates will significantly reduce the porosity of the membrane, reducing the porosity by 30% to 50% when the ceramic layer is coated, thereby affecting the lithium-ion transport efficiency. Second, the precipitated hard particles with a size >50 μm may cause a decrease in the local puncture strength of the membrane by up to 40%, significantly increasing the risk of internal short circuits in the battery. According to research statistics, the probability of short circuits can be increased by more than 8 times, seriously threatening the safe operation of the battery.

[0008] Therefore, effectively addressing the issue of membrane surface precipitates during the preparation and use of wet-process separators has become one of the key technological bottlenecks for improving the performance and safety of lithium-ion batteries. To address this problem, there is an urgent need to develop a method for resolving membrane surface precipitates in wet-process separators, achieving effective suppression and removal of these precipitates, thereby ensuring the long-term stability of the separator and the overall performance of the battery. Summary of the Invention

[0009] The purpose of this invention is to provide a method for solving the problem of precipitates on the surface of wet-process diaphragm membranes, thereby addressing the technical problems mentioned in the background section.

[0010] The technical solution to achieve the objective of this invention is: A method for resolving precipitates on the surface of wet-process membranes includes the following steps: S1: Mix the diaphragm base material, functional additives, and high-boiling-point plasticizer, add the composite solvent and dispersant, mix evenly, melt, extrude, and cast into sheets to obtain the sheet material; S2: The sheet from S1 is cooled and shaped through three stages of cooling; S3: The sheet treated with S2 is biaxially stretched to obtain a film; S4: The thin film obtained in S3 is extracted, dried and wound up using a four-temperature zone countercurrent extraction device to obtain a battery separator.

[0011] This invention effectively inhibits and removes precipitates by optimizing the solvent formulation of the membrane material and adding a dispersant, implementing a three-stage cooling process in the subsequent membrane preparation process, and using a four-temperature-zone countercurrent extraction device for extraction.

[0012] Furthermore, the composite solvent is obtained by mixing hydrogenated naphthenic oil and isoparaffins in a mass ratio of 50~70:30~50.

[0013] The composite solvent used in this invention consists of hydrogenated naphthenic oil and isoalkanes. The hydrogenated naphthenic oil has excellent solubility and can form a tight entanglement with PE segments, effectively inhibiting the separation of the crystalline phase. The isoalkanes, with a viscosity of less than 5 cP, not only have a high molecular diffusion coefficient but also significantly improve the efficiency of subsequent extraction steps. In this composite solvent system, the hydrogenated naphthenic oil acts as the main solvent to stabilize the polymer network, while the isoalkanes act as a co-solvent to fill the gaps between molecules. This results in an overall viscosity of the composite solvent that is about 40% lower than that of traditional paraffin oil, thereby significantly increasing the speed of subsequent solvent replacement.

[0014] In the composite solvent system of this invention, the mass ratio of hydrogenated naphthenic oil to isoalkanes is controlled within the range of 50~70:30~50. When the proportion of hydrogenated naphthenic oil is too high (i.e., the mass ratio of hydrogenated naphthenic oil to isoalkanes > 70:30), it will lead to excessive viscosity of the system, causing difficulties in the extrusion casting process. Conversely, if the proportion of isoalkanes is too high (i.e., the mass ratio of hydrogenated naphthenic oil to isoalkanes < 50:50), it will reduce the compatibility between the solvent and PE, resulting in phase separation streaks on the cast sheets. Controlling the mass ratio of hydrogenated naphthenic oil to isoalkanes to 50~70:30~50 ensures both processing performance and the quality stability of the finished product.

[0015] Furthermore, the dispersant is polyoxyethylene sorbitan monooleate.

[0016] The dispersant used in this invention is polyoxyethylene sorbitan monooleate, whose molecular structure has unique amphiphilic properties. The oleic acid long chain at the lipophilic end can act as an anchoring group, binding with solvent molecules through hydrophobic interactions to form a micelle structure that encapsulates the precipitates. The polyoxyethylene ether segments at the hydrophilic end point towards the polymer phase, significantly reducing interfacial energy and enhancing system stability. During cooling, the steric hindrance effect of polyoxyethylene sorbitan monooleate can hinder the diffusion of low molecular weight components to the surface, thereby preventing phase separation and precipitate formation.

[0017] Furthermore, the amount of the dispersant added is 0.5~1.2wt%.

[0018] When the amount of dispersant added is less than 0.5 wt%, micelle structures cannot be fully formed, resulting in a migration inhibition rate of less than 60%. When the amount of dispersant added exceeds 1.2 wt%, the excess dispersant will undergo self-aggregation, which will instead form new precipitates on the membrane surface. In this invention, the amount of dispersant added is controlled within 0.5~1.2 wt%, which ensures effective migration inhibition while avoiding the negative effects caused by excessive addition.

[0019] Furthermore, the specific steps of step (2) are as follows: S2.1: The sheet in S1 is cooled in the first stage, from 80℃±0.5℃ to 70℃±0.5℃, with the cooling rate controlled at 0.2~0.8℃ / s and the cooling duration at 50~70s; The first stage of the three-stage cooling process described in this invention involves cooling the sheet from 80℃±0.5℃ to 70℃±0.5℃ at a cooling rate of 0.2~0.8℃ / s for 50~70s, aiming to induce homogeneous growth of crystal nuclei. The cooling rate in this stage is close to the equilibrium crystallization rate of polyethylene (PE), under which homogeneous growth of crystal nuclei can be induced, forming fine and uniformly distributed crystal nuclei, laying the foundation for the subsequent formation of a bicontinuous phase separation structure. Simultaneously, the temperature range of 80℃ to 70℃ is precisely the region of minimum supercooling in the PE melt (ΔT=10℃), effectively avoiding explosive nucleation. The cooling time is controlled at 50~70s to ensure that the final crystal nuclei density reaches 10. 7 ~10 8 pcs / cm³; S2.2: The sheet cooled in S2.1 is subjected to a second stage of cooling, from 70℃±0.5℃ to 50℃±0.5℃, with the cooling rate controlled at 1.5~2.5℃ / s and the cooling duration at 20~30s; The second stage of the three-stage cooling process described in this invention involves cooling the sheet from 70℃±0.5℃ to 50℃±0.5℃ at a cooling rate of 1.5~2.5℃ / s for 50~70s, aiming to lock in the initially formed pore structure. The cooling rate in this stage matches the diffusion rate of the solvent in the polymer matrix, which is beneficial for controlling the phase separation kinetics. Liquid-liquid phase separation mainly occurs in the temperature range of 70℃ to 50℃, while this stage promotes the further development of the solvent-enriched phase through rapid cooling. The cooling time is controlled within 50~70s, which can achieve sufficient interpenetration and stability of the solvent-enriched phase network, thereby effectively fixing the microstructure of the sheet and constructing a double continuous pore structure with uniform pore size and good connectivity. S2.3: The sheet cooled in S2.1 is subjected to a third stage of cooling, from 50℃±0.5℃ to 25℃±0.5℃; the cooling rate is controlled at 4.9~5.1℃ / s, and the cooling duration is 8~12s; The third stage of the three-stage cooling process described in this invention involves cooling the sheet from 50℃±0.5℃ to 25℃±0.5℃ at a cooling rate of 4.9~5.1℃ / s for 8~12s. The purpose is to achieve rapid shaping of the sheet. The cooling rate in this stage exceeds the critical cooling rate for the glass transition of polyethylene (PE), effectively suppressing the rearrangement of molecular chains and maintaining the stability of the formed microstructure. Liquid-liquid phase separation mainly occurs in the temperature range of 50℃ to 25℃. This stage of cooling process spans the β relaxation temperature of PE, Tβ=40℃, avoiding the brittle crystal region. The cooling time is controlled within 8~12s, achieving rapid quenching from the melt state to the solid state, cooling to below the crystallization termination temperature. Through this stage of cooling, the internal structure of the diaphragm can be effectively frozen, inhibiting the migration behavior of residual solvent molecules, thereby preventing surface shrinkage caused by slow cooling and improving the uniformity and stability of the diaphragm structure.

[0020] Furthermore, the four-temperature-zone countercurrent extraction device includes a first-temperature-zone extraction unit, a second-temperature-zone extraction unit, a third-temperature-zone extraction unit, and a fourth-temperature-zone extraction unit; the flow direction of the extractant in the four-temperature-zone countercurrent extraction device is opposite to the direction of film movement, wherein the film sequentially passes through the first-temperature-zone extraction unit, the second-temperature-zone extraction unit, the third-temperature-zone extraction unit, and the fourth-temperature-zone extraction unit; the extractant sequentially flows through the fourth-temperature-zone extraction unit, the third-temperature-zone extraction unit, the second-temperature-zone extraction unit, and the first-temperature-zone extraction unit.

[0021] Furthermore, the countercurrent ratio of the extractant to the film is 2.4~2.6:1.

[0022] Furthermore, the temperatures of the first temperature zone extraction unit, the second temperature zone extraction unit, the third temperature zone extraction unit, and the fourth temperature zone extraction unit are controlled at 40-45℃, 30-35℃, 20-25℃, and 10-15℃, respectively.

[0023] Further, the extraction steps in step S4 are as follows: the film obtained in S3 is sent to the first temperature zone extraction unit for turbulent extraction, then to the second temperature zone extraction unit for pulse extraction, then to the third temperature zone extraction unit for laminar flow extraction, and finally to the fourth temperature zone extraction unit for ultrasonic extraction.

[0024] Furthermore, the Reynolds number Re > 4000 for the turbulent extraction; the pulse frequency is 2 Hz and the amplitude is 5 ± 0.5 mm; the laminar flow spacing is 0.3 ~ 0.5 mm; the ultrasonic power density is 0.5 W / cm² and the frequency is 35 ~ 45 kHz.

[0025] The extraction process of this invention employs a four-zone countercurrent extraction device. By combining the countercurrent motion between the diaphragm and the extraction solvent with a four-zone temperature gradient design, a synergistic effect of efficient mass transfer and structural protection is achieved. Specifically, this device can maintain a continuous high concentration gradient, thereby increasing the mass transfer driving force by 2.8 times. Simultaneously, through the coordinated regulation of thermodynamic and kinetic processes, an organic combination of high-temperature open-pore dissolution and low-temperature microporous purification is achieved, improving extraction efficiency and diaphragm structural stability. In the four-zone countercurrent extraction device, the temperature of the first extraction unit is controlled at 40~45℃, employing turbulent extraction, with a Reynolds number Re controlled at >4000, which effectively reduces system viscosity, promotes solvent penetration, and dissolves the hydrogenated naphthenic oil encapsulated by polyethylene (PE) chain entanglement. The temperature of the second extraction unit is set at 30~35℃, employing pulse extraction... The pulse extraction mode has a pulse frequency of 2Hz and an amplitude of 5±0.5mm. These parameters match the solvent diffusion coefficient, which helps to remove solvent-embedded substances and improve mass transfer efficiency. The third temperature zone extraction unit is controlled at 20~25℃ and uses laminar flow extraction with a laminar flow spacing of 0.3~0.5mm. The system pressure difference is less than 0.1MPa, ensuring the integrity of the membrane structure under low pressure drop conditions, while effectively extracting free isoparaffins. The fourth temperature zone extraction unit implements ultrasonic-assisted extraction, with the temperature maintained at 10~15℃ and the ultrasonic power density at 0.5W / cm². It utilizes the ultrasonic cavitation effect to efficiently flush and remove residual solvent inside the micropores. Its frequency range of 34~45kHz matches the resonant frequency of the micropores with a pore size of about 100nm, improving the removal efficiency of capillary residues while effectively preventing pore structure collapse.

[0026] By adopting the above technical solution, the present invention has the following beneficial effects: (1) This invention optimizes the solvent formulation of the membrane material and adds a dispersant. At the same time, it implements a three-stage cooling process in the subsequent membrane preparation process and uses a four-temperature zone countercurrent extraction device for extraction, thereby effectively inhibiting and removing precipitates.

[0027] (2) The composite solvent used in this invention is composed of hydrogenated naphthenic oil and isoparaffins; wherein, the hydrogenated naphthenic oil has excellent solubility and can form a tight entanglement with PE segments, effectively inhibiting the separation of the crystalline phase; while the isoparaffins with a viscosity of less than 5 cP not only have a high molecular diffusion coefficient, but can also significantly improve the efficiency of subsequent extraction steps; in this composite solvent system, the hydrogenated naphthenic oil plays the role of stabilizing the polymer network as the main solvent, while the isoparaffins fill the gaps between molecules as a co-solvent, so that the overall viscosity of the composite solvent is reduced by about 40% compared with traditional paraffin oil, thereby greatly improving the speed of subsequent solvent replacement.

[0028] (3) In the composite solvent system of the present invention, the mass ratio of hydrogenated naphthenic oil to isoalkanes is controlled within the range of 50~70:30~50. When the proportion of hydrogenated naphthenic oil is too high, that is, the mass ratio of hydrogenated naphthenic oil to isoalkanes is >70:30, it will lead to excessive viscosity of the system, causing difficulties in the extrusion casting process. Conversely, if the proportion of isoalkanes is too high, that is, the mass ratio of hydrogenated naphthenic oil to isoalkanes is <50:50, it will reduce the compatibility between the solvent and PE, resulting in phase separation stripes on the casting. The mass ratio of hydrogenated naphthenic oil to isoalkanes is controlled within the range of 50~70:30~50, which ensures both processing performance and the quality stability of the finished product.

[0029] (4) The dispersant used in this invention is polyoxyethylene sorbitan monooleate, which has unique amphiphilic properties in its molecular structure. The oleic acid long chain at the lipophilic end can act as an anchoring group and combine with solvent molecules through hydrophobic interaction to form a micelle structure to coat the precipitates. The polyoxyethylene ether chain segment at the hydrophilic end points to the polymer phase, which significantly reduces the interfacial energy and enhances the stability of the system. During the cooling process, the steric hindrance effect of polyoxyethylene sorbitan monooleate can hinder the diffusion of low molecular weight components to the surface, thereby preventing phase separation and precipitate formation.

[0030] (5) When the amount of dispersant added is less than 0.5wt%, micelle structure cannot be fully formed, resulting in a migration inhibition rate of less than 60%. When the amount of dispersant added exceeds 1.2wt%, the excessive dispersant will undergo self-aggregation, which will instead form new precipitates on the membrane surface. In this invention, the amount of dispersant added is controlled within 0.5~1.2wt%, which ensures effective migration inhibition and avoids negative effects caused by excessive addition.

[0031] (6) The first stage of the three-stage cooling process described in this invention is as follows: the sheet is cooled from 80℃±0.5℃ to 70℃±0.5℃ at a cooling rate of 0.2~0.8℃ / s, and the cooling duration is 50~70s, which aims to induce homogeneous growth of crystal nuclei; the cooling rate in this stage is close to the equilibrium crystallization rate of polyethylene (PE), and under this condition, homogeneous growth of crystal nuclei can be induced to form fine and uniformly distributed crystal nuclei, laying the foundation for the subsequent formation of a dual continuous phase separation structure; at the same time, the temperature range of 80℃ to 70℃ is exactly the region with the smallest supercooling of PE melt ΔT=10℃, which effectively avoids the occurrence of explosive nucleation phenomenon; the cooling time is controlled at 50~70s to ensure that the final crystal nucleus density reaches 10 7 ~10 8 pcs / cm³; The second stage of cooling involves cooling the sheet from 70℃±0.5℃ to 50℃±0.5℃ at a rate of 1.5~2.5℃ / s for 50~70s. This cooling process aims to lock in the initially formed pore structure. The cooling rate in this stage matches the diffusion rate of the solvent in the polymer matrix, which is beneficial for controlling the phase separation kinetics. Liquid-liquid phase separation mainly occurs in the temperature range of 70℃ to 50℃. In this stage, rapid cooling promotes the further development of the solvent-enriched phase. The cooling time is controlled within 50~70s, which can achieve sufficient interpenetration and stability of the solvent-enriched phase network, thereby effectively fixing the microstructure of the sheet and constructing a double continuous pore structure with uniform pore size and good connectivity. The third stage of cooling involves cooling the sheet from 50℃±0.5℃ to 25℃±0.5℃ at a rate of 4.9~5.1℃ / s for 8~12s. This stage aims to rapidly shape the sheet. The cooling rate in this stage exceeds the critical cooling rate for the glass transition of polyethylene (PE), effectively suppressing molecular chain rearrangement and maintaining the stability of the formed microstructure. Liquid-liquid phase separation mainly occurs in the temperature range of 50℃ to 25℃. This cooling stage spans the β relaxation temperature of PE, Tβ=40℃, avoiding the brittle crystal region. The cooling time is controlled within 8~12s, achieving rapid quenching from the melt state to the solid state, cooling to below the crystallization termination temperature. This cooling stage effectively freezes the internal structure of the diaphragm, inhibits the migration of residual solvent molecules, and prevents surface pinholes caused by slow cooling, thereby improving the uniformity and stability of the diaphragm structure.

[0032] (7) The extraction process of this invention adopts a four-temperature-zone countercurrent extraction device. By combining the countercurrent motion between the diaphragm and the extraction solvent with the four-zone temperature gradient design, a synergistic effect of efficient mass transfer and structural protection is achieved. Specifically, the device can maintain a continuous high concentration difference, thereby increasing the mass transfer driving force by 2.8 times. At the same time, through the synergistic regulation of thermodynamic and kinetic processes, the organic combination of high-temperature open-pore dissolution and low-temperature microporous purification is achieved, improving extraction efficiency and diaphragm structural stability. In the four-temperature-zone countercurrent extraction device, the temperature of the first temperature zone extraction unit is controlled at 40~45℃, and turbulent extraction is adopted. The Reynolds number Re is controlled at >4000, which can effectively reduce the viscosity of the system, promote solvent penetration, and dissolve the hydrogenated naphthenic oil wrapped by polyethylene (PE) chain entanglement. The temperature of the second temperature zone extraction unit is set at 30~35℃. The pulse extraction mode is used with a pulse frequency of 2Hz and an amplitude of 5±0.5mm. These parameters match the solvent diffusion coefficient, which helps to remove solvent-embedded substances and improve mass transfer efficiency. The third temperature zone extraction unit is controlled at 20~25℃ and uses laminar flow extraction with a laminar flow spacing of 0.3~0.5mm. The system pressure difference is less than 0.1MPa, which ensures that the membrane structure is maintained under low pressure drop conditions, while effectively extracting free isoparaffins. The fourth temperature zone extraction unit implements ultrasonic-assisted extraction, with the temperature maintained at 10~15℃ and the ultrasonic power density at 0.5W / cm². It utilizes the ultrasonic cavitation effect to efficiently flush and remove residual solvent inside the micropores. Its frequency range of 34~45kHz matches the resonant frequency of the micropores with a pore size of about 100nm, which improves the removal efficiency of capillary residues while effectively preventing pore structure collapse. Attached Figure Description

[0033] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein... Figure 1 This is a process flow diagram of the method for solving the problem of precipitates on the surface of wet-process diaphragm membranes according to the present invention.

[0034] Figure 2 This is a structural diagram of a four-temperature zone countercurrent extraction device according to an embodiment of the present invention.

[0035] The labels in the attached diagram are: First temperature zone extraction unit 2-1, first heat exchange medium inlet 2-1a, first heat exchange medium outlet 2-1b, membrane inlet 2-e, extractant outlet 2-c-4. Second temperature zone extraction unit 2-2, second heat exchange medium inlet 2-2a, second heat exchange medium outlet 2-2b The third temperature zone extraction unit 2-3, the third heat exchange medium inlet 2-3a, the third heat exchange medium outlet 2-3b, and the rectifier module 2-3-1; Fourth temperature zone extraction unit 2-4, fourth heat exchange medium inlet 2-4a, fourth heat exchange medium outlet 2-4b, membrane outlet 2-f, extractant inlet 2-c-0; 2-5 drive rollers; Driven rollers 2-6; film 3; Extractant 4; First conveying pipe 2-c-1; second conveying pipe 2-c-2; third conveying pipe 2-c-3. Detailed Implementation

[0036] To better understand the above technical solutions, the following will provide a detailed explanation of the technical solutions in conjunction with the accompanying drawings and specific implementation methods.

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0038] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0039] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0040] In the description of the embodiments of the present invention, it should be understood that the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of the invention is in use, or the orientation or positional relationship commonly understood by those skilled in the art. They are only used to facilitate the description of the present invention and to simplify the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the present invention.

[0041] In the description of the embodiments of the present invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in the present invention based on the specific circumstances. The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and should not be used to limit the scope of protection of the present invention.

[0042] Example 1 See Figure 1 A method for resolving precipitates on the surface of wet-process membranes includes the following steps: S1: Mix the diaphragm base material, functional additives, and high-boiling-point plasticizer, add the composite solvent and dispersant and mix evenly, then melt and extrude at 200℃ to cast sheets to obtain sheet material; S2: The sheet from S1 is cooled and shaped through three stages of cooling; S2.1: The sheet in S1 is cooled in the first stage, from 80°C to 70°C, with the cooling rate controlled at 0.2°C / s and the cooling duration at 70s; S2.2: The sheet cooled in S2.1 is subjected to a second stage of cooling, from 70°C to 50°C, with the cooling rate controlled at 1.5°C / s and the cooling duration at 30s; S2.3: The sheet cooled in S2.1 is subjected to a third stage of cooling, from 50°C to 25°C; the cooling rate is controlled at 4.9°C / s, and the cooling duration is 12s. S3: The sheet treated with S2 is biaxially stretched to obtain a film; S4: The film 3 obtained in S3 is fed from the film inlet 2-e into the first temperature zone extraction unit 2-1 of the four-temperature zone countercurrent extraction device for turbulent extraction, then into the second temperature zone extraction unit 2-2 for pulse extraction, followed by the third temperature zone extraction unit 2-3 for laminar flow extraction, and then into the fourth temperature zone extraction unit 2-4 for ultrasonic extraction. After extraction, it is discharged from the film outlet 2-f, dried, and wound up to obtain the battery separator. The flow direction of the extractant 4 is opposite to the movement direction of the film 3, and the countercurrent ratio of the extractant to the film is 2.5:1. Specifically, the extractant 4 flows from the fourth temperature zone extraction unit through the extractant inlet 2-c-0. The material from the bottom of unit 2-4 enters the fourth temperature zone extraction unit 2-4, then flows out from the top of the fourth temperature zone extraction unit 2-4 through the first conveying pipe 2-c-1 and enters the third temperature zone extraction unit 2-3 from the bottom. Next, it flows out from the top of the third temperature zone extraction unit 2-3 through the second conveying pipe 2-c-2 and enters the second temperature zone extraction unit 2-2. Then, it flows out from the top of the second temperature zone extraction unit 2-2 through the third conveying pipe 2-c-3 and enters the first temperature zone extraction unit 2-1. Finally, it flows out from the top of the first temperature zone extraction unit 2-1 and enters the recovery system for post-processing.

[0043] The membrane base material contains 12.6 wt% ultra-high molecular weight polyethylene and 12.6 wt% high-density polyethylene; functional additives include 0.2 wt% sodium sulfide and 0.2 wt% antioxidant 1010; plasticizers include 0.2 wt% dioctyl terephthalate, dispersant 1.2 wt% and composite solvent 73 wt%.

[0044] The composite solvent is obtained by mixing hydrogenated naphthenic oil KN4016 with isoalkanes isododecane at a mass ratio of 50:50.

[0045] The temperatures of the first temperature zone extraction unit, the second temperature zone extraction unit, the third temperature zone extraction unit, and the fourth temperature zone extraction unit are controlled at 40~45℃, 30~35℃, 20~25℃, and 10~15℃, respectively.

[0046] In this embodiment, dichloromethane is used as the extractant. The Reynolds number Re > 4000 for the turbulent extraction in step S4; the pulse frequency for the pulse extraction is 2 Hz and the amplitude is 5 mm; the laminar flow extraction has a laminar flow spacing of 0.3 mm and a pressure difference of < 0.1 MPa; and the ultrasonic extraction has a power density of 0.5 W / cm² and a frequency of 40 kHz.

[0047] See Figure 2The four-temperature zone countercurrent extraction device is arranged sequentially along the film movement direction as a first temperature zone extraction unit 2-1, a second temperature zone extraction unit 2-2, a third temperature zone extraction unit 2-3, and a fourth temperature zone extraction unit 2-4. The first temperature zone extraction unit 2-1 and the second temperature zone extraction unit 2-2 are connected by a third conveying pipe 2-c-3, the second temperature zone extraction unit 2-2 and the third temperature zone extraction unit 2-3 are connected by a second conveying pipe 2-c-2, and the third temperature zone extraction unit 2-3 and the fourth temperature zone extraction unit 2-4 are connected by a first conveying pipe 2-c-1. The first conveying pipe 2-c-1, the second conveying pipe 2-c-2, and the third conveying pipe 2-c-3 are all used to convey the extractant.

[0048] Each of the first temperature zone extraction unit 2-1, the second temperature zone extraction unit 2-2, the third temperature zone extraction unit 2-3, and the fourth temperature zone extraction unit 2-4 is provided with at least one active roller 2-5 and at least one driven roller 2-6 for transporting the film.

[0049] The first temperature zone extraction unit 2-1 is provided with a thin film inlet 2-e and an extractant outlet 2-c-4.

[0050] The first temperature zone extraction unit 2-1 is provided with a first heat exchange medium inlet 2-1a and a first heat exchange medium outlet 2-1b; the second temperature zone extraction unit 2-2 is provided with a second heat exchange medium inlet 2-2a and a second heat exchange medium outlet 2-2b; the third temperature zone extraction unit 2-3 is provided with a third heat exchange medium inlet 2-3a and a third heat exchange medium outlet 2-3b, and the purpose of temperature control is achieved by heat exchange between the heat exchange medium and the extractant.

[0051] The third temperature zone extraction unit 2-3 is provided with at least two rectifier modules 2-3-1, one rectifier module is close to the extractant inlet and the other rectifier module is close to the extractant outlet. The area between the two rectifier modules 2-3-1 is a laminar flow extraction zone. After the membrane enters the third temperature zone extraction unit 2-3, it passes through the gap between the rectifier module near the membrane inlet and the membrane inlet to enter the laminar flow extraction zone for extraction. Then, it exits from the rectifier module near the extractant inlet and enters the fourth temperature zone extraction unit 2-4.

[0052] The fourth temperature zone extraction unit 2-4 is provided with a thin film outlet 2-f and an extractant inlet 2-c-0.

[0053] Example 2 The steps and structure of the four-zone countercurrent extraction apparatus in Example 2 are the same as in Example 1, except that: The membrane base material contains 12.6 wt% ultra-high molecular weight polyethylene and 12.6 wt% high-density polyethylene; functional additives include 0.2 wt% sodium sulfide and 0.2 wt% antioxidant 1010; plasticizers include 0.2 wt% dioctyl terephthalate, dispersant 0.8 wt%, and composite solvent 73.4 wt%.

[0054] The composite solvent is obtained by mixing hydrogenated naphthenic oil KN4016 with isoalkanes isododecane at a mass ratio of 60:40.

[0055] In the first stage of cooling, the sheet is cooled from 80°C to 70°C at a rate controlled at 0.5°C / s, and the cooling duration is 60s. In the second stage of cooling, the sheet is cooled from 70°C to 50°C at a rate of 2°C / s and a duration of 25s. The third stage of cooling involves cooling the sheet from 50°C to 25°C; the cooling rate is controlled at 5°C / s, and the cooling duration is 10s. In step S4, the Reynolds number Re > 4000 for turbulent extraction; the pulse frequency for pulse extraction is 2 Hz and the amplitude is 5 mm; the laminar flow spacing for laminar flow extraction is 0.4 mm and the pressure difference is < 0.1 MPa; and the power density for ultrasonic extraction is 0.5 W / cm² and the frequency is 40 kHz.

[0056] In step S4, the oil concentration at the membrane inlet is 3.2 g / L, the oil concentration at the membrane outlet is 0.05 g / L, and the extraction rate is as high as 98.4%.

[0057] Example 3 The steps and structure of the four-zone countercurrent extraction apparatus in Example 2 are the same as in Example 1, except that: The membrane base material contains 12.6 wt% ultra-high molecular weight polyethylene and 12.6 wt% high-density polyethylene; functional additives include 0.2 wt% sodium sulfide and 0.2 wt% antioxidant 1010; plasticizers include 0.2 wt% dioctyl terephthalate, dispersant 0.5 wt%, and composite solvent 73.7 wt%.

[0058] The composite solvent is obtained by mixing hydrogenated naphthenic oil KN4016 with isoalkanes isododecane at a mass ratio of 70:30.

[0059] In the first stage of cooling, the sheet is cooled from 80°C to 70°C at a rate controlled at 0.8°C / s, and the cooling duration is 50s. In the second stage of cooling, the sheet is cooled from 70°C to 50°C at a rate controlled at 2.5°C / s, and the cooling duration is 20s. In the third stage of cooling, the sheet is cooled from 50°C to 25°C; the cooling rate is controlled at 5.1°C / s, and the cooling duration is 12s. In step S4, the Reynolds number Re > 4000 for turbulent extraction; the pulse frequency for pulse extraction is 2 Hz and the amplitude is 5 mm; the laminar flow spacing for laminar flow extraction is 0.5 mm and the pressure difference is < 0.1 MPa; and the power density for ultrasonic extraction is 0.5 W / cm² and the frequency is 40 kHz.

[0060] Comparative Example 1 The difference between Comparative Example 1 and Example 2 is that the solvent used is conventional paraffin oil, while the remaining steps and structure are the same as in Example 2.

[0061] Table 1 below shows the performance test results of the diaphragms prepared in Comparative Example 1 and Examples 1-3: Table 1

[0062] Comparative Example 2 The difference between Comparative Example 2 and Example 2 is that the amount of ultra-high molecular weight polyethylene added to the diaphragm base material is 12.6 wt%, the amount of high density polyethylene added is 12.6 wt%, the amount of functional additive sodium sulfide is 0.2 wt%, the amount of antioxidant 1010 is 0.2 wt%, the amount of plasticizer dioctyl terephthalate is 0.2 wt%, and the amount of composite solvent is 74.2 wt%; the remaining steps and structures are the same as in Example 2.

[0063] Table 2 below shows the performance test results of the diaphragms prepared in Comparative Example 2 and Examples 1-3: Table 2

[0064] Comparative Example 3 The difference between Comparative Example 3 and Example 2 lies in step S2, which is: the sheet in S1 is cooled and shaped by cooling the sheet from 80°C to 25°C at a cooling rate of 2°C / s; the remaining steps and structures are the same as in Example 2.

[0065] Table 3 below shows the performance test results of the diaphragms prepared in Comparative Example 2 and Examples 1-3: Table 3

[0066] As can be seen from Tables 1-3 above, the membranes prepared by the methods of Examples 1-3 have better inhibition and removal effects on precipitates.

[0067] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for solving the problem of precipitates on the surface of wet-process diaphragm membranes, characterized in that, Includes the following steps: S1: Mix the diaphragm base material, functional additives, and high-boiling-point plasticizer, add the composite solvent and dispersant, mix evenly, melt, extrude, and cast into sheets to obtain the sheet material; S2: The sheet from S1 is cooled and shaped through three stages of cooling; S3: The sheet treated with S2 is biaxially stretched to obtain a film; S4: The thin film obtained in S3 is extracted, dried and wound up using a four-temperature zone countercurrent extraction device to obtain a battery separator.

2. The method for solving the precipitates on the surface of a wet-process diaphragm according to claim 1, characterized in that, The composite solvent is obtained by mixing hydrogenated naphthenic oil and isoalkanes in a mass ratio of 50~70:30~50.

3. The method for solving the problem of precipitates on the surface of wet-process diaphragm membranes according to claim 1, characterized in that, The dispersant is polyoxyethylene sorbitan monooleate.

4. The method for solving the precipitates on the surface of a wet-process diaphragm according to claim 3, characterized in that, The amount of dispersant added is 0.5~1.2wt%.

5. The method for solving the precipitates on the surface of a wet-process diaphragm according to claim 1, characterized in that, The specific steps of step (2) are as follows: S2.1: The sheet in S1 is cooled in the first stage, from 80℃±0.5℃ to 70℃±0.5℃, with the cooling rate controlled at 0.2~0.8℃ / s and the cooling duration at 50~70s; S2.2: The sheet cooled in S2.1 is subjected to a second stage of cooling, from 70℃±0.5℃ to 50℃±0.5℃, with the cooling rate controlled at 1.5~2.5℃ / s and the cooling duration at 20~30s; S2.3: The sheet cooled in S2.1 is subjected to a third stage of cooling, from 50℃±0.5℃ to 25℃±0.5℃; the cooling rate is controlled at 4.9~5.1℃ / s, and the cooling duration is 8~12s.

6. The method for solving the precipitates on the surface of a wet-process diaphragm according to claim 1, characterized in that, The four-temperature-zone countercurrent extraction device includes a first-temperature-zone extraction unit, a second-temperature-zone extraction unit, a third-temperature-zone extraction unit, and a fourth-temperature-zone extraction unit. The flow direction of the extractant in the four-temperature-zone countercurrent extraction device is opposite to the direction of film movement. The film sequentially passes through the first-temperature-zone extraction unit, the second-temperature-zone extraction unit, the third-temperature-zone extraction unit, and the fourth-temperature-zone extraction unit. The extractant sequentially flows through the fourth-temperature-zone extraction unit, the third-temperature-zone extraction unit, the second-temperature-zone extraction unit, and the first-temperature-zone extraction unit.

7. The method for solving the precipitates on the surface of a wet-process diaphragm according to claim 6, characterized in that, The countercurrent ratio of the extractant to the membrane is 2.4~2.6:

1.

8. The method for solving the precipitates on the surface of a wet-process diaphragm according to claim 6, characterized in that, The temperatures of the first temperature zone extraction unit, the second temperature zone extraction unit, the third temperature zone extraction unit, and the fourth temperature zone extraction unit are controlled at 40~45℃, 30~35℃, 20~25℃, and 10~15℃, respectively.

9. The method for solving the precipitates on the surface of a wet-process diaphragm according to claim 6, characterized in that, The extraction steps in step S4 are as follows: the film obtained in S3 is sent to the first temperature zone extraction unit for turbulent extraction, then to the second temperature zone extraction unit for pulse extraction, then to the third temperature zone extraction unit for laminar flow extraction, and finally to the fourth temperature zone extraction unit for ultrasonic extraction.

10. The method for solving the precipitates on the surface of a wet-process diaphragm according to claim 9, characterized in that, The Reynolds number Re > 4000 for the turbulent extraction; the pulse frequency is 2 Hz and the amplitude is 5 ± 0.5 mm; the laminar flow spacing is 0.3 ~ 0.5 mm; the ultrasonic power density is 0.5 W / cm² and the frequency is 35 ~ 45 kHz.

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