Preparation method for porous polymer film, and porous polymer film and use thereof

Abstract

The present application belongs to the technical field of the preparation of separator materials, and specifically relates to a preparation method for a porous polymer film, and a porous polymer film and the use thereof. The method comprises: (1) acquiring a thin film containing a polymer and a plasticizer; (2) extracting the plasticizer from the thin film by using an extraction liquid so as to obtain a wet film; and (3) drying the wet film, wherein the dimensional shrinkage rate of the wet film in a TD direction during drying is 5%-20%. The porous polymer film prepared by using the method has high-temperature dimensional stability, and good air permeability and thickness uniformity.

Description

A method for preparing a polymer porous membrane, the polymer porous membrane, and its application. Technical Field

[0001] This application belongs to the field of membrane material preparation technology, specifically relating to a method for preparing a polymer porous membrane, the polymer porous membrane and its application, and more specifically, a method for preparing a porous membrane with good high-temperature dimensional stability, the porous membrane with good high-temperature dimensional stability and its application. Background Technology

[0002] Currently, wet-process lithium-ion battery separator products suffer from poor dimensional stability at high temperatures. This poor dimensional stability causes stress release after the separator is wound up, leading to poor winding quality, wrinkling, and cracking of the separator. This results in poor slitting quality, changes in the size and appearance after slitting, and other quality problems, which may even have a significant impact on the safety performance of the battery.

[0003] Existing technical approaches to improve poor high-temperature dimensional stability include using specific high molecular weight polyethylene (PE) and adjusting the stretch ratio, heat setting methods, or coating the base film surface. The first method reduces the strength of the diaphragm and causes excessive energy consumption, while the second method makes it difficult to balance dimensional stability with diaphragm permeability and pore structure.

[0004] Summary of the Invention

[0005] This application provides a method for preparing a polymer porous membrane with good high-temperature dimensional stability, the polymer porous membrane, and its application.

[0006] Therefore, this application provides the following technical solution.

[0007] The first aspect of this application provides a method for preparing a polymer porous membrane, comprising the following steps:

[0008] (1) Obtain a film containing polymer and plasticizer;

[0009] (2) The plasticizer in the film is extracted using an extraction solution to obtain a wet film;

[0010] (3) Dry the wet film, wherein the dimensional shrinkage rate of the wet film in the TD direction during the drying process is 5% to 20%. Wherein, the dimensional shrinkage rate refers to the percentage of the difference between the dimensional size of the film obtained in step (1) in the TD direction and the dimensional size of the wet film after drying in step (3) in the TD direction, compared with the dimensional size of the film obtained in step (1) in the TD direction.

[0011] Controlling the dimensional shrinkage rate of the wet film in the TD direction during drying to 5-20% can effectively reduce the thermal shrinkage rate of the polymer porous membrane without adjusting other process parameters, while also ensuring good air permeability and thickness uniformity. When the dimensional shrinkage rate of the wet film in the TD direction exceeds 20%, the pore structure collapses, affecting the air permeability of the final membrane and consequently its ion conductivity. Excessive dimensional shrinkage in the TD direction also affects the thickness uniformity in that direction. When the dimensional shrinkage rate of the membrane in the TD direction is less than 5%, stress release is insufficient, and the thermal shrinkage rate of the polymer porous membrane cannot be effectively reduced.

[0012] Optionally, the dimensional shrinkage rate of the wet film in the TD direction during the drying process is 5% to 15%. For example, it can be a range of 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, or any combination thereof.

[0013] Controlling the dimensional shrinkage rate of the wet film in the TD direction during the drying process to 5-15% can further facilitate the achievement of polymer porous membranes with good thermal shrinkage rate, air permeability and thickness uniformity.

[0014] In this application, there are no particular restrictions on the method for controlling the dimensional shrinkage rate of the wet film in the TD direction during the drying process. As long as the above-mentioned characteristic range can be met, the manufacturing method corresponding to the purpose can be freely selected.

[0015] To better control the dimensional shrinkage rate of the wet film in the TD direction during the drying process, as an optional technical solution of this application, the extraction liquid is defined to include an extractant, wherein the saturated vapor pressure of the extractant at 25°C is ≤10 kPa; thereby, the drying rate of the wet film during the drying process can be controlled, avoiding the extractant drying too quickly during the drying stage, which would cause the film to shrink too quickly and the shrinkage rate to be too large, resulting in the collapse of the pore structure and affecting the performance of the final polymer porous membrane.

[0016] For safety reasons, the extractant may optionally satisfy the following conditions: flash point temperature ≥ 45℃ at room temperature and pressure or no flash point at 25℃ and one standard atmosphere.

[0017] Optionally, the extractant is at least one selected from water, decane, laurylane, and 2,5-dichlorotoluene.

[0018] This application does not impose any special restrictions on the drying temperature; the commonly used temperature in this field can be freely selected according to the purpose to be achieved.

[0019] To better control the drying rate of the wet film and further ensure the drying effect, and to synergistically control the dimensional shrinkage rate of the film in the TD direction, the drying temperature is optionally 30℃~90℃;

[0020] Further optionally, the drying temperature is 40°C to 80°C. For example, it can be a range of 40°C, 45°C, 48°C, 50°C, 53°C, 58°C, 60°C, 63°C, 68°C, 70°C, 75°C, 80°C, or any combination thereof.

[0021] As another optional technical solution of this application, the extraction liquid is defined as an ionic liquid;

[0022] Adding ionic liquids to the extractant can better control the dimensional shrinkage rate of the wet membrane in the TD direction during drying. When the membrane is immersed in the extractant, the ionic liquid adheres to the membrane surface or partially enters the micropores of the membrane. Due to the interaction forces of the ionic liquid, it provides the membrane with a force that counteracts shrinkage during drying, thereby effectively controlling the dimensional shrinkage rate of the wet membrane in the TD direction. In addition, the addition of ionic liquids can further improve the ionic conductivity of the membrane.

[0023] It should be noted that, in addition to ionic liquids, the extractant may also include other extractants, such as dichloromethane, which are commonly used in the art; or the extractants mentioned above in this application. Optionally including the extractants mentioned above in this application can better synergize with ionic liquids to achieve better physical properties of the polymer porous membrane, such as thermal shrinkage rate, thickness uniformity, and air permeability, as well as superior ionic conductivity.

[0024] To mitigate the adverse effects of chelation between ionic liquids and electrolyte components, and to achieve a polymer porous membrane with both improved physical properties and ionic conductivity, as an optional technical solution in this application, the mass fraction of ionic liquid in the wet membrane is 0.1 wt% to 6 wt%, based on the mass of the dried wet membrane; for example, it can be a range of 0.1 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, 1.5 wt%, 1.8 wt%, 2.0 wt%, 2.2 wt%, 2.5 wt%, 2.8 wt%, 3.0 wt%, 3.2 wt%, 3.5 wt%, 3.8 wt%, 4.0 wt%, 4.5 wt%, 4.8 wt%, 5.0 wt%, 5.4 wt%, 5.8 wt%, 6.0 wt%, or any combination thereof.

[0025] Optionally, based on the mass of the dried wet membrane, the mass fraction of the ionic liquid in the wet membrane is 0.3 wt% to 1 wt%. For example, it can be a range of 0.3 wt%, 0.35 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1.0 wt%, or any combination thereof. Further controlling the ionic liquid content after drying the wet membrane better avoids adverse reactions between the ionic liquid and the electrolyte components, thereby better achieving the physical properties and ionic conductivity of the polymer porous membrane.

[0026] The drying temperature is not specifically limited; it can be adjusted according to the ionic liquid content to meet the conditions for the wet film TD dimensional shrinkage rate during the drying process. As an optional technical solution in this application, the drying temperature is 30℃~90℃.

[0027] Further optionally, the drying temperature is 40°C to 80°C. For example, it can be a range of 40°C, 45°C, 48°C, 50°C, 53°C, 58°C, 60°C, 63°C, 68°C, 70°C, 75°C, 80°C, or any combination thereof.

[0028] This application does not specifically limit the extraction method, but the number of extractions includes at least one extraction; this application may optionally perform multiple extractions, such as two or more extractions, three or more extractions, etc.

[0029] To better control the ionic liquid content after the wet film is dried, this application optionally specifies that the ionic liquid content in the extract does not exceed 6.5 wt% during the final extraction. For example, it can be a range of 6 wt%, 5.5 wt%, 5 wt%, 4.8 wt%, 4.5 wt%, 4 wt%, 3.8 wt%, 3.5 wt%, 3 wt%, 2.8 wt%, 2.5 wt%, 2 wt%, 1.9 wt%, 1.8 wt%, 1.5 wt%, 1.2 wt%, 1 wt%, 0.8 wt%, 0.5 wt%, 0.3 wt%, 0.1 wt%, or any combination thereof.

[0030] This application does not specifically limit the type of ionic liquid, and the type can be freely selected according to the intended purpose. Ionic liquids described in this application include at least one of the following: imidazole ionic liquids, pyridine ionic liquids, alkyl sulfonic acid ionic liquids, quaternary ammonium ionic liquids, quaternary phosphorus ionic liquids, pyrrolidine ionic liquids, and piperidine ionic liquids.

[0031] To better enable the application of ionic liquids in the extraction solution to achieve good membrane extraction effect, the ionic liquid may optionally be at least one selected from 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium acetate, sodium 1-octadecyl sulfonate, sodium 1-pentadecanyl sulfonate, 1-ethyl-3-methylimidazolium difluorosulfonylimide, 1-butyl-3-methyl difluorosulfonylimide, 1-ethyl-3-methyl ditrifluorosulfonylimide, 1-butyl-3-methyl ditrifluorosulfonylimide, dodecyl quaternary ammonium salt, octadecyl quaternary ammonium salt, 1-ethyl-3-methyl ditrifluorosulfonylimide salt imidazolium dinitrile salt, and N-alkylpyridine.

[0032] To ensure that the dimensional shrinkage rate in the TD direction during the wet film drying process is met, this application does not impose specific restrictions on other preparation methods and steps for polymer porous membranes, as long as the raw materials and other preparation methods and steps corresponding to the purpose can be freely selected according to the well-known or conventional operations in the field.

[0033] To better prepare polymer porous membranes with good overall performance, this application provides the following optional technical solutions.

[0034] Optionally, the polymer is a homopolymer, copolymer, or mixture of polymers thereof of at least one of propylene, ethylene, butene, pentene, methyl methacrylate, tetrafluoroethylene, and difluoroethylene.

[0035] Optionally, the polymer is at least one of polyethylene, polypropylene, and ethylene-propylene copolymer. Step (1) specifically includes: melt-blending the polymer and plasticizer, stretching, to obtain a film containing the polymer and plasticizer;

[0036] Optionally, the mass ratio of the polymer to the plasticizer is (15-50):(85-50);

[0037] Optionally, the temperature of the melt mixing is not lower than 160°C;

[0038] Optionally, the temperature of the melt mixing is not lower than 180°C;

[0039] Optionally, the temperature of the melt mixing is not higher than 300°C;

[0040] Optionally, the temperature of the melt mixing is not higher than 250°C;

[0041] Optionally, the stretching ratio is 15 to 40 times during the stretching process;

[0042] Optionally, during the stretching process, the stretching temperature is not lower than 60°C;

[0043] Optionally, during the stretching process, the stretching temperature is not lower than 80°C;

[0044] Optionally, the stretching temperature is not higher than 200°C during the stretching process;

[0045] Optionally, the stretching temperature is not higher than 140°C during the stretching process;

[0046] Optionally, the stretching ratio in the MD direction is 5 to 10 times, and the stretching ratio in the TD direction is 8 to 12 times.

[0047] After drying the wet film, step (3) also includes a shaping step;

[0048] Optionally, the shaping temperature is 90–135°C, and the shrinkage rate of the shaping is not higher than 0.9.

[0049] Optionally, the shaping shrinkage rate is not higher than 0.8.

[0050] In step (1), the stretching method is not specifically limited. For example, MD uniaxial stretching using a roller stretching machine, TD uniaxial stretching using a tenter frame, successive biaxial stretching using a combination of a roller stretching machine and a tenter frame, or simultaneous biaxial stretching using a simultaneous biaxial tenter frame or blow molding, etc. Here, the MD direction refers to the direction of film movement during diaphragm preparation; the TD direction refers to the direction perpendicular to the direction of film movement during diaphragm preparation.

[0051] In step (1), the polymer and plasticizer are melt-blended, cooled, and stretched to obtain a film containing the polymer and plasticizer. The cooling method is not specifically limited. For example, it can be a method of direct contact with cooling media such as cold air or cooling water, or a method of contact with rollers or extruders cooled by cooling media. From the point of view of excellent thickness control, the method of contact with rollers cooled by cooling media can be selected.

[0052] Plasticizers are organic compounds that form a homogeneous solution with polyolefins at temperatures below their boiling point. Examples include decahydronaphthalene, xylene, dioctyl phthalate, dibutyl phthalate, stearyl alcohol, oleyl alcohol, decanol, nonanol, diphenyl ether, n-decane, n-dodecane, and paraffin oil. Paraffin oil and dioctyl phthalate are optional.

[0053] In step (3), this application does not specify the extraction method, as long as the plasticizer in the film can be extracted. The extraction method can be single-stage extraction or multi-stage extraction. The extraction method that this application can choose is multi-stage overflow method.

[0054] Multi-stage overflow methods can be, but are not limited to, the listed methods. For example, multi-stage overflow methods include: having at least three gradients of extractant along the direction of film movement, which can reduce the circulation flow rate of the extractant and avoid excessive impact tension on the film surface caused by the flow of extractant, resulting in film deformation and adverse effects such as pore structure and thermal shrinkage of the final product. In addition, it can effectively save energy consumption costs in the extraction stage; wherein, during extraction, the overflow direction of the extractant is opposite to the direction of film movement.

[0055] This application does not impose a specific limitation on the extraction temperature, which can be selected according to the requirements for extracting plasticizers from the film. For example, the extraction temperature can be, but is not limited to, 30–55°C. Specifically, this application allows for extraction temperatures along the film's movement direction to be no higher than those of the preceding extraction. In step-by-step extraction, the temperature of each subsequent extraction is controlled to be no higher than that of the preceding extraction. As the plasticizer is extracted, the gradual reduction of plasticizer within the film causes the overall film structure to lose support. Therefore, the extraction temperature is adjusted accordingly as the amount of plasticizer inside the film decreases, better controlling the film's thermal shrinkage and thus better maintaining the film's microporous structure. Simultaneously, a suitable extraction temperature ensures better extraction effect and efficiency, enabling better and faster extraction of plasticizers from the film's interior.

[0056] Furthermore, the concentration of the ionic liquid in the extract of the subsequent extraction is not higher than the concentration of the ionic liquid in the extract of the previous extraction.

[0057] Optionally, step (2) may further include a first spraying step using a first spraying liquid, wherein the first spraying liquid includes the ionic liquid described in this application, during the extraction.

[0058] It should be noted that after extraction in step (2) and before drying in step (3), other steps can be added as needed, such as water washing, wetting, cross-linking, etc.

[0059] Optionally, step (2) further includes a water washing step after extraction. The washing solution includes an ionic liquid and a washing agent, the washing agent includes water, and the ionic liquid is the ionic liquid described in this application. The concentration of the ionic liquid in the washing solution is not higher than 10 wt%. A concentration of not more than 10 wt% in the washing solution can effectively control the diffusion rate of the ionic liquid extractant from the film into the washing solution, ensuring washing efficiency and washing quality.

[0060] Optionally, during the washing process, a multi-stage washing method can be adopted, wherein the concentration of the ionic liquid in the washing solution of each subsequent wash is no higher than 30 wt% of the concentration of the ionic liquid in the washing solution of the previous wash. Controlling the concentration of the ionic liquid in the washing solution to decrease sequentially can better control the washing efficiency. Furthermore, controlling the concentration of the ionic liquid in the washing solution of each subsequent wash to no higher than 30 wt% of the concentration of the ionic liquid in the washing solution of the previous wash can better control the residual content of the ionic liquid in the membrane while maintaining high washing efficiency. Secondly, it can also better avoid the collapse of the microporous structure caused by the ionic liquid extractant in the membrane being washed too quickly, thus affecting the quality of the microporous membrane.

[0061] Optionally, the water washing process may further include a step of using a second spray liquid, which includes water, for a second spraying.

[0062] Before performing step (3) of drying the wet membrane, a wetting step may be included, in which the wetting solution is applied. The type of wetting solution can be freely selected according to the desired purpose. For example, to better control the ionic liquid content of the polymer porous membrane, the wetting solution may include the ionic liquid described in this application and water, wherein the concentration of the ionic liquid in the wetting solution is 0.05–5 wt%. For example, to achieve modification or crosslinking of the porous membrane, the wetting solution may include commonly used modifying agents, initiators, and crosslinking agents in the art. Optionally, a crosslinking step may be performed after this wetting step to better achieve modification or crosslinking of the porous membrane.

[0063] A second aspect of this application provides a polymer porous membrane prepared by the above-described preparation method;

[0064] The polymer porous membrane exhibits a shrinkage rate ≤2.5% in the TD direction under heat treatment at 105℃ / 1H.

[0065] Optionally, the shrinkage rate of the polymer porous membrane in the TD direction is ≤2% under heat treatment at 105℃ / 1H;

[0066] Optionally, the shrinkage rate of the polymer porous membrane in the MD direction is ≤3% under heat treatment at 105℃ / 1H;

[0067] Optionally, the thickness difference of the polymer porous membrane in the TD direction is ≤1μm.

[0068] A third aspect of this application provides a battery comprising a separator, the separator comprising a polymer porous membrane prepared by the above-described preparation method or the above-described polymer porous membrane.

[0069] The technical solution of this application has the following advantages:

[0070] 1. The method for preparing a polymer porous membrane provided in this application includes (1) obtaining a film containing a polymer and a plasticizer; (2) extracting the plasticizer from the film using an extractant to obtain a wet film; and (3) drying the wet film, wherein the dimensional shrinkage rate of the wet film in the TD direction during the drying process is 5% to 20%. The polymer porous membrane prepared by this method has high-temperature dimensional stability and good air permeability and thickness uniformity. In preparing the polymer porous membrane, extracting the plasticizer from the film to obtain a wet film, drying the wet film, and controlling the dimensional shrinkage rate of the wet film in the TD direction to be 5% to 20% during the drying process can effectively reduce the thermal shrinkage rate of the polymer porous membrane without adjusting other process parameters, and make it have good air permeability and thickness uniformity.

[0071] 2. The method for preparing the polymer porous membrane provided in this application controls the saturated vapor pressure of the extractant at 25°C to be ≤10 kPa, which can control the drying rate of the wet membrane during the drying process and avoid the extractant drying rate being too fast during the drying stage, which would cause the membrane to shrink too quickly and have too large a shrinkage rate, resulting in the collapse of the pore structure and affecting the final membrane performance.

[0072] 3. The method for preparing the polymer porous membrane provided in this application, by adding an ionic liquid to the extraction solution, can better control the dimensional shrinkage rate of the membrane in the TD direction. When the membrane is immersed in the extraction solution, the ionic liquid adheres to the surface of the membrane or partially enters the micropores of the membrane. Due to the interaction forces of the ionic liquid, it provides the membrane with a force that counteracts shrinkage during the drying process, thereby effectively controlling the dimensional shrinkage rate of the membrane in the TD direction. In addition, the addition of the ionic liquid can further improve the ionic conductivity of the membrane. Attached Figure Description

[0073] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0074] Figure 1 is a scanning electron microscope image (magnification 20K) of the polymer porous membrane in Example 1 of this application. Detailed Implementation

[0075] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0076] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0077] Example 1

[0078] This embodiment provides a method for preparing a polymer porous membrane, including the following steps:

[0079] (1) Take polyethylene resin (weight average molecular weight 90W) and liquid paraffin oil (kinematic viscosity of 45 mmHg at 40℃) in a mass ratio of 23:77. 2 The mixture is added to a twin-screw extruder and melt-blended to form a melt. The extruder temperature is 200±5℃. The melt is extruded through a die and then cast and cooled at 20℃ to form a precursor film. The precursor film is then subjected to MD stretching and TD stretching to obtain a polyethylene film containing paraffin oil. The MD stretching temperature is 95℃ and the MD stretching ratio is 6 times. The TD stretching temperature is 110℃ and the TD stretching ratio is 8.5 times. The width of the stretched film is 4800mm.

[0080] (2) The plasticizer paraffin oil in the film from step (1) is extracted using a multi-stage overflow method to obtain a wet film; the polyethylene film containing paraffin oil is passed into an extraction tank to extract the paraffin oil in the film; wherein, along the direction of film movement, the multi-stage overflow method includes extraction tank 1, extraction tank 2, and extraction tank 3, with the temperatures of each extraction tank being 50℃, 40℃, and 35℃ respectively; the extract in extraction tanks 1-3 includes water and ionic liquid, and the concentrations of the ionic liquid in the extract in extraction tanks 1-3 are 5±0.2wt%, 2±0.2wt%, and 1±0.1wt% respectively, and the ionic liquid is 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid. The content of the ionic liquid is 0.8wt%, based on the total mass of polymer and ionic liquid in the wet film (i.e., the mass of the wet film after drying).

[0081] (3) Drying the wet film obtained in step (2); wherein the temperature of the wet film obtained in step (2) is 40℃, the width of the dried wet film is 4580mm, and the dimensional shrinkage rate in the TD direction is 5%. Then, it is shaped at a temperature of 110℃ with a shrinkage ratio of 0.7, and then wound up after cooling to room temperature.

[0082] The above method yields a polymer porous membrane with a thickness of 12 μm and a porosity of 38%.

[0083] Example 2

[0084] This embodiment provides a method for preparing a polymer porous membrane, including the following steps:

[0085] (1) Same as Example 1.

[0086] (2) Same as Example 1.

[0087] (3) The difference from Example 1 is that the drying temperature is 50°C, the width of the wet film after drying is 4320 mm, and the dimensional shrinkage rate in the TD direction is 10%.

[0088] The polymer porous membrane with a thickness of 12 μm and a porosity of 38% was prepared by the above method.

[0089] Example 3

[0090] This embodiment provides a method for preparing a polymer porous membrane, including the following steps:

[0091] (1) Same as Example 1.

[0092] (2) The difference from Example 1 is that the extraction tank is a single tank, the extraction liquid in the extraction tank includes water and ionic liquid, the concentration of ionic liquid in the extraction liquid is 1±0.1wt%, the ionic liquid is 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid, and the content of ionic liquid in the wet film is 0.8wt% based on the total mass of polymer and ionic liquid in the wet film (i.e. the mass of the wet film after drying).

[0093] (3) The difference from Example 1 is that the temperature of the wet film obtained in drying step (2) is 80°C, the width of the dried wet film is 4080 mm, and the dimensional shrinkage rate in the TD direction is 15%.

[0094] The above method yields a polymer porous membrane with a thickness of 12 μm and a porosity of 38%.

[0095] Example 4

[0096] This embodiment provides a method for preparing a polymer porous membrane, including the following steps:

[0097] (1) Same as Example 1.

[0098] (2) Difference from Example 1: The concentration of the ionic liquid in the extraction tank 3 is 0.5±0.1%, and the content of the ionic liquid is 0.3wt% based on the total mass of polymer and ionic liquid in the wet film (i.e. the mass of the wet film after drying).

[0099] (3) Difference from Example 1: The drying temperature is 80°C, the width of the dried wet film is 3850 mm, and the dimensional shrinkage rate in the TD direction is 20%.

[0100] The above method yields a polymer porous membrane with a thickness of 12 μm and a porosity of 38%.

[0101] Example 5

[0102] This embodiment provides a method for preparing a polymer porous membrane, including the following steps:

[0103] (1) Same as Example 1;

[0104] (2) The difference from Example 1 is that the extractant in extraction tanks 1-3 is water and does not contain ionic liquid. The polyethylene film containing paraffin oil is passed into the extraction tank to extract the paraffin oil in the film and extract the plasticizer paraffin oil in the film to obtain a wet film.

[0105] (3) The difference from Example 1 is that the drying temperature is 50°C, the width of the dried wet film is 4080 mm, and the dimensional shrinkage rate in the TD direction is 15%.

[0106] The above method yields a polymer porous membrane with a thickness of 12 μm and a porosity of 38%.

[0107] Example 6

[0108] This embodiment provides a method for preparing a polymer porous membrane, including the following steps:

[0109] (1) Same as Example 1.

[0110] (2) The difference from Example 1 is that the multi-stage overflow method includes extraction tank 1 and extraction tank 2. The extract in extraction tank 1-2 includes dichloromethane and ionic liquid. The concentration of ionic liquid in the extract in extraction tank 1-2 is 8±0.2wt% and 6±0.2wt% respectively. Based on the total mass of polymer and ionic liquid in the wet film (i.e. the mass of the wet film after drying), the content of ionic liquid in the wet film is 5.8wt%.

[0111] (3) The difference from Example 1 is that the drying temperature is 50°C, the width of the wet film after drying is 4320 mm, and the dimensional shrinkage rate in the TD direction is 10%.

[0112] The above method yields a polymer porous membrane with a thickness of 12 μm and a porosity of 38%.

[0113] Example 7

[0114] This embodiment provides a method for preparing a polymer porous membrane, including the following steps:

[0115] (1) Same as Example 1.

[0116] (2) Difference from Example 1: The extractant in extraction tanks 1-3 includes dichloromethane and ionic liquid. The concentrations of the ionic liquid in the extractant in extraction tanks 1-3 are 10±0.2wt%, 8±0.2wt%, and 5.5±0.2wt%, respectively. Based on the total mass of the polymer and ionic liquid in the wet film (i.e., the mass of the wet film after drying), the content of the ionic liquid in the wet film is 5.4wt%.

[0117] (3) The difference from Example 1 is that the temperature of the wet film obtained in drying step (2) is 60°C, the width of the dried wet film is 4080 mm, and the dimensional shrinkage rate in the TD direction is 15%.

[0118] The above method yields a polymer porous membrane with a thickness of 12 μm and a porosity of 38%.

[0119] Example 8

[0120] This embodiment provides a method for preparing a polymer porous membrane, including the following steps:

[0121] (1) Same as Example 1.

[0122] (2) Difference from Example 1: 2,5-dichloromethane was used as the extractant instead of the water in the three extraction tanks in Example 1, and everything else was the same.

[0123] (3) Difference from Example 1: The drying temperature is 70°C, the width of the dried wet film is 4080 mm, and the dimensional shrinkage rate in the TD direction is 15%.

[0124] The above method yields a polymer porous membrane with a thickness of 12 μm and a porosity of 38%.

[0125] Comparative Example 1

[0126] This comparative example provides a method for preparing a polymer porous membrane, comprising the following steps:

[0127] (1) Same as Example 1.

[0128] (2) The difference from Example 1 is that the extract in extraction tanks 1-3 includes dichloromethane and ionic liquid. The concentrations of ionic liquid in the extract in extraction tanks 1-3 are 12±0.2wt%, 9±0.2wt%, and 6.5±0.2wt%, respectively. Based on the total mass of polymer and ionic liquid in the wet film (i.e., the mass of the wet film after drying), the content of ionic liquid in the wet film is 6wt%.

[0129] (3) Drying the wet film obtained in step (2); wherein the drying temperature is 50℃, the width of the dried wet film is 4650mm, and the dimensional shrinkage rate in the TD direction is 3%.

[0130] The above method yields a polymer porous membrane with a thickness of 12 μm and a porosity of 38%.

[0131] Comparative Example 2

[0132] This comparative example provides a method for preparing a polymer porous membrane, comprising the following steps:

[0133] (1) Same as Example 1.

[0134] (2) The difference from Example 1 is that the extractant in the three extraction tanks is dichloromethane, which does not contain ionic liquid. The polyethylene film containing paraffin oil is passed into the extraction tanks 1-3 to extract the paraffin oil in the film and obtain a wet film.

[0135] (3) The difference from Example 1 is that the wet film obtained in drying step (2) is dried at a temperature of 20°C, the width of the dried wet film is 4650 mm, and the dimensional shrinkage rate in the TD direction is 3%.

[0136] The above method yields a polymer porous membrane with a thickness of 12 μm and a porosity of 38%.

[0137] Comparative Example 3

[0138] This comparative example provides a method for preparing a polymer porous membrane, comprising the following steps:

[0139] (1) Same as Example 1;

[0140] (2) The difference from Example 1 is that the extractant in the three extraction tanks is dichloromethane, which does not contain ionic liquid. The polyethylene film containing paraffin oil is passed into the three extraction tanks to extract the paraffin oil in the film and obtain a wet film.

[0141] (3) The difference from Example 1 is that the wet film obtained in drying step (2) is dried at a temperature of 65°C, the width of the dried wet film is 3600 mm, and the dimensional shrinkage rate in the TD direction is 25%. The above method produces a polymer porous membrane with a thickness of 12 μm and a porosity of 38%.

[0142] Comparative Example 4

[0143] This comparative example provides a method for preparing a polymer porous membrane, comprising the following steps:

[0144] (1) Same as Example 5.

[0145] (2) Same as Example 5.

[0146] (3) The difference from Example 5 is that the wet film obtained in drying step (2) is dried at a temperature of 95°C, the width of the dried wet film is 3600 mm, and the dimensional shrinkage rate in the TD direction is 25%.

[0147] The above method yields a polymer porous membrane with a thickness of 12 μm and a porosity of 38%.

[0148] Performance testing

[0149] The polymer porous membranes in the examples and comparative examples were subjected to the following performance tests, and the test results are shown in Table 1.

[0150] 1. Heat shrinkage, thickness, porosity, air permeability and ionic conductivity: Tested in accordance with the provisions of GB / T 36363-2018.

[0151] 2. Thickness range in the TD direction: Along the TD direction of the porous membrane, at intervals of at least 1 cm, perform thickness tests according to the above method, testing at least 5 points. The difference between the maximum and minimum thickness values ​​obtained is the thickness range in the TD direction.

[0152] Table 1. Performance results of porous membranes in each embodiment and comparative example.

[0153] Figure 1 is an electron microscope image of the polymer porous membrane prepared in Example 1. It can be seen that Example 1 prepared a polymer porous membrane with uniform pore distribution.

[0154] As can be seen from Table 1, the polymer porous membrane prepared in this application has good thermal shrinkage rate, air permeability and thickness uniformity.

[0155] Furthermore, based on the experimental results of Comparative Examples 1-4, it can be seen that when the dimensional shrinkage rate of the wet film in the TD direction during the drying process is less than 5% or more than 20%, it will affect the ionic conductivity of the polymer porous membrane. The shrinkage rate is relatively high at 105℃ / 1H, resulting in decreased high-temperature dimensional stability. When the shrinkage rate of the wet film in the TD direction is greater than 20%, the pore structure collapses, affecting the air permeability of the polymer porous membrane, and thus affecting the ionic conductivity. Moreover, excessive dimensional shrinkage of the wet film in the TD direction will also affect the thickness uniformity in the TD direction. When the shrinkage rate of the wet film in the TD direction is less than 5%, the stress release is insufficient, and the thermal shrinkage rate of the polymer porous membrane cannot be effectively reduced.

[0156] Example 4 shows that by controlling the drying temperature and ionic liquid content, the shrinkage rate of the wet film in the TD direction is 20%. From the results of Example 4, when the shrinkage rate of the wet film in the TD direction is 20%, compared with Examples 1-3, the air permeability, thickness uniformity and shrinkage rate at 105℃ / 1H of the polymer porous membrane all decrease to varying degrees, indicating that the dimensional shrinkage rate of the wet film in the TD direction during the drying process can be selected from 5% to 15%.

[0157] Based on the results of Examples 3 and 5, adding an ionic liquid to the extract can further reduce the heat recovery rate of the polymer porous membrane under the condition that the dimensional shrinkage rate in the TD direction is the same during the wet film drying process.

[0158] As can be seen from Examples 6-7, when dichloromethane is used as the extractant, by coordinating the control of the ionic liquid content and drying temperature to regulate the dimensional shrinkage rate of the wet film in the TD direction during the drying process, the resulting polymer porous material also has good air permeability, thickness uniformity, physical properties such as shrinkage rate at 105℃ / 1H, and ionic conductivity.

[0159] The experimental results above show that the polymer porous membrane prepared by the method of this application has high-temperature dimensional stability, as well as good air permeability and thickness uniformity. Furthermore, the addition of ionic liquid can further improve the ionic conductivity of the membrane.

[0160] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a polymer porous membrane, characterized in that, Includes the following steps: (1) Obtain a film containing polymer and plasticizer; (2) The plasticizer in the film is extracted using an extraction solution to obtain a wet film; (3) Drying the wet film, wherein the dimensional shrinkage rate of the wet film in the TD direction during the drying process is 5% to 20%.

2. The preparation method according to claim 1, characterized in that, During the drying process, the dimensional shrinkage rate of the wet film in the TD direction is 5% to 15%.

3. The preparation method according to claim 1, characterized in that, The extract includes an extractant, the extractant having a saturated vapor pressure ≤10 kPa at 25°C; Optionally, the extractant satisfies the following conditions: a flash point temperature ≥ 45℃ at room temperature and pressure or no flash point at 25℃ and one standard atmosphere; Optionally, the extractant is at least one selected from water, decane, laurylane, and 2,5-dichlorotoluene.

4. The preparation method according to any one of claims 1 to 3, characterized in that, The drying temperature is 30℃~90℃; Optionally, the drying temperature is 40°C to 80°C.

5. The preparation method according to any one of claims 1 to 4, characterized in that, The extraction solution includes an ionic liquid; Based on the mass of the dried wet film, the mass fraction of ionic liquid in the wet film is 0.1 wt% to 6 wt%. Optionally, based on the mass of the dried wet film, the mass fraction of the ionic liquid in the wet film is 0.3 wt% to 1 wt%.

6. The preparation method according to claim 5, characterized in that, In step (2), the number of extractions includes at least one extraction. Optionally, during the final extraction, the content of ionic liquid in the extract does not exceed 6.5 wt%.

7. The preparation method according to claim 5 or 6, characterized in that, The ionic liquid is at least one of imidazole ionic liquids, pyridine ionic liquids, alkyl sulfonic acid ionic liquids, quaternary ammonium ionic liquids, quaternary phosphorus ionic liquids, pyrrolidine ionic liquids, and piperidine ionic liquids; Optionally, the ionic liquid is at least one selected from 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium acetate, sodium 1-octadecyl sulfonate, sodium 1-pentadecanyl sulfonate, 1-ethyl-3-methylimidazolium difluorosulfonylimide, 1-butyl-3-methyl difluorosulfonylimide, 1-ethyl-3-methyl ditrifluorosulfonylimide, 1-butyl-3-methyl ditrifluorosulfonylimide, dodecyl quaternary ammonium salt, octadecyl quaternary ammonium salt, 1-ethyl-3-methyl ditrifluorosulfonylimide salt imidazolium dinitrile salt, and N-alkylpyridine. Optionally, the polymer is a homopolymer, copolymer, or mixture of polymers thereof of at least one of propylene, ethylene, butene, pentene, methyl methacrylate, tetrafluoroethylene, and difluoroethylene. Optionally, the polymer is at least one of polyethylene, polypropylene, and ethylene-propylene copolymer.

8. The preparation method according to claim 1, characterized in that, The specific steps (1) include: melt mixing of polymer and plasticizer, stretching, to obtain a film containing polymer and plasticizer; Optionally, the mass ratio of the polymer to the plasticizer is (15-50):(85-50); Optionally, the temperature of the melt mixing is not lower than 160°C; Optionally, the temperature of the melt mixing is not lower than 180°C; Optionally, the temperature of the melt mixing is not higher than 300°C; Optionally, the temperature of the melt mixing is not higher than 250°C; Optionally, the stretching ratio is 15 to 40 times during the stretching process; Optionally, during the stretching process, the stretching temperature is not lower than 60°C; Optionally, during the stretching process, the stretching temperature is not lower than 80°C; Optionally, the stretching temperature is not higher than 200°C during the stretching process; Optionally, the stretching temperature is not higher than 140°C during the stretching process; Optionally, the stretching ratio in the MD direction is 5 to 10 times, and the stretching ratio in the TD direction is 8 to 12 times.

9. The preparation method according to claim 1, characterized in that, After drying the wet film, step (3) also includes a shaping step; Optionally, the shaping temperature is 90–135°C, and the shrinkage rate of the shaping is not higher than 0.

9. Optionally, the shaping shrinkage rate is not higher than 0.

8.

10. The polymer porous membrane prepared by the preparation method according to any one of claims 1 to 9; The polymer porous membrane exhibits a shrinkage rate ≤2.5% in the TD direction under heat treatment at 105℃ / 1H. Optionally, the shrinkage rate of the polymer porous membrane in the TD direction is ≤2% under heat treatment at 105℃ / 1H; Optionally, the shrinkage rate of the polymer porous membrane in the MD direction is ≤3% under heat treatment at 105℃ / 1H; Optionally, the thickness difference of the polymer porous membrane in the TD direction is ≤1μm.

11. A battery, characterized in that, The battery includes a separator, which includes a polymer porous membrane prepared by the preparation method according to any one of claims 1 to 9 or a polymer porous membrane according to claim 10.

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