Microporous membrane as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of diaphragm material preparation, and particularly relates to a microporous membrane as well as a preparation method and application thereof. The microporous membrane comprises a polymer and an ionic liquid, and the microporous membrane comprises 0.01 wt%-5wt% of the ionic liquid, optionally 0.1 wt%-5wt% of the ionic liquid, based on the total mass of the microporous membrane. A proper amount of ionic liquid is introduced into the microporous membrane, the ionic liquid can be continuously or discontinuously adsorbed on the surfaces of the fibers in the microporous membrane, and the ionic liquid is adsorbed on the surfaces of the fibers in the microporous membrane through the oleophylic end of the ionic liquid, so that internal static electricity can be eliminated, and reduction of ionic conductivity can be better avoided; and the ion migration speed of the electrolyte is increased, and lithium dendrites are reduced. When the microporous membrane is used as a battery diaphragm, a proper amount of ionic liquid is added into the microporous membrane, so that the problem of electrostatic accumulation of the microporous membrane is solved, meanwhile, the microporous membrane has relatively low thermal shrinkage, and the problems that the battery capacity is influenced, the electrochemical safety is caused and the like are avoided.

Description

A microporous membrane and its preparation method and application Technical Field

[0001] The present application belongs to the technical field of diaphragm material preparation, and specifically relates to a microporous membrane and its preparation method and application. Background Art

[0002] Existing technology uses dichloromethane to extract the plasticizer from wet-laid films, and then dries the film to create a microporous membrane with a continuous microporous structure. During the drying process, the film is prone to shrinkage, with significant changes in pore size and porosity, and corresponding stress. This internal stress is gradually released after winding, resulting in a series of appearance problems caused by excessive thermal shrinkage. Currently, the main approaches to improving shrinkage include using specific low-molecular-weight polyethylene, adjusting the stretch ratio, heat setting, or coating the base film surface. However, these approaches can lead to new problems such as reduced membrane strength and excessive energy consumption, making it difficult to balance shrinkage with membrane properties such as air permeability, pore size, and porosity.

[0003] Secondly, after drying, the microporous membrane accumulates static electricity due to friction with rollers during operation or post-processing. Currently, static electricity is eliminated using an anti-static bar, but this method does not completely eliminate static electricity; it only eliminates static electricity on the surface of the microporous membrane. High shrinkage and static electricity accumulation can lead to membrane quality problems, difficulty in battery cell processing, and ultimately reduced battery cell yield.

[0004] Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present application is to overcome the defects of the diaphragms used in batteries in the prior art, such as high shrinkage and static electricity accumulation, thereby providing a microporous membrane and its preparation method and application.

[0006] During the research process, the applicant creatively discovered that when the existing technology uses dichloromethane to make the film into a film with a through microporous structure, the film is prone to size reduction and change because dichloromethane has a low boiling point (39.8°C at room temperature and pressure), absorbs a large amount of heat during evaporation, and evaporates too quickly and unevenly, instantly volatilizing the liquid dichloromethane that occupies the microporous structure inside the film. In a short period of time, the micropores of the film lose their support and shrink instantly under the tension of the roller, resulting in lateral and large-area size reduction, large changes in pore size and porosity, and corresponding stress. If the subsequent shaping effect is not good, the internal stress will be gradually released after winding, resulting in stress release of the film product after winding, causing problems such as poor winding quality, wrinkling, and ribs, as well as poor slitting quality and changes in size and appearance after slitting.

[0007] In addition, static electricity accumulates due to friction with the roller during operation or post-processing. The existing methods for improving static electricity can only eliminate static electricity on the surface of the membrane, but cannot eliminate static electricity continuously. As time goes by, the static electricity inside the membrane will jump to the surface, and as post-processing processes such as slitting and winding proceed, the microporous membrane will regenerate static electricity. Static electricity accumulation leads to uneven winding and wrinkling, as well as unstable tension during the assembly of battery cells, uneven winding or stacking of cells, especially for large-sized cells, where the stacking or winding effect is even worse. Based on this, the applicant has found that these problems can be effectively solved by introducing ionic liquids into the microporous membrane. To this end, the present application provides the following technical solutions.

[0008] In a first aspect, the present application provides a microporous membrane, the microporous membrane comprising a polymer and an ionic liquid, wherein the microporous membrane comprises 0.01 wt% to 5 wt% of the ionic liquid based on the total mass of the microporous membrane;

[0009] For example, it can be 0.02wt%, 0.05wt%, 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1.0wt%, 1.3wt%, 1.5wt%, 1.7wt%, 1.9wt%, 2.0wt%, 2.2wt%, 2.4wt%, 2.6wt%, 2.8wt%, 3.0wt%, 3.1wt%, 3.3wt%, 3.5wt%, 3.7wt%, 3.9wt%, 4.1wt%, 4.3wt%, 4.5wt%, 4.7wt%, 4.9wt%, 5.0wt% or a range consisting of any two of them.

[0010] Optionally, the microporous membrane contains 0.1 wt% to 5 wt% ionic liquid;

[0011] Optionally, the microporous membrane contains 0.3 wt% to 1.0 wt% of ionic liquid.

[0012] The microporous membrane comprises a plurality of fibers, which are interlaced and connected to form pores. The ionic liquid, through its oleophilic end, is attached to at least the surface of the fibers within the microporous membrane. The fibrils in the microporous membrane are obtained by stretching and aligning the polymer during film preparation. The extractant extracts the plasticizer from the film to form the pores.

[0013] The microporous membrane satisfies at least one of the following:

[0014] a) the average pore size of the microporous membrane is 20 to 70 nm;

[0015] b) the standard deviation of the pore size of the microporous membrane is 2.5 to 28 nm;

[0016] c) the absolute value of the difference between the sum of the average pore size and the standard deviation of the microporous membrane and the bubble point pore size in the microporous membrane does not exceed 15 nm, optionally the absolute value of the difference does not exceed 10 nm.

[0017] The average pore size of the microporous membrane can be, for example, 21 nm, 22 nm, 25 nm, 27 nm, 29 nm, 33 nm, 34 nm, 36 nm, 37 nm, 39 nm, 41 nm, 42 nm, 43 nm, 44 nm, 45 nm, 46 nm, 47 nm, 48 nm, 49 nm, 50 nm, 51 nm, 52 nm, 53 nm, 54 nm, 55 nm, 56 nm, 57 nm, 58 nm, 59 nm, 60 nm, 61 nm, 62 nm, 63 nm, 64 nm, 65 nm, 66 nm, 67 nm, 68 nm, 69 nm or a range consisting of any two of them.

[0018] The standard deviation of the pore size of the microporous membrane can be, for example, 2.9 nm, 3.2 nm, 3.8 nm, 4.2 nm, 4.6 nm, 4.9 nm, 5.5 nm, 5.8 nm, 6.3 nm, 6.5 nm, 6.8 nm, 7 nm, 7.2 nm, 7.8 nm, 8.2 nm, 8.6 nm, 9.0 nm, 9.4 nm, 9.8 nm, 10 nm, 10.2 nm, 10.6 nm, 11 nm, 11.5 nm, 11.8 nm, 12.2 nm, 12.8 nm, 13.2 nm, 13.6 nm, 14 nm, 14.4 nm, 14.8 nm, 15.2 nm, 15.6 nm, 16 nm, 16.4 nm, 16.8 nm, 17.2 nm, 17.8 nm, 18.2 nm, 18.8 nm, 19.5 nm, 19.8 nm, or a range consisting of any two thereof.

[0019] The absolute value of the difference between the sum of the average pore size and the standard deviation and the bubble point pore size in the microporous membrane can be, for example, 0.5 nm, 0.8 nm, 1.2 nm, 1.4 nm, 1.5 nm, 1.8 nm, 2.0 nm, 2.2 nm, 2.4 nm, 2.6 nm, 2.8 nm, 3.2 nm, 3.5 nm, 3.8 nm, 4.0 nm, 4.2 nm, 4.6 nm, 4.9 nm, 5.5 nm, 5.8 nm, 6. 4nm, 6.8nm, 7.0nm, 7.2nm, 7.5nm, 7.8nm, 8.2nm, 8.6nm, 9.0nm, 9.4nm, 9.8nm, 10.2nm, 10.6nm, 11nm, 11.5nm, 11.8nm, 12.2nm, 12.8nm, 13.2nm, 13.6nm, 14nm, 14.4nm, 14.8nm or a range consisting of any two thereof.

[0020] Optionally, the porosity of the microporous membrane is 30% to 70%. The porosity of the microporous membrane can be, for example, 33%, 36%, 39%, 42%, 45%, 48%, 50%, 53%, 56%, 59%, 62%, 65%, 68%, 70%, or any two thereof.

[0021] The ionic liquid is at least one of an imidazole ionic liquid, a pyridine ionic liquid, an alkylsulfonic acid ionic liquid, a quaternary ammonium ionic liquid, a quaternary phosphonium ionic liquid, a pyrrolidine ionic liquid and a piperidine ionic liquid;

[0022] Optionally, the ionic liquid is at least one of 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium acetate, 1-octadecylsulfonic acid sodium salt, 1-pentadecanesulfonic acid sodium salt, 1-ethyl-3-methylimidazolium bisfluorosulfonyl imide salt, 1-butyl-3-methylbisfluorosulfonyl imide salt, 1-ethyl-3-methylbistrifluorosulfonyl imide salt, 1-butyl-3-methylbistrifluorosulfonyl imide salt, dodecyl quaternary ammonium salt, octadecyl quaternary ammonium salt, 1-ethyl-3-methylbistrifluorosulfonyl imide salt, imidazole dinitrile salt and N-alkylpyridine;

[0023] Optionally, the polymer is a homopolymer, copolymer or a mixture of polymers of at least one of propylene, ethylene, butene, pentene, methyl methacrylate, tetrafluoroethylene and difluoroethylene;

[0024] Optionally, the polymer is at least one of polyethylene, polypropylene and ethylene-propylene copolymer.

[0025] A second aspect of the present application provides a method for preparing a microporous membrane, comprising the following steps:

[0026] (1) obtaining a film containing a plasticizer;

[0027] (2) extracting the plasticizer from the plasticizer-containing film using an extractant; wherein the extractant includes a first ionic liquid; the extracting solution includes the extractant and the plasticizer; and the concentration of the plasticizer in the extracting solution decreases along the moving direction of the plasticizer-containing film;

[0028] (3) Wash and shape.

[0029] The concentration of the plasticizer in the extract in step (2) is not higher than 7 wt %;

[0030] Optionally, in step (2), at least three extraction liquids with gradient concentrations are provided along the movement direction of the plasticizer-containing film; by providing more than three extraction liquids with gradient concentrations, the circulation flow rate of the extraction liquid can be reduced, thereby avoiding excessive impact tension on the film surface caused by the flow of the extraction liquid, resulting in deformation of the membrane surface and adverse effects on the pore structure and thermal shrinkage of the final product. In addition, it can also effectively save energy consumption costs in the extraction stage.

[0031] Optionally, in step (2), along the moving direction of the plasticizer-containing film, the concentration of the plasticizer in the latter extract is not higher than 85 wt % of the concentration of the plasticizer in the former extract;

[0032] Optionally, along the movement direction of the plasticizer-containing film, the plasticizer concentration in the latter extraction liquid is not higher than 65wt% of the plasticizer concentration in the former extraction liquid; the plasticizer concentration in the extraction liquid can be controlled to be 65wt%, which can control the concentration of the plasticizer in the extraction liquid to form a gradient difference, thereby better ensuring the extraction liquid flow impact tension that the film withstands during the extraction process, and at the same time better extracting the plasticizer from the micropores of the film.

[0033] Optionally, in step (2), the extraction temperature is 30-55°C;

[0034] Optionally, along the movement direction of the plasticizer-containing film, the temperature of the latter extraction is not higher than the temperature of the former extraction; the temperature of the latter extraction in the step extraction is controlled not to be higher than the temperature of the former extraction. As the plasticizer is extracted, the plasticizer inside the film gradually decreases, causing the overall structure of the film to lose support. Therefore, the extraction temperature is adjusted accordingly as the plasticizer inside the film decreases, thereby better controlling the thermal shrinkage of the film and thus better maintaining the microporous structure of the film. At the same time, the appropriate extraction temperature can ensure better extraction effect and extraction efficiency, and can better and more quickly extract the plasticizer inside the film.

[0035] Optionally, in step (2), when performing the extraction, the step further includes performing a first spraying step using a first spraying liquid;

[0036] Optionally, the first spray liquid includes a second ionic liquid.

[0037] Furthermore, when performing the extraction, a multi-stage overflow method can be used, and when performing the extraction, the overflow direction of the extract is opposite to the direction of movement of the film;

[0038] In the washing step of step (3), the washing liquid includes an ionic liquid and a washing agent, the washing agent includes water, and the concentration of the ionic liquid in the washing liquid is not higher than 10wt%; ensuring that the concentration of the ionic liquid in the washing liquid is not higher than 10wt% can effectively control the diffusion rate of the ionic liquid extractant in the film into the washing liquid, thereby ensuring the washing efficiency and washing quality.

[0039] Optionally, during the water washing, the step of spraying with a second spray liquid is further included, wherein the second spray liquid includes water;

[0040] Optionally, in the water washing step of step (3), the concentration of the ionic liquid in the washing solution decreases successively along the moving direction of the plasticizer-containing film;

[0041] Optionally, along the movement direction of the plasticizer-containing film, the concentration of the ionic liquid in the washing liquid of the subsequent washing is not higher than 30wt% of the ionic liquid concentration in the washing liquid of the previous washing; controlling the concentration of the ionic liquid in the washing liquid to decrease successively can better control the washing efficiency; further, controlling the concentration of the ionic liquid in the washing liquid of the subsequent washing is not higher than 30wt% of the ionic liquid concentration in the washing liquid of the previous washing, while maintaining high washing efficiency, better control the residual content of the ionic liquid in the film; secondly, it can also better avoid the ionic liquid extractant in the film being washed too quickly, causing the microporous structure to collapse, thereby affecting the quality of the microporous membrane.

[0042] In the washing step, the ionic liquid in the washing liquid is brought into the washing tank along the moving direction of the plasticizer-containing film during the preparation of the microporous membrane. Therefore, the ionic liquid in the washing liquid comes from the first ionic liquid and the second ionic liquid.

[0043] Optionally, after washing with water and before shaping in step (3), the step of infiltrating with an infiltration liquid is further included;

[0044] Optionally, the infiltration liquid includes a third ionic liquid and water;

[0045] Optionally, the concentration of the ionic liquid in the impregnation solution is 0.05-5 wt %.

[0046] The step (1) comprises at least one step of obtaining the plasticizer-containing film by a melt-kneading method and stretching the plasticizer-containing film;

[0047] Optionally, the stretching temperature is not less than 60°C;

[0048] Optionally, the stretching temperature does not exceed 200°C;

[0049] Optionally, the stretching temperature is not less than 80°C;

[0050] Optionally, the stretching temperature is 80 to 140°C;

[0051] Optionally, when stretching along the MD direction, the stretching ratio is 5 to 10 times;

[0052] Optionally, when stretching in the TD direction, the stretching ratio is 8 to 12 times. The MD direction refers to the direction of movement of the film when preparing the microporous membrane; the TD direction refers to the direction perpendicular to the direction of movement of the film when preparing the microporous membrane.

[0053] Optionally, the temperature of the melt mixing is not less than 160°C;

[0054] Optionally, the temperature of the melt mixing is not higher than 260°C;

[0055] Optionally, the temperature of the melt mixing is not less than 180°C;

[0056] Optionally, the melt mixing temperature is 180-250°C;

[0057] Optionally, the mass fraction of the plasticizer in the plasticizer-containing film is 50 to 85 wt%.

[0058] Optionally, the mass fraction of the plasticizer in the plasticizer-containing film is 60 to 80 wt%;

[0059] In the step (3), the shaping temperature is 100° C. to 135° C., and the shrinkage rate is not higher than 0.9, and can be optionally not higher than 0.8.

[0060] The first ionic liquid, the second ionic liquid and the third ionic liquid may be the same or different; the ionic liquid in the microporous membrane may be one kind or a mixture of multiple kinds.

[0061] The first ionic liquid, the second ionic liquid, and the third ionic liquid can all be selected from at least one of imidazole ionic liquids, pyridine ionic liquids, alkyl sulfonic acid ionic liquids, quaternary ammonium ionic liquids, quaternary phosphonium ionic liquids, pyrrolidine ionic liquids, and piperidine ionic liquids;

[0062] The first ionic liquid, the second ionic liquid, and the third ionic liquid can all be selected from at least one of 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium acetate, 1-octadecylsulfonic acid sodium salt, 1-pentadecanesulfonic acid sodium salt, 1-ethyl-3-methylimidazolium bisfluorosulfonyl imide salt, 1-butyl-3-methylbisfluorosulfonyl imide salt, 1-ethyl-3-methylbistrifluorosulfonyl imide salt, 1-butyl-3-methylbistrifluorosulfonyl imide salt, dodecyl quaternary ammonium salt, octadecyl quaternary ammonium salt, 1-ethyl-3-methylbistrifluorosulfonyl imide salt, imidazole dinitrile salt, and N-alkylpyridine.

[0063] Plasticizers are organic compounds that can form a uniform solution with polyolefins at temperatures below the boiling point. Specifically, the plasticizers can include decalin, xylene, dioctyl phthalate, dibutyl phthalate, stearyl alcohol, oleyl alcohol, decanol, nonanol, diphenyl ether, n-decane, n-dodecane, paraffin oil, etc. Among them, paraffin oil and dioctyl phthalate can be selected as plasticizers.

[0064] The above is an explanation of the optional manufacturing method of the microporous membrane of the present application. It is an optional preparation method that the applicant believes to be more conducive to economic benefits, product quality, etc. during the research process. However, as long as the obtained microporous membrane has the characteristics that meet the requirements of this application, there are no restrictions on the type of polymer, type of solvent, stretching method, extraction method, pore opening method, water washing method, infiltration method, method of applying ionic liquid to the microporous membrane, heat setting method, etc.

[0065] A third aspect of the present application provides a battery, the battery comprising a separator, the separator comprising the above-mentioned microporous membrane or the microporous membrane prepared by the above-mentioned preparation method.

[0066] The technical solution of this application has the following advantages:

[0067] 1. The microporous membrane provided by the present application comprises a polymer and an ionic liquid. Based on the total mass of the microporous membrane, the microporous membrane comprises 0.01 wt% to 5 wt% of the ionic liquid, optionally 0.1 wt% to 5 wt% of the ionic liquid, and further optionally 0.3 wt% to 1.0 wt% of the ionic liquid. The present application introduces an appropriate amount of ionic liquid into the microporous membrane, and the ionic liquid can be continuously or discontinuously adsorbed on the surface of the fibers inside the microporous membrane. The ionic liquid is adsorbed on the surface of the fibers inside the microporous membrane through its lipophilic end, and the hydrophilic end is a free end, which can not only eliminate internal static electricity, but also better avoid the reduction of ionic conductivity, while increasing the migration speed of electrolyte ions and reducing lithium dendrites. When the microporous membrane of the present application is used as a battery separator, adding an appropriate amount of ionic liquid to the microporous membrane is beneficial to improving its static electricity accumulation problem, while making the microporous membrane have lower thermal shrinkage, avoiding affecting battery capacity and causing problems such as electrochemical safety.

[0068] However, if the ionic liquid content in the microporous membrane is too high, its air permeability, tensile strength and thermal shrinkage will deteriorate. Secondly, because the ions in the ionic liquid are embedded in the positive and negative electrodes of the battery due to the charging effect, the discharge and deintercalation are incomplete or the diameter of the anions and cations in the ionic liquid is much larger than the diameter of the lithium ions, they occupy more limited space in the embedded electrode material or directly destroy the structure of the electrode material, resulting in a certain degree of capacity decay. Thirdly, because the anions and cations in the ionic liquid may chelate excessively with the components in the electrolyte, it will affect the viscosity of the electrolyte in the liquid lithium-ion battery to a certain extent and reduce the ionic conductivity.

[0069] 2. The microporous membrane provided herein, by controlling the pore structure of the microporous membrane, allows for more complete and uniform adsorption of ionic liquids within the membrane. Since the ionic liquid content will have a certain impact on the micropore structure, from the perspective of battery applications, controlling the pore structure of the microporous membrane can better synergize with the appropriate amount of ionic liquid to achieve good electrochemical performance.

[0070] 3. The preparation method of the microporous membrane provided by the present application can well adsorb ionic liquid on the surface of the fibers inside the microporous membrane, and control the content of ionic liquid in the microporous membrane, and can continuously release static electricity under the dynamic production of the film and the static storage, ensuring that the static electricity of the film is fully released, further ensuring good production quality and product quality. Compared with the existing technology using dichloromethane extraction process, the absolute value of the electrostatic force of the film obtained by the preparation method of the present application is below 800V. After being wound and left to stand for a period of time, the static electricity of the film will gradually reach a normal level, that is, there will be no obvious back-absorption phenomenon after the microporous membrane is pulled apart from the membrane roll. Under the same conditions, the static electricity of the film obtained by the prior art is difficult to be released, and the absolute value of its electrostatic force is basically between 1500v and 5000v. Even if the microporous membrane is left for one day and then pulled apart, there is still a significant back-absorption phenomenon.

[0071] Secondly, during the drying process, the film surface dries more evenly and the tension is more controllable, resulting in more uniform stress in all directions of the film. The film's dimensional changes before and after drying are minimal, further allowing for more complete stress relief during the shaping process. Thirdly, the preparation method is more environmentally friendly, generating no waste gas and requiring no waste gas recovery or treatment.

[0072] 4. The preparation method of the microporous membrane provided in this application adopts a multi-stage overflow method and controls the gradient concentration changes in the extraction, water washing and other processes, so that the content of the ionic liquid in the microporous membrane is controllable and the production efficiency is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the specific implementation methods or the description of the prior art. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0074] FIG1 is a scanning electron microscope image of the microporous membrane of Example 1 of the present application (magnification is 20,000 times). DETAILED DESCRIPTION

[0075] The following examples are provided to further better understand the present application, but are not limited to the best implementation mode described herein, and do not limit the content and protection scope of the present application. Any product identical or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with other prior arts shall fall within the scope of protection of the present application.

[0076] The term "longitudinal direction" used in this application is also called MD direction, which refers to the direction in which the equipment runs.

[0077] The term "transverse direction" used in this application is also referred to as TD direction, which refers to a direction perpendicular to the running direction of the device.

[0078] The term "±" as used herein means that a particular value includes variations thereof, which are normal fluctuations in actual processes.

[0079] If no specific experimental steps or conditions are specified in the examples, the conventional experimental steps or conditions described in the literature in this field can be used. If the manufacturer of the reagents or instruments is not specified, they are all commercially available conventional reagents.

[0080] Example 1

[0081] This embodiment provides a method for preparing a microporous membrane, comprising the following steps:

[0082] (1) Take polyethylene resin (weight average molecular weight 90W) and liquid paraffin oil (kinematic viscosity 45mm at 40℃) in a mass ratio of 23:77. 2 / s), adding it to a twin-screw extruder for melt mixing to form a melt, the extruder temperature is 200±5°C, the melt is extruded through a die head, the melt is cast and cooled at 20°C to form a precursor film, and the precursor film is sequentially subjected to MD stretching and TD stretching to obtain a polyethylene film containing paraffin oil; wherein the MD stretching temperature is 95°C, the MD stretching ratio is 6 times, the TD stretching temperature is 110°C, and the TD stretching ratio is 8.5 times.

[0083] (2) A multi-stage overflow method is used to pass a polyethylene film containing paraffin oil 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, extraction tank 3, extraction tank 4, extraction tank 5, and extraction tank 6, and the temperature of each extraction tank is 55°C, 50°C, 45°C, 40°C, 40°C, and 35°C, respectively; while the multi-stage extraction also includes a step of using a first spray liquid to perform a first spray on the film, and the concentration of the extractant in the extract of each extraction tank is controlled by an independent ionic liquid spray overflow rate. The purpose of the first spray is to supplement the extractant to ensure that the extracts in different extraction tanks are The concentration difference of the extractant improves the extraction efficiency. As the extraction proceeds, the extractant replaces the paraffin oil in the polyethylene film containing paraffin oil, the paraffin oil enters the extraction tank, and the paraffin oil and the extractant form an extract. The extracts in the extraction tanks 1-6 include the extractant and the paraffin oil. The concentrations of the paraffin oil in the extracts are 6±0.2wt%, 3.5±0.2wt%, 2±0.1wt%, 1.2±0.02wt%, 0.7±0.02wt%, and 0.2±0.01wt%, respectively. The extractant is 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid, and the first spray liquid is 1-octadecyl sodium sulfonate ionic liquid.

[0084] (3) The film is sequentially transported to the first water washing tank, the second water washing tank and the third water washing tank for water washing. The temperature of each water washing tank is 35°C, the concentration of the ionic liquid in the water washing liquid of the first water washing tank is 5.5±0.2wt%, the concentration of the ionic liquid in the water washing liquid of the second water washing tank is 1.3±0.1wt%, and the concentration of the ionic liquid in the water washing liquid of the third water washing tank is 0.2±0.02wt%. The water washing agent in the water washing liquid is water. While washing, the second spraying step is also included, and the second spraying step is performed using a second spraying liquid. The second spraying liquid is water. Each water washing tank is sprayed with independent pure water for the second spraying, and the concentration of the ionic liquid in the water washing liquid of the water washing tank is controlled by the overflow of the water spray. While performing the second spraying, the water will bring the ionic liquid in the film into the water washing tank to form a water washing liquid, that is, the water washing liquid includes the ionic liquid and water washed from the surface and micropores of the film.

[0085] After the water washing is completed, the film is transported to an infiltration tank at a temperature of 35° C. The infiltration solution includes octadecyl quaternary ammonium salt and water, and the concentration of the ionic liquid in the infiltration solution is 1 wt %.

[0086] The film is then transported to a drying oven, where low-temperature, high-speed airflow at 20-40°C, infrared radiation, and heat medium roller contact heating at 40-70°C are used in turn to remove moisture from the inner and outer surfaces of the film; it is then sent to a horizontal stretching oven for shaping and modification, with a shaping temperature of 110°C and a shrinkage ratio of 0.7. The film is then cooled to room temperature and rolled up.

[0087] This embodiment provides a microporous membrane, including polyethylene and ionic liquid. Based on the total mass of the microporous membrane, the concentration of the ionic liquid in the microporous membrane is 0.8 wt%, the thickness of the microporous membrane is 12 μm, the average pore diameter is 40 nm, the standard deviation is 6.5 nm, the bubble point pore diameter is 50 nm, and the porosity is 43%.

[0088] Example 2

[0089] This embodiment provides a method for preparing a microporous membrane, comprising the following steps:

[0090] (1) Same as Example 1.

[0091] (2) Same as Example 1.

[0092] (3) The difference from Example 1 is that the concentration of the ionic liquid in the impregnation solution is 3 wt%.

[0093] This embodiment provides a microporous membrane, including polyethylene and ionic liquid. Based on the total mass of the microporous membrane, the concentration of the ionic liquid in the microporous membrane is 2.7 wt%, the thickness of the microporous membrane is 12 μm, the average pore diameter is 40 nm, the standard deviation is 6.5 nm, the bubble point pore diameter is 48 nm, and the porosity is 41%.

[0094] Example 3

[0095] This embodiment provides a method for preparing a microporous membrane, comprising the following steps:

[0096] (1) Same as Example 1.

[0097] (2) Same as Example 1.

[0098] (3) The difference from Example 1 is that the concentration of the ionic liquid in the washing solution of the third washing is 0.1±0.02 wt %. After the washing is completed, the film is not treated with the impregnation solution. Other aspects are the same as in Example 1.

[0099] This embodiment provides a microporous membrane, including polyethylene and ionic liquid. Based on the total mass of the microporous membrane, the concentration of the ionic liquid in the microporous membrane is 0.06 wt%, the thickness of the microporous membrane is 12 μm, the average pore diameter is 40 nm, the standard deviation is 6.5 nm, the bubble point pore diameter is 52 nm, and the porosity is 44%.

[0100] Example 4

[0101] This embodiment provides a method for preparing a microporous membrane, comprising the following steps:

[0102] (1) The difference from Example 1 is that the mass ratio of polyethylene powder to liquid paraffin oil is 20:80, the MD stretching temperature is 95°C, the MD stretching ratio is 6 times, the TD stretching temperature is 118°C, and the TD stretching ratio is 9.5 times.

[0103] (2) The difference from Example 1 is that the extractant 1-ethyl-3-methylimidazolium tetrafluoroborate in Example 1 is replaced with 1-octadecylsulfonic acid sodium salt.

[0104] (3) The difference from Example 1 is that the ionic liquid octadecyl quaternary ammonium salt in the infiltration solution of Example 1 is replaced with N-alkylpyridine.

[0105] This embodiment provides a microporous membrane, including polyethylene and ionic liquid. Based on the total mass of the microporous membrane, the concentration of the ionic liquid in the microporous membrane is 0.8 wt%, the thickness of the microporous membrane is 16 μm, the average pore diameter is 42 nm, the standard deviation is 7 nm, the bubble point pore diameter is 53 nm, and the porosity is 48%.

[0106] Example 5

[0107] This embodiment provides a method for preparing a microporous membrane, comprising the following steps:

[0108] (1) Same as Example 1.

[0109] (2) The difference from Example 1 is that the multi-stage overflow method includes extraction tank 1, extraction tank 2, extraction tank 3, and extraction tank 4, and the temperature of each extraction tank is 50°C, 45°C, 40°C, and 35°C, respectively; the concentration of paraffin oil in the extracts of extraction tanks 1-4 is 3.25±0.2wt%, 1.2±0.1wt%, 0.6±0.02wt%, and 0.1±0.01wt%, respectively; the extractant in the extract is a mixed ionic liquid of octadecyl quaternary ammonium salt and 1-octadecyl sulfonic acid sodium salt in a mass ratio of 1:1. The first spray liquid is 1-pentadecane sulfonic acid sodium salt.

[0110] (3) The difference from Example 1 is that, during the water washing, the concentration of the ionic liquid in the water washing solution in the first water washing tank is 8.5 wt ± 0.2 wt %, the concentration of the ionic liquid in the water washing solution in the second water washing tank is 1.5 ± 0.2 wt %, and the concentration of the ionic liquid in the water washing solution in the third water washing tank is 0.27 ± 0.02 wt %;

[0111] When the infiltration was performed, the concentration of the ionic liquid in the infiltration solution was 4.5 wt %;

[0112] Other steps are the same as in Example 1.

[0113] This embodiment provides a microporous membrane, including polyethylene and ionic liquid. Based on the total mass of the microporous membrane, the concentration of the ionic liquid in the microporous membrane is 4.1 wt%, the thickness of the microporous membrane is 12 μm, the average pore diameter is 40 nm, the standard deviation is 6.5 nm, the bubble point pore diameter is 45 nm, and the porosity is 38%.

[0114] Example 6

[0115] This embodiment provides a method for preparing a microporous membrane, comprising the following steps:

[0116] (1) Same as Example 1;

[0117] (2) Same as Example 1;

[0118] (3) The difference from Example 1 is that the film is transported to a water washing tank for water washing. During the water washing, the concentration of the ionic liquid in the water washing solution is 5.3±0.2wt%, and the wetting step is not performed.

[0119] This embodiment provides a microporous membrane, including polyethylene and ionic liquid. Based on the total mass of the microporous membrane, the concentration of the ionic liquid in the microporous membrane is 4.9 wt%, the thickness of the microporous membrane is 12 μm, the average pore diameter is 40 nm, the standard deviation is 6.3 nm, the bubble point pore diameter is 44 nm, and the porosity is 38%.

[0120] Example 7

[0121] This embodiment provides a method for preparing a microporous membrane, comprising the following steps:

[0122] (1) Same as Example 1;

[0123] (2) Same as Example 1;

[0124] (3) The difference from Example 1 is that the concentration of the ionic liquid in the washing liquid of the first washing tank is 9±0.2wt%, the concentration of the ionic liquid in the washing liquid of the second washing tank is 3±0.1wt%, the concentration of the ionic liquid in the washing liquid of the third washing tank is 1±0.02wt%, and the infiltration step is not performed.

[0125] This embodiment provides a microporous membrane, including polyethylene and ionic liquid. Based on the total mass of the microporous membrane, the concentration of the ionic liquid in the microporous membrane is 0.8 wt%, the thickness of the microporous membrane is 12 μm, the average pore diameter is 36 nm, the standard deviation is 6.8 nm, the bubble point pore diameter is 40 nm, and the porosity is 38%.

[0126] Comparative Example 1

[0127] This comparative example provides a microporous membrane, comprising the following steps:

[0128] (1) Same as Example 1;

[0129] (2) A single extraction tank was used, the extraction tank temperature was 45°C, and the concentration of paraffin oil in the extract was 7±0.2 wt%.

[0130] (3) The difference from Example 1 is that the film is sent to a water washing tank for the first water washing and the second water washing. The ionic liquid concentration in the washing liquid of the first water washing is 12±0.2wt%, and the ionic liquid concentration in the washing liquid of the second water washing is 6.3±0.2wt%.

[0131] After the water washing is completed, the film is not subjected to the infiltration treatment, and the other operations are the same as those in Example 1.

[0132] This embodiment provides a microporous membrane, including polyethylene and ionic liquid. Based on the total mass of the microporous membrane, the concentration of the ionic liquid in the microporous membrane is 6 wt%, the thickness of the microporous membrane is 12 μm, the average pore diameter is 38 nm, the standard deviation is 10 nm, the bubble point pore diameter is 41 nm, and the porosity is 35%.

[0133] Comparative Example 2

[0134] This comparative example provides a method for preparing a microporous membrane, comprising the following steps:

[0135] (1) Same as Example 1.

[0136] (2) The difference from Example 1 is that a single extraction tank is used, the extraction tank temperature is 45° C., and the concentration of paraffin oil in the extract is 7.5±0.2 wt %.

[0137] (3) The difference from Example 1 is that the film is sent to a water washing tank for the first water washing and the second water washing. The ionic liquid concentration in the washing liquid of the first water washing is 40±0.2wt%, and the ionic liquid concentration in the washing liquid of the second water washing is 12±0.2wt%.

[0138] After the water washing is completed, the film is not subjected to the infiltration treatment, and the other operations are the same as those in Example 1.

[0139] This comparative example provides a microporous membrane comprising polyethylene and an ionic liquid. Based on the total mass of the microporous membrane, the concentration of the ionic liquid in the microporous membrane is 10 wt %, the thickness of the microporous membrane is 12 μm, the average pore size of the fiber pores is 36 nm, the standard deviation is 12.5 nm, the bubble point pore size is 41 nm, and the porosity is 31%.

[0140] Test example

[0141] This test example provides the performance test results of the microporous membranes provided in the examples and comparative examples, as follows:

[0142] (1) Thermal shrinkage and thickness of microporous membrane: refer to the provisions of GB / T 36363-2018.

[0143] (2) Static electricity accumulation of microporous membrane: The microporous membrane was placed in an environment with a temperature of 25°C and a humidity of 40% for 1 hour and 24 hours respectively, and then the electrostatic test was performed using a Keyence handheld electrostatic tester. The microporous membrane roll was pulled out 50 meters in the MD direction using insulating latex gloves and then the electrostatic test was performed. The test probe was perpendicular to the film surface at a height of about 3-5 cm. The test was performed every 10 meters in the MD direction, and a total of 10 data were tested. The test was performed every 10 meters in the TD direction, and a total of 10 data were tested. The 20 data of MD and TD were averaged to obtain the static electricity accumulation of the microporous membrane.

[0144] (3) Tensile strength of microporous membrane: in accordance with the provisions of GB / T 1040.3-2006.

[0145] (4) Average pore size, bubble point pore size, and standard deviation of the microporous membrane: measured at 25°C using a PMI instrument (Jiayun Co., Ltd., model CFP-1500AE) and a galwick (surface tension at 25°C: 15.9 dynes / cm) as the infiltration liquid. The pore size is expressed in nm.

[0146] (5) Ionic conductivity of microporous membrane: Cut 4 membrane samples with a diameter of 45 mm from a flat surface, soak the samples in an electrolyte (1.0 M LiPF6 in a 3:3:4 volume EC / EMC / DMC solvent) and seal and soak for 30 min; pour about 15 ml of 1 mol / L fresh electrolyte (1.0 M LiPF6 in a 3:3:4 volume EC / EMC / DMC solvent) into the surface resistance test fixture; place 1, 2, 3, and 4 membranes in the fixture for testing respectively; use the number of membrane layers as the horizontal axis and the membrane resistance as the vertical axis for linear fitting, and calculate the slope and fit of the straight line. When the fit is greater than 0.999, the slope is the surface resistance Q of the membrane, and the ionic conductivity is calculated according to the formula:

[0147] Wherein, d is the thickness of the microporous membrane, in μm;

[0148] Q is the surface resistance, in ohm·cm 2 ;

[0149] σ is the ionic conductivity, with the unit of S / cm.

[0150] (6) Capacity retention of microporous membrane: Graphite, conductive carbon black, and binder styrene-butadiene rubber latex were prepared in a mass ratio of 96:2:2 to form a negative electrode slurry, which was coated on copper foil to form a negative electrode sheet; lithium iron phosphate, conductive agent, and binder polyvinylidene fluoride were prepared in a mass ratio of 97:1:2 to form a positive electrode slurry, which was coated on aluminum foil to form a positive electrode sheet; 1.0 M LiPF6 was added to a 3:3:4 volume ratio of EC / EMC / DMC solvent to form an electrolyte;

[0151] The positive electrode sheet, microporous membrane and negative electrode sheet are wound into a battery cell, which is then placed in an aluminum-plastic packaging bag, injected with the above-mentioned electrolyte, and processed through packaging, formation and other processes to make a battery.

[0152] The battery was discharged at 1C / 1C rate at 25°C for 1000 cycles, and its capacity retention rate was calculated using the formula;

[0153] (7) Test method for ionic liquid content in microporous membrane: Take a 10 cm × 10 cm microporous membrane sample and weigh it M1. Place the microporous membrane sample in 100 ml of DCM solvent, ultrasonicate it at 100 Hz for 15 min, dry it at 80 °C for 5 min, and weigh M2. The ionic liquid content is calculated by the following formula:

[0154] Table 1 Performance test results of microporous membrane

[0155] FIG1 is an electron microscope image of the microporous membrane of Example 1 of the present application. From the image, it can be seen that the fibers formed by stretching and orientation in the microporous membrane are interlaced to form a pore structure.

[0156] From the experimental results in Table 1, it can be seen that the tensile strength of the microporous membrane of the present application is good (the tensile strength in the MD direction or the TD direction is 2590 kgf / cm 2 The thermal shrinkage is low (105℃ / 1H, the thermal shrinkage in MD direction is less than 3%, and the thermal shrinkage in TD direction is less than 2%), and the battery prepared by it also has good ionic conductivity (the conductivity is not less than 8×10 -4 S / cm) and capacity retention rate (the battery is cycled 1000 times and the capacity is not less than 85%).

[0157] Compared with Comparative Example 1, when the ionic liquid content in the microporous membrane is excessive, its thermal yield is higher and its tensile strength is lower, which, when used in a battery, will affect the battery's conductivity and capacity retention to a certain extent. Compared with Comparative Example 2, the microporous membrane obtained using a single-stage extraction process has a higher content of ionic liquid, which also affects its thermal yield and tensile strength, as well as the battery's conductivity and capacity retention.

[0158] Furthermore, according to the results of Example 1 and Example 7, when the concentration of the ionic liquid in the washing liquid of the latter water washing exceeds 30 wt % compared with the concentration of the ionic liquid in the washing liquid of the previous water washing, the pore structure of the obtained microporous membrane will collapse and the thermal shrinkage will increase, affecting the quality of the microporous membrane. This shows that the present application helps to further improve the structure and quality of the microporous membrane by controlling the concentration of the ionic liquid in the washing liquid of the latter water washing to not exceed 30 wt % of the concentration of the ionic liquid in the washing liquid of the previous water washing.

[0159] Obviously, the above embodiments are merely examples for clarity of explanation and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications can be made based on the above description. It is not necessary and impossible to enumerate all implementation methods here. Obvious variations or modifications arising therefrom remain within the scope of protection of the present invention.

Claims

1. A microporous membrane, characterized in that: The microporous membrane comprises a polymer and an ionic liquid, and based on the total mass of the microporous membrane, the microporous membrane comprises 0.01 wt% to 5 wt% of the ionic liquid; Optionally, the microporous membrane contains 0.1 wt% to 5 wt% ionic liquid; Optionally, the microporous membrane contains 0.3 wt% to 1.0 wt% of ionic liquid.

2. The microporous membrane according to claim 1, characterized in that The microporous membrane is provided with a plurality of fibers, and the plurality of fibers are staggered and connected to form pores. The ionic liquid is attached to at least the surface of the fibers inside the microporous membrane through its lipophilic end.

3. The microporous membrane according to claim 1 or 2, characterized in that: At least one of the following is met: a) the average pore size of the microporous membrane is 20 to 70 nm; b) the standard deviation of the pore size of the microporous membrane is 2.5 to 28 nm; c) the absolute value of the difference between the sum of the average pore size and the standard deviation of the microporous membrane and the bubble point pore size in the microporous membrane does not exceed 15 nm; Optionally, the absolute value of the difference does not exceed 10 nm.

4. The microporous membrane according to any one of claims 1 to 3, 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 phosphonium ionic liquids, pyrrolidine ionic liquids and piperidine ionic liquids; Optionally, the ionic liquid is at least one of 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium acetate, 1-octadecylsulfonic acid sodium salt, 1-pentadecanylsulfonic acid sodium salt, 1-ethyl-3-methylimidazolium bisfluorosulfonyl imide salt, 1-butyl-3-methylbisfluorosulfonyl imide salt, 1-ethyl-3-methylbistrifluorosulfonyl imide salt, 1-butyl-3-methylbistrifluorosulfonyl imide salt, dodecyl quaternary ammonium salt, octadecyl quaternary ammonium salt, 1-ethyl-3-methylbistrifluorosulfonyl imide salt, imidazole dinitrile salt and N-alkyl pyridine; Optionally, the polymer is a homopolymer, a copolymer or a mixture of polymers 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.

5. A method for preparing a microporous membrane, characterized in that: The following steps are involved: (1) obtaining a film containing a plasticizer; (2) extracting the plasticizer in the plasticizer-containing film using an extractant; wherein the extractant comprises a first ionic liquid; the extracting solution comprises the extractant and the plasticizer; and along the moving direction of the plasticizer-containing film, the concentration of the plasticizer in the extracting solution decreases successively; (3) Wash and shape.

6. The preparation method according to claim 5, characterized in that: The concentration of the plasticizer in the extract in step (2) is not higher than 7 wt %; Optionally, in the step (2), at least three extraction solutions with concentration gradients are provided along the moving direction of the plasticizer-containing film; Optionally, in step (2), along the moving direction of the plasticizer-containing film, the concentration of the plasticizer in the latter extract is not higher than 85 wt % of the concentration of the plasticizer in the former extract; Optionally, in step (2), the extraction temperature is 30 to 55°C; Optionally, along the moving direction of the plasticizer-containing film, the temperature of the latter extraction is not higher than the temperature of the former extraction; Optionally, in the step (2), when performing the extraction, the step further includes performing a first spraying step using a first spraying liquid; Optionally, the first spray liquid includes a second ionic liquid.

7. The preparation method according to any one of claims 5 to 6, characterized in that: In the water washing step of step (3), the water washing solution comprises an ionic liquid and a water washing agent, the water washing agent comprises water, and the concentration of the ionic liquid in the water washing solution is not higher than 10 wt %; Optionally, when performing the water washing, a step of spraying with a second spray liquid is also included, and the second spray liquid includes water; Optionally, in the water washing step of step (3), the concentration of the ionic liquid in the water washing solution decreases successively along the moving direction of the plasticizer-containing film; Optionally, along the moving direction of the plasticizer-containing film, the concentration of the ionic liquid in the washing solution of the latter washing is not higher than 30wt% of the concentration of the ionic liquid in the washing solution of the former washing; Optionally, after washing with water and before shaping in step (3), a step of soaking with soaking liquid is also included; Optionally, the infiltration liquid includes a third ionic liquid and water; Optionally, the concentration of the ionic liquid in the impregnation solution is 0.05-5 wt %.

8. The preparation method according to any one of claims 5 to 7, characterized in that: The step (1) comprises at least one step of obtaining the plasticizer-containing film by a melt-kneading method and stretching the plasticizer-containing film; Optionally, the stretching temperature is not less than 60°C; Optionally, the stretching temperature does not exceed 200°C; Optionally, the temperature of the melt mixing is not less than 160°C; Optionally, the temperature of the melt mixing is not higher than 260°C; Optionally, the mass fraction of the plasticizer in the plasticizer-containing film is 50 to 85 wt %.

9. The preparation method according to any one of claims 5 to 8, characterized in that: In the step (3), the shaping temperature is 100° C. to 135° C., and the shrinkage rate is not higher than 0.9, and can be optionally not higher than 0.

8.

10. A battery, characterized in that: The battery comprises a separator, and the separator comprises the microporous membrane according to any one of claims 1 to 4 or the microporous membrane prepared by the preparation method according to any one of claims 5 to 9.

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