Microporous film for composite current collector, preparation method therefor, and composite current collector

Abstract

The present application relates to a microporous film for a composite current collector, a preparation method therefor, and a composite current collector. The microporous film for a composite current collector comprises an ionic liquid. The mass percentage of the ionic liquid in the microporous film is 0.01%-6%, the air permeability of the microporous film is 10 sec / 100 cc-50 sec / 100 cc, and the longitudinal tensile strength of the microporous film is 3500 kgf / cm2-5000 kgf / cm2. The ionic liquid comprises a first ionic liquid, and the first ionic liquid has a hydrophilic-lipophilic balance value of <10.

Description

Microporous membranes for composite current collectors and their preparation methods and composite current collectors

[0001] Related applications

[0002] This application claims priority to PCT international patent application filed on January 24, 2025, with application number PCT / CN2025 / 074895, entitled "Microporous membranes and their preparation methods and applications", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of microporous membranes, and in particular to microporous membranes for composite current collectors, methods for their preparation, and composite current collectors. Background Technology

[0004] Currently, microporous membranes struggle to meet the increasingly demanding market requirements for membrane products. For example, in the field of composite current collectors, high mechanical strength, uniform pore size, and high temperature resistance are required to achieve low resistance and good electrochemical consistency. Therefore, there is an urgent need to develop a microporous membrane with excellent overall performance that meets the market requirements in these areas. Summary of the Invention

[0005] Based on this, some embodiments of this application provide a microporous membrane for composite current collectors, which has good comprehensive performance to meet the requirements for good application of the membrane in the field of composite current collectors.

[0006] In addition, some other embodiments of this application also provide a method for preparing a microporous membrane for a composite current collector and a composite current collector.

[0007] A microporous membrane for composite current collectors, the microporous membrane containing an ionic liquid, wherein the mass percentage of the ionic liquid in the microporous membrane is 0.01% to 6%, the air permeability of the microporous membrane is 10 sec / 100 cc to 50 sec / 100 cc, and the longitudinal tensile strength of the microporous membrane is 3500 kgf / cm². 2 ~5000 kgf / cm 2 The ionic liquid includes a first ionic liquid, wherein the hydrophilic-lipophilic balance value of the first ionic liquid is <10.

[0008] In some embodiments, the hydrophilic-lipophilic balance value of the first ionic liquid is 0 to 9.8.

[0009] In some embodiments, the hydrophilic-lipophilic balance value of the first ionic liquid is 3 to 7.

[0010] In some embodiments, the molecular weight of the first ionic liquid is 200 to 1000.

[0011] In some embodiments, the boiling point of the first ionic liquid is 200°C to 500°C.

[0012] In some embodiments, the viscosity of the first ionic liquid at 25°C is 20cp to 100cp.

[0013] In some embodiments, the mass percentage of the first ionic liquid in the microporous membrane is 0.01% to 6%.

[0014] In some embodiments, the mass percentage of the first ionic liquid in the microporous membrane is 0.1% to 5%.

[0015] In some embodiments, the microporous membrane further contains a second ionic liquid, the second ionic liquid having a hydrophilic-lipophilic balance value greater than that of the first ionic liquid.

[0016] In some embodiments, the difference between the hydrophilic-lipophilic balance value of the second ionic liquid and the hydrophilic-lipophilic balance value of the first ionic liquid is <10, and the difference can be selected as 1 to 9.

[0017] In some embodiments, the hydrophilic-lipophilic balance value of the second ionic liquid is 10 to 20.

[0018] In some embodiments, the boiling point of the second ionic liquid is 100°C to 500°C.

[0019] In some embodiments, the viscosity of the second ionic liquid at 25°C is 10 cp to 50 cp.

[0020] In some embodiments, the viscosity of the second ionic liquid at 25°C is 5 cp to 20 cp lower than that of the first ionic liquid at 25°C.

[0021] In some embodiments, the mass percentage of the second ionic liquid in the microporous membrane is 0.01% to 3%.

[0022] In some embodiments, the mass percentage of the second ionic liquid is 0.1% to 0.5%.

[0023] In some embodiments, the first ionic liquid and the second ionic liquid are each independently selected from one or more of imidazole salts, pyridine salts, quaternary ammonium salts, quaternary phosphonium salts, and pyrrolidines.

[0024] In some embodiments, the first ionic liquid and the second ionic liquid are each independently selected from 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium methyl sulfate, 1,3-dimethylimidazolium dimethyl phosphate, 1,3-dimethylimidazolium tetrafluoroborate, 3-dimethylimidazolium hexafluorophosphate, 1,3-dimethylimidazolium hexafluoroantimonate, 1,3-dimethylimidazolium trifluoromethanesulfonate, 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium perchlorate, 1,3-dimethylimidazolium The nitrate, 1,3-dimethylimidazolium methanesulfonate, 1,3-dimethylimidazolium p-toluenesulfonate, 1,3-dimethylimidazolium trifluoroacetate, 1-ethyl-3-methylimidazolium bromide, 1-ethyl-3-methylimidazolium iodide, ethyl 1-ethyl-3-methylimidazolium sulfate, methyl 1-ethyl-3-methylimidazolium sulfate, diethyl 1-ethyl-3-methylimidazolium phosphate, dimethyl 1-ethyl-3-methylimidazolium phosphate, tetrafluoroborate, hexafluorophosphate, and 1-ethyl-3-methylimidazolium hexafluoroantimonate are selected from one or more of these.

[0025] In some embodiments, the first ionic liquid comprises one or more of the following: 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium hexafluorophosphate, 1,3-dimethylimidazolium hexafluoroantimonate, 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium methanesulfonate, 1,3-dimethylimidazolium p-toluenesulfonate, 1-ethyl-3-methylimidazolium diethyl phosphate, 1-ethyl-3-methylimidazolium hexafluorophosphate, and 1-ethyl-3-methylimidazolium hexafluoroantimonate; and / or

[0026] The second ionic liquid includes one or more of the following: methyl 1,3-dimethylimidazolium sulfate, dimethyl 1,3-dimethylimidazolium phosphate, tetrafluoroborate, trifluoromethanesulfonate, perchlorate, nitrate, trifluoroacetate, ethyl-3-methylimidazolium bromide, iodide, ethyl 1-ethyl-3-methylimidazolium sulfate, methyl 1-ethyl-3-methylimidazolium sulfate, and dimethyl 1-ethyl-3-methylimidazolium phosphate.

[0027] In some embodiments, the microporous membrane includes a polymer comprising homopolymers or copolymers of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene and norbornene, thermoplastic polyurethane elastomers, polyolefin elastomers, polyethylene terephthalate, polyurethane, or mixtures of polymers thereof.

[0028] In some embodiments, the polymer includes one or more of polypropylene, polyethylene terephthalate, and ethylene-propylene-norbornene polymer.

[0029] In some embodiments, the polymer has a viscosity-average molecular weight of 2 million to 6 million.

[0030] In some embodiments, the microporous membrane further contains additives, including one or more of antioxidants, metal soaps, light stabilizers, antistatic agents, and antifogging agents.

[0031] In some embodiments, the additive accounts for 0.01% to 5% of the total mass of the microporous membrane.

[0032] In some embodiments, the additive accounts for 0.1% to 1% of the total mass of the microporous membrane.

[0033] In some embodiments, the microporous membrane comprises multiple layers of sub-membranes stacked together, with different polymers in adjacent sub-membranes, and both containing the first ionic liquid.

[0034] In some embodiments, the average pore size of the microporous membrane is 100 nm to 200 nm.

[0035] In some embodiments, the transverse tensile strength of the microporous membrane is 2500 kgf / cm². 2 ~5000 kgf / cm 2 .

[0036] In some embodiments, the puncture strength of the microporous membrane is 150 gf to 1000 gf.

[0037] In some embodiments, the porosity of the microporous membrane is 70% to 90%.

[0038] In some embodiments, the thickness of the microporous membrane is 2 μm to 10 μm.

[0039] In some embodiments, the thickness of the microporous membrane is 2 μm to 8 μm.

[0040] A method for preparing a microporous membrane for composite current collectors includes the following steps:

[0041] A mixture comprising polymer and ionic liquid is mixed, extruded, and cooled to form an intermediate film.

[0042] The intermediate membrane is stretched, extracted, and heat-set to obtain the microporous membrane.

[0043] The ionic liquid in the microporous membrane comprises 0.01% to 6% by mass, the microporous membrane has a permeability of 10 sec / 100 cc to 50 sec / 100 cc, and the longitudinal tensile strength of the microporous membrane is 3500 kgf / cm². 2 ~5000 kgf / cm 2 The ionic liquid includes a first ionic liquid, wherein the hydrophilic-lipophilic balance value of the first ionic liquid is <10.

[0044] In some embodiments, the hydrophilic-lipophilic balance value of the first ionic liquid is 0 to 9.8.

[0045] In some embodiments, the hydrophilic-lipophilic balance value of the first ionic liquid is 3 to 7.

[0046] In some embodiments, the molecular weight of the first ionic liquid is 200 to 1000.

[0047] In some embodiments, the boiling point of the first ionic liquid is 200°C to 500°C.

[0048] In some embodiments, the mass ratio of the polymer to the ionic liquid is (5-19):(81-95).

[0049] In some embodiments, the extractant in the extraction step includes dichloromethane; or...

[0050] In the extraction step, the extractant includes a mixed solvent of a second ionic liquid and water, wherein the hydrophilic-lipophilic balance value of the second ionic liquid is greater than that of the first ionic liquid.

[0051] In some embodiments, the difference between the hydrophilic-lipophilic balance value of the second ionic liquid and the hydrophilic-lipophilic balance value of the first ionic liquid is <10.

[0052] In some embodiments, the hydrophilic-lipophilic balance value of the second ionic liquid is 10 to 20.

[0053] In some embodiments, the boiling point of the second ionic liquid is 100°C to 500°C.

[0054] In some embodiments, the viscosity of the second ionic liquid at 25°C is 10 cp to 50 cp.

[0055] In some embodiments, the viscosity of the second ionic liquid at 25°C is 5 cp to 20 cp lower than that of the first ionic liquid at 25°C.

[0056] In some embodiments, the extractant comprises a mixed solvent of a second ionic liquid and water, wherein the mass percentage concentration of the second ionic liquid in the extractant is 5% to 10%.

[0057] In some embodiments, after the extraction step, the process further includes washing the extracted microporous membrane intermediate with water, separating it with a reverse osmosis membrane, and drying it.

[0058] In some embodiments, the water washing of the extracted microporous membrane intermediate includes a first-stage water washing and a second-stage water washing. The mass percentage concentration of the second ionic liquid in the water washing solution used in the first-stage water washing is 3% to 5%, and the mass percentage concentration of the second ionic liquid in the water washing solution used in the second-stage water washing is not higher than 1%, and the mass percentage concentration of the second ionic liquid in the water washing solution used in the second-stage water washing is not higher than 30 wt% of the mass percentage concentration of the second ionic liquid in the water washing solution used in the first-stage water washing.

[0059] In some embodiments, the temperature T of the mixed extrusion 挤 For T 挤 =Polymer melting point T m +(15℃~60℃).

[0060] In some embodiments, in the step of mixing and extruding a mixture comprising a polymer and an ionic liquid, the melt index of the polymer is 0.6 g / 10 min to 10 g / 10 min.

[0061] In some embodiments, the melt index of the polymer is 0.6 g / 10 min to 6 g / 10 min.

[0062] A composite current collector, the composite current collector comprising a support layer, the support layer comprising the microporous membrane described above or a microporous membrane prepared by the preparation method described above.

[0063] The microporous membranes of some embodiments of this application have a suitable ionic liquid content, which enables the microporous membrane to maintain good air permeability and tensile strength. When applied to the field of composite current collectors, the microporous membranes with a suitable ionic liquid content help to improve the electroplating effect in the current collector preparation process, thereby having a certain effect on controlling the moisture and resistance of the current collector surface.

[0064] The above-mentioned method for preparing microporous membranes uses ionic liquids, which have good compatibility with various polymers and can be applied to more polymer systems. Furthermore, the microporous membranes prepared by this method have better performance. Detailed Implementation

[0065] The technical solutions of this application will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0066] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0067] Unless otherwise stated or in case of conflict, the terms or phrases used in this application shall have the following meanings:

[0068] In this application, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" and "second" may explicitly or implicitly include at least one of those features.

[0069] In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise expressly and specifically limited.

[0070] In this application, "one or more" refers to any one, two, or more of the listed items. "Multiple" refers to any two or more of the listed items.

[0071] Unless otherwise specified, all percentage concentrations mentioned in this application refer to the final concentration. The final concentration refers to the proportion of the added component in the system after the addition of that component.

[0072] In this application, terms such as "further," "even more," "particularly," "for example," "like," "example," and "exemplary" are used for descriptive purposes to indicate a connection in the coverage of different technical solutions presented earlier and later, but should not be construed as limiting the preceding technical solution or restricting the scope of protection herein. Unless otherwise specified herein, A (e.g., B) indicates that B is a non-limiting example of A, and it can be understood that A is not limited to B.

[0073] In this application, "optionally," "optionally," and "optional" mean that something is optional, that is, it is selected from either "present" or "absent." If multiple "options" appear in a technical solution, unless otherwise specified and there are no contradictions or mutual constraints, each "option" is independent. In this application, descriptions such as "optionally contains" and "optionally includes" indicate "contains or does not contain." "Optional component X" indicates whether component X exists or does not exist, or whether component X is contained or not.

[0074] When a numerical range is disclosed in this application, the range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to an integer, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed in this application should be understood to include any and all subranges to which they are included.

[0075] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0076] The terms "comprising" and "having," and any variations thereof, used in the embodiments of this application, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the steps or units listed, but may optionally include steps or units not listed, or may optionally include other steps or components inherent to such processes, methods, products, or devices.

[0077] In this application, the reference to "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this application can be combined with other embodiments.

[0078] In this application, a microporous membrane refers to a membrane formed having a large number of micropores on its surface and interior, with at least some of the micropores being interconnected, allowing gas or liquid to pass through from one surface of the membrane to the other. The “surface” of the membrane refers to the surface with the largest surface area.

[0079] Currently, microporous membranes are struggling to meet the increasingly demanding requirements of the market for membrane products. For example, in the field of composite current collectors, comprehensive performance requirements such as thickness, porosity, air permeability, and mechanical strength are being put forward for microporous membranes. Therefore, it is urgent to develop a microporous membrane that can meet the market requirements of the above fields.

[0080] Researchers have creatively discovered that the overall performance of microporous membranes obtained through industrialization is difficult to improve significantly. One reason is the limitation of existing preparation processes, which mainly include wet and dry processes. The wet process employs thermally induced phase separation, requiring good compatibility between the pore-forming agent and the film-forming substrate. Currently, paraffin oil is widely used as the pore-forming agent in industrial production, and its compatible substrate material is polyethylene (PE). However, it is difficult to adapt to other polymer materials and apply them to industrial production. This is because the current paraffin oil co-extrusion system cannot meet the requirements for uniform mixing with polymer melts from different systems to obtain high-quality intermediate films. For example, a melt mixture of PE and paraffin oil with a low melt index (below 0.6 g / 10 min, 190℃, 2.16 kg) (PE mass percentage of the melt is 30%, also known as melt solids content) has poor melt flowability and cannot meet the requirements for cooling and film formation, thus failing to produce qualified, high-quality membrane products. To increase fluidity, existing solutions often involve reducing the melt solids content to below 15%. This leads to a significant amount of paraffin oil seeping out during phase separation, affecting subsequent manufacturing processes such as unstable or ineffective stretching. The resulting membranes are of extremely poor quality and fail to meet application requirements. Another solution is to continuously increase the extrusion temperature to improve melt fluidity; however, this causes polymer molecule degradation and paraffin oil volatilization, resulting in products that do not meet requirements. Dry processes are applicable to a wider range of polymer systems compared to wet processes, but microporous membranes produced by dry processes generally exhibit inferior heat resistance, thickness, pore size uniformity, and permeability compared to those produced by wet processes. For example, microporous membranes produced by dry uniaxial stretching have poor transverse strength, poor consistency, difficulty in achieving thinner membranes, and higher permeability. The dry biaxial stretching process has very high requirements for the preparation environment. If environmental control and the control of dust and burrs on the electrode are insufficient, the resulting microporous membrane will have a high short-circuit rate. Moreover, because this process is difficult to control the pore size, it is prone to uneven pore size distribution. Therefore, the performance improvement of microporous membranes obtained in current industrial applications is very limited. Traditional processes face significant limitations in ensuring that microporous membranes have a thin thickness range while also maintaining various properties such as mechanical strength.

[0081] Based on this, this application provides a microporous membrane with good overall performance, and a preparation method for obtaining a microporous membrane with good overall performance that is suitable for industrialization.

[0082] In a first aspect, this application provides a microporous membrane for composite current collectors, which contains an ionic liquid. The mass percentage of the ionic liquid in the microporous membrane is 0.01% to 6%, the air permeability of the microporous membrane is 10 sec / 100 cc to 50 sec / 100 cc, and the longitudinal tensile strength of the microporous membrane is 3500 kgf / cm². 2 ~5000 kgf / cm 2 Ionic liquids include first ionic liquids, and the hydrophilic-lipophilic balance value of first ionic liquids is <10.

[0083] Microporous membranes with appropriate ionic liquid content maintain good air permeability and tensile strength. When applied to composite current collectors, microporous membranes with appropriate ionic liquid content help improve the electroplating effect during the preparation of composite current collectors, thereby having a certain effect on controlling the moisture and resistance on the surface of composite current collectors.

[0084] The first ionic liquid is an ionic salt that is liquid below 100°C. The first ionic liquid can form a homogeneous phase with the polymer under high temperature conditions. Below 100°C, it will separate into an ionic liquid and a polymer solid phase, which is beneficial for preparation methods applicable to different polymer systems and can obtain microporous membranes with good uniformity.

[0085] The microporous membrane contains several fibrils, which are interwoven to form pores. Ionic liquids adhere to the surface of the fibrils inside the microporous membrane through their oleophilic ends. The fibrils in the microporous membrane are obtained by stretching and oriented the polymer during film preparation. The extractant extracts the pore-forming agent from the film to form pores.

[0086] In some embodiments, the longitudinal tensile strength (MD strength) of the microporous membrane is 3500 kgf / cm². 2 ~5000 kgf / cm 2 For example, the molecular weight density (MD) of a microporous membrane can be, but is not limited to, 3500 kgf / cm². 2 4000 kgf / cm 2 4500kgf / cm 2 5000 kgf / cm 2 Or the range formed by any two of these values.

[0087] In some embodiments, the air permeability of the microporous membrane is 10 sec / 100 cc to 50 sec / 100 cc. For example, the air permeability of the microporous membrane may be, but is not limited to, 10 sec / 100 cc, 20 sec / 100 cc, 30 sec / 100 cc, 40 sec / 100 cc, 50 sec / 100 cc, or any combination of these values.

[0088] In some embodiments, the hydrophilic-lipophilic balance (HLB) value of the first ionic liquid is 0 to 9.8. For example, the HLB value of the first ionic liquid can be, but is not limited to, any value or a range between any two values ​​from 0, 1, 2, 3, 3.2, 3.5, 3.6, 3.8, 4, 4.1, 4.3, 4.6, 4.7, 4.8, 4.9, 5, 5.1, 5.3, 5.6, 5.9, 6, 6.2, 6.6, 6.8, 7, 7.3, 8, 8.2, 8.4, 8.9, 9, 9.5, 9.8. A hydrophilic-lipophilic balance value within the above range indicates good lipophilicity, enabling better binding with polymers in microporous membranes.

[0089] Optionally, the hydrophilic-lipophilic balance value of the first ionic liquid is 3 to 7. Therefore, when applied to battery current collectors, controlling the HLB value within a certain range helps to further control the moisture content on the surface of the composite current collector, while also further improving the electroplating effect and assembly efficiency. The moisture content on the surface of the composite current collector can be tested using the Karl Fischer method (volume method), based on the volume of reagent consumed and the titration. The HLB value of the ionic liquid in this application can be determined and calculated using emulsification methods, critical micelle concentration, etc., and this application is not limited to these methods. Specifically, for example, turpentine oil (required HLB=16) and cottonseed oil (required HLB=6) can be mixed in a certain proportion to prepare an oil phase (covering the HLB range of 3 to 18); take 15g of oil phase + 5g of the sample to be tested + 80g of water, homogenize and emulsify (1500rpm, 5 minutes), let stand for 24 hours and observe the layering. The HLB value of the oil phase corresponding to the sample with the best stability is the HLB value of the sample to be tested; or, for example, use an HLB value measuring instrument (HLB-Master 3000) to measure it.

[0090] In some embodiments, the molecular weight of the first ionic liquid is 200 to 1000. For example, the molecular weight of the first ionic liquid can be, but is not limited to, any value or a range between any two of 200, 300, 400, 500, 600, 700, 800, 900, and 1000. The molecular weight of the ionic liquid can be determined directly by mass spectrometry. The sample is ionized using techniques such as electrospray ionization (ESI) or matrix-assisted laser desorption / ionization (MALDI), and the molecular ion peak of the ionic liquid is found by obtaining the mass spectrum of the sample using a mass spectrometer. The molecular weight is obtained based on the mass-to-charge ratio of the molecular ion peak. Of course, nuclear magnetic resonance, light scattering, and other methods can also be used for testing; this application is not limited to these methods.

[0091] The molecular weight of the first ionic liquid meets the above range, which is beneficial to further improve the pore size uniformity and mechanical strength of the microporous membrane. At the same time, when the microporous membrane is applied to the field of battery composite current collectors, it further improves the consistency of the electrochemical performance of the composite current collector. Specifically, the standard deviation value of the sheet resistance of the composite current collector surface can be used for characterization. For example, a composite current collector is made by electroplating a 1μm thick aluminum foil or copper foil on the surface of the microporous membrane. The standard deviation value is then measured at five positions around and in the center of the composite current collector using a four-probe resistance tester.

[0092] In some embodiments, the boiling point of the first ionic liquid is 200℃ to 500℃. Specifically, the boiling point of the first ionic liquid may be, but is not limited to, 200℃, 205℃, 210℃, 215℃, 220℃, 225℃, 230℃, 235℃, 240℃, 245℃, 250℃, 255℃, 275℃, 280℃, 285℃, 290℃, 295℃, 300℃, 320℃, 340℃, 350℃, 360℃, 380℃, 400℃, 420℃, 440℃, 450℃, 460℃, 480℃, 500℃, or a range of any two of these values. The boiling point can be determined by conventional testing methods, including but not limited to distillation, and can be obtained by referring to the standard GB / T 616-2006.

[0093] The first ionic liquid has a suitable boiling point, which further enhances the mechanical strength of the microporous membrane while ensuring that the microporous membrane has a good pore structure.

[0094] In some embodiments, the viscosity of the first ionic liquid at 25°C is 20 cp to 100 cp. The viscosity can be determined using existing viscosity testing methods, such as placing a sample treated in a constant-temperature water bath at 25°C for 1 hour into a rotational viscometer and taking the viscosity reading. Specifically, the viscosity of the first ionic liquid can be any value or a range between any two values ​​from 20 cp, 30 cp, 40 cp, 50 cp, 60 cp, 70 cp, 80 cp, 90 cp, to 100 cp.

[0095] The aforementioned first ionic liquid exhibits suitable viscosity and good fluidity at room temperature, which helps to further improve the pore uniformity of the microporous membrane while maintaining good thickness consistency. Therefore, when applied to composite current collectors, it helps to further improve the electroplating effect during the composite current collector preparation process, thus having a certain effect on controlling the moisture and resistance of the composite current collector surface. Specifically, the resistance of the composite current collector can be characterized by its surface sheet resistance. For example, a composite current collector is prepared by electroplating a 1μm thick copper or aluminum foil onto the surface of a microporous membrane. A four-probe resistance meter is used to measure the resistance at five locations around the perimeter and center of the composite current collector, and the average value is taken.

[0096] In some embodiments, the first ionic liquid is selected from one or more of ionic liquids such as imidazole salts, pyridine salts, quaternary ammonium salts, quaternary phosphonium salts, and pyrrolidines.

[0097] Specifically, the first ionic liquid is selected from 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium methyl sulfate, 1,3-dimethylimidazolium dimethyl phosphate, 1,3-dimethylimidazolium tetrafluoroborate, 1,3-dimethylimidazolium hexafluorophosphate, 1,3-dimethylimidazolium hexafluoroantimonate, 1,3-dimethylimidazolium trifluoromethanesulfonate, 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium perchlorate, 1,3-dimethylimidazolium nitrate, 1,3-dimethylimidazolium trifluoromethanesulfonate, 1,3-dimethylimidazolium perchlorate, 1,3-dimethylimidazolium nitrate, 1,3-dimethylimidazolium trifluoromethanesulfonyl nitrate, and 1,3-dimethylimidazolium trifluoromethanesulfonyl nitrate. One or more of the following: 1,3-dimethylimidazolium methanesulfonate, 1,3-dimethylimidazolium p-toluenesulfonate, 1,3-dimethylimidazolium trifluoroacetate, 1-ethyl-3-methylimidazolium bromide, 1-ethyl-3-methylimidazolium iodide, 1-ethyl-3-methylimidazolium ethyl sulfate, 1-ethyl-3-methylimidazolium methyl sulfate, 1-ethyl-3-methylimidazolium diethyl phosphate, 1-ethyl-3-methylimidazolium dimethyl phosphate, 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium hexafluorophosphate, and 1-ethyl-3-methylimidazolium hexafluoroantimonate.

[0098] It is understood that the above only lists some commonly used types of ionic liquids, but it is not limited to these. Other ionic liquids can also be used, as long as they meet the above conditions. They will not be listed exhaustively here.

[0099] In some embodiments, the first ionic liquid comprises one or more of the following: 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium hexafluorophosphate, 1,3-dimethylimidazolium hexafluoroantimonate, 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium methanesulfonate, 1,3-dimethylimidazolium p-toluenesulfonate, 1-ethyl-3-methylimidazolium diethyl phosphate, 1-ethyl-3-methylimidazolium hexafluorophosphate, and 1-ethyl-3-methylimidazolium hexafluoroantimonate.

[0100] In some embodiments, the mass percentage of the first ionic liquid in the microporous membrane is 0.01% to 6%. For example, the mass percentage of the first ionic liquid may be, but is not limited to, 0.01%, 0.02%, 0.05%, 0.1%, 0.12%, 0.15%, 0.18%, 0.2%, 0.22%, 0.25%, 0.28%, 0.3%, 0.32%, 0.35%, 0.38%, 0.4%, 0.42%, 0.45%, 0.48%, 0.5%, 0.6%, 0.8%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 6%, or a range of any two of these values. In some embodiments, the mass percentage of the first ionic liquid in the microporous membrane is 0.1% to 5%. Furthermore, in some embodiments, the mass percentage of the first ionic liquid in the microporous membrane is 0.1% to 0.5%. By controlling the content of the first ionic liquid, the internal resistance of the composite current collector can be reduced, and the energy density can be increased.

[0101] In some embodiments, the microporous membrane comprises a polymer, including homopolymers or copolymers of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene and norbornene, thermoplastic polyurethane elastomers (TPU), polyolefin elastomers (POE), polyethylene terephthalate (PET), polyurethane (PU), or mixtures of polymers thereof.

[0102] In some embodiments, the polymer includes one or more of polypropylene, polyethylene terephthalate (PET), and ethylene-propylene-norbornene polymers.

[0103] When applied to composite current collectors, in some embodiments, the polymer's viscosity-average molecular weight is 2 million to 6 million, which helps to further improve the mechanical strength of the microporous membrane and thus increase the energy density of the composite current collector. The viscosity-average molecular weight of the polymer can be determined using conventional methods, including but not limited to measuring the intrinsic viscosity at 135°C with decahydronaphthalene as a solvent according to GB1841-1980, and substituting this intrinsic viscosity into the following formula (Formula 1) to calculate the viscosity-average molecular weight: Viscosity-average molecular weight (Mv) = (5.34 × 10⁻⁶) / ( ... 4 )×[η] 1.49 (Formula 1).

[0104] The energy density of the composite current collector can be determined by the following steps:

[0105] 1) The positive electrode active material NCM811, conductive agent SP, conductive agent KS-6, and binder PVDF are mixed in solvent NMP at a mass ratio of 90:2:1:3 to prepare a positive electrode slurry. The slurry is then coated onto the aluminum foil surface of the composite current collector (the aluminum foil thickness on one side is 1μm). The slurry is then dried in an oven at 95℃ and rolled on a roller press to obtain the positive electrode.

[0106] 2) The active material artificial graphite, the conductive agent acetylene black, and the binder CMC / SBR (sodium carboxymethyl cellulose / styrene-butadiene rubber, with a mass ratio of 1:1) are added to water in a mass ratio of 90:5:5 and dispersed evenly to prepare a negative electrode slurry. This slurry is coated onto the copper foil surface of the composite current collector (the copper foil thickness on one side is 1 μm). After drying at 85°C in an oven, the negative electrode is obtained by rolling in a roller press.

[0107] 3) In a dry room with a dew point <-40℃, after stacking and packaging the positive electrode, separator, and negative electrode sheets, follow the formula of 2.5g / m 2 The electrolyte (Xinzhoubang, LBC3008A) is injected in a specific ratio to obtain the battery. The separator can be a commercially available battery separator, such as a 7μm thick polyethylene microporous membrane (38% porosity) from Shenzhen Xingyuan Material Technology Co., Ltd.

[0108] Test method: The battery is tested within a voltage range of 3.0V to 4.5V. The discharge plateau voltage is determined by cyclic voltammetry at a rate of 0.5C. Then, the battery is discharged at a constant current of 1C from a fully charged state, and the discharge capacity is measured. The mass energy density is calculated using the following formula:

[0109] In some embodiments, the viscosity-average molecular weight of the polymer can be between 2 million and 6 million. Specifically, the viscosity-average molecular weight of the polymer can be any value or a range between any two values ​​from 2 million, 2.1 million, 2.2 million, 2.3 million, 2.4 million, 2.5 million, 2.6 million, 2.7 million, 2.8 million, 2.9 million, 3 million, 3.1 million, 3.2 million, 3.3 million, 3.4 million, 3.5 million, 3.6 million, 3.7 million, 3.8 million, 3.9 million, 4 million, 4.1 million, 4.2 million, 4.3 million, 4.4 million, 4.5 million, 4.6 million, 4.7 million, 4.8 million, 4.9 million, 5 million, 5.1 million, 5.2 million, 5.3 million, 5.4 million, 5.5 million, 5.6 million, 5.7 million, 5.8 million, 5.9 million, to 6 million.

[0110] In some embodiments, the microporous membrane further contains a second ionic liquid, which is an ionic salt that is liquid below 50°C. The hydrophilic-lipophilic balance value of the second ionic liquid is greater than that of the first ionic liquid. This indicates that the second ionic liquid is more hydrophilic than the first ionic liquid, and the first ionic liquid is more lipophilic than the second ionic liquid. When applied to the field of composite current collectors, this helps to further improve the electroplating effect during the preparation process of composite current collectors, thereby further controlling the moisture and resistivity on the surface of the composite current collector.

[0111] In some embodiments, the difference between the hydrophilic-lipophilic balance value of the second ionic liquid and the hydrophilic-lipophilic balance value of the first ionic liquid is <10, and can be selected as 1 to 9. For example, the difference between the hydrophilic-lipophilic balance value of the second ionic liquid and the hydrophilic-lipophilic balance value of the first ionic liquid can be, but is not limited to, 9, 8, 7, 6, 5, 4, 3, 2, 1 or any combination of these values.

[0112] Specifically, the hydrophilic-lipophilic balance value of the second ionic liquid is 10–20. For example, the hydrophilic-lipophilic balance value of the second ionic liquid can be, but is not limited to, any value from 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or a range of any two of these values. Controlling the hydrophilic-lipophilic balance value of the second ionic liquid within a certain range is beneficial for further improving the permeability of the microporous membrane. When applied in the field of composite current collectors, it helps to further improve the electroplating effect during the preparation process of composite current collectors, thereby further controlling the moisture and resistance on the surface of the composite current collector to obtain better results.

[0113] Specifically, the hydrophilicity and lipophilicity of the second ionic liquid can also be determined by emulsification testing. For example, an oil phase can be prepared by mixing polyoxyethylene (required HLB=20) and cottonseed oil (required HLB=6) in a certain proportion; take 15g of oil phase + 5g of the sample to be tested + 80g of water, homogenize and emulsify (1500rpm, 5 minutes), let it stand for 24 hours and observe the layering. The HLB value of the oil phase corresponding to the sample with the best stability is the HLB value of the sample to be tested; or, for example, it can be measured using an HLB value measuring instrument (HLB-Master 3000).

[0114] In some embodiments, the boiling point of the second ionic liquid is 100°C to 500°C. For example, the boiling point of the second ionic liquid may be, but is not limited to, any value of 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, 205°C, 210°C, 215°C, 220°C, 225°C, 230°C, 235°C, 240°C, 245°C, 250°C, 255°C, 275°C, 280°C, 285°C, 290°C, 295°C, 300°C, 320°C, 340°C, 350°C, 360°C, 380°C, 400°C, 420°C, 440°C, 450°C, 460°C, 480°C, or 500°C, or a range of any two of these values. The above configuration helps to further improve the air permeability of the microporous membrane, and when applied to the field of composite current collectors, it helps to further reduce the resistance of the composite current collector.

[0115] In some embodiments, the viscosity of the second ionic liquid at 25°C is 10 cp to 50 cp. For example, the viscosity of the second ionic liquid can be any value or a range between any two values ​​from 10 cp, 12 cp, 15 cp, 18 cp, 20 cp, 22 cp, 25 cp, 28 cp, 30 cp, 32 cp, 35 cp, 38 cp, 40 cp, 42 cp, 45 cp, 48 cp, to 50 cp. Adopting the above setting is beneficial for further improving the air permeability of the microporous membrane, and thus, when applied in the field of composite current collectors, it is beneficial for further reducing the resistance of the composite current collector.

[0116] In some embodiments, the viscosity of the second ionic liquid at 25°C is 5 to 20 cp lower than that of the first ionic liquid at 25°C. Specifically, the viscosity of the second ionic liquid at 25°C can be any value or a range between any two values ​​that are lower than the viscosity of the first ionic liquid at 25°C by 5, 7, 9, 11, 13, 15, 17, 19, or 20 cp. By controlling the viscosity difference between the second and first ionic liquids, it is beneficial to further improve the pore size uniformity of the microporous membrane and better control its porosity. When applied to the field of composite current collectors, this helps to further reduce the resistance of the composite current collector.

[0117] In some embodiments, the mass percentage of the second ionic liquid in the microporous membrane is 0.01% to 3%. For example, the mass percentage of the second ionic liquid can be, but is not limited to, any value or a range of any two of the following: 0.01%, 0.02%, 0.05%, 0.1%, 0.12%, 0.15%, 0.18%, 0.2%, 0.22%, 0.25%, 0.28%, 0.3%, 0.32%, 0.35%, 0.38%, 0.4%, 0.42%, 0.45%, 0.48%, 0.5%, 0.6%, 0.8%, 1%, 1.5%, 2%, 2.5%, and 3%. When the mass percentage of the second ionic liquid in the microporous membrane is 0.01% to 3%, the mass percentage of the first ionic liquid is also 0.01% to 3%, optionally 0.1% to 3%.

[0118] In some embodiments, the mass percentage of the second ionic liquid in the microporous membrane is 0.1% to 0.5%. Controlling the mass percentage of the second ionic liquid within a certain range is beneficial for further improving the product's permeability. Therefore, when applied to composite current collectors, it helps to further enhance the electroplating effect during the composite current collector preparation process and better control the moisture and resistance on the composite current collector surface. When the mass percentage of the second ionic liquid in the microporous membrane is 0.1% to 0.5%, the mass percentage of the first ionic liquid is 0.1% to 5%.

[0119] It is understood that the mass percentage of the second ionic liquid in this application can be tested by the following method: Weigh a 10cm × 10cm microporous membrane sample (M1), place the sample in 100mL of DCM solvent, sonicate at 100Hz for 15min, dry at 80℃ for 5min, weigh again (M2), and calculate using the following formula:

[0120] In some embodiments, the second ionic liquid is selected from one or more of ionic liquids such as imidazole salts, pyridine salts, quaternary ammonium salts, quaternary phosphonium salts, and pyrrolidines.

[0121] Specifically, the second ionic liquid is selected from 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium methyl sulfate, 1,3-dimethylimidazolium dimethyl phosphate, 1,3-dimethylimidazolium tetrafluoroborate, 1,3-dimethylimidazolium hexafluorophosphate, 1,3-dimethylimidazolium hexafluoroantimonate, 1,3-dimethylimidazolium trifluoromethanesulfonate, 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium perchlorate, 1,3-dimethylimidazolium nitrate, 1,3-dimethylimidazolium trifluoromethanesulfonate, 1,3-dimethylimidazolium perchlorate, 1,3-dimethylimidazolium nitrate ... One or more of the following: 1,3-dimethylimidazolium methanesulfonate, 1,3-dimethylimidazolium p-toluenesulfonate, 1,3-dimethylimidazolium trifluoroacetate, 1-ethyl-3-methylimidazolium bromide, 1-ethyl-3-methylimidazolium iodide, 1-ethyl-3-methylimidazolium ethyl sulfate, 1-ethyl-3-methylimidazolium methyl sulfate, 1-ethyl-3-methylimidazolium diethyl phosphate, 1-ethyl-3-methylimidazolium dimethyl phosphate, 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium hexafluorophosphate, and 1-ethyl-3-methylimidazolium hexafluoroantimonate.

[0122] It is understood that the above only lists some commonly used types of second ionic liquids, but it is not limited to these. Other ionic liquids can also be used, as long as they meet the above conditions. They will not be listed exhaustively here.

[0123] In some embodiments, the second ionic liquid comprises one or more of the following: methyl 1,3-dimethylimidazolium sulfate, dimethyl 1,3-dimethylimidazolium phosphate, tetrafluoroborate, trifluoromethanesulfonate, perchlorate, nitrate, trifluoroacetate, ethyl-3-methylimidazolium bromide, iodide, ethyl 1-ethyl-3-methylimidazolium sulfate, methyl 1-ethyl-3-methylimidazolium sulfate, and dimethyl 1-ethyl-3-methylimidazolium phosphate.

[0124] It is understood that the second ionic liquid and the first ionic liquid can be of the same type, as long as they meet at least one of the above-defined ranges for HLB value, molecular weight, boiling point, and viscosity at 25°C. For example, the HLB value can be controlled by changing the structure of the anion and cation, such as by changing the alkyl chain length of the cation or introducing different functional groups, or by adding some high-boiling solvents, such as high-boiling ethers (ether solvents with a boiling point ≥300°C. Specifically, this may include one or more of ethylene glycol phenyl ether and propylene glycol phenyl ether).

[0125] In some embodiments, the microporous membrane further contains additives, including one or more of known additives such as antioxidants, metal soaps, light stabilizers, antistatic agents, and antifogging agents. Metal soaps refer to metal salts formed by the reaction of metals other than alkali metals, metal oxides, or salts with fatty acids, rosin acids, naphthenic acids, etc., such as, but not limited to, calcium stearate and zinc stearate.

[0126] In some embodiments, the mass of the additive in the microporous membrane is 0.01% to 5% of the total mass of the microporous membrane.

[0127] In some embodiments, the mass percentage of the additive in the microporous membrane is 0.1% to 1% of the total mass of the microporous membrane. For example, the mass percentage of the additive in the total mass of the microporous membrane may be, but is not limited to, 0.01%, 0.02%, 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, 0.95%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or any range of these values. By controlling the mass percentage of the additive, it is beneficial to ensure the film-forming properties and quality of the microporous membrane while achieving the desired effects from the additive.

[0128] It is understood that this application does not impose any particular limitation on the way the additive is added to the microporous membrane. For example, it can be added by mixing with the main film-forming material of the microporous membrane during the preparation of the microporous membrane, or by immersing the intermediate membrane in a solution containing the additive at a certain stage of the preparation of the microporous membrane, or by coating the microporous membrane, etc.

[0129] In some embodiments, the microporous membrane comprises multiple layers of submembranes stacked together, wherein adjacent submembranes contain different polymers and all contain a first ionic liquid. It is understood that the difference in polymers between adjacent submembranes can be due to different types of polymers or different viscosity-average molecular weights of the polymers. It is also understood that the first ionic liquid in each submembrane may be the same or different.

[0130] This application does not specifically limit the average pore size of the microporous membrane. However, based on considerations of mechanical strength and electrochemical safety, in some embodiments, the average pore size of the microporous membrane is 100 nm to 200 nm. For example, the average pore size of the microporous membrane can be any value or a range between any two values ​​between 100 nm, 105 nm, 110 nm, 115 nm, 120 nm, 125 nm, 130 nm, 135 nm, 140 nm, 145 nm, 150 nm, 155 nm, 160 nm, 165 nm, 170 nm, 175 nm, 180 nm, 185 nm, 190 nm, 195 nm, and 200 nm.

[0131] The average pore size of microporous membranes can be measured using a capillary porosity meter, such as a PMI instrument (Jia Yun Co., Ltd., CFP-1500AE model) at 25°C with a Galwick immersion solution (surface tension of 15.9 dynes / cm at 25°C). The pore size is expressed in nm. Alternatively, other industry-standard testing methods can also be used.

[0132] This application does not specifically limit the mechanical strength of the microporous membrane. However, considering factors such as heat resistance, processability, and electrochemical safety, in some embodiments, the transverse tensile strength (TD strength) of the microporous membrane is 2500 kgf / cm². 2 ~5000 kgf / cm 2 For example, the TD strength of a microporous membrane can be, but is not limited to, 2500 kgf / cm². 2 3000 kgf / cm 2 3500kgf / cm 2 4000 kgf / cm 2 4500kgf / cm 2 5000 kgf / cm 2 Or the range formed by any two of these values.

[0133] In some embodiments, the puncture strength of the microporous membrane is 150 gf to 1000 gf. For example, the puncture strength of the microporous membrane may be, but is not limited to, 150 gf, 200 gf, 250 gf, 300 gf, 350 gf, 400 gf, 450 gf, 500 gf, 550 gf, 600 gf, 650 gf, 700 gf, 750 gf, 800 gf, 850 gf, 900 gf, 950 gf, 1000 gf, or a range of any two of these values.

[0134] This invention does not specifically limit the porosity of the microporous membrane; however, based on considerations of mechanical strength and electrochemical safety, in some embodiments, the porosity of the microporous membrane is 70% to 90%. For example, the porosity of the microporous membrane may be, but is not limited to, 70%, 75%, 80%, 85%, 90%, or any combination of these values.

[0135] This application does not particularly limit the thickness of the microporous membrane. However, considering mechanical strength and internal resistance, in some embodiments, the thickness of the microporous membrane is 2 μm to 10 μm. For example, the thickness of the microporous membrane can be, but is not limited to, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, 8 μm, 10 μm, or any combination thereof. In some embodiments, the thickness of the microporous membrane is 2 μm to 8 μm.

[0136] It is understood that the tensile strength (including longitudinal and transverse tensile strength) in this application can refer to GB / T 36363-2018. When testing transverse tensile strength, a strip with a longitudinal width of 15 mm is cut and clamped at both ends of the fixture, and the tensile testing machine speed is set to 200 mm / min. When testing the longitudinal tensile strength of the microporous membrane, a strip with a transverse width of 15 mm is cut and tested, with other parameter settings consistent with the conditions for the transverse tensile strength test. Puncture strength can be tested according to GB / T 36363-2018. After flattening and clamping the microporous membrane sample, puncture it at a rate of 300 mm / min, and determine the puncture strength data. Furthermore, porosity, air permeability, and thickness can be tested according to GB / T 36363-2018. Of course, other test methods can also be used for the aforementioned test items.

[0137] Secondly, this application provides a method for preparing a microporous membrane for a composite current collector, comprising the following steps:

[0138] A mixture comprising polymer and ionic liquid is mixed, extruded, and cooled to form an intermediate film.

[0139] The intermediate membrane was stretched, extracted, and heat-set to obtain a microporous membrane;

[0140] The microporous membrane contains 0.01% to 6% ionic liquid by mass, has a permeability of 10 sec / 100 cc to 50 sec / 100 cc, and a longitudinal tensile strength of 3500 kgf / cm². 2 ~5000 kgf / cm 2 The ionic liquid includes a first ionic liquid, wherein the hydrophilic-lipophilic balance value of the first ionic liquid is <10.

[0141] In some embodiments, the mass percentage of the ionic liquid in the microporous membrane is 0.1% to 5%. Specifically, the mass percentage of the ionic liquid in the microporous membrane can be any value or a range between any two values ​​from 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3.0%, 3.1%, 3.2%, 3.3%, 3.4%, 3.5%, 3.6%, 3.7%, 3.8%, 3.9%, 4.0%, 4.1%, 4.2%, 4.3%, 4.4%, 4.5%, 4.6%, 4.7%, 4.8%, 4.9%, to 5.0%. Controlling the mass percentage of ionic liquid in microporous membranes within a certain range helps improve the overall performance of microporous membranes in composite current collector applications.

[0142] Extensive research has revealed that mixing ionic liquids with polymers, especially those containing a first ionic liquid, without the need for commonly used paraffin oils or white oils, achieves better compatibility with various polymers, making it applicable to a wider range of polymer systems, and resulting in microporous membranes with superior performance. Therefore, this application provides a novel method entirely different from traditional membrane fabrication systems, offering a new research direction and approach for wet processes and solving the limitation on the use of polymer systems in traditional processes.

[0143] In some embodiments, the hydrophilic-lipophilic balance (HLB) value of the first ionic liquid is 0 to 9.8. For example, the HLB value of the first ionic liquid can be, but is not limited to, any value or a range between any two values ​​from 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 9.5, to 9.8. In some embodiments, the HLB value of the first ionic liquid is 3 to 7. The HLB value is used to characterize the hydrophilicity and lipophilicity of a substance; the smaller the HLB value, the stronger the lipophilicity, and vice versa. Generally, the HLB value of paraffin is 0, and the HLB value of polyethylene glycol is 20, which are used as standards. When the HLB value of the first ionic liquid is within the above range, it has good lipophilicity and can have good high-temperature compatibility with polymers in microporous membranes, making the above preparation method applicable to more polymer systems. The HLB value of this application can be determined and calculated by emulsification, critical micelle concentration, etc., and this application is not limited thereto.

[0144] In some embodiments, the molecular weight of the first ionic liquid is 200 to 1000. For example, the molecular weight of the first ionic liquid can be, but is not limited to, any value or a range between any two of 200, 300, 400, 500, 600, 700, 800, 900, and 1000. A suitable molecular weight of the first ionic liquid allows for good high-temperature compatibility with polymers, thus enabling its application in high-temperature preparation processes of more polymer systems and resulting in higher-quality microporous membranes. The molecular weight of the ionic liquid can be directly determined using mass spectrometry. Specifically, the sample is ionized using techniques such as electrospray ionization (ESI) or matrix-assisted laser desorption / ionization (MALDI), and the molecular ion peak of the ionic liquid is found by obtaining the mass spectrum of the sample using a mass spectrometer. The molecular weight is then obtained based on the mass-to-charge ratio of the molecular ion peak. Of course, nuclear magnetic resonance, light scattering, and other methods can also be used for testing; this application is not limited to these methods.

[0145] In some embodiments, the boiling point of the first ionic liquid is 200°C to 500°C. The higher boiling point of the first ionic liquid allows it to better form a homogeneous phase with the polymer at high temperatures, which further facilitates the formation of intermediate films with uniform thickness and better film-forming properties through blending and extrusion. This helps to form microporous films with superior overall performance, meeting the needs of different application scenarios. Specifically, the boiling point of the first ionic liquid may be, but is not limited to, 200℃, 205℃, 210℃, 215℃, 220℃, 225℃, 230℃, 235℃, 240℃, 245℃, 250℃, 255℃, 260℃, 265℃, 270℃, 275℃, 280℃, 285℃, 290℃, 295℃, 300℃, 320℃, 340℃, 350℃, 360℃, 380℃, 400℃, 420℃, 440℃, 450℃, 460℃, 480℃, 500℃, or a range of any two of these values. The boiling point can be determined by conventional testing methods, including but not limited to distillation.

[0146] In some embodiments, the viscosity of the first ionic liquid at 25°C is 20 cp to 100 cp. Specifically, the viscosity of the first ionic liquid can be any value or a range between any two values ​​from 20 cp, 30 cp, 40 cp, 50 cp, 60 cp, 70 cp, 80 cp, 90 cp, and 100 cp. The aforementioned first ionic liquid has a suitable viscosity at room temperature, good fluidity, which is beneficial for mixing with polymers and for better control of thickness, porosity, and pore size during the preparation process. The viscosity can be determined using conventional testing methods, including but not limited to reading the viscosity after placing the sample treated in a constant-temperature water bath at 25°C for 1 hour into a rotational viscometer.

[0147] In some embodiments, the first ionic liquid is selected from one or more of imidazole salts, pyridine salts, quaternary ammonium salts, quaternary phosphonium salts, and pyrrolidines. It is understood that the ionic liquid may also have functionalized groups, such as hydroxyl, carboxyl-functionalized ionic liquids, ether groups, ester groups, amino groups, cyano groups, etc.

[0148] Specifically, the first ionic liquid is selected from 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium methyl sulfate, 1,3-dimethylimidazolium dimethyl phosphate, 1,3-dimethylimidazolium tetrafluoroborate, 1,3-dimethylimidazolium hexafluorophosphate, 1,3-dimethylimidazolium hexafluoroantimonate, 1,3-dimethylimidazolium trifluoromethanesulfonate, 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium perchlorate, 1,3-dimethylimidazolium nitrate, 1,3-dimethylimidazolium trifluoromethanesulfonate, 1,3-dimethylimidazolium perchlorate, 1,3-dimethylimidazolium nitrate, 1,3-dimethylimidazolium trifluoromethanesulfonyl nitrate, and 1,3-dimethylimidazolium trifluoromethanesulfonyl nitrate. One or more of the following: 1,3-dimethylimidazolium methanesulfonate, 1,3-dimethylimidazolium p-toluenesulfonate, 1,3-dimethylimidazolium trifluoroacetate, 1-ethyl-3-methylimidazolium bromide, 1-ethyl-3-methylimidazolium iodide, 1-ethyl-3-methylimidazolium ethyl sulfate, 1-ethyl-3-methylimidazolium methyl sulfate, 1-ethyl-3-methylimidazolium diethyl phosphate, 1-ethyl-3-methylimidazolium dimethyl phosphate, 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium hexafluorophosphate, and 1-ethyl-3-methylimidazolium hexafluoroantimonate.

[0149] It is understood that the above only lists some commonly used types of ionic liquids, but it is not limited to these. Other ionic liquids can also be used, as long as they meet the above conditions. They will not be listed exhaustively here.

[0150] In some embodiments, the mixture comprising the polymer and the ionic liquid may further include a high-boiling-point ether solvent, for example, an ether solvent with a boiling point ≥300°C. Specifically, the ether solvent may be one or more of ethylene glycol phenyl ether and propylene glycol phenyl ether. Adding the aforementioned high-boiling-point ether solvent is beneficial for further improving the compatibility between the ionic liquid and the polymer.

[0151] In some embodiments, the mass ratio of polymer to ionic liquid is (5–19):(81–95). Specifically, the mass ratio of polymer to ionic liquid can be any value or a range between any two of the following: 5:95, 7:93, 9:91, 11:89, 13:87, 15:85, 17:83, 19:81. Controlling the mass ratio of polymer to ionic liquid within a certain range helps to control the melt flow during processing, improve the quality of intermediate films, and result in better overall performance of the prepared microporous films, such as pore formation consistency and mechanical strength. Optionally, the mass ratio of polymer to ionic liquid is (10–19):(81–90).

[0152] Specifically, the ionic liquid accounts for 81% to 95% of the total mass of the ionic liquid and polymer, and the polymer accounts for 5% to 19% of the total mass of the ionic liquid and polymer. Optionally, the ionic liquid accounts for 81% to 90% of the total mass of the ionic liquid and polymer, and the polymer accounts for 10% to 19% of the total mass of the ionic liquid and polymer.

[0153] In some embodiments, the polymer is selected from homopolymers or copolymers of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, and norbornene, thermoplastic polyurethane elastomers (TPU), polyolefin elastomers (POE), polyethylene terephthalate (PET), polyurethane (PU), or mixtures of polymers thereof. In some embodiments, the polymer is selected from one or more of polypropylene, polyethylene terephthalate, and ethylene-propylene-norbornene polymers. The application of a first ionic liquid to the microporous membrane expands the types of microporous membranes available.

[0154] Researchers have creatively discovered that traditional wet-process microporous membrane preparation methods use liquid paraffin oil as a pore-forming agent. However, due to the poor compatibility of paraffin oil with different polymer melt systems, it can only effectively use polyethylene (PE) as the main raw material for microporous membranes. This limits the production of membranes using other polymer raw material systems and restricts further improvements in the performance of microporous membranes, hindering their application in various fields. Currently, some raw materials with melting points above 170℃ (such as PP) cannot be processed into a suitable melt in existing paraffin oil melt systems, making it difficult to produce microporous membranes. Alternatively, dry processes can only produce microporous membranes with poor performance consistency (e.g., porosity, strength). This application, through extensive research, has found that using a first ionic liquid eliminates the need for traditional pore-forming agents such as liquid or solid paraffin oil, achieving better compatibility with a wider range of polymers. This makes it applicable to more polymer systems, and the resulting microporous membranes exhibit superior performance.

[0155] In some embodiments, in the step of mixing and extruding a mixture comprising a polymer and an ionic liquid, the mixing of the polymer and the ionic liquid can be done in one step or in multiple steps.

[0156] It can be understood that a single mixing involves mixing all the components, including the polymer and the ionic liquid, at once, where the polymer and the ionic liquid can be one or more types. Multiple mixing involves mixing the polymer and ionic liquid in two or more separate batches, where the polymer and ionic liquid content is the same or different in each batch, and the types of polymer and ionic liquid can also be the same or different in each batch.

[0157] In some embodiments of this application, the multiple mixing steps can meet the needs of microporous membrane products for more diverse performance and more differentiated application scenarios.

[0158] When applied to composite current collectors, in some embodiments, the polymer has a viscosity-average molecular weight of 2 million to 6 million, which helps to further improve the mechanical strength of the microporous membrane and thus improve the energy density of the composite current collector.

[0159] In some embodiments, the viscosity-average molecular weight of the polymer can be between 2 million and 6 million. Specifically, the viscosity-average molecular weight of the polymer can be any value or a range between any two values ​​from 2 million, 2.1 million, 2.2 million, 2.3 million, 2.4 million, 2.5 million, 2.6 million, 2.7 million, 2.8 million, 2.9 million, 3 million, 3.1 million, 3.2 million, 3.3 million, 3.4 million, 3.5 million, 3.6 million, 3.7 million, 3.8 million, 3.9 million, 4 million, 4.1 million, 4.2 million, 4.3 million, 4.4 million, 4.5 million, 4.6 million, 4.7 million, 4.8 million, 4.9 million, 5 million, 5.1 million, 5.2 million, 5.3 million, 5.4 million, 5.5 million, 5.6 million, 5.7 million, 5.8 million, 5.9 million, to 6 million. Controlling the viscosity-average molecular weight of the polymer within the range specified in this application helps control melt flow during processing and improves the quality of intermediate films. The viscosity-average molecular weight of the polymer can be determined using conventional methods, including but not limited to determining the intrinsic viscosity at 135°C using decahydronaphthalene as a solvent according to GB1841-1980, and substituting this intrinsic viscosity into the following formula (Formula 1) to calculate the viscosity-average molecular weight: Viscosity-average molecular weight (Mv) = (5.34 × 10⁻⁶) / ( ... 4 )×[η] 1.49 (Formula 1).

[0160] It is understood that the methods for preparing microporous membranes in some embodiments of this application are also applicable to the preparation of microporous membranes with other molecular weights and other thicknesses.

[0161] Unlike traditional wet processes, this method uses ionic liquids mixed with polymers, which has less impact on polymer cooling and crystallization, results in more thorough phase separation, and allows for greater flexibility in adjusting the solid content during extrusion, leading to better melt quality. This makes it easier to adjust micropore size, porosity, and other parameters. For example, traditional wet and dry stretching techniques struggle to achieve porosities above 60% for PE and PP microporous membranes, typically remaining between 20% and 60%. In some embodiments of this application, the preparation method uses ionic liquids mixed with polymers without the addition of pore-forming agents such as paraffin oil, enabling the preparation of microporous membranes with higher porosities. For instance, the porosity of the microporous membrane can be adjusted between 10% and 90%, and further, between 60% and 90%. Traditional wet and dry stretching processes struggle to achieve average pore sizes exceeding 60 nm for PE and PP microporous membranes, typically ranging from 20 nm to 50 nm. Some embodiments of this application utilize preparation methods that can adjust the average pore size range of the microporous membrane between 20 nm and 200 nm to meet the performance requirements of different applications (e.g., mechanical strength, electrochemical safety). Furthermore, the average pore size of the microporous membrane can reach over 100 nm, for example, between 100 nm and 200 nm. For instance, the average pore size can be any value or a range between any two values ​​from 100 nm, 105 nm, 110 nm, 115 nm, 120 nm, 125 nm, 130 nm, 135 nm, 140 nm, 145 nm, 150 nm, 155 nm, 160 nm, 165 nm, 170 nm, 175 nm, 180 nm, 185 nm, 190 nm, 195 nm, to 200 nm. Furthermore, in some embodiments of this application, using an ionic liquid containing a first ionic liquid as a pore-forming agent can prepare microporous membranes with lower permeability, for example, permeability of 10 sec / 100cc to 50 sec / 100cc (the permeability of microporous membranes prepared by conventional wet and dry processes is 90 sec / 100cc to 500 sec / 100cc).

[0162] In some embodiments, during the step of mixing and extruding the polymer and ionic liquid, the mixing and extrusion temperature T 挤 For T 挤 =Polymer melting point T m +(15℃~60℃). Control the temperature of the mixed extrusion at the polymer melting point T. m + (15℃~60℃) helps to reduce the risk of polymer degradation at excessively high temperatures while ensuring processability. At the same time, a lower extrusion temperature means that it is easier to reach the phase separation temperature. Therefore, the production efficiency is higher. Under the same conditions, the production line can be faster and more efficient. The final microporous membrane has better overall performance and meets the performance requirements of microporous membranes for different application scenarios.

[0163] Specifically, if the polymer is a polyolefin, its mixing extrusion temperature can be 145°C to 300°C. In some embodiments, the polymer is polyethylene, and its mixing extrusion temperature can be 145°C to 200°C. In other embodiments, if the polymer is polyethylene terephthalate (PET), its mixing extrusion temperature can be 250°C to 300°C. For example, the mixing extrusion temperature can be, but is not limited to, 145°C, 150°C, 155°C, 160°C, 165°C, 170°C, 175°C, 180°C, 185°C, 190°C, 195°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, 270°C, 280°C, 290°C, 300°C, or any combination of these values. As mentioned earlier, traditional wet-process paraffin oil-polyethylene co-extrusion biaxial systems require extrusion temperatures set high above the polymer's melting point to increase fluidity. However, in some embodiments of this application, compared to traditional paraffin oil pore-forming agents, the extrusion temperature can be further reduced, resulting in higher fluidity at a relatively lower and suitable temperature, thus reducing the risk of polymer degradation due to increased temperature. For example, under the same conditions, the extrusion temperature can be reduced by 30–60°C, effectively reducing the risk of polymer oxidation or molecular weight degradation due to high temperatures. Furthermore, a lower extrusion temperature means it is easier to reach the phase separation temperature, resulting in higher production efficiency. Under the same conditions, the production line can achieve faster and more efficient production speeds.

[0164] In some embodiments, during the step of extruding the mixture comprising the polymer and the ionic liquid, the melt index of the polymer is 0.6 g / 10 min to 10 g / 10 min. In some embodiments, the melt index of the polymer is 0.6 g / 10 min to 6 g / 10 min. The melt index is tested at 190°C and 21.6 kg. The ionic liquid includes a first ionic liquid, which gives the blend a suitable melt index, resulting in good high-temperature fluidity of the melt, thereby ensuring uniform mixing of the blend, improving the quality of the intermediate film, and reducing processing difficulty. The polymer can also be selected from polymers with a melting point below 300°C, such as polyolefins, which can also form good interactions with the ionic liquid, further improving the processability of the microporous membrane.

[0165] In some embodiments, during the step of mixing and extruding the mixture comprising the polymer and the ionic liquid, additives may also be added. These additives include one or more known additives such as antioxidants, metal soaps like calcium stearate and zinc stearate, light stabilizers, antistatic agents, and antifogging agents. Adding additives can improve the properties of the microporous membrane, such as its antioxidant properties.

[0166] Specifically, the mass ratio of additive to polymer is 1:(20 to 10000). In some embodiments, the mass ratio of additive to polymer is 1:(100 to 1000). For example, the mass ratio of additive to polymer may be, but is not limited to, 1:20, 1:50, 1:80, 1:100, 1:150, 1:200, 1:250, 1:300, 1:350, 1:400, 1:450, 1:500, 1:550, 1:600, 1:650, 1:700, 1:750, 1:800, 1:850, 1:900, 1:950, 1:1000, 1:2000, 1:3000, 1:5000, 1:8000, 1:10000, or any range of two of these values. By controlling the mass ratio of additives to polymers, it is beneficial to ensure the film-forming properties and quality of microporous membranes while achieving the corresponding effects brought by additives.

[0167] It is understood that the additive is not limited to the step of mixing and extruding the mixture including the polymer and the ionic liquid. In other embodiments, the additive can also be added by immersing the intermediate membrane in a solution containing the additive at a certain stage of the preparation process of the microporous membrane, or by coating the microporous membrane. This application does not make any particular limitation.

[0168] Specifically, in some embodiments, the preparation method of some embodiments of this application may further include steps such as cooling and molding, stretching, extraction, and heat setting.

[0169] In some embodiments, during the cooling and forming step, the cooling method may be rapid roller contact cooling, or other cooling methods such as water cooling and air cooling, or a combination of two or more cooling methods, to cool and form the sheet. The cooling and forming temperature is 10℃ to 60℃. In some embodiments, the cooling and forming temperature is 10℃ to 30℃. For example, the cooling and forming temperature may be, but is not limited to, any value or a range of any two of the following: 10℃, 12℃, 14℃, 15℃, 16℃, 18℃, 20℃, 22℃, 24℃, 25℃, 26℃, 28℃, 30℃, 32℃, 34℃, 35℃, 36℃, 38℃, 40℃, 42℃, 44℃, 45℃, 46℃, 48℃, 50℃, 52℃, 54℃, 55℃, 56℃, 58℃, and 60℃. Controlling the cooling and molding temperature within the range specified in this application helps to achieve more thorough phase separation, improves the quality of the intermediate membrane, and can control the comprehensive properties of the final microporous membrane, such as pore size uniformity and mechanical strength, while also taking into account the economic efficiency of the process.

[0170] In some embodiments, the biaxial stretching process may involve first performing a longitudinal stretch with a stretch ratio of 3 to 19 times, followed by a transverse stretch with a stretch ratio of 5 to 19 times. In some embodiments, the longitudinal stretch ratio is 10 to 19 times. For example, the longitudinal stretch ratio may be, but is not limited to, any value from 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, and 19 times, or a range of any two of these values. In some embodiments, the transverse stretch ratio is 10 to 19 times. For example, the transverse stretch ratio may be, but is not limited to, any value from 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, and 19 times, or a range of any two of these values.

[0171] In other embodiments, the biaxial stretching process may also involve simultaneous transverse and longitudinal stretching, with a total stretching ratio, for example, ranging from 15 to 361 times. In some embodiments, the total stretching ratio is 100 to 361 times. For example, the total stretching ratio may be, but is not limited to, any value from 15, 30, 50, 80, 100, 120, 140, 160, 180, 200, 220, 240, 260, 280, 300, 320, 340, 360, and 361 times, or a range consisting of any two of these values. It is understood that controlling the biaxial stretching ratio within the range of this application allows the intermediate film after cooling and molding to be oriented in each stretching direction, thereby facilitating the control of the microporous film to obtain suitable mechanical strength or stiffness, and thus meeting the usage requirements of different application scenarios.

[0172] In some embodiments, the extraction temperature is 15°C to 50°C. For example, the extraction temperature can be, but is not limited to, any value among 15°C, 20°C, 25°C, 30°C, 35°C, 40°C, 45°C, and 50°C, or a range of any two of these values. Using the above settings is beneficial for improving extraction efficiency, controlling the content of ionic liquid and extractant on the final microporous membrane, controlling the pore uniformity of the microporous membrane, exhibiting suitable electrochemical safety, and thus meeting the usage requirements of different application scenarios.

[0173] In some embodiments, the stretching ratio for transverse heat setting is 1 to 2 times, and the heat setting temperature is 90 to 300°C. In some embodiments, the heat setting temperature is 90 to 180°C, and further, the heat setting temperature can be 90 to 145°C. For example, the stretching ratio for transverse heat setting can be, but is not limited to, any value from 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2 times, or a range of any two of these values. The heat-setting temperature can be, but is not limited to, any value or a range of any two of the following: 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, 120℃, 125℃, 130℃, 135℃, 140℃, 145℃, 150℃, 155℃, 160℃, 165℃, 170℃, 175℃, 180℃, 190℃, 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, 260℃, 270℃, 280℃, 290℃, and 300℃. By controlling the stretching ratio and temperature of the transverse heat-setting within the range of this application, the crystallinity of the microporous membrane is further improved through the dual effects of heating and molecular orientation, thus solidifying the microporous structure of the membrane and helping to enhance its overall performance, thereby meeting the needs of different application scenarios.

[0174] In some embodiments, the extractant in the extraction step includes dichloromethane. Traditional paraffin oil / polyethylene co-extrusion biaxial stretching processes primarily use low-boiling-point solvents such as dichloromethane as extractants to remove pore-forming agents causing phase separation in the film. In some embodiments of this application, the pore-forming agent includes a first ionic liquid, and conventionally used dichloromethane extractants can also be used for extraction, following conventional extraction processes. The extracted extractant is then directly separated by distillation. During the drying process, the gaseous dichloromethane is adsorbed or compressed, and finally condensed and recovered for reuse.

[0175] In other embodiments, the extraction step uses a mixed solvent of a second ionic liquid and water as the extractant. The second ionic liquid has a higher hydrophilic-lipophilic balance (HLP) value than the first ionic liquid. This indicates that the second ionic liquid is more hydrophilic than the first, and the first ionic liquid is more lipophilic than the second, thus enabling the extraction of the first ionic liquid. The extracted film intermediate is then dried to remove water, yielding a microporous film.

[0176] In some embodiments, the difference between the hydrophilic-lipophilic balance value of the second ionic liquid and that of the first ionic liquid is <10, and the difference can be selected as 1 to 9. In some embodiments, the hydrophilic-lipophilic balance value of the second ionic liquid is 10 to 20. With the above configuration, when the microporous membrane is applied in the field of composite current collectors, it is beneficial to reduce the resistance of the composite current collector. Specifically, the resistance of the composite current collector can be characterized by its surface sheet resistance. For example, a composite current collector can be made by electroplating a 1 μm thick copper or aluminum foil onto the surface of the microporous membrane, and the resistance can be measured at five locations around the composite current collector and at its center using a four-probe resistance meter, and the average value is taken.

[0177] In some embodiments, the boiling point of the second ionic liquid is 100°C to 500°C. Specifically, the boiling point of the second ionic liquid can be 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, 150°C, 155°C, 175°C, 180°C, 185°C, 190°C, 195°C, 200°C, 205°C, 210°C, 215°C, 220°C, 225°C, 230°C, etc. The temperature range is any value or any two values ​​from the following: ℃, 235℃, 240℃, 245℃, 250℃, 255℃, 275℃, 280℃, 285℃, 290℃, 295℃, 300℃, 320℃, 340℃, 350℃, 360℃, 380℃, 400℃, 420℃, 440℃, 450℃, 460℃, 480℃, and 500℃. Using this setting, the microporous membrane, when applied in the field of current collectors, helps to reduce the resistance of composite current collectors.

[0178] In some embodiments, the viscosity of the second ionic liquid at 25°C is 10 cp to 50 cp. Specifically, the viscosity of the second ionic liquid at 25°C can be any value or a range between any two of the following: 10 cp, 12 cp, 14 cp, 16 cp, 18 cp, 20 cp, 22 cp, 24 cp, 26 cp, 28 cp, 30 cp, 32 cp, 34 cp, 36 cp, 38 cp, 40 cp, 42 cp, 44 cp, 46 cp, 48 cp, and 50 cp. With this configuration, when the microporous membrane is applied in the field of composite current collectors, it is beneficial to reduce the resistance of the composite current collector.

[0179] In some embodiments, the viscosity of the second ionic liquid at 25°C is 5 to 20 cp lower than that of the first ionic liquid at 25°C. Specifically, the viscosity of the second ionic liquid at 25°C can be any value or a range between any two values ​​that are lower than the viscosity of the first ionic liquid at 25°C, such as 5, 7, 9, 11, 13, 15, 17, 19, or 20 cp. This configuration improves extraction efficiency and helps control the ionic liquid content on the surface of the microporous membrane. Furthermore, the interaction between the two ionic liquids and the polymer results in a microporous membrane with excellent overall performance, meeting the needs of various application scenarios.

[0180] In some embodiments, the second ionic liquid is selected from one or more of imidazole salts, pyridine salts, quaternary ammonium salts, quaternary phosphonium salts, pyrrolidines, and functionalized ionic liquids.

[0181] Specifically, the second ionic liquid is selected from 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium methyl sulfate, 1,3-dimethylimidazolium dimethyl phosphate, 1,3-dimethylimidazolium tetrafluoroborate, 1,3-dimethylimidazolium hexafluorophosphate, 1,3-dimethylimidazolium hexafluoroantimonate, 1,3-dimethylimidazolium trifluoromethanesulfonate, 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium perchlorate, 1,3-dimethylimidazolium nitrate, 1,3-dimethylimidazolium trifluoromethanesulfonate, 1,3-dimethylimidazolium perchlorate, 1,3-dimethylimidazolium nitrate ... One or more of the following: 1,3-dimethylimidazolium methanesulfonate, 1,3-dimethylimidazolium p-toluenesulfonate, 1,3-dimethylimidazolium trifluoroacetate, 1-ethyl-3-methylimidazolium bromide, 1-ethyl-3-methylimidazolium iodide, 1-ethyl-3-methylimidazolium ethyl sulfate, 1-ethyl-3-methylimidazolium methyl sulfate, 1-ethyl-3-methylimidazolium diethyl phosphate, 1-ethyl-3-methylimidazolium dimethyl phosphate, 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium hexafluorophosphate, and 1-ethyl-3-methylimidazolium hexafluoroantimonate.

[0182] It is understood that the above only lists some commonly used types of ionic liquids, but it is not limited to these. Other ionic liquids can also be used, as long as they meet the above conditions. They will not be listed exhaustively here.

[0183] In some embodiments, the mass percentage concentration of the second ionic liquid in the extractant is 5% to 10%. For example, the mass percentage concentration of the second ionic liquid in the extractant can be, but is not limited to, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, or any combination of these values. Adopting the above configuration helps to improve the demulsification efficiency during the extraction process, resulting in higher extraction efficiency. Simultaneously, it reduces the use of low-boiling-point organic extractants, lowers the organic waste gas generated during production, making the production process more environmentally friendly. Furthermore, it allows for control of the ionic liquid content on the final microporous membrane, enabling the microporous membrane to possess superior overall performance.

[0184] In some embodiments, after the extraction step, the method further includes: washing the extracted microporous membrane intermediate with water, separating it with a reverse osmosis membrane, and drying it.

[0185] The ionic liquid in the microporous membrane intermediate is removed by washing with water, and the first ionic liquid, the second ionic liquid and water can be separated by reverse osmosis membrane separation, which facilitates subsequent recycling and reuse.

[0186] It should be noted that after extraction and before drying, other steps can be added as needed, such as washing, wetting, and cross-linking.

[0187] In some embodiments, a washing step is further included after extraction. The washing solution includes an ionic liquid and a washing agent, the washing agent including water, and the ionic liquid being the second ionic liquid of this application. The mass percentage concentration of the second ionic liquid in the washing solution is not higher than 10%. A concentration of no more than 10% of the ionic liquid 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.

[0188] In some embodiments, a multi-stage washing method can be used during the washing process, 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 be 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 prevent the ionic liquid extractant in the membrane from being washed too quickly, which could cause the microporous structure to collapse and affect the quality of the microporous membrane.

[0189] In some embodiments, a water washing step is included after extraction. The water washing includes a first-stage water washing and a second-stage water washing. The mass percentage concentration of the second ionic liquid in the water washing solution used in the first-stage water washing is 3% to 5%, and the mass percentage concentration of the second ionic liquid in the water washing solution used in the second-stage water washing is not higher than 1%, and the mass percentage concentration of the second ionic liquid in the water washing solution used in the second-stage water washing is not higher than 30 wt% of the mass percentage concentration of the second ionic liquid in the water washing solution used in the first-stage water washing.

[0190] In some embodiments, the washing process further includes a step of performing a second spraying with a second spray liquid, the second spray liquid comprising water.

[0191] Before drying the wet membrane, a wetting step may be included, where 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 microporous membrane, the wetting solution may include the ionic liquid described in this application and water, wherein the mass percentage concentration of the ionic liquid in the wetting solution is 0.05% to 5%. 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. In some embodiments, a crosslinking step is performed after this wetting step to better achieve modification or crosslinking of the porous membrane. The mass percentage concentration of the ionic liquid in the wetting solution may be any value or a range between any two values ​​from 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, to 0.5%.

[0192] It is understood that the steps of cooling, forming, stretching, and heat setting can also be performed using existing methods in the art. For example, biaxial stretching can be performed separately in the transverse and longitudinal directions, or simultaneously in the transverse and longitudinal directions, which will not be elaborated here. In addition, the steps of cooling, forming, biaxial stretching, and heat setting can be performed using methods commonly used in the art, and can be carried out on the basis of existing wet process production lines, with high compatibility.

[0193] It is understood that after the heat setting step, there are also steps of winding and slitting, which can be done using existing methods in the field and will not be described in detail here.

[0194] In some embodiments of this application, since the pore-forming agent contains a first ionic liquid, an organic low-boiling-point solvent such as dichloromethane (DCM) can be selected for extraction and drying to remove the pore-forming agent. Alternatively, a suitable mixed solvent of a second ionic liquid and water can be selected, and a polar ionic liquid with demulsification efficiency can be added to the extraction environment. The pore-forming agent is then removed by washing with purified water. Because the drying portion of this extraction process contains only water, no organic waste gas is generated, making the production process more environmentally friendly. Furthermore, when adding additives such as antioxidants to the polymer and pore-forming agent blend system, using a mixed solvent of the second ionic liquid and water as the extractant is beneficial because the extractant has good hydrophilicity and is less likely to extract oil-soluble additives, allowing the additives to remain in the microporous membrane, which improves the performance of the microporous membrane.

[0195] Thirdly, this application provides a composite current collector, including a support layer, wherein the support layer includes the microporous membrane described in the first aspect or the microporous membrane prepared by the preparation method described in the second aspect.

[0196] It is understood that the composite current collector also includes a conductive layer, which can be a commonly used material in the field, such as copper foil or aluminum foil, and is not specifically limited here.

[0197] To make the objectives and advantages of this application clearer, the microporous membrane and its effects of this application are further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are only for explaining this application and should not be used to limit this application. Unless otherwise specified, the following embodiments do not include components other than unavoidable impurities. Unless otherwise specified, the drugs and instruments used in the embodiments are conventional choices in the art. Experimental methods in the embodiments that do not specify specific conditions are implemented according to conventional conditions, such as those described in literature, books, or methods recommended by the manufacturer.

[0198] Example 1

[0199] This embodiment provides a microporous membrane comprising a polymer and an ionic liquid. The ionic liquid includes a first ionic liquid and a second ionic liquid, with the first ionic liquid and the second ionic liquid comprising 0.1% and 0.15% by mass, respectively, in the microporous membrane. The polymer is PP, with a viscosity-average molecular weight of 2 million, a melt index of 0.7 g / 10 min, and a melting point of 165 °C. The first ionic liquid is 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide salt and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide salt in a mass ratio of 95:5, with an HLB value of 5.5, a molecular weight of 400, a boiling point of 350℃, and a viscosity of 35.6 cp at 25℃. The second ionic liquid is 1,3-dimethylimidazolium trifluoromethanesulfonate, 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium dimethyl phosphate salt, and 1-ethyl-3-methylimidazolium ethyl sulfate salt in a mass ratio of 45:33:12:10, with an HLB value of 13, a molecular weight of 260, a boiling point of 230℃, and a viscosity of 20 cp at 25℃. Details are shown in Tables 1 to 3.

[0200] The method for preparing the microporous membrane in this embodiment includes the following steps:

[0201] (1) The mixture comprising the polymer and the first ionic liquid is mixed and extruded at an extrusion temperature of 210°C and cooled and shaped at 20°C to obtain an intermediate film; the mass ratio of the polymer to the first ionic liquid is 15:85.

[0202] (2) The intermediate membrane is first stretched longitudinally by a ratio of 6, and then stretched transversely by a ratio of 8 to obtain the stretched intermediate membrane.

[0203] (3) The stretched intermediate membrane was extracted at 25°C using a second ionic liquid and water as the extractant. The second ionic liquid had a mass percentage concentration of 10%. After extraction, the membrane was washed with water and dried. The washing solution consisted of the second ionic liquid and water, and a two-stage washing process was used. The mass percentage concentration of the second ionic liquid was 4% in the first stage of washing and 0.3% in the second stage of washing.

[0204] (4) The intermediate membrane after extraction and drying is heat-set to obtain a microporous membrane. The stretching ratio of the transverse heat-setting is 1.5 times, and the heat-setting temperature is 125℃.

[0205] Some of the process parameters during the preparation process are shown in Table 4.

[0206] Examples 2-5

[0207] The microporous membranes of Examples 2-5 are similar to those of Example 1, except that the mass percentage of the first ionic liquid in the microporous membranes is different, namely 0.5%, 3%, 5%, and 5.5%.

[0208] The preparation methods of the microporous membranes in Examples 2-5 are similar to those in Example 1, except that the mass percentage concentration of the second ionic liquid in the extractant differs during the preparation process. Other parameters are the same as in Example 1 and will not be repeated here.

[0209] Examples 6-8

[0210] The microporous membranes of Examples 6-8 are similar to those of Example 1, except that the ionic liquid composition is different. Details are shown in the table below. Other parameters are the same as in Example 1 and will not be repeated.

[0211] Example 9

[0212] The microporous membrane of Example 9 is similar to that of Example 1, except that the microporous membrane does not contain a second ionic liquid.

[0213] The preparation method of the microporous membrane in Example 9 is similar to that in Example 1, except that dichloromethane is used as the extractant in the extraction process. Other parameters are the same as in Example 1 and will not be repeated.

[0214] Examples 10-12

[0215] The microporous membranes of Examples 10-12 are similar to those of Example 1, except that the mass percentage of the second ionic liquid is different.

[0216] The preparation methods of the microporous membranes in Examples 10-12 are similar to those in Example 1, except that the mass percentage concentration of the second ionic liquid differs during the second-stage water washing process. Details are shown in the table below. Other parameters are the same as in Example 1 and will not be repeated.

[0217] Example 13

[0218] The microporous membrane in this embodiment is similar to that in Example 1, except that the polymer is different. The polymer used in this embodiment is polyethylene terephthalate (PET), with a viscosity-average molecular weight of 2.5 million, a melt index of 0.7 g / 10 min, and a melting point of 255 °C.

[0219] The preparation method of the microporous membrane in this embodiment is similar to that in Example 1, except that the extrusion temperature is different. In this embodiment, the extrusion temperature is 275°C.

[0220] Example 14

[0221] The microporous membrane in this embodiment is similar to that in Example 1, except that the polymer is different. The polymer used in this embodiment is an ethylene-propylene-norbornene copolymer with a viscosity-average molecular weight of 2.5 million, a melt index of 0.6 g / 10 min, and a melting point of 200 °C.

[0222] The preparation method of the microporous membrane in this embodiment is similar to that in Example 1, except that the extrusion temperature is different. In this embodiment, the extrusion temperature is 250°C.

[0223] Comparative Example 1

[0224] Comparative Example 1 provides a microporous membrane similar to Example 1, except that it does not contain ionic liquid, uses paraffin oil as a pore-forming agent, and dichloromethane as an extractant.

[0225] The preparation steps of the microporous membrane in Comparative Example 1 are similar to those in Example 1, except that steps (1) and (3) are different. Steps (1) and (3) of Comparative Example 1 are as follows:

[0226] (1) The mixture including polymer PP and paraffin oil is mixed and extruded at an extrusion temperature of 210°C and cooled and shaped at 20°C to obtain an intermediate film; the mass ratio of polymer to paraffin oil is 15:85.

[0227] (3) Extract the stretched intermediate membrane at a temperature of 25°C using dichloromethane as the extractant, and then dry it.

[0228] Comparative Example 2

[0229] Comparative Example 2 provides a microporous membrane similar to the microporous membrane of Example 1, except that the mass percentage of the ionic liquid is too high. In Comparative Example 2, the mass percentages of the first ionic liquid and the second ionic liquid in the microporous membrane are 5.5% and 1%, respectively, and the total mass percentage of the ionic liquid is 6.5%. See Tables 1 to 3 for details.

[0230] The preparation method of the microporous membrane in Comparative Example 2 is similar to that in Example 1, except that the mass percentage concentration of the second ionic liquid in the extractant and the mass percentage concentration of the second ionic liquid in the second-stage water wash are different. Details are shown in the table below.

[0231] Comparative Example 3

[0232] Comparative Example 3 provides a microporous membrane similar to the microporous membrane of Example 1, except that the mass percentage of the ionic liquid is 0.15%, and it contains only the second ionic liquid and not the first ionic liquid. Everything else is the same as in Example 1 and will not be repeated.

[0233] The preparation method of the microporous membrane in Comparative Example 3 is similar to that in Example 1, except that step (1) is different. In step (1) of Comparative Example 3, the first ionic liquid in Example 1 is replaced with an equal mass of paraffin oil. The other steps are the same as in Example 1 and will not be repeated here.

[0234] Table 1

[0235] Table 2

[0236] Table 3

[0237] Table 4

[0238] The microporous membranes prepared in the above embodiments and comparative examples were characterized, and the results are shown in Table 5 below. Porosity, air permeability, and thickness were tested according to GB / T 36363-2018. Tensile strength (including longitudinal and transverse tensile strength) was tested according to GB / T 36363-2018. When testing transverse tensile strength, a strip with a longitudinal width of 15 mm was cut and clamped at both ends of the fixture, and the tensile testing machine speed was set to 200 mm / min. When testing the longitudinal tensile strength of the microporous membrane, a strip with a transverse width of 15 mm was cut and tested, with other parameters set consistent with the conditions for the transverse tensile strength test. Puncture strength was measured according to GB / T 36363-2018. The microporous membrane sample was flattened, clamped, and punctured at a rate of 300 mm / min.

[0239] Table 5 Characterization of the microporous membranes in each embodiment and comparative example

[0240] The microporous membranes prepared in the above embodiments and comparative examples were used as support layers in composite current collectors. The performance of the composite current collectors was tested, and the experimental data are shown in Table 6 below. A positive electrode composite current collector was prepared by electroplating a 1μm thick aluminum foil on each of the two surfaces of the microporous membrane, and a negative electrode composite current collector was prepared by electroplating a 1μm thick cylindrical foil on each of the two surfaces of the microporous membrane. The moisture content on the surface of the composite current collector was determined using the Karl Fischer method (volume method), based on the volume of reagent consumed and the titration. The resistance of the composite current collector was characterized by its surface sheet resistance. A four-probe resistance meter was used to measure the resistance at five locations around the perimeter and center of the composite current collector, and the average value was taken. The energy density of the composite current collector was determined by the following steps:

[0241] 1) The positive electrode active material NCM811, conductive agent SP, conductive agent KS-6, and binder PVDF are mixed in solvent NMP at a mass ratio of 90:2:1:3 to prepare a positive electrode slurry. The slurry is then coated onto the aluminum foil surface of the composite current collector (the aluminum foil thickness on one side is 1μm). The slurry is then dried in an oven at 95℃ and rolled on a roller press to obtain the positive electrode.

[0242] 2) The active material artificial graphite, the conductive agent acetylene black, and the binder CMC / SBR (sodium carboxymethyl cellulose / styrene-butadiene rubber, with a mass ratio of 1:1) are added to water in a mass ratio of 90:5:5 and dispersed evenly to prepare a negative electrode slurry. This slurry is coated onto the copper foil surface of the composite current collector (the copper foil thickness on one side is 1 μm). After drying at 85°C in an oven, the negative electrode is obtained by rolling in a roller press.

[0243] 3) In a dry room with a dew point <-40℃, after stacking and packaging the positive electrode, separator, and negative electrode sheets, follow the formula of 2.5g / m 2 The electrolyte (Xinzhoubang, LBC3008A) is injected in a specific ratio to obtain the battery. The separator can be a commercially available battery separator, such as a 7μm thick polyethylene microporous membrane (38% porosity) from Shenzhen Xingyuan Material Technology Co., Ltd.

[0244] Test method: The battery is tested within a voltage range of 3.0V to 4.5V. The discharge plateau voltage is determined by cyclic voltammetry at a rate of 0.5C. Then, the battery is discharged at a constant current of 1C from a fully charged state, and the discharge capacity is measured. The mass energy density is calculated using the following formula:

[0245] Table 6

[0246] As can be seen from the above experimental data, the microporous membrane prepared in the examples has a suitable ionic liquid content, maintains good air permeability and longitudinal tensile strength. When applied to the field of composite current collectors, it helps to improve the electroplating effect in the current collector preparation process, thereby having a certain effect on controlling the moisture and resistance on the surface of the current collector, and also helps to improve the battery energy density.

[0247] Specifically, Comparative Example 1 used a conventional process (paraffin oil + DCM) to prepare a microporous membrane, which did not contain ionic liquid, and its performance was inferior to that of the Example. In Comparative Example 2, the microporous membrane contained an excessive amount of ionic liquid, and its performance was inferior to that of the Example. In Comparative Example 3, paraffin oil was used as the pore-forming agent and a second ionic liquid as the extractant during the preparation of the microporous membrane, and the performance of the microporous membrane was inferior to that of the Example.

[0248] Furthermore, as can be seen from Examples 1-5, adjusting the content of the first ionic liquid can further reduce the moisture resistance of the composite current collector and improve its energy density. As can be seen from Examples 8 and 6-7, adjusting the HLB values ​​of the first and second ionic liquids is beneficial for further improving the moisture content, resistance, and energy density of the composite current collector. As can be seen from Examples 9-12 and Example 1, adding the second ionic liquid and adjusting its amount is beneficial for further improving the energy density of the composite current collector. As can be seen from Examples 13-14 and Example 1, replacing the polymer with PET and ethylene-propylene-norbornene copolymer results in slightly better tensile properties for PET and ethylene-propylene-norbornene copolymer than for PP.

[0249] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0250] The embodiments described above are merely illustrative of several implementation methods of this application, intended to facilitate a detailed understanding of the technical solutions of this application, but should not be construed as limiting the scope of protection of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the scope of protection of this application. It should be understood that technical solutions obtained by those skilled in the art based on the technical solutions provided in this application through logical analysis, reasoning, or limited experimentation are all within the scope of protection of the appended claims. Therefore, the scope of protection of this patent application should be determined by the content of the appended claims, and the specification can be used to interpret the content of the claims.

Claims

1. A microporous membrane for composite current collectors, wherein, The microporous membrane contains an ionic liquid, the mass percentage of which is 0.01% to 6%. The microporous membrane has a permeability of 10 sec / 100 cc to 50 sec / 100 cc and a longitudinal tensile strength of 3500 kgf / cm². 2 ~5000 kgf / cm 2 The ionic liquid includes a first ionic liquid, wherein the hydrophilic-lipophilic balance value of the first ionic liquid is <10.

2. The microporous membrane according to claim 1, wherein, The first ionic liquid satisfies one or more of the following conditions: (1) The hydrophilic-lipophilic balance value of the first ionic liquid is 0 to 9.8, and can be selected as 3 to 7; (2) The molecular weight of the first ionic liquid is 200 to 1000; (3) The boiling point of the first ionic liquid is 200℃~500℃; (4) The viscosity of the first ionic liquid at 25°C is 20cp~100cp; (5) In the microporous membrane, the mass percentage of the first ionic liquid is 0.01% to 6%, and optionally, the mass percentage of the first ionic liquid is 0.1% to 5%.

3. The microporous membrane according to claim 2, wherein, The ionic liquid further includes a second ionic liquid, the second ionic liquid having a hydrophilic-lipophilic balance value greater than that of the first ionic liquid.

4. The microporous membrane according to claim 3, wherein, The second ionic liquid satisfies one or more of the following conditions: (1) The difference between the hydrophilic-lipophilic balance value of the second ionic liquid and the hydrophilic-lipophilic balance value of the first ionic liquid is <10, and the difference can be selected as 1 to 9; (2) The hydrophilic-lipophilic balance value of the second ionic liquid is 10-20; (3) The boiling point of the second ionic liquid is 100℃~500℃; (4) The viscosity of the second ionic liquid at 25°C is 10cp~50cp; (5) The viscosity of the second ionic liquid at 25°C is 5cp to 20cp lower than that of the first ionic liquid at 25°C; (6) In the microporous membrane, the mass percentage of the second ionic liquid is 0.01% to 3%, and optionally, the mass percentage of the second ionic liquid is 0.1% to 0.5%.

5. The microporous membrane according to claim 3 or 4, wherein, The first ionic liquid and the second ionic liquid are each independently selected from one or more of imidazole salts, pyridine salts, quaternary ammonium salts, quaternary phosphonium salts, and pyrrolidines; Optionally, the first ionic liquid and the second ionic liquid are each independently selected from 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium methyl sulfate, 1,3-dimethylimidazolium dimethyl phosphate, 1,3-dimethylimidazolium tetrafluoroborate, 3-dimethylimidazolium hexafluorophosphate, 1,3-dimethylimidazolium hexafluoroantimonate, 1,3-dimethylimidazolium trifluoromethanesulfonate, 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium perchlorate, and 1,3-dimethylimidazolium nitrate. One or more of the following: 1,3-dimethylimidazolium methanesulfonate, 1,3-dimethylimidazolium p-toluenesulfonate, 1,3-dimethylimidazolium trifluoroacetate, 1-ethyl-3-methylimidazolium bromide, 1-ethyl-3-methylimidazolium iodide, 1-ethyl-3-methylimidazolium ethyl sulfate, 1-ethyl-3-methylimidazolium methyl sulfate, 1-ethyl-3-methylimidazolium diethyl phosphate, 1-ethyl-3-methylimidazolium dimethyl phosphate, 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium hexafluorophosphate, and 1-ethyl-3-methylimidazolium hexafluoroantimonate.

6. The microporous membrane according to claim 5, wherein, The first ionic liquid comprises one or more of the following: 1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium hexafluorophosphate, 1,3-dimethylimidazolium hexafluoroantimonate, 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, 1,3-dimethylimidazolium methanesulfonate, 1,3-dimethylimidazolium p-toluenesulfonate, 1-ethyl-3-methylimidazolium diethyl phosphate, 1-ethyl-3-methylimidazolium hexafluorophosphate, and 1-ethyl-3-methylimidazolium hexafluoroantimonate; and / or, The second ionic liquid includes one or more of the following: methyl 1,3-dimethylimidazolium sulfate, dimethyl 1,3-dimethylimidazolium phosphate, tetrafluoroborate, trifluoromethanesulfonate, perchlorate, nitrate, trifluoroacetate, ethyl-3-methylimidazolium bromide, iodide, ethyl 1-ethyl-3-methylimidazolium sulfate, methyl 1-ethyl-3-methylimidazolium sulfate, and dimethyl 1-ethyl-3-methylimidazolium phosphate.

7. The microporous membrane according to any one of claims 1 to 6, wherein, The microporous membrane includes a polymer, which includes homopolymers or copolymers of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene and norbornene, thermoplastic polyurethane elastomers, polyolefin elastomers, polyethylene terephthalate, polyurethane, or mixtures of polymers thereof. Optionally, the polymer includes one or more of polypropylene, polyethylene terephthalate, and ethylene-propylene-norbornene polymer; Optionally, the polymer has a viscosity-average molecular weight of 2 million to 6 million.

8. The microporous membrane according to any one of claims 1 to 7, wherein, The microporous membrane also contains additives, including one or more of antioxidants, metal soaps, light stabilizers, antistatic agents, and antifogging agents. Optionally, in the microporous membrane, the additive accounts for 0.01% to 5% of the total mass of the microporous membrane; alternatively, the additive accounts for 0.1% to 1% of the total mass of the microporous membrane. And / or, the microporous membrane comprises multiple layers of sub-membranes stacked together, with different polymers in adjacent sub-membranes, and both containing the first ionic liquid.

9. The microporous membrane according to any one of claims 1 to 8, wherein, The microporous membrane satisfies one or more of the following conditions: (1) The average pore size of the microporous membrane is 100 nm to 200 nm; (2) The transverse tensile strength of the microporous membrane is 2500 kgf / cm. 2 ~5000 kgf / cm 2 ; (3) The puncture strength of the microporous membrane is 150 gf to 1000 gf; (4) The porosity of the microporous membrane is 70% to 90%; (5) The thickness of the microporous membrane is 2μm to 10μm, and optionally, the thickness of the microporous membrane is 2μm to 8μm.

10. A method for preparing a microporous membrane for composite current collectors, wherein, Includes the following steps: A mixture comprising polymer and ionic liquid is mixed, extruded, and cooled to form an intermediate film. The intermediate membrane is stretched, extracted, and heat-set to obtain the microporous membrane. The ionic liquid in the microporous membrane comprises 0.01% to 6% by mass, the microporous membrane has a permeability of 10 sec / 100 cc to 50 sec / 100 cc, and the longitudinal tensile strength of the microporous membrane is 3500 kgf / cm². 2 ~5000 kgf / cm 2 The ionic liquid includes a first ionic liquid, wherein the hydrophilic-lipophilic balance value of the first ionic liquid is <10.

11. The method for preparing a microporous membrane according to claim 10, wherein, One or more of the following conditions must be met: (1) The hydrophilic-lipophilic balance value of the first ionic liquid is 0 to 9.8, and can be selected as 3 to 7; (2) The molecular weight of the first ionic liquid is 200 to 1000; (3) The boiling point of the first ionic liquid is 200℃~500℃; (4) The mass ratio of the polymer to the first ionic liquid is (5-19):(81-95).

12. The method for preparing a microporous membrane according to claim 10 or 11, wherein, In the extraction step, the extractant includes dichloromethane; or, In the extraction step, the extractant comprises a mixed solvent of a second ionic liquid and water, wherein the second ionic liquid satisfies one or more of the following conditions: (1) The hydrophilic-lipophilic balance value of the second ionic liquid is greater than that of the first ionic liquid; (2) The difference between the hydrophilic-lipophilic balance value of the second ionic liquid and the hydrophilic-lipophilic balance value of the first ionic liquid is <10, and the difference can be selected as 1 to 9; (3) The hydrophilic-lipophilic balance value of the second ionic liquid is 10-20; (4) The boiling point of the second ionic liquid is 100℃~500℃; (5) The viscosity of the second ionic liquid at 25°C is 10cp~50cp; (6) The viscosity of the second ionic liquid at 25°C is 5cp to 20cp lower than that of the first ionic liquid at 25°C.

13. The method for preparing a microporous membrane according to claim 12, wherein, The extractant comprises a mixed solvent of a second ionic liquid and water, wherein the mass percentage concentration of the second ionic liquid in the extractant is 5% to 10%. And / or, after the extraction step, the method further includes: washing the extracted microporous membrane intermediate with water, separating it with a reverse osmosis membrane, and drying it; Optionally, the extracted microporous membrane intermediate is washed with water, including a first-stage water wash and a second-stage water wash. The mass percentage concentration of the second ionic liquid in the washing solution of the first-stage water wash is 3% to 5%, and the mass percentage concentration of the second ionic liquid in the washing solution of the second-stage water wash is not higher than 1%, and the mass percentage concentration of the second ionic liquid in the washing solution of the second-stage water wash is not higher than 30 wt% of the mass percentage concentration of the second ionic liquid in the washing solution of the first-stage water wash.

14. The method for preparing a microporous membrane according to any one of claims 10 to 13, wherein, The temperature T of the mixed extrusion 挤 For T 挤 =Polymer melting point T m +(15℃~60℃); And / or, in the step of mixing and extruding a mixture comprising a polymer and an ionic liquid, the polymer has a melt index of 0.6 g / 10 min to 10 g / 10 min; optionally, the polymer has a melt index of 0.6 g / 10 min to 6 g / 10 min.

15. A composite current collector, wherein, The composite current collector includes a support layer, which comprises the microporous membrane according to any one of claims 1 to 9 or the microporous membrane prepared by the preparation method according to any one of claims 10 to 14.

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