Modification method of porous polymer membrane, modified porous polymer membrane and application of modified porous polymer membrane

Abstract

The invention relates to the technical field of lithium metal battery diaphragm materials, and discloses a modification method of a porous polymer membrane, a modified porous polymer membrane and application of the modified porous polymer membrane. The modification method of the porous polymer film comprises the following steps: dipping the porous polymer film in a photoinitiator solution, and then carrying out ultraviolet irradiation to obtain the porous polymer film with photo-initiation activity, the preparation method comprises the following steps: dipping a porous polymer membrane with photo-initiation activity in a mixed solution of a fluorine-containing monomer and an acrylate monomer, grafting through ultraviolet irradiation, and cleaning to obtain the modified porous polymer membrane. The modification process is simple, the material cost is low, and good universality is achieved; the modified porous polymer film is favorable for enhancing lithium ion transmission kinetics and uniform deposition / stripping on the surface of an electrode, promotes formation of a LiF-rich solid electrolyte interface (SEI) layer on the surface of a lithium metal negative electrode, is favorable for reducing the interface impedance of lithium ion transmission and improves the electrochemical performance of a lithium metal battery.

Description

Technical Field

[0001] The present application relates to the technical field of lithium metal battery separator materials, and in particular to a method for modifying a porous polymer membrane, a modified porous polymer membrane and applications thereof. Background Art

[0002] With the rapid development of the economy, the demand for advanced energy is growing. In order to meet the requirements of advanced equipment such as portable devices, electric vehicles and smart grids for energy storage devices, the development of high-performance energy storage devices with low cost, high energy density and long service life has become a hot topic of current research. Lithium-ion batteries, as the most mature secondary batteries, are widely used in various fields due to their advantages such as high energy density, high operating voltage, wide operating temperature and no memory effect. However, the content of lithium resources in the earth's crust is unevenly distributed, and as lithium resources are continuously consumed, their cost continues to rise, resulting in lithium-ion batteries being unable to meet the requirements of large-scale electrochemical energy storage.

[0003] Lithium metal batteries have the highest theoretical specific capacity (3860 mAh g -1 ) and ultra-low redox potential (-3.04V, relative to the standard hydrogen electrode), it is expected to become the next generation of high-performance electrochemical energy storage devices. However, lithium metal has a high reactivity and violent side reactions with the electrolyte, which can easily cause electrolyte exhaustion and shorten the life of the battery; at the same time, the low ion migration number of the electrolyte will easily lead to the formation of a large number of lithium dendrites during the lithium deposition process, causing the diaphragm to be pierced and the battery to short-circuit; in addition, due to the limitations of the preparation process, commercial polymer diaphragms have problems such as poor electrolyte wettability, unsatisfactory ionic conductivity, and single function. The interface impedance is large, which is not conducive to inhibiting the growth of lithium dendrites. The above bottleneck problems have restricted the further development of lithium metal batteries.

[0004] The development of new high-performance porous diaphragm materials is one of the keys to promote the development of lithium metal batteries. In recent years, in response to the problems of commercial porous polymer diaphragms, diaphragm coating modification strategies based on functional coatings have been developed. Although these strategies have alleviated the above problems to a certain extent, on the one hand, the material preparation and modification steps are complicated, and on the other hand, the hydrophobic surface of commercial diaphragms is not conducive to the uniform coating of functional coatings, resulting in poor wettability and difficulty in effectively reducing its interfacial impedance, which in turn affects the performance of lithium metal batteries. Summary of the invention

[0005] The present application provides a porous polymer membrane modification method, a modified porous polymer membrane and its application, aiming to solve the problems of complex modification process of existing porous polymer membrane coating and poor electrochemical performance of battery caused by high interface impedance of the modified porous polymer membrane.

[0006] In order to achieve the above objectives, the present application adopts the following technical solutions.

[0007] In a first aspect of the present application, a method for modifying a porous polymer membrane is provided, comprising:

[0008] S1, immersing the porous polymer film in a photoinitiator solution and irradiating it with ultraviolet light to obtain a porous polymer film with photoinitiator activity;

[0009] S2, immersing the porous polymer membrane with photoinitiating activity in a mixed solution of a fluorine-containing monomer and an acrylate monomer, irradiating it with ultraviolet light for grafting, and washing it to obtain a modified porous polymer membrane.

[0010] Preferably, the solute of the photoinitiator solution is a hydrogen abstraction type photoinitiator;

[0011] The solvents of the photoinitiator solution and the mixed solution are both polar organic solvents;

[0012] The polar organic solvent includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, 1,4-dioxane, acetonitrile or acetone.

[0013] More preferably, the hydrogen abstraction type photoinitiator includes any one of benzophenone, Michler's ketone, thioxanthone, isopropylthioxanthone or 2-ethylanthraquinone.

[0014] Preferably, the raw material of the porous polymer membrane includes at least one of polypropylene, polyethylene, polyimide, polyamide, polyvinylidene fluoride or cellulose.

[0015] Preferably, the fluorine-containing monomer includes at least one of pentafluorostyrene, 2,2,2-trifluoroethyl methacrylate, 2,2,3,3-tetrafluoropropyl methacrylate, 2,2,3,3-tetrafluoropropyl acrylate, 2,2,3,4,4,4-hexafluorobutyl methacrylate or 2,2,3,4,4,4-hexafluorobutyl acrylate;

[0016] and / or,

[0017] The acrylic acid ester monomer includes at least one of methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, acrylic acid, sec-butyl acrylate, n-propyl acrylate, cycloethyl acrylate, lauryl acrylate, 2-hydroxyethyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate or polyethylene glycol dimethacrylate.

[0018] Preferably, the concentration of the photoinitiator solution is 0.5-20 wt %; in the mixed solution, the ratio of the sum of the volumes of the fluorine-containing monomer and the acrylate monomer to the sum of the volumes of the fluorine-containing monomer, the acrylate monomer and the solvent is 10-90%.

[0019] Preferably, in S1, the immersion time is 5 to 30 minutes, and the ultraviolet light irradiation time is 5 to 15 minutes; in S2, the immersion time is 0.5 to 12 hours, and the ultraviolet light irradiation time is 5 to 30 minutes.

[0020] The second aspect of the present application provides a modified porous polymer membrane prepared by the above modification method.

[0021] The third aspect of the present application provides the use of the modified porous polymer membrane in a lithium metal battery.

[0022] In a fourth aspect of the present application, a lithium metal battery separator is provided, which comprises the above-mentioned modified porous polymer film.

[0023] Compared with the prior art, the beneficial effects of this application are:

[0024] The modification method of the present application directly grafts the modified components onto the surface of the porous polymer membrane in the form of random copolymerization by covalent bonding. The modification process is simple, the material cost is low, and it has the potential for large-scale production. It has good universality and can be used for various common commercial porous polymer membranes. By changing the types and proportions of modified components such as fluorine-containing monomers and acrylate monomers, modified porous polymer membranes with different properties can be obtained as needed.

[0025] In the random copolymer side chains grafted on the surface of the modified porous polymer membrane of the present application, the fluorine-containing monomer unit is rich in electronegative CF bonds, which is beneficial to enhancing the lithium ion transfer kinetics and uniform deposition / stripping on the electrode surface, while promoting the formation of a solid electrolyte interface (SEI) layer rich in LiF on the surface of the lithium metal negative electrode, thereby improving the electrochemical performance of the lithium metal battery; and the acrylic ester monomer unit provides rich ester functional groups, which is beneficial to enhancing the wettability of the porous polymer membrane to the electrolyte, thereby reducing the interfacial impedance of lithium ion transfer and further improving the electrochemical performance of the lithium metal battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments recorded in the present application, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0027] Figure 1This is the SEM spectrum of the modified porous polypropylene membrane prepared in Example 1;

[0028] Figure 2 This is the SEM spectrum of the modified porous polypropylene membrane prepared in Example 2;

[0029] Figure 3 This is the SEM spectrum of the modified porous polypropylene membrane prepared in Example 3;

[0030] Figure 4 The SEM spectrum of the commercial polypropylene separator of the comparative example;

[0031] Figure 5 The cycle performance test results of lithium-lithium symmetric batteries assembled with modified porous polypropylene membrane and commercial polypropylene separator;

[0032] Figure 6 The figure shows the cycle performance test results of lithium metal-lithium iron phosphate battery assembled with modified porous polypropylene membrane and commercial polypropylene separator. DETAILED DESCRIPTION

[0033] The technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0034] In the following description of this embodiment, the terms "include", "comprising", "having" and "containing" are all open terms, meaning including but not limited to.

[0035] In the following description of this embodiment, the term "and / or" is used to describe the association relationship of associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, B exists alone, and A and B exist at the same time. A and B can be singular or plural. The character " / " generally indicates that the associated objects before and after are in an "or" relationship.

[0036] In the following description of this embodiment, the term "at least one" refers to one or more, and "plurality" refers to two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b or c", or "at least one of a, b and c", can all represent: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, c can be single or multiple, respectively.

[0037] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a", "an" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings.

[0038] Those skilled in the art should understand that in the following description of the embodiments of the present application, the order of serial numbers does not mean the order of execution, some or all of the steps can be executed in parallel or sequentially, and the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0039] Those skilled in the art will appreciate that the numerical ranges in the embodiments of the present application are to be construed as specifically disclosing each intermediate value between the upper and lower limits of the scope. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present application. The upper and lower limits of these smaller ranges may be independently included or excluded in the scope.

[0040] Unless otherwise specified, the technical / scientific terms used herein have the same meanings as those generally understood by those skilled in the art to which this application belongs. Although this application only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of this application. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0041] In the first aspect, the present application provides a method for modifying a porous polymer membrane, which activates a commercial porous polymer membrane and then grafts a random copolymer side chain with a fluorine-containing monomer and an acrylate monomer as a structural unit onto its surface, thereby modifying an existing commercial porous polymer membrane. Specifically, the method includes:

[0042] S1, immersing the porous polymer film in a photoinitiator solution and irradiating it with ultraviolet light to obtain a porous polymer film with photoinitiator activity;

[0043] In the present application, the porous polymer membrane can be selected from commonly used commercial porous polymer membranes, such as porous polymer membranes prepared from polypropylene, polyethylene, polyimide, polyamide, polyvinylidene fluoride or cellulose, or composite porous polymer membranes prepared from two or more of the above materials.

[0044] In the present application, the solute of the photoinitiator solution is a hydrogen abstraction type photoinitiator, such as any one of benzophenone, Michler's ketone, thioxanthone, isopropyl thioxanthone or 2-ethylanthraquinone; the solvent of the photoinitiator solution is a polar organic solvent, and at least one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, 1,4-dioxane, acetonitrile or acetone can be selected.

[0045] In the present application, the concentration of the initiator solution is preferably 0.5 to 20 wt %.

[0046] In the present application, the porous polymer membrane is immersed in a photoinitiator solution and then irradiated with ultraviolet light of a wavelength of 365 nm, so that the CH bonds rich in the surface of the porous polymer membrane react with the hydrogen abstraction type photoinitiator, so that the porous polymer membrane has photoinitiating activity. Preferably, the immersion time is 5 to 30 minutes, and the ultraviolet light irradiation time is 5 to 15 minutes.

[0047] S2, immersing the porous polymer membrane with photoinitiating activity in a mixed solution of a fluorine-containing monomer and an acrylate monomer, irradiating it with ultraviolet light for grafting, and washing it to obtain a modified porous polymer membrane.

[0048] In the present application, a fluorine-containing monomer and an acrylate monomer are dissolved in a polar organic solvent to obtain a mixed solution. The usage ratio of the fluorine-containing monomer and the acrylate monomer is not limited, and the sum of the volumes of the fluorine-containing monomer and the acrylate monomer accounts for 10 to 90% of the total volume of the mixed solution. In the embodiments of the present application, the volume ratio of the fluorine-containing monomer and the acrylate monomer is preferably (1:4) to (4:1), and the volume percentage of the monomers in the mixed solution is preferably 50-80%.

[0049] Specifically, the fluorine-containing monomer includes at least one of pentafluorostyrene, 2,2,2-trifluoroethyl methacrylate, methacrylate-2,2,3,3-tetrafluoropropyl ester, acrylate-2,2,3,3-tetrafluoropropyl ester, methacrylate-2,2,3,4,4,4-hexafluorobutyl ester or acrylate-2,2,3,4,4,4-hexafluorobutyl ester; the acrylate monomer includes at least one of methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, acrylic acid, sec-butyl acrylate, n-propyl acrylate, cycloethyl acrylate, lauryl acrylate, 2-hydroxyethyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate or polyethylene glycol dimethacrylate. The polar organic solvent includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, 1,4-dioxane, acetonitrile or acetone.

[0050] In the present application, the immersion time of the porous polymer membrane with photoinitiating activity in the mixed solution is preferably 0.5 to 12 hours. After immersion, the porous polymer membrane is irradiated with ultraviolet light with a wavelength of 365 nm for 5 to 30 minutes to graft the fluorine-containing monomer and the acrylate monomer onto the surface of the porous polymer membrane. The modified porous polymer film is cleaned with the above-mentioned polar organic solvent to remove the residual monomer, thereby obtaining a modified porous polymer membrane grafted with a random copolymer.

[0051] The modification method of the present application directly grafts the modified components onto the surface of the porous polymer membrane in the form of random copolymerization in the form of covalent bonding. The modification process is simple, the material cost is low, and it is easy to mass produce. In addition, the modification method of the present application has good universality and can be used for various common commercial porous polymer membranes; by changing the types and proportions of the modified components such as fluorine-containing monomers and acrylate monomers, modified porous polymer membranes with different properties can be obtained as needed.

[0052] The modified porous polymer membrane obtained by the above modification method can be used as a lithium metal battery separator or for preparing a lithium metal battery separator for lithium metal batteries. In the random copolymer side chain grafted on the surface of the modified porous polymer membrane of the present invention, the fluorine-containing monomer unit is rich in electronegative CF bonds, which is conducive to enhancing the lithium ion transfer kinetics and uniform deposition / stripping on the electrode surface, while promoting the formation of a solid electrolyte interface (SEI) layer rich in LiF on the surface of the lithium metal negative electrode, thereby improving the electrochemical performance of the lithium metal battery; and the acrylic ester monomer unit provides rich ester functional groups, which is conducive to enhancing the wettability of the porous polymer membrane to the electrolyte, thereby reducing the interfacial impedance of lithium ion transfer and further improving the electrochemical performance of the lithium metal battery.

[0053] The present application also provides a lithium metal battery separator, which includes a modified porous polymer film. The lithium metal battery separator can enhance the lithium ion transmission kinetics and the uniform deposition / stripping on the electrode surface, and promote the formation of the SEI layer; on the other hand, it has good wettability to the electrolyte and low interface impedance, and can effectively improve the electrochemical performance of the lithium metal battery.

[0054] The present application is specifically described below through examples.

[0055] Example 1

[0056] This embodiment provides a method for modifying a porous polypropylene membrane, comprising:

[0057] S1, placing a commercial polypropylene membrane with a size of 4×20 cm in a container, adding 10 mL of a 10 wt% benzophenone DMF solution to the container, soaking the commercial polypropylene membrane, and shaking for 30 minutes. The immersed commercial polypropylene membrane is taken out, sandwiched between light-transmitting glass plates, irradiated under ultraviolet light with a wavelength of 365 nm for 5 minutes, taken out, and washed with an appropriate amount of DMF to obtain a porous polypropylene membrane with photoinitiated activity.

[0058] S2, placing a porous polypropylene membrane with photoinitiator activity in a container, adding a DMF solution of pentafluorostyrene and n-butyl acrylate, and shaking for 30 minutes. The sum of the volumes of pentafluorostyrene and n-butyl acrylate is 80 v / v% of the volume of the DMF solution, and the volume ratio of pentafluorostyrene to n-butyl acrylate is 1:4. The membrane after impregnation is fished out, sandwiched between light-transmitting glass plates, irradiated under ultraviolet light with a wavelength of 365 nm for 15 minutes, taken out, and washed with DMF to obtain a modified porous polypropylene membrane grafted with a random copolymer.

[0059] Example 2

[0060] This embodiment provides a method for modifying a porous polypropylene membrane, comprising:

[0061] S1, placing a commercial polypropylene membrane with a size of 4×20 cm in a container, adding 10 mL of a 10 wt% benzophenone DMF solution to the container, soaking the commercial polypropylene membrane, and shaking for 30 minutes. The immersed commercial polypropylene membrane is taken out, sandwiched between light-transmitting glass plates, irradiated under ultraviolet light with a wavelength of 365 nm for 5 minutes, taken out, and washed with an appropriate amount of DMF to obtain a porous polypropylene membrane with photoinitiated activity.

[0062] S2, placing a porous polypropylene membrane with photoinitiator activity in a container, adding a DMF solution of pentafluorostyrene and n-butyl acrylate, and shaking for 30 minutes. The sum of the volumes of pentafluorostyrene and n-butyl acrylate is 80 v / v% of the volume of the DMF solution, and the volume ratio of pentafluorostyrene to n-butyl acrylate is 1:1. The membrane after impregnation is fished out, sandwiched between light-transmitting glass plates, irradiated under ultraviolet light with a wavelength of 365 nm for 15 minutes, taken out, and washed with DMF to obtain a modified porous polypropylene membrane grafted with a random copolymer.

[0063] Example 3

[0064] This embodiment provides a method for modifying a porous polypropylene membrane, comprising:

[0065] S1, placing a commercial polypropylene membrane with a size of 4×20 cm in a container, adding 10 mL of a 10 wt% benzophenone DMF solution to the container, soaking the commercial polypropylene membrane, and shaking for 30 minutes. The immersed commercial polypropylene membrane is taken out, sandwiched between light-transmitting glass plates, irradiated under ultraviolet light with a wavelength of 365 nm for 5 minutes, taken out, and washed with an appropriate amount of DMF to obtain a porous polypropylene membrane with photoinitiated activity.

[0066] S2, placing a porous polypropylene membrane with photoinitiator activity in a container, adding a DMF solution of pentafluorostyrene and n-butyl acrylate, and shaking for 30 minutes. The sum of the volumes of pentafluorostyrene and n-butyl acrylate is 80 v / v% of the volume of the DMF solution, and the volume ratio of pentafluorostyrene to n-butyl acrylate is 4:1. The membrane after impregnation is fished out, sandwiched between light-transmitting glass plates, irradiated under ultraviolet light with a wavelength of 365 nm for 15 minutes, taken out, and washed with DMF to obtain a modified porous polypropylene membrane grafted with a random copolymer.

[0067] Example 4

[0068] This embodiment provides a method for modifying a porous polypropylene membrane, comprising:

[0069] S1, placing a commercial polypropylene membrane with a size of 4×20 cm in a container, adding 10 mL of a 10 wt% benzophenone DMF solution to the container, soaking the commercial polypropylene membrane, and shaking for 30 minutes. The immersed commercial polypropylene membrane is taken out, sandwiched between light-transmitting glass plates, irradiated under ultraviolet light with a wavelength of 365 nm for 5 minutes, taken out, and washed with an appropriate amount of DMF to obtain a porous polypropylene membrane with photoinitiated activity.

[0070] S2, placing a porous polypropylene membrane with photoinitiator activity in a container, adding a DMF solution of 2,2,3,3-tetrafluoropropyl acrylate and polyethylene glycol dimethacrylate, and shaking for 30 minutes. Among them, the sum of the volumes of 2,2,3,3-tetrafluoropropyl acrylate and polyethylene glycol dimethacrylate is 50v / v% of the volume of the DMF solution, and the volume ratio of 2,2,3,3-tetrafluoropropyl acrylate to polyethylene glycol dimethacrylate is 1:1. The membrane after immersion is fished out, sandwiched between light-transmitting glass plates, irradiated under ultraviolet light with a wavelength of 365nm for 15 minutes, taken out, and washed with DMF to obtain a modified porous polypropylene membrane grafted with a random copolymer.

[0071] Example 5

[0072] This embodiment provides a method for modifying a porous polyethylene membrane.

[0073] The difference between Example 5 and Example 2 is that the commercial polypropylene diaphragm is replaced by a commercial polyethylene diaphragm, and the rest is the same as Example 2.

[0074] Comparative Example

[0075] Commercial Polypropylene Diaphragm

[0076] The modified porous polypropylene membranes prepared in Examples 1-3 and the commercial polypropylene separators of the comparative example were tested for morphology, and their SEM images were as follows: Figure 1-Figure 4 As shown. Among them, Figure 1-3 They are SEM images of the modified porous polypropylene membranes prepared in Examples 1-3, Figure 4 This is the SEM image of commercial polypropylene separator.

[0077] from Figure 1-Figure 4 It can be seen that the modified porous polypropylene membranes prepared in Examples 1-3 all maintain the porous structure of the commercial polypropylene separator, ensuring that the transmission of lithium ions is not hindered.

[0078] The modified porous polypropylene membranes prepared in Examples 1-3 were subjected to EDS energy spectrum analysis, and the results are shown in Table 1. As the proportion of fluorine-containing monomers increases, the fluorine content in the final modified porous polypropylene membrane increases, indicating that the present application can adjust the proportion of functional monomers and then adjust the proportion of functional monomer units in the final modified porous polypropylene membrane to obtain modified porous polymer membranes with different properties.

[0079] Table 1 Atomic ratio of fluorine element in different modified porous polypropylene membranes

[0080] Fluorine atomic ratio (at%) Example 1 0.03 Example 2 4.70 Example 3 6.27

[0081] The modified porous polypropylene membranes of Examples 1-3 and the commercial polypropylene membranes of the comparative example were respectively used as battery separators to assemble lithium-lithium symmetrical batteries and perform cycle performance tests. The specific method is:

[0082] In a glove box with an argon atmosphere (water and oxygen content is less than 0.5ppm), the cut modified porous polypropylene membrane and commercial polypropylene membrane are used as separators to assemble CR2032 button batteries. The positive and negative electrodes are metal lithium sheets respectively; the concentration of the electrolyte is 1M, the solute is LiTFSI, the solvent is a mixed solvent of 1,3-dioxane and dimethyl ether with a volume ratio of 1:1, and 1.0% LiNO3 is added as an additive. The LandCT2001A battery test system is used to test the cycle stability of the lithium-lithium symmetric battery. The test results are as follows: Figure 5 shown.

[0083] Depend on Figure 5 It can be seen that the lithium-lithium symmetric button cell assembled with commercial polypropylene separator has a high conductivity at 5 mA cm -2、1mAh cm -2 After about 100 hours of cycling under the conditions, an obvious overpotential surge occurs. In comparison, the modified porous polypropylene membranes of Examples 1-3 all show better stability than commercial polypropylene membranes, especially the modified porous polypropylene membrane prepared in Example 2 can have a stability of more than 1400 hours.

[0084] The modified porous polypropylene membrane of Example 2 and the commercial polypropylene separator of the comparative example were used as battery separators to assemble lithium metal-lithium iron phosphate batteries for cycle performance testing. The specific method is:

[0085] Lithium iron phosphate, binder polyvinylidene fluoride, and conductive agent conductive carbon black Super P are mixed in a mass ratio of 8:1:1, and a small amount of N-methylpyrrolidone is added, and the mixture is stirred to form a slurry with a certain viscosity. Subsequently, the slurry is evenly coated on the surface of the aluminum foil using a coater, vacuum dried at 60°C, and then the electrode sheet with a diameter of 12 mm is cut; then, the battery is assembled in a glove box with an argon atmosphere (water and oxygen content are both less than 0.5ppm), with the lithium iron phosphate electrode sheet as the positive electrode, the metal lithium sheet as the negative electrode, and the electrolyte is LiPF6 dissolved in ethylene carbonate and diethyl carbonate in a volume ratio of 1:1, with a concentration of 1M, and 5wt% fluoroethylene carbonate additive is added at the same time; the modified porous polypropylene membrane of Example 2 and the commercial polypropylene diaphragm are used as diaphragms to assemble CR2032 button batteries. The LandCT2001A battery test system is used at 5C (1C = 170mAg -1 ) The cycle stability of the battery was tested at a current density of Figure 6 shown.

[0086] Depend on Figure 6 It can be seen that the specific capacity of the battery using the modified porous polypropylene membrane of Example 2 is significantly higher than that of the battery using the commercial polypropylene separator. After 600 cycles, the specific capacity of the battery using the commercial polypropylene separator decreased significantly; while the battery using the modified porous polypropylene membrane of Example 2 still maintained a high specific capacity after 1000 cycles. The modified porous polypropylene membrane prepared in Example 2 has better cycle stability than the commercial polypropylene separator.

[0087] The modified porous polypropylene membrane prepared in Example 4 and the modified porous polyethylene membrane prepared in Example 5 were tested for their performance, and they also had high cycle stability.

[0088] Although the present application has been described in detail in general terms and in specific embodiments in this specification, it is obvious to those skilled in the art that some modifications or improvements may be made to the present application. Therefore, these modifications or improvements made without departing from the spirit of the present application are within the scope of protection claimed in the present application.

Claims

1. A method for modifying a porous polymer membrane, characterized in that: include: S1, immersing the porous polymer film in a photoinitiator solution and irradiating it with ultraviolet light to obtain a porous polymer film with photoinitiator activity; S2, immersing the porous polymer membrane with photoinitiating activity in a mixed solution of a fluorine-containing monomer and an acrylate monomer, irradiating it with ultraviolet light for grafting, and washing it to obtain a modified porous polymer membrane.

2. The modification method according to claim 1, characterized in that: The solute of the photoinitiator solution is a hydrogen abstraction type photoinitiator; The solvents of the photoinitiator solution and the mixed solution are both polar organic solvents; The polar organic solvent includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, 1,4-dioxane, acetonitrile or acetone.

3. The modification method according to claim 2, characterized in that: The hydrogen abstraction type photoinitiator includes any one of benzophenone, Michler's ketone, thioxanthone, isopropylthioxanthone or 2-ethylanthraquinone.

4. The modification method according to claim 1, characterized in that: The raw material of the porous polymer membrane includes at least one of polypropylene, polyethylene, polyimide, polyamide, polyvinylidene fluoride or cellulose.

5. The modification method according to claim 1, characterized in that: The fluorine-containing monomer includes at least one of pentafluorostyrene, 2,2,2-trifluoroethyl methacrylate, 2,2,3,3-tetrafluoropropyl methacrylate, 2,2,3,3-tetrafluoropropyl acrylate, 2,2,3,4,4,4-hexafluorobutyl methacrylate or 2,2,3,4,4,4-hexafluorobutyl acrylate; and / or, The acrylic acid ester monomer includes at least one of methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, acrylic acid, sec-butyl acrylate, n-propyl acrylate, cycloethyl acrylate, lauryl acrylate, 2-hydroxyethyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate or polyethylene glycol dimethacrylate.

6. The modification method according to claim 2, characterized in that: The concentration of the photoinitiator solution is 0.5-20wt%; In the mixed solution, the ratio of the sum of the volumes of the fluorine-containing monomer and the acrylate monomer to the sum of the volumes of the fluorine-containing monomer, the acrylate monomer and the solvent is 10-90%.

7. The modification method according to claim 1, characterized in that: In S1, the immersion time is 5-30 min, and the UV irradiation time is 5-15 min; In S2, the immersion time is 0.5 to 12 hours, and the ultraviolet light irradiation time is 5 to 30 minutes.

8. A modified porous polymer membrane prepared by the modification method according to any one of claims 1 to 7.

9. Use of the modified porous polymer membrane according to claim 8 in lithium metal batteries.

10. A lithium metal battery separator, characterized in that: It comprises the modified porous polymer membrane according to claim 8.

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