Modified polyimide diaphragm as well as preparation method and application thereof

Abstract

The invention relates to a modified polyimide diaphragm as well as a preparation method and application thereof, and belongs to the technical field of battery materials, the modified polyimide diaphragm comprises a polyimide nanofiber membrane and a homopolymer or copolymer of 2-acrylamide-2-methylpropanesulfonic acid (AMPS) distributed on the surface of the polyimide nanofiber membrane and in pores of the polyimide nanofiber membrane. According to the modified polyimide diaphragm provided by the invention, the electrochemical performance is improved through the synergistic effect of multiple mechanisms such as physical barrier, electrostatic interaction, chemical adsorption and promotion of lithium ion transmission, and the shuttle problem of polysulfide in a lithium-sulfur battery and the dendritic crystal problem commonly existing in the lithium battery are effectively improved; therefore, the energy density and the cycling stability of the battery are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and particularly to a modified polyimide separator and its preparation method and application. Background Art

[0002] Due to its advantages such as high specific energy, high discharge voltage platform, and long cycle life, lithium-ion batteries have currently become the preferred system for secondary energy storage power sources and have been widely used in various fields such as electronic devices, electric vehicles, aerospace and military industries, and energy storage power stations. However, with the continuous increase in the requirement for energy density, traditional lithium secondary batteries with graphite as the negative electrode have almost reached their energy limit, and it is imperative to develop new secondary batteries with higher specific energy. Lithium-sulfur batteries and lithium-metal batteries with metallic lithium as the negative electrode, due to their high theoretical capacity and high power density, show great potential in high-energy-density battery systems and secondary energy systems and are regarded as powerful alternatives to traditional lithium-ion batteries.

[0003] However, the polysulfide shuttle effect caused by complex multi-electron electrochemical reactions inside lithium-sulfur batteries, as well as the low Coulomb efficiency of the battery when using traditional polyolefin separators, limit the exertion of their high-energy-density potential and seriously affect the energy density and cycle stability of lithium-sulfur batteries. At the same time, both lithium-sulfur batteries and lithium-metal batteries use pure metallic lithium sheets as the negative electrode. Different from the traditional graphite negative electrode that realizes charge and discharge cycles through the insertion and extraction process of lithium ions, the lithium-metal negative electrode relies on the deposition and stripping process of lithium, resulting in serious dendrite growth problems, consuming electrolyte and active lithium, reducing the efficiency of lithium deposition / stripping, and affecting the cycle stability of the battery. The uneven electric field distribution on the surface of the negative electrode leads to uneven deposition of lithium ions, promoting the formation of lithium dendrites; these dendrites will pierce the separator, causing internal short circuit of the battery, thereby triggering thermal runaway, fire, and explosion. Although the dendrite problem exists in various lithium battery systems, it is particularly serious in battery systems with lithium sheets as the negative electrode, seriously affecting battery life and safety and becoming a serious obstacle restricting the commercialization process of lithium-sulfur batteries and lithium-metal batteries.

[0004] To solve the above problems, the prior art introduces a surface functional layer on the surface of the separator. Although this method can to a certain extent inhibit the pore diffusion of polysulfides through the separator, the binding ability between the surface functional layer and the separator substrate is weak and it is easy to fall off, and it cannot effectively inhibit the polysulfide shuttle effect for a long time. At the same time, it cannot provide the function of inhibiting dendrite growth.

[0005] Therefore, there is an urgent need to develop a functional separator that can effectively inhibit polysulfide shuttle and inhibit lithium dendrite growth. Summary of the Invention

[0006] In view of one or more technical problems existing in the prior art, the present invention provides a modified polyimide separator and its preparation method and application. In the modified polyimide separator provided by the present invention, the homopolymer or copolymer of 2-acrylamide-2-methylpropanesulfonic acid (AMPS) has a strong binding strength with the polyimide nanofiber membrane and is not easily detached, which can effectively improve the strength of the separator. Through the synergistic action of multiple mechanisms such as physical barrier, electrostatic interaction, chemical adsorption and surface functional groups to promote lithium ion transport, the electrochemical performance is improved, and the shuttle problem of polysulfides in lithium-sulfur batteries and the lithium dendrite problem commonly existing in lithium batteries are effectively improved, thereby improving the energy density and cycle stability of the battery.

[0007] In the first aspect, the present invention provides a modified polyimide separator, which comprises a polyimide nanofiber membrane and a homopolymer or copolymer of 2-acrylamide-2-methylpropanesulfonic acid (AMPS) distributed on the surface and inside the pores of the polyimide nanofiber membrane. The copolymer is a binary copolymer or a multi-component copolymer, and the copolymerization component monomers of the copolymer are one or more of acrylamide (AM), acrylic acid (AA), N-vinylpyrrolidone, acrylonitrile, etc.

[0008] Preferably, the mass ratio of the homopolymer or copolymer of 2-acrylamide-2-methylpropanesulfonic acid (AMPS) to the polyimide nanofiber membrane is 0.5-4:1.

[0009] Preferably, the thickness of the modified polyimide separator is 2-50 μm;

[0010] The porosity of the modified polyimide separator is 45-85%;

[0011] The tensile strength of the modified polyimide separator is 40-140 MPa; and / or

[0012] The transverse and longitudinal thermal shrinkage rates of the modified polyimide separator at 300 °C are both less than 0.5%.

[0013] In the second aspect, the present invention provides a preparation method of the modified polyimide separator according to the first aspect, and the preparation method includes:

[0014] S1. Pretreat the polyimide nanofiber membrane with an alkaline solution to obtain a pretreated polyimide nanofiber membrane;

[0015] S2. In-situ generate a homopolymer or copolymer of 2-acrylamide-2-methylpropanesulfonic acid (AMPS) on the surface and inside the pores of the pretreated polyimide nanofiber membrane to obtain a modified polyimide separator of the homopolymer or copolymer of 2-acrylamide-2-methylpropanesulfonic acid (AMPS).

[0016] Preferably, the time for the pretreatment is 30 s to 5 min; and / or

[0017] the concentration of the alkaline solution is 0.1 to 2 mol / L; preferably, the alkaline substance is one or more of ammonia water, dimethylamine, diethylamine, potassium hydroxide, sodium hydroxide, lithium hydroxide, ethanolamine, triethylamine, dimethylaminoborane.

[0018] Preferably, a homopolymer or copolymer of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) is in-situ generated on the surface and inside the pores of the pretreated polyimide nanofiber membrane, including:

[0019] (ii) Prepare a monomer solution of copolymerization components in an equimolar ratio and mix it with the solution obtained in step (i) to obtain a precursor solution.

[0020] (iii) Immerse the pretreated polyimide nanofiber membrane in the precursor solution, and then place it in a mixed solution containing an initiator and a crosslinking agent for a polymerization crosslinking reaction;

[0021] (iiii) Repeat step (iii) multiple times to obtain a polyimide separator modified with a homopolymer or copolymer of 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

[0022] Preferably, the mass fraction of the precursor solution is 5 to 50%.

[0023] Preferably, in the mixed solution containing an initiator and a crosslinking agent, the initiator accounts for 0.01 to 0.5 wt%, and the crosslinking agent accounts for 0.01 to 0.5 wt%;

[0024] Preferably, the initiator is potassium persulfate; and / or

[0025] the crosslinking agent is N,N'-methylenebisacrylamide.

[0026] Preferably, the temperature of the polymerization crosslinking reaction is 30 to 80 °C; the time of the polymerization crosslinking reaction is 1 to 5 h.

[0027] The present invention in the third aspect provides an application of the modified polyimide separator described in the first aspect or the modified polyimide separator prepared by the preparation method described in the second aspect, including lithium-sulfur batteries, traditional lithium-ion batteries, sodium-ion batteries, and lithium metal batteries.

[0028] The present invention has at least the following beneficial effects compared with the prior art:

[0029] The modified polyimide separator provided by the present invention uses a polyimide nanofiber membrane as the base membrane, which has excellent chemical stability and high thermal stability. At the same time, its high specific surface area helps to improve the electrolyte wettability and improve the chemical properties of the battery. The homopolymer or copolymer of 2-acrylamide-2-methylpropanesulfonic acid (AMPS) is combined with the polyimide nanofibers through physical and chemical bonds, with strong binding strength and not easy to fall off. It can not only improve the strength of the separator but also regulate the pore structure of the polyimide nanofiber membrane to form a physical barrier to inhibit the diffusion of polysulfides. The synergistically introduced sulfonic acid groups are negatively charged and can have an electrostatic interaction (electrostatic repulsion) with polysulfide anions, reducing the diffusion of polysulfide anions in the electrolyte. At the same time, the sulfonic acid groups can bind to polysulfides through hydrogen bonds or chemical bonds to anchor the polysulfides and form stable compounds, further reducing the dissolution and migration of polysulfides, and thus effectively inhibiting the shuttle effect of polysulfides in lithium-sulfur batteries.

[0030] The modified polyimide separator provided by the present invention effectively improves the shuttle problem of polysulfides in lithium-sulfur batteries through the synergistic action of multiple mechanisms such as physical barrier, electrostatic interaction, chemical adsorption, and promotion of lithium-ion transport, thereby improving the energy density and cycle stability of the battery.

[0031] The modified polyimide separator modified with the homopolymer or copolymer of 2-acrylamide-2-methylpropanesulfonic acid (AMPS) provided by the present invention has good wettability and good ion conductivity, which is beneficial to promoting the rapid transport of lithium ions; at the same time, it can also promote the uniform deposition of lithium ions on the electrode surface, reduce the generation of lithium dendrites, thereby improving the performance of the battery and extending the life of the battery. Brief Description of the Drawings

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0033] Figure 1 It is an electron microscope image of the modified polyimide separator provided in Embodiment 1 of the present invention with a magnification of 5000 times;

[0034] Figure 2 It is an electron microscope image of the polyimide nanofiber membrane provided in Comparative Example 4 of the present invention with a magnification of 5000 times. Detailed Embodiments

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0036] In a first aspect, the present invention provides a modified polyimide separator, which includes a polyimide nanofiber membrane and a homopolymer or copolymer of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) distributed on the surface and inside the pores of the polyimide nanofiber membrane.

[0037] The modified polyimide separator provided by the present invention uses a polyimide nanofiber membrane as the base membrane, which has excellent chemical stability and high thermal stability. At the same time, its high specific surface area helps to improve the electrolyte wettability and improve the chemical properties of the battery. The homopolymer or copolymer of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) has a strong binding strength with the polyimide nanofiber membrane and is not easily detached. It can not only enhance the strength of the separator but also regulate the pore structure of the polyimide nanofiber membrane to form a physical barrier to inhibit the diffusion of polysulfides. The synergistically introduced sulfonic acid groups are negatively charged and can have an electrostatic interaction (electrostatic repulsion) with polysulfide anions, reducing the diffusion of polysulfide anions in the electrolyte. At the same time, the sulfonic acid groups can bind to polysulfides through hydrogen bonds or chemical bonds to anchor the polysulfides and form stable compounds, further reducing the dissolution and migration of polysulfides, and thus effectively inhibiting the shuttle effect of polysulfides in lithium-sulfur batteries.

[0038] The modified polyimide separator modified with the homopolymer or copolymer of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) provided by the present invention has good ionic conductivity, which is conducive to promoting the rapid transmission of lithium ions. At the same time, the chemical adsorption of sulfonic acid groups can also promote the uniform deposition of lithium ions on the electrode surface, reduce the formation of lithium dendrites, and thus improve the performance of the battery and extend the life of the battery.

[0039] In the modified polyimide separator provided by the present invention, the homopolymer or copolymer of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) has a strong binding strength with the polyimide nanofiber membrane and is not easily detached. It can effectively enhance the strength of the separator and improve the electrochemical performance through the synergistic action of multiple mechanisms such as physical barrier, electrostatic interaction, chemical adsorption, and promotion of lithium ion transmission. It effectively solves the problem of polysulfide shuttle in lithium-sulfur batteries and improves the redox reaction rate of the positive and negative active materials, thereby increasing the energy density and cycle stability of the battery.

[0040] The polyimide (PI) nanofiber membrane has excellent chemical stability, can remain stable in various chemical environments, and is not prone to chemical degradation, which is crucial for the long-term operation of the battery. The PI nanofiber membrane has high thermal stability and can remain stable at temperatures up to 500 °C, which means that under the high-temperature conditions that the battery may encounter, the PI nanofiber membrane can still maintain its structure and performance. The nanoscale size of the fibers in the PI nanofiber membrane has a larger surface area, which helps to improve the wettability of the electrolyte, the uniform distribution of the electrolyte, and the rapid transport of ions inside the battery, thereby improving the electrochemical performance of the battery.

[0041] According to some preferred embodiments, the mass ratio of the homopolymer or copolymer of 2-acrylamide-2-methylpropanesulfonic acid (AMPS) to the polyimide nanofiber membrane is 0.5-4:1 (for example, it can be 0.5:1, 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, or 4:1).

[0042] According to some preferred embodiments, the thickness of the modified polyimide separator is 2-50 μm (for example, it can be 2 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, or 50 μm);

[0043] The porosity of the modified polyimide separator is 45-85% (for example, it can be 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or 85%);

[0044] The tensile strength of the modified polyimide separator is 40-140 MPa (for example, it can be 40 MPa, 50 MPa, 60 MPa, 70 MPa, 80 MPa, 90 MPa, 100 MPa, 110 MPa, 120 MPa, 130 MPa, or 140 MPa); and / or

[0045] The transverse and longitudinal thermal shrinkage rates of the modified polyimide separator at 300 °C are both less than 0.5%.

[0046] In a second aspect, the present invention provides a method for preparing the modified polyimide separator described in the first aspect, and the preparation method includes:

[0047] S1. Pretreat the polyimide nanofiber membrane with an alkaline solution to obtain a pretreated polyimide nanofiber membrane;

[0048] S2. In-situ generate homopolymers or copolymers of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) on the surface and inside the pores of the pretreated polyimide nanofiber membrane to obtain a modified polyimide separator with homopolymers or copolymers of 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

[0049] The present invention first pretreats the polyimide nanofiber membrane with an alkaline solution to improve the hydrophilic property of the polyimide nanofiber membrane, and then in-situ generates homopolymers or copolymers of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) on the surface and inside the pores of the pretreated polyimide nanofiber membrane. The binding strength between the polyimide nanofiber membrane and the homopolymers or copolymers of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) is high and not easy to fall off. By regulating the pores of the separator to synergistically interact the sulfonic acid groups with polysulfides, the problem of polysulfide shuttle in lithium-sulfur batteries can be effectively solved.

[0050] It should be noted that the polyimide nanofiber membrane of the present invention is obtained by electrospinning and thermal imidization of a polyamic acid solution; specifically: a dianhydride and a diamine are mixed in a molar ratio of 1:1 to obtain a polyamic acid solution; the polyamic acid solution is used to prepare a polyamic acid nanofiber membrane by an electrospinning process; the polyamic acid nanofiber membrane is heat-treated at 300 °C for 2 hours to obtain a polyimide nanofiber membrane.

[0051] According to some preferred embodiments, the pretreatment time is 30 s to 5 min (for example, it can be 30 s, 1 min, 2 min, 3 min, 4 min or 5 min).

[0052] According to some preferred embodiments, the concentration of the alkaline solution is 0.1 to 2 mol / L; preferably, the alkaline substance is one or more of ammonia water, dimethylamine, diethylamine, potassium hydroxide, sodium hydroxide, lithium hydroxide, ethanolamine, triethylamine, dimethylaminoborane. It should be noted that the purpose of the alkaline solution here is only for pretreatment to cause partial ring-opening of the imide rings on the surface of the polyimide nanofibers and improve the hydrophilic property of the polyimide nanofiber membrane. There is no specific limitation on the type and concentration of the alkaline solution, and the type and concentration of the alkaline solution are not limited to the above range.

[0053] According to some preferred embodiments, in-situ generating homopolymers or copolymers of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) on the surface and inside the pores of the pretreated polyimide nanofiber membrane includes:

[0054] (i) Adjust the pH of the 2-acrylamide-2-methylpropanesulfonic acid aqueous solution to 3-7 (for example, it can be 3, 4, 5, 6, or 7) using an alkaline solution; prepare a copolymerization component monomer solution in an equimolar ratio and mix it with the above solution to obtain a precursor solution. By adjusting the pH of the 2-acrylamide-2-methylpropanesulfonic acid aqueous solution using an alkaline solution in the present invention, it is to prevent the self-polymerization of 2-acrylamide-2-methylpropanesulfonic acid, so as to in-situ generate homopolymers or copolymers of 2-acrylamide-2-methylpropanesulfonic acid (AMPS) on the surface and inside the pores of the polyimide nanofiber membrane during subsequent treatment processes.

[0055] (ii) After soaking the pretreated polyimide nanofiber membrane in the precursor solution, place it in a mixed solution containing an initiator and a crosslinking agent for polymerization crosslinking reaction;

[0056] (iii) Repeat step (ii) multiple times to obtain a modified polyimide separator with homopolymers or copolymers of 2-acrylamide-2-methylpropanesulfonic acid (AMPS). In the present invention, by adjusting the number of times of repeating step (ii) and combining with the mass fraction of the precursor solution, the pore structure of the obtained separator and the amount of sulfonic acid groups introduced are controlled to regulate the performance of the separator.

[0057] According to some preferred embodiments, the mass fraction of the 2-acrylamide-2-methylpropanesulfonic acid aqueous solution is 5-50% (for example, it can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, or 50%). The inventors found that when other conditions remain unchanged, if the mass fraction of the 2-acrylamide-2-methylpropanesulfonic acid aqueous solution is too small, the filling of homopolymers or copolymers of 2-acrylamide-2-methylpropanesulfonic acid (AMPS) inside the pores of the pretreated polyimide nanofiber membrane is too little, the pores are large, and the improvement of the separator strength and polysulfide shuttling is not obvious; if the mass fraction of the 2-acrylamide-2-methylpropanesulfonic acid aqueous solution is too large, the filling inside the pores of the pretreated polyimide nanofiber membrane is too much, the porosity is too low, the battery capacity is reduced, and the cycle life is poor.

[0058] According to some preferred embodiments, in the mixed solution containing an initiator and a crosslinking agent, the initiator accounts for 0.01-0.5 wt% (for example, it can be 0.01 wt%, 0.05 wt%, 0.10 wt%, 0.15 wt%, 0.20 wt%, 0.25 wt%, 0.30 wt%, 0.35 wt%, 0.40 wt%, 0.45 wt% or 0.50 wt%), and the crosslinking agent accounts for 0.01-0.5 wt% (for example, it can be 0.01 wt%, 0.05 wt%, 0.10 wt%, 0.15 wt%, 0.20 wt%, 0.25 wt%, 0.30 wt%, 0.35 wt%, 0.40 wt%, 0.45 wt% or 0.50 wt%);

[0059] Preferably, the initiator is potassium persulfate; and / or

[0060] the crosslinking agent is N,N'-methylenebisacrylamide.

[0061] According to some preferred embodiments, the temperature of the polymerization crosslinking reaction is 30-80 °C (for example, it can be 30 °C, 35 °C, 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, 65 °C, 70 °C or 80 °C); the time of the polymerization crosslinking reaction is 1-5 h (for example, it can be 1 h, 2 h, 3 h, 4 h or 5 h).

[0062] According to some preferred embodiments, the number of times of repeating step (ii) is 3-10 times.

[0063] The present invention provides a lithium-sulfur battery in a third aspect, including the modified polyimide separator described in the first aspect or the modified polyimide separator prepared by the preparation method described in the second aspect.

[0064] In order to more clearly illustrate the technical solutions and advantages of the present invention, the present invention will be further described below with reference to embodiments. The present invention does not specifically limit the sources of various reagents used in the examples and comparative examples, and they can be directly purchased or synthesized by oneself. The polyimide nanofiber membrane in the examples and comparative examples of the present invention can be prepared by the following preparation method, specifically including: mixing a dianhydride (pyromellitic dianhydride) and a diamine (4,4'-diaminodiphenyl ether) in a molar ratio of 1:1 to prepare a polyamic acid solution; using the above polyamic acid solution to prepare a polyamic acid nanofiber membrane through an electrospinning process; heat-treating the polyamic acid nanofiber membrane at 300 °C for 2 hours to obtain a polyimide nanofiber membrane.

[0065] Example 1

[0066] A preparation method of a modified polyimide separator, including:

[0067] S1. Immerse the polyimide nanofiber membrane in a 1 mol / L potassium hydroxide solution for 1 minute, then rinse with deionized water and dry to obtain a pretreated polyimide nanofiber membrane;

[0068] S2. Add 2-acrylamide-2-methylpropanesulfonic acid (AMPS) to deionized water to prepare an aqueous solution with a mass fraction of 10%, then dropwise add a 1 mol / L potassium hydroxide solution and control the pH to 5;

[0069] S3. Immerse the pretreated polyimide nanofiber membrane in the 2-acrylamide-2-methylpropanesulfonic acid aqueous solution adjusted with potassium hydroxide solution for 15 minutes, then place it in a mixed solution containing an initiator and a crosslinking agent (prepared by mixing a 0.1% mass fraction potassium persulfate (KPS) solution and a 0.1% mass fraction N,N'-methylenebisacrylamide (NMBA) solution in a mass ratio of 1:1), and react for 3 h under a 40 °C water bath condition, then rinse with deionized water.

[0070] S4. Repeat step S3 five times to obtain a polyimide separator modified with 2-acrylamide-2-methylpropanesulfonic acid (AMPS) homopolymer.

[0071] Example 2

[0072] A preparation method of a modified polyimide separator, comprising:

[0073] S1. Immerse the polyimide nanofiber membrane in a 0.1 mol / L potassium hydroxide solution for 5 minutes, then rinse with deionized water and dry to obtain a pretreated polyimide nanofiber membrane;

[0074] S2. Add 2-acrylamide-2-methylpropanesulfonic acid (AMPS) to deionized water to prepare an aqueous solution with a mass fraction of 5%, then dropwise add a 0.1 mol / L potassium hydroxide solution and control the pH to 3;

[0075] S3. Immerse the pretreated polyimide nanofiber membrane in a polyacrylamide precursor solution for 30 minutes, then place it in a mixed solution containing an initiator and a crosslinking agent (prepared by mixing a 0.01% mass fraction potassium persulfate (KPS) solution and a 0.01% mass fraction N,N'-methylenebisacrylamide (NMBA) solution in a mass ratio of 1:1), and react for 5 h under a 30 °C water bath condition, then rinse with deionized water.

[0076] S4. Repeat step S3 ten times to obtain a polyimide separator modified with 2-acrylamide-2-methylpropanesulfonic acid (AMPS) homopolymer.

[0077] Example 3

[0078] A preparation method of a modified polyimide separator, comprising:

[0079] S1. Immerse the polyimide nanofiber membrane in a 2 mol / L potassium hydroxide solution for 30 seconds, then rinse with deionized water and dry to obtain a pretreated polyimide nanofiber membrane;

[0080] S2. Add 2-acrylamido-2-methylpropanesulfonic acid (AMPS) to deionized water to prepare an aqueous solution with a mass fraction of 50%, then dropwise add a 2 mol / L potassium hydroxide solution and control the pH to 7;

[0081] S3. Immerse the pretreated polyimide nanofiber membrane in the 2-acrylamido-2-methylpropanesulfonic acid aqueous solution adjusted with potassium hydroxide solution for 5 minutes, then place it in a mixed solution containing an initiator and a crosslinking agent (prepared by mixing a 0.5% mass fraction potassium persulfate (KPS) solution and a 0.5% mass fraction N,N'-methylenebisacrylamide (NMBA) solution in a mass ratio of 1:1), and react for 1 h under a water bath condition of 80 °C, then rinse with deionized water.

[0082] S4. Repeat step S3 three times to obtain a polyimide separator modified with 2-acrylamido-2-methylpropanesulfonic acid (AMPS) homopolymer.

[0083] Example 4

[0084] A preparation method of a modified polyimide separator, comprising:

[0085] S1. Immerse the polyimide nanofiber membrane in a 1.5 mol / L potassium hydroxide solution for 2 minutes, then rinse with deionized water and dry to obtain a pretreated polyimide nanofiber membrane;

[0086] S2. Add 2-acrylamido-2-methylpropanesulfonic acid (AMPS) to deionized water to prepare an aqueous solution with a mass fraction of 30%, then dropwise add a 1.5 mol / L potassium hydroxide solution and control the pH to 6;

[0087] S3. Immerse the pretreated polyimide nanofiber membrane in the 2-acrylamido-2-methylpropanesulfonic acid aqueous solution adjusted with potassium hydroxide solution for 10 minutes, then place it in a mixed solution containing an initiator and a crosslinking agent (prepared by mixing a 0.3% mass fraction potassium persulfate (KPS) solution and a 0.3% mass fraction N,N'-methylenebisacrylamide (NMBA) solution in a mass ratio of 1:1), and react for 2 h under a water bath condition of 60 °C, then rinse with deionized water.

[0088] S4. Repeat step S3 seven times to obtain a polyimide separator modified with poly(2-acrylamido-2-methylpropanesulfonic acid) (AMPS).

[0089] Example 5

[0090] A method for preparing a modified polyimide separator, comprising:

[0091] S1. Immerse the polyimide nanofiber membrane in a 1.5 mol / L potassium hydroxide solution for 2 minutes, then rinse with deionized water and dry to obtain a pretreated polyimide nanofiber membrane;

[0092] S2. Add 2-acrylamido-2-methylpropanesulfonic acid (AMPS) to deionized water to prepare an aqueous solution with a mass fraction of 50%, then add dropwise a 1.5 mol / L potassium hydroxide solution and control the pH to 6;

[0093] S3. Dissolve N-isopropylacrylamide (IPAM) with an equimolar ratio in deionized water, mix it with the above solution, and add deionized water to make a precursor solution with a mass fraction of 30%.

[0094] S4. Immerse the pretreated polyimide nanofiber membrane in the precursor solution for 10 minutes, then place it in a mixed solution containing an initiator and a crosslinking agent (obtained by mixing a 0.3% potassium persulfate (KPS) solution and a 0.3% N,N'-methylenebisacrylamide (NMBA) solution in a mass ratio of 1:1), and react at 60 °C in a water bath for 2 h, then rinse with deionized water.

[0095] Comparative Example 1

[0096] It is basically the same as Example 4, except that: S2. Add 2-acrylamido-2-methylpropanesulfonic acid (AMPS) to deionized water to prepare an aqueous solution with a mass fraction of 2%, then add dropwise a 1.5 mol / L potassium hydroxide solution and control the pH to 6.

[0097] Comparative Example 2

[0098] It is basically the same as Example 4, except that: S2. Add 2-acrylamido-2-methylpropanesulfonic acid (AMPS) to deionized water to prepare an aqueous solution with a mass fraction of 60%, then add dropwise a 1.5 mol / L potassium hydroxide solution and control the pH to 6.

[0099] Comparative Example 3

[0100] It is basically the same as Example 4, except that: S2. Add 2-acrylamido-2-methylpropanesulfonic acid (AMPS) to deionized water to prepare an aqueous solution with a mass fraction of 10%.

[0101] Comparative Example 4

[0102] The separator is a polyimide nanofiber membrane.

[0103] Comparative Example 5

[0104] It is basically the same as Example 5, except that: S3. N-isopropylacrylamide (IPAM) in an equimolar ratio is dissolved in deionized water, mixed with the above solution, and deionized water is added to make a precursor solution with a mass fraction of 10%.

[0105] From Figure 1-2 It can be seen that compared with the polyimide nanofiber membrane, the modified polyimide separator provided by the present invention has a smaller porosity.

[0106] The performance data of the polyimide separators and the lithium-sulfur batteries containing the separators in the examples and comparative examples of the present invention are shown in Table 1, and the methods for each performance test are as follows:

[0107] Separator performance Porosity: Cut a sample with a size of 5 cm × 5 cm and weigh md, then immerse it completely in n-butanol (n-BuOH) for 4 h to make n-BuOH completely fill the micropores of the separator. Then take out the separator, quickly remove the excess liquid on the surface of the sample with filter paper, and quickly weigh the wet weight of the sample, denoted as mw. The mass of n-BuOH remaining in the micropores of the separator is the difference between the two (mw - md), denoted as mb, and the porosity = [(mb / ρb) / (mb / ρb + md / ρp)] × 100%, where ρb is the density of n-BuOH and ρp is the density of the separator.

[0108] Tensile strength: Use a cutter to cut out a standard dumbbell-shaped test specimen with a length and width of 20 mm and 4 mm respectively in the middle parallel part, measure the average thickness with a thickness gauge, and then test the tensile strength of the specimen at a tensile speed of 100 mm / min.

[0109] Puncture strength: Lay the separator flat in the clamp of a universal testing machine and clamp it tightly, and perform puncture at a rate of 100 mm / min. The puncture strength = maximum load (N) / average thickness of the separator (μm).

[0110] Battery performance

[0111] The electrolyte of the lithium-sulfur battery uses lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as the solute, with a concentration of 1 M, the additive is LiNO 3， with a concentration of 0.4 M, and the solvent is a mixed solvent of DOL (1,3-dioxolane) and DME (dimethyl ether) with a volume ratio of 1:1. Sulfur (S) is selected as the positive electrode active material, and the surface loading is 1.8 - 2.0 mg·cm -2, the dosage ratio of the electrolyte to the active material is 12 μL·mg -1 . The cyclic performance of the battery was tested using a Land test system. The battery structure was S|Separator|Li, and the voltage range was 1.7 - 2.8 V.

[0112] Table 1. Performance data of the separators and lithium - sulfur batteries containing the separators in the examples and comparative examples of the present invention

[0113]

[0114] As can be seen from Table 1, the modified polyimide nanofiber membrane provided by the present invention has high strength. The tensile strength is greater than 40 MPa, and can reach up to 136 MPa at most; the puncture strength is greater than 50 gf, and can reach up to 240 gf at most, which is significantly better than the unmodified polyimide nanofiber membrane. The battery containing this separator has excellent cyclic performance. At 0.5 C, the capacity retention rate of the battery after 300 cycles is greater than 55%, and can reach up to 78% at most, which is significantly better than the cyclic performance of the battery containing the unmodified polyimide nanofiber membrane. Compared with Example 4, the mass fraction of the 2 - acrylamido - 2 - methylpropanesulfonic acid aqueous solution used in Comparative Example 1 is too small, and the homopolymer of 2 - acrylamido - 2 - methylpropanesulfonic acid (AMPS) filled in the pores of the polyimide nanofiber membrane is too little, and the pores are large, so the improvement of the strength of the separator and the shuttle of polysulfides is not obvious. Therefore, the tensile strength and puncture strength of the separator are significantly reduced, and the cyclic performance of the battery becomes significantly worse. Compared with Example 4, the mass fraction of the 2 - acrylamido - 2 - methylpropanesulfonic acid aqueous solution used in Comparative Example 2 is too large, and too much is filled in the pores of the polyimide nanofiber membrane, and the porosity is too low, resulting in a reduction in battery capacity and poor cycle life. Compared with Example 4, in Comparative Example 3, an alkaline solution was not used to adjust the pH of the 2 - acrylamido - 2 - methylpropanesulfonic acid aqueous solution, and 2 - acrylamido - 2 - methylpropanesulfonic acid underwent self - polymerization, which was not conducive to the in - situ generation of the homopolymer of 2 - acrylamido - 2 - methylpropanesulfonic acid (AMPS) on the surface and inside the pores of the polyimide nanofiber membrane. The improvement of the separator pores was small, and the improvement of the strength of the separator and the shuttle of polysulfides was not obvious. Therefore, the tensile strength and puncture strength of the separator were significantly reduced, and the cyclic performance of the battery became significantly worse. Similarly, compared with Example 5, the mass fraction of the precursor solution used in Comparative Example 5 is too small, and the copolymer of 2 - acrylamido - 2 - methylpropanesulfonic acid filled in the pores of the polyimide nanofiber membrane is too little, and the law of each performance is the same as that of filling with the homopolymer.

[0115] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A modified polyimide separator, characterized in that, The modified polyimide separator includes a polyimide nanofiber membrane and a homopolymer or copolymer of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) distributed on the surface and inside the pores of the polyimide nanofiber membrane. The copolymer is a binary copolymer or a multi-component copolymer, and the copolymerization components of the copolymer are one or more of acrylamide (AM), acrylic acid (AA), N-vinylpyrrolidone, and acrylonitrile.

2. The modified polyimide separator according to claim 1, wherein The mass ratio of the homopolymer or copolymer of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) to the polyimide nanofiber membrane is 0.5 to 4:

1.

3. The modified polyimide separator according to claim 1, wherein The thickness of the modified polyimide separator is 2 to 50 μm; The porosity of the modified polyimide separator is 45 to 85%; The tensile strength of the modified polyimide separator is 40 to 140 MPa; and / or The transverse and longitudinal thermal shrinkage rates of the modified polyimide separator at 300 °C are both less than 0.5%.

4. A method for preparing the modified polyimide separator according to any one of claims 1-3, characterized in that, The preparation method includes: S1. Pretreat the polyimide nanofiber membrane with an alkaline solution to obtain a pretreated polyimide nanofiber membrane; S2. In-situ generate a homopolymer or copolymer of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) on the surface and inside the pores of the pretreated polyimide nanofiber membrane to obtain a 2-acrylamido-2-methylpropanesulfonic acid (AMPS) homopolymer or copolymer modified polyimide separator.

5. The preparation method according to claim 4, characterized in that, The time of the pretreatment is 30 s to 5 min; and / or The concentration of the alkaline solution is 0.1 to 2 mol / L; preferably, the alkaline substance is one or several of ammonia water, dimethylamine, diethylamine, potassium hydroxide, sodium hydroxide, lithium hydroxide, ethanolamine, triethylamine, and dimethylaminoborane.

6. The preparation method according to claim 4, characterized in that, In-situ generating a homopolymer or copolymer of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) on the surface and inside the pores of the pretreated polyimide nanofiber membrane includes: (i) Adjust the pH of the 2-acrylamido-2-methylpropanesulfonic acid aqueous solution to 3 to 7 with an alkaline solution; (ii) Prepare a monomer solution of the copolymerization components in an equimolar ratio and mix it with the solution obtained in step (i) to obtain a precursor solution; (iii) Immerse the pretreated polyimide nanofiber membrane in the precursor solution, and then place it in a mixed solution containing an initiator and a crosslinking agent for polymerization crosslinking reaction; (iiii) Repeat step (iii) once or multiple times to obtain a 2-acrylamido-2-methylpropanesulfonic acid (AMPS) homopolymer or copolymer modified polyimide separator.

7. The preparation method according to claim 6, characterized in that, The mass fraction of the precursor solution is 5 to 50%.

8. The preparation method according to claim 6, characterized in that, In the mixed solution containing an initiator and a crosslinking agent, the initiator accounts for 0.01 to 0.5 wt%, and the crosslinking agent accounts for 0.01 to 0.5 wt%; Preferably, the initiator is potassium persulfate; and / or The crosslinking agent is N,N'-methylenebisacrylamide.

9. The preparation method according to claim 6, characterized in that, The temperature of the polymerization crosslinking reaction is 30 to 80 °C; the time of the polymerization crosslinking reaction is 1 to 5 h.

10. A battery comprising the modified polyimide separator according to any one of claims 1-3, characterized in that, The battery includes at least a positive electrode and a separator, and the battery is at least one of a lithium-sulfur battery, a lithium-ion battery, a sodium-ion battery, and a lithium metal battery.

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