A covalent organic framework material, a preparation method and a modified lithium-sulfur battery separator, a preparation method and applications thereof

Abstract

The present application belongs to the field of lithium-sulfur batteries, and relates to a covalent organic framework material, a preparation method, a modified lithium-sulfur battery diaphragm, a preparation method and application, which are used to solve the problem of lithium-sulfur battery shuttle effect. The covalent organic framework material containing anions and rich regular pore structure is mixed with a conductive agent, a binder and a solvent, and then mechanically ball-milled into a fine and uniform slurry, which is coated on a commercial diaphragm to form a specially-made modified diaphragm. On the one hand, the negatively charged anion groups in the COF have a repulsive effect on the polysulfide anions in the electrolyte, thereby improving the electrochemical performance of the lithium-sulfur battery; on the other hand, the special regular pore structure can prevent the diffusion of polysulfides to prevent the loss of sulfur active material; thereby inhibiting the shuttle effect generated by the reaction of polysulfides with metallic lithium. The lithium-sulfur battery assembled at a current density of 0.2C has a discharge capacity of up to 610 mAh / g after 200 cycles, showing a high specific capacity.

Description

Technical Field

[0001] This invention belongs to the field of lithium-sulfur battery technology, and relates to a covalent organic framework material, its preparation method and modified lithium-sulfur battery separator, its preparation method and application. Background Technology

[0002] Due to population and economic growth, as well as advancements in lifestyle, the demand for energy has steadily increased over time. With rising energy consumption, concerns about environmental pollution caused by fossil fuel use have become increasingly serious. To alleviate these problems and reduce our dependence on fossil fuels, it is necessary to develop and adopt alternative energy technologies based on renewable energy sources, such as solar and wind power. However, solar and wind power are intermittent; therefore, the efficient and economical storage of electricity generated by renewable energy sources is crucial. Rechargeable batteries are one of the best options for energy storage. Rechargeable battery systems, such as lead-acid, nickel-cadmium, nickel-metal hydride, and lithium-ion batteries, have served humanity for over a century, finding applications in various fields, such as portable electronic devices and automobiles. As portable electronic products become increasingly complex, and the demand for sustainable energy supply and a low-carbon economy drives the rapid development of environmentally friendly and high-energy-density energy storage devices, there is a need to develop advanced rechargeable batteries. However, due to the "embedding" mechanism, the most advanced commercial lithium-ion batteries are approaching their theoretical limits, hindering their further development in energy-related fields. Achieving 500Wh / kg... -1 To achieve the aforementioned high energy density targets, it is necessary to explore new battery systems. Currently, the energy density of LIBS products is mainly between 120 and 220 Wh / kg. -1 Between these limits, through optimization of electrode materials, electrolytes, separators, binders, and current collectors, the actual energy density of LIBS is expected to reach its limit. Therefore, emerging high-energy-density battery systems are highly anticipated worldwide. With further expansion of market demand, high-energy-density, inexpensive, and sustainable batteries are urgently needed in both academia and industry.

[0003] Among numerous electrode materials, lithium metal anodes stand out due to their extremely high theoretical specific capacity (3860 mAh g / g). -1 It has attracted much attention due to its extremely negative reduction potential (-3.04V, compared to the standard hydrogen electrode). Regarding the positive electrode, Herbet and Ulam first proposed elemental sulfur as the electrode active material in 1962. The theoretical specific capacity of elemental sulfur is 1673 mAh g⁻¹. -1 Therefore, Li-S batteries can achieve an astonishing theoretical energy density (2500 Wh / kg). -1 ) and volumetric energy density (2800Wh / L) -1 ), 5 times that of traditional commercial LIBS (387mAh g) -1(LiCoO2 / C battery). Furthermore, due to the abundant reserves of sulfur on Earth, Li-S batteries are significantly cheaper than LIBS batteries. Sulfur is also a green and non-toxic material, meeting future demands for clean energy. If improvements in cycle life and battery performance can be achieved, Li-S batteries could also provide long-term storage for renewable energy sources such as wind and solar power.

[0004] However, some inherent problems with lithium-sulfur batteries limit their practical applications, such as the insulation properties of S / Li2S, the volume expansion of the S electrode, the shuttle effect, and slow electrode reaction kinetics. The basic function of the lithium-sulfur battery separator is to prevent direct contact between the positive and negative electrodes, thus preventing short circuits, while allowing lithium-ion transfer. However, lithium polysulfides generated during the lithium-sulfur battery reaction can cross the separator and react with the metallic lithium anode, producing a shuttle effect and leading to severe capacity decay. These problems can be effectively solved by modifying the basic separator with a functional layer.

[0005] COFs (Covalent Organic Frameworks) materials possess numerous advantages, including low density, ordered structure, large specific surface area, and tunable pore size and structure. As a separator in lithium-sulfur batteries, the uniform and ordered nanoporous structure of COFs can physically block the transmembrane diffusion of lithium polysulfides while simultaneously providing channels for lithium-ion conduction. COFs also possess abundant chemical functional groups, enabling them to chemically interact with polar lithium polysulfides through processes such as adsorption conversion and electrostatic repulsion, effectively suppressing the shuttle effect through chemical mechanisms. Top-down and bottom-up strategies can be used to functionalize COFs, achieving multifunctional synergy in lithium-sulfur battery separators. COFs exhibit good chemical stability, remaining stable in electrolyte environments and within the voltage range during battery charge and discharge. Therefore, COFs as a separator in lithium-sulfur batteries can effectively address the shuttle effect problem. Patent CN114784452A discloses a method for preparing lithium-sulfur battery separator materials using fluorine-containing covalent organic framework materials. A fluorinated COF-F material with a crystalline structure was prepared by a classic solvothermal reaction using 2,4,6-tris(4-aminophenyl)-1,3,5-triazine and 1,3,5-tris(3-fluoro-4-formylphenyl)benzene as precursors. This material was then mixed with a conductive agent and a binder in a specific ratio, coated onto a polymer separator, and dried to obtain a separator material for lithium-sulfur batteries. This material possesses triazine and fluorine functional groups, which can enhance the chemical bonding of polysulfides and significantly improve the cycle performance of lithium-sulfur batteries. However, no relevant literature or patents were found regarding the preparation of covalent organic framework materials for modifying lithium-sulfur battery separators using monomers 2,4,6-trimethylpyrantetrafluoroborate and 1,3,5-tris(4'-aldehyde[1,1'-biphenyl]-4-yl)benzene. Summary of the Invention

[0006] To address the technical problem of the shuttle effect in lithium-sulfur batteries, this invention proposes a covalent organic framework material, its preparation method, and a modified lithium-sulfur battery separator, along with its application. The covalent organic framework material prepared in this invention possesses anionic groups and a rich, regular pore structure. When used to modify lithium-sulfur battery separators, it offers several advantages. First, the negatively charged anionic groups in the covalent organic framework material repel polysulfide anions in the electrolyte, thereby improving the electrochemical performance of the lithium-sulfur battery. Second, its unique regular pore structure prevents the diffusion of polysulfides, thus preventing the loss of sulfur-active materials and suppressing the shuttle effect caused by the reaction of polysulfides with metallic lithium.

[0007] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0008] A method for preparing a covalent organic framework material, comprising the following steps:

[0009] (1) Add monomers 2,4,6-trimethylpyrantetrafluoroborate and 1,3,5-tris(4'-aldehyde[1,1'-biphenyl]-4-yl)benzene to a mixed solvent and cycle through liquid nitrogen freezing and vacuum evacuation at least twice;

[0010] (2) The substance obtained in step (1) is heated to react. After the reaction is completed, it is post-processed to obtain a covalent organic framework material, which is named COF-BF4.

[0011] In step (1), the molar ratio of monomer 2,4,6-trimethylpyrantetrafluoroborate and monomer 1,3,5-tris(4'-aldehyde[1,1'-biphenyl]-4-yl)benzene is 1:(1-2).

[0012] In step (1), the mixed solvent is a mixture of 1,4-dioxane, mesitylene and trifluoroacetic acid, and the volume ratio of 1,4-dioxane, mesitylene and trifluoroacetic acid is (1-5):(1-5):(0.1-5).

[0013] In step (1), the total mass of monomers 2,4,6-trimethylpyrantetrafluoroborate and 1,3,5-tris(4'-aldehyde[1,1'-biphenyl]-4-yl)benzene dissolved in each 100 mL of mixed solvent is 1 to 5 g.

[0014] The number of times the vacuum is pumped in step (1) is 2-5.

[0015] The heating reaction temperature in step (2) is 100-160℃, and the heating reaction time is 50-80h.

[0016] The covalent organic framework material prepared by the above method.

[0017] The preparation method of the lithium-sulfur battery separator modified with the above-mentioned covalent organic framework material is as follows: the covalent organic framework material is mixed with a conductive agent and a binder, a solvent is added, and after being uniformly ball-milled by mechanical means, it is coated on the separator and dried to obtain the lithium-sulfur battery separator modified with the covalent organic framework material.

[0018] The conductive agent is acetylene black, the binder is polyvinylidene fluoride, and the mass ratio of the covalent organic framework material to the conductive agent and the binder is 100:(1-40):(1-30).

[0019] The solvent is N-methylpyrrolidone, and the total mass of the covalent organic framework material, conductive agent, and binder dissolved and dispersed in 2 mL of solvent is 50–150 mg; the drying temperature is 30–80 °C, and the drying time is 6–24 h.

[0020] The lithium-sulfur battery separator prepared by the above method is modified with a covalent organic framework material.

[0021] Application of the covalent organic framework material-modified lithium-sulfur battery separator in the field of lithium-sulfur batteries.

[0022] The covalent organic framework material described in this invention was tested by XRD after synthesis, proving that it generated COF( Figure 2 (Left image); Subsequently, its UV performance was tested in the wavenumber range of 250-800, and a unique characteristic band was found, indicating that COF was successfully synthesized between monomers; this was further confirmed by SEM testing, where its spherical stacked morphology could be seen in the electron microscope image. Figure 2 The right figure demonstrates the successful synthesis of COF. As a significant property of porous materials, the porosity of the prepared COF was investigated by N2 adsorption-desorption analysis at 77 K. COF-BF4 showed a Brunauer-Emmett-Taylor (BET) specific surface area of ​​450 m² / g. 2 / g, and the prepared COFs, according to the classification of the International Union of Pure and Applied Chemistry, have adsorption and desorption curves that conform to typical IV isotherms, exhibiting mesoporous structure characteristics with a pore size of 2.89 nm.

[0023] The present invention has the following beneficial effects:

[0024] 1. This invention uses COF material with anionic groups (boron tetrafluoride group (-BF4)) and abundant regular pore structure on the surface of lithium-sulfur separator to obtain a lithium-sulfur battery separator that inhibits polysulfides.

[0025] 2. The lithium-sulfur membrane modified with covalent organic framework material prepared in this invention can prevent the diffusion of polysulfides and thus prevent the loss of sulfur active substances due to the special regular pore structure of COF material; thereby suppressing the shuttle effect generated by the reaction of polysulfides with metallic lithium.

[0026] 3. The lithium-sulfur separator modified with covalent organic framework material prepared in this invention has improved cycle life and enhanced electrochemical performance of lithium-sulfur batteries because the COF material is rich in boron tetrafluoride groups (-BF4). Furthermore, the negatively charged anionic groups in the covalent organic framework material have a repulsive effect on polysulfide anions in the electrolyte, thereby inhibiting the shuttle effect of polysulfide anions and further improving the electrochemical performance of lithium-sulfur batteries.

[0027] 4. This invention utilizes a COF material with anionic groups (boron tetrafluoride groups (-BF4)) and a rich, regular pore structure for use as a separator in lithium-sulfur batteries. The positive electrode is a sulfur-carbon composite material, the negative electrode is a lithium metal sheet, and the electrolyte is LiTFSI electrolyte. After 200 cycles at a current density of 0.2C, the lithium-sulfur battery assembled in this invention achieves a discharge capacity of 610 mAh / g, exhibiting a high specific capacity. Figure 3 ).

[0028] 5. The impedance of the stainless steel symmetrical battery assembled with the organic composite material prepared in this invention as the separator modification material is 1.23Ω, and the impedance of the Celgard separator is 1.74Ω, indicating that the organic composite material has a lower impedance as a battery separator material.

[0029] ( Figure 4 )

[0030] 6. The thermogravimetric analysis of the organic composite material prepared by this invention shows that the material has good thermal stability. Figure 5 ).

[0031] 7. The impedance of the organic composite material prepared in this invention before and after cycling: Before cycling at 0.2C, the impedance of the lithium-sulfur battery modified with COF-BF4 membrane was 15.84Ω, and the impedance of the lithium-sulfur battery modified with Celgard membrane was 18.58Ω. After 200 cycles at a current density of 0.2C, the impedance of the lithium-sulfur battery modified with COF-BF4 membrane was 19.69Ω, and the impedance of the lithium-sulfur battery modified with Celgard membrane was 26.79Ω. The lithium-sulfur battery modified with COF-BF4 membrane has a lower impedance. Figure 6 ).

[0032] 8. The electrolyte contact angle of the organic composite material COF-BF4 prepared in this invention is 7.9°, while that of the Celgard membrane is 43.4°, indicating that the COF-BF4 membrane has better electrolyte wettability. Figure 7 and Figure 8 ).

[0033] 9. The specific capacity of the organic composite material prepared in this invention at a 1C current density. As shown in the figure, the capacity of the COF-BF4 modified lithium-sulfur battery after the first cycle at 1C is 1043.5 mAh g⁻¹, and after 1000 cycles it is 516 mAh g⁻¹, with a capacity decay rate of 0.055% per week. The capacity of the Celgard modified lithium-sulfur battery after the first cycle at 1C is 862.7 mAh g⁻¹, and after 1000 cycles it is 294.4 mAh g⁻¹, with a capacity decay rate of 0.066% per week. The COF-BF4 modified lithium-sulfur battery exhibits better electrochemical performance. Figure 9 ). Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 This is a structural diagram of Embodiment 1 of the present invention.

[0036] Figure 2 The images show the XRD pattern (left) and SEM image (right) of the COF material obtained in Example 1 of this invention.

[0037] Figure 3 This is a specific capacity diagram of the lithium-sulfur battery prepared in Example 1 of the present invention at a current density of 0.2C.

[0038] Figure 4 Impedance diagram of the COF-BF4 modified stainless steel symmetric cell prepared in Example 1 of this invention.

[0039] Figure 5 This is a thermogravimetric diagram of the COF material obtained in Example 1 of the present invention.

[0040] Figure 6 The image shows the EIS diagrams of the lithium-sulfur battery modified with COF-BF4 separator obtained in Example 1 of this invention before and after 0.2C cycling.

[0041] Figure 7 The contact angle is the Celgard diaphragm obtained in Example 1 of the present invention.

[0042] Figure 8 The contact angle is the COF-BF4 membrane obtained in Example 1 of this invention.

[0043] Figure 9This is a specific capacity diagram of the lithium-sulfur battery prepared in Example 1 of the present invention at 1C current density. Detailed Implementation

[0044] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0045] Unless otherwise specified, all raw materials used in this invention are commercially available products.

[0046] Example 1

[0047] This embodiment describes a method for preparing a lithium-sulfur separator COF-BF4 based on a covalent organic framework material (e.g., ...). Figure 1 (As shown), the steps are as follows:

[0048] (1) First, weigh 10.5 mg of 2,4,6-trimethylpyrantetrafluoroborate as monomer 1, and weigh 30.94 mg of 1,3,5-tris(4'-aldehyde[1,1'-biphenyl]-4-yl)benzene as monomer 2 (the molar ratio of monomer 1 to monomer 2 is 1:1). Place monomer 1 and monomer 2 in a glass bottle. Add 1 mL of 1,4-dioxane, 1 mL of mesitylene and 0.4 mL of trifluoroacetic acid to the glass bottle as a mixed solvent. Finally, cycle the process three times, using liquid nitrogen freezing for 5 min and vacuum evacuation for 5 min.

[0049] (2) The substance obtained in step (1) was placed in a forced-air drying oven and heated at 120°C for 72 hours to terminate the reaction. Then, it was dried in a vacuum oven at 70°C for 12 hours to obtain the COF material. The XRD pattern of this material is attached. Figure 2 As can be seen from the figure, COF-BF4 material has crystallinity;

[0050] (3) Weigh 70 mg of the COF material obtained in step (2) and place it in a ball milling mortar; weigh 20 mg of acetylene black as a conductive agent and place it in a ball milling mortar; weigh 10 mg of polyvinylidene fluoride as a binder and place it in a ball milling mortar; finally, add 1.5 mL of N-methylpyrrolidone solvent to the ball milling mortar and ball mill evenly for 240 min.

[0051] (4) The mixture after ball milling in step (3) is coated onto a commercial separator. The coated separator is placed in a drying oven and dried at 50°C for 12 hours. The obtained COF lithium-sulfur separator COF-BF4 is placed in a lithium-sulfur battery to obtain a COF material modified lithium-sulfur battery.

[0052] The covalent organic framework material described in this invention was tested by XRD after synthesis, proving that it generated COF( Figure 2 (Left image); Subsequently, its UV performance was tested in the wavenumber range of 250-800, and a unique characteristic band was found, indicating that COF was successfully synthesized between monomers; this was further confirmed by SEM testing, where its spherical stacked morphology could be seen in the electron microscope image. Figure 2 The right figure demonstrates the successful synthesis of COF. As a significant property of porous materials, the porosity of the prepared COF was investigated by N2 adsorption-desorption analysis at 77 K. COF-BF4 showed a Brunauer-Emmett-Taylor (BET) specific surface area of ​​450 m² / g. 2 / g, and the prepared COFs, according to the classification of the International Union of Pure and Applied Chemistry, have adsorption and desorption curves that conform to typical IV isotherms, exhibiting mesoporous structure characteristics with a pore size of 2.89 nm.

[0053] Example 2

[0054] This embodiment describes a method for preparing a lithium-sulfur separator COF-BF4 modified with a covalent organic framework material. The steps are as follows:

[0055] (1) First, weigh 10.5 mg of 2,4,6-trimethylpyrantetrafluoroborate as monomer 1, and weigh 46.4 mg of 1,3,5-tris(4'-aldehyde[1,1'-biphenyl]-4-yl)benzene (the molar ratio of monomer 1 to monomer 2 is 1:1.5). Place monomer 1 and monomer 2 in a glass bottle. Add 1 mL of 1,4-dioxane, 1 mL of mesitylene and 0.4 mL of trifluoroacetic acid to the glass bottle as a mixed solvent. Finally, cycle the process three times, using liquid nitrogen freezing for 5 min and vacuum evacuation for 5 min.

[0056] (2) The substance obtained in step (1) was placed in a forced-air drying oven and heated at 100°C for 72 hours to terminate the reaction. Then it was dried in a vacuum oven at 70°C for 12 hours to obtain COF material.

[0057] (3) Weigh 70 mg of the COF material obtained in step (2) and place it in a ball milling mortar; weigh 20 mg of acetylene black as a conductive agent and place it in a ball milling mortar; weigh 10 mg of polyvinylidene fluoride as a binder and place it in a ball milling mortar; finally, add 2 mL of N-methylpyrrolidone solvent to the ball milling mortar and ball mill evenly for 240 min.

[0058] (4) The mixture after ball milling in step (3) is coated onto a commercial separator. The coated separator is placed in a drying oven and dried at 50°C for 12 hours. The obtained COF lithium-sulfur separator COF-BF4 is placed in a lithium-sulfur battery to obtain a COF material modified lithium-sulfur battery.

[0059] Compared with Example 1, in this example, the molar ratio of monomer 1 and monomer 2 in step (1) is 1:1.5, the temperature of the drying oven in step (2) is 100℃, and 2mL of N-methylpyrrolidone is used in step (3).

[0060] Example 3

[0061] This embodiment describes a method for preparing a lithium-sulfur separator COF-BF4 modified with a covalent organic framework material. The steps are as follows:

[0062] (1) First, weigh 10.5 mg of 2,4,6-trimethylpyrantetrafluoroborate as monomer 1, and weigh 30.94 mg of 1,3,5-tris(4'-aldehyde[1,1'-biphenyl]-4-yl)benzene (the molar ratio of monomer 1 to monomer 2 is 1:1). Place monomer 1 and monomer 2 in a glass bottle. Add 2 mL of 1,4-dioxane, 1 mL of mesitylene and 0.4 mL of trifluoroacetic acid to the glass bottle as a mixed solvent. Finally, cycle the process three times, using liquid nitrogen freezing for 5 min and vacuum evacuation for 5 min.

[0063] (2) The substance obtained in step (1) was placed in a forced-air drying oven and heated at 120°C for 72 hours to terminate the reaction. Then it was dried in a vacuum oven at 70°C for 12 hours to obtain COF material.

[0064] (3) Weigh 70 mg of the COF material obtained in step (2) and place it in a ball milling mortar; weigh 30 mg of acetylene black as a conductive agent and place it in a ball milling mortar; weigh 10 mg of polyvinylidene fluoride as a binder and place it in a ball milling mortar; finally, add 2 mL of N-methylpyrrolidone solvent to the ball milling mortar and ball mill evenly for 240 min.

[0065] (4) The mixture after ball milling in step (3) is coated onto a commercial separator. The coated separator is placed in a drying oven and dried at 70°C for 12 hours. The obtained COF lithium-sulfur separator COF-BF4 is placed in a lithium-sulfur battery to obtain a COF material modified lithium-sulfur battery.

[0066] Compared with Example 1, in this example, step (1) 1,4-dioxane is 2 mL, step (3) acetylene black is 30 mg, N-methylpyrrolidone is 2 mL, and the drying temperature of the coated diaphragm in step (4) is 70°C.

[0067] Example 4

[0068] This embodiment describes a method for preparing a lithium-sulfur separator COF-BF4 modified with a covalent organic framework material. The steps are as follows:

[0069] (1) First, weigh 10.5 mg of 2,4,6-trimethylpyrantetrafluoroborate as monomer 1, and weigh 61.8 mg of 1,3,5-tris(4'-aldehyde[1,1'-biphenyl]-4-yl)benzene as monomer 2 (the molar ratio of monomer 1 to monomer 2 is 1:2). Place monomer 1 and monomer 2 in a glass bottle. Add 1 mL of 1,4-dioxane, 1 mL of mesitylene and 0.4 mL of trifluoroacetic acid to the glass bottle as a mixed solvent. Finally, cycle the process three times, using liquid nitrogen freezing for 5 min and vacuum evacuation for 5 min.

[0070] (2) The substance obtained in step (1) was placed in a forced-air drying oven and heated at 150°C for 72 hours to terminate the reaction. Then it was dried in a vacuum oven at 70°C for 12 hours to obtain COF material.

[0071] (3) Weigh 70 mg of the COF material obtained in step (2) and place it in a ball milling mortar; weigh 20 mg of acetylene black as a conductive agent and place it in a ball milling mortar; weigh 10 mg of polyvinylidene fluoride as a binder and place it in a ball milling mortar; finally, add 1.5 mL of N-methylpyrrolidone solvent to the ball milling mortar and ball mill evenly for 240 min.

[0072] (4) The mixture after ball milling in step (3) is coated onto a commercial separator. The coated separator is placed in a drying oven and dried at 80°C for 12 hours. The obtained COF lithium-sulfur separator COF-BF4 is placed in a lithium-sulfur battery to obtain a COF material modified lithium-sulfur battery.

[0073] Compared with Example 1, in this example, the molar ratio of monomer 1 and monomer 2 in step (1) is 1:2, the temperature of the drying oven in step (2) is 150°C, and the drying temperature of the coated diaphragm in step (4) is 80°C.

[0074] Example 5

[0075] This embodiment describes a method for preparing a lithium-sulfur separator COF-BF4 modified with a covalent organic framework material. The steps are as follows:

[0076] (1) First, weigh 10.5 mg of 2,4,6-trimethylpyrantetrafluoroborate as monomer 1, and weigh 30.94 mg of 1,3,5-tris(4'-aldehyde[1,1'-biphenyl]-4-yl)benzene as monomer 2 (the molar ratio of monomer 1 to monomer 2 is 1:1). Place monomer 1 and monomer 2 in a glass bottle. Add 1 mL of 1,4-dioxane, 2 mL of mesitylene and 0.4 mL of trifluoroacetic acid to the glass bottle as a mixed solvent. Finally, cycle the process three times, using liquid nitrogen freezing for 5 min and vacuum evacuation for 5 min.

[0077] (2) The substance obtained in step (1) was placed in a forced-air drying oven and heated at 120°C for 72 hours to terminate the reaction. Then it was dried in a vacuum oven at 50°C for 12 hours to obtain COF material.

[0078] (3) Weigh 70 mg of the COF material obtained in step (2) and place it in a ball milling mortar; weigh 20 mg of acetylene black as a conductive agent and place it in a ball milling mortar; weigh 10 mg of polyvinylidene fluoride as a binder and place it in a ball milling mortar; finally, add 2 mL of N-methylpyrrolidone solvent to the ball milling mortar and ball mill evenly for 240 min.

[0079] (4) The mixture after ball milling in step (3) is coated onto a commercial separator. The coated separator is placed in a drying oven and dried at 80°C for 12 hours. The obtained COF lithium-sulfur separator COF-BF4 is placed in a lithium-sulfur battery to obtain a COF material modified lithium-sulfur battery.

[0080] Compared with Example 1, in this example, the amount of trimethylbenzene in step (1) is 2 mL, the temperature of the vacuum drying oven in step (2) is 50°C, the amount of N-methylpyrrolidone in step (3) is 2 mL, and the drying temperature of the coated diaphragm in step (4) is 80°C.

[0081] Example 6

[0082] This embodiment describes a method for preparing a lithium-sulfur separator COF-BF4 modified with a covalent organic framework material. The steps are as follows:

[0083] (1) First, weigh 10.5 mg of 2,4,6-trimethylpyrantetrafluoroborate as monomer 1, and weigh 46.4 mg of 1,3,5-tris(4'-aldehyde[1,1'-biphenyl]-4-yl)benzene (the molar ratio of monomer 1 to monomer 2 is 1:1.5). Place monomer 1 and monomer 2 in a glass bottle. Add 1 mL of 1,4-dioxane, 1 mL of mesitylene and 1 mL of trifluoroacetic acid to the glass bottle as a mixed solvent. Finally, cycle the process three times, using liquid nitrogen freezing for 5 min and vacuum evacuation for 5 min.

[0084] (2) The substance obtained in step (1) was placed in a forced-air drying oven and heated at 100°C for 72 hours to terminate the reaction. Then it was dried in a vacuum oven at 70°C for 12 hours to obtain COF material.

[0085] (3) Weigh 70 mg of the COF material obtained in step (2) and place it in a ball milling mortar; weigh 10 mg of acetylene black as a conductive agent and place it in a ball milling mortar; weigh 10 mg of polyvinylidene fluoride as a binder and place it in a ball milling mortar; finally, add 1.5 mL of N-methylpyrrolidone solvent to the ball milling mortar and ball mill evenly for 240 min.

[0086] (4) The mixture after ball milling in step (3) is coated onto a commercial separator. The coated separator is placed in a drying oven and dried at 50°C for 12 hours. The obtained COF lithium-sulfur separator COF-BF4 is placed in a lithium-sulfur battery to obtain a COF material modified lithium-sulfur battery.

[0087] Compared with Example 1, in this example, the molar ratio of monomer 1 and monomer 2 in step (1) is 1:1.5, 1 mL of trifluoroacetic acid is added, the temperature of the drying oven in step (2) is 100℃, and the amount of acetylene black in step (3) is 10 mg.

[0088] Example 7

[0089] This embodiment describes a method for preparing a lithium-sulfur separator COF-BF4 modified with a covalent organic framework material. The steps are as follows:

[0090] (1) First, weigh 10.5 mg of 2,4,6-trimethylpyrantetrafluoroborate as monomer 1, and weigh 30.94 mg of 1,3,5-tris(4'-aldehyde[1,1'-biphenyl]-4-yl)benzene as monomer 2 (the molar ratio of monomer 1 to monomer 2 is 1:1). Place monomer 1 and monomer 2 in a glass bottle. Add 3 mL of 1,4-dioxane, 1 mL of mesitylene and 1 mL of trifluoroacetic acid to the glass bottle as a mixed solvent. Finally, cycle the process three times, using liquid nitrogen freezing for 5 min and vacuum evacuation for 5 min.

[0091] (2) The substance obtained in step (1) was placed in a forced-air drying oven and heated at 120°C for 72 hours to terminate the reaction. Then it was dried in a vacuum oven at 50°C for 12 hours to obtain COF material.

[0092] (3) Weigh 70 mg of the COF material obtained in step (2) and place it in a ball milling mortar; weigh 20 mg of acetylene black as a conductive agent and place it in a ball milling mortar; weigh 10 mg of polyvinylidene fluoride as a binder and place it in a ball milling mortar; finally, add 1.5 mL of N-methylpyrrolidone solvent to the ball milling mortar and ball mill evenly for 240 min.

[0093] (4) The mixture after ball milling in step (3) is coated onto a commercial separator. The coated separator is placed in a drying oven and dried at 60°C for 12 hours. The obtained COF lithium-sulfur separator COF-BF4 is placed in a lithium-sulfur battery to obtain a COF material modified lithium-sulfur battery.

[0094] Compared with Example 1, in this example, in step (2), 3 mL of 1,4-dioxane and 1 mL of trifluoroacetic acid are used, the drying temperature of the vacuum drying oven in step (2) is 50°C, and the drying temperature of the coated diaphragm in step (4) is 60°C.

[0095] Example 8

[0096] This embodiment describes a method for preparing a lithium-sulfur separator COF-BF4 modified with a covalent organic framework material. The steps are as follows:

[0097] (1) First, weigh 10.5 mg of 2,4,6-trimethylpyrantetrafluoroborate as monomer 1, and weigh 49.5 mg of 1,3,5-tris(4'-aldehyde[1,1'-biphenyl]-4-yl)benzene as monomer 2 (the molar ratio of monomer 1 to monomer 2 is 1:1.6). Place monomer 1 and monomer 2 in a glass bottle. Add 2.5 mL of 1,4-dioxane, 2.5 mL of mesitylene and 0.5 mL of trifluoroacetic acid to the glass bottle as a mixed solvent. Finally, cycle the process three times, using liquid nitrogen freezing for 5 min and vacuum evacuation for 5 min.

[0098] (2) The substance obtained in step (1) was placed in a forced-air drying oven and heated at 160°C for 50 h to terminate the reaction. Then it was dried in a vacuum oven at 70°C for 12 h to obtain COF material.

[0099] (3) Weigh 70 mg of the COF material obtained in step (2) and place it in a ball milling mortar; weigh 28 mg of acetylene black as a conductive agent and place it in a ball milling mortar; weigh 21 mg of polyvinylidene fluoride as a binder and place it in a ball milling mortar; finally, add 4.76 mL of N-methylpyrrolidone solvent to the ball milling mortar and ball mill evenly for 240 min.

[0100] (4) The mixture after ball milling in step (3) is coated onto a commercial separator. The coated separator is placed in a drying oven and dried at 30°C for 24 hours. The obtained COF lithium-sulfur separator COF-BF4 is placed in a lithium-sulfur battery to obtain a COF material modified lithium-sulfur battery.

[0101] Compared with Example 1, in this example, the molar ratio of monomer 1 and monomer 2 in step (1) is 1:1.6, and the ratio of mixed solvent is changed; the temperature and time of the drying oven in step (2), the ratio of conductive agent, binder and solvent in step (3), and the drying temperature and time in step (4) are changed.

[0102] Example 9

[0103] This embodiment describes a method for preparing a lithium-sulfur separator COF-BF4 modified with a covalent organic framework material. The steps are as follows:

[0104] (1) First, weigh 10.5 mg of 2,4,6-trimethylpyrantetrafluoroborate as monomer 1, and weigh 59.5 mg of 1,3,5-tris(4'-aldehyde[1,1'-biphenyl]-4-yl)benzene as monomer 2 (the molar ratio of monomer 1 to monomer 2 is 1:1.9). Place monomer 1 and monomer 2 in a glass bottle. Add 0.2 mL of 1,4-dioxane, 0.2 mL of mesitylene and 1 mL of trifluoroacetic acid to the glass bottle as a mixed solvent. Finally, cycle the process twice, first freezing in liquid nitrogen for 5 min and then vacuuming for 5 min.

[0105] (2) The substance obtained in step (1) was placed in a forced-air drying oven and heated at 110°C for 80 hours to terminate the reaction. Then it was dried in a vacuum oven at 70°C for 12 hours to obtain COF material.

[0106] (3) Weigh 100 mg of the COF material obtained in step (2) and place it in a ball milling mortar; weigh 1 mg of acetylene black as a conductive agent and place it in a ball milling mortar; weigh 1 mg of polyvinylidene fluoride as a binder and place it in a ball milling mortar; finally, add 1.36 mL of N-methylpyrrolidone solvent to the ball milling mortar and ball mill evenly for 240 min.

[0107] (4) The mixture after ball milling in step (3) is coated onto a commercial separator. The coated separator is placed in a drying oven and dried at 80°C for 6 hours. The obtained COF lithium-sulfur separator COF-BF4 is placed in a lithium-sulfur battery to obtain a COF material modified lithium-sulfur battery.

[0108] Compared with Example 1, in this example, the molar ratio of monomer 1 and monomer 2 in step (1) is 1:1.9, the ratio of mixed solvent is changed, and the number of cycles is changed; the temperature and time of the drying oven in step (2), the ratio of conductive agent, binder and solvent in step (3), and the drying temperature and time in step (4) are changed.

[0109] Comparative Example

[0110] The comparative example in this case uses an unmodified commercial separator, which is then placed in a lithium-sulfur battery.

[0111] The following table shows the specific implementation conditions for Examples 1-7.

[0112] Table 1 Specific Implementation Conditions for Implementation 1-7

[0113]

[0114]

[0115] Application examples

[0116] 1. Assembly of lithium-sulfur batteries

[0117] The lithium metal electrode sheet of the above embodiment was punched into a circular piece with a diameter of 16 mm. A sulfur-carbon composite material was used as the positive electrode, and LiTFSI electrolyte was added. The CR-2032 coin cell was assembled and packaged using a packaging machine. After standing for more than 12 hours, the electrochemical performance was tested.

[0118] 2. Electrochemical performance testing

[0119] Figure 3 The graph shows the rate-long cycle performance of the organic composite material used as a separator material in lithium-sulfur batteries in Example 1 of this invention. As can be seen from the graph, after 200 cycles at a current density of 0.2C, the battery exhibits a discharge capacity of 610 mAh / g, confirming the good performance of this organic composite material as a separator material for lithium-sulfur batteries.

[0120] Figure 4 The figure shows the impedance diagram of the stainless steel symmetrical battery assembled using the organic composite material as the separator modification material in Example 1 of the present invention. It can be seen from the figure that the impedance of the separator modified with COF-BF4 is 1.23Ω and the impedance of the Celgard separator is 1.74Ω. The organic composite material COF-BF4 has a lower impedance as a battery separator material.

[0121] Figure 5 The thermogravimetric diagram of the organic composite material COF-BF4 shows that the material has good thermal stability.

[0122] Figure 6 The figures show the EIS diagrams of the COF-BF4 membrane-modified lithium-sulfur battery obtained in Example 1 of this invention before and after 0.2C cycling. As can be seen from the figures, before 0.2C cycling, the impedance of the COF-BF4 membrane-modified lithium-sulfur battery is 15.84Ω, and the impedance of the Celgard membrane-modified lithium-sulfur battery is 18.58Ω. After 200 cycles at a current density of 0.2C, the impedance of the COF-BF4 membrane-modified lithium-sulfur battery is 19.69Ω, and the impedance of the Celgard membrane-modified lithium-sulfur battery is 26.79Ω. The COF-BF4 membrane-modified lithium-sulfur battery exhibits lower impedance.

[0123] Figure 7 The contact angle of the Celgard diaphragm obtained in Example 1 of this invention is shown. Figure 8 The figure shows the contact angle of the COF-BF4 membrane obtained in Example 1 of this invention. As can be seen from the figure, the electrolyte contact angle of the COF-BF4 membrane is 7.9°, while that of the Celgard membrane is 43.4°. The COF-BF4 membrane exhibits better electrolyte wettability.

[0124] Figure 9This is a specific capacity graph of the lithium-sulfur battery prepared in Example 1 of the present invention at 1C current density. As can be seen from the graph, the capacity of the COF-BF4 modified lithium-sulfur battery after the first cycle at 1C is 1043.5 mAh g. -1 After 1000 cycles, the capacity is 516 mAh g. -1 The capacity decay rate was 0.055% per week. The Celgard-modified lithium-sulfur battery had a capacity of 862.7 mAhg after the first cycle at 1C. -1 After 1000 cycles, the capacity was 294.4 mAh g. -1 The capacity decay rate per week was 0.066%, and the COF-BF4 modified lithium-sulfur battery exhibited better electrochemical performance.

[0125] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a lithium-sulfur battery separator modified with a covalent organic framework material, characterized in that: A covalent organic framework material is mixed with a conductive agent and a binder, a solvent is added, and the mixture is mechanically ball-milled until homogeneous. The mixture is then coated onto a separator and dried to obtain a lithium-sulfur battery separator modified with a covalent organic framework material. The preparation method of the covalent organic framework material includes the following steps: (1) Add monomers 2,4,6-trimethylpyrantetrafluoroborate and 1,3,5-tris(4'-aldehyde[1,1'-biphenyl]-4-yl)benzene to a mixed solvent and cycle through liquid nitrogen freezing and vacuum evacuation at least twice; (2) The substance obtained in step (1) is heated to react. After the reaction is completed, it is post-processed to obtain a covalent organic framework material, which is named COF-BF4. In step (1), the molar ratio of monomer 2,4,6-trimethylpyrantetrafluoroborate and monomer 1,3,5-tris(4'-aldehyde[1,1'-biphenyl]-4-yl)benzene is 1:(1~2); the mixed solvent is a mixture of 1,4-dioxane, mesitylene and trifluoroacetic acid, and the volume ratio of 1,4-dioxane, mesitylene and trifluoroacetic acid is (1~5):(1~5):(0.1~5); The heating reaction temperature in step (2) is 100~160℃, and the heating reaction time is 50~80h.

2. The method for preparing a lithium-sulfur battery separator modified with a covalent organic framework material according to claim 1, characterized in that: In step (1), the total mass of monomers 2,4,6-trimethylpyrantetrafluoroborate and 1,3,5-tris(4'-aldehyde[1,1'-biphenyl]-4-yl)benzene dissolved in each 100 mL of mixed solvent is 1~5 g.

3. The method for preparing a lithium-sulfur battery separator modified with a covalent organic framework material according to claim 1, characterized in that: The conductive agent is acetylene black; the binder is polyvinylidene fluoride; the mass ratio of the covalent organic framework material to the conductive agent and the binder is 100:(1~40):(1~30).

4. The method for preparing a lithium-sulfur battery separator modified with a covalent organic framework material according to claim 3, characterized in that: The solvent is N-methylpyrrolidone; the total mass of the covalent organic framework material, conductive agent and binder dissolved and dispersed in 2 mL of solvent is 50~150 mg; the drying temperature is 30~80℃ and the drying time is 6~24 h.

5. The lithium-sulfur battery separator modified with a covalent organic framework material prepared by the method according to any one of claims 1-4.

6. The application of the lithium-sulfur battery separator modified with the covalent organic framework material as described in claim 5 in the field of lithium-sulfur batteries.

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