Preparation method of atomic-scale distribution bimetallic material

Abstract

The invention relates to a preparation method of an atomic-scale distribution bimetallic material. The method comprises the following steps: firstly, coordinating metal salts in copper, cobalt, zinc and manganese with amino porphyrin monomers to obtain two different amino metalloporphyrin monomers; the preparation method comprises the following steps: firstly, preparing an MBY-N4-NC monomer, reacting the MBY-N4-NC monomer with an aldehyde group monomer through a solvothermal method to synthesize bimetallic distribution COFs (MAX / MBY-COF), and then performing high-temperature pyrolysis on the COFs (MAX / MBY-COF) in a nitrogen atmosphere to obtain the COFs-derived atomic-scale distribution bimetallic material (MAX / MBY-N4-NC). The specific capacity of the lithium-sulfur battery assembled by the diaphragm modified by the bimetallic material obtained by the invention is 94% of the theoretical capacity, and the average capacity attenuation of each circle is only 0.016% after 1000 circles of circulation under the 2C condition.

Description

Technical Field

[0001] The present invention belongs to the field of new energy materials, and particularly relates to a preparation method of an atomically distributed bimetallic material derived from covalent organic frameworks (COFs) and its application in the field of lithium-sulfur batteries. Background Art

[0002] With the rapid development of social economy and the increasing demand for energy, the extensive use of traditional fossil energy has caused serious environmental pollution, severely restricting the sustainable development of society. Developing green and renewable energy has become an urgent problem to be solved. Since Sony commercialized lithium-ion batteries in 1991, lithium-ion batteries have been dominant in the market of portable electronic devices. However, because their energy density has approached the theoretical limit, they cannot meet the needs of modern society for high-energy-density energy storage devices. Therefore, developing a battery system with high energy density, high safety and low cost has become a key problem urgently needed to be solved in the development of new energy.

[0003] Lithium-sulfur batteries are considered to be the most promising next-generation energy storage systems because of their high theoretical specific capacity (1675 mAh g -1 ), high mass energy density (2600 Wh kg -1 ), and high volume energy density (2800 Wh L -1 ). Moreover, the active material sulfur also has the characteristics of rich reserves, low cost and environmental friendliness, making lithium-sulfur batteries the focus of research on energy storage devices. However, due to the multi-step, multi-phase and multi-electron conversion processes involved in the charge and discharge process of lithium-sulfur batteries and being limited by the essential characteristics of sulfur and various sulfides, in practical applications, it faces problems such as poor conductivity of the active material sulfur and solid-state discharge products, the shuttle effect of polysulfides, and sluggish reduction reaction kinetics, which inevitably lead to low sulfur utilization, irreversible capacity decay and low Coulomb efficiency. Therefore, using materials with high catalytic activity to functionalize the surface of the PP separator, enabling it to effectively reduce the reduction reaction barrier of polysulfides and promote the effective deposition of solid lithium sulfide, and restricting the dissolved polysulfides in the positive electrode region, thereby alleviating the shuttle effect of polysulfides to improve the comprehensive performance of lithium-sulfur batteries.

[0004] Materials with atomic-level metal distribution are widely used in the field of lithium-sulfur batteries due to their high utilization rate of catalytic active sites, uniform metal active centers, unique electronic structures, etc. Xiong et al. (P. Wang, B. Xi, Z. Zhang, M. Huang, J. Feng, and S. Xiong, Atomic tungsten on graphene with unique coordination enabling kinetically boosted lithium-sulfur batteries, Angew. Chem. Int. Ed. 2021, 60, 15563.) reported a novel W / NG material with single tungsten atoms immobilized on nitrogen-doped graphene through a self-template and self-reduction strategy. The local coordination environment of W atoms endows W / NG with the ability to enhance the polysulfide adsorption capacity and catalytic activity. Therefore, when coated on the separator surface and applied to lithium-sulfur batteries, high cycle stability and ultra-high rate performance can be obtained even within 1000 cycles. Chen et al. (X. Zhang, T. Yang, Y. Zhang, X. Wang, J. Wang, Y. Li, A. Yu, X. Wang, and Z. Chen, Single zinc atom aggregates: Synergetic interaction to boost fast polysulfide conversion in lithium-sulfur batteries, Adv. Mater. 2023, 35, 2208470.) reported a highly loaded Zn single-atom nanomaterial with uniformly distributed short atomic spacings (<1 nm) on an ordered nitrogen-doped carbon nanotube array. The synergistic effect induced by adjacent single atoms further promotes the rapid reaction of sulfur under high sulfur loading. When used as a cathode modification material in lithium-sulfur batteries, even after 100 cycles under a sulfur loading condition (mass-to-liquid ratio ≈ 3.7 mL g -2 ), the battery can also exhibit excellent cycle stability and a capacity of 5.6 mAh cm -1 ) and 5.6 mAh cm -2High areal capacity. However, since the electrochemical cycle of sulfur reduction not only involves multi-step conversion reactions, but also has to overcome the energy barriers in the liquid-solid phase conversion of soluble long-chain polysulfides and insoluble short-chain polysulfides in the battery electrolyte. Therefore, due to the differences in reaction paths and related phase transitions, single-atom materials, restricted by the uniqueness of their electronic structures and active centers, inevitably cannot accelerate all steps to the same extent during polysulfide conversion reactions, thus failing to completely reduce polysulfides to solid sulfides, leading to the free diffusion of polysulfides and further affecting the improvement of the comprehensive performance of lithium-sulfur batteries. Therefore, designing and synthesizing new multifunctional materials is the key to effectively promoting the complete reduction of polysulfides to solid sulfides, inhibiting the shuttle effect of polysulfides and the formation of lithium dendrites, and improving electrochemical performance.

[0005] Currently, there are no reports on the study of modifying lithium-sulfur battery separators with atomically distributed bimetallic materials. Therefore, developing atomically distributed bimetallic materials with high stability, simple preparation, and low cost for modifying lithium-sulfur battery separators has become the key to expanding the application in the field of lithium-sulfur batteries. Summary of the Invention

[0006] The object of the present invention is to provide a preparation method of a COFs-derived atomically distributed bimetallic material for the problems existing in the sulfur reduction process in lithium-sulfur batteries, and coat it on the surface of the separator for application in lithium-sulfur batteries. This method first coordinates metal salts of copper, cobalt, zinc, and manganese with aminoporphyrin monomers to obtain two different amino-metal porphyrin monomers; then reacts them with aldehyde monomers through a solvothermal method to synthesize bimetal-distributed COFs (M AX / M BY -COF), and then pyrolyzes it at high temperature in a nitrogen atmosphere to obtain a COFs-derived atomically distributed bimetallic material (M AX / M BY -N4-NC). The lithium-sulfur battery assembled with the separator modified by the bimetallic material obtained in the present invention has a specific capacity reaching 95% of the theoretical capacity, and the average capacity decay per cycle is only 0.016% after 1000 cycles under 2C conditions.

[0007] The technical solution of the present invention is as follows:

[0008] A preparation method of an atomically distributed bimetallic material, the method comprising the following steps:

[0009] (1) Synthesis of amino-metal porphyrin monomers

[0010] Synthesis of the first amino metal porphyrin monomer: Under an inert atmosphere, add amino porphyrin and the first metal salt into a container, add the mixed solution and ultrasonically treat for 10 - 30 minutes to obtain a dispersion; heat and react the obtained dispersion at 70 - 90 °C for 12 - 36 hours, cool to room temperature, and filter; wash with deionized water, collect the organic layer, remove water with anhydrous Na2SO4, and rotary evaporate for 1 - 3 hours to obtain the amino metal porphyrin monomer;

[0011] Among them, the molar ratio of amino porphyrin to the first metal salt is 1:3 - 6, and 100 - 300 mg of amino porphyrin is added to every 140 mL of the mixed solution; the composition of the mixed solution is methanol, chloroform, and N,N-dimethylformamide, and the volume ratio is methanol:chloroform:N,N-dimethylformamide = 1:3 - 5:1 - 3;

[0012] Synthesis of the second amino metal porphyrin monomer, other steps are the same as the first one, the difference is that the first metal salt is replaced by the second metal salt;

[0013] The first metal salt described above is copper chloride or zinc chloride; the second metal salt is cobalt chloride or manganese chloride;

[0014] The inert atmosphere described above is nitrogen or argon;

[0015] (2) Synthesis of the bimetal-distributed COF material (M AX / M BY -COF)

[0016] Add the two different amino metal porphyrin monomers, aldehyde monomer, organic solvent, and acetic acid aqueous solution obtained in the previous step into a pressure tube, ultrasonically treat the obtained mixture for 10 - 30 minutes, react at 100 - 200 °C for 2 - 4 days under closed and vacuum conditions, cool to room temperature, filter, wash, and perform Soxhlet extraction, and then obtain the bimetal-distributed COF material after vacuum treatment at 50 - 70 °C for 10 - 14 hours;

[0017] Among them, the molar ratio is (the sum of the two amino metal porphyrin monomers and the amino porphyrin monomer):aldehyde monomer = 1:2 - 5; the molar ratio is the first amino metal porphyrin monomer:the second amino metal porphyrin monomer = 1:4 - 6;

[0018] 1 - 20 mL of organic solvent is added to each mole of amino metal porphyrin; the volume ratio of the acetic acid solution to the organic solvent is 1:3 - 10;

[0019] (3) Synthesis of the COF-derived atomically distributed bimetal material (M AX / M BY -N4-NC)

[0020] Heat the bimetal-distributed COFs from room temperature to 700 - 900 °C and maintain for 1 - 3 hours under an argon atmosphere; after cooling to room temperature, stir the obtained black powder in an aqueous hydrochloric acid solution for 10 - 20 hours; then centrifuge, wash, and dry under vacuum to obtain a COF-derived atomically distributed bimetal material;

[0021] The amino metal porphyrin described in step (2) is amino copper porphyrin, amino cobalt porphyrin, amino zinc porphyrin, or amino manganese porphyrin;

[0022] The aldehyde monomer described in step (2) is 4,4'-biphenyldicarboxaldehyde or [1,1’:4’,1”-terphenyl]-4,4”-dicarboxaldehyde;

[0023] The organic solvent described in step (2) is a mixed solution of 1,2-dichlorobenzene and butanol with a volume ratio of 1:1 - 3, and the concentration of the acetic acid aqueous solution is 5 - 7 M;

[0024] The concentration of the aqueous hydrochloric acid solution described in step (3) is 0.05 - 0.15 M;

[0025] The heating rate described in step (3) is 3 - 8 °C min -1 ; Centrifugation is carried out at a rotation speed of 4000 - 6000 for 2 - 4 minutes; washing is carried out by washing 3 - 5 times with deionized water and ethanol respectively; the temperature of the vacuum drying is 50 - 70 °C and the time is 10 - 14 hours;

[0026] The application of the atomically distributed bimetal material based on COFs is used as a modification material for the polypropylene separator of a lithium-sulfur battery and as a separator for a lithium-sulfur battery.

[0027] Add the atomically distributed bimetal material and the binder polyvinylidene fluoride to the N-methylpyrrolidone solvent for ultrasonic dispersion, then filter and modify on the surface of the polypropylene separator, and dry under vacuum at 30 - 50 °C for 24 - 48 hours to obtain a separator modified with the atomically distributed bimetal material;

[0028] Among them, 15 - 25 mg of the atomically distributed bimetal material is added to every 40 - 60 mL of N-methylpyrrolidone;

[0029] The mass ratio of the atomically distributed bimetal material to the binder polyvinylidene fluoride is 8:2 - 1;

[0030] In the separator, the loading amount of the atomically distributed bimetal material is 0.1 - 0.3 mg cm -2 .

[0031] The lithium-sulfur battery has a positive electrode of a carbon nanotube-sulfur composite material; a negative electrode of a lithium sheet; a diaphragm of a polypropylene diaphragm modified with a bimetallic material distributed at the atomic level; and an electrolyte of a mixed solution of 1,3-dioxolane and dimethyl ether (volume ratio of 1:1) containing 1.0M lithium bis(trifluoromethanesulfonic acid imide);

[0032] The preparation method of the positive electrode is as follows: sulfur powder and carbon nanotubes are mixed in a mass ratio of 3 to 4:1, 10 to 20 mL of carbon disulfide is added, and the mixture is stirred for 15 to 45 minutes. After the carbon disulfide is completely volatilized, the mixture is kept at 155 to 160° C. for 8 to 12 hours to obtain a carbon nanotube-sulfur composite material; the obtained carbon nanotube-sulfur composite material, conductive carbon black and binder polyvinylidene fluoride (mass ratio 8:1:1) are ground for 30 to 60 minutes to fully mix them, 3 to 5 mL of N-methylpyrrolidone is dropped into the mixture, and the mixture is stirred for 40 to 80 minutes. The obtained slurry is coated on an aluminum foil, dried at 50 to 70° C. for 10 to 14 hours, and cut into a 14 mm diameter sheet with a sulfur loading of about 1 to 1.5 mg cm -2 The positive electrode of the lithium-sulfur battery.

[0033] The beneficial effects of the present invention are:

[0034] (1) The present invention provides a method for preparing a COFs-derived atomic-level distributed bimetallic material. The bimetallic sites provide an ideal platform for understanding the catalytic long-chain reaction and short-chain reaction transformation.

[0035] (2) The COFs-derived atomic-level distributed bimetallic material prepared by the present invention as a polypropylene diaphragm modification material for lithium-sulfur batteries showed a high performance of 1580 mAh g at 0.05C. -1 High specific capacity, even at 5C, 800mAh g -1 The specific capacity of 2.5 N·m2 / cm3 indicates that it can effectively catalyze the conversion of polysulfides and thus improve the sulfur utilization rate.

[0036] (3) The atomically distributed bimetallic material obtained by the present invention not only has fast electron transfer and good ion transfer, but also the bimetallic sites can produce a synergistic effect to achieve tandem catalysis, and can gradually catalyze the conversion of long-chain reactions and short-chain reactions at the same time, thereby regulating the conversion and deposition of polysulfides. Using it as a lithium-sulfur battery diaphragm modification material can effectively inhibit the shuttle effect of polysulfides, improve the cycle performance of lithium-sulfur batteries, and solve the problem that single atomic materials cannot completely reduce polysulfides to solid sulfides due to limited active centers, resulting in the free diffusion of polysulfides; the prepared COFs-derived atomically distributed bimetallic material as a polypropylene diaphragm modification material for lithium-sulfur batteries has an average capacity decay of only 0.016% per cycle after 1000 cycles under 2C conditions, indicating that it has good cycle stability and accelerates the process of commercial application of lithium-sulfur batteries. Brief Description of the Drawings

[0037] Figure 1 Schematic diagram for the preparation of Cu4 / Co 16 -N4-NC material in Example 1

[0038] Figure 2 Schematic diagram for the preparation of Cu4 / Co 16 -N4-NC modified polypropylene separator and rate performance diagram of the polypropylene separator in Comparative Example 1

[0039] Figure 3 Schematic diagram for the preparation of Cu4 / Co 16 -N4-NC modified polypropylene separator and cycle test diagram of the polypropylene separator in Comparative Example 1 Specific Embodiment

[0040] For a further understanding of the method of the present invention, specific descriptions will be made in the form of examples in conjunction with the drawings. The following examples are only for the specific preparation method of the present invention and do not limit the scope of the present invention.

[0041] Example 1

[0042] (1) Synthesis of Amino-Metal Porphyrin Monomer

[0043] Synthesis of Substance 1, i.e., synthesis of 5,10,15,20-tetra(4-aminophenyl)porphyrin copper (Cu-TAPP)

[0044] Under a nitrogen atmosphere, H2-TAPP (200 mg, 0.3 mmol) and CuCl2 (161 mg, 1.2 mmol) were added to a 250 mL three-necked round-bottom flask, and 20 mL of methanol, 90 mL of chloroform, and 30 mL of N,N-dimethylformamide were added. The mixture was ultrasonically treated for 10 minutes to make it disperse evenly. The obtained dispersion was heated at 80 °C for 24 hours to allow the reaction between the amino porphyrin and the metal salt. After the solution was cooled to room temperature, it was filtered; after washing with deionized water, the organic layer was collected, dehydrated with anhydrous Na2SO4, and the mixed solution of methanol, chloroform, and N,N-dimethylformamide was removed by rotary evaporation for 2 hours to obtain Cu-TAPP;

[0045] Synthesis of Substance 2, i.e., synthesis of 5,10,15,20-tetra(4-aminophenyl)porphyrin cobalt (Co-TAPP)

[0046] Other steps were the same as those for the synthesis of Substance 1, except that CuCl2 (161 mg, 1.2 mmol) was replaced by CoCl2·6H2O (285 mg, 1.2 mmol); Co-TAPP was obtained;

[0047] (2) Cu4 / Co 16Synthesis of -COF

[0048] Add H2-TAPP (134.6 mg, 0.20 mmol), Cu-TAPP (7.3 mg, 0.01 mmol), Co-TAPP (29.2 mg, 0.04 mmol) and 4,4'-biphenyldicarboxaldehyde (158 mg, 0.75 mmol) into a thick-walled pressure tube. Then add 1,2-dichlorobenzene (10 mL) and butanol (10 mL) into the tube. After ultrasonicating the resulting mixture for 10 minutes, the solute is fully dissolved. Then add aqueous acetic acid solution (5 mL, 6 M) into the tube and ultrasonicate for 10 minutes. After degassing by freeze-pumping-thawing cycle 3 times to remove air, it is sealed and reacted at 120 °C for 3 days under the condition of -0.1 MPa. After cooling to room temperature, it is filtered and washed 3 times with tetrahydrofuran and acetone respectively. The obtained product is placed in a Soxhlet extractor and Soxhlet extracted with 1,4-dioxane, tetrahydrofuran and acetone for 36 hours. Finally, it is vacuum dried at 60 °C for 12 hours to obtain Cu4 / Co 16 -COF;

[0049] (3) Cu4 / Co 16 Synthesis of Cu4 / Co-N4-NC

[0050] At a heating rate of 5 °C min -1 Heat the Cu4 / Co-COF obtained in the previous step from room temperature to 800 °C at a heating rate of 5 °C min and keep it for 2 hours under an argon atmosphere. After cooling to room temperature, stir the black powder in 50 mL of 0.1 M hydrochloric acid for 15 hours. Centrifuge at 5000 rpm for 3 minutes to collect the product, then wash it 3 times with deionized water and ethanol respectively, and vacuum dry it at 60 °C for 12 hours to obtain Cu4 / Co 16 -N4-NC; 16 -N4-NC;

[0051] Figure 1 In this invention, the schematic diagram of the preparation of the dual-metal material with atomic-level distribution of Cu4 / Co 16 -N4-COF is shown in Figure 1.

[0052] Example 2

[0053] (1) Synthesis of amino metal porphyrin monomer

[0054] Synthesis of Substance 1, i.e., synthesis of zinc 5,10,15,20-tetra(4-aminophenyl)porphyrin (Zn-TAPP)

[0055] Under a nitrogen atmosphere, H2-TAPP (200 mg, 0.3 mmol) and ZnCl2 (163 mg, 1.2 mmol) were added to a 250 mL three-necked round-bottom flask. 20 mL of methanol, 90 mL of chloroform, and 30 mL of N,N-dimethylformamide were added, and the mixture was ultrasonically treated for 10 minutes to disperse evenly. The resulting dispersion was heated at 80 °C for 24 hours to allow the reaction of the aminoporphyrin and the metal salt. After the solution was cooled to room temperature, it was filtered; washed with deionized water, the organic layer was collected, dried over anhydrous Na2SO4, and the mixed solution of methanol, chloroform, and N,N-dimethylformamide was removed by rotary evaporation for 2 hours to obtain Zn-TAPP;

[0056] Synthesis of Substance 2, namely 5,10,15,20-tetra(4-aminophenyl)porphyrin manganese (Mn-TAPP)

[0057] Other steps were the same as those for the synthesis of Substance 1, except that ZnCl2 (163 mg, 1.2 mmol) was replaced by MnCl2 (151 mg, 1.2 mmol); Mn-TAPP was obtained;

[0058] (2) Synthesis of Zn4 / Mn 16 -COF

[0059] H2-TAPP (134.6 mg, 0.20 mmol), Zn-TAPP (7.3 mg, 0.01 mmol), and Mn-TAPP (29.2 mg, 0.04 mmol) and [1,1’:4’,1”-terphenyl]-4,4”-dicarboxaldehyde (214 mg, 0.75 mmol) were added to a thick-walled pressure tube. Then 1,2-dichlorobenzene (10 mL) and butanol (10 mL) were added to the tube. The resulting mixture was ultrasonically treated for 10 minutes to fully dissolve the solute. Then an acetic acid aqueous solution (5 mL, 6 M) was added to the tube and ultrasonically treated for 10 minutes. After degassing by freeze-pumping-thawing cycle 3 times to remove air, it was sealed and reacted at 120 °C under -0.1 MPa for 3 days. After cooling to room temperature, it was filtered and washed 3 times with tetrahydrofuran and acetone respectively. The obtained product was placed in a Soxhlet extractor and Soxhlet extracted with 1,4-dioxane, tetrahydrofuran, and acetone for 36 hours. Finally, it was vacuum dried at 60 °C for 12 hours to obtain Zn4 / Mn 16 -COF;

[0060] (3) Synthesis of Zn4 / Mn 16 -N4-NC

[0061] At a heating rate of 5 °C / min -1 of the Zn4 / Mn obtained in the previous step 16-The COF was heated from room temperature to 800 °C and maintained for 2 hours under an argon atmosphere. After cooling to room temperature, the black powder was stirred in 50 mL of 0.1 M hydrochloric acid for 15 hours. The product was collected by centrifugation at 5000 rpm for 3 minutes, then washed 3 times with deionized water and ethanol respectively, and vacuum dried at 60 °C for 12 hours to obtain Zn4 / Mn 16 -N4-NC;

[0062] Example 3

[0063] The Cu4 / Co prepared in Example 1 16 -N4-NC was used as a separator modification layer and applied to lithium-sulfur batteries.

[0064] (1) Preparation of Cu4 / Co 16 -N4-NC modified separator:

[0065] 25 mg of Cu4 / Co 16 -N4-NC and 4 mg of binder polyvinylidene fluoride were added to 50 mL of N-methylpyrrolidone solvent for ultrasonic dispersion. After uniform dispersion, 5 mL of the mixed solution was taken and filtered to modify the surface of the polypropylene separator. Then the obtained modified separator was dried under vacuum at 40 °C for 36 hours, and cut into a diameter of 18 mm with a loading of 0.2 mg cm -2 of Cu4 / Co 16 -N4-NC modified separator.

[0066] (2) Preparation of carbon nanotube-sulfur composite:

[0067] 150 mg of sulfur powder and 50 mg of carbon nanotubes were ground and mixed, 15 mL of carbon disulfide was added, and stirred for 30 minutes. After the carbon disulfide completely volatilized, it was kept at 155 °C for 12 hours to obtain a carbon nanotube-sulfur composite; 160 mg of the carbon nanotube-sulfur composite, 20 mg of conductive carbon black and 20 mg of binder polyvinylidene fluoride were ground for 45 minutes to make them fully mixed, 3 mL of N-methylpyrrolidone was dropped in, and stirred for 60 minutes. The obtained slurry was coated on aluminum foil and dried at 60 °C for 12 hours, and cut into a diameter of 14 mm with a sulfur loading of about 1.2 mg cm -2 of the positive electrode sheet of the lithium-sulfur battery.

[0068] (3) Assembly of the battery:

[0069] For the assembled lithium-sulfur battery, the positive electrode is a carbon nanotube-sulfur composite; the negative electrode is a lithium sheet; the separator is Cu4 / Co 16-N4-NC modified polypropylene separator; the electrolyte is a mixed solution of 1,3-dioxolane and dimethyl ether (volume ratio 1:1) containing 1.0 M lithium bis(trifluoromethanesulfonyl)imide, and then an assembled button lithium-sulfur battery is obtained in a glove box filled with argon. Then, the battery rate performance graph and cycle test graph are obtained by testing in a Neware battery test cabinet BST-5V 20 mA as shown in Figure 2 And Figure 3 , at a current density of 0.05 C, a very high initial discharge specific capacity can be obtained, and excellent cycle stability can be exhibited at 2 C.

[0070] Example 4

[0071] Other steps are the same as in Example 3, except that an unmodified polypropylene separator is directly used to assemble the battery.

[0072] Figure 2 In, Cu4 / Co in Example 1 of the present invention 16 The lithium-sulfur battery assembled with the -N4-NC modified polypropylene separator and the battery assembled with the polypropylene separator are both tested using a Neware battery test cabinet BST-5V 20 mA. The lithium-sulfur battery assembled with the Cu4 / Co 16 -N4-NC modified separator can obtain an initial discharge specific capacity of 1580 mAh g at a current density of 0.05 C, and can still obtain a high specific capacity of 800 mAh g even at 5 C. -1 In contrast, the unmodified polypropylene separator in Comparative Example 1 can only reach an initial discharge specific capacity of 1114 mAh g at 0.05 C, and only exhibits a specific capacity of 461 mAh g at 5 C. -1 -1 -1 This shows that the separator modification material of the present invention effectively promotes the catalytic conversion of polysulfides, thereby improving the performance of the lithium-sulfur battery. -1

[0073] Figure 3 Figure 3 In, Cu4 / Co in Example 1 of the present invention 16 -N4-NC as the separator modification material of the lithium-sulfur battery has an average capacity decay of only 0.016% per cycle after 1000 cycles at 2 C, while the lithium-sulfur battery with the unmodified polypropylene separator in Comparative Example 1 has an average capacity decay of up to 0.055% per cycle. This shows that the separator modification material of the present invention has an obvious effect on improving the cycle stability of the lithium-sulfur battery, which provides effective guidance for the next commercial application of the lithium-sulfur battery.

[0074] Matters not covered in the present invention are well-known technologies.

Claims

1. A method for preparing an atomically distributed bimetallic material, characterized in that, The method includes the following steps: (1) Synthesis of amino metal porphyrin monomers Synthesis of the first amino metal porphyrin monomer: Under an inert atmosphere, add amino porphyrin and the first metal salt into a container, add a mixed solution and perform ultrasonic treatment for 10 - 30 minutes to obtain a dispersion; heat and react the obtained dispersion at 70 - 90 °C for 12 - 36 hours, cool to room temperature, and filter; wash with deionized water, collect the organic layer, remove water with anhydrous Na2SO4, and perform rotary evaporation for 1 - 3 hours to obtain the amino metal porphyrin monomer; Among them, the molar ratio of amino porphyrin to the first metal salt is 1:3 - 6, and 100 - 300 mg of amino porphyrin is added to every 140 mL of the mixed solution; the composition of the mixed solution is methanol, chloroform, and N,N - dimethylformamide, and the volume ratio is methanol:chloroform:N,N - dimethylformamide = 1:3 - 5:1 - 3; Synthesis of the second amino metal porphyrin monomer, other steps are the same as the first one, the difference is that the first metal salt is replaced by the second metal salt; The first metal salt described is copper chloride or zinc chloride; the second metal salt is cobalt chloride or manganese chloride; (2) Synthesis of bimetal - distributed COFs materials Add the two different amino metal porphyrin monomers, aldehyde - group monomer, organic solvent, and acetic acid aqueous solution obtained in the previous step into a pressure tube, perform ultrasonic treatment on the obtained mixture for 10 - 30 minutes, react at 100 - 200 °C for 2 - 4 days under closed and vacuum conditions, cool to room temperature, filter, wash, perform Soxhlet extraction, and then obtain the bimetal - distributed COF material after vacuum treatment at 50 - 70 °C for 10 - 14 hours; Among them, the molar ratio is: the sum of the two amino metal porphyrin monomers and the amino porphyrin monomer:aldehyde - group monomer = 1:2 - 5; The molar ratio is: the first amino metal porphyrin monomer:the second amino metal porphyrin monomer = 1:4 - 6; The aldehyde - group monomer described is 4,4'-biphenyl dialdehyde or [1,1’:4’,1”-terphenyl]-4,4”-dialdehyde; 1 - 20 mL of organic solvent is added per mole of amino metal porphyrin; the volume ratio of the acetic acid solution to the organic solvent is 1:3 - 10; (3) Synthesis of COFs - derived atomically - distributed bimetal materials Heat the bimetal - distributed COFs from room temperature to 700 - 900 °C and maintain it for 1 - N3 hours under an argon atmosphere; after cooling to room temperature, stir the obtained black powder in an aqueous hydrochloric acid solution for 10 - 20 hours; then centrifuge, wash, and perform vacuum drying to obtain the COFs - derived atomically - distributed bimetal material.

2. The method for preparing the atomically distributed bimetallic material according to claim 1, characterized in that, The inert atmosphere described in step (1) is nitrogen or argon.

3. The method for preparing the atomically distributed bimetallic material according to claim 1, characterized in that, The organic solvent described in step (2) is a mixed solution of 1,2 - dichlorobenzene and butanol, and its volume ratio is 1:1 - 3, and the concentration of the acetic acid aqueous solution is 5 - 7 M.

4. The method for preparing the atomically distributed bimetallic material according to claim 1, characterized in that, The concentration of the aqueous hydrochloric acid solution described in step (3) is 0.05 - 0.15 M; The heating rate in step (3) is 3 to 8 °C / min -1 ; centrifugation is carried out at a rotation speed of 4000 to 6000 for 2 to 4 minutes; washing is carried out by washing 3 to 5 times successively with deionized water and ethanol; the temperature of the vacuum drying is 50 to 70 °C and the time is 10 to 14 hours.

5. Application of the atomically - distributed bimetal material prepared by the method according to claim 1, characterized in that it is used as a modification material for the polypropylene separator of a lithium - sulfur battery and serves as the separator of the lithium - sulfur battery.

6. The application according to claim 5, characterized in that, The lithium-sulfur battery described above has a positive electrode made of a carbon nanotube-sulfur composite material, a negative electrode made of a lithium sheet, and a separator made of a polypropylene separator modified with a bimetallic material with atomic-level distribution. The electrolyte is a mixed solution of 1,3-dioxolane and dimethyl ether (volume ratio 1:1) containing 1.0 M lithium bis(trifluoromethanesulfonyl)imide. The method for preparing the polypropylene separator modified with the bimetallic material with atomic-level distribution includes the following steps: The bimetallic material with atomic-level distribution and the binder polyvinylidene fluoride are added to an N-methylpyrrolidone solvent for ultrasonic dispersion, and then suction filtration is carried out to modify the surface of the polypropylene separator, followed by drying under vacuum at 30 - 50 °C for 24 - 48 hours to obtain the separator modified with the bimetallic material with atomic-level distribution. Among them, 15 - 25 mg of the bimetallic material with atomic-level distribution is added to every 40 - 60 mL of N-methylpyrrolidone. The mass ratio of the bimetallic material with atomic-level distribution to the binder polyvinylidene fluoride is 8:2 - 1. In the separator, the loading amount of the atomically distributed bimetallic material is 0.1 to 0.3 mg cm -2 .

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