A high-s / n atomic content manganese monatomic catalyst, a preparation method and application thereof in lithium-sulfur batteries

Abstract

A high S / N atomic content manganese monatomic catalyst, a preparation method and its application in lithium-sulfur batteries, first, a sheet-shaped g-C3N4 template is prepared by pyrolysis, and a layer of polypyrrole is coated on the surface of the template by polymerization of pyrrole monomers to form a g-C3N4@PPy composite material. Because the surface of polypyrrole has a large number of nitrogen atoms, it can adsorb a large amount of Mn 2+ , forming a pyrolysis precursor g-C3N4@Mn-PPy. After mixing the precursor with sulfur powder and pyrolysis, acid washing and drying, the catalyst is obtained. Because the gas produced by the decomposition of the g-C3N4 template has an etching effect, it can control the pore structure of the catalyst. At the same time, during the pyrolysis process, the decomposition of the template and the evaporation of the sulfur powder can make the pyrolysis atmosphere rich in S / N atoms, fully contact with the precursor, increase the doping amount, and the S and N atomic content of the catalyst is high. Due to a large number of non-metallic atoms, the catalysis and adsorption of polysulfide of metal active sites are improved; at the same time, the conductivity of the PPy-derived nitrogen-doped carbon carrier is significantly improved. When the catalyst is used as a separator modification material in lithium-sulfur batteries, it exhibits excellent specific capacity and outstanding cycle stability.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of electrochemistry, and relates to a manganese single-atom catalyst with high S / N atomic content and a preparation method and application thereof, in particular to a preparation method of a manganese single-atom catalyst with high S / N atomic content and application of the manganese single-atom catalyst as a modification material to a lithium-sulfur battery to improve the catalysis and adsorption effect of the catalyst on polysulfides. BACKGROUND

[0002] As a new type of secondary battery, the lithium-sulfur battery has a theoretical specific capacity and energy density of 1675 mAh g -1 and 2600 Wh kg -1 , respectively; meanwhile, sulfur material, as the main material for the oxidation and reduction of the positive electrode of the battery, has the advantages of rich reserves, low cost and environmental friendliness. Therefore, the lithium-sulfur battery is a very promising secondary energy storage system. However, many problems greatly limit the commercialization process of the lithium-sulfur battery: (1) the poor conductivity of sulfur and its discharge products leads to low sulfur utilization; (2) about 80% of the volume change in the battery during the charging and discharging process; (3) the shuttle effect of polysulfides leads to rapid capacity fading and low coulombic efficiency. A series of problems lead to poor performance and safety of the battery, which need to be solved immediately.

[0003] A large number of solutions have been proposed for the above problems, and research shows that loading a catalyst with high conductivity and catalytic activity on the separator of the lithium-sulfur battery can effectively inhibit the shuttle effect of polysulfides and improve the performance of the battery. The catalyst loaded on one side of the separator can block the transmission process of polysulfides, forming a kind of “positive electrode protection area” to effectively prevent polysulfides from entering the negative electrode area. Meanwhile, due to the high conductivity of the catalyst, the utilization rate of Li2S / Li2S2, the discharge product on the surface of the catalyst, can be improved, and the performance of the battery can be significantly improved. However, the research on single-atom catalysts is still difficult to achieve ideal electrochemical performance.

[0004] When a metal single-atom catalyst is applied to a lithium-sulfur battery as a new type of catalyst, the excellent catalytic and anchoring effect of the metal active site can accelerate the conversion of polysulfides and limit their diffusion; meanwhile, the excellent conductivity of the carbon-based material can effectively improve the utilization rate of sulfur and its discharge products. However, the activity of the metal active site and the conductivity of the carrier still cannot meet the current application of the lithium-sulfur battery. The method of doping with non-metallic atoms can improve the properties of the active site and the carrier, and can improve the rate performance and cycle stability of the battery. Therefore, increasing the content of heteroatoms in the catalyst can effectively promote the commercialization process of the lithium-sulfur battery. At present, there is a lack of research on increasing the content of non-metallic atoms in single-atom catalysts, and it is necessary to explore a preparation process that can effectively increase the content of non-metallic atoms for the lithium-sulfur battery.

[0005] In order to increase the content of metal atoms in the catalyst, the application uses a gas phase doping method to prepare a manganese single-atom catalyst rich in S / N atoms. The process has the following advantages: (1) g-C3N4 capable of producing etching gas is used as a template, and the product is a porous sheet structure, which is beneficial to ion / electron transmission; (2) the template and sulfur source can produce gas rich in S / N atoms, which can fully contact and dope the precursor, thereby increasing the content of heterogeneous S / N atoms in the catalyst; (3) a large number of non-metal atoms improve the conductivity and catalytic / adsorption capacity of the catalyst, thereby improving the performance of the battery; (4) the preparation process is convenient, fast and low in cost, and can realize mass production. SUMMARY

[0006] In view of the deficiencies of the synthesis method in the prior art, the application proposes a strategy of gas phase doping to prepare a manganese single-atom catalyst rich in S / N atoms and its application in lithium-sulfur batteries. For the preparation of the catalyst, a sheet-shaped g-C3N4 template is first prepared by pyrolysis, and a layer of polypyrrole is coated on the surface of the template by polymerization of pyrrole monomers to form a g-C3N4@PPy composite material. Because the surface of the polypyrrole has a large number of nitrogen atoms, it can adsorb a large number of Mn 2+ , i.e. a pyrolysis precursor g-C3N4@Mn-PPy. The precursor is fully mixed with a certain amount of sulfur powder and then pyrolyzed, and the catalyst is obtained after acid washing and drying. Because the g-C3N4 template decomposes to produce gas with etching effect, it can control the pore structure of the catalyst. At the same time, during the pyrolysis process, the decomposition of the template and the evaporation of the sulfur powder can make the pyrolysis atmosphere rich in S / N atoms, fully contact with the precursor, increase the doping amount, and the S and N atom contents of the prepared catalyst are as high as 6.38at% and 12.20at% respectively. Due to a large number of non-metal atoms, the catalytic and adsorption polysulfide capacity of the metal active site is improved; at the same time, the conductivity of the PPy-derived nitrogen-doped carbon carrier is significantly improved. When the catalyst is used as a separator modification material for lithium-sulfur batteries, it exhibits excellent specific capacity and outstanding cycle stability.

[0007] Based on the above description, the technical scheme of the application is:

[0008] A manganese single-atom catalyst with high S / N atom content, which is prepared by pyrolyzing a g-C3N4@PPy precursor after adsorbing manganese ions and mixing with sulfur powder. Because the PPy has a high intrinsic nitrogen atom content and the nitrogen atom doping effect of the gas produced by the decomposition of the template can increase the nitrogen atom content of the catalyst. The sulfur atom-containing gas produced by the evaporation of the sulfur powder can fully contact with the precursor, thereby increasing the sulfur atom content. The S and N atom contents of the manganese single-atom catalyst can reach 5.50at%-6.30at% and 6.32at%-12.00at%.

[0009] A method for preparing a manganese single-atom catalyst with high S / N atom content includes the following steps:

[0010] Step 1: Synthesize g-C3N4 template

[0011] 1.1) Dicyandiamide was placed in a closed ceramic boat and kept at 550-650℃ for 4-6 hours in an air atmosphere with a heating rate of 2-5℃ / min. After naturally cooling to room temperature, the product was thoroughly ground to obtain block g-C3N4 powder.

[0012] 1.2) Place the block g-C3N4 in an open ceramic boat and keep it at 500-600℃ for 2-4 hours in an air atmosphere with a heating rate of 3-5℃ / min. After cooling to room temperature, the flake g-C3N4 powder is obtained.

[0013] Step 2: Synthesis of g-C3N4@PPy composite material

[0014] 2.1) At room temperature, place the flake g-C3N4 in deionized water and sonicate for 15-30 minutes to disperse it evenly. Under ice bath conditions, add pyrrole monomer dropwise to the above solution to obtain a uniformly dispersed mixed solution.

[0015] 2.2) At room temperature, ammonium persulfate (NH4)2S2O8 is slowly added to the mixed solution obtained in step 2.1) and stirring is continued for 6-10 hours. During this process, pyrrole monomers can react on the surface of the flake-like g-C3N4 under the action of ammonium persulfate initiator to form polypyrrole.

[0016] 2.3) The product was washed several times with deionized water and then freeze-dried for 12 hours to obtain g-C3N4@PPy composite material.

[0017] In step 2.1), 0.2-0.3 μL of pyrrole monomer is added to each 1 mL of solution.

[0018] In step 2.2), 1.5-2.0g of ammonium persulfate is added to every 1mL of the mixed solution.

[0019] Step 3: Synthesize a manganese single-atom catalyst with high S / N atom content

[0020] 3.1) At room temperature, the g-C3N4@PPy composite material was ultrasonically dispersed in deionized water to a concentration of 2-3 mg / mL. Manganese acetate [Mn(CH3COO)2] was then added to the solution and stirred for 6-10 hours. After washing several times with deionized water using a vacuum filtration method, the mixture was dried to obtain g-C3N4@Mn-PPy. During this process, the positively charged Mn... 2+ Ions are adsorbed onto the surface of the composite material due to electrostatic adsorption.

[0021] 3.2) After thoroughly mixing the product obtained in step 3.1) with sulfur powder, perform stepwise pyrolysis. Specifically: In an Ar atmosphere, heat to 500-600℃ at a heating rate of 2-3℃ / min and hold for 4-6 hours. During this process, sulfur powder evaporates to form a sulfur-rich atmosphere. Then heat to 900-950℃ at a heating rate of 4-6℃ / min and hold for 2-3 hours, followed by natural cooling. During this process, the g-C3N4 template decomposes to produce a nitrogen-rich etching gas.

[0022] 3.3) The product obtained in step 3.2) was acid-washed with 0.5-1 mol / L H2SO4 at 60-90℃ for 5-8 h, then washed with deionized water until neutral, and vacuum dried to obtain a manganese single-atom catalyst with high S / N atom content.

[0023] In step 3.1), the amount of manganese acetate added is 0.5-1 times the mass of the g-C3N4@PPy composite material.

[0024] In step 3.2), the amount of sulfur powder added is 1-1.5 times the mass of the g-C3N4@PPy composite material.

[0025] The application of a high S / N atom content manganese single-atom catalyst in lithium-sulfur batteries involves modifying commercial battery PP separators with the synthesized catalyst. The specific operational steps are as follows:

[0026] Step 1: Preparation of modified diaphragm

[0027] The prepared catalyst, carbon nanotubes and binder (PVDF) were mixed and ground thoroughly at a mass ratio of 8:1:1. The resulting mixture was then added to an appropriate amount of isopropanol and sonicated to ensure uniform dispersion. The catalyst was then loaded onto a PP membrane by vacuum filtration and dried at 60°C for 12 hours to obtain the modified membrane.

[0028] Step 2: Preparation of sulfur / carbon cathode

[0029] Sublimed sulfur and BP-2000 were ground thoroughly at a mass ratio of 75:25 and then kept at 155°C for 12 hours under an Ar atmosphere. The resulting powder was then mixed with Super P and PVDF at a mass ratio of 7:2:1 and ground thoroughly. NMP was then added and stirred for 12 hours. The resulting uniformly mixed slurry was coated onto aluminum foil and dried at 60°C for 12 hours.

[0030] Step 3: Assemble the lithium-sulfur battery

[0031] The prepared composite separator, sulfur / carbon cathode, and lithium sheet were assembled into a lithium-sulfur battery. The electrolyte was added at a rate of 25 μL to the cathode side and 15 μL to the anode side, with a sulfur loading of ~1 mg / cm³.2 .

[0032] The beneficial effects of this invention are as follows:

[0033] 1) The preparation process of this catalyst is convenient and quick, the raw material cost is low and environmentally friendly. On this basis, this process can effectively increase the non-metallic S / N atom content in single-atom catalysts, thus having a good benefit effect.

[0034] 2) To increase the content of non-metallic atoms in the catalyst, a gas-phase doping strategy is adopted. By selecting a decomposable template and evaporated sulfur powder as S and N sources, the doping step is completed during the pyrolysis process. This not only increases the content and uniformity of non-metallic atoms, but also avoids complex steps such as template removal, making it easy to carry out mass production.

[0035] 3) The abundant S / N atoms in this catalyst can, on the one hand, enhance the catalytic and polysulfide adsorption capabilities of the Mn-N active sites, improve the reaction kinetics of polysulfides, and suppress the shuttle effect; on the other hand, it can also improve the conductivity of the support, thereby increasing the utilization rate of sulfur and its discharge products. Based on these effects, the lithium-sulfur battery assembled with the modified separator exhibits excellent rate performance and cycle performance.

[0036] 4) The catalyst loading on the modified membrane is only 0.15 mg / cm³. 2 It achieves good performance even at low loading (the specific capacity decays by only 0.016% per cycle after 1600 cycles at a 2C current density), showing great promise for the application of this catalyst in batteries. Attached Figure Description

[0037] Figure 1 The image shows a scanning electron microscope (SEM) image of the catalyst prepared in Example 1.

[0038] Figure 2 The image shows a transmission electron microscope (TEM) image of the catalyst prepared in Example 1.

[0039] Figure 3 The X-ray absorption fine structure (XAFS) test results are shown for the catalyst prepared in Example 1.

[0040] Figure 4 The wavelet transform EXAFS signal of the catalyst prepared in Example 1 (the color depth in the figure represents the bonding concentration);

[0041] Figure 5 The rate performance of the lithium-sulfur battery with the catalyst prepared in Example 1;

[0042] Figure 6 The cycle performance of the lithium-sulfur battery prepared by the catalyst in Example 1 is shown. Detailed Implementation

[0043] The following specific implementation case further illustrates the preparation method of manganese single-atom catalysts rich in S / N atoms.

[0044] Implementation Case 1: Preparation of MnSNC Catalyst

[0045] Step 1: Synthesize g-C3N4 template

[0046] An appropriate amount of dicyandiamide was placed in a closed ceramic boat and kept at 550°C for 6 hours in air at a heating rate of 5°C / min. After naturally cooling to room temperature, the product was thoroughly ground to obtain blocky g-C3N4 powder. Subsequently, an appropriate amount of blocky g-C3N4 was placed in an open ceramic boat and kept at 600°C for 4 hours in air at a heating rate of 5°C / min. After cooling to room temperature, flake-like g-C3N4 powder was obtained.

[0047] Step 2: Synthesis of g-C3N4@PPy composite material

[0048] Flake-shaped g-C3N4 was placed in deionized water and ultrasonically dispersed to achieve a uniform concentration (1.25 g / mL). Under ice bath conditions, pyrrole monomer was added dropwise to the solution and stirred for a period of time (addition amount: 0.3 μL / mL). Subsequently, a certain amount of ammonium persulfate [(NH4)2S2O8, addition amount: 2.0 g / mL] was slowly added to the solution and stirring continued for 10 h. Finally, the product was washed several times with deionized water and freeze-dried for 12 h to obtain the g-C3N4@PPy composite material.

[0049] Step 3: Synthesize a manganese single-atom catalyst with high S / N atom content

[0050] The prepared g-C3N4@PPy composite material was ultrasonically dispersed in deionized water (concentration 3 mg / mL). Subsequently, manganese acetate [Mn(CH3COO)2] was added to the solution and stirred (addition amount 1 times the mass of the g-C3N4@PPy composite material). After washing several times with deionized water, the mixture was dried to obtain g-C3N4@Mn-PPy. The obtained product was thoroughly mixed with sulfur powder (sulfur powder addition amount 1.5 times the mass of the g-C3N4@PPy composite material) and subjected to stepwise pyrolysis: under an Ar atmosphere, the temperature was increased to 600℃ at a rate of 3℃ / min and held for 6 h; then increased to 950℃ at a rate of 6℃ / min and held for 3 h, followed by natural cooling. The obtained product was acid-washed with 1 mol / L H2SO4 at 90℃ for 8 h, washed with deionized water until neutral, and vacuum dried to obtain a manganese single-atom catalyst with high S / N atom content. The manganese single-atom catalyst described herein contains 6.38 at% S and 12.20 at% N atoms.

[0051] The morphology of the prepared catalyst is as follows Figure 1 As shown, due to the effect of the sheet-like g-C3N4 template, the obtained catalyst morphology is a sheet-like structure. Meanwhile, to demonstrate the internal structure of the sample, TEM tests were performed (e.g., ...). Figure 2 As shown, the catalyst contains a large amount of graphitic carbon structure. Simultaneously, the etching gas of the template leads to the formation of numerous porous structures. The coordination structure of manganese atoms was analyzed using X-ray absorption fine structure analysis. Figure 3 The results showed that Mn is fixed by N atoms and exists as a single atom without bonding with S atoms. The coordination mode of Mn with N atoms is MnN4 configuration. Furthermore, wavelet transform contour plots ( Figure 4 This also proves the conclusion.

[0052] The fourth step is the application of the obtained catalyst in lithium-sulfur batteries.

[0053] 1. Preparation of modified diaphragms

[0054] The prepared catalyst, carbon nanotubes and binder (PVDF) were mixed and ground thoroughly at a mass ratio of 8:1:1. The resulting mixture was then added to an appropriate amount of isopropanol and sonicated to ensure uniform dispersion. The catalyst was then loaded onto a PP membrane by vacuum filtration and dried at 60°C for 12 hours to obtain the modified membrane.

[0055] 2. Preparation of sulfur / carbon cathode

[0056] Sublimed sulfur and BP-2000 were ground thoroughly at a mass ratio of 75:25 and then kept at 155°C for 12 hours under an Ar atmosphere. The resulting powder was then mixed with Super P and PVDF at a mass ratio of 7:2:1 and ground thoroughly. NMP was then added and stirred for 12 hours. The resulting uniformly mixed slurry was coated onto aluminum foil and dried at 60°C for 12 hours.

[0057] 3. Assemble lithium-sulfur batteries

[0058] The prepared composite separator, sulfur / carbon cathode, and lithium sheet were assembled into a lithium-sulfur battery. The electrolyte was added at a rate of 25 μL to the cathode side and 15 μL to the anode side, with a sulfur loading of ~1 mg / cm³. 2 .

[0059] The assembled lithium-sulfur battery was used for electrochemical performance testing, and the results are as follows: Figure 5 , Figure 6As shown, at a current density of 0.1C, the battery's specific capacity in the first cycle reached 1526.3 mAh / g; when the current density increased to 5C, the specific capacity remained at 486.5 mAh / g; and at a current density of 2C, after 1600 cycles, the specific capacity decayed by only 0.016% per cycle. Rate and cycle stability tests indicate that the prepared catalyst possesses good conductivity and the ability to limit / catalyze polysulfides.

[0060] Implementation Case 2: Preparation of MnSNC Catalyst

[0061] Step 1: Synthesize g-C3N4 template

[0062] An appropriate amount of dicyandiamide was placed in a closed ceramic boat and kept at 650°C for 4 hours in air at a heating rate of 2°C / min. After naturally cooling to room temperature, the product was thoroughly ground to obtain blocky g-C3N4 powder. Subsequently, an appropriate amount of blocky g-C3N4 was placed in an open ceramic boat and kept at 500°C for 2 hours in air at a heating rate of 3°C / min. After cooling to room temperature, flake-like g-C3N4 powder was obtained.

[0063] Step 2: Synthesis of g-C3N4@PPy composite material

[0064] Flake-shaped g-C3N4 was placed in deionized water and ultrasonically dispersed to achieve a uniform concentration (0.75 g / mL). Under ice bath conditions, pyrrole monomer was added dropwise to the solution and stirred for a period of time (0.2 μL / mL). Subsequently, a certain amount of ammonium persulfate [(NH4)2S2O8, 1.5 g / mL] was slowly added to the solution, and stirring continued for 6 hours. Finally, the product was washed several times with deionized water and freeze-dried for 12 hours to obtain the g-C3N4@PPy composite material.

[0065] Step 3: Synthesize a manganese single-atom catalyst with high S / N atom content

[0066] The prepared g-C3N4@PPy composite material was ultrasonically dispersed in deionized water (concentration 2 mg / mL). Subsequently, manganese acetate [Mn(CH3COO)2] was added to the solution and stirred (addition amount 0.5 times the mass of the g-C3N4@PPy composite material). After washing several times with deionized water, the mixture was dried to obtain g-C3N4@Mn-PPy. The obtained product was thoroughly mixed with sulfur powder (sulfur powder addition amount 1 times the mass of the g-C3N4@PPy composite material) and subjected to stepwise pyrolysis: under an Ar atmosphere, the temperature was increased to 500℃ at a rate of 2℃ / min and held for 4 h; then increased to 900℃ at a rate of 4℃ / min and held for 2 h, followed by natural cooling. The obtained product was acid-washed with 0.5 mol / L H2SO4 at 60℃ for 5 h, washed with deionized water until neutral, and vacuum dried to obtain a manganese single-atom catalyst with high S / N atom content. The S and N atom contents in the manganese single-atom catalyst are 6.40 at% and 12.25 at%, respectively.

[0067] The fourth step is the application of the obtained catalyst in lithium-sulfur batteries.

[0068] 1. Preparation of modified diaphragms

[0069] The prepared catalyst, carbon nanotubes and binder (PVDF) were mixed and ground thoroughly at a mass ratio of 8:1:1. The resulting mixture was then added to an appropriate amount of isopropanol and sonicated to ensure uniform dispersion. The catalyst was then loaded onto a PP membrane by vacuum filtration and dried at 60°C for 12 hours to obtain the modified membrane.

[0070] 2. Preparation of sulfur / carbon cathode

[0071] Sublimed sulfur and BP-2000 were ground thoroughly at a mass ratio of 75:25 and then kept at 155°C for 12 hours under an Ar atmosphere. The resulting powder was then mixed with Super P and PVDF at a mass ratio of 7:2:1 and ground thoroughly. NMP was then added and stirred for 12 hours. The resulting uniformly mixed slurry was coated onto aluminum foil and dried at 60°C for 12 hours.

[0072] 3. Assemble lithium-sulfur batteries

[0073] The prepared composite separator, sulfur / carbon cathode, and lithium sheet were assembled into a lithium-sulfur battery. The electrolyte was added at a rate of 25 μL to the cathode side and 15 μL to the anode side, with a sulfur loading of ~1 mg / cm³. 2 .

[0074] The assembled lithium-sulfur battery was used for electrochemical performance testing. At a current density of 0.1C, the battery achieved a high specific capacity of 1541.2 mAh / g in the first cycle; when the current density increased to 5C, the specific capacity remained at 512.3 mAh / g; and at a current density of 2C, after 1600 cycles, the specific capacity decayed by only 0.014% per cycle. Rate and cycle stability tests demonstrated that the prepared catalyst possesses good conductivity and the ability to limit / catalyze polysulfides.

[0075] Implementation Case 3: Preparation of MnSNC Catalyst

[0076] Step 1: Synthesize g-C3N4 template

[0077] An appropriate amount of dicyandiamide was placed in a closed ceramic boat and kept at 600°C for 3 hours in air at a heating rate of 3°C / min. After naturally cooling to room temperature, the product was thoroughly ground to obtain blocky g-C3N4 powder. Subsequently, an appropriate amount of blocky g-C3N4 was placed in an open ceramic boat and kept at 550°C for 3 hours in air at a heating rate of 4°C / min. After cooling to room temperature, flake-like g-C3N4 powder was obtained.

[0078] Step 2: Synthesis of g-C3N4@PPy composite material

[0079] Flake-shaped g-C3N4 was placed in deionized water and ultrasonically dispersed to achieve a uniform concentration (1 g / mL). Under ice bath conditions, pyrrole monomer was added dropwise to the solution and stirred for a period of time (addition amount: 0.25 μL / mL). Subsequently, a certain amount of ammonium persulfate [(NH4)2S2O8, addition amount: 1.75 g / mL] was slowly added to the solution and stirring continued for 8 hours. Finally, the product was washed several times with deionized water and freeze-dried for 12 hours to obtain the g-C3N4@PPy composite material.

[0080] Step 3: Synthesize a manganese single-atom catalyst with high S / N atom content

[0081] The prepared g-C3N4@PPy composite material was ultrasonically dispersed in deionized water (concentration 2.5 mg / mL). Subsequently, manganese acetate [Mn(CH3COO)2] was added to the solution and stirred (addition amount 0.75 times the mass of the g-C3N4@PPy composite material). After washing several times with deionized water, the mixture was dried to obtain g-C3N4@Mn-PPy. The obtained product was thoroughly mixed with sulfur powder (sulfur powder addition amount 1.25 times the mass of the g-C3N4@PPy composite material) and subjected to stepwise pyrolysis: under an Ar atmosphere, the temperature was increased to 550℃ at a rate of 2.5℃ / min and held for 5 h; then increased to 920℃ at a rate of 4℃ / min and held for 2.5 h, followed by natural cooling. The obtained product was acid-washed with 0.75 mol / L H2SO4 at 80℃ for 7 h, washed with deionized water until neutral, and vacuum dried to obtain a manganese single-atom catalyst with high S / N atom content. The manganese single-atom catalyst contains 6.52 at% S and 12.20 at% N atoms, respectively.

[0082] The fourth step is the application of the obtained catalyst in lithium-sulfur batteries.

[0083] 1. Preparation of modified diaphragms

[0084] The prepared catalyst, carbon nanotubes and binder (PVDF) were mixed and ground thoroughly at a mass ratio of 8:1:1. The resulting mixture was then added to an appropriate amount of isopropanol and sonicated to ensure uniform dispersion. The catalyst was then loaded onto a PP membrane by vacuum filtration and dried at 60°C for 12 hours to obtain the modified membrane.

[0085] 2. Preparation of sulfur / carbon cathode

[0086] Sublimed sulfur and BP-2000 were ground thoroughly at a mass ratio of 75:25 and then kept at 155°C for 12 hours under an Ar atmosphere. The resulting powder was then mixed with Super P and PVDF at a mass ratio of 7:2:1 and ground thoroughly. NMP was then added and stirred for 12 hours. The resulting uniformly mixed slurry was coated onto aluminum foil and dried at 60°C for 12 hours.

[0087] 3. Assemble lithium-sulfur batteries

[0088] The prepared composite separator, sulfur / carbon cathode, and lithium sheet were assembled into a lithium-sulfur battery. The electrolyte was added at a rate of 25 μL to the cathode side and 15 μL to the anode side, with a sulfur loading of ~1 mg / cm³. 2 .

[0089] The assembled lithium-sulfur battery was used for electrochemical performance testing. At a current density of 0.1C, the battery achieved a high specific capacity of 1554.3 mAh / g in the first cycle; when the current density increased to 5C, the specific capacity remained at 540.2 mAh / g; and at a current density of 2C, after 1600 cycles, the specific capacity decayed by only 0.015% per cycle. Rate and cycle stability tests demonstrated that the prepared catalyst possesses good conductivity and the ability to limit / catalyze polysulfides.

[0090] Implementation Case 4: Preparation of SNC Catalysts

[0091] Step 1: Synthesize g-C3N4 template

[0092] An appropriate amount of dicyandiamide was placed in a closed ceramic boat and kept at 650°C for 4 hours in air at a heating rate of 2°C / min. After naturally cooling to room temperature, the product was thoroughly ground to obtain blocky g-C3N4 powder. Subsequently, an appropriate amount of blocky g-C3N4 was placed in an open ceramic boat and kept at 600°C for 2 hours in air at a heating rate of 3°C / min. After cooling to room temperature, flake-like g-C3N4 powder was obtained.

[0093] Step 2: Synthesis of g-C3N4@PPy composite material

[0094] Flake-shaped g-C3N4 was placed in deionized water and ultrasonically dispersed to achieve a uniform concentration (0.75 g / mL). Under ice bath conditions, pyrrole monomer was added dropwise to the solution and stirred for a period of time (0.2 μL / mL). Subsequently, a certain amount of ammonium persulfate [(NH4)2S2O8, 1.5 g / mL] was slowly added to the solution, and stirring continued for 6 hours. Finally, the product was washed several times with deionized water and freeze-dried for 12 hours to obtain the g-C3N4@PPy composite material.

[0095] Step 3: Synthesize a manganese single-atom catalyst with high S / N atom content

[0096] The prepared g-C3N4@PPy composite material was thoroughly mixed with sulfur powder (the amount of sulfur powder added was 1.25 times the mass of the g-C3N4@PPy composite material) and subjected to stepwise pyrolysis: Under an Ar atmosphere, the temperature was increased to 500℃ at a rate of 2℃ / min and held for 4 h; then increased to 900℃ at a rate of 4℃ / min and held for 2 h, followed by natural cooling. The resulting product was acid-washed with 0.5 mol / L H2SO4 at 60℃ for 5 h, washed with deionized water until neutral, and vacuum dried to obtain a catalyst with high S / N atom content. The S and N atom contents in the manganese single-atom catalyst were 6.45 at% and 12.30 at%, respectively.

[0097] The fourth step is the application of the obtained catalyst in lithium-sulfur batteries.

[0098] 1. Preparation of modified diaphragms

[0099] The prepared catalyst, carbon nanotubes and binder (PVDF) were mixed and ground thoroughly at a mass ratio of 8:1:1. The resulting mixture was then added to an appropriate amount of isopropanol and sonicated to ensure uniform dispersion. The catalyst was then loaded onto a PP membrane by vacuum filtration and dried at 60°C for 12 hours to obtain the modified membrane.

[0100] 2. Preparation of sulfur / carbon cathode

[0101] Sublimed sulfur and BP-2000 were ground thoroughly at a mass ratio of 75:25 and then kept at 155°C for 12 hours under an Ar atmosphere. The resulting powder was then mixed with Super P and PVDF at a mass ratio of 7:2:1 and ground thoroughly. NMP was then added and stirred for 12 hours. The resulting uniformly mixed slurry was coated onto aluminum foil and dried at 60°C for 12 hours.

[0102] 3. Assemble lithium-sulfur batteries

[0103] The prepared composite separator, sulfur / carbon cathode, and lithium sheet were assembled into a lithium-sulfur battery. The electrolyte was added at a rate of 25 μL to the cathode side and 15 μL to the anode side, with a sulfur loading of ~1 mg / cm³. 2 .

[0104] The assembled lithium-sulfur battery was used for electrochemical performance testing. At a current density of 0.1C, the battery achieved a high specific capacity of 1550.9 mAh / g in the first cycle; when the current density increased to 5C, the specific capacity remained at 530.8 mAh / g; and at a current density of 2C, after 1600 cycles, the specific capacity decayed by only 0.014% per cycle. Rate and cycle stability tests demonstrated that the prepared catalyst possesses good conductivity and the ability to limit / catalyze polysulfides.

[0105] Implementation Case 5: Preparation of NC Catalysts

[0106] Step 1: Synthesize g-C3N4 template

[0107] An appropriate amount of dicyandiamide was placed in a closed ceramic boat and kept at 550°C for 4 hours in air at a heating rate of 2°C / min. After naturally cooling to room temperature, the product was thoroughly ground to obtain blocky g-C3N4 powder. Subsequently, an appropriate amount of blocky g-C3N4 was placed in an open ceramic boat and kept at 500°C for 2 hours in air at a heating rate of 3°C / min. After cooling to room temperature, flake-like g-C3N4 powder was obtained.

[0108] Step 2: Synthesis of g-C3N4@PPy composite material

[0109] Flake-shaped g-C3N4 was placed in deionized water and ultrasonically dispersed to achieve a uniform concentration (0.75 g / mL). Under ice bath conditions, pyrrole monomer was added dropwise to the solution and stirred for a period of time (0.2 μL / mL). Subsequently, a certain amount of ammonium persulfate [(NH4)2S2O8, 1.5 g / mL] was slowly added to the solution, and stirring continued for 6 hours. Finally, the product was washed several times with deionized water and freeze-dried for 12 hours to obtain the g-C3N4@PPy composite material.

[0110] Step 3: Synthesize a manganese single-atom catalyst with high S / N atom content

[0111] The prepared g-C3N4@PPy composite material was subjected to stepwise pyrolysis: Under an Ar atmosphere, the temperature was increased to 500℃ at a rate of 2℃ / min and held for 4 h; then increased to 900℃ at a rate of 4℃ / min and held for 2 h, followed by natural cooling. The resulting product was acid-washed with 0.5 mol / L H2SO4 at 60℃ for 5 h, washed with deionized water until neutral, and vacuum dried to obtain the NC manganese single-atom catalyst. The S and N atom contents of the manganese single-atom catalyst were 6.32 at% and 12.15 at%, respectively.

[0112] The fourth step is the application of the obtained catalyst in lithium-sulfur batteries.

[0113] 1. Preparation of modified diaphragms

[0114] The prepared catalyst, carbon nanotubes and binder (PVDF) were mixed and ground thoroughly at a mass ratio of 8:1:1. The resulting mixture was then added to an appropriate amount of isopropanol and sonicated to ensure uniform dispersion. The catalyst was then loaded onto a PP membrane by vacuum filtration and dried at 60°C for 12 hours to obtain the modified membrane.

[0115] 2. Preparation of sulfur / carbon cathode

[0116] Sublimed sulfur and BP-2000 were ground thoroughly at a mass ratio of 75:25 and then kept at 155°C for 12 hours under an Ar atmosphere. The resulting powder was then mixed with Super P and PVDF at a mass ratio of 7:2:1 and ground thoroughly. NMP was then added and stirred for 12 hours. The resulting uniformly mixed slurry was coated onto aluminum foil and dried at 60°C for 12 hours.

[0117] 3. Assemble lithium-sulfur batteries

[0118] The prepared composite separator, sulfur / carbon cathode, and lithium sheet were assembled into a lithium-sulfur battery. The electrolyte was added at a rate of 25 μL to the cathode side and 15 μL to the anode side, with a sulfur loading of ~1 mg / cm³. 2 .

[0119] The assembled lithium-sulfur battery was used for electrochemical performance testing. At a current density of 0.1C, the battery achieved a high specific capacity of 1490.2 mAh / g in the first cycle; when the current density increased to 5C, the specific capacity remained at 469.3 mAh / g; and at a current density of 2C, after 1600 cycles, the specific capacity decayed by only 0.018% per cycle. Rate and cycle stability tests demonstrated that the prepared catalyst possesses good conductivity and the ability to limit / catalyze polysulfides.

[0120] The embodiments described above are merely illustrative of implementation methods of the present invention and should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A method for preparing a manganese single-atom catalyst with high S / N atom content, characterized in that, The manganese single-atom catalyst is obtained by pyrolysis of g-C3N4@PPy as a precursor, which is then mixed with sulfur powder after manganese ion adsorption. The S and N atom contents in the manganese single-atom catalyst can reach 5.50 at%-6.30 at% and 6.32 at%-12.00 at%, respectively. The specific preparation steps are as follows: Step 1: Prepare sheet-like g-C3N4 templates by pyrolysis; Step 2: Coating a layer of polypyrrole onto the template surface through the polymerization reaction of pyrrole monomers to synthesize g-C3N4@PPy composite material; Step 3: Synthesize a manganese single-atom catalyst with high S / N atom content 3.1) Adsorbing Mn into the composite material 2+ This forms the pyrolysis precursor g-C3N4@Mn-PPy; 3.2) After thoroughly mixing the product obtained in step 3.1) with sulfur powder, perform stepwise pyrolysis; specifically: in an Ar atmosphere, raise the temperature from room temperature to 500-600 ℃ and hold for 4-6 h; then raise the temperature to 900-950 ℃ and hold for 2-3 h, and then cool naturally. 3.3) The product obtained in step 3.2) was acid washed and dried to obtain the catalyst.

2. The method for preparing a manganese single-atom catalyst with high S / N atom content according to claim 1, characterized in that, The specific preparation steps are as follows: Step 1: Synthesize g-C3N4 template 1.1) Dicyandiamide was placed in a closed ceramic boat and kept at 550-650 ℃ for 4-6 h in an air atmosphere with a heating rate of 2-5 ℃ / min. After naturally cooling to room temperature, the product was thoroughly ground to obtain block g-C3N4 powder. 1.2) Take a block of g-C3N4 and place it in an open ceramic boat. Heat it to 500-600 ℃ in air and keep it at that temperature for 2-4 h. After cooling to room temperature, you will get flake g-C3N4 powder. Step 2: Synthesis of g-C3N4@PPy composite material 2.1) Place the flake g-C3N4 in deionized water and sonicate it to disperse it evenly. Under ice bath conditions, add pyrrole monomer dropwise to the above solution to obtain a uniformly dispersed mixed solution. 2.2) Slowly add ammonium persulfate (NH4)2S2O8 to the mixed solution obtained in step 2.1) and continue stirring for 6-10 h; 2.3) The g-C3N4@PPy composite material was obtained by washing with deionized water and freeze-drying. Step 3: Synthesize a manganese single-atom catalyst with high S / N atom content 3.1) The g-C3N4@PPy composite material was placed in deionized water and ultrasonically dispersed to make it uniform; then manganese acetate Mn(CH3COO)2 was added to the above solution and stirred for 6-10 h. After washing several times with deionized water, it was dried to obtain g-C3N4@Mn-PPy. 3.2) The product obtained in step 3.1) is thoroughly mixed with sulfur powder and then subjected to stepwise pyrolysis; 3.3) The product obtained in step 3.2) was acid-washed with H2SO4 at 60-90 °C, then washed with deionized water until neutral, and vacuum dried to obtain a manganese single-atom catalyst with high S / N atom content.

3. The method for preparing a manganese single-atom catalyst with high S / N atom content according to claim 2, characterized in that, In step 2.1), 0.2-0.3 μL of pyrrole monomer is added to every 1 mL of solution.

4. The method for preparing a manganese single-atom catalyst with high S / N atom content according to claim 2, characterized in that, In step 2.2), 1.5-2.0 g of ammonium persulfate is added to every 1 mL of the mixed solution.

5. The method for preparing a manganese single-atom catalyst with high S / N atom content according to claim 2, characterized in that, In step 3.1), the concentration of the g-C3N4@PPy composite material in deionized water is 2-3 mg / mL.

6. The method for preparing a manganese single-atom catalyst with high S / N atom content according to claim 2, characterized in that, In step 3.1), the amount of manganese acetate added is 0.5-1 times the mass of the g-C3N4@PPy composite material.

7. The method for preparing a manganese single-atom catalyst with high S / N atom content according to claim 2, characterized in that, In step 3.2), the amount of sulfur powder added is 1-1.5 times the mass of the g-C3N4@PPy composite material.

8. The application of a manganese single-atom catalyst with high S / N atom content in lithium-sulfur batteries, characterized in that, The catalyst synthesized by any of the preparation methods described in claims 1-7 is applied to lithium-sulfur batteries to modify commercial battery PP separators.

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