Preparation method of metal oxide-based composite cathode material for lithium-sulfur battery

Abstract

The present invention relates to a preparation method of a cathode material for a lithium-sulfur battery, and particularly to the preparation of a metal oxide-based composite cathode material for a lithium-sulfur battery. It includes the following steps: First, pretreatment of carbon nanotube materials; Second, preparation of a metal-organic framework / carbon nanotube precursor; Third, preparation of a cobalt-based nitrogen-doped porous carbon framework material; Fourth, preparation of a cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon carrier; Fifth, preparation of a lithium-sulfur battery composite cathode material. The metal oxide-based composite cathode material prepared by the present invention by combining high-temperature carbonization and hydrothermal reaction methods effectively solves the problems of poor conductivity of elemental sulfur materials, shuttle effect of polysulfides, and electrode volume expansion in lithium-sulfur batteries through the chemical adsorption of a porous carbon conductive framework and bipolar substances, and achieves excellent cycle stability performance through the combined compatibility of cerium dioxide, cobalt-nitrogen-carbon, and porous structures. At the same time, the prepared sulfur carrier material has rich raw materials and low cost, and the synthesis method is simple, so it is suitable for further commercialization on the cathode material of lithium-sulfur batteries.

Description

Technical Field

[0001] The present invention relates to the technical field of cathode materials for lithium-sulfur batteries, and particularly relates to a preparation method of a metal oxide-based composite cathode material for lithium-sulfur batteries. Background Art

[0002] With the rapid development of society, electric vehicles, intelligent networks, and mobile electronic devices have become an indispensable part of human society. However, the energy densities provided by some materials such as lithium cobaltate or lithium iron phosphate currently used in lithium-ion batteries (LiBs) as energy storage systems cannot meet people's needs. Therefore, there is an urgent need to seek other advanced battery energy storage devices with high energy density. Compared with traditional lithium-ion batteries, lithium-sulfur (Li-S) batteries have a higher theoretical energy density (2600 Wh kg -1 ). The inherent advantages of elemental sulfur, such as its low price, pollution-free nature, and abundant reserves, have attracted people's attention and research on lithium-sulfur batteries. However, there are some intractable problems in the commercialization process of lithium-sulfur batteries. For example, the conductivity of sulfur (S) and its reduction products (Li2S2 / Li2S) is very poor, which limits the utilization of active substances. The "shuttle effect" caused by the dissolution of polysulfides in the electrolyte during discharge leads to low Coulomb efficiency and rapid capacity decay of the battery, and the loss of active substances due to the volume change of the sulfur cathode during charge and discharge results in poor cycle stability of the battery. To address these drawbacks, carrier materials are usually introduced to composite with elemental sulfur to solve the above problems. Currently, the most common carrier material is to wrap elemental sulfur in various carbon materials to improve the problems existing in the sulfur cathode through the electronic conductivity and porosity of the carbon materials. However, the physical adsorption with carbon as the carrier material can only adsorb polysulfides through van der Waals forces, and the interaction is weak, and the phenomenon of polysulfide dissolution and shuttle still occurs, which is not conducive to the long-term stable cycling of lithium-sulfur batteries. Therefore, preparing a cathode active material carrier material that can effectively limit the shuttle of polysulfides is the key to the development of high-energy-density lithium-sulfur batteries (1. W.W. Sun, S.K. Liu, Y.J. Li, D.Q. Wang, Q.P. Guo, X.B. Hong, K. Xie, Z.Y. Ma, C.M. Zheng, S.Z. Xiong. Advanced Functional Materials, 2022, 32(43): 2205471. 2. Y. Yao, C.Y. Chang, R.G. Li, D. Guo, Z.X. Liu, X. Pu, J.Y. Zhai. Chemical Engineering Journal, 2022, 431: 134033). Summary of the Invention

[0003] The object of the present invention is to provide an efficient sulfur carrier material, a preparation method and an application thereof for lithium-sulfur batteries in an energy storage system. This carrier material can solve the technical problems of poor electrochemical performance of lithium-sulfur batteries caused by the dissolution and shuttling of polysulfides, low conductivity of elemental sulfur, and volume expansion.

[0004] To solve the above technical problems, the present invention provides the following technical solutions:

[0005] A metal-oxide-based composite cathode material for lithium-sulfur batteries, which specifically includes the following steps:

[0006] I. Pretreatment of carbon nanotubes

[0007] Mix carbon nanotubes and an acidic solution at a mass ratio of 0.1-1:0.01-1 by stirring at room temperature, then centrifuge and wash to obtain the pretreated carbon nanotube material;

[0008] II. Preparation of metal-organic framework / carbon nanotube precursor

[0009] Prepare a solution by mixing the carbon nanotubes treated in step I with a dispersant, cobalt(II) nitrate hexahydrate, 2-methylimidazole, and methanol at a mass ratio of 0.01-0.1:0.1-1:1-3:1-5:0.01-1, stir at room temperature, and after dissolution, centrifuge and wash to obtain the metal-organic framework / carbon nanotube precursor;

[0010] III. Preparation of cobalt-based nitrogen-doped porous carbon framework material

[0011] Put the metal-organic framework / carbon nanotube precursor obtained in step II into a tubular furnace with a protective gas for programmed temperature rise and calcination and carbonization treatment. After heat preservation, cool to room temperature to obtain the cobalt-based nitrogen-doped porous carbon framework material;

[0012] IV. Preparation of cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon carrier

[0013] Prepare a solution by mixing the cobalt-based nitrogen-doped porous carbon framework material obtained in step III, cerium(III) nitrate hexahydrate, and deionized water at a mass ratio of 0.1-1:0.01-1:0.01-0.1, stir, add a certain volume of alkaline solution, stir, transfer to a reaction kettle, heat, cool to room temperature, centrifuge and wash to obtain the cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon carrier;

[0014] V. Preparation of lithium-sulfur battery composite cathode material

[0015] After mixing the cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon support obtained in Step 4 with elemental sulfur at a mass ratio of 1:2-5, grinding, transferring it to a tubular furnace, and performing programmed heating in a protective gas, heating, heat preservation, and then cooling to room temperature, the composite cathode material for the lithium-sulfur battery can be obtained.

[0016] The acidic solution described in Step 1 is one or more of hydrochloric acid, sulfuric acid, or nitric acid, with a concentration of 0.05-1 mol / L. The carbon nanotubes treated with the acidic solution can grow some oxygen-containing functional groups on their surfaces, thereby increasing the polar effect of the carbon nanotubes and facilitating the adsorption of polysulfides.

[0017] Furthermore, the dispersant described in Step 2 is one or more of polyvinylpyrrolidone, polyethylene glycol, or sodium polyacrylate. One or more of polyvinylpyrrolidone, polyethylene glycol, or sodium polyacrylate as the dispersant in the solution can fully disperse the solute in the solution, facilitating the full reaction of the materials.

[0018] Furthermore, the stirring time of the mixed solution in Step 2 is 12-48 h.

[0019] Furthermore, the programmed heating rate in Step 3 is 1-5 °C / min, the calcination carbonization temperature is 600-900 °C, and the heat preservation time is 2-6 h.

[0020] Furthermore, the alkaline solution in Step 4 is one of sodium hydroxide or potassium hydroxide, and the added volume is 10%-40% of the mixed solution of the cobalt-based nitrogen-doped porous carbon framework and cerium nitrate hexahydrate.

[0021] Furthermore, the heating temperature in Step 4 is 150-180 °C, and the heat preservation time is 12-24 h.

[0022] Furthermore, in Step 5, the mass ratio of the synthesized support material to elemental sulfur is 1:2-5.

[0023] Furthermore, the programmed heating rate in Step 5 is 1-5 °C / min, the heating temperature is 150-155 °C, and the heat preservation time is 12-24 h.

[0024] Furthermore, the protective gas in Steps 3 and 5 is one of nitrogen, argon, or argon / hydrogen mixed gas.

[0025] Based on the above technical solutions, the following beneficial effects are achieved:

[0026] The present invention designs a metal-organic framework grown in-situ on carbon nanotubes as a precursor material for constructing a sulfur carrier. The electron transport channels formed by interconnected carbon nanotubes can promote the electron migration efficiency and enhance the conductivity of elemental sulfur. After carbonizing the precursor material, the cobalt-based nitrogen-doped porous carbon conductive framework derived therefrom and the cobalt-based nitrogen-doped porous carbon loaded with cerium dioxide formed by hydrothermal reaction of cerium hexahydrate nitrate have bipolar chemical adsorption, which can effectively adsorb polysulfides dissolved in the electrolyte to further improve the utilization rate of active substances. At the same time, the nitrogen-doped porous conductive carbon framework can effectively accommodate sulfur molecules and relieve the volume expansion during the sulfur lithiation process, thereby extending the cycle service life of the lithium-sulfur battery. Therefore, the lithium-sulfur battery assembled with the composite cathode material prepared by combining this carrier material with sulfur has a high initial discharge specific capacity of 1095.4 mAh / g at a rate of 0.1 C. After different rate cycles of 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C and then returning to a rate of 0.1 C, it still has a discharge specific capacity of 787.5 mAh / g, and the capacity retention rate is greater than 72%. At the same time, the Coulomb efficiency remains above 98% after 100 cycles at a rate of 0.5 C. In summary, the cobalt-based nitrogen-doped porous carbon loaded with cerium dioxide prepared by combining the high-temperature carbonization method and the hydrothermal reaction method in the present invention as a sulfur carrier material has the advantages of a continuous electron transport channel composed of carbon nanotubes and nitrogen-doped porous carbon and bipolar adsorption sites composed of cerium dioxide and cobalt-nitrogen-carbon, effectively solving the problems of poor conductivity of elemental sulfur materials, polysulfide shuttle effect, and electrode volume expansion in the energy storage system lithium-sulfur battery. The combined compatibility of the above cerium dioxide, cobalt-nitrogen-carbon, and porous structure realizes the long cycle life and high stability of the lithium-sulfur battery. At the same time, the prepared carrier material has rich raw materials and low cost, and the synthesis method is simple, so it is suitable for further commercialization on lithium-sulfur batteries in energy storage systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the modification results of the embodiments of the present invention, the drawings required for the description of the comparative examples and embodiments will be briefly introduced below.

[0028] Figure 1 X-ray diffraction pattern of the metal-organic framework / carbon nanotube precursor material prepared in the example;

[0029] Figure 2 X-ray diffraction pattern of the cobalt-based nitrogen-doped porous carbon material prepared in the example;

[0030] Figure 3 X-ray diffraction pattern of the cobalt-based nitrogen-doped porous carbon carrier material loaded with cerium dioxide prepared in the example;

[0031] Figure 4Electron micrograph of the metal-organic framework / carbon nanotube precursor material prepared for the example;

[0032] Figure 5 Electron micrograph of the cobalt-based nitrogen-doped porous carbon material prepared for the example;

[0033] Figure 6 Electron micrograph of the sulfur carrier material of the cobalt-based nitrogen-doped porous carbon framework loaded with cerium dioxide prepared for the example;

[0034] Figure 7 Charge and discharge curves of the lithium-sulfur battery assembled with the cobalt-based nitrogen-doped porous carbon loaded with cerium dioxide as the sulfur carrier material prepared for the example at different rates;

[0035] Figure 8 Rate performance diagram of the lithium-sulfur battery assembled with the cobalt-based nitrogen-doped porous carbon loaded with cerium dioxide as the sulfur carrier material prepared for the example;

[0036] Figure 9 Cyclic voltammogram of the lithium-sulfur battery assembled with the cobalt-based nitrogen-doped porous carbon loaded with cerium dioxide as the sulfur carrier material prepared for the example;

[0037] Figure 10 Cycling performance diagram of the lithium-sulfur battery assembled with the cobalt-based nitrogen-doped porous carbon loaded with cerium dioxide as the sulfur carrier material prepared for the example at 0.5 C rate, with Coulombic efficiency and discharge specific capacity from top to bottom in sequence;

[0038] Figure 11 Charge and discharge curves of the lithium-sulfur battery assembled with the cobalt-based nitrogen-doped porous carbon loaded with cerium dioxide as the sulfur carrier material prepared for Comparative Example 1 at different rates;

[0039] Figure 12 Rate performance diagram of the lithium-sulfur battery assembled with the cobalt-based nitrogen-doped porous carbon loaded with cerium dioxide as the sulfur carrier material prepared for Comparative Example 1;

[0040] Figure 13 Cycling performance diagram of the lithium-sulfur battery assembled with the cobalt-based nitrogen-doped porous carbon loaded with cerium dioxide as the sulfur carrier material prepared for Comparative Example 1 at 0.5 C rate, with Coulombic efficiency and discharge specific capacity from top to bottom in sequence;

[0041] Figure 14 Cycling performance diagram of the lithium-sulfur battery assembled with the cobalt-based nitrogen-doped porous carbon loaded with cerium dioxide as the sulfur carrier material prepared for Comparative Example 2 at 0.5 C rate, with Coulombic efficiency and discharge specific capacity from top to bottom in sequence;

[0042] Figure 15Cycling performance graph of the lithium-sulfur battery assembled with cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon prepared in Comparative Example 3 as the sulfur carrier material at a rate of 0.5. From top to bottom are the Coulombic efficiency and discharge specific capacity respectively;

[0043] Figure 16 Cycling performance graph of the lithium-sulfur battery assembled with cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon prepared in Comparative Example 4 as the sulfur carrier material at a rate of 0.5. From top to bottom are the Coulombic efficiency and discharge specific capacity respectively. Detailed implementation manners

[0044] The above content of the present invention will be further described in detail through the following examples. However, the subject matter of the present invention is not limited to the following examples only. All technologies implemented based on the above content of the present invention fall within the scope of the present invention.

[0045] Experimental drugs

[0046]

[0047] Experimental equipment

[0048]

[0049] The preparation process steps of the following comparative examples and examples will be further introduced in combination with the attached drawings. However, the protection scope of the present invention is not limited to the following examples only.

[0050] Example: A metal oxide-based composite cathode material for a lithium-sulfur battery in this example is prepared according to the following steps:

[0051] I. Pretreatment of carbon nanotube materials

[0052] Add 0.3 g of carbon nanotubes to 20 mL of nitric acid solution with a concentration of 15 mol / L and stir and mix at room temperature. After centrifugation and washing, perform drying treatment;

[0053] II. Preparation of metal-organic framework / carbon nanotube precursor

[0054] Take 0.1 g of the treated carbon nanotubes and 0.5 g of polyvinylpyrrolidone and dissolve them in 80 mL of methanol solution, then stir for 2 h. Add 1.75 g of cobalt(II) nitrate hexahydrate to the above solution and stir for 6 h, and then add 1.98 g of 2-methylimidazole and stir for 18 h. After centrifugation and washing, the metal-organic framework / carbon nanotube precursor can be obtained;

[0055] III. Preparation of cobalt-based nitrogen-doped porous carbon framework material

[0056] Put the metal-organic framework / carbon nanotube precursor obtained in Step 2 into a tube furnace with a protective gas. Sinter at 800 °C for 4 h at a heating rate of 5 °C / min, and then cool to room temperature to obtain the cobalt-based nitrogen-doped porous carbon framework material;

[0057] IV. Preparation of Cobalt-Based Nitrogen-Doped Porous Carbon Support Loaded with Cerium Dioxide

[0058] Disperse 0.2 g of the cobalt-based nitrogen-doped porous carbon framework material and 0.05 g of cerium nitrate hexahydrate in 40 mL of deionized water and stir for 6 hours. Then add 15 mL of sodium hydroxide solution with a concentration of 0.02 mol / L to the above solution and stir for 6 h. Then transfer the solution to a hydrothermal reactor, keep it at 180 °C for 16 h, cool to room temperature, centrifuge and wash to obtain the cobalt-based nitrogen-doped porous carbon framework support loaded with cerium dioxide;

[0059] V. Preparation of Composite Cathode Material

[0060] Mix the cobalt-based nitrogen-doped porous carbon framework support loaded with cerium dioxide obtained in Step 4 and elemental sulfur in a mass ratio of 1:3. After grinding, transfer it to a tube furnace and sinter at 155 °C for 12 h under nitrogen conditions to obtain the composite cathode material for the lithium-sulfur battery.

[0061] Comparative Example 1: A metal oxide-based composite cathode material for a lithium-sulfur battery in this example was prepared according to the following steps:

[0062] I. Pretreatment of Carbon Nanotube Material

[0063] Add 0.3 g of carbon nanotubes to 20 mL of nitric acid solution with a concentration of 15 mol / L, stir and mix at room temperature, and perform drying treatment after centrifugation and washing;

[0064] II. Preparation of Metal-Organic Framework / Carbon Nanotube Precursor

[0065] Take 0.1 g of the treated carbon nanotubes and 0.5 g of polyvinylpyrrolidone, dissolve them in 80 mL of methanol solution, stir for 2 h, then add 1.75 g of cobalt nitrate hexahydrate to the above solution, stir for 6 h, and then add 1.98 g of 2-methylimidazole and stir for 18 h. After centrifugation and washing, the metal-organic framework / carbon nanotube precursor can be obtained;

[0066] III. Preparation of Cobalt-Based Nitrogen-Doped Porous Carbon Framework Material

[0067] Put the metal-organic framework / carbon nanotube precursor obtained in Step 2 into a tube furnace with a protective gas. Sinter at 800 °C for 4 h at a heating rate of 5 °C / min, and then cool to room temperature to obtain the cobalt-based nitrogen-doped porous carbon framework material;

[0068] IV. Preparation of Cerium Dioxide-Loaded Cobalt-Based Nitrogen-Doped Porous Carbon Support

[0069] Disperse 0.2 g of cobalt-based nitrogen-doped porous carbon framework material and 0.01 g of cerium nitrate hexahydrate in 40 mL of deionized water and stir for 6 hours. Then, add 15 mL of sodium hydroxide solution with a concentration of 0.02 mol / L to the above solution and stir for 6 h. Then transfer the solution to a hydrothermal reactor, keep it at 180 °C for 16 h, cool to room temperature, and after centrifugation and washing, obtain the cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon framework support;

[0070] V. Preparation of Composite Cathode Material

[0071] Mix the cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon framework support obtained in step IV with elemental sulfur in a mass ratio of 1:3. After grinding, transfer it to a tube furnace and sinter it at 155 °C for 12 h under nitrogen atmosphere to obtain the composite cathode material for lithium-sulfur battery.

[0072] Comparative Example 2:

[0073] I. Pretreatment of Carbon Nanotube Material

[0074] Add 0.3 g of carbon nanotubes to 20 mL of nitric acid solution with a concentration of 15 mol / L, stir and mix at room temperature, and after centrifugation and washing, perform drying treatment;

[0075] II. Preparation of Metal-Organic Framework / Carbon Nanotube Precursor

[0076] Take 0.1 g of treated carbon nanotubes and 0.5 g of polyvinylpyrrolidone, dissolve them in 80 mL of methanol solution, stir for 2 h, then add 1.75 g of cobalt nitrate hexahydrate to the above solution and stir for 6 h, and then add 1.98 g of 2-methylimidazole and stir for 18 h. After centrifugation and washing, the metal-organic framework / carbon nanotube precursor can be obtained;

[0077] III. Preparation of Cobalt-Based Nitrogen-Doped Porous Carbon Framework Material

[0078] Put the metal-organic framework / carbon nanotube precursor obtained in step II into a tube furnace with a protective gas, sinter it at a heating rate of 5 °C / min to 800 °C for 4 h, and then cool to room temperature to obtain the cobalt-based nitrogen-doped porous carbon framework material;

[0079] IV. Preparation of Cerium Dioxide-Loaded Cobalt-Based Nitrogen-Doped Porous Carbon Support

[0080] Disperse 0.2 g of cobalt-based nitrogen-doped porous carbon framework material and 0.025 g of cerium nitrate hexahydrate in 40 mL of deionized water and stir for 6 h. Then add 15 mL of sodium hydroxide solution with a concentration of 0.02 mol / L to the above solution and stir for 6 h. Then transfer the solution to a hydrothermal autoclave, keep it at 180 °C for 16 h, cool to room temperature, and after centrifugation and washing, obtain a cobalt-based nitrogen-doped porous carbon framework support loaded with cerium dioxide;

[0081] V. Preparation of Composite Cathode Material

[0082] Mix the cobalt-based nitrogen-doped porous carbon framework material support loaded with cerium dioxide obtained in Step 4 with elemental sulfur in a mass ratio of 1:3. After grinding, transfer it to a tubular furnace and sinter at 155 °C for 12 h under nitrogen conditions to obtain the composite cathode material for the lithium-sulfur battery.

[0083] Comparative Example 3:

[0084] I. Pretreatment of Carbon Nanotube Material

[0085] Add 0.3 g of carbon nanotubes to 20 mL of nitric acid solution with a concentration of 15 mol / L, stir and mix at room temperature, and after centrifugation and washing, perform drying treatment;

[0086] II. Preparation of Metal-Organic Framework / Carbon Nanotube Precursor

[0087] Take 0.1 g of treated carbon nanotubes and 0.5 g of polyvinylpyrrolidone, dissolve them in 80 mL of methanol solution, stir for 2 h, then add 1.75 g of cobalt nitrate hexahydrate to the above solution and stir for 6 h, and then add 1.98 g of 2-methylimidazole and stir for 18 h. After centrifugation and washing, the metal-organic framework / carbon nanotube precursor can be obtained;

[0088] III. Preparation of Cobalt-Based Nitrogen-Doped Porous Carbon Framework Material

[0089] Put the metal-organic framework / carbon nanotube precursor obtained in Step 2 into a tubular furnace with a protective gas, heat it at a heating rate of 5 °C / min to 800 °C, sinter for 4 h, and then cool to room temperature to obtain the cobalt-based nitrogen-doped porous carbon framework material;

[0090] IV. Preparation of Cobalt-Based Nitrogen-Doped Porous Carbon Support Loaded with Cerium Dioxide

[0091] Disperse 0.2 g of cobalt-based nitrogen-doped porous carbon framework material and 0.075 g of cerium nitrate hexahydrate in 40 mL of deionized water and stir for 6 h. Then, add 15 mL of sodium hydroxide solution with a concentration of 0.02 mol / L to the above solution and stir for 6 h. Subsequently, transfer the solution to a hydrothermal autoclave, keep it at 180 °C for 16 h, and after centrifugation and washing, obtain a cobalt-based nitrogen-doped porous carbon framework support loaded with cerium dioxide;

[0092] V. Preparation of Composite Cathode Material

[0093] Mix the cobalt-based nitrogen-doped porous carbon framework material support loaded with cerium dioxide obtained in Step 4 with elemental sulfur in a mass ratio of 1:3. After grinding, transfer it to a tubular furnace and sinter at 155 °C for 12 h under nitrogen atmosphere to obtain the composite cathode material for the lithium-sulfur battery.

[0094] Comparative Example 4:

[0095] I. Pretreatment of Carbon Nanotube Material

[0096] Add 0.3 g of carbon nanotubes to 20 mL of nitric acid solution with a concentration of 15 mol / L, stir and mix at room temperature, and after centrifugation and washing, perform drying treatment;

[0097] II. Preparation of Metal-Organic Framework / Carbon Nanotube Precursor

[0098] Take 0.1 g of treated carbon nanotubes and 0.5 g of polyvinylpyrrolidone, dissolve them in 80 mL of methanol solution, stir for 2 h, then add 1.75 g of cobalt nitrate hexahydrate to the above solution and stir for 6 h, and then add 1.98 g of 2-methylimidazole and stir for 18 h. After centrifugation and washing, the metal-organic framework / carbon nanotube precursor can be obtained;

[0099] III. Preparation of Cobalt-Based Nitrogen-Doped Porous Carbon Framework Material

[0100] Put the metal-organic framework / carbon nanotube precursor obtained in Step 2 into a tubular furnace with a protective gas, sinter at a heating rate of 5 °C / min to 800 °C for 4 h, and then cool to room temperature to obtain the cobalt-based nitrogen-doped porous carbon framework material;

[0101] IV. Preparation of Cobalt-Based Nitrogen-Doped Porous Carbon Support Loaded with Cerium Dioxide

[0102] Disperse 0.2 g of cobalt-based nitrogen-doped porous carbon framework material and 0.1 g of cerium nitrate hexahydrate in 40 mL of deionized water and stir for 6 h. Then, add 15 mL of sodium hydroxide solution with a concentration of 0.02 mol / L to the above solution and stir for 6 h. Subsequently, transfer the solution to a hydrothermal autoclave, keep it at 180 °C for 16 h, and after centrifugation and washing, obtain a cobalt-based nitrogen-doped porous carbon framework support loaded with cerium dioxide;

[0103] V. Preparation of Composite Cathode Material

[0104] Mix the cobalt-based nitrogen-doped porous carbon framework support material loaded with cerium dioxide obtained in Step 4 with elemental sulfur in a mass ratio of 1:3. After grinding, transfer it to a tube furnace and sinter it at 155 °C for 12 h under nitrogen atmosphere to obtain the lithium-sulfur battery composite cathode material.

[0105] The only difference between the four groups of Comparative Examples 1 to 4 and the Example lies in the different cerium dioxide loading amounts. The lithium-sulfur batteries assembled with the cobalt-based nitrogen-doped porous carbon loaded with cerium dioxide prepared by the Example to Comparative Example 4 as the sulfur carrier material are tested as follows:

[0106] Perform performance characterization on the above Comparative Examples and Examples

[0107] 1) X-ray diffraction (XRD) test. Use an X-ray diffractometer with the model X’Pert PRO to characterize the phase structure of different samples. The test current and voltage are 40 mA and 40 mV respectively, and the scanning range is 10 - 90°.

[0108] 2) Scanning electron microscope (SEM) test. Use a scanning electron microscope with the model FEI sirion200 to characterize the microscopic morphology of different samples. The acceleration voltage is 0.2 - 30 kV, and the resolution is 20 kV.

[0109] 3) Cyclic voltammetry (CV) test. Use an electrochemical workstation with the model CHI760E to perform cyclic voltammetry tests on the lithium-sulfur battery composite cathode material in the range of 1.5 - 3 V at a scanning rate of 0.05 mV / s. Determine the potentials at which oxidation and reduction reactions occur in the electrochemical reaction based on the positions of the oxidation peak and reduction peak.

[0110] 4) Charge-discharge test. Use a Blue Power battery tester with the model CT2001A to perform different rate and cyclic charge-discharge tests on the lithium-sulfur battery assembled with the prepared novel composite cathode in the voltage range of 1.5 - 3 V. Evaluate the electrochemical performance of the novel composite cathode material based on the tested rate performance and cyclic stability,

[0111] Figure 1X-ray diffraction pattern of the material obtained by in-situ growth of metal-organic framework on carbon nanotubes prepared for the example. The crystal structure of the metal-organic framework has strong diffraction peaks at multiple low angles, while no diffraction peaks appear for carbon nanotubes due to their low content, which is consistent with the previous research results (X. Gao, S. Li, Y. Du, B. Wang. APL Materials, 2019, 7(9): 091115), indicating the successful preparation of the material obtained by in-situ growth of metal-organic framework on carbon nanotubes.

[0112] Figure 2 X-ray diffraction pattern of the cobalt-based nitrogen-doped porous carbon material prepared for the example. Obvious diffraction peaks at 44.2°, 51.3° and 75.7° correspond to the (111), (200) and (220) crystal planes of cubic cobalt (PDF#15-0806) respectively, indicating the successful conversion of cobalt ions into metallic cobalt after carbonization. In addition, the diffraction peak at 26.1° corresponds to the (002) crystal plane of graphite carbon, indicating the successful conversion of the organic ligand into graphite carbon (C. H. Chen, S. H. Lin, Y. J. Wu, J. T. Su, C. C. Cheng, P. Y. Cheng, Y. C. Ting, S. Y. Lu. Chemical Engineering Journal, 2022, 431: 133924).

[0113] Figure 3 X-ray diffraction pattern of the cobalt-based nitrogen-doped porous carbon sulfur carrier material loaded with cerium dioxide prepared for the example. The diffraction peaks of this material at 28.5°, 33.1°, 47.5° and 56.4° correspond to the (111), (200), (220) and (311) crystal planes of cubic cerium dioxide (PDF#81-0792), indicating the successful loading of cerium dioxide into the sulfur carrier material through hydrothermal reaction (W. J. Feng, J. Z. Chen, Y. P. Niu, W. Zhao, L. Zhang. Journal of Alloys and Compounds, 2022, 906: 164341).

[0114] Figure 4Electron micrograph of the material in which the metal-organic framework prepared for the example grows in-situ on carbon nanotubes. The metal-organic framework has a typical rhombic dodecahedron shape, and these rhombic dodecahedrons are connected to the carbon nanotubes. The electron transport channels formed by the carbon nanotubes can promote the electron migration efficiency and enhance the conductivity of elemental sulfur (W.J. Feng, W. Zhao, Z.J. Shi, J.Z. Chen. Journal of Materials Science: Materials in Electronics, 2022, 33(22): 17483-17492.).

[0115] Figure 5 Electron micrograph of the cobalt-based nitrogen-doped porous carbon framework material prepared for the example. The cobalt-based nitrogen-doped porous carbon particles are evenly distributed, indicating that the size of the cobalt-based nitrogen-doped porous carbon after carbonization treatment is significantly reduced.

[0116] Figure 6 Scanning electron micrograph of the cobalt-based nitrogen-doped porous carbon material loaded with cerium dioxide prepared for the example. It can be clearly seen that the cerium dioxide generated after the hydrothermal reaction is evenly distributed on the surface of the cobalt-based nitrogen-doped porous carbon framework material, which is more conducive to the adsorption of polysulfides and thus reduces the loss of active substances (X.C. Chen, L.B. Li, Y.H. Shan, D. Zhou, W.J. Cui, Y.M.Y, Zhao. Journal of Energy Chemistry, 2022, 70: 502-510.).

[0117] Figure 7 Initial charge-discharge curves of the lithium-sulfur battery using the cobalt-based nitrogen-doped porous carbon loaded with cerium dioxide as the sulfur carrier material prepared in the example at different rates. The initial discharge specific capacities at 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C are 1095.4 mAh / g, 860.4 mAh / g, 668.3 mAh / g, 560.1 mAh / g, and 456.1 mAh / g respectively. Compared with the data of the comparative example, the excellent discharge specific capacity of this lithium-sulfur battery is attributed to the effective synergistic adsorption of polysulfides by cerium dioxide and cobalt-nitrogen-carbon bipolar substances in the carrier material.

[0118] Figure 8The rate performance curve of a lithium-sulfur battery assembled with a cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon as a sulfur carrier material prepared in the examples. When the lithium-sulfur battery is cycled from 0.1 C to different rates and then returns to 0.1 C, it still has a discharge specific capacity of 787.5 mAh / g, and the capacity retention rate is greater than 72%, indicating that the lithium-sulfur battery with this sulfur carrier material has excellent rate performance. The excellent rate performance is attributed to the acceleration of the redox reaction rate during charge and discharge by cerium dioxide and cobalt-nitrogen-carbon bipolar substances in the carrier material.

[0119] Figure 9 The cyclic voltammetry curve of a lithium-sulfur battery assembled with a composite cathode prepared by mixing a cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon as a sulfur carrier material and elemental sulfur in a mass ratio of 1:3 at 0.05 mV / s in the examples. Two reduction peaks are observed at 2.28 V and 2.01 V, indicating that elemental sulfur is ultimately converted to solid-phase lithium sulfide after being transformed into long-chain polysulfides; the oxidation peak at 2.49 V is caused by the reversible reaction of solid-phase lithium sulfide through polysulfides and finally to elemental sulfur. The cyclic curve has good reversibility after three cycles, which means that the lithium-sulfur battery with a cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon framework as a sulfur carrier material has excellent cycle stability (C.C. Hu, X.Y. Zhang, H.P. Li, Y. Zhao. Solid State Sciences, 2022, 134: 107025).

[0120] Figure 10 The cycle performance curve of a lithium-sulfur battery assembled with a cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon as a sulfur carrier material prepared in the examples. At 0.5 C, it has a reversible discharge specific capacity of 557.2 mAh / g after 100 cycles, the capacity retention rate is 66.6%, and the Coulombic efficiency is above 98%. The excellent cycle performance is attributed to the uniform distribution of cerium dioxide on the surface of porous carbon in the carrier material, which promotes the conversion of polysulfides, and the porous carbon structure alleviates the volume expansion problem of elemental sulfur and discharge products.

[0121] Figure 11 The initial charge-discharge curves of a lithium-sulfur battery assembled with a cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon as a sulfur carrier material prepared in Comparative Example 1 at different rates. The initial discharge specific capacities at 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C are 997.7 mAh / g, 528.8 mAh / g, 355 mAh / g, 263.4 mAh / g, and 190.6 mAh / g, respectively. The discharge specific capacities at different rates are relatively low, and the polarization between the charge-discharge curves is serious, indicating that the prepared sulfur carrier material has low conductivity and cannot effectively adsorb polysulfides.

[0122] Figure 12The rate performance curve of the lithium-sulfur battery assembled with cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon as the sulfur carrier material prepared in Comparative Example 1. When the lithium-sulfur battery is cycled from 0.1C to different rates and then returns to 0.1C, the discharge specific capacity is 815.7 mAh / g, and the capacity retention rate is greater than 81%, indicating that the prepared sulfur carrier material has excellent rate performance.

[0123] Figure 13 The cycle performance curve of the lithium-sulfur battery assembled with cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon as the sulfur carrier material prepared in Comparative Example 1. After 100 cycles at 0.5C rate, the capacity is only 308.6 mAh / g, indicating that the sulfur carrier material prepared under this condition has poor electrochemical performance.

[0124] Figure 14 The cycle performance curve of the lithium-sulfur battery assembled with cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon as the sulfur carrier material prepared in Comparative Example 2. After 100 cycles at 0.5C rate, the reversible discharge specific capacity is 511.3 mAh / g, and the capacity retention rate is 63.5%.

[0125] Figure 15 The cycle performance curve of the lithium-sulfur battery assembled with cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon as the sulfur carrier material prepared in Comparative Example 3. After 100 cycles at 0.5C rate, the reversible discharge specific capacity is 825.1 mAh / g, and the capacity retention rate is 65.3%.

[0126] Figure 16 The cycle performance curve of the lithium-sulfur battery assembled with cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon as the sulfur carrier material prepared in Comparative Example 4. After 100 cycles at 0.5C rate, the reversible discharge specific capacity is 758.4 mAh / g, and the capacity retention rate is 53.1%. Compared with Figure 13 the cycle stability of Example 1 decreases, indicating that excessive loading of cerium dioxide may lead to a decrease in the conductivity of the carbon-based material and a reduction in the pores of the porous structure, resulting in a decrease in the ability of the carrier material to adsorb polysulfides.

[0127] In summary, the composite cathode material prepared by combining an appropriate cerium dioxide content with a cobalt-based porous carbon framework can effectively promote the conversion and adsorption of polysulfides, solve the problems of poor conductivity of the elemental sulfur active material in the cathode material of lithium-sulfur batteries, the shuttle effect of polysulfides, and volume expansion in energy storage systems, and provide an effective technical implementation scheme for achieving stable cycling and excellent rate performance of lithium-sulfur batteries. Therefore, based on the above advantages, a novel composite cathode material for lithium-sulfur batteries in this invention application has good application prospects in the energy storage systems of various electronic devices.

Claims

1. A preparation method of a metal oxide-based composite cathode material for a lithium-sulfur battery, characterized in that, It includes the following steps: I. Pretreatment of carbon nanotubes Mix carbon nanotubes and an acidic solution at a mass ratio of 0.1 - 1:0.01 - 1 by stirring at room temperature, then centrifuge and wash to obtain the pretreated carbon nanotube material; II. Preparation of metal-organic framework / carbon nanotube precursor Prepare a solution by mixing the carbon nanotubes treated in step I, a dispersant, cobalt(II) nitrate hexahydrate, 2-methylimidazole, and methanol at a mass ratio of 0.01 - 0.1:0.1 - 1:1 - 3:1 - 5:0.01 - 1, stir at room temperature, and after dissolution, centrifuge and wash to obtain the metal-organic framework / carbon nanotube precursor; III. Preparation of cobalt-based nitrogen-doped porous carbon framework material Put the metal-organic framework / carbon nanotube precursor obtained in step II into a tubular furnace with a protective gas and carry out programmed heating, calcination and carbonization treatment. After heat preservation, cool to room temperature to obtain the cobalt-based nitrogen-doped porous carbon framework material; IV. Preparation of cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon carrier Prepare a solution by mixing the cobalt-based nitrogen-doped porous carbon framework material obtained in step III, cerium(III) nitrate hexahydrate, and deionized water at a mass ratio of 0.1 - 1:0.01 - 1:0.01 - 0.1, stir, add a certain volume of an alkaline solution, stir, transfer to a reaction kettle, heat, cool to room temperature, centrifuge and wash to obtain the cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon carrier; V. Preparation of lithium-sulfur battery composite cathode material Mix the cerium dioxide-loaded cobalt-based nitrogen-doped porous carbon carrier obtained in step IV and elemental sulfur at a mass ratio of 1:2 - 5, grind, transfer to a tubular furnace, carry out programmed heating in a protective gas, heat, after heat preservation, cool to room temperature to obtain the lithium-sulfur battery composite cathode material.

2. The preparation method of a metal oxide-based composite cathode material for a lithium-sulfur battery according to claim 1, characterized in that The acidic solution described in step I is one or more of hydrochloric acid, sulfuric acid, or nitric acid, with a concentration of 0.05 - 1 mol / L.

3. The preparation method of a metal oxide-based composite cathode material for a lithium-sulfur battery according to claim 1, characterized in that The dispersant described in step II is one or more of polyvinylpyrrolidone, polyethylene glycol, or sodium polyacrylate.

4. The preparation method of a metal oxide-based composite cathode material for a lithium-sulfur battery according to claim 1, wherein The stirring time described in step II is 12 - 48 h.

5. The preparation method of a metal oxide-based composite cathode material for a lithium-sulfur battery according to claim 1, characterized in that The programmed heating rate described in step III is 1 - 5 °C / min, the calcination and carbonization temperature is 600 - 900 °C, and the heat preservation time is 2 - 6 h.

6. The preparation method of a metal oxide-based composite cathode material for a lithium-sulfur battery according to claim 1, wherein The alkaline solution described in step IV is one of sodium hydroxide or potassium hydroxide, with a concentration of 0.01 - 1 mol / L, and the added volume is 10% - 40% of the mixed solution of cobalt-based nitrogen-doped porous carbon framework and cerium(III) nitrate hexahydrate.

7. The preparation method of a metal oxide-based composite cathode material for a lithium-sulfur battery according to claim 1, characterized in that The heating temperature described in step IV is 150 - 180 °C, and the heat preservation time is 12 - 24 h.

8. The preparation method of a metal oxide-based composite cathode material for a lithium-sulfur battery according to claim 1, characterized in that The programmed heating rate described in step V is 1 - 5 °C / min, the heating temperature is 150 - 155 °C, and the heat preservation time is 12 - 24 h.

9. The preparation method of a metal oxide-based composite cathode material for a lithium-sulfur battery according to claim 1, characterized in that The protective gas in the tubular furnace described in steps III and V is one of nitrogen, argon, or argon / hydrogen mixture.

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