Preparation Of MOF-Derived Composites And Their Application In Lithium-Sulfur Batteries

With the rapid development of technology and society,various electronic equipment and power cars have increasingly higher requirements for energy storage systems.At the same time,increasing depletion of non-renewable energy and severe environmental issues make it increasingly urgent to develop a safe and efficient energy storage system.Since its commercialization,lithium-ion batteries（LIBs）have been favored because of their stability,long cycle life,and portability.However,the low current theoretical capacity and high cost of traditional cathode materials for LIBs limit the further commercial application.In recent years,elemental sulfur has received widespread attention due to its high theoretical capacity and energy density,abundant resources,environmental friendliness and low cost.However,when it is used as a cathode material for lithium-sulfur batteries（LSBs）,it still faces many challenges.For example,the poor conductivity of the active material sulfur and the final discharge products Li₂S/Li₂S₂,the significant volume change of the material during the cycles caused by the difference in material density,and the“shuttle effect”caused by the migration of lithium polysulfides between the positive and negative electrodes.In order to solve the above problems,we rationally design cathode composite material to effectively improve the electrochemical performance of LSBs.（1）ZIF-67 diamond dodecahedron precursor with uniformly dispersion and particle size was prepared by a simple room temperature static method.The hollow NiO-NiCo₂O₄ material was prepared by reflowing and calcining the ZIF-67 precursor.Polypyrrole particles were coated onto the surface of the NiO-NiCo₂O₄ material in an ice-water bath environment to obtain the NiO-NiCo₂O₄@PPy material.The S/NiO-NiCo₂O₄@PPy composite material was prepared by combining the NiO-NiCo₂O₄@PPy material with elemental sulfur.The materials were characterized by XRD,FTIR,XPS,TG,BET,FESEM,and TEM.The performance of LSBs has been effectively improved after modification.At 0.2 C,the specific discharge capacity of the S/NiO-NiCo₂O₄@PPy composite in the first cycle is 963 mAh g^-1,and the discharge capacity can still maintain 641mAh g^-1 after cycling,which is much higher than that of pure sulfur electrode.The material still has good cycling performance even at a high current density of 1 C.The effect of materials on improving battery performance was further analyzed by testing the material’s adsorption performance and observing the separators and electrodes before and after charging/discharging.The results show that the NiO-NiCo₂O₄ material can inhibit the dissolution and diffusion of lithium polysulfides through chemical interactions,thus improving the“shuttle effect”.The introduction of polypyrrole material can effectively improve the electrical conductivity of the composite material,making the electron transmission more smooth and unobstructed during the reaction.The hollow structure of material can improve the volume change problem of elemental sulfur during charging/discharging.Therefore,the NiO-NiCo₂O₄@PPy composite has played a positive role in improving the electrochemical performance of LSBs.（2）The ratio of cobalt nitrate and 2-methylimidazole was changed to obtain a smaller ZIF-67 precursor.The ZIF-67 material was mixed with graphene oxide（GO）prepared by the improved Hummer method,and the GO material was reduced to rGO material by refluxing and further heating.The obtained product was refluxed with nickel nitrate to prepare LDH/rGO material.The LDH/rGO material was mixed with a CS₂ solution in which sulfur was dissolved to prepare S/LDH/rGO composite material.The materials were characterized by XRD,Raman,BET,TG,FESEM,and TEM.The first-cycle discharge capacity of the S/LDH/rGO composite exhibits 958 mAh g^-1 at 0.2 C and 730 mAh g^-1 at 1 C,which is greatly improved compared to pure sulfur electrode.At the same time,photos of the separators and electrodes before and after the cycles were used to further explore the reasons for the excellent electrochemical performance of the composite material.The research shows that lithium polysulfides can be well adsorbed because of the presence of LDH material and the larger specific surface area of the material,thus improving the utilization of elemental sulfur.The rGO material can speed up electron transport and make the reaction smooth.Therefore,the S/LDH/rGO composite can effectively improve the electrochemical performance of LSBs.