Synthesis Of MS₂（M=Sn、Mo）@nitrogen-doped Carbon Micro-nanofibers/Li₂S₆ Composite Electrodes And Research On Their Electrochemical Behaviors

Lithium-sulfur（Li-S）batteries are considered as next-generation energy storage devices due to their high theoretical energy density(2600 Wh kg^-1),rich sulfur content,and low cost.However,the industrialization of Li-S batteries is blocked by the serve“shuttle effect”of polysulfides（Li₂S_x,4≤x≤8）produced during the charge-discharge processes with high sulfur loading(≥3 mg cm^-2),resulting in the low utilization rate of active materials,the invalid of lithium metal anode and poor electrochemical performance.This paper explores the above issues,and the works are as follows:The traditional electrodes with high sulfur loading which were prepared by scraper coating technique were prone to crack during the drying processes.In this work,polyacrylonitrile（PAN）,zeolitic imidazolate framework‐8（ZIF-8）and N,N-dimethylformamide（DMF）selected as the spinning solution precursor was used to prepare PAN/ZIF-8 composite micro-nanofibers by the electrospinning technology.After carbonization,nitrogen-doped carbon micro-nanofibers（N-CNFs）with three-dimensional porous structure were synthesized.Based on the Li₂S₆ solution,the electrochemical behaviors of N-CNFs/Li₂S₆ composite electrode with high sulfur loading（sulfur loading:4.74 mg）were investigated.The results illustrated that the functional N-CNFs significantly enhance the electronic conductivity of micro-nanofibers,reduce the internal resistance of the electrode,and improve the utilization rate of active materials.Meanwhile,nitrogen doping increases the active sites of traditional carbon fiber materials,effectively inhibits the"shuttle effect"of polysulfides and decreases the corrosion of the metal lithium negative electrode during the electrochemical reaction.Based on the above-mentioned optimized electrospinning processes to prepare three-dimensional N-CNFs,the transition metal sulfides—SnS₂ was coated on the surface of N-CNFs by combining with hydrothermal technology（SnS₂@N-CNFs）.Electrochemical behaviors of SnS₂@N-CNFs/Li₂S₆ composite electrode were studied.The results show that SnS₂@N-CNFs/Li₂S₆ composite electrode effectively inhibits the"shuttle effect"of polysulfides and improves the utilization of active materials and cycle stability of cells with high sulfur loading of 7.11 mg.The experimental results demonstrate that the first cycle of the cell has a discharge specific capacity of 1010.3mAh g^-1（7.18 mAh）at 0.2 C.After 150 cycles,the capacity decay rate of the cell is0.08%per cycle.The excellent electrochemical performance of SnS₂@N-CNFs/Li₂S₆composite electrode can be attributed to the synergistic effect of three-dimensional porous structure of composite micro/nanofiber and SnS₂ on polysulfides,which improves the discharge capacity of the cell.During the research,it was found that final product Li₂S₂/Li₂S of electrochemical discharge reaction were deposited on the surface of the composite micro-nanofibers with the high sulfur loading system,resulting in the poor conductivity of the composite fiber membrane and the restriction of adsorption sites.Therefore,ivy-structured MoS₂@N-CNFs micro-nanofibers were prepared by combining with electrospinning and hydrothermal synthesis technology,and the electrochemical behaviors of MoS₂@N-CNFs/Li₂S₆ composite electrode were studied with high sulfur loading of7.11 mg.Cyclic voltammetry measurement results show that ivy-structured MoS₂@N-CNFs accelerates the electrochemical reaction kinetics and reduces the polarization of the cell.After 250 cycles,discharge specific capacity is 747.3 mAh g^-1 at 0.2 C,effectively improving cycle life of the cell.Based on X-ray photoelectron spectroscopy（XPS）and density functional theory calculation（DFT）,it is shown that the"synergy effect"of MoS₂ and N-CNFs inhibits the migration of polysulfides,reduces the charge transfer resistance of the battery and increases the utilization rate of the active material.