Synthesis And Electrochemical Performance Of Metal-Chalcogen Compounds Based Li-S Cathode Materials

With the demand for energy is more than ever.The non-renewable nature of fossil fuels and environmental pollution have urged people to turn their attention to find new energy storage systems.Lithium-sulfur（Li-S）batteries are recognized as one of the most promising energy storage systems for the next generation because of their low price and high energy density.However,defects such as sulfur poor conductivity and shuttle effect during the electrochemical reaction process in Li-S batteries seriously imped their practical applications.In this degree thesis,four different structures based on Metal-Chalcogen compounds were designed and prepared for application in Li-S battery cathodes,and their effects on improving sulfur utilization,accelerating lithium ion diffusion and inhibiting lithium polysulfide（LiPSs）shuttle were clarified.（1）The coaxial nanocomposite sulfur host cathode material PPy@Mn3O4 was prepared by a simple stepwise method.The polypyrrole（PPy）with hollow structure conducive to the sulfur-confinement,shorts electron/ion transport distance and improves the sulfur utilization;meanwhile,the excellent LiPSs anchoring ability of Mn3O4 mitigates the shuttle effect and improves the battery cycling stability.As a result,the Li-S cell with PPy@Mn3O4-S cathode releases an initial discharge specific capacity of up to 1419.9 mAh g-1 at 0.1 C,continually a 50%retention rate and a low decay rate of 0.062%per cycle over 800 cycles at 1 C.（2）The 1T-MoS₂ hollow microspheres were synthesized by a Hydrothermal method,sequanly the 1T-MoS₂-S@PPy prepared base on 1T-MoS₂ used as a sulfur composite,which combined the high conductivity of PPy and the adsorption-catalytic conversion function of LiPSs of 1T-MoS₂ to improve the electrochemical performance of Li-S batteries.Therefore,the Li-S battery based on 1T-MoS₂-S@PPy delivered a high initial discharge specific capacity（1434 mAh g-1 at 0.1 C）,good rate performance（767 mAh g-1 at 3 C）and longterm cycling stability（decay rate 0.051%per cycle over 800 cycles at 1 C）.（3）A composite cathode interlayer material（CNTs@CeO₂）with intertwined CNTs and CeO₂ was fabricated by a simple solvothermal-self assembly method.The high-speed conductive network woven by CNTs facilitates electron rapid transport,the abundant active sites in CeO₂ cavities ensure the adsorption and catalytic conversion of LiPSs,the hollow structure of CeO₂ conducive to the rapid penetration of electrolyte and accelerates the transmission of Li ions,significantly enhancing the conversion of LiPSs and sulfur utilization.Benefiting from this,the Li-S cells with CNTs@CeO₂ interlayer delivered initial discharge specific capacity is as high as 1040.6 mAh g-1 at 0.2 C,and the capacity decay rate over 1000 cycles at 1 C is only 0.064%per cycle.（4）The hollow spherical CeO₂ was uniformly embedded in nitrogen-doped carbon nanofibers by electrostatic spinning to prepar a fibrous interconnected three-dimensional composite NCF@CeO₂ cathode interlayer,in which the skeleton provides a throughconducting grid to accelerate electron transfer at the interface of nitrogen doped carbon and CeO₂,the larger specific surface area guarantees sufficient interfacial reactions for sulfur fast-redox reactions,while the CeO₂ with hollow structure and abundant oxygen defects provides strong physicochemical adsorption of LiPSs.Meanwhile,the robust structure guarantees the structural integrity of the electrode,also can avoid the side reactions between electrolyte and catalyst by the embedded structure,achieving a balance between LiPSs shuttle inhibition and Li ions diffusion.The Li-S battery with this interlayer realeased a high initial discharge specific capacity of 1072.9 mAh g-1 at 0.2 C and a capacity decay rate of only 0.063%per cycle after 1000 cycles at 1C.