Synthesis And Electrochemical Performance Of Li₂S Cathode Materials For Li-S Batteries

To meet the effective,environmental,economic requirement of energy system for future society,the research and development of new-generation of green energy storage materials and devices have been the common concerns of scientists from all over the world.Among all the energy storage systems,Lithium/sulfur(Li-S)battery seems to be one of the most promising systems,due to the high energy density,low cost and environmental friendliness.However,the insulate nature of cathode materials,shuttle effect and volume change during the charge/discharge processes and dendrite growth of Li anodes result in fast capacity decay and battery failure,which hinder the industrialization of Li-S batteries.Coupled with non-lithium source anode,Li2S acting as cathode can effectively improve the safety of cell.However,there are still some problems such as insulation of cathode materials and the dissolution of polysulfides.In this paper,Li2S composite cathodes with coating structure were designed and synthesized,and nanostructure strategy was also adopted to improving the electro-conductivity of cathodes.The main research contents are as follows:(1)Carbon coated Li2S cathodes were synthesized by solvent evaporation and annealing treatment.According to the solubility difference between PVP and Li2S in THF,with the evaporation of THF,Li2S precipitates fist,followed with the precipitation of PVP.The electrochemical tests reveal good cyclic performance:the electrode delivers an initial discharge capacity of 865.4 mAh ·g-1 at 0.2C,after 100 cycles,the capacity decay to 593.1 mAh· g-1,the coulombic efficiency declines to 93%.(2)Core-shell nanostructured Li2S cathodes(Li2S@C)were also synthesized by combining solvent evaporation and chemical vapor deposition.Nanoscale PS spheres were used as nucleating agents,The PS spheres decomposed and evaporated during the annealing process,so there would be spare space inside the nano-Li2S@C composite.The deposited carbon shell can not only improving the electroconductivity of Li2S,but also effectively encapsulate the polysulfides,The as-prepared nano-Li2S@C cathodes showed good cyclic performances.Furthermore,the nanostructure of composites guarantee short diffusion distance of Li-ions and elevated rate performance.The electrode delivers an initial discharge capacity of 1121.9 mAh·g-1 at 0.2C,after 100 cycles,the capacity remains 804.2 mAh·g-1,the capacity retention is 71.7%,and the coulombic efficiency remains higher than 96%.Even at higher current densities of 1C and 2C,it still exhibits high capacities of 760.7 mAh·g-1 and 682.9 mAh·g-1.(3)Nanoscale Li2S cathodes with egg-like structure were synthesized by the same method with nanoscale Nanocarbon particles as nucleating agents.The synergistic effect of coated structure and nanocrystalliation design result in good cyclic and rate performance of egg-like Li2S cathodes.The electrode delivers an initial discharge capacity of 1083.5 mAh·9-1 at 0.2C,after 100 cycles,the capacity remains 893.6 mAh·g-1,the capacity retention is 82.5%,and the coulombic efficiency remains higher than 96%.Even at higher current densities of 1C and 2C,it still exhibits high capacities of 763.5 mAh·g-1 and 625.8 mAh·g-1.