Synthesis And Electrochemical Performance Study Of Co₉S₈-based Cathode Materials For Lithium-sulfur Batteries

Lithium–sulfur?Li-S?batteries have caused widespread concern due to their numerous advantages,such as high theoretical energy densities?1675 mAh/g?,environmental friendliness and low cost.The commercialization of Lithium–sulfur?Li-S?batteries is being hampered by the inherent insulation and volume expansion of sulfur,as well as the dissolution of polysulfide and resulting shuttle effect during charge and discharge.In order to overcome these problems,this paper starts from the cathode structure and materials of lithium-sulfur batteries,and uses Co₉S₈ namotubes as the matrix to prepare composite materials with conductive material and grapheme oxide.The main research contents are as follows:?1?Co₉S₈ nanotubes were prepared by a hydrothermal method,then Co₉S₈@S nanotube composites were synthesized by melt-diffusion method.The initial capacity of Co₉S₈@S nanotube composites cathode was 937 mAh/g and stabled at 650 mAh/g after100 cycles at 0.1 C.The specific capacities were 700.6,602.7,509.4,425.7 mAh/g at different C rates of 0.2,0.5,1,and 2,respectively.Hollow Co₉S₈ nanotubes can encapsulate intermediate polysulfide to weaken shuttle effect in physically and chemically;and can form effective conductive networks which can increase the overall electrical conductivity.?2?Co₉S₈@C composites were prepared with different concentrations of TEOS,resorcinol,formaldehyde.The microscopic appearance and structure were compared.Finally,the hollow core-shell structure Co₉S₈@C composites which has the highest conformity were prepared at the reactant concentration TEOS?0.62 ml?,resorcinol?0.125 g?,formaldehyde?0.17 ml?.The effects of two different sulphurization methods on the electrochemical performance of the battery were compared and the following conclusions were obtained.The melt-diffusion method is superior to CS₂ sulphurization method.The initial capacity of Co₉S₈@S@C composites cathode was 873.4 mAh/g and stabled at 360 mAh/g after 100 cycles at 0.1 C.The coulombic efficiency is relatively stable and remains above 98%.The specific capacities were 915,611,420,362,304mAh/g at different C rates of 0.1,0.2,0.5,1,and 2,respectively.More importantly,when the C rate return to 0.1,the specific capacity of the Co₉S₈@S@C nanotube composites electrode is restored to 550 mAh/g.?3?Graphene oxide was prepared by modified hummers,then the Co₉S₈@S@GO composites is ultrasonically treated.The initial capacity of Co₉S₈@S@GO composites cathode was 1036 mAh/g and stabled at 1023 mAh/g after 100 cycles at 0.1 C.the capacity retention rate is 98.7%.The initial capacity of Co₉S₈@S@GO composites cathode was 667 mAh/g and stabled at 631 mAh/g after 200 cycles at 1 C.The capacity retention rate is 94.6%.This shows that the battery has good stability.This is because the three-dimensional open structure of graphene oxide provides more paths for electron transport,and there is a certain gap between graphene oxide sheets to facilitate rapid and sufficient infiltration of the electrolyte and accommodate more elemental sulfur.