The Preparation And Application Of Porous Honeycomb-like Carbon Nitride And Cobalt Sulfide For Lithium-Sulfur Batteries

Lithium-sulfur?Li-S?battery has attracted increasing attention by researchers,which has many advantages,such as high specific capacity and energy density,the content of sulfur is abundance in the earth,low cost and environmental friendliness.Li-S battery is promising for next generation energy storage system.However,it has been found by researchers that Li-S battery also has many problems which limit their further development and application.The active material of sulfur is insulating,severe shuttle effect of lithium polysulfides during the cycling,obvious volume expansion and dendrites growth in lithium anode.To solve these problems,the researchers have proposed various approaches,such as design and develop various sulfur host materials,researching electrolytes,membranes,lithium anodes and so on.Developing host materials is regarded as one of the most effective methods to solve these problems.In view of the problems in the above-mentioned of Li-S battery,this thesis focused on the design and synthesis of sulfur host materials,the main contents are listed as follows:?1?SiO₂ was used as template which treated by polymine,and cyanamide as raw maternal during the formation of g-C₃N₄,combined with subsequent calcination method to prepare porous honeycomb-like C₃N₄?PHCN?material,and successfully synthesized the PHCN/rGO composite utilized it as sulfur hosts for Li-S batteries.PHCN/rGO can improve the conductivity of whole the electrodes,PHCN provides active sites adsorb lithium polysulfides by forming chemical bonding Li-N,the specific porous structure could provide physical adsorption and alleviate volume expansion,rGO could provide fast ion/electron pathways effectively.PHCN/rGO/S cathode exhibited an initial discharge capacity of 1061.1 mAh g'1 at 0.2 C and outstanding rate performance(495.1 mAh g^-1)at high current density of 5 C.And the specific capacity still retained 519 mAh g^-1 after 400 cycles at 1 C.Even at high sulfur loading(4.3 mg cm^-2)the capacity decay rate was only 0.16%per cycle at 0.5 C for 200 cycles.Benefiting from the unique structure of C₃N₄ and the synergistic effect of conductive rGO,the electrodes display excellent electrochemical performance.?2?Amorphous Co_1-xS precursor was synthesized via a solvothermal method,hollow Co_1-xS/C composite was prepared by coating polypyrrole at first and then calcination.Coi-xS/rGO composite was composed of Co_1-xS and graphene.Compared with other metallic compounds,metal sulfide has good conductivity.Moreover,the chemical bonds of metal sulfide is similar to those of lithium polysulfides,it is easy to interaction with lithium polysulfides so that effectively improve the utilization of active materials and retard the shuttle effect.In addition,conductive carbon materials provide good pathways for electrons and ions.The outstanding electrochemical performance even at large current density and long cycle of the electrodes could be mainly be attributed to the advantages of unique stuctrued materials.