Preparation Of Zinc Sulfide-Sulfur@Nitrogen-Doped Carbon Composite Materials And Their Lithium-Storage Performance Studies

The energy crisis and environmental problems rendered human beings to accelerate the development of renewable clean power sources.Lithium-sulfur batteries have received widespread attention due to their low cost,environmental friendliness,high theoretical specific capacity and energy density.However,the insulation nature of elemental sulfur and its discharge products lithium sulfide,determines that its extremely low electronic conductivity.At the same time,the"shuttle effect"from polysulfides during the charge and discharge process will cause irreversible loss of capacity.Moreover,volume expansion of sulfur particles during the discharge process would lead to destroying of positive electrode structure.These problems severely hinder the practical application of lithium-sulfur batteries.This paper aims to improve the electrochemical performance of sulfur cathodes through the physical limitations of the microporous structure of porous carbon materials,in-situ chemical adsorption and catalysis of zinc sulfide,and polypyrrole coating.The research content of this paper is as follows:(1)Using glucose as the carbon source,urea as the nitrogen source,and Na Cl crystal as the template,the nitrogen-doped porous carbon material is synthesized by freeze-drying and high-temperature carbonation,and the carbon/sulfur composite material is prepared by the molten sulfur loading method.By controlling the mass ratio of glucose to urea,the nitrogen doping amount of the porous carbon matrix is optimized.The synthesized carbon materials were characterized by XRD,SEM,BET and infrared spectroscopy,and the morphology and pore structure distribution of the carbon materials were analyzed.The electrochemical performance of S@C composite material was tested,and it was concluded that the electrochemical performance of S@NPC-40sample was the best.The S@NPC-40 composite specific can deliver an initial capacity of 1029 m Ah g^-1 at 0.1C rate,and the capacity retention rate is 39.9%after100 cycles.The discharge specific capacity at the rates of 0.1C,0.2C,0.5C,1.0C and2.0C are 959.4,605.3,476.1,336.6 and 176.4 m Ah g^-1,respectively.The results indicate that the modification of elemental sulfur by the nitrogen-doped porous carbon material NPC-40 can improve the rate capability and cycling performance of the sulfur cathode material to a certain extent.(2)The porous carbon NPC-40 was modified with ZnS by a hydrothermal method,and the ZnS-S@NPC-40 composite material was synthesized after sulfur loading by melting method.The modification effect of transition metal sulfide on the sulfur cathode was explored.The ZnS-S@NPC-40 composite material modified by ZnS can deliver an initial capacity of 1208 m Ah g^-1 at 0.1C rate,and displays a retention rate of49.2%after 100 cycles.The discharge specific capacity at the rates of 0.1C,0.2C,0.5C,1.0C and 2.0C are 1123.2,753.4,573.2,460.4 and 290.9 m Ah g^-1,respectively.The results indicate that ZnS in the ZnS-S@NPC-40 composite material not only has a chemical adsorption effect on polysulfides,which effectively inhibits the shuttle effect of polysulfides,but also has a certain catalytic effect to promote the reaction kinetics of polysulfides.As a result,the rate capability and cycling stability of the battery are significantly improved.(3)ZnS-S@NPC-40@PPy composite material was prepared by coating the surface of ZnS-S@NPC-40 composite material particles with polypyrrole,and the obtained composite material was analyzed by XRD,SEM and electrochemical performance.The composite can deliver an initial capacity of 1158 m Ah g^-1 at 0.1C rate,and retains 62.8%of the initial capacity after 100 cycles.At the rates of 0.1C,0.2C,0.5C,1.0C and 2.0C,the specific capacities are 1205.8,799.7,623.5,514.8 and 401.6m Ah g^-1,respectively.The results show that the polypyrrole coating can enhance the electronic conductivity of the ZnS-S@NPC-40 core,further reduces the loss of polysulfides during the charging and discharging processes.As a result,the rate capability and cycling performance of the composite are significantly improved furthermore.