Electrochemical Properties Of Carbon And Nitrogen Compounds As Sulfur Carriers For Lithium-sulfur Batteries

Lithium-sulfur（Li-S）batteries are a new generation of energy storage systems with high theoretical energy density.However,the low electrical conductivity of sulfur,volume expansion,and the shuttle effect of lithium polysulfides（Li PSs）limit its commercialization.In view of the above problems,designing a rational network structure and enhancing its surface chemisorption capacity can significantly improve the electrochemical performance of Li-S batteries.In this paper,carbon nitride-based cathode materials with different mass ratios are prepared by thermal reduction and Ullmann method using dicyandiamide and melamine chloride as raw materials,and their performance in Li-S batteries is studied.Using dicyandiamide as raw material,protonated carbon nitride/carbon nanotube（p CN/CNTs）composites are prepared by thermal reduction method and dissolution-precipitation method.The materials have interwoven CNTs networks to realize the physical adsorption of Li PSs,and the uniformly precipitated p CN nanosheets form a strong chemical interaction with Li PSs,which can effectively inhibit the shuttle of Li PSs.When the mass ratio of p CN:CNTs is1:1,the composite（marked as CC1:1）shows good electrochemical performance as sulfur carrier in Li-S battery.The initial discharge specific capacity of the cathode is 1201.0 m Ah·g^-1 at 0.5C,the capacity retention rate is 60%after 500 cycles,and the capacity decayed by 0.08%per cycle,showing good cycle stability.A novel carbon nitride（C₃N₃）material is prepared by Ullmann method using cyanuric chloride and copper foil as raw materials.The results showed that C₃N₃ dissolved in concentrated sulfuric acid（98%）and formed a solution with a concentration of 300 mg·m L^-1.It is shown that electrostatic repulsion and sulfate/bisulfate intercalation are two important synergistic factors for the successful dissolution of C₃N₃.The optimized C₃N₃（d C-S/S）cathodes recrystallized in concentrated sulfuric acid showed better electrochemical performance for Li-S batteries.The large number of pyridine nitrogen sites contained in it can effectively adsorb Li PSs and reduce the shuttle of Li PSs.The initial discharge specific capacity of the cathode is 448.0m Ah·g^-1 at 0.5C,and 392.6 m Ah·g^-1 after 500 cycles,and the capacity retention rate is 85.7%,showing good cycle performance.The C₃N₃ materials prepared by the Ullmann method can effectively improve the chemical adsorption of Li PSs,and provide a design idea for improving the chemical sulfur fixation of carbon nitride-based composites.Using C₃N₃ and CNTs as raw materials,new recrystallized carbon nitride/carbon nanotube（d-C₃N₃/CNTs）composites are prepared by dissolution and reprecipitation method.When the mass ratio of d-C₃N₃:CNTs is 1:1,the composites（denoted as d CC-1:1）exhibited better electrochemical performance for Li-S batteries.The interwoven CNTs network physically adsorbs the soluble Li PSs,and the abundant exposed pyridine nitrogen sites in the d-C₃N₃nanosheets play a strong chemical interaction for the soluble Li PSs.The initial discharge specific capacity of the cathode is 803.4 m Ah·g^-1 at 1C,605.0 m Ah·g^-1 after 500 cycles,the capacity retention rate is 75.3%,and the decay rate per cycle is 0.049%,showing good long-cycle performance.The paper has 34 figures,11 tables,and 108 references.