Preparation Of Co/N Doped Porous Carbon Composites And Its Application In Lithium Sulfur Batteries

Lithium-sulfur batteries（LSBs）are considered as a high specific energy secondary battery with great development prospects due to its high theoretical energy density（2600Wh/kg）,abundant active material sulfur reserves,low price and easy availability,and environmental friendliness.However,lithium sulfur batteries are faced with problems such as the slow conversion reaction kinetics of sulfur species on the positive side and the shuttle effect of polysulfides,which seriously hold off their commercialization.In recent years,various metal-based carbon materials with high conductivity and large specific surface area have been developed to promote the redox kinetics of sulfur and the adsorption of polysulfides.In particular,cobalt-based catalyst showed a better polysulfides adsorption–conversion ability,showing great potential for the application in LBSs.The work of the paper is divided into three parts:（1）CoSn（OH）₆ was prepared by coprecipitation method.Through a series of treatments such as dopamine coating,high temperature calcination and acid etching,the subnanometric Co Sn clusters into N-doped carbon nanoboxes（SN-Co Sn@C NBs）.The hollow and porous structure were able to host more sulfur,and promoted the charge transfer while buffering the potential volume expansion during the charge-discharge cycles.The Co/Sn active sites strengthened the chemisorption of polysulfide species and accelerated the redox reaction dynamics.The optimal SN-Co Sn@C NB composite electrode due to its unique structural characteristics and composition advantages,exhibited a discharge capacity of 1168.3 m Ah/g at 0.2 C current density.Even at 5 C current density,still has 557.3 m Ah/g initial discharge capacity,and 0.056%fading rate per cycle is available for 800 cycles.（2）To simplify the carbon material preparation process,Sub-nano Co clusters embedded 3D interconnected carbon material（Co-NFC）was prepared as a highly efficient sulfur cathode for Li-S batteries via a one-step pyrolysis method in the molten salt medium.The sulfur host with such structure can endow the material with good electrical conductivity and high specific surface area,and highly dispersed cobalt clusters provide active sites for the adsorption catalysis of polysulfides,making it have the dual functions of physical limitation and chemical adsorption.When discharging at0.2 C current density,the initial discharge specific capacity is 1027.5 m Ah/g.The capacity decay rate per cycle is only 0.061%of 500 cycles at 1 C.The preparation of metal-based porous carbon by a simple and convenient method provides a new idea to improve the performance of lithium-sulfur batteries.（3）In order to further enhance the adsorption catalytic activity of the materials,perylenequinone（6,12-dihydroxyperylene-1,7-dione,DPD）was adsorbed onto the carbon-supported Co substrate byπ-πbonds（DPD/Co-N-C）,utilized to improve polysulfide chemisorption and conversion on Co containing Li-S battery cathodes.The prepared carbon materials not only have cobalt cluster sites,but also the rich oxygen-containing functional groups in DPD can form Li-O bonds with polysulfides to covalently fix them,alleviate the shuttle effect and improve the sulfur utilization.The newly prepared DPD/Co-N-C composite provides dual chemisorption sites for sulfur fixation and catalytic conversion,when discharging at 0.2 C current density,the initial discharge specific capacity is 1096.5 m Ah/g,and the capacity decay rate per cycle is only 0.041%after 800 cycles at 1 C.The synergistic effect of covalent sulfur fixation and chemical sulfur fixation provides a new strategy for the development of high-efficiency sulfur cathode for lithium sulfur batteries.