Design And Synthesis Of Hollow Porous Carbon?materials And Their Application In Lithium-Sulfur Batteries

Rechargeable lithium-sulfur(Li-S)batteries with the theoretical specific energy(?2600 Wh kg-1)have attracted considerable attention due to its favorable prospect in future high-energy storage systems.Although sulfur has the advantages of high abundance,low cost,and nontoxicity,several inherent problems caused by the sulfur cathodes still prevent the practical applications of Li-S batterie.Firstly,the low conductivities of sulfur,the shuttling of the soluble polysulfide intermediates between cathode and anode,and the large volumetric change(?80%)between sulfur and Li2S lead to the low sulfur utilization and poor cycling stabilities.Secondly,most of sulfur cathodes are usually obtained with relatively low sulfur loading(<2 mg cm-2)and/or low sulfur content of the cathodes(?70 wt%),which cannot meet the demand for high-energy-density Li-S batteries.Finally,the imbalance between the areal sulfur loading and electrode thickness makes high-sulfur-loading high areal capacities accompanied with low volumetric energy densities.Therefore,how to construct high-performance Li-S batteries with high areal and volumetric capacities,long cycle life,and good rate capability still remains a great challenge.To address these issues,this work focuses on the high-performance Li-S batteries based on hollow porous carbon materials.The research covers three main parts and major results have been summarized as follow:(i)Carbon/sulfur cathodes;We developed a facile silica-assisted coating strategy for the controllable synthesis of(N-HPCB)N-HPCB with a large specific surface area and high nitrogen content.When used as the host material for sulfur cathode,the S/N-HPCB composite exhibited high capacity,rate capability and cycling stability.We also demonstrated that bowl-like structure can improve interconnection and packing density of-hollow porous structure,leading to the significantly enhanced electrochemical and volumetric energy density.(ii)Self-Supporting sulfur cathodes:We designed a novel yolk-shell nanostructure,graphene encapsulated in hollow mesoporous carbon nanosheet(G@HMCN),as a promising sulfur host to construct the self-supporting cathodes.With high surface area,large pore volume,and excellent dispersibility,G@HMCN is able to generate 2D carbon/sulfur composite nanosheets(G@HMCN/S)with high sulfur content.The co-assembly of G@HMCN/S and graphene further formed a free-standing,flexible,and closely packed G@HMCN/S-G hybrid paper with high sulfur content and high sulfur loading.When directly used as the cathodes,G@HMCN/S-G exhibited significantly enhanced performance for high-energy-density Li-S batteries.(iii)Modifying Li-S battery configuration:We successfully consturted a polysulfide barrier layer on the cathode side of separator by using G@HMCN as the barrier materials.The G@HMCN barrier layer can not only further alleviate the shuttle effect by the physical obstruction and/or adsorption of polysulfides,but also can act as the expanded current collectors to reduce the resistance while increase the utilization of active materials.Benefiting from G@HMCN barrier layer,the the capacity and cyclic performance of Li-S batteries with high sulfur content and high sulfur loading were significantly improved.