Design Of Hierarchically Carbon Materials For Lithium-Sulfur Batteries

Lithium-sulfur(Li-S)batteries are considered as the promising candidates for next generation rechargeable batteries due to the high theoretical energy density(2600 Wh kg-1)and the low cost of sulfur.However,the application of Li-S batteries is still limited by several issues associated with cathodes,including the insulating nature of sulfur,the large volumetric change between sulfur and Li2S and the dissolution and migration of polysulfide intermediates(i.e.,"shuttling effect").Since many Li-S cathodes are made with low sulfur content(<70 wt%)and low areal sulfur loading(<2 mg cm-2),there is a growing awareness that how to exert the energy density advantage is critical to the patricidal Li-S batteries.To improve the energy-density of Li-S batteries,self-supported cathodes have attracted increasing attention during the past few years,since self-supported structures can avoid the use of polymer binders and conductive agents.A common approach is to use carbon-based films(e.g.graphene foam and carbon nanofibers/nanotubes paper)to fabricate the free-standing C/S composite films which can be directly used as the Li-S cathodes.However,most of the carbon-based films do not possess some key characteristics(e.g.,high surface area and high polysulfide-binding ability)for the suppression of polysulfide shuttling.To address thesis issues,this work focus on the design of the self-supported electrodes by combining electrospinning technique and hard-templating methods.The as-prepared hierarchically structured carbon materials were successfully used to construct high energy-density Li-S batteries.The main research results were summarized as follow:(i)Two kind of hierarchically structured carbon materials including the vertically oriented graphene coated carbon nanofibers(CF@VOG)and hollow carbon nanofibers(HCF)were successfully synthesized by the eletrospunning method.CF@VOG can combine the advantages of 1 D,2 D and 3 D structures.Compared with carbon nanofibers,the large cavity of HCF can accommodate the volume change of the active materials.(ii)We developed a facile one-pot sol-gel method to controllable hollow carbon spheres based on the co-assembly of polybenzoxazine and silica.To improve the space utilization of hollow carbon nanofibers,yolk-shell structured carbon fibers(i.e.,hollow carbon spheres encapsulated in hollow carbon nanofibers)with high surface area and favorable conductive networks were further synthesized via a scalable templated electrospinning strategy.(iii)Yolk-shell structured carbon fibers were successfully used to fabricate self-supported carbon/sulfur cathode for high-sulfur-loading Li-S batteries.We demonstrated that the as-prepared yolk-shell structure not only allows carbon fibers to inherit the advantages of their structural units,but also can bring an interesting synergetic enhancement to further improve their performance toward sulfur loading and polysulfide binding.The self-supported cathode with 70 wt%and 4 mg cm-2 of sulfur delivers high capacity,favorable rate capability,and excellent cyclic stability.Even with an areal sulfur loading of 16 mg cm-2,the self-supported cathode still exhibits high sulfur utilization and good capacity retention.