Sulfur-rich Copolymers Grafted Graphene Sheets:Preparation And Application In Li-S Batteries

High-density and stable energy storage systems are urgently needed to satisfy the increasing market demands due to the rapid developments of electric vehicles,portable electronic devices and smart grids.As a new type of secondary battery,lithium-sulfur(Li-S)batteries have drawn considerable attention in both academic and industrial communities due to the high theoretical energy density(2600 Wh kg-1)as well as the high natural abundance,low cost,and good environmental compatibility of sulfur cathode.However,the practical uses of Li-S batteries are still handicapped by a number of problems,such as the “shuttling effect”,sluggish redox kinetics of polysulfide intermediates and volume expansion of sulfur during charging/discharging.Collectively,the shuttling effect and cathode pulverization originated from volume expansion of sulfur result in deteriorated cycling stabilities,the sluggish electrochemical redox kinetics leads to poor rate capabilities.Although the researchers have made tremendous attempts and put forward various promising solutions to solve the fundamental problems associated with Li-S batteries,the practical demands of contemporary batteries have not yet to be satisfied.In this paper,we propose an in-situ copolymerization strategy to solve aforementioned problems of Li-S batteries by covalently grafting sulfur-rich copolymers onto graphene.Firstly,the isopropenyl-rich graphene composites(RGO-g-IDBI)were obtained by modification of graphene using a 3-isopropenyl-?,?-dimethylbenzyl isocyanate(IDBI)monomer,then the “inverse vulcanization” methodology between elemental sulfur and isopropenyl group of IDBI was employed to prepare sulfur-rich copolymer covalently grafted graphene(RGO-g-poly(S-r-IDBI)),finally,the RGO-g-poly(S-r-IDBI)cathodes exhibit excellent electrochemical performance.As a result,RGO-g-poly(S-r-IDBI)cathodes show fast redox kinetics(such as small polarization,low overpotential,fast charge transfer and Li+ transmission),high sulfur utilization rate(the initial specific capacity is 1065 m A h g-1,which is 10% higher than elemental sulfur cathode at 0.1 C),enhanced rate capability(688 m A h g-1 for RGO-g-poly(S-r-IDBI)cathode vs.400 m A h g-1 for elemental sulfur cathode at 1 C),and outstanding long-term cycling stability(a capacity decay of 0.021% per cycle,less than one tenth of that measured for elemental sulfur cathode).Finally,we find that the excellent electrochemical performance of RGO-g-poly(S-r-IDBI)cathodes are closely related to the structure design of our cathode materials by further analysis of the batteries.(1)Uniform grafting of sulfur-rich copolymers onto conductive graphene scaffold facilitates the transport of electrons and Li+ in RGO-g-poly(S-r-IDBI)cathodes(Li+ diffusion coefficients were increased by nearly one order of magnitude when compared to the analogues prepared using elemental sulfur)due to the good compatibility of sulfur-rich copolymers and electrolyte.Thus accelerating the redox reaction kinetics,achieving high sulfur utilization and enhanced rate performance of sulfur-containing cathodes.(2)The covalent linkages between sulfur-rich copolymer networks and graphene framework can confine the polysulfides inside the cathodes and further suppress the dissolution and shuttle effectively.At the same time,due to the high mechanical strength of graphene and the flexibility of sulfur-rich copolymers,RGO-g-poly(S-r-IDBI)composites exhibit high stability of cathode and molecular structure,which can alleviate the volume expansion of sulfur and prevent the pulverization of the cathode effectively.Thus,RGO-g-poly(S-r-IDBI)cathodes show ultrastable cycling performance.