Functional Carbon-based Composites:Design And Their Application In Lithium-sulfur Batteries

Low cost and high energy density lithium-sulfur batteries show great promise for powering next-generation energy storage system.However,the slow oxidation-reduction reaction kinetics during the charging and discharging processes and the shuttle effect of soluble lithium polysulfide seriously reduce the utilization of sulfur and hinder its commercial application.In recent years,various researches devoted to solving the problem have been proposed and have made great progress.However,it is still a challenge to develop a sulfur hosts which has a suitable adsorption ability and catalyzes effect to lithium polysulfide.In this paper,carbon-based composites decorated with inorganic compounds was successfully prepared by ion exchange,in-situ vulcanization and acid etching methods.The paper aimes to develop a sulfur hosts with excellent structure,suitable components and economical efficiency.A series of electrochemical tests and theoretical calculations revealed the synergistic mechanism of adsorption and catalysis in lithium-sulfur batteries.The specific research content is as follows:(1)The preparation and electrochemical performance of highly dispersed Sb₂S₃nanodots embedded in nitrogen/sulfur dual-doped porous carbon nanosheets.Aiming to the problems such as uneven deposition of Li₂S during discharging process.Dimethylimidazole was used as the carbon source.Leaf-like nanosheets composing of highly dispersed Sb₂S₃ nanodots embedded in highly nitrogen(18.9%)and sulfur dual-doped porous carbon skeleton(NSCNLs@Sb₂S₃)is designed for efficient immobilization and catalysis of polysulfides conversion by in-situ vulcanization and cation exchange methods.The conductive carbon matrix effectively enhances the conductivity.Nitrogen/sulfur doping species and Sb₂S₃ nanodots provide abundant stable active sites to anchor lithium polysulfides.The symmetric cells cyclic voltammetry tests,Li₂S nucleation tests and ex-situ XPS tests reveal that the Sb₂S₃nanodots can boost the redox reaction kinetics and guide the polysulfide to uniformly deposit on carbon matrixes,leading to a stable electrode structure.Benefiting from the unique design of the structure and rational componential selection,the S-NSCNLs@Sb₂S₃ electrodes deliver a stable capacity after 800 cycles with a capacity fade rate of 0.029%per cycle at 1.0 C and an excellent rate capacity of 567 mAh g^-1 at4.0 C.(2)Fe₇S₈-MoS₂ heterostructure and few layers MoS₂-embeded N-/P-doping carbon nanocapsules as effect sulfur host.In view of the poor effect of a single inorganic metal compound on the conversion of lithium polysulfide,two pairs of coupling components are designed to adsorb and catalyze the practical structure of soluble lithium polysulfide.The Fe₇S₈-MoS₂ heterostructure on the surface of the nanocapsule greatly enhanced the adsorption capacity of lithium polysulfide and accelerated its conversion kinetics.Besides,the few layer MoS₂-embeded the hollow N-/P-doping carbon nanocapsule framework provides sufficient space and abundant adsorption sites to physically and chemically confine soluble lithium polysulfide,effectively suppressing their dissolution and shuttle effect.The synergistic coupling between the Fe₇S₈-MoS₂ heterojunction catalyst and the delicately designed carbon architecture enables excellent rate capability and cycle performances for Li-S batteries.The S/Fe₇S₈-MoS₂@MoS₂-NPC cathode under high mass loading of 4.02 mg cm^-2 shows an excellent cycling performance with898.3 mAh g^-1 after 200 cycles.