Rational Design Of Transition Metal Sulfides And Their Application In Lithium-Sulfur Batteries

Due to the rapid development of secondary batteries and electric vehicles,future battery development must have the advantages of high energy density,long-cycle stability,and low cost.Lithium-sulfur batteries offer a high theoretical energy density of 2600 Wh kg-1 or 2800 Wh L-1.Due to its abundant on earth,and low cost and environmental friendlinessof sulfur,it has attracted worldwide attention.However,the practical application of lithium-sulfur batteries still meets many challenges.First,the conductivity of sulfur is particularly poor,which limits the utilization of sulfur and reduces the rate capability of lithium-sulfur batteries.Second,the shuttle effect caused by the dissolution of lithium polysulfide induces a large loss of active materials,which eventually leads to a rapid capacity reduction.Finally,the huge volume change of sulfur during the lithiation/delithiation process may damage the cathode structure and cause a downhill cycle performance for the battery in the practical scene.Consequently,it is of vital importance to modify sulfur cathode to achieve high-performance LSBs.In order to solve the aforementioned problems,researchers have conducted a lot of efforts to develop new host materials for sulfur.Recently,nanostructured carbon materials have been proposed as host materials for sulfur.Because carbon with hollow structures can provide fast electronic pathways and physically capture polysulfides,the resulting carbon/sulfur composites extend cycle lifespan and increase reversible capacity.However,considering most carbon-based materials are non-polar,the interaction with polar polysulfides is very weak and only can be used as a physical confinement to hinder the diffusion of LiPSs.In order to further enhance the affinity ability for the sulfur host to anchor LiPSs and extend the cycle life of Li-S batteries,polar host materials are welcomed,such as metal oxides,sulfides,phosphides,nitrides,and derived Mxenes.Based on Lewis acid-base coordination and polar-polar interaction with LiPSs,strong anchoring effect and excellent battery performance can be achieved.Therefore,composite materials combining carbon materials and polar materials are favourable as sulfur hosts in LSBs.1)In this paper,a template-free direct current arc discharge method was used to synthesize TiSx(1.5<x<2)@HCN(hollow carbon nanocages)as an efficient sulfur host for lithium-sulfur batteries.TiSx@HCN can accommodate the volume change of active materials and suppress the shuttle effect of lithium polysulfides.Theoretical calculations show that the defective titanium sulfide accelerates the conversion of longchain lithium polysulfides to intermediate species and promotes the redox reaction.Therefore,the hybrid cathode complex with sulfur exhibits excellent electrochemical performance.A reversible capacity of 1118.6 mAh g-1 was obtained at a rate of 0.1 C with a slow capacity decay of 0.108%per cycle over 200 cycles.This synthesis method provides a simple and effective new idea for the synthesis of nanocomposites.2)In this paper,NbSx@N-HCN was synthesized by DC arc discharge method to improve the cycling performance of Li-S batteries.NbSx@N-HCN can effectively anchor lithium polysulfides with sufficient space to buffer the volume change of active species.Therefore,it effectively improves the cycle performance of lithium-sulfur batteries.The initial discharge capacity can reach 1092.6 mAh g-1 at a rate of 0.1C,and the battery still has a high specific capacity of 683.2 mAh g-1 after 100 cycles.Finally,the performance of organic sulfides is improved by encapsulating organic sulfides in NbSx@N-HCN and then making full use of its porous structure and Li-N interaction and metal sulfides to produce physisorption and chemisorption of organosulfur discharge products,respectively.The synthesis method was further expanded,and new ideas were provided for the application of this type of material.