Rational Design Of Molybdenum-based Composite Structure And Its Application In Lithium-sulfur Batteries

Lithium-sulfur batteries have become one of the most promising energy storage systems for commercial applications due to its high theoretical energy density,low cost and environmental friendliness of cathode sulfur.However,there are still many problems in lithium-sulfur batteries.The shuttle effect caused by polysulfide seriously weakens the cycle performance of the battery,especially when the sulfur load is high.In order to break the limitations of the lithium-sulfur battery system,the design and preparation of highly stable and long-cycle lithium-sulfur batteries has become a research hotspot in the field of energy storage.Compared with the cumbersome design of sulfur hosts,low-cost and high-efficiency separator modification is considered to be a feasible solution to many problems of lithium-sulfur batteries.In this paper,the modification of the separator effectively inhibits the shuttle effect caused by the excessive diffusion of polysulfides,promotes the transport of lithium ions,accelerates the reaction kinetics of sulfur,and significantly improves the overall performance of the battery.The main research contents are as follows:(1)A novel molybdenum single atom catalyst with unique Mo/N2O2-C sites immobilized on nitrogen-doped graphene(Mo/NG)was prepared by a self-designed structure-directed template strategy.Combined with experiments and theoretical calculations,it is proved that Mo/NG can effectively inhibit the shuttle of polysulfide on the positive side and promote its efficient conversion.On the anode side,the material has suitable lithium affinity and lower lithium ion diffusion energy barrier,which helps the uniform redistribution and diffusion of lithium ions on the separator,promotes the uniform deposition of lithium ions,and effectively inhibits the formation of lithium dendrites.Benefiting from the above advantages,the lithium-sulfur battery assembled by this material exhibits excellent electrochemical performance.(2)Bimetallic selenides exhibit better electrochemical performance than corresponding bimetallic oxides,sulfides and monometallic selenides due to their high electrical conductivity and synergistic effect between metals.In view of this,based on the above work,combined with further high-temperature selenization,a two-dimensional composite structure of bimetallic selenide Mo0.25Nb0.75Se2 nanosheets supported on nitrogen-doped graphene(Mo0.25Nb0.75Se2/NG)was designed and synthesized.Mo0.25Nb0.75Se2 ultrathin nanosheets have abundant active sites,which is beneficial to accelerate the reaction kinetics of sulfur.The high conductivity of the two-dimensional composite structure is beneficial to the transfer of electrons and ions.Benefiting from the above advantages,the corresponding battery exhibits excellent cycle performance and high rate performance.After 200 cycles at a current density of 0.5 C,the specific capacity is as high as 935.2 mAh g-1,and the specific capacity remains at 931.7 mAh g-1 at a current density of 6 C.