The Application Of Transition Metal Cobalt Doped Niobium Nitride And Molybdenum Selenide As Novel Sulfur Host Materials For Lithium-Sulfur Batteries

Lithium-sulfur(Li-S)battery has been considered as one of the most potential energy storage devices for replacing lithium-ion batteries.Because of its high energy density(2600 Wh kg-1)and theoretical specific capacity(1675 mAh g-1),the natural abundance of sulfur cathode and low cost,Li-S batteries have attracted widespread attention from researchers,however,they still have low utilization of the sulfur cathode,poor coulombic efficiency and rapid capacity fade,owing to the insulation of sulfur and the discharge products lithium sulfide,the dissolution of polysulfides in the electrolyte leading to shuttle effect and?80%volume change of the sulfur cathode during repeat charge and discharge processes.These problems have hindered the practical application of lithium-sulfur batteries.To solve the above issues,a controllable research design of sulfur host materials has been proposed,and combined with excellent adsorption ability of materials for polysulfides.In this thesis,Co-NbN porous spheres and core-shell MoSe2@C were synthesized and applied as sulfur host polar materials,and the electrochemical performance of materials has been studied in the Li-S batteries.The main research contents of the thesis are as following:(1)The porous cobalt doped niobium nitride spheres were successfully designed and synthesized as sulfur host material for lithium-sulfur batteries through solvothermal reaction and ammonia calcination methods.Its unique porous tunnel structure is capable of effectively accommodating volume expansion of sulfur during repeat charge-discharge process,porous Co-NbN spheres with plentiful active sites can trap polysulfides,and the synergistic effects between cobalt and niobium nitride can accelerates the electrochemistry kinetics.Thus,combining the above advantages,Co-NbN can significantly improve the electrochemical performance of lithium-sulfur batteries.With the sulfur content of 72%in the composite cathode,the battery delivers initial discharge capacity of 984 mAh g'1.After 800 cycles,a reversible capacity of 404.5 mAh g-1 is maintained with decay rate of 0.07%per cycle.It is worth noting that it exhibits excellent rate performance up to approximately 600 mAh g-1 at 1 C with a high sulfur load of 3.3 mg cm-2.Moreover,it exhibits excellent electrochemical performance under low electrolyte(E/S=8)and high sulfur loading of 5.6 mg cm-2.Therefore,the porous Co-NbN spheres as sulfur host material have exhibited great potential for advanced lithium sulfur batteries.(2)The uniform molybdenum-based nanorods precursor was synthesized via simple hydrothermal reaction.Then,the obtained precursor was coated carbon and calcined in an argon-hydrogen atmosphere to successfully synthesize MoSe2@C nanorods with a core-shell structure.The MoSe2@C is mixed with rGO as an effective sulfur host material.The hollow structure of the material can effectively alleviate volume expansion and shorten diffusion distance of lithium ions,the strong chemical interaction between polar MoSe2 particles and polysulfides suppresses shuttle effect by forming Mo-S and Li-Se bond,and carbon coating and mixed with rGO can improve the conductivity and the utilization rate of sulfur.The composite cathode at 0.2 C achieves initial discharge capacity of 1144 mAh g-1 and maintains a capacity of 740 mAh g'1 after 120 cycles.In addition,the cathode exhibits excellent electrochemical performance at large current density and high sulfur loading.