| Lithium-Sulfur?Li-S?batteries are regarded as one of the most promising solution for the next generation high-energy-storage systems,due to their high theoretical specific capacity,high energy density and low cost and environment-friendly.However,their practical application is hindered by low utilization of the sulfur,poor cycle life and rate capability induced by the low electrical conductivity,high solubility of the lithium polysulfides.In this thesis,transition metal nitride,vanadium nitride?VN?,was used as sulfur host to improve the electrochemical performances of sulfur cathode.Porous VN with different morphologies were prepared by hydrothermal method and nitridation method,and a novel porous laminated VN network was successfully prepared.VN has high conductivity,strong ability of chemically trapping polysulfides and high catalytic activity,meantime,porous laminated VN has the unique characteristics of large tap density,the porous laminated structure can accommodate more sulfur,which make Li-S batteries show excellent long-life and high-rate performance.The specific research works are as follows:1.Hydrothermal method was used to explore the effect of hydrothermal time,pH and the template on the morphology of vanadium oxide?VOx?.Then the porous VN of nanowires/bands,nanosheets and laminated nanosheets were prepared by nitridation method,and finally S@VN composites with different morphologies were prepared by melt diffusion method.The composite materials own good ability to limit the“shuttle effect”of polysulfides.and the abundant pore structure can buffer the volume expansion of sulfur and increase the loading of sulfur.It is found by electrochemical tests that the porous laminated VN showed a high specific capacity of 526.5 mA h g-11 after 200 cycles at the current density of 3350 mA g-1?2 C?,which is better than that of other morphologies.2.In order to optimize the pore size of porous laminated VN and its electrochemical performances as sulfur host for Li-S batteries,the optimal nitridation conditions were obtained by adjusting the temperature and time of nitridation reaction.The mechanism of the reaction and chemical interaction with polysulfides were further investigated.Through optimization and exploration,the porous laminated VN obtained at 600?for 2 h under NH3 can effectively play its role of adsorbing polysulfides,limiting the shuttle effect of polysulfides,improving the catalytic efficiency for transformation of polysulfides.The porous laminated S@VN cathode shows excellent specific capacity of 805.5 mA h g-11 after 200 cycles at the current of 3350 mA g-1?2 C?,even an extremely stable specific capacity of 344.3 mA h g-11 and 381.8 mA h g-11 after 500 long cycles at the current density of 8375 mA g-1?5 C?and 16750 mA g-1?10 C?respectively,which the capacity decay rates of each cycle were only 0.12%and 0.11%,respectively. |
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