High-performance Lithium-sulfur Battery Assembled With Vanadium Nitride-derived Cathode Sulfur Host Materials

With the demand for portable equipment,electric cars and renewable energy increase,the development of energy storage equipment with high-energy-density and long lifetime has captured considerable interest.Lithium-sulfur(Li-S)batteries are regarded as one of promising next-generation high-energy secondary batteries systems owing to their high theoretical specific capacity,low-cost and environmental protection of raw material sulfur.Nevertheless,the practical application of Li-S batteries is impeded by many problems,including the low conductivity of sulfur and its solid discharge products,the shuttle effect of polysulfides,and volume change of active sulfur during charging and discharging cycling.This dissertation aims at the above-mentioned problems and focuses on the structural design of sulfur cathode materials,a series of vanadium nitride(VN)-based composite with different structures and morphologies have been fabricated and utilized as sulfur host material in the cathode.The electrochemical performance of the assembled battery was further explored.The main research contents are as follows:1.A nitrogen-doped carbon-supported VN nanoparticle composite(VN@NPC/CNT)is obtained under the no ammonia condition through ion exchange and carbonization processes using carbon nanotubes(CNT)wrapped with imidazole-based ionic polymer(Im IP)as the precursor.Im IP not only acts as the sole source of carbon and nitrogen,but also effectively inhibits the aggregation of VN during the pyrolysis process,and obtains highly dispersed,ultra-fine VN nanoparticles.Polar VN has an adsorption effect on polysulfides,which can reduce the impact of the shuttle effect.Moreover,VN provides abundant catalytic active sites to adsorb sulfur species and catalyze its further conversion.The electrochemical activity of the VN@NPC/CNT-800-S electrode is enhanced,showing excellent cycling performance.At a high rate of 5 C,the specific capacity of the first cycle reached 726 m A h g^-1,and after 1200 cycles,it could still be kept at 417 m A h g^-1,and the capacity decay rate is as low as 0.035%.2.A self-supporting composite(CFP-VN)constructed of vanadium nitride nanoarrays is prepared by hydrothermal and nitriding process using carbon nanofiber as a conductive substrate.The conductive carbon body is closely connected with the VN array,the lithium-ion transmission is accelerated,and the enclosed space formed can effectively suppress the volume change problem during the charging/discharging process.The highly conductive structural framework and the catalysis effect of VN for the polysulfide conversion reaction increase the utilization of sulfur.The CFP-VN/S electrode with 7.0mg cm^-2sulfur loading obtained the specific capacity of 1112 m A h g^-1as well as an areal capacity of 7.8 m A h cm^-2at 0.1 C.The specific capacity and areal capacity can be maintained at 826 m A h g^-1and 5.8 m A h cm^-2respectively after the battery is cycled for150 cycles.