Preparation Of Conductive Vanadium-based Compounds（V₂O₃,VN） And Their Performance In Lithium-sulfur Battery

Lithium-sulfur batteries（LSBs）have attracted word-wide attentions in recent years due to its high theoretical energy density(2600 Wh kg^-1)and specific capacity(1675 mAh g^-1).The active substance of LSBs is sulfur,which is abundant,low in price,non-toxic,and eco-friendly.Therefore,LSBs have been considered to be the next generation secondary batteries.Different from the conventional lithium-ion batteries which is based on the intercalation reaction of lithium ions,the electrochemical process of LSBs is based on the phase transition reaction of sulfur species on the solid-electrolyte interface.However,both sulfur and the final discharge product Li₂S are insulator,which hinder the electron transfer.In addition,the intermediate polysulfide is soluble in the electrolyte and will migrate between the cathode and anode,which leads to the shuttle effect.To solve these problems,researchers have carried out exploration in many aspects,including the electrolyte,separators,and cathode material.The goal of this dissertation is to explore the controllable synthesis of V₂O₃and VN,which is metal conductive,via a simple solvothermal method followed by calcination in NH₃/Ar atmosphere and their application for LSBs.The details are summarized as follows:（1）Hierarchical V₂O₅ microflowers composed of thin nanosheets have been achieved by a solvothermal reaction first and then a low-temperature calcination process.The prepared V₂O₅ hollow spheres were reduced to V₂O₃ hollow spheres（V₂O₃-HBs）at 450^oC under NH₃/Ar atmosphere.The morphology and structure of the V₂O₃-HBs were characterized by transmission electron microscopy,scanning electron microscopy,powder X-ray diffraction and X-ray photoelectron spectroscopy.The powder of V₂O₃-HBs and sulfur was mixed directly without further thermal treatment and the mixture was used as the cathode of LSBs.The electrochemical test showed that the initial capacity reaches 1375 mAh g^-1 and the capacity after 100 cycles is 815 mAh g^-1 at 0.2C(1C=1675 mA g^-1);Even at 1C,the initial capacity reaches 710 mAh g^-1 and the capacity after 500 cycles is 530 mAh g^-1.It indicates that the addition of V₂O₃-HBs in the LSBs can effectively improve the cycle performance.In order to increase the amount of sulfur,the electrochemical properties of V₂O₃-HBs/S composite with different sulfur amounts were investigate.（2）VN hollow spheres（VN-HBs）were obtained via solvothermal method,followed by further annealing at 700^oC under NH₃/Ar atmosphere.The structure and morphology of the product were characterized by transmission electron microscopy,scanning electron microscopy and power X-ray Diffractometer.The VN-HBs were used as the sulfur host of LSBs.The batteries present a high initial discharge capacity of 1401 mAh g^-1 at 0.2C with a sulfur content of 60%and maintain at 770 mAh g^-1 after 100 cycles.Even at a high current density of1C,the initial capacity can still achieve at 725 mAh g^-1 and 438 mAh g^-1 after500 cycles,with an average capacity decay of only 0.079%.The electrochemical performances with high sulfur content of 70%and 80%were also investigated.