The Preparation And Electrochemical Performance Investigations Of Vanadium Nitride Based Spherical Nanomaterials And Composites

Lithium-sulfur(Li-S)battery has attracted increasing attention due to its high specific capacity and energy density and the abundance of sulfur.However,the problems such as the serious shuttle effect of polysulfide,the large volume effect during the cycling and the poor conductivity of sulfur limit its further development.Developing a new type of sulfur host material is one of the effective ways to solve these problems.The transition vanadium nitride(VN)is a promising material owing to its high conductivity,chemical/structural stability and strong interaction with the polysulfides.In addition,the commercial anode material of lithium ion battery is graphite and its low specific capacity cannot satisfy the increasing demand of high energy/power density of battery.Therefore,the development of a new type of anode material is very urgent.Compared with graphite,transition metal nitrides(belonging to the transformation mechanism)have high theoretical specific capacity,but the large volume effect during the cycling might cause poor stability of batteries and hinder the development of this type of materials.To resolve the above problems,the design,synthesis and electrochemical properties of VN based composites are studied in this thesis.The main contents are listed as follows:(1)It is an effective method to modify the electrode materials by designing nano-materials with special structure,element doping and carbon coating/compounding with other materials.Here we reported a simple template-free solvothermal method combined with subsequent calcination method to prepare cobalt-doped vanadium nitride(Co-VN)yolk-shell nanospheres and we investigated the formation process of this unique structure.Then we successfully synthesized the Co-VN@C nanospheres after carbon coating and applied them as sulfur hosts for Li-S batteries for the first time.Benefiting from the unique structural advantages and the synergistic effect of conductive VN,cobalt and nitrogen-doped carbon(NC),the Co-VN@C/S electrode displays excellent electrochemical performance.It is noted that the batteries display high specific capacity and excellent cycling performances even with high areal sulfur loading amount.(2)Uniform yolk-shell precursor of Co-VN nanospheres and the double-shell precursor of VN micro-nanospheres were fabricated through a solvothermal method for the first time.Then the Co-VN nanospheres and VN micro-nanospheres with shape remained structure were obtained after the process of nitrogenization.Then the PDDA-modified positive Co-VN nanospheres were combined with the negative graphene oxide.Finally,Co-VN/RGO and VN/RGO powder were obtained after freezing-dried and annealing in NH3.Owing to the unique structural advantages and the good electrical conductivity,high structural/chemical stability and high theoretical specific capacity of VN,the Co-VN/RGO and VN/RGO electrodes deliver excellent electrochemical performances even at large currents.