Preparation And Electrochemical Performances Of Vanadium-Based Metal Oxides/(Nitrides)

With the increasing demand for renewable and sustainable energy,the development of new energy storage devices has become a hot research topic.Supercapacitors have the characteristics of long cycle life and high power density,etc.,but their low energy density still cannot meet the needs of commercialization.Therefore,designing excellent supercapacitor electrode materials with high energy density has caused extensive research.Vanadium is rich in reserves and low cost.Additionally,vanadium-based materials have coordination polyhedron and variable valence state,which can make it an open structure to facilitate ion insertion and extraction.In this thesis,the vanadium-based metal oxides/nitrides are designed and synthesized.Their electrochemical performance were also ivestigated in detail.The main contents of the paper are as follows:The composite material of graphene/V₂O₅ with a good crosslinked structure was synthesized by thermal decomposition approach.Electrochemical measurements demonstrate that the composite with graphene loading of 3.7%（GV-15）has the highest specific capacity.The specific capacity of GV-15 is 509.4 C·g^-1 at a current density of 1 A·g^-1.An asymmetric supercapacitor using GV-15 as the positive electrode and graphene/FeOOH as the negative electrode has been assembled.It can achieve an energy density of 15.6 Wh kg^-1 and the capacity retention ratio is 91.1%after 10000 cycles,indicating excellent cycle stability.The V₂O₅/graphene composite was synthesized through a hydrothermal approach using glucose as the template.The crystallinity,particle size and microstructures of V₂O₅ were optimized by changing the calcination time of the precursor.In order to improve the conductivity of V₂O₅,graphene nanosheets were introduced to further improve the electrochemical performances.At a current density of 1 A·g^-1,the specific capacity is 482.8C·g^-1.An asymmetric supercapacitor has been assembled using graphene/V₂O₅ composite as the positive electrode and graphene/porous carbon as the negative electrode.It shows a specific capacity is 156.2 C·g^-1 at 1 A·g^-1 and the energy density is 34.7 Wh·kg^-1.Additionally,the asymmetric supercapacitor has excellent cycle stability with 91.9%of its initial capacity can be retained after 10000 cycles.Vanadium nitride was obtained through heat treatment of pyrrole intercalated V₂O₅precursor at high temperature.The effect of stirring time on the microstructure and electrochemical performances of the samples were investigated in detail.The specific capacity of VN sample prepared with stirring time of one week（VN-5）is 299.0 C·g^-1 at the current density of 2 A·g^-1.When VN-5 is used as the negative electrode and graphene/Ni S₂/Ni₂P is used as the positive electrode,the assembled asymmetric supercapacitor achieves a specific capacity is 270.0 C·g^-1 at 2 A·g^-1.The specific capacity remains 71.3 C·g^-1 when the current density reaches to 60 A·g^-1.The energy density of our fabricated asymmetric supercapacitor is 40.9Wh·kg^-1.Furthermore,the asymmetric supercapacitor exhibits excellent cycling stability with capacity retention of 94.8%after 10000 continuous cycles at 10 A·g^-1.