Study On The Preparation,structure And Electrochemical Properties Of Nano VO₂

Vanadium dioxide?VO₂?is a homogeneous material with various crystal structures,among which the common structures are VO₂?B?,VO₂?M?and VO₂?R?.This type of material has typical thermally induced phase transition characteristics.It can undergo reversible first-order phase transition near 341 K?68??,and change from a high-temperature metal phase?M phase?to a low-temperature semiconductor phase?R phase?.Before and after the phase transition,its optical,electrical and magnetic properties will change abruptly.Therefore,it has potential applications in the fields of optical switches,laser protective layer,smart windows and so on.On the other hand,VO₂?B?with a layered structure has higher charge-discharge specific capacity and good cycle stability when used as an electrode material for ion batteries.However,VO₂ materials still have the problems of high phase transition temperature,slow response speed and high production cost in applications such as smart windows.When used as a cathode material for lithium ion batteries,V⁴⁺in VO₂ will undergo disproportionation reaction during the charge and discharge process.In this way,the layered structure and the diffusion channels of ions are destroyed,resulting in a reduction in the electrical conductivity and structural stability of the material,which leads to a reduction in specific capacity and cycle stability.Nanometerization of material grain size and ion doping are effective methods to improve material performance.In this project,nano-VO₂?B?was prepared by hydrothermal method,and nano-VO₂?M?was prepared by annealing as a precursor.The preparation conditions and ion doping on nano-VO₂ microstructure,electrochemical performance and the main contents of the influence of phase change characteristics are as follows:1.Taking V₂O₅ as the source of vanadium and H₂C₂O₄ as the reducing agent,by controlling the concentration of V₂O₅ and the ratio of reductant to vanadium source,the effects of preparation conditions on the structure and electrochemical properties of VO₂?B?nanoparticles was explored.When V₂O₅ concentration is constant,the experimental results show that the grain size of VO₂?B?decreases first and then increases with the increase of H₂C₂O₄ concentration When the concentration ratio of H₂C₂O₄ to V₂O₅ is 1.50,VO₂?B?has the largest specific capacitance and better capacity retention,showing good electrochemical performance.2.A series of B-phase nano V_1-xM_xO₂?M=Mn,Mo,W?samples with different doping concentrations were prepared by hydrothermal method,and the effects of M ion doping concentration on the microstructure and electrochemical properties of the samples was studied.For V_1-xMn_xO₂ series samples,the size and specific capacitance of the nanorod samples decrease first and then increase with the increase of doping concentration.When x=0.020,the samples have the largest specific capacitance and show excellent cycle stability;the appropriate concentration of ion doping is conducive to improving the conductivity,specific capacitance and cycle reversibility of the samples.3.With VO₂?B?as precursor,a series of high-quality nano VO₂?M?samples were prepared by annealing.The effects of ion doping,annealing environment and annealing temperature on microstructure and phase transition characteristics were studied.The results show that for the undoped samples,when the annealing temperature is 475?,VO₂?B?begin to change into VO₂?M?,and the morphology of the samples also changes from nanorods to nanoballs.For the V_1-xW_xO₂ samples,the pure V_1-xW_xO₂ nano powder are successfully prepared by high temperature annealing at 600?,and the phase transition temperature of VO₂?M?can be significantly reduced by W-doping.With the increase of doping concentration,the phase transition temperature decreases gradually.In this paper,a series of VO₂?B?and VO₂?M?nano samples with good quality were obtained by optimizing the preparation process.The structure and properties of the samples were characterized by means of positron annihilation technology.The effects of ion doping,annealing environment,annealing temperature and other factors on the microstructure,surface morphology,defect characteristics,electrochemical properties and phase transition temperature of VO₂ system were studied,the experimental data are provided for the improvement and application of the electrochemical performance of nano VO₂.