Study On Controlled Synthesis And Energy Storage Of Vanadium Oxide Composite Micro-nano Structure

Supercapacitor is an energy storage device with fast charge and discharge（i.e.high power）and long cycle life.However,its energy storage mechanism limits its energy density(usually less than 50 Wh kg^-1).In order to expand the application range of supercapacitors,it is of great significance to explore the electrode materials of supercapacitors with high power density and energy density and long cycle life.Zinc ion batteries have low cost and abundant natural resources,low negative electrode potential（-0.763 V vs SHE）and high th eoretical specific capacity(820 m Ah g^-1).However,it is still a challenge to design high performance cathode materials with stable internal structure and fast diffusion channel of zinc ions.Vanadium oxide is an ideal cathode material for supercapacitors and zinc ion batteries because of its abundant valence states,crystal structure,wide layer spacing and high theoretical specific capacity.However,vanadium based materials have poor electrical conductivity and solubility,resulting in capacity decay and poor long-term cyclic stability,thus limiting their large-scale application.It is one of the most promising methods to improve the electrochemical performance of energy storage system to design the composition and micro-nano structure of materials reasonably and adjust the surface structure and electronic structure of materials.Based on this,the research content of this paper is as follows:（1）In the third chapter of the experimental system,in order to inhibit the dissolution of vanadium based material,and improve its energy storage performance.In this chapter,LDH shell coating strategy is adopted to reduce the contact between vanadium base material and electrolyte.At the same time,a hollow core-shell structure was constructed to alleviate the stacking effect of LDH material and improve the ion diffusion rate.Firstly,highly dispersed and uniform vanadium-triglyceride spheres were synthesized by solvothermal method,which were used as precursors to coat LDH shells and construct V₆O₁₁@Ni（OH）₂/Ni OOH hollow double-shell nanoflower structures.The shell thickness（density）was controlled by adjusting the reaction time,and a series of compounds were characterized and electrochemical properties were tested.It was found that V₆O₁₁@Ni（OH）₂/Ni OOH-6（VN-6）active materials with reaction time of 6 h showed good electrochemical properties in supercapacitors.When at 1 A g^-1,the specific capacitance of VN-6 is up to 1018.2 F g^-1,and the cycle stability of VN-6 remains at 123.8%after 6000 cycles.In addition,the energy density of VN-6//AC device reaches 24.3 Wh kg^-1,and after 5000 cycles,the cycle stability reaches 129.6%at 8 A g^-1current density.In this study,the stability of vanadium-based compounds was effectively improved by adjusting the component proportion and constructing complex hollow multi-shell structure,and high performance cathode materials for supercapacitor were obtained.（2）In chapter 4,in order to improve the electrical conductivity of vanadium-based compounds,anion substitution was adopted to construct oxygen-rich defect strategy to improve the surface electronic structure of vanadium-based compounds.Different from the traditional strategy of high temperature reduction to construct oxygen defects,solvothermal method is adopted to replace oxygen defects by S^2-,which can effectively alleviate the lattice shrinkage caused by decomposition of high temperature water of crystal.Oxygen deficient V₃O₇·H₂O（HVO-1）nanoneedle arrays were constructed on carbon cloth by in-situ growth method in order to reduce the use of additives and further improve their mass transfer and conductivity.The morphology and crystal characterization show that it is an amorphous structure with abundant oxygen vacancy and low band gap（0.69 e V）.When applied to zinc ion batteries,the specific capacitance reaches 496.5 m Ah g^-1at 0.3 A g^-1.Compared with the unsubstituted V₃O₇·H₂O（HVO-0）,the capacity is increased nearly 1 times.At the current density of 8 A g^-1,the capacity retention rate is 90%after 2000 cycles.XRD,XPS,HRTEM and other measurements show that there are obvious zinc ions embedded in the process of charging and discharging.Theoretical studies show that HVO-1 has higher adsorption energy of zinc ions than HVO-0,which indicates that the presence of oxygen-rich defects increases the reversible deinking ability of zinc ions.At the same time,the material is also suitable for supercapacitors,in a three-electrode system,the current density of 1 A g^-1,can reach 1240 F g^-1specific capacity.This study provides a new idea for anionic doping strategy in metal oxide zinc ion storage,and can be extended to other cathode materials for energy storage applications.