Construction And Electrochemical Properties Of Vanadium Oxide Electrode Materials

With the rapid consumption of fossil energy,the problem of environmental pollution and energy crisis are more and more serious,therefore,the clean energy,possessing the characteristics of non-pollution and renewability,attracts more and more attention.However,clean energies(i.e.,solar,wind and geothermal energy)are seriously affected by natural conditions,such as region,weather,time and so on,and thus possess the shortcomings of intermittently and regionally,making it difficult to directly use.Battery system,considered to be a crucial technology for energy conversion and storage and converting intermittent clean energy into steady electricity,occupies a vital position in the transformation and development of energy.Based on this,owing to the various crystal structure and abundant physical and chemical performance,vanadium oxide materials have been used as electrode materials in various ion batteries.However,the unstable structural stability and insufficient electronic conductivity of these materials result in the poor electrochemical performance,thus restricting their further development.It is found that reasonable micro-nano structure design of materials is an effective strategy to modified the vanadium oxide based electrode materials,which can not only realize rapid diffusion of ions,but also improve the structural stability of materials.Doping other ions and regulating the crystal structure of vanadium oxide can effectively improve the electrical conductivity of materials,so as to enhance the charge transfer efficiency of materials.In this thesis,the vanadium oxide electrode materials are optimized and modified based on these two strategies.The specific research content of this thesis mainly includes the following parts:(1)In order to improve the diffusion efficiency of Zn²⁺ions and the cyclic stability of electrode materials,a 3D flower-like structure(3D-NVO)composed of ammonium vanadate(NH₄V₄O₁₀,named as NVO)nanoribbon was designed and synthesized by microwave-assisted hydrothermal method in term of microstructure design The nanostructure can effectively shorten the diffusion distance of Zn²⁺ions and improve the ion diffusion efficiency,and the flower-like morphology enhances the stability of electrode material structure during discharge/charge process.In terms of crystal structure,NH₄⁺ions embedded between VO layers can act as"pillars",which can not only enlarge the layer spacing to promote the diffusion of Zn²⁺ions,but also stabilize the layered structure and improve the cyclic stability.Based on this,3D-NVO materials show excellent electrochemical properties in zinc ion batteries:high capacity of 485 m Ah/g at 0.1 A/g,excellent rate performance of 142 m Ah/g at10 A/g and stable cycle performance of more than 3000 cycles under 10 A/g.(2)In view of the problems of poor structural stability and slow Zn²⁺ions diffusion kinetics of hydrated vanadium pentoxide(V₂O₅·n H₂O,named as HVO)material,this thesis adopts one-step microwave assisted hydrothermal technology to synthesize hydrated vanadium pentoxide with pre-embedded K⁺ions(K_0.52V₂O₅·0.29H₂O,named as KHVO).K⁺ions and water molecules coexist in the VO layer.Both of them can act as"pillars",which can not only enlarge the layer spacing,improve the diffusion efficiency of Zn²⁺and achieve rapid ionic diffusion dynamics,but also stabilize the layered structure and improve the stability of the structure.In addition,the water molecules between the layers can also act as a“lubricant”,further promoting the diffusion of Zn²⁺ions.Based on this,the material shows good electrochemical performance in quasi-solid zinc ion battery:high reversible specific capacity of?300 m Ah/g at 100 m A/g,the great rate performance of 110 and 65 m Ah/g at 2 A/g and 5A/g,and excellent cycling performance of over 1500 cycles at 2 A/g.(3)In order to solve the problems of low electronic conductivity and slow K⁺and Zn²⁺ions diffusion kinetics of VO₂(B)material,microwave-assisted hydrothermal technology was used in this thesis to design the micromorphology of the sample and synthesize a microsphere with nanoribbon synapses on the surface.Moreover,VO₂(B)was doped with potassium.The results show that the micro-nano composite structure plays a positive role in improving the ions diffusion dynamics and structural stability of the materials.Potassium doping treatment can obviously improve the charge exchange rate of electrode materials,and thus improve the electrochemical performance of electrode materials.Based on this,the electrode material has high K⁺diffusion coefficient(3.9×10^-12 cm~2/s),large specific capacity(?420 m Ah/g)and excellent rate performance(up to 151 m Ah/g at 2 A/g)in potassium ion batteries.In the zinc ion battery,the electrode material shows a high zinc storage capacity(395 m Ah/g)under a low current condition,and also achieves 122 m Ah/g reversible capacity at an extremely high rate(50 A/g).