Preparation And Electrochemical Properties Of Nickel-based Vanadate Materials

The extensive use of fossil fuels has caused great pollution to the environment where we live in.Therefore,lithium ion batteries and supercapacitors（SCs）have attracted researchers’attention as a clean and renewable energy.SCs are widely studied due to their advantages involving in rapid charge and discharge,long lifetime and high specific energy in practical applications.The electrode materials exert influence on the performance of SCs to an extreme,so it is of great importance to select the appropriate electrode material and obtain the material with high electrochemical activity by adjusting the morphology and structure.The transition metal nickel-based vanadates and their composites have shown good electrochemical activity in numerous studies.However,the syntheses of most of the reported nickel-based vanadates are difficult and the processes are complex.Based on the above analyses,nickel-based vanadates with different structures were regulated by simple synthetic methods in this paper to obtain superior electrochemical activities.The main work is as follows:（1）NiV₂O₆ with different morphologies and structures were prepared by hydrothermal method combined with subsequent room temperature liquid phase synthesis method.On this basis,the effect of synthetic technology on the morphology,structure and performance of NiV₂O₆ was discussed,indicating that adjustment of the pH of the solution by controlling the amount of ammonia water has a great influence on the morphology and properties of the material.When used as the positive electrode material of the SC,the results illustrate that microsphere of NiV₂O₆ possesses the best performance.The specific capacity is 416.4 C/g at the current density of 1 A/g,and the specific capacity decreases by 15.4%after 3000 cycles（5 A/g）.The main reason is that the NiV₂O₆ with microspheres structure can provide more electrochemical active sites.The feasibility analysis of the practical application of the supercapattery assembled together with rGO shows good electrochemical performance,the energy density is 25.3 Wh/kg at the power density of 400.5 W/kg.（2）The Ni₃V₂O₈ materials were prepared by microwave-assisted method.Electron microscopy analyses show that all the materials were microspheres.According to the experimental results,the relationships between the microwave reaction conditions and the electrochemical properties of the prepared Ni₃V₂O₈materials were discussed and analyzed,indicating that the reaction time has a great influence on the properties of the material.The prepared materials were tested as supercapacitor materials,the results display that Ni₃V₂O₈ microspheres with the reaction time of 30 min possesses the best electrochemical performance,reaching a specific capacity of 413.4 C/g（the current density of 1 A/g）.When assembled into a supercapattery with rGO,which showed the energy density of 24.2 Wh/kg at the power density of 816 W/kg.（3）Based on the above work of Ni₃V₂O₈,Ni_xCo_3-xV₂O₈ microspheres structures formed by self-assembly of nanorods were prepared by introducing cobalt.The results of characterizations show that the introduction of cobalt transforms the morphology of the material from a pure microsphere to a microsphere structure self-assembled by nanorods,thus increasing the specific surface area of the material,shortening the diffusion path of ions/atoms,and buffering the volume change caused by the charging and discharging process.In addition,there are also interface effects and synergistic effects between multi-metals to improve capacitance performance.When tested as the electrode materials for SCs,the products showed that the comprehensive performance is the best when the ratio of nickel to cobalt is 2:2,the specific capacity is 394.2 C/g at 1 A/g and the rate capability is excellent（76.2%）.After 10,000 cycles,the final specific capacity is 82%of the original.Two electrode tests were conducted on the supercapattery assembled with rGO,and the results show that the energy density is31.8 Wh/kg at the power density of 800 W/kg.