Preparation Of Transition Metal Vanadate Energy Storage Materials And Study On Supercapacitor Performance

With the continuous development of economy and society,the pressure of energy supply is increasing day by day,which promotes the use and development of clean energy and high-performance energy storage devices.Among them,supercapacitors have become a research hotspot in the field of energy storage due to their advantages of high power density,long cycle life,fast charging and discharging speed,and low maintenance cost.However,the low energy density of supercapacitors seriously hinders their commercial applications and is an urgent problem to be solved.According to the calculation formula of energy density（E（28）0.5CV~2）,the energy density of supercapacitors is mainly limited by the specific capacitance C and the working voltage V.Therefore,increasing the energy density of supercapacitors can be achieved by increasing the specific capacitance of the electrode material and increasing the device operating voltage.In this thesis,a deep eutectic solvent（DES）was used as a reaction medium to prepare a CoV₂O₆material with a polyhedral structure,which exhibited good electrochemical performance as an electrode material.The mechanism analysis of the excellent cycling performance of the electrode was carried out through the tests of CV,EIS,XRD,XPS and SEM during the electrode cycle.In addition,in order to improve the working voltage of the device,the DES was also used as the electrolyte of SCs to improve its energy density.The details are as follows:1.Synthesize DES with malonic acid and choline chloride,and use it as a reaction solvent to prepare CoV₂O₆electrode material.Then CoV₂O₆was annealed at different temperatures to prepare electrodes with different morphologies.Experiments show that controlling the annealing temperature has a great influence on the morphology and properties of the material.At a current density of 1 A g^-1,the specific capacity is:CoV₂O₆-600(194.0 C g^-1)>CoV₂O₆-500(127.9 C g^-1)>CoV₂O₆-700(124.5 C g^-1).Among them,CoV₂O₆-600 has a polyhedral structure with a capacity retention rate of122.2%after 100,000 cycles.In addition,CoV₂O₆-600 was used as the positive electrode and activated carbon（AC）as the negative electrode.In the two-electrode system,the energy density of the CoV₂O₆-600//AC device can reach 20.6 Wh kg^-1at373.4 W kg^-1.The deep eutectic solvothermal method in this work provides a new idea for the synthesis of CoV₂O₆.2.CoV₂O₆-600 prepared based on DES has excellent cycling stability.The CV,EIS,XRD,XPS and SEM analyses before and after cycling and during cycling show that the excellent cycle life is attributed to:（i）CoV₂O₆-600 intercalates into the interstices of CoV₂O₆-600 polyhedron in order to accommodate different electrolyte ions in the early stage of cycling A slight displacement may occur,exposing more active sites,increasing the specific capacity after cycling,and the polyhedral structure of CoV₂O₆-600 is more stable,enabling it to have a long cycle life;（ii）On the surface of the CoV₂O₆-600polyhedron,a new Co OOH active phase was generated during the cycling process,and the synergistic effect of the new active phase and CoV₂O₆-600 resulted in its excellent cycling stability and performance.3.A series of DES-based electrolytes were prepared by adjusting different molar ratios between choline chloride and malonic acid as 1:0.5,1:1,and 1:1.5.DES-based supercapacitors have a voltage window of up to 1.8 V.DES-1 has the highest electrical conductivity(2.05 m S cm^-1),a specific capacitance of up to 154.6 C g^-1,and the highest energy density of 11.63 Wh kg^-1.The DES formed from choline chloride and malonic acid can not only provide a promising electrolyte replacement for rationally designed supercapacitors,but also is expected to be extended to other electrolytes for energy storage devices.