Study On Preparation And Modification Of Vanadium Sodium Fluorooxophosphate As Anode Material For Sodium Ion Batteries

Due to the scarcity of lithium resources,lithium-ion batteries cannot be used in the field of large-scale energy storage.It is one of the solutions that can meet the needs of large-scale energy storage field to manufacture sodium-ion batteries using the same main group of sodium as lithium.Among the cathode materials for sodium ion batteries,polyanionic sodium vanadium fluoride phosphate(NVPOF)has the advantages of high voltage and large specific capacity.It is one of the cathode materials with great potential for sodium ion batteries,but it has the disadvantage of low electronic conductivity.It is very important for the commercialization of NVPOF to study the modification of NVPOF and its charging and discharging mechanism.The cathode material NVPOF sodium ion battery was prepared by coprecipitate method.X-ray diffraction showed that the crystal structure of the synthesized material was tetragonal system,corresponding to I4/mmm(No.139)space group.NVPOF-SC material was obtained by carbon coating modification of NVPOF material using tannic acid as carbon source.Scanning electron microscope image showed that the modified material particles were more regular and evenly distributed.The electrochemical performance of the battery was tested.The test results showed that the specific capacity of the NVPOF electrode was about 110mAh/g,9mAh/g and 7mAh/g respectively at 0.1 C,5C and 10C current ratios.The specific capacity of NVPOF-SC was about 110mAh/g,53mAh/g and 42mAh/g respectively at 0.1C,5C and 10C current ratios,and the multiplier performance of the modified material is better.The results show that the capacity retention rate of NVPOF-SC is 74.63%,which is better than that of NVPOF electrode 58.07%.Electrochemical impedance spectroscopy shows that the charge transfer resistance of NVPOF and NVPOF-SC electrodes is 122.3Ω and 99.5Ω,respectively.The significantly reduced charge transfer resistance indicates that carbon coating can improve the electronic conductivity of materials.In order to further study the charge and discharge mechanism of the battery when NVPOF was used as the cathode material of the battery,the electrochemical model of the pseudo-two-dimensional sodium ion battery was established,and the simulation study of the cathode material of the sodium ion battery was carried out by comsol modeling software.The concentration distribution of sodium ions in the discharge process of sodium ion battery was studied.It was found that the cathode material with low solid phase diffusivity led to the accumulation of sodium ions on the surface of electrode material during the discharge process.The cathode material with high solid phase diffusivity had better consistency in the concentration of sodium embedded on the surface and inside electrode particles during the discharge process,which would make the battery have lower concentration polarization.It is helpful to reduce the occurrence of electrode side reaction.The effect of the diffusivity of cathode material on the battery performance was studied.It was found that the diffusivity had no significant effect on the capacity of the battery at a lower ratio(≤2C),but at a higher ratio,the material with a higher diffusivity had a larger capacity.