Study On Preparation And Mechanism Of Na₃V₂（PO₄）₂F₃ As Cathode Materials For Sodium-Ion Battery

At present,the mainstream lithium ion battery in the market is difficult to meet people’s needs due to the cost and safety restrictions.As a cutting-edge energy storage process,sodium-ion battery has many advantages over lithium battery,such as rich elements,wide distribution and low price.However,from the microscopic point of view,the ion radius of sodium-ion is relatively large and the electrochemical reaction potential is higher,so it is necessary to improve the structure stability and energy density of sodium-ion battery by some modification means.At present,a variety of sodium-ion cathode materials have been reported,including the layered structure of Na_xCo O₂ and Na_xMn O₂.In addition,phosphate compounds belong to polyanion type compounds,and there are（XO^m）^n-（X=P,S,Si,etc.）groups in the materials,and the X-O bond in the group can bring high structural stability and working voltage.In addition,because phosphate can bond with vanadium group to form NASICON channel,it brings strong diffusion ability of sodium-ions Among them,Na₃V₂（PO₄）₂F₃ has attracted extensive attention due to its combination of polyanion structure and NASICON structure.In this work,the manganese doped sodium vanadium fluorophosphate/carbon composite and the porous microspheres of sodium vanadium fluorophosphate were synthesized by hydrothermal method,and the crystal plane orientation growth was realized to obtain the high capacity and high rate of sodium vanadium fluorophosphate cathode materials.Specific research work and results are as follows:（1）A kind of manganese doped vanadium phosphate sodium fluoride generation/carbon composite material was synthesized by hydrothermal method and high temperature sintering.Specifically,citric acid aqueous solution is used as the solvent carbon source and reducing agent.By adjusting the manganese source(Mn²⁺,Mn³⁺,Mn⁴⁺),the valence state of doped Mn ion is changed,and then through the doping modification of vanadium site,the crystal stable sodium vanadium fluorophosphate is prepared.In the charge-discharge test of sodium-ion half battery,the discharge capacity of this material was 116.2 mA hg^-1,93.3 mA hg^-1,71.2 mA hg^-1,49.6 mA hg^-1,and 41.5 mA hg^-1 at the current density of 0.2C,0.5 C,1 C,2 C,and 5 C,respectively.At a high current density of 1 C,the specific discharge capacity was 56.8 mA hg^-1 after 500 cycles,and the capacity retention rate was 67.7%.（2）A porous microsphere of sodium vanadium fluorophosphate was synthesized by hydrothermal method using Polyvinylpyrrolidone（PVP）as surfactant.The morphology of the micron spheres can be changed by adjusting the amount of PVP.In the charge and discharge test for sodium-ion half battery,the discharge capacity of this material is 127.8 mA hg^-1,109.6 mA hg^-1,95.6 mA hg^-1 and 77.3 mA hg^-1 at the current density of 0.2 C,0.5 C,1C,2 C and 5C,respectively.At the high current density of 1 C,after 500 cycles,the specific discharge capacity is still 72.4 mA hg^-1,and the capacity retention rate is 72.4%.