Synthesis Of Sodium Vanadium Fluorophosphate Cathode For Sodium Battery And Performance Research

Because of its excellent rate performance,abundant raw material,and low cost,sodium ion batteries are all ideal replacement power sources for the widely used lithium ion batteries.The cathode material Na₃V₂（PO₄）₂F₃ with polyanion structure for sodium ion batteries has the advantages of stable crystal structure,unique three-dimensional ion diffusion channel,high working voltage,and so on,which is the focus of the majority of scientific researchers.However,its low electronic conductivity and easy generation of impurity phases during the synthesis process greatly limit its development prospects.Based on this,this thesis starts with the synthesis of Na₃V₂（PO₄）₂F₃ material,and explores a series of optimal synthesis conditions.On this basis,Na₃V₂（PO₄）₂F₃ is modified to adjust its structure,morphology and improve its electrochemical performance.（1）The one-step sol-gel method is used to synthesize Na₃V₂（PO₄）₂F₃/C cathode.The Na₃V₂（PO₄）₂F₃/C material is synthesized at different temperatures.The results show that the synthesized samples had the best electrochemical performance when the sintering temperature is 700℃.Different ratios of Na F to VPO₄ will transform the crystal phase of the material.When Na F:VPO₄ is 1.5:1 and the sintering time is 4 h,polytetrafluoroethylene（PVDF）is used as the carbon source and the carbon content is kept at 6 wt%,the synthesized Na₃V₂（PO₄）₂F₃/C sample with tarivorate structure is the pure phase and it delivers the best electrochemical performance.（2）The electrochemical performance of Na₃V₂（PO₄）₂F₃/C is improved by changing the added amount of surfactant cetyltrimethylammonium bromide（CTAB）.Due to the insoluble charactristic of PVDF,CTAB is used to disperse it.According to different CTAB concentration intervals,CTAB molecules in the aqueous solution can form micelles with the morphology of spherical and rod-shaped.Using CTAB micelles as the soft template,the Na₃V₂（PO₄）₂F₃/C sample with spherical-like particle morphology is successfully prepared,and its formation mechanism is analyzed.When the concentration of CTAB is 1.4×10^-2 mol?L^-1,the electrochemical performance of the synthesized sample is the best.Its initial reversible capacity is 121.3 m Ah?g^-1 at0.1 C.At a large rate of 10 C,the specific discharge capacity is as high as 99.7m Ah?g^-1.After 1000 charge-discharge cycles at 10 C,the capacity retention rate is91%.After cycling,the XRD diffraction peaks of the cathode film are consistent with the standard PDF card,but the intensity is slightly decreased,indicating that Na₃V₂（PO₄）₂F₃ can still maintain a stable crystal structure after cycling.