Study On Synthesis And Modification Of Na₃V₂（PO₄）₂F₃ As Cathode Material For Sodium-ion Batteries

Sodium vanadium fluorophosphate has been a kind of the most promising commercialized polyanionic cathode materials for sodium-ion batteries due to its high discharge potential platform,larger theoretical specific capacity and structural stability.However,in literature reports,Na₃V₂（PO₄）₂F₃ is mostly synthesized by high-energy-consuming solid-phase methods,which lose F element under decomposition at high temperature.Low electronic conductivity and poor cycle stability restrict the industrial application of Na₃V₂（PO₄）₂F₃.Therefore,in this paper,aiming at the synthesis process of Na₃V₂（PO₄）₂F₃ material and the capacity cycle degradation,the environment-friendly liquid-phase method is used to synthesize Na₃V₂（PO₄）₂F₃ material,exploring the effect of synthesis process for the crystallinity and Sodium Storage Performance of Na₃V₂（PO₄）₂F₃,comparing the reaction mechanism of the Na₃V₂（PO₄）₂F₃ material synthesized by the liquid-phase method and the solid-phase method in heat-treatment process,revealing the capacity decay mechanism of the Na₃V₂（PO₄）₂F₃ material during the battery charge and discharge cycle,and proposing the corresponding solution.The research content of this paper as follows:1)Optimize the process conditions of the reaction temperature,time,and type of phosphorus source for the hydrothermal synthesis of Na₃V₂（PO₄）₂F₃ materials.When the hydrothermal temperature is 180℃,the time is 16 h,and the reaction phosphorus source is Na₂HPO₄,the crystallinity and sodium storage performance of the synthesized sample are better.The crystallinity and sodium storage performance of samples synthesized by the sol-gel method are studied,and the sodium storage performance of the samples is significantly improved compared with the hydrothermal method.After 56 cycles at 0.2 C,the reversible capacity increased from 54.2 mAh g^-1 to 83.5 mAh g^-1.In-situ composite of CNTs was added during the synthesis,and it was found that the sodium storage performance was positively correlated with crystallinity.After the 600℃ annealing sample is composited with CNTs,when it is cycled for 120 cycles at a rate of 0.5 C,the capacity retention rate is increased from 60%to 86%,the capacity retention rate is increased from 63%to 87.7%after 150cycles at 1 C rate.2)Study the reaction mechanism in heat-treatment process of Na₃V₂（PO₄）₂F₃ synthesized by the liquid-phase method.The general rule that the material is prone to decomposition reaction at high temperature is obtained,and the comparison is verified by the solid-phase method.It is found that the annealing temperature of Na₃V₂（PO₄）₂F₃ material is higher than600℃,which will cause F decomposition.Na₃V₂（PO₄）₂F₃ materials synthesized by different methods have different period of the F element loss.The liquid-phase method mainly focuses on the loss in the early stage of the aqueous solution reaction process,while the solid-phase method is mainly manifested in the later high temperature heat treatment process.3)Study the capacity degration of Na₃V₂（PO₄）₂F₃ material during charge and discharge cycles,exploring the microstructure changes of pole pieces,diaphragms,and active materials behind the capacity attenuation,and revealing the mechanism of material spalling and current collector corrosion.The capacity decay mechanism of Na₃V₂（PO₄）₂F₃ material is that trace water induces F-dissolution to form Na₃V₂（PO₄）₂F_3-X（OH）_X and HF.HF corrodes the interface between the active material and the current collector,causing the active material to fall off and inactivate,increasing the polarization voltage and accelerating the capacity decay.Therefore,a protection strategy for the capacity attenuation of the fluorine-containing cathode material is obtained,and a TiO₂ layer is coated on the surface of the material to improve the cycle stability of the material.After 80 cycles at 1 C rate,the sample capacity retention rate increased by 27.4%after coating.