Research On Synthesis And Electrochemical Performance Of Na₃V₂（PO₄）₂O₂F Cathode For Sodium-ion Batteries

Sodium-ion batteries（SIBs）with high abundance and low cost are expected to become a new generation of energy storage techniques for large-scale energy storage systems.The development of cathode materials is an important step in the building of high energy density sodium-ion batteries.Among all the cathode materials,Na₃V₂（PO₄）₂O₂F（NVPOF）is receiving active study due to its high structural integrity,satisfactory work voltage,and outstanding energy density.However,its practical application is still impeded by the low intrinsic electronic conductivity.Herein,based on the hydrothermal synthesis of NVPOF,we explored the optimal hydrothermal process conditions.In addition,aiming at the poor conductivity of NVPOF,MXene nanosheets and carbon were used to composite NVPOF respectively,in order to obtain electrode materials with excellent electrochemical performance.Through systematic testing and analysis,the main results are as follows:（1）The results of the temperature and time gradient experiments of the hydrothermal process shows that the highest yield and best crystallinity of p-NVPOF can be obtained under the hydrothermal conditions of 180℃ and 12 h.The microstructure of p-NVPOF is stacked with bulk blocks messily.The electrochemical performance of p-NVPOF needs further improvement.（2）The NVPOF/MXene composite was synthesized by using single-layer MXene nanosheets with excellent electrical conductivity and large specific as the substrate for the growth of NVPOF.The composite retains the layered structure and the grain size of NVPOF tends to be nano-scale.Mxene nanosheets as conductive matrix can improve the electrical conductivity of the electrode material and the nano-scale particle size can shorten the diffusion distance of sodium ions.NVPOF/MXene exhibits a reversible capacity of 105.7 m A h g^-1 at a current density of 1 C and the capacity almost dont’t decay after 100 cycles.The improved rate performance and excellent cyclic stability indicate that compounding with MXene could enhance the electrochemical performance of NVPOF effectively.（3）Na₃V₂（PO₄）₂O₂F grown on the surface of the core carbon sphere was synthesized via a one-step hydrothermal method.Subsequently,the above particles coated with a uniform N-doped carbon layer（CS@NVPOF@NC）were successfully fabricated.This sandwiched structure built by core carbon sphere and carbon layer can significantly improve the electronic conductivity,boost sodium-ion diffusion kinetics and mitigate volume change of electrode upon cycling process.As a result,the as-prepared electrode exhibits a desirable specific capacity of 119.8 m A h g^-1 with an attractive capacity retention of 96%after 100 cycles at the current density of 1 C.The capacity decay per cycle is only 0.032%during 500 cycles at a high current density,indicating outstanding electrochemical performance.