Preparation And Electrochemical Properties Of Na₂FePO₄F/C Cathode Materials For Sodium-ion Batteries

Na₂FePO₄F had the advantages of high charge-discharge voltage,stable platform,large storage capacity and good chemical stability,because of its three-dimensional open frame structure.However,the material had low ion migration rate and poor cycle stability,which greatly limited its commercial application as a cathode material for sodium ion batteries.In this study,the high purity Na₂FePO₄F/C powder was synthesized by sol-gel method.The electrochemical properties of Na₂FePO₄F cathode materials were improved by optimizing the sol-gel process,controlling the morphology of the synthesized powders and the surface carbon coating.The composition and microstructure of the composites were analyzed by XRD,BET,SEM,TGA and Raman test.The electrochemical performance of all the composites at room temperature were investigated by constant current charging and discharging technique,cyclic voltammetry（CV）and AC impedance method（EIS）.The effects of calcination temperature and oxalic acid content on the structure and electrochemical properties of Na₂FePO₄F/C powders were studied in the sol-gel process.The results showed that the high purity Na₂FePO₄F/C electrode materials with good electrochemical performance could be synthesized at 600oC.The discharge capacity of it was 80.2 mAh g^-1 after 500 cycles in 0.5 C,maintaining the 87.75%of initial specific capacity(91.4 mAh g^-1).The morphology of the synthetic powder was regulated by adjusting the proportion of oxalic acid and raw materials in the sol-gel process.The Na₂FePO₄F/C cathode material with porous sponge like structure was prepared when the molar ratio of oxalic acid to raw material was 4:1.The porous sponge Na₂FePO₄F/C cathode material exhibited excellent electrochemical performance.The specific capacity of the first discharge at 0.1 C was 115.5 mAh g^-1.The specific capacity under2 C remained 73.9%of the initial specific capacity.At 0.5 C,the initial capacity was100.8 mAh g^-1.After 1000 cycles,it remained as high as 83.3 mAh g^-1 with only a 17.4%capacity loss.In order to further improve the electrochemical performance of porous Na₂FePO₄F/C materials,the effect of surface carbon coating on the structure and electrochemical properties of porous Na₂FePO₄F/C materials was studied.It was found that the 10.76 wt%glucose coated Na₂FePO₄F/C material exhibited a smaller particle size,more uniform pore distribution and good electrochemical performance,and the discharge specific capacity under 0.1 C-5 C was 110.6,106.5,100.3,92.7,81.8 and64.5 mAh g^-1,respectivelly.