Double-shelled hollow Na2FePO4F/C spheres cathode for high-performance sodium-ion batteries

In this study, the hierarchical hollow Na 2 FePO 4 F/C microspheres as high-performance cathode for sodium-ion batteries (SIBs) are developed by adjusting the reaction time of solvothermal synthesis. With prolonging solvothermal time, the structure of the microspheres gradually changes from urchin-like hollow structure to acanthosphere-like hollow structure and finally to double-shelled hollow structure. Dissolution–recrystallization mechanism is proposed to better understand the formation of the double-shelled hollow microspheres of Na 2 FePO 4 F/C. When evaluated as cathode materials for SIBs, the double-shelled hollow Na 2 FePO 4 F/C sample delivers a discharge capacity as high as 120.1 mAh g −1 at 0.1 C and maintains the capacity retention of 92.5% at 1 C after 200 cycles. For the purpose of explaining the improved electrochemical performance of the double-shelled hollow Na 2 FePO 4 F/C materials, all the electrodes are analyzed with cyclic voltammetry and electrochemical impedance spectroscopy. It is found that the excellent electrochemical performances are mainly attributed to its unique structure, which can enhance electronic and ionic conductivity during repeated Na + insertion/extraction processes.