Dual anionic substitution engineering for an advanced NASICON phosphate cathode in sodium-ion batteries

Sodium-ion batteries (SIBs) are prominently used for stationary energy storage due to the abundant resources and low cost of Na. The development of high-performance cathodes for SIBs will be a favorable choice for competing with the market-dominant lithium-ion batteries. Among the various cathodes, Na3V2(PO4)(2)O2-2xF1+2x (0 <= x <= 1) materials have become a preferred choice due to their superior structural stability, fast ion transport and high operating potential. Herein, a series of materials with various ratios of F- and O2- (F/O) are prepared via a high temperature solid-state method, and the tuning mechanism of different F/O ratios is studied in detail by analyzing the structural evolution, electrochemical performance and reaction kinetics of materials. The optimal F/O ratio material Na3V2(PO4)(2)O0.6F2.4 (x = 0.7) exhibits a favorable rate and cycling performance. The capacity at 20C is equivalent to that of the x = 0 material at 5C, and each cycle decay is 0.040% after 200 cycles at 0.5C. Moreover, the optimized F/O ratio material (x = 0.7) also demonstrates excellent reaction kinetics, and the Na apparent diffusion coefficient (D-app,D-Na) for the high potential region is about 10(-10)-10(-12) cm(2) s(-1). A systematic research of dual anion substitution in phosphates will be useful for the structural design and performance improvement of other cathode materials in SIBs.