Improved rate and cycling performances of Na₃V₂(PO₄)₂F₂O by Ti^3+/4+ doping with two oxidation states for sodium cathodes

Ti at the oxidation states of Ti³⁺ and Ti⁴⁺, was used to enhance the performance of Na₃V₂(PO₄)₂F₂O by partially substituting vanadium. After doping Ti, the crystallographic volume is decreased due to the less radii of Ti^3+/4+, and the valence of Ti is demonstrated identical to V. During sodium insertion in Ti-doped Na₃V₂(PO₄)₂F₂O, the two discharge plateaus split into three because of the rearrangement of local redox environment. Consequently, the optimized Na₃V_0.96Ti_0.04(PO₄)₂F₂O shows a specific capacity of 123 and 63 mA·h/g at 0.1C and 20C, respectively. After 350 cycles at 0.5C, the capacity is gradually reduced corresponding to a retention of 71.05%. The significantly improved performance is attributed to the rapid electrochemical kinetics, and showcases the strategy of replacing V^3+/4+ with Ti^3+/4+ for high-performance vanadium-based oxyfluorophosphates.