Unlocking fast and reversible sodium intercalation in NASICON Na4MnV(PO4)3 by fluorine substitution

The exploitation of high energy and high power densities cathode materials for sodium ion batteries is a challenge. Na-super-ionic-conductor (NASICON) Na4MnV(PO4)3 is one of promising high-performance and low-cost cathode materials, however, still suffers from not reaching the theoretical capacity, low rate capability, and poor cycling stability. In this work, we deploy a novel sodium-deficient NASICON fluorinated phosphate cathode material for sodium ion batteries which demonstrates, notably, high energy and high power densities concomitant with high sodium diffusion kinetics. The enhanced performance of this novel Na3.85⬜0.15MnV(PO3.95F0.05)3 cathode was evidenced by demonstrating a relatively high energy density of ∼380 Wh kg−1 at low rate with much improved rate capability compared to non-doped Na4MnV(PO4)3, and long cycling life over 2000 cycles at high current rates. The structural investigation during battery operation using in situ x-ray diffraction (XRD) reveals bi-phase mechanism with high structural reversibility. The combined XRD and 23Na nuclear magnetic resonance (NMR) analyses demonstrate that the sodium extraction/insertion from Na2 is faster than Na1 site. These findings open promising prospects for unlocking of high energy and high power densities of NASICON phosphate materials by fluorine substitution towards high-performance sodium ion batteries.