Boosting the multi-electron reaction capability of Na4MnCr(PO4)3 by a fluorine doping strategy for sodium-ion batteries

Phosphate cathode materials have received particular attention in recent years. However, phosphates generally deliver a low capacity, which limits the potential of sodium-ion batteries (SIBs) to achieve a high energy density. In this work, we have prepared an NASICON-type fluorine-doped Na3.85MnCr(PO3.95F0.05)3 (NMCPF-0.05). NMCPF-0.05 can deliver a discharge capacity of 125.1 mAh g−1 at 10 mA g−1 based on a three-electron transfer reaction associated with Mn4+/Mn2+ and Cr4+/Cr3+ redox couples, which is significantly superior to Na4MnCr(PO4)3 (117.8 mAh g−1). The density functional theory calculations and galvanostatic intermittent titration technique measurements demonstrate that the enhanced electrochemical activity of NMCPF-0.05 is attributed to the improvements of electronic conductivity and Na+ diffusion kinetics due to fluorine doping. The ex-situ X-ray diffraction patterns reveal that the Na intercalation/deintercalation of NMCPF-0.05 follows a solid-solution reaction mechanism. This study presents a viable strategy to boost the electrochemical activity of NASICON-type MnCr-based phosphate cathode materials with multi-electron reaction capability for high-energy-density SIBs.