Synergistical Engineering of Vacancy and Doping Enables High-Rate and Ultrastable Na4Fe2Mn(PO4)2P2O7 Cathode for Sodium Ion Batteries.

Iron-manganese-based polyanionic compounds (Na4Fe3-xMnx(PO4)2P2O7) have attracted extensive interest as cathode for sodium ion batteries (SIBs) owing to higher working voltage, and higher energy density compared to Na4Fe3(PO4)2P2O7. However, such cathode suffers from sluggish Na+ diffusion, severe voltage hysteresis, and large structural strain caused by Jahn-Teller distortion of Mn3+. Taking Na4Fe2Mn(PO4)2P2O7 (NFMPP) as an example, herein, a synergistic engineering strategy of vacancy and doping for constructing Na4Fe2Mn1-5y/3Nb2y/3□y(PO4)2P2O7 cathodes (□: vacancy) is developed where vacancies endow rigid MnO6 octahedra more flexible for boosting Na+ mobility while Nb doping stabilize crystal structure. Based on theoretical calculations and electrochemical characterizations, the optimized Na4Fe2Mn0.9Nb0.04□0.06(PO4)2P2O7 exhibits the lowest Na+ diffusion energy barrier, the best rate capability and cyclability at both room temperature and elevated temperature than those of NFMPP with only vacancy/doping. Such cathode shows a specific capacity of 63.6 mAh g-1 at 50 C, a capacity retention of 91.0% after 10 000 cycles at 10 C, and a capacity retention of 70.1% after 3 000 cycles at 10 C (50 °C) in half cell, and also delivers a reversible capacity of 96.5 mAh g-1 at 0.1 C and a capacity retention of 88.2% after 300 cycles in full cell.