Synergistical Engineering of Vacancy and Doping Enables High‐Rate and Ultrastable Na 4 Fe 2 Mn(PO 4 ) 2 P 2 O 7 Cathode for Sodium Ion Batteries

Abstract Iron‐manganese‐based polyanionic compounds (Na 4 Fe 3‐x Mn x (PO 4 ) 2 P 2 O 7 ) have attracted extensive interest as cathode for sodium ion batteries (SIBs) owing to higher working voltage, and higher energy density compared to Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 . However, such cathode suffers from sluggish Na + diffusion, severe voltage hysteresis, and large structural strain caused by Jahn‐Teller distortion of Mn 3+ . Taking Na 4 Fe 2 Mn(PO 4 ) 2 P 2 O 7 (NFMPP) as an example, herein, a synergistic engineering strategy of vacancy and doping for constructing Na 4 Fe 2 Mn 1‐5y/3 Nb 2y/3 □ y (PO 4 ) 2 P 2 O 7 cathodes (□: vacancy) is developed where vacancies endow rigid MnO 6 octahedra more flexible for boosting Na + mobility while Nb doping stabilize crystal structure. Based on theoretical calculations and electrochemical characterizations, the optimized Na 4 Fe 2 Mn 0.9 Nb 0.04 □ 0.06 (PO 4 ) 2 P 2 O 7 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.