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

Iron-manganese-based polyanionic compounds (Na₄Fe_3-xMn_x(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 Na₄Fe₂Mn_1-5y/3Nb_2y/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 Na₄Fe₂Mn_0.9Nb_0.04□_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.