Synergistical Engineering of Vacancy and Doping Enables High-Rate and Ultrastable NaFeMn(PO)PO Cathode for Sodium Ion Batteries.

Iron-manganese-based polyanionic compounds (NaFeMn(PO)PO) have attracted extensive interest as cathode for sodium ion batteries (SIBs) owing to higher working voltage, and higher energy density compared to NaFe(PO)PO. However, such cathode suffers from sluggish Na diffusion, severe voltage hysteresis, and large structural strain caused by Jahn-Teller distortion of Mn. Taking NaFeMn(PO)PO (NFMPP) as an example, herein, a synergistic engineering strategy of vacancy and doping for constructing NaFeMnNb□(PO)PO cathodes (□: vacancy) is developed where vacancies endow rigid MnO octahedra more flexible for boosting Na mobility while Nb doping stabilize crystal structure. Based on theoretical calculations and electrochemical characterizations, the optimized NaFeMnNb□(PO)PO 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 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 at 0.1 C and a capacity retention of 88.2% after 300 cycles in full cell.