Low-cost layered oxide cathode involving cationic and anionic redox with a complete solid-solution sodium-storage behavior

Iron/manganese-based layered transition-metal oxides have attracted numerous attentions as promising cathodes for sodium-ion batteries (SIBs) due to their high theoretical capacities and abundant reserves, yet they still suffer from detrimental phase transitions and fast capacity fading. Herein, we realize a complete solid-solution sodiation/desodiation behavior over a wide voltage range of 1.5–4.3 V in a novel P2-Na0.75Ca0.05Li0.15Fe0.2Mn0.6O2 (P2-NCLFMO) cathode material, where cationic and anionic redox reactions synergistically provide charge compensation. Li ions in transition-metal (TM) sites not only trigger the oxygen redox activity, but also increase the Na content for stabilizing the P2-structure at high voltage and weaken the Jahn-Teller effect by diluting the Mn3+ concentration; Ca ions in Na sites serve as “pillars” to stabilize the layered structure and alleviate the lattice oxygen release. The complete solid-solution reaction in the wide voltage range ensures both rapid Na+ diffusivity and small volume variation. Therefore, the P2-NCLFMO cathode shows outstanding rate capability (183 mAh g−1 at 0.1C in comparison with 49.9 mAh g−1 at 20C) and respectable cycling performance (76% capacity retention after 150 cycles at 1C). Additionally, the prototype Na-ion full battery constructed by the P2-NCLFMO cathode and hard carbon anode delivers a promising energy density of 246.3 Wh kg−1. This work provides a new platform for achieving high-energy and long-life layered oxide cathodes involving cationic and anionic redox by eliminating the irreversible phase transitions.