Properties of rich-Nae effect and reversible oxygen redox with a complete solid-solution sodium-storage behavior

P2-type Fe/Mn base oxide cathodes arouse much attention in sodium-ion batteries due to their high theoretical specific capacity, economic feasibility , and environmental friendliness . However, detrimental phase transitions and irreversible oxygen release result in continuous capacity decay, thereby posing a significant challenge to the simultaneous achievement of high capacity and high stability. To address this challenge, the optimal Na 0.67 Mg 0.06 Li 0.06 Fe 0.28 Mn 0.6 O 2 (NMLFM-2) cathode material , was developed in this study, which realizes a complete solid solution process and a stable oxygen redox reaction within the voltage window of 2–4.2 V. The introduction of Mg and Li realizes the “rich-Na e ” effect, which raises the valence state of Mn, thereby stabilizing the oxygen redox reaction and reducing the Jahn-Teller effect of Mn 3+ . The elimination of the unfavorable P2 to OP4 phase transition achieves a complete solid solution reaction, enhancing the structural stability and Na + fast reaction kinetics of the material. Thus, the NMLFM-2 cathode delivered a high initial specific capacity of 160.3 mAh g −1 at 0.1C within the voltage range of 2–4.2 V, realizing an impressive cycling performance with 89.3% capacity retention after 200 cycles at 1C. This study presents a novel pathway for simultaneously achieving structural stability and reversible oxygen redox, thereby enabling the trade-off between cathode capacity and stability in Fe/Mn-based layered oxides.