Na-site K pillar and Na-O-Mg configuration regulate lattice oxygen freedom in Fe/Mn-based cathodes enabling reversible anionic redox chemistry

The affordability of Fe/Mn-based cathodes renders them a highly competitive choice for sodium-ion batteries (SIBs). The path to commercialization for these cathodes is obstructed due to inherently slow kinetics and insufficient stability in ambient conditions. This work reports on the P2/O3-Na_0.67K_0.03Mn_0.5Fe_0.38Mg_0.12O₂ (NFM-KM2) cathode, where K/Mg co-doping confers outstanding sodium storage capacity and stability, with the mechanism elucidated in detail. A critical insight emerged from our analysis. Specifically, it regulated the lattice oxygen freedom to trigger and enhance the activity and reversibility of the anion redox reaction. In this way, K/Mg co-doping resolved the persistent challenge of harnessing anionic redox reaction (ARR): it demonstrated how modulating the oxygen’s local coordination and the lattice oxygen's degrees of freedom can effectively activate these reactions while ensuring their high-voltage reversibility. Leveraging this concept, NFM-KM2 integrated wonderful capacity (218.1 mAh g^-1 at 0.1 C), stability (93.0%/85.9%, 100/200 cycles), rate capability (92.1%), and energy density (416.7 Wh kg^-1). The study further deepened the understanding of structural evolution and charge compensation mechanisms. The findings elucidated how a specific local oxygen environment regulated lattice oxygen freedom to activate and sustain ARR, representing a paradigm shift in understanding inactive element doping and guiding innovative design of leverage reversible anionic redox chemistry in Fe/Mn-based systems.