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.

High-entropy oxides (HEOs) and medium-entropy oxides (MEOs) are new types of single-phase solid solution materials. MEOs have rarely been reported as positive electrode material for sodium-ion batteries (SIBs). In this study, we first proposed the concept of the application of MEOs in SIBs. P2-type 3-cation oxide Na_2/3Ni_1/3Mn_1/3Fe_1/3O₂ (NaNMF) and 4-cation oxide Na_2/3Ni_1/3Mn_1/3Fe_1/3-xAl_xO₂ (NaNMFA) were prepared using the solid-state method, rather than the doping technology. In addition, the importance of the concept of entropy stabilization in material performance and battery cycling was demonstrated by testing 3-cation (NaNMF) and 4-cation (NaNMFA) oxides in the same system. Thus, NaNMFA can provide a reversible capacity of about 125.6 mAh·g^-1 in the voltage range of 2-4.2 V, and has enhanced cycle stability. The capacity and decay law of the MEO batteries indicate that the configurational entropy (1.28 R (NaNMFA) > 1.10 R (NaNMF)) of the cationic system, is the main factor affecting the structural and cycle stability of the electrode material. This work emphasizes that the rational design of MEOs with novel structures and different electrochemically active elements may be the strategy for exploring high-performance SIB cathode materials in next-generation energy storage devices.