Interlayer–Surface Synergistic Regulation in High‐Entropy O3‐Type Layered Oxides Toward Structurally Robust and Air‐Stable Sodium‐Ion Batteries

O3‐type layered transition metal oxides offer high theoretical energy density and abundant Na + storage sites for sodium‐ion batteries but suffer from complex multistage phase transitions and poor air stability. Herein, a high‐entropy layered cathode NaNi 0.3 Mn 0.3 Fe 0.2 Ti 0.1 Cu 0.06 Mg 0.04 O 1.95 F 0.05 , was designed via cation–anion co‐doping and rational elemental tuning. The incorporation of Ti and Mg enhances lattice rigidity and alleviates local structural strain, while F effectively suppressing the Jahn–Teller distortion, thereby improving structural stability and mitigating multistage phase transitions through synergistic cation regulation. Furthermore, Mg/F regulation of the interlayer structure and surface chemistry induces moderate contraction of the Na interlayer spacing and a low‐polarity surface, thereby enhancing air stability while maintaining fast Na + transport kinetics. The cathode delivers a high reversible capacity of 130.8 mAh g −1 at 0.1 C, retains 85% capacity after 500 cycles at 2 C, and maintains 83 mAh g −1 at 10 C. Notably, after 30 days of air exposure, 88.6% of the initial capacity is preserved, and a full cell paired with a hard carbon anode achieves an energy density of 257.6 Wh kg −1 with excellent cycling stability. This work demonstrates an effective high‐entropy design strategy for developing O3‐type layered cathodes with improved structural stability and air stability.