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_0.3Mn_0.3Fe_0.2Ti_0.1Cu_0.06Mg_0.04O_1.95F_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.