Dual‐Site Mg/Zn Substitution in Fe–Mn–Ni Layered Oxides: High‐Entropy Engineering for Stable Oxygen Redox and Enhanced Sodium‐Ion Storage

The development of high‐performance O3‐type cathode materials for sodium‐ion batteries (SIBs) is hindered by structural instability and limited reversibility of oxygen redox reactions (ORR). Herein, a dual‐substitution strategy is proposed to synergistically activate stable ORR and structural reinforcement in NaFe 0.33 Mn 0.33 Ni 0.33 O 2 (FMN). Mg substitution induces anion redox activity, achieving a high initial capacity of 163.2 mAh g −1 , while Zn substitution stabilizes the host structure, enabling 71.6% capacity retention after 100 cycles. By integrating these effects through high‐entropy engineering, NaMg 0.1 Zn 0.15 Fe 0.11 Mn 0.4 Ni 0.23 O 2 (MZFMN) is synthesized, which exhibits a balanced electrochemical performance, with a high initial discharge capacity of 154.5 mAh g −1 and superior cyclability of 78.0% retention after 100 cycles. Mechanistic studies reveal that Mg facilitates reversible ORR, Zn mitigates phase transitions via covalent Zn‐O bonding, and the high‐entropy configuration suppresses irreversible structural degradation. This work establishes a paradigm for designing multifunctional cathodes by combining cation substitution and entropy‐driven stabilization, advancing SIBs toward practical energy storage applications.