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_0.33Mn_0.33Ni_0.33O₂ (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_0.1Zn_0.15Fe_0.11Mn_0.4Ni_0.23O₂ (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.