Achieving highly reversible anionic redox and negligible voltage decay by a high-entropy strategy in Na-ion layered...

Leveraging anionic redox in layered oxide cathodes offers a promising strategy to achieve high-energy-density sodium-ion batteries. Nevertheless, irreversible oxygen release remains a major challenge, as it undermines structural integrity and thus leads to severe voltage decay, contributing to rapid capacity loss. Herein, we propose a high-entropy configuration strategy to simultaneously enhance anionic redox reversibility and suppress voltage decay for Na-ion layered oxide cathodes. By engineering a multi-elemental configuration within the transition metal slabs of NaNi_0.4Fe_0.2Mn_0.4O₂ (NFM), the entropy-stabilized Na_0.9Li_0.05Ni_0.3Fe_0.1Cu_0.05 Mn_0.4Ti_0.1O_1.95F_0.05 (NFM-HEO) cathode exhibits significantly improved structural robustness and oxygen redox stability. It is demonstrated that the overoxidation of oxygen anions induces lattice oxygen loss and oxygen vacancy formation, leading to structural deterioration and voltage fading. The incorporation of a high-entropy configuration modulates the electronic structure and introduces strong CuO and TiO bonds, thereby enhancing oxygen electron localization and inhibiting oxygen evolution, which collectively mitigate oxygen vacancy formation and structural degradation. As a result, the entropy-engineered NFM-HEO achieves a high reversible capacity (153.6 mAh g^-1 at 0.1 C), outstanding cycling stability (83 % capacity retention at 1 C for 300 cycles), and minimal voltage decay (0.4 mV per cycle), showing highly anionic redox reversibility and negligible voltage decay. This work provides insights for the oxygen redox reversibility and paves a way for designing outstanding layered high-entropy cathode materials.