Medium‐Entropy Regulation Enables Phase‐Stable Layered Oxide Cathodes with Reversible Anionic Redox for Sodium‐Ion Batteries

ABSTRACT Layered transition‐metal (TM) oxides with anionic redox reactions are promising cathode candidates for sodium‐ion batteries because of their high theoretical capacity and cost effectiveness, but they still suffer from severe P‐to‐O phase transition, irreversible TM migration, and lattice oxygen release. Herein, we report a strategy of rational entropy regulation for circumventing these multiple issues by systematically investigating layered TM oxide cathodes with low‐, medium‐, and high‐entropy configurations. It reveals that compared with the counterparts, the medium‐entropy cathode not only mitigates the lattice strain by accommodating the changes of local interactions conferred by entropy‐driven stabilization within the TMO 2 slabs, but also facilitates the appropriate facet exposure to maintain sufficient interlayer Na + shielding within the single NaO 2 slab, together delaying the P‐to‐O phase transition onset and suppressing the neighboring O‐type stacking. Therefore, this moderate medium‐entropy configuration enables reversible dynamic TM migration, benefiting from the robust phase stability, as revealed by in situ high‐energy‐resolution fluorescence‐detected X‐ray absorption spectroscopy results, which further minimizes oxygen vacancy formation and inhibits irreversible oxygen release. As a result, enhanced electrochemical performances with a long‐enduring reversible anionic redox activity are achieved. Our work underscores the critical role of rational entropy regulation for achieving high‐performance layered TM oxide cathodes.