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

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 TMO₂ slabs, but also facilitates the appropriate facet exposure to maintain sufficient interlayer Na⁺ shielding within the single NaO₂ 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.