Strengthened π-type interaction in layered oxide cathodes with reversible anionic redox for sodium-ion batteries.

Activating anionic redox activity in P2-type layered oxide cathodes is a promising pathway to enhance the specific capacity for sodium-ion batteries (SIBs). However, the highly active anionic redox process arising from the non-bonding O 2p orbitals frequently leads to irreversible oxygen release and surface degradation, which severely limit the long-term cycling stability. Herein, we propose a strategy of strengthening transition metal-oxygen (TM-O) π-type interaction with a regulated local oxygen coordination environment by incorporating the Ru/Ru redox couple into P2-type NaLiMnO (NLMO) to achieve a reversible anionic redox reaction. Upon high-voltage charging, the formed Ru state with a half-filled t 4d electronic configuration establishes a strengthened π-type interaction with non-bonding O 2p orbitals within the Na-O-Li configuration compared to the inherently weaker Mn-O π-type interaction in NLMO. Such a strengthened π-type interaction effectively enhances anionic redox reversibility, suppresses irreversible oxygen release and realizes a complete solid-solution behavior with stable TMO octahedra throughout cycling. This preserved structural integrity also prevents crack formation and minimizes transition metal dissolution, thereby mitigating surface degradation. The resulting NaLiMnRuO (NLMRO) thus exhibits a reversible anionic redox activity with markedly improved cycling stability. Our work highlights that dynamically engineering potent π-type interaction during electrochemical cycling is a promising avenue for developing high-performance SIBs with cumulative cationic and anionic redox reactions.