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 Ru⁴⁺/Ru⁵⁺ redox couple into P2-type Na_0.6Li_0.2Mn_0.8O₂ (NLMO) to achieve a reversible anionic redox reaction. Upon high-voltage charging, the formed Ru⁵⁺ state with a half-filled t_2g 4d³ 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 TMO₆ octahedra throughout cycling. This preserved structural integrity also prevents crack formation and minimizes transition metal dissolution, thereby mitigating surface degradation. The resulting Na_0.6Li_0.2Mn_0.7Ru_0.1O₂ (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.