Unlocking Reversible Anionic Redox in Layered Oxides via a Cationic-Pair-Mediated Stabilization

The exploitation of anionic redox chemistry in P2-type layered oxides presents a pivotal pathway for boosting the energy density of sodium-ion batteries (SIBs). However, this process is intrinsically plagued by a fundamental trade-off: achieving high oxygen redox activity invariably triggers irreversible oxygen loss and severe structural degradation, including detrimental phase transitions and substantial volume variations. Here, we introduce a cationic-pair-mediated stabilization strategy via the cosubstitution of Li and Zn into the transition-metal layers of P2–Na 0.78 Ni 0.11 Li 0.12 Zn 0.1 Mn 0.67 O 2 (NNLZMO). We demonstrate that the formed Na–O–Li and Na–O–Zn configurations are not merely coexistent but function as an integrated synergistic pair. This pair electronically activates highly reversible anionic redox via the formation of the stabilized nonbonding O 2p states, concurrently impose structural confinement that suppresses Na + /vacancy ordering and blocks the irreversible P2–O2 phase transition. Consequently, the NNLZMO cathode undergoes a minimal-strain P2–Z phase transition with a negligible volume change of only 1.59% and achieves a high reversible capacity (174.62 mAh g –1 at 0.1C) and exceptional capacity retention (90.8% after 100 cycles at 1C) within a wide voltage window of 1.5–4.35 V. This work establishes the effective cationic-pair design principle for concurrently unlocking and stabilizing anionic redox in high-energy layered cathodes, paving the way for stable SIBS.