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_0.78Ni_0.11Li_0.12Zn_0.1Mn_0.67O₂ (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.