Unveiling the Complementary Manganese and Oxygen Redox Chemistry for Stabilizing the Sodium‐Ion Storage Behaviors of Layered Oxide Cathodes

Layered oxide cathodes for sodium‐ion batteries (SIBs) have drawn increasing attention owing to their fascinating additional capacity contributed by oxygen‐redox chemistry. Unfortunately, excessive oxygen redox incurs an irreversible oxygen release, deteriorating the cyclic stability and compromising the advantage of additional capacity. Significant efforts have been made so far to stabilize lattice oxygen, but the potential advantages associated with oxygen loss have been ignored. Herein, a complementary Mn and O redox mechanism is first revealed in a novel P2‐Na0.75Ca0.04[Li0.1Ni0.2Mn0.67]O2 cathode. The partial oxygen release activates more Mn3+/Mn4+ redox capacities upon cycling, which effectively compensate the capacity loss from O2‐/On‐ redox and thus enable an ultra‐stable cycling performance (capacity retentions of 102.1% and 95.2% over 100 and 200 cycles at 5 C, respectively). The fast Na+ diffusion kinetics of the cathode also ensure an exceptional rate capability (133.1 and 68.8 mAh g‐1 at 0.1 and 20 C, respectively). The electrode crystal/electronic structure evolution and charge compensation mechanism upon sodiation/desodiation have been elucidated via systematic operando measurements and theoretical computations. The complementary chemistry is a universal principle that stabilizes the Na‐storage behaviors of Mn‐based oxide cathodes, providing new opportunities for anionic redox to boost the energy density and cycling life of SIBs simultaneously. A complementary Mn and O redox mechanism, namely, the gradually activated Mn3+/Mn4+ redox capacities associated with partial oxygen release upon cycling could effectively compensate the capacity loss from O2–/On– redox, is first proposed to explain the admirable cycling stability of Mn‐based layered oxide cathodes for sodium‐ion batteries.