Jahn–Teller Distortion Triggers Unconventional Molecular O 2 Formation in P2‐Type Na 0.67 MnO 2 Cathodes

ABSTRACT Oxygen redox chemistry offers a pathway to high energy density in sodium‐ion batteries but is often accompanied by irreversible molecular O 2 formation, leading to voltage hysteresis and poor cycling stability. The origin of molecular O 2 formation is commonly attributed to high‐voltage charging yet remains debatable. Here, we uncover an overlooked mechanism in P2‐type Na 0.67 MnO 2 , where molecular O 2 formation is decoupled from the conventional high‐voltage paradigm and instead originates from Jahn–Teller distortion during discharge. Combined experimental and computational results reveal that Jahn–Teller‐induced lattice distortion shortens O–O distances and enhances the O–O dimerization tendency, leading to irreversible oxygen redox and structural degradation. This finding establishes a previously unrecognized distortion‐mediated pathway governing oxygen redox. To address this issue, an Al/Si co‐doping strategy is developed to regulate the electronic structure and reinforce the lattice framework. The co‐doping suppresses Jahn–Teller distortion, stabilizes O 2p states, and weakens the tendency toward O–O dimerization, thereby inhibiting molecular O 2 formation. Consequently, the optimized Na 0.67 Al 0.01 Si 0.01 Mn 0.99 O 2 exhibits superior cycling performance, retaining 95% capacity after 100 cycles within 4.4–2.0 V and 90% after 500 cycles within 4.0–2.0 V. These findings provide a mechanistic framework for oxygen redox chemistry and offer a practical design strategy for stabilizing high‐voltage manganese‐based sodium‐ion batteries.