Proton-Exchange Surface Engineering Enables Air-Stable Sodium Layered Oxide Cathodes for Carbon-Neutral Energy Storage.

O3-type layered oxides have emerged as one of the most promising cathode candidates for sodium-ion batteries, which are widely regarded as a key enabling technology for carbon-neutral grid-scale energy storage. However, their practical application is hindered by surface degradation upon air exposure and structural distortion from Mn3+. Herein, we develop a two-step surface engineering protocol that combines proton-exchange washing with a short postcalcination to treat O3-type cathodes. Proton-exchange treatment removes surface carbonates and oxidizes Mn3+ to widen the interlayers for enhanced Na+ diffusion and Jahn-Teller suppression, while the subsequent short calcination heals residual defects and locks in the refined structure. The optimally treated Na0.82Cu0.19Fe0.39Mn0.42O2 cathode delivers a reversible capacity of 110.2 mAh g-1 at 0.2 C with 92.3% retention after 100 cycles (substantially outperforming the pristine sample at 48.6%), while retaining its superior performance even after one month of air exposure. This cost‑effective and scalable surface engineering approach thus removes a critical industrial bottleneck for sodium‑ion batteries, marking a tangible step toward sustainable storage technologies for the zero‑carbon transition.