Buffered Fluorination Strategy for Stabilizing Sodium‐Rich O3‐Type Layered Oxides in High‐Humidity Environments for Sodium‐Ion Batteries

O3-type layered oxides have emerged as promising cathode candidates for sodium-ion batteries (SIBs). However, progressive capacity fade persists due to unstable cathode-electrolyte interphase (CEI) evolution and structural degradation during deep desodiation, compounded by moisture sensitivity that severely compromises processability. Herein, a buffered fluorination strategy derived from Na3FeF6 is demonstrated for engineering oxide surfaces in a high humidity, through which effectively mitigates structural collapse caused by moisture-induced sodium leaching via constant acidity and constant release rate of F-. The resultant armor-like artificial CEI imposes a domain-limiting effect, suppressing detrimental phase transitions during deep desodiation. Moreover, combining theoretical calculations and experiments reveals that surface fluorination can inhibit the lattice oxygen mobility, impede the iron dissolution, and alleviate the lattice distortion on the surface. In short, the results extend the concept of high-humidity surface engineering to the more challenging sodium-rich O3-type layered oxides.