Mechanistic Insights Into Anionic Doping in O3‑Type Na(NiFeMn) 1/3 O 2 Cathode

ABSTRACT The lack of mechanistic guidelines hinders rational anionic doping in layered oxide cathodes for sodium‐ion batteries (SIBs). Using O3‐type Na(NiFeMn) 1/3 O 2 as a model and combining experiments with density functional theory (DFT) calculations, we reveal that doping effects are governed by two intrinsic dopant properties: valence‐electron configuration and ionic radius. For radius‐matched dopants, electron‐donating F reduces Fe 3 + , enhancing high‐voltage and air stability, whereas electron‐withdrawing N oxidizes Ni 2 + , accelerating degradation. Oversized (Cl, Br) or mismatched (B) dopants cause structural collapse. This dual‐parameter framework enables predictive design of stable, high‐performance cathodes.