Mechanistic Insights Into Anionic Doping in O3‑Type Na(NiFeMn)_1/3O₂ Cathode

The lack of mechanistic guidelines hinders rational anionic doping in layered oxide cathodes for sodium-ion batteries (SIBs). Using O3-type Na(NiFeMn)_1/3O₂ 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.