Unlocking Fast Na+ Migration in F-Doped O3-Type Cathodes via First-Principles Calculations

O3-type layered transition-metal oxides are widely regarded as promising cathode materials for sodium-ion batteries due to their intrinsically high sodium content and favorable energy density. Nevertheless, their practical rate capability is hindered by sluggish Na+ transport and relatively high diffusion barriers. To address this issue, elemental substitution has emerged as an effective modification strategy. In this work, fluorine (F), characterized by strong electronegativity and a small ionic radius, is introduced to partially substitute oxygen in the bulk lattice of O3-type NaNi1/3Fe1/3Mn1/3O2 (NNFM). First-principles calculations demonstrate that F incorporation leads to an expansion of the interlayer spacing along the c-axis and a weakening of Na–O interactions, both of which facilitate Na+ migration. Among the considered configurations, Mn-adjacent substitution exhibits the lowest formation energy, indicating enhanced thermodynamic stability. Furthermore, electronic structure analysis reveals a reduced band gap (from 0.515 eV to 0.342–0.356 eV) and strengthened O-2p/Mn-3d orbital hybridization, contributing to improved electronic conductivity. These findings provide atomistic insights into F-induced modulation mechanisms and suggest an effective pathway for optimizing Na+ transport in O3-type cathodes.