First-principles Study of Iron Oxyfluorides and Lithiation of FeOF

First-principles studies of iron oxyfluorides in the FeF2 rutile framework (FeOxF2−x, 0 ≤ x ≤ 1) are performed using density functional theory (DFT) in the general gradient approximation (GGA) with a Hubbard U correction. Studies of O/F orderings reveal FeOF to be particularly stable compared to other FeOxF2−x (x 6= 1) structures, where FeF2–FeOF mixing is not energetically favored. The band gap of FeF2 is found to decrease as oxygen is substituted into its structure. The GGA+U electronic structure evolves from that of a Mott-Hubbard insulator (x = 0), to a charge transfer semiconductor (x = 1). Lithiation studies reveal that lithiation sites offering mixed O/F environments are the most stable. An insertion voltage plateau up to Li0.5FeOF on lithiation is found, in agreement with recent Li-ion battery experiments. The energetics of further lithiation with respect to conversion scenarios are discussed.