First-principles study of iron oxyfluorides and lithiation of FeOF

First-principles studies of iron oxyfluorides in the FeF[subscript 2] rutile framework (FeO[subscript x]F[subscript 2−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 FeO[subscript x]F[subscript 2−x] (x≠1) structures, where FeF[subscript 2]-FeOF mixing is not energetically favored. The band gap of FeF[subscript 2] 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 Li[subscript 0.5]FeOF on lithiation is found, in agreement with recent Li-ion battery experiments. The energetics of further lithiation with respect to conversion scenarios are discussed.United States. Dept. of Energy. Office of Basic Energy Sciences (Northeastern Center for Chemical Energy Storage Award DE-SC0001294