Comparative Study of Tavorite and Triplite LiFeSO₄F as Cathode Materials for Lithium Ion Batteries: Structure, Defect Chemistry, and Lithium Conduction Properties from Atomistic Simulation

To explore the possibility of LiFeSO₄F with two polymorphs (tavorite and triplite) as the cathode material for lithium ion batteries, structure, defect chemistry, and Li⁺ ion conduction properties are studied by atomistic simulation with empirical potential parameters. We investigate the correct structure of tavorite LiFeSO₄F, which was newly determined. The concentration of intrinsic defects in the tavorite form is very low in comparison with the triplite form. Configurations of FeO₄F₂ octahedra in the triplite form are in favor of corner-sharing connections over edge-sharing ones. Even though calculated migration energies prove that both isomorphs are Li⁺ ion conductors, the Li⁺ ions in the triplite LiFeSO₄F move in the restricted migration paths (one- or two-dimensional), whereas the tavorite isomorph has a continuous three-dimensional Li⁺ ion conducting network