LixFeF6 (x = 2, 3, 4) battery materials: structural, electronic and lithium diffusion properties

Abstract

Lithium iron fluoride materials have attracted recent interest as cathode materials for lithium ion batteries. The electrochemical properties of the high energy density Li_xFeF₆ (x = 2, 3, 4) materials have been evaluated using a combination of potential-based and DFT computational methods. Voltages of 6.1 V and 3.0 V are found for lithium intercalation from Li₂FeF₆ to α-Li₃FeF₆ and α-Li₃FeF₆ to Li₄FeF₆ respectively. The calculated density of states indicate that Li₂FeF₆ possesses metallic states that become strongly insulating after lithium intercalation to form α-Li₃FeF₆. The large energy gain associated with this metal–insulator transition is likely to contribute to the associated large voltage of 6.1 V. Molecular dynamics simulations of lithium diffusion in α-Li₃FeF₆ at typical battery operating temperatures indicate high lithium-ion mobility with low activation barriers. These results suggest the potential for good rate performance of lithium iron fluoride cathode materials.