Transition metal NaMF₃ compounds as model systems for studying the feasibility of ternary Li-M-F and Na-M-F single phases as cathodes for lithium-ion and sodium-ion batteries

The high electronegativity of fluorine suggests that fluorides may enable cathode materials for lithium-ion and sodium-ion batteries with higher voltage profiles for insertion or larger capacities for conversion. Iron has traditionally been studied as a redox center in fluorine-based cathodes, but the other metals have rarely been investigated. We demonstrate that the NaMF ₃ series (M = Mn, Fe, Co, Ni, Cu) offers a convenient means to compare the feasibility of the first row transition metals employed as fluorine-based cathodes for sodium-ion and lithium-ion batteries. NaFeF ₃ was found to be the only electrochemically active compound, which is able to undergo reversible Na⁺ reinsertion. This is ascribed to: (i) the chemical nature of the charged state FeF₃ which is the only fluorine acceptor and chemically the most stable compound among the other MF₃ and (ii) the existence of two FeF₃ polymorphs with closer structural resemblance to the parent NaFeF₃, which may contribute to the rigidity of the parent NaFeF₃ structure during desodiation.