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 NaMF3 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. NaFeF3 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.