Reversible Insertion in AFeF₃ (A = K⁺, NH₄⁺) Cubic Iron Fluoride Perovskites

The search for new cathode materials is primordial for alkali-ion battery systems, which are facing a constantly growing demand for high energy density storage devices. In quest of more performing active compounds on the positive side, anhydrous iron(III) fluoride demonstrated to be a good compromise in terms of high capacity, operating voltage, and low cost. However, its reaction toward lithium leads to complicated insertion/conversion reactions, which hinder its performances in Li-ion cells. Cycling this material against larger ions such as sodium and potassium is hard or simply impossible due to the size of the channels of the FeF3 framework impeding ions diffusion. Herein, we propose a strategy based on the use of cubic perovskite AFeF3 (A = K+, NH4+) as starting materials, allowing the straightforward insertion (after a first disinsertion of the alkali and/or NH4+ ion) of lithium within the structure and enabling the cycling toward larger alkali ions such as sodium and potassium. For example, a cubic KFeF3 perovskite, produced by a facile synthesis method, shows superior rate capability toward lithium retaining a capacity of up to 132 mA·h·g–1 at 5 C or of 120 mA·h·g–1 at 5 C toward sodium and enabling cycling toward potassium. Moreover, cubic NH4FeF3 perovskite is discussed for the first time as the suitable cathode material for alkali-ion batteries.