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.