Reversible Insertion in AFeF 3 (A = K + , NH 4 + ) 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 performant 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 towards 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 towards lithium retaining a capacity of up to 132mAh.g-1 at 5C, or of 120mAh.g-1 at 5C towards sodium and enabling cycling towards potassium. Moreover, cubic NH4FeF3 perovskite is discussed for the first time as suitable cathode material for alkali-ion batteries.