Unlocking iron metal as a cathode for sustainable Li-ion batteries by an anion solid solution

Traditional cathode chemistry of Li-ion batteries relies on the transport of Li-ions within the solid structures, with the transition metal ions and anions acting as the static components. Here, we demonstrate that a solid solution of F^- and PO₄^3- facilitates the reversible conversion of a fine mixture of iron powder, LiF, and Li₃PO₄ into iron salts. Notably, in its fully lithiated state, we use commercial iron metal powder in this cathode, departing from electrodes that begin with iron salts, such as FeF₃. Our results show that Fe-cations and anions of F^- and PO₄^3- act as charge carriers in addition to Li-ions during the conversion from iron metal to a solid solution of iron salts. This composite electrode delivers a reversible capacity of up to 368 mAh/g and a specific energy of 940 Wh/kg. Our study underscores the potential of amorphous composites comprising lithium salts as high-energy battery electrodes.