FeF3 as Reversible Cathode for All-Solid-State Fluoride Batteries

Abstract

Fluoride batteries are attracting intensive attention because they can provide a higher energy density than conventional lithium-ion batteries. Among various metal fluorides, FeF3 is a promising candidate for the cathode material of fluoride batteries because of its high theoretical capacity. In this report, the reversibility of an FeF3 cathode is investigated in conjunction with fluorite-type Ba0.6La0.4F2.4 as the electrolyte and Pb as the counter-electrode material. For the first time, the discharge–charge performance of a fluoride battery using FeF3 cathode is investigated. The initial discharge capacity is 579 mAh g−1, and a capacity of 461 mAh g−1 is retained at the 10th cycle. The reversible conversion reaction mechanism for FeF3 is clarified by X-ray diffraction and X-ray adsorption spectroscopy. The results revealed that FeF3 is reduced to FeF2 at the first-stage plateau and then to Fe metal at the second-stage plateau; they also reveal that the reverse process proceeded during charging. Ex situ scanning electron microscopy observations show that the morphology of the cathode changed reversibly and that, when the battery is in the discharged state, voids are present because of shrinkage of the electrode.