Reversible Charge/Discharge Reaction of a Ternary Metal Fluoride, Pb₂CuF₆: A Highly Conductive Cathode Material for Fluoride-Ion Batteries

Fluoride-ion all-solid-state batteries with high energy densities are promising replacements for lithium-ion batteries. Herein, we report Pb2CuF6, which has a fluorite-related structure, as a cathode material for use in fluoride-ion batteries. The Pb2CuF6 cathode composite reacted to charging and discharging without the addition of a solid electrolyte. It exhibited a capacity of 319 mA h cm–3 per electrode volume during the first discharge, exceeding that of the conventional CuF2 electrode. The discharge capacity of Pb2CuF6 remained at 24% of the theoretical capacity, even at 1 mA cm–2 (corresponding to a rate of 0.77C), and the Cu utilization ratio was approximately four times higher than that of a CuF2 cathode. β-PbF2, a fluoride-ion conductor, and metallic Cu were formed during the discharging process of Pb2CuF6; during the subsequent charging process, Pb2CuF6 was formed again. All products of the charge–discharge reaction were present in a nanoparticle form. Pb2CuF6 exhibited an ionic conductivity of 3.0 × 10–5 S cm–1 at 140 °C. The high-rate performance of Pb2CuF6 is attributed to both the high ionic conductivity of the reaction products and the formation of nanoparticles. These findings are beneficial for the material design to improve the charge–discharge performance of electrode materials for fluoride-ion batteries.