Composite anode for fluoride-ion batteries using alloy formation and phase separation in charge and discharge processes

Abstract

For the development of fluoride-ion batteries, new design criteria for anode materials must be established. Although LaF₃ is a possible anode material owing to its fluoride-ion conductivity, the redox potential of La/LaF₃ is too low (−2.4 V vs. Pb/PbF₂) and most electrolytes are decomposed. Here, we propose (In + LaF₃-based material) composite anodes to positively shift the redox potential via the reversible formation of an intermetallic phase. We first show that the redox potential of the (In + La_0.9Ba_0.1F_2.9) anode is higher than that of La/LaF₃ by 0.6 V, which can prevent electrolyte decomposition. Next, we demonstrate via the scanning transmission electron microscopy analysis of an (In + LaF₃) anode that defluorination and fluorination of LaF₃ occur with the formation and decomposition of In₃La, respectively. Such reversible formation of an intermetallic phase decreases the difference in the Gibbs free energy between the discharged and charged states, which decreases the electromotive force and results in a positive shift in the redox potential. Our results will provide a method to control the redox potentials of fluoride-ion battery anodes via reversible alloy formation, which is likely to considerably increase the number of available electrolytes and be a step towards developing practical fluoride-ion batteries.