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 + La0.9Ba0.1F2.9) 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.