Reconsidering Anode Materials for Fluoride-Ion Batteries – The Unexpected Roles of Carbide Formation

Carbon is a ubiquitous additive to enhance the electrical conductivity of battery electrodes. Although carbon is generally assumed to be inert, the poor reversibility seen in some fluoride-ion battery electrodes has not been explained or systematically explored. Here, we utilize the Materials Project database to assess electrode deactivation reactions that result in the formation of a metal carbide. Specifically, we compare the theoretical potentials of MFy reduction to either the corresponding metal or metal carbide MCx. We find that the formation of MCx is relevant in electrode materials that contain electropositive metals such as lanthanide fluorides but is unlikely to be important in electrodes that contain electronegative metals such as ZnF2, PbF2 or SnF2. The side reactions that form metal carbides therefore offer a possible explanation of the poor reversibility of lanthanide- or alkaline earth metal-based electrode materials. Finally, we highlight that the carbide formation process might be exploited for designing cheap anode systems with improved reversibility.