Solid Fluoride Electrolytes and Their Composite with Carbon: Issues and Challenges for Rechargeable Solid State Fluoride-Ion Batteries

Solid-state batteries relying on fluoride-ion shuttle are still at their early stage of development. Assessing the fluoride solid electrolyte’s electrochemical stability and its conduction properties in a mixture with carbon, as well as the possible interaction of fluoride-ion with carbon both during the electrode preparation and upon electrochemical reactions, are mandatory to enable future practical applications. Here, we discuss these points using LaF 3 doped with BaF 2 (La 0.95 Ba 0.05 F 2.95, LBF) as a benchmark solid fluoride electrolyte. We establish that lithium may be used as a pseudoreference electrode to assess the electrochemical stability window of LBF and support the experiment with thermodynamic calculations. We demonstrate the chemical compatibility of LBF with carbon upon ball-milling and investigate the electrical conductivity of the formed LBF-C composite. We use a LBF|LBF-C|LBF cell (in this configuration, LBF serves as electronically blocking electrode) to assess the ionic conductivity of the LBF-C composite. The results confirm that both electronic and ionic percolations are ensured within the LBF-C composite despite a noticeable decrease of the ionic conductivity. Finally, we use a Li|LBF|LBF-C cell to evaluate the electrochemical fluorination of the carbon in the LBF-C composite. Our results suggest an electrochemical activity of carbon with fluoride ions. The possible interactions of carbon with fluorides to form insulating carbon fluoride (CF x ) must be considered when determining the operating voltage of FIBs.