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

International audience ; 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 LaF3 doped with BaF2 (La0.95Ba0.05F2.95, LBF) as a benchmark solid fluoride electrolyte. We establish that lithium may be used as a pseudo-reference 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 insured 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 (CFx) must be considered when determining the operating voltage of FIBs.