Interfacial Stability of Iron (III) Fluoride Intercalation Cathodes

Iron (III) fluoride and its respective hydrates have recently shown promise as intercalation cathodes for lithium-ion batteries. These fluoride materials represent an alternative method to increasing insertion voltage compared to the commonly studied polyanion inductive effect. In comparison to oxides, only a few studies have been published identifying the changes that occur at the electrolyte-fluoride interface. In this work we investigated the various decomposition phases that form at the active interface on cycling of FeF₃ and pyrochlore FeF₃-0.5H₂O cathodes in various organic electrolytes. Due to the presence of framework fluoride at the interface, we examined the CEI formed both with the standard Gen2 electrolyte (3:7 EC/EMC, 1.2M LiPF₆) and the same solvent mix with the more stable salt at 1.0M concentration LiTFSI-based electrolyte. The CEI composition was investigated ex-situ through several techniques including x-ray photoelectron spectroscopy (XPS), glow discharge optical emission spectroscopy (GDOES), and scanning electron microscopy (SEM). Cycling stability of the two cathodes were compared and differences in CEI composition and thickness were investigated during cycling. This work extends our knowledge into interfacial stability of fluoride-based cathodes in common lithium-ion based electrolytes.