Understanding the Conversion Mechanism of Transition Metal Fluoride Cathodes: the Case of Monodisperse CoF2 Nanocrystals

Abstract Conversion-type transition metal fluorides (TMFs) are promising cathode materials for high-energy lithium-metal batteries, but their practical application is hindered by rapid capacity fade and pronounced voltage hysteresis. Among them, FeF₂ exhibits superior electrochemical reversibility, recently attributed to its topotactic phase evolution. Here, we advance the understanding of TMF cathodes by investigating the conversion mechanism of CoF₂, which—despite its similar physicochemical properties to FeF₂—suffers from greater voltage hysteresis and limited cycle life. We synthesise monodisperse, single-crystalline CoF₂ nanorods and employ high-resolution scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS) to probe their conversion behaviour at high spatial resolution. Our analysis reveals that the increased hysteresis during lithiation stems from the nucleation of metallic cobalt in a high-energy face-centred cubic phase, which promotes the formation of low-energy semi-coherent interfaces within the LiF matrix. These insights offer guiding principles to mitigate the key limitations of TMF-based cathodes and inform the design of next-generation conversion materials.