Understanding the reaction mechanism and performances of 3d transition metal cathodes for all-solid-state fluoride ion batteries

Abstract

Fluoride ion batteries (FIBs) are regarded as promising energy storage devices, and it is important and urgent to develop cathode materials with high energy densities for use in FIBs. However, systematic investigations of 3d transition metal/metal fluorides have been rarely reported thus far because of the restricted reversibility and unfavorable interfacial compatibility of 3d transition metal/metal fluorides with solid-state electrolytes. Herein, 3d transition metals are investigated by utilizing thin-film cells with LaF₃ substrates. The highly reversible (de)fluorinations of Cu, Co, and Ni are validated at various temperatures. High capacity utilizations of 79.5%, 100%, and 90.5% are obtained during the initial cycle at 150 °C. By combining results from X-ray absorption spectroscopy (XAS) and electrochemical characterization, the electrochemical behaviors of Cu, Co, and Ni, as well as experimental evidence of the two-phase transition mechanism during the M/MF₂ reaction are reported for the first time. This provides new insights required for future cathode designs for use in all-solid-state FIBs.