Fluoride Shuttle Battery Reactions of CuF2: Intermediate Phase for Defluorination

Fluoride shuttle batteries (FSBs) that use defluorination and fluorination of active materials are considered candidates for future batteries with high energy densities. Elucidation of the mechanisms of FSB reactions is the key to developing batteries with high performance. In this work, in situ Raman mapping and three-dimensional (3D) morphology analysis by laser scanning confocal microscopy (LSCM) during defluorination of CuF2 microparticles in a liquid electrolyte were conducted. Defluorination proceeded through a new intermediate phase of CuF2 with fluorine vacancies, characterized by green color and distinct Raman peaks. Results of UV-vis measurements and density functional theory (DFT) calculations indicate that electrons are doped into an unoccupied Cu 3d band above the band gap of the pristine CuF2 when a small amount of F- is extracted, resulting in photoabsorption of the intermediate phase in a red-to-infrared region and electronic conductivity. This intermediate phase is thought to promote defluorination of CuF2, which is initially an electronic insulator. The insulator-metal transition due to extraction of an anion has not so far been reported for simple metal halides. The results of the unique observation of spatiotemporal changes in the distribution of the intermediate phase provide important information for the development of active materials and also solid electrolytes for FSBs.