Fluoride Shuttle Battery Reactions of CuF₂: Intermediate Phase for Defluorination

Abstract

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 CuF 2 microparticles in a liquid electrolyte were conducted. Defluorination proceeded through a new intermediate phase of CuF 2 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 CuF 2 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 CuF 2 , 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.

Emergence of Two-Dimensional F-Induced Kagome Lattice on the Cu(111) Surface

“…The first report about the synthesis of CuF in the sphalerite structure has been done in 1933, but the discussion about the existence of the Cu­(I) fluoride crystal structure continues until today. − Another interesting aspect is the prediction of unusual properties of CuF due to its Weyl semimetal band structure, which might be relevant for spintronics technology. Currently, surface fluorination is receiving much attention due to the development of fluoride shuttle batteries, , which operate by defluorination and fluorination of active materials. Fluorine shuttle batteries are promising for future battery technology with a high energy density.…”