Author
Toshiro Yamanaka, Zempachi Ogumi, Office of Society-Academia Collaboration for Innovation, Kyoto University
Takeshi Abe, Graduate School of Engineering, Kyoto University
Keywords
Next-generation battery, Fluoride shuttle battery, in-situ Raman imaging, UV-VIS, DFT calculations, in-situ 3D analysis, laser scanning confocal microscopy.
Abstract
Fluoride shuttle batteries (FSBs), in which defluorination and fluorination of active materials are used as electrode reactions, are considered as candidates for future batteries with high energy densities. The expected energy densities of the FSB’s are several times higher than that of lithium ion batteries, and FSBs are currently one of main research subjects of national projects supported by New Energy and Industrial Technology Development Organization (NEDO) in Japan. Unveiling reaction mechanisms of FSBs is important for the developments of FSB’s with high performance. CuF2 is considered as one of the most promising active materials for positive electrodes of FSB’s. In this review, we present examples of elucidation of mechanisms of FSB reactions, i.e., defluorination of CuF2, by in-situ high-speed Raman mapping, in-situ 3D analysis and UV-VIS measurements. In-situ high-speed Raman imaging and in-situ 3D analysis by laser scanning confocal microscopy (LSCM) were conducted almost simultaneously. It was found that 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 DFT calculations indicate that the intermediate phase has electronic conductivity, thus promoting defluorination of CuF2 that is initially an electronic insulator. This is the first example of insulator-metal transition of metal halides due to extraction of an anion. Such results have very important implication for the developments of FSBs.