[2D01-04] Thin Film (TF)

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[2D03] Epitaxial cerium fluoride thin films synthesized via topochemical fluorination: Effects of Sr incorporation on fluoride-ion conductivity

*Akira Chikamatsu¹, Ayuka Nakano¹, Miku Hagiwara¹, Dai Kutsuzawa², Erika Fukushi³, Hiroyuki Oguchi³, Fumihiko Uesugi⁴, Tsukasa Katayama⁵, Yasushi Hirose⁶ (1. Ochanomizu University (Japan), 2. Central Research Institute of Electric Power Industry (Japan), 3. Shibaura Institute of Technology (Japan), 4. National Institute for Materials Science (Japan), 5. Tokyo University of Science (Japan), 6. Tokyo Metropolitan University (Japan))

Fluoride ion batteries (FIBs) offer high theoretical energy densities and rely on earth-abundant fluorine, but their performance is limited by poor charge-discharge behavior and high operating temperatures. In this study, we examined Sr substituted cerium fluoride epitaxial films-Sr_0.5Ce_0.5F_2.5 and Ce_0.975Sr_0.025F_3-δ-synthesized by topochemical fluorination of SrCeO₃ and Ce_0.975Sr_0.025O_2-δ. Complete structural conversion occurred above 350 ºC, ccompanied by reduction of Ce⁴⁺ to Ce³⁺ and full transformation into fluorite-type or tysonite-type structures. Scanning transmission electron microscopy with energy dispersive X-ray spectroscopy confirmed cation rearrangement and substitution of oxygen by fluorine. The films exhibit measurable fluoride ion conductivity: 7.4 x 10^-8 S cm^-1 at 323 K for Sr_0.5Ce_0.5F_2.5 and 1.1 x 10^-6 S cm^-1 at 300 K for Ce_0.975Sr_0.025F_3-δ, with activation energies of 0.61 and 0.51 eV, respectively. These results demonstrate that cerium based fluoride epitaxial films are promising model systems for studying F^- transport mechanisms and for guiding the design of new solid electrolytes for FIBs.

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