Rare-Earth Fluoridesulfide Compounds with Fluoride Ion Conducting Layers

All-solid-state fluoride-ion batteries (FIBs) are highly attracted attention due to high theoretical energy density and high safety compared with those of conventional lithium-ion batteries(LIBs)[1]. However, it is far from practical use because there is no solid electrolyte which exhibits high ionic conductivity and wide electrochemical potential window at room temperature like lithium-ion conductor. Most of the previously reported fluoride-ion conductors have been limited to the single-anion fluorides with fluorite-type, tysonite-type, and perovskite-type structures [2].<br/>In the field of material sciences, mixed-anion compounds with more than more anions have recently attracted attention. Compared with single-anion compounds such as oxides and fluorides, mixed-anion compounds have the possibility to exhibit innovative functions due to their specific crystal and coordination structure[3]. In mixed anion compounds, combinations of anion species with different ionic radius result in anion-order structures[4]. Fluorosulfides, which contains S²⁻ and F⁻ with largely different ionic radii, are ideal for creating anion-ordered structures suitable for fluoride-ion conduction. To our best knowledge, there is few reports on the fluoride-ion conductor materials containing mixed-anion compounds.<br/>In this study, we synthesized rare-earth fluorosulfides compounds by solid-state reaction under vacuum[5]. In the crystal structure of these fluorosulfides, the fluoride- and sulfide-ion layers are present across cation layers. The separation of fluoride and sulfide ions is attributed to the difference anionic size. The presence of multiple anions results in the formation of an anion-ordering two-dimensional crystal structure with fluoride ion conducting layers, which cannot be realized for single-anion compounds. The material development of fluoride ion conductors using flurosulfide compounds is expected to increase crystal structure variations with fluoride-ion conduction pathways.<br/>[1] M. A. Reddy, M. Fichtner, <i>J. Mater. Chem</i>, <b>21,</b> 17059-17062(2011).<br/>[2] K. Motohashi, T. Nakamura, Y. Kimura, Y. Uchimoto, K. Amezawa, <i>Solid State Ionics.</i>, <b>338</b>, 113-120(2019).<br/>[3] H. Kageyama, K. Hayashi, K. Maeda, J. P. Attfield, Z. Hiroi, J. M. Rondinelli, K. R. Poeppelmeier, <i>Nature Comm</i>, <b>9</b> 772(2018).<br/>[4] S. Tachibana, K. Ide, T. Tojigamori, Y. Yamamoto, H. Miki, H. Yamasaki, Y. Kotani, Y. Orikasa, <i>Chem. Lett.</i>, <b>50(1)</b>, 120-123(2021).<br/>[5] S. Tachibana, C. Zhong, K. Ide, H. Yamasaki, T. Tojigamori, H. Miki, T. Saito, T. Kamiyama, K. Shimoda, Y. Orikasa, Fluorosulfide La_2+xSr_1−xF_4+xS₂ with a Triple-Fluorite Layer Enabling Interstitial Fluoride-Ion Conduction, https://doi.org/10.1021/acs.chemmater.3c00188