Alkali-Rich Antiperovskite M<sub>3</sub>FCh (M = Li, Na; Ch = S, Se, Te): The Role of Anions in Phase Stability and Ionic Transport
Susumu Fujii, Shenghan Gao, Cédric Tassel, Tong Zhu, Thibault Broux, Koji Okada, Yuto Miyahara, Akihide Kuwabara, Hiroshi Kageyama
Abstract
To improve ionic conductivity, solid-state electrolytes with polarizable anions that weakly interact with mobile ions have received much attention, a recent example being lithium/sodium-rich antiperovskite M3HCh (M = Li, Na; Ch = S, Se, Te). Herein, in order to clarify the role of anions in antiperovskites, the M3FCh family, in which the polarizable H– anion at the octahedral center is replaced by the ionic F– anion, is investigated theoretically and experimentally. We unexpectedly found that the stronger attractive interaction between F– and M+ ions does not slow down the M+ ion diffusion, with the calculated energy barrier being as low as that of M3HCh. This fact suggests that the low-frequency rotational phonon modes of the octahedron of cubic M3FCh (and M3HCh) are intrinsic to facilitate the fast ionic diffusion. A systematic analysis further reveals a correlation between the tolerance factor t and the ionic transport: as t decreases within the cubic phase, the rotational mode becomes softer, resulting in the reduction of the migration energy. The cubic iodine-doped Li3FSe has a room-temperature ionic conductivity of 5 × 10–5 S/cm with a bulk activation energy of 0.18 eV.