Revealing Impacts of Anion Defect Species on Fluoride-Ion Conduction of Ruddlesden-Popper Oxyfluoride Ba₂ScO₃F

Mixed anion oxyfluorides are one of promising candidates of fast fluoride-ion conductor for all-solid-state fluoride-ion batteries. In order to establish scientific guidelines for further development of oxyfluoride-based solid electrolyte, understanding the true impact of anion defect species on ionic conduction is important. In this work, Ruddlesden-Popper oxyfluoride Ba₂ScO₃F, which can accept relatively high concentration of various types of anion defects, is selected as a target material to reveal defect functionalities. Oxide-ion vacancy (V_O∙∙), fluoride-ion vacancy (V_F∙) and interstitial fluoride-ion (F⁰_i) were introduced into Ba₂ScO₃F, and the influence of each anion defect on fluoride-ion conduction was investigated. The ionic conductivities in Ba₂ScO₃F were improved by introducing fluoride-ion defects (V_F∙ or F⁰_i), while not improved by introducing V_O∙∙. These suggest that the fluoride-ion migrates by interstitialcy diffusion through the rock-salt structure in Ruddlesden-Popper oxyfluorides, and tuning anion defects species can be rational and effective strategy for the development of fast fluoride-ion conductors based on mixed anion compounds.