Stability, Elastic Properties, and the Li Transport Mechanism of the Protonated and Fluorinated Antiperovskite Lithium Conductors.

Lithium-rich antiperovskites (APs) have attracted significant research attention due to their ionic conductivity above 1 mS cm at room temperature. However, recent experimental reports suggest that proton-free lithium-rich APs, such as LiOCl, may not be synthesized using conventional methods. While LiOHCl has a lower conductivity of about 0.1 mS cm at 100 °C, its partially fluorinated counterpart, Li(OH)FCl, is a significantly better ionic conductor. In this article, using density functional theory simulations, we show that it is easier to synthesize LiOHCl and two of its fluorinated variants, i.e., Li(OH)FCl and LiOHFCl, than LiOCl. The transport properties and electrochemical windows of LiOHCl and the fluorinated variants are also studied. The molecular dynamics simulations suggest that the greater conductivity of Li(OH)FCl is due to structural distortion of the lattice and correspondingly faster OH reorientation dynamics. Partially fluorinating the Cl site to obtain LiOHFCl leads to an even greater ionic conductivity without impacting the electrochemical window and synthesizability of the materials. This study motivates further research on the correlation between local structure distortion, OH dynamics, and increased Li mobility. Furthermore, it introduces LiOHFCl as a novel Li conductor.