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-1 at room temperature. However, recent experimental reports suggest that proton-free lithium-rich APs, such as Li3OCl, may not be synthesized using conventional methods. While Li2OHCl has a lower conductivity of about 0.1 mS cm-1 at 100 °C, its partially fluorinated counterpart, Li2(OH)0.9F0.1Cl, is a significantly better ionic conductor. In this article, using density functional theory simulations, we show that it is easier to synthesize Li2OHCl and two of its fluorinated variants, i.e., Li2(OH)0.9F0.1Cl and Li2OHF0.1Cl0.9, than Li3OCl. The transport properties and electrochemical windows of Li2OHCl and the fluorinated variants are also studied. The ab initio molecular dynamics simulations suggest that the greater conductivity of Li2(OH)0.9F0.1Cl is due to structural distortion of the lattice and correspondingly faster OH reorientation dynamics. Partially fluorinating the Cl site to obtain Li2OHF0.1Cl0.9 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 Li2OHF0.1Cl0.9 as a novel Li conductor.