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

Abstract

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)_0.9F_0.1Cl, 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)_0.9F_0.1Cl and Li₂OHF_0.1Cl_0.9, than Li₃OCl. The transport properties and electrochemical windows of Li₂OHCl and the fluorinated variants are also studied. The <i>ab initio</i> molecular dynamics simulations suggest that the greater conductivity of Li₂(OH)_0.9F_0.1Cl is due to structural distortion of the lattice and correspondingly faster OH reorientation dynamics. Partially fluorinating the Cl site to obtain Li₂OHF_0.1Cl_0.9 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 Li₂OHF_0.1Cl_0.9 as a novel Li conductor.