Exploring the soft cradle effect and ionic transport mechanisms in the LiMXCl4 superionic conductor family

Abstract

LiMXCl₄, a recently discovered lithium superionic conductor, achieves Li conductivity up to 12.4 mS/cm at room temperature. Notably, LiNbOCl₄ features flexible, rotating polyhedra, potentially explaining its high ionic conductivity. However, the generalizability of these findings across different chemistries and the direct link between polyhedra rotations and Li-ion mobility remain unclear. In this study, we explore various M-cation and X-anion substitutions in the LiMXCl₄ system, identifying fluoro-chlorides as promising for enhancing electrochemical stability while maintaining high ionic conductivity. Meyer-Neldel analysis on ab initio simulations reveals that LiMXCl₄ outperforms existing halide conductors, with projected conductivities of 10–100 mS/cm. Our probabilistic analysis of lithium-ion hops and small-angle tilting events reveals a “soft cradle effect,” where weakly bound M-octahedra tilt in conjunction with Li-ion hops, optimizing the energy landscape. This work provides fundamental insights into the factors driving high ionic conductivity in non-close-packed oxyhalide systems and suggests exciting directions for further improving these materials.