Thermodynamic stability and diffusion mechanism of LiMXCl4 superionic conductors

LiMXCl4 is a recently discovered lithium superionic conductor reported with a Li conductivity up to 12.4 mS/cm at room temperature. In this work, we explore various types of M-cation and X-anion substitutions in the LiMXCl4 system. We find that fluoro-chlorides may provide promising thermodynamic and electrochemical stability without compromising ionic conductivity. Ab-initio molecular dynamics simulations on seven substitutions and three lithium concentrations for each substitution suggest that even higher conductivity may be achieved in LiMXCl4 than has been reported. A Meyer-Neldel analysis comparing LiMXCl4, close-packed halides, and LaCl3-type systems demonstrates the potential of the LiMXCl4 family due to their high Meyer-Neldel energy, high prefactor, and low activation energy, projecting a range of conductivity of 10-100 mS/cm. An analysis of the correlation between lithium-ion hops and small-angle tilting events finds that LiMXCl4 systems exhibit a strong cradle effect where weakly bound M-octahedra often tilt their orientation in conjunction with a nearby Li-ion hop to flatten the lithium-ion energy landscape. Such an advantage originates from the fact that in the LiMXCl4 structure, one-dimensional M-octahedral chains are bound via weak van der Waals interactions which can accommodate for reduction in free volume via rotational correlation of the octahedra. Our work demonstrates an exciting direction towards further improving this class of materials in terms of ionic conductivity and electrochemical stability and provides a fundamental understanding of the factors that lead to high ionic conductivity in non-close-packed oxyhalide systems.