Superionic Amorphous Li₂ZrCl₆ and Li₂HfCl₆

Amorphous LiTaCl₆ and LiNbCl₆ have recently achieved conductivities greater than 10.0 mS·cm^-1 at room temperature, causing an explosion of interest. Here, we predict from molecular dynamics simulations with machine learning force fields that amorphous Li₂ZrCl₆ and Li₂HfCl₆ have even higher room temperature Li-ion diffusivities and double room temperature Li-ion conductivities of LiTaCl₆ and LiNbCl₆ to over 20 mS·cm^-1. Our analysis of anion motions and cation-anion coupling reveals that anion vibrations (M-Cl stretching and Cl-M-Cl bending) are critical for enabling fast Li-ion transport (contributing to 87% of total Li-ion diffusivity), while libration/rotation of MCl₆ ^2- octahedra contributes only marginally (13%). In addition, individual hopping of Li-ion dominates (60%) over concerted motion (40%). More importantly, we show that the superiority of Li₂ZrCl₆ and Li₂HfCl₆ to LiTaCl₆ and LiNbCl₆ stems from a reduction of high-frequency modes in the Li-ion vibrational density of states (VDOS) and from red shifts of M-Cl stretching and Cl-M-Cl bending modes in Cl-VDOS. The Li-VDOS center can be used as a descriptor to predict Li-ion diffusivity in amorphous halides. These insights advance our understanding of ionic transport in amorphous materials and highlight Li₂ZrCl₆ and Li₂HfCl₆ (especially the former for its low cost) as promising superionic solid electrolytes for all solid-state Li batteries.