Superionic Amorphous Li 2 ZrCl 6 and Li 2 HfCl 6

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.