Fundamental aspects of the structural and electrolyte properties of Li2OHCl from simulations and experiment

Solid-state electrolytes that are compatible with high-capacity electrodes are expected to enable the next generation of batteries. As a promising example, ${\mathrm{Li}}₂\mathrm{OHCl}$ was reported to have good ionic conductivity and to be compatible with a lithium metal anode even at temperatures above $100{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. In this work, we explore the fundamental properties of ${\mathrm{Li}}₂\mathrm{OHCl}$ by comparing simulations and experiments. Using calculations based on density functional theory, including both static and dynamic contributions through the quasiharmonic approximation, we model a tetragonal ground state, which is not observed experimentally. An ordered orthorhombic low-temperature phase was also simulated, agreeing with experimental structural analysis of the pristine electrolyte at room temperature. In addition, comparison of the ordered structures with simulations of the disordered cubic phase provide insight into the mechanisms associated with the experimentally observed abrupt increase in ionic conductivity as the system changes from its ordered orthorhombic to its disordered cubic phase. A large Haven ratio for the disordered cubic phase is inferred from the computed tracer diffusion coefficient and measured ionic conductivity, suggesting highly correlated motions of the mobile Li ions in the cubic phase of ${\mathrm{Li}}₂\mathrm{OHCl}$. We find that the OH bond orientations participate in gating the Li ion motions which might partially explain the predicted Li-Li correlations.