Understanding Li-Ion Dynamics in Lithium Hydroxychloride (Li2OHCl) Solid State Electrolyte via Addressing the Role of Protons

Low-melting-point solid-state electrolytes (SSE) are critically important for low-cost manufacturing of all-solid-state batteries. Lithium hydroxychloride (Li2OHCl) is a promising material within the SSE domain due to its low melting point (mp < 300 degrees C), cheap ingredients (Li, H, O, and Cl), and rapid synthesis. Another unique feature of this compound is the presence of Li vacancies and rotating hydroxyl groups which promote Li-ion diffusion, yet the role of the protons in the ion transport remains poorly understood. To examine lithium and proton dynamics, a set of solid-state NMR experiments are conducted, such as magic-angle spinning Li-7 NMR, static Li-7 and H-1 NMR, and spin-lattice T-1(Li-7)/T-1(H-1) relaxation experiments. It is determined that only Li+ contributes to long-range ion transport, while H+ dynamics is constrained to an incomplete isotropic rotation of the OH group. The results uncover detailed mechanistic understanding of the ion transport in Li2OHCl. It is shown that two distinct phases of ionic motions appear at low and elevated temperatures, and that the rotation of the OH group controls Li+ and H+ dynamics in both phases. The model based on the NMR experiments is fully consistent with crystallographic information, ionic conductivity measurements, and Born-Oppenheimer molecular dynamic simulations.