Tuning lithium-yttrium chloride local structure through coordination control and mixing during synthesis

Lithium halide-based solid electrolytes have high Li⁺ conductivity and can be synthesized through low-temperature aqueous solution routes. While Li₃InCl₆ can be readily synthesized through dehydration, other analogous materials, such as Li₃YCl₆, cannot. This difference may be due to differences in H₂O coordination strength, which leads to partial hydrolysis to form YOCl. In this work, we followed and compared Li₃YCl₆ synthesis using three different methods using in situ neutron diffraction. The data revealed that forming an ammonium halide complex intermediate is essential in synthesizing Li₃YCl₆ from an aqueous solution. In carefully examining the Li₃YCl₆ products, we found that changes in local structure follow on to drive significant differences in ionic transport and Li⁺ diffusivity as determined through diffusion NMR measurements. These changes were ascribed to the change in the correlative transport of Li⁺. This work provides insight into the reaction mechanisms involved in synthesizing halide solid electrolytes and highlights the importance of considering their synthetic and processing conditions to optimize their performance in all-solid-state batteries.