Influence of synthesis parameters on crystallization behavior and ionic conductivity of the Li₄PS₄I solid electrolyte

Superionic solid electrolytes are key to the development of advanced solid-state Li batteries. In recent years, various materials have been discovered, with ionic conductivities approaching or even exceeding those of carbonate-based liquid electrolytes used in high-performance Li-ion batteries. Among the different classes of inorganic solid electrolytes under study, lithium thiophosphates are one of the most promising due to their high Li-ion conductivity at room temperature and mechanical softness. Here, we report about the effect of synthesis parameters on the crystallization behavior and charge-transport properties of Li₄PS₄I. We show that thermally induced crystallization of Li₄PS₄I (P4/nmm), starting from the glassy phase 1.5Li₂S-0.5P₂S₅-LiI, adversely affects the material's conductivity. However, both conductivity and crystallization temperature can be significantly increased by applying pressure during the preparation.