Elucidation of the role of lithium iodide as an additive for the liquid-based synthesis of Li7P2S8I solid electrolyte

Liquid-phase synthesis for a sulfide-based solid electrolyte has been widely studied due to its great advantage of being a simpler and more cost-effective method compared with the conventional solid-phase synthesis, even it could induce homogeneous reactions in the solution. However, the physically and chemically stable phosphorus pentasulfide (P₂S₅) is barely soluble in various solvents; this has been a major problem for achieving a pure solution-phase dissolving solid electrolyte. Therefore, exploring an effective additive for liquid-phase synthesis would be worthwhile and could potentially lead to the discovery of new chemicals to produce qualified solid electrolytes. In this paper, lithium iodide's (LiI) dual role as a strong nucleophile as well as a major reactant source for producing Li₇P₂S₈I is first investigated. The nucleophilic additive of LiI has been proven to break the P-S bonds of P₂S₅, driving the insoluble P₂S₅ to become the soluble intermediate complex. Also, it is demonstrated that controlling the reaction times between the LiI and the P₂S₅ is key to achieving solution-based synthesis, and the role of LiI is investigated by conducting bonding analysis.