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.