Li-argyrodite phase Li₆PS₅Cl is a promising solid electrolyte (SE) with potential applications in all-solid-state batteries (ASSBs). Conventional SE synthesis methods such as high-energy ball-milling and solid-state synthesis require a significant amount of energy. Consequently, in recent years, several studies have been conducted on developing liquid phase methods for mass-producing SEs. One such liquid phase method uses tetrahydrofuran (THF, an aprotic solvent) and ethanol (EtOH, a protic solvent) to synthesize Li₆PS₅Cl. However, the synthesized SE contains impurities that are generated by reactions between EtOH and PS₄³⁻. In this study, we present a novel one-pot liquid phase method for synthesizing Li₆PS₅Cl using THF. The synthesized SE had a high ionic conductivity (2.03 mS·cm⁻¹) and low electronic conductivity (7.44 × 10⁻⁸ S·cm⁻¹). Notably, it had few impurities and was essentially composed of a single phase. Furthermore, an ASSB cell composed of LiNbO₃-coated LiNi_0.6Mn_0.2CoO₂ (NMC622)/Li₆PS₅Cl/Li-In, which contained the synthesized SE, exhibited a high discharge capacity of 156 mAh·g⁻¹. Therefore, the liquid phase method proposed herein can be used to synthesize SEs, and can help realize mass production and commercialization.