All-solid-state lithium-ion batteries (ASLBs) employing sulfide solid electrolytes are attractive next-generation rechargeable batteries that could offer improved safety and energy density. Recently, wet syntheses or processes for sulfide solid electrolyte materials have opened new opportunities to explore new materials and practical fabrication methods for ASLBs. Herein, a new wet-chemical route for the synthesis of Li-deficient Li3-xPS4 (0 ≤ x ≤ 0.3), enabled by dual solvents, is described for the first time. Owing to its miscibility with tetrahydrofuran and ability to dissolve elemental sulfur, o-xylene as a co-solvent facilitates the wet-chemical synthesis of Li3-xPS4. Li3-xPS4 (0 ≤ x ≤ 0.15) derived using dual solvents shows Li+ conductivity of ~0.2 mS cm-1 at 30 °C, in contrast to 0.034 mS cm-1 for a sample obtained using a conventional single solvent (tetrahydrofuran, x = 0.15). The evolution of the structure for Li3-xPS4 is also investigated by complementary analysis using X-ray diffraction, Raman, and X-ray photoelectron spectroscopy measurements. LiCoO2/Li-In ASLBs employing Li2.85PS4 obtained using dual solvents exhibit a reversible capacity of 130 mA h g-1 with good cycle retention at 30 °C, outperforming cells with Li2.85PS4 obtained using a conventional single solvent.