Wet-Chemical Tuning of Li_3−xPS₄ (0≤x≤0.3) Enabled by Dual Solvents for All-Solid-State Lithium-Ion Batteries

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 opportunities to explore new materials and practical fabrication methods for ASLBs. A new wet-chemical route for the synthesis of Li-deficient Li_3−xPS₄ (0≤x≤0.3) has been developed, which is enabled by dual solvents. Owing to its miscibility with tetrahydrofuran and ability to dissolve elemental sulfur, o-xylene as a cosolvent facilitates the wet-chemical synthesis of Li_3−xPS₄. Li_3−xPS₄ (0≤x≤0.15) derived by using dual solvents shows Li⁺ conductivity of approximately 0.2 mS cm⁻¹ at 30 °C, in contrast to 0.034 mS cm⁻¹ for a sample obtained by using a conventional single solvent (tetrahydrofuran, x=0.15). The evolution of the structure for Li_3−xPS₄ is also investigated by complementary analysis using X-ray diffraction, Raman, and X-ray photoelectron spectroscopy measurements. LiCoO₂/Li–In ASLBs employing Li_2.85PS₄ obtained by using dual solvents exhibit a reversible capacity of 130 mA h g⁻¹ with good cycle retention at 30 °C, outperforming cells with Li_2.85PS₄ obtained by using a conventional single solvent.