Improved ionic conductivity of Li2S-P2S5-LiI solid electrolytes synthesized by liquid-phase synthesis

Liquid-phase synthesis is a promising method for the large-scale production of sulfide-type solid electrolytes. However, solid electrolytes synthesized by liquid-phase methods exhibit lower conductivity values than those synthesized using solid-state synthesis (e.g., mechanical milling). Li2S-P2S5-LiI solid electrolytes are commonly used in all-solid-state battery applications owing to their high ionic conductivity and chemical stability of lithium. Previous studies have shown that Li2S-P2S5-LiI solid electrolytes (Li2S:P2S5:LiI = 3:1:1) synthesized using the liquid-phase method have lower conductivities (0.35–0.63 mS cm−1) than those synthesized via solid-state synthesis (1.35 mS cm−1). The ionic conductivity of Li2S-P2S5-LiI solid electrolytes obtained via liquid-phase synthesis may be improved by optimizing the drying conditions. Our study observed that the Li2S-P2S5-LiI solid electrolytes dried below 130 °C under vacuum and at 170 °C under ambient pressure exhibited an increased ionic conductivity of 1.0 mS cm−1, which is comparable to that of Li2S-P2S5-LiI solid electrolytes obtained via solid-state synthesis. X-ray diffraction patterns revealed that the intensities of the new crystalline phase were higher than those of the Li4PS4I phase; in addition, a lower weight loss was observed in the thermogravimetry curve measurements. These results indicated that the drying step at 170 °C under an ambient pressure could promote removal of organic solvent from complex and new phase formation, thereby contributing to improved ionic conductivity. Our study provides important guidance for the optimization of liquid-phase synthesis with the aim of achieving high ionic conductivity.