Quantitative analysis of crystallinity in an argyrodite sulfide-based solid electrolyte synthesized via solution processing

Liquid-phase synthesis is a useful technique for preparing argyrodite sulfide-based solid electrolytes, and the synthesis conditions such as heat treatment strongly affect the conductivity. Because the understanding of structural changes reveals crucial information about their properties, it is necessary to evaluate this change during heat treatment to determine the factors that affect the conductivity. In this study, X-ray diffraction measurements and transmission electron microscope observations reveal the effects of heat treatment on the crystallinities and ionic conductivities in the synthesis process of argyrodite electrolytes with tetrahydrofuran and ethanol. The amorphous material is in the main phase when heated at low temperatures below 200 °C and exhibits relatively low conductivities of ca. 2 × 10(−4) S cm(−1) despite precipitation of the argyrodite crystals. As the heat treatment temperature increases, the ratio of argyrodite crystals increases, involving nucleation and grain growth, leading to high conductivities of over 10(−3) S cm(−1). It is critical to control the ratio of the amorphous and crystal phases to achieve high conductivities in the synthesis of argyrodite electrolytes via liquid-phase processing.