Ionic conductivity and crystallization process in the Li2S–P2S5 glass electrolyte

Abstract

The ionic conductivity of the Li2S–P2S5 glass electrolyte systems strongly depends on the type of precipitated crystalline phases and heat-treatment conditions. To clarify the correlation between ionic conductivity and crystallization behavior, a specific observation technique to visualize the precipitated nanocrystallites via transmission electron microscopy (TEM), and a quantitative method for evaluating the crystallization degree are developed. The 75Li2S·25P2S5 glass exhibits a high conductivity of approximately 7.5 × 10−4 S/cm at a sintering temperature between 120 °C and 180 °C. Above 180 °C, however, conductivity sharply decreases to 1.8 × 10−4 S/cm. In situ TEM observation reveals that this is attributed to the crystallization of the glasses. The crystallization process is mainly characterized by the formation of β-Li3PS4 nanocrystallites precipitated above 180 °C whose size remains intact during further heating until 250 °C. As a result, to achieve high ionic conductivity in the 75Li2S·25P2S5 glass, it is important to realize an amorphous state with the presence of nanocrystallites that would yield a crystallinity degree lower than 5%.