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

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%.