Sulfide solid electrolytes are electrochemically stable in a wide potential window and exhibit high lithium ion conductivities of more than 10^-4 S cm^-1 at room temperature. In particular, the Li₂S-P₂S₅ glasses, which are served as the solid electrolyte, have been reported to crystallize above 473 K in the heating process from room temperature. Noted that lithium ion conductivities depend strongly on precipitated crystalline phases [1, 2]. In this study we have investigated microstructures of the Li₂S-P₂S₅ glasses by transmission electron microscopy (TEM). Figure 1 shows the high-resolution (HR) TEM image of the 75Li₂S-25P₂S₅ glass at room temperature, together with the corresponding electron diffraction (ED) pattern. Although a halo pattern due to the amorphous structure was observed in the ED pattern, crystalline nanoparticles with approximately 5x5 nm² were found to be randomly distributed in the amorphous state, as indicated by dotted circle regions (Fig. 1). Furthermore, in-situ TEM observation in the 75Li₂S-25P₂S₅ glasses revealed that the crystallization started at approximately 473 K and gradually proceeded as temperature increased from 473 K.

[1] F. Mizuno et al, Adv. Mater. 17 (2005) 918-921. [2] F. Mizuno et al, Solid State Ionics 177 (2006) 2721-2725.