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Direct observation of a non-crystalline state of Li2SP2S5 solid electrolytes

(a) The XRD pattern of the 80Li2S20P2S5 glass ceramics obtained by heat treatment. (b) HR-TEM image indicating the microstructure of the 80Li2S20P2S5 glass ceramics, together with a BF image in the inset. The image was obtained from the area indicated by the blue circle in the BF image.

There are two types of solid electrolytes which has been recently expected to be applied to all-solid-state batteries. One is the glasses characterized by an amorphous state. The other is the glass ceramics containing crystalline in an amorphous matrix. However, the non-crystalline state of glasses and glass ceramics is still an open question. It has been anticipated that sea-island and core-shell structures including crystalline nanoparticles have been proposed as candidate models for glass ceramics. Nevertheless, no direct observation has been conducted so far. Here we report the non-crystalline state of Li2SP2S5 glasses and glass ceramics, and the crystallization behavior of the glasses during heating via direct transmission electron microscopy (TEM) observation. High resolution TEM images clearly revealed the presence of crystalline nanoparticles in an amorphous region. Eventually we suggest that the precipitation and connection of crystalline nanoparticles in an amorphous matrix are key to achieving high ionic conductivity.

(full paper)