Disentangling Phase and Morphological Evolution During the Formation of the Lithium Superionic Conductor Li 10 GeP 2 S 12 .

The structural and morphological changes of the Lithium superionic conductor Li 10 GeP 2 S 12 , prepared via a widely used ball milling-heating method over a comprehensive heat treatment range (50 - 700 °C), are investigated. Based on the phase composition, the formation process can be distinctly separated into four zones: Educt, Intermediary, Formation, and Decomposition zone. It is found that instead of Li 4 GeS 4 -Li 3 PS 4 binary crystallization process, diversified intermediate phases, including GeS 2 in different space groups, multiphasic lithium phosphosulfides (Li x P y S z ), and cubic Li 7 Ge 3 PS 12 phase, are involved additionally during the formation and decomposition of Li 10 GeP 2 S 12 . Furthermore, the phase composition at temperatures around the transition temperatures of different formation zones shows a significant deviation. At 600 °C, Li 10 GeP 2 S 12 is fully crystalline, while the sample decomposed to complex phases at 650 °C with 30 wt.% impurities, including 20 wt.% amorphous phases. These findings over such a wide temperature range are first reported and may help provide previously lacking insights into the formation and crystallinity control of Li 10 GeP 2 S 12 .