Disentangling Phase and Morphological Evolution During the Formation of the Lithium Superionic Conductor Li10GeP2S12

The structural and morphological changes of the Lithium superionic conductor Li10GeP2S12, 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 Li4GeS4-Li3PS4 binary crystallization process, diversified intermediate phases, including GeS2 in different space groups, multiphasic lithium phosphosulfides (LixPySz), and cubic Li7Ge3PS12 phase, are involved additionally during the formation and decomposition of Li10GeP2S12. Furthermore, the phase composition at temperatures around the transition temperatures of different formation zones shows a significant deviation. At 600 °C, Li10GeP2S12 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 Li10GeP2S12.