Li-ions distributed among the interspaces, has been given as a reason for its high ionic conductivity. [2][3][4][5][6][7] While the calculated ionic conductivity of Li 10 GeP 2 S 12 is about 9 -13 mS cm -1 at room temperature, [5][6][7] and agrees well with the reported experimental value of 12 mS cm -1 , [1] various other researchers have stated a wide range of practical ionic conductivities at room temperature differing by up to an order of magnitude. [8] This observation can be rationalized by considering the variations in synthesis methods used to produce Li 10 GeP 2 S 12 , for example, with differences in reactants treatment, heating temperature, and time. [1,[9][10][11] This wide range of synthesis methods manifests significant differences in measured ionic conductivities due to crystallization and secondary phase formation variations. An essential part of the crystallization process is the structural and morphological changes that the reactants undergo. These changes will significantly influence the composition and crystallinity of the final product, leading to different electrochemical behaviors and physical properties.The crystallization process of Li 10 GeP 2 S 12 , using initial powders of Li 2 S, P 2 S 5 , and GeS 2 , has been previously investigated, where it was demonstrated that the crystallinity, as determinedThe 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 .

Li 10 GeP 2 S 12 is a phosphosulfide solid electrolyte that exhibits exceptionally high Li-ion conductivity, reaching a conductivity above 10 −3 S cm −1 at room temperature, rivaling that of liquid electrolytes. Herein, a method to produce glassy-ceramic Li 10 GeP 2 S 12 via a single-step utilizing high-energy ball milling was developed and systematically studied. During the high energy milling process, the precursors experience three different stages, namely, the 'Vitrification zone' where the precursors undergo homogenization and amorphization, 'Intermediary zone' where Li 3 PS 4 and Li 4 GeS 4 are formed, and the 'Product stage' where the desired glassy-ceramic Li 10 GeP 2 S 12 is formed after 520 min of milling. At room temperature, the as-milled sample achieved a high ionic conductivity of 1.07 × 10 −3 S cm −1 . It was determined via quantitative phase analyses (QPA) of transmission X-ray diffraction results that the as-milled Li 10 GeP 2 S 12 possessed a high degree of amorphization (44.4 wt %). To further improve the crystallinity and ionic conductivity of the Li 10 GeP 2 S 12 , heat treatment of the as-milled sample was carried out. The optimal heat-treated Li 10 GeP 2 S 12 is almost fully crystalline and possesses a room temperature ionic conductivity of 3.27 × 10 −3 S cm −1 , an over 200% increase compared to the glassy-ceramic Li 10 GeP 2 S 12 . These findings help provide previously lacking insights into the controllable preparation of Li 10 GeP 2 S 12 material.