Li 4 GeO 4 -based solid electrolytes can be synthesized at low temperatures, and their formability is improved by adding Li 2 SO 4 . Glass ceramic Li 4 GeO 4 exhibits a relatively high ionic conductivity of approximately 10 ¹6 S cm ¹1 at room temperature, which is higher than that of glass Li 4 GeO 4 . Thus, Li 4 GeO 4 has promising applications in all-solid-state lithium ion batteries. To understand the correlation between the ionic conductivity, formability, and microstructure of the synthesized materials, the microstructures and crystallization process of glass and glassceramic Li 4 GeO 4 and 80Li 4 GeO 4 •20Li 2 SO 4 (Li 3.6 Ge 0.8 S 0.2 O 4 ) were observed by transmission electron microscopy (TEM). Since Li 3.6 Ge 0.8 S 0.2 O 4 glass exhibits a halo diffraction pattern, the addition of Li 2 SO 4 to Li 4 GeO 4 stabilizes its amorphous phase. In addition, glass ceramic samples were found to be characterized by an amorphous state containing nanocrystallites with a crystallinity degree of approximately 40%, which improves the ionic conductivity of the material.