Ionic Conductivity of Lithium Germanium Phosphate Glass-Ceramics

In this study, the effect of induced crystallization on the electrical transport was studied in the mixed glass former glasses with the composition 40Li₂O–(60 – x)­P₂O₅–xGeO₂, x = 0–25 mol %, as potential solid electrolytes for Li-ion batteries. It has been of interest to investigate how various steps of crystallization influence electrical transport in prepared glass-ceramics. Structural properties of obtained glass-ceramics, which contain single to multicrystalline phases, are characterized by XRD, MAS NMR, and SEM and then correlated with electrical properties studied using impedance spectroscopy. For GeO₂-free glass-ceramics, a slight increase in the electrical conductivity is evidenced, whereas a conductivity decrease for glass-ceramics containing up to 20 mol % GeO₂ is related to the reduction of a number of lithium ions in the residual glassy phase, since the LiPO₃ crystalline phase is formed. The crystallization in the glass-ceramics with higher GeO₂ content causes an increase in the electrical conductivity. This increase is a result of two simultaneous contributions. One is the formation of crystallites with well-defined shapes, which creates easy conduction pathways for lithium ion transport within crystalline grains and along crystalline grain boundaries. And the second one is the increase of the predominantly phosphate amorphous phase for samples Ge-25.