Preparation and characterization of glasses in the LiI+Li2S+GeS2+Ga2S3 system

Abstract

Glass formation in the LiI+Li2S+GeS2+Ga 2S3 system has been investigated in order to optimize their properties for use in solid state lithium batteries. These glasses have been studied in an effort to obtain a new series of glasses that break the often observed relation that high ionic conductivity glasses are also those with the lowest Tg. Here, Ga2S3 was added to act as a glass intermediate where it is expected upon addition to the glass composition, to eliminate non-bridging sulfurs thereby increasing the glass-forming character, increasing the Li+ ion conductivity, as well increasing the Tg of the glass. The glass-forming range was determined for both ternary Li2S + GeS2 + Ga2S3 and quaternary (LiI doped) systems. As observed in other glasses, the addition of LiI lowered the melting temperature and Tg, but slightly improved (expanded) the glass-forming range. The ionic conductivity was measured for both the ternary and quaternary glasses and the largest improvement was observed with the addition of LiI, although other compositional characteristics, such as Ga2S3 content, are necessary to achieve the highest conductivity. DSC measurements showed that the highest Tgs, ∼300 °C, were observed for glasses with the highest concentration of Ga 2S3 and this suggests that the role of Ga 2S3 is that of a glass intermediate eliminating non-bridging sulfur units in the glass. This role is also supported by the conductivity measurements, where the conductivity increases with Ga 2S3 content at constant Li2S concentrations. Significantly, it was also observed that the addition of LiI improved both the conductivity and the thermal stability of the glass, increasing the T c-Tg values by up to ∼100 °C. Finally, the densities and molar volumes of the glasses showed a near linear decrease with the addition of Li2S, but were surprisingly uncorrelated with any other component, most notably LiI.