The comparative structure, properties, and ionic conductivity of LiI+Li2S+GeS2 glasses doped with Ga2S3 and La2S3

The well known and characterized fast ion conducting (FIC) LiI + Li2S + GeS2 glass-forming system has been further optimized for higher ionic conductivity and improved thermal and chemical stability required for next generation solid electrolyte applications by doping with Ga2S3 and La2S3. These trivalent dopants are expected to eliminate terminal and non-bridging sulfur (NBS) anions thereby increasing the network connectivity while at the same time increasing the Li+ ion conductivity by creating lower basicity [(Ga or La)S-4/2](-) anion sites. Consistent with the finding that the glass-forming range for the Ga2S3 doped compositions is larger than that for the La2S3 compositions, the addition of Ga2S3 is found to eliminate NBS units to create bridging sulfur (BS) units that not only gives an improvement to the thermal stability, but also maintains and in some cases increases the ionic conductivity. The compositions with the highest Ga2S3 content showed the highest T(g)s of similar to 325 degrees C. The addition of La2S3 to the base glasses, by comparison, is found to create NBS by forming high coordination octahedral LaS63- sites, but yet still improved the chemical stability of the glass in dry air and retained its high ionic conductivity and thermal stability. Significantly, at comparable concentrations of Li2S and Ga2S3 or La2S3, the La2S3-doped glasses showed the higher conductivities. The addition of the LiI to the glass compositions not only improved the glass-forming ability of the compositions, but also increased the ionic conductivity glasses. LiI concentrations from 0 to 40 mol% improved the conductivities of the Ga2S3 glasses from similar to 10(-5) to similar to 10(-3) (Omega cm)(-1) and of the La2S3 glasses from similar to 10(-4) to similar to 10(-3) (Omega cm)(-1) at room temperature. A maximum conductivity of similar to 10(-3) (Omega cm)(-1) at room temperature was observed for all of the glasses and this value is comparable to some of the best Li ion conductors in a sulfide glass system. Yet these new compositions are markedly more thermally and chemically stable than most Li+ ion conducting sulfide glasses. LiI additions decreased the T(g)s and T(c)s of the glasses, but increased the stability towards crystallization (T-c - T-g). (C) 2007 Elsevier B.V. All rights reserved.