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

Abstract The well known and characterized fast ion conducting (FIC) LiI + Li 2 S + GeS 2 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 Ga 2 S 3 and La 2 S 3 . 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 Ga 2 S 3 doped compositions is larger than that for the La 2 S 3 compositions, the addition of Ga 2 S 3 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 Ga 2 S 3 content showed the highest T g s of ∼325 °C. The addition of La 2 S 3 to the base glasses, by comparison, is found to create NBS by forming high coordination octahedral LaS 6 3− 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 Li 2 S and Ga 2 S 3 or La 2 S 3 , the La 2 S 3 -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 Ga 2 S 3 glasses from ∼10 −5 to ∼10 −3 (Ω cm) −1 and of the La 2 S 3 glasses from ∼10 −4 to ∼10 −3 (Ω cm) −1 at room temperature. A maximum conductivity of ∼10 −3 (Ω 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 ).