Correlation between local structure and ionic conductivity in GeS2-based oxy-sulfide glasses
Numerous studies of the ionic conductivities in oxide doped chalcogenaide glasses have shown the anomalous result that the ionic conductivity actually increases significantly (by more than a factor of 10 in some cases) by the initial addition of an oxide phase to a pure sulfide glass such as Li[subscript x]MO[subscript y] (Li[subscript x]MO[subscript y] = Li3PO4, Li4SiO4, Li3BO3, Li4GeO4) doped into the Li2S + SiS2 glass system. While this conductivity and physical property improvement is important to the battery application of these glasses as solid electrolytes, there has yet to be a definitive study of this anomalous conductivity increase in these glasses. It is not clear, for example, whether the added oxysalts work to increase the total number of cations in the glass or whether it works to increase their mobility, or perhaps both. To develop a better and more complete understanding of the anomalous conductivity increase in these oxy-sulfide glasses, our approach here is to use simper oxide additions where we simply substitute oxygen for sulfur without adding any Li cations to the glass so that if the conductivity does increase, then the conductivity increase can be more completely and thoroughly understood on the basis of purely structural (mobility) changes to the glass. In this study, GeO2 was added systematically to the Li2S + GeS2 glass system. The ionic conductivities, structures, and thermal and physical properties of Li2S + GeS2 + GeO2 glasses have been investigated. The relationship between the structural changes caused by the addition of GeO2 and ionic conductivities in Li2S + GeS2 + GeO2 glasses was correlated in an effort to determine the structural origin of the anomalous conductivity increase in these glasses. It was proposed that the "doorway" radius between adjacent cation sites forming a rigid framework of the glass network slightly increases with the addition of oxygen to the glass, which was the major cause in decreasing the activation energy and thereby increasing the conductivity.