Glass formation in non-oxide chalcogenide systems. Structural elucidation of Li 2SSiS 2LiI solid electrolytes by quantitative 29Si, 6Li and 7Li high resolution solid state NMR methods

Abstract

The local structure in Li 2SSiS 2LiIl glasses is investigated by solid state high resolution 6Li, 7Li, and 29Si NMR, using the technique of magic-angle sample spinning (MAS) in conjunction with absolute signal quantitation. Parallel studies of crystallized specimens indicate the presence of a crystalline compound with stoichiometry Li 2SiS 2, and a second phase with the likely stoichiometry Li 4SiS 4. Two distinct silicon environmentsare found in the latter phase. The 29Si MAS NMR spectra of crystalline lithium silicon sulfides are readily interpretable in terms of microstructures with differing fractions of non-bridging sulfur atoms attached to Si. In analogy to crystalline silicates, the 29Si resonance is shifted downfield with increasing numbers of non-bridging sulfur atoms, with an overall chemical shift range of ca. 30 ppm. In contrast, the 29Si MAS-NMR spectra of the glasses appear invariant over the entire region of glass formation, although the resolution would be sufficient to discern different distributions of microstructures if they weere present. Furthermore, in striking contrast to the oxide-analog system, no correspondence exists between the chemical shifts of the glasses and those observed in their crystalline counterparts. These highly unusual results suggest that the bonding concepts used to describe the local structures of oxidic glasses cannot be applied for the structural description of chalcogenide glasses, even if they are, as in the present case, stoichiometry-analog. Complementary information is provided by lithium NMR. The use of the rare isotope 6Li instead of 7Li offers the advantage of significantly improved spectroscopic resolution, and thus more accurate chemical shift determinations.