Fast Li⁺ ion conduction in chloro-borate glasses

Substitutions of Cl for 0 in lithium borate glases of the type Li₂O(LiCl)₂B₂O₃ (Li₂Z = 36.4 m/o) were found to result in systematically increasing Li ion conductivity, and decreasing Tg and density. The changes in conductivity were manifested in a linearly decreasing activation energy with fraction (LiCl)₂ rather than changes in the pre-exponential δ₀. In contrast to earliest studies which implied no BO network modification upon Cl addition, we find substantial changes as exemplified in Tg and density variations. The effect of Cl substitution was traced to a change in O/B ratio and is not in disagreement with Raman, IR and ESR spectra which show that C1 does not enter into the BO network. The conductivity results, analyzed in terms of both a random site and weak electrolyte model, favor the first but are inconclusive. In either case the observed structural changes must be accounted for. Liquid-liquid immiscibility limited the vitreous domain of glasses formed with LiCl. This resulted in a leveling off of various physical parameters with composition in the two-phase domain as observed in similar studies [5]. Results suggest that δ might continue to increase if chlorine content could be increased beyond the apparent glass solubility limit. These glasses were found to retain chlorine to high levels as reported elsewhere. Vaporization losses in these glasses are substantially reduced relative to silicates, for example, due to lower melt temperatures and shorter soak times.