Lithium Mobility in Borate and Phosphate Glass Networks

In order to improve our understanding of the Li-mobility in oxide glass networks with Li as the principle mobile particle, electrical conductivity and self-diffusivity of lithium was studied in two phosphate (0.2 Li2O center dot 0.8 P2O5, 0.3 Li2O center dot 0.2 MgO center dot 0.5 P2O5) and one borate (0.25 Li2O center dot 0.75 B2O3) glass compositions. Conductivity measurements provided information about ion dynamics while isotope-exchange experiments involving isotopically enriched Li diffusion glass couples provided information about long-range diffusivity of Li-isotopes through borate and phosphate networks. Due to the limitations of individual glass stabilities, the temperature range for selected experiments was very small, e.g. as in the case of Li-phosphate composition between 373 and 520 K. The activation energy for Li-migration derived from conductivity measurements was similar for Li-Mg-phosphate and Li-borate, 90.4 and 85.2 kJ/mol, while for pure Li-phosphate the value was 74.7 kJ/mol. In the case of self-diffusion, the activation energies were comparable with Li-Mg-phosphate having the highest value of 76.9 kJ/mol while Li-phosphate and Li-borate had almost the same value of 72.9 and 72.2 kJ/mol, respectively. In these glass compositions with similar Li-cation concentration, the differences in the mobility predominantly depend on structural arrangement of building units and the spatial distribution of negative potentials, as reflected in the value of H-R/f, i.e. the Haven ratio divided by the correlation factor, as a mean to better understand the diffusion mechanism in glass structures, where vacancy vs. interstitial diffusion cannot be clearly defined. For Li-phosphate almost unconstrained Li-migration was indicated with the H-R/f value of 0.98, while Li-Mg-phosphate had the most structural constraint on mobilized Li-cations, with the H-R/f value of 0.30. Findings are compared with silicate (Li2O center dot 3 SiO2) and aluminosilicate (Li2O center dot Al2O3 center dot 4 SiO2) glasses from our previous studies in order to elaborate the effect of network topology.