Insights from Local Network Structures and Localized Diffusion on the Ease of Lithium Ion Transport in Two Mixed Glass-Former Systems

The conductivity enhancement observed in the two glass systems A (40Li 2 O–(60– x )P 2 O 5 – x GeO 2 ) and B (40Li 2 O–10B 2 O 3 –(50– x )P 2 O 5 – x GeO 2 ) as the germanate content is increased from 0 to 25 mol % confirms the positive mixed glass-former effect (MGFE) in these systems. In this study, we further employ state-of-the-art NMR techniques along with Raman spectroscopy to probe the local network structures. We use the MIGRATION concept to model the experimental conductivity and permittivity spectra obtained from impedance spectroscopy to understand the scaling features of the spectra and to calculate the value of the spatial extent of the localized diffusion of the lithium ion. As x increases from 0 to 25 mol %, for system A, a continuous increase of the POGe cross-linkages as well as a continuous and strong modification of the phosphate network is observed. In system B, deeper analysis of the 31 P magic-angle spinning NMR experiments done using 2D 11 B/ 11 B homonuclear and 11 B/ 31 P heteronuclear NMR is used to determine the nature of the BOB linkages as well as that of the POB linkages. Modeling of the conductivity spectra shows that the shape parameter of the spectra remains the same for both systems and for all compositions, a feature typical for MGFE. Furthermore, correlating the trends of the spatial extent of localized diffusion with that gleaned from the local structures, we infer that, as the relative germanate content increases, in the ternary glass (system A) the ease of mobility of the Li + ion is enhanced while in the quaternary glass (system B, x > 0 mol %) it is somewhat hindered.