Spatial distribution of lithium ions in glasses studied by 7Li{6Li} spin echo double resonance

Abstract

This manuscript introduces ⁷Li{⁶Li} spin echo double resonance (SEDOR) spectroscopy as a novel approach for studying the spatial distribution of lithium ions in solid electrolytes. Theoretical simulations using density operator theory as well as experimental validation on the model compound lithium carbonate reveal that this method affords a selective measurement of ⁷Li–⁶Li heteronuclear dipole–dipole couplings. Dipolar second moments characterizing internuclear lithium–lithium interactions have been measured in lithium silicate (Li₂O)_x(SiO₂)_1−x, (0.1 ≤ x ≤ 0.4) and lithium borate (Li₂O)_x(B₂O₃)_1−x, (0.1 ≤ x ≤ 0.3) glasses. The results indicate that the spatial distributions of the lithium ions in these two glass systems are decidedly different. In the lithium silicate glass system, the results give clear evidence of strong cation clustering for x ≤ 0.3, providing quantitative support for a previously proposed model of a one-dimensional channel structure. In contrast, in the lithium borate glass system, the cations seem to be more or less randomly distributed. Nevertheless, an observed superlinear dependence of M₂(⁷Li–⁶Li) as a function of ion concentration indicates subtle changes of the lithium arrangement principles, which are discussed in relation to the previously proposed ring structure of borate glasses.