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

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 < or = x < or = 0.4) and lithium borate (Li2O)(x)(B2O3)(1-x), (0.1 < or = x < or = 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 < or = 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.

Site discrimination in mixed-alkali glasses studied by cross-polarization NMR.

Cation-cation interactions are thought to play a significant role in shaping the nonlinear compositional dependence of ionic conductivity, known as the mixed-alkali effect (MAE) in glassy solid electrolytes. For providing a structural rationale of this effect, the discrimination of various cation sites in mixed-alkali glasses is of interest. In the present study, cross-polarization (CP) experiments have been applied to glasses in the system [(Li(2)O)x(Na(2)O)(1-x)](0.3)[B(2)O(3)]0.7 to discriminate between alkali ions by virtue of different heteronuclear (7)Li-(23)Na dipole-dipole coupling strengths. Cross-polarization studies involving two types of quadrupolar nuclei (both (7)Li and (23)Na have a spin-quantum number I = 3/2) are complicated by spin state mixing under radio frequency irradiation and magic-angle spinning (MAS). Therefore careful validation and optimization protocols are reported for the model compound LiNaSO(4) prior to conducting the measurements on the glassy samples. (23)Na -->( 7)Li CP/MAS NMR spectra have been obtained on glasses containing the Na(+) ions as the dilute species. They reveal that those lithium species interacting particularly strongly with sodium ions have the same average (7)Li chemical shift as the entire lithium population; the symmetrical situation applies to the (23)Na nuclei at the sodium rich end of the composition range. On the other hand, a clear site discrimination is afforded by temperature-dependent static (23)Na -->( 7)Li CP experiments, indicating that the Li ions that are most strongly interacting with sodium ions are strongly immobilized. This finding provides the first direct experimental evidence for the proposed secondary mismatch concept invoked for explaining the strong MAE in the dilute foreign ion limit.