Lithium Doping of MgAl2O4 and ZnAl2O4 Investigated by High-Resolution Solid State NMR

Lithium-doped spinels could function as solid state electrolytes in batteries in which all components have the spinel structure. In this study we prepared lithium-doped MgAl2O4 and ZnAl2O4. Advanced solid state Li-7, Mg-25, and Al-27 NMR have been used to investigate the structure of these spinels and the structural changes that take place therein upon lithium doping. The spinel structure is well retained, although the amount of tetrahedral aluminum increases with increased lithium content. Using MQMAS experiments, the presence of two tetrahedral sites is discovered in both doped and undoped spinels. It is shown that the spinel structure of MgAl2O4 is heterogeneous, which leads to distributions in chemical shift and quadrupolar parameters. The heterogeneity is also shown in Mg-25 spectra obtained with the quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) and sideband selective Double Frequency Sweep-QCPMG (ssDFS-QCPMG) pulse sequences. Lithium mobility is measured using static variable temperature Li-7 NMR experiments. A low fraction of mobile lithium was found (3-4%) for magnesium spinels. The lithium doped ZnAl2O4 sample shows practically no mobility. This sample has significantly less disorder and less cation site inversion than the magnesium samples.