Alternative Approach to Increasing Li Mobility in Li-La-Nb/Ta Garnet Electrolytes

The solid-state lithium electrolytes Li5La3Ta2O12 and Li5La3Nb2O12 have been the focus of recent studies due to their chemically robust nature (against the lithium electrode; particularly the tantalate analogue) and high lithium conductivity. Structural characterization and Li-conductivity studies have shown that there are two populations of lithium cations, nominally called octahedral (less tightly bound) and tetrahedral sites (tightly bound), and the octahedrally coordinated lithium are considerably more mobile than the tetrahedrally coordinated lithium. In this study, we have documented two methods to preferentially vacate the tetrahedral or nonmobile lithium sites. This is accomplished by either Li2O volatility upon repeated mixing and heating solid-state processing steps or by aqueous H+-Li+ exchange, followed by removal of the exchanged-in H+ by water vaporization. The aqueous treatment produces the largest change in the tetrahedral (nonmobile) lithium sites concentration. The ability to ion-exchange the Li+ also suggests an aqueous instability of these phases that is not widely recognized. These processes are documented by Li-6 MAS NMR, infrared spectroscopy, and thermogravimetry. Combined Li-6 and Li-7 NMR studies show that aqueous treatment significantly increases the dynamics of the remaining Li sites. The NMR energy of activation (E) for lithium mobility in Li5-xLa3Ta2O12-x/2 decreases by similar to 54% for the long-T1 Li component and similar to 14% for the last-T-1 component following heat treatment, while subsequent aqueous treatment revealed a slow component with a similar E to the heat treated sample hut with the short-T-1 component showing an approximate doubling in the E-a.