Ion Dynamics in Solid Electrolytes: NMR Reveals the Elementary Steps of Li+ Hopping in the Garnet Li6.5La3Zr1.75Mo0.25O12

• Published in: Chemistry of materials 2015-10, Vol.27 (19), p.6571-6582
• Main Authors: Bottke, Patrick, Rettenwander, Daniel, Schmidt, Walter, Amthauer, Georg, Wilkening, Martin
• Format: Article
• Language: English
• ISSN: 0897-4756; 1520-5002
• DOI: 10.1021/acs.chemmater.5b02231

Garnet-type oxides are considered to belong to the most attractive solid Li+ electrolytes. This is due to their wide electrochemical stability window as well as their superior ionic conductivity, with a Li-ion transference number of almost one. Usually ionic conductivities are studied via impedance spectroscopy on a macroscopic length scale. Time-domain NMR methods, however, have been used much less extensively to shed light on the elementary hopping processes in highly conducting oxide garnets. Here, we used NMR relaxometry and stimulated echo NMR to study Li+ self-diffusion in Li6.5La3Zr1.75Mo0.25O12 (LLZMO), which served as a model compound to collect information on the 7Li spin dynamics. It turned out that NMR spin–lattice relaxation (SLR) recorded in both the laboratory and rotating frame of reference shows features that seem to be a universal fingerprint for fast conducting garnets that have been stabilized in their cubic modification. In contrast to Al-doped garnet-type Li7La3Zr2O12 that modifies the Li sublattice, in LLZMO the Li sublattice remains intact, offering the possibility to get to the bottom of Li-ion dynamics in LLZO-based garnets. Most importantly, whereas NMR SLR rates measured at 194.3 MHz reflect an almost universal behavior of local hoppings being thermally activated by only 0.151(3) eV, the spin-lock technique (33.3̅ kHz) gives evidence of two separate, overlapping rate peaks with activation energies on the order of 0.29 eV for the elementary steps of Li-ion hopping. This points to a less pronounced distribution of Li+ jump rates on the kilohertz time scale than has been observed for the Al-stabilized LLZO samples. The NMR results obtained also entail information on both the Li+ diffusion coefficients and the shape of the underlying motional correlation functions. The latter has been provided by 7Li NMR spin-alignment echo correlation spectroscopy that also shows the involvement of 24d and 96h sites in Li+ diffusion.