NMR Down to Cryogenic Temperatures: Accessing the Rate-Limiting Step of Li Transport in Argyrodite Electrolytes
Katharina Hogrefe, Florian Stainer, Nicolò Minafra, Wolfgang G. Zeier, Martin Wilkening
Abstract
High Resolution Image Download MS PowerPoint Slide Ion hopping processes in Li-containing argyrodite-type compounds are intensively studied because these materials might act as electrolytes with superionic transport properties. Such materials are urgently needed to realize liquid-free solid-state batteries. As in some of the frontrunners of this class of materials Li + diffusivity is extremely high, cryogenic temperatures are needed to completely freeze any thermally activated Li + motional processes. Here, we exposed Li 6.6 P 0.4 Ge 0.6 S 5 I, serving as a model substance, to temperatures as low as 9 K. By continuously increasing the temperature, we stepwise liberate the Li + ions and progressively switch on the various diffusion processes. Slow translational Li + jump processes were directly probed by sensing the associated spin fluctuations of the 7 Li spins in the frame of noncontact and, thus, nondestructive solid-state nuclear magnetic resonance (NMR) experiments. As an example, Li + ion exchange within the Li-rich cages in Li 6.6 P 0.4 Ge 0.6 S 5 I starts to affect magnetic dipolar interactions governing the NMR lines at temperatures as low as 100 K. At 163 and 116 K, the so-called spin-lock NMR relaxation rate passes through local maxima associated with extremely high Li + diffusivity that is governed by activation energies as low as 170 and 100 meV, respectively. Most importantly, we identified the rate peak at 163 K as the one reflecting the rate-limiting intercage Li + diffusion process that enables the ions to be transported over long distances. The corresponding Einstein–Smoluchowski diffusion coefficient excellently agrees with that indirectly probed by macroscopic conductivity spectroscopy.