NMR Down to Cryogenic Temperatures: Accessing the Rate-Limiting Step of Li Transport in Argyrodite Electrolytes

Abstract

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 Li6.6P0.4Ge0.6S5I, 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 7Li 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 Li6.6P0.4Ge0.6S5I 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.