Abstract
Lithium-rich argyrodites belong to a relatively new group of fast ion conducting solids. They might serve as powerful electrolytes in all-solid-state lithium-ion batteries being, from a medium-term point of view, the key technology when safe energy storage systems have to be developed. Spin-lattice relaxation (SLR) nuclear magnetic resonance (NMR) measurements carried out in the rotating frame of reference turned out to be the method of choice to study Li dynamics in argyrodites. When plotted as a function of the inverse temperature, the SLR rates log10(R1ρ) reveal an asymmetric diffusion-induced rate peak. The rate peak contains information on the Li jump rate, the activation energy of the hopping process as well as correlation effects. In particular, considering the high-temperature flank of the SLR NMR rate peak recorded in the rotating frame of reference, an activation energy of approximately 0.49 eV is found. This value represents long-range lithium jump diffusion in crystalline Li7PSe6. As an example, at 325 K the Li jump rate determined from SLR NMR is in the order of 1.4 × 10(5) s(-1). The pronounced asymmetry of the rate peak R1ρ(1/T) points to correlated Li motion. It is comparable to that which is typically found for structurally disordered materials showing a broad range of correlation times.
Highlights
During the last few years many Li containing solid electrolytes have been screened and modified for such purposes
Li dynamics of the compounds belonging to the class of Li7PX6 (X = S, Se) and Li6PX5X0 (X0 = Cl, Br, I), which are from a structural point of view closely related to the mineral Ag8GeS6, have mainly been investigated by impedance spectroscopy.[9,15,16,17,18,19]
Local electric structures and Li self-diffusion in a polycrystalline sample of Li7PSe6 have been examined by temperature-variable 7Li nuclear magnetic resonance (NMR) spectroscopy
Summary
During the last few years many Li containing solid electrolytes have been screened and modified for such purposes. Monitoring the complete diffusion-induced SLR NMR rate peak, which is obtained when the SLR rate R1r is plotted vs the inverse temperature 1/T, gives access to both the activation energy Ea and the pre-exponential factor t0À1 determining the underlying Arrhenius relation (see below). It contains information about correlation effects such as structural disorder and Coulomb interactions.[32] it might be useful to roughly evaluate to what extent shortrange Li motions influence the R1r rates measured and, ion transport parameters
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