Long-range Li+ dynamics in the lithium argyrodite Li7PSe6 as probed by rotating-frame spin–lattice relaxation NMR

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(R_1ρ) 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 × 10⁵ s⁻¹. The pronounced asymmetry of the rate peak R_1ρ(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.