Investigation of the Li-ion conduction behavior in the Li₁₀GeP₂S₁₂ solid electrolyte by two-dimensional T₁-spin alignment echo correlation NMR

Li₁₀GeP₂S₁₂ (LGPS) is the fastest known Li-ion conductor to date due to the formation of one-dimensional channels with a very high Li mobility. A knowledge-based optimization of such materials for use, for example, as solid electrolyte in all-solid-state batteries requires, however, a more comprehensive understanding of Li ion conduction that considers mobility in all three dimensions, mobility between crystallites and different phases, as well as their distributions within the material. The spin alignment echo (SAE) nuclear magnetic resonance (NMR) technique is suitable to directly probe slow Li ion hops with correlation times down to about 10⁻⁵ s, but distinction between hopping time constants and relaxation processes may be ambiguous. This contribution presents the correlation of the ⁷Li spin lattice relaxation (SLR) time constants (T₁) with the SAE decay time constant τ_c to distinguish between hopping time constants and signal decay limited by relaxation in the τ_c distribution. A pulse sequence was employed with two independently varied mixing times. The obtained multidimensional time domain data was processed with an algorithm for discrete Laplace inversion that does not use a non-negativity constraint to deliver 2D SLR–SAE correlation maps. Using the full echo transient, it was also possible to estimate the NMR spectrum of the Li ions responsible for each point in the correlation map. The signal components were assigned to different environments in the LGPS structure.