Studies on the Mechanisms for the Transport of Lithium Ions in Solid-State Composites Using a New Combination of Li NMR Methods

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In view of a high relevance in energy and information technologies, a large number of recent research studies have focused on the dynamics of ions in solid-state electrolytes. Despite these efforts, our understanding of ion transport in solids is still incomplete, limiting the possibilities of a knowledge-based design of new materials. The foundations for a fundamental understanding are laid by a comprehensive experimental characterization. Here, we propose to exploit that solid-state nuclear magnetic resonance is a powerful tool to investigate complex motions of ions in solid-state electrolytes. Specifically, we intend to use a broad spectrum of 6Li and 7Li NMR methods to ascertain dynamics of lithium ions on a large variety of time scales and length scales. For a study of dynamics on microscopic length scales, stimulated-echo experiments and field-cycling experiments will be combined for the first time. The former enable a direct measurement of correlation functions of the ionic motion, while the latter provide access to the spectral density of the ionic motion via a determination of the frequency dependence of the spin-lattice relaxation time T1. The time windows of both these techniques complement each other in an ideal way, allowing us to cover a time range of ca. 1 ps - 10 s. To study the transport of lithium ions on mesoscopic and macroscopic length scales, it is planned to measure self-diffusion coefficients by means of NMR experiments in static field gradients. Applying this new combination of 6Li und 7Li NMR methods to the lithium ionic motion on various time scales and length scales, we intend to trace back long-range transport and conductivity to elementary ionic jumps in a solid matrix.Our studies will focus on composites, which have attracted a great deal of attention in the quest for solid-state electrolytes with improved material properties. Specifically, we propose to investigate composites where the solid matrix exhibits distinguishable structures or compositions in different spatial regions. An important criterion for the selection of the systems is that an increase of structural heterogeneity is accompanied by an increase of the ionic conductivity. In detail, two interesting examples to be studied are (Li2S)_x(P2S5)_(1-x) glass-ceramics and (Li2S)_0.5-[(1-x)GeS2-xGeO2]_0.5 mixed-network former glasses. In order to ascertain the origin of the enhanced ionic conductivity of these composite materials, the mechanism for the ion transport will be ascertained in the light of the structural heterogeneity. This knowledge will be of great use for a future improvement of the electric conductivity of solid-state electrolytes.