Lithium ion conductors continue to attract huge attention mainly due to the need for improved Li ion batteries, although “post-Li” energy storage systems are also being explored. In any case, basic research dealing with, e.g., the elementary ion jump process, the dimensionality of diffusion or the influence of structural disorder – besides being a field in its own right – is indispensable for those applications, too.

Here, exemplary results of our group on Li ion conductors are reviewed. Interestingly, materials used as model systems for basic questions quite often are also potential battery materials, i.e. solid electrolytes or electrodes, and vice versa.

An early example of a cathode material has been the intercalation compound Li_xTiS₂ which turned out to be a model system for two-dimensional Li diffusion, involving a unique jump process over many decades, as was proven by a combination of nuclear magnetic resonance (NMR) techniques. Besides the layer-structured also the cubic modification as well as nanocrystalline and amorphous forms were examined with respect to dimensionality and disorder effects.. Anode materials studied by us primarily from a fundamental point of view have been Li_xC₆ (0<x≤1), Li_4+xTi₅O₁₂ (0<x<3) and, as one of the silicides, Li₁₂Si₇. In Li₁₂Si₇ very fast quasi one-dimensional Li diffusion was detected. Among the potential electrolytes for all-solid-state Li ion batteries, Li₇La₃Zr₂O₁₂ was investigated in some detail also with respect to the influence of doping and defects on Li diffusivity.

The examples to be presented extensively involve NMR, which now is an established materials science method for Li ion dynamics [1, 2], but also impedance spectroscopy [3], mass spectrometry [4] and neutron reflectometry [5] have been applied in the Li ion transport studies.

[1] C. V. Chandran, P. Heitjans, Solid-State NMR Studies of Lithium Ion Dynamics Across Materials Classes, Annual Reports on NMR Spectroscopy 89 (2016) 1-102.
[2] K. Volgmann, V. Epp, J. Langer, B. Stanje, J. Heine, S. Nakhal, M. Lerch, M. Wilkening, P. Heitjans, Solid-State NMR to Study Translational Li Ion Dynamics in Solids with Low-Dimensional Diffusion Pathways, Z. Phys. Chem. 231 (2017) 1215–1241.
[3] J. Rahn, E. Hüger, L. Dörrer, B. Ruprecht, P. Heitjans, H. Schmid, Li Self-Diffusion in Lithium Niobate Single Crystals at Low Temperatures, Phys. Chem. Chem. Phys. 116 (2012) 2427–2433.
[4] A.-M. Welsch, H. Behrens, I. Horn, S. Roß, P. Heitjans, Self-Diffusion of Lithium in LiAlSi₂O₆ Glasses Studied Using Mass Spectrometry, J. Phys. Chem. A 116 (2012) 309–318.
[5] F. Strauß, E. Hüger, P. Heitjans, T. Geue, J. Stahn, H. Schmidt, Li Permeation through Thin Lithium-Silicon Films for Battery Applications Investigated by Neutron Reflectometry, Energy Technology 12 (2016) 1582–1587.