Investigation of the Li-ion conduction behavior in the Li 10 GeP 2 S 12 solid electrolyte by two-dimensional T 1 -spin alignment echo correlation NMR M.C. Paulus a,b,⇑ , M.F. Graf a,b , P.P.R.M.L. Harks c , A. Paulus a , P.P.M. Schleker a,d , P.H.L. Notten a,c , R.-A. Eichel a,e , J. Granwehr a,b a Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung (IEK-9), D-52425 Jülich, Germany b RWTH Aachen University, Institut für Technische und Makromolekulare Chemie (ITMC), D-52074 Aachen, Germany c Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, NL-5600 MB Eindhoven, The Netherlands d Max-Planck-Institute for Chemical Energy Conversions, Mülheim an der Ruhr, Germany e RWTH Aachen University, Institut für Physikalische Chemie (IPC), D-52074 Aachen, Germany article info Article history: Received 9 March 2018 Revised 1 July 2018 Accepted 13 July 2018 Available online 17 July 2018 Keywords: Solid state NMR Relaxation-correlation NMR Inverse Laplace transform Solid state electrolytes Lithium-ion migration abstract Li 10 GeP 2 S 12 (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 under- standing of Li ion conduction that considers mobility in all three dimensions, mobility between crystal- lites 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 7 Li spin lattice relaxation (SLR) time constants (T 1 ) with the SAE decay time constant s c to distinguish between hopping time con- stants and signal decay limited by relaxation in the s 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. Ó 2018 Elsevier Inc. All rights reserved. 1. Introduction In state of the art Li ion batteries the electrolyte contains toxic and flammable organic liquids [1]. All-solid-state batteries, where the liquid electrolyte is replaced by glassy or crystalline inorganic solids, are a promising approach to achieve safer battery systems with higher volumetric energy densities [2]. One of the biggest challenges in these compounds is their relatively low Li ion con- ductivity compared to the liquid counterparts, but there are several promising candidates to close this gap. One of these candidates is the sulfide based Li 10 GeP 2 S 12 (LGPS) with a Li ion conductivity of 1.2 mScm 1 at room temperature [3], which is already competi- tive with liquid electrolytes. Li 10 GeP 2 S 12 represents the quasi- binary system Li 4 GeS 4 –Li 3 PS 4 , which crystallizes in the tetragonal space group P4 2 /nma with four lithium sites, shown in Fig. 1 [4]. The Li1 (16c) and Li3 (8f) sites are arranged in channels along the c-direction, whereas Li2 (4c) and Li4 (4d) reside in inter- channel positions [5]. This channel-like lattice structure can be considered the primary reason for the high ion mobility. An investigation of the solid-solution system Li 10+d Ge 1+d P 2-d S 12 by Kwon et al. [6] showed that the tetragonal modification of LGPS can only be achieved in the range of 0  d  0.35. Therefore, syn- thesizing pure-phase tetragonal LGPS is quite challenging and in most cases a significant amount of orthorhombic LGPS is obtained as secondary phase during the synthesis process. In several studies, the Li ion conductivity has been investigated using impedance spectroscopy. The highest conductivity in the LGPS solid solution system was found at d = 0.35 (6.1 mScm 3 ) [6], whereas Kamaya predicts the Li ion conductivity of the synthe- sized compound to be 1.2 mScm 3 at 27 °C. [3] The ion diffusion pathways were investigated by Weber et al. [7] using neutron diffraction measurements. The pathways have been recalculated https://doi.org/10.1016/j.jmr.2018.07.008 1090-7807/Ó 2018 Elsevier Inc. All rights reserved. ⇑ Corresponding author at: Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung (IEK-9), D-52425 Jülich, Germany. E-mail address: m.paulus@fz-juelich.de (M.C. Paulus). Journal of Magnetic Resonance 294 (2018) 133–142 Contents lists available at ScienceDirect Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr