The attractive long-term stability and safety of so-called all-solid-state batteries has given new impetus to developing powerful solid Li⁺ electrolytes. Post Li-ion batteries equipped with solid electrolytes may also be very attractive for hybrid electric vehicles as these systems are expected to withstand high operation temperatures. Moreover, they may serve as attractive components in Li-oxygen and Li-sulfur batteries. In addition to the various oxides and phosphates studied so far, see, e.g., refs. [1,2], sulfide-based materials have attracted particular attention due to their very high ionic conductivities [3,4].

Li₃PS₄ represents such a promising candidate, since its achievable Li-ion conductivity is comparable to that of liquid electrolytes. Phase-pure Li₃PS₄ was prepared by joint high-energy ball-milling of Li₂S and P₂S₅ for 35 hours and subsequent heat treatment at 260 °C for 17 hours. X-ray diffraction analysis corroborated the phase purity of the sample prepared. To study the elementary steps of ion hopping in Li₃PS₄ ⁷Li solid-state nuclear magnetic resonance (NMR) spectroscopy was employed. In particular, we used temperature-variable relaxometry measurements carried out in both the laboratory and rotating frame of reference to shed light on local as well as long-range ion dynamics. Line shape measurements revealed fast Li-ion exchange able to completely average homonuclear dipole-dipole interactions at temperatures much below ambient. This perfectly agrees with a recent NMR study focusing on field gradient experiments [5]. Moreover, it is in line with our spin-lattice relaxation (SLR) measurements performed at low temperatures: From the low temperature flank of the diffusion-induced SLR rates an activation energy as low as 0.1 eV was determined. Such a low value indicates fast, localized Li jumps present in Li₃PS₄. The high Li⁺ diffusivity also manifests itself in the appearance of an SLR spin-lock NMR rate peak that shows up well below room temperature.

[1] H. Buschmann, J. Dölle, S. Berendts, A. Kuhn, P. Bottke, M. Wilkening, P. Heitjans, A. Senyshyn, H. Ehrenberg, A. Lottnyk, V. Duppel, L. Kienle, J. Janek, Phys. Chem. Chem. Phys. 13 (2011) 19378

[2] V. Epp, Q. Ma, E.-M. Hammer, F. Tietz, M. Wilkening, Phys. Chem. Chem. Phys., 17 (2015) 32115

[3] M. Tatsumisago, M. Nagao, A. Hayashi, J. Asian Ceram. Soc., 1 (2013) 17

[4] V. Epp, O. Gün, H.-J. Deiseroth, M. Wilkening, J. Phys. Chem. Lett., 4 (2013) 2118

[5] K. Hayamizu, Y. Aihara, T. Watanabe, T. Yamada, S. Ito, N. Machida, Solid State Ion., DOI: 10.1016/j.ssi.2015.06.016