Li-Ion Dynamics in Li2S:LiI Composites – on the Effect of Solid-Solution Formation on Overall Li+ Transport

The enormous interest in developing powerful all-solid-state Li-ion batteries led to a boost in electrolyte research. Some Li-ion conductors have Ag-bearing analogs; the fast Ag + ion conductor, Ag 3 SI, could be one of such materials. So far, no clear evidence is available that the Li-counterpart, Li 3 SI, does exist. Here, we treated an equimolar mixture of Li 2 S and LiI in high-energy ball mills and followed structural changes via X-ray diffraction (XRD) and magic-angle spinning (MAS) 6,7 Li nuclear magnetic resonance (NMR) measurements. Differences in local structures as sensed by MAS NMR will be discussed for annealed and non-annealed samples, that is, powder samples directly obtained after finishing the ball-milling procedure. For nanocrystalline, non-annealed Li 2 S:LiI, the MAS NMR response is characterized by a broad distribution of NMR lines showing chemical shifts between that of pure Li 2 S and LiI, hence pointing to the formation of solid-solution like regions. Ion dynamics on different length scales was probed with variable-temperature, potentiostatic conductivity spectroscopy as well as solid-state NMR spin-lattice relaxation (SLR) measurements. Defect-rich nanocrystalline Li 2 S:LiI has to be characterized by an overall conductivity that is 2 orders of magnitude higher as compared to that of the microcrystalline, annealed reference sample. SLR NMR experiments supports this result and reveal lower activation energies for both the fast local hopping ion dynamics and long-range ion diffusivity in the sample directly obtained after ball milling. Finally, mixing-time dependent 6 Li 2D exchange NMR spectroscopy reveals even (very) slow Li + exchange between the Li 2 S-rich and LiI-rich regions in the nanocrystalline sample. Off-diagonal intensities are especially seen for mixing times longer than 80 ms.