Fast Li Ion Dynamics in Defect-Rich Nanocrystalline Li4PS4I─The Effect of Disorder on Activation Energies and Attempt Frequencies

Solid-state electrolytes with fast ionic transport properties will play a crucial role in future energy storage applications that rely on electrochemical reactions. The group of lithium thiophosphates appears to be especially promising as it includes compounds such as Li ₁₀GeP ₂S ₁₂, already exceeding the conductivity of conventional liquid electrolytes. Recently, the I-containing thiophosphate, Li ₄PS ₄I, belonging also to this group attracted great interest due to its theoretically high ionic conductivity that is assumed from the favorable 3D percolation pathways. However, earlier studies showed that ionic conductivity is lower than expected from these structural considerations. Here, we reinvestigated both long-range and short-range ion dynamics through nuclear magnetic resonance (NMR) measurements as well as by conductivity spectroscopy to gain more insights into the conduction mechanism. It turned out that by changing the morphology of Li ₄PS ₄I, that is, by going from the coarse-grained to the nanocrystalline form, a significant increase in ion dynamics is seen that is accompanied by a change in the Arrhenius prefactor and a clear decrease in activation energy [0.35 eV (nano) and 0.49 eV (micro)] for ionic conduction. We assume that the smaller energy barrier for the nanostructured sample, which was prepared by ball milling, originates form the increased number of defects introduced through mechanical treatment. In line with the reduction in activation energy, we clearly observed an increase in room-temperature conductivity from 2.9 × 10 ^-2 mS cm ^-1 (micro) to 0.47 mS cm ^-1 (nano), that is, by almost two orders of magnitude. Diffusion-induced spin-lattice relaxation ⁷Li NMR measurements reveal two distinct Li ion diffusion processes. While the slower process shows similar activation energies for both micro- and nanocrystalline Li ₄PS ₄I, the faster relaxation process displays, however, activation energies of 0.32 and 0.13 eV for the microcrystalline and the nanocrystalline sample, respectively. Additional spin-lock NMR measurements sense long-range ion transport (0.34-0.36 eV) and point to anisotropic ion conduction for both samples. Taken together, the combination of nuclear and non-nuclear methods operating on different time scales help characterize the relevant diffusion pathways in Li ₄PS ₄I finally leading to the superior behavior of nano-Li ₄PS ₄I in terms of through-going ionic conduction.