Understanding the Origin of Enhanced Li-Ion Transport in Nanocrystalline Argyrodite-Type Li6PS5I

Argyrodite-type Li 6 PS 5 X (X = Cl, Br) compounds are considered to act as powerful ionic conductors in next-generation all-solid-state lithium batteries. In contrast to Li 6 PS 5 Br and Li 6 PS 5 Cl compounds showing ionic conductivities on the order of several mS cm –1 , the iodine compound Li 6 PS 5 I turned out to be a poor ionic conductor. This difference has been explained by anion site disorder in Li 6 PS 5 Br and Li 6 PS 5 Cl leading to facile through-going, that is, long-range ion transport. In the structurally ordered compound, Li 6 PS 5 I, long-range ion transport is, however, interrupted because the important intercage Li jump-diffusion pathway, enabling the ions to diffuse over long distances, is characterized by higher activation energy than that in the sibling compounds. Here, we introduced structural disorder in the iodide by soft mechanical treatment and took advantage of a high-energy planetary mill to prepare nanocrystalline Li 6 PS 5 I. A milling time of only 120 min turned out to be sufficient to boost ionic conductivity by 2 orders of magnitude, reaching σ total = 0.5 × 10 –3 S cm –1 . We followed this noticeable increase in ionic conductivity by broad-band conductivity spectroscopy and 7 Li nuclear magnetic relaxation. X-ray powder diffraction and high-resolution 6 Li, 31 P MAS NMR helped characterize structural changes and the extent of disorder introduced. Changes in attempt frequency, activation entropy, and charge carrier concentration seem to be responsible for this increase.