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

Argyrodite-type Li6PS5X (X = Cl, Br) compounds are considered to act as powerful ionic conductors in next-generation allsolid-state lithium batteries. In contrast to Li6PS5Br and Li6PS5Cl compounds showing ionic conductivities on the order of several mS cm -1 , the iodine compound Li6PS5I turned out to be a poor ionic conductor. This difference has been explained by anion site disorder in Li6PS5Br and Li6PS5Cl leading to facile through-going, that is, long-range ion transport. In the structurally ordered compound, Li6PS5I, 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 Li6PS5I. A milling time of only 120 min turned out to be sufficient to boost ionic conductivity by 2 orders of magnitude, reaching sigma(total) = 0.5 X 10(-3) S cm(-1). We followed this noticeable increase in ionic conductivity by broad-band conductivity spectroscopy and Li-7 nuclear magnetic relaxation. X-ray powder diffraction and high-resolution Li-6, P-31 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.