RESUMO
Understanding the origins of fast ion transport in solids is important to develop new ionic conductors for batteries and sensors. Nature offers a rich assortment of rather inspiring structures to elucidate these origins. In particular, layer-structured materials are prone to show facile Li+ transport along their inner surfaces. Here, synthetic hectorite-type Li0.5[Mg2.5Li0.5]Si4O10F2, being a phyllosilicate, served as a model substance to investigate Li+ translational ion dynamics by both broadband conductivity spectroscopy and diffusion-induced 7Li nuclear magnetic resonance (NMR) spin-lattice relaxation experiments. It turned out that conductivity spectroscopy, electric modulus data, and NMR are indeed able to detect a rapid 2D Li+ exchange process governed by an activation energy as low as 0.35 eV. At room temperature, the bulk conductivity turned out to be in the order of 0.1 mS cm-1. Thus, the silicate represents a promising starting point for further improvements by crystal chemical engineering. To the best of our knowledge, such a high Li+ ionic conductivity has not been observed for any silicate yet.
RESUMO
Argyrodite-type Li6PS5X (X = Cl, Br) compounds are considered to act as powerful ionic conductors in next-generation all-solid-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 σtotal = 0.5 × 10-3 S cm-1. We followed this noticeable increase in ionic conductivity by broad-band conductivity spectroscopy and 7Li nuclear magnetic relaxation. X-ray powder diffraction and high-resolution 6Li, 31P 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.