Abstract
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.
Highlights
Reducing human greenhouse gas emissions to lessen the increase of global temperature is one of the biggest challenges that industrial societies are facing
Li6PS5I remains stable under the milling conditions chosen; we do not observe any chemical degradation when milling the compound under an inert gas atmosphere
The important intercage hopping processes occur less frequently, resulting in poor ionic direct current (DC) conductivities with a value on the order of 1 μS cm−1 under ambient conditions
Summary
Reducing human greenhouse gas emissions to lessen the increase of global temperature is one of the biggest challenges that industrial societies are facing. Li6PS5I was the authors first introduced by Deiseroth studied the structural details, ionic conductivity, and diffusion pathways in a sample prepared by a solid-state synthesis route. In contrast to anion-disordered Li6PS5X (X = Cl, Br), for the ordered counterpart with X = I, the important intercage diffusion step, being necessary to enable long-range ion transport rather than only local jump processes, is characterized by noticeably higher activation energy (see below).[46,67]. NMR helped us to further characterize the microstructure of the nanocrystalline sample Our investigation represents another application-oriented example where high-energy ballmilling was successfully applied to boost ion dynamics of an originally poor ionic conductor without changing its overall chemical composition. While for previous oxide examples[70−73] the final conductivities showed values in the μS range, mechanical treatment of Li6PS5I ensured that conductivities with values almost touching the mS regime were reached
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