We investigate the influence of Br−/S2− site-exchange on lithium diffusion in the agyrodite-type solid electrolyte Li6PS5Br by ab-initio molecular dynamics simulations. Based on the calculated trajectories a new mechanism for the internal lithium reorganization within the Li-cages around the 4d sites is identified. This reorganization mechanism is highly concerted and cannot be described by one single rotation axis only. Simulations with Br−/S2− defects reveal that Lii. interstitials are the dominant mobile charge carriers, which originate from Frenkel pairs. These are formed because BrS. defects on the 4d sites cause the transfer of one or even two Lii. to the neighboring 12 cages. The lithium interstitials then carry out intercage jumps via interstitial and interstitialcy mechanisms. With that, one single BrS. defect enables Li diffusion over an extended spatial area explaining why low degrees of site-exchange are sufficient to trigger superionic conduction. The vacant sites of the Frenkel pairs, namely VLi′, are mostly immobile and bound to the BrS. defect. To a lesser degree also SBr′ defects induce disturbances in the lithium distribution and act as sinks for lithium interstitials restricting the Lii. motion to the vicinity of the SBr′ defect.
@article{arxiv.2102.09873,
title = {Influence of Br$^{-}$/S$^{2-}$ site-exchange on Li diffusion mechanism in Li$_6$PS$_5$Br -- a computational study},
author = {Marcel Sadowski and Karsten Albe},
journal= {arXiv preprint arXiv:2102.09873},
year = {2021}
}