Influence of Br${}^{-}$/S${}^{2-}$ site-exchange on Li diffusion mechanism in Li${}_6$PS${}_5$Br -- a computational study

Abstract

We investigate the influence of Br${}^{-}$/S${}^{2-}$ site-exchange on lithium diffusion in the agyrodite-type solid electrolyte Li${}_6$PS${}_5$Br 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${}^{-}$/S${}^{2-}$ defects reveal that Li${}_i^{.}$ interstitials are the dominant mobile charge carriers, which originate from Frenkel pairs. These are formed because Br${}_{\text{S}}^{.}$ defects on the $4d$ sites cause the transfer of one or even two Li${}_i^{.}$ to the neighboring 12 cages. The lithium interstitials then carry out intercage jumps via interstitial and interstitialcy mechanisms. With that, one single Br${}_{\text{S}}^{.}$ 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 V${}_{\text{Li}}^{\prime }$, are mostly immobile and bound to the Br${}_{\text{S}}^{.}$ defect. To a lesser degree also S${}_{\text{Br}}^{\prime }$ defects induce disturbances in the lithium distribution and act as sinks for lithium interstitials restricting the Li${}_i^{.}$ motion to the vicinity of the S${}_{\text{Br}}^{\prime }$ defect.