Influence of Br-/S2- site-exchange on Li diffusion mechanism in Li6PS5Br: a computational study

We investigate how low degrees of Br-/S2- site-exchange influence the Li+ diffusion in the argyrodite-type solid electrolyte Li6PS5Br by ab initio molecular dynamics simulations. Based on the atomic trajectories of the defect-free material, a new mechanism for the internal Li+ reorganization within the Li+ cages around the 4d sites is identified. This reorganization mechanism is highly concerted and cannot be described by just one rotation axis. Simulations with Br-/S2- defects reveal that Li+ interstitials (Li-i.) are the dominant mobile charge carriers and originate from Frenkel pairs. These are formed because BrS. defects on the 4d sites donate one or even two Li-i. to the neighbouring cages. The Lii. 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 V-Li ', are mostly immobile and bound to the BrS. defect. Because SBr ' defects on 4a sites act as sinks for Lii. they seem to be beneficial only for the local Li+ transport. In their vicinity T4 tetrahedral sites start to get occupied. Because the Li+ transport was found to be rather confined if SBr ' and BrS. defects are direct neighbours, their relative arrangement seems to be crucial for effective long-range transport. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.

Automated summary

Ab initio molecular dynamics simulations were used to investigate the influence of low degrees of Br-/S2- site-exchange on Li+ diffusion in the argyrodite-type solid electrolyte Li6PS5Br. A new mechanism for internal Li+ reorganization was identified, with Li+ interstitials being the dominant mobile charge carriers originating from Frenkel pairs due to BrS. defects. The relative arrangement of SBr' and BrS. defects was found to be crucial for effective long-range transport of Li+.