ABSTRACT
Lithium thiophosphates are a promising class of solid electrolyte (SE) materials for all-solid-state batteries (ASSBs) due to their high Li-ion conductivity. Yet, the practical application of lithium thiophosphates is hindered by their chemical instability, which remains a prevalent challenge in the field. Oxygen substitution has been discussed in the literature as a promising strategy to enhance stability. Nevertheless, the lack of understanding of the role of synthesis strategy on the resulting structure-property relationship makes it difficult to predict and control the material's behaviour, limiting our ability to fully utilize oxygen substitution as a viable solution. Here, we show that not only the total oxygen content but also the oxygen distribution within the material affects the ion conductivity. By carefully analysing the local structure of oxy-sulfide glasses, we find that few highly oxygenated structural units like [PO4]3- and [PO3S]3- are more detrimental to the ionic conductivity than a larger amount of less substituted units like [POS3]3-. Further, we demonstrate how the oxygen distribution is connected to the synthesis in high-energy ball milling by comparing two different sets of precursor materials. The results may explain the deviations in the past literature. The findings should be transferable to other Li-thiophosphate materials and enable more directed design of new materials.
ABSTRACT
We investigate how low degrees of [Formula: see text] site-exchange influence the [Formula: see text] diffusion in the argyrodite-type solid electrolyte [Formula: see text] by ab initio molecular dynamics simulations. Based on the atomic trajectories of the defect-free material, a new mechanism for the internal [Formula: see text] reorganization within the [Formula: see text] cages around the [Formula: see text] sites is identified. This reorganization mechanism is highly concerted and cannot be described by just one rotation axis. Simulations with [Formula: see text] defects reveal that [Formula: see text] interstitials ([Formula: see text]) are the dominant mobile charge carriers and originate from Frenkel pairs. These are formed because [Formula: see text] defects on the [Formula: see text] sites donate one or even two [Formula: see text] to the neighbouring cages. The [Formula: see text] then carry out intercage jumps via interstitial and interstitialcy mechanisms. With that, one single [Formula: see text] defect enables [Formula: see text] 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 [Formula: see text], are mostly immobile and bound to the [Formula: see text] defect. Because [Formula: see text] defects on [Formula: see text] sites act as sinks for [Formula: see text] they seem to be beneficial only for the local [Formula: see text] transport. In their vicinity T4 tetrahedral sites start to get occupied. Because the [Formula: see text] transport was found to be rather confined if [Formula: see text] and [Formula: see text] 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'.
