Influence of oxygen distribution on the Li-ion conductivity in oxy-sulfide glasses - taking a closer look

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.