Na3PS3O as a Solid Electrolyte: Influence of the Synthesis Method on the Structural Unit Formation

To improve battery power density, sodium all-solid-state batteries, using a solid electrolyte and sodium metal (Na0) anode, are part of the next generation of electrochemical storage systems. Among potential solid electrolyte materials, Na3PS4, sodium ortho-thiophosphate, is attractive because it possesses high ionic conductivity and good compressibility. However, it reacts with Na0 and is unstable when exposed to moisture. To address these stability issues, oxy-thiophosphate compositions (Na3PS4-xOx, 1 < x < 3) are being considered. For instance, the Na3PS3O phase has been reported to be stable versus Na0. Here, we report on the formation of the Na3PS3O phase using aqueous or mechanochemical synthesis and the impact of the physical state on the conduction properties. The preparation of a pure Na3PS3O material is challenging, as other structural units than PS3O3- are always present, regardless of the synthesis method used. Crystalline Na3PS3O containing PS2O23- and PSO33- impurities can be synthesized from aqueous synthesis, or that containing P2O74- impurities can be obtained via mechanochemistry followed by annealing. A pure amorphous Na3PS3O phase could not be synthesized, as the direct ball-milling synthesis (regardless of the precursors used) led to the formation of a PS43- matrix containing diverse oxy-thiophosphate and phosphate species.