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The large-scale production of solid-state batteries necessitates the development of alternative routes for processing air-sensitive thiophosphate-based solid electrolytes. To set a basis for this, we investigate the chemical stability and ionic conductivity of the LGPS-type lithium-ion conductor tetra-Li7SiPS8 (LiSiPS) processed with various organic solvents. We elucidate the nature of colorful polysulfides that arise during solvent treatment and trace back their origin to the dissolution of the Li3PS4-type amorphous side phase typically present in LiSiPS. We find that water and alcohols decompose LiSiPS by the nucleophilic attack into oxygen-substituted thiophosphates and thioethers and propose a reaction mechanism for the latter. Moreover, we confirm that quaternary thiophosphates can be recrystallized from MeOH solutions upon subsequent high-temperature treatment. Aprotic solvents with donor numbers smaller than 15 kcal mol–1 are suitable for wet-processing quaternary thiophosphates because both the crystal structure of the electrolyte and a high ionic conductivity of >1 mS cm–1 are retained. Using anisole as a case study, we clarify that a residual water content of up to 800 ppm does not lead to a significant deterioration in the ionic conductivity when compared to dry solvents (≤5 ppm). Additionally, we observe a decrease in ionic conductivity with an increasing amount of the solvent residue, which depends not only on the donor number of the solvent but also on the vapor pressure and interactions between the solvent molecules and thiophosphate groups in the solid electrolyte. Thus, optimization of solvent-processing methods of thiophosphate electrolytes is a multifaceted challenge. This work provides transferable insights regarding the stability of LiSiPS against organic solvents that may enable competitive and large-scale thiophosphate-based solid electrolyte processing.
Hatz et al. (Mon,) studied this question.