Li₂S-P₂S₅ (LPS) glass-ceramic solid-state electrolytes (SSEs) have garnered substantial interest in solid-state battery research due to remarkable electrochemical properties and enhanced safety over conventional organic liquid electrolytes.[1, 2]   While conventional mechanical synthesis techniques for processing LPS SSEs are generally energy-intensive and time-consuming, recent advances in solution processing techniques have produced LPS SSEs with exceptional ionic conductivities and mechanical properties at low reaction temperatures and in a fraction of the amount of time required by mechanical processes.[3]  A major challenge lies in the limited-understanding of the underlying chemistries involved in solution processing, despite considerable efforts to elucidate full reaction mechanisms.[4] Here we contribute to the growing body of research investigating solvent and precursor interactions by exploring how the thiol functional group of the solvent ethyl mercaptan (EthSH) interacts with Li₂S and P₂S₅ under common reaction conditions to facilitate the formation of LPS SSEs.

Three common molar stoichiometries of LPS systems (50:50, 70:30, 75:25) and the individual precursors Li₂S and P₂S₅ were reacted in EthSH separately at room temperature for 24 hr with magnetic stirring, dried, and then annealed at 225 °C for 1 hr following common solution processing methodology.[5]  During each process step, subsamples of each reaction were collected and characterized electrochemically and physiochemically by AC electrical impedance (EIS), powder X-ray diffraction (PXRD), Raman shift, and nuclear magnetic resonance (NMR) spectroscopies, and scanning electron microscopy.

The low boiling point of ethyl mercaptan allows for rapid drying of the processed SSEs. However, the solvent does not complex with Li₂S and P₂S₅ when the Li₂S content of the LPS system is at or above 70 mol%, as other polar organic solvents do, nor does it complex with the individual precursors.   Instead, the 70:30 and 75:25 samples exhibited a moderate amount amorphous characteristic, commonly associated with solid-state reactions, and expressed XRD peaks assigned to Li₂S and Li₄P₂S₆.  At 50 mol% Li₂S, the ethyl mercaptan stabilized a Li₂S-P₂S₅/EthSH complex similar to Li₂P₂S₆ with a high order of crystallinity at room temperature, which was verified with PXRD, Raman, and SEM.  All three systems exhibited low or immeasurable ionic conductivity with EIS. To the best of our knowledge room temperature crystallization of any LPS system has not been reported before.