Solvent-Mediated Synthesis of Amorphous Li₃PS₄/Polyethylene Oxide Composite Solid Electrolytes with High Li⁺ Conductivity

Solvent-mediated routes have emerged as an effective, scalable, and low-temperature method to fabricate sulfide-based solid-state electrolytes. However, tuning the synthesis conditions to optimize the electrolyte’s morphology, structure, and electrochemical properties is still underexplored. Here, we report a new class of composite solid electrolytes (SEs) containing amorphous Li3PS4 synthesized in situ with a poly(ethylene oxide) (PEO) binder using a one-pot, solvent-mediated route. The solvent and thermal processing conditions have a dramatic impact on the Li3PS4 structure. Conducting the synthesis in tetrahydrofuran resulted in crystalline β-Li3PS4 whereas acetonitrile led to amorphous Li3PS4. Annealing at 140 °C increased the Li+ conductivity of an amorphous composite (Li3PS4 + 1 wt % PEO) by 3 orders of magnitude (e.g., from 4.5 × 10–9 to 8.4 × 10–6 S/cm at room temperature) because of: (i) removal of coordinated solvent and (ii) rearrangement of the polyanionic network to form P2S74– and PS43– moieties. The PEO content in these composites should be limited to 1–5 wt % to ensure reasonable Li+ conductivity (e.g., up to 1.1 × 10–4 S/cm at 80 °C) while providing enough binder to facilitate scalable processing. The results of this study highlight a new strategy to suppress crystallization in sulfide-based SEs,, which has important implications for solid-state batteries.