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 P₂S₇^4– and PS₄^3– 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. Here, the results of this study highlight a new strategy to suppress crystallization in sulfide-based SEs, which has important implications for solid-state batteries.