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

Abstract

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. The results of this study highlight a new strategy to suppress crystallization in sulfide-based SEs,, which has important implications for solid-state batteries.

Importance of multimodal characterization and influence of residual Li2S impurity in amorphous Li3PS4 inorganic electrolytes

“…Nanda et al and Tatsumisago et al have previously used XRD to conrm amorphous LPS formation where LPS is reported to exhibit one broad peak around 19 (observed in Sample 2). 43,46 The second peak at 30 in Sample 2 corresponds to the Kapton lm background (Fig. S2 †).…”

“…45,52,53 The visual color differences indicate likely unreacted precursors in Sample 1, and X-ray diffraction (XRD) was used to determine the differences. [38][39][40][41]43 Because Li 2 S crystallizes in the cubic space group Fm 3 m, weak reections within the amorphous LPS spectra corresponding to unreacted Li 2 S were expected. 43 However, as Fig.…”

“…[38][39][40][41]43 Because Li 2 S crystallizes in the cubic space group Fm 3 m, weak reections within the amorphous LPS spectra corresponding to unreacted Li 2 S were expected. 43 However, as Fig. 1a shows, several broad features are observed in both samples, yielding little insight into the chemical composition of both samples.…”

“…However, solution processing leads to LPSsolvent complexes that require temperatures as high as 100 C to remove the solvent and form pure LPS. 38,[41][42][43][44] Furthermore, solution processed LPS leads to ionic conductivities that are lower than those obtained using ball milling. 44 Finally, LPS stability in solvents is also of concern as recent reports have shown that Li 7 P 3 S 11 decomposes in polar solvents such as acetonitrile and dimethylformamide (DMF).…”

“…In addition, many literature reports on amorphous LPS synthesis use primarily X-ray diffraction to study phase and product purity even though XRD is a poor tool for amorphous compound characterization. 37,[43][44][45][46] Therefore, the inuence of unreacted precursors on amorphous LPS conductivity and electrochemical performance is unknown.…”