Design strategies for liquid-phase synthesis of sodium-based quaternary solid-state electrolytes

Abstract

The cost-effective and scalable production of solid-state electrolytes is essential for advancing all-solid-state sodium batteries as a safer, lower-cost alternative to Li-ion batteries. Herein, we explored liquid-phase synthesis methods for quaternary chalcogenide solid-state electrolytes (SSEs) with the formula Na₁₁Sn₂PnCh₁₂ (Pn = P, Sb, Ch = S, Se). We outline the key design principles, including reagent selection, solvent chemistry, and thermal treatment, while elucidating reaction mechanisms through systematic studies using XRD, Raman, and FTIR spectroscopy. We demonstrate that acetonitrile, an aprotic solvent with high polarity, enables the synthesis of Na₁₁Sn₂PS₁₂ via a simple solution process followed by heat-treatment, achieving high ionic conductivity (σ_Na⁺ = 0.2 mS cm⁻¹). The antimony analog, Na₁₁Sn₂SbS₁₂, can be produced in an aqueous solution facilitated by reactive bisulfide intermediates, yielding a highly crystalline chalcogenide at low temperatures with excellent transport properties (σ_Na⁺ = 0.4 mS cm⁻¹). For selenide analogs (e.g. Na₁₁Sn₂SbSe₁₂), alkahest amine–thiol solvents facilitate synthesis from elemental and binary precursors, marking the first report of its kind. Additionally, we identified Na₁₁Sn₂SbS₁₂ as the most promising candidate for utilization in sodium all-solid-state batteries owing to its wide electrochemical stability and strong compatibility with Na–Sn alloy anodes.