Advancing sulfide solid electrolytes via green Li₂S synthesis

We present a potentially eco-friendly, cost-efficient strategy for synthesizing high-purity Li₂S, a key precursor for sulfide-based solid electrolytes. While these electrolytes surpass conventional organic counterparts in both safety and performance, their widespread application is hindered by the high cost of Li₂S. Here, a solvent-free metathesis route is developed, in which thiourea serves as an S²⁻ donor to sulfurize LiOH, enabling scalable Li₂S production (∼100 g per batch) with significantly reduced projected costs. During the process, intermediates (H₂NCN, H₂O) are transformed into benign gases (CO₂, NH₃) that spontaneously leave the system, thereby driving Li₂S formation without ΔG_mix limitations. The as-synthesized Li₂S is successfully applied to prepare sulfide-based solid electrolytes such as Li₁₀GeP₂S₁₂ and argyrodite-Li_5.5PS_4.5Cl_1.5, achieving laboratory-scale (1 kg) production costs reduction of up to 27.5% and 92.9%, respectively. Furthermore, all-solid-state batteries employing Li_5.5PS_4.5Cl_1.5 demonstrate electrochemical performance comparable to those fabricate with commercial Li₂S. This scalable methodology thus may provide a proming pathway to bridge low-cost Li₂S synthesis with the practical deployment of sulfide-based solid electrolytes, which may accelerate the commercialization of high-performance all-solid-state batteries.