Elucidating the Electrochemical Activity of Electrolyte-Insoluble Polysulfide Species in Lithium-Sulfur Batteries

The direct synthesis of Li 2 S 2 , a proposed solid intermediate in the discharge of lithium-sulfur (Li-S) batteries, was accomplished by treating elemental lithium with sulfur in liquid ammonia at −41°C. The as-synthesized product was analyzed by X-ray photoelectron spectroscopy (XPS) as well as X-ray diffraction (XRD) and determined to be a mixture of crystalline Li 2 S, amorphous Li 2 S 2, and higher-order polysulfides (Li 2 S x , x > 2). Monitored filtration followed by a tailored electrochemical approach was used to successfully remove the higher-order polysulfides and yielded a powder, which was determined by XPS to be comprised of ∼9 mol% insoluble polysulfide species (mainly Li 2 S 2 ) and ∼91 mol% Li 2 S. This material was discharged galvanostatically in an electrochemical cell and, despite the lack of soluble polysulfide species, was shown to exhibit a discharge plateau at ∼2.1 V vs. Li/Li + . This result confirmed the electrochemical reducibility of electrolyte-insoluble polysulfides in Li-S batteries. Moreover, it was determined that the reduction of solid polysulfides was confined to areas where the sulfur-sulfur bonds were in intimate contact with the conductive current collector. Finally, it was observed that commercially available Li 2 S samples contain significant quantities of polysulfide-type impurities.