Using Raman Spectroscopy to Elucidate Conversion in Crosslinked Disulfide Polymers for Lithium-Sulfur Batteries

Abstract

The main advantages of Lithium-Sulfur batteries are the high theoretical specific capacity of elemental sulfur and the sustainability of sulfur-based materials over toxic, expensive heavy metals such as cobalt. Sulfur can theoretically achieve 1675 mAh g–1 through a conversion mechanism, but due to the parasitic polysulfide shuttle problem, the capacity rapidly fades. We demonstrate new crosslinked disulfides as promising cathode materials for Li–S cells that are designed to have only a single point of S–S scission, preventing the formation of polysulfides. The first model crosslinked disulfide system we synthesized was designed to maximize the ratio of S–S to the electrochemically inactive framework. The material contains a 1:1 ratio of S:C with a theoretical gravimetric capacity of 609 mAh g–1. Cells made with this crosslinked disulfide cathode material gain capacity through 100 cycles and have 98% capacity retention thereafter through 200 cycles, demonstrating promise for stable, long-term cycling. Coulombic efficiencies near 100% for every cycle, suggesting the suppression of polysulfide shuttle through molecular design. Further iterations of this crosslinked disulfide design aim to increase ionic and electronic conductivities to achieve higher gravimetric capacities that will lead these stable sulfur-based batteries closer to a commercially viable technology. Using crosslinked disulfides as cathode materials in liquid Li–S cells has not previously been explored in detail, so Raman spectroscopy provides an excellent handle to gain mechanistic insight into the conversion processes in these systems. Ex situ Raman confirms the proposed mechanism of disulfide bonds breaking to form a S–Li thiolate species upon discharge and reformation upon charge, and operando Raman provides more detailed understanding of the redox reactions occurring in the cell. Other characterization techniques, such as NMR and XPS provide structural information, in addition to SEM yielding macrostructural information, to piece together a detailed description of the disulfide behavior as a cathode material. Figure 1