Tri-sulfur radical trapping in lithium–sulfur batteries

Lithium-sulfur (Li–S) batteries have emerged as a next-generation battery technology owing to their prospects of high capacity and energy density. They, however, suffer from rapid capacity decay due to the shuttling of reaction intermediate species: Li polysulfides (LiPSs). One of the more important and intriguing PSs is the tri-sulfur radical ($S_3^{•-}$), observed mainly in high-donor number (DN) solvent-based electrolytes. Although this radical has been proposed to be crucial to full active material (AM) utilization, there is currently no direct evidence of the impact of $S_3^{•-}$ on cycling stability. To gain more insight into the role of the $S_3^{•-}$, we studied the use of radical traps in low and high DN solvent-based electrolytes by operando Raman spectroscopy. The traps were based on nitrone and iminium cation, and $S_3^{•-}$ was indeed successfully trapped in ex situ analysis. However, it was the ionic liquid-based trap, specifically pyridinium, that effectively suppressed $S_3^{•-}$ during battery operation. Overall, the PS formation was altered in the presence of the traps and we confirmed the impact of $S_3^{•-}$ formation on the Li–S battery redox reactions and show how the trapping correlates with Li–S battery performance. Therefore, stabilization of the $S_3^{•-}$ might be a path to improved Li–S batteries.