Monitoring Redox Pathways and Performance Limitations in Lithium‐Sulfur Batteries Using In Situ 7/6 Li and ³³ S NMR Spectroscopies

Lithium-sulfur (Li-S) batteries offer high capacity and reduced costs in comparison to the traditional lithium-ion systems. However, the complex series of redox mechanisms that occur in this battery chemistry and accompanying structural transformations are often associated with different routes for cell failure. Therefore, a fundamental understanding of the underlying mechanisms is essential to accelerate the development of these batteries. The combination of operando 6/7Li and ³³S NMR spectroscopy is reported for the first time, providing real-time structural information on the reaction pathways of the sulfur redox processes. The evolution of the polysulfides (poly-S) in the electrolyte and dendrite formation on the anode was monitored with ⁷Li and ⁶Li NMR spectroscopy. Via ³³S NMR experiments, the exact onset of Li₂S formation was determined. By following the evolution of poly-S species and Li₂S, we could track the entire redox pathway and identify performance-limiting mechanisms. The accumulation of soluble poly-S, resulting from an incomplete poly-S to S₈ reduction reaction during charge, was identified as one process leading to capacity fade, while degradation via a poly-S shuttle mechanism was negligible, at least during the first few cycles.