Monitoring Redox Pathways and Performance Limitations in Lithium-Sulfur Batteries Using In Situ 7/6Li and 33S 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.