Spatial Progression of Polysulfide Reactivity with Lithium Nitrate in Li-Sulfur Batteries

LiNO₃ is a common electrolyte additive in Li-S batteries, but its stabilizing effect is not well-understood due to the complex electrolyte chemistry. This complexity often hampers the clear characterization and interpretation of data. Herein, we explore the LiNO₃ reactivity with polysulfide through in operando sulfur K-edge spectroscopy, using a sulfur-free electrolyte with LiNO₃ as the sole salt. We reveal a spatially progressing chemical reaction influenced by the polysulfide concentration gradient. Polysulfides are electrochemically generated near the sulfur cathode, leading to a high local concentration. As a result, they are incompletely oxidized by LiNO₃ to sulfites, which are gradually further oxidized into sulfonates and sulfates. Conversely, polysulfides near the anode side have a lower local concentration as they are diffused from the cathode side, thus leading to more highly oxidized species like sulfonates and sulfates. These reaction products are stable during electrochemical cycling, suggesting their capabilities to passivate the electrodes and contribute to the cycling stabilities of Li-S batteries.