Polysulfide Speciation and Electrolyte Interactions in Lithium–Sulfur Batteries with in Situ Infrared Spectroelectrochemistry

Understanding redox mechanisms as well as interactions between redox species and electrolyte is critical for rational design of electrolyte/cathode systems for Li–S batteries. Here, we demonstrate in situ FT-IR with attenuated total reflection (ATR) to monitor both polysulfide (PS) speciation (Sx2–, 2 ≤ x ≤ 8) and triflate anion (electrolyte) coordination state while simultaneously discharging/charging a full battery coin cell. We report the concentration of various PS species as a function of voltage during cell discharge. In addition, we found that molecular-level changes occurred in the electrolyte salt anion in response to PS speciation. During discharge, PS dissolution increases total solute concentration, inducing anion interactions between low coordination state complexes—ion pairs and free ions—to form aggregate complexes. Under fast cyclic voltammetry sweep, less progressive formation of all PSs, due to diffusion limitations, resulted in a higher concentration of aggregates and PSs even upon completion of discharge. This new application of in situ FT-IR offers direct insight into dynamic interactions between electrolyte salt and polysulfides fundamental in developing Li–S systems.