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

Abstract

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.