Insight into Metal–Sulfur (Li/Na) Redox Reactions through Operando UV–Vis and RRDE Studies

Metal–sulfur (M–S) batteries, including Li–S and Na–S systems, are promising next-generation energy storage technologies due to their high energy density and cost-effectiveness. However, their practical application is limited by complex redox reactions and the dissolution of intermediate polysulfides in ether-based electrolytes, leading to capacity fading and poor cyclability. In this work, we employ a combined approach using operando UV–vis spectroscopy and rotating ring-disk electrode (RRDE) voltammetry to investigate the sulfur redox mechanisms in Li–S and Na–S batteries with 1 M LiTFSI and 1 M NaPF6 in DOL/DME (1:1) electrolytes, respectively. UV–vis spectroscopy has been used to determine the lithium polysulfide intermediates formed during the battery cycling in previously published reports. Our study extends its application to Na–S systems and, for the first time, integrates it with RRDE to directly detect and quantify soluble intermediates by calculating the number of electrons involved in the reduction processes in both systems (i.e., Li–S and Na–S).