Probing the lithium-sulfur redox reactions: A rotating-ring disk electrode study

Detailed mechanistic understanding of sulfur redox reactions is critical for developing efficient and stable lithium-sulfur batteries. Here, we employ the rotating-ring disk electrode technique to probe the reaction kinetics and reaction mechanism of lithium-sulfur redox reactions in dimethyl sulfoxide and 1,3-dioxolane:1,2-dimethoxyethane. We quantitatively determine the number of electrons involved in the reduction reactions and the specific activity of sulfur reduction reactions. We show that the electrochemical steps of sulfur reduction exhibit fast reaction kinetics and only account for approximately one-quarter of the total capacity (i.e., ≈4 e^-/S₈) within the short reaction time in RRDE experiments (seconds). The complete conversion of sulfur to Li₂S can only be accomplished via chemical (i.e., potential-independent) polysulfide recombination/dissociation reactions that generate electrochemically reducible polysulfides with long reaction time (hours) in a closed battery cell. The influence of the solvent's solvation power on the rate capability of the sulfur reduction/oxidation processes and the implications for lithium-sulfur batteries will be discussed.