Probing the Lithium–Sulfur Redox Reactions: A Rotating-Ring Disk Electrode Study

Abstract

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 − / S 8 ) within the short reaction time in RRDE experiments (seconds). The complete conversion of sulfur to Li 2 S 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.

Surface-enhanced redox chemistry of polysulphides on a metallic and polar host for lithium-sulphur batteries

“…This can be explained by the fact that where little or no interaction of LiPSs with support is present-as in the case of VC and graphite-the polysulphide species exist in rapid equilibria in solution. Disproportionation of intermediate-length LiPSs such as S 4 2 À to sulphur with lower and higher oxidation states is known to occur under these conditions 38,39 . Shifting of the equilibrium towards elemental sulphur formation is driven both by its insolubility and strong interaction with the carbon surface, thus accounting for the C-S 0 contribution.…”

“…Initial reduction is thought to give S 8 2 À that disproportionates in solution to elemental sulphur and S 6 2 À (refs 31,38,39). The soluble S 6 2 À can either undergo cleavage to form soluble, reducible S 3 À ; or further reduce to S 4 2 À and ultimately precipitate Li 2 S; and/or reduce and disproportionate to form a higher order polysulphide and Li 2 S. These solution-mediated reactions are highly complex 38,39 . Nonetheless, data clearly show the high concentration or supersaturation of LiPS before the end of discharge (Fig.…”

A chemistry and material perspective on lithium redox flow batteries towards high-density electrical energy storage

“…Very recently, Lu and co-workers employed the rotating-ring disk electrode technique and a lithium ion conducting solid electrolyte for elimination of the loss of active material to probe the reaction kinetics and reaction mechanism of lithium-sulphur redox reactions in DMSO and DOL/DME electrolytes. 140 They proposed that the electrochemical steps of sulphur reduction in liquid phase exhibited fast reaction kinetics and only accounted for approximately one-quarter of the total capacity within the short reaction time. The complete conversion of sulphur to Li 2 S could only be accomplished via chemical polysulfide recombination/ dissociation reactions that generated electrochemically reducible polysulfides with long reaction time in a closed cell (Fig.…”

A simple, experiment-based model of the initial self-discharge of lithium-sulphur batteries

“…However, in the pathway of sulphur reduction, lithium polysulphides of variable chain length, Li 2 S n , are formed [13][14][15][16][17][18][19][20][21]:…”