Time-Resolved In Situ Spectroelectrochemical Study on Reduction of Sulfur in N,N[sup ʹ]-Dimethylformamide

Abstract

Real-time in situ spectroelectrochemical studies have been carried out in N,NЈ-dimethyl formamide containing lithium trifluoromethane sulfonate as an electrolyte and the results are reported. The results indicate that the primary reduction product of the cyclic form of sulfur, S 8c 2Ϫ , undergoes an equilibrium reaction to its linear chain counterpart, S 8l 2Ϫ , which then dissociates into various products. These two dianions and S 3 Ϫ• were produced along with a minor product, S 4 2Ϫ , at the potential corresponding to the first electron transfer. These products were further reduced or dissociated to species including S 7 2Ϫ , S 6 2Ϫ , S 5 2Ϫ , S 4 2Ϫ , S 3 2Ϫ , S 2 2Ϫ , and S 2Ϫ at the second electron-transfer step as evidenced by the spectral shifts observed during electrolysis. The reduction reactions are generally chemically reversible, making it possible to use sulfur reduction as a cathode reaction for Li/S batteries.

Characterization of Polysulfide Radicals Present in an Ether‐Based Electrolyte of a Lithium–Sulfur Battery During Initial Discharge Using In Situ X‐Ray Absorption Spectroscopy Experiments and First‐Principles Calculations

“…a small amount of LiS 5 is still detected). Similarly, reaction ( 8) Figure 5. Comparison of the best fi t spectra from theory (dashed lines) and XAS measurements (solid lines) for each of the three voltages in Figure 1.…”

“…[ 8,10,12,17 ] Step II: Discharging the battery to 2.02 V, radical anions produced at the cathode in step I are no longer present at this step. This could, in principle, result from either electrochemical consumption or subsequent chemical reactions in the electrolyte.…”

Fabrication and electrochemical behavior of a lithium-sulfur cell with a TiO2-sulfur-carbon aerogel-based cathode

“…The reduction peaks at 2.3 V could be assigned to the transformation of elemental sulfur into higher-order lithium polysulfides, while the peaks at 2.0 V were attributable to the reduction of the higherorder lithium polysulfides to lower-order lithium polysulfides and lithium sulfide [25]. During the oxidation process, only the peaks at 2.5 V were observed; these corresponded to the transformation of LiS 2 /Li 2 S 2 into polysulfides [26].…”

Lithium/Sulfur Secondary Batteries: A Review

“…The polysulfides are either linear or cyclic in structure depending on the value of 'n' in S n x-. Thus, the chain form is stable for n <5, the cyclic form is stable for n = 8 or 9, and either the cyclic or chain form for n = 6 or 7 [7]. The formation of various polysulfides in different non-aqueous solvents as well as their characteristics have been investigated by researchers using chemical, electrochemical and spectroscopic methods, and different sulfur-reduction mechanisms have been proposed; however, some of them still remain controversial [8].…”