Polysulfide Radicals Appearance in Partially Discharged Lithium-Sulfur Battery Analyzed By First-Principles Interpretations of X-Ray Absorption Spectra

The presence and role of polysulfide radicals in the electrochemical processes of lithium sulfur batteries (Li-S) is currently being debated. In particular, the radical trisulfur anion, S₃^.-, has concurrently been purported as a key species or has been discounted as existing during battery operation.^1,2 While radical species could be formed via a direct electrochemical pathway, the prevailing assumption is formation via the dissociation of Li₂S₆ to LiS₃.³ It is unclear how critical the formation of polysulfide radical anions is to the remaining redox pathways.⁴ It is not known if the radicals are an essential intermediate for complete reduction of sulfur (or complete oxidation of Li₂S). In addition, the concentration of radical anions relative to polysulfide dianion species (Li₂S_x) formed during redox reactions has not yet been quantified.

Recently, X-ray absorption spectroscopy (XAS) has been utilized to probe Li-S reaction mechanisms.^2,4,5 If properly interpreted, sulfur K-edge XAS can provide powerful insights into the molecular species formed during the charge and discharge reactions of the Li-S battery.  The challenge of interpreting spectroscopic data of Li-S chemistry has been the lack of spectral standards for isolated polysulfide species. Due to possible spontaneous polysulfide disproportionation and the establishment of equilibrium mixtures in solution, establishing standards for specific molecular species is necessarily complicated, if not impossible. As a result, fingerprinting is frequently performed using solid analogs, which may induce errors in the analysis since the correspondence between the solid and solution phase XAS is by no means guaranteed. A recent advance towards establishing standards, and thus atomistic interpretations of XAS experimental measurements, has been the use of electronic structure methods based on density functional theory (DFT). In a previous work we demonstrated that the XAS of dissolved lithium polysulfide dianion species and various polysulfide radicals can be obtained via first-principles molecular dynamics and DFT spectral simulations.^7,8

In this work, XAS was performed on Li-S cells containing an ether-based electrolyte.  Cells were discharged to various depths and probed using XAS at the sulfur K-edge.  First principles calculations were performed to obtain XAS of single polysulfide molecules.  These theoretical spectra were then used to determine the distribution of polysulfide species present in the battery electrolyte.⁶ This analysis revealed the presence of polysulfide radical anions in significant concentrations after early stages of discharge. 

Whereas previous studies probed the sulfur cathode in an attempt to directly examine the charge and discharge reaction processes, this present study explores a different question, also pertinent to Li-S cells: if cell discharge were stopped at specific points in the discharge process, what intermediate species would be present in the battery electrolyte after ample time has been given for polysulfide dissolution to occur? This question is of some importance, as real-world consumer batteries will be stopped and started as the user chooses so that it would be useful to know what species can be expected in the battery electrolyte if dissolution were to occur. Our results provide further insight into the discharge and disproportionation reactions that take place in the electrolyte during cycling.

References

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