Lithium-sulfur batteries offer a tremendous increase in specific capacity compared to the current state-of-the-art lithium-ion batteries, with the sulfur cathode having a large theoretical capacity of 1672 mA h g^-1.¹ In practice, however, this large theoretical capacity is rarely achieved, particularly at high loadings of sulfur within the cathode, at low electrolyte amount, or at kinetically limiting high rates and low temperatures.^{2, 3} A primary reason for this is the highly insulating nature of sulfur and its lithium polysulfide and lithium sulfide (Li₂S_x, 1 < x <8) discharge products. During charge and discharge, reduction and oxidation of sulfur species are confined to a primarily surface-based reaction pathway, which limits the total utilizable capacity as insulating discharge products precipitate and drive the voltage down to the cell’s lower voltage limits.⁴

While many efforts have focused on limiting the solubility of intermediate polysulfide species, it has been reported that higher donor number solvents can offer a pathway to greater electrochemical utilization of sulfur through a solution-based reaction pathway, primarily driven by the S₃^.- radical.⁴ Here, we demonstrate that some higher donor number solvents, such as Dimethyl Sulfoxide (DMSO) and N,N-Dimethylacetamide (DMA), exhibit greater utilization and relative disproportionation to the S₃^·- radical, while others, like 1-methylimidazole (MeIm), show little presence of the S₃^·- radical and less first cycle discharge capacity. We investigate this unique phenomenon via ultraviolet-visible spectrophotometry at a high concentration of Li₂S₆ in each solvent. Relative concentration of each polysulfide species varies with overall concentration, and we choose to study relative speciation at high concentrations as this approximates the typical polysulfide concentrations found in electrolyte while cycling.

Favorable disproportionation to the S₃^·- radical can be strategically used as a framework to optimize electrolytes for increased capacity in lithium-sulfur cells through a solution-based reaction pathway. Intelligent use of these radical-inducing solvents in conjunction with the predominantly used Dioxolane (DOL) and Dimethoxyethane (DME) electrolytes can enable greater sulfur utilization in traditionally capacity limiting scenarios, such as low temperature, high rates, and large sulfur loadings.

References:

1. A. Manthiram, S. H. Chung and C. Zu, Adv. Mater., 27, 1980 (2015).
2. H. J. Peng, J. Q. Huang, X. B. Cheng and Q. Zhang, Adv. Energy Mater., 7, 1700260 (2017).
3. Y. V. Mikhaylik and J. R. Akridge, J. Electrochem. Soc., 150, A306 (2003).
4. M. Cuisinier, C. Hart, M. Balasubramanian, A. Garsuch and L. F. Nazar, Adv. Energy Mater., 5, 1401801 (2015).

Figure Caption:

Fig. 1. a) 0.1 mM of Li₂S₆ dissolved in the 4 solvent systems of interest. A clear blue color, indicative of the S₃^·- radical, is predominant in DMA and DMSO. b) First cycle discharge profiles of a sulfur cathode in 4 different electrolytes, where the solvents of interest are used with 50% DOL.