Mapping Polysulfides in Sodium-Sulfur Batteries

Sodium-sulfur (Na-S) batteries provide lithium-free alternatives to lithium-sulfur (Li-S) batteries. Na-S chemistry has been less studied. Thus, the types of polysulfides (PS) and their evolution during charge-discharge of Na-S batteries are not as well understood as those in the Li-S system. We, therefore, study the formation of different PS in tetraethylene glycol dimethyl ether-based electrolyte during battery operation using in situ Raman and ex situ ultraviolet-visible (UV-vis) spectroscopies. We start by making reference solutions with different ratios of sodium sulfide (Na₂S) to sulfur, ranging from pure Na₂S to Na₂S:7S, with the sulfur ratio increasing by one integer per solution. We then correlate the UV-vis and Raman peaks to PS species. Our galvanostatic charge-discharge (GCD) and cyclic voltammetry measurements show a total of ten features. Using ex situ UV-vis on aliquots and in situ Raman spectra from PS solutions at GCD voltage plateaus, we map out sodium polysulfide (NaPS) species at key stages of the charge-discharge cycle. We identify Na₂S₈, Na₂S₄, and Na₂S₂ as intermediates and Na₂S as the final product. We find that intermediate Na₂S₆ forms from disproportionation of Na₂S₈ and Na₂S₄. We also observe that intermediate PS can also dissociate into S₃^•- radical species, which contributes to loss of active material. Our results provide detailed insights into Na-S chemistry that will be helpful for the development of high performance and stable batteries.