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.