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.