Probing the Polysulfide Confinement in Two Different Sulfur Hosts for a Mg|S Battery Employing Operando Raman and Ex-Situ UV-Visible Spectroscopy.

We study here the Mg-polysulfide confinement inside two structurally different model porous materials, viz., toray carbon paper (TC) and multiwalled carbon nanotubes (CNT), using operando Raman and postcycling ex-situ UV-vis spectroscopy. Sulfur encapsulated inside CNT (CNT-S) and TC (TC-S) serves as S-cathodes in a rechargeable room temperature Mg|S battery. Operando Raman spectroscopy indicates the presence of higher-order Mg-polysulfides at the CNT cathode. This is due to the combination of their entrapment inside CNT and also possibly to their localization in the liquid electrolyte in the vicinity of CNT-S. This finding is directly correlated to the ex-situ UV-vis spectroscopy, which shows a lesser degree of Mg-polysulfide dissolution into the electrolyte solution. In comparison, TC-S, where sulfur is encapsulated within the open matrix formed by the nanofiber network of the carbon paper, displays poorer polysulfide confinement. The distinct differences in their abilities to confine the Mg-polysulfides are corroborated by battery performance. In the current density range (0.05-1) C, the battery with CNT-S displays much higher specific capacities, being nearly two times that of TC-S at 1 C.