Performance Enhancement and Mechanistic Studies of Room-Temperature Sodium–Sulfur Batteries with a Carbon-Coated Functional Nafion Separator and a Na2S/Activated Carbon Nanofiber Cathode

Operation of sodium–sulfur batteries at room temperature has been proposed and studied for about a decade, but polysulfide-shuttle through the traditional battery separator and low-utilization of the sulfur cathode commonly have been the major challenges. Also, because of the highly active nature of the sodium metal, the conventional room temperature sodium–sulfur (RT Na–S) battery concept with the sodium–metal anode and elemental sulfur cathode imposes serious safety concerns. To overcome the above difficulties, we present here a RT Na–S system with an advanced membrane-electrode-assembly (MEA) comprising a carbon-coated, presodiated Nafion membrane (Na-Nafion) and a sodium sulfide (Na2S) cathode. The Na-Nafion membrane provides a facile Na+-ion conductive path and serves as a cation-selective shield to prevent the migration of the polysulfides to the anode. The carbon coating on the Na-Nafion plays an upper-current-collector role and thereby improves the electrochemical utilization of the active Na2S. Employing Na2S as the cathode provides a pathway to develop the RT Na–S batteries with sodium–metal-free anodes. The RT Na–S battery with the above MEA exhibits remarkably enhanced capacity and cyclability in contrast to the Na–S batteries with the conventional electrolyte–separator configuration. Mechanistic studies reveal that the suppression of polysulfide migration through the Na-Nafion is due to size and electronic effects.