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.