Enhanced reversible capacity of sulfurized polyacrylonitrile cathode for room-temperature Na/S batteries by electrochemical activation

Low costs and high theoretical energy densities make room-temperature Na/S batteries attractive for large-scale applications. However, obtaining sulfur cathodes with high reversible capacities remains challenging. For sulfurized polyacrylonitrile (SPAN) cathode, we found the reaction mechanism between sodium and sulfur, and then developed a method to obtain high reversible capacity by electrochemical activation. During the first discharge (sodiation), one sodium per sulfur atom reacts irreversibly with the conjugated carbon backbone, which reduces the resistance of SPAN. Upon further sodiation, sodium reversibly reacts with free sulfur generated by cleaving C–S and S–S bonds in SPAN to form Na2S. After simple activation, i.e. further sodiation, 1.8 sodium atoms per sulfur atom, the reversible discharge capacity reaches 1502 mAh g−1-sulfur, similar to the theoretical capacity of Na2S, which is the highest value ever reported. After 100 cycles, the capacity remains at 1405 mAh g−1-sulfur; thus, the energy density of SPAN is 543 Wh kg−1, much higher than the theoretical value for lithium-ion batteries. The reported reaction mechanism and activation process provide new strategies for room-temperature Na/S batteries.