Architecting Freestanding Sulfur Cathodes for Superior Room-Temperature Na-S Batteries

Room-temperature sodium-sulfur (RT Na-S) batteries have attracted extensive attention because of their low cost and high specific energy. RT Na-S batteries, however, usually suffer from sluggish reaction kinetics, low reversible capacity, and short lifespans. Herein, it is shown that chain-mail catalysts, consisting of porous nitrogen doped carbon nanofibers (PCNFs) encapsulating Co nanoparticles (Co@PCNFs), can activate sulfur via electron engineering. The chain-mail catalysts Co@PCNFs with a micrograde hierarchical structure as a freestanding sulfur cathode (Co@PCNFs/S) can provide space for high mass loading of sulfur and polysulfides. The electrons can rapidly transfer from chain-mail catalysts to sulfur and polysulfides during discharge-charge processes, therefore boosting its conversion kinetics. As a result, this freestanding Co@PCNFs/S cathode achieves a high sulfur loading of 2.1 +/- 0.2 mg cm(-2), delivering a high reversible capacity of 398 mA h g(-1) at 0.5 C (1 C = 1675 mA g(-1)) over 600 cycles and superior rate capability of an average capacity of 240 mA h g(-1) at 5 C. Experimental results, combined with density functional theory calculations, demonstrate that the Co@PCNFs/S can efficiently improve the conversion kinetics between the polysulfides and Na2S via transferring electrons from Co to them, thereby realizing efficient sulfur redox reactions.