Covalent sulfur confined in mesoporous hollow carbon spheres for effective kinetic regulation of room-temperature sodium-sulfur batteries

Room-temperature sodium-sulfur batteries have attracted extensive attention as a cost-effective and high theoretical energy density next-generation energy storage systems. However, the insulating nature of sulfur, the sluggish reactivity of sulfur with sodium, and severe dissolution of polysulfide are the main challenges of this technology. Here, we propose a covalent sulfur bond breakage mechanism by bonding of sulfur chain onto the inner cavity of the sulfydryl-functionalized mesoporous hollow carbon spheres (MHCS) to efficiently promote the generation of short-chain sodium polysulfide and accelerate polysulfide redox kinetics. The covalent sulfur confined MHCS (S@MHCS) cathode delivers an unprecedented rate capability (250 mAh g-1 at 5 C) and exhibits excellent long-term cycling stability over 1000 cycles at 1 C with a high cathode loading of 5 mg cm-2. Moreover, the associated electrochemical reaction mechanism is also revealed by ex-situ X-ray photoelectron spectroscopy and theoretical calculation. This work introduces a novel sulfur cathode design to enhance the performance of room-temperature sodium-sulfur batteries.