Nitrogen and sulfur co-doped mesoporous hollow carbon spheres for high rate sodium ion storage

Sodium-ion batteries (SIBs) exhibit an similar storage mechanism to lithium ion batteries (LIBs). Since lithium is costly and sodium is abundant on earth, SIBs have potential to replace LIBs in many applications, especially large-scale energy storage. Carbonaceous materials have special advantages for sodium ion storage, including low cost, high capacity and high conductiviy. However, the storage properties of carbonaceous materials are expected to be improved to meet the development of SIBs. Structure design and heteroatoms doping are two effective approaches to enhance the capacity and rate performance of carbonaceous materials for sodium storage. In this paper, nitrogen and sulfur co-doped mesoporous hollow carbon spheres (NSHCSs) are synthesized through a facile pyrolysis process, using ionic liquid [EMIm]HSO₄ as the heteroatom source and phenolic resin as the carbon source. Owing to the hollow structure and element co-doping, the NSHCSs possess high surface area, abundant defects and enlarged interlayer spacing. The electrochemical measurements demonstrate the NSHCSs possess excellent sodium storage properties, with reversible capacities of 245.8, 229.6, 197.9, 169.7, 150.5, 123.0, and 86.2 mAh g⁻¹ at 0.1, 0.2, 0.5, 1, 2, 5, and 10 A g⁻¹, respectively. The NSHCSs also display superior capacity retention, with a capacity of 124.7 mAh g⁻¹ after 900 cycles at a high rate of 5 A g⁻¹.