Recent advances in producing hollow carbon spheres for use in sodium-sulfur and potassium-sulfur batteries

Sodium-sulfur (Na-S) and potassium-sulfur (K-S) batteries for use at room temperature have received widespread attention because of the abundance and low cost of their raw materials and their high energy density. However, their development is restricted by the shuttling of polysulfides, large volume expansion and poor conductivity. To overcome these obstacles, an effective approach is to use carbon-based materials with abundant space for the sulfur that has sulfiphilic sites to immobilize it, and a high electrical conductivity. Hollow carbon spheres (HCSs) with a controllable structure and composition are promising for this purpose. We consider recent progress in optimizing the electrochemical performance of Na-/K-S batteries by using these materials. First, the advantages of HCSs, their synthesis methods, and strategies for preparing HCSs/sulfur composite materials are reviewed. Second, the use of HCSs in Na-/K-S batteries, along with mechanisms underlying the resulting performance improvement, are discussed. Finally, prospects for the further development of HCSs for metal-S batteries are presented.