Recent progress on the design of hollow carbon spheres to host sulfur in room-temperature sodium-sulfur batteries

Room-temperature sodium-sulfur (RT-Na/S) batteries are an important class of rechargeable batteries with a high theoretical capacity of 1675 mAh g(-1) and energy density up to 1276 Wh kg(-1). Together with the abundant and cheap sodium and sulfur, RT-Na/S batteries are recognized as one of the attractive next-generation high-energy-density devices for large-scale energy storage. However, similar to the working principle in lithium-sulfur batteries, RT-Na/S batteries suffer from some essential problems such as the poor conductivity of elemental sulfur, the large volume change during charge-discharge cycling and the serious shuttle effect caused by sodium polysulfides dissolution. Moreover, the shuttle effect and volume variation seem more pronounced in RT-Na/S batteries, which further deteriorates their performances and seriously hinders the progress towards practical application. Recent years have witnessed a fast growth of designing porous carbons especially hollow carbon spheres (HCSs) as sulfur host to address these problems, due to their unique structural features such as special shape, large void space, permeable shell and facile functionalization. This review summarizes the recent progress of HCSs-based materials as the sulfur hosts in RT-Na/S batteries. Beginning with a brief introduction to RT-Na/S batteries, carbon hosts and design strategies for preparing HCSs, emphasis is then placed on manipulating the pore structure, heteroatom doping and metal species decoration with the aim to alleviate the "shuttle effect" and thus to improve the performance. Finally, perspectives on current challenge and future directions are also discussed.