Advances in sulfur-doped carbon materials for use as anodes in sodium-ion batteries

Sodium-ion batteries (SIBs) are regarded as one of the most promising candidates for the post-lithium-ion battery (LIB) era due to the abundance and low cost of sodium and their similar operating principles to LIBs. Because of their low sodium intercalation potential, high capacity, and good stability, carbon anode materials appear to be the key to practical applications. Heteroatom doping (e.g., sulfur, nitrogen, phosphorus, oxygen, boron doping) has proved to be an effective way of changing the physical and electrochemical properties of carbon materials to improve their energy storage. Among these, sulfur doping has been widely studied. The S atom has a large covalent radius to expand the interlayer spacing of carbons and thus increase the number of active sites for sodium storage. This review summarizes research progress in the design, synthesis, and electrochemical properties of sulfur-doped carbon anodes for SIBs, including the sodium storage mechanism, preparation strategies, and the way sulfur doping changes the structure of carbon materials, with the aim of improving its specific capacity, rate capability and cycle life in SIBs. Key problems of sulfur-doped carbon anodes are presented and possible solutions are considered.