Sulfur-doped carbon nanofibers as stable and high performance anode materials for sodium-ion batteries

Carbon nanofibers are deemed ideal sodium-ion battery (SIB) anode materials due to their unique structure, but their low capacity has limited further development. Heteroatom modification is one of the common methods to improve the electrochemical properties of carbon materials. Therefore, we prepared sulfur-doped carbon nanofiber (CNF-nS) composites by heat-treating sublimed sulfur and carbon nanofiber precursors under an argon atmosphere. When it serves as the anode of a sodium ion half-cell, the reversible discharge specific capacity of CNF-2S is 368.2 mA h g⁻¹, which is better than the 203.4 mA h g⁻¹ of the carbon nanofiber (CNF). The CNF-2S anode also has good high-rate cycling performance, and its discharge specific capacity is 122 mA h g⁻¹ at 10 A g⁻¹ after 1000 cycles. Furthermore, the CNF-2S anode and the Na₃V₂(PO₄)₃ (NVP) cathode were paired and assembled into a full cell, which displayed a satisfactory capacity of 182.6 mA h g⁻¹ after 200 cycles at 0.1 A g⁻¹. In short, the CNF-2S anode exhibits good sodium storage performance, because sulfur doping increases the layer spacing of the CNF, promotes the transfer of sodium ions, and enhances the electronic conductivity.