Advanced anode materials for sodium ion batteries

Sodium-ion batteries (SIBs) have received extensive attention as promising, cost-effective energy storage devices for the large-scale energy storage due to the abundance and low cost of sodium resources in the earth’s crust (2.83% for Na vs. 0.0065% for Li), and the similar working mechanism with lithium-ion batteries. Nevertheless, the severe volumetric expansion of most reported anodes during the sodiation/desodiation processes affects the cycling stability of SIBs due to the larger radius of sodium ion than that of lithium ion. Moreover, the reported anodes usually show limited specific capacity, which also inhibits the commercial application of SIBs. Therefore, designing high-performance anode materials is urgently needed and remains a major challenge for SIBs. In this thesis, the mesoporous sulfur and nitrogen co-doped carbon (S/N-C) nanofibers are firstly designed as the anode materials for SIBs based on the doping effect on improving the capacity. Then, WS2 nanosheets embedded in the lotus rhizome-like heteroatom-doped carbon nanofibers with abundant hierarchical tubes inside are designed for further improving the capacity. Last, the Fe7S8 modified sulfur, nitrogen-doped carbon (S/N-C) nanofibers are prepared to improve the capacity and cycling stability of SIBs. The mesoporous S/N-C nanofibers doped with amounts of N and S atoms (S/N-C) are fabricated by electrospinning technique. The contents of N and S in the S/N-C are 12.59% and 27.95%, respectively. Owing to the high contents of N and S in the S/N-C, the electrode delivered a high reversible capacity of 552.5 and 355.3 mA h g-1 at 0.1 and 5 A g-1, respectively. The excellent performance is attributed to the introduction of N and S in carbon nanofibers. It increases the active sites for Na+ storage and facilitate Na+ transfer, which is confirmed by in-situ Raman spectra and density functional theory (DFT) calculations. Moreover, the mesoporous S/N-C nanofibers can be wetted by the liquid electrolyte, which also facilitates the Na+ transport for increasing the rate performance. These indicate the as-synthesized S/N-C is a promising anode material for SIBs.