Stabilization residual oxygen reduces sulfur activity in hard carbon anode for sodium-ion batteries

Sulfur-doped hard carbon materials are considered as the most promising candidate for anodes of sodium-ion batteries, since they can expand carbon interlayer spacing and form a highly active C-S bond in the carbon skeleton. However, the multiple hard carbons contain a large number of oxygen-containing functional groups, which are carried by themselves or be heat-treated in the air. Therefore, in this work, we are using electrospinning technology, then one-step sulfur doping and carbonization to prepare the highly active sulfur-doped carbon nanofibers (S-CNFs). The results indicate that the S-CNFs electrode has larger interlayer spacing, more active sites and C-S bonds, which can stockpile the additional sodium ion in the electrode material. The S-CNFs maintain the comparatively higher reversible capacity of 401.9 mA h g(-1) after 100 cycles at 0.1 A g(-1) and excellent long cyclability of 258.3 mA h g(-1) after 1000 cycles at 5 A g(-1) with Coulombic efficiency closing to 100%. In addition, the results also proved that these oxygen-containing functional groups will combine with the sulfur to form S=O bonds of inactive sulfur, which can reduce the adsorption capacity of sodium ions and impede the charge transfer of the electrode.