The traditional wisdom holds that expanding the interlayer spacing of carbon-based materials to achieve more Na ions insertion via hetero-atom doping is an effective way to increase their sodium storage as the anode of sodium-ion batteries (SIBs). However, the relatively low diffusion control capacity indicates the contribution of capacity derived from insertion/extraction of Na ions and Na ion electrolyte complex is much smaller than expected. In this paper, it is demonstrated that although the interlayer spacing can be expanded by hetero-atom doping, the capacity increase mainly comes from the adsorption of Na ions and reversible reaction between -C-S_x-C- covalent chains and Na ions, rather than the insertion of Na ions or co-insertion of Na ions electrolyte complex. What’s more, the mechanism is exclusive to ether-based electrolytes and would be inhibited in its ester counterparts. Kinetic analysis verified that the synergetic effect of N/S co-doping could not only largely enhance the Na ion diffusion rate but also reduce the electrochemical impedance of nitrogen and sulfur co-doped 3D graphene framework (NSGFs). In addition, postmortem techniques, including SEM, ex-situ XPS, HTEM and ex-situ Raman spectra, all demonstrated an even and ultrathin inorganic-rich SEI film, and the extremely physicochemical stable structure of the 3D graphene matrix. Therefore, NSGFs electrodes exhibit a high specific capacity of 834.0 mAh g^-1 at 0.1 A g^-1 and remarkable rate performance of 261.1 mAh g^-1 at 5A g^-1 in ether electrolyte. Furthermore, the NSGFs electrode presented stable long-term cycling at a high current density of 1 A g^-1. This work sheds light on the mechanism of improving the electrochemical performance of carbon-based anode by heteroatom doping in SIBs and provides a new insight for designing high-performance anodes of SIBs.

Sodium-ion batteries,Graphene,Doped