ABSTRACT
The development of graphitic carbon materials as anodes of sodium-ion batteries (SIBs) is greatly restricted by their inherent low specific capacity. Herein, nitrogen and sulfur co-doped 3D graphene frameworks (NSGFs) were successfully synthesized via a simple and facile one-step hydrothermal method and exhibited high Na storage capacity in ether-based electrolytes. A systematic comparison was made between NSGFs, undoped graphene frameworks (GFs) and nitrogen-doped graphene frameworks (NGFs). It is demonstrated that the high specific capacity of NSGFs can be attributed to the free diffusion of Na ions within the graphene layer and reversible reaction between -C-Sx-C- covalent chains and Na ions thanks to the large interplanar distance and the dominant -C-Sx-C- covalent chains in NSGFs. NSGF anodes, therefore, exhibit a high initial coulombic efficiency (ICE) (92.8%) and a remarkable specific capacity of 834.0 mA h g-1 at 0.1 A g-1. Kinetic analysis verified that the synergetic effect of N/S co-doping not only largely enhanced the Na ion diffusion rate but also reduced the electrochemical impedance of NSGFs. Postmortem techniques, such as SEM, ex situ XPS, HTEM and ex situ Raman spectroscopy, all demonstrated the extremely physicochemically stable structure of the 3D graphene matrix and ultrathin inorganic-rich solid electrolyte interphase (SEI) films formed on the surface of NSGFs. Yet it is worth noting that the Na storage performance and mechanism are exclusive to ether-based electrolytes and would be inhibited in their carbonate ester-based counterparts. In addition, the corrosion of copper foils under the synergetic effect of S atoms and ether-based electrolytes was reported for the first time. Interestingly, by-products derived from this corrosion could provide additional Na storage capacity. This work sheds light on the mechanism of improving the electrochemical performance of carbon-based anodes by heteroatom doping in SIBs and provides a new insight for designing high-performance anodes of SIBs.
