Understanding the improved performance of sulfur-doped interconnected carbon microspheres for Na-ion storage

As one of the low-cost energy storage systems, Na-ion batteries (NIBs) have received tremendous attention. However, the performance of current anode materials still cannot meet the requirements of NIBs. In our work, we obtain sulfur-doped interconnected carbon microspheres (S-CSs) via a simple hydrothermal method and subsequent sulfurizing treatment. Our S-CSs exhibit an ultrahigh reversible capacity of 520 mAh g^–1 at 100 mA g^–1 after 50 cycles and an excellent rate capability of 257 mAh g^–1, even at a high current density of 2 A g^–1. The density functional theory calculations demonstrate that sulfur doping in carbon favors the adsorption of Na atom during the sodiation process, which is accountable for the performance enhancement. Furthermore, we also utilize operando Raman spectroscopy to analyze the electrochemical reaction of our S-CSs, which further highlights the sulfur doping in improving Na-ion storage performance.