A Promising Carbon/g-C3N4 Composite Negative Electrode for a Long-Life Sodium-Ion Battery

2D graphitic carbon nitride (g-C3N4) nanosheets are a promising negative electrode candidate for sodium-ion batteries (NIBs) owing to its easy scalability, low cost, chemical stability, and potentially high rate capability. However, intrinsic g-C3N4 exhibits poor electronic conductivity, low reversible Na-storage capacity, and insufficient cyclability. DFT calculations suggest that this could be due to a large Na+ ion diffusion barrier in the innate g-C3N4 nanosheet. A facile one-pot heating of a mixture of low-cost urea and asphalt is strategically applied to yield stacked multilayer C/g-C3N4 composites with improved Na-storage capacity (about 2 times higher than that of g-C3N4, up to 254 mAh g(-1)), rate capability, and cyclability. A C/g-C3N4 sodium-ion full cell (in which sodium rhodizonate dibasic is used as the positive electrode) demonstrates high Coulombic efficiency (ca. 99.8 %) and a negligible capacity fading over 14 000 cycles at 1 A g(-1).