A Promising Carbon/g‐C 3 N 4 Composite Negative Electrode for a Long‐Life Sodium‐Ion Battery

2D graphitic carbon nitride (g‐C 3 N 4 ) 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‐C 3 N 4 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‐C 3 N 4 nanosheet. A facile one‐pot heating of a mixture of low‐cost urea and asphalt is strategically applied to yield stacked multilayer C/g‐C 3 N 4 composites with improved Na‐storage capacity (about 2 times higher than that of g‐C 3 N 4 , up to 254 mAh g −1 ), rate capability, and cyclability. A C/g‐C 3 N 4 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 .