Theoretical investigation of g-C3N6 as an anode material for sodium-ion batteries for emergency energy in subway.

CONTEXT: Sodium-ion batteries (SIBs) are rich in resources and low in cost, exhibiting great potential for emergency energy storage in high-density public facilities such as subway stations. This study, based on density functional theory (DFT), systematically explores the theoretical performance of g-C3N6 and its sulfur-doped derivative (S-g-C3N6) as anode materials for SIBs. Electronic structure analysis revealed that pristine g-C3N6 is a direct-bandgap semiconductor with a bandgap of 2.601 eV, whereas S doping significantly reduced the bandgap to 0.326 eV. The most stable adsorption site for Na atoms on the g-C3N6 surface was found to be above the C atom, with an adsorption energy of -1.732 eV. Diffusion barrier calculations revealed that the minimum migration barriers for Na ions on the g-C3N6 and S-g-C3N6 surfaces are as low as 0.072 and 0.068 eV, respectively. The corresponding theoretical specific capacities reach 1339.2 and 1164.2 mAh/g, respectively. The average OCV was approximately 0.700 V, which is favorable for achieving a high energy density. In summary, g-C3N6-based materials, particularly after S doping modification, combine high specific capacity, excellent conductivity, and ultrafast ion diffusion kinetics, making them highly promising anode candidates intended for emergency energy storage in subway stations. METHODS: Both model optimization and property calculations were performed using the CASTEP module. The exchange-correlation interactions among electrons were treated using the PBE functional under the GGA. For the analysis of band structures and projected density of states, a more accurate HSE06 hybrid functional was further employed for correction to obtain more precise bandgap values. The cutoff energy was set to 600 eV. The maximum interatomic force was set below 0.001 eV/Å, the maximum displacement below 0.001 Å, and the internal stress below 0.001 GPa.