Room-temperature sodium-sulfur (RT Na-S) batteries, as one of effective candidates for next-generation high-energy-density battery systems, have the advantages of high theoretical energy density (i.e., 1274 W·h·kg^-1), high elemental abundance (i.e., S and Na) and low cost. However, the practical application of RT Na-S batteries is restricted due to the poor electronic conductivity of sulfur, sluggish reaction kinetics, and sodium polysulfides (NaPSs) shuttle effect. The related studies are performed on cathode materials of RT Na-S batteries. Among various materials, two-dimensional layered materials have attracted extensive attention due to their unique structures and physicochemical properties, showing a substantial promise for applications. Compared to conventional experimental research methods, first-principles computational techniques can assist in designing novel high-performance electrode materials in atomic and electronic scales. This paper was thus to investigate g-C₃N₄ as a catalyst for RT Na-S batteries based on the first-principles calculation methods. In addition, the chemical interactions between g-C₃N₄ and NaPSs, electronic structure, and reaction energy barriers were also analyzed.

Density functional theory (DFT) calculations were carried out by a software named Vienna ab initio simulation package (VASP). The exchange-correlation energy was described using the Perdew-Burke-Ernzerhof (PBE) functional within the framework of the generalized gradient approximation (GGA). A plane-wave cutoff energy of 500 eV was chosen. A vacuum layer larger than 20Å was used in the calculations to avoid the interlayer interactions. The convergence of energy and force criteria on the atoms were set to be 10^-5 eV and 0.02 eV·Å^-1, respectively. The DFT-D3 method was used to calculate the long-range van der Waals interactions. The 3×3×1 and 4×4×1 Monkhorst-Pack K-points were set in the first Brillouin zone for geometric optimization and calculation of density of states (DOS), respectively. The adsorption energy of NaPSs adsorbed on g-C₃N₄ monolayer was calculated by

The g-C₃N₄ studied belongs to a hexagonal crystal system, where C and N atoms are bonded in sp² hybridized configuration, forming a conjugated π-electron structure. The density of states (DOS) of g-C₃N₄ monolayer shows semiconductor characteristics with a band gap of 1.12 eV. The adsorption energy is a fundamental criterion for assessing whether a material can anchor polysulfides. The adsorption energies of NaPSs and S₈ on the g-C₃N₄ surface were calculated, and all the values range from -1.0 eV to -5.0 eV, indicating that g-C₃N₄ can be ideal candidates for anchoring NaPSs. The analysis of DOS and differential charge density of these adsorption systems shows that electron transfer occurs through Na-S and Na-N bonds and the band gap decreases, compared to the pristine g-C₃N₄ (except S₈ adsorption system), facilitating electron transfer and providing electrons for the redox processes of NaPSs. The Gibbs free energy calculation for the entire discharge process reveals that the energy barrier of the rate-determining step is only 0.70 eV. These results emphasize a pivotal role played by the g-C₃N₄ in accelerating the conversion of NaPSs.

Based on first-principles calculations, we systematically investigated the anchoring and catalytic behavior of g-C₃N₄ toward NaPSs. The results showed that g-C₃N₄ could have a great potential as a sulfur host and catalytic material for RT Na-S batteries. g-C₃N₄ had an anchoring effect on NaPSs, contributing to improved battery cycle life. g-C₃N₄ could effectively capture NaPSs from the electrolytes, suppressing the shuttle effect. And g-C₃N₄ accelerated the conversion kinetics of NaPSs, enhancing sulfur utilization. All these findings could underscore an immense potential of g-C₃N₄ in the design of RT Na-S battery cathodes.