First-principles study on the anchoring mechanisms of polysulfides by cyano/nitrogen vacancy modified g-C3N4

Carbon nitride (gCN), as a potential sulfur host for lithium‑sulfur (Li-S) battery cathodes, suffers from poor electrical conductivity, which can be effectively improved through modification. This study employs first-principles calculations based on density functional theory (DFT) to systematically investigate the adsorption behaviors of elemental sulfur (S 8 ) molecules and polysulfides (Li 2 S ₙ , n = 1, 2, 4, 6, 8) on gCN, cyan-modified gCN (gCN-Cyan), nitrogen vacancy-modified gCN (gCN-NV), and dual-modified gCN (gCN-DD), along with their lithium-ion diffusion properties, thermodynamic stability, and other related characteristics. The results show that the band gap of gCN-NV decreases to 1.109 eV, the electrical conductivity improves, and the band gap of gCN-Cyan increases. gCN-NV has the strongest adsorption performance (−3.83 ∼ −1.8 eV) for Li 2 S n and S 8 molecules. All four materials demonstrate high thermodynamic stability under both room and high-temperature conditions. Notably, the gCN-NV structure remains thermodynamically stable even after Li 2 S 4 adsorption (gCN-NV@Li 2 S 4 ). The formation of Li-N bonds in the structure of gCN-NV@Li 2 S 4 enhances the anchoring ability to soluble polysulfides and inhibits their shuttle effect. The diffusion energy barrier of Li + on gCN-NV is the lowest at 0.61 eV, which is lower than that of the other three materials. The Li 2 S 2 → Li 2 S step is identified as the rate-limiting step in the reaction process due to its maximum Gibbs free energy difference (Δ G ). The Δ G for each step are moderate (−8.16 → 0.42 → 0.01 → −0.04 → 1.41 eV) on gCN-NV, and a notable reduction in Δ G is observed for the Li 2 S 6 → Li 2 S 4 → Li 2 S 2 transition steps. Meanwhile, the gCN-NV surface demonstrates the lowest decomposition energy barrier for Li 2 S (2.00 eV) and the lowest migration energy barrier for Li 2 S 2 (1.49 eV). gCN-NV accelerates the catalytic conversion of Li 2 S n via a synergistic mechanism of strong adsorption and rapid conversion, providing theoretical support for the design of high-performance sulfur host materials for Li-S batteries. • Nitrogen vacancy modification in gCN improves its conductivity and enables strong adsorption of sulfur species. • gCN-NV exhibits the lowest lithium-ion diffusion barrier, the lowest decomposition energy barrier for Li 2 S. • gCN-NV combines strong polysulfide anchoring and rapid catalytic conversion for lithium sulfur batteries.