N/O dual coordination of cobalt single atom for fast kinetics sodium-sulfur batteries

Room-temperature sodium-sulfur batteries are promising grid-scale energy storage systems owing to their high energy density and low cost. However, their application is limited by the dissolution of long-chain sodium polysulfides and slow redox kinetics. To address these issues, a cobalt single-atom catalyst with N/O dual coordination was derived from a metal-organic framework precursor (denoted as Co–N₂O₂/MOFc) for sulfur storage. Theoretical analysis demonstrates that, compared with the Co–N₄ structure, the introduction of oxygen atoms can further tune the d-electron density of Co atoms via the coordinative effect, which enhances d-p hybridization after Na₂S_x adsorption on Co–N₂O₂/MOFc. This leads to higher adsorption energy for Na₂S_x, lower Gibbs free energy for the rate-limiting process and a decreased Na₂S decomposition energy barrier, thereby promoting the polysulfide conversion reaction kinetics. When used as a sulfur host, the Co–N₂O₂/MOFc/S cathode exhibits excellent performance with a capacity of 590 mAh·g⁻¹ (983 mAh·g⁻¹ normalized by the sulfur mass) after 100 cycles at 0.1 A·g⁻¹ and an excellent rate capability of 350 mAh·g⁻¹ at 10 A·g⁻¹.