Theoretical Insights into Single-Atom Catalysts for Improved Cathode Kinetics of Na−S and Na−Se Batteries

Rechargeable sodium–sulfur/selenium (Na–S/Se) batteries are gaining significant interest for large-scale energy storage applications due to their low-cost, high theoretical energy density, and environmentally friendly characteristics. However, the performance of these batteries is hindered due to the shuttle effect. Single-atom catalysts (SACs) play a crucial role in promoting Na₂S/Se redox process at the cathodes and in mitigating the shuttle behavior of sodium poly-sulfides/selenides (Na₂X_n, X = S and Se, n = 1−8). In this work, Co, Fe, Ir, Ni, Pd, Pt, and Rh transition metals supported in nitrogen-deficient graphitic carbon nitride (g-C₃N₄) are investigated as single-atom catalyst to explore the kinetics of sulfur/selenium reduction reactions and the catalytic decomposition of short-chain poly-sulfides/selenides using density functional theory (DFT) calculations. We find that the SACs-adsorbed on reduced g-C₃N₄ monolayers produce more adequate binding energies to trap higher-order Na₂X_n than pristine g-C₃N₄ surfaces. Additionally, the SACs reduce the free energies of the rate-determining step during discharge and present a lower decomposition barrier of Na₂Se during charging for the Na-Se electrode compared to the Na-S electrode. The underlying mechanism behind this fast kinetics is thoroughly examined using charge transfer, bond strength, and d-band center analysis. Our work demonstrates an effective strategy for designing single-atom catalysts SACs and offers solutions to the performance constraints caused by the shuttle effect in sodium-sulfur (Na-S) and sodium-selenium (Na-Se) batteries.