Single-Atom Catalysts for Improved Cathode Performance in Na–S Batteries: A Density Functional Theory (DFT) Study

The room-temperature sodium–sulfur (Na–S) batteries have attracted remarkable attention because of their promise to deliver high-capacity and low-cost earth-abundant sodium and sulfur. However, the practical development of Na–S batteries is hindered due to multiple challenges, including rapid capacity fading stems from the dissolution of intermediate sodium polysulfides (Na2Sn) and slow kinetics of electrochemical conversion reactions. In this study, we introduced novel transition-metal single-atom catalysts (SACs) on nitrogen-doped graphene (NG) to impede the dissolution of higher-order Na2Sn and improve otherwise sluggish kinetics of short-chain polysulfides. The density functional theory (DFT) calculations are used to elucidate the detailed interactions of the polysulfides on the SACs. The pristine and nitrogen-doped graphenes are also considered and found to provide ineffective anchoring for trapping polysulfides. However, the SACs embedded via monodispersed transition-metal atoms in NG (TM-NG where TM = Cr, Fe, and Co) exhibit adequate binding strength toward Na2Sn species. The calculated adsorption energies of soluble Na2Sn on SACs are superior compared to that of commonly used ether-type electrolyte solvents; thus, the SACs are predicted to serve as effective immobilizers for soluble Na2Sn to prevent shuttling. The enhanced binding strength for TM-doped substrates arises from the strong TM–S covalent interactions. The density of state (DOS) calculations reveal that both the pristine and polysulfide adsorbed TM-NG exhibits metallic behavior and illustrate the mechanisms of stronger polysulfide interactions originated from the hybridization of TM-3d and S-2p orbitals. Furthermore, the electron-deficient SACs are found to substantially reduce the Na2S decomposition barrier, which demonstrates effective electrocatalysis in favor of complete reversible conversion of polysulfides. Overall, the effectiveness of the SACs on preventing shuttle effect and improving the kinetics of electrocatalytic conversion of polysulfides unraveled in this study will lead to a paradigm shift in developing advanced Na–S batteries with highly efficient electrocatalysts for Na2Sn conversion.