Bifunctional Single Transition Metal Atom Catalyst-Embedded Carbon Nanoribbon Cathode Hosts for High-Performance Na-S Batteries: A DFT-Driven and ML-Assisted Design

The application of room-temperature sodium-sulfur (RT Na-S) batteries is severely hindered by two major roadblocks, namely, the shuttle effect of sodium polysulfides (NaPSs) and the slow kinetics of the sulfur reduction reaction (SRR). To resolve these issues, in this study, bifunctional single-atom catalysts (SACs) designed by anchoring transition metal (TM) atoms on 4-5-6-8 member-ringed carbon nanoribbons (CNRs) are proposed. The introduction of TM atoms tunes the binding strength of NaPSs to effectively suppress the shuttle effect. Gibbs’s free energy calculations reveal that TM-CNRs substantially improve the kinetics of the SRR by reducing the rate-determining energy barrier. For some of the TM-CNRs, the reduction can be as large as 97%. Based on the binding strength of NaPSs and the rate-determining activation barrier, SACs are categorized into strong, moderate, and weak cathode hosts. With suppressed sulfur poisoning activity, TM-CNRs reduce the decomposition barrier for Na2S by approximately a factor of three. The gradient boosting regression algorithm-based machine learning model reveals that a combined descriptor, β (ionization energy of TM × number of sulfur atoms in the target NaPS), governs the binding strength of NaPSs. This study, along with providing a fundamental understanding of TM-CNRs' catalytic activity, elucidates the suitability of moderate class TM-CNRs for RT Na-S batteries.