Single-atom catalysts based on C₂N for sulfur cathodes in Na-S batteries: a first-principles study

Several major roadblocks, including the "shuttle effect" caused by the dissolved higher-order sodium polysulfides (NaPSs), extremely poor conductivity of sulfur cathodes, and sluggish conversion kinetics of charging-discharging reactions, have hindered the commercialization of sodium-sulfur batteries (NaSBs). In our study, representative C₂N-based single-atom catalysts (SACs), TM@C₂N (TM = Fe, Ni and V), are proposed to improve the comprehensive performance of NaSBs. Based on first-principles calculations, we first discuss in detail the anchoring behavior of all adsorption systems, TM@C₂N/(S₈ and NaPSs). The results indicate that compared to pristine C₂N, TM@C₂N substrates exhibit a stronger capability to capture S₈/NaPSs clusters through physical/chemical binding, with V@C₂N showing the most outstanding capability ranging from -2.37 to -5.03 eV. The density of states analysis reveals that metallic properties can be well maintained before and after adsorption of polysulfides. More importantly, TM@C₂N configurations can greatly reduce the energy barriers of charging and discharging reactions, thereby accelerating the conversion efficiency of NaSBs. It is worth mentioning that V@C₂N has lower charge-discharge energy barriers and Na ion migration rates, since the embedded TM atom weakens the strong binding of Na⁺ in the N₆ cavity of C₂N. The intrinsic mechanism analysis reveals that the interaction between the d orbitals of V and the p orbitals of S leads to the weakening of Na-S bonds, which can not only effectively inhibit the shuttle effect, but also promote the dissociation of Na₂S. Overall, this work not only offers excellent catalytic materials, but also provides vital guidance for designing SACs in NaSBs.