Asymmetric Coordination Induced d-Band Center Upshift of Ti Single Atoms for Accelerated Sulfur Conversion in Aqueous...

In aqueous Zn-S batteries (AZSBs), sulfur cathodes based on conversion reactions offer ultrahigh theoretical capacity; however, sulfur redox chemistry is fundamentally constrained by sluggish solid-solid reaction kinetics. Here, a single-atom catalyst, characterized by a designed asymmetric Ti-N₃S coordination configuration embedded in a conductive carbon matrix, was precisely exploited and served as a catalyst enhancing the electrochemical activity of sulfur redox kinetics. Partial substitution of N ligands by S disrupts the symmetric Ti-N₄ coordination, leading to increased electron density in the Ti 3d orbitals. This electronic redistribution upshifts the d-band center of Ti, which elevates the energy of antibonding orbitals and reduces their occupation states. Consequently, Ti-S interactions are strengthened, and the adsorption capacity toward sulfur species is enhanced in AZSBs. Meanwhile, the interfacial charge transfer is accelerated, leading to a lower energy barrier for the reversible S/ZnS conversion in AZSBs. As a result, the S@Ti-N₃S/C cathode delivers a high initial capacity of 1375 mAh g^-1 at 0.2 A g^-1 and retains a reversible capacity of 570 mAh g^-1 after 300 cycles at 5 A g^-1 in AZSBs. This work highlights d-band center engineering via asymmetric coordination as an effective strategy for regulating sulfur conversion kinetics in AZSBs.