Asymmetric Zn–N₄–S configuration via second-shell sulfur doping for ultralow-polarization aqueous zinc–sulfur batteries

The development of aqueous zinc–sulfur batteries (AZSBs) is primarily plagued by the sluggish kinetics and poor reversibility of the solid–solid sulfur conversion reaction. Herein, we report a Zn single-atom catalyst embedded within a N, S-doped porous carbon matrix (Zn SAs/NSC) as an efficient sulfur host to tackle these challenges. A one-step pyrolysis of zeolitic imidazolate frameworks (ZIFs) with potassium thiosulfate (K₂S₂O₃) was employed to produce hierarchical pore structure with large specific surface area of 2135.5 m²·g⁻¹ and incorporation of sulfur atoms in the second coordination shell of Zn–N₄ sites, resulting in an asymmetric Zn–N₄–S configuration. When employed as a sulfur host, the Zn SAs/NSC-800-based cathode delivers a notable discharge capacity of 1610.6 mAh·g⁻¹ at 0.1 A·g⁻¹ with an ultralow polarization voltage of 0.276 V. It also exhibits good rate capability (1229.1 mAh·g⁻¹ at 5 A·g⁻¹) and long-term cycling stability (60% capacity retention after 1000 cycles). Density functional theory (DFT) calculations reveal that the Zn–N₄–S sites can enhance the adsorption energy of ZnS (−2.08 eV) and facilitate electron transfer, thereby reducing the energy barrier for the solid–solid conversion. This work demonstrates the significant roles of metal single-atom coordination microenvironment in enhancing the electrochemical performance of AZSBs.