Vacancy-Mediated Sulfur Exchange Between Catalysts and Polysulfides Accelerates Sulfur Redox Kinetics in...

Metal sulfides are promising sulfur electrocatalysts for lithium-sulfur (Li-S) batteries, yet their rational design is limited by an incomplete understanding of interfacial sulfur redox processes. Here, we reveal a vacancy-mediated sulfur-exchange mechanism that accelerates sulfur redox kinetics. Using Bi₂S₃ as a model catalyst, ³⁴S isotope-labeling reveals dynamic sulfur exchange between external polysulfides and vacancy-associated Bi-S lattice environments during cycling. Accessible sulfur vacancies act as exchange centers, enabling sulfur incorporation, migration, and local reconstruction. By tuning vacancy concentration, we establish a direct correlation between sulfur-vacancy density, sulfur-exchange extent, and catalytic sulfur conversion. Vacancy-rich Bi₂S₃ nearly doubles the electrochemically accessible interfacial area, with double-layer capacitance increasing from 1.04 to 2.18 mF cm^-2, and raises sulfur exchange from 0.71% to 1.05%. Consequently, Li₂S nucleation and precipitation are accelerated; the apparent Li₂S formation barrier decreases from 0.352 to 0.179 eV, and polarization and impedance growth are suppressed. A 2.5 Ah Li-S pouch cell delivers an energy density of ∼504 Wh kg^-1. These findings identify vacancy-mediated sulfur exchange as a mechanistic basis for designing high-performance metal sulfide catalysts for practical sulfur batteries.