Vacancy‐Mediated Sulfur Exchange Between Catalysts and Polysulfides Accelerates Sulfur Redox Kinetics in Lithium‐Sulfur Batteries

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 Bi 2 S 3 as a model catalyst, 34 S 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 Bi 2 S 3 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, Li 2 S nucleation and precipitation are accelerated; the apparent Li 2 S 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.