Confined Sulfur in Microporous Carbon Renders Superior Cycling Stability in Li/S Batteries

The use of sulfur in the next generation Li-ion batteries is currently precluded by its poor cycling stability caused by irreversible Li₂S formation and the dissolution of soluble polysulfides in organic electrolytes that leads to parasitic cell reactions. Here, a new C/S cathode material comprising short-chain sulfur species (predominately S₂) confined in carbonaceous subnanometer and the unique charge mechanism for the subnano-entrapped S₂ cathodes are reported. The first charge-discharge cycle of the C/S cathode in the carbonate electrolyte forms a new type of thiocarbonate-like solid electrolyte interphase (SEI). The SEI coated C/S cathode stably delivers ≈600 mAh g^-1 capacity over 4020 cycles (0.0014% loss cycle^-1) at ≈100% Coulombic efficiency. Extensive X-ray photoelectron spectroscopy analysis of the discharged cathodes shows a new type of S₂ species and a new carbide-like species simultaneously, and both peaks disappear upon charging. These data suggest a new sulfur redox mechanism involving a separated Li⁺/S^2- ion couple that precludes Li₂S compound formation and prevents the dissolution of soluble sulfur anions. This new charge/discharge process leads to remarkable cycling stability and reversibility.