A Four‐Electron Sulfur Electrode Hosting a Cu2+/Cu+ Redox Charge Carrier

The elemental sulfur electrode with Cu²⁺ as the charge carrier gives a four-electron sulfur electrode reaction through the sequential conversion of S↔CuS↔Cu₂ S. The Cu-S redox-ion electrode delivers a high specific capacity of 3044 mAh g^-1 based on the sulfur mass or 609 mAh g^-1 based on the mass of Cu₂ S, the completely discharged product, and displays an unprecedently high potential of sulfur/metal sulfide reduction at 0.5 V vs. SHE. The Cu-S electrode also exhibits an extremely low extent of polarization of 0.05 V and an outstanding cycle number of 1200 cycles retaining 72 % of the initial capacity at 12.5 A g^-1 . The remarkable utility of this Cu-S cathode is further demonstrated in a hybrid cell that employs an Zn metal anode and an anion-exchange membrane as the separator, which yields an average cell discharge voltage of 1.15 V, the half-cell specific energy of 547 Wh kg^-1 based on the mass of the Cu₂ S/carbon composite cathode, and stable cycling over 110 cycles.