A Bi-Directional Redox Mediator for High-Capacity Rechargeable Aqueous Zn─S Batteries

Aqueous Zn─S batteries are promising energy storage systems with high theoretical capacity and low cost. However, the direct conversion between S and ZnS at the cathode suffers extremely sluggish reaction kinetics, which further decay rapidly upon the increase of current density and active material loading. Facing these challenges, we herein present a bi-directional redox mediator strategy and identify a self-adjusted Cu-based candidate. It possesses two redox couples whose potentials are on either side of the thermodynamic redox potential of S/ZnS reaction while between its kinetic oxidation and reduction potentials. These alignments enable the bi-directional mediator to catalyze the charge and discharge processes using either redox couple, respectively. As a result, the sulfur cathode reaches a high capacity of 1629 mAh g −1 at 0.1 A g −1 . It also maintains 880 mAh g −1 after 500 cycles at 1 A g −1 , superior to rapid decay to 148 mAh g −1 after only 28 cycles with the mediator free cathode. Furthermore, with a high sulfur loading of 7.3 mg cm −2 and lean electrolyte of 7 µL mg S −1 , the Zn─S battery achieves a high areal capacity over 10 mAh cm −2 .