A Dual Redox Mediator Concept to Catalyze Both Reaction Directions of the Two-Electron Transfer Path of MnO2 Cathodes for Aqueous Zn Batteries.

The dissolution/deposition energy storage path of MnO2 cathodes involves two-electron transfer and provides doubled theoretical capacity of the typical one-electron transfer reaction. However, its reaction activity and contribution in aqueous Zn batteries are seriously limited. Herein, we present a dual redox mediator (RM) concept to catalyze both of the reduction/oxidation reaction directions of this two-electron transfer path. Uniquely, the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) compound features two pairs of redox couples with potentials below and above that of the MnO2 cathode. TEMPO also exhibits facile conversion kinetics during both electrochemical processes and chemical reactions with Mn-active materials via two-electron transfer. These properties allow TEMPO to catalyze the dissolution/deposition between MnO2/Mn2+ using either redox couple, respectively. The substituent group on the TEMPO molecule further determines its functioning stabilities. As a result, the optimal TEMPO dual RM enables the MnO2 cathode to achieve a high capacity of 517 mAh g-1 at 0.3 A g-1 and 80.4% capacity retention after 20 000 cycles at 10 A g-1 in typical Swagelok-type Zn batteries. The electrolytic-type battery also realizes a high areal capacity up to 80 mAh cm-2 with 105 mWh cm-2 energy density. This work provides an effective dual RM concept toward high-performance Zn-Mn batteries.