A Tiron Redox Mediator Enabling Dominant Two-Electron Transfer Energy Storage in MnO2 Cathode Materials for Aqueous Zn Batteries

Rechargeable aqueous Zn-MnO 2 batteries are promising for large-scale energy storage. A two-electron transfer reaction of MnO 2 /Mn 2+ is available for the cathode, delivering a high theoretical capacity of 616 mAh g −1 . However, this process requires protons, and its practical contribution in conventional mildly acidic zinc batteries is limited. Herein, an organic redox mediator of Tiron is demonstrated to realize dominant two-electron transfer energy storage in MnO 2 cathode materials. The reversible hydroquinone/benzoquinone redox couple of Tiron allows facile spontaneous chemical reaction with MnO 2 to generate Mn 2+ as well as electrochemical redox in zinc cells according to combined in situ/ex situ analysis. Theoretical calculations further reveal that Tiron reduces the energy required for Mn release from the MnO 2 lattice. Therefore, Tiron not only boosts the two-electron reaction of MnO 2 but also reactivates any dead materials. Thanks to these effects, the MnO 2 cathode achieves a high capacity of 525 mAh g −1 at 0.3 A g −1 , far exceeding 384 mAh g −1 in the baseline system. The lifespan also extends four times to 12 000 cycles at 5 A g −1 together with stable capacity retention. This work presents an effective strategy to encourage the high-capacity reaction path of MnO 2 cathodes in aqueous zinc batteries.