A Tiron Redox Mediator Enabling Dominant Two‐Electron Transfer Energy Storage in MnO 2 Cathode Materials for Aqueous Zn Batteries

Abstract 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.