Direct Proof of the Reversible Dissolution/Deposition of Mn 2+ /Mn 4+ for Mild‐Acid Zn‐MnO 2 Batteries with Porous Carbon Interlayers

Mild‐acid Zn‐MnO 2 batteries have been considered a promising alternative to Li‐ion batteries for large scale energy storage systems because of their high safety. There have been remarkable improvements in the electrochemical performance of Zn‐MnO 2 batteries, although the reaction mechanism of the MnO 2 cathode is not fully understood and still remains controversial. Herein, the reversible dissolution/deposition (Mn 2+ /Mn 4+ ) mechanism of the MnO 2 cathode through a 2e − reaction is directly evidenced using solution‐based analyses, including electron spin resonance spectroscopy and the designed electrochemical experiments. Solid MnO 2 (Mn 4+ ) is reduced into Mn 2+ (aq) dissolved in the electrolyte during discharge. Mn 2+ ions are then deposited on the cathode surface in the form of the mixture of the poorly crystalline Zn‐containing MnO 2 compounds through two‐step reactions during charge. Moreover, the failure mechanism of mild‐acid Zn‐MnO 2 batteries is elucidated in terms of the loss of electrochemically active Mn 2+ . In this regard, a porous carbon interlayer is introduced to entrap the dissolved Mn 2+ ions. The carbon interlayer suppresses the loss of Mn 2+ during cycling, resulting in the excellent electrochemical performance of pouch‐type Zn‐MnO 2 cells, such as negligible capacity fading over 100 cycles. These findings provide fundamental insights into strategies to improve the electrochemical performance of aqueous Zn‐MnO 2 batteries.