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.