Direct Proof of the Reversible Dissolution/Deposition of Mn2+/Mn4+ for Mild‐Acid Zn‐MnO2 Batteries with Porous Carbon Interlayers

Abstract

Abstract Mild‐acid Zn‐MnO2 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‐MnO2 batteries, although the reaction mechanism of the MnO2 cathode is not fully understood and still remains controversial. Herein, the reversible dissolution/deposition (Mn2+/Mn4+) mechanism of the MnO2 cathode through a 2e− reaction is directly evidenced using solution‐based analyses, including electron spin resonance spectroscopy and the designed electrochemical experiments. Solid MnO2 (Mn4+) is reduced into Mn2+ (aq) dissolved in the electrolyte during discharge. Mn2+ ions are then deposited on the cathode surface in the form of the mixture of the poorly crystalline Zn‐containing MnO2 compounds through two‐step reactions during charge. Moreover, the failure mechanism of mild‐acid Zn‐MnO2 batteries is elucidated in terms of the loss of electrochemically active Mn2+. In this regard, a porous carbon interlayer is introduced to entrap the dissolved Mn2+ ions. The carbon interlayer suppresses the loss of Mn2+ during cycling, resulting in the excellent electrochemical performance of pouch‐type Zn‐MnO2 cells, such as negligible capacity fading over 100 cycles. These findings provide fundamental insights into strategies to improve the electrochemical performance of aqueous Zn‐MnO2 batteries.

Keywords

• aqueous batteries
• mild acid electrolytes
• porous carbon interlayers
• reaction mechanisms
• Zn‐MnO2 batteries