Toward the Understanding of the Reaction Mechanism of Zn/MnO<sub>2</sub> Batteries Using Non-alkaline Aqueous Electrolytes
Sung Joo Kim, Daren Wu, Lisa M. Housel, Lijun Wu, Kenneth J. Takeuchi, Amy C. Marschilok, Esther S. Takeuchi, Yimei Zhu
Abstract
Zn/MnO<sub>2</sub> systems using non-alkaline aqueous electrolytes have attracted tremendous interest as rechargeable aqueous Zn ion batteries due to their safety and high specific capacities. Despite their promising electrochemical performance, however, their reaction mechanism has remained elusive. Here in this paper, we examined the structural evolution of cryptomelane α-MnO<sub>2</sub> cathode by ex situ transmission electron microscopy after electrochemical testing using two different non-alkaline aqueous Zn electrolytes with acetate and triflate salts of different pH values. We have discovered that the systems tested in both electrolytes exhibit a dissolution–deposition reaction mechanism through dissolution–deposition of Mn<sup>2+</sup> ions from/on the cathodes with a display of similar discharge/charge product formation. We have also found that the cell tested using the acetate electrolyte shows evidence of structural irreversibility that might contribute to its rapid capacity degradation. This finding offers an important insight into optimizing the cathode design for enhanced electrochemical function of aqueous Zn/MnO<sub>2</sub> batteries.