Revealing the Dominance of the Dissolution-Deposition Mechanism in Aqueous Zn-MnO 2 Batteries.

Zn-MnO 2 batteries have attracted extensive attention for grid-scale energy storage applications, however, the energy storage chemistry of MnO 2 in mild acidic aqueous electrolytes remains elusive and controversial. Using α-MnO 2 as a case study, we developed a methodology by coupling conventional coin batteries with customized beaker batteries to pinpoint the operating mechanism of Zn-MnO 2 batteries. This approach visually simulates the operating state of batteries in different scenarios and allows for a comprehensive study of the operating mechanism of aqueous Zn-MnO 2 batteries under mild acidic conditions. It is validated that the electrochemical performance can be modulated by controlling the addition of Mn 2+ to the electrolyte. The method is utilized to systematically eliminate the possibility of Zn 2+ and/or H + intercalation/de-intercalation reactions, thereby confirming the dominance of the MnO 2 /Mn 2+ dissolution-deposition mechanism. By combining a series of phase and spectroscopic characterizations, the compositional, morphological and structural evolution of electrodes and electrolytes during battery cycling is probed, elucidating the intrinsic battery chemistry of MnO 2 in mild acid electrolytes. Such a methodology developed can be extended to other energy storage systems, providing a universal approach to accurately identify the reaction mechanism of aqueous aluminum-ion batteries as well.