To Make a Painstaking Investigation: Revealing the Electrochemical Reactions in Aqueous Zn─Mn Batteries.

The rechargeable aqueous Zn||MnO batteries have been extensively explored, but the electrochemical reaction mechanisms, especially in terms of Mn/MnO dissolution/deposition and Zn/H intercalation chemistry, are still not fully understood. Herein, a Zn||MnO-based battery system is constructed and the variation of the battery composition is skillfully regulated by the separation of variables. The possibility of Zn/H intercalation chemistry is ruled out and the dominance of the dissolution/deposition mechanism is strongly demonstrated. This study confirms that the chemistry of the controversial double-discharge platform is a dissolution reaction, determined by different proton concentrations and zinc ions hydrolysis. Discharge Plateau I is the MnO dissolution dominated by the surplus H in the electrolyte, while Discharge Plateau II is the smooth discharge plateau resulting from the hydrolysis of Zn releasing protons when the proton concentration decreases to the point of Zn(OH) generation. This work provides a better understanding of the dissolution/deposition mechanism of Zn||MnO and paves the way for the practical application of manganese-based aqueous batteries. It also provides a comprehensive method to study the mechanism of electrochemical reactions.