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

The rechargeable aqueous Zn||MnO₂ batteries have been extensively explored, but the electrochemical reaction mechanisms, especially in terms of Mn²⁺/MnO₂ dissolution/deposition and Zn²⁺/H⁺ intercalation chemistry, are still not fully understood. Herein, a Zn||MnO₂-based battery system is constructed and the variation of the battery composition is skillfully regulated by the separation of variables. The possibility of Zn²⁺/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 MnO₂ dissolution dominated by the surplus H⁺ in the electrolyte, while Discharge Plateau II is the smooth discharge plateau resulting from the hydrolysis of Zn²⁺ releasing protons when the proton concentration decreases to the point of Zn(OH)₂ generation. This work provides a better understanding of the dissolution/deposition mechanism of Zn||MnO₂ 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.