Revisiting the Charging Mechanism of α-MnO 2 in Mildly Acidic Aqueous Zinc Electrolytes

In recent years, there have been extensive debates regarding the charging mechanism of MnO cathodes in aqueous Zn electrolytes. The discussion centered on several key aspects including the identity of the charge carriers contributing to the overall capacity, the nature of the electrochemical process, and the role of the zinc hydroxy films that are reversibly formed during the charging/discharging. Intense studies are also devoted to understanding the effect of the Mn additive on the performance of the cathodes. Nevertheless, it seems that a consistent explanation of the α-MnO charging mechanism is still lacking. To address this, a step-by-step analysis of the MnO cathodes is conducted. Valuable information is obtained by using in situ electrochemical quartz crystal microbalance with dissipation (EQCM-D) monitoring, supplemented by solid-state nuclear magnetic resonance (NMR), X-ray diffraction (XRD) in Characterization of Materials, and pH measurements. The findings indicate that the charging mechanism is dominated by the insertion of H O ions, while no evidence of Zn intercalation is found. The role of the Mn additive in promoting the generation of protons by forming MnOOH, enhancing the stability of Zn/α-MnO batteries is thoroughly investigated. This work provides a comprehensive overview on the electrochemical and the chemical reactions associated with the α-MnO electrodes, and will pave the way for further development of aqueous cathodes for Zn-ion batteries.