Accessing the Two-Electron Charge Storage Capacity of MnO2 in Mild Aqueous Electrolytes
Rechargeable batteries based on MnO2 cathodes, able to operate in mild aqueous electrolytes, have attracted remarkable attention due to their appealing features for the design of low-cost stationary energy storage devices. However, the charge/discharge mechanism of MnO2 in such media is still unclear and a matter of debate. Here, an in-depth quantitative spectroelectrochemical analysis of MnO2 thin-films provides a set of important new mechanistic insights. A major finding is that charge storage occurs through the reversible two electron faradaic conversion of MnO2 into water-soluble Mn2+ in the presence of a wide range of weak Brønsted acids, including the [Zn(H2O)6]2+ or [Mn(H2O)6]2+ complexes commonly present in aqueous Zn/MnO2 batteries. Furthermore, it is evidenced that buffered electrolytes loaded with Mn2+ are ideal to achieve highly reversible conversion of MnO2 with both high gravimetric capacity and remarkably stable charging/discharging potentials. In the most favorable case, a record gravimetric capacity of 450 mA·h·g-1 was obtained at a high rate of 1.6 A·g-1, with a coulombic efficiency close to 100% and a MnO2 utilization of 84%. Overall, the present results challenge the common view on MnO2 charge storage mechanism in mild aqueous electrolytes and underline the benefit of buffered electrolytes for high-performance rechargeable aqueous batteries.