Accessing the Two‐Electron Charge Storage Capacity of MnO 2 in Mild Aqueous Electrolytes

Abstract Rechargeable batteries based on MnO 2 cathodes, able to operate in mild aqueous electrolytes, have attracted attention due to their appealing features for the design of low‐cost stationary energy storage devices. However, the charge/discharge mechanism of MnO 2 in such media is still a matter of debate. Here, an in‐depth quantitative spectroelectrochemical analysis of MnO 2 thin‐films provides a set of unrivaled mechanistic insights. A major finding is that charge storage occurs through the reversible two‐electron faradaic conversion of MnO 2 into Mn 2+ in the presence of a wide range of weak Brønsted acids, including the [Zn(H 2 O) 6 ] 2+ or [Mn(H 2 O) 6 ] 2+ complexes present in aqueous Zn/MnO 2 batteries. Furthermore, it is shown that buffered electrolytes loaded with Mn 2+ are ideal to achieve highly reversible conversion of MnO 2 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 is obtained at a high rate of 1.6 A·g −1 , with a Coulombic efficiency close to 100% and a MnO 2 utilization of 84%. Overall, the present results challenge the common view on MnO 2 the charge storage mechanism in mild aqueous electrolytes and underline the benefit of buffered electrolytes for high‐performance rechargeable aqueous batteries.