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
2O)
6]
2+ or [Mn(H
2O)
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.
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