Publication 906

Adv. Energy Mater., 10 (23), 2000332, 2020 DOI:10.1002/aenm.202000332
Accessing the Two-Electron Charge Storage Capacity of MnO2 in Mild Aqueous Electrolytes

Université de Paris, Laboratoire d’Electrochimie Moléculaire, CNRS, F-75013 Paris, France

Rechargeable batteries based on MnO₂ 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 MnO₂ in such media is still a matter of debate. Here, an in-depth quantitative spectroelectrochemical analysis of MnO₂ 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 MnO₂ into Mn²⁺ in the presence of a wide range of weak Brønsted acids, including the [Zn(H₂O)₆]²⁺ or [Mn(H₂O)₆]²⁺ complexes present in aqueous Zn/MnO₂ batteries. Furthermore, it is shown 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 is 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 the charge storage mechanism in mild aqueous electrolytes and underline the benefit of buffered electrolytes for high-performance rechargeable aqueous batteries.