Energy storage provides flexibility to an energy system and is therefore key for the incorporation of renewable energy sources such as wind and solar into the grid. Aqueous Zn-MnO 2 batteries are promising candidates for grid-scale applications due their high theoretical capacity (616 mAh g-1) and the abundance of their components in the Earth's crust. However, they suffer from low cyclability, which is probably linked to the dramatic pH variations induced by the electrochemical conversion of MnO 2. These pH variations are known to trigger the precipitation/dissolution of zinc hydroxide sulfate (Zn 4 (OH) 6 SO 4 .xH 2 O, ZHS), which might have an influence on the conversion of MnO 2. Herein, optical reflectometry is used to image and quantitively monitor the MnO 2 electrode's charge and discharge in situ and under operation. It emphasizes how solid-phase ZHS rules the dynamics of both charge and discharge, providing a comprehensive picture of the mechanism at play in aqueous Zn-MnO 2 batteries. If the precipitation of ZHS might impede the MnO 2 electrode's discharge, it is a crucial pH buffer delaying the occurrence of the competing oxidation of water on charge.