Decoupling the Dynamics of Zinc Hydroxide Sulfate Precipitation/Dissolution in Aqueous Zn–MnO₂ Batteries by Operando Optical Microscopy: A Missing Piece of the Mechanistic Puzzle

Abstract 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–MnO₂ batteries are promising candidates for grid‐scale applications due to 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 MnO₂. These pH variations are known to trigger the precipitation/dissolution of zinc hydroxide sulfate (Zn₄(OH)₆SO₄ . xH₂O, (ZHS)), which might have an influence on the conversion of MnO₂. Herein, optical reflectometry is used to image and quantitatively monitor the MnO₂ 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–MnO₂ batteries. If the precipitation of ZHS might impede the MnO₂ electrode's discharge, it is a crucial pH buffer delaying the occurrence of the competing oxidation of water on charge.