Reversible Cl/Cl redox in a spinel MnO electrode.

A unique prospect of using halides as charge carriers is the possibility of the halides undergoing anodic redox behaviors when serving as charge carriers for the charge-neutrality compensation of electrodes. However, the anodic conversion of halides to neutral halogen species has often been irreversible at room temperature due to the emergence of diatomic halogen gaseous products. Here, we report that chloride ions can be reversibly converted to near-neutral atomic chlorine species in the MnO electrode at room temperature in a highly concentrated chloride-based aqueous electrolyte. Notably, the Zn cations inserted in the first discharge and trapped in the MnO structure create an environment to stabilize the converted chlorine atoms within the structure. Characterization results suggest that the Cl/Cl redox is responsible for the observed large capacity, as the oxidation state of Mn barely changes upon charging. Computation results corroborate that the converted chlorine species exist as polychloride monoanions, , [Cl] and [Cl], inside the Zn-trapped MnO, and the presence of polychloride species is confirmed experimentally. Our results point to the halogen plating inside electrode lattices as a new charge-storage mechanism.