Chloride Insertion Enhances the Electrochemical Oxidation of Iron Hydroxide Double-Layer Hydroxide into Oxyhydroxide in Alkaline Iron Batteries

Rechargeable alkaline iron batteries that constituteenvironmentallybenign electrolytes and earth-abundant industrial materials are desirablegreen solutions for large-scale energy storage. As one of the mostabundant metal elements in the earth's crust, iron (Fe) cansatisfy nearly all criteria for low-cost and safe battery electrodes.However, challenges in achieving reversible Fe redox impede theirextensive implementation in modern energy supply systems. This studyrevealed that Cl-anion insertion into Fe(OH)(2) layered doublehydroxide (LDH) formed a green rust intermediate phase with the formula[Fe-2 Fe-2+(1) (3+)(HO-)(6)](+)[Cl](-), which assisteda high Fe(OH)(2)/FeOOH conversion reaction (64.7%) and improvedcycling stability. This new iron redox chemistry was validated by operando X-ray diffraction, electrochemical testing, X-rayabsorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS)analysis, scanning transmission electron microscopy-energy-dispersiveX-ray spectroscopy (STEM-EDS) mapping, and molecular dynamics (MD)simulations. Our study provides new insight into designing LDH materialsfor high-capacity alkaline iron batteries.