Insights into the Interlayer Water-Induced Reversible Proton Insertion and Deinsertion in Ruddlesden–Popper Layered Fe Oxides

The widespread use of two-dimensional materials in aqueous rechargeable batteries makes it essential to elucidate their electrochemical behavior in water to ensure the safety and stability of the batteries for use in grid-level energy storage systems. However, the complexity of the hydration structure of ions and the presence of protons and hydroxide ions due to water ionization hinder the discovery of new active materials and the elucidation of the reaction mechanism. This study demonstrates the feasibility of Ruddlesden–Popper layered perovskite LaSr3Fe3O10−δ as a new active material, which is capable of electrochemical reduction and oxidation and the introduction of interlayer water. The operando X-ray diffraction and X-ray absorption results revealed that the Fe redox couple and the consequent introduction of protons and the interlayer water are responsible for the charge and discharge capacities. In addition, the capacity decreased in highly concentrated aqueous solutions with a few free water molecules. Furthermore, LaSr3Fe3O8(OH)2·mH2O synthesized via a two-step process involving chemical reduction and the introduction of interlayer water showed a clear decrease in capacity although the Fe valence was as low as that obtained by the electrochemical method. This study presents novel active materials for aqueous batteries and provides insights into the role of interlayer water frequently present in layered materials.