Proton Ion Exchange Reaction in Li₃IrO₄: A Way to New H_3+xIrO₄ Phases Electrochemically Active in Both Aqueous and Nonaqueous Electrolytes

Abstract Progress over the past decade in Li‐insertion compounds has led to a new class of Li‐rich layered oxide electrodes cumulating both cationic and anionic redox processes. Pertaining to this new class of materials are the Li/Na iridate phases, which present a rich crystal chemistry. This work reports on a new protonic iridate phase H_3+xIrO₄ having a layered structure obtained by room temperature acid‐leaching of Li₃IrO₄. This new phase shows reversible charge storage properties of 1.5 e⁻ per Ir atom with high rate capabilities in both nonaqueous (vs Li⁺/Li) and aqueous (vs capacitive carbon) media. It is demonstrated that Li‐insertion in carbonate LiPF₆‐based electrolyte occurs through a classical reduction process (Ir⁵⁺ ↔ Ir³⁺), which is accompanied by a well‐defined structural transition. In concentrated H₂SO₄ electrolyte, this work provides evidence that the overall capacity of 1.7 H⁺ per Ir results from two additive redox processes with the low potential one showing ohmic limitations. Altogether, the room temperature protonation approach, which can be generalized to various Li‐rich phases containing either 3d, 4d or 5d metals, offers great opportunities for the judicious design of attractive electrode materials.