Proton Ion Exchange Reaction in Li3 IrO4 : A Way to New H3+ x IrO4 Phases Electrochemically Active in Both Aqueous and Nonaqueous Electrolytes

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 H3+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.