Strong oxygen participation in the redox governing the structural and electrochemical properties of Na-rich layered oxide Na₂IrO₃

The recent revival of the Na-ion battery concept has prompted intense activities in the search for new Na-based layered oxide positive electrodes. The largest capacity to date was obtained for a Na-deficient layered oxide that relies on cationic redox processes only. To go beyond this limit, we decided to chemically manipulate these Na-based layered compounds in a way to trigger the participation of the anionic network. We herein report the electrochemical properties of a Na-rich phase Na₂IrO₃, which can reversibly cycle 1.5 Na⁺ per formula unit while not suffering from oxygen release nor cationic migrations. Such large capacities, as deduced by complementary XPS, X-ray/neutron diffraction and transmission electron microscopy measurements, arise from cumulative cationic and anionic redox processes occurring simultaneously at potentials as low as 2.7 V vs Na⁺/Na. The inability to remove more than 1.5 Na⁺ is rooted in the formation of an O1-type phase having highly stabilized Na sites as confirmed by DFT calculations, which could rationalize as well the competing metal/oxygen redox processes in Na₂IrO₃. This work will help to define the most fertile directions in the search for novel high energy Na-rich materials based on more sustainable elements than Ir.