Strong Oxygen Participation in the Redox Governing the Structural and Electrochemical Properties of Na-Rich Layered Oxide Na₂IrO₃

Abstract

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 Na 2 IrO 3 , 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 3.0 V. 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 Na 2 IrO 3. 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.

Editors' Choice—Practical Assessment of Anionic Redox in Li-Rich Layered Oxide Cathodes: A Mixed Blessing for High Energy Li-Ion Batteries

“…They range from extending these studies to other Li-rich layered oxide phases showing anionic redox activity, either without (O-O) pairs or with it but avoiding TM migration, to the elaboration of electrochemical and phenomenological models that can fully rationalize our experimental findings. Our results should warn researchers to also keep in mind practical applications in the pursuit of anionic redox which is gaining fast importance in electrochemical systems, 65 including Na-based cathodes [66][67][68] and cation-disordered cathodes. 69 Solving these issues is not insurmountable and we hope that our timely identification of these fundamental bottlenecks toward real-world applications will encourage direct targeting of anionic redox with fundamental studies and mitigation strategies for neutralizing the problems with LR-NMC materials in general and facilitating its utilization in practical high energy density Li-ion batteries.…”

Progress in High‐Voltage Cathode Materials for Rechargeable Sodium‐Ion Batteries

“…), which are composed of a stacking of alternative layers of Na and [Na 1/3 M′ 2/3 ]O 2 , have been reported to exhibit large capacities exceeding the theoretical value based on the cationic redox species. [36] This extra capacity is associated with additional oxygen redox contribution at high voltages. A recent work from Yamada and co-workers demonstrated the critical role of honeycomb-type cation ordering in Na 2 RuO 3 to trigger the oxygen redox process by evaluating the Na storage behaviors of two ordered and disordered polymorphs.…”

Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides

“…For example, Li-rich surface and near-surface regions often exhibit oxygen evolution and reconstruction during delithiation 24–28 , yet surface-sensitive X-ray photoelectron spectroscopy (XPS) is often used to infer the nature of bulk anion redox 5,9,11,29 . Likewise, correlations between spatially averaged X-ray absorption spectra (XAS) are often used to assess hybridization 6,7,10,12–14,30–32 , which assumes that redox chemistry occurs uniformly.…”