Fundamental Insights from a Single‐Crystal Sodium Iridate Battery

Abstract

Electrochemically driven chemical transformations play the key role in controlling storage of energy in chemical bonds and subsequent conversion to power electric vehicles and consumer electronics. The promise of coupling anionic oxygen redox with cationic redox to achieve a substantial increase in capacities has inspired research in a wide range of electrode materials. A key challenge is that these studies have focused on polycrystalline materials, where it is hard to perform precise structural determinations, especially related to the location of light atoms. Here a different approach is utilized and a highly ordered single crystal, Na2−xIrO3 is harnessed, to explore the role of defects and structural transformations in layered transition metal oxide materials on redox‐activity, capacity, reversibility, and stability. Within a combined experimental and theoretical framework, it is demonstrated that 1) it is possible to cycle Na2−xIrO3, offering proof of principle for single‐crystal based batteries 2) structural phase transitions coincide with Ir 4+/Ir 5+ redox couple with no evident contribution from anionic redox 3) strong irreversibility and capacity fade observed during cycling correlates with the Na + migration resulting in progressive growth of an electrochemically inert O3‐type NaIrO3 phase.

Quantum materials with strong spin–orbit coupling: challenges and opportunities for materials chemists

“…10,[34][35][36] In fact, the exact same materials are often under study by the two communities at the same time. 10,37,38 At the outset it is also worth noting that, although materials such as iridates have so far been the most popular for studies of d electrons with strong spin-orbit coupling, it is not only oxides that host such states but also e.g. halides 16 and intermetallics, 39 and in addition to compounds of the platinum group metals that we focus on there are a growing number of materials based on other 5d elements, such as rhenium and tantalum.…”

Unraveling Anionic Redox for Sodium Layered Oxide Cathodes: Breakthroughs and Perspectives

“…[ 152 ] Tepavcevic et al. [ 153 ] argued that the excess capacity of ordered single crystal Na 2‐ x IrO 3 can be attributed to neither Ir 5+ /Ir 6+ redox nor the pronounced change in the bulk oxygen oxidation reaction plausibly caused by impurities that inevitably populated the surface and interface of the material or other electrolyte contributions; thus, the additional capacity (120 mAh g −1 ) immediately disappears. During charging, Na 2 IrO 3 comprising two Na ions in the Na layer of the IrO 6 octahedral honey lattice changes to the Na 3 Ir 2 O 6 structure and then to O1‐type NaIrO 3 (Figure 7e,f).…”

“…Other helpful methods include XANES, EXAFS, sXAS, and hard XAS. Most reports have used XAS to characterize the electronic structures of cations and anions, such as NaMn 1/3 Fe 1/3 Ni 1/3 O 2 , [ 183 ] Na 2‐ x IrO 3 , [ 153 ] and Na 2/3 Mg 1/3 Mn 2/3 O 2 [ 114 ] EELS, which can be adapted to demonstrate electronic states, atomic compositions, surface properties, and chemical bonding of oxygen, is used to probe light elements. Similar to the XAS technique, O K‐edge EELS pre‐edge peaks reflect the oxidation number of oxygen ions.…”

Configuration‐dependent anionic redox in cathode materials

“…The formation of the unstable O 1 ‐type Na x IrO 3 results in structural distortion when more than 1.5 Na + ions are extracted (Figure 7A). However, only the Ir 4+ /Ir 5+ redox is involved in the highly ordered single‐crystal Na 2 IrO 3 107 . The capacity fading in the single‐crystal Na 2 IrO 3 is correlated with the Na + ions migration, resulting in the formation of O 3 ‐type Na 1 IrO 3 (Figure 7B).…”

“…Structural evolution during Na extraction from the single‐crystal Na 2 IrO 3 (B) 107 . Reproduced with permission 107 . Copyright 2020, Wiley‐VCH…”