Intrinsic Origin of Nonhysteretic Oxygen Capacity in Conventional Na-Excess Layered Oxides.

An intriguing redox chemistry via oxygen has emerged to achieve high-energy-density cathodes and has been intensively studied for practical use of anion-utilization oxides in A-ion batteries (A: Li or Na). However, in general, the oxygen redox disappears in the subsequent discharge with a large voltage hysteresis after the first charge process for A-excess layered oxides exhibiting anion redox. Unlike these hysteretic oxygen redox cathodes, the two Na-excess oxide models NaIrO and NaRuO unambiguously exhibit nonhysteretic oxygen capacities during the first cycle, with honeycomb-ordered superstructures. In this regard, the reaction mechanism in the two cathode models is elucidated to determine the origin of nonhysteretic oxygen capacities using first-principles calculations. First, the vacancy formation energies show that the thermodynamic instability in NaIrO increases at a lower rate than that in NaRuO upon charging. Second, considering that the strains of Ir-O and Ru-O bonding lengths are softened after the single-cation redox of Ru/Ru and Ir/Ir, the contribution in the oxygen redox from = 0.5 to 0.75 is larger in NaRuO than that in NaIrO. Third, the charge variations indicate a dominant cation redox activity via Ir(5d)-O(2p) for above 0.5 in NaIrO. Its redox participation occurred with the oxygen redox, opposite to the behavior in NaRuO. These three considerations imply that the chemical weakness of Ir(5d)-O(2p) leads to a more redox-active environment of Ir ions and reduces the oxygen redox activity, which triggers the nonhysteretic oxygen capacity during (de)intercalation. This provides a comprehensive guideline for design of reversible oxygen redox capacities in oxide cathodes for advanced A-ion batteries.