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 Na₂IrO₃ and Na₂RuO₃ 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 Na₂IrO₃ increases at a lower rate than that in Na₂RuO₃ upon charging. Second, considering that the strains of Ir-O and Ru-O bonding lengths are softened after the single-cation redox of Ru⁴⁺/Ru⁵⁺ and Ir⁴⁺/Ir⁵⁺, the contribution in the oxygen redox from x = 0.5 to 0.75 is larger in Na_1-Ru_0.5O_1.5 than that in Na_1-Ir_0.5O_1.5. Third, the charge variations indicate a dominant cation redox activity via Ir(5d)-O(2p) for x above 0.5 in Na_1-Ir_0.5O_1.5. Its redox participation occurred with the oxygen redox, opposite to the behavior in Na_1-Ru_0.5O_1.5. 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.