Determining Factors in Triggering Hysteretic Oxygen Capacities in Lithium-Excess Sodium Layered Oxides.

Oxygen redox (OR) reactions in sodium layered oxide cathodes have been studied intensively to harness their full potential in achieving high energy density for sodium-ion batteries (SIBs). However, OR triggers a large hysteretic voltage during discharge after the first charge process for OR-based oxides, and its intrinsic origin is unclear. Therefore, in this study, an in-depth reinvestigation on the fundamentals of the reaction mechanism in Na[Li1/3Mn2/3]O2 with a Mn/Li ratio (R) of 2 was performed to determine the factors that polarize the OR activity and to provide design rules leading to nonhysteretic oxygen capacity using first-principles calculations. Based on thermodynamic energies, the O2-/O22- and O2-/On- conditions reveal the monophasic (0.0 ≤ x ≤ 4/6) and biphasic (4/6 ≤ x ≤ 1.0) reactions in Na1-x[Li2/6Mn4/6]O2, but each stability at x = 5/6 is observed differently. The O-O bond population elucidates that the formation of an interlayer O-O dimer is a critical factor in triggering hysteretic oxygen capacity, whereas that in a mixed layer provides nonhysteretic oxygen capacity after the first charge. In addition, the migration of Li into the 4h site in the Na metallic layer contributes less to the occurrence of voltage hysteresis because of the suppression of the interlayer O-O dimer. These results are clearly elucidated using the combined-phase mixing enthalpies and chemical potentials during the biphasic reaction. To compare the Mn oxide with R = 2, Na1-x[Li1/6Mn5/6]O2 tuned with R = 5 was investigated using the same procedure, and all the impeding factors in restraining the nonhysteretic OR were not observed. Herein, we suggest two strategies based on three types of OR models: (i) exploiting the migration of Li ions for the suppression of the interlayer O-O dimer and (ii) modulating the Mn/Li ratio for controlling the OR participation, which provides an exciting direction for nonhysteretic oxygen capacities for SIBs and lithium-ion batteries.

Importance of Chemical Distortion on the Hysteretic Oxygen Capacity in Li-Excess Layered Oxides.

Nonhysteretic redox capacity is a critical factor in achieving high energy density without energy loss during cycling for rechargeable battery electrodes, which has been considered a major challenge in oxygen redox (OR) for Li-excess layered oxide cathodes for lithium-ion batteries (LIBs). Until recently, transition metal migration into the Li metal layer and the formation of O-O dimers have been considered major factors affecting hysteretic oxygen capacity. However, Li-excess layered oxides, particularly Ru oxides, exhibit peculiar voltage hysteresis that cannot be sufficiently described by only these factors. Therefore, this study aims to unlock the critical impeding factors in restraining the non-polarizing oxygen capacity of Li-excess layered oxides (herein, Li2RuO3) that exhibit reversible OR reactions. First, Li2RuO3 undergoes an increase in the chemical potential fluctuation as both the thermodynamic material instability and vacancy content increase. Second, the chemical compression of O-O bonds occurs at the early stage of the OR reaction (0.5 ≤ x ≤ 0.75) for Li1-xRu0.5O1.5, leading to flexible voltage hysteresis. Finally, in the range of 0.75 ≤ x ≤ 1.0, for Li1-xRu0.5O1.5, the formation of an O(2p)-O(2p)* antibonding state derived from the structural distortion of the RuO6 octahedron leads to the irreversibility of the OR reaction and enhanced voltage hysteresis. Consequently, our study unlocks the new decisive factor, namely, the structural distortion inducing the O(2p)-O(2p)* antibonding state, of the hysteretic oxygen capacity and provides insights into enabling the full potential of the OR reaction for Li-excess layered oxides for advanced LIBs.

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 Na1-xRu0.5O1.5 than that in Na1-xIr0.5O1.5. Third, the charge variations indicate a dominant cation redox activity via Ir(5d)-O(2p) for x above 0.5 in Na1-xIr0.5O1.5. Its redox participation occurred with the oxygen redox, opposite to the behavior in Na1-xRu0.5O1.5. 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.