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–/O2 2– and O2–/O n– 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.