Unlocking the Intrinsic Origin of the Reversible Oxygen Redox Reaction in Sodium-Based Layered Oxides

Unlike cathodes for lithium-ion batteries, oxygen redox (OR) processes at a high voltage ((Formula presented.) 4.2 V) during the first charge in sodium-ion batteries (SIBs) employ some Li-incorporated Mn oxides that is recovered during subsequent discharge. To determine the intrinsic origin, P2-type Na_0.6[Li_0.2Mn_0.8]O₂ exhibiting a reversible OR-induced two-phase reaction was investigated using experiments and first-principle calculations. First, operando X-ray diffraction results in reversible P2-Z phase transformations and thermodynamic analysis show the two-phase reaction features Li migration into the tetrahedral sites from the transition-metal layer in the latter phase. Second, Li-induced decoupling of the oxygen 2p-electron led to selective anion redox activity depending on the oxygen sites that are Li-rich (redox-active) and Mn-rich (redox-inactive) environments. Third, redox-active oxygen coordinated to the Li vacancy predominantly participates in the formation of peroxo-like dimers with distortion of the MnO₆ octahedron, as observed in the reversible extended X-ray absorption fine structure spectra during the OR reaction. Considering three physicochemical perspectives, we reveal that Li ions play a role in activating OR reactions and control OR participation in the charge-compensation process. Our findings suggest that the Li/Mn ratio is a critical factor for achieving a reversible OR reaction, and broaden the possibilities of exploiting OR to reach high-energy densities in next-generation SIBs.