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

Abstract

AbstractUnlike cathodes for lithium‐ion batteries, oxygen redox (OR) processes at a high voltage ( 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 Na0.6[Li0.2Mn0.8]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.