Tailoring Intrinsic Oxygen Vacancies in P3-Type Na0.66Mn0.66Mg0.33O1.93 to Enhance Oxygen Redox Activity.

Integrating oxygen redox reactions with transition metal redox reactions offers a promising strategy to double or triple the energy density for large-capacity battery cathodes. In this study, we have introduced intrinsic oxygen vacancies (V_O) into a P3 layered cathode to modulate the electronic structure of O atoms and enhance oxygen redox activity. Rietveld-refined X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy confirm the successful creation of V_O in Na_0.66Mn_0.66Mg_0.33O_1.93 (OV-NMM). Density functional theory (DFT) calculations reveal that V_O in OV-NMM positively enhances the antibonding interaction between Mn and O, directing excess electrons from O holes toward adjacent Mn t_2g and O 2p orbitals. This modification significantly improves the reversibility and accelerates Na⁺ transport kinetics for O2^-/O^- redox reactions. Ex situ synchrotron-based XRD demonstrates that V_O effectively eliminates the O3 phase, reduces Mg²⁺ migration, and suppresses irreversible structural changes. XAS of Mn K-edge and O K-edge further illustrate the advantageous role of oxygen vacancies in facilitating oxygen redox reactions. These findings highlight the potential of defect engineering, particularly V_O, to boost anionic redox activity for high-capacity energy storage applications.