Confining bulk molecular O₂ by inhibiting charge transfer on surface anions toward stable redox electrochemistry in layered oxide cathodes

Molecular O₂ has been clarified as an important O-oxidation model in anionic redox, the charge compensation provided by which pushes energy-density limits of layered oxide cathodes. However, how to confine the bulk-formed molecular O₂ and retard its conversation to free gaseous O₂ (↑), an origin of unstable redox electrochemistry, remains open questions. Here, we propose a strategy via tuning relative values of Mott–Hubbard U and charge-transfer energy Δ to suppress charge transfer on surface anions to confine the bulk molecular O₂. Supported by theoretical calculations, Nb⁵⁺ without 4d electrons which can enable U < < Δ is selected as a charge-transfer insulator. The thus Nb⁵⁺-surface-tailored model compound Na_0.67Fe_0.5Mn_0.5O₂ shows an oxygen-redox-inactive surface and well-confined bulk molecular O₂ as directly uncovered by soft X-ray absorption spectroscopy and 50 K-electron paramagnetic resonance results respectively. Meanwhile, a stable redox electrochemistry with enhanced cycling stability, inhibited voltage decay and eliminated P2-O2 phase transitions is observed because of the well-caged bulk O₂. More broadly, this work presents a versatile access to stabilize the important O-oxidation model, enriching approaches of stabilizing the anionic redox electrochemistry.