Anchoring Ligand Electron Enables Robust Metal-Oxygen Coordination Toward 4.5 V O3-Type Sodium-Ion Battery Cathodes

High-voltage operation enables sodium-sufficient O3-type layered oxides to approach the maximum achievable energy densities for practical sodium-ion batteries (SIBs). This high-voltage regime, however, induces structural degradation strongly correlated with oxygen redox activity, a mechanism still incompletely resolved. Using prototypical O3-type NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) as a model system, we identify the origin of this instability as a detrimental feedback loop between σ-type oxygen redox and cation migration. We thus propose an "anchoring ligand electron (ALE)" strategy, employing a multi-level screening protocol to identify optimal anchor agents that confine oxygen redox to stable π-type configurations with robust metal-oxygen coordination. The ALE-engineered NFM cathode mitigates excessive oxygen ligand electron transfer, achieving record capacity retention at an ultrahigh voltage of 4.5 V after 300 cycles. The superior cyclic stability is demonstrated to be closely associated with the stable π-type oxygen redox and suppressed metal-oxygen decoordination. This ALE strategy expands the optimization pathway toward ultrahigh-voltage and high-energy-density cathodes.