Ligand-Field Splitting Parameter Optimization Achieves Synergistic Enhancement of Transition Metal Redox Activity and Structural Stability in High-Voltage Sodium Layered Oxide Cathodes

Triggering oxygen anionic redox to achieve high-capacity Na_xTMO₂ faces a critical challenge because of the irreversible chemo-mechanical distortion and uncontrollable oxygen release at high voltage. To circumvent this issue, a strategy of stimulating transition metal (TM) redox activity based on the ligand-field splitting parameter (Δ) is proposed. Specifically, strongly polarized Mg−O−Fe configurations in the O3-NaNi_0.1Fe_0.2Mn_0.5Mg_0.2O₂ (O3-NaNFMMO) is constructed to effectively optimize the electron occupancy state of Fe 3d orbital by reducing its Δ, thereby stimulating the Fe redox activity while alleviating excessive oxygen redox. Additionally, the Mg pillar in Na sites ensures more extractable Na⁺ and suppresses the Na-free layers formation at high voltage, which can simultaneously improve the specific capacity and cycling stability. As a result, the designed cost-effective O3-NaNFMMO cathode delivers an outstanding specific capacity of 198 mAh g⁻¹ at 0.1 C and high-voltage cycling stability with 78% capacity retention after 1500 cycles at 5 C. Notably, the thermal degradation and air sensitivity, as the critical barriers to commercialization, are significantly suppressed in O3-NaNFMMO cathode. This work establishes a universal design principle for high-performance Na_xTMO₂ cathodes and offers a scalable pathway toward practical, cost-effective SIBs.