Tuning High-Voltage Cu-Fe Redox in P2-Type Layered Metal Oxide Cathodes for Sodium-Ion Batteries

P2-type layered oxides have gained significant attention in energy storage applications due to their superior rate capability and cycling performance. However, P2-type layered oxides suffer from low sodium content and irreversible phase transitions at high voltages, limiting reversible capacity and long-term durability. Although Cu-Mn-based systems offer a high operating voltage through the Cu²⁺/Cu³⁺ redox couple, the accessible voltage window remains narrow, resulting in restricted capacity. In this work, we employ a compositional engineering strategy by introducing Fe into the Cu-Mn framework to enhance both structural stability and redox activity. Fe incorporation enhances structural stability and redox activity by introducing additional cationic redox centers. The synergistic involvement of the Cu²⁺/Cu³⁺ and Fe³⁺/Fe⁴⁺ redox couples maintains a high average voltage (∼3.54 V) while improving electrochemical reversibility. The resulting P2-Na_0.7Cu_0.3Mn_0.6Fe_0.1O₂ cathode demonstrates a reversible capacity exceeding ∼90 mAh g^-1 when combined with CNTs, along with excellent cycling stability. Ex situ XRD analysis reveals reversible phase evolution, confirming enhanced structural robustness during cycling. This study highlights the effectiveness of Fe substitution in optimizing Cu-Mn cathode chemistry, offering a promising pathway for high-energy and durable sodium-ion battery systems.