Enhanced Oxygen Redox Activity and Structure Stability of P2‐Type Manganese‐Based Cathodes Through Medium‐Entropy Strategy

ABSTRACT Oxygen redox reaction offers a promising strategy to enhance the energy density of manganese‐based layered transition‐metal oxides, yet the associated multiple structural transitions and volume changes usually undermine long‐term stability. Entropy stabilization, which leverages elemental synergy to improve structural robustness, has emerged as a promising solution. Here, we integrate ionic potential into medium‐entropy design to guide element selection. Taking Na 0.67 Ni 0.33 Mn 0.67 O 2 as an example, doping with multiple low ionic potential elements elevates O 2p degeneracy, thereby enhancing and stabilizing high voltage oxygen redox reactions. A medium‐entropy P2‐type oxide, Na 0.8 Li 0.1 Ni 0.1 Cu 0.1 Ti 0.1 Mn 0.6 O 2 , demonstrates a high reversible capacity of 223.7 mAh g −1 and an energy density of 616.3 Wh kg −1 , while maintaining 87% capacity retention over 200 cycles. It markedly suppresses transition metal layer gliding and Jahn–Teller distortion, stabilizes Mn 3+ against disproportionation to preserve Mn redox activity and suppress voltage decay, while detrimental phase transitions are fully inhibited. This strategy simultaneously boosts reversible capacity and leverages entropy‐driven phase stabilization, offering a practical route toward next‐generation, high‐capacity, durable sodium‐ion batteries.