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

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_0.67Ni_0.33Mn_0.67O₂ 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_0.8Li_0.1Ni_0.1Cu_0.1Ti_0.1Mn_0.6O₂, 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 Mn³⁺ 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.