Synergistic cation-dual phase engineering in P2-type cathodes: Enabling reversible oxygen redox and high-energy...

P2-type layered oxides have emerged as promising cathode candidates for sodium-ion batteries (SIBs) owing to their favorable theoretical capacity and open ion diffusion channels. Nevertheless, the practical implementation of these materials faces significant challenges, particularly irreversible phase transformations (P2-O2 and P2-P2') and detrimental lattice oxygen evolution occurring at high states of charge (SoC). Herein, we developed a P2 phase cathode material through a dual Cu/Mg co-doping strategy coupled with controlled CuO secondary phase formation, which synergistically suppresses Jahn-Teller distortion and alleviates O^2- loss. Theoretical calculations combined with experimental characterizations reveal that the Cu²⁺ strengthen oxygen binding energy via robust CuO covalent interactions, thereby suppressing irreversible oxygen evolution, Mg²⁺ optimize the electronic structure through enhanced O 2p-Mg 2p orbital hybridization, enabling reversible O^2- redox activity. Moreover, the stable CuO secondary phase effectively enhances P2-phase structural stability. As a result, Na_0.67Mn_0.50Cu_0.34Mg_0.16O₂ (NMCM3) delivers a high reversible capacity of 144.5 mA h g^-1 at 0.1C, 91 % capacity retention after 50 cycles, and achieves 232.77 Wh kg^-1 energy density in full cells. This work establishes a cation biphasic coordination strategy that simultaneously addresses structural stability and O^2- redox reversibility, providing a viable pathway for developing high-energy-density cathode materials in SIBs.