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

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 loss. Theoretical calculations combined with experimental characterizations reveal that the Cu strengthen oxygen binding energy via robust CuO covalent interactions, thereby suppressing irreversible oxygen evolution, Mg optimize the electronic structure through enhanced O 2p-Mg 2p orbital hybridization, enabling reversible O redox activity. Moreover, the stable CuO secondary phase effectively enhances P2-phase structural stability. As a result, NaMnCuMgO (NMCM3) delivers a high reversible capacity of 144.5 mA h g at 0.1C, 91 % capacity retention after 50 cycles, and achieves 232.77 Wh kg energy density in full cells. This work establishes a cation biphasic coordination strategy that simultaneously addresses structural stability and O redox reversibility, providing a viable pathway for developing high-energy-density cathode materials in SIBs.