Stabilizing O3-type Na(NiFeMn)_1/3O₂ cathode for high-rate sodium storage through strategic copper-magnesium co-doping

O3-type layered oxides, offering the advantages of high specific capacity and low cost, are currently regarded as one of the most promising cathode materials for sodium-ion batteries (SIBs), demonstrating considerable potential in energy storage systems. However, undesirable phase transitions and sluggish sodium-ion diffusion severely impede their further development. Herein, synergistic chemical substitution was employed to remarkably enhance cycling stability while simultaneously boosting rate capability. The as-prepared NaNi_0.289Fe_0.284Mn_0.283Cu_0.097Mg_0.046O₂ (NFMCM) exhibited exceptional cycling stability with 86.1% capacity retention over 200 cycles at 1 C with an energy density of 400 Wh·kg⁻¹, significantly surpassing the pristine Na(NiFeMn)_1/3O₂ (NFM, 64.3%), while delivering a remarkable specific capacity exceeding 60 mAh·g⁻¹ at an ultrahigh rate of 10 C. Notably, the full cell configured with NFMCM and commercial hard carbon maintains 83.4% capacity retention over 300 cycles (266 Wh·kg⁻¹), highlighting substantial potential for practical implementation. Density functional theory (DFT) calculations reveal that NFMCM possesses an optimized charge environment around oxygen species, where the elevated transition metal sliding energy effectively suppresses the O3–P3 phase transition, thereby optimizing the electrochemical performance. This work provides novel insights into the design principles of O3-type cathode materials for SIBs.