P2-Na0.67Ni0.33Mn0.67O2 cathodes face commercialization barriers since irreversible P2–O2 phase transitions occur when charged above 4.0 V and Mn3+ degradation occurs if discharged to 2.0 V; this, along with moisture sensitivity, results in high costs for production and storage. To address these challenges, we realized the synergy of phase engineering and site-selective substitution through the rational design of a layered oxide, Na0.80Mn0.60Mg0.07Ni0.23Cu0.07Zn0.03O2 (P2/O3-Na8M). The site-selective substitution of Zn, as a pillar, at the Na sites prevents irreversible phase transitions and resists H2O insertion via the stronger attraction of Zn2+–O–Zn2+ than that of Na+–O–Na+. The accommodation of Cu/Mg in TM sites further enhances structural stability by suppressing oxygen loss through stronger Cu–O covalency than Ni–O, reducing Na+/H+ exchange, and stabilizing Na–O bonds by lowering the overall cationic potential of the structure, in addition to mitigating the Jahn–Teller distortions and enabling ultra-wide potential operation. Theoretical and experimental investigations reveal that the P2/O3 phase heterostructure lowers Na+ diffusion barrier, enabling P2/O3-Na8M to deliver 150 mAh g−1 at 0.1C with 80.8% capacity retention after 300 cycles at 5C within 1.8–4.4 V while preserving the structure and performance after 35 days of air/water exposure. The full cell can achieve a high energy density of 281.2 Wh kg−1 and offer 82.5% retention after 600 cycles at 1C.