Synergistic Perovskite Titanate Coating and Lattice Doping Toward Air‐Stable and Long‐Life O3‐Type NaNi _1/3 Fe _1/3 Mn _1/3 O ₂ Cathodes

O3-type layered oxides are promising cathode materials for sodium-ion batteries but suffer from structural instability, sluggish kinetics, and moisture sensitivity. This work proposes a synergistic modification of O3-NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM). Among various perovskite titanates (CaTiO₃, SrTiO₃, BaTiO₃) investigated, CaTiO₃ proves to be the most effective modifier. DFT calculations reveal that Ca²⁺ doping uniquely strengthens the Na-O interaction, fundamentally enhancing the air stability. The conformal CaTiO₃ coating serves as a robust physical barrier against humid air, significantly suppressing the formation of surface carbonates and residual alkali. Simultaneously, Ca²⁺ and Ti⁴⁺ are doped into Na and transition metal (TM) sites, respectively, which enlarges the Na⁺ layer spacing (from 3.701 to 3.812 Å), strengthens the TM─O framework, and mitigates irreversible phase transitions. As a result, the modified NFM@CTO06 cathode exhibits outstanding electrochemical performance, demonstrating a high capacity retention of 85.1% after 300 cycles at 1 C. Furthermore, it demonstrates reduced voltage hysteresis, enhanced Na⁺ diffusion kinetics, and significantly improved air stability. This surface-to-bulk strategy offers a feasible approach toward practical high-energy-density sodium-ion batteries.