Constructing Epitaxial Coating and Na‐Site Lattice Doping to Boost Sodium‐Storage Performance of Sodium Layered Oxides

O3‐type NaFe 1/3 Ni 1/3 Mn 1/3 O 2 (NFNM) is promising cathode materials for Na‐ion batteries (NIBs) owing to their high capacity and facile synthesis features. However, their practical application is hindered by insufficient electrochemical performance caused by poor air stability of materials and serious structure degradation during cycling. Herein, we introduce a synergistic surface and bulk yttrium modification strategy, which forms an epitaxially Y 2 O 3 coating on the surface and induces a selective doping of Y 3+ into Na layers in the bulk. The coating prevents moisture/CO 2 attack, preserving structure integrity and air stability. Meanwhile Y 3+ pillars widen Na + channels and suppress detrimental O3‐P3 phase transitions caused by transition metal‐layer sliding, thereby improving both Na + diffusion kinetics and structural stability. As a result, even after 7 days of air exposure, the modified NFNM with optimal yttrium content (3 mol %) retains an initial capacity of 130.4 mA h g −1 with 86.0% capacity retention after 200 cycles at 1 C (2–4 V), outperforming most of NFNM‐based cathodes. Such a synergistic surface and bulk modification strategy provides a facile and practical route to obtain air‐stable and durable NFNM‐based NIBs.