Enabling Electrochemical and Air Stability of O3‐Type Layered Oxide Cathode Materials Using High‐Ionic‐Conductivity Na 5 FeSi 4 O 12 Solid Electrolyte as Novel Inorganic Functional Coating for Durable Sodium‐Ion Batteries

O3-phase transition metal layered oxides are considered as promising cathode materials for sodium-ion batteries (SIBs) due to high specific capacity and simple preparation process. However, several critical bottlenecks such as severe irreversible phase transition, sluggish diffusion kinetics, deteriorated interfacial contact, and unsatisfactory air stability largely hinder their stable operation in SIBs. Herein, Na₅FeSi₄O₁₂ (NFS) solid electrolyte material with high room-temperature ionic conductivity, low sintering temperature, and outstanding chemical stability is introduced as a novel inorganic functional coating on the surface of the NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) cathode material to modify the electrochemical and air stability. The NFS coating can offer integrated functions for the electrode, such as enhanced cycling reversibility, improved Na⁺ transport kinetics, as well as suppressed interfacial side reactions through physical isolation. Consequently, the electrode delivers high specific capacity (123.5 mAh g^-1 at 1 C), enhanced cycling reversibility (95.3 mAh g^-1 after 300 cycles at 5 C), improved rate capability (60.9 mAh g^-1 at 10 C), and excellent air stability (works stably over 800 cycles at 1 C after an exposure of five days to air). This work paves the design and understanding of highly conductive Na₅FeSi₄O₁₂ coating for the modification of O3-type layered oxides cathode materials for efficient Na-storage.