Tailored Interphase Chemistry Enables Ultra-Stable O3-Type Sodium Layered Oxide Cathodes

Among all oxide-based cathode materials for sodium-ion batteries, the O3-type transition metal oxides are considered the most commercially viable due to their high capacity. However, their commercialization remains challenging due to interfacial instability and parasitic side reactions, upon both air exposure and electrochemical cycling. Here, we present a novel near-surface engineering strategy to construct a multifunctional calcium-based interphase on NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM). The resulting Ca-rich rock-salt surface layer features a robust Ca-O framework, which significantly enhances air stability while also improving electrode processability. During cycling, the engineered interphase facilitates favorable Na⁺ transport kinetics and effectively suppresses parasitic interfacial reactions, thereby contributing to an improved long-term electrochemical performance. As a result, the surface-engineered NFM delivers a superior rate capability of 117.5 mA h g^-1 at 5 C (600 mA g^-1) and an outstanding capacity retention of 83.3% after 500 cycles. This work offers a universal interphase design strategy for stabilizing highly reactive O3-type cathodes.