Defensive and Ion Conductive Surface Layer Enables High Rate and Durable O3‐type NaNi_1/3Fe_1/3Mn_1/3O₂ Sodium‐Ion Battery Cathode

Abstract Na‐based layered transition metal oxides with an O3‐type structure are considered promising cathodes for sodium‐ion batteries. However, rapid capacity fading, and poor rate performance caused by serious structural changes and interfacial degradation hamper their use. In this study, a NaPO 3 surface modified O3‐type layered NaNi 1/3 Fe 1/3 Mn 1/3 O 2 cathode is synthesized, with improved high‐voltage stability through protecting layer against acid attack, which is achieved by a solid‐gas reaction between the cathode particles and gaseous P 2 O 5 . The NaPO 3 nanolayer on the surface effectively stabilizes the crystal structure by inhibiting surface parasitic reactions and increasing the observed average voltage. Superior cyclic stability is exhibited by the surface‐modified cathode (80.1% vs 63.6%) after 150 cycles at 1 C in the wide voltage range of 2.0 V–4.2 V (vs Na + /Na). Moreover, benefiting from the inherent ionic conduction of NaPO 3 , the surface‐modified cathode presents excellent rate capability (103 mAh g −1 vs 60 mAh g −1 ) at 10 C. The outcome of this study demonstrates a practically relevant approach to develop high rate and durable sodium‐ion battery technology.