Defensive and Ion Conductive Surface Layer Enables High Rate and Durable O3-type NaNi_1/3 Fe_1/3 Mn_1/3 O₂ Sodium-Ion...

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 NaPO₃ surface modified O3-type layered NaNi_1/3 Fe_1/3 Mn_1/3 O₂ 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 P₂ O₅ . The NaPO₃ 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 NaPO₃ , 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.