Defensive and Ion Conductive Surface Layer Enables High Rate and Durable O3-type NaNi 1/3 Fe 1/3 Mn 1/3 O 2 Sodium-Ion Battery Cathode.

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.