Defensive and Ion Conductive Surface Layer Enables High Rate and Durable O3-type NaNi1/3 Fe1/3 Mn1/3 O2 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 NaPO3 surface modified O3-type layered NaNi1/3 Fe1/3 Mn1/3 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.