Undesired reactions between layered sodium transition-metal oxide cathodes and air impede their utilization in practical sodium-ion batteries. Consequently, a fundamental understanding of how layered oxide cathodes degrade in air is of paramount importance, but it has not been fully understood yet. Here a comprehensive study on a model material NaNi0.7Mn0.15Co0.15O2 reveals its reaction chemistry with air and the dynamic evolution of the degradation species upon air exposure. We find that besides the extraction of Na+ ions from the crystal lattice to form NaOH, Na2CO3, and Na2CO3·H2O in contact with air, nickel ions gradually dissolve from the bulk to form NiO and accumulate on the particle surface as revealed by subnanometer surface-sensitive time-of-flight secondary ion mass spectroscopy. The degradation species on the surface are insulating, leading to an increase in interfacial resistance and declined electrochemical performance. We also demonstrate a feasible surface coating strategy for suppressing the unfavorable degradation process. Understanding the degradation mechanism at a nanoscale can facilitate the future development of high-energy cathodes for sodium-ion batteries.
Keywords: Sodium-ion batteries; air sensitivity; high-Ni layered oxides; secondary ion mass spectroscopy; surface nanocoating; transition-metal dissolution.