Understanding the Air-Exposure Degradation Chemistry at a Nanoscale of Layered Oxide Cathodes for Sodium-Ion Batteries

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 NaNi_0.7Mn_0.15Co_0.15O₂ 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.