Origin of Air-Stability for Transition Metal Oxide Cathodes in Sodium-Ion Batteries

The air-sensitivity of transition metal oxide cathode materials (Na_xTMO₂, TM: transition metal) is a challenge for their practical application in sodium-ion batteries for large-scale energy storage. However, the deterioration mechanism of Na_xTMO₂ under ambient air is unclear, which hinders the precise design of air-stable Na_xTMO₂. Here, we revealed the origin of Na_xTMO₂ degradation by capturing the initial degradation status and microstructural evolution under ambient atmospheres with optimal humidity. It was found that the insertion of CO₂ into Na layers along (003) planes of Na_xTMO₂ led to initial growth of Na₂CO₃ nanoseeds between TM layers, which initiated fast structure degradation with surface cracks and extrusion of Na₂CO₃ out of Na_xTMO₂. The degradation extents and pathways for Na_xTMO₂ could be highly associated with crystal orientation, particle morphology, and ambient humidity. Interestingly, the deteriorated Na_xTMO₂ could be completely healed through optimal recalcination, showing even improved air-stability and electrochemical performance. This work provides a helpful perspective on the interfacical structure design of high-performance Na_xTMO₂ cathodes for sodium-ion batteries.