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

The air-sensitivity of transition metal oxide cathode materials (Na x TMO 2 , 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 x TMO 2 under ambient air is unclear, which hinders the precise design of air-stable Na x TMO 2 . Here, we revealed the origin of Na x TMO 2 degradation by capturing the initial degradation status and microstructural evolution under ambient atmospheres with optimal humidity. It was found that the insertion of CO 2 into Na layers along (003) planes of Na x TMO 2 led to initial growth of Na 2 CO 3 nanoseeds between TM layers, which initiated fast structure degradation with surface cracks and extrusion of Na 2 CO 3 out of Na x TMO 2 . The degradation extents and pathways for Na x TMO 2 could be highly associated with crystal orientation, particle morphology, and ambient humidity. Interestingly, the deteriorated Na x TMO 2 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 x TMO 2 cathodes for sodium-ion batteries.