Degradation Mechanism of O3-Type NaNi_1/3Fe_1/3Mn_1/3O₂Cathode Materials during Ambient Storage and Their in Situ Regeneration

Sodium-ion batteries (SIBs) hold great promise for low-cost energy storage. Despite the major advances made in the material preparation and battery performance, air instability has become a bottleneck for the storage and electrode fabrication of O3-type NaNi1/3Fe1/3Mn1/3O2 (NFM), but the underlying mechanism remains elusive. Here we discovered that NFM loses Na+ ions during ambient storage and Na2CO3 "fibers"sprout from the particle surface, which caused the performance decay. We further demonstrated a facile resintering strategy to regenerate the NFM in situ. This work highlights the importance of stringent humidity control and provides the basis for designing surface-modification strategies.