Progressive degradation and regeneration pathways in O3-NaNi1/3Fe1/3Mn1/3O2 under long-term low-humidity air exposure

This study systematically reveals the failure mechanisms of O3-type layered oxide NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) during long-term air exposure and their impact on electrochemical performance. Through multi-scale characterization, including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR), combined with electrochemical analysis, we demonstrate that Na+ migration and CO₂/H₂O synergistic reactions are the primary drivers of degradation. These processes induce the progressive growth of surface Na₂CO₃/HCO₃⁻ impurities, accompanied by lattice distortion (a-axis contraction by 0.94 % and c-axis expansion by 0.74 %) and partial Mn³⁺ oxidation (Mn⁴⁺ proportion increased to 52.6 %). These combined effects result in severe electrochemical deterioration: samples exposed for 15 days exhibit a 50.3 % initial capacity decay (from 129.9 to 64.6 mAh/g), and a tenfold decrease in Na⁺ diffusion coefficient. Notably, a 7-day exposure to a CO₂ atmosphere with trace moisture triggers surface carbonation reactions, emphasizing exposure duration as a critical variable under trace moisture conditions. Furthermore, this study proposes a ‘de-intercalation-framework reversibility’ mechanism: although water soaking induces 80 % Na⁺ extraction and partial TM-O framework distortion, low-temperature re-sodiation effectively repairs the lattice and restores the O3 structure. These findings provide theoretical insights and technical pathways for the failure prevention and regeneration of highly air-sensitive sodium-ion cathode materials.