Unveiling the Origin of Air Stability in Polyanion and Layered‐Oxide Cathode Materials for Sodium‐Ion Batteries and Their Practical Application Considerations

Abstract Although the electrochemical properties of various cathode candidates for sodium‐ion batteries (SIBs) are intensively studied on the material level, the evaluation and comparison from the perspective of future practical implementation are still lacking. In particular, air stability, which is often overlooked, will not only affect the electrode manufacturing processes but also cause detrimental structural change. Herein, two recently commercialized cathode products of NaNi 1/3 Fe 1/3 Mn 1/3 O 2 (NFM) and Na 3 V 2 (PO 4 ) 3 (NVP) are adopted to evaluate their stability in ambient atmospheres and investigate the structural degradation mechanisms. Theoretical calculations and experimental results demonstrate that the stability of NFM is significantly inferior to that of NVP during air exposure owing to the thermodynamically preferred chemical transformation of NFM. Furthermore, Raman mapping and X‐ray diffraction (XRD) measurements reveal the generation of alkaline, which accelerates the structural degradation of NFM. Accordingly, a simple second‐sintering strategy is proposed to enhance the air stability of NFM by removing impurities and retrieving surface‐precipitated Na + into the crystalline host. In situ XRD reveals the stabilized crystal structure after the second‐sintering treatment not only effectively inhibits the chemical transformation of NFM in air but also endows the improved cycling stability (a high capacity retention of 90.7% after 200 cycles).