Abstract: The air stability of cathode materials is a critical prerequisite for the commercialization of sodium-ion batteries (SIBs), as it affects electrode processing, capacity degradation, and other practical considerations. Although layered oxides are known for their acute air sensitivity, Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) cathode material exhibits relatively better air stability. However, prolonged air exposure still leads to degradation, with reported extraction of Na⁺, Fe²⁺, and PO₄^3- to form Na₃PO₄ and amorphous impurities. This degradation of the crystal structure deteriorates the electrochemical performance of the material. Nevertheless, the precise atmospheric triggers and detailed degradation mechanisms of NFPP remain unclear. Herein, we systematically elucidate the interaction mechanisms between NFPP and key atmospheric components. It reveals that significant degradation occurs only under the combined effect of oxygen and water vapor, under which Na⁺ and PO₄^3- are extracted to form Na₃PO₄ impurities. Simultaneously, the extracted Fe²⁺ is oxidized to Fe(III), forming an amorphous Fe₂O₃·zH₂O mixture. A re-sintering treatment can partially restore the crystal structure of the degraded NFPP, but the associated active material loss prevents full recovery of low-rate capacity, confirming the coexistence of reversible and irreversible degradation processes. These findings elucidate the air oxidation degradation mechanism of NFPP and provide theoretical guidance for its storage condition control and practical application.

Key words: Na₄Fe₃(PO₄)₂P₂O₇, Air stability, Oxidative degradation, Sodium-ion battery