Understanding the air sensitivity and deterioration mechanism of the Na4Fe3(PO4)2P2O7 cathode for Na-ion batteries

The air stability of electrode materials is a crucial factor for their practical application in rechargeable batteries. Na4Fe3(PO4)2P2O7 (NFPP) is a cathode material for sodium-ion batteries that has garnered increasing attention for long-life stationary energy storage. However, there is limited understanding regarding the air stability of NFPP. Here, we systematically investigated the chemical and structural stability and degradation mechanism of NFPP upon heating in ambient air. It is revealed that the extraction of Na+, Fe2+, and PO43− ions from NFPP is induced by moisture in ambient air, resulting in the formation of a composite amorphous phase and Na3PO4 small crystalline clusters on the NFPP surface. In addition, we observed a gradual enrichment of crystalline Na3PO4 through X-ray diffraction analysis during prolonged air exposure. The degraded NFPP leads to significantly reduced capacity and enlarged charge and discharge polarization. We demonstrate that optimal re-calcination under an inert atmosphere can effectively restore the mildly deteriorated NFPP material in terms of both structure and electrochemical performance. This research provides valuable insights for practical applications of polyanion cathodes in sodium-ion batteries.