Excellent stability of layered Na0.67Ni0.33Fe0.33Mn0.33O2 cathode materials with P2/O3 biphasic system in humid ambient air

Sodium ion batteries (SIBs) have shown broad application prospects in the field of energy storage due to their low cost, high safety, wide operating temperature range, fast charging, and environmental friendliness. As one of the key components of SIBs, layered oxide cathodes have significant advantages such as high specific capacity, feasible energy density, and high operating voltage, but they also face challenges of poor interface stability and insufficient air stability. In this work, a P2/O3 biphasic system Na_0.67Ni_0.33Fe_0.33Mn_0.33O₂ (Na0.67NFM) is synthesized by a sol-gel and subsequent solid phase calcination method. Furthermore, the structural characteristics of Na0.67NFM calcined at 900 °C (Na0.67NFM-900) are investigated, revealing that it maintains a high stability even after exposure in humid ambient air for 5 days. Although a small amount of Na₂CO₃ and NaHCO₃ is produced on the surface of Na0.67NFM-900, the main P2/O3 structure still maintains overall integrity into the internal bulk phase. Therefore, the electrochemical performance of Na0.67NFM cathode is not significantly affected after short-term air exposure. After 200 cycles at 1C, the capacity retention of Na0.67NFM-900 cathode still remains 72.3 %, which is almost comparable to that of the unexposed one (75.7 %). This work indicates that Na0.67NFM is a promising candidate cathode material with exceptional air stability for SIBs.