The abundant resource of sodium (Na) makes the Na-ion batteries a promising alternate to the Li-ion batteries in electric energy storage, especially when iron (Fe)-based oxide electrodes are used. Layer-structured Na_xFe_yM_1-yO₂ (M for Mn and Ni, etc.), for example, has drawn much attention and is one type of the most attractive cathode materials for the Na-ion batteries. However, the poor cycling performance hinders their applications and the underneath mechanism for their poor performances remains unclear. In this study, the structural transition of NaFeO₂ was characterized and the driving forces for the irreversible Fe migration and O oxidation were explored during Na extraction. The Fe migration from the Fe layer to the Na layer was observed for the first time at the atomic scale. The chemical states and coordination environment of the Fe ions within the Na layer were determined. The density functional theory (DFT) calculations were carried out to understand the Fe migration and O oxidation. These findings provide helpful revelation on constructing the structure and improving the performance of the NaFeO₂-structured cathode materials for the Na-ion batteries.

钠（Na）的丰富资源使Na离子电池在电能存储方面成为锂离子电池的有希望的替代品，尤其是在使用铁（Fe）基氧化物电极时。例如，层状的Na _x Fe _y M _1-y O ₂（M表示Mn和Ni等）引起了很多关注，并且是Na离子电池最有吸引力的正极材料之一。然而，不良的循环性能阻碍了它们的应用，并且其不良性能的底层机理仍然不清楚。在这项研究中，NaFeO ₂的结构转变表征了Na萃取过程中不可逆的Fe迁移和O氧化的驱动力。首次在原子尺度上观察到Fe从Fe层迁移到Na层。确定了Na层中Fe离子的化学状态和配位环境。进行了密度泛函理论（DFT）计算，以了解铁的迁移和O的氧化。这些发现为构造用于Na离子电池的NaFeO ₂结构的正极材料和改善其性能提供了有益的启示。