Core-shell structured FeO@C hollow nanospheres as a high-performance negative material for potassium-ion batteries.

FeO is considered a promising electrode for potassium-ion batteries (PIBs) applications due to their natural abundance, low cost and high theoretical capacity. However, FeO suffers from capacity decay and sluggish reaction kinetic during the electrochemical process. Herein, the unique core-shell FeO@C featured with hollow nanospheres FeO as core and amorphous carbon layer as protect shell, the optimal framework of FeO@C is proposed to improve the structural stability and promote K charge transport. Accordingly, the FeO@C delivers an extraordinary cycling performance (437 mAh g after 200 cycles at 0.2 A/g), which is a remarkable electrochemical performance in Fe-based oxides negative electrode. To further elucidate the K-ion storage mechanisms of FeO, an in-depth characterization of FeO phase transition and changes in the coordination environment of iron atoms using in operando synchrotron techniques. Results from this study proved that K/Fe displacement/reordering occurs in the FeO@C electrode, which leads to inverse FeO (maghemite and hematite) phase and turning into KFeO (0 < x < 2, intermediate rock-salt-like phase), finally converting into KO and metallic Fe. Furthermore, the MgKVO (MKVO)//FeO@C full cell was assembled to investigate the practical application. These results may provide theoretical support for modifying Fe-base metal oxides of PIBs.