Core-shell structured Fe₂O₃@C hollow nanospheres as a high-performance negative material for potassium-ion batteries

Fe₂O₃ is considered a promising electrode for potassium-ion batteries (PIBs) applications due to their natural abundance, low cost and high theoretical capacity. However, Fe₂O₃ suffers from capacity decay and sluggish reaction kinetic during the electrochemical process. Herein, the unique core-shell Fe₂O₃@C featured with hollow nanospheres Fe₂O₃ as core and amorphous carbon layer as protect shell, the optimal framework of Fe₂O₃@C is proposed to improve the structural stability and promote K⁺ charge transport. Accordingly, the Fe₂O₃@C delivers an extraordinary cycling performance (437 mAh g^-1 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 Fe₂O₃, an in-depth characterization of Fe₂O₃ phase transition and changes in the coordination environment of iron atoms using in operando synchrotron techniques. Results from this study proved that K⁺/Fe^2+/3+ displacement/reordering occurs in the Fe₂O₃@C electrode, which leads to inverse Fe₂O₃ (maghemite and hematite) phase and turning into K_xFe₂O₃ (0 < x < 2, intermediate rock-salt-like phase), finally converting into K₂O and metallic Fe. Furthermore, the Mg_0.008K_0.51V₂O₅ (MKVO)//Fe₂O₃@C full cell was assembled to investigate the practical application. These results may provide theoretical support for modifying Fe-base metal oxides of PIBs.