Electrochemical properties of uniquely structured Fe₂O₃ and FeSe₂/graphitic-carbon microrods synthesized by applying a metal-organic framework

Uniquely structured Fe₂O₃ and FeSe₂/graphitic-carbon (GC) microrods composed of hollow Fe₂O₃ and FeSe₂ nanospheres, respectively, were successfully prepared by applying a metal-organic framework (MOF, MIL-88) as the precursor and template. This strategy involves the fabrication of Fe@GC microrods by the thermal reduction of MIL-88 microrods followed by transformation into hollow Fe₂O₃ nanosphere aggregate (H-Fe₂O₃-NSA) microrods and hollow FeSe₂ nanosphere aggregate/GC (H-FeSe₂/GC) microrods by means of oxidation and selenization, respectively. During the post-treatment step, metallic Fe nanocrystals embedded in GC are converted into hollow metal compound nanospheres through nanoscale Kirkendall diffusion. This novel structure makes it possible to achieve a superior electrochemical performance by alleviating the volume variation and providing ample ion reaction sites. In addition, in the case of H-FeSe₂/GC, the carbon framework not only prevents the structural collapse but also ensures sufficient electron transport during repeated cycles. Thus, the H-Fe₂O₃-NSA and H-FeSe₂/GC microrods have high specific discharge capacities of 973 mA h g⁻¹ after 400 cycles at 1 A g⁻¹ and 587 mA h g⁻¹ after 100 cycles at 0.2 A g⁻¹ when applied as anode materials for lithium-ion and sodium-ion batteries, respectively.