Design and synthesis of bubble-nanorod-structured Fe₂O₃-Carbon nanofibers as advanced anode material for li-ion batteries

A structure denoted as a "bubble-nanorod composite" is synthesized by introducing the Kirkendall effect into the electrospinning method. Bubble-nanorod-structured Fe₂O₃-C composite nanofibers, which are composed of nanosized hollow Fe₂O₃ spheres uniformly dispersed in an amorphous carbon matrix, are synthesized as the target material. Post-treatment of the electrospun precursor nanofibers at 500°C under 10% H₂/Ar mixture gas atmosphere produces amorphous FeO_x-carbon composite nanofibers. Post-treatment of the FeO_x-carbon composite nanofibers at 300°C under air atmosphere produces the bubble-nanorod-structured Fe₂O₃-C composite nanofibers. The solid Fe nanocrystals formed by the reduction of FeO_x are converted into hollow Fe₂O₃ nanospheres during the further heating process by the well-known Kirkendall diffusion process. The discharge capacities of the bubble-nanorod-structured Fe₂O₃-C composite nanofibers and hollow bare Fe₂O₃ nanofibers for the 300th cycles at a current density of 1.0 A g^-1 are 812 and 285 mA h g^-1, respectively, and their capacity retentions measured from the second cycle are 84 and 24%, respectively. The hollow nanospheres accommodate the volume change that occurs during cycling. The unique structure of the bubble-nanorod-structured Fe₂O₃-C composite nanofibers results in their superior electrochemical properties by improving the structural stability during long-term cycling.