Superior Electrochemical Properties of Nanofibers Composed of Hollow CoFe₂O₄ Nanospheres Covered with Onion-Like Graphitic Carbon

Nanofibers composed of hollow CoFe₂O₄ nanospheres covered with onion-like carbon are prepared by applying nanoscale Kirkendall diffusion to the electrospinning process. Amorphous carbon nanofibers embedded with CoFe₂@onion-like carbon nanospheres are prepared by reduction of the electrospun nanofibers. Oxidation of the CoFe₂-C nanofibers at 300°C under a normal atmosphere produces porous nanofibers composed of hollow CoFe₂O₄ nanospheres covered with onion-like carbon. CoFe₂ nanocrystals are transformed into the hollow CoFe₂O₄ nanospheres during oxidation through a well-known nanoscale Kirkendall diffusion process. The discharge capacities of the carbon-free CoFe₂O₄ nanofibers composed of hollow nanospheres and the nanofibers composed of hollow CoFe₂O₄ nanospheres covered with onion-like carbon are 340 and 930 mA h g^-1, respectively, for the 1000th cycle at a current density of 1 g^-1. The nanofibers composed of hollow CoFe₂O₄ nanospheres covered with onion-like carbon exhibit an excellent rate performance even in the absence of conductive materials.