Enhanced Li-ion storage performance of novel tube-in-tube structured nanofibers with hollow metal oxide nanospheres covered with a graphitic carbon layer

One-dimensional (1D) nanofibers constructed with structurally stable nano-architecture and highly conductive carbon components can be employed to develop enhanced anodic materials for lithium-ion batteries. However, achieving an intricate combination of well-designed 1D-nanostructural materials and conductive carbon components for excellent lithium-ion storage capacity is a key challenge. In this study, novel and unique tube-in-tube structured nanofibers consisting of hollow metal oxide (CoFe₂O₄) nanospheres covered with a graphitic carbon (GC) layer were feasibly and successfully synthesized. A facile pitch solution infiltration method was applied to provide electrical conductivity in the tube-in-tube structure. Generally, mesophase pitch with liquid characteristics uniformly infiltrates the porous nanocrystals and transforms into graphitic layers around metallic CoFe₂ alloys during the reduction process. The oxidation process that follows produces the hollow CoFe₂O₄ nanosphere by the nanoscale Kirkendall effect and the GC layer by selective decomposition of amorphous carbon layers. Hollow CoFe₂O₄ nanospheres comprising tube-in-tube structured nanofibers and GC layers are formed by pitch-derived carbon; these have improved structural stability and electrical conductivity resulting in excellent cycling and characteristics. Tube-in-tube structured nanofibers consisting of hollow CoFe₂O₄@GC nanospheres showed excellent long-cycle performance (682 mA h g^-1 for the 1400th cycle at a high current density of 3.0 A g^-1) and excellent rate capability (355 mA h g^-1) even at an extremely high current density of 50 A g^-1.