Hierarchical structured composite materials consisting of NiCo2O4 and electrospun carbon fibers were fabricated through the coprecipitation of Ni-Co compounds on electrospun carbon fibers. Post thermal treatment converted Ni-Co compounds to NiCo2O4. We aimed to identify how different fiber diameters affect the structures and electrochemical performance of NiCo2O4/carbon fiber composites. Carbon fibers with diameters smaller than 400 nm resulted in the formation of NiCo2O4 needles, whereas those with diameters larger than 800 nm resulted in the development of NiCo2O4 flowers. The composites with needle-type NiCo2O4 exhibited richer mesopores and larger specific surface areas than fibers with flower-like NiCo2O4 did. When used as lithium-ion battery anodes, the needle-like and flower-like composites exhibited specific capacities of 639 and 498 mAh/g, respectively, at a current density of 200 mA/g after 100 cycles, which were much higher than pristine carbon fibers. The enhanced specific capacity resulted from the combinative effects of the efficient electron transfer in the carbon fiber networks and the high specific capacity of NiCo2O4. Furthermore, needle-like materials with large mesopore volume allowed for the accommodation of volume changes and favored fast lithium-ion transfer during continuous electrochemical reactions, leading to more favorable cycling stability and rate capability than the flower-like materials did.
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