Three-dimensional macroporous CNTs microspheres highly loaded with NiCo₂O₄ hollow nanospheres showing excellent lithium-ion storage performances

Three-dimensional macroporous carbon nanotubes microspheres highly loaded with phase-pure NiCo₂O₄ hollow nanospheres are synthesized by the spray pyrolysis process and are characterized for potential use in lithium-ion batteries. Polystyrene nanobead template and the nanoscale Kirkendall diffusion process are first combined and are applied to the spray pyrolysis process to form macroporous NiCo₂O₄/carbon nanotubes composite microspheres with extremely high rate performance as anode materials for lithium-ion batteries. Metallic NiCo₂/carbon nanotubes composite microspheres—formed as intermediate products—are transformed into composite microspheres of phase-pure NiCo₂O₄ hollow nanospheres and carbon nanotubes by the nanoscale Kirkendall diffusion process. The mean size of the hollow NiCo₂O₄ nanospheres decorated on the carbon nanotubes backbone is 28 nm. The macroporous NiCo₂O₄/carbon nanotubes composite microspheres have discharge capacities of 840, 748, 677, 591, 514, 451, 391, 337, and 289 mA h g⁻¹ at current densities of 0.5, 1, 2, 5, 10, 15, 20, 25, and 30 A g⁻¹, respectively. The discharge capacity of the macroporous NiCo₂O₄/carbon nanotubes microspheres for the 500th cycle at a current density of 3 A g⁻¹ is 572 mA h g⁻¹. The uniquely structured hollow NiCo₂O₄ nanosphere/carbon nanotubes composite microspheres have superior cycling and rate performances for lithium-ion storage.