Rational design of metal-organic framework-templated hollow NiCo₂O₄ polyhedrons decorated on macroporous CNT microspheres for improved lithium-ion storage properties

We report three-dimensional (3D) porous microspheres comprising interconnected carbon nanotubes (CNT) decorated with hollow NiCo₂O₄ polyhedrons (H-NCO/CNT) for high-performance lithium-ion batteries (LIBs). The rationally designed composites are successfully fabricated via the combination of spray-pyrolysis and solution-based methods. The macroporous CNT microsphere obtained by spray pyrolysis acts as a substrate for the growth of the zeolitic imidazolate framework-67 (ZIF-67) in ethanol solution. During ion exchange and subsequent oxidation processes, the ZIF-67 polyhedrons were converted into hollow NiCo₂O₄ polyhedrons consisting of small crystal domains. Rational design of such composite microspheres offers a highly conductive 3D porous network that simultaneously enables fast ion and electron diffusion deep inside the electrodes during cycling. In addition, the hollow polyhedron interiors can accommodate large volume changes and shorten the transport pathway for the ions and electrons. Owing to these structural advantages, high capacity, long cycle life, and excellent rate capability are achieved from H-NCO/CNT microspheres when applied as LIB anodes; the discharge capacity of H-NCO/CNT microspheres remained at 1673 mA h g⁻¹ after 200 cycles at a current density of 1.0 A g⁻¹. Even when cycled at a high current density of 20.0 A g⁻¹, a high capacity of 639 mA h g⁻¹ could be achieved.