Iron Oxide Nanoparticle‐Encapsulated CNT Branches Grown on 3D Ozonated CNT Internetworks for Lithium‐Ion Battery Anodes

Abstract Multidimensional hierarchical architecturing is a promising chemical approach to provide unique characteristics synergistically integrated from individual nanostructured materials for energy storage applications. Herein, hierarchical complex hybrid architectures of CNT‐on‐OCNT‐Fe are reported, where iron oxide nanoparticles are encapsulated inside carbon nanotube (CNT) branches grown onto the ozone‐treated surface of 3D CNT internetworked porous structures. The activated surface of the 3D ozonated CNT (OCNT) interacts with the iron oxide nanoparticles, resulting in different chemical environments of inner and outer tubes and large surface area. The mixed phases of iron oxide nanoparticles are confined by full encapsulation inside the conductive nanotubes and act as catalysts to vertically grow the CNT branches. This unique hierarchical architecture allows CNT‐on‐OCNT‐Fe to achieve a reasonable capacity of >798 mA h g⁻¹ at 50 mA g⁻¹, with outstanding rate capability (≈72% capacity retention at rates from 50 to 1000 mA g⁻¹) and cyclic stability (>98.3% capacity retention up to 200 cycles at 100 mA g⁻¹ with a coulombic efficiency of >97%). The improved rate and cyclic capabilities are attributed to the hierarchical porosity of 3D OCNT internetworks, the shielding of CNT walls for encapsulated iron oxide nanoparticles, and a proximate electronic pathway for the isolated nanoparticles.