Porous Co3O4 Nanotubes Derived From Co4(CO)12 Clusters on Carbon Nanotube Templates: A Highly Efficient Material For Li‐Battery Applications

Over the past decades, a worldwide effort has been made to search for alternative anode materials of lithium batteries for improving their energy density and safety. It has been found that 3d transition metal oxides such as nickel oxide, cobalt oxide, and iron oxide exhibit reversible capacities about three times larger than those of graphite (372 mAh g) at a relative low potential, which greatly spurs the rapid development in this field. Among them, cobalt oxides (Co3O4 and CoO) have shown the highest capacity (700 mAh g) and best cycle performance (93.4 % of initial capacity was retained after 100 cycles), compared with nickel oxide (NiO) and iron oxides (Fe2O3 and Fe3O4). [3] In recent years, the nanostructured materials have attracted great interest in the application of anode or cathode materials for lithium batteries because of their high surface-to-volume ratio and short path length for Li transport. As a result, it is believed that the Co3O4 nanomaterials can exhibit the superior Li-battery performance. Previously, several Co3O4 nanostructures such as nanoparticles, nanowires, and nanotubes were prepared by different methods. Among them, the nanotubes have been considered one of the most promising structures for lithium batteries due to their higher surface-to-volume ratio than other one-dimensional nanostructures such as nanowires and more difficult for aggregation in comparison with nanoparticles. By far, there is little literature about the synthesis and application of Co3O4 nanotubes for lithium batteries. For example, Chen et al. synthesized the Co3O4 nanotubes via the anodic aluminum oxide (AAO) template route and applied them for lithium batteries with the capacity of about 800 mAh g at the current density of 50 mA g. However, there are some disadvantages for the AAO template assisted approach to synthesize metal oxide nanotubes, which restrict their application in Libattery. Firstly, the mass production of metal oxide nanotubes prepared by such an approach is impracticable, which is one of the bottlenecks for their wide applications. Secondly, it is very difficult to completely remove the nanoporous alumina template. Thirdly, the diameters of metal oxide nanotubes prepared by such an approach are usually larger than 100 nm. Recently, carbon nanotubes (CNTs) have been considered to be an ideal template for the synthesis of metal oxide nanotubes due to the mass production, being easily removed and small diameter. For example, Liu et al. reported the synthesis of Fe2O3/CNTs core-shell nanostructures and polycrystalline Fe2O3 nanotubes by supercritical fluids approach using CNTs as templates. Unfortunately, the high temperature and pressure were needed in this approach. In addition, metal oxide/ CNTs core-shell nanostructures and metal oxide nanotubes were also achieved by CNT-template assisted chemical vapor deposition (CVD), which operated at high temperature and, moreover, only deposited oxides on the top surface of CNTs. Furthermore, metal oxide/CNTs core-shell nanostructures were also fabricated by chemical precipitation method. However, the formation of metal oxide nanoparticles in the solution or metal oxide with very large grain size on the surface of CNTs was inevitable in this approach, which made it difficult to form metal oxide nanotubes after oxidation of CNTs. Very recently, we have developed a novel approach to synthesizing the porous and polycrystalline In2O3 nanotubes by layer-by-layer assembly on CNT templates in combination with subsequent calcinations, which exhibit superior gas sensing performance. Herein, we report a novel sonochemistry method to synthesizing CNTs-CoOx nanocables derived from Co4(CO)12 clusters on CNT templates at room temperature and subsequent transformation into uniform porous Co3O4 nanotubes by the calcination. Moreover, the assynthesized porous Co3O4 nanotubes have been applied in anode materials for lithium batteries, which exhibit the superior performance and thus promising application. Firstly, Co4(CO)12 and CNTs were mixed in hexane and sonicated for 1 h at room temperature. During the sonication, Co4(CO)12 can be readily decomposed to Co and CO, as shown in Reaction 1. Secondly, Co atom can be rapidly oxidized into CoOx due to the oxygen atmosphere in the solution, as illustrated in Reaction 2. Thirdly, CoOx with positive charge can be compactly deposited on the surface of CNTs C O M M U N IC A IO N