Accurate Control of Multi-shelled Co3O4 Hollow Microspheres for High- Performance Anode Materials in Lithium Ion Batteries**

Thanks to their high energy density, light weight and long cycle life, rechargeable lithium ion batteries (LIBs) have become one of the dominant power sources for portable electronic devices. With the growing need for higher capacity and safety, numerous efforts have been made to develop alternative high-performance electrode materials for next-generation LIBs. As an example, the anode electrodes made of transition-metal oxides have been explored to exhibit reversible capacities about twoor three-times as large as those based on the conventional graphite materials (<372 mAh g ). Among various types of transition-metal oxides, cobalt oxide (Co3O4) receives special attention since it is expected to have a high capacity (about 1000 mAh g) and excellent cycling performance. Unfortunately, although the best capacity of Co3O4 to date has increased to 1450 mAh g by using Co3O4 micro-/nano-materials, the practical use of Co3O4 as anode electrodes for LIBs is still largely hindered due to their poor capacity retention upon cycling and/or low rate capability. To circumvent these issues, hollow micro-/nano-structures are used to replace the solid micro-/nanostructures as the LIB anode materials based on the following considerations: (1) The larger surface area of the hollow structures enables more access for Li ions due to an increased electrodeelectrolyte contact area. (2) The hollow interior provides additional free volume for alleviating the structural strain associated with repeated Li insertion/extraction processes, resulting in the improved cycling stability. (3) Properly prepared multi-shelled hollow micro-/nano-structures not only allow for significantly reduced diffusion paths for both Li ions and electrons (leading to a better rate capability), but also provide more lithium storage sites, resulting in a higher overall capacity. Recently, significant efforts have been devoted to the fabrication of hollow spheres with multiple shells. Different metal oxides with the multi-shelled hollow sphere structures have been synthesized based on the soft/hard template methods. It should be noted that these methods are only suitable for specific metal oxides and cannot be applied generally to a wide range of materials. Recently, we reported a facile and general strategy to prepare metal oxide hollow microspheres by using carbonaceous microspheres (CMSs) as the sacrificial templates. The number of shells can be controlled by adjusting the heating conditions and the concentration of metal ions. However, due to the large radii of hydrated cobalt ions, we failed to achieve control over the number of multi-shelled Co3O4 hollow microspheres.