Self-Supporting Cobalt Oxide Nanowire Anodes for Li-Ion Batteries Offer High Capacities and Rate Capabilities
|SEM images of Co3O4 nanowire arrays growing on Ti foil viewed when tilted by 40°. The inset shows the open tips of the nanowires. Click to enlarge.|
Researchers at Ohio State University (OSU) led by Professor Yiying Wu have developed a lithium-ion battery anode material from nanowire arrays of a cobalt oxide (Co3O4) that offers increased rate capabilities for high-powered applications, improves the cyclic properties in a rapid charge/discharge process, and increases the energy capacities.
As detailed earlier this year in the journal Nano Letters, at a current of 1C, the self-supported nanowire arrays maintain a stable capacity of 700 mAh/g after 20 discharge/charge cycles. When the current is increased to 50C, 50% of the capacity can be retained. OSU is offering the technology for licensing.
Wu’s team developed a template-free method to enable the large-area growth of the nanowires directly on a current-collecting titanium (Ti) substrate. No carbon or polymer additives are needed, which will save a mixing step.
The Co3O4 nanowire (NW) array reported in the journal shows a capacity close to twice that of the theoretical capacity (372 mAh/g) for graphite.
In previous literature references, there are only a few papers about free-standing NW array anodes prepared by the cumbersome template-synthesis method, including the carbon nanotube membrane (490 mAh/g), the SnO2 NW arrays (~700 mAh/g), and the Fe3O4/Cu composite NW arrays (~800 mAh/g). In addition, there are papers about random nanowire/nanotube anodes, such as multiwall carbon nanotubes (320 mAh/g), TiO2 (305 mAh/g), SnO2 (~400 mAh/g), Co3O4 (~500 mAh/g), CuO (~500 mAh/g), Fe2O3 (510 mAh/g), and MoO3 (150 mAh/g). By comparison, it is easy to tell that our Co3O4 NW arrays show one of the best capacities among the reported NW anode materials.—Wu (2008)
The team also tested the performance of the arrays at higher currents varied from 2 to 50C. Rate capability is an important parameter for many applications of batteries such as electric vehicles, which require fast discharge and/or charge rate.
The CO3O4 arrays can retain 85% capacity at 8C, 69% at 20C, and 50% at 50C relative to the capacity at 1C. By contrast, the capacity of non-self-supported NWs or powders decays much more sharply with the increase of current. The team also found that their NW arrays show good cyclability at high currents. After 20 cycles, the NW arrays can still maintain a capacity of 450 mAh/g at 20C and 240 mAh/g at 50C.
The researchers attributed the high rate capacity and rate capability of the nanowires arrays to the hierarchical architecture:
The NW array configuration can ensure that every NW participates in the electrochemical reaction, because every nanowire is in electric contact with the Ti substrate and also interfaced with the electrolyte solution.
The open space between neighboring NWs allows for easy diffusion of the electrolyte. This feature is particularly helpful for high power applications when the battery is charged or discharged at high current.
The NWs in this study are mesoporous with an average pore size of 3.3 nm and a BET surface area of 73.5 m2/g. The porosity will enhance the electrolyte/Co3O4 contact area, shorten the Li+ ion diffusion length in the NWs, and accommodate the strain induced by the volume change during the electrochemical reaction.
Yanguang Li, Bing Tan, and Yiying Wu; Mesoporous Co3O4 Nanowire Arrays for Lithium Ion Batteries with High Capacity and Rate Capability; Nano Lett., 8 (1), 265 -270, 2008. DOI: 10.1021/nl0725906