Massachusetts Institute of Technology
New method produced up to ten fold increase in power
CAMBRIDGE, Mass. -- Batteries might gain a boost
in power capacity as a result of a new finding from researchers at MIT.
They found that using carbon nanotubes for one of the battery's electrodes
produced a significant increase up to tenfold in the amount of power
it could deliver from a given weight of material, compared to a conventional
lithium-ion battery. Such electrodes might find applications in small portable
devices, and with further research might also lead to improved batteries
for larger, more power-hungry applications.
This "electrostatic self-assembly" process is important, Hammond explains, because ordinarily carbon nanotubes on a surface tend to clump together in bundles, leaving fewer exposed surfaces to undergo reactions. By incorporating organic molecules on the nanotubes, they assemble in a way that "has a high degree of porosity while having a great number of nanotubes present," she says.
Lithium batteries with the new material demonstrate some of the advantages of both capacitors, which can produce very high power outputs in short bursts, and lithium batteries, which can provide lower power steadily for long periods, Lee says. The energy output for a given weight of this new electrode material was shown to be five times greater than for conventional capacitors, and the total power delivery rate was 10 times that of lithium-ion batteries, the team says. This performance can be attributed to good conduction of ions and electrons in the electrode, and efficient lithium storage on the surface of the nanotubes.
In addition to their high power output, the carbon nanotube electrodes showed very good stability over time. After 1,000 cycles of charging and discharging a test battery, there was no detectable change in the material's performance.
The electrodes the team produced had thicknesses up to a few microns, and the improvements in energy delivery only were seen at high-power output levels.
In future work, the team aims to produce thicker electrodes and extend the improved performance to low-power outputs as well, they say. In its present form, the material might have applications for small, portable electronic devices, says Shao-Horn, but if the reported high power capability were demonstrated in a much thicker form with thicknesses of hundreds of microns rather than just a few it might eventually be suitable for other applications such as hybrid cars.
While the electrode material was produced by alternately dipping a substrate into two different solutions a relatively time-consuming process Hammond suggests that the process could be modified by instead spraying the alternate layers onto a moving ribbon of material, a technique now being developed in her lab. This could eventually open the possibility of a continuous manufacturing process that could be scaled up to high volumes for commercial production, and could also be used to produce thicker electrodes with a greater power capacity. "There isn't a real limit" on the potential thickness, Hammond says. "The only limit is the time it takes to make the layers," and the spraying technique can be up to 100 times faster than dipping, she says.
Lee says that while carbon nanotubes have been produced in limited quantities so far, a number of companies are currently gearing up for mass production of the material, which could help to make it a viable material for large-scale battery manufacturing.