A new type of high-power battery may help make larger hybrid vehicles a reality, according to a research paper published this week. A group of scientists at MIT have found a way to use carbon nanotubes to create a device that combines the strengths of batteries and capacitors, resulting in a battery than can both store a large amount of energy and put out a high rate of power. The ability to provide a better combination of high power and rapid discharge may help engineers tailor the batteries to a broader range of vehicles.

Batteries and capacitors have long occupied independent niches when it comes to storing electricity. Lithium batteries can store a significant amount of energy using chemical processes, but can only supply a low rate of power; capacitors can deliver a lot of power at once by eliminating the difference between two oppositely charged plates, but have low total energy storage.

Researchers have been trying to mitigate the shortcomings of both devices for some time, by either forcing higher rates of output from batteries or more storage from capacitors. They've achieved some success in increasing the rate of discharge from lithium batteries by shortening the distance that the ions diffuse to a few nanometers, but the output remained too low for many high-power applications. Similar efforts to adapt capacitors have yielded limited successes.

To get the functionality they were looking for, researchers needed a material that could quickly shuffle ions around the battery, but would also bond strongly to them, ensuring a higher release of energy when the ions are released. As is often the case in materials science, they needed to look no further than carbon nanotubes.

To construct an electrode for their new battery, the researchers created alternating layers of carbon nanotube sheets coated with carboxylic acid and amine functional groups—these can undergo charge transfer reactions with lithium ion charge carriers. Their addition also seems to roughen up the surface of the nanotubes, increasing the surface area available for reactions.

The researchers tested a battery that used the layered carbon nanotube electrode on the positive end, and a lithium electrode on the negative end. The power output of the batteries declined as the nanotube electrode's thickness increased, placing a ceiling on its numbers. But an electrode three micrometers thick could still deliver energies of 200 watt-hours per kilogram (a bit better than current-generation lithium batteries), and a power of 100 kilowatts per kilogram. They were able to match the energy of lithium ion batteries at lower power outputs, and at high power had better energy delivery than the nanoscale-diffusion lithium batteries.

While these numbers were impressive, batteries with pure lithium electrodes are not the norm. For a more realistic setup, researchers tried instead using a composite electrode made of lithium titanium oxide along with the carbon nanotube electrode. They found that these batteries had lower energy and power, but at 30 watt-hours per kilogram and 5 kilowatts per kilogram, their performance is several times better than the current generation of capacitors. The battery was also very resilient, showing no drop in performance even after 2,500 cycles.

The new battery doesn't best either capacitors or batteries at their respective strengths— it stores energy only about as well as any lithium ion battery, and supplies rushes of power as well as a capacitor. However, it may find use as a versatile middle-of-the-road device that has high storage and can supply bursts of power if needed.

Researchers hope that this new style of battery will eventually allow for larger hybrid vehicles that are less reliant on their gas engines to sustain a high power draw. Potential benefactors of the technology might include tractor trailers and buses.

The authors indicate that they plan to continue by verifying how the electrodes behave on larger scales, where "larger" means tens and hundred of micrometers. They also hope to develop ways to prevent some of the energy loss during charging and discharging. The new battery may also benefit from a new method of assembling multiwalled carbon nanotubes by spraying them on layer by layer, which may allow fine tuning of the voltage differences needed during charge and discharge.

Nature Nanotechnology, 2010. DOI: 10.1038/NNANO.2010.116 (About DOIs).