Nanotubes: Stepping Up Energy Storage

Photograph by Mark Thiessen, National Geographic

A ladybug perches on carbon nanotubes stretched between copper wires; some scientists believe these tiny straws—each about 10,000 times thinner than a human hair—hold big promise for better batteries to power plug-in cars.

Carbon is just one material in a whole pantry of ingredients being treated and mixed in new ways in hopes of concocting a battery that lasts longer, costs less, stores more energy, delivers greater bursts of power, and occupies less space than the lithium-ion batteries commonly used today. About a dozen new electric vehicle models are set to hit the road before the end of this year, but most analysts predict only slow market penetration of  EVs—due mainly to the high costs and unwieldiness of today's batteries.

"If you open up a laptop, most of the real estate is the battery," said Kristin Persson, a research chemist with the U.S. Department of Energy's Lawrence Berkeley National Lab. Similarly, in cars, "the battery takes up too much space." The T-shaped battery pack in General Motors' plug-in hybrid Chevy Volt, for example, weighs nearly 400 pounds and extends more than five feet down the length of the car, allowing room for only two passengers in the back seat.

But what if a smaller battery could deliver the same amount of energy? It would enable manufacturers of portable electronics and plug-in cars alike to "do other fancy stuff" with that space, said Persson, a co-founder of the magnesium-ion battery company Pellion Technologies. She also is a co-founder of the Materials Project, a joint effort by the Berkeley lab and the Massachusetts Institute of Technology to create a searchable database of the properties of tens of thousands of compounds, and to predict those of new compounds using supercomputing.

Think of battery cells as a sandwich. The two slices of bread are electrodes—one, called the cathode, has a positive charge. The other, known as the anode, has a negative charge. Squished between them like mayonnaise is an electrolyte made of organic solvents. Positively charged lithium ions shuttle back and forth through the electrolyte during charging and discharging. But each electrode has room to store only so many ions, limiting battery capacity. That's part of why the search is on for new ingredients—if the electrodes can hold more ions, the battery can store more energy.

The anodes in today's lithium-ion battereries are graphite—a form of carbon. But researchers believe that nanotubes, like those shown above, made of from one-atom-thick sheets of carbon rolled into hollow tubes, could support faster charging and discharging, thanks to more surface area.

The electrodes work as a team, so the battery's capacity is limited by the electrode with less room for charge. In today's batteries, that's the cathode.

So researchers have also begun working with oxygen-coated carbon nanotubes for cathodes. Others are experimenting with carbon nanotubes as a low-cost alternative to platinum catalysts found both in fuel cells and metal-air batteries.

"Carbon itself is very abundant, but you need to change the structure slightly to use it in batteries," Persson explained, noting it must be processed at very high temperatures. "The carbon we use in anodes is not the carbon we tend to dig out of the ground."

(Related: "Pictures: Lightest Material Ever Created?")

—Josie Garthwaite

http://news.nationalgeographic.com/news/energy/2012/09/pictures/120914-better-electric-car-battery/Published September 14, 2012