Higher-Capacity Lithium-Ion Batteries

By Kevin Bullis

Nanostructured electrodes and active materials could shrink batteries for portable electronics and electric vehicles.

Researchers in France have created lithium-ion battery electrodes with several times the energy capacity, by weight and volume, of conventional electrodes. The new electrodes could help shrink the size of cell-phone and laptop batteries, or else increase the length of time a device could run on a charge. What’s more, the nanotech methods used to make these electrodes could provide a simple and inexpensive way to structure new materials for next-generation batteries for plug-in hybrid and all-electric vehicles.

The key advance is the development of an inexpensive and simple way to organize tiny particles into a desired nanostructure, says Patrice Simon, a chemistry professor at the Université Paul Sabatier, who participated in the work along with other researchers at the university and Université Picardie Jules Verne.

In a conventional battery electrode, ions and electrons will move quickly into and out of the active material — allowing fast charging and discharging — only if the material is deposited in a very thin film. Thin films, however, limit the amount of active material that can be incorporated into a battery. For high-capacity batteries, engineers typically increase the thickness of the active material, trading off fast charging and high-power bursts for more energy storage.

This new nanostructure allows for both high power and high storage capacity. Active materials are applied in a very thin film to copper nanorods anchored to sheets of copper foil. This thin film allows for fast movement of ions and electrons — providing the power. At the same time, the high surface area of the forest of nanorods makes it possible to pack much more active material into an electrode than thin films typically allow, thus increasing energy capacity. The rods provide 50 square centimeters of surface area for every square centimeter of electrode.

In addition, the high ion and electron mobility of the thin layer makes it possible to use a new active material and a new chemical reaction for lithium-ion batteries. This new chemistry is attractive because it can accommodate far more lithium ions, and their electron counterparts, than the chemistry used now, thereby potentially storing more energy.

The new electrodes, which would be used as the negative electrodes in lithium-ion batteries, also showed the ability to retain their high capacity after being charged and discharged many times, suggesting that the electrodes may have a long useable lifetime, Simon says, although more extensive tests are needed to confirm this supposition.

Because this advance, described online this week in Nature Materials, applies so far to negative electrodes, the percentage increase in capacity over today’s batteries will depend on the capacity of the positive electrode as well. (See “Battery Breakthrough” for a description of one possible positive electrode candidate cited by the researchers.) The first applications of the technology will likely be extremely small batteries, Simon says. These could be useful for remote sensors or medical implants. Further applications will require increasing the size of the electrodes that the researchers can make, and also optimizing the active material they use.

The materials used in reported experiments are not energy efficient — about 20-25 percent of the energy used to charge them cannot be recovered while discharging them. This energy loss is not a big problem with cell-phone batteries, says Gerbrand Ceder, materials science and engineering professor at MIT. “Over the lifetime you probably spend a few pennies in charging the cell phone,” he says. But for larger energy applications, such as electric vehicles, this lack of efficiency could be costly, especially with high electricity prices. For this reason, the researchers are incorporating different high-capacity active materials into their nanostructured electrodes that do not have this energy efficiency problem.

In turning to nanotechnology to improve batteries, the French researchers are not unique. At least two companies, A123 Systems, in Watertown, MA, and Altair Nano, in Reno, NV, have made batteries that include electrodes with nanostructured active materials; and numerous research groups around the world are developing such electrodes. Simon describes his group’s process as being simpler and cheaper than many other methods for making nanostructures. It is also versatile, capable of being used with a variety of active materials, he says.

It could also be important for another key trend in battery research: the move away from flat layers of electrode materials to positive and negative electrodes that interpenetrate — a three-dimensional architecture that can improve the mobility of ions and electrodes, thereby increasing battery power. The French group is also now working on a three-dimensional battery, says Simon, that will combine their negative electrodes with a high-performance positive electrode.