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Building a Better Battery

by Nikhil Swaminathan

February 17, 2010

The future of powering electric vehicles.
These batteries use lithium ions as the electrolyte. A battery pack made of these cells, while more powerful than lead-acid and nickel-metal hydride batteries, is still 10 times weaker than an internal combustion engine of the same weight. Versions of these batteries are already used in in both the Tesla Roadster and Chevy Volt, as well as many electronic devices, such as laptops and cell phones. The knock on current lithium ion technology: It dispenses its stored energy slowly, so acceleration may be slow, and the batteries take several hours to charge. Also, while lithium is plentiful, it's not extensively mined, so it’s expensive to obtain. It may take up to 10 years for supply to catch up to projected demand.

Ultracapacitors

Ultracapacitors charge quickly and dispense their charge speedily (curing the slow acceleration problem that plagues some electric cars). They also last much longer than batteries—they can be recharged over and over again, whereas batteries eventually will not recharge. That's because ultracapacitors use electric fields, instead of slowly depleting chemicals, to get charges. They are already in use in short-run electric buses in Russia and garbage trucks in the United States. The downside: They only hold their charge for a limited time, so it's unlikely that ultracapacitors will become a viable option for powering a car alone. "I think ultracapacitors are a technology that's going to work with [battery] systems," says Savinell. However, one Texas-based company called EEStor says it has solved the storage problem, claiming its ultracapacitors will enable a small car to travel 250 miles on a single charge that only takes five minutes to complete.

Fuel Cells

Like batteries, fuel cells have cathodes and anodes and involve a chemical reaction, specifically making water and electrons (and thus electricity) by combining hydrogen with oxygen. The technology is simple enough, but the safety issues are the drag: The transport and onboard storage of highly explosive (remember the Hindenburg?) hydrogen gas could keep fuel cells from catching on. In addition, the catalysts needed to split hydrogen atoms into protons and electrons (like platinum, palladium, rhodium, nickel) are very expensive. "Fuel cells from a mobile standpoint are difficult," says NREL's Keyser. "Maybe in twenty five or thirty years down the road, we may be able to deal with all the storage issues, the transport issues, the infrastructure issues, the catalyst itself." Seemingly agreeing with Keyser's skepticism is the Obama administration, which cut $100 million from the federal hydrogen fuel cell program in 2009.

Redox Flow

Similar to fuel cells, redox flow batteries would require filling stations rather than plug-in capability. In this case, a charged electrolyte flows through the battery, producing electrons. After a while, the electrolyte loses its charge and needs to be pumped out and replaced. The electrolyte is typically made with vanadium, which is the 22nd most abundant element in the world. It's also very safe. "If you were to spill this on the road and light a cigarette near it, it's not going to go off like hydrogen," says Keyser. "The big thing with [redox flow batteries] is: Are you going to get the energy density or power density that you need for the car itself?" Right now, even lithium ion cells are several times more powerful than redox flow cells. German researchers, however, claim they have a method to increase the distance redox flow batteries can power a car by four to five times, rendering them roughly equal to lithium ion batteries.

Metal Air

Savinell and Keyser both point to metal air batteries as the technology of the future. This battery uses the oxygen in the air as its cathode, which means it doesn't need as much material and gets more energy for its weight. Depending on what material is used for the anode, metal air batteries could be anywhere from three times more powerful than lithium ion batteries of the same weight to as powerful as an internal combustion engine. IBM intends to bring these to market in five years for smaller electronics. "For lithium air, I think that's more ten to fifteen years down the road [to power a car]," says Keyser. "We're just starting to really look at that and understand all the benefits and the costs associated with lithium air batteries." One major barrier remains: When the oxygen reacts with the electrolyte to form ions, it also creates a solid that can gunk up the air intake, blocking the battery's function. Researchers are searching for an electrolyte that will produce the necessary ions but avoid the formation of this solid.

Illustrations by Will Etling.

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