The catalyst consists of nanoparticles of a gold and platinum alloy; in testing it was able to return 77 percent of the energy used to charge the battery as electricity when discharged. That’s up from the previously published record of about 70 percent, the researchers say. The work, which was reported online this week in the Journal of the American Chemical Society, suggests a new approach to lithium-air battery catalysts that could lead to the even higher efficiencies of 85 to 90 percent needed for commercial batteries.
Lithium-air batteries, which generate electricity by reacting lithium metal and oxygen from the air, are attractive for their potential to store vast amounts of energy. They could be a practical way to store more than three times as much energy, by weight, as today’s lithium-ion batteries, extending the range of electric vehicles, for example.
But prototype lithium-air batteries are plagued with problems. In addition to being very inefficient, they typically only last a few dozen charge and discharge cycles. They are also sluggish–only releasing their energy slowly–and prone to contamination by carbon dioxide and water. And the lithium metal used for one of the electrodes is dangerously reactive and eventually grows dendrites, which can lead to short circuits.
By improving the battery’s efficiency, the new catalyst research, led by Yang Shao-Horn and Hubert Gasteiger, professors of mechanical engineering, in collaboration with Kimberly Hamad-Schifferli, a professor of mechanical engineering and biological engineering, addresses one of their most serious problems. The catalysts could also help make such batteries longer lived.
When lithium-air batteries are discharged, the lithium metal reacts with oxygen to form lithium oxide and release electrons. When charged, oxygen is released and lithium metal reforms. The new catalysts promote these reactions, and so reduce the amount of energy wasted as the cells are charged and discharged. The gold atoms in the catalyst facilitate the combination of lithium and oxygen; the platinum helps the opposite reaction, freeing the oxygen.
In some ways the findings fly in the face of previous assumptions. Platinum, known for being one of the best catalysts for promoting the combination of hydrogen and oxygen in fuel cells, was one of the first materials tried for catalyzing lithium and oxygen in lithium-air batteries. But experiments showed that it actually did a poor job, so platinum was dropped.
The MIT researchers found that platinum is useful in lithium-air batteries, but for the opposite reaction–freeing oxygen from lithium oxide during charging. "Everyone knew that platinum was inactive for discharging the battery, but we showed that platinum was one of the best catalysts for charging," Shao-Horn says.
On the other hand, gold is typically considered a poor catalyst because it is inert, Shao-Horn says. Indeed, the MIT researchers had first used gold as a sort of control in experiments to measure reactions involving a poor catalyst. To their surprise, they found that gold does a good job of catalyzing the combination of lithium and oxygen–much better than platinum. (Toyota researchers had shown this previously, and issued a patent a few months before Shao-Horn’s group saw the effect.) Furthermore, the researchers found that both catalysts became more effective when they were combined as nanoparticles. "Together they work synergistically," Shao-Horn says.
In addition to improving efficiency, promoting these reactions could also potentially increase the number of times that lithium-air batteries can be recharged, by minimizing the accumulation of lithium oxide, which otherwise clogs up the battery. As they continue to develop lithium-air batteries, the MIT researchers will explore this possibility; they will study the gold platinum catalysts in more detail to understand how they work; and develop new catalysts with different combinations of materials.
The MIT researchers are also working to cut the cost of the catalyst by using less platinum and gold. One option is to coat nanoparticles made of cheaper materials with thin layers of these precious metals. Other researchers have demonstrated that inexpensive manganese oxide catalysts can be effective for lithium-air batteries, says Jean-Marie Tarascon, a professor at the Universite de Picardie Jules Verne in France. He says this material recently has been shown to produce even higher efficiencies than Shao-Horn’s catalysts.
The Search for Cheaper, Lighter Car Batteries
Jonathan Fahey, www.forbes.com
The Nissan Leaf, an electric car that will go on sale this fall, is priced at $33,000. It’s a $16,500 subcompact car that costs double that thanks to a battery estimated to cost $16,500. The eStar, an electric truck being developed by Navistar, will sell for $150,000 because it will tote a battery that costs at least $75,000. Cost isn’t the only problem. Both the Leaf and the eStar will be limited to 100 miles of driving on a charge.
Both vehicles are powered by the same kind of batteries that power your laptop, ones that shuttle lithium ions back and forth between two electrodes. The unattractiveness of electric vehicles boils down to two facts: Rechargeable batteries cost a lot and weigh a lot. A lithium-ion battery, at its best, packs 110 watt-hours of energy per pound. Gasoline has 6,000 watt-hours per pound. Now, a gasoline motor is inefficient, discarding 85% of the fuel’s energy–losing it to the transmission, wasting it on idling and discharging it as heat. Electric motors waste just 10%, but it still leaves gas with a 9-to-1 weight advantage.
While battery makers are making impressive progress beating down the cost of lithium-ion batteries and improving their performance for cars, battery makers and electric vehicle builders agree that the world needs something new for electric vehicles.
"No one expects the lithium-ion battery to even double [in energy density]," says Winfried Wilcke, a nanoscale-science manager at IBM Research. "And we need to do much better even than that."
Wilcke is the senior scientist in an IBM group that is trying to develop a battery that can do much better–more than seven times better, he hopes, or 800 watt-hours per pound. This would mean that a 125-pound battery would be competing with a 100-pound full gas tank.
The trick is to make use of something light and easily available: air. IBM and others, including carmakers like Toyota and the tiny 20-year-old PolyPlus of Berkeley, Calif., are working on what are known as metal-air batteries. One electrode is a metal (lithium is the most promising), but the other is air. This type of battery would be lighter for the simple fact that it doesn’t have to carry around one of its electrodes. The concept is, says Wilcke, a lot like burning gasoline, which is a dense energy source precisely because the oxygen it marries doesn’t have to be schlepped around.
The benefits of metal-air batteries have been known for decades, and zinc-air batteries are made by the millions to power small devices like hearing aids. But no one has figured out how to make them bigger and rechargeable–that’s why this is still a bit of a science project. Hope now rests with improvements in materials science, computer modeling and techniques used to observe the behavior of materials at the atomic scale.
The goal is a car battery that can push a family of four 500 miles down the road. IBM calls its program the Battery 500 Project. A bill introduced in the Senate recently would pay a $10 million prize to the developer of a commercially viable electrical car battery that can go 500 miles on a charge.
Most batteries are packaged with both the positive electrode (called the cathode during discharge) and the negative electrode (the anode). For a lithium-ion battery the anode, often made of graphite, stores lithium ions when charged. The battery also includes a cathode made from some mixture of lithium and cobalt, iron, oxygen or phosphorus that collects the ions the way a parking garage stores cars. That’s the problem: "The weight of the cars is much less than the weight of the building," says Wilcke. "The useful ions are dwarfed by the cathode material."
In a lithium-air battery the anode is pure lithium, the lightest metal in the periodic table, and almost all is used to produce power. The cathode, instead of some heavy metal mixture, is air. Lithium, extremely reactive, would meet with air and react with the oxygen at a lightweight porous carbon structure. It would create lithium peroxide and release two electrons that are diverted to a circuit to provide electric power.
Whether recharging can happen at all is a question. Wilcke says that his group, using a technique called differential electrochemical mass spectrometry, has proved that recharging is happening by detecting the creation of oxygen from lithium peroxide.
Another difficulty is that the lithium needs to be kept away from water, and air contains water vapor. PolyPlus, a company founded by Lawrence Berkeley National Laboratory scientists, thinks it has found an answer. It’s a thin ceramic membrane that envelops the lithium and allows lithium ions to pass through but not water molecules. "I don’t see how anyone’s going to commercialize lithium-air without using our technology," says Steven Visco, a founder and chief technology officer of PolyPlus.
Wilcke says that after a year of work IBM’s project is on track to produce a laboratory-size rechargeable battery by 2012 and an electric-vehicle-size demo battery by mid-decade. If you take only short trips, buy a Leaf while you’re waiting.
By Kevin Bullis, www.technologyreview.com