Lithium-air batteries offer three times the energy density
By Darren Quick
April 5, 2010
Lithium-air battery technology looks to have a big future. With the potential of providing energy densities up to three times that of the conventional lithium-ion batteries found in just about every portable consumer electronics device going around (not to mention the incoming wave of electric vehicles), many companies, including IBM and General Motors are pursuing work on lithium-air batteries. Now researchers at MIT have made a breakthrough that could help make the commercial development of lightweight rechargeable batteries a reality.
Lithium-air (also known as lithium-oxygen) batteries are similar in principle to lithium-ion batteries. However, lithium-air batteries electrochemically couple a lithium anode to atmospheric oxygen through a carbon-based air cathode instead of the heavy conventional compounds found in lithium-ion batteries. This means they are able to have higher energy density because of the lighter cathode and the fact that oxygen is freely available in the environment and doesn’t need to be stored in the battery.
Unfortunately lithium-air batteries haven’t become a commercial reality because there has been a lack of understanding of what kinds of electrode materials could promote the electrochemical reactions that take place in these batteries. Now a new study out of MIT reports that electrodes with gold or platinum as a catalyst show a much higher level of activity and thus a higher efficiency than simple carbon electrodes in these batteries. This new work sets the stage for further research that could lead to even better electrode materials, perhaps alloys of gold and platinum or other metals, or metallic oxides, and to less expensive alternatives.
Lead author of the paper published this week, doctoral student Yi-Chun Lu, explains that the MIT team has developed a method for analyzing the activity of different catalysts in the batteries, and now they can build on this research to study a variety of possible materials. “We’ll look at different materials, and look at the trends,” she says. “Such research could allow us to identify the physical parameters that govern the catalyst activity. Ultimately, we will be able to predict the catalyst behaviors. ”
With more and more portable gadgets finding their way into our daily lives, not to mention automakers turning to rechargeable batteries to power electric cars, the development of lightweight batteries that can deliver lots of energy are seen as an important advance. Even modest increases in a battery’s energy density rating – a measure of the amount of energy that can be delivered for a given weight – offers longer battery life for portable devices and greater range for electric vehicles.
The MIT team admits there are still a number of issues that need to be addressed before lithium-air batteries become a practical commercial product.
Lithium in metallic form, which is used in lithium-air batteries, is highly reactive in the presence of even minuscule amounts of water. This is not an issue in current lithium-ion batteries because carbon-based materials are used for the negative electrode. MIT associate professor of mechanical engineering and materials science, Shao-Horn, says the same battery principle can be applied without the need to use metallic lithium; graphite or some other more stable negative electrode materials could be used instead, she says, leading to a safer system.
But the biggest issue is developing a system that keeps its power through a sufficient number of charging and discharging cycles for it to be useful in vehicles or electronic devices.
Researchers also need to look into details of the chemistry of the charging and discharging processes, to see what compounds are produced and where, and how they react with other compounds in the system. “We’re at the very beginning” of understanding exactly how these reactions occur, Shao-Horn says.
Gholam-Abbas Nazri, a researcher at the GM Research & Development Center in Michigan, calls this research “interesting and important,” and says this addresses a significant bottleneck in the development of this technology: the need find an efficient catalyst. This work is “in the right direction for further understanding of the role of catalysts,” and it “may significantly contribute to the further understanding and future development of lithium-air systems,” he says.
While some companies working on lithium-air batteries have said they see it as a 10-year development program, Shao-Horn says it is too early to predict how long it may take to reach commercialization. “It’s a very promising area, but there are many science and engineering challenges to be overcome,” she says. “If it truly demonstrates two to three times the energy density” of today’s lithium-ion batteries, she says, the likely first applications will be in portable electronics such as computers and cell phones, which are high-value items, and only later would be applied to vehicles once the costs are reduced.
The MIT team’s research appears in the paper, “The Influence of Catalysts on Discharge and Charge Voltages of Rechargeable Li–Oxygen Batteries,” which is published in the journal Electrochemical and Solid-State Letters.
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