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A rechargeable, air-breathing battery that can store up to 10 times the energy of today's conventional lithium ion batteries could be just the breakthrough that makes electric cars practical—if it ever leaves the laboratory. Scientists worldwide are searching for the right combination of materials that could make a working lithium–air battery a reality, not to mention stand up to the rigors of repeated charging and discharging.
Today, however, chemists have unveiled a material that seems to do the trick: gold.
Researchers at the University of Saint Andrews in Scotland are the first to acknowledge that an electric car battery made of gold would not be practical. But by using gold in an experimental battery, they have taken a crucial step on the way to a viable mass-market lithium battery that can power an electric car for hundreds of kilometers between charges.
The Saint Andrews chemists, in a paper published in the journal Science on Friday, describe how an experimental lithium–air battery featuring an organic electrolyte (dimethyl sulfoxide) and a porous gold electrode maintained 95 percent capacity after 100 charge–discharge–recharge cycles.
Scientists are interested in lithium–air batteries because the current lithium ion technology has inherent storage capacity limitations that doom it as a long-term option for electric cars. Lithium ion batteries used in electric vehicles rely on a metal oxide or metal phosphate (typically cobalt, manganese or iron-based materials) cathode as a positive electrode, a carbon-based anode as a negative electrode and an electrolyte to conduct lithium ions from one electrode to the other. When the car is driven, the lithium ions flow from the anode to the cathode through the electrolyte and separator membrane. Charging the battery reverses the direction of ion flow. The limited number of ions that can be stored in these electrodes, however, has prompted researchers to look in new directions.
"Lithium ion batteries are in many ways the best we have right now in terms of energy density, and they'll be with us for quite some time, including in electric vehicles," according to Saint Andrews chemistry professor Peter Bruce. "But we already know that if we can double the energy storage in those batteries, that's going to be the limit of what's possible. Lithium ion batteries won't meet our needs moving forward, hence the interest in looking at alternatives such as lithium–air."
In principle, lithium–air batteries would collect oxygen from the air while the vehicle is in motion. That means they would not need heavy-metal oxides that add weight to the battery yet offer limited storage. The oxygen molecules react with lithium ions and electrons on the surface of a porous gold cathode to form lithium peroxide. This lithium peroxide formation during discharge leads to an electrical current that powers the car's motor. When charging, the reverse reaction takes place—the oxygen is released back into the atmosphere.
Thus far, the cathodes and electrolytes being tested for use in lithium–air batteries decompose and degrade to the point where there is little lithium peroxide formation or only partial formation and decomposition after only a few charge–discharge cycles, Bruce says. He and Saint Andrews colleagues Zhangquan Peng, Stefan Freunberger and Yuhui Chen had been searching for a reaction at the electrode that could be repeated over many cycles.
Given the need to keep battery costs down, the Saint Andrews team suggests that instead of all-gold electrodes, one option might be to coat carbon with gold and observe whether the results are comparable.
"We've demonstrated that the electrochemical reaction that needs to take place in an air battery does work and does seem to be reversible," Bruce says. He and his team turned to gold as a possible electrode material because the metal is a stable substrate for oxygen reduction in nonaqueous (made from a liquid other than water) systems. The use of a porous gold provides voids for incorporation of the solid lithium peroxide, something not available from planar (flat, nonporous) gold. "We don't really know what it is about the nanoporous gold that seems to give us this level of stability, more work needs to be done to determine this," he acknowledges.
Lithium–air battery research is also underway at Argonne National Laboratory, the Massachusetts Institute of Technology, IBM and elsewhere, so stay tuned.