IN THE AIR: In this screen capture from an IBM computational simulation, scientists study the simulations of the interaction of an organic solvent electrolyte (propylene carbonate) with lithium ions (white) and oxygen near a surface of Lithium-peroxide (the planar structure near the bottom of the screen). Image: Courtesy of IBM Research-Zurich
Researchers predict a new type of lithium battery under development could give an electric car enough juice to travel a whopping 800 kilometers before it needs to be plugged in again—about 10 times the energy that today's lithium ion batteries supply. It is a tantalizing prospect—a lighter, longer-lasting, air-breathing power source for the next generation of vehicles—if only someone could build a working model. Several roadblocks stand between these lithium–air batteries and the open road, however, primarily in finding electrodes and electrolytes that are stable enough for rechargeable battery chemistry.
IBM plans to take lithium–air batteries out of neutral by building a working prototype by the end of next year. The company announced Friday it has stepped up development efforts by adding two Japanese technology firms—chemical manufacturer Asahi Kasei Corp. and electrolyte maker Central Glass—to the IBM Battery 500 Project, a coalition IBM established in 2009 to accelerate the switch from gas to electric-powered vehicles among carmakers and their customers.
The lithium ion batteries used in today's 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.
Most fully charged lithium ion car batteries today will take an electric vehicle only 160 kilometers before petering out. (Nissan says its all-electric Leaf has a range of about 175 kilometers.) Plug-in electric vehicles such as the Chevy Volt have an even more limited range of up to 80 kilometers before its gas-powered motor must kick in.
The specifics of how lithium–air batteries will operate is still being determined, but the general principal is that, instead of using heavy metal oxides, oxygen would be collected from the air while an electric vehicle is in motion. The oxygen molecules react with lithium ions and electrons on the surface of a porous carbon 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 to the atmosphere. The anode, meanwhile, is made of lithium, the lightest metal. Without the need for heavy metals the battery would be several times lighter while being able to store more energy than its lithium ion cousin.
Although this works in a computer simulation, lithium–air batteries have specific requirements in practice that scientists are still trying to meet. "We found out pretty early in the project that the electrolytes currently used in lithium ion batteries do not work in lithium–air batteries because the oxygen in a lithium–air battery attacks and destroys the electrolyte," rendering it unable to conduct a charge, says Winfried Wilcke, Battery 500 Project's principle investigator. One solution, he adds, would be to use two different electrolytes, one for the cathode and a second for the anode, with a membrane in between to keep them from mixing.
That is where IBM's new partners come in. Asahi Kasei will develop a membrane the batteries can use to separate their electrolytes while allowing lithium ions to pass from the anode to the cathode. Central Glass will create a new class of electrolytes and high-performance additives specifically designed to improve lithium–air battery performance.
Another way to gauge the lithium–air battery's potential is to compare it to other batteries in terms of specific energy, or how much energy it produces in relation to its size. Whereas a conventional lead–acid car battery will produce up to 40 watt-hours per kilogram, a lithium ion battery maxes out at 250 watt-hours per kilogram. A lithium–air battery's potential far exceeds 1,400 watt-hours per kilogram. "I'm shooting for 1,000 watt-hours per kilogram, but we won't have real number for the energy density until we've built a larger prototype," Wilcke says.
The Battery 500 Project is not the only game in town when it comes to developing lithium–air batteries. Researchers at the Massachusetts Institute of Technology are developing a lithium–air battery with carbon nanofiber electrodes. And, Yangchuan Xing, an associate professor of chemical and biological engineering at the Missouri University of Science and Technology, received an Advanced Research Projects Agency–Energy (ARPA–E) award for $1.2 million last year to develop lithium–air batteries.
Wilcke estimates the lithium–air batteries might be ready for production by 2020 at the earliest, "if we don't find any show-stopping technology along the way." He adds: "The only thing I'm certain of is that it won't happen this decade."