The Virtual Battery

One way to improve the performance of a lithium battery is to try combining the lithium with other elements, as Yet-Ming Chiang did to create A123's batteries: he doped nanoparticles of the electrode material lithium iron phosphate with metal to create a material that charged and discharged 10 times as fast as lithium-ion batteries then on the market. But such phenomenal success with that approach is rare, says Gerbrand Ceder. "Cumulatively, tens of thousands of lithium compounds have been made," he says. "Out of that, there have been three or four interesting ones."

To improve the odds, Ceder tests out battery materials in a computer model before turning to the lab bench. "The value of virtual experimenting is that it tells you what is possible and what is not possible," he says. "Some argue it is too idealized, but that is its strength as well." A scientist using trial and error might keep trying something that is actually impossible, mistakenly assuming that faulty technique is the problem. Conversely, a shortcoming in an experimental setup might cause a potentially useful material to be rejected as hopeless. Computer modeling prevents such scenarios from holding up experimental progress.

This spring, after 14 years of systematically designing and applying methods for predicting properties such as the voltage of a battery material and the speed at which lithium ions will move through it, Ceder's group demonstrated on the bench a battery material that charges and discharges very fast. The process used to produce it is unusual. Ceder overlaid lithium phosphate on a nanostructured layer of lithium iron phosphate to create an electrode whose surface conducts lithium ions rapidly. Although he's still working on improving the material's energy density, the batteries he made can discharge in 10 seconds, putting them on a par with ultracapacitors. "We set out for excessive charge rates as proof of concept," he says. "Computation shows us that some of the materials people didn't think would work will work. It allows us to see the potential of things."

Big Energy in Small Packages

Chiang, meanwhile, is moving on from his success with A123's lithium batteries. "The high-power lithium-ion battery is a reality and is maturing at a fast rate," he says. "Now we understand power. Going forward, the bigger challenge is getting higher energy density."

Higher energy density is important for tiny batteries like the ones that power pacemakers and other implantable medical devices; the more total energy they can hold, the longer they last and the less frequently they need to be replaced. But achieving it is challenging, because the smaller a battery gets, the less energy per unit volume it can store. "Extrapolation of the existing technology will never get you there," says Chiang.

Seeking energy-dense batteries for microdevices, the U.S. Defense Advanced Research Projects Agency recently put out a call for a battery 10 cubic millimeters in volume with an energy density comparable to that of early lithium-ion batteries: 200 watt-hours per liter. "Getting a density comparable to that of today's lithium-ion technology in a microbattery is extremely difficult," says Chiang. Part of the problem is that as size decreases, the packaging and electrical connections account for an increasingly large portion of the battery. By redesigning the electrodes and packaging, however, he has made a five-cubic-millimeter battery that holds 650 watt-hours per liter--half the size and more than three times the energy density that the military sought.

Chiang is now applying those designs to bigger batteries. "We're using microbatteries as a way to test high-energy-density design concepts that apply to larger-scale storage," he says. One application for this research would be in plug-in or hybrid electric vehicles. The more energy their batteries can store, the farther they can travel without needing to recharge or dip into the gas tank.

That's not the only environmental benefit that might flow from developing batteries with greater energy density. The smaller a battery that stores a given amount of energy, the cheaper it will be. The cheaper the technology, the more of the batteries utility companies will be able to buy and use to incorporate renewable energy into the electrical grid. "If you need less battery for a given application, you can drive down the cost, which means wider adoption of these technologies," Chiang says. And that means we can at last start relying on green power sources to meet a significant fraction of our energy needs.