Today’s battery technology is adequate for electric vehicles with a range of more than 200 miles, but the batteries are still very expensive and require elaborate safety mechanisms. There are also concerns that they won’t last long enough to be attractive to most consumers.
But current research will double energy-storage capacity while also increasing the lifetime of batteries, improving safety, and cutting costs more than enough to make electric vehicles and plug-in hybrids practical for the mass market. At least these were the predictions of researchers presenting their latest work at the Materials Research Society (MRS) meeting in Boston this week. And although many significant challenges remain, an experimental type of rechargeable battery that’s like a fuel cell could increase battery storage that much more.
Stanley Whittingham, inventor of the first commercial lithium-ion battery and professor of chemistry, materials science, and engineering at the State University of New York, at Binghamton, says current research should make electric vehicles practical–with the following caveat: they’ll probably be used for trips of less than 100 miles. Those who want 300-to-400-mile ranges typical of gasoline-powered vehicles will need to turn to plug-in hybrids: vehicles much like today’s gas-electric hybrids, but with a much larger battery pack that makes it possible to go longer on electric power, thereby saving gas. These batteries could be partly charged by an onboard gas engine, but also by electricity from a wall socket.
Whittingham says that while he expects battery capacity to double, it’s not going to get much better than that. The real advances in batteries, he says, won’t be in energy capacity, but in safety, longevity, and cost. If electric vehicles are to be widespread, one of the most important goals of battery research must be to replace the cobalt now used in the lithium-ion batteries found in cell phones and laptops. “There’s just not enough [cobalt] in the world,” says Whittingham, who is working on mixed-metal electrodes, which require little to no cobalt.
One promising new type of battery, which actually has lower storage capacity than today’s lithium-ion batteries, could nevertheless prove a boon to plug-in hybrids. Lithium iron phosphate batteries use iron, a very cheap metal, instead of cobalt, and they have an inherently safe chemistry (see “Safer Lithium-Ion Batteries”). What’s more, they operate at a lower voltage that will extend the life of the electrolyte, and therefore the battery.
Yet-Ming Chiang, a MIT materials scientist, is developing even better versions of these batteries. Typically when designing batteries engineers have to choose between high-power batteries, such as those needed for power tools and hybrids, which deliver intense bursts of power, and high-energy batteries that pack less of a punch, but can deliver more total energy per charge. According to computer models created by Chiang’s lab and presented at the MRS meeting, it may be possible to remove this trade-off by producing nanostructured electrodes made by combining two different types of particles in a specific arrangement in the electrode. This could as much as double energy capacity for high-power applications, without the need to develop new materials, Chiang says.
A researcher at the MRS meeting described another experimental way of creating new electrode structures–a way that could increase energy capacity over existing batteries by four times or more. Peter Bruce, professor of chemistry at the University of St. Andrews, in Scotland, is reviving interest in a type of battery that is something like a fuel cell. This battery has been widely used in the past, but making it rechargeable has proved difficult. Ordinarily, a battery contains all the materials needed to carry out its current-creating chemical reactions. But in this design, one of the reactants, oxygen, can be harvested from the air. As in a fuel cell, in which hydrogen ions combine with oxygen to form water, lithium ions in this battery combine with oxygen to form lithium peroxide. Using oxygen makes it possible to eliminate many of the materials normally included in a battery, drastically cutting its weight. Based on his experiments, Bruce says that such batteries could store as much as 600 to 700 milliamp hours per gram (more than four times that of batteries today) while maintaining the ability to be charged and discharged for many cycles.
So far, Bruce has conducted his experiments using pure oxygen. A working battery would need to be equipped with a membrane, which could be a material similar to Gore-Tex that would seal out both water and carbon dioxide, he says. It might also need a valve to shut off the supply of oxygen to keep reactions from occurring when no current is needed.
Perhaps a bigger problem is the fact that the batteries lose about half of their energy to heat as they are discharged, Whittingham says. This creates a big heat-management issue, and it cuts into the energy-saving motivation for driving hybrids or electric vehicles. “If [Bruce] can be successful, it would be great,” he says. But even without such dramatic gains in energy capacity, current research could make batteries much more practical. “I expect the auto companies will be happy with two times [higher capacity] if it will last 10 years,” Whittingham says.
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