Ultra-High-Power Lithium-Ion Batteries
A lithium-ion battery electrode described this week in the journal Nature can deliver electricity several times faster than other such batteries. It could be particularly useful where rapid power bursts are needed, such as for laser weapons or hybrid race cars.
Test batteries based on the new electrode–developed by Gerbrand Ceder, a professor of materials science at MIT–can be discharged in 10 seconds. In comparison, the best high-power lithium-ion batteries today discharge in a minute and a half, and conventional lithium-ion batteries, such as those in laptops, can take hours to discharge. The new high rate, the researchers calculate, would allow a one-liter battery based on the material to deliver 25,000 watts, or enough power for about 20 vacuum cleaners.
This level of power output would put these batteries on par with ultracapacitors, gadgets that can rapidly discharge power but can’t carry much energy for their size, says John Miller, a vice president for systems and applications at Maxwell Technologies, a manufacturer of ultracapacitors, who wasn’t involved in the research. The new batteries would store nearly 10 times as much energy as an ultracapacitor of the same size. The combination of small size and extreme power could make the batteries particularly useful for race cars, he says. (Starting this year, new Formula One racing rules will allow race cars to store energy from braking to deliver very brief jolts of acceleration.)
To improve the batteries, the researchers modified an electrode material called lithium iron phosphate to allow electrons and ions to move in and out of it much more quickly. The advance is based on computer models that Ceder developed in 2004. The models suggested a way to improve conductivity by directing lithium ions toward particular faces of crystals within the material.
To exploit this, Ceder included extra lithium and phosphorus. This helps form a layer of lithium diphosphate, a material known for its high lithium-ion conductivity. He says that ions encountering the material are quickly shuttled to faces that can pull them in, allowing for very fast discharging.
The fast-discharging materials may also recharge quickly, raising the possibility of cell phones that charge in seconds, Ceder says, but this would require expensive chargers. Ric Fulop, vice president of business development at A123 Systems, a battery maker based in Watertown, MA, that has licensed Ceder’s new material, says that it could be useful for hybrids or for delivering the power needed for laser weapons. (Fulop notes that A123 is not developing batteries for the latter application.)
Other researchers have already modified lithium iron phosphate to achieve power levels high enough for power tools and for most hybrid vehicles. Indeed, iron phosphate batteries are already being sold by more than one battery maker for such applications. Ultimately, the energy capacity of lithium iron phosphate is lower than that of other lithium-ion battery materials, making Ceder’s advance of limited value, says Jeff Dahn, a professor of physics at Dalhousie University, in Halifax, Nova Scotia. This battery is good for acceleration, but not as much for long range. “A real breakthrough … would be a new positive electrode material with quantum-leap performance specs” in energy storage, Dahn says.
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