Lithium-Ion Battery Keeps Its Cool
Source: MIT Technology Review, Ucilia Wang (5/26/11)
"A new Li-ion battery holds much more energy than previous versions."
A new kind of lithium-ion battery holds much more energy than previous versions, while still working well at high temperatures. It could prove useful for hybrid and electric cars, where high-density batteries usually come with safety risks. A graphite current collector and imide salt are used in the battery's electrolyte. These materials enable the battery to last longer and withstand higher temperatures. The battery has an energy density of 225-watt hours per kilogram. This falls at the high-end range of laptop batteries, and roughly 50 percent higher than lithium-ion batteries used in electric vehicles.
Lithium-ion batteries are widely used in consumer electronics, but design changes are needed to make sure they work safely in electric or hybrid cars. Carmakers are typically forced to use lower-density batteries, and to use electronics and cooling systems to ensure the battery cells don't run too hot. Liquid cooling and thermal management electronics and software are employed to prevent overheating and other problems.
A cathode material such as lithium-iron phosphate is sometimes used for electric vehicle batteries because it can withstand high temperatures. The trade-off is that it has a relatively low energy density—around 140-watt hours per kilogram.
The battery replaces lithium hexafluorophosphate, one of the components of a lithium-ion battery, with imide salt. Unlike lithium hexafluorophosphate, it does not react with water inside the battery cell—a reaction that significantly degrades the cycle life of a battery. Lithium hexafluorophosphate also starts to decompose at room temperature and loses its effectiveness more significantly when the temperature hits 55 °C. Imide salt doesn't start to decompose at higher temperatures.