Lithium-Ion Battery Keeps Cool

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.

Leyden Energy uses a graphite current collector and imide salt in the battery's electrolyte. These materials enable the battery to last longer and withstand higher temperatures; Leyden has declined to discuss how it achieved higher energy densities.

The company says the battery has an energy density of 225 watt-hours per kilogram.

Lithium-Ion Battery Keeps Cool
This falls at the high-end range of laptop batteries, and roughly 50 percent higher than lithium-ion batteries used in electric vehicles. The four-year-old startup expects to see its cells used by a tablet PC maker later this year, says Leyden CEO Aakar Patel.

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.

Tesla Motors, for example, uses batteries that are similar to those found in laptops to power its Roadster sports car. The company uses liquid cooling and thermal management electronics and software 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.

Leyden focuses on the electrolyte and current collector, because the two affect the performance of the cathode and anode and help to determine the longevity and stability of a battery, says Patel. The results are batteries that work just as well in temperatures up to 60 °C, he adds.

Lithium-Ion Battery Keeps Cool
Leyden's 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.

But imide salt can cause trouble by corroding the aluminum current collector that is typically found in a battery cell. Graphite makes a good substitute because it is immune to this assault. "The key advance for Leyden is not the electrolyte.

Their magic is, they are not using aluminum as the current collector," says Venkat Srinivasan, a scientist at the Lawrence Berkeley National Laboratory who has seen the company's technology. "This change allowed them to change the electrolyte."

Patel says the batteries could use air cooling rather than liquid cooling, which would make them cheaper and lighter. The company is also developing battery management electronics and software to prevent overcharging or undercharging, problems that can compromise battery life, he says. Leyden recently received a $2.96 million grant from California on a project to produce 10 car battery packs per month.

Lithium-Ion Battery Keeps Cool
"Leyden's cell technology presents a very real advantage for a vehicle battery pack in terms of thermal management, life-cycle performance, and energy density," says Brian Wismann, director of product development at Brammo, a company that is developing electric motorcycles and is interested in using Leyden's technology.

Leyden's new technology will first show up in laptop battery. The company says these batteries will achieve over 1,000 charge and discharge cycles, compared with 300 cycles from a typical laptop battery, and will have a three-year warranty instead of the usual one-year warranty.
Source: Technologyreview

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