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New Separator Film May Advance Li-ion Battery Safety

February 26, 2008

Advances in separator materials may enable the safe use of lithium-ion batteries in electric and hybrid electric vehicles. Current hybrids use NiMH batteries, which lack the long-life needed to power the major systems on hybrid vehicles.

Li-ion batteries are commonly used in smaller devices, including phones, portable music players and laptops, but the technology poses serious safety issues. Li-ion batteries were at the center of several multi-billion dollar recalls that affected laptop manufacturers in 2006 and 2007. Several reports of battery fires - some leading to personal injuries - underscore the dangers inherent in the use of Li-ion technology.

Li-ion batteries rely on porous separator films that prevent the battery’s anode and cathode from reacting spontaneously. Failure in the separator films can cause a catastrophic runaway reaction that leads to an enormous, instantaneous discharge of energy and opens the potential for an explosion and/or fire.

Li-ion batteries operate somewhat differently in laptops and portable electronics than they would in automobiles, primarily because the laptop battery is designed to discharge steadily over time. In the automotive environment, the Li-ion battery would be responsible for storing large amounts of energy that would allow for controlled discharge while at the same time supplying instant power for accessories and systems whose power consumption varies widely during the vehicle’s operation. Given the amount of energy an automotive Li-ion battery must store, the safety risk of its use in an automobile is staggering.

To help mitigate the risk of catastrophic failure in automotive Li-ion batteries, Exxon Mobil recently introduced a new separator film designed specifically for automotive applications. The composite, multilayer separator film offers several different operational characteristics that combine to provide a safer separator for automotive batteries. For example, one layer is specifically designed to intervene on a runaway reaction, and shut the battery down before a catastrophic failure can occur.

One major problem with existing separator films is that a sharp rise in the internal temperature of the battery can induce a heat-related failure in an otherwise working separator film, triggering a runaway reaction. Exxon Mobile’s new film not only shuts the battery down when it reaches an internal temperature of 140° C, the film itself is designed to withstand thermal breakdown until it reaches 190° C. The combined action of the shutdown layer and the higher thermal stability of the film should prevent heat-related film failures altogether. Electrovaya has already announced that it plans to use the new separator film in its all-electric Maya-300 vehicle.

The flip-side of the safety question for automakers is one of liability. GM and Toyota are both racing to put a Li-ion powered hybrid on the market in 2009 or 2010, and both admit that finding answers to the safety questions posed by the use of Li-ion batteries will be a major determinant in their ability to succeed in the hybrid arena. Tackling the issue of separator films, and their performance during a catastrophic event will allow automakers to bring safer, more viable hybrid passenger vehicle programs to market.

Written by Hybrid Car Chat · Filed Under Hybrid Technology 

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