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Crash tests shed light on improving Li-ion used in EVs

06 Jun 2013

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Wierzbicki said that in order to know how a battery will deform in a crash, it?s important to "start from the smallest building block." In the case of lithium-ion battery packs, that building block is the "jellyroll", a single battery?s interior that is made of alternating anode and cathode layers, and a separating layer, all rolled up and encased in a protective tube of aluminium or steel.

The batteries work when lithium ions travel across each separating layer, creating a current. But when the separator is compromised by the forces generated by an impact, a battery can short-circuit, and possibly catch fire.

To test a battery's resilience, the team crushed batteries between metal plates in various orientations, and used metal spheres and rods to dent and deform individual cells. The tests were designed to mimic certain repercussions of a crash: batteries crushing each other, or parts of a battery pack piercing the individual batteries inside.

To prevent "catastrophic thermal runaway," the researchers ran each test on batteries that were 90 per cent discharged; the remaining 10 per cent charge still allowed measurement of sudden drops in voltage. In addition to voltage, Wierzbicki and Sahraei monitored battery temperature and structural deformation after impact.

Keeping ahead of thermal runaway
The researchers used their data to develop a computational model for how a single cylindrical lithium-ion battery deforms under various crash scenarios. The model, which the researchers validated with further experimental tests, accurately predicted battery indentation under a certain load or force.

"With the knowledge of how a battery reacts in a crash, you can design your battery pack to resist damage," Sahraei said. "When you have a better understanding of how the cells react, you may find you could reduce the weight of the battery pack by reducing the excessive protective structures around it."

Sahraei, Wierzbicki and their colleagues are continuing to study the physical limits of cylindrical lithium-ion batteries, as well as the pouch and prismatic batteries that are used to power vehicles like the Chevrolet Volt. Ultimately, the group hopes to scale up experiments to test the integrity of whole battery packs, and incorporate battery models into whole-vehicle simulations. To further explore new and safer designs, Wierzbicki is forming a battery consortium that will include lithium-ion battery manufacturers and car companies.

Per Onnerud, chief technology officer at energy-storage start-up, Cloteam, said the safety of electric vehicles' batteries will become a more pressing issue in the near future: To reduce carbon dioxide emissions, federal officials hope to dramatically increase sales of plug-in electric vehicles by 2020.

"In order for us as a society to realise these targets, the systems have to be intrinsically safe on the lowest-level component," said Onnerud, who did not participate in the research. "This is an important part of driving cost down. It all starts with the design."

While it's virtually impossible to design lithium-ion batteries to be risk-free, Wierzbicki said that models like his can help to reduce catastrophic outcomes in accidents involving electric vehicles.

"There?s a certain critical velocity at which bad things happen," Wierzbicki said. "Right now, thermal runaway might occur during a 20mph side collision. We'd like to increase that threshold to maybe 40mph. By doing this, maybe 95 per cent of accidents would be safe from the point of view of a battery exploding. But there will always be some collision—for example, a very fast car hits a tree or a post—and that?s not a survivable accident for people and also for batteries. So you cannot have absolute safety. But we can increase this safety."

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