The ability to make batteries lighter, cheaper and longer lasting is crucial to the development and adoption of next-generation electronics—from mobile phones and tablets to electric cars. Advances in lithium ion batteries have helped slim down smart phones and put cars like the Nissan Leaf and Chevy Volt on the road. Yet lithium can also be volatile and has been accused of causing electrical fires in gadgets and even Volt test vehicles. Investigating the failure of a lithium ion or any other battery is difficult because any post-mortem requires opening, and thus destroying, the battery to see inside. A new technique could sidestep that problem.
A team of researchers from Cambridge University in England, New York University (NYU) and Stony Brook University in New York say they have developed a way to use magnetic resonance imaging (MRI) to inspect batteries noninvasively. The researchers focused on better understanding how and why lithium deposits build up on electrodes and elsewhere after charging.
Normally it's best to avoid having any metal near an MRI, for fear of turning that metal into a projectile. (The MRI's powerful magnetic field will strongly attract any nearby metallic objects.*) In addition, a metal's conducting surfaces block radio frequency fields, so an MRI would not reveal much information about what's deep inside a metallic object.
Not a problem, the researchers reported Sunday in Nature Materials. (Scientific American is part of Nature Publishing Group.) An MRI's radio waves may not penetrate the metals in a battery, but they can scan and measure features on the battery's surface. These measurements can be used to recreate two- or three-dimensional digital images of the battery, including any lithium deposits that may have gathered on the battery's electrodes. Such deposits can contribute to overheating, battery failure and possibly even a fire or explosion, according to the researchers.
Researchers at Argonne National Laboratory in Illinois used nuclear magnetic resonance (NMR) spectroscopy in 2001 to first study the movement of lithium ions within a battery from the outside (pdf), but this work "does not offer the level of detail provided by our technique," says Alexej Jerschow, an NYU chemistry professor who contributed to the research. "For example, we were able to obtain 3-D images of a battery before and after charging."
MRI also proved more accurate than NMR, which doesn't provide detailed information about what's happening inside the battery. Scanning electron microscopy, another tool that has been used to study batteries, requires cutting a battery open. "Not only does one destroy the battery in the process, but also exposure to air alters the surfaces, so this technique does not really study the electrodes in their working condition," Jerschow says. "MRI is nondestructive, so you can take a functioning battery and take an image of it, much like one can take an MRI of the human body."
Jerschow and his colleagues are continuing to refine their approach to improve image resolution and reduce the amount of time it takes to obtain an image.
*Clarification (2/15/12): Neither the battery used during testing nor lithium itself is magnetic.
Image: MRI in the pristine (uncharged) state and after passing current. Courtesy of S. Chandrashekar, Nicole M. Trease, Hee Jung Chang, Lin-Shu Du, Clare P. Grey, Alexej Jerschow, and Nature Materials