Li-ion batteries blow up because they breed nanowire crystals

Boffins think 'dendrites' are the inevitable result of re-charges, which is awkward

Video For the first time a group of scientists has captured close-up images of mysterious finger-like growths known as dendrites that can lead to short circuits and fires in the lithium-ion batteries that power hordes of smartphones, laptops and other gadgetries.

By using cryo-electron microscopy (cryo-EM), researchers from Stanford University and the US National Accelerator Laboratory have revealed that the lithium metal dendrites are long six-sided crystals. Their study was published in Science on Thursday.

The growths can spark battery fires by causing a short circuit if they pierce through the separator, a membrane placed in between the cathode and anode. The resulting surge of current can lead to thermal runaway, whereby the increased heat coupled with flammable electrolyte fluid spark fires.

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Yazhang Li, lead author of the paper and a graduate student at Stanford University told The Register that the dendrites formed during the charging process of a lithium-ion battery.

“There are some proposed theories on how dendrites form but the exact mechanism is still unclear. This is because there has not yet been a technique that can study the native state of these dendrites at high resolution without damaging them.

“Our work using cryo-EM introduces a new way to study these delicate structures without damaging them and we have found that these dendrites grow as single-crystalline, faceted nanowire structures,” he explained.

The long crystals can range from about 100 nanometers to two micrometers in diameter, depending on the charging current.

The researchers examined thousands of lithium metal dendrites forming in different electrolytes and also the solid electrolyte interphase, a coating that forms on the metal electrodes.

Cryo-EM, a technique that won this year's Nobel Prize for Chemistry, was vital to studying the dendrites in more detail. Traditional transmission electron (TEM) microscopy techniques which involve aiming a beam of electrons through a sample destroyed the dendrites.

“TEM sample preparation is carried out in air, but lithium metal corrodes very quickly in air,” Li said. “Every time we tried to view lithium metal at high magnification with an electron microscope the electrons would drill holes in the dendrite or even melt it altogether.”

“It’s like focusing sunlight onto a leaf with a magnifying glass. But if you cool the leaf at the same time you focus the light on it, the heat will be dissipated and the leaf will be unharmed. That’s what we do with cryo-EM. When it comes to imaging these battery materials, the difference is very stark.”

Cryo-EM involves freezing the dendrites in liquid nitrogen before thinly slicing them to view under a microscope. By flash-freezing the batteries at different points of its charge and discharge cycle, scientists can see how it changes over time.

Li hopes that continuing the Cryo-EM experiments will allow scientists to make safer batteries.

“With our new technique using Cryo-EM, we get a close-up view of the nanostructure of lithium dendrites and the surface corrosion layer that forms during battery operation. We have found that these nanostructures are distinct under different conditions and by correlating these changes, we can provide key insights to design safer and longer-lasting batteries,” he concluded. ®

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