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Lithium-metal failure mechanism identified

Thu, 07/29/2021 - 13:12 -- Vic
Lithium metal failure mechanism identified

Scientists at in the US have found evidence of an internal mechanism that causes lithium-metal battery failure.

Lithium-metal cells hold 50% more energy than lithium-ion cells, but higher failure rates and safety problems have hindered commercialisation efforts— with direct evidence of the reasons for the failure never confirmed.

The team consisted of researchers from Sandia National Laboratories, working with Thermo Fisher Scientific, the University of Oregon and Lawrence Berkeley National Laboratory.

The first nano-scale images ever taken inside intact, lithium-metal coin batteries challenge prevailing theories.

The images were been published in the journal ACS Energy Letters.

When the team reviewed images of the batteries' insides, they expected to find needle-shaped deposits of lithium spanning the battery. 

Most battery researchers think that a lithium spike forms after repetitive cycling and that it punches through a plastic separator between the anode and the cathode, forming a bridge that causes a short. But lithium is a soft metal, so scientists have not understood how it could get through the separator.

Harrison's team found a surprising second culprit: a hard build-up formed as a by-product of the battery's internal chemical reactions.

Every time the battery recharged, the by-product, called solid electrolyte interphase, grew. 

Capping the lithium, it tore holes in the separator, creating openings for metal deposits to spread and form a short.

Together, the lithium deposits and the by-product were much more destructive than previously believed, acting less like a needle and more like a snowplough. 

Determining cause-of-death for a coin battery is surprisingly difficult because of its stainless-steel casing.

The shell limits what diagnostics, like X-rays, can see from the outside, while removing parts of the cell for analysis rips apart the battery's layers and distorts whatever evidence might be inside. 

Katie Jungjohann, a Sandia nano-scale-imaging scientist at the Center for Integrated Nanotechnologies and her collaborators used a microscope that has a laser to mill through a battery's outer casing.

They paired it with a sample holder that keeps the cell's liquid electrolyte frozen at temperatures between -148 and -184°F (-100 and -120°C). 

The laser creates an opening just large enough for a narrow electron beam to enter and bounce back onto a detector, delivering a high-resolution image of the battery's internal cross section with enough detail to distinguish the different materials.

The original demonstration instrument, which was the only such tool in the United States at the time, was built and still resides at a Thermo Fisher Scientific laboratory in Oregon. 

An updated duplicate now resides at Sandia and will be used broadly across Sandia to help solve many materials and failure-analysis problems.

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