How a polymer holds the key to safer Li-ion batteries

“When a battery fails, it fails very quickly, so it can go from being completely intact to being engulfed in flames and completely destroyed within a couple of seconds,” Martin Pham, a PhD student at UCL working in improving the safety and performance of lithium-ion batteries, explains.

Most cases of thermal runaway originate from an internal short circuit. That short generates an increasing amount of heat that can trigger a failure and spark fires. The temperature inside the battery can reach 800 degrees Celsius.

With the aim to study the thermal runaway in batteries, scientists from UCL, NREL, NASA, and the ESRF, tested a kind of rechargeable lithium-ion batteries called “18650 cells”. These are commercially ready and could be used in electric vehicles and in aerospace applications. They tested regular ones and another type with a unique feature: a polymer layer inserted in the current collector.

Current collectors in batteries conduct electricity to and from the negative and positive terminals. They are typically made of either aluminum or copper. The new current collector is composed of a layer of aluminium, then plastic, then aluminium, like an equivalent of an “inverted material used for crisp packets” (which is made of a layer of plastic, then aluminium, then plastic).

The manufacturers of this new structure initially conceived it as a way to cut the amount of metal used by 40%. This would make production cheaper for companies and would make collectors more robust in tensile stress. However, when the team tested them in the lab, they were in for a surprise.

“We did experiments in a lab by checking what happened when we perforated the battery with a nail”, says Donal Finegan, a scientist at NREL, “Driving a nail into a battery aims to capture the mechanical abuse a battery might experience in the case of an electric vehicle getting into a serious accident”.

The experiments showed that there was no short circuit after perforation. “It was actually very impressive, as the cell was still reading full voltage after being punctured with a nail, which is unprecedented”, he adds.

The researchers then went to the ESRF’s ID19 beamline, where they wanted to see why the nail did not trigger a short circuit and consequently, thermal runaway. “ID19 is the best beamline in the world for this kind of research, in terms of quality and frame rate”, adds Paul Shearing, Professor of Chemical Engineering at UCL.

The team used high-speed synchrotron X-ray radiography to study the batteries with standard collectors and with the polymer one during nail-penetration testing. This, combined with pre- and post-mortem X-ray computed tomography, provided the scientists with insights into the function of the current collectors. “Acquiring high-speed movies of battery failure pushes the limits but also outlines where we are unique: operando slow-motion records with hard X-rays of engineered devices”, explains Alexander Rack, scientist in charge of ID19.

The data showed that in the new batteries the polymer isolates the short circuit. “With normal batteries, we see the tearing of the collector, whereas with the new ones the polymer is more flexible and bends with the nail for a while before tearing.  When a short circuit occurs, the polymer current collector retreats from the hot region and isolates from the short circuit, preventing thermal runaway. Conceptually, when a plastic film is placed near a flame, the material tends to retreat”, explains Finegan.

The next step for the team is to check how robust the batteries are in other scenarios like against thermal abuse and against short circuiting it from inside.

Reference:

Pham et al, Cell Reports Physical Sciences (2021).

Text by Montserrat Capellas Espuny.

Video by Montserrat Capellas Espuny and Mark McGee.