Safety of Li-Ion Batteries Improved Using Shear-Thickening Electrolyte

As revolutionary as electric vehicles are, they are still susceptible to an Achilles heel — the very source that provides them power.

One of the typical types of batteries fitted in electric vehicles, lithium-ion batteries (Li-ion batteries) is vulnerable to catching on fire or bursting because of a crash or any other major impact applied onto the vehicle. The impact causes the internal short-circuit of electrodes. The small fire can spread all through the battery and to other areas of the car through "thermal runaway."

"Although significant efforts have been applied to the thermal management of the battery cells, battery fires and explosions in recent electrical car accidents pose significant concerns in public," said Yu Zhu, Ph.D., associate professor of polymer science at The University of Akron (UA). "In most cases, the battery ignited when it was not operated under normal use, such as through a large external impact, or crash."

Zhu and his team of graduate students in UA's College of Polymer Science and Polymer Engineering are aiming to enhance the safety of Li-ion batteries by developing a shear-thickening electrolyte — a substance that can turn thicker during impact, placed between the battery's anode and cathode that will be impact-resistant, thus not causing a fire or an explosion upon collision. Under usual circumstances, the novel electrolyte stays soft.

The team’s research, led by Zhu's Ph.D. student Kewei Liu, was recently reported in the Journal of Power Sources: "A shear thickening fluid based impact resistant electrolyte for safe Li-ion batteries."

In Li-ion batteries for use in electric vehicles, the cathode and anode are separated by a very soft membrane and a liquid electrolyte. Simply replacing a liquid electrolyte with its solid counterpart is still a challenging task because both electrodes are porous and they need liquid to fill pores and make contact. Our idea is you can still use a liquid-like electrolyte under a normal situation, but with a liquid that can improve its own mechanical strengths under impact. So, we developed a shear-thickening electrolyte.

Yu Zhu, Ph.D., Associate Professor of Polymer Science, UA

Imagine it to be like a starch and water mixture. When one puts their hand into it and slowly stirs the starch and water, a very little resistance is felt. However, if the stir rate is increased, one will clearly feel much more resistance. Actually, it is possible for a bowling ball to bounce off the surface of a water and cornstarch mixture, which acts like a solid during impact.

A liquid having such properties is referred to as a dilatant, a type of non-Newtonian fluid. If an electrolyte is also a dilatant, it will stop the battery from short-circuiting under external impact. However, creating a shear-thickening electrolyte is a lot more difficult than blending cornstarch and water, as the composition of the electrolyte is complex containing different ions, solvents, and numerous additives.

"In our preliminary research," Zhu said, "we demonstrated that a modified low-cost glass fiber filler can produce the shear-thickening electrolyte we're looking for, which is compatible with commercial Li-ion batteries and shows improved impact resistance."

Videos from Zhu's team show the bullet with varying speeds impacting the standard liquid electrolyte and shear-thickening electrolyte. The shear-thickening electrolyte absorbed the kinetic energy and decelerated the moving bullet considerably.

Compared to a conventional liquid electrolyte, the shear-thickening electrolyte will not significantly reduce the performance of Li-ion batteries. During an impact, the shear-thickening electrolyte will immediately behave like a solid and generate larger force to resist external impact because of the shear-thickening effect. This solution is complementary to external thermal management system of the battery pack, which often falls short in response to the abrupt impact.

Yu Zhu, Ph.D., Associate Professor of Polymer Science, UA

Zhu said the research on enhancing of Li-ion batteries is quite new, particularly for the use in electric vehicles. He continued that shear-thickening electrolytes can have other novel applications, such as in bulletproof energy storage devices.

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