The safety aspects of lithium-ion batteries – will my laptop, car, or iPhone catch fire? – are worthy to ponder in this day of lithium-ion battery popularity. The production and use of lithium-ion batteries for consumer electronics and the transportation sector, in particular, are on a significant rise thanks to their attractively high energy density. Although instances of these batteries catching fire are rare, they can happen, and for this reason scientists are researching fail-proof ways to shut down batteries in the presence of dangerous triggers.
Thermal management within lithium-ion batteries is critical. The individual cells, ranging from one to hundreds in a battery depending on the application, contain the necessary ingredients for a fire – a combustible electrolyte in proximity to an oxidant and reductant. A separator physically prevents the anode (negative electrode) and cathode (positive electrode) from touching and internally short-circuiting the cell. The separator also must house the electrolyte for lithium-ion transport. An additional functionality of the separator is to melt and "shut down" the battery by stopping current flow completely in the event of an abnormal temperature increase, which can lead to fire. Although traditional polyethylene-based separators melt at about 135 oC, successful shutdown is not guaranteed.
Researchers at the Center for Electrical Energy Storage (CEES): Tailored Interfaces are developing a cell additive that melts and halts all ion and electron transport within batteries. "The research is targeted to make lithium-ion batteries safer for the general public," says Scott White, professor of Aerospace Engineering at the University of Illinois, Urbana-Champaign and CEES lead researcher. "It's also a cool example of a development in one field being applied to another field. Original 'self-healing,' capsule-based research was supported by the Department of Defense and here we're transitioning and using the same concepts in Department of Energy battery research."
In their article, which appeared in the May 2012 issue of Advanced Energy Materials (with cover page), a team of CEES researchers described the synthesis of polyethylene and paraffin wax microspheres and their functionality. The addition of a small amount of microspheres did not impede the cell's normal performance at moderate temperatures. However, when the temperature of a cell was raised (110 oC for polyethylene, 65 oC for paraffin), the microspheres melted and effectively shut down cell operation, preventing thermal runaway.
Based on this success and borrowing from a background in "smart" materials research, scientists in CEES are looking for ways to shut down batteries using various triggers, such as cell voltage, resistance change, current flow, or reduction/oxidation event. "I'm excited about the possibilities for microcapsules," says Marta Baginska, a graduate student working with CEES and lead author of the Advanced Energy Materials paper. "With thermoresponsive microcapsule incorporation, we have proof–of-concept that microcapsules can function usefully in lithium-ion batteries. Now we can think about multifunctionality."
