Autonomic Shutdown and Safe Batteries
We are exploring the application of autonomic materials concepts to battery technology to improve their safety for use in consumer products and transportation. Li-ion batteries are vital energy storage devices due to their high specific energy density, lack of memory effect, and long cycle life. Li-ion cells are prone to failure due to low thermal tolerance of separator materials and susceptibility to short circuits which result in dangerous fires and explosions. We are developing new strategies for improving battery safety through autonomic shutdown and thermally triggered release of flame retardants.
Autonomic shutdown:
Our approach to autonomic shutdown is shown schematically in the image below. We incorporate thermally responsive capsules onto battery anodes during battery manufacturing. The microcapsules are triggered to undergo a melt transition at a critical (design) temperature producing a conformal coating that infuses the battery electrode and creating an ion-blocking layer, effectively shutting down the battery and preventing thermal runaway.
Autonomic shutdown of batteries using thermally responsive microcapsules. Capsules are deposited onto the battery anode and undergo a thermal (melt) transition at a critical temperature, creating a conformal, ion-blocking polymer layer on the anode shutting down the battery from normal operation and preventing thermal runaway.
Cross-sectional and overhead view (SEM) of battery anodes. Left column of images shows a normal graphitic anode. Middle column of images shows an anode coated with polyethylene (PE) microcapsules (10-50 μm). Right column of images shows the same electrode after exposure to 110°C to simulate overheating conditions. The PE capsules melt and infuse the electrode creating a conformal, ion-blocking polymer film on the anode. Baginska, M.; Blaiszik, B. J.; Merriman, R. J.; Sottos, N. R.; Moore, J. S.; White, S. R., Autonomic Shutdown of Lithium-Ion Batteries Using Thermoresponsive Microspheres. Advanced Energy Materials 2012, 2, (5), 583-590.
Encapsulated additives:
Additives are commonly used to improve battery performance, maintain cycle lifetime, and improve safety. We are exploring the encapsulation of battery additives to provide on-demand delivery of additive compounds when and where needed in the battery cell. Flame retardants are used to suppress the potential for battery fires, but their direct incorporation in electrolytes sacrifices battery performance. Through encapsulation coupled with thermally triggered microcapsules, the virgin battery performance can be maintained while release of the flame retardant at high temperature provides increased battery safety.
In situ polymerization of urea/formaldehyde capsules containing the flame retardant Tris(2-chloroethyl phosphate) (TCP).