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| Scanning electron microscope image of polyethylene microcapsules spin coated onto Li-ion battery anode. These microcapsules are thermally triggered during thermal runaway conditions and infuse the anode shutting down ion transport and preventing lithium fires. Photo credit: Marta Baginska. Click to enlarge. |
Researchers at the University of Illinois Urbana-Champaign are applying new materials and concepts integrated within the battery cell to enable a variety of critical features including fail-safe or autonomic shutdown, self-healing of battery performance, and greatly extended lifetimes. Prof. Scott R. White talked about his group’s research at the annual meeting of the American Association for the Advancement of Science (AAAS) as part of the “Pillars, Polymers, and Computers: Creative Approaches to Electrical Energy Storage” program.
The basic premise is to incorporate microcapsules containing a latent core material within the battery environment—within the electrolyte, embedded in the anode, or layered on the separator. Triggering of the microcapsule is accomplished by a variety of external stimuli (e.g. heat or mechanical force). Once triggered, the microcapsules release their payload in order to affect performance. The researchers have demonstrated conductivity restoration in systems using encapsulated carbon nanotubes, silver binders, graphite particles, and liquid metal.
A variety of complex damage mechanisms in Li-ion batteries and microelectronics can lead to a significant loss of conductivity, and eventual system failure, White noted. For Li-ion batteries, cracking, deterioration, and electrochemical pulverization occur during the massive volume changes associated with the intercalation/deintercalation of Li+ ions during charge and discharge. As this damage accumulates, there is a significant degradation of the efficiency, and eventually failure of the battery. Polymeric microcapsules containing a liquid metal (Ga, Ga-In, Ga-In-Sn) can release in response to the mechanical forces associated of these damage processes and upon release, locally restore electrical conductivity and system performance—i.e. self-heal.
Alternatively, thermoresponsive microcapsules can be designed to melt and/or polymerize at triggering temperatures that are well below unsafe conditions for batteries. Incorporating these microcapsules onto battery electrodes or within battery separators provides a fail-safe autonomic shutdown feature prior to electrical breakdown and potential fires.
Whatever approach and functionality is imparted to Li-ion batteries, the addition of the microencapsulated phase must not lead to degradation in battery performance, White said. Through proper engineering of the materials, capsule design, and integration this objective can also be achieved.
Previous work by the researchers involving autonomic healing polymers produced self-healing coatings, used for commercial applications such as paint, that automatically repair cracks and other failures using microencapsulated agents. The researchers will consider lessons learned from their past work and apply them to new developments in batteries.
Inspired by biological systems that routinely accomplish self-healing, thermal regulation, regeneration, and other autonomic responses, we believe that new materials and concepts integrated within the battery cell can enable a variety of critical features including fail-safe or autonomic shutdown, self-healing of battery performance, and greatly extended lifetimes.
—Scott White
Funding for the research comes from the Department of Energy’s Center for Electrical Energy Storage (CEES) effort.