They are a ubiquitous element of life in the 21^st Century, a starting point for any future energy independence from fossil fuels, and an absolute necessity for a planet connected by electronics. Batteries make smartphones, laptops, and electric vehicles possible. But their lifespan and expense can frustrate consumers, while safety issues are a growing concern, and the power they provide often seems unable to keep up with need.

Beckman Institute researchers Scott White, Jeff Moore, and Nancy Sottos are addressing these issues as part of an effort funded by the Department of Energy called the Center for Electrical Energy Storage (CEES). White was invited to speak about the research at the annual meeting of the American Association for the Advancement of Science (AAAS) as part of a program titled Pillars, Polymers, and Computers: Creative Approaches to Electrical Energy Storage. White’s talk, presented Sunday at the AAAS meeting in Washington, D.C., was titled Enabling Concepts for Safe, Self-Healing Li-Ion Batteries.

White, Sottos, and Moore are members of Beckman’s Autonomous Materials Systems group and have been pioneers in developing the area of self-healing materials. Now they are applying self-healing concepts to electrical energy storage issues.

In the abstract for the talk, White writes about the challenges of EES systems and the potential solutions offered by employing self-healing approaches: “Energy storage is one of the key technical challenges for the future, especially for electric vehicles. Batteries have been, and remain, the most versatile and widely used technology in response to this technical challenge. Future electric and hybrid vehicles require tremendous scientific and engineering advancements in terms of energy density, charging rates, and service life compared to the current state of art.

“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.”

According to the CEES website, its mission involves tackling the “scientific limitations of today’s electrochemical energy storage (EES) technologies” in order to understand the electrochemical processes powering these systems and to “design novel materials and interfacial structures to enable revolutionary improvement of these devices.”

The CEES has a research focus on lithium-ion batteries, the popular choice as an energy source for numerous electronic devices like the iPhone and laptops. Overcoming current battery technology limitations such as those involving reliability and lifespan, as well as safety concerns for both rechargeable lithium ion and non-rechargeable lithium metal batteries, is a major goal for researchers in this center.

The growing demand for ever-more powerful portable electronics and for hybrid and fully electrical vehicles (EVs) requires new battery technology that is safer, longer lasting, and perhaps even more powerful than today’s current systems. Self-healing materials offer a solution.

For example, autonomic shutdown of malfunctioning battery cells in EVs could serve as a fail-safe mechanism on the road, while autonomic repair of damage in lithium-ion batteries could lead to greater reliability in electronic devices, as well as prevent overheating and fires.

Previous work by the researchers involving autonomous polymer materials produced self-healing coatings (used for commercial applications such as paint) that automatically repair cracks and other failures using microencapsulated agents. This new research line required new approaches.

“We had to come up with completely new triggering mechanisms, materials that are used in self-healing,” White said. “That is all quite new stuff but the concepts flow from what we’ve demonstrated before in other materials systems. We are early on in the process and one of the first things that we did was basically to play around with lots of different approaches and different materials systems just to get an idea of what might work down the road.”

The Illinois researchers are experimenting with different microencapsulated systems to test their effectiveness at restoring conductivity and enhancing safety. The microencapsulated system works when stress induced by mechanical, chemical, or thermal factors ruptures the capsules, releasing a healing agent. The researchers have demonstrated conductivity restoration in systems using encapsulated carbon nanotubes, silver binders, graphite particles, and liquid metal.

Through the different approaches and materials used so far, the liquid metal method has shown the most promise. The core of the egg-like liquid metal microcapsules is a liquid gallium-indium alloy with a polymer surface. White said the method has demonstrated conductivity restoration within 40 microseconds of capsule rupture.

“It is immediate. We get 100 percent restoration of conductivity in a few microseconds,” he said. “This kind of performance that we see far outshines anything that we’ve tried before. It looks incredibly good right now.”

The microcapsule technology has another advantage for manufacturing purposes: it can be integrated into existing technologies.

“Another aspect of taking what we’ve done in the past and applying it here is making capsules over multiple length scales,” White said. “We can make them small enough to integrate with current battery designs so that it’s a seamless addition.”

The group is also dedicated to creating safer battery technology, including creating a method for applying self-healing coatings to the anode/separator/cathode elements of the electrochemical cell so a battery shuts down when overheated.

“We either coat the anode or the separator with solid microcapsules, which in this case are polyethylene and we do that through a process called spin-coating,” White said. “If you make a battery this way with our capsules integrated, you get the same kind of charge density cycling performance that you would see normally.

“What happens is that under a thermal cycle, if we heat it up to a critical point where it melts, then it coats the entire surface and the capacity of the battery drops to zero. It doesn’t conduct ions through that layer anymore, so the battery shuts down.”   

White said most of the work in the research line is focused toward increasing the performance of energy storage devices so hybrid and electric vehicles will become a common sight on roadways.

“The DOE is particularly challenging the centers that they have set up to do the necessary science and innovation to allow electric vehicles to become a reality,” he said. “We are challenged to improving energy storage to the level you need in order to have fully electric vehicles. Not only cars, but buses, airplanes, all kinds of things.”

White expects the technology they are working on to be applicable to other energy storage systems besides lithium ion batteries.

“The fundamental technology that we are developing works for lithium and will work for other battery systems as well,” he said. “Batteries of the future are going to be ones that adapt to their environment and have some kind of higher-level function to them.”

The research line has its genesis in a brainstorming session involving, White, Sottos, and Moore regarding future directions for their self-healing work. The triggering mechanisms to start the self-healing function in that earlier research were based on mechanical rupture.

“Capsules can be triggered to do that kind of release using a variety of methods, mechanical energy, or thermal energy, where you have a capsule that melts and releases,” White said. “And it does link into our ideas and work that is going on in mechanochemistry. In surveying the interrelations between all those fields, we at the same time said we’ve got capsules, and how about all these other sources of energy.

“So we were brainstorming about new functionalities for encapsulated components and at the same time Jeff was approach by the now-director (Michael Thackeray) of the center about putting together a proposal that would focus on batteries. They were particularly interested in recruiting us for our work on self-healing as a new aspect to be brought into their project.”

The efforts of the Beckman researchers are only about a year old, but papers on the research are being prepared and the early efforts look good.

“Some of the most promising ones we are running with right now,” White said. “I don’t know five years down the road what suite of materials and functionalities we are going to come up with. It’s really an exciting time because we get to play a lot.”