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Batteries — whether they’re powering a smartphone or storing energy on the grid — take a beating.
Repeated charging and discharging causes all kinds of wear and tear on the devices we increasingly rely on to keep our gadgets, cars and renewable energy sources running. But what if batteries could repair themselves automatically and fix on-the-fly the cracks that lead to dead laptop batteries, the limited range of electric cars and other modern woes?
That’s the idea behind the work of a team led by two professors at the University of Illinois Urbana-Champaign (UIUC). They’re taking self-healing materials research and applying it to a novel subject area: energy storage. The hope is that a better understanding of how nanoparticles bind and come undone will lead to more reliable, longer-lasting and higher-capacity batteries.
“The idea was to try to take some of the self-healing work we’ve done in plastics and bring it into the battery world, because batteries do have all these reliability issues,” says Nancy Sottos, a professor of materials science and engineering, and one of the lead researchers on the project. “There’s a lot of cracking and chemical changes that go on in the battery that are, in general, undesirable. And of course what you see in your devices is basically they’re just not charging anymore.”
A breakthrough in battery technology is a sort of Holy Grail in today’s era of mobile communications and distributed energy. Consumers demand more and more from their portable devices, and energy storage is seen as a key ingredient for widespread renewable energy deployment. In short, better batteries would make it easier for utilities and grid operators to manage the variable flows of power from intermittent wind and solar energy sources.
The UIUC team — led by Sottos and Scott White, a professor of aerospace engineering — introduces a unique nanoparticle composite material into a key part of lithium-ion batteries, the energy-storage technology that dominates personal electronics and plays an increasing role in transportation and electricity.
In May, the team published a study in the journal Advanced Energy Materials, finding that their experimental technology mitigated a lithium-ion battery’s typical deterioration, retaining 80 percent of its initial capacity after cycling through its charge 400 times.
Challenges with silicon
Lithium-ion batteries consist of a positively charged cathode and a negatively charged anode, separated by a salt-based solution called an electrolyte. When a battery powers a device, lithium ions stored in the anode move to the cathode, which then compels a current of electrons to flow externally through the attached device, giving it power. When a battery is charged, the inverse happens.
The challenge is that most of today’s lithium-ion batteries use a graphite-based anode, which does the job well, Sottos says, but is limited in terms of the amount of lithium ions (and thus energy) it can store. Many researchers are looking to silicon as a potential alternative because of certain attractive properties.
“Silicon is a really interesting material,” Sottos says. “It’s also very abundant, relatively inexpensive, environmentally sound, and silicon can store a lot more lithium and so you can get really high theoretical capacities with silicon.”
There’s a catch, though. Silicon expands as much as 400 percent by volume when it stores lithium ions, Sottos says. By comparison, graphite anodes expand by only about 6 percent. The expanding silicon eventually “self-pulverizes” under the strain, rendering the battery kaput. Others have tried using nanoscale silicon to overcome the problem, but these nanoparticles have a tendency to break away from the binding material that holds them together.
“If you don’t solve this problem, basically what happens is that you start out with this really high capacity that you were trying to get, and then it just sort of decays with time and your electrode doesn’t last very long,” Sottos says. “It’s holding back commercial use of high-capacity materials like silicon.”
More research needed
The UIUC team builds on this previous work by experimenting with “dynamic ionic bonding” between silicon nanoparticles and their polymer binder. Early results suggest that this novel bonding scheme holds together better than other silicon composite anodes.
“When it charges, if [the bond] pulls open, and debonds — which would normally cause a loss of electrical connection — basically, these bonds are going to reform and make that interface sound again,” Sottos says. “You’ll continue to have high performance of the electrode.”
Sottos says the team has filed a patent for the technology with the aim of possibly commercializing it down the line, but more research is needed in the interim. It’s just one part of an investigation into what Sottos calls “autonomous strategies for batteries.”
For example, Sottos’ team is working on batteries that would automatically shut down if they are close to catching on fire, or they might include time-release additives that perform maintenance on damaged components. This would be particularly useful for electric-vehicle batteries, where safety and longevity are at a premium.
“There’s a suite of ideas that we’re thinking about that could potentially lead to some interesting commercial applications,” Sottos says.