If electric cars are ever to drive hundreds of miles between charges—as they must to compete with gas-powered cars—their batteries will need to store much more energy. Unfortunately, several of the most promising high-capacity battery materials are prone to breaking in ways that would cut an electrified road trip short.
Now researchers at Stanford University have shown that mixing one such promising battery material, silicon microparticles, with self-healing polymers helps prevent a longer-lasting battery from failing. They say the self-healing polymers could stabilize other promising but damage-prone battery materials.
The self-healing battery’s negative electrode, or anode, combines silicon with polymers that act like chemical zippers, healing cracks that form when the battery is used and recharged.
The self-healing battery electrode has so far been tested with pure lithium metal as the positive electrode, because its storage capacity is much greater than that of any conventional cathode. The self-healing electrode itself has eight times the storage capacity of the carbon anodes found in a conventional rechargeable lithium-ion battery. If paired with a conventional cathode, it would create a battery that stored about 40 percent more energy. If paired with a correspondingly high-capacity cathode, total energy storage would be doubled or tripled.
While previous silicon batteries could only be discharged and recharged 10 times before breaking down, the self-healing battery weathers 100 charging cycles. But that’s still not enough, acknowledges Stanford materials scientist Yi Cui. “We need to go to 500 cycles for portable electronics, and a few thousand for electric vehicles,” Cui says.
Still, Cui’s approach may provide a new way forward for promising materials that have been stalled. “This points to a way to solve a general problem with high-capacity anodes,” says Paul Braun, a materials scientist at the University of Illinois at Urbana-Champaign who is not involved in the work.
Silicon anodes take in large amounts of lithium when the battery is charged, and release all that lithium as the battery is put to use. Such anodes can store a lot of energy in a small space, but their high capacity is a liability as far as the materials they’re made of are concerned: as large amounts of lithium enter and leave the battery, the silicon expands and contracts, cracking the anodes the first time they’re used; the same thing happens to anodes made of tin and germanium.
For the self-healing battery, Cui collaborated with another Stanford researcher, Zhenan Bao, who had previously developed self-healing electronic skin based on a stretchy, sticky polymer (see “Electric Skin that Rivals the Real Thing”).
When the polymer is fractured, it flows back together. The group mixed in some conductive carbon particles to ensure that the polymer, which isn’t conductive, wouldn’t impede the flow of electricity through the battery. This gooey mixture was then combined with silicon microparticles to make an anode. When the battery is charged and discharged, the silicon still expands, contracts, and fractures, but the polymer pulls everything back together. “Normally, once the anode cracks, you lose electrical contact,” says Cui. “The self-healing polymer ties the broken parts back together.”
There are other ways to deal with silicon’s tendency to crack. Cui’s group has experimented with nanostructured forms of silicon, including nanowires, that can withstand the strain of charging and recharging. Nanostructured silicon anodes like this are being developed by Amprius, a Sunnyvale, California company that Cui cofounded. However, researchers and companies are still learning about these nanomaterials. “It’s easy to get your hands on a small vial of nanostructured silicon, but to make 50 or 60 tons at a reasonable cost is a big problem that hasn’t been solved,” says Braun.
Cui says the combination of microparticles with the healing polymer could be less expensive and more practical for high-capacity batteries than approaches that require expensive nanomaterials. The silicon microparticles used in the self-healing battery demonstration can be bought off the shelf in large quantities and aren’t very expensive.
Nancy Sottos, a materials scientist at the University of Illinois at Urbana-Champaign, has developed yet another approach: Sottos mixes capsules of healing materials in with the battery materials. One such material is a bubble that bursts to release conductive metal to heal electrical connections in a damaged battery. Her group has made early proof-of-concept demonstrations using this method.
Yuegang Zhang, a battery researcher at the Lawrence Berkeley National Laboratory, says the Stanford self-healing binder shows promise for other kinds of high-capacity battery materials, such as tin. Zhang has taken a different approach in his own work, mixing tin nanostructures with stretchy, strong, conductive graphene to hold the anodes together. Noting the small number of times Cui’s silicon batteries can be recharged, he says, “silicon still has problems, but I like this idea.”
Now that they’ve made the first demonstration, Cui and Bao are working on fixes that would allow their self-healing silicon battery to go through more charge cycles. “We’re just starting,” Cui says.