The 787 Dreamliner’s battery problems have been a nightmare for Boeing and a reminder that bigger batteries require more-extensive safety measures. Technologies that use new battery chemistries and cooling techniques to prevent lithium-ion battery fires are in the works, but systems that monitor the batteries’ electrical behavior offer particular promise to catch future catastrophic conflagrations long before they happen.
Lithium-ion batteries can experience what’s called thermal runaway, in which some event leads to heating, which in turn leads to more heating, and so on. Part of the lithium-ion safety problem, says Elton Cairns, a faculty senior scientist at Lawrence Berkeley National Laboratory, in California, involves the electrolytes in the batteries. “We’re using mixtures of organic solvents that are quite flammable and quite volatile,” he says of today’s lithium-ion electrolytes. “In my view, that’s just asking for trouble.” And with enough heat, oxygen gets liberated from a battery’s metal-oxide anode. “There you’ve got all the makings of a fire,” he says. And a flame front that doesn’t need anything outside the battery to sustain itself is very hard to extinguish. That’s why lithium-ion battery fires can get so big (like the one that knocked out a U.S. Navy minisub in 2008), he says.
Nonflammable electrolytes are in the works. Cairns’s group has, in fact, patented one that it has successfully tested on prototype lithium-ion cells since 2006, he says. Cooling systems, such as the Chevy Volt’s dedicated battery radiator, can help prevent thermal runaway too, says Donald Sadoway, a professor of materials chemistry at MIT.
But cooling and chemistry will work a lot better if they’re combined with sensing and circuitry, according to Michael Pecht, director of the University of Maryland’s Center for Advanced Life Cycle Engineering. He says engineers need to work harder on smarter battery-management systems. Many such systems monitor only a battery’s current flow over time, he says. “It’s like the doctor just measuring your pulse and saying you’re healthy.”
Pecht says his group is developing battery-management systems that track current, voltage, mechanical strain, temperature, and other operating parameters. And once a battery’s use and performance run too far astray, the system warns that the battery needs servicing.
Sensing systems may, in fact, be able to detect bad batteries that have already passed factory tests. These parts suffer from an internal short circuit, a defect that is difficult to identify. As a private consultant to lithium-ion battery manufacturers and device makers that use those batteries, Brian Barnett, vice president of the Lexington, Mass.–based technology-development company TIAX, has examined many case studies of lithium-ion problems. “Frequently, the level of destruction was too great to determine what transpired,” he says. “However, when you could find a cause, overwhelmingly we discovered proof that there had been a foreign metal particle that had got into the cell.” What was particularly worrisome was that in “a couple hundred incidents, it showed that none of them occurred in the first three months,” he says. Many internal short circuits, in other words, cannot be detected at the factory.
The contaminants were often tiny shards of crimped, scraped, or flaked metal of various sizes that could be as small as tens of micrometers, Barnett says. Battery manufacturers already have many tricks—including using strong magnets and shrouded cutting areas—to keep contaminants out of battery assemblies. But, he says, the persistence of rare but catastrophic battery fires from cells made at even the best lithium-ion factories in the world suggests that some baseline level of contamination exists—and has to be rooted out in other ways.
Further experiments and computer models of these metal particles in lithium-ion batteries also revealed a likely mechanism for time-delayed thermal runaways. If the metal shard is near the cathode, Barnett says, the battery’s voltage oxidizes the contaminant particle, which is often iron, copper, nickel, or zinc. And the resulting nanoscale charged particles can then migrate across the battery’s microporous separator. The contaminant particles then reach the anode.
“At the voltages of the anode,” Barnett says, “you have a process that’s not so different than electroplating....It plates out. And when that process happens progressively over time, you get a metal deposit. It’s shaped largely like a dendrite. It starts to fill the holes in the separator, eventually making contact with the cathode. And it’s only at that moment when you get a short.”
Like an arterial plaque deposit that accumulates over time and leads to a heart attack, the dendrite builds and builds till weeks or months later it causes—in a worst-case scenario—a short that sparks a fire in the electrolyte.
But Barnett is hopeful that those one-in-5-million batteries containing dangerous metal contaminant particles can be detected. A smart battery-management system that uses some clever signal processing can uncover the weeks- or months-long process of forming the dangerous dendrite short circuits, he says. TIAX has developed a proprietary hardware and software package that engineers can add to battery-management units to uncover such developing hazards before they short and cause a thermal runaway, Barnett says, adding that the technology has already proved itself in the lab: His team has inserted prototypical contaminant particles into test batteries and detected their subtle signatures in the batteries’ otherwise-normal performance. The company began rolling out the technology in January.
Barnett and TIAX have been at the forefront of studying and detecting those rare batteries that may be catastrophically impaired, says Gerry Woolf, editor of the trade magazine Batteries & Energy Storage Technology. “This has been a problem that Brian has addressed the industry on for at least half a decade,” Woolf says. “It’s a hidden problem—the heart attack you don’t expect.”
James Barnes, development manager at the Department of Energy’s Office of Vehicle Technologies, says that while the DOE can’t endorse any specific new advance, the department has helped fund the TIAX technology. And, Barnes says, “there is no question that it would be very useful to be able to sense a developing failure in a battery before cell runaway occurs. Early detection would give time to take mitigating actions.”