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NO PUN intended, but we need to go a little deeper with the grounding of Boeing 787 Dreamliners, their fire and smoke emergencies having been attributed to the aircraft’s use of lithium-ion batteries. Some commentators blame “the same technology used in cars,” and then they cite fires with the Fisker Karma and Chevrolet Volt.
It’s more complicated than this.
The confusion comes from the fact that there’s a variety of lithium battery chemistries, each with a different cathode material and resulting performance tradeoffs.
What do these batteries have in common? As with all batteries, a lithium-ion rechargeable battery works by shuttling electrical charge between two electrodes, a positively charged cathode and negatively charged anode.
Fully charged, the lithium ions hang out at the anode. Flip a switch, and (negative) electrons in the anode are attracted through this external circuit where they perform work before joining the (positively charged) cathode. This electron travel causes the lithium ions to migrate through an electrolyte to the cathode. The electricity continues to flow until migration ceases—i.e., until the battery needs recharging.
In recharging, an applied external voltage sends electrons back from cathode to anode; the lithium ions do their thing by returning to the anode and teaming up with the electrons again.
This same routine occurs in any lithium-ion battery. Also, the anode material is typically carbon. But lithium chemistries diverge in their choice of cathode material—with plenty of tradeoffs.
A serious tradeoff—indeed, a hazard—is thermal runaway, the bête noir of lithium batteries. Charging any battery causes heat and, with some chemistries, too much heat can lead to higher and higher temperatures, even to the point of explosion.
A classic lithium battery story: A parachute tester finds himself plummeting downward when he sees another guy coming up toward him. The parachuter yells, “Hey, do you know anything about parachutes??” “No,” yells the guy heading up, “do you know anything about lithium batteries??”
Thermal runaway is profoundly related to cathode choice. And Mother Nature, sometimes being a real mother, rewards the more efficient cathode materials with higher propensities of thermal runaway. This is countered by complex techniques involving circuitry, materials, recharge strategies and failure modes.
Lithium cobalt oxide, LiCoO2, is a very efficient cathode material, with risk of thermal runaway mitigated by appropriate engineering. It has been the cathode of choice for batteries in mobile phones and laptops—thus, giving an excellent data base of battery use. (Early laptop fires, for example, were traced to impurities causing shorts, not thermal runaway per se.)
For reasons of high efficiency, the Boeing 787 Dreamliner’s batteries are also of the cobalt oxide variety.
By contrast, the only automotive application of cobalt oxide chemistry was with the Tesla Roadster. The Chevrolet Volt’s cathodes are lithium manganese spinel. The Fisker Karma has another conservative cathode choice, lithium iron phosphate. These automotive cathode choices trade some efficiency for other benefits, including avoidance of anything resembling thermal runaway.
The Volt fire occurred three weeks after a side-impact crash test. It was traced to a ruptured battery coolant line. Also contributing was a shortcoming of test protocol for such vehicles (a conventional car’s leaking gasoline tank, for example, wouldn’t have been left for three weeks).
One Fisker Karma fire was attributed to a faulty cooling fan, unrelated to the car’s battery pack. Others involved 17 Karmas that were submerged in Hurricane Sandy waters. The fires were traced to salt water intrusion of electrical control modules. ds
© Dennis Simanaitis, SimanaitisSays.com, 2013