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Lithium Ion for Aviation

It holds promise for higher capacity batteries, but also risks for fires and explosions. Owners should tread cautiously.

For the great wide world of transportation, the lithium-ion battery is the shining city on the hill, that pivotal bit of technology that will have us whizzing around in silent cars banishing the evils of carbon dioxide. For aviation, lithium-ion is both an enigma and an opportunity. To understand both, you need only to grasp three numbers: 50, 150 and 1700. The opportunity part resides in the first two numbers — a lead-acid battery’s energy density is about 50 Wh/kg, a third or less than that of the typical lithium-ion’s 150 Wh/kg. Now for the enigma. The 1700 is the Wh/kg energy content of gasoline, adjusted for the typical internal combustion engine’s 20 percent efficiency. The very best lithium-ion batteries can do at the moment is 400 Wh/kg and these don’t exist commercially yet. That means the practical electric airplane may be on the horizon, but it’s not around the corner.

Lithium batteries ought to be a slam dunk for one application, though: starting and main aircraft batteries. “They’re twice the capacity and half the weight,” says Skip Koss of Concorde Battery, a leading GA supplier. The only thing is, he says, they’ve been known to erupt in violent flames from time to time, thus Concorde isn’t satisfied that the technology is yet worth the risk.

The Cars Are Driving

The market for lithium-ion batteries in aviation is bifurcated and although not strong yet, it’s certain to go that way. What’s driving development, including capacity improvement, new chemistries and safety, is the emerging electric vehicle market. But two other segments are promising, too: grid batteries that are used for smoothing out and backing up commercial electrical grids and IT/data applications. Aviation will be a fraction of this, but it’s still seen as a growth market.

For aviation, primary batteries for electric airplanes have thus far been based largely — although not exclusively — on the lithium-cobalt-oxide technology that’s popular for notebook computers. Panasonic and Sony are big players in these markets and these batteries find their way into developmental airplanes through companies that buy the individual 3.6-volt cells and custom package them for specified voltages and amp-hour requirements.

The battery pack in the PC-Aero Electra One Solar electric airplane we saw in Germany last spring uses a 60-volt system that PC-Aero’s Calin Gologan told us is a compromise between safety and efficiency. Electric airplanes have been — and remain — hamstrung by battery energy density considerations. Even though the best mass-market lithium-ion technology is now capable of nearly 200 Wh/kg, twice that density would make them more marketable.

Panasonic, whose batteries Gologan uses in the Solar One, promised higher watt density batteries, but hasn’t delivered. “The technology exists. I think they don’t bring it for economical reasons. They don’t want to start to the production lines,” Gologan says. Meanwhile, PC-Aero has equipped its soon-to-be intro airplane with solar cells that can extend flight time by 50 percent. Gologan believes for the fly-for-fun market that electric airplanes will represent initially, 90 minutes to a couple of hours of slow-speed endurance is sufficient and he believes currently available batteries can do that. Obviously, if electric airplanes are to become serious contenders in, first, the training market and later personal transportation, better batteries are a must. Everyone in this field we spoke to tells us they’re sure these are coming, but no one knows when or what their capacity will be.

In Slovenia, innovative Pipistrel has been offering an electric version of its Taurus G2 motorglider since 2007. Total sales so far? One, although more are expected to ship this year. Current battery technology allows two climbs to 4000 feet, followed by 3.5 hours of charging, says Pipistrel’s Tine Tomazic, who takes claims of battery capacity doubling within five years with a pound of salt. “Since 2007, when we flew the prototype G2, the energy density went from 163 (Wh/kg) to 185 for the same power required,” he said. That’s not much progress in five years, although Pipistrel says it’s confident enough in better batteries to plan an all-electric version of its new Panthera and also an electric Alpha trainer. Meanwhile, Pipistrel is also developing a hybrid.

Where Are the Batteries?

With the industry stuck at the 200 Wh/kg barrier, if not practically a lot less, where and when is this new battery capacity supposed to appear? These turn out be difficult questions to pin down. We spoke with Bill Mitchell, VP of commercial solutions at A123, a startup company that has become a leader in EV batteries but who also sees potential in aviation. A123 is championing a lithium-iron-nanophosphate technology that it claims has good energy density and is more stable than the cobalt-oxide chemistries that currently dominant the market. The holy grail of lithium efficiency is to move lithium ions through the electrolytes and electrodes more efficiently and A123’s nano-size particles do that, says Mitchell. Further, because they vent when thermally stressed, they’re less volatile than oxide batteries, whose electrolytes of lithium salts dissolved in flammable organic solvents are highly susceptible to intense fires.
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