Can Lithium Deliver the Goods?
Laptop battery is bulked up for service in hybrids
4 November 2004 -- Ten new Federal Express hybrid-electric vans that took to the New York City road in October are carrying much more than the usual surprise packages and parcels. The vans are powered partly by lithium-ion batteries -- a scaled-up version of the batteries in cameras and laptops, not the nickel metal hydride (NiMH) batteries that have been the technology of choice in the pioneering hybrid cars sold by Toyota, Honda, and Ford. The Fed Ex trail is a test, one of several, of whether lithium ion batteries could displace NiMH in the hybrid vehicles of the future.
By growing consensus among specialists in the field, lithium batteries are markedly superior in some respects to the standard nickel-metal hydride alternative. ”You come up with a battery that's smaller and lighter,” says Michael Saft, U.S. marketing director for French battery giant Saft, in Bagnolet, France. ”We think that, long term, lithium is going to be the standard for the industry.”
Lithium batteries already surpass NiMH batteries in terms of power density, the metric of choice for hybrid vehicles, where electrical energy typically is needed in short bursts -- either to help start the car or to supplement traction from a gasoline or diesel engine. (For electric cars, which rely entirely on batteries, energy density is the more important measure of performance.)
But despite those power gains, there is one big drawback: lithium batteries are flammable, and a spate of recent accidents in cellphones, laptops, and electric bikes have fanned concerns about their suitability for the highway. ”I wouldn't predict the demise of NiMH batteries any time soon,” says Pete Savagian, engineering director for hybrid powertrain systems with General Motors Corp.
Lithium batteries work by shifting lithium ions between a graphite anode and a metal cathode in a liquid or polymer electrolyte. By devising thinner electrodes with high surface areas, lithium battery manufacturers have managed to improve their power output per unit volume at a rate of about 10 percent per year over the last five years, according to Alan Cocconi, chairman of AC Propulsion Inc., a developer of electric vehicle technology in San Dimas, Calif.
Battery supplier Hitachi Ltd., Tokyo, which provided prototype 40-kilowatt power packs for the FedEx trucks, says that the truck's batteries can deliver up to 2000 kW of power per kilogram -- at least 60 percent more power than the NiMH battery pack in Toyota's popular hybrid Prius sedan. And major automakers are now testing second-generation modules that deliver 2500 watts of power per kilogram, says Richard Kamioke, senior sales manager for Hitachi Automotive Products, a U.S. subsidiary.
Hitachi's competitors report similar improvements. Compact Power, based in Monument, Colo., an R and D subsidiary of Korea's LG, claims that its lithium polymer hybrid vehicle prototypes pack 3000 W per kilogram.
Because of that progress, lithium technology is beginning to edge beyond being shown around the auto show circuit. Toyota Motor Corp., Toyoto City, is selling lithium batteries as an option in one of its production cars in Japan, the subcompact Vitz. The Vitz's battery powers a beefed-up starter that gets the car rolling until the gas engine takes over, enabling the engine to shut off when the Vitz stops and then restart quickly on demand. The battery also absorbs some energy during braking.
The Vitz doesn't quite qualify as a hybrid, because the power from its battery never actually drives the car. The total energy stored in its battery is only about 180 watthours, one-eighth the energy of the Prius and only equal to the capacity of two or three laptop batteries. But as a result, this ”idle-stop” version of the Vitz averages 25.5 kilometers per liter of gas, an 8.5 percent boost in an already highly efficient car.
FedEx's project may be smaller -- a total of 18 hybrid prototypes will be deployed across the United States by year-end -- but it asks much more of the lithium battery. The truck's four 40-kW battery modules can actually propel it at low speeds, providing a 40-50 percent boost in fuel efficiency. That qualifies the vehicle as a ”full” hybrid, in contrast to so-called mild hybrids, in which electric power can only supplement the engine but not drive the vehicle independently.
Power isn't everything , of course. Automakers also want batteries that last longer than NiMH technology, cost less, and are safe. ”Fundamentally, we think [lithium] should last longer [than NiMH] and should be less expensive,” says Savagian. Analysts price lithium car batteries at more than US $1000 per kilowatthour versus an estimated $800 per kWh for NiMH, but they are betting that volume production will close that gap as it has for other electronics applications.
But safety is another matter. The organic electrolytes in many lithium ion batteries are carcinogenic and most employ metal oxide cathodes that are highly flammable when heated. A fire in one cell can set off an explosive chain reaction in the other cells in a module. ”When a phone battery blows up, it's not such a big deal, but with a hybrid battery, you've got a ton of energy,” says Lawrence Simmering, Ford's manager for advanced energy conversion and storage systems.
Simmering points out that lithium ion batteries from several would-be suppliers have repeatedly failed the stringent safety tests established by DaimlerChrysler, Ford, and GM. In one, a nail is shot into the battery; to pass the test, the battery must withstand the abuse without shorting. Simmering says battery developers are often surprised by the fireworks that the Big Three's tests elicit from their cells. ”We've got lots of interesting video,” he says.
Simmering's worst nightmare is a high-speed collision that deforms and short-circuits cells, which could lead to a fire or an explosion. Ironically, few companies have more experience with damaged lithium batteries than parcel carriers like FedEx. In August, handlers at FedEx's Memphis sorting facility removed a smoldering box of lithium batteries from a cargo plane, prompting an ongoing investigation by the U.S. National Transportation Safety Board, in Washington, D.C. The culprit was a pair of lithium modules destined for an electric vehicle.
All things considered, Savagian says it could be three to five years before lithium breaks into the hybrid market. He predicts that once it does, there will be a long period of competition between NiMH and lithium -- a battle that should help drive down the cost of hybrid vehicles and work to the benefit of consumers and the environment alike.