Lithium Batteries for Hybrid Cars
Hybrid cars need to travel farther in electric-only mode, and that means lithium-ion battery technologies have a lot riding on them
In late November General Motors announced plans to release a vehicle that will be able to go long distances in electric-only mode. It thus became the first U.S. company to commit to producing a so-called plug-in hybrid design—one that has batteries so capacious that they can be recharged not only by the engine but also from wall current in the garage. It represents the next way station along the path to an all-electric vehicle.
Troy Clarke, president of GM North America, told IEEE Spectrum that a plug-in version of the Saturn Vue Green Line sport-utility vehicle could hit dealer lots 24 months after the launch, in 2009, of a standard hybrid version using GM's "two-mode hybrid" transmission. He would not, however, commit to a specific date or even a year.
Tellingly, GM has not yet announced where it will get the lithium-ion batteries that any plug-in requires. Only such batteries--the kind used in laptops--pack enough energy to sustain electric-only mode for 32 kilometers (20 miles), the range generally regarded as necessary. In a statement released on 4 January, in the runup to the Detroit Motor Show, the company did say that it had agreed to support the battery technology programs of two joint ventures, and that it would also assess the technologies of other, unnamed companies.
Beyond plug-ins: the Volt
Although plug-in hybrids involve larger batteries, their fundamental design hardly varies from that of other, mechanical-drive cars. More radical is the ”series hybrid electric” car, which powers the wheels with electric motors and uses the onboard combustion engine only to run a backup generator that recharges the batteries as needed.
The Chevrolet Volt, unveiled to the press on 7 January at Detroit’s North American International Auto Show, is the first-ever series hybrid concept car shown by a major manufacturer. For an animated tour of its innards, click here. Its 1.0-liter, 3-cylinder turbocharged engine runs an onboard 53-kilowatt generator that recharges a 16-kilowatthour lithium-ion battery made of 80 four-volt cells. The battery pack’s volume is 100 L, one-third as much as the lead-acid batteries in GM’s 1990s-issue electric car, the EV1. GM’s targeted maximum weight for the pack is 180 kilograms (400 pounds). The company also wants the battery to last at least 10 years, through 4,000 full-discharge cycles.
The battery pack would charge in less than 6.5 hours, power a 120-kW electric motor delivering 320 newton-meters of peak torque, and go 64 km (40 miles) in all-electric mode on battery charge alone. The 12-gallon gasoline tank would add an additional 965 km (600 miles) to that range.
”We don’t have a battery pack yet,” said Tony Posawatz, the vehicle line director. He confirmed that the vehicle shown in Detroit doesn’t yet run.
Lithium ion: light and cheap
Everything thus depends on the pace of development of lithium-ion batteries. Right now they’re the only candidate for the job, because they store more than twice as much energy (110 to 130 watt hours per kilogram) as the next-best technology, the nickel-metal-hydride (NiMH) batteries in today’s gas-electric hybrids. The reason: lithium is the lightest solid element, so it’s easily portable. What’s more, it’s cheap.
To make lithium-ion batteries practical for mass-produced electric-drive vehicles, new technologies must increase the energy the batteries store and the speed with which they can discharge it. They must also lengthen cycle life to 15 years or 241 000 km (150 000 miles)—the average life of a vehicle. Finally, they must keep the cost as low as possible.
The technology has advanced quickly, says Mark Duvall, manager of technology development for electric transportation at the Electric Power Research Institute, in Palo Alto, Calif. He’s ”impressed and bullish” on the prospects for new lithium variants, some of which EPRI has tested to ascertain their cycle lives.
The first production car to use lithium-ion batteries was the Toyota Vitz CVT 4, a small car sold only in Japan. It used a four-cell, 12 ampere-hour lithium-ion battery pack to power its electric accessories and restart the engine after idle stop. More recently, Tesla Motors, in San Carlos, Calif., has offered the Tesla Roadster, an all-electric sports car that uses 6831 lithium-ion cells, each roughly the size of a double-A battery. They give the car up to 400 km (250 miles) of range, as well as the breathtaking acceleration of 0 to 100 kilometers per hour (0 to 60 miles per hour) in less than 4 seconds.
Why use so many little cells? First, because they’re readily available, and second, because current lithium technology is susceptible to thermal runaway—a problem underlined recently by flaming laptops—and larger cells mean greater risk. The Tesla’s 410-kg (900-pound) battery pack is stuffed not only with cells but also with sensors and control logic designed to detect and isolate any misbehaving cell.
Better batteries through chemistry
The cathodes of current lithium-ion batteries are made of lithium-cobalt metal oxide (LiCoO 2 ). This material is pricey, and it can become unstable and release oxygen if the cell is overcharged. One alternative is to replace the cobalt in the cathodes with iron phosphates, which won’t release oxygen under any charge and therefore will not burn.
A123Systems, in Watertown, Mass., first launched a lithium-ion phosphate battery this past fall in Black & Decker’s DeWalt power tools. A123Systems claims its batteries can be recharged 10 times as often as conventional lithium-ion designs, charge to 90 percent capacity in 5 minutes, and charge fully in less than 15 minutes. Conventional lithium-ion models, by contrast, can take twice as long.
In May, the company unveiled a battery pack it said could be ready for electric vehicle use within three years. It’s smaller than a carton of cigarettes and weighs barely 4.5 kg (10 lbs.), one-fifth as heavy as an equivalent NiMH battery. A123 is taking part in one of the two joint ventures to which GM has awarded battery development contracts. Its partner is Cobasys, of Orion, Michigan, itself a joint venture of Chevron Technology Ventures and Energy Conversion Devices Inc. GM's other contract is with a joint venture between Johnson Controls, of Milwaukee, and Saft Advanced Power Systems, of Paris.
Austin, Texas–based Valence Technology also uses iron-phosphate cathodes for its Saphion battery. The technology is used in the Segway, the self-stabilizing scooter, and in unofficial conversions that aim to increase the range of a Toyota Prius.
Customarily, the anode of a lithium-ion battery is made of graphite, which can store only a limited amount of energy. Researchers at Sandia National Laboratories, in Livermore, Calif., have developed anodes using a composite of graphite and silicon that can quadruple storage capacity.
Late this year, 3M Co., in St. Paul, Minn., will deliver still another kind of anode, based on amorphous silicon, which the company says will store twice the energy of today’s lithium batteries. Other researchers are trying to make anodes of alloys of lithium and two other metals, generally antimony mixed with either copper, manganese, or indium. Such three-metal alloys should also increase storage capacity.
Cells now being developed by Altair Nanotechnologies, based in Reno, Nev., switch the lithium from the cathode to the anode, forming a compound called lithium-titanate spinel (Li 4 Ti 5 O 12 ). The company says that the cells recharge in 3 minutes and deliver three times as much power as the conventional design, and at a great operating range of temperatures: –30 °C to 249 °C (–22 °F to 480 °F). It also says that its batteries can keep on ticking after 9000 recharging cycles, compared with 1000 for conventional cells. Altair’s battery, however, is not yet in production.
The big gamble
Once lithium batteries have met energy-storage, power-delivery, durability, and cost goals, a massive investment in manufacturing capacity will be needed to produce them in bulk for use in cars. But the market is crowded and competitive; close to a dozen manufacturers have announced new lithium battery technologies—with no guarantees that automakers will buy. And that number omits the in-house battery research that the major automakers themselves are conducting.
Take Toyota, which builds the lion’s share of hybrid vehicles globally. In 2005 it purchased General Motors’ share of Fuji Heavy Industries Ltd. (which manufactures Subarus)—in part, analysts suggest, to get Fuji’s share of its joint venture with Tokyo Electric Power to develop automotive lithium batteries. Subaru has already announced that in 2009 it will build and sell the R1e, an electric version of its tiny R1 urban car that will use lithium-ion batteries. Mitsubishi Motors, in Tokyo, will do much the same with its ”i” urban car, most likely using batteries from Litcel, its joint venture with TDK Corp.
Analysts estimate the price premium for today’s hybrids at roughly US $5000, some $3000 of which goes to cover the cost of a NiMH battery pack. At today’s gasoline and electricity prices, you’d need six to 10 years of operation to pay it back. But the analysts also say the hybrid premium could fall to $2000 in five years ($1200 or more of it the cost of lithium-ion batteries), which would allow for a three-year payback.
Electric-drive cars won’t be here this year—or next year—but they will arrive sooner than you might think
The payback period could be longer for a plug-in hybrid, because it would have larger, costlier batteries—though fuel mileage is hard to calculate. It all depends on how much of the mileage is covered in electric mode, with power taken from the grid, and how much in gasoline mode.
Powerful forces—global warming, possible carbon taxes, global political instability—seem to be lining up in ways that will bring us electric-drive cars that will be feasible and affordable for the first time ever. They won’t arrive this year, or next year…but they’ll be here sooner than you might think. It all comes down to one question: when will the lithium-ion batteries be ready?
About the Author
John Voelcker has written about automotive technology, home building, and other topics for 20 years. He covered software and microprocessor design for IEEE Spectrum from 1985 to 1990. A connoisseur of vintage British automobiles, he writes Spectrum ’s annual ”Top Ten Tech Cars” feature.