IN DREAMLIKE silence, an electric car strains up a steep hill and coasts down the other side. It glides round a bend, then gently slows to a halt at the traffic lights of a deserted town. The car is powered by a battery which is competing in a hotly contested race. The winner will provide the cleanest, quietest and most efficient source of power for a new generation of electric vehicles, whose numbers are expected to soar as the next century begins.
But for the moment, the car and its test route are merely simulations, conjured up by computer programs at Argonne National Laboratory near Chicago. Computers control electrical circuits that drain energy from the batteries as if they were supplying the power to push passenger vehicles over the hills and along the streets of an imaginary town. The room is cluttered with newly designed batteries from over a dozen development programmes, sent for evaluation at Argonne鈥檚 test laboratory, which is sponsored by both government and industry.
Tests like these are taking place all over the world, but the search for the ideal power source for electric vehicles is especially frenetic in the US. In 1990, the state of California decreed that by 1998, 2 per cent of the cars sold there by major motor manufacturers must emit no pollutants at all. The figure rises to 5 per cent for 2001 and 10 per cent for 2003. A dozen or so eastern states are considering similar measures.
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These rules are opposed by the car makers, who say they demand too much, too soon. But despite such protests, the companies are anxious not to be left behind. The potential rewards for a successful design are enormous. Consumer tests conducted by Ford, General Motors and Chrysler 鈥� the 鈥淏ig Three鈥� of the US auto industry 鈥� found that people liked the quietness of electric vehicles. And last December, thousands of ordinary drivers turned up at the 12th annual International Electric Vehicle Symposium in Anaheim, California, intrigued by the possibility that some day the car in their garage could be all-electric.
The motor manufacturers are keen to appear positive about electric cars. Michael McCabe, Ford鈥檚 marketing manager for electric vehicles, points out that Ford鈥檚 test fleet of 53 Ecostar minivans, powered by sodium-sulphur rechargeable batteries, has logged more than 400 000 kilometres. It has not all been trouble-free, however: two of the batteries caught fire last year.
A successful design could be used much more widely than in cars and vans. The motor industry envisages battery powered scooters and bicycles aimed not only at American and European buyers, but also those in China, India and Southeast Asia. 鈥淭he potential market is tantalisingly huge,鈥� says Vassilis Keramidas, executive director for the energy storage group at Bellcore, the New Jersey research organisation owned by a group of telecoms companies.
Wired for power
But the road to these glittering prizes is far from clear. No existing battery can give performance that rivals that of the traditional internal combustion engine. The battery the developers are striving for has to meet some daunting requirements. Paramount among these are the ability to store large amounts of energy for a given battery weight or volume and deliver bursts of high power for good acceleration, a long 鈥渃ycle life鈥�, allowing the battery to be recharged many times before it wears out, and low cost. In addition it must not be prone to explode, should be made of environmentally friendly materials, and must be rugged enough to survive 鈥渢he pothole from hell鈥�, as one researcher puts it.
Most modern batteries attempt to achieve these goals using principles which have changed little since the Italian physicist Alessandro Volta assembled the first battery almost 200 years ago. In its simplest form, a battery consists of two electrodes made of compounds that release energy when they react 鈥� except that instead of reacting directly they are connected through an electrolyte. Ions transferred in reactions between the electrodes and the electrolyte shift electrons from one electrode to the other, and the energy of the reactions can drive these electrons through an external circuit to provide electrical power. Ideally, the electrodes will be made of materials that are very reactive and very light, to yield a high rate of energy output (high power) for a relatively low weight. During recharging, an external voltage is applied across the battery terminals to reverse the reactions, and push the electrodes back up the electrochemical hill to their original chemical state.
Many batteries fulfil a few of the 鈥渋deal鈥� requirements, and sodium-sulphur batteries are among the leading contenders. These use a sodium and a sulphur electrode, both in a molten state, and a solid ceramic electrolyte called beta-alumina, which conducts sodium ions. The battery produces power when the positively charged sodium ions flow through the electrolyte, while their electrons flow through the external circuit. When these ions and electrons arrive at the other electrode they combine with sulphur to form sodium polysulphide. Sodium is a very light metal and is highly reactive, giving the batteries high peak power for a relatively low weight.
The sodium-sulphur battery in the Ecostar, for instance, can give a speed of 110 kilometres per hour, and the vans routinely travel 150 kilometres between charges. The batteries can be recharged around 800 times. But they are not as efficient as they could be: some of their energy has to be used to keep the molten electrodes at their operating temperature of about 370 C, and the insulating materials that encase the battery push its weight to over 300 kilograms. Cost is another problem. Each Ecostar battery costs $46 000, and McCabe does not foresee the price dropping below $15 000 even if the batteries were mass-produced.
Another contender is the familiar lead-acid battery which sparks conventional car engines into life. The electrodes are made of lead and lead dioxide, and both react with a sulphuric acid electrolyte to form lead sulphate. One lead-acid electrode pair or 鈥渃ell鈥� provides about 2 volts, so to obtain higher voltages a number have to be run in series.
Heavyweight contender
Lead-acid batteries for electric vehicles are relatively cheap, at roughly $10 000 each, but their performance shows some serious shortcomings. The batteries are heavy and bulky, so the range between charges for batteries of the highest acceptable weight and volume is typically less than 100 kilometres. And in the Argonne tests, lead-acid batteries survived fewer than half as many recharging cycles as the sodium-sulphur type, because the constant chemical reactions at the electrodes eventually cause them to disintegrate. Environmental worries also arise: millions of tonnes of lead would be needed to power the world鈥檚 buses, vans and commuter vehicles.
So what about the rechargeable batteries that work so well for flashlights and radios? Nickel-cadmium batteries, for instance, have electrodes made of nickel hydroxide and cadmium, which produce an energetic reaction involving an aqueous potassium hydroxide electrolyte. The downside is that cadmium is a toxic heavy metal that is found only as a trace element in various ores. Nothing like enough could be mined to manufacture batteries for a planetful of electric vehicles. For this reason, nickel-cadmium electric vehicle batteries are doomed, according to Jeffery Dahn, a battery researcher at Simon Fraser University in British Columbia, Canada. And nickel, while plentiful, is three to ten times as expensive in large quantities as manganese, lead, zinc and other common battery materials.
But whichever technology comes up tops, no electric power plant will match the versatility and power of the internal combustion engine. 鈥淲e are trying to compete with something that we have very little chance of imitating,鈥� says Jacob Jorn茅, a battery researcher at the University of Rochester in New York state. The weight of a petrol-engined vehicle鈥檚 fuel tank, when full, is largely that of the fuel itself, which burns to yield more than 10 000 watt-hours of energy per kilogram. But with batteries, the weight of metal electrodes and other components means that battery developers鈥� most optimistic projections for their future products are no better than 200 watt-hours per kilogram. Electric motors convert electrical energy to motion more efficiently than internal combustion engines convert petrol, but a kilogram of fuel can still propel a car much farther than can a 1-kilogram battery.
鈥淲e are used to an engine that does everything 鈥� shopping and a trip to California, says Robert Selman of the Illinois Institute of Technology in Chicago, who leads a battery development programme aimed at military applications. Most observers believe that early electric vehicles will typically be sold to multiple-car families who will, by contrast, use them for commuting and neighbourhood errands, leaving longer trips to a petrol or diesel-engined car.
Cash injection
Jorn茅 concludes that it would be a mistake to wait for the arrival of the 鈥渕agic battery鈥� before building practical electric vehicles. This view is shared by the United States Advanced Battery Consortium (USABC), an influential organisation funded by government and industry. By last summer, the USABC had committed $262 million to battery development. Its approach is to divide candidate batteries into near, mid and long-term niches, and support each group appropriately, with progressively more demanding targets. The emphasis is on high energy density, reasonable peak power, long cycle life and reduced cost.
The USABC鈥檚 frontrunner for the short term is the advanced lead-acid battery, a spruced-up relative of the conventional car battery. The company Electrosource of Austin in Texas claims that its batteries can withstand 900 charge cycles, drive a minivan over 125 kilometres between charges and deliver power comparable to that of a sodium-sulphur battery 鈥� all for a cost that will eventually fall to less than $3000. The electrodes are grid structures made of fibreglass with lead coatings, and the batteries weigh 40 per cent less than conventional lead-acid batteries of the same capacity.
Despite Electrosource鈥檚 apparent success, some researchers question whether batteries can ever maintain a consistently long cycle life if they rely on the lead-acid cycle, in which the electrode changes from one compound to another. 鈥淚t鈥檚 hard to hold the electrode together when it鈥檚 changing all the time,鈥� says Dahn. It is for this reason that 鈥渋ntercalation鈥� has become the buzzword in the battery business. The term is applied to what happens when layered electrodes made of carbon or metal oxides store 鈥� or intercalate 鈥� charged ions in the spaces between the layers. Small positive ions can slip between the layers of a graphite electrode, say, and form weak bonds within its structure. If the ions are then tempted out towards another electrode where they react or can bond more strongly, then the graphite structure adjusts only slightly as they leave, and the chemical changes are not great enough to bring about rapid breakdown of the electrodes. In future, devices made in this way could be 鈥渞obust for thousands and thousands of cycles鈥�, says Dahn.
This idea is employed in one of the battery types that the USABC favours for mid-term projects 鈥� the nickel-metalhydride battery. One developed by Ovonic Battery Company in Troy, Michigan, can withstand 1000 charging cycles. It also showed the highest ever peak power per unit weight in the latest round of testing at Argonne 鈥� the equivalent of around 80 horsepower (60 kilowatts) in a typical engine. The negative electrode is made of a complex alloy which soaks up and stores positive hydrogen ions in its intercalating lattice structure. The ions are attracted to the other electrode, which is made of nickel hydroxide, and a chemical reaction takes place there. Ovonic鈥檚 battery powered a very small passenger car called a Geo Metro over a record 345 kilometres on a single charge at last year鈥檚 American Tour de Sol, a rally for electric vehicles held in New England.
The company says it can cope with the high cost of nickel, but admits that there are still some problems. 鈥淚t鈥檚 just not that easy to come up with something that will have the power and durability you need,鈥� says Dennis Corrigan, director of advanced battery development at Ovonic. The battery slowly discharges when not in use, and when it gets too warm during strenuous use the electrodes鈥� lattice structure can break down. The company鈥檚 researchers believe that these problems can be tackled by tinkering with the proportions of the elements in the electrode. Each element confers different advantages and disadvantages. Zirconium, for instance, helps promote fast cell reactions, giving higher power, but can cause small-scale structural defects that hasten the breakdown of the lattice. Towards the end of 1995, a joint venture with General Motors called GM Ovonics will begin manufacturing 2000 batteries of this type per year.
A USABC favourite for the mid-term is the sodium-sulphur battery, which the consortium hopes will become better and cheaper as development continues. British company Silent Power of Runcorn in Cheshire, a division of the German-based energy and technology company RWE, has been awarded a $12.1 million contract to improve the design.
For the long term, the USABC is backing lithium batteries. The goal here is mass production before 2010. Lithium is the lightest metal and can be used to build battery cells with the highest energy densities because it gives up its electrons easily. Moreover, its small ions can fit cosily into the spaces within intercalation lattices.
The new generation of 鈥渞ocking chair鈥� batteries use intercalation compounds at both electrodes. Lithium ions are first stored within, say, a graphite electrode, where they are relatively weakly bound. In the discharge phase, the ions are attracted into the crevices of the other electrode, which is made of a compound such as manganese dioxide, where they are more tightly bound. During recharge, the lithium ions are drawn back into the graphite. The electrolyte鈥檚 main role is simply to conduct lithium ions, which 鈥渞ock鈥� back and forth between the electrodes as the battery is charged and discharged. Its chemistry does not change (see Diagram).
Sony Energytec, part of the Japanese electronics giant, caused a stir in 1992 when it began mass-producing a rechargeable, lithium rocking chair battery for use in portable electronic equipment such as camcorders and laptop computers. With a reported cycle life of 1200 charges and high resistance to mechanical shock, overcharging and other abuse, the battery is a good candidate for scaling up for use in electric vehicles. Following Sony鈥檚 success, the USABC awarded Duracell in Needham Massachusetts, and the German battery company Varta a $10 million joint contract to develop lithium batteries.
Despite these successes, lithium batteries still have their problems. Many designs suffer from low peak power because the electrolytes tend to have low conductivities for lithium ions, which slow down the output rate. There are also safety concerns about using such a reactive material. But these obstacles can be overcome, the companies say. Some groups are eyeing energy storage devices such as capacitors or flywheels to provide surges of power for rapid acceleration. 鈥淲e don鈥檛 see, from a scientific point of view, any show stoppers,鈥� says Keramidas, whose research group at Bellcore has its own lithium design. But scaling up the power to a level where they can be used for electric vehicles still poses problems. 鈥淚n all honesty, we haven鈥檛 done it yet,鈥� Keramidas admits.
So should we be discouraged if the ideal battery still seems years 鈥� possibly decades 鈥� away? 鈥淚t鈥檚 not surprising to me, because I鈥檝e spent 25 years working in the field,鈥� says Gary Henriksen, a project manager in Argonne鈥檚 Electrochemical Technology Division. Until 1990, when California decided to insist on a quota of nonpolluting vehicles, funding levels in the field had been at rock bottom since the end of the oil crisis. Even so, 鈥渋t鈥檚 not just a matter of throwing money at it,鈥� he says. 鈥淏attery development is evolutionary in nature. It takes both money and time.鈥� But given enough of both, he believes that several winning ideas will emerge from that silent test circuit not far from his office.