Picture the scene: in downtown New York City, all-electric cars glide through streets in a zero-emission transport revolution. Polluting, inefficient gasoline and diesel vehicles are nowhere to be seen – or heard. The only things getting in the way of these smooth, noiseless vehicles are the horse-drawn trams.
That’s right, we’re talking about the past. The electric car had its heyday over a century ago. Its brief reign came to an end in 1912, when gasoline-powered Cadillacs began to come fitted with starter motors. That did away with the inconvenient crank handle needed to get their engines going, and they could run for 100 miles or more on a tank on fuel. The all-electric car’s battery would run out before you reached the city limits. It was no contest.
Now, in the first decade of the 21st century, history is about to go into reverse. The climate crisis is prompting thoughts of an all-electric economy, of which electric cars will be a vital part. The idea has been taking shape in engineering labs and on the roads for a while (Â鶹´«Ă˝, 20 September 2008, p 26), and now fresh impetus is finally coming from on high. “Our dependency on oil is dangerous and short-sighted,” US energy secretary Stephen Chu wrote in Newsweek in April. “We must… move toward running new vehicles on electricity and to generating that electricity from clean, renewable sources like solar and wind power.”
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There’s just one rather large obstacle remaining – and it’s the same one that stalled the electric car 100 years ago. “In the end, it all comes down to the lowly battery,” says Donald Sadoway, who studies materials chemistry at the Massachusetts Institute of Technology. Though batteries have been around for more than 200 years (see time line), precious little research effort has gone into improving them. That’s changing fast. In May, the US government set aside $2 billion for developing advanced battery manufacturing methods and $400 million towards the electrification of transport. A combination of new computer modelling techniques, innovative thinking and this well-timed injection of cash are set to transform battery technology. “We’re poised for a step change,” Sadoway says.
Change is certainly overdue. The lead-acid batteries in today’s diesel and petrol-powered cars and trucks are essentially unchanged from the first rechargeable battery, invented by French physicist Gaston Planté in 1859. Like other rechargeable batteries, lead-acid cells consist of two electrodes, each of a different material, separated by a solid or liquid electrolyte through which electrically charged ions pass. On discharge, for example, when an electric motor is connected to the battery, electrons are drawn out of the negative electrode, causing it to releases lead ions into the electrolyte. Meanwhile, electrons flow into the positive electrode, creating an attractive force that draws lead-ions out of the electrolyte (see diagram). The process is reversible: pass a current through the battery in the opposite direction, and the ions are forced back to where they started, recharging the battery.
Lead-acid batteries have their virtues, but in terms of one crucial characteristic they are at the bottom of the pile: for the amount of energy they store they are both bulky and heavy. Nickel-cadmium cells, which were invented in 1899, hold significantly more energy than a lead-acid battery of the same size. Even better are nickel-metal hydride batteries, introduced in 1989, which increased this energy density by a further factor of nearly 3 (see diagram).
Weighty issue
The weight of the battery is also a problem, and here the biggest leap forward came when Sony jumped on the emerging lithium ion (Li-ion) technology to power its digital cameras. Lithium is the lightest of all the metals, and so offers a huge weight advantage in comparison with lead and the other metals used in battery electrodes. Li-ion batteries went on sale in consumer products in 1991 and still offer the highest commercially available energy density in terms of watt-hours per kilogram.
Li-ion batteries are usually built using three thin sheets in the form of a “rolled up sandwich”. There are two electrodes, one made from graphite, the other from a metal oxide, and a “separator” that acts to prevent the electrodes coming into direct contact with each other and shorting out. The electrolyte contains a lithium salt (LiPF6) in an organic solvent (a mixture of ethylene carbonate and dimethyl carbonate) that has special electro-chemical properties.
Thanks largely to Sony’s investment, this technology has the best performance – a combination of high energy-density, longevity and a high rate of charge and discharge – of all commercially available batteries. As a result, it has become the power behind consumer gadgets such as cellphones and laptop computers. It is also the battery of choice for the first generation of all-electric vehicles, though scaling them up to provide the necessary power brings a new set of problems.
In the two decades that Li-ion technology has been around, it has developed in leaps and bounds. The energy that can be stored by a battery of a given size has doubled, and researchers claim it can . The compactness of Li-ion batteries makes the heat generated during charge and discharge a potential problem, however, and car-makers have to find ways of dissipating it. What’s more, manufacturing flaws, such as loose metal particles that pierce the separator, can create a short circuit that can cause the batteries to catch fire.
Another widely cited worry is cost, and Sadoway is convinced that the cost of the lithium itself will rule out lithium-based batteries for mass-market cars. An electric family car needs to have range at least 220 kilometres and sell for $25,000 at most, he reckons. That needs a battery that costs no more than $100 per kilowatt-hour of capacity. “And you can’t get there with lithium,” he says. This analysis is challenged by a , a New York market research company, which concludes that the cost of lithium metal is not the main issue, as it accounts for only 3 per cent of the cost of the battery. This suggests that there is plenty of scope for improved manufacturing techniques to bring the battery cost down.
Car-makers certainly seem to be sticking with Li-ion technology. “The electric vehicle will only work with lithium-ion batteries,” says Mark Verbrugge, director of materials research for General Motors in Detroit, Michigan. He accepts that cost is an issue. “The first production is going to be very expensive,” he says. “Providing credit for customers will be a very important part of the equation.”
“The electric vehicle will only work with lithium-ion batteries”
Sadoway wants to take a different approach. He reckons that by radically altering the battery’s chemistry it should be possible to improve performance without incurring an unacceptable cost. Stripped to its basics, this approach simply involves finding the right combination of chemicals for the electrodes and electrolyte. The question is, how do you know which chemicals to go for when there’s a whole periodic table to choose from? It’s a bit like trying to guess which combination of chemicals will combine to make an effective drug to treat a particular ailment. “The number of possibilities is so large we can’t pull bottles off shelves and synthesise compounds to see how they might perform – that’s going to take too long,” Sadoway says.
So he is taking a leaf out of the pharmaceutical industry’s book. Companies looking for new molecules to act as a drug have long known that cooking up every likely candidate takes too much time and effort. Instead they run computer simulations of the chemistry involved. Sadoway thinks finding new battery technologies should be no different: why rely on guesswork and tinker with chemicals when you can let a computer do the work and explore more combinations at much greater speed and lower cost?
All that’s required is a working knowledge of basic quantum mechanics. Take the Schrödinger equation, which describes how electrons, protons and neutrons behave, apply it to the internal chemistry of a battery, then feed the information into a computer program. The computer crunches the numbers and tells you how your battery will perform in terms of voltage, power capacity, charging and discharge rates and so on.
Materials scientist Gerbrand Ceder, also of MIT, shares Sadoway’s enthusiasm for this approach. “You tell me the elements involved, and I can solve the Schrödinger equation for those materials,” he says. “We can calculate almost anything you can imagine. It doesn’t mean you can always make that battery, but it narrows the problem down quite a bit.”
The idea is already paying dividends. In March, Ceder stunned the world by showing how a subtle twist in design, prompted by computer models, speeded up 100-fold the charging rate for Li-ion batteries in which the cathode is made of lithium iron phosphate – a type sometimes used in batteries for power tools, for example. Though the energy density of these lithium iron phosphate batteries is still too low for transport applications, Ceder’s achievement shows what the computational approach can be capable of. And while in this case the leap was spectacular – in one aspect of performance at least – much more modest improvements will get us to the point where batteries are good enough energy stores to trump alternative power sources.
Sadoway is confident that improvements by a factor of 2 or 3 are achievable. In many Li-ion batteries, the cathode is made from lithium cobalt oxide, and he wants to investigate the effect of replacing the cobalt with another metal. Chromium, for example, has up to six active electrons compared with cobalt’s maximum of three, suggesting that substituting one for the other could double capacity at a stroke. A few innovations like this and low-cost high-performance batteries start to look feasible. Where a battery might have held 150 watt-hours per kilogram, it would suddenly hold 500, making batteries a better option than gasoline or diesel for powering cars, Sadoway says.
Something in the air
Peter Bruce, who researches energy storage systems at the University of St Andrews in the UK, is more circumspect about the breakthroughs computer modelling might bring. “In principle it is possible to calculate everything, but the difficulty is whether you can describe the system in sufficient detail,” he says. “It’s an exciting and interesting approach, but it’s not the only game in town.”
Bruce is a player in one of the other games: he is part of a team researching “air electrodes”. Where the anode of a Li-ion battery might normally be made from cobalt oxide, in these systems it is made of a highly porous form of carbon. Oxygen from the air fills the voids, and combines with lithium ions as they arrive at the electrode. The resulting lithium oxide is a lot lighter than the standard electrodes, and so packs much more energy per kilogram. “We get an eight to tenfold increase in energy density,” Bruce says.
The team freely admit that they are still a long way from producing commercial batteries with air electrodes. So far they have produced prototype cells and run promising initial test but, as Bruce says: “There are still plenty of technical issues to be addressed.”
The same can be said about another sophisticated avenue for battery research: nano-engineering of electrodes. This involves restructuring their properties at near-atomic scales to help them absorb and release lithium ions more efficiently. The hope is that this will speed up charging and discharging, as well as making the electrodes longer-lasting. But tinkering around in the lab is a long way from making them on a commercial scale. “Nanostructured materials are really very interesting, but we don’t yet know if they are going to be too expensive,” says , a Nevada-based battery-technology consultant.
Despite their potential, radical approaches like atomic modelling and nano-engineering remain a minority interest. The lion’s share of the investment in effort and cash – some 98 per cent – is still going into the search for incremental improvements to Li-ion batteries. Brodd reckons the new investment in Li-ion manufacturing might soon bring us to a tipping point at which these batteries become cost-effective for cars. “It will stimulate new developments. There really is going to be a new era for battery systems.”
- Michael Brooks is the author of 13 Things That Don’t Make Sense (Random House, 2008)
