Hooked on lithium
Jun 20th 2002
From The Economist print edition
Without the lithium-ion battery, introduced a decade ago, portable gadgets—from mobile phones and video cameras to laptops and palmtops—would have remained brick-like objects best left on the desk or at home. But the innovation would have floundered had electro-chemists in America not teamed up with a Japanese firm
THE Mobira Senator, launched in 1982 by Nokia, was the grand-daddy of today's mobile phones. It consisted of a small handset connected to a brick-like battery pack, with a hefty handle on top—a vital feature, since the whole thing weighed 9.8kg. Today, a typical mobile phone is a hundredth of this (ie, 100 grams or less) and can be tucked discreetly into a shirt pocket. This 99% weight reduction has been achieved largely through advances in battery technology. Above all, it is down to one particular breakthrough: the advent of the lithium-ion rechargeable battery.
Lithium-ion batteries are the foot-soldiers of the digital revolution. They power telephones, music players, digital cameras and laptops. They are amazingly small and light, and can store more energy in less space than any other type of rechargeable battery. The Ericsson T39 phone, for example, can run for a week on a lithium-ion battery the size of ten business cards stacked on top of one another. A nickel-metal-hydride battery of equivalent capacity would weigh twice as much and be about twice as big; nickel-cadmium and lead-acid batteries are heavier and larger still.
Lithium-ion batteries are also superior in that they do not suffer from “battery memory effect”, a loss of capacity that occurs when a battery is recharged before it is fully depleted. As a result, lithium-ion batteries now account for 63% of worldwide sales of portable batteries, compared with 23% for nickel-cadmium and 14% for nickel-metal-hydride.
But getting lithium-battery technology from the laboratory into millions of people's pockets was not easy, and took many years. Engineers had numerous problems to overcome. Having solved them, they are now working on even more advanced lithium batteries that will be more versatile still. Unless something entirely new comes along—microscopic fuel cells, for example, or more efficient photo-voltaic cells—the world's gadgets are likely to be running on lithium for some time to come.
Rechargeable batteries work by exploiting reversible electro-chemical reactions at the battery's positive and negative terminals (electrodes). The battery is charged by applying an electric current, which causes a reaction that puts the battery into a high-energy state. The battery can then be used as a power source. As it discharges, the electro-chemical reaction operates in reverse, releasing energy and returning the battery to its uncharged, low-energy state. Different battery types, such as lead-acid and nickel-cadmium, use different compounds and chemical reactions, but the basic principle is the same for each.
Lithium has particular appeal for use in batteries for two reasons, observes Michel Armand, a researcher at the University of Montreal and co-author of a recent paper in Nature about lithium batteries. First, it is the most electro-positive metal, which means it can be used to make batteries with higher terminal voltages (typically, four volts rather than 1.5 volts) than other designs. And, second, it is the lightest metal, having the capacity to store 3,860 amp-hours of charge per kilogram of weight compared with 260 amp-hours per kilogram for lead.
This means that lithium-based batteries have a high energy density, enabling them to pack a lot of energy into a small, light package. Accordingly, lithium batteries have been used since the 1970s to power watches, calculators and medical implants. But such batteries are non-rechargeable. Making a lithium battery that could be recharged was far from easy.
One early attempt, a research project launched in 1972 by Exxon, an American energy giant, used titanium sulphide as the positive electrode and pure lithium metal as the negative electrode. Titanium sulphide was chosen because it is an “intercalation” compound—a substance with a layered, crystalline structure that can absorb other particles between its layers, much as a sponge soaks up water. Discharging the battery causes charged atoms (ions) of lithium to break away from the negative electrode, swim through an intermediate liquid (electrolyte) and take up residence within the titanium-sulphide lattice, releasing energy in the process. Recharging the battery causes the lithium ions to swim back again, and to re-attach themselves to the negative electrode.
The drawback with this design, however, is that after repeated charge/discharge cycles the lithium does not form a perfectly smooth metal at the negative electrode. Instead, it assumes an uneven, “dendritic” form that is unstable and reactive, and may lead to explosions. This problem has never been completely solved, says Frank McLarnon, a researcher at the Lawrence Berkeley National Laboratory in California. Replacing the negative electrode with a lithium-aluminium alloy reduced the risk of explosion, but made the battery far less efficient and long-lived.
Rocking on
Instead, a new approach was tried, and was working well in the laboratory by the late 1980s. It involved using a second intercalation compound to play host to the lithium ions at the negative electrode. Discharging and recharging the battery then simply caused the lithium ions to move back and forth between the two intercalation compounds, releasing energy in one direction and absorbing it in the other. This to-and-fro technique became known as “rocking-chair” technology. The use of lithium in its ionic, rather than its metallic, state made the batteries much safer than previous designs—and explains why they are known as “lithium-ion” batteries.
By this time, a team of researchers led by John Goodenough, now at the University of Texas, had discovered a new family of intercalation compounds based on oxides of manganese, cobalt and nickel. The first commercial lithium-ion battery, launched by Sony in 1991, was a rocking-chair design that used lithium-cobalt-oxide for the positive electrode, and graphite (carbon) for the negative one.
Charging the battery causes lithium ions to move out of the cobalt-oxide lattice and slip between the sheets of carbon atoms in the graphite electrode—a state of higher potential energy. Discharging the battery causes them to move back again, releasing energy in the process (see illustration). This type of battery is now in widespread use, and Dr Goodenough was awarded the $450,000 Japan Prize in 2001 in recognition of his work.
As well as identifying the right combination of materials, Sony's key contribution, says Dr McLarnon, was to recognise that the battery must not be overcharged. If too many lithium ions are extracted from the cobalt-oxide lattice, it will disintegrate. Besides, packing too many lithium ions into the graphite lattice can result in small particles of lithium being formed, with possibly dangerous consequences. Sony implemented a number of safety measures, including the use of a porous polymer that melts together if the battery overheats, thus preventing ion transport and shutting the battery down. It also developed a special “smart” charger with software that prevents overcharging. Such chargers are now commonplace.
Since the commercialisation of lithium-ion batteries began in the 1990s, there have been a few minor improvements, and the cost has come down substantially. One clever tweak, which has made possible the ultra-thin batteries used to power mobile phones, was the replacement of the liquid electrolyte between the battery's two electrodes with a porous separator soaked with an electrolytic gel. This means the battery can be put together like a sandwich, making thin, flat designs possible.
The next step is to move from a gel to a solid polymer electrolyte, which will be thinner and lighter still. The problem, says Dr McLarnon, is that lithium ions diffuse far more slowly through a solid than through a gel or liquid, making the battery slow to charge and discharge—unless the polymer is made extremely thin.
Still in its infancy
After that? According to Dr Armand, rechargeable lithium-based battery technologies are still in their infancy, and there is much scope for improvement. A number of promising new materials are being developed, including new intercalation compounds and aerogels for the positive electrode, as well as novel alloys for the negative electrode. Some researchers have been working on new ways to prevent dendritic growth in lithium metal-based batteries, using special electrolytes.
If lithium-ion batteries are so light and powerful, why not use them in electric cars? The main problem, says Dr McLarnon, who is working in this area, is cost. A set of lithium-ion batteries capable of powering an electric car now costs around $10,000. Lead-acid batteries may be five times heavier, four times bulkier and far less efficient—the General Motors EV-1, for example, is powered by 533kg of lead-acid batteries that take eight hours to recharge—but they are also a lot cheaper.
But now that all-electric cars have fallen out of favour, since most of their alleged benefits can also be achieved by hybrid (petrol/electric) vehicles, lithium-ion is back in the running. Hybrid vehicles need a much smaller power pack, bringing the cost of a suitable lithium-ion battery down to below $1,000. Even so, says Dr McLarnon, there is still work to be done. The batteries may wear out after a few years of use, and they must be optimised to deliver, and absorb, sudden bursts of power, rather than large amounts of energy at a roughly constant rate, which is what consumer-electronics devices require. Dr McLarnon is optimistic that this can be achieved.
The development of the lithium-ion battery is an object lesson in how pure and applied research, driven by commercial interests, can generate the incremental improvements in a technology that are necessary for transforming it into a useful product. In this case, intercalation compounds were an offshoot of pure research into superconductivity. They were then picked up by Dr Goodenough and other researchers working on battery technology; and the final pieces of the puzzle were supplied by Sony. (Dr Goodenough, who did his original research at Oxford, says battery firms in the West rejected his approaches.)
There was no single “eureka” moment, but a series of gradual improvements—with the baton passed between a number of different groups. “These things zigzag back and forth,” says Dr Goodenough. “That's how innovation works.” The baton has now been passed to new researchers, seeking further improvements and applications, from video-phones to cars. Lithium-ion batteries, it seems, still have a long way to go.
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