In October, 47 teams from 25 nations will race from Darwin to Adelaide in the World Solar Challenge. The gruelling 3000-kilometre journey will test the mettle of drivers and solar-powered electric cars – as well as their traditional weak point of batteries.
That weak point may be about to change. Major advances in design and manufacture are resulting in new generations of batteries that not only perform better but last longer, are more reliable and make fewer environmental demands.
Batteries are electrochemical storage devices that work by turning chemical energy into electrical energy. They comprise one or more cells, each containing two electrodes – a negative and a positive – and a chemical called the electrolyte that transports charge between them.
In order for one electrode to receive more electrons than the other, different metals are used for the negative and positive electrodes. On battery discharge these are known respectively as the anode and cathode, with excess electrons flowing from the former to the latter. This is what generates an electric current.
In the case of lithium-ion batteries, used widely in domestic and other appliances, the positive electrodes generally incorporate cobalt, which must be mined and then processed at high temperature, releasing relatively large quantities of the greenhouse gas carbon dioxide.
Not so with the supposedly cheap and long-lasting aluminium-ion battery, developed by US scientists. Designed for the latest generation of smartphones, the battery is said to charge in about one minute.
The developers, whose research was published in the British journal Nature, claim their revolutionary device has the potential to replace lithium-ion batteries. Besides recharging much faster, the new aluminium battery is safer than existing lithium-ion batteries, which occasionally burst into flames, the researchers claimed.
"The new aluminium battery has two aspects that are much safer than traditional phone and laptop batteries," says Monash University chemistry Professor Douglas MacFarlane.
Aluminium, he points out, is a safer metal to use in batteries because it does not react violently if exposed to water.
"YouTube is full of videos of what might happen when lithium metal comes in contact with water," MacFarlane explains.
The second aspect of the new battery that makes it safer than current phone batteries is the electrolyte ferrying current between the electrodes.
In a lithium-ion battery, this electrolyte is based on a solvent that is quite flammable; so if the battery is damaged or mistreated, a fire or explosion can occur.
"In the new battery, the electrolyte is based on an ionic liquid – effectively a liquid salt," MacFarlane notes. "This is completely non-flammable and hence intrinsically safer."
Researchers are also introducing this electrolyte in lithium-ion batteries – so their safety features are likely to benefit phone batteries quite soon.
"One of the exciting things about the new aluminium batteries is their fast charging," he says. "This feature is also of importance in the much bigger batteries used in electric cars, where recharging the battery is a major limitation."
Trade-off
One drawback of the new batteries is that they do not pack as much energy as their lithium-ion counterparts. "So we will need to carry a heavier and bigger battery to get the same battery life; in the end this may be a trade-off many users will not want to make in a phone or a laptop," MacFarlane explains.
Ultimately, he believes, the main application may be for larger stationary storage purposes, such as for solar or wind energy.
The new Tesla battery for the home is an example of this application. "There is interest in installing larger versions in wind and solar farms to even out the variability of the energy flowing from these generators," MacFarlane says.
Sydney University chemistry Professor Anthony Vassallo agrees. "The new aluminium-ion battery is an important development in that rechargeable batteries based on aluminium have never been that successful – except by replacing the spent aluminium electrodes with fresh electrodes and regenerating the aluminium metal in a reprocessing facility."
Aluminium is an attractive battery material, he adds. "First it is very light. Second, it is abundant and cheap. Third, it can lose three electrons per atom, compared with lithium, which can only lose one.
"The problem with using aluminium, though, is that as it gives up its electrons it starts to form a new material with molecules in the electrolyte – and air. This is typically a gel. This makes the battery less and less useful as it discharges – and very difficult to recharge," Vassallo explains.
The new battery uses completely new chemistry for the electrolyte, avoiding these problems.
"This means the aluminium battery can now be cycled many thousands of times before it starts to fail," he says. "But the electrolyte needs to be water-free, so needs the same type of careful packaging that lithium batteries use.
"Another downside is that the energy the battery can store for a certain weight is quite low. Compare this to the lithium batteries in your phone or tablet that could store four times as much," Vassallo says.
"So, to get the same 'run time' you would need a battery that is at least three times heavier than lithium batteries. Most people wouldn't accept this."
On the plus-side, he notes, the battery can be recharged really quickly. "While this is really impressive, you wouldn't be able to use the standard battery charger to do this – as to get a full charge into the battery in a minute or so would mean a charging current of maybe 10 amps or more."
This, Vassallo says, would require special, and expensive, battery chargers – and would rule out charging a phone or tablet from a USB port."This is another example of the new battery technologies that are coming through now," Vassallo explains. "It is a very exciting time in battery science and engineering."
But, he cautions: "It takes at least five or 10 years for a new battery to be commercialised because of the huge investment in manufacturing and safety testing that is required."
Sun sense
Solar batteries consist of a large number of connected solar cells, which convert sunlight directly into electricity.
Researchers in the US have succeeded in building a solar cell and a battery in a single device. It achieves an energy saving of 20 per cent over traditional lithium-ion batteries, as a result of a unique solar panel on top of the battery.
The solar panel is a solid sheet rather than a mesh as in previous designs. Another key difference comes from the use of a water-based electrolyte inside the battery. Because water circulates inside it, the new design belongs to a relatively new class of "aqueous flow" batteries, as they are known.
"Solar-charged batteries, where the solar cell is integrated into the battery's outer casing, and connected internally to the battery cells, are becoming available," says MacFarlane.
There are numerous approaches to this but matching the solar cell output to what the battery needs is the challenge. "For larger-scale domestic solar storage, batteries inside the house are more suitable – and subject to less extremes of temperature. For example, the Tesla battery – but this is pretty expensive at the moment," he notes.
Vassallo, meanwhile, is working on a variety of promising new solar batteries. "They will make a big difference to the networks – but I can't see the economics of grid disconnection working anytime soon," he warns.
Plant power
Another promising line of research exploits the properties of certain plants, crops and biological wastes.
The climbing plant madder (Rubia tinctorum), for instance, has the potential to build a rechargeable battery that is greener than any other, say researchers at Rice University and the City College of New York in the US.
"We can accomplish the same function as the cobalt, used in lithium-ion batteries, by using a dye extracted from the madder plant," says lead researcher Dr Leela Mohana Reddy, formerly of Rice University. The organic dye, called purpurin, has been used since time immemorial for colouring fabrics.
With some tweaking, purpurin is turned into a highly effective, natural cathode for lithium-ion batteries. "What's more, the cathode can be produced at room temperature – something that couldn't be done with cobalt cathodes," Reddy explains.
"We are fascinated by the ability to develop alternative electrode technologies to replace conventional inorganic materials in batteries," he adds. "And we're now tapping into other plant molecules to further improve battery performance."
Organic batteries would suit a variety of purposes – from powering tomorrow's shelf of small electronic toys, such as robotic cats and dogs, to fleets of electric vehicles. But they are some way from commercial reality.
"We are working at a laboratory scale now, so it might take a couple of years before fully organic batteries are on the market," Reddy says.
Air power
In their relentless quest for greener, cleaner battery systems, meanwhile, German researchers are turning to the stuff we breathe – air.
Innovative lithium-air batteries promise to deliver three to five times higher energy-storage capacities than lithium-ion batteries, says Dr Wolfgang Schmidbauer, the project co-ordinator for the Glanz (stands for "anodes and cells protected by glass") project at Schott, a technology company in Mainz.
The battery is expected to achieve a specific energy of about 1000 watt-hours per kilogram, thus extending the range for electric cars.
This new battery relies on a special material of glass-ceramic powder, Schmidbauer explains. The size of its grains is ideal for conducting lithium ions and provides outstanding electrochemical resistance.
"Our material is bound inside an organic matrix," he says. "Together, both elements form a highly stable and dense membrane that separates the battery's two electrodes."
In all laboratory set-ups, synthetic air or oxygen is used for the reaction that takes place at the battery's cathode. "This is because real air has some impurities, such as carbon monoxide or carbon dioxide," Schmidbauer notes.
Not ready
The new battery is not yet ready for market. "We still have fundamental problems that need scientific research to be solved. And then there's engineering scale-up. So, all in all, it will take 10 to 20 years before the lithium-air battery is on sale."
Conducting ions in solid state materials is where the future lies, Schmidbauer believes. "We're optimistic that we'll be able to apply our expertise to a range of new applications."
High-energy batteries running on lithium-air, or lithium sulphur, for example, are well suited for robots or unmanned vehicles, he adds.
"One application, which I like personally, is in an unmanned aircraft which flies above regular altitudes and works as a network relay for internet and telecommunications purposes," Schmidbauer says.
The daylight energy for such a craft would be derived from solar cells, which also load the batteries; at nighttime, the batteries would supply the required energy. Because it would be a fully electric vehicle, such an aircraft could cruise for weeks or months at a stretch, he notes.
Future
Casting ahead, promising new devices include organic lithium-ion batteries, which are potentially more environmental friendly than conventional metal-based batteries because they use organic polymers, which are flexible plastics, instead of metals to provide electrical power.
In addition, there is much potential for supercapacitors – devices that store electronic charge for a short time, such as those used to keep car electronics running smoothly. In essence, they comprise two metallic plates separated by a dielectric, a material that is a poor conductor of electricity.
Supercapacitors have an important role to play in providing for the "surges in energy" or moments of high power demand, thereby extending the capacity and life of the battery.
"Mazda already has a capacitor at the core of their hybrid electric vehicle regenerative braking system," MacFarlane notes.
A Sydney company, meanwhile, is developing supercapacitors for several car applications. "There is even discussion of using them for very rapidly charging power banks on trams, every time they stop," MacFarlane says.
Combinations of such devices are also possible, says MacFarlane. "In a combined device, the supercapacitor deals with high power demands – fast charging and discharging – with the lithium-ion battery designed for optimal energy storage."
Vassallo has some doubts. "At first glance, you would think they will work well together, but combining batteries and supercapacitors can be a struggle and may not be necessary," he points out.
The voltage of a capacitor drops as energy is taken out, he adds. "But, for most batteries, the voltage doesn't change that much except at the extremes. So what happens is that without extra components to manage this, the full benefit of the capacitor is not obtained."
Lithium batteries are now very capable of rapid, high-power discharge so if needed they can provide this pulse power.
"Of course, not all lithium batteries are designed for this but many of the new one have really good high power capability," Vassallo says. "Some electric vehicles started using combination capacitors and batteries, but Tesla and most others find it isn't needed and just adds to cost and complexity."
Yet another prospect is the flow battery, a rechargeable device based on two chemical components that are dissolved in electrolyte liquid and separated by a membrane. The exchange of ions generates the flow of electric current. Among other things, these batteries can be quickly recharged by replacing the electrolyte liquid.
"Flow batteries – large, small and micro – will be the next major evolution of energy-storage technology and should be commonplace in 40 years' time," MacFarlane says. "Those based on 'air electrodes', which use oxygen in the air as a component in the battery reaction, will offer the highest energy density at the lowest cost."
Links
And Doug MacFarlane's research Or follow him on Twitter @DRMacFarlane
Learn more about the workings of electric and hybrid vehicles in the textbook Oxford Big Ideas Australian Curriculum Science 8 (Oxford University Press)
VCAA link
Please send bright ideas for new topics to pspinks@fairfaxmedia.com.au
