Excitement builds over new battery technology
Aluminium battery technology could provide a breakthrough alternative to conventional battery technologies such as lithium-ion, providing a safe, long-lasting, fast charging option in applications from small electronic devices right up to renewable energy storage.
From electric vehicles to smartphones, everyone is looking for that next big development in battery technology - the step change that will deliver faster charging times, longer operation and greater life-time. Currently lithium-ion is the technology of choice for performance applications, but the race is on to develop and commercialise superior alternatives, with lots of potential from the likes of aluminium-air, sodium-nickel and even super-capacitor technologies.
It would be grossly unfair to suggest that lithium-ion technology is beset with problems - and certainly the advantages of lithium-ion far outweigh its disadvantages - but there are some widely recognised downsides. The technology is somewhat fragile and requires protection circuitry to maintain safe operation. Aging is also a concern, and the issues surrounding transportation have been widely publicised.
Now researchers at Stanford University have developed an alternative battery technology that they say is fast charging, long lasting and inexpensive. They say the new aluminium-ion technology offers a safe alternative to many commercial batteries in wide use today, with headline features such as a 60 second charge time and a life time of over 7,500 charging cycles. "We have developed a rechargeable aluminium-ion battery that may replace existing storage devices, such as alkaline batteries, which are bad for the environment, and lithium-ion batteries, which occasionally burst into flames," says Hongjie Dai, a professor of chemistry at Stanford. "Our new battery won't catch fire, even if you drill through it."
Aluminium has long been an attractive material for batteries, mainly because of its low cost, low flammability and high-charge storage capacity. An aluminium-ion battery consists of two electrodes: a negatively charged anode made of aluminium and a positively charged cathode. "People have tried different kinds of materials for the cathode," Dai explains. "We accidentally discovered that a simple solution is to use graphite, which is basically carbon. In our study, we identified a few types of graphite material that give us very good performance." For the experimental battery, the Stanford team placed the aluminium anode and graphite cathode, along with an ionic liquid electrolyte, inside a flexible polymer-coated pouch. "The electrolyte is basically a salt that's liquid at room temperature, so it's very safe," says Stanford graduate student Ming Gong.
Dai adds: "Aluminium batteries are safer than conventional lithium-ion batteries used in millions of laptops and cell phones today. Lithium-ion batteries can be a fire hazard." As an example, he pointed to recent decisions by a number of airlines to ban bulk lithium-battery shipments on passenger planes.
"In our study, we have videos showing that you can drill through the aluminium battery pouch, and it will continue working for a while longer without catching fire," he continues. "But lithium batteries can go off in an unpredictable manner - in the air, the car or in your pocket. Besides safety, we have achieved major breakthroughs in aluminium-battery performance."
One example is ultra-fast charging. Smartphone owners know that it can take hours to charge a lithium-ion battery. But the Stanford team reported "unprecedented charging times" of down to 1 minute with the aluminium prototype.
Durability is another important factor. The Stanford team noted that aluminium batteries developed at other laboratories usually died after just 100 charge-discharge cycles. But the Stanford battery was able to withstand more than 7,500 cycles without any loss of capacity, with the researchers asserting that this was the first time an ultra-fast aluminium-ion battery had been constructed with stability over thousands of cycles. By comparison, they say, a typical lithium-ion battery lasts about 1,000 cycles.
"Another feature of the aluminium battery is flexibility," Gong adds. "You can bend it and fold it, so it has the potential for use in flexible electronic devices. Aluminium is also a cheaper metal than lithium."
Potential applications for the technology are wide ranging. In addition to small electronic devices, aluminium batteries could be used to store renewable energy on the electrical grid, says Dai. "The grid needs a battery with a long cycle life that can rapidly store and release energy," he explains. "Our latest unpublished data suggests that an aluminium battery can be recharged tens of thousands of times. It's hard to imagine building a huge lithium-ion battery for grid storage."
Aluminium-ion technology also offers an environmentally friendly alternative to disposable alkaline batteries, Dai adds. "Millions of consumers use 1.5V AA and AAA batteries. Our rechargeable aluminium battery generates about 2V of electricity. That's higher than anyone has achieved with aluminium."
But more improvements will be needed to match the voltage of lithium-ion batteries, Dai adds. "Our battery produces about half the voltage of a typical lithium battery. But improving the cathode material could eventually increase the voltage and energy density. Otherwise, our battery has everything else you'd dream that a battery should have: inexpensive electrodes, good safety, high-speed charging, flexibility and long cycle life. I see this as a new battery in its early days. It's quite exciting."
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