By Hayley Axford
Researchers from Stanford University have developed a rechargeable aluminium battery that could replace the existing storage devices such as alkaline and lithium-ion batteries. With a reported global smartphone audience of 1.75 billion people, it is widely known that the battery life of smartphones is generally poor. While lithium-ion batteries take hours to charge, the prototype aluminium battery would be able to charge your smartphone in 60 seconds!
Stanford researchers believe that the new technology offers a safer alternative to many commercial batteries that are used today. In a report from the university, Professor of Chemistry, Hongjie Dai explained that alkaline batteries are bad for the environment and lithium-ion batteries, which take longer to charge, occasionally burst into flames. Two American airline companies, United Airlines and Delta Air Lines, have effectively banned bulk lithium-battery shipments on passenger planes due to their potential fire hazard. Professor Dai claims that the new aluminium-ion battery will not catch fire, “even if you drill through it”.
Due to it being lightweight and inexpensive, aluminium has attracted the interest of battery engineers for many years; however a commercially viable product was never developed. One of the main challenges researchers faced during development was finding materials that are capable of producing sufficient voltage after repeated cycles of charging and discharging.
An aluminium-ion battery consists of two electrodes: a negatively charged anode made of aluminium and a positively charged cathode. While others have tried various kinds of materials for the cathode, Professor Dai said that they “accidentally discovered that a simple solution is to use graphite, which is basically carbon”. Dai added that in their study, they identified a few types of graphite material that gave a very good performance. The Stanford researchers involved in this experimental battery, placed the aluminium anode and graphite cathode, along with an ionic liquid electrolyte, inside a flexible polymer-coated pouch. Stanford graduate student, Ming Gong, who is also the co-lead author of the study, explained that “the electrolyte is basically a salt that’s liquid at room temperature”, which makes it very safe. This contrasts with the flammable electrolytes used in lithium-ion batteries.
When the battery discharges, aluminium dissolves at the anode, while aluminium-containing ions slide into the spaces between atomic graphite layers at the cathode. When it begins charging again, the reverse occurs, depositing metallic aluminium metal back on the anode.
Another interesting feature of the aluminium battery is that it is flexible. According to Gong, “You can bend it and fold it, so it has the potential for use in flexible electronic devices”. Aluminium is also appealing due to its low flammability and high-charge storage capacity. Larger aluminium batteries could be used for storing renewable energy on electrical grids, which need batteries with a long cycle life to store and release energy.
Previous experimental aluminium batteries that were developed in other laboratories found that the battery died after just 100 charge-discharge cycles, whereas the Stanford aluminium battery prototype is able to keep on going for 7, 500 cycles without any loss of capacity. The standard lithium-ion battery only kept going for 1, 000 cycles. Researchers noted that this was the first time that an ultra-fast aluminium battery had been constructed with stability over thousands of cycles.
Stanford’s battery generates about two volts of electricity, which is the highest ever achieved with aluminium, however, this is nearly half of the voltage generated from the typical lithium-ion battery used with smartphones. As the battery is still in its prototype phase, Professor Dai feels that this can be resolved by improving the cathode material.
Battery technology has been the slowest of all technologies to evolve over the last 100 years, hence the mixed reaction over the development of Stanford’s prototype battery. “Aluminium batteries are very difficult technology and I think their method of storing the charges inside the graphite is rather clever”, said Professor Clare Grey, a materials chemist at the University of Cambridge. Engineer and inventor, Elon Musk is amongst the sceptics, taking to Twitter to comment on the announcement of the aluminium battery. Musk tweeted: “Battery ‘breakthroughs’ need to state power *and* energy density (not the same thing), plus how long they last. They usually fail on energy.”
Before this version of the aluminium battery can be mass produced, the team of researchers will have to improve the cathode material to increase voltage and energy density. The team has admitted that there is still plenty of room for growth within the project, but are aware of the growth potential in a battery like this. Commenting on this, Professor Dai said: “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.”