Key to long life, according to a paper in Nature (‘An ultrafast rechargeable aluminium-ion battery‘), is using a three-dimensional graphitic-foam for the cathode.

“People have tried different kinds of materials for the cathode,” said Professor Hongjie Dai. “We accidentally discovered that a simple solution is to use graphite. In our study, we identified a few types of graphite material that give us very good performance.”

The anode is metallic aluminium, with an ionic liquid electrolyte, and the whole thing fits in a flexible polymer-coated pouch, which can be flexed in use.

Remarkably, cycle life for the whole cell, currently measured at 7,500 cycles, is under hard use conditions.

“The cycle life reported is under fast charge/discharge operation,” Gong told Electronics Weekly. Discharge depth was “almost 100%. We cut off the voltage at 0.5V to avoid side reactions, but there were negligible capacity below 0.5V.”

According to Gong, it can also be charged in a minute and discharged in a minute.

Terminal voltage is more Pb-acid that Li-ion. “Our battery produces about half the voltage of a typical lithium battery,” said Dai. “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.”

Energy density in the proof-of-concept cell is estimated to be 40Wh/kg. “We believe that it can further improved, like the lithium ion batteries in the early stages,” Gong told Electronics Weekly.

How difficult is it to make ‘three-dimensional graphitic-foam’?

At current stage, the process is slightly complicated, said Gong, but it is a scalable process. “We are currently looking for materials with similar performance but with easier and more scalable production.”

Personal electronics and grid storage are proposed applications – although, for phones, capacity would have to be improved by 5-10x.

“The grid needs a battery with a long cycle life that can rapidly store and release energy,” said Dai. “Our latest unpublished data suggest that an aluminium battery can be recharged tens of thousands of times.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.”

According to Nature: