new rival to the lithium-ion battery has been created that charges in under a minute and still performs almost perfectly after being recharged thousands of times. The new battery is based on aluminium instead of lithium, which should make it both cheaper and safer than their lithium-ion competitors. The US team behind the aluminium-ion battery say that the technology could find its way into the home, help store renewable energy for the power grid and even power vehicles.
The aluminium-ion battery is conceptually similar to the lithium-ion battery: when the battery is discharged atoms from a metal anode are oxidised, releasing electrons into the external circuit. When recharged, the electrons are driven back to the anode.
The aluminium-ion battery offers tantalising solutions to problems with lithium-ion ones. Aluminium, being the most abundant metal in the Earth's crust, is much cheaper than lithium and is also much less reactive so battery fires are unlikely to be a problem. Ionising aluminium also liberates three electrons compared with lithium's one, potentially giving the batteries a higher charge capacity. But aluminium-ion batteries have been plagued by numerous difficulties: the discharge voltages have often been as low as 0.55V and various cathodes trialled have disintegrated during repeated cycling, causing the lifetimes of the batteries of the batteries to drop to 85% or less within 100 cycles.
But now Hongjie Dai and colleagues at Stanford University in California unveil a prototype battery with a new graphite cathode that solves these problems. The group's discovery of the remarkable properties of graphite began with a stroke of luck. While investigating the potential of metal oxide films as cathodes, the researchers used graphite particles to increase their conductivity. When the composite films showed battery like behaviour, they decided to investigate further. In the Stanford team's aluminium-ion battery, aluminium chloride ions (AlCl4-) are stored at the cathode when the battery is charged. On discharge, the ions migrate to the anode where they combine with metallic aluminium to form Al2Cl7-. When the researchers investigated the graphite cathode’s chemistry they discovered that AlCl4- anions reversibly intercalate between the graphite layers.
Natural problem
However, natural graphite cathodes suffered from two problems: first, the battery's maximum charge and discharge rates were slowed by diffusion of the bulky AlCl4- ions between the atomic planes. Worse, when the ions did diffuse in, they caused 50-fold expansion of the graphite, pulling it apart into loosely-stacked atomic flakes. The team solved both problems with a graphitic foam they produced by depositing carbon onto nickel and then dissolving the metal in acid to produce a carbon structure with large atomic spaces that allowed the AlCl4- ions in and out rapidly with little or no damage.