Technology could increase lithium-ion batteries’ charge

Northwestern University engineers claim that they have developed technology that could improve rechargeable batteries.

The university said in a statement that the research team has created an electrode for lithium-ion batteries that allows them to hold a charge up to 10 times greater than current technology.

Batteries with the electrode are also reported to be able to charge 10 times faster than current batteries.

The researchers are reported to have achieved this by combining two chemical-engineering approaches.

‘Even after 150 charges, which would be one year or more of operation, the battery is still five times more effective than lithium-ion batteries on the market today,’ explained Prof Harold Kung from the McCormick School of Engineering and Applied Science.

Lithium-ion batteries charge through a chemical reaction in which lithium ions are sent between two ends of the battery, the anode and the cathode.

In current rechargeable batteries, the anode, which is made of multiple layers of carbon-based graphene sheets, can only accommodate one lithium atom for every six carbon atoms. To increase energy capacity, scientists have previously experimented with replacing the carbon with silicon, as silicon can accommodate much more lithium — four lithium atoms for every silicon atom. However, silicon expands and contracts dramatically in the charging process, causing fragmentation and losing its charge capacity rapidly.

Currently, the speed of a battery’s charge rate is hindered by the shape of the graphene sheets; they are extremely thin but, by comparison, very long. During the charging process, a lithium ion must travel all the way to the outer edges of the graphene sheet before entering and coming to rest between the sheets. Because it takes so long for lithium to travel to the middle of the graphene sheet, an ionic ’traffic jam’ occurs around the edges of the material.

Kung’s research team has combined two techniques to combat both of these problems. First, to stabilise the silicon in order to maintain maximum charge capacity, it sandwiched clusters of silicon between the graphene sheets. This allowed for a greater number of lithium atoms in the electrode, while utilising the flexibility of graphene sheets to accommodate the volume changes of silicon during use.

‘Now we almost have the best of both worlds,’ Kung said. ‘We have much higher energy density because of the silicon and the sandwiching reduces the capacity loss caused by the silicon expanding and contracting. Even if the silicon clusters break up, the silicon won’t be lost.’

Kung’s team also used a chemical oxidation process to create minuscule holes (10 to 20nm) in the graphene sheets, termed ‘in-plane defects’, so that the lithium ions would have a ‘shortcut’ into the anode and be stored there through their reaction with silicon.

The technology could pave the way for better batteries in mobile phones and iPods, as well as more efficient, smaller batteries for electric cars. The research team believes that the technology could be seen in the marketplace in the next three to five years.

A paper describing the research is published in the journal Advanced Energy Materials.