That dream battery could be closer to reality. Researchers at Northwestern University have created an electrode for lithium-ion batteries – rechargeable batteries like those found in cell phones and iPods – that allows the batteries to charge more quickly and hold a charge up to 10 times longer than current technology. Researchers say the technology could pave the way for more efficient, smaller batteries that could be useful in electric cars. [Zhao et al., Advanced Energy Materials, (2011), DOI: 10.1002/aenm.201100426]
“We have found a way to extend a lithium-ion battery’s charge life by 10 times at the beginning of the battery’s life,” said Professor Kung, lead author of the paper. “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.”
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. As energy in the battery is used, the lithium ions travel from the anode, through the electrolyte, and to the cathode; as the battery is recharged, they travel in the reverse direction.
Meanwhile, a battery’s charge rate – the speed at which it recharges – is limited by another factor: the speed at which the lithium ions can make their way from the electrolyte into the anode. Currently, that speed is hindered by the shape of the graphene sheets: They are extremely thin – just one carbon atom thick – 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. And because it takes so long for the lithium to travel to the middle of the graphene sheet, a sort of ionic traffic jam occurs around the edges of the material.
Now, Kung’s research team has combined two techniques to combat both these problems. First, to stabilize the silicon in order to maintain maximize charge capacity, they sandwiched clusters of silicon between the graphene sheets. This allowed for a greater number of lithium ions in the electrode while utilizing 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.”