IPhone 4S owners bemoaning their feeble battery life may have some hope, at least, for the future: Northwestern University engineers have developed a cellphone battery that charges in 15 minutes and stays charged for more than a week.
They've created an electrode for lithium-ion batteries that allows the batteries to hold a charge up to 10 times greater than current technology and charge 10 times faster.
"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," says lead author Harold H Kung.
As well as better batteries for cellphones and iPods, the technology could lead to more efficient, smaller batteries for electric cars, says the team. The technology could hit the market in the next three to five years.
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.
With current technology, the performance of a lithium-ion battery is limited. How long a battery can maintain its chargeis limited by how many lithium ions can be packed into the anode or cathode. Meanwhile, the speed at which it recharges is limited by the speed at which the lithium ions can make their way from the electrolyte into the anode.
In current rechargeable batteries, the anode - made of layers of carbon-based graphene sheets - can only accommodate one lithium atom for every six carbon atoms. Replacing the carbon with silicon means much more lithium can be accommodated. However, silicon expands and contracts dramatically in the charging process, causing fragmentation and losing its charge capacity rapidly.
Kung's research team has been able to stabilize the silicon in order to maintain maximum charge capacity, by sandwiching clusters of silicon between the graphene sheets. This allows for a greater number of lithium atoms 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."
The team also used a chemical oxidation process to create miniscule holes in the graphene sheets to give the lithium ions a 'shortcut' into the anode. This reduces the time it takes the battery to recharge by up to 10 times.