A team of engineers has created an
electrode for lithium-ion batteries -- rechargeable batteries such as those
found in cellphones and iPods -- that allows the batteries to hold a charge up
to 10 times greater than current technology. Batteries with the new electrode
also can charge 10 times faster than current batteries.
This research was all focused on the anode; next, the researchers will
begin studying changes in the cathode that could further increase effectiveness
of the batteries. They also will look into developing an electrolyte system
that will allow the battery to automatically and reversibly shut off at high
temperatures - a safety mechanism that could prove vital in electric car
applications.
The researchers combined two chemical
engineering approaches to address two major battery limitations -- energy
capacity and charge rate -- in one fell swoop. In addition to better batteries
for cellphones and iPods, the technology could pave the way for more efficient,
smaller batteries for electric cars.
The technology could be seen in the
marketplace in the next three to five years, the researchers said.
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 in two ways. Its energy
capacity -- how long a battery can maintain its charge -- is limited by the
charge density, or how many lithium ions can be packed into the anode or
cathode. 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.
In current rechargeable batteries, the
anode -- made of layer upon layer 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 -- 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 lithium to travel to the middle of the graphene
sheet, a sort of ionic traffic jam occurs around the edges of the material.
Now, research team has combined two
techniques to combat both these problems. First, to stabilize the silicon in
order to maintain maximum charge capacity, they sandwiched clusters of silicon
between the graphene sheets. This allowed 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. Thus much higher energy
density have been achieved 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.
Scientist also used a chemical
oxidation process to create miniscule holes (10 to 20 nanometers) in the
graphene sheets termed in-plane defects so the lithium ions would have a
"shortcut" into the anode and be stored there by reaction with
silicon. This reduced the time it takes the battery to recharge by up to 10
times.
