Cell phone batteries often heat up and at times, can burst into flames.
In most cases, the culprit behind such incidents can be traced back to lithium batteries.
Despite providing long-lasting electric currents that can keep devices powered up, lithium
batteries can internally short circuit, heating up the device.
Researchers at Texas A&M University have invented a technology that can prevent lithium batteries
from heating and failing.
Their carbon nanotube design for the battery’s conductive plate, called the anode, enables
the safe storage of a large quantity of lithium ions, thereby reducing the risk of fire.
Further, the researchers said that their new anode architecture will help lithium batteries
charge faster than current ­­commercially available batteries.
Their results are published in the journal Nano Letters.
When lithium batteries are in use, charged particles move between the battery’s two
compartments.
Electrons given up by lithium atoms move from one side of the battery to the other.
On the other hand, lithium ions travel the other direction.
When charging the battery, lithium ions and electrons go back to their original compartments.
Hence, the property of the anode, or the electrical conductor that houses lithium ions within
the battery, plays a decisive role in the battery’s properties.
A commonly used anode material is graphite.
In these anodes, lithium ions are inserted between layers of graphite.
However, this design limits the amount of lithium ions that can be stored within the
anode and even requires more energy to pull the ions out of the graphite during charging.
These batteries also have a more insidious problem.
Sometimes lithium ions do not evenly deposit on the anode.
Instead, they accumulate on the anode’s surface in chunks, forming tree-like structures,
called dendrites.
Over time, the dendrites grow and eventually pierce through the material that separates
the battery's two compartments.
This breach causes the battery to short circuit and can set the device ablaze.
Growing dendrites also affect the battery’s performance by consuming lithium ions, rendering
them unavailable for generating a current.
Another anode design involves using pure lithium metal instead of graphite.
Compared to graphite anodes, those with lithium metal have a much higher energy content per
unit mass or energy density.
But they too can fail in the same catastrophic way due to the formation of dendrites.
To address this problem, the researchers designed anodes using highly conductive, lightweight
materials called carbon nanotubes.
These carbon nanotube scaffolds contain spaces or pores for lithium ions to enter and deposit.
However, these structures do not bind to lithium ions favorably.
Hence, they made two other carbon nanotube anodes with slightly different surface chemistry
— one laced with an abundance of molecular groups that can bind to lithium ions and another
that had the same molecular groups but in a smaller quantity.
With these anodes, they built batteries to test the tendency to form dendrites.
As expected, the researchers found that scaffolds made with just carbon nanotubes did not bind
to lithium ions well.
Consequently, there was almost no dendrite formation, but the battery’s ability to
produce large currents was also compromised.
On the other hand, scaffolds with an excess of binding molecules formed many dendrites,
shortening the battery’s lifetime.
However, the carbon nanotube anodes with an optimum quantity of the binding molecules
prevented the formation of dendrites.
In addition, a vast quantity of lithium ions could bind and spread along the scaffold’s
surface, thereby boosting the battery’s ability to produce large, sustained currents.
The researcher said that their top-performing anodes handle currents five times more than
commercially-available lithium batteries.
This feature is particularly useful for large-scale batteries, such as those used in electric
cars, that require quick charging.