A
breakthrough in components for next-generation batteries could come from
special materials that transform their structure to perform better over time.
A team of
researchers at the U.S. Department of Energy’s Argonne National Laboratory, led
by Argonne nanoscientist Tijana Rajh and battery expert Christopher Johnson,
discovered that nanotubes composed of titanium dioxide can switch their phase
as a battery is cycled, gradually boosting their operational capacity.
Laboratory tests showed that new batteries produced with this material could be
recharged up to half of their original capacity in less than 30 sec.
By
switching out conventional graphite anodes for ones composed of the titanium
nanotubes, Rajh and her colleagues witnessed a surprising phenomenon. As the
battery cycled through several charges and discharges, its internal structure
began to orient itself in a way that dramatically improved the battery’s
performance.
“We
did not expect this to happen when we first started working with the material,
but the anode spontaneously adopted the best structure,” Rajh says.
“There’s an internal kind of plasticity to the system that allows it to
change as the battery gets cycled.”
According
to Argonne nanoscientist Hui Xiong, who worked
with Rajh to develop the new anode material, titanium dioxide seemed like it
would be unlikely to adequately substitute for graphite. “We started with
a material that we never thought would have provided a functional use, and it
turned into something that gave us the best result possible,” she says.
One of the
other researchers in Rajh’s group, Sanja Tepavcevic, has adopted a similar
approach to make a self-improving structure for a sodium-ion nanobattery.
“This
is highly unusual material behavior,” adds Jeff Chamberlain, an Argonne chemist who leads the laboratory’s energy storage
major initiative. “We’re seeing some nanoscale phase transitions that are
very interesting from a scientific standpoint, and it is the deeper
understanding of these materials’ behaviors that will unlock mysteries of
materials that are used in electrical energy storage systems.”
The reason
that titanium dioxide seemed like an implausible solution for battery
development lies in the amorphous nature of the material. Because amorphous
materials have no internal order, they lack the special electronic properties
of highly ordered crystalline materials. However, amorphous materials have not
been known to undergo such profound structural transformations during cycling,
according to Rajh. Most of the known battery materials undergo the opposite
transition: they start out as highly crystalline and pulverize to an amorphous
state upon cycling.
Having
anodes composed of titanium dioxide instead of graphite also improves the
reliability and safety of lithium-ion batteries. In certain cases, lithium can
work its way out of solution and deposit on the graphite anodes, causing a
dangerous chain reaction known as thermal runaway. “Every type of test
we’ve conducted on titanium anodes has shown them to be exceptionally
safe,” Chamberlain says.
The Argonne discovery came from
collaboration between two of the laboratory’s flagship user facilities: the Center
for Nanoscale Materials and the Advanced Photon Source. By combining
nanofabrication techniques with high-intensity X-rays to characterize the
nanotubes, the Argonne researchers were able to
quickly observe this unusual behavior.
