Argonne nanoscientist Tijana Rajh
Coming at Batteries From Every Angle
December 2011 / January 2012
Volume 9 Number 6
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 Argonne National Laboratory, led by 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 seconds.
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 said. “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 said. 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,” said 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 said.
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 state-of-the-art nanofabrication techniques with high-intensity X-rays to characterize the nanotubes, the researchers were able to quickly observe this unusual behavior.
Although lithium-ion technology dominates headlines in battery research and development, a new element is also making its presence known as a potentially powerful alternative: sodium Sodium-ion technology possesses a number of benefits that lithium-based energy storage cannot capture, explained Argonne’s Christopher Johnson, who is leading an effort to improve the performance of ambient-temperature sodium-based batteries.
Perhaps most importantl, sodium is far more naturally abundant than lithium, which makes sodium lower in cost and less susceptible to extreme price fluctuations as the battery market rapidly expands.
“Our research into sodium-ion technology came about because one of the things we wanted to do was to cover all of our bases in the battery world,” Johnson said. “We knew going in that the energy density of sodium would be lower, but these other factors helped us decide that these systems could be worth pursuing.”
Sodium ions are roughly three times as heavy as their lithium cousins, however, and their added heft makes it more difficult for them to shuttle back and forth between a battery’s electrodes. As a result, scientists have to be more particular about choosing proper battery chemistries that work well with sodium on the atomic level.
While some previous experiments have investigated the potential of high-temperature sodium-sulfur batteries, Johnson explained that room-temperature sodium-ion batteries have only begun to be explored. “It’s technologically more difficult and more expensive to go down the road of sodium-sulfur; we wanted to leverage the knowledge in lithium-ion batteries that we’ve collected over more than 15 years,” he said.
Because of their reduced energy density, sodium-ion batteries will not work as effectively for the transportation industry, as it would take a far heavier battery to provide the same amount of energy to power a car. However, in areas like stationary energy storage, weight is less of an issue, and sodium-ion batteries could find a wide range of applications.
“The big concerns for stationary energy storage are cost, performance and safety, and sodium-ion batteries would theoretically perform well on all of those measures,” Johnson said.
All batteries are composed of three distinct materials—a cathode, an anode and an electrolyte. Just as in lithium-ion batteries, each of these materials has to be tailored to accommodate the specific chemical reactions that will make the battery perform at its highest capacity. “You have to pick the right materials for each component to get the entire system to work the way it’s designed,” Johnson said.
To that end, Johnson has partnered with Rajh’s group to investigate how sodium ions are taken up by the titanium dioxide anodes. “The way that those nanotubes are made is very scalable—if you had large sheets of titanium metal, you can form the tubes in a large array,” Johnson said. “That would then enable you to create a larger battery.”
The next stage of the researchwould involve the exploration of aqueous, or water-based, sodium-ion batteries, which would have the advantage of being even safer and less expensive.
Jared Sagoff is a writer at Argonne National Laboratory.
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