Team visualizes complex electronic state

May 19, 2014 by David L. Chandler
The internal molecular structure of the electrode compound reveals what the researchers call the "superstructure." At right is a scanning transmission electron microscope image of the material, and, at left, the image is color-coded based on electrical properties: Each green dot stands for a stripe of manganese plus-4 ions; purple dots, for manganese plus-3 ions; and mixed color dots (green inside purple), for stripes with both ions. The ordering of stripes shows the cooperative Jahn-Teller distortion. Credit: Xin Li

A material called sodium manganese dioxide has shown promise for use in electrodes in rechargeable batteries. Now a team of researchers has produced the first detailed visualization—down to the level of individual atoms—of exactly how the material behaves during charging and discharging, in the process elucidating an exotic molecular state that may help in understanding superconductivity.

The new findings are reported this week in the journal Nature Materials, in a paper by MIT postdoc Xin Li, professors Young Lee and Gerbrand Ceder, also of MIT, and 12 others.

The phenomenon the team investigated—known as the cooperative Jahn-Teller effect —"is a basic piece of physics that has been well-known historically," explains Ceder, the R.P. Simmons Professor of Materials Science and Engineering. It describes how the positions of atoms in certain compounds can be slightly distorted, changing the material's electrical and magnetic properties.

"It is associated with a lot of interesting phenomena," Ceder says—so a better understanding could be useful both in advancing our knowledge of physics and in potential applications, from improved batteries to new kinds of electronics.

While the Jahn-Teller phenomenon is well-known, Ceder says it's a bit unusual to see it in battery compounds such as the sodium manganese dioxide now under investigation as a possible lower-cost substitute for the lithium-based electrodes in .

Such work when an electrical current pulls ions out of an electrode during charging, then returns them to the electrode as the battery is used. The arrangement of atoms within the material "is very ordered, and normally the ordering is driven by fairly standard physics," Ceder says. "But in this material, the order is completely driven by the Jahn-Teller effect."

Understanding how that difference affects charging and discharging could be important in guiding teams around the world who are seeking to improve the performance of such batteries, but it proved a daunting challenge for the MIT team.

The team combined density functional theory with technologies including electron diffraction; synchrotron X-ray diffraction; neutron diffraction; and aberration-corrected atomic-resolution scanning microscopy for direct visualization. Using these methods, the researchers showed that the material produces a "superstructure" governed by the Jahn-Teller effect; at very low temperatures, it produces a kind of "magnetic stripe sandwich," with alternating stripes of ferrimagnetic and antiferromagnetic atomic chains.

"This is fundamental work," Li says, to determine "any intrinsic capacity limits to sodium manganese dioxide"—such as how much charge it can hold, or how many times it can go through the charge-discharge cycle without degradation. The ultimate goal is to find out "how [to] make a higher-capacity sodium-ion battery electrode," Li says.

In addition to possible battery applications, the work led to the finding that sodium forms bands of magnetic domains at temperatures of 60 kelvins (-352 degrees Fahrenheit) or less. This finding, Li says, may be important to the emerging field of spin electronics, where the spin states of electrons, rather than their electrical charges, carry and store information.

Even before this new research, Li says, batteries made of this sodium-ion composition "showed comparable capacity to the commercial lithium-ion batteries," which are one of the leading technologies in production today. While no companies are now producing sodium-ion batteries, the technology has great potential: Sodium is more abundant, less expensive, and safer to work with than lithium.

"This is still fairly basic research," Li says, adding: "Understanding always pushes us forward, especially in this field. You only make progress by understanding these materials better."

Explore further: Silly Putty material inspires better batteries

More information: Direct visualization of the Jahn–Teller effect coupled to Na ordering in Na5/8MnO2, DOI: 10.1038/nmat3964

Related Stories

Disordered materials hold promise for better batteries

Jan 09, 2014

Lithium batteries, with their exceptional ability to store power per a given weight, have been a major focus of research to enable use in everything from portable electronics to electric cars. Now researchers ...

Silly Putty material inspires better batteries

May 15, 2014

Using a material found in Silly Putty and surgical tubing, a group of researchers at the University of California, Riverside Bourns College of Engineering have developed a new way to make lithium-ion batteries ...

Komaba Group reports sodium ion battery progress

Sep 28, 2012

(Phys.org)—Scientists with a common goal, to figure out an alternative to the lithium ion battery, the main power source of choice, are not giving up. The quarrel is not with the lithium ion battery's performance ...

Recommended for you

Pseudoparticles travel through photoactive material

Apr 23, 2015

Researchers of Karlsruhe Institute of Technology (KIT) have unveiled an important step in the conversion of light into storable energy: Together with scientists of the Fritz Haber Institute in Berlin and ...

From metal to insulator and back again

Apr 22, 2015

New work from Carnegie's Russell Hemley and Ivan Naumov hones in on the physics underlying the recently discovered fact that some metals stop being metallic under pressure. Their work is published in Physical Re ...

Electron spin brings order to high entropy alloys

Apr 22, 2015

Researchers from North Carolina State University have discovered that electron spin brings a previously unknown degree of order to the high entropy alloy nickel iron chromium cobalt (NiFeCrCo) - and may play ...

Expanding the reach of metallic glass

Apr 22, 2015

Metallic glass, a class of materials that offers both pliability and strength, is poised for a friendly takeover of the chemical landscape.

Electrons move like light in three-dimensional solid

Apr 22, 2015

Electrons were observed to travel in a solid at an unusually high velocity, which remained the same independent of the electron energy. This anomalous light-like behavior is found in special two-dimensional ...

Quantum model helps solve mysteries of water

Apr 20, 2015

Water is one of the most common and extensively studied substances on earth. It is vital for all known forms of life but its unique behaviour has yet to be explained in terms of the properties of individual ...

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

swordsman
not rated yet May 20, 2014
Excellent! This is a step in the right direction in understanding the relationships between atoms in the near field. Too bad the dimensions weren't shown, as they are extremely important.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.