Researchers at Cornell University in the USA have developed batteries made with an antiferroelectric material without using lead. The new energy cell can release electrical discharges as quickly as those made exclusively from the toxic metal.
There are only a few known antiferroelectric materials in nature, and most of them contain lead. This structural feature makes them not safe enough to be used in everyday applications, away from laboratories.
“Some technologies require a faster discharge of energy, such as cardiac defibrillators. To build this equipment, alternative materials are used, mainly antiferroelectric ones”, explains industrial chemistry professor Darrell Schlom, co-author of the study.
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Lead-free antiferroelectrics
Like ferroelectrics, antiferroelectric metals belong to a class of materials that have bits of information called dipoles, arranged in a common pattern. In ferroelectrics, the dipoles are oriented in the same direction, while in antiferroelectrics, they alternate up and down. As the adjacent dipoles cancel each other out, the material has no specific polarization.
During the experiments, the researchers tested thin films of bismuth ferrite — a ferroelectric with the highest polarization of any known material. They stacked alternating layers of bismuth ferrite with other insulators to create a structure a thousand times thinner than the diameter of a human hair.
As the ferroelectric layers of bismuth ferrite became thinner, they realized that this highly polarized material was turning into an antiferroelectric compound without polarization, giving rise to a state hitherto unknown to scientists.
“Usually, we always study the most stable form of a material — its lowest energy state. Using our thin-film technology, we found it possible to stabilize what normally has a higher, inaccessible energy state,” adds the professor of chemist Julia Mundy, lead author of the study.
electrostatic engineering
The behavior of these dipoles can be exemplified as a “battle of two giants”, with the original ferroelectric material trying to polarize neighboring insulating layers in the power cell and surrounding insulating layers trying to resist this constant polarization.
This new ground state was called a polymorph by researchers because it still has the same atoms seen in its primitive condition, with only a slight structural change caused by electrostatic forces applied to the system.
“For two decades, we’ve focused on altering energy landscapes and material properties by applying tension and stretching thin films onto a crystal substrate. Now, electrostatic engineering has provided a new tool to replace lead that is not mechanical, it is electrical”, concludes Professor Darrell Schlom.
