Materials that conduct electricity with ions rather than electrons are essential for batteries, fuel cells, and more exotic technologies. Providing enough space in the molecular structure for the movement of ions usually requires a disordered (non-crystalline) material. But a paper in the 10 April Physical Review Letters reports a crystalline compound with very high conductivity. It consists of positively-charged lithium ions flowing through a stacked structure of much larger, negatively-charged buckyballs (soccer-ball-shaped molecules). The results suggest a new category of crystalline materials for researchers to study as they design new devices.
Highly ordered crystals are usually not the best starting point for researchers who want to make new materials with high ionic conductivity. Ionic conductors consist of molecules or groups of atoms of one charge that form a network, with space between them for smaller ions of the opposite charge to flow. But researchers usually assume conductivity requires random imperfections in the crystal structure, such as gaps or defects, for the smaller ions to hop into and out of; otherwise there would be no ionic flow.
A team of researchers from Italy, Hungary, and the UK reasoned that buckyballs bonded into a crystal structure, like stacked fruit, would generate a material with big spaces in between the spheres. “To create large channels, we need large building blocks,” says team member Mauro Riccò of the University of Parma in Italy. And each buckyball can accommodate multiple negative charges, good for incorporating many positive ions. But which ion to use? Previous experiments found that sodium ions couldn’t move easily between the buckyballs.
The smaller lithium ion is a much better choice, Riccò and his colleagues report after completing a long characterization of their new compound, which consists of four lithium ions per buckyball. There was some uncertainty about whether lithium would work, because it forms strong ionic bonds with in some other compounds. But the team measured a conductivity of 0.01 siemens per centimeter at room temperature in 100-milligram pellets of or five times the conductivity of standard, non-crystalline ionic conductors, says Riccò.
To show the conductivity was the result of flowing lithium ions and not some breakdown of the material, the team used nuclear magnetic resonance (NMR) measurements to probe powdered . The data confirmed that the lithium ions were indeed circulating around the buckyballs. According to both sets of experimental data, the activation energy required to bump a lithium ion from one buckyball to another is 200 milli-electron-volts, or roughly the energy of a typical molecular vibration. Riccò says the conductivity occurs because the lithium ions are so small, and the negative buckyball charges–which could theoretically impede the lithium flow–are more spread out than would be the case for more compact molecules.
The material’s conductivity and activation energy put it in the class of “superionic” conductors, which have about the same conductivity as water. That’s still a factor of 10 shy of “advanced” superionic conductors, which so far are non-crystalline, but is still “respectably high,” says Stephen Bennington of the Rutherford Appleton Laboratory in Oxfordshire, England. He notes that advanced ionic conductors are being used in so-called supercapacitors to provide a burst of power in some hybrid cars and prototype energy storage devices.
Riccò says the Parma group is still characterizing to see if it could possibly serve as a material for future lithium ion batteries. Whether applications are forthcoming or not, Bennington says the discovery “means that many groups are likely to start looking at the large zoo of -based structures for their potentiality as ionic conductors.” Like ions in a conductor, unexpected new materials have a tendency to diffuse.