Solid electrolytes: Sodium-based high performer

04/27/2015

A new sodium-based material with an excellent ion conductivity is a promising cheap alternative to lithium-based electrolytes

Both the wide corridors between the B10H10 anions and the reorientational motion of these anions are thought to contribute to the exceptionally high sodium-ion conductivity of Na2B10H10.
Both the wide corridors between the B10H10 anions and the reorientational motion of these anions are thought to contribute to the exceptionally high sodium-ion conductivity of Na2B10H10.

© 2015 Shin-ichi Orimo

A sodium-based material highly promising for use as a solid electrolyte in rechargeable batteries has been discovered by an international team of researchers1. The material is inexpensive since it consists of common elements, and it exhibits an exceptionally high conductivity for sodium ions at temperatures above 110 degrees Celsius.

Solid electrolytes are superior to their liquid counterparts for use in rechargeable batteries because they do not leak or explode. Lithium-based solid electrolytes are currently the best performers, but the relative scarcity of lithium means that their price fluctuates with global availability. Consequently, researchers are searching for alternative materials made from more abundant elements.

Now, Atsushi Unemoto and Shin-ichi Orimo at the AIMR and Motoaki Matsuo of the Institute for Materials Research, along with other researchers at Tohoku University and overseas collaborators, have discovered a potential rival to lithium-based electrolytes ― a complex hydride containing the metals sodium and boron (Na2B10H10). The material is inexpensive as it consists of three abundant elements: hydrogen, sodium and boron. Most importantly, it can rapidly ferry sodium ions between the electrodes of a battery, making it attractive for high-power applications.

When the researchers heated the material from room temperature, the sodium-ion conductivity initially increased considerably, but it suddenly leapt by almost a hundredfold when the temperature reached about 110 degrees Celsius. This dramatic increase in conductivity was due to a change in the material’s structure from a tightly packed structure to one containing wide, open corridors through which charge-carrying sodium ions could readily travel. The resulting sodium-ion conductivity is over ten times higher than those of previously investigated sodium-based complex hydrides.

The researchers strongly suspect, however, that another mechanism also contributes to the excellent sodium conductivity of the material. They believe that the ‘reorientational motion’ of the anion columns in the structure in some way assists the sodium ions as they travel through the corridors (see image).

“We anticipated that the material would exhibit a high ionic conductivity,” explains Matsuo, “because we had found a strong correlation between the reorientational motion of complex anions and the mobility of cations in complex hydrides in previous studies.”

The researchers are keen to explore the potential of this material. “In the future, we hope to reduce the onset temperature for sodium-ion conduction in Na2B10H10 from its present 110 degrees Celsius to close to room temperature,” says Matsuo. “In the longer term, we aim to construct all-solid-state sodium rechargeable batteries by using the material as the electrolyte.”

References

  1. Udovic, T. J., Matsuo, M., Tang, W. S., Wu, H., Stavila, V., Soloninin, A. V., Skoryunov, R. V., Babanova, O. A., Skripov, A. V., Rush, J. J., Unemoto, A., Takamura, H. & Orimo, S.-i. Exceptional superionic conductivity in disordered sodium decahydro-closo-decaborate. Advanced Materials 26, 7622−7626 (2014). | article

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.