Nat. Commun. 10, 2662 (2019)

Ion transport through solids is a fundamental process for the function of many devices, such as fuel cells and batteries. While the past several decades have seen tremendous efforts in discovering new ion-conducting materials, an equally — if not more — important focus is on the fundamental understanding of their conduction mechanism. Now, Morimoto et al. apply terahertz (THz) spectroscopy to track the optical conductivity of an electrolyte at picosecond to nanosecond timescales, revealing otherwise inaccessible information about microscopic ion migration.

The researchers use yttria-stabilized zirconia, a model ionic conductor in which the oxygen vacancies oscillate thermally at a frequency of several THz before hopping to neighbouring sites. On application of THz time-domain spectroscopy, the measured THz conductivity increases monotonically across the frequency range and is confirmed to originate from the strong oscillation of the vacancies. Temperature dependence analysis allows the extraction of the activation energy, which reflects the intrinsic potential barrier the mobile species has to overcome.