Explaining the Conductivity of Ionic Liquids
In most rechargeable batteries, ions move between electrodes by traveling through electrolytic solutions, which can be volatile and flammable. Room-temperature ionic liquids (RTILs)—salts with melting points below 100 °C—could offer a less hazardous alternative, but their behavior under strong electric fields remains unclear. Now Yufeng Cheng and colleagues at Beihang University in China and the University of Seville in Spain have simulated ion transport in one of the mainstays of RTIL research, an ionic liquid known as [BMIM][TFSI] [1]. The team explained a distinct high-field regime that had been observed before and discovered unexpected dynamics at still higher fields.
The conductivity of an electrolytic solution varies with its diffusivity according to the Nernst-Einstein equation. But in an ionic liquid, some ions of opposite charge form neutral complexes, breaking that relationship. Then, the Nernst-Einstein equation must be modified to treat only the portion of the liquid that comprises unbound ions. Nevertheless, charge transport remains proportional to the strength of an applied electric field—provided that the field strength is below a certain value.
Using molecular dynamics simulations, Cheng and colleagues found the threshold to be about 0.2 V/nm. Strengthening the field beyond that point yielded a disproportionate rise in charge transport owing to nonlinear increases in the RTIL’s diffusivity and conductivity. Previously, this nonlinearity had been attributed to neutral complexes becoming distorted and aligning their electric dipoles, making way for increased diffusion along the field direction. Cheng and colleagues confirmed this mechanism, but they found that it saturated at about 1.6 V/nm, while the diffusivity and conductivity continued to rise. To explain the continued rise, they propose that increasing the field strength shortens the residence times of ions in neutral complexes.
–Marric Stephens
Marric Stephens is a Corresponding Editor for Physics Magazine based in Bristol, UK.
References
- Y. Cheng et al., “Molecular dynamics simulation on current-voltage characteristics of room temperature ionic liquids under strong electric field,” Phys. Rev. E 113, 025415 (2026).