First-Principles Identification of Iodine Exchange Mechanism in Iodide Ionic Liquid

Abstract

We investigated the microscopic mechanism of ion transport in iodide ionic liquid, using first-principles calculations. We show that the desorption barrier of polyiodides (I-3(-) or I-3(-)) from the cation is in a similar energy range as or higher than the barrier for the bond dissociation and ensued desorption of neutral iodine (I-2). This suggests that, instead of the physical diffusion of such a negatively charged multiatomic species, the exchange of neutral iodine (I-2) between the polyiodides can be an easier channel for the movement of polyiodide. For the transport of the monoiodide anion (I-), we suggest the contribution of the Grotthuss-type ion exchange through the intermediately formed even-member anion (I-2n(-)), in addition to drift and diffusion. As a result, we suggest that, instead of the commonly cited diffusion of the triiodide/iodide (I-3(-)/I-) redox couple, the exchange of neutral iodine (I-2) and the Grotthuss-type transport (I-) constitute the dominant ion transport mechanism.