Heterogeneous electron transfer of ferrocene in acetonitrile-LiTFSI highly concentrated electrolyte

Abstract

Highly concentrated electrolytes (HCE) are mixtures of equivalent or near-equivalent amounts of salt and solvent displaying a liquid phase at room temperature. HCE have intensively studied for application in energy storage devices with a particular focus on batteries since the demonstration of the lack of reactivity of lithium metal in an acetonitrile HCE. The lack of "free" solvent molecules in HCE is responsible for their stability. This feature also suggests that heterogeneous electron transfer (ET) in HCE could be different from conventional electrolytes because of the importance of solvent reorganization during ET. Thus, we investigated the heterogeneous electron transfer of the ferrocenium/ferrocene (Fc+/Fc) redox couple as a function of concentration of the salt Li bis(trifluoromethanesulfonyl)imide in acetonitrile, a model system for HCE. We show that while the diffusivity of Fc (Shoup-Szabo) follows the trend with viscosity (η) expected from the Stokes-Einstein relation over the entire concentration range, the ET rate constant (k0) variation with η on the other hand diverges from ideality. Using Raman spectroscopy in the solution and on the surface (EC-SERS), we show that the most likely cause for the difference in ET rate constant between dilute and highly concentrated electrolytes involves a strong coordination of the ferrocenium with the complexes found in HCE. This new knowledge highlights the importance of increasing fundamental research on the topic of electrochemistry in HCE.