Electron Transfer and Transport Properties of Redox Compounds in Highly Concentrated Electrolytes

Abstract

Highly concentrated electrolytes (HCE) are currently gaining the interest of the scientific community for battery applications. These electrolytes are obtained when the number of ions of a salt nears or equals that of the solvent molecules, while maintaining a liquid state. In such systems, the solvent molecules are all found as complexes with the cations, usually Li+. It is currently accepted that in HCE, no “free” molecules of solvent are found in the solution since they are entirely located in the sphere of solvation of the ions present in large quantities. This unique structure brings a greater electrochemical stability of the solvent and a higher Li+ transport number. These properties have stimulated research to apply HCE in several electrochemical systems. For instance, HCEs employing acetonitrile as the solvent have been applied in lithium metal batteries while acetonitrile with moderate salt concentration spontaneously decomposes on lithium. Despite the attractiveness of these electrolytes, the origin of the properties and the relationships between the structure and those properties are still poorly understood, and there is currently no study on heterogeneous electron transfer in such electrolyte. We therefore aim at determining if the HCE structure has an impact on electron transfert.The approach developed in our group is the study of HCE based on LiTFSI and acetonitrile, a well-studied system quite representative of the highly concentrated systems. The approach that will be presented is divided in two parts. The first part concerns the study of the physicochemical properties of HCE. This strategy highlights the factors that have a significant impact on the properties of electrolytes. One of these factors is the water content. The results demonstrate that the presence of water in the solutions has a limited impact on the physical properties of viscosity and density of the mixtures as long as the concentration remains at or below 1000 ppm. The water content has also an impact on the electrochemical stability window (ESW). The ESW of HCE, decreases from 5.45V to less than 2.5 V at 1000 ppm of water.The second part concerns the study of the electron transfer rate of redox couples in HCE. We investigated the solvating structure of highly concentrated electrolytes via the electrochemistry of redox molecules. The first redox couple is the Ferrocenium/Ferrocene (Fc+/Fc) to probe the impact of the electrolyte properties on electron transfer reactions and to validate the approaches used. In the highly concentrated system, the Fc+/Fc redox couple follows the same relation (Stokes-Einstein and Nicholson-Shain) as the ideally dilute system. Two others redox couple have been studied, the Ru(bpy)3 3+/2+ and Fe3+/2+, to investigate the effect of different charges and electron transfer mechanisms. The inner sphere electron transfer of the Fe3+/2 redox couple is more affected by the salt concentration than the outer sphere complex (Ru(bpy)3 3+/2+ and Fc). These results highlight the importance of structure and composition in the development of highly concentrated electrolytes.