Impact of Water on the Properties of Litfsi-Acetonitrile Superconcentrated Electrolytes

Abstract

Superconcentrated electrolytes are a new class of electrolyte that is of interest to the scientific community for battery applications. These electrolytes consist of an amount of salt greater than the amount of solvent, while remaining liquid. It is currently accepted that the reduction of the solvation sphere under these conditions results in a greater electrochemical stability of the solvent and a higher Li+ transport number. These properties are widely used in the literature For instance, superconcentrated electrolytes employing acetonitrile as the solvent have been applied in lithium metal batteries while acetonitrile with moderate salt concentration decomposes on lithium. Despite the attractiveness of the field to these electrolytes, their properties are still poorly understood, and the methods of analysis used vary between different studies. There is no standard protocol for their use and it becomes difficult to establish correlations between different studies. Our goal is to establish the required methods to standardize the methodology and fill the gaps in the field.To do so, we firstly studied superconcentrated electrolytes based on LiTFSI and acetonitrile. The approach taken 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 at the concentration remains at and below 1000 ppm. The water content has a significant impact on the electrochemical window of stability of the electrolyte, reducing it to less than 2 V at 1000 ppm. The solvating structure of superconcentrated electrolytes is also studied via the electrochemistry of the Ferrocene/Ferrocenium couple to probe the impact of the electrolyte properties on electron transfer reactions. These results highlight the importance of structure and composition in the development of superconcentrated electrolytes.