Dual breaking of ionic association in water-in-LiTFSI electrolyte for low temperature battery applications

Abstract

This paper deals with the low temperature performance of Li-ion aqueous battery in water-in-LiTFSI electrolyte. Two simultaneous approaches have been adopted to reduce the ionic interaction of lithium cation with TFSI anion. First, the LiTFSI eutectic mixture has been prepared with trifluoroacetamide (TFA) that shows liquid-like properties due to the strong interaction of amide-group with TFSI−. The eutectic behavior is explained by the interaction of trifluoroacetamide with bulky TFSI− anion that reduced the melting point of mixture to −55.3 °C. Despite such eutectic behavior, the low conductivity still remains an issue which is solved by increasing water content that improves the hydration degree of lithium. Molecular dynamic simulation (MD) studies show that the addition of water at low molar ratio is sufficient for lithium ion hydration and about five times improves the conductivity of electrolyte. A dual approach to break the ionic association in LiTFSI strongly influences the activation barrier for the movement of ions and is facilitated by water, which is fully utilized for the solvation of lithium and no free water is available to participate in faradaic reactions.