Tailoring the Properties of Lithium Ionic Liquids by Anion Mixing

Achieving carbon neutrality necessitates the development of high-performance lithium-ion secondary batteries, driven by the increasing demand for electric vehicles and renewable energy storage. Molten salt electrolytes have emerged as promising candidates for enhancing both safety and performance owing to their thermal stability and high lithium-ion transference numbers. In this study, we focus on the anionic structure of fluorosulfonyl-based compounds to design novel lithium salts with low melting points. By strategically mixing lithium bis(fluorosulfonyl)amide (LiFSA) and lithium bis(trifluoromethanesulfonyl)amide (LiTFSA), we identified compositions exhibiting melting points lower than those of the individual salts. The properties of the equimolar mixtures were compared with those of the chemically equivalent asymmetric anion salt; lithium (fluorosulfonyl) (trifluoromethanesulfonyl)amide (LiFTA). A eutectic mixture of LiFSA and LiTFSA with a 1:1 molar ratio exhibited a liquidus temperature (T_L) of 395–397 K, resulting in an ionic liquid where lithium cations are the dominant charge carriers. This mixture rapidly formed a supercooled liquid phase, and the ionic conductivity (σ) of Li[(FSA)_0.5(TFSA)_0.5] varied continuously across the supercooled and molten states. At 373 K, the ionic conductivity and lithium-ion transference number (t_Li^ABC) were 0.0139 mS cm^–1 and 0.96 under anion-blocking conditions, respectively. Furthermore, we comprehensively investigated the bulk properties and electrochemical characteristics of Li[(FSA)_0.5(TFSA)_0.5], benchmarking its performance against LiFTA. These findings provide critical insights into the design of next-generation molten salt electrolytes for advanced battery applications.