Perfluorosulfonylamide Binary Molten Salt with a Low Melting Point for Li-Ion Battery Electrolytes

Molten salt electrolytes have been considered to be the most promising electrolytes for next-generation lithium secondary batteries, owing to their high safety and wide electrochemical windows. Perfluorosulfonylamide-based lithium salts exhibit the lowest melting temperatures among all of the lithium salts. Nevertheless, their melting point is above 100 °C, and lowering their melting point is still a considerable challenge for their application as secondary battery electrolytes. In this study, we prepared binary lithium salts by mixing lithium bis(fluorosulfonyl)amide (LiFSA, Li[F–SO₂–N–SO₂–F]) and lithium (fluorosulfonyl)(trifluoromethylsulfonyl)amide (LiFTA, Li[F–SO₂–N–SO₂–CF₃]) using a ball mill and revealed a eutectic composition ratio of 35:65, a eutectic point of 76 °C, and a viscosity of 20800 mPa s^–1 at 100 °C. This binary lithium molten salt electrolyte exhibited a low melting point, a wide electrochemical potential window of 5.1 V, no Li⁺ concentration gradient, and successful charge and discharge of a conventional graphite negative electrode with high Coulombic efficiency at a 0.05C rate. Our findings further advance the development of high-voltage and high-power lithium secondary batteries.