Solvation structure of phosphonium ionic liquid/CH₃SCN mixture as electrolytes for Li-ion batteries: Infrared pump-probe spectroscopic studies

Lithium-ion battery (LIB) electrolytes based on room-temperature ionic liquids (RTIL) are promising as safe and sustainable LIB electrolytes. However, there are no reports on the solvation structure and dynamics of RTIL due to the existence of various solvation species in RTIL. Here, we investigated the solvation structure and dynamics of binary mixtures of the phosphonium ionic liquid, tributyl(2-methoxyethyl)phosphonium bis(trifluoromethanesulfonyl)imide (P₄₄₄₁₀₂TFSI), and methylthiocyanate (CH₃SCN), with ultrafast mid-IR Spectroscopy. The changes of vibrational and rotational dynamics of CH₃SCN∙∙∙Li⁺ complex and free CH₃SCN with an increase in CH₃SCN concentration suggest the presence of at least four solvation species, providing superior resolving power for various solvation species existing in complex LIB electrolytes. Our experimental results show that the environment of free CH₃SCN and CH₃SCN∙∙∙Li⁺ complex changes drastically with change in CH₃SCN mole fraction and Li salt concentration in the mixture. We expect that this can affect the Li-ion transport mechanism in the IL-based electrolytes.