Novel binary room-temperature complex system based on LiTFSI and 2-oxazolidinone and its characterization as electrolyte

Binary room-temperature complex system has been prepared based on lithium bis(trifluoromethane sulfonyl) imide (LiN(So₂CF₃) ₂, LiTFSI) and 2-oxazolidinone (C₃H₅NO ₂, OZO). Both LiTFSI and OZO are solid at room temperature, but their mixture is a liquid with a liquidus temperature about -58.4 °C at a molar ratio of 1:4.0 characterized by differential scanning calorimetry. Thermogravimetry analysis shows that the complex system possesses high thermal stability over a wide range of temperature. Infrared and Raman spectroscopic studies have been carried out to understand the interactions between the precursors, LiTFSI and OZO, of the complex system. It is shown that the OZO can coordinate with the Li⁺ cation and the TFSI^- anion via their polar groups (the C=0 and NH groups). Such strong interactions lead to the dissociation of LiTFSI and the breakage of the hydrogen bonds among the OZO molecules, resulting in the formation of the complex. To have a comprehensive understanding to the above interactions, quantum chemistry calculations with nonlocal density function theory have also been performed on the free ions or organic molecule by determining their Mulliken charges, equilibrium configuration, binding energy, and the coordination number of Li⁺ ion. The calculations indicate that the ionic conductivity of the LiTFSI-OZO complex with different molar ratios depends strongly on the ionic species in the complex system. Electrochemical performances of the complex electrolyte are evaluated with ac impedance spectroscopy, cyclic voltammetry (CV), and in a test electric double layer capacitor, respectively. The complex at a molar ratio of 1:4.5 exhibits the highest ionic conductivity due to the relatively large amount of "free" ions at room temperature. The analysis for the CV behavior indicates that the electrochemical stability window of the electrolyte is about 3V. This kind of complex system proves to be a promising candidate of electrolytes for supercapacitor and other electrochemical devices.