As-vacuum-deposited ionic liquid [emim][TFSA], Li salt [Li][TFSA] and their solution films on solid substrates were investigated through an in situ combination analysis of ellipsometry and impedance spectroscopy. The ellipsometry measurement was first successfully applied to monitor the thickness of thin film [emim][TFSA] ionic liquid during the deposition, while it was revealed that the thickness of thin film [Li][TFSA] increased after air exposure. The thickness increase of the [Li][TFSA] film is due to its deliquescent property and the air-exposed [Li][TFSA] film became ion conducting in contrast to its insulating solid state before the air exposure. The two-layered [emim][TFSA]-[Li][TFSA] films were found to form homogeneous solution films with the mole fractions of [Li][TFSA] up to 0.7, significantly higher than the corresponding bulk solubility limit. The temperature dependence of, as well as the air exposure effect on, the ionic conductivity of [emim][TFSA]-[Li][TFSA] films will also be discussed.
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