Ionic Conductivity and Thermal Stability of Polyethylene Glycol Based Polymer Electrolytes Containing Phosphonium Ionic Liquids

Abstract

Room-temperature ionic liquids (RTILs), i.e. organic molten salts with melting points below 100 ◦C, have been regarded as potential media in various chemical systems due to the fact that RTILs have unique physicochemical properties such as favorable solubility of organic and inorganic compounds, no measurable vapor pressure, high thermal stability, low flammability, etc. On the other hand, polyethylene glycol (PEG) is widely used as an electrolyte for lithium polymer secondary batteries from the viewpoint of considerable interaction with lithium ions. However, PEG does not show sufficient ionic conductivity because partial crystallization proceeds at temperatures below the melting point.1 In order to solve such a problem, various types of RTILs have been investigated for the PEG based polymer electrolytes; however, RTILs based on quaternary phosphonium cations have rarely been employed for the polymer application despite the fact that the phosphonium RTILs offer both high conductivity and thermal stability.2 In this work, we report the preparation of the PEG-based polymer electrolytes containing phosphonium RTILs, examining their physicochemical properties such as conductivities, thermal decomposition temperatures, etc. The preparation of the phosphonium RTILs was performed by aqueous ion exchange reaction of the precursor quaternary phosphonium halides with lithium bis(trifluoromethylsulfonly)amide (TFSA). The RTILs and PEG were mixed at a mass ratio of 4:6 to prepare the polymer electrolytes. The conductivities of the polymer electrolytes were evaluated by an ac impedance measurement. The thermal decomposition temperature of the polymer electrolytes obtained was measured by a thermogravimetric analysis. As shown in Fig. 2, it was found that relatively high conductivities were observed in phosphonium RTIL based polymer electrolytes when compared to those in the polymer electrolytes prepared by using the corresponding ammonium RTILs. This result seems to be due to the relatively high transport property of the phosphonium RTILs. The thermal decomposition behavior of the phosphonium RTIL based polymer electrolytes will be discussed. References D. Fauteux, A. Massucco, M. McLin, M. van Buren, J. Shi, Electrochim. Acta, 40, 2185 (1995). K. Tsunashima, M. Sugiya, Electrochem. Commun., 9, 2353 (2007). Figure 1