(472d) Solid Organic Electrolytes and Ionic Liquids, with Poly(ethylene glycol) and Semifluorinated Alkyl Side Chains, for Photovoltaic and Energy Storage Applications

Energy conversion and storage devices such as dye-sensitized solar cells and lithium ion batteries often use liquid electrolyte solutions for conduction of ions. The problems of volatility and flammability of the common organic solvents used in these electrolytes have been overcome using non-volatile ionic liquids. However, there is a great interest in the development of ion-conducting solid electrolytes because they circumvent sealant issues associated with liquids, which is a significant advantage in large scale processing of solar cells and batteries, especially on flexible substrates. Room temperature ionic conductivities of solid polymer electrolytes are usually below about 10^-5 S/cm. Most liquid electrolytes, on the other hand, have higher ionic conductivity values, of the order of 10^-3 S/cm. The effects of low ionic conductivity on device performance can be partly offset by using thin films of the solid electrolytes, but approaches that lead to an increase in the conductivity values are nevertheless desirable. Gel electrolytes have been prepared by dispersing liquid electrolytes within a solid polymer matrix, to combine the high ionic conductivity of the liquid with the rigidity of the solid [1,2]. Instead, we have sought to develop all-solid electrolytes that do not contain liquid components. This paper reports the synthesis and characterization of an imidazolium organic salt that contains polar poly(ethylene glycol) segments linked to non-polar perfluoroalkyl segments. The imidazolium salt is a low-modulus solid at room temperature. Properties of this salt were determined using electrochemical impedance spectroscopy, thermogravimetric analysis, differential scanning calorimetry, dynamic mechanical spectroscopy, and X-ray diffraction. The fluorinated imidazolium iodide was compared with non-fluorinated analogues, which were liquids at room temperature. The temperature dependence of ionic conductivity and viscoelastic properties was measured. The effects of addition of an inorganic electrolyte, namely lithium iodide, on ionic conductivity, were studied. Lithium iodide resulted in a lowering of ionic conductivity. A strategy to increase the ionic conductivity, without using liquid electrolytes, will be presented.

1) Freitas, F. S.; de Freitas, J. N.; Ito, B. I.; De Paoli, M.-A.; Nogueira, A. F. ACS Appl. Mater. Interfaces 2009, 1, 2870.

2) Wu, J.; Lan, Z.; Lin, J.; Huang, M.; Hao, S.; Sato, T.; Yin, S. Adv. Mater. 2007, 19, 4006.