Pit Sippel (University of Augsburg)

Ionic liquids for future energy storage applications – A dielectric study

Improving energy-storage and -conversion devices to achieve a more sustainable energy supply of tomorrow is one of the most challenging tasks of our time [1]. Many of these devices, e.g., batteries, super capacitors and fuel cells, utilize ion conductors as essential building blocks. Different types of ionic conductors including special crystalline materials, polymers or solvent-based electrolytes are employed for application. Another class of solvent-free electrolytes are ionic liquids, which often solidify via a glass transition. Due to their interesting physical and chemical properties like high ionic conductivity, thermal and chemical stability and low volatility, in the past decade ionic liquids have come into the focus of materials science [2]. These properties make them very attractive for application, improving the safety and stability of energy systems. Moreover, the vast number of available combinations of cations and anions forming ionic liquids provides many possibilities for finding compounds optimized for application. When considering this enormous variety of ionic liquids, further insight into the underlying physics is essential to find the optimal material. For this purpose, dielectric spectroscopy is an outstanding tool, which allows observing the dynamical properties of glassy matter over a huge frequency and temperature range [3]. It provides access to important quantities like dc-conductivity, glass-transition temperature, electrode polarization and intrinsic relaxation processes. Many of these physical properties of ionic liquids are dominated by their dynamics resembling that of glassy matter. Especially, the room temperature dc-conductivity, an important figure of merit for any electrochemical application, depends in a systematic way on the glass temperature and the so-called fragility [4]. This is revealed by investigating 13 ionic liquids via dielectric spectroscopy.

1. D. R. MacFarlane et al., Energy Environ. Sci. 7, 232 (2014).
2. M. Armand et al., Nat. Mat. 8, 621 (2009).
3. P. Lunkenheimer et al., Contemp. Phys. 41, 15 (2000).
4. P. Sippel et al., arXiv:1502.06851.

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