Ionic liquids are without doubt one of the most interesting materials for the replacement of organic solvent based electrolytes for both lithium-ion and lithium metal batteries [1]. Their improved physico-chemical properties, such as negligible vapour pressure, high thermal decomposition temperatures and wide electrochemical window, as compared to traditional battery electrolytes, make them attractive for these devices. However, their relatively high viscosities due to their elevated melting points and lower conductivity at sub-ambient temperatures have proven difficult to overcome without blending them with other organic-based materials.

A further challenge has appeared in recent times as researchers and companies try prototyping devices incorporating ionic liquid electrolytes – suitable separators [2, 3]. Due to the varying hydrophobic / hydrophilic nature and rapidly changing viscosity (as a function of temperature) of ionic liquid electrolytes, the choice of a suitable separator is extremely challenging. Traditional polyethylene and polypropylene separators are typically difficult to “wet” and as such, lead to relatively high internal resistance and low initial capacities as the cell components wet out. In most instances, researchers revert to Whatman glass fibre separators due to the ease of wetting because of the very open pores within the structure.

In this presentation, we will detail some of the approaches to developing separator materials specifically for ionic liquid electrolytes by previous workers and contrast and compare this with our own efforts. We will detail the impact of the choice of base separator material on wettability and device performance and then examine various methodologies used to enhance separator performance.

References

1. A. Lewandowski and A. Swiderska-Mocek, Journal of Power Sources 194: 601-609 (2009)

2. C. S. Stefan, D. Lemordant, B. Claude-Montigny and D. Violleau, Journal of Power Sources 189(2): 1174-1178 (2009)

3. M. Kirchhöfer, J. von Zamory, E. Paillard and S. Passerini, International Journal of Molecular Sciences 15(8): 14868-14890 (2014)

Acknowledgements

The authors acknowledge funding support provided by the ARC Research Training Centre for Naval Design and Manufacturing and the CSIRO Manufacturing High Performance Metal Industries Program.