Physical Properties and Interactions of Ionic Liquids and Ionic Liquid Li-salt mixtures

Doctoral thesis, 2012

Ionic liquids (ILs) is a new class of salts with melting points <100 °C. Typically, an ionic liquid consists of a large organic cation and a charge−delocalized inorganic or organic anion. The unique variability of the ions allows, for the design of a large number of different ionic liquids. However, the relation between the molecular structure, interactions, and the physical properties of ionic liquids is not fully understood. This thesis addresses the effect of cation and anion structure on fundamental physical properties of ionic liquids such as melting, glass transition, decomposition, and transport of ions and aggregates. The influence of the cation on these properties is investigated using several imidazolium cations for a fixed anion, bis(trifluoromethanesulfonyl)imide [TFSI]. Similarly, the anion influence is investigated using tetrafluoroborate [BF4¯], tetracyanoborate [B(CN)4¯] trifluoromethanesulfonamide [TFSAm], and phosphonate anions with the imidazolium and pyrrolidinium cations. Lithium salts doped ionic liquids are suitable for applications as electrolytes in rechargeable lithium batteries. However, it has been shown that the introduction of Li-salts into ionic liquids causes the viscosities to increase and the conductivities to decrease. In order to further develop and optimise these systems it is important to understand the structure and the interactions of the ionic liquid/Li-salt mixtures as well as how the properties might change when these mixtures are incorporated into e.g. a polymer membrane. These issues have been addressed in this thesis for LiTFSI doped TFSI anion based ionic liquids. This thesis also investigates the phase behaviour and transport properties of imidazolium and pyrrolidinium based ionic liquids doped with LiTFSI over a large concentration range, xLiTFSI/(1−x)IL, 0.01