Martin LORENZ1, Monika SCHÖNHOFF1
1Institute of Physical Chemistry, University of Münster, Münster, Germany
Today lithium-ion batteries are among the most important energy storage media. Problems of commonly used organic electrolytes, such as the high flammability, require alternatives, for example ionic liquid (IL)-based electrolytes. Their transport properties are, however, still limited due to poor lithium conductivities, crystallization and unfavorable lithium-anion clusters. Improvement may be achieved by following different strategies: One is increasing heterogeneity in the systems by combining two different anions, another option is the use of high lithium salt concentrations to reduce lithium-anion cluster formation. Furthermore, the introduction of lithium coordinating functionalities like ether groups may assist to break up undesirable lithium-anion interactions.
The present work aims to combine these three strategies to improve the lithium transport process. To this end, two series of electrolytes, based on the 1-ethyl-3-methylimidazolium (EMIM)- and the 1-methyl-1-(2-methoxyethyl)pyrrolidinium (Pyr12O1)-cation, respectively, are investigated. The electrolytes contain bis(fluorosulfonyl)imide (FSI)- and bis(trifluoromethanesulfonyl)imide (TFSI) anions and high amounts of LiFSI and LiTFSI salt, varying the total FSI/TFSI anion ratio. Resulting electrolytes are investigated using different experimental methods, e.g. impedance spectroscopy as well as nuclear magnetic resonance (NMR)-based techniques. The latter are used to determine different species-specific transport quantities, thus enabling a deeper understanding of transport mechanisms. Especially the application of electrophoretic NMR (eNMR) provides valuable information, giving direct access to the electrophoretic mobility and drift direction of charged species in an applied electric field.
The mobilities of all species are generally enhanced with increasing FSI fraction for both series, caused by the smaller size and less strong lithium coordination of the FSI ion compared to TFSI. Lithium mobilities are generally negative, in accordance with literature results for similar IL, demonstrating the transport of lithium in net negatively charged clusters. However, an exception from this is seen for the electrolyte of the Pyr12O1-series, where FSI is the only anion present. Here, in contrast to all previously investigated lithium salt in IL systems, clearly positive lithium mobilities are detected. Consequently, the combination of the rather weakly lithium coordinating FSI anion with the ether functionalized organic cation fundamentally changes the lithium transport mechanism. Despite containing only one ether-group, Pyr12O1 seems to significantly weaken lithium-FSI interactions, thus enabling new transport routes, which could favor the application of these IL-based electrolytes in battery applications.