Dual-Ionen-Zellen basierend auf der Interkalation von Bis(trifluoromethansulfonyl)imid in Graphit: Untersuchungen zu Ionentransportprozessen im Elektrolyten und zum Interkalationsmechanismus

The first part of this thesis discusses the influence of stray capacitances on artifacts in three-electrode electrochemical impedance measurements with ion-blocking electrodes. To this end, Fletcher’s model based on a three-terminal equivalent network was refined to account for ion-blocking electrodes. The model was verified by performing measurements on networks built from electrical elements and real electrochemical cells. The results show that positioning the reference electrode nearby the working electrode induces severe artifacts. In case of suitable experimental conditions artifacts can be minimized. The reference electrode should be centered between the working and counter electrode. Furthermore, a large ratio of the double-layer capacitance of the working electrode and the stray capacitances should be maintained. To this end, short and actively shielded cables should be used. The second part of this thesis deals with ion-transport processes in electrolytes for dual-ion cells. In a first subproject, the formation of concentration gradients in an exemplary dual-ion cell was investigated. This was done in an indirect fashion by measuring the electrolyte resistances of the full cell and the half-cells by means of impedance spectroscopy. Additionally, finite element simulations of an idealized electrochemical cell were performed. The simulations were able to qualitatively describe the measured changes of the electrolyte resistances. Therefore, they could be used to investigate the concentration profiles of all species in the electrolyte. During charging of the dual-ion cell there is a strong decrease of the mole fraction of the lithium salt near the lithium electrode. In contrast, near the graphite electrode there is a small increase of the mole fraction. This effect can be explained by the different mobilities of the ionic species. In the second subproject,transference numbers of Li+ in different electrolytes were measured by means of very low frequency impedance spectroscopy. The resulting transference numbers are much smaller than transport numbers obtained by pulsed field gradient nuclear magnetic resonance spectroscopy. To explain these differences, a theoretical model for the description of ion fluxes in a binary electrolyte with two univalent ionic species was developed by combining the Onsager reciprocal relations with linear response theory. The theoretical expressions account for all correlated movements of cations and anions. An equation to calculate transference numbers was derived from the resulting equations for the current densities. The model shows that in case of small transference numbers strong correlations between the movements of cations and anions must exist. Transference numbers obtained by PP experiments that were reported in literature, are much larger than the values obtained by in this work In the third part of this thesis, the intercalation TFSI– into graphite is investigated by means of in-situ Raman spectroscopy. The measurements were used to monitor changes of the graphite electrode during the first and second charge/discharge cycles of a dual-ion cell. It could be shown that at the highest investigated potentials, the obtained graphite intercalation compound exhibits a stage index between 2 and 3. In contrast to intercalation of Li+, no dilute stage 1 compound is formed at the beginning of TFSI– intercalation. Instead, a GIC with a distinct stage