Untersuchungen zum Ionentransport in konzentrierten Flüssigelektrolyten für Lithiumionenbatterien

Based on the work of Wohde et al., the objective of this research was the investigation of ion transport and its limitations within both liquid electrolytes and complete battery cells. Furthermore, interionic interactions and their influence on ion transport parameters should be examined more closely. Based on this, the theoretical model following the Onsager formalism should be extented in order to determine the Onsgar transport coefficients. Knowledge of these coefficients yields nearly all transport parameters as well as fundamental insight into the microscopic dynamics within the electrolyte solutions. In the first part of this thesis, the possibility of cation-anion-anticorrelations was added to the combined formalism of Onsager’s reciprocal relations and linear response theory. This may cause a lower lithium ion transference number under anion blocking conditions compared to the related transport number, as well. Based on this, the Onsager formalism was expanded by additional transport parameters forming a system of equations which can be used to calculate the Onsager transport coefficients as well as the thermodynamic factor inserting several measured variables. This system was used to analyse the well-known electrolyte G4/LiTFSI 1:1. Thus, a strong cation-anion-anticorrelation was found to cause the very low lithium ion transference number under anion blocking conditions. Combining these findings with the work of Kashyap et al. leads to the conclusion that in ideal QILs the lithium ion transference number under anion blocking conditions must be zero due to the total conservation of momentum. The presence of unbound solvent molecules or the exchange of complexing molecules among themselves may cause a transfer of momentum leading to a measurable transference number higher than zero. These results could be approved by MD simulations. The experimental prove is still missing at the moment due to great instabilities of the corresponding electrolytes combined with lithium metal. The second part discusses the reproduction of the above mentioned results using another QIL in addition with the attempt to eliminate conservation of momentum as a limitating factor of ion transport by adding a third ionic species and using IL/salt mixtures. The chosen electrolytes were characterised electrochemically by determining the ionic conductivity, the lithium ion transference number under anion blocking conditions and the salt diffusion coefficient. Analysis of the ionic conductivities shows an Arrhenius behaviour whereas a decrease with increasing salt concentration for the IL/salt mixtures can be seen. The QILs show ionic conductivities in the range of the concentrated IL/salt mix-tures. The examined salt diffusion coefficients exhibit the same trend. For the analysed QIL, a very low lithium ion transference number under anion blocking conditions could be determined according to the expectations. Thus, the insight granted by the first part of this thesis seems to be an intrinsic and universal feature of QILs. In contrast, the IL/salt mixtures show much higher transference numbers which are even increasing with increasing salt concentration. Also, their transport and transference numbers show only slight differences. Therefore, it is concluded that there are interionic interactions present in the IL/salt mixtures but they are not influencing the transport as much as it is done in case of the QILs. These findings lead to the conclusion that the conservation of momentum is not a limiting factor any more for ion transport under anion blocking conditions in an electrolyte system containing three ionic species. In the third part, the parameters which limit ion transport and the charging process within a typical commercial lithium ion battery cell were investigated. Using a mathematical model by Huang et al. it could be shown that the low frequency resistance of a commercial lithium ion battery cell is dominated by the lithium ion transport under anion blocking conditions within the composite electrodes. With a small change to the transmission line model it is possible to monitor the same low frequency limit. This can be achieved by substitution of the ionic conductivity by the lithium ion conductivity under anion blocking conditions as well as considering the chemical capacitance within the active particle phase. Thus, the overall cell resistance could be estimated for all the electrolytes even though they are not fully characterised and could not be used within the complete model. In the following, it could be shown that the QILs are very resistive and can only achieve charging rates of about 0.3C, whereas the IL/salt mixtures can be charged with rates up to nearly 1C. Therefore, the IL/salt mixtures may be used as alternative electrolytes within a lithium ion battery as long as fast charging options are not required.