Ladungs- und Massentransport in lithiumbasierten Batterien: Korrelierte Ionenbewegung und Elektronentransportmechanismen

In the present work, the charge and mass transport within two different battery systems are discussed. The main focus lies on ionic movements within concentrated liquid electrolytes for lithium-ion batteries and on the movement of electrons within the solid phase of Li2O2 as discharge product in lithium-oxygen batteries. In the case of lithium-ion batteries, the influence of ion correlations on their direction of movement and on the resulting transport properties was quantified and discussed. For the lithium-oxygen batteries, the main focus was the proper identification of electron transport limitations during oxygen reduction for the Li2O2 thin-layer formation. The first part of this thesis addresses the predominant correlations of cations and anions within lithium salt/glyme mixtures in varying molar ratios and with different anions as electrolytes for lithium-ion batteries. The understanding of those correlations is crucial for describing and optimizing charge transport processes in form of ion movements within the battery. For the first time, the three independent methodes of Very-Low-Frequency Impedance Spectroscopy (VLF-IS), Pulsed Field Gradient Nuclear Magnetic Resonance (PFG-NMR), and electrophoretic Nuclear Magnetic Resonance (eNMR) were combined in order to obtain the different lithium ion transference and transport numbers for quantification of the ONSAGER conductivity coefficients and their self- and distinct parts solely by using experimental data. In this way, the dependence of the mentioned transference and transport numbers on the dilution and on the mass ratio of the anions to the cations could be discussed. Furthermore, linear response theory expressions were derived, showing the relation between the different correlations as well as transport quantities and center-of-mass and dipole fluctuations for further description of the lithium ion transport under anion-blocking conditions within a battery. Different dependences were observed for the different quantities. Thus, in addition to dipole fluctuations special attention must be payed on center-of-mass fluctuations in future theoretical approaches. The discussed results within the second part of this thesis are part of the field of lithium-oxygen batteries. The focus lies on a detailed identification of the changing limitations for the electron transport through the layers for further discussing transport mechanisms. In this context, the transition of laterally heterogeneous to homogeneous growth of thin Li2O2 layers plays an important role, which is identified by redox probe experiments in this thesis. Within aprotic Li-O2 battery cells thin Li2O2 layers at different depths of discharge (1-15 h) were formed by potentiostatic discharge experiments. During layer formation, the literature known exponential increase of the charge-transfer resistance of the oxygen reduction was observed by electrochemical impedance spectroscopy. This exponential increase was used in literature as proof for transport limitations in dense layers, thus discussing different transport mechanisms through Li2O2 layers. For a more detailed insight on the dense character of the layers obtained in this work, the imaging methods of Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) were applyed. After 3 h of discharge, the images showed a homogeneous and completely covered surface. In addition, AFM scratching experiments resulted in mechanically verified layer thicknesses matching the layer thicknesses estimated using the charge flow. Next, redox probe experiments using the redox couple cobaltocenium/cobaltocene (Co(Cp)2+/ Co(Cp)2) were performed in form of Cyclovoltammetry (CV) as well as Electrochemical Impedance Spectroscopy (EIS) experiments. It was shown that after 3-15 h of discharge, the electrons were forced to choose the same transport path through the layer for both reduction processes. Based on a closer comparison of the obtained charge-transfer resistances, three transport regimes could be identified in dependence on the electrode surface coverage, defining the transition between the heterogeneous and homogeneous growth. In this way it was shown that solely with the charge-transfer resistance of the oxygen reduction, no assumption on the dense character of the layer can be made. Thus, it is possible that in literature transport mechanisms within the frame of the homogeneous growth were discussed although the heterogeneous growth was not completed.