Characterization of lithium-ion batteries and their components using NMR methods

With the growing demands for power and energy density of a lithium-ion batteries (LIB), which are addressed by specific improvement of the materials used, new questions arise, that can be answered by the versatile analytical tool of "nuclear magnetic resonance". The application of NMR methods for the characterization of experimental LIBs and their components is therefore the focus of this dissertation. The different scales of the electrochemical system are considered, which can be subdivided into electrode pair level, electrolyte volume and particle level according to the structure of a full electrochemical cell. With regard to the electrode pair level, the commonly used measuring methods such as EIS, scanning electron microscopy or atomic force microscopy are extended by "magnetic resonance imaging" (MRI) as imaging method, which was performed in this work on ${}^1$H and ${}^7$Li nuclei. Using 2D MRI, the electrolyte distribution in experimental cells can be investigated by intensity images. Furthermore, 1D profiles provide information on how the Li${}^{+}$ distribution between the electrodes changes due to passivation processes at the electrodes and charge transfer in charge and discharge cycles. In both cases, the time-dependent process was spatially resolved and examined with individually adjusted measurement parameters. On the one hand, the developed method has the advantage to address formation and the change of the electrolyte composition by measuring the relaxation properties. On the other hand, the intensity profiles can be used to study the gradients in ion concentration, which are caused by the charge transfer between the electrodes in an externally closed circuit with different current rates. The intensity gradients and the time-resolved measurement of the degradation of the concentration gradient provide insights into the properties of charge transport. The investigation of ion diffusion in the electrolyte – without external forces – can also be addressed by NMR. By means of diffusometry the effective diffusion coefficient of the Li${}^{+}$ ions was measured in an experimental cell, which is influenced by porosity and the morphological properties of the separator. In addition, the diffusion coefficients of the solvent molecules and both ions of the conducting salt were measured by ${}^1$H-, ${}^7$Li- and ${}^{19}$F-NMR in the temperature range T ∈ [263, ..., 313] K and concentration range c ∈ [0.05; 2] mol L${}^{-1}$. The empirical modeling of the diffusion coefficients considers the dependencies on temperature and concentration as well as the viscosity change near the phase transition at low temperatures. Finally, the sedimentation velocity of active material particles is illuminated on the particle level by a method developed in this work. The gravity-induced sedimentation can lead to phase separation, which has negative impact on the coating process and is important to known during formulation of new electrode pastes. Spin-echo based MRI allows the determination of the sedimentation rate in opaque suspensions and supports the development of new formulations for electrode pastes. To conclude, LIBs can be characterized with the versatile NMR methods under different aspects, whereby the chosen spatial resolution as well as certain acquisition parameters must match with the time and length scales of the process being observed. Charge transfer and ion diffusion were investigated microscopically and linked to macroscopic measurement parameters like the cell voltage. The results from the determination of the ion diffusion coefficients can be incorporated into simulation models for LIBs involving temperature and concentration dependent modeling. Thus, MRI and NMR diffusometry are promising approaches for a time dependent and spatially resolved characterization of processes in electrochemical systems.