Untersuchungen zu Transportlimitierungen in Batterieelektroden

In the first part of this dissertation, thin and dense Li2O2 layers were electrochemically grown on glassy carbon electrodes. The electronic and ionic transport of these layers was studied with a combined approach of electrochemical impedance spectroscopy and atomic force microscopy measurements. Impedance spectroscopy allowed the determination of the layers’ ionic and electronic resistances, as well as the layers’ geometric and double layer capacitances during growth. Finally, after finishing the electrochemical experiment, the thickness of the Li2O2 layers was determined by means of atomic force microscopy. As the final thickness of the layers and their geometric capacitance were known, the permittivity could be calculated. In return, this allowed the monitoring of the layers’ time-dependent growth, the thickness-dependent decrease of the ionic conductivity, and the time dependent-increase of the resistances. The exponential increase of both the ionic and the electronic resistances with the thicknessgave strong indication that the obstruction of the charge transport could not be explained based on the coverage of insulating Li 2O2 on the glassy carbon electrode. The thickness of the layers is proportional to the charge. The latter would be proportional to the surface coverage if the layers grew in a non-uniform way. Hence, one would expect a hyperbolic dependence of the electronic resistance on the thickness in the log-plane for a surface-coverage dependent scenario. Furthermore, by analyzing the slopes of the thickness-dependent increase of resistance, tunneling was ruled out as a mechanism for charge transport through the layers. However, it was pointed out that field-dependent, non-linear charge transport of both, lithium ions and electron holes, is a likely explanation for the observed phenomenon. Non-linear Lithium ion migration is caused by strong gradients in the electric field, whereas electron holes are subject to non-linear effects because of a strong gradient in chemical potential. In the second part of this dissertation, the ionic transport in a solid-state composite anode was studied. These anodes contained different volume fractions of the glassy solid electrolyte Li7P2S8I (bulk ionic conductivity σion,electrolyte = (0, 75 ± 0, 1) mS · cm−1), the active material lithium titanate (Li7Ti5O12), and the conducting additive Super C-65 (carbon black). While the volume fraction of the conducting additive was kept constant throughout the whole study, the volume fractions of the active material and the solid electrolyte were varied. The ionic transport was probed by transmission-linetype-measurements and by measurement of the stationary Li + -current under electron blocking conditions. For volume fractions of the solid electrolyte ε ≥ 0, 4 both methods were in good agreement. Furthermore, in this porosity region the tortuosity/porosity relation behaved as predicted by the Bruggeman relation. However, there was a strong deviation from the Bruggeman relation at lower volume fractions of the solid electrolyte ε. Moreover, the transmission-line model turned out to be inaccurate for low ε. In this publication it was demonstrated that all solid state lithium ion batteries can be built such that their performance is comparable to liquid electrolyte counterparts. The ionic conductivity of an all solid composite electrode can be as high as systems that employ commercial electrodes and liquid electrolytes, if highly conductive materials are used. In the third part of this dissertation, an all solid state composite cathode was examined with two different methods. First the stationary Li+-current through the electrode was measured under electron blocking conditions by means of impedance spectroscopy. Here, the ionic resistance of the composite cathode was determined. Using optical microscopy, the thickness of the electrode was determined and an effective tortuosity was calculated. Furthermore, the expected tortuosity for an electrode with spherical particles and the same volume fractions of electrolyte ε was calculated by means of the Bruggeman relation. The experimentally determined effective tortuosity τ ef f deviated by 25% from the calculated value. In order to find an explanation for this deviation, the electrode was examined by means of focused-ion-beam/scanning-electron-microscopy, which generated a tomography. The investigated volume was reconstructed in a computer-based approach. A geometrical analysis and simulations were conducted on the reconstructed volume. Random-walk simulations allowed the determination of the tracer diffusion coefficient in the electrolyte phase within the composite electrode. The effective tortuosity was determined by comparing diffusion inside the electrode with unhindered diffusion in the bulk electrolyte. The effective tortuosity found by simulation was in very good agreement with the experimentally determined one. However, there was a lot of void space in the reconstructed volume of the electrode. These voids were artificially filled with electrolyte in another random-walk simulation. This way the effective tortuosity was calculated for a void-free electrode. On this instance, the effective tortuosity was in good agreement with the Bruggeman relation, but was significantly smaller than the one that accounted for the voids. Furthermore, the geometric analysis of the reconstructed sample showed that important characteristic lengths are shortened by the presence of voids. Altogether, it was shown that voids significantly limit the power of all solid state composite electrode because highly tortuous conducting paths are created. This problem does not exist in liquid cells because the electrolyte permeates through the porous electrodes. In order to build all solid state batteries that outperform their liquid counterparts, the void problem must be solved.