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Electrode microstructure directly affects ion and electron transport and, in turn, has a strong correlation to battery performance. Understanding the separate yet complementary effects of ionic and electronic transport in cell behavior is a challenge. This work provides through a combination of experiments and modeling a better understanding of the relationship between three aspects of the cell: ion transport within the electrode, electrode uniformity, and cell degradation. The first part of this work compares two experimental methods that determine ion transport in terms of tortuosity, a dimensionless geometric factor. The polarization-interrupt and blocking-electrolyte methods measure effective diffusivity and conductivity, respectively. The tortuosity of several commercial-quality electrodes was measured using both methods, producing reasonable agreement between the two methods in most cases. Next, the effect of cell cycling on ionic and electronic transport of electrodes was investigated. Using the blocking electrolyte method, the tortuosity of electrode films at varying extents of cycling was determined. Variations in electronic resistivity were quantified by micro-scale measurements using a previously developed micro-four-line probe. The changes in tortuosity and electronic resistivity were investigated for a graphite anode and several cathode chemistries including LiCoO2, LiNixCoyMnzO2, LiFePO4, and blends of transition metal oxides. Clear evidence of changes in tortuosity and electronic resistivity was observed during cell formation and cycling. The magnitude of the changes strongly depended on the chemistry of electrodes and cycling conditions. The results indicate that, under normal cycling conditions, electronic resistivity increases while tortuosity unexpectedly decreases. However, accelerated cycling conditions (i.e. elevated temperature) can lead to both electronic resistivity and tortuosity increase. Finally, the interplay of electrode tortuosity heterogeneity and Li-plating was investigated. The Li-plating reaction was incorporated into a Newman-type model and validated using the voltage profile and capacity-loss data from experiments. The simulation result shows that a heterogeneous anode can cause non-uniform Li plating while cathode heterogeneity did not have a significant effect. The Li-plating profile across the thickness of the anode with cell cycling showed that Li tends to plate at the high tortuosity region near the separator. Unexpectedly, Li plating tends to shift to the current collector side upon a sufficient increase in porosity close to the separator. Simulated capacity loss vs. cycling data indicates that there is a feedback mechanism with cycling: as cycling continues the rate of Li plating for the high-tortuosity region decreases at the separator side and the other two regions will eventually catch up in terms of plating.

Solid oxide fuel cells (SOFCs) have gained renewed interest due to their high energy-conversion efficiency, new discovery of fossil fuel sources, and low greenhouse gas emission. However, performance degradation during long-term operation is one of the greatest challenges to overcome for commercialization of SOFCs. At intermediate temperatures, chromium (Cr) vapor species that form over chromia-forming alloy interconnect, can transport and deposit in the cathode, and poison the cathode performance. Although extensive studies have been conducted on the Cr-poisoning phenomena, the mechanism of cathode performance degradation still needs to be clarified. Therefore, there is an urgent need to understand the degradation mechanisms and develop corresponding mitigation strategies. In this research, anode-supported cells with (La,Sr)MnO3-based cathode were fabricated. The cells were electrochemically tested with and without the presence of chromia-forming alloy interconnect, and operating conditions including cathode atmosphere, current condition, and interconnect contact were varied independently. It was found that both humidity and cathodic current promote chromium poisoning. Microstructural characterizations also confirmed that larger amounts of chromium-containing deposits are present at the cathode/electrolyte interfaces of the cell tested with cathodic current and/or humidity. With the help of free energy minimization calculations, the equilibrium cell potentials for Cr vapor species reductions are estimated and found to be very close to the open-circuit potential of the cell. Combining the experimental and computational results, the roles of humidity and cathodic current in Cr-poisoning are evaluated, and a mechanism associated to Cr vapor species dissociation at the triple-phase-boundaries is proposed. To evaluate the Cr-poisoning effects on cell performance, an analytical polarization model is used for quantitatively separating the contribution of various cell polarizations. By curve-fitting the current-voltage traces to this model, the changes of cathode polarizations due to Cr-poisoning are quantified. Under normal operating conditions, the cathodic activation polarization is determined to be most negatively impacted by Cr-poisoning. Mitigation of the Cr-poisoning effects using a dense lab-developed CuMn1.8O4 spinel interconnect coating was demonstrated. Employing the spinel coated interconnect mesh in on-cell tests, it was found that both the degradation in cell performance and Cr deposition in the cathode are significantly mitigated.

Für derzeitige, industriell hergestellte Solarmodule werden Leistungsgarantien von 20 bis 25 Jahren gegeben. Das hat hohen Ansprüchen bezüglich ihrer Zuverlässigkeit, welche sich über ihre Effizienzabnahme pro Jahr definiert, zur Folge. Die Einführung neuer Technologien, wie z.B. die der PERC- (passivated emitter and rear cell) als Ersatz für die bislang dominierende BSF-Technologie (back surface field) hat eine umfangreiche Änderung der Metallisierung der Solarzellen nach sich gezogen, wodurch neue Degradationseffekte auftreten können. In der vorliegenden Arbeit werden die einzelnen Komponenten der Solarzellenmetallisierung und –verschaltung in Bezug auf ihren Beitrag zur Degradation untersucht. Die dafür notwendige beschleunigte Alterung erfolgte mittels Temperaturwechselbelastung zwischen -40 °C und +85 °C. Unterstützt durch die numerische Simulation konnte die Degradation der Rückseitenmetallisierung und Zellverschaltung im Detail charakterisiert und Lösungen zur Reduktion der Leistungsabnahme abgeleitet werden. Erstmals wurde dabei der Einfluss der AgAl-Legierung und des Druckkontaktwiderstandes auf den Serienwiderstand der Solarmodule untersucht.