Degradation Mechanisms of Solid Oxide Fuel Cell Cathodes

Oxidkeramische Brennstoffzellen (SOFCs) sind eine Schlüsseltechnologie, um den Übergang von fossilen Energieträgern auf erneuerbare Energien voranzutreiben. Die hohen Betriebstemperaturen und katalytischen Eigenschaften ermöglichen die Verwendung einer Vielzahl von Brennstoffen, und die Kommerzialisierung der SOFCs wird durch verschiedene Förderprogramme weltweit vorangetrieben, wobei die Langzeitstabilität der verwendeten Materialien einen Forschungsschwerpunkt darstellt. In dieser Dissertation werden Degradationsmechanismen auf der Kathodenseite sauerstoffleitender SOFCs beleuchtet. Der Einfluss von Chromoxid auf die Sauerstoffaufnahmefähigkeit des Elektrolytmaterials YSZ und des Kathodenmaterials LSCF wurde mittels Sauerstoff 18 Isotopendiffusion untersucht. Die Auswertung der mittels Sekundärionenmassenspektrometrie (SIMS) gemessenen Diffusionsprofile zeigte dabei die Abhängigkeit des Oberflächenaustauschkoeffizienten keff von der Cr2O3 Schichtdicke und dem Substratmaterial. Weiterhin wurde die Segregation von Sr aus dem Kathodenmaterial LSCF untersucht. Röntgenabsorptionsspektroskopie (XAS) wurde eingesetzt, um Sr an der Oberfläche von Sr im Bulk LSCF zu unterschieden. Zuerst wurde die Säure-Base Reaktion des SrO an der Oberfläche mit gasförmigem Chromoxid genutzt, um die Segregation von Sr in Abhängigkeit der angelegten Zellspannung zu messen. Ein fundamentaler Unterschied zwischen Kathoden und Anoden-Seite lässt dabei Schlussfolgerungen über den Sr Segregationsmechanismus zu. Danach wurde im Rahmen eines neu entwickelten Verfahrens, dem Röntgenabsorptionsmappings (XAM) bzw. der Röntgenabsorptionsmikroskopie, die Sr Segregation in Dünnschichtzellen mit LSCF Kathoden von 10 nm Dicke untersucht. Dabei wurden die Ortsinformationen mit den morphologischen Änderungen der Kathode verknüpft und der Zusammenhang der Sr Segregation mit den lokalen Sauerstoffaustauschvermögen der Versuchsapparatur nachgewiesen. Diese drei Themen zeigen die verschiedenen Arten der Degradation und die Vielfältigen Möglichkeiten auf diese zu untersuchen. Eine Vielzahl Oberflächenanalytischen Untersuchungsmethoden werden dabei kombiniert um ein möglichst komplettes Bild der Kathodendegradation Sauerstoffleitender SOFCs zu erhalten.Solid oxide fuel cells (SOFCs) are a key technology for the transition from fossil fuels to renewable energy sources. The relatively high operating temperatures and the anode’s catalytic activity enable the use of a broad variety of fuels which speeds up their commercialization. However, the issue of relatively high degradation and long-term stability is still an area of intense research, funded by various programs and agencies worldwide. In this Ph.D. thesis the degradation mechanisms of solid oxide fuel cell cathodes (oxygen conduction type) were investigated. First, the influence of chromia on the oxygen uptake of YSZ (an electrolyte material) and LSCF (a cathode material) was measured via oxygen 18 tracer diffusion. The acquired diffusion profiles measured by secondary ion mass spectroscopy (SIMS) showed the dependence of the surface exchange coefficient keff on the thickness of a Cr2O3 surface layer and the substrate material. Second, the Sr segregation of LSCF was investigated via x-ray absorption spectroscopy and Sr on the surface was distinguished from the bulk Sr via formation of strontium chromate. The acid-base reaction of gaseous chromia with the SrO on the surface of the sample was used to measure the Sr segregation as a function of applied bias potential. The fundamental difference in the dependence of the cathode and anode side’s segregation in the bias potential gave clues towards the underlying Sr segregation mechanism. Finally, a completely new method, x-ray absorption mapping (XAM) was used to measure the Sr segregation in symmetric model cells with cathode thicknesses of 10 nm. The acquired information about the morphological change of the cathode together with the spatial information across the whole sample surface made the connection of Sr segregation and oxygen exchange in the sample evident. These three topics show not only the various ways of degradation but also the numerous ways to approach these problems using a multitude of surface science techniques and combining them to draw a more complete picture of the cathode degradation

Acid leaching of Al- and Ta-substituted Li7La3Zr2O12 (LLZO) solid electrolyte

Solid-state batteries (SSBs) are promising next-generation batteries due to their potential for achieving high energy densities and improved safety compared to conventional lithium-ion batteries (LIBs) with a flammable liquid electrolyte. Despite their huge market potential, very few studies have investigated SSB recycling processes to recover and reuse critical raw metals for a circular economy. For conventional LIBs, hydrometallurgical recycling has been proven to be able to produce high-quality products, with leaching being the first unit operation. Therefore, it is essential to establish a fundamental understanding of the leaching behavior of solid electrolytes as the key component of SSBs with different lixiviants. This work investigates the leaching of the most promising Al- and Ta-substituted Li7La3Zr2O12 (LLZO) solid electrolytes in mineral acids (H2SO4 and HCl), organic acids (formic, acetic, oxalic, and citric acid), and water. The leaching experiments were conducted using actual LLZO production waste in 1 M of acid at 1:20 S/L ratio at 25 ◦C for 24 h. The results showed that strong acids, such as H2SO4, almost completely dissolved LLZO. Encouraging selective leaching properties were observed with oxalic acid and water. This fundamental knowledge of LLZO leaching behavior will provide the basis for future optimization studies to develop innovative hydrometallurgical SSB recycling processes

Elastic energy driven multivariant selection in martensites via quantum annealing

We demonstrate the use of quantum annealing for the selection of multiple martensite variants in a microstructure with long-range coherency stresses and external mechanical load. The general approach is illustrated for martensites with four different variants, based on the minimization of the linear elastic energy. The equilibrium variant distribution is then analysed under application of tensile and shear strains and for different values of the considered shear and tetragonal contributions of the different martensite variants. The interface orientations between different domains of variants can be explained using the perspective of the elastic energy anisotropy for regular stripe patterns. For random grain orientations, the response to an external elastic strain is weaker and variants changes can be interpreted based on the rotated eigenstrain tensor

Oxide ceramic electrolytes for all-solid-state lithium batteries – cost-cutting cell design and environmental impact

All-solid-state batteries are a hot research topic due to the prospect of high energy density and higher intrinsic safety, compared to conventional lithium-ion batteries. Of the wide variety of solid-state electrolytes currently researched, oxide ceramic lithium-ion conductors are considered the most difficult to implement in industrial cells. Although their high lithium-ion conductivity combined with a high chemical and thermal stability make them a very attractive class of materials, cost-cutting synthesis and scalable processing into full batteries remain to be demonstrated. Additionally, they are Fluorine-free and can be processed in air but require one or more high temperature treatment steps during processing counteracting their ecological benefits. Thus, a viable cell design and corresponding assessment of its ecological impact is still missing. To close this gap, we define a target cell combining the advantages of the two most promising oxidic electrolytes, lithium lanthanum zirconium oxide (LLZO) and lithium aluminium titanium phosphate (LATP). Even though it has not been demonstrated so far, the individual components are feasible to produce with state-of-the-art industrial manufacturing processes. This model cell then allows us to assess the environmental impact of the ceramic electrolyte synthesis and cell component manufacturing not just on an abstract level (per kg of material) but also with respect to their contributions to the final cell. The in-depth life cycle assessment (LCA) analysis revealed surprising similarities between oxide-based all-solid-state batteries and conventional Li-ion batteries. The overall LCA inventory on the material level is still dominated by the cathode active material, while the fabrication through ceramic manufacturing processes is a major contributor to the energy uptake. A clear path that identifies relevant research and development directions in terms of economic benefits and environmental sustainability could thus be developed to promote the competitiveness of oxide based all-solid-state batteries in the market

Holistic view on cation interdiffusion during processing and operation of garnet all-solid-state batteries

All-solid-state batteries based on the active cathode material LiCoO2 (LCO), a garnet-type Li7La3Zr2O12 (LLZO) electrolyte and a Li-metal anode are attracting a lot of attention as a robust and safe alternative to conventional lithium-ion batteries. The challenges in the practical realization of such cells are related to high-temperature sintering, which compacts the ceramic powder but also leads to undesirable material interactions such as cation interdiffusion and secondary phase formation. Even if high initial capacities can be achieved, the all-inorganic cells suffer from a strong capacity drop due to various degradation phenomena during processing and operation, which are not yet fully understood. In this study, the thermodynamic and kinetic aspects of co-sintering as well as the structural evolution of materials and interfaces during processing and operation of co-sintered LCO-LLZO cathodes are investigated in detail. A thermodynamic model for the interdiffusion of cations is derived and the effects of the diffusion of Al- and Co-ions, which occurs during the processing and cycling of the cells, are investigated. In LLZO, the diffusion of 0.13 Co per formula unit (pfu) has a negligible effect on ionic and electronic conductivity and electrochemical stability. In contrast, the substitution of 0.01 pfu Al and the induced disorder in the layer structure of LCO increases the polarization during cycling. All-inorganic cells fabricated with optimized sintering parameters to minimize interdiffusion between LCO and LLZO show good initial performance but similar degradation during cycling, as the used processing parameters result in a more porous microstructure leading to the development of cracks along the LLZO/LCO interface. The results obtained highlight the inherent instabilities of all-ceramic cathodes with unprotected LCO/LLZO interfaces, which require precise tuning of materials and processing parameters to achieve both high mechanical stability and low interdiffusion.</p

Thermal Recovery of the Electrochemically Degraded LiCoO₂/Li₇La₃Zr₂O₁₂:Al,Ta Interface in an All-Solid-State Lithium Battery

All-solid-state lithium batteries are promising candidates for next-generation energy storage systems. Their performance critically depends on the capacity and cycling stability of the cathodic layer. Cells with a garnet Li7La3Zr2O12 (LLZO) electrolyte can show high areal storage capacity. However, they commonly suffer from performance degradation during cycling. For fully inorganic cells based on LiCoO2 (LCO) as cathode active material and LLZO, the electrochemically induced interface amorphization has been identified as an origin of the performance degradation. This study shows that the amorphized interface can be recrystallized by thermal recovery (annealing) with nearly full restoration of the cell performance. The structural and chemical changes at the LCO/LLZO heterointerface associated with degradation and recovery were analyzed in detail and justified by thermodynamic modeling. Based on this comprehensive understanding, this work demonstrates a facile way to recover more than 80% of the initial storage capacity through a thermal recovery (annealing) step. The thermal recovery can be potentially used for cost-efficient recycling of ceramic all-solid-state batteries.</p

Oxide‐Based Solid‐State Batteries: A Perspective on Composite Cathode Architecture

The garnet-type phase Li$_7$La$_3$Zr$_2$O$_{12}$ (LLZO) attracts significant attention as an oxide solid electrolyte to enable safe and robust solid-state batteries (SSBs) with potentially high energy density. However, while significant progress has been made in demonstrating compatibility with Li metal, integrating LLZO into composite cathodes remains a challenge. The current perspective focuses on the critical issues that need to be addressed to achieve the ultimate goal of an all-solid-state LLZO-based battery that delivers safety, durability, and pack-level performance characteristics that are unobtainable with state-of-the-art Li-ion batteries. This perspective complements existing reviews of solid/solid interfaces with more emphasis on understanding numerous homo- and heteroionic interfaces in a pure oxide-based SSB and the various phenomena that accompany the evolution of the chemical, electrochemical, structural, morphological, and mechanical properties of those interfaces during processing and operation. Finally, the insights gained from a comprehensive literature survey of LLZO–cathode interfaces are used to guide efforts for the development of LLZO-based SSBs