• Energy Research

For a battery cell, both the porosity of the electrodes/separator and the transport distance of charged species can evolve due to mechanical deformation arising from either lithium intercalation-induced swelling and contraction of the active particles or externally applied mechanical loading. To describe accurately the coupling between mechanical deformation and the cell’s electrochemical response, we extend Newman’s DualFoil model to allow variable, non-uniform porosities in both electrodes and the separator, which are dynamically updated based on the electrochemical and mechanical states of the battery cell. In addition, the finite deformation theory from continuum mechanics is used to modify the electrochemical transport equations to account for the change of the charged species transport distance. The proposed coupled electrochemomechanical model is tested with a parameterized commercial cell. Our simulation results confirm that mass conservation is satisfied with the new formulation. We further show that mechanical effects have a significant impact on the cell’s electrochemical response at high charge/discharge rates.

AbstractIn the global quest to renounce from fossil fuels, a large demand for the renewable production of hydrogen via water electrolysis exists. In this context, the solid oxide electrolyzer (SOE) is an interesting technology due to its high efficiency resulting from elevated operating temperatures of up to 900 °C. Physical modeling plays a vital role in the development of SOEs, as it lowers experimental costs and provides insight where measurements reach limits. A main challenge for modeling SOEs is the multitude of physical effects, occurring and interacting on various spatial and temporal scales. This requires assumptions and simplifications, particularly when increasing scope and dimensions of a model. In this review, we discuss the different approaches currently available in literature.

Thermal and mechanical effects play a vital role in determining the electrochemical behavior of lithium-ion batteries (LIBs). Non-uniform temperature distribution and mechanical deformation can result in uneven electrochemical states, leading to spatially varying aging rates that significantly shorten cell lifetime. In order to improve simulation accuracy and thus the quality of computational battery design optimization, it is therefore essential to capture these coupled phenomena in a simulation model of a full battery cell. In this work, an electro-chemo-thermo-mechanical coupled framework is proposed to simulate LIBs in the three-dimensional space. In this new framework, a recently proposed one-dimensional electrochemical model, which includes the impact of mechanical deformation and local lithiation state on the effective transport properties of the charged species, is coupled with a three-dimensional thermomechanical model. A unique coupling scheme is proposed to handle information exchange between these two models. This framework allows us to accurately and efficiently study the behavior of three-dimensional cells with realistic geometry and resolve the spatial variation of interested fields. Two commercial cells are studied to show the performance of the newly proposed battery simulation framework.

Abstract An elementary 1-dimensional model is developed for a solid state lithium-ion battery having a single ion conductor electrolyte, a lithium metal negative electrode and a composite positive electrode. The battery topology is assumed to be of the layered variety, thereby justifying the 1-dimensional formulation. The governing equations for the electrochemical kinetics at the interface between the negative electrode and the electrolyte separator are stated, as are those for ion transport in the electrolyte. The positive electrode is assumed to be a particulate composite of storage material within a matrix of electrolyte. A mixture theory is developed for the positive electrode encompassing ion transport in the electrolyte matrix and storage and unstorage of lithium in the active material subject to electrochemical kinetics at the perimeter of the storage particles. Many simplifying assumptions are made with the advantage of them leading to closed-form or semi-closed-form solutions, including linearization of the equations governing the redox kinetics at interfaces in the battery between electrolyte and active material. An approximation is given for the concentration of lithium in the positive electrode of the battery during discharge, with the details depending on a length scale parameter that depends on the competition between the rate of lithium insertion into/extraction from the storage particles and the rate at which lithium ions are transported in the electrolyte. When the conductivity of the electrolyte is high and the redox reactions are relatively sluggish, this length scale parameter is comparable to the thickness of the positive electrode or larger than it. In that case lithium insertion into/extraction from storage particles occurs everywhere within an active zone of the positive electrode, but with the rates least at the current collector of the positive electrode. If the conductivity of the electrolyte is poor and the redox reactions rapid, the length scale for the solution is small compared to the thickness of the positive electrode and insertion into/extraction from storage particles occurs only in a narrow slice of the positive electrode. This slice moved along the positive electrode and separates a region of it that is completely filled/empty from a region of it that has not yet gained or lost any of its lithium. In all cases there will usually be a region of the positive electrode near the separator that is completely filled during discharge and completely empty during battery charging. Our results also give outcomes from which the internal resistance of the battery can be estimated.

Publiziert als Diskussionsbeiträge der Scientists for Future 5 (43 pp). Die Erstveröffentlichung (nur in Deutsch) erfolgte am 16. Dez. 2020, diese geringfügige Revision 1.1 (Deutsch und Englisch) am 16. Jan. 2021. GERMAN SUMMARY (English Summary further below): Die Zeit drängt. Ohne schnell wirksame Gegenmaßnahmen werden Erderhitzung und Biodiversitätsverlust Ausmaße annehmen, welche die Lebensweise von Menschen nicht abschätzbaren Risiken aussetzen. Obwohl die Herausforderungen weiten Teilen der Bevölkerung bewusst sind, werden dringend nötige Entscheidungen aufgeschoben oder nur teilweise umgesetzt. Eine Ursache hierfür sind fehlende Foren, in denen sich Bürger:innen mit Expert:innen austauschen und gemeinsam mögliche Szenarien und Lösungen erörtern können. Scientists for Future empfiehlt deshalb, mit geeigneten Formen von Bürger:innenversammlungen eine breite und demokratisch partizipative Beteiligung an Zukunftsgestaltung und -sicherung zu ermöglichen. Diese sollten auch unabhängig von einem Auftrag von Regierung oder Parlament initiiert werden. Wir rufen daher zu einem Gründungstreffen auf, um Planung und Durchführung einer Bürger:innenversammlung zum Thema Klima im Jahr 2021 zu ermöglichen. Eine sorgfältige Planung ist nötig, damit die Durchführung neutral und offen geschieht. Hierfür werden einige zentrale Kriterien beschrieben. ENGLISH SUMMARY: Time is pressing. Without quick and effective countermeasures, global warming and loss of biodiversity will assume proportions that expose people’s way of life to incalculable risks. Although large parts of the population are aware of the challenges, urgently needed decisions are postponed or only partially implemented. One reason for this is the lack of forums where citizens can discuss possible scenarios and solutions with experts. Therefore, Scientists for Future recommends enabling a broad and democratic participatory involvement in shaping and securing the future by appropriate forms of citizens’ assemblies. These should also be initiated independently of a mandate from government or parliament. We therefore call for a founding meeting to enable the planning and implementation of a citizens’ assembly on the topic of climate in 2021. A careful planning is necessary, so that the execution happens neutrally and openly. For this purpose, some central criteria are described. ___ Suggested citation: Hagedorn, G.; Baasch, S., Blöbaum, A., Brendel, H., Hardt, J.N., Heiland, S. Klins­mann, M., Matthies, E., Pfennig, A., West, C., Wipfler, B., et al., (2021). Scientists for Future empfiehlt eine reprä­sen­ta­tive Klima-Bürger:innen­ver­sammlung im Jahr 2021 / Scientists for Future recommends a representative Climate Citizens’ Assem­bly in 2021 (Version 1.1, in Deutsch/German & Englisch/English). Diskussionsbeiträge der Scientists for Future, 5, 23 pp. doi:10.5281/zenodo.4417265. Volume 5 of "Diskussionsbeiträge der Scientists for Future". This publication contains both the German original text and an English translation.