Lifetime assessment and degradation mechanisms in electric double-layer capacitors
Aachen / Institute for Power Electronics and Electrical Drives (ISEA) (2019) [Book, Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (xii, 150 Seiten) : Illustrationen, Diagramme
Electric double-layer capacitors are energy storage devices employed in renewable energy applications as well as electrification of transport. They are based on electrostatic charge separation and their properties and applications are actively researched and developed to help fostering the transition to an ecoconscious lifestyle. The high costs of these devices necessitate elaborate lifetime assessments to maximize the time of use and thus to lower life cycle costs and broaden their possible field of application. Today, the end-of-life is usually described by simple rules of thumb. This does not hold up the asymmetric influence of temperature and voltage as in-depth investigations showed. Therefore, this thesis follows two main goals: First, deduct novel dependencies of lifetime on operating conditions, which are in line with previous observations and allow for a more detailed analysis. Herein, the work relies on existing test data and elaborates on new lifetime linkages between different operating conditions. Equations for lifetime halving factors dependent on operating conditions are derived and serve as a more general approach for estimating lifetime. Results show, a temperature increase of 7 K at rated voltage halves the lifetime of a capacitor whereas this value drops to 5 K for an elevated voltage of 300 mV above nominal conditions. Similar relations are found for lifetime fade dependent on voltage and its correlation to elevated temperatures. Second, this thesis aims at broadening the basic knowledge of the deterioration mechanisms on the microscopic scale. Observed asymmetry of decreased energy storage capability on the one hand and decreased power capability on the other hand are understood in-depth. This investigation is enabled by a novel test setup for accelerated lifetime testing yielding time-dependent results. It allows the correlation of electrical performance parameters over lifetime to electrochemical and physical parameters. These are obtained during post-mortem examinations which include microscopy, single electrode impedance spectroscopy, energy dispersive and regular x-ray diffraction, Raman spectroscopy as well as thermos gravimetric analysis. The results clearly show differing degradation mechanisms for the two electrodes exhibiting a cover-layer formation on the positive electrode and delamination and corrosion on the negative electrode. Especially the newly formed layer on top of the positive electrode is extensively investigated and characterized. It consists of a fluorine containing polymer, also known as Polytetrafluoroethylene (PTFE). This layer is still permeable to ions to a certain extent but significantly decreases the power capability. The negative electrode on the other hand shows delamination due to current collector corrosion and binder decomposition. The formation of PTFE and delamination are the main driver for a fade in energy storage capability. Thus, previously not understood effects could be explained. Recovery phenomena observed during breaks in between aging tests, stem from the still permeable layer on the positive electrode. These phenomena significantly increase the time needed to store energy but still allow the storage process itself. Overall, this thesis explains the reason for the asymmetric deterioration and gives new guidelines for assessing the lifetime of organic based electric double-layer capacitors. The results enable the development of tailored electrodes to counter observed deterioration mechanisms and broaden their possible field of application. Especially an asymmetric electrode setup might prove key for countering the observed asymmetric aging.
Sauer, Dirk Uwe