Investigation and modeling of the microscopic causes for non-linear ageing behavior of carbon-based electrochemical double layer capacitors

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Within this research project, the ageing behavior of electrochemical double layer capacitors, below called supercaps, will be systematically investigated. The goal is to achieve a deeper understanding of the physico-chemical processes at each electrode during ageing. Based on the experimental results of post mortem analysis, a physic-chemical model has to be developed. This model has to represent the ageing processes and can be used for life time prediction.Nowadays, a rule of thumb is typically used for life time prediction like Lifetime is halved by 100 mV voltage increase respectively 10 K temperature increase. This rule of thumb is based on extrapolated data of life time test results that have been carried out at elevated temperatures and voltages. However, experimental results show that the ageing behavior of supercap cells cannot be explained in an adequate way by this simple pattern of explanation. With consequent extrapolation these cells would have a lifetime expectancy of more than hundred years at low temperatures respectively low voltages. Therefore, the mechanisms of action of the ageing processes and their dependency on potentials, current rates and temperatures have to be identified within this research project. Based on these results a significantly more precise model for lifetime prediction will be developed.Post mortem analysis will be conducted at commercial carbon-based supercap cells to analyze the physico-chemical ageing processes. For these investigations cells that have already been aged at different ageing conditions can be used. But additional ageing tests will be carried out as well. Thus, structural changes of the electrode surface, as well as on cathode and on anode side, will be detected and a material analysis will be carried out. The results will incorporate into a physic-chemical model that begins on particle level and is able to represent the complex processes within the pores of carbon electrodes as well as at the barrier layer between electrolyte and electrode. Post mortem analysis should be based on the opening of cells that have already been aged in calendric and cyclic ageing tests at the institute of the applicant. Since the ageing conditions of these cells are known it has to be investigated which chemical and electrochemical processes have been initiated and how they influence the cell performance. Furthermore, it has to be analyzed how regeneration processes that were observed in experimental investigations can be explained. During these tests the cell capacitance increased suddenly after a break during the aging test. Finally, by means of the developed physic-chemical a fundamental bechmark of operating strategies and load profiles regarding lifetime of supercaps has to be carried out.