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Ladezustandsbestimmung von Lithium-Schwefel-Zellen mit Hilfe von Impedanzmessungen

: Berendes, Erik
: Kowal, Julia; Socher, Sebastian

Berlin, 2016, XI, 128 pp.
Berlin, TU, Master Thesis, 2016
Master Thesis
Fraunhofer IVI ()
Batteriespeicher; Lithium-Schwefel-Zelle

State of charge estimation on LiS-batteries is a challenging task due to at OCV-curves, long relaxation times and the self-discharge behaviour. In this thesis a model has been developed which can be used to predict the state of charge of LiS-batteries during discharge by measuring the impedance at a single-point frequency of 4 kHz. The study was carried out on LiS-pouch-cells with Li- and Si-anode made by Fraunhofer-Institute for Material and Beam Technology. Battery impedance usually depends on several internal and external parameters. Therefore, electrochemical impedance spectroscopy was performed to investigate the dependence of the impedance on state of charge, state of health, temperature, relaxation time and C-rate in the frequency range of 10 kHz to 100 Hz. The measured impedance spectra were analysed to select a single-point frequency where the impedance is highly affected by the state of charge and slightly affected by the other investigated parameters. The phase of the impedance at a frequency of 4 kHz was identied as a good indicator which meets both requirements. For the model the dependence of the impedance on state of charge, C-rate and relaxation time were parametrized. The change of impedance caused by temperature and aging is considered by a reference alignment approach. The model was finally evaluated by following the automotive NEDC driving cycle and a temperature cycle between 10 to 40°C. As a result of the validation, far more accurate results can be achieved with the help of the impedance-based model compared to the OCV method.