Ageing assessment of supercapacitors during calendar life and power cycling tests

Supercapacitors, also known as ultracapacitors, are commonly based on porous activated carbon electrodes and electrostatic charge storage mechanisms. Carbon electrodes are supposed to be chemically and electrochemically inert and the electrostatic nature of the charge storage mechanism is highly reversible. These properties should assure that supercapacitors have an infinite shelf life. But in practice, supercapacitor cells exhibit performances fading when they are used for months. This paper deals with the performances fading of supercapacitors under calendar life and power cycling tests. The used impedance model is analytical and based on physical quantities as double layer capacitance, specific area and electrolyte conductivity but also on elementary electrodes dispersion. A periodic characterization based on impedance spectroscopy is done in order to quantify the impedance changes during ageing tests. The obtained results confirm that impedance real part is increasing and the capacitance is decreasing during the two ageing tests, nevertheless the way they change is different. The impedance real part increase is independent of the frequency in calendar life test while, in power cycling test, this increase is accentuated at low frequencies. This difference will be explained thanks to impedance model parameters evolution. This study confirms that the two ageing tests affect differently the supercapacitor performances.