Lifetime and Performance Assessment of Commercial Electric Double-Layer Capacitors Based on Cover Layer Formation

The lifetime and performance of energy storage systems are essential characteristics for a major success of clean energy innovations, especially regarding the automotive sector. In this area, electric double-layer capacitors represent the cutting edge of nonfaradaic, high power, and long lifetime energy storages. Usually, degradation is neglected while operating or it is assessed by referring to simple rules of thumb. To better address the aging effects, commercial specimens have been investigated in-depth. The works can be split into two parts: first, extensive accelerated aging for more than 3 years to statistically analyze the degradation trend and significantly improve the current rules. Second, cell opening and surface characterization of the electrodes to gain a profound understanding of the ongoing processes and to correlate aging mechanisms to the statistics of the first part. It is found that a prominent cover layer forms during degradation on the positive electrode scaling with lost capacitance. The used methods for in-depth characterization include microscopy, X-ray diffraction, and thermogravimetric analysis with subsequent mass spectrometry. The newly formed layer consists of poly(tetrafluoroethylene) (also known as Teflon) that is still passable by charge carriers, albeit with a longer time constant. Additionally, the negative electrode shows corrosion and loss of contact. The composition of the cover layer has not been known yet. Thus, materials and especially electrolyte development can benefit from the results.