Abstract
Degradation mechanism of electrochemical double layer capacitors on high-voltage exposure is examined. Capacitance loss for positive electrodes is significantly accelerated above 4.5V (vs. Li/Li+) whereas that of negative electrodes occurs on the electrochemical cycles down to 1.2V. The surface of positive electrodes is covered with decomposition products of propylene carbonate used as electrolyte solution, and therefore high-voltage exposure increases impedance of electrodes. In contrast, the degradation of negative electrodes is triggered by the decomposition reaction of electrolyte salts, resulting in the formation of radical species. It is proposed that the radical species attack fluorinated polymer used as a binder, leading to the defluorination of binders and thus increase in polarization on electrochemical cycles. (C) The Electrochemical Society of Japan, All rights reserved.