Accelerated aging of electric double-layer capacitor cells under increased cell voltage and temperature

Electric double-layer capacitors (EDLCs) have promising automotive applications owing to their high power density, long cycle life, and enhanced safety. To meet the stringent requirements for severe conditions and long lifetimes, their aging behavior under increased applied voltage and operating temperature should be explored. Herein, EDLC cells were evaluated by cycling and floating tests to determine the accelerated aging factors for aging indices (specific capacitance decrease and internal resistance increase) under increased voltage and temperature. The cells were assembled using well-known materials, namely YP-50F activated carbon, polytetrafluoroethylene binder, tetraethylammonium tetrafluoroborate/propylene carbonate non-aqueous electrolyte, and a paper-based separator. The aging tests were performed under standard (3.0 V/25 °C), high-voltage (3.5 V/25 °C), and high-temperature (3.0 V/60 °C) conditions. The specific decrease in specific capacitance and specific increase in internal resistance were proportional to the square roots of the number of cycles and floating time in the early stage of aging (>80 % capacitance decrease and <1.5-fold internal resistance increase). Through linear regression analyses of these relationships, the accelerated aging factors under increased voltage and temperature were determined. The cell voltage increase from 3.0 to 3.5 V accelerated the specific capacitance decrease and internal resistance increase 2.8 and 13.6 times, respectively, in the cycling test and 16.2 and 17.3 times, respectively, in the floating test. Accelerated aging induced by the temperature increase from 25 to 60 °C was moderate in comparison. The floating test under the high-voltage (3.5 V) condition at 25 °C shortened the testing time by ∼1/16.