Simulation study on the lifetime of electrochemical capacitors using the accelerated degradation test under temperature and voltage stresses

Several studies on the degradation mechanism of electrochemical capacitors have been conducted, but they have not predicted and analyzed the lifetime of the capacitors. A capacitor's life is normally considered to end when its capacity is reduced to 20% of the initial level or its ac equivalent series resistance (ESR) increases by 200% compared its initial value. Once the ac ESR reaches a failure state, it becomes impractical to use it as a factor to determine the failure of an electrochemical capacitor; hence, only the capacity is considered as a factor when determining a capacitor's failure. This method, which is used for small-scale electrochemical capacitors employed as capacitance backup, cannot be applied to new electrochemical capacitors with high output and low resistance. Therefore, we analyzed a failure mechanism by conducting a load-life test on high-output cylindrical-shaped electrochemical capacitors under different voltages (2.5, 2.7, and 2.9 V) and temperatures (333.15 K, 343.15 K, and 353.15 K) for 1000-1500 h, using the simulation software ALTA 8 PRO and Weibull ++8. By proposing an appropriate dc ESR as a life-determining factor for existing capacitors and by analyzing different failure conditions (150%, 180%, and 200%) through comparison, we found that a 180% increase in the dc ESR over its initial level is the most appropriate factor to be used as a failure condition for electrochemical capacitor performance. In addition, in the case of performance degradation due to voltage and temperature changes, an acceleration factor for each condition has been deduced, facilitating a preliminary performance evaluation w an accelerated life test.