A Novel Approach for Supercapacitors Degradation Characterization

Alon Oz, The Nancy and Stephen Grand Technion Energy Program, Tehcnion - Israel Institute of Technology, Haifa, Israel
Danny Gelman, Department Of Chemical Engineering, Tehcnion - Israel Institute Of Technology, Haifa, Israel
Sioma Baltianski, Department Of Chemical Engineering, Tehcnion - Israel Institute Of Technology, Haifa, Israel
Yoed Tsur, The Nancy And Stephen Grand Technion Energy Program, Tehcnion - Israel Institute Of Technology, Haifa, Israel

Supercapacitors (SCs) gain a lot of interest especially due to their high power density. The energy is stored by means of electrostatic charge separation, which allows SCs to withstand thousands of cycles. SCs are usually monitored by their capacitance and equivalent series resistance (ESR). Tracking these two makes it a bit difficult to detect the upcoming failure of the cell at a desired sensitivity, since both change at a relatively slow manner. Therefore, a more comprehensive characterization technique, such as electrochemical impedance spectroscopy (EIS) should be employed. EIS serves as an ideal characterization technique to monitor degradation, since it is nondestructive and can be employed during operation. The challenging task is the analysis; using our novel EIS analysis approach of SCs, this challenge is resolved.

Impedance Spectroscopy Genetic Programing (ISGP), which serves as the cornerstone of our analysis approach, utilizes evolutionary programming to find the most suitable distribution function of relaxation times (DFRT). The analysis yields a DFRT model comprised of linear combination of peaks. Each peak in the model has its own characteristic relaxation time and area, and can be assigned to one or more processes in the tested sample.  In the case of SCs, the area of each peak corresponds to its share of the total capacitance of the cell. Plotting the DFRT as a function of frequency and monitoring the changes in each peak's relaxation time and area at different stages of degradation tests, result in a much straightforward understanding of the affected processes.

In this study, we examined commercial SCs during accelerated degradation. The obtained DFRT model suggest that the most affected process is the transport of electrolyte ions at the porous electrode. We were also able to detect the upcoming failure of the cell earlier than available when only monitoring the capacitance and ESR.