Comparative Study of Degradation Mechanisms of Commercial Supercapacitors at High Temperatures Depending on Storage Conditions

This study examined the electrochemical and physicochemical degradation of commercial supercapacitors (SCs) at elevated temperatures depending on their voltages. After being stored in the charged state (2.5 V), the capacitance decreased rapidly to 80 % of its initial capacitance, and the resistance at 1 kHz increased continuously to 2.5 times its initial resistance. After being stored in the discharged state (∼0 V), the capacitance was almost constant, but the resistance increased more than four times. In the charged state, oxidation products were deposited in the meso−/micropores of the positive electrode. They reduced the specific surface area of the positive electrode, which led to a rapid decrease in the capacitance and an increase in the resistance of SCs. In the discharged state, the supersaturation and precipitation of electrolyte salt hindered ion transport in the macro−/mesopores, increased the charge transfer resistance (R_ct), and decreased the double−layer capacitance (C_dl). These phenomena were verified by electrochemical impedance spectroscopy, cyclic voltammetry, in situ FT−IR for the electrolyte, N₂ adsorption−desorption analysis, and FE−SEM image for each electrode. Finally, the SC with better performance and durability at high temperature was verified by applying organic solvent with a high boiling point but not so high dielectric constant.