Correlation Between Electrolyte Degradation Products and Overcharging Voltages in Supercapacitors

This study systematically investigated the correlation between electrolyte decomposition products and overcharging voltages in cylindrical acetonitrile-based supercapacitors. Using a combination of gas chromatography-mass spectrometry, nuclear magnetic resonance, and thermogravimetric analyses, we identified acetamide as a primary degradation marker, forming at voltages as low as 2.7 V and reaching peak concentration at 3.5 V before undergoing further transformation into N-ethyl acetamide and trimethylsilyl acetamide. Notably, at ≥3.9 V, trimethylsilyl acetamide becomes the dominant by-product due to interactions with silicon impurities in activated carbon electrodes, accelerating degradation mechanisms. These decomposition pathways significantly impair supercapacitor performance, leading to a reduction in capacitance, Coulombic efficiency, and energy efficiency by diminishing the effective surface area of the electrode. Furthermore, trace water generated at elevated voltages exacerbates these degradation reactions, further compromising stability. This work underscores the critical role of electrolyte purity and electrode material composition in mitigating performance deterioration. The findings provide fundamental insights into voltage-dependent degradation mechanisms, offering strategies to enhance the longevity, efficiency, and reliability of acetonitrile-based supercapacitors for high-power energy storage applications.