Method Development for Multidimensional Study of Thermal Aging in Ethyl Isopropyl Sulfone–Electrolyte Supercapacitors

Understanding electrolyte degradation in electric double‐layer capacitors (EDLCs) is essential for advancing high‐temperature energy storage technologies. In this study, we present a comprehensive methodology to investigate temperature‐induced electrolyte aging by correlating electrochemical behavior with molecular (bulk electrolyte) and interfacial (material surfaces) degradation processes. A custom‐built Swagelok‐type postmortem cell equipped with a quasi‐reference silver wire enables simultaneous monitoring of the individual electrode potentials during operation and postaging access to both the liquid electrolyte and electrode surfaces. This integrated design allows for direct linkage between electrochemical response, liquid‐phase degradation (via gas chromatography‐mass spectrometry), and surface chemistry (via X‐ray photoelectron spectroscopy). The methodology is validated across a matrix of electrolytes composed of the 1 , 1 ‐dimethylpyrrolidinium tetrafluoroborate (Pyr 11 BF 4 ) salt in either acetonitrile (ACN), the alternative solvent ethyl isopropyl sulfone (EiPS), and an ACN:EiPS 75:25, wt% mixture. All systems were subjected to accelerated aging through 24 h voltage float tests at 3.0 V and three temperatures (20°C, 40°C, and 65°C). By selecting PYR 11 BF 4 —known for its high electrochemical and thermal stability—the degradation pathways observed can be primarily attributed to solvent effects. This work highlights the critical link between solvent decomposition and electrochemical aging, demonstrating how multidimensional postmortem analysis can guide the development of high‐voltage, temperature‐stable EDLCs.