Unraveling thermal oscillations in electric double layer capacitors: Linking interfacial ion overscreening and dielectric decrement to device performance via multi-scale electro-thermal approach

Electric double layer capacitors (EDLCs) possess distinct advantages in energy storage applications due to the high power density and exceptional cycle life. However, oscillation of device temperature caused by reversible heat generation rate can significantly affect the safety and performance of EDLCs, especially under certain operation such as thermally insulated conditions and wide potential windows. While conventional thermal studies focus on macroscopic irreversible Joule heating, there is insufficient investigation into the underlying mechanisms in reversible heat generation rates that are essential for rational development. In this work, a multi-scale electro-thermal model incorporating near-surface non-ideal effects is developed and coupled with a device-level heat transfer equation to investigate the heat generation rate and temperature evolution. The model reproduces the endothermic-to-exothermic transition of device-level reversible heat generation rate for the first time. In addition, the heat of mixing driven by entropy change is the dominant component of reversible heat generation rate. The overscreening effect is identified as the origin of the initial endothermic behavior during charging, which subsequently transitions to an exothermic regime driven by electromigration dynamics. Parametric studies demonstrate that higher current and lower effective dielectric constant amplify reversible heat generation, while a wider voltage window causes the reversible heat rate profiles to saturate into a stable plateau. Finally, the device’s temperature evolution was simulated on the heat generation rates, allowing for an analysis of the temperature oscillations driven by reversible heat during galvanostatic charge–discharge cycling. This study elucidates the critical link between molecular-scale ion dynamics and macroscopic thermal behavior, offering theoretical guidance for optimal design and thermal management of EDLCs.