Investigating the self-discharge mechanism in electrical double-layer capacitors with low-concentration carbonate additives

Abstract

This research underscores the critical roles of dielectric properties, molecular structure, and sizes of carbonate additives influencing the self-discharge behavior and mobility of solvated ions within the electrical double-layer (EDL) structure. Diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethylene carbonate (EC) solvents at the 300/1000-ppm additive levels are examined for electrical double-layer capacitors (EDLCs) using 1 M TEABF₄/PC (tetraethylammonium tetrafluoroborate/propylene carbonate). The electrochemical characterizations conducted in both coin-cell two-electrode and Swagelok three-electrode systems reveal that the self-discharge behavior in EDLCs can be distinctly divided into two mechanisms: charge redistribution occurring at high cell voltages and diffusion-controlled processes dominating at low cell voltages. The variation in dielectric constants of these carbonate additives leads to distinct self-discharge behaviors. (i) DEC, with its low dielectric constant and large molecular size, increases the interfacial and diffusion impedance, effectively reducing the self-discharge rates during the diffusion-controlled domain. Conversely, DMC, characterized by the low interfacial resistance, accelerates the self-discharge rate due to the rapid ion diffusion in the diffusion-dominant region. (ii) EC, as demonstrated through the DFT calculations, enhances the solvation coordination due to its high dielectric constant, leading to the reduction in the interfacial resistance and the increase in the intermolecular interactions, which collectively hinder the diffusion of solvated ions and decelerate the self-discharge rate. Additionally, the oxidation degree of linear carbonate additives within the activated carbon is higher than those of cyclic carbonates, suggesting that linear additives may compromise the stability of the EDL structure.