Different shapes based on ionic liquid leading to a two-stage discharge process

Abstract

To depict whether the discharging process is equal to reverse charging in ionic liquid (IL)-based electrolytes, electrolyte systems consisting of [C_nmim][AOT] (n = 2, 4, 6 and 8) and systems of [C_nmim][BF₄] (n = 2, 4 and 6) were studied using molecular dynamics simulations. During the discharging process, two-stage characteristics could be found near the negative electrode. Unlike the conventional view, in the first stage, cations almost did not move away from the interface layer; in the second stage, some cations “moved back” to the interface layer in the opposite direction. Moreover, it was revealed that the unconventional phenomenon was driven by the shape change between the standing and lying imidazolium rings. Meanwhile, this study reported that in the first stage of discharge, the rapid decrease in energy was mainly due to the change in the shape of counterions near the negative electrode; in the second stage of discharge, the reason for the slow decrease in energy was overscreening and slow movement of counterions from the negative electrode, which was consistent with the results of the discharge experiments. It was revealed that the shallow discharge of the supercapacitor is mainly affected by the first stage. In addition, short-chain ILs had more advantages in shallow discharge because of their more instantaneous explosive power. Long-chain ILs were shown to be more persistent and smoother in the deep discharging process, and reduced self-discharge led to an effective extension of the discharge life. The present study can be applied to optimize design concepts and boost the development of IL-based electrolytes.