A binary ionogel electrolyte for the realization of an all solid-state electrical double-layer capacitor performing at low temperature.

Abstract

Solid-state electrolytes made of an ionic liquid (IL) confined in a solid matrix, also known as ionogels, have been proposed to prevent electrolyte leakage from electrical double-layer capacitors (EDLCs), and thereof improve their safety. However, making ionogel-based EDLCs performing with a reasonable power at low temperature is still a major challenge due to the high melting point of the confined IL. To overcome such limitations, this contribution discloses ionogel films prepared by encapsulating a binary mixture of 1‑ethyl‑3-methylimidazolium bis(fluorosulfonyl)imide and 1‑ethyl‑3-methylimidazolium tetrafluoroborate - [EMIm][BF4]0.5[FSI]0.5 - into a PVdF-HFP network. Contrary to ionogels prepared from either [EMIm][FSI] or [EMIm][BF4], which display freezing/melting, the crystallization of confined [EMIm][BF4]0.5[FSI]0.5 is quenched in the binary ionogel, which shows only a glass transition at -101 °C. This quenching enables an increased ionicity and ionic diffusion at the interface with the PVdF host network, leading the binary ionogel membrane to display higher ionic conductivity below -20 °C than the parent binary [EMIm][BF4]0.5[FSI]0.5 liquid. Laminate EDLCs built with a binary ionogel separator and hierarchical carbon electrodes operated ideally from -40 °C to room temperature, displaying ca. 4 times larger output energy than a cell implementing the same carbon electrodes together with the binary [EMIm][BF4]0.5[FSI]0.5 ionic liquid.