A Binary Ionogel Electrolyte for the Realization of an All Solid-State Electrical Double-Layer Capacitor Performing at Low Temperature.

Abstract

Over the last years, solid-state electrolytes made of an ionic liquid (IL) confined in a solid (inorganic or polymer) matrix, also known as ionogels, have been proposed to solve the leakage problems occurring at high temperatures in classical electrical double-layer capacitors (EDLCs) with an organic electrolyte, and thereof improve the safety. However, making ionogel-based EDLCs perform with reasonable power at low temperature is still a major challenge due to the high melting point of the confined IL. To overcome these limitations, the present contribution discloses ionogel films prepared in a totally oxygen/moisture-free atmosphere by encapsulating 70 wt % of an equimolar mixture of 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide and 1-ethyl-3-methylimidazolium tetrafluoroborate - [EMIm][BF₄]_0.5[FSI]_0.5 - into a poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) network. The further called "binary ionogel" films demonstrated a high flexibility and a good ionic conductivity of 5.8 mS cm^-1 at 20 °C. Contrary to the ionogels prepared from either [EMIm][FSI] or [EMIm][BF₄], displaying melting at T_m=-16 °C and -7 °C, respectively, 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 were built with a 100 μm thick binary ionogel separator and electrodes made from a hierarchical micro-/mesoporous MgO-templated carbon containing a reasonable proportion of mesopores to enhance the mass transport of ions, especially at low temperature where the ionic diffusion noticeably decreases. The EDLCs operated up to 3.0 V with ideal EDL characteristics from -40 °C to room temperature. Their output specific energy under a discharge power of 1 kW kg^-1 is ca. 4 times larger than with a cell implementing the same carbon electrodes together with the binary [EMIm][BF₄]_0.5[FSI]_0.5 liquid. Hence, this binary ionogel electrolyte concept paves the road for developing safe and flexible solid-state energy storage devices operating at subambient temperatures in extreme environments.