The formidable challenge of supercapacitors (SCs) is to achieve both high energy density and high power density simultaneously. Although the utilization of porous carbon materials for physical adsorption in electrochemical double-layer capacitors (EDLCs) applications exhibits promising prospects, attaining exceptional performance necessitates additional methodologies. Here, we propose an effective strategy for formulating redox electrolytes through the incorporation of organic solvents and LiBr into 1-ethyl-3-methylimidazole tetrafluoroborate (EMIMBF 4 ) electrolytes. Within this formulation, LiBr serves as a redox mediator, contributing Br − /Br 3 − redox pairs to the system. The incorporation of EMIMBF 4 -LiBr-DMSO in brominated redox EDLCs led to a significant enhancement in specific capacitance, reaching up to 39.72 F g −1 , representing a remarkable increase of 159 % compared to EDLCs without any redox media. This configuration achieved a high power density of 496.6 W kg −1 while maintaining an energy density of 34.48 Wh kg −1 .
Shiyao Tang,Rui Teng,Chunling Cao,Hongjian Fu,Xiaofei Wu,Chun hui Ma,Shouxin Liu,Wei Li
Gel polymer electrolytes (GPEs) have been extensively explored for safe and flexible supercapacitors (SCs). However, within eutectic-based GPEs, the electrolyte phase is most commonly implemented either as salt/hydrogen-bond donor (HBD)-type deep eutectic solvents (DES) or by using DES as the solvent/plasticizing medium in polymer matrices. These approaches often suffer from high viscosity and consequently limited ionic conductivity, hindering ion transport and device performance. Here, we propose the concept of an ionic-liquid-assisted eutectic electrolyte; the ionic liquid serves as an active eutectic component and directly participates in establishing the eutectic solvation structure for electrolyte design. On this basis, we develop a quasi-solid ion-gel electrolyte by blending inexpensive bioderived urea (hydrogen-bond donor) with 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4, hydrogen-bond acceptor) at an optimized 3:7 molar ratio, followed by incorporation into a polyvinylidene fluoride-hexafluoro propylene (PVDF-HFP) matrix via solution casting and vacuum drying. The PVDF-HFP/Urea/EMIMBF4 (PUE) gel achieves a high ionic conductivity of 3.52 mS cm–1, a wide electrochemical stability window of 2.6 V, and reliable operation up to 60 °C. Symmetric SCs based on activated carbon electrodes deliver a specific capacitance of 173.1 F g–1 at 1 A g–1, an energy density of 40.7 Wh kg–1, and 80.1% capacitance retention after 5000 cycles (with a 10% capacitance increase at 60 °C). Additionally, the formation of a binary eutectic mixture significantly lowers the solidification point, which also confers intrinsic flame retardancy to the PUE gel. This work offers a facile and effective route to high-safety, wide-temperature, and eco-friendly quasi-solid electrolytes for next-generation energy storage devices.
Developing an ionic-liquid (IL) type electrolyte with both high voltage window and wide temperature window, especially the low temperature range, is crucial to increase the energy density of the associated electrical double layer capacitors. We proposed the addition of γ-butyrolactone (GBL) in EMIMBF4 to form a binary electrolyte. The melting point (∼15 °C) of EMIMBF4 disappeared and the glass transition point decreased from −95 °C (for pure EMIMBF4) to −126 °C for the binary electrolyte. The binary electrolyte also exhibited improved ionic conductivity (26 and 0.31 mS cm−1 at 20 and −70 °C, respectively) and allowed an electrode of mesoporous graphene to exhibit a capacitance of 131 F g−1 and energy density of 61 W h kg−1 at −70 °C and 3.7 V, the highest values reported so far. NMR and ATR-IR characterization validated the strong interaction between GBL and EMIMBF4 for breaking the ion pairs of the latter but without forming the solvated ions, as in the system of EMMIBF4 with propylene carbonate.
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