Flame-Retardant High-Conductivity Urea/EMIMBF₄ Eutectic Ion Gel Electrolyte for Symmetric Supercapacitors

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.