A nitrogen-doped mesopore-dominated carbon electrode allied with anti-freezing EMIBF4-GBL electrolyte for superior low-temperature supercapacitors

Ionic liquids (ILs) show great promise to endow electric double-layer capacitors (EDLCs) with high energy density; however, their operation in practical deep-cold environments has been severely plagued by two major problems, namely (i) poor compatibility between the electrode material and the ILs and (ii) the ease of freezing of ILs. Here, we show that the combination of a nitrogen-doped mesopore-dominated hierarchical carbon (NMHC) electrode with plentiful ion-accessible adsorption sites (specific surface area and pore volume of 2637.4 m2 g-1 and 1.679 cm3 g-1) and anti-freezing EMIBF4-GBL electrolyte with high ion-conductivity (2.3 S cm-1 at -50 °C) can address this issue. Specifically, "H-bond breakage" in EMIBF4 ILs was proposed for the first time to understand the anti-freezing mechanism, as evidenced by Raman spectroscopy, 1H NMR spectroscopy, and density functional theory calculations. As a result, the combination of NMHC electrode with EMIBF4-GBL electrolyte enabled an impressive specific energy of 61 W h kg-1 at -50 °C, which was 10.7 times larger than that of commercial YP50. This work showed excellent electrode-electrolyte synergy and provides a new understanding of IL-based electrolytes for low-temperature energy storage.