Additive Engineering Enables Ionic-Liquid Electrolyte-Based Supercapacitors To Deliver Simultaneously High Energy and Power Density

Ionic liquid (IL) electrolytes with a high potential window are promising candidates to high-energy-density supercapacitors; however, they commonly suffer from serious kinetic barriers that lead to poor power density. In this work, we propose an additive engineering method to promote rapid dynamics of IL-based supercapacitors. Additive engineering is based on adding cetyltrimethylammonium bromide-grafted Ti3C2 MXene (Ti3C2-CTAB) into 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4), a typical IL electrolyte for supercapacitors. Remarkably, IL electrolytes show a considerable increase by 38% in ionic conductivity and great reduction in solid–liquid surface energy from 18.03 to 12.37 mN m–1. We prove that electrostatic force and hydrogen bonds generated from the interaction between Ti3C2-CTAB and EMIMBF4 facilitate considerable dissociation of electrolyte ion pairs and ion-transfer capability. Consequently, additive engineering-designed IL-based supercapacitors deliver simultaneously a high energy density of 28.3 Wh kg–1 and power density of 18.3 kW kg–1. The increased high-power characteristics are supported by a faster ion diffusion coefficient (1.50 × 10–12 vs 4.04 × 10–13 cm2 s–1) and shorter relaxation time (3.83 vs 6.81 s). In addition, additive engineering guarantees a stable cycling life of 83.6% capacitance retention after 9000 cycles at the depth potential window from 0 to 3.0 V.