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 ILbased supercapacitors. Additive engineering is based on adding cetyltrimethylammonium bromide-grafted Ti3C2 MXene (Ti3C2CTAB) 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 x 10-12 vs 4.04 x 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.