By choosing an ionic liquid 1-butyl-3-methylimidazolium iodide (BMIMI) as plasticizer and redox additive, along with a neutral Li2SO4 aqueous solution to achieve wide operating voltage, a redox-active poly(vinyl alcohol) (PVA)-Li2SO4-BMIMI gel polymer electrolyte (GPE) was prepared and used for the assemble of an activated carbon-based flexible supercapacitor. The electrochemical performances of the flexible supercapacitor with an optimized PVA-Li2SO4-BMIMI GPE were evaluated by cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy techniques. The mechanical performance of the flexible supercapacitor under different bending angles was also discussed. The obtained PVA-Li2SO4-BMIMI GPE exhibits the rarely found elongation at break of up to about 1200%. The flexible supercapacitor can reach a high energy density of 29.3 Wh kg(-1), which is attributed to the wide voltage (1.5 V) resulted from the strong solvation of Li+ cations and SO42- anions, and the pseudocapacitance contributed by Faradaic reactions of the I-based ions in the GPE. Moreover, the flexible supercapacitor shows good cyclic durability and superior mechanical performance. For comparison, the PVA-Li2SO4-BMIMCl GPE was also synthesized and then its performance was studied. (c) 2018 Elsevier Ltd. All rights reserved.