Cation-polymerized ionic liquids and non-ionic polymer monomers can be facile to chain into performance-enhanced conducting copolymers in combination with their respective features. A novel highly-flexible Br-based polycationic conducting copolymer is presented that shows both wide electrochemical window (6.0 V) and high ionic conductivity (2.04 x 10(-6) S.cm(-1) at 25 degrees C, 3.09 x 10(-4) S.cm(-1) at 60 degrees C), capable of potential solid polymer electrolyte. A 4.2 V flexible all-solid-state C/C supercapacitor is evaluated by using the poly(VAc-co-EVImBr) thin-film electrolyte. The exhibited superior charge-discharge capabilities at both 25 degrees C and 60 degrees C support strongly the applicable perspective of the proposed solid copolymer electrolyte.