An imidazolium-based polymerized ionic liquid (PIL), poly(1-ethyl-3-vinylimidazolium bis(trifluoromethanesulfonylimide)) is successfully synthesized via a new three-step process comprising the direct radical polymerization of the 1-vinylimidazole monomer, and subsequent quaternization reaction followed by an anion exchange procedure. Furthermore, polymer electrolytes are prepared by blending as-obtained PIL as the polymer host with an ionic liquid and LiTFSI salt. Electrochemical measurements demonstrate that compared with polymer electrolytes containing the PIL host synthesized by the conventional route, polymer electrolytes containing the PIL host obtained by new synthetic process exhibit significantly improved capacity and cycling performance, which is due to higher ionic liquid content.