With 1-methyl-2-pyrrolidinone (NMP) as the solvent, the biodegradable gel polymer electrolyte films are prepared based on poly(vinyl alcohol) (PVA), lithium bis(trifluoromethane)sulfonimide (LiTFSI), and 1-ethyl-3 methylimidazoliumbis(trifluoromethylsulfonyl)imide (EMITFSI) by means of solution casting. The films are characterized to evaluate their structural and electrochemical performance. The 60PVA-40LiTFSI + 10 wt.% EMITFSI system exhibits excellent mechanical properties and a high ionic transference number (0.995), indicating primary ionic conduction in the film. In addition, because of the flexibility of polymer chain segments, its relaxation time is as low as 5.30 × 10⁻⁷ s. Accordingly, a high ionic conductivity (3.6 × 10⁻³ S cm⁻¹) and a wide electrochemical stability window (~5 V) are obtained. The electric double-layer capacitor (EDLC) based on this electrolyte system shows a specific capacitance of 101 F g⁻¹ and an energy density of 10.3 W h kg⁻¹, even after 1000 charge-discharge cycles at a current density of 0.4 A g⁻¹ under a charging voltage of 2 V. All these excellent properties imply that the NMP-soluble 60PVA-40LiTFSI + 10 wt.% EMITFSI gel polymer electrolyte could be a promising electrolyte candidate for electrochemical device applications.