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Biodegradable polymer electrolyte films based on poly(ε-caprolactone) (PCL) in conjunction with lithium tetrafluoroborate (LiBF) salt and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF) ionic liquid were prepared by solution cast technique. The structural, morphological, thermal, and electrical properties of these films were examined using X-ray diffraction (XRD), optical microscopy (OM), differential scanning calorimetry (DSC), and impedance spectroscopy. The XRD and OM results reveal that the pure PCL possesses a semi-crystalline nature and its degree of crystallinity decreases with the addition of LiBF salt and EMIMBF ionic liquid. DSC analysis indicates that the melting temperature and enthalpy are apparently lower for the 40 wt% EMIMBF gel polymer electrolyte as compared with the others. The ambient temperature electrical conductivity increases with increasing EMIMBF concentration and reaches a high value of ∼2.83 × 10 S cm for the 85 PCL:15 LiBF + 40 wt% EMIMBF gel polymer electrolyte. The dielectric constant and ionic conductivity follow the same trend with increasing EMIMBF concentration. The dominant conducting species in the 40 wt% EMIMBF gel polymer electrolyte determined by Wagner's polarization technique are ions. The ionic conductivity of this polymer electrolyte (∼2.83 × 10 S cm) should be high enough for practical applications. [ABSTRACT FROM AUTHOR]