Solid polymer electrolytes (SPE) of poly (ethylene oxide) (PEO) - 50% epoxidized natural rubber (ENR50) polymer blend doped with lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) have achieved an optimum ionic conductivity value of 6.45 × 10−5 S cm−1 at 25 wt % of LiTFSI salt (room temperature). The chronoamperometry analysis indicated that the charge transport in polymer electrolyte is near-dominated by ion with lithium transference numbers (TLi +) of 0.29. Notably, this electrolyte exhibits a good electrochemical stability of 4.2 V (vs. Li/Li+). Further investigation by IR has indicated that polymer-salt complexation occurred between Li+ from dopant salt with oxygen from the ether group (C-O-C). These have weaken the vibrational mode of methylene group (CH2); scissoring and rocking mode on plane as well as twisting mode for out of the plane. The strong complexation between the cations with the polymeric segments have indirectly increased the thermal stability of the polymer electrolytes. Nevertheless, using the facile blending method, the addition of environment benign and natural material of ENR50 has improved the PEO-based electrolyte system.