The electrical conductivity and lithium ion transport properties of the polyethylene oxide (PEO) with lithium salt Li(CF3SO2)2N (LiTFSI) and a room temperature ionic liquid of N-methyl-N-propylpiperdinium bis(fluorosulfonyl)imide (PP13FSI) were examined as a function of the ionic liquid content. The electrical conductivity of PEO18LiTFSI–xPP13FSI increased and the lithium ion transport number decreased with increasing the ionic liquid content. The interface resistance between PEO18LiTFSI–xPP13FSI and lithium metal was dependent on x and the lowest interface resistance was observed in the range of x = 1.2–1.44. The critical current density of lithium deposition and dissolution on lithium metal was reduced and lithium dendrite formation was suppressed by the addition of PP13FSI into PEO18LiTFSI.