The effect of plasticizers (or solvent additives) on the transport and electrochemical properties of polymer electrolytes has been examined. First, the mobility of cations and anions of LiN(CF3SO2)(2) (LiTFSI) was increased by doping the PEO-based electrolytes with ethylene carbonate and poly(ethylene glycol dimethyl) ether. However, the cation mobility in PEO-based electrolytes doped with propylene carbonate was suppressed resulting in a lower cation transference number. Nevertheless, all the plasticized electrolytes have higher bulk conductivity than the non-plasticized, even in the case the cation transference number decreased. This is due to increased ion mobilities and/or higher degree of ion dissociation upon the introduction of plasticizers. A study of the electrochemical stability of the electrolytes, by linear sweep voltammetry, shows that the plasticizers have a negligible effect on the electrolyte stability windows. Finally, the electrolyte/lithium metal electrode interfacial impedance was monitored by A.C. impedance as a function of storage time. The formation and growth rate of the passivation layer was diminished in the presence of the plasticizers.