A poly(styrenesulfonate) lithium salt was tested as a single-ion conductor in a poly(ethylene oxide)/poly(ethylene glycol) dimethyl ether matrix. Impedance spectroscopy and voltage step polarization were used to characterize the composite electrolytes. A specific conductivity of about 7×10−8 S cm−1 was evaluated at 70°C. The very low conductivity was attributed to the poor solubility of Li+ in the polymer matrix. AlCl3 was added to the polymer electrolyte to increase the salt dissolution. The addition of the Lewis acid strongly increases the conductivity and a specific conductivity of about 4×10−6 S cm−1 was measured at 20°C. For temperature lower than 60°C, the specific conductivity dependence with increasing temperature follows an Arrhenius-type behavior. An activation energy of about 55.6 kJ mol−1 was calculated. A very similar activation energy (60.3 kJ mol−1) was found for the charge transfer resistance. The transport properties of the polymer electrolyte were tested by applying a d.c. voltage step to a symmetrical lithium cell. The current at the applied voltage decreased with time. The decrease was related to an increase in the cell resistance due to the continuous growth of a passivation layer on the lithium surface.