Polymer gel electrolytes formed by hydroxyethylcellulose (HEC) plasticized with glycerol and containing lithium perchlorate were studied by nuclear magnetic resonance spectroscopy (NMR) and complex impedance spectroscopy. In heavily plasticized samples, results show that the addition of solvent enhances the conductivity, which reaches 6×10−5 S/cm at room temperature. The 7Li NMR results indicate that this enhancement is associated with a decoupling of the lithium-ion dynamics from the local motion of the polymer host, with the increase in the mobility of the ionic species, and with the increase of the charge carrier concentration resulting from the salt dissociation after addition of the solvent. The temperature dependence of the lithium NMR spin-lattice relaxation is interpreted assuming that there are two distinct lithium-ion dynamics. The corresponding NMR relaxation processes are characterized by activation energies of 0.3 and 0.12eV, respectively.