Raman, infrared (IR), fluorescence, X-ray photoelectron (XPS), and nuclear magnetic resonance (NMR) spectra of various combinations of the components of the polyacrylonitrile (PAN)-based electrolytes have been studied, it is found that the so-called composite- or mixed-phase polymer electrolytes are not simple mixtures of electrolytic salt, plasticizer and polymer; instead there are strong interactions between each two of the components of the electrolytes and strong competitions between the plasticizer and PAN to associate with the Li+ ions though different systems may show different interaction characteristics. By analyzing the important roles of the plasticizer, a possible mechanism of the ion transport in the PAN-based polymer electrolyte is proposed.