The evolution of the microstructures in polymer electrolytes composed of propylene carbonate (PC), polyacrylonitrile (PAN), and lithium bis(trifluoromethane sulfone imide) (LiTFSI) has been characterized by Raman spectroscopy and the conduction behaviors of the electrolytes have been evaluated by ac impedance spectroscopy. The conductivity of the electrolyte depends strongly on the association of the lithium ions and ion clusters with the electron donor of the polymer. A transition from "salt-in-polymer" to "polymer-in-salt" electrolyte has been observed when the salt content is very high (PC:PAN:LiTFSI = 1:1:5). Based on these facts, the distinct roles of the polymer are proposed on the transportation of the ionic species in conventional "salt-in-polymer" electrolyte and in "polymer-in-salt" electrolytes.