Raman and AC Impedance Spectroscopic Studies on Roles of Polyacrylonitrile in Polymer Electrolytes

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Abstract
Polymer electrolytes composed of polyacrylonitrile (PAN), propylene carbonate (PC), and lithium bis(trifluoromethane sulfone imide) [LiTFSI, LiN(CF3SO2)2]LiN(CF3SO2)2] have been studied by Raman and ac impedance spectroscopy. It is found that the associations of Li+Li+ ions with the polymer and with the solvent are two separate procedures. This makes it possible to study the influences of the polymer and the salt content on the conductivity seperately. Impedance study shows that the conductivity of the electrolyte is strongly dependent on the salt content, and a transition from “salt-in-polymer” to “polymer-in-salt” electrolyte is observed when the salt content is very high (at PC:PAN:LiTFSI=1:1:4 molePC:PAN:LiTFSI=1:1:4 mole ratio). Raman spectroscopic study indicates that the interactions between the lithium ionic species and the polymer are striking after PAN is dissolved in the PC/LiTFSI solution. A new spectral component is observed at 2280 cm−12280 cm−1 involving the interaction between the polymer and ionic cluster [Lim+TFSIn−][Lim+TFSIn−] (m>n)(m>n) (at PC:PAN:LiTFSI=1:1:3)PC:PAN:LiTFSI=1:1:3) as well as the 2270 cm−12270 cm−1 peak due to the nitrile- Li+Li+ interaction. As the interaction between PAN and the ionic clusters and the transition of conduction behavior occur at a similar salt content, it is proposed that PAN plays distinct roles in the transportation of the ionic species in conventional “salt-in-polymer” electrolyte and in the novel “polymer-in-salt” electrolytes. © 2002 The Electrochemical Society. All rights reserved.