Characterization of some polyacrylonitrile-based electrolytes

Polyacrylonitrile (PAN)-based electrolytes have been prepared by encapsulating Li salt solutions obtained by dissolving LiN(SO2CF3)(2), LiAsF6, LiSO3CF3, or LiPF6 in a plasticizer mixture of ethylene carbonate (EC) and propylene carbonate (PC). The conductivity of these electrolytes is governed by a combination of factors including the relative amounts of PAN, EC, PC and the Li salt, and temperature. A conductivity of about 4.5 x 10(-3) Omega(-1) cm(-1) at 30 degrees C was exhibited by EC-rich electrolytes containing both LiN(SO2CF3)(2) and LiPF6. The cationic transference numbers, measured using a de polarization method coupled with impedance spectroscopy, were found to be between 0.2 and 0.3 for electrolytes containing LiN(CF3SO2)(2) and LiPF6. The tensile strength of the electrolytes was observed to increase from 60 to 140 lb/in.(2) upon incorporation of the thermally activated cross-linking agent ethylene glycol dimethacrylate. Cyclic voltammetry on Pt electrodes has shown that these electrolytes have an inherent oxidation stability window extending up to about 5 V versus Li+/Li. On Al, Ni, and Cu electrodes, however, oxidation was observed at lower potentials because of metal corrosion reactions. Copper was reversibly oxidized at 3.5 V, irrespective of the Li salt present in the electrolyte. On the other hand, the nature of the Li salt influenced the corrosion potentials of Al and Ni. Nickel was found to be stable up to 4.2 V in the presence of LiAsF6, whereas it was oxidized at about 3.5 and 4 V when the electrolyte contained LiN(SO2CF3)(2) and LiSO3CF3, respectively. Aluminum electrodes showed no oxidation currents up to 4.2 V when the salts were LiPF6 and LiAsF6 which contrasted the behavior with LiN(SO2CF3)(2) and LiSO3CF3 showing oxidation at less than or equal to 4 V. The electrochemical stability of the electrolytes was further verified in Li/LiMn2O4 cells.