Dielectric relaxation, ionic conductivity and thermal studies of the gel polymer electrolyte system PAN/EC/PC/LiTFSI

Dielectric relaxation, ionic conductivity and thermal properties have been measured for the gel polymer electrolyte system poly(acrylonitrile)/ethylene carbonate/propylene carbonate/lithium bis(trifluoromethanesulfone)imide (PAN/EC/PC/LiTFSI) and for its components in the frequency range from 1 MHz to 1.8 GHz and over a temperature range from −20 to 50 °C. DSC results suggest that EC/PC exists in two different environments within the gel network; as regions in which the EC/PC molecules subjected to pairing interactions by the CN group in PAN and also as regions consisting of “free” EC/PC molecules. Addition of PAN to the EC/PC/LiTFSI liquid electrolyte has increased the ionic conductivity. Out of the various PAN/LiTFSI composition ratios studied for the gel polymer electrolyte, the 6:1 composition ratio by weight gives the highest ionic conductivity. The room temperature (23 °C) conductivity of the gel electrolyte with this composition, PAN(15.4%)/EC(41.0%)/PC(41.0%)/LiTFSI(2.6%) (by weight) is 2.5×10−3 S cm−1. DSC results show that this composition has the most amorphous nature, above −105 °C. The ε″ spectra of gel electrolytes with various compositions show the presence of a high-frequency peak in the 0.5-GHz region attributed to the α relaxation process and a peak/shoulder in the 10-MHz region attributed to the ion-pair relaxation. Li+ ion transport probably takes place in the vicinity of the PAN chains and the ion-pair relaxation frequency appears to reflect the dynamic environment in which the cations migrate. However, the coupling between the conductivity and the α relaxations, attributed to EC/PC molecules, appears to be weak. A model has been presented according to which the Li+ ions in the gel electrolyte appears to be solvated by both PAN (through CN) and EC/PC.