NMR and Raman studies of a novel fast-ion-conducting polymer-in-salt electrolyte based on LiCF3SO3 and PAN

We report spectroscopic results from investigations of a novel solid polymeric fast-ion-conductor based on poly(acrylonitrile), (PAN, of repeat unit [CH2CH(CN)](n)), and the salt LiCF3SO3. From NMR studies of the temperature and concentration dependencies of Li-7- and H-1-NMR linewidths, we conclude that significant ionic motion occurs at temperatures close to the glass transition temperature of these polymer-in-salt electrolytes, in accordance with a recent report on the ionic conductivity. In the dilute salt-in-polymer regime, however, ionic motion appears mainly to be confined to local salt-rich domains, as determined from the dramatic composition dependence of the ionic conductivity. FT-Raman spectroscopy is used to directly probe the local chemical anionic environment, as well as the Li+-PAN interaction. The characteristic delta(s)(CF3) mode of the CF3SO3- anion at similar to 750-780 cm(-1) shows that the ionic substructure is highly complex. Notably, no spectroscopic evidence of free anions is found even at relatively salt-depleted compositions (e.g. N:Li similar to 60-10:1). A strong Li+-PAN interaction is manifested as a pronounced shift of the characteristic polymer C=N stretching mode, found at similar to 2244 cm(-1) in pure PAN, to similar to 2275 cm(-1) for Li+-coordinated (C=N moieties. Our proton-NMR data suggest that upon complexation of PAN with LiCF3SO3, the glass transition occurs at progressively lower temperatures. (C) 2000 Elsevier Science Ltd. All rights reserved.