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NMR and Raman studies of a novel fast-ion-conducting polymer-in-salt electrolyte based on LiCF3SO3 and PAN
journal contributionposted on 03.01.2000, 00:00 authored by A Ferry, L Edman, Maria ForsythMaria Forsyth, D MacFarlane, J Sun
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 7Li- and lH-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 δs(CF3) mode of the CF3SO3− anion at ~750–780 cm−l 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~60–10:1). A strong Li+–PAN interaction is manifested as a pronounced shift of the characteristic polymer C=N stretching mode, found at ~2244 cm−l in pure PAN, to ~2275 cm−l for Li+-coordinated C=N moieties. Our proton-NMR data suggest that upon complexation of PAN with LiCF3 SO3, the glass transition occurs at progressively lower temperatures.