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 ⁷Li- and ¹H-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 δ_S(CF₃) mode of the CF₃SO₃ ^- anion at ∼ 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 ∼ 60-10:1). A strong Li⁺-PAN interaction is manifested as a pronounced shift of the characteristic polymer CΞN stretching mode, found at ∼ 2244 cm^-1 in pure PAN, to ∼ 2275 cm^-1 for Li⁺-coordinated CΞN moieties. Our proton-NMR data suggest that upon complexation of PAN with LiCF₃SO₃, the glass transition occurs at progressively lower temperatures.