Crystalline and amorphous phases in the poly(ethylene oxide)-LiCF3SO3 system

A detailed vibrational spectroscopic study of the ionic species present in the (PEO)(x)LiTf system (x = 3-100) indicates that "free" ions and ion pair s are the dominant species present in the dilute salt region at room temper ature. When the salt concentration is increased to an O:M ratio of about 40 :1, a band due to an aggregate species first appears and is assigned to the triflate ion vibrating in the (PEO)(3)LiCF3SO3 compound. The amorphous pha se of (PEO)(3)LiCF3SO3 was studied by examining the temperature dependence of the spectra as the compound was heated through the melting transition at about 176 degrees C. Spectral behavior of the CH2 rocking modes and the CO C stretching mode indicated that disordering above the melting temperature resulted in a broad distribution of -O-C-C-O- torsional angles. However the basic lithium-polyether-backbone interaction was not been significantly ch anged for at least 15 degrees C above the transition temperature. Additiona l effects of thermal disordering were noted in the CF3 symmetric stretching mode and the antisymmetric stretching mode, reflecting an increase in the reorientational motion of CF3 end of the anion. These observations are cons istent with the large values of the fluorine atom isotropic temperature fac tors noted in an X-ray diffraction study, Although the phase diagram report s only two phases present at room temperature (crystalline PEO and the comp ound (PEO)(3)LiCF3SO3), the spectral data clearly show that there is an add itional phase present at intermediate salt concentrations which appears to be an amorphous PEO phase containing dissolved lithium triflate. If only cr ystalline PEO and the compound are present at equilibrium, the approach to the equilibrium state must be very slow indeed.