Crystalline and Amorphous Phases in the Poly(ethylene oxide)−LiCF₃SO₃ System

Abstract

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 pairs are the dominant species present in the dilute salt region at room temperature. 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)3LiCF3SO3 compound. The amorphous phase of (PEO)3LiCF3SO3 was studied by examining the temperature dependence of the spectra as the compound was heated through the melting transition at about 176 °C. Spectral behavior of the CH2 rocking modes and the COC 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 changed for at least 15 °C above the transition temperature. Additional 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 consistent with the large values of the fluorine atom isotropic temperature factors noted in an X-ray diffraction study. Although the phase diagram reports only two phases present at room temperature (crystalline PEO and the compound (PEO)3LiCF3SO3), the spectral data clearly show that there is an additional phase present at intermediate salt concentrations which appears to be an amorphous PEO phase containing dissolved lithium triflate. If only crystalline PEO and the compound are present at equilibrium, the approach to the equilibrium state must be very slow indeed.

Highly Conductive, Ionic Liquid-Based Polymer Electrolytes

“…• C, coinciding to the melting of the salt rich phase 2 and in agreement with the DSC results ( Figure 3A). The PEO-7 sample exhibits a knee around −20…”

“…1 Nevertheless, the performance of LPBs is limited by the low ionic conductivity of the solvent-free polymer electrolytes at room temperature as the polymeric host is manly crystalline (i.e., resulting in slow motion of the Li + active specie). 2 For instance, polyethylene oxide-(PEO) based electrolytes exhibit suitable conduction values (≥10 −4 S cm −1 ) for practical applications only above 65…”

Structure and ionic conductivity of ionic liquid embedded PEO- LiCF3SO3 polymer electrolyte

“…4(c) at 964 and 949 cm −1 are primarily due to the C-O stretching modes with a slight contribution arising from CH 2 rocking motion. 39,40 These three regions are sensitive to the interaction of cations with the polymer backbone and hence importance is given for studying the conformational changes of PEO with the addition of EMITf ionic liquid. In Fig.…”

Raman study of triblock polyether/LiCF3SO3 polymeric electrolytes

“…For very lowconcentration samples (2 and 5%) the decrease in the CF 3 SO 3 concentration is accompanied by an increase in the concentration of ion pairs. For compounds in the intermediate concentration range (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) and 30%) there is a saturation in the concentration of ion pairs, and the further decrease in the CF 3 SO 3 concentration is associated with the increase in the concentration of negative triplets. The positive triplets appear when the CF 3 SO 3 concentration tends to a constant value.…”