Microwave Dielectric Relaxation, Electrical Conductance, and Ultrasonic Relaxation of LiPF6 in Poly(ethylene oxide) Dimethyl Ether-500

Abstract

Microwave dielectric complex permittivity ε* = ε‘ − Jε‘ ‘ in the frequency range ∼1−130 GHz and at temperatures of 25, 40, and 55 °C for the system LiPF6 dissolved in poly(ethylene oxide) dimethyl ether of average molar mass 500 (PEO-500) are reported. The dielectric spectra are interpreted by a Cole−Davidson distribution function for both the solvent and the solutions. The activation parameters ΔH⧧ and ΔS⧧ for the dielectric process have been determined for both solvent and solutions. Audio frequency electrical conductance of solutions of LiPF6 in PEO-500, in the concentration range ∼5 × 10-4 to 1.0 M (M = mol/dm3) and temperatures 25, 40, and 55 °C, have been determined. The conductance data in the diluted range have been analyzed by two forms of the Fuoss-Onsager conductance theory. Despite the presence of a minimum in the conductance vs concentration functions, the presence of conducting triple ions has not been postulated, because the minimum is predicted by the functional form of the conductance theory at low solvent permittivities, for moderate ion-pair associations. Ultrasonic relaxation spectra in the frequency range ∼1 to ∼400 MHz and LiPF6 concentration range 0.1−0.5 M at 25 °C are reported and analyzed in terms of the sum of two debye relaxation processes. The upper one centered around 150−200 MHz and common to the solvent PEO-500 is attributed to polymer-chain rearrangements, also influenced by the presence of the electrolyte. The lower relaxation process in the low megahertz frequency range is interpreted in terms of formation of dimers-ion pairs according to the process 2LiPF6 ⇌ (LiPF6)2, thus rationalizing the presence of a maximum in the static dielectric permitivity vs concentration.