Electrochemical Properties for Ionic Liquid/Polymer Electrolyte Systems

Abstract

The ionic liquid (1-ethyl-3-methylimidazolium hexa-fluorouphophate) ([emim][PF6]) with different molecular weights of poly(ethylene oxide) (PEO) (MW = 4600; 10,000; 14,000; 20,000; 35,000, and 100,000) has been characterized at various temperatures and compositions using phase behaviors and ionic conductivity. A molecular thermodynamic model based on a combination of the previous theory (BH model) by Chang et al., a nonrandomness theory (NR model), and the Pitzer-Debye-Huckel theory modified by Guggenheim (PDH model) considered not only short-range specific interactions between the polymer and a cation of the ionic liquid (IL), but also long-range electrostatic forces between anions and cations within the IL. We have derived a new melting point depression theory based on this BH-NR-PDH model. We also established an ionic conductivity model, based on the Nernst-Einstein equation, in which the diffusion coefficient is derived from the BH-NR-PDH model. The proposed model takes into account that the mobility of cations in the IL and the motions of the polymer host by expressing the effective chemical potential as the sum of the chemical potentials of the polymer and the IL. To describe the segmental motion of the cation and polymer chain, the effective coordinated unit parameter is introduced. The derived coordinated unit parameter for each state is used to determine the ionic conductivities of the given systems. Quantitative results from the proposed model are in good agreement with experimental data. The results indicate that the molecular weight of the polymer and the surrounding temperature play important roles in determining eutectic points and ionic conductivities of the given systems.