Multivalent ions in polymer electrolytes

Abstract

The electrochemical, thermal and structural properties of polyethylene oxide (PEO) based polymer electrolytes containing multivalent ions were investigated. The phase diagram for the PEO:Ca(CF₃SO₃)₂ system was determined by x-ray diffraction and differential scanning calorimetry techniques. Precipitation of the salt from the system at high temperatures was directly observed by variable temperature x- ray diffraction. This was ascribed to a negative entropy of dissolution of the salt in the polymer. A new crystalline complex PEO₆Ca(CF₃SO₃)₂, which exhibits a phase transition between two polymorphic forms was observed. The temperature dependence of ionic conductivity was related to the phase diagram. Redox behaviour of the PEO:Nil₂ system was probed. Motion of the Ni(II) species through the system was extremely slow as evidenced by the low effective diffusion coefficient (1.82 x 10 ¹¹ cm ²s⁻¹) and cationic current fraction (F₊ < 0.1). Deposition of nickel from the polymer was characterised by instantaneous nucleation followed by three dimensional diffusion controlled growth. Investigation of the redox behaviour of the PE0:Eu(CF₃SO₃)₃ system indicated that reduction of Eu³⁺ followed an ec mechanism. Evidence was obtained for extremely slow diffusion of Eu3+ containing species (D[sub]eff ~ 3.66 x 10 ⁻¹⁶cm²s⁻¹) through the system and slow kinetics of electron transfer. Thermal studies of the PEO:Co(SCN)₂ system indicated that the glass transition temperature (Tg) was grossly elevated by the presence of Co(SCN)₂ in the polymer. The absence of a crystalline PEO:Co(SCN)₂ complex was ascribed to the high Tg which leads to slow crystallisation kinetics. UV-visible spectra indicated that the Co²⁺ ion was tetrahedrally coordinated in the system at low salt concentrations. The structure of the PEO₃NaClO₄ crystalline complex was reported as a subsidiary study.