Conductivity and nuclear magnetic resonance studies on polymer electrolytes based on poly(ethylene oxide)

Abstract

The thesis details studies relating to polymer electrolytes; the solid ionic conductors farmed by the dissolution of salts in suitable high molecular weight polymers. An outline of polymer electrolyte study is presented with respect to current understanding of the phase behaviour, morphology and conductance behaviour of the electrolyte materials. (In particular, those based upon the linear homopolymer poly(ethylene oxide), PEO.) An electrochemical study has been undertaken (₂98 K) involving a low molecular weight PEO analogue, PEO(₄00)e = CH₃C0₂(CH₂CH₂0) CO CH₃ (n = 8 - 9 ), containing LiCF₃SO₃ or LiClO₄. The study has shown that at low to medium salt concentrations in polyether media ion - ion interactions are important and are realized as ion association. The conductance vs. concentration behaviour has been modelled according to an equilibrium between single, ion pair and triple ion species where the concentration of simple (single) ions are small and decreasing, and above a total salt concentration of about 0.01 mol kg⁻¹, the majority of the current is carried by triple ion species of the form Li₂X- LiX₂ (X = CF₃SO₃ , CIO₄). Equilibrium constant data were obtained for single and triple ion formation (from neutral ion pairs). Determination of triple ion formation constants vs. temperature has shown that the triple ion formation process for LiCF₃SO₃ in PEO(₄00)e is an exothermic process, negative, whereas for LiClO₄ AH = 0 kJinal⁻¹. Using nuclear magnetic resonance (nmr), diffusion coefficients have been obtained for the oligomer chain in PEO(₄00)e and PEO(₄00)e.LiCF₃SO₃ solutions. The chain diffusion coefficients have been shown to give good agreement with those for salt diffusion, determined from conductance measurements via the Nernst - Einstein relation. An in - depth nmr investigation of the PEO.LICF₃SO₃ system (high molecular weight PEO) has shown that there is partition of lithium environments, probably within the salt rich crystalline phase (EQ/Li - ₃.5/1). Significant numbers of lithium nuclei are not observed with the nmr technique because they occupy environments of law symmetry. This was reinforced by other nmr measurements which suggested cation - anion proximity in the crystalline phase. A mixed salt system has been studied, PEO. LiCF₃SO₃. Nal, and it has been shown that the mixing of salts gave materials with superior conductivities to the relevant single salt systems (PEO. LiCF₃SO₃ and PEO.Nal) of the same overall salt content. Nmr has shown that the mixed salt effect was due to a larger amorphous (conducting) polymer phase and more potential charge carriers for the mixed salt in comparison to the single salt materials. A marked effect upon lithium motion was observed for PEO.LiCF₃SO₃ Nal system in comparison to PEO.LiCF₃SO₃ and it has been proposed that this was due to the observed lithium species becoming mobile at notably lower temperatures for the mixed salt system.