The electrochemistry of liquid polymer electrolytes
This thesis is concerned with ion-ion and ion-polymer interactions in liquid polymer electrolytes (LPE's). LPE's are useful analogues of visco-elastic polymer electrolytes (VEPE's) which might be used in high energy density batteries. The LPE's investigated comprise alkali metal salts dissolved in polyethers. Two types of polyether are studied, liquid copolymers of ethylene oxide (EO) and propylene oxide (PO), and poly(propylene oxide) (PPO). Separation of an EO-rich phase at low temperature or,for some iodide salt solutions, precipitation of the salt at high temperature, are two phenomena found to limit the temperature range in which some LPE's remain homogeneous. A new parameter, the diffusion number, is defined to reflect the relative diffusional mobilities of the cation and anion ion-constituents. This parameter is influenced by the motion of neutral ion aggregates (e.g. ion-pairs) and is thus fundamentally different from classical transference numbers, which indicate the net ion-constituent migration during electrolysis in the absence of diffusion. Cation transference numbers (t+) are measured using the Hittorf method. The results, showing t+ = 0.05 (+0.05), suggest that the cation is almost immobile during electrolysis and are significantly different from the results of experiments intended to determine transference numbers in VEPE's. The discrepancy is explained in terms of a model which stresses the importance of the non-conductive motion of neutral ion-pairs in polymer electrolytes. Non-conductive motion is demonstrated by diffusion coefficients determined by pulsed magnetic field gradient nuclear magnetic resonance. Studies of the conductivity and viscosity of LPE's demonstrate the influence of the cation and the polymer on the degree of dissociation of the dissolved salt. The conductivity of PPO-based electrolytes is shown to display a very large end-group effect. The mobility of the dissociated ions is found to be dependent on the anion and the nature of the polymer back-bone.