Ion transport in polymer electrolytes

Abstract

The ion-polymer and ion-ion interactions in polymer electrolytes based on high molecular weight, amorphous methoxy-linked PEO (PMEO) and lithium salts have been investigated by conductivity measurement, magic-angle spinning NMR (mas NMR) and pulsed field gradient NMR (pfgNMR) techniques. In the very dilute salt concentration region, ion pairing effects are dominant in these polymer electrolytes. Ion association is found to increase with temperature and salt concentration. Ion transport for these electrolytes is controlled both by segmental motion of the polymer and activation process, in which the former is important for the dilute concentration samples while the latter is important for the concentrated samples. The mass transport process in polymer electrolytes based on a zinc salt has been investigated by steady state dc polarisation and Hittorf techniques. Zinc ion constituents in these electrolytes are mobile with a limiting current fraction of about 0.2 at 80°C, and the transference number measured by the Hittorf method is less than 0.1. The main species in these electrolytes are proposed to be neutral mobile triples. The electrode-electrolyte interfaces in polymer electrolytes based on calcium and magnesium salts have been studied. Dc polarisation experiments for these polymer electrolytes were carried out using two electrode cells with the metal anode and mercury film amalgam cathode. The results of dc polarisation experiments suggest that calcium species are mobile in high molecular weight electrolytes, while magnesium species are immobile. The influence of the molecular weight of the polymer on the dynamics of cation constituents has been studied based on the experimental results of dc polarisation and pfg NMR, and on the theoretical analyses of the reptation theory and the Rouse model. It is found that the transport of the gravity centre of the polymer will influence the ion transport in polymer electrolytes based on PEO in a manner described by the Rouse model when the molecular weight of PEO is less than 3200.