STRUCTURE AND ELECTROCHEMISTRY OF POLYMER ELECTROLYTES

Two essential but often neglected topics in the field of polymer electrolytes are highlighted. The first concerns the structure of crystalline polymer electrolytes, from which much may be learnt about the principles governing polymer electrolyte structure in general. The crystal structures of PEO(3)-LiCF3SO3, PEO(3)-NaClO4, PEO(4)-KSCN, PEO(4)-NH4SCN and PEO(4)-RbSCN are discussed. These reveal that, contrary to the established view, cations ranging in size from Li+ to Rb+ are accommodated within the PEO helix, although the chain conformation changes with increasing cation size. The coordination number around the cations increases from 5, in the case of Li+, to 7 in the case of Rb+. In all examples studied, the immediate environment around the cation is composed of two anions with the remaining ligands being ether oxygens From the PEO chain. Each ion is associated with only one chain and thus there is no ionic cross-linking. This suggests the importance of intrachain rather than interchain cross-linking in polymer electrolyte systems. It appears that T-g may increase with salt content due mainly to intrachain rather than interchain cross-links. The location of the cation within the PEO helix in all systems reported here may have implications for the mechanism of ionic conduction. The second much-neglected topic is the interfacial electrochemistry of polymer electrolytes, particularly redox reactions in solid polymers at metal electrodes. We have shown that the combination of ac spectroscopy and ultramicroelectrodes provides an ideal tool with which to study interfacial reactions in polymer electrolytes. Impedance measurements have been made up to 10(9) Omega. These reveal that the electrochemistry of ferrocene on platinum electrodes is complex, whereas the Fe-3+/2+ couple appears, at least at low concentrations, to exhibit a simple one-step electrode reaction. The crucial role that the study of redox reactions in solid and liquid ether solvents can play in understanding the important topic of electron transfer in general is discussed.