COMPARISON OF THE CATION ENVIRONMENT IN POLYMER ELECTROLYTES BASED ON POLY(ETHYLENE OXIDE) AND TRANSITION-METAL BROMIDES

The local structure of polymer electrolyte systems based on poly(ethylene oxide), PEG, containing MBr(2) (M = Co, Ni, Cu and Zn) salts has been studied by UV-VIS spectroscopy and EXAFS. The results indicate the presence both of anionic complexes MBr(4)(2-) (and perhaps MBr(3)O(-)) for Co, Ni and Cu systems and the six-coordinate cationic species required for charge compensation, namely MO(6)(2+) and MO(4)(2+) (and perhaps MO(5)Br(+) and MO(3)Br(+)). In contrast to these transition-metal salts, for ZnBr2 the formation of a neutral salt-PEG complex, ZnBr2O2, is favoured. In the case of CoBr2-PEO and NiBr2-PEO systems, much better fits for EXAFS results are obtained when two oxygen shells are considered. This supports the view that the oxygen from the PEO backbone coordinates with these metal ions forming structural units approaching octahedral and tetrahedral geometry.