Comparison of the cation environment in polymer electrolytes based on poly(ethylene oxide) and transition-metal bromides

Abstract

The local structure of polymer electrolyte systems based on poly(ethylene oxide), PEO, containing MBr₂(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 MBr₄^2–(and perhaps MBr₃O^–) for Co, Ni and Cu systems and the six-coordinate cationic species required for charge compensation, namely MO₆²⁺ and MO₄²⁺(and perhaps MO₅Br⁺ and MO₃Br⁺). In contrast to these transition-metal salts, for ZnBr₂ the formation of a neutral salt–PEO 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 appoaching octachedral and tetrahedral geometry.