Plasticiser-induced local structure in polymer electrolytes

Abstract

Results are reported for EXAFS local structural studies of polymer electrolytes incorporating nickel bromide. The bench-mark system PEO₈ : NiBr₂ was studied at 25 and 170 °C [PEO = poly(ethylene oxide)]. Nickel was shown to be surrounded by both oxygen and bromine nearest neighbours, but at high temperature the number of oxygen neighbours was considerably reduced. For the more dilute PEO₁₀₀ : NiBr₂ there are no bromine nearest neighbours, indicating an absence of ion association. The local structure in solutions of nickel bromide in poly(ethylene glycol) is shown to be substantially different from the bench-mark system. For electrolyte films made from nickel bromide, PEO and a plasticiser it is shown that when the plasticiser is methyl end-capped poly(ethylene glycol) the local structure surrounding nickel is virtually unchanged from that of the bench-mark polymer electrolyte. When hydroxy end-capped poly(ethylene glycol) is used, the structure closely resembles that in the solution of the low molecular weight oligomer only. It is concluded that the choice of platiciser is important.