The effect of salt concentration on the local atomic structure and conductivity of PEO-based NiBr 2 electrolytes

Abstract

High molecular weight poly (ethylene oxide) (PEO) ( MW = 5 × 10 6) based NiBr 2 electrolytes of compositions, NiBr 2(PEO) n , n = 2, 4, 8, 16, 32, 100, where n is the number of oxygen per Ni(II) ion, were prepared by solution casting. An unusually high oxygen/metal ratio of 2 could be achieved with NiBr 2 salt. X-ray powder diffraction study and pair distribution function (PDF) analysis indicate that no crystalline PEO-salt complex phase exists until n≤16. At low salt concentration ( n> 16), the electrolyte consists of two components: amorphous PEO containing dissolved salt and crystalline PEO in which salt ions may exist as imperfections. The interchain correlation of PEO is virtually unaffected by dilute concentrations of salt. With increasing salt concentration, a new PEO-salt crystalline complex starts to form and the interchain correlation of PEO is drastically increased. These results are consistent with those obtained from differential scanning calorimetry (DSC) analysis and show a possible correlation between changes in local atomic structure and ionic conductivity.