A study of the local atomic structure and transport properties of poly(ethylene oxide) PEO-based nickel bromide electrolytes

Heng Cai, University of Pennsylvania


In poly(ethylene glycol) (PEG)-based NiBr$\sb2$ electrolytes, EXAFS and UV-visible spectroscopy were used to probe the immediate surroundings of the Ni(II) in the electrolytes. It was found that Ni(II) is partitioned between octahedral, (NiO$\sb6\rbrack\sp{2+}$, and tetrahedral, (NiBr$\sb4\rbrack\sp{2-}$, complexes. The equilibrium between the two Ni(II) forms shifts with salt concentration. At low NiBr$\sb2$ concentrations, Ni(II) exists principally as (NiO$\sb6\rbrack\sp{2+}$. With increasing NiBr$\sb2$ concentration, the equilibrium between (NiO$\sb6\rbrack\sp{2+}$ and (NiBr$\sb4\rbrack\sp{2-}$ shifts to favor the formation of the tetrahedral complex ion. This model suggests that, at low total NiBr$\sb2$ concentrations, conductivity occurs principally by the motion of free Br(-I) ions. With increasing total NiBr$\sb2$ concentration, so long as the Ni(II) ions are predominantly in octahedral complexes, the population of free Br(-I) increases but so does the viscosity of the electrolyte; presumably the result of the formation of pseudo-crosslinks between chains by Ni(II) ions. When Ni(II) ions begin to form tetrahedral complexes with Br(-I) ions, the free Br(-I) concentration begins to decrease, and the population of (NiBr$\sb4\rbrack\sp{2- }$ complex ions increases. At high total NiBr$\sb2$ concentrations, conductivity then results from the motion of (NiBr$\sb4\rbrack\sp{2-}$ through a very viscous medium and consequently is quite low. In poly(ethylene oxide) (PEO)-based NiBr$\sb2$ electrolytes, the local atomic arrangement around Br(-I) in NiBr$\sb2$(PEO)$\sb8$ was obtained for the first time. It was seen that, on average, there are one Ni(II) and two oxygens, at 2.48 and 2.58 A respectively, in the first atomic shell surrounding Br(-I). X-ray powder diffraction and Pair Distribution Function (PDF) analysis found no crystalline PEO-salt complex phase until n = 16. At low salt concentration (n $>$ 16), salt ions are likely to stay either in the amorphous region of PEO or in the crystalline domain as imperfections. The interchain correlation of PEO is virtually unaffected by dilute salt concentration. With increasing salt concentration, a new PEO-salt crystalline complex starts to form and the interchain correlation of PEO is drastically improved. These results are consistent with those obtained from DSC analysis and show a possible correlation between local atomic structure change and a change in the ionic conductivity behavior. (Abstract shortened by UMI.)

Subject Area

Materials science|Chemical engineering|Polymers

Recommended Citation

Cai, Heng, "A study of the local atomic structure and transport properties of poly(ethylene oxide) PEO-based nickel bromide electrolytes" (1992). Dissertations available from ProQuest. AAI9308542.