Nano‐ or Microclustering as an Explanation for the Unusual Thermal Behavior of Nickel Polymer Electrolytes

Abstract

Nickel polymer electrolytes have long been known to have unexpected properties. Careful studies of (i) conductivity as a function of temperature for PEO3:NiBr2 over the range 25 T 180°C; (ii) conductivity at 150°C of PEO:NiBr2 over the composition range 8 n 200; and (iii) complementary differential scanning calorimetry and variable-temperature polarizing microscopy (VTPM) measurements, are reported. In conjunction with extended x-ray absorption fine structure results reported earlier, these measurements suggest that the complicated dependence of conductivity as a function of temperature can be explained in terms of phase separation of the salt in the amorphous region followed by melting of salt-rich spherulites as the sample is heated from 140 to 180°C, without the need to invoke explanations based on the degree of hydration. It is also suggested that phase-separated NiBr2 particles are present at room temperature in all samples that have been preheated to 140°C in order to dry them and also in anhydrous concentrated films. It is possible to make dry films of PE015:NiBr2 but not of PEO6:NiBr2. Attempts to reproduce the remarkable enhancement of conductivity of rehydrated films of PEO8:NiBr2 observed by other workers were consistently unsuccessful.