Polymer electrolytes comprising oligomeric lithium borate salts and poly(ethylene oxide)

Polymer electrolytes comprising lithium borate salts and high molecular weight poly(ethylene oxide) PEO are prepared by casting from solution. The branched structure of borate salts, obtained in a two-step synthesis process, includes three oligomeric oxyethylene segments of various lengths n and a butyl group. The weight proportions of polymer and oligomeric salts are chosen so that they represent certain molar proportions of EO units (coming both from anion and from polymer matrix) to lithium: 50:1, 32:1, 16:1, and 10:1. Samples of the electrolytes are investigated by differential scanning calorimetry (DSC) and impedance spectroscopy in a wide range of temperature. For each electrolyte, characteristic parameters such as glass transition temperature Tg are calculated. It is found, that some systems – for example, electrolytes composed of borate salt with average length n = 7.5 and molar ratio EO:Li of 32:1 and 50:1 exhibit values of the glass transition temperature lower than the values obtained for pure PEO and pure borate salt, which is also reflected in characteristic frequency of the dielectric relaxations related to segmental chain motion. The low Tg partially compensates decrease in conductivity caused by crystallization. Interestingly, mixed PEO-borate salt systems also exhibit transference numbers higher than those of pure borate salt.