Novel Alternating Comblike Copolymer Electrolytes with Single Lithium Ionic Conduction

Abstract

Poly[lithium-N-(4-sulfophenyl)maleimide-co-methoxyoligo(oxyethylene) methacrylate], P(LiS-MOEn), with three different oligo(oxyethylene) side chains (i.e., three different n values) have been synthesized. The copolymers are of a new type of comblike, nearly alternating copolymer electrolytes, showing single lithium ionic conductivity. All the three copolymers show a glass transition at about -50 °C (Tg1). In addition, copolymers with n = 7 and 12 also exhibit a second glass transition in the temperature range of 30-50 °C (Tg2), while the copolymer with n = 16 shows an endothermic peak near room temperature. Tg1 is assigned to the oligo(oxyethylene) side chain, while Tg2 is attributed to the main chain of the copolymer domain. The melting point (Tm) is attributed to the endothermic change of the partially crystalline phase formed by the long oligoether side chain. When the temperature is below 50 °C, P(LiSMOEn)s with larger n values have a higher ionic conductivity. When the temperature is above 50 °C, however, P(LiSMOEn)s with smaller n values show higher ionic conductivity because of their higher salt concentration. The maximum ionic conductivity at 30 °C is 1.5 × 10-7 S cm-1 for n = 16. The temperature dependence of ionic conductivity indicates that the Arrhenius behavior is not obeyed. The ionic conduction follows a special dual Vogel-Tamman-Fulcher (VTF) behavior.