Phosphonium ionic liquids as greener electrolytes for poly(vinyl chloride)-based ionic conducting polymers

Abstract

Ionic liquid based ion-conducting polymers have been prepared and characterized by loading poly(vinyl chloride) (PVC) with one of two phosphonium-based ionic liquids (PhILs) (trihexyl(tetradecyl) phosphonium bis(trifluoromethylsulfonyl)imide, [P_14,6,6,6][Tf₂N] and trihexyl(tetradecyl) phosphonium chloride, [P_14,6,6,6][Cl]) and a commonly used PVC plasticizer (di-isononyl phthalate, DINP). Different proportions of each charged (PhILs) and non-charged (DINP) additive were used to evaluate the influence of PhIL ionicity on the ionic conductivity of the PVC-based electrolyte and to study the effect of the thermomechanical properties of PVC on the diffusivity of ionic charges in between PVC molecular chains, and consequently on the electrochemical properties of the polymer. Films were characterized for their chemical, morphological, thermomechanical and electrical properties. The results show that both PhIL ionicity and PhIL–PVC compatibility play a major role in decreasing the electrical resistivity of PVC films. The lowest film resistivity (0.4 kΩ cm), corresponding to an estimated electrical conductivity of ∼2.4 μS cm⁻¹, was observed for PVC films loaded with the highest tested amount of [P_14,6,6,6][Tf₂N] (45 wt% of PhIL at fixed DINP composition, 9 wt%). These films were also stable at temperatures up to 200 °C without using any further PVC thermal stabilizer. The polymer electrolytes presented in this work may be used as platforms to produce soft, safer and cost-effective ion-conducting materials by using non-volatile and electrochemically stable PhILs as liquid electrolytes incorporated into a cheap, stable and versatile polymer such as PVC.