Amphiphilic polymer gel electrolytes. 3. Influence of the ionophobic–ionophilic balance on the ion conductive properties

Abstract

Gel electrolytes based on amphiphilic copolymethacrylates containing ethylene oxide (EO) n side chains as ionophilic units and fluorocarbon (CF 2) 6 side chains as ionophobic units showed a substantial increase in ion conductivity, as compared to corresponding gels based on homopolymers of (EO) n methacrylate. The highest ion conductivity was found for a gel based on a copolymer composed of 24 wt% of (EO) 9 methacrylate and 76 wt% of fluorocarbon methacrylate. This gel contained 70 wt% of 1 M LiPF 6 γ-butyrolactone, and had an ion conductivity of 4.0 mS/cm at 20°C, and 1.0 mS/cm at −20°C. It was found that gels based on fluorocarbon-containing copolymers having (EO) 9 side chains had higher ion conductivities than gels based on copolymers containing (EO) 2 side chains. The study showed that it is important to control the ionophobic–ionophilic balance of the amphiphilic copolymer in order to achieve high ion conductivities in the gel electrolyte. It is very likely that the ionophobic side chains of the copolymer associated to form microdomains in the electrolyte. It is also likely that longer EO chains increased the stability of these microdomains in the electrolyte. However, strong lithium ion coordination to EO chains above a critical chain length may have suppressed the ion conductivity of the gels.