Single lithium-ion conducting poly(tetrafluorostyrene sulfonate) – polyether block copolymer electrolytes

Abstract

Solid single-ion conducting polymers continue to attract significant interest as electrolyte materials with great potential to improve safety and performance of energy storage devices. Still, their low conductivity is a significant hurdle presently preventing their application. Here, we report on highly conductive BAB triblock copolymers with A blocks of either poly(ethylene oxide) (PEO) or poly(ethylene oxide-co-propylene oxide) (PEOPO), and B blocks of poly(lithium 2,3,5,6-tetrafluorostyrene-4-sulfonate) (PPFSLi). The copolymers were readily synthesised by atom transfer radical polymerisation (ATRP) of 2,3,4,5,6-pentafluorostyrene from polyether macroinitiators, followed by quantitative thiolation using NaSH and subsequent oxidation to form the sulfonate anions. The copolymers possessed high thermal stability and their ionic content was conveniently controlled by the block ratio during the ATRP. Above the polyether melting point, PEO-based block copolymers with [O] : [Li] = [18] : [1] showed the highest conductivity, close to 1.4 × 10⁻⁵ S cm⁻¹ at 60 °C, while at lower temperatures, the PEOPO materials reached the highest conductivity, nearly 1.5 × 10⁻⁶ S cm⁻¹ at 20 °C. The high conductivity of the former copolymer suggests weak interactions of the lithium ions with the pentafluorosulfonate anions in combination with a high degree of Li⁺ dissociation facilitated by PEO. The results of the present study demonstrate that well-designed block copolymers containing lithium pentafluorostyrene sulfonate units can approach the levels of conductivity required for high-temperature lithium battery applications.