Electrolytes based on LiClO4 and branched PEG–boronate ester polymers for electrochromics

Structure–property relationships of polymers incorporating Lewis acidic boron in their branched chain architecture were investigated as a function of LiClO4 salt concentration. The polymers were synthesised from boron trioxide, and different poly(ethylene glycol)s (PEGs) and monomethyl ether end-capped PEGs. The PEG–boronate ester polymers based on PEGs having molecular weights equal to, or below, 300 g/mol were found to be amorphous, colourless, and highly transparent. Electrolytes derived from these polymers and LiClO4 reached ionic conductivities up to 10−4 S cm−1 at room temperature, and were thermally stable up to at least 150 °C. A number of electrolytes showed a combination of attractive properties which made them interesting for use in electrochromic devices.