High coloration performance of electrochromic devices assembled with electrolytes based on a branched boronate ester polymer and lithium perchlorate salt

An amorphous, colorless, and highly transparent polymer containing boron atoms in its branched chain architecture was synthesised from boron trioxide, poly(ethylene glycol) (PEG) and methoxy end-capped PEG. Electrolytes based on this polymer and different concentrations of lithium perchlorate (LiClO4) were prepared and characterised by impedance spectroscopy, calorimetry and thermogravimetry. The results showed a thermal stability up to at least 150 °C, and conductivities reaching 10−4 S cm−1 at room temperature. The latter value was much lower than that measured for a reference liquid electrolyte based on propylene carbonate (PC) and LiClO4. The boron-containing electrolytes were further evaluated in electrochromic devices fabricated using transparent and electronically conducting SnO2/F electrodes and electrochromically active tungsten oxide (WO3) films. Their performance was compared with corresponding devices assembled using liquid and gel electrolytes based on LiClO4 salt and PC. The best coloration performance was observed for the electrochromic device incorporating the boron-containing electrolyte having a molar ratio of [B]/[ClO4] equal to 3:1. The value of the coloration efficiency was 59 cm2 C−1 for this device. Cyclic voltammetry further showed that the performance of this device was not affected by continuous cycling, and that 34 times less injected electric charge was needed in order to obtain the same coloration level compared to the device containing the liquid electrolyte (0.31 mC and 10.5 mC, respectively). In addition, improved durability and optical properties were observed for the former device when the coloration voltage was increased to 4 V, at which the device showed a very high contrast ratio of 7700