Tungsten Oxide Electrochromic Windows with Lithium Polymer Electrolytes

Abstract

Electrochromic windows are built using WO3 as the electrochromic material and V2O5 as the counter electrode. Both are deposited onto ITO coated glass panes by vacuum evaporation and are amorphous to X-ray diffraction. The electrolytes are lithium conducting polymers constituted by diallyl amide by-product DaaRfSO3Li, (CH2=CH-CH2)2NCO-CF(CF3)SO3Li grafted in a cross-linked polyether NPC1000 (network poly(isobuthenyloligooxyethylene)), with a tLi+ close to unity, and poly(ethyleneglycol)-borate ester with LiCF3SO3, with a tLi+ close to 0.34. The electrochemical characterization of the electrodes is carried out by cyclic voltammetry, coulometric titration, and impedance spectroscopy, allowing the determination of the chemical diffusion coefficients of lithium into WO3 and V2O5. Potentiostatic cycling of the complete transmissive cells yield a transmission variation from 48% to 12% at ~630 nm with a response time of 10 seconds at room temperature. The main drawbacks are the residual coloration in the bleached state and the low exchange current densities at room temperature, which underline the need for good electrolyte/electrode contacts.