Sb–Cu–Li electrochromic mirrors

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Abstract

Switchable mirrors offer significant advantages over traditional electrochromic devices for control of incident light in architectural and aerospace applications due to their large dynamic ranges in both transmission and reflection in the visible and near infrared regimes. Here we describe construction and spectroscopic characterization of a complete electrochromic mirror device consisting of an antimony–copper alloy (40 at% Cu) active electrode coupled with an optically passive vanadium oxide counter electrode and a crosslinked polymer gel electrolyte. Transmittance and reflectance spectra in the visible–near IR (300–2500 nm) in both mirror and transparent states are reported. The photopic transmittance of the complete device varied from less than 3% to more than 20% during cycling, requiring about 40 min for complete switching in each direction. At the same time, the photopic reflectance varied from 40% to 25%. The crosslinked polymer improves the stability of the mirror electrode relative to that in a liquid electrolyte.

Introduction

Switchable mirrors based on the interconversion of metallic antimony or bismuth and semiconducting phases via lithiation and delithiation of the host materials in a non-aqueous electrolyte were recently reported [1]. Potential applications include solar heat control for buildings, motor vehicles and airplanes, data switching, and display technologies. The charge required to switch an antimony mirror electrode, its electronic conductivity in the transparent state, and its optical properties were improved by the addition of copper or silver [2]. Because the commonly employed transparent conducting oxide coatings are unstable at the switching potentials, mirror electrodes prepared to date have relied upon the intrinsic conductivity of the active material. Volume expansion and contraction during cycling contribute to loss of connectivity within the electrode and eventually produce isolated regions that switch slowly or not at all. One approach to mitigate this effect is to use a rigid electrolyte, which tends to confine the volume changes to the electrode plane. Crosslinked polymer gel electrolytes have been shown to be compatible with electrochromic materials [3]. Here we describe construction of complete self-contained devices with superior cycling stability incorporating a vanadium oxide counter electrode and UV crosslinked gel electrolyte. The optical properties of the mirror electrode have also been measured over a wider spectral range than that previously reported.

Section snippets

Electrode preparation and characterization

Sb–Cu alloy films 40 nm in thickness were deposited on 25 mm×37 mm×1 mm plain glass substrates by DC magnetron co-sputtering from separate 50 mm diameter targets angled 26° from normal, with target to substrate distances of 90 mm. The Sb:Cu mole ratio in the films was controlled at 3:2 by adjusting the sputtering power to the guns as determined from previous work [2]. Film thicknesses were measured by stylus profilometry (Dektak). Vanadium oxide films were prepared by spin coating 25 mm×37 mm×2.2 mm SnO2

Electrolyte properties

The conductivity of the 1 M LiPF6 in PC/EMC (1/1) electrolyte is 6.8 mS/cm [4]. Addition of polyethylene glycol diacrylate decreased the conductivity one order of magnitude to 0.673 mS/cm due to an increase in viscosity. Crosslinking, which significantly improved the mechanical properties of the electrolyte, reduced the conductivity only slightly, to 0.666 mS/cm (Fig. 1). The elastic modulus (E′) of the crosslinked gel (Fig. 2) was 1.1×106 dyn/cm2, with minimal frequency dependence. The loss modulus

Conclusions

A UV crosslinked gel polymer electrolyte was used to fabricate a self-contained switchable mirror based on transport of lithium ions between a sputtered Sb–Cu alloy mirror electrode and a sol–gel LixV2O5 counter electrode. The polymer provided excellent rigidity, adhesion, transparency, and conductivity. The stability of the mirror electrode was improved by the presence of the rigid polymer gel electrolyte. Further improvements in both stability and switching speed may be realized through the

Acknowledgements

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Building Technology, State and Community Programs, Office of Building Research and Standards of the US Department of Energy under Contract No. DE-AC03-76SF00098.

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